Advances in molecular mechanisms and therapeutic strategies for central nervous system diseases based on gut microbiota imbalance

Graphical abstract

Highlights

• •The unbalanced GM homeostasis has a reciprocal causation with the development of CNS diseases.
• •GM and it’s metabolites participate the regulation of CNS diseases via affecting the neuroinflammation level.
• •GM metabolites are essential for synergistic bidirectional communication between ENS and CNS.
• •The therapeutic strategies based on GM regulation are the good choices of the recovery of CNS diseases.

Abstract

Backgroud

Central nervous system (CNS) diseases pose a serious threat to human health, but the regulatory mechanisms and therapeutic strategies of CNS diseases need to be further explored. It has been demonstrated that the gut microbiota (GM) is closely related to CNS disease. GM structure disorders, abnormal microbial metabolites, intestinal barrier destruction and elevated inflammation exist in patients with CNS diseases and promote the development of CNS diseases. More importantly, GM remodeling alleviates CNS pathology to some extent.

Aim of review

Here, we have summarized the regulatory mechanism of the GM in CNS diseases and the potential treatment strategies for CNS repair based on GM regulation, aiming to provide safer and more effective strategies for CNS repair from the perspective of GM regulation.

Key scientific concepts of review

The abundance and composition of GM is closely associated with the CNS diseases. On the basis of in-depth analysis of GM changes in mice with CNS disease, as well as the changes in its metabolites, therapeutic strategies, such as probiotics, prebiotics, and FMT, may be used to regulate GM balance and affect its microbial metabolites, thereby promoting the recovery of CNS diseases.

Introduction

The human diseases research have always been conducted at the three levels of organism, cell and molecule. As we known that, the body is composed of cells, microbes and their metabolites, which seems to be an ecosystem. Thus, it needs pay more attention on the influence of microorganisms and metabolite production on the occurrence of human diseases. With the rapid development of sequencing technology and omics, it has been found that the gut microbiota (GM) has a diverse and complex relationship with central nervous system (CNS) diseases. CNS diseases, consisting of Parkinson's disease (PD), Alzheimer's disease (AD), Major depressive disorder (MDD), Autism spectrum disorder (ASD), Multiple sclerosis (MS) and Spinal cord injury (SCI), are major diseases that seriously affect human health. GM structural disorders, metabolite abnormalities, intestinal barrier disruption and upregulation of inflammatory factors exist in patients with CNS diseases, which promotes the development of CNS diseases (Table 1, Table 2). More importantly, GM remodeling, such as intermittent fasting (IF), probiotics, prebiotics, and fecal microbiota transplantation (FMT), can alleviate the development of CNS diseases to some extent (Fig. 1). The metabolites of GM regulate the development of CNS diseases through the intestinal barrier, inflammation, and other brain substances (Fig. 2). Current review summarizes the regulatory mechanism of GM on CNS diseases, and the therapeutic strategy of CNS diseases based on GM regulation (Fig. 3, Fig. 4), aiming to provide a new idea for the treatment of CNS diseases.

Table 1.

Altered gut microbiota in CNS patients and mice.

Diseases	Increased/Enriched GM	Decreased GM	Mechanisms	References
AD	Bacteroidetes, Escherichia/Shigella	Firmicutes, Eubacterium rectale, Bacteroides fragilis	Promoting plaque deposits in the cerebral cortex; Decreasing GABA, taurine, valine and BDNF; Increasing pro-inflammatory cytokines.	[78], [79], [80], [83], [84], [85]
PD	Enterobacteriaceae	Prevotellaceae, Lachnospiraceae	(1) Increasing intestinal permeability and inflammation; (2) Promoting abnormal aggregation of α-syn; (3) Promoting the TLR4/TNF-α pathway; (4) Upregulating IL-1β, CCL5 and CRP.	[96], [97], [98], [99], [100], [104], [105], [106]
MDD	Bacteroidetes, Actinobacteria, Proteobacteria	Firmicutes, Bifidobacterium, Lactobacillus	(1) Modulating inflammation-related pathways; (2) Increasing CCR2, IL-22ra2 and LPS; (3) Activating NF-κB pathway; (4) Damaging the tight junctions.	[49], [110], [111], [112], [113], [115], [116], [117]
ASD	Bacteroidetes, Desulfovibrio, Lactobacillus, Clostridium, Caloramator Sarcina, Clostridium clusters I and Clostridium clusters II	Firmicute, Bifidobacterium	(1) Increasing symptoms of enteritis, constipation, diarrhea and neuronal loss.	[123], [125]
MS	Proteobacteria, Methanobrevibacter, Akkermansia, Desulfovibrionacea	Bacteroidetes, Clostridium, Fecalibacterium, Prevotella	Increasing the sulfur metabolism, long-chain fatty acid content and LPS; Leading to intestinal leakage and a systemic immune response.	[132], [133], [134], [137]
SCI	Shigella Rikenell, Bacteroidesa, Staphylococcus, Mucispirillum	Lactobacillu, Allobaculum, Sutterella	Aggravating systemic inflammation; Destroying the intestinal barrier.	[142], [143]

Table 2.

The function of gut microbiota.

Sequence	Function	Gut microbiota	References
1	Positively correlating with Pro-inflammatory cytokines	Salmonella typhimurium, Clostridium difficile, Citrobacter rodentium, Escherichia coli, Legionella pneumophilia, Salmonella typhimurium, Vibrio cholerae, Shigella, Bacteroides, Rikenella, Staphylococcus and Mucispirillum	[8], [9], [55], [142]
2	Positively correlating with Anti-inflammatory cytokines	Dorea, Blautia, Sutterella, Bacteroides fragilis, Clostridium, Coprococcus, Lactobacillus rhamnosus, Pseudobutyrivibrio, Butyrivibrio and Allobaculum	[8], [9], [75], [98], [142], [152]
3	Promoting intestinal permeability	Bacteroides	[143]
4	Increasing intestinal barrier integrity	Prevotellaceae, Blautia, Coprococcus, Pseudobutyrivibrio, Butyrivibrio	[98], [99]
5	Producing short-chain fatty acids	Bacteroidetes, Firmicutes, Blautia, Coprococcus, Pseudobutyrivibrio, Butyrivibrio	[28], [30], [98], [144]
6	Producing Lipopolysaccharide	Burkholderiaceae, Staphylococcaceae, Porphyromonas gingivalis, Propionibacterium acnes, Bacteroides fragilis	[48]
7	Producing γ-aminobutyric acid	Bifidobacterium, Enterococcus, Lactobacillus, Bacteroides, Escherichia, Parabacteroides, Bifidobacterium dentium	[66], [67], [73]

Fig. 1.

The molecular mechanisms and therapeutic strategies for central nervous system diseases based on gut microbiota imbalance. CNS diseases leads to the dysbiosis of GM. Compared to the normal control, the GM dysbiosis produces more pro-inflammatory products and disrupts the microbial metabolites, such as LPS, SCFAs, and GABA. The disordered GM and metabolites not only stimulate the host to produce inflammatory factors, but also disrupt the intestinal permeability, and then lead to intestinal leakage, which affect the physiological and biochemical reactions of host through MGB axis. Lastly, the disordered GM damages the nerve cells in brain and spinal cord, and promotes the development of CNS diseases, such as AD, PD, or SCI. On the basis of in-depth analysis of GM changes with CNS disease, as well as the changes in its metabolites, therapeutic strategies, such as probiotics, prebiotics, and FMT, may be used to regulate GM balance and affect its microbial metabolites, thereby promoting the recovery of CNS diseases. In summary, there is a close relationship between CNS diseases and GM, and GM remodeling may be the effective strategy for CNS repair.

Fig. 2.

Fig. The repair mechanism of GM metabolites on CNS diseases.

Fig. 3.

The regulatory role of GM on development of CNS diseases.

Fig. 4.

The therapeutic strategies of CNS diseases based on GM regulation.

GM and CNS

GM

The gut is the largest digestive organ in the organism, which digests and absorbs nutrients, and subsequently provides the energy for survival and growth. During this process, the cells can interact with microorganisms in the gut and maintain the steady state of the intestinal microenvironment with the normal distribution and metabolism of intestinal microorganisms [1], [2]. There are trillions of microorganisms in the gut, including fungi, bacteria, viruses and bacteriophages. GM mainly refers to intestinal bacteria and includes some fungi. The GM keeps the health of host through regulating the metabolic level, integrity of epithelial cells, and function of immune system [3]. A large amount of bacteria exist on the skin, respiratory tract, oral cavity, digestive tract and gastrointestinal (GI) tract in the body, can in symbiosis with the host. The GM consists of more than 100 trillion microorganisms that mainly colonize in the colon (10¹¹-10¹² cells/ml). The GM is estimated to be made up of at least 1000 different known bacterial species and encodes more than 3 million genes, which is more than the human genome 100 times [4]. Additionally, there are four major phyla in the GM: Bacteroidetes, Firmicutes, Proteobacteria and Actinobacteria, and two secondary phyla: Verrucomicrobia and Fusobacteria. The diversity and abundance of GM is involved in maintaining the normal physiological status of host.

The GM is recognized as a metabolic organ that maintains homeostasis by regulating nutrition, the immune response, and systemic inflammation [5]. In vivo, GM not only regulates nutrient metabolism and vitamin synthesis, but also involves in the breakdown of drugs and other exogenous substances. Moreover, GM produces a series of metabolites and small molecules to regulate the process of cellular life. It is well known that GM is essential for the production of short chain fatty acids (SCFAs) after the digestion of dietary fiber and excreting intestinal gases, as well as the release of lactic acid and alcohols [5]. In CNS, SCFAs not only provide an energy source for cells, but also act as the signaling molecules to promote microglial maturation [6] Additionally, GM can regulate the balance between pro-inflammatory cytokines and anti-inflammatory factors by mediating the immune system [7].

GM homeostasis in CNS

The CNS is divided into the brain and spinal cord. CNS diseases generally refer to the phenomenon of neuronal death, axonal injury and demyelination of brain or spinal cord caused by mechanical stress, hyperglycemia, inflammation or hemorrhage. Common CNS diseases include AD, PD, MDD, ASD, MS and SCI. With the global population entering the aging process, the prevalence rate of CNS diseases shows a rapidly increasing trend and brings an enormous burden to the country. Thus, it is of great scientific significance to reveal the regulatory mechanism and therapeutic strategy for CNS diseases. It is well known that excessive intracellular stress, inflammation, autophagy and apoptosis are the key factors causing the difficult repair of CNS injury, especially the inflammatory microenvironment. After injury, the infiltration of neutrophils, glial cells and macrophages promote the release of inflammatory factors and trigger a serious inflammatory storms. The inflammatory response occurs throughout the whole process of CNS repair. Inflammatory factors promote the degeneration and necrosis of neurons and glial cells in injured areas, and axon atrophy due to Wallerian degeneration. Although peripheral inflammatory cells will form the first line of defense after injury, microglia are also activated and release series of inflammatory factors, such as tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), thereby amplifying the sustained inflammatory response in the injured center and surrounding uninjured areas, which results in the sustained cell death and extensive demyelination. Therefore, the intense inflammatory response in the microenvironment is a critical factor for the difficulty of nerve repair after CNS injury.

More interestingly, GM homeostasis participates in the regulation of CNS repair by affecting the balance of inflammatory cytokines (Fig. 1). Inflammation is closely related to the changes of GM abundance and diversity. For example, an increase in Clostridium and decreases in Sutterella, Dorea, and Blautia all enhance the elevated IL-6, IL-1, IL-17α, and interferon-γ (IFN-γ) levels, which circulate to the brain [8]. Bacteroides fragilis, as well as some members of Clostridium species have an anti-inflammatory effects via promoting the releases of anti-inflammatory cytokines (such as IL-10 and IL-13), while Salmonella typhimurium and Clostridium difficile induce the releases of pro-inflammatory cytokines [9].

The GM is called “enterocerebral organ” and “adult's second brain”, which participates in the regulation of autoimmune diseases, metabolic diseases and nervous system diseases [10]. The GM, enteric nervous system (ENS) and brain have a close relationship and constitute a system named the microbiota-gut-brain (MGB) axis [11]. VN is the longest and most widely distributed autonomic nerve, which originates from the brainstem and extends through the neck, lastly enters the chest and abdomen. As a parasympathetic nerve, it contains afferent fibers, of which 80 % are responsible for transmitting taste, visceral, and somatic sensations; and efferent fibers, of which 20 % are involved in the regulation of gastrointestinal, cardiac, and pulmonary functions [12]. The brain can connect with the intestinal tract through VN, immune system, metabolic processes of intestinal microorganisms and biochemical signals (such as hormones) [13]. On the one hand, the changes of intestinal microbial components directly affect the intestinal permeability and intestinal barrier function, and subsequently regulate GI epithelial cells and intestinal nervous system [14]. On the other hand, GM-mediated imbalance of MGB not only affects neurotransmitter receptors and neurotrophic factors [15], [16], but also destroys blood brain barrier (BBB) integrity and consequently results in excessive activation of microglial cells and destruction of dopaminergic neurons (DAn) [17]. In turn, the brain also influences the abundance and function of GM, and then regulates the intestinal functions, such as GI motility, digestive secretion and intestinal mucosal immune response [18]. It has been shown that sterile mice implanted with GM from depression exhibit the obvious depression-like behavior; moreover, the GM structure of sterile mice is highly similar to that of depression patients, indicating that depression-like behavior will be transmitted along with GM [19]. SCI mice also exhibit GM imbalance, intestinal barrier destruction, permeability change and SCFAs content decrease, which are reversed by FMT remodelling [20]. More importantly, the metabolites of GM, such as SCFAs and lipopolysaccharide (LPS), are essential for synergistic bidirectional communication between ENS and CNS, thereby directly or indirectly regulating CNS diseases (FigS. 1 and 2).

Additionally, it has been reported that bacterial extracellular vesicles (BEVs) is another mechanism for GM regulating the homeostasis of host. BEVs is the typically 20–300 nm diameter of vesicles, which is composed of lipid bilayer with multiple proteins and nucleic acids [21], [22]. They are the natural carriers of bacterial molecules, including LPS, peptidoglycans, lipids, proteins, nucleic acids, as well as pathogens [23]. They act as a transmission mechanism for allowing the long-distance delivery of bacterial active compounds [24]. BEVs have dual functions in regulating homeostasis of host. On the one hand, BEVs cause the neuroinflammation and damage neuronal function [25], [26]. On the other hand, BEVs are also beneficial to the pathophysiological regulation of host with promoting the intestinal barrier integrity. It has been demonstrated that the mice are orally administered BEVs deriving from Akkermansia muciniphila and exhibit the restored intestinal barrier, reduced recuitment of immune cells, and improved metabolic disorders after high-fat diet (HFD) treatment [27].

The role of GM metabolites in CNS

SCFAS

SCFAs are important metabolites produced by anaerobic fermentation of polysaccharides in the colon [28]. SCFAs are mainly made of acetic acid (40–60 %), butyric acid (15–20 %) and propionic acid (20–25 %). It has been shown that dietary fiber intake increases the abundances of Lachnospira, Akkermansia, Bifidobacterium, Lactobacillus, Faecalibacterium, and Dorea, which are bacteria that produce SCFAs [29]. Specifically, Bacteroidetes is responsible for the production of propionic acid and acetic acid, while Firmicutes produces large amounts of butyric acid [28], [30]. SCFAs are mainly absorbed by colonic cells through H⁺ dependent or Na⁺ dependent monocarboxylic acid transporters [31]. Then, SCFAs can bind with G protein-coupled receptors of colonic L-cells and small intestine L-cells in the gastrointestinal mucosa, immune system, and nervous system, subsequently mediating downstream signaling pathways [32]. In addition to mediate the gut against inflammatory factors, SCFAs contribute to maintain the intestinal barrier integrity by increasing tight junction (TJ) proteins [33]. More importantly, SCFAs also regulate the synthesis and secretion of neurotrophic factors, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), which affect mitochondrial function, immune response, and lipid metabolism in CNS. Neurotrophic factors, such as BDNF, play important roles in maintaining neuronal survival and synaptic function [34]. Meanwhile, SCFAs are agonists of aromatic hydrocarbon receptor (AHR) in brain and acts as ligand activated transcription factors to directly regulate microglia and astrocytes [35].

Thus, SCFAs have a close relationship with the neural function of brain (Fig. 1, Fig. 2). It has been reported that microbiogenic SCFAs are the key mediator of MGB, which regulates the denaturation and aggregation of amyloid in brain and further promotes amyloid β (Aβ) deposition by modulating the microglial phenotype [36]. In AD mice, butyrate treatment not only enhances the expression of learning- and memory-related genes by enhancing histone acetylation in the hippocampus, but also relieves the neuroinflammation, promotes neurotrophin secretion and ultimately improves memory function [37]. In 5 × FAD mice, butyrate injection significantly reduces the accumulation of Aβ in the brain [38]. A low content of SCFAs, especially butyrate, is significantly associated with decreased cognitive function in PD patients [39]. In addition, butyrate not only stabilizes hypoxia-inducible factor 1, a transcription factor, to coordinate barrier protection, but also activates genes expression encoding tight junction (TJ) components and proteins to enhance the barrier integrity [40], [41]. The decrease in SCFAs-producing bacteria adversely affects PD patients, including intestinal leakage, colonic inflammation, and α-synuclein (α-syn) deposition in the GI, and finally induces the activation of microglia and cognitive impairment in the brain [42], [43]. Vancomycin treatment results in GM disorders in type 1 diabetes (T1D) mice and then triggers a deficiency in acetic acid, decrease in synaptophysin, and impaired learning and memory in the hippocampus. Exogenous supplementation with acetate or FMT could alleviate T1D-associated cognitive decline [44]. Mechanism investigation demonstrates that acetic acid-associated G-coupled protein receptors (GPR41/GPR43) activation not only regulates the production of leptin and intestinal hormones, thereby improving energy and glucose homeostasis, but also inhibits ERK/JNK/NF-κB signal, thereby reducing neuroinflammation and improving cognition [45], [46]. In addition to formic acid, butyric acid and acetic acid, propionic acid also plays an important role in CNS diseases. The amount of propionic acid in the stool and blood of MS patients is significantly reduced, while the amount of butyric acid and acetic acid have not significant difference. Meanwhile, the abundance of bacteria that produce formic acid is decreased in the MS group [47]. Normalizing the morphology and function of regulatory T cells by supplementation with propionic acid in MS patients alleviates neurodegenerative diseases [47]. Therefore, maintaining SCFAs homeostasis has a important significance for neurological function.

LPS

As a component of the outer membrane of gram-negative bacteria, LPS can induce the immune responses in mammalian cells. The GM family, including Burkholderiaceae, Staphylococcaceae, Propionibacterium acnes, Porphyromonas gingivalis and Bacteroides fragilis, can produce LPS [48]. LPS penetrates the BBB, activates the specific pattern recognition receptors of innate immune system, and ultimately exerts adverse effects on nerve development and repair of brain [49]. It has been found that compared to healthy subjects, there are significantly increased gram-negative bacteria in the gut of ASD patients [50]. The gram-negative bacteria mediated-LPS can be absorbed into the bloodstream through damaged intestinal walls and activate Toll-like receptors (TLRs) in ENS and CNS [51], [52]. The TLRs family consists of 11 members that located on cell surface (TLR 1, TLR 2, TLR 4, TLR 5, TLR 6, and TLR 11) and lysosomes (TLR 3, TLR 7, TLR 8, and TLR 9). TLRs will respond to different antigens and further activate the MAP kinases and NF-κB, causing serious inflammation [53]. LPS or peptidoglycans are also reported to enter the bloodstream from the gut, and increase the levels of IgM and IgA, and then bind with TLR-2/4 complexes and cause a hyperimmune response, which increases pro-inflammatory cytokines and triggers the oxidative stress [54]. Gram-negative bacteria also activate the NF-κB, and promote IL-1β and IL-18 release through the atypical NOD-like receptor thermal protein domain associated protein 3 inflammasome pathway [55]. In addition, as a neurotoxin, LPS can cleave intercellular junction proteins, disrupt cellular barriers, and induce “intestinal leakage” [56]. The defective gut barrier leads to systemic exposure to LPS, which in turn induces the intestinal inflammation, microglial activation and neuroinflammation. LPS has been reported to bind with TLRs on microglia, and activate NF-κB, ultimately leading to neurological dysfunction [57].

It has been reported that the plasma LPS level in AD patients is three times higher than that in healthy controls, especially in gray matter, neocortical and hippocampal regions of AD brain [57], [58]. LPS-injected mice show serious deposition of Aβ protein and severe cognitive dysfunction [59]. Moreover, elevated LPS also leads to the increased malondialdehyde, nitrite and nitrate in the brain while decreasing glutathione levels, which is a common phenomenon in many mental disorders [60], [61]. In elderly individuals, plasma LPS and its related pro-inflammatory cytokine levels are positively correlated with the risk of Aβ protein degeneration [62]. The endotoxin hypothesis of PD also suggests that LPS is involved in the pathogenesis of PD. The elevated LPS level in the gut of rotenone-induced PD mice has been reported to mediate chronic inflammation through TLR4-MyD88 pathway [63]. The communication between LPS and cytokines leads to the neuroinflammation and spreading of α-syn, which exacerbates the neurodegeneration of PD [64]. In conclusion, LPS may be a crucial factor that links intestinal microbiota with CNS pathologies.

γ-Aminobutyric acid (GABA)

GABA, also known as aminobutyric acid, is a nonprotein amino acid found in prokaryotes and eukaryotes. As the major inhibitory neurotransmitter of neuroendocrine signaling molecules, GABA has the highest concentration in brain and plays an important role in host-microbiota communication [65]. The biological mechanisms of GABA production include putrescine degradation, glutamate decarboxylation, and arginine or ornithine degradation [66]. GM, such as Bifidobacterium, Enterococcus and Lactobacillus, are known to contain glutamate decarboxylase genes that convert L-monosodium L-glutamate into GABA in the GI tract, and ultimately inhibit neuronal activity [67]. Bacteroides, Escherichia and Parabacteroides are involved in the metabolism of GABA [66].

GABA is a key intermediate medium for GM regulating the physiological and biochemical responses of host. GABA is reported to regulate CNS function; in turn, the CNS regulates the sympathetic and parasympathetic branches or the hypothalamus–pituitary–adrenal (HPA) axis, thereby affecting the gastrointestinal or ENS status, and the colonization or metabolism of GM [68]. This explains the crosstalk between the microbiota and host in neuropsychiatric diseases. At 5 months of age, compared with infants with low risk of ASD, the infants with high risk of ASD have fewer Bifidobacterium, and more Clostridium and Klebsiella, which results in significantly lower GABA abundance in stool and elevated neuroimmune response [69]. In addition, the relative abundance of Bacteroides that produce GABA is negatively correlated with depression features of brain [66]. Series of clinical studies have demonstrated that there is a clear correlation between the decrease of GABA and the occurrence of AD [70]. And the increase in Blautia-dependent arginine metabolism GABA level in the intestine is associated with decrease of AD risk [71]. Moreover, as an essential growth factor, GABA induces the growth of GM, thereby regulating the GM structure under stress condition [66], [72]. Therefore, GABA may exert an important role in CNS diseases.

More interestingly, there is Bifidobacterium dentium-derived GABA that is only present in blood but not in the brain. It is known that Bifidobacterium dentium has an enzymatic mechanism for producing GABA from glutamate, glutamine and succinic acid. Germ-free mice transplanted with Bifidobacterium dentium show a significantly increased concentration of GABA in stool but not in the brain [73]. The VN is the main pathways into the CNS via MGB axis [74]. GABA has also been reported to modulate the CNS by VN and ENS. Lactobacillus rhamnosus (JB-1), a probiotic with anti-inflammatory properties, modulates the expression of central GABA receptor and then affects the emotional behavior of mice via VN [75]. Treatment with JB-1 strain for 4 weeks significantly increases GABA level in CNS of BALB/c mice and activates the GABA signaling pathway through VN [66], [76]. Additionally, the expression of GABA_B1b mRNA in the hippocampus is increased in mice fed with JB-1 strain, and GABA _Aα2 mRNA is decreased in the prefrontal cortex and amygdala, but there isn’t any neurochemical and depression related behavioral effect in vagectomy mice [75].

The regulatory role of GM on development of CNS diseases

AD

AD, an irreversible neurodegenerative disease, is characterized by Aβ protein deposition and Tau protein hyperphosphorylation [77]. AD patients exist the immoderate inflammatory response and poor neurogenesis in the hippocampus, which leads to progressive memory and cognitive impairment [77]. At present, there are a series of systematic studies on the pathogenesis of AD, but the therapeutic strategy for AD is still unclear. It has been reported that compared to normal healthy people, the GM of AD patients has dramatic changes with increases in Escherichia/Shigella and decreases in Eubacterium rectale and Bacteroides fragilis [78]. Moreover, compared with age-matched wild-type mice, AD mouse models, such as APP/PS1 mice, have reduced the Firmicutes level and increased Bacteroidetes level in the gut [79], [80]. However, 5 × FAD mice exhibit the increased abundance of Firmicutes and decreased abundance of Bacteroidetes [81], [82]. In conclusion, there are certain differences or even contradictions in GM changes in AD mice due to different animal models or different stages of AD pathogenesis.

GM changes may be a key causative factor for the development of AD. Administration of Bacteroides fragilis has been reported to trigger serious plaque deposits in the cerebral cortex of normal mice [83]. Additionally, C57BL/6 mice transplanted with the fecal microbiota from AD patients exhibit the reduced contents of GABA, taurine and valine in stool and subsequently promote the cognitive decline [84]. Moreover, the normal C57BL/6 mice transplanted with the fecal microbiota from 5 × FAD mice also exist the decreased BDNF level in hippocampus, and increased pro-inflammatory cytokines in colon and plasma, which promotes the memory impairment [85]. Thus, transplanting fecal microbiota from healthy donors into AD patients may be a potential strategy to alleviate the pathological process of AD patients. FMT has been reported to decrease the Aβ protein accumulation and Tau protein phosphorylation by modulating intestinal and systemic immune responses in AD mice [86]. The BBB is a dynamic structure that restricts the access of harmful substances from blood vessel into the brain [87]. Magnetic resonance imaging of AD patients and autopsy studies have demonstrated that BBB damage is significantly associated with learning and memory deficits [88]. GM disorder has been reported to promote the increases in inflammatory factors and amyloid protein, which enters the BBB, and promotes neuroinflammation and Aβ plaque formation in brain [89]. More interestingly, pathogenic bacteria will produce the amyloid protein “curli”, which forms biofilms and displays similarities with Aβ protein in the host [90]. The functional similarity of these proteins may promote the progression of neurological diseases through signaling cascades from gut to the brain [90]. The mechanism research further demonstrates that VN maybe an important way for bidirectional communication and mutual interference between brain and intestine. Blocking VN is reported to attenuate the C/EBPβ/δ-secreted enzyme signaling in the gut of AD patients and 3 × Tg mice, and then inhibit Aβ level and Tau pathology, and restore the learning and memory, suggesting that VN is a transport channel for protein aggregates from GI to the brain [91].

PD

PD is another common chronic neurodegenerative disease with an incidence rate second only to AD, which seriously affects the health and life quality of middle-aged people. The main pathological features of PD are the formation of α-syn and Lewy neurites, and the progressive loss of DAn in substantia nigra compact. PD patients have the following symptoms: 1) Dyskinesia, such as tremor, stiffness, and postural instability; 2) Nondyskinesia, such as rapid eye movement sleep disorder, anosmia, cognitive impairment and autonomic neuropathy; and 3) Autonomic nervous system and ENS disorders, such as hyperhidrosis, dysuria and postural hypotension [92]. In addition to genetic factors, disorders of protein clearance, apoptosis, mitochondrial dysfunction and neuroinflammation, GM disorders also have a close relationship with the development of PD [93], [94], [95]. The increased abundance of Enterobacteriaceae in intestines is positively correlated with the unstable posture of PD patients during exercise [96]. Moreover, there are significant decreases of beneficial flora, such as Lachnospiraceae and Prevotellaceae, in the stool of PD patients [97], [98]. Prevotellaceae is involved in the regulation of mucin expression and plays a key role in maintaining intestinal permeability [99]. Lachnospiraceae, such as Blautia, Pseudobutyrivibrio, Coprococcus, and Butyrivibrio, can promote the production of SCFAs, exert anti-inflammatory effect and relieve “intestinal leakage” in PD patients [98]. More importantly, GM dysbiosis in PD patients has been reported to increase intestinal permeability and intestinal inflammation and result in intestinal plexus exposure to LPS, which promotes abnormal aggregation of α-syn and PD development [100]. Therefore, it is worth further studying the relationship between GM disorders and PD development.

GM is reported to regulate the pathological and physiological development of PD through two pathways: the neural pathway and humoral pathway. Braak and his colleagues had suggested that PD pathology might begin in the digestive tract and spread from the gut to the brain via VN. Then, Braak's conclusion is confirmed by researchers who injected preformed fibrils (PFF) into the duodenum and pyloric muscle layer of mice to simulate the spread of pathological α-syn, and then excised VN to explore whether it was transmitted via neural pathways [101]. Humoral pathways are much more complex. CNS disease is associated with intestinal microecological barrier accompanied with abnormal immune responses. Abnormal immune responses not only increase intestinal inflammation, but also induce microglial activation and neuroinflammation, which are the central events in the pathogenesis of PD. It has been reported that GM-mediated inflammatory metabolites can bind with α-syn and transmit it into the brain through the VN [102]. More importantly, compared with wild-type animals, LPS exposure induces more pronounced symptoms of DAn loss, neuroinflammation, protein aggregation, and Lewy body formation in the striatum nigra neurons of transgenic mice that overexpress human A53T mutations [103]. These phenomena have also been proved in PD patients [103]. Moreover, elevated α-syn also induces microglial activation, which in turn enhances α-syn aggregation [102]. Mechanistic investigation has demonstrated that the GM of normal healthy mice significantly improves the motor dysfunction of PD mice by modulating the TLR4/TNF-α pathways in the colon and brain [104]. The levels of inflammatory molecules, such as IL-1β, CCL5 and CRP, are significantly increased in brain and cerebrospinal fluid, which is the most common disorder in the peripheral blood of PD patients [105], [106].

MDD

MDD, a psychiatric disorder, is characterized by depression, impaired cognitive function, and sleep or appetite disorders [107]. As a common high-incidence disease, MDD seriously affects the health status of patients and imposes a great burden on families and society. Epidemiological findings show that approximately 350 million people are affected by different degrees of depression [108]. Regarding the pathophysiological mechanism of depression, the monoamine hypothesis has been proposed with decreased concentrations of monoamine substances in synaptic gaps under depressed conditions, such as norepinephrine (NE), 5-hydroxytryptamine (5-HT) and dopamine [109]. This is one of the most common pathophysiological hypotheses of depression. Therefore, the therapeutic strategy for MDD mainly focuses on the regulation of monoamines level.

Recently, GM disorders have been found to be involved in the development of MDD pathologies. MDD patients exhibit the increases in Bacteroidetes, Actinobacteria and Proteobacteria, and decreases in Bifidobacterium, Firmicutes and Lactobacillus [110], [111]. There are similar results in MDD animal models [112]. Moreover, there is also significant difference in GM mediated-serum metabolites in the 60 MDD patients when comparing with 60 healthy controls, and 14 out of 17 different genera belonged to Firmicutes [113]. The functional predictive analysis further revealed that these differences are related with the inflammation-related pathways, suggesting that GM may be involved in the development of depression by modulating the inflammatory response of host [113].

GI inflammation has been reported to trigger the anxiety-like behavior and exert an adverse effect on CNS function [114]. Pro-inflammatory factors, such as CCR2, IL-22ra2 and NF-κB, could promote the development of pathophysiology of inflammatory bowel syndrome, thereby triggering systemic inflammatory responses and anxiety-like behaviors [49]. Moreover, there is significant increase of plasma LPS level in MDD patients when compared with that in healthy controls [115], [116]. LPS is believed to damage the tight junctions, increase BBB permeability, and ultimately promote the neuroinflammation [117]. Thus, intraperitoneal injection of LPS is also considered as a common method to establish animal models of depression. In the further therapeutic research, Bifidobacterium adolescentis NK98 and Lactobacillus reuteri NK33 from healthy human feces is found to decrease LPS level and alleviate depressive behavior in mice [118]. In addition, subdiaphragmatic vagotomy could also prevent depression-like behavior and change the composition of GM in mice after LPS administration, suggesting that LPS may regulate MGB-mediated depression-like behavior through VN [119]. Moreover, oral antibiotic therapy also effectively reduces the alcohol-induced microglial activation and relieves depressive-like behaviors, which strongly supports the effect of GM on MDD [120].

ASD

ASD is a kind of neurodevelopmental disorder syndrome with the symptoms of difficulty language communication, rigid behavior, social communication disorder and narrow interests [121]. ASD patients also have symptoms of enteritis, constipation, and diarrhea, which are closely related to behavioral problems and emotional disorders. Recently, approximately 2.2 %-2.7 % of children have ASD symptoms, and boys are more likely to have ASD than girls, which reaches 4:1 [122]. Recently, GM disorders have been demonstrated to closely relate to the development of ASD. Compared with healthy controls, ASD children exhibited significant changes in the abundance of GM with increases of Lactobacillus, Clostridium, Bacteroidetes, Desulfovibrio, Sarcina and Caloramator, and decreases of Bifidobacterium and Firmicutes [123]. Moreover, the GM in ASD children have non-spore-forming anaerobes and microaerobes, but not in the control group [124]. There are abundant histochemical Clostridium clusters I and II in the feces of ASD children, which results in a higher incidence of ASD [125]. Probiotics also have an effective role in the treatment of ASD. Probiotic treatment during pregnancy significantly reduces maternal immune activation-induced ASD behavior in offspring, rescues neuronal loss, and reduces GABA in the prefrontal cortex of adults [126]. Additionally, breastfeeding for at least 6 months promotes the development of a healthy GM for offspring and consequently reduces the risk of ASD [127], [128].

MS

MS is an autoimmune disease of CNS, in which T cells respond to antigens in their own myelin sheath, thereby promoting demyelination [129]. The clinical symptoms of MS include motor dysfunction and cognitive dysfunction. Immune cells (including cytotoxic CD8 + T cells, CD4 + T cells, B cells and macrophages)-mediated demyelination and axonal injury are the histopathological features of MS with the symptom of leukoencephalopathy, and which will develop into progressive downregulation of motor and cognitive abilities until irreversible loss [130]. Relapsing remitting multiple sclerosis is the most common type of MS(accounts for 87 % of MS cases), and commonly found in patients aged between 20 and 50 years [131]. During the pathological mechanistic study of MS, GM disorders are found in experimental autoimmune encephalomyelitis mouse model (a recognized mouse model of MS) and MS patients. Furthermore, the abundances of Bacteroidetes, Clostridium, Fecalibacterium and Prevotella are reported to significantly decrease in MS patients [132]. Moreover, there are significantly increased abundance of Methanobrevibacter and Akkermansia in different types of MS models [133], [134]. More interestingly, the genetic background of animals affects the change of GM abundance on MS diseases, suggesting that the genetic background of animals needs to be seriously considered when treating autoimmune diseases with probiotics [135].

Epidemiological studies shows that obesity significantly enhances the risk and severity of MS [136]. GM disorder is also a caused factor in obesity-induced the increased risk of MS. It has been found that HFD-induced obesity promotes the successful construction of MS animal model by promoting the enrichment of Proteobacteria and Desulfovibrionaceae, increasing the sulfur metabolism, long-chain fatty acid content and LPS, and finally leading to intestinal leakage and a systemic immune response [137]. It has also been reported that western salt-rich diets modulate the T-helper cell 17 axis by reducing Lactobacillus murinus and worsen the pathological process of MS disease [138]. In conclusion, GM disorder may be a key target for ameliorating MS pathologies.

SCI

SCI is a severe CNS trauma that leads to sensory and motor dysfunction. SCI has different pathological changes at different stages. The pathological changes in the acute (2–48 h) and subacute (14 d) stages of SCI include ion imbalance, lipid peroxidation, vascular injury, accumulation of excitatory neurotransmitters, free radical production, edema and apoptosis. In the chronic stage of SCI (1 month after injury), the main pathological features include chronic inflammation, neuronal loss and glial scar formation [139]. SCI destroys the myelin structures, exposes axons, and blocks nerve conduction at the site of lesion, and then oligodendrocytes are lost following it [140]. Thus, clearing myelin sheath fragments around injury site is an important event for nerve regeneration [141].

In addition to inflammation, cellular stress, autophagy and apoptosis, GM disorder is also an important regulatory factor in nerve repair after SCI. It has been reported that SCI induces increases of pro-inflammatory bacteria, including Shigella, Rikenella, Staphylococcus Bacteroides and Mucispirillum, while anti-inflammatory bacteria, such as Allobaculum, Lactobacillus and Sutterella, are decreased in SCI mice [142]. Excessive Bacteroides destroys the intestinal barrier and aggravates systemic inflammation of host [143]. Additionally, Firmicutes is also positively correlated with the motor function recovery. Firmicutes is significantly reduced in the gut of SCI mice and reversed by FMT treatment [55], [144]. Further mechanism study demonstrates that stimulating VN will upregulate alpha 7 nicotinic acetylcholine receptor (α7nAchR) of SCI rats, and then promote the polarization transformation of microglia from M1 type to M2 type, which significantly reduces inflammatory response [145]. During the treatment of SCI patients, stimulation of VN combined with rehabilitation training significantly improves the recovery of forelimb motor function in SCI [146]. It is speculate that stimulation of VN could regulate the role GM, then improve SCI recovery. Therefore, maintaining GM homeostasis and stimulate the VN may be the targets for SCI recovery.

The therapeutic strategies of CNS diseases based on GM regulation

Intermittent fasting (IF)

IF refers to a controlled dietary pattern through periodic fasting, alternate day fasting and time-limited feeding and maintains a good nutritional status. IF therapy has been recognized as a healthy dietary therapeutic method since the 1840 s, and has been attracted more and more attention of scientific researchers in the field of neuroscience. Epidemiological studies have found that individuals with reduced calorie intake have a lower risk of neurodegenerative diseases, such as AD and PD [147]. IF therapy not only significantly reduces Aβ deposition and reduces neuroinflammation, but also increases BDNF and postsynaptic density protein 95 (PSD 95) expression and inhibits oxidative stress, which consequently alleviates cognitive impairment [147], [148]. In a conclusion, there is a close relationship between IF and CNS diseases, and appropriate IF therapy may be beneficial for alleviating CNS diseases.

The GM is considered to be one of the important mediators in the interaction between IF and host [148]. The GM is highly responsive to changes in diet. Thus, IF may affect the abundance and species of GM and participate in the regulation of CNS repair. It has been reported that the abundance and species of GM will be altered within a day after changing the intake of nutrients in the diet of rodents and humans [149]. Moreover, fasting every other day significantly increases the abundance of Firmicutes in C57BL/6N mice, while other categories of microorganisms are decreased [150]. Additionally, feeding every other day can enrich the abundances of Lactobacillaceae, Bacteoidaceae and Prevotellaceae in fecal samples of C57BL/6J female mice [151]. Consistent with those in C57BL/6J mice, IF also significantly alters the abundance and of Bacteroidaceae, Lactobacillaceae and Prevotellaceae in MS mice [151]. Lactobacilliaceae is known as a typical probiotic that can alleviate the inflammation level [152]. In addition, enrichment of Prevotellaceae enhances the production of SCFAs, such as butyrate [144]. In summary, GM and it’s metabolites changes may be the main mechanism for the food intake regulating nerve recovery of CNS diseases.

Furthermore, the molecular mechanistic studies reveals that IF not only plays a neuroprotective role by enhancing the ketone pathway and regulating ketone metabolism in GM, but also significantly inhibits GM-mediated LPS biosynthesis and alleviates neuroinflammation [151]. IF has been reported to significantly reduce ischemia-associated increases in TNF-α and IL-6 levels in mice after ischemic injury [153]. Additionally, alternate day fasting also suppresses oxidative stress level in the hippocampus of chronic cerebral hypoperfusion rats [153]. Moreover, IF treatment also activates the Akt signaling pathway to inhibit NF-κB/JNK pathway, and increases PSD 95 expression in the hippocampus, and ultimately promotes the recovery of synaptic function in diabetic condition [154].

Prebiotics

Prebiotics refer to the indigestible food ingredients in the intestinal tract, such as fructooligosaccharides, galactooligosaccharides and xylooligosaccharides, which promote the growth and activity of various bacteria, and then exert a beneficial effect on host [155]. Compared with probiotics, prebiotics have the advantage of promoting the proliferation of multiple beneficial bacteria but not only benefit one kind of microbe [156]. Given their role in health, prebiotics have been classified as functional foods in the scientific literature [157]. Prebiotics, such as dietary polysaccharides, typically escape the digestion of host in the stomach or small intestine, and then reach the colon, where they are fermented and promote the probiotic growth [158]. These polysaccharides remarkably change GM struction and it’s metabolites [159]. SCFAs are known to be the final product of prebiotic fermentation and presumed to be the bridge for MGB [160]. Common prebiotics include fucoidan, mannan-oligosaccharides (MOS), and polymannuronic acid (PM).

Fucoidan, also known as fucoidan sulfate, is a common prebiotic that uniquely binds with sulfonic acid groups. Fucoidan is mainly found in brown algae and some marine invertebrates. Fucoidan supplementation affects the abundance of GM and it’s metabolites. Fucoidan significantly increases the abundance of beneficial bacteria, including Ruminococcaceae and Lactobacillus, and reduces pathogenic bacteria (especially Peptococcus) in the colon of diabetes mellitus (DM) mice [161]. Fucoidan has been reported to decrease the body weight, fasting blood glucose and insulin sensitivity of type 2 diabetes (T2D) through remodeling GM struction and further inducing the tauroursodeoxycholic acid level in the colon [162]. Additionally, fucoidan also enriches Parabacteroides goldsteinii to produce large amounts of succinic acid and activate the intestinal farnesoid X receptor signaling pathway, thereby improving lipid and glucose metabolism disorders [163]. Of course, besides GM, fucoidan also exerts it’s intrinsic anti-inflammatory activity to regulate the homeostasis of host [164].

MOS is one of prebiotics that extracted from the cell wall of plants, bacteria or yeasts [165]. MOS has also been reported to increase Lactobacillus and decrease Helicobacter, and then enhance butyrate formation during treating neural injury, which significantly reduces inflammation and strengthens the immune system [166], [167]. MOS (0.12 % in drinking water, w/v) has been reported to significantly reduce the accumulation of Aβ protein, balance the redox state, and inhibit the neuroinflammatory response of brain, which significantly improves cognitive function and attenuates anxiety and obsessive–compulsive behaviors [166]. In addition, MOS also upregulates the expression of NE by reducing corticosterone and corticotropin releasing hormone to ease HPA axis dysfunction [166].

PM is also one of the most widely studied prebiotics and has certain therapeutic potential for PD. Oral administration of PM alone or in combination with Lactobacillus rhamnosus GG (LGG) remarkably improves the expression of tyrosine hydroxylase gene and then reduces DAn loss in the striatum of PD mice [168]. Notably, the combination of PM and LGG has a better neuroprotective effect than PM or LGG alone. PM provides a neuroprotective effect by promoting SCFAs-mediated anti-inflammatory and anti-apoptotic mechanisms, while LGG increases abundance of Clostridium and then increases GDNF expression in the striatum.

Probiotics

Probiotics are active microorganisms that colonize in body and exert the beneficial effect on host by remodeling GM composition. Probiotic-associated GM balance can regulate the mucosal and systemic immune function to promote nutrient absorption and intestinal health. The concept of probiotics is first proposed by the microbiologist-Élie Metchnikoff, who is one of the groundbreaking researchers in the field of probiotics [169]. Traditionally, Lactobacilli, Bifidobacteria and other lactic acid-producing bacteria are primarily isolated from fermented dairy products and the fecal microbiome, which have been acted as probiotics [170]. Moreover, a large number of bacteria, including Roseburia intestinalis, Eubacterium spp., Faecalibacterium prausnitzii, Bacteroides spp. and Akkermansia muciniphila, also have the opportunity to be the next probiotic [171], [172].

Probiotics affect the GI tract, another organs and tissues by metabolizing GM-associated various compounds. Previous study demonstrates that probiotics can inhibit inflammatory response, reduce intestinal permeability, and inhibit endotoxemia by secreting antibacterial substances, such as organic acids [173]. Probiotics promote the fermentation of substrates to product a large variety of organic acids and volatile fatty acids, which lowers the pH level in gut and outcompete enteropathogens, such as Salmonella typhi, Helicobacter pylori, and Entamoeba histolytica [174]. In addition, probiotics have been found to inhibit oxidative reactive proteins (glutathione peroxidase, glutathione reductase and catalase) and pro-inflammatory cytokines (IFN-γ and iNOS) in the intestinal mucosa by altering the GM composition of DM mice [175]. Probiotics also affect the ENS by regulating GM composition and then regulate CNS function. For example, Lactobacillus reuteri enhances the afferent sensory nerves of intestinal motility and interacts with the gut-brain axis of rats, thereby selectively increasing the excitability of neurons in rats [176]. Probiotics, especially Bifidobacterium, are thought to exert a beneficial effect on improving cognitive function. Probiotics SLAB51 (a mixture of Streptococcus thermophilus, Bifidobacteria, and Lactobacillus) has been reported to significantly inhibit inflammatory factors in plasma, reverse the damaged ubiquitin proteasome system and autophagic level, and consequently relieve the deposition of Aβ protein in AD mice [177], [178].

However, this does not mean that probiotics are all good for the host. Notably, excessive intake of probiotics may also lead to serious side effects, such as sepsis, especially for critically ill, vulnerable populations of elderly and immunocompromised patients [179]. In addition, probiotics may lead to persistent long-term ecological imbalances after antibiotic treatment, which increases the risk of infectious diseases [180]. Moreover, serotonin syndrome is another notable side effect of probiotics, which is usually caused by the use of selective serotonin reuptake inhibitors (SSRIs) in depressed patients. Probiotics in combination with potent SSRIs will significantly increase the risk of serotonin syndrome [181]. Because the people with or at risk for AD are often elderly or even suffer from depression at the same time, they need to be more cautious during treatment with probiotics.

Fecal microbiota transplantation (FMT)

FMT is a technique for transferring intact and stable intestinal microbial communities from the stool of healthy donors to patients with a specific disease, which eventually restores GM dysbiosis and relieves the symptoms of disease [182]. It has been proven that FMT is highly safe that does not induce immune or rejected reactions in the host [183]. The FMT technique is first used in 1958 for the treatment of refractory and recurrent Clostridium difficile infections with a success rate of up to 90 %, and thus considered as an alternative to antibiotics [184]. Recently, FMT-mediated GM remodeling has been paid more and more attention in the field of CNS injury. It has been found that SCI mice transplanted with the intestinal microbial communities of normal mice have a better intestinal TJ ability and a much higher content of SCFAs in microflora metabolites, which significantly alleviates SCI-mediated inflammatory reaction and promotes SCI recovery [20]. Transplantation of fecal microbiota from normal C57BL/6J mice into a PD mouse model could increase the abundances of Firmicutes and Clostridiales, decrease the abundances of Proteobacteria, Turicibacterales and Enterobacteriales, and consequently alleviate GM disorders [185]. FMT remodeling has also been reported to significantly restore the abnormal content of SCFAs due to dysbacteriosis, thereby alleviating the dysfunction of PD mice [134]. FMT can also suppress the activation of astrocytes and microglia in the substantia nigra, and inhibit the TLR4/TNF-α inflammatory signaling pathway, thus alleviating the development of PD [186]. More interestingly, FMT donor with 8–10 weeks (age) have a better recovery of cognitive function in 32-week-old 5 × FAD mice than those receiving FMT donor for 32 weeks [187]. Thus, due to the different microflora structures caused by different ages, the age of the donor should be the key factor to consider during FMT.

In summary, FMT-associated GM remodeling may not be sufficient to influence the onset and development of CNS diseases, and its role on CNS recovery must consider some critical factors, such as the severity of disease and donor age. As we all known that, the pathogenic agent is unlikely to be a single microbe in most cases. Thus, compared to probiotics, FMT has it’s advantage on transplanting the intact healthy GM in the intestine to some extent. Due to its high safety and good efficacy, FMT has also been widely used to treat autoimmune enteritis, chronic nonalcoholic cirrhosis, epilepsy and autism spectrum disorders [188]. However, as a biological therapy, FMT also has its side effects for natural reaction of host during introduction of living microorganisms and their metabolites, including constipation, abdominal pain, diarrhea, and transient low fever [189]. Of course, these side effects may subside in a matter of days or weeks.

Western medicine

Metformin (Met)

Since the 1960 s, Met has been widely used to treat T2D and other metabolic diseases with the principle of inhibiting the production of liver glucose and increasing the utilization of peripheral glucose. In addition, Met has also been reported to exert significant neuroprotective roles in CNS diseases, such as Huntington’s disease (HD), PD, ischemic brain injury, and SCI [190]. In PD mice, Met not only activates the AMPK-BDNF signaling pathway to change the expression of astrocyte activity-related genes, but also alleviates mitochondrial damage and inhibits neuroinflammation, which significantly ameliorates the development of PD [191], [192]. Met treatment could also reverse autophagic level in microglia to enhance myelin debris clearance, and promote myelin regeneration and neural repair after SCI [141].

GM is reported to be an important target for Met promoting CNS diseases recovery. There were 22 enriched and 24 depleted microbes in Met-treated healthy mice, including increases of Prevotellaceae, Verrucomicrobiaceae, Rikenellaceae, and Porphyromonadaceae, and decreases of Rhodobacteriaceae and Lachnospiraceae [193]. Thus, Met treatment can reduce the expressions of LPS-binding proteins and the improve intestinal barrier function by altering GM composition in mice [194]. During the aging process, Met treatment can alleviate aging-related microbiota disorders by inhibiting Wnt signaling pathway, promote the beneficial metabolites, such as butyrate and taurine, reduce intestinal leakage and inflammation, and finally alleviate the aging process [195]. In addition, Met-associated GM remodeling can inhibit the microglia/macrophage activation and neuroinflammation in the brain of obese mice [191]. In a conclusion, Met-mediated GM changes may be the important regulatory factor for its neuroprotective function.

Antibiotics

Antibiotics are commonly used to prevent and treat bacterial infections in clinical practice. The animal and clinical studies have shown that the use of antibiotics accompanied with intestinal ecological disorders is closely related to CNS dysfunction [196], [197]. Antibiotic therapy has been linked to the development of neurological diseases, including depression, anxiety, psychosis and delirium [198]. It has been observed that antibiotic treatment drastically leads to permanent changes of GM struction, which inhibits the concentrations of neuromodulators in serum, such as tryptophan (Trp) and kynurenine, and decreases BDNF expression in the hippocampus, and consequently promotes the development of anxiety and cognitive impairment [199]. Moreover, different kinds of antibiotics have different effects on nervous diseases. For example, ampicillin increases the corticosterone level in serum, and then induces the anxiety-like behavior and impaired spatial memory in rats, while minocycline treatment reduces IL-6 and TNF-α levels and eventually relieves anxiety-like behaviors [200], [201]. In addition, minocycline significantly ameliorates mild stress-induced behavioral and cognitive deficits by inhibiting the release of high mobility group box 1 in microglia and neurons [202]. Although there are no significant effect on intestinal permeability or inflammatory response under ciprofloxacin and metronidazole exposure, short-term exposure of these two antibiotics will increase 5-HT level to ameliorate anxiety-like and depression-like behaviors [203].

Mechanism studies demonstrate that change of GM struction is also the key caused event for antibiotics regulating the development of CNS diseases. Oral administration of vancomycin and ampicillin has been shown to trigger intestinal ecological disorders and inhibit the expression of intestinal tight junction proteins, which will promote the increased LPS production to enter into bloodstream [204]. More importantly, LPS in the bloodstream not only enters the brain (especially into the hippocampus region) to promote neuroinflammation [205], but also activates microglia to trigger NF-κB and promote expressions of chemokines and cytokines in mice [206]. Thus, there is still some controversy about the use of antibiotics in the treatment of diseases. For example, long-term or repeated use of antibiotics may disrupt the normal microbiota, thereby increasing the risk of depression.

Melatonin

Melatonin (5-methoxy-n-acetamide) is an indole hormone secreted by the pineal gland. Trp is used as a raw material to synthesize melatonin under series of enzymatic reactions under regulating by circadian rhythm [207]. Except for the pineal gland, melatonin is also produced from the gut, skin and retina et al. And it mainly acts locally, but not circulates throughout the body [208]. In the gut, GM promote the production of SCFAs and then stimulate 5-HT secretion. 5-HT is further converted to N-acetyl-5-hydroxytryptamine and melatonin [209]. There is abundant melatonin in the GI tract, which is about 10–100 times than that in blood and 400 times than that in the pineal gland[208], [210]. Melatonin, in turn, regulates GM diversity and abundance with increasing abundance of Firmicutes and decreasing abundance of Bacteroidetes [211].

Melatonin has a neuroprotective role in CNS diseases based on GM regulation. Melatonin reduces the abundance of Clostridiales and increases the abundance of Lactobacillus in SCI mice, thereby promoting axonal regeneration and neural function recovery [212]. Melatonin also increases the abundance of Myxobacteria and Streptococcus mucophapgus, and then protects the intestinal barrier and reduces intestinal inflammation [213]. Additionally, physiological disorders of melatonin may also be a genetic and environmental risk factor for ASD development. There are abnormal release patterns of melatonin in plasma from ASD patients [214]. Moreover, the mothers of ASD children have also shown a less melatonin during pregnancy, which supports the idea that a lack of melatonin contributes to ASD development [215]. And melatonin supplementation during pregnancy or lactation has been found to restore the disordered GM state and rescue social deficits in the offspring of sodium valproate-induced ASD mice [216]. Therefore, melatonin may be a potential therapeutic agent for CNS diseases.

Traditional Chinese medicine (TCM)

There are many different structural types of compounds in TCM that are directly absorbed by the body. The TMC will reach the intestinal tract to interact with GM and then metabolize the chemical components of Chinese medicine into other substances. Of course, Chinese medicine components also regulate the composition and metabolism of GM. Due to the complex composition of TCM and the continuous development of 16S rRNA sequencing technology and metabolomics, the effects of TCM on host from the perspective of GM have been attracted more and more attention. The TCM, such as puerarin and Zi Shen Wan Fang (ZSWF), have good effects for regulating the GM to treat CNS diseases.

Puerarin is a bioactive ingredient extracted from pueraria lobata that has the properties of lowering blood pressure and blood lipids, and exerting anti-tumor, anti-oxidation and anti-inflammatory effects [217], [218]. Additionally, puerarin also has neuroprotective potential based on GM imbalance. It has been found that puerarin treatment significantly ameliorates depression-associated increasing abundance of Bacteroidetes, Proteobacteria, and Actinobacteria and decreasing abundance of Firmicutes [110], [219]. Moreover, puerarin significantly increases the beneficial bacteria (such as Bacteroidaceae and Prevotellaceae) and decreases the harmful bacteria(such as Lachnospiraceae, Helicobacteria and Desulfovibrionaceae) [219]. Puerarin (100 mg/kg) has been reported to exert its anti-depressant effect by relieving oxidative stress and neuroinflammation in the hippocampus [220]. Moreover, puerarin-mediated GM changes are mostly related to intestinal immune regulation. It has been demonstrated that puerarin inhibits IL-1β, IL-6 and TNF-α levels in the hippocampus and serum of rats after brain injury [221]. More importantly, puerarin also stimulates mucin secretion and goblet cell differentiation, which reconstructs the colonic mucus layer to relieve ulcerative colitis [222]. Thus, we speculate that the anti-depressant effect of puerarin may be achieved by directly or indirectly promoting anti-inflammatory bacteria.

ZSWF is an optimized prescription composed of Anemarrhenae Rhizoma, Phellodendri Chinensis Cortex, and Cistanches Herba. ZSWF significantly improves the imbalanced status of GM in the body. At the phylum level, ZSWF treatment decreases the relative abundance of Firmicutes and Proteobacteria in the gut of mice and significantly increases the abundance of Bacteroidetes. At the genus level, ZSWF treatment significantly increases the abundance of Bacteroides and Alistipes and reduces the abundance of Desulfoubrio, Dorea, and Allobaculum [223]. It has been observed that ZSWF significantly increases the expression of TJ proteins (ZO-1 and occludin) and mucoprotein 2 in the colon to protect the intestinal barrier integrity and improve immunoglobulin-a level in the colon content [223]. As we all known that, TCM has a long history and a wide variety. With the development of sequencing and omics, in-depth exploration of TCM’s efficacy and pharmacology at the microbial community level will undoubtedly contribute to the treatment of CNS diseases.

Conclusion and prospects

With the development of metabolomics and sequencing technology, CNS-related changes in the abundance and composition of GM have been attracted more and more attention. On the basis of in-depth analysis of GM changes during CNS disease, as well as the changes in its metabolites, the therapeutic strategies(such as probiotics, prebiotics, and FMT) may be used to regulate GM balance and affect its metabolic pathways and products, thereby promoting the recovery of CNS disease. However, this process is very complex, and the current technology is not fully mature. For example, 16S rRNA sequencing technology cannot distinguish between dead and active microbial communities, and 16S rRNA sequencing technology lacks resolution beyond the genus level. Therefore, it is difficult to identify microorganisms at the strain level, and depict the causal relationship between GM change and precise intervention strategies. However, these issues will eventually be overcame with the progress of basic research and further integration of big data. In summary, there is a very complex regulatory network in vivo. The molecular mechanism and therapeutic strategy of CNS diseases based on GM imbalance need to be further studied.

CRediT authorship contribution statement

Tao Wei’s role: Writing – original draft; the role of Yanren Zhang, Saiqun Nie, Li Fang and Bingbin Wang: assisted in the Writing – original draft; Jian Xiao’s role: Writing – review & editing/Funding acquisition; Yanqing Wu’s role: Writing – review & editing/Conceptualization/Funding acquisition. All authors have approved the final version of the manuscript.

Funding

This study was partially supported by a research grant from Zhejiang Provincial Natural Science Foundation (LY22H090007 and LZ23H060001), National Natural Science Foundation of China (82272254, 82172428, 81972150), Basic Scientific Research Project of Wenzhou (Y20220060) and Graduate Scientific Research Foundation of Wenzhou University (3162024003054).

Biographies

Tao Wei: A M.S. candidate of Wenzhou University, who has 2 publications on nerve injury repair field. The M.S. project is about the role of gut microbiota on spinal cord injury recovery.

Yanren Zhang: A M.S. candidate of Wenzhou University, who has 2 publications on nerve injury repair field. The M.S. project is about the role of lipid metabolism disorder on spinal cord injury recovery.

Bingbin Wang: A M.S. candidate of Wenzhou University, who has focused on the rersearch of neural development. The M.S. project is about the role of diabetes on neural tube defects.

Saiqun Nie: A M.S. candidate of Wenzhou University, who has focused on the rersearch of nerve injury repair. The M.S. project is about the role of microglial cells on spinal cord injury recovery.

Li Fang: A M.S. candidate of Wenzhou University, who has focused on the rersearch of nerve injury repair. The M.S. project is about the role of lipid metabolism disorder on spinal cord injury recovery.

Professor Jian Xiao: Ph.D., Dean of Graduate School, Wenzhou Medical University, National Outstanding Youth Fund gainer, who has committed himself to scientific research and applied Research for 20 years in nerve repair. He has launched a series of research and published 100 papers in nerve repair field.

Associate professor Yanqing Wu: Ph.D., visiting scholar of University of Maryland, master supervisor of Wenzhou University. She has mainly engaged in research in nerve injury repair field and published more than 60 papers in this field.