Gut: The gate and key to brain

Abstract

Brain science is the frontier of modern science, and new advances have been made in brain-like designs and brain-computer interfaces to simulate or develop brain functions. However, given that the brain is hermetically sealed within the skull, exploration and deciphering of the brain structure and functions are limited. Growing evidence suggests that the gut is not just a digestive organ. It not only provides essential nutrients and electrolytes for brain neurodevelopment and the maintenance of brain function, but it also transmits external environmental and intestinal wall signals from the intestinal lumen to the central nervous system through multiple pathways to regulate brain activity, function, and structure. A variety of gut–brain interaction pathways have been identified, including neural pathways, neuroimmune signaling, endocrine pathways, and biochemical messengers produced by gut microbes. Gut microbes interact with food and the gut to modulate gut–brain communication. The gut’s important role and potential in neurodevelopment, maintenance of normal function, and disease development make it an increasingly important area of research in brain science and neuropsychiatric disorders. The gut’s unique role in brain functions and its accessibility for research (compared to direct brain studies) establish it as a critical gate to understanding the mysteries of brain science. Crucially, intestinal nutrients and microbes provide two unique keys to unlock this gate—enabling neural regulation and novel treatments for neuropsychiatric diseases.

Introduction

At one point, it was understood that the brain controlled all parts of the body, that neurological and psychiatric disorders could be attributed to defects in the brain cells themselves, and that the gastrointestinal (GI) tract was an organ independent of the brain, only responsible for digesting food. However, in the 19th century, scientists began to recognize a connection between the gut and the brain when William Beaumont observed that certain emotions affecting the nervous system, such as anger or irritability, could cause a delay in the rate of digestion.^[1] The GI tract is 9–10 m in length;^[2] however, the total mucosal surface area is 250–400 m²,^[3] making it the largest and most direct gate for interaction and substance exchange between the body and the external environment. The GI tract contains 70–80% of the body’s immune cells, >100 million neurons, approximately 100,000 exogenous nerve endings, and up to 10¹⁴ microorganisms, which encode >100 times the number of genes encoded by the human genome.^[2,4] This suggests that the gut has great potential to maintain homeostasis and regulate neural functions in the human body. In recent years, the integration of multi-omics technology with brain imaging data and clinical parameters has provided comprehensive insights into gut–brain communication. In addition to providing essential nutrients, the gut can transmit signals to the brain through nervous, immune, endocrine, and metabolic pathways to regulate the development and function of the nervous system [Figure 1]. Furthermore, the gut is involved in the pathogenesis of neuropsychiatric diseases such as autism spectrum disorder (ASD), schizophrenia, Parkinson disease (PD), Alzheimer’s disease (AD), multiple sclerosis, depression, and anxiety.^[5,6] Recent evidence has demonstrated that restoring gut homeostasis offers a novel and promising approach to managing and preventing these diseases.^[7,8] The significance of the gut as a gate to the brain is continuously being reaffirmed. In this review, we aim to provide an overview of the important role of the gut in the regulation of the brain and to summarize recent advances in targeting the microbiota–gut–brain axis for the treatment of neuropsychiatric disorders, with a view to raise the attention to gut and brain sciences. We also aim to provide a reference for brain science and mechanistic studies of gut–brain comorbidities and explore new therapeutic approaches. Although the previous reviews have detailed the fundamental pathways of gut–brain communication, this article uniquely positions the gut as the essential gate to the brain and defines nutrients and gut microbiota as the two keys to unlock its regulatory potential. Furthermore, we provide a focused analysis of emerging therapeutic strategies, particularly fecal microbiota transplantation (FMT) for neuropsychiatric disorders, thereby bridging insights into gut–brain communication with therapeutic prospects. The central nervous system (CNS) also has a complex regulatory function in the gut in two-way gut–brain communication, but this has not been focused on in this review.

Figure 1.

Pathways of gut–brain communication. In addition to providing essential nutrients, the gut can transmit signals to the brain through nervous, immune, endocrine, and metabolic pathways to regulate the development and function of the nervous system. BAs: Bile acids; CCK: Cholecystokinin; GLP-1: Glucagon-like peptide-1; IL: Interleukin; PYY: Peptide YY; SCFAs: Short-chain fatty acids; TNF-α: Tumor necrosis factor alpha.

Gut’s Role in Providing Nutrients and Energy for Brain Development and Neuronal Activities

More than 90% of the body’s nutrients are absorbed and supplied by the digestive tract, providing energy for the body and having a profound effect on neurons and the development of the nervous system.

Glucose is the primary energy donor and the most important substrate for healthy brain function and development. Approximately 30% of circulating glucose is located in brain extracellular fluid.^[9] Most of the energy produced from glucose metabolism (70%) is used for neuronal signal transmission functions such as action potential, calcium activities, synaptic transmission, and glutamate cycling; the remaining part is involved in non-signaling activities, like axonal transport, resting potential, and cytoskeleton remodeling.^[10] Furthermore, glucose metabolism provides the carbon used for nucleic acids, fatty acids, and amino acids synthesis, and produces metabolites that are involved in the regulation of inflammatory and redox reactions.^[10,11] Altered glucose homeostasis interferes with the development of brain structure and cognitive abilities. In children, decreased glucose levels impair hippocampal development, mediate intracellular calcium toxicity and excitotoxic neuronal damage, and cause defects in consciousness and cognitive abilities, including intelligence, learning, memory, and verbal fluency.^[9] Hyperglycemia increases blood-brain barrier (BBB) permeability and triggers oxidative stress, leading to mitochondrial dysfunction and cellular damage.^[9,12]

Lipids constitute approximately 60% of the dry weight in the brain.^[13] They are the main components of the cell membrane and are implicated in the maintenance of membrane potential, neurotransmission, synaptic plasticity, and inflammation.^[13,14] The polyunsaturated fatty acid omega-3 is an important component of synthesized lipids and is essential for brain development and function. One study involving middle-aged participants free of clinical dementia found that higher omega-3 fatty acid concentrations were associated with increased hippocampal volume and improved cognitive function.^[15] Epidemiological studies have shown that children with attention deficit hyperactivity disorder (ADHD) and ASD have low erythrocyte omega-3 levels.^[16] Supplementation with omega-3 can help improve social interactions, communication, behavior, and attention in children with ADHD and ASD.^[17] Omega-3 deficiency in the brain may also be a risk factor for PD and depression.^[18] Moreover, omega-3 may contribute to neuroprotection through its anti-inflammatory effects and improved BBB and lymphatic functions.^[19,20,21] Because the human body is unable to synthesize omega-3 polyunsaturated fatty acids on its own, intestinal absorption is the only source. Therefore, the gut plays a key role in the regulation of brain development and function.

Similar to omega-3 fatty acids, nine essential amino acids, including lysine and tryptophan, must be absorbed by the gut and supplied to the brain and organs. Some of these amino acids are essential precursors for the synthesis of neurotransmitters in the brain and have important effects on nerve signaling.^[22] One study reported the critical roles of large neutral amino acids, most of which are essential amino acids, in brain development, through controlling the expression of SlC7A5, a transporter of essential large neutral amino acids in neuronal cells, thereby restricting their transport to neuronal cells.^[23] The lack of these amino acids in mouse neurons resulted in reduced brain size, microcephaly, and ASD-like behavioral changes.^[23] In mouse model with neurodegenerative disease, a low protein diet led to downregulated synaptic component expression and accelerated brain atrophy, while these abnormal phenotypes can be rescued by seven essential amino acids.^[24]

Other nutrients also affect neurological health and cognitive abilities. Vitamins are essential for normal physiological functions; however, the body cannot synthesize them endogenously. Vitamin B is involved in the brain’s energy production, DNA/RNA synthesis/repair, genomic and non-genomic methylation, and synthesis of neurochemicals and signaling molecules.^[25] Especially folic acid and vitamin B₁₂ may prevent CNS developmental disorders, mood disorders, and dementia.^[26,27] Notably, although it cannot fully meet human needs, the gut microbiota serves as a significant contributor to both the synthesis and bioavailability of folic acid and vitamin B₁₂.^[28,29] For example, Lactobacilli can synthesize vitamin B₁₂, which may directly increase vitamin B₁₂ levels in the body.^[28] Vitamins C and E act as antioxidants by inhibiting oxidative stress and may help ameliorate motor and cognitive decline during aging.^[30] Active vitamin D functions as a neurosteroid, promoting neuronal growth and maturation. It has a neuroprotective role as it inhibits excessive inflammatory neurovascular damage and attenuates amyloid pathology. An observational and Mendelian randomization study showed that lower vitamin D levels were associated with a lower total brain volume and the increased risk of dementia and stroke.^[31] Minerals are essential for building and maintaining the brain structure and realizing intercellular connections. Iron deficiency affects neuronal processes such as myelination and synaptic plasticity, and mediates alterations in the electrophysiological properties of neural circuitry and neurotransmitter systems.^[32,33] Volatile fatty acids produced by the gut microbiota can regulate iron absorption as a compensatory mechanism during iron deficiency.^[34] Magnesium deficiency may increase glutamatergic neurotransmission and lead to oxidative stress and neuronal cell death.^[35] Zinc is essential for neuronal migration and synapse formation.^[36] Zinc deficiency is common in patients with PD, AD, and depression.^[37] Gut microbiota act as a regulator of zinc absorption. Studies have demonstrated that Lactobacillus plantarum enhances zinc absorption and improves its bioavailability in children.^[38] Furthermore, several studies have confirmed that flavonoids and alpha-lipoic acids improve cognition and have neuroprotective effects.^[39,40]

In early life, gut microbial communities significantly affect the development and function of the nervous system through influencing the metabolic processes of key host nutrients. Reduced cord serum triglyceride was found in infants who were subsequently diagnosed with ASD, and the levels of cord serum triglyceride were correlated with the abundance of Bifidobacterium at 1 year.^[41] Meanwhile, a significant depletion of semi-essential amino acid L-arginine was found, which was positively correlated with the abundance of Roseburia, Coprococcus, and Akkermansia.^[41] Infants later diagnosed with ASD show decreased levels of neuroprotective riboflavin. This metabolic alteration is linked to Coprococcus, which regulates riboflavin processing.^[41]

Signaling Mechanisms from the Gut to the Brain Regulating Neuronal Activities

Neuronal pathways for gut–brain interactions

The GI tract is the largest and the most direct gate in the body and is in contact with the external environment. It is rich in GI neuronal cells that connect the GI tract to the brain, enabling the brain to sense changes in its external environment from the GI tract. The gut contains two neuroanatomical pathways that interact with the brain. One is the direct reciprocal exchange of information between the gut and the brain through the autonomic nervous system and the vagus nerve in the spinal cord; the other is through mutual communication between the enteric nervous system in the gut and the autonomic nervous system and the vagus nerve in the spinal cord.^[42] Both the enteric nervous system and the CNS originate from the epiblast of the early embryo.^[43] This common origin provides an anatomical basis for communication in the gut–brain neural pathway. The vagus nerve is the most important neuronal component in the gut–brain axis and is a key pathway for two-way gut–brain communication.^[44] Neurotransmitters produced by GI endocrine cells and microbiota, GI hormones, intestinal microbiota-derived metabolites, and immune factors communicate with the CNS via vagal afferent fibers. The ratio of afferent to efferent fibers in the vagus nerve is 9:1, and the overwhelming preponderance of afferent fibers highlights the predominant role and importance of signaling from the GI tract to the brain.^[45] Salmonella transmits infection signals to the CNS via the vagus nerve, causing abnormal activation of neurons and immune cells, and inducing anxiety-like behavior.^[46] Vagotomy significantly inhibits the infection-driven surge in pro-inflammatory cytokines and effectively reduces hippocampal microglial activation and neuroinflammation while mitigating anxiety-like behaviors in mice,^[46] confirming the critical role of the vagus nerve in gut–brain communication. Deposition of misfolded a-synuclein, a marker of PD, can initiate in the enteric nervous system and enter the brain via the vagal pathway, suggesting that the pathogenesis of PD is related to the neural pathway between the gut and the brain.^[47]

Neuroimmune signaling

The GI tract houses the largest population of immune cells in the body and is widely exposed to food-based and environmental antigens from the GI lumen, in addition to GI microorganisms and their derivatives. Communication between the gut and immune system contributes to the recognition of self and non-self antigens and protection against potentially harmful pathogens. Neuroimmune signaling is also involved in gut–brain axis signal transmission, which regulates brain behavior and function.^[1] Microbial-associated molecular patterns (MAMPs) are highly conserved microbial components. Two principal MAMPs, namely, lipopolysaccharide (LPS) and peptidoglycan (PGN), are sufficient to alter brain development and function.^[48] LPS can be recognized by the intestinal pattern recognition receptor (PRR) toll-like receptor, activating dendritic cells, neutrophils, and macrophages, and producing interleukin (IL)-1a, IL-1b, IL-6, and tumor necrosis factor alpha (TNF-α).^[49] These proinflammatory factors can cross the BBB, directly affecting brain function and disrupting its integrity.^[50,51] In addition, pro-inflammatory cytokines released by gut immune cells in response to microecological dysregulation can activate vagal afferent pathways and affect the brain regions associated with mood and behavior.^[52] An intraperitoneal injection of LPS in mice stimulated microglia activation in the hippocampus and induced neuroinflammation and cognitive deficits.^[53] PGN can cross the BBB and can be sensed by specific PRR of the innate immune system, with potential effects on brain development and behavior.^[54] In cellular immune pathways, gut immune cells can directly regulate neuroimmune homeostasis and the brain’s response to inflammation. Gut antigens stimulated B cells to differentiate into immunoglobulin A (IgA)⁺ plasma cells, which relocated to the brain and spinal cord to attenuate neuroinflammation in an IL-10-dependent manner in mice with experimental autoimmune encephalomyelitis.^[55] In contrast, in several disease contexts, the migration of gut immune cells activated by host- or microbial-derived antigens to the CNS worsens neuropathophysiological changes.^[56] Gut microbiota dysbiosis after acute cerebral ischemia induces polarization and activation of pro-inflammatory T helper 1 and T helper 17 cells in the gut.^[57] These intestinal T cells and monocytes migrate to the brain and exacerbate neuroinflammation.^[57] In a Drosophila model of AD, enterobacteria infection accelerated the disease progression by promoting immune cell migration into the brain, thereby provoking TNF-c Jun N-terminal kinases (JNK)-mediated neurodegeneration.^[58] In inflammatory bowel disease, the immune response triggered by gut microbiota can also lead to alterations in neurotransmitter metabolism and neuronal excitability,^[59] suggesting that the gut mucosal immune system is an important regulator of gut–brain communication.

Neuroactive signaling molecules modulated by the gut and microbiota

Shared neurochemical language between the gut and brain provides an important mechanism for gut–brain communication. Neuroactive signaling molecules, such as serotonin (5-HT) and gamma-aminobutyric acid (GABA), are extensively involved in information transmission between cells. An imbalance in this system has been shown to be significantly associated with mental health and cognitive and emotional issues. The gut and the microbes in its lumen can produce and release neuroactive substances that are homologous to brain neurotransmitters, which can alter the concentrations of neurotransmitters or their precursors in the circulation or in the brain.^[60] These signaling molecules can also act on intestinal afferent nerve endings to transmit signals to the brain, thereby directly or indirectly affecting CNS activity and cognitive function.^[52,61] In addition, gut microbiota can regulate the biosynthesis and metabolism of neurotransmitters in the brain.^[62] However, the underlying mechanisms have not been fully elucidated.

More than 90% of the body’s 5-HT is stored in the gut, primarily produced by enterochromaffin cells of the GI mucosa and enteric neuronal cells.^[63] Gut microbiota can influence this process through regulating the expression of host tryptophan hydroxylase-1.^[64] Some gut bacterial genera such as Escherichia and Streptococcus can directly synthesize 5-HT.^[64] Intestinal 5-HT cannot cross the BBB and thus does not directly affect 5-HT levels in the brain; however, it can indirectly participate in the regulation of brain activity. 5-hydroxytryptophan (5-HTP), a precursor of 5-HT, has been shown to be significantly decreased in the guts of patients with AD, and increased cognitive impairment is positively correlated with reduced 5-HTP.^[65] The nerve endings of the vagal afferents are located close to enterochromaffin cells and express 5-HT-specific receptor 5-HT₃R, suggesting that 5-HT influences brain function through the vagal pathway that connects the gut to the brain.^[66] Gut microbiota regulates 5-HT synthesis and metabolism in the brain. 5-HT-metabolizing bacterial species such as Clostridium and Lactobacillus are upregulated in children with ASD.^[67] Excessive 5-HT production by gut microbiota can deplete peripheral tryptophan availability, the precursor substance of 5-HT, affecting the synthesis of 5-HT in the brain, thus influencing mood and cognition.^[67]

Dopamine, a reward neurotransmitter, is involved in processes such as affection, memory, attention, and motivation.^[68] Moreover, it serves as a precursor of norepinephrine and epinephrine. Dysfunctional dopaminergic transmission is associated with severe CNS disorders, including PD and schizophrenia.^[69] More than 50% of the body’s dopamine is synthesized in the gut,^[70] including in enteric nervous system dopaminergic neurons and epithelial cells in the GI tract. Bacterial genera in the gut such as Bacillus, Escherichia, Staphylococcus, and Serratia also possess the ability to produce dopamine.^[71] While gut-derived dopamine cannot cross the BBB, levodopa, a precursor substance produced by gut bacteria, can enter the brain through the circulation and be converted to dopamine.^[72] In animal experiments, the use of antibiotics to disrupt the balance of the gut microbiota reduced the concentrations of dopamine, 5-HT, and their precursors in the hypothalamus.^[73] Furthermore, the peak serum concentration and bioavailability of levodopa can be increased through inhibiting levodopa decarboxylation by gut microbiota.^[74] This may enhance the therapeutic efficacy of levodopa in PD, reflecting the significant effect of the gut on dopamine metabolism in the brain.

GABA is a major inhibitory neurotransmitter of the CNS and the primary inhibitory mechanism involved in the negative feedback signaling pathway of the hypothalamic-pituitary-adrenal (HPA) axis; it is synthesized predominantly by GABAergic neurons that express glutamic acid decarboxylase.^[69] Dysfunction in GABAergic neurotransmission is associated with anxiety, depression, and cognitive disorders.^[69] Moreover, GABA can be produced in the gut by bacterial genera, such as Bacteroides, Parabacteroides, Eubacterium, Bifidobacterium, Lactobacillus, Escherichia, and Pseudomonas.^[22,71] Gut-derived GABA cannot cross the BBB but may act locally on the enteric nervous system or vagus nerve and transmit neural signals to the brain. It has been reported that Lactobacillus rhamnosus with GABA-producing capacity reduces stress-induced corticosterone and anxiety-and depression-related behavior.^[75,76] It also induces region-dependent alterations in the GABA receptor mRNA in the brain.^[75] These processes are vagus nerve-dependent.^[75] Another study found that after four weeks of supplementation with Lactobacillus rhamnosus, GABA levels in the brains of mice were elevated.^[77] It has been revealed through ¹³C-labeling that colonic fermentation metabolites of carbohydrates produced by gut microbiota can cross the BBB and affect the GABA metabolism cycles in the CNS, especially in the hypothalamus.^[78] This evidence confirms an interaction between the gut and GABA metabolism in the brain. Imbalances in the gut GABA metabolism are involved in the pathogenesis of neuropsychiatric disorders. An elevated level of microbial GABA degradation was observed in children with tic disorders, and the severity of motor or vocal tics positively correlated with the abundance of gut bacteria known to degrade GABA, such as Klebsiella pneumoniae.^[79]

Glutamate is the most abundant excitatory neurotransmitter in the brain, a precursor of GABA, and plays a key role in the regulation of neuronal excitability and synaptic plasticity.^[80] The production of glutamate in the brain relies on the coordinated action of neurons and astrocytes, using intermediate metabolites of glycolysis and phosphate-activated glutaminase from the hydrolytic deamination of glutamine.^[81] Moreover, glutamine can be synthesized by enteroendocrine cells and gut microbiota such as Bacteroides vulgatus, Lactobacillus plantarum, Lactobacillus paracasei, Lactococcus lactis, and Campylobacter jejuni.^[81,82] Despite its inability to cross the BBB, glutamate in the gut can communicate information to the brain via the vagus nerve. Neuropod cells are enteroendocrine cells that synapse with vagal neurons, which express the vesicular glutamate transporter protein-1 and release glutamate within milliseconds, thus, rapidly transmitting sensory stimuli from gut sugars to the brain.^[82,83] Glutamine, the precursor of glutamate, enters the brain through the transporters.^[84] Microbial synthesis by Lactobacillus, Bacteroides, Corynebacterium, Brevibacterium, and Campylobacter provides a source of glutamine,^[71] indicating the potential of gut microbes to regulate glutamate biosynthesis. Additionally, similar to the GABA metabolic cycle, colonic fermentation metabolites can cross the BBB and participate in glutamate biosynthesis.^[78]

Other neuroactive signaling molecules are also inextricably linked to the gut. Acetylcholine is the main neurotransmitter for cholinergic neurons. Gut microbes such as Lactobacillus plantarum, Bacillus subtilis, Escherichia coli, and Staphylococcus aureus can produce acetylcholine.^[22] Moreover, choline, the precursor substance of acetylcholine, is mainly derived from the gut and can be transported to the brain via carriers on capillary endothelial cells.^[85] Histamine and some trace amines also act as neurotransmitters and are considered important neuromodulators despite their low concentrations in the brain.^[86] Gut microbes can influence neural activity by regulating their metabolic processes.^[22,86] For example, gut Staphylococcus strains express staphylococcal aromatic amino acid decarboxylase, which decarboxylate its corresponding aromatic amino acid substrates and then synthesize three types of trace amines including tryptamine, tyramine, and phenethylamine.^[81]

Gut hormones and their influence on the brain

The GI tract is the largest endocrine organ in mammals. Along with the production of neurotransmitters, gut endocrine cells can also release GI peptides, such as cholecystokinin (CCK), glucagon-like peptide (GLP), and peptide YY (PYY), which affect CNS functions through both endocrine and paracrine functions. Endocrine signaling refers to the process in which signaling molecules circulate through the body and bind to receptors located in the CNS to transmit signals.^[87] In paracrine signaling, signaling molecules act on the afferent nerve endings of the vagus nerve in the GI tract, thereby influencing the CNS.^[87]

The neuropeptide Y (NPY) family comprises NPY, PYY, and pancreatic polypeptide (PP). PYY and PP are exclusively released by the endocrine cells of the digestive system, whereas NPY is widely expressed throughout the gut–brain axis, including intestinal neurons, primary afferent neurons, multiple neuronal pathways in the brain, and sympathetic neurons.^[88] The receptors of the NPY family mainly include Y₁, Y₂, Y₄, and Y₅.^[89] In addition to the gut, Y₁ and Y₂ receptors are widely present in the brain, whereas Y₄ and Y₅ receptors are distributed in the nucleus tractus solitarius, hypothalamus, and amygdala.^[90] The extensive distribution of these receptors along the gut–brain signaling pathway indicates that the NPY family plays a significant role in gut–brain communication. Gut-derived PYY and PP can cross the BBB via transmembrane diffusion and bind to homologous receptors in the postrema.^[91,92] Alternatively, they can activate homologous receptors located at afferent nerve endings of the vagus nerve and send signals to the brainstem.^[90] The NPY family has great potential for the regulation of emotions. The Y₄ receptor is primarily activated by gut-derived PP, and genetic deletion of Y₄ receptors impairs fear extinction, suggesting a potential role of gut PP-Y4 signaling in the pathogenesis of fear- and anxiety-related disorders.^[93] Similarly, enhanced depression or anxiety-like behaviors have been observed in mice lacking the NPY or PYY genes.^[94] Gut microbiota participate in the regulation of brain signaling by influencing the metabolism of gut hormones. Antibiotic-induced gut dysbiosis in mice has been shown to lead to an imbalance in the NPY system, impaired novel object recognition, and cognitive impairment.^[95]

Glucagon-like peptide-1 (GLP-1) is an incretin hormone whose receptors are widely distributed in brain regions important for metabolism, endocrine system, and stress, and is involved in the regulation of the HPA axis and stress response.^[96] GLP-1 has a short half-life and is rapidly degraded by the enzyme dipeptidyl peptidase IV after being released into the bloodstream.^[97] Due to this rapid degradation, GLP-1 cannot accumulate sufficiently to cross the BBB in biologically active concentrations. Consequently, GLP-1 predominantly transmits signals to the brain through local action on GLP-1 receptors located on vagal afferent neurons. GLP-1 levels in intestinal lymph are higher than those in plasma following stimulation of intestinal secretion of GLP-1, suggesting that the lymphatic system may be a signaling channel for GLP-1 in gut–brain communication.^[98] The neurotrophic and neuroprotective effects of GLP-1 receptor agonists and their potential therapeutic efficacy in neurodegenerative disorders such as PD have been characterized in several in vitro and in vivo preclinical studies.^[99] Studies have shown that an engineered strain consistently producing GLP-1 reduces LPS-induced memory disorders and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dyskinesia in mice.^[100] The probiotic Clostridium butyricum, which increases GLP-1 production and upregulates the expression of cerebral GLP-1 receptors, ameliorates MPTP-induced motor deficits and dopaminergic neuronal loss in mice.^[101] These findings highlight the beneficial effects of enhanced gut GLP-1 signaling in the CNS. GLP-1 production is strongly influenced by microbial metabolite short-chain fatty acids (SCFAs). SCFAs act as secretagogues by binding to transmembrane-free fatty acid receptors on L cells that secrete GLP-1.^[102] Compared with healthy individuals, patients with PD have a reduced ability to secrete GLP-1 into systemic circulation during meals and this change has been associated with dysbiosis of the gut flora and reduced SCFA levels.^[103]

CCK is secreted mainly in the upper small intestine and binds to CCK-a and CCK-b receptors, which are expressed mainly in the gut and brain, respectively.^[104] CCK can cross the BBB and exert its role in regulating appetite through binding to CKK-a receptors in the hypothalamus and hindbrain.^[104] It influences emotional behavior through binding to CKK-b receptors in the limbic regions.^[90] Furthermore, CCK can influence neurotransmitters, such as glutamate, dopamine, acetylcholine, and GABA, subsequently regulating brain function.^[81]

Corticotropin-releasing hormone (CRH) plays an important role in the HPA axis. It is abundantly expressed in the neurons of the hypothalamic paraventricular nucleus, mediating neural control of the release of pituitary adrenocorticotropic hormone (ACTH), and ultimately regulating glucocorticoids. CRH is also present in the colon and ileum and is released from enterochromaffin cells to participate in processes such as human stress and pressure regulation.^[105] Acute and chronic stress induce the release of gut CRH in mice, which regulates physiological processes under stress.^[106,107] However, the effects of gut CRH on the HPA axis and CNS activity have not been fully elucidated. Activation of the HPA axis can be modulated by gut microbiota. Corticosterone levels are elevated in germ-free and antibiotic-exposed mice, and Enterococcus faecalis can inhibit the activation of the HPA axis and affect social behavior in mice.^[108]

Gut bacterial metabolites and their influence on the brain

Approximately 2000 bacterial species have been identified in the human gut,^[80] and the genetic repertoire of the gut microbiome is estimated to include 232 million genes.^[61] The immense genetic and bioactive potential of the gut microbiota indicates its crucial role and prominent position in the human body. The gut microbiota transforms and metabolizes complex molecules from dietary and host sources, and the resulting metabolites such as SCFAs, bile acids, and tryptophan are widely involved in the regulation of neurodevelopment and neural activity.

SCFAs, the end products of the bacterial fermentation of polysaccharides, including butyrate, propionate, and acetate, serve as energy substrates for colonic epithelial cells.^[48] They can be transported from the intestine to the CNS and cross the BBB, thereby affecting neural function and development.^[48] SCFAs contribute to the maintenance of BBB integrity. Germ-free mice are unable to produce SCFAs, and thus develop impaired BBB permeability, as evidenced by the increased extravasation of Evans blue dye into the brain parenchyma and decreased levels of endothelial tight junction proteins, which can be reversed by supplementation with SCFAs.^[109] Using an in vitro BBB model, butyrate and propionate were found to promote the rearrangement of the actin cytoskeleton and tight junction proteins, thereby improving BBB integrity.^[110] Moreover, SCFAs can tighten the blood-cerebrospinal fluid barrier and maintain microglial activity and Aβ clearance in antibiotic-exposed mice, demonstrating therapeutic potential in AD.^[111] However, the mechanisms through which SCFAs improve BBB and blood-cerebrospinal fluid barrier integrity are not yet fully understood. In addition, SCFAs reduce neuroinflammation. Acetate exerts anti-neuroinflammatory effects through upregulation of G protein-coupled receptor-41, which is associated with inflammatory regulation, and inhibition of the extracellular signal-regulated kinase (ERK)/JNK/nuclear factor kappa B (NF-κB) pathway, significantly improving cognitive dysfunction in mice.^[112] Propionate and butyrate reduce the activation of microglia and decrease the levels of pro-inflammatory factors in germ-free mice through histone deacetylase inhibition, exerting neuroprotective effects.^[113] SCFAs regulate stress responses and emotional states. SCFAs can reduce the expression of genes involved in stress signaling in the hypothalamus, thereby improving stress responses and anxiety-related behaviors in mice.^[114] Acetate alters the levels of neurotransmitters, such as glutamine, glutamate, and GABA, in the hypothalamus.^[115] SCFAs are essential energy sources for neurons and glial cells in the CNS and contribute to brain development.^[116] Butyrate promotes neuronal growth and enhances synaptic plasticity, potentially affecting mood and cognition.^[52]

Bile acids (BAs) are the end products of cholesterol metabolism. Cholesterol is metabolized to primary BAs in the liver. Primary BAs enter the gut and generate secondary BAs such as deoxycholic acid (DCA), lithocholic acid, and ursodeoxycholic acid (UDCA) under the action of microbiota, which can bind to taurine and glycine to form conjugated BAs such as glycolithocholic acid (GLCA), taurolithocholic acid (TLCA), taurodeoxycholic acid (TDCA), and tauroursodeoxycholic acid (TUDCA).^[117] BAs from the gut or liver can cross the BBB to the brain via simple diffusion or through the BA transporter and are involved in the regulation of brain functions.^[118] The effects of BAs on the brain can be classified as either beneficial or detrimental. TUDCA is a strong inhibitor of mitochondrial apoptosis that induces neural stem cell proliferation, self-renewal, and neuronal conversion by improving the mitochondrial integrity and function of neural stem cells.^[119] It also reduces glial cell activation and the levels of pro-inflammatory cytokines and improves neuropathological phenotypes in animal models of various neurological diseases, such as AD and Huntington disease.^[118,120] UDCA is another BA that is considered beneficial for the human body. It inhibits the production of the pro-inflammatory cytokine IL-6 and nitric oxide in microglia.^[121] UCDA attenuates neuroinflammation and inhibits apoptosis by stabilizing mitochondrial membranes.^[122,123] A recent study showed that UDCA levels were lower in infants who were subsequently diagnosed with ASD, and this alteration correlated with the abundance of Bifidobacterium breve.^[41] DCA, GLCA, TLCA, and TDCA are cytotoxic BAs that are significantly associated with the disease state and cognitive impairment in patients with AD.^[124] DCA disrupts mitochondrial membranes, leading to an increase in reactive oxygen species (ROS), markers of inflammation, and apoptosis as well as decreased cell viability and DNA synthesis.^[124] In addition, DCA impairs learning and memory through blocking the activation of excitotoxic N-methyl-d-aspartic acid (NMDA) receptors.^[125]

Tryptophan metabolism involves the 5-HT, kynurenine, and indole pathways. The indole pathway is a microbial metabolic pathway for tryptophan, whereas the other two are host metabolic pathways that are either directly or indirectly regulated by gut microbes.^[126] In the indole pathway, tryptophan is metabolized by the gut flora into indole and its derivatives such as indole-3-aldehyde (IAld), indole-3-acetic-acid (IAA), and indole-3-propionic acid (IPA). Indole and its derivatives, such as IAA and IPA, can cross the BBB to exert anti-inflammatory, antioxidant, and neuroprotective effects, thereby influencing brain function and behavior.^[127,128] Lower serum concentrations of IAld are related to more severe depressive symptoms, suggesting that the tryptophan-indoles metabolism may be a crucial factor involved in the regulation of emotions.^[129] The aryl hydrocarbon receptor (AhR) is expressed in various neuronal cells, such as neurons, astrocytes, and microglia, and has important effects on neuronal proliferation, differentiation, and survival.^[130] Indole and its derivatives, IAA and IAld, activate AhR signaling in astrocytes and inhibit CNS inflammation.^[131] Neurogenesis is diminished in germ-free mice and in germ-free mice colonized with a single-gene TnaA knockout mutant Escherichia coli unable to produce indole.^[132] Supplementation with indole promotes neurogenesis in the hippocampus of adult mice through activation of AhR.^[132] Moreover, IPA mediates neuroprotection by modulating the cholinergic and redox regulatory systems, inflammatory stress, apoptotic response, and DNA damage in rats.^[133] Kynurenine is further metabolized to produce two neuromodulators, kynurenic acid (KYNA) and quinolinic acid (QUIN) .^[134] Both compounds, along with kynurenine, can cross the BBB and reach the CNS. QUIN induces excitotoxicity by activating NMDA receptors, increasing neuronal glutamate release, and inhibiting astrocytic glutamate reuptake.^[134] This process is closely associated with AD and other neurological diseases. KYNA is neuroprotective against QUIN-induced excitotoxicity.^[127] The effects of gut-derived 5-HT have previously been reviewed. The balance among the three tryptophan metabolic pathways plays a key role in regulating neural activity. One study found that, after transplanting the fecal microbiota of children with ASD into microbiota-free bees, the cognitive abilities of the latter significantly declined.^[135] Meanwhile, an imbalance in tryptophan metabolism was observed, indicating that melatonin and serotonin were significantly downregulated in the serotonin metabolism pathway, IA and IPA were downregulated in the indole pathway, and L-kynurenine was enriched in the kynurenine pathway.^[135] Association analysis showed that the differentially expressed genes closely related to these metabolites were involved in many important physiological processes, such as amino acid metabolism, fatty acid metabolism, neuroimmunity, and synaptic function in the brain, and were closely related to the realization of higher-order functions of the brain.^[135] Another study showed that Roseburia intestinalis inhibited the production of kynurenine and QUIN, increased 5-HT levels in the brain and colon, and improved synaptogenesis and neuroglial maintenance, thereby alleviating depressive behaviors in mice.^[136] Modulation of the conversion of tryptophan to indole, 5-HT, and kynurenine derivatives may emerge as targets for modulation of neural activity.

Therapeutic Approaches Targeting the Gut for Psychiatric and Neurological Disorders

Gut–brain interactions and their critical role in brain function provide the basis for using the gut as a target site for regulating and treating neuropsychiatric disorders. Strategies for altering the composition and activity of the gut microbiota, such as dietary modulation, probiotics, prebiotics, and FMT, can influence brain function and behavior, and play a therapeutic role in neuropsychiatric disorders including depression, anxiety, ASD, AD, and PD.^[8] In particular, FMT has demonstrated a distinctive research and development value in clinical trials in recent years.

The interaction between the gut microbiota and diet can significantly influence energy, inflammation, and immune regulation, and further affect neural activities through the gut–brain axis. Therefore, dietary intervention represents a safe and clinically feasible therapeutic approach, aligning with the traditional Chinese medical concept of ‘medicinal-food homology’. The Mediterranean diet, rich in fiber, polyphenols, and omega-3 fatty acids, promotes the growth of beneficial gut bacteria such as Clostridium leptum, Eubacterium rectale, Faecalibacterium prausnitzii, Bifidobacteria and Bacteroides, and improves metabolism.^[137,138] Adherence to a Mediterranean diet is associated with a lower risk of depression, anxiety, and cognitive decline.^[137,139] A ketogenic diet characterized by a low carbohydrate and high fat composition can prevent seizures in a microbiota-dependent manner.^[137] However, the underlying mechanism of action of gut microbiota in this process remains unclear. A thorough understanding of the crosstalk between gut microbiota and diet, along with personalized dietary approaches tailored to specific gut microbes, may substantially enhance therapeutic outcomes.

Current research increasingly prioritizes probiotic and prebiotic interventions targeting the modulation of the gut–brain axis. These microbial modulators can effectively re-establish beneficial gut microbiota and metabolic functions, enhance the integrity of the intestinal barrier and BBB, and attenuate systemic inflammation and glial activation, resulting in reduced neurodegenerative disease pathology.^[80] In a mouse model, Bifidobacterium breve reversed chronic stress-induced depression and anxiety symptoms.^[140] This effect may have been achieved through alleviating the hyperactive HPA response and inflammation by regulating glucocorticoid receptor expression.^[140] It also enhanced the expression of brain-derived neurotrophic factors.^[140] Chronic stress-induced gut microbial dysbiosis was restored, accompanied by increased SCFA and 5-HTP levels.^[140] Clinical trials have confirmed the effects of probiotics in improving cognitive function and preventing brain atrophy in older adult patients with suspected mild cognitive impairment,^[141] and enhancing the motor parameters of patients with PD.^[142] However, the underlying mechanism remains unclear.

In contrast to conventional probiotics, genetically engineered bacteria can specifically produce bioactive molecules and metabolites that potentially affect human health and exert specific effects on the host’s physiology and health. An engineered probiotic producing lactic acid can activate the natural anti-autoimmune pathways in dendritic cells, suppressing CNS inflammation and exerting neuroprotective effects in mice with experimental autoimmune encephalomyelitis.^[143] However, safety and ethical issues associated with engineered bacteria need to be fully addressed. The potential therapeutic effects of prebiotics are also interesting. Mannan oligosaccharides can reshape gut microbiota and enhance SCFA formation, concomitantly balancing cerebral redox homeostasis and suppressing neuroinflammatory responses.^[144] These coordinated mechanisms ultimately ameliorate cognitive deficits and attenuate anxiety- and obsessive-like behaviors in mice models of AD.^[144] Moreover, mannan oligosaccharides balance the HPA axis by decreasing corticosterone and CRH levels and upregulating norepinephrine expression.^[144] The modulatory effects of probiotics and prebiotics on different neuropsychiatric disorders remain to be validated using different combinations of preparations and by conducting appropriate clinical trials.

FMT is a gut microbiota intervention strategy that has been studied in recent years. It involves the transfer of processed microbial communities derived from healthy donor stools into a recipient’s GI tract.^[145] FMT can restore the gut microbiota in patients and is used to intervene in and treat gut diseases.^[145,146] The therapeutic effects of FMT on neuropsychiatric disorders have been partially investigated. FMT significantly improved psychological and GI symptoms and quality of life in patients with irritable bowel syndrome or functional constipation comorbid with anxiety and depression.^[147,148] Furthermore, FMT also improved GI symptoms and quality of life to some extent in patients with primary depression.^[149] FMT alleviates tic severity in children with Tourette syndrome (TS).^[150] This process is accompanied by remarkable changes in gut microbiota and amino acid metabolism.^[150] Moreover, FMT may reduce the pro-inflammatory immune response in some patients with TS.^[150] However, the mechanism through which FMT regulates the neural activity in the TS through the gut remains unclear. Animal studies suggested that the underlying mechanism may involve the promotion of 5-HT secretion.^[151] Children with ASD frequently present with significant GI symptoms and gut dysbiosis,^[67,152] and the severity of these symptoms is positively correlated with the severity of ASD.^[153] These findings substantiate the important regulatory role of the gut in ASD neurodevelopmental pathogenesis. In 2015, FMT was first performed in the treatment of ASD and showed remarkable effects, significantly reducing the Childhood Autism Rating Scale (CARS) score, a major clinical indicator.^[154] An open-label study demonstrated that FMT significantly ameliorated GI symptoms and ASD symptoms in children with ASD.^[155] Successful partial engraftment of donor microbiota was observed, concomitant with beneficial alterations in the gut environment, including enhanced overall bacterial diversity and increased abundance of taxa such as Bifidobacterium, Prevotella, and Desulfovibrio.^[155] Another study identified an association between reduced abundance of Eubacterium coprostanoligenes and response to FMT in ASD patients.^[156] Furthermore, the long-term efficacy of FMT for ASD was notable. Most improvements in both GI and ASD symptoms were sustained two years after treatment completion.^[157] Importantly, key alterations in the gut microbiota remained detectable during follow-up, including significantly increased bacterial diversity and elevated abundances of Bifidobacterium and Prevotella.^[157] Both AD and PD are accompanied with severe constipation symptoms that correlate with symptom severity in patients.^[158,159] One study reported that FMT could maintain and improve cognitive function in patients with mild cognitive impairment through altering gut microbiota structure and modulating lipid metabolism, but was less effective in patients with AD and more severe cognitive impairment.^[160] The preliminary exploration of FMT for PD has yielded encouraging results. In a randomized controlled trial, orally administered lyophilized FMT capsules increased the proportion of Firmicutes, reduced the proportion of Proteobacteria, and improved subjective motor and non-motor symptoms.^[161] Moreover, constipation and gut motility were improved.^[161] FMT infused via colonoscopy can also improve motor and non-motor symptoms of PD.^[162] FMT is also used to treat amyotrophic lateral sclerosis (ALS). One study reported the efficacy of FMT in two patients with late-onset classic ALS, who had a Japanese ALS severity classification of grade 5 and required tracheotomy and mechanical ventilation.^[163] After two rounds of FMT, both patients had significant improvement in respiratory function and were weaned off of mechanical ventilation; they also had improved muscle strength, which allowed for assisted standing and mobility^[163]. Other noteworthy treatment responses included improved swallowing function and reduced muscle fasciculations.^[163] The therapeutic mechanism may depend on increasing the abundance of beneficial bacteria such as Bacteroides and Faecalibacterium prausnitzii and altering metabolic pathways such as arginine and branched-chain amino acid biosynthesis.^[163] However, another study found that FMT failed to significantly slow the decline in the ALS Functional Rating Scale-Revised (ALSFRS-R) score in patients with sporadic ALS.^[164] No significant improvements were detected in respiratory function, muscle strength, autonomic nervous function, or cognitive abilities; observed benefits were limited to symptoms of constipation, depression, and anxiety, concomitant with an increase in Bifidobacterium.^[164] The above studies show the potential of FMT as a new treatment for neuropsychiatric disorders. However, further studies are needed to explore its long-term efficacy, safety, optimal treatment regimen, and strategies to overcome microbial colonization resistance.

Conclusion

Gut–brain interaction has emerged as a pivotal research focus in recent years. Accumulating clinical and preclinical evidence has substantiated the crucial role of the gut in maintaining systemic homeostasis and modulating neurophysiological processes. In addition to providing essential nutrients for neural development and functional maintenance, the gut influences neurological activity through multiple pathways, including neural circuits, neuroimmune signaling, neurotransmitters, gut hormones, and microbial-derived metabolites. Nutritional factors and gut microbiota constitute key regulatory components in gut-brain communication, acting on various nodal points along the gut–brain axis to mediate bidirectional interactions. Dysregulation of gut–brain interactions has been implicated in the pathogenesis of diverse neuropsychiatric disorders. Therapeutic strategies targeting the microbiota–gut–brain axis, particularly FMT, have gained increasing attention. FMT can effectively ameliorate gut microbial ecology and metabolic profiles, thereby exhibiting substantial therapeutic potential in neuropsychiatric conditions.

However, several critical questions remain unanswered. First, due to the high variability of the gut microbiota, the complexity of human neurological disorders, and the limitations of animal models in mimicking human diseases, the precise pathways and mechanisms underlying gut–brain communication are not yet fully elucidated. Second, while research has largely concentrated on the relationship between gut bacteria and the nervous system, the roles of viruses and fungi in the gut–brain axis remain significantly understudied. Furthermore, research on therapeutic interventions targeting the gut–brain axis, particularly concerning FMT for neuropsychiatric disorders, remains in preliminary stages. Lack of standardized protocols for donor screening, preparation methods, and dosing regimens results in inconsistent efficacy of FMT. Long-term data on neurological outcomes and safety is insufficient. Future research priorities should shift from purely observational studies to prospective studies, causal research, and in-depth molecular mechanistic explorations. Randomized controlled trials with larger cohorts are needed to validate clinical efficacy and establish standardized FMT protocols, thereby providing high-quality evidence for clinical application. Additionally, mechanistic studies to clarify FMT-modulated gut–brain axis signaling pathways are necessary. Integrated multi-omics analyses should be used to identify predictive biomarkers for treatment responses. Meanwhile, pioneering approaches to overcome colonization resistance are worthy of exploration.

The gut serves as a gate for understanding and exploring brain science, and nutrients and microbes offer two unique keys to unlocking this gate for regulating neural functions and treating neuropsychiatric diseases.

Conflicts of interest

None.