Bidirectional regulation of the brain–gut–microbiota axis following traumatic brain injury

Abstract

Traumatic brain injury is a prevalent disorder of the central nervous system. In addition to primary brain parenchymal damage, the enduring biological consequences of traumatic brain injury pose long-term risks for patients with traumatic brain injury; however, the underlying pathogenesis remains unclear, and effective intervention methods are lacking. Intestinal dysfunction is a significant consequence of traumatic brain injury. Being the most densely innervated peripheral tissue in the body, the gut possesses multiple pathways for the establishment of a bidirectional “brain–gut axis” with the central nervous system. The gut harbors a vast microbial community, and alterations of the gut niche contribute to the progression of traumatic brain injury and its unfavorable prognosis through neuronal, hormonal, and immune pathways. A comprehensive understanding of microbiota-mediated peripheral neuroimmunomodulation mechanisms is needed to enhance treatment strategies for traumatic brain injury and its associated complications. We comprehensively reviewed alterations in the gut microecological environment following traumatic brain injury, with a specific focus on the complex biological processes of peripheral nerves, immunity, and microbes triggered by traumatic brain injury, encompassing autonomic dysfunction, neuroendocrine disturbances, peripheral immunosuppression, increased intestinal barrier permeability, compromised responses of sensory nerves to microorganisms, and potential effector nuclei in the central nervous system influenced by gut microbiota. Additionally, we reviewed the mechanisms underlying secondary biological injury and the dynamic pathological responses that occur following injury to enhance our current understanding of how peripheral pathways impact the outcome of patients with traumatic brain injury. This review aimed to propose a conceptual model for future risk assessment of central nervous system-related diseases while elucidating novel insights into the bidirectional effects of the “brain–gut–microbiota axis.”

Introduction

Adverse outcomes in patients with traumatic brain injury (TBI) may persist for a long time and may include complications such as bacterial infections, gastrointestinal (GI) diseases, and further deterioration of neural transmission. These involve a complex range of pathophysiological mechanisms, including excitotoxicity, oxidative stress, inflammation, immune suppression, cell apoptosis, and autophagy (Hanscom et al., 2021). Despite attempts to provide effective defense strategies through animal experiments, the prognosis of patients with severe TBI remains unfavorable. Mounting evidence suggests that the gut microbiota plays a crucial role in maintaining health and managing disease in the host, particularly within the CNS (Benakis et al., 2016; Sundman et al., 2017). The concept of the “microbiota–gut–brain axis,” which governs bidirectional gut–brain communication, has been firmly established, facilitating the maintenance of physiological equilibrium (Sundman et al., 2017).

Peripheral influences play a pivotal role in regulating the physiological processes of the brain in both healthy and diseased states. It serves as the primary mode of signal transmission by integrating incoming and outgoing signals from neural, hormonal, and immune sources to establish a systematic connection between the gut–spinal cord–brain complex (Panther et al., 2022). Dysfunction of this axis occurs as a pathological consequence of disease onset. The role of gut microbiota in neurological and psychiatric disorders has gradually been uncovered, as gut dysbiosis disrupts the development and maturation of the CNS and enteric nervous system (ENS) by unfavorably affecting the host’s immune response (Warner, 2019; Chen et al., 2022). The feedback regulation involving the brain, gut, and microbiota is complex and closely interconnected, and several questions are still worth considering, such as whether CNS injury directly affects changes in gut microbiota and their metabolites, or if it indirectly acts through peripheral neuro-immune crosstalk. How alterations in the gut microbiota and neuronal activity influence the localization of initial injury, the signaling molecules involved, and the sites where potential enduring effects manifest all need to be clarified; therefore, this review on the bidirectional communication between gut microbiota and the CNS aims to clarify these concepts.

This article highlights the significance of the bidirectional “brain–gut–microbiota axis” in influencing the adverse prognosis and functional behavior post-TBI through the involvement of neuro-immune and endocrine factors. The present review utilizes TBI as a case to propose a theoretical framework for comprehending the potential mechanisms underlying risks after CNS injury. Our approach transcends the limited effects of alterations in the gut microbiome to emphasize the interconnections between anatomically independent systems, thereby establishing a foundation for effective clinical prevention and treatment.

Search Strategy

The relevant literature was retrieved by conducting an electronic search of the PubMed database between September 7 and October 22, 2023. During the search strategy and selection criteria, we employed the following keywords: “traumatic brain injury,” “central nervous system injury,” “brain–gut axis,” “sympathetic nervous system,” “vagus nerve,” “intestinal barrier,” “neuroinflammation,” “immunosuppression,” “neuroendocrine,” “enteric nervous system,” “sensory afferent nerve,” “host defense mechanisms post-traumatic pain,” and “post-traumatic depression.” Various combinations of these search terms were comprehensively utilized to access the literature. Additionally, we conducted a thorough review of relevant literature titles and abstracts to further enhance the screening process. Since CNS injury also includes various disease types, such as ischemic stroke, hemorrhagic stroke, and spinal cord injury. In this narrative review, we try to focus only on the study of TBI and its complications, along with the underlying pathogenic mechanisms.

Negative Impacts on Gut Microbiota Post–Traumatic Brain Injury

The occurrence of TBI leads to several secondary effects, and the disruption of the GI environment caused by CNS damage triggers an ecological imbalance, potentially leading to systemic inflammation and compromised neurological outcomes. A better comprehension of alterations in the composition of the gut microbiota following TBI and the underlying mechanisms of brain–gut interaction can enhance the functional prognosis of CNS injury.

Modify the composition and abundance of gut microbiota

CNS damage significantly affects the abundance and diversity of bacteria residing in the gut. Clinical trial data indicate that severe TBI results in a significant reduction in intestinal bacteria, with levels reducing to 1/1000 of those of healthy controls on the day of injury, particularly the levels of anaerobic bacteria and Lactobacillus spp. Moreover, 2 weeks post-TBI, the intestinal flora and concentration of major short-chain fatty acids (SCFAs) do not return to normal levels, while the population of harmful Enterococcus and Pseudomonas gradually increase (Hayakawa et al., 2011). Patients with chronic TBI exhibit enduring alterations in the composition of the gut microbiome, characterized by elevated levels of Actinobacteria, Firmicutes, and Ruminococcaceae and decreased levels of Bacteroidetes and Prevotella (Ma et al., 2017; Urban et al., 2020; Table 1). In mouse models, at 24 hours after controlled cortical impact, the populations of Lactobacillus gastricus, Ruminococcus flavus, and Eubacterium gastroensis deceased significantly, while the populations of Eubacterium sulci and Marwinia forme increased significantly (Treangen et al., 2018). Moreover, previous studies have reported increased abundance of Firmicutes, Proteobacteria, and Ruminococcus, as well as an increase in the population of Bacteroidetes, Lactobacillus, and Clostridium difficile 7 days post TBI in rodents (Treangen et al., 2018; Celorrio et al., 2021; Taraskina et al., 2022; Table 1). Additionally, the population of Bifidobacterium—one of the key microorganisms that promote the production of γ-aminobutyric acid (GABA) in the body to regulate the response of the gut–brain axis—increased significantly 7 days after a severe controlled cortical impact injury (Bao et al., 2023). Therefore, given that the severity of brain damage caused by various methods varies and changes in gut flora at different time points post-TBI exhibit some dissimilarities, it indicates heterogeneity in the cascade of molecular effects directly resulting from TBI.

Table 1.

Effect of traumatic brain injury on the gut microbiome

Species	Traumatic brain injury method	Study range	Effect of microbiota	Reference
Sprague–Dawley male rats	Closed head injury model of engineered rotational acceleration	Both 1 and 9 d post-injury	Firmicutes/Bacteroidetes ratio increased; Perturbations of Firmicutes lineages	Smith et al., 2024
C57BL/6 male and female mice	Controlled cortical impact injury	3 d or 7 wk post-injury	Lactobacillus is the majority; Dubosiella increased in male mice; Faecalibaculum increased in female mice	Holcomb et al., 2024
C57BL/6 male mice	Controlled cortical impact injury	3 d post-injury	Alphaproteobacteria, Actinobacteria, Gammaproteobacteria increased; Bacteroidia, Epsilonproteobacteria, 4C0d-2 decreased	Ma et al., 2019
Traumatic brain injury group: 21 males, 1 female; control group: 13 males, 5 females;	Type of traumatic brain injury incurred not reported	27−502 mon post-injury	Chronic traumatic brain injury increased Actinobacteria, Firmicutes, and Verrucomicrobia and decreased Bacteriodetes; family-level changes: chronic traumatic brain injury increased Unc03qxR and Unc02cwq and decreased Prevotellaceae; species-level changes: chronic traumatic brain injury increased Bacteroides thetaiotaomicron and decreased Prevotella copri, Prevotella spp., and Sutterella spp.	Urban et al., 2020
C57BL/6J mice, male, 6−8 wk	Controlled cortical impact	7 and 3−90 d post-injury	Depletion of Lactobacillus, Clostridium, and Bacteroides; Atopobium and Ruminococcus increased	Celorrio et al., 2021
Patients with traumatic brain injury in 2015 and provided fecal sample in 2020	Type of traumatic brain injury incurred not reported	5 yr post-injury	Traumatic brain injury induced the Corynebacterium genus and Alistipes genus increased; the Corynebacterium and Pseudomonas were detected significantly	Pyles et al., 2024

Influence the adhesion and colonization of the gut microbiota

The harmonious symbiosis of intestinal flora in vivo relies on its adhesion and colonization capabilities (Macpherson et al., 2023). The aberrant expansion and migration of intestinal endogenous bacteria during a diseased state constitute a significant risk factor for peripheral infection following TBI. Comparing the cross-colonization of microbiota from different donor sources in the body revealed that the host can select its own microbiota by facilitating microbial adhesion through antimicrobial peptides (AMPs) or mucopolysaccharides (Figure 1). Goblet cells secrete mucins, which are highly glycosylated products that are important for the selection of symbiotic bacteria by providing attachment sites for bacterial adhesins (Arike and Hansson, 2016). A previous study demonstrated that abnormal mucin glycosylation is a consequential outcome of infections and inflammation and serves as a crucial regulatory mechanism for the colonization of intestinal commensal bacteria within the body (Magalhães et al., 2015). Additionally, a specific spectrum of biofilms is formed by microorganisms that colonize the mucus layer of the intestinal epithelium, exhibiting selectivity in subsequent species adhesion (Engevik et al., 2021). Overall, the mechanisms coordinating host-microbiota colonization are vital for resisting infections by opportunistic pathogens.

Figure 1.

Adverse consequences of TBI on gut microbiota.

TBI induces alterations in the abundance and composition of gut microbiota, which may contribute to the pathogenesis of TBI sequelae by disrupting the symbiotic relationship between microbiota and intestinal epithelial cells. Epithelial cells play a crucial role in maintaining bacterial adhesion through mucin secretion and AMPs, while a unique mutual selection mechanism exists within the bacterial community. Moreover, during disease progression, commensal bacteria undergo degeneration and migration, leading to peripheral infection via intestinal bacterial translocation. Created with BioRender.com. AMPs: Antimicrobial peptides; TBI: traumatic brain injury.

Unleash the potential pathogenicity of the gut microbiota

A previous study shows that the gut microbiota may be capable of restoring CNS damage in neurological disorders (Howard et al., 2017). However, in most cases, a shift in the host’s commensal microbiota towards opportunistic pathogens can significantly impair the prognosis of TBI (Figure 1). After severe TBI, there is a significant increase in the abundance of Lachnoclostridium, Acinetobacter, Bacteroides, and Streptococcus in the gut of mice 3 hours post-injury. After 7 days, the relative abundance of Acinetobacter in the lungs increases to 16.2%, and an analysis of lung tissue microbiota revealed that 49.69% of microorganisms originated from the gut, ultimately leading to lung infection (Yang et al., 2022b). The disruption of the intestinal mucosal epithelial barrier serves as the primary mechanism for opportunistic pathogens to invade, encompassing various multicellular pathways such as downregulation of paracellular tight junction complexes, impaired secretion of AMPs by Paneth cells, and dysregulation of homeostasis in microfold cells within intestinal lymph nodes (Iacob et al., 2018; Drolia and Bhunia, 2019; Fekete and Buret, 2023). Hence, tracing the origins, functions, and changes of microorganisms in the brain, lungs, and urinary tract is an effective approach for preventing poor prognosis after TBI.

Top-Down Modulation of the Brain–Gut–Microbiota Axis: Dysregulation of the Microbiota Following Central Nervous System Injury

Gut–brain communication maintains the homeostasis of both the CNS and GI, and this influences the composition and function of the gut microbiota, as well as the integrity of the intestinal barrier. It also plays a role in immune monitoring within both visceral organs and the circulatory system through neuro-immune–endocrine efferent signals. These processes collectively shape a healthy ecological environment within the GI (Li et al., 2020; Neufeld et al., 2024; Zhang et al., 2024). An acute TBI implies a dynamic process wherein significant therapeutic opportunities arise from identifying and addressing potential delays. However, the precise interplay between the brain-gut-microbiota triad and its impact on TBI recovery remains elusive.

Autonomic nervous system perspectives for traumatic brain injury

Sympathetic nervous system disorders

Paroxysmal sympathetic hyperactivity is a primary etiology of secondary nerve injury in patients with TBI. Diffuse or focal brain injury disrupts the connectivity between one or more brain centers and cortical inhibitory centers, such as the hippocampus, amygdala, insular cortex, cingulate cortex, and other brainstem nuclei. This disruption leads to the interruption of descending inhibitory pathways, loss of inhibition of spinal sympathetic reflexes, increased excitatory interneurons activity, and increased sympathetic activity (Jafari et al., 2022). The SNS releases more neurotransmitters after TBI, including catecholamines, epinephrine, norepinephrine, and dopamine (Straub et al., 2006). These neurotransmitters trigger opposing pro-inflammatory and anti-inflammatory responses in the gut, depending on the receptors they bind to (Straub et al., 2000, 2002). The induction of sympathetic denervation by 6-hydroxydopamine results in an increase in activated CD68⁺CD86⁺ macrophages and monocytes within the intestinal mucosa. This leads to an up-regulation of proinflammatory, such as cytokines interleukin-1 (IL-1), tumor necrosis factor-α (TNF-α), and interferon gamma (IFN-γ), while down-regulating the expression of arginase-1 and CD163. Consequently, there is a decrease in intestinal epithelial barrier function and an enhancement of antimicrobial defense (Mallesh et al., 2022). The SNS also plays a regulatory role in the modulation of T helper 1 (Th1), Th17, and regulatory T cells (Tregs) within the intestinal environment (Kasper and Shoemaker, 2010; Figure 2). Demyelination occurs when proinflammatory Th1/Th17 cells migrate to the CNS (Lee et al., 2020; Lin et al., 2021). SNS-induced dendritic cells can partially counteract the effects of Th1/Th17 cells and restore homeostasis within the gut-brain axis under pathological conditions. Additionally, ablation of β-adrenergic receptors inhibits systemic immune responses (resulting in a decrease in the number of circulating CD4⁺ T cells) and also induces elevated levels of beneficial SCFAs within the colon.

Figure 2.

The impact of traumatic brain injury on the brain–gut–microbiome axis from a top-down perspective.

(1) After TBI, the SNS regulates the presence of CD68⁺CD86⁺ macrophages and monocytes in the gut, thereby playing a crucial role in maintaining intestinal barrier integrity and enhancing antimicrobial defense ability. (2) The activation of the VN leads to the stimulation of the descending cholinergic anti-inflammatory pathway. The release of ACh by VN binds to receptors on immune cells and inhibits inflammatory responses. In addition, ACh induced the activation of the WNT pathway contributes to AMP secretion and initiates host defense. The activation of VN suppresses the inflammatory response of myometrial macrophages via the JAK2/STAT3 signaling pathway. (3) TBI results in damage to the intestinal mucosal barrier, characterized by epithelial cell apoptosis and an increased population of microfold cells. (4) TBI induces activation of the HPA axis, resulting in the elevated release of ACTH and corticosterone levels, which subsequently leads to increased paracellular permeability of intestinal epithelial cells. (5) Moreover, TBI results in the hyperactivation of adrenergic neurons within the ENS and triggers the conversion of monocytes into an anti-inflammatory phenotype. (6) TBI results in peripheral immunosuppression, leading to a shift from Th1 cells towards Th2 cells and inducing spleen atrophy. Created with BioRender.com. ACh: Acetylcholine; ACTH: adrenocorticotropic hormone; AMP: antimicrobial peptide; ENS: enteric nervous system; HPA axis: hypothalamic–pituitary–epinephrine axis; JAK2: Janus kinase 2; SNS: sympathetic nervous system; STAT3: signal transducer and activator of transcription 3; VN: vagus nerve; WNT: canonical Wnt/β-catenin pathway.

The innervation effect of the SNS on the colon of mice gradually increases in a proximal-to-distal manner (Muller et al., 2020). A previous study has demonstrated an upregulation of proto-oncogene protein c-fos expression in both the celiac ganglion and superior mesenteric ganglion of germ-free (GF) mice, indicating evident neuronal activation (Muller et al., 2020). However, following fecal microbiota transplantation (FMT), there is a decrease in c-fos expression, suggesting that exogenous sympathetic nerves possess direct sensing capabilities and provide feedback responses to intestinal microorganisms. Following CNS injury, increased SNS activity in the gut results in increased permeability of the intestinal mucosal barrier and peripheral immunosuppression (Stanley et al., 2016). This is evidenced by the upregulation of triggering receptors expressed on myeloid cells 1 in inflammatory macrophages, leading to intestinal bacterial translocation and adverse outcomes associated with peripheral infection (Liu et al., 2019; Figure 2). Therefore, the SNS pathway is responsible for the disruption of the gut microbiota post-TBI.

Vagus nerve dysfunction

TBI leads to dysfunction of the autonomic nervous system. The vagus nerve (VN), which originates from the paraventricular nucleus (PVN) of the hypothalamus, serves as a primary component of the parasympathetic nervous system. Its descending nerve fibers project towards the pituitary gland and ventral tegmental area, where they influence both the hypothalamic–pituitary–adrenal (HPA) axis and dopamine system. Various studies have demonstrated that post-TBI, VN stimulation (VNS) can effectively restore forelimb movement and coordination capabilities in rats, thereby leading to improved neurological function scores (Pruitt et al., 2016; Du et al., 2024). The application of VNS can attenuate the upregulation of aquaporin protein-4, restore the integrity of the blood–brain barrier (BBB), induce angiogenesis, enhance metabolic activity in key regions, such as the forebrain, thalamus, and reticular structure, elicit a broad spectrum of cortical activation, facilitate brain arousal, and ultimately promote restoration of the injured brain area (Neren et al., 2016; Collins et al., 2021; Divani et al., 2023). Following neurological trauma, a cascade of secondary damage occurs, including excessive generation of free radicals, which surpasses the capacity of antioxidant mechanisms and leads to irreversible oxidative injury. VNS can safeguard against nerve injury in rats after TBI by suppressing oxidative stress, inflammation, and apoptosis, which is mediated through the nuclear factor kappa-B/nucleotide-binding domain-like receptor protein 3 signaling pathway (Tang et al., 2020). Furthermore, it has been discovered that the PVN of the VN nucleus synthesizes oxytocin, which exerts an inhibitory effect on microglial activation by downregulating extracellularly regulated protein kinases and P38 phosphorylation (Yuan et al., 2016). The findings imply that the activation of the VN plays a crucial role in ameliorating neurodevelopmental abnormalities and neurological impairments.

VN input is the main anti-inflammatory pathway in the gut, as it releases acetylcholine, which binds to muscarinic receptors, promotes proper differentiation, maintains peripherally derived Tregs (Nakata et al., 2022), and enhances the secretion of AMPs and lysozyme (Shi et al., 2014; Labed et al., 2018), while simultaneously inhibiting the release of inflammatory factors by macrophages (Stakenborg et al., 2019; Yang et al., 2021; Figure 2). This intricate mechanism serves as a crucial regulator of the immune ecological niche in the gut. VNS treatment can ameliorate the reduction of CD103⁺ cells in intestinal lymph nodes, increase the Treg/Th17 ratio, and promote intestinal tolerance to inflammation in rats with hemorrhagic shock after a traumatic injury (Morishita et al., 2015). The expression of GABA_A receptors in the dorsal nucleus of the neurons of the VN can be suppressed by electroacupuncture therapy. Furthermore, simultaneously stimulating the efferent pathway of the VN and activating the Janus kinase 2/signal transducer and activator of transcription 3 signaling pathway mediated by the α7 nicotinic acetylcholine receptor on intestinal macrophages inhibits intestinal inflammation (Yang et al., 2021; Figure 2). Additionally, the prophylactic administration of a 5-hydroxytryptamine receptor 4 inhibitor results in a significant reduction in intestinal inflammation. However, this anti-inflammatory effect is not observed in α7 nicotinic acetylcholine receptor knockout mice, indicating an interaction between 5-HT and the cholinergic anti-inflammatory pathway (Stakenborg et al., 2019).

Although the gut microbiota can communicate with the CNS through endocrine and immune pathways, vagal signaling is one of the fastest and most direct routes (Yuan et al., 2021). The VN remains intact following TBI, and sensory afferents drive motor output to establish a neural reflex circuitry. Clinical studies have demonstrated that in patients with heightened intestinal barrier permeability, brief stimulation of the VN prevents an increase in paracellular permeability and impedes the dissemination of pathogenic microorganisms (Reese, 1987; Krzyzaniak et al., 2012; Bonaz, 2022). The findings of infection-related studies suggest that VNS treatment effectively enhances the release of acetylcholine, which enables the host mucosa to effectively combat intestinal pathogens (Ramirez et al., 2019; Chu et al., 2021). On one hand, the egl-30–egl-8 axis, which encodes Gaq and phospholipase Cb in muscarinic receptors, has been found to confer resistance against Staphylococcus aureus (Labed et al., 2018). On the other hand, intestinal infection triggers the secretion of acetylcholine by ENS neurons, which in turn activates the downstream WNT/β-catenin pathway and initiates the barrier defense mechanisms by intestinal epithelial cells (Labed et al., 2018; Figure 2). Additionally, T cell-derived Ach can promote the transcription of Ifny and other genes, which effectively reduces the bacterial load of Citrobacter rodentium in the gut and establishes a stable host-symbiotic environment (Ramirez et al., 2019). Therefore, cholinergic signaling is a key mediator in maintaining the brain–gut–microbial ecological balance.

Neuroendocrine perspectives for traumatic brain injury

A significant risk factor for TBI is the presence of post-traumatic stress disorder, and clinical evidence suggests that approximately 25% of patients with TBI exhibit adrenal insufficiency resulting from the inhibition of HPA axis activation (Powner and Boccalandro, 2008). The occurrence of TBI results in hypopituitarism, which suppresses the stress response of the HPA axis and diminishes adrenocorticotropic hormone production, thereby impairing adrenal gland stimulation and reducing corticosterone synthesis (Figure 2). The apoptosis rate and Caspase-3 expression of adrenal cells are elevated during the acute stage of mild TBI, while the apoptotic protein expression in pituitary cells is increased during the chronic stage of mild TBI, indicating that apoptosis plays a role in the occurrence and progression of endocrine dysfunction in the HPA axis following TBI (Taheri et al., 2022).

Intestinal barrier damage is a pathological characteristic of intestinal complications resulting from post-traumatic stress. However, studies have demonstrated that probiotics actively participate in inhibiting the excessive reaction of the HPA axis, thereby reducing plasma adrenocorticotropic hormone and corticosterone concentrations. This ultimately preserves the paracellular permeability of colonic epithelial cells and plays a protective role in maintaining intestinal integrity (Ait-Belgnaoui et al., 2012; Figure 2). Gut microbiota can modulate hormone release by influencing the activity of key components of the limbic system, such as the amygdala, hippocampus, and thalamus (Bastiaanssen et al., 2019). Similarly, the HPA axis can impact changes in gut microbiota through endocrine factors (Sudo, 2014; Figure 2). Excessive activation of the β-adrenergic system can lead to cortisol depletion, which further perpetuates the pathological mechanism of chronic inflammation (Bugajski et al., 1995; Abdullahi et al., 2020; Zhao et al., 2022). It has been demonstrated that the disruption of the sympathetic–adrenal medullary system mediates immune suppression and increases susceptibility to infection in mice with high-level spinal cord injury (Prüss et al., 2017). Prophylactic adrenalectomy effectively mitigates the occurrence of glucocorticoid overdose and lymphocyte depletion induced by spinal cord injury; however, it is not protective against pneumonia.

Intestinal microenvironment perspectives for traumatic brain injury

Breach of intestinal barrier integrity

The bidirectional communication between the CNS and ENS is crucial for the regulation of various GI functions, including motility, secretion, blood flow, intestinal permeability, mucosal immune activity, and visceral sensation (Rhee et al., 2009; Liu et al., 2024). The functional and structural dysfunction of the GI tract after TBI has been demonstrated in both human subjects and experimental models (Bansal et al., 2010; Urban et al., 2020). Adrenergic hyperactivity mediates small intestinal mucosal injury and increased permeability in experimental rats 72 hours post-TBI (Yang et al., 2022b). This disruption results in the translocation of intestinal commensal bacteria, along with their by-products, such as lipopolysaccharide (LPS), leading to peripheral infection and potentially sepsis.

Apoptosis of intestinal epithelial cells and loss of tight junction proteins between epithelial cells result in increased gut barrier permeability after TBI, relying on the release of metalloproteinases and reactive oxygen species by neutrophils (Pan et al., 2019). Mitochondrial autophagy effectively attenuates oxidative stress-induced epithelial cell apoptosis through the inhibition of the extracellular regulated protein kinases/nuclear factor erythroid 2-related factor 2/heme oxygenase-1 signaling pathway (Liu et al., 2017), for which CCAAT/enhancer binding protein deta may be an upstream target (Banerjee et al., 2019). Moreover, gut microbial disorders are associated with mucus barrier breakdown, when the mucus layer thins, which exposes more antigens and bacteria to the epithelium (Paone and Cani, 2020). Ablation of recombinant forkhead box protein O1, which maintains goblet cell function, significantly increases susceptibility to GI inflammation (Chen et al., 2021). The polysaccharide structure of mucus facilitates the function of the intestinal immune barrier by binding to lectin-like proteins present in immune cells, such as the mucin 2 receptor complex, thereby inhibiting the proinflammatory effects of dendritic cells (Shan et al., 2013).

Once pathogens break through the physical and mucus barriers, innate immunity is first activated, followed by adaptive immunity. The host has evolved a large amount of mucosa-associated lymphoid tissue (enrichment of T and B lymphocytes) for host defense, which can stimulate the secretion of regenerating islet–derived protein 3 gamma by intestinal epithelial cells during Gram-negative bacteria activity (Abreu, 2010). Moreover, GF mice exhibit diminished sizes of Peyer’s patches, decreased production of immunoglobulin A, and lowered expression of Toll-like receptors (Shirakashi et al., 2022). The mucosa-associated lymphoid tissues in the GI encompass a unique subset of microfold cells that function as a conduit for specific microorganisms to enter and exit (Kobayashi et al., 2019; Figure 2). When the differentiation and maturation of microfold cells are impaired, it results in reduced B cell follicle size and diminished secretion of immunoglobulin A. Consequently, the initiation of adaptive immunity in the gut is hindered, thereby facilitating the escape of pathogenic microorganisms from the gut after central impairment (Nagashima et al., 2017).

Abnormalities of the enteric nervous system

Multiple genes implicated in CNS dysfunction, such as Chd8, Tcf4, Slc6a4, Shank3, and Nlgn3, also contribute to ENS dysfunction and ultimately result in functional GI disorders associated with CNS damage (Niesler and Rappold, 2021). The excessive activation of the adrenergic system has been demonstrated to result in impaired intestinal physiological function in TBI experiments. Noradrenergic neurons in the ENS exert their effects on intestinal monocytes via β2 adrenergic receptors, as they promote the differentiation of myometrial macrophages into an M2 phenotype (Gabanyi et al., 2016), suppress the production of Th1 cytokines by dendritic cells, and impede the differentiation of monocytes into Th1 cells in vivo (Panina-Bordignon et al., 1997; Ramer-Quinn et al., 1997; Figure 2). The ENS neurons induce goblet cell secretion of AMP and mucus by producing IL-18, thereby impeding the colonization of Salmonella bacteria to facilitate host defenses (Jarret et al., 2020). In addition, the absence of neuromodulatory mechanisms in enteric glial cells affects the immunoregulatory function of IL-22 release from ILC3 cells, which impairs the defense function of intestinal epithelial cells, leads to dysbiosis of the microflora, and increases susceptibility to intestinal infections (Sonnenberg et al., 2011; Ibiza et al., 2016).

Peripheral immunosuppression perceptions for traumatic brain injury

Stage I: burst of central inflammation

The immune system must appropriately respond to injury, and TBI is accompanied by a range of secondary effects that continuously pose challenges to the peripheral immune system. Damage to the CNS and BBB after TBI leads to leakage of brain-derived antigens and inflammatory mediators, leading to a systemic inflammatory reaction syndrome while triggering increased activity of resident innate immune cells within the brain. Additionally, circulating leukocytes, complement proteins, and inflammatory cytokines are activated. Following the activation of the myeloid cell system, neutrophils initially infiltrate the injured brain tissue, followed by varying degrees of natural killer cell and T cell aggregation on the ipsilateral side of the injury. These immune cells are recruited through multiple pathways, including lymphoid tissue, blood vessels, arachnoid membranes or the choroid plexus (Celorrio et al., 2024; Figure 2). The initiation of adaptive immune responses and the establishment of unique immune memory are facilitated by antigen presenting cells, which present antigenic peptides to immune cells (Jha et al., 2019). The SNS and the HPA axis act as mediators that coordinate peripheral immune responses with central inflammatory reactions (Wong et al., 2011; McCulloch et al., 2017).

Stage II: peripheral immunodeficiency

TBI stimulation leads to an elevation in the levels of inflammatory factors, such as TNF-α, IL-1β, as well as chemokines chemokine (C–X–C motif), ligand 1 protein (CXCL1), CXCL2, and CXCL3, among others. The expression of triggering receptors that are expressed on myeloid cells 1 on peripheral myeloid cells amplifies the pro-inflammatory response of both brain-derived and gut-derived immune components (Liu et al., 2019). When the proinflammatory response becomes excessive, the body will release anti-inflammatory factors as a compensatory mechanism; however, this may result in an increased susceptibility to peripheral infections (Celorrio et al., 2024). The primary mechanism underlying CNS injury-induced immunosuppression involves a transition from a Th1 to a Th2 lymphocyte phenotype, accompanied by reduced lymphocyte and NK cell counts in both the bloodstream and spleen, as well as compromised neutrophil and monocyte defense mechanisms (Prass et al., 2003; Faura et al., 2021; Figure 2). The occurrence of pulmonary edema and pneumonia with systemic immunosuppression has been observed 24 hours after experimental TBI (Vermeij et al., 2013). The immune dysfunction persists for several weeks following TBI, and studies conducted on mice with closed head injuries have confirmed a reduction in the number of peripheral blood monocytes within 24 hours post-injury, which continues for up to 1 month (Schwulst et al., 2013). Additionally, there is a shift towards an M2 phenotype characterized by anti-inflammatory abilities. Administration of Streptococcus pneumoniae infected TBI mice resulted in elevated levels of high mobility group box 1 in the peripheral circulation and elevated IL-1β and TNF-α in the cerebral cortex, whereas TNF-α in the lung was relatively reduced compared to that in the lungs of mice that were given Streptococcus pneumoniae infection with sham surgery (Doran et al., 2020). Furthemore, the expression of M2-like macrophage markers was increased, leading to the manifestation of more severe bacterial infections in the lungs, thereby confirming that TBI compromises monocyte immune function.

Role of the hypothalamic–pituitary–adrenal axis in peripheral immunity

Activation of the HPA axis following brain injury has been demonstrated to stimulate the secretion of glucocorticoids (Jehn et al., 2010), which subsequently modulates B cell differentiation and suppresses B cell lymphopoiesis (Igarashi et al., 2005). The administration of glucocorticoids in mice infected with Trypanosoma cruzi and Mycobacterium avium induces immune organ atrophy, which can be reversed to restore peripheral immune function to a state of normal physiological function by inhibiting the HPA axis (Majlessi et al., 2006; Pérez et al., 2007). In addition, mice exhibit evident signs of systemic immunosuppression, and atrophy of the spleen and thymus accompanied by lymphopenia have been observed during brain injury caused by Angiostrongylus cantonensis infection; however, the reduction in B and T cells was not attributed to apoptosis but rather resulted from impaired development following thymocyte activation and compromised B cell genesis (Chen et al., 2016; Qiao et al., 2023). Further preclinical and randomized controlled trials are required to elucidate the profound impact of alternative therapies on the activation of the HPA axis–induced immunosuppression, specifically focusing on their effects on the gut microbiota in patients with TBI.

Role of brain-spleen crosstalk on peripheral immunity

Acute brain injury results in spleen atrophy, highlighting the crucial role of the spleen in maintaining immune homeostasis both centrally and peripherally (Yu et al., 2021). Studies have shown that mice treated with LPS exhibit depressive-like phenotypes, resulting in abnormal expression of ionized calcium binding adapter molecule 1 and postsynaptic density protein 95 in the hippocampus, while denervation of the spleen effectively restores elevated plasma IL-6 and leads to significant changes in microbiota composition (Erny et al., 2015; Ma et al., 2022a). The SNS and HPA axis serve as mechanisms for brain–spleen communication, wherein norepinephrine binds to β2 adrenergic receptors and activates glucocorticoid receptors to directly suppress lymphocyte activity or cellular immunity (Figure 2). Furthermore, they enhance the secretion of anti-inflammatory cytokines or cortisol and even promote apoptosis of immune cells by interacting with VN, leading to increased synthesis of Th2 cytokines (Sharma et al., 2019; Celorrio et al., 2024). The far-reaching effects of neuro-immune regulatory mechanisms between the brain, spleen, and gut after TBI necessitate further elucidation.

Down-Top Alterations in the Brain–Gut–Microbiota Axis: Disruptions in Gut Microbiota Exacerbate Central Nervous System Symptoms

Post-traumatic pain is a common clinical manifestation after TBI. The inflammatory response and microbial disorders are the main causes of this pain. A study showed that inhibiting the release of substance P from transient receptor potential vanilloid 1 (TRPV1) mechanoreceptors can reduce damage-related Tau hyperphosphorylation and improve cognitive and behavioral disorders after TBI (Corrigan et al., 2021). Moreover, this hinders the up-regulation of TRPV1 in cerebral vascular endothelium cells induced by TBI, and assumes an anti-apoptotic role to protect the integrity of the BBB (Yang et al., 2019). It is evident that sensory afferents play a crucial role in the pathophysiology and pathology of TBI. However, recent studies have demonstrated that the depletion of the gut microbiota can effectively prevent or significantly suppress neuropathic pain induced by chronic nerve injury (Ma et al., 2022b; Chen et al., 2023). The transplantation of fresh fecal bacteria has been shown to restore the manifestation of neuropathic pain behavior (Ma et al., 2022b). This suggests that targeting the gut microbiota could be a promising therapeutic approach for treating neuropathic pain. The host’s response and coordination with gut bacterial pathogens is a highly intricate phenomenon that integrates multiple signaling pathways (Góralczyk-Bińkowska et al., 2022; Rastogi et al., 2022). Herein, we elucidate the underlying mechanisms of “down-top” signaling triggered by alterations in the microbiota after TBI and its unfavorable prognosis.

Sensory perception and defense of the gut microbiome

The role of microbes in the regulation of the CNS in neurological disorders has been well established. However, it is important to acknowledge that sensory afferents from the spinal cord and the vagal ganglion represent the two primary pathways through which information from the GI tract is transmitted to the CNS (Breit et al., 2018; Perna et al., 2021; Ma et al., 2022b). The cell bodies of these neurons are located in the dorsal root ganglia (DRG) and tuberous/jugular ganglia (Figure 3). Spinal afferents terminate at the dorsal horn of the spinal cord, while vagal afferents project to the nucleus of the solitary tract in the brainstem (Adidharma et al., 2022). The activation of nerve terminal nociceptors is commonly believed to be initiated by inflammatory mediators produced by immune cells (Chavan et al., 2017). This means that afferent signals are transmitted from the periphery to the spinal cord, while efferent impulses propagate from nerve axons back to nerve terminals, resulting in the local release of vasoactive mediators in the surrounding tissues. This phenomenon is known as “neurogenic inflammation” (Matsuda et al., 2019). Recent studies have demonstrated that sensory neurons directly respond to various bacterial products, including bacterial toxins and metabolites (Lai et al., 2017; Baral et al., 2018; Saelinger et al., 2019). Pain is elicited upon bacterial invasion of the body, and nociceptive neurons transmit this aversive sensation upwards through a variety of mechanical and chemical molecular sensors. Peripheral involvement in the prevention and control of bacterial infection has emerged as a prominent research focus in recent years. Therefore, we examined the potential mechanisms underlying host sensing and defense against microbial alterations, translocation, invasion, and infection following TBI.

Figure 3.

The potential mechanisms by which the brain senses and responds to microbial changes.

The ascending transmission involves vagal afferents terminating in the DMV and dorsal root afferents of the spinal cord, whose terminals are equipped with molecular sensors to detect inflammatory mediators and microbiota alterations. The secretions and metabolites produced by bacteria can also activate nociceptors and induce an increase in intracellular calcium flux, which in turn, stimulates the nociceptors to release neurotransmitters (such as CGRP) for signal transmission to the host brain. Additionally, EECs possess intrinsic sensory conduction properties, enabling the formation of neural circuits with vagal afferents to facilitate the transmission of alterations in the GI milieu upwards. Created with BioRender.com. CGRP: Calcitonin-gene–related peptide; DMV: dorsal nucleus of vagus nerve; EECs: enteroendocrine cells.

Vagal afferent perspectives

The chemoreceptors of vagal afferents play a crucial role in the transmission of glucose, fat, and appetite-related signals (Williams et al., 2016; Han et al., 2018; Berthoud et al., 2021). A previous study involving experimental bacterial infection models revealed that the microbial sensing ability of vagal afferents and oral administration of Helicobacter pylori or Salmonella typhimurium significantly activated the expression of c-fos in the vagal ganglia and nucleus of the solitary tract (Riley et al., 2013). Therefore, the vagal afferent pathway may serve as a potential monitoring mechanism in the progression of TBI-induced augmented intestinal barrier permeability and subsequent peripheral infection resulting from gut bacterial translocation.

The activation of vagal afferents contributes to the regulation of neuro-immune interactions through the expression of immune factor-related receptors, thereby maintaining body homeostasis. When degeneration of neurons in the dorsal nucleus of the VN impairs complete signal transmission within the VN circuit, there is an increase in the number of inflammatory monocytes and mature macrophages, resulting in a significant release of pro-inflammatory factors (such as TNF-α and CXCL1), thereby exacerbating intestinal inflammation (Wirtz et al., 2007; Qi et al., 2012). TRPV1⁺ nociceptors have been shown to regulate innate immune responses in fatal pneumonia caused by drug-resistant Staphylococcus aureus. Targeted depletion of these neurons by diphtheria toxin improves neutrophil and γδT cell responses, thereby promoting efficient bacterial clearance and improving host survival (Baral et al., 2018). After Staphylococcus aureus infection, Myd88^–/– mice exhibited heightened pain-like hypersensitivity at 72 hours post-infection. The aforementioned studies provide compelling evidence implicating bacterial activity in sensory signaling via immune activation (Hanke et al., 2012; Figure 3). Additionally, bacteria directly trigger calcium influx and action potentials in nociceptors through N-formylated peptides and pore-forming toxin α-hemolysin (Chiu, 2018; Figure 3), implying an unsuspected role in the host-pathogen interaction of GI dysfunction following CNS injury.

Neuropodocytes, a specific subset of enteroendocrine cells (EECs), actively participate in ascending signal transmission through the establishment of synaptic connections with the VN. Neuropodocytes exhibit electrical excitability and employ glutamate as the neurotransmitter for synaptic transmission with the vagal nerve, thereby establishing a neuroepithelial circuit that connects the GI lumen to the brainstem (Figure 3). This helps the brain to quickly perceive intestinal stimuli (Bellono et al., 2017; Kaelberer et al., 2018). The activation of the Gaolf/adenylate cyclase signaling cascade in EECs by isovaleric acid, a type of SCFA, promotes intracellular Ca²⁺ influx mediated by the sensory receptors Trpa1 and Olfr558 (Psichas et al., 2015; Bellono et al., 2017; Figure 3). The process activates intracellular signaling factors in EECs, including prominent molecular sensors, such as G protein-coupled receptors, Toll-like receptors, and microbial associated molecular pattern receptors (Lai et al., 2017; Kaelberer et al., 2020). When the EEC senses LPS, G protein-coupled receptors are activated, leading to the release of glucagon-like peptide-1, which compromises the integrity of the intestinal mucosa (Lebrun et al., 2017; Figure 3). In the context of TBI, the role of intestinal epithelial and immune cells in GI surveillance and the initiation of protective responses following gut barrier breakdown is well established, while the contribution of vagal afferents remains underestimated.

Spinal afferent nerve perspectives

Spinal afferent nerves undoubtedly play a crucial role in rapid signal transmission between the GI and CNS. Anterograde tracing experiments in mouse DRGs have revealed the presence of exogenous efferent terminals in the GI tract. These external neurons originate in the DRG of the spinal cord, project throughout the GI tract and relevant immune sites (lymph nodes, mucosa-associated lymphoid tissues), and are responsive to noxious stimuli, such as bacterial pathogens and toxins (Lai et al., 2017; Figure 3). The majority of spinal afferents projecting to the gut consist of slowly conducting unmyelinated C fibers, which are commonly characterized by the expression of transient receptor potential channels. These fibers often exhibit terminals that contain various peptides, including calcitonin-gene-related peptide (CGRP) or substance P. The activation of TRP channels triggers the release of peptides from efferent terminals through axonal reflexes, subsequently exerting their effects by binding to neuropeptide receptors on immune cells (Chiu et al., 2013; Meseguer et al., 2014; Figure 3).

Numerous studies have been conducted to explore the role of sensory nerves in the neuro-immune interactions of the peripheral tissue barrier (Talbot et al., 2015; Gabanyi et al., 2016; Liu et al., 2016). Skin nociceptors interact with IL-33 through the expression of growth stimulation-expressed gene 2 to induce immune activation (Liu et al., 2016). Activation of Nav1.8⁺ neurons in the DRG induces cytokine production by CD4 and ILC2 cells, including IL-5, via the vasoactive intestinal peptide/vasoactive intestinal peptide receptor 2 axis (Talbot et al., 2015). A study showed that colonization of the colonic epithelium of TRPV1^–/– mice by Citrobacter rodentium resulted in increased species abundance in fecal samples, which was inversely correlated with decreased expression of neutrophil chemokines and adhesion factors (Chiu et al., 2013). These findings provide strong evidence for the role of peripheral neural-immune crosstalk in enhancing barrier defense capabilities.

Nociceptors are capable of expressing Toll-like receptors and other pattern recognition receptors to detect microbe-associated molecular patterns present on the surfaces of bacteria (Peters and Emrick, 2023). With the discovery that intestinal Salmonella infection triggers nociceptive circuit activation to initiate the host’s defenses (Lai et al., 2020), an increasing number of studies have focused on the role of nociception and pain perception in the microbial domain. TRPV1⁺ nociceptive neurons transmit signals from enteral stimuli to the spinal cord in response to exogenous bacterial invasion, leading to the release of CGRP from peripheral terminals. The secretion of CGRP plays a crucial role in regulating the population of microfold cells within Peyer’s patches in the small intestine, thereby restricting the access points of pathogenic microorganisms entering or exiting the intestinal tract (Figure 3). Moreover, the sensory neurons that innervate the gut have a regulatory role in the tissue programming of macrophages at the distal level during Salmonella infection (Gabanyi et al., 2016). However, in experimental models of colitis or irritable bowel syndrome, TRP receptors exhibit exaggerated responses, which contribute to peripheral inflammation and nociception through the expression of related receptors on immune cells or induce local inflammation by releasing vasoactive substances (Kihara et al., 2003; Szitter et al., 2010; Guo et al., 2021; Perna et al., 2021). Hence, the combination of immunotherapy and nociceptor activation can synergistically enhance the host’s defenses and concurrently coordinate the immune response, thereby yielding improved defensive effects.

A recent study demonstrated that Nav1.8⁺ nociceptors transmit signals via the CGRP/receptor activity-modifying protein 1 signaling axis to intestinal goblet cells, thereby regulating mucus production and facilitating barrier protection (Yang et al., 2022a). Depletion of these nociceptors in mice results in a decrease in the mucus layer and dysbiosis of the gut microbiota. Although the capacity of DRG neurons to perceive and react to bacteria and their metabolites has been elucidated in the context of infection and inflammation-related disorders, the role of DRG neurons in TBI and its unfavorable prognosis remains unknown, thus representing a future avenue for exploring alternative therapies for secondary peripheral infection following TBI.

Microbiota and brain

The comprehension of the initiation events and outcome endpoints of microbial stimulation impacting neural circuits is a crucial objective in mitigating adverse outcomes following TBI. The neural pathways involved in the bidirectional brain-gut-microbe connection after TBI can either descend locally from the brain or ascend from nerve terminals to reach specific nuclei in the brain. Utilizing optogenetics and chemical genetics has facilitated our understanding of the relative contribution of these neural circuits to behavioral changes dependent on microbes.

The hypothalamus (which receives sensory and visceral information), PVN (involved in physiological homeostasis and stress regulation), central amygdala (associated with fear processing and memory formation), anterior cingulate cortex, supplementary somatosensory cortex, and insular cortex effectively respond to peripheral neuronal inputs and perform their respective functions. Activation of neurons in the gigantocellular paralateral nucleus/anterior ventrolateral medulla region of the brainstem, which innervate sympathetic preganglionic neurons projecting to the GI, is triggered by alterations in microbial composition, resulting in a deceleration of GI transport (Muller et al., 2020). The agouti-related peptide and pro-opiomelanocortin neurons in the arcuate nucleus of the hypothalamus project to the sympathetic preganglionic brain–derived neurotrophic factor neurons in the PVN and establish a neural circuit that regulates the plasticity of the sympathetic postganglionic nerve, thereby exerting direct control over energy homeostasis within the body (Wang et al., 2020). The insular cortex plays a crucial role in the integration of external immune responses, and its association with sympathetic innervation and visceral information processing highlights its significance in mice with colitis (Han et al., 2018; Koren et al., 2021). After LPS infection, the nucleus of the solitary tract-area postrema region of mice responded rapidly, with the activation of adenylate cyclase activating polypeptide 1 (ADCYAP1⁺) neurons playing a pivotal role in the brain’s perception mechanism (Ilanges et al., 2022). By suppressing ADCYAP1⁺ neurons, the manifestation of LPS-induced phenomena, such as anorexia, obesity, and acinesia can be significantly attenuated, thereby establishing a crucial connection between central processes and infection. Furthermore, vasopressin cells in the PVN can further diminish sucrose preference in both male and female mice during LPS infection, while specifically inducing social behavioral deficits in male mice (Sgritta et al., 2019; Whylings et al., 2021). These findings imply that sex hormones exert specific nuclear regulation on brain regions. Due to the cascading nature of multiple biological processes in TBI complications, drug interventions targeting a single nervous system have been shown to be ineffective in achieving therapeutic outcomes. The aforementioned active regions, which exhibit microbial effects on sensing, offer compelling evidence to elucidate the interplay between the central and peripheral nervous systems triggered by TBI. The intricate bidirectional regulation can be disentangled through the identification of distinct brain nuclei that fulfill separate functions in priming and outcome.

Consequences of Dysregulation in the Brain–Gut–Microbiota Axis

Exacerbation of neural inflammation

There is increasing evidence suggesting that the CNS and the immune system are closely interconnected. This functional interaction explains the occurrence of immune responses following CNS injury. Resident microglia activation and peripheral blood neutrophil recruitment in early TBI are followed by the infiltration of macrophages derived from lymphocytes and monocytes. Proinflammatory and anti-inflammatory cytokines promote each other to terminate the neuroinflammatory response after trauma, while chemokine signaling leads to the activation and recruitment of immune cells (Simon et al., 2017). Some risk factors establish a bias toward the immune response in the injured brain, such as altered microbiota, impaired immune surveillance, abnormal signal transmission, and the release of brain-derived antigens, which are freshly recognized by the immune system and then turn on the autoimmune response. Furthermore, antibiotic-induced modifications of the gut microbiota result in a reduction in ischemic brain injury. Specifically, dysbiosis hinders the migration of effector T cells from the GI tract to the leptomeninges following stroke. Within the brain, these cells contribute to heightened neuroinflammation. Gut dysbiosis also impacts the integrity and permeability of the BBB. When combined with the physical disruption caused by TBI, it facilitates easier passage of intestinal contents and the associated upregulation of proinflammatory immune responses into the CNS. This leads to heightened microglia activity, neuroinflammation, and neuropathology (Erny et al., 2015; Sun et al., 2016). TBI-induced gut dysbiosis may contribute to heightened activation of microglia following CNS injury (Witcher et al., 2021). The observed mitochondrial dysfunction in TBI may also be influenced by gut dysbiosis, which exerts an impact on mitochondrial function (Pandya et al., 2014).

Although the findings discussed so far detail the potential mechanisms by which altered microbial composition aggravates neurological damage, most of these are experimental studies; therefore, further clinical studies are needed to elucidate these interactions and confirm these scientific findings. Additionally, small molecules secreted by bacteria also support CNS function. Notably, microglial developmental defects in GF-conditioned FFAR2^–/– (SCFA receptor) mice lead to impaired brain connectivity and social behavioral alterations (Erny et al., 2015). Supplementation with SCFAs restores the expression of c-fos both peripherally and centrally, thereby ameliorating neuroinflammation. The attenuation of CNS inflammation is accompanied by a reduction in white matter demyelination and axonal loss, as propionic acid induces a population of gut-derived Tregs, promoting neuroprotection (Pandya et al., 2014). Overall, transplantation of SCFA is similar to the effects of gut probiotics, as it exerts a protective function on the injured brain through the gut-brain axis.

Abnormal neurotransmission

The development of depression following CNS injury is typically a slow process triggered by negative emotional responses that hinder the recovery of neurological function, including anxiety, depression, and social behavioral disorders (Margolis et al., 2021; Chen et al., 2022). Summarizing the underlying mechanisms of bidirectional communication in the “brain–gut–microbe” axis, makes it easy to understand that the interwoven networks of the nervous system and neurosecretory monoamines play an essential role in modulating emotions. The primary injury of TBI leads to the loss of hippocampal neurogenesis, transient activation and long-term inhibition of pain sensing in the sensory cortex, reduction of brain-derived neurotrophic factor release at the injury site, and disruption of the balance between excitatory and inhibitory neurotransmitters, which are risk factors for the interlinkage of multiple regions in the brain. For instance, cortical–striatal–pallidal–thalamic–cortical projection dysfunction mediates the development of post-stroke depression (Loubinoux et al., 2012). Moreover, the gut microbiota modulates the function of central GABAergic nerves (Jiang et al., 2015) and the metabolism of D-glutamate (Chang et al., 2020), which play a crucial role in emotional-behavioral functions, such as anxiety, cognition, and learning post-CNS injury. Furthermore, the gut microbiota play significant roles in the regulation of neurogenesis and neuroplasticity; however, their roles in depression following TBI remain uncertain. A study reported that hippocampal neurogenesis in adult GF mice exhibited significantly higher proliferation compared with that in conventional mice (Ogbonnaya et al., 2015). Furthermore, bacterial cell wall components have been reported to have the ability to traverse the maternal–fetal barrier, resulting in increased cortical nerve proliferation and cognitive impairment in the offspring (Humann et al., 2016). Antibiotic treatment decreases spatial and object recognition abilities in mice but could be restored by the administration of the probiotic VSL3 (Möhle et al., 2016). Using magnetic resonance imaging to monitor brain network connectivity in diseased states can explore potential therapeutic targets for functional changes or weakened connectivity.

Studies have demonstrated that pre-TBI restoration of the gut microbial community structure through fecal microbiota transfer can mitigate neurocognitive deficits in mice following TBI (Du et al., 2021; Davis et al., 2022; Medel-Matus et al., 2022). Within 90 days post-injury, FMT-treated TBI mice exhibited reduced cortical volume loss, preserved white matter connectivity, and increased microglial activation in the brain (Davis et al., 2022). These activated microglia facilitate the maintenance of functional connectivity among brain regions, including the amygdala, hippocampus, and prefrontal cortex. However, clinical TBI encompasses a spectrum of injury progression, ranging from mild to severe, diffuse to focal, and single to multiple occurrences. Therefore, the generalizability of FMT to the entire spectrum of TBI processes requires further investigation. Additionally, controlling for differences in age, sex hormones, and physical conditions poses significant challenges in FMT treatment.

Secondary peripheral infection

Experimental and clinical studies have shown that TBI can lead to severe peripheral organ and systemic functional impairments. The most common complications include respiratory and urinary tract infections caused by Gram-negative bacteria. While risk factors, such as hospitalization, drowsiness, swallowing difficulties, aspiration, and bladder dysfunction make patients susceptible to infection (Cook et al., 2020; Shepetovsky et al., 2021; Zhang et al., 2022), there is evidence supporting two independent risk factors (Sharma et al., 2019): gut bacterial translocation (Yang et al., 2022b) and immune suppression (Stephens et al., 2023). Although antibiotics are clinically administered for the treatment of infections following TBI, some patients treated with moxifloxacin experience persistent infections and exhibit elevated levels of plasma IL-10 (Chamorro et al., 2007). After conducting a comprehensive analysis of the aforementioned potential mechanisms, it has been postulated that catecholamines exhibit impaired regulatory effects on immune factors, consequently leading to the loss of inhibitory capacity of IL-10 towards IFN-γ. This subsequently leads to a shift from Th1 to Th2 responses, resulting in an inadequate resolution of bacterial invasions and infections. The neuroimmune regulatory systems constitute a dynamic and intricate network of interconnected responses and pathways that facilitate specific mechanisms for immune suppression and infection. The above chapters provide a cutting-edge review on the regulatory mechanisms of the “brain–gut” axis on microbiota and the effects of microbiota in the “gut–brain” axis. There is a pathological similarity in CNS injury–related diseases, such as stroke, spinal cord injury, and TBI; however, there is heterogeneity in the mechanisms of bidirectional communication between the brain and gut, and infections are not completely associated with neurodegeneration.

Conclusion

The microbial–gut–brain axis is a widely recognized concept in neuro-immunology. It encompasses ascending neural pathways that transmit information through the spinal cord and brain stem for host defense, and the descending ANS that regulates a range of GI functions by binding receptors to affect immunity and metabolism. Changes in microbial colonization or deficiency could affect CNS efficacy, orchestrate local and systemic immune homeostasis, and facilitate crosstalk between the ENS and the CNS (Table 1). These interactions highlight the value of bidirectional brain–gut communication after CNS injury. Although invisible lines connect these three parts, they still belong to different regional modules in terms of organizational structure. By investigating the interconnection between these nodes through biogenetic means, we aim to explore the unique properties of neuro-immunity while largely preserving anatomical structure and functionality. However, further research is needed to specifically elucidate the dynamic processes through which gut microbiota signals and their products traverse disease-affected states, such as the gut barrier, BBB, and leptomeninges.

It should be noted that this review does not primarily focus on examining the role played by mechanical barriers in systemic spread. Additionally, GI complications of CNS injury involve multiple branches, including the metabolic, endocrine, and immune systems, and each has distinct anatomical characteristics. Understanding the commonalities and specificities of potential signaling molecules in the peripheral system is a challenging task; however, it will ultimately contribute to defining the most applicable interventions in clinical practice.

Traditionally, the utilization of antibiotics, dietary supplementation, and FMT are beneficial in terms of facilitating the colonization of commensal bacteria, enhancing resistance against pathogenic microorganisms, preserving mucosal barrier integrity, and collectively shaping a healthy intestinal niche. Moreover, these approaches serve as effective strategies to maintain a harmonious host-microbe symbiotic balance (Figure 4). Almost all neurological diseases or injuries have a common feature: prolonged neuroinflammation. Studies comparing rodent microbiomes before and after exposure have found this inflammation to be a result of reciprocal causality. Given the intricate nature of the GI environment, it is imperative to further investigate the temporal alterations in pathways that regulate and perceive microbial changes through signal transmission.

Figure 4.

Timeline showing the role of brain–gut–microbiota axis in TBI in the literature.

GI: Gastrointestinal; TBI: traumatic brain injury.

Additional file: Open peer review report 1 ^{(81.5KB, pdf)} .

Funding Statement

Funding: This work was supported by the National Natural Science Foundation of China, No. 82174112 (to PZ); Science and Technology Project of Haihe Laboratory of Modern Chinese Medicine, No. 22HHZYSS00015 (to PZ); and State-Sponsored Postdoctoral Researcher Program, No. GZC20231925 (to LN).

Data availability statement:

Not applicable.