Sex biased gut dopamine signaling in multiple sclerosis

Abstract

Multiple sclerosis shows a strong sex bias, with unclear mechanisms. In this issue of Immunity, Peng et al elucidate a female-biased increase in intestinal dopamine signaling that diminishes protective Lactobacillus and exacerbates inflammation in a mouse model of multiple sclerosis.

Multiple sclerosis (MS) is an autoimmune disease in the central nervous system (CNS) that exhibits strong sex bias, with higher incidence, earlier onsets, and more frequent relapses in females.¹ However, the mechanisms underlying sex bias in MS remain largely unclear. Some gut commensal bacteria are sensitive to or utilize sex hormones.² Sex differences in the gut microbiome, such as lower abundance of Bacteroidetes in females,³ have been reported, which might likely become more pronounced when the gut microbiome is perturbed. Gut dysbiosis plays an important role in driving neuroimmune dysregulation that underlies neurological diseases.^4,5 Vice versa, inflammation in extraintestinal organs, including the CNS, may induce perturbation of the gut microbiome possibly via transient changes in gut permeability. The intestinal epithelium is the interface between gut bacteria/metabolites and the host, which may position the intestinal epithelium to be a critical converter and amplifier of sex-specific changes in the gut microbiome to sex bias in host response. This is elegantly demonstrated by Peng et al. using MOG-induced experimental autoimmune encephalitis (EAE), a mouse model of MS. Induction of CNS inflammation led to increased expression of dopamine receptor, Drd2, and Drd2-regulated lysozyme in the intestinal epithelium of female mice. This diminished lysozyme-sensitive Lactobacillus and Lactobacillus-derived anti-inflammatory N2-acetyl-L-lysine (NAL), which is able to pass through the gut epithelium and enter the circulation and CNS, ultimately resulting in more severe CNS inflammation in female mice (Figure 1).⁶

Figure 1.

A gut Drd2-Lysozyme-NAL axis in the female intestine is linked to microglia activation and increased CNS inflammation.

Peng et al. first demonstrated that Drd2 is expressed in both the colon and small intestinal epithelial cells, with highest expression near the crypt base. Following EAE induction, only female mice exhibited an increase in Drd2 expression in the intestinal epithelium. Conditional ablation of Drd2 specifically in the intestinal epithelium resulted in resistance to EAE, particularly among female mice; this was associated with reduced microglia activation and mononuclear cell infiltration and also downregulation of IL-17 signaling, IL-1β, and TNFα in the spinal cord. A gut-derived phenylethylamine (PEA), a D2-like receptor agonist, was found elevated in the stool of MS patients, particularly in female patients. Oral administration of PEA exacerbated the severity of EAE only in female mice, which could be reversed by Drd2 deficiency. They further elucidated that Drd2 colocalizes with lysozyme (Lyz1) in gut Paneth cells, and female mice lacking intestinal Drd2 had reduced Lyz1 expression within small intestinal IECs, suggesting a potential role for dopamine signaling in the regulation of Paneth cell function. Peng et al. further demonstrated that intestinal epithelial cell (IEC) Drd2 modulates Lyz1 activity through PKA (protein kinase A) signaling, and administration of lysozyme prior to EAE induction exacerbated EAE symptoms in a Drd2-, sex-, and gut microbiome-dependent manner.

Furthermore, Peng et al. observed an expansion of Lactobacillus in female mice lacking IEC Drd2, and metagenomic analysis identified Lactobacillus acidipiscis as the lysozyme-sensitive bacterium, which was validated through in vitro studies. Subsequently, untargeted metabolomic profiling of spinal cord samples from female mice lacking IEC Drd2 revealed elevated concentrations of N2-acetyl-L-lysine (NAL), which was found to be produced by Lactobacillus acidipiscis and exhibited potent anti-inflammatory properties both in vivo and in vitro. Interestingly, fecal levels of NAL decreased after administration of lysozyme, while treatment with NAL effectively alleviated EAE symptoms in a dose-dependent manner, particularly in female mice. NAL may limit EAE severity through inhibition of microglia activation. However, the mechanism underlying this effect remains unclear. The authors hypothesized that sphingosine-1-phosphoate (S1P) signaling implicated in MS pathology might be involved in NAL regulation during EAE.⁷ Indeed, administration of NAL reduced the expression of Sphk1, which catalyzes sphingosine conversion to S1P, and the proportion of MS-associated Sphk1+ microglia was reduced in NAL-treated EAE mice. These findings suggest that Lactobacillus acidipiscis-derived NAL exerts therapeutic effects on EAE severity in part by modulating sphingolipid metabolism specifically within microglia. In conclusion, this mouse study by Peng et al. has elucidated a gut bacteria-driven PEA-Drd2-Lysozyme-NAL-SIP axis with a female bias that may contribute to increased susceptibility of females to MS.

This study by Peng et al. reveals the gut microbiome as a driver of sex bias in MS and adds additional support to the emerging paradigm that the gut-brain axis is involved in the manifestation of some if not all immune-associated CNS diseases. However, there are some remaining questions for future inquiry. For example, it remains to be resolved how CNS inflammation leads to a more pronounced increase in the expression of Drd2 in the female intestine? There may be early events in the female intestine triggered by CNS inflammation that are linked to the expression of Drd2. For example, one of these events might be a change in Drd2 agonists in the gut, such as gut-derived PEA. The mechanisms underlying increased intestinal PEA in female MS patients remain elusive. Also, since sphingosine-1-phosphate is ubiquitously expressed and implicated in lymphocyte trafficking to the CNS, it would be interesting to investigate how NAL may potentially affect other aspects of immune response to CNS inflammation.

EAE induction disproportionally increases Drd2 in the female intestinal epithelium, leading to elevated lysozyme secretion from goblet cells and loss of lysozyme-sensitive Lactobacillus. This results in reduced levels of Lactobacillus-derived N2-acetyl-L-lysine to restrain the expression of Sphk1 and MS-associated Sphk1+ microglia, thereby increasing CNS inflammation.

Individuals with autoimmune diseases, such as MS, tend to display gastrointestinal symptoms, including altered gut microbiome and disrupted gut barrier function, which is sex dimorphic and consistent with the sex bias of autoimmune disease.^4,8 It remains unclear whether the gastrointestinal symptoms are consequential to autoimmune response originated from the primary affected organ, such as the CNS in the case of MS, or if preexisting gastrointestinal symptoms, due to genetic susceptibility or unhealthy diet, lead to gut inflammation that further fuels the autoimmune response. In either case, given the accumulating evidence that females are more prone to autoimmune response to pathogens or self-antigens, the threshold for inflammation-induced intestinal perturbation and symptoms might be lower in females. This may unfortunately further amplify gut inflammation and exacerbate autoimmune response in females. In support of this hypothesis, the study by Peng et al. demonstrates, at least in mice, that induction of CNS inflammation leads to more profound changes in dopamine signaling in the female intestine that culminate in loss of a gut bacteria-derived metabolite with an inhibitory effect on microglia activation and CNS inflammation. Can this vicious cycle in females with CNS autoimmune propensity be broken and reversed? Identifying the earliest events that link CNS inflammation and changes in the female intestine would be critical.

A conspicuous commonality between the intestine and brain is the neurotransmitters that are produced by both. The neurotransmitters in the intestine, such as dopamine and serotonin, appear to exert potent effects on both immune cells and IECs as well.⁹ In this way, gut-derived neurotransmitters may indirectly shape the response in the brain. Is it a coincidence that inflammation in brain triggers changes in the signaling of a neurotransmitter in the intestine? Has this gut-brain crosstalk been evolved to protect us? Understanding the intricate crosstalk between the gut and brain may unravel new opportunities to leverage modifications of the gut to prevent or treat CNS autoimmune diseases.