Gut Microbiota: a Player in Chronic Pain in Patients With Rheumatoid Arthritis?

There are currently many treatments available for patients with rheumatoid arthritis (RA). Although these treatments are very effective on inflammation, chronic pain persists in 20 to 30% of patients, even though clinical and biological remission seems to have been achieved, greatly reducing their daily quality of life. They are characterized in particular by a global lowering of pain thresholds, with a clinical picture of secondary fibromyalgia. Indeed, it seems that RA patients present central and peripheral hypersensitization of nociceptors, abnormalities in central pain processing and a defect in top-down inhibitory pain control mechanisms. It is also possible that RA patients with these inflammation-dissociated polyalgic pictures have a different pathophysiology of their disease than those with inflammation-only pain. A growing body of work shows that the gut microbiota (GM) affects pain pathways at multiple levels through the gut-brain axis. This axis relies, among others, on the afferent pathways of the vagus nerve. The microbiota modulates the activity of the vagus nerve by releasing neurotransmitters (gamma-aminobutyric acid, dopamine), by modulating immune cells (cytokines), by modulating the secretions of enteroendocrine cells (serotonin, cholecystokinins, glucagon-like peptide-1, peptide XY) or by releasing metabolites such as short-chain fatty acids, including butyrate. The latter is known to bind to several functional receptors present on vagal afferents. A decrease in Faecalibacterium has been associated with chronic pain and chronic fatigue.

RA is associated with dysbiosis of the gut microbiota, i.e. a change in the composition of the gut flora . Among the bacteria that are most often differentially represented between RA and control, Fecal bacteria are also less abundant in RA patients. The research team has conducted several experiments that clearly confirm the hypothesis that the microbiota of RA patients acts differently on the gut-brain axis than that of healthy subjects. Indeed, it is known that these RA patients present a defect in the control of parasympathetic activity and a link between dysbiosis of the GM and alteration of the autonomous nervous system (ANS) has already been shown in several pathologies. Considering that the GM is altered in RA and that alterations in the GM can modulate the ANS, the research team therefore hypothesized that the GM of RA patients is capable of altering cardiovascular function and particularly autonomic function. To demonstrate the role of the GM in ANS alterations, a mouse model with humanized microbiota by fecal microbiota transplantation (FMT) of RA patients or healthy subjects (age- and sex-matched controls) was developed and the research team showed that transfer of gut microbiota from RA patients toof mice previously treated with antibiotics alters the autonomic control of cardiac rhythm. Indeed, a decrease in vagal activity, objectified by the analysis of heart rate variability was observed, unpublished data. This confirms our hypothesis that the microbiota of RA patients acts differently on the gut-brain axis than that of healthy subjects. There is therefore a strong rationale for thinking that the microbiota of these patients could influence the central sensitization and control of pain and that RA patients with a fibromyalgia profile have a different microbiota from others.

Bacterial extracellular vesicles (EVs) are important vectors of communication between the gut microbiota and its host. They are the subject of increasing attention and could participate in the gut-brain axis. Indeed, some pathogenic EVs participate in the alteration of the intestinal barrier, pass into the bloodstream and can therefore act remotely from the intestine. Moreover, it has been shown that some bacteria (e.g. A. muciniphila and Faecalibacterium Prausnitzii) modulate the intestinal serotonin system mainly via their EVs. However, our preliminary experiments with stool transfer from RA patients suggest that serotonin (5-hydroxytryptamine 5 HT) is altered, as the research team showed that its expression is decreased in the intestine of mice transplanted with stool from RA patients (unpublished).

Hypothesis: RA patients have an altered gut microbiota or dysbiosis. Among the bacteria that are most often differentially found between RA and control, Faecalibacterium are less abundant in RA patients and a decrease in Faecalibacterium has also been associated with chronic pain patterns . The research team therefore hypothesizes that the intestinal microbiota of RA patients, and in particular the decrease in Faecalibacterium, would promote pain sensitization phenomena, the lack of top-down inhibitory control, and thus chronic pain despite the control of joint inflammation.

To test this hypothesis, the investigating team will compare the composition of the gut microbiota of patients with RA in inflammatory flare, RA in pain-free remission, RA without inflammation with chronic pain, healthy subjects without pain, and patients with fibromyalgia without inflammatory rheumatism. Microbial analysis by 16S ribosomal RNA gene sequencing and quantification of major microbiota-derived fecal metabolites (including short-chain fatty acids and tryptophan derivatives and vesicles

Next, investigators will also functionally evaluate the impact of the gut microbiota on pain sensitization. For this purpose, the research team will use in vitro experimental models (intestinal barrier models (Caco2) and neurons of vagal afferents (primary cultures obtained from murine jugular and nuchal ganglia). In an in vivo study, some faeces samples will be used to humanize the mouse microbiota and then evaluate the impact on tactile sensitivity (Von-Frey).