Mental health profoundly impacts inflammatory responses in the body

By Tarun Sai LomteReviewed by Lily Ramsey, LLMMay 30 2023

A recent study published in the Cell Journal uncovered mechanisms by which psychological stress affects bowel inflammation.

Study: The enteric nervous system relays psychological stress to intestinal inflammation. Image Credit: sdecoret/Shutterstock.com

Background

Psychological stress profoundly impacts inflammatory responses. The impact of psychological stress on the severity of the disease is particularly notable in inflammatory bowel disease (IBD).

Various epidemiologic studies support that stressful events can aggravate IBD flares. Still, the mechanistic basis underlying the stress-associated exacerbation of IBD is less understood.

The study and findings

In the present study, researchers explored the underlying mechanisms of the effects of psychological stress on enteric inflammation.

First, they induced colitis in a mouse model of chronic stress (restraint stress) and observed aggravation of intestinal inflammation. RNA sequencing uncovered striking gene expression changes.

Genes associated with type 2 immunity and antimicrobial peptides were downregulated, whereas IBD-associated genes and pro-inflammatory cytokines were upregulated.

Notably, stress experienced before colitis onset had the most potent effect on inflammation exacerbation. This suggests that stress might precondition the intestine for an enhanced inflammatory response during subsequent encounters with a colitogenic trigger.

Single-cell RNA sequencing of over 23,000 clusters of differentiation 45-positive (CD45⁺) leucocytes from colonic tissue of control and stressed mice identified 13 unique immune cell types.

In stressed mice, T cells, innate lymphoid cells, and monocytes or macrophages had the highest differentially expressed genes. Further, the team found that innate or adaptive lymphocytes did not drive bowel inflammatory response to stress.

Therefore, the researchers focused on myeloid cells, especially macrophages and monocytes. Sub-clustering of data revealed three monocytes (Mono 1-3) and two macrophages (Mac 1-2) subsets.

Pseudo-time trajectory analysis indicated monocyte accumulation in stressed mice. Depleting mice of C-C motif chemokine receptor 2-positive (CCR2⁺) monocytes protected them from stress-mediated exacerbation of colitis.

Further, tumor necrosis factor (TNF)-producing monocytes were highly accumulated in colons from stressed mice. Neutralizing TNF by a monoclonal antibody (mAb) protected mice from the stress-mediated effect.

Next, the researchers followed the transmission of psychological stress from the brain to the intestine. Restraint stress in mice significantly increased serum levels of corticosterone and noradrenaline.

The team inhibited the brain-mediated induction of the release of adrenal corticosterone, which diminished corticosterone levels and rendered the mice resistant to the effects of restraint stress on colitis.

Likewise, adrenalectomy or inhibiting glucocorticoid receptor (GR) signaling prevented the stress-induced exacerbation of colitis.

The researchers speculated that the GR signaling in myeloid cells was responsible for the stress-mediated effects on IBD. Nonetheless, mice without a GR gene, nuclear receptor subfamily 3 group C member 1 (Nr3c1), in myeloid cells were susceptible to stress effects similar to their littermate controls, suggesting an indirect result of glucocorticoids on monocytes.

Next, the team investigated whether chronic psychological stress impacted enteric nervous system (ENS) cells. To this end, Nr3c1 was deleted from enteric neurons and glia. This protected mice from the stress-mediated impact of colitis and prevented monocyte accumulation.

These findings collectively suggested that ENS could be a relay between glucocorticoids and intestinal inflammatory response.

Next, single-nucleus RNA sequencing of enteric neurons and glia were performed, and uniform manifold approximation and projection (UMAP) clusters were identified.

The sub-clustering analysis uncovered four unique transcriptional states. One state, termed enteric glia associated with psychological stress (eGAPS), was exclusive to stressed conditions.

Mice depleted of enteric glia were resistant to the effects of stress on colitis. Moreover, glia ablation also prevented monocyte accumulation in the colon. Next, the team generated and analyzed a pairwise interaction map based on single-nucleus and single-cell transcriptomes, which showed several hypothetical interactions between colonic myeloid cells and eGAPS.

The team focused on interactions between the Mono1 cluster and eGAPS because of high Tnf expression in these monocytes. Colony stimulating factor 1 (Csf1) was one of the mediators of this interaction.

Moreover, Csf1 expression was elevated in enteric glia upon stress. High colonic Csf1 expression depended on ENS GR signaling because Nr3c1 deletion blunted Csf1 induction upon stress.

Neutralizing CSF1 protein with a mAb conferred resistance to stress effects on colitis. Next, the researchers studied the neuronal compartment of the single-nucleus ENS dataset.

Nitrergic and cholinergic subsets of mature neurons were under-represented in the stressed group, whereas precursors were enriched. This implied that stress increased precursor-like neurons while reducing mature neurons.

Further experiments suggested that stress-induced a transition toward a less differentiated phenotype and decreased cholinergic and nitrergic neurons, leading to dysmotility.

Expression profiles of immature and mature enteric neurons were compared. Transforming-growth factor beta 2 (Tgfb2) gene was significantly associated with the precursor state.

Using a TGF-β-neutralizing antibody prevented the transition and restored intestinal motility in stressed mice. Finally, the researchers explored the relationship between psychological stress, bowel inflammation, and dysmotility in human IBD patients.

Patients with high chronic psychological stress were at a significantly elevated risk of developing IBD relative to those without stress.

Psychological stress was also linked to a more severe IBD post-diagnosis. Leucocytes, including monocytes, were increased in stressed patients. Patients with stress were more likely to report dissatisfaction with bowel habits and obstipation, develop ileus, and require surgery. The team investigated whether these changes were general or specific to IBD patients.

They studied populations in the United Kingdom (UK) Biobank who were disease-free or patients with irritable bowel syndrome or an extraintestinal disease (rheumatoid arthritis).

They identified that dysmotility was associated with stress in all populations, but elevated inflammatory markers and monocyte accumulation were only observed in the context of intestinal disease.

Conclusions

The authors identified cellular and molecular events linking stress perception to the aggravation of intestinal inflammation.

Overall, the study provided a mechanistic basis for brain effects on peripheral inflammation and identified ENS as a relay between psychological stress and intestinal inflammation, suggesting that stress management could prove valuable in IBD care.