Investigating Immunomodulatory Signaling Pathways Influenced by Substance-P

Abstract

The immune and the nervous systems engage in bidirectional communication under both physiological and pathological conditions. Over recent decades, classical neurotransmitters and neuropeptides have been recognized as key modulators of immune responses. Substance P (SP), a neuropeptide released primarily by nociceptive neurons, is well known for its role in pain sensation but has also been implicated in immune regulation. SP exerts its effects by binding to tachykinin receptors (TACR1, TACR2, and TACR3), which are expressed on various immune and non-immune cells. Previous work from our lab demonstrated that TACR1 antagonism in the Dextran-Sodium Sulfate (DSS)-induced colitis model results in a significant reduction in inflammatory Th17 cells while promoting regulatory CD4+ T-cell populations, thereby mitigating colitis severity. However, the mechanisms underlying SP-TACR1- mediated T-cell differentiation remain unclear. This thesis investigates (i) The signaling pathways underlying the SP-TACR1 interaction that can result in a Th17 development bias. (ii) The regulation of TACR1 expression in different immune cell populations and (iii) Expression pattern of different TACR1 isoforms in healthy human PBMCs. Based on our literature review, we hypothesized that SP-TACR1 signaling can directly induce phosphorylation of STAT3 at the Y705 residue, a key transcription factor regulating Th17 development. Our findings confirm this hypothesis, demonstrating that SP-induced STAT3 phosphorylation takes place in a TACR1-dependent manner in multiple immune cell populations. Furthermore, we show that this STAT3 phosphorylation is regulated by the mTOR complex. Notably, our studies reveal a feedforward inflammatory circuit wherein SP-TACR1 interaction induces IL-6 production from myeloid cells, which in turn enhances TACR1 expression and additively sustains STAT3 phosphorylation. Human PBMCs were revealed to have a heterogenous expression of full-length and truncated isoform of TACR1, in which the former was restricted to a few myeloid populations, whereas the expression of the latter was more widespread across lymphoid and myeloid populations. In summary, this thesis provides mechanistic insights into SP-TACR1 signaling in murine immune cells, highlighting its role in perpetuating Th17- driven inflammation, and lays out the expression pattern of TACR1 isoforms in Human PBMC samples. These findings enhance our understanding of signaling pathways modulated by neuroimmune interactions and their implications in inflammatory diseases.