Histamine is a potent mediator of inflammation and regulator of innate and adaptive immune responses, and autoimmune diseases such as experimental allergic encephalomyelitis (EAE), the principal autoimmune model of multiple sclerosis (MS). EAE is an inflammatory demyelinating disease of the central nervous system with a critical genetic component. We identified Bphs as a susceptibility locus for EAE and positional candidate gene cloning demonstrated Bphs to be the histamine H1 receptor (Hrh1/H1R). As in many autoimmune diseases, CD4+ T cells play a key role in MS and EAE through production of cytokines such as interferon-g and interleukin-17. T cell-specific expression of the susceptible (BphsS/H1RS) allele allowed for EAE development on a H1RKO background, while expression of the resistant (BphsR/H1RR) allele in the same fashion did not. H1R alleles differ by three amino acids in the third intracellular loop (H1RS: Pro263Val312Pro330;H1RR: Leu263Met312Ser330). This domain is associated with signal transduction, but may also be important in protein folding and trafficking, like the intracellular domains of other G-protein coupled receptors (GPCRs). In transfection studies, H1RR surface expression was substantially lower than H1RS, with H1RR exhibiting intracellular retention in the endoplasmic reticulum (ER).
In Aim 1 we will dissect the mechanism(s) responsible for the ER-retention of the H1RR allele and whether pharmacological chaperones can restore H1RR surface expression and affect disease in vivo. Pharmacological chaperones restore surface expression and function of polymorphic GPCRs in vitro, and knowledge gained from the proposed in vivo experiments may have broad implications for GPCR-diseases. We also have evidence of a functional, complementary mechanism that can overcome the ER retention process in BphsR mice. Compilation of Hrh1 sequence data and Bphs data from ~100 inbred mouse strains, revealed strains that were phenotypically BphsS, but bear the H1RR allele. Preliminary mapping studies linked the locus to Hrh1 and because it corrects BphsR, we refer to this gene as Bphs-enhancer (Bphse). SJL/J mice exhibit another Hrh1-linked phenotype, spontaneous histamine sensitivity (Shs). We hypothesize that Bphse and Shs reside within a functional linkage disequilibrium (LD)-domain linked to Hrh1 and may contain genes related to H1R folding, trafficking, or signaling.
In Aim 2 we will use a forward genetics approach to positionally clone and identify Bphse and Shs. H1R signaling through p38 mitogen activated protein kinase (MAPK) is important in EAE susceptibility. For many GPCRs sustained MAPK activation involves non-G-protein signaling through b-arrestins. Since we found no difference in G-protein activation of allelic H1R-Ga fusion proteins, this suggested that H1R signaling may also involve a novel non-G-protein b-arrestin pathway. Thus in Aim 3 we will examine whether non-G-protein signaling downstream of the H1R is required for EAE development. Non-G-protein signaling by GPCRs is an emerging paradigm and new pharmacologics are being developed to selectively target these pathways.
The function of G-protein coupled receptors (GPCRs) such as the histamine H1 receptor (H1R) can be regulated by their subcellular localization and the signaling pathways they elicit. Proper folding and cell surface expression of GPCRs is required for ligand binding and signaling. Mutations that lead to improper folding and/or intracellular trafficking comprise the largest class of GPCR mutations that result in disease. Our finding that H1R alleles controlling susceptibility to autoimmune disease exhibit differential cell surface expression and altered intracellular trafficking, with the resistant allele being retained within the endoplasmic reticulum, was the first demonstration that polymorphisms influencing GPCR trafficking and cell surface expression can regulate immune functions. Understanding the mechanisms leading to differences in the trafficking and cell surface expression of the H1R alleles, and how they can be manipulated in vivo using this naturally occurring mouse model, will undoubtedly aid in the development of new therapeutic strategies for diseases in which gain- or loss-of-function mutants leading to GPCR misfolding and/or improper intracellular trafficking are implicated.
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