The Role of Active Vitamin D on Reserved Intestinal Stem Cells

Abstract

Inflammatory bowel disease (IBD), a key risk factor for colorectal cancer, affects 2.39 million Americans and has no effective treatment. Current attempts include steroidal immunosuppressants, an inhibitor of immune trafficking to the gut, and therapies that aim to block gut-specific antigens. However, IBD patients often either do not respond or develop tolerance to these treatments. These limitations leave IBD patients in a cruel dilemma: either to pursue more aggressive and taxing therapies or suffer under IBD and risk further intestinal damage or the progression to colorectal cancer. A hopeful alternative to these immunosuppressive treatments is regenerative medicine. When the epithelium fails toregenerate due to inflammation, luminal bacteria infiltrate the epithelial barrier and exacerbate disease. Regenerative therapies aid regeneration to prevent progression of the disease. Recently, a novel therapy with calcitriol (an active metabolite of vitamin D) has shown promising results for ameliorating symptoms of a murine model of IBD. However, calcitriol is known to promote the differentiation of putative LGR5+ intestinal stem cells (LGR5+ ISCs), which are essential to long term-intestinal homeostasis, leaving the broader impact on cell populations unclear. This study focused on the marker genes of reserve ISC populations (Mex3a, Tert, Hopx, Bmi1) and mature epithelial cells (Alpi, Chga, Lyz1, Muc2). After a meta-analysis of bulk RNA-seq datasets of calcitriol-treated mice, this project found significant upregulation of the reserve ISC markers Mex3a, Tert, and Hopx and observed a non-significant increase for Bmi1 after treatment. After independent validation of Bmi1 in vivo in mice, an in vitro arm tested dose-dependent effects of calcitriol in intestinal organoids, and concluded that high concentrations were needed to activate Mex3a. These results on reserve stem cells suggest that calcitriol’s ameliorative effects in IBD may rely on reserve stem cells instead of LGR5+ ISCs. Future research on treatments for IBD could target reserve ISCs instead.

Keywords: Inflammatory Bowel Disease, Vitamin D, Calcitriol, Macrophage, Cell Therapy, Bioinformatics, Intestinal Organoids, qPCR, RNA-seq, Mice

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