Poster Presentation Australian Diabetes Society and the Australian Diabetes Educators Association Annual Scientific Meeting 2014

Islet α-cell specific gene expression (#248)

Caitlin M.D. Orr 1 , Lorna M Dickson 1 , Barton Wicksteed 1
  1. University of Chicago, Chicago, IL, United States

Although glucagon secretion is defective in diabetes, basic understanding of alpha-cell biology is poor. We are characterizing the mechanism by which α-cell specific gene expression is controlled. Currently transcription factors (TFs) such as Arx, MafB, Pax6, Brn4/Pou3f4, Cdx2/3 and Foxa2 are considered important for directing gene expression to the α-cells, but this is based upon the involvement of these factors in the developmental establishment of an α-cell population or in the expression of a single gene, the proglucagon gene. Our unbiased approach is to identify enhancer elements that confer α-cell specific gene expression and characterize these sequences to identify TFs that bind them to direct transcription to the α-cells. Enhancers are small distal regulatory elements than can be identified by characteristic histone modifications, such as histone 3 acetylation at lysine 27 (H3K27Ac) and monomethylation at lysine 4 (H3K4m1). In addition, enhancer elements often lie in evolutionarily conserved regions (ECRs). Here we identified enhancer elements in the genome of α-cells (αTC1) by association with H3K4m1 and ECRs. 25 million primary reads mapped to 74753 peaks, which when filtered against 15 other tissues, identified 4593 peaks unique to αTC cells. Of these, 1211 fell in ECRs and 350 mapped near genes expressed more strongly in α-cells versus beta-cells. Screening 21 of these 350 elements in luciferase assays showed 38% enhanced expression in α versus β-cells and 29% showed similar enhancement in both cell types. Two elements, one showing α-cell specificity and one expressing in α- and β-cells, that map near 2 α-cell genes (proglucagon and prohormone convertase 2) drove GFP expression in zebrafish islets with the same cell-type specificity. Further ChIP-seq experiments, currently underway will map known α-cell transcription factors (Arx, Pax6, MafB Foxa2) to determine whether these or novel TFs are associated with these sequences that direct gene expression to islet α-cells.