A concerted effort is being made to understand the molecular programs underlying ? cell formation and function in order to provide therapeutic insight into new diabetes treatment strategies. From these studies, the MafA transcription factor has been demonstrated to be both an essential regulator of mature ? cell function, and inactivated in mouse models of type 2 diabetes mellitus. MafA and the only other closely related large Maf protein expressed in islet cells, MafB, were originally characterized in our laboratory and elsewhere due to their respective significance in stimulating islet ? (i.e. MafA, insulin) and a cell (MafB, glucagon) hormone gene transcription. Significantly, MafA synthesis has been shown to be critical to the production of functional ? cells, with the presence of MafB in insulin+ cells associated with immature and relatively dysfunctional cells. Our analyses of MafA and MafB protein chimeras have shown that the C-terminal leucine- zipper (dimerization) region regulates the distinct endogenous insulin gene activation ability of MafA in cell-based assays, as well as in phosphorylation-dependent DNA binding in vitro. We hypothesize that these sequences also provide distinguishing activating properties to MafA in vivo. This hypothesis will be tested by analyzing the functional characteristics of transgenes expressing either a MafB/A chimera or MafB on islet ? cell function in MafA(panc mice, which lack MafA expression throughout the pancreas due to the actions of a transgenic Cre recombinase in MafAfl/fl mice. Strikingly, human islets were found to be unlike rodents by expressing MafA and MafB in ? cells. Since no other differences were observed in islet-enriched transcription factor distribution between species, our analysis of MafB transgenic mice will also provide insight into whether this factor contributes to observed changes in glucose sensing and insulin secretion characteristics between human and rodent islets. We hypothesize that the MafA C-terminal sequences create unique binding surfaces for transcriptional coregulatory proteins. Surprisingly, there is a paucity of information regarding the role these proteins play in islet ? cell development or function. We have implemented an 'in cell'chemical cross-linking strategy to isolate and characterize such molecules from mouse ? cells. Exciting preliminary data illustrate the ability of MafA to bind MLL2, Rbbp5, Ash2, Dpy30, and Wdr5, components of the euchromatin mixed lineage leukemia 2 (MLL2) histone 3 lysine 4 trimethylation complex. Experiments are proposed to determine how MLL2 influences MafA-mediated gene activation and islet ? cell function. The data generated here will provide valuable insight into the transcriptional regulatory mechanisms that will likely be impactful in the production of cellular therapeutics for diabetes treatment.
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