Identification of the Exocrine Pancreas Proteases as Novel Regulators of β-cell Production

Abstract

The rising rate of type 2 diabetes (T2D) has made it a major health burden. This can largely be attributed to high rates of obesity, one of the greatest contributing factors towards the development of T2D. Despite this, the majority of obese individuals are not diabetic. This raises the question of what mechanisms mediate differential susceptibility to T2D in the presence of obesity. This project seeks to address this question by studying the mechanisms of differential susceptibility to T2D in two rare obesity ciliopathies, Alström Syndrome (AS) and Bardet-Biedl Syndrome (BBS). Both of these ciliopathies are characterized by near-complete penetrance of early onset obesity, but they exhibit markedly different rates of T2D. AS patients are far more susceptible, with the majority developing T2D by age 20, compared to a much smaller subset of BBS patients that are afflicted. In light of the likely contribution of β-cells to T2D susceptibility, we set to identify mechanisms of β-cell production in them. Here we have shown β-cell loss in AS and increase in BBS as a likely contributor towards the discrepant rate of T2D. Next, RNA-Sequencing of these models identified several exocrine pancreas proteases that were differentially expressed between AS and BBS. Thus, we hypothesized that these proteases affect endocrine pancreas function. To test this, we overexpressed each protease in transgenic zebrafish embryos in which β-cells can be visualized. Overexpression significantly increased the β-cell number and rescued the loss of β-cells observed in animals depleted of alms1. We found ctrb1 overexpression led to increased β-cell proliferation in transgenic larvae and rescued the AS model reduction in proliferation. In addition, we found that the proteases are taken up by β-cells in vitro and induce proliferation in cultured β-cells and freshly isolated ex vivo islet cultures, suggesting this effect is conserved in mammalian systems. Finally, endogenous uptake of CTRB1 was found in both zebrafish and mouse models, suggesting a direct interaction between the exocrine and endocrine pancreas. These data support an important role for exocrine pancreatic enzymes in the modulation of β-cells in diabetes.