Novel Regulators of Beta Cell Proliferation and Function

Veterans Health Administration, Decatur PA

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Abstract

Diabetes is one of the leading causes of morbidity and mortality in the veteran population. The Veteran population are particularly susceptible for diabetes, with a staggering 70% of Veterans at risk for diabetes. 25% of all Veterans have diabetes, similar to that seen in older Americans from the general population. A failure to increase the β-cell proliferation and functional β-cell mass in response to increasing metabolic demand from insulin resistance associated with obesity and aging, underlies most causes of adult onset diabetes in veterans. It is, therefore, imperative to identify pathways that regulate functional β-cell mass that could be leveraged for treating β-cell failure and diabetes. Our data demonstrate that Tead1, a critical transcriptional effector of the mammalian Hippo pathway, is robustly expressed in mouse and human islets and has a non- redundant role in regulating β-cell proliferation and function. The mammalian Hippo-Tead1 pathway consists of a core kinase cascade, culminating with Lats1&2, inhibitory kinases, that phosphorylate coactivators, Yap and Taz, preventing their nuclear translocation and co-activation of Tead1-mediated transcription regulating cell proliferation and apoptosis. The premise for this proposal rests on our preliminary data that strongly suggest that Tead1 is the switch regulating the proliferation restriction, while promoting mature function in adult β-cells. Recent reports have found some contrasting results, wherein, (a) Yap-Tead1 act as an enhancer in many β- cell maturation genes in human embryonic pancreatic progenitors, (b) Yap has also been shown to inhibit endocrinogenesis in mice, and acquisition of mature function during differentiation of human iPS cells (hIPSCs), but (d) sufficient to induce proliferation in human islets, ex vivo. Preliminary data shows that β-cell specific Tead1 deletion leads to diabetes and glucose intolerance. Tead1-null islets display a decrease in expression of mature β-cell markers and a loss of glucose stimulated insulin secretion. Furthermore, our data indicates that embryonic Tead1 deletion in β-cells also led to profound diabetes suggesting that Tead1 is required in the endocrine progenitors for normal differentiation. To comprehensively test the regulation of β-cell differentiation, proliferation and function by the Hippo-tead1 pathway, we hypothesize that Tead1 regulates β-cell proliferation and acquisition of mature function via context-dependent co-factor specific, transcriptional regulation of a network of proliferation and mature-phenotype defining genes. The broad goal is to mechanistically delineate key pathways regulating functional β-cell mass that can be harnessed to promote human β-cell proliferation with preserved function, through genetic loss- and gain-of- function studies using in vivo mouse models and ex vivo mouse and human islets and human iPSCs. We will specifically 1. To test if enhancing Yap activity in adult β-cells in vivo is sufficient for β-cell proliferation and maintenance of mature function via co-activation of Tead1, in GOF studies (overexpressing constitutively nuclear phospho-mutant YapS112A specifically in adult mouse β-cells using a Tet-On system) in inducing proliferation and expansion of β-cells under high fat diet induced insulin resistant stat, identifying direct targets of Tead1 in β-cells under basal and proliferating states and delineate Tead1 dependent and independent Yap targets in β-cells. 2. Test if mammalian Hippo-Tead1 pathway is required for human β- cell differentiation and proliferation by assessing if Yap/Taz-Tead1 regulates iPSC differentiation into pancreatic progenitors and β-cells and identify the molecular targets of Tead1 in this process. We will also test if modulation of Tead1 activity improves β-cell function and proliferation in isolated human islets and if this regulatory role is impaired in diabetes. Collectively the proposed studies will critically address how the Hippo- Tead1 pathway regulates β-cell function and proliferation in human β-cells. With the significant rise in incidence of diabetes in veterans, there is an urgent need to develop novel therapies to reverse declining functional β-cell mass and successful completion of the proposed experiments will address this critical need.