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 f...