Intrinsic and extrinsic factors underlying β-cell quiescence during development and aging.

Aging is a universal process that is accompanied by the loss of proliferative potential of cells. However, factors governing this age-dependent decline in proliferation remain largely undefined. The pancreatic β-cells of the islets of Langerhans serve as a unique model to explore the effect of cellular age on proliferation and function within the same organ. During early juvenile stage, the zebrafish islet is rapidly expanding and newly differentiated β-cells are added to the pool of older β-cells that were formed during embryogenesis. In this thesis, using accurate reporters for cell-cycle stages and intra-cellular calcium sensors, it was shown that younger β-cells are more proliferative but less functional compared to older β-cells. Furthermore, as the animal ages, the overall rate of β-cell proliferation declines. Transcriptomic analysis of β-cells from young adult and older adult islets revealed that older cells display an inflammatory signature. Transgenic reporter line for inflammatory NF-kB activity showed that β-cells of younger islets display varying levels of NF-kB activity, which becomes homogenous in older β-cells. Furthermore, the cells with higher NF-kB-activity proliferate less compared to their neighbors with lower activity. Specifically, younger NF-kBhigh cells upregulate socs2, a negative regulator of proliferation that is also enriched in older β-cells. Interestingly, activated macrophages were observed infiltrating the islet during late juvenile stages, thus pointing to an important role of the microenvironment in activation of inflammatory signature in the islet. Overall, this study shows that cells of different ages co-exist within the same micro-organ. This age-related cellular heterogeneity governs the rate of proliferation of the tissue. The loss of cellular heterogeneity with age reduces the proliferative pool of the tissue. Finally, the expression of inflammatory NF-kB activity acts as a marker of this loss of proliferative heterogeneity