Perez, K. C.; Alexander, J.; Rahman, M. M.; Alsharif, H. A.; Liu, Y.; Kim, J.-A.; Hunter, C. S.; Nguyen, T. M.; Bhatnagar, S.

The transition from proliferative to functionally mature {beta}-cells is a critical developmental process, yet the molecular mechanisms that coordinate this shift remain poorly understood. Here, we identify Tomosyn-2 as a key regulator of {beta}-cell maturation. Tomosyn-2 expression declines with age in mouse islets, coinciding with enhanced biphasic glucose-stimulated insulin secretion (GSIS) and reduced {beta}-cell proliferation. Genetic deletion of Tomosyn-2 improves glucose tolerance, elevates plasma insulin levels, and augments islet insulin secretion, without altering systemic insulin sensitivity. Mechanistically, Tomosyn-2 interacts with syntaxin-1A (Stx1A) to inhibit insulin granule exocytosis by limiting SNARE complex formation. Transcriptomic and network analyses reveal that Tomosyn-2 loss reprograms gene expression to strengthen the coupling between insulin secretion and proliferative pathways. Its deletion also reduces {beta}-cell proliferation and mass expansion, suppresses cell cycle and Akt1 signaling, and promotes {beta}-cell identity, maturation, and altered islet architecture. These findings identify Tomosyn-2 as a crucial molecular switch that orchestrates the balance between proliferation and functional maturation during postnatal {beta}-cell development.