Sushant Bhatnagar

Stimulus‐coupled insulin secretion from the pancreatic islet β‐cells involves the fusion of insulin granules to the plasma membrane (PM) via SNARE complex formation—a cellular process key for maintaining whole‐body glucose homeostasis. Less is known about the role of endogenous inhibitors of SNARE complexes in insulin secretion. We show that an insulin granule protein synaptotagmin‐9 (Syt9) deletion in mice increased glucose clearance and plasma insulin levels without affecting insulin action compared to the control mice. Upon glucose stimulation, increased biphasic and static insulin secretion were observed from ex vivo islets due to Syt9 loss. Syt9 colocalizes and binds with tomosyn‐1 and the PM syntaxin‐1A (Stx1A); Stx1A is required for forming SNARE complexes. Syt9 knockdown reduced tomosyn‐1 protein abundance via proteasomal degradation and binding of tomosyn‐1 to Stx1A. Furthermore, Stx1A‐SNARE complex formation was increased, implicating Syt9‐tomosyn‐1‐Stx1A complex is inhibitory in insulin secretion. Rescuing tomosyn‐1 blocked the Syt9‐knockdown‐mediated increases in insulin secretion. This shows that the inhibitory effects of Syt9 on insulin secretion are mediated by tomosyn‐1. We report a molecular mechanism by which β‐cells modulate their secretory capacity rendering insulin granules nonfusogenic by forming the Syt9‐tomosyn‐1‐Stx1A complex. Altogether, Syt9 loss in β‐cells decreases tomosyn‐1 protein abundance, increasing the formation of Stx1A‐SNARE complexes, insulin secretion, and glucose clearance. These outcomes differ from the previously published work that identified Syt9 has either a positive or no effect of Syt9 on insulin secretion. Future work using β‐cell‐specific deletion of Syt9 mice is key for establishing the role of Syt9 in insulin secretion.

Brain‐specific angiogenesis inhibitor 3 (ADGRB3/BAI3) belongs to the family of adhesion G protein‐coupled receptors. It is most highly expressed in the brain where it plays a role in synaptogenesis and synapse maintenance. Genome‐wide association studies have implicated ADGRB3 in disorders such as schizophrenia and epilepsy. Somatic mutations in ADGRB3 have also been identified in cancer. To better understand the in vivo physiological role of ADGRB3 , we used CRISPR/Cas9 editing to generate a mouse line with a 7‐base pair deletion in Adgrb3 exon 10. Western blot analysis confirmed that homozygous mutants ( Adgrb3 ∆7/∆7 ) lack full‐length ADGRB3 expression. The mutant mice were viable and reproduced in Mendelian ratios but demonstrated reduced brain and body weights and deficits in social interaction. Measurements of locomotor function, olfaction, anxiety levels and prepulse inhibition were comparable between heterozygous and homozygous mutants and wild‐type littermates. Since ADGRB3 is also expressed in organs such as lung and pancreas, this new mouse model will facilitate elucidation of ADGRB3's role in non‐central nervous system‐related functions. Finally, since somatic mutations in ADGRB3 were identified in patients with several cancer types, these mice can be used to determine whether loss of ADGRB3 function contributes to tumour development.