MZIAUT, H. | bioRxiv Science

bioRxiv2026

Villin-SNAP25 interaction regulates insulin secretion downstream of ICA512 signaling

Pancreatic islet {beta}-cells step-up insulin production and secretion in response to the elevation of blood glucose levels. ICA512/PTPRN is a transmembrane cargo of the insulin secretory granules (SGs). Upon SG exocytosis, its Ca2+/calpain mediated cleavage at the plasma membrane generates an ICA512-cleaved cytosolic fragment (ICA512-CCF) which as a decoy phosphatase prolongs phospho-STAT5/3 activities, thereby enhancing the transcription of mRNAs for SG cargoes, including insulin and its own, to replenish SG stores. In addition, ICA512 positively regulates the expression of F-actin modifier villin. Villin, in turn, modulates the size of actin cages surrounding cortical SGs, hence regulating SG mobility and exocytosis. Here we show that villin controls the number of SG docking sites for exocytosis by directly interacting at low glucose with the t-SNARE SNAP-25, thus restricting in this condition SG access to fusion sites and insulin release. Replacement of ICA512-CCF N-terminal ubiquitin-acceptor lysine 609 (K609) with valine (V) stabilized ICA512-CCF in insulinoma cells and homozygous ICA512K609V mice and enhanced the levels of phospho-STAT5/3 and villin. In ICA512K609V female mice these molecular traits correlated with reduced body weight, improved insulin sensitivity, reduced basal insulin secretion and onset time for glucose stimulated insulin secretion. Taken together, these data demonstrates that SG exocytosis induced ICA512 retrograde signaling acts in concert with the novel SNARE complex regulator villin to plastically adapt {beta}-cell actin cytoskeleton and access to SG docking sites for optimal control of insulin secretion in response to variations in extracellular glucose levels. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=110 SRC="FIGDIR/small/699220v1_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@37d835org.highwire.dtl.DTLVardef@5389f1org.highwire.dtl.DTLVardef@855d98org.highwire.dtl.DTLVardef@1c3c639_HPS_FORMAT_FIGEXP M_FIG Graphical abstract Model of ICA512 and STAT3/5-dependent regulation of -cell function. At basal conditions, villin limits secretory granule (SG) docking by interacting with SNAP-25, restricting insulin release (1). Increased metabolic load triggers hyperglycemia-induced ICA512 processing and growth factor-mediated STAT3/5 phosphorylation (2-4), enhancing ISG mRNAs, granule biogenesis, and villin expression (5,6). Villin modulates SNARE protein availability, controlling SG docking and secretion. Together, ICA512 retrograde signaling and villin-mediated SNARE regulation dynamically remodel the -cell actin cytoskeleton, adjusting SG docking site access to ensure appropriate insulin secretion in response to fluctuating glucose levels. C_FIG

cell biology↗

bioRxiv2017

Mir-132 controls beta cell proliferation and survival in mouse model through the PTEN/AKT/FOXO3 signaling

Aim and hypothesismicroRNAs (miRNAs) play an integral role in maintaining beta cell function and identity. Deciphering their targets and precise role, however, remains a challenge. In this study we aimed to identify miRNAs and their downstream targets involved in regeneration of islet beta cells following partial pancreatectomy in mice.\n\nMethodsRNA from laser capture microdissected (LCM) islets of partially pancreatectomized and sham-operated mice were profiled with microarrays to identify putative miRNAs implicated in control of beta cell regeneration. Altered expression of selected miRNAs, including miR-132, was verified by RT-PCR. Potential targets of miR-132 were seleced through bioinformatic data mining. Predicted miR-132 targets were validated for their changed RNA and protein expression levels and signaling upon miR-132 knockdown or overexpression in MIN6 cells. The ability of miR-132 to foster beta cell proliferation in vivo was further assessed in pancreatectomized miR-132-/- and control mice.\n\nResultsPartial pancreatectomy significantly increased the number of BrdU+/insulin+ positive islet cells. Microarray profiling revealed 14 miRNAs, including miR-132 and -141, to be significantly upregulated in LCM islets of partially pancreatectomized compared to LCM islets of control mice. In the same comparison miR-760 was the only miRNA found to be downregulated. Changed expression of these miRNAs in islets of partially pancreatectomized mice was confirmed by RT-PCR only in the case of miR-132 and -141. Based on previous knowledge of its function, we chose to focus our attention on miR-132. Downregulation of miR-132 in MIN6 cells reduced proliferation while enhancing the expression of proapoptic genes, which was instead reduced in miR-132 overexpression MIN6 cells. Microarray profiling, RT-PCR and immunoblotting of miR-132 overexpressing MIN6 cells revealed their downregulated expression of Pten, with concomitant increased levels of pro-proliferative factors phospho-Akt and phospho-Creb as well as inactivation of pro-apoptotic Foxo3 via its phosphorylation. Finally, we show that regeneration of beta cells following partial pancreatectomy was reduced in miR-132-/- mice compared to control mice.\n\nConclusions/InterpretationsOur study provides compelling evidence for upregulation of miR-132 being critical for regeneration of mouse islet beta cells in vivo through downregulation of its target Pten. Hence, the miR-132/Pten/Akt/Foxo3 signaling pathway may represent a suitable target to enhance beta cell mass.\n\nResearch in ContextWhat is already known?\n\nO_LISeveral miRNAs, including miR-132, are known to regulate beta cell function and mass in several mouse models of diabetes db/db, ob/ob and high fat-diet.\nC_LI\n\nWhat is the key question?\n\nO_LIWhich are miRNAs implicated in control of beta cell regeneration upon partial pancreatectomy and how?\nC_LI\n\nWhat are the new findings?\n\nO_LImiR-132 is critical to promote regeneration of mouse beta cells in vivo following partial pancreatectomy\nC_LIO_LIIn vitro studies in mouse MIN6 cells indicate that miR-132 fosters beta cell proliferation by down-regulating the expression of phosphatase Pten, thereby tilting the balance between anti-apoptotic factor Akt and pro-apoptotic factor Foxo3 activities towards proliferation through regulation of their phosphorylation.\nC_LI\n\nHow might this impact on clinical practice in the foreseeable future?\n\nO_LIThese findings strengthen the rationale for targeting the expression of miR-132 to increase beta cell mass in vivo (type 2 diabetes) or ex-vivo (islet transplantation in type 1 diabetes) for the treatment of diabetes.\nC_LI

cell biology↗