SUMO1 enhances cAMP‐dependent exocytosis and glucagon secretion from pancreatic α‐cells

Abstract

Key pointsr SUMOylation is the reversible modification of proteins by the attachment of small ubiquitin-like modifier (SUMO) peptides, which in pancreatic β-cells inhibits insulin exocytosis and glucagon-like peptide-1 (GLP-1) receptor signalling.r We find in glucagon-secreting pancreatic α-cells that SUMOylation increases excitability and enhances exocytosis by increasing L-type Ca 2+ currents.r The ability of SUMOylation to facilitate α-cell exocytosis is cAMP-dependent, leading to enhanced adrenaline-stimulated glucagon secretion.r SUMO1 prevents inhibition of α-cell Na Abstract Post-translational modification by the small ubiquitin-like modifier-1 (SUMO1) limits insulin secretion from β-cells by inhibiting insulin exocytosis and glucagon-like peptide-1 (GLP-1) receptor signalling. The secretion of glucagon from α-cells is regulated in a manner opposite to that of insulin; it is inhibited by elevated glucose and GLP-1, and increased by adrenergic signalling. We therefore sought to determine whether SUMO1 modulates mouse and human α-cell function. Action potentials (APs), ion channel function and exocytosis in single α-cells from mice and humans, identified by glucagon immunostaining, and glucagon secretion from intact islets were measured. The effects of SUMO1 on α-cell function and the respective inhibitory and stimulatory effects of exendin 4 and adrenaline were examined. Upregulation of SUMO1 increased α-cell AP duration, frequency and amplitude, in part as a result of increased Ca 2+ channel activity that led to elevated exocytosis. The ability of SUMO1 to enhance α-cell exocytosis was cAMP-dependent and resulted from an increased L-type Ca 2+ current and a shift away from exocytosis dependent on non-L-type channels, an effect that was mimicked by knockdown of the deSUMOylating enzyme sentrin/SUMO-specific protease-1 (SENP1). Finally, although SUMO1 prevented GLP-1 receptor-mediated inhibition of α-cell Na + channels and single-cell exocytosis, it failed to prevent the exendin 4-mediated inhibition of glucagon secretion. Consistent with its cAMP dependence, however, SUMO1 enhanced α-cell exocytosis and glucagon secretion stimulated by adrenaline. Thus, by contrast with its inhibitory role in β-cell exocytosis, SUMO1 is a positive regulator of α-cell exocytosis and glucagon secretion under conditions of elevated cAMP.

Converting Adult Pancreatic Islet α Cells into β Cells by Targeting Both Dnmt1 and Arx

“…In electrophysiological studies, mouse α- and β-cells have long been distinguished by characteristic differences in the voltage-dependent inactivation of Na + channels (Gopel et al, 2000) and more recently by opposing glucose-dependent exocytotic responses to serial membrane depolarization (Ferdaoussi et al, 2015; Dai et al, 2014). Our single cell RNA-Seq studies reveal upregulation of genes mediating the β-cell Na + current ( Scn9a ) and contributing to β-cell glucose sensing ( Slc2a2 ) in the converted α-cells, but not in the unconverted α-cells from αiADKO mice (Figure 4e).…”

“…Glucose-stimulation amplifies the exocytotic response to membrane depolarization in β-cells (Ferdaoussi 2015) but suppresses the response in α-cells (Dai 2014). Accordingly, exocytosis in α-cells from the control mice is suppressed by a rise in glucose (n=16 cells) and amplified by a drop in glucose (n=21; Figure 5c–d), and this is also observed in the non-converted (Ins Neg ,YFP + ) α-cells from the αiADKO mice (n=24 and 13 cells; Figure 5e–f).…”

The Roles of SUMO in Metabolic Regulation

“…Conversely, overexpression of SENP1 reduces insulin secretion and impairs intracellular Ca 2+ handling by inducing cell death (Hajmrle et al 2014). SUMO-1 has an opposite effect on exocytosis in pancreatic α-cells: it enhances exocytosis and glucagon secretion in cAMP-dependent manner (Dai et al 2014). Recently, a metabolomics profiling of the insuli-noma cell line 832/13 demonstrated a significant glucose-induced changes in purine pathway intermediates: a decrease in inosine monophosphate (IMP) and an increase in adenylosuccinate (S-AMP).…”

Glucagon – the new ‘insulin’ in the pathophysiology of diabetes

“…Other recent reports include studies examining the role on glucagon regulation of pancreatic polypeptide [54] and of small ubiquitin-like modifier-1 [55], g-hydroxybutyrate and glycine [56], clock gene Rev-erb alpha [57], and growth factor receptorbound protein 10 gene [58].…”