A glucose-dependent spatial patterning of exocytosis in human β-cells is disrupted in type 2 diabetes

Abstract

Fu et al.Spatial organization of exocytosis in human b-cells 2 SUMMARY Impaired insulin secretion in type 2 diabetes (T2D) is linked to reduced insulin granule docking, disorganization of the exocytotic site, and an impaired glucose-dependent facilitation of insulin exocytosis. We show in b-cells from 80 human donors that the glucose-dependent amplification of exocytosis is disrupted in T2D. Spatial analyses of granule fusion, visualized by total internal reflection fluorescence (TIRF) microscopy in 24 of these donors, demonstrate that these are non-random across the surface of b-cells from donors with no diabetes (ND). The compartmentalization of events occurs within regions defined by concurrent or recent membrane-resident secretory granules. This organization, and the number of membraneassociated granules, is glucose-dependent and notably impaired in T2D b-cells. Mechanistically, multi-channel Kv2.1 clusters contribute to maintaining the density of membrane-resident granules and the number of fusion 'hotspots', while SUMOylation sites at the channel N-(K145) and C-terminus (K470) determine the relative proportion of fusion events occurring within these regions. Thus, a glucose-dependent compartmentalization of fusion, regulated in part by a structural role for Kv2.1, is disrupted in b-cells from donors with type 2 diabetes.Abstract:

ZIGIR, a Granule-Specific Zn2+ Indicator, Reveals Human Islet α Cell Heterogeneity

“…This highlights the potential of applying ZIGIR to tag and to image native secretory granules in live cells. Tracking the spatial and temporal dynamics of secretory granules has important implications in understanding the mechanisms regulating granule distribution, trafficking, and docking near the plasma membrane, and defects in these cellular processes have been observed in islet cells from diabetic donors (Fu et al, 2019;Gandasi et al, 2018;Rorsman and Renströ m, 2003;Rosengren et al, 2012). Traditionally, tagging and imaging of dense core granules employ fluorescent proteins fused with a granule marker such as phogrin, syncollin, pro-insulin, islet amyloid polypeptide, or neuropeptide Y (Barg et al, 2002;Ferri et al, 2019;Michael et al, 2004;Rutter et al, 2006;Tabei et al, 2013).…”

Insulin granule biogenesis and exocytosis

“…In isolated cells, polarized secretion is maintained to some extent. Spatial patterning of ISG recruitment and fusion occurs at polarized sites especially when the SNARE clusters also include Ca 2+ channels [134,151]. Local activation of integrin signalling mimics the vascular face of the cell and contribute to polarized secretion [152,153].…”

“…β-Cells from T2D organ donors exhibit decreased numbers of docked granules, at least in part due to downregulation of docking factors, such as and Syntaxin-1, Munc18, Munc13, Rim2, Rab3a, and Rabphilin3a [158], and recent advances in single-cell sequencing are likely to expand this list [159]. Moreover, ISG docking in T2D β-cells also loose spatial control and occurs at random locations at the plasma membrane, indicating the absence of a specific docking signal [151]. The reduced docking results in impaired ISG exocytosis in these cells [151,158].…”

“…Moreover, ISG docking in T2D β-cells also loose spatial control and occurs at random locations at the plasma membrane, indicating the absence of a specific docking signal [151]. The reduced docking results in impaired ISG exocytosis in these cells [151,158]. The ability of antidiabetic drugs, such as sulfonylureas, to restore biphasic insulin secretion indicates that β-cell defects also result from impaired electrical activity [160,161].…”

Human Pluripotent Stem Cells: A Unique Tool for Toxicity Testing in Pancreatic Progenitor and Endocrine Cells

“…Glucose-stimulated insulin secretion is amplified by other nutrients, such as fatty acids and amino acids, and binding of gut-derived hormones (GLP-1, GIP) to incretin receptors. Dysregulated glucose-induced insulin secretion is a well-established feature of T2D (83)(84)(85)(86)(87) and also reported in T1D patients prior to disease onset (88)(89)(90)(91)(92), suggesting a possible link between insulin secretory defects and activation of inappropriate autoimmune responses. Therefore, environmental contaminants that interfere with any aspect of the complex insulin secretory pathway (e.g., glucose sensing, mitochondrial metabolism, ion flux, exocytotic machinery, sensitivity to amplification signals) could adversely affect the fine-tuned ability of b-cells to couple insulin secretion with a nutrient secretagogue.…”