Nuclear translocation of an ICA512 cytosolic fragment couples granule exocytosis and insulin expression in β-cells

Islet cell autoantigen 512 (ICA512)/IA-2 is a receptor tyrosine phosphatase-like protein associated with the insulin secretory granules (SGs) of pancreatic β-cells. Here, we show that exocytosis of SGs and insertion of ICA512 in the plasma membrane promotes the Ca2+-dependent cleavage of ICA512 cytoplasmic domain by μ-calpain. This cleavage occurs at the plasma membrane and generates an ICA512 cytosolic fragment that is targeted to the nucleus, where it binds the E3-SUMO ligase protein inhibitor of activated signal transducer and activator of transcription-y (PIASy) and up-regulates insulin expression. Accordingly, this novel pathway directly links regulated exocytosis of SGs and control of gene expression in β-cells, whose impaired insulin production and secretion causes diabetes

The Complement Anaphylatoxin C5a Receptor Contributes to Obese Adipose Tissue Inflammation and Insulin Resistance

Obese adipose tissue (AT) inflammation contributes critically to development of insulin resistance. The complement anaphylatoxin C5a receptor (C5aR) has been implicated in inflammatory processes and as regulator of macrophage activation and polarization. However, the role of C5aR in obesity and AT inflammation has not been addressed. We engaged the model of diet-induced obesity and found that expression of C5aR was significantly upregulated in the obese AT, compared with lean AT. In addition, C5a was present in obese AT in the proximity of macrophage-rich crownlike structures. C5aR-sufficient and -deficient mice were fed a high-fat diet (HFD) or a normal diet (ND). C5aR deficiency was associated with increased AT weight upon ND feeding in males, but not in females, and with increased adipocyte size upon ND and HFD conditions in males. However, obese C5aR−/− mice displayed improved systemic and AT insulin sensitivity. Improved AT insulin sensitivity in C5aR−/− mice was associated with reduced accumulation of total and proinflammatory M1 macrophages in the obese AT, increased expression of IL-10, and decreased AT fibrosis. In contrast, no difference in β cell mass was observed owing to C5aR deficiency under an HFD. These results suggest that C5aR contributes to macrophage accumulation and M1 polarization in the obese AT and thereby to AT dysfunction and development of AT insulin resistance.This work was supported by grants from the National Institutes of Health (Grant AI068730 to J.D.L.), the Else-Kro¨ner Fresenius Stiftung (to T.C.), the Deutsche Forschungsgemeinschaft (to T.C.), and the German Federal Ministry of Education and Research to the German Center for Diabetes Research (to T.C.).Peer reviewe

Islet Cell Autoantigen of 69 kDa Is an Arfaptin-related Protein Associated with the Golgi Complex of Insulinoma INS-1 Cells

International audienceIslet cell autoantigen of 69 kDa (ICA69) is a cytosolic protein of still unknown function. Involvement of ICA69 in neurosecretion has been suggested by the impairment of acetylcholine release at neuromuscular junctions upon mutation of its homologue gene ric-19 in C. elegans. In this study, we have further investigated the localization of ICA69 in neurons and insulinoma INS-1 cells. ICA69 was enriched in the perinuclear region, whereas it did not co-localize with markers of synaptic vesicles/synaptic-like microvesicles. Confocal microscopy and subcellular fractionation in INS-1 cells showed co-localization of ICA69 with markers of the Golgi complex and, to a minor extent, with immature insulin-containing secretory granules. The association of ICA69 with these organelles was confirmed by immunoelectron microscopy. Virtually no ICA69 immunogold labeling was observed on secretory granules near the plasma membrane, suggesting that ICA69 dissociates from secretory granule membranes during their maturation. In silico sequence and structural analyses revealed that the N-terminal region of ICA69 is similar to the region of arfaptins that interacts with ARF1, a small GTPase involved in vesicle budding at the Golgi complex and immature secretory granules. ICA69 is therefore a novel arfaptin-related protein that is likely to play a role in membrane trafficking at the Golgi complex and immature secretory granules in neurosecretory cells

A Spatial Model of Insulin-Granule Dynamics in Pancreatic β-Cells.

Insulin secretion from pancreatic β-cells in response to sudden glucose stimulation is biphasic. Prolonged secretion in vivo requires synthesis, delivery to the plasma membrane (PM) and exocytosis of insulin secretory granules (SGs). Here, we provide the first agent-based space-resolved model for SG dynamics in pancreatic β-cells. Using recent experimental data, we consider a single β-cell with identical SGs moving on a phenomenologically represented cytoskeleton network. A single exocytotic machinery mediates SG exocytosis on the PM. This novel model reproduces the measured spatial organization of SGs and insulin secretion patterns under different stimulation protocols. It proposes that the insulin potentiation effect and the rising second-phase secretion are mainly due to the increasing number of docking sites on the PM. Furthermore, it shows that 6 min after glucose stimulation, the 'newcomer' SGs are recruited from a region within less than 600 nm from the PM

ICA512 signaling enhances pancreatic beta-cell proliferation by regulating cyclins D through STATs

Changes in metabolic demands dynamically regulate the total mass of adult pancreatic beta-cells to adjust insulin secretion and preserve glucose homeostasis. Glucose itself is a major regulator of beta-cell proliferation by inducing insulin secretion and activating beta-cell insulin receptors. Here, we show that islet cell autoantigen 512 (ICA512)/IA-2, an intrinsic tyrosine phosphatase-like protein of the secretory granules, activates a complementary pathway for beta-cell proliferation. On granule exocytosis, the ICA512 cytoplasmic domain is cleaved and the resulting cytosolic fragment (ICA512-CCF) moves into the nucleus where it enhances the levels of phosphorylated STAT5 and STAT3, thereby inducing insulin gene transcription and granule biogenesis. We now show that knockdown of ICA512 decreases cyclin D1 levels and proliferation of insulinoma INS-1 cells, whereas beta-cell regeneration is reduced in partially pancreatectomized ICA512-/- mice. Conversely, overexpression of ICA512-CCF increases both cyclin D1 and D2 levels and INS-1 cell proliferation. Up-regulation of cyclin D1 and D2 by ICA512-CCF is affected by knockdown of STAT3 and STAT5, respectively, whereas it does not require insulin signaling. These results identify ICA512 as a regulator of cyclins D and beta-cell proliferation through STATs and may have implication for diabetes therapy