Alterations of the Ca2＋ signaling pathway in pancreatic beta-cells isolated from db/db mice

Upon glucose elevation, pancreatic beta-cells secrete insulin in a Ca2＋-dependent manner. In diabetic animal models, different aspects of the calcium signaling path- way in beta-cells are altered, but there is no consensus regarding their relative contributions to the development of beta-cell dysfunction. In this study, we compared the increase in cytosolic Ca2＊ （[Ca2＊]~） via Ca2＋ influx, Ca2＊ mobilization from endoplasmic reticulum （ER） calcium stores, and the removal of Ca2＋ via multiple mechanisms in beta-cells from both diabetic db/db mice and non- diabetic C57BL/6J mice. We refined our previous quan- titative model to describe the slow [Ca2＋]i recovery after depolarization in beta-cells from db/db mice. According to the model, the activity levels of the two subtypes of the sarco-endoplasmic reticulum Ca2＋-ATPase （SERCA） pump, SERCA2 and SERCA3, were severely down-reg- ulated in diabetic cells to 65% and 0% of the levels in normal cells. This down-regulation may lead to a reduc- tion in the Ca2＋ concentration in the ER, a compensatory up-regulation of the plasma membrane Na＋/Ca2＋ exchanger （NCX） and a reduction in depolarizationevoked Ca2＋ influx. As a result, the patterns of glucosestimulated calcium oscillations were significantly different in db/db diabetic beta-cells compared with normal cells. Overall, quantifying the changes in the calcium signaling pathway in db/db diabetic beta-cells will aid in the development of a disease model that could provide insight into the adaptive transformations of beta-cell function during diabetes development.

HID-1 is a peripheral membrane protein primarily associated with the medial-and transGolgi apparatus

Caenorhabditis elegans hid-1 gene was first identified in a screen for mutants with a high-temperature-induced dauer formation(Hid)phenotype.Despite the fact that the hid-1 gene encodes a novel protein(HID-1)which is highly conserved from Caenorhabditis elegans to mammals,the domain structure,subcellular localization,and exact function of HID-1 remain unknown.Previous studies and various bioinformatic softwares predicted that HID-1 contained many transmembrane domains but no known functional domain.In this study,we revealed that mammalian HID-1 localized to the medial-and transGolgi apparatus as well as the cytosol,and the localization was sensitive to brefeldin A treatment.Next,we demonstrated that HID-1 was a peripheral membrane protein and dynamically shuttled between the Golgi apparatus and the cytosol.Finally,we verified that a conserved N-terminal myristoylation site was required for HID-1 binding to the Golgi apparatus.We propose that HID-1 is probably involved in the intracellular trafficking within the Golgi region.

hTAC internalizes via both clathrin-dependent and clathrin-independent endocytosis in mammalian cells

Dear Editor, Endocytosis is a crucial process employed by cells to internal- ize nutrients and turnover membrane components and is essential for many functions, including nutrient uptake, signal transduction, cytokinesis, morphogenesis, cell adhesion and migration. Endocytosis is classified as clathrin-dependent endocytosis （CDE） or clathrin-independent endocytosis （CIE） according to its dependence on clathrin. Several different CIE pathways have been proposed, including caveolin-dependent endocytosis, flotillin-dependent endocytosis, the clathrin-inde- pendent carrier pathway, ARF6-dependent endocytosis, phagocytosis, macropinocytosis, the IL2RI3 pathway （Doherty and McMahon, 2009）, the newly identified fast endophilin-me- diated endocytosis pathway （Boucrot et al., 2015） and the EGFR-NCE pathway （Caldieri et al., 2017）.

I-BAR protein IRSp53 regulates clathrin-independent endocytosis in a biphasic manner

Endocytosis is a fundamental cellular activity that plays crucial roles in a variety of biological processes,including nutrient uptake,signal transduction,immune response,morphogenesis,neurotransmission,cell migration and tissue development[1–3].Endocytosis has been classified into two subtypes:the

Bulk-like endocytosis plays an important role in the recycling of insulin granules in pancreatic beta cells

Although bulk endocytosis has been found in a number of neuronal and endocrine cells,the molecular mechanism and physiological function of bulk endocytosis remain elusive.In pancreatic beta cells,we have observed bulk-like endocytosis evoked both by flash photolysis and trains of depolarization.Bulk-like endocytosis is a clathrin-independent process that is facilitated by enhanced extracellular Ca^(2+) entry and suppressed by the inhibition of dynamin function.Moreover,defects in bulklike endocytosis are accompanied by hyperinsulinemia in primary beta cells dissociated from diabetic KKAy mice,which suggests that bulk-like endocytosis plays an important role in maintaining the exo-endocytosis balance and beta cell secretory capability.