Initially, the pandemic COVID-19, caused by SARS-CoV-2, was considered to be an exclusive lung disease, eventually leading to serious respiratory symptoms 1 . In the meantime, accumulating experimental and clinical studies have suggested that SARS-CoV-2 may also cause lesions in the kidneys, heart, brain, and gastrointestinal and endocrine organs [2][3][4][5][6][7] . SARS-CoV-2 tropism towards distinct tissues is governed by cellular factors expressed on target cells such as the viral entry receptor angiotensin-converting enzyme 2 (ACE2) 8 and the transmembrane serine protease 2 (TMPRSS2) 8 . ACE2 messenger RNA 9-13 and protein 12-14 expression within the islets of Langerhans has been reported, but not yet been shown, to allow SARS-CoV-2 entry 9,12,15 . Diabetes mellitus presents Janus like in 16 ): first, pre-existing diabetes is a highly prevalent comorbidity observed in 11-22% of patients and as such increases the risk of a severe disease, requiring more intense interventions and increasing mortality [17][18][19][20][21][22] . Second, SARS-CoV-2 infection seems to affect the exocrine pancreas, manifesting as pancreatitis in 32.5% of critically ill patients 23 , and pancreatic enlargement and abnormal amylase or lipase levels in 7.5-17% of patients 9,22 . Third, metabolic dysregulation has been observed in patients with COVID-19 as:(1) increased hyperglycaemia in patients with type 2 diabetes 24 ; (2) ketoacidosis in 2-6.4% of diabetic and non-diabetic patients 18,25 ; and (3), in case studies reporting ketoacidosis on SARS-CoV-2 infection, accompanied by (4) new-onset type 1 diabetes mellitus (T1DM) in the absence of autoantibodies [26][27][28] . In a cohort study of patients with diabetes, hyperglycaemia was reported in more than 50% of all cases, and almost a third experienced diabetic ketoacidosis 29 . Finally, a multicentre study found an 80% increase of new-onset T1DM in children during the COVID-19 pandemic 30 . In accordance, a recent meta-analysis summarizes that severe SARS-CoV-2 infects and replicates in cells of the human endocrine and exocrine pancreas

Endocytosis has a crucial role in many cellular processes. The best-characterized mechanism for endocytosis involves clathrin-coated pits [1], but evidence has accumulated for additional endocytic pathways in mammalian cells [2]. One such pathway involves caveolae, plasma-membrane invaginations defined by caveolin proteins. Plasma-membrane microdomains referred to as lipid rafts have also been associated with clathrin-independent endocytosis by biochemical and pharmacological criteria [3]. The mechanisms, however, of nonclathrin, noncaveolin endocytosis are not clear [4, 5]. Here we show that coassembly of two similar membrane proteins, flotillin1 and flotillin2 [6-8], is sufficient to generate de novo membrane microdomains with some of the predicted properties of lipid rafts [9]. These microdomains are distinct from caveolin1-positive caveolae, are dynamic, and bud into the cell. Coassembly of flotillin1 and flotillin2 into microdomains induces membrane curvature, the formation of plasma-membrane invaginations morphologically similar to caveolae, and the accumulation of intracellular vesicles. We propose that flotillin proteins are defining structural components of the machinery that mediates a clathrin-independent endocytic pathway. Key attributes of this machinery are the dependence on coassembly of both flotillins and the inference that flotillin microdomains can exist in either flat or invaginated states.

Flotillin-1 and flotillin-2 co-assemble into plasma membrane microdomains that are involved in the endocytosis of molecules such as glycosyl phosphatidylinositol (GPI)-linked proteins. Previous studies suggest that budding of flotillin microdomains from the plasma membrane is a tightly regulated process. Here, we demonstrate that endocytosis of flotillins is regulated by the Src family kinase Fyn. The Src kinase inhibitor PP2 prevents EGF-induced flotillin internalisation, and EGF-induced internalisation does not occur in SYF cells lacking Src, Yes and Fyn. Expression of Fyn, but not Src or Yes, restores EGF-induced internalisation in SYF cells. Expression of an active form of Fyn but not other Src kinases is sufficient to induce redistribution of flotillins from the plasma membrane to late endosomes and lysosomes. Using two partial Fyn constructs that form a functional kinase upon addition of rapamycin to cells, we show that flotillin internalisation from the plasma membrane occurs shortly after Fyn activation. Tyr160 in flotillin-1 and Tyr163 in flotillin-2 are directly phosphorylated by Fyn, and mutation of these residues to phenylalanine prevents Fyn-induced flotillin internalisation. Uptake of the GPI-linked protein CD59 is reduced by expression of the phenylalanine-mutated flotillins. These data establish uptake of flotillin microdomains as a tyrosine-kinase-regulated endocytic process.

In alveolar type II cells, the release of surfactant is considerably delayed after the formation of exocytotic fusion pores, suggesting that content dispersal may be limited by fusion pore diameter and subject to regulation at a postfusion level. To address this issue, we used confocal FRAP and N-(3-triethylammoniumpropyl)-4-(4-[dibutylamino]styryl) pyridinium dibromide (FM 1-43), a dye yielding intense localized fluorescence of surfactant when entering the vesicle lumen through the fusion pore (Haller, T., J. Ortmayr, F. Friedrich, H. Volkl, and P. Dietl. 1998. Proc. Natl. Acad. Sci. USA. 95:1579–1584). Thus, we have been able to monitor the dynamics of individual fusion pores up to hours in intact cells, and to calculate pore diameters using a diffusion model derived from Fick's law. After formation, fusion pores were arrested in a state impeding the release of vesicle contents, and expanded at irregular times thereafter. The expansion rate of initial pores and the probability of late expansions were increased by elevation of the cytoplasmic Ca2+ concentration. Consistently, content release correlated with the occurrence of Ca2+ oscillations in ATP-treated cells, and expanded fusion pores were detectable by EM. This study supports a new concept in exocytosis, implicating fusion pores in the regulation of content release for extended periods after initial formation.