Yoshihiro Akimoto

Deletion of CDKAL1 Affects Mitochondrial ATP Generation and First-Phase Insulin Exocytosis

Background A variant of the CDKAL1 gene was reported to be associated with type 2 diabetes and reduced insulin release in humans; however, the role of CDKAL1 in β cells is largely unknown. Therefore, to determine the role of CDKAL1 in insulin release from β cells, we studied insulin release profiles in CDKAL1 gene knockout (CDKAL1 KO) mice. Principal Findings Total internal reflection fluorescence imaging of CDKAL1 KO β cells showed that the number of fusion events during first-phase insulin release was reduced. However, there was no significant difference in the number of fusion events during second-phase release or high K+-induced release between WT and KO cells. CDKAL1 deletion resulted in a delayed and slow increase in cytosolic free Ca2+ concentration during high glucose stimulation. Patch-clamp experiments revealed that the responsiveness of ATP-sensitive K+ (KATP) channels to glucose was blunted in KO cells. In addition, glucose-induced ATP generation was impaired. Although CDKAL1 is homologous to cyclin-dependent kinase 5 (CDK5) regulatory subunit-associated protein 1, there was no difference in the kinase activity of CDK5 between WT and CDKAL1 KO islets. Conclusions/Significance We provide the first report describing the function of CDKAL1 in β cells. Our results indicate that CDKAL1 controls first-phase insulin exocytosis in β cells by facilitating ATP generation, KATP channel responsiveness and the subsequent activity of Ca2+ channels through pathways other than CDK5-mediated regulation.

Distribution and Function of Macrophage Galactose-type C-type Lectin 2 (MGL2/CD301b): EFFICIENT UPTAKE AND PRESENTATION OF GLYCOSYLATED ANTIGENS BY DENDRITIC CELLS*

Dendritic cells (DCs) express cell surface lectins that are potentially involved in the recognition, uptake, and presentation of glycosylated foreign substances. A unique calcium-type (C-type) lectin, the macrophage galactose (Gal)-type C-type lectin (MGL/CD301) expressed on DCs, is thought to participate in the recognition of molecules from both altered self and pathogens due to its monosaccharide specificity for Gal and N-acetylgalactosamine (GalNAc). Although mice have two MGL genes, Mgl1 and Mgl2, their distinct roles have not been previously explored. The present report characterizes the properties of MGL2 by examining its distribution and its role in antigen presentation by DCs. We generated an MGL2-specific monoclonal antibody and examined MGL2 expression in tissues by immunohistochemistry and in isolated cells by flow cytometry. The cells reactive with this antibody were shown to be a portion of MGL1-expressing cells, mostly conventional DCs. Internalization of soluble polyacrylamide polymers (PAA) with α-GalNAc residues (GalNAc-PAA) by bone marrow-derived DCs (BM-DCs) was mediated by MGL2, as revealed by a comparison of Mgl1−/− and Mgl2−/− BM-DCs with wild-type BM-DCs. Biotinylated GalNAc-PAA conjugated to streptavidin (SAv) was more efficiently presented to SAv-primed T cells by BM-DCs than β-N-acetylglucosamine-PAA conjugated to SAv or SAv alone as shown by thymidine uptake and cytokine production. This is the first report that demonstrates the involvement of GalNAc residues in antigen uptake and presentation by DCs that lead to CD4+ T cell activation.

Cell-surface arylsulfatase A and B on sinusoidal endothelial cells, hepatocytes, and Kupffer cells in mammalian livers

Arylsulfatase A (ARSA) and B (ARSB) have been regarded as lysosomal enzymes because of their hydrolytic activity on synthetic aromatic substrates and the lysosomal localization of their enzymatic activity. Using sea urchin embryos, we previously demonstrated that the bulk of ARS is located on the cell surface of the epithelium, colocalizing with sulfated polysaccharides, and that it does not exhibit enzymatic activity. To examine whether ARSA and ARSB exist on the cell surface in mammalian tissues, we raised antibodies against ARSA and ARSB and examined immunohistochemically their localization in the liver using light and electron microscopy. Here we show that mammalian ARSA and ARSB exist on the cell surface of sinusoidal endothelial cells, hepatocytes, and sinusoidal macrophages (Kupffer cells), as well as in the lysosome. They are also colocalized with heparan sulfate proteoglycan. These results suggest that ARSA and ARSB also may function in the cell surface of mammals. This is the first report to show cell-surface localization of ARS in mammalian somatic cells. The extracellular localization of ARS will provide new insight for human ARS deficiency disorders, such as metachromatic leukodystrophy and mucopolysaccharidosis VI.

Increased Apoptosis of Myoblasts in Drosophila Model for the Walker-Warburg Syndrome

Walker-Warburg syndrome, a progressive muscular dystrophy, is a severe disease with various kinds of symptoms such as muscle weakness and occasional seizures. The genes of protein O-mannosyltransferases 1 and 2 (POMT1 and POMT2), fukutin, and fukutin-related protein are responsible for this syndrome. In our previous study, we cloned Drosophila orthologs of human POMT1 and POMT2 and identified their activity. However, the mechanism of onset of this syndrome is not well understood. Furthermore, little is known about the behavioral properties of the Drosophila POMT1 and POMT2 mutants, which are called rotated abdomen (rt) and twisted (tw), respectively. First, we performed various kinds of behavioral tests and described in detail the muscle structures by using these mutants. The mutant flies exhibited abnormalities in heavy exercises such as climbing or flight but not in light movements such as locomotion. Defective motor function in mutants appeared immediately after eclosion and was exaggerated with aging. Along with motor function, muscle ultrastructure in the tw mutant was altered, as seen in human patients. We demonstrated that expression of RNA interference (RNAi) for the rt gene and the tw mutant was almost completely lethal and semi-lethal, respectively. Flies expressing RNAi had reduced lifespans. These findings clearly demonstrate that Drosophila POMT mutants are models for human muscular dystrophy. We then observed a high density of myoblasts with an enhanced degree of apoptosis in the tw mutant, which completely lost enzymatic activity. In this paper, we propose a novel mechanism for the development of muscular dystrophy: POMT mutation causes high myoblast density and position derangement, which result in apoptosis, muscle disorganization, and muscle cell defects.

Amino Acid Transporter LAT3 Is Required for Podocyte Development and Function

LAT3 is a Na+-independent neutral l-amino acid transporter recently isolated from a human hepatocellular carcinoma cell line. Although liver, skeletal muscle, and pancreas are known to express LAT3, the tissue distribution and physiologic function of this transporter are not completely understood. Here, we observed that glomeruli express LAT3. Immunofluorescence, confocal microscopy, and immunoelectron microscopy revealed that LAT3 localizes to the apical plasma membrane of podocyte foot processes. In mice, starvation upregulated glomerular LAT3, phosphorylated AKT1, reconstituted the actin network, and elongated foot processes. In the fetal kidney, we observed intense LAT3 expression at the capillary loops stage of renal development. Finally, zebrafish morphants lacking lat3 function showed collapsed glomeruli with thickened glomerular basement membranes. Permeability studies of the glomerular filtration barrier in these zebrafish morphants demonstrated a disruption of selective glomerular permeability. Our data suggest that LAT3 may play a crucial role in the development and maintenance of podocyte structure and function by regulating protein synthesis and the actin cytoskeleton.

Expression of galectin-1, a new component of slit diaphragm, is altered in minimal change nephrotic syndrome

Nephrin is an essential structural component of the glomerular slit diaphragm (SD), a highly organized intercellular junction that constitutes the ultrafiltration barrier of the kidney. Recent studies have identified two additional nephrin-interacting SD proteins (NEPH1 and NEPH2), suggesting that the zipper-like pattern of the SD is formed through complex intra- and intermolecular interactions of these proteins. However, the complexity of the SD structure suggests that additional SD components remain to be discovered. In this study, we identified galectin-1 (Gal-1) as a new component of the SD, binding to the ectodomain of nephrin. Using dual-immunofluorescence and confocal microscopy and dual-immunoelectron microscopy, we found Gal-1 colocalizing with the ectodomain of nephrin at the SD in normal human kidney. By immunoprecipitation and surface plasmon resonance, we confirmed a direct molecular interaction between Gal-1 and nephrin. Moreover, recombinant Gal-1 induced tyrosine phosphorylation of the cytoplasmic domain of nephrin and activation of the extracellular signal-regulated kinase 1/2 in podocytes. We also showed that podocytes are a major site of biosynthesis of Gal-1 in the glomerulus and that the normal expression patterns and levels of Gal-1 are altered in patients with minimal change nephrotic syndrome. Finally, in puromycin aminonucleoside-induced rat nephrosis, an apparent reduction in the levels of Gal-1 and nephrin around the onset of heavy proteinuria was also revealed. Our data present Gal-1 as a new extracellular ligand of nephrin localized at the glomerular SD, and provide further insight into the complex molecular organization, interaction, and structure of the SD, which is an active site of intracellular signaling necessary for podocyte function.

Expression of matrix metalloproteinases 2 and 9 and tissue inhibitors of matrix metalloproteinases 2 and 1 in the glomeruli of human glomerular diseases: the results of studies using immunofluorescence, in situ hybridization, and immunoelectron microscopy

BackgroundIt has been reported matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of matrix metalloproteinases (TIMPs), play important roles in the decomposition of the extracellular matrices of the glomerulus during the pathological processes in various glomerular diseases. Although the activity of these enzymes in cases of experimental glomerulonephritis has been described, the expression sites in the glomeruli of human renal diseases have been identified in only a few articles and remain controversial.MethodsThe expression of the gelatinase group of MMPs (MMP-2 and MMP-9) and their inhibitors (TIMP-2 and TIMP-1) were evaluated in 19 renal biopsies of several types of glomerular diseases by immunofluorescence (IF) labeling. In addition, several samples of immunoglobulin A nephropathy (IgAN) were also investigated by in situ hybridization (ISH) and immunoelectron microscopy (IEM).ResultsThe expression of MMP-2 was observed in all the cases examined by IF and ISH. TIMP-2 expression varied from negative to positive among 11 cases of IgAN, but was negative in the cases with lupus nephritis (LN) (nxa0=xa03), membranoproliferative glomerulonephritis (MPGN) (nxa0=xa02), and post-streptococcal glomerulonephritis (nxa0=xa01). However, it was weakly positive in the cases of diabetic nephropathy (DMN) (nxa0=xa02). MMP-2 was mainly observed along glomerular capillary loops (GCLs) and Bowman’s capsules, whereas TIMP-2 was found in the mesangial area. The expression of MMP-9 in cases of IgAN varied, and was local, not diffuse, if it was present. MMP-9 expression in cases of LN, MPGN, and DMN was diffuse, but the intensity of staining varied. MMP-9 was primarily expressed in the mesangium. TIMP-1 expression was negative in all cases except for those with IgAN. The localization of MMP-2 in patients with IgAN, which was investigated by IEM, was revealed to be mainly on the endothelial cell membranes of GCLs, podocyte membranes, the parietal cell membranes of Bowman’s capsules, and some on the membranes of mesangial cells.ConclusionThe study results suggest that the expression levels and patterns of MMPs and TIMPs are generally similar in several types of glomerular diseases, even though each case has a somewhat different distribution and intensity of expression. When these enzymes were present, their main sites were as follows: MMP-2 was found along glomerular basement membrane, TIMP-2 was located in the acellular mesangial area, MMP-9 was seen in the mesangium, and TIMP-1 was hardly detected. MMP-2 expression is clearly demonstrated to exist at the above-described sites by IEM in patients with IgAN.

Properties of L-type amino acid transporter 1 in epidermal ovarian cancer.

Hypothesis: To investigate the expression and the functional properties of L-type amino acid transporter 1 (LAT1) in human epithelial ovarian cancer to provide a basis for potential new therapies to control the growth and the metastasis of ovarian cancer. Methods: The material used comprised 63 surgically resected specimens obtained from female patients undergoing gynecologic surgery at Kyorin University School of Medicine (Tokyo, Japan). The expression of LAT1 in 53 cases of ovarian cancers was determined by Western blot and immunohistochemical staining, and results were compared with those of normal ovarian tissues (5 cases) and benign ovarian tumors (5 cases). Furthermore, we examined the effect of 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH), the classic inhibitor of system L on the survival, the migration, and the uptake of l-leucine by human epithelial ovarian cancer cell line (OVCAR-3). Results: The LAT1 was significantly up-regulated in various human epithelial ovarian cancers that was localized predominantly on their plasma membrane and in the plasma membrane of the ovarian cancer cell line in conjunction with 4F2hc via disulfide bonds. The BCH inhibited the proliferation and the migration of the OVCAR-3 cells and the uptake of [14C]l-leucine by these cells in a dose-dependent manner. The OVCAR-3 cells did not express LAT2, and the uptake of [14C]l-leucine by these cells was Na+-independent and almost completely inhibited by BCH. Thus, our findings indicated that most l-leucine uptake in OVCAR-3 cells was mediated by LAT1. Conclusions: The LAT1 plays significant roles in nutrition, proliferation, and migration of ovarian cancer. Then, LAT1 inhibition would be useful for anticancer therapy in suppressing tumor growth without affecting normal tissues.

Imaging exocytosis of single glucagon-like peptide-1 containing granules in a murine enteroendocrine cell line with total internal reflection fluorescent microscopy.

To analyze the exocytosis of glucagon-like peptide-1 (GLP-1) granules, we imaged the motion of GLP-1 granules labeled with enhanced yellow fluorescent protein (Venus) fused to human growth hormone (hGH-Venus) in an enteroendocrine cell line, STC-1 cells, by total internal reflection fluorescent (TIRF) microscopy. We found glucose stimulation caused biphasic GLP-1 granule exocytosis: during the first phase, fusion events occurred from two types of granules (previously docked granules and newcomers), and thereafter continuous fusion was observed mostly from newcomers during the second phase. Closely similar to the insulin granule fusion from pancreatic beta cells, the regulated biphasic exocytosis from two types of granules may be a common mechanism in glucose-evoked hormone release from endocrine cells.

Change in Long-Spacing Collagen in Descemet's Membrane of Diabetic Goto-Kakizaki Rats and Its Suppression by Antidiabetic Agents

We examined changes in the ultrastructure and localization of major extracellular matrix components, including 5 types of collagen (type I, III, IV, VI, and VIII), laminin, fibronectin, and heparan sulfate proteoglycan in Descemets membrane of the cornea of diabetic GK rats. In the cornea of diabetic GK rats, more long-spacing collagen fibrils were observed in Descemets membrane than in the membrane of the nondiabetic Wistar rats. Both GK and Wistar rats showed an age-dependent increase in the density of the long-spacing collagen. Immunoelectron microscopy showed that type VIII collagen was localized in the internodal region of the long-spacing collagen, which was not labelled by any of the other antibodies used. The antidiabetic agents nateglinide and glibenclamide significantly suppressed the formation of the long-spacing collagen in the diabetic rats. Long-spacing collagen would thus be a useful indicator for studying diabetic changes in the cornea and the effect of antidiabetic agents.