Podocyte-specific expression of a novel trans-Golgi protein Vear in human kidney.

BACKGROUND: Vear is a recently identified Golgi apparatus-associated protein. It has been suggested to be involved in vesicular trafficking between the Golgi and the vacuolar/lysosomal system. Proteins similar to Vear have also been shown to interact with activated ARF proteins (ADP ribosylation factor), and they are probably involved in membrane trafficking from the trans-Golgi network (TGN). We have previously shown that Vear is widely distributed in human tissues, with an especially high level of mRNA in the kidney. This study further characterizes the distribution and subcellular localization of Vear in normal adult kidney and shows its association with glomerulogenesis in fetal kidney. METHODS: Immunofluorescence and immunoelectron microscopy were used to study the expression of Vear in fetal and adult kidney. The expression of Vear in isolated glomeruli was shown by immunoblotting. The distribution of its mRNA was analyzed by using in situ and Northern hybridization. RESULTS: In situ hybridization and immunofluorescence microscopy showed that in the kidney, Vear is present in glomerular structures. By fluorescence microscopy, the immunoreactivity for Vear was found only in podocytes, as judged by its distinct colocalization with podocalyxin and vimentin, well-established marker proteins of podocytes. Its specific expression in the glomeruli versus other compartments of the kidney was also verified by Western blotting. By using immunogold electron microscopy, Vear was seen in the Golgi apparatus, tubulovesicular structures, and membranes adjacent to the Golgi complex. In fetal kidney, expression of Vear coincided with the formation of segmental structures of the glomeruli. It was first seen close to the undifferentiated luminal cells at the vesicular stage and increasingly in the differentiating podocytes at the more advanced stages of glomerulogenesis. CONCLUSIONS: In the kidney, Vear shows a distinct, specific, and developmentally regulated expression in glomerular podocytes. This suggests that Vear has a specific function in podocytes. It could be associated with the known high secretory and synthetic activity of the podocytes, especially the production of the basement membrane components, which are critically involved in the glomerulogenesis and the maintenance of the glomerular function.

Binding of GGA2 to the lysosomal enzyme sorting motif of the mannose 6-phosphate receptor.

The GGAs are a multidomain protein family implicated in protein trafficking between the Golgi and endosomes. Here, the VHS domain of GGA2 was shown to bind to the acidic cluster-dileucine motif in the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CI-MPR). Receptors with mutations in this motif were defective in lysosomal enzyme sorting. The hinge domain of GGA2 bound clathrin, suggesting that GGA2 could be a link between cargo molecules and clathrin-coated vesicle assembly. Thus, GGA2 binding to the CI-MPR is important for lysosomal enzyme targeting.

Vear, a novel Golgi-associated protein, is preferentially expressed in type I cells in skeletal muscle.

Vear is a novel Golgi-associated protein with a domain structure characteristic of many vesicular transport-associated proteins. It has been suggested that Vear is involved in vesicle transport through trans-Golgi. In this study, we have determined the localization of Vear in skeletal muscle. The staining for Vear in normal human muscle revealed a distribution pattern similar to that of type I fibers. We conclude that Vear is preferentially expressed in type I fibers in human muscle, presumably indicative of a specific function that remains to be identified.

Vear, a novel Golgi-associated protein with VHS and gamma-adaptin "ear" domains.

The molecular basis of the selectivity and the details of the vesicle formation in endocytic and secretory pathways are still poorly known and most probably involve as yet unidentified components. Here we describe the cloning, expression, and tissue and cell distribution of a novel protein of 67 kDa (called Vear) that bears homology to several endocytosis-associated proteins in that it has a VHS domain in its N terminus. It is also similar to gamma-adaptin, the heavy subunit of AP-1, in having in its C terminus a typical "ear" domain. In immunofluorescence microscopy, Vear was seen in the Golgi complex as judged by a typical distribution pattern, a distinct colocalization with the Golgi marker gamma-adaptin, and a sensitivity to treatment of cells with brefeldin A. In cell fractionation, Vear partitioned with the post-nuclear membrane fraction. In transfection experiments, hemagglutinin-tagged full-length Vear and truncated Vear lacking the VHS domain assembled on and caused compaction of the Golgi complex. Golgi association without compaction was seen with the ear domain of Vear, whereas the VHS domain alone showed a diffuse membrane- and vesicle-associated distribution. The Golgi association and the bipartite structure along with the differential targeting of its domains suggest that Vear is involved in heterotypic vesicle/suborganelle interactions associated with the Golgi complex. Tissue-specific function of Vear is suggested by its high level of expression in kidney, muscle, and heart.

EAST, an epidermal growth factor receptor- and Eps15-associated protein with Src homology 3 and tyrosine-based activation motif domains.

We describe the cloning and characterization of a new cytoplasmic protein designated epidermal growth factor receptor-associated protein with SH3- and TAM domains (EAST). It contains an Src homology 3 domain in its midregion and a tyrosine-based activation motif in its COOH terminus. Antibodies to EAST recognize a 68-kDa protein that is present in most chicken tissues. An epidermal growth factor (EGF)-dependent association between the EGF receptor (EGFR) and EAST was shown by reciprocal immunoprecipitation/immunoblotting studies with specific antibodies. Activated EGFR catalyzed the tyrosine phosphorylation of EAST, as judged by an in vitro kinase assay with both immunoprecipitated and purified EGFR. Immunoprecipitation/immunoblotting experiments also demonstrated an association between EAST and eps15, an EGFR substrate associated with clathrin-coated pits and vesicles, which is essential in the endocytotic pathway. The association between EAST and eps15 was not affected by EGF treatment. In immunofluorescence microscopy, EAST was shown to partially colocalize with clathrin. The sequence of the NH2-terminal portion of EAST shows a high degree of similarity with a group of proteins involved in endocytosis or vesicle trafficking. Thus, EAST is a novel signal transduction component probably involved in EGF signaling and in the endocytotic machinery.

Glutathione depletion induces glycogenolysis dependent ascorbate synthesis in isolated murine hepatocytes.

The relationship between glutathione deficiency, glycogen metabolism and ascorbate synthesis was investigated in isolated murine hepatocytes. Glutathione deficiency caused by various agents increased ascorbate synthesis with a stimulation of glycogen breakdown. Increased ascorbate synthesis from UDP-glucose or gulonolactone could not be further affected by glutathione depletion. Fructose prevented the stimulated glycogenolysis and ascorbate synthesis caused by glutathione consumption. Reduction of oxidised glutathione by dithiothreitol decreased the elevated glycogenolysis and ascorbate synthesis in diamide or menadione treated hepatocytes. Our results suggest that a change in GSH/GSSG ratio seems to be a sufficient precondition of altering glycogenolysis and a consequent ascorbate synthesis.