Exocytosis Proteins: Typical and Atypical Mechanisms of Action in Skeletal Muscle

Insulin-stimulated glucose uptake in skeletal muscle is of fundamental importance to prevent postprandial hyperglycemia, and long-term deficits in insulin-stimulated glucose uptake underlie insulin resistance and type 2 diabetes. Skeletal muscle is responsible for ~80% of the peripheral glucose uptake from circulation via the insulin-responsive glucose transporter GLUT4. GLUT4 is mainly sequestered in intracellular GLUT4 storage vesicles in the basal state. In response to insulin, the GLUT4 storage vesicles rapidly translocate to the plasma membrane, where they undergo vesicle docking, priming, and fusion via the high-affinity interactions among the soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) exocytosis proteins and their regulators. Numerous studies have elucidated that GLUT4 translocation is defective in insulin resistance and type 2 diabetes. Emerging evidence also links defects in several SNAREs and SNARE regulatory proteins to insulin resistance and type 2 diabetes in rodents and humans. Therefore, we highlight the latest research on the role of SNAREs and their regulatory proteins in insulin-stimulated GLUT4 translocation in skeletal muscle. Subsequently, we discuss the novel emerging role of SNARE proteins as interaction partners in pathways not typically thought to involve SNAREs and how these atypical functions reveal novel therapeutic targets for combating peripheral insulin resistance and diabetes.

Correlative microscopy and block-face imaging (CoMBI) method for both paraffin-embedded and frozen specimens

Correlative microscopy and block-face imaging (CoMBI), a method that we previously developed, is characterized by the ability to correlate between serial block-face images as 3-dimensional (3D) datasets and sections as 2-dimensional (2D) microscopic images. CoMBI has been performed for the morphological analyses of various biological specimens, and its use is expanding. However, the conventional CoMBI system utilizes a cryostat, which limits its compatibility to only frozen blocks and the resolution of the block-face image. We developed a new CoMBI system that can be applied to not only frozen blocks but also paraffin blocks, and it has an improved magnification for block-face imaging. The new system, called CoMBI-S, comprises sliding-type sectioning devices and imaging devices, and it conducts block slicing and block-face imaging automatically. Sections can also be collected and processed for microscopy as required. We also developed sample preparation methods for improving the qualities of the block-face images and 3D rendered volumes. We successfully obtained correlative 3D datasets and 2D microscopic images of zebrafish, mice, and fruit flies, which were paraffin-embedded or frozen. In addition, the 3D datasets at the highest magnification could depict a single neuron and bile canaliculus.

Identification of potential genomic biomarkers for Parkinson's disease using data pooling of gene expression microarrays

Aim: In this study, we aimed to identify potential diagnostic biomarkers Parkinson's disease (PD) by exploring microarray gene expression data of PD patients. Materials & methods: Differentially expressed genes associated with PD were screened from the GSE99039 dataset using weighted gene co-expression network analysis, followed by gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses, gene-gene interaction network analysis and receiver operator characteristics analysis. Results: We identified two PD-associated modules, in which genes from the chemokine signaling pathway were primarily enriched. In particular, CS, PRKCD, RHOG and VAMP2 directly interacted with known PD-associated genes and showed higher expression in the PD samples, and may thus be potential biomarkers in PD diagnosis. Conclusion: A DFG-analysis identified a four-gene panel (CS, PRKCD, RHOG, VAMP2) as a potential diagnostic predictor to diagnose PD.

Vesicular transport system in myotubes: ultrastructural study and signposting with vesicle-associated membrane proteins

Myofibers have characteristic membrane compartments in their cytoplasm and sarcolemma, such as the sarcoplasmic reticulum, T-tubules, neuromuscular junction, and myotendinous junction. Little is known about the vesicular transport that is believed to mediate the development of these membrane compartments. We determined the locations of organelles in differentiating myotubes. Electron microscopic observation of a whole myotube revealed the arrangement of Golgi apparatus, rough endoplasmic reticulum, autolysosomes, mitochondria, and smooth endoplasmic reticulum from the perinuclear region toward the end of myotubes and the existence of a large number of vesicles near the ends of myotubes. Vesicles in myotubes were further characterized using immunofluorescence microscopy to analyze expression and localization of vesicle-associated membrane proteins (VAMPs). VAMPs are a family of seven proteins that regulate post-Golgi vesicular transport via the fusion of vesicles to the target membranes. Myotubes express five VAMPs in total. Vesicles with VAMP2, VAMP3, or VAMP5 were found near the ends of the myotubes. Some of these vesicles are also positive for caveolin-3, suggesting their participation in the development of T-tubules. Our morphological analyses revealed the characteristic arrangement of organelles in myotubes and the existence of transport vesicles near the ends of the myotubes.

VAMP2 marks quiescent satellite cells and myotubes, but not activated myoblasts

We examined the expression and intracellular localization of vesicle-associated membrane protein 2 (VAMP2) during the differentiation of skeletal muscle cells by immunofluorescence microscopy. In isolated single myofibers, VAMP2 was expressed in quiescent satellite cells, downregulated in proliferating myoblastic cells, and re-expressed with differentiation. In the myoblastic cell line C2C12, VAMP2 was expressed at a low level in the proliferating stage, and then increased after differentiation into myotubes. Based on these results, we propose that VAMP2 can be used as a molecular marker for both quiescent satellite cells and myotubes, but not for proliferating myoblasts. We also found the partial colocalization of VAMP2 with transferrin- or Rab11-labeled vesicles in myotubes, suggesting a role of VAMP2 in the trafficking of recycling endosomes.