A novel function of Noc2 in agonist-induced intracellular Ca2+ increase during zymogen-granule exocytosis in pancreatic acinar cells

In vivo Roles of Rab27 and Its Effectors in Exocytosis

The monomeric GTPase Rab27 regulates exocytosis of a broad range of vesicles in multicellular organisms. Several effectors bind GTP-bound Rab27a and/or Rab27b on secretory vesicles to execute a series of exocytic steps, such as vesicle maturation, movement along microtubules, anchoring within the peripheral F-actin network, and tethering to the plasma membrane, via interactions with specific proteins and membrane lipids in a local milieu. Although Rab27 effectors generally promote exocytosis, they can also temporarily restrict it when they are involved in the rate-limiting step. Genetic alterations in Rab27-related molecules cause discrete diseases manifesting pigment dilution and immunodeficiency, and can also affect common diseases such as diabetes and cancer in complex ways. Although the function and mechanism of action of these effectors have been explored, it is unclear how multiple effectors act in coordination within a cell to regulate the secretory process as a whole. It seems that Rab27 and various effectors constitutively reside on individual vesicles to perform consecutive exocytic steps. The present review describes the unique properties and in vivo roles of the Rab27 system, and the functional relationship among different effectors coexpressed in single cells, with pancreatic beta cells used as an example.Key words: membrane trafficking, regulated exocytosis, insulin granules, pancreatic beta cells.

An epigenome-wide methylation study of healthy individuals with or without depressive symptoms

Major depressive disorder is a common psychiatric disorder that is thought to be triggered by both genetic and environmental factors. Depressive symptoms are an important public health problem and contribute to vulnerability to major depression. Although a substantial number of genetic and epigenetic studies have been performed to date, the detailed etiology of depression remains unclear and there are no validated biomarkers. DNA methylation is one of the major epigenetic modifications that play diverse roles in the etiology of complex diseases. In this study, we performed an epigenome-wide association study (EWAS) of DNA methylation on subjects with (N = 20) or without (N = 27) depressive symptoms in order to examine whether different levels of DNA methylation were associated with depressive tendencies. Employing methylation-array technology, a total of 363,887 methylation sites across the genomes were investigated and several candidate CpG sites associated with depressive symptoms were identified, especially annotated to genes linked to a G-protein coupled receptor protein signaling pathway. These data provide a strong impetus for validation studies using a larger cohort and support the possibility that G-protein coupled receptor protein signaling pathways are involved in the pathogenesis of depression.

Clinical Implications of Rabphillin-3A-Like Gene Alterations in Breast Cancer

For the rabphillin-3A-like (RPH3AL) gene, a putative tumor suppressor, the clinical significance of genetic alterations in breast cancers was evaluated. DNA and RNA were extracted from formalin-fixed, paraffin-embedded (FFPE) cancers and matching normal tissues. DNA samples were assessed for loss of heterozygosity (LOH) at the 17p13.3 locus of RPH3AL and the 17p13.1 locus of the tumor suppressor, TP53. RPH3AL was sequenced, and single nucleotide polymorphisms (SNPs) were genotyped. RNA samples were evaluated for expression of RPH3AL, and FFPE tissues were profiled for its phenotypic expression. Alterations in RPH3AL were correlated with clinicopathological features, LOH of TP53, and patient survival. Of 121 cancers, 80 had LOH at one of the RPH3AL locus. LOH of RHP3AL was associated with nodal metastasis, advanced stage, large tumor size, and poor survival. Although ~50% were positive for LOH at the RPH3AL and TP53 loci, 19 of 105 exhibited LOH only at the RPH3AL locus. Of these, 12 were non-Hispanic Caucasians (Whites), 15 had large tumors, and 12 were older (>50 years). Patients exhibiting LOH at both loci had shorter survival than those without LOH at these loci (log-rank, P = 0.014). LOH at the TP53 locus alone was not associated with survival. Analyses of RPH3AL identified missense point mutations in 19 of 125 cases, a SNP (C>A) in the 5’untranslated region at -25 (5’UTR-25) in 26 of 104, and a SNP (G>T) in the intronic region at 43 bp downstream to exon-6 (intron-6-43) in 79 of 118. Genotype C/A or A/A of the SNP at 5’UTR-25 and genotype T/T of a SNP at intron-6-43 were predominantly in Whites. Low levels of RNA and protein expression of RPH3AL were present in cancers relative to normal tissues. Thus, genetic alterations in RPH3AL are associated with aggressive behavior of breast cancers and with short survival of patients.

Multi-point scanning two-photon excitation microscopy by utilizing a high-peak-power 1042-nm laser

The temporal resolution of a two-photon excitation laser scanning microscopy (TPLSM) system is limited by the excitation laser beam's scanning speed. To improve the temporal resolution, the TPLSM system is equipped with a spinning-disk confocal scanning unit. However, the insufficient energy of a conventional Ti:sapphire laser source restricts the field of view (FOV) for TPLSM images to a narrow region. Therefore, we introduced a high-peak-power Yb-based laser in order to enlarge the FOV. This system provided three-dimensional imaging of a sufficiently deep and wide region of fixed mouse brain slices, clear four-dimensional imaging of actin dynamics in live mammalian cells and microtubule dynamics during mitosis and cytokinesis in live plant cells.

Two-photon excitation STED microscopy by utilizing transmissive liquid crystal devices

Transmissive liquid crystal devices (tLCDs) enable the modification of optical properties, such as phase, polarization, and laser light intensity, over a wide wavelength region at a high conversion efficiency. By utilizing tLCDs, we developed a new two-photon excitation stimulated emission depletion microscopy technique based on a conventional two-photon microscope. Spatial resolution was improved by compensating for phase shifts distributed in the optical path. Using this technique, we observed the fine structures of microtubule networks in fixed biological cells.

Ultrasensitive imaging of Ca2+ dynamics in pancreatic acinar cells of yellow cameleon-nano transgenic mice

Yellow Cameleons are genetically encoded Ca²⁺ indicators in which cyan and yellow fluorescent proteins and calmodulin work together as a fluorescence (Förster) resonance energy transfer Ca²⁺-sensor probe. To achieve ultrasensitive Ca²⁺ imaging for low resting Ca²⁺ or small Ca²⁺ transients in various organs, we generated a transgenic mouse line expressing the highest-sensitive genetically encoded Ca²⁺ indicator (Yellow Cameleon-Nano 15) in the whole body. We then focused on the mechanism of exocytotic events mediated by intracellular Ca²⁺ signaling in acinar cells of the mice with an agonist and observed them by two-photon excitation microscopy. In the results, two-photon excitation imaging of Yellow Cameleon-Nano 15 successfully visualized intracellular Ca²⁺ concentration under stimulation with the agonist at nanomolar levels. This is the first demonstration for application of genetically encoded Ca²⁺ indicators to pancreatic acinar cells. We also simultaneously observed exocytotic events and an intracellular Ca²⁺ concentration under in vivo condition. Yellow Cameleon-Nano 15 mice are healthy and no significant deteriorative effect was observed on physiological response regarding the pancreatic acinar cells. The dynamic range of 165% was calculated from R_max and R_min values under in vivo condition. The mice will be useful for ultrasensitive Ca²⁺ imaging in vivo.

Ca²⁺-regulated secretory granule exocytosis in pancreatic and parotid acinar cells

Protein secretion from acinar cells of the pancreas and parotid glands is controlled by G-protein coupled receptor activation and generation of the cellular messengers Ca(2+), diacylglycerol and cAMP. Secretory granule (SG) exocytosis shares some common characteristics with nerve, neuroendocrine and endocrine cells which are regulated mainly by elevated cell Ca(2+). However, in addition to diverse signaling pathways, acinar cells have large ∼1 μm diameter SGs (∼30 fold larger diameter than synaptic vesicles), respond to stimulation at slower rates (seconds versus milliseconds), demonstrate significant constitutive secretion, and in isolated acini, undergo sequential compound SG-SG exocytosis at the apical membrane. Exocytosis proceeds as an initial rapid phase that peaks and declines over 3 min followed by a prolonged phase that decays to near basal levels over 20-30 min. Studies indicate the early phase is triggered by Ca(2+) and involves the SG proteins VAMP2 (vesicle associated membrane protein2), Ca(2+)-sensing protein synatotagmin 1 (syt1) and the accessory protein complexin 2. The molecular details for regulation of VAMP8-mediated SG exocytosis and the prolonged phase of secretion are still emerging. Here we review the known regulatory molecules that impact the sequential exocytic process of SG tethering, docking, priming and fusion in acinar cells.

Rab27 effectors, pleiotropic regulators in secretory pathways

Rab27, a member of the small GTPase Rab family, is widely conserved in metazoan, and two Rab27 isoforms, Rab27A and Rab27B, are present in vertebrates. Rab27A was the first Rab protein whose dysfunction was found to cause a human hereditary disease, type 2 Griscelli syndrome, which is characterized by silvery hair and immunodeficiency. The discovery in the 21st century of three distinct types of mammalian Rab27A effectors [synaptotagmin-like protein (Slp), Slp homologue lacking C2 domains (Slac2), and Munc13-4] that specifically bind active Rab27A has greatly accelerated our understanding not only of the molecular mechanisms of Rab27A-mediated membrane traffic (e.g. melanosome transport and regulated secretion) but of the symptoms of Griscelli syndrome patients at the molecular level. Because Rab27B is widely expressed in various tissues together with Rab27A and has been found to have the ability to bind all of the Rab27A effectors that have been tested, Rab27A and Rab27B were initially thought to function redundantly by sharing common Rab27 effectors. However, recent evidence has indicated that by interacting with different Rab27 effectors Rab27A and Rab27B play different roles in special types of secretion (e.g. exosome secretion and mast cell secretion) even within the same cell type. In this review article, I describe the current state of our understanding of the functions of Rab27 effectors in secretory pathways.