Recycling of the dense-core vesicle membrane protein phogrin in Min6 beta-cells

Insulin secretion assays in an engineered MIN6 cell line

Insulin secretion from pancreatic beta cells is crucial for maintaining glucose homeostasis. The murine insulinoma derived MIN6 cell line is commonly used as a model for insulin secretion studies. However, its glucose responsiveness wanes with passaging, and insulin secretion is traditionally measured by expensive and time-consuming RIA or ELISA. We have developed a MIN6 subclone (MIN6-6) that allows for high throughput assay of insulin secretion in both population and single cells. In addition, MIN6-6 also expresses Cas9, permitting genome wide CRISPR screen of insulin secretion using a pooled sgRNA library. Here we provide methods for assaying insulin secretion both in bulk and in single cells in MIN6-6 cells, as well as for CRISPR screen of insulin secretion.•A highly glucose responsive beta cell reporter line (MIN6-6) with multiple engineered functionalities.•Allows for CRISPR/Cas9 mutagenesis, quantification of bulk insulin secretion by a straightforward nanoLuc assay and visualization of intracellular insulin granules.•Allows for en masse quantification of insulin granule exocytosis in individual cells under multiple conditions.

Common and female-specific roles of protein tyrosine phosphatase receptors N and N2 in mice reproduction

Simultaneous knockout of the neuroendocrine marker genes Ptprn and Ptprn2, which encode the protein tyrosine phosphatase receptors N and N2, causes infertility in female mice while males are fertile. To elucidate the mechanism of the sex-specific roles of Ptprn and Ptprn2 in mouse reproduction, we analyzed the effects of their double knockout (DKO) on the hypothalamic-pituitary-gonadal axis. In DKO females, delayed puberty and lack of ovulation were observed, complemented by changes in ovarian gene expression and steroidogenesis. In contrast, testicular gene expression, steroidogenesis, and reproductive organs development were not significantly affected in DKO males. However, in both sexes, pituitary luteinizing hormone (LH) beta gene expression and LH levels were reduced, as well as follicle-stimulating hormone beta gene and gonadotropin-releasing hormone (GnRH) gene, while the calcium-mobilizing and LH secretory actions of GnRH were preserved. Hypothalamic Gnrh1 and Kiss1 gene expression was also reduced in DKO females and males. In parallel, a significant decrease in the density of immunoreactive GnRH and kisspeptin fibers was detected in the hypothalamic arcuate nucleus of DKO females and males. The female-specific kisspeptin immunoreactivity in the rostral periventricular region of the third ventricle was also reduced in DKO females, but not in DKO males. These data indicate a critical role of Ptprn and Ptprn2 in kisspeptin-GnRH neuronal function and sexual dimorphism in the threshold levels of GnRH required to preserve reproductive functions.

Novel autoantibodies to the β-cell surface epitopes of ZnT8 in patients progressing to type-1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by autoimmune destruction of insulin-producing β-cells in pancreatic islets. Seroconversions to islet autoantibodies (IAbs) precede the disease onset by many years, but the role of humoral autoimmunity in the disease initiation and progression are unclear. In the present study, we identified a new IAb directed to the extracellular epitopes of ZnT8 (ZnT8ec) in newly diagnosed patients with T1D, and demonstrated immunofluorescence staining of the surface of human β-cells by autoantibodies to ZnT8ec (ZnT8ecA). With the assay specificity set on 99th percentile of 336 healthy controls, the ZnT8ecA positivity rate was 23.6% (74/313) in patients with T1D. Moreover, 30 children in a longitudinal follow up of clinical T1D development were selected for sequential expression of four major IAbs (IAA, GADA, IA-2A and ZnT8icA). Among them, 10 children were ZnT8ecA positive. Remarkably, ZnT8ecA was the earliest IAb to appear in all 10 children. The identification of ZnT8ec as a cell surface target of humoral autoimmunity in the earliest phase of IAb responses opens a new avenue of investigation into the role of IAbs in the development of β-cell autoimmunity.

Differential sorting behavior for soluble and transmembrane cargoes at the trans-Golgi network in endocrine cells

Regulated secretion of neuropeptides and peptide hormones by secretory granules (SGs) is central to physiology. Formation of SGs occurs at the trans-Golgi network (TGN) where their soluble cargo aggregates to form a dense core, but the mechanisms controlling the sorting of regulated secretory cargoes (soluble and transmembrane) away from constitutively secreted proteins remain unclear. Optimizing the use of the retention using selective hooks method in (neuro-)endocrine cells, we now quantify TGN budding kinetics of constitutive and regulated secretory cargoes. We further show that, by monitoring two cargoes simultaneously, it becomes possible to visualize sorting to the constitutive and regulated secretory pathways in real time. Further analysis of the localization of SG cargoes immediately after budding from the TGN revealed that, surprisingly, the bulk of two studied transmembrane SG cargoes (phogrin and VMAT2) does not sort directly onto SGs during budding, but rather exit the TGN into nonregulated vesicles to get incorporated to SGs at a later step. This differential behavior of soluble and transmembrane cargoes suggests a more complex model of SG biogenesis than anticipated.

EIPR1 controls dense-core vesicle cargo retention and EARP complex localization in insulin-secreting cells

Dense-core vesicles (DCVs) are secretory vesicles found in neurons and endocrine cells. DCVs package and release cargoes including neuropeptides, biogenic amines, and peptide hormones. We recently identified the endosome-associated recycling protein (EARP) complex and the EARP-interacting-protein EIPR-1 as proteins important for controlling levels of DCV cargoes in Caenorhabditis elegans neurons. Here we determine the role of mammalian EIPR1 in insulinoma cells. We find that in Eipr1 KO cells, there is reduced insulin secretion, and mature DCV cargoes such as insulin and carboxypeptidase E (CPE) accumulate near the trans-Golgi network and are not retained in mature DCVs in the cell periphery. In addition, we find that EIPR1 is required for the stability of the EARP complex subunits and for the localization of EARP and its association with membranes, but EIPR1 does not affect localization or function of the related Golgi-associated retrograde protein (GARP) complex. EARP is localized to two distinct compartments related to its function: an endosomal compartment and a DCV biogenesis-related compartment. We propose that EIPR1 functions with EARP to control both endocytic recycling and DCV maturation.

Genetic dissection of neuropeptide cell biology at high and low activity in a defined sensory neuron

Neuropeptides are ubiquitous modulators of behavior and physiology. They are packaged in specialized secretory organelles called dense core vesicles (DCVs) that are released upon neural stimulation. Whereas local recycling of synaptic vesicles has been investigated intensively, there are few studies on recycling of DCV proteins. We set up a paradigm to study DCVs in a neuron whose activity we can control. We validate our model by confirming many previous observations on DCV cell biology. We identify a set of genes involved in recycling of DCV proteins. We also find evidence that different mechanisms of DCV priming and exocytosis may operate at high and low neural activity.

Neuropeptides are ubiquitous modulators of behavior and physiology. They are packaged in specialized secretory organelles called dense core vesicles (DCVs) that are released upon neural stimulation. Unlike synaptic vesicles, which can be recycled and refilled close to release sites, DCVs must be replenished by de novo synthesis in the cell body. Here, we dissect DCV cell biology in vivo in a Caenorhabditis elegans sensory neuron whose tonic activity we can control using a natural stimulus. We express fluorescently tagged neuropeptides in the neuron and define parameters that describe their subcellular distribution. We measure these parameters at high and low neural activity in 187 mutants defective in proteins implicated in membrane traffic, neuroendocrine secretion, and neuronal or synaptic activity. Using unsupervised hierarchical clustering methods, we analyze these data and identify 62 groups of genes with similar mutant phenotypes. We explore the function of a subset of these groups. We recapitulate many previous findings, validating our paradigm. We uncover a large battery of proteins involved in recycling DCV membrane proteins, something hitherto poorly explored. We show that the unfolded protein response promotes DCV production, which may contribute to intertissue communication of stress. We also find evidence that different mechanisms of priming and exocytosis may operate at high and low neural activity. Our work provides a defined framework to study DCV biology at different neural activity levels.

Newly produced synaptic vesicle proteins are preferentially used in synaptic transmission

Aged proteins can become hazardous to cellular function, by accumulating molecular damage. This implies that cells should preferentially rely on newly produced ones. We tested this hypothesis in cultured hippocampal neurons, focusing on synaptic transmission. We found that newly synthesized vesicle proteins were incorporated in the actively recycling pool of vesicles responsible for all neurotransmitter release during physiological activity. We observed this for the calcium sensor Synaptotagmin 1, for the neurotransmitter transporter VGAT, and for the fusion protein VAMP2 (Synaptobrevin 2). Metabolic labeling of proteins and visualization by secondary ion mass spectrometry enabled us to query the entire protein makeup of the actively recycling vesicles, which we found to be younger than that of non‐recycling vesicles. The young vesicle proteins remained in use for up to ~ 24 h, during which they participated in recycling a few hundred times. They were afterward reluctant to release and were degraded after an additional ~ 24–48 h. We suggest that the recycling pool of synaptic vesicles relies on newly synthesized proteins, while the inactive reserve pool contains older proteins.

Control of insulin granule formation and function by the ABC transporters ABCG1 and ABCA1 and by oxysterol binding protein OSBP

In pancreatic β-cells, insulin granule membranes are enriched in cholesterol and are both recycled and newly generated. Cholesterol’s role in supporting granule membrane formation and function is poorly understood. ATP binding cassette transporters ABCG1 and ABCA1 regulate intracellular cholesterol and are important for insulin secretion. RNAi inter­ference–induced depletion in cultured pancreatic β-cells shows that ABCG1 is needed to stabilize newly made insulin granules against lysosomal degradation; ABCA1 is also involved but to a lesser extent. Both transporters are also required for optimum glucose-stimulated insulin secretion, likely via complementary roles. Exogenous cholesterol addition rescues knockdown-induced granule loss (ABCG1) and reduced secretion (both transporters). Another cholesterol transport protein, oxysterol binding protein (OSBP), appears to act proximally as a source of endogenous cholesterol for granule formation. Its knockdown caused similar defective stability of young granules and glucose-stimulated insulin secretion, neither of which were rescued with exogenous cholesterol. Dual knockdowns of OSBP and ABC transporters support their serial function in supplying and concentrating cholesterol for granule formation. OSBP knockdown also decreased proinsulin synthesis consistent with a proximal endoplasmic reticulum defect. Thus, membrane cholesterol distribution contributes to insulin homeostasis at production, packaging, and export levels through the actions of OSBP and ABCs G1 and A1.

BAIAP3, a C2 domain-containing Munc13 protein, controls the fate of dense-core vesicles in neuroendocrine cells

Dense-core vesicle (DCV) exocytosis is a SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor)-dependent anterograde trafficking pathway that requires multiple proteins for regulation. Several C2 domain-containing proteins are known to regulate Ca2+-dependent DCV exocytosis in neuroendocrine cells. In this study, we identified others by screening all (∼139) human C2 domain-containing proteins by RNA interference in neuroendocrine cells. 40 genes were identified, including several encoding proteins with known roles (CAPS [calcium-dependent activator protein for secretion 1], Munc13-2, RIM1, and SYT10) and many with unknown roles. One of the latter, BAIAP3, is a secretory cell-specific Munc13-4 paralog of unknown function. BAIAP3 knockdown caused accumulation of fusion-incompetent DCVs in BON neuroendocrine cells and lysosomal degradation (crinophagy) of insulin-containing DCVs in INS-1 β cells. BAIAP3 localized to endosomes was required for Golgi trans-Golgi network 46 (TGN46) recycling, exhibited Ca2+-stimulated interactions with TGN SNAREs, and underwent Ca2+-stimulated TGN recruitment. Thus, unlike other Munc13 proteins, BAIAP3 functions indirectly in DCV exocytosis by affecting DCV maturation through its role in DCV protein recycling. Ca2+ rises that stimulate DCV exocytosis may stimulate BAIAP3-dependent retrograde trafficking to maintain DCV protein homeostasis and DCV function.

Coupling of Insulin Secretion and Display of a Granule-resident Zinc Transporter ZnT8 on the Surface of Pancreatic Beta Cells

The islet-specific zinc transporter ZnT8 mediates zinc enrichment in the insulin secretory granules of the pancreatic beta cell. This granular zinc transporter is also a major self-antigen found in type 1 diabetes patients. It is not clear whether ZnT8 can be displayed on the cell surface and how insulin secretion may regulate the level of ZnT8 exposure to extracellular immune surveillance. Here we report specific antibody binding to the extracellular surface of rat insulinoma INS-1E cells that stably expressed a tagged human zinc transporter ZnT8. Flow cytometry analysis after fluorescent antibody labeling revealed strong correlations among the levels of ZnT8 expression, its display on the cell surface, and glucose-stimulated insulin secretion (GSIS). Glucose stimulation increased the surface display of endogenous ZnT8 from a basal level to 32.5% of the housekeeping Na⁺/K⁺ ATPase on the cell surface, thereby providing direct evidence for a GSIS-dependent surface exposure of the ZnT8 self-antigen. Moreover, the variation in tagged-ZnT8 expression and surface labeling enabled sorting of heterogeneous beta cells to subpopulations that exhibited marked differences in GSIS with parallel changes in endogenous ZnT8 expression. The abundant surface display of endogenous ZnT8 and its coupling to GSIS demonstrated the potential of ZnT8 as a surface biomarker for tracking and isolating functional beta cells in mixed cell populations.

GTP- and GDP-Dependent Rab27a Effectors in Pancreatic Beta-Cells

Small guanosine triphosphatases (GTPases) participate in a wide variety of cellular functions including proliferation, differentiation, adhesion, and intracellular transport. Conventionally, only the guanosine 5'-triphosphate (GTP)-bound small GTPase interacts with effector proteins, and the resulting downstream signals control specific cellular functions. Therefore, the GTP-bound form is regarded as active, and the focus has been on searching for proteins that bind the GTP form to look for their effectors. The Rab family small GTPase Rab27a is highly expressed in some secretory cells and is involved in the control of membrane traffic. The present study reviews recent progress in our understanding of the roles of Rab27a and its effectors in pancreatic beta-cells. In the basal state, GTP-bound Rab27a controls insulin secretion at pre-exocytic stages via its GTP-dependent effectors. We previously identified novel guanosine 5'-diphosphate (GDP)-bound Rab27-interacting proteins. Interestingly, GDP-bound Rab27a controls endocytosis of the secretory membrane via its interaction with these proteins. We also demonstrated that the insulin secretagogue glucose converts Rab27a from its GTP- to GDP-bound forms. Thus, GTP- and GDP-bound Rab27a regulate pre-exocytic and endocytic stages in membrane traffic, respectively. Since the physiological importance of GDP-bound GTPases has been largely overlooked, we consider that the investigation of GDP-dependent effectors for other GTPases is necessary for further understanding of cellular function.

PI3K regulates endocytosis after insulin secretion by mediating signaling crosstalk between Arf6 and Rab27a

In secretory cells, endocytosis is coupled to exocytosis to enable proper secretion. Although endocytosis is crucial to maintain cellular homeostasis before and after secretion, knowledge about secretagogue-induced endocytosis in secretory cells is still limited. Here, we searched for proteins that interacted with the Rab27a GTPase-activating protein (GAP) EPI64 (also known as TBC1D10A) and identified the Arf6 guanine-nucleotide-exchange factor (GEF) ARNO (also known as CYTH2) in pancreatic β-cells. We found that the insulin secretagogue glucose promotes phosphatidylinositol (3,4,5)-trisphosphate (PIP3) generation through phosphoinositide 3-kinase (PI3K), thereby recruiting ARNO to the intracellular side of the plasma membrane. Peripheral ARNO promotes clathrin assembly through its GEF activity for Arf6 and regulates the early stage of endocytosis. We also found that peripheral ARNO recruits EPI64 to the same area and that the interaction requires glucose-induced endocytosis in pancreatic β-cells. Given that GTP- and GDP-bound Rab27a regulate exocytosis and the late stage of endocytosis, our results indicate that the glucose-induced activation of PI3K plays a pivotal role in exocytosis-endocytosis coupling, and that ARNO and EPI64 regulate endocytosis at distinct stages.

PTPRN2 and PLCβ1 promote metastatic breast cancer cell migration through PI(4,5)P2-dependent actin remodeling

Altered abundance of phosphatidyl inositides (PIs) is a feature of cancer. Various PIs mark the identity of diverse membranes in normal and malignant cells. Phosphatidylinositol 4,5‐bisphosphate (PI(4,5)P₂) resides predominantly in the plasma membrane, where it regulates cellular processes by recruiting, activating, or inhibiting proteins at the plasma membrane. We find that PTPRN2 and PLCβ1 enzymatically reduce plasma membrane PI(4,5)P₂ levels in metastatic breast cancer cells through two independent mechanisms. These genes are upregulated in highly metastatic breast cancer cells, and their increased expression associates with human metastatic relapse. Reduction in plasma membrane PI(4,5)P₂ abundance by these enzymes releases the PI(4,5)P₂‐binding protein cofilin from its inactive membrane‐associated state into the cytoplasm where it mediates actin turnover dynamics, thereby enhancing cellular migration and metastatic capacity. Our findings reveal an enzymatic network that regulates metastatic cell migration through lipid‐dependent sequestration of an actin‐remodeling factor.

Day of the dead: pseudokinases and pseudophosphatases in physiology and disease

Pseudophosphatases and pseudokinases are increasingly viewed as integral elements of signaling pathways, and there is mounting evidence that they have frequently retained the ability to interact with cellular 'substrates', and can exert important roles in different diseases. However, these pseudoenzymes have traditionally received scant attention compared to classical kinases and phosphatases. In this review we explore new findings in the emerging pseudokinase and pseudophosphatase fields, and discuss their different modes of action which include exciting new roles as scaffolds, anchors, spatial modulators, traps, and ligand-driven regulators of canonical kinases and phosphatases. Thus, it is now apparent that pseudokinases and pseudophosphatases both support and drive a panoply of signaling networks. Finally, we highlight recent evidence on their involvement in human pathologies, marking them as potential novel drug targets.

Activated Cdc42-bound IQGAP1 determines the cellular endocytic site

Recruitment of specific molecules to a specific membrane site is essential for communication between specialized membranous organelles. In the present study, we identified IQGAP1 as a novel GDP-bound-Rab27a-interacting protein. We found that IQGAP1 interacts with GDP-bound Rab27a when it forms a complex with GTP-bound Cdc42. We also found that IQGAP1 regulates the endocytosis of insulin secretory membranes. Silencing of IQGAP1 inhibits both endocytosis and the glucose-induced redistribution of endocytic machinery, including Rab27a and its binding protein coronin 3. These processes can also be inhibited by disruption of the trimeric complex with dominant negative IQGAP1 and Cdc42. These results indicate that activation of Cdc42 in response to the insulin secretagogue glucose recruits endocytic machinery to IQGAP1 at the cell periphery and regulates endocytosis at this membrane site.

Islet autoantigens: structure, function, localization, and regulation

Islet autoantigens associated with autoimmune type 1 diabetes (T1D) are expressed in pancreatic β cells, although many show wider patterns of expression in the neuroendocrine system. Within pancreatic β cells, every T1D autoantigen is in one way or another linked to the secretory pathway. Together, these autoantigens play diverse roles in glucose regulation, metabolism of biogenic amines, as well as the regulation, formation, and packaging of secretory granules. The mechanism(s) by which immune tolerance to islet-cell antigens is lost during the development of T1D, remains unclear. Antigenic peptide creation for immune presentation may potentially link to the secretory biology of β cells in a number of ways, including proteasomal digestion of misfolded products, exocytosis and endocytosis of cell-surface products, or antigen release from dying β cells during normal or pathological turnover. In this context, we evaluate the biochemical nature and immunogenicity of the major autoantigens in T1D including (pro)insulin, GAD65, ZnT8, IA2, and ICA69.

Comprehensive description of the N-glycoproteome of mouse pancreatic β-cells and human islets

Cell surface N-glycoproteins provide a key interface of cells to their environment and therapeutic entry points for drug and biomarker discovery. Their comprehensive description denotes therefore a formidable challenge. The β-cells of the pancreas play a crucial role in blood glucose homeostasis, and disruption of their function contributes to diabetes. By combining cell surface and whole cell capturing technologies with high-throughput quantitative proteomic analysis, we report on the identification of a total of 956 unique N-glycoproteins from mouse MIN6 β-cells and human islets. Three-hundred-forty-nine of these proteins encompass potential surface N-glycoproteins and include orphan G-protein-coupled receptors, novel proteases, receptor protein kinases, and phosphatases. Interestingly, stimulation of MIN6 β-cells with glucose and the hormone GLP1, known stimulators of insulin secretion, causes significant changes in surface N-glycoproteome expression. Taken together, this β-cell N-glycoproteome resource provides a comprehensive view on the composition of β-cell surface proteins and expands the scope of signaling systems potentially involved in mediating responses of β-cells to various forms of (patho)physiologic stress and the extent of dynamic remodeling of surface N-glycoprotein expression associated with metabolic and hormonal stimulation. Moreover, it provides a foundation for the development of diabetes medicines that target or are derived from the β-cell surface N-glycoproteome.

Activation of steroid-sensitive TRPM3 channels potentiates glutamatergic transmission at cerebellar Purkinje neurons from developing rats

The functional implications of transient receptor potential melastatin 3 (TRPM3) activation, the most recently described member of the melastatin subfamily of cation permeable TRP channels, have begun to be elucidated in recent years. The discovery of TRPM3 activation by the steroid pregnenolone sulfate (PregS) has shed new light on the physiological role of this channel. For example, TRPM3 activation enhances insulin secretion from β pancreatic cells, induces contraction of vascular smooth muscle, and is also involved in the detection of noxious heat. Although TRPM3 expression has been detected in several regions of the developing and mature brain, little is known about the roles of TRPM3 in brain physiology. In this study, we demonstrate the abundant expression of TRPM3 steroid-sensitive channels in the developing cerebellar cortex. We also show that TRPM3-like channels are expressed at glutamatergic synapses in neonatal Purkinje cells. We recently showed that PregS potentiates spontaneous glutamate release onto neonatal Purkinje cells during a period of active glutamatergic synapse formation; we now show that this effect of PregS is mediated by TRPM3-like channels. Mefenamic acid, a recently discovered TRPM3 antagonist, blocked the effect of PregS on glutamate release. The PregS effect on glutamate release was mimicked by other TRPM3 agonists (nifedipine and epipregnanolone sulfate) but not by a TRMP3-inactive steroid (progesterone). Our findings identify TRPM3 channels as novel modulators of glutamatergic transmission in the developing brain.

Rab27a in pancreatic beta-cells, a busy protein in membrane trafficking

The small GTPases have the 'active' GTP-bound and 'inactive' GDP-bound states, and thereby act as a molecular switch in cells. Rab27a is a member of this family and exists in T-lymphocytes, melanocytes and pancreatic beta-cells. Rab27a regulates secretion of cytolytic granules from cytotoxic T-lymphocytes and intracellular transport of melanosomes in melanocytes. In pancreatic beta-cells, Rab27a controls pre-exocytotic stages of insulin secretion. A few GTP-dependent Rab27a effectors are known to mediate these cellular functions. We recently found that Rab27a also possesses the GDP-dependent effector coronin 3. Coronin 3 regulates endocytosis in pancreatic beta-cells through its interaction with GDP-Rab27a. These results imply that GTP- and GDP-Rab27a actively regulate distinct stages in the insulin secretory pathway. In this review, we provide an overview of the roles of both GTP- and GDP-Rab27a in pancreatic beta-cells.

Rab27a, actin and beta-cell endocytosis

The output and time-course of insulin release from pancreatic beta-cells are elegantly controlled. The secretory process comprises pre-exocytotic stages, exocytosis and post-exocytotic stages. The small GTPase Rab27a is known to regulate pre-exocytotic stages that determine the size of the readily-releasable pool of insulin granules. GTP-Rab27a and its specific effectors are responsible for this process like other GTPases. Recently, we searched for Rab27a-interacting proteins and identified coronin 3. Unexpectedly, coronin 3 only bound GDP-Rab27a and this interaction regulated post-exocytotic stages via reorganization of the actin cytoskeleton. Since glucose converts Rab27a from the GTP- to GDP-bound form, we suggested that Rab27a plays a crucial role in stimulus-endocytosis coupling in pancreatic beta-cells, and that this is the key molecule for membrane recycling of insulin granules. In this review, we provide an overview of the roles of Rab27a and its GTP- and GDP-dependent effectors in the insulin secretory pathway of pancreatic beta-cells.

Signaling from the secretory granule to the nucleus: Uhmk1 and PAM

Neurons and endocrine cells package peptides in secretory granules (large dense-core vesicles) for storage and stimulated release. Studies of peptidylglycine alpha-amidating monooxygenase (PAM), an essential secretory granule membrane enzyme, revealed a pathway that can relay information from secretory granules to the nucleus, resulting in alterations in gene expression. The cytosolic domain (CD) of PAM, a type 1 membrane enzyme essential for the production of amidated peptides, is basally phosphorylated by U2AF homology motif kinase 1 (Uhmk1) and other Ser/Thr kinases. Proopiomelanocortin processing in AtT-20 corticotrope tumor cells was increased when Uhmk1 expression was reduced. Uhmk1 was concentrated in the nucleus, but cycled rapidly between nucleus and cytosol. Endoproteolytic cleavage of PAM releases a soluble CD fragment that localizes to the nucleus. Localization of PAM-CD to the nucleus was decreased when PAM-CD with phosphomimetic mutations was examined and when active Uhmk1 was simultaneously overexpressed. Membrane-tethering Uhmk1 did not eliminate its ability to exclude PAM-CD from the nucleus, suggesting that cytosolic Uhmk1 could cause this response. Microarray analysis demonstrated the ability of PAM to increase expression of a small subset of genes, including aquaporin 1 (Aqp1) in AtT-20 cells. Aqp1 mRNA levels were higher in wild-type mice than in mice heterozygous for PAM, indicating that a similar relationship occurs in vivo. Expression of PAM-CD also increased Aqp1 levels whereas expression of Uhmk1 diminished Aqp1 expression. The outlines of a pathway that ties secretory granule metabolism to the transcriptome are thus apparent.

Regulation of insulin secretion by phosphatidylinositol-4,5-bisphosphate

The role of PIP(2) in pancreatic beta cell function was examined here using the beta cell line MIN6B1. Blocking PIP(2) with PH-PLC-GFP or PIP5KIgamma RNAi did not impact on glucose-stimulated secretion although susceptibility to apoptosis was increased. Over-expression of PIP5KIgamma improved cell survival and inhibited secretion with accumulation of endocytic vacuoles containing F-actin, PIP(2), transferrin receptor, caveolin 1, Arf6 and the insulin granule membrane protein phogrin but not insulin. Expression of constitutively active Arf6 Q67L also resulted in vacuole formation and inhibition of secretion, which was reversed by PH-PLC-GFP co-expression. PIP(2) co-localized with gelsolin and F-actin, and gelsolin co-expression partially reversed the secretory defect of PIP5KIgamma-over-expressing cells. RhoA/ROCK inhibition increased actin depolymerization and secretion, which was prevented by over-expressing PIP5KIgamma, while blocking PIP(2) reduced constitutively active RhoA V14-induced F-actin polymerization. In conclusion, although PIP(2) plays a pro-survival role in MIN6B1 cells, excessive PIP(2) production because of PIP5KIgamma over-expression inhibits secretion because of both a defective Arf6/PIP5KIgamma-dependent endocytic recycling of secretory membrane and secretory membrane components such as phogrin and the RhoA/ROCK/PIP5KIgamma-dependent perturbation of F-actin cytoskeleton remodelling.

Glucose-induced translocation of coronin 3 regulates the retrograde transport of the secretory membrane in the pancreatic beta-cells

GTP-Rab27a is known to regulate insulin exocytosis. We have recently reported that coronin 3, which paradoxically binds GDP-Rab27a, participates in endocytosis of the insulin secretory membrane. Here, we demonstrate that glucose stimulation caused redistribution of coronin 3 in the vicinity of the plasma membrane, which was mimicked by overexpression of the GDP-Rab27a mutant or the Rab27a GAP. Glucose-induced translocation of coronin 3 was inhibited by Rab27a knock-down. The internalized phogrin, an insulin granule associated protein, located near the plasma membrane by the dominant-negative coronin 3, but the protein at the outer surface of the plasma membrane was decreased. These results indicate that glucose recruits coronin 3 near the plasma membrane, and that it regulates the retrograde transport of the secretory membrane in pancreatic beta-cells.

Actin assembly controlled by GDP-Rab27a is essential for endocytosis of the insulin secretory membrane

We have recently reported that GDP-bound Rab27a regulates endocytosis of the insulin secretory membrane via its binding to coronin 3, an actin-binding protein. The aim of this study was to examine the participation of actin assembly in the Rab27a-dependent regulation of endocytosis using a pancreatic beta cell line, MIN6. Coronin 3 promoted F-actin bundling only in the presence of GDP-Rab27a. This effect was independent of coronin-3-binding to the actin-related proteins 2 and 3 (Arp2/3). Uptake of anti-phogrin-lumen antibody into MIN6 was inhibited by anti-coronin-3-C antibody which recognizes the actin-binding site. This inhibition was also observed with coronin-3-R28D, which lacks in actin binding. These results suggest that coronin 3 is a genuine GDP-Rab27a effector, and that controls endocytosis of the secretory membrane via modulating actin assembly in pancreatic beta-cells.

Rab11 and its effector Rip11 participate in regulation of insulin granule exocytosis

Rab GTPases and their effectors play important roles in membrane trafficking between cellular compartments in eukaryotic cells. In the present study, we examined the roles of Rab11B and its effectors in insulin secretion in pancreatic beta-cells. In the mouse insulin-secreting cell line MIN6, Rab11 was co-localized with insulin-containing granules, and over-expression of the GTP- or the GDP-bound form of Rab11B significantly inhibited regulated secretion, indicating involvement of Rab11B in regulated insulin secretion. To determine the downstream signal of Rab11-mediated insulin secretion, we examined the effects of various Rab11-interacting proteins on insulin secretion, and found that Rip11 is involved in cAMP-potentiated insulin secretion but not in glucose-induced insulin secretion. Analyses by immunocytochemistry and subcellular fractionation revealed Rip11 to be co-localized with insulin granules. The inhibitory effect of the Rip11 mutant was not altered in MIN6 cells lacking Epac2, which mediates protein kinase A (PKA)-independent potentiation of insulin secretion, compared with wild-type MIN6 cells. In addition, Rip11 was found to be phosphorylated by PKA in MIN6 cells. The present study shows that both Rab11 and its effector Rip11 participate in insulin granule exocytosis and that Rip11, as a substrate of PKA, regulates the potentiation of exocytosis by cAMP in pancreatic beta-cells.

Gene silencing of phogrin unveils its essential role in glucose-responsive pancreatic beta-cell growth

OBJECTIVE

Phogrin and IA-2, autoantigens in insulin-dependent diabetes, have been shown to be involved in insulin secretion in pancreatic beta-cells; however, implications at a molecular level are confusing from experiment to experiment. We analyzed biological functions of phogrin in beta-cells by an RNA interference technique.

RESEARCH DESIGN AND METHODS

Adenovirus-mediated expression of short hairpin RNA specific for phogrin (shPhogrin) was conducted using cultured beta-cell lines and mouse islets. Both glucose-stimulated insulin secretion and cell proliferation rate were determined in the phogrin-knockdown cells. Furthermore, protein expression was profiled in these cells. To see the binding partner of phogrin in beta-cells, coimmunoprecipitation analysis was carried out.

RESULTS

Adenoviral expression of shPhogrin efficiently decreased its endogenous expression in pancreatic beta-cells. Silencing of phogrin in beta-cells abrogated the glucose-mediated mitogenic effect, which was accompanied by a reduction in the level of insulin receptor substrate 2 (IRS2) protein, without any changes in insulin secretion. Phogrin formed a complex with insulin receptor at the plasma membrane, and their interaction was promoted by high-glucose stimulation that in turn led to stabilization of IRS2 protein. Corroboratively, phogrin knockdown had no additional effect on the proliferation of beta-cell line derived from the insulin receptor-knockout mouse.

CONCLUSIONS

Phogrin is involved in beta-cell growth via regulating stability of IRS2 protein by the molecular interaction with insulin receptor. We propose that phogrin and IA-2 function as an essential regulator of autocrine insulin action in pancreatic beta-cells.

Expression and function of IA-2 family proteins, unique neuroendocrine-specific protein-tyrosine phosphatases

IA-2 (also known as islet cell antigen ICA-512) and IA-2 beta (also known as phogrin, phosphatase homologue in granules of insulinoma) are major autoantigens in insulin-dependent diabetes mellitus (IDDM). Autoantibodies against both proteins are expressed years before clinical onset, and they become predictive markers for high-risk subjects. However, the role of these genes in the IDDM pathogenesis has been reported fairly negative by recent studies. IA-2 and IA-2 beta are type I transmembrane proteins that possess one inactive protein-tyrosine phosphatase (PTP) domain in the cytoplasmic region, and act as one of the constituents of regulated secretory pathways in various neuroendocrine cell types including pancreatic beta-cells. Existence of IA-2 homologues in different species suggests a fundamental role in neuroendocrine function. Studies of knockout animals have shown their involvement in maintaining hormone content, however, their specific steps in the secretory pathway IA-2 functions as well as their molecular mechanisms in the hormone content regulation are still unknown. More recent studies have suggested a novel function showing that they contribute to pancreatic beta-cell growth. This review attempts to show the possible biological functions of IA-2 family, focusing on their expression and localization in the neuroendocrine cells.

The GDP-dependent Rab27a effector coronin 3 controls endocytosis of secretory membrane in insulin-secreting cell lines

Rab27a is involved in the control of membrane traffic, a crucial step in the regulated secretion. Typically, the guanosine triphosphate (GTP)-bound form has been considered to be active and, therefore, searching for proteins binding to the GTP-form has been attempted to look for their effectors. Here, we have identified the actin-bundling protein coronin 3 as a novel Rab27a effector that paradoxically bound guanosine diphosphate (GDP)-Rab27a in the pancreatic beta-cell line MIN6. Coronin 3 directly bound GDP-Rab27a through its beta-propeller structure. The most important insulin secretagogue glucose promptly shifted Rab27a from the GTP- to GDP-bound form. Knockdown of coronin 3 by RNAi resulted in the inhibition of phogrin (an insulin-granule-associated protein) internalization and the uptake of FM4-64 (a marker of endocytosis). Similar results were reproduced by disruption of the coronin-3-GDP-Rab27a interaction with the dominant-negative coronin 3, and coexpression of the GDP-Rab27a mutant rescued these changes. Taken together, our results indicate that interaction of GDP-Rab27a and coronin 3 is important in stimulus-endocytosis coupling, and that GTP- and GDP-Rab27a regulates insulin membrane recycling at the distinct stages.

Role of the vesicular chloride transporter ClC-3 in neuroendocrine tissue

ClC-3 is an intracellular chloride transport protein known to reside on endosomes and synaptic vesicles. The endogenous protein has been notoriously difficult to detect in immunohistological experiments because of the lack of reliable antibodies. Using newly generated antibodies, we now examine its expression pattern at the cellular and subcellular level. In all tissues examined, immunostaining indicated that ClC-3 is a vesicular protein, with a prominent expression in endocrine cells like adrenal chromaffin cells and pancreatic islet cells. In line with a possible function of ClC-3 in regulating vesicle trafficking or exocytosis in those secretory cells, capacitance measurements and amperometry indicated that exocytosis of large dense-core vesicles (LDCVs) was decreased in chromaffin cells from ClC-3 knock-out mice. However, immunohistochemistry complemented with subcellular fractionation showed that ClC-3 is not detectable on LDCVs of endocrine cells, but localizes to endosomes and synaptic-like microvesicles in both adrenal chromaffin and pancreatic beta cells. This observation points to an indirect influence of ClC-3 on LDCV exocytosis in chromaffin cells, possibly by affecting an intracellular trafficking step.

Distinct roles of the C2A and the C2B domain of the vesicular Ca2+ sensor synaptotagmin 9 in endocrine beta-cells

Synaptotagmins form a family of calcium-sensor proteins implicated in exocytosis, and these vesicular transmembrane proteins are endowed with two cytosolic calcium-binding C2 domains, C2A and C2B. Whereas the isoforms syt1 and syt2 have been studied in detail, less is known about syt9, the calcium sensor involved in endocrine secretion such as insulin release from large dense core vesicles in pancreatic beta-cells. Using cell-based assays to closely mimic physiological conditions, we observed SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor)-independent translocation of syt9C2AB to the plasma membrane at calcium levels corresponding to endocrine exocytosis, followed by internalization to endosomes. The use of point mutants and truncations revealed that initial translocation required only the C2A domain, whereas the C2B domain ensured partial pre-binding of syt9C2AB to the membrane and post-stimulatory localization to endosomes. In contrast with the known properties of neuronal and neuroendocrine syt1 or syt2, the C2B domain of syt9 did not undergo calcium-dependent membrane binding despite a high degree of structural homology as observed through molecular modelling. The present study demonstrates distinct intracellular properties of syt9 with different roles for each C2 domain in endocrine cells.

A dominant mutation in Snap25 causes impaired vesicle trafficking, sensorimotor gating, and ataxia in the blind-drunk mouse

The neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex is essential for synaptic vesicle exocytosis, but its study has been limited by the neonatal lethality of murine SNARE knockouts. Here, we describe a viable mouse line carrying a mutation in the b-isoform of neuronal SNARE synaptosomal-associated protein of 25 kDa (SNAP-25). The causative I67T missense mutation results in increased binding affinities within the SNARE complex, impaired exocytotic vesicle recycling and granule exocytosis in pancreatic beta-cells, and a reduction in the amplitude of evoked cortical excitatory postsynaptic potentials. The mice also display ataxia and impaired sensorimotor gating, a phenotype which has been associated with psychiatric disorders in humans. These studies therefore provide insights into the role of the SNARE complex in both diabetes and psychiatric disease.

Insulin vesicle release: walk, kiss, pause ... then run

The mechanisms by which insulin-containing dense core secretory vesicles approach and finally fuse with the plasma membrane are of considerable current interest: defects in these processes may be one of the contributing factors to Type 2 diabetes. In this review, we discuss the molecular mechanisms involved in vesicle trafficking within the pancreatic beta-cell and the mechanisms whereby these may be regulated. We then go on to describe recent evidence that suggests that vesicle fusion at the plasma membrane is a partly reversible process ("kiss and run" or "cavity recapture"). We propose that vesicles may participate in a exo-endocytotic cycle in which a proportion of those that have already undergone an interaction with the plasma membrane may exchange exocytotic machinery with maturing vesicles.

Imaging of dynamic secretory vesicles in living pollen tubes of Picea meyeri using evanescent wave microscopy

Evanescent wave excitation was used to visualize individual, FM4-64-labeled secretory vesicles in an optical slice proximal to the plasma membrane of Picea meyeri pollen tubes. A standard upright microscope was modified to accommodate the optics used to direct a laser beam at a variable angle. Under evanescent wave microscopy or total internal reflection fluorescence microscopy, fluorophores localized near the surface were excited with evanescent waves, which decay exponentially with distance from the interface. Evanescent waves with penetration depths of 60 to 400 nm were generated by varying the angle of incidence of the laser beam. Kinetic analysis of vesicle trafficking was made through an approximately 300-nm optical section beneath the plasma membrane using time-lapse evanescent wave imaging of individual fluorescently labeled vesicles. Two-dimensional trajectories of individual vesicles were obtained from the resulting time-resolved image stacks and were used to characterize the vesicles in terms of their average fluorescence and mobility, expressed here as the two-dimensional diffusion coefficient D2. The velocity and direction of vesicle motions, frame-to-frame displacement, and vesicle trajectories were also calculated. Analysis of individual vesicles revealed for the first time, to our knowledge, that two types of motion are present, and that vesicles in living pollen tubes exhibit complicated behaviors and oscillations that differ from the simple Brownian motion reported in previous investigations. Furthermore, disruption of the actin cytoskeleton had a much more pronounced effect on vesicle mobility than did disruption of the microtubules, suggesting that actin cytoskeleton plays a primary role in vesicle mobility.

Insulin-Regulated Aminopeptidase Marks an Antigen-Stimulated Recycling Compartment in Mast Cells

Insulin-regulated aminopeptidase (IRAP) is a marker for insulin-sensitive recycling compartments of fat and muscle cells that contain the glucose transporter isoform GLUT4. Unlike GLUT4, IRAP is expressed in many other cell types. Thus, it is a potential marker for regulated recycling compartments that are analogous to GLUT4 vesicles. In bone marrow-derived mast cells, IRAP is highly expressed and localizes to an intracellular compartment different from secretory granules. Using cell-surface biotinylation, we determined that IRAP underwent rapid redistribution to the plasma membrane on antigen/immunoglobulin E (IgE) stimulation and was re-internalized within 30 min. When granule exocytosis was inhibited, by removing extracellular calcium, adding the protein kinase C inhibitor bisindolylmaleimide or the phosphatidylinositol 3-kinase inhibitor wortmannin, IRAP redistribution was still detected in stimulated cells. However, the redistribution of IRAP required intracellular calcium. By immunofluorescence, IRAP significantly co-localized with the transferrin receptor (TfR), a marker for constitutively recycling endosomes. However, antigen/IgE stimulation did not increase TfR on the cell surface, indicating that IRAP and TfR may follow different pathways to the plasma membrane. In rat peritoneal mast cells, the distributions of IRAP and TfR overlapped to only a limited extent, indicating that overlap may decrease with cell differentiation. We propose that IRAP vesicles represent a second IgE-sensitive exocytotic compartment in mast cells, which is regulated differently from secretory granules, and that these vesicles may be similar to GLUT4 vesicles.

Dense core vesicle proteins IA-2 and IA-2beta: metabolic alterations in double knockout mice

IA-2 and IA-2beta are members of the transmembrane protein tyrosine phosphatase family located in dense core vesicles of neuroendocrine cells, including the beta-cells of pancreatic islets. In the present study, by mating C57BL/6Nci IA-2(+/-) with IA-2beta(+/-) mice, we generated double knockout mice (IA-2(-/-)/IA-2beta(-/-)) to study the effect of the combined deletion of these two proteins on insulin secretion and blood glucose levels. The double knockout mice appeared healthy at birth and showed normal growth and development. Histological examination and immunostaining for insulin, glucagon, somatostatin, and pancreatic polypeptide revealed no difference between the double knockout and wild-type mice. Nonfasting blood glucose and insulin levels also were within the normal range. However, compared with the wild-type mice, the double knockout mice showed glucose intolerance and an absent first-phase insulin release curve. No evidence of insulin resistance was observed nor were there alterations in fasting blood glucose, insulin, or leptin levels in the double knockout mice maintained on a high-fat diet compared with the wild-type mice maintained on the same diet. In addition, to determine whether the combined deletion of IA-2 and IA-2beta played any role in the development of diabetes in NOD mice, we generated double knockout mice on the NOD/LtJ background. The incidence of diabetes in these mice was not significantly different than that in the wild-type mice. Taken together, our experiments show that the dense core vesicle proteins IA-2 and IA-2beta, alone or in combination, are involved in insulin secretion, but neither alone nor in combination are they required for the development of diabetes in NOD mice.

An extended tyrosine-targeting motif for endocytosis and recycling of the dense-core vesicle membrane protein phogrin

Integral membrane proteins of neuroendocine dense-core vesicles (DCV) appear to undergo multiple rounds of exocytosis; however, their trafficking and site of incorporation into nascent DCVs is unclear. Previous studies with phogrin (IA-2beta) identified sorting signals in the luminal domain that is cleaved post-translationally; we now describe an independent DCV targeting motif in the cytosolic domain that may function at the level of endocytosis and recycling. Pulse-chase radiolabeling and cell surface biotinylation experiments in the pituitary corticotroph cell line AtT20 showed that the mature 60/65 kDa form that resides in the DCV is generated by limited proteolysis in a post-trans Golgi network compartment with similar kinetics to the formation of the principal cargo, ACTH. Phogrin is exposed on the cell surface in response to stimuli and progressively internalized to a perinuclear compartment that overlaps with recycling endosomes marked by transferrin. Chimeric molecules of phogrin transmembrane and cytosolic sequences with the interleukin-2 receptor alpha chain (Tac) were sorted to DCVs through the action of an extended tyrosine-based motif Y(654)QELCRQRMA located in a 27aa sequence adjacent to the membrane-spanning domain. A 36aa domain terminating in this sequence conferred DCV localization to Tac in the absence of any other cytosolic or luminal phogrin components. The endocytosis and DCV targeting of phogrin Y(654) > A mutants correlated with the impaired binding of the phogrin cytosolic tail to the micro-subunit of the AP2 adaptor complex in vitro.

Selective nucleotide-release from dense-core granules in insulin-secreting cells

Secretory granules of insulin-secreting cells are used to store and release peptide hormones as well as low-molecular-weight compounds such as nucleotides. Here we have compared the rate of exocytosis with the time courses of nucleotide and peptide release by a combination of capacitance measurements, electrophysiological detection of ATP release and single-granule imaging. We demonstrate that the release of nucleotides and peptides is delayed by approximately 0.1 and approximately 2 seconds with respect to membrane fusion, respectively. We further show that in up to 70% of the cases exocytosis does not result in significant release of the peptide cargo, likely because of a mechanism that leads to premature closure of the fusion pore. Release of nucleotides and protons occurred regardless of whether peptides were secreted or not. These observations suggest that insulin-secreting cells are able to use the same secretory vesicles to release small molecules either alone or together with the peptide hormone.

Retrieval and reuse of pituitary secretory granule proteins

The pituitary contains professional secretory cells, devoting a large fraction of their energy to the synthesis of hormones that are stored for secretion in response to a complex mixture of inputs. Ba2+, a substitute for Ca2+, and phorbol ester, a mimic for diacylglycerol, have a synergistic effect on exocytosis. By using these secretagogues, we developed a paradigm in which phorbol ester potentiation of Ba2+-evoked exocytosis produces a robust secretory response in multiple pituitary cell types. Because cells subjected to this stimulatory paradigm remain healthy despite their greatly reduced hormone content, we used this paradigm to study the fate of granule membrane proteins. We examined the turnover of peptidylglycine alpha-amidating monooxygenase (PAM), a membrane enzyme involved in the final maturation of many peptides, and VAMP2, a vesicle soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE). The stability of recently synthesized PAM was increased by sustained exocytosis. Biotinylation studies established that the appearance of integral membrane PAM at the plasma membrane was stimulated along with hormone secretion. PAM biotinylated on the cell surface undergoes cleavage to yield soluble peptidylglycine-alpha-hydroxylating monooxygenase that can then be secreted in a regulated fashion. Consistent with a kiss-and-run or cavicapture mode of secretion (Taraska, J. W., Perrais, D., Ohara-Imaizumi, M., Nagamatsu, S., and Almers, W. (2003) Proc. Natl. Acad. Sci. U. S. A. 100, 2070-2075), biotinylated prolactin was also retained by the cells and later released in response to secretagogues. Thus, pituitary cells can retrieve and reuse components of the machinery involved in the final stages of exocytosis (the SNAREs) as well as soluble and membrane granule proteins.