Involvement of conventional kinesin in glucose-stimulated secretory granule movements and exocytosis in clonal pancreatic beta-cells

Microtubule-associated protein MAP7 promotes tubulin posttranslational modifications and cargo transport to enable osmotic adaptation

Cells remodel their cytoskeletal networks to adapt to their environment. Here, we analyze the mechanisms utilized by the cell to tailor its microtubule landscape in response to changes in osmolarity that alter macromolecular crowding. By integrating live-cell imaging, ex vivo enzymatic assays, and in vitro reconstitution, we probe the impact of cytoplasmic density on microtubule-associated proteins (MAPs) and tubulin posttranslational modifications (PTMs). We find that human epithelial cells respond to fluctuations in cytoplasmic density by modulating microtubule acetylation, detyrosination, or MAP7 association without differentially affecting polyglutamylation, tyrosination, or MAP4 association. These MAP-PTM combinations alter intracellular cargo transport, enabling the cell to respond to osmotic challenges. We further dissect the molecular mechanisms governing tubulin PTM specification and find that MAP7 promotes acetylation and inhibits detyrosination. Our data identify MAP7 in modulating the tubulin code, resulting in microtubule cytoskeleton remodeling and alteration of intracellular transport as an integrated mechanism of cellular adaptation.

Motor properties of Myosin 5c are modulated by tropomyosin isoforms and inhibited by pentabromopseudilin

Myosin 5c (Myo5c) is a motor protein that is produced in epithelial and glandular tissues, where it plays an important role in secretory processes. Myo5c is composed of two heavy chains, each containing a generic motor domain, an elongated neck domain consisting of a single α-helix with six IQ motifs, each of which binds to a calmodulin (CaM) or a myosin light chain from the EF-hand protein family, a coiled-coil dimer-forming region and a carboxyl-terminal globular tail domain. Although Myo5c is a low duty cycle motor, when two or more Myo5c-heavy meromyosin (HMM) molecules are linked together, they move processively along actin filaments. We describe the purification and functional characterization of human Myo5c-HMM co-produced either with CaM alone or with CaM and the essential and regulatory light chains Myl6 and Myl12b. We describe the extent to which cofilaments of actin and Tpm1.6, Tpm1.8 or Tpm3.1 alter the maximum actin-activated ATPase and motile activity of the recombinant Myo5c constructs. The small allosteric effector pentabromopseudilin (PBP), which is predicted to bind in a groove close to the actin and nucleotide binding site with a calculated ΔG of -18.44 kcal/mol, inhibits the motor function of Myo5c with a half-maximal concentration of 280 nM. Using immunohistochemical staining, we determined the distribution and exact localization of Myo5c in endothelial and endocrine cells from rat and human tissue. Particular high levels of Myo5c were observed in insulin-producing β-cells located within the pancreatic islets of Langerhans.

Glucose-stimulated KIF5B-driven microtubule sliding organizes microtubule networks in pancreatic beta cells

In pancreatic islet beta cells, molecular motors use cytoskeletal polymers microtubules as tracks for intracellular transport of insulin secretory granules. Beta-cell microtubule network has a complex architecture and is non-directional, which provide insulin granules at the cell periphery for rapid secretion response, yet to avoid over-secretion and subsequent hypoglycemia. We have previously characterized a peripheral sub-membrane microtubule array, which is critical for withdrawal of excessive insulin granules from the secretion sites. Microtubules in beta cells originate at the Golgi in the cell interior, and how the peripheral array is formed is unknown. Using real-time imaging and photo-kinetics approaches in clonal mouse pancreatic beta cells MIN6, we now demonstrate that kinesin KIF5B, a motor protein with a capacity to transport microtubules as cargos, slides existing microtubules to the cell periphery and aligns them to each other along the plasma membrane. Moreover, like many physiological beta-cell features, microtubule sliding is facilitated by a high glucose stimulus. These new data, together with our previous report that in high glucose sub-membrane MT array is destabilized to allow for robust secretion, indicate that MT sliding is another integral part of glucose-triggered microtubule remodeling, likely replacing destabilized peripheral microtubules to prevent their loss over time and beta-cell malfunction.

CAMSAP2 localizes to the Golgi in islet β-cells and facilitates Golgi-ER trafficking

Glucose stimulation induces the remodeling of microtubules, which potentiates insulin secretion in pancreatic β-cells. CAMSAP2 binds to microtubule minus ends to stabilize microtubules in several cultured clonal cells. Here, we report that the knockdown of CAMSAP2 in primary β-cells reduces total insulin content and attenuates GSIS without affecting the releasability of insulin vesicles. Surprisingly, CAMSAP2 knockdown does not change microtubule stability. Unlike in cultured insulinoma cells, CAMSAP2 in primary β-cells predominantly localizes to the Golgi apparatus instead of microtubule minus ends. This novel localization is specific to primary β- but not α-cells and is independent of microtubule binding. Consistent with its specific localization at the Golgi, CAMSAP2 promotes efficient Golgi-ER trafficking in primary β-cells. Moreover, primary β-cells and insulinoma cells likely express different CAMSAP2 isoforms. We propose that a novel CAMSAP2 isoform in primary β-cells has a non-canonical function, which promotes Golgi-ER trafficking to support efficient production of insulin and secretion.

Microtubules in Pancreatic β Cells: Convoluted Roadways Toward Precision

Pancreatic islet β cells regulate glucose homeostasis via glucose-stimulated insulin secretion (GSIS). Cytoskeletal polymers microtubules (MTs) serve as tracks for the transport and positioning of secretory insulin granules. MT network in β cells has unique morphology with several distinct features, which support granule biogenesis (via Golgi-derived MT array), net non-directional transport (via interlocked MT mesh), and control availability of granules at secretion sites (via submembrane MT bundle). The submembrane MT array, which is parallel to the plasma membrane and serves to withdraw excessive granules from the secretion hot spots, is destabilized and fragmented downstream of high glucose stimulation, allowing for regulated secretion. The origin of such an unusual MT network, the features that define its functionality, and metabolic pathways that regulate it are still to a large extent elusive and are a matter of active investigation and debate. Besides the MT network itself, it is important to consider the interplay of molecular motors that drive and fine-tune insulin granule transport. Importantly, activity of kinesin-1, which is the major MT-dependent motor in β cells, transports insulin granules, and has a capacity to remodel MT network, is also regulated by glucose. We discuss yet unknown potential avenues toward understanding how MT network and motor proteins provide control for secretion in coordination with other GSIS-regulating mechanisms.

Carnosine protects stimulus-secretion coupling through prevention of protein carbonyl adduction events in cells under metabolic stress

Type 2 diabetes is characterised by failure to control glucose homeostasis, with numerous diabetic complications attributable to the resulting exposure of cells and tissues to chronic elevated concentrations of glucose and fatty acids. This, in part, results from formation of advanced glycation and advanced lipidation end-products that are able to modify protein, lipid, or DNA structure, and disrupt normal cellular function. Herein we used mass spectrometry to identify proteins modified by two such adduction events in serum of individuals with obesity, type 2 diabetes, and gestational diabetes, along with similar analyses of human and mouse skeletal muscle cells and mouse pancreatic islets exposed to glucolipotoxic stress. We also report that carnosine, a histidine containing dipeptide, prevented 65-90% of 4-hydroxynonenal and 3-nitrotyrosine adduction events, and that this in turn preserved mitochondrial function and protected stimulus-secretion coupling in cells exposed to metabolic stress. Carnosine therefore offers significant therapeutic potential against metabolic diseases.

Microtubules and Gαo-signaling modulate the preferential secretion of young insulin secretory granules in islet β cells via independent pathways

For sustainable function, each pancreatic islet β cell maintains thousands of insulin secretory granules (SGs) at all times. Glucose stimulation induces the secretion of a small portion of these SGs and simultaneously boosts SG biosynthesis to sustain this stock. The failure of these processes, often induced by sustained high-insulin output, results in type 2 diabetes. Intriguingly, young insulin SGs are more likely secreted during glucose-stimulated insulin secretion (GSIS) for unknown reasons, while older SGs tend to lose releasability and be degraded. Here, we examine the roles of microtubule (MT) and Gαo-signaling in regulating the preferential secretion of young versus old SGs. We show that both MT-destabilization and Gαo inactivation results in more SGs localization near plasma membrane (PM) despite higher levels of GSIS and reduced SG biosynthesis. Intriguingly, MT-destabilization or Gαo-inactivation results in higher secretion probabilities of older SGs, while combining both having additive effects on boosting GSIS. Lastly, Gαo inactivation does not detectably destabilize the β-cell MT network. These findings suggest that Gαo and MT can modulate the preferential release of younger insulin SGs via largely parallel pathways.

Deficiency of Klc2 Induces Low-Frequency Sensorineural Hearing Loss in C57BL/6 J Mice and Human

The transport system in cochlear hair cells (HCs) is important for their function, and the kinesin family of proteins transports numerous cellular cargos via the microtubule network in the cytoplasm. Here, we found that Klc2 (kinesin light chain 2), the light chain of kinesin-1 that mediates cargo binding and regulates kinesin-1 motility, is essential for cochlear function. We generated mice lacking Klc2, and they suffered from low-frequency hearing loss as early as 1 month of age. We demonstrated that deficiency of Klc2 resulted in abnormal transport of mitochondria and the down-regulation of the GABAA receptor family. In addition, whole-genome sequencing (WGS) of patient showed that KLC2 was related to low-frequency hearing in human. Hence, to explore therapeutic approaches, we developed adeno-associated virus containing the Klc2 wide-type cDNA sequence, and Klc2-null mice delivered virus showed apparent recovery, including decreased ABR threshold and reduced out hair cell (OHC) loss. In summary, we show that the kinesin transport system plays an indispensable and special role in cochlear HC function in mice and human and that mitochondrial localization is essential for HC survival.

3D FIB-SEM reconstruction of microtubule-organelle interaction in whole primary mouse β cells

Microtubules play a major role in intracellular trafficking of vesicles in endocrine cells. Detailed knowledge of microtubule organization and their relation to other cell constituents is crucial for understanding cell function. However, their role in insulin transport and secretion is under debate. Here, we use FIB-SEM to image islet β cells in their entirety with unprecedented resolution. We reconstruct mitochondria, Golgi apparati, centrioles, insulin secretory granules, and microtubules of seven β cells, and generate a comprehensive spatial map of microtubule-organelle interactions. We find that microtubules form nonradial networks that are predominantly not connected to either centrioles or endomembranes. Microtubule number and length, but not microtubule polymer density, vary with glucose stimulation. Furthermore, insulin secretory granules are enriched near the plasma membrane, where they associate with microtubules. In summary, we provide the first 3D reconstructions of complete microtubule networks in primary mammalian cells together with evidence regarding their importance for insulin secretory granule positioning and thus their supportive role in insulin secretion.

Visualizing subcellular rearrangements in intact β cells using soft x-ray tomography

Characterizing relationships between cell structures and functions requires mesoscale mapping of intact cells showing subcellular rearrangements following stimulation; however, current approaches are limited in this regard. Here, we report a unique application of soft x-ray tomography to generate three-dimensional reconstructions of whole pancreatic β cells at different time points following glucose-stimulated insulin secretion. Reconstructions following stimulation showed distinct insulin vesicle distribution patterns reflective of altered vesicle pool sizes as they travel through the secretory pathway. Our results show that glucose stimulation caused rapid changes in biochemical composition and/or density of insulin packing, increased mitochondrial volume, and closer proximity of insulin vesicles to mitochondria. Costimulation with exendin-4 (a glucagon-like peptide-1 receptor agonist) prolonged these effects and increased insulin packaging efficiency and vesicle maturation. This study provides unique perspectives on the coordinated structural reorganization and interactions of organelles that dictate cell responses.

Insulin granule biogenesis and exocytosis

Insulin is produced by pancreatic β-cells, and once released to the blood, the hormone stimulates glucose uptake and suppresses glucose production. Defects in both the availability and action of insulin lead to elevated plasma glucose levels and are major hallmarks of type-2 diabetes. Insulin is stored in secretory granules that form at the trans-Golgi network. The granules undergo extensive modifications en route to their release sites at the plasma membrane, including changes in both protein and lipid composition of the granule membrane and lumen. In parallel, the insulin molecules also undergo extensive modifications that render the hormone biologically active. In this review, we summarize current understanding of insulin secretory granule biogenesis, maturation, transport, docking, priming and eventual fusion with the plasma membrane. We discuss how different pools of granules form and how these pools contribute to insulin secretion under different conditions. We also highlight the role of the β-cell in the development of type-2 diabetes and discuss how dysregulation of one or several steps in the insulin granule life cycle may contribute to disease development or progression.

The magnesium transporter NIPAL1 is a pancreatic islet-expressed protein that conditionally impacts insulin secretion

Type 2 diabetes is a chronic metabolic disease characterized by pancreatic β-cell dysfunction and peripheral insulin resistance. Among individuals with type 2 diabetes, ∼30% exhibit hypomagnesemia. Hypomagnesemia has been linked to insulin resistance through reduced tyrosine kinase activity of the insulin receptor; however, its impact on pancreatic β-cell function is unknown. In this study, through analysis of several single-cell RNA-sequencing data sets in tandem with quantitative PCR validation in both murine and human islets, we identified NIPAL1 (NIPA-like domain containing 1), encoding a magnesium influx transporter, as an islet-enriched gene. A series of immunofluorescence experiments confirmed NIPAL1's magnesium-dependent expression and that it specifically localizes to the Golgi in Min6-K8 cells, a pancreatic β-cell-like cell line (mouse insulinoma 6 clone K8). Under varying magnesium concentrations, NIPAL1 knockdown decreased both basal insulin secretion and total insulin content; in contrast, its overexpression increased total insulin content. Although the expression, distribution, and magnesium responsiveness of NIPAL1 in α-TC6 glucagonoma cells (a pancreatic α-cell line) were similar to the observations in Min6-K8 cells, no effect was observed on glucagon secretion in α-TC6 cells under the conditions studied. Overall, these results suggest that NIPAL1 expression is regulated by extracellular magnesium and that down-regulation of this transporter decreases glucose-stimulated insulin secretion and intracellular insulin content, particularly under conditions of hypomagnesemia.

Microtubules Regulate Localization and Availability of Insulin Granules in Pancreatic Beta Cells

Two key prerequisites for glucose-stimulated insulin secretion (GSIS) in β cells are the proximity of insulin granules to the plasma membrane and their anchoring or docking to the plasma membrane (PM). Although recent evidence has indicated that both of these factors are altered in the context of diabetes, it is unclear what regulates localization of insulin granules and their interactions with the PM within single cells. Here, we demonstrate that microtubule (MT)-motor-mediated transport dynamics have a critical role in regulating both factors. Super-resolution imaging shows that whereas the MT cytoskeleton resembles a random meshwork in the cells' interior, MTs near the cell surface are preferentially aligned with the PM. Computational modeling suggests two consequences of this alignment. First, this structured MT network preferentially withdraws granules from the PM. Second, the binding and transport of insulin granules by MT motors prevents their stable anchoring to the PM. These findings suggest the MT cytoskeleton may negatively regulate GSIS by both limiting the amount of insulin proximal to the PM and preventing or breaking interactions between the PM and the remaining nearby insulin granules. These results predict that altering MT network structure in β cells can be used to tune GSIS. Thus, our study points to the potential of an alternative therapeutic strategy for diabetes by targeting specific MT regulators.

Glucolipotoxicity Alters Insulin Secretion via Epigenetic Changes in Human Islets

Type 2 diabetes (T2D) is characterized by insufficient insulin secretion and elevated glucose levels, often in combination with high levels of circulating fatty acids. Long-term exposure to high levels of glucose or fatty acids impair insulin secretion in pancreatic islets, which could partly be due to epigenetic alterations. We studied the effects of high concentrations of glucose and palmitate combined for 48 h (glucolipotoxicity) on the transcriptome, the epigenome, and cell function in human islets. Glucolipotoxicity impaired insulin secretion, increased apoptosis, and significantly (false discovery rate <5%) altered the expression of 1,855 genes, including 35 genes previously implicated in T2D by genome-wide association studies (e.g., TCF7L2 and CDKN2B). Additionally, metabolic pathways were enriched for downregulated genes. Of the differentially expressed genes, 1,469 also exhibited altered DNA methylation (e.g., CDK1, FICD, TPX2, and TYMS). A luciferase assay showed that increased methylation of CDK1 directly reduces its transcription in pancreatic β-cells, supporting the idea that DNA methylation underlies altered expression after glucolipotoxicity. Follow-up experiments in clonal β-cells showed that knockdown of FICD and TPX2 alters insulin secretion. Together, our novel data demonstrate that glucolipotoxicity changes the epigenome in human islets, thereby altering gene expression and possibly exacerbating the secretory defect in T2D.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

PIWI-interacting RNAs as novel regulators of pancreatic beta cell function

AIMS/HYPOTHESIS

P-element induced Wimpy testis (PIWI)-interacting RNAs (piRNAs) are small non-coding RNAs that interact with PIWI proteins and guide them to silence transposable elements. They are abundantly expressed in germline cells and play key roles in spermatogenesis. There is mounting evidence that piRNAs are also present in somatic cells, where they may accomplish additional regulatory tasks. The aim of this study was to identify the piRNAs expressed in pancreatic islets and to determine whether they are involved in the control of beta cell activities.

METHODS

piRNA profiling of rat pancreatic islets was performed by microarray analysis. The functions of piRNAs were investigated by silencing the two main Piwi genes or by modulating the level of selected piRNAs in islet cells.

RESULTS

We detected about 18,000 piRNAs in rat pancreatic islets, many of which were differentially expressed throughout islet postnatal development. Moreover, we identified changes in the level of several piRNAs in the islets of Goto-Kakizaki rats, a well-established animal model of type 2 diabetes. Silencing of Piwil2 or Piwil4 genes in adult rat islets caused a reduction in the level of several piRNAs and resulted in defective insulin secretion and increased resistance of the cells to cytokine-induced cell death. Furthermore, overexpression in the islets of control animals of two piRNAs that are upregulated in diabetic rats led to a selective defect in glucose-induced insulin release.

CONCLUSIONS/INTERPRETATION

Our results provide evidence for a role of PIWI proteins and their associated piRNAs in the control of beta cell functions, and suggest a possible involvement in the development of type 2 diabetes.

DATA AVAILABILITY

Data have been deposited in Gene Expression Omnibus repository under the accession number GSE93792. Data can be accessed via the following link: https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?token=ojklueugdzehpkv&acc=GSE93792.

Adipose-specific deletion of Kif5b exacerbates obesity and insulin resistance in a mouse model of diet-induced obesity

Recent studies have shown that KIF5B (conventional kinesin heavy chain) mediates glucose transporter type 4 translocation and adiponectin secretion in 3T3-L1 adipocytes, suggesting an involvement of KIF5B in the homeostasis of metabolism. However, the in vivo physiologic function of KIF5B in adipose tissue remains to be determined. In this study, adipose-specific Kif5b knockout (F-K5bKO) mice were generated using the Cre-LoxP strategy. F-K5bKO mice had similar body weights to controls fed on a standard chow diet. However, F-K5bKO mice had hyperlipidemia and significant glucose intolerance and insulin resistance. Deletion of Kif5b aggravated the deleterious impact of a high-fat diet (HFD) on body weight gain, hepatosteatosis, glucose tolerance, and systematic insulin sensitivity. These changes were accompanied by impaired insulin signaling, decreased secretion of adiponectin, and increased serum levels of leptin and proinflammatory adipokines. F-K5bKO mice fed on an HFD exhibited lower energy expenditure and thermogenic dysfunction as a result of whitening of brown adipose due to decreased mitochondria biogenesis and down-regulation of key thermogenic gene expression. In conclusion, selective deletion of Kif5b in adipose tissue exacerbates HFD-induced obesity and its associated metabolic disorders, partly through a decrease in energy expenditure, dysregulation of adipokine secretion, and insulin signaling.-Cui, J., Pang, J., Lin, Y.-J., Gong, H., Wang, Z.-H., Li, Y.-X., Li, J., Wang, Z., Jiang, P., Dai, D.-P., Li, J., Cai, J.-P., Huang, J.-D., Zhang, T.-M. Adipose-specific deletion of Kif5b exacerbates obesity and insulin resistance in a mouse model of diet-induced obesity.

Ca2+Influx Does Not Trigger Glucose-Induced Traffic of the Insulin Granules and Alteration of Their Distribution

This study investigated mechanisms by which glucose increases readily releasable secretory granules via acting on preexocytotic steps, i.e., intracellular granule movement and granule access to the plasma membrane using a pancreatic beta-cell line, MIN6. Glucose-induced activation of the movement occurred at a substimulatory concentration with regard to insulin output. Glucose activation of the movement was inhibited by pretreatment with thapsigargin plus acetylcholine to suppress intracellular Ca2+ mobilization. Inhibitors of calmodulin and myosin light chain kinase also suppressed glucose activation of the movement. Simultaneous addition of glucose with Ca2+ channel blockers or the ATP-sensitive K+ channel opener diazoxide failed to suppress the traffic activation, and addition of these substances on top of glucose stimulation resulted in a further increase. Although stimulatory glucose had minimal changes in the intracellular granule distribution, inhibition of Ca2+ influx revealed increases by glucose of the granules in the cell periphery. In contrast, high K+ depolarization decreased the peripheral granules. Glucose-induced granule margination was abolished when the protein kinase C activity was downregulated. These findings indicate that preexocytotic control of insulin release is regulated by distinct mechanisms from Ca2+ influx, which triggers insulin exocytosis. The nature of the regulation by glucose may explain a part of potentiating effects of the hexose independent of the closure of the ATP-sensitive K+ channel.

Applications of phototransformable fluorescent proteins for tracking the dynamics of cellular components

In the past few decades, fluorescent proteins have revolutionized the field of cell biology. Phototransformable fluorescent proteins are capable of changing their excitation and emission spectra after being exposed to specific wavelength(s) of light. The majority of phototransformable fluorescent proteins have originated from marine organisms. Genetic engineering of these proteins has made available many choices for different colors, modes of conversion, and other biophysical properties. Their phototransformative property has allowed the highlighting and tracking of subpopulations of cells, organelles, and proteins in living systems. Furthermore, phototransformable fluorescent proteins have offered new methods for superresolution fluorescence microscopy and optogenetics manipulation of proteins. One of the major advantages of phototransformable fluorescent proteins is their applicability for visualizing newly synthesized proteins that are en route to their final destinations. In this paper, we will discuss the biological applications of phototransformable fluorescent proteins with special emphasis on the application of tracking membrane proteins in vertebrate photoreceptor cells.

A Syntenic Cross Species Aneuploidy Genetic Screen Links RCAN1 Expression to β-Cell Mitochondrial Dysfunction in Type 2 Diabetes

Type 2 diabetes (T2D) is a complex metabolic disease associated with obesity, insulin resistance and hypoinsulinemia due to pancreatic β-cell dysfunction. Reduced mitochondrial function is thought to be central to β-cell dysfunction. Mitochondrial dysfunction and reduced insulin secretion are also observed in β-cells of humans with the most common human genetic disorder, Down syndrome (DS, Trisomy 21). To identify regions of chromosome 21 that may be associated with perturbed glucose homeostasis we profiled the glycaemic status of different DS mouse models. The Ts65Dn and Dp16 DS mouse lines were hyperglycemic, while Tc1 and Ts1Rhr mice were not, providing us with a region of chromosome 21 containing genes that cause hyperglycemia. We then examined whether any of these genes were upregulated in a set of ~5,000 gene expression changes we had identified in a large gene expression analysis of human T2D β-cells. This approach produced a single gene, RCAN1, as a candidate gene linking hyperglycemia and functional changes in T2D β-cells. Further investigations demonstrated that RCAN1 methylation is reduced in human T2D islets at multiple sites, correlating with increased expression. RCAN1 protein expression was also increased in db/db mouse islets and in human and mouse islets exposed to high glucose. Mice overexpressing RCAN1 had reduced in vivo glucose-stimulated insulin secretion and their β-cells displayed mitochondrial dysfunction including hyperpolarised membrane potential, reduced oxidative phosphorylation and low ATP production. This lack of β-cell ATP had functional consequences by negatively affecting both glucose-stimulated membrane depolarisation and ATP-dependent insulin granule exocytosis. Thus, from amongst the myriad of gene expression changes occurring in T2D β-cells where we had little knowledge of which changes cause β-cell dysfunction, we applied a trisomy 21 screening approach which linked RCAN1 to β-cell mitochondrial dysfunction in T2D.

Disallowance of Acot7 in β-Cells Is Required for Normal Glucose Tolerance and Insulin Secretion

Encoding acyl-CoA thioesterase-7 (Acot7) is one of ∼60 genes expressed ubiquitously across tissues but relatively silenced, or disallowed, in pancreatic β-cells. The capacity of ACOT7 to hydrolyze long-chain acyl-CoA esters suggests potential roles in β-oxidation, lipid biosynthesis, signal transduction, or insulin exocytosis. We explored the physiological relevance of β-cell-specific Acot7 silencing by re-expressing ACOT7 in these cells. ACOT7 overexpression in clonal MIN6 and INS1(832/13) β-cells impaired insulin secretion in response to glucose plus fatty acids. Furthermore, in a panel of transgenic mouse lines, we demonstrate that overexpression of mitochondrial ACOT7 selectively in the adult β-cell reduces glucose tolerance dose dependently and impairs glucose-stimulated insulin secretion. By contrast, depolarization-induced secretion was unaffected, arguing against a direct action on the exocytotic machinery. Acyl-CoA levels, ATP/ADP increases, membrane depolarization, and Ca(2+) fluxes were all markedly reduced in transgenic mouse islets, whereas glucose-induced oxygen consumption was unchanged. Although glucose-induced increases in ATP/ADP ratio were similarly lowered after ACOT7 overexpression in INS1(832/13) cells, changes in mitochondrial membrane potential were unaffected, consistent with an action of Acot7 to increase cellular ATP consumption. Because Acot7 mRNA levels are increased in human islets in type 2 diabetes, inhibition of the enzyme might provide a novel therapeutic strategy.

Secretagogin affects insulin secretion in pancreatic β-cells by regulating actin dynamics and focal adhesion

Secretagogin (SCGN), a Ca²⁺-binding protein having six EF-hands, is selectively expressed in pancreatic β-cells and neuroendocrine cells. Previous studies suggested that SCGN enhances insulin secretion by functioning as a Ca²⁺-sensor protein, but the underlying mechanism has not been elucidated. The present study explored the mechanism by which SCGN enhances glucose-induced insulin secretion in NIT-1 insulinoma cells. To determine whether SCGN influences the first or second phase of insulin secretion, we examined how SCGN affects the kinetics of insulin secretion in NIT-1 cells. We found that silencing SCGN suppressed the second phase of insulin secretion induced by glucose and H₂O₂, but not the first phase induced by KCl stimulation. Recruitment of insulin granules in the second phase of insulin secretion was significantly impaired by knocking down SCGN in NIT-1 cells. In addition, we found that SCGN interacts with the actin cytoskeleton in the plasma membrane and regulates actin remodelling in a glucose-dependent manner. Since actin dynamics are known to regulate focal adhesion, a critical step in the second phase of insulin secretion, we examined the effect of silencing SCGN on focal adhesion molecules, including FAK (focal adhesion kinase) and paxillin, and the cell survival molecules ERK1/2 (extracellular-signal-regulated kinase 1/2) and Akt. We found that glucose- and H₂O₂-induced activation of FAK, paxillin, ERK1/2 and Akt was significantly blocked by silencing SCGN. We conclude that SCGN controls glucose-stimulated insulin secretion and thus may be useful in the therapy of Type 2 diabetes.

Beta-cell mitochondrial carriers and the diabetogenic stress response

Mitochondria play a central role in pancreatic beta-cells by coupling metabolism of the secretagogue glucose to distal events of regulated insulin exocytosis. This process requires transports of both metabolites and nucleotides in and out of the mitochondria. The molecular identification of mitochondrial carriers and their respective contribution to beta-cell function have been uncovered only recently. In type 2 diabetes, mitochondrial dysfunction is an early event and may precipitate beta-cell loss. Under diabetogenic conditions, characterized by glucotoxicity and lipotoxicity, the expression profile of mitochondrial carriers is selectively modified. This review describes the role of mitochondrial carriers in beta-cells and the selective changes in response to glucolipotoxicity. In particular, we discuss the importance of the transfer of metabolites (pyruvate, citrate, malate, and glutamate) and nucleotides (ATP, NADH, NADPH) for beta-cell function and dysfunction. This article is part of a Special Issue entitled: Mitochondrial Channels edited by Pierre Sonveaux, Pierre Maechler and Jean-Claude Martinou.

Microtubules Negatively Regulate Insulin Secretion in Pancreatic β Cells

For glucose-stimulated insulin secretion (GSIS), insulin granules have to be localized close to the plasma membrane. The role of microtubule-dependent transport in granule positioning and GSIS has been debated. Here, we report that microtubules, counterintuitively, restrict granule availability for secretion. In β cells, microtubules originate at the Golgi and form a dense non-radial meshwork. Non-directional transport along these microtubules limits granule dwelling at the cell periphery, restricting granule availability for secretion. High glucose destabilizes microtubules, decreasing their density; such local microtubule depolymerization is necessary for GSIS, likely because granule withdrawal from the cell periphery becomes inefficient. Consistently, microtubule depolymerization by nocodazole blocks granule withdrawal, increases their concentration at exocytic sites, and dramatically enhances GSIS in vitro and in mice. Furthermore, glucose-driven MT destabilization is balanced by new microtubule formation, which likely prevents over-secretion. Importantly, microtubule density is greater in dysfunctional β cells of diabetic mice.

The skeleton in the closet: actin cytoskeletal remodeling in β-cell function

Over the last few decades, biomedical research has considered not only the function of single cells but also the importance of the physical environment within a whole tissue, including cell-cell and cell-extracellular matrix interactions. Cytoskeleton organization and focal adhesions are crucial sensors for cells that enable them to rapidly communicate with the physical extracellular environment in response to extracellular stimuli, ensuring proper function and adaptation. The involvement of the microtubular-microfilamentous cytoskeleton in secretion mechanisms was proposed almost 50 years ago, since when the evolution of ever more sensitive and sophisticated methods in microscopy and in cell and molecular biology have led us to become aware of the importance of cytoskeleton remodeling for cell shape regulation and its crucial link with signaling pathways leading to β-cell function. Emerging evidence suggests that dysfunction of cytoskeletal components or extracellular matrix modification influences a number of disorders through potential actin cytoskeleton disruption that could be involved in the initiation of multiple cellular functions. Perturbation of β-cell actin cytoskeleton remodeling could arise secondarily to islet inflammation and fibrosis, possibly accounting in part for impaired β-cell function in type 2 diabetes. This review focuses on the role of actin remodeling in insulin secretion mechanisms and its close relationship with focal adhesions and myosin II.

Analysis of Kif5b expression during mouse kidney development

Recent studies showed that kidney-specific inactivation of Kif3a produces kidney cysts and renal failure, suggesting that kinesin-mediated intracellular transportation is important for the establishement and maintenance of renal epithelial cell polarity and normal nephron functions. Kif5b, one of the most conserved kinesin heavy chain, is the mouse homologue of the human ubiquitous Kinesin Heavy Chain (uKHC). In order to elucidate the role of Kif5b in kidney development and function, it is essential to establish its expression profile within the organ. Therefore, in this study, we examined the expression pattern of Kif5b in mouse kidney. Kidneys from embryonic (E) 12.5-, 16.5-dpc (days post coitus) mouse fetuses, from postnatal (P) day 0, 10, 20 pups and from adult mice were collected. The distribution of Kif5b was analyzed by immunostaining. The possible involvement of Kif5b in kidney development was investigated in conditional mutant mice by using a Cre-LoxP strategy. This study showed that the distribution of Kif5b displayed spatiotemporal changes during postnatal kidney development. In kidneys of new born mice, Kif5b was strongly expressed in all developing tubules and in the ureteric bud, but not in the glomerulus or in other early-developing structures, such as the cap mesenchyme, the comma-shaped body, and the S-shaped body. In kidneys of postnatal day 20 or of older mice, however, Kif5b was localized selectively in the basolateral domain of epithelial cells of the thick ascending loop of Henle, as well as of the distal convoluted tubule, with little expression being observed in the proximal tubule or in the collecting duct. Conditional knock-down of Kif5b in mouse kidney did not result in detectable morphological defects, but it did lead to a decrease in cell proliferation rate and also to a mislocalization of Na⁺/K⁺/-ATPase, indicating that although Kif5b is non-essential for kidney morphogenesis, it is important for nephron maturation.

Aged insulin granules display reduced microtubule-dependent mobility and are disposed within actin-positive multigranular bodies

Insulin secretion is key for glucose homeostasis. Insulin secretory granules (SGs) exist in different functional pools, with young SGs being more mobile and preferentially secreted. However, the principles governing the mobility of age-distinct SGs remain undefined. Using the time-reporter insulin-SNAP to track age-distinct SGs we now show that their dynamics can be classified into three components: highly dynamic, restricted, and nearly immobile. Young SGs display all three components, whereas old SGs are either restricted or nearly immobile. Both glucose stimulation and F-actin depolymerization recruit a fraction of nearly immobile young, but not old, SGs for highly dynamic, microtubule-dependent transport. Moreover, F-actin marks multigranular bodies/lysosomes containing aged SGs. These data demonstrate that SGs lose their responsiveness to glucose stimulation and competence for microtubule-mediated transport over time while changing their relationship with F-actin.

Cytoskeletal dependence of insulin granule movement dynamics in INS-1 beta-cells in response to glucose

For pancreatic β-cells to secrete insulin in response to elevated blood glucose, insulin granules retained within the subplasmalemmal space must be transported to sites of secretion on the plasma membrane. Using a combination of super-resolution STORM imaging and live cell TIRF microscopy we investigate how the organization and dynamics of the actin and microtubule cytoskeletons in INS-1 β-cells contribute to this process. GFP-labeled insulin granules display 3 different modes of motion (stationary, diffusive-like, and directed). Diffusive-like motion dominates in basal, low glucose conditions. Upon glucose stimulation no gross rearrangement of the actin cytoskeleton is observed but there are increases in the 1) rate of microtubule polymerization; 2) rate of diffusive-like motion; and 3) proportion of granules undergoing microtubule-based directed motion. By pharmacologically perturbing the actin and microtubule cytoskeletons, we determine that microtubule-dependent granule transport occurs within the subplasmalemmal space and that the actin cytoskeleton limits this transport in basal conditions, when insulin secretion needs to be inhibited.

Phospholipase C-related catalytically inactive protein (PRIP) controls KIF5B-mediated insulin secretion

We previously reported that phospholipase C-related catalytically inactive protein (PRIP)-knockout mice exhibited hyperinsulinemia. Here, we investigated the role of PRIP in insulin granule exocytosis using Prip-knockdown mouse insulinoma (MIN6) cells. Insulin release from Prip-knockdown MIN6 cells was higher than that from control cells, and Prip knockdown facilitated movement of GFP-phogrin-labeled insulin secretory vesicles. Double-immunofluorescent staining and density step-gradient analyses showed that the KIF5B motor protein co-localized with insulin vesicles in Prip-knockdown MIN6 cells. Knockdown of GABA_A-receptor-associated protein (GABARAP), a microtubule-associated PRIP-binding partner, by Gabarap silencing in MIN6 cells reduced the co-localization of insulin vesicles with KIF5B and the movement of vesicles, resulting in decreased insulin secretion. However, the co-localization of KIF5B with microtubules was not altered in Prip- and Gabarap-knockdown cells. The presence of unbound GABARAP, freed either by an interference peptide or by Prip silencing, in MIN6 cells enhanced the co-localization of insulin vesicles with microtubules and promoted vesicle mobility. Taken together, these data demonstrate that PRIP and GABARAP function in a complex to regulate KIF5B-mediated insulin secretion, providing new insights into insulin exocytic mechanisms.

Functional implications of the Golgi and microtubular network in gonadotropes

In contrast to the widely accepted images of the Golgi apparatus as a cup-like shape, the Golgi in pituitary gonadotropes is organized as a spherical shape in which the outer and inner faces are cis- and trans-Golgi elements, respectively. At the center of the spherical Golgi, a pair of centrioles is situated as a microtubule-organizing center from which radiating microtubules isotropically extend toward the cell periphery. This review focuses on the significance of the characteristic organization of the Golgi and microtubule network in gonadotropes, considering the roles of microtubule-dependent membrane transport in the formation and maintenance of the Golgi structure. Because the highly symmetrical organization of the Golgi is possibly perturbed in response to experimental treatments of gonadotropes, monitoring of the Golgi structure in gonadotropes under various experimental conditions will be a novel in vivo approach to elucidate the biogenesis of the Golgi apparatus.

Mitochondrial function and insulin secretion

In the endocrine fraction of the pancreas, the β-cell rapidly reacts to fluctuations in blood glucose concentrations by adjusting the rate of insulin secretion. Glucose-sensing coupled to insulin exocytosis depends on transduction of metabolic signals into intracellular messengers recognized by the secretory machinery. Mitochondria play a central role in this process by connecting glucose metabolism to insulin release. Mitochondrial activity is primarily regulated by metabolic fluxes, but also by dynamic morphology changes and free Ca(2+) concentrations. Recent advances of mitochondrial Ca(2+) homeostasis are discussed; in particular the roles of the newly-identified mitochondrial Ca(2+) uniporter MCU and its regulatory partner MICU1, as well as the mitochondrial Na(+)-Ca(2+) exchanger. This review describes how mitochondria function both as sensors and generators of metabolic signals; such as NADPH, long chain acyl-CoA, glutamate. The coupling factors are additive to the Ca(2+) signal and participate to the amplifying pathway of glucose-stimulated insulin secretion.

RNAi screening in primary human hepatocytes of genes implicated in genome-wide association studies for roles in type 2 diabetes identifies roles for CAMK1D and CDKAL1, among others, in hepatic glucose regulation

Genome-wide association (GWA) studies have described a large number of new candidate genes that contribute to of Type 2 Diabetes (T2D). In some cases, small clusters of genes are implicated, rather than a single gene, and in all cases, the genetic contribution is not defined through the effects on a specific organ, such as the pancreas or liver. There is a significant need to develop and use human cell-based models to examine the effects these genes may have on glucose regulation. We describe the development of a primary human hepatocyte model that adjusts glucose disposition according to hormonal signals. This model was used to determine whether candidate genes identified in GWA studies regulate hepatic glucose disposition through siRNAs corresponding to the list of identified genes. We find that several genes affect the storage of glucose as glycogen (glycolytic response) and/or affect the utilization of pyruvate, the critical step in gluconeogenesis. Of the genes that affect both of these processes, CAMK1D, TSPAN8 and KIF11 affect the localization of a mediator of both gluconeogenesis and glycolysis regulation, CRTC2, to the nucleus in response to glucagon. In addition, the gene CDKAL1 was observed to affect glycogen storage, and molecular experiments using mutant forms of CDK5, a putative target of CDKAL1, in HepG2 cells show that this is mediated by coordinate regulation of CDK5 and PKA on MEK, which ultimately regulates the phosphorylation of ribosomal protein S6, a critical step in the insulin signaling pathway.

Kinesin-1 and mitochondrial motility control by discrimination of structurally equivalent but distinct subdomains in Ran-GTP-binding domains of Ran-binding protein 2

The pleckstrin homology (PH) domain is a versatile fold that mediates a variety of protein–protein and protein–phosphatidylinositol lipid interactions. The Ran-binding protein 2 (RanBP2) contains four interspersed Ran GTPase-binding domains (RBD_{n= 1–4}) with close structural homology to the PH domain of Bruton's tyrosine kinase. The RBD₂, kinesin-binding domain (KBD) and RBD₃ comprise a tripartite domain (R₂KR₃) of RanBP2 that causes the unfolding, microtubule binding and biphasic activation of kinesin-1, a crucial anterograde motor of mitochondrial motility. However, the interplay between Ran GTPase and R₂KR₃ of RanBP2 in kinesin-1 activation and mitochondrial motility is elusive. We use structure–function, biochemical, kinetic and cell-based assays with time-lapse live-cell microscopy of over 260 000 mitochondrial-motility-related events to find mutually exclusive subdomains in RBD₂ and RBD₃ towards Ran GTPase binding, kinesin-1 activation and mitochondrial motility regulation. The RBD₂ and RBD₃ exhibit Ran-GTP-independent, subdomain and stereochemical-dependent discrimination on the biphasic kinetics of kinesin-1 activation or regulation of mitochondrial motility. Further, KBD alone and R₂KR₃ stimulate and suppress, respectively, multiple biophysical parameters of mitochondrial motility. The regulation of the bidirectional transport of mitochondria by either KBD or R₂KR₃ is highly coordinated, because their kinetic effects are accompanied always by changes in mitochondrial motile events of either transport polarity. These studies uncover novel roles in Ran GTPase-independent subdomains of RBD₂ and RBD₃, and KBD of RanBP2, that confer antagonizing and multi-modal mechanisms of kinesin-1 activation and regulation of mitochondrial motility. These findings open new venues towards the pharmacological harnessing of cooperative and competitive mechanisms regulating kinesins, RanBP2 or mitochondrial motility in disparate human disorders.

Mitochondrial signal transduction in pancreatic β-cells

In the endocrine fraction of the pancreas, the task of the beta-cell is to continuously and perfectly adjust insulin secretion to fluctuating blood glucose levels, thereby maintaining glycemia and nutrient homeostasis. This glucose sensing coupled to insulin exocytosis depends on transduction of metabolic signals into intracellular messengers recognized by the exocytotic machinery. Central to this metabolism-secretion coupling, mitochondrial signal transduction refers to both integration and generation of metabolic signals, connecting glucose sensing to insulin exocytosis. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, in the stimulation of insulin release. This review describes the role of mitochondria in metabolic signal transduction regulating insulin secretion.

MyRIP interaction with MyoVa on secretory granules is controlled by the cAMP-PKA pathway

Myosin- and Rab-interacting protein is not a classic receptor for MyoVa on large, dense-core secretory granules (SGs), but it aids in PKA-dependent phosphorylation of MyoVa-associated proteins on SGs in endocrine and neuroendocrine cells.

Myosin- and Rab-interacting protein (MyRIP), which belongs to the protein kinase A (PKA)–anchoring family, is implicated in hormone secretion. However, its mechanism of action is not fully elucidated. Here we investigate the role of MyRIP in myosin Va (MyoVa)-dependent secretory granule (SG) transport and secretion in pancreatic beta cells. These cells solely express the brain isoform of MyoVa (BR-MyoVa), which is a key motor protein in SG transport. In vitro pull-down, coimmunoprecipitation, and colocalization studies revealed that MyRIP does not interact with BR-MyoVa in glucose-stimulated pancreatic beta cells, suggesting that, contrary to previous notions, MyRIP does not link this motor protein to SGs. Glucose-stimulated insulin secretion is augmented by incretin hormones, which increase cAMP levels and leads to MyRIP phosphorylation, its interaction with BR-MyoVa, and phosphorylation of the BR-MyoVa receptor rabphilin-3A (Rph-3A). Rph-3A phosphorylation on Ser-234 was inhibited by small interfering RNA knockdown of MyRIP, which also reduced cAMP-mediated hormone secretion. Demonstrating the importance of this phosphorylation, nonphosphorylatable and phosphomimic Rph-3A mutants significantly altered hormone release when PKA was activated. These data suggest that MyRIP only forms a functional protein complex with BR-MyoVa on SGs when cAMP is elevated and under this condition facilitates phosphorylation of SG-associated proteins, which in turn can enhance secretion.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

The microtubule associated protein syntabulin is required for glucose-stimulated and cAMP-potentiated insulin secretion

Syntabulin is a microtubule-associated protein that mediates anterograde transport of vesicles to neuronal processes. Here, we found that syntabulin was expressed in mouse pancreas and insulin-secreting β-cells, and that it partially co-localized with microtubule and insulin-containing granules. The association of syntabulin with these organelles increased upon glucose stimulation. Knock-down of syntabulin by shRNA reduced both basal and glucose-stimulated insulin secretion, and diminished cAMP-Epac2 and cAMP-PKA potentiated insulin secretion. Additionally, syntabulin was preferentially phosphorylated by the Epac2 agonist 8-pCPT-2'-O-Me-cAMP, suggesting that syntabulin could be a novel effector of Epac2 and play a critical role in cAMP-enhanced insulin secretion.

PI3K p110α/Akt signaling negatively regulates secretion of the intestinal peptide neurotensin through interference of granule transport

Neurotensin (NT), an intestinal peptide secreted from N cells in the small bowel, regulates a variety of physiological functions of the gastrointestinal tract, including secretion, gut motility, and intestinal growth. The class IA phosphatidylinositol 3-kinase (PI3K) family, which comprised of p110 catalytic (α, β and δ) and p85 regulatory subunits, has been implicated in the regulation of hormone secretion from endocrine cells. However, the underlying mechanisms remain poorly understood. In particular, the role of PI3K in intestinal peptide secretion is not known. Here, we show that PI3K catalytic subunit, p110α, negatively regulates NT secretion in vitro and in vivo. We demonstrate that inhibition of p110α, but not p110β, induces NT release in BON, a human endocrine cell line, which expresses NT mRNA and produces NT peptide in a manner analogous to N cells, and QGP-1, a pancreatic endocrine cell line that produces NT peptide. In contrast, overexpression of p110α decreases NT secretion. Consistently, p110α-inhibition increases plasma NT levels in mice. To further delineate the mechanisms contributing to this effect, we demonstrate that inhibition of p110α increases NT granule trafficking by up-regulating α-tubulin acetylation; NT secretion is prevented by overexpression of HDAC6, an α-tubulin deacetylase. Moreover, ras-related protein Rab27A (a small G protein) and kinase D-interacting substrate of 220 kDa (Kidins220), which are associated with NT granules, play a negative and positive role, respectively, in p110α-inhibition-induced NT secretion. Our findings identify the critical role and novel mechanisms for the PI3K signaling pathway in the control of intestinal hormone granule transport and release.

Genes associated with MUC5AC expression in small airway epithelium of human smokers and non-smokers

Background

Mucus hypersecretion contributes to the morbidity and mortality of smoking-related lung diseases, especially chronic obstructive pulmonary disease (COPD), which starts in the small airways. Despite progress in animal studies, the genes and their expression pattern involved in mucus production and secretion in human airway epithelium are not well understood. We hypothesized that comparison of the transcriptomes of the small airway epithelium of individuals that express high vs low levels of MUC5AC, the major macromolecular component of airway mucus, could be used as a probe to identify the genes related to human small airway mucus production/secretion.

Methods

Flexible bronchoscopy and brushing were used to obtain small airway epithelium (10^th to 12^th order bronchi) from healthy nonsmokers (n=60) and healthy smokers (n=72). Affymetrix HG-U133 plus 2.0 microarrays were used to assess gene expression. Massive parallel sequencing (RNA-Seq) was used to verify gene expression of small airway epithelium from 5 nonsmokers and 6 smokers.

Results

MUC5AC expression varied 31-fold among the healthy nonsmokers. Genome-wide comparison between healthy nonsmokers (n = 60) grouped as “high MUC5AC expressors” vs “low MUC5AC expressors” identified 528 genes significantly up-regulated and 15 genes significantly down-regulated in the high vs low expressors. This strategy identified both mucus production and secretion related genes under control of a network composed of multiple transcription factors. Based on the literature, genes in the up-regulated list were used to identify a 73 “MUC5AC-associated core gene” list with 9 categories: mucus component; mucus-producing cell differentiation-related transcription factor; mucus-producing cell differentiation-related pathway or mediator; post-translational modification of mucin; vesicle transport; endoplasmic reticulum stress-related; secretory granule-associated; mucus secretion-related regulator and mucus hypersecretory-related ion channel. As a validation cohort, we assessed the MUC5AC-associated core gene list in the small airway epithelium of an independent set of healthy smokers (n = 72). There was up-regulation of MUC5AC in the small airway epithelium of smokers (2.3-fold, p < 10^-8) associated with a coordinated up-regulation of MUC5AC-associated core gene expression pattern in the small airway epithelium of smokers (p < 0.01). Deep sequencing confirmed these observations.

Conclusion

The identification of the genes associated with increased airway mucin production in humans should be useful in understanding the pathogenesis of airway mucus hypersecretion and identifying therapeutic targets.

Author summary

Mucus hypersecretion contributes to the morbidity and mortality of smoking-related lung diseases, especially chronic obstructive pulmonary disease (COPD), which starts in the small airways. Little is known about the gene networks associated with the synthesis and secretion of mucins in the human small airway epithelium. Taking advantage of the knowledge that MUC5AC is a major mucin secreted by the small airway epithelium, the expression of MUC5AC in small airway epithelium is highly regulated at the transcriptional level and our observation that healthy nonsmokers have variable numbers of MUC5AC⁺ secretory cells in the human small airway epithelium, we compared genome-wide gene expression of the small airway epithelium of high vs low MUC5AC expressors from 60 nonsmokers to identify the genes associated with MUC5AC expression. This novel strategy enabled identification of a 73 “MUC5AC-associated core gene” list with 9 categories, which control a series of processes from mucin biosynthesis to mucus secretion. The coordinated gene expression pattern of MUC5AC-associated core genes were corroborated in an independent cohort of 72 healthy smokers. Deep sequencing of small airway epithelium RNA confirmed these observations. This finding will be useful in identifying therapeutic targets to treat small airway mucus hypersecretion.

Partitioning and Exocytosis of Secretory Granules during Division of PC12 Cells

The biogenesis, maturation, and exocytosis of secretory granules in interphase cells have been well documented, whereas the distribution and exocytosis of these hormone-storing organelles during cell division have received little attention. By combining ultrastructural analyses and time-lapse microscopy, we here show that, in dividing PC12 cells, the prominent peripheral localization of secretory granules is retained during prophase but clearly reduced during prometaphase, ending up with only few peripherally localized secretory granules in metaphase cells. During anaphase and telophase, secretory granules exhibited a pronounced movement towards the cell midzone and, evidently, their tracks colocalized with spindle microtubules. During cytokinesis, secretory granules were excluded from the midbody and accumulated at the bases of the intercellular bridge. Furthermore, by measuring exocytosis at the single granule level, we showed, that during all stages of cell division, secretory granules were competent for regulated exocytosis. In conclusion, our data shed new light on the complex molecular machinery of secretory granule redistribution during cell division, which facilitates their release from the F-actin-rich cortex and active transport along spindle microtubules.

A unique ball-shaped Golgi apparatus in the rat pituitary gonadotrope: its functional implications in relation to the arrangement of the microtubule network

In polarized exocrine cells, the Golgi apparatus is cup-shaped and its convex and concave surfaces are designated as cis and trans faces, functionally confronting the rough endoplasmic reticulum and the cell surface, respectively. To clarify the morphological characteristics of the Golgi apparatus in non-polarized endocrine cells, the investigators immunocytochemically examined its precise architecture in pituitary gonadotropes, especially in relation to the arrangement of the intracellular microtubule network. The Golgi apparatus in the gonadotropes was not cup-shaped but ball-shaped or spherical, and its outer and inner surfaces were the cis and trans faces, respectively. Centrioles were situated at the center of the Golgi apparatus, from which radiating microtubules isotropically extended to the cell periphery through the gaps in the spherical wall of the Golgi stack. The shape of the Golgi apparatus and the arrangement of microtubules demonstrated in the present study could explain the microtubule-dependent movements of tubulovesicular carriers and granules within the gonadotropes. Furthermore, the spherical shape of the Golgi apparatus possibly reflects the highly symmetrical arrangement of microtubule arrays, as well as the poor polarity in the cell surface of pituitary gonadotropes.

Molecular mechanism of myosin Va recruitment to dense core secretory granules

The brain-spliced isoform of Myosin Va (BR-MyoVa) plays an important role in the transport of dense core secretory granules (SGs) to the plasma membrane in hormone and neuropeptide-producing cells. The molecular composition of the protein complex that recruits BR-MyoVa to SGs and regulates its function has not been identified to date. We have identified interaction between SG-associated proteins granuphilin-a/b (Gran-a/b), BR-MyoVa and Rab27a, a member of the Rab family of GTPases. Gran-a/b-BR-MyoVa interaction is direct, involves regions downstream of the Rab27-binding domain, and the C-terminal part of Gran-a determines exon specificity. MyoVa and Gran-a/b are partially colocalised on SGs and disruption of Gran-a/b-BR-MyoVa binding results in a perinuclear accumulation of SGs which augments nutrient-stimulated hormone secretion in pancreatic beta-cells. These results indicate the existence of at least another binding partner of BR-MyoVa that was identified as rabphilin-3A (Rph-3A). BR-MyoVa-Rph-3A interaction is also direct and enhanced when secretion is activated. The BR-MyoVa-Rph-3A and BR-MyoVa-Gran-a/b complexes are linked to a different subset of SGs, and simultaneous inhibition of these complexes nearly completely blocks stimulated hormone release. This study demonstrates that multiple binding partners of BR-MyoVa regulate SG transport, and this molecular mechanism is universally used by neuronal, endocrine and neuroendocrine cells.

T-cadherin (Cdh13) in association with pancreatic β-cell granules contributes to second phase insulin secretion

Glucose homeostasis depends on adequate control of insulin secretion. We report the association of the cell-adhesion and adiponectin (APN)-binding glycoprotein T-cadherin (Cdh13) with insulin granules in mouse and human β-cells. Immunohistochemistry and electron microscopy of islets in situ and targeting of RFP-tagged T-cadherin to GFP-labeled insulin granules in isolated β-cells demonstrate this unusual location. Analyses of T-cadherin-deficient (Tcad-KO) mice show normal islet architecture and insulin content. However, T-cadherin is required for sufficient insulin release in vitro and in vivo. Primary islets from Tcad-KO mice were defective in glucose-induced but not KCl-mediated insulin secretion. In vivo, second phase insulin release in T-cad-KO mice during a hyperglycemic clamp was impaired while acute first phase release was unaffected. Tcad-KO mice showed progressive glucose intolerance by 5 mo of age without concomitant changes in peripheral insulin sensitivity. Our analyses detected no association of APN with T-cadherin on β-cell granules although colocalization was observed on the pancreatic vasculature. These data identify T-cadherin as a novel component of insulin granules and suggest that T-cadherin contributes to the regulation of insulin secretion independently of direct interactions with APN.

Exophilin8 transiently clusters insulin granules at the actin-rich cell cortex prior to exocytosis

Exophilin8/MyRIP/Slac2-c is an effector protein of the small GTPase Rab27a and is specifically localized on retinal melanosomes and secretory granules. We investigated the role of exophilin8 in insulin granule trafficking. Exogenous expression of exophilin8 in pancreatic β cells or their cell line, MIN6, polarized (exophilin8-positive) insulin granules at the cell corners, where both cortical actin and the microtubule plus-end-binding protein, EB1, were present. Mutation analyses indicated that the ability of exophilin8 to act as a linker between Rab27a and myosin Va is essential for its granule-clustering activity. Moreover, exophilin8 and exophilin8-associated insulin granules were markedly stable and immobile. Total internal reflection fluorescence microscopy indicated that exophilin8 restricts the motion of insulin granules at a region deeper than that where another Rab27a effector, granuphilin, accumulates docked granules directly attached to the plasma membrane. However, the exophilin8-induced immobility of insulin granules was eliminated upon secretagogue stimulation and did not inhibit evoked exocytosis. Furthermore, exophilin8 depletion prevents insulin granules from being transported close to the plasma membrane and inhibits their fusion. These findings indicate that exophilin8 transiently traps insulin granules into the cortical actin network close to the microtubule plus-ends and supplies them for release during the stimulation.