Real-time bioluminescence imaging of a protein secretory pathway in living mammalian cells using Gaussia luciferase

Using photon counting and charge-coupled device (CCD) cameras, we have applied the method of real-time bioluminescence imaging to investigate protein trafficking in mammalian cells. In the living cells of Chinese hamster ovary and PC12D cells, exocytotic secretion of protein and protein targeting on the cell surface were visualized using the secreted Gaussia luciferase (GLase) as a reporter protein in a minute. After incubation of the cells with luciferin (coelenterazine) for 10min, luciferin was imported into the cells and the vesicle transport network in the cells could be shown by luminescence images of GLase activity. Further, we demonstrate that GLase with a heterologous signal peptide sequence is targeted to the cell surface in neuronally differentiated PC12D cells and luminescence signals could be detected in a few seconds.     

Video-rate bioluminescence imaging of matrix metalloproteinase-2 secreted from a migrating cell

Background

Matrix metalloproteinase-2 (MMP-2) plays an important role in cancer progression and metastasis. MMP-2 is secreted as a pro-enzyme, which is activated by the membrane-bound proteins, and the polarized distribution of secretory and the membrane-associated MMP-2 has been investigated. However, the real-time visualizations of both MMP-2 secretion from the front edge of a migration cell and its distribution on the cell surface have not been reported.

Methodology/Principal Findings

The method of video-rate bioluminescence imaging was applied to visualize exocytosis of MMP-2 from a living cell using Gaussia luciferase (GLase) as a reporter. The luminescence signals of GLase were detected by a high speed electron-multiplying charge-coupled device camera (EM-CCD camera) with a time resolution within 500 ms per image. The fusion protein of MMP-2 to GLase was expressed in a HeLa cell and exocytosis of MMP-2 was detected in a few seconds along the leading edge of a migrating HeLa cell. The membrane-associated MMP-2 was observed at the specific sites on the bottom side of the cells, suggesting that the sites of MMP-2 secretion are different from that of MMP-2 binding.

Conclusions

We were the first to successfully demonstrate secretory dynamics of MMP-2 and the specific sites for polarized distribution of MMP-2 on the cell surface. The video-rate bioluminescence imaging using GLase is a useful method to investigate distribution and dynamics of secreted proteins on the whole surface of polarized cells in real time.     

Glucose-stimulated insulin secretion is coupled to the interaction of actin with the t-SNARE (target membrane soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein) complex

The actin monomer sequestering agent latrunculin B depolymerized beta-cell cortical actin, which resulted in increased glucose-stimulated insulin secretion in both cultured MIN6 beta-cells and isolated rat islet cells. In perifused islets, latrunculin B treatment increased both first- and second-phase glucose-stimulated insulin secretion without any significant effect on total insulin content. This increase in secretion was independent of calcium regulation because latrunculin B also potentiated calcium-stimulated insulin secretion in permeabilized MIN6 cells. Confocal immunofluorescent microscopy revealed a redistribution of insulin granules to the cell periphery in response to glucose or latrunculin B, which correlated with a reduction in phalloidin staining of cortical actin. Moreover, the t-SNARE [target membrane soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor] proteins Syntaxin 1 and SNAP-25 coimmunoprecipitated polymerized actin from unstimulated MIN6 cells. Glucose stimulation transiently decreased the amount of actin coimmunoprecipitated with Syntaxin 1 and SNAP-25, and latrunculin B treatment fully ablated the coimmunoprecipitation. In contrast, the actin stabilizing agent jasplakinolide increased the amount of actin coimmunoprecipitated with the t-SNARE complex and prevented its dissociation upon glucose stimulation. These data suggest a mechanism whereby glucose modulates beta-cell cortical actin organization and disrupts the interaction of polymerized actin with the plasma membrane t-SNARE complex at a distal regulatory step in the exocytosis of insulin granules.     

DOC2b is a SNARE regulator of glucose-stimulated delayed insulin secretion

Insulin secretion is precisely regulated by blood glucose with unique biphasic pattern. The regulatory mechanism of the second-phase insulin release is unclear. In this study, we report that DOC2b (double C2 domain protein isoform b), a SNARE related protein, was associated with insulin vesicles and translocated to plasma membrane within several minutes upon high-glucose stimulation followed by an interaction with syntaxin4, but not syntaxin1. This binding specificity and the time course of DOC2b translocation were suitable for the regulation of second-phase insulin release. Increased DOC2b expression enhanced glucose-stimulated insulin secretion. In contrast, silencing DOC2b inhibited delayed release of insulin, without affecting rapid (∼7 min) phase secretion. Interestingly, DOC2b had no effects on KCl-triggered insulin release. These data suggest that DOC2b may be a regulator for delayed (second-phase) insulin secretion in MIN6 cells.     

The PDZ/coiled-coil domain containing protein PIST modulates insulin secretion in MIN6 insulinoma cells by interacting with somatostatin receptor subtype 5

The multi-domain protein PIST (protein interacting specifically with Tc10) interacts with the SSTR5 (somatostatin receptor 5) and is responsible for its intracellular localization. Here, we show that PIST is expressed in pancreatic beta-cells and interacts with SSTR5 in these cells. PIST expression in MIN6 insulinoma cells is reduced by somatostatin (SST). After stimulation with SST, SSTR5 undergoes internalization together with PIST. MIN6 cells over-expressing PIST display enhanced glucose-stimulated insulin secretion and a decreased sensitivity to SST-induced inhibition of insulin secretion. These data suggest that PIST plays an important role in insulin secretion by regulating SSTR5 availability at the plasma membrane.     

A direct interaction between Cdc42 and vesicle-associated membrane protein 2 regulates SNARE-dependent insulin exocytosis

In pancreatic beta cells, insulin granule exocytosis is regulated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein (SNAP) receptor) proteins, and this is coupled to cortical F-actin reorganization via the Rho family GTPase Cdc42 by an unknown mechanism. We investigated interactions among the target SNARE protein Syntaxin 1A and the vesicle-associated membrane SNARE protein (VAMP2) with Cdc42 and compared these structural interactions with their functional importance to glucose-stimulated insulin secretion in MIN6 beta cells. Subcellular fractionation analyses revealed a parallel redistribution of Cdc42 and VAMP2 from the granule fraction to the plasma membrane in response to glucose that temporally corresponded with the glucose-induced activation of Cdc42. Moreover, within these fractions Cdc42 and VAMP2 were found to co-immunoprecipitate under basal and glucose-stimulated conditions, suggesting that they moved as a complex. Furthermore, VAMP2 bound both GST-Cdc42-GTPgammaS and GST-Cdc42-GDP, indicating that the Cdc42-VAMP2 complex could form under both cytosolic GDP-bound Cdc42 and plasma membrane GTP-bound Cdc42 conformational conditions. In vitro binding analyses showed that VAMP2 bound directly to Cdc42 and that a heterotrimeric complex with Syntaxin 1A could also be formed. Deletion analyses of VAMP2 revealed that only the N-terminal 28 residues were required for Cdc42 binding. Expression of this 28-residue VAMP2 peptide in MIN6 beta cells resulted in the specific impairment of glucose-stimulated insulin secretion, indicating a functional importance for the Cdc42-VAMP2 interaction. Taken together, these data suggest a mechanism whereby glucose activates Cdc42 to induce the targeting of intracellular Cdc42-VAMP2-insulin granule complexes to Syntaxin 1A at the plasma membrane.     

Palmitate activates AMP-activated protein kinase and regulates insulin secretion from beta cells

AMP-activated protein kinase (AMPK) is an energy sensor that regulates cellular metabolism. Changes in AMPK activity contribute to the regulation of insulin secretion. Epidemiological evidence links the ingestion of saturated fatty acid with hyperinsulinemia. The aim of the present study was to examine the effects of palmitate on beta cell AMPK activity and insulin secretion. Isolated rat islets and MIN6 beta cells were treated acutely (5-60 min) or chronically (24 h) with palmitate. Insulin secretion, AMPK and acetyl CoA carboxylase phosphorylation were assessed. The acute effects of palmitate included AMPK activation and augmentation in insulin secretion. Activation of AMPK by 24h pretreatment with palmitate suppressed glucose-stimulated insulin secretion, but not the response of insulin secretion to combined stimuli of glucose and palmitate. This study demonstrated that palmitate availability affected beta cell AMPK activity. In beta cells, an increase in AMPK activity may be required for fatty acid-induced fatty acid oxidation and prevention of lipotoxicity.     

Novel mechanistic link between focal adhesion remodeling and glucose-stimulated insulin secretion

Actin cytoskeleton remodeling is well known to be positively involved in glucose-stimulated pancreatic β cell insulin secretion. We have observed glucose-stimulated focal adhesion remodeling at the β cell surface and have shown this to be crucial for glucose-stimulated insulin secretion. However, the mechanistic link between such remodeling and the insulin secretory machinery remained unknown and was the major aim of this study. MIN6B1 cells, a previously validated model of primary β cell function, were used for all experiments. Total internal reflection fluorescence microscopy revealed the glucose-responsive co-localization of focal adhesion kinase (FAK) and paxillin with integrin β1 at the basal cell surface after short term stimulation. In addition, blockade of the interaction between β1 integrins and the extracellular matrix with an anti-β1 integrin antibody (Ha2/5) inhibited short term glucose-induced phosphorylation of FAK (Tyr-397), paxillin (Tyr-118), and ERK1/2 (Thr-202/Tyr-204). Pharmacological inhibition of FAK activity blocked glucose-induced actin cytoskeleton remodeling and glucose-induced disruption of the F-actin/SNAP-25 association at the plasma membrane as well as the distribution of insulin granules to regions in close proximity to the plasma membrane. Furthermore, FAK inhibition also completely blocked short term glucose-induced activation of the Akt/AS160 signaling pathway. In conclusion, these results indicate 1) that glucose-induced activation of FAK, paxillin, and ERK1/2 is mediated by β1 integrin intracellular signaling, 2) a mechanism whereby FAK mediates glucose-induced actin cytoskeleton remodeling, hence allowing docking and fusion of insulin granules to the plasma membrane, and 3) a possible functional role for the Akt/AS160 signaling pathway in the FAK-mediated regulation of glucose-stimulated insulin secretion.     

Functional Analysis of Novel Candidate Regulators of Insulin Secretion in the MIN6 Mouse Pancreatic β Cell Line

Elucidating the regulation of glucose-stimulated insulin secretion (GSIS) in pancreatic β cells is important for understanding and treating diabetes. The pancreatic β cell line, MIN6, retains GSIS but gradually loses it in long-term culture. The MIN6 subclone, MIN6c4, exhibits well-regulated GSIS even after prolonged culture. We previously used DNA microarray analysis to compare gene expression in the parental MIN6 cells and MIN6c4 cells and identified several differentially regulated genes that may be involved in maintaining GSIS. Here we investigated the potential roles of six of these genes in GSIS: Tmem59l (Transmembrane protein 59 like), Scgn (Secretagogin), Gucy2c (Guanylate cyclase 2c), Slc29a4 (Solute carrier family 29, member 4), Cdhr1 (Cadherin-related family member 1), and Celsr2 (Cadherin EGF LAG seven-pass G-type receptor 2). These genes were knocked down in MIN6c4 cells using lentivirus vectors expressing gene-specific short hairpin RNAs (shRNAs), and the effects of the knockdown on insulin expression and secretion were analyzed. Suppression of Tmem59l, Scgn, and Gucy2c expression resulted in significantly decreased glucose- and/or KCl-stimulated insulin secretion from MIN6c4 cells, while the suppression of Slc29a4 expression resulted in increased insulin secretion. Tmem59l overexpression rescued the phenotype of the Tmem59l knockdown MIN6c4 cells, and immunostaining analysis indicated that the TMEM59L protein colocalized with insulin and GM130, a Golgi complex marker, in MIN6 cells. Collectively, our findings suggested that the proteins encoded by Tmem59l, Scgn, Gucy2c, and Slc29a4 play important roles in regulating GSIS. Detailed studies of these proteins and their functions are expected to provide new insights into the molecular mechanisms involved in insulin secretion.     

Brain-selective Kinase 2 (BRSK2) Phosphorylation on PCTAIRE1 Negatively Regulates Glucose-stimulated Insulin Secretion in Pancreatic β-Cells

Brain-selective kinase 2 (BRSK2) has been shown to play an essential role in neuronal polarization. In the present study, we show that BRSK2 is also abundantly expressed in pancreatic islets and MIN6 β-cell line. Yeast two-hybrid screening, GST fusion protein pull-down, and co-immunoprecipitation assays reveal that BRSK2 interacts with CDK-related protein kinase PCTAIRE1, a kinase involved in neurite outgrowth and neurotransmitter release. In MIN6 cells, BRSK2 co-localizes with PCTAIRE1 in the cytoplasm and phosphorylates one of its serine residues, Ser-12. Phosphorylation of PCTAIRE1 by BRSK2 reduces glucose-stimulated insulin secretion (GSIS) in MIN6 cells. Conversely, knockdown of BRSK2 by siRNA increases serum insulin levels in mice. Our results reveal a novel function of BRSK2 in the regulation of GSIS in β-cells via a PCTAIRE1-dependent mechanism and suggest that BRSK2 is an attractive target for developing novel diabetic drugs.     

Metabolic consequence of long-term exposure of pancreatic beta cells to free fatty acid with special reference to glucose insensitivity.

Long-term exposure of the pancreatic beta cells to free fatty acid (FFA) reportedly inhibits glucose-stimulated insulin secretion. We here studied the impact of FFA on glucose and lipid metabolism in pancreatic beta cells with special reference to insulin secretion. Pancreatic beta-cell line MIN6 was exposed to various concentrations of palmitate for 3 days. Glucose-stimulated insulin secretion and insulin content were decreased corresponding to the concentration of the palmitate exposed. Glycolytic flux and ATP synthesis was unchanged, but pyruvate-stimulated change in NAD(P)H concentration was decreased. Pyruvate carboxylase was decreased at the protein level, which was restored by the removal of palmitate or the inhibition of beta-oxidation. Intracellular content of triglyceride and FFA were elevated, beta-oxidation was increased, and de novo lipogenesis from glucose was decreased. NADPH content and citrate output into the medium, which reflected pyruvate malate shuttle flux, were decreased, but malic enzyme activity was unaffected. The malic enzyme inhibitor alone inhibited insulin response to glucose. In conclusion, long-term exposure of FFA to beta cells inhibits glucose-stimulated insulin secretion via the decreased NADPH contents due to the inhibition of pyruvate carboxylase and malate pyruvate shuttle flux.     

Mitochondrial functional state in clonal pancreatic beta-cells exposed to free fatty acids

Excessive free fatty acid (FFA) exposure represents a potentially important diabetogenic condition that can impair insulin secretion from pancreatic beta-cells. Because mitochondrial oxidative phosphorylation is a main link between glucose metabolism and insulin secretion, in the present work we investigated the effects of the FFA oleate (OE) on mitochondrial function in the clonal pancreatic beta-cell line, MIN6. Both the long term (72 h) and short term (immediately after application) impact of OE exposure on beta-cells was investigated. After 72 h of exposure to OE (0.4 mm, 0.5% bovine serum albumin) cells were washed and permeabilized, and mitochondrial function (respiration, phosphorylation, membrane potential formation, production of reactive oxygen species) was measured in the absence or presence of OE. MIN6 cells exposed to OE for 72 h showed impaired glucose-stimulated insulin secretion and decreased cellular ATP. Mitochondria in OE-exposed cells retained normal functional characteristics in FFA-free medium; however, they were significantly more sensitive to the acute uncoupling effect of OE treatment. The mitochondria of OE-exposed cells displayed increased depolarization caused by acute OE treatment, which is attributable to the elevation in the FFA-transporting function of uncoupling protein 2 and the dicarboxylate carrier. These cells also had an increased production of reactive oxygen species in complex I of the mitochondrial respiratory chain that could be activated by FFA. A high level of reduction of respiratory complex I augmented acute FFA-induced uncoupling in a way compatible with activation of mitochondrial uncoupling protein by intramitochondrial superoxide. A stronger augmentation was observed in OE-exposed cells. Together, these events may underlie FFA-induced depression of the ATP/ADP ratio in beta-cells, which accounts for the defective glucose-stimulated insulin secretion associated with lipotoxicity.     

YES,a Src Family Kinase,Is a Proximal Glucose-specific Activator of Cell Division Cycle Control Protein 42 (Cdc42) in Pancreatic Islet β Cells

Second-phase insulin secretion sustains insulin release in the face of hyperglycemia associated with insulin resistance, requiring the continued mobilization of insulin secretory granules to the plasma membrane. Cdc42, the small Rho family GTPase recognized as the proximal glucose-specific trigger to elicit second-phase insulin secretion, signals downstream to activate the p21-activated kinase (PAK1), which then signals to Raf-1/MEK/ERK to induce filamentous actin (F-actin) remodeling, to ultimately mobilize insulin granules to the plasma membrane. However, the steps required to initiate Cdc42 activation in a glucose-specific manner in β cells have remained elusive. Toward this, we identified the involvement of the Src family kinases (SFKs), based upon the ability of SFK inhibitors to block glucose-stimulated Cdc42 and PAK1 activation events as well as the amplifying pathway of glucose-stimulated insulin release, in MIN6 β cells. Indeed, subsequent studies performed in human islets revealed that SFK phosphorylation was induced only by glucose and within 1 min of stimulation before the activation of Cdc42 at 3 min. Furthermore, pervanadate treatment validated the phosphorylation event to be tyrosine-specific. Although RT-PCR showed β cells to express five different SFK proteins, only two of these, YES and Fyn kinases, were found localized to the plasma membrane, and of these two, only YES kinase underwent glucose-stimulated tyrosine phosphorylation. Immunodetection and RNAi analyses further established YES kinase as a proximal glucose-specific signal in the Cdc42-signaling cascade. Identification of YES kinase provides new insight into the mechanisms underlying the sustainment of insulin secretion via granule mobilization/replenishment and F-actin remodeling.     

Expression of CD38 with intracellular enzymatic activity: a possible explanation for the insulin release induced by intracellular cADPR

CD38 is a transmembrane glycoprotein expressed in multiple cell types, including pancreatic β cells. It can serve as an enzyme that catalyzes the metabolism of two different Ca(2+)-mobilizing compounds, cyclic adenosine diphosphoribose (cADPR) and nicotinic acid adenine dinucleotide phosphate. One of these metabolites, cADPR, is known to be involved in glucose-induced insulin secretion from pancreatic β cells. Although the essential role of CD38 for endogenous cADPR synthesis has been established, the relationship between the proposed extracellular enzymatic activity of CD38 and the intracellular Ca(2+) modulation caused by the intracellular cADPR accumulation has not yet been fully explained. For a better understanding of the role of CD38 in the insulin secretion machinery, analysis of the intracellular localization of this molecule in pancreatic β cells is essential. In an attempt to provide a method to probe the N-terminal and C-terminal of CD38 separately, we generated an insulin-secreting MIN6 murine pancreatic β cell line expressing a human CD38 bearing an N-terminal FLAG epitope tag. We found a weak but consistent expression of the FLAG epitope outside of the cells, indicating the presence of a small amount of CD38 with cytoplasmic enzymatic activity. MIN6 cells transfected with human CD38 exhibited increased glucose-induced insulin release. In addition, anti-FLAG cross-linking further enhanced the insulin release, suggesting that the N-terminal of CD38 expressed on the cell surface functions as a receptor for an unknown ligand and triggers positive signals for insulin secretion.     

Microarray Analysis of Novel Candidate Genes Responsible for Glucose-Stimulated Insulin Secretion in Mouse Pancreatic β Cell Line MIN6

Elucidating the regulation of glucose-stimulated insulin secretion (GSIS) in pancreatic islet β cells is important for understanding and treating diabetes. MIN6 cells, a transformed β-cell line derived from a mouse insulinoma, retain GSIS and are a popular in vitro model for insulin secretion. However, in long-term culture, MIN6 cells'' GSIS capacity is lost. We previously isolated a subclone, MIN6 clone 4, from the parental MIN6 cells, that shows well-regulated insulin secretion in response to glucose, glybenclamide, and KCl, even after prolonged culture. To investigate the molecular mechanisms responsible for preserving GSIS in this subclone, we compared four groups of MIN6 cells: Pr-LP (parental MIN6, low passage number), Pr-HP (parental MIN6, high passage number), C4-LP (MIN6 clone 4, low passage number), and C4-HP (MIN6 clone 4, high passage number). Based on their capacity for GSIS, we designated the Pr-LP, C4-LP, and C4-HP cells as “responder cells.” In a DNA microarray analysis, we identified a group of genes with high expression in responder cells (“responder genes”), but extremely low expression in the Pr-HP cells. Another group of genes (“non-responder genes”) was expressed at high levels in the Pr-HP cells, but at extremely low levels in the responder cells. Some of the responder genes were involved in secretory machinery or glucose metabolism, including Chrebp, Scgn, and Syt7. Among the non-responder genes were Car2, Maf, and Gcg, which are not normally expressed in islet β cells. Interestingly, we found a disproportionate number of known imprinted genes among the responder genes. Our findings suggest that the global expression profiling of GSIS-competent and GSIS-incompetent MIN6 cells will help delineate the gene regulatory networks for insulin secretion.     

Transcellular neuroligin-2 interactions enhance insulin secretion and are integral to pancreatic β cell function

Normal glucose-stimulated insulin secretion is dependent on interactions between neighboring β cells. Elucidation of the reasons why this cell-to-cell contact is essential will probably yield critical insights into β cell maturation and function. In the central nervous system, transcellular protein interactions (i.e. interactions between proteins on the surfaces of different cells) involving neuroligins are key mediators of synaptic functional development. We previously demonstrated that β cells express neuroligin-2 and that insulin secretion is affected by changes in neuroligin-2 expression. Here we show that the effect of neuroligin-2 on insulin secretion is mediated by transcellular interactions. Neuroligin-2 binds with nanomolar affinity to a partner on the β cell surface and contributes to the increased insulin secretion brought about by β cell-to-β cell contact. It does so in a manner seemingly independent of interactions with neurexin, a known binding partner. As in the synapse, transcellular neuroligin-2 interactions enhance the functioning of the submembrane exocytic machinery. Also, as in the synapse, neuroligin-2 clustering is important. Neuroligin-2 in soluble form, rather than presented on a cell surface, decreases insulin secretion by rat islets and MIN-6 cells, most likely by interfering with endogenous neuroligin interactions. Prolonged contact with neuroligin-2-expressing cells increases INS-1 β cell proliferation and insulin content. These results extend the known parallels between the synaptic and β cell secretory machineries to extracellular interactions. Neuroligin-2 interactions are one of the few transcellular protein interactions thus far identified that directly enhance insulin secretion. Together, these results indicate a significant role for transcellular neuroligin-2 interactions in the establishment of β cell function.     

5'-AMP-activated protein kinase controls insulin-containing secretory vesicle dynamics

Changes in 5'-AMP-activated protein kinase (AMPK) activity have recently been implicated in the control of insulin secretion by glucose (da Silva Xavier, G., Leclerc, I., Varadi, A., Tsuboi, T., Moule, S. K., and Rutter, G. A. (2003) Biochem. J. 371, 761-774). Here, we examine the possibility that activation of AMPK may regulate distal steps in insulin secretion, including vesicle movement and fusion with the plasma membrane. Vesicle dynamics were imaged in single pancreatic MIN6 beta-cells expressing lumen-targeted pH-insensitive yellow fluorescent protein, neuropeptide Y.Venus, or monomeric red fluorescent protein by total internal reflection fluorescence and Nipkow disc confocal microscopy. Overexpression of a truncated, constitutively active form of AMPK (AMPKalpha1, 1-312, T172D; AMPK CA), inhibited glucose-stimulated (30 versus 3.0 mM) vesicle movements, and decreased the number of vesicles docked or fusing at the plasma membrane, while having no effect on the kinetics of individual secretory events. Expression of the activated form of AMPK also prevented dispersal of the cortical actin network at high glucose concentrations. Monitored in permeabilized cells, where the effects of AMPK CA on glucose metabolism and ATP synthesis were bypassed, AMPK CA inhibited Ca2+ and ATP-induced insulin secretion, and decreased ATP-dependent vesicle movements. These findings suggest that components of the vesicle transport network, including vesicle-associated motor proteins, may be targets of AMPK in beta-cells, dephosphorylation of which is required for vesicle mobilization at elevated glucose concentrations.