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

GDP-bound Rab27a regulates clathrin disassembly through HSPA8 after insulin secretion

Clathrin-dependent endocytosis is a key process for secretory cells, in which molecules on the plasma membrane are both degraded and recycled in a stimulus-dependent manner. There are many reports showing that disruption of endocytosis is involved in the onset of various diseases. Recently, it has been reported that such disruption in pancreatic β-cells causes impaired insulin secretion and might be associated with the pathology of diabetes mellitus. Compared with exocytosis, there are few reports on the molecular mechanism of endocytosis in pancreatic β-cells. We previously reported that GDP-bound Rab27a regulates endocytosis through its GDP-dependent effectors after insulin secretion. In this study, we identified heat shock protein family A member 8 (HSPA8) as a novel interacting protein for GDP-bound Rab27a. HSPA8 directly bound GDP-bound Rab27a via the β2 region of its substrate binding domain (SBD). The β2 fragment was capable of inhibiting the interaction between HSPA8 and GDP-bound Rab27a, and suppressed glucose-induced clathrin-dependent endocytosis in pancreatic β-cells. The region also affected clathrin dynamics on purified clathrin-coated vesicles (CCVs). These results suggest that the interaction between GDP-bound Rab27a and HSPA8 regulates clathrin disassembly from CCVs and subsequent vesicle transport. The regulatory stages in endocytosis by HSPA8 differ from those for other GDP-bound Rab27a effectors. This study shows that GDP-bound Rab27a dominantly regulates each stage in glucose-induced endocytosis through its specific effectors in pancreatic β-cells.

GDP-Bound Rab27a Dissociates from the Endocytic Machinery in a Phosphorylation-Dependent Manner after Insulin Secretion

Glucose-stimulated insulin secretion is controlled by both exocytosis and endocytosis in pancreatic β-cells. Although endocytosis is a fundamental step to maintain cellular responses to the secretagogue, the molecular mechanism of endocytosis remains poorly defined. We have previously shown that in response to high concentrations of glucose, guanosine 5'-diphosphate (GDP)-bound Rab27a is recruited to the plasma membrane where IQ motif-containing guanosine 5'-triphosphatase (GTPase)-activating protein 1 (IQGAP1) is expressed, and that complex formation promotes endocytosis of secretory membranes after insulin secretion. In the present study, the regulatory mechanisms of dissociation of the complex were investigated. Phosphorylation of IQGAP1 on serine (Ser)-1443, a site recognized by protein kinase Cε (PKCε), inhibited the interaction of GDP-bound Rab27a with IQGAP1 in a Cdc42-independent manner. Glucose stimulation caused a translocation of PKCε from the cytosol to the plasma membrane. In addition, glucose-induced endocytosis was inhibited by the knockdown of IQGAP1 with small interfering RNA (siRNA). However, the expression of the non-phosphorylatable or phosphomimetic form of IQGAP1 could not rescue the inhibition, suggesting that a phosphorylation-dephosphorylation cycle of IQGAP1 is required for endocytosis. These results suggest that IQGAP1 phosphorylated by PKCε promotes the dissociation of the IQGAP1-GDP-bound Rab27a complex in pancreatic β-cells, thereby regulating endocytosis of secretory membranes following insulin secretion.

Advances in understanding the role of disease-associated proteins in spinal muscular atrophy

INTRODUCTION

Spinal muscular atrophy (SMA) is a neurodegenerative disorder characterized by alpha motor neuron loss in the spinal cord due to reduced survival motor neuron (SMN) protein level. While the genetic basis of SMA is well described, the specific molecular pathway underlying SMA is still not fully understood. Areas covered: This review discusses the recent advancements in understanding the molecular pathways in SMA using different omics approaches and genetic modifiers identified in both vertebrate and invertebrate systems. The findings that are summarized in this article were deduced from original articles and reviews with a particular focus on the latest advancements in the field. Expert commentary: The identification of genetic modifiers such as PLS3 and NCALD in humans or of SMA modulators such as Elavl4 (HuD), Copa, Uba1, Mapk10 (Jnk3), Nrxn2 and Tmem41b (Stasimon) in various SMA animal models improved our knowledge of impaired cellular pathways in SMA. Inspiration from modifier genes and their functions in motor neuron and neuromuscular junctions may open a new avenue for future SMA combinatorial therapies.

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

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

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

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

Interplay between Rab27a effectors in pancreatic β-cells

The small GTPase Rab27a is a member of the Rab family that is involved in membrane trafficking in various kinds of cells. Rab27a has GTP- and GDP-bound forms, and their interconversion regulates intracellular signaling pathways. Typically, only a GTP-bound GTPase binds its specific effectors with the resulting downstream signals controlling specific cellular functions. We previously identified novel Rab27a-interacting proteins. Surprisingly, some of these proteins interacted with GDP-bound Rab27a. The present study reviews recent progress in our understanding of the roles of Rab27a and its effectors in the secretory process. In pancreatic β-cells, GTP-bound Rab27a regulates insulin secretion at the pre-exocytotic stages via its GTP-specific effectors such as Exophilin8/Slac2-c/MyRIP and Slp4/Granuphilin. Glucose stimulation causes insulin exocytosis. Glucose stimulation also converts Rab27a from its GTP- to its GDP-bound form. GDP-bound Rab27a interacts with GDP-specific effectors and controls endocytosis of the secretory membrane. Thus, Rab27a cycling between GTP- and GDP-bound forms synchronizes with the recycling of secretory membrane to re-use the membrane and keep the β-cell volume constant.

HAP1 helps to regulate actin-based transport of insulin-containing granules in pancreatic β cells

Huntingtin-associated protein 1 (HAP1) is enriched in neurons and binds to polyglutamine-expanded huntingtin. It consists of two alternatively spliced isoforms, HAP1A and HAP1B, which differ only in their short C-terminal sequences. Both HAP1A and HAP1B have been also detected in pancreatic β cells, where the loss of HAP1 impairs glucose-stimulated insulin secretion. Here, we use time-lapse laser scanning confocal microscopy to provide direct evidence that HAP1A, but not HAP1B, co-localizes and co-migrates with insulin-containing vesicles and actin-based myosin Va motor protein in the INS-1 pancreatic β cell line. Knocking down HAP1 expression using small interfering RNA significantly inhibited actin-based transport of insulin vesicles following glucose stimulation. Co-immunoprecipitation experiments demonstrated interaction between HAP1A, myosin Va, and phogrin, a transmembrane protein in insulin-containing vesicles. Stimulating INS-1 cells with glucose increased the association of HAP1A with myosin Va, while silencing HAP1 expression reduced the association of myosin Va with phogrin after glucose stimulation, without affecting levels of myosin Va or actin. Our results provide real-time evidence in living cells that HAP1 may help regulate transport of insulin-containing secretory granules along cortical actin filaments. This also raises the possibility that HAP1 may play an important role in actin-based secretory vesicle trafficking in neurons.

Activated Cdc42-bound IQGAP1 determines the cellular endocytic site

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

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

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

Rab27a, actin and beta-cell endocytosis

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

Rab27a negatively regulates phagocytosis by prolongation of the actin-coating stage around phagosomes

Rab27a, a Rab family small GTPase, is involved in the exocytosis of secretory granules in melanocytes and cytotoxic T-cells. Rab27a mutations cause type 2 Griscelli syndrome, which is characterized by immunodeficiency, including uncontrolled macrophage activation known as hemophagocytic syndrome. However, the role of Rab27a in phagocytosis remains elusive. Here, using macrophage-like differentiated HL-60 cells and C3bi-opsonized zymosan as a pathogen-phagocyte model, we show that Rab27a negatively regulates complement-mediated phagocytic activity in association with F-actin remodeling. We found that transfection of Rab27a shRNA into HL-60 cells enhances complement-mediated phagocytosis. To clarify the mechanisms underlying the elevated phagocytosis in Rab27a knockdown cells, we analyzed the process of phagosome formation focusing on F-actin dynamics: F-actin assembly, followed by F-actin extension around the particles and the subsequent degradation of F-actin, leading to internalization of the particles enclosed in phagosomes. Microscopic analysis revealed that these actin-related processes, including F-actin coating and F-actin degradation, proceed more rapidly in Rab27a knockdown cells than in control HL-60 cells. Both elevated phagocytosis and accelerated F-actin remodeling were restored by expression of rescue-Rab27a and Rab27a-Q78L (GTP-bound form), but not by Rab27a-T23N (GDP-bound form). Furthermore, an increased accumulation of Coronin 1A surrounding F-actin coats was observed in Rab27a knockdown cells, suggesting that the function of Coronin 1A is related to the regulation of the F-actin coating. Our findings demonstrate that Rab27a plays a direct regulatory role in the nascent process of phagocytosis by prolongation of the stage of actin coating via suppression of Coronin 1A. This study may contribute to an explanation of the underlying mechanisms of excessive phagocytosis observed in Griscelli syndrome.