The Small GTPase RalA controls exocytosis of large dense core secretory granules by interacting with ARF6-dependent phospholipase D1

Epigenetic profiling reveals key genes and cis-regulatory networks specific to human parathyroids

In all terrestrial vertebrates, the parathyroid glands are critical regulators of calcium homeostasis and the sole source of parathyroid hormone (PTH). Hyperparathyroidism and hypoparathyroidism are clinically important disorders affecting multiple organs. However, our knowledge regarding regulatory mechanisms governing the parathyroids has remained limited. Here, we present the comprehensive maps of the chromatin landscape of the human parathyroid glands, identifying active regulatory elements and chromatin interactions. These data allow us to define regulatory circuits and previously unidentified genes that play crucial roles in parathyroid biology. We experimentally validate candidate parathyroid-specific enhancers and demonstrate their integration with GWAS SNPs for parathyroid-related diseases and traits. For instance, we observe reduced activity of a parathyroid-specific enhancer of the Calcium Sensing Receptor gene, which contains a risk allele associated with higher PTH levels compared to the wildtype allele. Our datasets provide a valuable resource for unraveling the mechanisms governing parathyroid gland regulation in health and disease.

Drebrin Regulates Collateral Axon Branching in Cortical Layer II/III Somatosensory Neurons

Proper cortical lamination is essential for cognition, learning, and memory. Within the somatosensory cortex, pyramidal excitatory neurons elaborate axon collateral branches in a laminar-specific manner that dictates synaptic partners and overall circuit organization. Here, we leverage both male and female mouse models, single-cell labeling and imaging approaches to identify intrinsic regulators of laminar-specific collateral, also termed interstitial, axon branching. We developed new approaches for the robust, sparse, labeling of Layer II/III pyramidal neurons to obtain single-cell quantitative assessment of axon branch morphologies. We combined these approaches with cell-autonomous loss-of-function (LOF) and overexpression (OE) manipulations in an in vivo candidate screen to identify regulators of cortical neuron axon branch lamination. We identify a role for the cytoskeletal binding protein drebrin (Dbn1) in regulating Layer II/III cortical projection neuron (CPN) collateral axon branching in vitro LOF experiments show that Dbn1 is necessary to suppress the elongation of Layer II/III CPN collateral axon branches within Layer IV, where axon branching by Layer II/III CPNs is normally absent. Conversely, Dbn1 OE produces excess short axonal protrusions reminiscent of nascent axon collaterals that fail to elongate. Structure-function analyses implicate Dbn1S142 phosphorylation and Dbn1 protein domains known to mediate F-actin bundling and microtubule (MT) coupling as necessary for collateral branch initiation upon Dbn1 OE. Taken together, these results contribute to our understanding of the molecular mechanisms that regulate collateral axon branching in excitatory CPNs, a key process in the elaboration of neocortical circuit formation.SIGNIFICANCE STATEMENT Laminar-specific axon targeting is essential for cortical circuit formation. Here, we show that the cytoskeletal protein drebrin (Dbn1) regulates excitatory Layer II/III cortical projection neuron (CPN) collateral axon branching, lending insight into the molecular mechanisms that underlie neocortical laminar-specific innervation. To identify branching patterns of single cortical neurons in vivo, we have developed tools that allow us to obtain detailed images of individual CPN morphologies throughout postnatal development and to manipulate gene expression in these same neurons. Our results showing that Dbn1 regulates CPN interstitial axon branching both in vivo and in vitro may aid in our understanding of how aberrant cortical neuron morphology contributes to dysfunctions observed in autism spectrum disorder and epilepsy.

V-ATPase modulates exocytosis in neuroendocrine cells through the activation of the ARNO-Arf6-PLD pathway and the synthesis of phosphatidic acid

Although there is mounting evidence indicating that lipids serve crucial functions in cells and are implicated in a growing number of human diseases, their precise roles remain largely unknown. This is particularly true in the case of neurosecretion, where fusion with the plasma membrane of specific membrane organelles is essential. Yet, little attention has been given to the role of lipids. Recent groundbreaking research has emphasized the critical role of lipid localization at exocytotic sites and validated the essentiality of fusogenic lipids, such as phospholipase D (PLD)-generated phosphatidic acid (PA), during membrane fusion. Nevertheless, the regulatory mechanisms synchronizing the synthesis of these key lipids and neurosecretion remain poorly understood. The vacuolar ATPase (V-ATPase) has been involved both in vesicle neurotransmitter loading and in vesicle fusion. Thus, it represents an ideal candidate to regulate the fusogenic status of secretory vesicles according to their replenishment state. Indeed, the cytosolic V1 and vesicular membrane-associated V0 subdomains of V-ATPase were shown to dissociate during the stimulation of neurosecretory cells. This allows the subunits of the vesicular V0 to interact with different proteins of the secretory machinery. Here, we show that V0a1 interacts with the Arf nucleotide-binding site opener (ARNO) and promotes the activation of the Arf6 GTPase during the exocytosis in neuroendocrine cells. When the interaction between V0a1 and ARNO was disrupted, it resulted in the inhibition of PLD activation, synthesis of phosphatidic acid during exocytosis, and changes in the timing of fusion events. These findings indicate that the separation of V1 from V0 could function as a signal to initiate the ARNO-Arf6-PLD1 pathway and facilitate the production of phosphatidic acid, which is essential for effective exocytosis in neuroendocrine cells.

Membrane interaction to intercellular spread of pathology in Alzheimer’s disease

Progressive development of pathology is one of the major characteristic features of neurodegenerative diseases. Alzheimer’s disease (AD) is the most prevalent among them. Extracellular amyloid-β (Aβ) plaques and intracellular tau neurofibrillary tangles are the pathological phenotypes of AD. However, cellular and animal studies implicate tau as a secondary pathology in developing AD while Aβ aggregates is considered as a trigger point. Interaction of Aβ peptides with plasma membrane (PM) seems to be a promising site of involvement in the events that lead to AD. Aβ binding to the lipid membranes initiates formation of oligomers of Aβ species, and these oligomers are known as primary toxic agents for neuronal toxicities. Once initiated, neuropathological toxicities spread in a “prion-like” fashion probably through the mechanism of intercellular transfer of pathogenic aggregates. In the last two decades, several studies have demonstrated neuron-to-neuron transfer of neurodegenerative proteins including Aβ and tau via exosomes and tunneling nanotubes (TNTs), the two modes of long-range intercellular transfer. Emerging pieces of evidence indicate that molecular pathways related to the biogenesis of exosomes and TNTs interface with endo-lysosomal pathways and cellular signaling in connection to vesicle recycling-imposed PM and actin remodulation. In this review, we discuss interactions of Aβ aggregates at the membrane level and its implications in intercellular spread of pathogenic aggregates. Furthermore, we hypothesize how spread of pathogenic aggregates contributes to complex molecular events that could regulate pathological and synaptic changes related to AD.

RILP inhibits proliferation, migration, and invasion of PC3 prostate cancer cells

RILP (Rab-interacting lysosomal protein) is a key regulator of lysosomal transport and a potential tumor suppressor. However, the role of RILP in prostate cancer and the underlying mechanism of RILP in regulating the proliferation, migration, and invasion of prostate cancer cells remain to be studied. In this study, we confirmed RalGDS (Ral guanine nucleotide dissociation stimulator) as the interaction partner of RILP in PC3 prostate cancer cells. Immunofluorescence microscopy showed that RILP recruits RalGDS to the lysosomal compartment. We found that RILP inhibits the activation of RalA and downstream effector RalBP1, and negatively regulates the downstream molecular phosphorylation of Ras. We showed that RILP inhibits the proliferation, migration, and invasion of PC3 prostate cancer cells, which may give rise to novel ideas for cancer treatment.

The RAL Enigma: Distinct Roles of RALA and RALB in Cancer

RALA and RALB are highly homologous small G proteins belonging to the RAS superfamily. Like other small GTPases, the RALs are molecular switches that can be toggled between inactive GDP-bound and active GTP-bound states to regulate diverse and critical cellular functions such as vesicle trafficking, filopodia formation, mitochondrial fission, and cytokinesis. The RAL paralogs are activated and inactivated by a shared set of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) and utilize similar sets of downstream effectors. In addition to their important roles in normal cell biology, the RALs are known to be critical mediators of cancer cell survival, invasion, migration, and metastasis. However, despite their substantial similarities, the RALs often display striking functional disparities in cancer. RALA and RALB can have redundant, unique, or even antagonistic functions depending on cancer type. The molecular basis for these discrepancies remains an important unanswered question in the field of cancer biology. In this review we examine the functions of the RAL paralogs in normal cellular physiology and cancer biology with special consideration provided to situations where the roles of RALA and RALB are non-redundant.

RalA and PLD1 promote lipid droplet growth in response to nutrient withdrawal

Lipid droplets (LDs) are dynamic organelles that undergo dynamic changes in response to changing cellular conditions. During nutrient depletion, LD numbers increase to protect cells against toxic fatty acids generated through autophagy and provide fuel for beta-oxidation. However, the precise mechanisms through which these changes are regulated have remained unclear. Here, we show that the small GTPase RalA acts downstream of autophagy to directly facilitate LD growth during nutrient depletion. Mechanistically, RalA performs this function through phospholipase D1 (PLD1), an enzyme that converts phosphatidylcholine (PC) to phosphatidic acid (PA) and that is recruited to lysosomes during nutrient stress in a RalA-dependent fashion. RalA inhibition prevents recruitment of the LD-associated protein perilipin 3, which is required for LD growth. Our data support a model in which RalA recruits PLD1 to lysosomes during nutrient deprivation to promote the localized production of PA and the recruitment of perilipin 3 to expanding LDs.

Structure and regulation of human phospholipase D

Mammalian phospholipase D (PLD) generates phosphatidic acid, a dynamic lipid secondary messenger involved with a broad spectrum of cellular functions including but not limited to metabolism, migration, and exocytosis. As a promising pharmaceutical target, the biochemical properties of PLD have been well characterized. This has led to the recent crystal structures of human PLD1 and PLD2, the development of PLD specific pharmacological inhibitors, and the identification of cellular regulators of PLD. In this review, we discuss the PLD1 and PLD2 structures, PLD inhibition by small molecules, and the regulation of PLD activity by effector proteins and lipids.

Ral GTPases promote breast cancer metastasis by controlling biogenesis and organ targeting of exosomes

Cancer extracellular vesicles (EVs) shuttle at distance and fertilize pre-metastatic niches facilitating subsequent seeding by tumor cells. However, the link between EV secretion mechanisms and their capacity to form pre-metastatic niches remains obscure. Using mouse models, we show that GTPases of the Ral family control, through the phospholipase D1, multi-vesicular bodies homeostasis and tune the biogenesis and secretion of pro-metastatic EVs. Importantly, EVs from RalA or RalB depleted cells have limited organotropic capacities in vivoand are less efficient in promoting metastasis. RalA and RalB reduce the EV levels of the adhesion molecule MCAM/CD146, which favors EV-mediated metastasis by allowing EVs targeting to the lungs. Finally, RalA, RalB, and MCAM/CD146, are factors of poor prognosis in breast cancer patients. Altogether, our study identifies RalGTPases as central molecules linking the mechanisms of EVs secretion and cargo loading to their capacity to disseminate and induce pre-metastatic niches in a CD146-dependent manner.

Phosphatidic acid: Mono- and poly-unsaturated forms regulate distinct stages of neuroendocrine exocytosis

Lipids have emerged as important actors in an ever-growing number of key functions in cell biology over the last few years. Among them, glycerophospholipids are major constituents of cellular membranes. Because of their amphiphilic nature, phospholipids form lipid bilayers that are particularly useful to isolate cellular content from the extracellular medium, but also to define intracellular compartments. Interestingly, phospholipids come in different flavors based on their fatty acyl chain composition. Indeed, lipidomic analyses have revealed the presence in cellular membranes of up to 50 different species of an individual class of phospholipid, opening the possibility of multiple functions for a single class of phospholipid. In this review we will focus on phosphatidic acid (PA), the simplest phospholipid, that plays both structural and signaling functions. Among the numerous roles that have been attributed to PA, a key regulatory role in secretion has been proposed in different cell models. We review here the evidences that support the idea that mono- and poly-unsaturated PA control distinct steps in hormone secretion from neuroendocrine cells.

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

Phosphatidic acid metabolism regulates neuroendocrine secretion but is not under the direct control of lipins

Phosphatidic acid (PA) produced by phospholipase D1 has been shown to contribute to secretory vesicle exocytosis in a large number of cell models. Among various hypotheses, PA may contribute to recruit and/or activate at the exocytotic site a set of proteins from the molecular machinery dedicated to secretion, but also directly influence membrane curvature thereby favoring membrane rearrangements required for membrane fusion. The release of informative molecules by regulated exocytosis is a tightly controlled process. It is thus expected that PA produced to trigger membrane fusion should be rapidly metabolized and converted in a lipid that does not present similar characteristics. PA-phosphatases of the lipin family are possible candidates as they convert PA into diacylglycerol. We show here that lipin 1 and lipin 2 are expressed in neuroendocrine cells where they are cytosolic, but also partially associated with the endoplasmic reticulum. Silencing of lipin 1 or 2 did not affect significantly either basal or evoked secretion from PC12 cells, suggesting that it is unlikely that conversion of PA into a secondary lipid by lipins might represent a regulatory step in exocytosis in neurosecretory cells. However, in agreement with a model in which PA-metabolism could contribute to prevent entering into exocytosis of additional secretory vesicles, ectopic expression of lipin1B-GFP in bovine chromaffin cells reduced the number of exocytotic events as revealed by carbon fiber amperometry recording. Furthermore, individual spike parameters reflecting fusion pore dynamics were also modified by lipin1B-GFP, suggesting that a tight control of PA levels represents an important regulatory step of the number and kinetic of exocytotic events.

Role of Phospholipase D-Derived Phosphatidic Acid in Regulated Exocytosis and Neurological Disease

Lipids play a vital role in numerous cellular functions starting from a structural role as major constituents of membranes to acting as signaling intracellular or extracellular entities. Accordingly, it has been known for decades that lipids, especially those coming from diet, are important to maintain normal physiological functions and good health. On the other side, the exact molecular nature of these beneficial or deleterious lipids, as well as their precise mode of action, is only starting to be unraveled. This recent improvement in our knowledge is largely resulting from novel pharmacological, molecular, cellular, and genetic tools to study lipids in vitro and in vivo. Among these important lipids, phosphatidic acid plays a unique and central role in a great variety of cellular functions. This review will focus on the proposed functions of phosphatidic acid generated by phospholipase D in the last steps of regulated exocytosis with a specific emphasis on hormonal and neurotransmitter release and its potential impact on different neurological diseases.

Synaptotagmin 5 regulates Ca2+-dependent Weibel-Palade body exocytosis in human endothelial cells

Elevations of intracellular free Ca²⁺ concentration ([Ca²⁺]_i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of von Willebrand factor (VWF) from endothelial cells; however, the identity of WPB-associated Ca²⁺-sensors involved in transducing acute increases in [Ca²⁺]_i into granule exocytosis remains unknown. Here, we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVECs) and is recruited to WPBs to regulate Ca²⁺-driven WPB exocytosis. Western blot analysis of HUVECs identified SYT5 protein, and exogenously expressed SYT5-mEGFP localised almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 (shSYT5) reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca²⁺-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca²⁺ sensor regulating Ca²⁺-dependent secretion of stored mediators from vascular endothelial cells.

Pituitary somatolactotropes evade an oncogenic response to Ras

Distinct cell types have been shown to respond to activated Ras signaling in a cell-specific manner. In contrast to its pro-tumorigenic role in some human epithelial cancers, oncogenic Ras triggers differentiation of pheochromocytoma cells and medullary thyroid carcinoma cells. Furthermore, we have previously demonstrated that in pituitary somatolactotropes, activated Ras promotes differentiation and is not sufficient to drive tumorigenesis. These findings demonstrate that lactotrope cells have the ability to evade the tumorigenic fate that is often associated with persistent activation of Ras/ERK signaling, and suggest that there may be differential expression of inhibitory signaling molecules or negative cell cycle regulators that act as a brake to prevent the tumorigenic effects of sustained Ras signaling. Here we aim to gain further insight into the mechanisms that allow GH4T2 cells to evade an oncogenic response to Ras. We show that Ral, but likely not menin, plays a key role in directing Ras-mediated differentiation of somatolactotropes, which may allow these cells to escape the tumorigenic fate that is often associated with activated Ras signaling. We also show that dominant negative Ras expression results in reduced GH4T2 cell proliferation and transformation, but does not influence differentiation. Taken together, the data presented here begin to shed light on the mechanisms by which pituitary somatolactotropes evade an oncogenic response to persistently activated Ras signaling and suggest that the architecture of the Ras signaling cascade in some endocrine cell types may be distinct from that of cells that respond to Ras in an oncogenic manner.

Extracellular vesicles: lipids as key components of their biogenesis and functions

Intercellular communication has been known for decades to involve either direct contact between cells or to operate via circulating molecules, such as cytokines, growth factors, or lipid mediators. During the last decade, we have begun to appreciate the increasing importance of intercellular communication mediated by extracellular vesicles released by viable cells either from plasma membrane shedding (microvesicles, also named microparticles) or from an intracellular compartment (exosomes). Exosomes and microvesicles circulate in all biological fluids and can trigger biological responses at a distance. Their effects include a large variety of biological processes, such as immune surveillance, modification of tumor microenvironment, or regulation of inflammation. Extracellular vesicles can carry a large array of active molecules, including lipid mediators, such as eicosanoids, proteins, and nucleic acids, able to modify the phenotype of receiving cells. This review will highlight the role of the various lipidic pathways involved in the biogenesis and functions of microvesicles and exosomes.

Weibel-Palade bodies at a glance

The vascular environment can rapidly alter, and the speed with which responses to both physiological and pathological changes are required necessitates the existence of a highly responsive system. The endothelium can quickly deliver bioactive molecules by regulated exocytosis of its secretory granules, the Weibel-Palade bodies (WPBs). WPBs include proteins that initiate both haemostasis and inflammation, as well those that modulate blood pressure and angiogenesis. WPB formation is driven by von Willebrand factor, their most abundant protein, which controls both shape and size of WPBs. WPB are generated in a range of sizes, with the largest granules over ten times the size of the smallest. In this Cell Science at a Glance and the accompanying poster, we discuss the emerging mechanisms by which WPB size is controlled and how this affects the ability of this organelle to modulate haemostasis. We will also outline the different modes of exocytosis and their polarity that are currently being explored, and illustrate that these large secretory organelles provide a model for how elements of secretory granule biogenesis and exocytosis cooperate to support a complex and diverse set of functions.

The Small GTPase Ral orchestrates MVB biogenesis and exosome secretion

Extracellular vesicles are novel mediators of cell-cell communication. They are present in all species and involved in physiological and pathological processes. One class of extracellular vesicles, the exosomes, originate from an endosomal compartment, the MultiVesicular Body (MVB), and are released from the cell upon fusion of the MVB with the plasma membrane. Although different molecular mechanisms have been associated with MVB biogenesis and exosome secretion, how they coordinate remains poorly documented. We recently found that the small GTPase Ral contributes to exosome release in nematodes and mammalian tumor cells. More specifically, we found that C. elegans RAL-1 is required for the biogenesis of MVBs, and later for MVB fusion with the plasma membrane. Here, we discuss our results in relationship with other factors involved in extracellular vesicle production such as the ESCRT complex and Phospholipase 1D. We propose models to explain Ral function in exosome secretion, its conservation in animals, and its possible role in tumor progression.

Phosphatidic acid and neurotransmission

Lipids play a vital role in the health and functioning of neurons and interest in the physiological role of neuronal lipids is certainly increasing. One neuronal function in which neuronal lipids appears to play key roles in neurotransmission. Our understanding of the role of lipids in the synaptic vesicle cycle and neurotransmitter release is becoming increasingly more important. Much of the initial research in this area has highlighted the major roles played by the phosphoinositides (PtdIns), diacylglycerol (DAG), and phosphatidic acid (PtdOH). Of these, PtdOH has not received as much attention as the other lipids although its role and metabolism appears to be extremely important. This lipid has been shown to play a role in modulating both exocytosis and endocytosis although its precise role in either process is not well defined. The currently evidence suggest this lipid likely participates in key processes by altering membrane architecture necessary for membrane fusion, mediating the penetration of membrane proteins, serving as a precursor for other important SV cycling lipids, or activating essential enzymes. In this review, we address the sources of PtdOH, the enzymes involved in its production, the regulation of these enzymes, and its potential roles in neurotransmission in the central nervous system.

ADP Ribosylation Factor 6 Regulates Neuronal Migration in the Developing Cerebral Cortex through FIP3/Arfophilin-1-dependent Endosomal Trafficking of N-cadherin

During neural development, endosomal trafficking controls cell shape and motility through the polarized transport of membrane proteins related to cell–cell and cell–extracellular matrix interactions. ADP ribosylation factor 6 (Arf6) is a critical small GTPase that regulates membrane trafficking between the plasma membrane and endosomes. We herein demonstrated that the knockdown of endogenous Arf6 in mouse cerebral cortices led to impaired neuronal migration in the intermediate zone and cytoplasmic retention of N-cadherin and syntaxin12 in migrating neurons. Rescue experiments with separation-of-function Arf6 mutants identified Rab11 family-interacting protein 3 (FIP3)/Arfophilin-1, a dual effector for Arf6 and Rab11, as a downstream effector of Arf6 in migrating neurons. The knockdown of FIP3 led to impaired neuronal migration in the intermediate zone and cytoplasmic retention of N-cadherin in migrating neurons, similar to that of Arf6, which could be rescued by the coexpression of wild-type FIP3 but not FIP3 mutants lacking the binding site for Arf6 or Rab11. These results suggest that Arf6 regulates cortical neuronal migration in the intermediate zone through the FIP3-dependent endosomal trafficking.

Ral-Arf6 crosstalk regulates Ral dependent exocyst trafficking and anchorage independent growth signalling

Integrin dependent regulation of growth factor signalling confers anchorage dependence that is deregulated in cancers. Downstream of integrins and oncogenic Ras the small GTPase Ral is a vital mediator of adhesion dependent trafficking and signalling. This study identifies a novel regulatory crosstalk between Ral and Arf6 that controls Ral function in cells. In re-adherent mouse fibroblasts (MEFs) integrin dependent activation of RalA drives Arf6 activation. Independent of adhesion constitutively active RalA and RalB could both however activate Arf6. This is further conserved in oncogenic H-Ras containing bladder cancer T24 cells, which express anchorage independent active Ral that supports Arf6 activation. Arf6 mediates active Ral-exocyst dependent delivery of raft microdomains to the plasma membrane that supports anchorage independent growth signalling. Accordingly in T24 cells the RalB-Arf6 crosstalk is seen to preferentially regulate anchorage independent Erk signalling. Active Ral we further find uses a Ral-RalBP1-ARNO-Arf6 pathway to mediate Arf6 activation. This study hence identifies Arf6, through this regulatory crosstalk, to be a key downstream mediator of Ral isoform function along adhesion dependent pathways in normal and cancer cells.

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Ral mediates Arf6 activation downstream of integrins and oncogenic Ras.

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Arf6 mediates Ral-exocyst dependent delivery of raft microdomains.

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Active Ral supports anchorage independent Arf6 activation in bladder cancer T24 cells.

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Ral-Arf6 crosstalk in T24 cells regulates anchorage independent Erk signalling.

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A Ral-RalBP1-ARNO-Arf6 pathway mediates the Ral-Arf6 crosstalk.

RAL-1 controls multivesicular body biogenesis and exosome secretion

Exosomes are secreted vesicles arising from the fusion of multivesicular bodies (MVBs) with the plasma membrane. Despite their importance in various processes, the molecular mechanisms controlling their formation and release remain unclear. Using nematodes and mammary tumor cells, we show that Ral GTPases are involved in exosome biogenesis. In Caenorhabditis elegans, RAL-1 localizes at the surface of secretory MVBs. A quantitative electron microscopy analysis of RAL-1-deficient animals revealed that RAL-1 is involved in both MVB formation and their fusion with the plasma membrane. These functions do not involve the exocyst complex, a common Ral guanosine triphosphatase (GTPase) effector. Furthermore, we show that the target membrane SNARE protein SYX-5 colocalizes with a constitutively active form of RAL-1 at the plasma membrane, and MVBs accumulate under the plasma membrane when SYX-5 is absent. In mammals, RalA and RalB are both required for the secretion of exosome-like vesicles in cultured cells. Therefore, Ral GTPases represent new regulators of MVB formation and exosome release.

RILP suppresses invasion of breast cancer cells by modulating the activity of RalA through interaction with RalGDS

RILP (Rab7-interacting lysosomal protein) is a key regulator for late endosomal/lysosomal trafficking, and probably a tumor suppressor in prostate cancer. However, the role of RILP in other cancers and the underlying mechanism for RILP in regulating the invasion of cancer cells remain to be investigated. In this study, we showed that overexpression of RILP in breast cancer cells inhibits the migration and invasion, whereas the depletion of RILP by RNAi-mediated knockdown promotes the migration and invasion. We identified RalGDS (Ral guanine nucleotide dissociation stimulator) as a novel interacting partner for RILP, and truncation analysis revealed the N-terminal region of RILP is responsible for interacting with the guanine nucleotide exchange factor (GEF) domain of RalGDS. Immunofluorescence microscopy revealed that RalGDS can be recruited to the late endosomal compartments by RILP. Further investigations indicated that the overexpression of RILP inhibits the activity of RalA, a downstream target of RalGDS. Our data suggest that RILP suppresses the invasion of breast cancer cells by interacting with RalGDS to inhibit its GEF activity for RalA.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

The cortical acto-Myosin network: from diffusion barrier to functional gateway in the transport of neurosecretory vesicles to the plasma membrane

Dysregulation of regulated exocytosis is linked to an array of pathological conditions, including neurodegenerative disorders, asthma, and diabetes. Understanding the molecular mechanisms underpinning neuroexocytosis including the processes that allow neurosecretory vesicles to access and fuse with the plasma membrane and to recycle post-fusion, is therefore critical to the design of future therapeutic drugs that will efficiently tackle these diseases. Despite considerable efforts to determine the principles of vesicular fusion, the mechanisms controlling the approach of vesicles to the plasma membrane in order to undergo tethering, docking, priming, and fusion remain poorly understood. All these steps involve the cortical actin network, a dense mesh of actin filaments localized beneath the plasma membrane. Recent work overturned the long-held belief that the cortical actin network only plays a passive constraining role in neuroexocytosis functioning as a physical barrier that partly breaks down upon entry of Ca(2+) to allow secretory vesicles to reach the plasma membrane. A multitude of new roles for the cortical actin network in regulated exocytosis have now emerged and point to highly dynamic novel functions of key myosin molecular motors. Myosins are not only believed to help bring about dynamic changes in the actin cytoskeleton, tethering and guiding vesicles to their fusion sites, but they also regulate the size and duration of the fusion pore, thereby directly contributing to the release of neurotransmitters and hormones. Here we discuss the functions of the cortical actin network, myosins, and their effectors in controlling the processes that lead to tethering, directed transport, docking, and fusion of exocytotic vesicles in regulated exocytosis.

RalA GTPase tethers insulin granules to L- and R-type calcium channels through binding α2 δ-1 subunit

RalA GTPase has been implicated in the regulated delivery of exocytotic vesicles to the plasma membrane (PM) in mammalian cells. We had reported that RalA regulates biphasic insulin secretion, which we have now determined to be contributed by RalA direct interaction with voltage-gated calcium (Cav ) channels. RalA knockdown (KD) in INS-1 cells and primary rat β-cells resulted in a reduction in Ca(2+) currents arising specifically from L-(Cav 1.2 and Cav 1.3) and R-type (Cav 2.3) Ca(2+) channels. Restoration of RalA expression in RalA KD cells rescued these defects in Ca(2+) currents. RalA co-immunoprecipitated with the Cav α2 δ-1 auxiliary subunit known to bind the three Cav s. Moreover, the functional molecular interactions between Cav α2 δ-1 and RalA on the PM shown by total internal reflection fluorescent microscopy/FRET analysis could be induced by glucose stimulation. KD of RalA inhibited trafficking of α2 δ-1 to insulin granules without affecting the localization of the other Cav subunits. Furthermore, we confirmed that RalA and α2 δ-1 functionally interact since RalA KD-induced inhibition of Cav currents could not be recovered by RalA when α2 δ-1 was simultaneously knocked down. These data provide a mechanism for RalA function in insulin secretion, whereby RalA binds α2 δ-1 on insulin granules to tether these granules to PM Ca(2+) channels. This acts as a chaperoning step prior to and in preparation for sequential assembly of exocyst and excitosome complexes that mediate biphasic insulin 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 B subunits of Shiga-like toxins induce regulated VWF secretion in a phospholipase D1-dependent manner

Shiga toxin (Stx) causes diarrhea-associated hemolytic uremic syndrome by damaging renal microvascular endothelium. The pentameric B subunits of Stx types 1 and 2 (Stx1B and Stx2B) are sufficient to stimulate acute VWF secretion from endothelial cells, but Stx1B and Stx2B exert distinct effects on Ca(2+) and cAMP pathways. Therefore, we investigated other signaling components in StxB-induced VWF exocytosis. Incubation of HUVECs with StxB transiently increased phospholipase D (PLD) activity. Inhibition of PLD activity or shRNA-mediated PLD1 knockdown abolished StxB-induced VWF secretion. In addition, treatment with StxB triggered actin polymerization, enhanced endothelial monolayer permeability, and activated RhoA. PLD activation and VWF secretion induced by Stx1B were abolished on protein kinase Cα (PKCα) inhibition or gene silencing but were only moderately reduced by Rho or Rho kinase inhibitors. Conversely, PLD activation and VWF exocytosis induced by Stx2B were reduced by Rho/Rho kinase inhibitors and dominant-negative RhoA, whereas attenuation of PKCα did not affect either process. Another PLD1 activator, ADP-ribosylation factor 6, was involved in VWF secretion induced by Stx1B or Stx2B, but not histamine. These data indicate that Stx1B and Stx2B induce acute VWF secretion in a PLD1-dependent manner but do so by differentially modulating PKCα, RhoA, and ADP-ribosylation factor 6.

Glycerolipid/free fatty acid cycle and islet β-cell function in health, obesity and diabetes

Pancreatic β-cells secrete insulin in response to fluctuations in blood fuel concentrations, in particular glucose and fatty acids. However, chronic fuel surfeit can overwhelm the metabolic, signaling and secretory capacity of the β-cell leading to its dysfunction and death - often referred to as glucolipotoxicity. In β-cells and many other cells, glucose and lipid metabolic pathways converge into a glycerolipid/free fatty acid (GL/FFA) cycle, which is driven by the substrates, glycerol-3-phosphate and fatty acyl-CoA, derived from glucose and fatty acids, respectively. Although the overall operation of GL/FFA cycle, consisting of lipolysis and lipogenesis, is "futile" in terms of energy expenditure, this metabolic cycle likely plays an indispensable role for various β-cell functions, in particular insulin secretion and excess fuel detoxification. In this review, we discuss the significance of GL/FFA cycle in the β-cell, its regulation and role in generating essential metabolic signals that participate in the lipid amplification arm of glucose stimulated insulin secretion and in β-cell growth. We propose the novel concept that the lipolytic segment of GL/FFA cycle is instrumental in producing signals for insulin secretion, whereas, the lipogenic segment generates signals relevant for β-cell survival/death and growth/proliferation.

The Epac-Rap1 signaling pathway controls cAMP-mediated exocytosis of Weibel-Palade bodies in endothelial cells

Endothelial cells contain specialized storage organelles called Weibel-Palade bodies (WPBs) that release their content into the vascular lumen in response to specific agonists that raise intracellular Ca(2+) or cAMP. We have previously shown that cAMP-mediated WPB release is dependent on protein kinase A (PKA) and involves activation of the small GTPase RalA. Here, we have investigated a possible role for another PKA-independent cAMP-mediated signaling pathway in the regulation of WPB exocytosis, namely the guanine nucleotide exchange factor Epac1 and its substrate, the small GTPase Rap1. Epinephrine stimulation of endothelial cells leads to Rap1 activation in a PKA-independent fashion. siRNA-mediated knockdown of Epac1 abolished epinephrine-induced activation of Rap1 and resulted in decreased epinephrine-induced WPB exocytosis. Down-regulation of Rap1 expression and prevention of Rap1 activation through overexpression of Rap1GAP effectively reduced epinephrine- but not thrombin-induced WPB exocytosis. Taken together, these data uncover a new Epac-Rap1-dependent pathway by which endothelial cells can regulate WPB exocytosis in response to agonists that signal through cAMP.

Arf6 regulates AP-1B-dependent sorting in polarized epithelial cells

The epithelial cell-specific clathrin adaptor complex AP-1B facilitates the sorting of various transmembrane proteins from recycling endosomes (REs) to the basolateral plasma membrane. Despite AP-1B's clear importance in polarized epithelial cells, we still do not fully understand how AP-1B orchestrates basolateral targeting. Here we identify the ADP-ribosylation factor 6 (Arf6) as an important regulator of AP-1B. We show that activated Arf6 pulled down AP-1B in vitro. Furthermore, interfering with Arf6 function through overexpression of dominant-active Arf6Q67L or dominant-negative Arf6D125N, as well as depletion of Arf6 with short hairpin RNA (shRNA), led to apical missorting of AP-1B-dependent cargos. In agreement with these data, we found that Arf6 colocalized with AP-1B and transferrin receptor (TfnR) in REs. In addition, we observed specific recruitment of AP-1B into Arf6-induced membrane ruffles in nonpolarized cells. We conclude that activated Arf6 directs membrane recruitment of AP-1B, thus regulating AP-1B's functions in polarized epithelial cells.

Control of secondary granule release in neutrophils by Ral GTPase

Neutrophil (polymorphonuclear leukocyte; PMN) inflammatory functions, including cell adhesion, diapedesis, and phagocytosis, are dependent on the mobilization and release of various intracellular granules/vesicles. In this study, we found that treating PMN with damnacanthal, a Ras family GTPase inhibitor, resulted in a specific release of secondary granules but not primary or tertiary granules and caused dysregulation of PMN chemotactic transmigration and cell surface protein interactions. Analysis of the activities of Ras members identified Ral GTPase as a key regulator during PMN activation and degranulation. In particular, Ral was active in freshly isolated PMN, whereas chemoattractant stimulation induced a quick deactivation of Ral that correlated with PMN degranulation. Overexpression of a constitutively active Ral (Ral23V) in PMN inhibited chemoattractant-induced secondary granule release. By subcellular fractionation, we found that Ral, which was associated with the plasma membrane under the resting condition, was redistributed to secondary granules after chemoattractant stimulation. Blockage of cell endocytosis appeared to inhibit Ral translocation intracellularly. In conclusion, these results demonstrate that Ral is a critical regulator in PMN that specifically controls secondary granule release during PMN response to chemoattractant stimulation.

Functional architecture of Weibel-Palade bodies

Weibel-Palade bodies (WPBs) are elongated secretory organelles specific to endothelial cells that contain von Willebrand factor (VWF) and a variety of other proteins that contribute to inflammation, angiogenesis, and tissue repair. The remarkable architecture of WPBs is because of the unique properties of their major constituent VWF. VWF is stored inside WPBs as tubules, but on its release, forms strikingly long strings that arrest bleeding by recruiting blood platelets to sites of vascular injury. In recent years considerable progress has been made regarding the molecular events that underlie the packaging of VWF multimers into tubules and the processes leading to the formation of elongated WPBs. Mechanisms directing the conversion of tightly packaged VWF tubules into VWF strings on the surface of endothelial cells are starting to be unraveled. Several modes of exocytosis have now been described for WPBs, emphasizing the plasticity of these organelles. WPB exocytosis plays a role in the pathophysiology and treatment of von Willebrand disease and may have impact on common hematologic and cardiovascular disorders. This review summarizes the major advances made on the biogenesis and exocytosis of WPBs and places these recent discoveries in the context of von Willebrand disease.

Ral isoforms are implicated in Fc gamma R-mediated phagocytosis: activation of phospholipase D by RalA

Phagocytosis is an essential element of the immune response permitting the elimination of pathogens, cellular debris, apoptotic cells, and tumor cells. Recently, both phospholipase D (PLD) isoforms, PLD1 and PLD2, were shown to be necessary for efficient FcgammaR-mediated phagocytosis. In this study, we investigated the role of a potential PLD regulator, the Ral GTPases RalA and RalB, in murine RAW 264.7 macrophages. Both Ral isoforms are expressed in macrophages and are transiently activated following FcgammaR stimulation. When Ral expression levels were varied using Ral mutants or interference RNA, phagocytosis assays revealed that Ral isoforms have antagonistic effects; RalA is a positive modulator, whereas RalB plays a negative role. We then focused on RalA and its possible relationship with PLD. The increase in PLD activity that occurs when phagocytosis is stimulated was inhibited in cells with reduced RalA protein, but it was unaffected by reduced levels of RalB. Furthermore, in macrophages transfected with dsRed-RalA and GFP-PLD1 or GFP-PLD2, RalA colocalized with PLD1 and PLD2 at the phagocytic cup during phagosome formation. Additional results obtained from immunoprecipitation of PLD from macrophages transfected with myc-RalA and hemagglutinin-tagged PLD1 or PLD2 indicated an enhanced interaction of RalA with both PLD isoforms during phagocytic stimulation. The increase in RalA and PLD1 interaction was transient and correlated with the time course of RalA activation. These findings reveal a novel pathway involving RalA and PLD in the regulation of FcgammaR-mediated phagocytosis.

Synthesis of fusogenic lipids through activation of phospholipase D1 by GTPases and the kinase RSK2 is required for calcium-regulated exocytosis in neuroendocrine cells

Exocytosis of hormones occurs through the fusion of large dense-core secretory vesicles with the plasma membrane. This highly regulated process involves key proteins such as SNAREs (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors) and also specific lipids at the site of membrane fusion. Among the different lipids required for exocytosis, our recent observations have highlighted the crucial role of PA (phosphatidic acid) in the late stages of membrane fusion in various exocytotic events. An RNAi (RNA interference) strategy coupled with the detection of PA in living cells has pointed to plasma membrane-associated PLD1 (phospholipase D1) as the main producer of PA in response to secretagogue stimulation. We have identified several GTPases which regulate the activation level of PLD1 in neuroendocrine cells. Finally, RSK2 (ribosomal S6 kinase 2) appears to phosphorylate and regulate the activity of PLD1 in a calcium-dependent manner. Altogether our results have unravelled a complex set of regulatory pathways controlling the synthesis of fusogenic lipids at the secretory granule fusion site by PLD1.

The GTPase RalA regulates different steps of the secretory process in pancreatic beta-cells

Background

RalA and RalB are multifuntional GTPases involved in a variety of cellular processes including proliferation, oncogenic transformation and membrane trafficking. Here we investigated the mechanisms leading to activation of Ral proteins in pancreatic β-cells and analyzed the impact on different steps of the insulin-secretory process.

Methodology/Principal Findings

We found that RalA is the predominant isoform expressed in pancreatic islets and insulin-secreting cell lines. Silencing of this GTPase in INS-1E cells by RNA interference led to a decrease in secretagogue-induced insulin release. Real-time measurements by fluorescence resonance energy transfer revealed that RalA activation in response to secretagogues occurs within 3–5 min and reaches a plateau after 10–15 min. The activation of the GTPase is triggered by increases in intracellular Ca²⁺ and cAMP and is prevented by the L-type voltage-gated Ca²⁺ channel blocker Nifedipine and by the protein kinase A inhibitor H89. Defective insulin release in cells lacking RalA is associated with a decrease in the secretory granules docked at the plasma membrane detected by Total Internal Reflection Fluorescence microscopy and with a strong impairment in Phospholipase D1 activation in response to secretagogues. RalA was found to be activated by RalGDS and to be severely hampered upon silencing of this GDP/GTP exchange factor. Accordingly, INS-1E cells lacking RalGDS displayed a reduction in hormone secretion induced by secretagogues and in the number of insulin-containing granules docked at the plasma membrane.

Conclusions/Significance

Taken together, our data indicate that RalA activation elicited by the exchange factor RalGDS in response to a rise in intracellular Ca²⁺ and cAMP controls hormone release from pancreatic β-cell by coordinating the execution of different events in the secretory pathway.

Cholesterol, regulated exocytosis and the physiological fusion machine

Exocytosis is a highly conserved and essential process. Although numerous proteins are involved throughout the exocytotic process, the defining membrane fusion step appears to occur through a lipid-dominated mechanism. Here we review and integrate the current literature on protein and lipid roles in exocytosis, with emphasis on the multiple roles of cholesterol in exocytosis and membrane fusion, in an effort to promote a more molecular systems-level view of the as yet poorly understood process of Ca2+-triggered membrane mergers.

betaPIX-activated Rac1 stimulates the activation of phospholipase D, which is associated with exocytosis in neuroendocrine cells

Rho GTPases are crucial regulators of actin cytoskeletal rearrangements and play important roles in many cell functions linked to membrane trafficking processes. In neuroendocrine cells, we have previously demonstrated that RhoA and Cdc42 mediate part of the actin remodelling and vesicular trafficking events that are required for the release of hormones by exocytosis. Here, we investigate the functional importance of Rac1 for the exocytotic reaction and dissect the downstream and upstream molecular events that might integrate it to the exocytotic machinery. Using PC12 cells, we found that Rac1 is associated with the plasma membrane and is activated during exocytosis. Silencing of Rac1 by siRNA inhibits hormone release, prevents secretagogue (high K(+))-evoked phospholipase D1 (PLD1) activation and blocks the formation of phosphatidic acid at the plasma membrane. We identify betaPix as the guanine nucleotide-exchange factor integrating Rac1 activation to PLD1 and the exocytotic process. Finally, we show that the presence of the scaffolding protein Scrib at the plasma membrane is essential for betaPix/Rac1-mediated PLD1 activation and exocytosis. As PLD1 has recently emerged as a promoter of membrane fusion in various exocytotic events, our results define a novel molecular pathway linking a Rho GTPase, Rac1, to the final stages of Ca(2+)-regulated exocytosis in neuroendocrine cells.

Phosphatidylinositol metabolism and membrane fusion

Membrane fusion underlies many cellular events, including secretion, exocytosis, endocytosis, organelle reconstitution, transport from endoplasmic reticulum to Golgi and nuclear envelope formation. A large number of investigations into membrane fusion indicate various roles for individual members of the phosphoinositide class of membrane lipids. We first review the phosphoinositides as membrane recognition sites and their regulatory functions in membrane fusion. We then consider how modulation of phosphoinositides and their products may affect the structure and dynamics of natural membranes facilitating fusion. These diverse roles underscore the importance of these phospholipids in the fusion of biological membranes.

ARF6 regulates the synthesis of fusogenic lipids for calcium-regulated exocytosis in neuroendocrine cells

An important role for specific lipids in membrane fusion has recently emerged, but regulation of their biosynthesis remains poorly understood. Among fusogenic lipids, phosphatidic acid and phosphoinositol 4,5-bisphosphate (PIP(2)) have been proposed to act at various steps of neurotransmitter and hormone exocytosis. Using real time FRET (fluorescence resonance energy transfer) measurements, we show here that the GTPase ARF6, potentially involved in the synthesis of these lipids, is activated at the exocytotic sites in PC12 cells stimulated for secretion. Depletion of endogenous ARF6 by siRNA dramatically inhibited secretagogue-evoked exocytosis. ARF6-siRNA greatly reduced secretagogue-evoked phospholipase D (PLD) activation and phosphatidic acid formation at the plasma membrane and moderately reduced constitutive levels of PIP(2) present at the plasma membrane in resting cells. Expression of an ARF6 insensitive to short interference RNA (siRNA) fully rescued secretion in ARF6-depleted cells. However, a mutated ARF6 protein specifically impaired in its ability to stimulate PLD had no effect. Finally, we show that the ARF6-siRNA-mediated inhibition of exocytosis could be rescued by an exogenous addition of lysophosphatidylcholine, a lipid that favors negative curvature on the inner leaflet of the plasma membrane. Altogether these data indicate that ARF6 is a critical upstream signaling element in the activation of PLD necessary to produce the fusogenic lipids required for exocytosis.

Acidic mammalian chitinase is secreted via an ADAM17/epidermal growth factor receptor-dependent pathway and stimulates chemokine production by pulmonary epithelial cells

Acidic mammalian chitinase (AMCase) is expressed in an exaggerated fashion in epithelial cells at sites of pulmonary T helper cell type 2 inflammation and plays important roles in the pathogenesis of anti-parasite and asthma-like responses. However, the mechanisms that control epithelial cell AMCase secretion and its effector responses have not been adequately defined. To address these issues, we used in vivo and in vitro experimental systems to define the pathways of epithelial AMCase secretion and its epithelial regulatory effects. Here we demonstrate that, in murine T helper cell type 2 modeling systems, AMCase colocalizes with the epidermal growth factor receptor (EGFR) and ADAM17 (a membrane disintegrin and metallopeptidase 17) in lung epithelial cells. In vitro cotransfection experiments in A549 cells demonstrated that AMCase and EGFR physically interact with each other. Cotransfection of AMCase and EGFR also increased, whereas EGFR inhibition decreased AMCase secretion. Interestingly, AMCase secretion was not significantly altered by treatment with EGF but was significantly decreased when the upstream EGFR transactivator ADAM17 was inhibited. AMCase secretion was also decreased when the EGFR-downstream Ras was blocked. Transfected and recombinant AMCase induced epithelial cell production of CCL2, CCL17, and CXCL8. These studies demonstrate that lung epithelial cells secrete AMCase via an EGFR-dependent pathway that is activated by ADAM17 and mediates its effects via Ras. They also demonstrate that the AMCase that is secreted feeds back in an autocrine and/or paracrine fashion to stimulate pulmonary epithelial cell chemokine production.

The ghost in the machine: small GTPases as spatial regulators of exocytosis

Temporal and spatial regulation of membrane-trafficking events is crucial to both membrane identity and overall cell polarity. Small GTPases of the Rab, Ral and Rho protein families have been implicated as important regulators of vesicle docking and fusion events. This review focuses on how these GTPases interact with the exocyst complex, which is a multisubunit tethering complex involved in the regulation of cell-surface transport and cell polarity. The Rab and Ral GTPases are thought to function in exocyst assembly and vesicle-tethering processes, whereas the Rho family GTPases seem to function in the local activation of the exocyst complex to facilitate downstream vesicle-fusion events. The localized activation of the exocyst by Rho GTPases is likely to have an important role in spatial regulation of exocytosis.

Clathrin-independent endocytosis: a unique platform for cell signaling and PM remodeling

There is increasing interest in endocytosis that occurs independently of clathrin coats and the fates of membrane proteins internalized by this mechanism. The appearance of clathrin-independent endocytic and membrane recycling pathways seems to vary with different cell types and cargo molecules. In this review we focus on studies that have been performed using HeLa and COS cells as model systems for understanding this membrane trafficking system. These endosomal membranes contain signaling molecules including H-Ras, Rac1, Arf6 and Rab proteins, and a lipid environment rich in cholesterol and PIP(2) providing a unique platform for cell signaling. Furthermore, activation of some of these signaling molecules (H-Ras, Rac and Arf6) can switch the constitutive form of clathrin-independent endocytosis into a stimulated one, associated with PM ruffling and macropinocytosis.

Drosophila RalA is essential for the maintenance of Jak/Stat signalling in ovarian follicles

Small GTPases of the Ras-like (Ral) family are crucial for signalling functions in both normal and cancer cells; however, their role in a developing organism is poorly understood. Here, we identify the Drosophila Ral homologue RalA as a new key regulator of polar-cell differentiation during oogenesis. Polar cells have a crucial role in patterning the egg chamber and in recruiting border cells, which undergo collective and guided migration. We show that RalA function is essential for the maintenance of anterior and posterior polar-cell fate and survival. RalA is required cell autonomously to control the expression of polar-cell-specific markers, including the Jak/Stat ligand Unpaired. The loss of RalA also causes a cell non-autonomous phenotype owing to reduced Jak/Stat signalling in neighbouring follicle cells. As a result, border-cell assembly and migration as well as the polarization of the oocyte are defective. Thus, RalA is required in organizing centres to control proper patterning and migration in vivo.