Rab27 and Rab3 sequentially regulate human sperm dense-core granule exocytosis

Spire-1 contributes to the invadosome and its associated invasive properties

Cancer cells have an increased ability to squeeze through extracellular matrix gaps that they create by promoting proteolysis of its components. Major sites of degradation are specialized micro-domains in the plasma membrane collectively named invadosomes where the Arp2/3 complex and formin proteins cooperate to spatio-temporally control actin nucleation and the folding of a dynamic F-actin core. At invadosomes, proper coupling of exo-endocytosis allows polarized delivery of proteases that facilitate degradation of ECM and disruption of the cellular barrier. We investigated the contribution of the actin nucleator Spire-1 to invadosome structure and function, using Src-activated cells and cancer cells. We found that Spire-1 is specifically recruited at invadosomes and is part of a multi-molecular complex containing Src kinase, the formin mDia1 and actin. Spire-1 interacts with the Rab3A GTPase, a key player in the regulation of exocytosis that is present at invadosomes. Finally, over- and under-expression of Spire-1 resulted in cells with an increased or decreased potential for matrix degradation, respectively, therefore suggesting a functional interplay of Spire-1 with both actin nucleation and vesicular trafficking that might impact on cell invasive and metastatic behavior.

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.

The secretion of von Willebrand factor from endothelial cells; an increasingly complicated story

von Willebrand factor (VWF) plays key roles in both primary and secondary hemostasis by capturing platelets and chaperoning clotting factor VIII, respectively. It is stored within the Weibel-Palade bodies (WPBs) of endothelial cells as a highly prothrombotic protein, and its release is thus necessarily under tight control. Regulating the secretion of VWF involves multiple layers of cellular machinery that act together at different stages, leading to the exocytic fusion of WPBs with the plasma membrane and the consequent release of VWF. This review aims to provide a snapshot of the current understanding of those components, in particular the members of the Rab family, acting in the increasingly complex story of VWF secretion.

Oligomerization of rab/effector complexes in the regulation of vesicle trafficking

Rabs comprise the largest member of the Ras superfamily of small GTPases with over 60 proteins in mammals and 11 proteins in yeast. Like all small GTPases, Rabs oscillate between an inactive GDP-bound conformation and an active GTP-bound state that is tethered to lipid membranes via a C-terminal prenylation site on conserved cysteine residues. In their active state, Rabs regulate various aspects of membrane trafficking, including vesicle formation, transport, docking, and fusion. The critical element of biological activity is the recruitment of cytosolic effector proteins to specific endomembranes by active Rabs. The importance of Rabs in cellular processes is apparent from their links to genetic disorders, immunodeficiency, cancer, and pathogen invasion. During the last decade, numerous structures of complexes have shed light on the molecular basis for Rab/effector specificity and their topological organization on subcellular membranes. Here, I review the known structures of Rab/effector complexes and their modes of oligomerization. This is followed by a brief discussion on the thermodynamics of effector recruitment, which has not been documented sufficiently in previous reviews. A summary of diseases associated with Rab/effector trafficking pathways concludes this chapter.

Rab26 modulates the cell surface transport of α2-adrenergic receptors from the Golgi

The molecular mechanisms underlying the transport from the Golgi to the cell surface of G protein-coupled receptors remain poorly elucidated. Here we determined the role of Rab26, a Ras-like small GTPase involved in vesicle-mediated secretion, in the cell surface export of α(2)-adrenergic receptors. We found that transient expression of Rab26 mutants and siRNA-mediated depletion of Rab26 significantly attenuated the cell surface numbers of α(2A)-AR and α(2B)-AR, as well as ERK1/2 activation by α(2B)-AR. Furthermore, the receptors were extensively arrested in the Golgi by Rab26 mutants and siRNA. Moreover, Rab26 directly and activation-dependently interacted with α(2B)-AR, specifically the third intracellular loop. These data demonstrate that the small GTPase Rab26 regulates the Golgi to cell surface traffic of α(2)-adrenergic receptors, likely through a physical interaction. These data also provide the first evidence implicating an important function of Rab26 in coordinating plasma membrane protein transport.

Munc18-1 controls SNARE protein complex assembly during human sperm acrosomal exocytosis

The spermatozoon is a very specialized cell capable of carrying out a limited set of functions with high efficiency. Sperm are then excellent model cells to dissect fundamental processes such as regulated exocytosis. The secretion of the single dense-core granule of mammalian spermatozoa relies on the same highly conserved molecules and goes through the same stages as exocytosis in other types of cells. In this study, we describe the presence of Munc18-1 in human sperm and show that this protein has an essential role in acrosomal exocytosis. We observed that inactivation of endogenous Munc18-1 with a specific antibody precluded the stabilization of trans-SNARE complexes and inhibited acrosomal exocytosis. Addition of recombinant Munc18-1 blocked secretion by sequestering monomeric syntaxin, an effect that was rescued by α-soluble NSF attachment protein. By electron microscopy, we observed that both the anti-Munc18-1 antibody and recombinant Munc18-1 inhibited the docking of the acrosome to the plasma membrane. In conclusion, our results indicate that Munc18-1 plays a key role in the dynamics of trans-SNARE complex assembly and/or stabilization, a process that is necessary for the docking of the outer acrosomal membrane to the plasma membrane and subsequent fusion pore opening.