A Tyrosine-based Motif Localizes a Drosophila Vesicular Transporter to Synaptic Vesicles in Vivo

Vesicular neurotransmitter transporters must localize to synaptic vesicles (SVs) to allow regulated neurotransmitter release at the synapse. However, the signals required to localize vesicular proteins to SVs in vivo remain unclear. To address this question we have tested the effects of mutating proposed trafficking domains in Drosophila orthologs of the vesicular monoamine and glutamate transporters, DVMAT-A and DVGLUT. We show that a tyrosine-based motif (YXXY) is important both for DVMAT-A internalization from the cell surface in vitro, and localization to SVs in vivo. In contrast, DVGLUT deletion mutants that lack a putative C-terminal trafficking domain show more modest defects in both internalization in vitro and trafficking to SVs in vivo. Our data show for the first time that mutation of a specific trafficking motif can disrupt localization to SVs in vivo and suggest possible differences in the sorting of VMATs versus VGLUTs to SVs at the synapse.     

Complexin 2 Modulates Vesicle-associated Membrane Protein (VAMP) 2-regulated Zymogen Granule Exocytosis in Pancreatic Acini

Complexins are soluble proteins that regulate the activity of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes necessary for vesicle fusion. Neuronal specific complexin 1 has inhibitory and stimulatory effects on exocytosis by clamping trans-SNARE complexes in a prefusion state and promoting conformational changes to facilitate membrane fusion following cell stimulation. Complexins are unable to bind to monomeric SNARE proteins but bind with high affinity to ternary SNARE complexes and with lower affinity to target SNARE complexes. Far less is understood about complexin function outside the nervous system. Pancreatic acini express the complexin 2 isoform by RT-PCR and immunoblotting. Immunofluorescence microscopy revealed complexin 2 localized along the apical plasma membrane consistent with a role in secretion. Accordingly, complexin 2 was found to interact with vesicle-associated membrane protein (VAMP) 2, syntaxins 3 and 4, but not with VAMP 8 or syntaxin 2. Introduction of recombinant complexin 2 into permeabilized acini inhibited Ca²⁺-stimulated secretion in a concentration-dependent manner with a maximal inhibition of nearly 50%. Mutations of the central α-helical domain reduced complexin 2 SNARE binding and concurrently abolished its inhibitory activity. Surprisingly, mutation of arginine 59 to histidine within the central α-helical domain did not alter SNARE binding and moreover, augmented Ca²⁺-stimulated secretion by 130% of control. Consistent with biochemical studies, complexin 2 colocalized with VAMP 2 along the apical plasma membrane following cholecystokinin-8 stimulation. These data demonstrate a functional role for complexin 2 outside the nervous system and indicate that it participates in the Ca²⁺-sensitive regulatory pathway for zymogen granule exocytosis.     

Differential Mechanisms of Shedding of the Glycosylphosphatidylinositol (GPI)-anchored NKG2D Ligands

Tumor cells release NKG2D ligands to evade NKG2D-mediated immune surveillance. The purpose of our investigation was to explore the cellular mechanisms of release used by various members of the ULBP family. Using biochemical and cellular approaches in both transfectant systems and tumor cell lines, this paper shows that ULBP1, ULBP2, and ULBP3 are released from cells with different kinetics and by distinct mechanisms. Whereas ULBP2 is mainly shed by metalloproteases, ULBP3 is abundantly released as part of membrane vesicles known as exosomes. Interestingly, exosomal ULBP3 protein is much more potent for down-modulation of the NKG2D receptor than soluble ULBP2 protein. This is the first report showing functionally relevant differences in the biochemistry of the three members of the ULBP family and confirms that in depth study of the biochemical features of individual NKG2D ligands will be necessary to understand and manipulate the biology of these proteins for therapy.     

Structure and Function of the Hair Cell Ribbon Synapse

Faithful information transfer at the hair cell afferent synapse requires synaptic transmission to be both reliable and temporally precise. The release of neurotransmitter must exhibit both rapid on and off kinetics to accurately follow acoustic stimuli with a periodicity of 1 ms or less. To ensure such remarkable temporal fidelity, the cochlear hair cell afferent synapse undoubtedly relies on unique cellular and molecular specializations. While the electron microscopy hallmark of the hair cell afferent synapse — the electron-dense synaptic ribbon or synaptic body — has been recognized for decades, dissection of the synapse’s molecular make-up has only just begun. Recent cell physiology studies have added important insights into the synaptic mechanisms underlying fidelity and reliability of sound coding. The presence of the synaptic ribbon links afferent synapses of cochlear and vestibular hair cells to photoreceptors and bipolar neurons of the retina. This review focuses on major advances in understanding the hair cell afferent synapse molecular anatomy and function that have been achieved during the past years.     

Synaptic proteins associate with a sub-set of lipid rafts when isolated from nerve endings at physiological temperature

Although the high presence of cholesterol in nerve terminals is well documented, specific roles of this lipid in transmitter release have remained elusive. Since cholesterol is a highly enriched component in the membrane microdomains known as lipid rafts, it is probable that these domains are very important in synaptic function. The extraction of lipid rafts using Brij 98 at 37 degrees C avoids the formation of nonspecific membrane aggregates at low temperature, allowing the isolation of more physiologically relevant lipid rafts. In the present work, we examine, by means of buoyancy analysis in sucrose gradients after solubilization of the membranes with Brij 98 or with Lubrol WX, the presence of proteins involved in exocytosis in detergent-resistant membranes (DRM) using rat brain synaptosomes as a neurological model. Significant proportions of the proteins tested in the present work, which are involved in neurotransmitter release, are found in Brij 98 raft fractions, demonstrating that significant pools of synaptic proteins are segregated in specific parts of the membrane at physiological temperature. On the other hand, Lubrol WX is unable to solubilize the major fraction of the proteins tested. Treatment of synaptosomes with methyl-beta-cyclodextrin (mbetaCD) causes alteration in the buoyancy properties of proteins initially present in Brij- as well as in Lubrol-resistant membranes, indicating the cholesterol-dependency of both kinds of microdomains. Finally, we detect the depolarization-induced enhancement of the cholesterol-dependent association of syntaxin 1 with Brij 98-rafts, under the same conditions in which prolonged neurotransmitter release is stimulated.     

ZD7288 inhibits exocytosis in an HCN-independent manner and downstream of voltage-gated calcium influx in pituitary lactotrophs

Pituitary lactotrophs fire action potentials spontaneously and the associated voltage-gated calcium influx is sufficient to maintain high prolactin release. Here we studied the role of hyperpolarization-activated cation channels in pacemaking activity, calcium signaling, and prolactin secretion in these cells. A slowly developing and hyperpolarization-activated inward current was identified but only in a fraction of lactotrophs. The current was blocked by ZD7288, a relatively specific blocker of these channels. However, the pacemaking activity increased in ZD7288-treated cells independently of the presence of this current. This in turn facilitated voltage-gated calcium influx and transiently stimulated prolactin secretion. Sustained ZD7288 application in concentrations that are commonly used to block the hyperpolarization-activated cation channels inhibited hormone release at elevated intracellular calcium concentrations. Agonist and Bay K 8644-stimulated prolactin release was also inhibited by ZD7288, indicating that this compound attenuates the exocytotic pathway downstream of calcium influx.     

Phospholipase C-related but catalytically inactive protein (PRIP) modulates synaptosomal-associated protein 25 (SNAP-25) phosphorylation and exocytosis

Exocytosis is one of the most fundamental cellular events. The basic mechanism of the final step, membrane fusion, is mediated by the formation of the SNARE complex, which is modulated by the phosphorylation of proteins controlled by the concerted actions of protein kinases and phosphatases. We have previously shown that a protein phosphatase-1 (PP1) anchoring protein, phospholipase C-related but catalytically inactive protein (PRIP), has an inhibitory role in regulated exocytosis. The current study investigated the involvement of PRIP in the phospho-dependent modulation of exocytosis. Dephosphorylation of synaptosome-associated protein of 25 kDa (SNAP-25) was mainly catalyzed by PP1, and the process was modulated by wild-type PRIP but not by the mutant (F97A) lacking PP1 binding ability in in vitro studies. We then examined the role of PRIP in phospho-dependent regulation of exocytosis in cell-based studies using pheochromocytoma cell line PC12 cells, which secrete noradrenalin. Exogenous expression of PRIP accelerated the dephosphorylation process of phosphorylated SNAP-25 after forskolin or phorbol ester treatment of the cells. The phospho-states of SNAP-25 were correlated with noradrenalin secretion, which was enhanced by forskolin or phorbol ester treatment and modulated by PRIP expression in PC12 cells. Both SNAP-25 and PP1 were co-precipitated in anti-PRIP immunocomplex isolated from PC12 cells expressing PRIP. Collectively, together with our previous observation regarding the roles of PRIP in PP1 regulation, these results suggest that PRIP is involved in the regulation of the phospho-states of SNAP-25 by modulating the activity of PP1, thus regulating exocytosis.     

Interplay between lipids and the proteinaceous membrane fusion machinery

For membrane fusion to occur, opposed lipid bilayers initially establish a fusion pore, often followed by complete mixing of the fusing membranes. Contemporary views suggest that during fusion lipid bilayers are continuous passive platforms that are disrupted and remodeled by catalytic proteins. Some models propose that even the architecture and composition of the fusion pore might be dominated by proteins rather than lipids. Hence, lipids have no regulatory contribution to this process; they simply adapt their shape passively for filling space between otherwise autonomous protein machineries.However, an increasing number of experimental findings indicate that membrane fusion critically depends on a variety of lipids and lipid derivatives. Therefore, a purely proteocentric view describes fusion mechanisms insufficiently. Instead, lipids have functions probably at different levels, as (i) a general influence on the propensity of lipid bilayers to fuse, (ii) a role in recruiting exocytotic proteins to the plasma membrane, (iii) a role in organizing membrane domains for fusion and (iv) direct regulatory effects on fusion protein complexes. In this review we have made an attempt to bring together the large body of evidence supporting a major role for lipids in membrane fusion either directly or indirectly.     

Phosphorylation of cysteine string protein on Serine 10 triggers 14-3-3 protein binding

Cysteine string protein (CSP) is a neuronal chaperone that maintains normal neurotransmitter exocytosis and is essential for preventing presynaptic neurodegeneration. CSP is phosphorylated in vivo on a single residue, Ser10, and this phosphorylation regulates its cellular functions, although the molecular mechanisms involved are unclear. To identify novel phosphorylation-specific binding partners for CSP, we used a pull-down approach using synthetic peptides and recombinant proteins. A single protein band was observed to bind specifically to a Ser10-phosphorylated CSP peptide (residues 4-14) compared to a non-phosphorylated peptide. This band was identified as 14-3-3 protein of various isoforms using mass spectrometry and Western blotting. PKA phosphorylation of full-length CSP protein stimulated 14-3-3 binding, and this was abolished in a Ser10-Ala mutant CSP, confirming the binding site as phospho-Ser10. As both CSP and 14-3-3 proteins are implicated in neurotransmitter exocytosis and neurodegeneration, this novel phosphorylation-dependent interaction may help maintain the functional integrity of the synapse.