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Jennifer R. Morgan - One of the best experts on this subject based on the ideXlab platform.
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α-Synuclein-112 Impairs Synaptic Vesicle Recycling Consistent With Its Enhanced Membrane Binding Properties.
Frontiers in cell and developmental biology, 2020Co-Authors: Lindsey G. Soll, Julia N. Eisen, Karina J. Vargas, Audrey T. Medeiros, Katherine Hammar, Jennifer R. MorganAbstract:Synucleinopathies are neurological disorders associated with α-synuclein overexpression and aggregation. While it is well-established that overexpression of wild type α-synuclein (α-syn-140) leads to cellular toxicity and neurodegeneration, much less is known about other naturally occurring α-synuclein splice isoforms. In this study we provide the first detailed examination of the Synaptic effects caused by one of these splice isoforms, α-synuclein-112 (α-syn-112). α-Syn-112 is produced by an in-frame excision of exon 5, resulting in deletion of amino acids 103-130 in the C-terminal region. α-Syn-112 is upregulated in the substantia nigra, frontal cortex, and cerebellum of parkinsonian brains and higher expression levels are correlated with susceptibility to Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple systems atrophy (MSA). We report here that α-syn-112 binds strongly to anionic phospholipids when presented in highly curved liposomes, similar to α-syn-140. However, α-syn-112 bound significantly stronger to all phospholipids tested, including the phosphoinositides. α-Syn-112 also dimerized and trimerized on isolated Synaptic membranes, while α-syn-140 remained largely monomeric. When introduced acutely to lamprey synapses, α-syn-112 robustly inhibited Synaptic Vesicle Recycling. Interestingly, α-syn-112 produced effects on the plasma membrane and clathrin-mediated Synaptic Vesicle endocytosis that were phenotypically intermediate between those caused by monomeric and dimeric α-syn-140. These findings indicate that α-syn-112 exhibits enhanced phospholipid binding and oligomerization in vitro and consequently interferes with Synaptic Vesicle Recycling in vivo in ways that are consistent with its biochemical properties. This study provides additional evidence suggesting that impaired Vesicle endocytosis is a cellular target of excess α-synuclein and advances our understanding of potential mechanisms underlying disease pathogenesis in the synucleinopathies.
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α-Synuclein-112 impairs Synaptic Vesicle Recycling consistent with its enhanced membrane binding properties
2020Co-Authors: Lindsey G. Soll, Julia N. Eisen, Karina J. Vargas, Audrey T. Medeiros, Katherine Hammar, Jennifer R. MorganAbstract:ABSTRACT Synucleinopathies are neurological disorders associated with α-synuclein overexpression and aggregation. While it is well established that overexpression of wild type α-synuclein (α-syn-140) leads to cellular toxicity and neurodegeneration, much less is known about other naturally occurring α-synuclein splice isoforms. In this study we provide the first detailed examination of the Synaptic effects caused by one of these splice isoforms, α-synuclein-112 (α-syn-112). α-Syn-112 is produced by an in-frame excision of exon 5, resulting in deletion of amino acids 103-130 in the C-terminal region. α-Syn-112 is upregulated in the substantia nigra, frontal cortex, and cerebellum of parkinsonian brains and is correlated with susceptibility to sporadic Parkinson’s disease (PD), dementia with Lewy bodies (DLB) and multiple systems atrophy (MSA). We report here that α-syn-112 binds strongly to anionic phospholipids when presented in highly-curved liposomes, similar to α-syn-140. However, α-syn-112 bound significantly stronger to all phospholipids tested, including the phosphoinositides. α-Syn-112 also dimerized and trimerized on isolated Synaptic membranes, while α-syn-140 remained largely monomeric. When introduced acutely to lamprey synapses, α-syn-112 robustly inhibited Synaptic Vesicle Recycling. Interestingly, α-syn-112 produced effects on the plasma membrane and clathrin-mediated Synaptic Vesicle endocytosis that were phenotypically intermediate between those caused by monomeric and dimeric α-syn-140. These findings indicate that α-syn-112 exhibits enhanced phospholipid binding and oligomerization in vitro and consequently interferes with Synaptic Vesicle Recycling in vivo in ways that are consistent with its biochemical properties. This study provides additional evidence suggesting that impaired Vesicle endocytosis is a cellular target of excess α-synuclein and advances our understanding of potential mechanisms underlying disease pathogenesis in the synucleinopathies.
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Acute increase of α-synuclein inhibits Synaptic Vesicle Recycling evoked during intense stimulation
Molecular biology of the cell, 2014Co-Authors: David J. Busch, Paul A. Oliphint, R. Walsh, Susan M. L. Banks, Wendy S. Woods, Jimin George, Jennifer R. MorganAbstract:Parkinson's disease is associated with multiplication of the α-synuclein gene and abnormal accumulation of the protein. In animal models, α-synuclein overexpression broadly impairs Synaptic Vesicle trafficking. However, the exact steps of the Vesicle trafficking pathway affected by excess α-synuclein and the underlying molecular mechanisms remain unknown. Therefore we acutely increased synuclein levels at a vertebrate synapse and performed a detailed ultrastructural analysis of the effects on preSynaptic membranes. At stimulated synapses (20 Hz), excess synuclein caused a loss of Synaptic Vesicles and an expansion of the plasma membrane, indicating an impairment of Vesicle Recycling. The N-terminal domain (NTD) of synuclein, which folds into an α-helix, was sufficient to reproduce these effects. In contrast, α-synuclein mutants with a disrupted N-terminal α-helix (T6K and A30P) had little effect under identical conditions. Further supporting this model, another α-synuclein mutant (A53T) with a properly folded NTD phenocopied the Synaptic Vesicle Recycling defects observed with wild type. Interestingly, the Vesicle Recycling defects were not observed when the stimulation frequency was reduced (5 Hz). Thus excess α-synuclein impairs Synaptic Vesicle Recycling evoked during intense stimulation via a mechanism that requires a properly folded N-terminal α-helix.
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PreSynaptic membrane retrieval and endosome biology: defining molecularly heterogeneous Synaptic Vesicles.
Cold Spring Harbor perspectives in biology, 2013Co-Authors: Jennifer R. Morgan, Heather Skye Comstra, Max Cohen, Victor FaundezAbstract:The release and uptake of neurotransmitters by Synaptic Vesicles is a tightly controlled process that occurs in response to diverse stimuli at morphologically disparate synapses. To meet these architectural and functional Synaptic demands, it follows that there should be diversity in the mechanisms that control their secretion and retrieval and possibly in the composition of Synaptic Vesicles within the same terminal. Here we pay particular attention to areas where such diversity is generated, such as the variance in exocytosis/endocytosis coupling, SNAREs defining functionally diverse Synaptic Vesicle populations and the adaptor-dependent sorting machineries capable of generating Vesicle diversity. We argue that there are various Synaptic Vesicle Recycling pathways at any given synapse and discuss several lines of evidence that support the role of the endosome in Synaptic Vesicle Recycling.
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Sniffing calcium from the outside: an extracellular calcium sensor for Synaptic Vesicle Recycling.
The Journal of Physiology, 2003Co-Authors: Jennifer R. MorganAbstract:In order for neurons to maintain communication with one another, following exocytosis, Synaptic Vesicles within the nerve terminal must be internalized from the plasma membrane and be refilled with neurotransmitter molecules for subsequent bouts of transmitter release. Calcium influx into the nerve terminal, which triggers exocytosis, may be an important temporal regulator of Vesicle Recycling. Several studies designed to address the calcium dependence of Synaptic Vesicle endocytosis have led to conflicting conclusions that intracellular calcium speeds up (Klingauf et al. 1998; Sankaranarayanan & Ryan, 2001), slows down (von Gersdorff & Matthews, 1994) or has little effect (Wu & Betz, 1996) on the rate of Vesicle Recycling. However, it is difficult to study the calcium dependence of endocytosis in isolation from exocytosis because these processes proceed concurrently and because exocytosis itself requires calcium influx. Thus, the role of calcium during Synaptic Vesicle Recycling is still unclear. In this issue of The Journal of Physiology, a study of snake motor boutons measures uptake of the membrane dye sulphorhodamine 101 (SR101) to show that calcium regulates Vesicle Recycling via an extracellular sensor (Teng & Wilkinson, 2003). The calcium dependence of Vesicle Recycling was revisited by temporally separating this process from the immediate effects of calcium influx and exocytosis. To do so, the preparation was cooled to 7 °C following stimulation in order to delay membrane retrieval well beyond the time when calcium had returned to resting levels and exocytosis was complete. The ‘endocytotic debt’ incurred by delaying endocytosis lasted for several hours and was recovered at any time by warming the preparation to room temperature. When the magnitude and time course of dye uptake were measured in the presence of increasing extracellular calcium concentrations, delayed endocytosis proceeded more rapidly. Large changes in extracellular calcium had no observable effect on resting intraterminal calcium levels, as measured by a calcium indicator that reports nanomolar calcium concentrations. Therefore, calcium is a positive regulator of the rate of membrane retrieval, and these experiments implicate an extracellular calcium sensor in Synaptic Vesicle Recycling. These same authors reported previously that delayed endocytosis is mediated by a temperature blockade of clathrin-coated Vesicle formation and uncoating (Teng & Wilkinson, 2000). Thus, the current results suggest that extracellular calcium might regulate the rate of coated Vesicle formation. This hypothesis is supported by the results of an electron microscope study of the lamprey giant synapse, in which Synaptic Vesicle Recycling from the plasma membrane was temporarily blocked by the removal of extracellular calcium following stimulation (Gad et al. 1998). Subsequent recovery of Synaptic Vesicles was triggered via a rapid burst in coated Vesicle formation by simply adding back 11 μM extracellular calcium. Although the interpretation was that minimal intracellular calcium is needed for Synaptic Vesicle Recycling, these experiments may have inadvertently measured the affinity of the putative extracellular calcium sensor. The mechanism by which the endocytotic calcium sensor translocates a signal across the plasma membrane to trigger coated Vesicle formation remains unclear. In the endocrine system, a G-protein-coupled extracellular calcium receptor regulates parathyroid hormone release by activating a cascade of pathways including protein kinase C and mitogen-activated protein kinase (Tfelt-Hansen et al. 2003), and the mechanisms for detecting extracellular calcium at synapses might be similar. During Synaptic activity, a nerve terminal is exposed to changes in both extracellular and intracellular calcium, and thus the extracellular calcium sensor may coordinate with intracellular calcium effectors to regulate the early and late stages of coated Vesicle formation. Calcium influx into the preSynaptic terminal is likely to decrease calcium transiently in the Synaptic cleft. As a result, Vesicle Recycling would be slowed until exocytosis is complete and extracellular calcium is returned to basal levels. Meanwhile, the entering calcium could trigger a cascade of events leading to the dephosphorylation of clathrin-associated proteins and their consequent assembly into multimolecular endocytotic complexes (Slepnev et al. 1998). Restoration of basal extracellular calcium levels could then trigger the extracellular sensor and speed up the early stages of endocytosis at a time when the intracellular endocytotic complexes are prepared to promote a burst of coated Vesicle formation. In this way, changes in both intracellular and extracellular calcium could regulate Synaptic Vesicle Recycling via coated Vesicle formation. The current study by Teng & Wilkinson (2003) also highlights several other features of Synaptic Vesicle endocytosis in snake motor boutons. First, only half of the membrane retrieval was mediated by delayed endocytosis, indicating that a second mode of endocytosis exists, as is seen in other nerve terminals (e.g. Aravanis et al. 2003; Gandhi & Stevens, 2003). In snake nerve terminals, the second mode of membrane retrieval is likely to be mediated by macropinocytosis, which resembles bulk endocytosis and from which coated Vesicles emanate (Teng & Wilkinson, 2000). Whether calcium affects the rate of macropinocytosis is unknown. Second, the ‘endocytotic debt’ incurred by delaying endocytosis was completely restored within 1 min, indicating that coated Vesicle formation can proceed rapidly. Finally, because removal of extracellular calcium never completely blocked delayed endocytosis, calcium must be a regulator rather than an essential component of membrane retrieval. Identification of the putative extracellular calcium sensor and the mechanism by which it regulates the speed of Synaptic Vesicle Recycling awaits further investigation.
Volker Haucke - One of the best experts on this subject based on the ideXlab platform.
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Modes and mechanisms of Synaptic Vesicle Recycling
Current opinion in neurobiology, 2016Co-Authors: Tolga Soykan, Tanja Maritzen, Volker HauckeAbstract:Neurotransmission requires the Recycling of Synaptic Vesicles (SVs) to replenish the SV pool, clear release sites, and maintain preSynaptic integrity. In spite of decades of research the modes and mechanisms of SV Recycling remain controversial. The identification of clathrin-independent modes of SV Recycling such as ultrafast endocytosis has added to the debate. Accumulating evidence further suggests that SV membrane retrieval and the reformation of functional SVs are separable processes. This may allow synapses to rapidly restore membrane surface area over a wide range of stimulations followed by slow reformation of release-competent SVs. One of the future challenges will be to pinpoint the exact mechanisms that link SV Recycling modes to Synaptic activity patterns at different synapses.
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Composition of isolated Synaptic boutons reveals the amounts of Vesicle trafficking proteins
Science, 2014Co-Authors: Benjamin G. Wilhelm, Sunit Mandad, Sven Truckenbrodt, Burkhard Rammner, Gala A. Claßen, Michael Krauss, K. Krohnert, Seong Joo Koo, Christina Schäfer, Volker HauckeAbstract:Synaptic Vesicle Recycling has long served as a model for the general mechanisms of cellular trafficking. We used an integrative approach, combining quantitative immunoblotting and mass spectrometry to determine protein numbers; electron microscopy to measure organelle numbers, sizes, and positions; and super-resolution fluorescence microscopy to localize the proteins. Using these data, we generated a three-dimensional model of an "average" synapse, displaying 300,000 proteins in atomic detail. The copy numbers of proteins involved in the same step of Synaptic Vesicle Recycling correlated closely. In contrast, copy numbers varied over more than three orders of magnitude between steps, from about 150 copies for the endosomal fusion proteins to more than 20,000 for the exocytotic ones.
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The tortoise and the hare revisited
eLife, 2013Co-Authors: Natalia L. Kononenko, Arndt Pechstein, Volker HauckeAbstract:Optogenetics and electron microscopy reveal an ultrafast mode of Synaptic Vesicle Recycling, adding a new twist to a 40-year-old controversy.
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adaptin endosomes for Synaptic Vesicle Recycling learning and memory
The EMBO Journal, 2010Co-Authors: Michael Kraus, Volker HauckeAbstract:EMBO J 29 8, 1318–1330 (2010); published online March042010 [PMC free article] [PubMed] The pathways by which neurotransmitter-filled preSynaptic Vesicles (SVs) are generated and recycled have been debated for a long time. Glyvuk et al (2010) in this issue of The EMBO Journal describe an unanticipated role for the clathrin adaptor AP-1 and in particular its σ1B subunit in SV Recycling. SV reformation is defective in σ1B-deficient mice, which instead accumulate large endosome-like vacuoles. These defects are paired with reduced motor coordination and long-term spatial memory. This work thus not only provides novel insights into the role of clathrin/AP-1 coats in SV Recycling from endosomes, but also unravels a molecular mechanism that may contribute to some forms of X-linked mental retardation. Synaptic transmission involves the fusion of neurotransmitter-filled Synaptic Vesicles (SVs) with the preSynaptic plasma membrane at the active zone. To sustain neurotransmission and to prevent expansion of the plasma membrane, SVs undergo a local cycle of reformation, whereby SV proteins and lipids are re-internalized and SVs are regenerated (Murthy and De Camilli, 2003; Ryan, 2006). SVs display a specific protein and lipid composition that enables them to store and release the neurotransmitter, to be targeted to or cluster near release sites, and to undergo multiple rounds of exo-endocytosis. Defective Vesicle cycling or exo-endocytic coupling has been linked to deficits in short-term Synaptic plasticity, memory, and cognitive performance, including mental retardation and schizophrenia in humans (Murthy and De Camilli, 2003). The question which mechanisms contribute to SV Recycling dates back to the early days of neurophysiology and the discovery by Katz that the neurotransmitter is released in predefined quanta. Are all SVs created equal? Do all SVs undergo full fusion with the plasmalemma? Are SVs solely regenerated from the plasma membrane proper or are endosomal intermediates involved? These fundamental questions (Ryan, 2006) have been difficult to answer, but a combination of genetic and optical tools seem well-suited to tackle them. This is precisely the path that Glyvuk et al (2010) have followed. The authors generated knockout (KO) mice lacking expression of AP-1/σ1B. X-chromosome-encoded σ1B is one out of three isogenes (termed A–C) for the tiny σ subunit of AP-1 (comprising γ1, β1, μ1 and σ1 adaptins) localized to the trans-Golgi network (TGN) and endosomes. In contrast to mice deficient in γ1 or μ1A adaptins, which suffer from early embryonic lethality, σ1B-KO mice survive to adulthood and develop normally, suggesting that σ1A and σ1C adaptins can partially compensate for σ1B function. However, σ1B-KO animals are hypoactive and show behavioral abnormalities such as balancing problems and impaired spatial learning. These defects are reminiscent of those seen in patients suffering from X-linked metal retardation due to premature STOP codons present in their σ1B gene (Tarpey et al, 2006). How can these phenotypes be explained at the cellular and molecular level? Surprisingly, Glyvuk et al (2010) found that preSynaptic function, in particular Recycling of SVs, was perturbed in σ1B-deficient animals. KO mice showed a severe reduction in the total number of SVs at hippocampal boutons at rest and upon activity. Instead, nerve terminals from σ1B-KO mice accumulated enlarged vacuoles, reminiscent of endosomes in non-neuronal cells. Frequently, these endosome-like structures contained clathrin-coated buds of undefined molecular identity, which did not appear to be connected to the plasmalemma (Figure 1). Most strikingly, replenishment of release-competent SVs after full depletion of the total Recycling SV pool was significantly slower than in wild-type littermates. Hence, the authors propose an unanticipated function for AP-1/σ1B-containing clathrin coats in SV reformation from endosomes. Moreover, the new study lends further support to the idea that bulk endocytosis (BE) and subsequent consumption of endosome-like vacuoles but not kiss-and-run (Zhang et al, 2009) represents the major alternative route for SV Recycling under conditions in which clathrin-mediated endocytosis (CME) becomes limiting. Figure 1 Hypothetical model for the role of different clathrin/AP complexes in SV Recycling. (A) SVs are clustered near the active zone (AZ). (B) In wild type SVs are recycled mainly by CME. Sustained high-level activity induces BE involving formation of vacuoles. ... Glyvuk et al (2010) argue that the CME of SV membranes represents a kinetic bottleneck of the Recycling pathway. Under conditions of sustained activity BE provides a compensatory mechanism to balance high exocytic load with matching endocytic activity. Vacuolar membrane invaginations are then consumed by undefined budding events that chop these membranes into small Vesicles that may re-enter the SV cycle (Figure 1A). It is this consumption step that the authors envision to depend on AP-1/σ1B (Figure 1B). The experimental evidence for this model at present remains indirect. AP-1, as its relative AP-2, is one of the major recruitment factors for clathrin and loss of either protein complex results in depletion of clathrin-coated pits from TGN/endosomes or the plasmalemma, respectively. Why then do AP-1/σ1B-KO mice accumulate clathrin-coated pits on endosome-like vacuoles? One possibility is that other σ1 isoproteins such as σ1A do a poor job in functionally replacing σ1B on endosomes. It is also possible that clathrin coats are formed by AP complexes other than AP-1 (Kim and Ryan, 2009). This question requires further study. Moreover, based on the observed SV depletion from resting terminals of σ1B-deficient neurons, an additional role for AP-1/σ1B in SV biogenesis at the TGN cannot be ruled out. An equally important open point is the origin and identity of the endosomal vacuoles found in σ1B-deficient preSynaptic terminals. The data suggest that they are derived from previously fused SV membranes and/or the plasma membrane. Similar vacuoles were seen at Drosophila neuromuscular synapses following acute inactivation of clathrin (Heerssen et al, 2008; Kasprowicz et al, 2008), a condition causing impaired neurotransmission and defective Recycling of SVs upon intense stimulation. Compensatory upregulation of BE then causes massive accumulation of vacuolar membranes inside stimulated boutons, which, however, lack morphologically recognizable coats, in contrast to σ1B mutants (Glyvuk et al, 2010). One might thus assume that the vacuoles observed in σ1B-KO mice result from BE. Whether such invaginations have fused with bona fide endosomes or solely represent a patch of internalized plasmalemma needs to be determined. The enigma remains: why do neurons require multiple routes for Recycling of functional SVs? Neurons have to respond to a broad spectrum of stimulus frequencies, yet, remain neurotransmission-competent over extended periods of time. Although the crucial role of CME in the maintenance of SV pools is undisputed, it is clear that this pathway operates with limited kinetics and sorting capacity that would limit exo-endocytic Vesicle cycling during intense stimulation. Non-specific bulk uptake of membrane in the vicinity of the release site may provide a kinetic relief from this bottleneck and allow neurons to maintain neurotransmission during high-level activity. However, BE may not come without a price. Bulk internalization of large membrane chunks will not suffice to maintain the morphological and biochemical characteristics of SVs. Thus, additional sorting steps downstream of the actual internalization event are needed and this may be precisely where AP-1/σ1B and, perhaps also AP-3, kick in. A potential path for future studies thus pertains to the question of whether σ1B-KO mice show defects in sorting of specific SV components and how this may relate to the phenotype in mice (Glyvuk et al, 2010) and by extension in human patients suffering from this form of X-linked mental retardation (Tarpey et al, 2006).
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stonins specialized adaptors for Synaptic Vesicle Recycling and beyond
Traffic, 2010Co-Authors: Tanja Maritzen, Volker Haucke, Jasmin PodufallAbstract:Stonins are a small family of evolutionarily conserved clathrin adaptor complex AP-2μ-related factors that may act as cargo-specific sorting adaptors in endocytosis and perhaps beyond. Whereas little is known about the localization and function of stonin 1, recent work suggests that stonin 2 serves as a linker between the endocytic proteins AP-2 and Eps15 and the calcium-sensing Synaptic Vesicle (SV) protein synaptotagmin 1. The molecular determinants involved in the recognition of SV cargo by the μ-homology domain of stonin 2 are evolutionarily conserved from worm to man, thereby identifying stonin 2 and its invertebrate homologs uncoordinated (UNC)-41 and stoned B as endocytic adaptors dedicated to the retrieval of surface-stranded SV proteins, most notably synaptotagmin. In this review, we summarize the current state of knowledge about mammalian stonins with a special focus on the role of stonin 2 in SV Recycling at preSynaptic nerve terminals.
Ege T. Kavalali - One of the best experts on this subject based on the ideXlab platform.
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persistence of quantal Synaptic Vesicle Recycling following dynamin depletion
bioRxiv, 2020Co-Authors: Ege T. Kavalali, Natali L. Chanaday, O Afuwape, M Kasap, Lisa M MonteggiaAbstract:Abstract Dynamins are GTPases required for pinching Vesicles off the plasma membrane once a critical curvature is reached during endocytosis. Here, we probed dynamin function in central synapses by depleting all three dynamin isoforms in postnatal hippocampal neurons. We found a decrease in the propensity of evoked neurotransmission as well as a reduction in Synaptic Vesicle numbers. Using the fluorescent reporter vGluT1-pHluorin, we observed that compensatory endocytosis after 20 Hz stimulation was arrested in ~40% of preSynaptic boutons, while remaining synapses showed only a modest effect suggesting the existence of a dynamin-independent endocytic pathway in central synapses. Surprisingly, we found that the retrieval of single Synaptic Vesicles, after either evoked or spontaneous fusion, was largely impervious to disruption of dynamins. Overall, our results suggest that classical dynamin-dependent endocytosis is not essential for retrieval of Synaptic Vesicle proteins after quantal single Synaptic Vesicle fusion.
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How do you recognize and reconstitute a Synaptic Vesicle after fusion
F1000Research, 2017Co-Authors: Natali L. Chanaday, Ege T. KavalaliAbstract:Synaptic Vesicle Recycling is essential for sustained and reliable neurotransmission. A key component of Synaptic Vesicle Recycling is the Synaptic Vesicle biogenesis process that is observed in synapses and that maintains the molecular identity of Synaptic Vesicles. However, the mechanisms by which Synaptic Vesicles are retrieved and reconstituted after fusion remain unclear. The complex molecular composition of Synaptic Vesicles renders their rapid biogenesis a daunting task. Therefore, in this context, kiss-and-run type transient fusion of Synaptic Vesicles with the plasma membrane without loss of their membrane composition and molecular identity remains a viable hypothesis that can account for the fidelity of the Synaptic Vesicle cycle. In this article, we discuss the biological implications of this problem as well as its possible molecular solutions.
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Synaptic Vesicle Recycling machinery components as potential therapeutic targets
Pharmacological Reviews, 2017Co-Authors: Ege T. KavalaliAbstract:PreSynaptic nerve terminals are highly specialized Vesicle-trafficking machines. Neurotransmitter release from these terminals is sustained by constant local Recycling of Synaptic Vesicles independent from the neuronal cell body. This independence places significant constraints on maintenance of Synaptic protein complexes and scaffolds. Key events during the Synaptic Vesicle cycle—such as exocytosis and endocytosis—require formation and disassembly of protein complexes. This extremely dynamic environment poses unique challenges for proteostasis at Synaptic terminals. Therefore, it is not surprising that subtle alterations in Synaptic Vesicle cycle-associated proteins directly or indirectly contribute to pathophysiology seen in several neurologic and psychiatric diseases. In contrast to the increasing number of examples in which preSynaptic dysfunction causes neurologic symptoms or cognitive deficits associated with multiple brain disorders, Synaptic Vesicle-Recycling machinery remains an underexplored drug target. In addition, irrespective of the involvement of preSynaptic function in the disease process, preSynaptic machinery may also prove to be a viable therapeutic target because subtle alterations in the neurotransmitter release may counter disease mechanisms, correct, or compensate for Synaptic communication deficits without the need to interfere with postSynaptic receptor signaling. In this article, we will overview critical properties of preSynaptic release machinery to help elucidate novel preSynaptic avenues for the development of therapeutic strategies against neurologic and neuropsychiatric disorders.
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Ca2+ Influx Slows Single Synaptic Vesicle Endocytosis
The Journal of neuroscience : the official journal of the Society for Neuroscience, 2011Co-Authors: Jeremy Leitz, Ege T. KavalaliAbstract:Ca²⁺-dependent Synaptic Vesicle Recycling is critical for maintenance of neurotransmission. However, uncoupling the roles of Ca²⁺ in Synaptic Vesicle fusion and retrieval has been difficult, as studies probing the role of Ca²⁺ in endocytosis relied on measurements of bulk Synaptic Vesicle retrieval. Here, to dissect the role of Ca²⁺ in these processes, we used a low signal-to-noise pHluorin-tagged vesicular probe to monitor single Synaptic Vesicle Recycling in rat hippocampal neurons. We show that Ca²⁺ increases Synaptic Vesicle fusion probability in the classical sense, but surprisingly decreases the rate of Synaptic Vesicle retrieval. This negative regulation of Synaptic Vesicle retrieval is blocked by the Ca²⁺ chelator, EGTA, as well as FK506, a specific inhibitor of Ca²⁺-calmodulin-dependent phosphatase calcineurin. The slow time course of aggregate Synaptic Vesicle retrieval detected during repetitive activity could be explained by a progressive decrease in the rate of Synaptic Vesicle retrieval during the stimulation train. These results indicate that Ca²⁺ entry during single action potentials slows the pace of subsequent Synaptic Vesicle Recycling.
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Cholesterol-dependent balance between evoked and spontaneous Synaptic Vesicle Recycling.
The Journal of physiology, 2006Co-Authors: Catherine R Wasser, Mert Ertunc, Ege T. KavalaliAbstract:Cholesterol is a prominent component of nerve terminals. To examine cholesterol's role in central neurotransmission, we treated hippocampal cultures with methyl-beta-cyclodextrin, which reversibly binds cholesterol, or mevastatin, an inhibitor of cholesterol biosynthesis, to deplete cholesterol. We also used hippocampal cultures from Niemann-Pick type C1-deficient mice defective in intracellular cholesterol trafficking. These conditions revealed an augmentation in spontaneous neurotransmission detected electrically and an increase in spontaneous Vesicle endocytosis judged by horseradish peroxidase uptake after cholesterol depletion by methyl-beta-cyclodextrin. In contrast, responses evoked by action potentials and hypertonicity were severely impaired after the same treatments. The increase in spontaneous Vesicle Recycling and the decrease in evoked neurotransmission were reversible upon cholesterol addition. Cholesterol removal did not impact on the low level of evoked neurotransmission seen in the absence of Synaptic Vesicle SNARE protein synaptobrevin-2 whereas the increase in spontaneous fusion remained. These results suggest that Synaptic cholesterol balances evoked and spontaneous neurotransmission by hindering spontaneous Synaptic Vesicle turnover and sustaining evoked exo-endocytosis.
Stephen J. Smith - One of the best experts on this subject based on the ideXlab platform.
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Retrograde regulation of Synaptic Vesicle endocytosis and Recycling
Nature Neuroscience, 2003Co-Authors: Kristina D Micheva, Joann Buchanan, Ronald W Holz, Stephen J. SmithAbstract:Sustained release of neurotransmitter depends upon the Recycling of Synaptic Vesicles. Until now, it has been assumed that Vesicle Recycling is regulated by signals from the preSynaptic bouton alone, but results from rat hippocampal neurons reported here indicate that this need not be the case. Fluorescence imaging and pharmacological analysis show that a nitric oxide (NO) signal generated postSynaptically can regulate endocytosis and at least one later step in Synaptic Vesicle Recycling. The proposed retrograde pathway involves an NMDA receptor (NMDAR)-dependent postSynaptic production of NO, diffusion of NO to a preSynaptic site, and a cGMP-dependent increase in preSynaptic phosphatidylinositol 4,5-biphosphate (PIP2). These results indicate that the regulation of Synaptic Vesicle Recycling may integrate a much broader range of neural activity signals than previously recognized, including postSynaptic depolarization and the activation of NMDARs at both immediate and nearby postSynaptic active zones.
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Synaptic Vesicle Recycling in synapsin I knock-out mice.
The Journal of cell biology, 1996Co-Authors: Timothy A. Ryan, Paul Greengard, L S Chin, Stephen J. SmithAbstract:The synapsins are a family of four neuron-specific phosphoproteins that have been implicated in the regulation of neurotransmitter release. Nevertheless, knock-out mice lacking synapsin Ia and Ib, family members that are major substrates for cAMP and Ca2+/ Calmodulin (CaM)-dependent protein kinases, show limited phenotypic changes when analyzed electrophysiologically (Rosahl, T.W., D. Spillane, M. Missler, J. Herz, D.K. Selig, J.R. Wolff, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1995. Nature (Lond.). 375: 488-493; Rosahl, T.W., M. Geppert, D. Spillane, D., J. Herz, R.E. Hammer, R.C. Malenka, and T.C. Sudhof. 1993. Cell. 75:661-670; Li, L., L.S. Chin, O. Shupliakov, L. Brodin, T.S. Sihra, O. Hvalby, V. Jensen, D. Zheng, J.O. McNamara, P. Greengard, and P. Andersen. 1995. Proc. Natl. Acad. Sci. USA. 92:9235-9239; see also Pieribone, V.A., O. Shupliakov, L. Brodin, S. Hilfiker-Rothenfluh, A.J. Czernik, and P. Greengard. 1995. Nature (Lond.). 375:493-497). Here, using the optical tracer FM 1-43, we characterize the details of Synaptic Vesicle Recycling at individual Synaptic boutons in hippocampal cell cultures derived from mice lacking synapsin I or wild-type equivalents. These studies show that both the number of Vesicles exocytosed during brief action potential trains and the total Recycling Vesicle pool are significantly reduced in the synapsin I-deficient mice, while the kinetics of endocytosis and Synaptic Vesicle repriming appear normal.
Erik M. Jorgensen - One of the best experts on this subject based on the ideXlab platform.
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μ2 adaptin facilitates but is not essential for Synaptic Vesicle Recycling in caenorhabditis elegans
Journal of Cell Biology, 2008Co-Authors: Kim Schuske, Shigeki Watanabe, Qiang Liu, Paul D Baum, Gian Garriga, Erik M. JorgensenAbstract:Synaptic Vesicles must be recycled to sustain neurotransmission, in large part via clathrin-mediated endocytosis. Clathrin is recruited to endocytic sites on the plasma membrane by the AP2 adaptor complex. The medium subunit (μ2) of AP2 binds to cargo proteins and phosphatidylinositol-4,5-bisphosphate on the cell surface. Here, we characterize the apm-2 gene (also called dpy-23), which encodes the only μ2 subunit in the nematode Caenorhabditis elegans. APM-2 is highly expressed in the nervous system and is localized to synapses; yet specific loss of APM-2 in neurons does not affect locomotion. In apm-2 mutants, clathrin is mislocalized at synapses, and Synaptic Vesicle numbers and evoked responses are reduced to 60 and 65%, respectively. Collectively, these data suggest AP2 μ2 facilitates but is not essential for Synaptic Vesicle Recycling.
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Studies of Synaptic Vesicle Endocytosis in the Nematode C. elegans
Traffic (Copenhagen Denmark), 2001Co-Authors: Todd W. Harris, Kim Schuske, Erik M. JorgensenAbstract:After Synaptic Vesicle exocytosis, Synaptic Vesicle proteins must be retrieved from the plasma membrane, sorted away from other membrane proteins, and reconstituted into a functional Synaptic Vesicle. The nematode Caenorhabditis elegans is an organism well suited for a genetic analysis of this process. In particular, three types of genetic studies have contributed to our understanding of Synaptic Vesicle endocytosis. First, screens for mutants defective in Synaptic Vesicle Recycling have identified new proteins that function specifically in neurons. Second, RNA interference has been used to quickly confirm the roles of known proteins in endocytosis. Third, gene targeting techniques have elucidated the roles of genes thought to play modulatory or subtle roles in Synaptic Vesicle Recycling. We describe a molecular model for Synaptic Vesicle Recycling and discuss how protein disruption experiments in C. elegans have contributed to this model.
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Mutations in Synaptojanin Disrupt Synaptic Vesicle Recycling
The Journal of cell biology, 2000Co-Authors: Todd W. Harris, Erika Hartwieg, H. Robert Horvitz, Erik M. JorgensenAbstract:Synaptojanin is a polyphosphoinositide phosphatase that is found at synapses and binds to proteins implicated in endocytosis. For these reasons, it has been proposed that synaptojanin is involved in the Recycling of Synaptic Vesicles. Here, we demonstrate that the unc-26 gene encodes the Caenorhabditis elegans ortholog of synaptojanin. unc-26 mutants exhibit defects in Vesicle trafficking in several tissues, but most defects are found at Synaptic termini. Specifically, we observed defects in the budding of Synaptic Vesicles from the plasma membrane, in the uncoating of Vesicles after fission, in the recovery of Vesicles from endosomes, and in the tethering of Vesicles to the cytoskeleton. Thus, these results confirm studies of the mouse synaptojanin 1 mutants, which exhibit defects in the uncoating of Synaptic Vesicles (Cremona, O., G. Di Paolo, M.R. Wenk, A. Luthi, W.T. Kim, K. Takei, L. Daniell, Y. Nemoto, S.B. Shears, R.A. Flavell, D.A. McCormick, and P. De Camilli. 1999. Cell. 99:179-188), and further demonstrate that synaptojanin facilitates multiple steps of Synaptic Vesicle Recycling.
