Motorneurons Require Cysteine String Protein-α to Maintain the Readily Releasable Vesicular Pool and Synaptic Vesicle Recycling

Decoding transcriptomic signatures of cysteine string protein alpha-mediated synapse maintenance

Synapse maintenance is essential for generating functional circuitry, and decrement in this process is a hallmark of neurodegenerative disease. Yet, little is known about synapse maintenance in vivo. Cysteine string protein α (CSPα), encoded by the Dnajc5 gene, is a synaptic vesicle chaperone that is necessary for synapse maintenance and linked to neurodegeneration. To investigate the transcriptional changes associated with synapse maintenance, we performed single-nucleus transcriptomics on the cortex of young CSPα knockout (KO) mice and littermate controls. Through differential expression and gene ontology analysis, we observed that both neurons and glial cells exhibit unique signatures in the CSPα KO brain. Significantly, all neuronal classes in CSPα KO brains show strong signatures of repression in synaptic pathways, while up-regulating autophagy-related genes. Through visualization of synapses and autophagosomes by electron microscopy, we confirmed these alterations especially in inhibitory synapses. Glial responses varied by cell type, with microglia exhibiting activation. By imputing cell-cell interactions, we found that neuron-glia interactions were specifically increased in CSPα KO mice. This was mediated by synaptogenic adhesion molecules, with the classical Neurexin1-Neuroligin 1 pair being the most prominent, suggesting that communication of glial cells with neurons is strengthened in CSPα KO mice to preserve synapse maintenance. Together, this study provides a rich dataset of transcriptional changes in the CSPα KO cortex and reveals insights into synapse maintenance and neurodegeneration.

Proximity labelling reveals effects of disease-causing mutation on the DNAJC5/cysteine string protein α interactome

Cysteine string protein α (CSPα), also known as DNAJC5, is a member of the DnaJ/Hsp40 family of co-chaperones. The name derives from a cysteine-rich domain, palmitoylation of which enables localization to intracellular membranes, notably neuronal synaptic vesicles. Mutations in the DNAJC5 gene that encodes CSPα cause autosomal dominant, adult-onset neuronal ceroid lipofuscinosis (ANCL), a rare neurodegenerative disease. As null mutations in CSP-encoding genes in flies, worms and mice similarly result in neurodegeneration, CSP is evidently an evolutionarily conserved neuroprotective protein. However, the client proteins that CSP chaperones to prevent neurodegeneration remain unclear. Traditional methods for identifying protein-protein interactions such as yeast 2-hybrid and affinity purification approaches are poorly suited to CSP, due to its requirement for membrane anchoring and its tendency to aggregate after cell lysis. Therefore, we employed proximity labelling, which enables identification of interacting proteins in situ in living cells via biotinylation. Neuroendocrine PC12 cell lines stably expressing wild type or L115R ANCL mutant CSP constructs fused to miniTurbo were generated; then the biotinylated proteomes were analysed by liquid chromatographymass spectrometry (LCMS) and validated by western blotting. This confirmed several known CSP-interacting proteins, such as Hsc70 and SNAP-25, but also revealed novel binding proteins, including STXBP1/Munc18-1. Interestingly, some protein interactions (such as Hsc70) were unaffected by the L115R mutation, whereas others (including SNAP-25 and STXBP1/Munc18-1) were inhibited. These results define the CSP interactome in a neuronal model cell line and reveal interactions that are affected by ANCL mutation and hence may contribute to the neurodegeneration seen in patients.

Decoding transcriptomic signatures of Cysteine String Protein alpha-mediated synapse maintenance

Synapse maintenance is essential for generating functional circuitry and decrement in this process is a hallmark of neurodegenerative disease. While we are beginning to understand the basis of synapse formation, much less is known about synapse maintenance in vivo. Cysteine string protein α (CSPα), encoded by the Dnajc5 gene, is a synaptic vesicle chaperone that is necessary for synapse maintenance and linked to neurodegeneration. To investigate the transcriptional changes associated with synapse maintenance, we performed single nucleus transcriptomics on the cortex of young CSPα knockout (KO) mice and littermate controls. Through differential expression and gene ontology analysis, we observed that both neurons and glial cells exhibit unique signatures in CSPα KO brain. Significantly all neurons in CSPα KO brains show strong signatures of repression in synaptic pathways, while upregulating autophagy related genes. Through visualization of synapses and autophagosomes by electron microscopy, we confirmed these alterations especially in inhibitory synapses. By imputing cell-cell interactions, we found that neuron-glia interactions were specifically increased in CSPα KO mice. This was mediated by synaptogenic adhesion molecules, including the classical Neurexin1-Neuroligin 1 pair, suggesting that communication of glial cells with neurons is strengthened in CSPα KO mice in an attempt to achieve synapse maintenance. Together, this study reveals unique cellular and molecular transcriptional changes in CSPα KO cortex and provides new insights into synapse maintenance and neurodegeneration.

NMJ-related diseases beyond the congenital myasthenic syndromes

Neuromuscular junctions (NMJs) are a special type of chemical synapse that transmits electrical stimuli from motor neurons (MNs) to their innervating skeletal muscle to induce a motor response. They are an ideal model for the study of synapses, given their manageable size and easy accessibility. Alterations in their morphology or function lead to neuromuscular disorders, such as the congenital myasthenic syndromes, which are caused by mutations in proteins located in the NMJ. In this review, we highlight novel potential candidate genes that may cause or modify NMJs-related pathologies in humans by exploring the phenotypes of hundreds of mouse models available in the literature. We also underscore the fact that NMJs may differ between species, muscles or even sexes. Hence the importance of choosing a good model organism for the study of NMJ-related diseases: only taking into account the specific features of the mammalian NMJ, experimental results would be efficiently translated to the clinic.

Neurotransmitter release progressively desynchronizes in induced human neurons during synapse maturation and aging

Rapid release of neurotransmitters in synchrony with action potentials is considered a key hardwired property of synapses. Here, in glutamatergic synapses formed between induced human neurons, we show that action potential-dependent neurotransmitter release becomes progressively desynchronized as synapses mature and age. In this solely excitatory network, the emergence of NMDAR-mediated transmission elicits endoplasmic reticulum (ER) stress leading to downregulation of key presynaptic molecules, synaptotagmin-1 and cysteine string protein α, that synchronize neurotransmitter release. The emergence of asynchronous release with neuronal maturity and subsequent aging is maintained by the high-affinity Ca2+ sensor synaptotagmin-7 and suppressed by the introduction of GABAergic transmission into the network, inhibition of NMDARs, and ER stress. These results suggest that long-term disruption of excitation-inhibition balance affects the synchrony of excitatory neurotransmission in human synapses.

Cysteine string protein alpha accumulates with early pre-synaptic dysfunction in Alzheimer’s disease

In Alzheimer’s disease, synapse loss causes memory and cognitive impairment. However, the mechanisms underlying synaptic degeneration in Alzheimer’s disease are not well understood. In the hippocampus, alterations in the level of cysteine string protein alpha, a molecular co-chaperone at the pre-synaptic terminal, occur prior to reductions in synaptophysin, suggesting that it is a very sensitive marker of synapse degeneration in Alzheimer’s. Here, we identify putative extracellular accumulations of cysteine string alpha protein, which are proximal to beta-amyloid deposits in post-mortem human Alzheimer’s brain and in the brain of a transgenic mouse model of Alzheimer’s disease. Cysteine string protein alpha, at least some of which is phosphorylated at serine 10, accumulates near the core of beta-amyloid deposits and does not co-localize with hyperphosphorylated tau, dystrophic neurites or glial cells. Using super-resolution microscopy and array tomography, cysteine string protein alpha was found to accumulate to a greater extent than other pre-synaptic proteins and at a comparatively great distance from the plaque core. This indicates that cysteine string protein alpha is most sensitive to being released from pre-synapses at low concentrations of beta-amyloid oligomers. Cysteine string protein alpha accumulations were also evident in other neurodegenerative diseases, including some fronto-temporal lobar dementias and Lewy body diseases, but only in the presence of amyloid plaques. Our findings are consistent with suggestions that pre-synapses are affected early in Alzheimer’s disease, and they demonstrate that cysteine string protein alpha is a more sensitive marker for early pre-synaptic dysfunction than traditional synaptic markers. We suggest that cysteine string protein alpha should be used as a pathological marker for early synaptic disruption caused by beta-amyloid.

Safeguarding Lysosomal Homeostasis by DNAJC5/CSPα-Mediated Unconventional Protein Secretion and Endosomal Microautophagy

Neuronal ceroid lipofuscinosis (NCL) is a collection of genetically inherited neurological disorders characterized by vision loss, seizure, brain death, and premature lethality. At the cellular level, a key pathologic hallmark of NCL is the build-up of autofluorescent storage materials (AFSM) in lysosomes of both neurons and non-neuronal cells. Molecular dissection of the genetic lesions underlying NCLs has shed significant insights into how disruption of lysosomal homeostasis may lead to lipofuscin accumulation and NCLs. Intriguingly, recent studies on DNAJC5/CSPα, a membrane associated HSC70 co-chaperone, have unexpectedly linked lipofuscin accumulation to two intimately coupled protein quality control processes at endolysosomes. This review discusses how deregulation of unconventional protein secretion and endosomal microautophagy (eMI) contributes to lipofuscin accumulation and neurodegeneration.

Adrenergic receptors control frequency-dependent switching of the exocytosis mode between "full-collapse" and "kiss-and-run" in murine motor nerve terminal

AIMS

Neurotransmitter release from the synaptic vesicles can occur through two modes of exocytosis: "full-collapse" or "kiss-and-run". Here we investigated how increasing the nerve activity and pharmacological stimulation of adrenoceptors can influence the mode of exocytosis in the motor nerve terminal.

METHODS

Recording of endplate potentials with intracellular microelectrodes was used to estimate acetylcholine release. A fluorescent dye FM1-43 and its quenching with sulforhodamine 101 were utilized to visualize synaptic vesicle recycling.

KEY FINDINGS

An increase in the frequency of stimulation led to a decrease in the rate of FM1-43 unloading despite the higher number of quanta released. High frequency activity promoted neurotransmitter release via the kiss-and-run mechanism. This was confirmed by experiments utilizing (I) FM1-43 dye quencher, that is able to pass into the synaptic vesicle via fusion pore, and (II) loading of FM1-43 by compensatory endocytosis. Noradrenaline and specific α2-adrenoreceptors agonist, dexmedetomidine, controlled the mode of synaptic vesicle recycling at high frequency activity. Their applications favored neurotransmitter release via full-collapse exocytosis rather than the kiss-and-run pathway.

SIGNIFICANCE

At the diaphragm neuromuscular junctions, neuronal commands are translated into contractions necessary for respiration. During stress, an increase in discharge rate of the phrenic nerve shifts the exocytosis from the full-collapse to the kiss-and-run mode. The stress-related molecule, noradrenaline, restricts neurotransmitter release in response to a high frequency activity, and prevents the shift in the mode of exocytosis through α2-adrenoceptor activation. This may be a component of the mechanism that limits overstimulation of the respiratory system during stress.

Autophagy in the Neuronal Ceroid Lipofuscinoses (Batten Disease)

The neuronal ceroid lipofuscinoses (NCLs), also referred to as Batten disease, are a family of neurodegenerative diseases that affect all age groups and ethnicities around the globe. At least a dozen NCL subtypes have been identified that are each linked to a mutation in a distinct ceroid lipofuscinosis neuronal (CLN) gene. Mutations in CLN genes cause the accumulation of autofluorescent lipoprotein aggregates, called ceroid lipofuscin, in neurons and other cell types outside the central nervous system. The mechanisms regulating the accumulation of this material are not entirely known. The CLN genes encode cytosolic, lysosomal, and integral membrane proteins that are associated with a variety of cellular processes, and accumulated evidence suggests they participate in shared or convergent biological pathways. Research across a variety of non-mammalian and mammalian model systems clearly supports an effect of CLN gene mutations on autophagy, suggesting that autophagy plays an essential role in the development and progression of the NCLs. In this review, we summarize research linking the autophagy pathway to the NCLs to guide future work that further elucidates the contribution of altered autophagy to NCL pathology.

Disorders of synaptic vesicle fusion machinery

The revolution in genetic technology has ushered in a new age for our understanding of the underlying causes of neurodevelopmental, neuromuscular and neurodegenerative disorders, revealing that the presynaptic machinery governing synaptic vesicle fusion is compromised in many of these neurological disorders. This builds upon decades of research showing that disturbance to neurotransmitter release via toxins can cause acute neurological dysfunction. In this review, we focus on disorders of synaptic vesicle fusion caused either by toxic insult to the presynapse or alterations to genes encoding the key proteins that control and regulate fusion: the SNARE proteins (synaptobrevin, syntaxin-1 and SNAP-25), Munc18, Munc13, synaptotagmin, complexin, CSPα, α-synuclein, PRRT2 and tomosyn. We discuss the roles of these proteins and the cellular and molecular mechanisms underpinning neurological deficits in these disorders.

A Drosophila model of neuronal ceroid lipofuscinosis CLN4 reveals a hypermorphic gain of function mechanism

The autosomal dominant neuronal ceroid lipofuscinoses (NCL) CLN4 is caused by mutations in the synaptic vesicle (SV) protein CSPα. We developed animal models of CLN4 by expressing CLN4 mutant human CSPα (hCSPα) in Drosophila neurons. Similar to patients, CLN4 mutations induced excessive oligomerization of hCSPα and premature lethality in a dose-dependent manner. Instead of being localized to SVs, most CLN4 mutant hCSPα accumulated abnormally, and co-localized with ubiquitinated proteins and the prelysosomal markers HRS and LAMP1. Ultrastructural examination revealed frequent abnormal membrane structures in axons and neuronal somata. The lethality, oligomerization and prelysosomal accumulation induced by CLN4 mutations was attenuated by reducing endogenous wild type (WT) dCSP levels and enhanced by increasing WT levels. Furthermore, reducing the gene dosage of Hsc70 also attenuated CLN4 phenotypes. Taken together, we suggest that CLN4 alleles resemble dominant hypermorphic gain of function mutations that drive excessive oligomerization and impair membrane trafficking.

Depalmitoylation by Palmitoyl-Protein Thioesterase 1 in Neuronal Health and Degeneration

Protein palmitoylation is the post-translational, reversible addition of a 16-carbon fatty acid, palmitate, to proteins. Protein palmitoylation has recently garnered much attention, as it robustly modifies the localization and function of canonical signaling molecules and receptors. Protein depalmitoylation, on the other hand, is the process by which palmitic acid is removed from modified proteins and contributes, therefore, comparably to palmitoylated-protein dynamics. Palmitoylated proteins also require depalmitoylation prior to lysosomal degradation, demonstrating the significance of this process in protein sorting and turnover. Palmitoylation and depalmitoylation serve as particularly crucial regulators of protein function in neurons, where a specialized molecular architecture and cholesterol-rich membrane microdomains contribute to synaptic transmission. Three classes of depalmitoylating enzymes are currently recognized, the acyl protein thioesterases, α/β hydrolase domain-containing 17 proteins (ABHD17s), and the palmitoyl-protein thioesterases (PPTs). However, a clear picture of depalmitoylation has not yet emerged, in part because the enzyme-substrate relationships and specific functions of depalmitoylation are only beginning to be uncovered. Further, despite the finding that loss-of-function mutations affecting palmitoyl-protein thioesterase 1 (PPT1) function cause a severe pediatric neurodegenerative disease, the role of PPT1 as a depalmitoylase has attracted relatively little attention. Understanding the role of depalmitoylation by PPT1 in neuronal function is a fertile area for ongoing basic science and translational research that may have broader therapeutic implications for neurodegeneration. Here, we will briefly introduce the rapidly growing field surrounding protein palmitoylation and depalmitoylation, then will focus on the role of PPT1 in development, health, and neurological disease.

Loss of postnatal quiescence of neural stem cells through mTOR activation upon genetic removal of cysteine string protein-α

Neural stem cells continuously generate newborn neurons that integrate into and modify neural circuitry in the adult hippocampus. The molecular mechanisms that regulate or perturb neural stem cell proliferation and differentiation, however, remain poorly understood. Here, we have found that mouse hippocampal radial glia-like (RGL) neural stem cells express the synaptic cochaperone cysteine string protein-α (CSP-α). Remarkably, in CSP-α knockout mice, RGL stem cells lose quiescence postnatally and enter into a high-proliferation regime that increases the production of neural intermediate progenitor cells, thereby exhausting the hippocampal neural stem cell pool. In cell culture, stem cells in hippocampal neurospheres display alterations in proliferation for which hyperactivation of the mechanistic target of rapamycin (mTOR) signaling pathway is the primary cause of neurogenesis deregulation in the absence of CSP-α. In addition, RGL cells lose quiescence upon specific conditional targeting of CSP-α in adult neural stem cells. Our findings demonstrate an unanticipated cell-autonomic and circuit-independent disruption of postnatal neurogenesis in the absence of CSP-α and highlight a direct or indirect CSP-α/mTOR signaling interaction that may underlie molecular mechanisms of brain dysfunction and neurodegeneration.

GPCR regulation of secretion

Modulation of neurotransmitter exocytosis by activated Gi/o coupled G-protein coupled receptors (GPCRs) is a universal regulatory mechanism used both to avoid overstimulation and to influence circuitry. One of the known modulation mechanisms is the interaction between Gβγ and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNAREs). There are 5 Gβ and 12 Gγ subunits, but specific Gβγs activated by a given GPCR and the specificity to effectors, such as SNARE, in vivo are not known. Although less studied, Gβγ binding to the exocytic fusion machinery (i.e. SNARE) provides a more direct regulatory mechanism for neurotransmitter release. Here, we review some recent insights in the architecture of the synaptic terminal, modulation of synaptic transmission, and implications of G protein modulation of synaptic transmission in diseases. Numerous presynaptic proteins are involved in the architecture of synaptic terminals, particularly the active zone, and their importance in the regulation of exocytosis is still not completely understood. Further understanding of the Gβγ-SNARE interaction and the architecture and mechanisms of exocytosis may lead to the discovery of novel therapeutic targets to help patients with various disorders such as hypertension, attention-deficit/hyperactivity disorder, post-traumatic stress disorder, and acute/chronic pain.

FUS causes synaptic hyperexcitability in Drosophila dendritic arborization neurons

Mutations in the nuclear localization signal of the RNA binding protein FUS cause both Frontotemporal Dementia (FTD) and Amyotrophic Lateral Sclerosis (ALS). These mutations result in a loss of FUS from the nucleus and the formation of FUS-containing cytoplasmic aggregates in patients. To better understand the role of cytoplasmic FUS mislocalization in the pathogenesis of ALS, we identified a population of cholinergic neurons in Drosophila that recapitulate these pathologic hallmarks. Expression of mutant FUS or the Drosophila homolog, Cabeza (Caz), in class IV dendritic arborization neurons results in cytoplasmic mislocalization and axonal transport to presynaptic terminals. Interestingly, overexpression of FUS or Caz causes the progressive loss of neuronal projections, reduction of synaptic mitochondria, and the appearance of large calcium transients within the synapse. Additionally, we find that overexpression of mutant but not wild type FUS results in a reduction in presynaptic Synaptotagmin, an integral component of the neurotransmitter release machinery, and mutant Caz specifically disrupts axonal transport and induces hyperexcitability. These results suggest that FUS/Caz overexpression disrupts neuronal function through multiple mechanisms, and that ALS-causing mutations impair the transport of synaptic vesicle proteins and induce hyperexcitability.

Newly produced synaptic vesicle proteins are preferentially used in synaptic transmission

Aged proteins can become hazardous to cellular function, by accumulating molecular damage. This implies that cells should preferentially rely on newly produced ones. We tested this hypothesis in cultured hippocampal neurons, focusing on synaptic transmission. We found that newly synthesized vesicle proteins were incorporated in the actively recycling pool of vesicles responsible for all neurotransmitter release during physiological activity. We observed this for the calcium sensor Synaptotagmin 1, for the neurotransmitter transporter VGAT, and for the fusion protein VAMP2 (Synaptobrevin 2). Metabolic labeling of proteins and visualization by secondary ion mass spectrometry enabled us to query the entire protein makeup of the actively recycling vesicles, which we found to be younger than that of non‐recycling vesicles. The young vesicle proteins remained in use for up to ~ 24 h, during which they participated in recycling a few hundred times. They were afterward reluctant to release and were degraded after an additional ~ 24–48 h. We suggest that the recycling pool of synaptic vesicles relies on newly synthesized proteins, while the inactive reserve pool contains older proteins.

Post-transcriptional Inhibition of Hsc70-4/HSPA8 Expression Leads to Synaptic Vesicle Cycling Defects in Multiple Models of ALS

Amyotrophic lateral sclerosis (ALS) is a synaptopathy accompanied by the presence of cytoplasmic aggregates containing TDP-43, an RNA-binding protein linked to ∼97% of ALS cases. Using a Drosophila model of ALS, we show that TDP-43 overexpression (OE) in motor neurons results in decreased expression of the Hsc70-4 chaperone at the neuromuscular junction (NMJ). Mechanistically, mutant TDP-43 sequesters hsc70-4 mRNA and impairs its translation. Expression of the Hsc70-4 ortholog, HSPA8, is also reduced in primary motor neurons and NMJs of mice expressing mutant TDP-43. Electrophysiology, imaging, and genetic interaction experiments reveal TDP-43-dependent defects in synaptic vesicle endocytosis. These deficits can be partially restored by OE of Hsc70-4, cysteine-string protein (Csp), or dynamin. This suggests that TDP-43 toxicity results in part from impaired activity of the synaptic CSP/Hsc70 chaperone complex impacting dynamin function. Finally, Hsc70-4/HSPA8 expression is also post-transcriptionally reduced in fly and human induced pluripotent stem cell (iPSC) C9orf72 models, suggesting a common disease pathomechanism.

The Establishment of a CSF-Contacting Nucleus "Knockout" Model Animal

To establish an entirely cerebrospinal fluid (CSF)-contacting nucleus-deficient model animal, we used cholera toxin B subunit (CB)- saporin (SAP), which is an analog of CB-HRP that specifically labels the CSF-contacting nucleus, to exclusively damage the nucleus. The effectiveness and specificity of the ablation were evaluated upon days 1-10 after CB-SAP microinjection into the brain ventricular system. The vital status, survival, and common physiological parameters of the model animals were also assessed during the experimental period. The results demonstrated that CB-SAP damaged only the CSF-contacting nucleus, but not other functional structures, in the brain. The complete ablation occurred by day 7 after CB-SAP microinjection. A model animal that had no CSF-contacting nucleus was established after survival beyond that time point. No obvious effects were observed in the vital status of the model animals, and their survival was ensured. The common physiological parameters of model animals were stable. The present study provides a method to establish a CSF-contacting nucleus "knockout" model animal, which is similar to a gene knockout model animal for studying this particular nucleus in vivo.

DNAJ Proteins in neurodegeneration: essential and protective factors

Maintenance of protein homeostasis is vitally important in post-mitotic cells, particularly neurons. Neurodegenerative diseases such as polyglutamine expansion disorders-like Huntington's disease or spinocerebellar ataxia (SCA), Alzheimer's disease, fronto-temporal dementia (FTD), amyotrophic lateral sclerosis (ALS) and Parkinson's disease-are often characterized by the presence of inclusions of aggregated protein. Neurons contain complex protein networks dedicated to protein quality control and maintaining protein homeostasis, or proteostasis. Molecular chaperones are a class of proteins with prominent roles in maintaining proteostasis, which act to bind and shield hydrophobic regions of nascent or misfolded proteins while allowing correct folding, conformational changes and enabling quality control. There are many different families of molecular chaperones with multiple functions in proteostasis. The DNAJ family of molecular chaperones is the largest chaperone family and is defined by the J-domain, which regulates the function of HSP70 chaperones. DNAJ proteins can also have multiple other protein domains such as ubiquitin-interacting motifs or clathrin-binding domains leading to diverse and specific roles in the cell, including targeting client proteins for degradation via the proteasome, chaperone-mediated autophagy and uncoating clathrin-coated vesicles. DNAJ proteins can also contain ER-signal peptides or mitochondrial leader sequences, targeting them to specific organelles in the cell. In this review, we discuss the multiple roles of DNAJ proteins and in particular focus on the role of DNAJ proteins in protecting against neurodegenerative diseases caused by misfolded proteins. We also discuss the role of DNAJ proteins as direct causes of inherited neurodegeneration via mutations in DNAJ family genes.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Differential regulation of synaptic AP-2/clathrin vesicle uncoating in synaptic plasticity

AP-1/σ1B-deficiency causes X-linked intellectual disability. AP-1/σ1B -/- mice have impaired synaptic vesicle recycling, fewer synaptic vesicles and enhanced endosome maturation mediated by AP-1/σ1A. Despite defects in synaptic vesicle recycling synapses contain two times more endocytic AP-2 clathrin-coated vesicles. We demonstrate increased formation of two classes of AP-2/clathrin coated vesicles. One which uncoats readily and a second with a stabilised clathrin coat. Coat stabilisation is mediated by three molecular mechanisms: reduced recruitment of Hsc70 and synaptojanin1 and enhanced μ2/AP-2 phosphorylation and activation. Stabilised AP-2 vesicles are enriched in the structural active zone proteins Git1 and stonin2 and synapses contain more Git1. Endocytosis of the synaptic vesicle exocytosis regulating Munc13 isoforms are differentially effected. Regulation of synaptic protein endocytosis by the differential stability of AP-2/clathrin coats is a novel molecular mechanism of synaptic plasticity.

Activity-Dependent Degradation of Synaptic Vesicle Proteins Requires Rab35 and the ESCRT Pathway

Synaptic vesicle (SV) pools must maintain a functional repertoire of proteins to efficiently release neurotransmitter. The accumulation of old or damaged proteins on SV membranes is linked to synaptic dysfunction and neurodegeneration. However, despite the importance of SV protein turnover for neuronal health, the molecular mechanisms underlying this process are largely unknown. Here, we have used dissociated rat hippocampal neurons to investigate the pathway for SV protein degradation. We find that neuronal activity drives the degradation of a subset of SV proteins and that the endosomal sorting complex required for transport (ESCRT) machinery and SV-associated GTPase Rab35 are key elements of this use-dependent degradative pathway. Specifically, neuronal activity induces Rab35 activation and binding to the ESCRT-0 protein Hrs, which we have identified as a novel Rab35 effector. These actions recruit the downstream ESCRT machinery to SV pools, thereby initiating SV protein degradation via the ESCRT pathway. Our findings show that the Rab35/ESCRT pathway facilitates the activity-dependent removal of specific proteins from SV pools, thereby maintaining presynaptic protein homeostasis.

SIGNIFICANCE STATEMENT

Synaptic transmission is mediated by the release of chemical neurotransmitters from synaptic vesicles (SVs). This tightly regulated process requires a functional pool of SVs, necessitating cellular mechanisms for removing old or damaged proteins that could impair SV cycling. Here, we show that a subset of SV proteins is degraded in an activity-dependent manner and that key steps in this degradative pathway are the activation of the small GTPase Rab35 and the subsequent recruitment of the endosomal sorting complex required for transport (ESCRT) machinery to SV pools. Further, we demonstrate that ESCRT-0 component Hrs is an effector of Rab35, thus providing novel mechanistic insight into the coupling of neuronal activity with SV protein degradation and the maintenance of functional SV pools.

The Role of Co-chaperones in Synaptic Proteostasis and Neurodegenerative Disease

Synapses must be preserved throughout an organism's lifespan to allow for normal brain function and behavior. Synapse maintenance is challenging given the long distances between the termini and the cell body, reliance on axonal transport for delivery of newly synthesized presynaptic proteins, and high rates of synaptic vesicle exo- and endocytosis. Hence, synapses rely on efficient proteostasis mechanisms to preserve their structure and function. To this end, the synaptic compartment has specific chaperones to support its functions. Without proper synaptic chaperone activity, local proteostasis imbalances lead to neurotransmission deficits, dismantling of synapses, and neurodegeneration. In this review, we address the roles of four synaptic chaperones in the maintenance of the nerve terminal, as well as their genetic links to neurodegenerative disease. Three of these are Hsp40 co-chaperones (DNAJs): Cysteine String Protein alpha (CSPα; DNAJC5), auxilin (DNAJC6), and Receptor-Mediated Endocytosis 8 (RME-8; DNAJC13). These co-chaperones contain a conserved J domain through which they form a complex with heat shock cognate 70 (Hsc70), enhancing the chaperone's ATPase activity. CSPα is a synaptic vesicle protein known to chaperone the t-SNARE SNAP-25 and the endocytic GTPase dynamin-1, thereby regulating synaptic vesicle exocytosis and endocytosis. Auxilin binds assembled clathrin cages, and through its interactions with Hsc70 leads to the uncoating of clathrin-coated vesicles, a process necessary for the regeneration of synaptic vesicles. RME-8 is a co-chaperone on endosomes and may have a role in clathrin-coated vesicle endocytosis on this organelle. These three co-chaperones maintain client function by preserving folding and assembly to prevent client aggregation, but they do not break down aggregates that have already formed. The fourth synaptic chaperone we will discuss is Heat shock protein 110 (Hsp110), which interacts with Hsc70, DNAJAs, and DNAJBs to constitute a disaggregase. Hsp110-related disaggregase activity is present at the synapse and is known to protect against aggregation of proteins such as α-synuclein. Congruent with their importance in the nervous system, mutations of these co-chaperones lead to familial neurodegenerative disease. CSPα mutations cause adult neuronal ceroid lipofuscinosis, while auxilin mutations result in early-onset Parkinson's disease, demonstrating their significance in preservation of the nervous system.

Neurons Export Extracellular Vesicles Enriched in Cysteine String Protein and Misfolded Protein Cargo

The fidelity of synaptic transmission depends on the integrity of the protein machinery at the synapse. Unfolded synaptic proteins undergo refolding or degradation in order to maintain synaptic proteostasis and preserve synaptic function, and buildup of unfolded/toxic proteins leads to neuronal dysfunction. Many molecular chaperones contribute to proteostasis, but one in particular, cysteine string protein (CSPα), is critical for proteostasis at the synapse. In this study we report that exported vesicles from neurons contain CSPα. Extracellular vesicles (EV’s) have been implicated in a wide range of functions. However, the functional significance of neural EV’s remains to be established. Here we demonstrate that co-expression of CSPα with the disease-associated proteins, polyglutamine expanded protein 72Q huntingtin^ex°ⁿ¹ or superoxide dismutase-1 (SOD-1^G93A) leads to the cellular export of both 72Q huntingtin^ex°ⁿ¹ and SOD-1^G93A via EV’s. In contrast, the inactive CSPα_HPD-AAA mutant does not facilitate elimination of misfolded proteins. Furthermore, CSPα-mediated export of 72Q huntingtin^ex°ⁿ¹ is reduced by the polyphenol, resveratrol. Our results indicate that by assisting local lysosome/proteasome processes, CSPα-mediated removal of toxic proteins via EVs plays a central role in synaptic proteostasis and CSPα thus represents a potential therapeutic target for neurodegenerative diseases.

CSPα, a Molecular Co-chaperone Essential for Short and Long-Term Synaptic Maintenance

Cysteine string protein α (CSPα) is a vesicle protein located in the presynaptic terminal of most synapses. CSPα is an essential molecular co-chaperone that facilitates the correct folding of proteins and the assembly of the exocytic machinery. The absence of this protein leads to altered neurotransmitter release and neurodegeneration in multiple model systems, from flies to mice. In humans, CSPα mutations are associated with the development of neuronal ceroid lipofuscinosis (NCL), a neurodegenerative disease characterized by intracellular accumulation of lysosomal material. Here, we review the physiological role of CSPα and the pathology resulting from the homozygous deletion of the gene or its mutations. In addition, we investigate whether long-term moderate reduction of the protein produces motor dysfunction. We found that 1-year-old CSPα heterozygous mice display a reduced ability to sustain motor unit recruitment during repetitive stimulation, which indicates that physiological levels of CSPα are required for normal neuromuscular responses in mice and, likely, in humans.

Vitamin D and/or calcium deficient diets may differentially affect muscle fiber neuromuscular junction innervation

INTRODUCTION

There is evidence that supports a role for Vitamin D (Vit. D) in muscle. The exact mechanism by which Vit. D deficiency impairs muscle strength and function is not clear.

METHODS

Three-week-old mice were fed diets with varied combinations of Vit. D and Ca²⁺ deficiency. Behavioral testing, genomic and protein analysis, and muscle histology were performed with a focus on neuromuscular junction (NMJ) -related genes.

RESULTS

Vit. D and Ca²⁺ deficient mice performed more poorly on given behavioral tasks than animals with Vit. D deficiency alone. Genomic and protein analysis of the soleus and tibialis anterior muscles revealed changes in several Vit. D metabolic, NMJ-related, and protein chaperoning and refolding genes.

CONCLUSIONS

These data suggest that detrimental effects of a Vit. D deficient or a Vit. D and Ca²⁺ deficient diet may be a result of differential alterations in the structure and function of the NMJ and a lack of a sustained stress response in muscles. Muscle Nerve 54: 1120-1132, 2016.

α-Synuclein Mutation Inhibits Endocytosis at Mammalian Central Nerve Terminals

α-Synuclein (α-syn) missense and multiplication mutations have been suggested to cause neurodegenerative diseases, including Parkinson's disease (PD) and dementia with Lewy bodies. Before causing the progressive neuronal loss, α-syn mutations impair exocytosis, which may contribute to eventual neurodegeneration. To understand how α-syn mutations impair exocytosis, we developed a mouse model that selectively expressed PD-related human α-syn A53T (h-α-synA53T) mutation at the calyx of Held terminals, where release mechanisms can be dissected with a patch-clamping technique. With capacitance measurement of endocytosis, we reported that h-α-synA53T, either expressed transgenically or dialyzed in the short term in calyces, inhibited two of the most common forms of endocytosis, the slow and rapid vesicle endocytosis at mammalian central synapses. The expression of h-α-synA53Tin calyces also inhibited vesicle replenishment to the readily releasable pool. These findings may help to understand how α-syn mutations impair neurotransmission before neurodegeneration.

SIGNIFICANCE STATEMENT

α-Synuclein (α-syn) missense or multiplication mutations may cause neurodegenerative diseases, such as Parkinson's disease and dementia with Lewy bodies. The initial impact of α-syn mutations before neuronal loss is impairment of exocytosis, which may contribute to eventual neurodegeneration. The mechanism underlying impairment of exocytosis is poorly understood. Here we report that an α-syn mutant, the human α-syn A53T, inhibited two of the most commonly observed forms of endocytosis, slow and rapid endocytosis, at a mammalian central synapse. We also found that α-syn A53T inhibited vesicle replenishment to the readily releasable pool. These results may contribute to accounting for the widely observed early synaptic impairment caused by α-syn mutations in the progression toward neurodegeneration.

Two for the Price of One: A Neuroprotective Chaperone Kit within NAD Synthase Protein NMNAT2

One of the most fascinating properties of the brain is the ability to function smoothly across decades of a lifespan. Neurons are nondividing mature cells specialized in fast electrical and chemical communication at synapses. Often, neurons and synapses operate at high levels of activity through sophisticated arborizations of long axons and dendrites that nevertheless stay healthy throughout years. On the other hand, aging and activity-dependent stress strike onto the protein machineries turning proteins unfolded and prone to form pathological aggregates associated with neurodegeneration. How do neurons protect from those insults and remain healthy for their whole life? Ali and colleagues now present a molecular mechanism by which the enzyme nicotinamide mononucleotide adenylyltransferase 2 (NMNAT2) acts not only as a NAD synthase involved in axonal maintenance but as a molecular chaperone helping neurons to overcome protein unfolding and protein aggregation.

This Primer examines recent research showing that a brain enzyme known to ameliorate excitoxicity and axonal degeneration also acts as a molecular chaperone to prevent protein aggregation.

DnaJ/Hsc70 chaperone complexes control the extracellular release of neurodegenerative-associated proteins

It is now known that proteins associated with neurodegenerative disease can spread throughout the brain in a prionlike manner. However, the mechanisms regulating the trans-synaptic spread propagation, including the neuronal release of these proteins, remain unknown. The interaction of neurodegenerative disease-associated proteins with the molecular chaperone Hsc70 is well known, and we hypothesized that much like disaggregation, refolding, degradation, and even normal function, Hsc70 may dictate the extracellular fate of these proteins. Here, we show that several proteins, including TDP-43, α-synuclein, and the microtubule-associated protein tau, can be driven out of the cell by an Hsc70 co-chaperone, DnaJC5. In fact, DnaJC5 overexpression induced tau release in cells, neurons, and brain tissue, but only when activity of the chaperone Hsc70 was intact and when tau was able to associate with this chaperone. Moreover, release of tau from neurons was reduced in mice lacking the DnaJC5 gene and when the complement of DnaJs in the cell was altered. These results demonstrate that the dynamics of DnaJ/Hsc70 complexes are critically involved in the release of neurodegenerative disease proteins.

The Notch ligand E3 ligase, Mind Bomb1, regulates glutamate receptor localization in Drosophila

The postsynaptic density (PSD) is a protein-rich network important for the localization of postsynaptic glutamate receptors (GluRs) and for signaling downstream of these receptors. Although hundreds of PSD proteins have been identified, many are functionally uncharacterized. We conducted a reverse genetic screen for mutations that affected GluR localization using Drosophila genes that encode homologs of mammalian PSD proteins. 42.8% of the mutants analyzed exhibited a significant change in GluR localization at the third instar larval neuromuscular junction (NMJ), a model synapse that expresses homologs of AMPA receptors. We identified the E3 ubiquitin ligase, Mib1, which promotes Notch signaling, as a regulator of synaptic GluR localization. Mib1 positively regulates the localization of the GluR subunits GluRIIA, GluRIIB, and GluRIIC. Mutations in mib1 and ubiquitous expression of Mib1 that lacks its ubiquitin ligase activity result in the loss of synaptic GluRIIA-containing receptors. In contrast, overexpression of Mib1 in all tissues increases postsynaptic levels of GluRIIA. Cellular levels of Mib1 are also important for the structure of the presynaptic motor neuron. While deficient Mib1 signaling leads to overgrowth of the NMJ, ubiquitous overexpression of Mib1 results in a reduction in the number of presynaptic motor neuron boutons and branches. These synaptic changes may be secondary to attenuated glutamate release from the presynaptic motor neuron in mib1 mutants as mib1 mutants exhibit significant reductions in the vesicle-associated protein cysteine string protein and in the frequency of spontaneous neurotransmission.

Expression profile of a Caenorhabditis elegans model of adult neuronal ceroid lipofuscinosis reveals down regulation of ubiquitin E3 ligase components

Cysteine string protein (CSP) is a chaperone of the Dnaj/Hsp40 family of proteins and is essential for synaptic maintenance. Mutations in the human gene encoding CSP, DNAJC5, cause adult neuronal ceroid lipofucinosis (ANCL) which is characterised by progressive dementia, movement disorders, seizures and premature death. CSP null models in mice, flies and worms have been shown to also exhibit similar neurodegenerative phenotypes. Here we have explored the mechanisms underlying ANCL disease progression using Caenorhaditis elegans mutant strains of dnj-14, the worm orthologue of DNAJC5. Transcriptional profiling of these mutants compared to control strains revealed a broad down-regulation of ubiquitin proteasome system (UPS)-related genes, in particular, components of multimeric RING E3 ubiquitin ligases including F-Box, SKR and BTB proteins. These data were supported by the observation that dnj-14 mutant worm strains expressing a GFP-tagged ubiquitin fusion degradation substrate exhibited decreased ubiquitylated protein degradation. The results indicate that disruption of an essential synaptic chaperone leads to changes in expression levels of UPS-related proteins which has a knock-on effect on overall protein degradation in C. elegans. The specific over-representation of E3 ubiquitin ligase components revealed in our study, suggests that proteins and complexes upstream of the proteasome itself may be beneficial therapeutic targets.

Different dynamin blockers interfere with distinct phases of synaptic endocytosis during stimulation in motoneurones

KEY POINTS

Neurotransmitter release requires a tight coupling between synaptic vesicle exocytosis and endocytosis with dynamin being a key protein in that process. We used imaging techniques to examine the time course of endocytosis at mouse motor nerve terminals expressing synaptopHluorin, a genetically encoded reporter of the synaptic vesicle cycle. We separated two sequential phases of endocytosis taking place during the stimulation train: early and late endocytosis. Freshly released synaptic vesicle proteins are preferentially retrieved during the early phase, which is very sensitive to dynasore, an inhibitor of dynamin GTPase activity. Synaptic vesicle proteins pre-existing at the plasma membrane before the stimulation are preferentially retrieved during the late phase, which is very sensitive to myristyl trimethyl ammonium bromide (MitMAB), an inhibitor of the dynamin-phospholipid interaction.

ABSTRACT

Synaptic endocytosis is essential at nerve terminals to maintain neurotransmitter release by exocytosis. Here, at the neuromuscular junction of synaptopHluorin (spH) transgenic mice, we have used imaging to study exo- and endocytosis occurring simultaneously during nerve stimulation. We observed two endocytosis components, which occur sequentially during stimulation. The early component of endocytosis apparently internalizes spH molecules freshly exocytosed. This component was sensitive to dynasore, a blocker of dynamin 1 GTPase activity. In contrast, this early component was resistant to myristyl trimethyl ammonium bromide (MiTMAB), a competitive agent that blocks dynamin binding to phospholipid membranes. The late component of endocytosis is likely to internalize spH molecules that pre-exist at the plasma membrane before stimulation starts. This component was blocked by MiTMAB, perhaps by impairing the binding of dynamin or other key endocytic proteins to phospholipid membranes. Our study suggests the co-existence of two sequential synaptic endocytosis steps taking place during stimulation that are susceptible to pharmacological dissection: an initial step, preferentially sensitive to dynasore, that internalizes vesicular components immediately after they are released, and a MiTMAB-sensitive step that internalizes vesicular components pre-existing at the plasma membrane surface. In addition, we report that post-stimulus endocytosis also has several components with different sensitivities to dynasore and MiTMAB.

Increased Expression of the Large Conductance, Calcium-Activated K+ (BK) Channel in Adult-Onset Neuronal Ceroid Lipofuscinosis

Cysteine string protein (CSPα) is a presynaptic J protein co-chaperone that opposes neurodegeneration. Mutations in CSPα (i.e., Leu115 to Arg substitution or deletion (Δ) of Leu116) cause adult neuronal ceroid lipofuscinosis (ANCL), a dominantly inherited neurodegenerative disease. We have previously demonstrated that CSPα limits the expression of large conductance, calcium-activated K+ (BK) channels in neurons, which may impact synaptic excitability and neurotransmission. Here we show by western blot analysis that expression of the pore-forming BKα subunit is elevated ~2.5 fold in the post-mortem cortex of a 36-year-old patient with the Leu116∆ CSPα mutation. Moreover, we find that the increase in BKα subunit level is selective for ANCL and not a general feature of neurodegenerative conditions. While reduced levels of CSPα are found in some postmortem cortex specimens from Alzheimer's disease patients, we find no concomitant increase in BKα subunit expression in Alzheimer's specimens. Both CSPα monomer and oligomer expression are reduced in synaptosomes prepared from ANCL cortex compared with control. In a cultured neuronal cell model, CSPα oligomers are short lived. The results of this study indicate that the Leu116∆ mutation leads to elevated BKα subunit levels in human cortex and extend our initial work in rodent models demonstrating the modulation of BKα subunit levels by the same CSPα mutation. While the precise sequence of pathogenic events still remains to be elucidated, our findings suggest that dysregulation of BK channels may contribute to neurodegeneration in ANCL.

Cysteine string protein (CSP) and its role in preventing neurodegeneration

Cysteine string protein (CSP) is a member of the DnaJ/Hsp40 family of co-chaperones that localises to neuronal synaptic vesicles. Its name derives from the possession of a string of 12–15 cysteine residues, palmitoylation of which is required for targeting to post-Golgi membranes. The DnaJ domain of CSP enables it to bind client proteins and recruit Hsc70 chaperones, thereby contributing to the maintenance of protein folding in the presynaptic compartment. Mutation of CSP in flies, worms and mice reduces lifespan and causes synaptic dysfunction and neurodegeneration. Furthermore, recent studies have revealed that the neurodegenerative disease, adult onset neuronal ceroid lipofuscinosis, is caused by mutations in the human CSPα-encoding DNAJC5 gene. Accumulating evidence suggests that the major mechanism by which CSP prevents neurodegeneration is by maintaining the conformation of SNAP-25, thereby facilitating its entry into the membrane-fusing SNARE complex. In this review, we focus on the role of CSP in preventing neurodegeneration and discuss how recent studies of this universal neuroprotective chaperone are being translated into potential novel therapeutics for neurodegenerative diseases.

Impaired intracellular trafficking defines early Parkinson's disease

Parkinson's disease (PD) is an insidious and incurable neurodegenerative disease, and represents a significant cost to individuals, carers, and ageing societies. It is defined at post-mortem by the loss of dopamine neurons in the substantia nigra together with the presence of Lewy bodies and Lewy neurites. We examine here the role of α-synuclein and other cellular transport proteins implicated in PD and how their aberrant activity may be compounded by the unique anatomy of the dopaminergic neuron. This review uses multiple lines of evidence from genetic studies, human tissue, induced pluripotent stem cells, and refined animal models to argue that prodromal PD can be defined as a disease of impaired intracellular trafficking. Dysfunction of the dopaminergic synapse heralds trafficking impairment.

Evidence that the presynaptic vesicle protein CSPalpha is a key player in synaptic degeneration and protection in Alzheimer's disease

Background

In Alzheimer’s disease synapse loss precedes neuronal loss and correlates best with impaired memory formation. However, the mechanisms underlying synaptic degeneration in Alzheimer’s disease are not well known. Further, it is unclear why synapses in AD cerebellum are protected from degeneration. Our recent work on the cyclin-dependent kinase 5 activator p25 suggested that expression of the multifunctional presynaptic molecule cysteine string protein alpha (CSPalpha) may be affected in Alzheimer’s disease.

Results

Using western blots and immunohistochemistry, we found that CSPalpha expression is reduced in hippocampus and superior temporal gyrus in Alzheimer’s disease. Reduced CSPalpha expression occurred before synaptophysin levels drop, suggesting that it contributes to the initial stages of synaptic degeneration. Surprisingly, we also found that CSPalpha expression is upregulated in cerebellum in Alzheimer’s disease. This CSPalpha upregulation reached the same level as in young, healthy cerebellum. We tested the idea whether CSPalpha upregulation might be neuroprotective, using htau mice, a model of tauopathy that expresses the entire wild-type human tau gene in the absence of mouse tau. In htau mice CSPalpha expression was found to be elevated at times when neuronal loss did not occur.

Conclusion

Our findings provide evidence that the presynaptic vesicle protein CSPalpha is a key player in synaptic degeneration and protection in Alzheimer’s disease. In the forebrain CSPalpha expression is reduced early in the disease and this may contribute to the initial stages of synaptic degeneration. In the cerebellum CSPalpha expression is upregulated to young, healthy levels and this may protect cerebellar synapses and neurons to survive. Accordingly, CSPalpha upregulation also occurs in a mouse model of tauopathy only at time when neuronal loss does not take place.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0096-z) contains supplementary material, which is available to authorized users.

Exocytosis and synaptic vesicle function

Synaptic vesicles release their vesicular contents to the extracellular space by Ca(2+)-triggered exocytosis. The Ca(2+)-triggered exocytotic process is regulated by synaptotagmin (Syt), a vesicular Ca(2+)-binding C2 domain protein. Synaptotagmin 1 (Syt1), the most studied major isoform among 16 Syt isoforms, mediates Ca(2+)-triggered synaptic vesicle exocytosis by interacting with the target membranes and SNARE/complexin complex. In synapses of the central nervous system, synaptobrevin 2, a major vesicular SNARE protein, forms a ternary SNARE complex with the plasma membrane SNARE proteins, syntaxin 1 and SNAP25. The affinities of Ca(2+)-dependent interactions between Syt1 and its targets (i.e., SNARE complexes and membranes) are well correlated with the efficacies of the corresponding exocytotic processes. Therefore, different SNARE protein isoforms and membrane lipids, which interact with Syt1 with various affinities, are capable of regulating the efficacy of Syt1-mediated exocytosis. Otoferlin, another type of vesicular C2 domain protein that binds to the membrane in a Ca(2+)-dependent manner, is also involved in the Ca(2+)-triggered synaptic vesicle exocytosis in auditory hair cells. However, the functions of otoferlin in the exocytotic process are not well understood. In addition, at least five different types of synaptic vesicle proteins such as synaptic vesicle protein 2, cysteine string protein α, rab3, synapsin, and a group of proteins containing four transmembrane regions, which includes synaptophysin, synaptogyrin, and secretory carrier membrane protein, are involved in modulating the exocytotic process by regulating the formation and trafficking of synaptic vesicles.

Caenorhabditis elegans dnj-14, the orthologue of the DNAJC5 gene mutated in adult onset neuronal ceroid lipofuscinosis, provides a new platform for neuroprotective drug screening and identifies a SIR-2.1-independent action of resveratrol

Adult onset neuronal lipofuscinosis (ANCL) is a human neurodegenerative disorder characterized by progressive neuronal dysfunction and premature death. Recently, the mutations that cause ANCL were mapped to the DNAJC5 gene, which encodes cysteine string protein alpha. We show here that mutating dnj-14, the Caenorhabditis elegans orthologue of DNAJC5, results in shortened lifespan and a small impairment of locomotion and neurotransmission. Mutant dnj-14 worms also exhibited age-dependent neurodegeneration of sensory neurons, which was preceded by severe progressive chemosensory defects. A focussed chemical screen revealed that resveratrol could ameliorate dnj-14 mutant phenotypes, an effect mimicked by the cAMP phosphodiesterase inhibitor, rolipram. In contrast to other worm neurodegeneration models, activation of the Sirtuin, SIR-2.1, was not required, as sir-2.1; dnj-14 double mutants showed full lifespan rescue by resveratrol. The Sirtuin-independent neuroprotective action of resveratrol revealed here suggests potential therapeutic applications for ANCL and possibly other human neurodegenerative diseases.

The synergistic effect between β-amyloid(1-42) and α-synuclein on the synapses dysfunction in hippocampal neurons

OBJECTIVE

This study was to explore the molecular mechanisms underpinning the synergetic effect between β-amyloid (Aβ) and α-synuclein (α-syn) on synapses dysfunction during the development of neurodegenerative disorders including Parkinson's disease (PD), dementia with Lewy bodies (DLB) and Alzheimer disease (AD).

METHODS

The primary cultured hippocampal neurons prepared from the fetal tissue of mice were divided into six groups and treated with DMSO, Aβ(42-1), α-syn, Aβ(1-42), α-syn plus Aβ(42-1) and α-syn plus Aβ(1-42), respectively. After incubation for 24 h, the synapsin I content was calculated by immunofluorescence and the synaptic vesicle recycling was monitored by FM1-43 staining. Furthermore, the expression of cysteine string protein-α (CSPα) detected by western blot was also conducted.

RESULTS

Either Aβ(1-42) or α-syn alone could induce a significant synapses dysfunction through reducing the content of synapsin I, inhibiting the synaptic vesicle recycling as well as down-regulating the expression of CSPα compared with the controls (P<0.05). However, simultaneous intervention with both α-syn and Aβ(1-42) aggravated these effects in cultured hippocampal neurons compared with the treatment with α-syn (synapsin I content: P<0.001; synaptic vesicle recycling: P=0.007; CSPα expression: P<0.001) or Aβ(1-42) (synapsin I number: P<0.001; synaptic vesicle recycling: P=0.007 CSPα expression: P<0.001) alone.

CONCLUSION

There was synergistic effect between Aβ and α-syn on synapses dysfunction through reducing the synapsin I content, inhibiting the synaptic vesicle recycling and down-regulating the expression of CSPα in several neurodegenerative diseases.

Synucleins regulate the kinetics of synaptic vesicle endocytosis

Genetic and pathological studies link α-synuclein to the etiology of Parkinson's disease (PD), but the normal function of this presynaptic protein remains unknown. α-Synuclein, an acidic lipid binding protein, shares high sequence identity with β- and γ-synuclein. Previous studies have implicated synucleins in synaptic vesicle (SV) trafficking, although the precise site of synuclein action continues to be unclear. Here we show, using optical imaging, electron microscopy, and slice electrophysiology, that synucleins are required for the fast kinetics of SV endocytosis. Slowed endocytosis observed in synuclein null cultures can be rescued by individually expressing mouse α-, β-, or γ-synuclein, indicating they are functionally redundant. Through comparisons to dynamin knock-out synapses and biochemical experiments, we suggest that synucleins act at early steps of SV endocytosis. Our results categorize α-synuclein with other familial PD genes known to regulate SV endocytosis, implicating this pathway in PD.

Oligomerization of Cysteine String Protein alpha mutants causing adult neuronal ceroid lipofuscinosis

Cysteine String Protein alpha (CSPα) is a palmitoylated, synaptic vesicle co-chaperone that is essential for neuroprotection. Two mutations in CSPα - L115R and L116Δ - cause adult neuronal ceroid lipofuscinosis (ANCL), a dominantly-inherited neurodegenerative disease. To elucidate the pathogenesis of ANCL, the intrinsic biochemical properties of human wildtype (WT) and disease mutant CSPα were examined. Mutant proteins purified from Escherichia coli exhibited high potency to oligomerize in a concentration, temperature, and time dependent manner, with L115R possessing the greatest potency. When freshly purified, ANCL mutant proteins displayed normal co-chaperone activity and substrate recognition similar to WT. However, co-chaperone activity was impaired for both CSPα mutants upon oligomerization. When WT and mutant CSPα were mixed together they co-oligomerized leading to an overall decrease of co-chaperone activity. The oligomerization properties of ANCL mutants were faithfully replicated in HEK 293T cells. Interestingly, the oligomers were covalently tagged by ubiquitination instead of palmitoylation. Taken together, ANCL mutations result in both a gain and partial loss-of-function.

Widespread sequence variations in VAMP1 across vertebrates suggest a potential selective pressure from botulinum neurotoxins

Botulinum neurotoxins (BoNT/A-G), the most potent toxins known, act by cleaving three SNARE proteins required for synaptic vesicle exocytosis. Previous studies on BoNTs have generally utilized the major SNARE homologues expressed in brain (VAMP2, syntaxin 1, and SNAP-25). However, BoNTs target peripheral motor neurons and cause death by paralyzing respiratory muscles such as the diaphragm. Here we report that VAMP1, but not VAMP2, is the SNARE homologue predominantly expressed in adult rodent diaphragm motor nerve terminals and in differentiated human motor neurons. In contrast to the highly conserved VAMP2, BoNT-resistant variations in VAMP1 are widespread across vertebrates. In particular, we identified a polymorphism at position 48 of VAMP1 in rats, which renders VAMP1 either resistant (I48) or sensitive (M48) to BoNT/D. Taking advantage of this finding, we showed that rat diaphragms with I48 in VAMP1 are insensitive to BoNT/D compared to rat diaphragms with M48 in VAMP1. This unique intra-species comparison establishes VAMP1 as a physiological toxin target in diaphragm motor nerve terminals, and demonstrates that the resistance of VAMP1 to BoNTs can underlie the insensitivity of a species to members of BoNTs. Consistently, human VAMP1 contains I48, which may explain why humans are insensitive to BoNT/D. Finally, we report that residue 48 of VAMP1 varies frequently between M and I across seventeen closely related primate species, suggesting a potential selective pressure from members of BoNTs for resistance in vertebrates.

Botulinum neurotoxins (BoNTs) target peripheral motor neurons and act by cleaving SNARE proteins, which are essential for neurotransmitter release from nerve terminals. SNARE proteins occur in multiple homologues and it has been difficult to determine which one is the physiologically relevant toxin target in motor nerve terminals among closely related SNARE homologues such as VAMP1 and VAMP2. Here we report that, in contrast to the highly conserved VAMP2, sequence variations in VAMP1 that confer resistance to BoNTs are widespread across vertebrates. In particular, residue 48 of VAMP1 is polymorphic between BoNT/D-sensitive residue M and BoNT/D-resistant residue I in rats. Taking advantage of this finding, we carried out an intra-species comparison, which showed that diaphragm motor nerve terminals from rats with I48 in VAMP1 were insensitive to BoNT/D as compared to those with M48. Since VAMP2 is conserved in rats, these data demonstrate that VAMP1 is the physiologically relevant toxin target in motor neurons. Interestingly, human VAMP1 encodes the BoNT/D-resistant residue I48, which may explain why humans are insensitive to BoNT/D. Finally, we found that residue 48 of VAMP1 switches frequently between M and I among 17 primate species, suggesting a potential selective pressure from BoNT/D for resistance in primates.

J protein mutations and resulting proteostasis collapse

Despite a century of intensive investigation the effective treatment of protein aggregation diseases remains elusive. Ordinarily, molecular chaperones ensure that proteins maintain their functional conformation. The appearance of misfolded proteins that aggregate implies the collapse of the cellular chaperone quality control network. That said, the cellular chaperone network is extensive and functional information regarding the detailed action of specific chaperones is not yet available. J proteins (DnaJ/Hsp40) are a family of chaperone cofactors that harness Hsc70 (heat shock cognate protein of 70 kDa) for diverse conformational cellular tasks and, as such, represent novel clinically relevant targets for diseases resulting from the disruption of proteostasis. Here we review incisive reports identifying mutations in individual J protein chaperones and the proteostasis collapse that ensues.

CSPα-chaperoning presynaptic proteins

Synaptic transmission relies on precisely regulated and exceedingly fast protein-protein interactions that involve voltage-gated channels, the exocytosis/endocytosis machinery as well as signaling pathways. Although we have gained an ever more detailed picture of synaptic architecture much remains to be learned about how synapses are maintained. Synaptic chaperones are “folding catalysts” that preserve proteostasis by regulating protein conformation (and therefore protein function) and prevent unwanted protein-protein interactions. Failure to maintain synapses is an early hallmark of several degenerative diseases. Cysteine string protein (CSPα) is a presynaptic vesicle protein and molecular chaperone that has a central role in preventing synaptic loss and neurodegeneration. Over the past few years, a number of different “client proteins” have been implicated as CSPα substrates including voltage-dependent ion channels, signaling proteins and proteins critical to the synaptic vesicle cycle. Here we review the ion channels and synaptic protein complexes under the influence of CSPα and discuss gaps in our current knowledge.

The large conductance, calcium-activated K+ (BK) channel is regulated by cysteine string protein

Large-conductance, calcium-activated-K(+) (BK) channels are widely distributed throughout the nervous system, where they regulate action potential duration and firing frequency, along with presynaptic neurotransmitter release. Our recent efforts to identify chaperones that target neuronal ion channels have revealed cysteine string protein (CSPα) as a key regulator of BK channel expression and current density. CSPα is a vesicle-associated protein and mutations in CSPα cause the hereditary neurodegenerative disorder, adult-onset autosomal dominant neuronal ceroid lipofuscinosis (ANCL). CSPα null mice show 2.5 fold higher BK channel expression compared to wild type mice, which is not seen with other neuronal channels (i.e. Cav2.2, Kv1.1 and Kv1.2). Furthermore, mutations in either CSPα's J domain or cysteine string region markedly increase BK expression and current amplitude. We conclude that CSPα acts to regulate BK channel expression, and consequently CSPα-associated changes in BK activity may contribute to the pathogenesis of neurodegenerative disorders, such as ANCL.

Cysteine string protein limits expression of the large conductance, calcium-activated K⁺ (BK) channel

Large-conductance, calcium-activated K⁺ (BK) channels are widely distributed throughout the nervous system and play an essential role in regulation of action potential duration and firing frequency, along with neurotransmitter release at the presynaptic terminal. We have previously demonstrated that select mutations in cysteine string protein (CSPα), a presynaptic J-protein and co-chaperone, increase BK channel expression. This observation raised the possibility that wild-type CSPα normally functions to limit neuronal BK channel expression. Here we show by Western blot analysis of transfected neuroblastoma cells that when BK channels are present at elevated levels, CSPα acts to reduce expression. Moreover, we demonstrate that the accessory subunits, BKβ4 and BKβ1 do not alter CSPα-mediated reduction of expressed BKα subunits. Structure-function analysis reveals that the N-terminal J-domain of CSPα is critical for the observed regulation of BK channels levels. Finally, we demonstrate that CSPα limits BK current amplitude, while the loss-of-function homologue CSPα_HPD-AAA increases BK current. Our observations indicate that CSPα has a role in regulating synaptic excitability and neurotransmission by limiting expression of BK channels.

Mutations in palmitoyl-protein thioesterase 1 alter exocytosis and endocytosis at synapses in Drosophila larvae

Infantile-onset neuronal ceroid lipofuscinosis (INCL) is a severe pediatric neurodegenerative disorder produced by mutations in the gene encoding palmitoyl-protein thioesterase 1 (Ppt1). This enzyme is responsible for the removal of a palmitate group from its substrate proteins, which may include presynaptic proteins like SNAP-25, cysteine string protein (CSP), dynamin, and synaptotagmin. The fruit fly, Drosophila melanogaster, has been a powerful model system for studying the functions of these proteins and the molecular basis of neurological disorders like the NCLs. Genetic modifier screens and tracer uptake studies in Ppt1 mutant larval garland cells have suggested that Ppt1 plays a role in endocytic trafficking. We have extended this analysis to examine the involvement of Ppt1 in synaptic function at the Drosophila larval neuromuscular junction (NMJ). Mutations in Ppt1 genetically interact with temperature sensitive mutations in the Drosophila dynamin gene shibire, accelerating the paralytic behavior of shibire mutants at 27 °C. Electrophysiological work in NMJs of Ppt1-deficient larvae has revealed an increase in miniature excitatory junctional potentials (EJPs) and a significant depression of evoked EJPs in response to repetitive (10 hz) stimulation. Endocytosis was further examined in Ppt1-mutant larvae using FM1-43 uptake assays, demonstrating a significant decrease in FM1-43 uptake at the mutant NMJs. Finally, Ppt1-deficient and Ppt1 point mutant larvae display defects in locomotion that are consistent with alterations in synaptic function. Taken together, our genetic, cellular, and electrophysiological analyses suggest a direct role for Ppt1 in synaptic vesicle exo- and endocytosis at motor nerve terminals of the Drosophila NMJ.

Tumor protein D52 controls trafficking of an apical endolysosomal secretory pathway in pancreatic acinar cells

Zymogen granule (ZG) formation in acinar cells involves zymogen cargo sorting from trans-Golgi into immature secretory granules (ISGs). ISG maturation progresses by removal of lysosomal membrane and select content proteins, which enter endosomal intermediates prior to their apical exocytosis. Constitutive and stimulated secretion through this mechanism is termed the constitutive-like and minor-regulated pathways, respectively. However, the molecular components that control membrane trafficking within these endosomal compartments are largely unknown. We show that tumor protein D52 is highly expressed in endosomal compartments following pancreatic acinar cell stimulation and regulates apical exocytosis of an apically directed endolysosomal compartment. Secretion from the endolysosomal compartment was detected by cell-surface antigen labeling of lysosome-associated membrane protein LAMP1, which is absent from ZGs, and had incomplete overlap with surface labeling of synaptotagmin 1, a marker of ZG exocytosis. Although culturing (16-18 h) of isolated acinar cells is accompanied by a loss of secretory responsiveness, the levels of SNARE proteins necessary for ZG exocytosis were preserved. However, levels of endolysosomal proteins D52, EEA1, Rab5, and LAMP1 markedly decreased with culture. When D52 levels were restored by adenoviral delivery, the levels of these regulatory proteins and secretion of both LAMP1 (endolysosomal) and amylase was strongly enhanced. These secretory effects were absent in alanine and aspartate substitutions of serine 136, the major D52 phosphorylation site, and were inhibited by brefeldin A, which does not directly affect the ZG compartment. Our results indicate that D52 directly regulates apical endolysosomal secretion and are consistent with previous studies, suggesting that this pathway indirectly regulates ZG secretion of digestive enzymes.