The dual phosphatase activity of synaptojanin1 is required for both efficient synaptic vesicle endocytosis and reavailability at nerve terminals

PERK-Mediated Unfolded Protein Response Activation and Oxidative Stress in PARK20 Fibroblasts

PARK20, an early onset autosomal recessive parkinsonism is due to mutations in the phosphatidylinositol-phosphatase Synaptojanin 1 (Synj1). We have recently shown that the early endosomal compartments are profoundly altered in PARK20 fibroblasts as well as the endosomal trafficking. Here, we report that PARK20 fibroblasts also display a drastic alteration of the architecture and function of the early secretory compartments. Our results show that the exit machinery from the Endoplasmic Reticulum (ER) and the ER-to-Golgi trafficking are markedly compromised in patient cells. As a consequence, PARK20 fibroblasts accumulate large amounts of cargo proteins within the ER, leading to the induction of ER stress. Interestingly, this stressful state is coupled to the activation of the PERK/eIF2α/ATF4/CHOP pathway of the Unfolded Protein Response (UPR). In addition, PARK20 fibroblasts reveal upregulation of oxidative stress markers and total ROS production with concomitant alteration of the morphology of the mitochondrial network. Interestingly, treatment of PARK20 cells with GSK2606414 (GSK), a specific inhibitor of PERK activity, restores the level of ROS, signaling a direct correlation between ER stress and the induction of oxidative stress in the PARK20 cells. All together, these findings suggest that dysfunction of early secretory pathway might contribute to the pathogenesis of the disease.

Critical role for Piccolo in synaptic vesicle retrieval

Loss of function of the active zone protein Piccolo has recently been linked to a disease, Pontocerebellar Hypoplasia type 3, which causes brain atrophy. Here, we address how Piccolo inactivation in rat neurons adversely affects synaptic function and thus may contribute to neuronal loss. Our analysis shows that Piccolo is critical for the recycling and maintenance of synaptic vesicles. We find that boutons lacking Piccolo have deficits in the Rab5/EEA1 dependent formation of early endosomes and thus the recycling of SVs. Mechanistically, impaired Rab5 function was caused by reduced synaptic recruitment of Pra1, known to interact selectively with the zinc finger domains of Piccolo. Importantly, over-expression of GTPase deficient Rab5 or the Znf1 domain of Piccolo restores the size and recycling of SV pools. These data provide a molecular link between the active zone and endosome sorting at synapses providing hints to how Piccolo contributes to developmental and psychiatric disorders.

Crosstalk between presynaptic trafficking and autophagy in Parkinson's disease

Parkinson's disease (PD) is a debilitating neurodegenerative disorder that profoundly affects one's motor functions. The disease is characterized pathologically by denervation of dopaminergic (DAergic) nigrostriatal terminal and degeneration of DAergic neurons in the substantia nigra par compacta (SNpc); however, the precise molecular mechanism underlying disease pathogenesis remains poorly understood. Animal studies in both toxin-induced and genetic PD models suggest that presynaptic impairments may underlie the early stage of DA depletion and neurodegeneration (reviewed in Schirinzi, T., et al. 2016). Supporting this notion, human genetic studies and genomic analysis have identified an increasing number of PD risk variants that are associated with synaptic vesicle (SV) trafficking, regulation of synaptic function and autophagy/lysosomal system (Chang, D., et al. 2017, reviewed in Trinh, J. & Farrer, M. 2013; Singleton, A.B., et al. 2013). Although the precise mechanism for autophagy regulation in neurons is currently unclear, many studies demonstrate that autophagosomes form at the presynaptic terminal (Maday, S. & Holzbaur, E.L. 2014; Vanhauwaert, R., et al. 2017; reviewed in Yue, Z. 2007). Growing evidence has revealed overlapping genes involved in both SV recycling and autophagy, suggesting that the two membrane trafficking processes are inter-connected. Here we will review emergent evidence linking SV endocytic genes and autophagy genes at the presynaptic terminal. We will discuss their potential relevance to PD pathogenesis.

Synaptojanin regulates Hedgehog signalling by modulating phosphatidylinositol 4-phosphate levels

In Hedgehog (Hh) signalling, Hh ligand concentration gradient is effectively translated into a spatially distinct transcriptional program to give precisely controlled context dependent developmental outcomes. In the absence of Hh, the receptor Patched (Ptc) inhibits the signal transducer Smoothened (Smo) by maintaining low phosphatidylinositol 4-phosphate (PI(4)P) levels. Binding of Hh to its receptor Ptc promotes PI(4)P production, which in turn activates Smo. Using wingdiscs of Drosophila melanogaster, this study shows that Synaptojanin (Synj), a dual phosphatase, modulates PI(4)P levels and affects Smo activation, and thereby functions as an additional regulatory step in the Hh pathway. Reducing the levels of Synj in the wing-discs caused enhancement of a Hh dominant gain-of-function Moonrat phenotype in the adult wings. Synj downregulation augmented Hh signalling, which was associated with elevated PI(4)P levels and Smo activation. Synj did not control the absolute pathway activity but rather fine-tuned the response since its downregulation increased expression of decapentaplegic (dpp), a low-threshold target of the pathway while the high-threshold targets remained unaffected. This is the first report that identifies Synj as a negative regulator of Hh signalling, implying its importance and an additional regulatory step in Hh signal transduction.

A Novel SYNJ1 Mutation in a Tunisian Family with Juvenile Parkinson's Disease Associated with Epilepsy

Mutations in SYNJ1 gene have been described in few families with juvenile atypical Parkinson disease (PD). This gene encodes for "Synaptojanin 1," an enzyme playing a major role in the phosphorylation and the recycling of synaptic vesicles. In this study, we report two siblings, from a consanguineous Tunisian family, presenting juvenile PD. Both siblings developed mild Parkinsonism at 16 and 21 years old respectively. One patient had generalized tonic-clonic seizures since the age of 7 years. There was no evidence of sleep or autonomic dysfunctions and psychiatric disorders in both cases, but they developed a moderate cognitive impairment. They kept a good respond to low doses of levodopa treatment with no dyskinesia or motor fluctuations. We designed an NGS-based screening of 22 currently most prevalent parkinsonism-associated genes. Genetic study revealed a novel compound heterozygous mutation (p.Leu1406Phefs*42 and p.Lys1321Glu) in SYNJ1 gene. The p.Lys1321Glu mutation is located in the proline-rich domain and leads to a significant change in the 3D structure of the protein (RMS = 12.58 Å). The p.Leu1406Phefs*42 mutation disrupt the AP2 binding sites and subsequently disable synaptic and vesicle endocytic recycling in neurons. This is the first report of mutation in the C-terminal domain of Synaptojanin 1 protein causing mild juvenile PD with generalized seizures, cognitive impairment, and good respond to levodopa treatment.

Genome-wide association study provides insights into genes related with horn development in Nelore beef cattle

The causal mutation for polledness in Nelore (Bos taurus indicus) breed seems to have appeared first in Brazil in 1957. The expression of the polled trait is known to be ruled by a few groups of alleles in taurine breeds; however, the genetic basis of this trait in indicine cattle is still unclear. The aim of this study was to identify genomic regions associated with the hornless trait in a commercial Nelore population. A total of 107,294 animals had phenotypes recorded and 2,238 were genotyped/imputed for 777k SNP. The weighted single-step approach for genome-wide association study (WssGWAS) was used to estimate the SNP effects and variances accounted for by 1 Mb sliding SNP windows. A centromeric region of chromosome 1 with 3.11 Mb size (BTA1: 878,631–3,987,104 bp) was found to be associated with hornless in the studied population. A total of 28 protein-coding genes are mapped in this region, including the taurine Polled locus and the IFNAR1, IFNAR2, IFNGR2, KRTAP11-1, MIS18A, OLIG1, OLIG2, and SOD1 genes, which expression can be related to the horn formation as described in literature. The functional enrichment analysis by DAVID tool revealed cytokine-cytokine receptor interaction, JAK-STAT signaling, natural killer cell mediated cytotoxicity, and osteoclast differentiation pathways as significant (P < 0.05). In addition, a runs of homozygosity (ROH) analysis identified a ROH island in polled animals with 2.47 Mb inside the region identified by WssGWAS. Polledness in Nelore cattle is associated with one region in the genome with 3.1 Mb size in chromosome 1. Several genes are harbored in this region, and they may act together in the determination of the polled/horned phenotype. Fine mapping the locus responsible for polled trait in Nelore breed and the identification of the molecular mechanisms regulating the horn growth deserve further investigation.

Parkinson's disease: convergence on synaptic homeostasis

Parkinson's disease, the second most common neurodegenerative disorder, affects millions of people globally. There is no cure, and its prevalence will double by 2030. In recent years, numerous causative genes and risk factors for Parkinson's disease have been identified and more than half appear to function at the synapse. Subtle synaptic defects are thought to precede blunt neuronal death, but the mechanisms that are dysfunctional at synapses are only now being unraveled. Here, we review recent work and propose a model where different Parkinson proteins interact in a cell compartment-specific manner at the synapse where these proteins regulate endocytosis and autophagy. While this field is only recently emerging, the work suggests that the loss of synaptic homeostasis may contribute to neurodegeneration and is a key player in Parkinson's disease.

Excess Synaptojanin 1 Contributes to Place Cell Dysfunction and Memory Deficits in the Aging Hippocampus in Three Types of Alzheimer's Disease

The phosphoinositide phosphatase synaptojanin 1 (SYNJ1) is a key regulator of synaptic function. We first tested whether SYNJ1 contributes to phenotypic variations in familial Alzheimer's disease (FAD) and show that SYNJ1 polymorphisms are associated with age of onset in both early- and late-onset human FAD cohorts. We then interrogated whether SYNJ1 levels could directly affect memory. We show that increased SYNJ1 levels in autopsy brains from adults with Down syndrome (DS/AD) are inversely correlated with synaptophysin levels, a direct readout of synaptic integrity. We further report age-dependent cognitive decline in a mouse model overexpressing murine Synj1 to the levels observed in human sporadic AD, triggered through hippocampal hyperexcitability and defects in the spatial reproducibility of place fields. Taken together, our findings suggest that SYNJ1 contributes to memory deficits in the aging hippocampus in all forms of AD.

Clathrin coat controls synaptic vesicle acidification by blocking vacuolar ATPase activity

Newly-formed synaptic vesicles (SVs) are rapidly acidified by vacuolar adenosine triphosphatases (vATPases), generating a proton electrochemical gradient that drives neurotransmitter loading. Clathrin-mediated endocytosis is needed for the formation of new SVs, yet it is unclear when endocytosed vesicles acidify and refill at the synapse. Here, we isolated clathrin-coated vesicles (CCVs) from mouse brain to measure their acidification directly at the single vesicle level. We observed that the ATP-induced acidification of CCVs was strikingly reduced in comparison to SVs. Remarkably, when the coat was removed from CCVs, uncoated vesicles regained ATP-dependent acidification, demonstrating that CCVs contain the functional vATPase, yet its function is inhibited by the clathrin coat. Considering the known structures of the vATPase and clathrin coat, we propose a model in which the formation of the coat surrounds the vATPase and blocks its activity. Such inhibition is likely fundamental for the proper timing of SV refilling.

Synapses in neurodegenerative diseases

Synapse is the basic structural and functional component for neural communication in the brain. The presynaptic terminal is the structural and functionally essential area that initiates communication and maintains the continuous functional neural information flow. It contains synaptic vesicles (SV) filled with neurotransmitters, an active zone for release, and numerous proteins for SV fusion and retrieval. The structural and functional synaptic plasticity is a representative characteristic; however, it is highly vulnerable to various pathological conditions. In fact, synaptic alteration is thought to be central to neural disease processes. In particular, the alteration of the structural and functional phenotype of the presynaptic terminal is a highly significant evidence for neural diseases. In this review, we specifically describe structural and functional alteration of nerve terminals in several neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Amyotrophic lateral sclerosis (ALS), and Huntington’s disease (HD).

Alteration of endosomal trafficking is associated with early-onset parkinsonism caused by SYNJ1 mutations

Recently, a new form of autosomal recessive early-onset parkinsonism (PARK20), due to mutations in the gene encoding the phosphoinositide phosphatase, Synaptojanin 1 (Synj1), has been reported. Several genes responsible for hereditary forms of Parkinson’s disease are implicated in distinct steps of the endolysosomal pathway. However, the nature and the degree of endocytic membrane trafficking impairment in early-onset parkinsonism remains elusive. Here, we show that depletion of Synj1 causes drastic alterations of early endosomes, which become enlarged and more numerous, while it does not affect the morphology of late endosomes both in non-neuronal and neuronal cells. Moreover, Synj1 loss impairs the recycling of transferrin, while it does not alter the trafficking of the epidermal growth factor receptor. The ectopic expression of Synj1 restores the functions of early endosomes, and rescues these trafficking defects in depleted cells. Importantly, the same alterations of early endosomal compartments and trafficking defects occur in fibroblasts of PARK20 patients. Our data indicate that Synj1 plays a crucial role in regulating the homeostasis and functions of early endosomal compartments in different cell types, and highlight defective cellular pathways in PARK20. In addition, they strengthen the link between endosomal trafficking and Parkinson’s disease.

DJ-1 deficiency impairs synaptic vesicle endocytosis and reavailability at nerve terminals

Mutations in DJ-1 (PARK7) are a known cause of early-onset autosomal recessive Parkinson's disease (PD). Accumulating evidence indicates that abnormalities of synaptic vesicle trafficking underlie the pathophysiological mechanism of PD. In the present study, we explored whether DJ-1 is involved in CNS synaptic function. DJ-1 deficiency impaired synaptic vesicle endocytosis and reavailability without inducing structural alterations in synapses. Familial mutants of DJ-1 (M26I, E64D, and L166P) were unable to rescue defective endocytosis of synaptic vesicles, whereas WT DJ-1 expression completely restored endocytic function in DJ-1 KO neurons. The defective synaptic endocytosis shown in DJ-1 KO neurons may be attributable to alterations in membrane cholesterol level. Thus, DJ-1 appears essential for synaptic vesicle endocytosis and reavailability, and impairment of this function by familial mutants of DJ-1 may be related to the pathogenesis of PD.

Dysfunction of autophagy and endosomal-lysosomal pathways: Roles in pathogenesis of Down syndrome and Alzheimer's Disease

Individuals with Down syndrome (DS) have an increased risk of early-onset Alzheimer's Disease (AD), largely owing to a triplication of the APP gene, located on chromosome 21. In DS and AD, defects in endocytosis and lysosomal function appear at the earliest stages of disease development and progress to widespread failure of intraneuronal waste clearance, neuritic dystrophy and neuronal cell death. The same genetic factors that cause or increase AD risk are also direct causes of endosomal-lysosomal dysfunction, underscoring the essential partnership between this dysfunction and APP metabolites in AD pathogenesis. The appearance of APP-dependent endosome anomalies in DS beginning in infancy and evolving into the full range of AD-related endosomal-lysosomal deficits provides a unique opportunity to characterize the earliest pathobiology of AD preceding the classical neuropathological hallmarks. Facilitating this characterization is the authentic recapitulation of this endosomal pathobiology in peripheral cells from people with DS and in trisomy mouse models. Here, we review current research on endocytic-lysosomal dysfunction in DS and AD, the emerging importance of APP/βCTF in initiating this dysfunction, and the potential roles of additional trisomy 21 genes in accelerating endosomal-lysosomal impairment in DS. Collectively, these studies underscore the growing value of investigating DS to probe the biological origins of AD as well as to understand and ameliorate the developmental disability of DS.

Endolysosomal dysfunction in Parkinson's disease: Recent developments and future challenges

Increasingly, genetic, cell biological, and in vivo work emphasizes the role of the endolysosomal system dysfunction in Parkinson's disease pathogenesis. Yet many questions remain about the mechanisms by which primary endolysosomal dysfunction causes PD as well as how the endolysosomal system interacts with α-synuclein-mediated neurotoxicity. We recently described a new mouse model of parkinsonism in which loss of the endolysosomal protein Atp13a2 causes behavioral, neuropathological, and biochemical changes similar to those present in human subjects with ATP13A2 mutations. In this Scientific Perspectives, we revisit the evidence implicating the endolysosomal system in PD, current hypotheses of disease pathogenesis, and how recent studies refine these hypotheses and raise new questions for future research. © 2016 International Parkinson and Movement Disorder Society.

PAKs in the brain: Function and dysfunction

p21-Activated kinases (PAKs) comprise a family of proteins covering a central role in signal transduction. They are downstream effectors of Rho GTPases and can affect a variety of processes in different cell types and tissues by remodeling the cytoskeleton and by promoting gene transcription and cell survival. Given the relevance of cytoskeletal organization in neuronal development as well as synaptic function and the importance of pro-survival signals in controlling neuronal cell fate, accumulating studies investigated the role of PAKs in the nervous system. In this review, we provide a critical overview of the role of PAKs in the nervous system, both in neuronal and non-neuronal cells, and discuss their potential link with neurodegenerative diseases.

Parkinson's Disease-Associated LRRK2 Hyperactive Kinase Mutant Disrupts Synaptic Vesicle Trafficking in Ventral Midbrain Neurons

Parkinson's disease (PD) is characterized pathologically by the selective loss of substantia nigra (SN) dopaminergic (DAergic) neurons. Recent evidence has suggested a role of LRRK2, linked to the most frequent familial PD, in regulating synaptic vesicle (SV) trafficking. However, the mechanism whereby LRRK2 mutants contribute to nigral vulnerability remains unclear. Here we show that the most common PD mutation LRRK2 G2019S impairs SV endocytosis in ventral midbrain (MB) neurons, including DA neurons, and the slowed endocytosis can be rescued by inhibition of LRRK2 kinase activity. A similar endocytic defect, however, was not observed in LRRK2 mutant neurons from the neocortex (hereafter, cortical neurons) or the hippocampus, suggesting a brain region-specific vulnerability to the G2019S mutation. Additionally, we found MB-specific impairment of SV endocytosis in neurons carrying heterozygous deletion of SYNJ1 (PARK20), a gene that is associated with recessive Parkinsonism. Combining SYNJ1^+/- and LRRK2 G2019S does not exacerbate SV endocytosis but impairs sustained exocytosis in MB neurons and alters specific motor functions of 1-year-old male mice. Interestingly, we show that LRRK2 directly phosphorylates synaptojanin1 in vitro, resulting in the disruption of endophilin-synaptojanin1 interaction required for SV endocytosis. Our work suggests a merge of LRRK2 and SYNJ1 pathogenic pathways in deregulating SV trafficking in MB neurons as an underlying molecular mechanism of early PD pathogenesis.SIGNIFICANCE STATEMENT Understanding midbrain dopaminergic (DAergic) neuron-selective vulnerability in PD is essential for the development of targeted therapeutics. We report, for the first time, a nerve terminal impairment in SV trafficking selectively in MB neurons but not cortical neurons caused by two PARK genes: LRRK2 (PARK8) and SYNJ1 (PARK20). We demonstrate that the enhanced kinase activity resulting from the most frequent G2019S mutation in LRRK2 is the key to this impairment. We provide evidence suggesting that LRRK2 G2019S and SYNJ1 loss of function share a similar pathogenic pathway in deregulating DAergic neuron SV endocytosis and that they play additive roles in facilitating each other's pathogenic functions in PD.

Parkinson Sac Domain Mutation in Synaptojanin 1 Impairs Clathrin Uncoating at Synapses and Triggers Dystrophic Changes in Dopaminergic Axons

Synaptojanin 1 (SJ1) is a major presynaptic phosphatase that couples synaptic vesicle endocytosis to the dephosphorylation of PI(4,5)P₂, a reaction needed for the shedding of endocytic factors from their membranes. While the role of SJ1's 5-phosphatase module in this process is well recognized, the contribution of its Sac phosphatase domain, whose preferred substrate is PI4P, remains unclear. Recently a homozygous mutation in its Sac domain was identified in early-onset parkinsonism patients. We show that mice carrying this mutation developed neurological manifestations similar to those of human patients. Synapses of these mice displayed endocytic defects and a striking accumulation of clathrin-coated intermediates, strongly implicating Sac domain's activity in endocytic protein dynamics. Mutant brains had elevated auxilin (PARK19) and parkin (PARK2) levels. Moreover, dystrophic axonal terminal changes were selectively observed in dopaminergic axons in the dorsal striatum. These results strengthen evidence for a link between synaptic endocytic dysfunction and Parkinson's disease.

Phosphorylation of Synaptojanin Differentially Regulates Endocytosis of Functionally Distinct Synaptic Vesicle Pools

The rapid replenishment of synaptic vesicles through endocytosis is crucial for sustaining synaptic transmission during intense neuronal activity. Synaptojanin (Synj), a phosphoinositide phosphatase, is known to play an important role in vesicle recycling by promoting the uncoating of clathrin following synaptic vesicle uptake. Synj has been shown to be a substrate of the minibrain (Mnb) kinase, a fly homolog of the dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A); however, the functional impacts of Synj phosphorylation by Mnb are not well understood. Here we identify that Mnb phosphorylates Synj at S1029 in Drosophila We find that phosphorylation of Synj at S1029 enhances Synj phosphatase activity, alters interaction between Synj and endophilin, and promotes efficient endocytosis of the active cycling vesicle pool (also referred to as exo-endo cycling pool) at the expense of reserve pool vesicle endocytosis. Dephosphorylated Synj, on the other hand, is deficient in the endocytosis of the active recycling pool vesicles but maintains reserve pool vesicle endocytosis to restore total vesicle pool size and sustain synaptic transmission. Together, our findings reveal a novel role for Synj in modulating reserve pool vesicle endocytosis and further indicate that dynamic phosphorylation and dephosphorylation of Synj differentially maintain endocytosis of distinct functional synaptic vesicle pools.

SIGNIFICANCE STATEMENT

Synaptic vesicle endocytosis sustains communication between neurons during a wide range of neuronal activities by recycling used vesicle membrane and protein components. Here we identify that Synaptojanin, a protein with a known role in synaptic vesicle endocytosis, is phosphorylated at S1029 in vivo by the Minibrain kinase. We further demonstrate that the phosphorylation status of Synaptojanin at S1029 differentially regulates its participation in the recycling of distinct synaptic vesicle pools. Our results reveal a new role for Synaptojanin in maintaining synaptic vesicle pool size and in reserve vesicle endocytosis. As Synaptojanin and Minibrain perturbations are associated with various neurological disorders, such as Parkinson's, autism, and Down syndrome, understanding mechanisms modulating Synaptojanin function provides valuable insights into processes affecting neuronal communication.

Autophagy in the presynaptic compartment in health and disease

Vijayan and Verstreken review the process of autophagy in the synapse and the role of autophagy in maintaining neuronal function.

Synapses are functionally distinct neuronal compartments that are critical for brain function, with synaptic dysfunction being an early pathological feature in aging and disease. Given the large number of proteins needed for synaptic function, the proliferation of defective proteins and the subsequent loss of protein homeostasis may be a leading cause of synaptic dysfunction. Autophagic mechanisms are cellular digestion processes that recycle cellular components and contribute to protein homeostasis. Autophagy is important within the nervous system, but its function in specific compartments such as the synapse has been unclear. Evidence from research on both autophagy and synaptic function suggests that there are links between the two and that synaptic homeostasis during aging requires autophagy to regulate protein homeostasis. Exciting new work on autophagy-modulating proteins that are enriched at the synapse has begun to link autophagy to synapses and synaptic dysfunction in disease. A better understanding of these links will help us harness the potential therapeutic benefits of autophagy in combating age-related disorders of the nervous system.

Phosphatidylinositol 5-phosphate 4-kinase regulates early endosomal dynamics during clathrin-mediated endocytosis

Endocytic turnover is essential for the regulation of the protein composition and function of the plasma membrane, and thus affects the plasma membrane levels of many receptors. In Drosophila melanogaster photoreceptors, photon absorption by the G-protein-coupled receptor (GPCR) rhodopsin 1 (Rh1; also known as NinaE) triggers its endocytosis through clathrin-mediated endocytosis (CME). We find that CME of Rh1 is regulated by phosphatidylinositol 5 phosphate 4-kinase (PIP4K). Flies lacking PIP4K show mislocalization of Rh1 on expanded endomembranes within the cell body. This mislocalization of Rh1 was dependent on the formation of an expanded Rab5-positive compartment. The Rh1-trafficking defect in PIP4K-depleted cells could be suppressed by downregulating Rab5 function or by selectively reconstituting PIP4K in the PI3P-enriched early endosomal compartment of photoreceptors. We also found that loss of PIP4K was associated with increased CME and an enlarged Rab5-positive compartment in cultured Drosophila cells. Collectively, our findings define PIP4K as a novel regulator of early endosomal homeostasis during CME.

Proteomic Analysis After Status Epilepticus Identifies UCHL1 as Protective Against Hippocampal Injury

Brief, non-harmful seizures (preconditioning) can temporarily protect the brain against prolonged, otherwise injurious seizures. Following focal-onset status epilepticus (SE) in preconditioned (tolerance) and sham-preconditioned (injury) mice, we screened for protein changes using a proteomic approach and identified several putative candidates of epileptic tolerance. Among SE-induced changes to both proteomic screens, proteins clustered in key regulatory pathways, including protein trafficking and cytoskeletal regulation. Downregulation of one such protein, ubiquitin carboxyl-terminal hydrolase isozyme L1 (UCHL1), was unique to injury and not evident in tolerance. UCHL1 inhibition decreased hippocampal ubiquitin, disrupted UPS function, interfered with seizure termination and exacerbated seizure-induced cell death. Though UCHL1 transcription was maintained after SE, we observed downregulation of the pro-translational antisense Uchl1 (AsUchl1) and confirmed that both AsUchl1 and rapamycin can increase UCHL1 expression in vivo. These data indicate that the post-transcriptional loss of UCHL1 following SE is deleterious to neuronal survival and may contribute to hyperexcitability, and are suggestive of a novel modality of rapamycin therapy.

Delineation of molecular pathway activities of the chronic antidepressant treatment response suggests important roles for glutamatergic and ubiquitin-proteasome systems

The aim of this study was to identify molecular pathways related to antidepressant response. We administered paroxetine to the DBA/2J mice for 28 days. Following the treatment, the mice were grouped into responders or non-responders depending on the time they spent immobile in the forced swim test. Hippocampal metabolomics and proteomics analyses revealed that chronic paroxetine treatment affects glutamate-related metabolite and protein levels differentially in the two groups. We found significant differences in the expression of N-methyl-d-aspartate receptor and neuronal nitric oxide synthase proteins between the two groups, without any significant alterations in the respective transcript levels. In addition, we found that chronic paroxetine treatment altered the levels of proteins associated with the ubiquitin-proteasome system (UPS). The soluble guanylate cyclase-β1, proteasome subunit α type-2 and ubiquitination levels were also affected in peripheral blood mononuclear cells from antidepressant responder and non-responder patients suffering from major depressive disorder. We submit that the glutamatergic system and UPS have a crucial role in the antidepressant treatment response in both mice and humans.

The SAC1 domain in synaptojanin is required for autophagosome maturation at presynaptic terminals

Presynaptic terminals are metabolically active and accrue damage through continuous vesicle cycling. How synapses locally regulate protein homeostasis is poorly understood. We show that the presynaptic lipid phosphatase synaptojanin is required for macroautophagy, and this role is inhibited by the Parkinson's disease mutation R258Q. Synaptojanin drives synaptic endocytosis by dephosphorylating PI(4,5)P₂, but this function appears normal in Synaptojanin^RQ knock-in flies. Instead, R258Q affects the synaptojanin SAC1 domain that dephosphorylates PI(3)P and PI(3,5)P₂, two lipids found in autophagosomal membranes. Using advanced imaging, we show that Synaptojanin^RQ mutants accumulate the PI(3)P/PI(3,5)P₂-binding protein Atg18a on nascent synaptic autophagosomes, blocking autophagosome maturation at fly synapses and in neurites of human patient induced pluripotent stem cell-derived neurons. Additionally, we observe neurodegeneration, including dopaminergic neuron loss, in Synaptojanin^RQ flies. Thus, synaptojanin is essential for macroautophagy within presynaptic terminals, coupling protein turnover with synaptic vesicle cycling and linking presynaptic-specific autophagy defects to Parkinson's disease.

Epsin1 modulates synaptic vesicle retrieval capacity at CNS synapses

Synaptic vesicle retrieval is an essential process for continuous maintenance of neural information flow after synaptic transmission. Epsin1, originally identified as an EPS15-interacting protein, is a major component of clathrin-mediated endocytosis. However, the role of Epsin1 in synaptic vesicle endocytosis at CNS synapses remains elusive. Here, we showed significantly altered synaptic vesicle endocytosis in neurons transfected with shRNA targeting Epsin1 during/after neural activity. Endocytosis was effectively restored by introducing shRNA-insensitive Epsin1 into Epsin1-depleted neurons. Domain studies performed on neurons in which domain deletion mutants of Epsin1 were introduced after Epsin1 knockdown revealed that ENTH, CLAP, and NPFs are essential for synaptic vesicle endocytosis, whereas UIMs are not. Strikingly, the efficacy of the rate of synaptic vesicle retrieval (the "endocytic capacity") was significantly decreased in the absence of Epsin1. Thus, Epsin1 is required for proper synaptic vesicle retrieval and modulates the endocytic capacity of synaptic vesicles.

Loss of SYNJ1 dual phosphatase activity leads to early onset refractory seizures and progressive neurological decline

SYNJ1 encodes a polyphosphoinositide phosphatase, synaptojanin 1, which contains two consecutive phosphatase domains and plays a prominent role in synaptic vesicle dynamics. Autosomal recessive inherited variants in SYNJ1 have previously been associated with two different neurological diseases: a recurrent homozygous missense variant (p.Arg258Gln) that abolishes Sac1 phosphatase activity was identified in three independent families with early onset parkinsonism, whereas a homozygous nonsense variant (p.Arg136*) causing a severe decrease of mRNA transcript was found in a single patient with intractable epilepsy and tau pathology. We performed whole exome or genome sequencing in three independent sib pairs with early onset refractory seizures and progressive neurological decline, and identified novel segregating recessive SYNJ1 defects. A homozygous missense variant resulting in an amino acid substitution (p.Tyr888Cys) was found to impair, but not abolish, the dual phosphatase activity of SYNJ1, whereas three premature stop variants (homozygote p.Trp843* and compound heterozygote p.Gln647Argfs*6/p.Ser1122Thrfs*3) almost completely abolished mRNA transcript production. A genetic follow-up screening in a large cohort of 543 patients with a wide phenotypical range of epilepsies and intellectual disability revealed no additional pathogenic variants, showing that SYNJ1 deficiency is rare and probably linked to a specific phenotype. While variants leading to early onset parkinsonism selectively abolish Sac1 function, our results provide evidence that a critical reduction of the dual phosphatase activity of SYNJ1 underlies a severe disorder with neonatal refractory epilepsy and a neurodegenerative disease course. These findings further expand the clinical spectrum of synaptic dysregulation in patients with severe epilepsy, and emphasize the importance of this biological pathway in seizure pathophysiology.

Arf6 and the 5'phosphatase of Synaptojanin 1 regulate autophagy in cone photoreceptors

Abnormalities in the ability of cells to properly degrade proteins have been identified in many neurodegenerative diseases. Recent work has implicated Synaptojanin 1 (SynJ1) in Alzheimer's disease and Parkinson's disease, although the role of this polyphosphoinositide phosphatase in protein degradation has not been thoroughly described. Here we dissected in vivo the role of SynJ1 in endolysosomal trafficking in zebrafish cone photoreceptors using a SynJ1-deficient zebrafish mutant, nrc^a14 . We found that loss of SynJ1 leads to specific accumulation of late endosomes and autophagosomes early in photoreceptor development. An analysis of autophagic flux revealed that autophagosomes accumulate due to a defect in maturation. In addition we found an increase in vesicles that are highly enriched for PI(3)P, but negative for an early endosome marker in nrc^a14 cones. A mutational analysis of SynJ1 enzymatic domains found that activity of the 5' phosphatase, but not the Sac1 domain, is required to rescue both aberrant late endosomes and autophagosomes. Finally, modulating activity of the PI(4,5)P₂ regulator, Arf6, rescued the disrupted trafficking pathways in nrc^a14 cones. Our study describes a specific role for SynJ1 in autophagosomal and endosomal trafficking and provides evidence that PI(4,5)P₂ participates in autophagy in a neuronal cell type.

DGKθ Catalytic Activity Is Required for Efficient Recycling of Presynaptic Vesicles at Excitatory Synapses

Synaptic transmission relies on coordinated coupling of synaptic vesicle (SV) exocytosis and endocytosis. While much attention has focused on characterizing proteins involved in SV recycling, the roles of membrane lipids and their metabolism remain poorly understood. Diacylglycerol, a major signaling lipid produced at synapses during synaptic transmission, is regulated by diacylglycerol kinase (DGK). Here, we report a role for DGKθ in the mammalian CNS in facilitating recycling of presynaptic vesicles at excitatory synapses. Using synaptophysin- and vGlut1-pHluorin optical reporters, we found that acute and chronic deletion of DGKθ attenuated the recovery of SVs following neuronal stimulation. Rescue of recycling kinetics required DGKθ kinase activity. Our data establish a role for DGK catalytic activity at the presynaptic nerve terminal in SV recycling. Altogether, these data suggest that DGKθ supports synaptic transmission during periods of elevated neuronal activity.

Phosphatidylinositolphosphate phosphatase activities and cancer

Signaling through the phosphoinositide 3-kinase pathways mediates the actions of a plethora of hormones, growth factors, cytokines, and neurotransmitters upon their target cells following receptor occupation. Overactivation of these pathways has been implicated in a number of pathologies, in particular a range of malignancies. The tight regulation of signaling pathways necessitates the involvement of both stimulatory and terminating enzymes; inappropriate activation of a pathway can thus result from activation or inhibition of the two signaling arms. The focus of this review is to discuss, in detail, the activities of the identified families of phosphoinositide phosphatase expressed in humans, and how they regulate the levels of phosphoinositides implicated in promoting malignancy.

Sac2/INPP5F is an inositol 4-phosphatase that functions in the endocytic pathway

The function of Sac2/INPP5F in the endocytic pathway and its activity as a 4-phosphatase suggest that Sac2/INPP5F and OCRL may cooperate in the sequential dephosphorylation of PI(4,5)P₂ in a partnership that mimics that of the two phosphatase modules of synaptojanin.

The recruitment of inositol phosphatases to endocytic membranes mediates dephosphorylation of PI(4,5)P₂, a phosphoinositide concentrated in the plasma membrane, and prevents its accumulation on endosomes. The importance of the conversion of PI(4,5)P₂ to PtdIns during endocytosis is demonstrated by the presence of both a 5-phosphatase and a 4-phosphatase (Sac domain) module in the synaptojanins, endocytic PI(4,5)P₂ phosphatases conserved from yeast to humans and the only PI(4,5)P₂ phosphatases in yeast. OCRL, another 5-phosphatase that couples endocytosis to PI(4,5)P₂ dephosphorylation, lacks a Sac domain. Here we show that Sac2/INPP5F is a PI4P phosphatase that colocalizes with OCRL on endocytic membranes, including vesicles formed by clathrin-mediated endocytosis, macropinosomes, and Rab5 endosomes. An OCRL–Sac2/INPP5F interaction could be demonstrated by coimmunoprecipitation and was potentiated by Rab5, whose activity is required to recruit Sac2/INPP5F to endosomes. Sac2/INPP5F and OCRL may cooperate in the sequential dephosphorylation of PI(4,5)P₂ at the 5 and 4 position of inositol in a partnership that mimics that of the two phosphatase modules of synaptojanin.

Synaptojanin cooperates in vivo with endophilin through an unexpected mechanism

Synaptojanin and endophilin represent a classic pair of endocytic proteins that exhibit coordinated action during rapid synaptic vesicle endocytosis. Current models suggest that synaptojanin activity is tightly associated with endophilin through high-affinity binding between the synaptojanin proline-rich domain (PRD) and the endophilin SH3 domain. Surprisingly, we find that truncated synaptojanin lacking the PRD domain sustains normal synaptic transmission, indicating that synaptojanin's core function in vivo resides in the remaining two domains that contain phosphoinositide-phosphatase activities: an N-terminal Sac1 phosphatase domain and a 5-phosphatase domain. We further show that the Sac1 domain plays an unexpected role in targeting synaptojanin to synapses. The requirement for Sac1 is bypassed by tethering the synaptojanin 5-phophatase to the endophilin membrane-bending Bin–Amphiphysin–Rvs (BAR) domain. Together, our results uncover an unexpected role for the Sac1 domain in vivo in supporting coincident action between synaptojanin and endophilin at synapses.

DOI:http://dx.doi.org/10.7554/eLife.05660.001

Nerve cells called neurons can rapidly carry information around the body. Each neuron forms connections called synapses with several other cells to build networks for information exchange. At most synapses, electrical activity in one neuron results in the release of chemicals called neurotransmitters from storage compartments called synaptic vesicles. The neurotransmitters leave the cell and cross the gap between the two neurons to activate the next cell.

After the neurotransmitters have been released, the synaptic vesicles need to be regenerated via a recycling process called endocytosis. This recycling process is very important for synapses to work properly, but it is not clear exactly how it occurs. Two of the proteins involved are called synaptojanin and endophilin. Synaptojanin is made up of three structural units (or ‘domains’), including the proline-rich domain and the Sac1 domain. It has been proposed that interactions between endophilin and the proline-rich domain of synaptojanin are essential for vesicle recycling.

Here, Dong et al. studied nematode worms that carry mutant forms of synaptojanin. The experiments show that the Sac1 domain, but not the proline-rich domain, is required for the synapses to work properly. However, the Sac1 domain is not required if synaptojanin is artificially linked to endophilin.

Dong et al.'s findings suggest that synaptojanin uses its Sac1 domains to work with endophilin. A future challenge will be to understand the details of how this cooperative action occurs.

DOI:http://dx.doi.org/10.7554/eLife.05660.002

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.

Calcineurin regulates the yeast synaptojanin Inp53/Sjl3 during membrane stress

During hyperosmotic shock, Saccharomyces cerevisiae adjusts to physiological challenges, including large plasma membrane invaginations generated by rapid cell shrinkage. Calcineurin, the Ca(2+)/calmodulin-dependent phosphatase, is normally cytosolic but concentrates in puncta and at sites of polarized growth during intense osmotic stress; inhibition of calcineurin-activated gene expression suggests that restricting its access to substrates tunes calcineurin signaling specificity. Hyperosmotic shock promotes calcineurin binding to and dephosphorylation of the PI(4,5)P2 phosphatase synaptojanin/Inp53/Sjl3 and causes dramatic calcineurin-dependent reorganization of PI(4,5)P2-enriched membrane domains. Inp53 normally promotes sorting at the trans-Golgi network but localizes to cortical actin patches in osmotically stressed cells. By activating Inp53, calcineurin repolarizes the actin cytoskeleton and maintains normal plasma membrane morphology in synaptojanin-limited cells. In response to hyperosmotic shock and calcineurin-dependent regulation, Inp53 shifts from associating predominantly with clathrin to interacting with endocytic proteins Sla1, Bzz1, and Bsp1, suggesting that Inp53 mediates stress-specific endocytic events. This response has physiological and molecular similarities to calcineurin-regulated activity-dependent bulk endocytosis in neurons, which retrieves a bolus of plasma membrane deposited by synaptic vesicle fusion. We propose that activation of Ca(2+)/calcineurin and PI(4,5)P2 signaling to regulate endocytosis is a fundamental and conserved response to excess membrane in eukaryotic cells.

Synaptojanin 2 is a druggable mediator of metastasis and the gene is overexpressed and amplified in breast cancer

Amplified HER2, which encodes a member of the epidermal growth factor receptor (EGFR) family, is a target of effective therapies against breast cancer. In search for similarly targetable genomic aberrations, we identified copy number gains in SYNJ2, which encodes the 5'-inositol lipid phosphatase synaptojanin 2, as well as overexpression in a small fraction of human breast tumors. Copy gain and overexpression correlated with shorter patient survival and a low abundance of the tumor suppressor microRNA miR-31. SYNJ2 promoted cell migration and invasion in culture and lung metastasis of breast tumor xenografts in mice. Knocking down SYNJ2 impaired the endocytic recycling of EGFR and the formation of cellular lamellipodia and invadopodia. Screening compound libraries identified SYNJ2-specific inhibitors that prevented cell migration but did not affect the related neural protein SYNJ1, suggesting that SYNJ2 is a potentially druggable target to block cancer cell migration.

Activity-dependent PI(3,5)P2 synthesis controls AMPA receptor trafficking during synaptic depression

Dynamic regulation of phosphoinositide lipids (PIPs) is crucial for diverse cellular functions, and, in neurons, PIPs regulate membrane trafficking events that control synapse function. Neurons are particularly sensitive to the levels of the low abundant PIP, phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2], because mutations in PI(3,5)P2-related genes are implicated in multiple neurological disorders, including epilepsy, severe neuropathy, and neurodegeneration. Despite the importance of PI(3,5)P2 for neural function, surprisingly little is known about this signaling lipid in neurons, or any cell type. Notably, the mammalian homolog of yeast vacuole segregation mutant (Vac14), a scaffold for the PI(3,5)P2 synthesis complex, is concentrated at excitatory synapses, suggesting a potential role for PI(3,5)P2 in controlling synapse function and/or plasticity. PI(3,5)P2 is generated from phosphatidylinositol 3-phosphate (PI3P) by the lipid kinase PI3P 5-kinase (PIKfyve). Here, we present methods to measure and control PI(3,5)P2 synthesis in hippocampal neurons and show that changes in neural activity dynamically regulate the levels of multiple PIPs, with PI(3,5)P2 being among the most dynamic. The levels of PI(3,5)P2 in neurons increased during two distinct forms of synaptic depression, and inhibition of PIKfyve activity prevented or reversed induction of synaptic weakening. Moreover, altering neuronal PI(3,5)P2 levels was sufficient to regulate synaptic strength bidirectionally, with enhanced synaptic function accompanying loss of PI(3,5)P2 and reduced synaptic strength following increased PI(3,5)P2 levels. Finally, inhibiting PI(3,5)P2 synthesis alters endocytosis and recycling of AMPA-type glutamate receptors (AMPARs), implicating PI(3,5)P2 dynamics in AMPAR trafficking. Together, these data identify PI(3,5)P2-dependent signaling as a regulatory pathway that is critical for activity-dependent changes in synapse strength.

Synaptojanin 1 mutation in Parkinson's disease brings further insight into the neuropathological mechanisms

Synaptojanin 1 (SYNJ1) is a phosphoinositide phosphatase highly expressed in nerve terminals. Its two phosphatase domains dephosphorylate phosphoinositides present in membranes, while its proline-rich domain directs protein-protein interactions with synaptic components, leading to efficient recycling of synaptic vesicles in neurons. Triplication of SYNJ1 in Down's syndrome is responsible for higher level of phosphoinositides, enlarged endosomes, and learning deficits. SYNJ1 downregulation in Alzheimer's disease models is protective towards amyloid-beta peptide (Aβ) toxicity. One missense mutation in one of SYNJ1 functional domains was recently incriminated in an autosomal recessive form of early-onset Parkinson's disease (PD). In the third decade of life, these patients develop progressive Parkinsonism with bradykinesia, dystonia, and variable atypical symptoms such as cognitive decline, seizures, and eyelid apraxia. The identification of this new gene, together with the fact that most of the known PD proteins play a role in synaptic vesicle recycling and lipid metabolism, points out that synaptic maintenance is a key player in PD pathological mechanisms. Studying PD genes as a network regulating synaptic activity could bring insight into understanding the neuropathological processes of PD and help identify new genes at fault in this devastating disorder.

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.

Inositol lipid phosphatases in membrane trafficking and human disease

The specific interaction of phosphoinositides with proteins is critical for a plethora of cellular processes, including cytoskeleton remodelling, mitogenic signalling, ion channel regulation and membrane traffic. The spatiotemporal restriction of different phosphoinositide species helps to define compartments within the cell, and this is particularly important for membrane trafficking within both the secretory and endocytic pathways. Phosphoinositide homoeostasis is tightly regulated by a large number of inositol kinases and phosphatases, which respectively phosphorylate and dephosphorylate distinct phosphoinositide species. Many of these enzymes have been implicated in regulating membrane trafficking and, accordingly, their dysregulation has been linked to a number of human diseases. In the present review, we focus on the inositol phosphatases, concentrating on their roles in membrane trafficking and the human diseases with which they have been associated.

A role of OCRL in clathrin-coated pit dynamics and uncoating revealed by studies of Lowe syndrome cells

Mutations in the inositol 5-phosphatase OCRL cause Lowe syndrome and Dent's disease. Although OCRL, a direct clathrin interactor, is recruited to late-stage clathrin-coated pits, clinical manifestations have been primarily attributed to intracellular sorting defects. Here we show that OCRL loss in Lowe syndrome patient fibroblasts impacts clathrin-mediated endocytosis and results in an endocytic defect. These cells exhibit an accumulation of clathrin-coated vesicles and an increase in U-shaped clathrin-coated pits, which may result from sequestration of coat components on uncoated vesicles. Endocytic vesicles that fail to lose their coat nucleate the majority of the numerous actin comets present in patient cells. SNX9, an adaptor that couples late-stage endocytic coated pits to actin polymerization and which we found to bind OCRL directly, remains associated with such vesicles. These results indicate that OCRL acts as an uncoating factor and that defects in clathrin-mediated endocytosis likely contribute to pathology in patients with OCRL mutations.

DOI:http://dx.doi.org/10.7554/eLife.02975.001

Oculo-Cerebro-Renal syndrome of Lowe (Lowe syndrome) is a rare genetic disorder that can cause cataracts, mental disabilities and kidney dysfunction. It is caused by mutations in the gene encoding OCRL, a protein that modifies a membrane lipid and that is found on membranes transporting molecules (cargo) into cells by a process known as endocytosis.

During endocytosis, the cell outer membrane is deformed into a pit that engulfs the cargo to be taken up by the cell. The pit then pinches off from the outer membrane to form a vesicle—a bubble-like compartment—inside the cell that transports the cargo to its destination. In one type of endocytosis, this process is mediated by a basket-like coat primarily made up from the protein clathrin that assembles at the membrane patch to be internalized. After the vesicle is released from the cell membrane, the clathrin coat is broken apart and its components are shed and recycled for use by new budding endocytic vesicles.

The OCRL protein had previously been observed associated to newly forming clathrin-coated vesicles, but the significance of this was not known. Now, Nández et al. have used a range of imaging and analytical techniques to further investigate the properties of OCRL, taking advantage of cells from patients with Lowe syndrome. These cells lack OCRL, and so allow the effect of OCRL's absence on cell function to be deduced. OCRL destroys the membrane lipid that helps to connect the clathrin coat to the membrane, and Nández et al. show that without OCRL the newly formed vesicle moves into the cell but fails to efficiently shed its clathrin coat. Thus, a large fraction of clathrin coat components remain trapped on the vesicles, reducing the amount of such components available to help new pits develop into vesicles. As a consequence, the cell has difficulty internalizing molecules.

Collectively, the findings of Nández et al. outline that OCRL plays a role in the regulation of endocytosis in addition to its previously reported actions in the control of intracellular membrane traffic. The results also help to explain some of the symptoms seen in Lowe syndrome patients.

DOI:http://dx.doi.org/10.7554/eLife.02975.002

Activity-dependent facilitation of Synaptojanin and synaptic vesicle recycling by the Minibrain kinase

Phosphorylation has emerged as a crucial regulatory mechanism in the nervous system to integrate the dynamic signalling required for proper synaptic development, function and plasticity, particularly during changes in neuronal activity. Here we present evidence that Minibrain (Mnb; also known as Dyrk1A), a serine/threonine kinase implicated in autism spectrum disorder and Down syndrome, is required presynaptically for normal synaptic growth and rapid synaptic vesicle endocytosis at the Drosophila neuromuscular junction (NMJ). We find that Mnb-dependent phosphorylation of Synaptojanin (Synj) is required, in vivo, for complex endocytic protein interactions and to enhance Synj activity. Neuronal stimulation drives Mnb mobilization to endocytic zones and triggers Mnb-dependent phosphorylation of Synj. Our data identify Mnb as a synaptic kinase that promotes efficient synaptic vesicle recycling by dynamically calibrating Synj function at the Drosophila NMJ, and in turn endocytic capacity, to adapt to conditions of high synaptic activity.

Synaptojanin 1 is required for endolysosomal trafficking of synaptic proteins in cone photoreceptor inner segments

Highly polarized cells such as photoreceptors require precise and efficient strategies for establishing and maintaining the proper subcellular distribution of proteins. The signals and molecular machinery that regulate trafficking and sorting of synaptic proteins within cone inner segments is mostly unknown. In this study, we show that the polyphosphoinositide phosphatase Synaptojanin 1 (SynJ1) is critical for this process. We used transgenic markers for trafficking pathways, electron microscopy, and immunocytochemistry to characterize trafficking defects in cones of the zebrafish mutant, nrc(a14) , which is deficient in phosphoinositide phosphatase, SynJ1. The outer segments and connecting cilia of nrc(a14) cone photoreceptors are normal, but RibeyeB and VAMP2/synaptobrevin, which normally localize to the synapse, accumulate in the nrc(a14) inner segment. The structure of the Endoplasmic Reticulum in nrc(a14) mutant cones is normal. Golgi develop normally, but later become disordered. Large vesicular structures accumulate within nrc(a14) cone photoreceptor inner segments, particularly after prolonged incubation in darkness. Cone inner segments of nrc (a14) mutants also have enlarged acidic vesicles, abnormal late endosomes, and a disruption in autophagy. This last pathway also appears exacerbated by darkness. Taken altogether, these findings show that SynJ1 is required in cones for normal endolysosomal trafficking of synaptic proteins.

Sorting of the vesicular GABA transporter to functional vesicle pools by an atypical dileucine-like motif

Increasing evidence indicates that individual synaptic vesicle proteins may use different signals, endocytic adaptors, and trafficking pathways for sorting to distinct pools of synaptic vesicles. Here, we report the identification of a unique amino acid motif in the vesicular GABA transporter (VGAT) that controls its synaptic localization and activity-dependent recycling. Mutational analysis of this atypical dileucine-like motif in rat VGAT indicates that the transporter recycles by interacting with the clathrin adaptor protein AP-2. However, mutation of a single acidic residue upstream of the dileucine-like motif leads to a shift to an AP-3-dependent trafficking pathway that preferentially targets the transporter to the readily releasable and recycling pool of vesicles. Real-time imaging with a VGAT-pHluorin fusion provides a useful approach to explore how unique sorting sequences target individual proteins to synaptic vesicles with distinct functional properties.