Parkinson's disease gene, Synaptojanin1, dysregulates the surface maintenance of the dopamine transporter

Missense mutations of PARK20/SYNJ1 (synaptojanin1/Synj1) have been linked to complex forms of familial parkinsonism, however, the molecular and cellular changes associated with dopaminergic dysfunction remains unknown. We now report fast depletion of evoked dopamine (DA) and altered maintenance of the axonal dopamine transporter (DAT) in the Synj1+/- neurons. While Synj1 has been traditionally known to facilitate the endocytosis of synaptic vesicles, we demonstrated that axons of cultured Synj1+/- neurons exhibit an increase of total DAT but a reduction of the surface DAT, which could be exacerbated by neuronal activity. We revealed that the loss of surface DAT is specifically associated with the impaired 5'-phosphatase activity of Synj1 and the hyperactive downstream PI(4,5)P2-PKCβ pathway. Thus, our findings provided important mechanistic insight for Synj1-regulated DAT trafficking integral to dysfunctional DA signaling in early parkinsonism.

TRPM2 as a conserved gatekeeper determines the vulnerability of DA neurons by mediating ROS sensing and calcium dyshomeostasis

Different dopaminergic (DA) neuronal subgroups exhibit distinct vulnerability to stress, while the underlying mechanisms are elusive. Here we report that the transient receptor potential melastatin 2 (TRPM2) channel is preferentially expressed in vulnerable DA neuronal subgroups, which correlates positively with aging in Parkinson's Disease (PD) patients. Overexpression of human TRPM2 in the DA neurons of C. elegans resulted in selective death of ADE but not CEP neurons in aged worms. Mechanistically, TRPM2 activation mediates FZO-1/CED-9-dependent mitochondrial hyperfusion and mitochondrial permeability transition (MPT), leading to ADE death. In mice, TRPM2 knockout reduced vulnerable substantia nigra pars compacta (SNc) DA neuronal death induced by stress. Moreover, the TRPM2-mediated vulnerable DA neuronal death pathway is conserved from C. elegans to toxin-treated mice model and PD patient iPSC-derived DA neurons. The vulnerable SNc DA neuronal loss is the major symptom and cause of PD, and therefore the TRPM2-mediated pathway serves as a promising therapeutic target against PD.

Synaptojanin1 modifies endolysosomal parameters in cultured ventral midbrain neurons

The accumulation of α-syn enriched protein aggregates is thought to arise from dysfunction in degradation systems within the brain. Recently, missense mutations of SYNJ1 encoding the SAC1 and 5'-phosphatase domains have been found in families with hereditary early-onset Parkinsonism. Previous studies showed that Synj1 haploinsufficiency (Synj1+/-) leads to accumulation of the autophagy substrate p62 and pathological α-syn proteins in the midbrain (MB) and striatum of aged mice. In this study, we aim to investigate the neuronal degradation pathway using the Synj1+/- MB culture from mouse pups of mixed sex as a model. Our data show that GFP-LC3 puncta formation and cumulative mKeima puncta formation are unaltered at baseline in Synj1+/- MB neurons. However, GFP-LAMP1 puncta is reduced with a similar decrease in endogenous proteins, including LAMP1, LAMP2, and LAMP2A. The LAMP1 vesicles are hyperacidified with enhanced enzymatic activity in Synj1+/- MB neurons. Using a combination of light and electron microscopy, we show that endolysosomal changes are primarily associated with a lack of SAC1 activity. Consistently, expressing the SYNJ1 R258Q mutant in N2a cells reduces the lysosome number. Interestingly, the endolysosomal defects in Synj1+/- neurons does not impact the clearance of exogenously expressed wild-type α-syn; however, the clearance of α-syn A53T was impaired in the axons of Synj1+/- MB neurons. Taken together, our results suggest axonal vulnerability to endolysosomal defects in Synj1 deficient MB neurons.Significance StatementIn the study, Zhu et al. discovered a previously uncharacterized role of Synj1 in regulating endolysosomal number, protein, and acidity in ventral midbrain neurons. These alterations are associated with a specific impairment in the clearance of α-syn A53T, but not WT α-syn in axons, suggesting an essential role of Synj1 in axonal degradative capacity under pathological stress. This work in cultured mammalian neurons complements recent research efforts in Drosophila, C. elegans and zebra fish, and provides a novel insight for the role Synj1 in neuronal degradation.

Mini-review: Synaptojanin 1 and its implications in membrane trafficking

This mini-review aims to summarize a growing body of literature on synaptojanin 1 (Synj1), a phosphoinositide phosphatase that was initially known to have a prominent role in synaptic vesicle recycling. Synj1 is coded by the SYNJ1 gene, whose mutations and variants are associated with an increasing number of neurological disorders. To better understand the mechanistic role of Synj1 in disease pathogenesis, we review details of phosphoinositide signaling pathways and the reported involvement of Synj1 in membrane trafficking with a specific focus on Parkinson’s disease (PD). Recent studies have tremendously advanced our understanding of Synj1 protein structure and function while broadening our view of how Synj1 regulates synaptic membrane trafficking and endosomal trafficking in various organisms and cell types. A growing body of evidence points to inefficient membrane trafficking as key pathogenic mechanisms in neurodegenerative diseases associated with abnormal Synj1 expression. Despite significant progress made in the field, the mechanism by which Synj1 connects to trafficking, signaling, and pathogenesis is lacking and remains to be addressed.

Synj1 haploinsufficiency causes dopamine neuron vulnerability and alpha-synuclein accumulation in mice

Synaptojanin1 (synj1) is a phosphoinositide phosphatase with dual SAC1 and 5'-phosphatase enzymatic activities in regulating phospholipid signaling. The brain-enriched isoform has been shown to participate in synaptic vesicle (SV) recycling. More recently, recessive human mutations were identified in the two phosphatase domains of SYNJ1, including R258Q, R459P and R839C, which are linked to rare forms of early-onset Parkinsonism. We now demonstrate that Synj1 heterozygous deletion (Synj1+/-), which is associated with an impaired 5'-phosphatase activity, also leads to Parkinson's disease (PD)-like pathologies in mice. We report that male Synj1+/- mice display age-dependent motor function abnormalities as well as alpha-synuclein accumulation, impaired autophagy and dopaminergic terminal degeneration. Synj1+/- mice contain elevated 5'-phosphatase substrate, PI(4,5)P2, particularly in the midbrain neurons. Moreover, pharmacological elevation of membrane PI(4,5)P2 in cultured neurons impairs SV endocytosis, specifically in midbrain neurons, and further exacerbates SV trafficking defects in Synj1+/- midbrain neurons. We demonstrate down-regulation of SYNJ1 transcript in a subset of sporadic PD brains, implicating a potential role of Synj1 deficiency in the decline of dopaminergic function during aging.

Synaptojanin1 deficiency upregulates basal autophagosome formation in astrocytes

Macroautophagy dysregulation is implicated in multiple neurological disorders, such as Parkinson's disease. While autophagy pathways are heavily researched in heterologous cells and neurons, regulation of autophagy in the astrocyte, the most abundant cell type in the mammalian brain, is less well understood. Missense mutations in the Synj1 gene encoding Synaptojanin1 (Synj1), a neuron-enriched lipid phosphatase, have been linked to Parkinsonism with seizures. Our previous study showed that the Synj1 haploinsufficient (Synj1^+/−) mouse exhibits age-dependent autophagy impairment in multiple brain regions. Here, we used cultured astrocytes from Synj1-deficient mice to investigate its role in astrocyte autophagy. We report that Synj1 is expressed in low levels in astrocytes and represses basal autophagosome formation. We demonstrate using cellular imaging that Synj1-deficient astrocytes exhibit hyperactive autophagosome formation, represented by an increase in the size and number of GFP-microtubule-associated protein 1A/1B-light chain 3 structures. Interestingly, Synj1 deficiency is also associated with an impairment in stress-induced autophagy clearance. We show, for the first time, that the Parkinsonism-associated R839C mutation impacts autophagy in astrocytes. The impact of this mutation on the phosphatase function of Synj1 resulted in elevated basal autophagosome formation that mimics Synj1 deletion. We found that the membrane expression of the astrocyte-specific glucose transporter GluT-1 was reduced in Synj1-deficient astrocytes. Consistently, AMP-activated protein kinase activity was elevated, suggesting altered glucose sensing in Synj1-deficient astrocytes. Expressing exogenous GluT-1 in Synj1-deficient astrocytes reversed the autophagy impairment, supporting a role for Synj1 in regulating astrocyte autophagy via disrupting glucose-sensing pathways. Thus, our work suggests a novel mechanism for Synj1-related Parkinsonism involving astrocyte dysfunction.

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.

Vesicular glutamate transporter 1 orchestrates recruitment of other synaptic vesicle cargo proteins during synaptic vesicle recycling

A long standing question in synaptic physiology is how neurotransmitter-filled vesicles are rebuilt after exocytosis. Among the first steps in this process is the endocytic retrieval of the transmembrane proteins that are enriched in synaptic vesicles (SVs). At least six types of transmembrane proteins must be recovered, but the rules for how this multiple cargo selection is accomplished are poorly understood. Among these SV cargos is the vesicular glutamate transporter (vGlut). We show here that vGlut1 has a strong influence on the kinetics of retrieval of half of the known SV cargos and that specifically impairing the endocytosis of vGlut1 in turn slows down other SV cargos, demonstrating that cargo retrieval is a collective cargo-driven process. Finally, we demonstrate that different cargos can be retrieved in the same synapse with different kinetics, suggesting that additional post-endocytic sorting steps likely occur in the nerve terminal.

Genetic causes of Parkinson's disease and their links to autophagy regulation

Genetic studies over the past 15 years have revolutionized our understanding towards the etiology of Parkinson's disease (PD). These studies have discovered many disease-linked genetic loci (PARK 1 to 18), which are now being interrogated for cellular pathways contributing to PD. Various pathogenic pathways were proposed but validation of each pathway awaits rigorous experimental testing. Here we review recent progress in understanding the influence of disease risk genes on cellular functions, specifically, autophagy pathways. Autophagy is a cell self-eating, lysosomal degradation system that plays an important role in cell homeostasis and survival. Neurons are post-mitotic cells and particularly vulnerable to the impairment of autophagic degradation due to their inability to redistribute damaged proteins and organelles to daughter cells. Emerging evidence has implicated dysfunctional autophagy in a growing number of neurodegenerative diseases including PD. We will also discuss the prospect of intervening autophagy pathways as a potential strategy to treat PD.

Motile axonal mitochondria contribute to the variability of presynaptic strength

One of the most notable characteristics of synaptic transmission is the wide variation in synaptic strength in response to identical stimulation. In hippocampal neurons, approximately one-third of axonal mitochondria are highly motile, and some dynamically pass through presynaptic boutons. This raises a fundamental question: can motile mitochondria contribute to the pulse-to-pulse variability of presynaptic strength? Recently, we identified syntaphilin as an axonal mitochondrial-docking protein. Using hippocampal neurons and slices of syntaphilin knockout mice, we demonstrate that the motility of axonal mitochondria correlates with presynaptic variability. Enhancing mitochondrial motility increases the pulse-to-pulse variability, whereas immobilizing mitochondria reduces the variability. By dual-color live imaging at single-bouton levels, we further show that motile mitochondria passing through boutons dynamically influence synaptic vesicle release, mainly by altering ATP homeostasis in axons. Thus, our study provides insight into the fundamental properties of the CNS to ensure the plasticity and reliability of synaptic transmission.

Snapin facilitates the synchronization of synaptic vesicle fusion

Synaptic vesicle (SV) fusion is a fine-tuned process requiring a concert of fusion machineries. Using cortical neurons from snapin-deficient mice, we reveal a role for Snapin in facilitating synchronous release. In addition to reduced frequency of miniature excitatory postsynaptic currents (mini-EPSCs) and smaller release-ready vesicle pool (RRP) size, snapin deficiency results in EPSCs with multiple peaks and increased rise and decay times, reflecting "desynchronized" SV fusion. These defects impair both synaptic precision and efficacy during sustained neurotransmission. Transient expression of Snapin not only rescues the slowed kinetics of EPSCs, but also further accelerates the rate found in wild-type neurons. Furthermore, expression of Snapin-C66A, a dimerization-defective mutant with impaired interactions with SNAP-25 and Synaptotagmin, reduces the RRP size but exhibits less effect on synchronized fusion. Our studies provide mechanistic insights into a dual role of Snapin in enhancing the efficacy of SV priming and in fine-tuning synchronous SV fusion.

Docking of axonal mitochondria by syntaphilin controls their mobility and affects short-term facilitation

Proper distribution of mitochondria within axons and at synapses is critical for neuronal function. While one-third of axonal mitochondria are mobile, a large proportion remains in a stationary phase. However, the mechanisms controlling mitochondrial docking within axons remain elusive. Here, we report a role for axon-targeted syntaphilin (SNPH) in mitochondrial docking through its interaction with microtubules. Axonal mitochondria that contain exogenously or endogenously expressed SNPH lose mobility. Deletion of the mouse snph gene results in a substantially higher proportion of axonal mitochondria in the mobile state and reduces the density of mitochondria in axons. The snph mutant neurons exhibit enhanced short-term facilitation during prolonged stimulation, probably by affecting calcium signaling at presynaptic boutons. This phenotype is fully rescued by reintroducing the snph gene into the mutant neurons. These findings demonstrate a molecular mechanism for controlling mitochondrial docking in axons that has a physiological impact on synaptic function.

Syntabulin-kinesin-1 family member 5B-mediated axonal transport contributes to activity-dependent presynaptic assembly

The mechanism by which microtubule-based axonal transport regulates activity-dependent presynaptic plasticity in developing neurons remains mostly unknown. Our previous studies established that syntabulin is an adaptor capable of conjoining the kinesin family member 5B (KIF5B) motor and syntaxin-1. We now report that the complex of syntaxin-1-syntabulin-KIF5B mediates axonal transport of the active zone (AZ) components essential for presynaptic assembly. Syntabulin associates with AZ precursor carriers and colocalizes and comigrates with green fluorescent protein (GFP)-Bassoon-labeled AZ transport cargos within developing axons. Knock-down of syntabulin or disruption of the syntaxin-1-syntabulin-KIF5B complex impairs the anterograde transport of GFP-Bassoon out of the soma and reduces the axonal densities of synaptic vesicle (SV) clusters and FM4-64 [N-(3-triethylammoniumpropyl)-4-(p-dibutylaminostyryl)pyridinium, dibromide] loading. Furthermore, syntabulin loss of function results in a reduction in both the amplitude of postsynaptic currents and the frequency of asynchronous quantal events, and abolishes the activity-induced recruitment of new GFP-Bassoon into the axons and subsequent coclustering with SVs. Consequently, syntabulin loss of function blocks the formation of new presynaptic boutons during activity-dependent synaptic plasticity in developing neurons. These studies establish that a kinesin motor-adaptor complex is critical for the anterograde axonal transport of AZ components, thus contributing to activity-dependent presynaptic assembly during neuronal development.

SNAP-29-mediated modulation of synaptic transmission in cultured hippocampal neurons

Identifying the molecules that regulate both the recycling of synaptic vesicles and the SNARE components required for fusion is critical for elucidating the molecular mechanisms underlying synaptic plasticity. SNAP-29 was initially isolated as a syntaxin-binding and ubiquitously expressed protein. Previous studies have suggested that SNAP-29 inhibits SNARE complex disassembly, thereby reducing synaptic transmission in cultured superior cervical ganglion neurons in an activity-dependent manner. However, the role of SNAP-29 in regulating synaptic vesicle recycling and short-term plasticity in the central nervous system remains unclear. In the present study, we examined the effect of SNAP-29 on synaptic transmission in cultured hippocampal neurons by dual patch clamp whole-cell recording, FM dye imaging, and immunocytochemistry. Our results demonstrated that exogenous expression of SNAP-29 in presynaptic neurons significantly decreased the efficiency of synaptic transmission after repetitive firing within a few minutes under low and moderate frequency stimulations (0.1 and 1 Hz). In contrast, SNAP-29 did not affect the density of synapses and basal synaptic transmission. Whereas neurotransmitter release was unaffected during intensive stimulation, recovery after synaptic depression was impaired by SNAP-29. Furthermore, knockdown of SNAP-29 expression in neurons by small interfering RNA increased the efficiency of synaptic transmission during repetitive firing. These findings suggest that SNAP-29 acts as a negative modulator for neurotransmitter release, probably by slowing recycling of the SNARE-based fusion machinery and synaptic vesicle turnover.

Immunoreceptor tyrosine-based inhibitory motif of the IL-4 receptor associates with SH2-containing phosphatases and regulates IL-4-induced proliferation

Immunoreceptor tyrosine-based inhibitory motifs (ITIM) have been implicated in the negative modulation of immunoreceptor signaling pathways. The IL-4R alpha-chain (IL-4Ralpha) contains a putative ITIM in the carboxyl terminal. To determine the role of ITIM in the IL-4 signaling pathway, we ablated the ITIM of IL-4Ralpha by deletion and site-directed mutagenesis and stably expressed the wild-type (WT) and mutant hIL-4Ralpha in 32D/insulin receptor substrate-2 (IRS-2) cells. Strikingly, 32D/IRS-2 cells expressing mutant human (h)IL-4Ralpha were hyperproliferative in response to IL-4 compared with cells expressing WT hIL-4Ralpha. Enhanced tyrosine phosphorylation of Stat6, but not IRS-2, induced by hIL-4 was observed in cells expressing mutant Y713F. Using peptides corresponding to the ITIM of hIL-4Ralpha, we demonstrate that tyrosine-phosphorylated peptides, but not their nonphosphorylated counterparts, coprecipitate SH2-containing tyrosine phosphatase-1, SH2-containing tyrosine phosphatase-2, and SH2-containing inositol 5'-phosphatase. The in vivo association of SH2-containing inositol 5'-phosphatase with IL-4Ralpha was verified by coimmunoprecipitation with anti-IL-4Ralpha Abs. These results demonstrate a functional role for ITIM in the regulation of IL-4-induced proliferation.

Positive regulation of interleukin-4-mediated proliferation by the SH2-containing inositol-5'-phosphatase

The SH2-containing inositol 5'-phosphatase (SHIP) is tyrosine-phosphorylated in response to cytokines such as interleukin (IL)-3, granulocyte-macrophage colony-stimulating factor, and macrophage colony-stimulating factor. SHIP has been shown to modulate negatively these cytokine signalings; however, a potential role in IL-4 signaling remains uncharacterized. It has been recently shown that IL-4 induces tyrosine phosphorylation of SHIP, implicating the phosphatase in IL-4 processes. Tyrosine kinases, Jak1 and Jak3, involved in IL-4 signaling can associate with SHIP, yet only Jak1 can tyrosine-phosphorylate SHIP when co-expressed. In functional studies, cells overexpressing wild type SHIP are found to be hyperproliferative in response to IL-4 in comparison to parental cells. In contrast, cells expressing catalytically inactive form, SHIP(D672A), show reduced proliferation in response to IL-4. These changes in IL-4-induced proliferation correlate with alterations in phosphatidylinositol 3,4,5-triphosphate levels. However, no differential activation of STAT6, Akt, IRS-2, or p70(S6k), in response to IL-4, was observed in these cells. These data suggest that the catalytic activity of SHIP acts in a novel manner to influence IL-4 signaling. In addition, these data support recent findings that suggest there are uncharacterized signaling pathways downstream of phosphatidylinositol 3,4,5-triphosphate.

IL-4 receptor mutations

IL-4 plays an important role in regulating immune responses. Distinct signaling pathways, including those for gene activation and cell differentiation and those for cell proliferation and protection from apoptosis, are initiated from the receptor complex for IL-4 following ligand-receptor engagement. Several advances have been made in our understanding of how distinct functions of IL-4 are mediated. Most of these studies employed artificial mutations of the IL-4-receptor alpha chain using site-directed mutagenesis and/or deletional mutation. In addition, naturally occurring mutations of the IL-4-receptor alpha chain have been identified and implicated as a genetic predisposition for allergic disorders. The results of these studies suggest a modular organization of the receptor and an independent regulation of gene activation and cell growth.

The role of recombination signal sequences in the preferential joining by deletion in DH-JH recombination and in the ordered rearrangement of the IgH locus

The bias favoring deletion over inversion in DH-JH rearrangement has been known for years, but the underlying mechanism has yet to be fully defined. It has been suggested that the ratio of deletion/inversion is determined by the combined effect of two factors: (i) the relative strengths of 5' and 3' recombination signal sequences (RSS) of a DH segment, and (ii) the efficiency with which the deletional product (one joint) forms relative to the inversional product (two joints). In this study, we analyzed for the first time the effect of factor 1 alone on the biased 3' RSS utilization in DH-JH joining by using deletional plasmids in an extrachromosomal substrate V(D)J recombination assay. It was found that the 3' RSS and associated coding end (12 bp) mediate recombination more efficiently than the 5' RSS/coding end DH-JH plasmids. These results demonstrate that the effect of the RSS/coding end alone can account, at least partially, for the predominant deletion in DH-JH recombination. The potential effect of the relative strength of RSS and associated coding end on the ordered rearrangement of DH-JH followed by VH to DH-JH was also assessed. When recombination frequencies of D-->J (3' DH to J3) were compared with frequencies of V-->D (VHPJ14 to 3' DH or VHOX2 to 3' DH), it was found that V-->D joining was, if anything, more efficient than D-->J joining. Therefore, if all three segments were accessible, RSS/coding end effects would not contribute to the ordered rearrangement of the IgH locus.