UNC-16, a JNK-signaling scaffold protein, regulates vesicle transport in C. elegans

The role of kinesin-1 in neuronal dense core vesicle transport, locomotion and lifespan regulation in C. elegans

Fast axonal transport is crucial for neuronal function and is driven by kinesins and cytoplasmic dynein. Here, we investigated the role of kinesin-1 in dense core vesicle (DCV) transport in C. elegans, using mutants in the kinesin light chains (klc-1 and klc-2) and the motor subunit (unc-116) expressing an ida-1::gfp transgene that labels DCVs. DCV transport in both directions was greatly impaired in an unc-116 mutant and had reduced velocity in a klc-2 mutant. In contrast, the speed of retrograde DCV transport was increased in a klc-1 mutant whereas anterograde transport was unaffected. We identified striking differences between the klc mutants in their effects on worm locomotion and responses to drugs affecting neuromuscular junction activity. We also determined lifespan, finding that unc-116 mutant was short-lived whereas the klc single mutant lifespan was wild type. The ida-1::gfp transgenic strain was also short-lived, but surprisingly, klc-1 and klc-2 extended the ida-1::gfp lifespan beyond that of wild type. Our findings suggest that kinesin-1 not only influences anterograde and retrograde DCV transport but is also involved in regulating lifespan and locomotion, with the two kinesin light chains playing distinct roles.

UNC-16 interacts with LRK-1 and WDFY-3 to regulate the termination of axon growth

In humans, MAPK8IP3 (also known as JIP3) is a neurodevelopmental disorder-associated gene. In Caenorhabditis elegans, the UNC-16 ortholog of the MAPK8IP3 protein can regulate the termination of axon growth. However, its role in this process is not well understood. Here, we report that UNC-16 promotes axon termination through a process that includes the LRK-1 (LRRK-1/LRRK-2) kinase and the WDFY-3 (WDFY3/Alfy) selective autophagy protein. Genetic analysis suggests that UNC-16 promotes axon termination through an interaction between its RH1 domain and the dynein complex. Loss of unc-16 function causes accumulation of late endosomes specifically in the distal axon. Moreover, we observe synergistic interactions between loss of unc-16 function and disruptors of endolysosomal function, indicating that the endolysosomal system promotes axon termination. We also find that the axon termination defects caused by loss of UNC-16 function require the function of a genetic pathway that includes lrk-1 and wdfy-3, 2 genes that have been implicated in autophagy. These observations suggest a model where UNC-16 promotes axon termination by interacting with the endolysosomal system to regulate a pathway that includes LRK-1 and WDFY-3.

LRK-1/LRRK2 and AP-3 regulate trafficking of synaptic vesicle precursors through active zone protein SYD-2/Liprin-α

Synaptic vesicle proteins (SVps) are transported by the motor UNC-104/KIF1A. We show that SVps travel in heterogeneous carriers in C. elegans neuronal processes, with some SVp carriers co-transporting lysosomal proteins (SV-lysosomes). LRK-1/LRRK2 and the clathrin adaptor protein complex AP-3 play a critical role in the sorting of SVps and lysosomal proteins away from each other at the SV-lysosomal intermediate trafficking compartment. Both SVp carriers lacking lysosomal proteins and SV-lysosomes are dependent on the motor UNC-104/KIF1A for their transport. In lrk-1 mutants, both SVp carriers and SV-lysosomes can travel in axons in the absence of UNC-104, suggesting that LRK-1 plays an important role to enable UNC-104 dependent transport of synaptic vesicle proteins. Additionally, LRK-1 acts upstream of the AP-3 complex and regulates its membrane localization. In the absence of the AP-3 complex, the SV-lysosomes become more dependent on the UNC-104-SYD-2/Liprin-α complex for their transport. Therefore, SYD-2 acts to link upstream trafficking events with the transport of SVps likely through its interaction with the motor UNC-104. We further show that the mistrafficking of SVps into the dendrite in lrk-1 and apb-3 mutants depends on SYD-2, likely by regulating the recruitment of the AP-1/UNC-101. SYD-2 acts in concert with AP complexes to ensure polarized trafficking & transport of SVps.

UNC-16 interacts with LRK-1 and WDFY-3 to regulate the termination of axon growth

MAPK8IP3 (unc-16/JIP3) is a neurodevelopmental-disorder associated gene that can regulate the termination of axon growth. However, its role in this process is not well understood. Here, we report that UNC-16 promotes axon termination through a process that includes the LRK-1(LRRK-1/LRRK-2) kinase and the WDFY-3 (WDFY3/Alfy) selective autophagy protein. Genetic analysis suggests that UNC-16 promotes axon termination through an interaction between its RH1 domain and the dynein complex. Loss of unc-16 function causes accumulation of late endosomes specifically in the distal axon. Moreover, we observe synergistic interactions between loss of unc-16 function and disruptors of endolysosomal function, indicating that the endolysosomal system promotes axon termination. We also find that the axon termination defects caused by loss of UNC-16 function require the function of a genetic pathway that includes lrk-1 and wdfy-3, two genes that have been implicated in autophagy. These observations suggest a model where UNC-16 promotes axon termination by interacting with the endolysosomal system to regulate a pathway that includes LRK-1 and WDFY-3.

A plant flavonol and genetic suppressors rescue a pathogenic mutation associated with kinesin in neurons

KIF1A, a microtubule-based motor protein responsible for axonal transport, is linked to a group of neurological disorders known as KIF1A-associated neurological disorder (KAND). Current therapeutic options for KAND are limited. Here, we introduced the clinically relevant KIF1A(R11Q) variant into the Caenorhabditis elegans homolog UNC-104, resulting in uncoordinated animal behaviors. Through genetic suppressor screens, we identified intragenic mutations in UNC-104's motor domain that rescued synaptic vesicle localization and coordinated movement. We showed that two suppressor mutations partially recovered motor activity in vitro by counteracting the structural defect caused by R11Q at KIF1A's nucleotide-binding pocket. We found that supplementation with fisetin, a plant flavonol, improved KIF1A(R11Q) worms' movement and morphology. Notably, our biochemical and single-molecule assays revealed that fisetin directly restored the ATPase activity and processive movement of human KIF1A(R11Q) without affecting wild-type KIF1A. These findings suggest fisetin as a potential intervention for enhancing KIF1A(R11Q) activity and alleviating associated defects in KAND.

Compound heterozygous variants in MAPK8IP3 were detected in severe congenital hypotonia mimicking lethal spinal muscular atrophy

Mitogen-activated protein kinase 8-interacting protein 3 gene (MAPK8IP3) encodes the c-Jun-amino-terminal kinase-interacting protein 3 (JIP3) and is involved in retrograde axonal transport. Heterozygous de novo pathogenic variants in MAPK8IP3 result in a neurodevelopmental disorder with or without brain abnormalities and possible axonal peripheral neuropathy. Whole-exome sequencing was performed on an individual presenting with severe congenital muscle hypotonia of neuronal origin mimicking lethal spinal muscular atrophy. Compound heterozygous rare variants (a splice and a missense) were detected in MAPK8IP3, inherited from the healthy parents. Western blot analysis in a muscle biopsy sample showed a more than 60% decrease in JIP3 expression. Here, we suggest a novel autosomal recessive phenotype of a lower motor neuron disease caused by JIP3 deficiency.

Kinesin-1 autoinhibition facilitates the initiation of dynein cargo transport

The functional significance of Kinesin-1 autoinhibition has been unclear. Kinesin-1 transports multiple cargoes including cytoplasmic dynein to microtubule plus ends. From a genetic screen for Aspergillus mutants defective in dynein-mediated early endosome transport, we identified a kinesin-1 mutation kinAK895* at the C-terminal IAK motif involved in autoinhibition. The kinA∆IAK and kinAK895E mutants exhibited a similar defect in dynein-mediated early endosome transport, verifying the importance of kinesin-1 autoinhibition in dynein-mediated transport. Kinesin-1 autoinhibition is not critical for dynein accumulation at microtubule plus ends or for the secretory vesicle cargoes of kinesin-1 to reach the hyphal tip. However, it facilitates dynein to initiate early endosome transport. This is unrelated to a direct competition between dynein and kinesin-1 on early endosomes because kinesin-3 rather than kinesin-1 drives the plus-end-directed early endosome movement. This effect of kinesin-1 autoinhibition on dynein-mediated early endosome transport is related to cargo adapter-mediated dynein activation but at a step beyond the switching of dynein from its autoinhibited conformation.

Active zone protein SYD-2/Liprin-α acts downstream of LRK-1/LRRK2 to regulate polarized trafficking of synaptic vesicle precursors through clathrin adaptor protein complexes

Synaptic vesicle proteins (SVps) are thought to travel in heterogeneous carriers dependent on the motor UNC-104/KIF1A. In C. elegans neurons, we found that some SVps are transported along with lysosomal proteins by the motor UNC-104/KIF1A. LRK-1/LRRK2 and the clathrin adaptor protein complex AP-3 are critical for the separation of lysosomal proteins from SVp transport carriers. In lrk-1 mutants, both SVp carriers and SVp carriers containing lysosomal proteins are independent of UNC-104, suggesting that LRK-1 plays a key role in ensuring UNC-104-dependent transport of SVps. Additionally, LRK-1 likely acts upstream of the AP-3 complex and regulates the membrane localization of AP-3. The action of AP-3 is necessary for the active zone protein SYD-2/Liprin-α to facilitate the transport of SVp carriers. In the absence of the AP-3 complex, SYD-2/Liprin-α acts with UNC-104 to instead facilitate the transport of SVp carriers containing lysosomal proteins. We further show that the mistrafficking of SVps into the dendrite in lrk-1 and apb-3 mutants depends on SYD-2, likely by regulating the recruitment of the AP-1/UNC-101. We propose that SYD-2 acts in concert with both the AP-1 and AP-3 complexes to ensure polarized trafficking of SVps.

Neuronal IRE-1 coordinates an organism-wide cold stress response by regulating fat metabolism

Cold affects many aspects of biology, medicine, agriculture, and industry. Here, we identify a conserved endoplasmic reticulum (ER) stress response, distinct from the canonical unfolded protein response, that maintains lipid homeostasis during extreme cold. We establish that the ER stress sensor IRE-1 is critical for resistance to extreme cold and activated by cold temperature. Specifically, neuronal IRE-1 signals through JNK-1 and neuropeptide signaling to regulate lipid composition within the animal. This cold-response pathway can be bypassed by dietary supplementation with unsaturated fatty acids. Altogether, our findings define an ER-centric conserved organism-wide cold stress response, consisting of molecular neuronal sensors, effectors, and signaling moieties, which control adaptation to cold conditions in the organism. Better understanding of the molecular basis of this stress response is crucial for the optimal use of cold conditions on live organisms and manipulation of lipid saturation homeostasis, which is perturbed in human pathologies.

Discovering autoinhibition as a design principle for the control of biological mechanisms

Autoinhibition is a design principle realized in many molecular mechanisms in biology. After explicating the notion of a design principle and showing that autoinhibition is such a principle, we focus on how researchers discovered instances of autoinhibition, using research establishing the autoinhibition of the molecular motors kinesin and dynein as our case study. Research on kinesin and dynein began in the fashion described in accounts of mechanistic explanation but, once the mechanisms had been discovered, researchers discovered that they exhibited a second phenomenon, autoinhibition. The discovery of autoinhibition not only reverses the pattern in terms of which philosophers have understood mechanism discovery but runs counter to the one phenomenon-one mechanism principle assumed to relate mechanisms and the phenomena they explain. The ubiquity of autoinhibition as a design principle, therefore, necessitates a philosophical understanding of mechanisms that recognizes how they can participate in more than one phenomenon. Since mechanisms with this design are released from autoinhibition only when they are acted on by control mechanisms, we advance a revised account of mechanisms that accommodates attribution of multiple phenomena to the same mechanism and distinguishes them from other processes that control them.

De novo mutations in KIF1A-associated neuronal disorder (KAND) dominant-negatively inhibit motor activity and axonal transport of synaptic vesicle precursors

KIF1A is a kinesin superfamily motor protein that transports synaptic vesicle precursors in axons. Cargo binding stimulates the dimerization of KIF1A molecules to induce processive movement along microtubules. Mutations in human Kif1a lead to a group of neurodegenerative diseases called KIF1A-associated neuronal disorder (KAND). KAND mutations are mostly de novo and autosomal dominant; however, it is unknown if the function of wild-type KIF1A motors is inhibited by heterodimerization with mutated KIF1A. Here, we have established Caenorhabditis elegans models for KAND using CRISPR-Cas9 technology and analyzed the effects of human KIF1A mutation on axonal transport. In our C. elegans models, both heterozygotes and homozygotes exhibited reduced axonal transport. Suppressor screening using the disease model identified a mutation that recovers the motor activity of mutated human KIF1A. In addition, we developed in vitro assays to analyze the motility of heterodimeric motors composed of wild-type and mutant KIF1A. We find that mutant KIF1A significantly impaired the motility of heterodimeric motors. Our data provide insight into the molecular mechanism underlying the dominant nature of de novo KAND mutations.

JIP3 interacts with dynein and kinesin-1 to regulate bidirectional organelle transport

The MAP kinase and motor scaffold JIP3 prevents excess lysosome accumulation in axons of vertebrates and invertebrates. How JIP3's interaction with dynein and kinesin-1 contributes to organelle clearance is unclear. We show that human dynein light intermediate chain (DLIC) binds the N-terminal RH1 domain of JIP3, its paralog JIP4, and the lysosomal adaptor RILP. A point mutation in RH1 abrogates DLIC binding without perturbing the interaction between JIP3's RH1 domain and kinesin heavy chain. Characterization of this separation-of-function mutation in Caenorhabditis elegans shows that JIP3-bound dynein is required for organelle clearance in the anterior process of touch receptor neurons. Unlike JIP3 null mutants, JIP3 that cannot bind DLIC causes prominent accumulation of endo-lysosomal organelles at the neurite tip, which is rescued by a disease-associated point mutation in JIP3's leucine zipper that abrogates kinesin light chain binding. These results highlight that RH1 domains are interaction hubs for cytoskeletal motors and suggest that JIP3-bound dynein and kinesin-1 participate in bidirectional organelle transport.

Analyzing the Impact of Gene Mutations on Axonal Transport in Caenorhabditis Elegans

The development and functions of neurons are supported by axonal transport. Axonal transport is a complex process whose regulation involves multiple molecules, such as microtubules, microtubule-associated proteins, kinases, molecular motors, and motor binding proteins. Gain of function and loss of function mutations of genes that encode these proteins often lead to human axonal neuropathy. Caenorhabditis elegans provides a powerful genetic system to study the consequences of gene mutations for axonal transport. Here, we discuss advantages and limitations of using C. elegans, propose standard criteria, and describe methods to analyze the impact of gene mutations on axonal transport in C. elegans. To obtain solid conclusions, it is necessary to image single neurons in vivo labeled by a specific promoter and to confirm that a mutation changes the localization of a cargo. The motility parameters of the transported cargo should then be analyzed in the mutant. This method enables the axonal transport of proteins and organelles, such as synaptic vesicle precursors and mitochondria, to be analyzed.

Intragenic suppressors of unc-104 ( e1265 ) identify potential roles of the conserved stalk region

UNC-104 and its mammalian ortholog, KIF1A, are microtubule motor proteins required for moving synaptic vesicle precursors from neuronal cell bodies to presynaptic sites. These motor proteins consist of N-terminal motor domain, followed by a neck region, three coiled-coil domains and a FHA domain, and a C-terminal PH domain. Between the coiled-coil 3 and the PH domain is a large uncharacterized region called stalk. In C. elegans unc-104 ( e1265 ), a partial loss of function mutant, synaptic vesicles are retained in the cell body and absent from presynaptic sites. unc-104 ( e1265 ) contains amino acid substitution D1497N in the PH domain and the mutant proteins show reduced PI(4,5)P(2) binding. Through genetic suppressor screening, we identified amino acid substitutions in a conserved region of the stalk that cause intragenic suppression of unc-104 ( e1265 ). Currently, little is known about the functions of the stalk region. Our findings imply potential compensatory or antagonistic interaction between the stalk region and the cargo binding PH domain.

MAPK signaling and a mobile scaffold complex regulate AMPA receptor transport to modulate synaptic strength

Synaptic plasticity depends on rapid experience-dependent changes in the number of neurotransmitter receptors. Previously, we demonstrated that motor-mediated transport of AMPA receptors (AMPARs) to and from synapses is a critical determinant of synaptic strength. Here, we describe two convergent signaling pathways that coordinate the loading of synaptic AMPARs onto scaffolds, and scaffolds onto motors, thus providing a mechanism for experience-dependent changes in synaptic strength. We find that an evolutionarily conserved JIP-protein scaffold complex and two classes of mitogen-activated protein kinase (MAPK) proteins mediate AMPAR transport by kinesin-1 motors. Genetic analysis combined with in vivo, real-time imaging in Caenorhabditis elegans revealed that CaMKII is required for loading AMPARs onto the scaffold, and MAPK signaling is required for loading the scaffold complex onto motors. Our data support a model where CaMKII signaling and a MAPK-signaling pathway cooperate to facilitate the rapid exchange of AMPARs required for early stages of synaptic plasticity.

Injury-induced inflammatory signaling and hematopoiesis in Drosophila

Inflammatory response in Drosophila to sterile (axenic) injury in embryos and adults has received some attention in recent years, and most concentrate on the events at the injury site. Here we focus on the effect sterile injury has on the hematopoietic organ, the lymph gland, and the circulating blood cells in the larva, the developmental stage at which major events of hematopoiesis are evident. In mammals, injury activates Toll-like receptor/NF-κB signaling in macrophages, which then express and secrete secondary, proinflammatory cytokines. In Drosophila larvae, distal puncture injury of the body wall epidermis causes a rapid activation of Toll and Jun kinase (JNK) signaling throughout the hematopoietic system and the differentiation of a unique blood cell type, the lamellocyte. Furthermore, we find that Toll and JNK signaling are coupled in their activation. Secondary to this Toll/JNK response, a cytokine, Upd3, is induced as a Toll pathway transcriptional target, which then promotes JAK/STAT signaling within the blood cells. Toll and JAK/STAT signaling are required for the emergence of the injury-induced lamellocytes. This is akin to the derivation of specialized macrophages in mammalian systems. Upstream, at the injury site, a Duox- and peroxide-dependent signal causes the activation of the proteases Grass and SPE, needed for the activation of the Toll-ligand Spz, but microbial sensors or the proteases most closely associated with them during septic injury are not involved in the axenic inflammatory response.

Neuronal endolysosomal transport and lysosomal functionality in maintaining axonostasis

Lysosomes serve as degradation hubs for the turnover of endocytic and autophagic cargos, which is essential for neuron function and survival. Deficits in lysosome function result in progressive neurodegeneration in most lysosomal storage disorders and contribute to the pathogenesis of aging-related neurodegenerative diseases. Given their size and highly polarized morphology, neurons face exceptional challenges in maintaining cellular homeostasis in regions far removed from the cell body where mature lysosomes are enriched. Neurons therefore require coordinated bidirectional intracellular transport to sustain efficient clearance capacity in distal axonal regions. Emerging lines of evidence have started to uncover mechanisms and signaling pathways regulating endolysosome transport and maturation to maintain axonal homeostasis, or “axonostasis,” that is relevant to a range of neurologic disorders. In this review, we discuss recent advances in how axonal endolysosomal trafficking, distribution, and lysosomal functionality support neuronal health and become disrupted in several neurodegenerative diseases.

JIP3 links lysosome transport to regulation of multiple components of the axonal cytoskeleton

Lysosome axonal transport is important for the clearance of cargoes sequestered by the endocytic and autophagic pathways. Building on observations that mutations in the JIP3 (MAPK8IP3) gene result in lysosome-filled axonal swellings, we analyzed the impact of JIP3 depletion on the cytoskeleton of human neurons. Dynamic focal lysosome accumulations were accompanied by disruption of the axonal periodic scaffold (spectrin, F-actin and myosin II) throughout each affected axon. Additionally, axonal microtubule organization was locally disrupted at each lysosome-filled swelling. This local axonal microtubule disorganization was accompanied by accumulations of both F-actin and myosin II. These results indicate that transport of axonal lysosomes is functionally interconnected with mechanisms that control the organization and maintenance of the axonal cytoskeleton. They have potential relevance to human neurological disease arising from JIP3 mutations as well as for neurodegenerative diseases associated with the focal accumulations of lysosomes within axonal swellings such as Alzheimer’s disease.

Rafiq et al. report that disruption of JIP3-dependent control of axonal lysosome transport in human neurons results in unexpected changes to the organization of multiple cytoskeletal proteins. This study provides new insights that improve our understanding of intellectual disabilities caused by mutations in JIP3, and are relevant for neurodegenerative diseases associated with accumulations of lysosomes such as the Alzheimer’s disease

Doublecortin and JIP3 are neural-specific counteracting regulators of dynein-mediated retrograde trafficking

Mutations in the microtubule (MT)-binding protein doublecortin (DCX) or in the MT-based molecular motor dynein result in lissencephaly. However, a functional link between DCX and dynein has not been defined. Here, we demonstrate that DCX negatively regulates dynein-mediated retrograde transport in neurons from Dcx^-/y or Dcx^-/y;Dclk1^-/- mice by reducing dynein's association with MTs and disrupting the composition of the dynein motor complex. Previous work showed an increased binding of the adaptor protein C-Jun-amino-terminal kinase-interacting protein 3 (JIP3) to dynein in the absence of DCX. Using purified components, we demonstrate that JIP3 forms an active motor complex with dynein and its cofactor dynactin with two dyneins per complex. DCX competes with the binding of the second dynein, resulting in a velocity reduction of the complex. We conclude that DCX negatively regulates dynein-mediated retrograde transport through two critical interactions by regulating dynein binding to MTs and regulating the composition of the dynein motor complex.

Kinesin-3 mediated axonal delivery of presynaptic neurexin stabilizes dendritic spines and postsynaptic components

The functional properties of neural circuits are defined by the patterns of synaptic connections between their partnering neurons, but the mechanisms that stabilize circuit connectivity are poorly understood. We systemically examined this question at synapses onto newly characterized dendritic spines of C. elegans GABAergic motor neurons. We show that the presynaptic adhesion protein neurexin/NRX-1 is required for stabilization of postsynaptic structure. We find that early postsynaptic developmental events proceed without a strict requirement for synaptic activity and are not disrupted by deletion of neurexin/nrx-1. However, in the absence of presynaptic NRX-1, dendritic spines and receptor clusters become destabilized and collapse prior to adulthood. We demonstrate that NRX-1 delivery to presynaptic terminals is dependent on kinesin-3/UNC-104 and show that ongoing UNC-104 function is required for postsynaptic maintenance in mature animals. By defining the dynamics and temporal order of synapse formation and maintenance events in vivo, we describe a mechanism for stabilizing mature circuit connectivity through neurexin-based adhesion.

Probiotics Interactions and the Modulation of Major Signalling Pathways in Host Model Organism Caenorhabditis elegans

Microorganisms live in the human digestive system and the gut microbiome constitutes part of our prime determining component for healthy aging and wellness. Gut microbiota has broad influences on its host, beginning from the digestion of food and nutrients absorption to protective roles against invading pathogens and host immune system regulation. Dysbiosis of the gut microbial composition has been linked to numerous diseases and there is a need to have a better grasp on what makes a 'good' gut microbiome. Caenorhabditis elegans (C. elegans) model organism is considered as a well-suited in-vivo model system and, is at the frontline of probiotic research because of its well-defined characteristics and prolific nature. Most importantly, C. elegans feeds on bacteria, which speeds up manipulations and investigations in probiotics research tremendously. With its unique salient features of short lifespan, and ease of propagation, different unknown probiotics biological roles can be measured at an organism level with precision in the form of worm's stress responses, survivability, and lifespan. In this review, new insights on the different mechanisms underlying the establishment of probiotics regulations of conserved signalling pathways such as p38 MAPK/SKN-1, DAF-2/DAF-16, and JNK-1/DAF-16 is highlighted based on information obtained from C. elegans studies. Along with the current state of knowledge and the uniqueness of C. elegans as a model organism, explorations of its future contribution and scope in synthetic biology and probiotics engineering strains are also addressed. This is expected to strengthen our understanding of probiotics roles and to facilitate novel discovery and applications, for specific therapeutics against age-related disorders and various pathophysiological conditions.

UNC-16 alters DLK-1 localization and negatively regulates actin and microtubule dynamics in Caenorhabditis elegans regenerating neurons

Neuronal regeneration after injury depends on the intrinsic growth potential of neurons. Our study shows that UNC-16, a Caenorhabditis elegans JIP3 homolog, inhibits axonal regeneration by regulating initiation and rate of regrowth. This occurs through the inhibition of the regeneration-promoting activity of the long isoform of DLK-1 and independently of the inhibitory short isoform of DLK-1. We show that UNC-16 promotes DLK-1 punctate localization in a concentration-dependent manner limiting the availability of the long isoform of DLK-1 at the cut site, minutes after injury. UNC-16 negatively regulates actin dynamics through DLK-1 and microtubule dynamics partially via DLK-1. We show that post-injury cytoskeletal dynamics in unc-16 mutants are also partially dependent on CEBP-1. The faster regeneration seen in unc-16 mutants does not lead to functional recovery. Our data suggest that the inhibitory control by UNC-16 and the short isoform of DLK-1 balances the intrinsic growth-promoting function of the long isoform of DLK-1 in vivo. We propose a model where UNC-16's inhibitory role in regeneration occurs through both a tight temporal and spatial control of DLK-1 and cytoskeletal dynamics.

Synaptic Vesicle Precursors and Lysosomes Are Transported by Different Mechanisms in the Axon of Mammalian Neurons

BORC is a multisubunit complex previously shown to promote coupling of mammalian lysosomes and C. elegans synaptic vesicle (SV) precursors (SVPs) to kinesins for anterograde transport of these organelles along microtubule tracks. We attempted to meld these observations into a unified model for axonal transport in mammalian neurons by testing two alternative hypotheses: (1) that SV and lysosomal proteins are co-transported within a single type of “lysosome-related vesicle” and (2) that SVPs and lysosomes are distinct organelles, but both depend on BORC for axonal transport. Analyses of various types of neurons from wild-type rats and mice, as well as from BORC-deficient mice, show that neither hypothesis is correct. We find that SVPs and lysosomes are transported separately, but only lysosomes depend on BORC for axonal transport in these neurons. These findings demonstrate that SVPs and lysosomes are distinct organelles that rely on different machineries for axonal transport in mammalian neurons.

De Pace et al. show that lysosomes and synaptic vesicle precursors (SVPs) are distinct organelles that move separately from the soma to the axon in rat and mouse neurons. Moreover, they demonstrate that the BLOC-1-related complex (BORC) is required for the transport of lysosomes but not SVPs in mouse neurons.

Multiple signaling pathways are essential for synapse formation induced by synaptic adhesion molecules

Little is known about the cellular signals that organize synapse formation. To explore what signaling pathways may be involved, we employed heterologous synapse formation assays in which a synaptic adhesion molecule expressed in a nonneuronal cell induces pre- or postsynaptic specializations in cocultured neurons. We found that interfering pharmacologically with microtubules or actin filaments impaired heterologous synapse formation, whereas blocking protein synthesis had no effect. Unexpectedly, pharmacological inhibition of c-jun N-terminal kinases (JNKs), protein kinase-A (PKA), or AKT kinases also suppressed heterologous synapse formation, while inhibition of other tested signaling pathways-such as MAP kinases or protein kinase C-did not alter heterologous synapse formation. JNK and PKA inhibitors suppressed formation of both pre- and postsynaptic specializations, whereas AKT inhibitors impaired formation of post- but not presynaptic specializations. To independently test whether heterologous synapse formation depends on AKT signaling, we targeted PTEN, an enzyme that hydrolyzes phosphatidylinositol 3-phosphate and thereby prevents AKT kinase activation, to postsynaptic sites by fusing PTEN to Homer1. Targeting PTEN to postsynaptic specializations impaired heterologous postsynaptic synapse formation induced by presynaptic adhesion molecules, such as neurexins and additionally decreased excitatory synapse function in cultured neurons. Taken together, our results suggest that heterologous synapse formation is driven via a multifaceted and multistage kinase network, with diverse signals organizing pre- and postsynaptic specializations.

GRDN-1/Girdin regulates dendrite morphogenesis and cilium position in two specialized sensory neuron types in C. elegans

Primary cilia are located at the dendritic tips of sensory neurons and house the molecular machinery necessary for detection and transduction of sensory stimuli. The mechanisms that coordinate dendrite extension with cilium position during sensory neuron development are not well understood. Here, we show that GRDN-1, the Caenorhabditis elegans ortholog of the highly conserved scaffold and signaling protein Girdin/GIV, regulates both cilium position and dendrite extension in the postembryonic AQR and PQR gas-sensing neurons. Mutations in grdn-1 disrupt dendrite outgrowth and mislocalize cilia to the soma or proximal axonal segments in AQR, and to a lesser extent, in PQR. GRDN-1 is localized to the basal body and regulates localization of HMR-1/Cadherin to the distal AQR dendrite. However, knockdown of HMR-1 and/or loss of SAX-7/LICAM, molecules previously implicated in sensory dendrite development in C. elegans, do not alter AQR dendrite morphology or cilium position. We find that GRDN-1 localization in AQR is regulated by UNC-116/Kinesin-1, and that correspondingly, unc-116 mutants exhibit severe AQR dendrite outgrowth and cilium positioning defects. In contrast, GRDN-1 and cilium localization in PQR is modulated by LIN-44/Wnt signaling. Together, these findings identify upstream regulators of GRDN-1, and describe new cell-specific roles for this multifunctional protein in sensory neuron development.

The axonal endolysosomal and autophagic systems

Neurons, because of their elaborate morphology and the long distances between distal axons and the soma as well as their longevity, pose special challenges to autophagy and to the endolysosomal system, two of the main degradative routes for turnover of defective proteins and organelles. Autophagosomes sequester cytoplasmic or organellar cargos by engulfing them into their lumen before fusion with degradative lysosomes enriched in neuronal somata and participate in retrograde signaling to the soma. Endosomes are mainly involved in the sorting, recycling, or lysosomal turnover of internalized or membrane-bound macromolecules to maintain axonal membrane homeostasis. Lysosomes and the multiple shades of lysosome-related organelles also serve non-degradative roles, for example, in nutrient signaling and in synapse formation. Recent years have begun to shed light on the distinctive organization of the autophagy and endolysosomal systems in neurons, in particular their roles in axons. We review here our current understanding of the localization, distribution, and growing list of functions of these organelles in the axon in health and disease and outline perspectives for future research.

A Presynaptic Perspective on Transport and Assembly Mechanisms for Synapse Formation

Neurons are highly polarized cells with a single axon and multiple dendrites derived from the cell body to form tightly associated pre- and postsynaptic compartments. As the biosynthetic machinery is largely restricted to the somatodendritic domain, the vast majority of presynaptic components are synthesized in the neuronal soma, packaged into synaptic precursor vesicles, and actively transported along the axon to sites of presynaptic biogenesis. In contrast with the significant progress that has been made in understanding synaptic transmission and processing of information at the post-synapse, comparably little is known about the formation and dynamic remodeling of the presynaptic compartment. We review here our current understanding of the mechanisms that govern the biogenesis, transport, and assembly of the key components for presynaptic neurotransmission, discuss how alterations in presynaptic assembly may impact nervous system function or lead to disease, and outline key open questions for future research.

Molecular mechanisms governing axonal transport: a C. elegans perspective

Axonal transport is integral for maintaining neuronal form and function, and defects in axonal transport have been correlated with several neurological diseases, making it a subject of extensive research over the past several years. The anterograde and retrograde transport machineries are crucial for the delivery and distribution of several cytoskeletal elements, growth factors, organelles and other synaptic cargo. Molecular motors and the neuronal cytoskeleton function as effectors for multiple neuronal processes such as axon outgrowth and synapse formation. This review examines the molecular mechanisms governing axonal transport, specifically highlighting the contribution of studies conducted in C. elegans, which has proved to be a tractable model system in which to identify both novel and conserved regulatory mechanisms of axonal transport.

Structural characterization of the RH1-LZI tandem of JIP3/4 highlights RH1 domains as a cytoskeletal motor-binding motif

JIP3 and JIP4 (JNK-interacting proteins 3 and 4) are adaptors for cargo recruitment by dynein/dynactin and kinesin1 motors. Both are dimers that are stabilised by two sections of leucine zipper coiled coils. The N-terminal Leucine Zipper I (LZI) belongs to a section that binds dynein-DLIC and kinesin1-KHC, whilst the medial Leucine Zipper II (LZII) binds dynactin-p150glued and kinesin1-KLC. Structural data is available for the LZII, but the LZI section is still uncharacterized. Here we characterize the N-terminal part of JIP3/4 which consists of an RH1 (RILP homology 1) domain followed by the LZI coiled coil using bioinformatical, biophysical and structural approaches. The RH1-LZI tandem of JIP3 associates as a high affinity homodimer exhibiting elongated alpha-helical fold. 3D homology modelling of the RH1-LZI tandem reveals that the kinesin1-KHC binding site mainly overlaps with the RH1 domain. A sequence comparison search indicates that only one other protein family has RH1 domains similar to those of JIP3/4, the RILP (Rab-interacting lysosomal protein) family which consists of adaptor proteins linking Rab GTPases to cytoskeletal motors. RILPL2 is recruited through its RH1 domain by the myosin 5a motor. Here, we showed that the RH1 domain of JIP3 also interacts with myosin 5 A in vitro, highlighting JIP3/4 as possible myosin 5a adaptors. Finally, we propose that JIP3/4 and RILP family members define a unique RH1/RH2-architecture adaptor superfamily linking cytoskeletal motors and Rab GTPases.

The Conserved IgSF9 Protein Borderless Regulates Axonal Transport of Presynaptic Components and Color Vision in Drosophila

Normal brain function requires proper targeting of synaptic-vesicle (SV) and active-zone components for presynaptic assembly and function. Whether and how synaptogenic signals (e.g., adhesion) at axo-dendritic contact sites promote axonal transport of presynaptic components for synapse formation, however, remain unclear. In this study, we show that Borderless (Bdl), a member of the conserved IgSF9-family trans-synaptic cell adhesion molecules, plays a novel and specific role in regulating axonal transport of SV components. Loss of bdl disrupts axonal transport of SV components in photoreceptor R8 axons, but does not affect the transport of mitochondria. Genetic mosaic analysis, transgene rescue and cell-type-specific knockdown indicate that Bdl is required both presynaptically and postsynaptically for delivering SV components in R8 axons. Consistent with a role for Bdl in R8 axons, loss of bdl causes a failure of R8-dependent phototaxis response to green light. bdl interacts genetically with imac encoding for a member of the UNC-104/Imac/KIF1A-family motor proteins, and is required for proper localization of Imac in R8 presynaptic terminals. Our results support a model in which Bdl mediates specific axo-dendritic interactions in a homophilic manner, which upregulates the Imac motor in promoting axonal transport of SV components for R8 presynaptic assembly and function.SIGNIFICANCE STATEMENT Whether and how synaptogenic adhesion at axo-dendritic contact sites regulates axonal transport of presynaptic components remain unknown. Here we show for the first time that a trans-synaptic adhesion molecule mediates specific interactions at axo-dendritic contact sites, which is required for upregulating the UNC-104/Imac/KIF1A motor in promoting axonal transport of synaptic-vesicle components for presynaptic assembly and function.

The Kinesin-3 motor, KLP-4, mediates axonal organization and cholinergic signaling in Caenorhabditis elegans

Microtubule plus-end directed trafficking is dominated by kinesin motors, yet kinesins differ in terms of cargo identity, movement rate, and distance travelled. Functional diversity of kinesins is especially apparent in polarized neurons, where long distance trafficking is required for efficient signal transduction-behavioral response paradigms. The Kinesin-3 superfamily are expressed in neurons and are hypothesized to have significant roles in neuronal signal transduction due to their high processivity. Although much is known about Kinesin-3 motors mechanistically in vitro, there is little known about their mechanisms in vivo. Here, we analyzed KLP-4, the Caenorhabditis elegans homologue of human KIF13A and KIF13B. Like other Kinesin-3 superfamily motors, klp-4 is highly expressed in the ventral nerve cord command interneurons of the animal, suggesting it might have a role in controlling movement of the animal. We characterized an allele of klp-4 that contains are large indel in the cargo binding domain of the motor, however, the gene still appears to be expressed. Behavioral analysis demonstrated that klp-4 mutants have defects in locomotive signaling, but not the strikingly uncoordinated movements such as those found in unc-104/KIF1A mutants. Animals with this large deletion are hypersensitive to the acetylcholinesterase inhibitor aldicarb but are unaffected by exogenous serotonin. Interestingly, this large klp-4 indel does not affect gross neuronal development but does lead to aggregation and disorganization of RAB-3 at synapses. Taken together, these data suggest a role for KLP-4 in modulation of cholinergic signaling in vivo and shed light on possible in vivo mechanisms of Kinesin-3 motor regulation.

A Model of Hereditary Sensory and Autonomic Neuropathy Type 1 Reveals a Role of Glycosphingolipids in Neuronal Polarity

Hereditary sensory and autonomic neuropathy Type 1 (HSAN1) is a rare autosomal dominantly inherited neuropathy, clinically characterized by a loss of distal peripheral sensory and motoneuronal function. Mutations in subunits of serine palmitoyltransferase (SPT) have been linked to the majority of HSAN1 cases. SPTs catalyze the condensation of l-serine with palmitoyl-CoA, the first committed and rate-limiting step in de novo sphingolipid biosynthesis. Despite extensive investigation, the molecular pathogenesis of HSAN1 remains controversial. Here, we established a Caenorhabditis elegans (C. elegans) model of HSAN1 by generating a sptl-1(c363g) mutation, encoding SPTL-1(C121W) and equivalent to human SPTLC1^C133W, at the C. elegans genomic locus through CRISPR. The sptl-1(c363g) homozygous mutants exhibited the same larval lethality and epithelial polarity defect as observed in sptl-1(RNAi) animals, suggesting a loss-of-function effect of the SPTL-1(C121W) mutation. sptl-1(c363g)/+ heterozygous mutants displayed sensory dysfunction with concomitant neuronal morphology and axon-dendrite polarity defects, demonstrating that the C. elegans model recapitulates characteristics of the human disease. sptl-1(c363g)-derived neuronal defects were copied in animals with defective sphingolipid biosynthetic enzymes downstream of SPTL-1, including ceramide glucosyltransferases, suggesting that SPTLC1^C133W contributes to the HSAN1 pathogenesis by limiting the production of complex sphingolipids, including glucosylceramide. Overexpression of SPTL-1(C121W) led to similar epithelial and neuronal defects and to reduced levels of complex sphingolipids, specifically glucosylceramide, consistent with a dominant-negative effect of SPTL-1(C121W) that is mediated by loss of this downstream product. Genetic interactions between SPTL-1(C121W) and components of directional trafficking in neurons suggest that the neuronal polarity phenotype could be caused by glycosphingolipid-dependent defects in polarized vesicular trafficking.SIGNIFICANCE STATEMENT The symptoms of inherited metabolic diseases are often attributed to the accumulation of toxic intermediates or byproducts, no matter whether the disease-causing enzyme participates in a biosynthetic or a degradation pathway. By showing that the phenotypes observed in a C. elegans model of HSAN1 disease could be caused by loss of a downstream product (glucosylceramide) rather than the accumulation of a toxic byproduct, our work provides new insights into the origins of the symptoms of inherited metabolic diseases while expanding the repertoire of sphingolipid functions, specifically, of glucosylceramides. These findings not only have their most immediate relevance for neuroprotective treatments for HSAN1, they may also have implications for a much broader range of neurologic conditions.

Maturation and Clearance of Autophagosomes in Neurons Depends on a Specific Cysteine Protease Isoform, ATG-4.2

In neurons, defects in autophagosome clearance have been associated with neurodegenerative disease. Yet, the mechanisms that coordinate trafficking and clearance of synaptic autophagosomes are poorly understood. Here, we use genetic screens and in vivo imaging in single neurons of C. elegans to identify mechanisms necessary for clearance of synaptic autophagosomes. We observed that autophagy at the synapse can be modulated in vivo by the state of neuronal activity, that autophagosomes undergo UNC-16/JIP3-mediated retrograde transport, and that autophagosomes containing synaptic material mature in the cell body. Through forward genetic screens, we then determined that autophagosome maturation in the cell body depends on the protease ATG-4.2, but not the related ATG-4.1, and that ATG-4.2 can cleave LGG-1/Atg8/GABARAP from membranes. Our studies revealed that ATG-4.2 is specifically necessary for the maturation and clearance of autophagosomes and that defects in transport and ATG-4.2-mediated maturation genetically interact to enhance abnormal accumulation of autophagosomes in neurons.

Yeast two-hybrid screening identifies MPZ-1 and PTP-1 as candidate scaffolding proteins of metabotropic glutamate receptors in Caenorhabditis elegans

The metabotropic glutamate receptors (mGluRs) are a class of G-protein-coupled receptor that undergo extensive interactions with scaffolding proteins, and this is intrinsic to their function as an important group of neuromodulators at glutamatergic synapses. The Caenorhabditis elegans nervous system expresses three metabotropic glutamate receptors, MGL-1, MGL-2 and MGL-3. Relatively little is known about how the function and signalling of these receptors is organised in C. elegans. To identify proteins that scaffold the MGL-1 receptor, we have conducted a yeast two-hybrid screen. Three of the interacting proteins, MPZ-1, NRFL-1 and PTP-1, displayed motifs characteristic of mammalian mGluR scaffolding proteins. Using cellular co-expression criterion, we show mpz-1 and ptp-1 exhibited overlapping expression patterns with subsets of mgl-1 neurons. This included neurones in the pharyngeal nervous system that control the feeding organ of the worm. The mGluR agonist L-CCG-I inhibits the activity of this network in wild-type worms, in an MGL-1 and dose-dependent manner. We utilised L-CCG-I to identify if MGL-1 function was disrupted in mutants with deletions in the mpz-1 gene. The mpz-1 mutants displayed a largely wild-type response to L-CCG-I, suggesting MGL-1 signalling is not overtly disrupted consistent with a non-obligatory modulatory function in receptor scaffolding. The selectivity of the protein interactions and overlapping expression identified here warrant further investigation of the functional significance of scaffolding of metabotropic glutamate receptor function.

Insights into Kinesin-1 Activation from the Crystal Structure of KLC2 Bound to JIP3

Kinesin-1 transports numerous cellular cargoes along microtubules. The kinesin-1 light chain (KLC) mediates cargo binding and regulates kinesin-1 motility. To investigate the molecular basis for kinesin-1 recruitment and activation by cargoes, we solved the crystal structure of the KLC2 tetratricopeptide repeat (TPR) domain bound to the cargo JIP3. This, combined with biophysical and molecular evolutionary analyses, reveals a kinesin-1 cargo binding site, located on KLC TPR1, which is conserved in homologs from sponges to humans. In the complex, JIP3 crosslinks two KLC2 TPR domains via their TPR1s. We show that TPR1 forms a dimer interface that mimics JIP3 binding in all crystal structures of the unbound KLC TPR domain. We propose that cargo-induced dimerization of the KLC TPR domains via TPR1 is a general mechanism for activating kinesin-1. We relate this to activation by tryptophan-acidic cargoes, explaining how different cargoes activate kinesin-1 through related molecular mechanisms.

Role of Mitogen Activated Protein Kinase Signaling in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by insufficient dopamine production due to the loss of 50% to 70% of dopaminergic neurons. A shortage of dopamine, which is predominantly produced by the dopaminergic neurons within the substantia nigra, causes clinical symptoms such as reduction of muscle mass, impaired body balance, akinesia, bradykinesia, tremors, postural instability, etc. Lastly, this can lead to a total loss of physical movement and death. Since no cure for PD has been developed up to now, researchers using cell cultures and animal models focus their work on searching for potential therapeutic targets in order to develop effective treatments. In recent years, genetic studies have prominently advocated for the role of improper protein phosphorylation caused by a dysfunction in kinases and/or phosphatases as an important player in progression and pathogenesis of PD. Thus, in this review, we focus on the role of selected MAP kinases such as JNKs, ERK1/2, and p38 MAP kinases in PD pathology.

Motoneuron Wnts regulate neuromuscular junction development

The neuromuscular junction (NMJ) is a synapse between motoneurons and skeletal muscles to control motor behavior. Unlike extensively investigated postsynaptic differentiation, less is known about mechanisms of presynaptic assembly. Genetic evidence of Wnt in mammalian NMJ development was missing due to the existence of multiple Wnts and their receptors. We show when Wnt secretion is abolished from motoneurons by mutating the Wnt ligand secretion mediator (Wls) gene, mutant mice showed muscle weakness and neurotransmission impairment. NMJs were unstable with reduced synaptic junctional folds and fragmented AChR clusters. Nerve terminals were swollen; synaptic vesicles were fewer and mislocated. The presynaptic deficits occurred earlier than postsynaptic deficits. Intriguingly, these phenotypes were not observed when deleting Wls in muscles or Schwann cells. We identified Wnt7A and Wnt7B as major Wnts for nerve terminal development in rescue experiments. These observations demonstrate a necessary role of motoneuron Wnts in NMJ development, in particular presynaptic differentiation.

UNC-16/JIP3 and UNC-76/FEZ1 limit the density of mitochondria in C. elegans neurons by maintaining the balance of anterograde and retrograde mitochondrial transport

We investigate the role of axonal transport in regulating neuronal mitochondrial density. We show that the density of mitochondria in the touch receptor neuron (TRN) of adult Caenorhabditis elegans is constant. Mitochondrial density and transport are controlled both by the Kinesin heavy chain and the Dynein-Dynactin complex. However, unlike in other models, the presence of mitochondria in C. elegans TRNs depends on a Kinesin light chain as well. Mutants in the three C. elegans miro genes do not alter mitochondrial density in the TRNs. Mutants in the Kinesin-1 associated proteins, UNC-16/JIP3 and UNC-76/FEZ1, show increased mitochondrial density and also have elevated levels of both the Kinesin Heavy and Light Chains in neurons. Genetic analyses suggest that, the increased mitochondrial density at the distal end of the neuronal process in unc-16 and unc-76 depends partly on Dynein. We observe a net anterograde bias in the ratio of anterograde to retrograde mitochondrial flux in the neuronal processes of unc-16 and unc-76, likely due to both increased Kinesin-1 and decreased Dynein in the neuronal processes. Our study shows that UNC-16 and UNC-76 indirectly limit mitochondrial density in the neuronal process by maintaining a balance in anterograde and retrograde mitochondrial axonal transport.

The C-terminal of CASY-1/Calsyntenin regulates GABAergic synaptic transmission at the Caenorhabditis elegans neuromuscular junction

The C. elegans ortholog of mammalian calsyntenins, CASY-1, is an evolutionarily conserved type-I transmembrane protein that is highly enriched in the nervous system. Mammalian calsyntenins are strongly expressed at inhibitory synapses, but their role in synapse development and function is still elusive. Here, we report a crucial role for CASY-1 in regulating GABAergic synaptic transmission at the C. elegans neuromuscular junction (NMJ). The shorter isoforms of CASY-1; CASY-1B and CASY-1C, express and function in GABA motor neurons where they regulate GABA neurotransmission. Using pharmacological, behavioral, electrophysiological, optogenetic and imaging approaches we establish that GABA release is compromised at the NMJ in casy-1 mutants. Further, we demonstrate that CASY-1 is required to modulate the transport of GABAergic synaptic vesicle (SV) precursors through a possible interaction with the SV motor protein, UNC-104/KIF1A. This study proposes a possible evolutionarily conserved model for the regulation of GABA synaptic functioning by calsyntenins.

GABA acts as a major inhibitory neurotransmitter in both vertebrate and invertebrate nervous systems. Despite the potential deregulation of GABA signaling in several neurological disorders, our understanding of the genetic factors that regulate GABAergic synaptic transmission has just started to evolve. Here, we identify a role for a cell adhesion molecule, CASY-1, in regulating GABA signaling at the C. elegans NMJ. We show that the mutants in casy-1 have reduced number of GABA vesicles at the synapse resulting in less GABA release from the presynaptic GABAergic motor neurons. Further, we show that the shorter isoforms of the casy-1 gene; casy-1b and casy-1c that carry a potential kinesin-motor binding domain are responsible for maintaining GABAergic signaling at the synapse. We show a novel interaction of the CASY-1 isoforms with the C- terminal of the UNC-104/KIF1A motor protein that mediates the trafficking of GABAergic synaptic vesicle precursors to the synapse, thus maintaining normal inhibitory signaling at the NMJ.

A MIG-15/JNK-1 MAP kinase cascade opposes RPM-1 signaling in synapse formation and learning

The Pam/Highwire/RPM-1 (PHR) proteins are conserved intracellular signaling hubs that regulate synapse formation and axon termination. The C. elegans PHR protein, called RPM-1, acts as a ubiquitin ligase to inhibit the DLK-1 and MLK-1 MAP kinase pathways. We have identified several kinases that are likely to form a new MAP kinase pathway that suppresses synapse formation defects, but not axon termination defects, in the mechanosensory neurons of rpm-1 mutants. This pathway includes: MIG-15 (MAP4K), NSY-1 (MAP3K), JKK-1 (MAP2K) and JNK-1 (MAPK). Transgenic overexpression of kinases in the MIG-15/JNK-1 pathway is sufficient to impair synapse formation in wild-type animals. The MIG-15/JNK-1 pathway functions cell autonomously in the mechanosensory neurons, and these kinases localize to presynaptic terminals providing further evidence of a role in synapse development. Loss of MIG-15/JNK-1 signaling also suppresses defects in habituation to repeated mechanical stimuli in rpm-1 mutants, a behavioral deficit that is likely to arise from impaired glutamatergic synapse formation. Interestingly, habituation results are consistent with the MIG-15/JNK-1 pathway functioning as a parallel opposing pathway to RPM-1. These findings indicate the MIG-15/JNK-1 pathway can restrict both glutamatergic synapse formation and short-term learning.

JIP3 localises to exocytic vesicles and focal adhesions in the growth cones of differentiated PC12 cells

The JNK-interacting protein 3 (JIP3) is a molecular scaffold, expressed predominantly in neurons, that serves to coordinate the activation of the c-Jun N-terminal kinase (JNK) by binding to JNK and the upstream kinases involved in its activation. The JNK pathway is involved in the regulation of many cellular processes including the control of cell survival, cell death and differentiation. JIP3 also associates with microtubule motor proteins such as kinesin and dynein and is likely an adapter protein involved in the tethering of vesicular cargoes to the motors involved in axonal transport in neurons. We have used immunofluorescence microscopy and biochemical fractionation to investigate the subcellular distribution of JIP3 in relation to JNK and to vesicular and organelle markers in rat pheochromocytoma cells (PC12) differentiating in response to nerve growth factor. In differentiated PC12 cells, JIP3 was seen to accumulate in growth cones at the tips of developing neurites where it co-localised with both JNK and the JNK substrate paxillin. Cellular fractionation of PC12 cells showed that JIP3 was associated with a subpopulation of vesicles in the microsomal fraction, distinct from synaptic vesicles, likely to be an anterograde-directed exocytic vesicle pool. In differentiated PC12 cells, JIP3 did not appear to associate with retrograde endosomal vesicles thought to be involved in signalling axonal injury. Together, these observations indicate that JIP3 may be involved in transporting vesicular cargoes to the growth cones of PC12 cells, possibly targeting JNK to its substrate paxillin, and thus facilitating neurite outgrowth.

UNC-16/JIP3 regulates early events in synaptic vesicle protein trafficking via LRK-1/LRRK2 and AP complexes

JIP3/UNC-16/dSYD is a MAPK-scaffolding protein with roles in protein trafficking. We show that it is present on the Golgi and is necessary for the polarized distribution of synaptic vesicle proteins (SVPs) and dendritic proteins in neurons. UNC-16 excludes Golgi enzymes from SVP transport carriers and facilitates inclusion of specific SVPs into the same transport carrier. The SVP trafficking roles of UNC-16 are mediated through LRK-1, whose localization to the Golgi is reduced in unc-16 animals. UNC-16, through LRK-1, also enables Golgi-localization of the μ-subunit of the AP-1 complex. AP1 regulates the size but not the composition of SVP transport carriers. Additionally, UNC-16 and LRK-1 through the AP-3 complex regulates the composition but not the size of the SVP transport carrier. These early biogenesis steps are essential for dependence on the synaptic vesicle motor, UNC-104 for axonal transport. Our results show that UNC-16 and its downstream effectors, LRK-1 and the AP complexes function at the Golgi and/or post-Golgi compartments to control early steps of SV biogenesis. The UNC-16 dependent steps of exclusion, inclusion and motor recruitment are critical for polarized distribution of neuronal cargo.

Synaptic vesicles (SVs) have a defined composition and size at the synapse. The multiple synaptic vesicle proteins (SVPs) found on these vesicle membranes are synthesized at and trafficked out of the cell body in distinct transport carriers. However, we do not yet understand how different SVPs are sorted and trafficked to the synapse. We show that UNC-16/JIP3 plays a critical role, in a series of essential steps, to ensure proper membrane composition and size of the ensuing SVP carrier exiting the cell body. These processes are “exclusion” of resident Golgi enzymes followed by the “inclusion” of synaptic vesicle proteins in the same transport carrier. Regulation of composition and size seems to occur independently of each other and depends on two distinct AP complexes acting downstream to LRK-1. Our study further indicates that the composition of the transport carrier formed is important for the recruitment of motors and consequently for the polarized localization of SVPs.

Convergent Transcriptional Programs Regulate cAMP Levels in C. elegans GABAergic Motor Neurons

Both transcriptional regulation and signaling pathways play crucial roles in neuronal differentiation and plasticity. Caenorhabditis elegans possesses 19 GABAergic motor neurons (MNs) called D MNs, which are divided into two subgroups: DD and VD. DD, but not VD, MNs reverse their cellular polarity in a developmental process called respecification. UNC-30 and UNC-55 are two critical transcription factors in D MNs. By using chromatin immunoprecipitation with CRISPR/Cas9 knockin of GFP fusion, we uncovered the global targets of UNC-30 and UNC-55. UNC-30 and UNC-55 are largely converged to regulate over 1,300 noncoding and coding genes, and genes in multiple biological processes, including cAMP metabolism, are co-regulated. Increase in cAMP levels may serve as a timing signal for respecification, whereas UNC-55 regulates genes such as pde-4 to keep the cAMP levels low in VD. Other genes modulating DD respecification such as lin-14, irx-1, and oig-1 are also found to affect cAMP levels.

Microtubule-dependent ribosome localization in C. elegans neurons

Subcellular localization of ribosomes defines the location and capacity for protein synthesis. Methods for in vivo visualizing ribosomes in multicellular organisms are desirable in mechanistic investigations of the cell biology of ribosome dynamics. Here, we developed an approach using split GFP for tissue-specific visualization of ribosomes in Caenorhabditis elegans. Labeled ribosomes are detected as fluorescent puncta in the axons and synaptic terminals of specific neuron types, correlating with ribosome distribution at the ultrastructural level. We found that axonal ribosomes change localization during neuronal development and after axonal injury. By examining mutants affecting axonal trafficking and performing a forward genetic screen, we showed that the microtubule cytoskeleton and the JIP3 protein UNC-16 exert distinct effects on localization of axonal and somatic ribosomes. Our data demonstrate the utility of tissue-specific visualization of ribosomes in vivo, and provide insight into the mechanisms of active regulation of ribosome localization in neurons.

Signal transduction cascades in axon regeneration: insights from C. elegans

Axon regeneration after nerve injury is a conserved biological process in many animals, including humans. The nematode Caenorhabditis elegans (C. elegans) has recently emerged as a genetically tractable model for studying regenerative responses in neurons. Extensive studies over several years using this organism have revealed a number of intrinsic and extrinsic signal transduction cascades that regulate axon regeneration, and these are found to be conserved from worms to humans. Further studies have demonstrated that these cascades consist of several signaling networks that ultimately merge into the c-Jun N-terminal kinase (JNK) cascade. In this review, we describe some recent insights into the signaling cascades controlling axon regeneration in C. elegans and describe their conserved roles in other organisms including mammals.

Inactivation of GABAA receptor is related to heat shock stress response in organism model Caenorhabditis elegans

The mechanisms underlying oxidative stress (OS) resistance are not completely clear. Caenorhabditis elegans (C. elegans) is a good organism model to study OS because it displays stress responses similar to those in mammals. Among these mechanisms, the insulin/IGF-1 signaling (IIS) pathway is thought to affect GABAergic neurotransmission. The aim of this study was to determine the influence of heat shock stress (HS) on GABAergic activity in C. elegans. For this purpose, we tested the effect of exposure to picrotoxin (PTX), gamma-aminobutyric acid (GABA), hydrogen peroxide, and HS on the occurrence of a shrinking response (SR) after nose touch stimulus in N2 (WT) worms. Moreover, the effect of HS on the expression of UNC-49 (GABAA receptor ortholog) in the EG1653 strain and the effect of GABA and PTX exposure on HSP-16.2 expression in the TJ375 strain were analyzed. PTX 1 mM- or H2O2 0.7 mM-exposed worms displayed a SR in about 80 % of trials. GABA exposure did not cause a SR. HS prompted the occurrence of a SR as did PTX 1 mM or H2O2 0.7 mM exposure. In addition, HS increased UNC-49 expression, and PTX augmented HSP-16.2 expression. Thus, the results of the present study suggest that oxidative stress, through either H2O2 exposure or application of heat shock, inactivates the GABAergic system, which subsequently would affect the oxidative stress response, perhaps by enhancing the activity of transcription factors DAF-16 and HSF-1, both regulated by the IIS pathway and related to hsp-16.2 expression.

KIF1A/UNC-104 Transports ATG-9 to Regulate Neurodevelopment and Autophagy at Synapses

Autophagy is a cellular degradation process important for neuronal development and survival. Neurons are highly polarized cells in which autophagosome biogenesis is spatially compartmentalized. The mechanisms and physiological importance of this spatial compartmentalization of autophagy in the neuronal development of living animals are not well understood. Here we determine that, in Caenorhabditis elegans neurons, autophagosomes form near synapses and are required for neurodevelopment. We first determine, through unbiased genetic screens and systematic genetic analyses, that autophagy is required cell autonomously for presynaptic assembly and for axon outgrowth dynamics in specific neurons. We observe autophagosome biogenesis in the axon near synapses, and this localization depends on the synaptic vesicle kinesin, KIF1A/UNC-104. KIF1A/UNC-104 coordinates localized autophagosome formation by regulating the transport of the integral membrane autophagy protein, ATG-9. Our findings indicate that autophagy is spatially regulated in neurons through the transport of ATG-9 by KIF1A/UNC-104 to regulate neurodevelopment.

Regulation of Gap Junction Dynamics by UNC-44/ankyrin and UNC-33/CRMP through VAB-8 in C. elegans Neurons

Gap junctions are present in both vertebrates and invertebrates from nematodes to mammals. Although the importance of gap junctions has been documented in many biological processes, the molecular mechanisms underlying gap junction dynamics remain unclear. Here, using the C. elegans PLM neurons as a model, we show that UNC-44/ankyrin acts upstream of UNC-33/CRMP in regulation of a potential kinesin VAB-8 to control gap junction dynamics, and loss-of-function in the UNC-44/UNC-33/VAB-8 pathway suppresses the turnover of gap junction channels. Therefore, we first show a signal pathway including ankyrin, CRMP, and kinesin in regulating gap junctions.