Expanded genetic screening in Caenorhabditis elegans identifies new regulators and an inhibitory role for NAD+ in axon regeneration.

The mechanisms underlying axon regeneration in mature neurons are relevant to the understanding of normal nervous system maintenance and for developing therapeutic strategies for injury. Here, we report novel pathways in axon regeneration, identified by extending our previous function-based screen using the C. elegans mechanosensory neuron axotomy model. We identify an unexpected role of the nicotinamide adenine dinucleotide (NAD+) synthesizing enzyme, NMAT-2/NMNAT, in axon regeneration. NMAT-2 inhibits axon regrowth via cell-autonomous and non-autonomous mechanisms. NMAT-2 enzymatic activity is required to repress regrowth. Further, we find differential requirements for proteins in membrane contact site, components and regulators of the extracellular matrix, membrane trafficking, microtubule and actin cytoskeleton, the conserved Kelch-domain protein IVNS-1, and the orphan transporter MFSD-6 in axon regrowth. Identification of these new pathways expands our understanding of the molecular basis of axonal injury response and regeneration

Expanded Genetic Screening in \u3cem\u3eCaenorhabditis elegans\u3c/em\u3e Identifies New Regulators and an Inhibitory Role for NAD\u3csup\u3e+\u3c/sup\u3e in Axon Regeneration

The mechanisms underlying axon regeneration in mature neurons are relevant to the understanding of normal nervous system maintenance and for developing therapeutic strategies for injury. Here, we report novel pathways in axon regeneration, identified by extending our previous function-based screen using the C. elegans mechanosensory neuron axotomy model. We identify an unexpected role of the nicotinamide adenine dinucleotide (NAD+) synthesizing enzyme, NMAT-2/NMNAT, in axon regeneration. NMAT-2 inhibits axon regrowth via cell-autonomous and non-autonomous mechanisms. NMAT-2 enzymatic activity is required to repress regrowth. Further, we find differential requirements for proteins in membrane contact site, components and regulators of the extracellular matrix, membrane trafficking, microtubule and actin cytoskeleton, the conserved Kelch-domain protein IVNS-1, and the orphan transporter MFSD-6 in axon regrowth. Identification of these new pathways expands our understanding of the molecular basis of axonal injury response and regeneration

Context specific regulation of synaptogenesis in C. elegans

Synapses are the functional connections between neurons and their targets.Synapse degeneration or dysfunction underlie several neurodevelopmental and neurodegenerative disorders ranging from autism to Alzheimer’s disease, making the study of synapse formation and maintenance highly relevant. Various types of synapses exist within the human nervous system, including en passant synapses, which form along the axon, and terminal synapses, which form at the ends of axons. The nervous system of the nematode C. elegans also forms en passant and terminal synapses, which when coupled with a transparent body, a known connectome and ease of genetic manipulation, makes it an ideal model for the study of synaptogenesis. In the first part of my dissertation, I elucidate the cellular mechanisms that regulate a unique synapse rewiring paradigm in C. elegans motor neurons, where pre-exisiting en passant synapses are completely eliminated and new synapses are formed without any alteration in overall axon morphology. My studies have identified a novel role for dynamic microtubules in facilitating this process, through the modulation of synaptic vesicle transport during remodeling. I also provide in vivo evidence for how molecular motors kinesin and dynein, which move towards opposite ends of the microtubule, engage in a tug-of-war to determine the overall direction of motion of synaptic vesicles. In addition, I uncovered an unexpected role for intermediate filament proteins in regulating microtubule dynamics during synapse formation, and a novel kinase that regulates synapse maintenance after remodeling. Finally, I explore how post-translational modifications of tubulin alter long distance synaptic vesicle transport to terminal synapses, using the mechanosensory neurons in C. elegans as a model. Taken together, my dissertation enhances our current understanding of the different pathways regulating synapse dynamics

Context specific regulation of synaptogenesis in C. elegans

Synapses are the functional connections between neurons and their targets.Synapse degeneration or dysfunction underlie several neurodevelopmental and neurodegenerative disorders ranging from autism to Alzheimer’s disease, making the study of synapse formation and maintenance highly relevant. Various types of synapses exist within the human nervous system, including en passant synapses, which form along the axon, and terminal synapses, which form at the ends of axons. The nervous system of the nematode C. elegans also forms en passant and terminal synapses, which when coupled with a transparent body, a known connectome and ease of genetic manipulation, makes it an ideal model for the study of synaptogenesis. In the first part of my dissertation, I elucidate the cellular mechanisms that regulate a unique synapse rewiring paradigm in C. elegans motor neurons, where pre-exisiting en passant synapses are completely eliminated and new synapses are formed without any alteration in overall axon morphology. My studies have identified a novel role for dynamic microtubules in facilitating this process, through the modulation of synaptic vesicle transport during remodeling. I also provide in vivo evidence for how molecular motors kinesin and dynein, which move towards opposite ends of the microtubule, engage in a tug-of-war to determine the overall direction of motion of synaptic vesicles. In addition, I uncovered an unexpected role for intermediate filament proteins in regulating microtubule dynamics during synapse formation, and a novel kinase that regulates synapse maintenance after remodeling. Finally, I explore how post-translational modifications of tubulin alter long distance synaptic vesicle transport to terminal synapses, using the mechanosensory neurons in C. elegans as a model. Taken together, my dissertation enhances our current understanding of the different pathways regulating synapse dynamics

Neural circuit rewiring: insights from DD synapse remodeling.

Nervous systems exhibit many forms of neuronal plasticity during growth, learning and memory consolidation, as well as in response to injury. Such plasticity can occur across entire nervous systems as with the case of insect metamorphosis, in individual classes of neurons, or even at the level of a single neuron. A striking example of neuronal plasticity in C. elegans is the synaptic rewiring of the GABAergic Dorsal D-type motor neurons during larval development, termed DD remodeling. DD remodeling entails multi-step coordination to concurrently eliminate pre-existing synapses and form new synapses on different neurites, without changing the overall morphology of the neuron. This mini-review focuses on recent advances in understanding the cellular and molecular mechanisms driving DD remodeling

Neural circuit rewiring: insights from DD synapse remodeling

Nervous systems exhibit many forms of neuronal plasticity during growth, learning and memory consolidation, as well as in response to injury. Such plasticity can occur across entire nervous systems as with the case of insect metamorphosis, in individual classes of neurons, or even at the level of a single neuron. A striking example of neuronal plasticity in C. elegans is the synaptic rewiring of the GABAergic Dorsal D-type motor neurons during larval development, termed DD remodeling. DD remodeling entails multi-step coordination to concurrently eliminate pre-existing synapses and form new synapses on different neurites, without changing the overall morphology of the neuron. This mini-review focuses on recent advances in understanding the cellular and molecular mechanisms driving DD remodeling

Differential regulation of polarized synaptic vesicle trafficking and synapse stability in neural circuit rewiring in Caenorhabditis elegans.

Neural circuits are dynamic, with activity-dependent changes in synapse density and connectivity peaking during different phases of animal development. In C. elegans, young larvae form mature motor circuits through a dramatic switch in GABAergic neuron connectivity, by concomitant elimination of existing synapses and formation of new synapses that are maintained throughout adulthood. We have previously shown that an increase in microtubule dynamics during motor circuit rewiring facilitates new synapse formation. Here, we further investigate cellular control of circuit rewiring through the analysis of mutants obtained in a forward genetic screen. Using live imaging, we characterize novel mutations that alter cargo binding in the dynein motor complex and enhance anterograde synaptic vesicle movement during remodeling, providing in vivo evidence for the tug-of-war between kinesin and dynein in fast axonal transport. We also find that a casein kinase homolog, TTBK-3, inhibits stabilization of nascent synapses in their new locations, a previously unexplored facet of structural plasticity of synapses. Our study delineates temporally distinct signaling pathways that are required for effective neural circuit refinement