Kota Mizumoto
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Graduate Student Supervision
Doctoral Student Supervision
Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.
Precise synaptic connection of neurons with their targets is essential for the proper functioning of the nervous system. A plethora of signaling pathways act in concert to mediate the precise spatial arrangement of synaptic connections. One method of ensuring proper wiring in the nervous system is through neuronal tiling. Neuronal tiling is a phenomenon in which neurons project axons and dendrites without overlapping with those of neighbouring neurons within the same class. While neuronal tiling is observed in several types of neurons across species, the molecular mechanisms remain less studied. My thesis aims to uncover the molecular mechanisms underlying neuronal tiling using C. elegans as a model organism, in which neuronal tiling is present in the GABAergic motor neurons. I found that axonal tiling between two neighbouring GABAergic motor neurons is regulated by a Wnt morphogen. However, disrupting axonal tiling did not affect presynaptic tiling between the two neurons. I showed that presynaptic tiling is controlled by a gap junction protein, UNC-9, which is localized at the presynaptic tiling border between neighbouring DD neurons. Strikingly, the gap junction channel activity of UNC- 9 is dispensable for its function in controlling tiled presynaptic patterning. While gap junctions are crucial for the proper functioning of the nervous system as channels, my findings uncovered the novel channel-independent role of UNC-9 in presynaptic patterning.
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Master's Student Supervision
Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.
Developmental neurite pruning is a phenomenon widely observed in different organisms including humans. Through this process, neurons selectively remove exuberant neurites by pruning to form a proper neurocircuit. Some neurites are pruned base on the competition of neuronal input, while others undergo stereotyped pruning which is controlled by morphogenic cues.We found that in Caenorhabditis elegans, a cholinergic motor neuron, PDB, undergoes stereotyped neurite pruning. During PDB development, we observed two posterior branches that are stereotypically pruned. Time-lapse imaging showed that these posterior branches are retracted while the anterior branch is extending. We also found a posteriorly expressed Wnt, LIN-44, and its receptor LIN-17/Frizzled (Fz) are responsible for the pruning of the posterior neurites. In lin-44 and lin-17 mutant animals, the posterior neurites often failed to be pruned. Furthermore, we discovered that the activation of LIN-44/Wnt is gradient independent, and membrane-tethered lin-44 is sufficient to induce asymmetrical posterior neurite pruning. LIN-17 and its downstream DSH-1/Dishevelled (Dsh/Dvl) proteins are recruited to the posterior neurites while either wildtype or membrane-tethered lin-44 is expressed. Our results showed a novel contact-dependent role of Wnt in asymmetric neurite pruning.
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Animal locomotion and behaviour are ultimately controlled by the precise neuronal circuit formation at the level of synaptic connection. Mutations in the genes that specify individual neuronal cell fate (or cell fate determinants) alter synaptic connections and circuit wiring which results in the malfunction of the nervous system. It is however not fully understood if the defects in these mutants are merely due to a consequence of cell fate transformation, or the cell fate determinants have specific functions in synapse pattern formation. Here we identify a novel role for a homeobox transcription factor UNC-4 and its co-repressor UNC-37/Groucho, in tiled synapse pattern formation of the cholinergic motor neurons (DA8 and DA9) in Caenorhabditis elegans. In unc-4 and unc-37 mutant animals, we observed large overlap between the synaptic domains of DA8 and DA9. Strikingly, we show using temperature-sensitive mutants and auxin-inducible degron system that unc-4 is not required during embryonic development when DA neurons cell fate is set but is required post-embryonically. In contrast, unc-37 is required embryonically and post-embryonically in DA neurons for a tiled synaptic innervation. Our result reveals a novel post-cell fate determination role of homeobox gene in neuronal pattern formation.
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Fine motor coordination depends on the precise synaptic connection between individual motor neurons and muscles. Recent studies have revealed the roles of extracellular signals such as Wnt, Netrin, and Semaphorin in synapse specificity. Little is known about their intracellular mechanisms in synapse patterning.In C. elegans, DA class motor neurons form en passant synapses along their axon on the dorsal nerve cord. Each DA neuron innervates a unique and tiled segment of muscle field by restricting its synapse to a distinct subaxonal domain - a phenomenon we term synaptic tiling. SEMAs/Semaphorins and their receptor PLX-1/Plexin were previously shown to be critical for the tiled synaptic innervation pattern between two neighboring neurons DA8 and DA9. Recently, structural and biochemical studies have predicted that mammalian Plexin acts as a GTPase activating protein (GAP) for Rap small GTPases.In this study, among three rap genes in the C. elegans genome, rap-2 is found to be required for synaptic tiling and functions through cycling between GTP- and GDP-bound forms. The genetic study has illustrated that rap-2 acts downstream of plx-1 to regulate synaptic tiling, supporting that PLX-1 acts as a RapGAP to regulate the spatial activity of RAP-2. MIG-15 is identified as an effector of RAP-2 in synaptic tiling. mig-15 mutants display severe synaptic tiling defects due to the increased synapse number of DA8 and DA9.iiiMIG-15 overexpression experiments demonstrated that MIG-15 controls both the length of synaptic domain and the number of synapses, while Plexin and RAP-2 define the length of the synaptic domain. PLX-1 overexpression experiments indicated that PLX-1 specifies synapse distribution via RAP-2 small GTPase and MIG-15 kinase. Overall, this study identified two novel components of Plexin signaling in the spatial regulation of synaptic pattern formation.
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