Intrinsic mechanisms controlling the subcellular localization, size, and number of synapses [electronic resource]

Nervous system function requires the structural and functional integrity of chemical synapses, the fundamental units of neural communication. Synapse formation requires the packaging of synaptic vesicle (SV) and active zone (AZ) proteins into vesicular cargoes in the cell soma, their long-distance microtubule-dependent transport down the axon and finally, their clustering and assembly into functional complexes at specific sites. The subcellular localization, size, and number of synapses diverse greatly between different types of neurons and are critical determinants of the identity and strength of neural connections. Compared to our knowledge of extrinsic signals and cell-surface molecules that regulate synapse formation and synaptic specificity, less is known about the intrinsic regulatory mechanisms that regulate presynaptic protein transport and assembly to achieve proper spatial distribution of synapses. We have taken advantage of the powerful genetics in Caenorhabditis elegans to investigate this question. Using in vivo time-lapse fluorescence microscopy in a motor neuron DA9 in C. elegans, we show that SV and AZ proteins exhibit extensive co-trafficking and undergo frequent pauses during axonal transport. At the pause sites, the balance between the capture and dissociation of mobile transport packets determines the extent of presynaptic protein clustering. Through forward genetic screens, we identified a molecular regulatory network that controls the capture and dissociation events to dictate the spatial distribution of presynaptic specializations. First, we identified an Arf-like small G protein, ARL-8, that regulates synapse distribution by inhibiting premature clustering of presynaptic cargoes during axonal transport. arl-8 mutants accumulate presynaptic specializations within the proximal axon of several neuronal classes, with a corresponding failure to assemble presynapses distally. Time-lapse imaging analysis revealed that presynaptic cargoes in arl-8 mutants exhibit a decreased tendency to dissociate from immotile clusters and an increased probability of being captured by immotile clusters. We further performed genetic suppressor screens to isolate molecules that functionally interact with arl-8 in presynaptic patterning. We report that loss-of-function mutations in a JKK-1/JNK-1 MAP kinase pathway and several active zone assembly proteins strongly and partially suppress the abnormal distribution of presynaptic proteins in arl-8 mutants. Time-lapse imaging analysis demonstrated that the JNK kinase pathway and active zone assembly molecules inhibit presynaptic protein dissociation. In addition, we found that the kinesin motor UNC-104/KIF1A controls the capture efficiency. Furthermore, UNC-104/KIF1A functions as an effector of ARL-8 by binding specifically to the GTP-bound form of ARL-8. Together, these findings revealed novel intrinsic mechanisms that control the subcellular localization, size, and number of synapses by coordinating axonal transport with presynaptic assembly.

Keywords: Biology.
Advisor: Shen Kang
McConnell Susan K. advisor. Thesis advisor
Nachury Maxence
Nelson W. J. (W. James) advisor. Thesis advisor