From synapse to circuit [electronic resource] : how to build polarized structures during neural development in C. elegans
Keywords: Biology.

The establishment of polarity is a fundamental process of neural development at multiple levels from synaptogenesis to building up neural circuits. At the circuit level, extrinsic cues, serving as attractive or repulsive signals, guide the pathfinding of axons, regulate the morphogenesis of dendritic arbors, and mediate synapse formation between specific pre- and postsynaptic partners at particular loci. Within a neuron, on the other hand, intrinsic mechanisms instruct the proper polarized subcellular distribution of microtubules, synaptic vesicles, neurotransmitter receptors and channels, etc. The establishments of polarized structures at both levels together ensure the unidirectional signal transmission in the complex neural network and orderly functional nervous system. The nematode Caenorhabditis elegans, with only 302 neurons whose cell fates, developmental processes and wiring partners well-identified, provides us with a good model organism to understand how polarized structures are built up at both the circuit and cellular levels. At the circuit level, we investigated the synaptic specificity in the C. elegans egg-laying circuit, where presynaptic neurons select one type of muscles, the vm2, as targets and form synapses on the dendritic spine-like muscle arms. Using forward genetic approaches, we found that the Notch-Delta signaling pathway was required to distinguish the target and non-target muscles. APX-1/Delta acts in the surrounding tissues, including the non-target muscle vm1, to activate LIN-12/Notch in the target muscle vm2. LIN-12 cell-autonomously promotes the expression of UNC-40/DCC and MADD-2 in vm2 for muscle arm formation and guidance. Ectopic expression of UNC-40/DCC in the non-target vm1 is sufficient to induce the polarized extension of muscle arms from the non-target vm1. Therefore, intercellular signaling via LIN-12/Notch instructs the formation of dendritic spine-like muscle arms and the specific postsynaptic target selection. We also investigated the polarity establishment at the subcellular level. In particular, we asked how intrinsic sorting machineries separate axonal and dendritic proteins, target them to their specific domains, and achieve polarized protein distributions in the axon and the dendrite. We identified compartment specific di-leucine motifs that are necessary and sufficient to target proteins to either the axon or the dendrite. We showed that the axonal di-leucine motifs are recognized by AP-3, a clathrin-associated adaptor protein (AP) complex. In contrast, dendritic di-leucine motifs are recognized by a different AP, named AP-1. Using both genetics and biochemical approaches, we found that the axonal di-leucine motifs bind to AP-3 with higher affinity than to AP-1, which underlies the sorting specificity. We also showed that axonal and dendritic proteins are packaged and transported on different cargo vesicles derived from the trans-Golgi network (TGN). AP-3 and AP-1 complexes are selectively required for forming the axonal and dendritic vesicles from the TGN, respectively. Thus, the AP-3 and AP-1 dependent sorting machineries instruct the properly polarized distributions of axonal and dendritic cargoes, support the efficient neurotransmission, and ensure normal neuronal activity. In summary, we explored mechanisms for building up the polarized structures at both the circuit level and subcellular levels of the nervous system. Extrinsic and intrinsic cues both contribute to the establishment of neural polarity, which in turn forms the fundamental basis of neural function.