Colloquium on the Brain and Cognition with Kang Shen, PhD, HHMI, Stanford University

Physiological importance of phase separation: a case study in synapse formation

This seminar will be held virtually. Please click here to join.

Synapse formation during neuronal development is critical to establish neural circuits and a nervous system1. Every presynapse builds a core active zone structure where ion channels are clustered and synaptic vesicles are released2. While the composition of active zones is well characterized2,3, how active zone proteins assemble together and recruit synaptic release machinery during development is not clear. Here, we find core active zone scaffold proteins SYD-2/Liprin-α and ELKS-1 phase separate during an early stage of synapse development, and later mature into a solid structure. We directly test the in vivo function of phase separation with mutants specifically lacking this activity. These mutant SYD-2 and ELKS-1 proteins remain enriched at synapses, but are defective in active zone assembly and synapse function. The defects are rescued with the introduction of a phase separation motif from an unrelated protein. In vitro, we reconstitute the SYD-2 and ELKS-1 liquid phase scaffold and find it is competent to bind and incorporate downstream active zone components. The fluidity of SYD-2 and ELKS-1 condensates is critical for efficient mixing and incorporation of active zone components. These data reveal that a developmental liquid phase of scaffold molecules is essential for synaptic active zone assembly before maturation into a stable final structure.

The Shen laboratory studies the neuronal cell biological questions using the nematode C. elegans as a model system. We have studied in vivo synapse formation and synaptic specificity by labeling defined synapses in single cell resolution and performed forward genetic screens to identify genes required for synapse formation and synaptic target selection. In the recent years, we have also extensively studied the molecular mechanisms of dendrite branching. Through forward genetics analysis, we have identified a ligand receptor complex that is specifically required for dendrite branching but not for axon development. This complex contains three ligands that are made from two tissues, skin and muscle. The dendrite receptor is only activated when all three ligands are presents. The coincidence detection afforded by this complex precisely targets the sensory dendrite to innervate the space between muscle and skin. We have also identified several mechanisms that regulate dendrite morphology during development and by environment through controlling the level of the receptor. We have also established molecular tools to systematically study the organization of microtubules in neurons. This series of genetic and cell biological studies has provided insights of sensory dendrite morphogenesis.

Host

Steven Flavell

Organizer

Brittany Greenough