What are the molecular and cellular steps that govern presynaptic assembly and determine presynaptic function?
SYNAPSE DEVELOPMENT
MODELING PATIENT MUTATIONS IN PRESYNAPTIC CALCIUM CHANNELS
WHAT IS THE ROLE OF CELL ADHESION MOLECULES SUCH AS NEUREXIN IN PRESYNAPTIC ASSEMBLY?
The presynaptic scaffold protein SYD-1 coordinates synapse assembly through interactions with PIP2 and subsequently recruits downstream synapse components, including neurexin. Early in synapse development, SYD-2/Liprin-α and ELKS-1 form phase-separated condensates that recruit and concentrate downstream active zone proteins, enabling them to participate in the multivalent interactions necessary to build and organize functional presynapses (panel 1). Following the expression of SYD-2 and ELKS (but prior to most other active zone proteins), SYD-1 protein becomes detectable at nascent presynapses. SYD-1 is recruited to the plasma membrane through interactions between its C2 and PDZ domains and PIP2 phospholipids (panel 2). SYD-1 may recruit the phase-separated condensate through interactions with SYD-2 and ELKS-1 (panel 2), and coordinates the recruitment of NRX-1/neurexin to presynapses through PDZ-based interactions (panel 3). The γ /short isoform of NRX-1 is the isoform predominantly expressed by the C. elegans nervous system and functions in synapse stabilization (Frankel et al., 2025).
SYD-1(C2) with full (PIP2 containing) membrane. SYD-1’s C2 domain is predicted to bind directly to the membrane
We have found that a recently-identified, conserved short isoform of neurexin, called gamma-neurexin, which does not contain any of the canonical extracellular binding domains, can nonetheless mediate presynaptic assembly and maturation. Our current studies are focused on elucidating the role of this short but seemingly redundant isoform in synapse development.
Moreover, we have found that the intracellular domain of neurexin alone is sufficient for presynaptic assembly. To understand the contribution of neurexin’s intracellular domain to presynaptic organization, we are undertaking proximity labeling approaches to identify neurexin’s intracellular binding partners (Profes et al, 2024).
SYNAPSE ASSEMBLY AT DISTINCT STAGES OF NEURONAL DEVELOPMENT
Detection of synaptic proteins at early stages of synapse formation in the embryonic nerve ring.
NON-CANONICAL ROLES FOR CELL ADHESION MOLECULES
We have identified non-canonical roles in synapse assembly for a pair of CAMs, SYG-1 and SYG-2. We have found that both SYG-1 and SYG-2 are expressed and function in the same presynaptic cell, suggesting they may interact in cis, contrary to their previously described role in the C. elegans HSN neuron. Furthermore, we have found that the intracellular domain of SYG-2 alone is sufficient for inducing presynaptic assembly, in contrast to the accepted idea that the extracellular domains of CAMs are required for their function. Using CRISPR transgenesis, we can identify the critical sub-domains of SYG-1 and SYG-2 required for their function in presynaptic assembly.
MODELING HUMAN PATIENT CALCIUM CHANNEL MUTATIONS IN C. ELEGANS
Secondary structure of UNC-2/CaV2α.
UNC-2 splice isoforms differ in the position of their I-II loop.
Some GOF (gain-of-function) mutations lead to increased reversal frequency. The video on the left shows wild-type animals. The video on the right is a GOF mutant, displaying increased reversal frequency.