title: Similar but unique
title: shared and individual roles for gamma-Pcdhs in cortical development
titleType: Subtitle
Creators: name: Steffen, David M.
Publisher: name: University of Iowa
Year: 2024
Subjects: Cell Adhesion
Dendritic Arborization
Synapse Maturation
Synaptogenesis
public abstract

description: Cell adhesion molecules (CAMs) are key players in the formation of neural circuits during development. The Gamma-Protocadherins (Gamma-Pcdhs), a family of 22 CAMs encoded by the Pcdhg gene cluster, are known to play important roles in dendrite arborization, axon targeting, and synapse development. This dissertation focuses on the discovery of new mechanisms by which these proteins act. We showed previously that multiple Gamma-Pcdhs interact physically with the autism-associated CAM Neuroligin-1 (Nlg1) and inhibit the latter's ability to promote excitatory synapse maturation. Here, we show that Gamma-Pcdhs can also interact physically with the related Neuroligin-2 (Nlg2), and inhibit this CAM's ability to promote inhibitory synapse development. In an artificial synapse assay, Gamma-Pcdhs co-expressed with Nlg2 in non-neuronal cells reduce inhibitory presynaptic maturation in contacting hippocampal axons. Mice lacking the Gamma-Pcdhs from the forebrain (including the cortex, the hippocampus, and portions of the amygdala) exhibit increased inhibitory synapse density and increased co-localization of Nlg2 with inhibitory postsynaptic markers in vivo. These Pcdhg mutants also exhibit defective social affiliation and an anxiety-like phenotype in behavioral assays. Together, these results suggest that Gamma-Pcdhs negatively regulate Nlgs to limit synapse density in a manner that is important for normal behavior.Next, we ventured to further investigate the extent to which individual Gamma-Pcdh family members play unique vs. common roles in neurodevelopment. We demonstrated previously that the Gamma-Pcdh-C3 isoform (Gamma-C3), via its unique “variable” cytoplasmic domain (VCD), interacts in cultured cells with Axin1, a Wnt-pathway scaffold protein that regulates the differentiation and morphology of neurons. Here, we confirm that Gamma-C3 and Axin1 interact in the cortex in vivo and show that a new mouse line specifically lacking Gamma-C3 exhibits disrupted Axin1 localization to synaptic fractions, without obvious changes in dendritic spine density or morphology. However, Gamma-C3 knockout mice exhibit severely decreased dendritic complexity of cortical pyramidal neurons that is not observed in mouse lines lacking several other Gamma-Pcdh isoforms. Combining knockout with rescue constructs in cultured cortical neurons, we show that Gamma-C3 promotes dendritic arborization through an Axin1-dependent mechanism mediated through its VCD. These data identify a novel mechanism though which Gamma-C3 uniquely regulates the formation of cortical circuitry.
description: The dissertation work collected here, in context with prior Weiner lab results, provides several new insights into Gamma-Pcdh functions in neurodevelopment: 1) through extracellular interactions, multiple Gamma-Pcdh isoforms can inhibit the function of other synaptic CAMs in a way that controls the density of both inhibitory and excitatory synapses; 2) on the other hand, isoform-specific roles govern neurodevelopment, with Gamma-C3 playing a dominant role in dendrite arborization while Gamma-C4 is required for neuronal survival; and 3) isoform-specific VCDs can influence neuronal development via unique cytoplasmic partners. Future work should focus on furthering our understanding of shared versus unique roles for members of the Gamma-Pcdh family.