Sweet, R. | bioRxiv Science

bioRxiv2022

CACNB4 overexpression decreases dendritic spine density in sex-specific manner

The canonical voltage-gated calcium channel (VGCC) subunit complex is comprised of the 1 subunit, the ion permeable channel, plus three auxiliary subunits: {beta}, 2{delta} and {gamma}. {beta} is the most extensively studied auxiliary subunit and is necessary for proper forward trafficking of the 1 subunit to the plasma membrane. 1 subunits mediate voltagedependent movement of calcium ions into the cytoplasm of neurons, including at dendritic sites, where increased intracellular calcium initiates signaling cascades that shape structural and functional plasticity of dendritic spines. Genetic studies strongly implicate calcium signaling dysfunction in the etiology of neurodevelopmental disorders including schizophrenia. Dendritic spine density (DSD) is significantly decreased in schizophrenia in primary auditory cortex where DSD is driven by loss of small spines, and small spine loss is associated with increased peptide levels of ALFDFLK found in the VGCC {beta} subunit {beta}4. Overexpessing CACNB4 to increase {beta}4 levels selectively reduced small spine density in cortical neuron cultures. The studies described herein set out to validate this in vitro observation in an intact mammalian system within a neurodevelopmental context. We overexpressed CACNB4 in neurodevelopment and assessed DSD and morphology in cerebral cortex of male and female mice at an adult timpoint. We then characterized {beta} protein levels and {beta}4 protein-protein interactions in male and female mouse cortex. Overexpression selectively reduced small dendritic spine density but this effect was present only in female mice and did not appear to result from estrous stage. Instead, the sex-dependent effect on DSD corresponded to sex differences in the {beta}4 interactome of male versus female mice: the VGCC {beta} subunit {beta}1b was significantly enriched in the {beta}4 interactome of brain tissue of male mice, and thus may have served to mitigate VGCC overexpression-mediated spine loss in male mice.

neuroscience↗

bioRxiv2021

A Kalirin Missense Mutation Enhances Dendritic RhoA Signaling and Leads to Regression of Cortical Dendritic Arbors Across Development

Normally, dendritic size is established prior to adolescence then remains relatively constant into adulthood due to a homeostatic balance between growth and retraction pathways. However, schizophrenia is characterized by accelerated reductions of cerebral cortex gray matter volume and onset of clinical symptoms during adolescence, with reductions in layer 3 pyramidal neuron dendritic length, complexity, and spine density identified in multiple cortical regions postmortem. Nogo receptor 1 (NGR1) activation of the GTPase RhoA is a major pathway restricting dendritic growth in the cerebral cortex. We show that the NGR1 pathway is stimulated by OMGp and requires the Rho guanine nucleotide exchange factor, Kalirin-9 (KAL9). Using a genetically encoded RhoA sensor, we demonstrate that a naturally occurring missense mutation in Kalrn, KAL-PT, that was identified in a schizophrenia cohort, confers enhanced RhoA activitation in neuronal dendrites compared to wildtype KAL. In mice containing this missense mutation at the endogenous locus there is an adolescent-onset reduction in dendritic length and complexity of layer 3 pyramidal neurons in the primary auditory cortex. Tissue density of dendritic spines was also reduced. Early adult mice with these structural deficts exhibited impaired detection of short gap durations. These findings provide a neuropsychiatric model of disease capturing how a mild genetic vulnerability may interact with normal developmental processes such that pathology only emerges around adolescence. This interplay between genetic susceptibility and normal adolescent development, both of which possess inherent individual variability, may contribute to heterogeneity seen in phenotypes in human neuropsychiatric disease. SIGNIFICANCE STATEMENTDendrites are long branching processes on neurons that contain small processes called spines that are the site of connections with other neurons, establishing cortical circuitry. Dendrites have long been considered stable structures, with rapid growth prior to adolescence followed by maintenance of size into adulthood. However, schizophrenia is characterized by accelerated reductions of cortical gray matter volume and onset of clinical symptoms during adolescence, with reductions in dendritic length present when examined after death. We show that dendrites retain the capacity for regression, and that a mild genetic vulnerability in a regression pathway leads to onset of structural impairments in previously formed dendrites across adolescence. This suggests that targeting specific regression pathways could potentially lead to new therapeutics for schizophrenia.

neuroscience↗