δ-Catenin controls astrocyte morphogenesis via layer-specific astrocyte-neuron cadherin interactions.

Astrocytes control the formation of specific synaptic circuits via cell adhesion and secreted molecules. Astrocyte synaptogenic functions are dependent on the establishment of their complex morphology. However, it is unknown if distinct neuronal cues differentially regulate astrocyte morphogenesis. δ-Catenin was previously thought to be a neuron-specific protein that regulates dendrite morphology. We found δ-catenin is also highly expressed by astrocytes and required both in astrocytes and neurons for astrocyte morphogenesis. δ-Catenin is hypothesized to mediate transcellular interactions through the cadherin family of cell adhesion proteins. We used structural modeling and biochemical analyses to reveal that δ-catenin interacts with the N-cadherin juxtamembrane domain to promote N-cadherin surface expression. An autism-linked δ-catenin point mutation impaired N-cadherin cell surface expression and reduced astrocyte complexity. In the developing mouse cortex, only lower-layer cortical neurons express N-cadherin. Remarkably, when we silenced astrocytic N-cadherin throughout the cortex, only lower-layer astrocyte morphology was disrupted. These findings show that δ-catenin controls astrocyte-neuron cadherin interactions that regulate layer-specific astrocyte morphogenesis.

GEARBOCS: An Adeno Associated Virus Tool for In Vivo Gene Editing in Astrocytes.

In the mammalian central nervous system (CNS), astrocytes are indispensable for brain development, function, and health. However, non-invasive tools to study astrocyte biology and function in vivo have been limited to genetically modified mice. CRISPR/Cas9-based genome engineering enables rapid and precise gene manipulations in the CNS. Here, we developed a non-invasive astrocyte-specific method utilizing a single AAV vector, GEARBOCS (Gene Editing in AstRocytes Based On CRISPR/Cas9 System). We verified GEARBOCS' specificity to mouse cortical astrocytes and demonstrated its utility for three types of gene manipulations: knockout (KO); tagging (TagIN); and reporter gene knock-in (Gene-TRAP) strategies. We deployed GEARBOCS to determine whether cortical astrocytes express Vamp2 protein. The presence of Vamp2-positive vesicles in cultured astrocytes is well-established, however, Vamp2 protein expression in astrocytes in vivo has proven difficult to ascertain due to its overwhelming abundance in neurons. Using GEARBOCS, we delineated the in vivo astrocytic Vamp2 expression and found that it is required for maintaining excitatory and inhibitory synapse numbers in the visual cortex. GEARBOCS strategy provides fast and efficient means to study astrocyte biology in vivo.