Astrocytes control excitatory synaptogenesis by secreting thrombospondins (TSPs), which function via their neuronal receptor, the calcium channel subunit α2δ-1. α2δ-1 is a drug target for epilepsy and neuropathic pain; thus the TSP-α2δ-1 interaction is implicated in both synaptic development and disease pathogenesis. However, the mechanism by which this interaction promotes synaptogenesis and the requirement for α2δ-1 for connectivity of the developing mammalian brain are unknown. In this study, we show that global or cell-specific loss of α2δ-1 yields profound deficits in excitatory synapse numbers, ultrastructure, and activity and severely stunts spinogenesis in the mouse cortex. Postsynaptic but not presynaptic α2δ-1 is required and sufficient for TSP-induced synaptogenesis in vitro and spine formation in vivo, but an α2δ-1 mutant linked to autism cannot rescue these synaptogenesis defects. Finally, we reveal that TSP-α2δ-1 interactions control synaptogenesis postsynaptically via Rac1, suggesting potential molecular mechanisms that underlie both synaptic development and pathology.

The small size of dendritic spines belies the elaborate role they play in excitatory synaptic transmission and ultimately complex behaviors. The cytoskeletal architecture of the spine is predominately composed of actin filaments. These filaments, which at first glance might appear simple, are also surprisingly complex. They dynamically assemble into different structures and serve as a platform for orchestrating the elaborate responses of the spine during spinogenesis and experience-dependent plasticity. Multiple mutations associated with human neurodevelopmental and psychiatric disorders involve genes that encode regulators of the synaptic cytoskeleton. A major, unresolved question is how the disruption of specific actin filament structures leads to the onset and progression of complex synaptic and behavioral phenotypes. This review will cover established and emerging mechanisms of actin cytoskeletal remodeling and how this influences specific aspects of spine biology that are implicated in disease.Dendritic spines are morphologically diverse, actin-rich protrusions emerging from dendritic shafts that serve as the receiving sites for the majority of excitatory synaptic transmission in the brain. First described over a century ago by Ramón y Cajal, dendritic spines typically consist of a spine head, which can range in size from 0.5 to 2 m in length, and thin spine neck (ϳ0.2 m thick) (Fig. 1). Their essential function is to compartmentalize biochemical and electrical signals in response to synaptic activation (1). Dendritic spines are supported by an underlying cytoskeleton that is almost exclusively composed of actin filaments, which can be up to ϳ200 nm long (2). Remodeling of this densely packed actin governs most, if not all, dendritic spine physiology. This includes spine formation and maintenance, synaptic adhesion, receptor endocytosis and exocytosis, and synaptic plasticity (Fig. 1). Further, the actin cytoskeleton drives dendritic spine turnover and morphological changes that occur in vivo in response to experience (3). Finally, aberrations in dendritic spine morphology and density are linked to a variety of neurological disorders such as schizophrenia (SZ) 2 and intellectual disability (ID) (4). Recent studies link de novo mutations associated with increased risk of complex psychiatric disorders such as SZ to genes encoding regulators of the post-synaptic actin cytoskeleton (5) (see Fig. 3). Together these findings strongly imply that proper maintenance of the spine actin cytoskeleton is critical for spine functionality and neuronal connectivity. This review will focus on the nuts and bolts of actin dynamics in spines as well as recent developments in the modulation of the synaptic cytoskeleton in two crucial dendritic spine processes whose disturbances are linked with synapse pathologies: synaptic adhesion and synaptic plasticity.

Excitatory synapse formation during development involves the complex orchestration of both structural and functional alterations at the postsynapse. However, the molecular mechanisms that underlie excitatory synaptogenesis are only partially resolved, in part because the internal machinery of developing synapses is largely unknown. To address this, we apply a chemicogenetic approach, in vivo biotin identification (iBioID), to discover aspects of the proteome of nascent synapses. This approach uncovered sixty proteins, including a previously uncharacterized protein, CARMIL3, which interacts in vivo with the synaptic cytoskeletal regulator proteins SrGAP3 (or WRP) and actin capping protein. Using new CRISPR-based approaches, we validate that endogenous CARMIL3 is localized to developing synapses where it facilitates the recruitment of capping protein and is required for spine structural maturation and AMPAR recruitment associated with synapse unsilencing. Together these proteomic and functional studies reveal a previously unknown mechanism important for excitatory synapse development in the developing perinatal brain.