Highlights d High-resolution transcriptome map of 40,000 cells from developing human brain d Cell-type-specific transcription factor (TF) expression and TF-gene networks d Defines intermediate cell transition states during early neurogenesis d Implicates specific cell types in neuropsychiatric disorders

Alternative precursor-mRNA splicing is a key mechanism for regulating gene expression in mammals and is controlled by specialized RNA-binding proteins. The misregulation of splicing is implicated in multiple neurological disorders. We describe recent mouse genetic studies of alternative splicing that reveal its critical role in both neuronal development and the function of mature neurons. We discuss the challenges in understanding the extensive genetic programmes controlled by proteins that regulate splicing, both during development and in the adult brain.

SUMMARY Proteins of the Rbfox family act with a complex of proteins called the Large Assembly of Splicing Regulators, LASR. We find that Rbfox interacts with LASR via its C-terminal domain (CTD) and this domain is essential for its splicing activity. In addition to LASR recruitment, a low complexity (LC) sequence within the CTD contains repeated tyrosines that mediate higher-order assembly of Rbfox/LASR and are required for splicing activation by Rbfox. This sequence spontaneously aggregates in solution to form fibrous structures and hydrogels, suggesting an assembly similar to the insoluble cellular inclusions formed by FUS and other proteins in neurologic disease. Unlike the pathological aggregates, we find that assembly of the Rbfox CTD plays an essential role in its normal splicing function. Rather than simple recruitment of individual regulators to a target exon, alternative splicing choices also depend on the higher-order assembly of these regulators within the nucleus.

Dysfunction of the neuronal RNA binding protein RBFOX1 has been linked to epilepsy and autism spectrum disorders. Rbfox1 loss in mice leads to neuronal hyper-excitability and seizures, but the physiological basis for this is unknown. We identify the vSNARE protein Vamp1 as a major Rbfox1 target. Vamp1 is strongly downregulated in Rbfox1 Nes-cKO mice due to loss of 3' UTR binding by RBFOX1. Cytoplasmic Rbfox1 stimulates Vamp1 expression in part by blocking microRNA-9. We find that Vamp1 is specifically expressed in inhibitory neurons, and that both Vamp1 knockdown and Rbfox1 loss lead to decreased inhibitory synaptic transmission and E/I imbalance. Re-expression of Vamp1 selectively within interneurons rescues the electrophysiological changes in the Rbfox1 cKO, indicating that Vamp1 loss is a major contributor to the Rbfox1 Nes-cKO phenotype. The regulation of interneuron-specific Vamp1 by Rbfox1 provides a paradigm for broadly expressed RNA-binding proteins performing specialized functions in defined neuronal subtypes.

Inhibiting the interaction between ß-amyloid (Aß) and a neuronal cell surface receptor, LilrB2, has been suggested as a potential route for treating Alzheimer’s disease (AD). Supporting this approach, AD-like symptoms are reduced in mouse models following genetic depletion of the LilrB2 homolog. In its pathogenic, oligomeric state, Aß binds to LilrB2, triggering a pathway to synaptic loss. Here we identified the LilrB2 binding moieties of Aß ( 16 KLVFFA 21 ) and identified its binding site on LilrB2 from a crystal structure of LilrB2 immunoglobulin domains D1D2 complexed to small molecules that mimic phenylalanine residues. In this structure, we observed two pockets that can accommodate the phenylalanine sidechains of KLVFFA. These pockets were confirmed to be 16 KLVFFA 21 binding sites by mutagenesis. Rosetta docking revealed a plausible geometry for the Aß-LilrB2 complex and assisted with the structure-guided selection of small molecule inhibitors. These molecules inhibit Aß-LilrB2 interactions in vitro and on the cell surface and reduce Aß cytotoxicity, which suggests these inhibitors are potential therapeutic leads against AD.