The CAP1 represses MRTF-SRF–dependent gene expression in the mouse cerebral cortex by an actin-dependent mechanism

Serum response factor (SRF) is a ubiquitously expressed transcription factor that is essential for brain development and function. SRF activity is controlled by two competing classes of coactivators, myocardin-related transcription factors (MRTF) and ternary complex factors, which introduce specificity into gene expression programs. Here, we explored the MRTF-mediated regulatory mechanism in mouse cortical neurons. Using gene-reporter assays and pharmacological and genetic approaches in isolated mouse cortical neurons, we found that cyclase-associated protein 1 (CAP1) repressed neuronal MRTF-SRF activity. CAP1 promoted cytosolic retention of MRTF by controlling cytosolic G-actin levels that required its helical folded domain and its CARP domain. This function of CAP1 was not redundant with that of its homolog CAP2 and was independent of cofilin1 and actin-depolymerizing factor. Deep RNA sequencing and mass spectrometry in cerebral cortex lysates from CAP1 knockout (CAP1-KO) mice supported the in vivo relevance for the CAP1-actin-MRTF-SRF signaling axis. Our study identified CAP1 as a repressor of neuronal gene expression and led to the identification of likely MRTF-SRF target genes in the developing cerebral cortex, whose dysregulation may contribute to impaired formation of neuronal networks in CAP1-KO mice. Together with our previous studies that implicated CAP1 in actin dynamics in axonal growth cones or excitatory synapses, we established CAP1 as a crucial actin regulator in neurons.