Mouse scalp development requires Rac1 and SRF for the maintenance of mechanosensing mesenchyme

Regulation of essential cellular responses like proliferation, migration, and differentiation is crucial for normal development. Rac1, a ubiquitously expressed small GTPase, executes these responses under the regulation of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GTPases). Mutations in specific GEFs (DOCK6) and GTPases (AHGAP31) that regulate Rac1 are associated with Adams-Oliver syndrome (AOS), a developmental syndrome characterized by congenital scalp defects and limb truncations. Genetic ablation of Rac1 in the mouse embryonic limb ectoderm results in limb truncation. However, the etiology of Rac1-associated cranial defects is unknown. To investigate the origin and nature of cranial defects, we used a mesenchymal Cre line (Pdgfra-Cre) to delete Rac1 in cranial mesenchyme. Rac1-KO mice died perinatally and lacked the apical portion of the calvarium and overlying dermis, resembling cranial defects seen in severe cases of AOS. In control embryos, -smooth muscle actin (SMA) expression was spatially restricted to the apical mesenchyme, suggesting a mechanical interaction between the growing brain and the overlying mesenchyme. In Rac1-KO embryos there was reduced proliferation of apical mesenchyme, and reduced expression of SMA and its regulator, serum response factor (SRF). Remarkably, Srf-KO mice generated with Pdgfra-Cre recapitulated the cranial phenotype observed in Rac1-KO mice. Together, these data suggest a model where Rac1 and SRF are critical to maintaining apical fibroblasts in a mechano-sensitive and proliferative state needed to complete cranial development.