Slit1 -a MET target gene in the embryonic limbs, prevents premature differentiation during mammalian myogenesis

Skeletal myogenesis requires precise spatio-temporal regulation of molecular signals during prenatal and postnatal development. Prenatal myogenesis in mouse commences at embryonic day (E) 9.5, characterized by the expression of PAX3 -a key myogenic regulator, and its target MET, in the embryonic muscle progenitor cells (EMPCs). These EMPCs delaminate from dermomyotome in the somites and migrate to designated areas such as the developing embryonic limbs and diaphragm. The trajectory of their migration is directed and limited by spatio-temporal availability of the MET ligand (HGF). Given its periodic expression during embryonic myogenesis and its recent association with familial arthrogryposis, it is important to identify additional non-migratory functions of MET signaling. We find conditional loss of Met in the Pax3 /somitic lineage affects survival in early neonatal stages because amuscular diaphragms cause respiratory distress. Impaired progenitor migration in conditional Met knockouts (cMet ^KO ) results in highly dysplastic muscles in neonates compared to muscleless limbs observed in previous mutants. Additionally, using cMet ^KO embryos (E11.5) we identify Slit1 as a novel target of MET that is downregulated in the mutant limb buds. Pharmacological modulation with SU11274, in vitro, confirms Slit1 as a MET responsive gene, which if knocked down in myoblasts leads to precocious myogenic differentiation. Similarly, cMet ^KO embryos, having reduced Slit1 expression, show greater myotomal differentiation and compact organization, compared to wildtype embryos. While Slit1 emerges as a MET target that represses precocious switch to myogenic differentiation, molecular intricacies of MET-mediated regulation of Slit1 and their spatio-temporal dynamic in fine-tuning myogenesis in the nascent limbs needs further examination.