Integrin signaling downregulates filopodia in muscle-tendon attachment

Cells need to sense their environment to ensure accurate targeting to specific destinations. This occurs in developing muscles, which need to attach to tendon cells before muscle contractions can begin. Elongating myotube tips form filopodia, which are presumed to have sensory roles, and are later suppressed upon building the attachment site. Here, we use live imaging and quantitative image analysis of lateral transverse (LT) myotubes in Drosophila to show that filopodia suppression occurs as a result of integrin signaling. Loss of the integrin subunits αPS2 and βPS increased filopodia number and length at stages when they are normally suppressed. Conversely, inducing integrin signaling, achieved by expression of constitutively dimerised βPS cytoplasmic domain (diβ), prematurely suppressed filopodia. We discovered that the integrin signal is transmitted through the protein Git (G-protein receptor coupled interacting protein) and its downstream kinase Pak (p21-activated kinase). Absence of these proteins causes profuse filopodia and prevents filopodial inhibition by diβ. Thus, integrin signaling terminates the exploratory behaviour of myotubes seeking tendons, enabling the actin machinery to focus on forming a strong attachment and assembling the contractile apparatus.