Structure/Function Analysis of Type IIa Receptor Tyrosine Phosphatases on Drosophila Muscle Integrity

Abstract

The Lar-family of receptor protein tyrosine phosphatases (RPTPs); including Lar, RPTPσ and RPTPδ, are utilized in signal transduction pathways during neural development. Fundamentally, the processes of axon guidance and synaptogenesis are carried out by rearrangements of the actin cytoskeleton for extension and maturation of structures. To determine whether this regulation is conserved in other tissues, we utilized interdisciplinary approaches for a structure/function analysis of Lar-RPTPs in the Drosophila musculature. We find that the single fly ortholog, Dlar, is localized to the muscle costamere in wandering L3 larvae. The costamere has important functions in muscle integrity and force transmission during contractions. Further, depletion of Dlar in the musculature causes aberrant sarcomeric actin patterning and mislocalization of the major transmembrane receptor of the costamere, integrin dimers. Ablation of two additional proteins from the musculature; including the guanine nucleotide exchange factor, Trio, and the basement membrane protein, Glutactin (Glt), results in similar disruptions to the muscle architecture. Thus, Trio and Glt provide links to actin through the Rho family of small GTPases and the BM membrane which is intimately involved in integrin signaling. We show that the actin cytoskeletal aberrations cause deficits in larval locomotor function. Additionally, we find that the cytosolic domains of Dlar are particularly important for muscle function and have implications in integrin signaling versus physical receptor/ligand interactions which agree with x-ray crystallographic analysis of the FN4-6 repeats. A proteomic approach was utilized to find novel binding partners and resulted in identification of basement membrane proteins as conserved Lar-RPTP ligands. Finally, we offer a model for Dlar signaling that results in the low affinity integrin conformation induced by inside-out signals arising from actin remodeling.