αPS2 integrin-mediated muscle attachment in Drosophila requires the ECM protein Thrombospondin

Abstract

During Drosophila embryogenesis, the attachment of somatic muscles to epidermal tendon cells requires heterodimeric PS-integrin proteins (alpha- and beta-subunits). The alpha-subunits are expressed complementarily, either tendon cell- or muscle-specific, whereas the beta-integrin subunit is expressed in both tissues. Mutations of beta-integrin cause a severe muscle detachment phenotype, whereas alpha-subunit mutations have weaker or only larval muscle detachment phenotypes. Furthermore, mutations of extracellular matrix (ECM) proteins known to act as integrin binding partners have comparatively weak effects only, suggesting the presence of additional integrin binding ECM proteins required for proper muscle attachment. Here, we report that mutations in the Drosophila gene thrombospondin (tsp) cause embryonic muscle detachment. tsp is specifically expressed in both developing and mature epidermal tendon cells. Its initial expression in segment border cells, the tendon precursors, is under the control of hedgehog-dependent signaling, whereas tsp expression in differentiated tendon cells depends on the transcription factor encoded by stripe. In the absence of tsp activity, no aspect of muscle pattern formation as well as the initial contact between muscle and tendon cells nor muscle-to-muscle attachments are affected. However, when muscle contractions occur during late embryogenesis, muscles detach from the tendon cells. The Tsp protein is localized to the tendon cell ECM where muscles attach. Genetic interaction studies indicate that Tsp specifically interacts with the alphaPS2 integrin and that this interaction is needed to withstand the forces of muscle contractions at the tendon cells.

Tendon development and musculoskeletal assembly: emerging roles for the extracellular matrix

“…In Drosophila, Sr promotes the transcription of the single fly thrombospondin (Tsp) gene in tenocytes. Tsp binds αPS2 (If)/βPS (Mys) Itg heterodimers on fly muscle and tendon cell surfaces and patterns muscle attachments (Chanana et al, 2007;Subramanian et al, 2007). Vertebrates have at least five thrombospondins, with diverse functions in cell migration, vasculogenesis, wound healing and cancer (Adams and Lawler, 2004;Bornstein et al, 2004;Kyriakides et al, 1999;Mustonen et al, 2012).…”

Connecting muscles to tendons: tendons and musculoskeletal development in flies and vertebrates

“…Development 137 (17) REVIEW Development 137 (17) PS2PS heterodimer interacts with Tsp (Chanana et al, 2007;Subramanian et al, 2007) and Tiggrin (Fogerty et al, 1994). Laminin and Tsp are secreted from the tendon cells, and Tiggrin is secreted from the muscle cell.…”

An O-Glycosyltransferase Promotes Cell Adhesion during Development by Influencing Secretion of an Extracellular Matrix Integrin Ligand

“…ECM ligands are a diverse group of molecules that are secreted by cells, creating local environments and matrices that modulate cell interactions. Mutations in integrins or their ECM ligands have been shown to disrupt cell adhesion, resulting in wing blistering (1, 3-5, 7, 9 -12), muscle detachment (3,13,14), and defects in dorsal closure (15) in Drosophila, as well as the human disease epidermolysis bullosa, in which the epidermal and dermal skin layers are separated (16,17). Loss of proper cell adhesive properties is also a hallmark of cancer progression and metastasis.…”