A Critical Function for the Actin Cytoskeleton in Targeted Exocytosis of Prefusion Vesicles during Myoblast Fusion

Abstract

Myoblast fusion is an essential step during muscle differentiation. Previous studies in Drosophila have revealed a signaling pathway that relays the fusion signal from the plasma membrane to the actin cytoskeleton. However, the function for the actin cytoskeleton in myoblast fusion remains unclear. Here we describe the characterization of solitary (sltr), a component of the myoblast fusion signaling cascade. sltr encodes the Drosophila ortholog of the mammalian WASP-interacting protein. Sltr is recruited to sites of fusion by the fusion-competent cell-specific receptor Sns and acts as a positive regulator for actin polymerization at these sites. Electron microscopy analysis suggests that formation of F-actin-enriched foci at sites of fusion is involved in the proper targeting and coating of prefusion vesicles. These studies reveal a surprising cell-type specificity of Sltr-mediated actin polymerization in myoblast fusion, and demonstrate that targeted exocytosis of prefusion vesicles is a critical step prior to plasma membrane fusion.

FuRMAS: triggering myoblast fusion in Drosophila

“…A systematic analysis of these vesicles in wild-type embryos suggests that this complex consists of an average of 1.4 vesicles per complex in an individual section (Estrada et al, 2007). Kim et al (2007) also described variable numbers of vesicles, which were also not all paired, a finding that is supported by Doberstein et al (1997). These observations raise two key questions: (1) Why are the vesicle numbers so different?…”

“…Doberstein et al (1997) suggested that these vesicles form a prefusion complex that can consist of up to 15 vesicles (40 nm in diameter) per contact site. From the Golgi to the membrane, they might be transported by F-actin, which disintegrates after prefusion complex formation (Kim et al, 2007). A systematic analysis of these vesicles in wild-type embryos suggests that this complex consists of an average of 1.4 vesicles per complex in an individual section (Estrada et al, 2007).…”

“…F-actin and D-Titin are known to accumulate in fc/growing myotubes and fcms, while Blow is concentrated in fcms (Kesper et al, 2007;Richardson et al, 2007). The Factin plugs at the site of fusion have also been termed actin foci (Kim et al, 2007;Richardson et al, 2007). However, before cell adhesion rings (Kesper et al, 2007) and actin plugs/foci (Richardson et al, 2007) can be established, successful cell adhesion must take place.…”

“…In Drosophila, heterologous cell adhesion between a founder cell (fc) and a fusion-competent myoblast (fcm) is the precondition for myoblast fusion followed by local F-actin accumulation and branching (Kesper et al, 2007;Kim et al, 2007;Richardson et al, 2007). It has been proposed that a signalling pathway initiated by successful cell adhesion between fcs and fcms relays fusion signals from the cell membrane to the actin cytoskeleton (reviewed by Chen and Olson, 2004).…”

“…It has been proposed that a signalling pathway initiated by successful cell adhesion between fcs and fcms relays fusion signals from the cell membrane to the actin cytoskeleton (reviewed by Chen and Olson, 2004). Recent data show that cell adhesion leads to the assembly of a multiprotein complex at the site of fusion (Kesper et al, 2007;Kim et al, 2007;Richardson et al, 2007), which was named the FuRMAS, a specific Fusion-Restricted Myogenic-Adhesive Structure, that contains F-actin at the sites of cell contact as plugs/foci (Kesper et al, 2007).…”