ketteandblown fuseinteract genetically during the second fusion step of myogenesis inDrosophila

Abstract

Drosophila myoblast fusion proceeds in two steps. The first one gives rise to small syncytia, the muscle precursor cells, which then recruit further fusion competent myoblasts to reach the final muscle size. We have identified Kette as an essential component for myoblast fusion. In kette mutants, founder cells and fusion-competent myoblasts are determined correctly and overcome the very first fusion. But then, at the precursor cell stage, fusion is interrupted. At the ultrastructural level, fusion is characterised by cellcell recognition, alignment, formation of prefusion complexes, electron dense plaques and membrane breakdown. In kette mutants, electron dense plaques of aberrant length accumulate and fusion is interrupted owing to a complete failure of membrane breakdown. Furthermore, we show that kette interacts genetically with blown fuse (blow) which is known to be required to proceed from prefusion complexes to the formation of the electron dense plaques. Interestingly, a surplus of Kette can replace Blow function during myogenesis. We propose a model in which Dumbfounded/Sticks and stones-dependent cell adhesion is mediated over Rolling Pebbles, Myoblast city, Crk, Blown fuse and Kette, and thus induces membrane fusion.

FuRMAS: triggering myoblast fusion in Drosophila

“…For example, Beckett and Baylies (2007) carefully counted the number of nuclei found in distinct muscle precursors in many known fusion mutants. Among these, only in myoblast city (mbc) mutants (Rushton et al, 1995;Erickson et al, 1997;Galletta et al, 1999) no fusion occurs at all as has been reported previously (Doberstein et al, 1997;Schröter et al, 2004Schröter et al, , 2006Menon et al, 2005). Interestingly, in agreement with previously published data, Beckett and Baylies (2007) found different numbers of nuclei in blow, kette, schizo/loner, mbc, and rols mutants as well as states of fusion between the segments of the individual mutants and concluded that there are no genetic differences between the two-phases of myoblast fusion, but rather only a delay in the fusion process itself.…”

“…5). In addition, fusion-relevant proteins also participate in a number of other developmental processes such as muscle attachment (Schröter et al, 2004;Schä fer et al, 2007), sarcomere assembly (e.g., Kreisköther et al, 2006) and visceral fusion (Klapper et al, 2001(Klapper et al, , 2002San Martin and Bate, 2001), Malpighian tubule organization (Denholm et al, 2003;Pü tz et al, 2005), garland cell development (Weavers et al, 2009), as well as eye development (e.g., Carthew, 2007;Fischbach et al, 2009). Thus, because of the multifunctional roles of fusion-relevant proteins, care should be taken when interpreting biochemical protein interaction assays, and the findings need to be correlated to the particular biological function.…”

“…By contrast, Beckett and Baylies (2007) endorsed a twotemporal-phase model of myoblast fusion and proposed that there are no genetic differences. However, it should be noted that the data leading to the proposal of the two-temporalphase (Beckett and Baylies, 2007) and two-phase (Rau et al, 2001;Schröter et al, 2004;Menon et al, 2005) models are not contradictory, but differences arise due to interpretation. For example, Beckett and Baylies (2007) carefully counted the number of nuclei found in distinct muscle precursors in many known fusion mutants.…”

“…In addition, several fusion-relevant proteins are maternally contributed, and with respect to fusion efficiency, all zygotic loss-offunction mutants with a strong maternal contribution of the corresponding mRNA are hard to interpret. For example, kette mutants often form small syncytia (Schröter et al, 2004;Beckett and Baylies, 2007), indicating that maternally provided Kette might be central to successful initial fusion events. Finally, recent studies on differential regulation of F-actin reorganization during the first and second fusion phases (Massarwa et al, 2007;Berger et al, 2008) add weight to the model of genetic differences in addition to temporal differences (see Key Molecular Players as Components of the Fusion-Restricted Myogenic-Adhesive Structure [FuRMAS] section).…”

“…For the first fusion phase, only myblast city (mbc) mutants have been analysed at the ultrastructural level and no vesicles nor electron-dense plaques were detected (Doberstein et al, 1997;Schröter et al, 2006). Both sns 15 (previously named rost 15 ; Paululat et al, 1995) and kette loss-of-function mutants accumulate aberrant electron-dense plaques that are considerably longer and thinner than in wild-type and these two mutants do not progress to the final membrane vesiculation stage (Doberstein et al, 1997;Schröter et al, 2004). Note that it is still not known whether sns 15 is a null mutant or not.…”