SCAR/WAVE and Arp2/3 are critical for cytoskeletal remodeling at the site of myoblast fusion

Keywords: cell-cell fusion, myoblast fusion, muscle, actin, Kette/Nap1, SCAR, Arp2/3

Drosophila genetics

Stocks were grown under standard conditions. Stocks used were twist promoter-GFP-actin (gift of H.A. Müller), rP298-lacZ (Nose et al., 1998), twist-CD2 (Dunin-Borkowski and Brown, 1995), apME-GFP (this study), apME-NLS::eGFP (this study), apME-NLS::dsRed (this study), Df(1)w67k30 [deficiency removing duf and rst, (Lefevre and Green, 1972; Ruiz-Gomez et al., 2000)], kette^J4–48 (Hummel et al., 2000), ants/rols^T627 (Chen and Olson, 2001), loner^T1032 (Chen et al., 2003), mbc^C1 (Rushton et al., 1995), sns^XB3 (Bour et al., 2000), blow¹ (Doberstein et al., 1997), Rac1^J11Rac2^Δmtl^Δ (Hakeda-Suzuki et al., 2002), SCAR^Δ37 (Zallen et al., 2002), SCAR^k13811 (Spradling et al., 1999, Berkeley Drosophila Genome Project), Arp3^EP3640 (Rorth, 1996), and D-WIP^D30 (Massarwa et al., 2007). Mutants were balanced and identified using CyO P[w⁺wg^en11lacZ], TM3 Sb¹ Dfd-lacZ or TTG [TM3, twi-GAL4, UAS-2xeGFP, (Halfon et al., 2002)]. Germline clones (Chou and Perrimon, 1996) were generated by heat shock of hs-FLP; ovoD, FRT40A/SCAR^k13811, FRT40A larvae. Germline clone females were mated to SCAR^k13811/CyO at 20–22°C to create SCAR^k13811 maternal/zygotic embryos (Zallen et al., 2002).

Germline transformation and constructs

apME-GFP, apME-NLS::eGFP (gifts of Z. Kambris) and apME-NLS::dsRed (this study) DNA were constructed by cloning the apterous mesodermal enhancer 680 into pGreenH-Pelican, pH-Stinger and pRedH-Stinger [(Barolo et al., 2000; Barolo et al., 2004; Capovilla et al., 2001); Berkeley Drosophila Genome Project], which respectively contain cytoplasmic eGFP, eGFP and dsRed.T4 downstream of a nuclear localization signal. Constructs were injected using established protocols (Beckett and Baylies, 2006).

Immunohistochemistry

Embryos were collected at 25°C on apple juice/agar plates and were fixed as described previously (Beckett and Baylies, 2006) except embryos were fixed in 4% EM grade paraformaldehyde (Electron Microscopy Sciences) in 0.2M Sodium Phosphate for Kette and Scar staining. Embryos were mounted in Prolong Gold (Molecular Probes) for fluorescent stainings or araldite otherwise. Antibodies were preabsorbed (PA) where noted and used at the indicated final dilutions: mouse anti-β-galactosidase (1:1000, Promega), chicken anti-β-galactosidase (1:1000, Cappel), rabbit anti-Lameduck (PA, 1:250) (Duan et al., 2001), rabbit anti-Kette (1:1000) (Hummel et al., 2000), guinea pig anti-Scar (1:500) (Zallen et al., 2002), mouse anti-GFP (1:400, Clontech), mouse anti-Rols7 (1:4000) (Menon and Chia, 2001), rat anti-Loner (PA, 1:300) (Chen et al., 2003), mouse anti-Rac1 (1:200, BD Biosciences), rat anti-Myoblast city (PA, 1: 100) (Erickson et al., 1997), rat anti-Sticks and Stones (Sns, 1:100) (Bour et al., 2000), rabbit anti-Blow (PA, 1:500) (Doberstein et al., 1997), rabbit anti-Slouch (PA, 1:200 Beckett and Baylies, 2007) and rabbit anti-myosin heavy chain (Mhc, 1:10000, a gift from D. Kiehart). Biotinylated secondary antibodies (Vector Laboratories and Jackson ImmunoResearch) and the Vectastain ABC kit (Vector Laboratories) were applied for non-fluorescent Mhc stainings. Additionally, TSA amplification (PerkinElmer Life Sciences) was applied for Kette, Scar, Loner, Sns and Rac1. We used Alexa488, Alexa555 and Alexa647-conjugated fluorescent secondary antibodies and Alexa546 and Alexa647-conjugated phalloidin (Invitrogen). Fluorescent images were acquired on a Zeiss LSM 510 confocal scanning system mounted on an Axiovert 100M microscope with a 63× 1.2NA C-Apochromat water objective. For confocal microscopy, pinholes were set to capture an optical slice of 1.1µm. Non-fluorescent images were acquired on a Zeiss Axiophot microscope. Images were processed using Adobe Photoshop. 3D reconstruction was created using Improvision Volocity software.

Live imaging

Detailed explanation of live imaging is provided as supplemental material. Briefly, embryos were mounted on glass-bottom Petri dishes (MatTek Cultureware). Timelapse sequences were acquired using the confocal system described above.

Foci size and duration measurements

Area was measured using the overlay function of the Zeiss LSM software (Supplemental Figure S2). Foci were measured in the optical slice where they had the greatest radius and where FC/FCM adhesion was verified with specific cell labeling. The edges of foci were determined by using the range indicator function of the software and setting the edge where there is a clear change from signal to background. Measurements were acquired in the linear range of intensity and no relevant pixels were saturated. Duration was calculated as the time between when a focus appeared in a sequence and when it disappeared from detection. Foci were only included for duration measurements if there were optical slices above and below throughout the sequence to ensure actual dissolution and rule out cell/foci movement. Statistical analysis was performed with Microsoft Excel.

Actin rearrangements during the period of myoblast fusion

To define the behavior of the actin cytoskeleton during myoblast fusion [stages 12–15, 7.5–13hr AEL; (Bate, 1990; Beckett and Baylies, 2007)], fixed wild-type embryos were stained with phalloidin to label filamentous actin (F-actin) and with specific reagents that distinguish FCs/myotubes and FCMs. rp298-lacZ expresses β-galactosidase in the nuclei of FCs (Nose et al., 1998), while Lmd is an FCM-specific transcription factor which has both nuclear and cytoplasmic expression [(Duan et al., 2001; Nose et al., 1998), Figure 1A–B, Supplemental Figure S1A–B]. Hence, the relevant myoblast cell types, their arrangements with respect to one another and changes in actin cytoskeleton could be identified during fusion.

In addition to a uniform accumulation of F-actin at the myoblast cell cortex (Figure 1A–B, Supplemental Figure. S1A–B), specific F-actin structures were observed. First, F-actin-based filopodia and lamellopodia were observed in both FCs and FCMs. FCMs extended filopodial projections directionally towards an FC/myotube (Supplemental Figure S1A). These data are consistent with the migration of FCMs from internal layers externally to fuse with myotubes (Beckett and Baylies, 2007). As recognition occurred between an FC/myotube and FCM, the FCMs assumed a tear-drop shape, and this change in cell shape was reflected in the actin cytoskeleton (Supplemental Figure S1B). At the sites of adhesion between an FC and/or myotube and FCMs, we observed a striking accumulation of F-actin in foci (Figure 1B, Supplemental Movie 1; Kim et al., 2007; Kesper et al, 2007). These foci were most often spherical and localized across both cell types (Figure 1C). To better characterize actin foci, we measured their size (Supplemental Figure S2). In wild-type embryos, these foci ranged in size from 0.7–4.5µm², averaging 1.9µm² (n = 100 foci; Table 1). The range observed in these fixed preparations most likely reflects the dynamic nature of the actin foci (see below). Actin foci were not observed in non-mesodermal tissues nor were they detected in mesodermal cells before or after the stages when fusion takes place (Supplemental Figure S3A,C). Furthermore, actin foci are present prior to pore formation in myotube/FCM membranes, as cytoplasmic GFP expressed specifically in myotubes does not leak across foci into the cytoplasm of adherent FCMs (Figure 1D). Thus, using these labeling techniques, a series of actin cytoskeletal behaviors that occur during muscle/myotube formation have been identified: actin-rich protrusive structures, cell shape changes and an accumulation of F-actin at the adhesion site between FCs/myotubes and FCMs.

Live imaging reveals dynamics of actin-based behaviors and the site of myoblast fusion

To reveal the dynamics of these actin-based behaviors and their contribution to the fusion process, time-lapse analysis of live embryos was employed. A GFP-actin fusion protein was expressed in all myoblasts under the control of the twist promoter (twip) (Verkhusha et al., 1999). Phalloidin staining of twip-GFP-actin embryos confirmed that both methods revealed the same range of actin-based rearrangements described above (Supplemental Figure S1C–E, Supplemental Figure S4). In particular, GFP-actin foci were observed at FC/myotube and FCM adhesion sites with a similar size and shape to the phalloidin-stained F-actin foci (mean: 1.8µm², range: 0.7–4.7 µm², n=25, Figure 1E, Supplemental Figure S4). Live imaging revealed that an actin focus at the adhesion site builds into the spherical structure observed in fixed embryos (Figure 1B). Actin foci are dynamic structures: the lifetime of actin foci ranged from 5.7 to 29.5 minutes with the average actin focus present for 11.9 minutes at all stages in which myoblast fusion takes place (n=50). Actin foci build to their maximum size in less than 2 minutes and, following their duration, completely dissolve in less than 1 minute.

We next tested whether the dynamic accumulation and dissolution of actin foci marked the site of fusion. Using our live imaging approach, we captured single fusion events of FCMs with a specific myotube (Figure 1E, Supplemental Figure S5, Supplemental Movie 2–Supplemental Movie 3). To assure that we were measuring myoblast fusion, we took advantage of the well established observation that, upon fusion of an FCM to a specific FC/myotube, the incorporated naïve FCM nucleus is programmed to the specific muscle identity and becomes labeled with the FC-specific identity genes, such as slouch or apterous (Baylies et al., 1998; Capovilla et al., 2001; Frasch, 1999). We therefore generated transgenic flies carrying both twip-GFP-actin, which is expressed in all FCs and FCMs, and an apterousME-NLS-dsRed construct, which is expressed in a specific subset of FCs, their growing myotubes and resultant muscles. In this way, we monitored single fusion events with 4D live imaging (a 3D Z-stack imaged over time) by following myoblast arrangements in time and space and by the accompanying incorporation of an additional dsRed nucleus into the apterousME-NLS-dsRed myotube. In all fusion events observed (>50), an ordered sequence of events takes place. First, an actin focus forms at the adhesion site between a fusing myotube, labeled with apterousME-NLS-dsRed, and an FCM. Next, the actin focus dissolves, followed by myoblast fusion and detection of dsRed in the newly added nucleus (≤ 5 minutes; Figure 1E, Supplemental Movie 2). Identical data were obtained using apterousME-NLS-eGFP (Supplemental Figure S5, Supplemental Movie 3). In no case was fusion observed in the absence of actin focus formation and dissolution. Based on these studies, we concluded that the actin focus marks the site of myoblast fusion and that dissolution of the actin focus directly precedes a fusion event. In addition, these data revealed the dynamics of actin foci formation and dissolution and myoblast fusion in vivo.

Mutations in known fusion genes affect foci differently

Since we identified a specific actin rearrangement that is directly linked to the fusion site, we next addressed whether mutations in the genes linked to myoblast fusion led to defects in actin foci number or morphology. Actin foci in mutant embryos were first examined in fixed preparations using phalloidin staining at stage 14, the period where the maximum number of fusion events takes place (Beckett and Baylies, 2007). The mutants fell into three classes based on actin foci number and size: 1) decreased number/normal size, 2) increased number/normal size, and 3) increased number/increased size (Table 1, Figure 2, Supplemental Figure S2 and data not shown).

Class one mutant embryos had fewer foci. Disruption of genes involved in FC-FCM recognition and adhesion, such as duf, rst and sns (Figure 2B and data not shown), led to embryos with no actin foci. These data indicated that adhesion between FCs/myotubes and FCMs is critically required to initiate actin nucleation and formation of the F-actin focus. Additionally, disruption of rols led to a decrease in the number of actin foci, although those that did form were of normal size (Figure 2C, Table 1 and data not shown). Antibodies to Sns and Rols colocalized with the actin foci (Figure 3A–B), consistent with the actin foci marking the fusion site.

One mutant showed wild-type size actin foci with increased numbers (class 2): loner (Figure 2D; Table 1 and data not shown). The increase in the number of foci was consistent with a block in myoblast fusion in this mutant background. Despite Loner’s reported FC-specific expression (Chen et al., 2003), we found consistent expression of Loner within myotubes and FCMs, localized near foci but never overlapping (Figure 3C and data not shown). These data, together with live imaging data (see below), suggested that, although Loner activity is required for the progression of fusion, it regulates fusion independently of actin foci.

A third class - Rac (Rac1, Rac2, Mtl triple mutants), kette, blow and mbc - showed enlarged foci, as well as increased numbers of actin foci (Figure 2E–H and data not shown). Rac localization was punctate throughout the cell, with partial overlap with F-actin foci (Figure 3D). The protein products encoded by the other genes were enriched at the sites of actin foci formation both in the myotubes and in adhering FCMs (Figure 3E–G). Expression of these proteins in both myotubes and FCMs is consistent with published data (Erickson et al., 1997; Schroter et al., 2006; Schroter et al., 2004). Enlarged foci were seen in these mutants from the earliest stages of fusion (data not shown), and foci persisted after fusion is complete in wild-type embryos (Supplemental Figure S3F). FCMs often clustered together at the side of adhesion to FCs in these mutants. However, distinct actin foci were often still discernable for an individual FC-FCM combination. In kette mutants, the average size of an actin focus is 3.4µm², with a range of 1.2–8.3µm², [n= 100; Table 1]. Likewise mutations in blow, mbc and Rac had more and larger F-actin foci than wild-type (Table 1 and data not shown).

Taken together, we now grouped the known fusion mutants into three distinct classes with respect to actin focus size, based on our analysis of fixed embryos. If adhesion between FC or myotube and FCMs is impaired, no or fewer numbers of wild-type foci were found (class 1). If there is a block in fusion after adhesion of an FCM to FC/Myotube, increased numbers of foci were found, consistent with a failure in the fusion process (classes 2 and 3). The size of the focus was, however, distinct in these two classes, with wild-type-sized foci found in class 2 mutant embryos and enlarged foci found in class 3. In addition, our finding of these two classes indicated that enlarged foci are not simply a consequence of a fusion block (Discussion).

Live imaging of mutants

To further understand the regulation of the actin foci at fusion sites, we performed live imaging analysis on representative mutants from the three classes of fusion mutants. Live imaging analysis of rols mutants, which are capable of some fusion, indicated that fusion always follows actin focus formation and dissolution, similar to wild-type (data not shown). Live imaging of loner mutants indicated that while actin foci form normally, they never dissolve, correlating with the increased numbers of foci observed and a complete fusion block (Beckett and Baylies, 2007; data not shown). These data, taken together with the wild-type size of actin foci in loner mutants, suggested that the Loner-ARF6 pathway regulates fusion independently of actin foci.

Of the class of mutations that lead to the formation of abnormally large actin foci, Kette, is most directly linked to actin polymerization (Schroter et al., 2004). Kette (also known as Nap1) is a member of an evolutionarily conserved complex, which regulates the activity of SCAR/WAVE. SCAR/WAVE, in turn, activates Arp2/3 dependent actin polymerization (Ibarra et al., 2005; Smith and Li, 2004; Vartiainen and Machesky, 2004). Live imaging of kette mutant embryos revealed that GFP-actin foci were larger, as seen in the fixed preparations, and persisted for significantly longer in kette embryos than wild-type (average 36.3 minutes compared to 11.9 minutes in wild-type, n = 25; Figure 4A–B, Supplemental Movie 4). This measure of actin focus duration in the kette mutant embryos is most likely an underestimate, as actin foci were always still present at the end of an imaging sequence (Supplemental Figure S3F). Despite the changes seen in actin foci size and number in kette mutant embryos, the foci still formed at sites of FC/FCM adhesion. These results indicated that the fusion defect in kette mutant embryos, and we would suggest, in the other members of this class (data not shown) is correlated with a failure in the dissolution of actin foci at sites of fusion.

Localization of fusion proteins in the different foci classes

We next examined the localization of members of the known fusion proteins in the three classes of fusion mutants (Figure 5). In fusion mutants that have no actin foci (i.e. sns), the fusion proteins that normally localize to the myoblast fusion site lost their polarized localization. Instead, Blow and Mbc became cortically distributed, while Kette was punctate in the cytoplasm (Figure 5A and data not shown). This suggested that adhesion is required for proper localization of this subset of fusion proteins that co-localize with the actin foci. In fusion mutants with normally sized actin foci (loner), the localization of this subset of fusion proteins is indistinguishable from wild-type (Figure 5B and data not shown). Finally, in fusion mutants with enlarged actin foci (kette, mbc, blow, Rac), this subset of the known fusion proteins continued to colocalize with F-actin and were present at high levels throughout the abnormally large actin foci at sites of myoblast adhesion (Figure 5C and data not shown). Taken together, these data indicated that the formation of an actin focus correlates with the proper localization of a specific subset of the known fusion proteins. Enlarged actin foci were associated with an accumulation of this subset of known fusion proteins that normally localize at the site of myoblast fusion. Interestingly, the localization of Loner, which does not colocalize with actin foci in wild-type embryos, was not altered in any class of mutants at the time of foci formation (data not shown).

Sltr/D-WIP is reported to regulate the formation of actin foci specifically in FCMs (Kim et al., 2007). Therefore, we tested if foci were asymmetrically disrupted in fusion mutants with enlarged actin foci. Use of an antibody against the FC/myotube-specific protein Rols indicated that the enlarged actin foci localize across both cell types in mbc, blow and kette mutant embryos. (Figure 5D–F). These data suggested that, unlike Sltr/D-WIP, these gene products are not required specifically in one cell type.

SCAR loss of function leads to a fusion block and prevents actin focus dissolution

To further understand the mechanism underlying the foci dissolution defect in kette mutants, we examined the function of SCAR/WAVE in myoblast fusion. The regulation of SCAR/WAVE by Kette/Nap1 has been studied in a variety of systems. Depending on the context, the Kette/Nap1 complex is thought to negatively or positively regulate the activity of SCAR/WAVE (Bogdan and Klambt, 2003; Eden et al., 2002; Ibarra et al., 2006; Kunda et al., 2003; Rogers et al., 2003). Therefore, we examined the final pattern of the embryonic musculature in SCAR loss of function mutants. Embryos homozygous for a null mutation of SCAR exhibited a moderate, but completely penetrant, myoblast fusion defect (Figure 6B). Embryos with reduced maternal and zygotic SCAR contributions, however, had a more severe myoblast fusion defect than removal of zygotic SCAR alone, with increased numbers of free myoblasts and thinner muscles (Figure 6C). Krüppel-expressing myotubes were most often mononucleate in maternal/zygotic Scar mutants, with an occasional binucleate cell, suggesting a severe fusion block (1.24 nuclei/myotube ± 0.43 S.D., n=22 hemisegments; data not shown). Altogether, these data indicated that SCAR is critical for myoblast fusion.

We next examined actin foci in SCAR mutants. Analysis of F-actin foci in zygotic SCAR mutants revealed a range in foci size from wild-type to enlarged, with an average size being 2.3µm² (Figure 6E, Table 1). SCAR maternal/zygotic mutants showed a dramatic increase in the size and number of actin foci, to a similar level as found in kette mutant embryos. The average size of a mutant focus was 3.4µm² (Figure 6F, Table 1). Similar to Kette, SCAR had overlapping localization with actin foci during myoblast fusion (Figure 6H). However, SCAR protein levels were virtually undetectable in kette mutant embryos, and the residual SCAR protein was not properly localized to actin foci (Figure 6I). These results suggested that, in the context of myoblast fusion, Kette functions as a positive regulator of the localization and stability of SCAR. In addition, the enlarged, persistent actin foci phenotypes in kette and SCAR mutants suggested that this pathway is essential, not for the formation of the actin focus, but for a critical reorganization required for its dissolution. Moreover, the use of Rols as a marker of FCs/myotubes indicated that the enlarged actin foci localize across both cell types in SCAR mutants, similar to other mutants with enlarged foci (Figure 6J).

Arp2/3 is required for fusion

The requirement of SCAR for proper myoblast fusion and actin focus dissolution prompted us to examine the role of the Arp2/3 complex in these processes. Examination of a zygotic loss-of-function allele of Arp3/Arp66B, an essential component of the Arp2/3 complex, revealed a moderate myoblast fusion defect, similar to the defect seen in a SCAR zygotic loss-of-function (Figure 6D). Likewise foci size was similar to that observed in SCAR zygotic mutants (Figure 6G, Table 1). As with SCAR, this relatively mild defect could be due to the presence of maternally contributed Arp3. However, further analysis of the Arp2/3 complex is complicated by an earlier requirement, as germline clones with available reagents do not develop to the stages of muscle development (Zallen et al., 2002). Nevertheless these data suggested that Arp2/3, like Kette and Scar, is required for actin focus dissolution.

Insight to cellular models of fusion

Transmission electron microscopy (TEM) analysis (Doberstein et al., 1997) has suggested that there are distinct events in the fusion process: after recognition and adhesion, the membranes between an FC and an FCM align, paired vesicles carrying electron dense material are then recruited to these sites (termed the ‘prefusion complex’), these vesicles then release the electron dense material, leading to plaque formation. Subsequently, the plasma membrane breaks down, leading to cytoplasmic continuity between the cells. TEM studies indicated that, in kette mutant embryos, myoblast fusion is blocked at plaque formation (Schroter et al., 2004). Similarly our data indicate that in kette mutant embryos, fusion is blocked due to enlarged actin foci at sites of fusion. It is thus tempting to equate the actin foci with the enlarged plaques seen by TEM. However, there is a strong argument against this conclusion. We see enlarged actin foci in a number of fusion mutants, including mbc and blow. Whereas all show enlarged foci, TEM analysis reveals that only kette mutant embryos show a block at plaque formation. mbc mutant embryos show a block prior to the recruitment of the electron dense vesicles, whereas embryos mutant for blow show a block at the prefusion complex, with no plaque formation detected (Doberstein et al., 1997). Hence actin foci cannot be equated with the electron dense plaques.

It is consistent, however, with both data sets that the subcellular events observed by TEM, namely membrane alignment, formation of the prefusion complex and plaque formation, happen concurrently with the actin focus formation that we report here. We see that the aligned membranes are still intact when a focus is detected (Figure 1). Recent TEM work suggests that actin is important for the targeting of vesicles (Kim et al., 2007). How the diffuse actin observed in these studies relates to the dynamic but concentrated accumulation of F-actin into foci at the plasma membrane in our study remains in question. Lastly, these observations highlight the larger question of how the events we detected with confocal microscopy relate to the events distinguished by TEM.

Kette/Nap1 positively regulates SCAR/WAVE-Arp2/3 to allow focus dissolution

Important to the interpretation of the mechanisms contributing to enlarged foci is our finding that SCAR/WAVE and Arp2/3 are also required for myoblast fusion. Reduction of maternal and zygotic contributions of SCAR/WAVE leads to a block in myoblast fusion, like that seen in kette mutant embryos. Reduction of zygotic Arp3 also leads to a block in fusion, although analysis of complete loss of Arp2/3 activity is precluded by the reagents currently available. Together these proteins provide an important direct link to an actin polymerization event at the site of fusion.

There has been some controversy as to how the Kette/Nap1 complex regulates SCAR/WAVE. In biochemical assays, the Kette/Nap1 complex negatively regulates SCAR/WAVE, holding it in an inactive state until the complex is activated by Rac (Eden et al., 2002). In support of this view, reducing the dose of SCAR can partially rescue the kette phenotype in the central nervous system of the Drosophila embryo (Bogdan and Klambt, 2003). In contrast, the Kette/Nap1 complex in Drosophila tissue culture cells positively regulates SCAR/WAVE by correctly localizing and stabilizing SCAR (Kunda et al., 2003; Rogers et al., 2003). Our data support a positive regulation of SCAR by the Kette complex in the context of myoblast fusion. In kette mutant embryos, SCAR protein levels are reduced significantly and the residual protein is not localized properly. It has been suggested that the differences in SCAR regulation by the Kette complex may be a reflection of the relative differences in the amounts of SCAR and the components of the regulatory complex in different contexts (Kunda et al., 2003).

The actin focus phenotype in SCAR and Arp3 mutants has also given mechanistic insight to the role of the Kette-SCAR-Arp2/3 pathway controlling its behavior. If SCAR or Arp2/3 was required for the polymerization of actin leading to foci formation, we would have expected smaller or absent actin foci in SCAR and Arp3 mutants. Instead, we find that, similar to kette mutants, enlarged foci are present in SCAR and Arp3 mutants. These data suggest that an enlarged actin focus results from the loss of a Kette-SCAR-Arp2/3-dependent actin polymerization event required for actin foci dissolution. This actin polymerization event is presumably transient, as we have not been able to detect a site of post-focus actin polymerization, other than cortical actin in our imaging assays. Aberrant actin accumulation in the absence of Kette/NAP1 and SCAR has been seen both in Drosophila (Hummel et al., 2000; Kunda et al., 2003; Zallen et al., 2002) and in Dictyostelium (Ibarra et al., 2006). These observations suggest that the Arp2/3-dependent actin polymerization machinery can function more generally in the regulation of actin cytoskeletal organization and is capable of both forming and dissolving visible F-actin structures. An additional activator of Arp2/3, WASP, may also have a potential role in the regulation of actin foci. Recent studies have indicated that WASP plays an essential role in myoblast fusion, although the regulation of actin foci was not tested (Massarwa et al., 2007; Schafer et al., 2007). Analysis of mutants in the WASP regulator, Solitary/D-WIP, indicates that actin foci do form (Kim et al, 2007). We find that the actin foci are approximately wildtype size in these mutants (Table 1).

A revised molecular model of cell fusion

On a molecular level, the current model placed the intracellular signaling events downstream of recognition into two distinct pathways that converge on cytoskeletal rearrangements required for fusion. Our data distinguish among the identified fusion genes with respect to actin reorganization at the site of fusion. Moreover our data suggests new relationships between the fusion mutants, leading to a revision of the existing model (Figure 7).

Downstream of recognition and adhesion, actin foci form in the myotube and FCM. Mbc-Rac activities and Blow-Kette-SCAR-Arp2/3 proteins act to promote FCM cell shape change and target actin reorganization, leading to the dissolution of the actin focus in both FCMs and myotubes. Moreover, Blow and Kette have similar protein localization and actin focus phenotype, consistent with observed genetic interactions (Schroter et al., 2004). Interestingly, absence of any one of these gene products does not prohibit localization of the other members of this genetic pathway at the fusion site. Rac has been found to regulate the Kette complex in several contexts (Eden et al., 2002; Steffen et al., 2004), adding further support to this aspect of our model. Based on previous studies in a number of systems (Ibarra et al., 2005; Machesky and Insall, 1998; Vartiainen and Machesky, 2004), the target of SCAR/WAVE activity is the Arp2/3 complex. Our data support this view: mutants in both SCAR and Arp3 show fusion defects. While we do not yet know how all the biochemical activities of these proteins are coordinated, the sum of all these proteins’ activities is to dissolve the actin focus, through actin reorganization in both the FCM and myotube. Our data suggests that dissolution of the focus is required for fusion to proceed and would be coupled to membrane breakdown and cytoplasmic mixing between the two cells.

A second pathway - Loner-ARF6 - also contributes to myoblast fusion. However this pathway does not appear to directly regulate actin foci, despite the block in myoblast fusion. Unlike mutants in the Mbc-Rac-Kette-SCAR-Arp2/3 pathway, where actin foci size is dramatically increased, foci size remains wild-type in loner mutants. Our protein localization studies are consistent with the actin foci data: Loner does not colocalize with the actin foci, but is most often found near actin foci. Likewise, analysis of the subset of known fusion components that colocalize with the foci do not change in loner mutants. We predict that this pathway is required for additional behaviors, either upstream or downstream of the foci, necessary for fusion, such as myoblast searching or migration, microtubule rearrangements or other subcellular functions such as membrane trafficking.

Our data provides new insight to the function of Rols in myoblast fusion. Rols is proposed to serve as an adapter (Chen and Olson, 2001; Chen and Olson, 2004) between the recognition and adhesion protein Duf and Mbc, a GEF for Rac (Hasegawa et al., 1996; Kiyokawa et al., 1998; Nolan et al., 1998), linking adhesion to cytoskeletal rearrangements. Biochemical data from overexpression studies in Drosophila S2 cells suggest a direct interaction between Rols and Mbc (Chen and Olson, 2001). Our data, however, would indicate that this relationship is not required for foci formation and dissolution. Rols appears not to be necessary for the recruitment of Mbc to the fusion site, as average foci size is wild-type in rols mutants whereas it is enlarged in mbc mutants. Moreover, we find that localization of Rols and Mbc is not identical at foci (data not shown). Our data supports the alternative model in which Rols is required for efficient Duf recruitment to the FC membrane (Menon et al., 2005). The drastically reduced number of actin foci in rols mutants suggests difficulties in myoblast recognition/adhesion. Those actin foci that do appear have wild-type size and correlate with fusion events, consistent with a model of reduced efficiency of fusion in rols mutants.

Our model leaves the function of the actin focus unresolved. Actin rearrangements can be linked to the active organization of membrane domains (Liu and Fletcher, 2006), membrane and protein trafficking (Egea et al., 2006; Kaksonen et al., 2006; Qualmann and Kessels, 2002; Stamnes, 2002), and structural support. Our data indicates that formation and dissolution of actin foci are essential for the progression of fusion. We have never observed a fusion event that has not been linked to an actin focus. The identification of the site of fusion, a particular actin structure at this site, new methods of analysis and key regulators of this structure open avenues of study for the process of cell-cell fusion in invertebrate and vertebrate biology.

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