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In the Drosophila embryo, body wall muscles are formed by the fusion of two cell types, Founder Cells (FCs) and Fusion Competent Myoblasts (FCMs). Using an enhancer derived from the Dmef2 gene ([C/D]*), we report the first GAL4 driver specifically expressed in FCMs. We have determined that this GAL4 driver causes expression in a subset of FCMs and, upon fusion, in developing myotubes from stage 14 onwards. In addition, we have shown that using this Dmef2-5x[C/D]*-GAL4 driver to express dominant negative Rac in only FCMs causes a partial fusion block. This novel GAL4 driver will provide a useful reagent to study Drosophila myoblast fusion and muscle differentiation.
Dmef2 is the sole Drosophila member of the Myocyte Enhancing Factor 2 (MEF2) family of transcription factors (Nguyen et al., 1994). Dmef2 is required for the differentiation of mesodermal tissues within the developing Drosophila embryo, specifically the body wall, visceral and cardiac muscles (Bour et al., 1995; Lilly et al., 1995; Lin et al., 1996; Ranganayakulu et al., 1995). During the early stages of mesoderm development, Dmef2 is expressed in all mesodermal cells and its expression is controlled by another mesodermal transcription factor, Twist (Twi) (Cripps et al., 1998). Dmef2 expression continues in somatic, visceral and cardiac progenitors and is maintained throughout the development of these tissues (Bour et al., 1995; Lilly et al., 1994; Ranganayakulu et al., 1995). Previous studies identified enhancers responsible for the expression of Dmef2 in the two cell types that form body wall muscles, the Founder Cells (FCs) and Fusion Competent Myoblasts (FCMs) (Nguyen and Xu, 1998). FCs determine the identity of individual muscles. Upon fusion to FCs, FCMs become entrained to the FCs individual developmental program (Baylies et al., 1998; Beckett and Baylies, 2006a). Further dissection of an FCM-specific enhancer (I-E) identified a 55bp sequence ([C/D]*) that is necessary and sufficient to direct expression of a lacZ reporter gene in FCMs. This [C/D]* enhancer is bound directly by the FCM-specific transcription factor Lame duck (Lmd, also known as Myoblasts incompetent and Gleeful), requires lmd for its expression and therefore mediates FCM-specific expression of Dmef2 (Duan et al., 2001; Furlong et al., 2001; Ruiz-Gomez et al., 2002) . We have cloned multiple copies of this [C/D]* enhancer element upstream of the GAL4 gene to create a novel GAL4 driver (Dmef2-5x[C/D]*-GAL4) that we characterize here.
We crossed flies homozygous for the Dmef2-5x[C/D]*-GAL4 driver to flies carrying a UAS-GFP transgene and analyzed GFP expression in the resulting embryos. We detected mesoderm-specific GFP expression from stage 14 onwards (10.5 hrs after egg laying [AEL]; Fig. 1). GFP expression is first detected during early to mid stage 14 in a small number of ventral mesodermal cells (Fig. 1A). This initiation of reporter gene expression is later than that observed using a direct lacZ fusion (Duan et al., 2001) and presumably is due to the delay caused by the GAL4/UAS system. From late stage 14 and throughout stage 15 (11.5-13 hrs AEL), GFP expression is detected along the dorsal-ventral (D-V) axis of the embryo in both growing myotubes and surrounding cells (Fig. 1B). Based on their location, it is likely that these surrounding cells are FCMs. GFP expression is also detected in the final body wall muscles at stage 16 (13-16 hrs AEL; Fig. 1C-D). However, when we analyzed these embryos we found that GFP expression was not detected in all muscles and GFP levels varied between muscles. This differential muscle expression is variable between hemisegments and embryos, and is quantified in Table 1. For example, Fig. 1D shows that GFP is not expressed in all LT muscles, and in those in which it is expressed, it is not expressed at the same level. We also found that GFP expression is rarely detected in the VT1 and VA3 muscles. These muscles are both small in size, with the VT1 muscle, for example, containing 2-4 nuclei at stage 16 (Bate, 1990; Beckett and Baylies, 2007). The absence of GFP expression could be due to the late initiation of GFP expression in FCMs as well as the small number of fusion events occurring in these muscles. The expression of GFP in mesodermal cells surrounding the growing myotubes and the varying levels of GFP expression in the final muscles suggested that this the Dmef2-5x[C/D]*-GAL4 driver is expressed in FCMs and that, upon fusion to myotubes, is also expressed in these cells. The variation in GFP levels in the final muscles between hemisegments and embryos also suggested that the Dmef2-5x[C/D]*-GAL4 driver is not expressed in all FCMs, consistent with the temporal delay caused by the GAL4/UAS system, and that an individual FCM does not always fuse to the same muscle.
To confirm that the Dmef2-5x[C/D]*-GAL4 driver is expressed in FCMs and, subsequently, upon fusion, in myotubes, we used confocal microscopy. We crossed Dmef2-5x[C/D]*-GAL4 flies to flies carrying a UAS-lacZ transgene and stained the resulting embryos with antibodies against β-galactosidase (βgal) and Krüppel (Kr), an FC identity transcriptional regulator (Fig. 2A; Ruiz-Gomez et al., 1997). We detected ßgal expression in a subset of mesodermal cells from stage 14, similar to the results that we obtained using the GFP reporter described above (Fig. 2A). Kr is expressed in the nuclei of a subset of FCs and developing myotubes (Ruiz-Gomez et al., 1997). We detected Kr expression in the developing LT2 and LT4 muscles at stage 14 (Fig. 2A'). In each myotube, while both nuclei are Kr-positive, only one nucleus also expresses ßgal (Fig. 2A"). ßgal is never detected in Kr-expressing FCs prior to fusion (data not shown), therefore the co-expression observed at stage 14 must be due to fusion. In agreement with this, we detected ßgal expression in a small number of surrounding cells that are presumably FCMs.
To confirm that the ßgal-expressing cells not expressing FC markers are indeed FCMs, we performed colocalization experiments using an antibody against the FCM-specific protein Lmd (Duan et al., 2001). For this experiment, we crossed flies containing both the Dmef2-5x[C/D]*-GAL4 driver and the rp298-lacZ transgene, which is expressed in the nuclei of FCs and myotubes (Nose et al., 1998), to flies carrying a UAS-2xeGFP transgene. We then stained the resulting embryos with antibodies against Lmd, GFP and ßgal and analyzed them using confocal microscopy (Fig. 2B). In these embryos, we detected GFP expression in a subset of Lmd-expressing FCMs at fixed time points (Fig. 2B-C). We quantified this expression and determined that approximately 47% of Lmd-positive cells (n=229) in each hemisegment at a single point in time co-express GFP, indicating that they express the Dmef2-5x[C/D]*-GAL4 driver. We observed Lmd-expressing FCMs surrounding rp298-lacZ expressing myotubes as previously described (Beckett and Baylies, 2007; Duan et al., 2001), a subset of which co-expressed GFP (Fig. 2C). GFP was also detected in the rp298-lacZ expressing myotubes (Fig. 2C). These data confirmed that the Dmef2-5x[C/D]*-GAL4 driver is expressed in a subset of FCMs, at the time points examined, and continues to be expressed upon fusion with the developing myotubes during the final stages of muscle development.
We have shown that the Dmef2-5x[C/D]*-GAL4 driver leads to reporter expression in subsets of FCMs and myotubes from stage 14 until the end of muscle development. To determine if this GAL4 driver could be used to analyze events that occur during these stages, we tested whether overexpression of dominant negative Rac (dnRac) in FCMs would affect muscle development. Rac activity is required for myoblast fusion as loss of Rac function (Rac1,Rac2, Mtl triple mutants) lead to a myoblast fusion block (Hakeda-Suzuki et al., 2002; Richardson et al., in revision). Overexpression of dnRac throughout the mesoderm in both FCs and FCMs phenocopies the loss of function phenotype, resulting in a fusion block (Luo et al., 1994). We therefore asked whether expression of the dnRac construct only in FCMs would lead to a fusion block. We crossed flies carrying the Dmef2-5x[C/D]*-GAL4 driver to flies carrying a UAS-dnRac transgene and assayed the final muscle pattern of the resulting embryos using an antibody against Myosin heavy chain (Mhc). In these embryos, we observed a small number of unfused myoblasts indicating a partial fusion block (Fig. 3B). To confirm that this lack of fusion was due to expression of dnRac using the Dmef2-5x[C/D]*-GAL4 driver, we repeated the experiment using flies carrying both UAS-dnRac and UAS-GFP transgenes. We stained the resulting embryos with antibodies against Mhc and GFP and analyzed them using confocal microscopy. We observed the same partial fusion block and confirmed that the majority of unfused myoblasts are GFP-positive (Fig. 3C). We also observed reduced levels of GFP in the muscles of these embryos, indicating that the GFP expression observed in the final muscles of Dmef2-5x[C/D]*-GAL4 > UAS-GFP embryos shown in Figure 1 is truly due to the fusion of GFP-expressing FCMs with developing myotubes.
To conclude, we have constructed and characterized a novel FCM-GAL4 driver that is expressed specifically in a subset of FCMs at the time points examined and, post-fusion, in the majority of developing myotubes from stage 14 until the end of muscle development. We have further shown that this tool can be used to block fusion and in a parallel study to block muscle attachment (Beckett and Baylies, submitted). To our knowledge, this is the first FCM-specific GAL4 driver to be described and will provide an extremely useful tool for the study of fusion and late events during muscle differentiation.
Fly strains used were: Dmef2-5x[C/D]*-GAL4 (this study), UAS-GFP (Bloomington), UAS-2xeGFP (a gift from A. Michelson), UAS-lacZ (Bloomington), rp298-lacZ (Nose et al., 1998), UAS-RacL89.6 (Luo et al., 1994). All experiments were performed at 25°C using standard genetic crosses.
A 55bp sequence ([C/D*]) located in the upstream region of Dmef2 was previously identified as a minimal enhancer sufficient for FCM expression (Duan et al., 2001). 5 copies of the [C/D*] fragment were ligated with uniform directionality from annealed oligos with AvaI sites and then cloned into the NotI site of the pDA188 vector (kindly provided by Konrad Basler). The constructed plasmid was used to generate Dmef2-5x[C/D*]-GAL4 transgenic flies using established protocols (Nguyen and Xu, 1998).
All embryos were fixed using 4% paraformaldhyde/heptane. Antibodies were preabsorbed 1:10 against fixed wild type embryos (PA) or in combination with the TSA system (TSA; PerkinElmer Life Sciences) where stated. For DAB staining, the antibody dilutions used were: mouse anti-GFP (1:200; PA; Clonetech) and rabbit anti-Mhc (1:20,000; TSA; a gift from D. Keihart). Biotinylated secondary antibodies were used in combination with Vector Elite ABC kit (Vector Laboratories, CA). Specimens were embedded in Araldite and images captured using an Axiocam camera (Zeiss) and processed using Adobe Photoshop 7.0. For fluorescent staining, the antibody dilutions used were: mouse anti-ßgal (1:2000; Promega), chicken anti-ßgal (1:1000; Abcam), guinea pig anti-Kr (1:500; PA; a gift from J. Reinitz), mouse anti-GFP (1:200; PA; Clonetech), rabbit anti-Lmd (1:250; PA) and rabbit anti-Mhc (1:1000; a gift from D. Kiehart). Alexa488, Alexa555 and Alexa647 conjugated secondary antibodies were used (1:400; Molecular Probes) and embryos were mounted in Vectashield (Vector Laboratories, CA). Embryos were analyzed using a Zeiss LSM 510 confocal scanning system. Images were exported from Zeiss LSM software and processed using Adobe Photoshop 7.0.
We wish to thank J. Reinitz, A. Michelson, K. Basler and D. Kiehart for reagents. This work was supported by the Sloan Kettering Institute and NIH grants (GM586989, GM78318 to M.K.B and AR4628 to H.T.N).
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