Regulators determining the identities of specific heart and somatic muscles in Drosophila
Evolutionary Cell and Developmental Biology (Zoology)
Final Report Abstract
In this project we have discovered novel processes of muscle development during postembryonic development in Drosophila and gained new insights into the role of the Tbx1 T-box transcription factor Org-1 in them. Most strikingly, we have described a novel process of muscle development in which specific terminally-differentiated syncytial muscles, called alary muscles, completely dedifferentiate and fragment into mononucleated myoblasts. These myoblasts then become respecified as muscle founder cells, which fuse with unrelated myoblasts to form new syncytia that differentiate into other types of muscles, namely the ventral longitudinal muscles of the adult heart. Hence, these ventral longitudinal muscles arise through a very unusual process of transdifferentiation. Previously, fragmentation of syncytial muscles into mononucleated myoblast had never been observed during normal development, although this process is reminiscent of analogous events known to occur during muscle regeneration upon limb amputation of in certain amphibian species. We identified and examined several regulators and regulatory pathways that control this process of transdifferentiation and in particular its first phase, which involves dedifferentiation and fragmentation of syncytial alary muscles. We showed that org-1 and its downstream target tailup (tup) initiate the process of dedifferentiation and syncytium fragmentation, and that Hox genes provide the spatial and ecdysone hormone signaling the temporal cues that activate org-1 in these alary muscles. FGF signaling is crucial for specifying the dedifferentiated myoblasts as founder cells of the ventral longitudinal heart muscles, and the metamorphosing heart tube acts to attract the newlygenerated VLM precursors towards it. In a selective RNAi screen for additional regulators we found a number of genes required for different aspects and successive steps of ventral longitudinal muscle formation. Focusing on genes with the strongest knock-down phenotypes, we showed that the Hippo pathway effector Yorkie and its DNA binding partner Scalloped are essential, downstream or org-1, for promoting anterior alary muscle dedifferentiation and fragmentation, and that the Hippo kinases must be inactive for allowing these factors to fulfill this function. An additional essential signaling pathway for promoting alary muscle dedifferentiation and fragmentation involves Drosophila Jun kinase (dJNK), which acts in part via the Hippo pathway and, more prominently, in parallel to it via activating the AP-1 transcription factor complex. As candidates of important transcriptional outputs of these signal pathways we identified Myc, which regulates dedifferentiation and fragmentation, and the Argonaute family protein Piwi, which appears necessary for the respecification of the fragmented myoblasts. Another lineage in which org-1 is expressed from embryonic to adult stages is the one derived from embryonic lateral adult muscle progenitors (AMPs), which proliferate during metamorphosis and were thought to give rise to the lateral set of lateral adult abdominal muscles, which continue to express org-1 and org-1 reporters. Analogously, ventral and dorsal AMPs expressing the muscle identity genes slouch and tailup were thought to give rise to ventral and dorsal adult abdominal muscles, respectively. To test these predictions and to determine whether these muscle identity genes are required for proper development of the respective set of adult abdominal muscles we examined these muscles in flies upon knock-downs and with mutations of org-1 and slouch. As we did not observe any phenotypes, we considered the possibility that AMPs from neighboring dorsoventral domains unaffected by the respective lesion could have the potential to compensate. Indeed, genetic lineage tracing of the org-1 and slouch AMP descendants demonstrated that there is lineage mixing. Because the descendants from ventral, lateral, and presumably dorsal AMPs each contribute to the forming abdominal muscles of all three dorsoventral muscle types during normal development we conclude that extrinsic signals play major roles in providing their particular identities.
Publications
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(2015). Org-1-dependent lineage reprogramming generates the ventral longitudinal musculature of the Drosophila heart. Curr Biol. 25, 488-494
Schaub, C., März, J., Reim, I., Frasch, M.
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(2016). Dedifferentiation, Redifferentiation, and Transdifferentiation of Striated Muscles During Regeneration and Development. Curr Top Dev Biol. 116, 331-355
Frasch, M.
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(2017). T-Box Genes in Drosophila Mesoderm Development. Curr Top Dev Biol. 122, 161-193
Reim, I., Frasch, M., Schaub, C.
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(2019). Yorkie and JNK revert syncytial muscles into myoblasts during Org-1 dependent lineage reprogramming
Schaub, C., Rose, M., Frasch, M.
