Yorkie and JNK revert syncytial muscles into myoblasts during Org-1 dependent lineage reprogramming

Lineage reprogramming has become a prominent focus in research since it was demonstrated that lineage restricted transcription factors can be used in vitro for direct reprogramming [1]. Recently, we reported that the ventral longitudinal musculature (VLM) of the adult Drosophila heart arises in vivo by direct lineage reprogramming from alary muscles (AM), a process which starts with dedifferentiation and fragmentation of syncytial alary muscles into mononucleate myoblasts. Central upstream activators of the genetic program regulating the development of VLMs from alary muscles are the T-box factor Org-1 (Drosophila Tbx1) and the LIM homeodomain factor Tup (Drosophila Islet1) [2]. However, the events downstream of Org-1 and Tup that exert dedifferentiation and fragmentation of alary muscles have been unknown. In the present report, we shed light on the initiation of this first step of transdifferentiation and show that AM lineage specific activation of Yorkie (Yki), the transcriptional co-activator of the transcription factor Scalloped (Sd), has a key role in initiating AM lineage reprogramming. An additional necessary input comes from active dJNK signaling, which contributes to the inactivation of the Hippo kinase cascade upstream of Yki and furthermore activates dJun. The synergistic activities of the Yki/Sd and dJun/dFos (AP-1) transcriptional activator complexes in the absence of Hippo activity initiate AM dedifferentiation and lead to the expression of Myc and piwi, which are crucial for different aspects of AM transdifferentiation. Our results provide new insights into the mechanisms that mediate muscle lineage plasticity during a cellular reprogramming process occurring in vivo.\n\nHighlightsO_LIDirect lineage reprogramming of alary muscles depends on Yorkie and JNK\nC_LIO_LIYorkie and JNK mediate reversal of syncytial muscle cell fate\nC_LIO_LIYki/Sd and AP-1 induce alary muscle dedifferentiation synergistically\nC_LIO_LIYki dependent Myc induces and Piwi mediates reprogramming of alary muscles\nC_LI