Muscle Cell Fate Choice Requires the T-Box Transcription Factor Midline in Drosophila

Abstract

Drosophila Midline (Mid) is an ortholog of vertebrate Tbx20, which plays roles in the developing heart, migrating cranial motor neurons, and endothelial cells. Mid functions in cell-fate specification and differentiation of tissues that include the ectoderm, cardioblasts, neuroblasts, and egg chambers; however, a role in the somatic musculature has not been described. We identified mid in genetic and molecular screens for factors contributing to somatic muscle morphogenesis. Mid is expressed in founder cells (FCs) for several muscle fibers, and functions cooperatively with the T-box protein H15 in lateral oblique muscle 1 and the segment border muscle. Mid is particularly important for the specification and development of the lateral transverse (LT) muscles LT3 and LT4, which arise by asymmetric division of a single muscle progenitor. Mid is expressed in this progenitor and its two sibling FCs, but is maintained only in the LT4 FC. Both muscles were frequently missing in mid mutant embryos, and LT4-associated expression of the transcription factor Krüppel (Kr) was lost. When present, LT4 adopted an LT3-like morphology. Coordinately, mid misexpression caused LT3 to adopt an LT4-like morphology and was associated with ectopic Kr expression. From these data, we concluded that mid functions first in the progenitor to direct development of LT3 and LT4, and later in the FCs to influence whichever of these differentiation profiles is selected. Mid is the first T-box factor shown to influence LT3 and LT4 muscle identity and, along with the T-box protein Optomotor-blind-related-gene 1 (Org-1), is representative of a new class of transcription factors in muscle specification. KEYWORDS midline; Tbx20; founder cell; muscle identity; differentiation T HE body wall muscles of the Drosophila larva are composed of a stereotypic pattern that includes 30 unique muscle fibers (each a single myotube) per abdominal hemisegment (Bate 1990). These muscles form during embryogenesis, during which time they take on unique characteristics such as size, shape, pattern of innervation, and epidermal attachment Baylies et al. 1998;Frasch 1999;De Joussineau et al. 2012). The unique identity of each muscle fiber is conferred through overlapping signaling pathways and cell autonomous transcription factors that specify muscle progenitors Baylies et al. 1998;Frasch 1999;Tixier et al. 2010), which then undergo asymmetric cell division to generate founder cells (FCs) (Bate 1990;Carmena et al. 1998). A single FC initiates the formation of multinucleate syncytia by fusing with fusion competent myoblasts (FCMs) (Rochlin et al. 2010;Abmayr and Pavlath 2012). FCs dictate the number of cell fusion events that take place, leading to a characteristic number of nuclei for each muscle in the pattern . These fusing nuclei adopt the transcriptional profile of the original FC nucleus (Bourgouin et al. 1992;D'Alessio and Frasch 1996;Ruiz-Gomez et al. 1997;Jagla et al. 1998;Keller et al. 1998;Nose et al. 1998;Crozatier and Vincent 1999;Knirr et al. 1999;D...

Intrinsic control of muscle attachment sites matching

“…This leads to a new model where iTF code refinement at each step of muscle identity specification, PMC >PC, PC >FC and FC >syncytial nuclei, is driven by a separate CRM. Distribution of the PC-identity information into two CRMs further supports the idea that iTF regulation at the PC stage is nodal to muscle identity specification ( Carmena et al, 1998 ; Dubois et al, 2016 ; Enriquez et al, 2012 ; Jagla et al, 2002 ; Nose et al, 1998 ; Kumar et al, 2015 ). Our former analysis started to decrypt the combinatorial control of each dorso-lateral muscle identity, which involves at least eight different iTFs, in addition to Nau/MRF [ Dubois et al, 2007 ].…”

A single-cell transcriptome atlas of the adult muscle precursors uncovers early events in fiber-type divergence inDrosophila

“…We also found lateral muscles scarcer (lms) expression restricted to DFM precursors, as previously reported (Muller et al, 2010). Other DFM-specific markers are genes whose roles were described in other types of muscles, including midline (mid) (Kumar et al, 2015), Anaplastic lymphoma kinase (Alk) (Englund et al, 2003;Lee et al, 2003) and wing blister (wb) (Martin et al, 1999) that are involved in embryonic myogenesis, and sprout (sty), which regulates maturation of adult founder cells in the abdomen (Dutta et al, 2005). Concordantly, the wb reporter is expressed at much higher levels in DFM precursors than in IFM precursors ( Figure 3E).…”

A cell atlas of adult muscle precursors uncovers early events in fibre‐type divergence in Drosophila

“…We also found lateral muscles scarcer ( lms ) expression restricted to DFM precursors, as previously reported (Muller et al , ). Other DFM‐specific markers are genes whose roles were described in other types of muscles, including midline ( mid ) (Kumar et al , ), Anaplastic lymphoma kinase ( Alk ) (Englund et al , ; Lee et al , ) and wing blister ( wb ) (Martin et al , ), which are involved in embryonic myogenesis, and sprout ( sty ), which regulates maturation of adult founder cells in the abdomen (Dutta et al , ). Finally, the panel of DFM markers included arginine kinase (Argk ), Neurotactin ( Nrt ), Amalgam ( Ama ) and Tenascin accessory ( Ten‐a ), whose function in muscle has not been investigated yet.…”