Pitx2 is an upstream activator of extraocular myogenesis and survival

Abstract

The transcription factors required to initiate myogenesis in branchial arch- and somite-derived muscles are known, but the comparable upstream factors required during extraocular muscle development have not been identified. We show Pax7 is dispensable for extraocular muscle formation, whereas Pitx2 is cell-autonomously required to prevent apoptosis of the extraocular muscle primordia. The survival requirement for Pitx2 is stage-dependent and ends following stable activation of genes for the muscle regulatory factors (e.g. Myf5, MyoD), which is reduced in the absence of Pitx2. Further, PITX2 binds and activates transcription of the Myf5 and MyoD promoters, indicating these genes are direct targets. Collectively, these data demonstrate PITX2 is required at several steps in the development of extraocular muscles, acting first as an anti-apoptotic factor in pre-myogenic mesoderm, and subsequently to activate the myogenic program in these cells. Thus, Pitx2 is the first demonstrated upstream activator of myogenesis in the extraocular muscles.

Musculin and TCF21 coordinate the maintenance of myogenic regulatory factor expression levels during mouse craniofacial development

“…In the case of Pitx2 -/-, which has also been implicated in extraocular muscle specification, direct association with the Myf5 and MyoD minimal promoters has been shown in C2C12 cells and in a mouse extraocular muscle primary cell line (Zacharias et al, 2011). Our data allow us to place MSC (and probably TCF21) directly upstream of both Myf5 and MyoD in the genetic network controlling branchiomeric myogenesis.…”

Control of stomach smooth muscle development and intestinal rotation by transcription factor BARX1

“…Table 1); such TFs are ideal candidates for an intermediary role. Six2 in particular has important functions in myogenesis, acting upstream of Myod (Relaix et al, 2013a), with a well-defined requirement in forming distal stomach muscle (Self et al, 2009) and Pitx2 controls extra-ocular myogenesis (Zacharias et al, 2011a). Isl1 and Pitx2 are further implicated in abdominal laterality; both TFs are restricted to the left side and a complementary factor, Tbx18 , is confined to the right side (Davis et al, 2008; Kurpios et al, 2008; Logan et al, 1998; Ryan et al, 1998).…”

An eye on the head: the development and evolution of craniofacial muscles

“…Furthermore, Pitx2 has been shown to act downstream of Pax3 in somitic myogenesis (L'Honore et al, 2010). Pitx2 directly binds the promoters of Myf5 and Myod, and mice that lack Pitx2 in the mesoderm that gives rise to EOMs fail to develop these muscles (Zacharias et al, 2011). However, Pitx2 does not activate Myod expression in Myf5:Mrf4 double mutants, which lack EOMs, in spite of its continued expression in mutant EOM founder cells (Sambasivan et al, 2009), although it does so in limb muscle progenitors (L'Honore et al, 2010) suggesting that other co-factors play critical roles with Pitx2 in EOMs.…”

“…Therefore, these genes could act both cell autonomously, as well as non-cell autonomously, to affect muscle development. That these genes also act cell autonomously to control myogenesis in head mesoderm has been supported by conditional knockout studies in mice (Aggarwal et al, 2010;Dastjerdi et al, 2007;Dong et al, 2006;Grifone and Kelly, 2007;Zacharias et al, 2011). However, additional conditional gene ablation studies are needed to assess cell-cell interactions in regulation of muscle cell fates.…”