The homeobox gene Arx is a novel positive regulator of embryonic myogenesis

Abstract

Skeletal muscle fibers form in overlapping, but distinct phases that depend on the generation of temporally different lineages of myogenic cells. During primary myogenesis (E10.5-E12.5 in the mouse), embryonic myoblasts fuse homotypically to generate primary fibers, whereas during later development (E14.5-E17.5), fetal myoblasts differentiate into secondary fibers. How these myogenic waves are regulated remains largely unknown. Studies have been hampered by the lack of markers which would distinguish embryonic from fetal myoblast populations. We show here that the homeobox gene Arx is strongly expressed in differentiating embryonic muscle, downstream of myogenic basic helix-loop-helix (bHLH) genes. Its expression progressively decreases during development. When overexpressed in the C2C12 myogenic cell line, Arx enhances differentiation. Accordingly, it stimulates the transcriptional activity from the Myogenin promoter and from multimerized E-boxes when co-expressed with MyoD and Mef2C in CH310T1/2. Furthermore, Arx co-immunoprecipitates with Mef2C, suggesting that it participates in the transcriptional regulatory network acting in embryonic muscle. Finally, embryonic myoblasts isolated from Arx-deficient embryos show a delayed differentiation in vivo together with an enhanced clonogenic capacity in vitro. We propose here that Arx acts as a novel positive regulator of embryonic myogenesis by synergizing with Mef2C and MyoD and by establishing an activating loop with Myogenin.

Caenorhabditis elegans aristaless/Arx gene alr-1 restricts variable gene expression

“…Aristaless-like proteins are thought to act as either transcriptional activators or transcriptional repressors (29,30,(33)(34)(35)(36)(37). In C. elegans, Melkman and Sengupta (20) found that alr-1 blocked expression of the DD motor neuron marker flp-13 in VD motor neurons and both down-regulated and up-regulated lin-11 expression in the AWA and ASG neurons, respectively.…”

Conserved and divergent functions of Nfix in skeletal muscle development during vertebrate evolution

“…6G-I), these secondary fibers do not form in nfixa-MO-injected embryos at 3 dpf, whereas they are present in rescued embryos. Moreover, we injected embryos with nfixa-MO and treated them with cyclopamine, which inhibits Hh signaling and blocks primary slow fiber formation (Biressi et al, 2008;Chen et al, 2002). Cyclopamine-treated control embryos normally developed secondary slow muscle fibers at the dorsal and ventral extremes of the myotome, whereas in nfixa-MO-injected embryos the secondary fibers were not formed.…”

ARX/Arx is expressed in germ cells during spermatogenesis in both marsupial and mouse

“…Subsequently, ARX orthologs were detected in the forebrain of chickens (Cobos et al 2005), Xenopus (El-Hodiri et al 2003, Kelly et al 2005, Seufert et al 2005, and Caenorhabditis elegans (Melkman & Sengupta 2005). The ARX protein contains a Q (50) paired-type homeodomain, a C-terminal aristaless domain, and an N-terminal octapeptide and acts as a nuclear transcriptional regulator to activate or inhibit gene expression (Seufert et al 2005, McKenzie et al 2007, Biressi et al 2008, Fullenkamp & El-Hodiri 2008, Colasante et al 2009). The ARX gene encodes a 562-amino acid protein (Ohira et al 2002, Stromme et al 2002 with four characteristic polyalanine (PolyA) tracts where most of the ARX mutations occur (Gecz et al 2006).…”

“…Deficiency of Arx results in a loss of mature endocrine a-cells with a concomitant increase in the numbers of b-and d-cells (Collombat et al 2003), suggesting that Arx is required for the specification and differentiation of a-cells (Collombat et al 2003. Arx is strongly expressed in differentiating embryonic muscle and is progressively decreased during myogenesis, acting as a positive regulator of the differentiation and specification of skeletal muscle fibers (Biressi et al 2008).…”

“…The X-linked aristaless homeobox gene, ARX, belongs to the family of paired-type homeodomain transcription factors that have essential roles in the development of the forebrain, olfactory bulb, pancreas, skeletal muscle, and gonads in mice and humans (Kitamura et al 2002, Heller et al 2005, Yoshihara et al 2005, Biressi et al 2008. ARX was initially identified as an important regulator of forebrain development in mice and zebrafish (Schneitz et al 1993, Miura et al 1997.…”