Prdm1‐ and Sox6‐mediated transcriptional repression specifies muscle fibre type in the zebrafish embryo

Abstract

The zebrafish u‐boot (ubo) gene encodes the transcription factor Prdm1, which is essential for the specification of the primary slow‐twitch muscle fibres that derive from adaxial cells. Here, we show that Prdm1 functions by acting as a transcriptional repressor and that slow‐twitch‐specific muscle gene expression is activated by Prdm1‐mediated repression of the transcriptional repressor Sox6. Genes encoding fast‐specific isoforms of sarcomeric proteins are ectopically expressed in the adaxial cells of ubotp39 mutant embryos. By using chromatin immunoprecipitation, we show that these are direct targets of Prdm1. Thus, Prdm1 promotes slow‐twitch fibre differentiation by acting as a global repressor of fast‐fibre‐specific genes, as well as by abrogating the repression of slow‐fibre‐specific genes.

Alkali‐like myosin light chain‐1 (myl1) is an early marker for differentiating fast muscle cells in zebrafish

“…5). This observation is in agreement with the established Prdm1 function in repressing the fast program in adaxial cells (von Hofsten et al, 2008), and further verifies myl1 as an excellent fast specific marker. Also, as a consequence of myod/myf5 down regulation, myl1 expression was depleted (Fig.…”

“…Staging of embryos (up to the 15-somite stage) was done by counting the number of somites as in Kimmel et al (1995). The transgenic line Tg(mylz2:GFP) i135 was previously described in von Hofsten et al (2008). Embryos older than the 15-somite stage were staged by hours or days post-fertilization development in 28.5 C. Embryos older than 1 dpf were treated with 0.003% phenylthiourea (PTU) to prevent pigmentation.…”

“…The muscle precursors lateral to the adaxial cells will become the fasttwich fibres. Both slow and fast muscle precursors require the expression of the muscle regulatory factors, such as MyoD, Myf5, and myogenin (Coutelle et al, 2001;Groves et al, 2005;Hammond et al, 2007;Hinits et al, 2009), and the decision to become fast or slow is determined by the transcriptional repressor Prdm1, which is induced by hedgehog and acts by directly repressing genes that encode fast fibre-specific sarcomeric proteins in the adaxial cells (Roy et al, 2001, Baxendale et al, 2004von Hofsten et al, 2008). Prdm1 is also repressing the expression of sox6 in the adaxial cells and Sox6 is in turn repressing the slow specific genes in the fast domain (von Hofsten et al, 2008).…”

“…Also, a commonly used protein marker for fast muscle fibres is the F310 antibody (DSHB), which was originally generated by Crow and Stockdale (1986). The antibody recognizes the mouse myosin light chain 1 and 3f (LC1f/3f) and has been used as a marker for fast fibres in several studies, including those in zebrafish (Kelly et al, 1997;Zammit et al, 2008;Elworthy et al, 2008;Liew et al, 2008;von Hofsten et al, 2008), where it can be detected at the 15-somite stage and onwards and coincides with fast fibre formation Liew et al, 2008).…”

Myotome adaptability confers developmental robustness to somitic myogenesis in response to fibre number alteration

“…Recombineered BAC transgenic. pfeffer tm236b (Odenthal et al, 1996) Mutation in csf1ra, which reduces the number of Pax7-expressing xanthophores Tg(pax7a:EGFP) t32239Tg ;pfe tm236/tm236 (Alsheimer, 2012) Bred onto pfeffer tm236b to aid observation of MPCs Tg(−2.2mylz2:GFP) i135 (von Hofsten et al, 2008) Marks fast muscle fibres …”