RESEARCH HIGHLIGHTS
Developmental biology Slow twitch, fast twitch
Zebrafish study reveals a gene regulatory network responsible for establishing and maintaining muscle fiber lineage
Understanding how cells maintain their determined state once specified during embryogenesis is a major aim of developmental biology. Philip Ingham at the A*STAR Institute of Molecular and Cell Biology and co-workers have uncovered a gene regula- tory network underlying the stability of skeletal muscle fiber- type differentiation1
. During early development, cells derived from a tissue layer
called mesoderm develop into muscle progenitor cells, which subsequently differentiate into two distinct muscle-fiber types: slow-twitch fibers, which contract slowly and work for a long time without getting tired; and fast-twitch fibers, which contract quickly but use more energy and tire quickly. The transcriptional regulator Sox6 plays an important role in
fast-twitch fiber specification. In the zebrafish embryo, transcrip- tion of Sox6 is specifically repressed in slow-twitch progenitors by the transcription factor Prdm1a, the gene for which is in turn activated by the signalling molecule Sonic hedgehog (Shh) secreted from the notochord, the flexible rod-shaped precursor of the inter-vertebral discs found in all vertebrate embryos. Notably, the expression of Prdm1a is only transient, raising the question as to how Sox6 repression is maintained in slow-twitch fibers post-embryonically. “We took advantage of the high resolution imaging afforded by the zebrafish to analyze the spatial regulation of Sox6 expression,” says Ingham. By generating transgenic zebrafish lines, the researchers identi-
fied regulatory sequences upstream of the Sox6 gene that play a crucial role in determining whether muscle progenitors differenti- ate into fast- or slow-twitch muscle fibers. They showed that this regulatory element specifically drives Prdm1a-dependent Sox6 expression in fast-twitch progenitors. However, the same element became transcriptionally active in the slow-twitch fibers of larval stages. Further experiments revealed that the endogenous Sox6 gene is also activated in larval slow-twitch fibers, whereas the Sox6 protein remains restricted to fast-twitch fibers. “This surprising finding suggested to us that the expression
of Sox6 is regulated partly at the level of translation rather than just transcriptionally as expected,” says Ingham. The research- ers next showed that Prdm1a induces the specific expression
A*STAR RESEARCH OCTOBER 2011– MARCH 2012
Cross section through a 6-day-old transgenic zebrafish larva showing the fast twitch specific expression of GFP under the control of the endogenous Sox6 promoter (transcriptional) and 3’UTR sequences (translational); the slow twitch fibers are outlined in red; the nuclei are stained blue.
in slow-twitch progenitors of a microRNA molecule encoded by a non-coding region of another gene. They found that this microRNA, called miR-499, in turn repressed the translation of Sox6 in these cells. “This ‘double-hit’ mechanism involving both transcriptional
and translational repression, sets up a feedback loop that ensures Sox6 is not expressed even after Prdm1a protein dissipates, thereby maintaining as well as initiating the slow-twitch muscle lineage,” says Ingham. “Similar mechanisms are likely to control the specification of other cell types.”
■
1. Wang, X. et al. Prdm1a and miR-499 act sequentially to restrict Sox6 activity to the fast twitch muscle lineage in the zebrafish embryo. Development 138, 4399–4404 (2011).
5
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96