Temporal regulation of the muscle gene cascade by Macho1 and Tbx6 transcription factors in Ciona intestinalis

Abstract

SummaryFor over a century, muscle formation in the ascidian embryo has been representative of 'mosaic' development. The molecular basis of muscle-fate predetermination has been partly elucidated with the discovery of Macho1, a maternal zinc-finger transcription factor necessary and sufficient for primary muscle development, and of its transcriptional intermediaries Tbx6b and Tbx6c. However, the molecular mechanisms by which the maternal information is decoded by cis-regulatory modules (CRMs) associated with muscle transcription factor and structural genes, and the ways by which a seamless transition from maternal to zygotic transcription is ensured, are still mostly unclear. By combining misexpression assays with CRM analyses, we have identified the mechanisms through which Ciona Macho1 (Ci-Macho1) initiates expression of Ci-Tbx6b and Ci-Tbx6c, and we have unveiled the cross-regulatory interactions between the latter transcription factors. Knowledge acquired from the analysis of the Ci-Tbx6b CRM facilitated both the identification of a related CRM in the Ci-Tbx6c locus and the characterization of two CRMs associated with the structural muscle gene fibrillar collagen 1 (CiFCol1). We use these representative examples to reconstruct how compact CRMs orchestrate the muscle developmental program from pre-localized ooplasmic determinants to differentiated larval muscle in ascidian embryos.

Genomic Strategies for Embryonic Development

“…Fig. 3.1(A3.2) completes the chain of causality with the cis-regulatory demonstration that the Macho1 transcription factor is required to activate a zygotic muscle regulatory gene, tbx6 (Kugler et al, 2010).…”

“…For example, the B5.1 blastomere gives rise to endoderm, mesenchyme, and tail muscle, the latter due to the Macho1 input. (A3.2), Cis-regulatory function of Macho1 in fifth and seventh cleavages B lineage muscle cell progenitors (from Kugler et al (2010) Ochoa-Espinosa et al (2005)). The mRNA is the product of the 15 polyploid nurse cells and during oogenesis is pumped into the oocyte through ring canals at the anterior end, where it is attached to the cytoskeleton until translation after fertilization.…”

The identification of transcription factors expressed in the notochord of Ciona intestinalis adds new potential players to the brachyury gene regulatory network

“…Mutation of Lmx1b leads to aberrant collagen fibril formation in mouse corneas (Pressman et al, 2000) and also to a decrease in the levels of type IV collagens in glomerular basement membranes of the kidney (Morello et al, 2001). Given the involvement of Lmx1b in the expression and maturation of collagen in different tissues and the expression of various collagen genes in the Ciona notochord (Wada et al, 2006;Kugler et al, 2010), it is possible that Ci-Lmx-like could play an analogous role in the formation of the notochordal sheath, a possibility that has yet to be explored in any chordate.…”

The maternal muscle determinant in the ascidian egg

“…The two consensus sequences do not match perfectly, but it is evident that macho‐1 binds to guanine‐rich sequences. The genome sequence of Ciona is available, and several candidate‐binding sites have been found in the 5' upstream regions of macho‐1 downstream genes 18…”