Transcriptional autoregulation of zebrafish tbx6 is required for somite segmentation

Abstract

The presumptive somite boundary in the presomitic mesoderm (PSM) is defined by the anterior border of the expression domain of Tbx6 protein. During somite segmentation, the expression domain of Tbx6 is regressed by Ripply-meditated degradation of Tbx6 protein. Although the expression of zebrafish tbx6 remains restricted to the PSM, the transcriptional regulation of tbx6 remains poorly understood. Here, we show that the expression of zebrafish tbx6 is maintained by transcriptional autoregulation. We find that a proximallocated cis-regulatory module, TR1, which contains two putative Tbox sites, is required for somite segmentation in the intermediate body and for proper expression of segmentation genes. Embryos with deletion of TR1 exhibit significant reduction of tbx6 expression at the 12-somite stage, although its expression is initially observed. Additionally, Tbx6 is associated with TR1 and activates its own expression in the anterior PSM. Furthermore, the anterior expansion of tbx6 expression in ripply gene mutants is suppressed in a TR1dependent manner. The results suggest that the autoregulatory loop of zebrafish tbx6 facilitates immediate removal of Tbx6 protein through termination of its own transcription at the anterior PSM.

Loss of Fgf-responsive Pea3 transcription factors results in ciliopathy-associated phenotypes during early zebrafish development

“…Indeed, we found similar defects in somite boundary formation in our 3etv mutants that in some cases lead to the fusion of adjacent somites, a phenotype identical to that caused by blocking FGF signaling. This fused somite phenotype is also similar to phenotypes observed in ripply1, ripply2, and tbx6 mutants (Ban et al, 2019;Kinoshita et al, 2018;Windner et al, 2015), suggesting that FGF is acting through etv4, etv5a, and etv5b to regulate the expression these genes. Consistent with this, we found that expression of ripply1, ripply2 and tbx6 are down regulated in 3etv mutants, providing a genetic link between 3Etv function and somite boundary formation.…”

“…4B). Transcription factors tbx6, ripply1, and ripply2 are critical for the formation of distinct somites; in the absence of these factors xirp2a expression is severely disrupted at somite boundaries and adjacent somites are fused together (Ban et al, 2019;Kinoshita et al, 2018). To see if the somite fusion in 3etv mutants could be attributed to reduced expression of tbx6 or ripply1/2 genes we looked at their expression in our 10 ss RNA-seq dataset and found that these genes were also reduced in 3etv mutants compared to wild-type ( Fig.…”

Cell Rearrangement Generates Pattern Emergence as a Function of Temporal Morphogen Exposure

“…Under these culture conditions, cells downregulated expression of tbx16 and upregulated their expression of tbx6 over a period of 6 hours, as measured by HCR on fixed timepoint samples imaged at high magnification (Figure 1C). Further examination using a tbx6::GFP reporter Ban et al (2019) revealed that GFP fluorescence increased in only a proportion of cells and that the GFP signal appeared synchronously after approximately 200 minutes (3.5 hours) in culture (Figure 1D,E).…”

Role of somite patterning in the formation of Weberian apparatus and pleural rib in zebrafish

“…5A,C,E). Although zebrafish tbx6 À/À embryos lack somite boundaries (van Eeden et al 1996;Nikaido et al 2002), tbx6 À/À larvae possessed irregular but robust myotome boundaries (van Eeden et al 1996;Windner et al 2012;Windner et al 2015;Ban et al 2019), which are thought to be recovered by the medial-to-lateral migration of slow muscle progenitor cells (Devoto et al 1996;Henry et al 2005). By comparing the ossified ribs with the myotomes, we found that the irregular patterning of distal ribs in tbx6 À/À larvae coincided well with the irregular myotome boundaries (Fig.…”

“…In mice and chicken, cells in the rostral and caudal halves within a somite are known to subsequently differentiate into different regions of the vertebral column (Aoyama & Asamoto, ; Saga & Takeda, ). In tbx6 ‐deficient zebrafish embryos, the rostral half property is lost, and instead, the caudal half property is expanded (caudalization) (Nikaido et al ; Ban et al ). Although subsequent differentiation of rostral and caudal somitic cells into the future vertebrae is not strictly lineage‐restricted in zebrafish (Morin‐Kensicki et al ), our results suggest that the rostral region of a somite preferentially contributes to the formation of anterior vertebrae including tripus, lateral processes and transformer process of vertebra 4, and further suggest the differential contribution of the rostral and caudal compartments of a somite to the anterior vertebrae in zebrafish.…”