Fibroblast growth factor 10 gene regulation in the second heart field by Tbx1, Nkx2-5, and Islet1 reveals a genetic switch for down-regulation in the myocardium

Abstract

During cardiogenesis, Fibroblast Growth Factor (Fgf10) is expressed in the anterior second heart field. Together with Fibroblast growth factor 8 (Fgf8), Fgf10 promotes the proliferation of these cardiac progenitor cells that form the arterial pole of the heart. We have identified a 1.7-kb region in the first intron of Fgf10 that is necessary and sufficient to direct transgene expression in this cardiac context. The 1.7-kb sequence is directly controlled by T-box transcription factor 1 (Tbx1) in anterior second heart field cells that contribute to the outflow tract. It also responds to both NK2 transcription factor related, locus 5 (Nkx2-5) and ISL1 transcription factor, LIM/homeodomain (Islet1), acting through overlapping sites. Mutation of these sites reduces transgene expression in the anterior second heart field where the Fgf10 regulatory element is activated by Islet1 via direct binding in vivo. Analysis of the response to Nkx2-5 loss-and Isl1 gain-of-function genetic backgrounds indicates that the observed up-regulation of its activity in Nkx2-5 mutant hearts, reflecting that of Fgf10, is due to the absence of Nkx2-5 repression and to up-regulation of Isl1, normally repressed in the myocardium by Nkx2-5. ChIP experiments show strong binding of Nkx2-5 in differentiated myocardium. Molecular and genetic analysis of the Fgf10 cardiac element therefore reveals how key cardiac transcription factors orchestrate gene expression in the anterior second heart field and how genes, such as Fgf10, normally expressed in the progenitor cell population, are repressed when these cells enter the heart and differentiate into myocardium. Our findings provide a paradigm for transcriptional mechanisms that underlie the changes in regulatory networks during the transition from progenitor state to that of the differentiated tissue.mouse embryo | transcriptional regulation

A common Shox2-Nkx2-5 antagonistic mechanism primes the pacemaking cell fate in the pulmonary vein myocardium and sinoatrial node

“…However, although the inhibitory effect of Nkx2-5 over Isl1, Tbx3 and Hcn4 has been documented (Cambier et al, 2014;Mommersteeg et al, 2007b;Prall et al, 2007), the cis-regulatory elements associated with these genes that mediate such effects remain to be characterized. It is unclear whether Nkx2-5 inhibits the expression of these pacemaker genes directly, although it inhibits cardiac progenitor genes directly (Watanabe et al, 2012). In our current studies, we could not identify co-occupancy of Shox2 and Nkx2-5 on the regulatory elements of any of these genes.…”

Genetic Networks Governing Heart Development

“…Furthermore, in the absence of extracellular signals, enhancers occupied by positive TFs may be repressed owing to the presence of negative cofactors that modify the surrounding chromatin (default repression) (Barolo and Posakony 2002;Mbodj et al 2013). Cardiac-kernel TFs often act as both activators and repressors (Prall et al 2007;Watanabe et al 2012), and are themselves subject to signal-dependent posttranscriptional modifications affecting activity and stability (Liang et al 2001;Elliott et al 2010). …”

Akt1 signaling coordinates BMP signaling and β-catenin activity to regulate second heart field progenitor development

“…Lineage tracing and retrospective clonal analysis have identified two populations of cardiac progenitors during early mouse heart development (Kelly et al, 2001;Vincent and Buckingham, 2010;Watanabe et al, 2012). These two pools of cardiac progenitors are localized in the first heart field (FHF, or primary heart field) and the second heart field (SHF, or anterior heart field) (Bruneau, 2008;Harvey, 2002;Olson, 2006).…”

“…Starting at embryonic day (E) 8.5, the migration of SHF progenitors from the splanchnic mesoderm (SM) and pharyngeal mesoderm (PM) into the linear heart tube is essential for heart development in mice (Watanabe et al, 2012). Genetic studies in mice have revealed that disruption of SHF formation and migration severely impaired heart development.…”

“…SHF progenitors exhibit continued proliferation and a delay in differentiation (Srivastava, 2006;Watanabe et al, 2012). Fgf8 and Fgf10, the first molecular marker of the murine SHF, are the two important regulators that promote SHF proliferation (Ilagan et al, 2006;Kelly et al, 2001;Park et al, 2006;Watanabe et al, 2010Watanabe et al, , 2012.…”