Chamber identity programs drive early functional partitioning of the heart

Abstract

The vertebrate heart muscle (myocardium) develops from the first heart field (FHF) and expands by adding second heart field (SHF) cells. While both lineages exist already in teleosts, the primordial contributions of FHF and SHF to heart structure and function remain incompletely understood. Here we delineate the functional contribution of the FHF and SHF to the zebrafish heart using the cis-regulatory elements of the draculin (drl) gene. The drl reporters initially delineate the lateral plate mesoderm, including heart progenitors. Subsequent myocardial drl reporter expression restricts to FHF descendants. We harnessed this unique feature to uncover that loss of tbx5a and pitx2 affect relative FHF versus SHF contributions to the heart. High-resolution physiology reveals distinctive electrical properties of each heart field territory that define a functional boundary within the single zebrafish ventricle. Our data establish that the transcriptional program driving cardiac septation regulates physiologic ventricle partitioning, which successively provides mechanical advantages of sequential contraction.

An exclusive cellular and molecular network governs intestinal smooth muscle cell differentiation in vertebrates

“…Our data provide evidence that identifies the LPM as the lineage that gives rise to SMCs in the GI tract of zebrafish embryos by combining reporter transgene imaging and genetic lineage-tracing experiments using the LPM-expressed drl:creERT2 (Mosimann et al, 2015). Our lineage-tracing results provide the first genetic confirmation in vertebrate that smooth muscle cells in the gut region are derived from lateral mesodermal organ precursors.…”

“…The LPM is patterned early into distinct regions that will give rise to precursors of kidney, heart, endothelium, hematopoietic and limb cell fates (Davidson and Zon, 2004;Gering et al, 2003;Mosimann et al, 2015). Although previous work has suggested that iSMCs arise from the lateral plate mesoderm (LPM), genetic demonstration for this origin is still missing in a vertebrate model (Roberts et al, 1998).…”

“…, Tg(-6.4drl:creERT2) and ubi:Switch lines have been described previously (Mosimann et al, 2015;Mosimann and Zon, 2011;Ober et al, 2006;Rohr et al, 2006;Yin et al, 2010;Zhou et al, 2011;Alvers et al, 2014). The generation of the Tg(acta2:mCherry) uto5 and Tg(tagln:EGFP) uto37 lines is described below.…”

“…As a second and independent approach to link iSMCs to an LPM origin, we performed Cre/lox-mediated lineage tracing in the Tg(drl:creERT2) line, which uniquely expresses tamoxifeninducible Cre recombinase in all presumptive LPM precursors already during late epiboly (Mosimann et al, 2015). We crossed drl: creERT2 with the ubiquitous GFP-to-mCherry loxP lineage trace transgene ubi:Switch (Mosimann and Zon, 2011) and induced Cre activity at late epiboly/tailbud stages, when drl transgene expression is confined to presumptive LPM cells.…”

Early frameshift alleles of zebrafish <em>tbx5a</em> that fail to develop the heartstrings phenotype

“…We injected the sgRNA complexed with Cas9 protein as solubilized RNPs [11] at sub-optimal concentration to achieve viable mosaicism (see Methods for details) in the multicolor Tg(lmo2:dsRED2;drl:EGFP;myl7:mCyan) reporter background, subsequently abbreviated as RGB. In RGB embryos, dsRED2 labels endothelial, hematopoietic, and endocardial progenitors (lmo2) in red [14], EGFP marks all lateral plate mesoderm lineages (drl) including pectoral fins in green [15], and AmCyan reveals the differentiated cardiomyocytes (myl7 ) in blue [16]; consequently, RGB enables in vivo imaging of all cardiovascular and additional LPM lineages over the first 3 days of development. From F0 outcrosses that transmitted mutant tbx5a alleles, we genotyped adult F1 zebrafish for the presence of mutated tbx5a alleles by tail clipping, PCR, sequencing, and CrispRVariants analysis [17].…”