Isl2b regulates anterior second heart field development in zebrafish

Abstract

After initial formation, the heart tube grows by addition of second heart field progenitor cells to its poles. The transcription factor Isl1 is expressed in the entire second heart field in mouse, and Isl1-deficient mouse embryos show defects in arterial and venous pole development. The expression of Isl1 is conserved in zebrafish cardiac progenitors; however, Isl1 is required for cardiomyocyte differentiation only at the venous pole. Here we show that Isl1 homologues are expressed in specific patterns in the developing zebrafish heart and play distinct roles during cardiac morphogenesis. In zebrafish, isl2a mutants show defects in cardiac looping, whereas isl2b is required for arterial pole development. Moreover, Isl2b controls the expression of key cardiac transcription factors including mef2ca, mef2cb, hand2 and tbx20. The specific roles of individual Islet family members in the development of distinct regions of the zebrafish heart renders this system particularly well-suited for dissecting Islet-dependent gene regulatory networks controlling the behavior and function of second heart field progenitors in distinct steps of cardiac development.

Continuous addition of progenitors forms the cardiac ventricle in zebrafish

“…At 26 hpf, when the differentiating cardiomyocytes in the linear heart tube represent FHF derivatives (Mosimann et al, 2015;de Pater et al, 2009), we detected tbx1 reporter expression in most of the differentiated ventricular cardiomyocytes and additionally in two MHC-negative domains at the IFT and OFT (Figure 2A). At 26 hpf, we detected tbx1:EGFP;Isl1 double-positive cells at the IFT of the linear heart tube ( Figure 2B,C), consistent with inferred SHF identity of IFT cells (Caputo et al, 2015;de Pater et al, 2009;Witzel et al, 2017).…”

“…tbx1 reporter-expressing cells at the IFT concomitantly expressed Isl1, confirming their SHF signature ( Figure 2B,C). We did not detect any Isl1 expression at the OFT, in accordance with other studies in zebrafish implicating Isl1 only in IFT development (Caputo et al, 2015;de Pater et al, 2009;Witzel et al, 2017). While we here focused on the development of the arterial pole, these observations warrant application of our tbx1 reporter transgenics for the elucidation of zebrafish IFT formation.…”

“…The zebrafish heart therefore recapitulates key processes of multi-chambered heart formation (Bakkers, 2011). 3 Lineage tracing, several molecular markers including ltbp3, isl1, and mef2c, or the maturation speed of fluorescent reporters have been used to describe late differentiating SHF-like myocardium (Hami et al, 2011;Lazic and Scott, 2011;de Pater et al, 2009;Witzel et al, 2017Witzel et al, , 2012Zhou et al, 2011).…”

Cardiac septation in heart development and evolution

“…The ltbp3 ‐positive cells give rise to OFT and the distal ventricle, and the knockdown of ltbp3 causes multi‐lineage cardiovascular defects in the ventricle and OFT. The SHF in zebrafish is regulated by tbx1 and isl2b , indicating that the molecular mechanisms of SHF development are conserved between zebrafish and mammals (Nevis et al., ; Witzel, Cheedipudi, Gao, Stainier, & Dobreva, ). A recent study showed that in zebrafish tbx5 and pitx2 , key regulators of ventricular septum, control the relative contribution of FHF and SHF to the heart tube, and a correct ratio of each heart field derived‐cells and the establishment of precise physiologic boundary between these two fates are important for normal cardiac function (Mosimann et al., ).…”

Loss of embryonic neural crest cardiomyocytes causes adult hypertrophic cardiomyopathy

“…In contrast, atrial NC-Cm spatial distribution did not appear as stereotypical as the ventricle. NC-Cm contribution to the inflow tract led us to ask whether the NC-Cms are also integrated into the secondary heart field and whether they contribute to the pacemaker cells of the developing heart 10 . The secondary heart field marker Isl1/2 co-localized with NC-Cms in the proximal ventricle area and a single cell at the inflow tract, suggesting that NC-Cms may also contribute to a subset of pacemaker cells (Extended Fig.2).…”