5 Retinoic Acid Signaling and Heart Development
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increased proliferation and differentiation of SHF progenitors (Bertrand et al. 2011).
Although Hoxa1 KO mice do not have overt cardiac defects, loss of a single Hoxb1
allele in Hoxa1 mutants produces OFT defects similar to Hoxb1 alone. While loss of
Hox genes produces SHF defects consistent with RA signaling, we have found that in
zebrafish embryos, excess Hox gene expression through overexpression of Hoxb5b, a
direct RA target, can eliminate CMs through posteriorization of the ALPM, which is
similar to excess RA treatment (Waxman et al. 2008; Waxman and Yelon 2009). Thus,
Hox genes are key mediators of RA signaling that promote proper heart development.
Although Hox genes are key effectors of RA, their loss thus far cannot explain
the full spectrum of defects found from loss or gain of RA signaling. Hence, other
effectors within a RA signaling-dependent gene regulatory network must mediate
aspects of RA signaling that regulate cardiac progenitor development. Factors that
may mediate RA signaling are Nuclear receptor subfamily 2 group F proteins (Nr2f;
formerly called Chicken ovalbumin upstream promoting transcription factors (CoupTF)), which are orphan nuclear receptors. Nr2f1 and Nr2f2 have been shown to have
conserved responsiveness across vertebrate phyla (Jonk et al. 1994; van der Wees
et al. 1996; Pereira et al. 2000; Lin et al. 2011; Love and Prince 2012; Laursen
et al. 2013; Dohn et al. 2019). Our recent study of Nr2f1a and Nr2f2 redundancy
in zebrafish indicates that they may have requirements within the ALPM restricting
CM specification (Dohn et al. 2019). A requirement for Nr2f2 in establishing atrial
identity has been known for two decades, as Nr2f2 KO mice have atrial differentiation defects (Pereira et al. 1999; Wu et al. 2013). Furthermore, in the differentiation
of atrial-like cells from human embryonic stem cells, NR2F1 and NR2F2 are upregulated upon treatment with exogenous RA (Devalla et al. 2015). While RA is part of
protocols used to differentiate atrial-like cells from embryonic and human inducedpluripotent stem cells (Gassanov et al. 2008; Zhang et al. 2011; Devalla et al. 2015),
as discussed above, a strict requirement for RA in atrial development from animal
models has yet to support this role. Collectively, these data imply that an RA-Nr2f
regulatory network may be part of the regulatory network that limits cardiac specification within the ALPM and other contexts, such as the induction of atrial-like cells
in cell culture.
Effectors of RA Signaling During Later Aspects of Heart Development
Much of what has been highlighted here focuses on RA signaling’s role in early patterning of the cardiac progenitor field within the ALPM and OFT defects resulting
from improper SHF development. However, RA signaling has reiterative requirements in heart development, with a key role in formation of the epicardium, diversification of epicardial-derived fibroblasts, and consequent myocardial expansion
(Moss et al. 1998; Niederreither et al. 2001; von Gise et al. 2011; Braitsch et al.
2012; El Robrini et al. 2016).
In the epicardium of mouse and quail embryos, Aldh1a2 is observed by E11.5
and stage 18, respectively (Moss et al. 1998; Xavier-Neto et al. 2000). In chicken
embryos, maintenance of both Wilm’s Tumor Gene 1 (wt1) and aldh1a2 expression
137
increased proliferation and differentiation of SHF progenitors (Bertrand et al. 2011).
Although Hoxa1 KO mice do not have overt cardiac defects, loss of a single Hoxb1
allele in Hoxa1 mutants produces OFT defects similar to Hoxb1 alone. While loss of
Hox genes produces SHF defects consistent with RA signaling, we have found that in
zebrafish embryos, excess Hox gene expression through overexpression of Hoxb5b, a
direct RA target, can eliminate CMs through posteriorization of the ALPM, which is
similar to excess RA treatment (Waxman et al. 2008; Waxman and Yelon 2009). Thus,
Hox genes are key mediators of RA signaling that promote proper heart development.
Although Hox genes are key effectors of RA, their loss thus far cannot explain
the full spectrum of defects found from loss or gain of RA signaling. Hence, other
effectors within a RA signaling-dependent gene regulatory network must mediate
aspects of RA signaling that regulate cardiac progenitor development. Factors that
may mediate RA signaling are Nuclear receptor subfamily 2 group F proteins (Nr2f;
formerly called Chicken ovalbumin upstream promoting transcription factors (CoupTF)), which are orphan nuclear receptors. Nr2f1 and Nr2f2 have been shown to have
conserved responsiveness across vertebrate phyla (Jonk et al. 1994; van der Wees
et al. 1996; Pereira et al. 2000; Lin et al. 2011; Love and Prince 2012; Laursen
et al. 2013; Dohn et al. 2019). Our recent study of Nr2f1a and Nr2f2 redundancy
in zebrafish indicates that they may have requirements within the ALPM restricting
CM specification (Dohn et al. 2019). A requirement for Nr2f2 in establishing atrial
identity has been known for two decades, as Nr2f2 KO mice have atrial differentiation defects (Pereira et al. 1999; Wu et al. 2013). Furthermore, in the differentiation
of atrial-like cells from human embryonic stem cells, NR2F1 and NR2F2 are upregulated upon treatment with exogenous RA (Devalla et al. 2015). While RA is part of
protocols used to differentiate atrial-like cells from embryonic and human inducedpluripotent stem cells (Gassanov et al. 2008; Zhang et al. 2011; Devalla et al. 2015),
as discussed above, a strict requirement for RA in atrial development from animal
models has yet to support this role. Collectively, these data imply that an RA-Nr2f
regulatory network may be part of the regulatory network that limits cardiac specification within the ALPM and other contexts, such as the induction of atrial-like cells
in cell culture.
Effectors of RA Signaling During Later Aspects of Heart Development
Much of what has been highlighted here focuses on RA signaling’s role in early patterning of the cardiac progenitor field within the ALPM and OFT defects resulting
from improper SHF development. However, RA signaling has reiterative requirements in heart development, with a key role in formation of the epicardium, diversification of epicardial-derived fibroblasts, and consequent myocardial expansion
(Moss et al. 1998; Niederreither et al. 2001; von Gise et al. 2011; Braitsch et al.
2012; El Robrini et al. 2016).
In the epicardium of mouse and quail embryos, Aldh1a2 is observed by E11.5
and stage 18, respectively (Moss et al. 1998; Xavier-Neto et al. 2000). In chicken
embryos, maintenance of both Wilm’s Tumor Gene 1 (wt1) and aldh1a2 expression