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E. Perl and J. S. Waxman
zebrafish embryos, cardiac progenitors arise from a more posterior region of the
ALPM, which in wild-type embryos normally harbors forelimb progenitors (Waxman et al. 2008). Moreover, we found that a continuous band of cells marked by the
gene encoding the transcription factor t-box 5 (tbx5) comprises a population of cardiac and forelimb progenitors that subsequently separate anteriorly and posteriorly
into their respective organ fields. RA signaling appears to partition the adjacent fates
within this tbx5
+ population, as all the tbx5
+ cells contribute to the heart field in RA
signaling deficient embryos, consistent with their posterior expansion.
Mechanistically, there is evidence that one of the primary functions of RA signaling in coordinating fate choice is through the repression of FGF signaling. In
aldh1a2/nls zebrafish and aldh1a2 mouse mutants, fgf8a and Fgf8, respectively,
which mark cardiac progenitors, are expanded, suggesting that this mechanism is
conserved across other vertebrates (Ryckebusch et al. 2008; Sirbu et al. 2008; Zhao
et al. 2009; Kumar and Duester 2014; Kumar et al. 2016). In zebrafish, inhibition of
Fgf8a is able to restore forelimb development in RA signaling-deficient embryos,
supporting the necessity of RA signaling to limit FGF signaling within the ALPM
for proper heart and forelimb development (Zhao et al. 2009; Sorrell and Waxman
2011).
While loss of RA signaling indicates there is a trade-off between the cardiac and
forelimb progenitor fields, increases in RA signaling affect fate decisions between
cardiac and adjacent anterior progenitors within the ALPM. For instance, lineagetracing in Cyp26-deficient embryos showed a modest increase in RA signaling is
capable of promoting an anterior shift in the cardiac field at the expense of more
anterior, adjacent cranial vasculature progenitors (Rydeen and Waxman 2014).
Altogether, the current data support the hypothesis that RA promotes fate decisions between the cardiac and adjacent anterior and posterior progenitor fields in the
ALPM, which is congruous with its larger role in A-P patterning along the body axis,
rather than directly between atrial and ventricular progenitors within a naïve cardiac
progenitor field.
Current State of the Field
Amalgamating the Consequences of RA Perturbation on Heart
Development
Although we still do not fully understand RA’s role as a morphogen within the
ALPM, given that it remains to be seen whether it might regulate specification of
the FHF, many of the more recent studies investigating the role of RA during heart
development have shifted away from manipulating RA levels in situ to investigating
how perturbations in its transport, synthesis, or degradation might influence cardiac
development. Similarly, much of our current understanding about how RA influences
cardiac development has been gained from utilizing better tools to manipulate its
E. Perl and J. S. Waxman
zebrafish embryos, cardiac progenitors arise from a more posterior region of the
ALPM, which in wild-type embryos normally harbors forelimb progenitors (Waxman et al. 2008). Moreover, we found that a continuous band of cells marked by the
gene encoding the transcription factor t-box 5 (tbx5) comprises a population of cardiac and forelimb progenitors that subsequently separate anteriorly and posteriorly
into their respective organ fields. RA signaling appears to partition the adjacent fates
within this tbx5
+ population, as all the tbx5
+ cells contribute to the heart field in RA
signaling deficient embryos, consistent with their posterior expansion.
Mechanistically, there is evidence that one of the primary functions of RA signaling in coordinating fate choice is through the repression of FGF signaling. In
aldh1a2/nls zebrafish and aldh1a2 mouse mutants, fgf8a and Fgf8, respectively,
which mark cardiac progenitors, are expanded, suggesting that this mechanism is
conserved across other vertebrates (Ryckebusch et al. 2008; Sirbu et al. 2008; Zhao
et al. 2009; Kumar and Duester 2014; Kumar et al. 2016). In zebrafish, inhibition of
Fgf8a is able to restore forelimb development in RA signaling-deficient embryos,
supporting the necessity of RA signaling to limit FGF signaling within the ALPM
for proper heart and forelimb development (Zhao et al. 2009; Sorrell and Waxman
2011).
While loss of RA signaling indicates there is a trade-off between the cardiac and
forelimb progenitor fields, increases in RA signaling affect fate decisions between
cardiac and adjacent anterior progenitors within the ALPM. For instance, lineagetracing in Cyp26-deficient embryos showed a modest increase in RA signaling is
capable of promoting an anterior shift in the cardiac field at the expense of more
anterior, adjacent cranial vasculature progenitors (Rydeen and Waxman 2014).
Altogether, the current data support the hypothesis that RA promotes fate decisions between the cardiac and adjacent anterior and posterior progenitor fields in the
ALPM, which is congruous with its larger role in A-P patterning along the body axis,
rather than directly between atrial and ventricular progenitors within a naïve cardiac
progenitor field.
Current State of the Field
Amalgamating the Consequences of RA Perturbation on Heart
Development
Although we still do not fully understand RA’s role as a morphogen within the
ALPM, given that it remains to be seen whether it might regulate specification of
the FHF, many of the more recent studies investigating the role of RA during heart
development have shifted away from manipulating RA levels in situ to investigating
how perturbations in its transport, synthesis, or degradation might influence cardiac
development. Similarly, much of our current understanding about how RA influences
cardiac development has been gained from utilizing better tools to manipulate its