5 Retinoic Acid Signaling and Heart Development
129
specification in mouse embryocarcinoma and embryonic stem cells (Edwards and
McBurney 1983; Rudnicki and McBurney 1987; Osmond et al. 1991; Wobus et al.
1994; Yutzey et al. 1994). Thus, this propensity for high concentrations of RA to
abolish all CM specification is likely conserved in vertebrates.
It must be noted that, despite the consequences of high RA signaling, the effects
of increased RA signaling can be more nuanced, and the level of excess RA signaling
can cause dramatically different, seemingly paradoxical, effects on atrial and ventricular specification that are not the converse of a loss of RA signaling (Fig. 5.4). For
instance, treatment of zebrafish embryos with low concentrations of RA can promote
both atrial and ventricular specification (Fig. 5.4; D’Aniello et al. 2013). Treatment
of zebrafish embryos with progressively increasing, yet still intermediate, concentrations can independently affect atrial or ventricular specification, which parallels
the degree of induced posteriorization along the A-P axis across germ layers. Lowerintermediate concentrations of RA can promote atrial specification without affecting ventricular progenitor specification, while slightly higher concentrations start to
repress ventricular specification without affecting atrial specification (Fig. 5.4). Interestingly, in zebrafish, cardiac defects due to the genetic loss of the cytochrome p450
enzyme CYP26 proteins, Cyp26a1 and Cyp26c1, which are necessary to degrade RA
within the early embryo, recapitulate the defects observed with lower-intermediate
RA treatment (Hernandez et al. 2007; Uehara et al. 2007; Rydeen and Waxman 2014,
2016).
We have postulated that these concentration-dependent effects of RA on cardiac
specification are due to a flattening of RA signaling, since multiple studies have
shown that RA signaling acts as a morphogen within the zebrafish embryo (White
et al. 2007; Schilling et al. 2012; Shimozono et al. 2013; Samarut et al. 2015; Sosnik
et al. 2016),.
Mechanism Underlying RA-Mediated Restriction of Heart Field
Specification: Influence on Cardiac and Adjacent Progenitor Fields
Current data challenge the conclusion that RA directly affects the specification of
cardiac chamber progenitors themselves. Given the differential effects observed with
varying increases in embryonic RA signaling, one can understand how the initial
observations based on chamber morphology were interpreted as promoting atrial
identity at the expense of ventricular identity. Indeed, we do not interpret that RA
signaling promotes a tradeoff between atrial and ventricular identity within a cardiac
progenitor field. Rather, the restriction of the cardiac field by RA appears to be a
consequence of RA’s broader role in patterning the ALPM along the A-P embryonic
axis.
More recent studies have demonstrated that RA signaling regulates the specification of cardiac progenitors by influencing fate decisions between the anterior
cardiac progenitor and posterior forelimb progenitors fields (Waxman et al. 2008;
Zhao et al. 2009; Sorrell and Waxman 2011; Cunningham et al. 2013). Specifically,
caged-fluorescein mediated lineage tracing showed that in RA signaling-deficient
129
specification in mouse embryocarcinoma and embryonic stem cells (Edwards and
McBurney 1983; Rudnicki and McBurney 1987; Osmond et al. 1991; Wobus et al.
1994; Yutzey et al. 1994). Thus, this propensity for high concentrations of RA to
abolish all CM specification is likely conserved in vertebrates.
It must be noted that, despite the consequences of high RA signaling, the effects
of increased RA signaling can be more nuanced, and the level of excess RA signaling
can cause dramatically different, seemingly paradoxical, effects on atrial and ventricular specification that are not the converse of a loss of RA signaling (Fig. 5.4). For
instance, treatment of zebrafish embryos with low concentrations of RA can promote
both atrial and ventricular specification (Fig. 5.4; D’Aniello et al. 2013). Treatment
of zebrafish embryos with progressively increasing, yet still intermediate, concentrations can independently affect atrial or ventricular specification, which parallels
the degree of induced posteriorization along the A-P axis across germ layers. Lowerintermediate concentrations of RA can promote atrial specification without affecting ventricular progenitor specification, while slightly higher concentrations start to
repress ventricular specification without affecting atrial specification (Fig. 5.4). Interestingly, in zebrafish, cardiac defects due to the genetic loss of the cytochrome p450
enzyme CYP26 proteins, Cyp26a1 and Cyp26c1, which are necessary to degrade RA
within the early embryo, recapitulate the defects observed with lower-intermediate
RA treatment (Hernandez et al. 2007; Uehara et al. 2007; Rydeen and Waxman 2014,
2016).
We have postulated that these concentration-dependent effects of RA on cardiac
specification are due to a flattening of RA signaling, since multiple studies have
shown that RA signaling acts as a morphogen within the zebrafish embryo (White
et al. 2007; Schilling et al. 2012; Shimozono et al. 2013; Samarut et al. 2015; Sosnik
et al. 2016),.
Mechanism Underlying RA-Mediated Restriction of Heart Field
Specification: Influence on Cardiac and Adjacent Progenitor Fields
Current data challenge the conclusion that RA directly affects the specification of
cardiac chamber progenitors themselves. Given the differential effects observed with
varying increases in embryonic RA signaling, one can understand how the initial
observations based on chamber morphology were interpreted as promoting atrial
identity at the expense of ventricular identity. Indeed, we do not interpret that RA
signaling promotes a tradeoff between atrial and ventricular identity within a cardiac
progenitor field. Rather, the restriction of the cardiac field by RA appears to be a
consequence of RA’s broader role in patterning the ALPM along the A-P embryonic
axis.
More recent studies have demonstrated that RA signaling regulates the specification of cardiac progenitors by influencing fate decisions between the anterior
cardiac progenitor and posterior forelimb progenitors fields (Waxman et al. 2008;
Zhao et al. 2009; Sorrell and Waxman 2011; Cunningham et al. 2013). Specifically,
caged-fluorescein mediated lineage tracing showed that in RA signaling-deficient