5 Retinoic Acid Signaling and Heart Development
135
indicating that there is some functional redundancy between these enzymes (Uehara
et al. 2007).
The earliest cardiac defects observed in cyp26-deficient zebrafish embryos is
an increase in atrial specification at the expense of adjacent anterior endothelial
progenitors due to an anterior shift in the cardiac progenitor field residing in the
ALPM, as has been covered above (Rydeen and Waxman 2014). However, OFT
development is highly sensitive to increases in RA. We have also shown that CYP26
deficiency can produce OFT defects independent of any early patterning defects
resulting from: (1) a failure of SHF progenitors to join the OFT, instead contributing
to the pharyngeal arch arteries, and (2) a loss of differentiated FHF ventricular CMs
due to disrupted cell polarity and extrusion from the heart tube due to changes in the
extracellular matrix (Rydeen and Waxman 2016). Therefore, while RA degradation
is necessary to promote proper patterning of cardiac and vascular progenitors, OFT
development is particularly sensitive to even modest increases in embryonic RA.
Contribution of Retinoic Acid Receptors Defects to Cardiac
Defects
RA is a ligand for nuclear receptors (RARs) that produce a direct transcriptional output. Canonical RA signaling is integrated through RARs that, upon heterodimerization with retinoid X receptors (RXRs), bind retinoic acid response elements (RAREs)
located in the promoters of target genes (Samarut et al. 2015). In mice, there are 3
RAR subtypes (-α, -β, and -γ), each with numerous isoforms.
There appears to be significant functional redundancy between RARs, given that
mutation of a single isoform in mice does not result in cardiac defects. However, double KOs of RARα with RARβ, RARγ or RXRα exhibit a range of OFT defects, including double outlet right ventricle (DORV), persistent truncus arteriosus (PTA), atrioventricular septal defects (AVSDs), and aortic arch defects (Fig. 5.5 and Table 5.1)
(Mendelsohn et al. 1994; Dickman et al. 1997; Ghyselinck et al. 1998; Dupé et al.
1999; Li et al. 2010). Interestingly, initial studies with RXRα KO mice indicated
their ventricular CMs undergo premature differentiation, which was partially recapitulated in RARα KO mice with VAD (Li et al. 1993; Sucov et al. 1994; Kastner
et al. 1994; Gruber et al. 1996; Kastner et al. 1997; Chen et al. 1998).
More recent work examining the OFT defects of RARα; RARβ double mutants
have shown that inappropriate TGF-β signaling accounts for the septation defects as
well as the preceding failure of the SHF to differentiate (Li et al. 2010). Interestingly,
ectopic expression of RA signaling
leads to defects in OFT cushion development via the direct repression of tbx2—
and consequently TGF-β signaling (Sakabe et al. 2012). Together, these findings may
support a broad antagonistic relationship between RA and TGF-β signaling in OFT
development.
135
indicating that there is some functional redundancy between these enzymes (Uehara
et al. 2007).
The earliest cardiac defects observed in cyp26-deficient zebrafish embryos is
an increase in atrial specification at the expense of adjacent anterior endothelial
progenitors due to an anterior shift in the cardiac progenitor field residing in the
ALPM, as has been covered above (Rydeen and Waxman 2014). However, OFT
development is highly sensitive to increases in RA. We have also shown that CYP26
deficiency can produce OFT defects independent of any early patterning defects
resulting from: (1) a failure of SHF progenitors to join the OFT, instead contributing
to the pharyngeal arch arteries, and (2) a loss of differentiated FHF ventricular CMs
due to disrupted cell polarity and extrusion from the heart tube due to changes in the
extracellular matrix (Rydeen and Waxman 2016). Therefore, while RA degradation
is necessary to promote proper patterning of cardiac and vascular progenitors, OFT
development is particularly sensitive to even modest increases in embryonic RA.
Contribution of Retinoic Acid Receptors Defects to Cardiac
Defects
RA is a ligand for nuclear receptors (RARs) that produce a direct transcriptional output. Canonical RA signaling is integrated through RARs that, upon heterodimerization with retinoid X receptors (RXRs), bind retinoic acid response elements (RAREs)
located in the promoters of target genes (Samarut et al. 2015). In mice, there are 3
RAR subtypes (-α, -β, and -γ), each with numerous isoforms.
There appears to be significant functional redundancy between RARs, given that
mutation of a single isoform in mice does not result in cardiac defects. However, double KOs of RARα with RARβ, RARγ or RXRα exhibit a range of OFT defects, including double outlet right ventricle (DORV), persistent truncus arteriosus (PTA), atrioventricular septal defects (AVSDs), and aortic arch defects (Fig. 5.5 and Table 5.1)
(Mendelsohn et al. 1994; Dickman et al. 1997; Ghyselinck et al. 1998; Dupé et al.
1999; Li et al. 2010). Interestingly, initial studies with RXRα KO mice indicated
their ventricular CMs undergo premature differentiation, which was partially recapitulated in RARα KO mice with VAD (Li et al. 1993; Sucov et al. 1994; Kastner
et al. 1994; Gruber et al. 1996; Kastner et al. 1997; Chen et al. 1998).
More recent work examining the OFT defects of RARα; RARβ double mutants
have shown that inappropriate TGF-β signaling accounts for the septation defects as
well as the preceding failure of the SHF to differentiate (Li et al. 2010). Interestingly,
ectopic expression of RA signaling
leads to defects in OFT cushion development via the direct repression of tbx2—
and consequently TGF-β signaling (Sakabe et al. 2012). Together, these findings may
support a broad antagonistic relationship between RA and TGF-β signaling in OFT
development.