5 Retinoic Acid Signaling and Heart Development
127
an expansion of AMHC1-expressing cells (Osmond et al. 1991; Yutzey et al. 1994).
Zebrafish treated with RA also appeared to have smaller ventricles (Stainier and
Fishman 1992). Furthermore, a single pulse of RA given to female mice at 7.5 dayspost coitus induced malformations ranging from reduced ventricle size to complete
absence of the OFT and ventricles in their offspring (Xavier-Neto et al. 1999). In
vitro studies also suggested that moderate concentrations (typically around 0.1 μM)
of RA preferentially induced stem cells to differentiate into cells with an atrial identity
(marked by atrial-specific ion channels) (Gassanov et al. 2008; Zhang et al. 2011;
Devalla et al. 2015).
A Second Hypothesis: Restriction of Heart Field Specification
The simplicity of the hypothesis described in the previous section makes it an attractive model for describing the role of RA signaling in cardiac progenitor specification.
However, new knowledge synthesized from the generation of new ALPM fate maps
in additional vertebrate models, the availability of new tools for precise counting
of CMs, and the examination of FHF and SHF markers provide compelling support
for an alternative hypothesis—that RA restricts overall CM progenitor specification,
rather than promoting fate decisions between the cardiac progenitors themselves
(Fig. 5.2).
Foremost, fate maps in zebrafish embryos and newer fate maps in chick embryos
suggest that the relationship between atrial and ventricular progenitors is not delineated by the A-P axis. Caged-fluorescein-mediated fate mapping in zebrafish showed
that atrial and ventricular progenitors lie in a medio-lateral orientation within the
ALPM (Keegan et al. 2004; Schoenebeck et al. 2007; Bakkers 2011). Subsequent
fate-maps in chick embryos corroborated the idea that the orientation of atrial and
ventricular cells is not along the A-P axis; findings that suggest the position of these
progenitors is conserved across multiple vertebrates (Abu-Issa and Kirby 2008).
A series of studies started in zebrafish demonstrated that RA signaling is required
to limit the size of the cardiac progenitor field. Accordingly, zebrafish mutants for
aldh1a2, called neckless (nls), and embryos treated with pharmacological inhibitors
of RA signaling were shown to have larger hearts (Fig. 5.3) (Keegan et al. 2005;
Waxman et al. 2008). Newly developed transgenic fish lines that allowed for counting
individual CMs and caged-fluorescein-mediated lineage tracing of the ALPM showed
that RA signaling limits the number of both atrial and ventricular CMs by restricting
the posterior boundary of these progenitor populations within the ALPM (Fig. 5.2)
(Waxman et al. 2008).
Work in Xenopus (Collop et al. 2006) and mice (Ryckebusch et al. 2008; Sirbu
et al. 2008; Lin et al. 2010) also showed that RA signaling restricted cardiac specification. Use of Aldh1a2 mutant mice demonstrated that, minimally, RA signaling is
necessary to limit the posterior extent of the SHF (Ryckebusch et al. 2008). It is still
not known if RA only restricts SHF specification or whether it also influences FHF
specification (Ryckebusch et al. 2008; Sirbu et al. 2008).
127
an expansion of AMHC1-expressing cells (Osmond et al. 1991; Yutzey et al. 1994).
Zebrafish treated with RA also appeared to have smaller ventricles (Stainier and
Fishman 1992). Furthermore, a single pulse of RA given to female mice at 7.5 dayspost coitus induced malformations ranging from reduced ventricle size to complete
absence of the OFT and ventricles in their offspring (Xavier-Neto et al. 1999). In
vitro studies also suggested that moderate concentrations (typically around 0.1 μM)
of RA preferentially induced stem cells to differentiate into cells with an atrial identity
(marked by atrial-specific ion channels) (Gassanov et al. 2008; Zhang et al. 2011;
Devalla et al. 2015).
A Second Hypothesis: Restriction of Heart Field Specification
The simplicity of the hypothesis described in the previous section makes it an attractive model for describing the role of RA signaling in cardiac progenitor specification.
However, new knowledge synthesized from the generation of new ALPM fate maps
in additional vertebrate models, the availability of new tools for precise counting
of CMs, and the examination of FHF and SHF markers provide compelling support
for an alternative hypothesis—that RA restricts overall CM progenitor specification,
rather than promoting fate decisions between the cardiac progenitors themselves
(Fig. 5.2).
Foremost, fate maps in zebrafish embryos and newer fate maps in chick embryos
suggest that the relationship between atrial and ventricular progenitors is not delineated by the A-P axis. Caged-fluorescein-mediated fate mapping in zebrafish showed
that atrial and ventricular progenitors lie in a medio-lateral orientation within the
ALPM (Keegan et al. 2004; Schoenebeck et al. 2007; Bakkers 2011). Subsequent
fate-maps in chick embryos corroborated the idea that the orientation of atrial and
ventricular cells is not along the A-P axis; findings that suggest the position of these
progenitors is conserved across multiple vertebrates (Abu-Issa and Kirby 2008).
A series of studies started in zebrafish demonstrated that RA signaling is required
to limit the size of the cardiac progenitor field. Accordingly, zebrafish mutants for
aldh1a2, called neckless (nls), and embryos treated with pharmacological inhibitors
of RA signaling were shown to have larger hearts (Fig. 5.3) (Keegan et al. 2005;
Waxman et al. 2008). Newly developed transgenic fish lines that allowed for counting
individual CMs and caged-fluorescein-mediated lineage tracing of the ALPM showed
that RA signaling limits the number of both atrial and ventricular CMs by restricting
the posterior boundary of these progenitor populations within the ALPM (Fig. 5.2)
(Waxman et al. 2008).
Work in Xenopus (Collop et al. 2006) and mice (Ryckebusch et al. 2008; Sirbu
et al. 2008; Lin et al. 2010) also showed that RA signaling restricted cardiac specification. Use of Aldh1a2 mutant mice demonstrated that, minimally, RA signaling is
necessary to limit the posterior extent of the SHF (Ryckebusch et al. 2008). It is still
not known if RA only restricts SHF specification or whether it also influences FHF
specification (Ryckebusch et al. 2008; Sirbu et al. 2008).