8 Fetal Alcohol Spectrum Disorder: Embryogenesis Under …
207
information about the relative effects of retinoid and alcohol substrates. However,
it does not provide information on potential competitive actions of upstream substrates. Rescue experiments using retinol or retinaldehyde more directly address the
competition model, as they themselves still have to undergo one or two oxidation
reactions, respectively, in the presence of ethanol or acetaldehyde. Retinol, retinaldehyde, and RA have all been used to rescue the effects of alcohol exposure (Yelin et al.
2005; Marrs et al. 2010; Muralidharan et al. 2015; Shabtai et al. 2018; Shukrun et al.
2019). These rescue experiments were monitored by morphological analysis, gene
expression changes, and in some cases, RA reporter constructs. In all of these experiments, developmental malformations were induced with ethanol and rescued with
the different retinoids.
In further support of the ethanol-retinol competition model, ethanol was shown
to inhibit retinol oxidation both in vivo (Raskin et al. 1976) and in vitro (Pocker and
Raymond 1980). Further work in the late 1990s and early 2000s corroborated the idea
that ethanol competed with retinol for ADH1, ADH3 or ADH4 (Deltour et al. 1999;
Molotkov and Duester 2002). These observations supported the hypothesis that the
FASD phenotype is an outcome of a competition between ethanol and retinol, for
the available ADH activity that results in a reduction in RA production (Fig. 8.5)
(Shabtai and Fainsod 2018).
However, the ethanol-retinol-competition for ADH model was questioned by
in vitro enzymatic studies which showed that rodent ADH has a higher preference for
retinol than ethanol, a finding that contested the proposed competition at the ADH
level (Reynier 1969; Plapp et al. 2001). Moreover, ethanol only inhibited retinol oxidation by members of the ADH family at alcohol concentrations that were in the high
end of what is measured in the blood of highly intoxicated individuals (Kedishvili
et al. 1998; Schuckit 1998; Plapp et al. 2001).
More recent studies have highlighted the importance of members of the SDR family in RA production. In particular, retinol dehydrogenase 10 (RDH10, SDR16C4),
a member of the SDR family, has been shown to be a major player in the production
of retinaldehyde at gastrula stages in the developing vertebrate embryo (Strate et al.
2009; Sandell et al. 2012). Mice mutant for rdh10 exhibit mid-embryogenesis lethality, a phenotype that can be rescued by maternal supplementation with retinaldehyde
(Rhinn et al. 2011; Kumar et al. 2012). In addition, pharmacological inhibition of
RDH activity significantly reduces RA levels, while inhibition of ADH activity has
no effect on this signal (Fig. 8.5) (Shabtai et al. 2018). These new findings support
the idea that members of the SDR family (and not the ADH family) catalyze most
retinaldehyde production during early embryonic stages. Such data suggest that the
competition between alcohol and retinol might take place on a different enzymatic
activity.
207
information about the relative effects of retinoid and alcohol substrates. However,
it does not provide information on potential competitive actions of upstream substrates. Rescue experiments using retinol or retinaldehyde more directly address the
competition model, as they themselves still have to undergo one or two oxidation
reactions, respectively, in the presence of ethanol or acetaldehyde. Retinol, retinaldehyde, and RA have all been used to rescue the effects of alcohol exposure (Yelin et al.
2005; Marrs et al. 2010; Muralidharan et al. 2015; Shabtai et al. 2018; Shukrun et al.
2019). These rescue experiments were monitored by morphological analysis, gene
expression changes, and in some cases, RA reporter constructs. In all of these experiments, developmental malformations were induced with ethanol and rescued with
the different retinoids.
In further support of the ethanol-retinol competition model, ethanol was shown
to inhibit retinol oxidation both in vivo (Raskin et al. 1976) and in vitro (Pocker and
Raymond 1980). Further work in the late 1990s and early 2000s corroborated the idea
that ethanol competed with retinol for ADH1, ADH3 or ADH4 (Deltour et al. 1999;
Molotkov and Duester 2002). These observations supported the hypothesis that the
FASD phenotype is an outcome of a competition between ethanol and retinol, for
the available ADH activity that results in a reduction in RA production (Fig. 8.5)
(Shabtai and Fainsod 2018).
However, the ethanol-retinol-competition for ADH model was questioned by
in vitro enzymatic studies which showed that rodent ADH has a higher preference for
retinol than ethanol, a finding that contested the proposed competition at the ADH
level (Reynier 1969; Plapp et al. 2001). Moreover, ethanol only inhibited retinol oxidation by members of the ADH family at alcohol concentrations that were in the high
end of what is measured in the blood of highly intoxicated individuals (Kedishvili
et al. 1998; Schuckit 1998; Plapp et al. 2001).
More recent studies have highlighted the importance of members of the SDR family in RA production. In particular, retinol dehydrogenase 10 (RDH10, SDR16C4),
a member of the SDR family, has been shown to be a major player in the production
of retinaldehyde at gastrula stages in the developing vertebrate embryo (Strate et al.
2009; Sandell et al. 2012). Mice mutant for rdh10 exhibit mid-embryogenesis lethality, a phenotype that can be rescued by maternal supplementation with retinaldehyde
(Rhinn et al. 2011; Kumar et al. 2012). In addition, pharmacological inhibition of
RDH activity significantly reduces RA levels, while inhibition of ADH activity has
no effect on this signal (Fig. 8.5) (Shabtai et al. 2018). These new findings support
the idea that members of the SDR family (and not the ADH family) catalyze most
retinaldehyde production during early embryonic stages. Such data suggest that the
competition between alcohol and retinol might take place on a different enzymatic
activity.