6 Retinoic Acid Signaling and Development of the Respiratory System
161
Targeting the Cellular Uptake of VA
In-situ hybridization of Stra6, the gene encoding a membrane protein that facilitates
the uptake of VA into some cells, shows expression of Stra6 in the lung mesenchyme
at the earliest stage of lung development in mice and quail (Bouillet et al. 1997;
Fernandes-Silva et al. 2017). This finding is consistent with the need for VA at the
embryonic and pseudoglandular phases of development. The expression of Stra6 at
later stages of development has not been explored.
Human fetuses from consanguineous families with STRA6 missence and nonsense mutations (Matthew-Wood syndrome) exhibit a broad spectrum of malformations, including lung hypoplasia/agenesis and diaphragmatic hernia with variable
penetrance (Golzio et al. 2007; Kawaguchi et al. 2007; Pasutto et al. 2007). These
severe defects recapitulate the abnormalities observed in VAD rat embryos (Wilson
et al. 1953).
Stra6-null mice that have been generated demonstrate ocular defects (Amengual
et al. 2014). Interestingly, outside of having diminishing VA stores within the lung,
there is no gross lung phenotype. Though the reason for the different lung phenotype
between human and mice with Stra6 mutations are not clear, the authors theorize
that the variability in maternal VA status and delivery to the fetus may explain the
variability of the phenotype. It is also speculated that additional genetic alterations
along the RA signaling pathway may be present in humans with Matthew-Wood
syndrome (Berry et al. 2013).
Targeting RA-Synthesizing Enzymes
The major RA-synthesizing enzyme, RALDH2, is expressed in primary lung buds at
the onset of mouse lung development, suggesting that RA signaling is ubiquitously
activated in the lung primordium (Malpel et al. 2000). The use of reporter mice
carrying a β 2 -RARE-lacZ (RARE-lacZ) transgene that is activated when RA interacts
with its receptors (Rossant et al. 1991) also revealed that there is extensive RA activity
in the foregut at the time of lung specification at E9.0 (early embryonic phase). Such
observations suggested that RA may control pathways critical for the initiation of
lung morphogenesis.
Inhibition of RALDH activity with diethylaminobenzaldehyde (DEAB) in mouse
foregut explants 24 h before lung bud morphogenesis (E8.5) has been shown to lead
to lung agenesis (unpublished data, Chen). Mouse embryos deficient in Rdh10, the
gene that encodes a dehydrogenase that catalyzes the first step of RA biosynthesis
in vertebrates, die before E13.5 and exhibit defective limb buds and lung bud agenesis or hypoplastic lungs (Sandell et al. 2007). Similarly, mouse embryos deficient
in Aldh1a2, the gene that encodes RALDH2 that catalyzes the second step of RA,
die at midgestation. These embryos exhibit truncated posterior region, lack of limb
buds, dilated heart, and an open neural tube (Niederreither et al. 1999). Note that
161
Targeting the Cellular Uptake of VA
In-situ hybridization of Stra6, the gene encoding a membrane protein that facilitates
the uptake of VA into some cells, shows expression of Stra6 in the lung mesenchyme
at the earliest stage of lung development in mice and quail (Bouillet et al. 1997;
Fernandes-Silva et al. 2017). This finding is consistent with the need for VA at the
embryonic and pseudoglandular phases of development. The expression of Stra6 at
later stages of development has not been explored.
Human fetuses from consanguineous families with STRA6 missence and nonsense mutations (Matthew-Wood syndrome) exhibit a broad spectrum of malformations, including lung hypoplasia/agenesis and diaphragmatic hernia with variable
penetrance (Golzio et al. 2007; Kawaguchi et al. 2007; Pasutto et al. 2007). These
severe defects recapitulate the abnormalities observed in VAD rat embryos (Wilson
et al. 1953).
Stra6-null mice that have been generated demonstrate ocular defects (Amengual
et al. 2014). Interestingly, outside of having diminishing VA stores within the lung,
there is no gross lung phenotype. Though the reason for the different lung phenotype
between human and mice with Stra6 mutations are not clear, the authors theorize
that the variability in maternal VA status and delivery to the fetus may explain the
variability of the phenotype. It is also speculated that additional genetic alterations
along the RA signaling pathway may be present in humans with Matthew-Wood
syndrome (Berry et al. 2013).
Targeting RA-Synthesizing Enzymes
The major RA-synthesizing enzyme, RALDH2, is expressed in primary lung buds at
the onset of mouse lung development, suggesting that RA signaling is ubiquitously
activated in the lung primordium (Malpel et al. 2000). The use of reporter mice
carrying a β 2 -RARE-lacZ (RARE-lacZ) transgene that is activated when RA interacts
with its receptors (Rossant et al. 1991) also revealed that there is extensive RA activity
in the foregut at the time of lung specification at E9.0 (early embryonic phase). Such
observations suggested that RA may control pathways critical for the initiation of
lung morphogenesis.
Inhibition of RALDH activity with diethylaminobenzaldehyde (DEAB) in mouse
foregut explants 24 h before lung bud morphogenesis (E8.5) has been shown to lead
to lung agenesis (unpublished data, Chen). Mouse embryos deficient in Rdh10, the
gene that encodes a dehydrogenase that catalyzes the first step of RA biosynthesis
in vertebrates, die before E13.5 and exhibit defective limb buds and lung bud agenesis or hypoplastic lungs (Sandell et al. 2007). Similarly, mouse embryos deficient
in Aldh1a2, the gene that encodes RALDH2 that catalyzes the second step of RA,
die at midgestation. These embryos exhibit truncated posterior region, lack of limb
buds, dilated heart, and an open neural tube (Niederreither et al. 1999). Note that