| pmid | 10570467 | |||
|---|---|---|---|---|
| title | Distinct functions for Aldh1 and Raldh2 in the control of ligand production for embryonic retinoid signaling pathways. | |||
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| journal | Dev Genet | |||
| year | 1999 | |||
| full_text_available | true | |||
| full_text_extraction_method | html | |||
| pmcid | PMC4342002 | |||
| doi | 10.1002/(SICI)1520-6408(1999)25:4<353::AID-DVG9>3.0.CO;2-G |
Distinct functions for Aldh1 and Raldh2 in the control of ligand production for embryonic retinoid signaling pathways.
Authors: Haselbeck RJ, Hoffmann I, Duester G Journal: Dev Genet (1999) DOI: 10.1002/(SICI)1520-6408(1999)25:4<353::AID-DVG9>3.0.CO;2-G PMC: PMC4342002
- Dev Genet. 1999;25(4):353-64. doi: 10.1002/(SICI)1520-6408(1999)25:4<353::AID-DVG9>3.0.CO;2-G.
Distinct functions for Aldh1 and Raldh2 in the control of ligand production for embryonic retinoid signaling pathways.
Haselbeck RJ(1), Hoffmann I, Duester G.
Author information: (1)Gene Regulation Program, Burnham Institute, La Jolla, CA 92037, USA.
During vertebrate embryogenesis retinoic acid (RA) synthesis must be spatiotemporally regulated in order to appropriately stimulate various retinoid signaling pathways. Various forms of mammalian aldehyde dehydrogenase (ALDH) have been shown to oxidize the vitamin A precursor retinal to RA in vitro. Here we show that injection of Xenopus embryos with mRNAs for either mouse Aldh1 or mouse Raldh2 stimulates RA synthesis at low and high levels, respectively, while injection of human ALDH3 mRNA is unable to stimulate any detectable level of RA synthesis. This provides evidence that some members of the ALDH gene family can indeed perform RA synthesis in vivo. Whole-mount immunohistochemical analyses of mouse embryos indicate that ALDH1 and RALDH2 proteins are localized in distinct tissues. RALDH2 is detected at E7.5-E10.5 primarily in trunk tissue (paraxial mesoderm, somites, pericardium, midgut, mesonephros) plus transiently from E8.5-E9.5 in the ventral optic vesicle and surrounding frontonasal region. ALDH1 is first detected at E9.0-E10. 5 primarily in cranial tissues (ventral mesencephalon, dorsal retina, thymic primordia, otic vesicles) and in the mesonephros. As previous findings indicate that embryonic RA is more abundant in trunk rather than cranial tissues, our findings suggest that Raldh2 and Aldh1 control distinct retinoid signaling pathways by stimulating high and low RA biosynthetic activities, respectively, in various trunk and cranial tissues.
Copyright 1999 Wiley-Liss, Inc.
DOI: 10.1002/(SICI)1520-6408(1999)25:4<353::AID-DVG9>3.0.CO;2-G PMCID: PMC4342002 PMID: 10570467 [Indexed for MEDLINE]
Abstract
During vertebrate embryogenesis retinoic acid (RA) synthesis must be spatiotemporally regulated in order to appropriately stimulate various retinoid signaling pathways. Various forms of mammalian aldehyde dehydrogenase (ALDH) have been shown to oxidize the vitamin A precursor retinal to RA in vitro. Here we show that injection of Xenopus embryos with mRNAs for either mouse Aldh1 or mouse Raldh2 stimulates RA synthesis at low and high levels, respectively, while injection of human ALDH3 mRNA is unable to stimulate any detectable level of RA synthesis. This provides evidence that some members of the ALDH gene family can indeed perform RA synthesis in vivo. Whole-mount immunohistochemical analyses of mouse embryos indicate that ALDH1 and RALDH2 proteins are localized in distinct tissues. RALDH2 is detected at E7.5–E10.5 primarily in trunk tissue (paraxial mesoderm, somites, pericardium, midgut, mesonephros) plus transiently from E8.5–E9.5 in the ventral optic vesicle and surrounding frontonasal region. ALDH1 is first detected at E9.0–E10.5 primarily in cranial tissues (ventral mesencephalon, dorsal retina, thymic primordia, otic vesicles) and in the mesonephros. As previous findings indicate that embryonic RA is more abundant in trunk rather than cranial tissues, our findings suggest that Raldh2 and Aldh1 control distinct retinoid signaling pathways by stimulating high and low RA biosynthetic activities, respectively, in various trunk and cranial tissues.
INTRODUCTION
Vitamin A (retinol) plays a role in the regulation of vertebrate embryonic development through its active metabolite retinoic acid (RA) [reviewed by Hofmann and Eichele, 1994 ; Gudas et al. , 1994 ; Maden, 1994 ]. RA functions as a ligand for several nuclear retinoic acid receptors (RARs) which function as ligand-dependent transcription factors in conjunction with retinoid-X receptors (RXRs), with which they form heterodimers [reviewed by Kastner et al. , 1994 ; Mangelsdorf et al. , 1994 ]. Essential roles for these receptors in vitamin A function have been confirmed in mice carrying targeted mutations of RARs and RXRs [ Lohnes et al. , 1994 ; Mendelsohn et al. , 1994 ; Luo et al. , 1996 ; Mascrez et al. , 1998 ]. A major challenge remaining in the study of retinoid signaling is the elucidation of how production of the ligand is spatiotemporally regulated in various tissues during development to provide the correct amount of RA for each discrete receptor-mediated signaling event.
The major biosynthetic pathway of RA from retinol involves two sequential steps catalyzed by several candidate enzymes [reviewed by Duester, 1996 ; Napoli, 1996 ]. First, reversible dehydrogenation of retinol into retinal is catalyzed by either cytosolic retinol dehydrogenases, which are members of the alcohol dehydrogenase (ADH) family, or by microsomal retinol dehydrogenases, which are members of the short-chain dehydrogenase/reductase family (SDR). Mice carrying a targeted Adh4 null mutation have defects in retinol utilization during vitamin A deficiency [ Deltour et al. , 1996b ] and Adh1 −/− mice display significant reductions in the ability to metabolize a dose of retinol to RA [ Deltour et al. , 1999a ]. Thus, at least two forms of ADH are involved in metabolizing retinol to retinal in vivo.
In the second step of the metabolic pathway, retinal is irreversibly oxidized to RA by cytosolic retinal dehydrogenases, which are members of the aldehyde dehydrogenase (ALDH) family. Examination of the second step of RA synthesis in vitro originally led to the identification of calf liver ALDH as an enzyme able to oxidize retinal to RA [ Futterman, 1962 ]. Subsequent studies revealed an extensive family of mammalian ALDHs, with the original form active for RA synthesis corresponding to class I ALDH (ALDH1) in the human [ Dockham et al. , 1992 ; Yoshida et al. , 1992 ], as well as its mouse ALDH1 homolog previously known as AHD2 [ Lee et al. , 1991 ; McCaffery et al. , 1992 ], and its rat ALDH1 homolog also known as RALDH or RalDH-I [ Labrecque et al. , 1995 ; Penzes et al. , 1997 ; Kathmann and Lipsky, 1997 ]. Two additional classes of ALDH, i.e., mitochondrial ALDH2 and cytosolic ALDH3, have been shown to have no RA synthetic activity in vitro [ Yoshida et al. , 1992 ]. The discovery that mouse ALDH1 is localized during embryogenesis in the dorsal retina (a retinoid-target tissue) provided impetus to examine this enzyme as a potential embryonic RA synthetic enzyme [ McCaffery et al. , 1991 ]. Further studies in the mouse culminated in the discovery of an additional RA synthetic enzyme identified as a distinct class of ALDH sharing approximately 70% amino acid sequence identity with ALDH1; this form has been named RALDH2 in mouse [ Zhao et al. , 1996 ], human [ Ono et al. , 1998 ], and chick [ Sockanathan and Jessell, 1998 ], and RalDH-II in the rat [ Wang et al. , 1996 ].
During early mouse embryogenesis, there exists a posterior preference for initial RA synthesis [ Hogan et al. , 1992 ]. This may help establish the anteroposterior axis during early development due to regulation of members of the Hox gene family, which are RA-inducible and differentially expressed along the anteroposterior axis with expression only in posterior tissues (trunk and posterior hindbrain) [ Shimeld, 1996 ]. In addition to anteroposterior patterning, RA does play a role in cranial development, as demonstrated by embryonic vitamin A-deficiency studies [ Maden et al. , 1996 ; Dickman et al. , 1997 ] as well as gene knockouts of RARs [ Lohnes et al. , 1994 ] and Raldh2 [ Niederreither et al. , 1999 ]. Thus, RA must exist in cranial locations as well as trunk locations. Localization of RA in mouse embryos has been determined using either a transgenic RA reporter mouse line [ Rossant et al. , 1991 ] or a whole-embryo explant RA bioassay [ Ang et al. , 1996a ] which employs an RA reporter cell line that was originally used to identify RA in the neural tube [ Wagner et al. , 1992 ]. In both assays, RA is initially detected at E7.5, limited to posterior tissues, and then from E8.5–E10.5 RA is still abundant in the trunk, but it is now also detected in the optic vesicles and surrounding frontonasal region. Raldh2 mRNA transcripts are initially expressed in the posterior mesoderm of mouse embryos at E7.5, with additional expression in the heart at E8.25, transient expression in the optic vesicles at E8.5, and a continued high level of expression in the trunk from stages E8.5–E10.5 and in the spinal cord by E12.5 [ Zhao et al. , 1996 ; Niederreither et al. , 1997 ; Moss et al. , 1998 ]. Raldh2 −/− null mutant mice die at midgestation, displaying no detectable RA in the trunk and frontonasal region at late E8.5, but with some RA still detectable in the optic vesicles [ Niederreither et al. , 1999 ]. Thus, Raldh2 appears to be responsible for essentially all of the RA synthesis occurring in the trunk and frontonasal regions by late E8.5, but not for all RA synthesis occurring in the optic vesicles. A role for mouse Aldh1 in optic vesicle RA synthesis is suggested based on its expression in the dorsal retina at E9.5 [ McCaffery et al. , 1991 ]; however, a more complete analysis of Aldh1 is needed to determine the full extent of its role in RA synthesis.
A direct demonstration of the abilities of Aldh1 and Raldh2 to catalyze embryonic RA synthesis by overexpression in an in vivo setting has not been previously reported. Also, the localization of both ALDH1 and RALDH2 proteins during embryogenesis has not been adequately addressed in order to identify tissues where the enzymes actually exist and RA synthesis can thus be expected to occur. Here, we examined mouse Aldh1 and Raldh2 for their ability to function in RA synthesis in vivo by expression in Xenopus embryos. Our results provide firm evidence that both genes stimulate RA synthesis when expressed in Xenopus , and these findings provide insight as to their relative abilities. In addition, we used specific antibodies directed against ALDH1 and RALDH2 to demonstrate distinct patterns of protein localization for these two enzymes in mouse cranial and trunk embryonic tissues, thus defining distinct domains where Aldh1 and Raldh2 may influence retinoid signaling during development.
DISCUSSION
Regulated production of the ligand RA is undoubtedly a key event controlling the function of retinoid receptors during vertebrate development. Differences in the rate of RA synthesis from one embryonic tissue to another is likely to contribute to the establishment of RA gradients that affect the overall control of development by retinoids. Although it is clear that some embryonic tissues contain more RA than others, it is less clear how these differences are generated. Here we have explored the role of the aldehyde dehydrogenase gene family in embryonic RA synthesis. We found that mouse Aldh1 and Raldh2 both stimulate both stimulate RA synthesis when expressed in Xenopus embryos, with Raldh2 being more efficient than Aldh1 . The distinct localization patterns of ALDH1 and RALDH2 proteins in mouse embryos and adult tissues suggest that these two RA synthetic enzymes initially establish differential cranial and trunk RA synthesis and function in distinct retinoid signaling events during development.