Recombinant class I aldehyde dehydrogenases specific for all-trans- or 9-cis-retinal

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

Retinoic acid (RA) is a vital morphogen for early patterning and organogenesis in the developing embryo. RA is a diffusible, lipophilic molecule that signals via nuclear RA receptor heterodimeric units that regulate gene expression by interacting with RA response elements in promoters of a significant number of genes. For precise RA signaling, a robust gradient of the morphogen is required. The developing embryo contains regions that produce RA, and specific intracellular concentrations of RA are created through local degradation mediated by Cyp26 enzymes. In order to elucidate the mechanisms by which RA executes precise developmental programs, the kinetics of RA metabolism must be clearly understood. Recent advances in techniques for endogenous RA detection and quantification have paved the way for mechanistic studies to shed light on downstream gene expression regulation coordinated by RA. It is increasingly coming to light that RA signaling operates not only at precise concentrations but also employs mechanisms of degradation and feedback inhibition to self-regulate its levels. A global gradient of RA throughout the embryo is often found concurrently with several local gradients, created by juxtaposed domains of RA synthesis and degradation. The existence of such local gradients has been found especially critical for the proper development of craniofacial structures that arise from the neural crest and the cranial placode populations. In this review, we summarize the current understanding of how local gradients of RA are established in the embryo and their impact on craniofacial development.

Kinetic characterization and regulation of the human retinaldehyde dehydrogenase 2 enzyme during production of retinoic acid

Retinoic acid (RA) is an important regulator of embryogenesis and tissue homoeostasis. Perturbation of RA signalling causes developmental disorders, osteoarthritis, schizophrenia and several types of tumours. RA is produced by oxidation of retinaldehyde from vitamin A. The main enzyme producing RA in the early embryo is retinaldehyde dehydrogenase 2 (RALDH2, ALDH1A2). In the present study we describe in depth the kinetic properties and regulation of the human RALDH2 (hRALDH2) enzyme. We show that this enzyme produces RA using in vivo and in vitro assays. We studied the naturally occurring all-trans-, 9-cis- and 13-cis-retinaldehyde isomers as substrates of hRALDH2. Based on the values measured for the Michaelis-Menten constant Km and the maximal rate Vmax, in vitro hRALDH2 displays the same catalytic efficiency for their oxidation. We characterized two known inhibitors of the vertebrate RALDH2 and determined their kinetic parameters on hRALDH2. In addition, RA was studied as a possible inhibitor of hRALDH2 and a regulator of its activity. We show that hRALDH2 is not inhibited by its oxidation product, all-trans-RA, suggesting the absence of a negative feedback regulatory loop. Expression of the Raldh2 gene is known to be regulated by RA itself, suggesting that the main regulation of the hRALDH2 activity level is transcriptional.

ALDH1B1 is a potential stem/progenitor marker for multiple pancreas progenitor pools

Aldehyde dehydrogenase (ALDH) genes are increasingly associated with stem/progenitor cell status but their role in the maintenance of pluripotency remains uncertain. In a screen conducted for downstream Ngn3 target genes using ES derived pancreas progenitors we identified Aldh1b1, encoding a mitochondrial enzyme, as one of the genes strongly up regulated in response to Ngn3 expression. We found both by in situ hybridization and immunofluorescence using a specific antibody that ALDH1B1 is exclusively expressed in the emerging pancreatic buds of the early embryo (9.5 dpc) in a Pdx1 dependent manner. Around the time of secondary transition, ALDH1B1 expression was restricted in the tip tripotent progenitors of the branching epithelium and in a subset of the trunk epithelium. Expression in the latter was Ngn3 dependent. Subsequently, ALDH1B1 expression persisted only in the tip cells that become restricted to the exocrine lineage and declined rapidly as these cells mature. In the adult pancreas we identified rare ALDH1B1(+) cells that become abundant following pancreas injury in either the caerulein or streptozotocin paradigms. Blocking ALDH catalytic activity in pancreas embryonic explants resulted in reduced size of the explants and accelerated differentiation suggesting for the first time that ALDH activity may be necessary in the developing pancreas for the maintenance and expansion of progenitor pools.

Functional properties and substrate characterization of human CYP26A1, CYP26B1, and CYP26C1 expressed by recombinant baculovirus in insect cells

INTRODUCTION

The cytochrome P450 CYP26 family of retinoic acid (RA) metabolizing enzymes, comprising CYP26A1, CYP26B1, and CYP26C1 is critical for establishing patterns of RA distribution during embryonic development and retinoid homeostasis in the adult. All three members of this family can metabolize all trans-RA. CYP26C1 has also been shown to efficiently metabolize the 9-cis isomer of RA.

METHODS

We have co-expressed each of the CYP26 enzymes along with the NADPH-cytochrome P450 oxidoreductase using a baculovirus/Sf9 insect cell expression system to determine the enzymatic activities of these enzymes in cell free preparations and have established an in vitro binding assay to permit comparison of binding affinities of the three CYP26 enzymes.

RESULTS

We demonstrated that the expressed enzymes can efficiently coordinate heme, as verified by spectral-difference analysis. All CYP26s efficiently metabolized all-trans-RA to polar aqueous-soluble metabolites, and in competition experiments exhibited IC(50) values of 16, 27, and 15nM for CYP26A1, B1, and C1 respectively for all-trans-RA. Furthermore, this metabolism was blocked with the CYP inhibitor ketoconazole. CYP26C1 metabolism of all trans-RA could also be effectively competed with 9-cis RA, with IC(50) of 62nM, and was sensitive to ketoconazole inhibition.

DISCUSSION

CYP26 enzymes are functionally expressed in microsomal fractions of insect cells and stably bind radiolabeled RA isomers with affinities respecting their substrate specificities. We demonstrated that compared to CYP26A and CYP26B, only CYP26C1 was able to bind with high affinity to 9-cis-RA. These assays will be useful for the screening of synthetic substrates and inhibitors of CYP26 enzymes and may be applicable to other cytochrome P450s and their respective substrates.

Aldehyde dehydrogenase 1A3 is transcriptionally activated by all-trans-retinoic acid in human epidermal keratinocytes

Retinoids are regulators of keratinocyte differentiation in the epidermis and important therapeutics in dermatology. The formation of the most active retinoid, all-trans-retinoic acid (RA) by oxidation of retinal is catalyzed by aldehyde dehydrogenases (ALDH), of which ALDH1A3 has been shown to be most efficient. Here we investigated the expression of ALDH1A3 in epidermal cultures. Three alternatively spliced mRNAs of ALDH1A3 were detected in skin cultures with the conventionally spliced mRNA being predominant. Among a panel of ALDH genes, only ALDH1A3 was upregulated by RA in primary keratinocytes. RA increased the expression of ALDH1A3 also in organotypic human skin cultures and in an epidermal explant in vitro whereas no upregulation was detected in dermal fibroblasts and HeLa cells. Our results indicate that the regulation of the retinoic acid metabolism in the epidermis involves transcriptional activation of ALDH1A3, possibly representing a positive feedback loop, which enhances the effect of exogenous RA.

Structural features of the NAD-dependent in situ retinoic acid supply system in esophageal mucosa

BACKGROUND

We previously reported that an NAD-dependent in situ retinoic acid supply system, which comprises some isoforms of alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) and provides retinoic acid from retinol via a 2-step oxidation process, exists in the rat esophagus. Herein, their isoforms responsible for the pathway and its localization in the rat esophagus was examined.

METHODS

The expressions of mRNAs of various isoforms of ADH and ALDH were examined in the fraction mainly comprising mucosal layer of the rat esophagus by RT-PCR. Expression levels of Class IV ADH and ALDH 1A1 were compared between the fractions and that mainly comprising muscle layer of the rat esophagus by quantitative PCR. The catalytic activities producing retinoic acid from retinal were compared between the 2 fractions and its optimum pH was also determined.

RESULTS

Classes I, III, and IV ADHs and ALDHs 1A1 and 3A1 were predominant isoforms in the rat esophageal mucosa. The expression levels of mRNA of Class IV ADH and ALDH 3A1 were significantly higher in the mucosal than in the muscle layer. Consistently, the catalytic activities producing retinoic acid from retinal were significantly higher in the former than the latter. The optimum pH of the process was 9.0.

CONCLUSIONS

Considering the affinities for retinol and retinal of ADHs and ALDHs expressed in the rat esophagus, the NAD-dependent in situ retinoic acid supply system in the rat esophagus is thought to comprise Class IV ADH and ALDH 1A1. In the rat esophagus, the system exists predominantly in the mucosal layer.

Retinoic acid counteracts developmental defects in the substantia nigra caused by Pitx3 deficiency

Selective neuronal loss in the substantia nigra (SNc), as described for Parkinson's disease (PD) in humans and for Pitx3 deficiency in mice, highlights the existence of neuronal subpopulations. As yet unknown subset-specific gene cascades might underlie the observed differences in neuronal vulnerability. We identified a developmental cascade in mice in which Ahd2 (Aldh1a1) is under the transcriptional control of Pitx3. Interestingly, Ahd2 distribution is restricted to a subpopulation of the meso-diencephalic dopaminergic (mdDA) neurons that is affected by Pitx3 deficiency. Ahd2 is involved in the synthesis of retinoic acid (RA), which has a crucial role in neuronal patterning, differentiation and survival in the brain. Most intriguingly, restoring RA signaling in the embryonic mdDA area counteracts the developmental defects caused by Pitx3 deficiency. The number of tyrosine hydroxylase-positive (TH+) neurons was significantly increased after RA treatment in the rostral mdDA region of Pitx3-/- embryos. This effect was specific for the rostral part of the developing mdDA area, and was observed exclusively in Pitx3-/- embryos. The effect of RA treatment during the critical phase was preserved until later in development, and our data suggest that RA is required for the establishment of proper mdDA neuronal identity. This positions Pitx3 centrally in a mdDA developmental cascade linked to RA signaling. Here, we propose a novel mechanism in which RA is involved in mdDA neuronal development and maintenance, providing new insights into subset-specific vulnerability in PD.

Kinetic properties of chimeric class I aldehyde dehydrogenases for retinal isomers

Retinal dehydrogenase type 1 (RALDH1) catalyzes the oxidation of all-trans and 9-cis retinal to the respective retinoic acids (RAs), whereas another member of the aldehyde dehydrogenase (ALDH) family, the phenobarbital-induced aldehyde dehydrogenase (PB-ALDH), is very poorly active. We have previously generated chimeras between these 2 enzymes that displayed selectivity for retinal isomers in crude bacterial extracts. Here we have characterized the kinetic properties of the corresponding purified recombinant proteins. The all-trans selective chimera RALDH-131 converted all-trans retinal to all-trans RA with 2.9-fold lower efficiency than the wild-type RALDH1 and had only residual activity with 9-cis retinal. The converse chimera PB-131 was specific for 9-cis retinal, with no residual activity for all-trans retinal. MgCl2 inhibited the activities of RALDH1 and PB-131, but not of RALDH-131, suggesting that amino acids 132-510 in RALDH are necessary for inhibition by MgCl2. These data demonstrate that the chimeric enzymes act as retinal isomer-selective ALDHs, and suggest that these enzymes may be useful to study the roles of cis RA isomers in embryogenesis and differentiation in vivo.

Localization of retinaldehyde dehydrogenases and retinoid binding proteins to sustentacular cells, glia, Bowman's gland cells, and stroma: potential sites of retinoic acid synthesis in the postnatal rat olfactory organ

Work from our laboratory suggests that retinoic acid (RA) influences neuron development in the postnatal olfactory epithelium (OE). The studies reported here were carried out to identify and localize retinaldehyde dehydrogenase (RALDH) expression in postnatal rat OE to gain a better understanding of potential in vivo RA synthesis sites in this continuously regenerating tissue. RALDH 1, 2, and 3 mRNAs were detected in postnatal rat olfactory tissue by RT-PCR analysis, but RALDH 1 and 2 transcripts were predominant. RALDH 1 immunoreactivity was localized to sustentacular cells in the OE and to Bowman's gland cells, and GFAP(+)/p75(-) olfactory ensheathing cells (OECs) in the underlying lamina propria (LP). RALDH 2 did not colocalize with RALDH 1, but appeared to be expressed in GFAP(-)/RALDH 1(-) OECs as well as in unidentified structures in the LP. Cellular RA binding protein (CRABP II) colocalized with RALDH 1. Cellular retinol/retinaldehyde binding protein (CRBP I) was localized to RALDH 1(+) sites in the OE and LP and RALDH 2(+) sites, primarily surrounding nerve fiber bundles in the LP. Vitamin A deficiency altered RALDH 1, but not RALDH 2 protein expression. The isozymes and binding proteins exhibited random variability in levels and areas of expression both within and between animals. These findings support the hypothesis that RA is synthesized in the postnatal OE (catalyzed by RALDH 1) and underlying LP (differentially catalyzed by RALDH 1 and RALDH 2) at sites that could influence the development, maturation, targeting, and/or turnover of olfactory receptor neurons throughout the olfactory organ.

Quantitative Analysis of Liver Protein Expression During Hibernation in the Golden-mantled Ground Squirrel

Mammals that enter deep hibernation experience extreme reductions in body temperature and in metabolic, respiratory, and heart rates for several weeks at a time. Survival of these extremes likely entails a highly regulated network of tissue- and time-specific gene expression patterns that remain largely unknown. To date, studies to identify differentially-expressed genes have employed a candidate gene approach or in a few cases broader unbiased screens at the RNA level. Here we use a proteomic approach to compare and identify differentially expressed liver proteins from two seasonal stages in the golden-mantled ground squirrel (summer and entrance into torpor) using two-dimensional gels followed by MS/MS. Eighty-four two-dimensional gel spots were found that quantitatively alter with the hibernation season, 68 of which gave unambiguous identifications based on similarity to sequences in the available mammalian database. Based on what is known of these proteins from prior research, they are involved in a variety of cellular processes including protein turnover, detoxification, purine biosynthesis, gluconeogenesis, lipid metabolism and mobility, ketone body formation, cell structure, and redox balance. A number of the enzymes found to change seasonally are known to be either rate-limiting or first enzymes in a metabolic pathway, indicating key roles in metabolic control. Functional roles are proposed to explain the changes seen in protein levels and their potential influence on the phenotype of hibernation.

A novel human cytochrome P450, CYP26C1, involved in metabolism of 9-cis and all-trans isomers of retinoic acid

Retinoids are potent regulators of cell proliferation, cell differentiation, and morphogenesis and are important therapeutic agents in oncology and dermatology. The gene regulatory activity of endogenous retinoids is effected primarily by retinoic acid isomers (all-trans and 9-cis) that are synthesized from retinaldehyde precursors in a broad range of tissues and act as ligands for nuclear retinoic acid receptors. The catabolism of all-trans-retinoic acid (atRA) is an important mechanism of controlling RA levels in cell and tissues. We have previously identified two cytochrome P450s, P450RAI-1 and P450RAI-2 (herein named CYP26A1 and CYP26B1), which were shown to be responsible for catabolism of atRA both in the embryo and the adult. In this report, we describe the identification, molecular cloning, and substrate characterization of a third member of the CYP26 family, named CYP26C1. Transiently transfected cells expressing CYP26C1 convert atRA to polar water-soluble metabolites similar to those generated by CYP26A1 and -B1. Competition studies with all-trans, 13-cis, and 9-cis isomers of retinoic acid demonstrated that atRA was the preferred substrate for CYP26C1. Although CYP26C1 shares extensive sequence similarity with CYP26A1 and CYP26B1, its catalytic activity appears distinct from those of other CYP26 family members. Specifically, CYP26C1 can also recognize and metabolize 9-cis-RA and is much less sensitive than the other CYP26 family members to the inhibitory effects of ketoconazole. CYP26C1 is not widely expressed in the adult but is inducible by RA in HPK1a, transformed human keratinocyte cell lines. This third CYP26 member may play a specific role in catabolizing both all-trans and 9-cis isomers of RA.

Retinoid activation of retinoic acid receptor but not retinoid X receptor is sufficient to rescue lethal defect in retinoic acid synthesis

Two isomers of retinoic acid (RA) may be necessary as ligands for retinoid signaling: all-trans-RA for RA receptors (RARs) and 9-cis-RA for retinoid X receptors (RXRs). This was explored by using retinaldehyde dehydrogenase (Raldh)2-/- mouse embryos lacking mesodermal RA synthesis that display early growth arrest unless rescued by all-trans-RA administration. Because isomerization of all-trans-RA to 9-cis-RA can occur, it is unclear whether both ligands are needed for rescue. We show here that an RAR-specific ligand can rescue Raldh2-/- embryos as efficiently as all-trans-RA, whereas an RXR-specific ligand has no effect. Further, whereas all-trans-RA was detected in embryos, 9-cis-RA was undetectable unless a supraphysiological dose of all-trans-RA was administered, revealing that 9-cis-RA is of pharmacological but not physiological significance. Because 9-cis-RA is undetectable and unnecessary for Raldh2-/- rescue, and others have shown that 4-oxo-RA is unnecessary for mouse development, all-trans-RA emerges as the only ligand clearly necessary for retinoid receptor signaling.

Enzymatic characterization of recombinant mouse retinal dehydrogenase type 1

Retinal dehydrogenases (RALDHs) convert retinal into retinoic acids (RAs), which are important signaling molecules in embryogenesis and tissue differentiation. We expressed mouse RALDH type 1 (mRALDH1) in Escherichia coli and studied the kinetic properties of the recombinant enzyme for retinal substrates. Purified recombinant mRALDH1 catalyzed the oxidation of all-trans and 9-cis retinal but not 13-cis retinal, and exhibited two pH optimums, 7.8 and 9.4, for all-trans and 9-cis retinal substrates, respectively. The K(m) for all-trans retinal (11.6 micro M) was 3-fold higher than for 9-cis retinal (3.59 micro M). However, the conversion efficiencies of either all-trans or 9-cis retinal to the respective RAs were similar. MgCl(2) inhibited the oxidation of both all-trans and 9-cis retinal. Chloral hydrate and acetaldehyde competitively suppressed all-trans retinal oxidation with inhibition constants (K(i)) of 4.99 and 49.4 micro M, respectively. Retinol, on the other hand, blocked the reaction uncompetitively. These data extend the kinetic characterization of mRALDH1, provide insight into the possible role of this enzyme in the biogenesis of RAs, and should give useful information on the determination of amino acid residues that play crucial roles in the catalysis of all-trans and 9-cis retinal.

Cloning of monkey RALDH1 and characterization of retinoid metabolism in monkey kidney proximal tubule cells

All-trans and 9-cis retinoic acids function as ligands for retinoic acid receptors (RARs and RXRs), which are ligand-dependent transcription factors and play important roles in development and cellular differentiation. Several retinal dehydrogenases are likely to contribute to the production of all-trans and 9-cis RAs in vivo, but their respective roles in different tissues are still poorly characterized. We have previously characterized and cloned from kidney tissues the rat retinal dehydrogenase type 1 (RALDH1), which oxidizes all-trans and 9-cis retinal with high efficiency but is inactive with 13-cis retinal. Here we have characterized the retinal-oxidizing activity in monkey JTC12 cells, which are derived from kidney proximal tubules. In vitro assay of cell lysates revealed the presence of a NAD+-dependent dehydrogenase that catalyzed the oxidation of all-trans, 9-cis, and 13-cis retinal. Northern blot analysis of JTC12 RNAs and cloning by reverse transcription-polymerase chain reaction demonstrated expression of a monkey homolog of RALDH1. Bacterially expressed JTC12 RALDH1 catalyzed conversion of all three retinal isomers, with a higher catalytic efficiency for 9-cis retinal than for all-trans and 13-cis retinal. Accordingly, live JTC12 produced 9-cis retinoic acid more efficiently than all-trans retinoic acid from their respective retinal precursors. Only metabolites corresponding to the same steric conformation were formed from 9-cis or all-trans retinal, indicating a lack of detectable isomerizing activity in JTC12 cells.

Characterization of the rat RALDH1 promoter. A functional CCAAT and octamer motif are critical for basal promoter activity

Retinal dehydrogenase type 1 (RALDH1) catalyzes the oxidation of retinal to retinoic acid (RA), a metabolite of vitamin A important for embryogenesis and tissue differentiation. Rat RALDH1 is expressed to high levels in developing kidney, and in stomach, intestine epithelia. To understand the mechanisms of the transcriptional regulation of rat RALDH1, we cloned a 1360-base pair (bp) 5'-flanking region of RALDH1 gene. Using luciferase reporter constructs transfected into HEK 293 and LLCPK (kidney-derived) cells, basal promoter activity was associated with sequences between -80 and +43. In this minimal promoter region, TATA and CCAAT cis-acting elements as well as SP1, AP1 and octamer (Oct)-binding sites were present. The CCAAT box and Oct-binding site, located between positions -72 and -68 and -56 and -49, respectively, were shown by deletion analysis and site-directed mutation to be critical for promoter activity. Nuclear extracts from kidney cells contain proteins specifically binding the Oct and CCAAT sequences, resulting in the formation of six complexes, while different patterns of complexes were observed with non-kidney cell extracts. Gel shift assays using either single or double mutations of the Oct and CCAAT sequences as well as super shift assays demonstrated single and double occupancy of these two sites by Oct-1 and CBF-A. In addition, unidentified proteins also bound the Oct motif specifically in the absence of CBF-A binding. These results demonstrate specific involvement of Oct and CCAAT-binding proteins in the regulation of RALDH1 gene.