Development of aldehyde dehydrogenase and alcohol dehydrogenase in mouse eye: evidence for light-induced changes

Ethanol toxicity differs depending on the time of day

Ethanol is one of the most commonly abused drugs and consequently its toxic and psychoactive effect has been widely investigated, although little is known about the time-dependent effects of this drug. In the present research zebrafish was used to assess daily rhythms in ethanol toxicity and behavioural effects, as well as the temporal pattern of expression of key genes involved in ethanol detoxification in the liver (adh8a, adh5, aldh2.1 and aldh2.2). Our results showed marked differences in the mortality rate of zebrafish larvae depending on the time of day of the exposure to 5% ethanol for 1h (82% and 6% mortality in the morning and at night, respectively). A significant daily rhythm was detected with the acrophase located at "zeitgeber" time (ZT) = 04:22 h. Behavioural tests exposing zebrafish to 1% ethanol provoked a major decrease in swimming activity (68-84.2% reduction) at ZT2, ZT6 and ZT10. In contrast, exposure at ZT18 stimulated swimming activity (27% increase). During the day fish moved towards the bottom of the tank during ethanol exposure, whereas at night zebrafish increased their activity levels right after the exposure to ethanol. Genes involved in ethanol detoxification failed to show significant daily rhythms in LD, although all of them exhibited circadian regulation in constant darkness (DD) with acrophases in phase and located at the end of the subjective night. Taken altogether, this research revealed the importance of considering the time of day when designing and carrying out toxicological and behavioural tests to investigate the effects of ethanol, as the adverse effects of this drug were more marked when fish were exposed in the morning than at night.

Aldehyde dehydrogenase inhibitors: a comprehensive review of the pharmacology, mechanism of action, substrate specificity, and clinical application

Aldehyde dehydrogenases (ALDHs) belong to a superfamily of enzymes that play a key role in the metabolism of aldehydes of both endogenous and exogenous derivation. The human ALDH superfamily comprises 19 isozymes that possess important physiological and toxicological functions. The ALDH1A subfamily plays a pivotal role in embryogenesis and development by mediating retinoic acid signaling. ALDH2, as a key enzyme that oxidizes acetaldehyde, is crucial for alcohol metabolism. ALDH1A1 and ALDH3A1 are lens and corneal crystallins, which are essential elements of the cellular defense mechanism against ultraviolet radiation-induced damage in ocular tissues. Many ALDH isozymes are important in oxidizing reactive aldehydes derived from lipid peroxidation and thereby help maintain cellular homeostasis. Increased expression and activity of ALDH isozymes have been reported in various human cancers and are associated with cancer relapse. As a direct consequence of their significant physiological and toxicological roles, inhibitors of the ALDH enzymes have been developed to treat human diseases. This review summarizes known ALDH inhibitors, their mechanisms of action, isozyme selectivity, potency, and clinical uses. The purpose of this review is to 1) establish the current status of pharmacological inhibition of the ALDHs, 2) provide a rationale for the continued development of ALDH isozyme-selective inhibitors, and 3) identify the challenges and potential therapeutic rewards associated with the creation of such agents.

Corneal crystallins and the development of cellular transparency

Past studies have established that the cornea like the lens abundantly expresses a few water-soluble enzyme/proteins in a taxon specific fashion. Based on these similarities it has been proposed that the lens and the cornea form a structural unit, the 'refracton', that has co-evolved through gene sharing to maximize light transmission and refraction to the retina. Thus far, the analogy between corneal crystallins and lens crystallins has been limited to similarities in the abundant expression, with few reports concerning their structural function. This review covers recent studies that establish a clear relationship between expression of corneal crystallins and light scattering from corneal stromal cells, i.e. keratocytes, that support a structural role for corneal crystallins in the development of transparency similar to that of lens crystallins that would be consistent with the 'refracton' hypothesis.

Preferential transcription of rabbit Aldh1a1 in the cornea: implication of hypoxia-related pathways

Here we examine the molecular basis for the known preferential expression of rabbit aldehyde dehydrogenase class 1 (ALDH1A1) in the cornea. The rabbit Aldh1a1 promoter-firefly luciferase reporter transgene (-3519 to +43) was expressed preferentially in corneal cells in transfection tests and in transgenic mice, with an expression pattern resembling that of rabbit Aldh1a1. The 5' flanking region of the rabbit Aldh1a1 gene resembled that in the human gene (60.2%) more closely than that in the mouse (46%) or rat (51.5%) genes. We detected three xenobiotic response elements (XREs) and one E-box consensus sequence in the rabbit Aldh1a1 upstream region; these elements are prevalent in other highly expressed corneal genes and can mediate stimulation by dioxin and repression by CoCl(2), which simulates hypoxia. The rabbit Aldh1a1 promoter was stimulated fourfold by dioxin in human hepatoma cells and repressed threefold by CoCl(2) treatment in rabbit corneal stromal and epithelial cells. Cotransfection, mutagenesis, and gel retardation experiments implicated the hypoxia-inducible factor 3alpha/aryl hydrocarbon nuclear translocator heterodimer for Aldh1a1 promoter activation via the XREs and stimulated by retinoic acid protein 13 for promoter repression via the E-box. These experiments suggest that XREs, E-boxes, and PAS domain/basic helix-loop-helix transcription factors (bHLH-PAS) contribute to preferential rabbit Aldh1a1 promoter activity in the cornea, implicating hypoxia-related pathways.

Alcohol dehydrogenases and aldehyde dehydrogenases among inbred strains of mice: multiplicity, development, genetic studies and metabolic roles

Alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) are the major enzymes responsible for the metabolism of alcohols and aldehydes in the body. Both exist as a family of isozymes in mammals, and have been extensively studied in animal models, particularly among inbred strains of mice. Mouse ADH exists as at least three major classes, which are predominantly localized in liver (classes I and III), and in stomach/cornea (class IV). Mouse ALDH exhibits extensive multiplicity, several forms of which have been characterized, including ALDH1 (liver cytoplasmic/class 1 isozyme); ALDH2 (liver mitochondrial/class 2.); ALDH3 (stomach cytosolic/class 3); ALDH4 (liver microsomal/class 3); and ALDH5 (testis cytosolic/class 3). Biochemical, genetic and molecular genetic analyses have been performed on several of these enzymes, including studies on variant forms of ADH and ALDH. Distinct metabolic roles are proposed, based upon their tissue and subcellular distribution characteristics and the biochemical properties for these enzymes.

Enigma of the abundant water-soluble cytoplasmic proteins of the cornea: the "refracton" hypothesis

It is accepted that the taxon-specific, multifunctional crystallins (small heat-shock proteins and enzymes) serve structural roles contributing to the transparent and refractive properties of the lens. The transparent cornea also accumulates unexpectedly high proportions of taxon-specific, multifunctional proteins particularly, but not only, in the epithelium. For example, aldehyde dehydrogenase 3 (ALDH3) is the main water-soluble protein in corneal epithelial cells of most mammals (but ALDH1 predominates in the rabbit), whereas gelsolin predominates in the zebrafish corneal epithelium. Moreover, some invertebrates (e.g., squid and scallop) accumulate proteins in their corneas that are similar to their lens crystallins. Pax-6, among other transcription factors, is implicated in development and tissue-specific gene expression of the lens and cornea. Environmental factors appear to influence gene expression in the cornea, but not the lens. Although no direct proof exists, the diverse, abundant corneal proteins may have evolved a crystallinlike role, in addition to their enzymatic or cytoskeletal functions, by a gene sharing mechanism similar to the lens crystallins. Consequently, it is proposed that the cornea and lens be considered as a single refractive unit, called here the "refracton," to emphasize their similarities and common function.

Transketolase gene expression in the cornea is influenced by environmental factors and developmentally controlled events

PURPOSE

Transketolase (TKT) has been proposed to be a corneal crystallin, and its gene and protein are abundantly expressed in the corneal epithelium of several mammals. A marked up-regulation of TKT gene expression coincides with the time of eyelid opening in the mouse. Here, we examined whether exposure to incident light contributes to the up-regulation of TKT gene expression during cornea maturation.

METHODS

Mice were raised in either standard light/dark cycling conditions or total darkness. In some cases, subcutaneous injections of epidermal growth factor (EGF) were given beginning on the day of birth to induce early eyelid opening. RNA was prepared from the corneas of mothers and pups and subjected to Northern blot analyses. In addition, the relative levels of TKT mRNA and/or enzyme activity were examined in the corneas of human, bovine, rat, chicken, and zebrafish.

RESULTS

TKT mRNA levels were 2.1-fold higher in the corneas of 25-day-old mouse pups ( 12 days after eyelid opening) that had been born and raised in light/dark conditions compared to pups born and raised in total darkness. By contrast, the level of TKT mRNA in the mature corneas of adult mice maintained in the dark for 2-8 weeks did not vary greatly from those of mice maintained in light/dark conditions. Interestingly, TKT mRNA levels in the corneas of dark-raised mice, although reduced, did exhibit the increase characteristically observed before and after eyelid opening. In addition, TKT mRNA levels were elevated fivefold in the corneas of 28-day-old mice raised in darkness and injected with EGF compared to uninjected mice also deprived of light. The EGF-injected mice opened their eyes 3 days early, and their corneal epithelium did not grossly differ from that of control mice. TKT mRNA and/or enzyme activity was found to be much higher in the corneas than in other tissues of humans, bovines, and rats but was extremely low in the corneas of chicken and zebrafish.

CONCLUSION

Our studies suggest that both exposure to incident light and events surrounding the process of eyelid opening play a role in the up-regulation of TKT gene expression observed during corneal maturation in mice. Light appears to play a less important role in the mature cornea in maintaining high levels of TKT gene expression. The low levels of TKT in the cornea of chicken and zebrafish support the notion that TKT acts as a taxon-specific enzyme-crystallin in mammals. The involvement of environmental signals for this putative, mammalian cornea crystallin contrasts with the purely developmental signals involved in the up-regulation of the crystallin genes of the lens.

Enhancer-independent promoter activity of the mouse alphaB-crystallin/small heat shock protein gene in the lens and cornea of transgenic mice

The alphaB-crystallin/small heat shock protein gene is expressed very highly in the mouse eye lens and to a lesser extent in many other nonocular tissues, including the heart, skeletal muscle and brain. Previously we showed in transgenic mice that lens-specific alphaB-crystallin promoter activity is directed by a proximal promoter fragment (-164/+44) and that non-lens promoter activity depends on an upstream enhancer (-427/-259) composed of at least 5 cis-control elements. Here we have used truncated alphaB-crystallin promoter-CAT transgenes to test by biphasic CAT assays and/or histochemistry for specific expression in the cornea and lens. Deletion either of 87 bp (-427/-340) from the 5' end of the alphaB-crystallin enhancer or of the whole enhancer (-427/-258) abolished alphaB-crystallin promoter activity in all tissues except the lens and corneal epithelium when examined by the biphasic CAT assay in 4-5-week-old transgenic mice. These truncations also lowered promoter strength in the lens. The -426/+44-CAT, -339/+44-CAT and -164/+44-CAT (previously thought to be lens-specific in transgenic mice) transgenes were all expressed in the 4-6-week-old corneal epithelium when examined histochemically. Immunohistochemical staining confirmed the presence of endogenous alphaB-crystallin in the mature corneal epithelial cells. CAT gene expression driven by the alphaB-crystallin promoter with or without the enhancer was evident in the embryonic and 4-6-week-old lens. By contrast, activity of the alphaB-crystallin promoter/enhancer-CAT transgene was not detectable in the corneal epithelium before birth. Taken together, these results indicate that the intact enhancer of the alphaB-crystallin/small heat shock protein gene is required for promoter activity in all tissues tested except the lens and cornea.

Multifunctional lens crystallins and corneal enzymes. More than meets the eye

The abundant water-soluble proteins, called crystallins, of the transparent, refractive eye lens have been recruited from metabolic enzymes and stress-protective proteins by a process called "gene sharing." Many crystallins are also present at lower concentration in nonocular tissues where they have nonrefractive roles. The complex expression pattern of the mouse alpha B-crystallin/small heat shock protein gene is developmentally controlled at the transcriptional level by a combinatorial use of shared and lens-specific regulatory elements. A number of crystallin genes, including that for alpha B-crystallin, are activated by Pax-6, a conserved transcription factor for eye evolution. Aldehyde dehydrogenase class 3 and transketolase are metabolic enzymes comprising extremely high proportions of the water-soluble proteins of the cornea and may have structural as well as enzymatic roles, reminiscent of lens enzyme-crystallins. Inductive processes appear to be important for the corneal-preferred expression of these enzymes. The use of the same protein for entirely different functions by a gene-sharing mechanism may be a general strategy based on evolutionary tinkering at the level of gene regulation.

The mouse transketolase (TKT) gene: cloning, characterization, and functional promoter analysis

The transketolase (TKT) gene is expressed 30-50 times more highly in the mature mouse cornea than in other tissues. Here, we have cloned and characterized the 30- to 40-kb single-copy mouse TKT gene. Sequence analysis supports the suggestion that present-day TKT and TKT-like genes arose from the duplication of a single common ancestral gene. A 6-bp polymorphism is present between different mouse strains in the noncoding region of exon 2. 5' RACE and primer extension analyses indicated that two regions separated by 630 bp are used as transcription initiation sites; both mRNAs appear to use a common initiator ATG codon. The minor distal transcription initiation site, preceded by a TATA sequence, is utilized in liver and is followed by an untranslated exon (exon 1). The major proximal transcription initiation site lies within intron 1, is used in cornea and liver, lacks a TATA sequence, is GC rich, and initiates at multiple sites within a 10-bp span, resembling the promoters of other housekeeping genes. In transfected cornea and lens cell lines, the -49/+90 fragment fused to the CAT gene acted as a minimal promoter, with higher activity noted for the -510/+91 fragment. TKT mRNA levels increased sixfold in the mouse cornea in vivo within 1-2 days of eye opening and were elevated in a lens cell line exposed to H2O2 or the glutathione-specific oxidizing agent diamide and in whole newborn mouse eyes incubated in the presence of light, consistent with multiple consensus stress-inducible control sequences in the TKT promoter regions. Taken together, these observations suggest that oxidative stress may play a role in the regulation of this gene in the cornea.

Aldehyde dehydrogenase class 3 expression: identification of a cornea-preferred gene promoter in transgenic mice

Aldehyde dehydrogenase class 3 (ALDH3) constitutes 20-40% of the total water-soluble proteins in the mammalian cornea. Here, we show by Northern blot analysis that ALDH3 expression in the mouse is at least 500-fold higher in the cornea than in any other tissue examined, with very low levels of expression detected in the stomach, urinary bladder, ocular lens, and lung. Histochemical localization reveals that this exceptional level of expression in the mouse cornea occurs in the anterior epithelial cells and that little ALDH3 is present in the keratocytes or corneal endothelial cells. A 13-kbp mouse ALDH3 promoter fragment containing >12 kbp of the 5' flanking sequence, the 40-bp untranslated first exon, and 29 bp of intron 1 directed cat reporter gene expression to tissues that express the endogenous ALDH3 gene, except that transgene promoter activity was higher in the stomach and bladder than in the cornea. By contrast, when driven by a 4.4-kbp mouse ALDH3 promoter fragment [1,050-bp 5' flanking region, exon 1, intron 1 (3.4 kbp), and 7 bp of exon 2] expression of the cat reporter gene was confined to the corneal epithelial cells, except for very low levels in the liver, effectively reproducing the corneal expression pattern of the endogenous ALDH3 gene. These results indicate that tissue-specific expression of ALDH3 is determined by positive and negative elements in the 5' flanking region of the gene and suggests putative silencers located in intron 1. We demonstrate regulatory sequences capable of directing cornea-specific gene expression, affording the opportunity for genetic engineering in this transparent tissue.

A genetic basis for corneal sensitivity to ultraviolet light among recombinant SWXJ inbred strains of mice

PURPOSE

To examine a possible genetic basis for corneal sensitivity to UV-B light exposure.

METHODS

To this end, adult male mice from the 14 SWXJ recombinant inbred albino strains (originating from SJL/J and SWR/J parental strains) were subjected to ultraviolet (UV) radiation exposure of 0.078 J/cm2 and photographed four days post-exposure, to assess corneal opacity and the possible correlation with corneal aldehyde dehydrogenase (ALDH) activity, alcohol dehydrogenase (ADH) activity and soluble protein content.

RESULTS

Those recombinant strains that exhibited the SWR/J strain phenotype of having low levels of ALDH and decreased soluble protein levels also exhibited greater levels of corneal clouding after UV-exposure than the other strains, which exhibited "normal" levels of both ALDH activity and soluble protein in the cornea.

CONCLUSIONS

These data support an hypothesis for a major role for ALDH in assisting the cornea to protect the eye against UV-induced tissue damage.