Cubozoan jellyfish: an Evo/Devo model for eyes and other sensory systems

Cnidaria are the most basal phylum containing a well-developed visual system located on specialized sensory structures (rhopalia) with eyes and statocyts. We have been exploring the cubozoan jellyfish, Tripedalia cystophora. In addition to containing simple photoreceptive ocelli, each rhopalium in Tridedalia has a large and small complex, camera-type eye with a cellular lens containing three distinct families of crystallins which apparently serve non-lenticular functions. Thus, Tridpedalia recruited crystallins by a gene sharing strategy as have mollusks and vertebrates. Tripedalia has a single Pax gene, PaxB, which encodes a structural and functional Pax 2/5/8-like paired domain as well as an octapeptide and Pax6-like homeodomain. PaxB binds to and activates Tripedalia crystallin promoters (especially J3-crystallin) and the Drosophila rhodopsin rh6 gene in transfection tests and induces ectopic eyes in Drosophila. In situ hybridization showed that PaxB and crystallin genes are expressed in the lens, retina and statocysts. We suggest from these results that an ancestral PaxB gene was a primordial gene in eye evolution and that eyes and ears (mechanoreceptors) may have had a common evolutionary origin. Thus, the numerous structural and molecular features of Tridpalia rhopalia indicate that ancient cubozoan jellyfish are fascinating models for evo/devo insights into eyes and other sensory systems.

Abundant corneal gelsolin in Zebrafish and the 'four-eyed' fish, Anableps anableps: possible analogy with multifunctional lens crystallins

The cornea accumulates high proportions (can be up to 50%) of taxon-specific, water-soluble, cytoplasmic proteins (often enzymes) that have been considered analogous to the multifunctional lens crystallins. We have shown that gelsolin (an actin-severing protein) is the major water-soluble corneal protein of the zebrafish (Danio rerio) and the 'four-eyed' fish (Anableps anableps). Each Anableps eye contains one lens, an aquatic ventral cornea with an epithelium comprising 5-7 cell layers, and an air-exposed flatter dorsal cornea with an epithelium comprising >20 cell layers and appreciably enriched with glycogen. Gelsolin accounts for 38 and 21% of the dorsal and ventral cornea, respectively, suggesting that the abundance of gelsolin in the cornea is not incompatible with its function in air. The thicker, glycogen-enriched, air-exposed dorsal cornea may protect against UV irradiation and desiccation. Gelsolin comprises approximately 50% of the 5 cell-layer thick aquatic corneal epithelium of zebrafish. Reported zebrafish ESTs have indicated the presence of a second gelsolin gene in this species. We show by RT-PCR that the abundant corneal gelsolin (also expressed weakly in lens) (C/L-gelsolin) is also expressed in early development and differs from a ubiquitously expressed gelsolin (U-gelsolin) that is not specialized for cornea. Microinjection tests showed that overexpression of C/L-gelsolin dorsalizes the embryo and can lead to axis duplication, while interruption of C/L-gelsolin expression with a specific morpholino oligonucleotide ventralizes the embryo and interferes with brain and eye development. The evidence that C/L-gelsolin participates in the bone morphogenetic protein (BMP)/Smad dorsal-ventral signaling pathway is reviewed. Finally, we speculate that soluble C/L-gelsolin:actin complexes in the cornea may be analogous to soluble alphaA:alphaB-crystallin complexes in the lens. Together, our data are consistent with an analogy between the abundance of gelsolin in fish corneas and taxon-specific multifunctional crystallins in lenses.

Vertebrate-like ??-crystallins in the ocular lenses of a copepod

The diverse crystallins are water-soluble proteins that are responsible for the optical properties of cellular lenses of animal eyes. While all vertebrate lenses contain physiological stress-related alpha- and betagamma-crystallins, some also contain taxon-specific, often enzyme-related crystallins. To date, the alpha- and betagamma-crystallins have been found only in vertebrate lenses. Here we report lenses from an invertebrate, the pontellid copepod Anomalocera ornata, accumulate betagamma-crystallin family members as judged by immunocytochemistry, western immunoblotting and microsequencing. Our data provide the first example of betagamma-crystallin members in an invertebrate lens, establishing that the use of this protein family as lens crystallins is not confined to vertebrates.

Role of Aldehyde Dehydrogenase Isozymes in the Defense of Rat Lens and Human Lens Epithelial Cells against Oxidative Stress

PURPOSE

4-Hydroxynonenal (HNE), a metastable lipid peroxidation product, is highly toxic to various cell types if not detoxified. Because of its constant exposure to light, the ocular lens continuously generates reactive oxygen species which, under conditions of oxidative stress, may lead to excessive lipid peroxidation and consequent formation of lipid-derived aldehydes (LDAs) such as HNE. The contribution of various isozymes of aldehyde dehydrogenase (ALDH) to the oxidation of LDAs has never been systematically investigated in the lens. The present study was undertaken to ascertain the role of ALDH1A1 and -3A1 in HNE metabolism and HNE-induced toxicity in cultured human lens epithelial cells (HLECs) and in rat and mouse lenses.

METHODS

The metabolism of 3H-HNE was studied in ALDH3A1-knockout mouse lens and in HLECs transfected with ALDH1A1- or -3A1-specific antisense RNA and short interfering (Si)RNA. Appropriate controls were used, including wild-type mouse lens, scrambled oligonucleotides, and a transfection reagent. Transfected HLECs were exposed to oxidative stress (Fenton reaction) or HNE (30 microM) for 3 hours. Toxicity parameters, such as cell viability, apoptosis, and protein-HNE adducts and oxidation of exogenously added 3H-HNE were measured. Rat lenses were transfected with the SiRNA specific to ALDH1A1, and oxidation of 3H-HNE and the susceptibility of the transfected lenses to oxidation-induced opacification were measured.

RESULTS

Rat lenses transfected with ALDH1A1-specific SiRNA, or cultured in the presence of the ALDH inhibitor cyanamide/disulfiram and subjected to oxidative stress displayed accelerated loss of transparency and a diminished capacity to oxidize HNE. Similarly, inhibition of ALDH1A1 in HLECs by ALDH1A1-specific antisense RNA or SiRNA was associated with decreased oxidation of 3H-HNE and increased susceptibility of the cells to oxidative damage, including apoptosis. Furthermore, 3H-HNE metabolism and HNE-induced toxicity were not affected in ALDH3A1-specific SiRNA- or antisense RNA-treated rat lenses, HLECs, or ALDH3A1-null mouse lenses.

CONCLUSIONS

The results suggest that, under oxidative stress, HNE produced in the lens epithelium can cause toxicity and thus contribute to oxidation-induced cataractogenesis. Furthermore, the studies indicate that ALDH1A1 is a critical isozyme for maintaining clarity in human, rat, and mouse lenses.

Postnatal gene expression in the normal mouse cornea by SAGE

PURPOSE

To provide a detailed gene expression profile of the normal postnatal mouse cornea.

METHODS

Serial analysis of gene expression (SAGE) was performed on postnatal day (PN)9 and adult mouse (6 week) total corneas. The expression of selected genes was analyzed by in situ hybridization.

RESULTS

A total of 64,272 PN9 and 62,206 adult tags were sequenced. Mouse corneal transcriptomes are composed of at least 19,544 and 18,509 unique mRNAs, respectively. One third of the unique tags were expressed at both stages, whereas a third was identified exclusively in PN9 or adult corneas. Three hundred thirty-four PN9 and 339 adult tags were enriched more than fivefold over other published nonocular libraries. Abundant transcripts were associated with metabolic functions, redox activities, and barrier integrity. Three members of the Ly-6/uPAR family whose functions are unknown in the cornea constitute more than 1% of the total mRNA. Aquaporin 5, epithelial membrane protein and glutathione-S-transferase (GST) omega-1, and GST alpha-4 mRNAs were preferentially expressed in distinct corneal epithelial layers, providing new markers for stratification. More than 200 tags were differentially expressed, of which 25 mediate transcription.

CONCLUSIONS

In addition to providing a detailed profile of expressed genes in the PN9 and mature mouse cornea, the present SAGE data demonstrate dynamic changes in gene expression after eye opening and provide new probes for exploring corneal epithelial cell stratification, development, and function and for exploring the intricate relationship between programmed and environmentally induced gene expression in the cornea.

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.

Mafs, Prox1, and Pax6 can regulate chicken betaB1-crystallin gene expression

During lens fiber cell differentiation, the regulation of crystallin gene expression is coupled with dramatic morphological changes. Here we report that Mafs, Prox1, and Pax6, which are essential transcription factors for normal lens development, bind to three functionally important cis elements, PL1, PL2, and OL2, in the chicken betaB1-crystallin promoter and may cooperatively direct the transcription of this lens fiber cell preferred gene. Gel shift assays demonstrated that Mafs bind to the MARE-like sequences in the PL1 and PL2 elements, whereas Prox1, a sequence-specific DNA-binding protein like its Drosophila homolog Prospero, interacts with the OL2 element. Furthermore, Pax6, a known repressor of the chicken betaB1-crystallin promoter, binds to all three of these cis elements. In transfection assays, Mafs and Prox1 activated the chicken betaB1-crystallin promoter; however, their transactivation ability was repressed when co-transfected with Pax6. Taken together with the known spatiotemporal expression patterns of Mafs, Prox1, and Pax6 in the developing lens, we propose that Pax6 occupies and represses the chicken betaB1-crystallin promoter in lens epithelial cells, and is displaced by Prox1 and Mafs, which activate the promoter, in differentiating cortical fiber cells.

Role of Pax genes in eye evolution: a cnidarian PaxB gene uniting Pax2 and Pax6 functions

PaxB from Tripedalia cystophora, a cubomedusan jellyfish possessing complex eyes (ocelli), was characterized. PaxB, the only Pax gene found in this cnidarian, is expressed in the larva, retina, lens, and statocyst. PaxB contains a Pax2/5/8-type paired domain and octapeptide, but a Pax6 prd-type homeodomain. Pax2/5/8-like properties of PaxB include a DNA binding specificity of the paired domain, activation and inhibitory domains, and the ability to rescue spa(pol), a Drosophila Pax2 eye mutant. Like Pax6, PaxB activates jellyfish crystallin and Drosophila rhodopsin rh6 promoters and induces small ectopic eyes in Drosophila. Pax6 has been considered a "master" control gene for eye development. Our data suggest that the ancestor of jellyfish PaxB, a PaxB-like protein, was the primordial Pax protein in eye evolution and that Pax6-like genes evolved in triploblasts after separation from Cnidaria, raising the possibility that cnidarian and sophisticated triploblastic eyes arose independently.

Serum albumin in mammalian cornea: implications for clinical application

PURPOSE

To compare the abundance and spatial distribution of serum albumin in the mouse and bovine cornea.

METHODS

Serum albumin from cornea was separated from transketolase by SDS-PAGE (+/-dithiothreitol [DTT]) and identified by peptide sequencing and immunoblot analyses. The fractional content of serum albumin was determined in water-soluble extracts of cornea by imaging analyses after SDS-PAGE. Serum albumin was localized in cornea by immunohistochemistry and by SDS-PAGE analyses of samples from separated epithelium and stroma.

RESULTS

SDS-PAGE (-DTT) resolved mouse serum albumin and transketolase and indicated that serum albumin was 13% of the water-soluble protein in whole mouse corneas. By contrast, corneal epithelial fractions contained little (<1%) serum albumin. Immunohistochemistry indicated that mouse serum albumin was present throughout the stroma between collagen lamellae. Immunohistochemical analyses of bovine cornea yielded similar results. In addition, immunohistochemistry for serum albumin revealed positive staining in a small number of basal epithelial cells next to Bowman's membrane, and greater staining in the anterior-peripheral stroma as well as immediately adjacent to Descemet's membrane.

CONCLUSIONS

Mouse and bovine cornea have a similar content and spatial distribution of serum albumin. The appreciable serum albumin in the cornea documented here and elsewhere raise the possibility that it contributes to the physiological or optical functions of the cornea. Moreover, serum albumin's ability to bind drugs suggests that mice corneas could be exploited to study drug-serum albumin interactions in vivo and to test the usefulness of serum albumin as a drug carrier for corneal disorders.

Requirement for Pax6 in corneal morphogenesis: a role in adhesion

The Pax6 transcription factor functions early during embryogenesis to control key steps in brain, pancreas, olfactory and ocular system development. A requirement for Pax6 in proper formation of lens, iris and retina is well documented. By examining the corneas of heterozygous Small eye (SEY) mice, this report shows that Pax6 is also necessary for normal corneal morphogenesis. In particular, the epithelial component of the postnatal and adult SEY (+/-) cornea is thinner owing to a reduction in the number of cell layers, despite a tenfold increase in the proliferative index and no change in TUNEL labeling. Ultrastructural views revealed large gaps between corneal epithelial cells and a change in the appearance of desmosomes, suggesting that adhesion abnormalities contribute to the corneal phenotype of SEY (+/-) mice. Western blot analysis and immunofluorescence showed equivalent amounts and normal localization of E-cadherin in SEY (+/-) corneas, and the actin cytoskeleton appeared normal as judged by phalloidin staining. By contrast, the levels of desmoglein, beta-catenin and gamma-catenin were reduced in the SEY (+/-) cornea. In addition, the amount of keratin-12 mRNA and protein, the major intermediate filament, was reduced in SEY (+/-) corneal epithelium as shown by in situ hybridization and immunohistochemistry. Finally, the SEY (+/-) corneal epithelium adheres less well than wild-type when challenged with gentle rubbing using a microsponge. In conclusion, our results indicate that cellular adhesion is compromised in the SEY (+/-) corneal epithelium and suggests a role for Pax6 in the proper generation and maintenance of the adult cornea.

BIR-1, a Caenorhabditis elegans homologue of Survivin, regulates transcription and development

bir-1, a Caenorhabditis elegans inhibitor-of-apoptosis gene homologous to Survivin is organized in an operon with the transcription cofactor C. elegans SKIP (skp-1). Because genes arranged in operons are frequently linked functionally, we have asked whether BIR-1 also functions in transcription. bir-1 inhibition resulted in multiple developmental defects that overlapped with C. elegans SKIP loss-of-function phenotypes: retention of eggs, dumpy, movement defects, and lethality. bir-1 RNA-mediated interference decreased expression of several gfp transgenes and the endogenous genes dpy-7 and hlh-1. Immunoblot analysis revealed decreased phosphoacetylated histones in bir-1 RNA-mediated interference-treated worms. In a heterologous transfection system, BIR-1 augments thyroid hormone-regulated transcription and has an additive effect with SKIP. These results show that BIR-1 functions in the regulation of transcription and development.

Gelsolin is a dorsalizing factor in zebrafish

The gene for gelsolin (an actin-binding, cytoskeletal regulatory protein) was shown earlier to be specialized for high corneal expression in adult zebrafish. We show here that zebrafish gelsolin is required for proper dorsalization during embryogenesis. Inhibition of gelsolin expression by injecting fertilized eggs with a specific morpholino oligonucleotide resulted in a range of concentration-dependent ventralized phenotypes, including those lacking a brain and eyes. These were rescued by coinjection of zebrafish gelsolin or chordin (a known dorsalizing agent) mRNAs, or human gelsolin protein. Moreover, injection of gelsolin mRNA or human gelsolin protein by itself dorsalized the developing embryos, often resulting in axis duplication. Injection of the gelsolin-specific morpholino oligonucleotide enhanced the expression of Vent mRNA, a ventral marker downstream of bone morphogenetic proteins, whereas injection of gelsolin mRNA enhanced the expression of chordin and goosecoid mRNAs, both dorsal markers. Our results indicate that gelsolin also modulates embryonic dorsalventral pattern formation in zebrafish.

Crystallin genes: specialization by changes in gene regulation may precede gene duplication

The crystallins account for 80-90% of the water-soluble proteins of the transparent lens. These diverse proteins are responsible for the optical properties of the lens and have been recruited from metabolic enzymes and stress proteins. They often differ among species (i.e. are taxon-specific) and may be expressed outside of the lens where they have non-refractive roles (a situation we call gene sharing). Crystallin recruitment has occurred by changes in gene regulation resulting in high lens expression. Duck lactate dehydrogenase/epsilon-crystallin and alpha-enolase/tau-crystallin are each encoded in single-copy genes, consistent with these enzymes acquiring a crystallin role, without loss of their nonlens metabolic function, by a change in gene regulation in the absence of gene duplication. The small heat shock protein/alpha-crystallins and avian argininosuccinate lyase/delta-crystallins were also recruited by a change in gene regulation leading to high lens expression, except this was followed by a gene duplication with further lens specialization of the alphaA and the delta1 (in chickens) crystallin genes. Cephalopod (squid and octopus) S-crystallins were recruited from glutathione S-transferase apparently after duplication of the original gene encoding the enzyme, although this remains uncertain. We speculate that one of the new genes (glutathione S-transferase/S11-crystallin) specialized for lens expression by a change in gene regulation and subsequently duplicated many times to form the lens-specialized, multiple S-crystallins that lack enzymatic activity. That similar transcription factors (e.g. Pax-6, retinoic acid receptors, maf, Sox, AP-1, CREB) regulate different crystallin genes suggest that common features of lens-specific expression have played a pivotal role for recruiting the diverse, multifunctional proteins as crystallins.

Orientation-dependent influence of an intergenic enhancer on the promoter activity of the divergently transcribed mouse Shsp/alpha B-crystallin and Mkbp/HspB2 genes

The mouse Shsp/alphaB-crystallin and Mkbp/HspB2 genes are closely linked and divergently transcribed. In this study, we have analyzed the contribution of the intergenic enhancer to Shsp/alphaB-crystallin and Mkbp/HspB2 promoter activity using dual-reporter vectors in transient transfection and transgenic mouse experiments. Deletion of the enhancer reduced Shsp/alphaB-crystallin promoter activity by 30- and 93-fold and Mkbp/HspB2 promoter activity by 6- and 10-fold in transiently transfected mouse lens alpha-TN4 and myoblast C2C12 cells, respectively. Surprisingly, inversion of the enhancer reduced Shsp/alphaB-crystallin promoter activity by 17-fold, but did not affect Mkbp/HspB2 promoter activity in the transfected cells. In contrast, enhancer activity was orientation-independent in combination with a heterologous promoter in transfected cells. Transgenic mouse experiments established the orientation dependence and Shsp/alphaB-crystallin promoter preference of the intergenic enhancer in its native context. The orientation dependence and preferential effect of the Shsp/alphaB-crystallin enhancer on the Shsp/alphaB-crystallin promoter provide an example of adaptive changes in gene regulation accompanying the functional diversification of duplicated genes during evolution.

Functional and structural characterization of the human gene BHLHB5, encoding a basic helix-loop-helix transcription factor

The genes encoding basic helix-loop-helix (bHLH) transcription factors have been implicated in many aspects of neural development, including cell growth, differentiation, and cell migration. Using both genomic and cDNA mouse and human clones encoding a neural-specific bHLH protein, human BHLHB5 was cloned and mapped to a region on chromosome 8q13 that segregates with Duane syndrome. Genomic sequence analysis of human BHLHB5 and mouse Bhlhb5 revealed that they contain a single exon encoding 381- and 355-amino-acid bHLH proteins, respectively. Multiple amino acid sequence alignments of the Bhlhb5 family members revealed several conserved motifs and an identical 147-amino-acid carboxy-terminal region that contains a 60-amino-acid bHLH domain. A 27-bp trinucleotide repeat (CAG)(9) encoding polyserine was found in human BHLHB5, but only one CAG was found at the corresponding position in the mouse Bhlhb5 and hamster BETA3 genes. Northern blot analysis of human BHLHB5 revealed brain-specific expression with the highest abundance in the cerebellum. Mouse Bhlhb5 can strongly repress a human PAX6 promoter.

Transketolase haploinsufficiency reduces adipose tissue and female fertility in mice

Transketolase (TKT) is a ubiquitous enzyme used in multiple metabolic pathways. We show here by gene targeting that TKT-null mouse embryos are not viable and that disruption of one TKT allele can cause growth retardation ( approximately 35%) and preferential reduction of adipose tissue ( approximately 77%). Other TKT(+/-) tissues had moderate ( approximately 33%; liver, gonads) or relatively little ( approximately 7 to 18%; eye, kidney, heart, brain) reductions in mass. These mice expressed a normal level of growth hormone and reduced leptin levels. No phenotype was observed in the TKT(+/-) cornea, where TKT is especially abundant in wild-type mice. The small female TKT(+/-) mice mated infrequently and had few progeny (with a male/female ratio of 1.4:1) when pregnant. Thus, TKT in normal mice appears to be carefully balanced at a threshold level for well-being. Our data suggest that TKT deficiency may have clinical significance in humans and raise the possibility that obesity may be treated by partial inhibition of TKT in adipose tissue.

Disruption of anterior segment development by TGF-β1 overexpression in the eyes of transgenic mice

Previous experiments showed that transgenic mice expressing a secreted self-activating transforming growth factor (TGF) -beta1 did not show a phenotype in the lens and cornea until postnatal day 21, when anterior subcapsular cataracts, sporadic thickening of the corneal stroma, and thinning of the corneal epithelium were noted (Srinivasan et al., 1998). To examine the effects of higher concentrations of TGF-beta1 on the lens and cornea, we constructed transgenic mice harboring the strong, lens-specific chicken betaB1-crystallin promoter driving an activated porcine TGF-beta1 gene. In contrast to the earlier study, the transgenic mice had microphthalmic eyes with closed eyelids. Already at embryonic day (E) 13.5, the future cornea of the transgenic mice was threefold thicker than that of wild-type littermates due to increased proliferation of corneal stromal mesenchyme cells. Staining of fibronectin and thrombospondin-1 was increased in periocular mesenchyme. At E17.5, the thickened transgenic corneal stroma was vascularized and densely populated by abundant star-shaped, neural cell adhesion molecule-positive cells of mesenchymal appearance surrounded by irregular swirls of collagen and extracellular matrix. The corneal endothelium, anterior chamber, and stroma of iris/ciliary body did not develop, and the transgenic cornea was opaque. Fibronectin, perlecan, and thrombospondin-1 were elevated, whereas type VI collagen decreased in the transgenic corneal stroma. Stromal mesenchyme cells expressed alpha-smooth muscle actin as did lens epithelial cells and cells of the retinal pigmented epithelium. By E17.5, lens fiber cells underwent apoptotic cell death that was followed by apoptosis of the entire anterior lens epithelium between E18.5 and birth. Posteriorly, the vitreous humor was essentially absent; however, the retina appeared relatively normal. Thus, excess TGF-beta1, a mitogen for embryonic corneal mesenchyme, severely disrupts corneal and lens differentiation. Our findings profoundly contrast with the mild eye phenotype observed with presumably lower levels of ectopic TGF-beta and illustrate the complexity of TGF-beta utilization and the importance of dose when assessing the effects of this growth factor.

Adaptive evolution of small heat shock protein/alpha B-crystallin promoter activity of the blind subterranean mole rat, Spalax ehrenbergi

Blind mole rats have degenerated subcutaneous eyes that are visually nonfunctional. In this investigation, we have compared the tissue specificity of the small heat shock protein (shsp)/alphaB-crystallin promoter of the mole rat superspecies, Spalax ehrenbergi, with that of the mouse. Earlier experiments showed that mouse shsp/alphaB-crystallin promoter/enhancer activity is high in the lens and moderate in the heart and skeletal muscle of transgenic mice. Here, we show in transgenic mouse experiments using the firefly luciferase reporter gene that, despite relatively few changes in sequence, the mole rat shsp/alphaB-crystallin promoter/enhancer has selectively lost lens activity after 13.5 days of embryogenesis (E13.5). The ratios of mole rat/mouse promoter activity were 0.01 for lens, 1.7 for heart, and 13.6 for skeletal muscle in 8-wk-old transgenic mice. Our data indicate that the shsp/alphaB-crystallin promoter/enhancer has undergone adaptive changes corresponding to the subterranean evolution of the blind mole rat. We speculate that selective pressures on metabolic economy may have contributed to these tissue-specific modifications of promoter/enhancer function during adaptation to life underground.

Ectopic expression of AP-2alpha transcription factor in the lens disrupts fiber cell differentiation

AP-2alpha is a developmentally important transcription factor which has been implicated in the regulation of cell growth, programmed cell death, and differentiation. To investigate the specific function of AP-2alpha in differentiation of the lens, AP-2alpha was expressed in the differentiating lens fiber cells under control of the alphaA-crystallin promoter. Normally, AP-2alpha is selectively expressed in lens epithelial cells and expression terminates at the lens equator, where epithelial cells terminally differentiate into fiber cells. Ectopic expression of the AP-2alpha gene in the fiber cell compartment resulted in bilateral cataracts and microphthalmia in mice by 2 weeks of age. Histological evaluation of embryonic and adult transgenic lenses revealed a significant reduction in lens size and anterior shifting of the transitional zone. Two aspects of fiber cell differentiation were also blocked, including the migration of newly formed fiber cells and an inhibition in fiber cell denucleation. Correlated with these defects were expanded expression of E-cadherin in the lens transitional zone and reduced expression of the fiber cell-specific protein MIP (major intrinsic protein). Together, these data demonstrate that AP-2alpha acts as a negative regulator of terminal fiber cell differentiation through the regulation of genes involved in cell adhesion and migration.

Structurally normal corneas in aldehyde dehydrogenase 3a1-deficient mice

We have constructed an ALDH3a1 null mouse to investigate the role of this enzyme that comprises nearly one-half of the total water-soluble protein in the mouse corneal epithelium. ALDH3a1-deficient mice are viable and fertile, have a corneal epithelium with a water-soluble protein content approximately half that of wild-type mice, and contain no ALDH3a1 as determined by zymograms and immunoblots. Despite the loss of protein content and ALDH3a1 activity, the ALDH3a1(-/-) mouse corneas appear indistinguishable from wild-type corneas when examined by histological analysis and electron microscopy and are transparent as determined by light and slit lamp microscopy. There is no evidence for a compensating protein or enzyme. Even though the function of ALDH3a1 in the mouse cornea remains unknown, our data indicate that its enzymatic activity is unnecessary for corneal clarity and maintenance, at least under laboratory conditions.