Molecular analyses of carbonic anhydrase-II expression and regulation in the developing chicken lens

The expression of carbonic anhydrase-II (CA-II) in the developing chicken lens was examined and compared with that in the retina of the chicken embryo. CA-II expression was measured by immunohistochemistry and radioimmunoassay during development, and CA-II mRNA was quantified by Northern blot and densitometric scanning and localized by in situ hybridization. A functional promoter of the chicken CA-II gene was identified by transfection of primary embryonic chicken lens epithelial cells and analyzed in deletion mutants. The results establish that CA-II makes up about 0.1% of the total soluble protein of the embryonic chicken lens, an amount insufficient to make it a candidate for an enzyme crystallin in this species. Lens fiber differentiation coincided with a loss of CA-II mRNA and protein; by contrast, CA-II persisted in the epithelial cells of the embryonic and mature lens. This and previous studies showed that CA-II amounts to as much as 3% of the protein of the embryonic chicken retina and follows a different developmental time course of expression; like the lens, CA-II decreases until day 10 in the embryonic retina, but, unlike the lens, it increases thereafter and plateaus at hatching. Progressive deletions of the 5' flanking regions (from position -1314 to +32) of the CA-II gene fused to the bacterial chloramphenicol acetyltransferase (CAT) reporter gene resulted in a gradual loss of promoter activity, consistent with an additive effect of putative cis-regulatory elements found in many crystallin genes. These experiments provide the foundation for a molecular analysis of the developmental and differential regulation of the CA-II gene in lens and retina.

Taxon-specific recruitment of enzymes as major soluble proteins in the corneal epithelium of three mammals, chicken, and squid

Studies of others have shown that class 3 aldehyde dehydrogenase is a major component of the epithelial cells of the mammalian cornea. Here we demonstrate by peptide sequencing that other major proteins of the corneal epithelium are also identical or related to enzymes in the human, mouse, kangaroo, chicken, and squid. Aldehyde dehydrogenase class 3 was found to be the major protein of human, mouse, and kangaroo corneal epithelial cells. Peptidyl prolyl cis-trans isomerase (cyclophilin) or a homologue thereof is strikingly abundant in the corneal epithelial cells of chicken, but not mammals, and appears to be absent from the cornea of squid. By contrast, enolase or its homologue is relatively abundant in both the mammalian and chicken corneal epithelial cells. In some instances, abundant enzymes are common to cornea and lens in the same species--for example, arginino-succinate lyase/delta 1-crystallin in the chicken and glutathione S-transferase-like protein in the squid; in other cases, the abundant proteins in the cornea have not been found as lens crystallins in any species--for example, aldehyde dehydrogenase class 3 and cyclophilin. These data suggest that enzymes and certain enzyme-crystallins have been recruited as major corneal proteins in a taxon-specific manner and may serve structural rather than, or as well as, enzymatic roles in corneal epithelial cells.

Characterization of squid crystallin genes. Comparison with mammalian glutathione S-transferase genes

Previous experiments have indicated that the crystallins of the squid lens (S-crystallins) are evolutionarily related to glutathione S-transferases (GST) (EC 2.5.1.18). Here we confirm by peptide sequencing that the crystallins of the lens of the squid Ommastrephes sloani pacificus comprise a family of GST-like proteins. Squid lens extracts showed 400 times less GST activity than those of liver using 1-chloro-2,4-dinitrobenzene as a substrate, suggesting that the abundant GST-like crystallins lack enzymatic activity. Four different cDNAs (pSL20-1, pSL18, pSL11, and pSL4) showed 20-25% similarity in homologous regions with mammalian GST polypeptides. pSL20-1, pSL18, and pSL4 each encode an S-crystallin with a unique internal peptide that is unrelated to mammalian GSTs or any other sequence in GenBank. The S-crystallin family is encoded in a minimum of 9-10 genes, and the exon-intron structures of at least two of these (SL20-1 and SL11) are similar to those of the mammalian GST genes. The SL20-1 gene has six exons, with the its unique internal peptide encoded precisely in exon 4; the SL11 gene lacks a unique internal peptide and has five exons. Experiments using bacterial chloramphenicol acetyltransferase as a reporter gene showed that at least 84 and 111 base pairs of 5'-flanking sequence are needed for function of the SL20-1 and SL11 promoters, respectively, in a transfected rabbit lens epithelial cell line (N/N1003A). Within these regions each has a putative TATA box and an upstream AP-1 site overlapping with antioxidant responsive-like elements, which are regulatory elements in the rat GST Ya and quinone reductase genes responsive to oxidative stress.

Crystallins of the octopus lens. Recruitment from detoxification enzymes

The eye lens crystallins of the octopus Octopus dofleini were identified by sequencing abundant proteins and cDNAs. As in squid, the octopus crystallins have subunit molecular masses of 25-30 kDa, are related to mammalian glutathione S-transferases (GST), and are encoded in at least six genes. The coding regions and deduced amino acid sequences of four octopus lens cDNAs are 75-80% identical, while their non-coding regions are entirely different. Deduced amino acid sequences show 52-57% similarity with squid GST-like crystallins, but only 20-25% similarity with mammalian GST. These data suggest that the octopus and squid lens GST-like crystallin gene families expanded after divergence of these species. Northern blot hybridization indicated that the four octopus GST-like crystallin genes examined are lens-specific. Lens extracts showed about 40 times less GST activity using 1-chloro-2,4-dinitrobenzene as substrate than liver extracts of the octopus, indicating that the major GST-like crystallins are specialized for a lens structural role. A prominent 59-kDa crystallin polypeptide, previously observed in octopus but not squid and called omega-crystallin (Chiou, S.-H. (1988) FEBS Lett. 241, 261-264), has been identified as an aldehyde dehydrogenase. Since cytoplasmic aldehyde dehydrogenase is a major protein in elephant shrew lenses (eta-crystallin; Wistow, G., and Kim, H. (1991) J. Mol. Evol. 32, 262-269) the octopus aldehyde dehydrogenase crystallin provides the first example of a similar enzyme-crystallin in vertebrates and invertebrates. The use of detoxification stress proteins (GST and aldehyde dehydrogenase) as cephalopod crystallins indicates a common strategy for recruitment of enzyme-crystallins during the convergent evolution of vertebrate and invertebrate lenses. For historical reasons we propose that the octopus GST-like crystallins, like those of the squid, are called S-crystallins.

The alpha A-crystallin gene: conserved features of the 5'-flanking regions in human, mouse, and chicken

Approximately 2 kb of 5'-flanking sequences of the lens-specific alpha A-crystallin genes from human and mouse are presented and compared with similar regions of the chicken gene. A repetitive element was found approximately 1 kb upstream from the coding sequences of the alpha A-crystallin gene in all three species (Alu in human, B2 in mouse, and CR1 in chicken), suggesting that they may have an important functional or structural role. Despite the ability of alpha A-crystallin promoters to function across species, dot matrix analyses show only limited similarity among the 600 bp 5' to the structural genes of these three species. The human 5'-flanking sequence is more similar to that of the mouse and chicken than the mouse and chicken are to each other. Numerous short sequences (8-13 bp) are common to all three genes but are distributed differently in each species. The locations and conservation of these sequence motifs suggest functional roles, possibly as cis-regulatory elements of transcription. One motif is similar to the alpha A-CRYBP1 binding site implicated earlier in the transcriptional regulation of the mouse alpha A-crystallin gene, and other motifs correspond to sites previously mapped by methylation interference studies in the mouse alpha A-crystallin promoter. The modular arrangement of conserved sequence motifs is consistent with evolutionary changes occurring at the level of gene regulation.

Expression of duck lens delta-crystallin cDNAs in yeast and bacterial hosts. Delta 2-crystallin is an active argininosuccinate lyase

The major soluble protein in the lenses of most birds and reptiles is delta-crystallin. In chickens and ducks the delta-crystallin gene has duplicated, and in the duck both genes contribute to the protein in the lens, while in the chicken lens there is a great preponderance of the delta 1 gene product. Purified delta-crystallin has previously been shown to possess the enzymatic activity of argininosuccinate lyase. In order to determine the enzymatic properties of the two duck delta-crystallins their corresponding cDNA molecules were placed in yeast and bacterial expression plasmids. In Saccharomyces cerevisiae, the activity of each crystallin was assessed by transformation of the expression plasmids into a strain deficient for argininosuccinate lyase activity. The ability of the resulting yeast to grow on arginine deficient medium was used as a measure of enzymatic activity. Yeast expressing the duck delta 2-crystallin protein grew rapidly, while those expressing delta 1-crystallin failed to grow. Enzyme activity measurements confirmed the presence of activity in the delta 2-crystallin-expressing yeast, and no detectable activity could be demonstrated in the delta 1-crystallin-expressing yeast. Northern blotting of RNA from the transformed yeast revealed equal levels of mRNA species from the two constructs. For further analysis, the delta 2-crystallin cDNA was placed in the bacterial expression plasmid, pET-3d. The delta 2-crystallin protein produced in Escherichia coli was purified to homogeneity and analyzed to determine the kinetic properties. A Km of 0.35 mM was determined for argininosuccinate and a Vm of 3.5 mumols/min/mg was determined. These data demonstrate that, following duplication of the primordial argininosuccinate lyase gene, one of the genes maintained its role as an enzyme (delta 2-crystallin) while also serving as a crystallin and the other has evolved to specialize as a structural protein in the lens (delta 1-crystallin), presumably losing most or all of its catalytic capacity.

Expression of the murine alpha B-crystallin gene in lens and skeletal muscle: identification of a muscle-preferred enhancer

The alpha B-crystallin gene is expressed at high levels in lens and at lower levels in some other tissues, notably skeletal and cardiac muscle, kidney, lung, and brain. A promoter fragment of the murine alpha B-crystallin gene extending from positions -661 to +44 and linked to the bacterial chloramphenicol acetyltransferase (CAT) gene showed preferential expression in lens and skeletal muscle in transgenic mice. Transfection experiments revealed that a region between positions -426 and -257 is absolutely required for expression in C2C12 and G8 myotubes, while sequences downstream from position -115 appear to be determinants for lens expression. In association with a heterologous promoter, a -427 to -259 fragment functions as a strong enhancer in C2C12 myotubes and less efficiently in myoblasts and lens. Gel shift and methylation interference studies demonstrated that nuclear proteins from C2C12 myoblasts and myotubes specifically bind to the enhancer.

Structure and expression of the duck alpha-enolase/tau-crystallin-encoding gene

In the duck, the glycolytic enzyme, alpha-enolase (alpha ENO) and the lens structural protein, tau-crystallin (tau CRY), are products of the same gene, an example of protein multi-functionality. We report that duck alpha ENO/tau CRY mRNA levels are developmentally regulated: alpha ENO/tau CRY mRNA levels in the lens increase over those in the liver by embryonic day 14 and, within the lens, are higher in the lens epithelium than in fiber cells. We determined the structure of the duck alpha ENO/tau CRY-encoding gene (alpha ENO/tau CRY), sequenced 1 kb of 5'-flanking region, and demonstrated that this region contains a functional promoter. The gene is 13 kb in size and is composed of twelve exons; the exon organization is identical to that of mammalian enolase-encoding genes. A fragment of 5'-flanking region (-803/+3) containing three CCAAT boxes and a TATA box was able to activate transcription of a heterologous reporter gene when transfected into cultured lens cells. However, in spite of greater quantities of alpha ENO/tau CRY mRNA and protein in the lens, the promoter was equally active in primary cultures of embryonic lens, liver and fibroblast cells. Since the cultured cells unexpectedly lost the restricted pattern of alpha ENO/tau CRY mRNA levels observed in vivo, evaluation of the promoter's tissue specificity was precluded.

Gene conversion and splice-site slippage in the argininosuccinate lyases/delta-crystallins of the duck lens: members of an enzyme superfamily

Argininosuccinate lyase(ASL)/delta-crystallin is a prominent example of an enzyme-crystallin with roles as both a catalyst and a major structural component of the eye lens in birds and reptiles. In chicken it appears that gene duplication and separation of function may have occurred with one gene product acting primarily as a crystallin and one primarily as an enzyme. However, two delta-crystallin-encoding genes are abundantly expressed in the lens of the embryonic duck (Anas platyrhynchos) which has extremely high ASL activity. Here the isolation and sequence analysis of full length cDNA clones for both duck delta-crystallins are described. The two delta-crystallins are highly similar (94% identical in predicted aa sequence), probably as a result of gene conversion. However, the cDNA for duck delta 2-crystallin contains an in-frame insertion of two codons, probably the result of a recent intron boundary slippage. ASL/delta-crystallin belongs to a superfamily of lyases, including fumarases, aspartases and adenylosuccinate lyase which possess some highly conserved blocks of aa sequence. There may be some clues to the tertiary structures of these conserved motifs in otherwise unrelated proteins for which three-dimensional structures are known.

Species-specific lens activation of the thymidine kinase promoter by a single copy of the mouse alpha A-CRYBP1 site and loss of tissue specificity by multimerization

One copy of the mouse alpha A-crystallin gene alpha A-CRYBP1 site activated the thymidine kinase (tk) promoter in a mouse lens epithelial cell line but not in primary chicken lens cells; multiple copies further activated the tk promoter and extended expression to fibroblasts, B cells, and chicken lens cultures. The loss of lens specificity by multimerization may place selective constraints on the number of alpha A-CRYBP1 sites in the alpha A-crystallin promoter.

Differential expression of the two delta-crystallin/argininosuccinate lyase genes in lens, heart, and brain of chicken embryos

Chicken delta-crystallin/argininosuccinate lyase (ASL), a major enzyme-crystallin of the embryonic lens, is encoded by two similar, tandemly arranged genes (delta 1 and delta 2). We show here by the polymerase chain reaction (PCR) and in situ hybridization that although the amount of mRNA for each of the delta-crystallins increases in the lens epithelial and fiber cells during development of the embryonic chicken, the delta 1 mRNA accumulates preferentially in the fiber cells. The delta 1/delta 2 mRNA ratio actually decreased from 16.5 +/- 7 to 6.5 +/- 1 in the central epithelial cells while it increased from 20 +/- 10 to 95 +/- 5 in the fibers between 6 and 14 days of development. By contrast, the heart and brain of 4- to 8-day-old embryonic chickens showed 10(3) to 10(4) times less delta-crystallin mRNA per microgram of total RNA than the lens, with a delta 1/delta 2 mRNA ratio of only 0.20 to 0.30. The tissue-specific differences in the relative expression of the two delta-crystallin genes suggest that the delta 2 polypeptide is principally responsible for ASL activity and that the delta 1 polypeptide is specialized for lens transparency. The trace amounts of delta 1 mRNA in the heart and brain raise the possibility that the delta 1 polypeptide contributes to or modulates ASL activity of the native tetrameric protein. Transfection experiments in which we used the pSVOCAT plasmid demonstrated that both delta-crystallin genes contain an enhancer in their third intron. The promoter and enhancer of each delta-crystallin gene were functionally comparable in the pSVOCAT plasmid when tested in primary embryonic lens epithelial cells, suggesting that the differential expression of the two delta-crystallin genes in the lens requires additional cis-regulatory sequences, post-transcriptional mechanisms, or both.

Human alpha B-crystallin gene and preferential promoter function in lens

alpha B-Crystallin, first identified as a structural component of the vertebrate eye lens, is expressed at high levels in lens and at lower levels in a number of other tissues, most notably cardiac and skeletal muscle, kidney, and brain. We have cloned and sequenced the human alpha B-crystallin gene and show that it is structurally similar to its hamster homolog. We have also identified its transcription initiation site in human lens RNA. Functional analysis of a promoter fragment extending from -537 to +21 (relative to the transcription initiation site) and fused to the bacterial chloramphenicol acetyltransferase gene suggests that this fragment contains regulatory elements that function preferentially, but not exclusively, in lens. In contrast, this fragment is apparently insufficient to promote transcription in glial cells, as this construct functioned poorly in a glioblastoma-astrocytoma cell line (U-373MG) that synthesizes high levels of the endogenous alpha B-crystallin gene product.

Evidence for neutral and selective processes in the recruitment of enzyme-crystallins in avian lenses

In apparent contrast to most other tissues, the ocular lenses in vertebrates show striking differences in protein composition between taxa, most notably in the recruitment of different enzymes as major structural proteins. This variability appears to be the result of at least partially neutral evolutionary processes, although there is also evidence for selective modification in molecular structure. Here we describe a bird, the chimney swift (Chaetura pelagica), that lacks delta-crystallin/argininosuccinate lyase, usually the major crystallin of avian lenses. Clearly, delta-crystallin is not specifically required for a functionally effective avian lens. Furthermore the lens composition of the swift is more similar to that of the related hummingbirds than to that of the barn swallow (Hirundo rustica), suggesting that phylogeny is more important than environmental selection in the recruitment of crystallins. However differences in epsilon-crystallin/lactate dehydrogenase-B sequence between swift and hummingbird and other avian and reptilian species suggest that selective pressures may also be working at the molecular level. These differences also confirm the close relationship between swifts and hummingbirds.

Regulation of the mouse alpha A-crystallin gene: isolation of a cDNA encoding a protein that binds to a cis sequence motif shared with the major histocompatibility complex class I gene and other genes

We have shown by site-directed mutagenesis that the sequence between positions -69 and -40 of the mouse alpha A-crystallin gene is crucial for tissue-specific gene expression in a transfected mouse lens epithelial cell line transformed with the early region of simian virus 40. Gel retardation experiments with synthetic oligodeoxynucleotides revealed a mouse lens nuclear protein which bound specifically to the palindromic sequence 5'-GGGAAATCCC-3' at positions -66 to -57 in the alpha A-crystallin promoter. By screening a bacteriophage lambda gt11 expression library of the transformed lens cells, we isolated a 2.5-kilobase-pair cDNA encoding a fusion protein which bound to this sequence and to the regulatory element of the major histocompatibility complex (MHC) class I gene. This cDNA hybridized to a 10-kilobase-pair polyadenylated RNA present in many different tissues, including lens. It encoded a protein, tentatively called alpha A-CRYBP1, containing at least two zinc fingers. alpha A-CRYBP1 is either homologous or very similar to the human nuclear proteins MBP-1 (Baldwin et al., Mol. Cell. Biol. 10:1406-1414, 1990), PRDII-BFI (Fan and Maniatis, Genes Dev. 4:29-42, 1990), and HIV-EP1 (Maekawa et al., J. Biol. Chem. 264:14591-14593, 1989), which bind to regulatory elements of the MHC class I, beta interferon, and human immunodeficiency virus genes, respectively. Our results suggest that the lens-specific alpha A-crystallin, MHC class I, beta interferon and other genes have a similar cis-acting DNA regulatory motif that shares alpha A-CRYBPI, MBP-1, PRDII-BF1, HIV-EP1, or other closely related proteins as trans-acting factors.

Molecular biology: recent studies on enzyme/crystallins and alpha-crystallin gene expression

Jin H. Kinoshita has had a major role in fostering investigations on the molecular biology of the eye during his tenure as Scientific Director of the National Eye Institute (N.E.I.). His appreciation and support of molecular biology have been the basis for many advances in this area in the N.E.I. and have led to a fertile breeding ground for molecular studies on the normal and diseased eye. Our recent studies have shown that the taxon-specific crystallins have been recruited from numerous metabolic enzymes; they are expressed in the lens as abundant structural proteins and in other tissues as enzymes, a situation we call gene sharing. Although not taxon-specific, alpha B-crystallin, structurally related to the small heat-shock proteins, is expressed in diverse tissues; by contrast, alpha A-crystallin appears to be lens-specific. Recombinant DNA studies indicate that the two alpha-crystallin genes use different promoter sequences and trans factors for their expression. Thus, the ubiquitous alpha-crystallin genes may provide some insight into the mechanisms used to recruit metabolic enzymes as taxon-specific lens crystallins.

Regulation of vimentin gene expression in the ocular lens

Vimentin expression in the lens is striking due to the reported mesenchymal preference of vimentin and the epithelial origin of the lens. The amount of chicken vimentin mRNA levels determined by Northern blot analysis increased 3-fold from 7 to 14 days of embryonic lens development and then decreased 10-fold at 16 days of development, suggesting that post-transcriptional processes may contribute to the level of cytoplasmic vimentin mRNA during lens development. To analyze the mechanisms governing vimentin gene expression in the lens at the level of transcription, a series of chicken vimentin 5'-flanking region deletions were fused to the bacterial CAT gene and transfected into fibroblasts and lens cultures derived from three species. The -160 to +1 sequence conferred equal promoter activity in cultured chicken lens epithelial cells and fibroblasts. The -321 to -160 sequences increased promoter activity in all cultures, but more strongly in fibroblasts than in lens cells. Sequence elements in the region -608 to -321 repressed promoter activity in lens cells and fibroblasts. Promoter activity was partially restored in fibroblasts but not in lens cells by -767 to -608 sequences. Vimentin gene expression in the lens thus appears to be controlled by multiple positive- and negative-acting elements in its 5'-flanking sequence.

Regulation of the murine alpha A-crystallin promoter in transgenic mice

To identify sequences necessary for lens-specific gene expression, lines of transgenic mice were generated which contain murine alpha A-crystallin promoter sequences [-111 to +46 (alpha 111), -88 to +46 (alpha 88), and -34 to +46 (alpha 34)] fused to the bacterial chloramphenicol acetyltransferase (CAT) gene and CAT expression was analyzed. Mice carrying the alpha 111-CAT or the alpha 88-CAT fusion transgene expressed CAT exclusively in lens, except for one line containing alpha 111-CAT, which expressed low levels of CAT in several nonlenticular tissues. Transcription from these promoters in lens initiated at the same site as the endogenous alpha A-crystallin promoter. In one line of mice alpha 88-CAT transgene became active in the lens during embryonic development at approximately the same time that the alpha A-crystallin gene normally begins to be expressed. In contrast, the alpha 34-CAT fusion transgene, containing the TATA box but no sequences further upstream, was inactive in transgenic mice. Our data suggest that 134 bp of sequence (-88 to +46) in the murine alpha A-crystallin gene is sufficient to provide lens specificity, although we cannot rule out the possibility that other sequences also contribute to promoter function.

Tissue-specific expression of the chicken alpha A-crystallin gene in cultured lens epithelia and transgenic mice

The present experiments show that the single gene for the lens-specific protein alpha A-crystallin of chickens and mice uses a different subset of cis- and trans-acting regulatory elements for expression in transfected embryonic chicken lens epithelial cells. A chicken alpha A-crystallin-chloramphenicol acetyltransferase (CAT) fusion gene required 162 base pairs whereas the murine alpha A-crystallin-CAT fusion gene required only 111 base pairs of 5'-flanking sequences for efficient tissue-specific expression in the transfected chicken lens cells. Gel retardation and competition experiments were performed using embryonic chicken lens nuclear extract and oligodeoxynucleotides identical to the 5'-flanking region of the chicken (-170/-111) and murine (-111/-88 and -88/-55) alpha A-crystallin gene. The results indicated that these homologous promoters use different nuclear factors for function. Methylation interference analysis identified a dyad of symmetry (CTGGTTCCCACCAG) at position -153 to -140 in the chicken alpha A-crystallin promoter which binds one or more lens nuclear factors. Gel mobility shift experiments using nuclear extracts of brain, reticulocytes, and muscle of embryonic chickens or HeLa cells suggested that the factor(s) binding to the chicken alpha A-crystallin gene promoter sequences are not lens specific. Despite differences in the functional and protein-binding properties of the alpha A-crystallin gene promoter of chickens and mice, expression of the chicken alpha A-crystallin-CAT fusion gene in transgenic mice was lens specific, consistent with a common underlying mechanism for expression of the alpha A-crystallin gene in chickens and mice.

Assignment of the alpha B-crystallin gene to human chromosome 11

Using a human alpha B-crystallin genomic probe and human-mouse somatic cell hybrids, the human alpha B-gene was assigned to chromosome 11 and further corroborated by in situ hybridization to normal metaphase chromosomes. This assignment confirmed and regionally mapped the locus to q22.3-23.1.

Lens crystallins and their genes: diversity and tissue-specific expression

The soluble proteins--or crystallins--that constitute the bulk of the cellular, transparent eye lens are encoded by a surprisingly diverse group of genes. Several crystallin genes generate further heterogeneity by producing more than one polypeptide, in which they use different mechanisms. Some crystallin genes are lens specific (e.g., alpha A and gamma), while others show only lens preference (alpha B and enzyme/crystallins); all the crystallin genes are temporally and spatially regulated in the developing lens. Transfection and transgenic mouse experiments, identifying DNA regulatory elements in the 5' flanking region and in one case (delta) in an intron, point to transcriptional control as the primary basis for the tissue- and differentiation-specific expression of crystallin genes. Crystallin promoters have been used to target foreign genes to the lens in transgenic and chimeric mice. Such gene transfer experiments have been used to create tumors and ablate specific cells in the lens. The identification of trans-acting factors responsible for crystallin gene expression has begun but is in its infancy. The many mechanisms leading to the diversity and precise regulation of crystallins show that the lens is, in addition to a favorable tissue for studying differential gene expression, a fascinating portrait of molecular evolution.