The presence of extralenticular crystallins and its relationship with transdifferentiation to lens

Exploring Early-Stage Retinal Neurodegeneration in Murine Pigmentary Glaucoma: Insights From Gene Networks and miRNA Regulation Analyses

Purpose

Glaucoma is a group of heterogeneous optic neuropathies characterized by the progressive degeneration of retinal ganglion cells. However, the underlying mechanisms have not been understood completely. We aimed to elucidate the genetic network associated with the development of pigmentary glaucoma with DBA/2J (D2) mouse model of glaucoma and corresponding genetic control D2-Gpnmb (D2G) mice carrying the wild type (WT) Gpnmb allele.

Methods

Retinas isolated from 13 D2 and 12 D2G mice were subdivided into 2 age groups: pre-onset (1-6 months: samples were collected at approximately 1-2, 2-4, and 5-6 months) and post-onset (7-15 months: samples were collected at approximately 7-9, 10-12, and 13-15 months) glaucoma were compared. Differential gene expression (DEG) analysis and gene-set enrichment analyses were performed. To identify micro-RNAs (miRNAs) that target Gpnmb, miRNA expression levels were correlated with time point matched mRNA expression levels. A weighted gene co-expression network analysis (WGCNA) was performed using the reference BXD mouse population. Quantitative real-time PCR (qRT-PCR) was used to validate Gpnmb and miRNA expression levels.

Results

A total of 314 and 86 DEGs were identified in the pre-onset and post-onset glaucoma groups, respectively. DEGs in the pre-onset glaucoma group were associated with the crystallin gene family, whereas those in the post-onset group were related to innate immune system response. Of 1329 miRNAs predicted to target Gpnmb, 3 miRNAs (miR-125a-3p, miR-3076-5p, and miR-214-5p) were selected. A total of 47 genes demonstrated overlapping with the identified DEGs between D2 and D2G, segregated into their time-relevant stages. Gpnmb was significantly downregulated, whereas 2 out of 3 miRNAs were significantly upregulated (P < 0.05) in D2 mice at both 3-and 10-month time points.

Conclusions

These findings suggest distinct gene-sets involved in pre-and post-glaucoma in the D2 mouse. We identified three miRNAs regulating Gpnmb in the development of murine pigmentary glaucoma.

Identification of novel differentially expressed genes in retinas of STZ-induced long-term diabetic rats through RNA sequencing

BACKGROUND

The aim of this research was to investigate the retinal transcriptome changes in long-term streptozotocin (STZ)-induced rats' retinas using RNA sequencing (RNA-seq), to explore the molecular mechanisms of diabetic retinopathy (DR), and to identify novel targets for the treatment of DR by comparing the gene expression profile we obtained.

METHODS

In this study, 6 healthy male SD rats were randomly divided into wild-type (WT) group and streptozotocin (STZ)-induced group, 3 rats each group. After 6 months, 3 normal retina samples and 3 DM retina samples (2 retinas from the same rat were considered as 1 sample) were tested and differentially expressed genes (DEGs) were measured by RNA-seq technology. Then, we did Gene Ontology (GO) enrichment analysis and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway analysis and validated the results of RNA-seq through qRT-PCR.

RESULTS

A total of 118 DEGs were identified, of which 72 were up-regulated and 46 were down-regulated. The enriched GO terms showed that 3 most significant enrichment terms were binding (molecular function), cell part (cellular component), and biological regulation (biological process). The results of the KEGG pathway analysis revealed a significant enrichment in cell adhesion molecules, PI3K-Akt signaling pathway, and allograft rejection, etc. CONCLUSION: Our research has identified specific DEGs and also speculated their potential functions, which will provide novel targets to explore the molecular mechanisms of DR.

CRYβA3/A1-Crystallin Knockout Develops Nuclear Cataract and Causes Impaired Lysosomal Cargo Clearance and Calpain Activation

βA3/A1-crystallin is an abundant structural protein of the lens that is very critical for lens function. Many different genetic mutations have been shown to associate with different types of cataracts in humans and in animal models. βA3/A1-crystallin has four Greek key-motifs that organize into two crystallin domains. It shown to bind calcium with moderate affinity and has putative calcium-binding site. Other than in the lens, βA3/A1 is also expressed in retinal astrocytes, retinal pigment epithelial (RPE) cells, and retinal ganglion cells. The function of βA3/A1-crystallin in the retinal cell types is well studied; however, a clear understanding of the function of this protein in the lens has not yet been established. In the current study, we generated the βA3/A1-crystallin knockout (KO) mouse and explored the function of βA3/A1-crystallin in lens development. Our results showed that βA3-KO mice develop congenital nuclear cataract and exhibit persistent fetal vasculature condition. At the cellular level KO lenses show defective lysosomal clearance and accumulation of nuclei, mitochondria, and autophagic cargo in the outer cortical region of the lens. In addition, the calcium level and the expression and activity of calpain-3 were increased in KO lenses. Taken together, these results suggest the lack of βA3-crystallin function in lenses, alters calcium homeostasis which in turn causes lysosomal defects and calpain activation. These defects are responsible for the development of nuclear cataract in KO lenses.

RNA sequencing reveals retinal transcriptome changes in STZ-induced diabetic rats

The present study aimed to investigate changes in retinal gene expression in streptozotocin (STZ)‑induced diabetic rats using next‑generation sequencing, utilize transcriptome signatures to investigate the molecular mechanisms of diabetic retinopathy (DR), and identify novel strategies for the treatment of DR. Diabetes was chemically induced in 10‑week‑old male Sprague‑Dawley rats using STZ. Flash‑electroretinography (F‑ERG) was performed to evaluate the visual function of the rats. The retinas of the rats were removed to perform high throughput RNA sequence (RNA‑seq) analysis. The a‑wave, b‑wave, oscillatory potential 1 (OP1), OP2 and ∑OP amplitudes were significantly reduced in the diabetic group, compared with those of the control group (P<0.05). Furthermore, the implicit b‑wave duration 16 weeks post‑STZ induction were significantly longer in the diabetic rats, compared with the control rats (P<0.001). A total of 868 genes were identified, of which 565 were upregulated and 303 were downregulated. Among the differentially expressed genes (DEGs), 94 apoptotic genes and apoptosis regulatory genes, and 19 inflammatory genes were detected. The results of the KEGG pathway significant enrichment analysis revealed enrichment in cell adhesion molecules, complement and coagulation cascades, and antigen processing and presentation. Diabetes alters several transcripts in the retina, and RNA‑seq provides novel insights into the molecular mechanisms underlying DR.

Functions of crystallins in and out of lens: roles in elongated and post-mitotic cells

The vertebrate lens evolved to collect light and focus it onto the retina. In development, the lens grows through massive elongation of epithelial cells possibly recapitulating the evolutionary origins of the lens. The refractive index of the lens is largely dependent on high concentrations of soluble proteins called crystallins. All vertebrate lenses share a common set of crystallins from two superfamilies (although other lineage specific crystallins exist). The α-crystallins are small heat shock proteins while the β- and γ-crystallins belong to a superfamily that contains structural proteins of uncertain function. The crystallins are expressed at very high levels in lens but are also found at lower levels in other cells, particularly in retina and brain. All these proteins have plausible connections to maintenance of cytoplasmic order and chaperoning of the complex molecular machines involved in the architecture and function of cells, particularly elongated and post-mitotic cells. They may represent a suite of proteins that help maintain homeostasis in such cells that are at risk from stress or from the accumulated insults of aging.

A crystallin gene network in the mouse retina

The present study was designed to examine the regulation of crystallin genes and protein in the mouse retina using the BXD recombinant inbred (RI) strains. Illumina Sentrix BeadChip Arrays (MouseWG-6v2) were used to analyze mRNA levels in 75 BXD RI strains along with the parental strains (C57Bl/6J and DBA/2J), and the reciprocal crosses in the Hamilton Eye Institute (HEI) Retina Dataset (www.genenetwork.org). Protein levels were investigated using immunoblots to quantify levels of proteins and indirect immunohistochemistry to define the distribution of protein. Algorithms in the Genomatix program were used to identify transcription factor binding sites common to the regulatory sequences in the 5' regions of co-regulated set of crystallin and other genes as compared to a set of control genes. As subset of genes, including many encoding lens crystallins is part of a tightly co-regulated network that is active in the retina. Expression of this crystallin network appears to be binary in nature, being expressed either at relatively low levels or being highly upregulated. Relative to a control set of genes, the 5' regulatory sequences of the crystallin network genes show an increased frequency of a set of common transcription factor-binding sites, the most common being those of the Maf family. Chromatin immunoprecipitation of human lens epithelial cells (HLEC) and rat retinal ganglion cells (RGC) confirmed the functionality of these sites, showing that MafA binds the predicted sites of CRYGA and CRYGD in HLE and CRYAB, CRYGA, CRYBA1, and CRYBB3 in RGC cells. In the retina there is a highly correlated group of genes containing many members of the α- β- and γ-crystallin families. These genes can be dramatically upregulated in the retina. One transcription factor that appears to be involved in this coordinated expression is the MAF family transcription of factors associated with both lens and extralenticular expression of crystallin genes.

A novel mutation in CRYBB1 associated with congenital cataract-microcornea syndrome: the p.Ser129Arg mutation destabilizes the βB1/βA3-crystallin heteromer but not the βB1-crystallin homomer

Congenital cataract-microcornea syndrome (CCMC) is a clinically and genetically heterogeneous condition characterized by lens opacities and microcornea. It appears as a distinct phenotype of heritable congenital cataract. Here we report a large Chinese family with autosomal dominant congenital cataract and microcornea. Evidence for linkage was detected at marker D22S1167 (LOD score [Z]=4.49, recombination fraction [θ]=0.0), which closely flanks the â-crystallin gene cluster locus. Direct sequencing of the candidate âB1-crystallin gene (CRYBB1) revealed a c.387C>A transversion in exon 4, which cosegregated with the disease in the family and resulted in the substitution of serine by arginine at codon 129 (p.Ser129Arg). A comparison of the biophysical properties of the recombinant β-crystallins revealed that the mutation impaired the structures of both βB1-crystallin homomer and βB1/βA3-crystallin heteromer. More importantly, the mutation significantly decreased the thermal stability of βB1/βA3-crystallin but not βB1-crystallin. These findings highlight the importance of protein-protein interactions among β-crystallins in maintaining lens transparency, and provide a novel insight into the molecular mechanism underlying the pathogenesis of human CCMC. © 2011 Wiley-Liss, Inc.

Expression of βA3/A1-crystallin in the developing and adult rat eye

Crystallins are very abundant structural proteins of the lens and are also expressed in other tissues. We have previously reported a spontaneous mutation in the rat βA3/A1-crystallin gene, termed Nuc1, which has a novel, complex, ocular phenotype. The current study was undertaken to compare the expression pattern of this gene during eye development in wild type and Nuc1 rats by in situ hybridization (ISH) and immunohistochemistry (IHC). βA3/A1-crystallin expression was first detected in the eyes of both wild type and Nuc1 rats at embryonic (E) day 12.5 in the posterior portion of the lens vesicle, and remained limited to the lens fibers throughout fetal life. After birth, βA3/A1-crystallin expression was also detected in the neural retina (specifically in the astrocytes and ganglion cells) and in the retinal pigmented epithelium (RPE). This suggested that βA3/A1-crystallin is not only a structural protein of the lens, but has cellular function(s) in other ocular tissues. In summary, expression of βA3/A1-crystallin is controlled differentially in various eye tissues with lens being the site of greatest expression. Similar staining patterns, detected by ISH and IHC, in wild type and Nuc1 animals suggest that functional differences in the protein, rather than changes in mRNA/protein level of expression, likely account for developmental abnormalities in Nuc1.

The cellular and molecular bases of vertebrate lens regeneration

Lens regeneration takes place in some vertebrates through processes of cellular dedifferentiation and transdifferentiation, processes by which certain differentiated cell types can give rise to others. This review describes the principal forms of lens regeneration that occur in vivo as well as related in vitro systems of transdifferentiation. Classic experimental studies are reviewed that define the tissue interactions that trigger these events in vivo. Recent molecular analyses have begun to identify the genes associated with these processes. These latter studies generally reveal tremendous similarities between embryonic lens development and lens regeneration. Different models are proposed to describe basic molecular pathways that define the processes of lens regeneration and transdifferentiation. Finally, studies are discussed suggesting that fibroblast growth factors play key roles in supporting the process of lens regeneration. Retinoids, such as retinoic acid, may also play important roles in this process.

Gene expression profiles of the collecting duct in the mouse renal inner medulla

Gene expression profiles of the collecting duct in the mouse renal inner medulla.

BACKGROUND

Gene expression profiles, constructed from 1000 to 2000 cloned cDNA sequences, depict their relative abundance of expression in a tissue. Establishing such a profile for mouse inner renal medullary collecting ducts (IMCDs), we compared expression patterns with those in other tissues including proximal tubule.

METHODS

A nonbiased 3'-end cDNA library was prepared from microdissected mouse IMCDs. Single-pass sequencing of 2000 randomly selected cDNA clones collected short sequences (approximate length, 250 bp) following poly (A), called gene signatures (GS). Identical sequences were considered a single GS. GS occurrence was quantitated to yield a list of expressed genes indicating their abundance.

RESULTS

Among 2000 clones, 1613 types of transcripts were found in IMCDs; 155 were identical or homologous to reported genes. The gene most expressed in IMCDs was alphaB-crystallin, a small stress (heat-shock) protein that is also a major structural protein in the ocular lens. According to Northern analysis, renal expression of this mRNA was induced by dehydration, presumably via tissue hypertonicity. However, expression did not change with acute NaCl loading. Also, a new member of the glutathione-S-transferase family was identified by comparing the IMCD expression profile with those of other tissues.

CONCLUSION

With our database of genes expressed in mouse IMCDs, we are devising an IMCD-specific microarray to study gene-expression responses to various physiologic alterations.

Developmental regulation of the chicken beta B1-crystallin promoter in transgenic mice

The cis-elements responsible for the high-level, lens-specific expression of the chicken beta B1-crystallin gene were investigated by generating mice harboring beta B1-crystallin promoter/chloramphenicol acetyl transferase (CAT) transgenes. Deletion of promoter sequences -434/-153 and -152/-127 as well as site-directed mutagenesis of the PL1 (-116/-102) and Pl2 (-90/-76) elements significantly decreased CAT gene expression in the lenses of adult transgenic mice. Transfection studies using multimerized PL1 and PL2 elements fused to the chicken beta-actin basal promoter indicated that PL1 is a general activating element while PL2 is involved in the lens-specificity of the chicken beta B1-crystallin promoter. CAT histochemistry demonstrated that the chicken beta B1-crystallin promoter (-434/+30) was active in both primary and secondary lens fiber cells from 12.5 days post coitum (dpc) until adulthood. Activity of the -152/+30/CAT transgene was relatively low and confined to the primary lens fiber cells of 16.5 dpc mice. Together, these data suggest that the reduced activity of this promoter in the adult lens is due both to this developmentally restricted expression pattern and a reduction in promoter activity. RNA hybridization studies demonstrated that the chicken beta B1-crystallin/CAT (-434/+30) transgene was expressed at similar levels in the same cells as the endogenous mouse beta B1-crystallin gene in 16.5 dpc transgenic mouse embryos. These data show a strict conservation of the lens-specific spatial and temporal regulation of the chicken and mouse beta B1-crystallin genes.

Differential expression of the two delta-crystallin genes in lens and non-lens tissues: shift favoring delta 2 expression from embryonic to adult chickens

Chicken argininosuccinate lyase (ASL)/delta-crystallin, a lens enzyme-crystallin, is encoded in two linked genes (delta 1 and delta 2); only the delta 2 polypeptide contains ASL activity. Here we have quantified delta 1- and delta 2-crystallin mRNA in the lens, cornea, neural retina, heart, and brain at different stages of embryonic development and in 1-wk-old and 1-yr-old chickens by the polymerase chain reaction using internal delta 1 and delta 2 RNA standards. The delta 1/delta 2 mRNA ratio differed for every tissue and was regulated during development. In the embryo there was more delta 1 than delta 2 mRNA in the lens (50-100 times), cornea (3-4 times), and neural retina (2-20 times), about equal amounts of delta 1 and delta 2 mRNA in the heart, and more delta 2 mRNA in the brain (15 times). delta 1-Crystallin mRNA differentially decreased in every tissue after hatching; by contrast, the delta 2 mRNA remained about the same except for the lens, where it decreased 50-fold between 1 wk and 1 yr after hatching. In the 1-yr-old chicken, the delta 2/delta 1 mRNA ratios were 7 in the lens, 175 in the cornea, 22 in the neural retina, 107 in the heart, and 136 in the brain, indicating that delta 2-crystallin is strongly favored in all adult tissues of the chicken. The excess of delta 1 to delta 2 mRNA in the embryonic lens, cornea, and neural retina is intriguing, and suggests some connection with developing transparent eye tissues. Finally, we raise the possibility that expression of both delta-crystallin genes may create tetrameric ASL isoenzymes (perhaps with different specific activities). The unexpected predominance of delta 2 mRNA in the 1-yr-old lens suggests that both the enzymatic and refractive functions of ASL/delta-crystallin are operative and spatially separated, with the enzymatic role present in the cortical fibers and the refractive role in the center of the lens.

Evidence for the extralenticular expression of members of the beta-crystallin gene family in the chick and a comparison with delta-crystallin during differentiation and transdifferentiation

The beta-crystallins are major water soluble proteins of vertebrate lens fibre cells and have previously been regarded as lens-specific proteins: however beta B2-and beta A3/A1-crystallin RNAs are transcribed and beta-crystallin polypeptides are detectable in the developing chick retina. The beta-crystallin RNA is transcribed in a subpopulation of retina cells and the number of transcribing cells and the level of beta-crystallin polypeptides increase during the differentiation of the retina. Several tissues express beta-crystallin polypeptides, but individual tissues are characterised by qualitative and quantitative differences in the beta- and delta-crystallin polypeptides expressed. The expression of beta-crystallins appears to be non-random as defined by tissue distribution, cellular localisation and ontogeny, implying a function for extralenticular beta-crystallins and a complex mechanism for the regulation of their expression.

Independent regulation of two coexpressed delta-crystallin genes in chick lens and nonlens tissues

It is known that delta-crystallin is super-abundant in the early chick lens, but it is found at lower levels in certain other tissues. Ninety-nine percent of the lens delta-crystallin poly(A)+ RNA is from the delta 1-crystallin gene. We report here that the delta 1- and delta 2-crystallin genes are both transcribed in the chick lens and retina throughout embryonic development and that both RNAs are found in embryo adenohypophysis and epiphysis and in day-old posthatch chick tibiofemoral chondrocytes and striated muscle. delta 1-crystallin RNA is more abundant in lens tissues, while delta 2-crystallin RNA is more abundant in all nonlens tissues. However, delta 1-crystallin RNA is processed more efficiently than delta 2-crystallin RNA in all early embryonic tissues examined. A comparison of lens epithelium and fibers established that levels of delta 2-crystallin RNA are the same but those of delta 1-crystallin RNA are over 100-fold higher in fibers compared to epithelial cells. The evidence implies independent regulation both of transcription and of post-transcriptional events for these two genes.

Ageing in the chick lens: in vitro studies

In principle, ageing may be due to the interaction of several factors, including the accumulation of random changes both genomic and non-genomic, secondary changes in a tissue contingent upon the changing function of other tissues, and programmed non-random changes in the tissue-specific expression of various genes. The use of a single tissue comprising one cell type only, in which the major gene products are well defined, in which there is a well attested series of developmental and age-related changes in cell properties and gene expression and which can be studied and compared in vivo and in vitro, offers advantages for investigation of these questions. The vertebrate eye lens possesses these advantages. The crystallins (proteins expressed at super-abundant levels in the lens) are well characterised. The lens epithelial cells (LEC) grow readily and can differentiate into the lens fibre cells in vitro, and, finally, such terminally differentiated cells may also be derived, by a process of transdifferentiation, from neural retina cells (NRC) in vitro. Thus the effect on ageing changes of the tissue of origin may also be studied. This article reviews our previous studies on long-term changes in growth potential, differentiation capacity and crystallin expression of chick lens cells in ageing cultures, their overall similarity to events in vivo and the effect on ageing changes of genotypes affecting the growth rate. It presents new information on these genetic aspects, and on crystallin expression in long-term ageing cultures of transdifferentiated neural retina, and compares the behaviour of ageing chick lens cells with that reported for mammals.

The cellular eye lens and crystallins of cubomedusan jellyfish

The ultrastructure and major soluble proteins of the transparent eye lens of two cubomedusan jellyfish, Tripedalia cystophora and Carybdea marsupialis, have been examined. Each species has two complex eyes (one large and one small) on four sensory structures called rhopalia. The lenses consist of closely spaced cells with few organelles. The lens is situated next to the retina, with only an acellular layer separating it from the photoreceptors. SDS-PAGE showed that the large lens of C. marsupialis has only two crystallin polypeptide bands (with molecular masses of approximately 20,000 and 35,000 daltons), while that of T. cystophora has three bands (two with a molecular mass near 20,000 daltons and one with a molecular mass near 35,000 daltons). Interestingly, the small lens of T. cystophora appears to be markedly deficient in or lack the lower molecular weight proteins. The crystallins behaved as monomeric proteins by FPLC and showed no immunological reaction with antisera of the major squid crystallin, chicken delta-crystallin or mouse gamma-crystallin in western immunoblots. Very weak reactions were found with antimouse alpha- and beta-crystallin sera. The 35,000 dalton crystallin of T. cystophora was purified and called J1-crystallin. It contained relatively high leucine (13%) and tyrosine (9%) and low methionine (2%). Several tryptic peptides were sequenced. Weak sequence similarities were found with alpha- and beta-crystallins, which may account for some of the apparent weak immunological cross-reactivity with these vertebrate crystallins. A polyclonal antiserum made in rabbits from a synthetic peptide of J1-crystallin reacted strongly with J1-crystallin of T. cystophora and C. marsupialis in immunoblots; by contrast, no reaction was obtained with the lower molecular weight crystallins from these jellyfish, with the squid crystallin, or with any crystallins from the frog or human lens. Thus, despite the structural similarities between the cubomedusan, squid and vertebrate lenses, their crystallins appear very different.

alpha B subunit of lens-specific protein alpha-crystallin is present in other ocular and non-ocular tissues

alpha-Crystallin, a tissue specific structural protein of the ocular lens, is known to be composed of two subunits, alpha A and alpha B. By using a specific antibody in an immunoblotting procedure we have found that one of the subunits, alpha B is present in a number of non-lenticular tissues including the retina, heart, skeletal muscle, skin, brain, spinal cord and lungs. Interestingly, in the rat, this protein is present in significantly higher concentrations in adult than in fetal tissues and, with the exception of the lens, fetal and adult heart has the highest concentration among the tissues examined. That the protein in question is, in fact, alpha B, was confirmed a) by the remarkable similarity of Staphylococcus aureus protease peptide maps of the protein in the heart and purified alpha-crystallin and b) by the sequence analysis of a rat heart cDNA clone identified by the alpha B antibody. Based on these observations we conclude that while alpha A has a tissue-specific role, alpha B is a polypeptide of independent function not restricted to the ocular lens.

Conditions permitting the homotopic expression of lens-specific crystallin genes

delta-Crystallin is a major soluble protein of the avian and reptilian lens, and its expression is highly tissue-specific in development. In order to understand regulatory mechanisms for tissue-specific expression of delta-crystallin gene, several experimental systems were established in a heterologous combination of the chicken gene and mouse cells. The expression was ectopic in various cell types differentiated in teratomas derived from mouse teratocarcinoma or embryonic stem cells which were transformed to carry the chicken delta-crystallin genes. Cells of the same transformed lines of embryonic stem cells expressed the chicken gene homotopically in chimeric embryos produced by injecting them into the blastocysts. The homotopic expression also occurred in experimental systems consisting of the heterologous introduction of the gene (1) into various mouse cells in primary cultures, and (2) into male pronuclei of mouse fertilized eggs.

Gene sharing by delta-crystallin and argininosuccinate lyase

The lens structural protein delta-crystallin and the metabolic enzyme argininosuccinate lyase (ASL; L-argininosuccinate arginine-lyase, EC 4.3.2.1) have striking sequence similarity. We have demonstrated that duck delta-crystallin has enormously high ASL activity, while chicken delta-crystallin has lower but significant activity. The lenses of these birds had much greater ASL activity than other tissues, suggesting that ASL is being expressed at unusually high levels as a structural component. In Southern blots of human genomic DNA, chicken delta 1-crystallin cDNA hybridized only to the human ASL gene; moreover, the two chicken delta-crystallin genes accounted for all the sequences in the chicken genome able to cross-hybridize with a human ASL cDNA, with preferential hybridization to the delta 2 gene. Correlations of enzymatic activity and recent data on mRNA levels in the chicken lens suggest that ASL activity depends on expression of the delta 2-crystallin gene. The data indicate that the same gene, at least in ducks, encodes two different functions, an enzyme (ASL) and a structural protein (delta-crystallin), although in chickens specialization and separation of functions may have occurred.

Expression of the delta-crystallin genes in the embryonic chicken lens

The amounts of lens mRNA derived from the delta 1- and delta 2-crystallin genes of the chicken were determined by primer extension experiments using gene-specific synthetic oligonucleotides. The primer extended products were sequenced to establish the identity of the resulting cDNAs. The results indicated that most of the delta-crystallin mRNA in the 14-day-old embryonic lens contained transcripts derived from the delta 1 gene. Importantly, however, about 1-2% of the extended products were derived from delta 2 mRNA. Although not quantitative, the primer extension experiments suggested that the delta 1/delta 2 mRNA ratio may differ in the lens fiber cells during development between 6 days of embryogenesis and 3 weeks after hatching. These data provide the first demonstration for the presence of delta 2-crystallin mRNA in the chicken lens and raise the possibility that the two linked, extremely similar delta-crystallin genes are differentially regulated during development.

The influence of the genotype on the process of ageing of chick lens cells in vitro

We reported previously that changes in crystallin expression in differentiating long-term primary cultures of lens cells from five different chick genotypes are similar to those which occur in vivo between hatching and the 8-week-old adult. These changes followed a similar program in all genotypes but occurred more rapidly in cells from the fast-growing than from the slow-growing genotypes. The present study examines ageing changes in lens cell populations from the same five genotypes, over a 4-6 month period, using long-term serial subcultures. The capacity for lentoid differentiation was progressively lost, but the rate of loss was inversely related to the intrinsic growth rate of the cells of these genotypes, occurring at the first passage in the slowest-growing strain, while fifth passage cells of the fastest-growing strain still retained some lentoid-forming capacity. The rate of loss of crystallin expression was also inversely related to the genetic growth rate, but the sequence of changes appears to be nonrandom, since it was broadly similar in all genotypes, starting with a preferential loss of delta-crystallin, as occurs in vivo; although alpha- and beta-crystallins were undetectable in late dedifferentiated cultures, the capacity of the cells for their synthesis was still present. Cultures from both fast-growing genotypes eventually showed senescence, but those from all three slow-growing genotypes underwent transformation. The major cell component in late cultures of all genotypes was actin.

Homology of delta crystallin and argininosuccinate lyase

1. Delta crystallin, a major lens protein characteristic of birds and reptiles, is homologous to argininosuccinate lyase; 57% of the residues in chicken delta crystallin and human lyase are identical. 2. Even more similar (62% identical residues) to the human lyase is the sequence translated from the presumably inactive delta-2 gene of the delta crystallin locus. 3. As both delta crystallin and lyase are synthesized in birds only during the embryonic and juvenile stages, the persistence of delta crystallin in the adult lens appears to be paedomorphic. 4. Possible correlations of the origins of delta crystallin with other events in sauropsid evolution are proposed.

Recruitment of enzymes as lens structural proteins

Crystallins, the principal components of the lens, have been regarded simply as soluble, structural proteins. It now appears that the major taxon-specific crystallins of vertebrates and invertebrates are either enzymes or closely related to enzymes. In terms of sequence similarity, size, and other physical characteristics delta-crystallin is closely related to argininosuccinate lyase, tau-crystallin to enolase, and SIII-crystallin to glutathione S-transferase; moreover, it has recently been demonstrated that epsilon-crystallin is an active lactate dehydrogenase. Enzymes may have been recruited several times as lens proteins, perhaps because of the developmental history of the tissue or simply because of evolutionary pragmatism (the selection of existing stable structures for a new structural role).