We analyzed the gamma-crystallin genes CRYGB, CRYGC, and CRYGS in the dog and tested single nucleotide polymorphisms (SNPs) for linkage and association with primary noncongenital cataract (CAT) in the dachshund, a popular dog breed. The crystallin genes may be involved in the pathogenesis of canine CAT as shown in humans and mice.

The crystallins were discovered more than 100 years ago by Mörner (1893. Untersuchungen der Proteinsubstanzen in den lichtbrechenden Medien des Auges. Z. Physiol. Chem. 18, 61-106) as the main structural proteins of the vertebrate eye lens. Since that time the major mammalian crystallins referred to as alpha-, beta-, and gamma-crystallins were characterized with respect to their genetic organization, regulation of their expression pattern and participation in several diseases. In recent years, more and more crystallins have also been identified outside the lens. Evolutionary analysis has demonstrated the relationship of crystallins to proteins involved in protection against stress. The alpha-crystallins form large complexes up to 1Mio Da; they are built up by two subunits referred to as alphaA- and alphaB-crystallins. These subunits are encoded by individual genes, Cryaa and Cryab being localized on different chromosomes and members of the small heat-shock protein family. The alphaA-crystallin is considered to be a molecular chaperone. It is expressed mainly in the lens - mutations in the Cryaa gene lead to recessive or dominant cataracts. In contrast, the alphaB-crystallin is rather ubiquitously expressed; mutations in the Cryab gene are associated with a broad variety of neurological, cardiac and muscular disorders. The beta/gamma-crystallin super family is encoded by at least 14 genes; the proteins are characterized by four Greek key motifs. In mammals, these genes are not only organized as individual genes (Cryba1, Cryba2, Crygf, Crygs, CrygN), but also in duplets (Cryba4-Crybb1 and Crybb2-Crybb3) and in one major cluster (Cryga-Cryge). The various Cryb and Cryg genes are considered to have been evolved by various duplications of the Greek key encoding units. The two main families are distinguished by the fact that each Greek key motif in the Cryb genes is encoded by one exon, whereas two motifs are encoded by one single exon in the Cryg genes. An intermediate between these subfamilies is CrygN encoding the first two Greek key motifs by individual exons, but the others by one single exon. Mutations in the Cryb/Cryg genes lead mainly to an opacification of the eye lens. In some Cryg mutants evidence was presented that the formation of large amyloid-like intranuclear inclusions containing the altered gamma-crystallins is a key event in cataract formation. Cataract formation, caused by Cryg mutations is further characterized by stopping the secondary lens fiber differentiation as indicated by the presence of remnants of cell nuclei, which are usually degraded in secondary fiber cells. Moreover, additional clinical features are being increasingly reported since these crystallins are found outside the eye: the betaB2-crystallin (previously referred to the basic principle crystallin) is also involved in neurogenesis and male infertility. For some of the beta/gamma-crystallins, Ca(2+)-binding properties have been discussed; however, it is an unsolved question whether these crystallins serve as Ca(2+) stores in vivo. Enzyme crystallins are enzymes, which have been recruited to the lens and are expressed there in high concentrations. The mu- and zeta-crystallins (gene symbols: Crym and Cryz, respectively) are discussed as examples for mammals. Mutations in the human CRYM gene lead to non-syndromic deafness, and mutations in the Cryz gene of guinea pigs cause cataracts.

Nuclear receptor coactivator 6 (NCOA6) is a multifunctional protein implicated in embryonic development, cell survival, and homeostasis. An 81-amino acid fragment, dnNCOA6, containing the N-terminal nuclear receptor box (LXXLL motif) of NCOA6, acts as a dominant-negative (dn) inhibitor of NCOA6. Here, we expressed dnNCOA6 in postmitotic transgenic mouse lens fiber cells. The transgenic lenses showed reduced growth; a wide spectrum of lens fiber cell differentiation defects, including reduced expression of gamma-crystallins; and cataract formation. Those lens fiber cells entered an alternate proapoptotic pathway, and the denucleation (karyolysis) process was stalled. Activation of caspase-3 at embryonic day (E)13.5 was followed by double-strand breaks (DSBs) formation monitored via a biomarker, gamma-H2AX. Intense terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) signals were found at E16.5. Thus, a window of approximately 72 h between these events suggested prolonged though incomplete apoptosis in the lens fiber cell compartment that preserved nuclei in its cells. Genetic experiments showed that the apoptotic-like processes in the transgenic lens were both p53-dependent and p53-independent. Lens-specific deletion of Ncoa6 also resulted in disrupted lens fiber cell differentiation. Our data demonstrate a cell-autonomous role of Ncoa6 in lens fiber cell differentiation and suggest novel insights into the process of lens fiber cell denucleation and apoptosis.

The Liver X Receptors (LXRs) play important roles in multiple metabolic pathways, including fatty acid, cholesterol, carbohydrate and energy metabolism. To expand the knowledge of the functions of LXR signaling during embryonic development, we performed a whole-genome microarray analysis of Lxr target genes in zebrafish larvae treated with either one of the synthetic LXR ligands T0901317 or GW3965. Assessment of the biological processes enriched by differentially expressed genes revealed a prime role for Lxr in regulating lipid metabolic processes, similarly to the function of LXR in mammals. In addition, exposure to the Lxr ligands induced changes in expression of genes in the neural retina and lens of the zebrafish eye, including the photoreceptor guanylate cyclase activators and lens gamma crystallins, suggesting a potential novel role for Lxr in modulating the transcription of genes associated with visual function in zebrafish. The regulation of expression of metabolic genes was phenotypically reflected in an increased absorption of yolk in the zebrafish larvae, and changes in the expression of genes involved in visual perception were associated with morphological alterations in the retina and lens of the developing zebrafish eye. The regulation of expression of both lipid metabolic and eye specific genes was sustained in 1 month old fish. The transcriptional networks demonstrated several conserved effects of LXR activation between zebrafish and mammals, and also identified potential novel functions of Lxr, supporting zebrafish as a promising model for investigating the role of Lxr during development.

The human eye lens is composed of fiber cells packed with crystallins up to 450 mg/ml. Human gammaD-crystallin (HgammaD-Crys) is a monomeric, two-domain protein of the lens central nucleus. Both domains of this long lived protein have double Greek key beta-sheet folds with well packed hydrophobic cores. Three mutations resulting in amino acid substitutions in the gamma-crystallin buried cores (two in the N-terminal domain (N-td) and one in the C-terminal domain (C-td)) cause early onset cataract in mice, presumably an aggregated state of the mutant crystallins. It has not been possible to identify the aggregating precursor within lens tissues. To compare in vivo cataract-forming phenotypes with in vitro unfolding and aggregation of gamma-crystallins, mouse mutant substitutions were introduced into HgammaD-Crys. The mutant proteins L5S, V75D, and I90F were expressed and purified from Escherichia coli. WT HgammaD-Crys unfolds in vitro through a three-state pathway, exhibiting an intermediate with the N-td unfolded and the C-td native-like. L5S and V75D in the N-td also displayed three-state unfolding transitions, with the first transition, unfolding of the N-td, shifted to significantly lower denaturant concentrations. I90F destabilized the C-td, shifting the overall unfolding transition to lower denaturant concentrations. During thermal denaturation, the mutant proteins exhibited lowered thermal stability compared with WT. Kinetic unfolding experiments showed that the N-tds of L5S and V75D unfolded faster than WT. I90F was globally destabilized and unfolded more rapidly. These results support models of cataract formation in which generation of partially unfolded species are precursors to the aggregated cataractous states responsible for light scattering.

The vertebrate ocular lens is a simple and continuously growing tissue. Growth factor-mediated receptor tyrosine kinases (RTKs) are believed to be required for lens cell proliferation, differentiation and survival. The signaling pathways downstream of the RTKs remain to be elucidated. Here, we demonstrate the important role of Ras in lens development by expressing a dominant-negative form of Ras (dn-Ras) in the lens of transgenic mice. We show that lens in the transgenic mice was smaller and lens growth was severely inhibited as compared to the wild-type lens. However, the lens shape, polarity and transparency appeared normal in the transgenic mice. Further analysis showed that cell proliferation is inhibited in the dn-Ras lens. For example, the percentage of 5-bromo-2'-deoxyuridine (BrdU)-labeled cells in epithelial layer was about 2- to 3-fold lower in the transgenic lens than in the wild-type lens, implying that Ras activity is required for normal cell proliferation during lens development. We also found a small number of apoptotic cells in both epithelial and fiber compartment of the transgenic lens, suggesting that Ras also plays a role in cell survival. Interestingly, although there was a delay in primary fiber cell differentiation, secondary fiber cell differentiation was not significantly affected in the transgenic mice. For example, the expression of beta- and gamma-crystallins, the marker proteins for fiber differentiation, was not changed in the transgenic mice. Biochemical analysis indicated that ERK activity, but not Akt activity, was significantly reduced in the dn-Ras transgenic lenses. Overall, our data imply that the RTK-Ras-ERK signaling pathway is essential for cell proliferation and, to a lesser extent, for cell survival, but not for crystallin gene expression during fiber differentiation. Thus, some of the fiber differentiation processes are likely mediated by RTK-dependent but Ras-independent pathways.

Cataract is a clinically and genetically heterogeneous disorder of the ocular lens and an important cause of visual impairment. The aim of this study was to map and identify the gene underlying autosomal dominant cataract segregating in a four-generation family, determine the lens expression profile of the identified gene, and test for its association with age-related cataract in a case-control cohort.