Alpha-crystallins are involved in specific interactions with the murine gamma D/E/F-crystallin-encoding gene

Transcriptional burst fraction and size dynamics during lens fiber cell differentiation and detailed insights into the denucleation process

Genes are transcribed in irregular pulses of activity termed transcriptional bursts. Cellular differentiation requires coordinated gene expression; however, it is unknown whether the burst fraction (i.e. the number of active phases of transcription) or size/intensity (the number of RNA molecules produced within a burst) changes during cell differentiation. In the ocular lens, the positions of lens fiber cells correlate precisely with their differentiation status, and the most advanced cells degrade their nuclei. Here, we examined the transcriptional parameters of the β-actin and lens differentiation-specific α-, β-, and γ-crystallin genes by RNA fluorescent in situ hybridization (FISH) in the lenses of embryonic day (E) E12.5, E14.5, and E16.5 mouse embryos and newborns. We found that cellular differentiation dramatically alters the burst fraction in synchronized waves across the lens fiber cell compartment with less dramatic changes in burst intensity. Surprisingly, we observed nascent transcription of multiple genes in nuclei just before nuclear destruction. Nuclear condensation was accompanied by transfer of nuclear proteins, including histone and nonhistone proteins, to the cytoplasm. Although lens-specific deletion of the chromatin remodeler SWI/SNF-related matrix-associated actin-dependent regulator of chromatin subfamily A member 5 (Smarca5/Snf2h) interfered with denucleation, persisting nuclei remained transcriptionally competent and exhibited changes in both burst intensity and fraction depending on the gene examined. Our results uncover the mechanisms of nascent transcriptional control during differentiation and chromatin remodeling, confirm the burst fraction as the major factor adjusting gene expression levels, and reveal transcriptional competence of fiber cell nuclei even as they approach disintegration.

Probing the changes in gene expression due to α-crystallin mutations in mouse models of hereditary human cataract

The mammalian eye lens expresses a high concentration of crystallins (α, β and γ-crystallins) to maintain the refractive index essential for lens transparency. Crystallins are long-lived proteins that do not turnover throughout life. The structural destabilization of crystallins by UV exposure, glycation, oxidative stress and mutations in crystallin genes leads to protein aggregation and development of cataracts. Several destabilizing mutations in crystallin genes are linked with human autosomal dominant hereditary cataracts. To investigate the mechanism by which the α-crystallin mutations Cryaa-R49C and Cryab-R120G lead to cataract formation, we determined whether these mutations cause an altered expression of specific transcripts in the lens at an early postnatal age by RNA-seq analysis. Using knock-in mouse models previously generated in our laboratory, in the present work, we identified genes that exhibited altered abundance in the mutant lenses, including decreased transcripts for Clic5, an intracellular water channel in Cryaa-R49C heterozygous mutant lenses, and increased transcripts for Eno1b in Cryab-R120G heterozygous mutant lenses. In addition, RNA-seq analysis revealed increased histones H2B, H2A, and H4 gene expression in Cryaa-R49C mutant lenses, suggesting that the αA-crystallin mutation regulates histone expression via a transcriptional mechanism. Additionally, these studies confirmed the increased expression of histones H2B, H2A, and H4 by proteomic analysis of Cryaa-R49C knock-in and Cryaa;Cryab gene knockout lenses reported previously. Taken together, these findings offer additional insight into the early transcriptional changes caused by Cryaa and Cryab mutations associated with autosomal dominant human cataracts, and indicate that the transcript levels of certain genes are affected by the expression of mutant α-crystallin in vivo.

New insight into the dynamical system of αB-crystallin oligomers

α-Crystallin possesses a dynamic quaternary structure mediated by its subunit dynamics. Elucidation of a mechanism of subunit dynamics in homo-oligomers of αB-crystallin was tackled through deuteration-assisted small-angle neutron scattering (DA-SANS) and electrospray ionization (ESI) native mass spectrometry (nMS). The existence of subunit exchange was confirmed with DA-SANS, and monomers liberated from the oligomers were observed with nMS. With increasing temperature, an increase in both the exchange rate and monomer population was observed despite the absence of oligomer collapse. It is proposed that transiently liberated subunits, namely, "traveling subunits," play a role in subunit exchange. Moreover, we propose that protein function is regulated by these traveling subunits.

A knock-in mouse model for the R120G mutation of αB-crystallin recapitulates human hereditary myopathy and cataracts

An autosomal dominant missense mutation in αB-crystallin (αB-R120G) causes cataracts and desmin-related myopathy, but the underlying mechanisms are unknown. Here, we report the development of an αB-R120G crystallin knock-in mouse model of these disorders. Knock-in αB-R120G mice were generated and analyzed with slit lamp imaging, gel permeation chromatography, immunofluorescence, immunoprecipitation, histology, and muscle strength assays. Wild-type, age-matched mice were used as controls for all studies. Both heterozygous and homozygous mutant mice developed myopathy. Moreover, homozygous mutant mice were significantly weaker than wild-type control littermates at 6 months of age. Cataract severity increased with age and mutant gene dosage. The total mass, precipitation, and interaction with the intermediate filament protein vimentin, as well as light scattering of αB-crystallin, also increased in mutant lenses. In skeletal muscle, αB-R120G co-aggregated with desmin, became detergent insoluble, and was ubiquitinated in heterozygous and homozygous mutant mice. These data suggest that the cataract and myopathy pathologies in αB-R120G knock-in mice share common mechanisms, including increased insolubility of αB-crystallin and co-aggregation of αB-crystallin with intermediate filament proteins. These knock-in αB-R120G mice are a valuable model of the developmental and molecular biological mechanisms that underlie the pathophysiology of human hereditary cataracts and myopathy.

Differential protein expression in the honey bee head after a bacterial challenge

Insect immune proteins and peptides induced during bacterial infection are predominantly synthesized by the fat body or by haemocytes and released into the hemolymph. However, tissues other than the "immune-related" ones are thought to play a role in bacteria-induced responses. Here we report a proteomic study of honey bee heads designed to identify the proteins that are differentially expressed after bacterial challenge in a major body segment not directly involved in insect immunity. The list of identified proteins includes structural proteins, an olfactory protein, proteins involved in signal transduction, energy housekeeping, and stress responses, and also two major royal jelly proteins. This study revealed a number of bacteria-induced responses in insect head tissue directly related to typical functions of the head, such as exocrine secretion, memory, and senses in general.

Crystallins in the eye: Function and pathology

Crystallins are the predominant structural proteins in the lens that are evolutionarily related to stress proteins. They were first discovered outside the vertebrate eye lens by Bhat and colleagues in 1989 who found alphaB-crystallin expression in the retina, heart, skeletal muscles, skin, brain and other tissues. With the advent of microarray and proteome analysis, there is a clearer demonstration that crystallins are prominent proteins both in the normal retina and in retinal pathologies, emphasizing the importance of understanding crystallin functions outside of the lens. There are two main crystallin gene families: alpha-crystallins, and betagamma-crystallins. alpha-crystallins are molecular chaperones that prevent aberrant protein interactions. The chaperone properties of alpha-crystallin are thought to allow the lens to tolerate aging-induced deterioration of the lens proteins without showing signs of cataracts until older age. alpha-crystallins not only possess chaperone-like activity in vitro, but can also remodel and protect the cytoskeleton, inhibit apoptosis, and enhance the resistance of cells to stress. Recent advances in the field of structure-function relationships of alpha-crystallins have provided the first clues to their underlying roles in tissues outside the lens. Proteins of the betagamma-crystallin family have been suggested to affect lens development, and are also expressed in tissues outside the lens. The goal of this paper is to highlight recent work with lens epithelial cells from alphaA- and alphaB-crystallin knockout mice. The use of lens epithelial cells suggests that crystallins have important cellular functions in the lens epithelium and not just the lens fiber cells as previously thought. These studies may be directly relevant to understanding the general cellular functions of crystallins.

Structure function relationship among α-crystallin related small heat shock proteins

A sequence alignment is presented which permits the detection of the sequence and structural homology among alpha-crystallin subunits, alphaA and alphaB, and distantly related sHsps, MjHsp16.5 and wheat Hsp16.9. This alignment shows that homology extends beyond the alpha-crystallin domain. Variations in the polydisperse quaternary structure appear, in part, dependent upon the N-terminal 18 and 19 amino acids that are essential for subunit interactions in polydisperse sHsps. The hydrophobic sequence that follows these N-terminal amino acids shares a number of aromatic amino acids and has significant homology with MjHsp16.5. In the second exon of alpha-crystallin, sequence homology is concentrated in a region with chaperone and ANS binding sites. It is clear that the binding site for ANS and its derivative, bis-ANS, requires both positively charged amino acids and hydrophobic interactions. Therefore, its binding is not a true measure of hydrophobic surface exposure. The limited homology and secondary structure in the following C-terminal sequences is related to the pattern of association of other sHsp subunits and/or functional differences. Our study suggests that alphaA has evolved in the lens to chaperone exposed beta-sheet edges of the betagamma crystallins and their proteolytic fragments. Also, both time and a harsh environment such as that in the lens interior, beta-sheet proteins would naturally generate beta-sheet edges. The interaction between such edges results in insoluble, abnormal protein aggregation and in the lens, light scattering elements that cause cataract.

Morphological characterization of the Alpha A- and Alpha B-crystallin double knockout mouse lens

BACKGROUND

One approach to resolving some of the in vivo functions of alpha-crystallin is to generate animal models where one or both of the alpha-crystallin gene products have been eliminated. In the single alpha-crystallin knockout mice, the remaining alpha-crystallin may fully or partially compensate for some of the functions of the missing protein, especially in the lens, where both alpha A and alpha B are normally expressed at high levels. The purpose of this study was to characterize gross lenticular morphology in normal mice and mice with the targeted disruption of alpha A- and alpha B-crystallin genes (alpha A/BKO).

METHODS

Lenses from 129SvEvTac mice and alpha A/BKO mice were examined by standard scanning electron microscopy and confocal microscopy methodologies.

RESULTS

Equatorial and axial (sagittal) dimensions of lenses for alpha A/BKO mice were significantly smaller than age-matched wild type lenses. No posterior sutures or fiber cells extending to the posterior capsule of the lens were found in alpha A/BKO lenses. Ectopical nucleic acid staining was observed in the posterior subcapsular region of 5 wk and anterior subcapsular cortex of 54 wk alpha A/BKO lenses. Gross morphological differences were also observed in the equatorial/bow, posterior and anterior regions of lenses from alpha A/BKO mice as compared to wild mice.

CONCLUSION

These results indicated that both alpha A- and alpha B-crystallin are necessary for proper fiber cell formation, and that the absence of alpha-crystallin can lead to cataract formation.

Alpha-crystallin-type heat shock proteins: socializing minichaperones in the context of a multichaperone network

Alpha-crystallins were originally recognized as proteins contributing to the transparency of the mammalian eye lens. Subsequently, they have been found in many, but not all, members of the Archaea, Bacteria, and Eucarya. Most members of the diverse alpha-crystallin family have four common structural and functional features: (i) a small monomeric molecular mass between 12 and 43 kDa; (ii) the formation of large oligomeric complexes; (iii) the presence of a moderately conserved central region, the so-called alpha-crystallin domain; and (iv) molecular chaperone activity. Since alpha-crystallins are induced by a temperature upshift in many organisms, they are often referred to as small heat shock proteins (sHsps) or, more accurately, alpha-Hsps. Alpha-crystallins are integrated into a highly flexible and synergistic multichaperone network evolved to secure protein quality control in the cell. Their chaperone activity is limited to the binding of unfolding intermediates in order to protect them from irreversible aggregation. Productive release and refolding of captured proteins into the native state requires close cooperation with other cellular chaperones. In addition, alpha-Hsps seem to play an important role in membrane stabilization. The review compiles information on the abundance, sequence conservation, regulation, structure, and function of alpha-Hsps with an emphasis on the microbial members of this chaperone family.

Antagonistic action of Six3 and Prox1 at the gamma-crystallin promoter

Gamma-crystallin genes are specifically expressed in the eye lens. Their promoters constitute excellent models to analyse tissue-specific gene expression. We investigated murine CRYGE/f promoters of different length in lens epithelial cell lines. The most active fragment extends from position -219 to +37. Computer analysis predicts homeodomain and paired-domain binding sites for all rodent CRYGD/e/f core promoters. As examples, we analysed the effects of Prox1 and Six3, which are considered important transcription factors involved in lens development. Because of endogenous Prox1 expression in N/N1003A cells, a weak stimulation of CRYGE/f promoter activity was found for PROX1. In contrast, PROX1 stimulated the CRYGF promoter 10-fold in CD5A cells without endogenous PROX1. In both cell lines Six3 repressed the CRYGF promoter to 10% of its basal activity. Our cell transfection experiments indicated that CRYG expression increases as Six3 expression decreases. Prox1 and Six3 act antagonistically on regulation of the CRYGD/e/f promoters. Functional assays using randomly mutated gammaF-crystallin promoter fragments define a Six3-responsive element between -101 and -123 and a Prox1-responsive element between -151 and -174. Since Prox1 and Six3 are present at the beginning of lens development, expression of CRYGD/e/f is predicted to remain low at this time. It increases as Six3 expression decreases during ongoing lens development.

Bovine lens crystallins do contain helical structure: a circular dichroism study

In order to settle a recent discussion on the secondary structure of lens crystallins, we have measured the circular dichroism (CD) spectra of alpha-, beta(H)-, and beta(L)-crystallin from 178 to 250 nm and of gamma-crystallin from 168 to 250 nm. The results were analysed by means of a newly developed algorithm that almost doubles the reliability of secondary structure prediction and that allows discrimination between alpha- and 3(10)-helical, and between extended and polyproline beta-type structure. The results indicate that the crystallins studied contain a non-negligible amount of alpha-helical structure, although at least 50% of it is in the form of single and/or distorted loops. In alpha-crystallin, which is related to the chaperones, the helical content is lower than in beta- and gamma-crystallin. In some cases, the helices may play a role in DNA binding by the crystallins.

Ectopic expression of alpha B-crystallin in Chinese hamster ovary cells suggests a nuclear role for this protein

alpha B-crystallin (alpha B) is known to be a cytosolic, small heat shock-like multimeric protein that has anti-aggregation, chaperone-like properties. The expression of the alpha B-crystallin gene is developmentally regulated and is induced by a variety of stress stimuli. Importantly, alpha B-crystallin expression is enhanced during oncogenic transformation of cells, in a number of tumors, and most notably, in many neurodegenerative disorders, including Alzheimer's disease and multiple sclerosis. Other than its perceived role as a structural protein in the ocular lens, the actual function of alpha B-crystallin in cellular physiology remains unknown. We have stably transfected CHO cells with an inducible alpha B-cDNA-MMTV-promoter construct that allows the synthesis of recombinant alpha B-crystallin only upon exposure of these cells to dexamethasone. Using immunostaining and conventional and confocal microscopy, we have examined the subcellular distribution of the ectopically expressed alpha B-crystallin. We find that in addition to being in the cytoplasm, the protein resides in the nuclear interior in the interphase nucleus. Double labeling with anti alpha B-crystallin and anti-tubulin, concanavallin, and wheat germ agglutinin, respectively, revealed that during cell division alpha B-crystallin is excluded from condensed chromatin and the nascent nuclei. However, the protein again appears in the newly formed nuclei after the completion of cytokinesis suggesting a conditional, regulatory role for alpha B-crystallin in the nucleus.

Interaction of DNA with bovine lens alpha-crystallin: its functional implications

Under normal conditions, lens aggregates of alpha-crystallin subunits, alpha A and alpha B, are found in the cytoplasm. However, during stress in nonlenticular tissues, alpha B translocates to the nucleus. A sequence study revealed that both subunits share a consensus sequence with other DNA binding proteins. These observations prompted us to investigate DNA binding with alpha-crystallin by UV-mediated photo-crosslinking. The data show that both single and double stranded DNA crosslink mainly with tetramers of alpha-crystallin subunits. The formation of tetramers appears to modify alpha-crystallin interactive properties and, therefore, its induction may have functional significance. These observations suggest that alpha-crystallin may have a nuclear function which includes DNA binding.

Refinement of 3D structure of bovine lens alpha A-crystallin

In absence of 3D structures for alpha-crystallin subunits, alpha A and alpha B, we utilized a number of experimental and molecular modeling techniques to generate working 3D models of these polypeptides (Farnsworth et al., 1994. In Molecular Modeling: From Virtual Tools to Real Problems (Eds. Kumosinski, T.F. and Liebman, M.N.) ACS Symposium Series 576, Ch. 9:123-134, 1994, ACS Books, Washington DC). The refinement of the initial bovine alpha A model was achieved using a more accurate estimation of secondary structure by new/updated methods for analyzing the far UV-CD spectra and by neural network secondary structure predictions in combination with database searches. The spectroscopic study reveals that alpha-crystallin is not an all beta-sheet protein but contains approximately 17% alpha-helices, approximately 33% beta-structures and approximately 50% turns and coils. The refinement of the alpha A structure results in an elongate, asymmetric amphipathic molecule. The hydrophobic N-terminal domain imparts the driving force for subunit aggregation while the more flexible, polar C-terminal domain imparts aggregate solubility. In our quaternary structure of the aggregate, the monomer is the minimal cooperative subunit. In bovine alpha A, the highly negatively charged C-terminal domain has three small positive areas which may participate in dimer or tetramer formation of independently expressed C-terminal domains. The electrostatic potential of positive areas is modulated and become more negative with phosphorylation and ATP binding. The refined bovine alpha A model was used to construct alpha A models for the human, chick and dogfish shark. A high degree of conservation of the three dimensional structure and the electrostatic potential was observed. Our proposed open micellar quaternary structure correlates well with experimental data accumulated over the past several decades. The structure is also predictive of the more recent data.

Phosphorylations of alpha A- and alpha B-crystallin

In addition to being refractive proteins in the vertebrate lens, the two alpha-crystallin polypeptides (alpha A and alpha B) are also molecular chaperones that can protect proteins from thermal aggregation. The alpha B-crystallin polypeptide, a functional member of the small heat shock family, is expressed in many tissues in a developmentally regulated fashion, is stress-inducible, and is overexpressed in many degenerative diseases and some tumors indicating that it plays multiple roles. One possible clue to alpha-crystallin functions is the fact that both polypeptides are phosphorylated on serine residues by cAMP-dependent and cAMP-independent mechanisms. The cAMP-independent pathway is an autophosphorylation that has been demonstrated in vitro, depends on magnesium and requires cleavage of ATP. Disaggregation of alpha A-, but not alpha B-crystallin into tetramers results in an appreciable increase in autophosphorylation activity, reminiscent of other heat shock proteins, and suggests the possibility that changes in the aggregation state of alpha A-crystallin are involved in yet undiscovered signal transduction pathways. The alpha-crystallin polypeptides differ with respect to their abilities to undergo cAMP-dependent phosphorylation, with preference given to the alpha B-crystallin chain. These differences and complexities in alpha-crystallin phosphorylations, coupled with the differences in expression patterns of the two alpha-crystallin polypeptides, are consistent with the idea that each polypeptide has distinctive structural and metabolic roles.

Subunit exchange of alphaA-crystallin

alpha-Crystallin, the major protein in the mammalian lens, is a molecular chaperone that can bind denaturing proteins and prevent their aggregation. Like other structurally related small heat shock proteins, each alpha-crystallin molecule is composed of an average of 40 subunits that can undergo extensive reorganization. In this study we used fluorescence resonance energy transfer to monitor the rapid exchange of recombinant alpha-crystallin subunits. We labeled alphaA-crystallin with stilbene iodoacetamide (4-acetamido-4'-((iodoacetyl)amino)stilbene-2,2'-disulfonic acid), which serves as an energy donor and with lucifer yellow iodoacetamide, which serves as an energy acceptor. Upon mixing the two populations of labeled alphaA-crystallin, we observed a reversible, time-dependent decrease in stilbene iodoacetamide emission intensity and a concomitant increase in lucifer yellow iodoacetamide fluorescence. This result is indicative of an exchange reaction that brings the fluorescent alphaA-crystallin subunits close to each other. We further showed that the exchange reaction is strongly dependent on temperature, with a rate constant of 0.075 min-1 at 37 degrees C and an activation energy of 60 kcal/mol. The subunit exchange is independent of pH and calcium concentration but decreases at low and high ionic strength, suggesting the involvement of both ionic and hydrophobic interactions. It is also markedly reduced by the binding of large denatured proteins. The degree of inhibition is directly proportional to the molecular mass and the amount of bound polypeptide, suggesting an interaction of several alphaA-crystallin subunits with multiple binding sites of the denaturing protein. Our findings reveal a dynamic organization of alphaA-crystallin subunits, which may be a key factor in preventing protein aggregation during denaturation.

Effects of temperature and concentration on bovine lens alpha-crystallin secondary structure: a circular dichroism spectroscopic study

Elucidation of the structure of alpha-crystallin, the major protein in all vertebrate lenses, is important for understanding its role in maintaining transparency and its function in other tissues under both normal and pathological conditions. Progress toward a unified consensus concerning the tertiary and quaternary structures of alpha-crystallin depends, in part, on an accurate estimation of its secondary structure. For the first time, three algorithms, SELCON, K2D and CONTIN were used to analyze far ultra-violet circular dichroism (UV-CD) spectra of bovine lens alpha-crystallin to estimate the secondary structure and to determine the effects of temperature and concentration. Under all experimental conditions tested, the analyses show that alpha-crystallin contains 14% alpha-helix, 35% beta-sheet and the remainder, random coil and turns. The results suggest that alpha-crystallin is best classified as a mixed protein. In addition, increased temperature and concentration of alpha-crystallin result in increased alpha-helices with a compensatory decrease in beta-sheets. Such structural alterations in alpha-crystallin may be functionally important during terminal differentiation of the lens fiber cells that is accompanied by increased protein concentrations and its role as a chaperone-like protein.

Targeted disruption of the mouse alpha A-crystallin gene induces cataract and cytoplasmic inclusion bodies containing the small heat shock protein alpha B-crystallin

alpha A-crystallin (alpha A) and alpha B-crystallin (alpha B) are among the predominant proteins of the vertebrate eye lens. In vitro, the alpha-crystallins, which are isolated together as a high molecular mass aggregate, exhibit a number of properties, the most interesting of which is their ability to function as molecular chaperones for other proteins. Here we begin to examine the in vivo functions of alpha-crystallin by generating mice with a targeted disruption of the alpha A gene. Mice that are homozygous for the disrupted allele produce no detectable alpha A in their lenses, based on protein gel electrophoresis and immunoblot analysis. Initially, the alpha A-deficient lenses appear structurally normal, but they are smaller than the lenses of wild-type littermates. alpha A-/- lenses develop an opacification that starts in the nucleus and progresses to a general opacification with age. Light and transmission electron microscopy reveal the presence of dense inclusion bodies in the central lens fiber cells. The inclusions react strongly with antibodies to alpha B but not significantly with antibodies to beta- or gamma-crystallins. In addition, immunoblot analyses demonstrate that a significant portion of the alpha B in alpha A-/- lenses shifts into the insoluble fraction. These studies suggest that alpha A is essential for maintaining lens transparency, possibly by ensuring that alpha B or proteins closely associated with this small heat shock protein remain soluble.

A new cat reporter gene vector designed for rapid and efficient cloning of PCR products

A cat reporter gene plasmid was constructed, which can be used very efficiently to clone PCR-derived promotor and enhancer fragments from genomic DNA. The new vector system pEK0CAT combines the efficiency in cloning with the approved low background of the pBLCAT6 vector. Additionally, the plasmid pEKSVCAT was constructed including the SV40 early promoter/enhancer to efficiently drive the cat reporter gene in particular cell lines. It can be used to optimize transfection conditions and as an internal positive control.

Genetic aspects of embryonic eye development in vertebrates

The vertebrate eye comprises tissues from different embryonic origins, e.g., iris and ciliary body are derived from the wall of the diencephalon via optic vesicle and optic cup. Lens and cornea, on the other hand, come from the overlying surface ectoderm. The timely action of transcription factors and inductive signals ensure the correct development of the different eye components. Establishing the genetic basis of eye defects has been an important tool for the detailed analysis of this complex process. One of the main control genes for eye development was discovered by the analysis of the allelic series of the Small eye mouse mutants and characterized as Pax6. It is involved in the interaction between the optic cup and the overlaying ectoderm. The central role for Pax6 in eye development is conserved throughout the animal kingdom as the murine Pax6 gene induces ectopic eyes in transgenic Drosophila despite the obvious diverse organization of the eye in the fruit fly compared to vertebrates. In human, mutations in the PAX6 gene are responsible for aniridia and Peter's anomaly. In addition to Pax6, other mutations affecting the interaction of the optic cup and the lens placode have been documented in the mouse. For the differentiation of the retina from the optic cup several genes are responsible: Mi leads to microphthalmia, if mutated, and encodes for a transcription factor, which is expressed in the melanocytes of the pigmented layer of the retina. In addition, further genes are implicated in the correct development of the retina, e.g., Chx10, Dlx1, GH6, Msx1 and -2, Otx1 and -2, or Wnt7b. Mutations within the retinoblastoma gene (RB1) are responsible for retinal tumors. Knock-out mutants of RB1 exhibit a block of lens differentiation prior to the retinal defect. Besides the influence of Rb1, the lens differentiates under the influence of growth factors (e.g., FGF, IGF, PDGF, TGF), and specific genes become activated encoding cytoskeletal proteins (e.g., filensin, phakinin, vimentin), structural proteins (e.g., crystallins) or membrane proteins (e.g., Mip). The optic nerve originates from the neural retina; ganglion cells grow to the optic stalk, forming the optic nerve. Its retrograde walk to the brain through the rudiment of the optic stalk depends on the correct Pax2 expression.