Is the lens canned?

The ocular lens somehow remains pellucid despite bombardment by ultraviolet radiation and endogenous hydrogen peroxide (present in the humoral fluids which bathe this tissue). The lens and adjacent aqueous and vitreous humors contain exceptionally high concentrations of reducing substances, particularly ascorbic acid, thought to be important in lenticular oxidant defense. However, in the presence of traces of transition metals, or when exposed to ultraviolet radiation, ascorbic acid readily reacts with oxygen, yielding hydrogen peroxide, and damaging lens crystallins. We propose the alternative hypothesis that the real antioxidant function of ascorbic acid, particularly that in the aqueous and vitreous humors, may be effecting the conversion of oxygen to water. Because the lens lacks a blood supply, coupled reactions of ascorbic acid with oxygen in the humoral fluid spaces should produce a metabolically sustained anaerobiosis. If so, nature may have preinvented the process of canning, wherein food (or in this case, the lens) is preserved by a combination of sterility and anoxia.     

The molecular refractive function of lens γ-Crystallins

γ-Crystallins constitute the major protein component in the nucleus of the vertebrate eye lens. Present at very high concentrations, they exhibit extreme solubility and thermodynamic stability to prevent scattering of light and formation of cataracts. However, functions beyond this structural role have remained mostly unclear. Here, we calculate molecular refractive index increments of crystallins. We show that all lens γ-crystallins have evolved a significantly elevated molecular refractive index increment, which is far above those of most proteins, including nonlens members of the βγ-crystallin family from different species. The same trait has evolved in parallel in crystallins of different phyla, including S-crystallins of cephalopods. A high refractive index increment can lower the crystallin concentration required to achieve a suitable refractive power of the lens and thereby reduce their propensity to aggregate and form cataracts. To produce a significant increase in the refractive index increment, a substantial global shift in amino acid composition is required, which can naturally explain the highly unusual amino acid composition of γ-crystallins and their functional homologues. This function provides a new perspective for interpreting their molecular structure.     

Thermal and acid denaturation of bovine lens α-crystallin

The chaperone-like protein α-crystallin is a ～35 subunit hetero-oligomer consisting of αA and αB subunits in a 3:1 molar ratio and has the function of maintaining eye lens transparency. We studied the thermal denaturation of α-crystallin by differential scanning calorimetry (DSC), circular dichroism (CD), and dynamic light scattering (DLS) as a function of pH. Our results show that between pH 7 and 10 the protein undergoes a reversible thermal transition. However, the thermodynamic parameters obtained by DSC are inconsistent with the complete denaturation of an oligomeric protein of the size of α-crystallin. Accordingly, the CD data suggest the presence of extensive residual secondary structure above the transition temperature. Within the pH range from 4 to 7 the increased aggregation propensity around the isoelectric point (pI ～ 6) precludes observation of a thermal transition. As pH decreases below 4 the protein undergoes a substantial unfolding. The secondary structure content of the acid-denatured state shows little sensitivity to heating. We propose that the thermal transition above pH 7 and the acid-induced transition at ambient temperature result in predominant denaturation of the αB subunit. Although the extent of denaturation of the αA subunit cannot be estimated from the current data, the existence of a native-like conformation is suggested by the preserved association of the subunits and the chaperone-like activity. A key difference between the thermal and the acid denaturation is that the latter is accompanied by dissociation of αB subunits from the remaining αA-oligomer, as supported by DLS studies.     

High-affinity binding of NADPH to camel lens ζ-crystallin

Fluorescence spectrum of camel lens ζ-crystallin, a major protein in the lens of camelids and histicomorph rodents, showed maximum emission at 315 nm. This emission maximum is blue shifted compared to most proteins, including α-crystallin, and appeared to be due to tryptophan in highly hydrophobic environment. Interaction of NADPH with ζ-crystallin quenched the protein fluorescence and enhanced the fluorescence of bound NADPH. Analysis of fluorescence quenching suggested high-affinity interaction between NADPH and ζ-crystallin with an apparent K_m<0.45 μM. This value is at least an order of magnitude lower than that suggested by activity measurements. Analysis of NADPH fluorescence showed a biphasic curve representing fluorescence of free- and bound-NADPH. The intersection between free- and bound-NADPH closely paralleled the enzyme concentration, suggesting one mole of NADPH was bound per subunit of the enzyme. Phenanthrenequinone (PQ), the substrate of ζ-crystallin, also was able to quench the fluorescence of ζ-crystallin, albeit weaker than NADPH. Quantitative analysis suggested that ζ-crystallin had low affinity for PQ in the absence of NADPH, and PQ binding induced significant conformational changes in ζ-crystallin.     

Temperature-dependent coaggregation of eye lens αB- and β-crystallins

Crystallin is essential not only for the maintenance of eye lens transparency, but also in the biology of other tissues. Eye lens α-crystallin exists as a heteropolymer composed of two homologous subunits, αA and αB. Despite the critical role of α-crystallin in many tissues, little is known regarding structural and functional significance of the two subunits. Herein, we describe a unique feature of αB-crystallin. At high temperatures (>70 °C) not only αB-crystallin aggregates but also enhances the aggregation of other lens proteins. Intriguingly, αB-crystallin-mediated coaggregation at and above 70 °C involves β- but not γ-crystallin. Further, αA-crystallin, but not a mutant (F71L) αA-crystallin, prevented aggregation of αB-crystallin and also reduced coaggregation of αB- and β-crystallin. These studies explain the rationale for the existence of α-crystallin heteropolymer with αA subunit as a major partner that is vital for lens transparency and provide insights into αB-crystallin-induced coaggregation which may have a bearing in some pathological conditions where αB-crystallin is overexpressed.     

Interaction of newly synthesized α-crystallin with isolated lens plasma membranes

Translation of lens polyribosomes in a reticulocyte cell-free system results mainly in the synthesis of the water-soluble crystallins. After incubation of the translation products with isolated lens fiber plasma membranes, the newly synthesized α-crystallin interacts with this fraction and becomes water-insoluble. Urea extraction of the reisolated plasma membranes shows that part of the polymeric α-crystallin, in particular the αA chains, becomes urea-insoluble. When the membranes were isolated under conditions that stabilize complex formation with the cytoskeleton, only αA₂ seems to interact with this complex. In contrast, interaction with β- and γ-crystallin could not be observed.     

Is the cytoskeleton-plasma membrane complex involved in lens protein biosynthesis?

Calf lens fiber cells contain a population of polyribosomes that direct, at leastin vitro, the synthesis of a specific plasma membrane protein MP26. This protein may serve as a marker in terminal differentiation, since it is absent in the lens epithelium but appears in lens fiber plasma membranes. The MP26 manufacturing polyribosomes are found to be associated with a structural complex in which also the cytoskeleton and plasma membranes participate. They can be released from the complex by treatment with DNAse I. This result presumably reflects the involvement of actin in the linkage of the MP26 synthesizing polyribosomes to the cytoskeleton-membrane complex.     

Do epidermal lens cells facilitate the absorptance of diffuse light?

Many understory plants rely on diffuse light for photosynthesis because direct light is usually scattered by upper canopy layers before it strikes the forest floor. There is a considerable gap in the literature concerning the interaction of direct and diffuse light with leaves. Some understory plants have well-developed lens-shaped epidermal cells, which have long been thought to increase the absorption of diffuse light. To assess the role of epidermal cell shape in capturing direct vs. diffuse light, we measured leaf reflectance and transmittance with an integrating sphere system using leaves with flat (Begonia erythrophylla, Citrus reticulata, and Ficus benjamina) and lens-shaped epidermal cells (B. bowerae, Colocasia esculenta, and Impatiens velvetea). In all species examined, more light was absorbed when leaves were irradiated with direct as opposed to diffuse light. When leaves were irradiated with diffuse light, more light was transmitted and more was reflected in both leaf types, resulting in absorptance values 2-3% lower than in leaves irradiated with direct light. These data suggest that lens-shaped epidermal cells do not aid the capture of diffuse light. Palisade and mesophyll cell anatomy and leaf thickness appear to have more influence in the capture and absorption of light than does epidermal cell shape.     

Outer membrane β-barrel structure prediction through the lens of AlphaFold2

Most proteins found in the outer membrane of gram-negative bacteria share a common domain: the transmembrane β-barrel. These outer membrane β-barrels (OMBBs) occur in multiple sizes and different families with a wide range of functions evolved independently by amplification from a pool of homologous ancestral ββ-hairpins. This is part of the reason why predicting their three-dimensional (3D) structure, especially by homology modeling, is a major challenge. Recently, DeepMind's AlphaFold v2 (AF2) became the first structure prediction method to reach close-to-experimental atomic accuracy in CASP even for difficult targets. However, membrane proteins, especially OMBBs, were not abundant during their training, raising the question of how accurate the predictions are for these families. In this study, we assessed the performance of AF2 in the prediction of OMBBs and OMBB-like folds of various topologies using an in-house-developed tool for the analysis of OMBB 3D structures, and barrOs. In agreement with previous studies on other membrane protein classes, our results indicate that AF2 predicts transmembrane β-barrel structures at high accuracy independently of the use of templates, even for novel topologies absent from the training set. These results provide confidence on the models generated by AF2 and open the door to the structural elucidation of novel transmembrane β-barrel topologies identified in high-throughput OMBB annotation studies or designed de novo.     

Cataract-linked γD-crystallin mutants have weak affinity to lens chaperones α-crystallins

To test the hypothesis that α-crystallin chaperone activity plays a central role in maintenance of lens transparency, we investigated its interactions with γ-crystallin mutants that cause congenital cataract in mouse models. Although the two substitutions, I4F and V76D, stabilize a partially unfolded γD-crystallin intermediate, their affinities to α-crystallin are marginal even at relatively high concentrations. Detectable binding required further reduction of γD-crystallin stability which was achieved by combining the two mutations. Our results demonstrate that mutants and possibly age-damaged γ-crystallin can escape quality control by lens chaperones rationalizing the observation that they nucleate protein aggregation and lead to cataract.     

Amino acid sequences around the cysteine residue of calf lens α-crystallin

1. Calf lens alpha-crystallin was carboxymethylated with radioactive sodium iodoacetate to label the thiol group. 2. The protein was then digested with trypsin or alternatively fractionated in urea to obtain the acidic (A) chains, which were then digested with trypsin. Either procedure gave two radioactive peptides containing carboxymethylcysteine. 3. These two peptides were closely related: the longer form contained 28 amino acid residues, and the shorter lacked two residues at the N-terminal end of the longer form. 4. The amino acid sequence of the peptides have been determined. 5. No evidence for the presence of more than one cysteine residue/chain was found. 6. The question of the molecular weight of the chains is discussed.     

Calpain II induced insolubilization of lens β-crystallin polypeptides may induce cataract

Addition of calpain II (EC 3.4.22.17) to soluble proteins from 10-day-old rat lens caused an increase in turbidity and production of water-insoluble protein. The insolubilization increased with higher concentrations of both lens protein and calpain II, it could be prevented by the cysteine protease inhibitor E-64; it required at least 0.5 mM Ca²⁺, it was limited to 6% of the soluble protein present and resulted from precipitation β-crystallin polypeptides. When compared by two-dimensional electrophoresis, the insoluble β-crystallin polypeptides produced by calpain II were similar to insoluble β-crystallin polypeptides found incataractous lenses. Trypsin also caused insolubilization of β-crystallin polypeptides, but these polypeptides were unlike polypeptides produced during cataract formation. These data suggested that the loss of solubility was due to a specific removal of N/or C-terminal extensions from β-crystallin polypeptides by calpain II, and that a similar process may occur in vivo during cataract formation. It is hypothesized that the insoluble protein produced by calpain II causes cataract by increasing light scatter in the lens.     

Preliminary X-ray crystallographic study of the turkey lens protein, δ-crystallin

The structural protein, δ-crystallin, has been purified and crystallized from adult turkey lens. The crystals are orthorhombic and normally belong to space group P2₁2₁2 with unit cell dimensions $\text{a = 99.9(2)}\text{A}\text{?}\text{, b = 133.4(3)}\text{A}\text{?}\text{and}\text{c = 69.1(2)}\text{A}\text{?}$. This corresponds to two molecules of molecular weight approximately 200,000 per unit cell. A second crystal form has also been found in which b and c increase to 135.4(3) Å and 140.0(3) Å. respectively, indicating four molecules per unit cell.     

Physicochemical characterization of γ-crystallins from bovine lens—Hydrodynamic and biochemical properties

A detailed hydrodynamic study has been made on the γ-crystallin of the bovine lens. Sedimentation study indicates that γ-crystallin shows a nearly gaussian peak throughout the course of sedimentation at high speed, using a synthetic boundary cell. The diffusion and sedimentation coefficients are 10.3×10^?7 cm²/sec and 2.51 S, respectively. The weight-average molecular weight of the unfractionated γ-crystallin calculated from sedimentation equilibrium is 21,800. The four major subfractions of γ-crystallin show similar hydrodynamic properties with an intrinsic viscosity of 2.50 ml/g and a Stokes radius of 21 Å. The distinct electrophoretic mobilities exhibited by the four subfractions show gel-concentration dependence and similar slopes in the Ferguson plot, indicative of being charge isomers of the same molecular species. Amino acid analysis of these four subfractions corroborated the conclusions that these γ-crystallin polypeptides are closely related and comprise a multigene family of crystallins. Based on the sedimentation and intrinsic viscosity data, γ-crystallin can be modeled as a prolate ellipsoid with an axial ratio of approximately 3.0 and a hydration factor of 0.27 g water per gram protein. The circular dichroism data for γ-crystallins showed a minimum at about 217 nm, characteristic of a β-sheet conformation. These structural characteristics are in good accord with those derived from X-ray diffraction data for γ-crystallin II.     

Degradation of δ-crystallin mRNA in the lens fiber cells of the chicken

δ-Crystallin is the principal protein synthesized in the embryonic chicken lens. After hatching δ-crystallin synthesis decreases and eventually ceases. We have determined when the δ-crystallin messenger RNA (mRNA) disappears from the lens fiber cells during the first year of age by cell-free translation of lens RNA in a reticulocyte lysate, RNA blot (Northern) hybridization, and in situ hybridization. The hybridization was performed with a nick-translated, cloned δ-crystallin cDNA (pδCr2). δ-Crystallin mRNA was present in the lens until 3 months of age and disappeared between the third and fifth month after hatching. The in situ hybridization experiments indicated that the δ-crystallin mRNA was present throughout the lens fiber mass until 1 month after hatching and was greatly reduced in the cortical fiber cells thereafter. In contrast to earlier stages, then, the cortical fiber cells differentiating at the lens equator after about 1 month of age do not accumulate δ-crystallin mRNA. The data also indicate that the maximal half-life of functional δ-crystallin mRNA in the posthatched chicken lens is about 2 months.     

Are mouse lens epithelial cells more sensitive to γ-irradiation than lymphocytes?

In this pilot study we compared for the first time the radiation sensitivity of mouse lens epithelial cells (LECs) and mouse lymphocytes. We freshly prepared LECs and lymphocytes and irradiated them with γ-rays (¹³⁷Cs; doses ranging from 0.25 to 2 Gy). DNA damage and repair were evaluated by alkaline comet assay and γH2AX foci assay. Using the comet assay, we observed a dose-dependent increase in DNA damage in both cell types. The faster formation of single- and double-strand breaks in LECs of C57BL/6 mice at doses below 1 Gy needs to be confirmed in other mouse strains. Immunofluorescence for γH2AX foci showed a higher degree of lesions in LECs from C57BL/6J mice compared to those of JF1 mice and to lymphocytes of both strains. Correspondingly, repair of DNA damage proceeded faster in LECs of C57BL/6J mice compared to LECs of JF1 mice and lymphocytes of both strains. It is obvious that the lymphocytes of both strains repaired DNA lesions more slowly than the corresponding LECs. In conclusion, our results demonstrate that LECs of C57Bl/6 mice show a steeper dose–response than lymphocytes in both types of experiments. It shows that both test systems are able to be used also at doses below 0.25 Gy. The observed difference in DNA repair between the LECs from C57BL/6J mice compared to the LECs from JF1 mice and to the lymphocytes of both strains warrants further experiments to identify the underlying molecular mechanisms.     

Is the C-terminal arm of lens gap junction channel protein the channel gate?

Lens gap junction channels are studied in a reconstituted system obtained by incorporating into liposomes, with or without calmodulin, the lens junction protein (MIP26) and its trypsin-cleaved product (MIP21) that lacks the C-terminal arm. Channel permeability is studied with an osmotic swelling assay. MIP26 and MIP21 liposomes swell in sucrose or polyethyleneglycol with or without Ca++ indicating the presence of large channels. Without Ca++, MIP26 and MIP21 liposomes swell in both permeants. With Ca++, MIP26-calmodulin liposomes do not swell in either permeant, indicating complete channel closure, while MIP21-calmodulin liposomes swell in sucrose but not in polyethyleneglycol. This suggests that the C-terminal arm participates in channel gating.     

The amino acid sequence of γ-crystallin (fraction II) from calf lens

The amino acid sequence of gamma-crystallin (fraction II) from calf lens has been determined; this indicates it to be a single-chain polypeptide of 165 amino acid residues.     

A cell polarity protein aPKCλ is required for eye lens formation and growth

The organisation of individual cells into a functional three-dimensional tissue is still a major question in developmental biology. Modulation of epithelial cell shape is a critical driving force in forming tissues. This is well illustrated in the eye lens where epithelial cells elongate extensively during their differentiation into fibre cells. It is at the lens equator that epithelial cells elongate along their apical-basal axis. During this process the elongating epithelial cells and their earliest fibre cell derivatives remain anchored at their apical tips, forming a discrete region or modiolus, which we term the lens fulcrum. How this is achieved has received scant attention and is little understood. Here, we show that conditional depletion of aPKCλ, a central effector of the PAR polarity complex, disrupts the apical junctions in elongating epithelial cells so that the lens fulcrum fails to form. This results in disorganised fibre cell alignment that then causes cataract. Interestingly, aPKCλ depletion also promotes epithelial-mesenchymal transition of the lens epithelial cells, reducing their proliferation, leading ultimately to a small lens and microphthalmia. These observations indicate that aPKCλ, a regulator of polarity and apical junctions, is required for development of a lens that is the correct size and shape.     

Cell kinetic status of mouse lens epithelial cells lacking αA- and αB-crystallin

alphaA- and alphaB-crystallins are small heat shock proteins and molecular chaperones that are known to prevent non-specific aggregation of denaturing proteins. Recent work indicates that alphaA-/- lens epithelial cells grow at a slower rate than wild-type cells, and cultured alphaB-/- cells demonstrate increased hyperproliferation and genomic instability, suggesting that these proteins may exert a direct effect on the cell cycle kinetics, and influence cell proliferation. However, the cell cycle parameters of alphaA/alphaBKO (double knockout) cells have not been analyzed. Here we investigate the cell cycle kinetics of synchronized mouse lens epithelial cultures derived from wild-type and alphaA/alphaB double knockout (alphaA/alphaBKO) mice using BrdU labeling of proliferating cells, and flow cytometric analysis. We also provide data on the changing pattern of expression of HSP25, a small heat shock protein in alphaA/alphaBKO and wild-type cells during the cell cycle. Using serum starvation to synchronize cells in the quiescent G0 phase, and restimulation with serum followed by BrdU labeling and flow cytometry, the data indicated that as compared to wild-type cells, a <50% smaller fraction of the alphaA/alphaBKO cells entered the DNA synthetic S phase of the cell cycle. Furthermore, there was a delay in cell cycle transit through S phase in alphaA/alphaBKO cells, suggesting that although capable of entering S phase, the alphaA/alphaBKO cells are blocked in G1 phase, and are delayed in their cell cycle progression. Immunoblot analysis with antibodies to the small heat shock protein HSP25 indicated that although HSP25 increased in G1 phase of wild-type cells, and remained elevated on further progression through the cell cycle, HSP25 accumulation was delayed to S phase in alphaA/alphaBKO cells. These data can be interpreted to indicate that mouse lens epithelial cell progression through the cell cycle is significantly affected by expression of alphaA and alphaB-crystallin.