High-resolution X-ray crystal structures of human gammaD crystallin (1.25 A) and the R58H mutant (1.15 A) associated with aculeiform cataract

Several human cataracts have been linked to mutations in the gamma crystallin gene. One of these is the aculeiform cataract, which is caused by an R58H mutation in gammaD crystallin. We have shown previously that this cataract is caused by crystallization of the mutant protein, which is an order of magnitude less soluble than the wild-type. Here, we report the very high-resolution crystal structures of the mutant and wild-type proteins. Both proteins crystallize in the same space group and lattice. Thus, a strict comparison of the protein-protein and protein-water intermolecular interactions in the two crystal lattices is possible. Overall, the differences between the mutant and wild-type structures are small. At position 58, the mutant protein loses the direct ion-pair intermolecular interaction present in the wild-type, due to the differences between histidine and arginine at the atomic level; the interaction in the mutant is mediated by water molecules. Away from the mutation site, the mutant and wild-type lattice structures differ in the identity of side-chains that occupy alternate conformations. Since the interactions in the crystal phase are very similar for the two proteins, we conclude that the reduction in the solubility of the mutant is mainly due to the effect of the R58H mutation in the solution phase. The results presented here are also important as they are the first high-resolution X-ray structures of human gamma crystallins.     

Lens crystallins and their microbial homologs: structure, stability, and function.

abg-Crystallins are the major protein components in the vertebrate eye lens--a as a molecular chaperone and b and g as structural proteins. Surprisingly, the latter two share some structural characteristics with a number of microbial stress proteins. The common denominator is not only the Greek key topology of their polypeptide chains but also their high intrinsic stability, which, in certain microbial crystallin homologs, is further enhanced by high-affinity Ca2+-binding. Recent studies of natural and mutant vertebrate bg-crystallins as well as spherulin 3a from Physarum polycephalum and Protein S from Myxococcus xanthus allowed the correlation of structure and stability of crystallins to be elucidated in some detail. From the thermodynamic point of view, stability increments come from (1) local interactions involved in the close packing of the cooperative units, (2) the all-b secondary structure of the Greek-key motif, (3) intramolecular interactions between domains, (4) intermolecular domain interactions, including 3D domain swapping and (v) excluded volume effects due to "molecular crowding" at the high cellular protein concentrations. Apart from these contributions to the Gibbs free energy of stability, significant kinetic stabilization originates from the high activation energy barrier determining the rate of unfolding from the native to the unfolded state. From the functional point of view, the high stability is responsible for the long-term transparency of the eye lens, on the one hand, and the stress resistance of the microorganisms in their dormant state on the other. Local structural perturbations due to chemical modification, wrong protein interactions, or other irreversible processes may lead to protein aggregation. A leading cataract hypothesis is that only after a-crystallin, a member of the small heat-shock protein family, is titrated out does pathological opacity occur. Understanding the structural basis of protein stability in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.     

用蛋白质印迹转移技术分析正常及硒性白内障大鼠晶状体及房水中蛋白质的性质

本文用蛋白质印迹转移技术分析了正常及硒性白内障大鼠晶状体及房水中蛋白质的性质。结果表明,晶状体中的脲溶性蛋白质可被抗α及抗γ晶体蛋白血清识别,提示α及γ晶体蛋白均为脲溶性蛋白质的主要成份。患白内障时房水中的蛋白质含量明显增加,且主要被抗γ血清识别,而被抗α血清识别的成份很少,表明在大鼠硒性白内障形成过程中,有较多低分子量蛋白质漏出到房水中,且其主要成份为γ晶体蛋白。此外,我们还发现正常及硒性白内障大鼠晶状体膜蛋白质与抗α及抗γ血清起反应的程度及分布有所不同,提示晶状体细胞膜与晶体蛋白之间存在着相互作用。     

Lens Crystallins and Their Microbial Homologs: Structure,Stability, and Function

Referee: Franz Schmid, Biochemicshes Laboratorium, Universitaet Bayeuth, D-95440 Bayeuth, Germany

abg-Crystallins are the major protein components in the vertebrate eye lens — a as a molecular chaperone and b and g as structural proteins. Surprisingly, the latter two share some structural characteristics with a number of microbial stress proteins. The common denominator is not only the Greek key topology of their polypeptide chains but also their high intrinsic stability, which, in certain microbial crystallin homologs, is further enhanced by high-affinity Ca²⁺-binding. Recent studies of natural and mutant vertebrate bg-crystallins as well as spherulin 3a from Physarum polycephalum and Protein S from Myxococcus xanthus allowed the correlation of structure and stability of crystallins to be elucidated in some detail. From the thermo-dynamic point of view, stability increments come from (1) local interactions involved in the close packing of the cooperative units, (2) the all-b secondary structure of the Greek-key motif, (3) intramolecular interactions between domains, (4) intermolecular domain interactions, including 3D domain swapping and (v) excluded volume effects due to “molecular crowding” at the high cellular protein concentrations. Apart from these contributions to the Gibbs free energy of stability, significant kinetic stabilization originates from the high activation energy barrier determining the rate of unfolding from the native to the unfolded state. From the functional point of view, the high stability is responsible for the long-term transparency of the eye lens, on the one hand, and the stress resistance of the microorganisms in their dormant state on the other. Local structural perturbations due to chemical modification, wrong protein interactions, or other irreversible processes may lead to protein aggregation. A leading cataract hypothesis is that only after a-crystallin, a member of the small heat-shock protein family, is titrated out does pathological opacity occur. Understanding the structural basis of protein stability in the healthy eye lens is the route to solve the enormous medical and economical problem of cataract.     

The Structure and Stability of the Disulfide-Linked γS-Crystallin Dimer Provide Insight into Oxidation Products Associated with Lens Cataract Formation

The reducing environment in the eye lens diminishes with age, leading to significant oxidative stress. Oxidation of lens crystallin proteins is the major contributor to their destabilization and deleterious aggregation that scatters visible light, obscures vision, and ultimately leads to cataract. However, the molecular basis for oxidation-induced aggregation is unknown. Using X-ray crystallography and small-angle X-ray scattering, we describe the structure of a disulfide-linked dimer of human γS-crystallin that was obtained via oxidation of C24. The γS-crystallin dimer is stable at glutathione concentrations comparable to those in aged and cataractous lenses. Moreover, dimerization of γS-crystallin significantly increases the protein’s propensity to form large insoluble aggregates owing to non-cooperative domain unfolding, as is observed in crystallin variants associated with early-onset cataract. These findings provide insight into how oxidative modification of crystallins contributes to cataract and imply that early-onset and age-related forms of the disease share comparable development pathways.     

Simulation and modeling of crowding effects on the thermodynamic and kinetic properties of proteins with atomic details

Recent experimental studies of protein folding and binding under crowded solutions suggest that crowding agents exert subtle influences on the thermodynamic and kinetic properties of the proteins. While some of the crowding effects can be understood qualitatively from simple models of the proteins, quantitative rationalization of these effects requires an atomistic representation of the protein molecules in modeling their interactions with crowders. A computational approach, known as postprocessing, has opened the door for atomistic modeling of crowding effects. This review summarizes the applications of the postprocessing approach for studying crowding effects on the thermodynamics and kinetics of protein folding, conformational transition, and binding. The integration of atomistic modeling with experiments in crowded solutions promises new insight into biochemical processes in cellular environments.     

Lens aging: Effects of crystallins

The primary function of the eye lens is to focus light on the retina. The major proteins in the lens—α, β, and γ-crystallins—are constantly subjected to age-related changes such as oxidation, deamidation, truncation, glycation, and methylation. Such age-related modifications are cumulative and affect crystallin structure and function. With time, the modified crystallins aggregate, causing the lens to increasingly scatter light on the retina instead of focusing light on it and causing the lens to lose its transparency gradually and become opaque. Age-related lens opacity, or cataract, is the major cause of blindness worldwide. We review deamidation, and glycation that occur in the lenses during aging keeping in mind the structural and functional changes that these modifications bring about in the proteins. In addition, we review proteolysis and discuss recent observations on how crystallin fragments generated in vivo, through their anti-chaperone activity may cause crystallin aggregation in aging lenses. We also review hyperbaric oxygen treatment induced guinea pig and ‘humanized’ ascorbate transporting mouse models as suitable options for studies on age-related changes in lens proteins.     

Lactate Dehydrogenase Like Crystallin: A Potentially Protective Shield for Indian Spiny-Tailed Lizard (Uromastyx hardwickii) Lens Against Environmental Stress?

Taxon specific lens crystallins in vertebrates are either similar or identical with various metabolic enzymes. These bifunctional crystallins serve as structural protein in lens along with their catalytic role. In the present study, we have partially purified and characterized lens crystallin from Indian spiny-tailed lizard (Uromastyx hardwickii). We have found lactate dehydrogenase (LDH) activity in lens indicating presence of an enzyme crystallin with dual functions. Taxon specific lens crystallins are product of gene sharing or gene duplication phenomenon where a pre-existing enzyme is recruited as lens crystallin in addition to structural role. In lens, same gene adopts refractive role in lens without modification or loss of pre-existing function during gene sharing phenomenon. Apart from conventional role of structural protein, LDH activity containing crystallin in U. hardwickii lens is likely to have adaptive characteristics to offer protection against toxic effects of oxidative stress and ultraviolet light, hence justifying its recruitment. Taxon specific crystallins may serve as good models to understand structure–function relationship of these proteins.     

Non-oxidative modification of lens crystallins by kynurenine: a novel post-translational protein modification with possible relevance to ageing and cataract

In humans, the crystallin proteins of the ocular lens become yellow-coloured and fluorescent with ageing. With the development of senile nuclear cataract, the crystallins become brown and additional fluorophores are formed. The mechanism underlying crystallin colouration is not known but may involve interaction with kynurenine-derived UV filter compounds. We have recently identified a sulphur-linked glutathionyl-3-hydroxykynurenine glucoside adduct in the lens and speculated that kynurenine may also form adducts with GSH and possibly with nucleophilic amino acids of the crystallins (e.g. Cys). Here we show that kynurenine modifies calf lens crystallins non-oxidatively to yield coloured (365 nm absorbing), fluorescent (Ex 380 nm/Em 450-490 nm) protein adducts. Carboxymethylation and succinylation of crystallins inhibited kynurenine-mediated modification by approx. 90%, suggesting that Cys, Lys and possibly His residues may be involved. This was confirmed by showing that kynurenine formed adducts with GSH as well as with poly-His and poly-Lys. NMR studies revealed that the novel poly-Lys-kynurenine covalent linkage was via the epsilon-amino group of the Lys side chain and the betaC of the kynurenine side chain. Analysis of tryptic peptides of kynurenine-modified crystallins revealed that all of the coloured peptides contained either His, Cys or an internal Lys residue. We propose a novel mechanism of kynurenine-mediated crystallin modification which does not require UV light or oxidative conditions as catalysts. Rather, we suggest that the side chain of kynurenine-derived lens UV filters becomes deaminated to yield an alpha,beta-unsaturated carbonyl which is highly susceptible to attack by nucleophilic amino acid residues of the crystallins. The inability of the lens fibre cells to metabolise their constituent proteins results in the accumulation of coloured/fluorescent crystallins with age.     

Protein oxidation and loss of protease activity may lead to cataract formation in the aged lens

Over 95% of the dry mass of the eye lens consists of specialized proteins called crystallins. Aged lenses are subject to cataract formation, in which damage, cross-linking, and precipitation of crystallins contribute to a loss of lens clarity. Cataract is one of the major causes of blindness, and it is estimated that over 50,000,000 people suffer from this disability. Damage to lens crystallins appears to be largely attributable to the effects of UV radiation and/or various active oxygen species (oxygen radicals, ¹O₂, H₂O₂, etc.). Photooxidative damage to lens crystallins is normally retarded by a series of antioxidant enzymes and compounds. Crystallins which experience mild oxidative damage are rapidly degraded by a system of lenticular proteases. However, extensive oxidation and cross-linking severely decrease proteolytic susceptibility of lens crystallins. Thus, in the young lens the combination of antioxidants and proteases serves to prevent crystallin damage and precipitation in cataract formation. The aged lens, however, exhibits diminished antioxidant capacity and decreased proteolytic capabilities. The loss of proteolytic activity may actually be partially attributable to oxidative damage which proteases (like any other protein)_can sustain. We propose that the rate of crystallin damage increases as antioxidant capacity declines with age. The lower protease activity of aged lens cells may be insufficient to cope with such rates of crystallin damage, and denatured crystallins may begin to accumulate. As the concentration of oxidatively denatured crystallins rises, cross-linking reactions may produce insoluble aggregates which are refractive to protease digestion. Such a scheme could explain many events which are known to contribute to cataract formation, as well as several which have appeared to be unrelated. This hypothesis is also open to experimental verification and intervention.     

Amyloid fibril formation by lens crystallin proteins and its implications for cataract formation

The alpha-, beta-, and gamma-crystallins are the major structural proteins within the eye lens and are responsible for its exceptional stability and transparency. Under mildly denaturing conditions, all three types of bovine crystallin assemble into fibrillar structures in vitro. Characterization by transmission electron microscopy, dye binding assays, and x-ray fiber diffraction shows that these species have all of the characteristics of fibrils associated with the family of amyloid diseases. Moreover, the full-length proteins are incorporated into the fibrils, (i.e. no protein cleavage is required for these species to form), although for the gamma-crystallins some fragmentation occurs under the conditions employed in this study. Our findings indicate that the inherent stability of the beta-sheet supramolecular structure adopted by the crystallins in the eye lens and the chaperone ability of alpha-crystallin must be crucial for preventing fibril formation in vivo. The crystallins are very stable proteins but undergo extensive post-translational modification with age that leads to their destabilization. The ability of the crystallins to convert into fibrils under destabilizing conditions suggests that this process could contribute to the development of cataract with aging.     

Biophysical chemistry of the ageing eye lens

This review examines both recent and historical literature related to the biophysical chemistry of the proteins in the ageing eye, with a particular focus on cataract development. The lens is a vital component of the eye, acting as an optical focusing device to form clear images on the retina. The lens maintains the necessary high transparency and refractive index by expressing crystallin proteins in high concentration and eliminating all large cellular structures that may cause light scattering. This has the consequence of eliminating lens fibre cell metabolism and results in mature lens fibre cells having no mechanism for protein expression and a complete absence of protein recycling or turnover. As a result, the crystallins are some of the oldest proteins in the human body. Lack of protein repair or recycling means the lens tends to accumulate damage with age in the form of protein post-translational modifications. The crystallins can be subject to a wide range of age-related changes, including isomerisation, deamidation and racemisation. Many of these modification are highly correlated with cataract formation and represent a biochemical mechanism for age-related blindness.     

Identification of Subunit-Subunit Interaction Sites in αA-WT Crystallin and Mutant αA-G98R Crystallin Using Isotope-Labeled Cross-Linker and Mass Spectrometry

Cataract is characterized by progressive protein aggregation and loss of vision. α-Crystallins are the major proteins in the lens responsible for maintaining transparency. They exist in the lens as highly polydisperse oligomers with variable numbers of subunits, and mutations in α-crystallin are associated with some forms of cataract in humans. Because the stability of proteins is dependent on optimal subunit interactions, the structural transformations and aggregation of mutant proteins that underlie cataract formation can be understood best by identifying the residue-specific inter- and intra-subunit interactions. Chemical crosslinking combined with mass spectrometry is increasingly used to provide structural insights into intra- and inter-protein interactions. We used isotope-labeled cross-linker in combination with LC-MS/MS to determine the subunit–subunit interaction sites in cataract-causing mutant αA-G98R crystallin. Peptides cross-linked by isotope-labeled (heavy and light forms) cross-linkers appear as doublets in mass spectra, thus facilitating the identification of cross-linker–containing peptides. In this study, we cross-linked wild-type (αA-WT) and mutant (αA-G98R) crystallins using the homobifunctional amine-reactive, isotope-labeled (d₀ and d₄) cross-linker–BS²G (bis[sulfosuccinimidyl]glutarate). Tryptic in-solution digest of cross-linked complexes generates a wide array of peptide mixtures. Cross-linked peptides were enriched using strong cation exchange (SCX) chromatography followed by both MS and MS/MS to identify the cross-linked sites. We identified a distinct intermolecular interaction site between K88 — K99 in the β5 strand of the mutant αA-G98R crystallin that is not found in wild-type αA-crystallin. This interaction could explain the conformational instability and aggregation nature of the mutant protein that results from incorrect folding and assembly.     

Effect of crowding on protein-protein association rates: fundamental differences between low and high mass crowding agents

Physiological media constitutes a crowded environment that serves as the field of action for protein-protein interaction in vivo. Measuring protein-protein interaction in crowded solutions can mimic this environment. In this work we follow the process of protein-protein association and its rate constants (k(on)) of the beta-lactamase (TEM)-beta-lactamase inhibitor protein (BLIP) complex in crowded solution using both low and high molecular mass crowding agents. In all crowded solutions (0-40% (w/w) of ethylene glycol (EG), poly(ethylene glycol) (PEG) 200, 1000, 3350, 8000 Da Ficoll-70 and Haemaccel the measured absolute k(on), but not k(off) values, were found to be slower as compared to buffer. However, there is a fundamental difference between low and high mass crowding agents. In the presence of low mass crowding agents and Haemaccel k(on) depends inversely on the solution viscosity. In high mass polymer solutions k(on) changes only slightly, even at viscosities 12-fold higher than water. The border between low and high molecular mass polymers is sharp and is dictated by the ratio between the polymer length (L) and its persistence length (Lp). Polymers that are long enough to form a flexible coil (L/Lp > 2) behave as high molecular mass polymers and those who are unable to do so (L/Lp < 2) behave as low molecular mass polymers. We concluded that although polymers solution are crowded, this property is not uniform; i.e. there are areas in the solution that contain bulk water, and in these areas proteins can diffuse and associate almost as if they were in diluted environment. This porous medium may be taken as mimicking some aspects of the cellular environment, where many of the macromolecules are organized along membranes and the cytoskeleton. To determine the contribution of electrostatic attraction between proteins in crowded milieu, we followed k(on) of wt-TEM and three BLIP analogs with up to 100-fold increased values of k(on) due to electrostatic steering. Faster associating BLIP variants keep their relative advantage in all crowded solutions, including Haemaccel. This result suggests that faster associating protein complexes keep their advantage also in complex environment.     

Competition between protein folding and aggregation with molecular chaperones in crowded solutions: insight from mesoscopic simulations

The living cell is inherently crowded with proteins and macromolecules. To avoid aggregation of denatured proteins in the living cell, molecular chaperones play important roles. Here we introduce a simple model to describe crowded protein solutions with chaperone-like species based on a dynamic density functional theory. As predicted by others, our simulations show that macromolecular crowding enhances the association of proteins and chaperones. However, when the intrinsic folding rate of the protein is slow, it is possible that crowding also enhances aggregation of proteins. The results of simulation suggest that, when the concentration of the crowding agent is as high as that in the cell, the association of the protein and unbound chaperone becomes correlated with the aggregation process, and that the protein-bound chaperones efficiently destroy the potential nuclei of aggregates and thus prevent the aggregation.     

Epidermal fucosylation of cell surface glycoprotein

The formation of covalently linked, high molecular weight protein aggregates has been thought to play an important role in opacification of the human lens. Antisera were used in Western blot analysis to demonstrate the involvement of all major classes of lens proteins (alpha, beta and gamma crystallin; the major intrinsic membrane polypeptide) in covalent aggregation. Of these classes, aggregation of gamma and beta crystallins via intermolecular disulfide bonding and aggregation of the major intrinsic membrane polypeptide via intermolecular, non-disulfide bonding were more pronounced in cataractous as compared with normal lenses.     

Crystallin distribution patterns in concentric layers from toad eye lenses

Protein distribution patterns across eye lenses from the Asiatic toad Bufo gargarizans were investigated and individual crystallin classes characterised. Special fractionation that follows the growth mode of the lens was used to yield nine fractions corresponding to layers laid down at different chronological (developmental) stages. Proportions of soluble and insoluble crystallins within each fraction were measured by Bradford assay. Water‐soluble proteins in all fractions were separated by size‐exclusion HPLC and constituents of each class further characterised by electrophoresis, RP‐HPLC and MS analysis. In outer lens layers, α‐crystallin is the most abundant soluble protein but is not found in soluble proteins in the lens centre. Water‐soluble β‐crystallins also decrease from their highest level in the outer lens to negligible mounts in the central lens. The proportion of soluble γ‐crystallin increases significantly towards the lens centre where this is the only soluble protein present. Insoluble protein levels increase significantly towards the lens centre. In B. gargarizans lenses, as with other anurans, the predominant water‐soluble protein class is γ‐crystallin. No taxon‐specific crystallins were found. The relationship between the protein distribution patterns and the functional properties of the lens this species is discussed.     

Effect of chronic hyperglycemia on crystallin levels in rat lens

Crystallins are the major structural proteins in the vertebrate eye lens that contribute to lens transparency. Although cataract, including diabetic cataract, is thought to be a result of the accumulation of crystallins with various modifications, the effect of hyperglycemia on status of crystallin levels has not been investigated. This study evaluated the effect of chronic hyperglycemia on crystallin levels in diabetic cataractous rat lens. Diabetes was induced in rats by injecting streptozotocin and maintained on hyperglycemia for a period of 10 weeks. At the end, levels of α-, β-, γ-crystallins and phosphoforms of αB-crystallins (αBC) were analyzed by immunoblotting. Further, solubility of crystallins and phosphoforms of αBC was analyzed by detergent soluble assay. Chronic diabetes significantly decreased the protein levels of α-, β- and αA-crystallins (αAC) in both soluble and insoluble fraction of lens. Whereas γ-crystallin levels were decreased and αBC levels were increased in lens soluble fraction with no change in insoluble fraction in diabetic rat lens. Although, diabetes activated the p38MAPK signaling cascade by increasing the p-p38MAPK in lens, the phosphoforms of αBC were decreased in soluble fraction with a concomitant increase in insoluble fraction of diabetic lens when compared to the controls. Moreover, diabetes strongly enhances the degradation of crystallins and phosphoforms of αBC in lens. Taken together, the decreased levels of crystallins and insolubilization of phosphoforms of αBC under chronic hyperglycemia could be one of the underlying factors in the development of diabetic cataract.     

Crowding effects on the formation and maintenance of nuclear bodies: insights from molecular-dynamics simulations of simple spherical model particles

The physics of structure formation and maintenance of nuclear bodies (NBs), such as nucleoli, Cajal bodies, promyelocytic leukemia bodies, and speckles, in a crowded nuclear environment remains largely unknown. We investigate the role of macromolecular crowding in the formation and maintenance of NBs using computer simulations of a simple spherical model, called Lennard-Jones (LJ) particles. LJ particles form a one-phase, dilute fluid when the intermolecular interaction is weaker than a critical value, above which they phase separate and form a condensed domain. We find that when volume-exclusive crowders exist in significant concentrations, domain formation is induced even for weaker intermolecular interactions, and the effect is more pronounced with increasing crowder concentration. Simulation results show that a previous experimental finding that promyelocytic leukemia bodies disappear in the less-crowded condition and reassemble in the normal crowded condition can be interpreted as a consequence of the increased intermolecular interactions between NB proteins due to crowding. Based on further analysis of the simulation results, we discuss the acceleration of macromolecular associations that occur within NBs, and the delay of diffusive transport of macromolecules within and out of NBs when the crowder concentration increases. This study suggests that in a polydisperse nuclear environment that is enriched with a variety of macromolecules, macromolecular crowding not only plays an important role in the formation and maintenance of NBs, but also may perform some regulatory functions in response to alterations in the crowding conditions.     

Crystallins in Retinal Ganglion Cell Survival and Regeneration

Crystallins are heterogeneous proteins classified into alpha, beta, and gamma families. Although crystallins were first identified as the major structural components of the ocular lens with a principal function to maintain lens transparency, further studies have demonstrated the expression of these proteins in a wide variety of tissues and cell types. Alpha crystallins (alpha A and alpha B) share significant homology with small heat shock proteins and have chaperone-like properties, including the ability to bind and prevent the precipitation of denatured proteins and to increase cellular resistance to stress-induced apoptosis. Stress-induced upregulation of crystallin expression is a commonly observed phenomenon and viewed as a cellular response mechanism against environmental and metabolic insults. However, several studies reported downregulation of crystallin gene expression in various models of glaucomatous nerodegeneration suggesting that that the decreased levels of crystallins may affect the survival properties of retinal ganglion cells (RGCs) and thus, be associated with their degeneration. This hypothesis was corroborated by increased survival of axotomized RGCs in retinas overexpressing alpha A or alpha B crystallins. In addition to RGC protective functions of alpha crystallins, beta and gamma crystallins were implicated in RGC axonal regeneration. These findings demonstrate the importance of crystallin genes in RGC survival and regeneration and further in-depth studies are necessary to better understand the mechanisms underlying the functions of these proteins in healthy RGCs as well as during glaucomatous neurodegeneration, which in turn could help in designing new therapeutic strategies to preserve or regenerate these cells.