Structural evidence of human nuclear fiber compaction as a function of ageing and cataractogenesis

Association of nuclear cataract prevalence with UV radiation and heat load in lens of older people -five city study

Epidemiological studies have reported that the frequency of nuclear cataracts (NUCs) is high among the elderly and in tropical countries. Ultraviolet (UV) irradiation and lens temperature are considered as key physical contributors, although their precise quantification is difficult. The aim of this study is to investigate the association of NUC prevalence with UV irradiation and heat load. First, we assessed the lens temperature using thermodynamic modeling considering the thermophysiological response. We then conducted a multivariate linear regression analysis for the epidemiological analysis of NUC prevalence across five cities. A strong correlation was observed between NUC prevalence and the combined effects of UV irradiation and cumulative equivalent minutes at 43 °C (CEM43°C) derived from the computed lens temperature (adjusted R2 = 0.933, p < 0.0001). Heat load significantly contributed to the prevalence at 52%, surpassing the contributions of UV irradiation (31%) and the decline in DNA repair capacity in the lens (17%). These results suggested that both UV radiation and heat load are associated with NUC, with heat load contributing more. Our findings provided important implications for future interventions, particularly in the context of global warming.

The effect of fibre cell remodelling on the power and optical quality of the lens

Vertebrate eye lenses are uniquely adapted to form a refractive index gradient (GRIN) for improved acuity, and to grow slowly in size despite constant cell proliferation. The mechanisms behind these adaptations remain poorly understood. We hypothesize that cell compaction contributes to both. To test this notion, we examined the relationship between lens size and shape, refractive characteristics and the cross-sectional areas of constituent fibre cells in mice of different ages. We developed a block-face imaging method to visualize cellular cross sections and found that the cross-sectional areas of fibre cells rose and then decreased over time, with the most significant reduction occurring in denucleating cells in the adult lens cortex, followed by cells in the embryonic nucleus. These findings help reconcile differences between the predictions of lens growth models and empirical data. Biomechanical simulations suggested that compressive forces generated from continuous deposition of fibre cells could contribute to cellular compaction. However, optical measurements revealed that the GRIN did not mirror the pattern of cellular compaction, implying that compaction alone cannot account for GRIN formation and that additional mechanisms are likely to be involved.

Aggrelyte-2 promotes protein solubility and decreases lens stiffness through lysine acetylation and disulfide reduction: Implications for treating presbyopia

Aging proteins in the lens become increasingly aggregated and insoluble, contributing to presbyopia. In this study, we investigated the ability of aggrelyte-2 (N,S-diacetyl-L-cysteine methyl ester) to reverse the water insolubility of aged human lens proteins and to decrease stiffness in cultured human and mouse lenses. Water-insoluble proteins (WI) of aged human lenses (65-75 years) were incubated with aggrelyte-2 (500 μM) for 24 or 48 h. A control compound that lacked the S-acetyl group (aggrelyte-2C) was also tested. We observed 19%-30% solubility of WI upon treatment with aggrelyte-2. Aggrelyte-2C also increased protein solubility, but its effect was approximately 1.4-fold lower than that of aggrelyte-2. The protein thiol contents were 1.9- to 4.9-fold higher in the aggrelyte-2- and aggrelyte-2C-treated samples than in the untreated samples. The LC-MS/MS results showed Nε -acetyllysine (AcK) levels of 1.5 to 2.1 nmol/mg protein and 0.6 to 0.9 nmol/mg protein in the aggrelyte-2- and aggrelyte-2C-treated samples. Mouse (C57BL/6J) lenses (incubated for 24 h) and human lenses (incubated for 72 h) with 1.0 mM aggrelyte-2 showed significant decreases in stiffness with simultaneous increases in soluble proteins (human lenses) and protein-AcK levels, and such changes were not observed in aggrelyte-2C-treated lenses. Mass spectrometry of the solubilized protein revealed AcK in all crystallins, but more was observed in α-crystallins. These results suggest that aggrelyte-2 increases protein solubility and decreases lens stiffness through acetylation and disulfide reduction. Aggrelyte-2 might be useful in treating presbyopia in humans.

A Review of Lens Biomechanical Contributions to Presbyopia

Purpose: Presbyopia-the progressive loss of near focus with age-is primarily a result of changes in lens biomechanics. In particular, the shape of the ocular lens in the absence of zonular tension changes significantly throughout adulthood. Contributors to this change in shape are changes in lens biomechanical properties, continuous volumetric growth lens, and possibly remodeling of the lens capsule. Knowledge in this area is growing rapidly, so the purpose of this mini-review was to summarize and synthesize these gains.Methods: We review the recent literature in this field.Results: The mechanisms governing age-related changes in biomechanical properties remains unknown. We have recently shown that lens growth may be driven by zonular tension. The same mechanobiological mechanism driving lens growth may also lead to remodeling of the capsule, though this remains to be demonstrated.Conclusions: This mini-review focuses on identifying mechanisms which cause these age-related changes, suggesting future work which may elucidate these mechanisms, and briefly discusses ongoing efforts to develop a non-surgical approach for therapeutic management of presbyopia. We also propose a simple model linking lens growth and biomechanical properties.

Generation of Lens Progenitor Cells and Lentoid Bodies from Pluripotent Stem Cells: Novel Tools for Human Lens Development and Ocular Disease Etiology

In vitro differentiation of human pluripotent stem cells (hPSCs) into specialized tissues and organs represents a powerful approach to gain insight into those cellular and molecular mechanisms regulating human development. Although normal embryonic eye development is a complex process, generation of ocular organoids and specific ocular tissues from pluripotent stem cells has provided invaluable insights into the formation of lineage-committed progenitor cell populations, signal transduction pathways, and self-organization principles. This review provides a comprehensive summary of recent advances in generation of adenohypophyseal, olfactory, and lens placodes, lens progenitor cells and three-dimensional (3D) primitive lenses, "lentoid bodies", and "micro-lenses". These cells are produced alone or "community-grown" with other ocular tissues. Lentoid bodies/micro-lenses generated from human patients carrying mutations in crystallin genes demonstrate proof-of-principle that these cells are suitable for mechanistic studies of cataractogenesis. Taken together, current and emerging advanced in vitro differentiation methods pave the road to understand molecular mechanisms of cataract formation caused by the entire spectrum of mutations in DNA-binding regulatory genes, such as PAX6, SOX2, FOXE3, MAF, PITX3, and HSF4, individual crystallins, and other genes such as BFSP1, BFSP2, EPHA2, GJA3, GJA8, LIM2, MIP, and TDRD7 represented in human cataract patients.

Roles of Eph-Ephrin Signaling in the Eye Lens Cataractogenesis, Biomechanics, and Homeostasis

The eye lens is responsible for fine focusing of light onto the retina, and its function relies on tissue transparency and biomechanical properties. Recent studies have demonstrated the importance of Eph-ephrin signaling for the maintenance of life-long lens homeostasis. The binding of Eph receptor tyrosine kinases to ephrin ligands leads to a bidirectional signaling pathway that controls many cellular processes. In particular, dysfunction of the receptor EphA2 or the ligand ephrin-A5 lead to a variety of congenital and age-related cataracts, defined as any opacity in the lens, in human patients. In addition, a wealth of animal studies reveal the unique and overlapping functions of EphA2 and ephrin-A5 in lens cell shape, cell organization and patterning, and overall tissue optical and biomechanical properties. Significant differences in lens phenotypes of mouse models with disrupted EphA2 or ephrin-A5 signaling indicate that genetic modifiers likely affect cataract phenotypes and progression, suggesting a possible reason for the variability of human cataracts due to Eph-ephrin dysfunction. This review summarizes the roles of EphA2 and ephrin-A5 in the lens and suggests future avenues of study.

EphA2 Affects Development of the Eye Lens Nucleus and the Gradient of Refractive Index

Purpose

Our studies in mouse eye lenses demonstrate that ephrin-A5 and EphA2 are needed for normal epithelial cells and lens transparency. We sought to determine whether EphA2 and ephrin-A5 are important for lens morphometrics, nucleus formation, and refractive index.

Methods

We performed tissue morphometric measurements, electron microscopy, Western blots, and interferometric measurements using an X-ray synchrotron beam source to measure the gradient of refractive index (GRIN) to compare mouse lenses with genetic disruption of EphA2 or ephrin-A5.

Results

Morphometric analysis revealed that although there is no change in the overall lens volume, there is a change in lens shape in both EphA2-/- lenses and ephrin-A5-/- lenses. Surprisingly, EphA2-/- lenses had small and soft lens nuclei different from hard lens nuclei of control lenses. SEM images revealed changes in cell morphology of EphA2-/- fiber cells close to the center of the lens. Inner EphA2-/- lens fibers had more pronounced tongue-and-groove interdigitations and formed globular membrane morphology only in the deepest layers of the lens nucleus. We did not observe nuclear defects in ephrin-A5-/- lenses. There was an overall decrease in magnitude of refractive index across EphA2-/- lenses, which is most pronounced in the nucleus.

Conclusions

This work reveals that Eph-ephrin signaling plays a role in fiber cell maturation, nuclear compaction, and lens shape. Loss of EphA2 disrupts the nuclear compaction resulting in a small lens nucleus. Our data suggest that Eph-ephrin signaling may be required for fiber cell membrane reorganization and compaction and for establishing a normal GRIN.

RNA-binding proteins and post-transcriptional regulation in lens biology and cataract: Mediating spatiotemporal expression of key factors that control the cell cycle, transcription, cytoskeleton and transparency

Development of the ocular lens - a transparent tissue capable of sustaining frequent shape changes for optimal focusing power - pushes the boundaries of what cells can achieve using the molecular toolkit encoded by their genomes. The mammalian lens contains broadly two types of cells, the anteriorly located monolayer of epithelial cells which, at the equatorial region of the lens, initiate differentiation into fiber cells that contribute to the bulk of the tissue. This differentiation program involves massive upregulation of select fiber cell-expressed RNAs and their subsequent translation into high amounts of proteins, such as crystallins. But intriguingly, fiber cells achieve this while also simultaneously undergoing significant morphological changes such as elongation - involving about 1000-fold length-wise increase - and migration, which requires modulation of cytoskeletal and cell adhesion factors. Adding further to the challenges, these molecular and cellular events have to be coordinated as fiber cells progress toward loss of their nuclei and organelles, which irreversibly compromises their potential for harnessing genetically hardwired information. A long-standing question is how processes downstream of signaling and transcription, which may also participate in feedback regulation, contribute toward orchestrating these cellular differentiation events in the lens. It is now becoming clear from findings over the past decade that post-transcriptional gene expression regulatory mechanisms are critical in controlling cellular proteomes and coordinating key processes in lens development and fiber cell differentiation. Indeed, RNA-binding proteins (RBPs) such as Caprin2, Celf1, Rbm24 and Tdrd7 have now been described in mediating post-transcriptional control over key factors (e.g. Actn2, Cdkn1a (p21Cip1), Cdkn1b (p27Kip1), various crystallins, Dnase2b, Hspb1, Pax6, Prox1, Sox2) that are variously involved in cell cycle, transcription, cytoskeleton maintenance and differentiation in the lens. Furthermore, deficiencies of these RBPs have been shown to result in various eye and lens defects and/or cataract. Because fiber cell differentiation in the lens occurs throughout life, the underlying regulatory mechanisms operational in development are expected to also be recruited for the maintenance of transparency in aged lenses. Indeed, in support of this, TDRD7 and CAPRIN2 loci have been linked to age-related cataract in humans. Here, I will review the role of key RBPs in the lens and their importance in understanding the pathology of lens defects. I will discuss advances in RBP-based gene expression control, in general, and the important challenges that need to be addressed in the lens to define the mechanisms that determine the epithelial and fiber cell proteome. Finally, I will also discuss in detail several key future directions including the application of bioinformatics approaches such as iSyTE to study RBP-based post-transcriptional gene expression control in the aging lens and in the context of age-related cataract.

Prevalence, ethnic differences and risk factors of primary angle-closure glaucoma in a multiethnic Chinese adult population: the Yunnan Minority Eye Study

PURPOSE

To describe the prevalence and risk factors of primary angle-closure glaucoma (PACG) and to explore nationality difference in Chinese.

METHODS

The Yunnan Minority Eye Study was conducted in a rural multiethnic area in Yunnan province and included 6546 participants aged over 50 years. PACG was diagnosed based on International Society of Geographical and Epidemiologic Ophthalmology criteria by experienced ophthalmologists. Multivariate regression modelling was conducted to examine risk factors for PACG. Principal component analyis (PCA) was performed to evaluate the effects of ethnicity on PACG.

RESULTS

The overall prevalence of PACG was 0.7% (95% CIs: 0.5% to 0.9%). PCA indicated that ethnicity is significantly related to the presence of PACG (p<0.001) after controlling for other risk factors. In addition, higher PACG prevalence was also correlated with older age (60-69 years group (OR: 3.47; 95% CI: 1.11 to 10.84; p<0.05) and 70-79 years group (OR: 4.71; 95% CI: 1.40 to 15.86; p<0.05) as compared with 50-59 years group), higher intraocular pressures (OR: 1.26; 95% CI: 1.17 to 1.36; p<0.001), shorter axial lengths (OR: 0.42; 95% CI: 0.32 to 0.56; p<0.001) and thinner central corneal thicknesses (OR: 0.89; 95% CI: 0.81 to 0.99; p<0.05).

CONCLUSIONS

This multiethnic study on Chinese adults living in the same geographical location indicated that ethnicity is a significant risk factor for PACG. However, there were still some of the effects of ethnic differences on the prevalence of PACG that could not be explained and further studies should take culture and lifestyle factors into account.

The importance of the epithelial fibre cell interface to lens regeneration in an in vivo rat model and in a human bag-in-the-lens (BiL) sample

Human lens regeneration and the Bag-in-the-Lens (BIL) surgical treatment for cataract both depend upon lens capsule closure for their success. Our studies suggest that the first three days after surgery are critical to their long-term outcomes. Using a rat model of lens regeneration, we evidenced lens epithelial cell (LEC) proliferation increased some 50 fold in the first day before rapidly declining to rates observed in the germinative zone of the contra-lateral, un-operated lens. Cell multi-layering at the lens equator occurred on days 1 and 2, but then reorganised into two discrete layers by day 3. E- and N-cadherin expression preceded cell polarity being re-established during the first week. Aquaporin 0 (AQP0) was first detected in the elongated cells at the lens equator at day 7. Cells at the capsulotomy site, however, behaved very differently expressing the epithelial mesenchymal transition (EMT) markers fibronectin and alpha-smooth muscle actin (SMA) from day 3 onwards. The physical interaction between the apical surfaces of the anterior and posterior LECs from day 3 after surgery preceded cell elongation. In the human BIL sample fibre cell formation was confirmed by both histological and proteome analyses, but the cellular response is less ordered and variable culminating in Soemmerring's ring (SR) formation and sometimes Elschnig's pearls. This we evidence for lenses from a single patient. No bow region or recognisable epithelial-fibre cell interface (EFI) was evident and consequently the fibre cells were disorganised. We conclude that lens cells require spatial and cellular cues to initiate, sustain and produce an optically functional tissue in addition to capsule integrity and the EFI.

Aquaporins Have Regional Functions in Development of Refractive Index in the Zebrafish Eye Lens

Purpose

In the eye lens, cytosolic protein concentrations increase progressively from the periphery to the center, contributing to the gradient of refractive index (GRIN). Aquaporins are membrane proteins of lens fiber cells that regulate water transport and adhesion and interact with cytoskeletal proteins. This study investigates how these membrane proteins contribute to proper development of the lens GRIN.

Methods

Loss-of-function deletions of aqp0a and/or aqp0b in zebrafish were generated using CRISPR/Cas9 gene editing. Lenses of single aqp0a^−/− mutants, single aqp0b^−/− mutants, and double aqp0a^−/−/aqp0b^−/− mutants from larval to elderly adult stages were measured using x-ray Talbot interferometry at SPring8 in Japan. The three-dimensional GRIN profiles in two orthogonal cross-sectional planes of each lens were analyzed and compared with in vivo images and previous results obtained from wild-type lenses.

Results

Single aqp0a^−/− mutants tended to show asymmetric GRIN profiles, with the central plateau regions shifted anteriorly. Single aqp0b^−/− mutants had smooth, symmetric GRIN profiles throughout development until spoke opacities appeared in several extremely old samples. Double aqp0a^−/−/aqp0b^−/− mutants showed lower magnitude GRIN profiles, as well as dips in the central plateau region.

Conclusions

These findings suggest that Aqp0a and Aqp0b have region-specific functions in the lens: Aqp0a is active peripherally, regulating centralization of the plateau region, and this function cannot be compensated for by Aqp0b. In the lens center, either Aqp0a or Aqp0b is required for formation of the plateau region, as well as for the GRIN to reach its maximum magnitude in mature lenses.

Deformations and Ruptures in Human Lenses With Cortical Cataract Subjected to Ex Vivo Simulated Accommodation

Purpose

Human cortical opacities are most commonly accompanied by changes in lens fiber structure in the equatorial region at the lens nucleus–cortex interface. Cortex and nucleus have different elastic properties, which change with age. We therefore subjected ex vivo lenses to simulated accommodation and studied the internal deformations to better understand the mechanism of cortical cataract formation.

Methods

Nine human donor lenses (33–88 years old) were tested using a bespoke radial stretching device for anterior eye segments. Seven of the lenses exhibited cortical cataracts. The other two lenses, without cataract, were used as controls. Frontal and cross-sectional images of the lens obtained during stretching facilitated measurements on equatorial lens diameter and central lens thickness in the stretched and unstretched states.

Results

Stretching caused the lens equatorial diameter to increase in all cases. Conversely, the lens central thickness showed no systematic variation during stretching. For four of the lenses with cortical cataract, ruptures were observed during stretching at the nucleus–cortex boundary adjacent to the cortical cataracts. Ruptures were not observed in the control lenses or in the three other lenses with cortical cataract.

Conclusions

Internal ruptures can occur in aged ex vivo lenses subjected to simulated disaccommodation. These ruptures occur at the nucleus–cortex interface; at this location, a significant stiffness discontinuity is expected to develop with age. It is hypothesized that ruptures occur in in vivo lenses during accommodation—or attempted accommodation.

Morphometric analysis of the lens in human aniridia and mouse Small eye

Congenital aniridia is caused by heterozygous mutations in the PAX6 gene. In this disease, congenital iris and foveal hypoplasia is associated with juvenile onset cataract, glaucoma, and corneal keratopathy. In rodents, Pax6 mutations result in a congenital reduction in ocular size that is not typically described in human aniridia. Here, the ocular morphometry of aniridia patients is compared with the lens phenotype of Pax6^+/tm1/Pgr mice to reveal whether there are species differences in Pax6 regulation of lens development and homeostasis. Ultrasound biometry (UBM) revealed that eleven percent of aniridia patients exhibited mild microphthalmia while the anterior chamber depth of aniridic eyes was significantly reduced from 6 months of age onward. Although aniridic lens thickness was normal from birth, it was significantly decreased in aniridic lenses older than 30. Notably, 86% of aniridic lenses exhibited cataractous changes in this cohort. In addition, a significant proportion of aniridia patients develop lens subluxation as they age associated with reduced lens diameter as measured by anterior segment optical coherence tomography (AS-OCT). Analysis of young adult Pax6^+/tm1/Pgr mouse lenses by micro-computed tomography (microCT), bright field and dark field imaging revealed that they are reduced in size but did not exhibit overt cataracts at this age. Overall, this study reveals that congenital microphthalmia as assessed by axial length, or microphakia, as assessed by lens thickness, are not typical in human aniridia, although these are primary manifestations of Pax6 mutations in mice, suggesting that PAX6 regulates some aspects of lens development differently between these species.

In vivo SS-OCT imaging of crystalline lens sutures

We demonstrate in vivo three-dimensional (3-D) visualization of crystalline lens sutures in healthy eyes using swept source optical coherence tomography (SS-OCT). Volumetric data sets of the crystalline lenses were acquired and processed to obtain enhanced contrast projection images and to extract suture patterns in both anterior and posterior lens. The results presented different types of the sutures including Y-sutures, simple and complex star sutures. Age-related changes in suture arrangement were characterized quantitatively. Crystalline lens suture imaging with SS-OCT might be a useful tool in fundamental studies on development and ageing of human lens.

Age-related changes in eye lens biomechanics, morphology, refractive index and transparency

Life-long eye lens function requires an appropriate gradient refractive index, biomechanical integrity and transparency. We conducted an extensive study of wild-type mouse lenses 1-30 months of age to define common age-related changes. Biomechanical testing and morphometrics revealed an increase in lens volume and stiffness with age. Lens capsule thickness and peripheral fiber cell widths increased between 2 to 4 months of age but not further, and thus, cannot account for significant age-dependent increases in lens stiffness after 4 months. In lenses from mice older than 12 months, we routinely observed cataracts due to changes in cell structure, with anterior cataracts due to incomplete suture closure and a cortical ring cataract corresponding to a zone of compaction in cortical lens fiber cells. Refractive index measurements showed a rapid growth in peak refractive index between 1 to 6 months of age, and the area of highest refractive index is correlated with increases in lens nucleus size with age. These data provide a comprehensive overview of age-related changes in murine lenses, including lens size, stiffness, nuclear fraction, refractive index, transparency, capsule thickness and cell structure. Our results suggest similarities between murine and primate lenses and provide a baseline for future lens aging studies.

Tropomyosin 3.5 protects the F-actin networks required for tissue biomechanical properties

Tropomyosins (Tpms) stabilize F-actin and regulate interactions with other actin-binding proteins. The eye lens changes shape in order to focus light to transmit a clear image, and thus lens organ function is tied to its biomechanical properties, presenting an opportunity to study Tpm functions in tissue mechanics. Mouse lenses contain Tpm3.5 (also known as TM5NM5), a previously unstudied isoform encoded by Tpm3, which is associated with F-actin on lens fiber cell membranes. Decreased levels of Tpm3.5 lead to softer and less mechanically resilient lenses that are unable to resume their original shape after compression. While cell organization and morphology appear unaffected, Tmod1 dissociates from the membrane in Tpm3.5-deficient lens fiber cells resulting in reorganization of the spectrin-F-actin and α-actinin-F-actin networks at the membrane. These rearranged F-actin networks appear to be less able to support mechanical load and resilience, leading to an overall change in tissue mechanical properties. This is the first in vivo evidence that a Tpm protein is essential for cell biomechanical stability in a load-bearing non-muscle tissue, and indicates that Tpm3.5 protects mechanically stable, load-bearing F-actin in vivoThis article has an associated First Person interview with the first author of the paper.

RNA-binding proteins in eye development and disease: implication of conserved RNA granule components

The molecular biology of metazoan eye development is an area of intense investigation. These efforts have led to the surprising recognition that although insect and vertebrate eyes have dramatically different structures, the orthologs or family members of several conserved transcription and signaling regulators such as Pax6, Six3, Prox1, and Bmp4 are commonly required for their development. In contrast, our understanding of posttranscriptional regulation in eye development and disease, particularly regarding the function of RNA-binding proteins (RBPs), is limited. We examine the present knowledge of RBPs in eye development in the insect model Drosophila as well as several vertebrate models such as fish, frog, chicken, and mouse. Interestingly, of the 42 RBPs that have been investigated for their expression or function in vertebrate eye development, 24 (~60%) are recognized in eukaryotic cells as components of RNA granules such as processing bodies, stress granules, or other specialized ribonucleoprotein (RNP) complexes. We discuss the distinct developmental and cellular events that may necessitate potential RBP/RNA granule-associated RNA regulon models to facilitate posttranscriptional control of gene expression in eye morphogenesis. In support of these hypotheses, three RBPs and RNP/RNA granule components Tdrd7, Caprin2, and Stau2 are linked to ocular developmental defects such as congenital cataract, Peters anomaly, and microphthalmia in human patients or animal models. We conclude by discussing the utility of interdisciplinary approaches such as the bioinformatics tool iSyTE (integrated Systems Tool for Eye gene discovery) to prioritize RBPs for deriving posttranscriptional regulatory networks in eye development and disease. WIREs RNA 2016, 7:527-557. doi: 10.1002/wrna.1355 For further resources related to this article, please visit the WIREs website.

The molecular mechanisms underlying lens fiber elongation

Lens fiber cells are highly elongated cells with complex membrane morphologies that are critical for the transparency of the ocular lens. Investigations into the molecular mechanisms underlying lens fiber cell elongation were first reported in the 1960s, however, our understanding of the process is still poor nearly 50 years later. This review summarizes what is currently hypothesized about the regulation of lens fiber cell elongation along with the available experimental evidence, and how this information relates to what is known about the regulation of cell shape/elongation in other cell types, particularly neurons.

The cause and consequence of fiber cell compaction in the vertebrate lens

Fiber cells of the ocular lens are arranged in a series of concentric shells. New growth shells are added continuously to the lens surface and, as a consequence, the preexisting shells are buried. To focus light, the refractive index of the lens cytoplasm must exceed that of the surrounding aqueous and vitreous humors, and to that end, lens cells synthesize high concentrations of soluble proteins, the crystallins. To correct for spherical aberration, it is necessary that the crystallin concentration varies from shell-to-shell, such that cellular protein content is greatest in the center of the lens. The radial variation in protein content underlies the critical gradient index (GRIN) structure of the lens. Only the outermost shells of lens fibers contain the cellular machinery necessary for protein synthesis. It is likely, therefore, that the GRIN (which spans the synthetically inactive, organelle-free zone of the lens) does not result from increased levels of protein synthesis in the core of the lens but is instead generated through loss of volume by inner fiber cells. Because volume is lost primarily in the form of cell water, the residual proteins in the central lens fibers can be concentrated to levels of >500 mg/ml. In this short review, we describe the process of fiber cell compaction, its relationship to lens growth and GRIN formation, and offer some thoughts on the likely nature of the underlying mechanism.

Deficiency of the RNA binding protein caprin2 causes lens defects and features of Peters anomaly

BACKGROUND

It was recently demonstrated that deficiency of a conserved RNA binding protein (RBP) and RNA granule (RG) component Tdrd7 causes ocular defects including cataracts in human, mouse and chicken, indicating the importance of posttranscriptional regulation in eye development. Here we investigated the function of a second conserved RBP/RG component Caprin2 that is identified by the eye gene discovery tool iSyTE.

RESULTS

In situ hybridization, Western blotting and immunostaining confirmed highly enriched expression of Caprin2 mRNA and protein in mouse embryonic and postnatal lens. To gain insight into its function, lens-specific Caprin2 conditional knockout (cKO) mouse mutants were generated using a lens-Cre deleter line Pax6GFPCre. Phenotypic analysis of Caprin2(cKO/cKO) mutants revealed distinct eye defects at variable penetrance. Wheat germ agglutinin staining and scanning electron microscopy demonstrated that Caprin2(cKO/cKO) mutants have an abnormally compact lens nucleus, which is the core of the lens comprised of centrally located terminally differentiated fiber cells. Additionally, Caprin2(cKO/cKO) mutants also exhibited at 8% penetrance a developmental defect that resembles a human condition called Peters anomaly, wherein the lens and the cornea remain attached by a persistent stalk.

CONCLUSIONS

These data suggest that a conserved RBP Caprin2 functions in distinct morphological events in mammalian eye development.

Crystalline lens and refractive development

Individual refractive errors usually change along lifespan. Most children are hyperopic in early life. This hyperopia is usually lost during growth years, leading to emmetropia in adults, but myopia also develops in children during school years or during early adult life. Those subjects who remain emmetropic are prone to have hyperopic shifts in middle life. And even later, at older ages, myopic shifts are developed with nuclear cataract. The eye grows from 15 mm in premature newborns to approximately 24 mm in early adult years, but, in most cases, refractions are maintained stable in a clustered distribution. This growth in axial length would represent a refractive change of more than 40 diopters, which is compensated by changes in corneal and lens powers. The process which maintains the balance between the ocular components of refraction during growth is still under study. As the lens power cannot be measured in vivo, but can only be calculated based on the other ocular components, there have not been many studies of lens power in humans. Yet, recent studies have confirmed that the lens loses power during growth in children, and that hyperopic and myopic shifts in adulthood may be also produced by changes in the lens. These studies in children and adults give a picture of the changing power of the lens along lifespan. Other recent studies about the growth of the lens and the complexity of its internal structure give clues about how these changes in lens power are produced along life.

The penny pusher: a cellular model of lens growth

PURPOSE

The mechanisms that regulate the number of cells in the lens and, therefore, its size and shape are unknown. We examined the dynamic relationship between proliferative behavior in the epithelial layer and macroscopic lens growth.

METHODS

The distribution of S-phase cells across the epithelium was visualized by confocal microscopy and cell populations were determined from orthographic projections of the lens surface.

RESULTS

The number of S-phase cells in the mouse lens epithelium fell exponentially, to an asymptotic value of approximately 200 cells by 6 months. Mitosis became increasingly restricted to a 300-μm-wide swath of equatorial epithelium, the germinative zone (GZ), within which two peaks in labeling index were detected. Postnatally, the cell population increased to approximately 50,000 cells at 4 weeks of age. Thereafter, the number of cells declined, despite continued growth in lens dimensions. This apparently paradoxical observation was explained by a time-dependent increase in the surface area of cells at all locations. The cell biological measurements were incorporated into a physical model, the Penny Pusher. In this simple model, cells were considered to be of a single type, the proliferative behavior of which depended solely on latitude. Simulations using the Penny Pusher predicted the emergence of cell clones and were in good agreement with data obtained from earlier lineage-tracing studies.

CONCLUSIONS

The Penny Pusher, a simple stochastic model, offers a useful conceptual framework for the investigation of lens growth mechanisms and provides a plausible alternative to growth models that postulate the existence of lens stem cells.

Intact and N- or C-terminal end truncated AQP0 function as open water channels and cell-to-cell adhesion proteins: end truncation could be a prelude for adjusting the refractive index of the lens to prevent spherical aberration

BACKGROUND

Investigate the impact of natural N- or C-terminal post-translational truncations of lens mature fiber cell Aquaporin 0 (AQP0) on water permeability (Pw) and cell-to-cell adhesion (CTCA) functions.

METHODS

The following deletions/truncations were created by site-directed mutagenesis (designations in parentheses): Amino acid residues (AA) 2-6 (AQP0-N-del-2-6), AA235-263 (AQP0-1-234), AA239-263 (AQP0-1-238), AA244-263 (AQP0-1-243), AA247-263 (AQP0-1-246), AA250-263 (AQP0-1-249) and AA260-263 (AQP0-1-259). Protein expression was studied using immunostaining, fluorescent tags and organelle-specific markers. Pw was tested by expressing the respective complementary ribonucleic acid (cRNA) in Xenopus oocytes and conducting osmotic swelling assay. CTCA was assessed by transfecting intact or mutant AQP0 into adhesion-deficient L-cells and performing cell aggregation and adhesion assays.

RESULTS

AQP0-1-234 and AQP0-1-238 did not traffic to the plasma membrane. Trafficking of AQP0-N-del-2-6 and AQP0-1-243 was reduced causing decreased membrane Pw and CTCA. AQP0-1-246, AQP0-1-249 and AQP0-1-259 mutants trafficked properly and functioned normally. Pw and CTCA functions of the mutants were directly proportional to the respective amount of AQP0 expressed at the plasma membrane and remained comparable to those of intact AQP0 (AQP0-1-263).

CONCLUSIONS

Post-translational truncation of N- or C-terminal end amino acids does not alter the basal water permeability of AQP0 or its adhesive functions. AQP0 may play a role in adjusting the refractive index to prevent spherical aberration in the constantly growing lens.

GENERAL SIGNIFICANCE

Similar studies can be extended to other lens proteins which undergo post-translational truncations to find out how they assist the lens to maintain transparency and homeostasis for proper focusing of objects on to the retina.

Beta-1 integrin is important for the structural maintenance and homeostasis of differentiating fiber cells

β1-Integrin is a heterodimeric transmembrane protein that has roles in both cell-extra-cellular matrix and cell-cell interactions. Conditional deletion of β1-integrin from all lens cells during embryonic development results in profound lens defects, however, it is less clear whether this reflects functions in the lens epithelium alone or whether this protein plays a role in lens fibers. Thus, a conditional approach was used to delete β1-integrin solely from the lens fiber cells. This deletion resulted in two distinct phenotypes with some lenses exhibiting cataracts while others were clear, albeit with refractive defects. Analysis of "clear" conditional knockout lenses revealed that they had profound defects in fiber cell morphology associated with the loss of the F-actin network. Physiological measurements found that the lens fiber cells had a twofold increase in gap junctional coupling, perhaps due to differential localization of connexins 46 and 50, as well as increased water permeability. This would presumably facilitate transport of ions and nutrients through the lens, and may partially explain how lenses with profound structural abnormalities can maintain transparency. In summary, β1-integrin plays a role in maintaining the cellular morphology and homeostasis of the lens fiber cells.

Ultrastructural analysis of the human lens fiber cell remodeling zone and the initiation of cellular compaction

The purpose is to determine the nature of the cellular rearrangements occurring through the remodeling zone (RZ) in human donor lenses, identified previously by confocal microscopy to be about 100 μm from the capsule. Human donor lenses were fixed with 10% formalin followed by 4% paraformaldehyde prior to processing for transmission electron microscopy. Of 27 fixed lenses, ages 22, 55 and 92 years were examined in detail. Overview electron micrographs confirmed the loss of cellular organization present in the outer cortex (80 μm thick) as the cells transitioned into the RZ. The transition occurred within a few cell layers and fiber cells in the RZ completely lost their classical hexagonal cross-sectional appearance. Cell interfaces became unusually interdigitated and irregular even though the radial cell columns were retained. Gap junctions appeared to be unaffected. After the RZ (40 μm thick), the cells were still irregular but more recognizable as fiber cells with typical interdigitations and the appearance of undulating membranes. Cell thickness was irregular after the RZ with some cells compacted, while others were not, up to the zone of full compaction in the adult nucleus. Similar dramatic cellular changes were observed within the RZ for each lens regardless of age. Because the cytoskeleton controls cell shape, dramatic cellular rearrangements that occur in the RZ most likely are due to alterations in the associations of crystallins to the lens-specific cytoskeletal beaded intermediate filaments. It is also likely that cytoskeletal attachments to membranes are altered to allow undulating membranes to develop.

Myopic refractive shift represents dense nuclear sclerosis and thin lens in lenticular myopia

BACKGROUND

It is not rare to meet unilateral nuclear sclerotic cataracts with myopic refractive changes (lenticular myopia) compared with the fellow eye in the ophthalmic examination of patients with decreased visual acuity. To determine the relationship between the myopic refractive changes and interocular differences of parameters, we investigated the interocular differences of ocular parameters between a lenticular myopic eye and the fellow eye.

METHOD

This retrospective study included 68 eyes of 34 patients, who showed unilateral lenticular myopia. We compared the dimensions of ocular component, such as anterior chamber depth, anterior chamber volume, lens thickness, vitreous chamber depth, lens position, lens density of nuclear sclerosis, anterior lens curvature and myopic refractive changes (spherical equivalent refraction) between the lenticular myopic eye and the myopic refractive change were examined.

RESULTS

Statistically significant differences were found between the lenticular myopic eye and the fellow eye for anterior chamber depth (p = 0.015) anterior chamber volume (p = 0.031), lens thickness (p < 0.001), lens density of the nuclear sclerosis (p < 0.001) and the spherical equivalent myopic refractive changes (p < 0.001). Based on univariate analysis, the interocular difference in spherical equivalent refraction was significantly correlated with interocular differences of the density of the nuclear sclerosis (r = 0.79, p < 0.001), lens thickness (r = -0.70, p < 0.001) and vitreous chamber depth (r = 0.43, p = 0.012). Based on multiple regression analysis, the interocular difference in spherical equivalent refraction was significantly correlated with interocular differences of density of nuclear sclerosis (p < 0.001) and lens thickness (p = 0.007).

CONCLUSION

The difference in myopic spherical change reflects the differences in the severity of nuclear sclerosis and lens thickness between the lenticular myopic eye and the fellow eye.

Nested shell optical model of the lens of the human eye

A nested shell model of the human lens is developed based on the known anatomical construction of the lens, on the known way in which the lens grows throughout its life, on the measured characteristics of the lens surfaces as a function of the age of the lens, on the measured changes in the shape of the lens during accommodation, and on measured material characteristics of the lens materials, such as density and index of refraction throughout. The observed changes in central surface curvature and thickness force the shell thicknesses to vary in a predictable way and in turn force the shell surface asphericity to take certain values. Thus, in addition to giving the shape of each shell, the model predicts the change expected in the asphericity of the lens surfaces as the lens ages and adds cortical cell layers. Two examples are given, one for a 25-year-old lens and one for a 40-year-old lens, to show how the cortical layers change their shapes throughout the cortex and over time as the lens ages. The performance of the model of this paper is compared to that of two other nested shell models, one where the layers have constant thickness and one where the lens posterior is fixed within the eye over time, to show the superior performance of this model with respect to maintaining a constant refractive error for the eye as the lens ages and grows.

Sphingolipid distribution changes with age in the human lens

The formation of an internal barrier to the diffusion of small molecules in the lens during middle age is hypothesized to be a key event in the development of age-related nuclear (ARN) cataract. Changes in membrane lipids with age may be responsible. In this study, we investigated the effect of age on the distribution of sphingomyelins, the most abundant lens phospholipids. Human lens sections were initially analyzed by MALDI mass spectrometry imaging. A distinct annular distribution of the dihydrosphingomyelin, DHSM (d18:0/16:0), in the barrier region was observed in 64- and 70-year-old lenses but not in a 23-year-old lens. An increase in the dihydroceramide, DHCer (d18:0/16:0), in the lens nucleus was also observed in the older lenses. These findings were supported by ESI mass spectrometry analysis of lipid extracts from lenses dissected into outer, barrier, and nuclear regions. A subsequent analysis of 18 lenses ages 20-72 years revealed that sphingomyelin levels increased with age in the barrier region until reaching a plateau at approximately 40 years of age. Such changes in lipid composition will have a significant impact on the physical properties of the fiber cell membranes and may be associated with the formation of a barrier.

Tissue localization and solubilities of αA-crystallin and its numerous C-terminal truncation products in pre- and postcataractous ICR/f rat lenses

PURPOSE

To investigate the tissue distribution and solubilities of various αA-crystallin truncation products in the cataractous ICR/f rat model.

METHODS

Rat lenses from precataractous (21-day) and postcataractous (100-day) ICR/f rats were sectioned and applied to a matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) target plate. Mass spectrometry images were collected to obtain a macromolecular profile of the abundant lens proteins. Separately, age-matched lenses were extracted into water-soluble (WS) and water-insoluble/urea-soluble (WI-US) fractions and subjected to MALDI-TOF mass spectrometry to correlate the protein solubilities with the imaging data. Protein identities were assigned by using a top-down proteomics approach on a high-resolution mass spectrometer.

RESULTS

Ten novel αA-crystallin truncation products were identified, along with six previously known αA-crystallin truncation products. Nearly all truncations exhibited nuclear localization, with larger truncated products displaying a ringlike localization that progressed outward toward the extranuclear, cortical region. The distributions were similar in both ages with the only significant difference being the amount of tissue area encompassed by a particular species with increasing age. Almost all nuclear products fractionated into the WI-US fraction, whereas the five largest extranuclear species exhibited mixed solubility.

CONCLUSIONS

A successful methodology for the sectioning and imaging of pre- and postcataractous ICR/f rat lenses has been established. Data collected from these analyses indicate that there are multiple αA-crystallin truncation products present in both pre- and postcataractous rats. Furthermore, these species have defined lenticular localizations and unique solubilities that may be a consequence of lens development and protein function within the lens environment.

On the growth and internal structure of the human lens

Growth of the human lens and the development of its internal features are examined using in vivo and in vitro observations on dimensions, weights, cell sizes, protein gradients and other properties. In vitro studies have shown that human lens growth is biphasic, asymptotic until just after birth and linear for most of postnatal life. This generates two distinct compartments, the prenatal and the postnatal. The prenatal growth mode leads to the formation of an adult nuclear core of fixed dimensions and the postnatal, to an ever-expanding cortex. The nuclear core and the cortex have different properties and can readily be physically separated. Communication and adhesion between the compartments is poor in older lenses. In vivo slit lamp examination reveals several zones of optical discontinuity in the lens. Different nomenclatures have been used to describe these, with the most common recognizing the embryonic, foetal, juvenile and adult nuclei as well as the cortex and outer cortex. Implicit in this nomenclature is the idea that the nuclear zones were generated at defined periods of development and growth. This review examines the relationship between the two compartments observed in vitro and the internal structures revealed by slit lamp photography. Defining the relationship is not as simple as it might seem because of remodeling and cell compaction which take place, mostly in the first 20 years of postnatal life. In addition, different investigators use different nomenclatures when describing the same regions of the lens. From a consideration of the dimensions, the dry mass contents and the protein distributions in the lens and in the various zones, it can be concluded that the juvenile nucleus and the layers contained within it, as well as most of the adult nucleus, were actually produced during prenatal life and the adult nucleus was completed within 3 months after birth, in the final stages of the prenatal growth mode. Further postnatal growth takes place entirely within the cortex. It can also be demonstrated that the in vitro nuclear core corresponds to the combined slit lamp nuclear zones. In view of the information presented in this review, the use of the terms foetal, juvenile and adult nucleus seems inappropriate and should be abandoned.

Femtosecond laser induced flexibility change of human donor lenses

BACKGROUND

According to the Helmholtz theory of accommodation the loss of accommodation amplitude is caused by the growing sclerosis of the crystalline lens, whereas the ciliary muscle and the lens capsule are mainly uneffected by age. A permanent treatment method for presbyopia which offers a dynamic accommodation ability is a recent field of study. The concept followed in this paper uses femtosecond laser pulses to potentially overcome the loss of deformation ability of the crystalline lens by creating gliding planes inside the lens tissue to improve its flexibility.

METHODS

The aim of the study is to show that the flexibility of human donor lenses can be increased by applying tightly focused near infrared femtosecond laser pulses into the lens tissue. Thereby the tissue is separated by the photodisruption effect. A certain pattern of gliding planes is cut inside the tissue of 41 human donor lenses and the deformation ability of the lenses are compared using the Fisher spinning test before and after laser treatment.

RESULTS

The laser treatment results in an increased deformation ability of the crystalline lens. The lens a-p thickness increases on average by 97 microm+/-14 microm after the treatment. The Fisher spinning test shows an increase of 16% in deformation ability of the lens at a rotational speed of 1620 rpm.

CONCLUSION

The creation of gliding planes with a fs laser inside the crystalline lens tissue can change the deformation ability of the lens. This might be an indication for a possible method to treat presbyopia in future.

Microindentation of the Young Porcine Ocular Lens

Debate regarding the mechanisms of how the eye changes focus (accommodation) and why this ability is lost with age (presbyopia) has recently been rejoined due to the advent of surgical procedures for the correction of presbyopia. Due to inherent confounding factors in both in vivo and in vitro measurement techniques, mechanical modeling of the behavior of the ocular lens in accommodation has been attempted to settle the debate. However, a paucity of reliable mechanical property measurements has proven problematic in the development of a successful mechanical model of accommodation. Instrumented microindentation was utilized to directly measure the local elastic modulus and dynamic response at various locations in the lens. The young porcine lens exhibits a large modulus gradient with the highest modulus appearing at the center of the nucleus and exponentially decreasing with distance. The loss tangent was significantly higher in the decapsulated lens and the force waveform amplitude decreased significantly upon removal of the lens capsule. The findings indicate that localized measurements of the lens’ mechanical properties are necessary to achieve accurate quantitative parameters suitable for mechanical modeling efforts. The results also indicate that the lens behaves as a crosslinked gel rather than as a collection of individual arched fiber cells.

Morphology of age-related cuneiform cortical cataracts: the case for mechanical stress

We evaluated the gross morphology, location, and fiber cell architecture of equatorial cortical opacities in the aging human lens. Using dark-field stereomicroscopy, we photographed donor lenses in toto and as thick slices. In addition, we investigated the details of the fiber cell architecture using fluorescent staining for membranes and by scanning electron microscopy. We then combined our data with data from recent studies on lens viscoelasticity. We found that small cortical and cuneiform opacities are accompanied by changes in fiber structure and architecture mainly in the equatorial border zone between the lens nucleus and cortex. Because the lens cortex and nucleus have different viscoelastic properties in young and old lenses, we hypothesize that external forces during accommodation cause shear stress predominantly in this border zone. The location of the described changes suggests that these mechanical forces may cause fiber disorganization, small cortical opacities, and ultimately, cuneiform cataracts.

Age-related compaction of lens fibers affects the structure and optical properties of rabbit lenses

BACKGROUND

The goal of this investigation was to correlate particular age-related structural changes (compaction) to the amount of scatter in rabbit lenses and to determine if significant fiber compaction occurred in the nuclear and inner cortical regions.

METHODS

New Zealand White rabbits at 16-20 months old (adult; n = 10) and at 3.5-4 years old (aged; n = 10) were utilized for this study. Immediately after euthanising, scatter was assessed in fresh lenses by low power helium-neon laser scan analysis. Scatter data was analyzed both for whole lenses and regionally, to facilitate correlation with morphometric data. After functional analysis, lenses were fixed and processed for scanning electron microcopy (SEM; right eyes) and light microscopy (LM; left eyes). Morphometric analysis of SEM images was utilized to evaluate compaction of nuclear fibers. Similarly, measurements from LM images were used to assess compaction of inner cortical fibers.

RESULTS

Scatter was significantly greater in aged lenses as compared to adult lenses in all regions analyzed, however the difference in the mean was slightly more pronounced in the inner cortical region. The anterior and posterior elliptical angles at 1 mm (inner fetal nucleus) were significantly decreased in aged vs. adult lenses (anterior, p = 0.040; posterior, p = 0.036). However, the average elliptical angles at 2.5 mm (outer fetal nucleus) were not significantly different in adult and aged lenses since all lenses examined had comparable angles to inner fetal fibers of aged lenses, i.e. they were all compacted. In cortical fibers, measures of average cross-sectional fiber area were significantly different at diameters of both 6 and 7 mm as a function of age (p = 0.011 and p = 0.005, respectively). Accordingly, the estimated fiber volume was significantly decreased in aged as compared to adult lenses at both 6 mm diameter (p = 0.016) and 7 mm diameter (p = 0.010).

CONCLUSION

Morphometric data indicates that inner cortical fibers undergo a greater degree of age-related compaction than nuclear fibers. Increased scatter appears to be only tentatively correlated with regions of fiber compaction, suggesting that it is simply one of an array of factors that contribute to the overall decreased transparency in aged rabbit lenses.

The effects of toxicological agents on the optics and mitochondria of the lens and the mitochondria of the corneal epithelium

This review describes how the morphology and distribution of the mitochondria of the epithelium and the superficial fibre layers of the lens were studied using confocal scanning laser microscopy. This research was correlated with an effort to use the optical properties of the intact lens in culture as a proxy for the cornea in measuring ocular toxicity. In turn, this work led to the confocal study of the in vitro and then the in vivo cornea and their possible use in using confocal microscopy to evaluate the effect of various treatments on the integrity of the surface of the eye. Finally, confocal examination of the mitochondria of the lens has provided an avenue to the study of mitochondrial dynamics.

Stiffness gradient in the crystalline lens

BACKGROUND

While the overall stiffness of the lens has been measured in a number of studies, the knowledge about the stiffness distribution within the lens is still limited. The purpose of this study was to determine the stiffness gradient in the human crystalline lens. A secondary purpose was to determine whether the stiffness gradient depends on age.

METHODS

The local dynamic stiffness was measured in 10 human crystalline lenses (age range: 19 to 78 years). The lenses were stored at -70 degrees C before being measured. The influence of freezing on the mechanical properties has been determined in a previous study. A small oscillating probe was used to measure the local dynamic shear modulus as a measure of lens stiffness. The measurements were taken in the cross-sectional plane through the lens equator.

RESULTS

The local dynamic shear modulus varied with location for all tested lenses. The central stiffness of the oldest lens (78 years) was 10(4) times higher than the youngest (19 years) lens. The equatorial stiffness of the oldest lens was 10(2) times higher than the youngest lens. For the older lenses, the centre was 5.8-210 times stiffer than the periphery, as opposed to earlier results described by Fisher (1971), who found that the periphery was up to 3 times softer than the centre for lenses younger than 70-years-old. For the three youngest lenses (19 to 49 years), the periphery was 2.2-16.6 times stiffer than the centre.

CONCLUSIONS

The dynamic stiffness of the crystalline lens varies with location within the lens. The stiffness gradient depends on the age of the lens. The results of the 10 lenses indicate that the stiffness of both centre and periphery increase with age, but at a different rate.

Sensitivity study of human crystalline lens accommodation

A nonlinear axisymmetric finite element method (FEM) analysis was employed to determine the critical geometric and material properties that affect human accommodation. In this model, commencing at zero, zonular traction on all lens profiles resulted in central lenticular surface steepening and peripheral surface flattening, with a simultaneous increase in central lens thickness and central optical power. An age-related decline in maximum zonular tension appears to be the most likely etiology for the decrease in accommodative amplitude with age.

X-ray imaging in advanced studies of ophthalmic diseases

Microscopic characterization of pathological tissues has one major intrinsic limitation, the small sampling areas with respect to the extension of the tissues. Mapping possible changes on vast tissues and correlating them with large ensembles of clinical cases is not a feasible procedure for studying most diseases, as for instance vision loss related diseases and, in particular, the cataract. Although intraocular lens implants are successful treatments, cataract still is a leading public-health issue that grows in importance as the population increases and life expectancy is extended worldwide. In this work we have exploited the radiation-tissue interaction properties of hard x-rays--very low absorption and scattering--to map distinct lesions on entire eye lenses. At the used synchrotron x-ray photon energy of 20 keV (wavelength lambda=0.062 nm), scattering and refraction are angular resolved effects. It allows the employed x-ray image technique to efficiently characterize two types of lesions in eye lenses under cataractogenesis: distributions of tiny scattering centers and extended areas of fiber cell compaction. The data collection procedure is relatively fast; allowing dozens of samples to be totally imaged (scattering, refraction, and mass absorption images) in a single day of synchrotron beam time. More than 60 cases of canine cataract, not correlated to specific causes, were investigated in this first application of x-rays to image entire lenses. Cortical opacity cases, or partial opacity, could be related to the presence of calcificated tissues at the cortical areas, clearly visible in the images, whose elemental contents were verified by micro x-ray fluorescence as very rich in calcium. Calcificated tissues were also observed at nuclear areas in some cases of hypermature cataract. Total opacity cases without distinguishable amount of scattering centers consist in 70% of the analyzed cases, where remarkable fissure marks owing to extended areas of fiber cell compaction are diagnosed.

The lens of the eye as a focusing device and its response to stress

The continued peripheral growth of the lens, resulting in the concentration of older tissue toward the center, has the important optical consequence of producing a lens of variable refractive index. An approach consisting of the projection of fine laser beams through excised lenses in physiological solution has been used for in vitro study of lens optical quality. By varying the separation of the incident beams and/or the wavelength characteristics of the laser used, lens refractive properties and relative transparency may be examined. In the review provided, these optical properties are correlated to lens suture anatomy, lens mitochondrial morphology and function and the function of lens heat shock proteins. In addition, lens spherical aberration is evaluated as a function of accommodation. This work can be highlighted as follows: Mammalian lens suture morphology has a direct impact on lens optical function and, while suture structure of mammalian and avian lenses are very different, they both show an age-related deterioration in morphology and focusing ability. The distribution and appearance of mitochondria of the lens epithelium and superficial fiber cells are similar in all vertebrates. Lens mitochondrial integrity is correlated to lens focusing ability, suggesting a correlation between lens optical properties and lens metabolic function. The induction of cold cataract measured optically in cultured mammalian lenses is enhanced by thermal (heat) shock and this effect is prevented by inhibiting heat shock protein production. Finally, lens accommodative function can be studied by measuring lens refractive change using a physiological model involving an intact accommodative apparatus.

Presbyopia: the first stage of nuclear cataract?

Presbyopia, the inability to accommodate, affects almost everyone at middle age. Recently, it has been shown that there is a massive increase in the stiffness(1) of the lens with age and, since the shape of the lens must change during accommodation, this could provide an explanation for presbyopia. In this review, we propose that presbyopia may be the earliest observable symptom of age-related nuclear (ARN) cataract. ARN cataract is a major cause of world blindness. The genesis of ARN cataract can be traced to the onset of a barrier within the lens at middle age. This barrier restricts the ability of small molecules, such as antioxidants, to penetrate into the centre of the lens leaving the proteins in this region susceptible to oxidation and post-translational modification. Major protein oxidation and colouration are the hallmarks of ARN cataract. We postulate that the onset of the barrier, and the hardening of the nucleus, are intimately linked. Specifically, we propose that progressive age-dependent hardening of the lens nucleus may be responsible for both presbyopia and ARN cataract.