Cell cycle synchrony in the developing chicken lens epithelium

Insights into the biochemical and biophysical mechanisms mediating the longevity of the transparent optics of the eye lens

In the human eye, a transparent cornea and lens combine to form the "refracton" to focus images on the retina. This requires the refracton to have a high refractive index "n," mediated largely by extracellular collagen fibrils in the corneal stroma and the highly concentrated crystallin proteins in the cytoplasm of the lens fiber cells. Transparency is a result of short-range order in the spatial arrangement of corneal collagen fibrils and lens crystallins, generated in part by post-translational modifications (PTMs). However, while corneal collagen is remodeled continuously and replaced, lens crystallins are very long-lived and are not replaced and so accumulate PTMs over a lifetime. Eventually, a tipping point is reached when protein aggregation results in increased light scatter, inevitably leading to the iconic protein condensation-based disease, age-related cataract (ARC). Cataracts account for 50% of vision impairment worldwide, affecting far more people than other well-known protein aggregation-based diseases. However, because accumulation of crystallin PTMs begins before birth and long before ARC presents, we postulate that the lens protein PTMs contribute to a "cataractogenic load" that not only increases with age but also has protective effects on optical function by stabilizing lens crystallins until a tipping point is reached. In this review, we highlight decades of experimental findings that support the potential for PTMs to be protective during normal development. We hypothesize that ARC is preventable by protecting the biochemical and biophysical properties of lens proteins needed to maintain transparency, refraction, and optical function.

Influence of Circadian Rhythm in the Eye: Significance of Melatonin in Glaucoma

Circadian rhythm and the molecules involved in it, such as melanopsin and melatonin, play an important role in the eye to regulate the homeostasis and even to treat some ocular conditions. As a result, many ocular pathologies like dry eye, corneal wound healing, cataracts, myopia, retinal diseases, and glaucoma are affected by this cycle. This review will summarize the current scientific literature about the influence of circadian patterns on the eye, focusing on its relationship with increased intraocular pressure (IOP) fluctuations and glaucoma. Regarding treatments, two ways should be studied: the first one, to analyze if some treatments could improve their effect on the ocular disease when their posology is established in function of circadian patterns, and the second one, to evaluate new drugs to treat eye pathologies related to the circadian rhythm, as it has been stated with melatonin or its analogs, that not only could be used as the main treatment but as coadjutant, improving the circadian pattern or its antioxidant and antiangiogenic properties.

A comparative analysis of αA- and αB-crystallin expression during the cell cycle in primary mouse lens epithelial cultures

AlphaA- and alphaB-crystallins are small heat shock proteins and molecular chaperones that prevent non-specific aggregation of denaturing proteins. Previous work in our laboratory has shown that lens epithelial cells derived from alphaA-/- mice exhibit slower growth, whereas alphaB-/- lens epithelial cells hyperproliferate at a higher rate in culture [Andley et al., J. Biol. Chem. 273 (1998) 31252; FASEB J. 15 (2001) 221]. Although both have been implicated in apoptosis and cell proliferation, direct analysis of their expression during the cell cycle has not been investigated. This study was undertaken to define the expression levels of alphaA and alphaB-crystallins during the cell cycle. Primary lens epithelial cell cultures derived from wild type mice were synchronized by serum starvation, and pulsed with bromodeoxyuridine (BrdU) at different times after re-stimulation with serum. Dual parameter flow cytometric studies with BrdU and propidium iodide (PI)-labeled cells were performed. Cells entered S phase 14 hr after serum re-stimulation. The duration of the S phase was 6 hr, and the total cell cycle transit time was between 24-27 hr. Enhanced expression of cyclin A, a protein essential for DNA synthesis was used as an additional marker to define the initiation of the S phase. Immunoblotting analysis demonstrated that the expression of alphaA and alphaB-crystallin was up to 10-fold higher in cells synchronized in G0 phase than in G1 phase. The levels of the proteins increased three-fold again as the cells entered the S phase and progressed to mitosis, but did not rise to the levels observed in G0 phase. This increase in expression of alphaA-crystallin resulted in part from enhanced synthesis during the S phase, as shown by an increase in [35S]methionine-labeling and immunoprecipitation of the radiolabeled alphaA-crystallin. The results were further confirmed by flow cytometric analysis using DNA content and alphaA-crystallin expression. The increase in alphaB-crystallin in S phase was paralleled by an increase in gene expression as shown by real-time RT-PCR analysis. These results demonstrate for the first time that in lens epithelial cells, alphaA and alphaB-crystallin levels are modulated during the cell cycle. Since the absence of alphaA and alphaB- crystallin in lens epithelial cells has been associated with disturbance of the tubulin cytoskeleton during mitosis, and with increased cell death or genomic instability, our results indicating that the alphaA- and alphaB-crystallin expression increases prior to mitosis are significant. The differential expression of these crystallins in the cell cycle may be important for optimal lens epithelial growth and lens transparency.

Role for alpha 6 integrin during lens development: Evidence for signaling through IGF-1R and ERK

We show that alpha 6 integrin function was required for normal lens cell differentiation by using an antisense construct to suppress alpha 6 integrin expression. To elucidate the mechanism by which this integrin functions in the regulation of the lens cell differentiation process, we determined the molecular composition of alpha 6 integrin signaling complexes at distinct stages of differentiation in vivo. Because both alpha 6 integrin and insulin-like growth factor-1 (IGF-1) have been implicated in signaling lens cell differentiation, we examined the possibility that they formed a signaling complex in the embryonic lens. Coprecipitation analysis revealed that alpha 6 integrin/IGF-1 receptor complexes were present and that their association was greatest in the equatorial zone, the region of the embryonic lens in which lens cells proliferate and then initiate their differentiation. These results provide in vivo support for the formation of integrin/growth factor receptor signaling complexes. We also found that extracellular signal-regulated kinase (ERK), a downstream effector of both integrin and growth factor receptor signaling pathways, was associated with the alpha 6 integrin signaling complexes in the embryonic lens. This result was supported by our findings that activated ERK, in addition to its nuclear location, localized to lens cell membranes in specific regions of cell-matrix and cell-cell contact. A connection between integrin ligand engagement and ERK activation was shown in vitro after lens cell attachment to laminin. These results demonstrate that alpha 6 integrin function is required for the early stages of lens cell differentiation most likely through its association with the IGF-1 receptor and the activation of ERK.

FGF signaling in chick lens development

The prevailing concept has been that an FGF induces epithelial-to-fiber differentiation in the mammalian lens, whereas chick lens cells are unresponsive to FGF and are instead induced to differentiate by IGF/insulin-type factors. We show here that when treated for periods in excess of those used in previous investigations (>5 h), purified recombinant FGFs stimulate proliferation of primary cultures of embryonic chick lens epithelial cells and (at higher concentrations) expression of the fiber differentiation markers delta-crystallin and CP49. Surprisingly, upregulation of proliferation and delta-crystallin synthesis by FGF does not require activation of ERK kinases. ERK function is, however, essential for stimulation of delta-crystallin expression in response to insulin or IGF-1. Vitreous humor, the presumptive source of differentiation-promoting activity in vivo, contains a factor capable of diffusing out of the vitreous body and inducing delta-crystallin and CP49 expression in chick lens cultures. This factor binds heparin with high affinity and increases delta-crystallin expression in an ERK-insensitive manner, properties consistent with an FGF but not insulin or IGF. Our findings indicate that differentiation in the chick lens is likely to be mediated by an FGF and provide the first insights into the role of the ERK pathway in growth factor-induced signal transduction in the lens.

Temporal and spatial growth patterns in the normal and cataractous human lens

This study presents a computational model of the growth of the normal human lens and the induction of spoke-like cortical cataract in the aging lens. The anterior lens is modelled as a 2-D disk with a circumferential germinative zone. Lens cortical fibre cells in the same generation cover the surface in three identical 120 deg growth wedge-shaped sectors, with centre cardinal fibres at the 90, 210 and 330 deg meridians. In the foetal lens all primary fibre cells begin to elongate simultaneously. Anterior migration is spatially asynchronous, where the centre fibre begins to move towards the anterior pole first. The fibres at the end of the sector move last in the anterior direction. Fibre elongation advanced at constant speed until the boundary of the sector is reached. Spatio-temporal asynchrony and random fluctuations were increased for the adult lens. The model foetal lens evolved Y-shaped sutures anteriorly, and an inverted Y-shaped posteriorly. Fibre length varied periodically with meridional angle. The adult lens displayed irregular growth. If clusters of germinative cells are caused to opacify the resultant opacities are predominantly spoke-shaped. The model mimics crystalline lens fibre growth to the extent of successfully evolving lens sutures. Fluctuations in lens mass are consistent with an ordered pattern of growth. Lens senescence includes a progressive loss of spatio-temporal synchrony in fibre migration from the germinative zone. Peripheral light focusing by the anterior eye is a possible explanation for the nasal predilection and cuneiform shape of age-related cortical cataract.

Changes in cyclin dependent kinase expression and activity accompanying lens fiber cell differentiation

Previous studies from this laboratory have shown that differentiating lens fiber cells contain two active cyclin dependent kinases (Cdks), Cdk1 and Cdk5. The present study was undertaken to explore the expression and regulation of six additional members of the Cdk family (Cdk2, Cdk3, Cdk4, Cdk6, Cdk7 and Cdk8) during lens differentiation. Differentiating lens fiber cells were separated from lens epithelial cells by microdissection of developing rat lenses [embryonic day 16 (E16) to postnatal day 8 (P8)] and Cdk expression was assessed by RT-PCR and immunoblotting. Two Cdks (Cdk3 and Cdk6) were not expressed in lens fiber cells or epithelial cells during this developmental period. In the lens epithelium, we detected proteins and mRNAs corresponding to all other Cdks examined (Cdk2, Cdk4, Cdk7, Cdk8) throughout this developmental period. Epithelial cells showed significant Cdk2 activity, which decreased with developmental age, but no significant activity was detected for Cdk4, Cdk7, or Cdk8. Fiber cells contained all four Cdk proteins and the corresponding Cdk mRNAs except for Cdk2 mRNA. None of the Cdks examined showed significant kinase activity in fiber cells. Immunoprecipitates of Cdk2 and Cdk4 from fiber cells contained p57(kip2), supporting the view that this Cdk inhibitor blocks the activity of these Cdks in lens fibers. In contrast, p57(kip2)did not co-immunoprecipitate with Cdk5 from lens fibers. These findings suggest that the differential affinity of p57(kip2)for members of the Cdk family may provide a mechanism for specific regulation of individual Cdks during fiber cell differentiation.

Lens cytoskeleton and transparency: a model

The function of the cytoskeleton in lens was first considered when cytoplasmic microtubules were observed in elongating fibre cells of the chick lens nearly 40 years ago. Since that time, tubulin, actin, vimentin and intermediate filaments have been identified and found to function in mitosis, motility and cellular morphology during lens cell differentiation. A role for the cytoskeleton in accommodation has been proposed and modification of the cytoskeletal proteins has been observed in several cataract models. Recently, a progressive increase in protein aggregation and lens opacification was found to correspond with the loss of cytoskeletal protein in the selenite model for cataract. In the present report a model is proposed for the role of tubulin, actin, vimentin, spectrin and the lens-specific filaments, filensin and CP49, in the establishment and maintenance of transparent lens cell structure.

Cell growth patterns and lens geometry: a quantitative study from three-dimensional reconstructions

The growth of the lens of the sea lamprey, Petromyzon marinus, was studied over the 5 years of larval development. Whole lenses (25) and Golgi-impregnated cells (393) were reconstructed with computer-assisted microscopy. Several cellular geometric parameters (length, width, curvature, surface, volume, shape) were correlated with the position of the cell's base on the lens capsular perimeter. Based on these correlations, the cells formed four groups that correspond to the central anterior, germinative, transitional and cortical fiber zones. A fifth zone, containing nuclear fiber cells, never stained. Lens growth is exponential during the 5 years. The anterior epithelium increases in size and in cell number by cell growth and division. The posterior mass increases in cell number by recruitment and increases in size by cell growth. A model is proposed to account for the size and shape of the lens based upon the coupling of anterior and posterior growth patterns. Four zonal boundaries are defined by changes in cell growth patterns. With growth, cells are subsumed into adjacent zones and zonal boundaries move away from the lens center. We find no support for the suggestion that cells migrate centrally.