Actin filament patterns in mouse lens epithelium: a study of the effects of aging, injury, and genetics

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.

Methodologies to unlock the molecular expression and cellular structure of ocular lens epithelial cells

The transparent ocular lens in the anterior chamber of the eye is responsible for fine focusing of light onto the retina. The lens is entirely cellular with bulk of the tissue composed of fiber cells, and the anterior hemisphere of the lens is covered by a monolayer of epithelial cells. Lens epithelial cells are important for maintaining fiber cell homeostasis and for continual growth of the lens tissue throughout life. Cataracts, defined as any opacity in the lens, remain the leading cause of blindness in the world. Following cataract surgery, lens epithelial cells can undergo a process of epithelial-to-mesenchymal transition (EMT), leading to secondary cataracts due to posterior capsular opacification (PCO). Since the epithelial cells make up only a small fraction of the lens, specialized techniques are required to study lens epithelial cell biology and pathology. Studies using native lens epithelial cells often require pooling of samples to obtain enough cells to make sufficient samples for traditional molecular biology techniques. Here, we provide detailed protocols that enable the study of native mouse lens epithelial cells, including immunostaining of the native lens epithelium in flat mounts, extraction of RNA and proteins from pairs of lens epithelial monolayers, and isolation of lens epithelial cells for primary culture. These protocols will enable researchers to gain better insight on representative molecular expression and cellular structure of lens epithelial cells. We also provide comparative data between native, primary culture, and immortalized lens epithelial cells and discuss the advantages and disadvantages of each technique presented.

Three-dimensional data capture and analysis of intact eye lenses evidences emmetropia-associated changes in epithelial cell organization

Organ and tissue development are highly coordinated processes; lens growth and functional integration into the eye (emmetropia) is a robust example. An epithelial monolayer covers the anterior hemisphere of the lens, and its organization is the key to lens formation and its optical properties throughout all life stages. To better understand how the epithelium supports lens function, we have developed a novel whole tissue imaging system using conventional confocal light microscopy and a specialized analysis software to produce three-dimensional maps for the epithelium of intact mouse lenses. The open source software package geometrically determines the anterior pole position, the equatorial diameter, and three-dimensional coordinates for each detected cell in the epithelium. The user-friendly cell maps, which retain global lens geometry, allow us to document age-dependent changes in the C57/BL6J mouse lens cell distribution characteristics. We evidence changes in epithelial cell density and distribution in C57/BL6J mice during the establishment of emmetropia between postnatal weeks 4-6. These epithelial changes accompany a previously unknown spheroid to lentoid shape transition of the lens as detected by our analyses. When combined with key findings from previous mouse genetic and cell biological studies, we suggest a cytoskeleton-based mechanism likely underpins these observations.

Surgical stress and cytoskeletal changes in lens epithelial cells following manual and femtosecond laser-assisted capsulotomy

AIM

To study the effect of mechanical stress on the cytoskeleton in lens epithelial cells following conventional phacoemulsification surgery (CPS) and femtosecond laser-assisted cataract surgery (FLACS).

METHODS

The cytoskeleton of the epithelial cells of the anterior lens capsules (ALC) removed by CPS and FLACS was examined by immunohistochemistry. Expression of the intermediate filament, glial fibrillary acidic protein (GFAP), and glutamine synthetase (GS) immunoreactivity were detected. In order to map the actin network of cells, fluorescently labeled phalloidin was used. The samples were examined using confocal laser scanning microscopy.

RESULTS

GFAP expression was visible in a larger number of the epithelial cells after CPS compared to FLACS. In CPS sample's epithelial cells, GFAP immunoreactivity indicated robust morphological change. Regarding the actin filaments, the presence of tubular elements connecting epithelial cells, regular actin pattern and marked cortical network after CPS were found. Following FLACS, the actin cytoskeleton of the epithelial cells remained densely structured, and the tubular elements were undetectable, however, the above-mentioned regular actin pattern and the marked cortical network were visible.

CONCLUSION

The conventional removal of the ALC induces more robust changes of the cytoskeleton of the lens epithelial cells.

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.

The lens actin filament cytoskeleton: Diverse structures for complex functions

The eye lens is a transparent and avascular organ in the front of the eye that is responsible for focusing light onto the retina in order to transmit a clear image. A monolayer of epithelial cells covers the anterior hemisphere of the lens, and the bulk of the lens is made up of elongated and differentiated fiber cells. Lens fiber cells are very long and thin cells that are supported by sophisticated cytoskeletal networks, including actin filaments at cell junctions and the spectrin-actin network of the membrane skeleton. In this review, we highlight the proteins that regulate diverse actin filament networks in the lens and discuss how these actin cytoskeletal structures assemble and function in epithelial and fiber cells. We then discuss methods that have been used to study actin in the lens and unanswered questions that can be addressed with novel techniques.

Ultrastructure of UVR-B-induced cataract and repair visualized with electron microscopy

PURPOSE

The aim of the study is to investigate and visualize the ultrastructure of cataract morphology and repair, after in vivo exposure to double threshold dose UVR-B in the C57BL/6 mouse lens.

METHODS

Twenty-six-week-old C57BL/6 mice received in vivo double threshold dose (6.4 kJ/m2) UVR-B for 15 min. The radiation output of the UVR-source had λMAX at 302.6 nm. After a latency period of 1, 2, 4 and 8 days following UVR-B exposure, the induced cataract was visualized with electron microscopy techniques. Induced, cataract was quantified as forward lens light scattering. Damage to the lens epithelium and the anterior cortex was investigated with light microscopy in toluidine blue-stained semi-thin sections, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and dark field illumination photography.

RESULTS

UVR-B-exposed lenses developed anterior subcapsular and/or cortical and nuclear cataract after 1 day. Lens light scattering peaked 2 days after exposure. Lens epithelial cell damage was seen in TEM as apoptotic cells, apoptotic bodies, nuclear chromatin condensation, and swollen and disrupted anterior cortex fibres throughout the sections of the whole anterior lens surface. These morphologic changes were also visualized with SEM. Within 8 days, anterior subcapsular cataract was repaired towards the anterior sutures.

CONCLUSION

UVR-B exposure of double cataract threshold dose induces a subtotal loss of epithelial cells across the whole anterior surface of the lens. This damage to the epithelium is repaired by epithelial cell movement from the equator towards the lens sutures, thus in retrograde direction to regular epithelial cell differentiation.

The Cdk5 activator P39 specifically links muskelin to myosin II and regulates stress fiber formation and actin organization in lens

Cyclin dependent kinase 5 (Cdk5), a proline-directed serine/threonine kinase, requires p39 for its enzymatic activity, and is implicated in cytoskeletal organization and contraction in numerous cell types. The C-terminus of p39 binds muskelin, a multi-domain scaffolding protein known to affect cytoskeletal organization, but the mechanisms by which muskelin affects cytoskeletal organization remain unclear. The present study sought to determine whether p39 might serve as an adaptor protein that links muskelin to stress fibers and to investigate the possible biological relevance of such an interaction. Double immunoprecipitation showed that muskelin, p39, and myosin II are components of a single intracellular complex, and suppressing p39 abrogated the interaction between muskelin and the myosin subunits, demonstrating that p39 is required to link muskelin to myosin II. Muskelin is colocalized with myosin regulatory light chain (MRLC) and on stress fibers. The suppression of muskelin reduced Rho-GTP, MRLC phosphorylation, disrupted stress fiber organization, and promoted cell migration, all of which closely mimic the effect of Cdk5 inhibition. Moreover, suppressing muskelin and inhibiting Cdk5 together have no additional effect, indicating that muskelin plays an important role in Cdk5-dependent signaling. p39 is necessary and sufficient for Cdk5-dependent regulation of MRLC phosphorylation, as suppression of p39, but not p35, reduces MRLC phosphorylation. Together, these results demonstrate that p39 specifically links muskelin to myosin II and consequently, to stress fibers and reveal a novel role for muskelin in regulating myosin phosphorylation and cytoskeletal organization.

Actin filament organization regulates the induction of lens cell differentiation and survival

The actin cytoskeleton has the unique capability of integrating signaling and structural elements to regulate cell function. We have examined the ability of actin stress fiber disassembly to induce lens cell differentiation and the role of actin filaments in promoting lens cell survival. Three-dimensional mapping of basal actin filaments in the intact lens revealed that stress fibers were disassembled just as lens epithelial cells initiated their differentiation in vivo. Experimental disassembly of actin stress fibers in cultured lens epithelial cells with either the ROCK inhibitor Y-27632, which destabilizes stress fibers, or the actin depolymerizing drug cytochalasin D induced expression of lens cell differentiation markers. Significantly, short-term disassembly of actin stress fibers in lens epithelial cells by cytochalasin D was sufficient to signal lens cell differentiation. As differentiation proceeds, lens fiber cells assemble actin into cortical filaments. Both the actin stress fibers in lens epithelial cells and the cortical actin filaments in lens fiber cells were found to be necessary for cell survival. Sustained cytochalasin D treatment of undifferentiated lens epithelial cells suppressed Bcl-2 expression and the cells ultimately succumbed to apoptotic cell death. Inhibition of Rac-dependent cortical actin organization induced apoptosis of differentiating lens fiber cells. Our results demonstrate that disassembly of actin stress fibers induced lens cell differentiation, and that actin filaments provide an essential survival signal to both lens epithelial cells and differentiating lens fiber cells.

Cytological and immunocytochemical approaches to the study of corneal endothelial wound repair

The vertebrate corneal endothelium represents a unique model system for investigating many cellular aspects of wound repair within an organized tissue in situ. The tissue exists as a cell monolayer that resides upon its own natural basement membrane that can be prepared as a flat mount to observe the entire cell population. Thus, it readily avails itself to many cytological and immunocytochemical methods at both the light microscopic and ultrastructural levels. In addition, the tissue is easily explanted into organ culture where further investigations can be carried out. These techniques have enabled investigators to use many approaches to explore function and changes in response to injury. In vivo, the endothelium acts as a transport tissue to actively pump Na+ and bicarbonate ions from the corneal stroma into the aqueous humor to control corneal transparency. Physiological findings indicate that fluid diffuses back into the stroma, across the endothelium, and thus hydration is said to be controlled by a pump-leak mechanism. Ultrastructural investigations, some employing horseradish peroxidase and lanthanum, have established the morphological basis for this mechanism as apical focal junctions that are not the classical tight junctions and do not constitute a complete zona occludens. Along with these apical focal junctions are gap junctions that appear identical to their counterparts in other cell types. Cytochemical studies localized both Na+K(+)-ATPase and carbonic anhydrase, the main pump enzymes associated with corneal hydration, to the lateral plasma membranes. Corneal endothelial cells of noninjured tissue do not traverse the cell cycle and are considered to be in the "Go" phase of the cell cycle as determined by microfluorometric analysis with DNA binding dyes such as auramin O and pararosaniline-Feulgen. However, injury can initiate cell cycle transverse and histochemical and cytological methods have been used to understand the tissue's response. Classical histochemical studies revealed that increased staining was observed for metabolic (NADase and NADPase) and lysosomal enzymes in cells bordering the wound area. The use of radiolabelled agents has further lead to an understanding of the endothelial wound response. Autoradiographic analyses of 3H-actinomycin D incorporation indicated that injury initiates changes in chromatin leading to increased binding levels of the drug in cells surrounding the wound. This change suggests that those cells undergo heightened macromolecular synthesis and this was confirmed by examining 3H-uridine and 3H-thymidine incorporation. The major mechanism involved in corneal endothelial repair is cell migration. Cytochemical and immunocytochemical investigations have allowed investigators an opportunity to gain some insight into changes that occur during this cellular process.(ABSTRACT TRUNCATED AT 400 WORDS)

Microtubules rich in post-translationally modified alpha-tubulin form distinct arrays in frog lens epithelial cells

Isolated frog lens epithelia were stained with antibodies against tyrosinated, detyrosinated or acetylated alpha-tubulin and observed by several means including a scanning confocal microscope. The most prominent feature of Rana pipiens lens cells was a primary cilium close to the apical surface of the cells above the centrosome. This structure was associated with microtubules rich in modified alpha-tubulin. The cilium was less pronounced but still discernible in the cells of another species R. ridibunda. In both species, the modified (acetylated or detyrosinated) microtubules formed arrays spatially distinct from the unmodified (tyrosinated) microtubules. The modified microtubules formed a basket of microtubules with a curly distribution around the nucleus while the tyrosinated array consisted predominantly of rather straighter microtubules running from the apical centrosome to the cell periphery, down the lateral sides of the cells and across the basal surface adjacent to the lens capsule and basement membrane. It is concluded that the organization of modified microtubules previously described for several types of cultured cells may represent a remnant of the three-dimensional perinuclear array of such microtubules described here for the cells of an intact epithelium.

Actin cytoskeletal network in aging and cancer

The cytoskeleton is being recognized as an important modulator of metabolic functions of the cell. The actin cytoskeletal network, in particular, is involved in events regulating cell proliferation and differentiation. The state of actin in a variety of cell types is regulated by signals arising from the cell surface through a wide spectrum of interactions. In this review, we explore the role of actin cytoskeletal network in a series of events which are known to influence cell proliferation and differentiation. These include interaction of actin network with extracellular matrix proteins, cell surface membranes, second messengers, cytoplasmic enzymes and the nucleus. Because of the involvement of the actin network in such diverse interactions, we propose that alterations in the actin cytoskeletal function may be an important aspect of generalized decrease in cellular functions associated with aging. Preliminary data indicate that alterations in the cytoskeletal network do occur in cells obtained from older individuals. Alterations in actin state are also reported during malignant transformation of cells in culture, and in naturally occurring tumors. Taken together, the existing data seem to suggest that changes in the actin cytoskeletal network may be a part of the aging process as well as malignant transformation. Therefore, the study of the actin cytoskeletal network and its regulation has the potential to yield important information regarding cellular senescence and neoplastic transformation.

Diamide induces reversible changes in morphology, cytoskeleton and cell-cell coupling in lens epithelial cells

The isolated frog lens epithelium can be maintained with its cell shape, cytoskeletal organization and membrane electrophysiological characteristics intact for more than 24 hr. Perifusion with the permeant oxidant diamide (1 mM) led to drastic, but reversible, changes in all the above parameters. After a 20 min exposure to diamide, the regular polygonal arrangement of the epithelial cells become increasingly disrupted as the cells reorganized and a 'rosette' pattern formed. The cells at the edges of the rosette pulled apart from one another while those in the centre maintained a relatively normal appearance. Blebs formed on the apical surface of all of the cells on prolonged exposure and the internal structure was also found to be severely disrupted. The cytoplasm became granular, vacuolated and the nucleus had a banded, non-homogeneous appearance. Phalloidin staining of F-actin microfilaments revealed that there was a general disruption of organization, with actin losing its association with the membrane. The microtubule array, organized around the centrosome, was also severely disrupted although microtubules were still discernible in most cells. During exposure to diamide the membrane potential depolarized and both electrical and dye coupling, which are normally extremely efficient in these cells, were disturbed. If the epithelium was exposed to 1 mM diamide for more than 45 min then all of the above changes were irreversible and cell death followed. If exposure was restricted to less than 30 min, then all of the above changes occurred and, in fact, progressed for over 1 hr; but if the epithelium was perifused for a further 20 hr in control medium, then most of the changes were reversible.

Appearance of alpha-smooth muscle actin in human eye lens cells of anterior capsular cataract and in cultured bovine lens-forming cells

Using light and electron-microscopic immunolocalization techniques, and gel electrophoresis combined with immunoblotting, we have examined the expression of cytoskeletal proteins in normal human fetal, child and adult lenses, in human anterior capsular cataract and in bovine lens cells in vivo and in vitro. In this report, we focus our observations on the pattern of actin-isoform expression during normal and pathological situations in vivo and culture conditions. We have noted that cells of developing and mature human lenses as well as bovine lens cells in situ contain only beta- and gamma-actins. In contrast, alpha-smooth muscle (alpha-sm) actin, an isoform typical of smooth muscle differentiation, was demonstrated in bovine lens cells at different times of culture. Moreover, the multilayered cells observed in the subcapsular zone of human anterior capsular cataract were characterized by the presence of alpha-sm actin. Thus, extensive changes in actin-isoform expression take place in lens cells growing in culture and may also occur during cataractogenesis. The biological meaning of the appearance of a marker of myoid differentiation in the ectodermally derived lens-forming cells is discussed.

Whole-mount preparations of mouse lens epithelium for the fluorescent cytological study of actin

A technique is given for the preparation of a sheet of epithelial cells from the capsule of the crystalline lens. A new method is described for fixation and staining with fluorescent phalloidin or actin antibody in order to localize the actin cytoskeleton in this tissue. Optical section of the preparation resolves such actin features as apical polygonal arrays, sequestered actin bundles, perinuclear actin aggregates, observed here for the first time, and filamentous networks in the basal region of the cell. This method is superior to previous ones in its ability to preserve actin-abundant sectors distinctively.

Comparative study of actin filament patterns in lens epithelial cells. Are these determined by the mechanisms of lens accommodation?

Actin filament patterns in lens epithelia from animals of various taxonomic groups were studied using rhodamine phalloidin fluorescence microscopy of epithelial whole mounts and transmission electron microscopy of tangential sections. The results were compared with the accommodative mechanism operating in each case as reported in the literature. Lenses that accommodate by deformation of the anterior surface, in squirrel, chipmunk, rabbit, monkey and human, showed polygonal arrays (PAs) at the apical end of the epithelial cells. Lenses that translate as a whole, in shark, bony fish and frog, showed stress fibers (SFs) at the basal or apical end of the cells. No specialized actin pattern was seen in turtle and bird, which have lenses that are squeezed into an anterior lenticonus; cat, where the lens is translated forward; or rat, cow and most mice, which have no defined accommodation. In exception, certain strains of laboratory mice did show sequestered actin bundles (SABs) and/or PAs. Based on our findings, we conclude that PAs, which resemble geodesic domes, do not take an active part in near-point accommodation; but like SFs, may serve to resist overextension by internal pressure of the fiber mass or by zonular tension.