Telomerase activity in lens epithelial cells of normal and cataractous lenses

Adult stem cells in the eye: Identification, characterisation, and therapeutic application in ocular regeneration - A review

Adult stem cells, present in various parts of the human body, are undifferentiated cells that can proliferate and differentiate to replace dying cells within tissues. Stem cells have specifically been identified in the cornea, trabecular meshwork, crystalline lens, iris, ciliary body, retina, choroid, sclera, conjunctiva, eyelid, lacrimal gland, and orbital fat. The identification of ocular stem cells broadens the potential therapeutic strategies for untreatable eye diseases. Currently, stem cell transplantation for corneal and conjunctival diseases remains the most common stem cell-based therapy in ocular clinical management. Lens epithelial stem cells have been applied in the treatment of paediatric cataracts. Several early-phase clinical trials for corneal and retinal regeneration using ocular stem cells are also underway. Extensive preclinical studies using ocular stem cells have been conducted, showing encouraging outcomes. Ocular stem cells currently demonstrate great promise in potential treatments of eye diseases. In this review, we focus on the identification, characterisation, and therapeutic application of adult stem cells in the eye.

Inhibition of posterior capsule opacification by adenovirus-mediated delivery of short hairpin RNAs targeting TERT in a rabbit model

PURPOSE

Posterior capsule opacification (PCO) is the most common postoperative complication after cataract surgery and cannot yet be eliminated. Here, we investigated the inhibitory effects of telomerase reverse transcriptase (TERT) gene silencing on PCO in a rabbit model.

METHODS

After rabbit lens epithelial cells (LECs) were treated with adenovirus containing short hairpin RNAs (shRNA) targeting TERT (shTERT group), adenovirus containing scramble nonsense control shRNA (shNC group) or PBS (control group), quantitative real-time polymerase chain reaction and Western blotting were used to measure the expression levels of TERT, and a scratch assay was performed to assess the LEC migration. New Zealand white rabbits underwent sham cataract surgery followed by an injection of adenovirus carrying shTERT into their capsule bag. The intraocular pressure and anterior segment inflammation were evaluated on certain days, and EMT markers (α-SMA and E-cadherin) were evaluated by Western blotting and immunofluorescence. The telomerase activity of the capsule bag was detected by ELISA. At 28 days postoperatively, haematoxylin and eosin staining of the cornea and iris and electron microscopy of the posterior capsule were performed.

RESULTS

Application of shTERT to LECs downregulated the expression levels of TERT mRNA and protein. The scratch assay results showed a decrease in the migration of LECs in the shTERT group. In vivo, shTERT decreased PCO formation after cataract surgery in rabbits and downregulated the expression of EMT markers, as determined by Western blotting and immunofluorescence. In addition, telomerase activity was suppressed in the capsule bag. Despite slight inflammation in the iris, histologic results revealed no toxic effects in the cornea and iris.

CONCLUSION

TERT silencing effectively reduces the migration and proliferation of LECs and the formation of PCO. Our findings suggest that TERT silencing may be a potential preventive strategy for PCO.

The lens epithelium as a major determinant in the development, maintenance, and regeneration of the crystalline lens

The crystalline lens is a transparent and refractive biconvex structure formed by lens epithelial cells (LECs) and lens fibers. Lens opacity, also known as cataracts, is the leading cause of blindness in the world. LECs are the principal cells of lens throughout human life, exhibiting different physiological properties and functions. During the embryonic stage, LECs proliferate and differentiate into lens fibers, which form the crystalline lens. Genetics and environment are vital factors that influence normal lens development. During maturation, LECs help maintain lens homeostasis through material transport, synthesis and metabolism as well as mitosis and proliferation. If disturbed, this will result in loss of lens transparency. After cataract surgery, the repair potential of LECs is activated and the structure and transparency of the regenerative tissue depends on postoperative microenvironment. This review summarizes recent research advances on the role of LECs in lens development, homeostasis, and regeneration, with a particular focus on the role of cholesterol synthesis (eg., lanosterol synthase) in lens development and homeostasis maintenance, and how the regenerative potential of LECs can be harnessed to develop surgical strategies and improve the outcomes of cataract surgery (Fig. 1). These new insights suggest that LECs are a major determinant of the physiological and pathological state of the lens. Further studies on their molecular biology will offer possibility to explore new approaches for cataract prevention and treatment.

Many Functions of Telomerase Components: Certainties, Doubts, and Inconsistencies

A growing number of studies have evidenced non-telomeric functions of "telomerase". Almost all of them, however, investigated the non-canonical effects of the catalytic subunit TERT, and not the telomerase ribonucleoprotein holoenzyme. These functions mainly comprise signal transduction, gene regulation and the increase of anti-oxidative systems. Although less studied, TERC (the RNA component of telomerase) has also been shown to be involved in gene regulation, as well as other functions. All this has led to the publication of many reviews on the subject, which, however, are often disseminating personal interpretations of experimental studies of other researchers as original proofs. Indeed, while some functions such as gene regulation seem ascertained, especially because mechanistic findings have been provided, other ones remain dubious and/or are contradicted by other direct or indirect evidence (e.g., telomerase activity at double-strand break site, RNA polymerase activity of TERT, translation of TERC, mitochondrion-processed TERC). In a critical study of the primary evidence so far obtained, we show those functions for which there is consensus, those showing contradictory results and those needing confirmation. The resulting picture, together with some usually neglected aspects, seems to indicate a link between TERT and TERC functions and cellular stemness and gives possible directions for future research.

Research on Werner Syndrome: Trends from Past to Present and Future Prospects

A rare and autosomal recessive premature aging disorder, Werner syndrome (WS) is characterized by the early onset of aging-associated diseases, including shortening stature, alopecia, bilateral cataracts, skin ulcers, diabetes, osteoporosis, arteriosclerosis, and chromosomal instability, as well as cancer predisposition. WRN, the gene responsible for WS, encodes DNA helicase with a 3′ to 5′ exonuclease activity, and numerous studies have revealed that WRN helicase is involved in the maintenance of chromosome stability through actions in DNA, e.g., DNA replication, repair, recombination, and epigenetic regulation via interaction with DNA repair factors, telomere-binding proteins, histone modification enzymes, and other DNA metabolic factors. However, although these efforts have elucidated the cellular functions of the helicase in cell lines, they have not been linked to the treatment of the disease. Life expectancy has improved for WS patients over the past three decades, and it is hoped that a fundamental treatment for the disease will be developed. Disease-specific induced pluripotent stem (iPS) cells have been established, and these are expected to be used in drug discovery and regenerative medicine for WS patients. In this article, we review trends in research to date and present some perspectives on WS research with regard to the application of pluripotent stem cells. Furthermore, the elucidation of disease mechanisms and drug discovery utilizing the vast amount of scientific data accumulated to date will be discussed.

Hallmarks of lens aging and cataractogenesis

Lens homeostasis and transparency are dependent on the function and intercellular communication of its epithelia. While the lens epithelium is uniquely equipped with functional repair systems to withstand reactive oxygen species (ROS)-mediated oxidative insult, ROS are not necessarily detrimental to lens cells. Lens aging, and the onset of pathogenesis leading to cataract share an underlying theme; a progressive breakdown of oxidative stress repair systems driving a pro-oxidant shift in the intracellular environment, with cumulative ROS-induced damage to lens cell biomolecules leading to cellular dysfunction and pathology. Here we provide an overview of our current understanding of the sources and essential functions of lens ROS, antioxidative defenses, and changes in the major regulatory systems that serve to maintain the finely tuned balance of oxidative signaling vs. oxidative stress in lens cells. Age-related breakdown of these redox homeostasis systems in the lens leads to the onset of cataractogenesis. We propose eight candidate hallmarks that represent common denominators of aging and cataractogenesis in the mammalian lens: oxidative stress, altered cell signaling, loss of proteostasis, mitochondrial dysfunction, dysregulated ion homeostasis, cell senescence, genomic instability and intrinsic apoptotic cell death.

Analysis of lens epithelium telomere length in age-related cataract

We aimed to investigate the associations among lens epithelium telomere length (LETL), cataract types, and systemic pro-senescence factors in patients with age-related cataract. In this prospective study, the general demographic factors, body mass index, smoking history, depression, hypertension, diabetes, various psychological measures, and uncorrected distant visual acuity of patients with age-related cataract were recorded. Lens Opacities Classification System III (LOCS III) scores and lens density measured by Scheimpflug imaging were used to evaluate the cataracts. LETL was measured by real-time polymerase chain reaction. Correlations among these parameters were analyzed. The LOCS III nuclear opalescence (NO) score was associated with age (β = 0.053, P < 0.001) and Patient Health Questionnaire-9 score (β = -0.042, P = 0.004). Smoking was identified as a risk factor affecting LOCS III NO score (odds ratio = 1.546, 95% confidence interval, 1.128-2.119), but not the LOCS III cortical or posterior subcapsular scores. LETLs showed a weak association with systemic factors and LOCS III scores, and a significantly moderate correlation with the average objective lens densities of different regions measured by Scheimpflug imaging (r values ranged from -0.278 to -0.523, P < 0.05). However, there was no correlation between the LETLs and the maximum lens densities. The groups with a relatively low lens density had longer LETLs. In Conclusion, being an age-related disease, cortical cataract was also associated with "aging of the lens epithelium." Notably, lens epithelium activity rarely showed systemic effects. Thus, future studies should emphasize the importance of the telomeric system in cataractous process and aging.

Long-term prevention of capsular opacification after lens-refilling surgery in a rabbit model

Purpose

To reduce capsular opacification by a peri‐surgical treatment of the lens capsule with drugs in an in vivo rabbit model. Lens‐refilling surgery is a potential therapeutic intervention to treat patients with a cataract lens. The lens material is replaced with an injectable (bio)polymer that retains the natural mechanical and optical lens properties, therewith allowing accommodation. The occurrence of capsular opacification mediated by lens epithelial cells negatively affects accommodation and vision and should be avoided in this lens restoration approach.

Methods

An in vivo rabbit animal model was used with lens replacement with a silicone‐based gel‐like polymer and concurrent treatment of the lens epithelium with drugs. A case‐study approach was applied as both drug combinations and implantation times were varied. The following drugs were investigated for their potential to prevent capsular opacification long‐term: actinomycin D, methotrexate, paclitaxel and Tween‐20. All were administered in a hyaluronic acid vehicle. The rabbits were clinically followed for periods up to 4 years postimplantation. Eyes, corneas and lenses were analysed post‐mortem using MRI and confocal microscopy.

Results

Treatment combinations containing actinomycin D generally led to the least appearance of capsular fibrosis. The use of Tween‐20 or paclitaxel without actinomycin D resulted in much earlier and pronounced fibrotic responses. The aspect of capsular opacification was highly variable in individual animals. Application of the drugs in a hyaluronic acid vehicle appeared to be a safe method that spared the corneal endothelium.

Conclusion

The feasibility of long‐term prevention of fibrosis over a period of more than 4 years has been demonstrated in lens refilling in the rabbit model.

Effects of ionizing radiation on telomere length and telomerase activity in cultured human lens epithelium cells

PURPOSE

To investigate the effects of ionizing radiation on telomere length and telomerase activity in human lens epithelial cells. There are studies suggesting evidence of telomere length in association with opacity of the lens; however, these studies have been conducted on Canine Lens cells. Our study was designed to understand further the effects of different doses of ionizing radiation on telomere length and telomerase activity in cultured human lens epithelium cells from three Donors.

MATERIALS AND METHODS

For this study, embryonic human lens epithelial (HLE) cells from three donors, obtained commercially were cultured. Telomere length and telomerase activity were measured after each passage until cells stopped growing in culture. This was repeated on irradiated (0.001 Gy, 0.01 Gy, 0.02 Gy, 0.1 Gy, 1 Gy and 2 Gy) cells. DNA damage response using the H2AX and telomere dysfunction foci assays were also examined at 30 mins, 24 hours, 48 hours and 72 hours postirradiation.

RESULTS AND CONCLUSION

We have demonstrated genetic changes in telomere length and oxidative stress, which may be relevant to cataractogenesis. Our study shows that in control cells telomere length increases as passage increases. We have also demonstrated that telomere length increases at higher doses of 1.0 Gy and 2.0 Gy. However, telomerase activity decreases dose dependently and as passages increase. These results are not conclusive and further studies ex vivo measuring lens opacity and telomere length in the model would be beneficial in a bigger cohort, hence confirming a link between telomere length, cataractogenesis and genetic factors.

Lens Epithelial Apoptosis and Cell Proliferation in Human Age-Related Cortical Cataract

Purpose

To probe the presence of apoptosis in the epithelium of human lenses with age-related cortical cataract as well as to assess cell proliferation, a predicted consequence of apoptotic cell death, in this specific cell population.

Methods

DNA fragmentation was assessed using terminal digoxigenin-labeled dUTP nick end labeling (TUNEL) in capsulotomy specimens obtained from patients who underwent either extracapsular cataract extraction for the removal of adult-onset cortical cataract (n=27) or clear lens extraction for the correction of high myopia (n=25). Cell proliferation was assayed in 23 epithelia of cataractous lenses, and 20 epithelia of non-cataractous lenses with the proliferation marker MIB1, a monoclonal antibody against the nuclear antigen Ki-67 that is detected throughout the cell cycle but is absent in the resting (G0) cell.

Results

TUNEL staining was observed in 25 (92.6%) specimens of cataractous lenses, whereas cells undergoing apoptosis were identified in 2 (8%) of the epithelia from non-catarac-tous lenses. Only two MIB1-positive samples were detected, one of which was a capsule obtained during intracapsular cataract extraction.

Conclusions

The epithelium of human lenses with cortical cataract undergoes low rate apoptotic death. This limited epithelial apoptosis is unlikely to result in any significant cell density decrease since epithelial gaps are likely to be replaced by cell proliferation at the germinative zone of the anterior lens capsule. Nevertheless, the accumulation of small-scale epithelial losses during lifetime may induce alterations in lens fiber formation and homeostasis and result in loss of lens transparency.

The lens growth process

The factors that regulate the size of organs to ensure that they fit within an organism are not well understood. A simple organ, the ocular lens serves as a useful model with which to tackle this problem. In many systems, considerable variance in the organ growth process is tolerable. This is almost certainly not the case in the lens, which in addition to fitting comfortably within the eyeball, must also be of the correct size and shape to focus light sharply onto the retina. Furthermore, the lens does not perform its optical function in isolation. Its growth, which continues throughout life, must therefore be coordinated with that of other tissues in the optical train. Here, we review the lens growth process in detail, from pioneering clinical investigations in the late nineteenth century to insights gleaned more recently in the course of cell and molecular studies. During embryonic development, the lens forms from an invagination of surface ectoderm. Consequently, the progenitor cell population is located at its surface and differentiated cells are confined to the interior. The interactions that regulate cell fate thus occur within the obligate ellipsoidal geometry of the lens. In this context, mathematical models are particularly appropriate tools with which to examine the growth process. In addition to identifying key growth determinants, such models constitute a framework for integrating cell biological and optical data, helping clarify the relationship between gene expression in the lens and image quality at the retinal plane.

Uveal Spindle Cell Tumor of Blue-Eyed Dogs: An Immunohistochemical Study

Immunohistochemical techniques were used to investigate the origin of a spindle cell tumor in the anterior uveal tract of dogs and the influence of ultraviolet radiation on the development of this tumor. Thirteen tumors were identified from the 4,007 canine ocular samples examined at the Comparative Ocular Pathology Laboratory of Wisconsin between 1978 and 2005. Siberian Husky and Siberian Husky mix dogs were overrepresented (10/13 dogs, overall median age 10 years). Light microscopic evaluation (all dogs) and electron microscopy (2 dogs) were performed. Immunohistochemical staining included alpha-smooth muscle actin (SMA), vimentin, S-100, desmin, glial fibrillary acidic protein (GFAP), Melan A, microphthalmic transcription factor (MITF-1), protein gene product 9.5 (PGP 9.5), laminin, gadd45, p53, proliferating cell nuclear antigen (PCNA), anti-UVssDNA (antibody for detection of (6–4)-dipyrimidine photoproducts), and telomerase reverse transcriptase (TERT). All tumors occurred in the iris with or without ciliary body involvement and were composed of spindle cells arranged in fascicles and whorls (variable Antoni A and B behavior). All tumors were positive when immunostained for vimentin and S-100. Nine of 13 tumors exhibited GFAP immunopositivity. All tumors were negative for SMA, desmin, Melan A, and MITF-1. Tumors were variably positive for PGP 9.5, laminin, gadd45, p53, PCNA, anti-UVssDNA, and TERT. Electron microscopy revealed intermittent basal laminae between cells. These tumors are morphologically and immunohistochemically most consistent with schwannoma. The relationship between spindle cell tumors of the anterior uvea of dogs, altered neural crest, blue iris color, and ultraviolet radiation has not yet been fully elucidated.

Morphological and proliferative studies on ex vivo cultured human anterior lens epithelial cells - relevance to capsular opacification

PURPOSE

To determine the structural characteristics of lens epithelial cells (LECs) found on the anterior portion of the lens capsule and their pluripotency, proliferating and migrating potential when grown ex vivo with relevance to posterior capsular opacification (PCO) after cataract surgery.

METHODS

The explants of anterior portion of the lens capsule consisting of monolayer of LECs were obtained from uneventful cataract surgery and were cultivated under adherent conditions. The size and shape of the outgrowing cells were recorded by scanning electron microscopy (SEM), while their migration and proliferation potential were followed using light microscopy. Positivity for proliferation (Ki-67)- and pluripotency (Sox2)-specific markers were tested by immunofluorescent staining.

RESULTS

The proliferation and migration of anterior portion of the lens capsule's LECs filling up the denuded and reverse side regions of the lens capsule as well as their growth on glass culture surfaces could be followed by light microscopy and SEM, while the distribution of LECs and their morphology could be analysed in detail by SEM. The expression of Ki-67 and Sox2 in LECs growing adherently on human anterior portion of the lens capsule could also be detected.

CONCLUSIONS

Classic light microscopy and SEM can be used to show that human anterior portion of the lens capsule harbours LECs that can proliferate and migrate, suggesting their pluripotency or putative stem cell nature. Similarly, morphological techniques can be used to study PCO and the effect different drugs or physical treatments have against PCO development.

Cytomorphometric study of epithelial cells in normal and cataractous human lenses in relation with hyperglycemia

The aim of the study is to evaluate and correlate the morphology and cell density of epithelial cells adhering to lens capsule surgically removed from the anterior central region with lens clarity and type of cataract present in patients with or without type 2 diabetes. Capsulorhexis specimens were obtained from patients who had undergone phacoemulsification cataract surgery. All the samples were centrifuged and stained by the aid of Papanicolaou technique and were observed under light microscope. We determinated the mean cell density, the degree of epithelial damage, and morphological indicators of cells such as cell area and the nucleus-plasma ratio. Patients with cataract demonstrated a statistical significant decrease in cell density and an heterogeneous cell picture in which enlarged cells dominated. In addition, type 2 diabetics with cataract had a significantly even lower mean epithelial cell density by the presence of larger cell area with smaller nucleus-plasma ratio. More pronounced alterations in the lens epithelium were correlated not only with the presence of cortical cataract, increased fasting blood sugar, and increased HbA1c but also with the prolonged duration of diabetes and the co-existence of diabetic retinopathy. It seems that density and morphology of the anterior lens epithelial cells determine the lens epithelium damage which is more profound in hyperglycemia and in cortical cataracts. The changes in lens epithelium seem to play an important role in cataractogenesis.

Gender and cataract--the role of estrogen

There is evidence from epidemiologic data that cataract is more common in women than men. This is not solely due to a higher rate of cataract extraction in women, as is the case in the western world, but several population-based studies show that females have a higher prevalence of lens opacities, especially cortical. There is no firm evidence that lifestyle-related factors are the cause of this gender discrepancy. Focus has therefore been directed towards the role of estrogen in cataract formation. Although data on endogenous and exogenous estrogen involvement in cataractogenesis are conflicting, some studies have indicated that hormone therapy may decrease the risk of cataract and thus be protective. It has been hypothesized that the decrease in estrogen at menopause cause increased risk of cataract in women, i.e. not strictly the concentration of estrogen, but more the withdrawal effect. Estrogens are known to exert several anti-aging effects that may explain the longer lifespan in women, including metabolically beneficial effects, neuroprotection, preservation of telomeres and anti-oxidative properties. Since oxidative stress is considered important in cataractogenesis, studies have investigated the effects of estrogens on lens epithelial cells in culture or in animal models. Several investigators have found protection by physiological concentrations of 17β-estradiol against oxidative stress induced by H2O2 in cultured lens epithelial cells. Although both main types of estrogen receptors, ERα and ERβ, have been demonstrated in lens epithelium, most studies so far indicate that the estrogen-mediated protection in the lens is exerted through non-genomic, i.e. receptor-independent mechanisms, possibly through phosphorylation of extracellular signal-regulated kinase (ERK1/ERK2), a member of the mitogen-activated protein kinase (MAPK)-signaling pathway. Further studies are needed, both epidemiologic as to the role of hormone therapies, and laboratory studies regarding molecular estrogen-mediated mechanisms, in order to comprehend the role of estrogens on cataract formation.

Sustained-release celecoxib from incubated acrylic intraocular lenses suppresses lens epithelial cell growth in an ex vivo model of posterior capsule opacity

PURPOSE

To determine whether celecoxib (CXB) can be released from incubated intraocular lenses (IOLs) sufficiently to inhibit lens epithelial cell (LEC) growth in an ex vivo model of posterior capsule opacification (PCO).

MATERIALS

LEC growth was evaluated for 14 days in canine lens capsules (LCs) that had been exposed to media containing 20 μM CXB for 1-5 days. After the incubation of hydrophilic and hydrophobic IOLs in CXB solution, the determination of the in vitro release of CXB from the IOLs was performed for up to 28 days. The incubated and nonincubated IOLs were evaluated in the ex vivo model of PCO, and the rate of LEC growth was evaluated over 28 days.

RESULTS

The treatment of LCs with 20 μM CXB for 4 and 5 days completely inhibited LEC growth. LEC repopulation did not occur after the removal of CXB. IOLs incubated in CXB for 24 h resulted in a sustained release of CXB in vitro at levels theoretically sufficient to inhibit PCO. LCs in the ex vivo model of PCO treated with acrylic IOLs incubated in CXB had significantly suppressed LEC ingrowth compared with untreated and IOL-only LCs.

CONCLUSIONS

A 4-day treatment of LCs with a concentration of 20 μM CXB may effectively prevent PCO. IOLs incubated in CXB for 24 h resulted in a sustained release of CXB in vitro at levels sufficient to inhibit LEC growth in the ex vivo model of PCO. Further studies are needed to determine whether CXB-incubated IOLs can effectively prevent the development of PCO in vivo.

Coevolution of telomerase activity and body mass in mammals: from mice to beavers

Telomerase is repressed in the majority of human somatic tissues. As a result human somatic cells undergo replicative senescence, which plays an important role in suppressing tumorigenesis, and at the same time contributes to the process of aging. Repression of somatic telomerase activity is not a universal phenomenon among mammals. Mice, for example, express telomerase in somatic tissues, and mouse cells are immortal when cultured at physiological oxygen concentration. What is the status of telomerase in other animals, beyond human and laboratory mouse, and why do some species evolve repression of telomerase activity while others do not? Here we discuss the data on telomere biology in various mammalian species, and a recent study of telomerase activity in a large collection of wild rodent species, which showed that telomerase activity coevolves with body mass, but not lifespan. Large rodents repress telomerase activity, while small rodents maintain high levels of telomerase activity in somatic cells. We discuss a model that large body mass presents an increased cancer risk, which drives the evolution of telomerase suppression and replicative senescence.

Ultraviolet irradiation up-regulates telomerase transcription and activity in lens epithelial cells

PURPOSE

Ultraviolet irradiation (UVR) increases telomerase activity in various cell types including skin, a sun-exposed organ. The lens is also continually exposed to UVR and we hypothesized that lenses exposed to UVR would have increased telomerase activity, with up-regulated TERT and TR, the two main components of the telomerase holoenzyme. To evaluate whether the cornea would protect lenses from such changes, whole globes, as well as isolated lenses, were exposed to UVR, and lenses were evaluated for changes in telomerase activity.

METHODS

There were three parts to this project. The first part of this experiment evaluated freshly harvested normal adult canine lenses exposed to 0, 300, 600, or 1200 J/m(2) UVR, and then allowed to recover for 1, 2, 3 and 4 h. Since only 600 J/m(2) UVR increased telomerase activity, four more postexposure recovery time-points for this UVR dose were evaluated: 10 min, 30 min, 8 h and 24 h. The second part of this experiment used freshly enucleated whole canine globes exposed to 0, 50, 100, 150, 300, 600 or 1200 J/m(2) and incubated overnight; lens epithelial cells (LEC) were evaluated for telomerase activity. The third part evaluated lenses that were exposed to 0 or 600 J/m(2) UVR, and then allowed to recover for 8 and 24 h, before TERT and TR mRNA levels were measured.

RESULTS

Isolated lenses exposed to 600 J/m(2) UVR had significantly higher telomerase activity than unexposed controls and other UVR doses, at all time-points except 24 h postexposure. Lenses from whole globes exposed to UVR showed a dose-dependent increase in telomerase activity except at 50 J/m(2) and 1200 J/m(2). Isolated lenses exposed to 600 J/m(2) UVR and then allowed to recover for 8 and 24 h significantly up-regulated TERT and TR mRNAs compared to unexposed control lenses.

CONCLUSIONS

Telomerase activity is regulated at both the transcriptional and post-translational levels in canine LEC. Previous work in our laboratory showed dose, time, and age-dependent changes in telomerase activity in the lens. The present study showed that TERT and TR mRNA transcription was increased for up to 24 h following an acute dose of UVR. Both telomerase activity and TERT and TR mRNA levels were elevated until 24 h post-UVR exposure, TERT in combination with TR functions in proliferation-related telomerase activity, but TERT alone has an anti-apoptotic function and its up-regulation may protect LEC from the acute effects of UVR. We are continuing to evaluate the mechanisms by which telomerase is regulated in normal and cataractous LEC.

A hierarchy of proliferative cells exists in mouse lens epithelium: implications for lens maintenance

PURPOSE

To determine the distribution of slow-cycling cells, which are detected as label-retaining cells (LRCs), in mouse lens epithelium during postnatal development.

METHODS

Pregnant BALB/c mice were injected intraperitoneally (twice daily) with tritiated thymidine (3H-TdR), beginning at 17 days of gestation until birth. At birth, the in utero-labeled neonatal mice were injected subcutaneously with 3H-TdR (twice daily) for 3 days. Mice were killed weekly for the first month and then at 3-week intervals up to 18.5 weeks (chase periods). Eyes were removed and processed for autoradiography. In living mice, small scrape wounds were made on the anterior surface of the lens of mice that had been "chased" for 18.5 weeks. Twenty-four hours later, wounded mice received a single injection of BrdU.

RESULTS

Immediately after the in utero/postnatal labeling period, 100% of the lens epithelial cells incorporated 3H-TdR, and all were heavily labeled. With time, the number of LRCs declined so that only 13% of the lens epithelial cells were labeled at 18.5 weeks. At this time the heaviest labeled cells were exclusively found in the central zone and represented 2% to 3% of the total LRCs. In contrast, lightly labeled cells were found in both the central and germinative zones. After wounding, the heavily labeled LRCs incorporated BrdU, indicating that these cells were healthy and could be recruited to proliferate.

CONCLUSIONS

The heavily labeled LRCs, located exclusively in the central region, represent cells that divide very infrequently during homeostasis (putative stem cells); on perturbation, these cells can proliferate. The lightly labeled LRCs, located in the central and germinative zones, cycle more frequently than the heavily labeled ones. These LRCs may be phenotypically indistinguishable from stem cells and maintain the normal proliferative needs of the lens. A third population of actively cycling cells exists primarily in the germinative zone and represents the transit amplifying cells, which have a limited proliferative potential.

Development of an accommodating intra-ocular lens--in vitro prevention of re-growth of pig and rabbit lens capsule epithelial cells

Cataract surgery is routinely performed to replace the clouded lens by a rigid polymeric intra-ocular lens unable to accommodate. By implanting a silicone gel into an intact capsular bag the accommodating properties of the natural lens can be maintained or enhanced. The implantation success of accommodating lenses is hampered by the occurrence of capsular opacification (PCO) due to lens epithelial cell (LEC) growth. In order to prevent LEC proliferation, a treatment regime using actinomycin D, cycloheximide and water was developed. The effectiveness of treatment was analyzed using an in vitro, MTT-based cell culture system and an ex vivo pig eye model in which the implanted lens-in-the-bag is cultured as a whole. LEC were exposed to treatment solutions for 5 min, then the cells were allowed to recover and to re-colonize the substratum. MTT conversion by cells was transiently inhibited by cycloheximide dissolved in water and by water alone. Exposure to actinomycin D resulted in a lasting inhibition of MTT conversion and consequently cell proliferation. These in vitro data could not be fully reproduced in the ex vivo pig eye model due to essential differences between both models. Treatment with actinomycin D containing solutions, however, resulted in a nearly complete absence of cells on the capsular wall. The pig eye model is a promising approach to further evaluate the effects of peri-surgical treatment during the accommodating intra-ocular lens implantation.

Telomerase activity with concurrent loss of cell cycle regulation in feline post-traumatic ocular sarcomas

Paraffin wax-embedded ocular globes of cats with post-traumatic ocular sarcomas were examined for the presence of TERT, the active subunit of telomerase. The latter is a ribonucleoprotein complex essential for immortalization and expressed by most malignant tumours, germ line cells, lens epithelial cells, and some stem cells. Due to the frequent loss of cell cycle control with the increased expression of telomerase activity, post-traumatic ocular sarcomas were also examined for loss of p16 expression and alterations in p53, the findings being related to mitotic score, tumour grade, and proliferating cell nuclear antigen. These sarcomas expressed telomerase at a high frequency (62.5%); in addition, the majority showed alterations in cell cycle control, as evaluated by lack of p16 immunolabelling (66.7%). Alterations in p53 were the sole mechanism by which cell cycle control was dysregulated in only two tumours expressing TERT (13%). These findings suggest that p16, and not p53, represents the primary mechanism by which post-traumatic ocular sarcomas that express telomerase activity escape cell cycle control.

The effects of oxidative stress on telomerase activity and other stress-related proteins in lens epithelial cells

Telomerase is a ribonucleoprotein complex responsible for maintaining the ends of chromosomes and for repair of DNA strand breaks. While telomerase activity is generally found in cells that have unlimited proliferative potential such as neoplastic cells, germline cells and some stem cells, lens epithelial cells (though not highly proliferative) have telomerase activity. Our previous studies indicated that lens epithelial cells express high levels of telomerase despite their low proliferative potential, thus we hypothesized that telomerase expression protects lens epithelial cells from oxidative stress. We also determined levels of the stress proteins gadd45 and p16 and the stress and proliferation-related protein, proliferating cell nuclear antigen (PCNA). In acute studies, lenses were exposed to TBHP for 0-120 min. In recovery studies, lenses were exposed to TBHP for 1 hr, then allowed to recover for up to 18 hr. In acute studies, telomerase activity was increased, p16 initially decreased then normalized, PCNA levels did not change significantly even in the overnight recovery groups, and gadd45 was decreased in some TBHP exposed groups. In recovery studies, telomerase activity was increased in all groups, gadd45 decreased then became elevated, and p16 levels were decreased at later recovery times. PCNA levels remained constant during the studies, indicating that there was no change in proliferation. These studies showed that elevated telomerase activity did not correlate with increased proliferation in lens epithelial cells; instead, increased telomerase activity was associated with increased levels of the stress-related protein gadd45 only in the later recovery times. These findings support the hypothesis that telomerase plays a protective rather than a proliferative role in lens epithelial cells.

Molecular cloning of the canine telomerase reverse transcriptase gene and its expression in neoplastic and non-neoplastic cells

OBJECTIVE

To perform molecular cloning of the canine telomerase reverse transcriptase (TERT) gene and determine its expression in neoplastic and non-neoplastic cells.

SAMPLE POPULATION

9 canine tumor cell lines derived from various neoplasms, 16 primary canine tumors, and tissues from 15 normal canine organs.

PROCEDURE

Tumor cell lines were derived from canine tumors that included osteosarcoma, mammary gland adenocarcinoma, melanoma, acute lymphoblastic leukemia, lymphoma, and mastocytoma and a canine primary fibroblast culture. Canine TERT complementary DNA (cDNA) was amplified by use of polymerase chain reaction (PCR) and sequenced. Expression of TERT mRNA was examined by reverse transcription (RT)-PCR assay. Telomerase activity was measured by use of the telomeric repeat amplification protocol assay.

RESULTS

The canine TERT cDNA clone was 237 base pairs in length and contained a central region encoding the reverse transcriptase motif 2. Expression of TERT mRNA was detected in canine tumor cell lines that had telomerase activity but not in telomerase-negative canine primary fibroblasts. The TERT mRNA was detected in 13 of 16 canine tumor tissues and several normal tissues such as liver, ovary, lymph node, and thymus. A significant correlation between TERT expression level and telomerase activity was noted.

CONCLUSIONS AND CLINICAL RELEVANCE

Expression of TERT mRNA was closely associated with telomerase activity in neoplastic cells as well as some non-neoplastic cells from dogs. In addition to telomerase activity, expression of TERT mRNA can be used as a marker of tumor cells.

Telomerase: a potential diagnostic and therapeutic tool in canine oncology

In recent years there has been considerable interest in telomerase as a target for therapeutic intervention in oncology. This largely stems from the vast number of studies that have demonstrated expression and activity of the enzyme telomerase in the majority of human cancer tissues with little or no activity detectable in normal somatic tissues. These studies have led to an interest in the role of telomerase in cancers associated with domesticated species, in particular tumors that affect dogs. This article reviews the biology of telomerase and the biological significance of telomerase activity in canine tumors and discusses the clinical implications of telomerase expression in canine cancers with regard to therapeutics and diagnostics.

Age-related telomere shortening occurs in lens epithelium from old rats and is slowed by caloric restriction

We have investigated whether the average relative telomere length of lens epithelial cells (LECs) from brown Norway rats decreases with the age of the donor animal, and whether chronic caloric restriction (CR) of the rats delays the telomere shortening. Our previous studies have demonstrated that clonal proliferative potential of rodent LECs as well as the in vivo rate of DNA synthesis decreases with age and that this decrease is slowed by chronic lifelong caloric restriction (CR). In order to determine if telomeric shortening might be involved in this loss of proliferative potential, we examined relative telomeric lengths in young, old ad lib fed (AL), and old calorically restricted (CR) brown Norway rats. We used fluorescence in situ hybridization with a peptide nucleic acid probe (PNA) complementary to the telomeric repeat sequence to quantitate relative telomere lengths in LECs in lens sections (TELO-FISH). Control experiments demonstrated that the PNA probe binding was restricted almost entirely to the terminal portions of the rat chromosomes with less than 5% bound at interstitial sites in typical metaphase spreads. The relative telomere lengths of interphase human fibroblast standards, as determined by TELO-FISH, were in good agreement with terminal restriction fragment analyses of the same standards and with literature values for rat cells. The average telomere lengths of interphase nuclei in the old AL rat LECs were found to be 21% shorter than paired young AL controls (P < 0.01 by Wilcoxian signed rank test). The calorically restricted old rats had less telomere erosion (12%) than the old AL group (P < 0.05). Although it is not clear whether such moderate telomeric erosion can limit cell division in rodent LECs, the telomeric shortening correlated well with previous studies demonstrating reduced clonal, replicative potential, and reduced rates of in vivo DNA replication in LECs from old rodents and a delay in this attenuation in animals on chronic CR.

Ultraviolet irradiation- and dimethyl sulfoxide-induced telomerase activity in ovarian epithelial cell lines

Information about telomerase regulation is incomplete, especially since various studies suggest complexity in telomerase regulation. Given the important association between telomerase and cancer, it is imperative to design and develop a model system in which telomerase activity can be regulated and studied. We employed ultraviolet (UV) radiation or dimethyl sulfoxide (DMSO) to transiently induce telomerase activity in a telomerase-positive cell line and, most importantly, in a telomerase-negative cell line. UV- or DMSO-induced telomerase activity was associated with increased hTRT, but not hTR, mRNA transcription in the telomerase-negative cells. However, no changes in hTRT or hTR mRNA transcription were noted with UV- or DMSO-induced telomerase activity in the telomerase-positive cells. Inhibition of protein synthesis or the phosphotidyl inositol 3-kinase (PI3K) pathway suppressed telomerase induction and/or activity in all cell lines examined, suggesting telomerase activity was dependent on protein synthesis and PI3K-mediated phosphorylation. Furthermore, enhanced telomerase activity was limited to UV and DMSO, since a variety of chemotherapeutic agents failed to induce telomerase activity. Therefore, our data provide a useful culture model system to study telomerase regulation in telomerase-negative and -positive cell lines and from which to obtain information about telomerase as a target for cancer intervention.