Colitz C M, Davidson M G, McGAHAN M C
Companion Animal and Special Species Medicine, College of Veterinary Medicine, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC, 27606, USA.
Exp Eye Res. 1999 Dec;69(6):641-9. doi: 10.1006/exer.1999.0739.
Telomerase is a ribonucleoprotein responsible for maintaining telomere length, preventing chromosomal degradation and recombination, and repairing DNA strand breaks. These activities are believed to be important in preventing cell senescence. Telomerase activity is normally found in germinal, neoplastic and stem cells, but not any ocular tissue studied to date. The epithelium of the crystalline lens is comprised of a population of cells with diverse mitotic potential including the germinative epithelium which contains cells with the potential for unlimited replicative capacity, equatorial cells which terminally differentiate into lens fibers, and the central epithelium which are considered to be quiescent and nonreplicative under normal circumstances. We speculated that the germinative region of lens epithelial cells might have telomerase activity, and that dysregulation of its activity might be associated with cataractogenesis. We investigated these hypotheses in lens capsule specimens from normal and cataractous dogs and from cultures of canine lens epithelial cells using standard assays for telomerase activity and telomere length. Telomerase activity was found in normal canine lens epithelial cells in the central, germinative and equatorial regions of the anterior lens capsule at equivalent levels. Similar findings were made in feline and murine lens epithelial cells, indicating that the presence of telomerase activity in the lens was not species specific. Lens fiber cells, corneal epithelium and endothelium and nonpigmented ciliary epithelium were telomerase negative. Telomerase activity and telomere lengths were significantly greater in lens epithelia from cataractous lenses when compared with normal lenses. Since telomerase activity is associated with an immortal phenotype, the presence of telomerase activity in the lens epithelial cells may function to prevent conversion to senescence. It was, therefore, difficult to explain why these cells cannot be passaged more than four times in culture. We found that telomerase activity and telomere lengths gradually decreased with increased passages until telomerase activity was no longer present at passage two. Consistent with these findings, there were no senescent cells present on the lens capsule when the lens was initially dissected for culture, but an increasing number of cells were senescent with each passage, correlating well with the loss of telomerase activity. Telomerase activity is likely important in the germinative epithelium to maintain its proliferative potential and prevent cell senescence. Telomerase may function in the quiescent, central lens to maintain telomeres damaged by oxidative stress and ultraviolet light exposure, thereby preventing accelerated loss of these elements which triggers cell senescence. It remains to be determined if the increase in telomerase activity in lens epithelial cells from cataractous lenses is a primary dysregulation that may have a role in the development of the cataract, or is secondary to cataract formation.
端粒酶是一种核糖核蛋白，负责维持端粒长度，防止染色体降解和重组，并修复DNA链断裂。这些活动被认为对防止细胞衰老很重要。端粒酶活性通常存在于生殖细胞、肿瘤细胞和干细胞中，但迄今为止在任何研究过的眼组织中都未发现。晶状体上皮由一群具有不同有丝分裂潜能的细胞组成，包括具有无限复制能力的生发上皮、终末分化为晶状体纤维的赤道细胞，以及在正常情况下被认为是静止且不复制的中央上皮。我们推测晶状体上皮细胞的生发区域可能具有端粒酶活性，其活性失调可能与白内障的发生有关。我们使用端粒酶活性和端粒长度的标准检测方法，对正常和白内障犬的晶状体囊膜标本以及犬晶状体上皮细胞培养物进行了研究，以验证这些假设。在前晶状体囊膜的中央、生发和赤道区域的正常犬晶状体上皮细胞中发现了同等水平的端粒酶活性。在猫和小鼠的晶状体上皮细胞中也有类似发现，表明晶状体中端粒酶活性的存在并非物种特异性。晶状体纤维细胞、角膜上皮和内皮以及无色素睫状体上皮均为端粒酶阴性。与正常晶状体相比，白内障晶状体的晶状体上皮中端粒酶活性和端粒长度显著更高。由于端粒酶活性与永生表型相关，晶状体上皮细胞中端粒酶活性的存在可能起到防止细胞衰老转化的作用。因此，很难解释为什么这些细胞在培养中传代不能超过四次。我们发现，随着传代次数增加，端粒酶活性和端粒长度逐渐降低，直到传代两次时端粒酶活性不再存在。与这些发现一致的是，在最初解剖用于培养的晶状体囊膜上没有衰老细胞，但随着每次传代，衰老细胞数量增加，这与端粒酶活性的丧失密切相关。端粒酶活性可能在生发上皮中很重要，以维持其增殖潜能并防止细胞衰老。端粒酶可能在静止的晶状体中央发挥作用，以维持因氧化应激和紫外线照射而受损的端粒，从而防止这些引发细胞衰老的元件加速丢失。晶状体上皮细胞中端粒酶活性的增加是可能在白内障发生中起作用的原发性失调，还是白内障形成的继发性结果，仍有待确定。
