The fundamental abnormality occurring in the conventional outflow pathway associated with elevated intraocular pressure and therefore, increased risk of developing glaucoma, an age-related disease affecting more than 70 million people world wide still remains obscure. However, data from several laboratories, including ours, support a key role of reactive oxygen species, both present in the aqueous humor as well as generated during the normal aging process within the outflow pathway, in the pathogenesis of glaucoma. Autophagy, a lysosomal pathway responsible for the degradation of long-lived proteins and organelles, has emerged as an important cellular homeostatic mechanism that is part of the early protective cellular response against oxidative stress. A general decline in autophagic activity has been observed in several tissues with aging and in age-related disorders. A corollary question is whether autophagy function declines with age in the outflow pathway tissue, and if so, whether this could contribute to the susceptibility to disease. Our preliminary data show that exposure of primary cultures of trabecular meshwork (TM) cells to chronic oxidative stress causes profound changes in the lysosomal degradative pathway, including: (1) Increased lysosomal mass and lysosomal enzymes protein content, (2) increased autophagic vacuoles content, (3) upregulated levels of LC3-II, (4) accumulation of intralysosomal oxidized material and damaged mitochondria, and (5) decreased cathepsin activities. In addition, stressed cultures showed elevated senescence-associated- ?-galactosidase (SA-?-gal), a marker found to be also upregulated in the TM from glaucoma donors. We hypothesize that aging of the outflow pathway is accompanied by a decline in the autophagic degradative capacity, thus leading to the inefficient removal of oxidized components and to the intracellular accumulation of nonfunctional aberrant cellular components, which reduce the ability of TM cells to respond against additional stressors of the autophagic pathway (i.e. mutant myocilin, pigment) further compromises the autophagic cellular function, thus promoting the secretion of autophagolysosomes into the extracellular space, which can contribute to the abnormal deposition observed in glaucoma. To test this hypothesis, we will investigate (1) whether aging of TM cells is associated with a decrease in autophagic flux in vitro and in vivo; (2), whether the experimentally-induced decreased in autophagic capacity in TM cells results in the accumulation of damaged proteins and organelles, as well as extracellular matrix vesicles;and (3), whether autophagy dysfunction is associated with the acquisition of a glaucoma phenotype, including the presence of extracellular matrix vesicles, in human eyes and in established mice glaucoma models.
The objective of this grant proposal is to investigate whether aging of the outflow pathway tissue is associated with malfunction of the cellular machinery responsible for the degradation of waste material, thus resulting in accumulation of, so called, biological garbage. We believe that, similar to what has been described in other age-related diseases, including Alzheimer's disease, Parkinson's disease, and atherosclerosis, the accumulation of that waste material may negatively affect other essential cellular functions and thus contribute to the pathology of Primary Open Angle Glaucoma (POAG). Understanding the mechanisms underlying a decline in the degradative capacity with aging in the outflow pathway might open new avenues for therapeutic target strategies for POAG
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|Xu, Ping; Lin, Yizhi; Porter, Kristine et al. (2014) Ascorbic acid modulation of iron homeostasis and lysosomal function in trabecular meshwork cells. J Ocul Pharmacol Ther 30:246-53|
|Porter, Kristine; Nallathambi, Jeyabalan; Lin, Yizhi et al. (2013) Lysosomal basification and decreased autophagic flux in oxidatively stressed trabecular meshwork cells: implications for glaucoma pathogenesis. Autophagy 9:581-94|
|Porter, Kristine; Lin, Yizhi; Liton, Paloma B (2013) Cathepsin B is up-regulated and mediates extracellular matrix degradation in trabecular meshwork cells following phagocytic challenge. PLoS One 8:e68668|
|Porter, Kristine M; Epstein, David L; Liton, Paloma B (2012) Up-regulated expression of extracellular matrix remodeling genes in phagocytically challenged trabecular meshwork cells. PLoS One 7:e34792|