Abstract
Cells in the trabecular meshwork (TM), a tissue responsible for draining aqueous humor out of the eye, are known to be highly phagocytic. Phagocytic activity in TM cells is thought to play an important role in outflow pathway physiology. However, the molecular mechanisms triggered by phagocytosis in TM cells are unknown. Here we investigated the effects of chronic phagocytic stress on lysosomal function using different phagocytic ligands (E. coli, carboxylated beads, collagen I-coated beads, and pigment). Lysotracker red co-localization and electron micrographs showed the maturation of E. coli- and collagen I-coated beads-containing phagosomes into phagolysosomes. Maturation of phagosomes into phagolysosomes was not observed with carboxylated beads or pigment particles. In addition, phagocytosis of E. coli and collagen I-coated beads led to increased lysosomal mass, and the specific up-regulation and activity of cathepsin B (CTSB). Higher levels of membrane-bound and secreted CTSB were also detected. Moreover, in vivo zymography showed the intralysosomal degradation of ECM components associated with active CTSB, as well as an overall increased gelatinolytic activity in phagocytically challenged TM cells. This increased gelatinolytic activity with phagocytosis was partially blocked with an intracellular CTSB inhibitor. Altogether, these results suggest a potential role of phagocytosis in outflow pathway tissue homeostasis through the up-regulation and/or proteolytic activation of extracellular matrix remodeling genes.
Highlights
The trabecular meshwork (TM) is a tiny tissue located in the anterior segment of the eye, between the cornea and the sclera, which is involved in maintaining proper levels of intraocular pressure (IOP)
PTM cells exposed to phagocytic challenge to either FITC-labeled E. coli or Fluoresbrite® Blue (BB) carboxylate microspheres for three days were incubated with 100 nM lysotracker red (LTR) in fresh media for one hour at 37 oC, 5% CO2
Our results indicate that phagocytic challenge of porcine TM cells to E. coli up-regulates the expression of cathepsin B (CTSB)
Summary
The trabecular meshwork (TM) is a tiny tissue located in the anterior segment of the eye, between the cornea and the sclera, which is involved in maintaining proper levels of intraocular pressure (IOP). It has been historically believed that MMPs are the major proteases involved in extracellular matrix (ECM) degradation, novel research data seem to contradict this central dogma and suggest that, while MMPs might play a critical regulatory role in ECM metabolism, other proteases or the coordinated action of several types of proteases are responsible for the bulk matrix degradation [20]. According to their catalytic mechanisms, proteases are classified into six different groups: serine proteases, threonine proteases, cysteine proteases, aspartate proteases, glutamic acid proteases, and metalloproteinases [21]. Our data support a novel role of phagocytic function in TM tissue homeostasis
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