Changes of lysosomal enzymes in the ciliary epithelium by foreign bodies

Fluctuation of lysosomal phospholipase A2 in experimental autoimmune uveitis in rats

Intraocular inflammation leads to oxidative stress and may generate lipid oxidation products. The present study was conducted to elucidate the pathophysiological roles of the lysosomal phospholipase A2 (LPLA2), a phospholipid-degrading enzyme, and the production of oxidized phospholipids (oxPLs) in autoimmune uveitis using a rat model. Lewis rats were immunized with a bovine interphotoreceptor retinoid-binding protein (bIRBP) peptide with complete Freund's adjuvant (CFA) to induce experimental autoimmune uveitis (EAU). The aqueous humor (AH) and serum were collected every week for 4 weeks from the immunized rats. The LPLA2 activity of the AH and serum was detected using liposomes consisting of 1,2-dioleoylphosphatidylglycerol/N-acetylsphingosine as the substrate under acidic conditions. Immunohistochemical analysis was performed using antibodies against LPLA2 and oxPLs. The ocular inflammation was exacerbated at 2 weeks after immunization. The LPLA2 activity in the rat AH was increased by EAU induction, and was concomitant with the extent of inflammation in the anterior chamber (AC). In contrast, the LPLA2 activity in the rat serum was not influenced by EAU induction. At 2 weeks after immunization, immunoreactivity of LPLA2 was observed in infiltrated macrophages in the AC and vitreous cavity of the EAU rats. Furthermore, immunoreactivity of oxPLs was observed in the infiltrated macrophages of EAU rat eyes. These results demonstrated that the LPLA2 activity of the AH is augmented with the inflammation in the AC. The high expression of LPLA2 and production of oxPLs are found in the infiltrated macrophages in the acute inflammation of EAU rats. The present findings suggest the connection between LPLA2 activity and oxPL metabolism in the inflammation sites in the eye.

Augmentation of Lysosomal Phospholipase A2 Activity in the Anterior Chamber in Glaucoma

AIM

The lysosomal enzyme in the anterior chamber has a crucial role in the digestion of the insoluble materials in the aqueous humor (AH). The dysfunction of AH filtration in the trabecular meshwork (TM) causes increasing AH outflow resistance in the TM. Those insoluble objects, including phospholipids, should be digested in the TM for normal outflow. The present study was conducted to explore the involvement of lysosomal phospholipase A2 (LPLA2), a phospholipid-degrading enzyme, of the AH in glaucoma using clinical AH specimens.

MATERIALS AND METHODS

One hundred and twenty-five AH specimens were collected from patients. The measurement of LPLA2 activity in the AH was carried out using liposomes consisting of phosphatidylglycerol and N-acetylsphingosine (NAS). The correlation between the LPLA2 activity in the AH and ocular diseases was investigated.

RESULTS

The human AH showed both transacylation of NAS and the release of fatty acids under acidic conditions but not at a neutral pH, which is consistent with the known properties of LPLA2. The LPLA2 activity in the AH was not affected by age or systemic disease. A comparison between ocular diseases showed that the AH specimens obtained from patients with glaucoma had significantly higher LPLA2 activity than the other ocular disease groups.

DISCUSSION

The present findings suggest that the ascended level of LPLA2 activity in the AH of glaucoma patients is associated with the development of glaucoma.

The shape and distribution of lysosomes and endocytosis in the ciliary epithelial cells of rats

The shape and distribution of lysosomes in the ciliary epithelium of rat eyes were examined by electron microscopy combined with acid phosphatase (ACPase) cytochemistry and three-dimensional observation of 2 microns-thick sections. ACPase activity was cytochemically localized in lysosomes and trans Golgi cisternae in the non-pigmented epithelial (NPE) and pigmented epithelial (PE) cells. In NPE cells, it was shown three-dimensionally, that most lysosomes had an elongate form, up to 5 microns in length, and a diameter of 70-100 nm. These elongate lysosomes (nematolysosomes) were predominantly located in the basal region of the cells. In contrast, PE cells had spherical lysosomes distributed at random throughout the cytoplasm. However, no nematolysosomes were seen in the PE cells. When the isolated ciliary processes were incubated in a medium containing horseradish peroxidase (HRP), HRP was incorporated into the nematolysosome-like structures by pinocytosis from the basal surface of the NPE cells. These findings suggest that nematolysosomes are associated with the pinocytotic activity of NPE cells. The pinocytosis-nematolysosomal route may be involved in the uptake and degradation of macromolecules from the aqueous humor in the posterior chamber.

Histochemical survey of the anterior segment of the normal human foetal and adult eye

The distributions of the lysosomal enzymes [acid phosphatase (AP), N-acetyl-beta-D-glucosaminidase (NAG), beta-glucuronidase (beta-Gluc), beta-galactosidase (beta-Gal), dipeptidylpeptidase II (DPP II)] and of the membrane-bound proteases [aminopeptidase M (APM), aminopeptidase A (APA), gamma-glutamyltransferase (GGT), dipeptidylpeptidase IV (DPP IV)] were investigated in the normal human adult and foetal anterior segment by histochemical methods. The distribution of these hydrolases varied between ocular tissues. The most active enzymes in the adult corneal epithelium and endothelium were AP, beta-Gluc, NAG, beta-Gal and GGT; in the keratocytes, APM, APA, beta-Gluc and GGT predominated. The adult trabecular meshwork cells were stained by AP, beta-Gluc, NAG, APM, GGT, DPP II and DPP IV. The enzymes AP, beta-Gluc, APM and APA, however, displayed greater activity in the endothelium of Schlemm's canal. The adult ciliary epithelium stained strongly for all lysosomal hydrolases; GGT was the most active protease here. Differences in enzyme activity were noted in some tissues when foetal and adult anterior segments were compared. There appeared to be a decrease in the activity of some enzymes with age and post-mortem delay greater than 24 h. The function(s) of each enzyme and their possible roles in the respective tissues are discussed.

Acidic lens protein degrading activity in bovine ciliary body

Biochemical studies of acidic lens protein degrading activity in the bovine ciliary body were performed. The activity showed two peaks, Fractions A and B, on Sephadex G-75 column chromatography. Fraction A contained cathepsin D and thiol proteinase activities and was inhibited by pepstatin and leupeptin. Fraction B contained thiol proteinase activity and was inhibited by leupeptin. The proportion of peptide released from the lens protein by Fractions A and B was higher than those from bovine serum albumin and casein. Lens protein may be a good assay substrate for cathepsin D and lysosomal thiol proteinase in the ciliary body.

Lysosomal enzymes in ocular tissues and diseases

Lysosomal enzymes are distributed widely in various ocular tissues. Among these tissues, the uvea and retina show the higher enzyme activities of acid phosphates, beta-blucuronidase, alpha-fucosidase, alpha-mannosidase, arylsulfatase, cathepsin D, cathepsin B and others. The particular role of lysosomal enzymes in the pathogenic processes of ocular diseases such as storage disease, uveitis, retinal degeneration, retinal detachment, corneal dystrophy and glaucoma is strongly suggested. The enzymes also have additional importance in ocular physiopathology.