Reduced TIGR/myocilin protein in the monkey ciliary muscle after topical prostaglandin F_2α treatment

Purpose. Mutations in the trabecular meshwork inducible glucocorticoid response (TIGR) gene, also known as myocilin, have recently been linked to some forms of glaucoma. Recent studies have shown that TIGR protein also is expressed in the ciliary muscle. Because uveoscleral outflow, which traverses the ciliary muscle, is increased by prostaglandins (PGs), the present study assessed whether topical PGs alter the amount of TIGR protein within the ciliary muscle. Methods. Vehicle was topically applied to one eye, and 2 μg PGF_2α-isopropyl ester (PGF_2α-IE) was applied to the other eye of cynomolgus monkeys twice daily for 5 days. Pressure reductions of 5 mm Hg in the PGF_2α-IE-treated eyes were confirmed. The eyes were then fixed and paraffin sections were cut from each eye. The distribution of TIGR protein in the ciliary muscle was determined by confocal scanning laser microscopy. Additional sections were immunostained with a polyclonal antibody to recombinant TIGR protein or with a polyclonal antibody to a synthetic peptide corresponding to the leucine zipper region within the TIGR protein. Staining intensity in the ciliary muscle was assessed by measuring optical density (OD) along two line segments overlying the ciliary muscle, by using a high-resolution imaging densitometer. Results. TIGR protein immunoreactivity was observed in ciliary muscle fibers throughout the ciliary muscle. Extracellular TIGR immunoreactivity colocalized with collagen type IV immunoreactivity. Intracellular staining also was present. Immunoreactivity was less intense in the sections from the PGF_2aα-IE-treated eyes compared with the vehicle-treated eyes. This was reflected in the reduction of mean OD scores in each monkey. Overall, the reduction of mean OD scores in the treated eyes was 42.1% ± 9.9% (P < 0.005) with the anti-recombinant TIGR antibody and 27.3% ± 10.4% with the anti-TIGR peptide antibody (P < 0.005). Conclusions. TIGR protein immunoreactivity was present both intracellularly and extracellularly in the ciliary muscle of the cynomolgus monkey. This suggests that extracellular TIGR protein is in contact with aqueous humor in the uveoscleral outflow pathway. Moreover, IOP-lowering topical PGF_2α-IE treatment decreases the amount of TIGR protein in the ciliary muscle.