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Swarup S. Swaminathan, Dong-Jin Oh, Min Hyung Kang, Haiyan Gong, Douglas J. Rhee; Modulation of Aqueous Humor Resistance and Outflow by SPARC. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6620.
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To determine the role of Secreted Protein Acidic and Rich in Cysteine (SPARC), a matricellular protein known to modulate extracellular matrix (ECM) content, on the aqueous outflow pattern in the trabecular meshwork (TM) of the mouse.
The eyes of wild-type (WT, N=14) and SPARC-knockout (KO, N=14) mice between 5 and 8 weeks old were studied. IOP was measured between 4 and 7 minutes with the TonoLab rebound tonometer under general anesthesia. 1-uL of 1:50 solution containing 20-nm fluorescent tracer microbeads was then injected transcorneally through a 35G needle into the anterior chamber at a rate of 4 nL/s using a microsyringe pump. Lubricating drops were applied to the eyes to prevent dehydration. The mice were sacrificed by anesthetic overdose and the eyes enucleated 45 min after the tracer injection. Flat mounts of the eyes were prepared, and fluorescence microscopy was used to evaluate the microbead distribution in the TM. Images from all eye sections were used to quantify the total length (TL) of the TM, the tracer-decorated length (L) of the TM, and the percent effective filtration length (PEFL = L/TL) from multiple images.
The IOP was significantly lower in KO mice (12.6 ± 2.5 mm Hg, p<0.005) compared to WT mice (16.3 ± 2.4 mm Hg). Segmental outflow, or non-continuous circumferential flow, was observed in the WT mice. However, continuous outflow was observed in SPARC KO mice. PEFL was significantly increased in SPARC KO mice (70.8% ± 11.9%, p<0.0005) compared to WT mice (54.7% ± 9.0%). A significant negative correlation was found between PEFL and IOP (R² = 0.6, N=11 pairs). When one outlier was identified and removed, R² increased to 0.73.
We demonstrate a significant increase in PEFL in SPARC KO mice, with an overall change from segmental to continuous outflow. Furthermore, we re-confirm a lower IOP for SPARC KO mice (1). Our data demonstrate that PEFL is lower in mice with higher IOPs. The significant increase in outflow area in SPARC KO mice supports the hypothesis that SPARC is an important modulator of aqueous humor outflow, and thus IOP.1. Haddadin R, et al. IOVS. 2009; 50(8): 3771-7.
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