pH-dependent changes in interphotoreceptor matrix domains surrounding cone photoreceptors

The cone matrix sheath is a biochemically and structurally distinct domain of the retinal interphotoreceptor matrix (IPM). In the present study, the insoluble components of the cone matrix sheath were extracted from bovine retina, and examined for pH-dependent morphological changes by light and electron microscopy. After soaking unfixed retinas in 25 mM HEPES-buffered solution (pH 2.0), sheets of aqueous-insoluble IPM containing cone matrix sheaths were physically dissociated from the inner retina. The insoluble IPM preparations were treated with 1.0% Triton X-100 to extract contaminating photoreceptor outer segment membrane, and exposed to HEPES-buffered solutions at pH 2.0, 4.5, 7.0, or 9.5. The cone matrix sheath, specifically stained with peanut agglutinin, appeared finely granular at pH 2.0, but was markedly condensed at pH 4.5. Cone sheath morphology became dispersed and appeared as a network of thick fibrils at pH 7.0, and further dispersed fibrillar and granular elements at pH 9.5. These findings suggest that pH changes in the interphotoreceptor space can alter the molecular organization of the IPM and influence the function of the IPM in the transport of nutrients and metabolites, and in retinal adhesion.