Utilization of metabolic energy in treatment of ocular surface disorders: polyphosphate as an energy source for corneal epithelial cell proliferation

Impaired regeneration of the corneal epithelium, as found in many ocular surface diseases, is a major clinical problem in ophthalmology. We hypothesized that corneal epithelial regeneration can be promoted by the physiological, energy-delivering as well as "morphogenetically active" polymer, inorganic polyphosphate (polyP). Corneal limbal explants (diameter, 4 mm) were cultivated on collagen-coated well plates in the absence or presence of polyP (chain length, similar to 40 P-i units; 50 mu g ml(-1)) or human platelet lysate (hp-lysate; 5% v/v). Cell outgrowth and differentiation were analyzed after staining with DRAQ5 (nuclei) and rhodamine phalloidin (cytoskeleton), as well as by environmental scanning electron microscopy (ESEM). Cell growth/viability of hCECs was assessed by XTT assay. The expression of SDF-1 was quantitated by qRT-PCR. Exposure to hp-lysate (also containing polyP) increased cell migration already at day 1. Even stronger was the effect of polyP. This effect was blocked by a mast cell serine protease. The formation of cell multilayers was enhanced by hp-lysate or even more by polyP. ESEM revealed continuous cell junctions and prominent microvilli on the surface of adjacent cells exposed to polyP; those structures were only rarely seen in the controls. The hp-lysate and, more potently, polyP increased the proliferation of hCECs, as well as SDF-1 expression. The findings indicate the potential usefulness of the natural polymer, polyP, for topical treatment of corneal epithelial defects. Future studies are directed to develop suitable formulations of polyP, such as biomimetic polyP nano/microparticles showing an adjustable release kinetics.