Our multi-disciplinary and multi-institutional application addresses dry eye syndromes from the perspective of the new human prosecretory mitogen 'lacritin' discovered by us. Lacritin is restrictively expressed in lacrimal and meibomian glands and in the cornea and conjunctiva, where it appears to be capable of protecting epithelia of the lacrimal-corneal axis against inflammation-associated cell death. Increased levels of proinflammatory cytokine TNFa in tears of dry eye patients is associated with damage to ocular surface epithelia. Indeed, adding TNFa to cultured human corneal epithelial cells promotes death via caspases-8 and -3. Recently we observed that caspase activation and death is completely prevented by inclusion of 10 nM lacritin. This observation has been reinforced by lacritin deletion analysis that reveals a cytoprotective site within lacritin's C-terminus. Lacritin's utilizes a unique cell targeting mechanism: heparanase unblocks a lacritin binding site within the N-terminal ectodomain of cell surface syndecan-1. Bound lacritin is then likely presented to a GPCR. Since heparanase has a lower pH optimum, it is possible that lacritin is protective against the hypothetical sudden low pH 'danger signal' thought to underlie the initiation of primary Sjogren's syndrome dry eye in some individuals. In rabbit preclinical studies, topical application of lacritin promotes increased tear flow for at least 4 hr without toxicity - even over 30 days of continuous treatment. In cell culture, lacritin stimulates tear secretion by lacrimal acinar cells - the same cells from which it is secreted. It also promotes corneal epithelial MUC16 and lacritin expression. Since lacritin is a natural tear protein, this suggests mechanisms of upstream and downstream autocrine stimulation that prolong lacritin's cytoprotective and prosecretory effects. Tear proteins likely function as bioactive complexes. These activities can be harnessed by recombinant protein engineering. Our working hypothesis is that lacritin is naturally protective against dry eye inflammation. Our immediate goal is to optimize lacritin's cytoprotective activity and understand its mechanism of action.
Our first aim i s to engineer the smallest and most cytoprotective form of lacritin.
Our second aim i s to work out biological pathways that underlie its cytoprotective activity in a search for treatment synergies or counterindications.
Our third aim i s to preclinically test topically applied or genetically induced lacritin in animal models of dry eye. ? ? ?
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