'Lacritin' is an ocular specific prosecretory and prosurvival factor in human tears. Selective lacritin deficiency has been reported in human dry eye tears. Here we test the hypothesis that lacritin is a key regulator of corneal sensory nerve activity for basal tearing, and of ocular surface metabolism for survival against stress. Important implications of this hypothesis are that deficiency of active lacritin may initiate dry eye disease and that topical lacritin may be therapeutic. We recently reported on treating mice suffering from aqueous deficient dry eye disease. Autoimmune regulator (Aire)-deficient [Aire-/-] mice dosed with lacritin displayed enhanced tear flow and ocular surface health. Lacritin also reduced lacrimal gland inflammation. On normal rabbit eyes, we found that a single lacritin dose was effective for hours. Our new electrophysiological studies in rats reveal that topical lacritin enhances the basal tear stimulatory 'dry response' of corneal sensory nerves. The dry response is regulated by TRPM8 cation channels that exquisitely sense slight corneal cooling as a consequence of the natural tendency of the eye to dessicate. Truncating 25 amino acids from lacritin's C- terminus ('C-25') abrogated dry response stimulation and tearing, implying that the capacity for cell targeting resides within this domain. We recently found that the domain generated synthetically as KQFIENGSEFAQKLLKKFS ('N-94/C-6') is as potent as lacritin, and interacts specifically with the cell surface heparan sulfate proteoglycan 'syndecan-1', and with the adrenergic 2C receptor. This discovery suggests a neural regulatory mechanism since adrenergic 2C and/or 2A receptors modulate TRPM8 activity. We just reported how lacritin restores ocular surface function in corneal epithelial cells stressed with inflammatory cytokines. Through a signaling pathway dependent of the longevity factor 'Forkhead Box 03' (FOXO3) and on the little known 'ATG101', lacritin stimulates mitochondrial oxidative phosphorylation and autophagy. Autophagy is a common quality control mechanism in cells. Binding partners of ATG101 suggest mechanisms by which lacritin accelerates autophagy and restores mitochondrial function (respectively through autophagy mediator ATG13, and mitochondrial ETFA). Our working hypothesis is that lacritin is essential for normal tearing, and ocular surface and glandular health. Our immediate goal is to elucidate unifying mechanisms of lacritin stimulated tearing and ocular surface health.
Our specific aims are to (1) stabilize N-94/C-6 or smaller synthetic form of lacritin, (2) characterize how topical lacritin promotes basal tearing, and (3) resolve how lacritin restores ocular surface health. University of Virginia Charlottesville Virginia James Madison University Harrisonburg Virginia University of California, San Francisco San Francisco California Thomas Jefferson University Philadelphia Pennsylvania
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