In terrestrial mammals, the tear film plays a vital role in maintaining the health of the ocular surface, by protecting against toxic challenges in the external environment, and thereby preserving visual acuity. These protective functions are critically dependent upon the integrity of the tear film structure, which comprises three layers: an underlying mucin foundation, a protein-rich aqueous component, and an overlying lipid layer. In a previous study, we determined that the tears of marine mammals do contain proteins, but that the tear film, examined using interferometry, appears to lack the lipid layer found in terrestrial mammals. This finding is significant in that it indicates that sea mammals use an alternative component, in lieu of lipid, to promote the stability of the tear film. We have also shown previously that there is a high likelihood that mucins, large glycoproteins, are present in the tears of marine mammals. We hypothesize that proteins secreted over the ocular surface, including mucins, provide stability to the tear film.
In this study, we sought to determine: first, whether the tears of sea mammals, including the Pinnipedia (sea lions, seals) and Cetacea (dolphins), contain similar levels and types of proteins of lower molecular weight (15-250 kDa); and second, to determine whether such proteins were analogous to the major proteins identified in terrestrial mammal tears.
Tear samples from sea lions, seals, and dolphins were subjected to polyacrylamide gel electrophoresis to separate proteins. Protein banding patterns from marine and terrestrial mammals were similar. However, when protein bands from seals and sea lions were subjected to trypsin digestion for protein identification, subsequent proteomic analysis of samples did not yield any matches with known tear proteins of terrestrial mammals. These findings suggest that, whereas marine mammal tears contain similar concentrations, and ranges of molecular weights, of proteins, sea mammal tear proteins may be distinct from those of land mammals. The presence of a wide variety of proteins in marine mammal ocular secretions supports our hypothesis that proteins, in lieu of lipid, provide stability to the tear film.
We thank Dr. Sam Ridgway (U.S. Navy Marine Mammal Program, San Diego, CA), and Dolphin Quest Oahu for generously providing ocular secretion samples from bottlenose dolphins. We also thank Ed Latson, MS, DVM, Central Park Aquatic Health, and the Aquarium of Niagara, Niagara Falls, NY for donations of seal and sea lion tears. Pablo Argueso, Sandra Spurr-Michaud, and Stephen Richards (S.ER.I. & H.M.S., Boston, MA) provided excellent technical advice. This work was supported by grants from Arey's Pond Boat Yard, Inc. and N.I.H. [RO1EY05612].