Human tear viscosity: an interactive role for proteins and lipids

Area of Science:

• Biophysics
• Ocular Surface Science

Background:

• Human tear viscosity is crucial for ocular lubrication and surface integrity.
• Tear rheology is complex, exhibiting non-Newtonian (shear-thinning) behavior not fully explained by mucin or individual proteins.
• Tear lipocalin (TL) is a major tear protein known to bind lipids, yet free lipids are not typically found in collected tears.

Purpose of the Study:

• To investigate the presence of free lipids in human tears.
• To elucidate the contribution of tear proteins, particularly tear lipocalin (TL), to tear viscosity.
• To examine the role of TL's lipid-binding state (apo-TL vs. holo-TL) and oligomerization on viscosity.

Main Methods:

• Rheological analysis of whole tears and protein solutions.
• Detection of free lipids in tears.
• Recombinant tear lipocalin (apo-TL and holo-TL) characterization using SDS-PAGE and analytical ultracentrifugation (AU).
• Comparison with beta-lactoglobulin.

Main Results:

• No free lipids were detected in whole human tears.
• Protein combinations including lysozyme, lactoferrin, or apo-TL were shear-thinning; holo-TL exhibited Newtonian (linear) behavior.
• Apo-TL was monomeric, while holo-TL showed some dimerization; beta-lactoglobulin displayed a monomer-dimer equilibrium.

Conclusions:

• Hetero-protein interactions significantly contribute to human tear viscosity.
• Lipid-binding-induced structural changes in TL, leading to oligomerization, play a key role in modulating tear rheology.
• Tear viscosity is primarily governed by protein interactions rather than free lipids.