Protein aging: truncation of aquaporin 0 in human lens regions is a continuous age-dependent process

The human lens is ideal for the study of macromolecular aging because cells in the centre, along with their constituent proteins, are present for our entire lives. We examined the major membrane protein, aquaporin 0 (AQP0), in regions of the lens formed at different times during our lifespan, to determine if similar changes could be detected and if they were progressive. Membrane fractions from three concentric lens regions were examined by SDS-PAGE coupled with densitometry, and Western blotting, to assess the time course of truncation. The overall extent of modification was also examined by MALDI mass spectrometry of the undigested proteins. In all regions, AQP0 became progressively more truncated, specifically by the loss of a 2kDa intracellular C-terminal peptide. The proteolysis increased steadily in all regions such that half of the AQP0 in the barrier region (that part of the lens formed immediately after birth) had been cleaved by age 40-50. MALDI mass spectrometry revealed that in all regions, AQP0 not only was shortened, it also became progressively more heterogeneous with age. Since the lens interior is devoid of active enzymes, it is very likely that the cleavage of AQP0 is chemically induced. We speculate that the loss of this C-terminal peptide 'spacer' may allow occlusion of AQP0 pores on the cytoplasmic face of the fibre cell membranes. Once a significant proportion of AQP0 has been cleaved, this occlusion may contribute to the formation of the lens permeability barrier that develops at middle age.