Loss or defects of a protein previously shown to play a key a role in cataract, the clouding of the lens that commonly strikes people in their seventies, has now been shown to contribute to presbyopia, the forty-something phenomenon when reading a menu by candlelight and threading a needle become newfound challenges.

Near vision can get blurry with age because the eye’s lens loses its flexibility. The lens normally changes its shape (by bending or flattening) to focus light onto the back of the eye, but with age, it often becomes stiff and less able to squeeze itself into the shape required for close-up vision.

Previously, studies showed that mice with mutations of the protein aquaporin zero (AQP0) developed cataracts. Now an NEI-funded study led by Kulandaiappan Varadaraj, Ph.D., of Stony Brook University of New York, shows that AQP0 also contributes to lens flexibility. This suggests that mutations or age-related changes affecting AQP0 may be a root cause of presbyopia.

The bulk of the lens is made up of fiber cells that contain mostly water and proteins called crystallins, which contribute to the transparent and light-focusing qualities of the lens. The fiber cells – and the crystallins within them – are densely packed into an orderly pattern, creating a nearly glass-like structure that light passes through.

Using mouse models, Varadaraj and his colleagues found that mice with AQP0 mutations had lenses with fiber cells in disarray, haphazardly arranged with gaps in between cells, compared with the compact, brick wall-like architecture of lens fiber cells in mice with normal AQP0.

The researchers propose that two key functions of AQP0 proteins contribute to the ideal degree of stiffness in the lens. AQP0 acts as a channel for water, allowing it to circulate and nourish the fiber cells, and dispose of waste. The protein, which is present on the surface of fiber cells, may also act as a kind of Velcro, binding the cells to each other so they are tightly packed with minimal space in between.

These two functions may be instrumental in regulating stiffness from the lens’ outer edge to the center. Lens stiffening likely occurs because throughout our lifetimes new fiber cells are continuously generated on the outer edges of the lens. To make room for these new cells and to adjust the focusing ability, older fiber cells lose most of their water content and biological working machinery and get compacted into the lens center. As more and more old cells are pushed inward, the center expands and becomes stiffer.

Age-related damage and/or mutations in AQP0 cause it to stop functioning properly, and the ever-increasing area of stiffness in the center of the lens eventually makes it difficult for the overall lens to change shape, especially for focusing on objects close-up.

Moving forward, Varadaraj and his team developed a new mutant mouse model to investigate on a molecular level how AQP0 contributes to lens stiffness. The model will enable them to screen the 263 amino acids (protein building blocks) that make up AQP0 in order to identify the specific part of the protein involved in lens stiffness.