Previously we have highlighted proteins expressed in the human neural retina. This week's article emphasizes the cellular structure and molecular dynamics of the lens.

The main function of the lens is to focus light on the retina. The passage of light through the cornea, lens and vitreous all the way to the retinal layer of the eye is only possible due to transparency of the tissue. Although the lens is very protein-rich, light absorption and light scattering in the lens is minimal.

The lens comprises non-diving lens cells which are mainly composed of ordered proteins called crystallins. Since structures such as cell organelles absorb light, the lens cells lack organelles completely. During embryogenesis, the cells from the outermost cell layer replicate, differentiate and are programmed to eliminate all their organelles.

The only molecules in these cells are the structural crystallin proteins, which do not absorb light. The crystallins constitute more than 90 percent of the total protein content in the lens.

The crystallin proteins are expressed in three major isoforms, alpha, beta and gamma crystallins and are known to have long lifespans. Alpha-crystallin have chaperone activity and can remodel and protect the cytoskeleton. The beta and gamma crystallin on the other hand are involved in development of the eye.

The lens has an array of different mechanisms to withstand insult such as protein degradation, cellular stress and other damaging factors that could affect the order, folding and function of the crystallins. Both synthesis of crystallin proteins and the replication of new lens cells decreases drastically with age. As time progresses, the crystallins accumulate more damage which ultimately leads to aggregation and disruption of the transparent ability of the lens. This results in a cloudy, blurry and impaired vision known as cataract.

We highlight one of the beta crystallin proteins CRYBB2 expressed in the lens. Within the Human Protein Atlas 53 genes are shown to be expressed in the human eye. Visit the Eye proteome page for more information about the human eye and its proteins.

Feria Hikmet Noraddin