The eye proteome

Eyes are responsible for the detection of light and its subsequent conversion of visual stimuli to neuronal signals that are transported to the visual centers of the brain. Protein expression of selected genes with literature indicating function related to different structures in the eye, excluding retina, were analyzed by using whole section of eye. Table 1 includes the full list of genes that have been analyzed so far. For information about gene and protein expression in the retina, please visit the The retina proteome.

Table 1. Following 29 genes have been analyzed in eye.

The mammalian lens consists predominantly of crystallin proteins, which are divided into alpha, beta, and gamma families. Crystallins are extremely stable, water-soluble proteins located in the cytoplasm of lens fiber cells. They maintain the transparency and refractive index of the lens. An example of a crystallin is crystallin beta B2 (CRYBB2) (Figure 1). Mutation in this gene is associated with congenital cataract.

Figure 1. Immunohistochemical staining of human eye using an antibody toward CRYBB2 shows strong positivity in lens.

The space between the retina and lens is filled by a transparent gelatinous mass called the vitreous humour, which is encapsulated in a hyaloid membrane consisting of collagen. Our analysis shows that several genes encode proteins expressed in the hyaloid membrane: namely, collagen type IX alpha 2 chain (COL9A2) and collagen type IX alpha 3 chain COL9A3, which are also expressed in hyaline cartilage, and opticin (OPTC). COL9A2 and COL9A3 each encode one of the three alpha chains of type IX collagen. OPTC may bind collagen fibrils and play an important role in their structural organization.

COL9A2 - cartilage
COL9A2 - eye

Figure 2. Immunohistochemical staining of human eye using an antibody toward OPTC shows strong positivity in vitreous of the eye.

Keratin 12 (KRT12) is a protein expressed in the corneal epithelium, which covers the front part of the eye (Figure 3). This protein may be involved in the maintenance of the normal function and structural integrity of the corneal epithelium.

Figure 3. Immunohistochemical staining of human eye using an antibody toward KRT12 shows strong positivity in corneal epithelium.

Eye anatomy, function and histology

The eyes are responsible for the detection of light and its transformation to neuronal signals which are then sent to the visual centers of the brain. The eye globe is enveloped in a fibrous corneoscleral layer. The anterior portion includes a translucent surface called the cornea, while the rest is made up of sclera, a white-colored fibrous layer to which the outer eye muscles are attached. The front of the cornea is covered with a non-keratinized stratified corneal epithelium. Underneath the corneoscleral coat lies the choroid, a layer containing blood and lymphatic vessels. Its main function is to provide nutrients to the cells of the eye. Light enters the eye via the pupil, which is a round opening surrounded by the iris. The iris consists of a pigmented fibrovascular layer that allows for the contraction or dilation of the pupil, as well as a layer of pigmented epithelial cells. The size of the pupil affects the amount of light that enters the eye. Light is refracted by the lens, a transparent disc suspended by ciliary muscle fibers. Elongated fiber cells are the predominant component of the lens while the anterior side is lined with epithelial cells. The refracted light is focused onto the retina, the innermost layer of the eye, which transforms light into nerve signals that are then transmitted by the optical nerve to the brain. The space between retina and lens is filled up by a transparent gelatinous mass called the vitreous humour. The vitreous humour consists of 99 percent water and is contained in a thin hyaloid membrane.