as a result of the filtering properties of macular pigment; this reduction is estimated at approximately 40%, but varies from 3 to 100% between individuals.

APOPTOSIS AND RETINAL

DEGENERATION

There is convincing evidence that genetic defects in retinitis pigmentosa can lead indirectly to cell death. This is evident, for example, in cone loss in patients with retinitis pigmentosa due to mutations in the rhodopsin gene, which is expressed only in rods. It is of interest that in these and other retinal degenerations, photoreceptor cell death occurs by apoptosis. These observations imply that therapeutic modification of the process leading to apoptosis or the apoptotic pathway itself may modify the course of these disorders.

Apoptosis is a common final pathway in retinal degeneration in humans and in animal models (Figure 3.3). Although biochemical aspects are similar in all apoptotic cells (nuclear chromatin condensation, cytoplasmic shrinking, dilated endoplasmic reticulum, membrane blebbing, and internucleosomal DNA fragmentation), different molecular events lead to cell death.

Caspases (cysteine aspartate-specific proteases) are the first proteases identified that coordinate and execute the apoptotic process in many apoptotic systems. These proteins are synthesized as inactive zymogens and are activated by proteolytic cleavage to form a tetramer. Two major pathways, leading to the degradation of key survival proteins, have been described for caspase activation: the extrinsic pathway, initiated by ligand binding to a death receptor and the intrinsic pathway, involving the release of cytochrome c from the mitochondrial intermembrane space into the cytosol.

In retinal degeneration, caspases, calpains, and (LEI)-DNase II have been shown to be activated during photoreceptor cell death in several models.9 Caspase-1 and -2 were detected in the retinal outer nuclear layer during degeneration in Royal College of Surgeons (RCS) rats, and inhibition of caspase-3 preserved the retina of S334ter rats and tubby mice. In rd mice, calpain mediates apoptosis through caspase-3 activation, and caspase-3 inhibitors preserve the retina.

An unexpected finding was observed during early attempts at retinal transplantation for photoreceptor rescue. Focal injury to the retina appears to protect nearby photoreceptors from degeneration. This was

clearly illustrated in the RCS rat in which mechanical injury produced by an injection of saline into the subretinal space or into the vitreous or even insertion of a needle without injection led to protection of photoreceptors near the wound. This protection is not restricted to genetically determined retinal degeneration. Similar photoreceptor rescue by mechanical injury was observed in light-induced retinal damage in the rat. Injury-induced photoreceptor rescue extends beyond the immediate vicinity of the lesion, suggesting that diffusible factors may be involved.

As mechanical injury to the eye increases the expression of basic fibroblast growth factor and ciliary neurotrophic factor (CNTF) in the rat retina, it is logical to assume that these agents may be responsible, at least in part, for this protection.

CNTF was found to rescue photoreceptors from degeneration when delivered continuously after adeno-associated virus vector transduction; however, it reduces scotopic (rod-dominated) and photopic (conedominated) electroretinographic (ERG) amplitudes, which are typically a measure of retinal function. This effect appears to occur in a dosedependent manner. The continuous delivery of CNTF also results in a reduction of some or all ERG responses in normal mice, rabbits, and rats. Intraocular CNTF injection reduces vision in normal rats. These paradoxical properties of growth factors might limit their therapeutic use.10

VISUAL CYCLE

The visual cycle has at least two interlinked components: the retinoid cycle and the electrical amplification cascade.

RETINOID CYCLE

Outer segment of photoreceptors

Visual phototransduction is the photochemical reaction that take place when light (photon) is converted to an electric signal in the retina. Rhodopsin, the visual pigment in the rods, is a membrane protein located in the outer segments of the rods. It contains a chromophore (11-cis retinal) that is covalently bound to opsin. On photoactivation, 11-cis retinal undergoes photoisomerization to all-trans retinal. While still covalently bound to opsin, the compound undergoes a number of conformational intermediates, collectively termed metarhodopsin. It is suggested that metarhodopsin II is important in inactivation of electrical activity.

Retinal pigment epithelium

All-trans retinal together with the opsin then diffuses into the interphotoreceptor matrix and enters the RPE, where they are hydrolyzed to all-trans retinal and opsin. The all-trans retinal is flipped across the membrane by ABCA4 (formerly called ABCR) transporter and then reduces to all-trans retinol, which is chaperoned by cellular retinolbinding protein and is esterified by lecithin-retinol acetyl transferase (LRAT). The ester is then chaperoned by RPE65. The all-trans retinol is isomerized by retinoid isomerase and retinoid hydrolase to 11 cis-retinol.

An alternate cycle exists when some of the all-trans retinal remains chaperoned by the opsin and is reduced by all-trans retinol dehydrogenase to release all-trans retinol. In both cases, all-trans retinol moves from the RPE into the outer segment.