Bolin Cai

A2E, a byproduct of the visual cycle

A substantial portion of the lipofuscin that accumulates with age and in some retinal disorders in retinal pigment epithelial (RPE) cells, forms as a consequence of light-related vitamin A recycling. Major constituents of RPE lipofuscin are the di-retinal conjugate A2E and its photoisomers. That the accretion of A2E has consequences for the cell, with the adverse effects of A2E being attributable to its amphiphilic structure and its photoreactivity, is consistent with evidence of an association between atrophic age-related macular degeneration (AMD) and excessive lipofuscin accumulation.

A2E, a fluorophore of RPE lipofuscin: can it cause RPE degeneration?

In atrophic age-related macular degeneration (AMD) and Stargardt disease, the death of retinal pigment epithelial (RPE) cell death leads to photoreceptor cell degeneration and visual impairment. Nevertheless, the cause of RPE atrophy is poorly understood. One factor that may place RPE cells at risk is the accumulation of critical levels of lipofuscin. Indeed, several lines of evidence indicate that the excessive accumulation of lipofuscin by RPE cells is significant in terms of the etiology of AMD. Firstly, histological analyses of human donor eyes (Wing et al., 1978; Weiter et al., 1986), in addition to fundus spectrophotometry (Delori et al., 1995a; Delori et al., 2001), and confocal ophthalmoscopy (von Ruckmann et al., 1997), have shown that RPE cells overlying the macula, with the exception of RPE in the cone-rich fovea, exhibit the most pronounced age-related accumulation of fluorescent material. Lipofuscin levels in RPE cells are also topographically correlated with histopathological indicators of AMD (Feeney-Burns et al., 1984; Dorey et al., 1989) and with the loss of photoreceptor cells in aged eyes (Dorey et al., 1989). Interestingly, increased fundus autofluorescence at the borders of geographic atrophy is considered to represent an enhanced accumulation of RPE lipofuscin and to implicate the latter in the disease process (Holz et al., 1999; Holz et al., 2001). While the amassing of lipofuscin by RPE is a feature of aging, excessive accretion also occurs in Stargardt disease, some forms of retinitis pigmentosa and cone-rod dystrophy (Weingeist et al., 1982; Rabb et al., 1986; Lopez et al., 1990; Delori et al., 1995b; Kennedy et al., 1995).

A2E, A Fluorophore of RPE Lipofuscin, Can Destabilize Membrane

Studies of Stargardt disease suggest a role for RPE lipofuscin in the RPE cell atrophy that characterizes macular degeneration. The best known constituent of RPE lipofuscin is the pyridinium bisretinoid, A2E (Eldred and Lasky, 1993; Parish et al., 1998). Amongst the properties of A2E that may be damaging to the RPE cell is its ability to destabilize cell membranes (Sparrow et al., 1999). A hydrophilic head group combined with a pair of hydrophobic side-arms are the structural correlates of this behavior. This amphiphilic structure accounts for the tendency of A2E to aggregate (Sakai et al., 1996; De and Sakmar, 2002), a behavior first recognized in deuterated chloroform (CDCl3), the broadening of the 1H NMR signal indicating that the protonated pyridinium moieties of A2E were closely packed within the interior of micelles while the hydrophobic chains contacted the solvent. Further evidence of the detergent-like behaviour of A2E has been revealed in experiments demonstrating the ability of A2E to induce concentration-dependent membrane leakage (Sparrow et al., 1999). In studies employing unilamellar vesicles, it has also been shown that A2E, at critical micellar concentrations, can solubilize membranes (De and Sakmar, 2002).

Involvement of c-Abl, p53 and the MAP kinase JNK in the cell death program initiated in A2E-laden ARPE-19 cells by exposure to blue light

The lipofuscin fluorophore A2E has been shown to mediate blue light-induced damage to retinal pigmented epithelial (RPE) cells. To understand the events that lead to RPE cell apoptosis under these conditions, we explored signaling pathways upstream of the cell death program. Human RPE cells (ARPE-19) that had accumulated A2E were exposed to blue light to induce apoptosis and the involvement of the transcription factors p53 and c-Abl and the mitogen activated protein kinases p38 and JNK were examined. We found that A2E/blue light caused upregulation and phosphorylation of c-Abl, and upregulation of p53. Pretreatment with the c-Abl inhibitor STI571 and transfection with siRNA specific to c-Abl and p53 prior to irradiation reduced A2E/blue light-induced cell death. Gene and protein expression of JNK and p38 was upregulated in response to A2E/blue light. Treatment with the JNK inhibitor SP600125 before irradiation resulted in increase in cell death whereas inhibition of p38 with SB203580 had no effect. This study indicates that c-Abl and p53 are important for execution of the cell death program initiated in A2E-laden RPE cells exposed to blue light, while JNK might play an anti-apoptotic role.