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Restoration of Lysosomal pH in RPE Cells from Cultured Human and ABCA4^–/– Mice: Pharmacologic Approaches and Functional Recovery

¹From the Departments of Physiology and ²Ophthalmology, University of Pennsylvania, Philadelphia, Pennsylvania.

PURPOSE. Degradation of engulfed material is primarily mediated by lysosomal enzymes that function optimally within a narrow range of acidic pH values. RPE cells are responsible for daily degradation of photoreceptor outer segments and are thus particularly susceptible to perturbations in lysosomal pH. The authors hypothesized that elevated lysosomal pH levels could slow enzyme activity and encourage accumulation of partially digested material. Consequently, treatment to lower perturbed lysosomal pH levels may enhance degradative activity.

METHODS. A high-throughput screening assay was developed to quantify the lysosomal pH of fresh mouse and cultured ARPE-19 cells. The effect of lysosomal pH on outer segment clearance was determined.

RESULTS. Lysosomal pH is elevated in RPE cells from ABCA4 knockout mice and in cultured human ARPE-19 cells exposed to N-retinylidene-N-retinylethanolamine (A2E), tamoxifen, or chloroquine. The lysosomal pH of fresh RPE cells from ABCA4^–/– mice and of chemically compromised RPE cells was reacidified by elevating intracellular cAMP directly. Compromised lysosomal pH was also restored by stimulating A_2A adenosine or β-adrenergic receptors, consistent with G_s-protein coupling of these receptors. Restoring lysosomal pH with these treatments enhanced photoreceptor outer segment clearance, demonstrating functional relevance consistent with an enhancement of degradative enzyme activity.

CONCLUSIONS. Elevation of lysosomal pH in RPE cells interferes with the degradation of outer segments and may contribute to the pathologies associated with A2E. Pharmacologic elevation of cAMP can restore an acid pH and improve degradative function.