Optineurin tunes outside-in signaling to regulate lysosome biogenesis and phagocytic clearance in the retina II
Balancing the competing demands of phagolysosomal degradation and autophagy is a significant challenge for phagocytic tissues. Yet, how this plasticity is accomplished in health and disease is poorly understood. In the retina, circadian phagocytosis and degradation of photoreceptor outer segments by the postmitotic retinal pigment epithelium (RPE) is essential for healthy vision. Disrupted autophagy due to mTOR overactivation in the RPE is associated with blinding macular degenerations; however, outer segment degradation is unaffected in these diseases, indicating that distinct mechanisms regulate these clearance mechanisms. Here, using advanced imaging and mouse models, we identify optineurin as a key regulator that tunes phagocytosis and lysosomal capacity to meet circadian demands and helps prioritize outer segment clearance by the RPE in macular degenerations. High-resolution live-cell imaging implicates optineurin in scissioning outer segment tips prior to engulfment, analogous to microglial trogocytosis of neuronal processes. Optineurin is essential for recruiting LC3, which anchors outer segment phagosomes to microtubules and facilitates phagosome maturation and fusion with lysosomes. This dynamically activates transcription factor EB (TFEB) to induce lysosome biogenesis in an mTOR-independent, TRPML1 (transient receptor potential-mucolipin 1)-dependent manner. RNAseq analyses show that expression of TFEB target genes temporally tracks with optineurin recruitment, and that lysosomal and autophagy genes are controlled by distinct transcriptional programs in the RPE. The unconventional plasma membrane-to-nucleus signaling mediated by optineurin ensures outer segment degradation under conditions of impaired autophagy in macular degeneration models. Independent regulation of these critical clearance mechanisms would help safeguard metabolic fitness of the RPE through the organismal lifespan. Overall design: To investigate transcriptomic changes in WT and Abca4-/- mouse RPE, we collected total RNA and performed bulk RNA sequencing. Samples were collected at 5 h after light onset. 3 mice (replicates) were included for each genotype.