Age-associated Tissue Organization Shifts Retinal Pigmented Epithelium Actomyosin Plasticity in Phagocytosis

Mechanical homeostasis in healthy tissues relies on a dynamic balance of mechanical properties essential for maintaining tissue integrity and function. Over time, this integrity decays due to structural reorganization from extracellular remodelling or an imbalance of cell death and proliferation. This is particularly challenging in postmitotic tissues such as the retina, where lack of proliferation requires age-dependent structural adaptation. The retinal pigment epithelium exemplifies this adaptation, compensating for apoptotic events through morphological reconfiguration and cytoskeletal remodelling. To explore the connection between age-related structural changes and mechanical homeostasis, we developed an in vitro model simulating cell density reduction observed in the human retinal pigmented epithelium during ageing. This model is characterized by reduced cell height, shortened apical microvilli, and alterations in the actin cytoskeleton. The resulting age-associated phenotype leads to significant biomechanical changes, including tissue stiffening, enhanced junctional contractility, and apical cell cortex remodelling. Transcriptional profiling revealed notable shifts in actin-associated gene expression, indicating reduced plasticity when remodelling is necessary for phagocytosis of photoreceptor outer segments. Structurally aged monolayers exhibit changes in phagocytic activity that mirror those seen in aged individuals. Additionally, we observed an impaired capacity for apico-lateral remodelling while internalizing photoreceptor outer segments. This altered activity can be partially corrected by regulating actin network remodelling capacity with inhibitors targeting the actin nucleators Arp2/3 and formins. Overall, our findings suggest that age-related structural changes in the postmitotic outer retina shift its mechanical homeostasis, impacting physiological functions that are crucial for healthy vision. Overall design: RNAseq profiling of 3 weeks mature hiPSCs-derived RPE (iCell-RPE, iRPE) cells 4 days after cell density reduction after on demand apoptosis via FKBP/casp8 activation (AgeD for Apoptosis-genereated Density) and time-matched control hiPSCs-derived RPE cells (Ctrl)