Cell loss disrupts mechanical homeostasis to drive retinal pigment epithelium ageing-like phenotype in vitro

Tissue homeostasis relies on mechanical feedback loops balanced by cell loss and proliferation. However, maintaining this balance becomes particularly challenging in postmitotic tissues, where alternative mechanisms replace compensatory proliferation. In the postmitotic retinal pigment epithelium (RPE), these mechanisms include significant structural adaptations over time. Yet, how these adaptations relate to epithelial mechanical homeostasis and age-associated functional decline remains poorly understood. To establish the relationship between structural changes, mechanical homeostasis and function, we developed an in vitro reductionistic model mimicking age-related reduction in RPE cell density. Inducing large-scale apoptosis in postmitotic stem cell-derived RPE monolayers recapitulates structural hallmarks of aged tissue, such as reduced cell height, shortened microvilli and cytoskeletal reorganisation. This new structure acquires a new mechanical equilibrium, evidenced by tissue stiffening and enhanced junctional contractility. Functionally, the monolayers display impaired vision-supporting phagocytosis of photoreceptor outer segments. Mechanistically, modulation of actin nucleators, Arp2/3 and formins, demonstrates that apicolateral monolayer deformation is critical for phagocytosis and may be compromised in aged RPE. Our findings suggest that a shift in mechanical homeostasis due to cell loss is a major driver of age-related RPE functional decline. Importantly, we show that structural remodelling in ageing alone can compromise tissue function, independent of other stressors.