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

Tissue homeostasis emerges from mechanical feedback loops balanced by cell loss and proliferation, a balance that in postmitotic tissues must be maintained without compensatory proliferation. Yet how these tissues preserve mechanical homeostasis and how this challenges function in ageing remains unclear. To establish the relationship between cell density, mechanical homeostasis, and function, we induced age-mimicking cell loss in a postmitotic retinal pigment epithelium (RPE) in vitro. This model recapitulates key structural hallmarks of RPE ageing, including reduced cell height, shortened microvilli and cytoskeletal reorganisation. The density-reduced RPE establishes a new mechanical equilibrium characterised by tissue stiffening and increased junctional contractility. Functionally, these monolayers exhibit impaired phagocytosis of photoreceptor outer segments due to compromised apicolateral plasticity, which is mechanistically linked to the modulation of actin nucleators, Arp2/3 and formins. Altogether, our findings show that a cell loss-induced shift in mechanical homeostasis drives age-related RPE dysfunction, demonstrating that structural remodelling and mechanics alone can compromise tissue function in ageing.