Damaged motile cilia undergo phagolysosomal clearance during regeneration

The remodelling and regeneration of motile cilia are widespread yet poorly understood cellular responses to stress. Whether components from damaged cilia are reutilised during regeneration remains unclear. Here, we apply acute, non-lethal stress to the multiciliated eukaryote Tetrahymena thermophila to investigate the fate of damaged motile cilia during regeneration. We find that damaged axonemes accumulate intracellularly as coiled structures, termed c-rings, which are sequestered within phagosome-associated membrane compartments and subsequently associate with acidic compartments for clearance. Imaging, quantitative proteomics and genetic perturbations reveal a phagolysosomal route for processing damaged axonemes, accompanied by recruitment of autophagy-associated machinery. Remarkably, ciliary regeneration proceeds without detectable c-ring accumulation, even when new protein synthesis is inhibited, indicating that reutilisation of internalised axonemal material is not essential for ciliogenesis. Our findings uncover an unexpected mechanism for clearing damaged motile cilia and reveal how ciliary degradation and regeneration can operate as distinct cellular processes.