Glutamylation imbalance leads to photoreceptor degeneration

The stereotypic structure of microtubules, assembled from conserved /{beta}-tubulin dimers is subject to a complex diversity of Post-translational Modifications (PTMs). PTMs are predicted to fine-tune microtubule properties and interactions with other proteins, thus allowing microtubules to perform specific functions. Cilia accumulate several types of tubulin PTMs, such as polyglutamylation, polyglycylation, detyrosination and acetylation, whose functions are not yet fully understood. Recently, mutations of AGBL5, coding for the deglutamylating enzyme CCP5, have been associated to retinitis pigmentosa, suggesting that perturbation of polyglutamylation leads to the degeneration of photoreceptor cells. However, the molecular mechanisms underlying this degeneration remain unknown. Here, using super-resolution Ultrastructure Expansion Microscopy in mouse and human photoreceptor cells, we found that most tubulin PTMs are accumulated at the level of the connecting cilium, a structure linking the outer and inner segments of photoreceptor cells. Using mouse models with increased glutamylation (Ccp5-/- and Ccp1-/-), or loss of tubulin acetylation (Atat1-/-), we demonstrated that aberrant glutamylation, but not loss of acetylation, resulted in perturbed molecular architecture of the outer segment, with the loss of the bulge region and destabilization of the distal axoneme. Concurrently, we observed a substantial impairment in tubulin glycylation and intraflagellar transport. Altogether our results indicate that glutamylation plays a crucial role in the maintenance of the molecular architecture of the outer segment and point to tubulin PTM imbalance as possible culprit in retinal degeneration.