Tubulin hyperglutamylation causes retinal degeneration by impairing the structural integrity of cilia and protein mistrafficking

Tubulins undergo various posttranslational modifications (PTMs) that confer diverse functions to microtubules. Tubulin polyglutamylation is dynamically regulated by glutamylation and deglutamylation. AGBL5 is a deglutamylase that specifically removes glutamate at the branching point. Mutations in AGBL5 in humans are associated with retinitis pigmentosa, but the underlying mechanism remains undefined. Here, we generated an Aglb5 knockout mouse that showed tubulin hyperglutamylation in photoreceptors, resulting in progressive retinal degeneration. RNA-seq analysis revealed the altered ciliary function in the Agbl5 knockout. Transmission electron microscopy indicated an impaired inner scaffold in the connecting cilium (CC). Consequently, phototransduction proteins were mislocalized or downregulated in mutant rod and cone photoreceptors. Disk membranes in photoreceptor outer segments were disorganized. Immunofluorescence revealed that the recruitment of IFT88, kinesin-II, and dynein-2 to the CC was affected, implicating defective intraflagellar transport (IFT). Collectively, these findings demonstrate that tubulin glutamylation homeostasis regulated by AGBL5 is critical for photoreceptor survival by maintaining the structural integrity of the CC and normal protein trafficking through IFT.