Centriole and transition zone structures in photoreceptor cilia revealed by cryo-electron tomography

Abstract

Primary cilia mediate sensory signaling in multiple organisms and cell types but have structures adapted for specific roles. Structural defects in them lead to devastating diseases known as ciliopathies in humans. Key to their functions are structures at their base: the basal body, the transition zone, the "Y-shaped links" and the "ciliary necklace". We have used cryo-electron tomography with subtomogram averaging and conventional TEM to elucidate the structures associated with the basal region of the "connecting cilia" of rod outer segments in mouse retina. The longitudinal variations in microtubule (MT) structures and the lumenal scaffold complexes connecting them have been determined, as well as membrane-associated transition zone structures: Y-shaped links connecting MT to the membrane, and ciliary beads connected to them that protrude from the cell surface and form a necklace-like structure. These results represent a clearer structural scaffold onto which molecules, identified genetics, proteomics, and superresolution fluorescence, can be placed in our emerging model of photoreceptor sensory cilia. SummaryCryo-electron tomography and subtomogram averaging reveal new structural features at the base of the light sensing cilia of retinal rods. These include the basal body, the Y-links between axoneme and membrane, and the ciliary necklace of the transition zone.