Cilia are complex evolutionary conserved eukaryotic structures that, projecting from cell surfaces, perform a variety of biological roles. Cilia are traditionally classified into motile or sensory and hundreds of proteins take part in their composition. This set of genes coding for ciliary components is known as the ciliome. Mutations in the ciliome underlie an ever-growing group of highly pleiotropic multisystemic diseases globally termed as ciliopathies. These diseases are characterized, among other symptoms, by mental retardation, sensory defects and/or metabolic disorders. Despite an estimated 1 in 1,000 people affected by these diseases, the molecular bases of the ciliopathies are still poorly understood. Proper cilium assembly and functionality requires the tightly co-regulated expression of ciliary components; however, little is known about the regulatory logic controlling ciliome transcription. Most ciliome genes are shared between motile and sensory cilia. RFX transcription factors (TFs) have an evolutionarily conserved role in the transcriptional regulation of both motile and sensory ciliome. In vertebrates, transcription of the motile ciliome is also directly regulated by FoxJ1, a Forkhead (FKH) TF. However, to date, TFs working together with RFX in the transcription of the sensory ciliome are unknown in any organism. We have identified FKH-8, a FKH TF, as a terminal selector of the sensory ciliome in C. elegans.
fkh-8 expression is restricted to the sixty ciliated sensory neurons of C. elegans, it binds the regulatory regions of the sensory ciliome, it is required for correct ciliome gene expression and cilium morphology and acts synergistically with DAF-19/RFX TF. Accordingly,
fkh-8 mutants display a wide range of behavioral defects in a plethora of sensory mediated paradigms, including olfaction, gustation, and mechano-sensation. Importantly, ciliome expression defects in
fkh-8 mutants can be rescued by mammalian FoxJ1 but not other FKH TFs. Thus, we have identified, for the first time, a TF that acts together with RFX TFs in the direct regulation of the sensory ciliome. Moreover, together with previous work, our results show that FKH and RFX TFs act together in the regulation of both motile and sensory cilia, suggesting this regulatory logic could be an ancient trait pre-dating functional sub-specialization of cilia. Finally, our results could help better understand the biological basis of orphan ciliopathies.