MKS5 and CEP290 Dependent Assembly Pathway of the Ciliary Transition Zone

Abstract

Cilia have a unique diffusion barrier (“gate”) within their proximal region, termed transition zone (TZ), that compartmentalises signalling proteins within the organelle. The TZ is known to harbour two functional modules/complexes (Meckel syndrome [MKS] and Nephronophthisis [NPHP]) defined by genetic interaction, interdependent protein localisation (hierarchy), and proteomic studies. However, the composition and molecular organisation of these modules and their links to human ciliary disease are not completely understood. Here, we reveal Caenorhabditis elegans CEP-290 (mammalian Cep290/Mks4/Nphp6 orthologue) as a central assembly factor that is specific for established MKS module components and depends on the coiled coil region of MKS-5 (Rpgrip1L/Rpgrip1) for TZ localisation. Consistent with a critical role in ciliary gate function, CEP-290 prevents inappropriate entry of membrane-associated proteins into cilia and keeps ARL-13 (Arl13b) from leaking out of cilia via the TZ. We identify a novel MKS module component, TMEM-218 (Tmem218), that requires CEP-290 and other MKS module components for TZ localisation and functions together with the NPHP module to facilitate ciliogenesis. We show that TZ localisation of TMEM-138 (Tmem138) and CDKL-1 (Cdkl1/Cdkl2/Cdkl3/Cdlk4 related), not previously linked to a specific TZ module, similarly depends on CEP-290; surprisingly, neither TMEM-138 or CDKL-1 exhibit interdependent localisation or genetic interactions with core MKS or NPHP module components, suggesting they are part of a distinct, CEP-290-associated module. Lastly, we show that families presenting with Oral-Facial-Digital syndrome type 6 (OFD6) have likely pathogenic mutations in CEP-290-dependent TZ proteins, namely Tmem17, Tmem138, and Tmem231. Notably, patient fibroblasts harbouring mutated Tmem17, a protein not yet ciliopathy-associated, display ciliogenesis defects. Together, our findings expand the repertoire of MKS module-associated proteins—including the previously uncharacterised mammalian Tmem80—and suggest an MKS-5 and CEP-290-dependent assembly pathway for building a functional TZ.

Homozygous variants in KIAA1549, encoding a ciliary protein, are associated with autosomal recessive retinitis pigmentosa

“…TMEM17 is a part of the Meckel syndrome (MKS) protein complex located in the ciliary transition zone, in which it facilitates cilium formation. Also, pathogenic variants in genes encoding proteins in this complex are known to cause (severe) ciliopathies 21 . TMEM17 pathogenic variants have been reported to cause oral-facial-digital syndrome type 6 (MIM: #277170) 21.…”

“…Also, pathogenic variants in genes encoding proteins in this complex are known to cause (severe) ciliopathies 21 . TMEM17 pathogenic variants have been reported to cause oral-facial-digital syndrome type 6 (MIM: #277170) 21. The MKS complex contains both cytoplasmic and transmembrane proteins and functions as a barrier preventing rapid diffusion of transmembrane proteins between cilia and plasma membranes 22.…”

Eupatilin rescues ciliary transition zone defects to ameliorate ciliopathy-related phenotypes

“…TCTN1 and TMEM216 of the MKS complex mediate the link between the transition zone and the ciliary membrane (4,24). Although CEP290 is known to be essential for the assembly of the MKS complex at the transition zone in Caenorhabditis elegans (23), the transition zone localization of TCTN1 and TMEM216 was apparently not disrupted in CEP290…”

“…These results demonstrate that eupatilin treatment improves the function of cone photoreceptors in Cep290-mutant retinas. CEP290 forms a complex with the ciliopathy protein NPHP5 and also interacts with other transition zone components including the Meckel syndrome (MKS) protein complex (20,23). TCTN1 and TMEM216 of the MKS complex mediate the link between the transition zone and the ciliary membrane (4,24).…”

Drosophila sensory cilia lacking MKS proteins exhibit striking defects in development but only subtle defects in adults

“…Studies, primarily in worms, indicate that there is a modular hierarchy of TZ assembly with NPHP8 (also known as RGRIP1L and MKS5) (Jensen et al, 2015; Li et al, 2016) initially recruiting Cep290 to form the central cylinder of the TZ (Schouteden et al, 2015) and the MKS and NPHP modules then cooperating to form the Y-links (Williams et al, 2008, 2011). Only when co-mutating or co-depleting components of both MKS and NPHP modules at the same time can a strong effect be observed (Dawe et al, 2007; Tammachote et al, 2009; Weatherbee et al, 2009; Williams et al, 2008, 2011; Yee et al, 2015).…”