Hypomorphic CEP290/NPHP6 mutations result in anosmia caused by the selective loss of G proteins in cilia of olfactory sensory neurons

Abstract

Cilia regulate diverse functions such as motility, fluid balance, and sensory perception. The cilia of olfactory sensory neurons (OSNs) compartmentalize the signaling proteins necessary for odor detection; however, little is known regarding the mechanisms of protein sorting/entry into olfactory cilia. Nephrocystins are a family of ciliary proteins likely involved in cargo sorting during transport from the basal body to the ciliary axoneme. In humans, loss-offunction of the cilia-centrosomal protein CEP290/NPHP6 is associated with Joubert and Meckel syndromes, whereas hypomorphic mutations result in Leber congenital amaurosis (LCA), a form of early-onset retinal dystrophy. Here, we report that CEP290 -LCA patients exhibit severely abnormal olfactory function. In a mouse model with hypomorphic mutations in CEP290 [retinal dystrophy-16 mice (rd16)], electro-olfactogram recordings revealed an anosmic phenotype analogous to that of CEP290 -LCA patients. Despite the loss of olfactory function, cilia of OSNs remained intact in the rd16 mice. As in wild type, CEP290 localized to dendritic knobs of rd16 OSNs, where it was in complex with ciliary transport proteins and the olfactory G proteins G olf and G␥13. Interestingly, we observed defective ciliary localization of G olf and G␥13 but not of G protein-coupled odorant receptors or other components of the odorant signaling pathway in the rd16 OSNs. Our data implicate distinct mechanisms for ciliary transport of olfactory signaling proteins, with CEP290 being a key mediator involved in G protein trafficking. The assessment of olfactory function can, therefore, serve as a useful diagnostic tool for genetic screening of certain syndromic ciliary diseases.nephrocystin ͉ olfaction

Bardet–Biedl syndrome proteins are required for the localization of G protein-coupled receptors to primary cilia

“…The fact that another membranebound ciliary protein, ACIII, localizes normally in Bbs2 Ϫ/Ϫ and Bbs4 Ϫ/Ϫ neurons indicates that the BBS proteins mediate ciliary localization of distinct signaling proteins. Several studies have found that defects in ciliary protein transport can be associated with mislocalization of some signaling proteins but not others, suggesting that there are multiple mechanisms for trafficking proteins to the cilium (22)(23)(24)(25). Interestingly, hypomorphic mutations in CEP290/NPHP6, which are known to cause the early onset retinopathy Leber congenital amaurosis (26), are associated with defective ciliary localization of olfactory G proteins but not other olfactory signaling proteins, including odorant GPCRs and ACIII (25).…”

“…Several studies have found that defects in ciliary protein transport can be associated with mislocalization of some signaling proteins but not others, suggesting that there are multiple mechanisms for trafficking proteins to the cilium (22)(23)(24)(25). Interestingly, hypomorphic mutations in CEP290/NPHP6, which are known to cause the early onset retinopathy Leber congenital amaurosis (26), are associated with defective ciliary localization of olfactory G proteins but not other olfactory signaling proteins, including odorant GPCRs and ACIII (25). It is possible that CEP290/NPHP6 and the BBS proteins are part of separate ciliary protein transport mechanisms in neurons, with the BBS proteins specifically mediating GPCR transport to cilia.…”

Functional expression of the olfactory signaling system in the kidney

“…These ciliopathies include diseases such as Bardet-Biedl Syndrome (BBS) (24,25) and Joubert and Meckel syndromes (26). Diseases such as BBS hint at a connection between the nose and the kidney because BBS patients present with both anosmia (the inability to smell) and renal cysts, as well as obesity.…”

Cerebellar development and disease

“…Since the photoreceptor outer segment is a giant modified cilium, the genetic evidence suggests a defect in ciliary function. Similarly, there is failure to transport G proteins into the cilia of olfactory sensory neurons rd16 mutant mice resulting in anosmia [40]. Finally, one JSRD patient with a proven CEP290 mutation displayed complete situs inversus.…”