Numb Regulates the Polarized Delivery of Cyclic Nucleotide-Gated Ion Channels in Rod Photoreceptor Cilia

Abstract

The development and maintenance of protein compartmentalization is essential for neuronal function. A striking example is observed in light-sensing photoreceptors, in which the apical sensory cilium is subdivided into an inner and outer segment, each containing specific proteins essential for vision. It remains unclear, however, how such polarized protein localization is regulated. We report here that the endocytic adaptor protein Numb localizes to the inner, but not the outer segment of mouse photoreceptor cilia. Rod photoreceptorspecific inactivation of numb in vivo leads to progressive photoreceptor degeneration, indicating an essential role for Numb in photoreceptor cell biology. Interestingly, we report that loss of Numb in photoreceptors does not affect the localization of outer segment disk membrane proteins, such as rhodopsin, Peripherin-rds, Rom-1, and Abca4, but significantly disrupts the localization of the rod cyclic nucleotide-gated (Cng) channels, which accumulates on the inner segment plasma membrane in addition to its normal localization to the outer segments. Mechanistically, we show that Numb interacts with both subunits of the Cng channel and promotes the trafficking of Cnga1 to the recycling endosome. These results suggest a model in which Numb prevents targeting of Cng channels to the inner segment, by promoting their trafficking through the recycling endosome, where they can be sorted for specific delivery to the outer segment. This study uncovers a novel mechanism regulating polarized protein delivery in light-sensing cilia, raising the possibility that Numb plays a part in the regulation of protein trafficking in other types of cilia.

The Late Endosomal Pathway Regulates the Ciliary Targeting of Tetraspanin Protein Peripherin 2

“…In agreement with our current finding, several PRPH2-binding proteins are known LE components (e.g., melanoregulin, V-ATPase; Damek-Poprawa et al, 2009;Zulliger et al, 2015). Moreover, an endosomal trafficking regulator Numb has been shown to regulate the outer segment expression of cGMP-gated channel, a protein that interacts with PRPH2 (Couturier et al, 2014;Ramamurthy et al, 2014).…”

Casz1 controls higher-order nuclear organization in rod photoreceptors

“…To distinguish between these possibilities, we introduced a transgene driving Cre under the control of a Rhodopsin regulatory element ( LMOP ) ( 27 ) into our floxed Casz1 conditional mutant line ( 21 , 28 ). Using recombination reporter alleles, we previously confirmed that the LMOP::Cre transgene is specifically expressed in rod photoreceptors beginning at around postnatal day (P)9 ( 27 , 29 ). We harvested control and Casz1 rod-specific conditional knockout ( Rod-cKO ) retinas and confirmed via immunohistochemistry that Casz1 protein staining was lost specifically in rods ( Fig.…”

Specialized Cilia in Mammalian Sensory Systems

“…Often, only one mutation in a ciliary protein leads to deficient formation and dysfunction of cilia, resulting in sensory impairments. Our group is currently focusing on the characterization of proteins located at the basal body complex of sensory cilia, e.g., Pericentrin ( Figure 6 D–J) [ 18 , 107 , 108 , 109 ]. The basal body complex, which is enclosed by the pericentriolar material (PCM), seems to be the organizing center of protein transport into and out of the cilium and might be the key to connect ciliary function to the phenotypes of numerous ciliopathies.…”