The role of primary cilia in the development and disease of the retina

Figures & data

Figure 1. Schematic representation of a rod photoreceptor cell and localization of ciliary proteins.(A) The schematic represents the rod photoreceptor cell outer segment, connecting cilium, inner segment, outer fiber, cell body, inner fiber and synaptic terminus. A number of key components of the ciliary apparatus are color coded and indicated. The IFT complex A (blue) and complex B (red) are represented in the magnified inset. A retinal pigmentary epithelial (RPE) cell is shown in gray at the top. (B) Confocal microscopy images of an immunofluorescent stained P20 mouse retinal cryosection showing the stratified layers of the retina. Cilium transition zone and basal body protein MKS1 is stained in green, and a novel interactant of MKS1, RNF34, is stained in red. These proteins localize to the base of the connecting cilium, as shown by the arrowheads in the enlarged insets. (C) Confocal microscopy image of a human adult retinal pigment epithelium (ARPE19) cell overexpressing enhanced-GFP-tagged lebercilin and immunostained with an antibody against acetylated α tubulin, which marks the axoneme of the cilium. Lebercilin can be seen in a punctuate pattern along the axoneme. Scale bar = 10μm.

Table 1. The most comprehensively characterized photoreceptor proteins mutated in human disease

Gene	Localization	Function	Retinal phenotype in animal model	Human disease (type)
AHI1	connecting cilium	thought to play a role in RAB8-mediated polarized vesicular trafficking	Ahi1 mutant mice do not develop OS, or develop abnormal OS leading to subsequent photoreceptor degeneration, associated with defects in vesicular trafficking of transducin and Rom-1, and a decrease in Rab8 expression. Opsin is also mislocalized throughout the photoreceptor is Ahi mutant mice. Defect in lamination of retina in zebrafish morphants.	JBTS(3)
ALMS1	basal body	thought to play a role in transport along the photoreceptor axoneme, possibly in endosome recycling	gene-trap Alms1 mutant mice accumulate vesicles in their IS and rhodopsin is mislocalized to the OS, leading to retinal degeneration	ALMS(1), LCA
ARL6	IS, OS, ONL	regulates the BBSome	a specific isoform of BBS3 is expressed in the human retina, and when this long isoform is knocked down in zebrafish, green opsin is mislocalized, with functional consequences on vision. Bbs3L mutant mice develop disorganized IS	BBS(3), RP(55)
BBS4	IS and OPL	component of the BBSome, a regulatory submodule of IFT	Bbbs4 null mice develop grossly normal photoreceptors at an early age but exhibit defective IFT. This leads to mislocalization of specific phototransduction proteins (rhodopsin, transducin and arrestin) but not the structural photoreceptor proteins peripherin or rom-1, which subsequently causes photoreceptor degeneration	BBS(4)
CC2D2A	connecting cilium	extension of connecting cilium membrane through Rab8-mediated vesicular trafficking	cc2d2a zebrafish morphants develop disorganized photoreceptor OS and accumulation of opsins and vesicles in IS. Cc2d2a mutant mice have microphthalmia	COACH, MKS(6), JBTS(9), RP
CEP290	base of connecting cilium	plays a role in IFT, essential for normal mislocalization of rhodopsin and arrestin but not essential for CC structure	mislocalization of rhodopsin and arrestin in rd16 mouse. rdAc Abyssian cat. cep290 morphant zebrafish reveal no gross lamination defects, but have statistically significant reduction in visual function.	BBS(14), LCA(10), JBTS(5), NPHP(6), MKS(4), SLS(6)
LCA5	connecting cilium	plays a role in connecting the IFT core machinery to proteins involved in selecting and recruiting cargo110	in mice lacking wild-type lebercilin, cone and rod opsins are mislocalized and the phototransduction G-protein transducin partially mislocalizes in response to light.	LCA(5)
MAK	base of connecting cilium	kinase, phosphorylates RP1 to regulate extension of ciliary axoneme to control CC and OS length	loss of Mak in mice leads to increased length of photoreceptors and retinal degeneration	RP(62)
MKKS	connecting cilium	chaperonin-like BBS protein, thought to regulate BBSome assembly	late-onset photoreceptor degeneration is observed in Bbs6 mutant mice	MKKS, BBS(6)
MYO7A	base of connecting cilium, at the site of disc morphogenesis	plays a role in transport of proteins from the IS to the OS for incorporation into discs	opsin accumulates in the IS of Myo7a mutant mouse	USH(1B), LCA
NPHP1	connecting cilium, especially around the basal body	thought to play role in the transport of specific proteins along the photoreceptor	Nphp1 mutant mice exhibit defects in sorting of proteins between the photoreceptor IS and OS, and compromised IFT	NPHP(1), SLS(1), JBTS(4)
NPHP4	connecting cilium. Proximal to RPGRIP1, RPGR and SDCCAG8	interacts with CEP290 and RPGRIP1.	Nphp4 mutant mice mislocalize rhodopsin and ROM-1 to the IS and INL, do not develop normal OS despite developing normal CC. The photoreceptors degenerate rapidly. Synaptic ribbons develop normally but degenerate. Mislocalization of synaptic vesicle protein and	NPHP(4), SLS(4), CORS
			post-synaptic density protein. CORD in Nphp4 wire-haired daschunds.
TMEM67	exact localization in photoreceptors not known	required for membrane disc assembly and rhodopsin transport	dysmorphic, misaligned membrane discs, and mislocalized rhodopsin, arrestin, and transducin, leading to rapid photoreceptor degeneration in bpck mice. CC structurally normal but rhodopsin mislocalized to IS and ONL in bpck mice.	COACH, JBTS(6), MKS(3), NPHP(11)
TOPORS	basal body	E3 ubiquitin ligase function. Role in photoreceptors unclear	morpholino silencing in zebrafish affects retinal development, OS fail to form	RP(31)
TTC8	connecting cilium	component of the BBSome, a regulatory submodule of IFT	retinal degeneration and mislocalization of rhodopsin to IS in Bbs8 mutant mice	BBS(8), RP(51)
RP1	photoreceptor axoneme	binds to singlet microtubules of the axoneme, especially at the point of disc membrane formation	In RP1 mutant mice, disc membranes are abnormally organized and discs do not stack correctly.	RP(1)
RP1L1	photoreceptor axoneme	interacts with RP1, thought to play similar role to RP1	abnormal OS morphology and photosensitivity in RP1L1 mice	OCMD
RP2	basal body	acts as the ARL3 GTPase activating protein. Plays a role in trafficking of vesicles from Golgi to cilium	small eyes and retinal degeneration in rp2 zebrafish morphants	RP(2)
RPGR	connecting cilium	contains a domain with homology to the RCC1 guanine nucleotide exchange factor (GEF) for Ran GTPase. This RCC1-like domain in RPGR acts as a GTP/GDP exchange factor for RAB8, a GTPase important for vesicular trafficking to the primary cilium	mislocalization of cone opsins in the cell body and synapses; reduced levels of rhodopsin in rods; leading to photoreceptor degeneration in Rpgr knockout mice	RP(3), CORD(X1)
RPGRIP1	distal part of the photoreceptor axoneme in rod and cone OS in humans, CC in mice.	transprt along the photoreceptor. Recruitment of RPGR, NPHP4 and SDCCAG8 to the CC	mice lacking RPGRIP1 develop grossly oversized OS discs. Rpgrip1nmf247 mice lack RPGR, NPHP4 and SDCCAG8 at the CC.	LCA(6), CORD(13)
TTC21B	photoreceptor axoneme	intraflagellar transport A complex protein	Ttc21b mutant mice have defective photoreceptor development	ATD(4), NPHP(12)
USH1B (myosin7a)	site of disc morphogenesis. Ribbon synapse. RPE. Apical IS collar.	molecular motor. plays a role in transport of proteins from the IS to the OS for incorporation into discs and RPE65 transport	opsin accumulates in the IS of Shaker1 (Myo7a mutant) mouse. Myo7a mutant mice have lower levels of RPE65, the RPE isomerase that has a key role in the retinoid cycle. Eyes not studied in mariner zebrafish mutant.	USH(1B)
USH1C (harmonin)	at the apical IS collar and ribbon synapse	structural protein, functions in the docking and loading of IFT cargos	deaf circular (Dfcr/Ush1c) mice do not undergo retinal degeneration and have normal synaptic ultrastructure and ERGs. Mice with knock-in of c.216G > A cryptic splice site mutation in Exon 3 of Ush1c have progressive loss of rods between 6 and 12 mo of age	USH(1C)
USH2A (usherin)	at the apical IS collar, basal body of connecting cilium	structural protein, functions in the docking and loading of IFT cargos, essential for long-term structural maintenance of photoreceptors. Interacts with lebercilin and ninein-like protein	mice lacking Ush2a develop late-onset progressive photoreceptor degeneration	USH(2A)
USH2C (GPR98, VLGR1)		G-protein coupled receptor. Exact function in photoreceptors not known	mild, late onset abnormalities in retinal function in Vlgr1/del7TM mice	USH(2C)