A role for rhodopsin in a signal transduction cascade that regulates membrane trafficking and photoreceptor polarity

Novel functions of photoreceptor guanylate cyclases revealed by targeted deletion

Targeted deletion of membrane guanylate cyclases (GCs) has yielded new information concerning their function. Here, we summarize briefly recent results of laboratory generated non-photoreceptor GC knockouts characterized by complex phenotypes affecting the vasculature, heart, brain, kidney, and other tissues. The main emphasis of the review, however, addresses the two GCs expressed in retinal photoreceptors, termed GC-E and GC-F. Naturally occurring GC-E (GUCY2D) null alleles in human and chicken are associated with an early onset blinding disorder, termed "Leber congenital amaurosis type 1" (LCA-1), characterized by extinguished scotopic and photopic ERGs, and retina degeneration. In mouse, a GC-E null genotype produces a recessive cone dystrophy, while rods remain functional. Rod function is supported by the presence of GC-F (Gucy2f), a close relative of GC-E. Deletion of Gucy2f has very little effect on rod and cone physiology and survival. However, a GC-E/GC-F double knockout (GCdko) phenotypically resembles human LCA-1 with extinguished ERGs and rod/cone degeneration. In GCdko rods, PDE6 and GCAPs are absent in outer segments. In contrast, GC-E(-/-) cones lack proteins of the entire phototransduction cascade. These results suggest that GC-E may participate in transport of peripheral membrane proteins from the endoplasmic reticulum (ER) to the outer segments.

What drives cell morphogenesis: a look inside the vertebrate photoreceptor

Vision mediating photoreceptor cells are specialized light-sensitive neurons in the outer layer of the vertebrate retina. The human retina contains approximately 130 million of such photoreceptors, which enable images of the external environment to be captured at high resolution and high sensitivity. Rod and cone photoreceptor subtypes are further specialized for sensing light in low and high illumination, respectively. To enable visual function, these photoreceptors have developed elaborate morphological domains for the detection of light (outer segments), for changing cell shape (inner segments), and for communication with neighboring retinal neurons (synaptic terminals). Furthermore, rod and cone subtypes feature unique morphological variations of these specialized characteristics. Here, we review the major aspects of vertebrate photoreceptor morphology and key genetic mechanisms that drive their formation. These mechanisms are necessary for cell differentiation as well as function. Their defects lead to cell death.

Calnexin improves the folding efficiency of mutant rhodopsin in the presence of pharmacological chaperone 11-cis-retinal

The lectin chaperone calnexin (Cnx) is important for quality control of glycoproteins, and the chances of correct folding of a protein increase the longer the protein interacts with Cnx. Mutations in glycoproteins increase their association with Cnx, and these mutant proteins are retained in the endoplasmic reticulum. However, until now, the increased interaction with Cnx was not known to increase the folding of mutant glycoproteins. Because many human diseases result from glycoprotein misfolding, a Cnx-assisted folding of mutant glycoproteins could be beneficial. Mutations of rhodopsin, the glycoprotein pigment of rod photoreceptors, cause misfolding resulting in retinitis pigmentosa. Despite the critical role of Cnx in glycoprotein folding, surprisingly little is known about its interaction with rhodopsin or whether this interaction could be modulated to increase the folding of mutant rhodopsin. Here, we demonstrate that Cnx preferentially associates with misfolded mutant opsins associated with retinitis pigmentosa. Furthermore, the overexpression of Cnx leads to an increased accumulation of misfolded P23H opsin but not the correctly folded protein. Finally, we demonstrate that increased levels of Cnx in the presence of the pharmacological chaperone 11-cis-retinal increase the folding efficiency and result in an increase in correct folding of mutant rhodopsin. These results demonstrate that misfolded rather than correctly folded rhodopsin is a substrate for Cnx and that the interaction between Cnx and mutant, misfolded rhodopsin, can be targeted to increase the yield of folded mutant protein.

The R7 RGS protein family: multi-subunit regulators of neuronal G protein signaling

G protein-coupled receptor signaling pathways mediate the transmission of signals from the extracellular environment to the generation of cellular responses, a process that is critically important for neurons and neurotransmitter action. The ability to promptly respond to rapidly changing stimulation requires timely inactivation of G proteins, a process controlled by a family of specialized proteins known as regulators of G protein signaling (RGS). The R7 group of RGS proteins (R7 RGS) has received special attention due to their pivotal roles in the regulation of a range of crucial neuronal processes such as vision, motor control, reward behavior, and nociception in mammals. Four proteins in this group, RGS6, RGS7, RGS9, and RGS11, share a common molecular organization of three modules: (i) the catalytic RGS domain, (ii) a GGL domain that recruits G beta(5), an outlying member of the G protein beta subunit family, and (iii) a DEP/DHEX domain that mediates interactions with the membrane anchor proteins R7BP and R9AP. As heterotrimeric complexes, R7 RGS proteins not only associate with and regulate a number of G protein signaling pathway components, but have also been found to form complexes with proteins that are not traditionally associated with G protein signaling. This review summarizes our current understanding of the biology of the R7 RGS complexes including their structure/functional organization, protein-protein interactions, and physiological roles.

Depleting Rac1 in mouse rod photoreceptors protects them from photo-oxidative stress without affecting their structure or function

In nonphagocytic cells, Rac1 is a component of NADPH oxidase that produces reactive oxygen species [Ushio-Fukai M (2006) Sci STKE 2006:re8]. Rac1 is expressed abundantly in mammalian retinal photoreceptors, where it is activated in response to light stimuli [Balasubramanian N, Slepak VZ (2003) Curr Biol 13:1306-1310]. We used Cre-LoxP conditional gene targeting to knock down Rac1 expression in mouse rod photoreceptors and found protection against light-induced photoreceptor death compared with WT litter-mates. We also found a similar protective effect on rods using apocynin, which inhibits NADPH oxidase activity. These results implicate both neuronal Rac1 and NADPH oxidase in cell death in this model of CNS degeneration. Studies in which dominant-mutants of Rac1 were expressed in transgenic Drosophila species demonstrated that Rac1 is a key regulator of photoreceptor morphogenesis and polarity [Chang HY, Ready DF (2000) Science 290:1978-1980]. However, we found that diminished Rac1 expression in mouse rods had no effect on retinal structure or function examined by light microscopy, electron microscopy, rhodopsin measurement, electroretinogram activity, and visual acuity, indicating rod outer segment morphogenesis proceeded normally in Rac1 conditional knockout mice. The lack of structural or functional effect of Rac1 depletion on photoreceptors, but protection under conditions of stress, indicate that the Rac1 pathway warrants exploration as a target for therapy in retinal neurodegenerative diseases.

Syntaxin 3 and SNAP-25 pairing, regulated by omega-3 docosahexaenoic acid, controls the delivery of rhodopsin for the biogenesis of cilia-derived sensory organelles, the rod outer segments

The biogenesis of cilia-derived sensory organelles, the photoreceptor rod outer segments (ROS), is mediated by rhodopsin transport carriers (RTCs). The small GTPase Rab8 regulates ciliary targeting of RTCs, but their specific fusion sites have not been characterized. Here, we report that the Sec6/8 complex, or exocyst, is a candidate effector for Rab8. We also show that the Qa-SNARE syntaxin 3 is present in the rod inner segment (RIS) plasma membrane at the base of the cilium and displays a microtubule-dependent concentration gradient, whereas the Qbc-SNARE SNAP-25 is uniformly distributed in the RIS plasma membrane and the synapse. Treatment with omega-3 docosahexaenoic acid [DHA, 22:6(n-3)] causes increased co-immunoprecipitation and colocalization of SNAP-25 and syntaxin 3 at the base of the cilium, which results in the increased delivery of membrane to the ROS. This is particularly evident in propranolol-treated retinas, in which the DHA-mediated increase in SNARE pairing overcomes the tethering block, including dissociation of Sec8 into the cytosol. Together, our data indicate that the Sec6/8 complex, syntaxin 3 and SNAP-25 regulate rhodopsin delivery, probably by mediating docking and fusion of RTCs. We show further that DHA, an essential polyunsaturated fatty acid of the ROS, increases pairing of syntaxin 3 and SNAP-25 to regulate expansion of the ciliary membrane and ROS biogenesis.

Loss of the cholesterol-binding protein prominin-1/CD133 causes disk dysmorphogenesis and photoreceptor degeneration

Prominin-1/CD133 (Prom-1) is a commonly used marker of neuronal, vascular, hematopoietic and other stem cells, yet little is known about its biological role and importance in vivo. Here, we show that loss of Prom-1 results in progressive degeneration of mature photoreceptors with complete loss of vision. Despite the expression of Prom-1 on endothelial progenitors, photoreceptor degeneration was not attributable to retinal vessel defects, but caused by intrinsic photoreceptor defects in disk formation, outer segment morphogenesis, and associated with visual pigment sorting and phototransduction abnormalities. These findings shed novel insight on how Prom-1 regulates neural retinal development and phototransduction in vertebrates.

Early synaptic defects in tulp1-/- mice

PURPOSE

Mutations in the photoreceptor-specific tubby-like protein 1 (TULP1) underlie a form of autosomal recessive retinitis pigmentosa. To investigate the role of Tulp1 in the photoreceptor synapse, the authors examined the presynaptic and postsynaptic architecture and retinal function in tulp1(-/-) mice

METHODS

The authors used immunohistochemistry to examine tulp1(-/-) mice before retinal degeneration and made comparisons with wild-type (wt) littermates and retinal degeneration 10 (rd10) mice, another model of photoreceptor degeneration that has a comparable rate of degeneration. Retinal function was characterized with the use of electroretinography.

RESULTS

In wt mice, Tulp1 is localized to the photoreceptor synapse. In the tulp1(-/-) synapse, the spatial relationship between the ribbon-associated proteins Bassoon and Piccolo are disrupted, and few intact ribbons are present. Furthermore, bipolar cell dendrites are stunted. Comparable abnormalities are not seen in rd10 mice. The leading edge of the a-wave had normal kinetics in tulp1(-/-) mice but reduced gain in rd10 mice. The b-wave intensity-response functions of tulp1(-/-) mice are shifted to higher intensities than in wt mice, but those of rd10 mice are not.

CONCLUSIONS

Photoreceptor synapses and bipolar cell dendrites in tulp1(-/-) mice display abnormal structure and function. A malformation of the photoreceptor synaptic ribbon is likely the cause of the dystrophy in bipolar cell dendrites. The association of early-onset, severe photoreceptor degeneration preceded by synaptic abnormalities appears to represent a phenotype not previously described. Not only is Tulp1 critical for photoreceptor function and survival, it is essential for the proper development of the photoreceptor synapse.

Expression of a novel OSPGYRP (rice proline-, glycine- and tyrosine-rich protein) gene, which is involved in vesicle trafficking, enhanced cold tolerance in E. coli

A novel OSPGYRP gene encoding a rice proline-, glycine- and tyrosine-rich protein was isolated from cold-stress treated rice seedlings using suppression subtractive hybridization. Both amino acid sequence analysis and subcellular localization confirm that OsPGYRP is a novel protein involved in vesicle trafficking. The expression of the OSPGYRP gene was induced by cold, salt, and osmotic stress. In addition, expression of the OSPGYRP gene in E. coli increased the resistance to cold stress. These results show that OsPGYRP is a novel protein involved in vesicle trafficking and plays an important role in plant adaptation to stress.

Ciliary targeting motif VxPx directs assembly of a trafficking module through Arf4

Dysfunctions of primary cilia and cilia-derived sensory organelles underlie a multitude of human disorders, including retinal degeneration, yet membrane targeting to the cilium remains poorly understood. Here, we show that the newly identified ciliary targeting VxPx motif present in rhodopsin binds the small GTPase Arf4 and regulates its association with the trans-Golgi network (TGN), which is the site of assembly and function of a ciliary targeting complex. This complex is comprised of two small GTPases, Arf4 and Rab11, the Rab11/Arf effector FIP3, and the Arf GTPase-activating protein ASAP1. ASAP1 mediates GTP hydrolysis on Arf4 and functions as an Arf4 effector that regulates budding of post-TGN carriers, along with FIP3 and Rab11. The Arf4 mutant I46D, impaired in ASAP1-mediated GTP hydrolysis, causes aberrant rhodopsin trafficking and cytoskeletal and morphological defects resulting in retinal degeneration in transgenic animals. As the VxPx motif is present in other ciliary membrane proteins, the Arf4-based targeting complex is most likely a part of conserved machinery involved in the selection and packaging of the cargo destined for delivery to the cilium.

R9AP and R7BP: traffic cops for the RGS7 family in phototransduction and neuronal GPCR signaling

RGS (regulator of G protein signaling) proteins have emerged as crucial regulators, effectors and integrators in G-protein-coupled receptor (GPCR) signaling networks. Many RGS proteins accelerate GTP hydrolysis by Galpha subunits, thereby regulating G protein activity, whereas certain RGS proteins also transduce Galpha signals to downstream targets. Particularly intriguing are members of the RGS7 (R7) family (RGS6, RGS7, RGS9 and RGS11), which heterodimerize with Gbeta5. In Caenorhabditis elegans, R7-Gbeta5 heterodimers regulate synaptic transmission, anesthetic action and behavior. In vertebrates, they regulate vision, postnatal development, working memory and the action of psychostimulants or morphine. Here we highlight R9AP and R7BP, a related pair of recently identified SNARE-like R7-family binding proteins, which regulate intracellular trafficking, expression and function of R7-Gbeta5 heterodimers in retina and brain. Emerging understanding of R7BP and R9AP promises to provide new insights into neuronal GPCR signaling mechanisms relevant to the causes and treatment of neurological disorders.

The outer segment serves as a default destination for the trafficking of membrane proteins in photoreceptors

Photoreceptors are compartmentalized neurons in which all proteins responsible for evoking visual signals are confined to the outer segment. Yet, the mechanisms responsible for establishing and maintaining photoreceptor compartmentalization are poorly understood. Here we investigated the targeting of two related membrane proteins, R9AP and syntaxin 3, one residing within and the other excluded from the outer segment. Surprisingly, we have found that only syntaxin 3 has targeting information encoded in its sequence and its removal redirects this protein to the outer segment. Furthermore, proteins residing in the endoplasmic reticulum and mitochondria were similarly redirected to the outer segment after removing their targeting signals. This reveals a pattern where membrane proteins lacking specific targeting information are delivered to the outer segment, which is likely to reflect the enormous appetite of this organelle for new material necessitated by its constant renewal. This also implies that every protein residing outside the outer segment must have a means to avoid this "default" trafficking flow.

Targeting of RGS7/Gbeta5 to the dendritic tips of ON-bipolar cells is independent of its association with membrane anchor R7BP

Complexes of regulator of G-protein signaling (RGS) proteins with G-protein beta5 (Gbeta5) subunits are essential components of signaling pathways that regulate the temporal characteristics of light-evoked responses in vertebrate retinal photoreceptors and ON-bipolar cells. Recent studies have found that RGS/Gbeta5 complexes bind to a new family of adapter proteins, R9AP (RGS9 anchor protein) and R7 family binding protein (R7BP), that in case of the RGS9/Gbeta5 complex were shown to determine its precise subcellular targeting to either the outer segment of photoreceptors or postsynaptic structures of striatal neurons, respectively. In this study, we establish that another trimeric complex consisting of RGS7, Gbeta5, and R7BP subunits is specifically targeted to the dendritic tips of retinal bipolar cells. However, examination of the mechanisms of complex targeting in vivo surprisingly revealed that the delivery of RGS7/Gbeta5 to the dendrites of ON-bipolar cells occurs independently of its association with R7BP. These findings provide a new mechanism for adapter-independent targeting of RGS/Gbeta5 complexes.

The translocation of signaling molecules in dark adapting mammalian rod photoreceptor cells is dependent on the cytoskeleton

In vertebrate rod photoreceptor cells, arrestin and the visual G-protein transducin move between the inner segment and outer segment in response to changes in light. This stimulus dependent translocation of signalling molecules is assumed to participate in long term light adaptation of photoreceptors. So far the cellular basis for the transport mechanisms underlying these intracellular movements remains largely elusive. Here we investigated the dependency of these movements on actin filaments and the microtubule cytoskeleton of photoreceptor cells. Co-cultures of mouse retina and retinal pigment epithelium were incubated with drugs stabilizing and destabilizing the cytoskeleton. The actin and microtubule cytoskeleton and the light dependent distribution of signaling molecules were subsequently analyzed by light and electron microscopy. The application of cytoskeletal drugs differentially affected the cytoskeleton in photoreceptor compartments. During dark adaptation the depolymerization of microtubules as well as actin filaments disrupted the translocation of arrestin and transducin in rod photoreceptor cells. During light adaptation only the delivery of arrestin within the outer segment was impaired after destabilization of microtubules. Movements of transducin and arrestin required intact cytoskeletal elements in dark adapting cells. However, diffusion might be sufficient for the fast molecular movements observed as cells adapt to light. These findings indicate that different molecular translocation mechanisms are responsible for the dark and light associated translocations of arrestin and transducin in rod photoreceptor cells.

Retinal laminar architecture in human retinitis pigmentosa caused by Rhodopsin gene mutations

PURPOSE

To determine the underlying retinal micropathology in subclasses of autosomal dominant retinitis pigmentosa (ADRP) caused by rhodopsin (RHO) mutations.

METHODS

Patients with RHO-ADRP (n = 17, ages 6-73 years), representing class A (R135W and P347L) and class B (P23H, T58R, and G106R) functional phenotypes, were studied with optical coherence tomography (OCT), and colocalized visual thresholds were determined by dark- and light-adapted chromatic perimetry. Autofluorescence imaging was performed with near-infrared light. Retinal histology in hT17M-rhodopsin mice was compared with the human results.

RESULTS

Class A patients had only cone-mediated vision. The outer nuclear layer (ONL) thinned with eccentricity and was not detectable within 3 to 4 mm of the fovea. Scotomatous extracentral retina showed loss of ONL, thickening of the inner retina, and demelanization of RPE. Class B patients had superior-inferior asymmetry in function and structure. The superior retina could have normal rod and cone vision, normal lamination (including ONL) and autofluorescence of the RPE melanin; laminopathy was found in the scotomas. With Fourier-domain-OCT, there was apparent inner nuclear layer (INL) thickening in regions with ONL thinning. Retinal regions without ONL had a thick hyporeflective layer that was continuous with the INL from neighboring regions with normal lamination. Transgenic mice had many of the laminar abnormalities found in patients.

CONCLUSIONS

Retinal laminar abnormalities were present in both classes of RHO-ADRP and were related to the severity of colocalized vision loss. The results in human class B and the transgenic mice support the following disease sequence: ONL diminution with INL thickening; amalgamation of residual ONL with the thickened INL; and progressive retinal remodeling with eventual thinning.

Nrl-knockout mice deficient in Rpe65 fail to synthesize 11-cis retinal and cone outer segments

PURPOSE

To define rod and cone function further in terms of visual cycle mechanism, the retinal phenotype resulting from Rpe65 (retinoid isomerase I) deficiency in Nrl(-)(/)(-) mice having a single class of photoreceptors resembling wild-type cones was characterized and outcomes of retinoid supplementation evaluated.

METHODS

Rpe65(-)(/)(-)/Nrl(-)(/)(-) mice were generated by breeding Rpe65(-)(/)(-) and Nrl(-)(/)(-) strains. Retinal histology, protein expression, retinoid content, and electroretinographic (ERG) responses were evaluated before and after treatment with 11-cis retinal by intraperitoneal injection. Results Retinas of young Rpe65(-)(/-)/Nrl(-)(/-) mice exhibited normal lamination, but lacked intact photoreceptor outer segments at all ages examined. Rpe65, Nrl, and rhodopsin were not detected, and S-opsin and M/L-opsin levels were reduced. Retinyl esters were the only retinoids present. In contrast, Nrl(-)(/)(-) mice exhibited decreased levels of retinaldehydes and retinyl esters, and elevated levels of retinols. ERG responses were elicited from Rpe65(-)(/-)/Nrl(-)(/-) mice only at the two highest intensities over a 4-log-unit range. Significant retinal thinning and outer nuclear layer loss occurred in Rpe65(-)(/-)/Nrl(-)(/-) mice with aging. Administration of exogenous 11-cis retinal did not rescue retinal morphology or markedly improve ERG responses.

CONCLUSIONS

The findings provide clarification of reported cone loss of function in Rpe65(-)(/-)/Nrl(-)(/-) mice, now showing that chromophore absence results in destabilized cone outer segments and rapid retinal degeneration. The data support the view that rod-dominant retinas do not have a cone-specific mechanism for 11-cis retinal synthesis and have potential significance for therapeutic strategies for rescue of cone-rich retinal regions affected by disease in the aging human population.

Rpe65-/- and Lrat-/- mice: comparable models of leber congenital amaurosis

PURPOSE

The Rpe65-/- mouse, used as a model for Leber congenital amaurosis, has slow rod degeneration and rapid cone loss, presumably because of the mistrafficking of cone opsins. This animal does not generate 11-cis retinal, and both cone loss and rod response are restored by 11-cis retinal administration. Similarly, the Lrat-/- mouse does not produce 11-cis retinal. The authors sought to determine whether the same effects on rod and cone opsins in the Rpe65-/- mouse are also present in the Lrat-/- mouse, thereby establishing that these changes can be attributed to the lack of 11-cis retinal rather than to some unknown function of RPE65.

METHODS

Rod and cone opsins were localized by immunohistochemical methods. Functional opsin levels were determined by regeneration with 11-cis retinal. Isorhodopsin levels were determined from pigment extraction. Opsin phosphorylation was determined by mass spectrometry.

RESULTS

Rods in both models degenerated slowly. Regenerable rod opsin levels were similar over the 6-month time course investigated, rod opsin was phosphorylated at a low level (approximately 10%), and minimal 9-cis retinal was generated by a nonphotic process, giving a trace light response. In both models, S-opsin and M/L-opsin failed to traffic to the cone outer segments appropriately, and rapid cone degeneration occurred. Cone opsin mistrafficking in both models was arrested on 11-cis retinal administration.

CONCLUSIONS

These data show that the Lrat-/- and Rpe65-/- mice are comparable models for studies of Leber congenital amaurosis and that the destructive cone opsin mistrafficking is caused by the lack of 11-cis retinal.

A novel form of transducin-dependent retinal degeneration: accelerated retinal degeneration in the absence of rod transducin

PURPOSE

Rhodopsin mutations account for approximately 25% of human autosomal dominant retinal degenerations. However, the molecular mechanisms by which rhodopsin mutations cause photoreceptor cell death are unclear. Mutations in genes involved in the termination of rhodopsin signaling activity have been shown to cause degeneration by persistent activation of the phototransduction cascade. This study examined whether three disease-associated rhodopsin substitutions Pro347Ser, Lys296Glu, and the triple mutant Val20Gly, Pro23His, Pro27Leu (VPP) caused degeneration by persistent transducin-mediated signaling activity.

METHODS

Transgenic mice expressing each of the rhodopsin mutants were crossed onto a transducin alpha-subunit null (Tr(alpha)(-/-)) background, and the rates of photoreceptor degeneration were compared with those of transgenic mice on a wild-type background.

RESULTS

Mice expressing VPP-substituted rhodopsin had the same severity of degeneration in the presence or absence of Tr(alpha). Unexpectedly, mice expressing Pro347Ser- or Lys296Glu-substituted rhodopsins exhibited faster degeneration on a Tr(alpha)(-/-) background. To test whether the absence of alpha-transducin contributed to degeneration by favoring the formation of stable rhodopsin/arrestin complexes, mutant Pro347Ser(+), Tr(alpha)(-/-) mice lacking arrestin (Arr(-/-)) were analyzed. Rhodopsin/arrestin complexes were found not to contribute to degeneration.

CONCLUSIONS

The authors hypothesized that the decay of metarhodopsin to apo-opsin and free all-trans-retinaldehyde is faster with Pro347Ser-substituted rhodopsin than it is with wild-type rhodopsin. Consistent with this, the lipofuscin fluorophores A2PE, A2E, and A2PE-H(2), which form from retinaldehyde, were elevated in Pro347Ser transgenic mice.

Targeting proteins to the ciliary membrane

Most vertebrate cell types display solitary nonmotile cilia on their surface that serve as cellular antennae to sense the extracellular environment. These organelles play key roles in the development of mammals by coordinating the actions of a single cell with events occurring around them. Severe defects in cilia lead to midgestational lethality in mice while more subtle defects lead to pathology in most organs of the body. These pathologies range from cystic diseases of the kidney, liver, and pancreas, to retinal degeneration, to bone and skeletal defects, hydrocephaly, and obesity. The sensory functions of cilia rely on proteins localized specifically to the ciliary membrane. Even though the ciliary membrane is a subdomain of the plasma membrane and is continuous with the plasma membrane, cells have the ability to specifically localize proteins to this domain. In this chapter, we will review what is currently known about the structure and function of the ciliary membrane. We will further discuss ongoing work to understand how the ciliary membrane is assembled and maintained, and discuss protein machinery that is thought to play a role in sorting or trafficking proteins to the ciliary membrane.

A model for transport of membrane-associated phototransduction polypeptides in rod and cone photoreceptor inner segments

We discuss putative mechanisms of membrane protein transport in photoreceptors based on Pde6d and Gucy2e/Gucy2f knockout mice. Knockout of the Pde6d gene encoding PrBP/delta, a prenyl binding protein present in the retina at relatively high levels, was shown to impair transport of G-protein coupled receptor kinase 1 (GRK1) and cone phosphodiesterase alpha' subunit (PDE6alpha') to the rod and cone outer segments. Other prenylated proteins are minimally affected, suggesting some specificity of interaction. Knockout of the Gucy2e gene encoding guanylate cyclase 1 (GC1) disrupted transport of G-protein coupled receptor kinase 1 (GRK1), cone PDE6alpha', cone transducin alpha and gamma subunits (cTalpha and cTgamma) to the cone outer segments, while a GC1/GC2 double knockout prevented transport of rod PDE6, but not transducin, GRK1, or rhodopsin, to the rod outer segments. These knockout phenotypes suggest that PrBP/delta functions in extracting prenylated proteins from the endoplasmic reticulum (ER) where they dock after prenylation, and that GC-bearing membranes may co-transport peripheral membrane proteins in vesicles. We conclude that distinct pathways have evolved in rods and cones for transport of integral and peripherally membrane-associated proteins.

A novel Usher protein network at the periciliary reloading point between molecular transport machineries in vertebrate photoreceptor cells

The human Usher syndrome (USH) is the most frequent cause of combined deaf-blindness. USH is genetically heterogeneous with at least 12 chromosomal loci assigned to three clinical types, USH1-3. Although these USH types exhibit similar phenotypes in human, the corresponding gene products belong to very different protein classes and families. The scaffold protein harmonin (USH1C) was shown to integrate all identified USH1 and USH2 molecules into protein networks. Here, we analyzed a protein network organized in the absence of harmonin by the scaffold proteins SANS (USH1G) and whirlin (USH2D). Immunoelectron microscopic analyses disclosed the colocalization of all network components in the apical inner segment collar and the ciliary apparatus of mammalian photoreceptor cells. In this complex, whirlin and SANS directly interact. Furthermore, SANS provides a linkage to the microtubule transport machinery, whereas whirlin may anchor USH2A isoform b and VLGR1b (very large G-protein coupled receptor 1b) via binding to their cytodomains at specific membrane domains. The long ectodomains of both transmembrane proteins extend into the gap between the adjacent membranes of the connecting cilium and the apical inner segment. Analyses of Vlgr1/del7TM mice revealed the ectodomain of VLGR1b as a component of fibrous links present in this gap. Comparative analyses of mouse and Xenopus photoreceptors demonstrated that this USH protein network is also part of the periciliary ridge complex in Xenopus. Since this structural specialization in amphibian photoreceptor cells defines a specialized membrane domain for docking and fusion of transport vesicles, we suggest a prominent role of the USH proteins in cargo shipment.