What drives cell morphogenesis: a look inside the vertebrate photoreceptor

The effects of ush2a gene knockout on vesicle transport in photoreceptors

USH2A (Usher syndrome type 2A) gene mutations are the predominant cause of Usher syndrome type 2, characterized by sensorineural hearing loss and retinitis pigmentosa (RP), and also significant contributors to non-syndromic RP. To date, there is a lack of definitive therapeutic interventions to mitigate the associated disorders caused by USH2A mutations, and the precise pathogenic mechanisms underlying their onset remain unclear. In the present study, we utilized the ush2a knockout zebrafish model to investigate the pathological mechanisms of RP. In late-stage ush2a-/- zebrafish, the outer segments of rods displayed shortened length and decreased number. Anomalous vesicle accumulation was observed at the junction between the inner and outer segments, accompanied by reduced expression and structural damage of actin filaments in the photoreceptor cells. Furthermore, we discovered that Rab8 expression was downregulated and exhibited aberrant localization in ush2a-/- zebrafish. Additionally, we identified an interaction between USH2A and Rab8. Therefore, the knockout of ush2a may potentially affect vesicle transport through the regulation of Rab8, providing a novel target for maintaining the survival of photoreceptor cells. These findings also contribute to our understanding of the potential molecular pathogenesis underlying RP caused by USH2A gene mutations.

Control of protein and lipid composition of photoreceptor outer segments-Implications for retinal disease

Vision is arguably our most important sense, and its loss brings substantial limitations to daily life for affected individuals. Light is perceived in retinal photoreceptors (PRs), which are highly specialized neurons subdivided into several compartments with distinct functions. The outer segments (OSs) of photoreceptors represent highly specialized primary ciliary compartments hosting the phototransduction cascade, which transforms incoming light into a neuronal signal. Retinal disease can result from various pathomechanisms originating in distinct subcompartments of the PR cell, or in the retinal pigment epithelium which supports the PRs. Dysfunction of primary cilia causes human disorders known as "ciliopathies", in which retinal disease is a common feature. This chapter focuses on PR OSs, discussing the mechanisms controlling their complex structure and composition. A sequence of tightly regulated sorting and trafficking events, both upstream of and within this ciliary compartment, ensures the establishment and maintenance of the adequate proteome and lipidome required for signaling in response to light. We discuss in particular our current understanding of the role of ciliopathy proteins involved in multi-protein complexes at the ciliary transition zone (CC2D2A) or BBSome (BBS1) and how their dysfunction causes retinal disease. While the loss of CC2D2A prevents the fusion of vesicles and delivery of the photopigment rhodopsin to the ciliary base, leading to early OS ultrastructural defects, BBS1 deficiency results in precocious accumulation of cholesterol in mutant OSs and decreased visual function preceding morphological changes. These distinct pathomechanisms underscore the central role of ciliary proteins involved in multiple processes controlling OS protein and lipid composition.

Overexpression of Rhodopsin or Its Mutants Leads to Energy Metabolism Dysfunction in 661w Cells

Purpose

Retinitis pigmentosa (RP) is a heterogeneous group of inherited disorders characterized by photoreceptor degeneration. The rhodopsin gene (RHO) is the most frequent cause of autosomal dominant RP (ADRP), yet it remains unclear how RHO mutations cause heterogeneous phenotypes. Energy failure is a main cause of the secondary cone death during RP progression; however, its role in primary rod death induced by ADRP RHO mutants is unknown.

Methods

Three RHO missense mutations were chosen from different clinical classes. Wild-type (WT) RHO and its mutants, P23H (class B1), R135L (class A), and G188R (class B2), were overexpressed in 661w cells, a mouse photoreceptor cell line, and their effects on oxidative phosphorylation (OXPHOS) and aerobic glycolysis were compared separately.

Results

Here, we report that energy failure is an early event in the cell death caused by overexpression of WT RHO and its mutants. RHO overexpression leads to OXPHOS deficiency, which might be a result of mitochondrial loss. Nonetheless, only in WT RHO and P23H groups, energy stress triggers AMP-activated protein kinase activation and metabolic reprogramming to increase glycolysis. Metabolic reprogramming impairment in R135L and G188R groups might be the reason why energy failure and cell injury are much more severe in those groups.

Conclusions

Our results imply that overexpression of RHO missense mutants have distinct impacts on the two energy metabolic pathways, which might be related to their heterogeneous phenotypes.

Variable phenotypes and penetrance between and within different zebrafish ciliary transition zone mutants

Meckel syndrome, nephronophthisis, Joubert syndrome and Bardet-Biedl syndrome are caused by mutations in proteins that localize to the ciliary transition zone (TZ). The phenotypically distinct syndromes suggest that these TZ proteins have differing functions. However, mutations in a single TZ gene can result in multiple syndromes, suggesting that the phenotype is influenced by modifier genes. We performed a comprehensive analysis of ten zebrafish TZ mutants, including mks1, tmem216, tmem67, rpgrip1l, cc2d2a, b9d2, cep290, tctn1, nphp1 and nphp4, as well as mutants in ift88 and ift172. Our data indicate that variations in phenotypes exist between different TZ mutants, supporting different tissue-specific functions of these TZ genes. Further, we observed phenotypic variations within progeny of a single TZ mutant, reminiscent of multiple disease syndromes being associated with mutations in one gene. In some mutants, the dynamics of the phenotype became complex with transitory phenotypes that are corrected over time. We also demonstrated that multiple-guide-derived CRISPR/Cas9 F0 'crispant' embryos recapitulate zygotic null phenotypes, and rapidly identified ciliary phenotypes in 11 cilia-associated gene candidates (ankfn1, ccdc65, cfap57, fhad1, nme7, pacrg, saxo2, c1orf194, ttc26, zmynd12 and cfap52).

Insights Gained From Zebrafish Models for the Ciliopathy Joubert Syndrome

Cilia are quasi-ubiquitous microtubule-based sensory organelles, which play vital roles in signal transduction during development and cell homeostasis. Dysfunction of cilia leads to a group of Mendelian disorders called ciliopathies, divided into different diagnoses according to clinical phenotype constellation and genetic causes. Joubert syndrome (JBTS) is a prototypical ciliopathy defined by a diagnostic cerebellar and brain stem malformation termed the “Molar Tooth Sign” (MTS), in addition to which patients display variable combinations of typical ciliopathy phenotypes such as retinal dystrophy, fibrocystic renal disease, polydactyly or skeletal dystrophy. Like most ciliopathies, JBTS is genetically highly heterogeneous with ∼40 associated genes. Zebrafish are widely used to model ciliopathies given the high conservation of ciliary genes and the variety of specialized cilia types similar to humans. In this review, we compare different existing JBTS zebrafish models with each other and describe their contributions to our understanding of JBTS pathomechanism. We find that retinal dystrophy, which is the most investigated ciliopathy phenotype in zebrafish ciliopathy models, is caused by distinct mechanisms according to the affected gene. Beyond this, differences in phenotypes in other organs observed between different JBTS-mutant models suggest tissue-specific roles for proteins implicated in JBTS. Unfortunately, the lack of systematic assessment of ciliopathy phenotypes in the mutants described in the literature currently limits the conclusions that can be drawn from these comparisons. In the future, the numerous existing JBTS zebrafish models represent a valuable resource that can be leveraged in order to gain further insights into ciliary function, pathomechanisms underlying ciliopathy phenotypes and to develop treatment strategies using small molecules.

Faim knockout leads to gliosis and late-onset neurodegeneration of photoreceptors in the mouse retina

Fas Apoptotic Inhibitory Molecule protein (FAIM) is a death receptor antagonist and an apoptosis regulator. It encodes two isoforms, namely FAIM-S (short) and FAIM-L (long), both with significant neuronal functions. FAIM-S, which is ubiquitously expressed, is involved in neurite outgrowth. In contrast, FAIM-L is expressed only in neurons and it protects them from cell death. Interestingly, FAIM-L is downregulated in patients and mouse models of Alzheimer's disease before the onset of neurodegeneration, and Faim transcript levels are decreased in mouse models of retinal degeneration. However, few studies have addressed the role of FAIM in the central nervous system, yet alone the retina. The retina is a highly specialized tissue, and its degeneration has proved to precede pathological mechanisms of neurodegenerative diseases. Here we describe that Faim depletion in mice damages the retina persistently and leads to late-onset photoreceptor death in older mice. Immunohistochemical analyses showed that Faim knockout (Faim^-/- ) mice present ubiquitinated aggregates throughout the retina from early ages. Moreover, retinal cells released stress signals that can signal to Müller cells, as shown by immunofluorescence and qRT-PCR. Müller cells monitor retinal homeostasis and trigger a gliotic response in Faim^-/- mice that becomes pathogenic when sustained. In this regard, we observed pronounced vascular leakage at later ages, which may be caused by persistent inflammation. These results suggest that FAIM is an important player in the maintenance of retinal homeostasis, and they support the premise that FAIM is a plausible early marker for late photoreceptor and neuronal degeneration.

Regeneration in Stentor coeruleus

We often think about regeneration in terms of replacing missing structures, such as organs or tissues, with new structures generated via cell proliferation and differentiation. But at a smaller scale, single cells, themselves, are capable of regenerating when part of the cell has been removed. A classic model organism that facilitates the study of cellular regeneration in the giant ciliate Stentor coeruleus. These cells, which can grow to more than a millimeter in size, have the ability to survive after extensive wounding of their surface, and are able to regenerate missing structures. Even a small piece of a cell can regenerate a whole cell with normal geometry, in a matter of hours. Such regeneration requires cells to be able to trigger organelle biogenesis in response to loss of structures. But subcellular regeneration also relies on intracellular mechanisms to create and maintain global patterning within the cell. These mechanisms are not understood, but at a conceptual level they involve processes that resemble those seen in animal development and regeneration. Here we discuss single-celled regeneration in Stentor from the viewpoint of standard regeneration paradigms in animals. For example, there is evidence that regeneration of the oral apparatus in Stentor follows a sender-receiver model similar to crustacean eyestalk regeneration. By drawing these analogies, we find that many of the concepts already known from the study of animal-scale regeneration and development can be applied to the study of regeneration at the cellular level, such as the concepts of determination, induction, mosaic vs. regulative development, and epimorphosis vs. morphallaxis. We propose that the similarities may go beyond analogy, and that some aspects of animal development and regeneration may have evolved by exploiting pre-existing subcellular developmental strategies from unicellular ancestors.

Identification of additional outer segment targeting signals in zebrafish rod opsin

In vertebrate photoreceptors, opsins are highly concentrated in a morphologically distinct ciliary compartment known as the outer segment (OS). Opsin is synthesized in the cell body and transported to the OS at a remarkable rate of 100 to 1000 molecules per second. Opsin transport defects contribute to photoreceptor loss and blindness in human ciliopathies. Previous studies revealed that the rhodopsin C-terminal tail, of 44 amino acids, is sufficient to mediate OS targeting in Xenopus photoreceptors. Here, we show that, although the Xenopus C-terminus retains this function in zebrafish, the homologous zebrafish sequence is not sufficient to target opsin to the OS. This functional difference is largely caused by a change of a single amino acid present in Xenopus but not in other vertebrates examined. Furthermore, we find that sequences in the third intracellular cytoplasmic loop (IC3) and adjacent regions of transmembrane helices 6 and 7 are also necessary for opsin transport in zebrafish. Combined with the cytoplasmic tail, these sequences are sufficient to target opsin to the ciliary compartment.

Biochemistry and physiology of zebrafish photoreceptors

All vertebrates share a canonical retina with light-sensitive photoreceptors in the outer retina. These photoreceptors are of two kinds: rods and cones, adapted to low and bright light conditions, respectively. They both show a peculiar morphology, with long outer segments, comprised of ordered stacks of disc-shaped membranes. These discs host numerous proteins, many of which contribute to the visual transduction cascade. This pathway converts the light stimulus into a biological signal, ultimately modulating synaptic transmission. Recently, the zebrafish (Danio rerio) has gained popularity for studying the function of vertebrate photoreceptors. In this review, we introduce this model system and its contribution to our understanding of photoreception with a focus on the cone visual transduction cascade.

CCDC66 frameshift variant associated with a new form of early-onset progressive retinal atrophy in Portuguese Water Dogs

Aberrant photoreceptor function or morphogenesis leads to blinding retinal degenerative diseases, the majority of which have a genetic aetiology. A variant in PRCD previously identified in Portuguese Water Dogs (PWDs) underlies prcd (progressive rod-cone degeneration), an autosomal recessive progressive retinal atrophy (PRA) with a late onset at 3–6 years of age or older. Herein, we have identified a new form of early-onset PRA (EOPRA) in the same breed. Pedigree analysis suggested an autosomal recessive inheritance. Four PWD full-siblings affected with EOPRA diagnosed at 2–3 years of age were genotyped (173,661 SNPs) along with 2 unaffected siblings, 2 unaffected parents, and 15 unrelated control PWDs. GWAS, linkage analysis and homozygosity mapping defined a 26-Mb candidate region in canine chromosome 20. Whole-genome sequencing in one affected dog and its obligatory carrier parents identified a 1 bp insertion (CFA20:g.33,717,704_33,717,705insT (CanFam3.1); c.2262_c.2263insA) in CCDC66 predicted to cause a frameshift and truncation (p.Val747SerfsTer8). Screening of an extended PWD population confirmed perfect co-segregation of this genetic variant with the disease. Western blot analysis of COS-1 cells transfected with recombinant mutant CCDC66 expression constructs showed the mutant transcript translated into a truncated protein. Furthermore, in vitro studies suggest that the mutant CCDC66 is mislocalized to the nucleus relative to wild type CCDC66. CCDC66 variants have been associated with inherited retinal degenerations (RDs) including canine and murine ciliopathies. As genetic variants affecting the primary cilium can cause ciliopathies in which RD may be either the sole clinical manifestation or part of a syndrome, our findings further support a role for CCDC66 in retinal function and viability, potentially through its ciliary function.

An optimized protocol for retina single-cell RNA sequencing

Purpose

Single-cell RNA sequencing (scRNA-seq) is a powerful technique used to explore gene expression at the single cell level. However, appropriate preparation of samples is essential to obtain the most information out of this transformative technology. Generating high-quality single-cell suspensions from the retina is critical to preserve the native expression profile that will ensure meaningful transcriptome data analysis.

Methods

We modified the conditions for rapid and optimal dissociation of retina sample preparation. We also included additional filtering steps in data analysis for retinal scRNA-seq.

Results

We report a gentle method for dissociation of the mouse retina that minimizes cell death and preserves cell morphology. This protocol also results in detection of higher transcriptional complexity. In addition, the modified computational pipeline leads to better-quality single-cell RNA-sequencing data in retina samples. We also demonstrate the advantages and limitations of using fresh versus frozen retinas to prepare cell or nuclei suspensions for scRNA-seq.

Conclusions

We provide a simple yet robust and reproducible protocol for retinal scRNA-seq analysis, especially for comparative studies.

Structural but Not Functional Alterations in Cones in the Absence of the Retinal Disease Protein Retinitis Pigmentosa 2 (RP2) in a Cone-Only Retina

X-linked retinitis pigmentosa 2 (XLRP2) patients and Rp2 ^null mice exhibit severe cone photoreceptor degeneration. However, due to the paucity of cones in mammalian model systems, it is not clear how cones respond to the loss of RP2. Here we have used the Nrl^-/- mice, which develop a rodless and short wavelength (S) opsin-containing cone-only retina, to generate Rp2 ^null::Nrl^-/- double knock out (Rp2-DKO) mice. We found that the ciliary axoneme and the outer segments (OSs) of the cones were significantly longer with disorganized membrane infoldings as compared to the Nrl^-/- mice. Additionally, we found misregulation in the expression of the genes related to ophthalmic disease, cell trafficking, and stress-response in the Rp2-DKO mice prior to the onset of cone degeneration. Surprisingly, the loss of RP2 did not affect progressive photoreceptor dysfunction of the Nrl^-/- mice and the trafficking of S opsin. Our data suggest that RP2 is a negative regulator of cone OS length but does not affect S-opsin trafficking and S-cone function. Our studies also provide a cone-only platform to design cone-targeted therapeutic strategies for X-linked RP2.

Cas9/sgRNA selective targeting of the P23H Rhodopsin mutant allele for treating retinitis pigmentosa by intravitreal AAV9.PHP.B-based delivery

P23H is the most common mutation in the RHODOPSIN (RHO) gene leading to a dominant form of retinitis pigmentosa (RP), a rod photoreceptor degeneration that invariably causes vision loss. Specific disruption of the disease P23H RHO mutant while preserving the wild-type (WT) functional allele would be an invaluable therapy for this disease. However, various technologies tested in the past failed to achieve effective changes and consequently therapeutic benefits. We validated a CRISPR/Cas9 strategy to specifically inactivate the P23H RHO mutant, while preserving the WT allele in vitro. We, then, translated this approach in vivo by delivering the CRISPR/Cas9 components in murine Rho+/P23H mutant retinae. Targeted retinae presented a high rate of cleavage in the P23H but not WT Rho allele. This gene manipulation was sufficient to slow photoreceptor degeneration and improve retinal functions. To improve the translational potential of our approach, we tested intravitreal delivery of this system by means of adeno-associated viruses (AAVs). To this purpose, the employment of the AAV9-PHP.B resulted the most effective in disrupting the P23H Rho mutant. Finally, this approach was translated successfully in human cells engineered with the homozygous P23H RHO gene mutation. Overall, this is a significant proof-of-concept that gene allele specific targeting by CRISPR/Cas9 technology is specific and efficient and represents an unprecedented tool for treating RP and more broadly dominant genetic human disorders affecting the eye, as well as other tissues.

The ciliopathy protein TALPID3/KIAA0586 acts upstream of Rab8 activation in zebrafish photoreceptor outer segment formation and maintenance

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous microtubule-based organelles involved in signal transduction. Cilia are anchored inside the cell through basal bodies (BBs), modified centrioles also acting as microtubule-organization centers. Photoreceptors (PRs) are sensory neurons, whose primary cilium forms a highly specialized compartment called the outer segment (OS) responsible for sensing incoming light. Thus, ciliopathies often present with retinal degeneration. Mutations in KIAA0586/TALPID3 (TA3) cause Joubert syndrome, in which 30% of affected individuals develop retinal involvement. To elucidate the function of TALPID3 in PRs, we studied talpid3 zebrafish mutants and identified a progressive retinal degeneration phenotype. The majority of PRs lack OS development due to defects in BB positioning and docking at the apical cell surface. Intracellular accumulation of the photopigment opsin leads to PR cell death of moderate severity. Electroretinograms demonstrate severe visual impairement. A small subset of PRs display normally docked BBs and extended OSs through rescue by maternally-deposited Talpid3. While localization of the small GTPase Rab8a, which plays an important role in BB docking, appears unaffected in talpid3-/- PRs, overexpression of constitutively active Rab8a rescues OS formation, indicating that the role of Ta3 in early ciliogenesis lies upstream of Rab8a activation in PRs.

Loss-of-function of the ciliopathy protein Cc2d2a disorganizes the vesicle fusion machinery at the periciliary membrane and indirectly affects Rab8-trafficking in zebrafish photoreceptors

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous organelles involved in transduction of environmental signals such as light sensation in photoreceptors. Concentration of signal detection proteins such as opsins in the ciliary membrane is achieved by RabGTPase-regulated polarized vesicle trafficking and by a selective barrier at the ciliary base, the transition zone (TZ). Dysfunction of the TZ protein CC2D2A causes Joubert/Meckel syndromes in humans and loss of ciliary protein localization in animal models, including opsins in retinal photoreceptors. The link between the TZ and upstream vesicle trafficking has been little explored to date. Moreover, the role of the small GTPase Rab8 in opsin-carrier vesicle (OCV) trafficking has been recently questioned in a mouse model. Using correlative light and electron microscopy and live imaging in zebrafish photoreceptors, we provide the first live characterization of Rab8-mediated trafficking in photoreceptors in vivo. Our results support a possibly redundant role for both Rab8a/b paralogs in OCV trafficking, based on co-localization of Rab8 and opsins in vesicular structures, and joint movement of Rab8-tagged particles with opsin. We further investigate the role of the TZ protein Cc2d2a in Rab8-mediated trafficking using cc2d2a zebrafish mutants and identify a requirement for Cc2d2a in the latest step of OCV trafficking, namely vesicle fusion. Progressive accumulation of opsin-containing vesicles in the apical portion of photoreceptors lacking Cc2d2a is caused by disorganization of the vesicle fusion machinery at the periciliary membrane with mislocalization and loss of the t-SNAREs SNAP25 and Syntaxin3 and of the exocyst component Exoc4. We further observe secondary defects on upstream Rab8-trafficking with cytoplasmic accumulation of Rab8. Taken together, our results support participation of Rab8 in OCV trafficking and identify a novel role for the TZ protein Cc2d2a in fusion of incoming ciliary-directed vesicles, through organization of the vesicle fusion machinery at the periciliary membrane.

Ciliopathies are human disorders caused by dysfunction of primary cilia, ubiquitous organelles involved in transduction of environmental signals to the cells. Concentration and regulation of signal detection proteins in the ciliary membrane is therefore tightly regulated through polarized vesicle trafficking and through a selective barrier at the ciliary base called the transition zone (TZ). Dysfunction of TZ proteins leads to human ciliopathies and to aberrant localization of ciliary proteins in animal models. In this work, we use zebrafish retinal photoreceptors as a model to explore the relationship between the TZ and upstream vesicle trafficking. Relying on modern technologies such as correlative light and electron microscopy and live imaging of fluorescently-tagged proteins, we identify a role for the TZ protein CC2D2A in organizing the components required for vesicle fusion at the periciliary membrane. We also characterize the movement dynamics of vesicles carrying light-detection proteins (opsins) towards the ciliary compartment of photoreceptors in vivo and provide novel data in support of the recently questioned involvement of the small GTPase Rab8 in opsin-carrier vesicle trafficking in photoreceptors.

Rpgrip1 is required for rod outer segment development and ciliary protein trafficking in zebrafish

Mutations in the RPGR-interacting protein 1 (RPGRIP1) gene cause recessive Leber congenital amaurosis (LCA), juvenile retinitis pigmentosa (RP) and cone-rod dystrophy. RPGRIP1 interacts with other retinal disease-causing proteins and has been proposed to have a role in ciliary protein transport; however, its function remains elusive. Here, we describe a new zebrafish model carrying a nonsense mutation in the rpgrip1 gene. Rpgrip1homozygous mutants do not form rod outer segments and display mislocalization of rhodopsin, suggesting a role for RPGRIP1 in rhodopsin-bearing vesicle trafficking. Furthermore, Rab8, the key regulator of rhodopsin ciliary trafficking, was mislocalized in photoreceptor cells of rpgrip1 mutants. The degeneration of rod cells is early onset, followed by the death of cone cells. These phenotypes are similar to that observed in LCA and juvenile RP patients. Our data indicate RPGRIP1 is necessary for rod outer segment development through regulating ciliary protein trafficking. The rpgrip1 mutant zebrafish may provide a platform for developing therapeutic treatments for RP patients.

Apico-basal Polarity Determinants Encoded by crumbs Genes Affect Ciliary Shaft Protein Composition, IFT Movement Dynamics, and Cilia Length

One of the most obvious manifestations of polarity in epithelia is the subdivision of the cell surface by cell junctions into apical and basolateral domains. crumbs genes are among key regulators of this form of polarity. Loss of crumbs function disrupts the apical cell junction belt and crumbs overexpression expands the apical membrane size. Crumbs proteins contain a single transmembrane domain and localize to cell junction area at the apical surface of epithelia. In some tissues, they are also found in cilia. To test their role in ciliogenesis, we investigated mutant phenotypes of zebrafish crumbs genes. In zebrafish, mutations of three crumbs genes, oko meduzy/crb2a, crb3a, and crb2b, affect cilia length in a subset of tissues. In oko meduzy (ome), this is accompanied by accumulation of other Crumbs proteins in the ciliary compartment. Moreover, intraflagellar transport (IFT) particle components accumulate in the ciliary shaft of ome;crb3a double mutants. Consistent with the above, Crb3 knockdown in mammalian cells affects the dynamics of IFT particle movement. These findings reveal crumbs-dependent mechanisms that regulate the localization of ciliary proteins, including Crumbs proteins themselves, and show that crumbs genes modulate intraflagellar transport and cilia elongation.

Photoreceptor Cilia and Retinal Ciliopathies

Photoreceptors are sensory neurons designed to convert light stimuli into neurological responses. This process, called phototransduction, takes place in the outer segments (OS) of rod and cone photoreceptors. OS are specialized sensory cilia, with analogous structures to those present in other nonmotile cilia. Deficient morphogenesis and/or dysfunction of photoreceptor sensory cilia (PSC) caused by mutations in a variety of photoreceptor-specific and common cilia genes can lead to inherited retinal degenerations (IRDs). IRDs can manifest as isolated retinal diseases or syndromic diseases. In this review, we describe the structure and composition of PSC and different forms of ciliopathies with retinal involvement. We review the genetics of the IRDs, which are monogenic disorders but genetically diverse with regard to causality.

The cytoplasmic tail of rhodopsin triggers rapid rod degeneration in kinesin-2 mutants

Photoreceptor degeneration can lead to blindness and represents the most common form of neural degenerative disease worldwide. Although many genes involved in photoreceptor degeneration have been identified, the underlying mechanisms remain to be elucidated. Here we examined photoreceptor development in zebrafish kif3a and kif3b mutants, which affect two subunits of the kinesin-2 complex. In both mutants, rods degenerated quickly, whereas cones underwent a slow degeneration process. Notably, the photoreceptor defects were considerably more severe in kif3a mutants than in kif3b mutants. In the cone photoreceptors of kif3a mutants, opsin proteins accumulated in the apical region and formed abnormal membrane structures. In contrast, rhodopsins were enriched in the rod cell body membrane and represented the primary reason for rapid rod degeneration in these mutants. Moreover, removal of the cytoplasmic tail of rhodopsin to reduce its function, but not decreasing rhodopsin expression levels, prevented rod degeneration in both kif3a and kif3b mutants. Of note, overexpression of full-length rhodopsin or its cytoplasmic tail domain, but not of rhodopsin lacking the cytoplasmic tail, exacerbated rod degeneration in kif3a mutants, implying an important role of the cytoplasmic tail in rod degeneration. Finally, we showed that the cytoplasmic tail of rhodopsin might trigger rod degeneration through activating the downstream calcium signaling pathway, as drug treatment with inhibitors of intracellular calcium release prevented rod degeneration in kif3a mutants. Our results demonstrate a previously unknown function of the rhodopsin cytoplasmic domain during opsin-triggered photoreceptor degeneration and may open up new avenues for managing this disease.

An inducible amphipathic helix within the intrinsically disordered C terminus can participate in membrane curvature generation by peripherin-2/rds

Peripherin-2/rds is required for biogenesis of vertebrate photoreceptor outer segment organelles. Its localization at the high-curvature rim domains of outer segment disk membranes suggests that it may act to shape these structures; however, the molecular function of this protein is not yet resolved. Here, we apply biochemical, biophysical, and imaging techniques to elucidate the role(s) played by the protein's intrinsically disordered C-terminal domain and an incipient amphipathic α-helix contained within it. We investigated a deletion mutant lacking only this α-helix in stable cell lines and Xenopus laevis photoreceptors. We also studied a soluble form of the full-length ∼7-kDa cytoplasmic C terminus in cultured cells and purified from Escherichia coli The α-helical motif was not required for protein biosynthesis, tetrameric subunit assembly, tetramer polymerization, localization at disk rims, interaction with GARP2, or the generation of membrane curvature. Interestingly, however, loss of the helical motif up-regulated membrane curvature generation in cellulo, introducing the possibility that it may regulate this activity in photoreceptors. Furthermore, the incipient α-helix (within the purified soluble C terminus) partitioned into membranes only when its acidic residues were neutralized by protonation. This suggests that within the context of full-length peripherin-2/rds, partitioning would most likely occur at a bilayer interfacial region, potentially adjacent to the protein's transmembrane domains. In sum, this study significantly strengthens the evidence that peripherin-2/rds functions directly to shape the high-curvature rim domains of the outer segment disk and suggests that the protein's C terminus may modulate membrane curvature-generating activity present in other protein domains.

Loss of ift122, a Retrograde Intraflagellar Transport (IFT) Complex Component, Leads to Slow, Progressive Photoreceptor Degeneration Due to Inefficient Opsin Transport

In the retina, aberrant opsin transport from cell bodies to outer segments leads to retinal degenerative diseases such as retinitis pigmentosa. Opsin transport is facilitated by the intraflagellar transport (IFT) system that mediates the bidirectional movement of proteins within cilia. In contrast to functions of the anterograde transport executed by IFT complex B (IFT-B), the precise functions of the retrograde transport mediated by IFT complex A (IFT-A) have not been well studied in photoreceptor cilia. Here, we analyzed developing zebrafish larvae carrying a null mutation in ift122 encoding a component of IFT-A. ift122 mutant larvae show unexpectedly mild phenotypes, compared with those of mutants defective in IFT-B. ift122 mutants exhibit a slow onset of progressive photoreceptor degeneration mainly after 7 days post-fertilization. ift122 mutant larvae also develop cystic kidney but not curly body, both of which are typically observed in various ciliary mutants. ift122 mutants display a loss of cilia in the inner ear hair cells and nasal pit epithelia. Loss of ift122 causes disorganization of outer segment discs. Ectopic accumulation of an IFT-B component, ift88, is observed in the ift122 mutant photoreceptor cilia. In addition, pulse-chase experiments using GFP-opsin fusion proteins revealed that ift122 is required for the efficient transport of opsin and the distal elongation of outer segments. These results show that IFT-A is essential for the efficient transport of outer segment proteins, including opsin, and for the survival of retinal photoreceptor cells, rendering the ift122 mutant a unique model for human retinal degenerative diseases.

Metabolic and redox signaling in the retina

Visual perception by photoreceptors relies on the interaction of incident photons from light with a derivative of vitamin A that is covalently linked to an opsin molecule located in a special subcellular structure, the photoreceptor outer segment. The photochemical reaction produced by the photon is optimal when the opsin molecule, a seven-transmembrane protein, is embedded in a lipid bilayer of optimal fluidity. This is achieved in vertebrate photoreceptors by a high proportion of lipids made with polyunsaturated fatty acids, which have the detrimental property of being oxidized and damaged by light. Photoreceptors cannot divide, but regenerate their outer segments. This is an enormous energetic challenge that explains why photoreceptors metabolize glucose through aerobic glycolysis, as cancer cells do. Uptaken glucose produces metabolites to renew that outer segment as well as reducing power through the pentose phosphate pathway to protect photoreceptors against oxidative damage.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Analysis of cilia structure and function in zebrafish

Cilia are microtubule-based protrusions on the surface of most eukaryotic cells. They are found in most, if not all, vertebrate organs. Prominent cilia form in sensory structures, the eye, the ear, and the nose, where they are crucial for the detection of environmental stimuli, such as light and odors. Cilia are also involved in developmental processes, including left-right asymmetry formation, limb morphogenesis, and the patterning of neurons in the neural tube. Some cilia, such as those found in nephric ducts, are thought to have mechanosensory roles. Zebrafish proved very useful in genetic analysis and imaging of cilia-related processes, and in the modeling of mechanisms behind human cilia abnormalities, known as ciliopathies. A number of zebrafish defects resemble those seen in human ciliopathies. Forward and reverse genetic strategies generated a wide range of cilia mutants in zebrafish, which can be studied using sophisticated genetic and imaging approaches. In this chapter, we provide a set of protocols to examine cilia morphology, motility, and cilia-related defects in a variety of organs, focusing on the embryo and early postembryonic development.

Unexpected Roles for Ciliary Kinesins and Intraflagellar Transport Proteins

Transport of proteins in the ciliary shaft is driven by microtubule-dependent motors, kinesins. Prior studies suggested that the heterotrimeric ciliary kinesin may be dispensable for certain aspects of transport in specialized cilia of vertebrate photoreceptor cells. To test this possibility further, we analyzed the mutant phenotype of the zebrafish kif3a gene, which encodes the common motor subunit of heterotrimeric ciliary kinesins. Cilia are absent in all organs examined, leading to the conclusion that kif3a is indispensable for ciliogenesis in all cells, including photoreceptors. Unexpectedly, kif3a function precedes ciliogenesis as ciliary basal bodies are mispositioned in mutant photoreceptors. This phenotype is much less pronounced in intraflagellar transport (IFT) mutants and reveals that kif3a has a much broader role than previously assumed. Despite the severity of their basal body phenotype, kif3a mutant photoreceptors survive longer compared to those in IFT mutants, which display much weaker basal body mispositioning. This effect is absent in kif3a;IFT double mutants, indicating that IFT proteins have ciliary transport-independent roles, which add to the severity of their photoreceptor phenotype. kif3a is dispensable for basal body docking in otic vesicle sensory epithelia and, surprisingly, short cilia form in mechanosensory cristae even in the absence of kif3a In contrast to Kif3a, the functions of the Kif3c-related protein, encoded by the kif3c-like (kif3cl) gene, and the homodimeric ciliary kinesin, kif17, are dispensable for photoreceptor morphogenesis. These studies demonstrate unexpected new roles for both ciliary heterotrimeric kinesins and IFT particle genes and clarify the function of kif17, the homodimeric ciliary kinesin gene.

Discs of mammalian rod photoreceptors form through the membrane evagination mechanism

Ultrastructural analysis of membrane organization in mouse rod cells demonstrates that new photoreceptor discs are contiguous evaginations of the outer segment plasma membrane, thereby resolving the debate of whether discs are formed through membrane evagination or intracellular vesicular fusion.

Photoreceptor discs are membrane organelles harboring components of the visual signal transduction pathway. The mechanism by which discs form remains enigmatic and is the subject of a major controversy. Classical studies suggest that discs are formed as serial plasma membrane evaginations, whereas a recent alternative postulates that discs, at least in mammalian rods, are formed through intracellular vesicular fusion. We evaluated these models in mouse rods using methods that distinguish between the intracellular vesicular structures and plasma membrane folds independently of their appearance in electron micrographs. The first differentiated membranes exposed to the extracellular space from intracellular membranes; the second interrogated the orientation of protein molecules in new discs. Both approaches revealed that new discs are plasma membrane evaginations. We further demonstrated that vesiculation and plasma membrane enclosure at the site of new disc formation are artifacts of tissue fixation. These data indicate that all vertebrate photoreceptors use the evolutionary conserved membrane evagination mechanism to build their discs.

The Ciliopathy Protein CC2D2A Associates with NINL and Functions in RAB8-MICAL3-Regulated Vesicle Trafficking

Ciliopathies are a group of human disorders caused by dysfunction of primary cilia, ubiquitous microtubule-based organelles involved in transduction of extra-cellular signals to the cell. This function requires the concentration of receptors and channels in the ciliary membrane, which is achieved by complex trafficking mechanisms, in part controlled by the small GTPase RAB8, and by sorting at the transition zone located at the entrance of the ciliary compartment. Mutations in the transition zone gene CC2D2A cause the related Joubert and Meckel syndromes, two typical ciliopathies characterized by central nervous system malformations, and result in loss of ciliary localization of multiple proteins in various models. The precise mechanisms by which CC2D2A and other transition zone proteins control protein entrance into the cilium and how they are linked to vesicular trafficking of incoming cargo remain largely unknown. In this work, we identify the centrosomal protein NINL as a physical interaction partner of CC2D2A. NINL partially co-localizes with CC2D2A at the base of cilia and ninl knockdown in zebrafish leads to photoreceptor outer segment loss, mislocalization of opsins and vesicle accumulation, similar to cc2d2a-/- phenotypes. Moreover, partial ninl knockdown in cc2d2a-/- embryos enhances the retinal phenotype of the mutants, indicating a genetic interaction in vivo, for which an illustration is found in patients from a Joubert Syndrome cohort. Similar to zebrafish cc2d2a mutants, ninl morphants display altered Rab8a localization. Further exploration of the NINL-associated interactome identifies MICAL3, a protein known to interact with Rab8 and to play an important role in vesicle docking and fusion. Together, these data support a model where CC2D2A associates with NINL to provide a docking point for cilia-directed cargo vesicles, suggesting a mechanism by which transition zone proteins can control the protein content of the ciliary compartment.

MicroRNAs are essential for differentiation of the retinal pigmented epithelium and maturation of adjacent photoreceptors

Dysfunction of the retinal pigmented epithelium (RPE) results in degeneration of photoreceptors and vision loss and is correlated with common blinding disorders in humans. Although many protein-coding genes are known to be expressed in RPE and are important for its development and maintenance, virtually nothing is known about the in vivo roles of non-coding transcripts. The expression patterns of microRNAs (miRNAs) have been analyzed in a variety of ocular tissues, and a few were implicated to play role in RPE based on studies in cell lines. Here, through RPE-specific conditional mutagenesis of Dicer1 or Dgcr8 in mice, the importance of miRNAs for RPE differentiation was uncovered. miRNAs were found to be dispensable for maintaining RPE fate and survival, and yet they are essential for the acquisition of important RPE properties such as the expression of genes involved in the visual cycle pathway, pigmentation and cell adhesion. Importantly, miRNAs of the RPE are required for maturation of adjacent photoreceptors, specifically for the morphogenesis of the outer segments. The alterations in the miRNA and mRNA profiles in the Dicer1-deficient RPE point to a key role of miR-204 in regulation of the RPE differentiation program in vivo and uncover the importance of additional novel RPE miRNAs. This study reveals the combined regulatory activity of miRNAs that is required for RPE differentiation and for the development of the adjacent neuroretina.

No evidence for a genetic blueprint: The case of the "complex" mammalian photoreceptor

Despite the intensity of the search for genes causing inherited retinal degenerations over the past 3 decades, of the approximately 200 disease genes identified to date, all appear to be ordinary housekeeping genes specifying proteins playing basic structural and functional roles in the mature photoreceptor cells. No genes or genetic elements have been identified which can be construed as having a specific morphogenic role, directing the development of the cytoarchitecture of any particular retinal cell. The evidence suggests that the cytoarchitecture of the retinal photoreceptors, although enormously complex, arises from the self-organization of the cells constituents without any regulation or direction from an external genetic blueprint.

Ultrastructural visualization of trans-ciliary rhodopsin cargoes in mammalian rods

Background

Cilia are vital to various cellular and sensory functions. The pathway by which ciliary membrane proteins translocate through the transition zone is not well understood. Direct morphological characterization of ciliary cargoes in transit remains lacking. In the vertebrate photoreceptor, rhodopsin is synthesized and transported from the inner segment to the disc membranes of the outer segment (OS), which is a modified cilium. To date, the membrane topology of the basal OS and the mechanisms by which rhodopsin is transported through the transition zone (i.e., connecting cilium) and by which nascent disc membranes are formed remain controversial.

Results

Using an antibody recognizing its cytoplasmic C-terminus, we localize rhodopsin on both the plasma membrane and lumen of the connecting cilium by immuno-electron microscopy (EM). We also use transmission EM to visualize the electron-dense enzymatic products derived from the rhodopsin-horseradish peroxidase (HRP) fusion in transfected rodent rods. In the connecting cilium, rhodopsin is not only expressed in the plasma membrane but also in the lumen on two types of membranous carriers, long smooth tubules and small, coated, filament-bound vesicles. Additionally, membrane-bound rhodopsin carriers are also found in close proximity to the nascent discs at the basal OS axoneme and in the distal inner segment. This topology-indicative HRP-rhodopsin reporter shows that the nascent basalmost discs and the mature discs have the same membrane topology, with no indication of evagination or invagination from the basal OS plasma membranes. Serial block face and focus ion beam scanning EM analyses both indicate that the transport carriers enter the connecting cilium lumen from either the basal body lumen or cytoplasmic space between the axonemal microtubules and the ciliary plasma membrane.

Conclusions

Our results suggest the existence of multiple ciliary gate entry pathways in rod photoreceptors. Rhodopsin is likely transported across the connecting cilium on the plasma membrane and through the lumens on two types of tubulovesicular carriers produced in the inner segment. Our findings agree with a previous model that rhodopsin carriers derived from the cell body may fuse directly onto nascent discs as they grow and mature.

Electronic supplementary material

The online version of this article (doi:10.1186/s13630-015-0013-1) contains supplementary material, which is available to authorized users.

Zebrafish as a systems toxicology model for developmental neurotoxicity testing

The developing brain is extremely sensitive to many chemicals. Exposure to neurotoxicants during development has been implicated in various neuropsychiatric and neurological disorders, including autism spectrum disorder, attention deficit hyperactive disorder, schizophrenia, Parkinson's disease, and Alzheimer's disease. Although rodents have been widely used for developmental neurotoxicity testing, experiments using large numbers of rodents are time-consuming, expensive, and raise ethical concerns. Using alternative non-mammalian animal models may relieve some of these pressures by allowing testing of large numbers of subjects while reducing expenses and minimizing the use of mammalian subjects. In this review, we discuss some of the advantages of using zebrafish in developmental neurotoxicity testing, focusing on central nervous system development, neurobehavior, toxicokinetics, and toxicodynamics in this species. We also describe some important examples of developmental neurotoxicity testing using zebrafish combined with gene expression profiling, neuroimaging, or neurobehavioral assessment. Zebrafish may be a systems toxicology model that has the potential to reveal the pathways of developmental neurotoxicity and to provide a sound basis for human risk assessments.

Retinal phototransduction

Vision is perhaps the most important of all our senses, and gives us an immense amount of information regarding the outside world. The initial format in which this information reaches the retina are photons; particles of energy radiation of a given wavelength emitted or reflected from our surroundings. The brain itself however, perceives information in electrical signals via action potentials and changes in electrochemical gradients. The processes involved in the transduction of photons into electrical potentials will be the focus of this article. This review article summarizes the recent advances in understanding these complex pathways and provides an overview of the main molecules involved in the neurobiology of vision.

Transgenic expression of constitutively active RAC1 disrupts mouse rod morphogenesis

PURPOSE

Dominant-active RAC1 rescues photoreceptor structure in Drosophila rhodopsin-null mutants, indicating an important role in morphogenesis. This report assesses the morphogenetic effect of activated RAC1 during mammalian rod photoreceptor development using transgenic mice that express constitutively active (CA) RAC1.

METHODS

Transgenic mice were generated by expressing CA RAC1 under control of the Rhodopsin promoter, and morphological features of the photoreceptors were evaluated by histology, immunohistochemistry, and transmission electron microscopy. Function was evaluated by electroretinography. Potential protein partners of CA RAC1 were identified by co-immunoprecipitation of retinal extracts.

RESULTS

Constitutively active RAC1 expression in differentiating rods disrupted outer retinal lamination as early as postnatal day (P)6, and many photoreceptor cell nuclei were displaced apically into the presumptive subretinal space. These photoreceptors did not develop normal inner and outer segments and had abnormal placement of synaptic elements. Some photoreceptor nuclei were also mislocalized into the inner nuclear layer. Extensive photoreceptor degeneration was subsequently observed in the adult animal. Constitutively active RAC1 formed a complex with the polarity protein PAR6 and with microtubule motor dynein in mouse retina. The normal localization of the PAR6 complex was disrupted in CA RAC1-expressing rod photoreceptors.

CONCLUSIONS

Constitutively active RAC1 had a profound negative effect on mouse rod cell viability and development. Rod photoreceptors in the CA RAC1 retina exhibited a defect in polarity and migration. Constitutively active RAC1 disrupted rod morphogenesis and gave a phenotype resembling that found in the Crumbs mutant. PAR6 and dynein are two potential downstream effectors that may be involved in CA RAC1-mediated defective mouse photoreceptor morphogenesis.

The retinal phenotype of Usher syndrome: pathophysiological insights from animal models

The Usher syndrome (USH) is the most prevalent cause of inherited deaf-blindness. Three clinical subtypes, USH1-3, have been defined, and ten USH genes identified. The hearing impairment due to USH gene defects has been shown to result from improper organisation of the hair bundle, the sound receptive structure of sensory hair cells. In contrast, the cellular basis of the visual defect is less well understood as this phenotype is absent in almost all the USH mouse models that faithfully mimic the human hearing impairment. Structural and molecular interspecies discrepancies regarding photoreceptor calyceal processes and the association with the distribution of USH1 proteins have recently been unravelled, and have led to the conclusion that a defect in the USH1 protein complex-mediated connection between the photoreceptor outer segment and the surrounding calyceal processes (in both rods and cones), and the inner segment (in rods only), probably causes the USH1 retinal dystrophy in humans.

Synaptojanin 1 is required for endolysosomal trafficking of synaptic proteins in cone photoreceptor inner segments

Highly polarized cells such as photoreceptors require precise and efficient strategies for establishing and maintaining the proper subcellular distribution of proteins. The signals and molecular machinery that regulate trafficking and sorting of synaptic proteins within cone inner segments is mostly unknown. In this study, we show that the polyphosphoinositide phosphatase Synaptojanin 1 (SynJ1) is critical for this process. We used transgenic markers for trafficking pathways, electron microscopy, and immunocytochemistry to characterize trafficking defects in cones of the zebrafish mutant, nrc(a14) , which is deficient in phosphoinositide phosphatase, SynJ1. The outer segments and connecting cilia of nrc(a14) cone photoreceptors are normal, but RibeyeB and VAMP2/synaptobrevin, which normally localize to the synapse, accumulate in the nrc(a14) inner segment. The structure of the Endoplasmic Reticulum in nrc(a14) mutant cones is normal. Golgi develop normally, but later become disordered. Large vesicular structures accumulate within nrc(a14) cone photoreceptor inner segments, particularly after prolonged incubation in darkness. Cone inner segments of nrc (a14) mutants also have enlarged acidic vesicles, abnormal late endosomes, and a disruption in autophagy. This last pathway also appears exacerbated by darkness. Taken altogether, these findings show that SynJ1 is required in cones for normal endolysosomal trafficking of synaptic proteins.

Myosin 3A kinase activity is regulated by phosphorylation of the kinase domain activation loop

Class III myosins are unique members of the myosin superfamily in that they contain both a motor and kinase domain. We have found that motor activity is decreased by autophosphorylation, although little is known about the regulation of the kinase domain. We demonstrate by mass spectrometry that Thr-178 and Thr-184 in the kinase domain activation loop and two threonines in the loop 2 region of the motor domain are autophosphorylated (Thr-908 and Thr-919). The kinase activity of MYO3A 2IQ with the phosphomimic (T184E) or phosphoblock (T184A) mutations demonstrates that kinase activity is reduced 30-fold as a result of the T184A mutation, although the Thr-178 site only had a minor impact on kinase activity. Interestingly, the actin-activated ATPase activity of MYO3A 2IQ is slightly reduced as a result of the T178A and T184A mutations suggesting coupling between motor and kinase domains. Full-length GFP-tagged T184A and T184E MYO3A constructs transfected into COS7 cells do not disrupt the ability of MYO3A to localize to filopodia structures. In addition, we demonstrate that T184E MYO3A reduces filopodia elongation in the presence of espin-1, whereas T184A enhances filopodia elongation in a similar fashion to kinase-dead MYO3A. Our results suggest that as MYO3A accumulates at the tips of actin protrusions, autophosphorylation of Thr-184 enhances kinase activity resulting in phosphorylation of the MYO3A motor and reducing motor activity. The differential regulation of the kinase and motor activities allows for MYO3A to precisely self-regulate its concentration in the actin bundle-based structures of cells.

Ablation of the X-linked retinitis pigmentosa 2 (Rp2) gene in mice results in opsin mislocalization and photoreceptor degeneration

PURPOSE

Mutations in the RP2 gene are associated with 10% to 15% of X-linked retinitis pigmentosa (XLRP), a debilitating disorder characterized by the degeneration of retinal rod and cone photoreceptors. The molecular mechanism of pathogenesis of photoreceptor degeneration in XLRP-RP2 has not been elucidated, and no treatment is currently available. This study was undertaken to investigate the pathogenesis of RP2-associated retinal degeneration.

METHODS

We introduced loxP sites that flank exon 2, a mutational hotspot in XLRP-RP2, in the mouse Rp2 gene. We then produced Rp2-null allele using transgenic mice that expressed Cre-recombinase under control of the ubiquitous CAG promoter. Electroretinography (ERG), histology, light microscopy, transmission electron microscopy, and immunofluorescence microscopy were performed to ascertain the effect of ablation of Rp2 on photoreceptor development, function, and protein trafficking.

RESULTS

Although no gross abnormalities were detected in the Rp2(null) mice, photopic (cone) and scotopic (rod) function as measured by ERG showed a gradual decline starting as early as 1 month of age. We also detected slow progressive degeneration of the photoreceptor membrane discs in the mutant retina. These defects were associated with mislocalization of cone opsins to the nuclear and synaptic layers and reduced rhodopsin content in the outer segment of mutant retina prior to the onset of photoreceptor degeneration.

CONCLUSIONS

Our studies suggest that RP2 contributes to the maintenance of photoreceptor function and that cone opsin mislocalization represents an early step in XLRP caused by RP2 mutations. The Rp2(null) mice should serve as a useful preclinical model for testing gene- and cell-based therapies.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

Kinesin-2 family motors in the unusual photoreceptor cilium

This review focuses on recent advances in the understanding of kinesin-2 family motors in vertebrate photoreceptor development. Zebrafish photoreceptors develop by the 3rd day of embryogenesis, making it possible to study mutant phenotypes without the use of conditional alleles. Recent work using a zebrafish kif3b mutant allele validates the concept that the heterotrimeric kinesin II motor is generally required for ciliogenesis. In zebrafish photoreceptors, however, loss of kif3b function delays but does not block cilium formation. This is thought to occur because both kif3b or kif3c can dimerize with kif3a and function redundantly. The second ciliary kinesin thought to function in photoreceptor cells is kif17. Prior work has shown that either morpholino knockdown of this gene or the overexpression of its dominant negative form can reduce or delay photoreceptor cilium development without any evident impact on ciliogenesis in general. This has led to the idea that kif17 may play an important role only in some specialized cilium types, such the one in photoreceptor cells. In a recently identified kif17 mutant, however, photoreceptor outer segments are formed by 5 dpf and an obvious delay of outer segment formation is seen only at the earliest stage analyzed (3 dpf). This work suggests that kif17 plays a significant role mainly at an early stage of photoreceptor development. Taken together, these studies lead to an intriguing concept that as they differentiate photoreceptors alter their kinesin repertoire.

Localization of Usher 1 proteins to the photoreceptor calyceal processes, which are absent from mice

The mechanisms underlying retinal dystrophy in Usher syndrome type I (USH1) remain unknown because mutant mice lacking any of the USH1 proteins-myosin VIIa, harmonin, cadherin-23, protocadherin-15, sans-do not display retinal degeneration. We found here that, in macaque photoreceptor cells, all USH1 proteins colocalized at membrane interfaces (i) between the inner and outer segments in rods and (ii) between the microvillus-like calyceal processes and the outer segment basolateral region in rods and cones. This pattern, conserved in humans and frogs, was mediated by the formation of an USH1 protein network, which was associated with the calyceal processes from the early embryonic stages of outer segment growth onwards. By contrast, mouse photoreceptors lacked calyceal processes and had no USH1 proteins at the inner-outer segment interface. We suggest that USH1 proteins form an adhesion belt around the basolateral region of the photoreceptor outer segment in humans, and that defects in this structure cause the retinal degeneration in USH1 patients.

Light-driven calcium signals in mouse cone photoreceptors

Calcium mediates various neuronal functions. The complexity of neuronal Ca²⁺ signaling is well exemplified by retinal cone photoreceptors, which, with their distinct compartmentalization, offer unique possibilities for studying the diversity of Ca²⁺ functions in a single cell. Measuring subcellular Ca²⁺ signals in cones under physiological conditions is not only fundamental for understanding cone function, it also bears important insights into pathophysiological processes governing retinal neurodegeneration. However, due to the proximity of light-sensitive outer segments to other cellular compartments, optical measurements of light-evoked Ca²⁺ responses in cones are challenging. We addressed this problem by generating a transgenic mouse (HR2.1:TN-XL) in which both short- and middle-wavelength-sensitive cones selectively express the genetically encoded ratiometric Ca²⁺ biosensor TN-XL. We show that HR2.1:TN-XL allows recording of light-evoked Ca²⁺ responses using two-photon imaging in individual cone photoreceptor terminals and to probe phototransduction and its diverse regulatory mechanisms with pharmacology at subcellular resolution. To further test this system, we asked whether the classical, nitric oxide (NO)-soluble guanylyl-cyclase (sGC)-cGMP pathway could modulate Ca²⁺ in cone terminals. Surprisingly, NO reduced Ca²⁺ resting levels in mouse cones, without evidence for direct sGC involvement. In conclusion, HR2.1:TN-XL mice offer unprecedented opportunities to elucidate light-driven Ca²⁺ dynamics and their (dys)regulation in cone photoreceptors.

Who drives the ciliary highway?

Cilia are protrusions on the surface of cells. They are frequently motile and function to propel cells in an aqueous environment or to generate fluid flow. Equally important is the role of immotile cilia in detecting environmental changes or in sensing extracellular signals. The structure of cilia is supported by microtubules, and their formation requires microtubule-dependent motors, kinesins, which are thought to transport both structural and signaling ciliary proteins from the cell body into the distal portion of the ciliary shaft. In multicellular organisms, multiple kinesins are known to drive ciliary transport, and frequently cilia of a single cell type require more than one kinesin for their formation and function. In addition to kinesin-2 family motors, which function in cilia of all species investigated so far, kinesins from other families contribute to the transport of signaling proteins in a tissue-specific manner. It is becoming increasingly obvious that functional relationships between ciliary kinesins are complex, and a good understanding of these relationships is essential to comprehend the basis of biological processes as diverse as olfaction, vision, and embryonic development.

Kinesin-2 family in vertebrate ciliogenesis

The differentiation of cilia is mediated by kinesin-driven transport. As the function of kinesins in vertebrate ciliogenesis is poorly characterized, we decided to determine the role of kinesin-2 family motors--heterotrimeric kinesin-II and the homodimeric Kif17 kinesin--in zebrafish cilia. We report that kif17 is largely dispensable for ciliogenesis; kif17 homozygous mutant animals are viable and display subtle morphological defects of olfactory cilia only. In contrast to that, the kif3b gene, encoding a heterotrimeric kinesin subunit, is necessary for cilia differentiation in most tissues, although exceptions exist, and include photoreceptors and a subset of hair cells. Cilia of these cell types persist even in kif3b/kif17 double mutants. Although we have not observed a functional redundancy of kif3b and kif17, kif17 is able to substitute for kif3b in some cilia. In contrast to kif3b/kif17 double mutants, simultaneous interference with kif3b and kif3c leads to the complete loss of photoreceptor and hair cell cilia, revealing redundancy of function. This is in agreement with the idea that Kif3b and Kif3c motor subunits form complexes with Kif3a, but not with each other. Interestingly, kif3b mutant photoreceptor cilia differentiate with a delay, suggesting that kif3c, although redundant with kif3b at later stages of differentiation, is not active early in photoreceptor ciliogenesis. Consistent with that, the overexpression of kif3c in kif3b mutants rescues early photoreceptor cilia defects. These data reveal unexpected diversity of functional relationships between vertebrate ciliary kinesins, and show that the repertoire of kinesin motors changes in some cilia during their differentiation.

Axonemal positioning and orientation in three-dimensional space for primary cilia: what is known, what is assumed, and what needs clarification

Two positional characteristics of the ciliary axoneme--its location on the plasma membrane as it emerges from the cell, and its orientation in three-dimensional (3D) space--are known to be critical for optimal function of actively motile cilia (including nodal cilia), as well as for modified cilia associated with special senses. However, these positional characteristics have not been analyzed to any significant extent for primary cilia. This review briefly summarizes the history of knowledge of these two positional characteristics across a wide spectrum of cilia, emphasizing their importance for proper function. Then the review focuses what is known about these same positional characteristics for primary cilia in all major tissue types where they have been reported. The review emphasizes major areas that would be productive for future research for understanding how positioning and 3D orientation of primary cilia may be related to their hypothesized signaling roles within different cellular populations.

The centrosomal protein pericentrin identified at the basal body complex of the connecting cilium in mouse photoreceptors

Background

Pericentrin (Pcnt), a conserved protein of the pericentriolar material, serves as a multifunctional scaffold for numerous proteins and plays an important role in microtubule organization. Recent studies indicate that Pcnt mutations are associated with a range of diseases including primordial dwarfism and ciliopathies. To date, three Pcnt splice variants from orthologous genes in mice and humans are known.

Principal Findings

We generated a specific Pcnt antiserum detecting all known Pcnt splice variants and examined the cellular and subcellular distribution of Pcnt in ciliated tissues of the mouse, the olfactory epithelium and the retina. For the first time, we identified Pcnt and its centrosomal interaction partners at the basal body complex of mouse retinal photoreceptors. Photoreceptors are morphologically and functionally subdivided into the light sensitive outer segment and the inner segment comprising the metabolic function of the cell. The two compartments are linked via a modified, specialized, non-motile cilium, the connecting cilium. Here, Pcnt colocalized with the whole protein machinery responsible for transport processes between the two compartments. Surprisingly, photoreceptors expressed a small Pcnt splice transcript – most likely a modified variant of Pcnt S – which was not present in receptor neurons of the olfactory epithelium.

Conclusions

Our findings suggest distinct functional roles of several Pcnt variants in different ciliated tissues and sensory neurons, like the olfactory epithelium and the retina of the mouse. The individual patchwork of different Pcnt splice transcripts seems to reflect the complexity of Pcnt function, an assumption corroborated by the heterogeneous clinical manifestations associated with mutations in the Pcnt gene.

In situ visualization of protein interactions in sensory neurons: glutamic acid-rich proteins (GARPs) play differential roles for photoreceptor outer segment scaffolding

Vertebrate photoreceptors initiate vision via a G-protein-mediated signaling cascade organized within a specialized cilium, the outer segment (OS). The membranous "stacked pancake" architecture of this organelle must be partially renewed daily to maintain cell function and viability; however, neither its static structure nor renewal process is well described in molecular terms. Glutamic acid-rich proteins (GARPs), including the cyclic nucleotide-gated cation channel (CNGB1) and GARP2 (a CNGB1 splice-variant), are proposed to contribute to OS organization in concert with peripherin/rds (P/rds), a retinal tetraspanin. We developed and applied an in situ fluorescence complementation approach that offers an unprecedented glimpse at the formation, trafficking, and localization of GARP-P/rds interactions in transgenic Xenopus laevis rod photoreceptors. Interactions for these (and other) proteins could be readily visualized using confocal microscopy. Nearly all associations, including CNGB1-P/rds interaction, were initiated within inner segments (ISs) before trafficking to OSs. In contrast, GARP2-P/rds interactions were only observed downstream, at or near sites of disk morphogenesis. These results suggest that GARP2-P/rds interaction participates directly in structuring disk stacks but CNGB1-P/rds interaction does not and instead serves mainly to localize plasma membrane ion channels. Altogether, the results lead us to propose that differential interaction of GARPs with P/rds may contribute to the broad phenotypic heterogeneity produced by inherited defects in P/rds. Analogous experiments applied to the synaptic protein RIBEYE suggest that monomers can oligomerize at the level of the IS before ribbon assembly and demonstrate the general applicability of this strategy for in situ analysis of protein interactions in sensory neurons.