The carboxyl terminal mutational hotspot of the ciliary disease protein RPGRORF15 (retinitis pigmentosa GTPase regulator) is glutamylated in vivo

Excessive tubulin glutamylation leads to progressive cone-rod dystrophy and loss of outer segment integrity

Mutations in Cytosolic Carboxypeptidase-like Protein 5 (CCP5) are associated with vision loss in humans. To decipher the mechanisms behind CCP5-associated blindness, we generated a novel mouse model lacking CCP5. In this model, we found that increased tubulin glutamylation led to progressive cone-rod dystrophy, with cones showing a more pronounced and earlier functional loss than rod photoreceptors. The observed functional reduction was not due to cell death, levels, or the mislocalization of major phototransduction proteins. Instead, the increased tubulin glutamylation caused shortened photoreceptor axonemes and the formation of numerous abnormal membranous whorls that disrupted the integrity of photoreceptor outer segments (OS). Ultimately, excessive tubulin glutamylation led to the progressive loss of photoreceptors, affecting cones more severely than rods. Our results highlight the importance of maintaining tubulin glutamylation for normal photoreceptor function. Furthermore, we demonstrate that murine cone photoreceptors are more sensitive to disrupted tubulin glutamylation levels than rods, suggesting an essential role for axoneme in the structural integrity of the cone outer segment. This study provides valuable insights into the mechanisms of photoreceptor diseases linked to excessive tubulin glutamylation.

Retinal Ciliopathies and Potential Gene Therapies: A Focus on Human iPSC-Derived Organoid Models

The human photoreceptor function is dependent on a highly specialised cilium. Perturbation of cilial function can often lead to death of the photoreceptor and loss of vision. Retinal ciliopathies are a genetically diverse range of inherited retinal disorders affecting aspects of the photoreceptor cilium. Despite advances in the understanding of retinal ciliopathies utilising animal disease models, they can often lack the ability to accurately mimic the observed patient phenotype, possibly due to structural and functional deviations from the human retina. Human-induced pluripotent stem cells (hiPSCs) can be utilised to generate an alternative disease model, the 3D retinal organoid, which contains all major retinal cell types including photoreceptors complete with cilial structures. These retinal organoids facilitate the study of disease mechanisms and potential therapies in a human-derived system. Three-dimensional retinal organoids are still a developing technology, and despite impressive progress, several limitations remain. This review will discuss the state of hiPSC-derived retinal organoid technology for accurately modelling prominent retinal ciliopathies related to genes, including RPGR, CEP290, MYO7A, and USH2A. Additionally, we will discuss the development of novel gene therapy approaches targeting retinal ciliopathies, including the delivery of large genes and gene-editing techniques.

AAV-RPGR Gene Therapy Rescues Opsin Mislocalisation in a Human Retinal Organoid Model of RPGR-Associated X-Linked Retinitis Pigmentosa

Variants within the Retinitis Pigmentosa GTPase regulator (RPGR) gene are the predominant cause of X-Linked Retinitis Pigmentosa (XLRP), a common and severe form of inherited retinal disease. XLRP is characterised by the progressive degeneration and loss of photoreceptors, leading to visual loss and, ultimately, bilateral blindness. Unfortunately, there are no effective approved treatments for RPGR-associated XLRP. We sought to investigate the efficacy of RPGRORF15 gene supplementation using a clinically relevant construct in human RPGR-deficient retinal organoids (ROs). Isogenic RPGR knockout (KO)-induced pluripotent stem cells (IPSCs) were generated using established CRISPR/Cas9 gene editing methods targeting RPGR. RPGR-KO and isogenic wild-type IPSCs were differentiated into ROs and utilised to test the adeno associated virus (AAV) RPGR (AAV-RPGR) clinical vector construct. The transduction of RPGR-KO ROs using AAV-RPGR successfully restored RPGR mRNA and protein expression and localisation to the photoreceptor connecting cilium in rod and cone photoreceptors. Vector-derived RPGR demonstrated equivalent levels of glutamylation to WT ROs. In addition, treatment with AAV-RPGR restored rhodopsin localisation within RPGR-KO ROs, reducing mislocalisation to the photoreceptor outer nuclear layer. These data provide mechanistic insights into RPGRORF15 gene supplementation functional potency in human photoreceptor cells and support the previously reported Phase I/II trial positive results using this vector construct in patients with RPGR-associated XLRP, which is currently being tested in a Phase III clinical trial.

Clinical characteristics of high myopia in female carriers of pathogenic RPGR mutations: a case series and review of the literature

BACKGROUND

RPGR mutations are the most common cause of X-linked retinitis pigmentosa (XLRP). High myopia has been described as a very frequent feature among affected female carriers of XLRP. However, the clinical phenotype of female patients presenting with X-linked RPGR-related high myopia has not been well described.

MATERIALS AND METHODS

Retrospective case series of four female patients with RPGR mutations and a diagnosis of high myopia, who presented to two academic eye centers. Clinical data, including age, family history, visual acuity, refractive error, dilated fundus exam, fundus photography, optical coherence tomography, electroretinography, and results of genetic testing, were collected.

RESULTS

Three RPGR variants identified in the present study have not been previously associated with myopia in female carriers. One variant (c.2405_2406delAG, p.Glu802Glyfs *32) has been previously associated with a myopic phenotype in a female patient. Patients became symptomatic between the first and sixth decades of life. Myopia-associated tilted optic discs and posterior staphyloma were present in all patients. Two patients presented with intraretinal migration of the retinal pigment epithelium.

CONCLUSION

RPGR-related high myopia has been associated with mutations in exons 1-14 and ORF15 in heterozygous females. There is a wide range of visual function among carriers. Although the exact mechanism of RPGR-related high myopia is still unclear, continued molecular diagnosis and description of phenotypes remain a crucial step in understanding the impact of RPGR mutations on visual function in female XLRP carriers.

Impaired glutamylation of RPGRORF15 underlies the cone-dominated phenotype associated with truncating distal ORF15 variants

Pathogenic variants in the Retinitis pigmentosa GTPase regulator (RPGR) gene lead to a clinically severe form of X-linked retinal dystrophy. However, it remains unclear why some variants cause a predominant rod, while others result in a cone-dominated phenotype. Post-translational glutamylation of the photoreceptor-specific RPGR^ORF15 isoform by the TTLL5 enzyme is essential for its optimal function in photoreceptors, and loss of TTLL5 leads to retinal dystrophy with a cone phenotype. Here we show that RPGR retinal disease, studied in a single cohort of 116 male patients, leads to a clear progressive shift from rod- to cone-dominating phenotype as the RPGR^ORF15 variant location approaches the distal part of the Open Reading Frame 15 (ORF15) region. The rod photoreceptor involvement on the contrary diminishes along the RGPR sequence, and the variants associated with the cone only phenotype are located predominantly in the very distal part, including the C-terminal basic domain. Moreover, these distal truncating RPGR^ORF15 variants disrupt the interaction with TTLL5 and lead to a significant impairment of RPGR glutamylation. Thus, consistent with the phenotype of TTLL5 pathogenic variants, our study shows that RPGR^ORF15 variants, which disrupt its basic domain and the interaction with TTLL5, also impair RPGR glutamylation and lead to the cone phenotype. This has implications for ongoing gene therapy clinical trials where the application of RPGR with impaired glutamylation may be less effective in treating RGPR dystrophies and may even convert a rod-cone dystrophy into a cone dystrophy phenotype.

Emerging gene therapy products for RPGR-associated X-linked retinitis pigmentosa

INTRODUCTION

Mutations in the RPGR gene are responsible for one of the most prevalent and severe types of retinitis pigmentosa. Gene therapy has shown great promise to treat inherited retinal diseases, and currently, four RPGR gene therapy vectors are being evaluated in clinical trials.

AREAS COVERED

This manuscript reviews the gene therapy products that are in development for X-linked retinitis pigmentosa caused by mutations in RPGR, and the challenges that scientists and clinicians have faced.

EXPERT OPINION

The development of a gene therapy product for RPGR-associated retinal degeneration has been a great challenge due to the incomplete understanding of the underlying genetics and mechanism of action of RPGR, and on the other hand, due to the instability of the RPGR gene. Three of the four gene therapy vectors currently in clinical trials include a codon-optimized version of the human RPGR sequence, and the other vector contains a shortened version of the human RPGR. To date, the only Phase I/II results published in a peer-reviewed journal demonstrate a good safety profile and an improvement in the visual field using a codon optimized version of RPGRORF15.

Identification of a novel RPGR mutation associated with X-linked cone-rod dystrophy in a Chinese family

Background

Cone-rod dystrophy (CORD) is a group of inherited retinal dystrophies, characterized by decreased visual acuity, color vision defects, photophobia, and decreased sensitivity in the central visual field. Our study has identified a novel pathogenic variant associated with X-linked cone-rod dystrophy (XLCORD) in a Chinese family.

Methods

All six family members, including the proband, affected siblings, cousins and female carriers, have underwent thorough ophthalmic examinations. The whole exome sequencing was performed for the proband, followed by Sanger sequencing for spilt-sample validation. A mammalian expression vector (AAV-MCS) with mutated retinitis pigmentosa GTPase regulator (RPGR) sequence was expressed in HEK293 T cells. The mutated protein was verified by Western blotting and immunohistochemistry.

Results

A novel mutation in the RPGR gene (c.2383G > T, p.E795X) is identified to be responsible for CORD pathogenesis.

Conclusions

Our findings have expanded the spectrum of CORD-associated mutations in RPGR gene and serve as a basis for genetic diagnosis for X-linked CORD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12886-021-02166-0.

Rare Co-Occurrence of Visual Snow in a Female Carrier With RPGRORF15-Associated Retinal Disorder

X-linked retinitis pigmentosa (XLRP), a rare form of retinitis pigmentosa (RP), is predominantly caused by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene. Affected males often present with severe phenotypes and early disease onset. In contrast, female carriers are usually asymptomatic or show stationary phenotypes. Herein, we reported an 8-year-old female carrier, a daughter of a confirmed RP father with RPGR mutation, with an early onset of progressive cone-rod pattern retinal dystrophy. Additionally, the carrier experienced visual snow-like symptom as long as she recalled. Ophthalmological examination showed the reduction of visual acuity and attenuation of photoreceptor functions since the age of 5 years. Further analysis revealed a heterozygous pathogenic variant of the RPGR gene and a random X-inactivation pattern. Although she harboured an identical RPGR variant as the father, there were phenotypic intrafamilial variations. The information on the variety of genotypic and phenotypic presentations in XLRP carriers is essential for further diagnosis, management, and monitoring of these cases, including the design of future gene therapy trials.

Gene therapy reforms photoreceptor structure and restores vision in NPHP5-associated Leber congenital amaurosis

The inherited childhood blindness caused by mutations in NPHP5, a form of Leber congenital amaurosis, results in abnormal development, dysfunction, and degeneration of photoreceptors. A naturally occurring NPHP5 mutation in dogs leads to a phenotype that very nearly duplicates the human retinopathy in terms of the photoreceptors involved, spatial distribution of degeneration, and the natural history of vision loss. We show that adeno-associated virus (AAV)-mediated NPHP5 gene augmentation of mutant canine retinas at the time of active degeneration and peak cell death stably restores photoreceptor structure, function, and vision with either the canine or human NPHP5 transgenes. Mutant cone photoreceptors, which failed to form outer segments during development, reform this structure after treatment. Degenerating rod photoreceptor outer segments are stabilized and develop normal structure. This process begins within 8 weeks after treatment and remains stable throughout the 6-month posttreatment period. In both photoreceptor cell classes mislocalization of rod and cone opsins is minimized or reversed. Retinal function and functional vision are restored. Efficacy of gene therapy in this large animal ciliopathy model of Leber congenital amaurosis provides a path for translation to human treatment.

The Emerging Roles of Axonemal Glutamylation in Regulation of Cilia Architecture and Functions

Cilia, which either generate coordinated motion or sense environmental cues and transmit corresponding signals to the cell body, are highly conserved hair-like structures that protrude from the cell surface among diverse species. Disruption of ciliary functions leads to numerous human disorders, collectively referred to as ciliopathies. Cilia are mechanically supported by axonemes, which are composed of microtubule doublets. It has been recognized for several decades that tubulins in axonemes undergo glutamylation, a post-translational polymodification, that conjugates glutamic acid chains onto the C-terminal tail of tubulins. However, the physiological roles of axonemal glutamylation were not uncovered until recently. This review will focus on how cells modulate glutamylation on ciliary axonemes and how axonemal glutamylation regulates cilia architecture and functions, as well as its physiological importance in human health. We will also discuss the conventional and emerging new strategies used to manipulate glutamylation in cilia.

RPGR isoform imbalance causes ciliary defects due to exon ORF15 mutations in X-linked retinitis pigmentosa (XLRP)

Mutations in retinitis pigmentosa GTPase regulator (RPGR) cause severe retinal ciliopathy, X-linked retinitis pigmentosa. Although two major alternatively spliced isoforms, RPGRex1-19 and RPGRORF15, are expressed, the relative importance of these isoforms in disease pathogenesis is unclear. Here, we analyzed fibroblast samples from eight patients and found that all of them form longer cilia than normal controls, albeit to different degrees. Although all mutant RPGRORF15 messenger RNAs (mRNAs) are unstable, their steady-state levels were similar or higher than those in the control cells, suggesting there may be increased transcription. Three of the fibroblasts that had higher levels of mutant RPGRORF15 mRNA also exhibited significantly higher levels of RPGRex1-19 mRNA. Four samples with unaltered RPGRex1-19 levels carried mutations in RPGRORF15 that resulted in this isoform being relatively less stable. Thus, in all cases, the RPGRex1-19/RPGRORF15 isoform ratio was increased, and this was highly correlative to the cilia extension defect. Moreover, overexpression of RPGRex1-19 (mimicking the increase in RPGRex1-19 to RPGRORF15 isoform ratio) or RPGRORF15 (mimicking reduction of the ratio) resulted in significantly longer or shorter cilia, respectively. Notably, the cilia length defect appears to be attributable to both the loss of the wild-type RPGRORF15 protein and to the higher levels of the RPGRex1-19 isoform, indicating that the observed defect is due to the altered isoform ratios. These results suggest that maintaining the optimal RPGRex1-9 to RPGRORF15 ratio is critical for cilia growth and that designing strategies that focus on the best ways to restore the RPGRex1-19/RPGRORF15 ratio may lead to better therapeutic outcomes.

Association of a Novel Intronic Variant in RPGR With Hypomorphic Phenotype of X-Linked Retinitis Pigmentosa

Importance

Pathogenic variants in retinitis pigmentosa GTPase regulator (RPGR) gene typically lead to a severe form of X-linked retinitis pigmentosa, which is associated with early severe vision loss.

Objective

To investigate an X-linked retinal degeneration family with atypical preservation of visual acuity in the presence of a novel deep intronic splice site RPGR c.779-5T>G variant.

Design, Setting, and Participants

In this case series, 3 members of an X-linked retinal degeneration family were studied by in-depth phenotyping and genetic screening at a single center. Data were collected and analyzed from November 2018 to March 2020.

Main Outcomes and Measures

Data were collected on full ophthalmic history, examination, and retinal imaging. A full retinitis pigmentosa gene panel was analyzed by next-generation sequencing. The pathogenicity of the RPGR c.779-5T>G variant was assessed by in silico splice prediction tools and by purpose-designed in vitro splicing assay.

Results

An 84-year-old man was referred with clinical diagnosis of choroideremia and possible inclusion into a gene therapy trial. He presented with late-stage retinal degeneration and unusually preserved visual acuity (78 and 68 ETRDS letters) that clinically resembled choroideremia. His 23-year-old grandson was still in early stages of degeneration but showed a very different clinical picture, typical of retinitis pigmentosa. Next-generation sequencing identified a sole RPGR c.779-5T>G variant of undetermined pathogenicity in both cases. The daughter of the proband showed an RPGR carrier phenotype and was confirmed to carry the same variant. The molecular analysis confirmed that the RPGR c.779-5T>G variation reduced the efficiency of intron splicing compared with wild type, leading to a population of mutant and normal transcripts. The predicted consequences of the pathogenic variant are potential use of an alternative splice acceptor site or complete skipping of exon 8, resulting in truncated forms of the RPGR protein with different levels of glutamylation.

Conclusions and Relevance

These results support the importance of careful interpretation of inconsistent clinical phenotypes between family members. Using a molecular splicing assay, a new pathogenic variant in a noncoding region of RPGR was associated with a proportion of normal and hypomorphic RPGR, where cones are likely to survive longer than expected, potentially accounting for the preserved visual acuity observed in this family.

RPGR gene therapy presents challenges in cloning the coding sequence

Introduction: Currently, there are three Phase I/II clinical trials based on gene therapy ongoing to test different AAV.RPGR or deleted RPGR vectors on patients affected by X-linked retinitis pigmentosa. These three vectors differ in the adeno-associated viral (AAV) vector capsid used, and the coding sequences: two contain codon optimized versions of RPGR which give the full-length protein, whilst the third uses a wild-type sequence that contains a large deletion encoding part of the functional domain of the RPGR protein.Areas covered: This review approaches the different studies that have led to the initiation of three different clinical trials for RPGR related X-linked retinitis pigmentosa.Expert opinion: The development of a gene therapy vector to deliver a normal copy of the RPGR gene into the photoreceptors has presented a challenge for the scientific community. The instability of its sequence and the fact that its function is not well understood can lead to the production of a nonfunctional or deleterious protein for the human retina. Since the RPGR protein undergoes post-translational glutamylation in the protein domain that may be particularly affected by gene instability, a functional assay of glutamylation is essential to verify the correct coding sequence.

Retinal Phenotype in the rd9 Mutant Mouse, a Model of X-Linked RP

Retinal degeneration 9 (rd9) mice carry a mutation in the retina specific “Retinitis Pigmentosa GTPase Regulator (RPGR)” Open Reading Frame (ORF) 15 gene, located on the X chromosome and represent a rare model of X-linked Retinitis Pigmentosa (XLRP), a common and severe form of retinal degeneration (Wright et al., 2010; Tsang and Sharma, 2018). The rd9 RPGR-ORF15 mutation in mice causes lack of the protein in photoreceptors and a slow degeneration of these cells with consequent decrease in Outer Nuclear Layer (ONL) thickness and amplitude of ERG responses, as previously described (Thompson et al., 2012). However, relative rates of rod and cone photoreceptor loss, as well as secondary alterations occurring in neuronal and non-neuronal retinal cell types of rd9 mutants remain to be assessed. Aim of this study is to extend phenotype analysis of the rd9 mouse retina focusing on changes occurring in cells directly interacting with photoreceptors. To this purpose, first we estimated rod and cone survival and its degree of intraretinal variation over time; then, we studied the morphology of horizontal and bipolar cells and of the retinal pigment epithelium (RPE), extending our observations to glial cell reactivity. We found that in rd9 retinas rod (but not cone) death is the main cause of decrease in ONL thickness and that degeneration shows a high degree of intraretinal variation. Rod loss drives remodeling in the outer retina, with sprouting of second-order neurons of the rod-pathway and relative sparing of cone pathway elements. Remarkably, despite cone survival, functional defects can be clearly detected in ERG recordings in both scotopic and photopic conditions. Moderate levels of Muller cells and microglial reactivity are sided by striking attenuation of staining for RPE tight junctions, suggesting altered integrity of the outer Blood Retina Barrier (BRB). Because of many features resembling slowly progressing photoreceptor degeneration paradigms or early stages of more aggressive forms of RP, the rd9 mouse model can be considered a rare and useful tool to investigate retinal changes associated to a process of photoreceptor death sustained throughout life and to reveal disease biomarkers (e.g., BRB alterations) of human XLRP.

Molecular Strategies for RPGR Gene Therapy

Mutations affecting the Retinitis Pigmentosa GTPase Regulator (RPGR) gene are the commonest cause of X-linked and recessive retinitis pigmentosa (RP), accounting for 10%-20% of all cases of RP. The phenotype is one of the most severe amongst all causes of RP, characteristic for its early onset and rapid progression to blindness in young people. At present there is no cure for RPGR-related retinal disease. Recently, however, there have been important advances in RPGR research from bench to bedside that increased our understanding of RPGR function and led to the development of potential therapies, including the progress of adeno-associated viral (AAV)-mediated gene replacement therapy into clinical trials. This manuscript discusses the advances in molecular research, which have connected the RPGR protein with an important post-translational modification, known as glutamylation, that is essential for its optimal function as a key regulator of photoreceptor ciliary transport. In addition, we review key pre-clinical research that addressed challenges encountered during development of therapeutic vectors caused by high infidelity of the RPGR genomic sequence. Finally, we discuss the structure of three current phase I/II clinical trials based on three AAV vectors and RPGR sequences and link the rationale behind the use of the different vectors back to the bench research that led to their development.

More Than Meets the Eye: Current Understanding of RPGR Function

This article summarizes the recent advances in our understanding of a major retinal disease gene RPGR (retinitis pigmentosa GTPase regulator), mutations in which are associated with majority of X-linked forms of retinal degenerations. A great deal of work has been done to uncover the ciliary localization of RPGR and its interacting proteins in the retina. However, the molecular mechanisms of action of RPGR in the photoreceptors are still unclear. Recent studies have begun to shed light on the intracellular pathways in which RPGR is likely involved. The deregulation of such pathways may underlie the pathogenesis of severe retinal degeneration associated with RPGR. With the recent advances in the gene augmentation therapy for RPGR-associated disease, there is a lot of excitement in the field. Patients with RPGR mutations, however, present with clinically heterogeneous manifestations. It is therefore imperative to examine the function of RPGR in detail, so that we can design patient-oriented therapeutic strategies for this disease.

Toward genome editing in X-linked RP-development of a mouse model with specific treatment relevant features

Genome editing represents a powerful tool to treat inherited disorders. Highly specific endonucleases induce a DNA double strand break near the mutant site, which is subsequently repaired by cellular DNA repair mechanisms that involve the presence of a wild type template DNA. In vivo applications of this strategy are still rare, in part due to the absence of appropriate animal models carrying human disease mutations and knowledge of the efficient targeting of endonucleases. Here we report the generation and characterization of a new mouse model for X-linked retinitis pigmentosa (XLRP) carrying a point mutation in the mutational hotspot exon ORF15 of the RPGR gene as well as a recognition site for the homing endonuclease I-SceI. Presence of the genomic modifications was verified at the RNA and protein levels. The mutant protein was observed at low levels. Optical coherence tomography studies revealed a slowly progressive retinal degeneration with photoreceptor loss starting at 9 months of age, paralleling the onset of functional deficits as seen in the electroretinogram. Early changes to the outer retinal bands can be used as biomarker during treatment applications. We further show for the first time efficient targeting using the I-SceI enzyme at the genomic locus in a proof of concept in photoreceptors following adeno-associated virus mediated gene transfer in vivo. Taken together, our studies not only provide a human-XLRP disease model but also act as a platform to design genome editing technology for retinal degenerative diseases using the currently available endonucleases.

Gene therapy for the treatment of X-linked retinitis pigmentosa

INTRODUCTION

X-linked retinitis pigmentosa caused by mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene is the most common form of recessive RP. The phenotype is characterised by its severity and rapid disease progression. Gene therapy using adeno-associated viral vectors is currently the most promising therapeutic approach. However, the construction of a stable vector encoding the full-length RPGR transcript has previously proven to be a limiting step towards gene therapy clinical trials. Recently however, a codon optimised version of RPGR has been shown to increase the stability and fidelity of the sequence, conferring a therapeutic effect in murine and canine animal models.

AREAS COVERED

This manuscript reviews the natural history of X-linked retinitis pigmentosa and the research performed from the discovery of the causative gene, RPGR, to the preclinical testing of potential therapies that have led to the initiation of three clinical trials.

EXPERT OPINION

X-linked retinitis pigmentosa is an amenable disease to be treated by gene therapy. Codon optimisation has overcome the challenge of designing an RPGR vector without mutations, and with a therapeutic effect in different animal models. With the RPGR gene therapy clinical trials still in the early stages, the confirmation of the safety, tolerability and potency of the therapy is still ongoing.

Identification of DmTTLL5 as a Major Tubulin Glutamylase in the Drosophila Nervous System

Microtubules (MTs) play crucial roles during neuronal life. They are formed by heterodimers of alpha and beta-tubulins, which are subjected to several post-translational modifications (PTMs). Amongst them, glutamylation consists in the reversible addition of a variable number of glutamate residues to the C-terminal tails of tubulins. Glutamylation is the most abundant MT PTM in the mammalian adult brain, suggesting that it plays an important role in the nervous system (NS). Here, we show that the previously uncharacterized CG31108 gene encodes an alpha-tubulin glutamylase acting in the Drosophila NS. We show that this glutamylase, which we named DmTTLL5, initiates MT glutamylation specifically on alpha-tubulin, which are the only glutamylated tubulin in the Drosophila brain. In DmTTLL5 mutants, MT glutamylation was not detected in the NS, allowing for determining its potential function. DmTTLL5 mutants are viable and we did not find any defect in vesicular axonal transport, synapse morphology and larval locomotion. Moreover, DmTTLL5 mutant flies display normal negative geotaxis behavior and their lifespan is not altered. Thus, our work identifies DmTTLL5 as the major enzyme responsible for initiating neuronal MT glutamylation specifically on alpha-tubulin and we show that the absence of MT glutamylation is not detrimental for Drosophila NS function.

Glutamylation Regulates Transport, Specializes Function, and Sculpts the Structure of Cilia

Ciliary microtubules (MTs) are extensively decorated with post-translational modifications (PTMs), such as glutamylation of tubulin tails. PTMs and tubulin isotype diversity act as a "tubulin code" that regulates cytoskeletal stability and the activity of MT-associated proteins such as kinesins. We previously showed that, in C. elegans cilia, the deglutamylase CCPP-1 affects ciliary ultrastructure, localization of the TRP channel PKD-2 and the kinesin-3 KLP-6, and velocity of the kinesin-2 OSM-3/KIF17, whereas a cell-specific α-tubulin isotype regulates ciliary ultrastructure, intraflagellar transport, and ciliary functions of extracellular vesicle (EV)-releasing neurons. Here we examine the role of PTMs and the tubulin code in the ciliary specialization of EV-releasing neurons using genetics, fluorescence microscopy, kymography, electron microscopy, and sensory behavioral assays. Although the C. elegans genome encodes five tubulin tyrosine ligase-like (TTLL) glutamylases, only ttll-11 specifically regulates PKD-2 localization in EV-releasing neurons. In EV-releasing cephalic male (CEM) cilia, TTLL-11 and the deglutamylase CCPP-1 regulate remodeling of 9+0 MT doublets into 18 singlet MTs. Balanced TTLL-11 and CCPP-1 activity fine-tunes glutamylation to control the velocity of the kinesin-2 OSM-3/KIF17 and kinesin-3 KLP-6 without affecting the intraflagellar transport (IFT) kinesin-II. TTLL-11 is transported by ciliary motors. TTLL-11 and CCPP-1 are also required for the ciliary function of releasing bioactive EVs, and TTLL-11 is itself a novel EV cargo. Therefore, MT glutamylation, as part of the tubulin code, controls ciliary specialization, ciliary motor-based transport, and ciliary EV release in a living animal. We suggest that cell-specific control of MT glutamylation may be a conserved mechanism to specialize the form and function of cilia.

BBSome function is required for both the morphogenesis and maintenance of the photoreceptor outer segment

Genetic mutations disrupting the structure and function of primary cilia cause various inherited retinal diseases in humans. Bardet-Biedl syndrome (BBS) is a genetically heterogeneous, pleiotropic ciliopathy characterized by retinal degeneration, obesity, postaxial polydactyly, intellectual disability, and genital and renal abnormalities. To gain insight into the mechanisms of retinal degeneration in BBS, we developed a congenital knockout mouse of Bbs8, as well as conditional mouse models in which function of the BBSome (a protein complex that mediates ciliary trafficking) can be temporally inactivated or restored. We demonstrate that BBS mutant mice have defects in retinal outer segment morphogenesis. We further demonstrate that removal of Bbs8 in adult mice affects photoreceptor function and disrupts the structural integrity of the outer segment. Notably, using a mouse model in which a gene trap inhibiting Bbs8 gene expression can be removed by an inducible FLP recombinase, we show that when BBS8 is restored in immature retinas with malformed outer segments, outer segment extension can resume normally and malformed outer segment discs are displaced distally by normal outer segment structures. Over time, the retinas of the rescued mice become morphologically and functionally normal, indicating that there is a window of plasticity when initial retinal outer segment morphogenesis defects can be ameliorated.

The BBSome is a protein complex that regulates ciliary trafficking in primary cilia, and mutations that impair BBSome function cause Bardet-Biedl Syndrome (BBS). BBS patients have retinal degeneration leading to blindness, but the disease pathophysiology has not been fully elucidated. In this study, we found that the BBSome is necessary for the structural organization of photoreceptor outer segments, and that the loss of different functional BBSome subunits causes outer segment malformation. Using a mouse model that allows the temporal inactivation of the BBSome, we inactivated BBSome function after the outer segment had formed normally. We found that the BBSome is required for both the initial formation and the continual maintenance of outer segment structures throughout life. In addition, using a mouse model that allows the temporal restoration of the BBSome, we restored BBSome function in immature photoreceptors and show that the malformed outer segment discs are displaced distally by normally formed outer segment structures. This finding indicates that when gene function is restored in immature retinas shortly after initial outer segment malformation, morphologically normal outer segments and a functionally normal retina can still result. This study has important implications for the timing of treatment of human retinal diseases.

Codon-Optimized RPGR Improves Stability and Efficacy of AAV8 Gene Therapy in Two Mouse Models of X-Linked Retinitis Pigmentosa

X-linked retinitis pigmentosa (XLRP) is generally a severe form of retinitis pigmentosa, a neurodegenerative, blinding disorder of the retina. 70% of XLRP cases are due to mutations in the retina-specific isoform of the gene encoding retinitis pigmentosa GTPase regulator (RPGR^ORF15). Despite successful RPGR^ORF15 gene replacement with adeno-associated viral (AAV) vectors being established in a number of animal models of XLRP, progression to human trials has not yet been possible. The inherent sequence instability in the purine-rich region of RPGR^ORF15 (which contains highly repetitive nucleotide sequences) leads to unpredictable recombination errors during viral vector cloning. While deleted RPGR may show some efficacy in animal models, which have milder disease, the therapeutic effect of a mutated RPGR variant in patients with XLRP cannot be predicted. Here, we describe an optimized gene replacement therapy for human XLRP disease using an AAV8 vector that reliably and consistently produces the full-length correct RPGR protein. The glutamylation pattern in the RPGR protein derived from the codon-optimized sequence is indistinguishable from the wild-type variant, implying that codon optimization does not significantly alter post-translational modification. The codon-optimized sequence has superior stability and expression levels in vitro. Significantly, when delivered by AAV8 vector and driven by the rhodopsin kinase promoter, the codon-optimized RPGR rescues the disease phenotype in two relevant animal models (Rpgr^−/y and C57BL/6J^Rd9/Boc) and shows good safety in C57BL6/J wild-type mice. This work provides the basis for clinical trial development to treat patients with XLRP caused by RPGR mutations.

Cilia - The sensory antennae in the eye

Cilia are hair-like projections found on almost all cells in the human body. Originally believed to function merely in motility, the function of solitary non-motile (primary) cilia was long overlooked. Recent research has demonstrated that primary cilia function as signalling hubs that sense environmental cues and are pivotal for organ development and function, tissue hoemoestasis, and maintenance of human health. Cilia share a common anatomy and their diverse functional features are achieved by evolutionarily conserved functional modules, organized into sub-compartments. Defects in these functional modules are responsible for a rapidly growing list of human diseases collectively termed ciliopathies. Ocular pathogenesis is common in virtually all classes of syndromic ciliopathies, and disruptions in cilia genes have been found to be causative in a growing number of non-syndromic retinal dystrophies. This review will address what is currently known about cilia contribution to visual function. We will focus on the molecular and cellular functions of ciliary proteins and their role in the photoreceptor sensory cilia and their visual phenotypes. We also highlight other ciliated cell types in tissues of the eye (e.g. lens, RPE and Müller glia cells) discussing their possible contribution to disease progression. Progress in basic research on the cilia function in the eye is paving the way for therapeutic options for retinal ciliopathies. In the final section we describe the latest advancements in gene therapy, read-through of non-sense mutations and stem cell therapy, all being adopted to treat cilia dysfunction in the retina.

RPGR, a prenylated retinal ciliopathy protein, is targeted to cilia in a prenylation- and PDE6D-dependent manner

RPGR (retinitis pigmentosa GTPase regulator) is a ciliary protein associated with several forms of inherited retinal degenerative diseases. PDE6D is a ubiquitously expressed prenyl-binding protein and involved in ciliary targeting of prenylated proteins. The current working model for the RPGR function depicts that RPGR acts as a scaffold protein to recruit cargo-loaded PDE6D to primary cilia. Here, we present evidence demonstrating an alternative relationship between RPGR and PDE6D, in which RPGR is a cargo of PDE6D for ciliary targeting. We found that the constitutive isoform of RPGR, which is prenylated, requires prenylation for its ciliary localization. We also found that there are at least two independent ciliary targeting signals in RPGR: one within the N-terminal region that contains the RCC1-like domain and the other near the prenylation site at the C-terminus. Ablation of PDE6D blocked ciliary targeting of RPGR. Our study indicates that prenylated RPGR is one of the cargos of PDE6D for ciliary trafficking and provides insight into the mechanisms by which RPGR is targeted to cilia.

Summary: RPGR is a ciliary protein that functions as a scaffold to recruit cargo-loaded PDE6D to cilia. Our study shows that RPGR is also a cargo of PDE6D.