Gene Therapy for Glaucoma by Ciliary Body Aquaporin 1 Disruption Using CRISPR-Cas9

CRISPR-Cas9-mediated deletion of carbonic anhydrase 2 in the ciliary body to treat glaucoma

The carbonic anhydrase 2 (Car2) gene encodes the primary isoenzyme responsible for aqueous humor (AH) production and plays a major role in the regulation of intraocular pressure (IOP). The CRISPR-Cas9 system, based on the ShH10 adenovirus-associated virus, can efficiently disrupt the Car2 gene in the ciliary body. With a single intravitreal injection, Car2 knockout can significantly and sustainably reduce IOP in both normal mice and glaucoma models by inhibiting AH production. Furthermore, it effectively delays and even halts glaucomatous damage induced by prolonged high IOP in a chronic ocular hypertension model, surpassing the efficacy of clinically available carbonic anhydrase inhibitors such as brinzolamide. The clinical application of CRISPR-Cas9 based disruption of Car2 is an attractive therapeutic strategy that could bring additional benefits to patients with glaucoma.

AAV-mediated gene therapies for glaucoma and uveitis: are we there yet?

Glaucoma and uveitis are non-vascular ocular diseases which are among the leading causes of blindness and visual loss. These conditions have distinct characteristics and mechanisms but share a multifactorial and complex nature, making their management challenging and burdensome for patients and clinicians. Furthermore, the lack of symptoms in the early stages of glaucoma and the diverse aetiology of uveitis hinder timely and accurate diagnoses, which are a cause of poor visual outcomes under both conditions. Although current treatment is effective in most cases, it is often associated with low patient adherence and adverse events, which directly impact the overall therapeutic success. Therefore, long-lasting alternatives with improved safety and efficacy are needed. Gene therapy, particularly utilising adeno-associated virus (AAV) vectors, has emerged as a promising approach to address unmet needs in these diseases. Engineered capsids with enhanced tropism and lower immunogenicity have been proposed, along with constructs designed for targeted and controlled expression. Additionally, several pathways implicated in the pathogenesis of these conditions have been targeted with single or multigene expression cassettes, gene editing and silencing approaches. This review discusses strategies employed in AAV-based gene therapies for glaucoma and non-infectious uveitis and provides an overview of current progress and future directions.

Future directions of glaucoma treatment: emerging gene, neuroprotection, nanomedicine, stem cell, and vascular therapies

PURPOSE OF REVIEW

The aim of this article is to summarize current research on novel gene, stem cell, neuroprotective, nanomedicine, and vascular therapies for glaucoma.

RECENT FINDINGS

Gene therapy using viral vectors and siRNA have been shown to reduce intraocular pressure by altering outflow and production of aqueous humor, to reduce postsurgical fibrosis with few adverse effects, and to increase retinal ganglion cell (RGC) survival in animal studies. Stem cells may treat glaucoma by replacing or stimulating proliferation of trabecular meshwork cells, thus restoring outflow facility. Stem cells can also serve a neuroprotective effect by differentiating into RGCs or preventing RGC loss via secretion of growth factors. Other developing neuroprotective glaucoma treatments which can prevent RGC death include nicotinamide, the NT-501 implant which secretes ciliary neurotrophic factor, and a Fas-L inhibitor which are now being tested in clinical trials. Recent studies on vascular therapy for glaucoma have focused on the ability of Rho Kinase inhibitors and dronabinol to increase ocular blood flow.

SUMMARY

Many novel stem cell, gene, neuroprotective, nanomedicine, and vascular therapies have shown promise in preclinical studies, but further clinical trials are needed to demonstrate safety and efficacy in human glaucomatous eyes. Although likely many years off, future glaucoma therapy may take a multifaceted approach.

Genes as drugs for glaucoma: latest advances

PURPOSE OF REVIEW

To provide the latest advances on the future use of gene therapy for the treatment of glaucoma.

RECENT FINDINGS

In preclinical studies, a number of genes have been shown to be able to reduce elevated intraocular pressure (IOP), and to exert neuroprotection of the retinal ganglion cells. These genes target various mechanisms of action and include among others: MMP3 , PLAT, IκB, GLIS, SIRT, Tie-2, AQP1. Some of these as well as some previously identified genes ( MMP3, PLAT, BDNF, C3, TGFβ, MYOC, ANGPTL7 ) are starting to move onto drug development. At the same time, progress has been made in the methods to deliver and control gene therapeutics (advances in these areas are not covered in this review).

SUMMARY

While preclinical efforts continue in several laboratories, an increasing number of start-up and large pharmaceutical companies are working on developing gene therapeutics for glaucoma ( Sylentis, Quetera/Astellas, Exhaura, Ikarovec, Genentech, Regeneron, Isarna, Diorasis Therapeutics ). Despite the presence of generic medications to treat glaucoma, given the size of the potential world-wide market (∼$7B), it is likely that the number of companies developing glaucoma gene therapies will increase further in the near future.

The concept of gene therapy for glaucoma: the dream that has not come true yet

Gene therapies, despite of being a relatively new therapeutic approach, have a potential to become an important alternative to current treatment strategies in glaucoma. Since glaucoma is not considered a single gene disease, the identified goals of gene therapy would be rather to provide neuroprotection of retinal ganglion cells, especially, in intraocular-pressure-independent manner. The most commonly reported type of vector for gene delivery in glaucoma studies is adeno-associated virus serotype 2 that has a high tropism to retinal ganglion cells, resulting in long-term expression and low immunogenic profile. The gene therapy studies recruit inducible and genetic animal models of optic neuropathy, like DBA/2J mice model of high-tension glaucoma and the optic nerve crush-model. Reported gene therapy-based neuroprotection of retinal ganglion cells is targeting specific genes translating to growth factors (i.e., brain derived neurotrophic factor, and its receptor TrkB), regulation of apoptosis and neurodegeneration (i.e., Bcl-xl, Xiap, FAS system, nicotinamide mononucleotide adenylyl transferase 2, Digit3 and Sarm1), immunomodulation (i.e., Crry, C3 complement), modulation of neuroinflammation (i.e., erythropoietin), reduction of excitotoxicity (i.e., CamKIIα) and transcription regulation (i.e., Max, Nrf2). On the other hand, some of gene therapy studies focus on lowering intraocular pressure, by impacting genes involved in both, decreasing aqueous humor production (i.e., aquaporin 1), and increasing outflow facility (i.e., COX2, prostaglandin F2α receptor, RhoA/RhoA kinase signaling pathway, MMP1, Myocilin). The goal of this review is to summarize the current state-of-art and the direction of development of gene therapy strategies for glaucomatous neuropathy.

AAV for Gene Therapy in Ocular Diseases: Progress and Prospects

Owing to the promising therapeutic effect and one-time treatment advantage, gene therapy may completely change the management of eye diseases, especially retinal diseases. Adeno-associated virus (AAV) is considered one of the most promising viral gene delivery tools because it can infect various types of tissues and is considered as a relatively safe gene delivery vector. The eye is one of the most popular organs for gene therapy, since its limited volume is suitable for small doses of AAV stably transduction. Recently, an increasing number of clinical trials of AAV-mediated gene therapy are underway. This review summarizes the biological functions of AAV and its application in the treatment of various ocular diseases, as well as the characteristics of different AAV delivery routes in clinical applications. Here, the latest research progresses in AAV-mediated gene editing and silencing strategies to modify that the genetic ocular diseases are systematically outlined, especially by base editing and prime editing. We discuss the progress of AAV in ocular optogenetic therapy. We also summarize the application of AAV-mediated gene therapy in animal models and the difficulties in its clinical transformation.

Gene therapies and gene product-based drug candidates for normalizing and preserving tissue functions in animal models of ocular hypertension and glaucoma

More than 76 million people worldwide are afflicted with the neurodegenerative eye diseases described and grouped together as glaucoma. A common feature amongst the many forms of glaucoma is chronically elevated intraocular pressure (IOP) within the anterior chamber of the eye that physically damages the retina, optic nerve and parts of the brain connected with visual perception. The mediators of the contusing raised IOP responsible for such damage and loss of vision include locally released inflammatory agents, tissue remodeling enzymes and infiltrating immune cells which damage the retinal ganglion cell (RGC) axons and eventually kill a significant number of the RGCs. Additional culprits include genetic defects of the patient that involve aberrations in receptors, enzymes and/or endogenous ligands and possible over- or under-production of the latter. Other genetic abnormalities may include issues with signal transduction machinery within key cells of critical tissues in the front (e.g. trabecular meshwork [TM] and Schlemm's canal [SC]) and back of the eye (e.g. retinal ganglion cells and their axons). Genome-wide associated studies (GWAS) coupled with next generation sequencing have provided powerful linkage of certain gene defects and polymorphic variants to the onset and progression of diseases of the tissues involved in fluid dynamics in the TM and SC, and many retinal elements (lamina cribosa, optic nerve head) at the back of the eye which cause ocular hypertension (OHT) and glaucomatous optic neuropathy (GON), respectively. Despite the availability of some drugs, fluid drainage microshunts and full surgical techniques to lower and control intraocular pressure, the major modifiable biomarker of open-angle and other forms of glaucoma, their side-effect profiles, less than optimum effectiveness and short duration of action present opportunities to clinically manage the glaucomas with next generation of treatments with high therapeutic indices, including gene therapies. Thus, identification, characterization and deployment of genetic data coupled with traditional drug discovery and novel gene replacement, gene editing and genetic engineering technologies may provide some solutions to the aforementioned problems. These aspects will be discussed in this article.

Adeno-associated virus vectors for retinal gene therapy in basic research and clinical studies

Retinal degenerative diseases, including glaucoma, age-related macular degeneration, diabetic retinopathy, and a broad range of inherited retinal diseases, are leading causes of irreversible vision loss and blindness. Gene therapy is a promising and fast-growing strategy to treat both monogenic and multifactorial retinal disorders. Vectors for gene delivery are crucial for efficient and specific transfer of therapeutic gene(s) into target cells. AAV vectors are ideal for retinal gene therapy due to their inherent advantages in safety, gene expression stability, and amenability for directional engineering. The eye is a highly compartmentalized organ composed of multiple disease-related cell types. To determine a suitable AAV vector for a specific cell type, the route of administration and choice of AAV variant must be considered together. Here, we provide a brief overview of AAV vectors for gene transfer into important ocular cell types, including retinal pigment epithelium cells, photoreceptors, retinal ganglion cells, Müller glial cells, ciliary epithelial cells, trabecular meshwork cells, vascular endothelial cells, and pericytes, via distinct injection methods. By listing suitable AAV vectors in basic research and (pre)clinical studies, we aim to highlight the progress and unmet needs of AAV vectors in retinal gene therapy.

Genetic rodent models of glaucoma in representing disease phenotype and insights into the pathogenesis

Genetic rodent models are widely used in glaucoma related research. With vast amount of information revealed by human studies about genetic correlations with glaucoma, use of these models is relevant and required. In this review, we discuss the glaucoma endophenotypes and importance of their representation in an experimental animal model. Mice and rats are the most popular animal species used as genetic models due to ease of genetic manipulations in these animal species as well as the availability of their genomic information. With technological advances, induction of glaucoma related genetic mutations commonly observed in human is possible to achieve in rodents in a desirable manner. This approach helps to study the pathobiology of the disease process with the background of genetic abnormalities, reveals potential therapeutic targets and gives an opportunity to test newer therapeutic options. Various genetic manipulation leading to appearance of human relevant endophenotypes in rodents indicate their relevance in glaucoma pathology and the utility of these rodent models for exploring various aspects of the disease related to targeted mutation. The molecular pathways involved in the pathophysiology of glaucoma leading to elevated intraocular pressure and the disease hallmark, apoptosis of retinal ganglion cells and optic nerve degeneration, have been extensively explored in genetic rodent models. In this review, we discuss the consequences of various genetic manipulations based on the primary site of pathology in the anterior or the posterior segment. We discuss how these genetic manipulations produce features in rodents that can be considered a close representation of disease phenotype in human. We also highlight several molecular mechanisms revealed by using genetic rodent models of glaucoma including those involved in increased aqueous outflow resistance, loss of retinal ganglion cells and optic neuropathy. Lastly, we discuss the limitations of the use of genetic rodent models in glaucoma related research.

Therapeutic applications of CRISPR/Cas9 gene editing technology for the treatment of ocular diseases

Ocular diseases are a highly heterogeneous group of phenotypes, caused by a spectrum of genetic variants and environmental factors that exhibit diverse clinical symptoms. As a result of its anatomical location, structure and immune privilege, the eye is an ideal system to assess and validate novel genetic therapies. Advances in genome editing have revolutionized the field of biomedical science, enabling researchers to understand the biology behind disease mechanisms and allow the treatment of several health conditions, including ocular pathologies. The advent of clustered regularly interspaced short palindromic repeats (CRISPR)-based gene editing facilitates efficient and specific genetic modifications in the nucleic acid sequence, resulting in permanent changes at the genomic level. This approach has advantages over other treatment strategies and is promising for the treatment of various genetic and non-genetic ocular conditions. This review provides an overview of the CRISPR/CRISPR-associated protein 9 (Cas9) system and summarizes recent advances in the therapeutic application of CRISPR/Cas9 for the treatment of various ocular pathologies, as well as future challenges.

Genome editing in the treatment of ocular diseases

Genome-editing technologies have ushered in a new era in gene therapy, providing novel therapeutic strategies for a wide range of diseases, including both genetic and nongenetic ocular diseases. These technologies offer new hope for patients suffering from previously untreatable conditions. The unique anatomical and physiological features of the eye, including its immune-privileged status, size, and compartmentalized structure, provide an optimal environment for the application of these cutting-edge technologies. Moreover, the development of various delivery methods has facilitated the efficient and targeted administration of genome engineering tools designed to correct specific ocular tissues. Additionally, advancements in noninvasive ocular imaging techniques and electroretinography have enabled real-time monitoring of therapeutic efficacy and safety. Herein, we discuss the discovery and development of genome-editing technologies, their application to ocular diseases from the anterior segment to the posterior segment, current limitations encountered in translating these technologies into clinical practice, and ongoing research endeavors aimed at overcoming these challenges.

Elevated Intraocular Pressure and Glaucomatous Optic Neuropathy: Genes to Disease Mechanisms, Therapeutic Drugs, and Gene Therapies

This review article focuses on the pathogenesis of and genetic defects linked with chronic ocular hypertension (cOHT) and glaucoma. The latter ocular disease constitutes a group of ocular degenerative diseases whose hallmark features are damage to the optic nerve, apoptotic demise of retinal ganglion cells, disturbances within the brain regions involved in visual perception and considerable visual impairment that can lead to blindness. Even though a number of pharmaceuticals, surgical and device-based treatments already exist addressing cOHT associated with the most prevalent of the glaucoma types, primary open-angle glaucoma (POAG), they can be improved upon in terms of superior efficacy with reduced side-effects and with longer duration of activity. The linkage of disease pathology to certain genes via genome-wide associated studies are illuminating new approaches to finding novel treatment options for the aforementioned ocular disorders. Gene replacement, gene editing via CRISPR-Cas9, and the use of optogenetic technologies may replace traditional drug-based therapies and/or they may augment existing therapeutics for the treatment of cOHT and POAG in the future.

Recently Approved Drugs for Lowering and Controlling Intraocular Pressure to Reduce Vision Loss in Ocular Hypertensive and Glaucoma Patients

Serious vision loss occurs in patients affected by chronically raised intraocular pressure (IOP), a characteristic of many forms of glaucoma where damage to the optic nerve components causes progressive degeneration of retinal and brain neurons involved in visual perception. While many risk factors abound and have been validated for this glaucomatous optic neuropathy (GON), the major one is ocular hypertension (OHT), which results from the accumulation of excess aqueous humor (AQH) fluid in the anterior chamber of the eye. Millions around the world suffer from this asymptomatic and progressive degenerative eye disease. Since clinical evidence has revealed a strong correlation between the reduction in elevated IOP/OHT and GON progression, many drugs, devices, and surgical techniques have been developed to lower and control IOP. The constant quest for new pharmaceuticals and other modalities with superior therapeutic indices has recently yielded health authority-approved novel drugs with unique pharmacological signatures and mechanism(s) of action and AQH drainage microdevices for effectively and durably treating OHT. A unique nitric oxide-donating conjugate of latanoprost, an FP-receptor prostaglandin (PG; latanoprostene bunod), new rho kinase inhibitors (ripasudil; netarsudil), a novel non-PG EP2-receptor-selective agonist (omidenepag isopropyl), and a form of FP-receptor PG in a slow-release intracameral implant (Durysta) represent the additions to the pharmaceutical toolchest to mitigate the ravages of OHT. Despite these advances, early diagnosis of OHT and glaucoma still lags behind and would benefit from further concerted effort and attention.

Gene therapy strategies for glaucoma from IOP reduction to retinal neuroprotection: progress towards non-viral systems

Glaucoma is the result of the gradual death of retinal ganglion cells (RGCs) whose axons form the optic nerve. Elevated intraocular pressure (IOP) is a major risk factors thatcontributes to RGC apoptosis and axonal loss at the lamina cribrosa, resulting in progressive reduction and eventual anterograde-retrograde transport blockade of neurotrophic factors. Current glaucoma management mainly focuses on pharmacological or surgical lowering of IOP, to manage the only modifiable risk factor. Although IOP reduction delays disease progression, it does not address previous and ongoing optic nerve degeneration. Gene therapy is a promising direction to control or modify genes involved in the pathophysiology of glaucoma. Both viral and non-viral gene therapy delivery systems are emerging as promising alternatives or add-on therapies to traditional treatments for improving IOP control and provide neuroprotection. The specific spotlight on non-viral gene delivery systems shows further progress towards improving the safety of gene therapy and implementing neuroprotection by targeting specific tissues and cells in the eye and specifically in the retina.

The application and progression of CRISPR/Cas9 technology in ophthalmological diseases

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated nuclease (Cas) system is an adaptive immune defence system that has gradually evolved in bacteria and archaea to combat invading viruses and exogenous DNA. Advances in technology have enabled researchers to enhance their understanding of the immune process in vivo and its potential for use in genome editing. Thus far, applications of CRISPR/Cas9 genome editing technology in ophthalmology have included gene therapy for corneal dystrophy, glaucoma, congenital cataract, Leber's congenital amaurosis, retinitis pigmentosa, Usher syndrome, fundus neovascular disease, proliferative vitreoretinopathy, retinoblastoma and other eye diseases. Additionally, the combination of CRISPR/Cas9 genome editing technology with adeno-associated virus vector and inducible pluripotent stem cells provides further therapeutic avenues for the treatment of eye diseases. Nonetheless, many challenges remain in the development of clinically feasible retinal genome editing therapy. This review discusses the development, as well as mechanism of CRISPR/Cas9 and its applications and challenges in gene therapy for eye diseases.

RNA-targeting strategies as a platform for ocular gene therapy

Genetic medicine is offering hope as new therapies are emerging for many previously untreatable diseases. The eye is at the forefront of these advances, as exemplified by the approval of Luxturna® by the United States Food and Drug Administration (US FDA) in 2017 for the treatment of one form of Leber Congenital Amaurosis (LCA), an inherited blindness. Luxturna® was also the first in vivo human gene therapy to gain US FDA approval. Numerous gene therapy clinical trials are ongoing for other eye diseases, and novel delivery systems, discovery of new drug targets and emerging technologies are currently driving the field forward. Targeting RNA, in particular, is an attractive therapeutic strategy for genetic disease that may have safety advantages over alternative approaches by avoiding permanent changes in the genome. In this regard, antisense oligonucleotides (ASO) and RNA interference (RNAi) are the currently popular strategies for developing RNA-targeted therapeutics. Enthusiasm has been further fuelled by the emergence of clustered regularly interspersed short palindromic repeats (CRISPR)-CRISPR associated (Cas) systems that allow targeted manipulation of nucleic acids. RNA-targeting CRISPR-Cas systems now provide a novel way to develop RNA-targeted therapeutics and may provide superior efficiency and specificity to existing technologies. In addition, RNA base editing technologies using CRISPR-Cas and other modalities also enable precise alteration of single nucleotides. In this review, we showcase advances made by RNA-targeting systems for ocular disease, discuss applications of ASO and RNAi technologies, highlight emerging CRISPR-Cas systems and consider the implications of RNA-targeting therapeutics in the development of future drugs to treat eye disease.

Aquaporins in Eye

The major part of the eye consists of water. Continuous movement of water and ions between the ocular compartments and to the systemic circulation is pivotal for many physiological functions in the eye. The movement of water facilitates removal of the many metabolic products of corneal-, ciliary body-, lens-, and retinal metabolism, while maintaining transparency in the optical compartments. Transport across the corneal epithelium and endothelium maintains the corneal transparency. Also, aqueous humor is continuously secreted by the epithelia of the ciliary body and maintains the intraocular pressure. In the retina, water is transported into the vitreous body and across the retinal pigment epithelium to regulate the extracellular environment and the hydration of the retina. Aquaporins are a major contributor in the water transport throughout the eye.

Transcriptome analysis of AAV-induced retinopathy models expressing human VEGF, TNF-α, and IL-6 in murine eyes

Retinopathies are multifactorial diseases with complex pathologies that eventually lead to vision loss. Animal models facilitate the understanding of the pathophysiology and identification of novel treatment options. However, each animal model reflects only specific disease aspects and understanding of the specific molecular changes in most disease models is limited. Here, we conducted transcriptome analysis of murine ocular tissue transduced with recombinant Adeno-associated viruses (AAVs) expressing either human VEGF-A, TNF-α, or IL-6. VEGF expression led to a distinct regulation of extracellular matrix (ECM)-associated genes. In contrast, both TNF-α and IL-6 led to more comparable gene expression changes in interleukin signaling, and the complement cascade, with TNF-α-induced changes being more pronounced. Furthermore, integration of single cell RNA-Sequencing data suggested an increase of endothelial cell-specific marker genes by VEGF, while TNF-α expression increased the expression T-cell markers. Both TNF-α and IL-6 expression led to an increase in macrophage markers. Finally, transcriptomic changes in AAV-VEGF treated mice largely overlapped with gene expression changes observed in the oxygen-induced retinopathy model, especially regarding ECM components and endothelial cell-specific gene expression. Altogether, our study represents a valuable investigation of gene expression changes induced by VEGF, TNF-α, and IL-6 and will aid researchers in selecting appropriate animal models for retinopathies based on their agreement with the human pathophysiology.

Prostaglandin-based rAAV-mediated glaucoma gene therapy in Brown Norway rats

Prostaglandin analogs are first-line treatments for open angle glaucoma and while effective at lowering intraocular pressure, they are undermined by patient non-compliance, causing atrophy of the optic nerve and severe visual impairment. Herein, we evaluate the safety and efficacy of a recombinant adeno-associated viral vector-mediated gene therapy aimed at permanently lowering intraocular pressure through de novo biosynthesis of prostaglandin F2α within the anterior chamber. This study demonstrated a dose dependent reduction in intraocular pressure in normotensive Brown Norway rats maintained over 12-months. Crucially, therapy could be temporarily halted through off-type riboswitch activation, reverting intraocular pressure to normal. Longitudinal multimodal imaging, electrophysiology, and post-mortem histology revealed the therapy was well tolerated at low and medium doses, with no major adverse effects to anterior chamber health, offering a promising alternative to current treatment strategies leading to clinically relevant reductions in intraocular pressure without the need for adherence to a daily treatment regimen.

Gene regulatory and gene editing tools and their applications for retinal diseases and neuroprotection: From proof-of-concept to clinical trial

Gene editing and gene regulatory fields are continuously developing new and safer tools that move beyond the initial CRISPR/Cas9 technology. As more advanced applications are emerging, it becomes crucial to understand and establish more complex gene regulatory and editing tools for efficient gene therapy applications. Ophthalmology is one of the leading fields in gene therapy applications with more than 90 clinical trials and numerous proof-of-concept studies. The majority of clinical trials are gene replacement therapies that are ideal for monogenic diseases. Despite Luxturna's clinical success, there are still several limitations to gene replacement therapies including the size of the target gene, the choice of the promoter as well as the pathogenic alleles. Therefore, further attempts to employ novel gene regulatory and gene editing applications are crucial to targeting retinal diseases that have not been possible with the existing approaches. CRISPR-Cas9 technology opened up the door for corrective gene therapies with its gene editing properties. Advancements in CRISPR-Cas9-associated tools including base modifiers and prime editing already improved the efficiency and safety profile of base editing approaches. While base editing is a highly promising effort, gene regulatory approaches that do not interfere with genomic changes are also becoming available as safer alternatives. Antisense oligonucleotides are one of the most commonly used approaches for correcting splicing defects or eliminating mutant mRNA. More complex gene regulatory methodologies like artificial transcription factors are also another developing field that allows targeting haploinsufficiency conditions, functionally equivalent genes, and multiplex gene regulation. In this review, we summarized the novel gene editing and gene regulatory technologies and highlighted recent translational progress, potential applications, and limitations with a focus on retinal diseases.

CRISPR/Cas9—A Promising Therapeutic Tool to Cure Blindness: Current Scenario and Future Prospects

CRISPR-based targeted genome editing is bringing revolutionary changes in the research arena of biological sciences. CRISPR/Cas9 has been explored as an efficient therapeutic tool for the treatment of genetic diseases. It has been widely used in ophthalmology research by using mouse models to correct pathogenic mutations in the eye stem cells. In recent studies, CRISPR/Cas9 has been used to correct a large number of mutations related to inherited retinal disorders. In vivo therapeutic advantages for retinal diseases have been successfully achieved in some rodents. Current advances in the CRISPR-based gene-editing domain, such as modified Cas variants and delivery approaches have optimized its application to treat blindness. In this review, recent progress and challenges of the CRISPR-Cas system have been discussed to cure blindness and its prospects.

Degeneration of retina-brain components and connections in glaucoma: Disease causation and treatment options for eyesight preservation

Eyesight is the most important of our sensory systems for optimal daily activities and overall survival. Patients who experience visual impairment due to elevated intraocular pressure (IOP) are often those afflicted with primary open-angle glaucoma (POAG) which slowly robs them of their vision unless treatment is administered soon after diagnosis. The hallmark features of POAG and other forms of glaucoma are damaged optic nerve, retinal ganglion cell (RGC) loss and atrophied RGC axons connecting to various brain regions associated with receipt of visual input from the eyes and eventual decoding and perception of images in the visual cortex. Even though increased IOP is the major risk factor for POAG, the disease is caused by many injurious chemicals and events that progress slowly within all components of the eye-brain visual axis. Lowering of IOP mitigates the damage to some extent with existing drugs, surgical and device implantation therapeutic interventions. However, since multifactorial degenerative processes occur during aging and with glaucomatous optic neuropathy, different forms of neuroprotective, nutraceutical and electroceutical regenerative and revitalizing agents and processes are being considered to combat these eye-brain disorders. These aspects form the basis of this short review article.

From Bench to Bed: The Current Genome Editing Therapies for Glaucoma

Glaucoma is a group of optic neuropathies featured by degeneration of retinal ganglion cells and loss of their axons in the optic nerve. The only currently approved therapies focus on lowering intraocular pressure with medication and surgery. Over the previous few decades, technological advances and research progress regarding pathogenesis has brought glaucomatous gene therapy to the forefront. In this review, we discuss the three current genome editing methods and potential disease mechanisms of glaucoma. We further summarize different genome editing strategies that are being developed to target a number of glaucoma-related genes and pathways from four aspects including strategies to lower intraocular pressure, neuroprotection, RGC and optic nerve neuro-regeneration, and other strategies. In summary, genome therapy is a promising therapy for treating patients with glaucoma and has great potential to be widely applied in clinical practice.

Newer advances in medical management of glaucoma

The burden of irreversible vision loss from Glaucoma continues to rise. While the disease pathogenesis is not well understood, intraocular pressure (IOP) is the only modifiable risk factor identified to prevent glaucomatous vision loss. Medical management remains the first-line of treatment in most adult glaucomas and the evolution of medical therapy for glaucoma has followed an exponential curve. This review tracks the rapid development of new medications and drug delivery systems in the recent years. Introduction of Rho kinase inhibitors with an entirely new mechanism of action from that of the currently used anti glaucoma medications has been a significant milestone. Latanoprostene Bunod is a novel, single molecule which provides two active metabolites that work through two different pathways for reducing intra ocular pressure. Bimatoprost implants and travoprost punctum plugs attempt to ease chronic medication use in glaucoma patients. Nanotechnology is an evolving route of drug delivery. Role of cannabinoids in medical management of glaucoma remain equivocal. The relatively short term effect on IOP, the risks of developing tolerance and side effects impacting patients' neurocognitive health greatly outweigh the potential benefit. Research on Latrunculin B, Adenosine receptor agonists, Specific gene silencing and Stem cell therapy are poised to make an impact on glaucoma treatment. While there is some evidence to support the role of Brimonidine in neuroprotection, further research is needed to clarify the role of Memantine and Neurotrophins. Evidence for benefit from dietary supplementation with Alpha lipoic acid, Forskolin , and Ginko Biloba is limited.

Characterization and Validation of In Vitro and In Vivo Models to Investigate TNF-α-Induced Inflammation in Retinal Diseases

Purpose

Inflammation is implicated in the etiology of diverse retinopathies including uveitis, age-related macular degeneration or diabetic retinopathy. Tumor necrosis factor alpha (TNF-α) is a well-known proinflammatory cytokine that is described as a biomarker for inflammation in diverse retinopathies and therefore emerged as an interesting target to treat inflammation in the eye by neutralizing anti-TNF-α antibodies.

Methods

Recently, we have demonstrated that Adeno-associated virus (AAV)–mediated expression of human TNF-α in the murine eye induces retinal inflammation including vasculitis and fibrosis, thereby mimicking human disease-relevant pathologies. In a proof-of-mechanism study, we now tested whether AAV-TNF-α induced pathologies can be reversed by neutralizing TNF-α antibody treatment.

Results

Strikingly, a single intravitreal injection of the TNF-α antibody golimumab reduced AAV-TNF-α–induced retinal inflammation and retinal thickening. Furthermore, AAV-TNF-α–mediated impaired retinal function was partially rescued by golimumab as revealed by electroretinography recordings. Finally, to study TNF-α-induced vasculitis in human in vitro cell culture assays, we established a monocyte-to-endothelium adhesion co-culture system. Indeed, also in vitro TNF-α induced monocyte adhesion to human retinal endothelial cells, which was prevented by golimumab.

Conclusions

Overall, our study describes valuable in vitro and in vivo approaches to study the function of TNF-α in retinal inflammation and demonstrated a preclinical proof-of-mechanism treatment with golimumab.

Translational Relevance

The AAV-based model expressing human TNF-α allows us to investigate TNF-α–driven pathologies supporting research in mechanisms of retinal inflammation.

The Intertwined Roles of Oxidative Stress and Endoplasmic Reticulum Stress in Glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide, and the burden of the disease continues to grow as the global population ages. Currently, the only treatment option is to lower intraocular pressure. A better understanding of glaucoma pathogenesis will help us to develop novel therapeutic options. Oxidative stress has been implicated in the pathogenesis of many diseases. Oxidative stress occurs when there is an imbalance in redox homeostasis, with reactive oxygen species producing processes overcoming anti-oxidant defensive processes. Oxidative stress works in a synergistic fashion with endoplasmic reticulum stress, to drive glaucomatous damage to trabecular meshwork, retinal ganglion cells and the optic nerve head. We discuss the oxidative stress and endoplasmic reticulum stress pathways and their connections including their key intermediary, calcium. We highlight therapeutic options aimed at disrupting these pathways and discuss their potential role in glaucoma treatment.

Current situation and progress of drugs for reducing intraocular pressure

Glaucoma, the most common cause of irreversible blindness worldwide, usually causes characteristic optic nerve damage. Pathological intraocular pressure (IOP) elevation is a major risk factor. Drug reduction of IOP is the preferred treatment for clinicians because it can delay the progression of disease. However, the traditional IOP-lowering drugs currently used by patients may be poorly tolerated. Therefore, in recent years, some new drugs have been put into clinical application or in clinical phase I–III studies. They have a better IOP-lowering effect and fewer adverse reactions. Because glaucoma is a chronic disease, drugs need to be administered continuously for a long time. For patients, good compliance and high drug bioavailability have a positive effect on the prognosis of the disease. Therefore, clinicians and scientists have developed drug delivery systems to solve this complex problem. In addition, natural compounds and dietary supplements have a good effect of reducing IOP, and they can also protect the optic nerve through antioxidant action. We summarize the current traditional drugs, new drugs, sustained-release drug delivery systems, and complementary drugs and outline the mechanism of action and clinical effects of these drugs on glaucoma and their recent advances.

Therapeutic Drugs and Devices for Tackling Ocular Hypertension and Glaucoma, and Need for Neuroprotection and Cytoprotective Therapies

Damage to the optic nerve and the death of associated retinal ganglion cells (RGCs) by elevated intraocular pressure (IOP), also known as glaucoma, is responsible for visual impairment and blindness in millions of people worldwide. The ocular hypertension (OHT) and the deleterious mechanical forces it exerts at the back of the eye, at the level of the optic nerve head/optic disc and lamina cribosa, is the only modifiable risk factor associated with glaucoma that can be treated. The elevated IOP occurs due to the inability of accumulated aqueous humor (AQH) to egress from the anterior chamber of the eye due to occlusion of the major outflow pathway, the trabecular meshwork (TM) and Schlemm’s canal (SC). Several different classes of pharmaceutical agents, surgical techniques and implantable devices have been developed to lower and control IOP. First-line drugs to promote AQH outflow via the uveoscleral outflow pathway include FP-receptor prostaglandin (PG) agonists (e.g., latanoprost, travoprost and tafluprost) and a novel non-PG EP2-receptor agonist (omidenepag isopropyl, Eybelis^®). TM/SC outflow enhancing drugs are also effective ocular hypotensive agents (e.g., rho kinase inhibitors like ripasudil and netarsudil; and latanoprostene bunod, a conjugate of a nitric oxide donor and latanoprost). One of the most effective anterior chamber AQH microshunt devices is the Preserflo^® microshunt which can lower IOP down to 10–13 mmHg. Other IOP-lowering drugs and devices on the horizon will be also discussed. Additionally, since elevated IOP is only one of many risk factors for development of glaucomatous optic neuropathy, a treatise of the role of inflammatory neurodegeneration of the optic nerve and retinal ganglion cells and appropriate neuroprotective strategies to mitigate this disease will also be reviewed and discussed.

Long-Term Decrease of Intraocular Pressure in Rats by Viral Delivery of miR-146a

Purpose

To evaluate the effects of miR-146a in trabecular meshwork (TM) cells and on intraocular pressure (IOP) in vivo via viral delivery of miR-146a to the anterior chamber of rat eyes.

Methods

Human TM cells were transfected with miR-146 mimic or inhibitor. Some cells from each group were then subjected to cyclic mechanical stress (CMS). Other cells from each group had no force applied. Gene expression was then analyzed by quantitative polymerase chain reaction (qPCR). Replication-deficient adenovirus and lentivirus expressing miR-146a were inoculated into the anterior segment of Brown Norway rat eyes. IOP was monitored by rebound tonometry, visual acuity was evaluated by optokinetic tracking (OKT), and inflammation markers in the anterior segment were examined by slit-lamp, qPCR, and semi-thin sections.

Results

miR-146 affected the expression of genes potentially involved in outflow homeostasis at basal levels and under CMS. Both lentiviral and adenoviral vectors expressing miR-146a resulted in sustained decreases in IOP ranging from 2.6 to 4.4 mmHg. Long term follow-up of rats injected with lentiviral vectors showed a sustained effect on IOP of 4.4 ± 2.9 mmHg that lasted until rats were sacrificed more than 8 months later. Eyes showed no signs of inflammation, loss of visual acuity, or other visible abnormalities.

Conclusions

Intracameral delivery of miR-146a can provide a long-term decrease of IOP in rats without signs of inflammation or other visible adverse effects.

Transitional Relevance

The IOP-lowering effects of miR-146 observed in rats provides a necessary step toward the development of an effective gene therapy for glaucoma in humans.

A Review on Mesenchymal Stem Cells for Treatment of Retinal Diseases

Mesenchymal Stem Cells (MSCs) have been studied extensively for the treatment of several retinal diseases. The therapeutic potential of MSCs lies in its ability to differentiate into multiple lineages and secretome enriched with immunomodulatory, anti-angiogenic and neurotrophic factors. Several studies have reported the role of MSCs in repair and regeneration of the damaged retina where the secreted factors from MSCs prevent retinal degeneration, improve retinal morphology and function. MSCs also donate mitochondria to rescue the function of retinal cells and exosomes secreted by MSCs were found to have anti-apoptotic and anti-inflammatory effects. Based on several promising results obtained from the preclinical studies, several clinical trials were initiated to explore the potential advantages of MSCs for the treatment of retinal diseases. This review summarizes the various properties of MSCs that help to repair and restore the damaged retinal cells and its potential for the treatment of retinal degenerative diseases.

Glaucoma Heritability: Molecular Mechanisms of Disease

Glaucoma is one of the world’s leading causes of irreversible blindness. A complex, multifactorial disease, the underlying pathogenesis and reasons for disease progression are not fully understood. The most common form of glaucoma, primary open-angle glaucoma (POAG), was traditionally understood to be the result of elevated intraocular pressure (IOP), leading to optic nerve damage and functional vision loss. Recently, researchers have suggested that POAG may have an underlying genetic component. In fact, studies of genetic association and heritability have yielded encouraging results showing that glaucoma may be influenced by genetic factors, and estimates for the heritability of POAG and disease-related endophenotypes show encouraging results. However, the vast majority of the underlying genetic variants and their molecular mechanisms have not been elucidated. Several genes have been suggested to have molecular mechanisms contributing to alterations in key endophenotypes such as IOP (LMX1B, MADD, NR1H3, and SEPT9), and VCDR (ABCA1, ELN, ASAP1, and ATOH7). Still, genetic studies about glaucoma and its molecular mechanisms are limited by the multifactorial nature of the disease and the large number of genes that have been identified to have an association with glaucoma. Therefore, further study into the molecular mechanisms of the disease itself are required for the future development of therapies targeted at genes leading to POAG endophenotypes and, therefore, increased risk of disease.

Current progress and limitations of AAV mediated delivery of protein therapeutic genes and the importance of developing quantitative pharmacokinetic/pharmacodynamic (PK/PD) models

While protein therapeutics are one of the most successful class of drug molecules, they are expensive and not suited for treating chronic disorders that require long-term dosing. Adeno-associated virus (AAV) mediated in vivo gene therapy represents a viable alternative, which can deliver the genes of protein therapeutics to produce long-term expression of proteins in target tissues. Ongoing clinical trials and recent regulatory approvals demonstrate great interest in these therapeutics, however, there is a lack of understanding regarding their cellular disposition, whole-body disposition, dose-exposure relationship, exposure-response relationship, and how product quality and immunogenicity affects these important properties. In addition, there is a lack of quantitative studies to support the development of pharmacokinetic-pharmacodynamic models, which can support the discovery, development, and clinical translation of this delivery system. In this review, we have provided a state-of-the-art overview of current progress and limitations related to AAV mediated delivery of protein therapeutic genes, along with our perspective on the steps that need to be taken to improve clinical translation of this therapeutic modality.

Looking into the future: Gene and cell therapies for glaucoma

Glaucoma is a complex group of optic neuropathies that affects both humans and animals. Intraocular pressure (IOP) elevation is a major risk factor that results in the loss of retinal ganglion cells (RGCs) and their axons. Currently, lowering IOP by medical and surgical methods is the only approved treatment for primary glaucoma, but there is no cure, and vision loss often progresses despite therapy. Recent technologic advances provide us with a better understanding of disease mechanisms and risk factors; this will permit earlier diagnosis of glaucoma and initiation of therapy sooner and more effectively. Gene and cell therapies are well suited to target these mechanisms specifically with the potential to achieve a lasting therapeutic effect. Much progress has been made in laboratory settings to develop these novel therapies for the eye. Gene and cell therapies have already been translated into clinical application for some inherited retinal dystrophies and age-related macular degeneration (AMD). Except for the intravitreal application of ciliary neurotrophic factor (CNTF) by encapsulated cell technology for RGC neuroprotection, there has been no other clinical translation of gene and cell therapies for glaucoma so far. Possible application of gene and cell therapies consists of long-term IOP control via increased aqueous humor drainage, including inhibition of fibrosis following filtration surgery, RGC neuroprotection and neuroregeneration, modification of ocular biomechanics for improved IOP tolerance, and inhibition of inflammation and neovascularization to prevent the development of some forms of secondary glaucoma.

High-Performance Near-Infrared Fluorescent Secondary Antibodies for Immunofluorescence

A broad array of imaging and diagnostic technologies employs fluorophore-labeled antibodies for biomarker visualization, an experimental technique known as immunofluorescence. Significant performance advantages, such as higher signal-to-noise ratio, are gained if the appended fluorophore emits near-infrared (NIR) light with a wavelength >700 nm. However, the currently available NIR fluorophore antibody conjugates are known to exhibit significant limitations, including low chemical stability and photostability, weakened target specificity, and low fluorescence brightness. These fluorophore limitations are resolved by employing a NIR heptamethine cyanine dye named s775z whose chemical structure is very stable, charge-balanced, and sterically shielded. Using indirect immunofluorescence for imaging and visualization, a secondary IgG antibody labeled with s775z outperformed IgG analogues labeled with the commercially available NIR fluorophores, IRDye 800CW and DyLight800. Comparison experiments include three common techniques: immunocytochemistry, immunohistochemistry, and western blotting. Specifically, the secondary IgG labeled with s775z was 3-8 times brighter, 3-6 times more photostable, and still retained excellent target specificity when the degree of antibody labeling was high. The results demonstrate that antibodies labeled with s775z can emit total photon counts that are 1-2 orders of magnitude higher than those currently possible, and thus enable unsurpassed performance for NIR fluorescence imaging and diagnostics. They are especially well suited for analytical applications that require sensitive NIR fluorescence detection or use modern photon-intense methods that require high photostability.

Altered Iris Aquaporin Expression and Aqueous Humor Osmolality in Glaucoma

Purpose

Aquaporins (AQPs) facilitate transmembrane osmotic water transport and may play a role in iris fluid conductivity, which is implicated in the pathophysiology of glaucoma. In this study, we compared the iris expression of AQPs and aqueous osmolality between primary angle closure glaucoma (PACG), primary open-angle glaucoma (POAG), and nonglaucoma eyes.

Methods

AQP1-5 transcripts from a cohort of 36 PACG, 34 POAG and 26 nonglaucoma irises were measured by quantitative real-time PCR. Osmolality of aqueous humor from another cohort of 49 PACG, 50 POAG, and 50 nonglaucoma eyes were measured using an osmometer. The localization of AQP1 in both glaucoma and nonglaucoma irises was determined by immunofluorescent analysis.

Results

Of the five AQP genes evaluated, AQP1 and AQP2 transcripts were significantly upregulated in both PACG (3.48- and 8.07-fold, respectively) and POAG (3.12- and 11.58-fold, respectively) irises relative to nonglaucoma counterparts. The aqueous osmolalities of PACG (303.68 mmol/kg) and POAG (300.79 mmol/kg) eyes were significantly lower compared to nonglaucoma eyes (312.6 mmol/kg). There was no significant difference in expression of AQP transcripts or aqueous osmolality between PACG and POAG eyes.

Conclusions

PACG and POAG eyes featured significant increase in AQP1 and AQP2 expression in the iris and reduced aqueous osmolality compared to nonglaucoma eyes. These findings suggest that the iris may be involved in altered aqueous humor dynamics in glaucoma pathophysiology. Because PACG did not differ from POAG in both properties studied, it is likely that they are common to glaucoma disease in general.

Cellular senescence in the aging retina and developments of senotherapies for age-related macular degeneration

Age-related macular degeneration (AMD), a degenerative disease in the central macula area of the neuroretina and the supporting retinal pigment epithelium, is the most common cause of vision loss in the elderly. Although advances have been made, treatment to prevent the progressive degeneration is lacking. Besides the association of innate immune pathway genes with AMD susceptibility, environmental stress- and cellular senescence-induced alterations in pathways such as metabolic functions and inflammatory responses are also implicated in the pathophysiology of AMD. Cellular senescence is an adaptive cell process in response to noxious stimuli in both mitotic and postmitotic cells, activated by tumor suppressor proteins and prosecuted via an inflammatory secretome. In addition to physiological roles in embryogenesis and tissue regeneration, cellular senescence is augmented with age and contributes to a variety of age-related chronic conditions. Accumulation of senescent cells accompanied by an impairment in the immune-mediated elimination mechanisms results in increased frequency of senescent cells, termed “chronic” senescence. Age-associated senescent cells exhibit abnormal metabolism, increased generation of reactive oxygen species, and a heightened senescence-associated secretory phenotype that nurture a proinflammatory milieu detrimental to neighboring cells. Senescent changes in various retinal and choroidal tissue cells including the retinal pigment epithelium, microglia, neurons, and endothelial cells, contemporaneous with systemic immune aging in both innate and adaptive cells, have emerged as important contributors to the onset and development of AMD. The repertoire of senotherapeutic strategies such as senolytics, senomorphics, cell cycle regulation, and restoring cell homeostasis targeted both at tissue and systemic levels is expanding with the potential to treat a spectrum of age-related diseases, including AMD.

Novel Drug Delivery Systems Fighting Glaucoma: Formulation Obstacles and Solutions

Glaucoma is considered to be one of the biggest health problems in the world. It is the main cause of preventable blindness due to its asymptomatic nature in the early stages on the one hand and patients' non-adherence on the other. There are several approaches in glaucoma treatment, whereby this has to be individually designed for each patient. The first-line treatment is medication therapy. However, taking into account numerous disadvantages of conventional ophthalmic dosage forms, intensive work has been carried out on the development of novel drug delivery systems for glaucoma. This review aims to provide an overview of formulation solutions and strategies in the development of in situ gel systems, nanosystems, ocular inserts, contact lenses, collagen corneal shields, ocular implants, microneedles, and iontophoretic devices. The results of studies confirming the effectiveness of the aforementioned drug delivery systems were also briefly presented.

Medical anti-glaucoma therapy: Beyond the drop

Barriers to effective medical therapy are numerous and include difficulties with effective and sustained control of intraocular pressure (IOP) and adherence to prescribed anti-glaucoma drop regimens. In an effort to circumvent these challenges, a number of new anti-glaucoma therapies with sustained effects have emerged. Methods for sustained delivery of prostaglandin analogs are being intensely investigated and many are in human clinical trials. Intracameral devices include the following: Allergan's Durysta^™ Bimatoprost SR, Envisia Therapeutics' ENV515 travoprost implant, Glaukos' iDose^™ , Ocular Therapeutix's OTX-TIC travoprost implant, and Santen's polycaprolactone implant with PGE2-derivative DE-117. Other prostaglandin-based technologies include Allergan's bimatoprost ring (placed in the conjunctival fornix), Ocular Therapeutics' OTX-TP intracanalicular travoprost implant, subconjunctival latanoprost in a liposomal formulation, and the PGE2 derivative PGN 9856-isopropyl ester that is applied to the periorbital skin. Exciting breakthroughs in gene therapy include using viral vectors to correct defective genes such as MYOC or to modulate gonioimplant fibrosis, CRISPR technology to edit MYOC or to alter aquaporin to reduce aqueous humor production, and siRNA technology to silence specific genes. Stem cell technology can repopulate depleted tissues or, in the case of Neurotech's Renexus® NT-501 intravitreal implant, serve as a living drug delivery device that continuously secretes neurotrophic factors. Other unique approaches involve nanotechnology, nasal sprays that deliver drug directly to the optic nerve and noninvasive alternating current stimulation of surviving cells in the optic nerve. Over time these modalities are likely to challenge the preeminent role that drops currently play in the medical treatment of glaucoma in animals.

Fibulin-3 knockout mice demonstrate corneal dysfunction but maintain normal retinal integrity

Fibulin-3 (F3) is an extracellular matrix glycoprotein found in basement membranes across the body. An autosomal dominant R345W mutation in F3 causes a macular dystrophy resembling dry age-related macular degeneration (AMD), whereas genetic removal of wild-type (WT) F3 protects mice from sub-retinal pigment epithelium (RPE) deposit formation. These observations suggest that F3 is a protein which can regulate pathogenic sub-RPE deposit formation in the eye. Yet the precise role of WT F3 within the eye is still largely unknown. We found that F3 is expressed throughout the mouse eye (cornea, trabecular meshwork (TM) ring, neural retina, RPE/choroid, and optic nerve). We next performed a thorough structural and functional characterization of each of these tissues in WT and homozygous (F3^-/-) knockout mice. The corneal stroma in F3^-/- mice progressively thins beginning at 2 months, and the development of corneal opacity and vascularization starts at 9 months, which worsens with age. However, in all other tissues (TM, neural retina, RPE, and optic nerve), gross structural anatomy and functionality were similar across WT and F3^-/- mice when evaluated using SD-OCT, histological analyses, electron microscopy, scotopic electroretinogram, optokinetic response, and axonal anterograde transport. The lack of noticeable retinal abnormalities in F3^-/- mice was confirmed in a human patient with biallelic loss-of-function mutations in F3. These data suggest that (i) F3 is important for maintaining the structural integrity of the cornea, (ii) absence of F3 does not affect the structure or function of any other ocular tissue in which it is expressed, and (iii) targeted silencing of F3 in the retina and/or RPE will likely be well-tolerated, serving as a safe therapeutic strategy for reducing sub-RPE deposit formation in disease. KEY MESSAGES: • Fibulins are expressed throughout the body at varying levels. • Fibulin-3 has a tissue-specific pattern of expression within the eye. • Lack of fibulin-3 leads to structural deformities in the cornea. • The retina and RPE remain structurally and functionally healthy in the absence of fibulin-3 in both mice and humans.

Aquaporin 1 (AQP1) Expression in Healthy Dog Tears

Simple Summary

The characterisation of tear proteins is very important for scientists and clinicians, as it enhances their understanding of ocular physiological phenomena that sometimes evolve into diseases. Recently, ophthalmic research has been focused on aquaporins (AQPs), a family of water channel proteins that are largely ubiquitous in body tissues and are known for their role in water and small solute transport across cell membranes. Based on AQPs’ presumable role in the eye, the aim of the present study was to investigate the expression of aquaporin-1 (AQP1) by Western blot analysis in canine eye tears. To this end, we collected tears from both eyes of 15 healthy dogs by employing two tear collection methods: Schirmer tear strips (STS) and ophthalmic sponges (OS). Moreover, ocular parameters such as Schirmer tear test 1 (STT 1), intraocular pressure (IOP), and tear film break up time (BUT) were measured, and fluorescein and lissamine green staining were performed to uncover possible correlations among the aforementioned parameters. Our results showed that the expression of AQP1 in tears collected by both methods and expressed as multiple bands (measured by densitometry) was higher for the tears collected by OS than for those collected by STS. This work forms the basis of future studies aiming to understand and establish the involvement of AQPs in the production and secretion of tears.

Abstract

Aquaporins (AQPs) are a family of thirteen membrane proteins that play an essential role in the transport of fluids across the cell plasma membrane. Recently, the expression of AQPs in different ocular tissues and their involvement in the pathophysiology of eye diseases, have garnered attention. Considering that literature on AQP expression in the lacrimal glands and their secretion is scarce, we aimed to characterise AQP1 expression in the tears of healthy dogs using two tear collection methods (Schirmer tear strips (STS) and ophthalmic sponges (OS)). Fifteen healthy dogs, free of ophthalmic diseases, were included in the study. Tear collection was performed by using STS in one eye and OS in the other. After the extraction of proteins from the tears, the expression of AQP1 was analysed by Western blotting. AQP1 was expressed as a band of 28 kDa. In addition, differences were observed in the expression of AQP1 and in the correlation between tear volume and protein concentration, in tears collected by the two different methods. Our results suggest that AQP1 has a specific role in tear secretion; further research is required to assess its particular role in the function of the ocular surface in eye physiology and pathology.