Taurine: a promising nutraceutic in the prevention of retinal degeneration

Taurine is considered a non-essential amino acid because it is synthesized by most mammals.However,dietary intake of taurine may be necessary to achieve the physiological levels required for the development,maintenance,and function of certain tissues.Taurine may be especially important for the retina.The concentration of taurine in the retina is higher than that in any other tissue in the body and taurine deficiency causes retinal oxidative stress,apoptosis,and degeneration of photoreceptors and retinal ganglion cells.Low plasma taurine levels may also underlie retinal degeneration in humans and therefore,taurine administration could exert retinal neuroprotective effects.Taurine has antioxidant,anti-apoptotic,immunomodulatory,and calcium homeostasis-regulatory properties.This review summarizes the role of taurine in retinal health and disease,where it appears that taurine may be a promising nutraceutical.

Optimal transcorneal electrical stimulation parameters for preserving photoreceptors in a mouse model of retinitis pigmentosa

Retinitis pigmentosa is a hereditary retinal disease that affects rod and cone photoreceptors,leading to progressive photoreceptor loss.Previous research supports the beneficial effect of electrical stimulation on photoreceptor survival.This study aims to identify the most effective electrical stimulation parameters and functional advantages of transcorneal electrical stimulation(tcES)in mice affected by inherited retinal degeneration.Additionally,the study seeked to analyze the electric field that reaches the retina in both eyes in mice and post-mortem humans.In this study,we recorded waveforms and voltages directed to the retina during transcorneal electrical stimulation in C57BL/6J mice using an intraocular needle probe with rectangular,sine,and ramp waveforms.To investigate the functional effects of electrical stimulation on photoreceptors,we used human retinal explant cultures and rhodopsin knockout(Rho^(-/-))mice,demonstrating progressive photoreceptor degeneration with age.Human retinal explants isolated from the donors’eyes were then subjected to electrical stimulation and cultured for 48 hours to simulate the neurodegenerative environment in vitro.Photoreceptor density was evaluated by rhodopsin immunolabeling.In vivo Rho^(-/-)mice were subjected to two 5-day series of daily transcorneal electrical stimulation using rectangular and ramp waveforms.Retinal function and visual perception of mice were evaluated by electroretinography and optomotor response(OMR),respectively.Immunolabeling was used to assess the morphological and biochemical changes of the photoreceptor and bipolar cells in mouse retinas.Oscilloscope recordings indicated effective delivery of rectangular,sine,and ramp waveforms to the retina by transcorneal electrical stimulation,of which the ramp waveform required the lowest voltage.Evaluation of the total conductive resistance of the post-mortem human compared to the mouse eyes indicated higher cornea-to-retina resistance in human eyes.The temperature recordings during and after electrical stimulation indicated no significant temperature change in vivo and only a subtle temperature increase in vitro(~0.5-1.5°C).Electrical stimulation increased photoreceptor survival in human retinal explant cultures,particularly at the ramp waveform.Transcorneal electrical stimulation(rectangular+ramp)waveforms significantly improved the survival and function of S and M-cones and enhanced visual acuity based on the optomotor response results.Histology and immunolabeling demonstrated increased photoreceptor survival,improved outer nuclear layer thickness,and increased bipolar cell sprouting in Rho^(-/-)mice.These results indicate that transcorneal electrical stimulation effectively delivers the electrical field to the retina,improves photoreceptor survival in both human and mouse retinas,and increases visual function in Rho^(-/-)mice.Combined rectangular and ramp waveform stimulation can promote photoreceptor survival in a minimally invasive fashion.

Bone marrow-derived mononuclear stem cells in the treatment of retinal degenerations

Retinal degenerative diseases affecting the outer retina in its many forms(inherited,acquired or induced)are characterized by photoreceptor loss,and represent currently a leading cause of irreversible vision loss in the world.At present,there are very few treatments capable of preventing,recovering or reversing photoreceptor degeneration or the secondary retinal remodeling,which follows photoreceptor loss and can also cause the death of other retinal cells.Thus,these diseases are nowadays one of the greatest challenges in the field of ophthalmological research.Bone marrow derived-mononuclear stem cell transplantation has shown promising results for the treatment of photoreceptor degenerations.These cells may have the potential to slow down photoreceptor loss,and therefore should be applied in the early stages of photoreceptor degenerations.Furthermore,because of their possible paracrine effects,they may have a wide range of clinical applications,since they can potentially impact on several retinal cell types at once and photoreceptor degenerations can involve different cells and/or begin in one cell type and then affect adjacent cells.The intraocular injection of bone marrow derived-mononuclear stem cells also enhances the outcomes of other treatments aimed to protect photoreceptors.Therefore,it is likely that future investigations may combine bone marrow derived-mononuclear stem cell therapy with other systemic or intraocular treatments to obtain greater therapeutic effects in degenerative retinal diseases.

Deletion of prominin-1 in mice results in disrupted photoreceptor outer segment protein homeostasis

AIM:To illustrate the underlying mechanism how prominin-1(also known as Prom1)mutation contribute to progressive photoreceptor degeneration.METHODS:A CRISPR-mediated Prom1 knockout(Prom1-KO)mice model in the C57BL/6 was generated and the photoreceptor degeneration phenotypes by means of structural and functional tests were demonstrated.Immunohistochemistry and immunoblot analysis were performed to reveal the localization and quantity of related outer segment(OS)proteins.RESULTS:The Prom1-KO mice developed the photoreceptor degeneration phenotype including the decreased outer nuclear layer(ONL)thickness and compromised electroretinogram amplitude.Immunohistochemistry analysis revealed impaired trafficking of photoreceptor OS proteins.Immunoblot data demonstrated decreased photoreceptor OS proteins.CONCLUSION:Prom1 deprivation causes progressive photoreceptor degeneration.Prom1 is essential for maintaining normal trafficking and normal quantity of photoreceptor OS proteins.The new light is shed on the pathogenic mechanism underlying photoreceptor degeneration caused by Prom1 mutation.

The endoplasmic reticulum membrane protein complex subunit Emc6 is essential for rhodopsin localization and photoreceptor cell survival

The endoplasmic reticulum(ER)membrane protein complex(EMC)is responsible for monitoring the biogenesis and synthetic quality of membrane proteins with tail-anchored or multiple transmembrane domains.The EMC subunit EMC6 is one of the core members of EMC and forms an enclosed hydrophilic vestibule in cooperation with EMC3.Despite studies demonstrating that deletion of EMC3 led to rhodopsin mislocalization in rod photoreceptors of mice,the precise mechanism leading to the failure of rhodopsin trafficking remains unclear.Here,we generated the first rod photoreceptor-specific knockout of Emc6(RKO)and cone photoreceptor-specific knockout of Emc6(CKO)mouse models.Deficiency of Emc6 in rod photoreceptors led to progressive shortening of outer segments(OS),impaired visual function,mislocalization and reduced expression of rhodopsin,and increased gliosis in rod photoreceptors.In addition,CKO mice displayed the progressive death of cone photoreceptors and abnormal localization of cone opsin protein.Subsequently,proteomics analysis of the RKO mouse retina illustrated that several cilium-related proteins,particularly anoctamin-2(ANO2)and transmembrane protein 67(TMEM67),were significantly down-regulated prior to OS degeneration.Detrimental rod photoreceptor cilia and mislocalized membrane disc proteins were evident in RKO mice.Our data revealed that in addition to monitoring the synthesis of rhodopsin-dominated membrane disc proteins,EMC6 also impacted rod photoreceptors'ciliogenesis by regulating the synthesis of membrane proteins associated with cilia,contributing to the mislocalization of membrane disc proteins.

The m^(6)A reader YTHDC2 maintains visual function and retinal photoreceptor survival through modulating translation of PPEF2 and PDE6B

Inherited retinal dystrophies (IRDs) are major causes of visual impairment and irreversible blindness worldwide, while the precise molecular and genetic mechanisms are still elusive. N6-methyladenosine (m^(6)A) modification is the most prevalent internal modification in eukaryotic mRNA. YTH domain containing 2 (YTHDC2), an m^(6)A reader protein, has recently been identified as a key player in germline development and human cancer. However, its contribution to retinal function remains unknown. Here, we explore the role of YTHDC2 in the visual function of retinal rod photoreceptors by generating rod-specific Ythdc2 knockout mice. Results show that Ythdc2 deficiency in rods causes diminished scotopic ERG responses and progressive retinal degeneration. Multi-omics analysis further identifies Ppef2 and Pde6b as the potential targets of YTHDC2 in the retina. Specifically, via its YTH domain, YTHDC2 recognizes and binds m^(6)A-modified Ppef2 mRNA at the coding sequence and Pde6b mRNA at the 5′-UTR, resulting in enhanced translation efficiency without affecting mRNA levels. Compromised translation efficiency of Ppef2 and Pde6b after YTHDC2 depletion ultimately leads to decreased protein levels in the retina, impaired retinal function, and progressive rod death. Collectively, our finding highlights the importance of YTHDC2 in visual function and photoreceptor survival, which provides an unreported elucidation of IRD pathogenesis via epitranscriptomics.

A cascade model of information processing and encoding for retinal prosthesis

Retinal prosthesis offers a potential treatment for individuals suffering from photoreceptor degeneration diseases.Establishing biological retinal models and simulating how the biological retina convert incoming light signal into spike trains that can be properly decoded by the brain is a key issue.Some retinal models have been presented,ranking from structural models inspired by the layered architecture to functional models originated from a set of specific physiological phenomena.However,Most of these focus on stimulus image compression,edge detection and reconstruction,but do not generate spike trains corresponding to visual image.In this study,based on stateof-the-art retinal physiological mechanism,including effective visual information extraction,static nonlinear rectification of biological systems and neurons Poisson coding,a cascade model of the retina including the out plexiform layer for information processing and the inner plexiform layer for information encoding was brought forward,which integrates both anatomic connections and functional computations of retina.Using MATLAB software,spike trains corresponding to stimulus image were numerically computed by four steps：linear spatiotemporal filtering,static nonlinear rectification,radial sampling and then Poisson spike generation.The simulated results suggested that such a cascade model could recreate visual information processing and encoding functionalities of the retina,which is helpful in developing artificial retina for the retinally blind.

The splicing factor Prp31 is essential for photoreceptor development in Drosophila

Retinitis pigmentosa is a leading cause of blindness and a progressive retinal disorder,affecting millions of people worldwide.This disease is characterized by photoreceptor degeneration,eventually leading to complete blindness.Autosomal dominant(adRP)has been associated with mutations in at least four ubiquitously expressed genes encoding pre-mRNA splicing factors—Prp3,Prp8,Prp31 and PAP1.Biological function of adRPassociated splicing factor genes and molecular mechanisms by which mutations in these genes cause cell-type specific photoreceptor degeneration in humans remain to be elucidated.To investigate the in vivo function of these adRP-associated splicing factor genes,we examined Drosophila in which expression of fly Prp31 homolog was down-regulated.Sequence analyses show that CG6876 is the likely candidate of Drosophila melanogaster Prp31 homolog(DmPrp31).Predicted peptide sequence for CG6876 shows 57%similarity to the Homo sapiens Prp31 protein(HsPrp31).Reduction of the endogenous Prp31 by RNAi-mediated knockdown speci-fically in the eye leads to reduction of eye size or complete absence of eyes with remarkable features of photoreceptor degeneration and recapitulates the bimodal expressivity of human Prp31 mutations in adRP patients.Such transgenic DmPrp31RNAi flies provide a useful tool for identifying genetic modifiers or interacting genes for Prp31.Expression of the human Prp31 in these animals leads to a partial rescue of the eye phenotype.Our results indicate that the Drosophila CG6876 is the fly ortholog of mammalian Prp31 gene.

Prototype chemical synapse chip for spatially patterned neurotransmitter stimulation of the retina ex vivo

Biomimetic stimulation of the retina with neurotransmitters,the natural agents of communication at chemical synapses,could be more effective than electrical stimulation for treating blindness from photoreceptor degenerative diseases.Recent studies have demonstrated the feasibility of neurotransmitter stimulation by injecting glutamate,a primary retinal neurotransmitter,into the retina at isolated single sites.Here,we demonstrate spatially patterned multisite stimulation of the retina with glutamate,offering the first experimental evidence for applicability of this strategy for translating visual patterns into afferent neural signals.To accomplish pattern stimulation,we fabricated a special microfluidic device comprising an array of independently addressable microports connected to tiny on-chip glutamate reservoirs via microchannels.The device prefilled with glutamate was interfaced with explanted rat retinas placed over a multielectrode array(MEA)with the retinal ganglion cells(RGC)contacting the electrodes and photoreceptor surface contacting the microports.By independently and simultaneously activating a subset of the microports with modulated pressure pulses,small boluses of glutamate were convectively injected at multiple sites in alphabet patterns over the photoreceptor surface.We found that the glutamate-driven RGC responses recorded through the MEA system were robust and spatially laid out in patterns strongly resembling the injection patterns.The stimulations were also highly localized with spatial resolutions comparable to or better than electrical retinal prostheses.Our findings suggest that surface stimulation of the retina with neurotransmitters in pixelated patterns of visual images is feasible and an artificial chemical synapse chip based on this approach could potentially circumvent the limitations of electrical retinal prostheses.