Gene-Based Therapeutics for Inherited Retinal Diseases

Inherited retinal diseases (IRDs) are a heterogenous group of orphan eye diseases that typically result from monogenic mutations and are considered attractive targets for gene-based therapeutics. Following the approval of an IRD gene replacement therapy for Leber’s congenital amaurosis due to RPE65 mutations, there has been an intensive international research effort to identify the optimal gene therapy approaches for a range of IRDs and many are now undergoing clinical trials. In this review we explore therapeutic challenges posed by IRDs and review current and future approaches that may be applicable to different subsets of IRD mutations. Emphasis is placed on five distinct approaches to gene-based therapy that have potential to treat the full spectrum of IRDs: 1) gene replacement using adeno-associated virus (AAV) and nonviral delivery vectors, 2) genome editing via the CRISPR/Cas9 system, 3) RNA editing by endogenous and exogenous ADAR, 4) mRNA targeting with antisense oligonucleotides for gene knockdown and splicing modification, and 5) optogenetic approaches that aim to replace the function of native retinal photoreceptors by engineering other retinal cell types to become capable of phototransduction.

Can the Dynamic Colouration and Patterning of Bluelined Goatfish (Mullidae; Upeneichthys lineatus) Be Perceived by Conspecifics?

Bluelined goatfish (<i>Upeneichthys lineatus</i>) exhibit dynamic body colour changes and transform rapidly from a pale, buff/white, horizontally banded pattern to a conspicuous, vertically striped, red pattern when foraging. This red pattern is potentially an important foraging signal for communication with conspecifics, provided that <i>U. lineatus</i> can detect and discriminate the pattern. Using both physiological and behavioural experiments, we first examined whether <i>U. lineatus</i> possess visual pigments with sensitivity to long (“red”) wavelengths of light, and whether they can discriminate the colour red. Microspectrophotometric measurements of retinal photoreceptors showed that while <i>U. lineatus</i>lack visual pigments dedicated to the red part of the spectrum, their pigments likely confer some sensitivity in this spectral band. Behavioural colour discrimination experiments suggested that <i>U. lineatus</i>can distinguish a red reward stimulus from a grey distractor stimulus of variable brightness. Furthermore, when presented with red stimuli of varying brightness they could mostly discriminate the darker and lighter reds from the grey distractor. We also obtained anatomical estimates of visual acuity, which suggest that <i>U. lineatus</i> can resolve the contrasting bands of conspecifics approximately 7 m away in clear waters. Finally, we measured the spectral reflectance of the red and white colouration on the goatfish body. Visual models suggest that <i>U. lineatus</i> can discriminate both chromatic and achromatic differences in body colouration where longer wavelength light is available. This study demonstrates that <i>U. lineatus</i> have the capacity for colour vision and can likely discriminate colours in the long-wavelength region of the spectrum where the red body pattern reflects light strongly. The ability to see red may therefore provide an advantage in recognising visual signals from conspecifics. This research furthers our understanding of how visual signals have co-evolved with visual abilities, and the role of visual communication in the marine environment.

Specialization of the photoreceptor transcriptome by Srrm3-dependent microexons is required for outer segment maintenance and vision

ABSTRACTRetinal photoreceptors differ in their transcriptomic profiles from other neuronal subtypes, likely as a reflection of their unique cellular morphology and function in the detection of light thorough the ciliary outer segment. We discovered a new layer of this molecular specialization by revealing that the vertebrate retina expresses the largest number of tissue-enriched microexons of all tissue types. A subset of these microexons is included exclusively in photoreceptor transcripts, particularly in genes involved in cilia biogenesis and in vesicle-mediated transport. This microexon program is regulated by Srrm3, a paralog of the neural microexon regulator Srrm4. Despite both proteins positively regulate retina microexons in vitro, only Srrm3 is highly expressed in mature photoreceptors and its deletion in zebrafish results in widespread downregulation of microexon inclusion, severe photoreceptor alterations and blindness. These results shed light into photoreceptor’s transcriptomic specialization and functionality, uncovering new cell type-specific roles for Srrm3 and microexons with implication for retinal diseases.

Receptor guanylyl cyclase (RGC) family in GtoPdb v.2021.3

The mammalian genome encodes seven guanylyl cyclases, GC-A to GC-G, that are homodimeric transmembrane receptors activated by a diverse range of endogenous ligands. These enzymes convert guanosine-5'-triphosphate to the intracellular second messenger cyclic guanosine-3',5'-monophosphate (cyclic GMP). GC-A, GC-B and GC-C are expressed predominantly in the cardiovascular system, skeletal system and intestinal epithelium, respectively. GC-D and GC-G are found in the olfactory neuropepithelium and Grueneberg ganglion of rodents, respectively. GC-E and GC-F are expressed in retinal photoreceptors.

Visual Cortex Engagement in Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a family of inherited disorders caused by the progressive degeneration of retinal photoreceptors. There is no cure for RP, but recent research advances have provided promising results from many clinical trials. All these therapeutic strategies are focused on preserving existing photoreceptors or substituting light-responsive elements. Vision recovery, however, strongly relies on the anatomical and functional integrity of the visual system beyond photoreceptors. Although the retinal structure and optic pathway are substantially preserved at least in early stages of RP, studies describing the visual cortex status are missing. Using a well-established mouse model of RP, we analyzed the response of visual cortical circuits to the progressive degeneration of photoreceptors. We demonstrated that the visual cortex goes through a transient and previously undescribed alteration in the local excitation/inhibition balance, with a net shift towards increased intracortical inhibition leading to improved filtering and decoding of corrupted visual inputs. These results suggest a compensatory action of the visual cortex that increases the range of residual visual sensitivity in RP.

Differentiation of Mesenchymal Stem Cells into Photoreceptor-like Cells under the Influence of a Temporary Increase in miRNA-182, -183 Expression

It is found that the death of retinal photoreceptors is the main cause of retinal degeneration, while there is not an effective treatment protocol. Data of preclinical and clinical trials indicates that the stem cell therapy is a useful way of treating retinal degeneration problems. On the other hand, previous works found that miRNA-182, -183 significantly affected the photoreceptor maturation and maintenance in animal models. The present study aimed to investigate the impact of a temporary increase in miRNA-182, -183 expression on the differentiation of human bone marrow mesenchymal stem cells (hBMSCs) into photoreceptor-like cells. To this end, miRNA-182, -183 was transfected into hBMSCs; then, qRT-PCR was performed to measure the expression levels of miRNA-182, -183 and some retina-specific genes such as OTX2, NRL, PKCα, and recoverin. CRX and rhodopsin (RHO) levels were also measured through qRT-PCR and immunocytochemistry We indicated that the transfection of hBMSCs with miRNA-182, -183 using the Lipofectamine induce differentiation and progenitor’s genes expression consisted of CRX, OTX2, PKC, Recoverin, NRL and RHO. Moreover, the upregulation expression of transcription factors, CRX and RHO, indicated that miRNA-182, -183 could serve as crucial functions in the differentiation of hBMSCs into photoreceptor-like cells. The findings may provide a new strategy to improve the usage of hBMSCs as a treatment for the retinal dysfunction.

Regulatory Mechanisms of Retinal Photoreceptors Development at Single Cell Resolution

Photoreceptors are critical components of the retina and play a role in the first step of the conversion of light to electric signals. With the discovery of the intrinsically photosensitive retinal ganglion cells, which regulate non-image-forming visual processes, our knowledge of the photosensitive cell family in the retina has deepened. Photoreceptor development is regulated by specific genes and proteins and involves a series of molecular processes including DNA transcription, post-transcriptional modification, protein translation, and post-translational modification. Single-cell sequencing is a promising technology for the study of photoreceptor development. This review presents an overview of the types of human photoreceptors, summarizes recent discoveries in the regulatory mechanisms underlying their development at single-cell resolution, and outlines the prospects in this field.

Insights on the Regeneration Potential of Müller Glia in the Mammalian Retina

Müller glia, the major glial cell types in the retina, maintain retinal homeostasis and provide structural support to retinal photoreceptors. They also possess regenerative potential that might be used for retinal repair in response to injury or disease. In teleost fish (such as zebrafish), the Müller glia response to injury involves reprogramming events that result in a population of proliferative neural progenitors that can regenerate the injured retina. Recent studies have revealed several important mechanisms for the regenerative capacity of Müller glia in fish, which may shed more light on the mechanisms of Müller glia reprogramming and regeneration in mammals. Mammalian Müller glia can adopt stem cell characteristics, and in response to special conditions, be persuaded to proliferate and regenerate, although their native regeneration potential is limited. In this review, we consider the work to date revealing the regenerative potential of the mammalian Müller glia and discuss whether they are a potential source for cell regeneration therapy in humans.

Negative regulation of melatonin secretion by melatonin receptors in ovine pinealocytes

Melatonin (MLT) is a biological modulator of circadian and seasonal rhythms and reproduction. The photoperiodic information is detected by retinal photoreceptors and transmitted through nerve transmissions to the pineal gland, where MLT is synthesized and secreted at night into the blood. MLT interacts with two G protein-coupled receptors, MT1 and MT2. The aim of our work was to provide evidence for the presence of MLT receptors in the ovine pineal gland and define their involvement on melatonin secretion. For the first time, we identified the expression of MLT receptors with the specific 2-[125I]-MLT agonistic radioligand in ovin pinealocytes. The values of Kd and Bmax are 2.24 ± 1.1 nM and 20 ± 6.8 fmol/mg. MLT receptors are functional and inhibit cAMP production and activate ERK1/2 through pertussis toxin-sensitive Gi/o proteins. The MLT receptor antagonist/ inverse agonist luzindole increased cAMP production (189 ± 30%) and MLT secretion (866 ± 13%). The effect of luzindole on MLT secretion was additive with the effect of well-described activators of this pathway such as the β-adrenergic agonist isoproterenol and the α-adrenergic agonist phenylephrine. Co-incubation of all three compounds increased MLT secretion by 1236 ± 199%. These results suggest that MLT receptors are involved in the negative regulation of the synthesis of its own ligand in pinealocytes. While adrenergic receptors promote MLT secretion, MLT receptors mitigate this effect to limit the quantity of MLT secreted by the pineal gland.

Temporal vision: measures, mechanisms and meaning

ABSTRACT Time is largely a hidden variable in vision. It is the condition for seeing interesting things such as spatial forms and patterns, colours and movements in the external world, and yet is not meant to be noticed in itself. Temporal aspects of visual processing have received comparatively little attention in research. Temporal properties have been made explicit mainly in measurements of resolution and integration in simple tasks such as detection of spatially homogeneous flicker or light pulses of varying duration. Only through a mechanistic understanding of their basis in retinal photoreceptors and circuits can such measures guide modelling of natural vision in different species and illuminate functional and evolutionary trade-offs. Temporal vision research would benefit from bridging traditions that speak different languages. Towards that goal, I here review studies from the fields of human psychophysics, retinal physiology and neuroethology, with a focus on fundamental constraints set by early vision.