Effects of GABA receptor antagonists on thresholds of P23H rat retinal ganglion cells to electrical stimulation of the retina

The mechanistic functional landscape of retinitis pigmentosa: a machine learning-driven approach to therapeutic target discovery

BACKGROUND

Retinitis pigmentosa is the prevailing genetic cause of blindness in developed nations with no effective treatments. In the pursuit of unraveling the intricate dynamics underlying this complex disease, mechanistic models emerge as a tool of proven efficiency rooted in systems biology, to elucidate the interplay between RP genes and their mechanisms. The integration of mechanistic models and drug-target interactions under the umbrella of machine learning methodologies provides a multifaceted approach that can boost the discovery of novel therapeutic targets, facilitating further drug repurposing in RP.

METHODS

By mapping Retinitis Pigmentosa-related genes (obtained from Orphanet, OMIM and HPO databases) onto KEGG signaling pathways, a collection of signaling functional circuits encompassing Retinitis Pigmentosa molecular mechanisms was defined. Next, a mechanistic model of the so-defined disease map, where the effects of interventions can be simulated, was built. Then, an explainable multi-output random forest regressor was trained using normal tissue transcriptomic data to learn causal connections between targets of approved drugs from DrugBank and the functional circuits of the mechanistic disease map. Selected target genes involvement were validated on rd10 mice, a murine model of Retinitis Pigmentosa.

RESULTS

A mechanistic functional map of Retinitis Pigmentosa was constructed resulting in 226 functional circuits belonging to 40 KEGG signaling pathways. The method predicted 109 targets of approved drugs in use with a potential effect over circuits corresponding to nine hallmarks identified. Five of those targets were selected and experimentally validated in rd10 mice: Gabre, Gabra1 (GABARα1 protein), Slc12a5 (KCC2 protein), Grin1 (NR1 protein) and Glr2a. As a result, we provide a resource to evaluate the potential impact of drug target genes in Retinitis Pigmentosa.

CONCLUSIONS

The possibility of building actionable disease models in combination with machine learning algorithms to learn causal drug-disease interactions opens new avenues for boosting drug discovery. Such mechanistically-based hypotheses can guide and accelerate the experimental validations prioritizing drug target candidates. In this work, a mechanistic model describing the functional disease map of Retinitis Pigmentosa was developed, identifying five promising therapeutic candidates targeted by approved drug. Further experimental validation will demonstrate the efficiency of this approach for a systematic application to other rare diseases.

Pre- and postsynaptic alterations in the visual cortex of the P23H-1 retinal degeneration rat model

P23H rats express a variant of rhodopsin with a mutation that leads to loss of visual function with similar properties as human autosomal dominant retinitis pigmentosa (RP). The advances made in different therapeutic strategies to recover visual system functionality reveal the need to know whether progressive retina degeneration affects the visual cortex structure. Here we are interested in detecting cortical alterations in young rats with moderate retinal degeneration, and in adulthood when degeneration is severer. For this purpose, we studied the synaptic architecture of the primary visual cortex (V1) by analyzing a series of pre- and postsynaptic elements related to excitatory glutamatergic transmission. Visual cortices from control Sprague Dawley (SD) and P23H rats at postnatal days 30 (P30) and P230 were used to evaluate the distribution of vesicular glutamate transporters VGLUT1 and VGLUT2 by immunofluorescence, and to analyze the expression of postsynaptic density protein-95 (PSD-95) by Western blot. The amount and dendritic spine distribution along the apical shafts of the layer V pyramidal neurons, stained by the Golgi-Cox method, were also studied. We observed that at P30, RP does not significantly affect any of the studied markers and structures, which suggests in young P23H rats that visual cortex connectivity seems preserved. However, in adult rats, although VGLUT1 immunoreactivity and PSD-95 expression were similar between both groups, a narrower and stronger VGLUT2-immunoreactive band in layer IV was observed in the P23H rats. Furthermore, RP significantly decreased the density of dendritic spines and altered their distribution along the apical shafts of pyramidal neurons, which remained in a more immature state compared to the P230 SD rats. Our results indicate that the most notable changes in the visual cortex structure take place after a prolonged retinal degeneration period that affected the presynaptic thalamocortical VGLUT2-immunoreactive terminals and postsynaptic dendritic spines from layer V pyramidal cells. Although plasticity is more limited at these ages, future studies will determine how reversible these changes are and to what extent they can affect the visual system’s functionality.

Activation of retinal ganglion cells using a biomimetic artificial retina

Objective.Biomimetic protein-based artificial retinas offer a new paradigm for restoring vision for patients blinded by retinal degeneration. Artificial retinas, comprised of an ion-permeable membrane and alternating layers of bacteriorhodopsin (BR) and a polycation binder, are assembled using layer-by-layer electrostatic adsorption. Upon light absorption, the oriented BR layers generate a unidirectional proton gradient. The main objective of this investigation is to demonstrate the ability of the ion-mediated subretinal artificial retina to activate retinal ganglion cells (RGCs) of degenerated retinal tissue.Approach. Ex vivoextracellular recording experiments with P23H line 1 rats are used to measure the response of RGCs following selective stimulation of our artificial retina using a pulsed light source. Single-unit recording is used to evaluate the efficiency and latency of activation, while a multielectrode array (MEA) is used to assess the spatial sensitivity of the artificial retina films.Main results.The activation efficiency of the artificial retina increases with increased incident light intensity and demonstrates an activation latency of ∼150 ms. The results suggest that the implant is most efficient with 200 BR layers and can stimulate the retina using light intensities comparable to indoor ambient light. Results from using an MEA show that activation is limited to the targeted receptive field.Significance.The results of this study establish potential effectiveness of using an ion-mediated artificial retina to restore vision for those with degenerative retinal diseases, including retinitis pigmentosa.

Retinitis Pigmentosa and Retinal Degenerations: Deciphering Pathways and Targets for Drug Discovery and Development

Inherited retinal diseases (IRDs) are a group of retinopathies generally caused by genetic mutations. Retinitis pigmentosa (RP) represents one of the most studied IRDs. RP leads to intense vision loss or blindness resulting from the degeneration of photoreceptor cells. To date, RP is mainly treated with palliative supplementation of vitamin A and retinoids, gene therapies, or surgical interventions. Therefore, a pharmacologically based therapy is an urgent need requiring a medicinal chemistry approach, to validate molecular targets able to deal with retinal degeneration. This Review aims at outlining the recent research efforts in identifying new drug targets for RP, especially focusing on the neuroprotective role of the Wnt/β-catenin/GSK3β pathway and apoptosis modulators (in particular PARP-1) but also on growth factors such as VEGF and BDNF. Furthermore, the role of spatiotemporally expressed G protein-coupled receptors (GPR124) in the retina and the emerging function of histone deacetylase inhibitors in promoting retinal neuroprotection will be discussed.

Effects of Dopamine D2-Like Receptor Antagonists on Light Responses of Ganglion Cells in Wild-Type and P23H Rat Retinas

In animal models of retinitis pigmentosa the dopaminergic system in the retina appears to be dysfunctional, which may contribute to the debilitated sight experienced by retinitis pigmentosa patients. Since dopamine D2-like receptors are known to modulate the activity of dopaminergic neurons, I examined the effects of dopamine D2-like receptor antagonists on the light responses of retinal ganglion cells (RGCs) in the P23H rat model of retinitis pigmentosa. Extracellular electrical recordings were made from RGCs in isolated transgenic P23H rat retinas and wild-type Sprague-Dawley rat retinas. Intensity-response curves to flashes of light were evaluated prior to and during bath application of a dopamine D2-like receptor antagonist. The dopamine D2/D3 receptor antagonists sulpiride and eticlopride and the D4 receptor antagonist L-745,870 increased light sensitivity of P23H rat RGCs but decreased light sensitivity in Sprague-Dawley rat RGCs. In addition, L-745,870, but not sulpiride or eticlopride, reduced the maximum peak responses of Sprague-Dawley rat RGCs. I describe for the first time ON-center RGCs in P23H rats that exhibit an abnormally long-latency (>200 ms) response to the onset of a small spot of light. Both sulpiride and eticlopride, but not L-745,870, reduced this ON response and brought out a short-latency OFF response, suggesting that these cells are in actuality OFF-center cells. Overall, the results show that the altered dopaminergic system in degenerate retinas contributes to the deteriorated light responses of RGCs.

Hearing impairment in the P23H-1 retinal degeneration rat model

The transgenic P23H line 1 (P23H-1) rat expresses a variant of rhodopsin with a mutation that leads to loss of visual function. This rat strain is an experimental model usually employed to study photoreceptor degeneration. Although the mutated protein should not interfere with other sensory functions, observing severe loss of auditory reflexes in response to natural sounds led us to study auditory brain response (ABR) recording. Animals were separated into different hearing levels following the response to natural stimuli (hand clapping and kissing sounds). Of all the analyzed animals, 25.9% presented auditory loss before 50 days of age (P50) and 45% were totally deaf by P200. ABR recordings showed that all the rats had a higher hearing threshold than the control Sprague-Dawley (SD) rats, which was also higher than any other rat strains. The integrity of the central and peripheral auditory pathway was analyzed by histology and immunocytochemistry. In the cochlear nucleus (CN), statistical differences were found between SD and P23H-1 rats in VGluT1 distribution, but none were found when labeling all the CN synapses with anti-Syntaxin. This finding suggests anatomical and/or molecular abnormalities in the auditory downstream pathway. The inner ear of the hypoacusic P23H-1 rats showed several anatomical defects, including loss and disruption of hair cells and spiral ganglion neurons. All these results can explain, at least in part, how hearing impairment can occur in a high percentage of P23H-1 rats. P23H-1 rats may be considered an experimental model with visual and auditory dysfunctions in future research.

Focal electrical stimulation of major ganglion cell types in the primate retina for the design of visual prostheses

Electrical stimulation of retinal neurons with an advanced retinal prosthesis may eventually provide high-resolution artificial vision to the blind. However, the success of future prostheses depends on the ability to activate the major parallel visual pathways of the human visual system. Electrical stimulation of the five numerically dominant retinal ganglion cell types was investigated by simultaneous stimulation and recording in isolated peripheral primate (Macaca sp.) retina using multi-electrode arrays. ON and OFF midget, ON and OFF parasol, and small bistratified ganglion cells could all be activated directly to fire a single spike with submillisecond latency using brief pulses of current within established safety limits. Thresholds for electrical stimulation were similar in all five cell types. In many cases, a single cell could be specifically activated without activating neighboring cells of the same type or other types. These findings support the feasibility of direct electrical stimulation of the major visual pathways at or near their native spatial and temporal resolution.

Activation of ganglion cells in wild-type and P23H rat retinas with a small subretinal electrode

Electronic retinal prostheses are being developed for people who become blind due to loss of photoreceptors from the disease retinitis pigmentosa. Previously, we reported on the responses of RGCs in the P23H rat (a model of retinitis pigmentosa) and the Sprague-Dawley (SD) rat to stimulation with a 400-μm diameter electrode (Jensen and Rizzo, 2011). With recent clinical trials now utilizing smaller (50-200 μm) electrodes, I sought to investigate the electrically evoked responses of RGCs in P23H and SD rat retinas with a smaller (125-μm diameter) electrode. Here, I report on the electrically evoked spike activity from RGCs that arose from stimulation of the retinal neural network. With biphasic current pulses of 1 ms per phase, the thresholds for activation of SD rat RGCs ranged from 0.52 to 2.8 μA; thresholds of P23H rat RGCs ranged from 1.2 to 7.8 μA. Median thresholds of RGCs were 1.4 μA in SD rats and 2.5 μA in P23H rats. These thresholds measurements were obtained with the recording electrode placed over the stimulating electrode. I also examined how thresholds of RGCs change as a function of distance (100-500 μm) from the center of the stimulating electrode. The median threshold currents of RGCs were much higher in P23H rats for all distances. What was striking was that the thresholds for activation of RGCs in P23H rat retinas rose much more rapidly. When the recording electrode was only 100-200 μm from the center of the stimulating electrode, the median threshold current of P23H rat RGCs rose by 460%. In contrast, the median threshold current of SD rat RGCs increased only 29%. I also investigated the contribution of photoreceptors to the electrically evoked responses of ON-center RGCs in SD rat retinas by examining the change in RGC thresholds when photoreceptor input to ON bipolar cells was blocked with the mGluR6 antagonist CPPG. At 500-600 μm, CPPG suppressed the light-evoked responses of the RGCs and at the same time increased the amount of current needed to generate an electrically evoked response. Similar to what was observed with SD rat RGCs, CPPG suppressed the light-evoked responses of ON-center P23H rat RGCs. However, the stimulation thresholds were not significantly altered. In conclusion, the data show that the threshold currents for indirect stimulation of both SD and P23H rat RGCs with a 125-μm diameter electrode are much lower than what we found previously with a 400-μm diameter electrode. To achieve high resolution vision with a multielectrode array, the spread of activation of RGCs needs to be limited. Our findings indicate that the spread of activation of RGCs is more confined in the degenerate retina. Lastly, my findings indicate that photoreceptors contribute to the lower stimulation thresholds of RGCs in normal, healthy retinas.