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In the eye, cells from the retinal pigment epithelium (RPE) facing the neurosensory retina exert several functions that are all crucial for long-term survival of photoreceptors (PRs) and vision. Among those, RPE cells phagocytose under a circadian rhythm photoreceptor outer segment (POS) tips that are constantly subjected to light rays and oxidative attacks. The MerTK tyrosine kinase receptor is a key element of this phagocytic machinery required for POS internalization. Recently, we showed that MerTK is subjected to the cleavage of its extracellular domain to finely control its function. In addition, monocytes in retinal blood vessels can migrate inside the inner retina and differentiate into macrophages expressing MerTK, but their role in this context has not been studied yet. We thus investigated the ocular phenotype of MerTK cleavage-resistant (MerTKCR) mice to understand the relevance of this characteristic on retinal homeostasis at the RPE and macrophage levels. MerTKCR retinae appear to develop and function normally, as observed in retinal sections, by electroretinogram recordings and optokinetic behavioral tests. Monitoring of MerTKCR and control mice between the ages of 3 and 18 months showed the development of large degenerative areas in the central retina as early as 4 months when followed monthly by optical coherence tomography (OCT) plus fundus photography (FP)/autofluorescence (AF) detection but not by OCT alone. The degenerative areas were associated with AF, which seems to be due to infiltrated macrophages, as observed by OCT and histology. MerTKCR RPE primary cultures phagocytosed less POS in vitro, while in vivo, the circadian rhythm of POS phagocytosis was deregulated. Mitochondrial function and energy production were reduced in freshly dissected RPE/choroid tissues at all ages, thus showing a metabolic impairment not present in macrophages. RPE anomalies were detected by electron microscopy, including phagosomes retained in the apical area and vacuoles. Altogether, this new mouse model displays a novel phenotype that could prove useful to understanding the interplay between RPE and PRs in inflammatory retinal degenerations and highlights new roles for MerTK in the regulation of the energetic metabolism and the maintenance of the immune privilege in the retina.

Evaluating biocompatibility is a core essential step to introducing a new material as a candidate for brain-machine interfaces. Foreign body reactions often result in glial scars that can impede the performance of the interface. Having a high conductivity and large electrochemical window, graphene is a candidate material for electrical stimulation with retinal prosthesis. In this study, non-functional devices consisting of chemical vapor deposition (CVD) graphene embedded onto polyimide/SU-8 substrates were fabricated for a biocompatibility study. The devices were implanted beneath the retina of blind P23H rats. Implants were monitored by optical coherence tomography (OCT) and eye fundus which indicated a high stability in vivo up to 3 months before histology studies were done. Microglial reconstruction through confocal imaging illustrates that the presence of graphene on polyimide reduced the number of microglial cells in the retina compared to polyimide alone, thereby indicating a high biocompatibility. This study highlights an interesting approach to assess material biocompatibility in a tissue model of central nervous system, the retina, which is easily accessed optically and surgically.

Many neural interfaces used for therapeutic applications are based on extracellular electrical stimulation to control cell polarization and thus functional activity. Amongst them, retinal implants have been designed to restore visual perception in blind patients affected by photoreceptor degeneration diseases, such as age-related macular degeneration (AMD) or retinitis pigmentosa (RP). While designing such a neural interface, several aspects must be taken into account, like the stimulation efficiency related to the current distribution within the tissue, the bio-interface optimization to improve resolution and tissue integration, and the material biocompatibility associated with long-term aging. In this study, we investigate the use of original microelectrode geometries for subretinal stimulation. The proposed structures combine the use of 3D wells with protuberant mushroom shaped electrode structures in the bottom, implemented on a flexible substrate that allows the in vivo implantation of the devices. These 3D microelectrode structures were first modeled using finite element analysis. Then, a specific microfabrication process compatible with flexible implants was developed to create the 3D microelectrode structures. These structures were tested in vivo to check the adaptation of the retinal tissue to them. Finally, preliminary in vivo stimulation experiments were performed.

Photophobia may arise from various causes and frequently accompanies numerous ocular diseases. In modern highly illuminated world, complaints about greater photosensitivity to blue light increasingly appear. However, the pathophysiology of photophobia is still debated. In the present work, we investigated in vivo the role of various neural pathways potentially implicated in blue-light aversion. Moreover, we studied the light-induced neuroinflammatory processes on the ocular surface and in the trigeminal pathways. Adult male C57BL/6J mice were exposed either to blue (400–500 nm) or to yellow (530–710 nm) LED light (3 h, 6 mW/cm2). Photosensitivity was measured as the time spent in dark or illuminated parts of the cage. Pharmacological treatments were applied: topical instillation of atropine, pilocarpine or oxybuprocaine, intravitreal injection of lidocaine, norepinephrine or “blocker” of the visual photoreceptor transmission, and intraperitoneal injection of a melanopsin antagonist. Clinical evaluations (ocular surface state, corneal mechanical sensitivity and tear quantity) were performed directly after exposure to light and after 3 days of recovery in standard light conditions. Trigeminal ganglia (TGs), brainstems and retinas were dissected out and conditioned for analyses. Mice demonstrated strong aversion to blue but not to yellow light. The only drug that significantly decreased the blue-light aversion was the intraperitoneally injected melanopsin antagonist. After blue-light exposure, dry-eye-related inflammatory signs were observed, notably after 3 days of recovery. In the retina, we observed the increased immunoreactivity for GFAP, ATF3, and Iba1; these data were corroborated by RT-qPCR. Moreover, retinal visual and non-visual photopigments distribution was altered. In the trigeminal pathway, we detected the increased mRNA expression of cFOS and ATF3 as well as alterations in cytokines’ levels. Thus, the wavelength-dependent light aversion was mainly mediated by melanopsin-containing cells, most likely in the retina. Other potential pathways of light reception were also discussed. The phototoxic message was transmitted to the trigeminal system, inducing both inflammation at the ocular surface and stress in the retina. Further investigations of retina-TG connections are needed.