Objective
Considering the demonstrated implication of the retina in Parkinson disease (PD) pathology and the importance of dopaminergic cells in this tissue, we aimed to analyze the state of the ...dopaminergic amacrine cells and some of their main postsynaptic neurons in the retina of PD.
Methods
Using immunohistochemistry and confocal microscopy, we evaluated morphology, number, and synaptic connections of dopaminergic cells and their postsynaptic cells, AII amacrine and melanopsin‐containing retinal ganglion cells, in control and PD eyes from human donors.
Results
In PD, dopaminergic amacrine cell number was reduced between 58% and 26% in different retinal regions, involving a decline in the number of synaptic contacts with AII amacrine cells (by 60%) and melanopsin cells (by 35%). Despite losing their main synaptic input, AII cells were not reduced in number, but they showed cellular alterations compromising their adequate function: (1) a loss of mitochondria inside their lobular appendages, which may indicate an energetic failure; and (2) a loss of connexin 36, suggesting alterations in the AII coupling and in visual signal transmission from the rod pathway.
Interpretation
The dopaminergic system impairment and the affection of the rod pathway through the AII cells may explain and be partially responsible for the reduced contrast sensitivity or electroretinographic response described in PD. Also, dopamine reduction and the loss of synaptic contacts with melanopsin cells may contribute to the melanopsin retinal ganglion cell loss previously described and to the disturbances in circadian rhythm and sleep reported in PD patients. These data support the idea that the retina reproduces brain neurodegeneration and is highly involved in PD pathology. ANN NEUROL 2020;88:893–906
Retinal neurodegenerative diseases involve a scenario of inflammation and cell death that leads to morphological alterations and visual impairment. Non-ocular inflammatory processes could affect ...neurodegenerative retinal disorders and their progression, at least in part by activating microglial cells and releasing pro-inflammatory cytokines. Our purpose was to study the consequences of a systemic inflammatory process in the progression of retinal degeneration in P23H rats, a retinitis pigmentosa (RP) model. In order to induce a mild chronic systemic inflammation, we administered low doses of lipopolysaccharide (LPS) from age P20 to P60 to dystrophic P23H rats and healthy SD rats. Visual responsiveness was assessed by electroretinography (ERG). The morphological state of the retinas was analyzed by fluorescent immunohistochemistry (IHC), evaluating the number, morphology, and connectivity of different neuronal populations by means of cell type-specific markers. Microglia density, distribution, and degree of activation were evaluated by IHC and flow cytometry. The expression levels of inflammation- and apoptosis-related genes were analyzed by qRT-PCR arrays. Low-dose LPS administration did not induce significant functional or morphological changes in the retina of SD rats, although at the molecular level, we detected expression changes in genes related to apoptosis. Otherwise, systemic injection of LPS into P23H rats induced a further deterioration in the ERG response, with greater loss of photoreceptors and worsening of synaptic connectivity, accompanied by increasing numbers of microglial cells, which also showed a more intense activation state. Several inflammation- and apoptosis-related genes were upregulated. Our results indicate that chronic exacerbation of the inflammatory response in response to LPS accelerates neurodegeneration in dystrophic P23H rats, suggesting that in patients with ocular neurodegenerative diseases, peripheral damage, as a systemic infection or chronic inflammatory process, could accelerate disease progression, and should be taken into account in order to select an appropriate therapy to revert, block or slow-down the degenerative process.
Retinitis pigmentosa includes a group of progressive retinal degenerative diseases that affect the structure and function of photoreceptors. Secondarily to the loss of photoreceptors, there is a ...reduction in retinal vascularization, which seems to influence the cellular degenerative process. Retinal macroglial cells, astrocytes, and Müller cells provide support for retinal neurons and are fundamental for maintaining normal retinal function. The aim of this study was to investigate the evolution of macroglial changes during retinal degeneration in P23H rats.
Homozygous P23H line-3 rats aged from P18 to 18 months were used to study the evolution of the disease, and SD rats were used as controls. Immunolabeling with antibodies against GFAP, vimentin, and transducin were used to visualize macroglial cells and cone photoreceptors.
In P23H rats, increased GFAP labeling in Müller cells was observed as an early indicator of retinal gliosis. At 4 and 12 months of age, the apical processes of Müller cells in P23H rats clustered in firework-like structures, which were associated with ring-like shaped areas of cone degeneration in the outer nuclear layer. These structures were not observed at 16 months of age. The number of astrocytes was higher in P23H rats than in the SD matched controls at 4 and 12 months of age, supporting the idea of astrocyte proliferation. As the disease progressed, astrocytes exhibited a deteriorated morphology and marked hypertrophy. The increase in the complexity of the astrocytic processes correlated with greater connexin 43 expression and higher density of connexin 43 immunoreactive puncta within the ganglion cell layer (GCL) of P23H vs. SD rat retinas.
In the P23H rat model of retinitis pigmentosa, the loss of photoreceptors triggers major changes in the number and morphology of glial cells affecting the inner retina.
Optical coherence tomography (OCT) and OCT angiography (OCTA) have been a technological breakthrough in the diagnosis, treatment, and follow-up of many retinal diseases, thanks to its resolution and ...its ability to inform of the retinal state in seconds, which gives relevant information about retinal degeneration.
In this review, we present an immunohistochemical description of the human and mice retina and we correlate it with the OCT bands in health and pathological conditions. Here, we propose an interpretation of the four outer hyperreflective OCT bands with a correspondence to retinal histology: the first and innermost band as the external limiting membrane (ELM), the second band as the cone ellipsoid zone (EZ), the third band as the outer segment tips phagocytosed by the pigment epithelium (PhaZ), and the fourth band as the mitochondria in the basal portion of the RPE (RPEmitZ). The integrity of these bands would reflect the health of photoreceptors and retinal pigment epithelium.
Moreover, we describe how the vascular plexuses vary in different regions of the healthy human and mice retina, using OCTA and immunohistochemistry. In humans, four, three, two or one plexuses can be observed depending on the distance from the fovea. Also, specific structures such as vascular loops in the intermediate capillary plexus, or spider-like structures of interconnected capillaries in the deep capillary plexus are found. In mice, three vascular plexuses occupy the whole retina, except in the most peripheral retina where only two plexuses are found. These morphological issues should be considered when assessing a pathology, as some retinal diseases are associated with structural changes in blood vessels.
Therefore, the analysis of OCT bands and OCTA vascular plexuses may be complementary for the diagnosis and prognosis of retinal degenerative processes, useful to assess therapeutic approaches, and it is usually correlated to visual acuity.
Retinitis pigmentosa is a heterogeneous group of inherited neurodegenerative retinal disorders characterized by a progressive peripheral vision loss and night vision difficulties, subsequently ...leading to central vision impairment. Chronic microglia activation is associated with various neurodegenerative diseases including retinitis pigmentosa. The objective of this study was to quantify microglia activation in the retina of P23H rats, an animal model of retinitis pigmentosa, and to evaluate the therapeutic effects of TUDCA (tauroursodeoxycholic acid), which has been described as a neuroprotective compound.
For this study, homozygous P23H line 3 and Sprague-Dawley (SD) rats were injected weekly with TUDCA (500 mg/kg, ip) or vehicle (saline) from 20 days to 4 months old. Vertical retinal sections and whole-mount retinas were immunostained for specific markers of microglial cells (anti-CD11b, anti-Iba1 and anti-MHC-II). Microglial cell morphology was analyzed and the number of retinal microglial was quantified.
Microglial cells in the SD rat retinas were arranged in regular mosaics homogenously distributed within the plexiform and ganglion cell layers. In the P23H rat retina, microglial cells increased in number in all layers compared with control SD rat retinas, preserving the regular mosaic distribution. In addition, a large number of amoeboid CD11b-positive cells were observed in the P23H rat retina, even in the subretinal space. Retinas of TUDCA-treated P23H animals exhibited lower microglial cell number in all layers and absence of microglial cells in the subretinal space.
These results report novel TUDCA anti-inflammatory actions, with potential therapeutic implications for neurodegenerative diseases, including retinitis pigmentosa.
Melanopsin-containing retinal ganglion cells (mRGCs) represent a third class of retinal photoreceptors involved in regulating the pupillary light reflex and circadian photoentrainment, among other ...things. The functional integrity of the circadian system and melanopsin cells is an essential component of well-being and health, being both impaired in aging and disease. Here we review evidence of melanopsin-expressing cell alterations in aging and neurodegenerative diseases and their correlation with the development of circadian rhythm disorders. In healthy humans, the average density of melanopsin-positive cells falls after age 70, accompanied by age-dependent atrophy of dendritic arborization. In addition to aging, inner and outer retinal diseases also involve progressive deterioration and loss of mRGCs that positively correlates with progressive alterations in circadian rhythms. Among others, mRGC number and plexus complexity are impaired in Parkinson's disease patients; changes that may explain sleep and circadian rhythm disorders in this pathology. The key role of mRGCs in circadian photoentrainment and their loss in age and disease endorse the importance of eye care, even if vision is lost, to preserve melanopsin ganglion cells and their essential functions in the maintenance of an adequate quality of life.
Retinitis pigmentosa (RP) is a blinding neurodegenerative disease of the retina that can be affected by many factors. The present study aimed to analyze the effect of different environmental light ...intensities in rd10 mice retina.
C57BL/6J and rd10 mice were bred and housed under three different environmental light intensities: scotopic (5 lux), mesopic (50 lux), and photopic (300 lux). Visual function was studied using electroretinography and optomotor testing. The structural and morphological integrity of the retinas was evaluated by optical coherence tomography imaging and immunohistochemistry. Additionally, inflammatory processes and oxidative stress markers were analyzed by flow cytometry and western blotting.
When the environmental light intensity was higher, retinal function decreased in rd10 mice and was accompanied by light-dependent photoreceptor loss, followed by morphological alterations, and synaptic connectivity loss. Moreover, light-dependent retinal degeneration was accompanied by an increased number of inflammatory cells, which became more activated and phagocytic, and by an exacerbated reactive gliosis. Furthermore, light-dependent increment in oxidative stress markers in rd10 mice retina pointed to a possible mechanism for light-induced photoreceptor degeneration.
An increase in rd10 mice housing light intensity accelerates retinal degeneration, activating cell death, oxidative stress pathways, and inflammatory cells. Lighting intensity is a key factor in the progression of retinal degeneration, and standardized lighting conditions are advisable for proper analysis and interpretation of experimental results from RP animal models, and specifically from rd10 mice. Also, it can be hypothesized that light protection could be an option to slow down retinal degeneration in some cases of RP.
Microglia act as the resident immune cells of the central nervous system, including the retina. In response to damaging stimuli microglia adopt an activated state, which can progress into a ...phagocytic phenotype and play a potentially harmful role by eliciting the expression and release of pro-inflammatory cytokines. The aim of the present study was to assess longitudinal changes in microglia during retinal degeneration in the homozygous P23H rat, a model of dominant retinitis pigmentosa. Microglial phenotypes, morphology and density were analyzed by immunohistochemistry, flow cytometry, and cytokine antibody array. In addition, we performed electroretinograms to evaluate the retinal response. In the P23H retina, sclera, choroid and ciliary body, inflammatory cells increased in number compared with the control at all ages analyzed. As the rats became older, a higher number of amoeboid MHC-II(+) cells were observed in the P23H retina, which correlated with an increase in the expression of pro-inflammatory cytokines. These findings suggest that, in the P23H model, retinal neuroinflammation persists throughout the rat's life span even after photoreceptor depletion. Therefore, the inclusion of anti-inflammatory drugs at advanced stages of the neurodegenerative process may provide better retinal fitness so the remaining cells could still be used as targets of cellular or gene therapies.
This review focuses on retina degeneration occurring during glaucoma, age-related macular degeneration (AMD), diabetic retinopathy (DR), and retinitis pigmentosa (RP), and on the potential ...therapeutic use of triads of repositioned medicines, addressed to distinct but complementary targets, to prevent, delay or stop retina cell death. Although myriad pathogenic mechanisms have been implicated in these disorders, common signaling pathways leading to apoptotic cell death to all of them, and to all neurodegenerative diseases are (i) calcium dyshomeostasis/excitotoxicity; (ii) oxidative stress/mitochondrial dysfunction, and (iii) neuroinflammation/P2X7 receptor activation. From a therapeutic point of view, it is relevant to consider the multitarget approach based on the use of combined medicines acting on complementary pathogenic mechanisms that has been highly successful in the treatment of chronic diseases such as cancer, AIDS, pain, hypertension, Parkinson’s disease, cardiac failure, depression, or the epilepsies as the basic mechanisms of cell death do not differ between the different CNS degenerative diseases. We suggest the multi-target therapy approach could be more effective compared with single-drug treatments. Used at doses lower than standard, these triads may also be safer and more efficient. After the establishment of a proof-of-concept in animal models of retinal degeneration, potential successful preclinical trials of such combinations may eventually drive to test this concept in clinical trials in patients, first to evaluate the safety and efficacy of the drug combinations in humans and then their therapeutic advantages, if any, seeking the prevention and/or the delay of retina degeneration and blindness.
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•Eye diseases leading to retinal degeneration have pathogenic pathways common with neurodegenerative diseases.•Common pathogenic pathways in neurodegeneration are excitotoxicity, oxidative stress and inflammation.•Combined drug triads acting on different targets may afford more effective neuroprotection.
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