The role of glial cells and the complement system in retinal diseases and Alzheimer’s disease: common neural degeneration mechanisms

Glaucoma: from pathogenic mechanisms to retinal glial cell response to damage

Glaucoma is a neurodegenerative disease of the retina characterized by the irreversible loss of retinal ganglion cells (RGCs) leading to visual loss. Degeneration of RGCs and loss of their axons, as well as damage and remodeling of the lamina cribrosa are the main events in the pathogenesis of glaucoma. Different molecular pathways are involved in RGC death, which are triggered and exacerbated as a consequence of a number of risk factors such as elevated intraocular pressure (IOP), age, ocular biomechanics, or low ocular perfusion pressure. Increased IOP is one of the most important risk factors associated with this pathology and the only one for which treatment is currently available, nevertheless, on many cases the progression of the disease continues, despite IOP control. Thus, the IOP elevation is not the only trigger of glaucomatous damage, showing the evidence that other factors can induce RGCs death in this pathology, would be involved in the advance of glaucomatous neurodegeneration. The underlying mechanisms driving the neurodegenerative process in glaucoma include ischemia/hypoxia, mitochondrial dysfunction, oxidative stress and neuroinflammation. In glaucoma, like as other neurodegenerative disorders, the immune system is involved and immunoregulation is conducted mainly by glial cells, microglia, astrocytes, and Müller cells. The increase in IOP produces the activation of glial cells in the retinal tissue. Chronic activation of glial cells in glaucoma may provoke a proinflammatory state at the retinal level inducing blood retinal barrier disruption and RGCs death. The modulation of the immune response in glaucoma as well as the activation of glial cells constitute an interesting new approach in the treatment of glaucoma.

Complement Factor H Loss in RPE Cells Causes Retinal Degeneration in a Human RPE-Porcine Retinal Explant Co-Culture Model

Age-related Macular degeneration (AMD) is a degenerative disease of the macula affecting the elderly population. Treatment options are limited, partly due to the lack of understanding of AMD pathology and the lack of suitable research models that replicate the complexity of the human macula and the intricate interplay of the genetic, aging and lifestyle risk factors contributing to AMD. One of the main genetic risks associated with AMD is located on the Complement Factor H (CFH) gene, leading to an amino acid substitution in the Factor H (FH) protein (Y402H). However, the mechanism of how this FH variant promotes the onset of AMD remains unclear. Previously, we have shown that FH deprivation in RPE cells, via CFH silencing, leads to increased inflammation, metabolic impairment and vulnerability toward oxidative stress. In this study, we established a novel co-culture model comprising CFH silenced RPE cells and porcine retinal explants derived from the visual streak of porcine eyes, which closely resemble the human macula. We show that retinae exposed to FH-deprived RPE cells show signs of retinal degeneration, with rod cells being the first cells to undergo degeneration. Moreover, via Raman analyses, we observed changes involving the mitochondria and lipid composition of the co-cultured retinae upon FH loss. Interestingly, the detrimental effects of FH loss in RPE cells on the neuroretina were independent of glial cell activation and external complement sources. Moreover, we show that the co-culture model is also suitable for human retinal explants, and we observed a similar trend when RPE cells deprived of FH were co-cultured with human retinal explants from a single donor eye. Our findings highlight the importance of RPE-derived FH for retinal homeostasis and provide a valuable model for AMD research.

Glaucoma and neuroinflammation: An overview

Glaucoma is an optic neuropathy characterized by well-defined optic disc morphological changes (i.e., cup enlargement, neuroretinal border thinning, and notching, papillary vessel modifications) consequent to retinal ganglion cell loss, axonal degeneration, and lamina cribrosa remodeling. These modifications tend to be progressive and are the main cause of functional damage in glaucoma. Despite the latest findings about the pathophysiology of the disease, the exact trigger mechanisms and the mechanism of degeneration of retinal ganglion cells and their axons have not been completely elucidated. Neuroinflammation may play a role in both the development and the progression of the disease as a result of its effects on retinal environment and retinal ganglion cells. We summarize the latest findings about neuroinflammation in glaucoma and examine the connection between risk factors, neuroinflammation, and retinal ganglion cell degeneration.

The value of nutritional supplements in treating Age-Related Macular Degeneration: a review of the literature

PURPOSE

To describe and evaluate the value of nutritional supplements in the management of age-related macular degeneration (AMD) through a review of the current literature.

METHODS

An extensive literature search was performed, and key research articles exploring AREDS and AREDS-2 formulations, genetics, omega fatty acids, calcium and folic acid in high-risk women were reviewed. PubMed and Web of Science databases were used for generating articles to review.

RESULTS

The AREDS and AREDS-2 trials, while difficult to validate, show support for antioxidant supplementation in reducing AMD progression in Caucasian populations. While genetic guided personalized medicine has been studied mainly with complement factor H and age-related maculopathy susceptibility 2 risk alleles, the data have not been reproducible. Women at a higher risk of cardiovascular disease may benefit from antioxidant therapies in preventing AMD. Omega 3 fatty acid supplementation has been widely supported through observational studies; however, randomized controlled trials have not shown benefit in disease progression. Calcium exposure has been linked to increased mechanisms in cell death and may be detrimental to older individuals with AMD.

CONCLUSION

The data regarding nutritional supplements in preventing AMD progression are inconclusive, and therefore recommendations should be based on risk factors and demographic data.

Retinal glial changes in Alzheimer's disease - A review

Alzheimer's disease (AD) is a neurodegenerative dementia characterized by the deposition of extracellular β-amyloid (Aβ) plaques and the presence of neurofibrillary tangles. Until now, the techniques used to analyze these deposits have been difficult to access, invasive, and expensive. This leads us to consider new access routes to the central nervous system (CNS), allowing us to diagnose the disease before the first symptoms appear. Recent studies have shown that microglial and macroglial cell activation could play a role in the development of this disease. Glial cells in the CNS can respond to various damages, such as neurodegenerative pathologies, with morphological and functional changes. These changes are a common feature in neurodegenerative diseases, including AD. The retina is considered an extension of the CNS and has a population of glial cells similar to that of the CNS. When glial cells are activated, various molecules are released and changes in glial cell expression occur, which can be indicators of neuronal damage. The objective of this review is to compile the most relevant findings in the last 10 years relating to alterations in the eye in AD, and the role that glial cells play in the degenerative process in the retina in the context of neurodegeneration.

Subretinal macrophages produce classical complement activator C1q leading to the progression of focal retinal degeneration

BACKGROUND

The role of the alternative complement pathway and its mediation by retinal microglia and macrophages, is well-established in the pathogenesis of Age-Related Macular Degeneration (AMD). However, the contribution of the classical complement pathway towards the progression of retinal degenerations is not fully understood, including the role of complement component 1q (C1q) as a critical activator molecule of the classical pathway. Here, we investigated the contribution of C1q to progressive photoreceptor loss and neuroinflammation in retinal degenerations.

METHODS

Wild-type (WT), C1qa knockout (C1qa^-/-) and mice treated with a C1q inhibitor (ANX-M1; Annexon Biosciences), were exposed to photo-oxidative damage (PD) and were observed for progressive lesion development. Retinal function was assessed by electroretinography, followed by histological analyses to assess photoreceptor degeneration. Retinal inflammation was investigated through complement activation, macrophage recruitment and inflammasome expression using western blotting, qPCR and immunofluorescence. C1q was localised in human AMD donor retinas using immunohistochemistry.

RESULTS

PD mice had increased levels of C1qa which correlated with increasing photoreceptor cell death and macrophage recruitment. C1qa^-/- mice did not show any differences in photoreceptor loss or inflammation at 7 days compared to WT, however at 14 days after the onset of damage, C1qa^-/- retinas displayed less photoreceptor cell death, reduced microglia/macrophage recruitment to the photoreceptor lesion, and higher visual function. C1qa^-/- mice displayed reduced inflammasome and IL-1β expression in microglia and macrophages in the degenerating retina. Retinal neutralisation of C1q, using an intravitreally-delivered anti-C1q antibody, reduced the progression of retinal degeneration following PD, while systemic delivery had no effect. Finally, retinal C1q was found to be expressed by subretinal microglia/macrophages located in the outer retina of early AMD donor eyes, and in mouse PD retinas.

CONCLUSIONS

Our data implicate subretinal macrophages, C1q and the classical pathway in progressive retinal degeneration. We demonstrate a role of local C1q produced by microglia/macrophages as an instigator of inflammasome activation and inflammation. Crucially, we have shown that retinal C1q neutralisation during disease progression may slow retinal atrophy, providing a novel strategy for the treatment of complement-mediated retinal degenerations including AMD.

Pleiotropic Effects of Risk Factors in Age-Related Macular Degeneration and Seemingly Unrelated Complex Diseases

Age-related macular degeneration (AMD) is a complex disease with both environmental and genetic factors influencing disease risk. Genome-wide case-control association studies, candidate gene analyses, and epidemiological studies reinforced the notion that AMD is predominantly a disease of an impaired complement system and an altered high-density lipoprotein (HDL) metabolism. Recent reports demonstrated the pleiotropic role of the complement system and HDL in complex diseases such as cardiovascular disease, autoimmune disorders, cancer, and Alzheimer's disease. In light of these findings, we explore current evidence for a shared genetic and environmental risk of AMD and unrelated complex diseases based on epidemiological studies. Shared risk factors may indicate common pathways in disease pathology and thus may have implications for novel treatment options of AMD pathology.

C5a Increases the Injury to Primary Neurons Elicited by Fibrillar Amyloid Beta

C5aR1, the proinflammatory receptor for C5a, is expressed in the central nervous system on microglia, endothelial cells, and neurons. Previous work demonstrated that the C5aR1 antagonist, PMX205, decreased amyloid pathology and suppressed cognitive deficits in two Alzheimer's Disease (AD) mouse models. However, the cellular mechanisms of this protection have not been definitively demonstrated. Here, primary cultured mouse neurons treated with exogenous C5a show reproducible loss of MAP-2 staining in a dose-dependent manner within 24 hr of treatment, indicative of injury to neurons. This injury is prevented by the C5aR1 antagonist PMX53, a close analog of PMX205. Furthermore, primary neurons derived from C5aR1 null mice exhibited no MAP-2 loss after exposure to the highest concentration of C5a tested. Primary mouse neurons treated with both 100 nM C5a and 5 µM fibrillar amyloid beta (fAβ), to model what occurs in the AD brain, showed increased MAP-2 loss relative to either C5a or fAβ alone. Blocking C5aR1 with PMX53 (100 nM) blocked the loss of MAP2 in these primary neurons to the level seen with fAβ alone. Similar experiments with primary neurons derived from C5aR1 null mice showed a loss of MAP-2 due to fAβ treatment. However, the addition of C5a to the cultures did not enhance the loss of MAP-2 and the addition of PMX53 to the cultures did not change the MAP-2 loss in response to fAβ. Thus, at least part of the beneficial effects of C5aR1 antagonist in AD mouse models may be due to protection of neurons from the toxic effects of C5a.

C1q propagates microglial activation and neurodegeneration in the visual axis following retinal ischemia/reperfusion injury

Background

C1q represents the initiating protein of the classical complement cascade, however recent findings indicate pathway independent roles such as developmental pruning of retinal ganglion cell (RGC) axons. Furthermore, chronic neuroinflammation, including increased expression of C1q and activation of microglia and astrocytes, appears to be a common finding among many neurodegenerative disease models. Here we compare the effects of a retinal ischemia/reperfusion (I/R) injury on glial activation and neurodegeneration in wild type (WT) and C1qa-deficient mice in the retina and superior colliculus (SC). Retinal I/R was induced in mice through elevation of intraocular pressure to 120 mmHg for 60 min followed by reperfusion. Glial cell activation and population changes were assessed using immunofluorescence. Neuroprotection was determined using histological measurements of retinal layer thickness, RGC counts, and visual function by flash electroretinography (ERG).

Results

Retinal I/R injury significantly upregulated C1q expression in the retina as early as 72 h and within 7 days in the superficial SC, and was sustained as long as 28 days. Accompanying increased C1q expression was activation of microglia and astrocytes as well as a significantly increased glial population density observed in the retina and SC. Microglial activation and changes in density were completely ablated in C1qa-deficient mice, interestingly however there was no effect on astrocytes. Furthermore, loss of C1qa significantly rescued I/R-induced loss of RGCs and protected against retinal layer thinning in comparison to WT mice. ERG assessment revealed early preservation of b-wave amplitude deficits from retinal I/R injury due to C1qa-deficiency that was lost by day 28.

Conclusions

Our results for the first time demonstrate the spatiotemporal changes in the neuroinflammatory response following retinal I/R injury at both local and distal sites of injury. In addition, we have shown a role for C1q as a primary mediator of microglial activation and pathological damage. This suggests developmental mechanisms of C1q may be re-engaged during injury response, modulation of which may be beneficial for neuroprotection.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0089-0) contains supplementary material, which is available to authorized users.

Low serum ficolin-3 levels are associated with severity and poor outcome in traumatic brain injury

Background

Ficolin-mediated activation of the lectin pathway of complement contributes to the complement-independent inflammatory processes of traumatic brain injury. Lower serum ficolin-3 levels have been demonstrated to be highly associated with unfavorable outcome after ischemic stroke. This prospective observatory study was designed to investigate the relationships between serum ficolin-3 levels and injury severity and clinical outcomes after severe traumatic brain injury.

Methods

Serum ficolin-3 levels of 128 patients and 128 healthy controls were measured by sandwich immunoassays. An unfavorable outcome was defined as Glasgow Outcome Scale score of 1–3. Study endpoints included mortality at 1 week and 6 months and unfavorable outcome at 6 months after head trauma. Injury severity was assessed by Glasgow Coma Scale score. Multivariate logistic models were structured to evaluate the relationships between serum ficolin-3 levels and study endpoints and injury severity.

Results

Compared with the healthy controls, serum ficolin-3 levels on admission were statistically decreased in patients with severe traumatic brain injury. Serum ficolin-3 levels were independently correlated with Glasgow Coma Scale scores. Ficolin-3 was also identified as an independent prognostic predictor for 1-week mortality, 6-month mortality, and 6-month unfavorable outcome. Under receiver operating characteristics curves, ficolin-3 has similar prognostic predictive values for all study endpoints compared with Glasgow Coma Scale scores.

Conclusions

It was proposed that lower serum ficolin-3 levels, correlated with injury severity, had the potential to be the useful, complementary tool to predict short- or long-term clinical outcomes after severe traumatic brain injury.

High levels of serum mannose-binding lectins are associated with the severity and clinical outcomes of severe traumatic brain injury

BACKGROUND

Mannose-binding lectin (MBL) is a key component of innate immunity. The expression of cortical MBL is up-regulated after clinical and experimental head trauma. This study aimed to assess the association of serum MBL levels with injury severity and long-term clinical outcomes after severe traumatic brain injury (STBI).

METHODS

Serum MBL levels were measured in 122 patients and 100 healthy controls. Multivariate analyses were used to analyze the relationship between serum MBL levels and trauma severity reflected by Glasgow Coma Scale scores as well as between serum MBL levels and 6-month mortality and unfavorable outcome (Glasgow Outcome Scale score: 1-3). A receiver operating characteristic (ROC) curve was structured to evaluate the prognostic predictive performance of serum MBL levels.

RESULTS

Compared with healthy controls, serum MBL levels of patients were markedly elevated. Using multivariate analyses, serum MBL levels were found to be associated closely with Glasgow Coma Scale (GCS) scores and MBL emerged as an independent predictor for 6-month mortality and unfavorable outcome. Under ROC curve, serum MBL levels and GCS scores possessed similar prognostic predictive values.

CONCLUSION

Increased serum level of MBL was independently associated with head trauma severity and long-term clinical outcomes of STBI.

Progressive inflammatory pathology in the retina of aluminum-fed 5xFAD transgenic mice

At least 57 murine transgenic models for Alzheimer's disease (Tg-AD) have been developed to overexpress the 42 amino acid amyloid-beta (Aβ42) peptide in the central nervous system (CNS). These 'humanized murine Tg-AD models' have greatly expanded our understanding of the contribution of Aβ42 peptide-mediated pro-inflammatory neuropathology to the AD process. A number of independent laboratories using different amyloid-overexpressing Tg-AD models have shown that supplementation of murine Tg-AD diets and/or drinking water with aluminum significantly enhances Aβ42 peptide-mediated inflammatory pathology and AD-type cognitive change compared to animals receiving control diets. In humans AD-type pathology appears to originate in the limbic system and progressively spreads into primary processing and sensory regions such as the retina. In these studies, for the first time, we assess the propagation of Aβ42 and inflammatory signals into the retina of 5xFAD Tg-AD amyloid-overexpressing mice whose diets were supplemented with aluminum. The two most interesting findings were (1) that similar to other Tg-AD models, there was a significantly accelerated development of Aβ42 and inflammatory pathology in 5xFAD Tg-AD mice fed aluminum; and (2) in aluminum-supplemented animals, markers for inflammatory pathology appeared in both the brain and the retina as evidenced by an evolving presence of Aβ42 peptides, and accompanied by inflammatory markers - cyclooxygenase-2 (COX-2) and C-reactive protein (CRP). The results indicate that in the 5xFAD Tg-AD model aluminum not only enhances an Aβ42-mediated inflammatory degeneration of the brain but also appears to induce AD-type pathology in an anatomically-linked primary sensory area that involves vision.

Using genetic mouse models to gain insight into glaucoma: Past results and future possibilities

While all forms of glaucoma are characterized by a specific pattern of retinal ganglion cell death, they are clinically divided into several distinct subclasses, including normal tension glaucoma, primary open angle glaucoma, congenital glaucoma, and secondary glaucoma. For each type of glaucoma there are likely numerous molecular pathways that control susceptibility to the disease. Given this complexity, a single animal model will never precisely model all aspects of all the different types of human glaucoma. Therefore, multiple animal models have been utilized to study glaucoma but more are needed. Because of the powerful genetic tools available to use in the laboratory mouse, it has proven to be a highly useful mammalian system for studying the pathophysiology of human disease. The similarity between human and mouse eyes coupled with the ability to use a combination of advanced cell biological and genetic tools in mice have led to a large increase in the number of studies using mice to model specific glaucoma phenotypes. Over the last decade, numerous new mouse models and genetic tools have emerged, providing important insight into the cell biology and genetics of glaucoma. In this review, we describe available mouse genetic models that can be used to study glaucoma-relevant disease/pathobiology. Furthermore, we discuss how these models have been used to gain insights into ocular hypertension (a major risk factor for glaucoma) and glaucomatous retinal ganglion cell death. Finally, the potential for developing new mouse models and using advanced genetic tools and resources for studying glaucoma are discussed.

Serum vitamin D status is associated with the presence but not the severity of primary open angle glaucoma

OBJECTIVES

Vitamin D is involved in visual health and function. Our objective was to determine whether age-related vitamin D insufficiency was associated with the presence and the severity of primary open angle glaucoma (POAG) in a case-control study of older adults.

STUDY DESIGN

Case-control study.

MAIN OUTCOME MEASURES

One hundred fifty cases diagnosed with moderate-to-severe POAG (mean, 75.1 ± 8.5 years; 42.0% female) and 164 healthy controls (mean, 73.0 ± 7.9 years; 59.8% female) were included. POAG diagnosis was based on classical diagnostic criteria of optic nerve cupping and/or RNFL thinning, measured with optical coherence tomography. Severe POAG was defined as Humphrey visual field mean deviation (MD) worse than -12 dB. Vitamin D insufficiency was defined as serum 25OHD ≤ 75 nmol/L. Age, gender, mean arterial pressure, vitamin D supplementation, visual acuity, and intraocular pressure were used as potential confounders.

RESULTS

POAG cases had lower mean serum 25OHD concentration than controls (42.9 ± 25.7 nmol/L versus 49.4 ± 29.5 nmol/L, P=0.039) and a greater prevalence of vitamin D insufficiency (90.7% versus 82.3%, P=0.032). Increased mean serum 25OHD concentrations were associated with lower POAG frequency, even after adjustment for potential confounders (OR=0.89 per 10 nmol/L of 25OHD, P=0.045). Similarly, vitamin D insufficiency was associated with POAG (OR=2.09, P=0.034). Among POAG cases, no 25OHD difference was observed between moderate and severe POAG cases (respectively, 39.2 ± 23.3 nmol/L versus 45.1 ± 26.7 nmol/L, P=0.188); and no between-group difference regarding the prevalence of vitamin D insufficiency (88.9% versus 94.0%, P=0.313).

CONCLUSIONS

Decreased serum 25OHD concentration was associated with POAG. There was no 25OHD difference between moderate and severe POAG.