Glaucoma and the brain: Trans-synaptic degeneration, structural change, and implications for neuroprotection

Altered coupling of cerebral blood flow and functional connectivity strength in visual and higher order cognitive cortices in primary open angle glaucoma

Primary open-angle glaucoma (POAG) has been suggested to be a neurodegenerative disease associated with altered cerebral vascular hemodynamics and widespread disruption of neuronal activity within the visual, working memory, attention and executive networks. We hypothesized that disturbed neurovascular coupling in visual and higher order cognitive cortices exists in POAG patients and correlates with glaucoma stage and visual field defects. Through multimodal magnetic resonance imaging, we evaluated the cerebral blood flow (CBF)-functional connectivity strength (FCS) correlations of the whole gray matter and CBF/FCS ratio per voxel for all subjects. Compared with normal controls, POAG patients showed reduced global CBF-FCS coupling and altered CBF/FCS ratios, predominantly in regions in the visual cortex, salience network, default mode network, and dorsal attentional network. The CBF/FCS ratio was negatively correlated with glaucoma stage, and positively correlated with visual field defects in the lingual gyrus in POAG patients. Moreover, early brain changes were detected in early POAG. These findings indicate neurovascular coupling dysfunction might exist in the visual and higher order cognitive cortices in POAG patients and its clinical relevance. The results may contribute to the monitoring of POAG progression and provide insight into the pathophysiology of the neurodegenerative process in POAG.

Citicoline Modulates Glaucomatous Neurodegeneration Through Intraocular Pressure-Independent Control

Glaucoma is a neurodegenerative disease that causes progressive, irreversible vision loss. Currently, intraocular pressure (IOP) is the only modifiable risk factor for glaucoma. However, glaucomatous degeneration may continue despite adequate IOP control. Therefore, there exists a need for treatment that protects the visual system, independent of IOP. This study sought, first, to longitudinally examine the neurobehavioral effects of different magnitudes and durations of IOP elevation using multi-parametric magnetic resonance imaging (MRI), optokinetics and histology; and, second, to evaluate the effects of oral citicoline treatment as a neurotherapeutic in experimental glaucoma. Eighty-two adult Long Evans rats were divided into six groups: acute (mild or severe) IOP elevation, chronic (citicoline-treated or untreated) IOP elevation, and sham (acute or chronic) controls. We found that increasing magnitudes and durations of IOP elevation differentially altered structural and functional brain connectivity and visuomotor behavior, as indicated by decreases in fractional anisotropy in diffusion tensor MRI, magnetization transfer ratios in magnetization transfer MRI, T1-weighted MRI enhancement of anterograde manganese transport, resting-state functional connectivity, visual acuity, and neurofilament and myelin staining along the visual pathway. Furthermore, 3 weeks of oral citicoline treatment in the setting of chronic IOP elevation significantly reduced visual brain integrity loss and visual acuity decline without altering IOP. Such effects sustained after treatment was discontinued for another 3 weeks. These results not only illuminate the close interplay between eye, brain, and behavior in glaucomatous neurodegeneration, but also support a role for citicoline in protecting neural tissues and visual function in glaucoma beyond IOP control.

Progression of Visual Pathway Degeneration in Primary Open-Angle Glaucoma: A Longitudinal Study

Background: Primary open-angle glaucoma (POAG) patients exhibit widespread white matter (WM) degeneration throughout their visual pathways. Whether this degeneration starts at the pre- or post-geniculate pathways remains unclear. In this longitudinal study, we assess the progression of WM degeneration exhibited by the pre-geniculate optic tracts (OTs) and the post-geniculate optic radiations (ORs) of POAG patients over time, aiming to determine the source and pattern of spread of this degeneration.

Methods: Diffusion-weighted MRI scans were acquired for 12 POAG patients and 14 controls at two time-points 5.4 ± 2.1 years apart. Fiber density (FD), an estimate of WM axonal density, was computed for the OTs and ORs of all participants in an unbiased longitudinal population template space. First, FD was compared between POAG patients and the controls at time-point 1 (TP1) and time-point 2 (TP2) independently. Secondly, repeated measures analysis was performed for FD change in POAG patients between the two time-points. Finally, we compared the rate of FD change over time between the two groups.

Results: Compared to the controls, POAG patients exhibited significantly lower FD in the left OT at TP1 and in both OTs and the left OR at TP2. POAG patients showed a significant loss of FD between the time-points in the right OT and both ORs, while the left OR showed a significantly higher rate of FD loss in POAG patients compared to the controls.

Conclusions: We find longitudinal progression of neurodegenerative WM changes in both the pre- and post-geniculate visual pathways of POAG patients. The pattern of changes suggests that glaucomatous WM degeneration starts at the pre-geniculate pathways and then spreads to the post-geniculate pathways. Furthermore, we find evidence that the trans-synaptic spread of glaucomatous degeneration to the post-geniculate pathways is a prolonged process which continues in the absence of detectable pre-geniculate degenerative progression. This suggests the presence of a time window for salvaging intact post-geniculate pathways, which could prove to be a viable therapeutic target in the future.

Tryptophan Pathway Abnormalities in a Murine Model of Hereditary Glaucoma

BACKGROUND

It has been shown that a possible pathogenetic mechanism of neurodegeneration in the mouse model of glaucoma (DBA/2J) may be an alteration of kynurenic acid (KYNA) in the retina. This study aimed to verify the hypothesis that alterations of tryptophan (TRP) metabolism in DBA/2J mice is not limited to the retina.

METHODS

Samples of the retinal tissue and serum were collected from DBA/2J mice (6 and 10 months old) and control C57Bl/6 mice of the same age. The concentration of TRP, KYNA, kynurenine (KYN), and 3-hydroxykynurenine (3OH-K) was measured by HPLC. The activity of indoleamine 2,3-dioxygenase (IDO) was also determined as a KYN/TRP ratio.

RESULTS

TRP, KYNA, L-KYN, and 3OH-K concentration were significantly lower in the retinas of DBA/2J mice than in C57Bl/6 mice. 3OH-K concentration was higher in older mice in both strains. Serum TRP, L-KYN, and KYNA concentrations were lower in DBA/2J than in age-matched controls. However, serum IDO activity did not differ significantly between compared groups and strains.

CONCLUSIONS

Alterations of the TRP pathway seem not to be limited to the retina in the murine model of hereditary glaucoma.

Vision, attention, and driving

Safe driving demands the coordination of multiple sensory and cognitive functions, such as vision and attention. Patients with neurologic or ophthalmic disease are exposed to selective pathophysiologic insults to driving-critical systems, placing them at a higher risk for unsafe driving and restricted driving privileges. Here, we evaluate how vision and attention contribute to unsafe driving across different patient populations. In ophthalmic disease, we focus on macular degeneration, glaucoma, diabetic retinopathy, and cataract; in neurologic disease, we focus on Alzheimer's disease, Parkinson's disease, and multiple sclerosis. Unsafe driving is generally associated with impaired vision and attention in ophthalmic and neurologic patients, respectively. Furthermore, patients with ophthalmic disease experience some degree of impairment in attention. Similarly, patients with neurologic disease experience some degree of impairment in vision. While numerous studies have demonstrated a relationship between impaired vision and unsafe driving in neurologic disease, there remains a dearth of knowledge regarding the relationship between impaired attention and unsafe driving in ophthalmic disease. In summary, this chapter confirms-and offers opportunities for future research into-the contribution of vision and attention to safe driving.

"Association of APOE gene polymorphisms with primary open angle glaucoma in Brazilian patients"

Background: Primary open-angle glaucoma (POAG) is a multifactorial disease that affects 65.5 million people worldwide. In addition to the genetic variants already established as indicators of greater risk for POAG, the apolipoprotein (APOE) gene has been studied in some populations, with controversial results. The aim of this study is to investigate the frequency of the genetic variants of APOE in the Brazilian population, and to evaluate the association between these polymorphisms and the risk of POAG. Methods: APOE variants (rs429358; rs7412) were genotyped in 402 POAG patients and 401 controls. We evaluated the association between APOE genetic variants and the risk for POAG, as well as the correlation between the requirement of glaucoma surgery and the APOE polymorphisms. Results: Among the three APOE gene isoforms, we found a low frequency of APOE alleles ε2 (7.34%) and ε4 (11.76%), but a high frequency of ε3 (80.88%) in our population. When compared to ε3ε3 reference genotype, ε2 allele-carriers (OR = 1.516; p-value = 0.04) and ε2ε3 genotype (OR = 1.655; p-value = 0.02) were associated with a greater risk for POAG. An additive genetic model confirmed the influence of the ε2 allele in the risk of POAG in this sample of the Brazilian population (OR = 1.502; p-value = 0.04). There was no significant association between the analyzed genotypes and the requirement or number of glaucoma surgeries (p > .05). Conclusion: Brazilian individuals carrying the APOEε2 allele may be at an increased risk for the development of POAG.

A 3D Model of Human Trabecular Meshwork for the Research Study of Glaucoma

Glaucoma is a multifactorial syndrome in which the development of pro-apoptotic signals are the causes for retinal ganglion cell (RGC) loss. Most of the research progress in the glaucoma field have been based on experimentally inducible glaucoma animal models, which provided results about RGC loss after either the crash of the optic nerve or IOP elevation. In addition, there are genetically modified mouse models (DBA/2J), which make the study of hereditary forms of glaucoma possible. However, these approaches have not been able to identify all the molecular mechanisms characterizing glaucoma, possibly due to the disadvantages and limits related to the use of animals. In fact, the results obtained with small animals (i.e., rodents), which are the most commonly used, are often not aligned with human conditions due to their low degree of similarity with the human eye anatomy. Although the results obtained from non-human primates are in line with human conditions, they are little used for the study of glaucoma and its outcomes at cellular level due to their costs and their poor ease of handling. In this regard, according to at least two of the 3Rs principles, there is a need for reliable human-based in vitro models to better clarify the mechanisms involved in disease progression, and possibly to broaden the scope of the results so far obtained with animal models. The proper selection of an in vitro model with a "closer to in vivo" microenvironment and structure, for instance, allows for the identification of the biomarkers involved in the early stages of glaucoma and contributes to the development of new therapeutic approaches. This review summarizes the most recent findings in the glaucoma field through the use of human two- and three-dimensional cultures. In particular, it focuses on the role of the scaffold and the use of bioreactors in preserving the physiological relevance of in vivo conditions of the human trabecular meshwork cells in three-dimensional cultures. Moreover, data from these studies also highlight the pivotal role of oxidative stress in promoting the production of trabecular meshwork-derived pro-apoptotic signals, which are one of the first marks of trabecular meshwork damage. The resulting loss of barrier function, increase of intraocular pressure, as well the promotion of neuroinflammation and neurodegeneration are listed as the main features of glaucoma. Therefore, a better understanding of the first molecular events, which trigger the glaucoma cascade, allows the identification of new targets for an early neuroprotective therapeutic approach.

Brimonidine-LAPONITE® intravitreal formulation has an ocular hypotensive and neuroprotective effect throughout 6 months of follow-up in a glaucoma animal model

Intravitreal administration is widely used in ophthalmological practice to maintain therapeutic drug levels near the neuroretina and because drug delivery systems are necessary to avoid reinjections and sight-threatening side effects. However, currently there is no intravitreal treatment for glaucoma. The brimonidine-LAPONITE® formulation was created with the aim of treating glaucoma for extended periods with a single intravitreal injection. Glaucoma was induced by producing ocular hypertension in two rat cohorts: [BRI-LAP] and [non-bri], with and without treatment, respectively. Eyes treated with brimonidine-LAPONITE® showed lower ocular pressure levels up to week 8 (p < 0.001), functional neuroprotection explored by scotopic and photopic negative response electroretinography (p = 0.042), and structural protection of the retina, retinal nerve fibre layer and ganglion cell layer (p = 0.038), especially on the superior-inferior axis explored by optical coherence tomography, which was corroborated by a higher retinal ganglion cell count (p = 0.040) using immunohistochemistry (Brn3a antibody) up to the end of the study (week 24). Furthermore, delayed neuroprotection was detected in the contralateral eye. Brimonidine was detected in treated rat eyes for up to 6 months. Brimonidine-LAPONITE® seems to be a potential sustained-delivery intravitreal drug for glaucoma treatment.

Association of ocular diseases with schizophrenia, bipolar disorder, and major depressive disorder: a retrospective case-control, population-based study

BACKGROUND

Psychiatric disorders and ocular neurovascular diseases may share a similar pathophysiological route of vascular structures or neurological changes. The aim of this study is to investigate the association between ocular neurovascular diseases and the risk of major psychiatric disorders.

METHODS

This was a retrospective case-control, population-based study including patients aged ≥20 and were diagnosed between 1997 and 2013. Ocular neurovascular diseases diagnosed between 1997 and 2006 and newly diagnosed psychiatric disorders including bipolar disorder (BD), major depressive disorder (MDD), and schizophrenia between 2007 and 2013 were registered. Patients were propensity-score matched with control groups without psychiatric disorders in each cohort based on selected covariates.

RESULTS

A total of one million sampled patients in the database were categorized based on their diagnoses; 2243 (37.4% men) were categorized into the BD group, 10,110 (35.2% men) into the MDD group, and 1623 (43.1% men) into the schizophrenia group. In the BD group, all glaucoma (OR 1.49, [1.18-1.89]), open-angle glaucoma (OR 2.08, [1.34-3.24]), and closed-angle glaucoma (OR 2.12, [1.36-3.33]) showed statistical significance of risk. In the MDD group, age-related macular degeneration (OR 1.33, [1.13-1.57]), all glaucoma (OR 1.24, [1.11-1.37]), open-angle glaucoma (OR 1.47, [1.21-1.80]), and dry eye syndrome (OR 1.22, [1.13-1.31]) were associated with a significantly higher risk. In the schizophrenia group, only all glaucoma (OR 1.47, [1.02-2.11]), glaucoma suspect (OR 1.88, [1.01-3.49]), and open-angle glaucoma (OR 2.19, [1.13-4.26]) showed statistical significance.

CONCLUSIONS

In this population-based study, ocular neurovascular diseases, especially glaucoma, were associated with increased risks of BD, MDD, and schizophrenia.

Common aspects between glaucoma and brain neurodegeneration

Neurodegeneration can be defined as progressive cell damage to nervous system cells, and more specifically to neurons, which involves morphologic alterations and progressive loss of function until cell death. Glaucoma exhibits many aspects of neurodegenerative disease. This review examines the pathogenesis of glaucoma, comparing it with that of Alzheimer's disease (AD) and Parkinson's disease (PD), highlighting their common features. Indeed, in all three diseases there are not only the same types of pathogenic events, but also similarities of temporal cadences that determine neuronal damage. All three age-related illnesses have oxidative damage and mitochondrial dysfunction as the first pathogenic steps. The consequence of these alterations is the death of visual neurons in glaucoma, cognitive neurons in AD and regulatory motor neurons (substantia nigra) in PD. The study of these common pathogenic events (oxidative stress, mitochondrial dysfunction, protein degradation, apoptosis and autophagy) leads us to consider common therapeutic strategies for the treatment and prevention of these diseases. Also, examination of the genetic aspects of the pathways involved in neurodegenerative processes plays a key role in shedding light on the details of pathogenesis and can suggest new treatments. This review discusses the common molecular aspects involved in these three oxidative-stress and age-related diseases.

Brain networks reorganization and functional disability in glaucoma

Glaucoma is an optic neuropathy characterized by progressive loss of retinal ganglion cells with associated structural and functional changes of the optic nerve head and retinal nerve fiber layer. However, recent studies employing advanced neuroimaging techniques confirmed that glaucomatous damage is not limited to the eye but extends to the brain, affecting it also beyond the central visual pathways and disrupting brain network organization. We therefore posit that, while visual field changes play an important role in glaucoma-induced disability, central nervous pathways and mechanisms may play an important role in sustaining functional and daily living disability caused by the disease. Here we to summarize the current state of the art on the involvement of central brain circuits and possibly related disabilities in patients with glaucoma.

Increased Equivalent Input Noise in Glaucomatous Central Vision: Is it Due to Undersampling of Retinal Ganglion Cells?

Purpose

Recent evidence shows that macular damage is common even in early stages of glaucoma. Here we investigated whether contrast sensitivity loss in the central vision of glaucoma patients is due to an increase in equivalent input noise (Neq), a decrease in calculation efficiency, or both. We also examined how retinal undersampling resulting from loss of retinal ganglion cells (RGCs) may affect Neq and calculation efficiency.

Methods

This study included 21 glaucoma patients and 23 age-matched normally sighted individuals. Threshold contrast for orientation discrimination was measured with a sinewave grating embedded in varying levels of external noise. Data were fitted to the linear amplifier model (LAM) to factor contrast sensitivity into Neq and calculation efficiency. We also correlated macular RGC counts estimated from structural (spectral-domain optical coherence tomography) and functional (standard automated perimetry Swedish interactive thresholding algorithm 10-2) data with either Neq or efficiency. Furthermore, using analytical and computer simulation approach, the relative effect of retinal undersampling on Neq and efficiency was evaluated by adding the RGC sampling module into the LAM.

Results

Compared with normal controls, glaucoma patients exhibited a significantly larger Neq without significant difference in efficiency. Neq was significantly correlated with Pelli-Robson contrast sensitivity and macular RGC counts. The results from analytical derivation and model simulation further demonstrated that Neq can be expressed as a function of internal noise and retinal sampling.

Conclusions

Our results showed that equivalent input noise is significantly elevated in glaucomatous vision, thereby impairing foveal contrast sensitivity. Our findings further elucidated how undersampling at the retinal level may increase equivalent input noise.

The Neurovascular Unit in Glaucomatous Neurodegeneration

Glaucoma is a neurodegenerative disease of the visual system and leading cause of blindness worldwide. The disease is associated with sensitivity to intraocular pressure (IOP), which over a large range of magnitudes stresses retinal ganglion cell (RGC) axons as they pass through the optic nerve head in forming the optic projection to the brain. Despite clinical efforts to lower IOP, which is the only modifiable risk factor for glaucoma, RGC degeneration and ensuing loss of vision often persist. A major contributor to failure of hypotensive regimens is the multifactorial nature of how IOP-dependent stress influences RGC physiology and structure. This stress is conveyed to the RGC axon through interactions with structural, glial, and vascular components in the nerve head and retina. These interactions promote pro-degenerative pathways involving biomechanical, metabolic, oxidative, inflammatory, immunological and vascular challenges to the microenvironment of the ganglion cell and its axon. Here, we focus on the contribution of vascular dysfunction and breakdown of neurovascular coupling in glaucoma. The vascular networks of the retina and optic nerve head have evolved complex mechanisms that help to maintain a continuous blood flow and supply of metabolites despite fluctuations in ocular perfusion pressure. In healthy tissue, autoregulation and neurovascular coupling enable blood flow to stay tightly controlled. In glaucoma patients evidence suggests these pathways are dysfunctional, thus highlighting a potential role for pathways involved in vascular dysfunction in progression and as targets for novel therapeutic intervention.

Magnetic Resonance Imaging for Glaucoma Evaluation

The damage caused by glaucoma has been extensively evaluated at the level of the retina and optic nerve head. Many advances have been shown in this field in the last decades. Recent studies have also proved degenerative changes in the brain involving the intracranial optic nerve, lateral geniculate nucleus, and visual cortex. Moreover, these brain abnormalities are also correlated with clinical, optic nerve head, and visual field findings. In this review, we critically evaluate the existing literature studying the use of magnetic resonance imaging in glaucoma, and we discuss issues related to how magnetic resonance imaging results should be incorporated into our clinical practice.

Retinal neurodegeneration in patients with end-stage renal disease assessed by spectral-domain optical coherence tomography

Spectral-domain optical coherence tomography (SD-OCT) represents a reliable tool for retinal layer volume and thickness measurement. The aim of this study was to evaluate retinal changes indicating neurodegenerative processes in patients with end-stage renal disease (ESRD) compared to healthy controls. This was a cross-sectional, single-center study comprising 32 ESRD patients and 38 controls. Sectoral retinal nerve fiber layer (RNFL) thickness and retinal layer volumes were obtained by SD-OCT. Age- and gender-adjusted retinal layer volumes such as total retinal volume (p = 0.037), ganglion cell layer volume (GCL, p = 0.003), ganglion cell layer - inner plexiform layer volume (GCL-IPL, p = 0.005) and inner retinal layer volume (IRL, p = 0.042) of the right eye were lower in ESRD patients. Inner plexiform layer volume of both eyes (IPL, right eye: p = 0.017; left eye: 0.044) was reduced, as was RNFL thickness in the temporal superior sector (right eye: p = 0.016). A subgroup analysis excluding patients with diabetes revealed that GCL (p = 0.014) and GCL-IPL volume of the right eye (p = 0.024) and temporal superior sector of the RNFL scan (p = 0.021) in ESRD patients were still significantly thinner. We observed a decrease in several retinal layer volumes and temporal RNFL thickness indicative of retinal neurodegenerative processes in patients with ESRD.

Heavy metal toxicity and the aetiology of glaucoma

Despite recent advances, our understanding of the aetiological mechanisms underlying glaucoma remains incomplete. Heavy metals toxicity has been linked to the development of neurodegenerative diseases and various ocular pathologies. Given the similarities in pathophysiology between glaucoma and some neurodegenerative disorders, it is plausible that heavy metal toxicity may play a role in the development of glaucoma. Heavy metal exposure may be occupational, or through water or dietary contamination. In this report, we review mechanisms for systemic and neurotoxicity for arsenic, cadmium, chromium, cobalt, lead, mercury, and manganese, and weigh the evidence for an association between glaucoma and the accumulation of heavy metals either in ocular tissues or in the central nervous system.

NLRP3 Deficiency Attenuates Secondary Degeneration of Visual Cortical Neurons Following Optic Nerve Injury

In the visual pathway, optic nerve (ON) injury may cause secondary degeneration of neurons in distal regions, such as the visual cortex. However, the role of the neuroinflammatory response in regulating secondary impairment in the visual cortex after ON injury remains unclear. The NOD-like receptor family pyrin domain containing 3 (NLRP3) is an important regulator of neuroinflammation. In this study, we established a mouse model of unilateral ON crush (ONC) and showed that the expression of NLRP3 was significantly increased in the primary visual cortex (V1) as a response to ONC and that the NLRP3 inflammasome was activated in the contralateral V1 1 days-14 days after ONC. Ablation of the NLRP3 gene significantly decreased the trans-neuronal degeneration within 14 days. Visual electrophysiological function was improved in NLRP3^-/- mice. Taken together, these findings suggest that NLRP3 is a potential therapeutic target for protecting visual cortical neurons against degeneration after ON injury.

Reduced Cerebral Blood Flow in the Visual Cortex and Its Correlation With Glaucomatous Structural Damage to the Retina in Patients With Mild to Moderate Primary Open-angle Glaucoma

PURPOSE

Altered ocular and cerebral vascular autoregulation and vasoreactivity have been demonstrated in patients with primary open-angle glaucoma (POAG). In the present study, we investigated the correlations between reduced cerebral blood flow (CBF) in early and higher-tier visual cortical areas and glaucomatous changes in the retinas of patients with mild to moderate POAG.

PATIENTS AND METHODS

3-dimensional pseudocontinuous arterial spin labelling magnetic resonance imaging at 3 T was performed in 20 normal controls and 15 mild to moderate POAG patients. Regions of interest were selected based on the Population-Average, Landmark- and Surface-based (PALS) atlas of the human cerebral cortex. Arterial spin labelling-measured CBF values were extracted in the early and higher-tier visual cortical areas and were compared between patients and controls using a 2-sample t test. Pearson correlation analyses were used to assess the correlations between reduced CBF and cup-to-disc ratio, retinal nerve fiber layer thickness, and ganglion cell complex thickness.

RESULTS

Reduced CBF in early visual cortical areas (V1, V2, and ventral posterior area) and in the higher-tier visual left lateral occipital cortex was presented in mild to moderate POAG patients compared with controls. Furthermore, reduced CBF of the right areas V2 and ventral posterior area was correlated with cup-to-disc ratio, total ganglion cell complex thickness, and average retinal nerve fiber layer thickness.

CONCLUSIONS

In conclusion, the complex pathologic progress of POAG includes abnormal cerebral perfusion within the visual cortex since the mild to moderate disease stages. The association of cerebral perfusion changes with alterations of the optic disc and the retina may contribute to the early diagnosis of POAG.

Widespread brain reorganization perturbs visuomotor coordination in early glaucoma

Glaucoma is the world's leading cause of irreversible blindness, and falls are a major public health concern in glaucoma patients. Although recent evidence suggests the involvements of the brain toward advanced glaucoma stages, the early brain changes and their clinical and behavioral consequences remain poorly described. This study aims to determine how glaucoma may impair the brain structurally and functionally within and beyond the visual pathway in the early stages, and whether these changes can explain visuomotor impairments in glaucoma. Using multi-parametric magnetic resonance imaging, glaucoma patients presented compromised white matter integrity along the central visual pathway and around the supramarginal gyrus, as well as reduced functional connectivity between the supramarginal gyrus and the visual occipital and superior sensorimotor areas when compared to healthy controls. Furthermore, decreased functional connectivity between the supramarginal gyrus and the visual brain network may negatively impact postural control measured with dynamic posturography in glaucoma patients. Taken together, this study demonstrates that widespread structural and functional brain reorganization is taking place in areas associated with visuomotor coordination in early glaucoma. These results implicate an important central mechanism by which glaucoma patients may be susceptible to visual impairments and increased risk of falls.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

Axon injury signaling and compartmentalized injury response in glaucoma

Axonal degeneration is an active, highly controlled process that contributes to beneficial processes, such as developmental pruning, but also to neurodegeneration. In glaucoma, ocular hypertension leads to vision loss by killing the output neurons of the retina, the retinal ganglion cells (RGCs). Multiple processes have been proposed to contribute to and/or mediate axonal injury in glaucoma, including: neuroinflammation, loss of neurotrophic factors, dysregulation of the neurovascular unit, and disruption of the axonal cytoskeleton. While the inciting injury to RGCs in glaucoma is complex and potentially heterogeneous, axonal injury is ultimately thought to be the key insult that drives glaucomatous neurodegeneration. Glaucomatous neurodegeneration is a complex process, with multiple molecular signals contributing to RGC somal loss and axonal degeneration. Furthermore, the propagation of the axonal injury signal is complex, with injury triggering programs of degeneration in both the somal and axonal compartment. Further complicating this process is the involvement of multiple cell types that are known to participate in the process of axonal and neuronal degeneration after glaucomatous injury. Here, we review the axonal signaling that occurs after injury and the molecular signaling programs currently known to be important for somal and axonal degeneration after glaucoma-relevant axonal injuries.

In vivo imaging of structural, metabolic and functional brain changes in glaucoma

Glaucoma, the world’s leading cause of irreversible blindness, is a condition for which elevated intraocular pressure is currently the only modifiable risk factor. However, the disorder can continue to progress even at reduced intraocular pressure. This indicates additional key factors that contribute to the etiopathogenesis. There has been a growing amount of literature suggesting glaucoma as a neurodegenerative disease of the visual system. However, it remains debatable whether the observed pathophysiological conditions are causes or consequences. This review summarizes recent in vivo imaging studies that helped advance the understanding of early glaucoma involvements and disease progression in the brains of humans and experimental animal models. In particular, we focused on the non-invasive detection of early structural and functional brain changes before substantial clinical visual field loss in glaucoma patients; the eye-brain interactions across disease severity; the metabolic changes occurring in the brain’s visual system in glaucoma; and, the widespread brain involvements beyond the visual pathway as well as the potential behavioral relevance. If the mechanisms of glaucomatous brain changes are reliably identified, novel neurotherapeutics that target parameters beyond intraocular pressure lowering can be the promise of the near future, which would lead to reduced prevalence of this irreversible but preventable disease.

Subcortical visual pathway may be a new way for early diagnosis of glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide, which is now viewed as a disease of brain with pathogenesis not fully understood. The main diagnostic methods are examining the retinal nerve fiber layer through optical coherence tomography and investigating visual field defect, but these methods present disadvantages in clinical practice. Studies have shown that patients with glaucoma often suffer negative emotion like anxiety and depression which is related to abnormal or reduced amygdala. Moreover, selective reduction of fMRI responses to transient achromatic stimuli in the superficial layer of the superior colliculus was found in the early glaucoma patients. By summarizing previous studies, we developed a hypothesis: superior colliculus-pulvinar-amygdala subcortical visual pathway may be involved in the incidence or progression of glaucoma. Validating this hypothesis would further clarify the mechanism of glaucoma and lead to the development of a more sensitive method for making an early diagnosis of glaucoma.

Inflammation and blood-brain barrier breach remote from the primary injury following neurotrauma

BACKGROUND

Following injury to the central nervous system, increased microglia, secretion of pro- and anti-inflammatory cytokines, and altered blood-brain barrier permeability, a hallmark of degeneration, are observed at and immediately adjacent to the injury site. However, few studies investigate how regions remote from the primary injury could also suffer from inflammation and secondary degeneration.

METHODS

Adult female Piebald-Viral-Glaxo (PVG) rats underwent partial transection of the right optic nerve, with normal, age-matched, unoperated animals as controls. Perfusion-fixed brains and right optic nerves were harvested for immunohistochemical assessment of inflammatory markers and blood-brain barrier integrity; fresh-frozen brains were used for multiplex cytokine analysis.

RESULTS

Immediately ventral to the optic nerve injury, immunointensity of both the pro-inflammatory biomarker inducible nitric oxide synthase (iNOS) and the anti-inflammatory biomarker arginase-1 (Arg1) increased at 7 days post-injury, with colocalization of iNOS and Arg1 immunoreactivity within individual cells. CD11b+ and CD45+ cells were increased 7 days post-injury, with altered BBB permeability still evident at this time. In the lower and middle optic tract and superior colliculus, IBA1+ resident microglia were first increased at 3 days; ED1+ and CD11b+ cells were first increased in the middle and upper tract and superior colliculus 7 days post-injury. Increased fibrinogen immunoreactivity indicative of altered BBB permeability was first observed in the contralateral upper tract at 3 days and middle tract at 7 days post-injury. Multiplex cytokine analysis of brain homogenates indicated significant increases in the pro-inflammatory cytokines, IL-2 and TNFα, and anti-inflammatory cytokine IL-10 1 day post-injury, decreasing to control levels at 3 days for TNFα and 7 days for IL-2. IL-10 was significantly elevated at 1 and 7 days post-injury with a dip at 3 days post-injury.

CONCLUSIONS

Partial injury to the optic nerve induces a complex remote inflammatory response, characterized by rapidly increased pro- and anti-inflammatory cytokines in brain homogenates, increased numbers of IBA1+ cells throughout the visual pathways, and increased CD11b+ and ED1+ inflammatory cells, particularly towards the synaptic terminals. BBB permeability can increase prior to inflammatory cell infiltration, dependent on the brain region.