Neurovascular dysfunction in glaucoma

Vascular resistance indices are higher in the superior than inferior optic nerve head and retina

Retinal vascular resistance is of interest in glaucoma research, as a potential link between retinal ganglion cell loss and observed phenomena including disrupted vascular autoregulation, altered biomechanical stiffness, and impaired neurovascular coupling. It can now be assessed in vivo, using laser speckle flowgraphy. However, continued progress in the field requires better understanding of its physiology. In this study, we test the hypothesis of homogeneity of vascular resistance indices between regions of the retina: specifically, between superior and inferior hemifields. The resistivity index (maximum flow minus minimum flow, as a proportion of the maximum) and pulsatility index (maximum minus minimum, as a proportion of the mean) were measured in major vessels within the optic nerve head, in the remaining tissue within the optic nerve head, and in peripapillary branch retinal arteries, separated in each case into superior and inferior quadrants. This was performed in 378 eyes of 189 participants with suspected, early or moderate glaucoma; and in 99 eyes of 50 participants without any ocular pathology. In the glaucoma cohort, the resistivity index was on average 9% higher superiorly than inferiorly in vessels within the optic nerve head; 8% higher superiorly in remaining tissue; and 8% higher superiorly in peripapillary vessels (all p < 0.001). The pulsatility index was on average 11% higher superiorly in all three locations (all p < 0.001). Average flow was slightly higher superiorly in major vessels in the nerve head, but higher inferiorly elsewhere. In the healthy control cohort, resistivity index was higher superiorly by 10% in vessels and 8% in tissue within the optic nerve head; pulsatility index was 12% and 10% higher superiorly respectively (all p < 0.001). The fact that these differences were similar between the two cohorts suggests that they are not caused by the disease process. However, it is notable that glaucomatous loss most frequently occurs first in the superior visual field, corresponding with the inferior retina. The finding that vascular resistance indices are consistently higher in the superior retina warrants further investigation, both for its causes and consequences.

Pericyte-to-Endothelial Cell Communication via Tunneling Nanotubes Is Disrupted by a Diol of Docosahexaenoic Acid

The pericyte coverage of microvessels is altered in metabolic diseases, but the mechanisms regulating pericyte-endothelial cell communication remain unclear. This study investigated the formation and function of pericyte tunneling nanotubes (TNTs) and their impact on endothelial cell metabolism. TNTs were analyzed in vitro in retinas and co-cultures of pericytes and endothelial cells. Using mass spectrometry, the influence of pericytes on endothelial cell metabolism was examined. TNTs were present in the murine retina, and although diabetes was associated with a decrease in pericyte coverage, TNTs were longer. In vitro, pericytes formed TNTs in the presence of PDGF, extending toward endothelial cells and facilitating mitochondrial transport from pericytes to endothelial cells. In experiments with mitochondria-depleted endothelial cells displaying defective TCA cycle metabolism, pericytes restored the mitochondrial network and metabolism. 19,20-Dihydroxydocosapentaenoic acid (19,20-DHDP), known to disrupt pericyte-endothelial cell junctions, prevented TNT formation and metabolic rescue in mitochondria-depleted endothelial cells. 19,20-DHDP also caused significant changes in the protein composition of pericyte-endothelial cell junctions and involved pathways related to phosphatidylinositol 3-kinase, PDGF receptor, and RhoA signaling. Pericyte TNTs contact endothelial cells and support mitochondrial transfer, influencing metabolism. This protective mechanism is disrupted by 19,20-DHDP, a fatty acid mediator linked to diabetic retinopathy.

Loneliness and social isolation are associated with an increased risk of glaucoma: a UK Biobank cohort study

BACKGROUND

Loneliness and social isolation have been found to be associated with various health-related outcomes. Our study aimed to evaluate the association of loneliness and social isolation with the risk of glaucoma.

METHODS

A total of 373,330 participants from the UK Biobank without glaucoma at recruitment were included in this study. Self-reported questionnaires were used to define loneliness and social isolation. Incident glaucoma events were identified by hospital inpatient admissions and self-reported data. COX proportional hazards models adjusted for sociodemographic, lifestyle, and health-related factors were used to estimate hazard ratios (HRs) and 95% CIs.

RESULTS

During a median follow-up of 13.1 (interquartile range: 12.3-13.9) years, 6,489 participants developed glaucoma. After adjusting for confounding factors, loneliness (yes vs. no: adjusted HR: 1.16; 95% CI: 1.04-1.30; P = 0.009) and social isolation (yes vs. no: adjusted HR: 1.08; 95% CI: 1.01-1.16; P = 0.033) were associated with an increased risk of glaucoma.

CONCLUSIONS

In this population-based prospective cohort study, loneliness and social isolation were associated with a higher risk of glaucoma.

New Insights in ATP Synthesis as Therapeutic Target in Cancer and Angiogenic Ocular Diseases

Lactate and ATP formation by aerobic glycolysis, the Warburg effect, is considered a hallmark of cancer. During angiogenesis in non-cancerous tissue, proliferating stalk endothelial cells (ECs) also produce lactate and ATP by aerobic glycolysis. In fact, all proliferating cells, both non-cancer and cancer cells, need lactate for the biosynthesis of building blocks for cell growth and tissue expansion. Moreover, both non-proliferating cancer stem cells in tumors and leader tip ECs during angiogenesis rely on glycolysis for pyruvate production, which is used for ATP synthesis in mitochondria through oxidative phosphorylation (OXPHOS). Therefore, aerobic glycolysis is not a specific hallmark of cancer but rather a hallmark of proliferating cells and limits its utility in cancer therapy. However, local treatment of angiogenic eye conditions with inhibitors of glycolysis may be a safe therapeutic option that warrants experimental investigation. Most types of cells in the eye such as photoreceptors and pericytes use OXPHOS for ATP production, whereas proliferating angiogenic stalk ECs rely on glycolysis for lactate and ATP production. (J Histochem Cytochem XX.XXX-XXX, XXXX).

Addressing neurodegeneration in glaucoma: Mechanisms, challenges, and treatments

Glaucoma is the leading cause of irreversible blindness globally. The disease causes vision loss due to neurodegeneration of the retinal ganglion cell (RGC) projection to the brain through the optic nerve. Glaucoma is associated with sensitivity to intraocular pressure (IOP). Thus, mainstay treatments seek to manage IOP, though many patients continue to lose vision. To address neurodegeneration directly, numerous preclinical studies seek to develop protective or reparative therapies that act independently of IOP. These include growth factors, compounds targeting metabolism, anti-inflammatory and antioxidant agents, and neuromodulators. Despite success in experimental models, many of these approaches fail to translate into clinical benefits. Several factors contribute to this challenge. Firstly, the anatomic structure of the optic nerve head differs between rodents, nonhuman primates, and humans. Additionally, animal models do not replicate the complex glaucoma pathophysiology in humans. Therefore, to enhance the success of translating these findings, we propose two approaches. First, thorough evaluation of experimental targets in multiple animal models, including nonhuman primates, should precede clinical trials. Second, we advocate for combination therapy, which involves using multiple agents simultaneously, especially in the early and potentially reversible stages of the disease. These strategies aim to increase the chances of successful neuroprotective treatment for glaucoma.

Stem cell factor protects against chronic ischemic retinal injury by modulating on neurovascular unit

Retinal ischemia is a significant factor in various vision-threatening diseases, but effective treatments are currently lacking. This study explores the potential of stem cell factor (SCF) in regulating the neurovascular unit as a therapeutic intervention for retinal ischemic diseases. A chronic retinal ischemia model was established in Brown Norway rats using bilateral common carotid artery occlusion (BCCAO). Subsequent SCF treatment resulted in a remarkable recovery of retinal function, as indicated by electroretinogram, light/dark transition test, and optokinetic head tracking test results. Histological examination demonstrated a significant increase in the number of retinal neurons and an overall thickening of the retina. Immunofluorescence confirmed these findings and further demonstrated that SCF treatment regulated retinal remodeling. Notably, SCF treatment ameliorated the disrupted expression of synaptic markers in the control group's BCCAO rats and suppressed the activation of Müller cells and microglia. Retinal whole-mount analysis revealed a significant improvement in the abnormalities in retinal vasculature following SCF treatment. Transcriptome sequencing analysis revealed that SCF-induced transcriptome changes were closely linked to the Wnt7 pathway. Key members of the Wnt7 pathway, exhibited significant upregulation following SCF treatment. These results underscore the protective role of SCF in the neurovascular unit of retinal ischemia rats by modulating the Wnt7 pathway. SCF administration emerges as a promising therapeutic strategy for retinal ischemia-related diseases, offering potential avenues for future clinical interventions.

Glaucoma, Pseudoexfoliation and Hearing Loss: A Systematic Literature Review

Purpose: To investigate the relationship between glaucoma, pseudoexfoliation and hearing loss (HL). Methods: A systematic literature search following PRISMA guidelines was conducted using the PubMed, Embase, Scopus and Cochrane databases from 1995 up to 28 August 2023. Results: Thirty studies out of the 520 records screened met the inclusion criteria and were included. Most articles (n = 20) analysed the association between pseudoexfoliation syndrome (XFS) and HL, showing XFS patients to have higher prevalence of sensorineural hearing loss (SNHL) at both speech frequencies (0.25, 0.5, 1 and 2 kHz), and higher frequencies (4 and 8 kHz) compared to controls in most cases. No significant differences in prevalence or level of HL between XFS and pseudoexfoliative glaucoma (XFG) were detected in most studies. Eight articles analysed the relationship between primary open-angle glaucoma (POAG) and HL. Overall, a positive association between the two conditions was highlighted across all studies except for two cases. Similarly, articles focusing on NTG and HL (n = 4) showed a positive association in most cases. The role of autoimmunity and, in particular, the presence of antiphosphatidylserine antibodies (APSA) in patients with NTG and HL suggested an underlying autoimmune or vascular mechanism contributing to their pathogenesis. Only one study analysed the relationship between angle-closure glaucoma (ACG) and HL, showing higher incidence of ACG in patients with SNHL compared to normal hearing controls. Conclusions: Most studies detected an association between XFS and HL as well as POAG/NTG/ACG and HL, suggesting the presence of a similar pathophysiology of neurodegeneration. However, given the strength of the association of XFS with HL, it remains unclear whether the presence of XFG is further associated with SNHL. Further research specifically targeted to assess the correlation between glaucoma, XFS and HL is warranted to provide a more comprehensive understanding of this association.

Loss of Sarm1 reduces retinal ganglion cell loss in chronic glaucoma

Glaucoma is one of the leading causes of irreversible blindness worldwide and vision loss in the disease results from the deterioration of retinal ganglion cells (RGC) and their axons. Metabolic dysfunction of RGC plays a significant role in the onset and progression of the disease in both human patients and rodent models, highlighting the need to better define the mechanisms regulating cellular energy metabolism in glaucoma. This study sought to determine if Sarm1, a gene involved in axonal degeneration and NAD+ metabolism, contributes to glaucomatous RGC loss in a mouse model with chronic elevated intraocular pressure (IOP). Our data demonstrate that after 16 weeks of elevated IOP, Sarm1 knockout (KO) mice retain significantly more RGC than control animals. Sarm1 KO mice also performed significantly better when compared to control mice during optomotor testing, indicating that visual function is preserved in this group. Our findings also indicate that Sarm1 KO mice display mild ocular developmental abnormalities, including reduced optic nerve axon diameter and lower visual acuity than controls. Finally, we present data to indicate that SARM1 expression in the optic nerve is most prominently associated with oligodendrocytes. Taken together, these data suggest that attenuating Sarm1 activity through gene therapy, pharmacologic inhibition, or NAD+ supplementation, may be a novel therapeutic approach for patients with glaucoma.

Orchestrating Blood Flow in the Retina: Interpericyte Tunnelling Nanotube Communication

The retina transforms light into electrical signals, which are sent to the brain via the optic nerve to form our visual perception. This complex signal processing is performed by the retinal neuron and requires a significant amount of energy. Since neurons are unable to store energy, they must obtain glucose and oxygen from the bloodstream to produce energy to match metabolic needs. This process is called neurovascular coupling (NVC), and it is based on a precise mechanism that is not totally understood. The discovery of fine tubular processes termed tunnelling nanotubes (TNTs) set a new type of cell-to-cell communication. TNTs are extensions of the cellular membrane that allow the transfer of material between connected cells. Recently, they have been reported in the brain and retina of living mice, where they connect pericytes, which are vascular mural cells that regulate vessel diameter. Accordingly, these TNTs were termed interpericyte tunnelling nanotubes (IPTNTs), which showed a vital role in blood delivery and NVC. In this chapter, we review the involvement of TNTs in NVC and discuss their implications in retinal neurodegeneration.