Microtubule Imaging Reveals Cytoskeletal Deficit Predisposing the Retinal Ganglion Cell Axons to Atrophy in DBA/2J

Impaired axonal transport at the optic nerve head contributes to neurodegeneration in a novel Cre-inducible mouse model of myocilin glaucoma

Elevation of intraocular pressure (IOP) due to trabecular meshwork (TM) dysfunction, leading to neurodegeneration, is the pathological hallmark of primary open-angle glaucoma (POAG). Impaired axonal transport is an early and critical feature of glaucomatous neurodegeneration. However, a robust mouse model that replicates these human POAG features accurately has been lacking. We report the development and characterization of a novel Cre-inducible mouse model expressing a DsRed-tagged Y437H mutant of human myocilin (Tg.CreMYOCY437H). A single intravitreal injection of HAd5-Cre induced selective MYOC expression in the TM, causing TM dysfunction, reducing outflow facility, and progressively elevating IOP in Tg.CreMYOCY437H mice. Sustained IOP elevation resulted in significant retinal ganglion cell (RGC) loss and progressive axonal degeneration in Cre-induced Tg.CreMYOCY437H mice. Notably, impaired anterograde axonal transport was observed at the optic nerve head before RGC degeneration, independent of age, indicating that impaired axonal transport contributes to RGC degeneration in Tg.CreMYOCY437H mice. In contrast, axonal transport remained intact in ocular hypertensive mice injected with microbeads, despite significant RGC loss. Our findings indicate that Cre-inducible Tg.CreMYOCY437H mice replicate all glaucoma phenotypes, providing an ideal model for studying early events of TM dysfunction and neuronal loss in POAG.

Differential protection by nicotinamide in a mouse model of glaucoma DBA/2J revealed by second-harmonic generation microscopy

Glaucoma is a blinding disease where the retinal ganglion cells and their axons degenerate. Degradation of axonal microtubules is thought to play a critical role in the pathogenesis, but the mechanism is unknown. Here we investigate whether microtubule disruption in glaucoma can be alleviated by metabolic rescue. The integrity of axonal microtubules and the morphology of the retinal nerve fibers were evaluated by second-harmonic generation microscopy in a mouse model of glaucoma, DBA/2J, which received a dietary supplement of nicotinamide (NAM) for reducing metabolic stress. It was compared with control DBA/2J, which did not receive NAM, and non-glaucomatous DBA/2J-Gpnmb+. We found that the morphology of the retinal nerve fibers, but not axonal microtubules, are significantly protected by NAM. The decoupling is analogous to microtubule deficit, a glaucoma pathology in which axonal microtubules exhibit rapid degradation compared to the morphology of the retinal nerve fibers. Understanding microtubule deficit could provide insights into the divergent responses to NAM. From co-registered images of second-harmonic generation and immunofluorescence, it was determined that microtubule deficit was not due to a shortage of tubulins. Furthermore, microtubule deficit colocalized with the sectors in which the retinal ganglion cells were disconnected from the brain, suggesting that microtubule disruption is associated with axonal transport deficit in glaucoma. Together, our data suggests significant role axonal microtubules play in glaucomatous degeneration, offering a new opportunity for neuroprotection.

Protective effects of nicotinamide in a mouse model of glaucoma DBA/2 studied by second-harmonic generation microscopy

Glaucoma is a blinding disease where the retinal ganglion cells and their axons degenerate. Degradation of axonal microtubules is thought to play a critical role in the pathogenesis, but the mechanism is unknown. Here we investigate whether microtubule disruption in glaucoma can be alleviated by metabolic rescue. The morphology and integrity of microtubules of the retinal nerve fibers were evaluated by second-harmonic generation microscopy in a mouse model of glaucoma, DBA/2, which received a dietary supplement of nicotinamide to reduce metabolic stress. It was compared with control DBA/2, which did not receive nicotinamide, and non-glaucomatous DBA/2-Gpnmb+. We found that morphology but not microtubules are significantly protected by nicotinamide. Furthermore, from co-registered images of second-harmonic generation and immunofluorescence, it was determined that microtubule deficit was not due to a shortage of tubulins. Microtubule deficit colocalized with the sectors in which the retinal ganglion cells were disconnected from the brain, indicating that microtubule disruption is associated with axonal transport deficit in glaucoma. Together, our data suggests significant role axonal microtubules play in glaucomatous degeneration, offering a new opportunity for neuroprotection.

Assessment of Peripapillary Retinal Nerve Fiber Layer Optical Density Ratios in Healthy Eyes Using Optical Coherence Tomography with Different Analytical Radii

PURPOSE

To explore the characteristics and determinants of peripapillary retinal nerve fiber layer (RNFL) optical density (OD) by optical coherence tomography (OCT) in healthy eyes with varied analytical radii.

METHODS

Peripapillary OCT scans centered at the optic disc of 150 eyes from 150 healthy subjects (64 males and 86 females) were included. Under 5 analytical circles with different radii (1.45 mm, 1.7 mm, 1.95 mm, 2.2 mm and 2.45 mm), the circumpapillary circular cross-sectional images were exported for further analysis using Image J. Peripapillary RNFL and retinal pigment epithelium (RPE) OD in different quadrants and clock-hours were obtained. RNFL optical density ratio (ODR) was then calculated as RNFL OD divided by RPE OD. A linear mixed-effects model analysis was performed to assess the relationship between RNFL ODR and analytical radius, accounting for axial length, age, spherical equivalent, thickness and image score.

RESULTS

The RNFL ODRs had a double-hump pattern with peaks in the superior and inferior quadrants and troughs in the temporal and nasal areas. In the linear mixed-effects model analysis, a trend of decreasing mean RNFL ODR with increasing analytical radius was found (0.9227 ± 0.0689, 0.9063 ± 0.0620, 0.8916 ± 0.0552, 0.8729 ± 0.0553 and 0.8575 ± 0.0564 respectively, p = 0.034). RNFL ODR values was negatively correlated with age (p < 0.001), positively correlated with corresponding RNFL thickness (p < 0.001). No significant correlation was found between RFNL ODR and image score, axial length and spherical equivalent.

CONCLUSIONS

RNFL ODR profile showed a comparable double-hump configuration with RNFL thickness. RNFL ODR values tended to decrease with larger analytical circles and older age, and increase with corresponding RNFL thickness. These factors should be considered when interpreting RNFL ODR in glaucoma assessment.

The Application of MicroRNAs in Glaucoma Research: A Bibliometric and Visualized Analysis

Glaucoma is similar to a neurodegenerative disorder and leads to global irreversible loss of vision. Despite extensive research, the pathophysiological mechanisms of glaucoma remain unclear, and no complete cure has yet been identified for glaucoma. Recent studies have shown that microRNAs can serve as diagnostic biomarkers or therapeutic targets for glaucoma; however, there are few bibliometric studies that focus on using microRNAs in glaucoma research. Here, we have adopted a bibliometric analysis in the field of microRNAs in glaucoma research to manifest the current tendencies and research hotspots and to present a visual map of the past and emerging tendencies in this field. In this study, we retrieved publications in the Web of Science database that centered on this field between 2007 and 2022. Next, we used VOSviewer, CiteSpace, Scimago Graphica, and Microsoft Excel to present visual representations of a co-occurrence analysis, co-citation analysis, tendencies, hotspots, and the contributions of authors, institutions, journals, and countries/regions. The United States was the main contributor. Investigative Ophthalmology and Visual Science has published the most articles in this field. Over the past 15 years, there has been exponential growth in the number of publications and citations in this field across various countries, organizations, and authors. Thus, this study illustrates the current trends, hotspots, and emerging frontiers and provides new insight and guidance for searching for new diagnostic biomarkers and clinical trials for glaucoma in the future. Furthermore, international collaborations can also be used to broaden and deepen the field of microRNAs in glaucoma research.

Genetic Factors Implicated in the Investigation of Possible Connections between Alzheimer's Disease and Primary Open Angle Glaucoma

Both Alzheimer's disease (AD) and primary open angle glaucoma (POAG) are diseases of primary global neurodegeneration with complex pathophysiologies. Throughout the published literature, researchers have highlighted similarities associated with various aspects of both diseases. In light of the increasing number of findings reporting resemblance between the two neurodegenerative processes, scientists have grown interested in possible underlying connections between AD and POAG. In the search for explanations to fundamental mechanisms, a multitude of genes have been studied in each condition, with overlap in the genes of interest between AD and POAG. Greater understanding of genetic factors can drive the research process of identifying relationships and elucidating common pathways of disease. These connections can then be utilized to advance research as well as to generate new clinical applications. Notably, AD and glaucoma are currently diseases with irreversible consequences that often lack effective therapies. An established genetic connection between AD and POAG would serve as the basis for development of gene or pathway targeted strategies relevant to both diseases. Such a clinical application could be of immense benefit to researchers, clinicians, and patients alike. This paper aims to summarize the genetic associations between AD and POAG, describe common underlying mechanisms, discuss potential areas of application, and organize the findings in a review.

Short-Term Omega-3 Supplementation Modulates Novel Neurovascular and Fatty Acid Metabolic Proteome Changes in the Retina and Ophthalmic Artery of Mice with Targeted Cyp2c44 Gene Deletion

Cytochrome P450 (CYP) gene mutations are a common predisposition associated with glaucoma. Although the molecular mechanisms are largely unknown, omega-3 polyunsaturated fatty acids (ω-3 PUFA) and their CYP-derived bioactive mediators play crucial roles in the ocular system. Here, we elucidated the proteome and cell-signalling alterations attributed to the main human CYP2C gene deficiency using a homologous murine model (Cyp2c44^−/−), and unravelled the effects of acute ω-3 PUFA supplementation in two ocular vascular beds comprising the retrobulbar ophthalmic artery (OA) and retina (R). Male Cyp2c44^−/− mice (KO) and their floxed littermates (WT) were gavaged daily for 7 days with 0.01 mL/g of ω-3 PUFA composed of menhaden fish oil. Another group in respective strains served as vehicle-treated controls. OA and R were isolated at day 8 post-treatment (n = 9/group) and subjected to mass spectrometry (MS)-based proteomics and in silico bioinformatics analyses. Cyp2c44^−/− resulted in significant detrimental proteome changes associated with compromised vascular integrity and degeneration in the OA and R, respectively. However, notable changes in the OA after ω-3 PUFA intake were associated with the maintenance of intercellular junctional and endothelial cell functions, as well as activation of the fatty acid metabolic pathway in the KO mice. Conversely, ω-3 PUFA supplementation profoundly influenced the regulation of a large majority of retinal proteins involved in the preservation of neuronal and phototransduction activities in WT mice, namely synaptophysin, phosducin and guanylate cyclase-1, while significantly abrogating degenerative processes in the KO mice via the regulation of, namely, synaptotagmin-1 and beta-crystallin B2. In gist, this study demonstrated that dietary supplementation with ω-3 PUFA for a short period of seven days regulated specific neuro-vasculoprotective mechanisms to preserve the functionality of the OA and R in the absence of Cyp2c44. The potential adjunct use of ω-3 PUFA for glaucoma therapy needs further investigation.

H-ABC tubulinopathy revealed by label-free second harmonic generation microscopy

Hypomyelination with atrophy of the basal ganglia and cerebellum is a recently described tubulinopathy caused by a mutation in the tubulin beta 4a isoform, expressed in oligodendrocytes. The taiep rat is the only spontaneous tubulin beta 4a mutant available for the study of this pathology. We aimed to identify the effects of the tubulin mutation on freshly collected, unstained samples of the central white matter of taiep rats using second harmonic generation microscopy. Cytoskeletal differences between the central white matter of taiep rats and control animals were found. Nonlinear emissions from the processes and somata of oligodendrocytes in tubulin beta 4a mutant rats were consistently detected, in the shape of elongated structures and cell-like bodies, which were never detected in the controls. This signal represents the second harmonic trademark of the disease. The tissue was also fluorescently labeled and analyzed to corroborate the origin of the nonlinear signal. Besides enabling the description of structural and molecular aspects of H-ABC, our data open the door to the diagnostic use of nonlinear optics in the study of neurodegenerative diseases, with the additional advantage of a label-free approach that preserves tissue morphology and vitality.

Axonal architecture of the mouse inner retina revealed by second harmonic generation

We describe a novel method for visualizing the network of axons in the unlabeled fresh wholemount retina. The intrinsic radiation of second harmonic generation (SHG) was utilized to visualize single axons of all major retinal neurons, i.e., photoreceptors, horizontal cells, bipolar cells, amacrine cells, and the retinal ganglion cells. The cell types of SHG+ axons were determined using transgenic GFP/YFP mice. New findings were obtained with retinal SHG imaging: Müller cells do not maintain uniformly polarized microtubules in the processes; SHG+ axons of bipolar cells terminate in the inner plexiform layer (IPL) in a subtype-specific manner; a subset of amacrine cells, presumably the axon-bearing types, emits SHG; and the axon-like neurites of amacrine cells provide a cytoskeletal scaffolding for the IPL stratification. To demonstrate the utility, retinal SHG imaging was applied to testing whether the inner retina is preserved in glaucoma, using DBA/2 mice as a model of glaucoma and DBA/2-Gpnmb+ as the nonglaucomatous control. It was found that the morphology of the inner retina was largely intact in glaucoma and the presynaptic compartments to the retinal ganglion cells were uncompromised. It proves retinal SHG imaging as a promising technology for studying the physiological and diseased retinas in 3D.

Adaptive optics scanning laser ophthalmoscopy may support early diagnosis of glaucoma

Purpose:

To compare image characteristics of retinal nerve fiber layer (RNFL) between glaucoma patients and healthy controls using adaptive optics scanning laser ophthalmoscopy (AOSLO).

Methods:

This was a cross-sectional pilot study with two groups: a glaucoma group with patients with moderate or severe glaucoma as per the Hodapp–Parrish–Anderson classification system and a control group with healthy individuals. The optic nerve damage in moderate glaucoma was predominantly located in only one hemisphere; the other hemisphere was un- or minimally affected on optical coherence tomography and automated perimetry and is referred to as early glaucoma. The structure of RNFL bundles and gain (%) in RNFL images with mean pixel values between 15 and 35 were analyzed. Imaging was performed one degree away from the optic disc margin at two and four cardinal clock positions in the glaucoma and control groups, respectively. The field of view was 1.3° at 2.3 m resolution. We studied one eye per participant.

Results:

There were 11 glaucoma patients and 7 healthy controls. Imaging was successful at 88% of the locations in controls and early glaucoma; the reflectivity differed significantly (0.51 and 0.56, respectively, P < 0.001) but not the structure of RNFL bundles (Cohen’s Kappa 0.11) between them. In patients with moderate and severe glaucoma, imaging was successful only at 46% of the locations; RNFL bundles were not discernible, and RNFL reflectivity did not differ from those with early glaucoma (P < 0.11).

Conclusion:

The recorded gain (%) of RNFL images obtained using AOSLO could be an objective indicator of early glaucoma.

Evidence-based understanding of disc hemorrhage in glaucoma

Disc hemorrhage is a characteristic finding that is highly associated with glaucoma development or progression. Consequently, the literature commonly designates disc hemorrhage as a "risk factor" for glaucoma progression; however, the exact cause-and-effect relationship or mechanism remains unclear. In this review, we discuss the emerging evidence that disc hemorrhage is a secondary development that follows glaucomatous damage. As our understanding of disc hemorrhage has progressed in recent decades, we suggest the terminology be changed from "risk factor" to "indicator" of ongoing glaucomatous development or progression for a more accurate description, better indication of the clinical implications and, ultimately, a better guide for future research.

Harmonic Generation Microscopy 2.0: New Tricks Empowering Intravital Imaging for Neuroscience

Optical harmonic generation, e.g., second- (SHG) and third-harmonic generation (THG), provides intrinsic contrasts for three-dimensional intravital microscopy. Contrary to two-photon excited fluorescence (TPEF), however, they have found relatively specialized applications, such as imaging collagenous and non-specific tissues, respectively. Here we review recent advances that broaden the capacity of SHG and THG for imaging the central nervous system in particular. The fundamental contrast mechanisms are reviewed as they encode novel information including molecular origin, spectroscopy, functional probes, and image analysis, which lay foundations for promising future applications in neuroscience.