Estimation of axon counts in a rat model of glaucoma: comparison of fixed-pattern sampling with targeted sampling

DNA and RNA oxidative damage in the retina is associated with ganglion cell mitochondria

This study examines retinas from a rat glaucoma model for oxidized nucleosides 8OHdG and 8OHG, biomarkers for oxidative damage of DNA and RNA, respectively. Immunohistochemical data indicate a predominant localization of 8OHdG/8OHG in retinal ganglion cells (RGCs). The levels for these oxidized DNA/RNA products were 3.2 and 2.8 fold higher at 1 and 2 weeks after intraocular pressure elevation compared to control retinas, respectively. 8OHdG/8OHG were almost exclusively associated with mitochondrial DNA/RNA: ~ 65% of 8OHdG/8OHG were associated with RNA isolated from mitochondrial fraction and ~ 35% with DNA. Furthermore, we analyzed retinas of the rd10 mouse, a model for retinitis pigmentosa, with severe degeneration of photoreceptors to determine whether high levels of 8OHdG/8OHG staining intensity in RGCs of control animals is related to the high level of mitochondrial oxidative phosphorylation necessary to support light-evoked RGC activity. No significant difference in 8OHdG/8OHG staining intensity between control and rd10 mouse retinas was observed. The results of this study suggest that high levels of 8OHdG/8OHG in RGCs of wild-type animals may lead to cell damage and progressive loss of RGCs observed during normal aging, whereas ocular hypertension-induced increase in the level of oxidatively damaged mitochondrial DNA/RNA could contribute to glaucomatous neurodegeneration.

Histomorphometry in Peripheral Nerve Regeneration: Comparison of Different Axon Counting Methods

BACKGROUND

Histomorphometry quantitatively evaluates nerve regeneration. Total myelinated fiber count (TMFC) is most accurately obtained manually across full nerve cross-sections, but most researchers opt for automated, sampled analysis. Few of the numerous techniques available have been validated. The goal of this study was to compare common histomorphometric methods (full manual [FM], sampled manual [SM], and sampled automatic [SA]) to determine their reliability and consistency.

MATERIAL AND METHODS

Twenty-four rats underwent sciatic nerve (SN) repair with 20mm isografts; SNs distal to the graft were analyzed. TMFC was manually determined in each full cross-section. Counts were also extrapolated from sampled fields, both manually and automatically with ImageJ software. Myelinated fiber diameter, axon diameter, and myelin sheath thickness were measured manually in full and sampled fields; G-ratio was calculated. Repeated-measures MANOVA, Spearman correlation, and Wilcoxon signed-rank tests were performed. A systematic review of histomorphometry in rat SN repair was performed to analyze the variability of techniques in the literature.

RESULTS

FM TMFC was 13,506 ± 4,217. Both sampled methods yielded significantly different TMFCs (SM:14.4 ± 13.4%, P< 0.001; SA:21.8 ± 44.7%, P = 0.037). All three methods strongly correlated with each other, especially FM and SM (r_s = 0.912, P< 0.001). FM fiber diameter, axon diameter, and myelin sheath thickness did not differ from SM (P = 0.493, 0.209, and 0.331, respectively). 65% of papers used sampling; 78% utilized automated or semi-automated analysis. Software, sampling, and histomorphometric parameters varied widely.

CONCLUSION

SM and SA analysis are reliable with standardized, systematic sampling. Transparency is essential to allow comparison of data; meanwhile, researchers must be cognizant of the wide variety of methodologies in the literature.

Inducible rodent models of glaucoma

Glaucoma is one of the leading causes of vision impairment worldwide. In order to further understand the molecular pathobiology of this disease and to develop better therapies, clinically relevant animal models are necessary. In recent years, both the rat and mouse have become popular models in glaucoma research. Key reasons are: many important biological similarities shared among rodent eyes and the human eye; development of improved methods to induce glaucoma and to evaluate glaucomatous damage; availability of genetic tools in the mouse; as well as the relatively low cost of rodent studies. Commonly studied rat and mouse glaucoma models include intraocular pressure (IOP)-dependent and pressure-independent models. The pressure-dependent models address the most important risk factor of elevated IOP, whereas the pressure-independent models assess "normal tension" glaucoma and other "non-IOP" related factors associated with glaucomatous damage. The current article provides descriptions of these models, their characterizations, specific techniques to induce glaucoma, mechanisms of injury, advantages, and limitations.

QuPath Automated Analysis of Optic Nerve Degeneration in Brown Norway Rats

Purpose

A novel application of QuPath open-source digital analysis software is used to provide in-depth morphological analysis of progressive optic nerve (ON) degeneration in rats.

Methods

QuPath software was adapted to assess axon and gliotic morphology in toluidine blue-stained, Brown Norway rat ON light micrographs. QuPath axon numbers, density, size distributions, and gliotic areas were obtained from test images and ON cross-sections separated by damage grade. QuPath results were compared with manual counting, AxonJ, and electron microscopy axon estimates.

Results

QuPath-derived axon number, density, and diameter decreased with increasing ON damage. Axon density negatively correlated with gliotic areas in test images (R2 = 0.759; P < 0.0001; N = 40) and in ON cross-sections (R2 = 0.803; P < 0.0004; N = 10). Although axon losses occurred across most axon diameters, large axons were more susceptible to degeneration. The exception was swollen axons > 2 µm, which increased in moderately but not severely damaged images. QuPath axon counts correlated strongly with manual counts of test images (R2 = 0.956; P < 0.0001). QuPath outperformed AxonJ on test images and total ON axon counts. Compared to electron microscopy analysis, QuPath undercounted ON axons; however, correlation between the methods was robust (R2 = 0.797; P < 0.001; N = 10).

Conclusions

QuPath analysis reliably identified axon loss, axon morphology changes, and gliotic expansion that occurred in degenerating ONs.

Translational Relevance

QuPath is a valuable tool for rapid, automated, analysis of healthy and degenerating ONs. Reproducible preclinical studies for new glaucoma treatments depend on unbiased in-depth analysis of ON pathology. This was provided by the QuPath approach.

Efficient determination of axon number in the optic nerve: A stereological approach

Quantifying the number of axons in the optic nerve is of interest in many research questions. Here, we show that a stereological method allows simple, efficient, precise and unbiased determination of the total axon number in the murine optic nerve. Axons in semi-thin optic nerve cross sections from untreated eyes (n = 21) and eyes subjected to retinal damage by intravitreous NMDA injections (n = 32) or PBS controls (n = 5) were manually identified, counted and digitally labeled by hand. A stereological procedure was empirically tested with systematic combinations of different sampling methods (simple random sampling without replacement, systematic uniform random sampling, stratified random sampling) and sampling parameters. Extensive numerical Monte Carlo experiments were performed to evaluate their large-sample properties. Our results demonstrate reliable determination of total axon number and superior performance compared to other methods at a small fraction of the time required for a full manual count. We specify suitable sampling parameters for the adoption of an efficient stereological sampling scheme, give empirical estimates of the additionally introduced sampling variance to facilitate experimental planning, and offer AxonCounter, an easy-to-use plugin implementing these stereological methods for the multi-platform image processing application NIH ImageJ.

Expression and activation of mitogen-activated protein kinases in the optic nerve head in a rat model of ocular hypertension

BACKGROUND

Glaucoma is a leading cause of irreversible blindness manifesting as an age-related, progressive optic neuropathy with associated retinal ganglion cell (RGC) loss. Mitogen-activated protein kinases (MAPKs: p42/44 MAPK, SAPK/JNK, p38 MAPK) are activated in various retinal disease models and likely contribute to the mechanisms of RGC death. Although MAPKs play roles in the development of retinal pathology, their action in the optic nerve head (ONH), where the initial insult to RGC axons likely resides in glaucoma, remains unexplored.

METHODS

An experimental paradigm representing glaucoma was established by induction of chronic ocular hypertension (OHT) via laser-induced coagulation of the trabecular meshwork in Sprague-Dawley rats. MAPKs were subsequently investigated over the following days for expression and activity alterations, using RT-PCR, immunohistochemistry and Western immunoblot.

RESULTS

p42/44 MAPK expression was unaltered after intraocular pressure (IOP) elevation, but there was a significant activation of this enzyme in ONH astrocytes after 6-24 h. Activated SAPK/JNK isoforms were present throughout healthy RGC axons but after IOP elevation or optic nerve crush, they both accumulated at the ONH, likely due to RGC axon transport disruption, and were subject to additional activation. p38 MAPK was expressed by a population of microglia which were significantly more populous following IOP elevation. However it was only significantly activated in microglia after 3 days, and then only in the ONH and optic nerve; in the retina it was solely activated in RGC perikarya.

CONCLUSIONS

In conclusion, each of the MAPKs showed a specific spatio-temporal expression and activation pattern in the retina, ONH and optic nerve as a result of IOP elevation. These findings likely reflect the roles of the individual enzymes, and the cells in which they reside, in the developing pathology following IOP elevation. These data have implications for understanding the mechanisms of ocular pathology in diseases such as glaucoma.

Investigations into Retinal Pathology in the Early Stages of a Mouse Model of Alzheimer's Disease

There is increasing recognition that visual performance is impaired in early stages of Alzheimer’s disease (AD); however, no consensus exists as to the mechanisms underlying this visual dysfunction, in particular regarding the timing, nature, and extent of retinal versus cortical pathology. If retinal pathology presents sufficiently early, it offers great potential as a source of novel biomarkers for disease diagnosis. The current project utilized an array of immunochemical and molecular tools to perform a characterization of retinal pathology in the early stages of disease progression using a well-validated mouse model of AD (APP_SWE/PS1_ΔE9). Analytical endpoints included examination of aberrant amyloid and tau in the retina, quantification of any neuronal degeneration, delineation of cellular stress responses of neurons and particularly glial cells, and investigation of oxidative stress. Brain, eyes, and optic nerves were taken from transgenic and wild-type mice of 3 to 12 months of age and processed for immunohistochemistry, qPCR, or western immunoblotting. The results revealed robust expression of the human APP transgene in the retinas of transgenic mice, but a lack of identifiable retinal pathology during the period when amyloid deposits were dramatically escalating in the brain. We were unable to demonstrate the presence of amyloid plaques, dystrophic neurites, neuronal loss, macro- or micro-gliosis, aberrant cell cycle re-entry, oxidative stress, tau hyperphosphorylation, or upregulations of proinflammatory cytokines or stress signaling molecules in the retina. The overall results do not support the hypothesis that detectable retinal pathology occurs concurrently with escalating amyloid deposition in the brains of APP_SWE/PS1_ΔE9 mice.

Evidence Supporting an Association Between Expression of Major Histocompatibility Complex II by Microglia and Optic Nerve Degeneration During Experimental Glaucoma

AIM

We acquired age-matched and sex-matched Sprague-Dawley rats from 2 independent breeding establishments. Serendipitously, we observed that constitutive, and bacterial toxin-induced, expression of major histocompatibility complex (MHC) class II RT1B chain in the uveal tract was much lower in one of the cohorts. Activated microglia are known to upregulate MHC II RT1B expression during optic nerve (ON) degeneration induced by raised intraocular pressure (IOP). We investigated whether, in a model of experimental glaucoma, microglial upregulation of MHC II RT1B was less efficacious and ON degeneration correspondingly less severe in the cohort of rats with low MHC II RT1B expression.

METHODS

Experimental glaucoma was induced by lasering the trabecular meshwork using a standard protocol. After 2 weeks of elevated IOP, retinal ganglion cells (RGC) survival, ON degeneration, and microglial responses were determined in both cohorts of rats.

RESULTS

Raised IOP-induced expression of MHC II RT1B by microglia was muted in the "Low" cohort compared with the "High" cohort. Axonal degeneration, RGC loss, and microgliosis were all significantly lower in the cohort of rats with low basal and induced expression of MHC II RT1B, despite both cohorts displaying IOP responses that were indistinguishable in terms of peak IOP and IOP exposure.

CONCLUSIONS

Expression of MHC II RT1B by activated microglia in the ON during experimental glaucoma was associated with more severe RGC degeneration. Further studies are needed to elucidate the role of MHC II during experimental glaucoma.

Dramatic Effect of Oral CSF-1R Kinase Inhibitor on Retinal Microglia Revealed by In Vivo Scanning Laser Ophthalmoscopy

This report provides sound evidence that the small molecule pharmaceutical PLX5622, a highly selective CSF-1R kinase inhibitor, crosses the blood-retina barrier and suppresses microglia activity. Members of this class of drug are in advanced clinical development stages and may represent a novel approach to modulate ocular inflammatory processes.

Automated Axon Counting in Rodent Optic Nerve Sections with AxonJ

We have developed a publicly available tool, AxonJ, which quantifies the axons in optic nerve sections of rodents stained with paraphenylenediamine (PPD). In this study, we compare AxonJ's performance to human experts on 100x and 40x images of optic nerve sections obtained from multiple strains of mice, including mice with defects relevant to glaucoma. AxonJ produced reliable axon counts with high sensitivity of 0.959 and high precision of 0.907, high repeatability of 0.95 when compared to a gold-standard of manual assessments and high correlation of 0.882 to the glaucoma damage staging of a previously published dataset. AxonJ allows analyses that are quantitative, consistent, fully-automated, parameter-free, and rapid on whole optic nerve sections at 40x. As a freely available ImageJ plugin that requires no highly specialized equipment to utilize, AxonJ represents a powerful new community resource augmenting studies of the optic nerve using mice.

Human umbilical cord blood mononuclear cells and chorionic plate-derived mesenchymal stem cells promote axon survival in a rat model of optic nerve crush injury

The use of mesenchymal stem cells (MSCs) in cell therapy in regenerative medicine has great potential, particularly in the treatment of nerve injury. Umbilical cord blood (UCB) reportedly contains stem cells, which have been widely used as a hematopoietic source and may have therapeutic potential for neurological impairment. Although ongoing research is dedicated to the management of traumatic optic nerve injury using various measures, novel therapeutic strategies based on the complex underlying mechanisms responsible for optic nerve injury, such as inflammation and/or ischemia, are required. In the present study, a rat model of optic nerve crush (ONC) injury was established in order to examine the effects of transplanting human chorionic plate-derived MSCs (CP‑MSCs) isolated from the placenta, as well as human UCB mononuclear cells (CB-MNCs) on compressed rat optic nerves. Expression markers for inflammation, apoptosis, and optic nerve regeneration were analyzed, as well as the axon survival rate by direct counting. Increased axon survival rates were observed following the injection of CB‑MNCs at at 1 week post-transplantation compared with the controls. The levels of growth-associated protein-43 (GAP‑43) were increased after the injection of CB‑MNCs or CP‑MSCs compared with the controls, and the expression levels of hypoxia-inducible factor-1α (HIF-1α) were also significantly increased following the injection of CB-MNCs or CP-MSCs. ERM-like protein (ERMN) and SLIT-ROBO Rho GTPase activating protein 2 (SRGAP2) were found to be expressed in the optic nerves of the CP‑MSC-injected rats with ONC injury. The findings of our study suggest that the administration of CB‑MNCs or CP‑MSCs may promote axon survival through systemic concomitant mechanisms involving GAP‑43 and HIF‑1α. Taken together, these findings provide further understanding of the mechanisms repsonsible for optic nerve injury and may aid in the development of novel cell-based therapeutic strategies with future applications in regenerative medicine, particularly in the management of optic nerve disorders.

Glial coverage in the optic nerve expands in proportion to optic axon loss in chronic mouse glaucoma

Within the white matter, axonal loss by neurodegeneration is coupled to glial cell changes in gene expression, structure and function commonly termed gliosis. Recently, we described the highly variable expansion of gliosis alebosco@neuro.utah.edu in degenerative optic nerves from the DBA/2J mouse model of chronic, age-related glaucoma. Here, to estimate and compare the levels of axonal loss with the expansion of glial coverage and axonal degeneration in DBA/2J nerves, we combined semiautomatic axon counts with threshold-based segmentation of total glial/scar areas and degenerative axonal profiles in plastic cross-sections. In nerves ranging from mild to severe degeneration, we found that the progression of axonal dropout is coupled to an increase of gliotic area. We detected a strong correlation between axon loss and the aggregate coverage by glial cells and scar, whereas axon loss did not correlate with the small fraction of degenerating profiles. Nerves with low to medium levels of axon loss displayed moderate glial reactivity, consisting of hypertrophic astrocytes, activated microglia and normal distribution of oligodendrocytes, with minimal reorganization of the tissue architecture. In contrast, nerves with extensive axonal loss showed prevalent rearrangement of the nerve, with loss of axon fascicle territories and enlarged or almost continuous gliotic and scar domains, containing reactive astrocytes, oligodendrocytes and activated microglia. These findings support the value of optic nerve gliotic expansion as a quantitative estimate of optic neuropathy that correlates with axon loss, applicable to grade the severity of optic nerve damage in mouse chronic glaucoma.

Effect of subconjunctival glucose on retinal ganglion cell survival in experimental retinal ischaemia and contrast sensitivity in human glaucoma

PURPOSE

This study aims to evaluate the effect of subconjunctival glucose on the retinal ganglion cells (RGCs) in experimental retinal ischaemia and contrast sensitivity in humans with primary open-angle glaucoma (POAG).

METHODS

First, we measured the intravitreal concentration of glucose at various time points after a subconjunctival injection of 100 μl of 50% glucose to Sprague-Dawley rats. Next, treatment and control groups received 50% subconjunctival glucose and iso-osmotic (8%) saline, respectively, 1 h prior to a unilateral ischaemic retinal injury; 7 days later, the damage profiles were compared using RGC and axon counts. Subsequently, we conducted a double-blind, crossover, pilot clinical study in seven eyes of five pseudophakic subjects with severe POAG. Subjects received either 0.3 mL of 50% glucose subconjunctivally or iso-osmotic (8%) saline, then vice versa after a 2-3 week 'wash-out' period; change in contrast sensitivity from baseline was the primary outcome.

RESULTS

Subconjunctival glucose preserved approximately 60% of Brn3a-positive RGCs in all retinal zones compared with an 80% loss in control retinas, and rescued approximately 40% of the axonal loss. In the human trial, the contrast sensitivity at 12 cycles/degree was 0.24 log units greater than baseline (95% confidence interval 0.12-0.36; P < 0.001).

CONCLUSIONS

Subconjunctival glucose partially protects RGC somata and axons against an ischaemic insult and temporarily recovers contrast sensitivity in patients with severe POAG. Although an unlikely therapeutic strategy for POAG, the findings motivate further bioenergetic-based research in glaucoma and other optic nerve and retinal diseases, where energy failure may be part of the pathogenesis.

Semi-automated counting method of axons in transmission electron microscopic images

OBJECTIVE

To evaluate the accuracy of a new semi-automated method for counting axons in transmission electron microscopic (TEM) images.

PROCEDURES

Optic nerve cross sections were obtained from both eyes of Sprague Dawley rats after unilateral induction of chronic ocular hypertension. TEM images (3000× magnification) of cross sections were evaluated by both semi-automated and manual counting methods. The semi-automated counting method was performed using ImageJ software after simple image optimization, and the resulting estimate of axon damage was compared with semiquantitative damage grading scale from light microscopic (LM) images.

RESULTS

Axon counts obtained from the semi-automated method were strongly correlated with those obtained from the manual counting method (Pearson's correlation coefficient r = 0.996, P < 0.001) and from the full manual count from LM images (Spearman's ρ = 0.973, P < 0.001). The semi-automated method measured axonal damage with an error of 0.94 ± 3.16% (mean ± standard deviation), with worse axonal damage associated with greater error. Interobserver and intra-observer variability in axons counts were low (Spearman's ρ = 0.999, P < 0.005). The results of the semi-automated counting method were highly correlated with semiquantitative damage grading scale (Spearman's ρ = 0.965, P < 0.001).

CONCLUSION

Results of our semi-automated method for counting axons in TEM images were strongly correlated with those of conventional counting methods and showed excellent reproducibility.

A rat experimental model of glaucoma incorporating rapid-onset elevation of intraocular pressure

Glaucoma is a chronic disease that causes structural and functional damage to retinal ganglion cells (RGC). The currently employed therapeutic options are not sufficient to prevent vision loss in patients with glaucoma; therefore, there is a need to develop novel therapies, which requires the creation of functional, repeatable and easy-to-utilize animal models for use in pre-clinical studies. The currently available models ensure only low to moderate damage in optic nerves, with high variation in the outcomes and poor repeatability. We have developed an effective and reproducible rat glaucoma model based on a previous idea for a "Bead Model" in mice, which could be useful in future glaucoma research. Additionally, in an attempt to achieve rapid elevation of Intraocular Pressure (IOP), we included an initial "high-pressure injury" as part of this method, which serves as the equivalent of a severe glaucoma attack. These modifications made it possible to achieve longer lasting IOP elevation with chronic damage of retinal ganglion cells.