Severe, Early Axonal Degeneration Following Experimental Anterior Ischemic Optic Neuropathy

Perivascular neurons instruct 3D vascular lattice formation via neurovascular contact

The vasculature of the central nervous system is a 3D lattice composed of laminar vascular beds interconnected by penetrating vessels. The mechanisms controlling 3D lattice network formation remain largely unknown. Combining viral labeling, genetic marking, and single-cell profiling in the mouse retina, we discovered a perivascular neuronal subset, annotated as Fam19a4/Nts-positive retinal ganglion cells (Fam19a4/Nts-RGCs), directly contacting the vasculature with perisomatic endfeet. Developmental ablation of Fam19a4/Nts-RGCs led to disoriented growth of penetrating vessels near the ganglion cell layer (GCL), leading to a disorganized 3D vascular lattice. We identified enriched PIEZO2 expression in Fam19a4/Nts-RGCs. Piezo2 loss from all retinal neurons or Fam19a4/Nts-RGCs abolished the direct neurovascular contacts and phenocopied the Fam19a4/Nts-RGC ablation deficits. The defective vascular structure led to reduced capillary perfusion and sensitized the retina to ischemic insults. Furthermore, we uncovered a Piezo2-dependent perivascular granule cell subset for cerebellar vascular patterning, indicating neuronal Piezo2-dependent 3D vascular patterning in the brain.

Impact of Acute Ocular Hypertension on Retinal Ganglion Cell Loss in Mice

Purpose

To assess the correlation between intraocular pressure (IOP) levels and retinal ganglion cell (RGC) loss across different fixed-duration episodes of acute ocular hypertension (AOH).

Methods

AOH was induced in Thy1-YFP-H transgenic mice by inserting a needle connected to a saline solution container into the anterior chamber. Thirty-one groups were tested, each comprising three to five mice exposed to IOP levels ranging from 50 to 110 mm Hg in 5/10 mm Hg increments for 60/90/120 minutes and a sham control group. The YFP-expressing RGCs were quantified by confocal scanning laser ophthalmoscopy, whereas peripapillary ganglion cell complex thickness was measured using spectral-domain optical coherence tomography. Changes in RGC count and GCCT were determined from values measured 30 days after AOH relative to baseline (before AOH).

Results

In the 60-minute AOH groups, RGC loss varied even when IOP was increased up to 110 mm Hg (36.8%-68.2%). However, for longer durations (90 and 120 minutes), a narrow range of IOP levels (60-70 mm Hg for 90-minute duration; 55-65 mm Hg for 120-minute duration) produced a significant difference in RGC loss, ranging from <25% to >90%. Additionally, loss of YFP-expressing RGCs was comparable to that of total RGCs in the same retinas.

Conclusions

Reproducible RGC loss during AOH depends on precise durations and IOP thresholds. In the current study, the optimal choice is an AOH protocol set at 70 mm Hg for a duration of 90 minutes.

Translational Relevance

This study can assist in determining the optimal duration and intensity of IOP for the effective utilization of AOH models.

The Molecular Mechanisms Involved in Axonal Degeneration and Retrograde Retinal Ganglion Cell Death

Axonal degeneration is a pathologic change common to multiple retinopathies and optic neuropathies. Various pathologic factors, such as mechanical injury, inflammation, and ischemia, can damage retinal ganglion cell (RGC) somas and axons, eventually triggering axonal degeneration and RGC death. The molecular mechanisms of somal and axonal degeneration are distinct but also overlap, and axonal degeneration can result in retrograde somal degeneration. While the mitogen-activated protein kinase pathway acts as a central node in RGC axon degeneration, several newly discovered molecules, such as sterile alpha and Toll/interleukin-1 receptor motif-containing protein 1 and nicotinamide mononucleotide adenylyltransferase 2, also play a critical role in this pathological process following different types of injury. Therefore, we summarize the types of injury that cause RGC axon degeneration and retrograde RGC death and important underlying molecular mechanisms, providing a reference for the identification of targets for protecting axons and RGCs.

The Susceptibility of Retinal Ganglion Cells to Optic Nerve Injury is Type Specific

Retinal ganglion cell (RGC) death occurs in many eye diseases, such as glaucoma and traumatic optic neuropathy (TON). Increasing evidence suggests that the susceptibility of RGCs varies to different diseases in an RGC type-dependent manner. We previously showed that the susceptibility of several genetically identified RGC types to N-methyl-D-aspartate (NMDA) excitotoxicity differs significantly. In this study, we characterize the susceptibility of the same RGC types to optic nerve crush (ONC). We show that the susceptibility of these RGC types to ONC varies significantly, in which BD-RGCs are the most resistant RGC type while W3-RGCs are the most sensitive cells to ONC. We also show that the survival rates of BD-RGCs and J-RGCs after ONC are significantly higher than their survival rates after NMDA excitotoxicity. These results are consistent with the conclusion that the susceptibility of RGCs to ONC varies in an RGC type-dependent manner. Further, the susceptibilities of the same types of RGCs to ONC and NMDA excitotoxicity are significantly different. These are valuable insights for understanding of the selective susceptibility of RGCs to various pathological insults and the development of a strategy to protect RGCs from death in disease conditions.

Prospective analysis of macular and optic disc changes after non-arteritic anterior ischemic optic neuropathy

PURPOSE

To prospectively analyse macular and optic disc changes after the occurrence of non-arteritic anterior ischemic optic neuropathy (NAION) and study possible predictors of final visual outcome.

METHODS

Patients with NAION underwent a complete ophthalmic examination, including spectral-domain optical coherence tomography of the macula and optic nerve head. The examination was repeated 1, 3, 6, 9 and 12 months after onset. Final visual prognosis was evaluated by visual field (VF) and best-corrected visual acuity (BCVA) at the final visit. Data within the NAION group were analysed over the course of the disease and compared to a disease-free control group at each visit.

RESULTS

Twenty-two eyes with NAION and 43 eyes from a control group were included. The retinal nerve fiber layer (RNFL) was significantly thicker in NAION eyes than controls at presentation (P=0.00), and significantly decreased during the next 3 months after presentation (P=0.02). The ganglion cell+inner plexiform layer (GCIPL) was thinner in the NAION group throughout the course of the disease (all P<0.05). Although the acute NAION eyes had significantly lower cup/disc ratios and higher neuroretinal and disc sizes (all P=0.00), there were no significant differences between groups from the third month onwards (all P>0.05). The best predictors of BCVA and VF were GCIPL at 3 months of follow-up (r²=0.32; P=0.03) and RNFL at 6 months of follow-up (r²=0.41; P=0.01) respectively.

CONCLUSIONS

RNFL and optic disc changes occur during the first 3 months after the onset of NAION, whereas GCIPL is affected soon after the onset of symptoms. GCIPL and RNFL are useful predictors of final visual outcome.

A Small Molecule TrkB Neurotrophin Receptor Partial Agonist as Possible Treatment for Experimental Nonarteritic Anterior Ischemic Optic Neuropathy

PURPOSE

Brain-derived neurotrophic factor (BDNF) and activation of its high affinity receptor tropomyosin kinase (Trk) B promote retinal ganglion cells (RGCs) survival following injury. In this study, we tested the effects of LM22A-4, a small molecule TrkB receptor-specific partial agonist, on RGC survival in vitro and in experimental nonarteritic anterior ischemic optic neuropathy (AION), the most common acute optic neuropathy in those older than 50 years.

METHODS

We assessed drug effects on immunopanned, cultured RGCs and calculated RGC survival and assessed TrkB receptor activation by mitogen-activated protein (MAP) kinase translocation. To assess effects in vivo, we induced murine AION and treated the animals with one intravitreal injection and three-week systemic treatment. We measured drug effects using serial spectral-domain optical coherence tomography (OCT) and quantified retinal Brn3A+ RGC density three weeks after ischemia.

RESULTS

In vitro, LM22A-4 significantly increased the survival of cultured RGCs at day 2 (95% CI control: 8.4-13.6; LM22A-4: 23.7-30.3; BDNF: 24.3-29.9; P ≤ 0.0001), similar to the effect of the endogenous TrkB receptor ligand BDNF. There was also significant nuclear and cytoplasmic translocation of MAP kinase (95% CI control: 0.9-6.8; LM22A-4: 38.8-84.4; BDNF: 64.0-93.0; P = 0.0002), a known downstream event of TrkB receptor activation. Following AION, LM22A-4 treatment led to significant preservation of the ganglion cell complex (95% CI: AION-PBS: 66.8-70.7%; AION-LM22A-4: 70.0-73.1; P = 0.03) and total retinal thickness (95% CI: AION-PBS: 185-196%; AION-LM22A-4: 195-203; P = 0.002) as measured by OCT compared with non-treated eyes. There was also significant rescue of the Brn3A+ RGC density on morphometric analysis of whole mount retinae (95% CI control: 2360-2629; AION-PBS: 1647-2008 cells/mm2; AION-LM22A-4: 1958-2216 cells/mm2; P = 0.02).

CONCLUSIONS

TrkB receptor partial agonist LM22A-4 promoted survival of cultured RGCs in vitro by TrkB receptor activation, and treatment in vivo led to increased survival of RGCs after optic nerve ischemia, providing support that LM22A-4 may be effective therapy to treat ischemic optic neuropathy.

ABBREVIATIONS

AION: anterior ischemic optic neuropathy, BDNF: Brain-derived neurotrophic factor, GCC: ganglion cell complex, MAP: mitogen-activated protein, OCT: spectral-domain optical coherence tomography, OD: right eye, ON: optic nerve, ONH: optic nerve head, OS: left eye, RGC: retinal ganglion cell; Trk: tropomyosin kinase.

The Effect of Combined Systemic Erythropoietin and Steroid on Non-arteritic Anterior Ischemic Optic Neuropathy: A Prospective Study

BACKGROUND

To investigate the effect of combined intravenous (IV) erythropoietin (EPO) and corticosteroid as well as systemic steroid alone for the treatment of non-arteritic anterior ischemic optic neuropathy (NAION).

METHODS

In this prospective interventional comparative case series, 113 consecutive patients diagnosed with recent onset (less than 14 days) NAION were included. Patients were categorized into three groups. 40 patients received systemic IV corticosteroid combined with recombinant human erythropoietin (rhEPO) (group 1), 43 patients received systemic corticosteroid alone (group 2), and 30 patients were enrolled as the control group (group 3). Functional and structural outcomes were analyzed 3 and 6 months after treatment. Best corrected visual acuity (BCVA) was the main outcome, and mean deviation (MD) and peripaillary retinal nerve fiber layer thickness (PRNFLT) were secondary outcome measures.

RESULTS

The mean BCVA at the time of presentation was 0.98 (±0.65), 0.96 (±0.67), and 1.02 (±0.63) log MAR in groups 1, 2, and 3, respectively (P = 0.95). At month 3, the corresponding values were 0.73 (±0.45), 0.76 (±0.49), and 0.8 (±0.45) log MAR (P = 0.80), and at the 6-month follow-up, they were 0.76 (±0.45), 0.71 (±0.4), and 0.71 (±0.46) log MAR, respectively (P = 0.87). There was no statistically significant difference in BCVA between months 3 and 6, which implies stabilization of the visual acuity by month 3. Considering the visual field, within 6 months of follow-up after disease onset, the MD index improved in all groups with no statistically significant differences between them (P = 0.82). PRNFLT at presentation was 178 (±60), 186 (±59), and 166 (±57) micrometers in groups 1, 2, and 3, respectively (P= 0.99), which decreased to 77 (±16), 83 (±22), and 73 (±11), respectively, at final visit (P = 0.14) Conclusion: We found no beneficial effect of either systemic steroid alone or combined with EPO in the visual outcome of NAION patients.

Distinguishing ischaemic optic neuropathy from optic neuritis by ganglion cell analysis

PURPOSE

To determine whether a pattern of altitudinal ganglion cell loss, as detected and measured by optical coherence tomography (OCT), can be used to distinguish non-arteritic ischaemic optic neuropathy (NAION) from optic neuritis (ON) during the acute phase, and whether the rate or severity of ganglion cell loss differs between the two diseases.

METHODS

We performed a retrospective, case-control study of 44 patients (50 eyes) with ON or NAION and 44 age-matched controls. Non-arteritic ischaemic optic neuropathy and ON patients had OCT at presentation and four consecutive follow-up visits. Controls had OCT at one point in time. The ganglion cell complex (GCC) was evaluated in the macula, and the retinal nerve fibre layer (RNFL) was evaluated in the peripapillary region. Ganglion cell complex thickness, RNFL thickness and GCC mean superior and inferior hemispheric difference were compared between NAION and ON patients at each time-point using unpaired t-tests and between disease and control subjects at first measurement using paired t-tests.

RESULTS

Mean time from onset of symptoms to initial presentation was 10.7 ± 6.6 days in NAION and 11.7 ± 8.6 days in ON (p = 0.67). There was a significantly greater vertical hemispheric difference in GCC thickness in NAION patients than ON patients at all time-points (5.5-10.7 μm versus 3.1-3.6 μm, p = 0.01-0.049). Mean GCC thickness was significantly decreased at less than 2 weeks after onset in NAION compared to age-matched controls (72.1 μm versus 82.1 μm, p < 0.001), as well as in ON compared to age-matched controls (74.3 μm versus 84.5 μm, p < 0.001). Progression and severity of GCC and RNFL loss did not differ significantly between NAION and ON.

CONCLUSION

A quantitative comparison of mean superior and inferior hemispheric GCC thickness with OCT may be used to distinguish NAION from ON.

Early applications of granulocyte colony-stimulating factor (G-CSF) can stabilize the blood-optic-nerve barrier and ameliorate inflammation in a rat model of anterior ischemic optic neuropathy (rAION)

Granulocyte colony-stimulating factor (G-CSF) was reported to have a neuroprotective effect in a rat model of anterior ischemic optic neuropathy (rAION model). However, the therapeutic window and anti-inflammatory effects of G-CSF in a rAION model have yet to be elucidated. Thus, this study aimed to determine the therapeutic window of G-CSF and investigate the mechanisms of G-CSF via regulation of optic nerve (ON) inflammation in a rAION model. Rats were treated with G-CSF on day 0, 1, 2 or 7 post-rAION induction for 5 consecutive days, and a control group were treated with phosphate-buffered saline (PBS). Visual function was assessed by flash visual evoked potentials at 4 weeks post-rAION induction. The survival rate and apoptosis of retinal ganglion cells were determined by FluoroGold labeling and TUNEL assay, respectively. ON inflammation was evaluated by staining of ED1 and Iba1, and ON vascular permeability was determined by Evans Blue extravasation. The type of macrophage polarization was evaluated using quantitative real-time PCR (qRT-PCR). The protein levels of TNF-α and IL-1β were analyzed by western blotting. A therapeutic window during which G-CSF could rescue visual function and retinal ganglion cell survival was demonstrated at day 0 and day 1 post-infarct. Macrophage infiltration was reduced by 3.1- and 1.6-fold by G-CSF treatment starting on day 0 and 1 post-rAION induction, respectively, compared with the PBS-treated group (P<0.05). This was compatible with 3.3- and 1.7-fold reductions in ON vascular permeability after G-CSF treatment compared with PBS treatment (P<0.05). Microglial activation was increased by 3.8- and 3.2-fold in the early (beginning treatment at day 0 or 1) G-CSF-treated group compared with the PBS-treated group (P<0.05). Immediate (within 30 mins of infarct) treatment with G-CSF also induced M2 microglia/macrophage activation. The cytokine levels were lower in the group that received immediate G-CSF treatment compared to those in the later G-CSF treatment group (P<0.05). Early treatment with G-CSF stabilized the blood-ON barrier to reduce macrophage infiltration and induced M2 microglia/macrophage polarization to decrease the expressions of pro-inflammatory cytokines in this rAION model.

Time Course of Macular and Peripapillary Inner Retinal Thickness in Non-arteritic Anterior Ischaemic Optic Neuropathy Using Spectral-Domain Optical Coherence Tomography

To report a time course of the ganglion cell complex (GCC) and circumpapillary retinal nerve fibre layer (cpRNFL) thicknesses using spectral-domain optical coherence tomography in patients with non-arteritic anterior ischaemic optic neuropathy (NAION), five patients with unilateral NAION were studied (the average age of 66.8 ± 7.8 years old). Forty-one age-matched normal controls were also enrolled. The GCC and cpRNFL thicknesses were measured at the initial visit and at 1, 3, 6, and 12 months using RTVue-100. The GCC thickness and the cpRNFL thickness of the patients were compared with those of the normal controls. The GCC thickness in the NAION patients was 96.49 μm at the initial visit, 84.28 μm at 1 month, 74.26 μm at 3 months, 71.23 μm at 6 months, and 69.51 μm at 12 months. The values at 1, 3, 6, and 12 months were significantly reduced (p < 0.01). The cpRNFL thickness at the initial visit was significantly increased, whereas the values at 6 and 12 months were significantly reduced (p < 0.01). The GCC thickness is more useful for the detection of retinal ganglion cell loss at an early stage than the cpRNFL thickness, because the GCC thickness is unaffected by optic disc swelling at the initial visit, unlike the cpRNFL thickness.

Assessment of retinal ganglion cell damage in glaucomatous optic neuropathy: Axon transport, injury and soma loss

Glaucoma is a disease characterized by progressive axonal pathology and death of retinal ganglion cells (RGCs), which causes structural changes in the optic nerve head and irreversible vision loss. Several experimental models of glaucomatous optic neuropathy (GON) have been developed, primarily in non-human primates and, more recently and commonly, in rodents. These models provide important research tools to study the mechanisms underlying glaucomatous damage. Moreover, experimental GON provides the ability to quantify and monitor risk factors leading to RGC loss such as the level of intraocular pressure, axonal health and the RGC population. Using these experimental models we are able to gain a better understanding of GON, which allows for the development of potential neuroprotective strategies. Here we review the advantages and disadvantages of the relevant and most often utilized methods for evaluating axonal degeneration and RGC loss in GON. Axonal pathology in GON includes functional disruption of axonal transport (AT) and structural degeneration. Horseradish peroxidase (HRP), rhodamine-B-isothiocyanate (RITC) and cholera toxin-B (CTB) fluorescent conjugates have proven to be effective reporters of AT. Also, immunohistochemistry (IHC) for endogenous AT-associated proteins is often used as an indicator of AT function. Similarly, structural degeneration of axons in GON can be investigated via changes in the activity and expression of key axonal enzymes and structural proteins. Assessment of axonal degeneration can be measured by direct quantification of axons, qualitative grading, or a combination of both methods. RGC loss is the most frequently quantified variable in studies of experimental GON. Retrograde tracers can be used to quantify RGC populations in rodents via application to the superior colliculus (SC). In addition, in situ IHC for RGC-specific proteins is a common method of RGC quantification used in many studies. Recently, transgenic mouse models that express fluorescent proteins under the Thy-1 promoter have been examined for their potential to provide specific and selective labeling of RGCs for the study of GON. While these methods represent important advances in assessing the structural and functional integrity of RGCs, each has its advantages and disadvantages; together they provide an extensive toolbox for the study of GON.