Persistence of intact retinal ganglion cell terminals after axonal transport loss in the DBA/2J mouse model of glaucoma

A novel three-dimensional method for detailed analysis of RGC central projections under acute ocular hypertension

Normal perception of visual information relies not only on the quantity and quality of retinal ganglion cells (RGCs), but also on the integrity of the visual pathway, within which RGC central projection predominates. However, the exact changes of RGC central projection under particular pathological conditions remain to be elucidated. Here, we report a whole-brain clearing method modified from iDISCO for 3D visualization of RGC central projection. The CTB-labeled RGC central projection was visualized three-dimensionally with minimized both fluorescence quenching and the time taken. For observation of RGC axonal degeneration pattern under pathological conditions, we took acute ocular hypertension (AOH) as an example. Mice were intracamerally irrigated, and fluorescent signal in brain subregions where RGC axons projected to were quantified. The novel methodology is well-applied for rapid clearing and observation of RGC central projection in C57BL/6J, showing damaged RGC central projection on the AOH side and the most statistically significant degeneration in the superior colliculi (SC). Detailed analysis also revealed a distinct injury pattern among lateral geniculate nuclei (LGN) subregions, with the parvocellular part of the pregeniculate nuclei (PrGPC) being more vulnerable compared with the magnocellular part (PrGMC). The intracranial retrograde labeling of RGC subgroups based on brain damage variation showed PrGPC-projecting RGCs (Plgn RGC) being smaller than PrGMC-projecting RGCs (Mlgn RGC) in size and less in number, yet more vulnerable in terms of degeneration under AOH. Our data revealed the methodology for visualizing selective neuronal vulnerability under AOH, and in the meantime provided novel approach for future mechanisms exploration regarding RGC degeneration.

Enhanced Synaptic Inhibition in the Dorsolateral Geniculate Nucleus in a Mouse Model of Glaucoma

Elevated intraocular pressure (IOP) triggers glaucoma by damaging the output neurons of the retina called retinal ganglion cells (RGCs). This leads to the loss of RGC signaling to visual centers of the brain such as the dorsolateral geniculate nucleus (dLGN), which is critical for processing and relaying information to the cortex for conscious vision. In response to altered levels of activity or synaptic input, neurons can homeostatically modulate postsynaptic neurotransmitter receptor numbers, allowing them to scale their synaptic responses to stabilize spike output. While prior work has indicated unaltered glutamate receptor properties in the glaucomatous dLGN, it is unknown whether glaucoma impacts dLGN inhibition. Here, using DBA/2J mice, which develop elevated IOP beginning at 6-7 months of age, we tested whether the strength of inhibitory synapses on dLGN thalamocortical relay neurons is altered in response to the disease state. We found an enhancement of feedforward disynaptic inhibition arising from local interneurons along with increased amplitude of quantal inhibitory synaptic currents. A combination of immunofluorescence staining for the γ-aminobutyric acid (GABA)A-α1 receptor subunit, peak-scaled nonstationary fluctuation analysis, and measures of homeostatic synaptic scaling pointed to an ∼1.4-fold increase in GABA receptors at postsynaptic inhibitory synapses, although several pieces of evidence indicate a nonuniform scaling across inhibitory synapses within individual relay neurons. Together, these results indicate an increase in inhibitory synaptic strength in the glaucomatous dLGN, potentially pointing toward homeostatic compensation for disruptions in network and neuronal function triggered by increased IOP.

Nonuniform scaling of synaptic inhibition in the dorsolateral geniculate nucleus in a mouse model of glaucoma

Elevated intraocular pressure (IOP) triggers glaucoma by damaging the output neurons of the retina called retinal ganglion cells (RGCs). This leads to the loss of RGC signaling to visual centers of the brain such as the dorsolateral geniculate nucleus (dLGN), which is critical for processing and relaying information to the cortex for conscious vision. In response to altered levels of activity or synaptic input, neurons can homeostatically modulate postsynaptic neurotransmitter receptor numbers, allowing them to scale their synaptic responses to stabilize spike output. While prior work has indicated unaltered glutamate receptor properties in the glaucomatous dLGN, it is unknown whether glaucoma impacts dLGN inhibition. Here, using DBA/2J mice, which develop elevated IOP beginning at 6-7 months of age, we tested whether the strength of inhibitory synapses on dLGN thalamocortical relay neurons is altered in response to the disease state. We found an enhancement of feed-forward disynaptic inhibition arising from local interneurons along with increased amplitude of quantal inhibitory synaptic currents. A combination of immunofluorescence staining for the GABA A -α1 receptor subunit, peak-scaled nonstationary fluctuation analysis, and measures of homeostatic synaptic scaling indicated this was the result of an approximately 1.4-fold increase in GABA receptor number at post-synaptic inhibitory synapses, although several pieces of evidence strongly indicate a non-uniform scaling across inhibitory synapses within individual relay neurons. Together, these results indicate an increase in inhibitory synaptic strength in the glaucomatous dLGN, potentially pointing toward homeostatic compensation for disruptions in network and neuronal function triggered by increased IOP.

Significance Statement

Elevated eye pressure in glaucoma leads to loss of retinal outputs to the dorsolateral geniculate nucleus (dLGN), which is critical for relaying information to the cortex for conscious vision. Alterations in neuronal activity, as could arise from excitatory synapse loss, can trigger homeostatic adaptations to synaptic function that attempt to maintain activity within a meaningful dynamic range, although whether this occurs uniformly at all synapses within a given neuron or is a non-uniform process is debated. Here, using a mouse model of glaucoma, we show that dLGN inhibitory synapses undergo non-uniform upregulation due to addition of post-synaptic GABA receptors. This is likely to be a neuronal adaptation to glaucomatous pathology in an important sub-cortical visual center.

Loss of Retinogeniculate Synaptic Function in the DBA/2J Mouse Model of Glaucoma

Retinal ganglion cell (RGC) axons comprise the optic nerve and carry information to the dorsolateral geniculate nucleus (dLGN), which is then relayed to the cortex for conscious vision. Glaucoma is a blinding neurodegenerative disease that commonly results from intraocular pressure (IOP)-associated injury leading to RGC axonal pathology, disruption of RGC outputs to the brain, and eventual apoptotic loss of RGC somata. The consequences of elevated IOP and glaucomatous pathology on RGC signaling to the dLGN are largely unknown yet are likely to contribute to vision loss. Here, we used anatomic and physiological approaches to study the structure and function of retinogeniculate (RG) synapses in male and female DBA/2J (D2) mice with inherited glaucoma before and after IOP elevation. D2 mice showed progressive loss of anterograde optic tract transport to the dLGN and vGlut2 labeling of RGC axon terminals while patch-clamp measurements of RG synaptic function showed that synaptic transmission was reduced in 9-month and 12-month D2 mice because of the loss of individual RGC axon inputs. TC neuron dendrites had reduced Sholl complexity at 12 months, suggestive of delayed reorganization following reduced synaptic input. There was no detectable change in RGC density in 11- to 12-month D2 retinas, quantified as the number of ganglion cell layer-residing somata immuno-positive for NeuN and immuno-negative for the amacrine marker choline acetyltransferase (ChAT). Thus, observed synaptic defects appear to precede RGC somatic loss. These findings identify glaucoma-associated and IOP-associated deficits in an important subcortical RGC projection target, shedding light on processes linking IOP to vision loss.

Age-related ultrastructural changes in the lateral cortex of the inferior colliculus

Temporal precision, a key component of sound and speech processing in the inferior colliculus (IC), depends on a balance of inhibition and excitation, and this balance degrades during aging. The cause of disrupted excitatory-inhibitory balance in aging is unknown, however changes at the synapse are a likely candidate. We sought to determine whether synaptic changes occur in the lateral cortex of the IC (IClc), a multimodal nucleus that processes lemniscal, intrinsic, somatosensory, and descending auditory input. Using electron microscopic techniques across young, middle age and old Fisher Brown Norway rats, our results demonstrate minimal loss of synapses in middle age, but significant (∼28%) loss during old age. However, in middle age, targeting of GABAergic dendrites by GABAergic synapses is increased and the active zones of excitatory synapses (that predominantly target GABA-negative dendrites) are lengthened. These synaptic changes likely result in a net increase of excitation in the IClc during middle age. Thus, disruption of excitatory-inhibitory balance in the aging IClc may be due to synaptic changes that begin in middle age.

Influences of Glaucoma on the Structure and Function of Synapses in the Visual System

Significance: Glaucoma is an age-related neurodegenerative disorder of the visual system associated with sensitivity to intraocular pressure (IOP). It is the leading irreversible cause of vision loss worldwide, and vision loss results from damage and dysfunction of the retinal output neurons known as retinal ganglion cells (RGCs). Recent Advances: Elevated IOP and optic nerve injury triggers pruning of RGC dendrites, altered morphology of excitatory inputs from presynaptic bipolar cells, and disrupted RGC synaptic function. Less is known about RGC outputs, although evidence to date indicates that glaucoma is associated with altered mitochondrial and synaptic structure and function in RGC-projection targets in the brain. These early functional changes likely contribute to vision loss and might be a window into early diagnosis and treatment. Critical Issues: Glaucoma affects different RGC populations to varying extents and along distinct time courses. The influence of glaucoma on RGC synaptic function as well as the mechanisms underlying these effects remain to be determined. Since RGCs are an especially energetically demanding population of neurons, altered intracellular axon transport of mitochondria and mitochondrial function might contribute to RGC synaptic dysfunction in the retina and brain as well as RGC vulnerability in glaucoma. Future Directions: The mechanisms underlying differential RGC vulnerability remain to be determined. Moreover, the timing and mechanisms of RGCs synaptic dysfunction and degeneration will provide valuable insight into the disease process in glaucoma. Future work will be able to capitalize on these findings to better design diagnostic and therapeutic approaches to detect disease and prevent vision loss. Antioxid. Redox Signal. 37, 842-861.

Intrinsically photosensitive retinal ganglion cells in glaucoma

Glaucoma is a group of eye diseases afflicting more than 70 million people worldwide. It is characterized by damage to retinal ganglion cells (RGCs) that ultimately leads to the death of the cells and vision loss. The diversity of RGC types has been appreciated for decades, and studies, including ours, have shown that RGCs degenerate and die in a type-specific manner in rodent models of glaucoma. The type-specific loss of RGCs results in differential damage to visual and non-visual functions. One type of RGC, the intrinsically photosensitive retinal ganglion cell (ipRGC), expressing the photopigment melanopsin, serves a broad array of non-visual responses to light. Since its discovery, six subtypes of ipRGC have been described, each contributing to various image-forming and non-image-forming functions such as circadian photoentrainment, the pupillary light reflex, the photic control of mood and sleep, and visual contrast sensitivity. We recently demonstrated a link between type-specific ipRGC survival and behavioral deficits in a mouse model of chronic ocular hypertension. This review focuses on the type-specific ipRGC degeneration and associated behavioral changes in animal models and glaucoma patients. A better understanding of how glaucomatous insult impacts the ipRGC-based circuits will have broad impacts on improving the treatment of glaucoma-associated non-visual disorders.

Alterations in Retinal Signaling Across Age and Sex in 3xTg Alzheimer’s Disease Mice

Background:

Visual disturbances often precede cognitive dysfunction in patients with Alzheimer’s disease (AD) and may coincide with early accumulation of amyloid-β (Aβ) protein in the retina. These findings have inspired critical research on in vivo ophthalmic Aβ imaging for disease biomarker detection but have not fully answered mechanistic questions on how retinal pathology affects visual signaling between the eye and brain.

Objective:

The goal of this study was to provide a functional and structural assessment of eye-brain communication between retinal ganglion cells (RGCs) and their primary projection target, the superior colliculus, in female and male 3xTg-AD mice across disease stages.

Methods:

Retinal electrophysiology, axonal transport, and immunofluorescence were used to determine RGC projection integrity, and retinal and collicular Aβ levels were assessed with advanced protein quantitation techniques.

Results:

3xTg mice exhibited nuanced deficits in RGC electrical signaling, axonal transport, and synaptic integrity that exceeded normal age-related decrements in RGC function in age- and sex-matched healthy control mice. These deficits presented in sex-specific patterns among 3xTg mice, differing in the timing and severity of changes.

Conclusion:

These data support the premise that retinal Aβ is not just a benign biomarker in the eye, but may contribute to subtle, nuanced visual processing deficits. Such disruptions might enhance the biomarker potential of ocular amyloid and differentiate patients with incipient AD from patients experiencing normal age-related decrements in visual function.

The Role of Axonal Transport in Glaucoma

Glaucoma is a neurodegenerative disease that affects the retinal ganglion cells (RGCs) and leads to progressive vision loss. The first pathological signs can be seen at the optic nerve head (ONH), the structure where RGC axons leave the retina to compose the optic nerve. Besides damage of the axonal cytoskeleton, axonal transport deficits at the ONH have been described as an important feature of glaucoma. Axonal transport is essential for proper neuronal function, including transport of organelles, synaptic components, vesicles, and neurotrophic factors. Impairment of axonal transport has been related to several neurodegenerative conditions. Studies on axonal transport in glaucoma include analysis in different animal models and in humans, and indicate that its failure happens mainly in the ONH and early in disease progression, preceding axonal and somal degeneration. Thus, a better understanding of the role of axonal transport in glaucoma is not only pivotal to decipher disease mechanisms but could also enable early therapies that might prevent irreversible neuronal damage at an early time point. In this review we present the current evidence of axonal transport impairment in glaucomatous neurodegeneration and summarize the methods employed to evaluate transport in this disease.

Structural and functional plasticity in the dorsolateral geniculate nucleus of mice following bilateral enucleation

Within the nervous system, plasticity mechanisms attempt to stabilize network activity following disruption by injury, disease, or degeneration. Optic nerve injury and age-related diseases can induce homeostatic-like responses in adulthood. We tested this possibility in the thalamocortical (TC) neurons in the dorsolateral geniculate nucleus (dLGN) using patch-clamp electrophysiology, optogenetics, immunostaining, and single-cell dendritic analysis following loss of visual input via bilateral enucleation. We observed progressive loss of vGlut2-positive retinal terminals in the dLGN indicating degeneration post-enucleation that was coincident with changes in microglial morphology indicative of microglial activation. Consistent with the decline of vGlut2 puncta, we also observed loss of retinogeniculate (RG) synaptic function assessed using optogenetic activation of RG axons while performing whole-cell voltage clamp recordings from TC neurons in brain slices. Surprisingly, we did not detect any significant changes in the frequency of miniature post-synaptic currents (mEPSCs) or corticothalamic feedback synapses. Analysis of TC neuron dendritic structure from single-cell dye fills revealed a gradual loss of dendrites proximal to the soma, where TC neurons receive the bulk of RG inputs. Finally, analysis of action potential firing demonstrated that TC neurons have increased excitability following enucleation, firing more action potentials in response to depolarizing current injections. Our findings show that degeneration of the retinal axons/optic nerve and loss of RG synaptic inputs induces structural and functional changes in TC neurons, consistent with neuronal attempts at compensatory plasticity in the dLGN.

Absence of Cytochrome P450-1b1 Increases Susceptibility of Pressure-Induced Axonopathy in the Murine Retinal Projection

Mutations in the cytochrome P450-1B1 (Cyp1b1) gene is a common genetic predisposition associated with various human glaucomas, most prominently in primary congenital glaucoma (PCG). The role of Cyp1b1 in the eye is largely unknown, however, its absence appears to drive the maldevelopment of anterior eye structures responsible for aqueous fluid drainage in murine models. Nevertheless, vision loss in glaucoma ultimately results from the structural and functional loss of retinal ganglion cells (RGCs). Cyp1b1’s influence in the development and support of retinal ganglion cell structure and function under normal conditions or during stress, such as elevated ocular pressure; the most common risk factor in glaucoma, remains grossly unknown. Thus, to determine the role of Cyp1b1 in normal retinal projection development we first assessed the strucutrual integrity of RGCs in the retina, optic nerve, and superior colliculus in un-manipulated (naïve) Cyp1b1-knockout (Cyp1b1^–/–) mice. In addition, in a separate cohort of Cyp1b1^–/– and wildtype mice, we elevated and maintained intraocular pressure (IOP) at glaucomatous levels for 5-weeks, after which we compared RGC density, node of Ranvier morphology, and axonal transport between the genotypes. Our results demonstrate that naïve Cyp1b1^–/– mice develop an anatomically intact retinal projection absent of overt glaucomatous pathology. Following pressure elevation, Cyp1b1^–/– accelerated degradation of axonal transport from the retina to the superior colliculus and altered morphology of the nodes of Ranvier and adjacent paranodes in the optic nerves. Together this data suggests the absence Cyp1b1 expression alone is insufficient to drive murine glaucomatous pathology, however, may increase the vulnerability of retinal axons to disease relevant elevations in IOP.

Adaptive responses to neurodegenerative stress in glaucoma

Glaucoma causes loss of vision through degeneration of the retinal ganglion cell (RGC) projection to the brain. The disease is characterized by sensitivity to intraocular pressure (IOP) conveyed at the optic nerve head, through which RGC axons pass unmyelinated to form the optic nerve. From this point, a pathogenic triumvirate comprising inflammatory, oxidative, and metabolic stress influence both proximal structures in the retina and distal structures in the optic projection. This review focuses on metabolic stress and how the optic projection may compensate through novel adaptive mechanisms to protect excitatory signaling to the brain. In the retina and proximal nerve head, the unmyelinated RGC axon segment is energy-inefficient, which leads to increased demand for adenosine-5'-triphosphate (ATP) at the risk of vulnerability to Ca²⁺-related metabolic and oxidative pressure. This vulnerability may underlie the bidirectional nature of progression. However, recent evidence highlights that the optic projection in glaucoma is not passive but rather demonstrates adaptive processes that may push back against neurodegeneration. In the retina, even as synaptic and dendritic pruning ensues, early progression involves enhanced excitability of RGCs. Enhancement involves depolarization of the resting membrane potential and increased response to light, independent of RGC morphological type. This response is axogenic, arising from increased levels and translocation of voltage-gated sodium channels (NaV) in the unmyelinated segment. During this same early period, large-scale networks of gap-junction coupled astrocytes redistribute metabolic resources to the optic projection stressed by elevated IOP to slow loss of axon function. This redistribution may reflect more local remodeling, as astrocyte processes respond to focal metabolic duress by boosting glycogen turnover in response to axonal activity in an effort to promote survival of the healthiest axons. Both enhanced excitability and metabolic redistribution are transient, indicating that the same adaptive mechanisms that apparently serve to slow progression ultimately may be too expensive for the system to sustain over longer periods.

Neuronal and Synaptic Plasticity in the Visual Thalamus in Mouse Models of Glaucoma

Homeostatic plasticity plays important role in regulating synaptic and intrinsic neuronal function to stabilize output following perturbations to circuit activity. In glaucoma, a neurodegenerative disease of the visual system commonly associated with elevated intraocular pressure (IOP), the early disease is associated with altered synaptic inputs to retinal ganglion cells (RGCs), changes in RGC intrinsic excitability, and deficits in optic nerve transport and energy metabolism. These early functional changes can precede RGC degeneration and are likely to alter RGC outputs to their target structures in the brain and thereby trigger homeostatic changes in synaptic and neuronal properties in those brain regions. In this study, we sought to determine whether and how neuronal and synaptic function is altered in the dorsal lateral geniculate nucleus (dLGN), an important RGC projection target in the thalamus, and how functional changes related to IOP. We accomplished this using patch-clamp recordings from thalamocortical (TC) relay neurons in the dLGN in two established mouse models of glaucoma—the DBA/2J (D2) genetic mouse model and an inducible glaucoma model with intracameral microbead injections to elevate IOP. We found that the intrinsic excitability of TC neurons was enhanced in D2 mice and these functional changes were mirrored in recordings of TC neurons from microbead-injected mice. Notably, many neuronal properties were correlated with IOP in older D2 mice, when IOP rises. The frequency of miniature excitatory synaptic currents (mEPSCs) was reduced in 9-month-old D2 mice, and vGlut2 staining of RGC synaptic terminals was reduced in an IOP-dependent manner. These data suggest that glaucoma-associated changes to neuronal excitability and synaptic inputs in the dLGN might represent a combination of both stabilizing/homeostatic plasticity and pathological dysfunction.

A broad perspective on the molecular regulation of retinal ganglion cell degeneration in glaucoma

Glaucoma is a complex neurodegenerative disease involving RGC axons, somas, and synapses at dendrites and axon terminals. Recent research advancements in the field have revealed a bigger picture of glaucomatous neurodegeneration that encompasses multiple stressors, multiple injury sites, multiple cell types, and multiple signaling pathways for asynchronous degeneration of RGCs during a chronic disease period. Optic nerve head is commonly viewed as the critical site of injury in glaucoma, where early injurious insults initiate distal and proximal signaling for axonal and somatic degeneration. Despite compartmentalized processes for degeneration of RGC axons and somas, there are intricate interactions between the two compartments and mechanistic overlaps between the molecular pathways that mediate degeneration in axonal and somatic compartments. This review summarizes the recent progress in the molecular understanding of RGC degeneration in glaucoma and highlights various etiological paths with biomechanical, metabolic, oxidative, and inflammatory components. Through this growing body of knowledge, the glaucoma community moves closer toward causative treatment of this blinding disease.

Early-Stage Ocular Hypertension Alters Retinal Ganglion Cell Synaptic Transmission in the Visual Thalamus

Axonopathy is a hallmark of many neurodegenerative diseases including glaucoma, where elevated intraocular pressure (ocular hypertension, OHT) stresses retinal ganglion cell (RGC) axons as they exit the eye and form the optic nerve. OHT causes early changes in the optic nerve such as axon atrophy, transport inhibition, and gliosis. Importantly, many of these changes appear to occur prior to irreversible neuronal loss, making them promising points for early diagnosis of glaucoma. It is unknown whether OHT has similarly early effects on the function of RGC output to the brain. To test this possibility, we elevated eye pressure in mice by anterior chamber injection of polystyrene microbeads. Five weeks post-injection, bead-injected eyes showed a modest RGC loss in the peripheral retina, as evidenced by RBPMS antibody staining. Additionally, we observed reduced dendritic complexity and lower spontaneous spike rate of On-αRGCs, targeted for patch clamp recording and dye filling using a Opn4-Cre reporter mouse line. To determine the influence of OHT on retinal projections to the brain, we expressed Channelrhodopsin-2 (ChR2) in melanopsin-expressing RGCs by crossing the Opn4-Cre mouse line with a ChR2-reporter mouse line and recorded post-synaptic responses in thalamocortical relay neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus evoked by stimulation with 460 nm light. The use of a Opn4-Cre reporter system allowed for expression of ChR2 in a narrow subset of RGCs responsible for image-forming vision in mice. Five weeks following OHT induction, paired pulse and high-frequency stimulus train experiments revealed that presynaptic vesicle release probability at retinogeniculate synapses was elevated. Additionally, miniature synaptic current frequency was slightly reduced in brain slices from OHT mice and proximal dendrites of post-synaptic dLGN relay neurons, assessed using a Sholl analysis, showed a reduced complexity. Strikingly, these changes occurred prior to major loss of RGCs labeled with the Opn4-Cre mouse, as indicated by immunofluorescence staining of ChR2-expressing retinal neurons. Thus, OHT leads to pre- and post-synaptic functional and structural changes at retinogeniculate synapses. Along with RGC dendritic remodeling and optic nerve transport changes, these retinogeniculate synaptic changes are among the earliest signs of glaucoma.

Nodes of Ranvier in Glaucoma

Retinal ganglion cell axons of the DBA/2J mouse model of glaucoma, a model characterized by extensive neuroinflammation, preserve synaptic contacts with their subcortical targets for a time after onset of anterograde axonal transport deficits, axon terminal hypertrophy, and cytoskeletal alterations. Though retrograde axonal transport is still evident in these axons, it is unknown if they retain their ability to transmit visual information to the brain. Using a combination of in vivo multiunit electrophysiology, neuronal tract tracing, multichannel immunofluorescence, and transmission electron microscopy, we report that eye-brain signaling deficits precede transport loss and axonal degeneration in the DBA/2J retinal projection. These deficits are accompanied by node of Ranvier pathology - consisting of increased node length and redistribution of the voltage-gated sodium channel Na_v1.6 that parallel changes seen early in multiple sclerosis (MS) axonopathy. Further, with age, axon caliber and neurofilament density increase without corresponding changes in myelin thickness. In contrast to these findings in DBA/2J mice, node pathologies were not observed in the induced microbead occlusion model of glaucoma - a model that lacks pre-existing inflammation. After one week of systemic treatment with fingolimod, an immunosuppressant therapy for relapsing-remitting MS, DBA/2J mice showed a substantial reduction in node pathology and mild effects on axon morphology. These data suggest that neurophysiological deficits in the DBA/2J may be due to defects in intact axons and targeting node pathology may be a promising intervention for some types of glaucoma.

Structural and Functional Rescue of Chronic Metabolically Stressed Optic Nerves through Respiration

Axon degeneration can arise from metabolic stress, potentially a result of mitochondrial dysfunction or lack of appropriate substrate input. In this study, we investigated whether the metabolic vulnerability observed during optic neuropathy in the DBA/2J (D2) model of glaucoma is due to dysfunctional mitochondria or impaired substrate delivery to axons, the latter based on our observation of significantly decreased glucose and monocarboxylate transporters in D2 optic nerve (ON), human ON, and mice subjected to acute glaucoma injury. We placed both sexes of D2 mice destined to develop glaucoma and mice of a control strain, the DBA/2J-Gpnmb⁺, on a ketogenic diet to encourage mitochondrial function. Eight weeks of the diet generated mitochondria, improved energy availability by reversing monocarboxylate transporter decline, reduced glial hypertrophy, protected retinal ganglion cells and their axons from degeneration, and maintained physiological signaling to the brain. A robust antioxidant response also accompanied the response to the diet. These results suggest that energy compromise and subsequent axon degeneration in the D2 is due to low substrate availability secondary to transporter downregulation.SIGNIFICANCE STATEMENT We show axons in glaucomatous optic nerve are energy depleted and exhibit chronic metabolic stress. Underlying the metabolic stress are low levels of glucose and monocarboxylate transporters that compromise axon metabolism by limiting substrate availability. Axonal metabolic decline was reversed by upregulating monocarboxylate transporters as a result of placing the animals on a ketogenic diet. Optic nerve mitochondria responded capably to the oxidative phosphorylation necessitated by the diet and showed increased number. These findings indicate that the source of metabolic challenge can occur upstream of mitochondrial dysfunction. Importantly, the intervention was successful despite the animals being on the cusp of significant glaucoma progression.

The effect of trabeculectomy on serum brain-derived neurotrophic factor levels in primary open-angle glaucoma

PURPOSE

We aimed to investigate the effect of trabeculectomy on serum brain-derived neurotrophic factor (BDNF) levels. Our secondary goal was to compare serum and aqueous humor (AH) BDNF levels in primary open-angle glaucoma (POAG) and control subjects.

METHODS

This prospective, cross-sectional study consists of 20 eyes of with advanced-stage POAG who had trabeculectomy and 19 eyes of age- and sex-matched control healthy subjects who had cataract surgery. Serum and AH samples were obtained preoperatively in trabeculectomy group and control subjects. Serum samples were obtained at the third postoperative month in both groups.

RESULTS

The aqueous humor and serum levels of BDNF at the surgery day were found to be strongly positive correlated (r = 0.868; p < 0.001). Serum and AH BDNF levels of POAG cases were significantly lower than control subjects at the surgery day (respectively p = 0.038, p = 0.011). In POAG cases, serum BDNF levels significantly increased at the third month after trabeculectomy while there was not a significant difference in control subjects with cataract surgery (p < 0.001; p = 0.717 respectively).

CONCLUSION

Trabeculectomy was found to have a positive effect on serum BDNF levels in POAG cases.

Anterograde Tract Tracing for Assaying Axonopathy and Transport Deficits in Glaucoma

Whether to stage degeneration or investigate early pathology in glaucoma, examination of axonal structure and function is essential. There are a wide variety of methods available to investigators using animal models of glaucoma, with varying utilities depending on the questions asked. Here, we describe the use of anterograde neuronal tract tracing using cholera toxin B (CTB) for the determination of axon transport integrity of the retinofugal projection. This method reveals the structure of the retinal axons as well as the functional integrity of anterograde transport systems.