Assessment of the uniform field electroretinogram for mouse retinal ganglion cell functional analysis.

PURPOSE: The uniform field electroretinogram (UF-ERG) has been suggested as an alternative to the pattern electroretinogram (PERG) for non-invasive assessment of retinal ganglion cell (RGC) function in primates. We evaluated the validity of the UF-ERG to assess mouse RGC activity in vivo. METHODS: Unilateral optic nerve crush (ONC) was performed on adult C57BL/6J mice. Contralateral eyes with uncrushed optic nerves and eyes from surgically naive mice served as experimental controls. Electrophysiological visual assessment was performed at 12 weeks post-ONC. Flash-mediated visual-evoked cortical potentials (VEPs) were measured to confirm the robustness of the ONC procedure. Full-field flash ERGs were used to interrogate photoreceptor and retinal bipolar cell function. RGC function was assessed with pattern ERGs. Summed onset and offset UF-ERG responses to alternating dark and light uniform field flash stimuli of different intensities and wavelengths were recorded from ONC and control eyes, and relative differences were compared to the PERG results. Following electrophysiological analysis, RGC loss was monitored by immunohistochemical staining of the RGC marker protein, RBPMS, in post-mortem retinal tissues. RESULTS: ONC dramatically impacts RGC integrity and optic nerve function, demonstrated by reduced RGC counts and near complete elimination of VEPs. ONC did not affect scotopic ERG a-wave and b-wave amplitudes, while PERG amplitudes of eyes subjected to ONC were reduced by approximately 50% compared to controls. Summation of ON and OFF UF-ERG responses did not reveal statistically significant differences between ONC and control eyes, regardless of visual stimulus. CONCLUSIONS: PERG responses are markedly impaired upon ONC, while UF-ERG responses are not significantly affected by surgical trauma to RGC axons in mice. The more closely related pattern and uniform field ERGs recorded in primates suggests species-specific differences in RGC features or subpopulations corresponding to PERG and UF-ERG response generators, limiting the utility of the UF-ERG for mouse RGC functional analysis.

Potential roles of branched-chain amino acids in neurodegeneration.

Despite having disease-specific pathologic features and symptoms, neurodegenerative diseases share common mechanisms, such as excitotoxicity, neuroinflammation, and neurotransmitter dysregulation. Although the common underlying cause of these neurodegenerative processes has yet to be identified, accumulating evidence suggests that branched-chain amino acids (BCAAs) could be involved. BCAAs have been shown to not only influence the central levels of neurotransmitters but also to induce excitotoxicity, hyperexcitability, inflammation, and oxidative stress. Furthermore, emerging evidence indicates that BCAA metabolism may be dysregulated in major neurodegenerative diseases, namely Alzheimer's and Parkinson's diseases and amyotrophic lateral sclerosis. In this review, we identified the neurodegenerative mechanisms of BCAAs and outlined their potential roles in neurodegenerative diseases, suggesting that targeting BCAA metabolism may represent a new approach to identifying new therapeutic targets for multifaceted neurodegenerative diseases.

Methylenetetrahydrofolate reductase deficiency alters cellular response after ischemic stroke in male mice.

Objective: Elevated homocysteine concentrations are a risk factor for stroke. A common genetic polymorphism in methylenetetrahydrofolate reductase (MTHFR 677 C→T) results in elevated levels of homocysteine. MTHFR plays a critical role in the synthesis of S-adenosylmethionine (SAM), a global methyl donor. Our previous work has demonstrated that Mthfr+/- mice, which model the MTHFR polymorphism in humans, are more vulnerable to ischemic damage. The aim of this study was to investigate the cellular mechanisms by which the MTHFR-deficiency changes the brain in the context of ischemic stroke injury.Methods: In the present study, three-month-old male Mthfr+/- and wild-type littermate mice were subjected to photothrombosis (PT) damage. Four weeks after PT damage, animals were tested on behavioral tasks, in vivo imaging was performed using T2-weighted MRI, and brain tissue was collected for histological analysis.Results: Mthfr+/- animals used their non-impaired forepaw more to explore the cylinder and had a larger damage volume compared to wild-type littermates. In brain tissue of Mthfr+/- mice methionine adenosyltransferase II alpha (MAT2A) protein levels were decreased within the damage hemisphere and increased levels in hypoxia-induced factor 1 alpha (HIF-1α) in non-damage hemisphere. There was an increased antioxidant response in the damage site as indicated by higher levels of nuclear factor erythroid 2-related factor 2 (Nrf2) in neurons and astrocytes and neuronal superoxide dismutase 2 (SOD2) levels.Conclusions: Our results suggest that Mthfr+/- mice are more vulnerable to PT-induced stroke damage through the regulation of the cellular response. The increased antioxidant response we observed may be compensatory to the damage amount.

Harnessing Astrocytes and Müller Glial Cells in the Retina for Survival and Regeneration of Retinal Ganglion Cells.

Astrocytes have been associated with the failure of axon regeneration in the central nervous system (CNS), as it undergoes reactive gliosis in response to damages to the CNS and functions as a chemical and physical barrier to axon regeneration. However, beneficial roles of astrocytes have been extensively studied in the spinal cord over the years, and a growing body of evidence now suggests that inducing astrocytes to become more growth-supportive can promote axon regeneration after spinal cord injury (SCI). In retina, astrocytes and Müller cells are known to undergo reactive gliosis after damage to retina and/or optic nerve and are hypothesized to be either detrimental or beneficial to survival and axon regeneration of retinal ganglion cells (RGCs). Whether they can be induced to become more growth-supportive after retinal and optic nerve injury has yet to be determined. In this review, we pinpoint the potential molecular pathways involved in the induction of growth-supportive astrocytes in the spinal cord and suggest that stimulating the activation of these pathways in the retina could represent a new therapeutic approach to promoting survival and axon regeneration of RGCs in retinal degenerative diseases.

PI 3-kinase delta enhances axonal PIP3 to support axon regeneration in the adult CNS.

Peripheral nervous system (PNS) neurons support axon regeneration into adulthood, whereas central nervous system (CNS) neurons lose regenerative ability after development. To better understand this decline whilst aiming to improve regeneration, we focused on phosphoinositide 3-kinase (PI3K) and its product phosphatidylinositol (3,4,5)-trisphosphate (PIP3 ). We demonstrate that adult PNS neurons utilise two catalytic subunits of PI3K for axon regeneration: p110α and p110δ. However, in the CNS, axonal PIP3 decreases with development at the time when axon transport declines and regenerative competence is lost. Overexpressing p110α in CNS neurons had no effect; however, expression of p110δ restored axonal PIP3 and increased regenerative axon transport. p110δ expression enhanced CNS regeneration in both rat and human neurons and in transgenic mice, functioning in the same way as the hyperactivating H1047R mutation of p110α. Furthermore, viral delivery of p110δ promoted robust regeneration after optic nerve injury. These findings establish a deficit of axonal PIP3 as a key reason for intrinsic regeneration failure and demonstrate that native p110δ facilitates axon regeneration by functioning in a hyperactive fashion.

One-carbon metabolism supplementation improves outcome after stroke in aged male MTHFR-deficient mice.

The prevalence of stroke increases with age and the ability to absorb all nutrients from our diets decreases with age. Nutrition is a modifiable risk factor for stroke, which is a leading cause of death and disability in world-wide. Deficiencies in one­carbon metabolism, including in methyltetrahydrofolate reductase (MTHFR), have been linked to increased risk of stroke. The Mthfr+/- mice mouse model mimic the phenotype of the MTHFR677C➔T polymorphism, such as elevated levels of homocystine. Using this mouse model, the aim of this study was to investigate the impact of dietary supplementation with 5-methylTHF, vitamin B12, and choline after ischemic stroke. Male Mthfr+/- and wildtype littermate control mice were aged (~1.5-year-old) and were placed on control diet (CD) 4-weeks prior to sensorimotor cortex damage using photothrombosis (PT), a model for ischemic stroke. Post-operatively, one group of Mthfr+/- and wildtype littermate mice were placed on 5-methylTHF, vitamin B12, and choline supplemented diet (SD). Four weeks after PT and SD motor function was assessed using the accelerating rotarod, forepaw asymmetry, and ladder beam walking tasks. Total homocysteine and cysteine levels were measured in blood. Brain tissue was processed to assess lesion volume and investigate biochemical and molecular changes. After PT and SD, Mthfr+/- mice were able to stay on the accelerating rotarod longer and used their impaired forepaw to explore more when compared to CD animals. Furthermore, total homocysteine levels in plasma and lesion volume were reduced in Mthfr+/+ and Mthfr+/- SD mice. Within the damage site, there were reduced levels of apoptotic cell death and increased neuroprotective cellular response in the brains of SD treated Mthfr+/- mice. This study reveals a critical role for one­carbon supplementation, with 5-methylTHF, vitamin B12, and choline, in supporting improvement after ischemic stroke damage.

Paraquat Exposure Increases Oxidative Stress Within the Dorsal Striatum of Male Mice With a Genetic Deficiency in One-carbon Metabolism.

Paraquat is an herbicide that is commonly used worldwide. Exposure to paraquat results in Parkinson's disease (PD)-like symptoms including dopaminergic cell loss. Nutrition has also been linked in the pathogenesis of PD, such as reduced levels of folic acid, a B-vitamin, and component of one-carbon metabolism. Within one-carbon metabolism, methylenetetrahydrofolate reductase (MTHFR) catalyzes the irreversible conversion of 5, 10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. A polymorphism in MTHFR (677 C&→T) has been reported in 5%-15% of North American and European human populations. The MTHFR polymorphism is also prevalent in PD patients. The goal of this study was to investigate the impact of paraquat-induced PD-like pathology in the context of reduced levels of MTHFR. Three-month-old male Mthfr+/- mice, which model the MTHFR polymorphism observed in humans, were administered intraperitoneal injections of paraquat (10 mg/kg) or saline 6 times over 3 weeks. At the end of paraquat treatment, motor and memory function were assessed followed by collection of brain tissue for biochemical analysis. Mthfr+/- mice treated with paraquat showed impaired motor function. There was increased microglial activation within the substantia nigra (SN) of Mthfr+/- mice treated with paraquat. Additionally, all Mthfr+/- mice that were treated with paraquat showed increased oxidative stress within the dorsal striatum, but not the SN. The present results show that paraquat exposure increases PD-like pathology in mice deficient in one-carbon metabolism.

The Role of Neuronal Pentraxin 2 (NP2) in Regulating Glutamatergic Signaling and Neuropathology.

Pentraxins are a superfamily of evolutionarily conserved proteins that are characterized by their multimeric architecture and their calcium-dependent binding. They can be broadly grouped into two subfamilies: short pentraxins and long pentraxins. Pentraxins regulate many processes in the brain as well as the periphery. Neuronal pentraxin 2 (NP2/NPTX2), also known as neuronal activity-regulated pentraxin (Narp), is an immediate-early gene that has been shown to play a critical role in guiding synaptic plasticity. NP2 has been previously linked to excitatory neurotransmission, based on its ability to aggregate excitatory receptors in the central nervous system. The mechanisms mediating the effects of NP2 on excitatory neurotransmission remain unclear and warrants further investigation. This review article focuses on the biological features of NP2 and discusses the literature supporting a role for NP2 and other pentraxins in glutamatergic signaling. An analysis of evidence around the role of pentraxins in neuropathology is also reviewed.

A genetic deficiency in folic acid metabolism impairs recovery after ischemic stroke.

Stroke is a leading cause of disability and death world-wide and nutrition is a modifiable risk factor for stroke. Metheylenetetrahydrofolate reductase (MTHFR) is an enzyme involved in the metabolism of folic acid, a B-vitamin. In humans, a polymorphism in MTHFR (677C→T) is linked to increased risk of stroke, but the mechanisms remain unknown. The Mthfr+/- mice mimic a phenotype described in humans at bp677. Using this mouse model, the aim of this study was to investigate the impact of MTHFR deficiency on stroke outcome. Male Mthfr+/- and wildtype littermate control mice were aged (~1.5-year-old) and trained on the single pellet reaching task. After which the sensorimotor cortex was then damaged using photothrombosis (PT), a model for ischemic stroke. Post-operatively, animals were tested for skilled motor function, and brain tissue was processed to assess cell death. Mthfr+/- mice were impaired in skilled reaching 2-weeks after stroke but showed some recovery at 5-weeks compared to wild types after PT damage. Within the ischemic brain, there was increased expression of active caspase-3 and reduced levels of phospho-AKT in neurons of Mthfr+/- mice. Recent data suggests that astrocytes may play a significant role after damage, the impact of MTHFR and ischemic investigated the impact of MTHFR-deficiency on astrocyte function. MTHFR-deficient primary astrocytes showed reduced cell viability after exposure to hypoxia compared to controls. Increased immunofluorescence staining of active caspase-3 and hypoxia-inducible factor 1-alpha were also observed. The data suggest that MTHFR deficiency decreases recovery after stroke by reducing neuronal and astrocyte viability.

It's All about Timing: The Involvement of Kir4.1 Channel Regulation in Acute Ischemic Stroke Pathology.

An acute ischemic stroke is characterized by the presence of a blood clot that limits blood flow to the brain resulting in subsequent neuronal loss. Acute stroke threatens neuronal survival, which relies heavily upon proper function of astrocytes. Neurons are more susceptible to cell death when an astrocyte is unable to carry out its normal functions in supporting the neuron in the area affected by the stroke (Rossi et al., 2007; Takano et al., 2009). For example, under normal conditions, astrocytes initially swell in response to changes in extracellular osmotic pressure and then reduce their regulatory volume in response to volume-activated potassium (K+) and chloride channels (Vella et al., 2015). This astroglial swelling may be overwhelmed, under ischemic conditions, due to the increased levels of glutamate and extracellular K+ (Lai et al., 2014; Vella et al., 2015). The increase in extracellular K+ contributes to neuronal damage and loss through the initiation of harmful secondary cascades (Nwaobi et al., 2016). Reducing the amount of extracellular K+ could, in theory, limit or prevent neuronal damage and loss resulting in an improved prognosis for individuals following ischemic stroke. Kir4.1, an inwardly rectifying K+ channel, has demonstrated an ability to regulate the rapid reuptake of this ion to return the cell to basal levels allowing it to fire again in rapid transmission (Sibille et al., 2015). Despite growing interest in this area, the underlying mechanism suggesting that neuroprotection could occur through modification of the Kir4.1 channel's activity has yet to be described. The purpose of this review is to examine the current literature and propose potential underlying mechanisms involving Kir4.1, specially the mammalian target of rapamycin (mTOR) and/or autophagic pathways, in the pathogenesis of ischemic stroke. The hope is that this review will instigate further investigation of Kir4.1 as a modulator of stroke pathology.

Recombinant growth differentiation factor 11 influences short-term memory and enhances Sox2 expression in middle-aged mice.

Previous evidence suggests that a significant decline in cognitive ability begins during middle-age and continues to deteriorate with increase in age. Recent work has demonstrated the potential rejuvenation impact of growth differentiation factor-11 (GDF-11) in aged mice. We carried out experiments to evaluate the impact of a single dose of recombinant (rGDF-11) on short-term visual and spatial memory in middle-aged male mice. On the novel object recognition task, we observed middle-aged mice treated rGDF-11 showed improved performance on the novel object recognition task. However, middle-aged mice did not show increased expression of phosphorylated-Smad2/3, a downstream effector of GDF-11. We noted however that the expression of the transcription factor, Sox2 was increased within the dentate gyrus. Our data suggest that a single injection of rGDF-11 contributes to improvements in cognitive function of middle-aged animals, which may be critical in the preservation of short-term memory capacity in old age.

B-vitamin and choline supplementation increases neuroplasticity and recovery after stroke.

Folates are B-vitamins that play an important role in brain function. Dietary and genetic deficiencies in folate metabolism result in elevated levels of homocysteine which have been linked to increased risk of developing a stroke. Reducing levels of homocysteine before or after a stroke through B-vitamin supplementation has been a focus of many clinical studies, however, the results remain inconsistent. Animal model systems provide a powerful mechanism to study and understand functional impact and mechanisms through which supplementation affects stroke recovery. The aim of this study was to understand the role of B-vitamins in stroke pathology using in vivo and in vitro mouse models. The first objective assessed the impact of folate deficiency prior to ischemic damage followed by B-vitamins and choline supplementation. Ischemic damage targeted the sensorimotor cortex. C57Bl/6 wild-type mice were maintained on a folic acid deficient diet for 4weeks prior to ischemic damage to increased levels of plasma homocysteine, a risk factor for stroke. Post-operatively mice were placed on a B-vitamin and choline supplemented diet for a period of four weeks, after which motor function was assessed in mice using the rotarod, ladder beam and forepaw asymmetry tasks. The second objective was to determine how a genetic deficiency in methylenetetrahydrofolate reductase (MTHFR), an enzyme involved in folate metabolism, increases vulnerability to stroke. Primary cortical neurons were isolated from Mthfr+/+, Mthfr+/- and Mthfr-/- embryos and were exposed to in vitro models of stroke which include hypoxia or oxygen glucose deprivation. Cell viability was measured 24-h after exposure stroke like conditions in vitro. In supplemented diet mice, we report improved motor function after ischemic damage compared to mice fed a control diet after ischemic damage. Within the perilesional cortex, we show enhanced proliferation, neuroplasticity and anti-oxidant activity in mice fed the supplemented diet. A genetic MTHFR deficiency resulted in neurodegeneration after exposure to in vitro models of stroke, by activating apoptosis promoting p53-dependent mechanisms. These results suggest that one-carbon metabolism plays a significant role in recovery after stroke and MTHFR deficiency contributes to poor recovery from stroke.

Retinal regeneration mechanisms linked to multiple cancer molecules: A therapeutic conundrum.

Over the last decade, a large number of research articles have been published demonstrating regeneration and/or neuroprotection of retinal ganglion cells following manipulation of specific genetic and molecular targets. Interestingly, of the targets that have been identified to promote repair following visual system damage, many are genes known to be mutated in different types of cancer. This review explores recent literature on the potential for modulating cancer genes as a therapeutic strategy for visual system repair and looks at the potential clinical challenges associated with implementing this type of therapy. We also discuss signalling mechanisms that have been implicated in cancer and consider how similar mechanisms may improve axonal regeneration in the optic nerve.

Preparation of embryonic retinal explants to study CNS neurite growth.

This protocol outlines the preparation of embryonic mouse retinal explants, which provides an effective technique to analyze neurite outgrowth in central nervous system (CNS) neurons. This validated ex vivo system, which displays limited neuronal death, is highly reproducible and particularly amenable to manipulation. Our previously published studies involving embryonic chick or adult mouse retinal explants were instrumental in the preparation of this protocol; aspects of these previous techniques were combined, adopted and optimized. This protocol thus permits more efficient analysis of neurite growth. Briefly, the retina is dissected from the embryonic mouse eye using precise techniques that take into account the small size of the embryonic eye. The approach applied ensures that the retinal ganglion cell (RGC) layer faces the adhesion substrate on coated cover slips. Neurite growth is clear, well-delineated and readily quantifiable. These retinal explants can therefore be used to examine the neurite growth effects elicited by potential therapeutic agents.

tPA promotes cortical neuron survival via mTOR-dependent mechanisms.

Tissue plasminogen activator (tPA) is a thrombolytic agent commonly used in the treatment of ischemic stroke. While the thrombolytic effects of tPA have been well established, the impact of this blood-brain barrier (BBB) crossing drug on neurons is not known. Given the widespread use of tPA in the clinical setting and the strict therapeutic window established for effective use of the drug, we examined the molecular mechanisms mediating the impact of tPA on postnatal cortical neurons isolated from the mouse brain. Dissociated postnatal primary cortical neurons were treated with tPA and the effects on neuron survival were evaluated. Pharmacological inhibitors of several signaling pathways previously implicated in neuroprotection (mTOR, JAK/STAT, MAPK and PKA-dependent mechanisms) were used to pinpoint the mechanistic effectors of tPA on neuron survival in vitro. We report here that tPA treatment results in a time-dependent neuroprotective effect on postnatal cortical neurons that relies predominantly on Janus kinase (JAK) and mammalian target of rapamycin (mTOR) signaling mechanisms. Taken together, these data suggest that tPA promotes neuroprotection in a temporally-regulated manner and that both JAK and mTOR signaling effectors are critical mediators of this neuroprotective effect. The results suggest the possibility of targeting these defined mechanisms to potentially expand the therapeutic window for tPA.

Granulocyte macrophage colony-stimulating factor treatment results in recovery of motor function after white matter damage in mice.

Clinical stroke usually results from a cerebral ischaemic event, and is frequently a debilitating condition with limited treatment options. A significant proportion of clinical strokes result from specific damage to the subcortical white matter (SWM), but currently there are few animal models available to investigate the pathogenesis and potential therapeutic strategies to promote recovery. Granulocyte macrophage colony-stimulating factor (GM-CSF) is a cytokine that has been previously shown to promote neuroprotective effects after brain damage; however, the mechanisms mediating this effect are not known. Here, it is reported that GM-CSF treatment results in dramatic functional improvement in a white matter model of stroke in mice. SWM stroke was induced in mice by unilateral injections of the vasoconstrictor, endothelin-1 (ET-1). The results reveal that ET-1-induced stroke impairs skilled motor function on the single pellet-reaching task and results in forelimb asymmetry, in adult mice. Treatment with GM-CSF, after stroke, restores motor function and abolishes forelimb asymmetry. The results also indicate that GM-CSF promotes its effects by activating mammalian target of rapamycin signalling mechanisms in the brain following stroke injury. Additionally, a significant increase in GM-CSF receptor expression was found in the ipsilateral hemisphere of the ET-1-injected brain. Taken together, the present study highlights the use of an under-utilized mouse model of stroke (using ET-1) and suggests that GM-CSF treatment can attenuate ET-1-induced functional deficits.

"GAG-ing with the neuron": The role of glycosaminoglycan patterning in the central nervous system.

Proteoglycans (PGs) are a diverse family of proteins that consist of one or more glycosaminoglycan (GAG) chains, covalently linked to a core protein. PGs are major components of the extracellular matrix (ECM) and play critical roles in development, normal function and damage-response of the central nervous system (CNS). GAGs are classified based on their disaccharide subunits, into the following major groups: chondroitin sulfate (CS), heparan sulfate (HS), heparin (HEP), dermatan sulfate (DS), keratan sulfate (KS) and hyaluronic acid (HA). All except HA are modified by sulfation, giving GAG chains specific charged structures and binding properties. While significant neuroscience research has focused on the role of one PG family member, chondroitin sulfate proteoglycan (CSPG), there is ample evidence in support of a role for the other PGs in regulating CNS function in normal and pathological conditions. This review discusses the role of all the identified PG family members (CS, HS, HEP, DS, KS and HA) in normal CNS function and in the context of pathology. Understanding the pleiotropic roles of these molecules in the CNS may open the door to novel therapeutic strategies for a number of neurological conditions.

Granulocyte macrophage colony-stimulating factor promotes regeneration of retinal ganglion cells in vitro through a mammalian target of rapamycin-dependent mechanism.

Lack of regeneration in the adult central nervous system (CNS) is a major hurdle that limits recovery from neurological ailments. Although accumulating research suggests the possibility of axon regeneration by targeting intrinsic signaling mechanisms, it remains a matter of controversy whether functional recovery can be achieved by manipulating aspects of molecular signaling. Recent studies have shown that granulocyte macrophage colony-stimulating factor (GM-CSF) may be an effective means of targeting repair following CNS injury; how this molecule is able to produce this effect is not known. Indeed, GM-CSF has been shown to promote neuronal survival, potentially through activation of as yet unknown cytokine-dependent signals and potentially through regulation of antiapoptotic mechanisms. It is well established that the loss of intrinsic regenerative ability is highly correlated with development of CNS neurons. We therefore designed experiments, using a well-established in vitro retinal ganglion cell (RGC) culture system, to evaluate the effect of GM-CSF on axon growth and cell survival and define possible mechanisms involved in GM-CSF-mediated effects in vitro. Several developmental stages were evaluated, with particular focus placed on stages at which axon growth is known to be significantly diminished. Our results reveal that GM-CSF not only promotes axon growth in postnatal RGCs but also enhances cell survival through a mammalian target of rapamycin (mTOR)-dependent mechanism.