Metabolomic changes in the mouse retina after optic nerve injury

Association between glaucoma susceptibility with combined defects in mitochondrial oxidative phosphorylation and fatty acid beta oxidation

Glaucoma is one of the leading causes of visual impairment and blindness worldwide, and is characterized by the progressive damage of retinal ganglion cells (RGCs) and the atrophy of the optic nerve head (ONH). The exact cause of RGC loss and optic nerve damage in glaucoma is not fully understood. The high energy demands of these cells imply a higher sensitivity to mitochondrial defects. Moreover, it has been postulated that the optic nerve is vulnerable towards damage from oxidative stress and mitochondrial dysfunction. To investigate this further, we conducted a pooled analysis of mitochondrial variants related to energy production, specifically focusing on oxidative phosphorylation (OXPHOS) and fatty acid β-oxidation (FAO). Our findings revealed that patients carrying non-synonymous (NS) mitochondrial DNA (mtDNA) variants within the OXPHOS complexes had an almost two-fold increased risk of developing glaucoma. Regarding FAO, our results demonstrated that longer-chain acylcarnitines (AC) tended to decrease, while shorter-chain AC tended to increase in patients with glaucoma. Furthermore, we observed that the knocking down cpt1a (a key rate-limiting enzyme involved in FAO) in zebrafish induced a degenerative process in the optic nerve and RGC, which resembled the characteristics observed in glaucoma. In conclusion, our study provides evidence that genes encoding mitochondrial proteins involved in energy metabolisms, such as OXPHOS and FAO, are associated with glaucoma. These findings contribute to a better understanding of the molecular mechanisms underlying glaucoma pathogenesis and may offer potential targets for therapeutic interventions in the future.

How is the P2X7 receptor signaling pathway involved in epileptogenesis?

Epilepsy, a condition characterized by spontaneous recurrent epileptic seizures, is among the most prevalent neurological disorders. This disorder is estimated to affect approximately 70 million people worldwide. Although antiseizure medications are considered the first-line treatments for epilepsy, most of the available antiepileptic drugs are not effective in nearly one-third of patients. This calls for the development of more effective drugs. Evidence from animal models and epilepsy patients suggests that strategies that interfere with the P2X7 receptor by binding to adenosine triphosphate (ATP) are potential treatments for this patient population. This review describes the role of the P2X7 receptor signaling pathways in epileptogenesis. We highlight the genes, purinergic signaling, Pannexin1, glutamatergic signaling, adenosine kinase, calcium signaling, and inflammatory response factors involved in the process, and conclude with a synopsis of these key connections. By unraveling the intricate interplay between P2X7 receptors and epileptogenesis, this review provides ideas for designing potent clinical therapies that will revolutionize both prevention and treatment for epileptic patients.

Reduced glutathione level in the aqueous humor of patients with primary open-angle glaucoma and normal-tension glaucoma

Glaucoma is a leading cause of blindness worldwide in older people. Profiling the aqueous humor, including the metabolites it contains, is useful to understand physiological and pathological conditions in the eye. In the current study, we used mass spectrometry (MS) to characterize the aqueous humor metabolomic profile and biological features of patients with glaucoma. Aqueous humor samples were collected during trabeculectomy surgery or cataract surgery and analyzed with global metabolomics. We included 40 patients with glaucoma (32 with POAG, 8 with NTG) and 37 control subjects in a discovery study. VIP analysis revealed five metabolites that were elevated and three metabolites that were reduced in the glaucoma patients. The identified metabolomic profile had an area under the receiver operating characteristic curve of 0.953. Among eight selected metabolites, the glutathione level was significantly decreased in association with visual field defects. Moreover, in a validation study to confirm the reproducibility of our findings, the glutathione level was reduced in NTG and POAG patients compared with a cataract control group. Our findings demonstrate that aqueous humor profiling can help to diagnose glaucoma and that various aqueous humor metabolites are correlated with clinical parameters in glaucoma patients. In addition, glutathione is clearly reduced in the aqueous humor of glaucoma patients with both IOP-dependent and IOP-independent disease subtypes. These findings indicate that antioxidant agents in the aqueous humor reflect glaucomatous optic nerve damage and that excessive oxidative stress may be involved in the pathogenesis of glaucoma.

The GPR84 molecule is a mediator of a subpopulation of retinal microglia that promote TNF/IL-1α expression via the rho-ROCK pathway after optic nerve injury

Resident microglia are important to maintain homeostasis in the central nervous system, which includes the retina. The retinal microglia become activated in numerous pathological conditions, but the molecular signatures of these changes are poorly understood. Here, using an approach based on FACS and RNA-seq, we show that microglial gene expression patterns gradually change during RGC degeneration induced by optic nerve injury. Most importantly, we found that the microglial cells strongly expressed Tnf and Il1α, both of which are known to induce neurotoxic reactive astrocytes, and were characterized by Gpr84high -expressing cells in a particular subpopulation. Moreover, ripasudil, a Rho kinase inhibitor, significantly blunted Gpr84 expression and cytokine induction in vitro and in vivo. Finally, GPR84-deficient mice prevented RGC loss in optic nerve-injured retina. These results reveal that Rho kinase-mediated GPR84 alteration strongly contribute to microglial activation and promote neurotoxicity, suggesting that Rho-ROCK and GPR84 signaling may be potential therapeutic targets to prevent the neurotoxic microglial phenotype induced by optic nerve damage, such as occurs in traumatic optic neuropathy and glaucoma.

Quantitative transportomics identifies Kif5a as a major regulator of neurodegeneration

Many neurons in the adult central nervous system, including retinal ganglion cells (RGCs), degenerate and die after injury. Early axon protein and organelle trafficking failure is a key component in many neurodegenerative disorders yet changes to axoplasmic transport in disease models have not been quantified. We analyzed early changes in the protein ‘transportome’ from RGC somas to their axons after optic nerve injury and identified transport failure of an anterograde motor protein Kif5a early in RGC degeneration. We demonstrated that manipulating Kif5a expression affects anterograde mitochondrial trafficking in RGCs and characterized axon transport in Kif5a knockout mice to identify proteins whose axon localization was Kif5a-dependent. Finally, we found that knockout of Kif5a in RGCs resulted in progressive RGC degeneration in the absence of injury. Together with expression data localizing Kif5a to human RGCs, these data identify Kif5a transport failure as a cause of RGC neurodegeneration and point to a mechanism for future therapeutics.

Exploring the systemic delivery of a poorly water-soluble model drug to the retina using PLGA nanoparticles

During the drug development process, many pharmacologically active compounds are discarded because of poor water solubility, but nanoparticle-based formulations are increasingly proposed as a solution for this problem. We therefore studied the distribution of nanoparticulate carriers and the delivery of their poorly water-soluble cargo to a structure of the central nervous system, the retina, under naive and pathological conditions. The lipophilic fluorescent dye coumarin 6 (Cou6) was encapsulated into poly(lactic-co-glycolic acid) PLGA nanoparticles (NPs). After intravenous administration in rats, we analyzed the distribution of cargo Cou6 and of the NP carrier covalently labeled with Cy5.5 in healthy animals and animals with optic nerve crush (ONC). In vivo real-time retina imaging revealed that Cou6 was rapidly released from PLGA NPs and penetrated the inner blood-retina barrier (BRB) within 15 min and PLGA NPs were gradually eliminated from the retinal blood circulation. Ex vivo microscopy of retinal flat mounts indicated that the Cou6 accumulated predominantly in the extracellular space and to a lesser extent in neurons. While the distribution of Cou6 in healthy animals and post ONC was comparable at early time point post-operation, the elimination of the NPs from the vessels was faster on day 7 post ONC. These results demonstrate the importance of considering different kinetics of nano-carrier and poorly water-soluble cargo, emphasizing the critical role of their parenchymal distribution, i.e. cellular/extracellular, and function of different physiological and pathological conditions.

Identification of biomarkers to diagnose diseases and find adverse drug reactions by metabolomics

Metabolomics has been widely used for investigating the biological functions of disease expression and has the potential to discover biomarkers in circulating biofluids or tissue extracts that reflect in phenotypic changes. Metabolic profiling has advantages because of the use of unbiased techniques, including multivariate analysis, and has been applied in pharmacological studies to predict therapeutic and adverse reactions of drugs, which is called pharmacometabolomics (PMx). Nuclear magnetic resonance (NMR)- and mass spectrometry (MS)-based metabolomics has contributed to the discovery of recent disease biomarkers; however, the optimal strategy for the study purpose must be selected from many established protocols, methodologies and analytical platforms. Additionally, information on molecular localization in tissue is essential for further functional analyses related to therapeutic and adverse effects of drugs in the process of drug development. MS imaging (MSI) is a promising technology that can visualize molecules on tissue surfaces without labeling and thus provide localized information. This review summarizes recent uses of MS-based global and wide-targeted metabolomics technologies and the advantages of the MSI approach for PMx and highlights the PMx technique for the biomarker discovery of adverse drug effects.

Lipid Landscape of the Human Retina and Supporting Tissues Revealed by High-Resolution Imaging Mass Spectrometry

The human retina provides vision at light levels ranging from starlight to sunlight. Its supporting tissues regulate plasma-delivered lipophilic essentials for vision, including retinoids. The macula is an anatomic specialization for high-acuity and color vision that is also vulnerable to prevalent blinding diseases. The retina's exquisite architecture comprises numerous cell types that are aligned horizontally, yielding structurally distinct cell, synaptic, and vascular layers that are visible in histology and in diagnostic clinical imaging. MALDI imaging mass spectrometry (IMS) is now capable of uniting low micrometer spatial resolution with high levels of chemical specificity. In this study, a multimodal imaging approach fortified with accurate multi-image registration was used to localize lipids in human retina tissue at laminar, cellular, and subcellular levels. Multimodal imaging results indicate differences in distributions and abundances of lipid species across and within single cell types. Of note are distinct localizations of signals within specific layers of the macula. For example, phosphatidylethanolamine and phosphatidylinositol lipids were localized to central RPE cells, whereas specific plasmalogen lipids were localized to cells of the perifoveal RPE and Henle fiber layer. Subcellular compartments of photoreceptors were distinguished by PE(20:0_22:5) in the outer nuclear layer, PE(18:0_22:6) in outer and inner segments, and cardiolipin CL(70:5) in the mitochondria-rich inner segments. Several lipids, differing by a single double bond, have markedly different distributions between the central fovea and the ganglion cell and inner nuclear layers. A lipid atlas, initiated in this study, can serve as a reference database for future examination of diseased tissues.

The role of neuroinflammation in the pathogenesis of glaucoma neurodegeneration

The chapter is a review enclosed in the volume "Glaucoma: A pancitopatia of the retina and beyond." No cure exists for glaucoma. Knowledge on the molecular and cellular alterations underlying glaucoma neurodegeneration (GL-ND) includes innovative and path-breaking research on neuroinflammation and neuroprotection. A series of events involving immune response (IR), oxidative stress and gene expression are occurring during the glaucoma course. Uveitic glaucoma (UG) is a prevalent acute/chronic complication, in the setting of chronic anterior chamber inflammation. Managing the disease requires a team approach to guarantee better results for eyes and vision. Advances in biomedicine/biotechnology are driving a tremendous revolution in ophthalmology and ophthalmic research. New diagnostic and imaging modalities, constantly refined, enable outstanding criteria for delimiting glaucomatous neurodegeneration. Moreover, biotherapies that may modulate or inhibit the IR must be considered among the first-line for glaucoma neuroprotection. This review offers the readers useful and practical information on the latest updates in this regard.

Aqp9 Gene Deletion Enhances Retinal Ganglion Cell (RGC) Death and Dysfunction Induced by Optic Nerve Crush: Evidence that Aquaporin 9 Acts as an Astrocyte-to-Neuron Lactate Shuttle in Concert with Monocarboxylate Transporters To Support RGC Function and Survival

Aquaporin 9 (AQP9) is an aquaglyceroporin that can transport lactate. Accumulating evidence suggests that astrocyte-to-neuron lactate shuttle (ANLS) plays a critical role in energy metabolism in neurons, including retinal ganglion cells (RGCs). To test the hypothesis that AQP9, in concert with monocarboxylate transporters (MCTs), participates in ANLS to maintain function and survival of RGCs, Aqp9-null mice and wild-type (WT) littermates were subjected to optic nerve crush (ONC) with or without intravitreal injection of an MCT2 inhibitor. RGC density was similar between the Aqp9-null mice and WT mice without ONC, while ONC resulted in significantly more RGC density reduction in the Aqp9-null mice than in the WT mice at day 7. Positive scotopic threshold response (pSTR) amplitude values were similar between the two groups without ONC, but were significantly more reduced in the Aqp9-null mice than in the WT mice 7days after ONC. MCT2 inhibitor injection accelerated RGC death and pSTR amplitude reduction only in the WT mice with ONC. Immunolabeling revealed that both RGCs and astrocytes expressed AQP9, that ONC predominantly reduced astrocytic AQP9 expression, and that MCTs 1, 2, and 4 were co-localized with AQP9 at the ganglion cell layer. These retinal MCTs were also co-immunoprecipitated with AQP9 in the WT mice. ONC decreased the co-immunoprecipitation of MCTs 1 and 4, but did not impact co-immunoprecipitation of MCT2. Retinal glucose transporter 1 expression was increased in Aqp9-null mice. Aqp9 gene deletion reduced and increased the intraretinal L-lactate and D-glucose concentrations, respectively. Results suggest that AQP9 acts as the ANLS to maintain function and survival of RGCs.

Metabolic basis of neuronal vulnerability to ischemia; an in vivo untargeted metabolomics approach

Understanding the root causes of neuronal vulnerability to ischemia is paramount to the development of new therapies for stroke. Transient global cerebral ischemia (tGCI) leads to selective neuronal cell death in the CA1 sub-region of the hippocampus, while the neighboring CA3 sub-region is left largely intact. By studying factors pertaining to such selective vulnerability, we can develop therapies to enhance outcome after stroke. Using untargeted liquid chromatography-mass spectrometry, we analyzed temporal metabolomic changes in CA1 and CA3 hippocampal areas following tGCI in rats till the setting of neuronal apoptosis. 64 compounds in CA1 and 74 in CA3 were found to be enriched and statistically significant following tGCI. Pathway analysis showed that pyrimidine and purine metabolism pathways amongst several others to be enriched after tGCI in CA1 and CA3. Metabolomics analysis was able to capture very early changes following ischemia. We detected 6 metabolites to be upregulated and 6 to be downregulated 1 hour after tGCI in CA1 versus CA3. Several metabolites related to apoptosis and inflammation were differentially expressed in both regions after tGCI. We offer a new insight into the process of neuronal apoptosis, guided by metabolomic profiling that was not performed to such an extent previously.

Nrf2 Suppresses Oxidative Stress and Inflammation in App Knock-In Alzheimer's Disease Model Mice

Nrf2 (NF-E2-related-factor 2) is a stress-responsive transcription factor that protects cells against oxidative stresses. To clarify whether Nrf2 prevents Alzheimer's disease (AD), AD model App^{NL-G-F/NL-G-F} knock-in (App^NLGF ) mice were studied in combination with genetic Nrf2 induction model Keap1^FA/FA mice. While App^NLGF mice displayed shorter latency to escape than wild-type mice in the passive-avoidance task, the impairment was improved in App^NLGF ::Keap1^FA/FA mice. Matrix-assisted laser desorption ionization-mass spectrometry imaging revealed that reduced glutathione levels were elevated by Nrf2 induction in App^NLGF ::Keap1^FA/FA mouse brains compared to App^NLGF mouse brains. Genetic Nrf2 induction in App^NLGF mice markedly suppressed the elevation of the oxidative stress marker 8-OHdG and Iba1-positive microglial cell number. We also determined the plasmalogen-phosphatidylethanolamine (PlsPE) level as an AD biomarker. PlsPE containing polyunsaturated fatty acids was decreased in the App^NLGF mouse brain, but Nrf2 induction attenuated this decline. To evaluate whether pharmacological induction of Nrf2 elicits beneficial effects for AD treatment, we tested the natural compound 6-MSITC [6-(methylsulfinyl)hexyl isothiocyanate]. Administration of 6-MSITC improved the impaired cognition of App^NLGF mice in the passive-avoidance task. These results demonstrate that the induction of Nrf2 ameliorates cognitive impairment in the AD model mouse by suppressing oxidative stress and neuroinflammation, suggesting that Nrf2 is an important therapeutic target of AD.

In vivo and in vitro knockout system labelled using fluorescent protein via microhomology-mediated end joining

Gene knockout is important for understanding gene function and genetic disorders. The CRISPR/Cas9 system has great potential to achieve this purpose. However, we cannot distinguish visually whether a gene is knocked out and in how many cells it is knocked out among a population of cells. Here, we developed a new system that enables the labelling of knockout cells with fluorescent protein through microhomology-mediated end joining-based knock-in. Using a combination with recombinant adeno-associated virus, we delivered our system into the retina, where the expression of Staphylococcus aureus Cas9 was driven by a retina ganglion cell (RGC)-specific promoter, and knocked out carnitine acetyltransferase (CAT). We evaluated RGCs and revealed that CAT is required for RGC survival. Furthermore, we applied our system to Keap1 and confirmed that Keap1 is not expressed in fluorescently labelled cells. Our system provides a promising framework for cell type-specific genome editing and fluorescent labelling of gene knockout based on knock-in.