Limited Energy Supply in Müller Cells Alters Glutamate Uptake

Small-molecule agonist AdipoRon alleviates diabetic retinopathy through the AdipoR1/AMPK/EGR4 pathway

BACKGROUND

Diabetes mellitus (DM) is a progressive disease that involves multiple organs due to increased blood glucose, and diabetic retinopathy (DR) is the main complication of DM in the eyes and causes irreversible vision loss. In the pathogenesis of diabetic vascular disease, oxidative stress caused by hyperglycemia plays an important role in Müller cell impairment. In recent years, AdipoRon, an adiponectin analog that demonstrated important physiological functions in obesity, diabetes, inflammation, and cardiovascular diseases, demonstrated cellular protection from apoptosis and reduced inflammatory damage through a receptor-dependent mechanism. Here, we investigated how AdipoRon reduced oxidative stress and apoptosis in Müller glia in a high glucose environment.

RESULTS

By binding to adiponectin receptor 1 on Müller glia, AdipoRon activated 5' adenosine monophosphate-activated protein kinase (AMPK)/acetyl-CoA carboxylase phosphorylation downstream, thereby alleviating oxidative stress and eventual apoptosis of cells and tissues. Transcriptome sequencing revealed that AdipoRon promoted the synthesis and expression of early growth response factor 4 (EGR4) and inhibited the cellular protective effects of AdipoRon in a high-glucose environment by reducing the expression of EGR4. This indicated that AdipoRon played a protective role through the EGR4 and classical AMPK pathways.

CONCLUSIONS

This provides a new target for the early treatment of DR.

Is fat the future for saving sight? Bioactive lipids and their impact on glaucoma

Glaucoma is characterized by a continuous loss of retinal ganglion cells. The cause of glaucoma is associated with an increase in intraocular pressure (IOP), but the underlying pathophysiology is diverse and, in most cases, unknown. There is an indisputable unmet need to identify new pathways involved in glaucoma pathogenesis. Increasing evidence suggests that bioactive lipids may be critical in the development and progression of glaucoma. Preclinical and clinical bioactive lipid targets exist and are being developed. In this review, we aim to shed light on the potential of bioactive lipids for the prevention, diagnosis, prognosis, and treatment of glaucoma by asking the question "is fat the future for saving sight".

Autophagy in the retinal neurovascular unit: New perspectives into diabetic retinopathy

Diabetic retinopathy (DR) is one of the most prevalent retinal disorders worldwide, and it is a major cause of vision impairment in individuals of productive age. Research has demonstrated the significance of autophagy in DR, which is a critical intracellular homeostasis mechanism required for the destruction and recovery of cytoplasmic components. Autophagy maintains the physiological function of senescent and impaired organelles under stress situations, thereby regulating cell fate via various signals. As the retina's functional and fundamental unit, the retinal neurovascular unit (NVU) is critical in keeping the retinal environment's stability and supporting the needs of retinal metabolism. However, autophagy is essential for the normal NVU structure and function. We discuss the strong association between DR and autophagy in this review, as well as the many kinds of autophagy and its crucial physiological activities in the retina. By evaluating the pathological changes of retinal NVU in DR and the latest advancements in the molecular mechanisms of autophagy that may be involved in the pathophysiology of DR in NVU, we seek to propose new ideas and methods for the prevention and treatment of DR.

Metabolism in Retinopathy of Prematurity

Retinopathy of prematurity is defined as retinal abnormalities that occur during development as a consequence of disturbed oxygen conditions and nutrient supply after preterm birth. Both neuronal maturation and retinal vascularization are impaired, leading to the compensatory but uncontrolled retinal neovessel growth. Current therapeutic interventions target the hypoxia-induced neovessels but negatively impact retinal neurons and normal vessels. Emerging evidence suggests that metabolic disturbance is a significant and underexplored risk factor in the disease pathogenesis. Hyperglycemia and dyslipidemia correlate with the retinal neurovascular dysfunction in infants born prematurely. Nutritional and hormonal supplementation relieve metabolic stress and improve retinal maturation. Here we focus on the mechanisms through which metabolism is involved in preterm-birth-related retinal disorder from clinical and experimental investigations. We will review and discuss potential therapeutic targets through the restoration of metabolic responses to prevent disease development and progression.

Fatty acid oxidation and photoreceptor metabolic needs

Photoreceptors have high energy demands and a high density of mitochondria that produce ATP through oxidative phosphorylation (OXPHOS) of fuel substrates. Although glucose is the major fuel for CNS brain neurons, in photoreceptors (also CNS), most glucose is not metabolized through OXPHOS but is instead metabolized into lactate by aerobic glycolysis. The major fuel sources for photoreceptor mitochondria remained unclear for almost six decades. Similar to other tissues (like heart and skeletal muscle) with high metabolic rates, photoreceptors were recently found to metabolize fatty acids (palmitate) through OXPHOS. Disruption of lipid entry into photoreceptors leads to extracellular lipid accumulation, suppressed glucose transporter expression, and a duel lipid/glucose fuel shortage. Modulation of lipid metabolism helps restore photoreceptor function. However, further elucidation of the types of lipids used as retinal energy sources, the metabolic interaction with other fuel pathways, as well as the cross-talk among retinal cells to provide energy to photoreceptors is not fully understood. In this review, we will focus on the current understanding of photoreceptor energy demand and sources, and potential future investigations of photoreceptor metabolism.

A2AR Antagonists Upregulate Expression of GS and GLAST in Rat Hypoxia Model

Background

The aim of this study was to research the effects of glutamine synthetase (GS) and glutamate aspartate transporter (GLAST) in rat Müller cells and the effects of an adenosine A_2AR antagonist (SCH 442416) on GS and GLAST in hypoxia both in vivo and in vitro.

Methods

This study used RT-PCR and Western blotting to quantify the expressions of GS and GLAST under different hypoxic conditions as well as the expressions of GS and GLAST at different drug concentrations. A cell viability assay was used to assess drug toxicity.

Results

mRNA and protein expression of GS and GLAST in hypoxia Group 24 h was significantly increased. mRNA and protein expressions of GS and GLAST both increased in Group 1 μM SCH 442416 compared with other groups. One micromolar SCH 442416 could upregulate GS and GLAST's activity in hypoxia both in vivo and in vitro.

Conclusions

Hypoxia activates GS and GLAST in rat retinal Müller cells in a short time in vitro. (2) A_2AR antagonists upregulate the activity of GS and GLAST in hypoxia both in vivo and in vitro.

Current Medical Therapy and Future Trends in the Management of Glaucoma Treatment

Glaucoma is a neurodegenerative disease characterized by progressive loss of retinal ganglion cells and their axons. Lowering of intraocular pressure (IOP) is currently the only proven treatment strategy for glaucoma. However, some patients show progressive loss of visual field and quality of life despite controlled IOP which indicates that other factors are implicated in glaucoma. Therefore, approaches that could prevent or decrease the rate of progression and do not rely on IOP lowering have gained much attention. Effective neuroprotection has been reported in animal models of glaucoma, but till now, no neuroprotective agents have been clinically approved. The present update provides an overview of currently available IOP-lowering medications. Moreover, potential new treatment targets for IOP-lowering and neuroprotective therapy are discussed. Finally, future trends in glaucoma therapy are addressed, including sustained drug delivery systems and progress toward personalized medicine.

Lactate: More Than Merely a Metabolic Waste Product in the Inner Retina

The retina is an extension of the central nervous system and has been considered to be a simplified, more tractable and accessible version of the brain for a variety of neuroscience investigations. The optic nerve displays changes in response to underlying neurodegenerative diseases, such as stroke, multiple sclerosis, and Alzheimer's disease, as well as inner retinal neurodegenerative disease, e.g., glaucoma. Neurodegeneration has increasingly been linked to dysfunctional energy metabolism or conditions in which the energy supply does not meet the demand. Likewise, increasing lactate levels have been correlated with conditions consisting of unbalanced energy supply and demand, such as ischemia-associated diseases or excessive exercise. Lactate has thus been acknowledged as a metabolic waste product in organs with high energy metabolism. However, in the past decade, numerous beneficial roles of lactate have been revealed in the central nervous system. In this context, lactate has been identified as a valuable energy substrate, protecting against glutamate excitotoxicity and ischemia, as well as having signaling properties which regulate cellular functions. The present review aims to summarize and discuss protective roles of lactate in various model systems (in vitro, ex vivo, and in vivo) reflecting the inner retina focusing on lactate metabolism and signaling in inner retinal homeostasis and disease.

Potential metabolic markers in glaucoma and their regulation in response to hypoxia

PURPOSE

To assess novel differences in serum levels of glucose, lactate and amino acids in patients with normal-tension glaucoma (NTG) compared to age-matched controls, at baseline and in response to universal hypoxia.

METHODS

Twelve patients diagnosed with NTG and eleven control subjects underwent normobaric hypoxia for 2 hr. Peripheral venous blood samples were taken at baseline, during hypoxia and in the recovery phase. Serum glucose and lactate levels were measured by a blood gas analyser. Amino acids were analysed by high-performance liquid chromatography.

RESULTS

Baseline levels of lactate and total amino acids were significantly lower in patients with NTG compared to healthy controls. No differences were seen in blood glucose levels between the two groups. Lactate levels remained unchanged during hypoxia in the control group, but increased in patients with NTG. In the recovery phase, total amino acid levels were reduced in the control group, whereas no changes were found in patients with NTG.

CONCLUSION

Reduced serum levels of lactate and total amino acids were identified as potential markers for NTG. Moreover, significant differential regulatory patterns of certain amino acids were found in patients with NTG compared to control subjects. Overall, our results suggest a link between systemic energy metabolites and NTG and support a novel understanding of glaucoma as an inner retinal manifestation of a systemic condition.

Glutamate transport system as a key constituent of glutamosome: Molecular pathology and pharmacological modulation in chronic pain

Neural uptake of glutamate is executed by the structurally related members of the SLC1A family of solute transporters: GLAST/EAAT1, GLT-1/EAAT2, EAAC1/EAAT3, EAAT4, ASCT2. These plasma membrane proteins ensure supply of glutamate, aspartate and some neutral amino acids, including glutamine and cysteine, for synthetic, energetic and signaling purposes, whereas effective removal of glutamate from the synaptic cleft shapes excitatory neurotransmission and prevents glutamate toxicity. Glutamate transporters (GluTs) possess also receptor-like properties and can directly initiate signal transduction. GluTs are physically linked to other glutamate signaling-, transporting- and metabolizing molecules (e.g., glutamine transporters SNAT3 and ASCT2, glutamine synthetase, NMDA receptor, synaptic vesicles), as well as cellular machineries fueling the transmembrane transport of glutamate (e.g., ion gradient-generating Na/K-ATPase, glycolytic enzymes, mitochondrial membrane- and matrix proteins, glucose transporters). We designate this supramolecular functional assembly as 'glutamosome'. GluTs play important roles in the molecular pathology of chronic pain, due to the predominantly glutamatergic nature of nociceptive signaling in the spinal cord. Down-regulation of GluTs often precedes or occurs simultaneously with development of pain hypersensitivity. Pharmacological inhibition or gene knock-down of spinal GluTs can induce/aggravate pain, whereas enhancing expression of GluTs by viral gene transfer can mitigate chronic pain. Thus, functional up-regulation of GluTs is turning into a prospective pharmacotherapeutic approach for the management of chronic pain. A number of novel positive pharmacological regulators of GluTs, incl. pyridazine derivatives and β-lactams, have recently been introduced. However, design and development of new analgesics based on this principle will require more precise knowledge of molecular mechanisms underlying physiological or aberrant functioning of the glutamate transport system in nociceptive circuits. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Essential Roles of Lactate in Müller Cell Survival and Function

Müller cells are pivotal in sustaining retinal ganglion cells, and an intact energy metabolism is essential for upholding Müller cell functions. The present study aimed to investigate the impact of lactate on Müller cell survival and function. Primary mice Müller cells and human Müller cell lines (MIO-M1) were treated with or without lactate (10 or 20 mM) for 2 and 24 hours. Simultaneously, Müller cells were incubated with or without 6 mM of glucose. L-lactate exposure increased Müller cell survival independently of the presence of glucose. This effect was abolished by the addition of the monocarboxylate inhibitor 4-cinnamic acid to the treatment media, whereas survival continued to increase in response to addition of D-lactate during glucose restriction. ATP levels decreased over time in MIO-M1 cells and remained stable over time in primary Müller cells. Lactate was preferably metabolized in MIO-M1 cells compared to glucose, and 10 mM of L-Lactate exposure prevented complete glycogen depletion in MIO-M1 cells. Glutamate uptake increased after 2 hours and decreased after 24 hours in glucose-restricted Müller cells compared to cells with glucose supplement. The addition of 10 mM of lactate to the treatment media increased glutamate uptake in glucose supplemented and restricted cells. In conclusion, lactate is a key component in maintaining Müller cell survival and function. Hence, lactate administration may be of great future interest, ultimately leading to novel therapies to rescue retinal ganglion cells.

A Perspective on the Müller Cell-Neuron Metabolic Partnership in the Inner Retina

The Müller cells represent the predominant macroglial cell in the retina. In recent decades, Müller cells have been acknowledged to be far more influential on neuronal homeostasis in the retina than previously assumed. With its unique localization, spanning the entire retina being interposed between the vessels and neurons, Müller cells are responsible for the functional and metabolic support of the surrounding neurons. As a consequence of major energy demands in the retina, high levels of glucose are consumed and processed by Müller cells. The present review provides a perspective on the symbiotic relationship between Müller cells and inner retinal neurons on a cellular level by emphasizing the essential role of energy metabolism within Müller cells in relation to retinal neuron survival.

Hypoxia-Inducible Factor-1α Target Genes Contribute to Retinal Neuroprotection

Hypoxia-inducible factor (HIF) is a transcription factor that facilitates cellular adaptation to hypoxia and ischemia. Long-standing evidence suggests that one isotype of HIF, HIF-1α, is involved in the pathogenesis of various solid tumors and cardiac diseases. However, the role of HIF-1α in retina remains poorly understood. HIF-1α has been recognized as neuroprotective in cerebral ischemia in the past two decades. Additionally, an increasing number of studies has shown that HIF-1α and its target genes contribute to retinal neuroprotection. This review will focus on recent advances in the studies of HIF-1α and its target genes that contribute to retinal neuroprotection. A thorough understanding of the function of HIF-1α and its target genes may lead to identification of novel therapeutic targets for treating degenerative retinal diseases including glaucoma, age-related macular degeneration, diabetic retinopathy, and retinal vein occlusions.

Mitochondrial function in Müller cells - Does it matter?

Growing evidence suggests that mitochondrial dysfunction might play a key role in the pathogenesis of age-related neurodegenerative inner retinal diseases such as diabetic retinopathy and glaucoma. Therefore, the present review provides a perspective on the impact of functional mitochondria in the most predominant glial cells of the retina, the Müller cells. Müller cells span the entire thickness of the neuroretina and are in close proximity to retinal cells including the retinal neurons that provides visual signaling to the brain. Among multiple functions, Müller cells are responsible for the removal of neurotransmitters, buffering potassium, and providing neurons with essential metabolites. Thus, Müller cells are responsible for a stable metabolic dialogue in the inner retina and their crucial role in supporting retinal neurons is indisputable. Müller cell functions require considerable energy production and previous literature has primarily emphasized glycolysis as the main energy provider. However, recent studies highlight the need of mitochondrial ATP production to upheld Müller cell functions. Therefore, the present review aims to provide an overview of the current evidence on the impact of mitochondrial functions in Müller cells.

Glia-Neuron Interactions in the Retina Can Be Studied in Cocultures of Müller Cells and Retinal Ganglion Cells

Glia-neuron partnership is important for inner retinal homeostasis and any disturbances may result in retinal ganglion cell (RGC) death. Müller cells support RGCs with essential functions such as removing excess glutamate and providing energy sources. The aim was to explore the impact of Müller cells on RGC survival. To investigate the Müller cell/RGC interactions we developed a coculture model, in which primary Müller cells were grown in inserts on top of pure primary RGC cultures. The impact of starvation and mitochondrial inhibition on the Müller cell ability to protect RGCs was studied. Moreover, the ability of Müller cells to remove glutamate from the extracellular space was investigated. RGC survival was evaluated by cell viability assays and glutamate uptake was assessed by kinetic uptake assays. We demonstrated a significantly increased RGC survival in presence of untreated and prestarved Müller cells. Additionally, prestarved Müller cells significantly increased RGC survival after mitochondrial inhibition. Finally, we revealed a significantly increased ability to take up glutamate in starved Müller cells. Overall, our study confirms essential roles of Müller cells in RGC survival. We suggest that targeting Müller cell function could have potential for future treatment strategies to prevent blinding neurodegenerative retinal diseases.

Lactate Transport and Receptor Actions in Retina: Potential Roles in Retinal Function and Disease

In retina, like in brain, lactate equilibrates across cell membranes via monocarboxylate transporters and in the extracellular space by diffusion, forming a basis for the action of lactate as a transmitter of metabolic signals. In the present paper, we argue that the lactate receptor GPR81, also known as HCAR1, may contribute importantly to the control of retinal cell functions in health and disease. GPR81, a G-protein coupled receptor, is known to downregulate cAMP both in adipose and nervous tissue. The receptor also acts through other down-stream mechanisms to control functions, such as excitability, metabolism and inflammation. Recent publications predict effects of the lactate receptor on neurodegeneration. Neurodegenerative diseases in retina, where the retinal ganglion cells die, notably glaucoma and diabetic retinopathy, may be linked to disturbed lactate homeostasis. Pilot studies reveal high GPR81 mRNA in retina and indicate GPR81 localization in Müller cells and retinal ganglion cells. Moreover, monocarboxylate transporters are expressed in retinal cells. We envision that lactate receptors and transporters could be useful future targets of novel therapeutic strategies to protect neurons and prevent or counteract glaucoma as well as other retinal diseases.

Towards axonal regeneration and neuroprotection in glaucoma: Rho kinase inhibitors as promising therapeutics

Due to a prolonged life expectancy worldwide, the incidence of age-related neurodegenerative disorders such as glaucoma is increasing. Glaucoma is the second cause of blindness, resulting from a slow and progressive loss of retinal ganglion cells (RGCs) and their axons. Up to now, intraocular pressure (IOP) reduction is the only treatment modality by which ophthalmologists attempt to control disease progression. However, not all patients benefit from this therapy, and the pathophysiology of glaucoma is not always associated with an elevated IOP. These limitations, together with the multifactorial etiology of glaucoma, urge the pressing medical need for novel and alternative treatment strategies. Such new therapies should focus on preventing or retarding RGC death, but also on repair of injured axons, to ultimately preserve or improve structural and functional connectivity. In this respect, Rho-associated coiled-coil forming protein kinase (ROCK) inhibitors hold a promising potential to become very prominent drugs for future glaucoma treatment. Their field of action in the eye does not seem to be restricted to IOP reduction by targeting the trabecular meshwork or improving filtration surgery outcome. Indeed, over the past years, important progress has been made in elucidating their ability to improve ocular blood flow, to prevent RGC death/increase RGC survival and to retard axonal degeneration or induce proper axonal regeneration. Within this review, we aim to highlight the currently known capacity of ROCK inhibition to promote neuroprotection and regeneration in several in vitro, ex vivo and in vivo experimental glaucoma models.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Present and New Treatment Strategies in the Management of Glaucoma

Glaucoma is a neurodegenerative disease characterized by retinal ganglion cell (RGC) death and axonal loss. It remains a major cause of blindness worldwide. All current modalities of treatment are focused on lowering intraocular pressure (IOP), and it is evident that increased IOP is an important risk factor for progression of the disease. However, it is clear that a significant number of glaucoma patients show disease progression despite of pressure lowering treatments. Much attention has been given to the development of neuroprotective treatment strategies, but the identification of such has been hampered by lack of understanding of the etiology of glaucoma. Hence, in spite of many attempts no neuroprotective drug has yet been clinically approved. Even though neuroprotection is without doubt an important treatment strategy, many glaucoma subjects are diagnosed after substantial loss of RGCs. In this matter, recent approaches aim to rescue RGCs and regenerate axons in order to restore visual function in glaucoma. The present review seeks to provide an overview of the present and new treatment strategies in the management of glaucoma. The treatment strategies are divided into current available glaucoma medications, new pressure lowering targets, prospective neuroprotective interventions, and finally possible neuroregenrative strategies.