Inorganic Polyphosphate Regulates AMPA and NMDA Receptors and Protects Against Glutamate Excitotoxicity via Activation of P2Y Receptors

Polyphosphate Nanoparticles: Balancing Energy Requirements in Tissue Regeneration Processes

Nanoparticles of a particular, evolutionarily old inorganic polymer found across the biological kingdoms have attracted increasing interest in recent years not only because of their crucial role in metabolism but also their potential medical applicability: it is inorganic polyphosphate (polyP). This ubiquitous linear polymer is composed of 10-1000 phosphate residues linked by high-energy anhydride bonds. PolyP causes induction of gene activity, provides phosphate for bone mineralization, and serves as an energy supplier through enzymatic cleavage of its acid anhydride bonds and subsequent ATP formation. The biomedical breakthrough of polyP came with the development of a successful fabrication process, in depot form, as Ca- or Mg-polyP nanoparticles, or as the directly effective polymer, as soluble Na-polyP, for regenerative repair and healing processes, especially in tissue areas with insufficient blood supply. Physiologically, the platelets are the main vehicles for polyP nanoparticles in the circulating blood. To be biomedically active, these particles undergo coacervation. This review provides an overview of the properties of polyP and polyP nanoparticles for applications in the regeneration and repair of bone, cartilage, and skin. In addition to studies on animal models, the first successful proof-of-concept studies on humans for the healing of chronic wounds are outlined.

The Protein Scaffolding Functions of Polyphosphate

Inorganic polyphosphate (polyP), one of the first high-energy compound on earth, defies its extreme compositional and structural simplicity with an astoundingly wide array of biological activities across all domains of life. However, the underlying mechanism of such functional pleiotropy remains largely elusive. In this review, we will summarize recent studies demonstrating that this simple polyanion stabilizes protein folding intermediates and scaffolds select native proteins. These functions allow polyP to act as molecular chaperone that protects cells against protein aggregation, as pro-amyloidogenic factor that accelerates both physiological and disease-associated amyloid formation, and as a modulator of liquid-liquid phase separation processes. These activities help to explain polyP's known roles in bacterial stress responses and pathogenicity, provide the mechanistic foundation for its potential role in human neurodegenerative diseases, and open a new direction regarding its influence on gene expression through condensate formation. We will highlight critical unanswered questions and point out potential directions that will help to further understand the pleiotropic functions of this ancient and ubiquitous biopolymer.

PRUNE1 and NME/NDPK family proteins influence energy metabolism and signaling in cancer metastases

We describe here the molecular basis of the complex formation of PRUNE1 with the tumor metastasis suppressors NME1 and NME2, two isoforms appertaining to the nucleoside diphosphate kinase (NDPK) enzyme family, and how this complex regulates signaling the immune system and energy metabolism, thereby shaping the tumor microenvironment (TME). Disrupting the interaction between NME1/2 and PRUNE1, as suggested, holds the potential to be an excellent therapeutic target for the treatment of cancer and the inhibition of metastasis dissemination. Furthermore, we postulate an interaction and regulation of the other Class I NME proteins, NME3 and NME4 proteins, with PRUNE1 and discuss potential functions. Class I NME1-4 proteins are NTP/NDP transphosphorylases required for balancing the intracellular pools of nucleotide diphosphates and triphosphates. They regulate different cellular functions by interacting with a large variety of other proteins, and in cancer and metastasis processes, they can exert pro- and anti-oncogenic properties depending on the cellular context. In this review, we therefore additionally discuss general aspects of class1 NME and PRUNE1 molecular structures as well as their posttranslational modifications and subcellular localization. The current knowledge on the contributions of PRUNE1 as well as NME proteins to signaling cascades is summarized with a special regard to cancer and metastasis.

Inflammatory Factor IL1α Induces Aberrant Astrocyte Proliferation in Spinal Cord Injury Through the Grin2c/Ca2+/CaMK2b Pathway

Spinal cord injury (SCI) is one of the most devastating traumas, and the aberrant proliferation of astrocytes usually causes neurological deficits. However, the mechanism underlying astrocyte over-proliferation after SCI is unclear. Grin2c (glutamate ionotropic receptor type 2c) plays an essential role in cell proliferation. Our bioinformatic analysis indicated that Grin2c and Ca2+ transport functions were inhibited in astrocytes after SCI. Suppression of Grin2c stimulated astrocyte proliferation by inhibiting the Ca2+/calmodulin-dependent protein kinase 2b (CaMK2b) pathway in vitro. By screening different inflammatory factors, interleukin 1α (IL1α) was further found to inhibit Grin2c/Ca2+/CaMK2b and enhance astrocyte proliferation in an oxidative damage model. Blockade of IL1α using neutralizing antibody resulted in increased Grin2c expression and the inhibition of astrocyte proliferation post-SCI. Overall, this study suggests that IL1α promotes astrocyte proliferation by suppressing the Grin2c/Ca2+/CaMK2b pathway after SCI, revealing a novel pathological mechanism of astrocyte proliferation, and may provide potential targets for SCI repair.

Inorganic polyphosphate: from basic research to diagnostic and therapeutic opportunities in ALS/FTD

Inorganic polyphosphate (polyP) is a simple, negatively charged biopolymer with chain lengths ranging from just a few to over a thousand ortho-phosphate (Pi) residues. polyP is detected in every cell type across all organisms in nature thus far analyzed. Despite its structural simplicity, polyP has been shown to play important roles in a remarkably broad spectrum of biological processes, including blood coagulation, bone mineralization and inflammation. Furthermore, polyP has been implicated in brain function and the neurodegenerative diseases amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), Alzheimer's disease and Parkinson's disease. In this review, we first address the challenges associated with identifying mammalian polyP metabolizing enzymes, such as Nudt3, and quantifying polyP levels in brain tissue, cultured neural cells and cerebrospinal fluid. Subsequently, we focus on recent studies that unveil how the excessive release of polyP by human and mouse ALS/FTD astrocytes contributes to these devastating diseases by inducing hyperexcitability, leading to motoneuron death. Potential implications of elevated polyP levels in ALS/FTD patients for innovative diagnostic and therapeutic approaches are explored. It is emphasized, however, that caution is required in targeting polyP in the brain due to its diverse physiological functions, serving as an energy source, a chelator for divalent cations and a scaffold for amyloidogenic proteins. Reducing polyP levels, especially in neurons, might thus have adverse effects in brain functioning. Finally, we discuss how activated mast cells and platelets also can significantly contribute to ALS progression, as they can massively release polyP.

Inorganic polyphosphate regulates functions of thymocytes via activation of P2X purinoreceptors

Inorganic polyphosphate (polyP) is an ancient polymer, which was proven to be a signalling molecule in the mammalian brain, mediating the communication between astrocytes via activation of P2Y1 purinoreceptors and modulating the activity of neurons. There is very limited information regarding the ability of polyP to transmit the information as an agonist of purinoreceptors in other cells and tissues. Here, we show that application of polyP to the suspension of primary thymocytes increases the concentration of intracellular calcium. PolyP evoked calcium signal was dependent on the presence of P2X inhibitors but not P2Y1 inhibitor. PolyP dependent increase in intracellular calcium concentration caused mild mitochondrial depolarization, which was dependent on inhibitors of purinoreceptors, extracellular calcium and inhibitor of mitochondrial calcium uniporter but wasn't dependent on cyclosporin A. Application of polyP modulated cell volume regulation machinery of thymocytes in calcium dependent manner. Molecular docking experiments revealed that polyP can potentially bind to several types of P2X receptors with binding energy similar to ATP - natural agonist of P2X purinoreceptors. Further molecular dynamics simulations with P2X4 showed that binding of one molecule of polyP dramatically increases permeability of this receptor-channel for water molecules. Thus, in this research we for the first time showed that polyP can interact with P2X receptors in thymocytes and modulate physiological processes.

ATP-mediated signalling in the central synapses

ATP released from the synaptic terminals and astrocytes can activate neuronal P2 receptors at a variety of locations across the CNS. Although the postsynaptic ATP-mediated signalling does not bring a major contribution into the excitatory transmission, it is instrumental for slow and diffuse modulation of synaptic dynamics and neuronal firing in many CNS areas. Neuronal P2X and P2Y receptors can be activated by ATP released from the synaptic terminals, astrocytes and microglia and thereby can participate in the regulation of synaptic homeostasis and plasticity. There is growing evidence of importance of purinergic regulation of synaptic transmission in different physiological and pathological contexts. Here, we review the main mechanisms underlying the complexity and diversity of purinergic signalling and purinergic modulation in central neurons.

Sodium Hexametaphosphate Serves as an Inducer of Calcium Signaling

In bacteria, polymers of inorganic phosphates, particularly linear polyphosphate, are used as alternative phosphate donors for adenosine triphosphate production. A six-chain form of sodium metaphosphate, sodium hexametaphosphate (SHMP), is believed to have no physiological functions in mammalian cells. In this study, we explored the possible effects of SHMP on mammalian cells, using mouse oocytes, which are useful for observing various spatiotemporal intracellular changes. Fertilization-competent oocytes were isolated from the oviducts of superovulated mice and cultured in an SHMP-containing medium. In the absence of co-incubation with sperm, SHMP-treated oocytes frequently formed pronuclei and developed into two-cell embryos owing to the increase in calcium concentration in the cytoplasm. We discovered an intriguing role for SHMP as an initiator of calcium rise in mouse oocytes, presumably in a wide variety of mammalian cells.

Carbon monoxide neurotoxicity is triggered by oxidative stress induced by ROS production from three distinct cellular sources

Carbon monoxide (CO) poisoning is one of the leading causes of toxic mortality and morbidity. We have studied the generation of reactive oxygen species in cortical neurons in culture in response to toxic doses of CO exposure. Fluorescence microscopy was used to measure the rate of free radical generation, lipid peroxidation, GSH level and also mitochondrial metabolism. We have found that toxic concentrations of CO released from CORM-401 induced mitochondrial depolarisation and inhibition of NADH dependent respiration to a lesser degree than when compared to ischaemia. Energy collapse was not observed within 40 min of CO exposure. We have found that CO induces the generation of reactive oxygen species resulting in lipid peroxidation and a decrease in GSH via three different mechanisms: from mitochondria during the first minutes of CO exposure, from xanthine oxidase at around 20 min exposure due to energy deprivation, and considerable ROS production from NADPH oxidase in the post CO exposure period (re-oxygenation). Inhibition of these different phases with mitochondrial antioxidants, inhibitors of xanthine oxidase, or NADPH oxidase, protected neurons and astrocytes against CO-induced oxidative stress and cell death. The most profound effect was seen during NADPH oxidase inhibition. Thus, oxidative stress has a remarkably significant role in CO-induced neuronal cell death and preventing its occurrence during reoxygenation is of great importance in the consideration of a positive, neurologically protective therapeutic outcome for CO exposed patients.

P2X7 Receptor and Purinergic Signaling: Orchestrating Mitochondrial Dysfunction in Neurodegenerative Diseases

Mitochondrial dysfunction is one of the basic hallmarks of cellular pathology in neurodegenerative diseases. Since the metabolic activity of neurons is highly dependent on energy supply, nerve cells are especially vulnerable to impaired mitochondrial function. Besides providing oxidative phosphorylation, mitochondria are also involved in controlling levels of second messengers such as Ca2+ ions and reactive oxygen species (ROS). Interestingly, the critical role of mitochondria as producers of ROS is closely related to P2XR purinergic receptors, the activity of which is modulated by free radicals. Here, we review the relationships between the purinergic signaling system and affected mitochondrial function. Purinergic signaling regulates numerous vital biological processes in the CNS. The two main purines, ATP and adenosine, act as excitatory and inhibitory neurotransmitters, respectively. Current evidence suggests that purinergic signaling best explains how neuronal activity is related to neuronal electrical activity and energy homeostasis, especially in the development of Alzheimer's and Parkinson's diseases. In this review, we focus on the mechanisms underlying the involvement of the P2RX7 purinoreceptor in triggering mitochondrial dysfunction during the development of neurodegenerative disorders. We also summarize various avenues by which the purine signaling pathway may trigger metabolic dysfunction contributing to neuronal death and the inflammatory activation of glial cells. Finally, we discuss the potential role of the purinergic system in the search for new therapeutic approaches to treat neurodegenerative diseases.

P2Y1 Receptor as a Catalyst of Brain Neurodegeneration

Different brain disorders display distinctive etiologies and pathogenic mechanisms. However, they also share pathogenic events. One event systematically occurring in different brain disorders, both acute and chronic, is the increase of the extracellular ATP levels. Accordingly, several P2 (ATP/ADP) and P1 (adenosine) receptors, as well as the ectoenzymes involved in the extracellular catabolism of ATP, have been associated to different brain pathologies, either with a neuroprotective or neurodegenerative action. The P2Y1 receptor (P2Y1R) is one of the purinergic receptors associated to different brain diseases. It has a widespread regional, cellular, and subcellular distribution in the brain, it is capable of modulating synaptic function and neuronal activity, and it is particularly important in the control of astrocytic activity and in astrocyte–neuron communication. In diverse brain pathologies, there is growing evidence of a noxious gain-of-function of P2Y1R favoring neurodegeneration by promoting astrocyte hyperactivity, entraining Ca2+-waves, and inducing the release of glutamate by directly or indirectly recruiting microglia and/or by increasing the susceptibility of neurons to damage. Here, we review the current evidence on the involvement of P2Y1R in different acute and chronic neurodegenerative brain disorders and the underlying mechanisms.

Endoplasmic Reticulum in Metaplasticity: From Information Processing to Synaptic Proteostasis

The ER (endoplasmic reticulum) is a Ca²⁺ reservoir and the unique protein-synthesizing machinery which is distributed throughout the neuron and composed of multiple different structural domains. One such domain is called EMC (endoplasmic reticulum membrane protein complex), pleiotropic nature in cellular functions. The ER/EMC position inside the neurons unmasks its contribution to synaptic plasticity via regulating various cellular processes from protein synthesis to Ca²⁺ signaling. Since presynaptic Ca²⁺ channels and postsynaptic ionotropic receptors are organized into the nanodomains, thus ER can be a crucial player in establishing TMNCs (transsynaptic molecular nanocolumns) to shape efficient neural communications. This review hypothesized that ER is not only involved in stress-mediated neurodegeneration but also axon regrowth, remyelination, neurotransmitter switching, information processing, and regulation of pre- and post-synaptic functions. Thus ER might not only be a protein-synthesizing and quality control machinery but also orchestrates plasticity of plasticity (metaplasticity) within the neuron to execute higher-order brain functions and neural repair.

Excessive release of inorganic polyphosphate by ALS/FTD astrocytes causes non-cell-autonomous toxicity to motoneurons

Non-cell-autonomous mechanisms contribute to neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), in which astrocytes release unidentified factors that are toxic to motoneurons (MNs). We report here that mouse and patient iPSC-derived astrocytes with diverse ALS/FTD-linked mutations (SOD1, TARDBP, and C9ORF72) display elevated levels of intracellular inorganic polyphosphate (polyP), a ubiquitous, negatively charged biopolymer. PolyP levels are also increased in astrocyte-conditioned media (ACM) from ALS/FTD astrocytes. ACM-mediated MN death is prevented by degrading or neutralizing polyP in ALS/FTD astrocytes or ACM. Studies further reveal that postmortem familial and sporadic ALS spinal cord sections display enriched polyP staining signals and that ALS cerebrospinal fluid (CSF) exhibits increased polyP concentrations. Our in vitro results establish excessive astrocyte-derived polyP as a critical factor in non-cell-autonomous MN degeneration and a potential therapeutic target for ALS/FTD. The CSF data indicate that polyP might serve as a new biomarker for ALS/FTD.

Engineered Neutral Phosphorous Dendrimers Protect Mouse Cortical Neurons and Brain Organoids from Excitotoxic Death

Nanoparticles are playing an increasing role in biomedical applications. Excitotoxicity plays a significant role in the pathophysiology of neurodegenerative diseases, such as Alzheimer’s or Parkinson’s disease. Glutamate ionotropic receptors, mainly those activated by N-methyl-D-aspartate (NMDA), play a key role in excitotoxic death by increasing intraneuronal calcium levels; triggering mitochondrial potential collapse; increasing free radicals; activating caspases 3, 9, and 12; and inducing endoplasmic reticulum stress. Neutral phosphorous dendrimers, acting intracellularly, have neuroprotective actions by interfering with NMDA-mediated excitotoxic mechanisms in rat cortical neurons. In addition, phosphorous dendrimers can access neurons inside human brain organoids, complex tridimensional structures that replicate a significant number of properties of the human brain, to interfere with NMDA-induced mechanisms of neuronal death. Phosphorous dendrimers are one of the few nanoparticles able to gain access to the inside of neurons, both in primary cultures and in brain organoids, and to exert pharmacological actions by themselves.

Mitochondrial Inorganic Polyphosphate (polyP) Is a Potent Regulator of Mammalian Bioenergetics in SH-SY5Y Cells: A Proteomics and Metabolomics Study

Inorganic polyphosphate (polyP) is an ancient, ubiquitous, and well-conserved polymer which is present in all the studied organisms. It is formed by individual subunits of orthophosphate which are linked by structurally similar bonds and isoenergetic to those found in ATP. While the metabolism and the physiological roles of polyP have already been described in some organisms, including bacteria and yeast, the exact role of this polymer in mammalian physiology still remains poorly understood. In these organisms, polyP shows a co-localization with mitochondria, and its role as a key regulator of the stress responses, including the maintenance of appropriate bioenergetics, has already been demonstrated by our group and others. Here, using Wild-type (Wt) and MitoPPX (cells enzymatically depleted of mitochondrial polyP) SH-SY5Y cells, we have conducted a comprehensive study of the status of cellular physiology, using proteomics and metabolomics approaches. Our results suggest a clear dysregulation of mitochondrial physiology, especially of bioenergetics, in MitoPPX cells when compared with Wt cells. Moreover, the effects induced by the enzymatic depletion of polyP are similar to those present in the mitochondrial dysfunction that is observed in neurodegenerative disorders and in neuronal aging. Based on our findings, the metabolism of mitochondrial polyP could be a valid and innovative pharmacological target in these conditions.

Activation of RAGE leads to the release of glutamate from astrocytes and stimulates calcium signal in neurons

The receptor for advanced glycation end products (RAGE) is a signal receptor first shown to be activated by advanced glycation end products, but also by a variety of signal molecules, including pathological advanced oxidation protein products and β-amyloid. However, most of the RAGE activators have multiple intracellular targets, making it difficult to unravel the exact pathway of RAGE activation. Here, we show that the cell-impermeable RAGE fragment sequence (60-76) of the V-domain of the receptor is able to activate RAGE present on the plasma membrane of neurons and, preferentially, astrocytes. This leads to the exocytosis of vesicular glutamate transporter vesicles and the release of glutamate from astrocytes, which stimulate NMDA and AMPA/kainate receptors, resulting in calcium signals predominantly in neurons. Thus, we show a specific mechanism of RAGE activation by the RAGE fragment and propose a mechanism by which RAGE activation can contribute to the neuronal-astrocytic communication in physiology and pathology.

Targeting Polyphosphate Kinases in the Fight against Pseudomonas aeruginosa

Polyphosphate (polyP) is a universally conserved molecule that plays critical roles in managing bacterial stress responses, in addition to biofilm formation and virulence. The enzymes that make polyphosphate molecules are called polyphosphate kinases (PPKs). Since these enzymes are not conserved in higher eukaryotes, PPKs make excellent therapeutic targets. In a recent paper in mBio, Neville and colleagues described gallein, a commercially available G-protein antagonist, as a novel dual-specificity inhibitor against two families of PPK enzymes in Pseudomonas aeruginosa. In this commentary, we discuss the impact of this discovery, outline potential challenges of implementing gallein use in the clinic, and describe how gallein will serve as a fantastic new tool to further fundamental PPK and polyP research in bacteria.

Mitochondrial calcium at the synapse

Mitochondria are dynamic organelles, which serve various purposes, including but not limited to the production of ATP and various metabolites, buffering ions, acting as a signaling hub, etc. In recent years, mitochondria are being seen as the central regulators of cellular growth, development, and death. Since neurons are highly specialized cells with a heavy metabolic demand, it is not surprising that neurons are one of the most mitochondria-rich cells in an animal. At synapses, mitochondrial function and dynamics is tightly regulated by synaptic calcium. Calcium influx during synaptic activity causes increased mitochondrial calcium influx leading to an increased ATP production as well as buffering of synaptic calcium. While increased ATP production is required during synaptic transmission, calcium buffering by mitochondria is crucial to prevent faulty neurotransmission and excitotoxicity. Interestingly, mitochondrial calcium also regulates the mobility of mitochondria within synapses causing mitochondria to halt at the synapse during synaptic transmission. In this review, we summarize the various roles of mitochondrial calcium at the synapse.

Dopamine controls neuronal spontaneous calcium oscillations via astrocytic signal

Dopamine is a neuromodulator and neurotransmitter responsible for a number of physiological processes. Dysfunctions of the dopamine metabolism and signalling are associated with neurological and psychiatric diseases. Here we report that in primary co-culture of neurons and astrocytes dopamine-induces calcium signal in astrocytes and suppress spontaneous synchronous calcium oscillations (SSCO) in neurons. Effect of dopamine on SSCO in neurons was dependent on calcium signal in astrocytes and could be modified by inhibition of dopamine-induced calcium signal or by stimulation of astrocytic calcium rise with ATP. Ability of dopamine to suppress SSCO in neurons was independent on D1- or D2- like receptors but dependent on GABA and alpha-adrenoreceptors. Inhibitor of monoaminoxidase bifemelane blocked effect of dopamine on astrocytes but also inhibited the effect dopamine on SSCO in neurons. These findings suggest that dopamine-induced calcium signal may stimulate release of neuromodulators such as GABA and adrenaline and thus suppress spontaneous calcium oscillations in neurons.

Inorganic Polyphosphate in Host and Microbe Biology

Inorganic polyphosphate (polyP) is produced by both bacteria and their eukaryotic hosts, and it appears to play multiple important roles in the interactions between those organisms. However, the detailed mechanisms of how polyP synthesis is regulated in bacteria, and how it influences both bacterial and host biology, remain largely unexplored. In this review, we examine recent developments in the understanding of how bacteria regulate the synthesis of polyP, what roles polyP plays in controlling virulence in pathogenic bacteria, and the effects of polyP on the mammalian immune system, as well as progress on developing drugs that may be able to target bacterial polyP synthesis as novel means of treating infectious disease.

Influence of energy deficiency on the subcellular processes of Substantia Nigra Pars Compacta cell for understanding Parkinsonian neurodegeneration

Parkinson's disease (PD) is the second most prominent neurodegenerative disease around the world. Although it is known that PD is caused by the loss of dopaminergic cells in substantia nigra pars compacta (SNc), the decisive cause of this inexorable cell loss is not clearly elucidated. We hypothesize that "Energy deficiency at a sub-cellular/cellular/systems level can be a common underlying cause for SNc cell loss in PD." Here, we propose a comprehensive computational model of SNc cell, which helps us to understand the pathophysiology of neurodegeneration at the subcellular level in PD. The aim of the study is to see how deficits in the supply of energy substrates (glucose and oxygen) lead to a deficit in adenosine triphosphate (ATP). The study also aims to show that deficits in ATP are the common factor underlying the molecular-level pathological changes, including alpha-synuclein aggregation, reactive oxygen species formation, calcium elevation, and dopamine dysfunction. The model suggests that hypoglycemia plays a more crucial role in leading to ATP deficits than hypoxia. We believe that the proposed model provides an integrated modeling framework to understand the neurodegenerative processes underlying PD.

Is there a link between inorganic polyphosphate (polyP), mitochondria, and neurodegeneration?

Mitochondrial dysfunction - including increased apoptosis, calcium and protein dyshomeostasis within the organelle, and dysfunctional bioenergetics and oxidative status - is a common, early feature in all the major neurodegenerative diseases, including Alzheimer's Disease (AD) and Parkinson's Disease (PD). However, the exact molecular mechanisms that drive the organelle to dysfunction and ultimately to failure in these conditions are still not well described. Different authors have shown that inorganic polyphosphate (polyP), an ancient and well-conserved molecule, plays a key role in the regulation of mitochondrial physiology under basal conditions. PolyP, which is present in all studied organisms, is composed of chains of orthophosphates linked together by highly energetic phosphoanhydride bonds, similar to those found in ATP. This polymer shows a ubiquitous distribution, even if a high co-localization with mitochondria has been reported. It has been proposed that polyP might be an alternative to ATP for cellular energy storage in different organisms, as well as the implication of polyP in the regulation of many of the mitochondrial processes affected in AD and PD, including protein and calcium homeostasis. Here, we conduct a comprehensive review and discussion of the bibliography available regarding the role of polyP in the mitochondrial dysfunction present in AD and PD. Taking into account the data presented in this review, we postulate that polyP could be a valid, innovative and, plausible pharmacological target against mitochondrial dysfunction in AD and PD. However, further research should be conducted to better understand the exact role of polyP in neurodegeneration, as well as the metabolism of the polymer, and the effect of different lengths of polyP on cellular and mitochondrial physiology.

Variability of mitochondrial energy balance across brain regions

Brain is not homogenous and neurons from various brain regions are known to have different vulnerabilities to mitochondrial mutations and mitochondrial toxins. However, it is not clear if this vulnerability is connected to different energy metabolism in specific brain regions. Here, using live-cell imaging, we compared mitochondrial membrane potential and nicotinamide adenine dinucleotide (NADH) redox balance in acute rat brain slices in different brain regions and further detailed the mitochondrial metabolism in primary neurons and astrocytes from rat cortex, midbrain and cerebellum. We have found that mitochondrial membrane potential is higher in brain slices from the hippocampus and brain stem. In primary co-cultures, mitochondrial membrane potential in astrocytes was lower than in neurons, whereas in midbrain cells it was higher than in cortex and cerebellum. The rate of NADH production and mitochondrial NADH pool were highest in acute slices from midbrain and midbrain primary neurons and astrocytes. Although the level of adenosine tri phosphate (ATP) was similar among primary neurons and astrocytes from cortex, midbrain and cerebellum, the rate of ATP consumption was highest in midbrain cells that lead to faster neuronal and astrocytic collapse in response to inhibitors of ATP production. Thus, midbrain neurons and astrocytes have a higher metabolic rate and ATP consumption that makes them more vulnerable to energy deprivation.

Inorganic polyphosphate is produced and hydrolyzed in F0F1-ATP synthase of mammalian mitochondria

Inorganic polyphosphate (polyP) is a polymer present in all living organisms. Although polyP is found to be involved in a variety of functions in cells of higher organisms, the enzyme responsible for polyP production and consumption has not yet been identified. Here, we studied the effect of polyP on mitochondrial respiration, oxidative phosphorylation and activity of F₀F₁-ATPsynthase. We have found that polyP activates mitochondrial respiration which does not coupled with ATP production (V₂) but inhibits ADP-dependent respiration (V₃). Moreover, PolyP can stimulate F₀F₁-ATPase activity in the presence of ATP and, importantly, can be hydrolyzed in this enzyme instead of ATP. Furthermore, PolyP can be produced in mitochondria in the presence of substrates for respiration and phosphate by the F₀F₁-ATPsynthase. Thus, polyP is an energy molecule in mammalian cells which can be produced and hydrolyzed in the mitochondrial F₀F₁-ATPsynthase.

Environmental and Nutritional "Stressors" and Oligodendrocyte Dysfunction: Role of Mitochondrial and Endoplasmatic Reticulum Impairment

Oligodendrocytes are myelinating cells of the central nervous system which are generated by progenitor oligodendrocytes as a result of maturation processes. The main function of mature oligodendrocytes is to produce myelin, a lipid-rich multi-lamellar membrane that wraps tightly around neuronal axons, insulating them and facilitating nerve conduction through saltatory propagation. The myelination process requires the consumption a large amount of energy and a high metabolic turnover. Mitochondria are essential organelles which regulate many cellular functions, including energy production through oxidative phosphorylation. Any mitochondrial dysfunction impacts cellular metabolism and negatively affects the health of the organism. If the functioning of the mitochondria is unbalanced, the myelination process is impaired. When myelination has finished, oligodendrocyte will have synthesized about 40% of the total lipids present in the brain. Since lipid synthesis occurs in the cellular endoplasmic reticulum, the dysfunction of this organelle can lead to partial or deficient myelination, triggering numerous neurodegenerative diseases. In this review, the induced malfunction of oligodendrocytes by harmful exogenous stimuli has been outlined. In particular, the effects of alcohol consumption and heavy metal intake are discussed. Furthermore, the response of the oligodendrocyte to excessive mitochondrial oxidative stress and to the altered regulation of the functioning of the endoplasmic reticulum will be explored.

Oxytocin Receptor Signaling in Vascular Function and Stroke

The oxytocin receptor (OXTR) is a G protein-coupled receptor with a diverse repertoire of intracellular signaling pathways, which are activated in response to binding oxytocin (OXT) and a similar nonapeptide, vasopressin. This review summarizes the cell and molecular biology of the OXTR and its downstream signaling cascades, particularly focusing on the vasoactive functions of OXTR signaling in humans and animal models, as well as the clinical applications of OXTR targeting cerebrovascular accidents.

Dexmedetomidine Attenuates Glutamate-Induced Cytotoxicity by Inhibiting the Mitochondrial-Mediated Apoptotic Pathway

Background

Glutamate (GLU) is the most excitatory amino acid in the central nervous system and plays an important role in maintaining the normal function of the nervous system. During cerebral ischemia, massive release of GLU leads to neuronal necrosis and apoptosis. It has been reported that dexmedetomidine (DEX) possesses anti-oxidant and anti-apoptotic properties. The objective of this study was to investigate the effects of DEX on GLU-induced neurotoxicity in PC12 cells.

Material/Methods

PC12 cells were treated with 20 mM GLU to establish an ischemia-induced injury model. Cell viability was accessed by MTT assay. MDA content and SOD activity were analyzed by assay kits. Apoptosis rate, ROS production, intracellular Ca²⁺ concentration, and MMP were evaluated by flow cytometry. Western blot analysis was performed to analyze expressions of caspase-3, caspase-9, cyt-c, bax, and bcl-2.

Results

PC12 cells treated with GLU exhibited reduced cell viability and increased apoptosis rates, which were ameliorated by pretreatment with DEX. DEX significantly increased SOD activity, reduced content of MDA, and decreased production of ROS in PC12 cells. In addition, DEX clearly reduced the level of intracellular Ca²⁺ and attenuated the decline of MMP. Moreover, DEX notably reduced expressions of caspase-3, caspase-9, cyt-c, and bax and increased expression of bcl-2.

Conclusions

Our findings suggest that DEX can protect PC12 cells against GLU-induced cytotoxicity, which may be attributed to its anti-oxidative property and reduction of intracellular calcium overload, as well as its ability to inhibit the mitochondria-mediated apoptotic pathway.