Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses

A novel in silico approach for predicting unbound brain-to-plasma ratio using machine learning-based support vector regression

The blood-brain barrier (BBB) functions as a vital protective mechanism, restricting the entry of substances and xenobiotics into the central nervous system (CNS). Consequently, BBB penetration is a critical aspect of absorption, distribution, metabolism, elimination, and toxicity (ADME/Tox) considerations in drug discovery and development as it is essential to minimize CNS-associated side effects in systemically targeted drugs and to enhance efficacy in CNS-targeted therapeutics. In this study, an in silico model utilizing the novel machine learning-based hierarchical support vector regression (HSVR) scheme was developed to predict the unbound brain-to-plasma concentration ratio (Kp,uu,brain) values using a diverse dataset of compounds with known BBB penetration properties. The HSVR model leverages a hierarchical framework to capture the complex relationships between molecular descriptors and BBB penetration mechanisms that can otherwise be insurmountably difficult for traditional methods or other machine learning algorithms. These complexities arise from the fact that BBB penetration can be governed by various factors, including passive diffusion and active influx and efflux transport processes. The accuracy, predictivity, robustness of HSVR were rigorously validated using comprehensive valuation metrics and stringent validation criteria. Its practical application was further substantiated through a mock test. Comparative analyses revealed that the HSVR model outperforms existing published models both quantitatively and qualitatively, providing a reliable tool for early-stage drug discovery and development. The adoption of this model has the potential to significantly streamline BBB penetration assessments, minimizing reliance on in vivo studies while expediting the identification of viable CNS drug candidates or systemic drug candidates prone to CNS-related challenges. This approach aligns seamlessly with the "fail early and fail fast" paradigm of modern drug discovery, enhancing both efficiency and cost-effectiveness.

Multifunctional Carbon Dots In Situ Confined Hydrogel for Optical Communication, Drug Delivery, pH Sensing, Nanozymatic Activity, and UV Shielding Applications

Inspired by the emerging potential of photoluminescent hydrogels, this work unlocks new avenues for advanced biosensing, bioimaging, and drug delivery applications. Carbon quantum dots (CDs) are deemed particularly promising among various optical dyes, for enhancing polymeric networks with superior physical and chemical properties. This study presents the synthesis of CDs derived from Prunella vulgaris, a natural plant resource, through a single-step hydrothermal process, followed by their uniform integration into hydrogel matrices via an in situ free radical graft polymerization. The resulting CD-integrated hydrogels exhibit multifunctionality in biomedical applications, featuring a diffusion-controlled drug release mechanism, permit concurrent delivery of photoluminescent CDs and therapeutic agents, enabling real-time monitoring over 32 h. In addition, these hydrogels function as a broad-range optical pH sensor (pH 3-11), provide robust ultraviolet (UV) shielding, and demonstrate nanozyme-like peroxidase activity. Critically, biocompatibility tests confirm their non-cytotoxicity toward fibroblast cells, establishing these hydrogels as promising candidates for diverse biomedical applications. These include advanced wound dressings that monitor the healing process and detect infection through pH sensing, and promote healing through the nanozymatic activity, all while maintaining a moist wound microenvironment. These hydrogels demonstrate exceptional suitability for advanced smart drug delivery, effective UV-blocking, and as innovative platforms for in vivo sensing and bioimaging.

Role of astrocytes connexins - pannexins in acute brain injury

Acute brain injuries (ABIs) encompass a broad spectrum of primary injuries such as ischemia, hypoxia, trauma, and hemorrhage that converge into secondary injury where some mechanisms show common determinants. In this regard, astroglial connexin and pannexin channels have been shown to play an important role. These channels are transmembrane proteins sharing similar topology and form gateways between adjacent cells named gap junctions (GJs) and pores into unopposed membranes named hemichannels (HCs). In astrocytes, GJs and HCs enable intercellular communication and have active participation in normal brain physiological processes, such as calcium waves, synapsis modulation, regional blood flow regulation, and homeostatic control of the extracellular environment, among others. However, after acute brain injury, astrocytes can change their phenotype and modify the activity of both channels and hemichannels, which can result in the amplification of danger signals, increased mediators of inflammation, and neuronal death, contributing to the expansion of brain damage and neurological deterioration. This is known as secondary brain damage. In this review, we discussed the main biological mechanism of secondary brain damage with a particular focus on astroglial connexin and pannexin participation during acute brain injuries.

Measuring cerebral enzymatic activity, brain pH and extracranial muscle metabolism with hyperpolarized 13C-pyruvate

Hyperpolarized carbon-13 (13C) magnetic resonance imaging (MRI) has shown promise for non-invasive assessment of the cerebral metabolism of [1-13C]pyruvate in both healthy volunteers and patients. The exchange of pyruvate to lactate catalysed by lactate dehydrogenase (LDH) and that of pyruvate flux to bicarbonate through pyruvate dehydrogenase (PDH) are the most widely studied reactions in vivo. Here we show the potential of the technique to probe additional enzymatic activity within the brain. Approximately 50 s after intravenous injection of hyperpolarized pyruvate, high-flip-angle pulses were used to detect cerebral 13C-labelled carbon dioxide (13CO2), in addition to the 13C-bicarbonate (H13CO3 -) subsequently formed by carbonic anhydrase (CA). Brain pH measurements, which were weighted towards the extracellular compartment, were calculated from the ratio of H13CO3 - to 13CO2 in seven volunteers using the Henderson-Hasselbalch equation, demonstrating an average pH ± SD of 7.40 ± 0.02, with inter-observer reproducibility of 0.04. In addition, hyperpolarized [1-13C]aspartate was also detected, demonstrating irreversible pyruvate carboxylation to oxaloacetate by pyruvate carboxylase (PC) and subsequent transamination by aspartate aminotransferase (AST), with the average flux being on average 11% ± 3% of that through PDH. A hyperpolarized [1-13C]alanine signal was also detected, but this was localized to extracranial muscle tissue in keeping with skeletal alanine aminotransferase (ALT) activity. The results demonstrate the potential of hyperpolarized 13C-MRI to assess cerebral and extracerebral [1-13C]pyruvate metabolism in addition to LDH and PDH activity. Non-invasive measurements of brain pH could be particularly important in assessing cerebral pathology given the wide range of disease processes that alter acid-base balance.

pH regulating mechanisms of astrocytes: A critical component in physiology and disease of the brain

Strict homeostatic control of pH in both intra- and extracellular compartments of the brain is fundamentally important, primarily due to the profound impact of free protons ([H+]) on neuronal activity and overall brain function. Astrocytes, crucial players in the homeostasis of various ions in the brain, actively regulate their intracellular [H+] (pHi) through multiple membrane transporters and carbonic anhydrases. The activation of astroglial pHi regulating mechanisms also leads to corresponding alterations in the acid-base status of the extracellular fluid. Notably, astrocyte pH regulators are modulated by various neuronal signals, suggesting their pivotal role in regulating brain acid-base balance in both health and disease. This review presents the mechanisms involved in pH regulation in astrocytes and discusses their potential impact on extracellular pH under physiological conditions and in brain disorders. Targeting astrocytic pH regulatory mechanisms represents a promising therapeutic approach for modulating brain acid-base balance in diseases, offering a potential critical contribution to neuroprotection.

FXTAS Neuropathology Includes Widespread Reactive Astrogliosis and White Matter Specific Astrocyte Degeneration

OBJECTIVE

Fragile X-associated tremor/ataxia syndrome (FXTAS) is a late-onset progressive genetic neurodegenerative disorder that occurs in FMR1 premutation carriers. The temporal, spatial, and cell-type specific patterns of neurodegeneration in the FXTAS brain remain incompletely characterized. Intranuclear inclusion bodies are the neuropathological hallmark of FXTAS, which are largest and occur most frequently in astrocytes, glial cells that maintain brain homeostasis. Here, we characterized neuropathological alterations in astrocytes in multiple regions of the FXTAS brain.

METHODS

Striatal and cerebellar sections from FXTAS cases (n = 12) and controls (n = 12) were stained for the astrocyte markers glial fibrillary acidic protein (GFAP) and aldehyde dehydrogenase 1L1 (ALDH1L1) using immunohistochemistry. Reactive astrogliosis severity, the prevalence of GFAP+ fragments, and astrocyte density were scored. Double label immunofluorescence was utilized to detect co-localization of GFAP and cleaved caspase-3.

RESULTS

FXTAS cases showed widespread reactive gliosis in both grey and white matter. GFAP staining also revealed remarkably severe astrocyte pathology in FXTAS white matter - characterized by a significant and visible reduction in astrocyte density (-38.7% in striatum and - 32.2% in cerebellum) and the widespread presence of GFAP+ fragments reminiscent of apoptotic bodies. White matter specific reductions in astrocyte density were confirmed with ALDH1L1 staining. GFAP+ astrocytes and fragments in white matter were positive for cleaved caspase-3, suggesting that apoptosis-mediated degeneration is responsible for reduced astrocyte counts.

INTERPRETATION

We have established that FXTAS neuropathology includes robust degeneration of astrocytes, which is specific to white matter. Because astrocytes are essential for maintaining homeostasis within the central nervous system, a loss of astrocytes likely further exacerbates neuropathological progression of other cell types in the FXTAS brain. ANN NEUROL 2024;95:558-575.

Effects of carbonic anhydrase activity on the excitability of hippocampal axons during high-frequency firing

Epileptic activity is known to cause a lowering of intraneuronal pH, which has been suggested to serve as a feedback signal to terminate seizures. The mechanism of such signaling is unclear, but likely involves an altered function of several types of ligand- and voltage-gated channels in postsynaptic membranes caused by increasing cytosolic and extracellular [H+]. In addition, axonal conduction properties may be altered by endogenous pH signals, but this has not been investigated. In the present study, we have recorded the axonal compound action potential (fiber volley) in hippocampal slices in the presence of glutamatergic and GABAergic antagonists. During high-frequency stimulation (HFS) of the Schaffer collaterals, the fiber volley was depressed and its latency from stimulus to peak increased. In the CA1 stratum radiatum these changes were enhanced when the carbonic anhydrase inhibitor acetazolamide (1 mM) was co-perfused. The enhancing effect of acetazolamide was absent after lowering of [Ca2+] in the perfusion medium. Acetazolamide had no detectable effect on HFS-evoked fiber volleys recorded from a more proximal site along the Schaffer collaterals (at the CA2-CA3 border) or from axons in the alveus of CA1. Intracellular acidification imposed by washout of NH4Cl (5 mM) had qualitatively similar effects on fiber volleys evoked at low frequency as those observed with acetazolamide during HFS in CA1 stratum radiatum. The results suggest that carbonic anhydrase-dependent pH regulation counteracts activity-induced reduction of the excitability of Schaffer collateral axons in CA1. A possible influence from local synaptic terminals on this effect is discussed.

Exogenous lactate administration: A potential novel therapeutic approach for neonatal hypoxia-ischemia

Neonatal hypoxic-ischemic encephalopathy (HIE) is the primary reason for neonatal mortality and prolonged disablement. Currently, hypothermia is the only approved clinical treatment available for HIE. However, hypothermia's limited therapeutic efficacy and adverse effects suggest an urgent need to advance our knowledge of its molecular pathogenesis and develop novel therapies. The leading cause of HIE is impaired cerebral blood flow and oxygen deprivation-initiated primary and secondary energy failure. Lactate was traditionally regarded as a marker of energy failure or a waste product of anaerobic glycolysis. Recently, the beneficial aspects of lactate as supplementary energy for neurons have been demonstrated. Under the conditions of HI, lactate supports various functions of neuronal cells, including learning and memory formation, motor coordination, and somatosensory. Furthermore, lactate contributes to the regeneration of blood vessels and has shown its beneficial effects on the immune system. This review first introduces the hypoxic or ischemic events-induced fundamental pathophysiological changes in HIE and then discusses the potential neuroprotective properties of lactate for the treatment and prevention of HIE. Finally, we discuss the possible protective mechanisms of lactate in the context of the pathological features of perinatal HIE. We conclude that exogenous and endogenous lactate exert neuroprotective effects in HIE. Lactate administration may be a potential approach to treating HIE injury.

Upregulation of carbonic anhydrase 1 beneficial for depressive disorder

Carbonic Anhydrase 1 (CAR1) is a zinc-metalloenzyme that catalyzes the hydration of carbon dioxide, and the alteration of CAR1 has been implicated in neuropsychiatric disorders. However, the mechanism underlying the role of CAR1 in major depressive disorder (MDD) remains largely unknown. In this study, we report the decreased level of CAR1 in MDD patients and depression-like model rodents. We found that CAR1 is expressed in hippocampal astrocytes and CAR1 regulates extracellular bicarbonate concentration and pH value in the partial hilus. Ablation of the CAR1 gene increased the activity of granule cells via decreasing their miniature inhibitory postsynaptic currents (mIPSC), and caused depression-like behaviors in CAR1-knockout mice. Astrocytic CAR1 expression rescued the deficits in mIPSCs of granule cells and reduced depression-like behaviors in CAR1 deficient mice. Furthermore, pharmacological activation of CAR1 and overexpression of CAR1 in the ventral hippocampus of mice improved depressive behaviors. These findings uncover a critical role of CAR1 in the MDD pathogenesis and its therapeutic potential.

Effects of pH alterations on stress- and aging-induced protein phase separation

Upon stress challenges, proteins/RNAs undergo liquid–liquid phase separation (LLPS) to fine-tune cell physiology and metabolism to help cells adapt to adverse environments. The formation of LLPS has been recently linked with intracellular pH, and maintaining proper intracellular pH homeostasis is known to be essential for the survival of organisms. However, organisms are constantly exposed to diverse stresses, which are accompanied by alterations in the intracellular pH. Aging processes and human diseases are also intimately linked with intracellular pH alterations. In this review, we summarize stress-, aging-, and cancer-associated pH changes together with the mechanisms by which cells regulate cytosolic pH homeostasis. How critical cell components undergo LLPS in response to pH alterations is also discussed, along with the functional roles of intracellular pH fluctuation in the regulation of LLPS. Further studies investigating the interplay of pH with other stressors in LLPS regulation and identifying protein responses to different pH levels will provide an in-depth understanding of the mechanisms underlying pH-driven LLPS in cell adaptation. Moreover, deciphering aging and disease-associated pH changes that influence LLPS condensate formation could lead to a deeper understanding of the functional roles of biomolecular condensates in aging and aging-related diseases.

Oxygen control: the often overlooked but essential piece to create better in vitro systems

Variations in oxygen levels play key roles in numerous physiological and pathological processes, but are often not properly controlled in in vitro models, introducing a significant bias in experimental outcomes. Recent developments in microfluidic technology have introduced a paradigm shift by providing new opportunities to better mimic physiological and pathological conditions, which is achieved by both regulating and monitoring oxygen levels at the micrometre scale in miniaturized devices. In this review, we first introduce the nature and relevance of oxygen-dependent pathways in both physiological and pathological contexts. Subsequently, we discuss strategies to control oxygen in microfluidic devices, distinguishing between engineering approaches that operate at the device level during its fabrication and chemical approaches that involve the active perfusion of fluids oxygenated at a precise level or supplemented with oxygen-producing or oxygen-scavenging materials. In addition, we discuss readout approaches for monitoring oxygen levels at the cellular and tissue levels, focusing on electrochemical and optical detection schemes for high-resolution measurements directly on-chip. An overview of different applications in which microfluidic devices have been utilized to answer biological research questions is then provided. In the final section, we provide our vision for further technological refinements of oxygen-controlling devices and discuss how these devices can be employed to generate new fundamental insights regarding key scientific problems that call for emulating oxygen levels as encountered in vivo. We conclude by making the case that ultimately emulating physiological or pathological oxygen levels should become a standard feature in all in vitro cell, tissue, and organ models.

Hydrogen Ion Dynamics as the Fundamental Link between Neurodegenerative Diseases and Cancer: Its Application to the Therapeutics of Neurodegenerative Diseases with Special Emphasis on Multiple Sclerosis

The pH-related metabolic paradigm has rapidly grown in cancer research and treatment. In this contribution, this recent oncological perspective has been laterally assessed for the first time in order to integrate neurodegeneration within the energetics of the cancer acid-base conceptual frame. At all levels of study (molecular, biochemical, metabolic, and clinical), the intimate nature of both processes appears to consist of opposite mechanisms occurring at the far ends of a physiopathological intracellular pH/extracellular pH (pHi/pHe) spectrum. This wide-ranging original approach now permits an increase in our understanding of these opposite processes, cancer and neurodegeneration, and, as a consequence, allows us to propose new avenues of treatment based upon the intracellular and microenvironmental hydrogen ion dynamics regulating and deregulating the biochemistry and metabolism of both cancer and neural cells. Under the same perspective, the etiopathogenesis and special characteristics of multiple sclerosis (MS) is an excellent model for the study of neurodegenerative diseases and, utilizing this pioneering approach, we find that MS appears to be a metabolic disease even before an autoimmune one. Furthermore, within this paradigm, several important aspects of MS, from mitochondrial failure to microbiota functional abnormalities, are analyzed in depth. Finally, and for the first time, a new and integrated model of treatment for MS can now be advanced.

Acidic pH reduces agonist efficacy and responses to synaptic-like glycine applications in zebrafish α1 and rat α1β recombinant glycine receptors

Many pentameric ligand-gated ion channels are modulated by extracellular pH. Glycine receptors (GlyRs) share this property, but it is not well understood how they are affected by pH changes. Whole cell experiments on HEK293 cells expressing zebrafish homomeric α1 GlyR confirmed previous reports that acidic pH (6.4) reduces GlyR sensitivity to glycine, whereas alkaline pH (8.4) has small or negligible effects. In addition to that, at pH 6.4 we observed a reduction in the maximum responses to the partial agonists β-alanine and taurine relative to the full agonist glycine. In cell-attached single-channel recording, low pH reduced agonist efficacy, as the maximum open probability decreased from 0.97, 0.91 and 0.66 to 0.93, 0.57 and 0.34 for glycine, β-alanine and taurine, respectively, reflecting a threefold decrease in efficacy equilibrium constants for all three agonists. We also tested the effect of pH 6.4 in conditions that replicate those at the native synapse, recording outside-out currents elicited by fast application of millisecond pulses of agonists on α1 and α1β GlyR, at a range of intracellular chloride concentrations. Acidic pH reduced the area under the curve of the currents, by reducing peak amplitude, slowing activation and speeding deactivation. Our results show that acidification of the extracellular pH by one unit, as may occur in pathological conditions such as ischaemia, impairs GlyR gating and is likely to reduce the effectiveness of glycinergic synaptic inhibition. KEY POINTS: Extracellular pH in the central nervous system (CNS) is known to shift towards acidic values during pathophysiological conditions such as ischaemia and seizures. Acidic extracellular pH is known to affect GABAergic inhibitory synapses, but its effect on signals mediated by glycine receptors (GlyR) is not well characterised. Moderate acidic conditions (pH 6.4) reduce the maximum single channel open probability of recombinant homomeric GlyRs produced by the neurotransmitter glycine or other agonists, such as β-alanine and taurine. When glycine was applied with a piezoelectric stepper to outside out patches, to simulate its fast rise and short duration at the synapse, responses became shorter and smaller at pH 6.4. The effect was also observed with physiologically low intracellular chloride and in mammalian heteromeric GlyRs. This suggests that acidic pH is likely to reduce the strength of inhibitory signalling at glycinergic synapses.

Persistent CO2 reactivity deficits are associated with neurological dysfunction up to one year after repetitive mild closed head injury in adolescent mice

Cerebrovascular reactivity (CVR) deficits in adolescents with concussion may persist after resolution of neurological symptoms. Whether or not CVR deficits predict long term neurological function is unknown. We used adolescent mice closed head injury (CHI) models (54 g, 107 cm or 117 cm drop height), followed by blood oxygenation level dependent (BOLD)-functional MRI with CO₂ challenge to assess CVR and brain connectivity. At one week, 3HD 107 cm mice showed delayed BOLD responses (p = 0.0074), normal striatal connectivity_, and an impaired respiratory rate response to CO₂ challenge (p = 0.0061 in ΔRmax). The 107 cm group developed rotarod deficits at 6 months (p = 0.02) and altered post-CO₂ brain connectivity (3-fold increase in striatum to motor cortex correlation coefficient) by one year, but resolved their CVR and respiratory rate impairments, and did not develop cognitive or circadian activity deficits. In contrast, the 117 cm group had persistent CVR (delay time: p = 0.016; washout time: p = 0.039) and circadian activity deficits (free-running period: 23.7 hr in sham vs 23.9 hr in 3HD; amplitude: 0.15 in sham vs 0.2 in 3HD; peak activity: 18 in sham vs 21 in 3HD) at one year. Persistent CVR deficits after concussion may portend long-term neurological dysfunction. Further studies are warranted to determine the utility of CVR to predict chronic neurological outcome after mild traumatic brain injury.

Computational modeling predicts ephemeral acidic microdomains in the glutamatergic synaptic cleft

At chemical synapses, synaptic vesicles release their acidic contents into the cleft, leading to the expectation that the cleft should acidify. However, fluorescent pH probes targeted to the cleft of conventional glutamatergic synapses in both fruit flies and mice reveal cleft alkalinization rather than acidification. Here, using a reaction-diffusion scheme, we modeled pH dynamics at the Drosophila neuromuscular junction as glutamate, ATP, and protons (H⁺) were released into the cleft. The model incorporates bicarbonate and phosphate buffering systems as well as plasma membrane calcium-ATPase activity and predicts substantial cleft acidification but only for fractions of a millisecond after neurotransmitter release. Thereafter, the cleft rapidly alkalinizes and remains alkaline for over 100 ms because the plasma membrane calcium-ATPase removes H⁺ from the cleft in exchange for calcium ions from adjacent pre- and postsynaptic compartments, thus recapitulating the empirical data. The extent of synaptic vesicle loading and time course of exocytosis have little influence on the magnitude of acidification. Phosphate but not bicarbonate buffering is effective at suppressing the magnitude and time course of the acid spike, whereas both buffering systems are effective at suppressing cleft alkalinization. The small volume of the cleft levies a powerful influence on the magnitude of alkalinization and its time course. Structural features that open the cleft to adjacent spaces appear to be essential for alleviating the extent of pH transients accompanying neurotransmission.

High-Throughput and Real-Time Monitoring of Single-Cell Extracellular pH Based on Polyaniline Microarrays

Real-time monitoring of extracellular pH (pHe) at the single-cell level is critical for elucidating the mechanisms of disease development and investigating drug effects, with particular importance in cancer cells. However, there are still some challenges for analyzing and measuring pHe due to the strong heterogeneity of cancer cells. Thus, it is necessary to develop a reliable method with good selectivity, reproducibility, and stability for achieving the pHe heterogeneity of cancer cells. In this paper, we report a high-throughput, real-time measuring technique based on polyaniline (PANI) microelectrode arrays for monitoring single-cell pHe. The PANI microelectrode array not only has a high sensitivity (57.22 mV/pH) ranging from pH 6.0 to 7.6 but also exhibits a high reliability (after washing, the PANI film was still smooth, dense, and with a sensitivity of 55.9 mV/pH). Our results demonstrated that the pHe of the cancer cell region is lower than that of the surrounding blank region, and pHe changes of different cancer cells exhibit significant cellular heterogeneity during cellular respiration and drug stimulation processes.

Effects of lactate and carbon monoxide interactions on neuroprotection and neuropreservation

Lactate, historically considered a waste product of anerobic metabolism, is a metabolite in whole-body metabolism needed for normal central nervous system (CNS) functions and a potent signaling molecule and hormone in the CNS. Neuronal activity signals normally induce its formation primarily in astrocytes and production is dependent on anerobic and aerobic metabolisms. Functions are dependent on normal dynamic, expansive, and evolving CNS functions. Levels can change under normal physiologic conditions and with CNS pathology. A readily combusted fuel that is sshuttled throughout the body, lactate is used as an energy source and is needed for CNS hemostasis, plasticity, memory, and excitability. Diffusion beyond the neuron active zone impacts activity of neurons and astrocytes in other areas of the brain. Barriergenesis, function of the blood-brain barrier, and buffering between oxidative metabolism and glycolysis and brain metabolism are affected by lactate. Important to neuroprotection, presence or absence is associated with L-lactate and heme oxygenase/carbon monoxide (a gasotransmitter) neuroprotective systems. Effects of carbon monoxide on L-lactate affect neuroprotection - interactions of the gasotransmitter with L-lactate are important to CNS stability, which will be reviewed in this article.

Decreased Brain pH and Pathophysiology in Schizophrenia

Postmortem studies reveal that the brain pH in schizophrenia patients is lower than normal. The exact cause of this low pH is unclear, but increased lactate levels due to abnormal energy metabolism appear to be involved. Schizophrenia patients display distinct changes in mitochondria number, morphology, and function, and such changes promote anaerobic glycolysis, elevating lactate levels. pH can affect neuronal activity as H⁺ binds to numerous proteins in the nervous system and alters the structure and function of the bound proteins. There is growing evidence of pH change associated with cognition, emotion, and psychotic behaviors. Brain has delicate pH regulatory mechanisms to maintain normal pH in neurons/glia and extracellular fluid, and a change in these mechanisms can affect, or be affected by, neuronal activities associated with schizophrenia. In this review, we discuss the current understanding of the cause and effect of decreased brain pH in schizophrenia based on postmortem human brains, animal models, and cellular studies. The topic includes the factors causing decreased brain pH in schizophrenia, mitochondria dysfunction leading to altered energy metabolism, and pH effects on the pathophysiology of schizophrenia. We also review the acid/base transporters regulating pH in the nervous system and discuss the potential contribution of the major transporters, sodium hydrogen exchangers (NHEs), and sodium-coupled bicarbonate transporters (NCBTs), to schizophrenia.

Genetic architecture and phenotypic landscape of deafness and onychodystrophy syndromes

Deafness and onychodystrophy syndromes are a group of phenotypically overlapping syndromes, which include DDOD syndrome (dominant deafness-onychodystrophy), DOORS syndrome (deafness, onychodystrophy, osteodystrophy, mental retardation and seizures) and Zimmermann-Laband syndrome (gingival hypertrophy, coarse facial features, hypoplasia or aplasia of nails and terminal phalanges, intellectual disability, and hypertrichosis). Pathogenic variants in four genes, ATP6V1B2, TBC1D24, KCNH1 and KCNN3, have been shown to be associated with deafness and onychodystrophy syndromes. ATP6V1B2 encodes a component of the vacuolar H⁺-ATPase (V-ATPase) and TBC1D24 belongs to GTPase-activating protein, which are all involved in the regulation of membrane trafficking. The overlapping clinical phenotype of TBC1D24- and ATP6V1B2- related diseases and their function with GTPases or ATPases activity indicate that they may have some physiological link. Variants in genes encoding potassium channels KCNH1 or KCNN3, underlying human Zimmermann-Laband syndrome, have only recently been recognized. Although further analysis will be needed, these findings will help to elucidate an understanding of the pathogenesis of these disorders better and will aid in the development of potential therapeutic approaches. In this review, we summarize the latest developments of clinical features and molecular basis that have been reported to be associated with deafness and onychodystrophy disorders and highlight the challenges that may arise in the differential diagnosis.

Protective effects of carbonic anhydrase inhibition in brain ischaemia in vitro and in vivo models

Ischaemic stroke is a leading cause of death and disability. One of the major pathogenic mechanisms after ischaemia includes the switch to the glycolytic pathway, leading to tissue acidification. Carbonic anhydrase (CA) contributes to pH regulation. A new generation of CA inhibitors, AN11-740 and AN6-277 and the reference compound acetazolamide (ACTZ) were investigated in two models of brain ischaemia: in rat hippocampal acute slices exposed to severe oxygen, glucose deprivation (OGD) and in an in vivo model of focal cerebral ischaemia induced by permanent occlusion of the middle cerebral artery (pMCAo) in the rat. In vitro, the application of selective CAIs significantly delayed the appearance of anoxic depolarisation induced by OGD. In vivo, sub-chronic systemic treatment with AN11-740 and ACTZ significantly reduced the neurological deficit and decreased the infarct volume after pMCAo. CAIs counteracted neuronal loss, reduced microglia activation and partially counteracted astrocytes degeneration inducing protection from functional and tissue damage.

Role of Carbonic Anhydrase in Cerebral Ischemia and Carbonic Anhydrase Inhibitors as Putative Protective Agents

Ischemic stroke is a leading cause of death and disability worldwide. The only pharmacological treatment available to date for cerebral ischemia is tissue plasminogen activator (t-PA) and the search for successful therapeutic strategies still remains a major challenge. The loss of cerebral blood flow leads to reduced oxygen and glucose supply and a subsequent switch to the glycolytic pathway, which leads to tissue acidification. Carbonic anhydrase (CA, EC 4.2.1.1) is the enzyme responsible for converting carbon dioxide into a protons and bicarbonate, thus contributing to pH regulation and metabolism, with many CA isoforms present in the brain. Recently, numerous studies have shed light on several classes of carbonic anhydrase inhibitor (CAI) as possible new pharmacological agents for the management of brain ischemia. In the present review we summarized pharmacological, preclinical and clinical findings regarding the role of CAIs in strokes and we discuss their potential protective mechanisms.

Neuronal oscillator robustness to multiple global perturbations

Neuronal activity depends on ion channels and biophysical processes that are strongly and differentially sensitive to physical variables such as temperature and pH. Nonetheless, neuronal oscillators can be surprisingly resilient to perturbations in these variables. We study a three-neuron pacemaker ensemble that drives the pyloric rhythm of the crab, Cancer borealis. These crabs routinely experience a number of global perturbations, including changes in temperature and pH. Although pyloric oscillations are robust to such changes, for sufficiently large deviations the rhythm reversibly breaks down. As temperature increases beyond a tipping point, oscillators transition to silence. Acidic pH deviations also show tipping points, with a reliable transition first to tonic spiking, then to silence. Surprisingly, robustness to perturbations in pH only moderately affects temperature robustness. Consistent with high animal-to-animal variability in biophysical circuit parameters, tipping points in temperature and pH vary across animals. However, the ordering and discrete classes of transitions at critical points are conserved. This implies that qualitative oscillator dynamics are preserved across animals despite high quantitative parameter variability. A universal model of bursting dynamics predicts the existence of these transition types and the order in which they occur.

NMDAR in cultured astrocytes: Flux-independent pH sensor and flux-dependent regulator of mitochondria and plasma membrane-mitochondria bridging

Glutamate N-methyl-D-aspartate (NMDA) receptor (NMDAR) is critical for neurotransmission as a Ca²⁺ channel. Nonetheless, flux-independent signaling has also been demonstrated. Astrocytes express NMDAR distinct from its neuronal counterpart, but cultured astrocytes have no electrophysiological response to NMDA. We recently demonstrated that in cultured astrocytes, NMDA at pH6 (NMDA/pH6) acting through the NMDAR elicits flux-independent Ca²⁺ release from the Endoplasmic Reticulum (ER) and depletes mitochondrial membrane potential (mΔΨ). Here we show that Ca²⁺ release is due to pH6 sensing by NMDAR, whereas mΔΨ depletion requires both: pH6 and flux-dependent NMDAR signaling. Plasma membrane (PM) NMDAR guard a non-random distribution relative to the ER and mitochondria. Also, NMDA/pH6 induces ER stress, endocytosis, PM electrical capacitance reduction, mitochondria-ER, and -nuclear contacts. Strikingly, it also produces the formation of PM invaginations near mitochondria along with structures referred to here as PM-mitochondrial bridges (PM-m-br). These and earlier data strongly suggest PM-mitochondria communication. As proof of the concept of mass transfer, we found that NMDA/pH6 provoked mitochondria labeling by the PM dye FM-4-64FX. NMDA/pH6 caused PM depolarization, cell acidification, and Ca²⁺ release from most mitochondria. Finally, the MCU and microtubules were not involved in mΔΨ depletion, while actin cytoskeleton was partially involved. These findings demonstrate that NMDAR has concomitant flux-independent and flux-dependent actions in cultured astrocytes.

Label-Free in Situ pH Monitoring in a Single Living Cell Using an Optical Nanoprobe

Intracellular pH plays critical roles in cell and tissue functions during processes such as metabolism, proliferation, apoptosis, ion transportation, endocytosis, muscle contraction and so on. It is thus an important biomarker that can readily be used to monitor the physiological status of a cell. Thus, disrupted intracellular pH may serve as an early indicator of cell dysfunction and deterioration. Various methods have been developed to detect cellular pH, such as pH-sensitive labeling reagents with fluorescent or Raman signals. However, excessive cellular uptake of these reagents will not only disrupt cell viability but also compromise effective long-term monitoring. Here, we present a novel fiber-optic fluorescent nanoprobe with a high spatial resolution for label-free, subcellular pH sensing. The probe has a fast response time (~20 seconds) with minimum invasiveness and excellent pH resolution (0.02 pH units) within a biologically relevant pH environment ranging from 6.17 to 8.11. Its applicability was demonstrated on cultured A549 lung cancer cells, and its efficacy was further testified in two typical cytotoxic cases using carbonylcyanide 3-chlorophenyl hydrazine, titanium dioxide, and nanoparticles. The probe can readily detect the pH variations among cells under toxin/nanoparticles administration, enabling direct monitoring of the early onset of physiological or pathological events with high spatiotemporal resolution. This platform has excellent promise as a minimum invasive diagnostic tool for pH-related cellular mechanism studies, such as inflammation, cytotoxicity, drug resistance, carcinogenesis, stem cell differentiation and so on.

Helicid Reverses Lipopolysaccharide-Induced Inflammation and Promotes GDNF Levels in C6 Glioma Cells through Modulation of Prepronociceptin

Helicid suppresses inflammatory factors and protects nerve cells in the hippocampus of rats with depression, but the mechanisms underlying its protective effects are unclear at present. In this investigation, we conducted gene silencing, Helicid intervention and rescue experiments to explore the protective actions of PNOC, the prepronociceptin gene known to regulate inflammatory processes, and Helicid on a C6 cell model of inflammation induced by LPS. Collective data from Western blots, ELISA, immunofluorescence and flow cytometry experiments showed that PNOC silencing or administration of Helicid led to reduced inflammatory factor levels, oxidative stress and expression of glial fibrillary acidic protein (GFAP), along with increased glial cell lines-derived neurotrophic factor (GDNF) expression. Furthermore, expression of p-Akt in the Akt signaling pathway was increased. Interestingly, overexpression of PNOC in the Helicid treatment group partially reversed the Helicid-induced changes in the above biochemical indexes. Our collective results provide strong evidence of Helicid-mediated regulation of the Akt signaling pathway through PNOC to improve cell inflammation and oxidative stress.

Novel insights into astrocyte-mediated signaling of proliferation, invasion and tumor immune microenvironment in glioblastoma

Glioblastoma (GBM) continues to be the most aggressive cancer of the brain. The dismal prognosis is largely attributed to the microenvironment surrounding tumor cells. Astrocytes, the main component of the GBM microenvironment, play several fundamental physiological roles in the central nervous system. During the development of GBM, tumor-associated astrocytes (TAAs) directly contact GBM cells, which activate astrocytes to form reactive astrocytes, facilitating tumor progression, proliferation and migration through multiple well-understood signaling pathways. Notably, TAAs also influence GBM cell behaviors via suppressing immune responses and enhancing the chemoradiotherapy resistance of tumor cells. These new activities are closely linked with the treatment and prognosis of GBM. In this review, we discuss recent advances regarding new functions of reactive astrocytes, including TAA-cancer cell interactions, mechanisms involved in immunosuppressive regulation, and chemoradiotherapy resistance. It is expected that these updated experimental or clinical studies of TAAs may provide a promising approach for GBM treatment in the near future.

Seizure initiation in infantile spasms vs. focal seizures: proposed common cellular mechanisms

Infantile spasms (IS) and seizures with focal onset have different clinical expressions, even when electroencephalography (EEG) associated with IS has some degree of focality. Oddly, identical pathology (with, however, age-dependent expression) can lead to IS in one patient vs. focal seizures in another or even in the same, albeit older, patient. We therefore investigated whether the cellular mechanisms underlying seizure initiation are similar in the two instances: spasms vs. focal. We noted that in-common EEG features can include (i) a background of waves at alpha to delta frequencies; (ii) a period of flattening, lasting about a second or more - the electrodecrement (ED); and (iii) often an interval of very fast oscillations (VFO; ~70 Hz or faster) preceding, or at the beginning of, the ED. With IS, VFO temporally coincides with the motor spasm. What is different between the two conditions is this: with IS, the ED reverts to recurring slow waves, as occurring before the ED, whereas with focal seizures the ED instead evolves into an electrographic seizure, containing high-amplitude synchronized bursts, having superimposed VFO. We used in vitro data to help understand these patterns, as such data suggest cellular mechanisms for delta waves, for VFO, for seizure-related burst complexes containing VFO, and, more recently, for the ED. We propose a unifying mechanistic hypothesis - emphasizing the importance of brain pH - to explain the commonalities and differences of EEG signals in IS versus focal seizures.

Neuronal Glutamatergic Synaptic Clefts Alkalinize Rather Than Acidify during Neurotransmission

The dogma that the synaptic cleft acidifies during neurotransmission is based on the corelease of neurotransmitters and protons from synaptic vesicles, and is supported by direct data from sensory ribbon-type synapses. However, it is unclear whether acidification occurs at non-ribbon-type synapses. Here we used genetically encoded fluorescent pH indicators to examine cleft pH at conventional neuronal synapses. At the neuromuscular junction of female Drosophila larvae, we observed alkaline spikes of over 1 log unit during fictive locomotion in vivo. Ex vivo, single action potentials evoked alkalinizing pH transients of only ∼0.01 log unit, but these transients summated rapidly during burst firing. A chemical pH indicator targeted to the cleft corroborated these findings. Cleft pH transients were dependent on Ca²⁺ movement across the postsynaptic membrane, rather than neurotransmitter release per se, a result consistent with cleft alkalinization being driven by the Ca²⁺/H⁺ antiporting activity of the plasma membrane Ca²⁺-ATPase at the postsynaptic membrane. Targeting the pH indicators to the microenvironment of the presynaptic voltage gated Ca²⁺ channels revealed that alkalinization also occurred within the cleft proper at the active zone and not just within extrasynaptic regions. Application of the pH indicators at the mouse calyx of Held, a mammalian central synapse, similarly revealed cleft alkalinization during burst firing in both males and females. These findings, made at two quite different non-ribbon type synapses, suggest that cleft alkalinization during neurotransmission, rather than acidification, is a generalizable phenomenon across conventional neuronal synapses.SIGNIFICANCE STATEMENT Neurotransmission is highly sensitive to the pH of the extracellular milieu. This is readily evident in the neurological symptoms that accompany systemic acid/base imbalances. Imaging data from sensory ribbon-type synapses show that neurotransmission itself can acidify the synaptic cleft, likely due to the corelease of protons and glutamate. It is not clear whether the same phenomenon occurs at conventional neuronal synapses due to the difficulties in collecting such data. If it does occur, it would provide for an additional layer of activity-dependent modulation of neurotransmission. Our findings of alkalinization, rather than acidification, within the cleft of two different neuronal synapses encourages a reassessment of the scope of activity-dependent pH influences on neurotransmission and short-term synaptic plasticity.

Acid-Sensing Ion Channels: Novel Mediators of Cerebral Vascular Responses

RATIONALE

Precise regulation of cerebral blood flow is critical for normal brain function. Insufficient cerebral blood flow contributes to brain dysfunction and neurodegeneration. Carbon dioxide (CO2), via effects on local acidosis, is one of the most potent regulators of cerebral blood flow. Although a role for nitric oxide in intermediate signaling has been implicated, mechanisms that initiate CO2-induced vasodilation remain unclear.

OBJECTIVE

Acid-sensing ion channel-1A (ASIC1A) is a proton-gated cation channel that is activated by extracellular acidosis. Based on work that implicated ASIC1A in the amygdala and bed nucleus of the stria terminalis in CO2-evoked and acid-evoked behaviors, we hypothesized that ASIC1A might also mediate microvascular responses to CO2.

METHODS AND RESULTS

To test this hypothesis, we genetically and pharmacologically manipulated ASIC1A and assessed effects on CO2-induced dilation of cerebral arterioles in vivo. Effects of inhalation of 5% or 10% CO2 on arteriolar diameter were greatly attenuated in mice with global deficiency in ASIC1A (Asic1a-/-) or by local treatment with the ASIC inhibitor, psalmotoxin. Vasodilator effects of acetylcholine, which acts via endothelial nitric oxide synthase were unaffected, suggesting a nonvascular source of nitric oxide may be key for CO2 responses. Thus, we tested whether neurons may be the cell type through which ASIC1A influences microvessels. Using mice in which Asic1a was specifically disrupted in neurons, we found effects of CO2 on arteriolar diameter were also attenuated.

CONCLUSIONS

Together, these data are consistent with a model wherein activation of ASIC1A, particularly in neurons, is critical for CO2-induced nitric oxide production and vasodilation. With these findings, ASIC1A emerges as major regulator of microvascular tone.

Hydrogen Ion Dynamics of Cancer and a New Molecular, Biochemical and Metabolic Approach to the Etiopathogenesis and Treatment of Brain Malignancies

The treatment of cancer has been slowly but steadily progressing during the last fifty years. Some tumors with a high mortality in the past are curable nowadays. However, there is one striking exception: glioblastoma multiforme. No real breakthrough has been hitherto achieved with this tumor with ominous prognosis and very short survival. Glioblastomas, being highly glycolytic malignancies are strongly pH-dependent and driven by the sodium hydrogen exchanger 1 (NHE1) and other proton (H⁺) transporters. Therefore, this is one of those pathologies where the lessons recently learnt from the new pH-centered anticancer paradigm may soon bring a promising change to treatment. This contribution will discuss how the pH-centric molecular, biochemical and metabolic perspective may introduce some urgently needed and integral novel treatments. Such a prospective therapeutic approach for malignant brain tumors is developed here, either to be used alone or in combination with more standard therapies.

Levetiracetam mediates subtle pH-shifts in adult human neocortical pyramidal cells via an inhibition of the bicarbonate-driven neuronal pH-regulation - Implications for excitability and plasticity modulation

The intracellular pH (pHi) of mammalian central neurons is tightly regulated and small pHi-fluctuations can fine-tune inter-/intracellular signaling, excitability, and synaptic plasticity. The research-gap about the pHi-regulation of human brain neurons is addressed here by testing possible influences of the anticonvulsant levetiracetam (LEV). BCECF-AM-loaded neocortical pyramidal cells were fluorometrically investigated in slice-preparations of tissue resected from the middle temporal gyrus of five adults with intractable temporal-lobe epilepsy. Recovery-slope from intracellular acidification following an ammonium prepulse (APP) was used to measure the pHi-regulation. Among twenty pyramidal cells exposed to 50 μM LEV, the resting pHi (7.09 ± 0.14) was lowered in eight (40%) neurons, on average by 0.02 ± 0.011 pH-units. In three (15%) and nine (45%) neurons, a minimal alkaline shift (0.017 ± 0.004 pH-units) and no pHi-shift occurred, respectively. The LEV-induced pHi-shifts were positively correlated with the resting pHi (r = 0.6, p = 0.006, n = 20). In five neurons, which all had responded on LEV with an acidification before, the recovery from APP-acidification was significantly delayed during LEV (p < 0.001). This inhibitory LEV-effect on pHi-regulation i) was similar to that of 200 μM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (n = 2) and ii) did not occur under nominal bicarbonate-free conditions (n = 2). Thus, LEV lowered the pHi of human neocortical pyramidal cells most likely by a weakening of the transmembrane HCO3(-)-mediated acid-extrusion. This might contribute to LEV's anticonvulsive potency. Neurons with more acidic resting pHi-values showed a minimal alkalization upon LEV providing a mechanism for paradoxical proconvulsive LEV-effects rarely observed in epilepsy patients. The significance of these subtle pHi-shifts for cortical excitability and plasticity is discussed.

Extracellular mild acidosis decreases the Ca2+ permeability of the human NMDA receptors

NMDA receptors (NMDARs) are glutamate-gated ion channels involved in excitatory synaptic transmission and in others physiological processes such as synaptic plasticity and development. The overload of Ca²⁺ ions through NMDARs, caused by an excessive activation of receptors, leads to excitotoxic neuronal cell death. For this reason, the reduction of Ca²⁺ flux through NMDARs has been a central focus in finding therapeutic strategies to prevent neuronal cell damage. Extracellular H⁺ are allosteric modulators of NMDARs. Starting from previous studies showing that extracellular mild acidosis reduces NMDA-evoked whole cell currents, we analyzed the effects of this condition on the NMDARs Ca²⁺ permeability, measured as "fractional calcium current" (P_f, i.e. the percentage of the total current carried by Ca²⁺ ions), of human NMDARs NR1/NR2A and NR1/NR2B transiently transfected in HeLa cells. Extracellular mild acidosis significantly reduces P_f of both human NR1/NR2A and NR1/NR2B NMDARs, also decreasing single channel conductance in outside out patches for NR1/NR2A receptor. Reduction of Ca²⁺ flux through NMDARs was also confirmed in cortical neurons in culture. A comparative analysis of both NMDA evoked Ca²⁺ transients and whole cell currents showed that extracellular H⁺ differentially modulate the permeation of Na⁺ and Ca²⁺ through NMDARs. Our data highlight the synergy of two distinct neuroprotective mechanisms during acidosis: Ca²⁺ entry through NMDARs is lowered due to the modulation of both open probability and Ca²⁺ permeability. Furthermore, this study provides the proof of concept that it is possible to reduce Ca²⁺ overload in neurons modulating the NMDAR Ca²⁺ permeability.

Novel hybrid action of GABA mediates inhibitory feedback in the mammalian retina

The stream of visual information sent from photoreceptors to second-order bipolar cells is intercepted by laterally interacting horizontal cells that generate feedback to optimize and improve the efficiency of signal transmission. The mechanisms underlying the regulation of graded photoreceptor synaptic output in this nonspiking network have remained elusive. Here, we analyze with patch clamp recording the novel mechanisms by which horizontal cells control pH in the synaptic cleft to modulate photoreceptor neurotransmitter release. First, we show that mammalian horizontal cells respond to their own GABA release and that the results of this autaptic action affect cone voltage-gated Ca2+ channel (CaV channel) gating through changes in pH. As a proof-of-principle, we demonstrate that chemogenetic manipulation of horizontal cells with exogenous anion channel expression mimics GABA-mediated cone CaV channel inhibition. Activation of these GABA receptor anion channels can depolarize horizontal cells and increase cleft acidity via Na+/H+ exchanger (NHE) proton extrusion, which results in inhibition of cone CaV channels. This action is effectively counteracted when horizontal cells are sufficiently hyperpolarized by increased GABA receptor (GABAR)-mediated HCO3- efflux, alkalinizing the cleft and disinhibiting cone CaV channels. This demonstrates how hybrid actions of GABA operate in parallel to effect voltage-dependent pH changes, a novel mechanism for regulating synaptic output.

Ionic signalling in astroglia beyond calcium

Astrocytes are homeostatic and protective cells of the central nervous system. Astroglial homeostatic responses are tightly coordinated with neuronal activity. Astrocytes maintain neuronal excitability through regulation of extracellular ion concentrations, as well as assisting and modulating synaptic transmission by uptake and catabolism of major neurotransmitters. Moreover, they support neuronal metabolism and detoxify ammonium and reactive oxygen species. Astroglial homeostatic actions are initiated and controlled by intercellular signalling of ions, including Ca²⁺ , Na⁺ , Cl^- , H⁺ and possibly K⁺ . This review summarises current knowledge on ionic signals mediated by the major monovalent ions, which occur in microdomains, as global events, or as propagating intercellular waves and thereby represent the substrate for astroglial excitability.

Protons as Messengers of Intercellular Communication in the Nervous System

In this review, evidence demonstrating that protons (H⁺) constitute a complex, regulated intercellular signaling mechanisms are presented. Given that pH is a strictly regulated variable in multicellular organisms, localized extracellular pH changes may constitute significant signals of cellular processes that occur in a cell or a group of cells. Several studies have demonstrated that the low pH of synaptic vesicles implies that neurotransmitter release is always accompanied by the co-release of H⁺ into the synaptic cleft, leading to transient extracellular pH shifts. Also, evidence has accumulated indicating that extracellular H⁺ concentration regulation is complex and implies a source of protons in a network of transporters, ion exchangers, and buffer capacity of the media that may finally establish the extracellular proton concentration. The activation of membrane transporters, increased production of CO₂ and of metabolites, such as lactate, produce significant extracellular pH shifts in nano- and micro-domains in the central nervous system (CNS), constituting a reliable signal for intercellular communication. The acid sensing ion channels (ASIC) function as specific signal sensors of proton signaling mechanism, detecting subtle variations of extracellular H⁺ in a range varying from pH 5 to 8. The main question in relation to this signaling system is whether it is only synaptically restricted, or a volume modulator of neuron excitability. This signaling system may have evolved from a metabolic activity detection mechanism to a highly localized extracellular proton dependent communication mechanism. In this study, evidence showing the mechanisms of regulation of extracellular pH shifts and of the ASICs and its function in modulating the excitability in various systems is reviewed, including data and its role in synaptic neurotransmission, volume transmission and even segregated neurotransmission, leading to a reliable extracellular signaling mechanism.

Aging is associated with a mild acidification in neocortical human neurons in vitro

The intracellular pH (pHi) in the cytosol of mammalian central neurons is tightly regulated and small pHi-fluctuations are deemed to modulate inter-/intracellular signaling, excitability, and synaptic plasticity. The resting pHi of young rodent hippocampal pyramidal neurons is known to decrease alongside aging for about 0.1 pH-units. There is no information about the relationship between age and pHi of human central neurons. We addressed this knowledge gap using 26 neocortical slices from 12 patients (1-56-years-old) who had undergone epilepsy surgery. For fluorometric recordings, the slice-neurons were loaded with the pHi-sensitive dye BCECF-AM. We found that the pyramidal cells' resting pHi (n = 26) descended linearly alongside aging (r = - 0.71, p < 0.001). This negative relationship persisted, when the sample was confined to specific brain regions (i.e., middle temporal gyrus, 23 neurons, r = - 0.68, p < 0.001) or pathologies (i.e., hippocampus sclerosis, 8 neurons, r = - 0.78, p = 0.02). Specifically, neurons (n = 9, pHi 7.25 ± 0.12) from young children (1.5 ± 0.46-years-old) were significantly more alkaline than neurons from adults (n = 17, 38.53 ± 12.38 years old, pHi 7.08 ± 0.07, p < 0.001). Although the samples were from patients with different pathologies the results were in line with those from the rodent hippocampal pyramidal neurons. Like a hormetin, the age-related mild pHi-decrease might contribute to neuroprotection, e.g., via limiting excitotoxicity. On the other hand, aging cortical neurons could become more vulnerable to metabolic overstress by a successive pHi-decrease. Certainly, its impact for the dynamics in short and long-term synaptic plasticity and, ultimately, learning and memory provides a challenge for further research.

Metabolic regulation of synaptic activity

Brain tissue is bioenergetically expensive. In humans, it composes approximately 2% of body weight and accounts for approximately 20% of calorie consumption. The brain consumes energy mostly for ion and neurotransmitter transport, a process that occurs primarily in synapses. Therefore, synapses are expensive for any living creature who has brain. In many brain diseases, synapses are damaged earlier than neurons start dying. Synapses may be considered as vulnerable sites on a neuron. Ischemic stroke, an acute disturbance of blood flow in the brain, is an example of a metabolic disease that affects synapses. The associated excessive glutamate release, called excitotoxicity, is involved in neuronal death in brain ischemia. Another example of a metabolic disease is hypoglycemia, a complication of diabetes mellitus, which leads to neuronal death and brain dysfunction. However, synapse function can be corrected with “bioenergetic medicine”. In this review, a ketogenic diet is discussed as a curative option. In support of a ketogenic diet, whereby carbohydrates are replaced for fats in daily meals, epileptic seizures can be terminated. In this review, we discuss possible metabolic sensors in synapses. These may include molecules that perceive changes in composition of extracellular space, for instance, ketone body and lactate receptors, or molecules reacting to changes in cytosol, for instance, KATP channels or AMP kinase. Inhibition of endocytosis is believed to be a universal synaptic mechanism of adaptation to metabolic changes.

Measurement of the Extracellular pH of Adherently Growing Mammalian Cells with High Spatial Resolution Using a Voltammetric pH Microsensor

There are only a few tools suitable for measuring the extracellular pH of adherently growing mammalian cells with high spatial resolution, and none of them is widely used in laboratories around the world. Cell biologists very often limit themselves to measuring the intracellular pH with commercially available fluorescent probes. Therefore, we built a voltammetric pH microsensor and investigated its suitability for monitoring the extracellular pH of adherently growing mammalian cells. The voltammetric pH microsensor consisted of a 37 μm diameter carbon fiber microelectrode modified with reduced graphene oxide and syringaldazine. While graphene oxide was used to increase the electrochemically active surface area of our sensor, syringaldazine facilitated pH sensing through its pH-dependent electrochemical oxidation and reduction. The good sensitivity (60 ± 2.5 mV/pH unit), reproducibility (coefficient of variation ≤3% for the same pH measured with 5 different microsensors), and stability (pH drift around 0.05 units in 3 h) of the built voltammetric pH sensors were successfully used to investigate the acidification of the extracellular space of both cancer cells and normal cells. The results indicate that the developed pH microsensor and the perfected experimental protocol based on scanning electrochemical microscopy can reveal details of the pH regulation of cells not attainable with pH sensors lacking spatial resolution or which cannot be reproducibly positioned in the extracellular space.

Acid-Sensing Ion Channels as Potential Therapeutic Targets in Neurodegeneration and Neuroinflammation

Acid-sensing ion channels (ASICs) are a family of proton-sensing channels that are voltage insensitive, cation selective (mostly permeable to Na⁺), and nonspecifically blocked by amiloride. Derived from 5 genes (ACCN1-5), 7 subunits have been identified, 1a, 1b, 2a, 2b, 3, 4, and 5, that are widely expressed in the peripheral and central nervous system as well as other tissues. Over the years, different studies have shown that activation of these channels is linked to various physiological and pathological processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis to name a few, so their potential as therapeutic targets is increasing. This review focuses on recent advances that have helped us to better understand the role played by ASICs in different pathologies related to neurodegenerative diseases, inflammatory processes, and pain.

Role of astrocyte connexin hemichannels in cortical spreading depression

Cortical spreading depression (CSD) is an intriguing phenomenon consisting of massive slow brain depolarizations that affects neurons and glial cells. It has been recognized since 1944, but its pathogenesis has only been uncovered during the last decade. Acute brain injuries can be further complicated by CSD in >50% of severe cases. This phenomenon is repetitive and produces a metabolic overload that increments secondary damage. Propagation of CSD is known to be linked to excitotoxicity, but the mechanisms associated with its initiation remain less understood. It has been shown that CSD can be initiated by increases in extracellular [K⁺] ([K⁺]_e), and animal models use high [K⁺]_e to promote CSD. Connexin hemichannel activity increases due to high [K⁺]_e and low extracellular [Ca²⁺], conditions that occur after brain injury. Moreover, glial cell gap junction channels are fundamental in controlling extracellular medium composition, particularly in maintaining normal extracellular glutamate and K⁺ concentrations through "spatial buffering". However, the role of astrocytic gap junctions under tissue stress can change to damage spread in the acute damage zone whereas the reduced communication in adjacent zone would reduce cell dead propagation. Here, we review the main findings associated with CSD, and discuss the possible involvement of astrocytic connexin-based channels in secondary damage propagation. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Activation of master virulence regulator PhoP in acidic pH requires the Salmonella-specific protein UgtL

Acidic conditions, such as those inside phagosomes, stimulate the intracellular pathogen Salmonella enterica to activate virulence genes. The sensor PhoQ responds to a mildly acidic pH by phosphorylating, and thereby activating, the virulence regulator PhoP. This PhoP/PhoQ two-component system is conserved in a subset of Gram-negative bacteria. PhoQ is thought to be sufficient to activate PhoP in mildly acidic pH. However, we found that the Salmonella-specific protein UgtL, which was horizontally acquired by Salmonella before the divergence of S. enterica and Salmonella bongori, was also necessary for PhoQ to activate PhoP under mildly acidic pH conditions but not for PhoQ to activate PhoP in response to low Mg²⁺ or the antimicrobial peptide C18G. UgtL increased the abundance of phosphorylated PhoP by stimulating autophosphorylation of PhoQ, thereby increasing the amount of the phosphodonor for PhoP. Deletion of ugtL attenuated Salmonella virulence and further reduced PhoP activation in a strain bearing a form of PhoQ that is not responsive to acidic pH. These data suggest that when Salmonella experiences mildly acidic pH, PhoP activation requires PhoQ to detect pH and UgtL to amplify the PhoQ response. Our findings reveal how acquisition of a foreign gene can strengthen signal responsiveness in an ancestral regulatory system.

Neuroprotective potential of astroglia

Astroglia are the homoeostatic cells of the central nervous system, which participate in all essential functions of the brain. Astrocytes support neuronal networks by handling water and ion fluxes, transmitter clearance, provision of antioxidants, and metabolic precursors and growth factors. The critical dependence of neurons on constant support from the astrocytes confers astrocytes with intrinsic neuroprotective properties. On the other hand, loss of astrocytic support or their pathological transformation compromises neuronal functionality and viability. Manipulating neuroprotective functions of astrocytes is thus an important strategy to enhance neuronal survival and improve outcomes in disease states. © 2017 The Authors Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Hypoxia-induced brain cell damage in male albino wistar rat

The biochemical markers of rat under low oxygen concentration, including brain water level, lactic acid, necrosis and Na+-K+-ATPase, was detected to analyze the hypoxia-induced brain damage, and to analyze the mechanism of brain injury. Histopathological alteration in brain tissue induced by hypoxia were investigated through hematoxylin and eosin stain (HE). Hypoxia induced factor-1a (HIF-1a) expression level in the brain was carried out using immunohistochemistry. Lactic acid level was positively correlated with the level of hypoxia, while concentration-dependent decrease in total Na+-K+-ATPase activity was noted. Hypoxia induced rathad a significant difference on brain water content compared to controls. The level of necrosis and lactic acid level was increased, and the decrease of Na+-K+-ATPase activity was observed. Histopathological examination of brain confirmed that there was neuronal cell death in hippocampal region. HIF-1a expression increased the hypoxia adaptation capability of the rat through the expressions of genes. Lactic acid, Na+-K+-ATPase and HIF-1a plays an important role in brain injury.

Cellular acidification as a new approach to cancer treatment and to the understanding and therapeutics of neurodegenerative diseases

During the last few years, the understanding of the dysregulated hydrogen ion dynamics and reversed proton gradient of cancer cells has resulted in a new and integral pH-centric paradigm in oncology, a translational model embracing from cancer etiopathogenesis to treatment. The abnormalities of intracellular alkalinization along with extracellular acidification of all types of solid tumors and leukemic cells have never been described in any other disease and now appear to be a specific hallmark of malignancy. As a consequence of this intracellular acid-base homeostatic failure, the attempt to induce cellular acidification using proton transport inhibitors and other intracellular acidifiers of different origins is becoming a new therapeutic concept and selective target of cancer treatment, both as a metabolic mediator of apoptosis and in the overcoming of multiple drug resistance (MDR). Importantly, there is increasing data showing that different ion channels contribute to mediate significant aspects of cancer pH regulation and etiopathogenesis. Finally, we discuss the extension of this new pH-centric oncological paradigm into the opposite metabolic and homeostatic acid-base situation found in human neurodegenerative diseases (HNDDs), which opens novel concepts in the prevention and treatment of HNDDs through the utilization of a cohort of neural and non-neural derived hormones and human growth factors.

Human Neuronal Calcium Sensor-1 Protein Avoids Histidine Residues To Decrease pH Sensitivity

pH is highly regulated in mammalian central nervous systems. Neuronal calcium sensor-1 (NCS-1) can interact with numerous target proteins. Compared to that in the NCS-1 protein of Caenorhabditis elegans, evolution has avoided the placement of histidine residues at positions 102 and 83 in the NCS-1 protein of humans and Xenopus laevis, possibly to decrease the conformational sensitivity to pH gradients in synaptic processes. We used all-atom molecular dynamics simulations to investigate the effects of amino acid substitutions between species on human NCS-1 by substituting Arg102 and Ser83 for histidine at neutral (R102H and S83H) and acidic pHs (R102Hp and S83Hp). Our cumulative 5 μs simulations revealed that the R102H mutation slightly increases the structural flexibility of loop L2 and the R102Hp mutation decreases protein stability. Community network analysis illustrates that the R102H and S83H mutations weaken the interdomain and strengthen the intradomain communications. Secondary structure contents in the S83H and S83Hp mutants are similar to those in the wild type, whereas the global structural stabilities and salt-bridge probabilities decrease. This study highlights the conformational dynamics effects of the R102H and S83H mutations on the local structural flexibility and global stability of NCS-1, whereas protonated histidine decreases the stability of NCS-1. Thus, histidines at positions 102 and 83 may not be compatible with the function of NCS-1 whether in the neutral or protonated state.

Acute neuroinflammation provokes intracellular acidification in mouse hippocampus

Background

Maintaining pH levels within the physiological norm is an important component of brain homeostasis. However, in some pathological or physiological conditions, the capacity of the pH regulatory system could be overpowered by various factors resulting in a transient or permanent alteration in pH levels. Such changes are often observed in pathological conditions associated with neuroinflammation. We hypothesized that neuroinflammation itself is a factor affecting pH levels in neural tissue. To assess this hypothesis, we examined the effects of acute LPS-induced neuroinflammation on intra- and extracellular pH (pHi and pHo) levels in the CA1 region of mouse hippocampus.

Methods

Acute neuroinflammation was induced using two approaches: (1) in vivo by i.p. injections of LPS (5 mg/kg) and (2) in vitro by incubating hippocampal slices of naïve animals in the LPS-containing media (1 μg/mL, 1 h at 35 °C). Standard techniques were used to prepare hippocampal slices. pHi was measured using ratiometric pH-sensitive fluorescent dye BCECF-AM. pHo was assessed using calibrated pH-sensitive micropipettes. The presence of neuroinflammation was verified with immunohistochemistry (IL-1β and Iba1) and ELISA (IL-1β and TNF-α).

Results

A significant reduction of pHi was observed in the slices of the LPS-injected 3-month-old (LPS 7.13 ± 0.03; Sal 7.22 ± 0.03; p = 0.043, r = 0.43) and 19-month-old (LPS 6.78 ± 0.08; Sal 7.13 ± 0.03; p = 0.0001, r = 0.32) mice. In contrast, the levels of pHo within the slice, measured in 19-month-old animals, were not affected (LPS 7.27 ± 0.02; Sal 7.26 ± 0.02; p = 0.6, r = 0.13). A reduction of pHi was also observed in the LPS-treated slices during the interval 3.5–7 h after the LPS exposure (LPS 6.92 ± 0.07; Veh 7.28 ± 0.05; p = 0.0001, r = 0.46).

Conclusions

Acute LPS-induced neuroinflammation results in a significant intracellular acidification of the CA1 neurons in mouse hippocampus, while the pHo remains largely unchanged. Such changes may represent a specific protective reaction of neural tissue in unfavorable external conditions or be a part of the pathological process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0747-8) contains supplementary material, which is available to authorized users.