Metabolic coupling between glia and neurons

Astrocytes as brain interoceptors

Astrocytes form a vascular-neuronal interface and provide CNS neural networks with essential structural and metabolic support. They embrace all penetrating arterioles and capillaries, enwrap multiple neuronal somata, thousands of individual synapses, and upon activation release gliotransmitters (ATP, glutamate and D-serine) capable of modulating neuronal activity. The aim of this brief report is to review recent data implicating astrocytes in the brain mechanisms responsible for the detection of different sensory modalities and transmitting sensory information to the relevant neural networks controlling vital behaviours. The concept of astrocytes as brain interoceptors is strongly supported by our recent data obtained from studies of the central nervous mechanisms underlying the chemosensory control of breathing. At the level of the medulla oblongata, astrocytes indeed act as functional central respiratory chemoreceptors, sensing changes in the arterial blood and brain levels of /pH and then imparting these changes on the activity of the respiratory network to induce adaptive changes in lung ventilation.

Changes in cytosolic glucose level in ATP stimulated live astrocytes

Astrocytes which lie between brain capillaries and neuronal terminals are the primary site of glucose uptake and have a key role in coupling synaptic activity to glucose utilization in the central nervous system (CNS). We used a fluorescence resonance energy transfer (FRET) based approach to monitor cytosolic glucose in astrocytes. We determined the effect of increasing extracellular glucose concentrations on FRET ratio as a measure of increased cytosolic glucose in astrocytes. By briefly raising extracellular glucose concentration, astrocytes responded promptly by increased cytosolic glucose levels, which was manifested by decreased time-dependent FRET ratio. The FRET ratio fall-time recorded at low extracellular D-glucose concentration change (from 0 to 0.5 mM) was 53 s, whereas 17 s was recorded by raising extracellular concentration of D-glucose from 0 to 10 mM, which is likely due to facilitated d-glucose entry along the increased D-glucose gradient across the plasmalemma. The relationship between the extracellular glucose concentration and the FRET ratio change is limited to the maximal ratio change, where the D-glucose plasma membrane permeability is balanced by the cytosolic utilization. We measured the effect of extracellular ATP, an important extracellular messenger for astrocyte-to-astrocyte communication, on intracellular glucose concentration. The results show that stimulation of astrocytes with ATP (1 mM) decreases cytosolic glucose concentration with a time constant of ∼145 s. The mechanism of this change is discussed.

The relationship between the acute cerebral metabolic response to citalopram and chronic citalopram treatment outcome

OBJECTIVES

Given the challenges in the clinical management of geriatric depression, research over the past decade has focused on developing early neurobiological markers of antidepressant treatment response. This study tested the hypothesis that lower baseline glucose metabolism and greater acute cerebral metabolic responses to a single, intravenous (IV) dose of the selective serotonin reuptake inhibitor (SSRI) citalopram would be associated with greater improvement of depressive symptoms after 12 weeks of citalopram treatment in geriatric depression.

METHODS

sixteen geriatric depressed patients underwent two scans to measure cerebral glucose metabolism after administration of either a saline placebo or citalopram infusion (40 mg, IV). Then, the patients were treated with the oral citalopram medication for 12 weeks.

RESULTS

greater improvement of depressive symptoms was associated with lower baseline metabolism in anterior cingulate, superior, middle, and inferior frontal gyri (bilaterally), inferior parietal lobule (bilaterally), precuneus (right), insula (left), parahippocampal gyrus (right), caudate (bilaterally), and putamen (left) regions. Greater improvement of depressive symptoms was associated with greater reductions in metabolism after acute citalopram administration in similar brain regions, including additional posterior cortical regions.

CONCLUSIONS

lower baseline cerebral metabolism and greater decreases with acute citalopram administration are associated with better antidepressant response to chronic citalopram treatment. These data are consistent with previous studies of total sleep deprivation and suggest that dynamic, early adaptive changes or normalization of cerebral metabolism may represent early neurobiological markers of chronic SSRI treatment response in geriatric depression.

Role and treatment of mitochondrial DNA-related mitochondrial dysfunction in sporadic neurodegenerative diseases

Several sporadic neurodegenerative diseases display phenomena that directly or indirectly relate to mitochondrial function. Data suggesting altered mitochondrial function in these diseases could arise from mitochondrial DNA (mtDNA) are reviewed. Approaches for manipulating mitochondrial function and minimizing the downstream consequences of mitochondrial dysfunction are discussed.

Sodium MRI

Sodium ((23)Na) imaging has a place somewhere between (1)H-MRI and MR spectroscopy (MRS). Like MRS it potentially provides information on metabolic processes, but only one single resonance of ionic (23)Na is observed. Therefore pulse sequences do not need to code for a chemical shift dimension, allowing (23)Na images to be obtained at high resolutions as compared to MRS. In this chapter the biological significance of sodium in the brain will be discussed, as well as methods for observing it with (23)Na-MRI. Many vital cellular processes and interactions in excitable tissues depend on the maintenance of a low intracellular and high extracellular sodium concentration. Healthy cells maintain this concentration gradient at the cost of energy. Leaky cell membranes or an impaired energy metabolism immediately leads to an increase in cytosolic total tissue sodium. This makes sodium a biomarker for ischemia, cancer, excessive tissue activation, or tissue damage as might be caused by ablation therapy. Special techniques allow quantification of tissue sodium for the monitoring of disease or therapy in longitudinal studies or preferential observation of the intracellular component of the tissue sodium. New methods and high-field magnet technology provide new opportunities for (23)Na-MRI in clinical and biomedical research.

Monitoring astrocyte calcium microdomains with improved membrane targeted GCaMP reporters

Astrocytes are involved in synaptic and cerebrovascular regulation in the brain. These functions are regulated by intracellular calcium signalling that is thought to reflect a form of astrocyte excitability. In a recent study, we reported modification of the genetically encoded calcium indicator (GECI) GCaMP2 with a membrane-tethering domain, Lck, to generate Lck-GCaMP2. This GECI allowed us to detect novel microdomain calcium signals. The microdomains were random and 'spotty' in nature. In order to detect such signals more reliably, in the present study we further modified Lck-GCaMP2 to carry three mutations in the GCaMP2 moiety (M153K, T203V within EGFP and N60D in the CaM domain) to generate Lck-GCaMP3. We directly compared Lck-GCaMP2 and Lck-GCaMP3 by assessing their ability to monitor several types of astrocyte calcium signals with a focus on spotty microdomains. Our data show that Lck-GCaMP3 is between two- and four-times better than Lck-GCaMP2 in terms of its basal fluorescence intensity, signal-to-noise and its ability to detect microdomains. The use of Lck-GCaMP3 thus represents a significantly improved way to monitor astrocyte calcium signals, including microdomains, and will facilitate detailed exploration of their molecular mechanisms and physiological roles.

Neuroglial roots of neurodegenerative diseases?

Neuroglia is critically important for controlling the brain homeostasis and for mounting the brain defence against pathological insults. Here, we overview recent data about the role of neuroglia in various types of neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, fronto-temporal dementia, Wernicke encephalopathy, amyotrophic lateral sclerosis and immunodeficiency virus-1-associated dementia). In all these forms of neurodegeneration, astroglia undergoes complex morphological and functional changes. The early and mid-term stages of neurodegenerative processes, and specifically of Alzheimer's disease, are associated with generalised atrophy of astroglia, whereas the later stages are characterised with an astrogliosis and microglial activation linked to neuropathological lesions such as senile plaques. Atrophic changes in astroglia may contribute to the initial cognitive deficits due to reduced glial synaptic coverage and decreased neuroprotection.

Abeta42 neurotoxicity is mediated by ongoing nucleated polymerization process rather than by discrete Abeta42 species

The identification of toxic Aβ species and/or the process of their formation is crucial for understanding the mechanism(s) of Aβ neurotoxicity in Alzheimer disease and also for the development of effective diagnostic and therapeutic interventions. To elucidate the structural basis of Aβ toxicity, we developed different procedures to isolate Aβ species of defined size and morphology distribution, and we investigated their toxicity in different cell lines and primary neurons. We observed that crude Aβ42 preparations, containing a monomeric and heterogeneous mixture of Aβ42 oligomers, were more toxic than purified monomeric, protofibrillar fractions, or fibrils. The toxicity of protofibrils was directly linked to their interactions with monomeric Aβ42 and strongly dependent on their ability to convert into amyloid fibrils. Subfractionation of protofibrils diminished their fibrillization and toxicity, whereas reintroduction of monomeric Aβ42 into purified protofibril fractions restored amyloid formation and enhanced their toxicity. Selective removal of monomeric Aβ42 from these preparations, using insulin-degrading enzyme, reversed the toxicity of Aβ42 protofibrils. Together, our findings demonstrate that Aβ42 toxicity is not linked to specific prefibrillar aggregate(s) but rather to the ability of these species to grow and undergo fibril formation, which depends on the presence of monomeric Aβ42. These findings contribute significantly to the understanding of amyloid formation and toxicity in Alzheimer disease, provide novel insight into mechanisms of Aβ protofibril toxicity, and important implications for designing anti-amyloid therapies.

The effect of glycogen phosphorolysis on basal glutaminergic transmission

Astrocytic glycogen metabolism sustains neuronal activity but its impact on basal glutamatergic synaptic transmission is not clear. To address this issue, we have compared the effect of glycogen breakdown inhibition on miniature excitatory postsynaptic currents (mEPSCs) in rat hippocampal pure neuronal culture (PNC) and in astrocyte-neuronal co-cultures (ANCC). Amplitudes of mEPSC in ANCC were nearly twice as large as in PNC with no difference in current kinetics. Inhibition of glycogen phosphorylase reduced mEPSC amplitude by roughly 40% in ANCC being ineffective in PNC. Altogether, these data indicate that astrocyte-neuronal interaction enhances basal mEPSCs in ANCC mainly due to astrocytic glycogen metabolism.

Environmental impoverishment and aging alter object recognition, spatial learning, and dentate gyrus astrocytes

Environmental and age-related effects on learning and memory were analysed and compared with changes observed in astrocyte laminar distribution in the dentate gyrus. Aged (20 months) and young (6 months) adult female albino Swiss mice were housed from weaning either in impoverished conditions or in enriched conditions, and tested for episodic-like and water maze spatial memories. After these behavioral tests, brain hippocampal sections were immunolabeled for glial fibrillary acid protein to identify astrocytes. The effects of environmental enrichment on episodic-like memory were not dependent on age, and may protect water maze spatial learning and memory from declines induced by aging or impoverished environment. In the dentate gyrus, the number of astrocytes increased with both aging and enriched environment in the molecular layer, increased only with aging in the polymorphic layer, and was unchanged in the granular layer. We suggest that long-term experience-induced glial plasticity by enriched environment may represent at least part of the circuitry groundwork for improvements in behavioral performance in the aged mice brain.

Transcriptional evidence for the "Reverse Warburg Effect" in human breast cancer tumor stroma and metastasis: similarities with oxidative stress, inflammation, Alzheimer's disease, and "Neuron-Glia Metabolic Coupling"

Caveolin-1 (-/-) null stromal cells are a novel genetic model for cancer-associated fibroblasts and myofibroblasts. Here, we used an unbiased informatics analysis of transcriptional gene profiling to show that Cav-1 (-/-) bone-marrow derived stromal cells bear a striking resemblance to the activated tumor stroma of human breast cancers. More specifically, the transcriptional profiles of Cav-1 (-/-) stromal cells were most closely related to the primary tumor stroma of breast cancer patients that had undergone lymph-node (LN) metastasis. This is consistent with previous morphological data demonstrating that a loss of stromal Cav-1 protein (by immuno-histochemical staining in the fibroblast compartment) is significantly associated with increased LN-metastasis. We also provide evidence that the tumor stroma of human breast cancers shows a transcriptional shift towards oxidative stress, DNA damage/repair, inflammation, hypoxia, and aerobic glycolysis, consistent with the "Reverse Warburg Effect". Finally, the tumor stroma of "metastasis-prone" breast cancer patients was most closely related to the transcriptional profiles derived from the brains of patients with Alzheimer's disease. This suggests that certain fundamental biological processes are common to both an activated tumor stroma and neuro-degenerative stress. These processes may include oxidative stress, NO over-production (peroxynitrite formation), inflammation, hypoxia, and mitochondrial dysfunction, which are thought to occur in Alzheimer's disease pathology. Thus, a loss of Cav-1 expression in cancer-associated myofibroblasts may be a protein biomarker for oxidative stress, aerobic glycolysis, and inflammation, driving the "Reverse Warburg Effect" in the tumor micro-environment and cancer cell metastasis.

Astrocytes in Alzheimer's disease

The circuitry of the human brain is formed by neuronal networks embedded into astroglial syncytia. The astrocytes perform numerous functions, providing for the overall brain homeostasis, assisting in neurogenesis, determining the micro-architecture of the grey matter, and defending the brain through evolutionary conserved astrogliosis programs. Astroglial cells are engaged in neurological diseases by determining the progression and outcome of neuropathological process. Astrocytes are specifically involved in various neurodegenerative diseases, including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, and various forms of dementia. Recent evidence suggest that early stages of neurodegenerative processes are associated with atrophy of astroglia, which causes disruptions in synaptic connectivity, disbalance in neurotransmitter homeostasis, and neuronal death through increased excitotoxicity. At the later stages, astrocytes become activated and contribute to the neuroinflammatory component of neurodegeneration.

Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca(2+) or Na(+) signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Phosphorylation status of pyruvate dehydrogenase distinguishes metabolic phenotypes of cultured rat brain astrocytes and neurons

Glucose metabolism in nervous tissue has been proposed to occur in a compartmentalized manner with astrocytes contributing largely to glycolysis and neurons being the primary site of glucose oxidation. However, mammalian astrocytes and neurons both contain mitochondria, and it remains unclear why in culture neurons oxidize glucose, lactate, and pyruvate to a much larger extent than astrocytes. The objective of this study was to determine whether pyruvate metabolism is differentially regulated in cultured neurons versus astrocytes. Expression of all components of the pyruvate dehydrogenase complex (PDC), the rate-limiting step for pyruvate entry into the Krebs cycle, was determined in cultured astrocytes and neurons. In addition, regulation of PDC enzymatic activity in the two cell types via protein phosphorylation was examined. We show that all components of the PDC are expressed in both cell types in culture, but that PDC activity is kept strongly inhibited in astrocytes through phosphorylation of the pyruvate dehydrogenase alpha subunit (PDH alpha). In contrast, neuronal PDC operates close to maximal levels with much lower levels of phosphorylated PDH alpha. Dephosphorylation of astrocytic PDH alpha restores PDC activity and lowers lactate production. Our findings suggest that the glucose metabolism of astrocytes and neurons may be far more flexible than previously believed.

Role of glia in epilepsy-associated neuropathology, neuroinflammation and neurogenesis

The black reaction allowed Golgi to describe with amazing detail the morphology of glial cells as well as their proximal location and intimate connections with neurons and blood vessels. Based on this location, Golgi hypothesized that glial cells were functional units in the nervous system and were not merely a structural support medium. Relatively recent advances have confirmed the importance of glial cells in nervous system function and disease. The occurrence of gliosis is considered the hallmark of damaged tissue. Gliosis can differentially influence disease development and it is a prevailing characteristic of temporal lobe epilepsy. Its presence in the epileptic hippocampi might contribute to hyperexcitability, the development of aberrant neurogenic changes and inflammatory processes related to seizures. Considering the accumulating data regarding the pathological role of glial cells in epilepsy, novel therapeutic approaches that target glial cells are being explored. Such therapeutic approaches directed to glial cells present a novel perspective for the management of refractory pathologies.

Spatial correlation between brain aerobic glycolysis and amyloid-β (Aβ ) deposition

Amyloid-β (Aβ) plaque deposition can precede the clinical manifestations of dementia of the Alzheimer type (DAT) by many years and can be associated with changes in brain metabolism. Both the Aβ plaque deposition and the changes in metabolism appear to be concentrated in the brain's default-mode network. In contrast to prior studies of brain metabolism which viewed brain metabolism from a unitary perspective that equated glucose utilization with oxygen consumption, we here report on regional glucose use apart from that entering oxidative phosphorylation (so-called "aerobic glycolysis"). Using PET, we found that the spatial distribution of aerobic glycolysis in normal young adults correlates spatially with Aβ deposition in individuals with DAT and cognitively normal participants with elevated Aβ, suggesting a possible link between regional aerobic glycolysis in young adulthood and later development of Alzheimer pathology.

Differential effects of Na+-K+ ATPase blockade on cortical layer V neurons

Sodium-potassium ATPase ('Na(+)-K(+) ATPase') contributes to the maintenance of the resting membrane potential and the transmembrane gradients for Na(+) and K(+) in neurons. Activation of Na(+)-K(+) ATPase may be important in controlling increases in intracellular sodium during periods of increased neuronal activity. Down-regulation of Na(+)-K(+) ATPase activity is implicated in numerous CNS disorders, including epilepsy. Although Na(+)-K(+) ATPase is present in all neurons, little is known about its activity in different subclasses of neocortical cells. We assessed the physiological properties of Na(+)-K(+) ATPase in fast-spiking (FS) interneurons and pyramidal (PYR) cells to test the hypothesis that Na(+)-K(+) ATPase activity would be relatively greater in neurons that generated high frequency action potentials (the FS cells). Whole-cell patch clamp recordings were made from FS and PYR neurons in layer V of rat sensorimotor cortical slices maintained in vitro using standard techniques. Bath perfusion of Na(+)-K(+) ATPase antagonists (ouabain or dihydro-ouabain) induced either a membrane depolarization in current clamp, or inward current under voltage clamp in both cell types. PYR neurons were divided into two subpopulations based on the amplitude of the voltage or current shift in response to Na(+)-K(+) ATPase blockade. The two PYR cell groups did not differ significantly in electrophysiological properties including resting membrane potential, firing pattern, input resistance and capacitance. Membrane voltage responses of FS cells to Na(+)-K(+) ATPase blockade were intermediate between the two PYR cell groups (P < 0.05). The resting Na(+)-K(+) ATPase current density in FS interneurons, assessed by application of blockers, was 3- to 7-fold larger than in either group of PYR neurons. Na(+)-K(+) ATPase activity was increased either through direct Na(+) loading via the patch pipette or by focal application of glutamate (20 mM puffs). Under these conditions FS interneurons exhibited the largest increase in Na(+)-K(+) ATPase activity. We conclude that resting Na(+)-K(+) ATPase activity and sensitivity to changes in internal Na(+) concentration vary between and within classes of cortical neurons. These differences may have important consequences in pathophysiological disorders associated with down-regulation of Na(+)-K(+) ATPase and hyperexcitability within cortical networks.

Recent advances in (patho)physiology of astroglia

Our view of astrocytes in the operation of the brain is changing dramatically over the last 3 decades. Astroglial calcium excitability controls the release of gliotransmitters, which can occur at the tripartite synapse. Astrocytes not only modulate synaptic transmission by releasing and taking up transmitters, but also receiving neuronal signals that act upon astrocytic plasma membrane receptors. This process represents the bidirectional neurone-glia communication. Additionally, astrocytes play role in the regulation of blood flow as well as ion and water homeostasis. Many of the brain dysfunctions are primary astropathies, including hepatic encephalopathy and Alexander disease, while other brain malfunctions, such as epilepsy and Alzheimer disease, may have substantial astrocytic contribution. Thus, these star-shaped cells by their roles in (patho)physiology of the brain seem to live up to the expectation one can have from their given name - astrocyte.

Mitochondrial dysfunction in glaucoma and emerging bioenergetic therapies

The similarities between glaucoma and mitochondrial optic neuropathies have driven a growing interest in exploring mitochondrial function in glaucoma. The specific loss of retinal ganglion cells is a common feature of mitochondrial diseases - not only the classic mitochondrial optic neuropathies of Leber's Hereditary Optic Neuropathy and Autosomal Dominant Optic Atrophy - but also occurring together with more severe central nervous system involvement in many other syndromic mitochondrial diseases. The retinal ganglion cell, due to peculiar structural and energetic constraints, appears acutely susceptible to mitochondrial dysfunction. Mitochondrial function is also well known to decline with aging in post-mitotic tissues including neurons. Because age is a risk factor for glaucoma this adds another impetus to investigating mitochondria in this common and heterogeneous neurodegenerative disease. Mitochondrial function may be impaired by either nuclear gene or mitochondrial DNA genetic risk factors, by mechanical stress or chronic hypoperfusion consequent to the commonly raised intraocular pressure in glaucomatous eyes, or by toxic xenobiotic or even light-induced oxidative stress. If primary or secondary mitochondrial dysfunction is further established as contributing to glaucoma pathogenesis, emerging therapies aimed at optimizing mitochondrial function represent potentially exciting new clinical treatments that may slow retinal ganglion cell and vision loss in glaucoma.

Metabolic response of the cerebral cortex following gentle sleep deprivation and modafinil administration

STUDY OBJECTIVES

The main energy reserve of the brain is glycogen, which is almost exclusively localized in astrocytes. We previously reported that cerebral expression of certain genes related to glycogen metabolism changed following instrumental sleep deprivation in mice. Here, we extended our investigations to another set of genes related to glycogen and glucose metabolism. We also compared the effect of instrumentally and pharmacologically induced prolonged wakefulness, followed (or not) by 3 hours of sleep recovery, on the expression of genes related to brain energy metabolism.

DESIGN

Sleep deprivation for 6-7 hours.

SETTING

Animal sleep research laboratory.

PARTICIPANTS

Adults OF1 mice.

INTERVENTIONS

Wakefulness was maintained by "gentle sleep deprivation" method (GSD) or by administration of the wakefulness-promoting drug modafinil (MOD) (200 mg/kg i.p.).

MEASUREMENTS AND RESULTS

Levels of mRNAs encoding proteins related to energy metabolism were measured by quantitative real-time PCR in the cerebral cortex. The mRNAs encoding protein targeting to glycogen (PTG) and the glial glucose transporter were significantly increased following both procedures used to prolong wakefulness. Glycogenin mRNA levels were increased only after GSD, while neuronal glucose transporter mRNA only after MOD. These effects were reversed after sleep recovery. A significant enhancement of glycogen synthase activity without any changes in glycogen levels was observed in both conditions.

CONCLUSIONS

These results indicate the existence of a metabolic adaptation of astrocytes aimed at maintaining brain energy homeostasis during the sleep-wake cycle.

Energy metabolism of the visual system

The visual system is one of the most energetically demanding systems in the brain. The currency of energy is ATP, which is generated most efficiently from oxidative metabolism in the mitochondria. ATP supports multiple neuronal functions. Foremost is repolarization of the membrane potential after depolarization. Neuronal activity, ATP generation, blood flow, oxygen consumption, glucose utilization, and mitochondrial oxidative metabolism are all interrelated. In the retina, phototransduction, neurotransmitter utilization, and protein/organelle transport are energy-dependent, yet repolarization-after-depolarization consumes the bulk of the energy. Repolarization in photoreceptor inner segments maintains the dark current. Repolarization by all neurons along the visual pathway following depolarizing excitatory glutamatergic neurotransmission preserves cellular integrity and permits reactivation. The higher metabolic activity in the magno- versus the parvo-cellular pathway, the ON- versus the OFF-pathway in some (and the reverse in other) species, and in specialized functional representations in the visual cortex all reflect a greater emphasis on the processing of specific visual attributes. Neuronal activity and energy metabolism are tightly coupled processes at the cellular and even at the molecular levels. Deficiencies in energy metabolism, such as in diabetes, mitochondrial DNA mutation, mitochondrial protein malfunction, and oxidative stress can lead to retinopathy, visual deficits, neuronal degeneration, and eventual blindness.

Decline in mitochondrial bioenergetics and shift to ketogenic profile in brain during reproductive senescence

BACKGROUND

We have previously demonstrated that mitochondrial bioenergetic deficits precede Alzheimer's pathology in the female triple transgenic Alzheimer's (3xTgAD) mouse model. Herein, we sought to determine the impact of reproductive senescence on mitochondrial function in the normal non-transgenic (nonTg) and 3xTgAD female mouse model of AD.

METHODS

Both nonTg and 3xTgAD female mice at 3, 6, 9, and 12 months of age were sacrificed and mitochondrial bioenergetic profile as well as oxidative stress markers were analyzed.

RESULTS

In both nonTg and 3xTgAD mice, reproductive senescence paralleled a significant decline in PDH, and Complex IV cytochrome c oxidase activity and mitochondrial respiration. During the reproductive senescence transition, both nonTg and 3xTgAD mice exhibited greater individual variability in bioenergetic parameters suggestive of divergent bioenergetic phenotypes. Following transition through reproductive senescence, enzymes required for long-chain fatty acid (HADHA) and ketone body (SCOT) metabolism were significantly increased and variability in cytochrome c oxidase (Complex IV) collapsed to cluster at a approximately 40% decline in both the nonTg and 3xTgAD brain which was indicative of alternative fuel generation with concomitant decline in ATP generation.

CONCLUSIONS

These data indicate that reproductive senescence in the normal nonTg female brain parallels the shift to ketogenic/fatty acid substrate phenotype with concomitant decline in mitochondrial function and exacerbation of bioenergetic deficits in the 3xTgAD brain.

GENERAL SIGNIFICANCE

These findings provide a plausible mechanism for increased life-time risk of AD in postmenopausal women and suggest an optimal window of opportunity to prevent or delay decline in bioenergetics during reproductive senescence.

Where the thoughts dwell: the physiology of neuronal-glial "diffuse neural net"

The mechanisms underlying the production of thoughts by exceedingly complex cellular networks that construct the human brain constitute the most challenging problem of natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for cognition. These neuronal networks however are embedded into much larger and probably more complex network formed by neuroglia. The latter, although being electrically silent, employ many different mechanisms for intercellular signalling. It appears that astrocytes can control synaptic networks and in such a capacity they may represent an integral component of the computational power of the brain rather than being just brain "connective tissue". The fundamental question of whether neuroglia is involved in cognition and information processing remains, however, open. Indeed, a remarkable increase in the number of glial cells that distinguishes the human brain can be simply a result of exceedingly high specialisation of the neuronal networks, which delegated all matters of survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition.

A genetically targeted optical sensor to monitor calcium signals in astrocyte processes

Calcium signaling is studied as a potential form of astrocyte excitability that may control astrocyte involvement in synaptic and cerebrovascular regulation. Fundamental questions remain unanswered about astrocyte calcium signaling, as current methods can not resolve calcium in small volume compartments, such as near the cell membrane and in distal cell processes. We modified the genetically encoded calcium sensor GCaMP2 with a membrane-tethering domain, Lck, increasing the level of Lck-GCaMP2 near the plasma membrane tenfold as compared with conventional GCaMP2. Using Lck-GCaMP2 in rat hippocampal astrocyte-neuron cocultures, we measured near-membrane calcium signals that were evoked pharmacologically or by single action potential-mediated neurotransmitter release. Moreover, we identified highly localized and frequent spontaneous calcium signals in astrocyte somata and processes that conventional GCaMP2 failed to detect. Lck-GCaMP2 acts as a genetically targeted calcium sensor for monitoring calcium signals in previously inaccessible parts of astrocytes, including fine processes.

Amyloid-beta aggregates cause alterations of astrocytic metabolic phenotype: impact on neuronal viability

Amyloid-beta (Abeta) peptides play a key role in the pathogenesis of Alzheimer's disease and exert various toxic effects on neurons; however, relatively little is known about their influence on glial cells. Astrocytes play a pivotal role in brain homeostasis, contributing to the regulation of local energy metabolism and oxidative stress defense, two aspects of importance for neuronal viability and function. In the present study, we explored the effects of Abeta peptides on glucose metabolism in cultured astrocytes. Following Abeta(25-35) exposure, we observed an increase in glucose uptake and its various metabolic fates, i.e., glycolysis (coupled to lactate release), tricarboxylic acid cycle, pentose phosphate pathway, and incorporation into glycogen. Abeta increased hydrogen peroxide production as well as glutathione release into the extracellular space without affecting intracellular glutathione content. A causal link between the effects of Abeta on glucose metabolism and its aggregation and internalization into astrocytes through binding to members of the class A scavenger receptor family could be demonstrated. Using astrocyte-neuron cocultures, we observed that the overall modifications of astrocyte metabolism induced by Abeta impair neuronal viability. The effects of the Abeta(25-35) fragment were reproduced by Abeta(1-42) but not by Abeta(1-40). Finally, the phosphoinositide 3-kinase (PI3-kinase) pathway appears to be crucial in these events since both the changes in glucose utilization and the decrease in neuronal viability are prevented by LY294002, a PI3-kinase inhibitor. This set of observations indicates that Abeta aggregation and internalization into astrocytes profoundly alter their metabolic phenotype with deleterious consequences for neuronal viability.

Integrated brain circuits: astrocytic networks modulate neuronal activity and behavior

The past decade has seen an explosion of research on roles of neuron-astrocyte interactions in the control of brain function. We highlight recent studies performed on the tripartite synapse, the structure consisting of pre- and postsynaptic elements of the synapse and an associated astrocytic process. Astrocytes respond to neuronal activity and neurotransmitters, through the activation of metabotropic receptors, and can release the gliotransmitters ATP, d-serine, and glutamate, which act on neurons. Astrocyte-derived ATP modulates synaptic transmission, either directly or through its metabolic product adenosine. d-serine modulates NMDA receptor function, whereas glia-derived glutamate can play important roles in relapse following withdrawal from drugs of abuse. Cell type-specific molecular genetics has allowed a new level of examination of the function of astrocytes in brain function and has revealed an important role of these glial cells that is mediated by adenosine accumulation in the control of sleep and in cognitive impairments that follow sleep deprivation.

Modulation of somatosensory-evoked cortical blood flow changes by GABAergic inhibition of the nucleus basalis of Meynert in urethane-anaesthetized rats

Vascular changes associated with brain functions are thought to be tightly coupled with neuronal activity through neuronal glucose consumption or the local release of vasoactive agents. In contrast, another view suggests that cortical blood flow is strongly regulated by the nucleus basalis of Meynert (NBM), independently of regional metabolism. Thus, although cortical regional cerebral blood flow (rCBF) variations induced by somatosensory stimulation are strongly linked to neuronal activity, they may also be partly controlled by the NBM. In the present study, cortical rCBF alterations in response to innocuous brushing of the hindlimb (HL) were investigated by laser speckle contrast imaging. The contribution of NBM to these changes was examined after injection of the GABAergic agonist muscimol into the right NBM, allowing comparison of somatosensory-evoked cortical rCBF modifications before and after NBM inactivation. As expected, HL brushing elicited a robust rCBF increase in the contralateral parietal cortex (PC), over the representation of the HL. However, these alterations were decreased, by approximately 40%, in the hemisphere ipsilateral to muscimol inactivation of NBM, whereas vehicle injection did not produce any significant variation. The results demonstrate that cortical rCBF changes induced by somatosensory stimulation are partly regulated by NBM.

The pentose-phosphate pathway in neuronal survival against nitrosative stress

Neurons are thought to be particularly vulnerable cells against reactive oxygen and nitrogen species (RONS) damage (nitrosative stress), due in part to their weak antioxidant defense and low ability to compensate energy homeostasis. Intriguingly, nitrosative stress efficiently stimulates the rate of the antioxidant pentose-phosphate pathway (PPP), which generates NADPH a necessary cofactor for the reduction of glutathione disulfide. In fact, inhibition of PPP sensitizes cultured neurons to glutathione oxidation and apoptotic death, whereas its stimulation confers resistance to nitrosative stress. Furthermore, we recently described that neurons can preferentially use glucose through the PPP by inhibiting glycolysis, which is achieved by continuously degrading the glycolytic positive-effector protein, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 (Pfkfb3) by the action of the E3 ubiquitine ligase anaphase-promoting complex/cyclosome (APC/C)(Cdh1). These results suggest that the antioxidant fragility of neurons may be compensated by the PPP at the expense of inhibiting bioenergetic glycolysis.

A Model of Indispensability of a Large Glial Layer in Cerebrovascular Circulation

We formulate the problem of oxygen delivery to neural tissue as a problem of association. Input to a pool of neurons in one brain area must be matched in space and time with metabolic inputs from the vascular network via the glial network. We thus have a model in which neural, glial, and vascular layers are connected bidirectionally, in that order. Connections between neuro-glial and glial-vascular stages are trained by an unsupervised learning mechanism such that input to the neural layer is sustained by the precisely patterned delivery of metabolic inputs from the vascular layer via the glial layer. Simulations show that the capacity of such a system to sustain patterns is weak when the glial layer is absent. Capacity is higher when a glial layer is present and increases with the layer size. The proposed formulation of neurovascular interactions raises many intriguing questions about the role of glial cells in cerebral circulation.

Morphological abnormalities of the thalamus in youths with attention deficit hyperactivity disorder

OBJECTIVE

The role of the thalamus in the genesis of attention deficit hyperactivity disorder (ADHD) remains poorly understood. The authors used anatomical MRI to examine the morphology of the thalamus in youths with ADHD and healthy comparison youths.

METHOD

The authors examined 46 youths with ADHD and 59 comparison youths 8-18 years of age in a cross-sectional case-control study. Conventional volumes and measures of surface morphology of the thalamus served as the main outcome measures.

RESULTS

A mixed-effects model comparing whole thalamic volumes revealed no significant differences between groups. Maps of the thalamic surface revealed significantly smaller regional volumes bilaterally in the pulvinar in youths with ADHD relative to comparison subjects. Post hoc analyses showed that ADHD patients who received stimulants (N=31) had larger conventional thalamic volumes than untreated youths with ADHD, and maps of the thalamic surface showed enlargement over the pulvinar in those receiving stimulants. Smaller regional volumes in the right lateral and left posterior thalamic surfaces were associated with more severe hyperactivity symptoms, whereas larger regional volumes in the right medial thalamic surfaces were associated with more severe symptoms of inattention.

CONCLUSION

These findings demonstrate reduced pulvinar volumes in youths with ADHD and indicate that this same area is relatively enlarged in patients treated with stimulants compared to those untreated. Associations of hyperactivity scores with smaller regional volumes on the lateral thalamic surface and inattention scores with larger regional volumes on the medial thalamic surface suggest the differential involvement of thalamic subcircuits in the pathogenesis of differing ADHD symptoms.

Myelination and the trophic support of long axons

In addition to their role in providing myelin for rapid impulse propagation, the glia that ensheath long axons are required for the maintenance of normal axon transport and long-term survival. This presumably ancestral function seems to be independent of myelin membrane wrapping. Here, I propose that ensheathing glia provide trophic support to axons that are metabolically isolated, and that myelin itself might cause such isolation. This glial support of axonal integrity may be relevant for a number of neurological and psychiatric diseases.

A microperfused incubator for tissue mimetic 3D cultures

High density, three-dimensional (3D) cultures present physical similarities to in vivo tissue and are invaluable tools for pre-clinical therapeutic discoveries and development of tissue engineered constructs. Unfortunately, the use of dense cultures is hindered by intra-culture transport limits allowing just a few layer thick cultures for reproducible studies. In order to overcome diffusion limits in intra-culture nutrient and gas availability, a simple scalable microfluidic perfusion platform was developed and validated. A novel perfusion approach maintained laminar flow of nutrients through the culture to meet metabolic need, while removing depleted medium and catabolites. Velocity distributions and 3D flow patterns were measured using microscopic particle image velocimetry. The effectiveness of forced convection laminar perfusion was confirmed by culturing 700 microm thick neural-astrocytic (1:1) constructs at cell density approaching that of the brain (50,000 cells/mm(3)). At the optimized flow rate of the nutrient medium, the culture viability reached 90% through the full construct thickness at 2 days of perfusion while unperfused controls exhibited widespread cell death. The membrane aerated perfusion platform was integrated within a miniature, imaging accessible enclosure enabling temperature and gas control of the culture environment. Temperature measurements demonstrated fast feedback response to environmental changes resulting in the maintenance of the physiological temperature within 37 +/- 0.2 degrees C. Reproducible culturing of tissue equivalents within dynamically controlled environments will provide higher fidelity to in vivo function in an in vitro accessible format for cell-based assays and regenerative medicine.

Elevated activity of the crayfish stretch receptor neuron increases resistance of surrounding glial cells to apoptosis induced by photodynamic treatment

Neuroglial interaction is very important for functioning and survival of nerve and glial cells. In the present work, we studied the influence of the intense neuronal activity on survival of the isolated crayfish stretch receptor neuron and surrounding glial cells subjected to photodynamic treatment, which induces intense oxidative stress. In the experimental group, neurons were stimulated by multiple extensions of the receptor muscle for 1h so that the firing rate did not fall below 10-15 Hz, whereas in the control group, the receptor muscles were relaxed and neurons were silent. After stimulation, the preparations were photosensitized with alumophthalocyanine Photosens and irradiated by 670 nm laser diode. The isolated stretch receptors were stained with propidium iodide and Hoechst 33342, which reveal the nuclei of the necrotic and the apoptotic cells, respectively. The level of apoptosis of photosensitized glial cells was significantly lower in the experimental group compared to the resting control. Necrosis of neurons and glial cells was not significantly influenced. Therefore, elevated neuronal activity increased the resistance of the surrounding glial cells to photoinduced apoptosis. This could be attributed to the depletion of the energetic resources, which are transferred from glia into the neuron to support its firing, or to the neurotrophic neuron-to-glia signaling.

Filming the glial dreams: real-time imaging of cannabinoid receptor trafficking in astrocytes

How does the brain process incoming information and produce thoughts? These questions represent, to all likelihood, the most challenging matters ever faced by natural sciences, matters which may never be fully comprehended. The evolution of the nervous system that, in about billion of years, brought into existence the human brain progressed through an ever-increasing complexity of neural networks. This evolution began from the diffuse nervous system, in which primordial neurons were able to sense the environmental inputs and convey them to effector organs and to the neighbouring neurons. At the next evolutionary stage the conglomerates of neuronal cell bodies, the ganglia, appeared, thus forming the primitive centralized nervous system. The developments which ensued went through a continuous increase in complexity of neuronal conglomerates, which eventually formed the central nervous system, which attained maximal perfection in mammals. In this issue of ASN NEURO, Osborne et al. have described details of real-time imaging of cannabinoid receptor trafficking in astrocytes, a technique that will help to elucidate the role of these receptors in the ever-increasing complex neural networks.

Functional and molecular interactions between aquaporins and Na,K-ATPase

The water channel aquaporin 4 (AQP4) is abundantly expressed in astrocytes and provides a mechanism by which water permeability of the plasma membrane can be regulated. Astrocytes play a key role in the clearance of both potassium (K(+)) and glutamate released during neuronal activity. Emerging evidence suggests that AQP4 facilitates K(+) clearance by astrocytes and contributes to recovery of neuronal excitability. Here we report that AQP4 can assemble with its regulator metabotropic glutamate receptor 5 (mGluR5) and with Na,K-ATPase; the enzyme responsible for active K(+) transport and for establishing the electrochemical gradient across the cell plasma membrane. We have, by use of pull down assays in rat brain tissue, identified the segment in the AQP4 NH(2)-terminus containing the amino acid residues 23-32 as the site for interaction with Na,K-ATPase catalytic subunit and with mGluR5. Mutagenesis studies revealed that the AQP4 amino acids K27 and W30 are of key importance for interaction with both Na,K-ATPase and mGluR5. To confirm that interaction also occurs within intact cells, we have performed fluorescence resonance energy transfer (FRET) studies in primary astrocytes derived from rat striatum. The results indicate close proximity of wild type AQP4 and Na,K-ATPase in the plasma membrane of rat astrocytes. FRET efficiencies observed with the mutants AQP4 K27A and AQP4 W30A were significantly lower, highlighting the importance of these residues for the interaction between AQP4 and Na,K-ATPase. We conclude that AQP4/Na,K-ATPase/mGluR5 can form a macromolecular complex/transporting microdomain in astrocytes. This complex may be of functional importance for the regulation of water and K(+) homeostasis in the brain, as well as for neuron-astrocyte metabolic crosstalk.

An interaction of oxytocin receptors with metabotropic glutamate receptors in hypothalamic astrocytes

Hypothalamic astrocytes play a critical role in the regulation and support of many different neuroendocrine events, and are affected by oestradiol. Both nuclear and membrane oestrogen receptors (ERs) are expressed in astrocytes. Upon oestradiol activation, membrane-associated ER signals through the type 1a metabotropic glutamate receptor (mGluR1a) to induce an increase of free cytoplasmic calcium concentration ([Ca(2+)](i)). Because the expression of oxytocin receptors (OTRs) is modulated by oestradiol, we tested whether oestradiol also influences oxytocin signalling. Oxytocin at 1, 10, and 100 nm induced a [Ca(2+)](i) flux measured as a change in relative fluorescence [DeltaF Ca(2+) = 330 +/- 17 relative fluorescent units (RFU), DeltaF Ca(2+) = 331 +/- 22 RFU, and DeltaF Ca(2+) = 347 +/- 13 RFU, respectively] in primary cultures of female post-pubertal hypothalamic astrocytes. Interestingly, OTRs interacted with mGluRs. The mGluR1a antagonist, LY 367385 (20 nm), blocked the oxytocin (1 nm)-induced [Ca(2+)](i) flux (DeltaF Ca(2+) = 344 +/- 19 versus 127 +/- 11 RFU, P < 0.001). Conversely, the mGluR1a receptor agonist, (RS)-3,5-dihydroxyphenyl-glycine (100 nm), increased the oxytocin (1 nm)-induced [Ca(2+)](i) response (DeltaF Ca(2+) = 670 +/- 31 RFU) compared to either compound alone (P < 0.001). Because both oxytocin and oestradiol rapidly signal through the mGluR1a, we treated hypothalamic astrocytes sequentially with oxytocin and oestradiol to determine whether stimulation with one hormone affected the subsequent [Ca(2+)](i) response to the second hormone. Oestradiol treatment did not change the subsequent [Ca(2+)](i) flux to oxytocin (P > 0.05) and previous oxytocin exposure did not affect the [Ca(2+)](i) response to oestradiol (P > 0.05). Furthermore, simultaneous oestradiol and oxytocin stimulation failed to yield a synergistic [Ca(2+)](i) response. These results suggest that the OTR signals through the mGluR1a to release Ca(2+) from intracellular stores and rapid, nongenomic oestradiol stimulation does not influence OTR signalling in astrocytes.

Neuroglia in neurodegeneration

Neuroglial cells are fundamental for control of brain homeostasis and they represent the intrinsic brain defence system. All forms in neuropathology therefore inevitably involve glia. The neurodegenerative diseases disrupt connectivity within brain circuits affecting neuronal-neuronal, neuronal-glial and glial-glial contacts. In addition neurodegenerative processes trigger universal and conserved glial reactions represented by astrogliosis and microglial activation. The complex of recently acquired knowledge allows us to regard the neurodegenerative diseases as primarily gliodegenerative processes, in which glial cells determine the progression and outcome of neuropathological process.

Impaired active avoidance learning in infant rats appears to be related to insufficient metabolic recruitment of the lateral septum

The temporal dissociation between early information acquisition and output of complex behaviors is a common principle during development. Thus, although infant rats are not able to generate sufficient avoidance behavior during two-way active avoidance (TWA) training they obviously deposit a certain "memory trace" (Schäble, Poeggel, Braun, & Gruss, 2007). The ontogeny of learning is probably mirrored by the maturing functionality of different basal forebrain regions. Two of the basal forebrain regions involved in TWA learning are the medial septum/diagonal band of Broca (MS/DB), which is essential for the encoding and retrieval of memory and the lateral septum (LS) that plays a role in the generation of behavior. Mapping 2-fluoro-deoxy-glucose utilization in freely behaving animals, the aim of this study was to assess the functional recruitment of the MS/DB and LS in infant (P17-P21) and adolescent (P38-P42) rats during the first (acquisition) and fifth (retrieval) TWA training. Metabolic activity in the MS/DB was similar in both age groups during acquisition and retrieval indicating that this region is already mature in the infant rat. In contrast, metabolic activity in the LS was generally lower in the infant rats suggesting that this region is not yet fully functional during P17 and P21. This insufficient recruitment may be one reason for the poor TWA performance of infant rats. Finally, the LS displayed significantly higher activity during acquisition than during retrieval indicating that the highest amount of energy is consumed during the initial learning phase.

The cytosolic redox state of astrocytes: Maintenance, regulation and functional implications for metabolite trafficking

Astrocytes have important functions in the metabolism of the brain. These cells provide neurons with metabolic substrates for energy production as well as with precursors for neurotransmitter and glutathione synthesis. Both the metabolism of astrocytes and the subsequent supply of metabolites from astrocytes to neurons are strongly affected by alterations in the cellular redox state. The cytosolic redox state of astrocytes depends predominantly on the ratios of the oxidised and reduced partners of the redox pairs NADH/NAD(+), NADPH/NADP(+) and GSH/GSSG. The NADH/NAD(+) pair is predominately in the oxidised state to accept electrons that are produced during glycolysis. In contrast, the redox pairs NADPH/NADP(+) and GSH/GSSG are biased towards the reduced state under unstressed conditions to provide electrons for reductive biosyntheses and antioxidative processes, respectively. In this review article we describe the metabolic processes that maintain the redox pairs in their desired redox states in the cytosol of astrocytes and discuss the consequences of alterations of the normal redox state for the regulation of cellular processes and for metabolite trafficking from astrocytes to neurons.

Probing for local activity-related modulation of the infrared backscattering of the brain cortex

The possibility to measure the metabolic activity of the brain cortex, with submillimeter spatial and subsecond temporal resolution, would open up enticing scenarios in addressing basic issues on the relation between different structural components of brain signal processing, and in providing an operational pathway to interaction with (dis)functional signal patterns. In the present article, we report the description of a simple system that allows the detection of the minute changes that occur in the optical backscattering of the cortex as a metabolic response to external stimuli. The simplicity of the system is compatible with scalability to an implantable probe. We validate the system on an animal model, and we propose an algorithm to extract meaningful data from the measured signal. We thus show the detection of individual haemodynamic cortical responses to individual stimulation events, and we provide operational considerations on the signal structure.