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Astrocyte-Endotheliocyte Axis in the Regulation of the Blood-Brain Barrier. Neurochem Res 2021; 46:2538-2550. [PMID: 33961207 DOI: 10.1007/s11064-021-03338-6] [Citation(s) in RCA: 21] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/09/2021] [Revised: 05/01/2021] [Accepted: 05/03/2021] [Indexed: 12/18/2022]
Abstract
The evolution of blood-brain barrier paralleled centralisation of the nervous system: emergence of neuronal masses required control over composition of the interstitial fluids. The barriers were initially created by glial cells, which employed septate junctions to restrict paracellular diffusion in the invertebrates and tight junctions in some early vertebrates. The endothelial barrier, secured by tight and adherent junctions emerged in vertebrates and is common in mammals. Astrocytes form the parenchymal part of the blood-brain barrier and commutate with endothelial cells through secretion of growth factors, morphogens and extracellular vesicles. These secreted factors control the integrity of the blood-brain barrier through regulation of expression of tight junction proteins. The astrocyte-endotheliocyte communications are particularly important in various neurological diseases associated with impairments to the blood-brain barrier. Molecular mechanisms supporting astrocyte-endotheliocyte axis in health and disease are in need of detailed characterisation.
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Díaz EF, Labra VC, Alvear TF, Mellado LA, Inostroza CA, Oyarzún JE, Salgado N, Quintanilla RA, Orellana JA. Connexin 43 hemichannels and pannexin-1 channels contribute to the α-synuclein-induced dysfunction and death of astrocytes. Glia 2019; 67:1598-1619. [PMID: 31033038 DOI: 10.1002/glia.23631] [Citation(s) in RCA: 26] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2018] [Revised: 04/08/2019] [Accepted: 04/09/2019] [Indexed: 01/22/2023]
Abstract
Diverse studies have suggested that cytoplasmic inclusions of misfolded α-synuclein in neuronal and glial cells are main pathological features of different α-synucleinopathies, including Parkinson's disease and dementia with Lewy bodies. Up to now, most studies have focused on the effects of α-synuclein on neurons, whereas the possible alterations of astrocyte functions and neuron-glia crosstalk have received minor attention. Recent evidence indicates that cellular signaling mediated by hemichannels and pannexons is critical for astroglial function and dysfunction. These channels constitute a diffusional route of communication between the cytosol and the extracellular space and during pathological scenarios they may lead to homeostatic disturbances linked to the pathogenesis and progression of different diseases. Here, we found that α-synuclein enhances the opening of connexin 43 (Cx43) hemichannels and pannexin-1 (Panx1) channels in mouse cortical astrocytes. This response was linked to the activation of cytokines, the p38 MAP kinase, the inducible nitric oxide synthase, cyclooxygenase 2, intracellular free Ca2+ concentration ([Ca2+ ]i ), and purinergic and glutamatergic signaling. Relevantly, the α-synuclein-induced opening of hemichannels and pannexons resulted in alterations in [Ca2+ ]i dynamics, nitric oxide (NO) production, gliotransmitter release, mitochondrial morphology, and astrocyte survival. We propose that α-synuclein-mediated opening of astroglial Cx43 hemichannels and Panx1 channels might constitute a novel mechanism involved in the pathogenesis and progression of α-synucleinopathies.
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Affiliation(s)
- Esteban F Díaz
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Valeria C Labra
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Tanhia F Alvear
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Luis A Mellado
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Carla A Inostroza
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Juan E Oyarzún
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Nicole Salgado
- Unidad de Microscopía Avanzada UC, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile
| | - Rodrigo A Quintanilla
- Escuela de Medicina, Centro de Investigación y Estudio del Consumo de Alcohol en Adolescentes (CIAA), Santiago, Chile.,Laboratory of Neurodegenerative Diseases, Universidad Autónoma de Chile, Santiago, Chile
| | - Juan A Orellana
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile.,Escuela de Medicina, Centro de Investigación y Estudio del Consumo de Alcohol en Adolescentes (CIAA), Santiago, Chile
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3
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Abstract
Mechanisms for elimination of metabolites from ISF include metabolism, blood-brain barrier transport and non-selective, perivascular efflux, this last being assessed by measuring the clearance of markers like inulin. Clearance describes elimination. Clearance of a metabolite generated within the brain is determined as its elimination rate divided by its concentration in interstitial fluid (ISF). However, the more frequently measured parameter is the rate constant for elimination determined as elimination rate divided by amount present, which thus depends on both the elimination processes and the distribution of the metabolite in the brain. The relative importance of the various elimination mechanisms depends on the particular metabolite. Little is known about the effects of sleep on clearance via metabolism or blood-brain barrier transport, but studies with inulin in mice comparing perivascular effluxes during sleep and wakefulness reveal a 4.2-fold increase in clearance. Amongst the important brain metabolites considered, CO2 is eliminated so rapidly across the blood-brain barrier that clearance is blood flow limited and elimination quickly balances production. Glutamate is removed from ISF primarily by uptake into astrocytes and conversion to glutamine, but also by transport across the blood-brain barrier. Both lactate and amyloid-β are eliminated by metabolism, blood-brain barrier transport and perivascular efflux and both show decreased production, decreased ISF concentration and increased perivascular clearance during sleep. Taken altogether available data indicate that sleep increases perivascular and non-perivascular clearances for amyloid-β which reduces its concentration and may have long-term consequences for the formation of plaques and cerebral arterial deposits.
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Affiliation(s)
- Stephen B Hladky
- Department of Pharmacology, University of Cambridge, Cambridge, CB2 1PD, UK.
| | - Margery A Barrand
- Department of Pharmacology, University of Cambridge, Cambridge, CB2 1PD, UK
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Abudara V, Retamal MA, Del Rio R, Orellana JA. Synaptic Functions of Hemichannels and Pannexons: A Double-Edged Sword. Front Mol Neurosci 2018; 11:435. [PMID: 30564096 PMCID: PMC6288452 DOI: 10.3389/fnmol.2018.00435] [Citation(s) in RCA: 31] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2018] [Accepted: 11/08/2018] [Indexed: 01/18/2023] Open
Abstract
The classical view of synapses as the functional contact between presynaptic and postsynaptic neurons has been challenged in recent years by the emerging regulatory role of glial cells. Astrocytes, traditionally considered merely supportive elements are now recognized as active modulators of synaptic transmission and plasticity at the now so-called "tripartite synapse." In addition, an increasing body of evidence indicates that beyond immune functions microglia also participate in various processes aimed to shape synaptic plasticity. Release of neuroactive compounds of glial origin, -process known as gliotransmission-, constitute a widespread mechanism through which glial cells can either potentiate or reduce the synaptic strength. The prevailing vision states that gliotransmission depends on an intracellular Ca2+/exocytotic-mediated release; notwithstanding, growing evidence is pointing at hemichannels (connexons) and pannexin channels (pannexons) as alternative non-vesicular routes for gliotransmitters efflux. In concurrence with this novel concept, both hemichannels and pannexons are known to mediate the transfer of ions and signaling molecules -such as ATP and glutamate- between the cytoplasm and the extracellular milieu. Importantly, recent reports show that glial hemichannels and pannexons are capable to perceive synaptic activity and to respond to it through changes in their functional state. In this article, we will review the current information supporting the "double edge sword" role of hemichannels and pannexons in the function of central and peripheral synapses. At one end, available data support the idea that these channels are chief components of a feedback control mechanism through which gliotransmitters adjust the synaptic gain in either resting or stimulated conditions. At the other end, we will discuss how the excitotoxic release of gliotransmitters and [Ca2+]i overload linked to the opening of hemichannels/pannexons might impact cell function and survival in the nervous system.
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Affiliation(s)
- Verónica Abudara
- Departamento de Fisiología, Facultad de Medicina, Universidad de la República, Montevideo, Uruguay
| | - Mauricio A Retamal
- Centro de Fisiología Celular e Integrativa, Facultad de Medicina, Clínica Alemana Universidad del Desarrollo, Santiago, Chile.,Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, TX, United States.,Programa de Comunicación Celular en Cáncer, Instituto de Ciencias e Innovación en Medicina, Santiago, Chile
| | - Rodrigo Del Rio
- Laboratory of Cardiorespiratory Control, Departamento de Fisiología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile.,Centro de Envejecimiento y Regeneración, Pontificia Universidad Católica de Chile, Santiago, Chile.,Centro de Excelencia en Biomedicina de Magallanes, Universidad de Magallanes, Punta Arenas, Chile
| | - Juan A Orellana
- Departamento de Neurología, Escuela de Medicina and Centro Interdisciplinario de Neurociencias, Facultad de Medicina, Pontificia Universidad Católica de Chile, Santiago, Chile.,Centro de Investigación y Estudio del Consumo de Alcohol en Adolescentes, Santiago, Chile
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5
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Wadle SL, Augustin V, Langer J, Jabs R, Philippot C, Weingarten DJ, Rose CR, Steinhäuser C, Stephan J. Anisotropic Panglial Coupling Reflects Tonotopic Organization in the Inferior Colliculus. Front Cell Neurosci 2018; 12:431. [PMID: 30542265 PMCID: PMC6277822 DOI: 10.3389/fncel.2018.00431] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2018] [Accepted: 10/31/2018] [Indexed: 12/17/2022] Open
Abstract
Astrocytes and oligodendrocytes in different brain regions form panglial networks and the topography of such networks can correlate with neuronal topography and function. Astrocyte-oligodendrocyte networks in the lateral superior olive (LSO)-an auditory brainstem nucleus-were found to be anisotropic with a preferred orientation orthogonally to the tonotopic axis. We hypothesized that such a specialization might be present in other tonotopically organized brainstem nuclei, too. Thus, we analyzed gap junctional coupling in the center of the inferior colliculus (IC)-another nucleus of the auditory brainstem that exhibits tonotopic organization. In acute brainstem slices obtained from mice, IC networks were traced employing whole-cell patch-clamp recordings of single sulforhodamine (SR) 101-identified astrocytes and concomitant intracellular loading of the gap junction-permeable tracer neurobiotin. The majority of dye-coupled networks exhibited an oval topography, which was preferentially oriented orthogonal to the tonotopic axis. Astrocyte processes showed preferentially the same orientation indicating a correlation between astrocyte and network topography. In addition to SR101-positive astrocytes, IC networks contained oligodendrocytes. Using Na+ imaging, we analyzed the capability of IC networks to redistribute small ions. Na+ bi-directionally diffused between SR101-positive astrocytes and SR101-negative cells-presumably oligodendrocytes-showing the functionality of IC networks. Taken together, our results demonstrate that IC astrocytes and IC oligodendrocytes form functional anisotropic panglial networks that are preferentially oriented orthogonal to the tonotopic axis. Thus, our data indicate that the topographic specialization of glial networks seen in IC and LSO might be a general feature of tonotopically organized auditory brainstem nuclei.
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Affiliation(s)
- Simon L Wadle
- Animal Physiology Group, Department of Biology, University of Kaiserslautern, Kaiserslautern, Germany
| | - Vanessa Augustin
- Animal Physiology Group, Department of Biology, University of Kaiserslautern, Kaiserslautern, Germany
| | - Julia Langer
- Institute of Neurobiology, Heinrich Heine University Düsseldorf, Düsseldorf, Germany
| | - Ronald Jabs
- Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany
| | - Camille Philippot
- Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany
| | - Dennis J Weingarten
- Animal Physiology Group, Department of Biology, University of Kaiserslautern, Kaiserslautern, Germany
| | - Christine R Rose
- Institute of Neurobiology, Heinrich Heine University Düsseldorf, Düsseldorf, Germany
| | - Christian Steinhäuser
- Institute of Cellular Neurosciences, Medical Faculty, University of Bonn, Bonn, Germany
| | - Jonathan Stephan
- Animal Physiology Group, Department of Biology, University of Kaiserslautern, Kaiserslautern, Germany
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6
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Apawu AK, Curley SM, Dixon AR, Hali M, Sinan M, Braun RD, Castracane J, Cacace AT, Bergkvist M, Holt AG. MRI compatible MS2 nanoparticles designed to cross the blood-brain-barrier: providing a path towards tinnitus treatment. NANOMEDICINE-NANOTECHNOLOGY BIOLOGY AND MEDICINE 2018; 14:1999-2008. [PMID: 29665440 DOI: 10.1016/j.nano.2018.04.003] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/13/2017] [Revised: 03/17/2018] [Accepted: 04/06/2018] [Indexed: 10/17/2022]
Abstract
Fundamental challenges of targeting specific brain regions for treatment using pharmacotherapeutic nanoparticle (NP) carriers include circumventing the blood-brain-barrier (BBB) and tracking delivery. Angiopep-2 (AP2) has been shown to facilitate the transport of large macromolecules and synthetic nanoparticles across the BBB. Thus, conjugation of AP2 to an MS2 bacteriophage based NP should also permit transport across the BBB. We have fabricated and tested a novel MS2 capsid-based NP conjugated to the ligand AP2. The reaction efficiency was determined to be over 70%, with up to two angiopep-2 conjugated per MS2 capsid protein. When linked with a porphyrin ring, manganese (Mn2+) remained stable within MS2 and was MRI detectable. Nanoparticles were introduced intracerebroventricularly or systemically. Systemic delivery yielded dose dependent, non-toxic accumulation of NPs in the midbrain. Design of a multifunctional MRI compatible NP platform provides a significant step forward for the diagnosis and treatment of intractable brain conditions, such as tinnitus.
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Affiliation(s)
- Aaron K Apawu
- Wayne State University School of Medicine, 540 E Canfield St, Detroit, MI, United States
| | - Stephanie M Curley
- SUNY Polytechnic Institute, Colleges of Nanoscale Science and Engineering, 257 Fuller Rd., Albany, NY, United States
| | - Angela R Dixon
- Wayne State University School of Medicine, 540 E Canfield St, Detroit, MI, United States
| | - Mirabela Hali
- Wayne State University School of Medicine, 540 E Canfield St, Detroit, MI, United States
| | - Moaz Sinan
- Wayne State University School of Medicine, 540 E Canfield St, Detroit, MI, United States
| | - Rod D Braun
- Wayne State University School of Medicine, 540 E Canfield St, Detroit, MI, United States
| | - James Castracane
- SUNY Polytechnic Institute, Colleges of Nanoscale Science and Engineering, 257 Fuller Rd., Albany, NY, United States
| | - Anthony T Cacace
- Wayne State University, Department of Communication Sciences & Disorders, 207 Rackham, 60 Farnsworth, Detroit, MI, United States
| | - Magnus Bergkvist
- SUNY Polytechnic Institute, Colleges of Nanoscale Science and Engineering, 257 Fuller Rd., Albany, NY, United States
| | - Avril Genene Holt
- Wayne State University School of Medicine, 540 E Canfield St, Detroit, MI, United States; John D. Dingell VA Medical Center, 4646 John R St, Detroit, MI, United States.
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7
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Abitbol JM, Kelly JJ, Barr KJ, Allman BL, Laird DW. Mice harbouring an oculodentodigital dysplasia-linked Cx43 G60S mutation have severe hearing loss. J Cell Sci 2018; 131:jcs.214635. [DOI: 10.1242/jcs.214635] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2017] [Accepted: 03/27/2018] [Indexed: 01/22/2023] Open
Abstract
Given the importance of connexin43 (Cx43) function in the central nervous system and sensory organ processing we proposed that it would also be crucial in auditory function. To that end, hearing was examined in two mouse models of oculodentodigital dysplasia that globally express GJA1 (Cx43) mutations resulting in mild or severe loss of Cx43 function. Although Cx43I130T/+ mutant mice with ∼50% Cx43 channel function did not have any hearing loss, Cx43G60S/+ mutant mice with ∼20% Cx43 channel function had severe hearing loss. There was no evidence of inner ear sensory hair cell loss, suggesting that the Cx43-linked hearing loss lies downstream in the auditory pathway. Since evidence suggests that Cx26 function is essential for hearing and may be protective against noise-induced hearing loss, we challenged Cx43I130T/+ mice with a loud noise and found that they had similar susceptibility to noise-induced hearing loss as controls suggesting that decreased Cx43 function does not sensitize the mice for environmentally-induced hearing loss. Taken together, this study suggests that Cx43 plays an important role in baseline hearing and is essential for auditory processing.
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Affiliation(s)
| | - John J. Kelly
- University of Western Ontario, London, Ontario, Canada
| | - Kevin J. Barr
- University of Western Ontario, London, Ontario, Canada
| | | | - Dale W. Laird
- University of Western Ontario, London, Ontario, Canada
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8
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Connexin 43-Mediated Astroglial Metabolic Networks Contribute to the Regulation of the Sleep-Wake Cycle. Neuron 2017; 95:1365-1380.e5. [PMID: 28867552 DOI: 10.1016/j.neuron.2017.08.022] [Citation(s) in RCA: 131] [Impact Index Per Article: 18.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/25/2017] [Revised: 06/29/2017] [Accepted: 08/14/2017] [Indexed: 01/13/2023]
Abstract
Astrocytes produce and supply metabolic substrates to neurons through gap junction-mediated astroglial networks. However, the role of astroglial metabolic networks in behavior is unclear. Here, we demonstrate that perturbation of astroglial networks impairs the sleep-wake cycle. Using a conditional Cre-Lox system in mice, we show that knockout of the gap junction subunit connexin 43 in astrocytes throughout the brain causes excessive sleepiness and fragmented wakefulness during the nocturnal active phase. This astrocyte-specific genetic manipulation silenced the wake-promoting orexin neurons located in the lateral hypothalamic area (LHA) by impairing glucose and lactate trafficking through astrocytic networks. This global wakefulness instability was mimicked with viral delivery of Cre recombinase to astrocytes in the LHA and rescued by in vivo injections of lactate. Our findings propose a novel regulatory mechanism critical for maintaining normal daily cycle of wakefulness and involving astrocyte-neuron metabolic interactions.
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9
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Hladky SB, Barrand MA. Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles. Fluids Barriers CNS 2016; 13:19. [PMID: 27799072 PMCID: PMC5508927 DOI: 10.1186/s12987-016-0040-3] [Citation(s) in RCA: 168] [Impact Index Per Article: 21.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2016] [Accepted: 09/01/2016] [Indexed: 12/24/2022] Open
Abstract
The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.
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Affiliation(s)
- Stephen B. Hladky
- Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge, CB2 1PD UK
| | - Margery A. Barrand
- Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge, CB2 1PD UK
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10
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Dienel GA, Cruz NF. Aerobic glycolysis during brain activation: adrenergic regulation and influence of norepinephrine on astrocytic metabolism. J Neurochem 2016; 138:14-52. [DOI: 10.1111/jnc.13630] [Citation(s) in RCA: 99] [Impact Index Per Article: 12.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/06/2016] [Revised: 03/24/2016] [Accepted: 03/31/2016] [Indexed: 12/17/2022]
Affiliation(s)
- Gerald A. Dienel
- Department of Cell Biology and Physiology; University of New Mexico; Albuquerque; New Mexico USA
- Department of Neurology; University of Arkansas for Medical Sciences; Little Rock Arkansas USA
| | - Nancy F. Cruz
- Department of Neurology; University of Arkansas for Medical Sciences; Little Rock Arkansas USA
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11
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Astrocyte physiopathology: At the crossroads of intercellular networking, inflammation and cell death. Prog Neurobiol 2015; 130:86-120. [PMID: 25930681 DOI: 10.1016/j.pneurobio.2015.04.003] [Citation(s) in RCA: 139] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/24/2014] [Revised: 04/15/2015] [Accepted: 04/20/2015] [Indexed: 12/11/2022]
Abstract
Recent breakthroughs in neuroscience have led to the awareness that we should revise our traditional mode of thinking and studying the CNS, i.e. by isolating the privileged network of "intelligent" synaptic contacts. We may instead need to contemplate all the variegate communications occurring between the different neural cell types, and centrally involving the astrocytes. Basically, it appears that a single astrocyte should be considered as a core that receives and integrates information from thousands of synapses, other glial cells and the blood vessels. In turn, it generates complex outputs that control the neural circuitry and coordinate it with the local microcirculation. Astrocytes thus emerge as the possible fulcrum of the functional homeostasis of the healthy CNS. Yet, evidence indicates that the bridging properties of the astrocytes can change in parallel with, or as a result of, the morphological, biochemical and functional alterations these cells undergo upon injury or disease. As a consequence, they have the potential to transform from supportive friends and interactive partners for neurons into noxious foes. In this review, we summarize the currently available knowledge on the contribution of astrocytes to the functioning of the CNS and what goes wrong in various pathological conditions, with a particular focus on Amyotrophic Lateral Sclerosis, Alzheimer's Disease and ischemia. The observations described convincingly demonstrate that the development and progression of several neurological disorders involve the de-regulation of a finely tuned interplay between multiple cell populations. Thus, it seems that a better understanding of the mechanisms governing the integrated communication and detrimental responses of the astrocytes as well as their impact towards the homeostasis and performance of the CNS is fundamental to open novel therapeutic perspectives.
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12
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Dienel GA, Cruz NF. Contributions of glycogen to astrocytic energetics during brain activation. Metab Brain Dis 2015; 30:281-98. [PMID: 24515302 PMCID: PMC4130810 DOI: 10.1007/s11011-014-9493-8] [Citation(s) in RCA: 70] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/29/2013] [Accepted: 01/21/2014] [Indexed: 12/11/2022]
Abstract
Glycogen is the major store of glucose in brain and is mainly in astrocytes. Brain glycogen levels in unstimulated, carefully-handled rats are 10-12 μmol/g, and assuming that astrocytes account for half the brain mass, astrocytic glycogen content is twice as high. Glycogen turnover is slow under basal conditions, but it is mobilized during activation. There is no net increase in incorporation of label from glucose during activation, whereas label release from pre-labeled glycogen exceeds net glycogen consumption, which increases during stronger stimuli. Because glycogen level is restored by non-oxidative metabolism, astrocytes can influence the global ratio of oxygen to glucose utilization. Compensatory increases in utilization of blood glucose during inhibition of glycogen phosphorylase are large and approximate glycogenolysis rates during sensory stimulation. In contrast, glycogenolysis rates during hypoglycemia are low due to continued glucose delivery and oxidation of endogenous substrates; rates that preserve neuronal function in the absence of glucose are also low, probably due to metabolite oxidation. Modeling studies predict that glycogenolysis maintains a high level of glucose-6-phosphate in astrocytes to maintain feedback inhibition of hexokinase, thereby diverting glucose for use by neurons. The fate of glycogen carbon in vivo is not known, but lactate efflux from brain best accounts for the major metabolic characteristics during activation of living brain. Substantial shuttling coupled with oxidation of glycogen-derived lactate is inconsistent with available evidence. Glycogen has important roles in astrocytic energetics, including glucose sparing, control of extracellular K(+) level, oxidative stress management, and memory consolidation; it is a multi-functional compound.
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Affiliation(s)
- Gerald A Dienel
- Department of Neurology, University of Arkansas for Medical Sciences, Slot 500, 4301 W. Markham St., Little Rock, AR, 72205, USA,
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13
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Charron G, Doudnikoff E, Canron MH, Li Q, Véga C, Marais S, Baufreton J, Vital A, Oliet SHR, Bezard E. Astrocytosis in parkinsonism: considering tripartite striatal synapses in physiopathology? Front Aging Neurosci 2014; 6:258. [PMID: 25309435 PMCID: PMC4174038 DOI: 10.3389/fnagi.2014.00258] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/08/2014] [Accepted: 09/10/2014] [Indexed: 01/30/2023] Open
Abstract
The current concept of basal ganglia organization and function in physiological and pathophysiological conditions excludes the most numerous cells in the brain, i.e., the astrocytes, present with a ratio of 10:1 neuron. Their role in neurodegenerative condition such as Parkinson's disease (PD) remains to be elucidated. Before embarking into physiological investigations of the yet-to-be-identified "tripartite" synapses in the basal ganglia in general and the striatum in particular, we therefore characterized anatomically the PD-related modifications in astrocytic morphology, the changes in astrocytic network connections and the consequences on the spatial relationship between astrocytic processes and asymmetric synapses in normal and PD-like conditions in experimental and human PD. Our results unravel a dramatic regulation of striatal astrocytosis supporting the hypothesis of a key role in (dys) regulating corticostriatal transmission. Astrocytes and their various properties might thus represent a therapeutic target in PD.
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Affiliation(s)
- Giselle Charron
- Institut des Maladies Neurodégénératives, Université de Bordeaux, UMR 5293 Bordeaux, France ; CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France
| | - Evelyne Doudnikoff
- Institut des Maladies Neurodégénératives, Université de Bordeaux, UMR 5293 Bordeaux, France ; CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France
| | - Marie-Helene Canron
- Institut des Maladies Neurodégénératives, Université de Bordeaux, UMR 5293 Bordeaux, France ; CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France
| | - Qin Li
- Institute of Laboratory Animal Sciences, Chinese Academy of Medical Science and Peking Union Medical College Beijing, China
| | - Céline Véga
- CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France ; UFR Sciences de la Vie, University Pierre et Marie Curie (UPMC) Paris, France
| | - Sebastien Marais
- Bordeaux Imaging Center, UMS 3420, Université de Bordeaux Bordeaux, France ; CNRS, Bordeaux Imaging Center, UMS 3420 Bordeaux, France ; INSERM, Bordeaux Imaging Center, US 004 Bordeaux, France
| | - Jérôme Baufreton
- Institut des Maladies Neurodégénératives, Université de Bordeaux, UMR 5293 Bordeaux, France ; CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France
| | - Anne Vital
- Institut des Maladies Neurodégénératives, Université de Bordeaux, UMR 5293 Bordeaux, France ; CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France
| | - Stéphane H R Oliet
- Neurocentre Magendie, U862, Institut National de la Santé et de la Recherche Médicale Bordeaux, France
| | - Erwan Bezard
- Institut des Maladies Neurodégénératives, Université de Bordeaux, UMR 5293 Bordeaux, France ; CNRS, Institut des Maladies Neurodégénératives, UMR 5293 Bordeaux, France ; Institute of Laboratory Animal Sciences, Chinese Academy of Medical Science and Peking Union Medical College Beijing, China
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14
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Hertz L, Gibbs ME, Dienel GA. Fluxes of lactate into, from, and among gap junction-coupled astrocytes and their interaction with noradrenaline. Front Neurosci 2014; 8:261. [PMID: 25249930 PMCID: PMC4158791 DOI: 10.3389/fnins.2014.00261] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/09/2014] [Accepted: 08/04/2014] [Indexed: 01/06/2023] Open
Abstract
Lactate is a versatile metabolite with important roles in modulation of brain glucose utilization rate (CMRglc), diagnosis of brain-injured patients, redox- and receptor-mediated signaling, memory, and alteration of gene transcription. Neurons and astrocytes release and accumulate lactate using equilibrative monocarboxylate transporters that carry out net transmembrane transport of lactate only until intra- and extracellular levels reach equilibrium. Astrocytes have much faster lactate uptake than neurons and shuttle more lactate among gap junction-coupled astrocytes than to nearby neurons. Lactate diffusion within syncytia can provide precursors for oxidative metabolism and glutamate synthesis and facilitate its release from endfeet to perivascular space to stimulate blood flow. Lactate efflux from brain during activation underlies the large underestimation of CMRglc with labeled glucose and fall in CMRO2/CMRglc ratio. Receptor-mediated effects of lactate on locus coeruleus neurons include noradrenaline release in cerebral cortex and c-AMP-mediated stimulation of astrocytic gap junctional coupling, thereby enhancing its dispersal and release from brain. Lactate transport is essential for its multifunctional roles.
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Affiliation(s)
- Leif Hertz
- Laboratory of Brain Metabolic Diseases, Institute of Metabolic Disease Research and Drug Development, China Medical University Shenyang, China
| | - Marie E Gibbs
- Drug Discovery Biology, Monash Institute of Pharmaceutical Sciences, Monash University Clayton, VIC, Australia
| | - Gerald A Dienel
- Department of Neurology, University of Arkansas for Medical Sciences Little Rock, AR, USA
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15
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Dye coupling and immunostaining of astrocyte-like glia following intracellular injection of fluorochromes in brain slices of the grasshopper, Schistocerca gregaria. Methods Mol Biol 2014; 1082:99-113. [PMID: 24048929 DOI: 10.1007/978-1-62703-655-9_7] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Injection of fluorochromes such as Alexa Fluor(®) 568 into single cells in brain slices reveals a network of dye-coupled cells to be associated with the central complex. Subsequent immunolabeling shows these cells to be repo positive/glutamine synthetase positive/horseradish peroxidase negative, thus identifying them as astrocyte-like glia. Dye coupling fails in the presence of n-heptanol indicating that dye spreads from cell to cell via gap junctions. A cellular network of dye-coupled, astrocyte-like, glia surrounds and infiltrates developing central complex neuropils. Intracellular dye injection techniques complement current molecular approaches in analyzing the functional properties of such networks.
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16
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Takano T, He W, Han X, Wang F, Xu Q, Wang X, Oberheim Bush NA, Cruz N, Dienel GA, Nedergaard M. Rapid manifestation of reactive astrogliosis in acute hippocampal brain slices. Glia 2013; 62:78-95. [PMID: 24272704 DOI: 10.1002/glia.22588] [Citation(s) in RCA: 63] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/17/2013] [Revised: 09/26/2013] [Accepted: 09/30/2013] [Indexed: 01/31/2023]
Abstract
A flurry of studies over the past decade has shown that astrocytes play a more active role in neural function than previously recognized. Hippocampal slices prepared from young rodent pups have served as a popular model for studying the pathways by which astrocytes participate in synaptic transmission. It is, however, not known how well astrocytes tolerate traumatic injury and hypoxia, which are unavoidable when preparing acute slices. We here showed that astrocytes exhibit striking changes in expression of several receptors and structural proteins, including re-expression of the developmental marker nestin within 90 min following preparation of live vibratome slices. Moreover, immunoelectron microscopy showed a 2.7-fold loss of astrocytic processes in acute hippocampal slices prepared from glial fibrillary acidic protein-green fluorescent protein reporter mice. A sharp decrease in the number of mitochondria was also noted in acute slices, concurrently with an increase in mitochondrial size. Glycogen content decreased 3-fold upon slice preparation and did not recover despite stable recordings of field excitatory postsynaptic current. Analysis of Ca(2+) signaling showed that astrocytic responses to purine receptor and mGluR5 agonists differed in slice versus in vivo. These observations suggest that the functional properties and the fine structure of astrocytes in slices may be reflective of early stages of reactive gliosis and should be confirmed in vivo when possible.
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Affiliation(s)
- Takahiro Takano
- Division of Glial Disease and Therapeutics, Center for Translational Neuromedicine, University of Rochester Medical Center, Rochester, New York
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17
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Astrocytic energetics during excitatory neurotransmission: What are contributions of glutamate oxidation and glycolysis? Neurochem Int 2013; 63:244-58. [PMID: 23838211 DOI: 10.1016/j.neuint.2013.06.015] [Citation(s) in RCA: 84] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2013] [Revised: 06/19/2013] [Accepted: 06/24/2013] [Indexed: 12/23/2022]
Abstract
Astrocytic energetics of excitatory neurotransmission is controversial due to discrepant findings in different experimental systems in vitro and in vivo. The energy requirements of glutamate uptake are believed by some researchers to be satisfied by glycolysis coupled with shuttling of lactate to neurons for oxidation. However, astrocytes increase glycogenolysis and oxidative metabolism during sensory stimulation in vivo, indicating that other sources of energy are used by astrocytes during brain activation. Furthermore, glutamate uptake into cultured astrocytes stimulates glutamate oxidation and oxygen consumption, and glutamate maintains respiration as well as glucose. The neurotransmitter pool of glutamate is associated with the faster component of total glutamate turnover in vivo, and use of neurotransmitter glutamate to fuel its own uptake by oxidation-competent perisynaptic processes has two advantages, substrate is supplied concomitant with demand, and glutamate spares glucose for use by neurons and astrocytes. Some, but not all, perisynaptic processes of astrocytes in adult rodent brain contain mitochondria, and oxidation of only a small fraction of the neurotransmitter glutamate taken up into these structures would be sufficient to supply the ATP required for sodium extrusion and conversion of glutamate to glutamine. Glycolysis would, however, be required in perisynaptic processes lacking oxidative capacity. Three lines of evidence indicate that critical cornerstones of the astrocyte-to-neuron lactate shuttle model are not established and normal brain does not need lactate as supplemental fuel: (i) rapid onset of hemodynamic responses to activation delivers oxygen and glucose in excess of demand, (ii) total glucose utilization greatly exceeds glucose oxidation in awake rodents during activation, indicating that the lactate generated is released, not locally oxidized, and (iii) glutamate-induced glycolysis is not a robust phenotype of all astrocyte cultures. Various metabolic pathways, including glutamate oxidation and glycolysis with lactate release, contribute to cellular energy demands of excitatory neurotransmission.
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18
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Escartin C, Rouach N. Astroglial networking contributes to neurometabolic coupling. FRONTIERS IN NEUROENERGETICS 2013; 5:4. [PMID: 23637659 PMCID: PMC3636502 DOI: 10.3389/fnene.2013.00004] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 01/31/2013] [Accepted: 03/18/2013] [Indexed: 12/21/2022]
Abstract
The strategic position of astrocytic processes between blood capillaries and neurons, provided the early insight that astrocytes play a key role in supplying energy substrates to neurons in an activity-dependent manner. The central role of astrocytes in neurometabolic coupling has been first established at the level of single cell. Since then, exciting recent work based on cellular imaging and electrophysiological recordings has provided new mechanistic insights into this phenomenon, revealing the crucial role of gap junction (GJ)-mediated networks of astrocytes. Indeed, astrocytes define the local availability of energy substrates by regulating blood flow. Subsequently, in order to efficiently reach distal neurons, these substrates can be taken up, and distributed through networks of astrocytes connected by GJs, a process modulated by neuronal activity. Astrocytic networks can be morphologically and/or functionally altered in the course of various pathological conditions, raising the intriguing possibility of a direct contribution from these networks to neuronal dysfunction. The present review upgrades the current view of neuroglial metabolic coupling, by including the recently unravelled properties of astroglial metabolic networks and their potential contribution to normal and pathological neuronal activity.
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Affiliation(s)
- Carole Escartin
- CEA DSV I2BM MIRCen and CNRS URA2210, Fontenay-aux-Roses Paris, France
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19
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Kreft M, Lukšič M, Zorec TM, Prebil M, Zorec R. Diffusion of D-glucose measured in the cytosol of a single astrocyte. Cell Mol Life Sci 2013; 70:1483-92. [PMID: 23224430 PMCID: PMC11113596 DOI: 10.1007/s00018-012-1219-7] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2012] [Revised: 11/18/2012] [Accepted: 11/22/2012] [Indexed: 01/26/2023]
Abstract
Astrocytes interact with neurons and endothelial cells and may mediate exchange of metabolites between capillaries and nerve terminals. In the present study, we investigated intracellular glucose diffusion in purified astrocytes after local glucose uptake. We used a fluorescence resonance energy transfer (FRET)-based nano sensor to monitor the time dependence of the intracellular glucose concentration at specific positions within the cell. We observed a delay in onset and kinetics in regions away from the glucose uptake compared with the region where we locally super-fused astrocytes with the D-glucose-rich solution. We propose a mathematical model of glucose diffusion in astrocytes. The analysis showed that after gradual uptake of glucose, the locally increased intracellular glucose concentration is rapidly spread throughout the cytosol with an apparent diffusion coefficient (D app) of (2.38 ± 0.41) × 10(-10) m(2) s(-1) (at 22-24 °C). Considering that the diffusion coefficient of D-glucose in water is D = 6.7 × 10(-10) m(2) s(-1) (at 24 °C), D app determined in astrocytes indicates that the cytosolic tortuosity, which hinders glucose molecules, is approximately three times higher than in aqueous solution. We conclude that the value of D app for glucose measured in purified rat astrocytes is consistent with the view that cytosolic diffusion may allow glucose and glucose metabolites to traverse from the endothelial cells at the blood-brain barrier to neurons and neighboring astrocytes.
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Affiliation(s)
- Marko Kreft
- LN-MCP, Faculty of Medicine, Institute of Pathophysiology, University of Ljubljana, Zaloška cesta 4, 1000, Ljubljana, Slovenia.
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20
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Cruz NF, Ball KK, Froehner SC, Adams ME, Dienel GA. Regional registration of [6-(14)C]glucose metabolism during brain activation of α-syntrophin knockout mice. J Neurochem 2013; 125:247-59. [PMID: 23346911 DOI: 10.1111/jnc.12166] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2012] [Revised: 01/02/2013] [Accepted: 01/20/2013] [Indexed: 12/16/2022]
Abstract
α-Syntrophin is a component of the dystrophin scaffold-protein complex that serves as an adaptor for recruitment of key proteins to the cytoplasmic side of plasma membranes. α-Syntrophin knockout (KO) causes loss of the polarized localization of aquaporin4 (AQP4) at astrocytic endfeet and interferes with water and K(+) homeostasis. During brain activation, release of ions and metabolites from endfeet is anticipated to increase perivascular fluid osmolarity, AQP4-mediated osmotic water flow from endfeet, and metabolite washout from brain. This study tests the hypothesis that reduced levels of endfoot AQP4 increase retention of [(14)C]metabolites during sensory stimulation. Conscious KO and wild-type mice were pulse-labeled with [6-(14)C] glucose during unilateral acoustic stimulation or bilateral acoustic plus whisker stimulation, and label retention was assayed by computer-assisted brain imaging or analysis of [(14)C]metabolites in extracts, respectively. High-resolution autoradiographic assays detected a 17% side-to-side difference (p < 0.05) in inferior colliculus of KO mice, not wild-type mice. However, there were no labeling differences between KO and wild-type mice for five major HPLC fractions from four dissected regions, presumably because of insufficient anatomical resolution. The results suggest a role for AQP4-mediated water flow in support of washout of metabolites, and underscore the need for greater understanding of astrocytic water and metabolite fluxes.
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Affiliation(s)
- Nancy F Cruz
- Department of Neurology, University of Arkansas for Medical Sciences, Little Rock, Arkansas, USA
| | - Kelly K Ball
- Department of Neurology, University of Arkansas for Medical Sciences, Little Rock, Arkansas, USA
| | - Stanley C Froehner
- Department of Physiology and Biophysics, University of Washington, Seattle, Washington, USA
| | - Marvin E Adams
- Department of Physiology and Biophysics, University of Washington, Seattle, Washington, USA
| | - Gerald A Dienel
- Department of Neurology, University of Arkansas for Medical Sciences, Little Rock, Arkansas, USA
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21
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The unfolded protein response to endoplasmic reticulum stress in cultured astrocytes and rat brain during experimental diabetes. Neurochem Int 2013; 62:784-95. [PMID: 23411409 DOI: 10.1016/j.neuint.2013.02.009] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2012] [Revised: 01/29/2013] [Accepted: 02/04/2013] [Indexed: 12/21/2022]
Abstract
Oxidative-nitrosative stress and inflammatory responses are associated with endoplasmic reticulum (ER) stress in diabetic retinopathy, raising the possibility that disturbances in ER protein processing may contribute to CNS dysfunction in diabetics. Upregulation of the unfolded protein response (UPR) is a homeostatic response to accumulation of abnormal proteins in the ER, and the present study tested the hypothesis that the UPR is upregulated in two models for diabetes, cultured astrocytes grown in 25mmol/L glucose for up to 4weeks and brain of streptozotocin (STZ)-treated rats with diabetes for 1-7months. Markers associated with translational blockade (phospho-eIF2α and apoptosis (CHOP), inflammatory response (inducible nitric oxide synthase, iNOS), and nitrosative stress (nuclear translocation of glyceraldehyde-3-phosphate dehydrogenase, GAPDH) were not detected in either model. Nrf2 was present in nuclei of low- and high-glucose cultures, consistent with oxidative stress. Astrocytic ATF4 expression was not altered by culture glucose concentration, whereas phospho-IRE and ATF6 levels were higher in low- compared with high-glucose cultures. The glucose-regulated chaperones, GRP78 and GRP94, were also expressed at higher levels in low- than high-glucose cultures, probably due to recurrent glucose depletion between feeding cycles. In STZ-rat cerebral cortex, ATF4 level was transiently reduced at 4months, and p-IRE levels were transiently elevated at 3months. However, GRP78 and GRP94 expression was not upregulated, and iNOS, amyloid-β, and nuclear accumulation of GAPDH were not evident in STZ-diabetic brain. High-glucose cultured astrocytes and STZ-diabetic brain are relatively resistant to diabetes-induced ER stress, in sharp contrast with cultured retinal Müller cells and diabetic rodent retina.
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22
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Abstract
Metabolic signals are used for imaging and spectroscopic studies of brain function and disease and to elucidate the cellular basis of neuroenergetics. The major fuel for activated neurons and the models for neuron–astrocyte interactions have been controversial because discordant results are obtained in different experimental systems, some of which do not correspond to adult brain. In rats, the infrastructure to support the high energetic demands of adult brain is acquired during postnatal development and matures after weaning. The brain's capacity to supply and metabolize glucose and oxygen exceeds demand over a wide range of rates, and the hyperaemic response to functional activation is rapid. Oxidative metabolism provides most ATP, but glycolysis is frequently preferentially up-regulated during activation. Underestimation of glucose utilization rates with labelled glucose arises from increased lactate production, lactate diffusion via transporters and astrocytic gap junctions, and lactate release to blood and perivascular drainage. Increased pentose shunt pathway flux also causes label loss from C1 of glucose. Glucose analogues are used to assay cellular activities, but interpretation of results is uncertain due to insufficient characterization of transport and phosphorylation kinetics. Brain activation in subjects with low blood-lactate levels causes a brain-to-blood lactate gradient, with rapid lactate release. In contrast, lactate flooding of brain during physical activity or infusion provides an opportunistic, supplemental fuel. Available evidence indicates that lactate shuttling coupled to its local oxidation during activation is a small fraction of glucose oxidation. Developmental, experimental, and physiological context is critical for interpretation of metabolic studies in terms of theoretical models.
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23
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Abstract
Potential roles for lactate in the energetics of brain activation have changed radically during the past three decades, shifting from waste product to supplemental fuel and signaling molecule. Current models for lactate transport and metabolism involving cellular responses to excitatory neurotransmission are highly debated, owing, in part, to discordant results obtained in different experimental systems and conditions. Major conclusions drawn from tabular data summarizing results obtained in many laboratories are as follows: Glutamate-stimulated glycolysis is not an inherent property of all astrocyte cultures. Synaptosomes from the adult brain and many preparations of cultured neurons have high capacities to increase glucose transport, glycolysis, and glucose-supported respiration, and pathway rates are stimulated by glutamate and compounds that enhance metabolic demand. Lactate accumulation in activated tissue is a minor fraction of glucose metabolized and does not reflect pathway fluxes. Brain activation in subjects with low plasma lactate causes outward, brain-to-blood lactate gradients, and lactate is quickly released in substantial amounts. Lactate utilization by the adult brain increases during lactate infusions and strenuous exercise that markedly increase blood lactate levels. Lactate can be an 'opportunistic', glucose-sparing substrate when present in high amounts, but most evidence supports glucose as the major fuel for normal, activated brain.
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A cellular network of dye-coupled glia associated with the embryonic central complex in the grasshopper Schistocerca gregaria. Dev Genes Evol 2012; 222:125-38. [PMID: 22460819 DOI: 10.1007/s00427-012-0394-8] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2011] [Accepted: 03/11/2012] [Indexed: 12/25/2022]
Abstract
The central complex of the grasshopper (Schistocerca gregaria) brain comprises a modular set of neuropils, which develops after mid-embryogenesis and is functional on hatching. Early in embryogenesis, Repo-positive glia cells are found intermingled among the commissures of the midbrain, but then redistribute as central complex modules become established and, by the end of embryogenesis, envelop all midbrain neuropils. The predominant glia associated with the central body during embryogenesis are glutamine synthetase-/Repo-positive astrocyte-like glia, which direct extensive processes (gliopodia) into and around midbrain neuropils. We used intracellular dye injection in brain slices to ascertain whether such glia are dye-coupled into a communicating cellular network during embryogenesis. Intracellular staining of individual cells located at any one of four sites around the central body revealed a population of dye-coupled cells whose number and spatial distribution were stereotypic for each site and comparable at both 70 and 100% of embryogenesis. Subsequent immunolabeling confirmed these dye-coupled cells to be astrocyte-like glia. The addition of n-heptanol to the bathing saline prevented all dye coupling, consistent with gap junctions linking the glia surrounding the central body. Since dye coupling also occurred in the absence of direct intersomal contacts, it might additionally involve the extensive array of gliopodia, which develop after glia are arrayed around the central body. Collating the data from all injection sites suggests that the developing central body is surrounded by a network of dye-coupled glia, which we speculate may function as a positioning system for the developing neuropils of the central complex.
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25
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Lutz SE, Raine CS, Brosnan CF. Loss of astrocyte connexins 43 and 30 does not significantly alter susceptibility or severity of acute experimental autoimmune encephalomyelitis in mice. J Neuroimmunol 2012; 245:8-14. [PMID: 22342190 DOI: 10.1016/j.jneuroim.2012.01.007] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2011] [Revised: 12/29/2011] [Accepted: 01/17/2012] [Indexed: 11/15/2022]
Abstract
We showed previously that mice deficient in astrocyte gap junctions Cx43 and Cx30 exhibit white matter vacuolation and hypomyelination. In this study we tested the hypothesis that loss of astrocytic gap junction proteins leads to exacerbation of the primary demyelinating diseases, using experimental autoimmune encephalomyelitis (EAE) as a model system. To test for this, Cx43 floxed mice were crossed with GFAP:Cre, Cx30 null mice to generate mice lacking astrocytic expression of both Cx43 and Cx30 (dKO). EAE was induced using myelin oligodendrocyte glycoprotein (MOG(35-55)) peptide, and mice were monitored for acute expression of disease. No statistically significant difference in clinical or pathological expression of EAE was observed. Lesion load and susceptibility of different areas of the CNS to inflammation were similar in all genotypes. Moreover, no differences were noted in blood-brain barrier (BBB) permeability, tissue wet weight, axonal pathology, gliosis or demyelination during acute disease. These data show that loss of the astrocytic connexins, Cx43 and Cx30, and the white matter pathology observed in these mice does not statistically affect clinical or pathological expression of EAE and show that astrocyte gap junctions do not regulate autoimmune inflammation and associated BBB disruption in acute EAE.
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Affiliation(s)
- Sarah E Lutz
- Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY, USA.
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26
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Holmes WM, Lopez-Gonzalez MR, Gallagher L, Deuchar GA, Macrae IM, Santosh C. Novel MRI detection of the ischemic penumbra: direct assessment of metabolic integrity. NMR IN BIOMEDICINE 2012; 25:295-304. [PMID: 21751274 DOI: 10.1002/nbm.1748] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/17/2010] [Revised: 04/12/2011] [Accepted: 04/17/2011] [Indexed: 05/31/2023]
Abstract
We describe a novel magnetic resonance imaging technique to directly assess the metabolic integrity of penumbral tissue following stroke. For ischemically stressed tissue to be salvageable, it has to be capable of recovering aerobic metabolism (in place of anaerobic metabolism) on reperfusion. We probed ischemic brain tissue by altering the rate of oxygen delivery using a challenge of 100% oxygen ventilation. Any change from anaerobic to aerobic metabolism should alter the rate of lactate production and hence, levels of tissue lactate. Stroke was induced by permanent middle cerebral artery occlusion in rats. In Series 1 (n = 6), changes in tissue lactate during and following 100% oxygen challenge were monitored using (1)H magnetic resonance spectroscopy (MRS). Diffusion weighted imaging (DWI) and perfusion weighted imaging (PWI) were used to locate MRS voxels within the ischemic core, the homotopic contralateral striatum and within PWI/DWI mismatch (i.e. presumed penumbra). After 20 min of oxygen, lactate signal change was -16.1 ± 8.8% (mean ± SD) in PWI/DWI mismatch, +2.8 ± 5.1% in the ischemic core, and -0.6 ± 7.6% in the contralateral striatum. Return to air ventilation for 20 min resulted in a reversal, with lactate increasing by 46 ± 25.3% in the PWI/DWI mismatch, 6.6 ± 6.2% in the ischemic core, and -5 ± 11.4% in the contralateral striatum. In Series 2 (n = 6), a novel form of spectroscopic imaging was used to acquire lactate change maps to spatially identify regions of lactate change within the ischemic brain. This technique has potential clinical utility by identifying tissue that displays anaerobic metabolism capable of recovering aerobic metabolism when oxygen delivery is increased, which could provide a more precise assessment of penumbra.
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Affiliation(s)
- William M Holmes
- Glasgow Experimental MRI Centre, Division of Clinical Neuroscience, Faculty of Medicine, University of Glasgow, G61 1QH, Scotland, UK.
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27
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Ball KK, Harik L, Gandhi GK, Cruz NF, Dienel GA. Reduced gap junctional communication among astrocytes in experimental diabetes: contributions of altered connexin protein levels and oxidative-nitrosative modifications. J Neurosci Res 2011; 89:2052-67. [PMID: 21567444 DOI: 10.1002/jnr.22663] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2010] [Revised: 02/11/2011] [Accepted: 03/17/2011] [Indexed: 12/11/2022]
Abstract
Experimental diabetes increases production of reactive oxygen-nitrogen species and inhibits astrocytic gap junctional communication in tissue culture and brain slices from streptozotocin (STZ)-diabetic rats by unidentified mechanisms. Relative connexin (Cx) protein levels were assessed by Western blotting using extracts from cultured astrocytes grown in high (25 mmol/liter) or low (5.5 mmol/liter) glucose for 2-3 weeks and STZ-diabetic rat brain. Chemiluminescent signals for diabetic samples were normalized to those of controls on the same blot and same protein load. Growth in high glucose did not alter relative Cx26 level, whereas Cx30 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were reduced by ∼30%, and Cx43 increased ∼1.9-fold. In the inferior colliculus of STZ-diabetic rats, Cx30 and Cx43 levels in three of four rats were half those of controls, whereas GAPDH and actin were unaffected. Diabetes did not affect levels of Cx30, Cx43, or GAPDH in cerebral cortex, but actin level rose 24%. Cx43 was predominantly phosphorylated in control and diabetic samples, so the reduced dye transfer is not due to overall dephosphorylation of Cx43. Astrocytic growth in high glucose reduced the dye-labeled area by 75%, but 10 min of treatment with dithiothreitol restored normal dye transfer. In contrast, nitric oxide donors inhibited dye transfer among astrocytes grown in low glucose by 50-65% within 1 hr. Thus, modifications arising from oxidative-nitrosative stress, not altered connexin levels, may underlie the reduced dye transfer among severely hyperglycemic cultured astrocytes, whereas both oxidative-nitrosative stress and regionally selective down-regulation of connexin protein content may affect gap junctional communication in the brains of STZ-diabetic rats.
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Affiliation(s)
- Kelly K Ball
- Department of Neurology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA
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28
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Giaume C. Astroglial Wiring is Adding Complexity to Neuroglial Networking. FRONTIERS IN NEUROENERGETICS 2010; 2. [PMID: 20922057 PMCID: PMC2948443 DOI: 10.3389/fnene.2010.00129] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 07/03/2010] [Accepted: 08/23/2010] [Indexed: 01/05/2023]
Abstract
Astrocytes are organized as networks of communicating cells due to their high expression level of connexins, the molecular constituents of gap junction channels. Based on their permeability properties for ions and small signaling molecules such astroglial wiring interferes with neuronal activity and survival. In this paper, I identify and discuss which future technical and conceptual progress or advances should be achieved in order to better understand how neuroglial networking contributes to brain functions and dysfunctions.
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29
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Astrocytic gap junctional communication is reduced in amyloid-β-treated cultured astrocytes, but not in Alzheimer's disease transgenic mice. ASN Neuro 2010; 2:e00041. [PMID: 20730033 PMCID: PMC2922840 DOI: 10.1042/an20100017] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/26/2010] [Revised: 07/22/2010] [Accepted: 07/23/2010] [Indexed: 12/21/2022] Open
Abstract
Alzheimer's disease is characterized by accumulation of amyloid deposits in brain, progressive cognitive deficits and reduced glucose utilization. Many consequences of the disease are attributed to neuronal dysfunction, but roles of astrocytes in its pathogenesis are not well understood. Astrocytes are extensively coupled via gap junctions, and abnormal trafficking of metabolites and signalling molecules within astrocytic syncytia could alter functional interactions among cells comprising the neurovascular unit. To evaluate the influence of amyloid-beta on astrocyte gap junctional communication, cultured astrocytes were treated with monomerized amyloid-β(1-40) (1 μmol/l) for intervals ranging from 2 h to 5 days, and the areas labelled by test compounds were determined by impaling a single astrocyte with a micropipette and diffusion of material into coupled cells. Amyloid-β-treated astrocytes had rapid, sustained 50-70% reductions in the area labelled by Lucifer Yellow, anionic Alexa Fluor® dyes and energy-related compounds, 6-NBDG (a fluorescent glucose analogue), NADH and NADPH. Amyloid-β treatment also caused a transient increase in oxidative stress. In striking contrast with these results, spreading of Lucifer Yellow within astrocytic networks in brain slices from three regions of 8.5-14-month-old control and transgenic Alzheimer's model mice was variable, labelling 10-2000 cells; there were no statistically significant differences in the number of dye-labelled cells among the groups or with age. Thus amyloid-induced dysfunction of gap junctional communication in cultured astrocytes does not reflect the maintenance of dye transfer through astrocytic syncytial networks in transgenic mice; the pathophysiology of Alzheimer's disease is not appropriately represented by the cell culture system.
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Key Words
- 6-NBDG, 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-6-deoxyglucose
- A350, Alexa Fluor® 350
- A568, Alexa Fluor® 568
- APP, amyloid-β precursor protein
- Cx, connexin
- DCF, dichlorofluorescein
- DMEM, Dulbecco's modified Eagle's medium
- FBS, fetal bovine serum
- GFAP, glial fibrillary acidic protein
- H2DCF-DA, carboxydihydrodichlorofluorescein diacetate
- L-LME, L-leucine methyl ester hydrochloride
- ROS, reactive oxygen species
- SR101, sulforhodamine 101
- STZ, streptozotocin
- aCSF, artificial cerebral spinal fluid
- amyloid protein
- astrocyte
- connexin
- dBcAMP, dibutyryl cAMP
- dye transfer
- gap junction
- metabolite trafficking
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Sharma R, Fischer MT, Bauer J, Felts PA, Smith KJ, Misu T, Fujihara K, Bradl M, Lassmann H. Inflammation induced by innate immunity in the central nervous system leads to primary astrocyte dysfunction followed by demyelination. Acta Neuropathol 2010; 120:223-36. [PMID: 20532539 PMCID: PMC2892605 DOI: 10.1007/s00401-010-0704-z] [Citation(s) in RCA: 124] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2010] [Revised: 05/25/2010] [Accepted: 05/25/2010] [Indexed: 11/26/2022]
Abstract
Primary loss and dysfunction of astrocytes may trigger demyelination, as seen in neuromyelitis optica, an inflammatory disease of the central nervous system. In most patients affected by this disease, injury to astrocytes is initiated by the action of autoantibodies targeting aquaporin 4 (AQP-4), a water channel on astrocytes. We show here that damage of astrocytes and subsequent demyelination can also occur in the absence of autoantibody-mediated mechanisms. Following injection of lipopolysaccharide into the white matter initial microglia activation is followed by a functional disturbance of astrocytes, mainly reflected by retraction of astrocytic foot processes at the glia limitans and loss of AQP-4 and connexins, which are involved in the formation of gap junctions between astrocytes and oligodendrocytes. Demyelination and oligodendrocyte degeneration in this model follows astrocyte pathology. Similar structural abnormalities were also seen in a subset of active lesions in multiple sclerosis. Our studies suggest that astrocyte injury may be an important early step in the cascade of lesion formation in brain inflammation.
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MESH Headings
- Animals
- Aquaporin 4/metabolism
- Astrocytes/immunology
- Astrocytes/pathology
- Cells, Cultured
- Central Nervous System/pathology
- Coculture Techniques/methods
- Disease Models, Animal
- Encephalomyelitis, Autoimmune, Experimental/etiology
- Encephalomyelitis, Autoimmune, Experimental/immunology
- Encephalomyelitis, Autoimmune, Experimental/pathology
- Gene Expression Regulation/drug effects
- Gene Expression Regulation/immunology
- Humans
- Immunity, Innate
- Membrane Proteins/metabolism
- Microscopy, Confocal/methods
- Multiple Sclerosis/immunology
- Multiple Sclerosis/pathology
- Nerve Tissue Proteins/metabolism
- Neuroglia/drug effects
- Polysaccharides/pharmacology
- Rats
- Time Factors
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Affiliation(s)
- Rakhi Sharma
- Centre for Brain Research, Medical University of Vienna, Spitalgasse 4, 1090 Vienna, Austria
| | - Marie-Therese Fischer
- Centre for Brain Research, Medical University of Vienna, Spitalgasse 4, 1090 Vienna, Austria
| | - Jan Bauer
- Centre for Brain Research, Medical University of Vienna, Spitalgasse 4, 1090 Vienna, Austria
| | - Paul A. Felts
- Institute of Neurology, University College London, London, UK
- Centre for Anatomy and Human Identification, University of Dundee, Dundee, Scotland, UK
| | | | - Tatsuro Misu
- Tohoku University Graduate School of Medicine, Sendai, Japan
| | - Kazuo Fujihara
- Tohoku University Graduate School of Medicine, Sendai, Japan
| | - Monika Bradl
- Centre for Brain Research, Medical University of Vienna, Spitalgasse 4, 1090 Vienna, Austria
| | - Hans Lassmann
- Centre for Brain Research, Medical University of Vienna, Spitalgasse 4, 1090 Vienna, Austria
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Hyperglycaemia and diabetes impair gap junctional communication among astrocytes. ASN Neuro 2010; 2:e00030. [PMID: 20396375 PMCID: PMC2839462 DOI: 10.1042/an20090048] [Citation(s) in RCA: 69] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2009] [Revised: 01/03/2010] [Accepted: 01/05/2010] [Indexed: 01/21/2023] Open
Abstract
Sensory and cognitive impairments have been documented in diabetic humans and
animals, but the pathophysiology of diabetes in the central nervous system is
poorly understood. Because a high glucose level disrupts gap junctional
communication in various cell types and astrocytes are extensively coupled by
gap junctions to form large syncytia, the influence of experimental diabetes on
gap junction channel-mediated dye transfer was assessed in astrocytes in tissue
culture and in brain slices from diabetic rats. Astrocytes grown in
15–25 mmol/l glucose had a slow-onset, poorly reversible decrement in
gap junctional communication compared with those grown in 5.5 mmol/l glucose.
Astrocytes in brain slices from adult STZ (streptozotocin)-treated rats at
20–24 weeks after the onset of diabetes also exhibited reduced dye
transfer. In cultured astrocytes grown in high glucose, increased oxidative
stress preceded the decrement in dye transfer by several days, and gap
junctional impairment was prevented, but not rescued, after its manifestation by
compounds that can block or reduce oxidative stress. In sharp contrast with
these findings, chaperone molecules known to facilitate protein folding could
prevent and rescue gap junctional impairment, even in the presence of elevated
glucose level and oxidative stress. Immunostaining of Cx (connexin) 43 and 30,
but not Cx26, was altered by growth in high glucose. Disruption of astrocytic
trafficking of metabolites and signalling molecules may alter interactions among
astrocytes, neurons and endothelial cells and contribute to changes in brain
function in diabetes. Involvement of the microvasculature may contribute to
diabetic complications in the brain, the cardiovascular system and other
organs.
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Key Words
- 4-PBA, 4-phenylbutyric acid
- 6-NBDG, 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxyglucose
- Cx, connexin
- DCF, 2′,7′-dichlorodihydrofluorescein
- DIC, differential interference contrast
- DMEM, Dulbecco's modified Eagle's medium
- ER, endoplasmic reticulum
- FBS, fetal bovine serum
- LYCH, Lucifer Yellow CH
- LYVS, Lucifer Yellow VS
- MnTBAP, manganese(III) tetrakis (4-benzoic acid) porphyrin chloride
- NA, numerical aperture
- NOS, nitric oxide synthase
- PKC, protein kinase C
- RNS, reactive nitrogen species
- ROS, reactive oxygen species
- STZ, streptozotocin
- TMAO, trimethylamine N-oxide dihydrate
- TUDCA, tauroursodeoxycholic acid
- aCSF, artificial cerebrospinal fluid
- astrocyte
- carboxy-DCF-DA, carboxy DCF diacetate
- connexin (Cx)
- dBcAMP, dibutyryl cAMP
- diabetes
- gap junction
- hyperglycaemia
- l-NAME, l-Nω-nitro-l-arginine methyl ester
- streptozotocin
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Giaume C, Koulakoff A, Roux L, Holcman D, Rouach N. Astroglial networks: a step further in neuroglial and gliovascular interactions. Nat Rev Neurosci 2010; 11:87-99. [DOI: 10.1038/nrn2757] [Citation(s) in RCA: 562] [Impact Index Per Article: 40.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
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Ball KK, Cruz NF, Mrak RE, Dienel GA. Trafficking of glucose, lactate, and amyloid-beta from the inferior colliculus through perivascular routes. J Cereb Blood Flow Metab 2010; 30:162-76. [PMID: 19794399 PMCID: PMC2801760 DOI: 10.1038/jcbfm.2009.206] [Citation(s) in RCA: 71] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
Metabolic brain imaging is widely used to evaluate brain function and disease, and quantitative assays require local retention of compounds used to register changes in cellular activity. As labeled metabolites of [1- and 6-(14)C]glucose are rapidly released in large quantities during brain activation, this study evaluated release of metabolites and proteins through perivascular fluid flow, a pathway that carries solutes from brain to peripheral lymphatic drainage sites. Assays with [3,4-(14)C]glucose ruled out local oxidation of glucose-derived lactate as a major contributor of label loss. Brief infusion of [1-(14)C]glucose and D-[(14)C]lactate into the inferior colliculus of conscious rats during acoustic stimulation labeled the meninges, consistent with perivascular clearance of [(14)C]metabolites from interstitial fluid. Microinfusion of Evans blue albumin and amyloid-beta(1-40) (Abeta) caused perivascular labeling in the inferior colliculus, labeled the surrounding meninges, and Abeta-labeled-specific blood vessels in the caudate and olfactory bulb and was deposited in cervical lymph nodes. Efflux of extracellular glucose, lactate, and Abeta into perivascular fluid pathways is a normal route for clearance of material from the inferior colliculus that contributes to underestimates of brain energetics. Convergence of 'watershed' drainage to common pathways may facilitate perivascular amyloid plaque formation and pathway obstruction in Alzheimer's disease.
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Affiliation(s)
- Kelly K Ball
- Department of Neurology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA
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Gandhi GK, Cruz NF, Ball KK, Dienel GA. Astrocytes are poised for lactate trafficking and release from activated brain and for supply of glucose to neurons. J Neurochem 2009; 111:522-36. [PMID: 19682206 DOI: 10.1111/j.1471-4159.2009.06333.x] [Citation(s) in RCA: 116] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
Brain is a highly-oxidative organ, but during activation, glycolytic flux is preferentially up-regulated even though oxygen supply is adequate. The biochemical and cellular basis of metabolic changes during brain activation and the fate of lactate produced within brain are important, unresolved issues central to understanding brain function, brain images, and spectroscopic data. Because in vivo brain imaging studies reveal rapid efflux of labeled glucose metabolites during activation, lactate trafficking among astrocytes and between astrocytes and neurons was examined after devising specific, real-time, sensitive enzymatic fluorescent assays to measure lactate and glucose levels in single cells in adult rat brain slices. Astrocytes have a 2- to 4-fold faster and higher capacity for lactate uptake from extracellular fluid and for lactate dispersal via the astrocytic syncytium compared to neuronal lactate uptake from extracellular fluid or shuttling of lactate to neurons from neighboring astrocytes. Astrocytes can also supply glucose to neurons as well as glucose can be taken up by neurons from extracellular fluid. Astrocytic networks can provide neuronal fuel and quickly remove lactate from activated glycolytic domains, and the lactate can be dispersed widely throughout the syncytium to endfeet along the vasculature for release to blood or other brain regions via perivascular fluid flow.
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Affiliation(s)
- Gautam K Gandhi
- Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA
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Gandhi GK, Cruz NF, Ball KK, Theus SA, Dienel GA. Selective astrocytic gap junctional trafficking of molecules involved in the glycolytic pathway: impact on cellular brain imaging. J Neurochem 2009; 110:857-69. [PMID: 19457076 DOI: 10.1111/j.1471-4159.2009.06173.x] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2023]
Abstract
To assess the specificity of metabolite trafficking among gap junction-coupled astrocytes, we developed novel, real-time, single-cell enzymatic fluorescence assays to assay cell-to-cell transfer of unlabeled glycolytic intermediates and report (i) highly restricted transfer of glucose-6-phosphate (P) and two analogs, deoxyglucose (DG)-6-P, and 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-DG-6-P, compared with DG and 2- and 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-DG, (ii) extensive junctional diffusion of glyceraldehyde-3-P, NADH, and NADPH plus three anionic fluorescent dyes used as internal standards for transfer assays, and (iii) stimulation of gap junctional communication by increased intracellular Na(+) that also evokes metabolic responses in nearby coupled astrocytes. Thus, dye transfer does not predict gap junctional permeability of endogenous metabolites. Intracellular retention of flux-regulating compounds (e.g. glucose-6-P) may be necessary for local metabolic control, whereas 'syncytial sharing' may dissipate the work load on peri-synaptic astrocytes. Imaging of brain functional activity depends on local accumulation of exogenous or endogenous signals, and DG-6-P is trapped in the cell where it is phosphorylated, whereas rapid dispersal of cytoplasmic NAD(P)H and labeled glucose metabolites throughout the astrocytic syncytium can interfere with cellular assessment of neuron-astrocyte relationships in autoradiographic, fluorescence microscopic, and magnetic resonance spectroscopic studies.
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Affiliation(s)
- Gautam K Gandhi
- Department of Physiology and Biophysics, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA
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Stout RF, Spray DC, Parpura V. Astrocytic 'power-grid': Delivery upon neuronal demand. CELLSCIENCE 2009; 5:34-43. [PMID: 19756252 PMCID: PMC2743111] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
Gap junctions can connect the cytosolic compartments of adjacent astroglia. They allow intercellular flux of low-molecular weight (< ~ 1 kDa) compounds, including metabolites and second messengers. Only recently, however, has it been proposed that gap junctions may serve an additional role in the astrocytic metabolic network which maintains synaptic transmission. The brain seems to be using a strategy analogous to power-grid systems used in modern societies to supply energy; the astrocytic 'power-grid' can deliver the metabolic energy to neurons as needed. Such an astroglial energy grid is malleable and can change the size and shape in response to metabolic activity of neuronal network to deliver energy from the root source of energy of the brain, the blood glucose, to neurons.
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Affiliation(s)
- Randy F Stout
- Dept. of Neurobiology, Ctr. for Glial Biology in Medicine, Atomic Force Microscopy & Nanotechnology Laboratories, Civitan International Research Center, Evelyn F. McKnight Brain Institute, University of Alabama, Birmingham, AL 35294, USA
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Dienel GA, Cruz NF. Imaging brain activation: simple pictures of complex biology. Ann N Y Acad Sci 2009; 1147:139-70. [PMID: 19076439 DOI: 10.1196/annals.1427.011] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Elucidation of biochemical, physiological, and cellular contributions to metabolic images of brain is important for interpretation of images of brain activation and disease. Discordant brain images obtained with [(14)C]deoxyglucose and [1- or 6-(14)C]glucose were previously ascribed to increased glycolysis and rapid [(14)C]lactate release from tissue, but direct proof of [(14)C]lactate release from activated brain structures is lacking. Analysis of factors contributing to images of focal metabolic activity evoked by monotonic acoustic stimulation of conscious rats reveals that labeled metabolites of [1- or 6-(14)C]glucose are quickly released from activated cells as a result of decarboxylation reactions, spreading via gap junctions, and efflux via lactate transporters. Label release from activated tissue accounts for most of the additional [(14)C]glucose consumed during activation compared to rest. Metabolism of [3,4-(14)C]glucose generates about four times more [(14)C]lactate compared to (14)CO(2) in extracellular fluid, suggesting that most lactate is not locally oxidized. In brain slices, direct assays of lactate uptake from extracellular fluid demonstrate that astrocytes have faster influx and higher transport capacity than neurons. Also, lactate transfer from a single astrocyte to other gap junction-coupled astrocytes exceeds astrocyte-to-neuron lactate shuttling. Astrocytes and neurons have excess capacities for glycolysis, and oxidative metabolism in both cell types rises during sensory stimulation. The energetics of brain activation is quite complex, and the proportion of glucose consumed by astrocytes and neurons, lactate generation by either cell type, and the contributions of both cell types to brain images during brain activation are likely to vary with the stimulus paradigm and activated pathways.
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Affiliation(s)
- Gerald A Dienel
- Department of Neurology, University of Arkansas for Medical Sciences, Little Rock, AR 72205, USA.
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