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GlialSignaling

Advanced Seminars in Behavioral Neuroendocrinology

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Cliff – 20 September 2013
Emerging role for astroglial networks in information processing: from synapse to behavior U Pannasch and N Rouach 2013 Trends in Neuroscience71: 405–417

  1. Glial signaling is due in part to the existence of Astroglial Networks
  2. Similar to Neuronal Circuits
  3. Finely organized with Anatomical/Functional Compartments
  4. 2 main types of Glia: Microglia & Macroglia
  5. Microglia are phagocytes derived from macrophages
  6. Macrogliaare support cells

1)surround neuronal dendrites, soma, axonsand synapses

2)Glia is Greek for Glue

  1. Glia outnumber neurons 10-50X

1)Involved in housekeeping, removal of debris, but much more

  1. Subtypes of Macroglia are involved in Glial Signaling
  2. Schwann Cells
  3. Small cells in the peripheral NS
  4. Insulate axons

1)→ ↑ efficiency and speed of neuronal signal (action potential)

2)1 internode / Schwann cell

  1. Oligodendrocytes
  2. Small cells in the central NS
  3. Insulate axons → ↑ efficiency and speed of neuronal signal (action potential)

1)15 axonal internodes each

  1. Astrocytes
  2. Star shaped soma
  3. Long processes with end feet

1)Very similar to neurons

2)End feet interface with neurons

3)End feet interface with synapses → tripartite synapse

  1. Create the Blood-Brain Barrier (BBB)

1)Enwrapping blood vessels in the brain

2)Tight junctions maintain the integrity of the brains neurochemistry

3)Specialized transporters are required to cross the BBB

  1. Astroglial Networks allow for inter-Glial Signaling
  2. Astroglia express connexins (Cx)
  3. Gap Junction (GJ) channels are made from Cxs
  4. Cxs are basic channel proteins

1)Evolutionary precursor for ion channels + ionotropic receptors (ligand-gated)

  1. Cx30 + Cx43 ≈ 50% each
  2. Mediate large astrocyte ensembles
  1. Network Circuits include 100s of astrocytes
  2. Confer selective and preferential inter-astroglial connections

1)Not all neighboring astrocytes are connected by GJs

  1. Functionally distinct glial populations
  1. Short-term regulation via molecules effecting GJ coupling
  1. Astroglial Networks are functionally plastic, regulated by neuronal activity
  2. Neurotransmitters, cannabinoids, Endothelins
  3. Influence GJ permeability/selectivity
  4. Glial Signaling + Neuronal signaling are enhanced by the tripartite synapse
  5. Synaptic cleft wrapped by astrocytic end feet
  6. Controls synaptic boundaries
  7. Limits volume
  8. 3-way interactions
  9. Astrocyte synergism
  10. Active role in neurotransmission
  11. Sense neuronal inputs

1)Via ion channels

2)Neurotransmitter receptors

3)Neurotransmitter transporters

  1. Display dynamic signaling
  1. Astroglial Networks + tripartite synapse
  2. tripartite synapse astrocytes take up neurotransmitters

Justin Smith– 27 September 2013

FGF2 blocks PTSD symptoms via an astrocyte-based mechanismL Xia, M Zhai, L Wang, D Miao, Xia Zhua, W Wang2013 Behavioural Brain Res

  1. Glial signaling includes production and release of FGF2
  2. Astrocytes produce FGF2
  3. Astrocytes are the largest population of cells in the hippocampus
  4. More astrocytes than neurons in the brain
  5. FGF2is one of a family of22 FGF proteins(aka bFGF or FGFβ)
  6. FGF2 is a single polypeptide chain
  7. FGFs are a part of a larger family of Growth Factor (GF) or Neurotrophins
  8. FGFs stimulate neuronal mitosis = neuronal proliferation, survival, and repair
  9. 5 human FGF receptors

1)4 FGF receptors in rats

a)FGFR1 is the receptor for FGF2

  1. FGF2 is released by stress and glucocorticoids
  2. FGF2 stimulates mechanisms associated with learning and memory
  3. FGF2→ ↑ Glu release
  4. → ↑ AMPA GluR1 subunit (AMPA trafficking)
  5. → ↑ or ↓ NMDA activity
  6. → ↑ L voltage-gated Ca++ channels (LVGCCs)
  7. → ↑ neuronal [CA++]
  8. → ↑ LTP
  9. FGF2→ ↑ cAMP, PKC, pMAPK → ↑ FGF2
  10. → ↑ pCREB → ↑ CRE → ↑ gene transcription
  11. → ↑ neurogenesis, cell survival, cell differentiation
  1. Stress and trauma elicit → ↓ Astrocyte activationin hippocampus
  2. PTSD type single prolonged stress model → ↓ GFAP expression in astrocytes
  3. GFAP is glial fibrillary acidic protein – found only in glial cells
  4. Hippocampal astrocyte activation is stimulated by FGF2
  5. PTSD-like SPStress → ↓ Astrocyte GFAP activationis rescued by FGF2
  6. FGF2 does not ∆ neuronal protein (NeuN) expression
  7. Systemic FGF2 reduces anxiety
  8. Open field and elevated plus maze
  9. FGF2 reduced CS cue stimulated fear conditioned freezing
  10. FGF2 reduced generalized fear responsiveness - freezing to non-CS cue/context
  11. FGF2 also reduces reinstatement of fear conditioningafter extinction

1)Cue- or stress-precipitated relapse of fear conditioning

a)Original trauma or novel stress stimulate retrieval of fearful memory

  1. FGF2reduces renewal of fear conditioning after extinction

1)Renewal is reinstatement of fear conditioning in a novel context

James Robertson - 4 October 2013

Forebrain engraftment by human glial progenitor cells enhances synaptic plasticity and learning in adult miceX Han, M Chen, F Wang, M Windrem, S Wang, S Shanz, Q Xu, NA Oberheim, L Bekar, S Betstadt, AJ Silva, T Takano, Steven A Goldman, M Nedergaard2013 Stem Cell12: 342–353

  1. Glial Signaling influences LTP in hippocampal neurons
  2. Ca++-clamped Astrocytes blocks neuronal LTP in hippocampus
  3. Ca++ from astrocytes are necessary for neuronal LTP
  4. Astrocytes release d-serine
  5. Glial metabolic poison FAC blocks LTP
  6. d-serine synthesis inhibitor HOAsp blocks LTP

1)only after removing extant d-serine

  1. Astrocytes and d-serine necessary for neuronal LTP
  1. d-serine binds the Gly site onneuronal NMDA receptors → ↑ LTP
  1. Astrocyte signaling → ↑ LTP is limited to local astrocyte networks
  2. Astrocytes are required for some types of synaptic plasticity (like LTP)
  3. Human Astrocytes (hAstrocytes) are larger and have more branching than non-primates
  4. hAstrocytes have faster propagation velocity
  5. Human Astrocytes engrafted into mouse brain retain human morphology in mouse brain
  6. hAstrocytes in mouse chimera also have faster propagation velocity
  7. Mice chimeras with hAstrocytes→ ↑↑ LTP in hippocampus
  8. Compared to allografted or wild-type mice
  9. Slopes of fEPSPs were increased in chimeric mice
  10. DPCPX A1 receptor agonist did not block the ↑↑ LTP
  11. Astrocyte release of ATP or adenosine are not stimulating LTP
  12. Mice chimeras have more hippocampal TNFα expression
  13. TNFαstimulate increased fEPSP slopes and AMPA GluR1 (GluA1) subunit in normal mice
  14. Chimeric mice have ↑ hippocampal GluR1 AMPA receptor subunits
  15. TNFα synthesis inhibitor thalidomide reduces chimeric expression of TNFα and GluR1
  16. Limits GluR1 trafficking by → ↓ Ser831 phosphorylation of GluR1 subunits
  17. Reducing TNFα inhibits chimeric hippocampal ↑↑ LTP
  18. Chimeric mice with hAstrocytes → ↑ cognitive performance
  19. On tests for Auditory Fear Conditioning, Contextual Fear Conditioning, Barnes Maze, Object-Location Memory Task
  20. Chimeric ↑ cognitive performance blocked by → ↓ TNFα
  21. Human Astrocytes release TNFα to → ↑ LTP and ↑ cognitive performance

Justin Achua – 11 October 2013

ATP-sensitive potassium channel-mediated lactate effect on orexin neurons: implications for brain energetics during arousalMP Parsons, M Hirasawa2010, Journal of Neuroscience30: 8061– 8070

  1. Glial signaling mediates energy transfer to neurons
  2. Monocarboxylate transporters (MCT) move lactate from astrocytes to neurons
  3. MCT1 expels lactate into the extraneuronal space
  4. MCT1 is oriented toward exocytosis
  5. MCT1 are found in astrocytes
  6. MCT2 takes up lactate into neurons
  7. MCT2are oriented toward endocytosis
  8. MCT2are found in neurons
  9. Astrocytes generate lactate
  10. Glutamate (Glu) bursts stimulate lactate production
  11. Astrocytesmetabolizes glucose to lactate
  12. All glucose is taken into the brain by end feet of astrocytes
  13. Astrocytes provide lactate for orexin (Orx; aka hypocretin) neurons
  14. Orx cell bodies are in the DMH/PeF
  15. Co-express glutamate (Glu)
  16. Orx is important for feeding, arousal, sleep-wake cycles
  17. Orx cells are active only during waking that includes movement
  18. = active waking, less firing during quiet waking
  19. Becomes active just before active waking
  20. Predicts awakening
  21. Not during slow wave sleep, REM sleep (paradoxical sleep)
  22. Not during the transistion to SWS or REM
  23. Orx cells can use lactate for an energy source
  24. Orx cell firing blocked by lack of energy is restored by lactate
  25. Lack of energy by removing glucose
  26. Blocking MCTs blocks lactate transfer and inhibits Orx cell firing
  27. Lack of tetrodotoxin (TTX) inhibition demonstrates a postsynaptic effect
  28. Glial toxin (fluoroacetate, FAC) blocks actetate metabolism in astrocytes
  29. Only Astrocytes can use acetate as an energy source
  30. FAC + glucose will not permit Orx cell firing
  31. FAC + lactate restores Orx cell firing
  32. Orx cell firing blocked by lack of energy is also restored by acetate
  33. Acetate converted to lactate provides energy to neurons
  34. Astrocytes are providing the energy for neuron
  35. the energy source for Orx neurons in the brain is lactate from astrocytes
  36. Is the energy source for all neurons in the brain is lactate from astrocytes?
  37. Astrocyte lactate enhances Orx cell firing frequency
  38. ATP-sensitive potassium channels (K-ATP) inhibit Orx firing rate
  39. K-ATP channels close with ↑ [ATP]
  40. ↑ ATP → ↓ K-ATP → ↓ K+ efflux
  41. K-ATP comprised of Kir6.1 and SUR1 subunits in Orx cells
  42. ↓ ATP → ↑ K-ATP → ↑ K+ currentout → ↓ Orx firing rate
  43. ↓ ATP + SUR1 blocker → ↓ K-ATP → ↓ K+ currentout → ↑ Orx firing rate
  44. K+ channel blocker → ↓ K-ATP → ↑ Orx firing rate
  45. Kir6.2 are more sensitive to glucose; Kir6.2 are expressed in glucosensing neurons
  46. BlockingK-ATP channels increases Orx firing
  47. Blocking MCT decreases Orx firing rate
  48. Blocking KATPrestores Orx firing due to MCT inhibition
  49. KATPchannels are sensitive to MCT derived lactate
  50. KATPchannels allow Orx cells to be sensitive to lactate availability
  51. High lactate availability reduces K-ATPchannels inhibition of Orx firing
  52. ↑lactate → ↑ ATP → ↓ K-ATP → ↓ K+ currentout → ↑ Orx firing rate
  53. ↑ lactate → ↑ Orx firing rate
  54. Orexin neurons are lactate sensors
  55. ↑ Orx firing → ↑ feeding → ↑ glucose → ↑ astrocyte uptake → ↑lactate → ↑ Orx firing
  56. SCN output → ↑ astrocyte activity + ↑ Orx firing → ↑ HPA + ↑ NE/sympathetic/Epi → ↑ arousal + ↑ plasma glucose → ↑ astrocyte uptake → ↑ lactate transfer → ↑ Orx firing → ↑ feeding → ↑ glucose → ↑ astrocyte uptake → ↑lactate → ↑ Orx firing → ↑ feeding → ↑ glucose → ↑ glucosensor firing in hypothalamus and brainstem → ↑ satiety → ↓ feeding→ ↓ plasma glucose → ↓ astrocyte uptake → ↓ lactate → ↓ Orx firing

Zhang Yufeng – 18 October 2013

Higher transport and metabolism of glucose in astrocytes compared with neurons: a multiphoton study of hippocampal and cerebellar tissue slicesP Jakoby, E Schmidt, I Ruminot, R Gutiérrez, LF Barros, JW Deitmer 2013, Cerebral Cortex doi:10.1093/cercor/bhs309

  1. Glialcells control energy metabolism in the brain
  2. Energy requirements of the brain are high
  3. Brain = 2% of mass, but 20-28% of energy use
  4. Neurons don’t produce or store much glycogen
  5. Brain metabolizes less lipids or fatty acids than other sources
  6. Lipids provide more energy than any other substrate
  7. Lipids contain 2X energy of carbohydrates
  8. 4 potential reasons for lower brain use of lipids
  9. slow passage of fatty acids across the blood–brain barrier (BBB)
  10. NO - fatty acids diffuse rapidly across BBB
  11. Moved by diffusion rather than transporter
  12. low enzymatic capacity in neurons for fatty acid degradation
  13. YES – low oxidation of long-chain fatty acids
  14. Low translocation rate – low carnitine activity
  15. Low enzymatic capacity of the β-oxidation pathway
  16. Harmful side effects of long-chain fatty acids in the mitochondrial ATP synthesis
  17. YES – fatty acids produce oxidative stress in brain
  18. Fatty acids → ↓ electron transport chain → ↑ reactive oxygen species (ROS)
  19. Fatty acids ∆ mitochondrial internal membrane depolarization
  20. Fatty acids → ↑ mitochondrial permeability → ↑ proton leakage → ↑ROS
  21. Fatty acid oxidation is also too slow to match neuronal ATP requirements
  22. YES – fatty acids oxidation → ↑ neural hypoxia
  23. Glial cells do produce and store more glycogen
  24. Glial glycogen can be mobilized for neuronal use
  25. Astrocytes respond to metabolic demands
  26. Astrocytes are territorial cells
  27. Astrocyteprocesses and end feet don’t overlap with those of other astrocytes
  28. Astrocyte processes surround specific synapses
  29. Astrocyte end feet absorb blood glucose
  30. Astrocyte Ca++ waves → ↑ metabolic waves
  31. Neurons release Glu→ ↑mGluR - Astrocyte → ↑ Ca++ → ↑ Ca++wave
  32. ↑ Ca++ → ↑ phospholipase A (PLA) ↑ arachidonic acid (AA) → ↑ prostaglandin E2
  33. ↑ prostaglandin E2→ ↑arteriole dilation → ↑ blood delivery
  34. ↑ blood delivery → ↑end feet absorb blood glucose
  35. a-e processes move in a way following Ca++ wave along astrocytes & neurons
  36. Astrocytes and Neurons exhibit metabolic specializations
  37. Neurons sustain a high rate of oxidative metabolism compared to glial cells
  38. Neurons do not make much use of glycolysis
  39. ↓↓ 6-phosphofructose-2-kinase/fructose-2,6- bisphosphatase-3
  40. ↓↓ activation of the glycolytic enzyme phosphofructokinase-1 (PFK)
  41. Neural cells don’t use Fatty Acid as fuel
  42. Neurons can use glucose, lactate, pyruvate, glutamate, glutamine as energy substrates
  43. But Neurons take up glucose much more slowly than astrocytes
  44. And take up less total glucose
  45. Neurons rely on Astrocytes for antioxidant protection
  46. Astrocytes take up glucose
  47. Astrocytes take up glucose much more rapidly than neurons
  48. Take up much more total glucose
  49. True in brain regions tested: hippocampus and cerebellum
  50. Glucose transporters 1 and 3 equivalent in those areas
  51. Astrocytes have a greater metabolic plasticity than neurons
  52. Glycogen is the largest energy reserve of the brain
  53. Much less glycogen in brain than liver
  54. Glycogen has been found to be almost exclusively localized in astrocytes
  55. Astrocytes have a high glycolytic rate
  56. Glycolysis ends with lactate
  57. Speicialized transporters, MCT1 or MCT4 → transfer ↑ lactate to extracellular fluid
  58. Lactate is available for neuronal energy metabolism
  59. Lactate is taken up by neurons using MCT2
  60. Astrocytes have higher levels of various antioxidant molecules
  61. Blocking hippocampal MCT1/4 → ↓ long-term memory
  62. Adding lactate after Blocking hippocampal MCT1/4restores long-term memory
  63. Blocking hippocampal MCT2 → ↓ long-term memory (not restored by lactate)
  64. Astroglial glucose/lactate + MCT1/4and neuronal MCT2together are necessary for complex brain functions like learning and memory

Debra Perkins-Hicks – 25 October 2013

Chemokine contribution to neuropathic pain: respective induction of CXCL1 and CXCR2 in spinal cord astrocytes and neuronsZ-J Zhang, D-L Cao, X Zhang, R-R Ji, Y-J Gao 2013 Pain

  1. Glial Signaling may include production and release of chemokines
  2. chemotactic cytokines
  3. induce directed chemotaxis
  4. direct their movements according to certain chemicals in their environment
  5. stimulate chemotaxis of microglia
  6. Four families of chemokines
  7. CXC – have cysteine – random amino acid - cysteine
  8. CXCL1 and its receptor CXCR2
  9. Released mostly by astrocytes
  10. Also made and released by neurons
  11. Cause chronic pain following nerve injury
  12. CC
  13. CX3C
  14. C (XC)
  15. Glial-neuronal interactions
  16. Chemokines are involved in development and maintenance of neuropathic pain
  17. Astrocyte-neuron interactions are important for neuropathic pain
  18. development and maintenance in spinal cord
  19. spinal nerve ligation (SNL) → ↑ TNFα mRNA
  20. SNL → 1 day ↑↑ TNFα → 3 days ↑ TNFα → 21 days ↑ TNFα
  21. TNFα = tumor necrosis factor alpha
  22. ↑ TNFα → ↑ JNK(MAPK)→ rapidly ↑ CXCL1 in astrocytes
  23. SNL → 3 days ↑ CXCL1 → 10 days ↑↑ CXCL1 → 21 days ↑ CXCL1
  24. Same pattern for mRNA and protein
  25. TNFα antagonist (decoy receptor; etanercept) blocks ↑ CXCL1 at 3 days
  26. JNK antagonist (SP600125) blocks ↑ CXCL1 at 8-10 days
  27. JNKs = c-Jun N-terminal kinases
  28. JNKs are part of the MAPK 2nd messenger family
  29. Phosphorylate the c-Jun transcription factor at on Ser-63 and Ser-73 within its transcriptional activation domain
  30. MAPK = mitogen activated protein kinases
  31. TNFα antagonist → ↓ mechanical and heat pain responses
  32. JNK antagonist → ↓ mechanical and heat pain responses
  33. SNL → CXCL1 antibody → ↓ mechanical and heat pain responses
  34. SNL → CXCL1 shRNA → ↓ mechanical and heat pain responses
  35. CXCL1 → ↑ mechanical and heat pain responses
  36. CXCR2 antagonist → ↓ mechanical and heat pain responses
  37. Higher dose (20 μg) at 1, 3, 7h
  38. CXCR2 antagonist = SB225002
  39. AstrocyteCXCL1 → ↑ neuron CXCR2 → ↑ pERK → ↑ pCREB → CRE on DNA → ↑ cfos
  40. CXCR2 antagonist → blocks ↑ pERK, ↑ pCREB, and ↑ cfos
  41. SNL → neuron + astrocyte ↑ TNFα → ↑ JNK → ↑ CXCL1 → ↑ neuron CXCR2 → ↑ pERK → ↑ pCREB → CRE on DNA → ↑ cfos → ↑ neuronal pain signaling
  42. Astrocytes contribute to pain signaling throughCXCL1 → ↑ neuronal CXCR2

Mohsan Ali – 1 November 2013

Astrocyte-derived adenosine and A1 receptor activity contribute to sleep loss-induced deficits in hippocampal synaptic plasticity and memory in miceC Florian, CG Vecsey, MM Halassa, PG Haydon, Ted Abel2011Journal of Neuroscience31: 6956–6962

  1. Glial Signaling includes Astrocyte release of ATP and Adenosine
  2. ATP can be converted (hydrolyzed) to adenosine
  3. Adenosine binds to A1 receptors
  4. A1 → ↑ Gi1/2/3 or Go → ↓ AC (adenylate cyclase) → ↓ cAMP → ↓ PKA
  5. Sleep deprivation stimulates Astrocyte release of Adenosine
  6. and accumulation in cortex
  7. caffeine inhibits the effects of sleep deprivation by blocking adenosine A1 receptors
  8. During sleep deprivation, sleep homeostat increases the drive to sleep
  9. drive to sleep = Sleep pressure
  10. Sleep deprivation causes cognitive deficits
  11. Sleep deprivation → ↓ L-LTP
  12. Sleep comes in 2 stages: REM and NREM (paradoxical, slow wave)
  13. REM is characterized by high frequency, low amplitude EEG waves
  14. Rapid Eye Movement
  15. Sleep paralysis
  16. NREM is characterized by low frequency, high amplitude EEG waves
  17. Slow wave sleep = Slow Oscillation
  18. Up state – when action potential firing occurs
  19. Down state – absence of synaptic inputs; membrane resting potential
  20. Sleep pressure is measured by increased slow wave activity
  21. Sleep walking and talking occur during NREM
  22. Inhibition of Glial Signaling by blocking exocytosis→ ↓ adenosine
  23. Dominant-negative SNARE (dnSNARE) reduces binding of the SNAP-SNARE release complex for ATP or adenosine
  24. dnSNARE transfected into the promoter of GFAP
  25. GFAP only expressed in astrocytes
  26. dnSNARE for synaptobrevin protein
  27. Sleep deprivation → ↓ LTP → reversed by dnSNARE
  28. Sleep deprivation → ↑ sleep pressure → ↑ slow oscillation power → reversed by dnSNARE
  29. ↑ Adenosine accumulation → ↑ A1 receptor → ↑ sleep pressure
  30. Conditional inhibition of Glial Signalingvia tet-off-dnSNARE → ↓ adenosine
  31. Doxycycline inhibition times dnSNARE reduction of Adenosine release
  32. Sleep deprivation → ↓ L-LTP → reversed by tet-off-dnSNARE just before deprivation
  33. Sleep deprivation → ↓ L-LTP → reversed by A1 receptor antagonist
  34. Sleep deprivation → ↑ Adenosine accumulation → ↑ A1R activity → ↓ L-LTP
  35. tet-off-dnSNARE → ↓ adenosine (or ATP) release by Astrocytes
  36. Sleep deprivation → ↓ novel placement of object memory
  37. Sleep deprivation → ↓ spatial object memory reversed by tet-off-dnSNARE inhibiton of Astrocyte adenosine release
  38. Sleep deprivation → ↓ spatial object memory reversed by A1 receptor antagonist
  39. Sleep deprivation follows prolonged ↑ neuronal activity (↑ Glu release)→ ↑ Astrocyte uptake and binding of Glu → ↑ mGluR → ↑ Gp/ PLC / IP3 → ↑Ca++ → ↑ ATP/Adenosine release → ATP → hydrolyzed → Adenosine → ↑ adenosine accumulation→ ↑ A1 receptor activity → ↑ sleep pressure →↓ neuronal activity → ↓ cognitive function
  40. In the short-term waking → ↑ ATP → ↑ P2R → ↑ Gp/Gs/ion channel→ ↑ neuronal activity
  41. Long-term caffeine → ↑ A1 receptor number → ↓ cognitive function
  42. Sleep → ↓ neuronal activity (↓Glu release)→ ↓ Astrocyte uptake and mGluR binding → ↓ adenosine accumulation→ ↓ A1 receptor activity → ↓ sleep pressure → ↑ cognitive function
  43. acute caffeine → ↓ A1 receptor number → ↑ cognitive function

Jayandra Chiluwal – 6December 2013