Lecture 14: Astrocytes

Question 1

Astrocyte features

Answer 1

“Star” shaped, long branched processes
Most numerous glia in grey matter
Non-excitable support cells, but able to communicate - able to communicate with other astrocytes and critically with neutrons
Discovered around 1860 and they are now only being appreciated for the multiple functional roles
brain is 2% of the body’s weight but consumes 25% of the bodies energy utilisation so to help fuel the brain. astrocytes supplies the trophic support to neurons and other brain cells

Question 2

The many functions of astrocytes

Answer 2

1 - modulation of communication
2 - nutrient transport from the blood to neuron - cerebral blood flow
3 - supporting myelin coverage of neurons
4 - physical structuring of the brain - contributes to the scaffolding of the brain
5 - glial scare - repair/scar

Question 3

What is critical to astrocyte function?

Answer 3

morphology of astrocytes is critical to its function

size, structure, length of processes, end feet

relatively large cell with these important and extensive processes which can communicate with neurons, the vasculature, the interstitial volume and the bordering astrocytes as well and they communicate with these other cells with gap junctions that they have in their end feet

Question 4

Astrocytes visually/structurally distinguished by

Answer 4

Cytoplasm
́well developed cytoskeleton
́dominated by intermediate filaments (glial fibrillary acidic protein (GFAP) - most commonly used astrocytic marker, major intermediate filament proteins and they are depositing within the cytoplasm, they are important in astrocytic structure, integrity and aiding in its movement and shape change
or S100 = calcium binding protein ß (S100ß) - it is a calcium binding peptide in the cytoplasm, it is abundant in astrocytes cytoplasm and nuclei and it is involved in cell cycle regulation and cytoskeletal modification as well
́also microtubules and actin/mitochondria
́Many gap junctions - important for communication

́distal endfeet - lots of gap junctions which can dilate on the vasculature
́Dilated on BVs

Question 5

GFAP

Answer 5

stains astrocytes

dominated by intermediate filaments (glial fibrillary acidic protein (GFAP) - most commonly used astrocytic marker, major intermediate filament proteins and they are depositing within the cytoplasm, they are important in astrocytic structure, integrity and aiding in its movement and shape change

stained only one layer of the cortex in this example

Question 6

S100B

Answer 6

stains astrocytes

S100 = calcium binding protein ß (S100ß) - it is a calcium binding peptide in the cytoplasm, it is abundant in astrocytes cytoplasm and nuclei and it is involved in cell cycle regulation and cytoskeletal modification as well

stained across all layers in example

Question 7

staining with GFAP and S100B shows

Answer 7

highlights the heterogeneity of astrocytes

potentially in up to 9 subtypes of astrocytes

Question 8

astrocyte types list

Answer 8

fibrous (white matter)

protoplasmic (grey matter)

Question 9

Fibrous astrocytes in what matter

Answer 9

white matter

Question 10

Protoplasmic astrocytes in what matter

Answer 10

grey matter

Question 11

Fibrous astrocytes

Answer 11

white matter
radial glia - radially arranged in white matter and more specialised forms
large sparse processes (spreads quite far)

Question 12

Protoplasmic astrocytes

Answer 12

grey matter
majority of astrocytes
envelops synapses - short highly branched processes

many short or long processes - some branching distally, dilated end feet
form/shape specific to location and to function

Question 13

Functions of both fibrous and protoplasmic astrocytes

Answer 13

metabolic support, mechanical support, blood brain barrier, response to injury

Question 14

Astrocytes have exclusive territories

Answer 14

Each astrocyte covers a specific territory that interfaces with the microvasculature and that might include thousands of synapses

Discrete region of interaction of the fine terminal processes (yellow)

Question 15

Physical structuring of the brain and astrocytes …

Answer 15

astrocytes have exclusive territories
astrocytes have ordered arrangements with minimal overlap
astrocytes are dynamic

Question 16

Astrocytes have ordered arrangements with

Answer 16

minimum overlap (in a scaffold type arrangment) 
many gap junctions 
we know that these astrocytes would touch many epithelial cells, arteries, neurons, other glial cells etc - in the hippocampus an astrocyte can synapse up to 100000 times therefore have a wide range of contacts 
has the ordered arrangement in the brain which contributes to the physical structure of the brain

Question 17

How can astrocytes pick up information and then pass it on

Answer 17

communicate via Ca2+ waves
Ca2+ binding protein - store and release Ca2+
Because of their ordered arrangement they actually have a astroglial network - can propagated calcium waves over long distances in response to stimulation and similar to neurons they can release gliotransmitters in a calcium dependent manner

Question 18

Gap junctions associated with astrocytes list

Answer 18

hemichannel
intercellular gap junction
reflexive gap junction

Question 19

Hemichannel gap junction

Answer 19

gap junction to release into extracellular space

Question 20

Intercellular gap junction

Answer 20

two astrocytes communicating to each other via a gap junction

Question 21

Reflexive gap junction

Answer 21

gap junction onto itself

Question 22

Gap junction made up of proteins called

Answer 22

connexins

Question 23

Astrocytes are dynamic in response to

Answer 23

stimulation and environment, similar to microglia in this way

astrocytic processes show spontaneous morphological changes in a matter of minutes

Question 24

lamellipodia vs filopodia

Answer 24

Lamellipodia are based upon a thin sheet-like branched network of actin filaments, whereas filopodia are highly organised and tightly cross-linked long bundles of unidirectional and parallel actin

Question 25

astrocytes listen and talk to

Answer 25

synapses (modulation of communication)

Question 26

Synapses are enveloped by the

Answer 26

astrocytes which means that they can respond quickly to information that has come from the neuron

Question 27

Astrocytes are intimately associated with ...

Answer 27

synapses | therefore astroctyres have a function of the modulation of synapses

Question 28

astrocytes and the modulation of communication

Answer 28

astrocytes are intimately associated with synapses tripartite synapses glutamate uptake and recycling astrocytes control CNS ionic homeostasis gliotransmitters Synchronisation of neurons by astrocytic glutamate release

Question 29

tripartite synapses

Answer 29

modulation of communication presynaptic, postsynaptic, astrocyte Close morphological relationship between astrocytes and synapses as well as the functional expression of the relevant receptors, we have the tripartite synapse and according to this concept the synapse can be divided into three equally important parts presynaptic terminal, postsynaptic neural membrane and the surrounding astrocyte Presynaptic terminal releases neurotransmitters and these go to the posts synaptic terminal and also the perisynaptic astroglial membrane as well and this leads to the generation of a post synaptic potential in the neuron but also a calcium signal in the astrocytes which can trigger the release of gliotransmitters from the astrocytes which can then in turn signal back on to the presynaptic and postsynaptic neuron membranes as well not all synapses are tripartite but a lot of them are, it is a critical communication network in the brain

Question 30

modulation of communication - glutamate uptake and recycling

Answer 30

Neurons depend on astrocytes for glutamine for synthesis of glutamate, GABA and aspartate Main function of astrocytes at the synapse is the uptake of glutamate and recycling it, neurons cannot make their own amino acid neurotransmitters like glutamate (excitatory), or GABA (inhibitory) or Aspartate, so what astrocytes can do it\s that they can take up the glutamate from the synaptic cleft once it is released from the presynaptic neuron and majority (approx 80%) is taken up by the astrocyte and they then convert the glutamate to glutamine and it is the glutamine that recycled to the presynaptic neuron from which they can make new glutamate, gaba or aspartate 20% goes to the post synaptic neuron It is important for maintaining the low resting glutamate concentration at the synapse so that you can have continued neurotransmission and brain function GLAST and Glt1 are transporters found in the astrocytes

Question 31

Glutamate transporters found in astrocytes ...

Answer 31

``` GLAST = EAAT1 Glt1 = EAAT2 ```

Question 32

EAAT1 also called

Answer 32

GLAST

Question 33

EAAT2 also called

Answer 33

Glt1

Question 34

percentage of glutamate taken up by astrocytes

Answer 34

80%

Question 35

percentage of glutamate taken up by the post-synaptic neuron

Answer 35

20%

Question 36

Astrocytes control CNS ionic homeostasis ...

Answer 36

Astrocytes have many of the same sensitive ion channels and receptors of neurons but they lack the properties to fire action potentials but however these ion channels and membrane ransporters allow the glia to indirectly sense the level of neural activity at the synapse by monitoring the changes in the chemical environment shared by these cell types astrocytes actively buffer potassium ions have all the proteins a neighbouring neuron has voltage gated channels + neurotransmitter receptors

Question 37

Astrocytes 'talk' to synapses evidence ...

Answer 37

synaptic-like micro-vesicles (SLMVs)- not as many vesicles as you find in the presynaptic terminal, from here release gliotransmitters ́release gliotransmitters via exocytosis - Ca2+ release from endoplasmic reticulum - calcium releases from the ER induces the gliotransmitters release via exocytosis ́have ion channels and receptors in membrane Allows astrocytes to act on neurons and vessels and other glial cells

Question 38

Gliotransmitters released from glia ..

Answer 38

Gliotransmitters facilitate neuronal communication between neurons and other glial cells and are usually induced by Ca2+ signaling from endoplasmic reticulum glutamate, ATP, adenosine, cytokines, D-serine, eicosanoids, proteins and peptides, chemokine (attract cells to their territories, repair)

Question 39

Astrocytes can modulate neuronal activity - glutamate

Answer 39

Glutamate release presynaptically ́activates ionotropic glutamate receptors - in the postsynaptic neuron ́activates metabotropic receptors – astrocyte following the activation of these receptors we have increase in gliotransmitters such as glutamate being released from the astrocyte to the postsynaptic neuron

Question 40

Astrocytes can modulate neuronal activity - gliotransmitter

Answer 40

́Gliotransmitter release from astrocyte to post-synaptic neuron ́activates extrasynaptic NR2B-containing NMDA receptors to trigger slow inward currents (SICs) – this can increase the local neuronal excitability at the synapse ́changes membrane potential

Question 41

Tripartite synapse components are acting ...

Answer 41

with and on each other in response to one another

Question 42

gap junctions and astrocytes

Answer 42

made of connexins, different types | allow communication between the neuron and other astrocytes for example

Question 43

steps for tripartite communication

Answer 43

1 - release of glutamate - from presynaptic terminal 2 - glutamate taken up by astrocyte - glutamate as well as binding to the postsynaptic terminal is going to also be taken up by the astrocyte and then it is converted to glutamine 3 - convert to glutamine - which can then be recycled back into the presynaptic terminal 4- the release of gliotransmitter - this binding can also cause the release of gliotransmitters which can act on the postsynaptic terminal 5 - glutamine diffuses through gap junctions of astrocytic networks - Glutamine can also be taken up by neighbouring astrocytes and glutamine can diffuse and permeate through the gap junctions of the astrocytic network and then this trafficking may result in the release of gliotransmitters at a distant synapse and therefore affecting the activity of the underlying neural network (one synapse and astrocyte can transmit information to other parts of the neuronal network) 6 - release of gliotransmitter at distant synapses 7- or extracellular release signals propagated through gap junctions and extracellular ATP = astrocytes can use ATP as a signal which can induce signals in other astrocytes can have the release of gliotransmitters at the neuronal-glial synapse or you can have astrocyte propagation as a result of the tripartite synapse which can affect the post synaptic neuron

Question 44

How far can the astrocyte influence spread? Can something happening at synapse one affect other distant synapses

Answer 44

1 - “Astrocytes are ideally placed to mediate both local and wide-spread heterosynaptic effects on synaptic plasticity as a single astrocyte likely ensheaths multiple neuronal somata, hundreds of dendrites [53] and thousands of individual synapses ” 2 - Absolutely because we have the astrocytic neural network communicating with other synapses as well so these astrocytes can provide very long range communication and therefore this has influence over synaptic p[lasticity as well

Question 45

Synchronisation of neurons by astrocytic ...

Answer 45

glutamate release in which the release of glutamate from one single astrocyte onto several dendrites on different neurons can lead to synchronisation of different neurons

Question 46

Synchronisation of neurons by astrocytic glutamate release

Answer 46

Astrocyte (green) contacts a number of neurons across 100μm Results in neuronal synchronization of these neurons Known as a neuronal domain Domains of neighbouring astrocytes do not overlap* so it shows you that you have this synchronisation of particular neurons under the communication of astrocytes as well so the neural network is critical to the neuron synchrony that we see in the brain

Question 47

SBEM =

Answer 47

serial block-face imaging

Question 48

serial block-face imaging

Answer 48

type of electron microscope Way of retrieving high resolution, 3D images of a sample New advanced method of using TEM Images it then cuts it and images agai etc, can cut through slices and space images apart as small as 15 nanometers and then all of the slices are complied into a 3D image so you get construction of 3D volume future of TEM, helps you understand the connection between these cells at a very basic level and the information we get here is going to be vital to our understanding of function as well

Question 49

Cerebral blood flow, nutrient transport from blood to neuron and astrocytes

Answer 49

Astrocytes get energy by controlling blood flow

Question 50

Astrocytes get energy by

Answer 50

controlling blood flow - have food processes that are sitting on the blood vessels which is critical to their control of blood flow as they are critical to the vasodilation and vasoconstriction control of the blood vessel gap junctions between astrocytes between end feet at blood vessel

Question 51

Increase in brain activity =

Answer 51

increase in blood flow activity to that area

Question 52

how do we detect increase in brain activity

Answer 52

functional MRI (fMRI)

Question 53

How does the vascular system know that the nearby neurons need more glucose/energy

Answer 53

Dependent on neuron activity because astrocytes are very responsive to neuron activity and transmission and it is their responsiveness/ reaction to that that increase blood flow to that area as well

Question 54

Neurons separated from blood vessels by

Answer 54

cytoplasm of astrocytes

Question 55

Neural activity leads to

Answer 55

vasodilation - oxygen delivery which is shown through PET and fMRI

Question 56

Cerebral blood flow is coordinated by astrocytes...

Answer 56

glutamate receptors - glutamate binds and causes Ca2+ wave that goes to the end feet and then vasodilators are released from the end feet of the astrocytes calcium transient to end feet release of vasodilators

Question 57

Summary of cerebral blood flow and astrocytes

Answer 57

́ Astrocytes ensheath synaptic contacts ́ Astrocytic processes have glutamate receptors ́ Same astrocyte or astrocyte network - endfeet on blood vessel ́Trigger/stimulus ́Transient local increase in intracellular Ca2+ ́Ca2+ propagated to astrocytic endfeet via endoplasmic reticulum ́ Endfeet at BVs – high connexin (Cx) expression ́ Astrocytes release vasoactive metabolites Ready to receive the signal and the glutamate, not only is glutamate recycled but it some astrocytes or astrocyte netowroks have their end feet on the blood vessels and so this signal from the glutamate will cause a transient local increase in intracellular calcium concentration which will be propagated to the astrocytic end feet through the ER and then at the end feet there is high Connexin expression and therefore gap junctions and the astrocytes can release vasoactive metabolites to alter the blood flow at the very local level

Question 58

Astrocyte- neuron lactate shuttle

Answer 58

Astrocytes are critical to bringing glucose to neurons as well so when the glucose demand exceeds supply, astrocytes can convert their glycogen stores into glucose and lactate which can then spread through the gap junctions to fuel distant neurons to be used as energy, can be distant or near Glucose can come directly from the capillaries with the opening by the astrocytes and be taken up by glucose transport but glucose can also be imported (described above)

Question 59

What is the involvement of astrocytes in delivering energy to neurons?

Answer 59

Glucose is coming through the glucose transporters into the astrocytes and is being converted to pyruvate which is then being converted to lactate and then through the MCTs (which we also saw in the oligodendrocytes) I.e. monocarboxylate transporters which are the extracellular membrane channels can transport the lactate into the neurons as well and this then becomes an energy source for neurons This is really dependent on the metabolic activity of the neuron because it is dependent on the glutamate activating the intracellular signalling network in the astrocyte because one glutamate directly correlates with the transportation of one glucose molecule

Question 60

What do astrocytes have to deliver energy to neurons?

Answer 60

MCT4 and gap junctions

Question 61

Oligodendrocytes and astrocytes contribute to neuronal energy supplies...

Answer 61

Oligodendrocytes also provide lactate as an energy source through MCT channels (MCT1 in the myelin membrane and MCT2 in the axon membrane then lactate can be transferred into energy) can also see that astrocytes support oligodendrocytes because they can ferry glucose, enter glycolysis that can form ATP

Question 62

Glucose and lactate fluxes between blood vessels, astrocytes, oligodendrocytes and neurons

Answer 62

Oligodendrocytes and also astrocytes can take up glucose or lactate from the blood and glucose can be directly taken up by neurons as well of course Into the astrocytes and oligodendrocyte…. Glucose comes in through the glucose transporters and the lactate comes in through the MCT channels, the glucose can be converted into puruvate and then lactate and then can be directly sent to the neuron via the MCT channels and can also be directed to the myelin sheath to support the axon and also the energy levels as well at the axon

Question 63

Astrocyte dysfunction in AD

Answer 63

1. Reactive astrocytes become Aβ producers. - produce even more amyloid beta than we would expect because of the morphological and pathological changes in the brain 2. Aβ increases Ca2+ signaling in astrocytes. - have tight synchronisation of astrocytes in the network and now you are going to be having some misfiring and missignalling in the strictures as well 3. Aβ inhibits astrocyte glutamate uptake - so increased concentration of glutamate present in the synaptic cleft and also will actually inhibit the amount of glutamate available then from your transmutation as well 4. Glutamate excitotoxicity increases Ca2+ signaling in neurons, leading to inhibition of LTP and memory loss - glutamate excitotoxicity because more glutamate around which will affect the signalling of neurons leading to the inhibition of long term potentiation and memory loss 5. Impaired astrocytic glucose and reduced lactate release- more amyloid beta around and because of astrocytes being affected, the astrocytes therefore have impaired release of glucose within the astrocytes which reduces the excitatory release 6. Increased spontaneous Ca2+ signaling induces abnormal vascular responses in blood vessels - not going to get as much glucose or lactate entering the cell as a result