Lecture #9

Question 1

what are the two main pathogenic hypothesis for the induction of neurodegeneration?

Answer 1

the amyloid cascade hypothesis and the Tau hyperphsophorylation hypothesis

Question 2

describe the amyloid cascade hypothesis:

Answer 2

it has been demonstrated in animal models and brain tissues from patents that there can be the accumulation of extracellular β amyloid (βA) that forms tangles, derived by not regulating proteolytic cleave of amyloid precursor protein

accumulation is toxic to neurons affecting their survival

Question 3

describe the tau hyperphosphorylation cascade:

Answer 3

Tau is a protein usually acclimated in the axonal shaft, they hyperphosphorylation induces the aggregation of the protein and tis tau deposit inside the cells affects Thea axonal transport and therefore induces neuronal death

Question 4

what is the most famous transgenic mouse model to study AD?

Answer 4

5XFAD → APP and PSEN1 are added to genome 5

Question 5

what are microglia associated with neurodegeneration called?

Answer 5

disease associated microglia (DAMs)

Question 6

are DAMs protective or detrimental for the CNS?

Answer 6

literature is very contradictory:

papers support the idea that microglia help the CNS maintain homeostasis (KO models with Trem2)

other papers say that DAMs and Trem2 are detrimental for the homeostasis of the CNS

Question 7

describe the formation of the CNS in development:

Answer 7

neurons and microglia are derived from neuroepithelium soon after the closure of the neural tube and the thickening of the neuroepithelial radial glial layer

later on we have the generation of the germinal niche, the ventricular zone and from asymmetric cell division of radial glia of neuroepithelial cells we have the generation of all the neurons in the CNS

Question 8

describe the process of astroglial genesis in rodents:

Answer 8

intermediate progenitors of astrocytes are derived from an assymmetric cell division of the neuroepithelial cells, usually they lose their apical connection with the membrane, then they retract the basal connection and they migrate from the germinal niche to the cortical plate were they start to differentiate into astrocyte progenitors

Question 9

what is one way that astrocytes are similar to microglia?

Answer 9

they are self-sustaining: some astrocytes can proliferated during the normal post-natal life and replace dying cells

Question 10

what are the two subdivisions of astrocytes?

Answer 10

star-shaped astrocytes and elongated astrocytes

Question 11

name the two different types of star-shaped astrocyte:

Answer 11

• protoplasmic astrocytes: typically accumulating in gray matter in the cortical plate where we have neurons
• fibrous astrocytes: in the white matter

Question 12

where are both types of elongated astrocytes found?

Answer 12

muller cells in the retina and Bergmann glia in the cerebellum

Question 13

what happens after the elongation of the axonal shaft?

Answer 13

formation of myelin that is fundamental for the saltatory conduction of the impulses

Question 14

which portions of the CNS makes up the white and grey matter?

Answer 14

myelin is white while in the regions where we have the cell body of neurons the color is grey

Question 15

what is the territory of an astrocyte called?

Answer 15

domain - a region where there is the cell body and a cytoplasmic arborization

Question 16

what is used to track astrocytes in the CNS?

Answer 16

GFAP (glial fibrillary acid protein) → is not an absolute marker of astrocyte because it is usually expressed by specific subsets of astrocytes or in general by reactive astrocytes

normal antibody and immunohistochemistry are used or S100β

Question 17

describe the ratio of astrocytes to neurons in humans, rodents, and worms:

Answer 17

worms: 6:1
rodents: 3:1
humans: 1:1

suggests that the role of astrocytes increases in importance with the sophistication of the CNS

Question 18

describe the connection difference in astrocytes between mice and humans:

Answer 18

a single protoplasmic astrocyte in rodents can connect to around 48 cytoplasmic bundles and can cover up to 20,000 synapses

in humans a single astrocyte can cover 2 million synapses

Question 19

during development, what are astrocytes essential for developing?

Answer 19

the BBB

Question 20

what is the membrane potential for neurons?

Answer 20

-90 mV

Question 21

what is the rule the governs the voltage and current in astrocytes?

Answer 21

Ohm’s rule: V=RI

Question 22

what is the rule the governs the voltage and current in astrocytes?

Answer 22

Ohm’s rule: V=RI

Question 22

what is the rule the governs the voltage and current in astrocytes?

Answer 22

Ohm’s rule: V=RI

Question 22

what is the rule the governs the voltage and current in astrocytes?

Answer 22

Ohm’s rule: V=RI

Question 22

what is the rule the governs the voltage and current in astrocytes?

Answer 22

Ohm's rule: V=RI

Question 23

why are astrocytes considered Ohmic cells?

Answer 23

they don't have voltage gated channels opening at specific cell membrane potentials → they respond to the variation of cell membrane with the simple Ohm rule they are not involved in the synaptic / electrochemical transmission of signals

Question 24

describe the IV curve in neurons vs astrocytes:

Answer 24

a line in astrocytes, and a curve in neurons

Question 25

what are astrocytes connected by, and what does this allow for?

Answer 25

gap junctions → allows for the diffusion of small molecules important for spreading information from one astrocyte to another

Question 26

how are astrocytes similar to microglia in terms of spatial orientation?

Answer 26

they never overlap

Question 27

how do astrocytes enter into contact with vessels?

Answer 27

cytoplasmic hands and feet, that is fundamental for the blood brain barrier → enter in contact with each other through gap junctions that are at the end of the cytoplasmic bundle, but in physiological conditions they never overlap

Question 28

what are the 5 key functions of astrocytes?

Answer 28

1. regulate the molecular homeostasis of molecules and ions 2. regulate the pH of the extracellular space and the concentration of glucose 3. regulate neurogenesis 4. regulate the water content in the brain and the BBB 5. regulation of neurotransmitters in the cleft

Question 29

describe the tripartite synapse:

Answer 29

astrocytes collaborate with the system wrapping the individual synapses make up of the presynaptic terminal, the postsynaptic terminal, and the wrapping astrocytes

Question 30

what are astrocytes able to release into the extracellular space?

Answer 30

a small amount of soluble signal → TNF-⍺ (classical Proinflammatory cytokine involved in the inflammation of neurons) in front of neurons in order to regulate the distribution and total amount of glutamate receptor in the postsynaptic terminal

Question 31

how are astrocytes involved in synaptic communication?

Answer 31

they are able to quickly remove the neurotransmitter released by the presynaptic terminal in the cleft to avoid new stimulation

Question 32

what other function do astrocytes perform besides wrapping the synapses?

Answer 32

they also seal the synapses → preventing the neurotransmitter in the cleft from exiting by simple diffusion and entering the extracellular space where there is another synapse

Question 33

what can happen if potassium accumulates outside the cell?

Answer 33

too much potassium can reduce the membrane potential of neurons, reducing the distance between the resting membrane potential and the critical threshold, meaning that a subtle stimulus from a synapse could be enough to fire the neuron

Question 34

how do astrocytes help to remove excess potassium?

Answer 34

use specific side channels that require energy to operate and in a very efficient way the potassium that is accumulating here can be removed

Question 35

what do astrocytes need to function?

Answer 35

ATP

Question 36

what neurotransmitter is used by man synapses, and what happens if there is too much?

Answer 36

glutamate - we don't want the spillover of glutamate because it can activate other synapses we do not want to be activated → excitotoxicity astrocytes sealing the synapses to prevent the contamination by non-synaptically related glutamate allows for the faster removal of glutamate from the cleft causing a rapid replenishment of glutamate in neurons

Question 37

in what case is glutamate not toxic in excess?

Answer 37

if we have an enzyme that transforms glutamate into glutamine

Question 38

describe the glutamate - glutamine shuttle:

Answer 38

astrocytes can capture glutamate, transform it into something nontoxic (glutamine) release this molecule into the extracellular space allowing the neurons to capture the glutamine, transform it back to glutamate that is available for a second round of transmission

Question 39

how do astrocytes make energy with glucose?

Answer 39

the can capture glucose in the circulation and through glycolysis convert it into pyruvate and lactic acid

Question 40

how do astrocytes release the lactic acid created?

Answer 40

through the transporter MCT-2 into the extracellular space MCT-1 is also expressed which allows lactate to cross from the extracellular space inside to the presynaptic terminal and use the Krebbs cycle to convert lactate into ATP

Question 41

how can glucose be captured by neurons?

Answer 41

GLUT1 and GLUT3

Question 42

describe the lactate shuttle and glucose homeostasis:

Answer 42

increased neuronal activity means increased energy requirement → a huge amount of glutamate is released and astrocytes start to capture it and convert it to glutamate they need to acquire a huge amount of glucose and therefore they need to produce a huge amount of lactate → lactate is moving from astrocytes to neurons and therefore we have a perfect cycle

Question 43

what is the "plan B" for astrocytes when glucose is not available?

Answer 43

astrocytes can provide glycogen from its own energetic reserve

Question 44

what are the function of BMECs? (brain microvessle endothelial cells)

Answer 44

connected by tight junctions and help avoid the spillover of molecules from the blood circulation

Question 45

what cells are around the BMECs, and what is their function?

Answer 45

pericytes - play an important role in the regulation of the BBB homeostasis, integrity, macrophage activity, and modulate blood flow

Question 46

what are the function of perivascular macrophages?

Answer 46

like microglia - important in inflammatory disorders because they seem be the first type of activation of neuron in disorders

Question 47

how is BMEC metabolism important?

Answer 47

they can destroy toxic molecules and prevent molecules from moving across the blood, as well as important to keep neurons functionally separate from the rest of the body

Question 48

how do astrocytes regulate blood circulation?

Answer 48

they can induce vasodilation and vasoconstriction

Question 49

how is vasodilation induced?

Answer 49

when glutamate enters the neuron, we also have the accumulation of calcium in the region outside of the presynaptic terminal (too much calcium = over activation of the system) in response to the calcium arachidonic acid is related and can be converted into prostaglandins by Cox-1, inducing vasodilation

Question 50

how is vasoconstriction induced?

Answer 50

under the same conditions as before, arachidonic acid can be converted into 2-HETE which is hydroxyeicosatetraenois acid, which interacts with P450 of BMEC which induces vasodilation

Question 51

what types of channels do astrocytes express on their hands and feet, and which is the most famous?

Answer 51

aquaporin channels - aquaporin 4

Question 52

what do the acquaporinis allow the astrocytes to do?

Answer 52

absorb water in order to shape the size of the extracellular space to modulate the size of the synapse

Question 53

describe the activation of astrocytes:

Answer 53

they can be activated along a gradient

Question 54

what is the first and easiest thing to look for in astrocyte activation?

Answer 54

GFAP expression when we go into regions not far from the tissue damage, these cells up regulate GFAP and start to become hypertrophic → by measuring the size of the astrocytes we can see that the cell bodies start to increase

Question 55

as astrocytes become hypertrophic, what do they form?

Answer 55

they start to proliferate and overlap producing a glial scar

Question 56

how is the glial scar detrimental?

Answer 56

there is no space to repair or restore neuronal functionality

Question 57

what is the goal of the glial scar?

Answer 57

to confine the damage

Question 58

what is the classical marker of astrocytes that have gone into senescence?

Answer 58

β-GAL like activity → neurons and microglia can sense this factors and therefore astrocytes can contribute to a general inflammatory condition of the aging brain