Lecture 28- Glial development I

Question 1

What are glia?

Answer 1

• Essentially a catch-all category for more or less everything in the nervous system excluding neurons • Glia are an extremely diverse set of cells: • Different developmental origins • Different functional roles • Diverse morphologies and profiles of gene/protein expression

Question 2

What is the percentage of glia in the brain of evolutionarily different organisms?

Answer 2

-With increase in size and complexity of nervous system comes a huge increase in the % of the brain made up of glia -nematode: few -drosophila: 25% -mouse: 65% -human: 50-90%

Question 3

What are the types of glia?

Answer 3

1.CNS: • Astrocytes • Oligodendrocytes • Microglia • Radial glia • Ependymal cells 2.PNS: • Schwann cells • Satellite cells • Olfactory ensheathing glia • Enteric glia

Question 4

What are some characteristics of the Schwann cells?

Answer 4

-PNS -surround axons in PNS -are responsible for myelination of peripheral axons -participate in repair process after injury

Question 5

What are some characteristics of Oligodendrocytes?

Answer 5

-CNS -myelinate CNS axons -provide structural framework

Question 6

What are some characteristics of Astrocytes?

Answer 6

-CNS -maintain blood-brain barrier -provide structural support -regulate ion, nutrient and dissolved gas concentrations -absorb and recycle neurotransmitters -form scar tissue after injury -regulate blood flow -regulate synpatogenesis

Question 7

What are some characteristics of Microglia?

Answer 7

-CNS -immune cells of the CNS -remove cell debris, wastes and pathogenes by phagocytosis -present antigens to T cells in disease -participate in synaptic pruning? (maybe) -macrophages of the brain

Question 8

How do oligodendrocytes make the myelin sheaths on axons?

Answer 8

-the oligodendrocytes will extend processes that will wrap around an axon to insulate -all oligodendrocytes are myelinating

Question 9

What are two types of Schwann cells?

Answer 9

-Myelinating (large diameter axons) -Non-myelinating (small diameter axons!Remak bundles)= these provide protection to the axons

Question 10

What can the Schwann cell do when there is an injury?

Answer 10

-Can become phagocytic in presence of injury

Question 11

How do Schwann cells myelinate the PNS axons?

Answer 11

-secrete components of the basil lamina and extracellular matrix

Question 12

What are the most abundant cells of the human CNS?

Answer 12

-astrocytes

Question 13

What are the two types of astrocytes?

Answer 13

-fibrous astrocytes (white matter) -protoplasmic astrocyte (grey matter)

Question 14

What are the roles of of astrocytes?

Answer 14

• Express high levels of glutamate transporters: prevent excessive glutamate build-up around synapses
• Similarly, buffer extracellular K+ to prevent it interfering with propagation of action potentials
• Link synapses with blood vessels to couple neuronal activity with regional blood flow/glucose delivery
• one of the main roles= have glutamate transporters= clean up the neurotransmitter so it doesn’t become toxic
• also clean up K+ -link synapses to blood flow= some of the processes signal to blood vessel to increase blood in the region etc.
• fMRI really looks at the activity of the astrocytes= the blood flow at the receiving end of the astrocytes

Question 15

What is the tripartite synapse?

Answer 15

• Astrocytes also believed to form part of the “tripartite synapse”
• Astrocytes have processes adjacent to synapses • Upon neurotransmitter release astrocytes also activated (as evidenced by increased intracellular Ca2+ levels)
• This in turn can lead to release of neuromodulatory substances such as glutamate, ATP and adenosine
• when synapses release transmitter= astrocytes can respond, can regulate the blood flow an dcan release ATP and adenosine
• modulate synaptic function

Question 16

What was the experiment with injecting human astrocytes into mice?

Answer 16

-took out human astrocytes and injected to mice= they were better in learning, memory than the control mice that had mice astrocytes injected =suggests the astrocytes are important in learning -• Engraftment of human astrocytes into mice has recently been shown to improve their performance in some learning tasks • How much of our cognitive processing is due to glia?

Question 17

What are some more roles for microglia?

Answer 17

• Also first systematically described by Pio del Rio Hortega • Described as “microglia” to distinguish them from “macroglia” (astrocytes) –physiological role= phacoytosis of debri, dead cells etc. -microglia swicth between two states, resting and active (ameboid) state= the one where phagocytosis happens -• Major physiological role is the phagocytosis of cellular debris and pathogens • Are thought to be the antigen presenting cells of the CNS (interact with T-cells in injury) -• Highly reactive to disruptions to normal homeostasis or injury, however:

Question 18

What are the two states of the microglia?

Answer 18

1. Ramified 2. Amoeboid

Question 19

What are the stages in neural cell birth?

Answer 19

• Pulse-chase labelling studies using tritiated thymidine or retrovirus to birth date cells led to the model of sequential specification of neurons, then glia
• seems that there are stages in development when different cells are born
• first neurons hen glia
• neurogenesis, then astrogliogenesis and then oligodendrogenesis

Question 20

How does the shift from specification of neurons to glia occur?

Answer 20

• This shift is due to changes in both extracellular gliogenic signals and the instrinsic properties of the neural stem cells
• the shift is due to the environement of the brain -intrinsic change in the stem cells in the brain, at later stages= only glia born

Question 21

What is the overly simplistic model for the development of specification of neurons vs glia?

Answer 21

-early stages= neurogenic stage, later gliogenic stage

Question 22

What is the current model of the specification of neurons vs glia?

Answer 22

• Current model more complex, however, and needs to take into account a number of different stem cell and progenitor populations
• this is the total model, do not have to remember this in detail
• even early on get glial progenitor cells
• at first neural stem cells giving rise mainly to neurons, later both neurons and intermediate prglial progenitor cells and later shoft to the glial only

Question 23

What is the rest of the lecture about?

Answer 23

-generation of astrocytes and microglia

Question 24

What do astrocytes arise from?

Answer 24

• Astrocytes thought to arise directly from radial glial cells during development
• radial glial cell gives rise to astrocytes

Question 25

Where in the CNS does the specification of astrocytes occur?

Answer 25

Specification of astrocytes occurs in multiple CNS regions (both dorsoventral and rostrocaudal)

-astrocytes are developed by each level of the developing spinal cord

Question 26

When does the specification of astrocytes occur?

Answer 26

The specification of astrocytes overlaps both spatially and temporally with generation of neurons.

Question 27

How can you see the specification of astrocytes from multiple dorso-ventral domains and what does it mean?

Answer 27

-This specification of astrocytes from multiple dorso-ventral domains can be seen using a genetic fate mapping strategy -shows that astrocytes are given rise to by every level of the neural tube -thus it is not the region of the spinal cord that determines if it is an astorcytes or neuron

Question 28

What determines the specification of astrocytes vs a neuronal fate?

Answer 28

• The extrinsic factors that promote astrogliogenesis are poorly defined, but several signalling systems have been identified:
  1. • Delta-Notch pathway
  2. • LIF/CNTF/CT-1 -JAK/STAT pathway
  3. • BMP-SMAD pathway

Question 29

What is the role of delta/notch pathway in determining the astrocyte fate?

Answer 29

-did this pathway already -• Lateral inhibition via the Delta/Notch system partially responsible for setting up a mixed population of neural/astrocyte progenitors -Proneural transcription factors including: Neurogenin,Mash1 and! Olig2, promote Jagged/Delta expression -This in turn activates Notch in adjacent cells -Notch in turn inhibits proneural transcription factors and increases expression of Hes and NFIA (1 in there) to maintain cells as progenitors/astrocytes

Question 30

What does Hes do?

Answer 30

Hes= keep cells in immature state, prevents from becoming a neuron

Question 31

What does NFIA do?

Answer 31

-one of the ways notch makes astrocytes occur is by activating the NFIA • NFIA (induced by Notch) is an important promoter of astrogliogenesis • Usually induced around the time that astrocytes start being generated • siRNA knockdown of NFIA gene in developing chick spinal cord leads to loss of GLAST expression (astrocyte marker) -the experiment showing this: look at the picture!

Question 32

How do the •LIF/CNTF/CT-1- JAK/STAT and BMP-SMAD pathways contribute to astrocyte specification?

Answer 32

• Lif induces JAK/STAT to go inti the nucleus and change gene transcription, turns on astrocyte genes
• BMP binds and activate SMAD that then affects gene transcription as well
• These factors lead to activation of astrocyte gene promoters, which become accessible through Notch
• mediated changes in DNA structure

Question 33

When is it easier for cells to become astrocytes rather than neurons?

Answer 33

• cell become more capable of become astrocytes rather than neurons when the genes for astrocytes are not methylated any longer= so easier to access by the pathways
• this happens later in embryonic development

Question 34

What is the composite model for astrocyte specification?

Answer 34

• Composite model has Notch and JAK/STAT pathways interacting with transcription factors and chromatin state to induce astrocyte specification
• these pathways are active once you have developed neurons
• neurons produce jagged and delta and lif CNTF and CT1 all of which activate the pathways leading to astrocyte specification
• model of lateral inhibiton= the cell fated to be neurons will signal surrounding cells to become astrocytes

Question 35

Are microglia very different from other cells in the nervous system?

Answer 35

-yes very different to the other cells of the nervous system -they look different and are an immune cell type

Question 36

What’s been the burning question about microglia for a while?

Answer 36

• Embryonic origins of microglia hotly debated for a long time • Neuroectodermal origins? • Generated by monocytes migrating into the CNS during development? • Constant exchange with macrophages in the peripheral blood? -these are the theories that were considered for microglial origin -macrophages and microglial are super closely related, hard to tell apart

Question 37

What is the origin of the microglia?

Answer 37

-yolk sac origin

Question 38

How did they discover the origin of the microglia?

Answer 38

• discovered in 2010 using genetic fate mapping techniques in mice -embryonic microglia detectable by CX3CR1 expression from E9.5:
• transgenic mice, looking at where the microglia are coming from
• the mesenchymal cells migrate to the developing brain
• they also put in a dye into cells of the yolk sac and saw if these move into the embryo
• Activation of lacZ reporter expression in yolk sac cells at E7.25 resulted in lacZ+ microglia in the brain at E9.5= proof of the microglial origin
• coming from the developing yolk sac -label some of the cells of the yolk cells blue, they will remain blue
• then see that the embryo has them in so the microglia are likely derived from the yolk sac
• hemopodic stem cells of the yolk sac are the origin!

Question 39

Do people get microglia (macrophages) from the periphery during their lifetime? How was this anwered?

Answer 39

-Using irradiation experiments in newborn mice and parabiosis experiments to replace circulating myeloid cells, Ginhoux and colleagues demonstrated that >95% adult microglia remained host-derived –the experiment= shows that the macrophages from the blood circulation show that they don’t go into the brain from the periphery, instead have a self-replenishing population in the brain -These experiments, along with observations that the microglia were highly proliferative through life, show that microglia are derived from primative yolk sac myeloid progenitors that form a self-replenishing population in the brain

Question 40

Summary?

Answer 40

• Glia are a large proportion of the brain in larger/more complex animals • Oligodendrocytes and Schwann cells: Myelinate/ ensheath axons to increase the speed and efficiency of conduction • Astrocytes: Buffer extracellular neurotransmitter and ion concentrations as well as linking neuronal activity to blood flow • Microglia: The “macrophages of the CNS” • Developmentally, neurons tend to be born prior to glia, though in reality there are multiple overlapping progenitor populations • Shift from neurogenesis to astrogliogenesis occurs: • Due to lateral inhibition from neurons via Delta/Notch • Through astrogliogenic signals including CNTF and the BMPs • These converge to induce expression of pro-astrocyte genes such as NFIA and GFAP • Microglia are myeloid cells that migrate from the yolk sac into the brain during embyronic development (microglia are important in the refinement of synaptic connections too, mop up the unneeded synapses)