Oligodendrocytes

Question 1

What is the consequence of an incorrect ratio of oligodendrocytes to neurones during development?

Answer 1

Dysmyelination during development usually leads to mental retardation and/or death (leukodystrophies/leukoencephalopathies).

Question 2

What axons would benefit from myelination?

Answer 2

All, except those under 1 micron in diameter.

Question 3

Why is myelination important?

Answer 3

Myelination is important for:
• Maintaining speed of conduction
• Electrical insulation, and thus making it more energy efficient
• Allows diameters of axons to be much smaller (space saving). Without myelin, our spinal chord would be the diameter of a log.

Question 4

What are the steps in oligodendrocyte development?

Answer 4

The steps in the development of oligodendrocytes are well defined, requiring a series of highly orchestrated events:
Proliferation → Migration → Differentiation → Axon contact → Myelination

Question 5

What molecular marker is used to identify oligodendrocyte progenitors?

Answer 5

Perinatal, late and adult progenitors are PDGFaR+.

Platelet Derived Growth Factor Alpha-Receptor.

Question 6

Where are oligodendrocyte progenitor cells generated in the spinal cord?

Answer 6

In the sub ventricular zone cells of the brain and spinal cord, in the pMN domain around E12.5.

However, oligodendrocytes could have potentially arisen from another area. Nkx6 NO leads to oligodendrogenesis from the dorsal regions.

Question 7

Describe the experiment that shows oligodendrocyte induction by ventralising signals

Answer 7

A. During normal spinal cord development, PDGF R+ cell [PDGF receptor- positive cell - oligodendrocyte precursor cells] foci appear in a specific region of the ventral ventricular zone following motoneurone differentiation.

B. When caudal regions of the Danforth short-tail mutant mouse (lacking a notochord and floor plate) → motoneurones and oligodendrocyte precursors fail to develop

C.Grafting ectopic notochord (at the top) induces ectopic floor plate, motoneurones and oligodendrocyte precursors.

Question 8

Describe the main spinal cord patterning during development

Answer 8

Patterning occurs when morphogens act on cells, producing gradients. It so happens that there are only two main morphogens that guide the development of the spinal chord:
• Sonic Hedgehog Protein (SHH) produced by floor plate, ventral to dorsal gradient
• Bone Morphogenic Protein (BMP) produced by roof plate, dorsal to ventral gradient

As these migrate towards each other, they form patterns of different cell zones [see right]. Different neuronal populations arise as a result of the combinatorial effects of several transcription factors and morphogen concentrations.

Question 9

What set of genes are affected by spinal cord patterning?

Answer 9

Expression of two sets of genes/transcription factors occur based on position i.e. SHH concentration gradient.
Class I genes are expressed predominantly in the dorsal half, while Class II genes are expressed mainly ventrally. It is the overlap of these various genes (transcription factors) that determine which cell types are generated.

Question 10

What transcription factor combination leads to motor neurone development?

Answer 10

low Pax6, high Olig2

Question 11

How are motor neurone and oligodendcyte development linked?

Answer 11

Motor neurones are produced first in their particular domains (pMN) of the spinal cord, and at some point this ceases (E4.0 in chicks). Oligodendrocytes are produced shortly after, in the same site.

Question 12

What causes the switch in production from motor neurone cells to oligodendrocytes?

Answer 12

A number of genes called proneural genes, are important in neuronal generation: neurogenin 1 & 2 (Ngn1&2). These genes are downregulated in ventral neuroepithelium of neural tube, once motor neurone production ceases. Once Ngn is downregulated, glial cell production occurs in domains expressing the Olig2 gene.

At the time neurogenins start to disappear, Nkx2.2 co-expression aids in the development in oligodendrocytes.

Question 13

What genes are vital in oligodendrocyte development and what roles do they play?

Answer 13

Olig1 has a role in oligodendrocyte maturation. Olig1 NO causes uncoupling of myelogenesis and axonal recognition.

Olig2 is necessary for oligodendrocytes. Olig2 NO mice entirely lack oligodendrocyte lineage cells in the spinal cord (but not brain).

Olig1+2 NO mice have no oligodendrocytes anywhere in the CNS.

Question 14

What transcription factors regulate Olig1 and 2 expression?

Answer 14

Olig1 & 2 expression is regulated indirectly by Shh, via Nkx 6.1 and 6.2.

Question 15

Where are oligodendrocyte progenitor cells generated in the forebrain?

Answer 15

However, in the brain, there are three key domains (multiple origins):
• Ventral Domain [red] – cells expressing Nkx-2.1
• Medial Domain [blue] – cells expressing Gsh2
• Dorsal Domain [green] –cells expressing Emx1

(Interestingly, this domain also gives rise to neurons projecting to cerebral cortex)

Question 16

Describe how oligodendrocyte generation in the forebrain occurs in waves.

Answer 16

• Nkx2.1-Cre mice showed Nkx-2.1 (ventral most precursors) were produced around E12.5 from the medial ganglionic eminence. These cells populate the entire cortex by E18.
• These cells are joined by a second wave at E16-18 derived from the lateral ganglionic eminence (Gsh2 expressing cells) while ventral contribution (from cells expressing Nkx-2.1) decreases.
• After E18, third wave (cells developing dorsally) are evident

• Mixed colour (green and red), suggest that the cells arise from dorsal and ventral regions (mixed picture)

Interestingly, the cells derived during wave 1 disappear after birth, and are gradually eliminated from all other parts of the brain (i.e. they are developed, and then are taken over by other cells). All oligodendrocyte populations appear to function in the same way. Ablation of one population leads to replacement by another with no functional consequences.

Question 17

How do oligodendrocyte progenitors migrate to their final site?

Answer 17

Progenitors need to migrate to where they need to be. Migration of oligodendrocyte precursor cells can occur over large distances, e.g. optic nerve (large percentage of the optic nerve and optic tracts are myelinated).
• Oligodendrocytes are derived from around the chiasm and have to migrate a fairly large distance.

As any cell develops, it needs two types of influences. These are intrinsic and extrinsic signals.

Question 18

What extrinsic signals affect oligodendrocyte migration, survival, proliferation and myelin formation?

Answer 18

• Platelet derived growth factor (PDGF) promotes migration, survival and proliferation
• Fibroblast growth factor 2 (FGF2) also promotes (slight migration/shuffling) & proliferation, however, it inhibits differentiation
• Neuregulin b1 (NRGβ1), produced by neurones and axons promote proliferation, target dep survival & myelin formation.

Question 19

At what points do what signals affect oligodendrocyte maturation?

Answer 19

At the perinatal progenitor stage, the cells are responsive to the mitogen [mit] PDGF, which expands the population to migrate away. Perinatal progenitors are highly motile.

They then become unresponsive to PDGF, and become responsive to the FGF (late progenitor phase). FGF causes these cells to divide. At this stage with multiple processes, the cells continue to divide but not migrate (population continues to divide in a localised area and push each other away).

Question 20

What determines the final number of oligodendrocyte cells?

Answer 20

While increasing the levels of PDGFa from normal levels (in wildtype (wt) mice) to that of transgenic (tg) and double-transgenic (tg/tg) mice results in increased oligodendrocyte progenitor cells.

However, the number of mature oligodendrocytes in all three cases remain the same (irrespective of original factor of PDGFa).This suggests that the final number of oligodendrocytes is determined by factors other than PDGF.
• Axonal contact is the most likely factor determining survival of oligodendrocytes
• Oligodendrocyte progenitor numbers eventually default to a “correct” number irrespective of the number of progenitors that develop - and it is the number of oligodendrocytes that determine the level of myelination occurring
• Apoptosis therefore plays a key role in restoring the default value of oligodendrocytes.

This is a key principle in neurodevelopment. We overproduce cells, synapses, progenitors, before there is a culling of these to their final numbers.

Question 21

What key factors lead to oligodendrocyte proliferative arrest and differentiation?

Answer 21

In order to get oligodendrocyte precursors to a stage where they can produce myelin, proliferative arrest needs to occur. However cell cycle exit is not sufficient to induce differentiation. It is the bioavailability of transcription activators and a decrease in inhibitors which lead to differentiation.
• Key Activators: Sox10, Olig1/2, HdAc, Mash1
• Key Inhibitors: Sox5, Sox6, Hes5, Id4, Tcf4

Key transcription factor Yingyang1 (YY1) is a key regulator of oligodendrocyte differentiation. YY1 does not directly act on myelin production, but indirectly represses the inhibitors of the myelin genes. This leads to a stimulation of myelination. Experiments by He et al 2007 ablated YY1 gene, leading to defective myelination.

Question 22

What protein is a key nuclear protein allowing terminal differentiation in oligodendrocytes?

Answer 22

Myelin gene regulatory factor (MRF) is a key nuclear protein that is expressed specifically in post-mitotic oligodendrocytes - Emery et al 2009.

Question 23

What is a factor that aids in the process of oligodendrocyte contact and survival?

Answer 23

One of the factors which aid this process is Neural Cell Adhesion Molecule (NCAM), which stimulates oligodendrocyte survival.

Question 24

What mechanism leads to axonal myelination?

Answer 24

1) Target innervation and electrical activity in axon cause the release of ATP
2) ATP stimulates astrocytes to produce and secrete LIF (leukaemia inhibitory factor) that stimulate oligodendrocytes to produce myelin
3) Axons can directly stimulate oligodendrocytes through cell adhesion molecules (NCAM, L1)
4) Inhibitory molecules are Downregulated (Notch, PSA- NCAM, Lingo-1)
5) Multiple axo-glial signals result in ensheathment

Question 25

Describe myeline ensheathment by oligodendrocytes

Answer 25

The oligodendrocyte process first contacts the axon. The leading process tucks under and extends around
the axon in multiple wraps.
• Cytoplasm filled areas are inner and outer loops
• Unlike Schwann cells, oligodendrocytes do not wrap their cell bodies around the axon, but multiple processes ensheath several axons in the CNS

Myelin grows in 2 directions:
• Obviously one is to keep wrapping around (can make up to 50 layers)
• But also grows outwards laterally

Question 26

What protein aids in the adhesion of oligodendrocyte membrane around the axons?

Answer 26

myelin basic protein (MBP) causes the adhesion of the membranes, collapsing the intracellular space.

Question 27

When do OPCs proliferate until?

Answer 27

OPCs continue to proliferate and generate myelinating OL well into adulthood