N371: Oligos + microglia

Where do microglia come from?

Yolk sac early during embryogenesis (before the BBB closes)

Are new microglia generated after birth?

No, adult cells are maintained through slow self renewal

How do monocytes differ from microglia in origin?

Monocytes are continuously replaced from bone marrow stem cells

Key conclusion from fate mapping?

Microglia are not replaced in adulthood

What defines Homeostatic (surveillant) microglia?

They are always active! Constant process movement and environmental sensing. Have lots of receptors → responsive to many different factors

How are microglia “reactive”?

Injury → release of ATP (“find me” signals) → activates P2Y12 Receptor on microglia → microglia respond → upregulation of UDP (“eat me” signals) → Phagocytosis by microglia

Blocking P2Y12R causes what?

Slower microglial response

What happens if microglia can’t respond to injury?

Worse damage. Microglia injury response is protective

What signal is released immediately after injury?

ATP (“find me” signal)

What signal promotes phagocytosis?

UDP (“eat me” signal)

How does tamoxifen block microglial extension?

Blocks Cl⁻ influx = microglia cannot xtend processes

Effect of latrunculin B on microglia?

Disrupts actin polymerization = microglia paralyzed = no outgrowth

What does ablation do to microglia?

Removes microglia near lesion site = who is gonna respond to injury? 🥹

What is synaptic refinement?

Circuits become more specific and tuned

What is synaptic pruning?

Removal of unnecessary synapses by microglia

When does pruning occur?

During a critical developmental window

What molecular system tags synapses for removal?

Complement pathway! C3 tags weak or inactive synapses. Recognized by C3R on microglia. (only a litt

What does TTX do in the context of pruning?

blocks neuronal firing → synapses weaker → tagged for removal → more eating

What does forskolin do in the context of pruning?

increases neuronal activity → stronger synapses → not tagged → no eating

What makes microglia more surveillant?

Express two pore K+ channels (K2P) that alllow them to change their behaviour/morphology.

What happens if microglial K2P channels are blocked?

Cannot monitor neuronal activity properly = less surveillant (less exploration) + less ramified (fewer, thicker, shorter branches)

What is the whisker trimming model used for?

Studying activity-dependent plasticity.

Cut whiskers = reduced neuronal firing = microglia were less responsive/surveillant

What happens when microglia are removed in the context of epilespy?

Excessive neuronal activity increases

What is CSF1R

Receptor on microglia that is required for their survival. Inhibit = they die

How do microglia regulate activity?

Excessive activity → release of K+ and ATP → microglia sense it with P2Y12R and convert ATP → adenosine via CD39/CD73 → adenosine suppressive function

What happens to microgliaCD39/CD72/A1R (Ado receptors)

Will make seizures worse

What is the oligodendrocyte lineage?

OPC → differentiating oligodendrocyte → mature oligodendrocyte

What are OPCs?

Immature progenitor cells in the oligodendrocyte lineage. Have progenitor markers, do NOT produce myelin

What happens during OPC differentiation?

Loss of OPC markers + gain of oligodendrocyte markers

What triggers myelin formation?

OPC makes contact with an axon

What defines a mature oligodendrocyte?

Fully differentiated cell with myelin sheaths. DO NOT make new myelin

What is capacitance?

How much eparation between charges?

How does myelin affect capacitance?

= low capacitance = less anions on the surface will attract less cations = more cations available to depolarize = more separation = faster AP

How does NO myelin affect capacitance?

= high capacitance = more anions on the surface will attract more cations = less cations available to depolarize other = less separation = slower AP

How does NO myelin affect resistance?

= low resistance (current not trapped) = higher transmembrane current = slower AP

How does myelin affect resistance?

= high resistance (fat traps current, less cations leak out) = lower transmembrane current = faster AP

Relationship between myelin and conduction velocity?

Positive linear for most axon diameters. EXCEPT in axon diameters less than one micrometer; these conduct slower when myelinated

How much OPC differentiation over time even when fully myelinated?

OPCs continue to differentiate! Myelin is dynamic and maintined over time (because oligos die.. need new ones to maintain sheath)

When does an OPC decide to myelinate?

During a brief decision window. If it fails to encorporate a functional myelin sheath, the cell dies!

What is Myrf?

A transcription factor needed for OPC differentiation + myelin production

Loss of Myrf causes what?

Failure to form functional myelinating oligodendrocytes

Mice on complex wheel experiement?

OPC-CreER x Floxed Myrf mouse (instead of reporter line). on = no myelin

Can produce myelin = better at the task

Less myelin = less able to do the task

BUT also: if mice learned the wheel BEFORE Myrf deletion, deleting Myrf after did NOT make them forget the skill!

What is MG2?

OPC receptor regulating differentiation. Loss causes reduced differentiation → less myelin

What happened in the loss of function morris water maze?

MG2 deletion / reduced myelin → take longer to find the platform (impaired visuo-spatial memory, decreased performance) 

What happened in the gain of function morris water maze?

boosting myelination → better performance + faster learning + better recall, etc.. YAY 😸

Are OPCs passive stem-like cells?

No — OPCs are electrically responsive to nearby neuronal firing

What is the main receptor type at OPC synapses?

AMPARs

What is the likely role of OPC synapses?

Fine-tuning differentiation and circuit integration

How did they prove that OPCs are responsive to nearby neuronal activity?

stimulated axons, recorded from OPCs using patch clamp

Results: PSPs in OPCs, meaning that OPCs were receiving synaptic input!

What is the mechanism of the relationship between NMDAR and oligo:

Activity causes glutamate release → - glutamate goes into thin layer between myelin sheath in axon → stimulates AMPAR and NMDAR present in myelin → Ca2+ increase inside → increases glucose transporters → oligos can take in more glucose → glucose converted into lactate into the axon (Because axons can rapidly convert lactate into ATP)

Why can’t a single AMPAR subunit knockout eliminate OPC synapses?

Why can’t a single AMPAR subunit knockout eliminate OPC synapses?

What was the effect of AMPAR loss on oligodendrocyte production?

Only a subtle reduction = they are not essential for early OPC differentiation or survival

What key finding showed myelin responds to activity?

Ca²⁺ signals inside myelin increase during stimulation = Myelin is electrically and functionally responsive

What does TTX block in oligos?

blocks voltage-gated Na⁺ channels → blocks APs → reduced myelin Ca2+ signals

What does NBQX block in oligos?

blocks AMPA receptors → reduced myelin Ca2+ signals

What does DCKA block in oligos?

blocks NMDA receptors → reduced myelin Ca2+ signals

What is activity-dependent metabolic coupling?

Active neurons trigger metabolic support from oligodendrocytes (increased glucose uptake in oligos)

What happens if NMDARs are removed from oligodendrocytes?

They will no longer increase glucose uptake = metabolic coupling breaks

What happens if oligodendrocyte glucose transport is blocked during activity?

Axonal firing fails (signal amplitude drops)