GLIA Study Notes

Neurons: Cellular components of the nervous system.
- Glial Cells include:
- Astrocytes
- Oligodendrocytes
- Schwann Cells
- Microglia

Definition: Star-like cells discovered by Otto Deiters in 1860; named 'astrocyte' by Mihaly von Lenhossek in 1895.
Main Functions:
- Ensheathe blood vessels and neurons.
- Provide metabolic support.
- Engage in transmitter uptake.
- Involved in gliotransmission.

Astrocytes' Role in Energy Dynamics:
- Inputs:
- Pre-synaptic neuron releases glutamate.
- Transport of glucose and water facilitated by astrocytes.
- Processes:
- Glycolysis produces 2 ATP from glucose.
- Full oxidation of glucose via the mitochondria yields a total of 38 ATP.
  - Relevant Equation:
  - $ext{Glucose} + 6O<em>2 ightarrow 6CO</em>2 + 6H_2O + 38 ext{ATP}$
- Output:
- Production of glutamine and lactate; lactate mainly fuels neurons.
- Astrocytes respond to synaptic activity by increasing blood flow, which improves glucose and O₂ delivery based on metabolic demand.

Synaptic Association:
- A single astrocyte can wrap around up to 140,000 synapses in rodents and up to 2 million in humans.

Importance of GLT-1:
- Definition: Glutamate transporter 1 in rodents; termed GLAST (glutamate aspartate transporter 1) in humans.
- Functions:
- Eliminates over 80% of neuronal and glial extracellular glutamate to prevent excitotoxicity.
- Primarily localized in perisynaptic processes, recycling ~75% of glutamate every 20 seconds.
- Excess glutamate can activate postsynaptic receptors (NMDA-R and mGluR5) and presynaptic autoreceptors (mGluR2/3), leading to cellular overstimulation.

Key Processes:
- Astrocytes are the primary producers of lactate, providing energy sources for neurons via the astrocyte-neuron lactate shuttle.
- Mechanism:
- Synaptic glutamate uptake by GLT-1 stimulates blood glucose transfer into astrocytes via GLUT1.
- Glucose converts into lactate and is ultimately released back for neuronal use via monocarboxylate transporters (MCTs).
- Significance: Essential for maintaining Long-Term Potentiation (LTP) and overall neuronal excitability.

Glutamate to Glutamine Cycle:
- GLT-1 and GLAST convert synaptic glutamate into glutamine to be recycled back into the synapse for neuronal use.
- Blood-derived glucose is also converted into D-serine, which serves as a co-transporter for NMDA-R, maintaining glutamate homeostasis overall.

Inhibitory Synapses:
- Astrocytes express GABA-A and GABA-B receptors.
- GABA-B receptor stimulation leads to increased astroglial calcium flux and glutamate release.
- Synaptic GABA uptake is conducted via GABA transporters (GAT 1 and 3), wherein GABA is converted back to glutamine.
- The primary aim is to maintain the inhibitory tone through these processes.

Challenge to Canonical Microglial Structure:
- An evolving perspective emphasizes the coexistence of several functional states rather than a simple dichotomy between 'good' and 'bad' microglia.
- Key States:
- Resting: Ramified microglia
- Activated States:
  - M1 (Pro-inflammatory)
  - M2 (Anti-inflammatory)
- Integration of diverse molecular signatures (epigenetic, proteomic, transcriptomic, and metabolomic) is essential for understanding microglial functionality.

Key Historical Figures:
- 1899: Franz Nissl identified rod-shaped cells related to brain pathologies.
- 1913: Cajal presented the first morphological description of microglia.
- Pío del Río-Hortega later refined methods to identify and describe microglial characteristics.
- 1966: Reports confirmed rodent microglial proliferation post nerve injury (facial nerve axotomy).

Functional Role:
- Microglia influence synaptic strengthening through dendritic spine contact formation, facilitating behavioral performance in various tasks associated with learning and memory.
- Associative plasticity genes linked to microglial activation implicate their role in LTP and cortical development.
- Microglial secretion of Brain-Derived Neurotrophic Factor (BDNF) plays a critical role in enhancing both dendritic spine formation and memory tasks.

Microglia Activation:
- Activation relates to Toll-like receptor 4 (TLR4) that triggers the release of various pro-inflammatory and neurotrophic factors, influencing neuronal function and plasticity.
- Example Factors:
- TNF-α enhances postsynaptic AMPA-R calcium permeability and reduces GABA-A receptor activity.
- IL-1β's effect is linked to contributing to LTP via NMDA receptor modulations.
- BDNF impacts GABAergic neuron excitability although precise mechanisms remain elusive.

Cooperation:
- Reactive astrocytes and microglia engage in a bidirectional communication loop to sustain synaptic stability and metabolic interactions.
- Certain cytokines reduce astrocytic GLT-1 expression, promoting glutamate release that can modify both glial and neuronal activity.

Oligodendrocytes:
- Located in the Central Nervous System (CNS), capable of myelinating multiple axons simultaneously.
Schwann Cells:
- Located in the Peripheral Nervous System (PNS), myelinating single axonal segments only.

Myelination Process:
- Myelinating cells form concentric layers around axons known as internodes.
- Variation exists in internode size and length, generally favoring larger axons for myelination due to their role in rapid signal conduction in neural circuits.

Oligodendrocyte Lineage and Function:
- Developmental states include:
- Oligodendrocyte precursor cells (OPCs)
- Immature pre-myelinating oligodendrocytes
- Mature myelinating oligodendrocytes
- Heterogeneity is observed in terms of developmental origin, morphology, and myelination functions, influenced by factors including age, sex, and transcriptional activity.

Transformation Process involves:
- OPC differentiation leading to new oligodendrocytes, extension of myelin sheaths, and remodeling existing myelin.
- The life cycle includes proliferation, differentiation, and eventual integration into the neural network.

Next Week: Discussion on ethics and research methods in clinical populations.
- Considerations for how to formulate complex neuroscience inquiries utilizing both clinical and preclinical models.