Notes on Microglial Development and Dynamics

BIOL2051: Lineage and Development of Glial Cells III: Microglia

Introduction and Quick History

• Microglia are considered to be macrophages and represent one of several immune cell populations in the brain.

Microglial Development

• Microglial development has been studied over a century, with significant advancements made in understanding their lineage and function.

• Historical Timeline:

  • 1880: Nissl staining technique was developed by Franz Nissl, aiding in the visualization of microglial cells.

  • 1897: Victor Babeş observed the activation of microglia during a rabies case and noted their clustering in various viral infections.

  • 1920: Pío del Río Hortega, a student of Santiago Ramón y Cajal, named these cells "microglia". Later characterized their response to brain lesions (1927) and noted the presence of microglia in white matter regions in 1932.

  • 1988: Hickey and Kimura demonstrated that perivascular microglial cells originate from bone marrow and express major histocompatibility complex (MHC) class II proteins crucial for antigen presentation.

The Immune Census of the Brain

• Techniques utilized in studying brain immune cells include:

  • Single-cell dissociation of tissue from naive C57BL/6 mice.

  • Use of mass cytometry (CyTOF) and cell staining for cell identification.

Immunological Structure of the Brain

• Identification of various immune cell subpopulations in the brain:

  • Microglia: The primary resident macrophages within the brain parenchyma.

  • Dendritic Cells: Sub-classified into Type A and B dendritic cells.

  • T Cells: CD4 and CD8 subsets present.

  • Other Cell Types: Include NK cells, B cells, and myeloid subpopulations (Gr-1+).

Types of Macrophages in the Brain

• Subpopulations include:

  • Meningeal Macrophages: Located in the meninges.

  • Choroid Plexus Macrophages: Found in the choroid plexus.

  • Perivascular Macrophages: Situated in perivascular spaces close to blood vessels.

Characteristics of Microglia

• Microglia are characterized by:

  • Ramified Morphology: Displaying a mosaic distribution across brain parenchyma

  • Morphological Differences: Greater ramified structures in grey matter vs. bipolar shapes in white matter.

  • Density Variation: Differences in cell density across brain regions, with each microglial cell covering an average volume of approximately $50000 ext{mm}^3$.

  • Immune Sensors: Equipped with various immune receptors enabling responses to homeostatic disruptions.

Microglial Functions

• Key roles of microglia include:

  • Immunosurveillance: Monitoring the brain environment for potential threats.

  • Synaptic Pruning: Involved in the removal of excess synapses during development.

  • Neuromodulation: Modulating neuronal signalling.

  • Phagocytosis: Engaging in the clearance of apoptotic cells and debris.

Developmental Pathways

• Microglial cells arise from erythromyeloid progenitors (EMPs) derived from the yolk sac, colonizing the brain without relay through the liver. This process occurs earlier than in other organs.

Key Factors in Microglial Specification

• Environmental Factors:

  • CSF1, IL34, and TGFβ are crucial for shaping and maintaining microglial identity.

• Regulatory Transcription Factors: Include PU.1, C/EBPs, RUNX1, and IRF8 that drive specification and fate acquisition in microglia.

Gene Expression and Diversity

• Microglia express stage-specific genes reflective of their developmental state. Analysis through single-cell RNA sequencing reveals:

  • Variation across different developmental stages, showcasing a dynamic functional profile.

Statistical Analysis and Findings

• Observation of gene expression diversity across lifespan with distinct patterns during prenatal, adulthood, and aging phases.

• Microglial turnover is maintained throughout life through a balance of proliferation and apoptosis. The intrinsic mechanisms provide a turnover of about $6.6$ times during mouse lifespan.

Summary

1. The immune system in the brain is complex and highly diverse.

2. Microglia play critical roles in brain development and homeostasis.

3. Microglia develop from yolk sac progenitors through a systematic program.

4. The majority of brain macrophages remain stable and are not replaced during adulthood and healthy aging.

Open Questions and Future Directions

• Current research emphasizes identifying unanswered questions regarding microglial functions and their implications in neuroinflammatory disorders and neurodegenerative diseases.

• Emerging insights into transcriptional diversity and functional roles of microglia warrant further investigation to understand their contributions to brain health and disease.

BIOL2051: Lineage and Development of Glial Cells III: Microglia

Introduction and Functional Significance

• Microglia are the primary resident macrophages of the Central Nervous System (CNS), representing roughly $10\%$ to $15\%$ of all cells within the brain.

• Unlike macroglia (astrocytes and oligodendrocytes) which are of neuroectodermal origin, microglia are derived from the mesoderm, specifically the yolk sac.

• They act as the principal immune effectors, maintaining brain homeostasis and responding to pathological insults.

Detailed Historical Context

• 1880: Franz Nissl developed the Nissl staining (using cresyl violet), which allowed for the first granular visualization of glial nuclei, though at the time they were indistinguishable from other non-neuronal cells.

• 1897: Victor BabeŒ identified "Stäbchenzellen" (rod cells) in rabies cases, providing the first evidence of microglial morphological transformation during neuroinflammation.

• 1920: Pío del Río Hortega, using silver carbonate staining, successfully distinguished microglia from astrocytes and oligodendrocytes. He termed them the "third element" of the CNS and correctly proposed their mesenchymal origin and migratory nature.

• 1988: Hickey and Kimura used bone marrow chimeric models to identify perivascular cells, establishing that while most microglia are long-lived and self-renewing, certain perivascular subpopulations could be replenished from the bone marrow under specific conditions.

The Immune Census and Methodology

• Modern understanding relies on high-dimensional single-cell technologies:

  • Single-cell RNA sequencing (scRNA-seq): Defines transcriptionally distinct states of microglia across development and disease.

  • Mass Cytometry (CyTOF): Uses antibodies conjugated to rare earth metal isotopes to quantify over $40$ different proteins simultaneously per cell, allowing for the mapping of the brain's immune landscape.

Immunological Structure and Subpopulations

• Parenchymal Microglia: The only immune cells found within the brain tissue itself under healthy conditions. They are highly ramified and non-overlapping, creating a grid-like surveillance network.

• Non-Parenchymal Macrophages (CAMs - CNS-Associated Macrophages):

  • Meningeal Macrophages: Reside in the dura, arachnoid, and pia mater to scan the Cerebrospinal Fluid (CSF).

  • Choroid Plexus Macrophages: Positioned at the blood-CSF barrier.

  • Perivascular Macrophages: Reside in the Virchow-Robin spaces, monitoring the entry of solutes and cells from the blood.

• Other Immune Populations: Natural Killer (NK) cells, $B$ cells, and $T$ cells ($CD4+$ and $CD8+$) are typically restricted to the meninges or the vasculature in the healthy brain.

Microglial Characteristics and Morphology

• Ramified (Homeostatic) State: Characterized by a small cell body and long, highly branched processes that are constantly moving to survey the environment.

• Morphological Heterogeneity:

  • Grey Matter: Microglia show more complex, radially symmetric branching.

  • White Matter: Microglia often exhibit elongated, bipolar shapes aligned with axonal tracts.

• Coverage: Each microglial cell manages a discrete territory of approximately $50000 \text{mm}^3$ (in mouse models), ensuring no part of the parenchyma is left unmonitored.

Key Physiological Functions

• Immunosurveillance: Microglial processes scan the entire brain parenchyma every $数$ hours.

• Synaptic Pruning: Microglia refine neural circuits by phagocytosing "weak" synapses during the postnatal "critical period." This is often mediated by the classical complement pathway (e.g., $C1q$ and $C3$ tagging synapses for removal).

• Neuromodulation: They secrete factors like Brain-Derived Neurotrophic Factor (BDNF), which supports LTP (Long-term Potentiation) and neuronal plasticity.

• Debris Clearance: Rapidly migrate to sites of injury via ATP gradients (detected by $P2Y12$ receptors) to clear apoptotic debris.

Developmental Origin and Specification

• Yolk Sac Progenitors: Microglia arise from Erythromyeloid Progenitors (EMPs) in the yolk sac during the first wave of hematopoiesis (around embryonic day $E8.0$ in mice).

• Colonization: These progenitors migrate into the developing brain primordium via the blood before the blood-brain barrier fully closes. This occurs much earlier than the colonization of other tissues by definitive macrophages.

• Transcription Factor Network:

  • PU.1: Essential for the initial myeloid lineage commitment.

  • IRF8: Crucial for the transition from macrophage progenitor to a dedicated microglial identity.

Factors Shaping Microglial Identity

• Survival Signals: The cytokine CSF1 (Colony Stimulating Factor 1) and its receptor CSF1R are mandatory for microglial survival.

• CNS-Specific Cues: IL-34 (produced by neurons) and TGFβ (produced by astrocytes) are essential for maintaining the homeostatic signature of microglia.

Gene Expression and Life Cycle

• Transcriptional Diversity: Single-cell analysis has identified "Disease-Associated Microglia" (DAM), which downregulate homeostatic genes (like $P2Y12$) and upregulate phagocytic genes (like $TREM2$ and $ApoE$) in response to amyloid plaques or neurodegeneration.

• Turnover and Longevity: Microglia are long-lived cells. In mice, they proliferate and die at a slow rate that allows for approximately $6.6$ complete population turnovers over a lifetime. In humans, some microglia may persist for several decades.

Summary of Modern Paradigms

1. Self-Renewal: In a healthy adult brain, the microglial pool is maintained by local proliferation, not by recruitment from blood monocytes.

2. Diversity: Microglia are not a monolithic population; they exhibit regional and state-specific molecular signatures.

3. Dual Roles: While protective, chronic microglial activation is implicated in neurodegenerative pathologies such as Alzheimer's and Parkinson's disease.