Microglia

Microglia are a group of neuroglial cells that constitute about 15% of all brain cells. These brain-resident immune cells play a crucial role in modulating the central nervous system's (CNS) development, activity, and plasticity. As the resident macrophages of the CNS, microglia perform diverse and essential functions in both the developing and adult brain, during homeostasis and disease.

Microglia's functions are supported by intricate bidirectional communication with other brain cells under steady-state conditions and with infiltrating peripheral immune cells during perturbations. They are highly motile, allowing them to interact with all cells in the CNS to mediate normal development, homeostasis, and general brain physiology.

Key Characteristics

• Microglia are the main myeloid cells in the brain.


• They are macrophage-like cells of distinct developmental origin that express the chemokine receptor CX3CR1.


• Microglia respond to neuronal activation by suppressing neuronal activity.


• They colonize the CNS early in embryonic development, comprising between 5% and 10% of the total number of CNS cells.

Visualization and Markers

The marker IBA-1, which is upregulated in reactive microglia, is often used to visualize these cells.

Trophic Support and Maintenance

Microglia processes provide trophic support to neurons, model synaptic architecture during development, and remove dying cells and cell debris. Known trophic support molecules for microglia include:

• Colony-stimulating factor 1 (CSF-1)


• IL-34


• TGF-beta 2


• Cholesterol

Protective Roles

Microglia, originating from mesodermal cells, clear neurotoxic proteins and protect neurons from damaging exogenous molecules, toxins, infectious agents, or pathogens. They are primarily responsible for mapping the parenchyma.

Pathological Activation

Excess and persistent microglial activation is associated with neuroinflammation, neuronal energetic deterioration, and the progression of neurodegenerative diseases of aging.

In summary, microglia are essential for maintaining CNS health through their roles in development, homeostasis, and protection against disease. Their ability to communicate with other brain cells and respond to changes in the CNS environment underscores their importance in both normal and pathological conditions.

References:
Katsumoto et al. 2014 J. Immunol. 193: 2615-2621
Kocur et al. 2015 Acta Neuropathol. Commun. 3: 20
Matcovitch-Natan et al. 2016 Science 353: aad8670
Keren-Shaul et al. 2017 Cell 169: 1276-1290
Lan et al. 2017 Nat Rev Neurol. 13(7): 420-433
Badimon et al. 2020
Castellanii & Schwartz 2020 Trends in immunology 41: 794
Cunnane et al. 2020
Liddelow et al. 2020 Trends in Immunology 41: 820
Borst et al. 2021 Immunity 54: 2194-2208
Rosenberg & Rao 2021 iScience 24: 102863
Buckley & McGavern 2022 Immunol Rev 306: 58-75
Naomi et al. 2022 Nutrients 14: 20
Šimončičová et al. 2022 Trends in Pharmacological Sciences, August 43(8): 669
Zhou et al. 2022b Aging Cell 21: e13599

see alsO.
Alzheimer's Disease (AD) & Microglia
Cholesterol & Microglia
Immunity & Microglia
Microglia & Neuroinflammation
Microglia & Transforming growth factor beta (TGF-beta)
Microglia & Transforming growth factor beta 2 (TGF-beta 2)