Nanoscale Surveillance of the Brain by Microglia via cAMP-Regulated Filopodia

Area of Science:

• Neuroscience
• Immunology
• Cell Biology

Background:

• Microglia, the brain's resident immune cells, continuously survey the brain parenchyma using large, motile processes to maintain homeostasis and detect pathology.
• The mechanisms underlying microglia's dynamic sensing capabilities at different scales are not fully understood.

Purpose of the Study:

• To investigate the role of filopodia in microglial sensing and their regulation by intracellular signaling pathways.
• To elucidate how cyclic AMP (cAMP) signaling controls microglial morphology and surveillance strategies.

Main Methods:

• Utilized advanced microscopy techniques to observe microglial morphology and dynamic behaviors.
• Manipulated intracellular cyclic AMP (cAMP) levels using pharmacological agents (norepinephrine, nitric oxide donors, phosphodiesterase inhibitors, P2Y12 receptor agonists) and G protein-coupled receptor activation.
• Examined cytoskeletal dynamics and filopodia formation in primary microglia.

Main Results:

• Demonstrated that microglia employ thin, actin-dependent filopodia for fast, nanoscale sensing in discrete brain regions, distinct from their larger processes.
• Showed that increased cAMP levels promote filopodia formation while collapsing large processes.
• Revealed that activation of G_i-coupled P2Y12 receptors inhibits filopodia but promotes large process extension.
• Confirmed these cAMP-mediated morphological changes in primary ramified microglia, indicating intrinsic regulation.

Conclusions:

• Microglial nanoscale surveillance of the brain parenchyma relies on localized increases in cyclic AMP (cAMP) to drive filopodia formation.
• Intracellular cAMP levels act as a critical switch, controlling the balance between filopodia-mediated nanoscale sensing and large process-mediated surveillance.
• Modulating cAMP signaling alters microglial immunosurveillance polarity and scale, impacting brain homeostasis maintenance.