Intravital calcium imaging of meningeal macrophages reveals niche-specific dynamics and aberrant responses to brain hyperexcitability.

Abstract

The meninges, which envelop and protect the brain, host a dense network of resident macrophages with diverse roles in regulating homeostasis and neuroinflammation. Despite their importance, we have a limited understanding of their behavior in vivo. Many dynamic cellular functions of macrophages involve intracellular Ca²⁺ signaling. However, virtually nothing is known about the spatiotemporal Ca²⁺ dynamics of meningeal macrophages in vivo. We developed a chronic intravital two-photon imaging approach and related computational analysis tools to interrogate meningeal macrophage Ca²⁺ dynamics, at subcellular resolution, in a novel Pf4-Cre:Ai162 conditional GCaMP6s reporter mouse model. Using imaging in awake mice, we characterized Ca²⁺ activity in meningeal macrophages at steady state and in response to cortical spreading depolarization (CSD), an aberrant pro-inflammatory brain hyperexcitability event implicated in migraine, traumatic brain injury, and stroke. In homeostatic meninges, macrophages in the dural perivascular niche exhibited several Ca²⁺ dynamic features, including event duration and signal frequency spectrum, distinct from those localized to the interstitial, non-perivascular niche. Simultaneous tracking of macrophage Ca²⁺ dynamics and local vasomotion revealed a subset of dural perivascular macrophages whose activity was coupled to locomotion-driven diameter fluctuations of their associated vessels. Most perivascular and non-perivascular meningeal macrophages displayed propagating intracellular Ca²⁺ activity and synchronized intercellular Ca²⁺ elevations, potentially driven by extrinsic factors. In response to CSD, the majority of perivascular and non-perivascular meningeal macrophages showed a persistent decrease in Ca²⁺ activity, while a smaller subset displayed Ca²⁺ elevations. Mechanistically, calcitonin gene-related peptide receptor signaling mediated the increase but not the decrease in CSD-mediated Ca²⁺ signaling. Collectively, our results highlight a previously unknown diversity of Ca²⁺ dynamics in meningeal macrophages at steady state and in response to an aberrant brain hyperexcitability event linked to neuroinflammation.