Morphotype-specific calcium signaling in human microglia

Key functions of Ca²⁺ signaling in rodent microglia include monitoring the brain state as well as the surrounding neuronal activity and sensing the danger or damage in their vicinity. Microglial Ca²⁺ dyshomeostasis is a disease hallmark in many mouse models of neurological disorders but the Ca²⁺ signal properties of human microglia remain unknown.

We developed a novel genetically-encoded ratiometric Ca²⁺ indicator, targeting microglial cells in the freshly resected human tissue, organotypically cultured tissue slices and analyzed in situ ongoing Ca²⁺ signaling of decades-old microglia dwelling in their native microenvironment.

The data revealed marked compartmentalization of Ca²⁺ signals, with signal properties differing across the compartments and resident morphotypes. The basal Ca²⁺ levels were low in ramified and high in ameboid microglia. The fraction of cells with ongoing Ca²⁺ signaling, the fraction and the amplitude of process Ca²⁺ signals and the duration of somatic Ca²⁺ signals decreased when moving from ramified via hypertrophic to ameboid microglia. In contrast, the size of active compartments, the fraction and amplitude of somatic Ca²⁺ signals and the duration of process Ca²⁺ signals increased along this pathway.