Microglia represent a specialized population of microphages-like cells in the central nervous system involved in inflammatory responses. It has been observed that microglia freely extend and retract their processes to survey the brain and rapidly direct their processes towards sites of injury. However, the role of microglial surveillance and directed process of motility remains poorly understood.
Genetic depletion of the P2Y12R receptor transiently delays microglial-directed motility to laser ablation without affecting baseline motility, which is potentially due to compensation by other G-protein coupled receptors. Therefore, the group hypothesized that inhibiting the Gi pathway in microglia would lead to a sustained inhibition in microglia dynamics.
The authors made MgPTX mice for microglia-specific Gi inhibition. Using in vivo two-photon time-lapse imaging they found dramatically reduced microglia surveillance in MgPTX mice compared to littermate control mice. Lack of P2Y12R delayed but did not abolish the microglia response to laser ablation. In contrast, in MgPTX mice, direct process of motility was abolished even two hours after laser ablation. Thus, inhibition of Gi signaling in microglia impaired both microglia surveillance and lesion-induced directed motility.
Interestingly, MgPTX mice developed spontaneous seizures that decreased survival. This result suggested that microglial dynamics modulate neuronal network synchrony.
To investigate the role of microglia in hyperexcitability, the authors monitored motility of microglia and calcium dynamics with two-photon microscopy in barrel cortex upon whiskers stimulation. Repetitive whisker stimulation increased microglia surveillance in MgWT mice but not in MgPTX mice. This lack of microglial response was associated with a cumulative increase in evoked neuronal calcium transients and prolonged decay times of evoked calcium transients. Whisker-stimulus-induced neuronal activity in MgPTX mice was hypersynchrony, as assessed by quantification of synchronized neuronal firing and network burst activity. This result suggests that microglia-neuron interactions prevent network hypersynchrony.
The authors described a new role for microglia motility to maintain brain network synchronization within physiological range and to prevent hyperexcitability. This function of microglia is controlled by microglia-specific Gi pathway. Strategies to modulate Gi in microglia may hold promise to harness hyperactivity in neurological disorders with impaired microglial dynamics.
The in vivo two-photon imaging, ablation and glutamate uncaging experiments were conducted using a two-photon imaging system from Bruker Corp. (Multiphoton Microscopes, Madison, WI).