Microglial calcium signaling is attuned to neuronal activity in awake mice

Abstract

Microglial calcium signaling underlies a number of key physiological and pathological processes in situ, but has not been studied in vivo in awake mice. Using multiple GCaMP6 variants targeted to microglia, we assessed how microglial calcium signaling responds to alterations in neuronal activity across a wide range. We find that only a small subset of microglial somata and processes exhibited spontaneous calcium transients in a chronic window preparation. However, hyperactive shifts in neuronal activity (kainate status epilepticus and CaMKIIa Gq DREADD activation) triggered increased microglial process calcium signaling, often concomitant with process extension. Additionally, hypoactive shifts in neuronal activity (isoflurane anesthesia and CaMKIIa Gi DREADD activation) also increased microglial process calcium signaling. Under hypoactive neuronal conditions, microglia also exhibited process extension and outgrowth with greater calcium signaling. Our work reveals that microglia have highly distinct microdomain signaling, and that processes specifically respond to bi-directional shifts in neuronal activity through increased calcium signaling.

Deficiency of Microglial Autophagy Increases the Density of Oligodendrocytes and Susceptibility to Severe Forms of Seizures

“…A growing body of evidence demonstrates that microglia influence neuronal excitability and seizure susceptibility ( Badimon et al, 2020 ; Umpierre et al, 2020 ; Wu et al, 2020 ). We found that excessive activation of mTOR in TSC1-deficient mice leads to severe spontaneous seizures, involving a noninflammatory mechanism ( Zhao et al, 2018 ).…”

Changing Functional Signatures of Microglia along the Axis of Brain Aging

“…During application of exogenous glutamate or NMDA (N-Methyl-D-aspartate), microglial processes extend towards neurons; and this recruitment is dependent on activation of neuronal NMDA receptors and accompanying Ca 2+ influx, followed by ATP release from neurons and purinergic P2Y12 receptor-mediated stimulation on microglia [ 22 , 33 ]. Interestingly, a recent in vivo study has shown that a hyper- as well as a hypoactive state of the neural network can increase process outgrowth [ 34 ], indicating that any abnormal neuronal activity, irrespective of the direction of this abnormality, increases microglial process dynamics. Consistently, the dynamics of microglial surveillance is also changed by different kinds of anesthesia, known to modify neuronal activity [ 35 , 36 ].…”

“…Intracellular Ca 2+ signaling is actively involved into both sensor and effector functions of microglia [ 34 , 43 , 44 , 45 ]. Due to technical reasons, however, it has long been challenging to study microglial Ca 2+ signaling in vivo.…”

“…Change in the activity state of the surrounding neural network represents one of such disturbances of tissue homeostasis. Consistently, both an increase (caused, for example, by kainate-induced status epilepticus or activation of excitatory Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)) and a decrease (induced by isoflurane anesthesia or inhibitory DREADDs) in neuronal activity increased intracellular Ca 2+ signaling in microglia [ 34 ]. The latter Ca 2+ signals also compartmentalized to microglial processes, similar to the laser damage-induced Ca 2+ signals described above [ 47 ].…”

“…As non-excitable cells, microglia rely on intracellular Ca 2+ signaling to trigger their executive functions, e.g., increased processing and release of pro-inflammatory cytokines like IL-1β, IL-18 [ 63 ], TNF-α [ 64 , 65 ], or nitric oxide [ 66 ]. The latter might signal in an auto-/paracrine manner to sensitize microglia [ 34 , 44 , 45 ], which, in turn, might react much more vividly in case of subsequent pathology. Indeed, although basal levels of typical pro-inflammatory cytokines (TNF-α, IL-6, IL-1β) were similar in microglia from young and middle-aged mice, peripheral LPS challenge led to higher expression levels of these cytokines in brains of middle-aged mice [ 67 ].…”