Microglial calcium signal acts as a rapid sensor of single neuron damage in vivo

Abstract

In the healthy adult brain microglia, the main immune-competent cells of the CNS, have a distinct (so-called resting or surveying) phenotype. Resting microglia can only be studied in vivo since any isolation of brain tissue inevitably triggers microglial activation. Here we used in vivo two-photon imaging to obtain a first insight into Ca(2+) signaling in resting cortical microglia. The majority (80%) of microglial cells showed no spontaneous Ca(2+) transients at rest and in conditions of strong neuronal activity. However, they reliably responded with large, generalized Ca(2+) transients to damage of an individual neuron. These damage-induced responses had a short latency (0.4-4s) and were localized to the immediate vicinity of the damaged neuron (< 50 μm cell body-to-cell body distance). They were occluded by the application of ATPγS as well as UDP and 2-MeSADP, the agonists of metabotropic P2Y receptors, and they required Ca(2+) release from the intracellular Ca(2+) stores. Thus, our in vivo data suggest that microglial Ca(2+) signals occur mostly under pathological conditions and identify a Ca(2+) store-operated signal, which represents a very sensitive, rapid, and highly localized response of microglial cells to brain damage. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Spontaneous Ca 2+ transients in mouse microglia

“…Previous studies described spontaneous Ca 2+ transients in microglia in vivo [18,19], and these events are similar as the ones described in our study. First, the percentages of 'active' microglia, i.e.…”

“…those cells that display at least one transient during the recording period, were similar. Eichhoff et al [18] found that around 22% of the microglia of 2-4 months old CX 3 CR1 +/GFP mice were spontaneously active, whereas we found transients in 25.2 ± 4.1% of adult (P60-P70) microglia. Second, rise and decay kinetics of microglial Ca 2+ transients in vitro and in vivo resemble each other and are in the range of 10-13 s [18].…”

“…From their appearance, kinetics and pharmacology, the events described here resemble all characteristics of those 4 transients that were previously described in vivo [18,19]. Since there were no other cell types in our preparations, and since the spontaneous Ca 2+ transients were not dependent on the microglia culturing density, we suggest that they originate from intrinsic microglial signaling.…”

“…Similarly, we recorded these events in purified microglial cultures, and these events were not dependent on the cell density of the cultures. Previous studies conducted in vivo on microglia also reported spontaneous Ca 2+ transients [18,19], but were actually not suitable to distinguish between external, non-microglial and intrinsic microglial triggering of these events. In the present study we could show these responses in different isolated microglial preparations, under physiological and pathophysiological conditions, therefore suggesting that this type of activity represents an intrinsic property of microglia.…”

“…These elevations were later attributed to an ATP-and P2X 7 -dependent astrocyte-microglia interaction [17]. More recently, spontaneous calcium transients in microglia were also reported in vivo and in situ [18,19] but in contrast, the signaling mechanism was dependent on intracellular Ca 2+ stores and independent from astrocytic or neuronal Ca 2+ waves. The authors concluded that the spontaneous Ca 2+ transients may be triggered by neuronal, damage-induced ATP release.…”