Transgenic mice expressing a cameleon fluorescent Ca²⁺ indicator in astrocytes and Schwann cells allow study of glial cell Ca²⁺ signals in situ and in vivo

Glial cell Ca²⁺ signals play a key role in glial-neuronal and glial-glial network communication. Numerous studies have thus far utilized cell-permeant and injected Ca²⁺ indicator dyes to investigate glial Ca²⁺ signals in vitro and in situ. Genetically encoded fluorescent Ca²⁺ indicators have emerged as novel probes for investigating cellular Ca²⁺ signals. We have expressed one such indicator protein, the YC 3.60 cameleon, under the control of the S100β promoter and directed its expression predominantly in astrocytes and Schwann cells. Expression of YC 3.60 extended into the entire cellular cytoplasmic compartment and the fine terminal processes of protoplasmic astrocytes and Schwann cell Cajal bands. In the brain, all the cells known to express S100β in the adult or during development, expressed YC 3.60. While expression was most extensive in astrocytes, other glial cell types that express S100β, such as NG2 and CNP-positive oligodendrocyte progenitor cells (OP cells), microglia, and some of the large motor neurons in the brain stem, also contained YC 3.60 fluorescence. Using a variety of known in situ and in vivo assays, we found that stimuli known to elicit Ca²⁺ signals in astrocytes caused substantial and rapid Ca²⁺ signals in the YC 3.60-expressing astrocytes. In addition, forepaw stimulation while imaging astrocytes through a cranial window in the somatosensory cortex in live mice, revealed robust evoked and spontaneous Ca²⁺ signals. These results, for the first time, show that genetically encoded reporter is capable of recording activity-dependent Ca²⁺ signals in the astrocyte processes, and networks.