Abstract
NG2 cells, the fourth type of glia in the mammalian CNS, receive synaptic input from neurons. The function of this innervation is unknown yet. Postsynaptic changes in intracellular Ca2+-concentration ([Ca2+]i) might be a possible consequence. We employed transgenic mice with fluorescently labeled NG2 cells to address this issue. To identify Ca2+-signaling pathways we combined patch-clamp recordings, Ca2+-imaging, mRNA-transcript analysis and focal pressure-application of various substances to identified NG2-cells in acute hippocampal slices. We show that activation of voltage-gated Ca2+-channels, Ca2+-permeable AMPA-receptors, and group I metabotropic glutamate-receptors provoke [Ca2+]i-elevations in NG2 cells. The Ca2+-influx is amplified by Ca2+-induced Ca2+-release. Minimal electrical stimulation of presynaptic neurons caused postsynaptic currents but no somatic [Ca2+]i elevations, suggesting that [Ca2+]i elevations in NG2 cells might be restricted to their processes. Local Ca2+-signaling might provoke transmitter release or changes in cell motility. To identify structural prerequisites for such a scenario, we used electron microscopy, immunostaining, mRNA-transcript analysis, and time lapse imaging. We found that NG2 cells form symmetric and asymmetric synapses with presynaptic neurons and show immunoreactivity for vesicular glutamate transporter 1. The processes are actin-based, contain ezrin but not glial filaments, microtubules or endoplasmic reticulum. Furthermore, we demonstrate that NG2 cell processes in situ are highly motile. Our findings demonstrate that gray matter NG2 cells are endowed with the cellular machinery for two-way communication with neighboring cells.
Highlights
In addition to astrocytes, oligodendrocytes, and microglia, NG2 cells are recognized as a fourth glial cell type in the CNS [1,2]
Cell identification and basic electrophysiological properties Cell identification in the hippocampus was based on EYFP or EGFP fluorescence, morphology, and physiological criteria as reported previously [18,22,23]
We further investigated presence and localization of vGLUT1 and vGLUT2 protein in gray matter NG2 cells in hippocampal slices by applying high resolution fluorescence microscopy, subsequent to patch-clamp recording and biocytin filling
Summary
Oligodendrocytes, and microglia, NG2 cells are recognized as a fourth glial cell type in the CNS [1,2]. NG2 cells are unique among glial cells in receiving synaptic input (reviewed by [2,6]), but the physiological impact of this innervation is unknown It remains unclear whether pre-synaptic transmitter release generates Ca2+-elevations in post-synaptic NG2 cells, which might evoke cellular motility or release of neuroactive substances. This ignorance is quite astonishing in view of the increasing knowledge of glia-mediated modulation of CNS signaling, such as astrocyte-neuron interactions which gave rise to the tripartite synapse concept [7,8,9]. It is known for more than a decade that ‘complex’
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