An EGFP-expressing astrocyte in the cortex of a one-week-old mouse. On one side of this highly polarized cell, an astroglial endfoot contacts a brain capillary (arrow), and on the other side, its highly branched processes enwrap the numerous synapses of the brain parenchyma. Image courtesy of F. Kirchhoff, Max Planck Institute of Experimental Medicine, Göttingen, Germany. See online supplementary information S1 (movie).

Recent imaging studies have indicated that dendritic spines can be highly motile during synaptic development and plasticity, and according to a new report in the European Journal of Neuroscience, astrocytic processes are probably just as dynamic.

Astrocytes are no longer considered to be a passive glue that connects and supports neurons, and there is growing evidence that they are important modulators of synaptic transmission. Astrocytic processes have been shown to be intimately associated with synapses, so they are also ideally placed to synchronize neuronal network activity.

In the new study, Hirrlinger, Hülsmann and Kirchhoff used time-lapse imaging in slices of mouse brainstem to monitor the movements of astrocytic processes in the vicinity of active synapses. To visualize astrocytes and their processes, the authors used a transgenic mouse line, in which enhanced green fluorescent protein (EGFP) was expressed under the control of the promoter for the astrocyte-specific marker glial fibrillary acidic protein (GFAP). They tagged active synapses by staining the tissue with FMI-43, a fluorescent dye that is taken up by recycling neurotransmitter vesicles.

The authors were able to identify astrocytic processes that were in close contact with active synapses. These processes showed two types of movement: they formed lamellipodia-like structures that could glide across the surfaces of neurons, and they extended processes that resembled filopodia.

What might be the functional significance of astrocytic-process motility at synapses? Hirrlinger et al. suggest that it might contribute to the appropriate presentation of important regulatory molecules. For example, one of the ways in which astrocytes are thought to regulate neurotransmission is through the release and uptake of glutamate, and the reorientation of astrocytic processes might allow the correct positioning of glutamate transporters and release sites.

Thanks to these new findings, the idea that astrocytes have an active role in synaptic function takes on a more literal meaning.