Structural plasticity of perisynaptic astroglial processes as a determinant of synaptic properties

Cátia Domingos1, Franziska Müller2, Polina Gulakova3, Martin Schwarz4, Susanne Schoch-McGovern3, Evgeni Ponimaskin2, Andre Zeug2, Christian Henneberger1

1 Institute of Cellular Neurosciences, University of Bonn Medical School, Bonn, Germany
2 Cellular Neurophysiology, Hannover Medical School, Hannover, Germany
3 Institute of Neuropathology, University of Bonn Medical School, Bonn, Germany
4 Department of Epileptology, University of Bonn Medical School, Bonn, Germany

The tripartite synapse is a dynamic structure. Unpublished data from our lab demonstrate that perisynaptic astrocytes processes undergo rapid structural changes through remodeling of the actin cytoskeleton. Because interactions between astrocytes and neurons on the synaptic level can profoundly alter synaptic transmission and its plasticity, the latter is likely to be modified by structural changes of perisynaptic processes. However, it is largely unknown if that is indeed the case.

To test this hypothesis, we specifically manipulated astrocyte morphology by altering the activity of actin polymerization regulatory proteins: Ras homolog gene family member A (RhoA) GTPase and LIM kinase. Mutant variants of RhoA and the LIM kinase inhibitor S3 peptide were specifically over-expressed in astrocytes using stereotaxic injections of recombinant viruses (rAAV) in dorsal hippocampus. We quantified the effect on astrocyte morphology and synaptic transmission and plasticity in acute hippocampal slices using two-photon fluorescence microscopy and electrophysiological approaches.

We detected bidirectional changes of astrocyte morphology, as revealed by the analysis of astrocytic volume fraction. A reduction of astrocyte process abundance (i.e. a decrease of the volume fraction) was observed for constitutively active and native RhoA astrocytic over-expression whereas the opposite was found for S3 peptide over-expression.

On a functional level, preliminary electrophysiological recordings of field potentials in response to activation of the CA3-CA1 pathway did not reveal a major difference between astrocytic overexpression of constitutively active RhoA-CA and control conditions (input/output relationship, paired-pulse behavior, long-term potentiation and depression).

Together these techniques enable us to explore how the signaling of the RhoA GTPase pathway shapes the spatial configuration of the tripartite synapse and what its effect on synaptic transmission and plasticity is