Studies on astrocyte function : potential roles in brain water homeostasis and neuroprotection

Abstract

Astrocytes are essential in brain homeostasis and function, including maintenance of water and ion balance. Astrocytes express the water channel aquaporin 4 (AQP4), implicated in both physiological functions and injury processes associated with brain edema, a common consequence of brain diseases. As part of the tripartite synapse astrocytes are tightly coupled to normal brain function via neuron-astrocyte interactions and by providing metabolic support to neurons as well as controlling extracellular potassium and glutamate. The overall aim was to explore the regulation of astrocyte water permeability and study aspects of astrocyte function of relevance for the interplay between astrocytes and neurons in physiology and ischemia.

The molecular mechanisms involved in short term regulation of astrocyte AQP4 were investigated by exploring the effects of glutamate and potassium on astrocyte water permeability. Glutamate was found to significantly increase astrocyte water permeability via activation of group I metabotropic glutamate receptors (mGluRs), an effect attributable to an effect on AQP4. An essential conclusion in this study is that AQP4 can be short-term regulated via gating. The evidence supports that this effect is dependent on phosphorylation, that the AQP4 serine 111 residue is a molecular target for the regulation and that this residue can be phosphorylated by particular protein kinases directly.

Next we showed that elevations in extracellular potassium increase astrocyte water permeability via a cAMP-dependent mechanism involving AQP4. The role of AQP4 serine 111 in the regulation of astrocyte water permeability was confirmed in this study. A prolonged upregulation of astrocyte water permeability was dependent on Kir-channel function. The effect could be modulated by calcium when such signaling was triggered by high extracellular potassium. The findings point to a functional coupling between water transport and potassium handling in astrocytes. Hence, as fundamental ‘messengers’ from neurons, glutamate and potassium were found to regulate astrocyte water permeability. The results indicate that astrocyte water permeability can be dynamically regulated in response to neuronal activity and that modulation of astrocyte signaling is dependent on both dose and duration of exposure to its regulators.

Due to its neuroprotective potential, the effects of EPO on astrocyte function were examined with regard to water permeability and aspects of astrocyte metabolism. EPO was found to counteract the glutamate-induced upregulation of astrocyte water permeability and significantly reduce neurological symptoms in an animal model of brain edema. It was shown that EPO modifies mGluR-mediated intracellular calcium signaling. In oxygen-glucose deprivation, a cellular model of ischemia, EPO was found to enhance astrocyte glutamate uptake. The effect was depended on the sodium pump Na,K-ATPase and coupled to intracellular pH regulation. The evidence also suggested that astrocyte metabolism is enhanced by EPO under oxygen-glucose-free conditions, a finding that indirectly supports a therapeutic potential of EPO or EPO analogs in ischemia.

Taken together, our data indicate a dynamic role for astrocytes in the regulation of brain water and ion homeostasis. Upregulation of AQP4 water permeability facilitates water transport across the plasma membrane. In conditions associated with perturbed brain water balance, this may accelerate or attenuate brain edema depending on the phase of edema formation or resolution. EPO protects against water overload by modulation of astrocyte water permeability. Moreover, by enhanced astrocyte metabolism and restored astrocyte function in ischemia, EPO should favor local homeostasis and promote neuroprotection via astrocytes.