AQP4e-based orthogonal arrays regulate rapid cell volume changes in astrocytes
- 1Laboratory of Neuroendocrinology — Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, 1000 Ljubljana, Slovenia
- 2Celica BIOMEDICAL, Tehnološki park 24, 1000 Ljubljana, Slovenia
- Correspondence should be addressed to Robert Zorec or Jernej Jorgačevski, Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, 1000 Ljubljana, Slovenia. E-mail: robert.zorec{at}mf.uni-lj.si, jernej.jorgacevski{at}mf.uni-lj.si
-
Author contributions: M.L., B.R., and J.J. performed research; M.L. and B.R. analyzed data; M.L., M.P., B.R., J.J., and R.Z. wrote the paper; M.P., J.J., and R.Z. designed research; R.Z. contributed unpublished reagents/analytic tools.
Abstract
Water channel aquaporin 4 (AQP4) plays a key role in the regulation of water homeostasis in the brain. It is predominantly expressed in astrocytes at blood—brain and blood—liquor interfaces. Although several AQP4 isoforms have been identified in the mammalian brain, two, AQP4a (M1) and AQP4c (M23), have been confirmed to cluster into plasma membrane supramolecular structures, termed orthogonal arrays of particles (OAPs) and to enhance water transport through the plasma membrane. However, the role of the newly described water-conductive mammalian isoform AQP4e is unknown. Here, the dynamics of AQP4e aggregation into OAPs and its role in the regulation of astrocyte water homeostasis have been studied. Using super-resolution structured illumination, atomic force, and confocal microscopies, the results revealed that, in female rat astrocytes, AQP4e isoform co-localizes with OAPs, affecting its structural dynamics. In hypoosmotic conditions, which elicit cell edema, OAP formation was considerably enhanced by overexpressed AQP4e. Moreover, the kinetics of the cell swelling and of the regulatory volume decrease (RVD) was faster in astrocytes overexpressing AQP4e, compared with untransfected controls. Furthermore, the increase in maximal cell volume elicited by hypoosmotic stimulation was significantly smaller in AQP4e overexpressing astrocytes. For the first time, this study demonstrates an active role of AQP4e in the regulation of OAP structural dynamics and in water homeostasis.
SIGNIFICANCE STATEMENT
Water channel AQP4 plays a key role in the regulation of water homeostasis in the brain. To date, only AQP4a and AQP4c isoforms have been confirmed to enhance water transport through plasmalemma and to cluster into orthogonal arrays of particles (OAPs). We here studied the dynamics, aggregation and role in the regulation of astrocyte water homeostasis of the newly described water-conductive mammalian isoform AQP4e. Our main findings are: • Brain edema mimicking hypoosmotic conditions stimulate the formation of new OAPs with larger diameters, due to the incorporation of additional cytoplasmic AQP4 channels and the redistribution of AQP4 channels of the existing OAPs. • AQP4e affects the dynamics of cell swelling and regulatory volume decrease in astrocytes exposed to hypoosmotic conditions.
Footnotes
-
The authors declare no competing financial interests.
-
This work was supported by the Research Agency of Slovenia, grant numbers P3 0310, J3 6790, and J3 7605. We gratefully acknowledge Prof. Antonio Frigeri (University of Bari, Italy) and Prof. Vanda A. Lennon (Mayo Clinic Rochester, USA) for providing NMO-IgG antibodies and Prof. Ole Petter Ottersen for the generous gift of AQP4e-GFP plasmid.
- Received March 22, 2017.
- Revision received July 31, 2017.
- Accepted September 23, 2017.
- Copyright © 2017 the authors
