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Indirect Role of AQP4b and AQP4d Isoforms in Dynamics of Astrocyte Volume and Orthogonal Arrays of Particles

by Marjeta Lisjak 1,†, Maja Potokar 1,2,†, Robert Zorec 1,2 and Jernej Jorgačevski 1,2,*
1
Laboratory of Neuroendocrinology–Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, 1000 Ljubljana, Slovenia
2
Celica Biomedical, Tehnološki park 24, 1000 Ljubljana, Slovenia
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Cells 2020, 9(3), 735; https://doi.org/10.3390/cells9030735
Received: 17 January 2020 / Revised: 10 March 2020 / Accepted: 13 March 2020 / Published: 17 March 2020
(This article belongs to the Special Issue Astrocytes in Space and Time)
Water channel aquaporin 4 (AQP4) plays a key role in the regulation of water homeostasis in the central nervous system (CNS). It is predominantly expressed in astrocytes lining blood–brain and blood–liquor boundaries. AQP4a (M1), AQP4c (M23), and AQP4e, present in the plasma membrane, participate in the cell volume regulation of astrocytes. The function of their splicing variants, AQP4b and AQP4d, predicted to be present in the cytoplasm, is unknown. We examined the cellular distribution of AQP4b and AQP4d in primary rat astrocytes and their role in cell volume regulation. The AQP4b and AQP4d isoforms exhibited extensive cytoplasmic localization in early and late endosomes/lysosomes and in the Golgi apparatus. Neither isoform localized to orthogonal arrays of particles (OAPs) in the plasma membrane. The overexpression of AQP4b and AQP4d isoforms in isoosmotic conditions reduced the density of OAPs; in hypoosmotic conditions, they remained absent from OAPs. In hypoosmotic conditions, the AQP4d isoform was significantly redistributed to early endosomes, which correlated with the increased trafficking of AQP4-laden vesicles. The overexpression of AQP4d facilitated the kinetics of cell swelling, without affecting the regulatory volume decrease. Therefore, although they reside in the cytoplasm, AQP4b and AQP4d isoforms may play an indirect role in astrocyte volume changes. View Full-Text
Keywords: aquaporin 4 (AQP4); AQP4b; AQP4d; AQPe4; endosome; lysosome; Golgi apparatus; astrocyte; structured illumination microscopy; brain edema aquaporin 4 (AQP4); AQP4b; AQP4d; AQPe4; endosome; lysosome; Golgi apparatus; astrocyte; structured illumination microscopy; brain edema
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Lisjak, M.; Potokar, M.; Zorec, R.; Jorgačevski, J. Indirect Role of AQP4b and AQP4d Isoforms in Dynamics of Astrocyte Volume and Orthogonal Arrays of Particles. Cells 2020, 9, 735.

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