Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, 1000 Ljubljana, Slovenia.
Celica Biomedical, Tehnološki park 24, 1000 Ljubljana, Slovenia.
Cells. 2020 Mar 17;9(3):735. doi: 10.3390/cells9030735.
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.
水通道蛋白 4（AQP4）在中枢神经系统（CNS）的水稳态调节中发挥关键作用。它主要在沿血脑和血脑脊液边界排列的星形胶质细胞中表达。存在于质膜中的 AQP4a（M1）、AQP4c（M23）和 AQP4e 参与星形胶质细胞的细胞体积调节。其剪接变体 AQP4b 和 AQP4d 的功能，预测存在于细胞质中，目前尚不清楚。我们研究了 AQP4b 和 AQP4d 在原代大鼠星形胶质细胞中的细胞分布及其在细胞体积调节中的作用。AQP4b 和 AQP4d 同工型在早期和晚期内体/溶酶体和高尔基体中表现出广泛的细胞质定位。这两种同工型都没有定位于质膜中的正交排列颗粒（OAP）。在等渗条件下过表达 AQP4b 和 AQP4d 同工型会降低 OAP 的密度；在低渗条件下，它们仍然不存在于 OAP 中。在低渗条件下，AQP4d 同工型明显重定位到早期内体，这与含有 AQP4 的囊泡的运输增加相关。AQP4d 的过表达促进了细胞肿胀的动力学，而不影响调节性体积减少。因此，尽管它们位于细胞质中，但 AQP4b 和 AQP4d 同工型可能在星形胶质细胞体积变化中发挥间接作用。
