Expression Of Aquaporins-4 And Its Relation To Brain Water Content In The Developmental Mouse Brain

Previous studies indicate that water transport protein aquaporin-4 is involved in the formation and maintenance of brain water homeostasis. Nevertheless, developmental feature of aquaporin-4 in the whole brain and its relation to brain water content during postnatal life remains unknown. Western blot and densitometry analysis showed that aquaporin-4 expression in CD1 mouse brain was only 21.3% of the adult level one week after birth, but significantly increased to 67.4% at the second week, and reached 88.6% of adult level by 4 weeks. Consistent with increased aquaporin-4 expression, dynamic reduction in brain water content occurred during postnatal life. Statistical analysis demonstrated a negative partial correlation between aquaporin-4 expression and brain water content levels during postnatal development (rP2P = 0.92 and P = 0.002). There was no difference in brain water content between aquaporin-4 null and wild-type mice at the first week after birth. However, aquaporin-4 null mice showed greater brain water content compared with littermate controls from the second week up to adult stage. Moreover, we investigated the spatio-temporal immunolocalization of AQP4 in postnatal CD1 mouse brain. Semiquantitative image analysis revealed that AQP4 expression in the brain stem and hypothalamus was earlier than that in the cerebral cortex and cerebellum. Mature feature of AQP4 localization in perivascular astrocyte processes and basal lateral membrane of ependymal cells was completed at approximately 2 week after birth. These results characterize developmental feature of aquaporin-4 in the postnatal brain and highlight a vital role of aquaporin-4 in the maturation of brain water homeostasis. Ependymal cells form the walls of the ventricles, and take part in the production of cerebrospinal fluid. Aquaporin-4 (AQP4), one isoform of water channels mainly distributed in the brain, is restricted to basolateral plasma membranes of ependymal cells. The highly polarized expression of AQP4 suggests it may be involved in maintaining the structural and functional integrity of the ependyma. This hypothesis was validated by use of adult AQP4 knockout mice generated by Fan et al. (J Neurosci Res. 2005 Nov 15;82(4):458-64). Histological analysis showed disorganized ependymal layer of the lateral ventricle and aqueduct in AQP4-deficiency mice. A majority (92.7%) of null mice displayed reduced lateral ventricular volume, while a small fraction (7.3%) had enlarged or normal ventricular size with a narrow aqueduct. Immunohistochemistry demonstrated that AQP4 deletion resulted in decreased expression of gap junction protein connexin43 in the ependymal cells. Electron microscopy confirmed junctional complex absence at basolateral membranes of ependymocytes. Moreover, AQP4 knockout mice showed decreased cerebrospinal fluid production and increased brain water content compared with wild-type mice. These results highlight a key role of AQP4 in maintaining the structure and function of the ependyma. In addition, variable profiles of ventricle system in adult AQP4 null mice indicate functional AQP4 polymorphisms.