Glial-conditional deletion of aquaporin-4 ( Aqp4 ) reduces blood–brain water uptake and confers barrier function on perivascular astrocyte endfeet

Abstract

Tissue-and cell-specific deletion of the Aqp4 gene is required to differentiate between the numerous pools of aquaporin-4 (AQP4) water channels. A glial-conditional Aqp4 knockout mouse line was generated to resolve whether astroglial AQP4 controls water exchange across the blood-brain interface. The conditional knockout was driven by the glial fibrillary acidic protein promoter. Brains from conditional Aqp4 knockouts were devoid of AQP4 as assessed by Western blots, ruling out the presence of a significant endothelial pool of AQP4. In agreement, immunofluorescence analysis of cryostate sections and quantitative immunogold analysis of ultrathin sections revealed no AQP4 signals in capillary endothelia. Compared with litter controls, glial-conditional Aqp4 knockout mice showed a 31% reduction in brain water uptake after systemic hypoosmotic stress and a delayed postnatal resorption of brain water. Deletion of astroglial Aqp4 did not affect the barrier function to macromolecules. Our data suggest that the blood-brain barrier (BBB) is more complex than anticipated. Notably, under certain conditions, the astrocyte covering of brain microvessels is rate limiting to water movement. edema | electron microscopy | homeostasis | membrane | swelling M ore than 100 y have elapsed since it was first shown that some solutes present in blood are retained by brain capillaries, pointing to the existence of a barrier function at the bloodbrain interface. This early finding naturally inspired a discussion as to what could constitute the morphological substrate of this barrier (1). With the advent of the electron microscope it became clear that the blood-brain interface is composed of endothelia and pericytes, surrounded by a basal lamina and perivascular endfeet of astrocytes. After decades of intense debate, a concept emerged that the functional barrier resides at the level of the endothelial cells (2). This was consistent with morphological data, which clearly showed that endothelia were continuous and coupled by tight junctions (3). The perivascular endfeet, on the other hand, were not coupled by tight junctions and were portrayed as a discontinuous layer with spacious clefts separating the individual processes.Discussions on the morphological substrate of barrier function have been focused on solutes (4), and the field has not yet matured to provide a consistent view regarding what cellular structures, if any, restrict water movement between blood and brain. Following the discovery of water channels, it became clear that the major brain water channel AQP4 is implicated in water transport at the blood-brain interface. Thus, global Aqp4 knockout significantly limited the development of brain edema, attesting to the importance of AQP4 water channels (4). As AQP4 is strongly expressed in the perivascular endfeet (5), the interest in these processes was rekindled also in the context of their possible barrier function.Specifically, it was proposed that the endfeet could restrict water flow, most significantly in pathophysiological se...

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“…Brain water content was measured according to the wet/dry mass method 27. Mice were sacrificed via cervical dislocation.…”

Cytoprotective effects of growth factors: BDNF more potent than GDNF in an organotypic culture model of Parkinson's disease

“…This role has been demonstrated in studies where Aqp4 or components of the DAPC are deleted (292,306,310). Moreover, glial-conditional Aqp4 knockouts demonstrate delayed postnatal reabsorption of water, resulting in slightly higher brain water content in Aqp4 knockout animals (164). Although removal of AQP4 itself does not result in impaired water homeostasis (292,311,312), AQP4 has been demonstrated to play a key role in several pathological conditions associated with water accumulation in brain tissue, known as brain edema.…”

“…Further, they are involved in neuronal survival by stimulating synaptogenesis, neurogenesis and release of signaling molecules such as transmitters, antioxidants and neurotrophic factors, including glial cell-derived neurotrophic factor (GDNF) and brainderived neurotrophic factor (BDNF). Astrocytes further secrete factors that can stimulate both cell survival and death, such as cytokines and inflammatory molecules, and ROS (162)(163)(164)(165)(166)(167)(168)(169)(170)(171). To execute all these functions, astrocytes form neurovascular units, where their terminal processes, endfeet, contact the vasculature and other processes enwrapping neuronal synapses (163,172).…”

Blood–Brain Barrier Disruption, Vascular Impairment, and Ischemia/Reperfusion Damage in Diabetic Stroke

“…Astrocyte–endothelial cell interactions have also been shown to be essential in regulating brain water content and electrolyte balance under normal and pathological conditions 48, 49. Ischemic injury to the brain activates astrocytes, and reactive astrocytes can exert a deleterious role (secrete proinflammatory cytokines, inhibit axonal regeneration, infarct expansion) in the acute phase after stroke while exerting a protective role (neurite sprouting, synapse formation, rebuild BBB, secrete neurotrophic factors) in the chronic phase after stroke 50, 51.…”

“…Middle‐aged rats induced with DM exhibit a significant decrease of paravascular Aquaporin‐4 expression in the hippocampus 55. The interaction between the astrocytes, Aquaporin‐4, and endothelial cells regulates brain water content as well as poststroke edema resolution (ie, transport of water via bulk flow from the brain parenchyma to the vascular, intraventricular, and subarachnoid compartments) 49, 56. Aquaporin‐4 has been implicated in water uptake into the brain tissue during the evolution of cytotoxic edema, as well as in water clearance after vasogenic edema 21, 49, 57.…”