The Dual Role of AQP4 in Cytotoxic and Vasogenic Edema Following Spinal Cord Contusion and Its Possible Association With Energy Metabolism via COX5A

Abstract

Spinal cord edema, mainly including vasogenic and cytotoxic edema, influences neurological outcome after spinal cord contusion (SCC). Aquaporin 4 (AQP4) is the most ubiquitous water channel in the central nervous system (CNS), which is a rate-limiting factor in vasogenic edema expressing in brain injury, and it contributes to the formation of cytotoxic edema locating in astrocytes. However, little is known about the regulatory mechanism of AQP4 within vasogenic and cytotoxic edema in SCC, and whether the regulation mechanism of AQP4 is related to Cytochrome coxidase (COX5A) affecting energy metabolism. Therefore, the SCC model is established by Allen’s method, and the degree of edema and neuronal area is measured. The motor function of rats is evaluated by the Basso, Beattie, and Bresnahan (BBB) scoring system. Meanwhile, AQP4 and COX5A are detected by real-time quantitative PCR (qRT-PCR) and western blot (WB). The localization of targeted protein is exhibited by immunohistochemical staining (IHC) and immunofluorescence (IF). Additionally, the methodology of AQP4 lentivirus-mediated RNA interference (AQP4-RNAi) is used to reveal the effect on edema of SCC and the regulating molecular mechanism. Firstly, we observe that the tissue water content increases after SCC and decreases after the peak value of tissue water content at 3 days ( P < 0.05) with abundant expression of AQP4 protein locating around vascular endothelial cells (VECs), which suggests that the increasing AQP4 promotes water reabsorption and improves vasogenic edema in the early stage of SCC. However, the neuronal area is larger than in the sham group in the 7 days ( P < 0.05) with the total water content of spinal cord decrease. Meanwhile, AQP4 migrates from VECs to neuronal cytomembrane, which indicates that AQP4 plays a crucial role in aggravating the formation and development of cytotoxic edema in the middle stages of SCC. Secondly, AQP4-RNAi is used to elucidate the mechanism of AQP4 to edema of SCC. The neuronal area shrinks and the area of cytotoxic edema reduces after AQP4 downregulation. The BBB scores are significantly higher than in the vector group after AQP4-RNAi at 5, 7, and 14 ( P < 0.05). There is a relationship between AQP4 and COX5A shown by bioinformatics analysis. After AQP4 inhibition, the expression of COX5A is significantly upregulated in the swelling astrocytes. Therefore, the inhibition of AQP4 expression reduces cytotoxic edema in SCC and improves motor function, which may be associated with upregulation of COX5A via affecting energy metabolism. Moreover, it is not clear how the inhibition of AQP4 directly causes the upregulation of COX5A.

High-altitude cerebral edema: its own entity or end-stage acute mountain sickness?

“…While clearly the glymphatic system is highly complex, a major therapeutic target may be the aquaporin water channels, particularly AQP4, as they are strongly expressed at the borders between brain parenchyma and major fluid compartments, including astrocyte foot processes (brain-blood), glia limitans (brain-subarachnoid CSF), as well as ependymal cells and subependymal astrocytes (brain-ventricular CSF) (121). This pattern of water channel distribution, potential interaction of AQP4 with neighboring ionic channels / gap junctions and reliance on the normal function of the sodium potassium pump, suggests that AQP4 transcriptomes are both crucial to brain wide water homeostasis and vulnerable to reduced cellular energy metabolism; thus, altered expression can affect both the development and resolution of edema (19,51,115). Indeed, deletion of AQP4 increases cerebral extracellular volume via blunting of downstream glymphatic efflux (56), which results in a significantly greater rise in intracranial pressure post infusion of saline into brain tissue (96).…”

Reactive Astrogliosis: Implications in Spinal Cord Injury Progression and Therapy

“…Although reactive astrocytes do have some positive effects, the abnormal functions of them are more notable. At present, the negative effects of reactive astrocytes mainly include the following: (a) production of proinflammatory chemokines and cytokines [ 26 , 47 , 48 ]; (b) generation of Ca2+ signals [ 49 , 50 ]; (c) upregulation of aquaporin 4 (AQP4), leading to cytotoxic edema in spinal cord injury and ischemia [ 51 ]; (d) elevation of neuroexcitatory glutamate levels [ 52 , 53 ]; (e) promotion of proliferation and migration or formation of an immunosuppressive environment in glioblastoma [ 8 , 54 ]; (f) contribution to induction and maintenance of chronic pain [ 7 ]; and (g) maintenance and promotion of neurodegenerative diseases [ 6 ]. Due to the context-dependent nature of reactive astrocytes, descriptions of their role from different reports of the same disease model may also be different, or they may even be reported to have opposite regulatory effects.…”

Blood–Brain Barrier and Neurovascular Unit In Vitro Models for Studying Mitochondria-Driven Molecular Mechanisms of Neurodegeneration

“…Some authors suggest that aquaporins may function as a regulator of mitochondrial functions in other cell types [ 70 ], but whether it is applicable for brain cells remains to be examined. However, astroglial mitochondria inner membrane is equipped with AQP9, which is permeable for water and several organic molecules, particularly for lactate, presumably transferring this glycolytic metabolite into the mitochondria in conditions associated with OXPHOS alterations [ 71 ].…”