The Importance of AQP4 in Capillary Perfusion Dynamics during Brain Edema: A Two Photon Microscopy Study on Awake Mice

During brain edema (BE), elevated intracranial pressure (ICP) compresses capillaries and reduces cerebral blood flow (CBF) by decreasing perfusion pressure. The reduction in CBF and the increase in ICP during BE may affect individual capillary perfusion. It has previously been shown that the water channel aquaporin‐4 (AQP4) is an important mediator in the development of BE. In this study we wanted to test the hypothesis that the deletion of AQP4 may result in improved capillary flow during BE. Aim of the study To quantify pathological changes and the effect of BE on capillary perfusion in C57Bl/6 (WT) and AQP4‐KO mice using a novel model of mild‐BE by water intoxication (mild‐WI). Methods Mild‐WI (1.8 mg/ml NaCl, 10% body weight, i.p. infusion over 20 sec.) was performed in 33 mice and the effects measured by two protocols. 1) Multiphoton microscopy (TPM). We prepared mice (N = 8 WT, 7 AQP4‐KO) for awake‐TPM by surgical implantation of a chronic cranial window (left parietal bone, barrel cortex) and a head bar (occipital bone). We measured erythrocyte velocity (RBCv) by linescan and tail‐i.v. injection of fluorescent tracer (TexasRed) before and during mild‐WI. 2) regional CBF was monitored by Laser Doppler (LD) and ICP by pressure‐telemetry implant PA‐C10 (DSI) (N = 10 WT, 8 AQP4‐KO). After surgical recovery, mice were trained to be head‐fixed for 6 days. Mild‐WI was studied for 1 hour in both experimental groups. Brain water content was measured post‐mortem by wet/dry method and compared to saline‐loaded‐mice (9 mg/ml NaCl, 10% body weight). All experiments followed regulation from Norwegian Food Safety Authority. Results Brain water content was significatively higher compared to respective saline‐mice control in WT than AQP4‐KO (1.17 ± 0.1 and 0.83 ± 0.1, % difference, p=0.037). 20 min after mild‐WI, rCBF was more reduced from baseline in WT than AQP4‐KO (reduction: 50% and 36%, p=0.0115). ICP increased linearly after WI before reaching a plateau (approx. 30 min) and without inducing brain stem herniation. Plateau values of ICP were higher in WT than AQP4‐KO (27.5 ± 4.4 and 19.4 ± 5 mmHg). TPM of individual capillaries showed a consistent decrease of RBCv from baseline (WT: 1.16±0.6, AQP4: 1.04 ± 0.6 mm/sec, Mean ± SD) during the development of BE (10 and 30 from mild‐WI: WT (Δ)= 0.24 ± 0.5, 0.27 ± 0.6, AQP4‐KO (Δ) = 0.08 ± 0.58, 0.19 ± 0.37 mm/sec). The drop (%) in RBCv was higher in WT than AQP4‐KO (Median at 10, 30 min: WT= −24.7 and −33.8. AQP4‐KO: −15.9 and −22.5). Conclusions In AQP4‐KO mice, the development of mild‐BE showed a slower kinetics (lower rCBF decrease and lower ICP plateau) and lower severity (less increased brain water content) compared to WT. Accordingly, capillary flow measured on individual vessels was less reduced by BE in AQP4‐KO mice. Together, these results suggest that AQP4 may influence capillary perfusion during mild‐BE. Support or Funding Information Financial support was received from: Letten Foundation; Korningsfonden; Research Council of Norway (grant no. 249988), Aarhus University, and the Danish Medical Research Council This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .