Cerebral Microcirculation, Perivascular Unit, and Glymphatic System: Role of Aquaporin-4 as the Gatekeeper for Water Homeostasis

In the past, water homeostasis of the brain was understood as a certain quantitative equilibrium of water content between intravascular, interstitial, and intracellular spaces governed mostly by hydrostatic effects i.e., strictly by physical laws. The recent achievements in molecular bioscience have led to substantial changes in this regard. Some new concepts elaborate the idea that all compartments involved in cerebral fluid homeostasis create a functional continuum with an active and precise regulation of fluid exchange between them rather than only serving as separate fluid receptacles with mere passive diffusion mechanisms, based on hydrostatic pressure. According to these concepts, aquaporin-4 (AQP4) plays the central role in cerebral fluid homeostasis, acting as a water channel protein. The AQP4 not only enables water permeability through the blood-brain barrier but also regulates water exchange between perivascular spaces and the rest of the glymphatic system, described as pan-cerebral fluid pathway interlacing macroscopic cerebrospinal fluid (CSF) spaces with the interstitial fluid of brain tissue. With regards to this, AQP4 makes water shift strongly dependent on active processes including changes in cerebral microcirculation and autoregulation of brain vessels capacity. In this paper, the role of the AQP4 as the gatekeeper, regulating the water exchange between intracellular space, glymphatic system (including the so-called neurovascular units), and intravascular compartment is reviewed. In addition, the new concepts of brain edema as a misbalance in water homeostasis are critically appraised based on the newly described role of AQP4 for fluid permeation. Finally, the relevance of these hypotheses for clinical conditions (including brain trauma and stroke) and for both new and old therapy concepts are analyzed.

TNF-α Impairs Pericyte-Mediated Cerebral Microcirculation via the NF-κB/iNOS Axis After Experimental Traumatic Brain Injury

Background Secondary structural and functional abnormalities of the neurovascular unit are important pathological mechanisms following traumatic brain injury (TBI). The tumor necrosis factor α (TNF-α)/nuclear factor-κB (NF-κB) pathway regulates neuroinflammation and oxidative damage, which may act as triggers for pathological processes after TBI. However, the role of TNF-α/NF-κB in pericyte-mediated cerebral microcirculation are currently unknown. Methods We assessed the activity and mechanisms of the TNF-α/NF-κB signaling axis on pericyte-mediated microcirculation using the mouse controlled cortical impact model and BV2 cells. Immunofluorescent staining and western blot analysis were used to detect activation of the TNF-α/NF-κB signaling pathway and the expression of inducible nitric oxide synthase (iNOS) to evaluate the effects of the TNF-α specific inhibitor infliximab (IFX). Modified neurological severity scores, Garcia test, Nissl staining, and TUNEL staining were employed to determine the neuroprotective effects of IFX supplementation. The relative blood flow values in the capillary areas surrounding the impinging lesion were observed by Laser speckle contrast imaging. The impact of IFX on pericyte markers was assessed to evaluate whether pericyte damage was dependent on the TNF-α/NF-κB/iNOS axis to gain further insight into the mechanisms underlying the development of the microcirculation disturbance after TBI. Results Microglia were activated after TBI, and the expression of NF-κB, iNOS, a disintegrin and metalloproteinase 17, inflammatory factors, and free radicals increased around the injury areas. After lipopolysaccharide treatment, the expression of TNF-α and downstream NF-κB/iNOS in BV2 cells was significantly upregulated. Pharmacological inhibition of TNF-α via IFX significantly reduced NF-κB p65 phosphorylation and nuclear translocation and downregulated iNOS expression. Meanwhile, we found that specific inhibition of TNF-α reversed pericyte marker loss, and improved pericyte function and cerebral microcirculation perfusion after TBI, which could attenuate inflammation and oxidative damage, reduce neuronal cell damage and apoptosis, and play a neuroprotective role. Conclusion The results of this study suggested that microglia activated and released TNF-α after TBI, which promoted neuroinflammation and oxidative stress by activating downstream NF-κB/iNOS signals, and this led to pericyte-mediated disturbance of the cerebral microcirculation, which may be one of the vital mechanisms of secondary injury in TBI.

Propionate ameliorates diabetes-induced neurological dysfunction through regulating the PI3K-AKT-eNOS signaling pathway

Diabetes-entailed disorder of cognition, deemed as "diabetes-entailed brain malfunction", finds its confirmation from numerous literature. Current evidence supports that oxidative stress, neuronal apoptosis, and cerebral microcirculation weakness are associated with cognition deficits induced by diabetes. The present study explores the effect of propionate on neurological deficits, cerebral blood flow, and oxidative stress in diabetic mice. Propionate could markedly improve neurological function, which was correlated with its capabilities of stimulating NO production, increasing cerebral microcirculation, suppressing oxidative stress and reducing neuron loss in the hippocampus. In addition, the results of western blotting indicated that the brain-protective function of propionate in STZ-induced T1DM mice is related to PI3K/AKT/eNOS signaling pathway. To sum up, cure by using ester or salt of propionic acid weakens brain-related blood circulation in the microvascular system, programmed cell death of the hippocampus and nerve-related malfunction based on a mouse-modeled diabetic disease. Thus, ester or salt of propionic acid, utilized for preserving comestible at present, is potentially advantageous to the amelioration of cognition-related decay entailed by diabetic disease.

Visualization and Quantification of the Cerebral Microcirculation using Contrast-enhanced Ultrasound Particle Tracking Velocimetry

Noninvasive measurements of the regional microvascular perfusion might lead to sensitive biomarkers for the changes in intracranial hemodynamics that could guide timely surgical interventions for neonatal brain injuries. The current work utilizes a clinically available contrast enhanced ultrasound (CEUS) system and particle tracking velocimetry to perform ultrasound localization microscopy for measuring the microcirculation in piglets. A new deep learning method based on U-net is proposed for enhancing noisy raw CEUS images and detecting the microbubbles. Subsequently, the bubbles are tracked using a Kalman filter based method, which incorporates conditions of spatio-temporal consistency in flow direction and globally optimizes the assignment of bubbles to trajectories. Based on analysis of synthetic data, the U-net results demonstrate significant improvement in the processing speed and localization accuracy over a conventional blind deconvolution method. Visualization of the microvasculature is performed by superposing the bubble trajectories, enabling depiction of a complex micro-vessel network, where neighboring vessels separated by 40 µm can be distinguished. The corresponding perfusion map shows the velocity distribution in these vessels. Based on the current frame rate (44 fps), speeds in the 0.1 to 12 cm/s range can be well captured. These methods show promise as potential clinical tools for bedside measurement of cerebral microcirculation.

Levosimendan increases brain tissue oxygen levels after cardiopulmonary resuscitation independent of cardiac function and cerebral perfusion

Prompt reperfusion is important to rescue ischemic tissue; however, the process itself presents a key pathomechanism that contributes to a poor outcome following cardiac arrest. Experimental data have suggested the use of levosimendan to limit ischemia–reperfusion injury by improving cerebral microcirculation. However, recent studies have questioned this effect. The present study aimed to investigate the influence on hemodynamic parameters, cerebral perfusion and oxygenation following cardiac arrest by ventricular fibrillation in juvenile male pigs. Following the return of spontaneous circulation (ROSC), animals were randomly assigned to levosimendan (12 µg/kg, followed by 0.3 µg/kg/min) or vehicle treatment for 6 h. Levosimendan-treated animals showed significantly higher brain PbtO2 levels. This effect was not accompanied by changes in cardiac output, preload and afterload, arterial blood pressure, or cerebral microcirculation indicating a local effect. Cerebral oxygenation is key to minimizing damage, and thus, current concepts are aimed at improving impaired cardiac output or cerebral perfusion. In the present study, we showed that NIRS does not reliably detect low PbtO2 levels and that levosimendan increases brain oxygen content. Thus, levosimendan may present a promising therapeutic approach to rescue brain tissue at risk following cardiac arrest or ischemic events such as stroke or traumatic brain injury.

Vascular Biology

The article summarizes recent studies investigating mechanisms of collateral flow variability in stroke and the role of pericytes in cerebral microcirculation and stroke.

Increased beat-to-beat variability of cerebral microcirculatory perfusion during atrial fibrillation: a near-infrared spectroscopy study

Aims Atrial fibrillation (AFib) is associated with cognitive decline/dementia, independently from clinical strokes or transient ischaemic attacks (TIA). Recent in silico data suggested that AFib may induce transient critical haemodynamic events in the cerebral microcirculation. The aim of this study is to use non-invasive spatially resolved cerebral near-infrared spectroscopy (SRS-NIRS) to investigate in vivo beat-to-beat microcirculatory perfusion during AFib and after sinus rhythm (SR) restoration. Methods and results Cerebral SRS-NIRS with high-frequency sampling (20 Hz) and non-invasive systemic haemodynamic monitoring were recorded before and after elective electrical cardioversion (ECV) for AFib or atrial flutter (AFL). To assess beat-to-beat effects of the rhythm status, the frequency distribution of inter-beat differences in tissue haemoglobin index (THI), a proxy of microcirculatory cerebral perfusion, was compared before and after SR restoration. Fifty-three AFib/AFL patients (mean age 69 ± 8 years, 79% males) were ultimately enrolled. Cardioversion was successful in restoring SR in 51 (96%) patients. In front of a non-significant decrease in arterial blood pressure extreme events between pre- and post-ECV measurements, a significant decrease of both hypoperfusive and hyperperfusive/hypertensive microcirculatory events was observed after SR restoration (P < 0.001 and P = 0.041, respectively). Conclusion The present is the first in vivo demonstration that SR restoration by ECV significantly reduces the burden of extreme single-beat haemodynamic events in cerebral microcirculation. Future studies are needed to assess whether SR maintenance might slow long-term AFib-correlated cognitive decline/dementia.