Real-time monitoring of changes in brain extracellular sodium and potassium concentrations and intracranial pressure after selective vasopressin-1a receptor inhibition following focal traumatic brain injury in rats

Aquaporin 4 in Traumatic Brain Injury: From Molecular Pathways to Therapeutic Target

Traumatic brain injury (TBI) is known as an acute degenerative pathology of the central nervous system, and has been shown to increase brain aquaporin 4 (AQP4) expression. Various molecular mechanisms affect AQP4 expression, including neuronal high mobility group box 1, forkhead box O3a, vascular endothelial growth factor, hypoxia-inducible factor-1 α (HIF-1 α) sirtuin 2, NF-κB, Malat1, nerve growth factor and Angiotensin II receptor type 1. In addition, inhibition of AQP4 with FK-506, MK-801 (indirectly by targeting N-methyl-D-aspartate receptor), inactivation of adenosine A2A receptor, levetiracetam, adjudin, progesterone, estrogen, V1aR inhibitor, hypertonic saline, erythropoietin, poloxamer 188, brilliant blue G, HIF-1alpha inhibitor, normobaric oxygen therapy, astaxanthin, epigallocatechin-3-gallate, sesamin, thaliporphine, magnesium, prebiotic fiber, resveratrol and omega-3, as well as AQP4 gene silencing lead to reduced edema upon TBI. This review summarizes current knowledge and evidence on the relationship between AQP4 and TBI, and the potential mechanisms involved.

Emerging therapeutic targets for cerebral edema

INTRODUCTION

Cerebral edema is a key contributor to death and disability in several forms of brain injury. Current treatment options are limited, reactive, and associated with significant morbidity. Targeted therapies are emerging based on a growing understanding of the molecular underpinnings of cerebral edema.

AREAS COVERED

We review the pathophysiology and relationships between different cerebral edema subtypes to provide a foundation for emerging therapies. Mechanisms for promising molecular targets are discussed, with an emphasis on those advancing in clinical trials, including ion and water channels (AQP4, SUR1-TRPM4) and other proteins/lipids involved in edema signaling pathways (AVP, COX2, VEGF, and S1P). Research on novel treatment modalities for cerebral edema [including recombinant proteins and gene therapies] is presented and finally, insights on reducing secondary injury and improving clinical outcome are offered.

EXPERT OPINION

Targeted molecular strategies to minimize or prevent cerebral edema are promising. Inhibition of SUR1-TRPM4 (glyburide/glibenclamide) and VEGF (bevacizumab) are currently closest to translation based on advances in clinical trials. However, the latter, tested in glioblastoma multiforme, has not demonstrated survival benefit. Research on recombinant proteins and gene therapies for cerebral edema is in its infancy, but early results are encouraging. These newer modalities may facilitate our understanding of the pathobiology underlying cerebral edema.

Traumatic Brain Injury: Ultrastructural Features in Neuronal Ferroptosis, Glial Cell Activation and Polarization, and Blood-Brain Barrier Breakdown

The secondary injury process after traumatic brain injury (TBI) results in motor dysfunction, cognitive and emotional impairment, and poor outcomes. These injury cascades include excitotoxic injury, mitochondrial dysfunction, oxidative stress, ion imbalance, inflammation, and increased vascular permeability. Electron microscopy is an irreplaceable tool to understand the complex pathogenesis of TBI as the secondary injury is usually accompanied by a series of pathologic changes at the ultra-micro level of the brain cells. These changes include the ultrastructural changes in different parts of the neurons (cell body, axon, and synapses), glial cells, and blood–brain barrier, etc. In view of the current difficulties in the treatment of TBI, identifying the changes in subcellular structures can help us better understand the complex pathologic cascade reactions after TBI and improve clinical diagnosis and treatment. The purpose of this review is to summarize and discuss the ultrastructural changes related to neurons (e.g., condensed mitochondrial membrane in ferroptosis), glial cells, and blood–brain barrier in the existing reports of TBI, to deepen the in-depth study of TBI pathomechanism, hoping to provide a future research direction of pathogenesis and treatment, with the ultimate aim of improving the prognosis of patients with TBI.

Insights into Cell Surface Expression, Supramolecular Organization, and Functions of Aquaporin 4 Isoforms in Astrocytes

Aquaporin 4 (AQP4) is the most abundant water channel in the central nervous system (CNS). Its expression is confined to non-neuronal glial cells, predominantly to astrocytes that represent a heterogeneous glial cell type in the CNS. The membrane of astrocyte processes, which align brain capillaries and pia, is particularly rich in AQP4. Several isoforms of AQP4 have been described; however, only some (AQP4a (M1), AQP4 c (M23), AQP4e, and AQP4ex) have been identified in the plasma membrane assemblies of astrocytes termed orthogonal arrays of particles (OAPs). Intracellular splicing isoforms (AQP4b, AQP4d, AQP4f, AQP4-Δ4) have been documented, and most of them are postulated to have a role in the cell surface distribution of the plasma membrane isoforms and in the formation of OAPs in murine and human astrocytes. Although OAPs have been proposed to play various roles in the functioning of astrocytes and CNS tissue as a whole, many of these still need to be described. OAPs are studied primarily from the perspective of understanding water permeability regulation through the plasma membrane and of their involvement in cell adhesion and in the dynamics of astrocytic processes. This review describes the cellular distribution of various AQP4 isoforms and their implications in OAP assembly, which is regulated by several intracellular and extracellular proteins.

Evidence of early vasogenic edema following minor head impact that can be reduced with a vasopressin V1a receptor antagonist

BACKGROUND

Does minor head impact without signs of structural brain damage cause short-term changes in vasogenic edema as measured by an increase apparent diffusion coefficient (ADC) using diffusion weighted imaging? If so, could the increase in vasogenic edema be treated with a vasopressin V1a receptor antagonist? We hypothesized that SRX251, a highly selective V1a antagonist, would reduce vasogenic edema in response to a single minor head impact.

METHODS

Lightly anesthetized male rats were subjected to a sham procedure or a single hit to the forehead using a closed skull, momentum exchange model. Animals recovered in five min and were injected with saline vehicle (n = 8) or SRX251 (n = 8) at 15 min post head impact and again 7-8 hrs later. At 2 h, 6 h, and 24 h post injury, rats were anesthetized and scanned for increases in ADC, a neurological measure of vasogenic edema. Sham rats (n = 6) were exposed to anesthesia and scanned at all time points but were not hit or treated. Images were registered to and analyzed using a 3D MRI rat atlas providing site-specific data on 150 different brain areas. These brain areas were parsed into 11 major brain regions.

RESULTS

Untreated rats with brain injury showed a significant increase in global brain vasogenic edema as compared to sham and SRX251 treated rats. Edema peaked at 6 h in injured, untreated rats in three brain regions where changes in ADC were observed, but returned to sham levels by 24 h. There were regional variations in the time course of vasogenic edema and drug efficacy. Edema was significantly reduced in cerebellum and thalamus with SRX251 treatment while the basal ganglia did not show a response to treatment.

CONCLUSION

A single minor impact to the forehead causes regional increases in vasogenic edema that peak at 6 h but return to baseline within a day in a subset of brain regions. Treatment with a selective V1a receptor antagonist can reduce much of the edema.

Treating head injury using a novel vasopressin 1a receptor antagonist

Arginine vasopressin (AVP) is a chemical signal in the brain that influences cerebral vascular resistance and brain water permeability. Increases in AVP contribute to the pathophysiology of brain edema following traumatic brain injury (TBI). These effects are mediated through AVP V1a receptors that are expressed in cortical and subcortical brain areas. This exploratory study characterizes the effects of a novel, V1a receptor antagonist, AVN576, on behavioral and magnetic resonance imaging (MRI) measures after severe TBI. Male Sprague Dawley rats were impacted twice producing contusions in the forebrain, putative cerebral edema, and cognitive deficits. Rats were treated with AVN576 after initial impact for 5 days and then tested for changes in cognition. MRI was used to assess brain injury, enlargement of the ventricles, and resting state functional connectivity. Vehicle treated rats had significant deficits in learning and memory, enlarged ventricular volumes, and hypoconnectivity in hippocampal circuitry. AVN576 treatment eliminated the enlargement of the lateral ventricles and deficits in cognitive function while increasing connectivity in hippocampal circuitry. These data corroborate the extensive literature that drugs selectively targeting the AVP V1a receptor could be used to treat TBI in the clinic.

Intracranial Pressure Monitoring in Experimental Traumatic Brain Injury: Implications for Clinical Management

Traumatic brain injury (TBI) is often associated with long-term disability and chronic neurological sequelae. One common contributor to unfavorable outcomes is secondary brain injury, which is potentially treatable and preventable through appropriate management of patients in the neurosurgical intensive care unit. Intracranial pressure (ICP) is currently the predominant neurological-specific physiological parameter used to direct the care of severe TBI (sTBI) patients. However, recent clinical evidence has called into question the association of ICP monitoring with improved clinical outcome. The detailed cellular and molecular derangements associated with intracranial hypertension (IC-HTN) and their relationship to injury phenotype and neurological outcomes are not completely understood. Various animal models of TBI have been developed, but the clinical applicability of ICP monitoring in the pre-clinical setting has not been well-characterized. Linking basic mechanistic studies in translational TBI models with investigation of ICP monitoring that more faithfully replicates the clinical setting will provide clinical investigators with a more informed understanding of the pathophysiology of IC-HTN, thus facilitating development of improved therapies for sTBI patients.

Immunohistochemical Evaluation of Aquaporin-4 and its Correlation with CD68, IBA-1, HIF-1α, GFAP, and CD15 Expressions in Fatal Traumatic Brain Injury

Traumatic brain injury (TBI) is one of the leading causes of death and disability worldwide. Our understanding of its pathobiology has substantially increased. Following TBI, the following occur, edema formation, brain swelling, increased intracranial pressure, changes in cerebral blood flow, hypoxia, neuroinflammation, oxidative stress, excitotoxicity, and apoptosis. Experimental animal models have been developed. However, the difficulty in mimicking human TBI explains why few neuroprotective strategies, drawn up on the basis of experimental studies, have translated into improved therapeutic strategies for TBI patients. In this study, we retrospectively examined brain samples in 145 cases of death after different survival times following TBI, to investigate aquaporin-4 (AQP4) expression and correlation with hypoxia, and neuroinflammation in human TBI. Antibodies anti-glial fibrillary acid protein (GFAP), aquaporin-4 (AQP4), hypoxia induced factor-1α (HIF-1α), macrophage/phagocytic activation (CD68), ionized calcium-binding adapter molecule-1 (IBA-1), and neutrophils (CD15) were used. AQP4 showed a significant, progressive increase between the control group and groups 2 (one-day survival) and 3 (three-day survival). There were further increases in AQP4 immunopositivity in groups 4 (seven-day survival), 5 (14-dayssurvival), and 6 (30-day survival), suggesting an upregulation of AQP4 at 7 to 30 days compared to group 1. GFAP showed its highest expression in non-acute cases at the astrocytic level compared with the acute TBI group. Data emerging from the HIF-1α reaction showed a progressive, significant increase. Immunohistochemistry with IBA-1 revealed activated microglia starting three days after trauma and progressively increasing in the next 15 to 20 days after the initial trauma. CD68 expression demonstrated basal macrophage and phagocytic activation mostly around blood vessels. Starting from one to three days of survival after TBI, an increase in the number of CD68 cells was progressively observed; at 15 and 30 days of survival, CD68 showed the most abundant immunopositivity inside or around the areas of necrosis. These findings need to be developed further to gain insight into the mechanisms through which brain AQP4 is upregulated. This could be of the utmost clinicopathological importance.

Pathophysiology and treatment of cerebral edema in traumatic brain injury

Cerebral edema (CE) and resultant intracranial hypertension are associated with unfavorable prognosis in traumatic brain injury (TBI). CE is a leading cause of in-hospital mortality, occurring in >60% of patients with mass lesions, and ∼15% of those with normal initial computed tomography scans. After treatment of mass lesions in severe TBI, an important focus of acute neurocritical care is evaluating and managing the secondary injury process of CE and resultant intracranial hypertension. This review focuses on a contemporary understanding of various pathophysiologic pathways contributing to CE, with a subsequent description of potential targeted therapies. There is a discussion of identified cellular/cytotoxic contributors to CE, as well as mechanisms that influence blood-brain-barrier (BBB) disruption/vasogenic edema, with the caveat that this distinction may be somewhat artificial since molecular processes contributing to these pathways are interrelated. While an exhaustive discussion of all pathways with putative contributions to CE is beyond the scope of this review, the roles of some key contributors are highlighted, and references are provided for further details. Potential future molecular targets for treating CE are presented based on pathophysiologic mechanisms. We thus aim to provide a translational synopsis of present and future strategies targeting CE after TBI in the context of a paradigm shift towards precision medicine. This article is part of the Special Issue entitled "Novel Treatments for Traumatic Brain Injury".

Aquaporins in Brain Edema and Neuropathological Conditions

The aquaporin (AQP) family of water channels are a group of small, membrane-spanning proteins that are vital for the rapid transport of water across the plasma membrane. These proteins are widely expressed, from tissues such as the renal epithelium and erythrocytes to the various cells of the central nervous system. This review will elucidate the basic structure and distribution of aquaporins and discuss the role of aquaporins in various neuropathologies. AQP1 and AQP4, the two primary aquaporin molecules of the central nervous system, regulate brain water and CSF movement and contribute to cytotoxic and vasogenic edema, where they control the size of the intracellular and extracellular fluid volumes, respectively. AQP4 expression is vital to the cellular migration and angiogenesis at the heart of tumor growth; AQP4 is central to dysfunctions in glutamate metabolism, synaptogenesis, and memory consolidation; and AQP1 and AQP4 adaptations have been seen in obstructive and non-obstructive hydrocephalus and may be therapeutic targets.

Potassium Aspartate Attenuates Brain Injury Induced by Controlled Cortical Impact in Rats Through Increasing Adenosine Triphosphate (ATP) Levels, Na+/K+-ATPase Activity and Reducing Brain Edema

BACKGROUND Potassium aspartate (PA), as an electrolyte supplement, is widely used in clinical practice. In our previous study, we found PA had neuroprotective effects against apoptosis after cerebral ischemia/reperfusion in rats. In this study, we examine whether PA has protective effects on traumatic brain injury (TBI). MATERIAL AND METHODS TBI was induced by controlled cortical impact (CCI) in rats. Vehicle treatment (control) or PA treatment was administered intraperitoneally at 30 minutes after CCI. The modified neurological severity score (mNSS) and cortical lesion volume were examined. Brain edema and blood-brain barrier (BBB) integrity were measured, as well as brain ATP contents, lactic acid levels, and Na+/K+-ATPase activities. RESULTS We found that CCI induced cortical injury in rats. Acute PA treatment at the dose of 62.5 mg/kg and 125 mg/kg significantly improved neurological deficits (p<0.05 and p<0.001, respectively) and decreased the cortical lesion volume (p<0.05 and p<0.001, respectively) compared with vehicle-only treatment. PA treatment at the dose of 125 mg/kg attenuated brain edema and ameliorated BBB integrity. In addition, PA treatment significantly reduced the loss of ATP (p<0.01), reduced lactic acid levels (p<0.001), and increased the activity of Na+/K+-ATPase (p<0.01). CONCLUSIONS Our results indicate PA has neuroprotective effects on TBI through increasing ATP levels, Na+/K+-ATPase activity, and reducing brain edema. It provides experimental evidence for the clinical application of PA.

Effects of Different Doses of Levetiracetam on Aquaporin 4 Expression in Rats with Brain Edema Following Fluid Percussion Injury

Background

This study was designed to investigate the effects of different doses of levetiracetam on aquaporin 4 (AQP4) expression in rats after fluid percussion injury.

Material/Methods

Sprague-Dawley rats were randomly divided into 4 groups: sham operation group, traumatic brain injury group, low-dose levetiracetam group, and high-dose levetiracetam group. Brain edema models were established by fluid percussion injury, and intervened by the administration of levetiracetam. Samples from the 4 groups were collected at 2, 6, 12, and 24 h, and at 3 and 7 days after injury. Histological observation was performed using hematoxylin-eosin staining and immunohistochemical staining. AQP4 and AQP4 mRNA expression was detected using Western blot assay and RT-PCR. Brain water content was measured by the dry-wet method.

Results

Compared with the traumatic brain injury group, brain water content, AQP4 expression, and AQP4 mRNA expression were lower in the levetiracetam groups at each time point and the differences were statistically significant (P<0.05). The intervention effects of high-dose levetiracetam were more apparent.

Conclusions

Levetiracetam can lessen brain edema from fluid percussion injury by down-regulating AQP4 and AQP4 mRNA expression. There is a dose-effect relationship in the preventive effect of levetiracetam within a certain extent.

Sodium and Potassium Relating to Parkinson's Disease and Traumatic Brain Injury

Alkali metals, especially sodium and potassium, are plentiful and vital in biological systems. They take on important roles in health and disease. Such roles include the regulation of homeostasis, osmosis, blood pressure, electrolytic equilibria, and electric current. However, there is a limit to our present understanding; the ions have a great ability and capacity for action in health and disease, much greater than our current understanding. For the regulation of physiological homeostasis, there is a crucial regulator (renin-angiotensin system, RAS), found at both peripheral and central levels. Misregulation of the Na(+)-K(+) pump, and sodium channels in RAS are important for the understanding of disease progression, hypertension, diabetes, and neurodegenerative diseases, etc. In particular, RAS displays direct or indirect interaction important to Parkinson's disease (PD). In this chapter, the relationship between the regulation of sodium/potassium concentration and PD was sought. In addition, some recent biochemical and clinical findings are also discussed that help describe sodium and potassium in the context of traumatic brain injury (TBI). TBI is caused from the heavy striking of the head; this strongly affects ion flux in the affected tissue (brain) and damages cellular regulation systems. Thus, inappropriate concentrations of ions (hyper- and hyponatremia, and hyper- and hypokalemia) will perturb homeostasis giving rise to important and far reaching effects. These changes also impact osmotic pressure and the concentration of other metal ions, such as the calcium(II) ion.

Selective vasopressin-1a receptor antagonist prevents brain edema, reduces astrocytic cell swelling and GFAP, V1aR and AQP4 expression after focal traumatic brain injury

A secondary and often lethal consequence of traumatic brain injury is cellular edema that we posit is due to astrocytic swelling caused by transmembrane water fluxes augmented by vasopressin-regulated aquaporin-4 (AQP4). We therefore tested whether vasopressin 1a receptor (V1aR) inhibition would suppress astrocyte AQP4, reduce astrocytic edema, and thereby diminish TBI-induced edematous changes. V1aR inhibition by SR49059 significantly reduced brain edema after cortical contusion injury (CCI) in rat 5h post-injury. Injured-hemisphere brain water content (n=6 animals/group) and astrocytic area (n=3/group) were significantly higher in CCI-vehicle (80.5±0.3%; 18.0±1.4 µm(2)) versus sham groups (78.3±0.1%; 9.5±0.9 µm(2)), and SR49059 blunted CCI-induced increases in brain edema (79.0±0.2%; 9.4±0.8µm(2)). CCI significantly up-regulated GFAP, V1aR and AQP4 protein levels and SR49059 suppressed injury induced up regulation (n=6/group). In CCI-vehicle, sham and CCI-SR49059 groups, GFAP was 1.58±0.04, 0.47±0.02, and 0.81±0.03, respectively; V1aR was 1.00±0.06, 0.45±0.05, and 0.46±0.09; and AQP4 was 2.03±0.34, 0.49±0.04, and 0.92±0.22. Confocal immunohistochemistry gave analogous results. In CCI-vehicle, sham and CCI-SR49059 groups, fluorescence intensity of GFAP was 349±38, 56±5, and 244±30, respectively, V1aR was 601±71, 117.8±14, and 390±76, and AQP4 was 818±117, 158±5, and 458±55 (n=3/group). The results support that edema was predominantly cellular following CCI and documented that V1aR inhibition with SR49059 suppressed injury-induced up regulation of GFAP, V1A and AQP4, blunting edematous changes. Our findings suggest V1aR inhibitors may be potential therapeutic tools to prevent cellular swelling and provide treatment for post-traumatic brain edema.