Dynamic in vivo imaging of cerebral blood flow and blood-brain barrier permeability

Visualization of Blood-Brain Barrier Disruption in Septic Mice with the New Method Based on in Vivo Imaging Technology

BACKGROUND

Dynamic monitoring of the blood-brain barrier (BBB) functional status in septic mice can help to explore the pathological mechanisms. Therefore, we proposed a new method for monitoring BBB permeability and applied it to the detection of sepsis models.

METHODS

The new method involves the construction of an optical cranial window and in vivo imaging. We performed dynamic monitoring of BBB permeability and cerebral blood flow (CBF) in cecal ligation puncture (CLP) and endotoxemia (lipopolysaccharide [LPS]) mice.

RESULTS

The sensitivity and accuracy of this method were higher than those of Evans blue evaluation. The increase of BBB permeability in the group of CLP mice was relatively mild and correlated with overall survival, and the damage was irreversible. Contrarily, BBB damage in the LPS group was more acute and severe, unrelated to overall survival, but recoverable. The CBF decreased significantly in both model mouse groups 24 h after modeling, but only the CBF proportion decrease in the LPS group was significantly correlated with an increase in BBB permeability. Within 24 h after both models were established, the decrease in blood flow in the digestive organs occurred earlier than in the brain and kidneys, and the decrease in small intestine blood flow in the LPS group progressed faster.

CONCLUSIONS

We have successfully demonstrated the feasibility of our novel method to detect BBB permeability in mice. Our results revealed a significant difference in the BBB permeability change trend between the CLP and LPS model mice when survival curves were consistent. Notably, the CLP-model mice demonstrated a closer resemblance to clinical patients. Our findings suggest that early-stage brain tissue hypoperfusion has a greater impact on BBB function damage in endotoxemia mice, which is related to the faster progression of blood flow redistribution.

Rodent brain extraction and dissection: A comprehensive approach

The neuroscience is continuously expanding field, and conducting experiments serves as one of the most effective approaches to enhance and broad our understanding of this fascinating field. Most of the lab work in neuroscience involves the use of animal models such as rats and mice for experiments dedicated to monitoring cerebral changes. The study:•Introduces a practical method for brain extraction without perfusion with paraformaldehyde prioritizing brain integrity and avoiding damage.•Offers a detailed, step-by-step dissection guide for different brain regions, including the hippocampus, cerebral cortex, corpus striatum, thalamus, cerebellum, and medial prefrontal cortex, from rodent brains, accompanied by high-resolution images that provide anatomical clarity.•Presents enhanced reliability, precision, and detailed anatomical descriptions.Conclusion: This study has introduced a reliable technique for brain extraction that eliminates the need for paraformaldehyde perfusion. Furthermore, a comprehensive methodology has been presented for extracting different brain regions from rodent brains.

Mechanisms of blood-retinal barrier disruption related to intraocular inflammation and malignancy

Blood-retinal barrier (BRB) disruption is a common accompaniment of intermediate, posterior and panuveitis causing leakage into the retina and macular oedema resulting in vision loss. It is much less common in anterior uveitis or in patients with intraocular lymphoma who may have marked signs of intraocular inflammation. New drugs used for chemotherapy (cytarabine, immune checkpoint inhibitors, BRAF inhibitors, EGFR inhibitors, bispecific anti-EGFR inhibitors, MET receptor inhibitors and Bruton tyrosine kinase inhibitors) can also cause different types of uveitis and BRB disruption. As malignant disease itself can cause uveitis, particularly from breast, lung and gastrointestinal tract cancers, it can be clinically difficult to sort out the cause of BRB disruption. Immunosuppression due to malignant disease and/or chemotherapy can lead to infection which can also cause BRB disruption and intraocular infection. In this paper we address the pathophysiology of BRB disruption related to intraocular inflammation and malignancy, methods for estimating the extent and effect of the disruption and examine why some types of intraocular inflammation and malignancy cause BRB disruption and others do not. Understanding this may help sort and manage these patients, as well as devise future therapeutic approaches.

Memantine inhibits cortical spreading depolarization and improves neurovascular function following repetitive traumatic brain injury

Cortical spreading depolarization (CSD) is a promising target for neuroprotective therapy in traumatic brain injury (TBI). We explored the effect of NMDA receptor antagonism on electrically triggered CSDs in healthy and brain-injured animals. Rats received either one moderate or four daily repetitive mild closed head impacts (rmTBI). Ninety-three animals underwent craniectomy with electrocorticographic (ECoG) and local blood flow monitoring. In brain-injured animals, ketamine or memantine inhibited CSDs in 44 to 88% and 50 to 67% of cases, respectively. Near-DC/AC-ECoG amplitude was reduced by 44 to 75% and 52 to 67%, and duration by 39 to 87% and 61 to 78%, respectively. Daily memantine significantly reduced spreading depression and oligemia following CSD. Animals (N = 31) were randomized to either memantine (10 mg/kg) or saline with daily neurobehavioral testing. Memantine-treated animals had higher neurological scores. We demonstrate that memantine improved neurovascular function following CSD in sham and brain-injured animals. Memantine also prevented neurological decline in a blinded, preclinical randomized rmTBI trial.

A fluorescein angiography-based computer-aided algorithm for assessing the retinal vasculature in diabetic retinopathy

OBJECTIVE

To present a fluorescein angiography (FA)‒based computer algorithm for quantifying retinal blood flow, perfusion, and permeability, in patients with diabetic retinopathy (DR). Secondary objectives were to quantitatively assess treatment efficacy following panretinal photocoagulation (PRP) and define thresholds for pathology based on a new retinovascular function (RVF) score for quantifying disease severity.

METHODS

FA images of 65 subjects (58 patients and 7 healthy volunteers) were included. Dye intensity kinetics were derived using pixel-wise linear regression as a measure of retinal blood flow, perfusion, and permeability. Maps corresponding to each measure were then generated for each subject and segmented further using an ETDRS grid. Non-parametric statistical analyses were performed between all ETDRS subfields. For 16 patients, the effect of PRP was measured using the same parameters, and an amalgam of RVF was used to create an RVF index. For ten post-treatment patients, the change in FA-derived data was compared to the macular thickness measured using optical coherence tomography.

RESULTS

Compared to healthy controls, patients had significantly lower retinal and regional perfusion and flow, as well as higher retinal permeability (p < 0.05). Moreover, retinal flow was inversely correlated with permeability (R = -0.41; p < 0.0001). PRP significantly reduced retinal permeability (p < 0.05). The earliest marker of DR was reduced retinal blood flow, followed by increased permeability. FA-based RVF index was a more sensitive indicator of treatment efficacy than macular thickness.

CONCLUSIONS

Our algorithm can be used to quantify retinovascular function, providing an earlier diagnosis and an objective characterisation of disease state, disease progression, and response to treatment.

A companion to the preclinical common data elements and case report forms for in vivo rodent neuroimaging: A report of the TASK3-WG3 Neuroimaging Working Group of the ILAE/AES Joint Translational Task Force

The International League Against Epilepsy/American Epilepsy Society (ILAE/AES) Joint Translational Task Force established the TASK3 working groups to create common data elements (CDEs) for various aspects of preclinical epilepsy research studies, which could help improve the standardization of experimental designs. In this article, we discuss CDEs for neuroimaging data that are collected in rodent models of epilepsy, with a focus on adult rats and mice. We provide detailed CDE tables and case report forms (CRFs), and with this companion manuscript, we discuss the methodologies for several imaging modalities and the parameters that can be collected.

Craniotomy for acute monitoring of pial vessels in the rodent brain

A growing awareness for vascular contribution to pathogenesis of brain diseases increases the need for techniques that allow high-resolution imaging and quantification of changes in function and structure of cerebral microvessels. Cerebral vessels are very sensitive structures, making them vulnerable for injury. In addition, they are uniquely characterized with the blood-brain barrier, and an extra caution is required during procedures that involve engagement of cerebral vessels (i.e., craniotomy).

Using state of the art facilities, including 3D intravital microscope, we describe here in details:•

The steps and equipment required for drilling a craniotomy and removing of the dura, while keeping brain parenchyma and vessels intact. This enables long duration of live and direct monitoring of pial vessels and imaging of BBB permeability.

•

We present the craniotomy procedure that relevant and compatible with imaging pial vessels and monitoring the blood-brain barrier in small rodents.

Visualization of blood-brain barrier disruption with dual-wavelength high-resolution photoacoustic microscopy

The blood-brain barrier (BBB) strictly regulates the substance exchange between the vascular network and the central nervous system, and plays a critical role in maintaining normal brain homeostasis. Impaired BBB is often accompanied with the emergence of cerebral diseases and probably further leads to severe neuroinflammation or even neurological degeneration. Hence, there is an urgent need to precisely monitor the impaired BBB to understand its pathogenesis and better guide the enactment of therapeutic strategies. However, there is a lack of high-resolution imaging techniques to visualize and evaluate the large-scale BBB disruption in pre-clinical and clinical aspects. In this study, we propose a dual-wavelength photoacoustic imaging (PAI) methodology that simultaneously reveals the abnormal microvasculature and impaired BBB within the cerebral cortex. In in vivo studies, BBB disruption in both mice and rats were induced by local hot-water stimulation and unilateral carotid arterial perfusion of hyperosmolar mannitol, respectively. Subsequently, the exogenous contrast agent (CA) was injected into the microcirculation via the tail vein, and photoacoustic (PA) images of the microvasculature and leaked CA within the cerebral cortex were obtained by dual-wavelength photoacoustic microscopy to evaluate the BBB disruption. Besides, analysis of distribution and concentration of leaked CA in lesion region was further conducted to quantitatively reveal the dynamic changes of BBB permeability. Furthermore, we exploited this approach to investigate the reversibility of BBB disruption within the two distinct models. Based on the experimental results, this new proposed approach presents excellent performance in visualizing microvasculature and leaked CA, and enabling it possesses great potential in evaluating the abnormal microvasculature and impaired BBB result from cerebrovascular diseases.

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.

Brainstem and Cortical Spreading Depolarization in a Closed Head Injury Rat Model

Traumatic brain injury (TBI) is the leading cause of death in young individuals, and is a major health concern that often leads to long-lasting complications. However, the electrophysiological events that occur immediately after traumatic brain injury, and may underlie impact outcomes, have not been fully elucidated. To investigate the electrophysiological events that immediately follow traumatic brain injury, a weight-drop model of traumatic brain injury was used in rats pre-implanted with epidural and intracerebral electrodes. Electrophysiological (near-direct current) recordings and simultaneous alternating current recordings of brain activity were started within seconds following impact. Cortical spreading depolarization (SD) and SD-induced spreading depression occurred in approximately 50% of mild and severe impacts. SD was recorded within three minutes after injury in either one or both brain hemispheres. Electrographic seizures were rare. While both TBI- and electrically induced SDs resulted in elevated oxidative stress, TBI-exposed brains showed a reduced antioxidant defense. In severe TBI, brainstem SD could be recorded in addition to cortical SD, but this did not lead to the death of the animals. Severe impact, however, led to immediate death in 24% of animals, and was electrocorticographically characterized by non-spreading depression (NSD) of activity followed by terminal SD in both cortex and brainstem.

The role of spreading depolarizations and electrographic seizures in early injury progression of the rat photothrombosis stroke model

Spreading depolarization (SD) and seizures are pathophysiological events associated with cerebral ischemia. Here, we investigated their role for injury progression in the cerebral cortex. Cerebral ischemia was induced in anesthetized male Wistar rats using the photothrombosis (PT) stroke model. SD and spontaneous neuronal activity were recorded in the presence of either urethane or ketamine/xylazine anesthesia. Blood-brain barrier (BBB) permeability, cerebral perfusion, and cellular damage were assessed through a cranial window and repeated intravenous injection of fluorescein sodium salt and propidium iodide until 4 h after PT. Neuronal injury and early lesion volume were quantified by stereological cell counting and manual and automated assessment of ex vivo T2-weighted magnetic resonance imaging. Onset SDs originated at the thrombotic core and invaded neighboring cortex, whereas delayed SDs often showed opposite propagation patterns. Seizure induction by 4-aminopyridine caused no increase in lesion volume or neuronal injury in urethane-anesthetized animals. Ketamine/xylazine anesthesia was associated with a lower number of onset SDs, reduced lesion volume, and neuronal injury despite a longer duration of seizures. BBB permeability increase inversely correlated with the number of SDs at 3 and 4 h after PT. Our results provide further evidence that ketamine may counteract the early progression of ischemic injury.

Assessment of Blood-Brain Barrier Permeability Using Miniaturized Fluorescence Microscopy in Freely Moving Rats

We report here the method of visualization of brain microcirculation and assessment of blood-brain barrier (BBB) permeability changes using the miniature integrated fluorescence microscope (i.e., miniscope) technology in awake, freely moving rats. The imaging cannula is implanted in the brain area of interest of anesthetized adult rats. After recovery and habituation, sodium fluorescein, a low-molecular-weight tracer, is injected i.v. Fluorescence intensity in the vicinity of microvessels, as an indicator of BBB permeability, is then recorded in vivo via the miniscope for extended periods of time. The method can be used to assess the changes in BBB permeability produced by pharmacologic agents; in this case, the drug of interest is administered after sodium fluorescein. An increase in the sodium fluorescein extravasation in brain microcirculation demonstrates an increase in BBB permeability. The method described here allows a high-resolution visualization of real-time changes in BBB permeability in awake, freely moving rats.

Longitudinal Characterization of Blood-Brain Barrier Permeability after Experimental Traumatic Brain Injury by In Vivo 2-Photon Microscopy

Vasogenic brain edema (VBE) formation remains an important factor determining the fate of patients with traumatic brain injury (TBI). The spatial and temporal development of VBE, however, remains poorly understood because of the lack of sufficiently sensitive measurement techniques. To close this knowledge gap, we directly visualized the full time course of vascular leakage after TBI by in vivo 2-photon microscopy (2-PM). Male C57BL/6 mice (n = 6/group, 6-8 weeks old) were assigned randomly to sham operation or brain trauma by controlled cortical impact. A cranial window was prepared, and tetramethylrhodamine-dextran (TMRM, MW 40,000 Da) was injected intravenously to visualize blood plasma 4 h, 24 h, 48 h, 72 h, or seven days after surgery or trauma. Three regions with increasing distance to the primary contusion were investigated up to a depth of 300 μm by 2-PM. No TMRM extravasation was detected in sham-operated mice, while already 4 h after TBI vascular leakage was significantly increased (p < 0.05 vs. sham) and reached its maximum at 48 h after injury. Vascular leakage was most pronounced in the vicinity of the contusion. The rate of extravasation showed a biphasic pattern, peaking 4 h and 48-72 h after trauma. Taken together, longitudinal quantification of vascular leakage after TBI in vivo demonstrates that VBE formation after TBI develops in a biphasic manner suggestive of acute and delayed mechanisms. Further studies using the currently developed dynamic in vivo imaging modalities are needed to investigate these mechanisms and potential therapeutic strategies in more detail.

TMS-Induced Controlled BBB Opening: Preclinical Characterization and Implications for Treatment of Brain Cancer

Proper neuronal function requires strict maintenance of the brain's extracellular environment. Therefore, passage of molecules between the circulation and brain neuropil is tightly regulated by the blood-brain barrier (BBB). While the BBB is vital for normal brain function, it also restricts the passage of drugs, potentially effective in treating brain diseases, into the brain. Despite previous attempts, there is still an unmet need to develop novel approaches that will allow safe opening of the BBB for drug delivery. We have recently shown in experimental rodents and in a pilot human trial that low-frequency, high-amplitude repetitive transcranial magnetic stimulation (rTMS) allows the delivery of peripherally injected fluorescent and Gd-based tracers into the brain. The goals of this study were to characterize the duration and safety level of rTMS-induced BBB opening and test its capacity to enhance the delivery of the antitumor growth agent, insulin-like growth factor trap, across the BBB. We employed direct vascular and magnetic resonance imaging, as well as electrocorticography recordings, to assess the impact of rTMS on brain vascular permeability and electrical activity, respectively. Our findings indicate that rTMS induces a transient and safe BBB opening with a potential to facilitate drug delivery into the brain.

The potential role of microvascular pathology in the neurological manifestations of coronavirus infection

Human coronaviruses are highly pathogenic viruses that pose a serious threat to human health. Examples include the severe acute respiratory syndrome outbreak of 2003 (SARS-CoV-1), the Middle East Respiratory Syndrome (MERS-CoV) outbreak of 2012, and the current SARS-CoV-2 (COVID-19) pandemic. Herein, we review the neurological manifestations of coronaviruses and discuss the potential pathogenic role of blood-brain barrier dysfunction. We present the hypothesis that pre-existing vascular damage (due to aging, cardiovascular disease, diabetes, hypertension or other conditions) facilitates infiltration of the virus into the central nervous system (CNS), increasing neuro-inflammation and the likelihood of neurological symptoms. We also discuss the role of a neuroinflammatory cytokine profile in both blood-brain barrier dysfunction and macrovascular disease (e.g. ischemic stroke and thromboembolism). Future studies are needed to better understand the involvement of the microvasculature in coronavirus neuropathology, and to test the diagnostic potential of minimally-invasive screening tools (e.g. serum biomarkers, fluorescein retinal angiography and dynamic-contrast MRI).

Development of Noninvasive in Vivo Approach to Assess Vascular Permeability in Inflammation Using Fluorescence Imaging

INTRODUCTION

In vivo fluorescence imaging can quantify vascular permeability without requiring sacrifice of animals. However, use of this noninvasive approach for vascular permeability assessment in remote organ injury caused by systemic inflammatory disease has not been reported.

METHODS

Evans blue (EB) and Genhance 750 fluorescent dye were mixed and injected into mice. The lung as a remote organ and the footpad as a noninvasive observational site were assessed in a cecal ligation and puncture (CLP)-induced systemic inflammation mouse model and compared with sham and hydrocortisone pretreated (CLP + HC) mouse models. Extraction of EB in harvested tissues was assessed as a conventional indicator of vascular permeability. Fluorescent intensities in the footpad or harvested lung were assessed and their correlation was analyzed to investigate this novel, noninvasive approach for estimation of lung vascular permeability.

RESULTS

Fluorescent intensity in the footpad and harvested lung in the CLP group was significantly higher than in the other groups (footpad, sham vs. CLP, P < 0.0001; CLP vs. CLP + HC, P = 0.0004; sham vs. CLP + HC, P = 0.058; lung, sham vs. CLP, P < 0.0001; CLP vs. CLP + HC, P < 0.0001; sham vs. CLP + HC, P = 0.060). The fluorescent intensity in the footpad was strongly correlated with that in the lung (r = 0.95).

CONCLUSIONS

This fluorescent technique may be useful for vascular permeability assessment based on EB quantification. Footpad fluorescent intensity was strongly correlated with that in the lung, and may be a suitable indicator in noninvasive estimation of lung vascular permeability.

In vivo monitoring blood-brain barrier permeability using spectral imaging through optical clearing skull window

The blood-brain barrier (BBB) plays a key role in the health of the central nervous system. Opening the BBB is very important for drug delivery to brain tissues to enhance the therapeutic effect on brain diseases. It is necessary to in vivo monitor the BBB permeability for assessing drug release with high resolution; however, an effective method is lacking. In this work, we developed a new method that combined spectral imaging with an optical clearing skull window to in vivo dynamically monitor BBB opening caused by 5-aminolevulinic acid (5-ALA)-mediated photodynamic therapy (PDT), in which the Evans blue dye (EBd) acted as an indicator of the BBB permeability. Using this method, we effectively monitored the cerebrovascular EBd leakage process. Moreover, the analysis of changes in the vascular and extravascular EBd concentrations demonstrated that the PDT-induced BBB opening exhibited spatiotemporal differences in the cortex. This spectral imaging method based on the optical clearing skull window provides a low-cost and simply operated tool for in vivo monitoring BBB opening process. This has a high potential for the visualization of drug delivery to the central nervous system. Thus, it is of tremendous significance in brain disease therapy. Monitoring the changes in PDT-induced BBB permeability by evaluating the EBd concentration using an optical clearing skull window. (A) Entire brains and coronal sections following treatment of PDT with/without an optical clearing skull window after injection of EBd. (B) Typical EBd distribution maps before and after laser irradiation captured by the spectral imaging method. (Colorbar represents the EBd concentration).

Functional morphology of the blood-brain barrier in health and disease

The adult quiescent blood–brain barrier (BBB), a structure organised by endothelial cells through interactions with pericytes, astrocytes, neurons and microglia in the neurovascular unit, is highly regulated but fragile at the same time. In the past decade, there has been considerable progress in understanding not only the molecular pathways involved in BBB development, but also BBB breakdown in neurological diseases. Specifically, the Wnt/β-catenin, retinoic acid and sonic hedgehog pathways moved into the focus of BBB research. Moreover, angiopoietin/Tie2 signalling that is linked to angiogenic processes has gained attention in the BBB field. Blood vessels play an essential role in initiation and progression of many diseases, including inflammation outside the central nervous system (CNS). Therefore, the potential influence of CNS blood vessels in neurological diseases associated with BBB alterations or neuroinflammation has become a major focus of current research to understand their contribution to pathogenesis. Moreover, the BBB remains a major obstacle to pharmaceutical intervention in the CNS. The complications may either be expressed by inadequate therapeutic delivery like in brain tumours, or by poor delivery of the drug across the BBB and ineffective bioavailability. In this review, we initially describe the cellular and molecular components that contribute to the steady state of the healthy BBB. We then discuss BBB alterations in ischaemic stroke, primary and metastatic brain tumour, chronic inflammation and Alzheimer’s disease. Throughout the review, we highlight common mechanisms of BBB abnormalities among these diseases, in particular the contribution of neuroinflammation to BBB dysfunction and disease progression, and emphasise unique aspects of BBB alteration in certain diseases such as brain tumours. Moreover, this review highlights novel strategies to monitor BBB function by non-invasive imaging techniques focussing on ischaemic stroke, as well as novel ways to modulate BBB permeability and function to promote treatment of brain tumours, inflammation and Alzheimer’s disease. In conclusion, a deep understanding of signals that maintain the healthy BBB and promote fluctuations in BBB permeability in disease states will be key to elucidate disease mechanisms and to identify potential targets for diagnostics and therapeutic modulation of the BBB.

Retinal and Cortical Blood Flow Dynamics Following Systemic Blood-Neural Barrier Disruption

To consider whether imaging retinal vasculature may be used as a marker for cortical vessels, we compared fluorescein angiography flow dynamics before and after pharmacological disruption of blood-neural barriers. Sodium fluorescein (1%, 200 μl/kg) was intravenously delivered in anesthetized adult Long Evans rats (n = 44, brain = 18, retina = 26). In the brain cohort, a cranial window was created to allow direct visualization of surface cortical vessels. Video fluorescein angiography was captured using a rodent retinal camera at 30 frames/second and fluorescence intensity profiles were evaluated for the time to reach 50% brightness (half-rise), 50% decay (half-fall), and the plateau level of remnant fluorescence (offset, %). Cortical vessels fluoresced earlier (artery half-rise: 5.6 ± 0.2 s) and decayed faster (half-fall: 10.3 ± 0.2 s) compared to retinal vasculature. Cortical vessels also had a considerably higher offset, particularly in the capillaries/extravascular space (41.4 ± 2.7%) whereas pigment in the retina reduces such residual fluorescence. In a sub-cohort of animals, sodium deoxycholate (DOC, 0.06 M dissolved in sterile saline, 1 mL) was delivered intravenously to cause simultaneous disruption of the blood-brain and blood-retinal barriers. A separate group received saline as vehicle control. Fluorescein angiography was re-measured at 6 and 24 h after drug infusion and evaluated by comparing flow dynamics to the upper quartile (75%) of the control group. Retinal vasculature was more sensitive to DOC-induced disruption with a higher fluorescence offset at 6 h (47.3 ± 10.6%). A delayed effect was seen in cortical vessels with a higher offset evident only at 24 h (65.6 ± 10.1%). Here we have developed a method to quantitatively compare fluorescein angiography dynamics in the retina and superficial cortical vessels. Our results show that systemic disruption of blood-neural barriers causes vascular leakage in both tissues but earlier in the retina suggesting that pharmacological blood-neural barrier disruption may be detected earlier in the eye than in cortical vasculature.

Glutamate-Mediated Blood-Brain Barrier Opening: Implications for Neuroprotection and Drug Delivery

The blood-brain barrier is a highly selective anatomical and functional interface allowing a unique environment for neuro-glia networks. Blood-brain barrier dysfunction is common in most brain disorders and is associated with disease course and delayed complications. However, the mechanisms underlying blood-brain barrier opening are poorly understood. Here we demonstrate the role of the neurotransmitter glutamate in modulating early barrier permeability in vivo Using intravital microscopy, we show that recurrent seizures and the associated excessive glutamate release lead to increased vascular permeability in the rat cerebral cortex, through activation of NMDA receptors. NMDA receptor antagonists reduce barrier permeability in the peri-ischemic brain, whereas neuronal activation using high-intensity magnetic stimulation increases barrier permeability and facilitates drug delivery. Finally, we conducted a double-blind clinical trial in patients with malignant glial tumors, using contrast-enhanced magnetic resonance imaging to quantitatively assess blood-brain barrier permeability. We demonstrate the safety of stimulation that efficiently increased blood-brain barrier permeability in 10 of 15 patients with malignant glial tumors. We suggest a novel mechanism for the bidirectional modulation of brain vascular permeability toward increased drug delivery and prevention of delayed complications in brain disorders.

SIGNIFICANCE STATEMENT

In this study, we reveal a new mechanism that governs blood-brain barrier (BBB) function in the rat cerebral cortex, and, by using the discovered mechanism, we demonstrate bidirectional control over brain endothelial permeability. Obviously, the clinical potential of manipulating BBB permeability for neuroprotection and drug delivery is immense, as we show in preclinical and proof-of-concept clinical studies. This study addresses an unmet need to induce transient BBB opening for drug delivery in patients with malignant brain tumors and effectively facilitate BBB closure in neurological disorders.

ZW800-1 for Assessment of Blood-Brain Barrier Disruption in a Photothrombotic Stroke Model

Background: Since it is known that serum albumin-bound dyes can cross the blood-brain barrier (BBB) after ischemia, Evans Blue dye is commonly used to assess BBB disruption because of its rapid binding to serum albumin. In addition, indocyanine green (ICG), a clinically available dye, binds to serum proteins that could also be used for assessment of BBB impairment. Unlike these near-infrared (NIR) dyes, zwitterionic NIR fluorophore (ZW800-1) shows no serum binding, ultralow non-specific tissue uptake, and rapid elimination from the body via renal filtration. In this study, we report the use of ZW800-1 as a NIR fluorescence imaging agent for detecting BBB disruption in rat stroke models. Methods: Three types of NIR fluorophores, Evans Blue, ICG, and ZW800-1, were administered intraperitoneally into rat photothrombotic stroke models by using 4% concentration of each NIR dye. The NIR fluorescence signals in the infarcted brain tissue and biodistribution were observed in real-time using the Mini-FLARE^® imaging system up to 24 h post-injection. Results: ZW800-1 provided successful visualization of the ischemic injury site in the brain tissue, while the remaining injected dye was clearly excreted from the body within a certain period of time. Although Evans Blue and ICG provided mapping of the infarcted brain lesions, they exhibited high non-specific uptake in most of the tissues and organs and persisted in the body over 24 h post-injection. Conclusion: Our results suggest the promising application of ZW800-1 as a new strategy in BBB experiments and future therapeutic development.

Blood brain barrier: An overview on strategies in drug delivery, realistic in vitro modeling and in vivo live tracking

Blood brain barrier (BBB) is a group of astrocytes, neurons and endothelial cells, which makes restricted passage of various biological or chemical entities to the brain tissue. It gives protection to brain at one hand, but at the other hand it has very selective permeability for bio-actives and other foreign materials and is one of the major challenges for the drug delivery. Nanocarriers are promising to cross BBB utilizing alternative route of administration such as intranasal and intra-carotid drug delivery which bypasses BBB. In future more optimized drug delivery system can be achieved by compiling the best routes with the best carriers. Single photon emission tomography (SPECT) and different brain-on-a-chip in vitro models are being very reliable to study live in vivo tracking of BBB and its pathophysiology, respectively. In the current review we have tried to exploit mechanistically all these to understand and manage the various BBB disruptions in diseased condition along with crossing the hurdles occurring in drug or gene delivery across BBB.

Identification of the Vascular Source of Vasogenic Brain Edema following Traumatic Brain Injury Using In Vivo 2-Photon Microscopy in Mice

Vasogenic brain edema due to vascular leakage is one of the most important factors determining the clinical outcome of patients following acute brain injury. To date, performing a detailed in vivo quantification of vascular leakage has not been possible. Here, we used in vivo 2-photon microscopy (2-PM) to determine the spatial (3D) and temporal development of vasogenic brain edema following traumatic brain injury (TBI) in mice; in addition, we identified the vessel types involved in vascular leakage. Thirteen male Tie2-GFP mice (6-8 weeks old) were subjected to controlled cortical impact (CCI) or a sham operation; subsequently, a cranial window was prepared adjacent to the injury site, and tetramethylrhodamine-dextran (TMRM, 40 mg/kg, MW 40,000) was injected intravenously to visualize blood plasma leakage. Parenchymal fluorescence intensity was monitored in three regions for 2-4 h post-CCI, reaching from the surface of the brain to a depth of 300 μm, and TMRM leakage was measured as an increase in TMRM fluorescence intensity outside the vessel lumen and in the parenchyma. In the CCI group, vascular leakage was detected in all investigated regions as early as 2.5 h post-injury. This leakage increased over time and was more pronounced proximal to the primary contusion. Both arterioles and venules contributed similarly to brain edema formation and their contribution was independent of vessel size; however, capillaries were the major contributor to leakage. In summary, using 2-PM to perform in vivo 3D deep-brain imaging, we found that TBI induces vascular leakage from capillaries, venules, and arterioles. Thus, all three vessel types are involved in trauma-induced brain edema and should be considered when developing novel therapies for preventing vasogenic brain edema.

Practical Applications of in Vivo and ex Vivo MRI in Toxicologic Pathology Using a Novel High-performance Compact MRI System

Magnetic resonance imaging (MRI) is widely used in preclinical research and drug development and is a powerful noninvasive method for assessment of phenotypes and therapeutic efficacy in murine models of disease. In vivo MRI provides an opportunity for longitudinal evaluation of tissue changes and phenotypic expression in experimental animal models. Ex vivo MRI of fixed samples permits a thorough examination of multiple digital slices while leaving the specimen intact for subsequent conventional hematoxylin and eosin (H&E) histology. With the advent of new compact MRI systems that are designed to operate in most conventional labs without the cost, complexity, and infrastructure needs of conventional MRI systems, the possibility of MRI becoming a practical modality is now viable. The purpose of this study was to investigate the capabilities of a new compact, high-performance MRI platform (M2™; Aspect Imaging, Israel) as it relates to preclinical toxicology studies. This overview will provide examples of major organ system pathologies with an emphasis on how compact MRI can serve as an important adjunct to conventional pathology by nondestructively providing 3-dimensional (3-D) digital data sets, detailed morphological insights, and quantitative information. Comparative data using compact MRI for both in vivo and ex vivo are provided as well as validation using conventional H&E.

Analyzing the blood-brain barrier: the benefits of medical imaging in research and clinical practice

A dysfunctional BBB is a common feature in a variety of brain disorders, a fact stressing the need for diagnostic tools designed to assess brain vessels' permeability in space and time. Biological research has benefited over the years various means to analyze BBB integrity. The use of biomarkers for improper BBB functionality is abundant. Systemic administration of BBB impermeable tracers can both visualize brain regions characterized by BBB impairment, as well as lead to its quantification. Additionally, locating molecular, physiological content in regions from which it is restricted under normal BBB functionality undoubtedly indicates brain pathology-related BBB disruption. However, in-depth research into the BBB's phenotype demands higher analytical complexity than functional vs. pathological BBB; criteria which biomarker based BBB permeability analyses do not meet. The involvement of accurate and engineering sciences in recent brain research, has led to improvements in the field, in the form of more accurate, sensitive imaging-based methods. Improvements in the spatiotemporal resolution of many imaging modalities and in image processing techniques, make up for the inadequacies of biomarker based analyses. In pre-clinical research, imaging approaches involving invasive procedures, enable microscopic evaluation of BBB integrity, and benefit high levels of sensitivity and accuracy. However, invasive techniques may alter normal physiological function, thus generating a modality-based impact on vessel's permeability, which needs to be corrected for. Non-invasive approaches do not affect proper functionality of the inspected system, but lack in spatiotemporal resolution. Nevertheless, the benefit of medical imaging, even in pre-clinical phases, outweighs its disadvantages. The innovations in pre-clinical imaging and the development of novel processing techniques, have led to their implementation in clinical use as well. Specialized analyses of vessels' permeability add valuable information to standard anatomical inspections which do not take the latter into consideration.

Quantitative spatial and temporal analysis of fluorescein angiography dynamics in the eye

Purpose

We describe a novel approach to analyze fluorescein angiography to investigate fluorescein flow dynamics in the rat posterior retina as well as identify abnormal areas following laser photocoagulation.

Methods

Experiments were undertaken in adult Long Evans rats. Using a rodent retinal camera, videos were acquired at 30 frames per second for 30 seconds following intravenous introduction of sodium fluorescein in a group of control animals (n = 14). Videos were image registered and analyzed using principle components analysis across all pixels in the field. This returns fluorescence intensity profiles from which, the half-rise (time to 50% brightness), half-fall (time for 50% decay) back to an offset (plateau level of fluorescence). We applied this analysis to video fluorescein angiography data collected 30 minutes following laser photocoagulation in a separate group of rats (n = 7).

Results

Pixel-by-pixel analysis of video angiography clearly delineates differences in the temporal profiles of arteries, veins and capillaries in the posterior retina. We find no difference in half-rise, half-fall or offset amongst the four quadrants (inferior, nasal, superior, temporal). We also found little difference with eccentricity. By expressing the parameters at each pixel as a function of the number of standard deviation from the average of the entire field, we could clearly identify the spatial extent of the laser injury.

Conclusions

This simple registration and analysis provides a way to monitor the size of vascular injury, to highlight areas of subtle vascular leakage and to quantify vascular dynamics not possible using current fluorescein angiography approaches. This can be applied in both laboratory and clinical settings for in vivo dynamic fluorescent imaging of vasculature.

Monitoring stroke progression: in vivo imaging of cortical perfusion, blood-brain barrier permeability and cellular damage in the rat photothrombosis model

Focal cerebral ischemia is among the main causes of death and disability worldwide. The ischemic core often progresses, invading the peri-ischemic brain; however, assessing the propensity of the peri-ischemic brain to undergo secondary damage, understanding the underlying mechanisms, and adjusting treatment accordingly remain clinically unmet challenges. A significant hallmark of the peri-ischemic brain is dysfunction of the blood-brain barrier (BBB), yet the role of disturbed vascular permeability in stroke progression is unclear. Here we describe a longitudinal in vivo fluorescence imaging approach for the evaluation of cortical perfusion, BBB dysfunction, free radical formation and cellular injury using the photothrombosis vascular occlusion model in male Sprague Dawley rats. Blood-brain barrier dysfunction propagated within the peri-ischemic brain in the first hours after photothrombosis and was associated with free radical formation and cellular injury. Inhibiting free radical signaling significantly reduced progressive cellular damage after photothrombosis, with no significant effect on blood flow and BBB permeability. Our approach allows a dynamic follow-up of cellular events and their response to therapeutics in the acutely injured cerebral cortex.

Losartan prevents acquired epilepsy via TGF-β signaling suppression

OBJECTIVE

Acquired epilepsy is frequently associated with structural lesions after trauma, stroke, and infections. Although seizures are often difficult to treat, there is no clinically applicable strategy to prevent the development of epilepsy in patients at risk. We have recently shown that vascular injury is associated with activation of albumin-mediated transforming growth factor β (TGF-β) signaling, and followed by local inflammatory response and epileptiform activity ex vivo. Here we investigated albumin-mediated TGF-β signaling and tested the efficacy of blocking the TGF-β pathway in preventing epilepsy.

METHODS

We addressed the role of TGF-β signaling in epileptogenesis in 2 different rat models of vascular injury, combining in vitro and in vivo biochemical assays, gene expression, and magnetic resonance and direct optical imaging for blood-brain barrier permeability and vascular reactivity. Long-term electrocorticographic recordings were acquired in freely behaving animals.

RESULTS

We demonstrate that serum-derived albumin preferentially induces activation of the activin receptor-like kinase 5 pathway of TGF-β receptor I in astrocytes. We further show that the angiotensin II type 1 receptor antagonist, losartan, previously identified as a blocker of peripheral TGF-β signaling, effectively blocks albumin-induced TGF-β activation in the brain. Most importantly, losartan prevents the development of delayed recurrent spontaneous seizures, an effect that persists weeks after drug withdrawal.

INTERPRETATION

TGF-β signaling, activated in astrocytes by serum-derived albumin, is involved in epileptogenesis. We propose losartan, a drug approved by the US Food and Drug Administration, as an efficient antiepileptogenic therapy for epilepsy associated with vascular injury.

Fluoxetine induces vasodilatation of cerebral arterioles by co-modulating NO/muscarinic signalling

Ischaemic stroke patients treated with Selective Serotonin Reuptake Inhibitors (SSRI) show improved motor, cognitive and executive functions, but the underlying mechanism(s) are incompletely understood. Here, we report that cerebral arterioles in the rat brain superfused with therapeutically effective doses of the SSRI fluoxetine showed consistent, dose-dependent vasodilatation (by 1.2 to 1.6-fold), suppressible by muscarinic and nitric oxide synthase (NOS) antagonists [atropine, NG-nitro-l-arginine methyl ester (l-NAME)] but resistant to nicotinic and serotoninergic antagonists (mecamylamine, methylsergide). Fluoxetine administered 10–30 min. following experimental vascular photo-thrombosis increased arterial diameter (1.3–1.6), inducing partial, but lasting reperfusion of the ischaemic brain. In brain endothelial b.End.3 cells, fluoxetine induced rapid muscarinic receptor-dependent increases in intracellular [Ca²⁺] and promoted albumin- and eNOS-dependent nitric oxide (NO) production and HSP90 interaction. In vitro, fluoxetine suppressed recombinant human acetylcholinesterase (rhAChE) activity only in the presence of albumin. That fluoxetine induces vasodilatation of cerebral arterioles suggests co-promotion of endothelial muscarinic and nitric oxide signalling, facilitated by albumin-dependent inhibition of serum AChE.

Novel fluorescein angiography-based computer-aided algorithm for assessment of retinal vessel permeability

Purpose

To present a novel method for quantitative assessment of retinal vessel permeability using a fluorescein angiography-based computer algorithm.

Methods

Twenty-one subjects (13 with diabetic retinopathy, 8 healthy volunteers) underwent fluorescein angiography (FA). Image pre-processing included removal of non-retinal and noisy images and registration to achieve spatial and temporal pixel-based analysis. Permeability was assessed for each pixel by computing intensity kinetics normalized to arterial values. A linear curve was fitted and the slope value was assigned, color-coded and displayed. The initial FA studies and the computed permeability maps were interpreted in a masked and randomized manner by three experienced ophthalmologists for statistical validation of diagnosis accuracy and efficacy.

Results

Permeability maps were successfully generated for all subjects. For healthy volunteers permeability values showed a normal distribution with a comparable range between subjects. Based on the mean cumulative histogram for the healthy population a threshold (99.5%) for pathological permeability was determined. Clear differences were found between patients and healthy subjects in the number and spatial distribution of pixels with pathological vascular leakage. The computed maps improved the discrimination between patients and healthy subjects, achieved sensitivity and specificity of 0.974 and 0.833 respectively, and significantly improved the consensus among raters for the localization of pathological regions.

Conclusion

The new algorithm allows quantification of retinal vessel permeability and provides objective, more sensitive and accurate evaluation than the present subjective clinical diagnosis. Future studies with a larger patients’ cohort and different retinal pathologies are awaited to further validate this new approach and its role in diagnosis and treatment follow-up. Successful evaluation of vasculature permeability may be used for the early diagnosis of brain microvascular pathology and potentially predict associated neurological sequelae. Finally, the algorithm could be implemented for intraoperative evaluation of micovascular integrity in other organs or during animal experiments.

Imaging blood-brain barrier dysfunction in animal disease models

The blood-brain barrier (BBB) is a highly complex structure, which separates the extracellular fluid of the central nervous system (CNS) from the blood of CNS vessels. A wide range of neurologic conditions, including stroke, epilepsy, Alzheimer's disease, and brain tumors, are associated with perturbations of the BBB that contribute to their pathology. The common consequence of a BBB dysfunction is increased permeability, leading to extravasation of plasma constituents and vasogenic brain edema. The BBB impairment can persist for long periods, being involved in secondary inflammation and neuronal dysfunction, thus contributing to disease pathogenesis. Therefore, reliable imaging of the BBB impairment is of major importance in both clinical management of brain diseases and in experimental research. From landmark studies by Ehrlich and Goldman, the use of dyes (probes) has played a critical role in understanding BBB functions. In recent years methodologic advances in morphologic and functional brain imaging have provided insight into cellular and molecular interactions underlying BBB dysfunction in animal disease models. These imaging techniques, which range from in situ staining to noninvasive in vivo imaging, have different spatial resolution, sensitivity, and capacity for quantitative and kinetic measures of the BBB impairment. Despite significant advances, the translation of these techniques into clinical applications remains slow. This review outlines key recent advances in imaging techniques that have contributed to the understanding of BBB dysfunction in disease and discusses major obstacles and opportunities to advance these techniques into the clinical realm.

Evaluation of laser speckle contrast imaging as an intrinsic method to monitor blood brain barrier integrity

The integrity of the blood brain barrier (BBB) can contribute to the development of many brain disorders. We evaluate laser speckle contrast imaging (LSCI) as an intrinsic modality for monitoring BBB disruptions through simultaneous fluorescence and LSCI with vertical cavity surface emitting lasers (VCSELs). We demonstrated that drug-induced BBB opening was associated with a relative change of the arterial and venous blood velocities. Cross-sectional flow velocity ratio (veins/arteries) decreased significantly in rats treated with BBB-opening drugs, ≤0.81 of initial values.

Stimulation of the sphenopalatine ganglion induces reperfusion and blood-brain barrier protection in the photothrombotic stroke model

Purpose

The treatment of stroke remains a challenge. Animal studies showing that electrical stimulation of the sphenopalatine ganglion (SPG) exerts beneficial effects in the treatment of stroke have led to the initiation of clinical studies. However, the detailed effects of SPG stimulation on the injured brain are not known.

Methods

The effect of acute SPG stimulation was studied by direct vascular imaging, fluorescent angiography and laser Doppler flowmetry in the sensory motor cortex of the anaesthetized rat. Focal cerebral ischemia was induced by the rose bengal (RB) photothrombosis method. In chronic experiments, SPG stimulation, starting 15 min or 24 h after photothrombosis, was given for 3 h per day on four consecutive days. Structural damage was assessed using histological and immunohistochemical methods. Cortical functions were assessed by quantitative analysis of epidural electro-corticographic (ECoG) activity continuously recorded in behaving animals.

Results

Stimulation induced intensity- and duration-dependent vasodilation and increased cerebral blood flow in both healthy and photothrombotic brains. In SPG-stimulated rats both blood brain-barrier (BBB) opening, pathological brain activity and lesion volume were attenuated compared to untreated stroke animals, with no apparent difference in the glial response surrounding the necrotic lesion.

Conclusion

SPG-stimulation in rats induces vasodilation of cortical arterioles, partial reperfusion of the ischemic lesion, and normalization of brain functions with reduced BBB dysfunction and stroke volume. These findings support the potential therapeutic effect of SPG stimulation in focal cerebral ischemia even when applied 24 h after stroke onset and thus may extend the therapeutic window of currently administered stroke medications.

A novel algorithm for the assessment of blood-brain barrier permeability suggests that brain topical application of endothelin-1 does not cause early opening of the barrier in rats

There are a number of different experimental methods for ex vivo assessment of blood-brain barrier (BBB) opening based on Evans blue dye extravasation. However, these methods require many different steps to prepare the brain and need special equipment for quantification. We here report a novel, simple, and fast semiquantitative algorithm to assess BBB integrity ex vivo. The method is particularly suitable for cranial window experiments, since it keeps the spatial information about where the BBB opened. We validated the algorithm using sham controls and the established model of brain topical application of the bile salt dehydrocholate for early BBB disruption. We then studied spreading depolarizations in the presence and the absence of the vasoconstrictor endothelin-1 and found no evidence of early BBB opening (three-hour time window). The algorithm can be used, for example, to assess BBB permeability ex vivo in combination with dynamic in vivo studies of BBB opening.

Vascular pathology and blood-brain barrier disruption in cognitive and psychiatric complications of type 2 diabetes mellitus

Vascular pathology is recognized as a principle insult in type 2 diabetes mellitus (T2DM). Co-morbidities such as structural brain abnormalities, cognitive, learning and memory deficits are also prevailing in T2DM patients. We previously suggested that microvascular pathologies involving blood-brain barrier (BBB) breakdown results in leakage of serum-derived components into the brain parenchyma, leading to neuronal dysfunction manifested as psychiatric illnesses. The current postulate focuses on the molecular mechanisms controlling BBB permeability in T2DM, as key contributors to the pathogenesis of mental disorders in patients. Revealing the mechanisms underlying BBB dysfunction and inflammatory response in T2DM and their role in metabolic disturbances, abnormal neurovascular coupling and neuronal plasticity, would contribute to the understanding of the mechanisms underlying psychopathologies in diabetic patients. Establishing this link would offer new targets for future therapeutic interventions.