Blood Brain Barrier Dysfunction in Neurodegenerative Diseases: Molecular Mechanisms, Transport Systems, and Disease-Specific Alterations

Background and Purpose: Arterial spin-labeling (ASL) MRI is sensitive for detecting hyperemic lesions (HLs) in patients with acute ischemic stroke (AIS). We evaluated whether HLs could predict blood-brain barrier (BBB) disruption and hemorrhagic transformation (HT) in AIS patients. Methods: In a retrospective study, ASL was performed within 6 hours of symptom onset before revascularization treatment in 25 patients with anterior circulation large vessel occlusion on baseline MR angiography. All patients underwent angiographic procedures intended for endovascular therapy and a noncontrast CT scan immediately after treatment. BBB disruption was defined as a hyperdense lesion present on the posttreatment CT scan. A subacute MRI or CT scan was performed during the subacute phase to assess HTs. The relationship between HLs and BBB disruption and HT was examined using the Alberta Stroke Program Early Computed Tomography Score (ASPECTS) locations in the symptomatic hemispheres. Results: A HL was defined as a region where CBF relative ≥1.4 (CBF relative =CBF HL /CBF contralateral ). HLs, BBB disruption and HT were found in 9, 15, and 15 patients, respectively. Compared with the patients without HLs, the patients with HLs had a higher incidence of both BBB disruption (100% versus 37.5%, P=0.003) and HT (100% versus 37.5%, P=0.003). Based on the ASPECTS locations, 21 regions of interests (ROIs) displayed HLs. Compared with the ROIs without HLs, the ROIs with HLs had a higher incidence of both BBB disruption (42.8% versus 3.9%, P<0.001) and HT (85.7% versus 7.8%, P<0.001). Conclusion: HLs detected on pretreatment ASL maps may enable the prediction and localization of subsequent BBB disruption and HT.

Blood brain barrier (BBB) disruption is an integral feature of numerous neurological diseases with infectious, inflammatory, neoplastic and vascular components. It is of particular interest in the immune mediated neurological diseases multiple sclerosis (MS), acute disseminated encephalomyelitis (ADEM), and acute hemorrhagic leukoencephalitis (AHLE). In particular, AHLE is associated with very high mortality. A fundamental question in these diseases is the extent inflammatory immune cells contribute to CNS vascular permeability. This lack of understanding currently undermines therapeutic approaches to ameliorate uncontrolled BBB disruption. In this review, we highlight the current experimental model systems available to address the contribution of inflammatory cells in BBB disruption. The contribution of T cells and other immune cell subsets in altering blood brain barrier tight junctions in the experimental autoimmune encephalomyelitis (EAE), lipopolysaccharide (LPS), and virus induced CNS vascular permeability model systems is also addressed. Results obtained from the use of these model systems have put forward a defined role for neutrophils, CD4 and CD8 T cells and vascular endothelial growth factor (VEGF) in BBB disruption. Based on these findings, we describe models in which immune cells engage and alter CNS cell types of the neurovascular unit and define future avenues of research. Acute disseminating encephalomyelitis (ADEM) and acute hemorrhagic leukoencephalitis (AHLE) are demyelinating diseases that usually present shortly after infections or vaccination (Callen et al., 2009; Gibbs et al., 2005). ADEM typically has an overall favorable prognosis, with 60-80% of patients showing a complete recovery (Dale et al., 2000; Gibbs et al., 2005; Rust, 2000; Tenembaum et al., 2002). AHLE, which is also known as Hurst’s disease, has been considered a severe form of ADEM and has a very poor prognosis, usually resulting in death in 2-14 days (Geerts et al., 1991; Hart & Earle 1975; Pinto et al., 2011; Posey et al., 1994; Suchowersky et al., 1983). This disease is characterized by extensive BBB disruption and microhemorrhage formation. Typical acute symptoms of AHLE include fever, malaise, and headache, followed by a rapid progression of multifocal neurologic signs

The pathogenesis of the fatal neurodegenerative condition amyotrophic lateral sclerosis (ALS) remains to be fully understood. Blood–brain barrier damage (BBBD) has been implicated as an exacerbating factor in several neurodegenerative conditions, including ALS. Therefore, this cross-sectional study used the novel D50 progression model to assess the clinical relevance of BBBD within a cohort of individuals with either ALS (n = 160) or ALS mimicking conditions (n = 31). Routine laboratory parameters in cerebrospinal fluid (CSF) and blood were measured, and the ratio of CSF to serum albumin levels (Qalb) was used as a proxy measure of BBBD. In the univariate analyses, Qalb levels correlated weakly with disease aggressiveness (as indicated by individual D50 values) and physical function (as measured by ALS Functional Rating Scale). However, after adjustment for cofactors in the elastic net regularization, only having limb-onset disease was associated with BBBD. The results reported here emphasize the clinical heterogeneity of ALS and the need for additional longitudinal and multi-modal studies to fully clarify the extent and effect of BBBD in ALS.

Blood-Brain Barrier (BBB) disruption is an important pathophysiological process of acute ischemic stroke (AIS), resulting in devastating malignant brain edema and hemorrhagic transformation. The rapid activation of immune cells plays a critical role in BBB disruption after ischemic stroke. Infiltrating blood-borne immune cells (neutrophils, monocytes, and T lymphocytes) increase BBB permeability, as they cause microvascular disorder and secrete inflammation-associated molecules. In contrast, they promote BBB repair and angiogenesis in the latter phase of ischemic stroke. The profound immunological effects of cerebral immune cells (microglia, astrocytes, and pericytes) on BBB disruption have been underestimated in ischemic stroke. Post-stroke microglia and astrocytes can adopt both an M1/A1 or M2/A2 phenotype, which influence BBB integrity differently. However, whether pericytes acquire microglia phenotype and exert immunological effects on the BBB remains controversial. Thus, better understanding the inflammatory mechanism underlying BBB disruption can lead to the identification of more promising biological targets to develop treatments that minimize the onset of life-threatening complications and to improve existing treatments in patients. However, early attempts to inhibit the infiltration of circulating immune cells into the brain by blocking adhesion molecules, that were successful in experimental stroke failed in clinical trials. Therefore, new immunoregulatory therapeutic strategies for acute ischemic stroke are desperately warranted. Herein, we highlight the role of circulating and cerebral immune cells in BBB disruption and the crosstalk between them following acute ischemic stroke. Using a robust theoretical background, we discuss potential and effective immunotherapeutic targets to regulate BBB permeability after acute ischemic stroke.

Background and purpose: Our previous study found adoptively transferred regulatory T cell (Treg) protected against ischemic stroke via attenuating blood brain barrier (BBB) disruption. Here, we examined the role of CCR5, a chemokine receptor in the Tregs’ recruitment and protection to the ischemic BBB after stroke. Methods: Transient focal cerebral ischemia was induced in widetype C57/BL6 or CCR5(-/-) mice by unilateral middle cerebral artery occlusion (MCAO) for 60 minutes. Tregs (2x10 6 /mouse) in 200 μl PBS or PBS control were injected intravenously at 2 hours after MCAO. Chemo-attractant migration of Tregs to the ischemic BBB was examined by two photon in vivo imaging and cell migration test in vitro. RT-PCR, Immunofluorescence staining and confocal microscopy were used to assess the chemokine expression, endothelial ICAM-1 expression and peripheral immune cell infiltration following cerebral ischemic injury. In vivo endogenous IgG leakage and in vitro BBB integrity were examined to determine the important role of CCR5 in Tregs’ protection on the ischemic BBB. Results: We found that cerebral ischemia reduced Treg number in the peripheral but the expression of CCR5 on Tregs in the blood was increased. In the ischemic brain, the expression of CCR5 ligands were significantly upregulated, and the expression of CCL5 co-localized with ischemic brain endothelial cells. Genetic depletion of CCR5 on the donor Tregs significantly impaired the transferred Tregs’ ability to migrate to the ischemic BBB and attenuated Tregs’ protection against BBB disruption and peripheral immune cell infiltration as determined by two photon in vivo imaging, immunofluorescence staining and MMP-9 ELISA. In vitro migration test further revealed that depletion of CCR5 hindered Tregs to migrate across endothelial cell layer. Conclusions: In conclusion, CCR5 plays an indispensible role in Tregs’ recruitment and protection against ischemic BBB disruption. Therapeutic strategies targeting CCR5 warrants further investigation to improve Treg based cell therapy in stroke treatment.

Background: Focal ischemia is thought to have global effects on the brain. Chronic cerebrovascular disease is associated with blood-brain barrier (BBB) disruption. We aimed to determine if ischemic stroke has a remote effect on the BBB. Methods: Patients with acute ischemic stroke who were imaged with MR perfusion weighted imaging (PWI) on presentation and again 24 hours later were included in the study. PWI source images were used to measure BBB disruption as a percentage of the signal that is due to gadolinium leakage. BBB was averaged within a region of interest (ROI) consisting of supratentorial white matter (WM) contralateral to the ischemic stroke. The ROI was defined using a group-averaged diffusion tensor imaging (DTI) fractional anisotropy map. For infratentorial strokes the ROI consisted of bilateral supratentorial WM. For bilateral strokes, the ROI consisted of WM in the less affected hemisphere after removal of the infarct. Segmentation algorithms were used to define infarct volume in mL on DTI at 24 hours and WM hyperintensity volume (WMHvol) on the initial FLAIR image. Average percent BBB disruption was compared with age, baseline modified Rankin scale (mRS), NIHSS, stroke volume, and WMHvol using linear regression. Results: 44 patients were included with a median age of 74, 39% female. Median NIHSS was 4; mean stroke volume was 10 mL. The baseline mRS ranged from 0-3 with a median of 0. The median BBB was 0.86% (IQR:0.72%-1.09%). Higher average BBB disruption measured in the WM remote from the acute stroke was associated with larger stroke volumes (p=0.006, r 2 =0.17, beta 0.410, CI:0.003-0.018), larger WMHvol (p<0.001, r 2 =0.26, beta 0.51, CI: 0.10-0.32), and higher NIHSS (p=0.039, r 2 =0.01, beta 0.312, CI:0.002-0.064) but not with age (p=0.447), sex (p=0.729), or baseline mRS (p=0.093). In multiple linear regression stroke volume (p=0.008) and WMHvol (p<0.001) were independently associated with remote disruption of the BBB but not NIHSS (p=0.684). Conclusions: Focal ischemia resulted in a proportional disruption of the BBB in the WM remote from the acute lesion. This effect was independent of the severity of WMHvol, a measure of cerebral small vessel disease. These findings support the concept of ischemic stroke having global effects on the brain.

The intersection of circadian rhythms and the blood-brain barrier with drug efficacy and delivery in neurological disorders.

Traumatic brain injury (TBI) is a leading global cause of disability and mortality, with blood-brain barrier (BBB) disruption exacerbating secondary injury. Transglutaminase 2 (TGM2), a multifunctional enzyme implicated in neuroinflammation and extracellular matrix remodeling, remains underexplored in TBI-related BBB dysfunction. This study elucidates the role of TGM2 in BBB disruption following TBI and explores its therapeutic potential in mitigating BBB damage. The controlled cortical impact (CCI) and oxygen-glucose deprivation (OGD) models were used to establish invivo and invitro TBI models in mice. The experimental approaches comprised RNA sequencing, Western blot analysis, RT-qPCR, immunofluorescence staining, and behavioral assessments. TGM2 expression peaked at 48 h after TBI, predominantly in brain endothelial cells, correlating with BBB disruption (reduced tight junction proteins, increased edema). TGM2 knockdown may attenuate MMP-9 via the IL-17 pathway, restoring BBB integrity. Invivo, TGM2 inhibition reduced Evans blue leakage, upregulated CLAUDIN-5/ZO-1, and improved motor coordination, balance, and spatial memory. TGM2 is a key molecule affecting the BBB after TBI. Inhibition of TGM2 can alleviate the blood-brain barrier and neurological deficits after TBI, acting through the IL-17-MMP-9 axis.

Ischemic stroke results in blood-brain barrier (BBB) disruption, during which the reciprocal interaction between ischemic neurons and components of the BBB appears to play a critical role. However, the underlying mechanisms for BBB protection remain largely unknown. In this study, we found that Serpina3n, a serine protease inhibitor, was significantly upregulated in the ischemic brain, predominantly in ischemic neurons from 6 hours to 3 days after stroke. Using neuron-specific adeno-associated virus (AAV), intranasal delivery of recombinant protein, and immune-deficient Rag1−/− mice, we demonstrated that Serpina3n attenuated BBB disruption and immune cell infiltration following stroke by inhibiting the activity of granzyme B (GZMB) and neutrophil elastase (NE) secreted by T cells and neutrophils. Furthermore, we found that intranasal delivery of rSerpina3n significantly attenuated the neurologic deficits after stroke. In conclusion, Serpina3n is a novel ischemic neuron-derived proteinase inhibitor that counterbalances BBB disruption induced by peripheral T cell and neutrophil infiltration after ischemic stroke. These findings reveal a novel endogenous protective mechanism against BBB damage with Serpina3n being a potential therapeutic target in ischemic stroke.

The blood-brain barrier (BBB) is a fundamental component of the central nervous system (CNS). Its functional and structural integrity is vital to maintain the homeostasis of the brain microenvironment by controlling the passage of substances and regulating the trafficking of immune cells between the blood and the brain. The BBB is primarily composed of highly specialized microvascular endothelial cells. These cells’ special features and physiological properties are acquired and maintained through the concerted effort of hemodynamic and cellular cues from the surrounding environment. This complex multicellular system, comprising endothelial cells, astrocytes, pericytes, and neurons, is known as the neurovascular unit (NVU). The BBB strictly controls the transport of nutrients and metabolites into brain parenchyma through a tightly regulated transport system while limiting the access of potentially harmful substances via efflux transcytosis and metabolic mechanisms. Not surprisingly, a disruption of the BBB has been associated with the onset and/or progression of major neurological disorders. Although the association between disease and BBB disruption is clear, its nature is not always evident, specifically with regard to whether an impaired BBB function results from the pathological condition or whether the BBB damage is the primary pathogenic factor prodromal to the onset of the disease. In either case, repairing the barrier could be a viable option for treating and/or reducing the effects of CNS disorders. In this review, we describe the fundamental structure and function of the BBB in both healthy and altered/diseased conditions. Additionally, we provide an overview of the potential therapeutic targets that could be leveraged to restore the integrity of the BBB concomitant to the treatment of these brain disorders.

The blood-brain barrier (BBB) acts as a structural and functional barrier for brain homeostasis. This review highlights the pathological contribution of BBB dysfunction to neuroimmunological diseases, including multiple sclerosis (MS), neuromyelitis optica spectrum disorder (NMOSD), myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD), autoimmune encephalitis (AE), and paraneoplastic neurological syndrome (PNS). The transmigration of massive lymphocytes across the BBB caused by the activation of cell adhesion molecules is involved in the early phase of MS, and dysfunction of the cortical BBB is associated with the atrophy of gray matter in the late phase of MS. At the onset of NMOSD, increased permeability of the BBB causes the entry of circulating AQP4 autoantibodies into the central nervous system (CNS). Recent reports have shown the importance of glucose-regulated protein (GRP) autoantibodies as BBB-reactive autoantibodies in NMOSD, which induce antibody-mediated BBB dysfunction. BBB breakdown has also been observed in MOGAD, NPSLE, and AE with anti-NMDAR antibodies. Our recent report demonstrated the presence of GRP78 autoantibodies in patients with MOGAD and the molecular mechanism responsible for GRP78 autoantibody-mediated BBB impairment. Disruption of the BBB may explain the symptoms in the brain and cerebellum in the development of PNS, as it induces the entry of pathogenic autoantibodies or lymphocytes into the CNS through autoimmunity against tumors in the periphery. GRP78 autoantibodies were detected in paraneoplastic cerebellar degeneration and Lambert-Eaton myasthenic syndrome, and they were associated with cerebellar ataxia with anti-P/Q type voltage-gated calcium channel antibodies. This review reports that therapies affecting the BBB that are currently available for disease-modifying therapies for neuroimmunological diseases have the potential to prevent BBB damage.

The blood-brain barrier (BBB) is composed of specific tight junction proteins and transporters expressed on the lining of endothelial cells of the vasculature in the brain. The structural and functional integrity of the BBB is one of the most critical factors for maintaining brain homeostasis and is mainly regulated by complex interactions between various cell types, such as endothelial cells, pericytes, and astrocytes, which are shaped by their differential responses to changes in microenvironments. Alterations in these cellular components have been implicated in neurodegenerative disorders. Although it has long been considered that BBB dysfunction is a mere ramification of pathological phenomena, emerging evidence supports its critical role in the pathogenesis of various disorders. In epilepsy, heightened BBB permeability has been found to be associated with increased occurrence of spontaneous seizures. Additionally, exaggerated inflammatory responses significantly correlate with increased BBB permeability during healthy aging. Furthermore, it has been previously reported that BBB disruption can be an early marker for predicting cognitive impairment in the progression of Alzheimer's disease. We herein review a potential role of the major cellular components of the BBB, with a focus on the contribution of BBB disruption, in neurodegenerative disease progression.

The potential interactive effects between subarachnoid hemorrhage (SAH) and blood brain barrier (BBB) disruption were studied in a rat model. Experimental subarachnoid hemorrhage was produced in twenty rats (experimental group) by the intracisternal injection of blood. In ten additional rats (control group), saline was administered in place of blood. Analysis of mean blood pressure (MBP), intracranial pressure (ICP) and cerebral perfusion pressure (CPP) demonstrated an increase in ICP and MBP and a drop in CPP in all animals following intracisternal injection. Subsequent infusion of the left internal carotid artery with sodium dehydrocholate resulted in blood-brain barrier (BBB) disruption in both groups as evidenced by Evans blue staining of the infused cortex. The extent of BBB disruption was significantly greater in the control group than the experimental group. Analysis of the experimental group demonstrated that animals with the lowest pre-SAH MBP and the lowest CPP during the maximum blood pressure response to SAH demonstrated the greatest resistance to experimental BBB disruption. The possibility of ischemia as a contributing factor in BBB protection subsequent to SAH is discussed.

Blood-brain barrier (BBB) disruption is a key pathological event following traumatic brain injury (TBI), yet its molecular and spatial characteristics remain incompletely understood. Here, we developed a dual dye-labeling system to assess the temporal and spatial dynamics of BBB permeability following controlled cortical impact (CCI) injury in EphA4f/f VE-Cadherin-CreERT2 (KO) and EphA4f/f (WT) mice. By tracking Evans Blue Dye (EBD), sodium fluorescein (NaFl), and IgG deposition, we reveal distinct patterns of extravasation in the injured cortex and hippocampus. NaFl, a small-molecule tracer, continues to extravasate for 7 days post-injury, whereas EBD leakage diminishes after 4 days. Notably, EC-specific EphA4 KO mice exhibit a protective role in BBB integrity. To further investigate BBB regulation, we integrated spatial transcriptomics with dye quantification, revealing that EphA4 EC ablation upregulates key BBB-related genes (Tjp2, Tjp3, Cldn1, and Ocln) and neuroprotective genes (Nr4a1 and Npas4). Notably, Wnt signaling genes are upregulated in the KO cortex, and we demonstrate that inhibition of Frizzled-4 (FZD4)/Wnt attenuates BBB protection in KO mice. Importantly, direct pharmacological activation of Wnt signaling with the FZD4 agonist FZM1.8 reduces lesion volume and BBB disruption. Overall, these findings demonstrate the effectiveness of spatial transcriptomics and dual-dye labeling in uncovering region-specific transcriptional changes associated with BBB disruption following CCI injury and in assessing the influence of EphA4/Wnt signaling. Wnt signaling emerges as a promising pathway for BBB protection and repair following TBI, offering a potential strategy to mitigate secondary brain injury.

The relationship between peripheral and central biomarkers in mild cognitive impairment (MCI), and the potential role of blood-brain barrier (BBB) dysfunction in this process, remain unclear. MCI, an intermediate state between normal aging and dementia, is characterized by early neuroinflammation and neuronal injury, yet how systemic markers reflect central pathology is poorly understood. In this study, we enrolled 74 participants, including 37 MCI patients and 37 cognitively normal controls. Based on the CSF/serum albumin ratio, subjects were classified into four groups-NC (cognitively normal with intact BBB), NMCI (MCI with intact BBB), BC (cognitively normal with BBB disruption), and BMCI (MCI with BBB disruption)-and further grouped as BBB-intact or BBB-disrupted. Serum and cerebrospinal fluid (CSF) levels of interleukin-4 (IL-4), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α), glial fibrillary acidic protein (GFAP), and neurofilament light (Nf-L) were measured using enzyme-linked immunosorbent assay. Spearman correlation analysis was applied to examine peripheral-central associations. No significant correlations were observed in NC or NMCI groups. A moderate serum-CSF GFAP correlation was found in the BC group (r = 0.446, P = 0.033), which became markedly stronger in the BMCI group (r = 0.753, P < 0.001). In the BBB-disrupted group, significant correlations were detected for GFAP (r = 0.652, P < 0.001), IL-4 (r = 0.412, P = 0.003), IL-6 (r = 0.296, P = 0.035), and TNF-α (r = 0.352, P = 0.011), with GFAP showing the strongest association. In contrast, within the BBB-intact group, only serum-CSF IL-6 correlation reached significance (r = 0.469, P = 0.024). These findings suggest that BBB disruption markedly enhances peripheral-central biomarker associations, especially for GFAP, highlighting the regulatory role of BBB integrity in linking systemic inflammation, neuronal injury, and MCI pathophysiology.