Blood-brain barrier permeability in Alzheimer's disease: a case-control MRI study

Young blood-mediated cerebromicrovascular rejuvenation through heterochronic parabiosis: enhancing blood-brain barrier integrity and capillarization in the aged mouse brain

Age-related cerebromicrovascular changes, including blood-brain barrier (BBB) disruption and microvascular rarefaction, play a significant role in the development of vascular cognitive impairment (VCI) and neurodegenerative diseases. Utilizing the unique model of heterochronic parabiosis, which involves surgically joining young and old animals, we investigated the influence of systemic factors on these vascular changes. Our study employed heterochronic parabiosis to explore the effects of young and aged systemic environments on cerebromicrovascular aging in mice. We evaluated microvascular density and BBB integrity in parabiotic pairs equipped with chronic cranial windows, using intravital two-photon imaging techniques. Our results indicate that short-term exposure to young systemic factors leads to both functional and structural rejuvenation of cerebral microcirculation. Notably, we observed a marked decrease in capillary density and an increase in BBB permeability to fluorescent tracers in the cortices of aged mice undergoing isochronic parabiosis (20-month-old C57BL/6 mice [A-(A)]; 6 weeks of parabiosis), compared to young isochronic parabionts (6-month-old, [Y-(Y)]). However, aged heterochronic parabionts (A-(Y)) exposed to young blood exhibited a significant increase in cortical capillary density and restoration of BBB integrity. In contrast, young mice exposed to old blood from aged parabionts (Y-(A)) rapidly developed cerebromicrovascular aging traits, evidenced by reduced capillary density and increased BBB permeability. These findings underscore the profound impact of systemic factors in regulating cerebromicrovascular aging. The rejuvenation observed in the endothelium, following exposure to young blood, suggests the existence of anti-geronic elements that counteract microvascular aging. Conversely, pro-geronic factors in aged blood appear to accelerate cerebromicrovascular aging. Further research is needed to assess whether the rejuvenating effects of young blood factors could extend to other age-related cerebromicrovascular pathologies, such as microvascular amyloid deposition and increased microvascular fragility.

Human brain clearance imaging: Pathways taken by magnetic resonance imaging contrast agents after administration in cerebrospinal fluid and blood

Over the last decade, it has become evident that cerebrospinal fluid (CSF) plays a pivotal role in brain solute clearance through perivascular pathways and interactions between the brain and meningeal lymphatic vessels. Whereas most of this fundamental knowledge was gained from rodent models, human brain clearance imaging has provided important insights into the human system and highlighted the existence of important interspecies differences. Current gold standard techniques for human brain clearance imaging involve the injection of gadolinium-based contrast agents and monitoring their distribution and clearance over a period from a few hours up to 2 days. With both intrathecal and intravenous injections being used, which each have their own specific routes of distribution and thus clearance of contrast agent, a clear understanding of the kinetics associated with both approaches, and especially the differences between them, is needed to properly interpret the results. Because it is known that intrathecally injected contrast agent reaches the blood, albeit in small concentrations, and that similarly some of the intravenously injected agent can be detected in CSF, both pathways are connected and will, in theory, reach the same compartments. However, because of clear differences in relative enhancement patterns, both injection approaches will result in varying sensitivities for assessment of different subparts of the brain clearance system. In this opinion review article, the "EU Joint Programme - Neurodegenerative Disease Research (JPND)" consortium on human brain clearance imaging provides an overview of contrast agent pharmacokinetics in vivo following intrathecal and intravenous injections and what typical concentrations and concentration-time curves should be expected. This can be the basis for optimizing and interpreting contrast-enhanced MRI for brain clearance imaging. Furthermore, this can shed light on how molecules may exchange between blood, brain, and CSF.

Associations between regional blood-brain barrier permeability, aging, and Alzheimer's disease biomarkers in cognitively normal older adults

BACKGROUND

Increased blood-brain barrier permeability (BBBp) has been hypothesized as a feature of aging that may lead to the development of Alzheimer's disease (AD). We sought to identify the brain regions most vulnerable to greater BBBp during aging and examine their regional relationship with neuroimaging biomarkers of AD.

METHODS

We studied 31 cognitively normal older adults (OA) and 10 young adults (YA) from the Berkeley Aging Cohort Study (BACS). Both OA and YA received dynamic contrast-enhanced MRI (DCE-MRI) to quantify Ktrans values, as a measure of BBBp, in 37 brain regions across the cortex. The OA also received Pittsburgh compound B (PiB)-PET to create distribution volume ratios (DVR) images and flortaucipir (FTP)- PET to create partial volume corrected standardized uptake volume ratios (SUVR) images. Repeated measures ANOVA assessed the brain regions where OA showed greater BBBp than YA. In OA, Ktrans values were compared based on sex, Aβ positivity status, and APOE4 carrier status within a composite region across the areas susceptible to aging. We used linear models and sparse canonical correlation analysis (SCCA) to examine the relationship between Ktrans and AD biomarkers.

RESULTS

OA showed greater BBBp than YA predominately in the temporal lobe, with some involvement of parietal, occipital and frontal lobes. Within an averaged ROI of affected regions, there was no difference in Ktrans values based on sex or Aβ positivity, but OA who were APOE4 carriers had significantly higher Ktrans values. There was no direct relationship between averaged Ktrans and global Aβ pathology, but there was a trend for an Ab status by tau interaction on Ktrans in this region. SCCA showed increased Ktrans was associated with increased PiB DVR, mainly in temporal and parietal brain regions. There was not a significant relationship between Ktrans and FTP SUVR.

DISCUSSION

Our findings indicate that the BBB shows regional vulnerability during normal aging that overlaps considerably with the pattern of AD pathology. Greater BBBp in brain regions affected in aging is related to APOE genotype and may also be related to the pathological accumulation of Aβ.

Novel targets and therapies of metformin in dementia: old drug, new insights

Dementia is a devastating disorder characterized by progressive and persistent cognitive decline, imposing a heavy public health burden on the individual and society. Despite numerous efforts by researchers in the field of dementia, pharmacological treatments are limited to relieving symptoms and fail to prevent disease progression. Therefore, studies exploring novel therapeutics or repurposing classical drugs indicated for other diseases are urgently needed. Metformin, a first-line antihyperglycemic drug used to treat type 2 diabetes, has been shown to be beneficial in neurodegenerative diseases including dementia. This review discusses and evaluates the neuroprotective role of metformin in dementia, from the perspective of basic and clinical studies. Mechanistically, metformin has been shown to improve insulin resistance, reduce neuronal apoptosis, and decrease oxidative stress and neuroinflammation in the brain. Collectively, the current data presented here support the future potential of metformin as a potential therapeutic strategy for dementia. This study also inspires a new field for future translational studies and clinical research to discover novel therapeutic targets for dementia.

Advances in Antioxidant Nanomedicines for Imaging and Therapy of Alzheimer's Disease

Significance: Reactive oxygen species (ROS) are crucial signaling molecules in the regulation of numerous physiological activities including the formation and function of the central nervous system (CNS). So far, many functional antioxidant nanomedicines with ROS scavenging capability to reduce oxidative stress in Alzheimer's disease (AD) have been developed for both imaging and therapy of AD. Recent Advances: This review focuses on the most recent advances in antioxidant nanomedicines such as ROS-scavenging nanoparticles (NPs), NPs with intrinsic antioxidant activity, and drug-loaded antioxidant NPs for AD theranostics. In addition to antioxidant nanomedicines, the emerging phototherapy treatment paradigms and the promising preclinic drug carriers, such as exosomes and liposomes, are also introduced. Critical Issues: In general, excessive generation of ROS can cause lipid peroxidation, oxidative DNA, as well as protein damage, aggravating pathogenic alterations, accumulation of amyloid-beta plaques and neurofibrillary tangles in the brain. These negative factors further cause cell death, which is the beginning of AD. Future Directions: We anticipate that this review will help researchers in the area of preclinical research and clinical translation of antioxidant nanomedicines for AD imaging and therapy.

Developmental priming of early cerebrovascular ageing: Implications across a lifetime

INTRODUCTION

Neurological conditions such as Alzheimer's disease and stroke represent a substantial health burden to the world's ageing population. Cerebrovascular dysfunction is a key contributor to these conditions, affecting an individual's risk profile, age of onset, and severity of neurological disease. Recent data shows that early-life events, such as maternal health during pregnancy, birth weight and exposure to environmental toxins can 'prime' the vascular system for later changes. With age, blood vessels can become less flexible and more prone to damage. This can lead to reduced blood flow to the brain, which is associated with cognitive decline and an increased risk of stroke and other cerebrovascular diseases. These in turn increase the risk of vascular dementia and Alzheimer's disease.

OBJECTIVES

We aim to explore how early life factors influence cerebrovascular health, ageing and disease.

METHODS

We have reviewed recently published literature from epidemiological studies, clinical cases and basic research which explore mechanisms that contribute to cerebrovascular and blood-brain barrier dysfunction, with a particularly focus on those that assess contribution of early-life events or vascular priming to subsequent injury.

RESULTS

Perinatal events have been linked to acute cerebrovascular dysfunction and long-term structural reorganisation. Systemic disease throughout the lifetime that produce inflammatory or oxidative stress may further sensitise the cerebrovasculature to disease and contribute to neurodegeneration.

CONCLUSIONS

By identifying these early-life determinants and understanding their mechanisms, scientists aim to develop strategies for preventing or mitigating cerebrovascular ageing-related issues.

Developing blood-brain barrier arterial spin labelling as a non-invasive early biomarker of Alzheimer's disease (DEBBIE-AD): a prospective observational multicohort study protocol

INTRODUCTION

Loss of blood-brain barrier (BBB) integrity is hypothesised to be one of the earliest microvascular signs of Alzheimer's disease (AD). Existing BBB integrity imaging methods involve contrast agents or ionising radiation, and pose limitations in terms of cost and logistics. Arterial spin labelling (ASL) perfusion MRI has been recently adapted to map the BBB permeability non-invasively. The DEveloping BBB-ASL as a non-Invasive Early biomarker (DEBBIE) consortium aims to develop this modified ASL-MRI technique for patient-specific and robust BBB permeability assessments. This article outlines the study design of the DEBBIE cohorts focused on investigating the potential of BBB-ASL as an early biomarker for AD (DEBBIE-AD).

METHODS AND ANALYSIS

DEBBIE-AD consists of a multicohort study enrolling participants with subjective cognitive decline, mild cognitive impairment and AD, as well as age-matched healthy controls, from 13 cohorts. The precision and accuracy of BBB-ASL will be evaluated in healthy participants. The clinical value of BBB-ASL will be evaluated by comparing results with both established and novel AD biomarkers. The DEBBIE-AD study aims to provide evidence of the ability of BBB-ASL to measure BBB permeability and demonstrate its utility in AD and AD-related pathologies.

ETHICS AND DISSEMINATION

Ethics approval was obtained for 10 cohorts, and is pending for 3 cohorts. The results of the main trial and each of the secondary endpoints will be submitted for publication in a peer-reviewed journal.

Regional differences in the link between water exchange rate across the blood-brain barrier and cognitive performance in normal aging

The blood-brain barrier (BBB) undergoes functional changes with aging which may contribute to cognitive decline. A novel, diffusion prepared arterial spin labeling-based MRI technique can measure the rate of water exchange across the BBB (kw) and may thus be sensitive to age-related alterations in water exchange at the BBB. However, studies investigating relationships between kw and cognition have reported different directions of association. Here, we begin to investigate the direction of associations between kw and cognition in different brain regions, and their possible underpinnings, by evaluating links between kw, cognitive performance, and MRI markers of cerebrovascular dysfunction and/or damage. Forty-seven healthy older adults (age range 61-84) underwent neuroimaging to obtain whole-brain measures of kw, cerebrovascular reactivity (CVR), and white matter hyperintensity (WMH) volumes. Additionally, participants completed uniform data set (Version 3) neuropsychological tests of executive function (EF) and episodic memory (MEM). Voxel-wise linear regressions were conducted to test associations between kw and cognitive performance, CVR, and WMH volumes. We found that kw in the frontoparietal brain regions was positively associated with cognitive performance but not with CVR or WMH volumes. Conversely, kw in the basal ganglia was negatively associated with cognitive performance and CVR and positively associated with regional, periventricular WMH volume. These regionally dependent associations may relate to different physiological underpinnings in the relationships between kw and cognition in neocortical versus subcortical brain regions in older adults.

Alzheimer's disease: The role of T lymphocytes in neuroinflammation and neurodegeneration

Alzheimer's disease, the leading cause of progressive cognitive decline globally, has been reported to be enhanced by neuroinflammation. Brain-resident innate immune cells and adaptive immune cells work together to produce neuroinflammation. Studies over the past decade have established the neuroimmune axis present in Alzheimer's disease; the crosstalk between adaptive and innate immune cells within and outside the brain is crucial to the onset and progression of Alzheimer's disease. Although the role of the adaptive immune system in Alzheimer's disease is not fully understood, it has been hypothesized that the brain's immune homeostasis is significantly disrupted, which greatly contributes to neuroinflammation. Brain-infiltrating T cells possess proinflammatory phenotypes and activities that directly contribute to neuroinflammation. The pro-inflammatory activities of the adaptive immune system in Alzheimer's disease are characterized by the upregulation of effector T cell activities and the downregulation of regulatory T cell activities in the brain, blood, and cerebrospinal fluid. In this review, we discuss the major impact of T lymphocytes on the pathogenesis and progression of Alzheimer's disease. Understanding the role and mechanism of action of T cells in Alzheimer's disease would significantly contribute to the identification of novel biomarkers for diagnosing and monitoring the progression of the disease. This knowledge could also be crucial to the development of immunotherapies for Alzheimer's disease.

Prospective T1 mapping to assess gadolinium retention in brain after intrathecal gadobutrol

PURPOSE

A possible pathway behind gadolinium retention in brain is leakage of contrast agents from blood to cerebrospinal fluid and entry into brain along perivascular (glymphatic) pathways. The object of this study was to assess for signs of gadolinium retention in brain 4 weeks after intrathecal contrast enhanced MRI.

METHODS

We prospectively applied standardized T1 mapping of the brain before and 4 weeks after intrathecal administration of 0.5 mmol gadobutrol in patients under work-up of cerebrospinal fluid circulation disorders. Due to methodological limitations, a safety margin for percentage change in T1 time was set to 3%. Region-wise differences were assessed by pairwise comparison using t-tests and forest plots, and statistical significance was accepted at .05 level (two-tailed).

RESULTS

In a cohort of 76 participants (mean age 47.2 years ± 17.9 [standard deviation], 47 women), T1 relaxation times remained unchanged in cerebral cortex and basal ganglia 4 weeks after intrathecal gadobutrol. T1 was reduced from 1082 ± 46.7 ms to 1070.6 ± 36.5 ms (0.98 ±2.9%) (mean [standard deviation]) (p=0.001) in white matter, thus within the pre-defined 3% safety margin. The brain stem and cerebellum could not be assessed due to poor alignment of posterior fossa structures at scans from different time points.

CONCLUSION

Gadolinium retention was not detected in the cerebral hemispheres 4 weeks after an intrathecal dose of 0.5 mmol gadobutrol, implying that presence of contrast agents in cerebrospinal fluid is of minor importance for gadolinium retention in brain.

Effects of intermittent fasting on cognitive health and Alzheimer's disease

OBJECTIVE

Caloric restriction by intermittent fasting produces several metabolic changes, such as increased insulin sensitivity and use of ketone bodies as energy sources. In humans, intermittent fasting has been studied in hypertension, diabetes, and related conditions, but, to date, not as a strategy to reduce the risk of emergent dementia. In this scoping review, the relevance of intermittent fasting as a potential preventive intervention for Alzheimer's dementia is explored.

BACKGROUND

The beneficial effects of calorie restriction have been documented in animals and humans. Decreased oxidative stress damage and attenuated inflammatory responses are associated with intermittent fasting. These changes have a favorable impact on the vascular endothelium and stress-induced cellular adaptation.

RESULTS

Physiological alterations associated with fasting have profound implications for pathological mechanisms associated with dementias, particularly Alzheimer's disease. Compared with ad libitum feeding, caloric restriction in animals was associated with a reduction in β-amyloid accumulation, which is the cardinal pathological marker of Alzheimer's disease. Animal studies have demonstrated synaptic adaptations in the hippocampus and enhanced cognitive function after fasting, consistent with these theoretical frameworks. Furthermore, vascular dysfunction plays a crucial role in Alzheimer's disease pathology, and intermittent fasting promotes vascular health.

CONCLUSIONS

These observations lead to a hypothesis that intermittent fasting over the years will potentially reverse or delay the pathological process in Alzheimer's disease.

Receptor-mediated drug delivery of bispecific therapeutic antibodies through the blood-brain barrier

Therapeutic antibody drug development is a rapidly growing sector of the pharmaceutical industry. However, antibody drug development for the brain is a technical challenge, and therapeutic antibodies for the central nervous system account for ~3% of all such agents. The principal obstacle to antibody drug development for brain or spinal cord is the lack of transport of large molecule biologics across the blood-brain barrier (BBB). Therapeutic antibodies can be made transportable through the blood-brain barrier by the re-engineering of the therapeutic antibody as a BBB-penetrating bispecific antibody (BSA). One arm of the BSA is the therapeutic antibody and the other arm of the BSA is a transporting antibody. The transporting antibody targets an exofacial epitope on a BBB receptor, and this enables receptor-mediated transcytosis (RMT) of the BSA across the BBB. Following BBB transport, the therapeutic antibody then engages the target receptor in brain. RMT systems at the BBB that are potential conduits to the brain include the insulin receptor (IR), the transferrin receptor (TfR), the insulin-like growth factor receptor (IGFR) and the leptin receptor. Therapeutic antibodies have been re-engineered as BSAs that target the insulin receptor, TfR, or IGFR RMT systems at the BBB for the treatment of Alzheimer's disease and Parkinson's disease.

Non-contrast estimate of blood-brain barrier permeability in humans using arterial spin labeling and magnetization transfer at 7 T

Blood-brain barrier (BBB) dysfunction is associated with a number of central nervous system diseases. This study demonstrates the application of a novel noninvasive technique to measure the BBB permeability in the human brain at 7 T. The technique exploits the fact that, when tissue macromolecules are saturated by off-resonance RF pulse, the intravascular and the extravascular (tissue) water experience different magnetization transfer effects. This principle was combined with arterial spin labeling to distinguish between the intravascular and the tissue water, and was used to calculate perfusion, water extraction fraction (E), and BBB permeability surface area product for water (PS). Simultaneous coregistered magnetization transfer ratio maps were also generated that can provide valuable additional information. Eighteen healthy volunteers (seven females), age = 27 ± 11 years and weight = 65 ± 9 kg, participated in the study. Average perfusion was 67 ± 5 and 29 ± 4 ml/100 g/min (p < 0.05); and E was 0.921 ± 0.025 and 0.962 ± 0.015 (p < 0.05) in the gray matter (GM) and the white matter (WM), respectively. PS was higher in the GM (171 ± 20 ml/100 g/min) compared with the WM (95 ± 18 ml/100 g/min) (p < 0.05). The parameters exhibited good reliability with test re-test experiments. The sensitivity of this technique was demonstrated by 200 mg caffeine intake, which resulted in a decrease in the resting PS by ~31%.

Virus-Like Cytosolic and Cell-Free Oxidatively Damaged Nucleic Acids Likely Drive Inflammation, Synapse Degeneration, and Neuron Death in Alzheimer's Disease

Oxidative stress, inflammation, and amyloid-β are Alzheimer's disease (AD) hallmarks that cause each other and other AD hallmarks. Most amyloid-β-lowering, antioxidant, anti-inflammatory, and antimicrobial AD clinical trials failed; none stopped or reversed AD. Although signs suggest an infectious etiology, no pathogen accumulated consistently in AD patients. Neuropathology, neuronal cell culture, rodent, genome-wide association, epidemiological, biomarker, and clinical studies, plus analysis using Hill causality criteria and revised Koch's postulates, indicate that the virus-like oxidative damage-associated molecular-pattern (DAMP) cytosolic and cell-free nucleic acids accumulated in AD patients' brains likely drive neuroinflammation, synaptotoxicity, and neurotoxicity. Cytosolic oxidatively-damaged mitochondrial DNA accumulated outside mitochondria dose-dependently in preclinical AD and AD patients' hippocampal neurons, and in AD patients' neocortical neurons but not cerebellar neurons or glia. In oxidatively-stressed neural cells and rodents' brains, cytosolic oxidatively-damaged mitochondrial DNA accumulated and increased antiviral and inflammatory proteins, including cleaved caspase-1, interleukin-1β, and interferon-β. Cytosolic double-stranded RNA and DNA are DAMPs that induce antiviral interferons and/or inflammatory proteins by oligomerizing with various innate-immune pattern-recognition receptors, e.g., cyclic GMP-AMP synthase and the nucleotide-binding-oligomerization-domain-like-receptor-pyrin-domain-containing-3 inflammasome. In oxidatively-stressed neural cells, cytosolic oxidatively-damaged mitochondrial DNA caused synaptotoxicity and neurotoxicity. Depleting mitochondrial DNA prevented these effects. Additionally, cell-free nucleic acids accumulated in AD patients' blood, extracellular vesicles, and senile plaques. Injecting cell-free nucleic acids bound to albumin oligomers into wild-type mice's hippocampi triggered antiviral interferon-β secretion; interferon-β injection caused synapse degeneration. Deoxyribonuclease-I treatment appeared to improve a severe-AD patient's Mini-Mental Status Exam by 15 points. Preclinical and clinical studies of deoxyribonuclease-I and a ribonuclease for AD should be prioritized.

Contributions of blood-brain barrier imaging to neurovascular unit pathophysiology of Alzheimer's disease and related dementias

The blood-brain barrier (BBB) plays important roles in the maintenance of brain homeostasis. Its main role includes three kinds of functions: (1) to protect the central nervous system from blood-borne toxins and pathogens; (2) to regulate the exchange of substances between the brain parenchyma and capillaries; and (3) to clear metabolic waste and other neurotoxic compounds from the central nervous system into meningeal lymphatics and systemic circulation. Physiologically, the BBB belongs to the glymphatic system and the intramural periarterial drainage pathway, both of which are involved in clearing interstitial solutes such as β-amyloid proteins. Thus, the BBB is believed to contribute to preventing the onset and progression for Alzheimer's disease. Measurements of BBB function are essential toward a better understanding of Alzheimer's pathophysiology to establish novel imaging biomarkers and open new avenues of interventions for Alzheimer's disease and related dementias. The visualization techniques for capillary, cerebrospinal, and interstitial fluid dynamics around the neurovascular unit in living human brains have been enthusiastically developed. The purpose of this review is to summarize recent BBB imaging developments using advanced magnetic resonance imaging technologies in relation to Alzheimer's disease and related dementias. First, we give an overview of the relationship between Alzheimer's pathophysiology and BBB dysfunction. Second, we provide a brief description about the principles of non-contrast agent-based and contrast agent-based BBB imaging methodologies. Third, we summarize previous studies that have reported the findings of each BBB imaging method in individuals with the Alzheimer's disease continuum. Fourth, we introduce a wide range of Alzheimer's pathophysiology in relation to BBB imaging technologies to advance our understanding of the fluid dynamics around the BBB in both clinical and preclinical settings. Finally, we discuss the challenges of BBB imaging techniques and suggest future directions toward clinically useful imaging biomarkers for Alzheimer's disease and related dementias.

The Role of ERK1/2 Pathway in the Pathophysiology of Alzheimer's Disease: An Overview and Update on New Developments

Alzheimer's disease (AD) is the most common neurodegenerative disorder worldwide. Several findings suggest that correcting the dysregulated signaling pathways may offer a potential therapeutic approach in this disease. Extracellular signal-regulated kinase 1/2 (ERK1/2), a member of the mitogen-activated protein kinase family, plays a major role in regulation of cell proliferation, autophagy process, and protein synthesis. The available literature suggests dysregulated ERK1/2 in AD patients with potential implications in the multifaceted underlying pathologies of AD, including amyloid-β plaque formation, tau phosphorylation, and neuroinflammation. In this regard, in the current review, we aim to summarize the reports on the potential roles of ERK1/2 in AD pathophysiology.

Changes in hippocampal volume during a preceding 10-year period do not correlate with cognitive performance and hippocampal blood‒brain barrier permeability in cognitively normal late-middle-aged men

Hippocampal blood-brain barrier (BBB) permeability may increase in normal healthy ageing and contribute to neurodegenerative disease. To examine this hypothesis, we investigated the correlation between blood-brain barrier (BBB) permeability, regional brain volume, memory functions and health and lifestyle factors in The Metropolit 1953 Danish Male Birth Cohort. We used dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with a gadolinium-based contrast agent to assess BBB permeability in 77 participants in the cohort. BBB permeability was measured as K_i values in the hippocampus, thalamus and white matter. Over a 10-year period, we observed progressive atrophy of both the left and right hippocampus (p = 0.001). There was no significant correlation between current BBB permeability and hippocampal volume, prior atrophy or cognition. The hippocampus volume ratio was associated with better visual and verbal memory scores (p < 0.01). Regional BBB differences revealed higher K_i values in the hippocampus and white matter than in the thalamus (p < 0.001). Participants diagnosed with type II diabetes had significantly higher BBB permeability in the white matter (p = 0.015) and thalamus (p = 0.016), which was associated with a higher Fazekas score (p = 0.024). We do not find evidence that BBB integrity is correlated with age-related hippocampal atrophy or cognitive functions. The association between diabetes, white matter hyperintensities and increased BBB permeability is consistent with the idea that cerebrovascular disease compromises BBB integrity. Our findings suggest that the hippocampus is particularly prone to age-related atrophy, which may explain some of the cognitive changes that accompany older age, but this prior atrophy is not correlated with current BBB permeability.

Arterial Spin Labelling-Based Blood-Brain Barrier Assessment and Its Applications

The brain relies on the blood-brain barrier (BBB) for the selective absorption of nutrients and the exclusion of other big molecules from the circulating blood. Therefore, the integrity of BBB is critical to brain health, and assessing BBB condition is of great clinical importance. BBB is often examined using exogenous tracers that can travel across the BBB, but the tracers might cause severe side effects. To avoid the use of external tracers, researchers have used magnetically labeled arterial blood as the endogenous tracer to assess the water permeability of BBB as a surrogate index of BBB. This paper reviews the three major types of Arterial Spin Labelling (ASL) based BBB water permeability assessment techniques and their applications in brain diseases such as Alzheimer's Disease.

Evaluation of blood-brain barrier integrity by the analysis of dynamic contrast-enhanced MRI - a comparison of quantitative and semi-quantitative methods

Disruption of the blood-brain barrier (BBB) is a key feature of various brain disorders. To assess its integrity a parametrization of dynamic magnetic resonance imaging (DCE MRI) with a contrast agent (CA) is broadly used. Parametrization can be done quantitatively or semi-quantitatively. Quantitative methods directly describe BBB permeability but exhibit several drawbacks such as high computation demands, reproducibility issues, or low robustness. Semi-quantitative methods are fast to compute, simply mathematically described, and robust, however, they do not describe the status of BBB directly but only as a variation of CA concentration in measured tissue. Our goal was to elucidate differences between five semi-quantitative parameters: maximal intensity (Imax), normalized permeability index (NPI), and difference in DCE values between three timepoints: baseline, 5 min, and 15 min (delta5-0, delta15-0, delta15-5) and two quantitative parameters: transfer constant (Ktrans) and an extravascular fraction (Ve). For the purpose of comparison, we analyzed DCE data of four patients 12-15 days after the stroke with visible CA enhancement. Calculated parameters showed abnormalities spatially corresponding with the ischemic lesion, however, findings in individual parameters morphometrically differed. Ktrans and Ve were highly correlated. Delta5-0 and delta15-0 were prominent in regions with rapid CA enhancement and highly correlated with Ktrans. Abnormalities in delta15-5 and NPI were more homogenous with less variable values, smoother borders, and less detail than Ktrans. Moreover, only delta15-5 and NPI were able to distinguish vessels from extravascular space. Our comparison provides important knowledge for understanding and interpreting parameters derived from DCE MRI by both quantitative and semi-quantitative methods.

The Impairment of Blood-Brain Barrier in Alzheimer's Disease: Challenges and Opportunities with Stem Cells

Alzheimer’s disease (AD) is the most common neurodegenerative disorder and its prevalence is increasing. Nowadays, very few drugs effectively reduce AD symptoms and thus, a better understanding of its pathophysiology is vital to design new effective schemes. Presymptomatic neuronal damage caused by the accumulation of Amyloid β peptide and Tau protein abnormalities remains a challenge, despite recent efforts in drug development. Importantly, therapeutic targets, biomarkers, and diagnostic techniques have emerged to detect and treat AD. Of note, the compromised blood-brain barrier (BBB) and peripheral inflammation in AD are becoming more evident, being harmful factors that contribute to the development of the disease. Perspectives from different pre-clinical and clinical studies link peripheral inflammation with the onset and progression of AD. This review aims to analyze the main factors and the contribution of impaired BBB in AD development. Additionally, we describe the potential therapeutic strategies using stem cells for AD treatment.

Lead Exposure in Developmental Ages Promotes Aβ Accumulation by Disturbing Aβ Transportation in Blood-Cerebrospinal Fluid Barrier/Blood-Brain Barriers and Impairing Aβ Clearance in the Liver

Environmental lead exposure is closely related to the progression of Alzheimer's disease (AD). Our previous study has shown that exposure to lead could result in the cholesterol unbalance and increase amyloid-beta (Aβ) generation in the brain. However, the potential effect of lead exposure on Aβ transportation is poorly reported. In this study, we sought to explore whether lead exposure in developmental ages impaired the integrity of BCSFB and BBB, two highly vascularized structures in the brain in a rat model. The Aβ clearance in the liver was also assessed. Our results showed that lead treatment in developmental ages increased the number of TUNEL-positive apoptotic cells in rat choroid plexus and microvessels. Moreover, lead exposure markedly increased pro-inflammatory factors expression including TNF-α and IL-1β in rat choroid plexus and microvessels. Interestingly, lead treatment increased the expression of AQP-1 and reduced the expression of TTR, two key proteins associated with the functions of choroid plexus and microvessels. Additionally, the expressions of ABCB1, LRP-1, and RAGE, three major receptors responsible for Aβ transportation, were disturbed by developmental lead exposure. All these pathologies resulted in Aβ_1-40 deposition within BCSFB and BBB and malfunctions of these two vascularized structures. Finally, we found that lead treatment remarkably inhibited the gene expression of LRP-1, which is responsible for Aβ endocytosis, in the liver tissue of the rat model. Collectively, our results provide the first evidence that developmental lead exposure induces Aβ deposition in BCSFB and BBB and impairs Aβ clearance in the liver, which would ultimately disturb Aβ transportation via choroid plexus/brain microvessels and facilitate Aβ deposition in the brain.

In vivo quantitative imaging of hippocampal inflammation in autoimmune neuroinflammatory conditions: a systematic review

The hippocampus is a morphologically complex region of the brain limbic system centrally involved in important cognitive, affective, and behavioural regulatory roles. It has exquisite vulnerability to neuroinflammatory processes, with some of its subregions found to be specific sites of neuroinflammatory pathology in ex-vivo studies. Optimizing neuroimaging correlates of hippocampal neuroinflammation would enable the direct study of functional consequences of hippocampal neuroinflammatory pathology, as well as the definition of therapeutic end-points for treatments targeting neuroinflammation, and their related affective or cognitive sequelae. However, in vivo traditional imaging of the hippocampus and its subregions is fraught with difficulties, due to methodological challenges deriving from its unique anatomical characteristics. The main objective of this review is to provide a current update on the characterization of quantitative neuroimaging correlates of hippocampal neuroinflammation by focusing on three prototypical autoimmune neuro-inflammatory conditions [multiple sclerosis (MS), systemic lupus erythematosus (SLE), and autoimmune encephalitis (AE)]. We focused on studies employing TSPO-targeting positron emission tomography (PET), quantitative magnetic resonance imaging (MRI), and spectroscopy techniques assumed to be sensitive to neuroinflammatory tissue changes. We found 18 eligible studies (14, 2, and 2 studies in MS, AE, and SLE, respectively). Across conditions, the largest effect was seen in TSPO PET and diffusion-weighted MRI studies. No study examined neuroinflammation-related changes at the hippocampal subfield level. Overall, results were largely inconsistent due to heterogeneous imaging methods, small sample sizes, and different population studies. We discuss how these data could inform future study design and conclude by suggesting further methodological directions aimed at improving the precision and sensitivity of neuroimaging techniques to characterize hippocampal neuroinflammatory pathology in the human brain.

Engagement of vascular early response genes typifies mild cognitive impairment

INTRODUCTION

Molecular responses in the brains of persons with mild cognitive impairment (MCI), the earliest transitional state between normal aging and early Alzheimer's disease (AD), are poorly understood.

METHODS

We examined AD-related neuropathology and transcriptome changes in the neocortex of individuals with MCI relative to controls and temporal responses to the mild hypoxia in mouse brains.

RESULTS

Subsets of vascular early response to hypoxia genes were upregulated in MCI prior to the buildup of AD neuropathology. Early activation of pro-angiogenic hypoxia-inducible factor signaling in response to mild hypoxia was detected in mouse brains similar to those that were altered in MCI. Protracted responses to hypoxia were characterized by activation of phosphoinositide 3-kinase (PI3K)-protein kinase B (Akt)-the mammalian target of rapamycin (mTOR) pathways in brain microvessel isolates.

DISCUSSION

These findings suggest that cerebrovascular remodeling is an important antecedent to the development of dementia and a component of the homeostatic response to reduced oxygen tension in aging prior to the onset of AD.

Blood-brain barrier leakage in Alzheimer's disease: From discovery to clinical relevance

Alzheimer's disease (AD) is the most common form of dementia. AD brain pathology starts decades before the onset of clinical symptoms. One early pathological hallmark is blood-brain barrier dysfunction characterized by barrier leakage and associated with cognitive decline. In this review, we summarize the existing literature on the extent and clinical relevance of barrier leakage in AD. First, we focus on AD animal models and their susceptibility to barrier leakage based on age and genetic background. Second, we re-examine barrier dysfunction in clinical and postmortem studies, summarize changes that lead to barrier leakage in patients and highlight the clinical relevance of barrier leakage in AD. Third, we summarize signaling mechanisms that link barrier leakage to neurodegeneration and cognitive decline in AD. Finally, we discuss clinical relevance and potential therapeutic strategies and provide future perspectives on investigating barrier leakage in AD. Identifying mechanistic steps underlying barrier leakage has the potential to unravel new targets that can be used to develop novel therapeutic strategies to repair barrier leakage and slow cognitive decline in AD and AD-related dementias.

Imaging subtle leaks in the blood-brain barrier in the aging human brain: potential pitfalls, challenges, and possible solutions

Recent studies using dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) with gadolinium-based contrast agents (GBCA) have demonstrated subtle blood-brain barrier (BBB) leaks in the human brain during normal aging, in individuals with age-related cognitive dysfunction, genetic risk for Alzheimer's disease (AD), mild cognitive impairment, early AD, cerebral small vessel disease (SVD), and other neurodegenerative disorders. In these neurological conditions, the BBB leaks, quantified by the unidirectional BBB GBCA tracer's constant Ktrans maps, are typically orders of magnitude lower than in brain tumors, after stroke and/or during relapsing episodes of multiple sclerosis. This puts extra challenges for the DCE-MRI technique by pushing calculations towards its lower limits of detectability. In addition, presently, there are no standardized multivendor protocols or evidence of repeatability and reproducibility. Nevertheless, subtle BBB leaks may critically contribute to the pathophysiology of cognitive impairment and dementia associated with AD or SVD, and therefore, efforts to improve sensitivity of detection, reliability, and reproducibility are warranted. A larger number of participants scanned by different MR scanners at different clinical sites are sometimes required to detect differences in BBB integrity between control and at-risk groups, which impose additional challenges. Here, we focus on these new challenges and propose some approaches to normalize and harmonize DCE data between different scanners. In brief, we recommend specific regions to be used for the tracer's vascular input function and DCE data processing and how to find and correct negative Ktrans values that are physiologically impossible. We hope this information will prove helpful to new investigators wishing to study subtle BBB damage in neurovascular and neurodegenerative conditions and in the aging human brain.

Memantine in the Prevention of Radiation-Induced Brain Damage: A Narrative Review

Simple Summary

Decline in cognitive function is a major problem for patients undergoing whole-brain radiotherapy (WBRT). Scientific interest has increased due to the high dropout rate of patients in the first months after WBRT and the early onset of cognitive decline. Therefore, the study of antiglutamatergic pharmacological prophylaxis and hippocampal-sparing WBRT techniques has been deepened based on the knowledge of the mechanisms of hyperglutamatergic neurotoxicity and the role of some hippocampal areas in cognitive decline. In order to provide a summary of the evidence in this field, and to foster future research in this setting, this literature review presents current evidence on the prevention of radiation-induced cognitive decline and particularly on the role of memantine.

Abstract

Preserving cognitive functions is a priority for most patients with brain metastases. Knowing the mechanisms of hyperglutamatergic neurotoxicity and the role of some hippocampal areas in cognitive decline (CD) led to testing both the antiglutamatergic pharmacological prophylaxis and hippocampal-sparing whole-brain radiotherapy (WBRT) techniques. These studies showed a relative reduction in CD four to six months after WBRT. However, the failure to achieve statistical significance in one study that tested memantine alone (RTOG 0614) led to widespread skepticism about this drug in the WBRT setting. Moreover, interest grew in the reasons for the strong patient dropout rates in the first few months after WBRT and for early CD onset. In fact, the latter can only partially be explained by subclinical tumor progression. An emerging interpretation of the (not only) cognitive impairment during and immediately after WBRT is the dysfunction of the limbic and hypothalamic system with its immune and hormonal consequences. This new understanding of WBRT-induced toxicity may represent the basis for further innovative trials. These studies should aim to: (i) evaluate in greater detail the cognitive effects and, more generally, the quality of life impairment during and immediately after WBRT; (ii) study the mechanisms producing these early effects; (iii) test in clinical studies, the modern and advanced WBRT techniques based on both hippocampal-sparing and hypothalamic-pituitary-sparing, currently evaluated only in planning studies; (iv) test new timings of antiglutamatergic drugs administration aimed at preventing not only late toxicity but also acute effects.

Aberrant Cerebral Iron Trafficking Co-morbid With Chronic Inflammation: Molecular Mechanisms and Pharmacologic Intervention

The redox properties that make iron an essential nutrient also make iron an efficient pro-oxidant. Given this nascent cytotoxicity, iron homeostasis relies on a combination of iron transporters, chaperones, and redox buffers to manage the non-physiologic aqueous chemistry of this first-row transition metal. Although a mechanistic understanding of the link between brain iron accumulation (BIA) and neurodegenerative diseases is lacking, BIA is co-morbid with the majority of cognitive and motor function disorders. The most prevalent neurodegenerative disorders, including Alzheimer's Disease (AD), Parkinson's Disease (PD), Multiple System Atrophy (MSA), and Multiple Sclerosis (MS), often present with increased deposition of iron into the brain. In addition, ataxias that are linked to mutations in mitochondrial-localized proteins (Friedreich's Ataxia, Spinocerebellar Ataxias) result in mitochondrial iron accumulation and degradation of proton-coupled ATP production leading to neuronal degeneration. A comorbidity common in the elderly is a chronic systemic inflammation mediated by primary cytokines released by macrophages, and acute phase proteins (APPs) released subsequently from the liver. Abluminal inflammation in the brain is found downstream as a result of activation of astrocytes and microglia. Reasonably, the iron that accumulates in the brain comes from the cerebral vasculature via the microvascular capillary endothelial cells whose tight junctions represent the blood-brain barrier. A premise amenable to experimental interrogation is that inflammatory stress alters both the trans- and para-cellular flux of iron at this barrier resulting in a net accumulation of abluminal iron over time. This review will summarize the evidence that lends support to this premise; indicate the mechanisms that merit delineation; and highlight possible therapeutic interventions based on this model.

The Role of Heparin and Glycocalyx in Blood-Brain Barrier Dysfunction

The blood-brain barrier (BBB) functions as a dynamic boundary that protects the central nervous system from blood and plays an important role in maintaining the homeostasis of the brain. Dysfunction of the BBB is a pathophysiological characteristic of multiple neurologic diseases. Glycocalyx covers the luminal side of vascular endothelial cells(ECs). Damage of glycocalyx leads to disruption of the BBB, while inhibiting glycocalyx degradation maintains BBB integrity. Heparin has been recognized as an anticoagulant and it protects endothelial glycocalyx from destruction. In this review, we summarize the role of glycocalyx in BBB formation and the therapeutic potency of heparin to provide a theoretical basis for the treatment of neurological diseases related to BBB breakdown.

Analysis of plasma proteins using 2D gels and novel fluorescent probes: in search of blood based biomarkers for Alzheimer's disease

BACKGROUND

The Australian Imaging and Biomarker Lifestyle (AIBL) study of aging is designed to aid the discovery of biomarkers. The current study aimed to discover differentially expressed plasma proteins that could yield a blood-based screening tool for Alzheimer's disease.

METHODS

The concentration of proteins in plasma covers a vast range of 12 orders of magnitude. Therefore, to search for medium to low abundant biomarkers and elucidate mechanisms of AD, we immuno-depleted the most abundant plasma proteins and pre-fractionated the remaining proteins by HPLC, prior to two-dimensional gel electrophoresis. The relative levels of approximately 3400 protein species resolved on the 2D gels were compared using in-gel differential analysis with spectrally resolved fluorescent protein detection dyes (Zdyes™). Here we report on analysis of pooled plasma samples from an initial screen of a sex-matched cohort of 72 probable AD patients and 72 healthy controls from the baseline time point of AIBL.

RESULTS

We report significant changes in variants of apolipoprotein E, haptoglobin, α1 anti-trypsin, inter-α trypsin inhibitor, histidine-rich glycoprotein, and a protein of unknown identity. α1 anti-trypsin and α1 anti-chymotrypsin demonstrated plasma concentrations that were dependent on APOE ε4 allele dose. Our analysis also identified an association with the level of Vitamin D binding protein fragments and complement factor I with sex. We then conducted a preliminary validation study, on unique individual samples compared to the discovery cohort, using a targeted LC-MS/MS assay on a subset of discovered biomarkers. We found that targets that displayed a high degree of isoform specific changes in the 2D gels were not changed in the targeted MS assay which reports on the total level of the biomarker.

CONCLUSIONS

This demonstrates that further development of mass spectrometry assays is needed to capture the isoform complexity that exists in theses biological samples. However, this study indicates that a peripheral protein signature has potential to aid in the characterization of AD.

Discerning the Role of Blood Brain Barrier Dysfunction in Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common form of neurodegenerative disease. The predominant characteristics of AD are the accumulation of amyloid-β (Aβ) and hyperphosphorylated tau in the brain. Blood brain barrier (BBB) dysfunction as one of the causative factors of cognitive impairment is increasingly recognized in the last decades. However, the role of BBB dysfunction in AD pathogenesis is still not fully understood. It remains elusive whether BBB dysfunction is a consequence or causative fact of Aβ pathology, tau pathology, neuroinflammation, or other conditions. In this review, we summarized the major findings of BBB dysfunction in AD and the reciprocal relationships between BBB dysfunction, Aβ pathology, tau pathology, and neuroinflammation. In addition, the implications of BBB dysfunction in AD for delivering therapeutic drugs were presented. Finally, we discussed how to better determine the underlying mechanisms between BBB dysfunction and AD, as well as how to explore new therapies for BBB regulation to treat AD in the future.

Robust Multi-TE ASL-Based Blood-Brain Barrier Integrity Measurements

Multiple echo-time arterial spin labelling (multi-TE ASL) offers estimation of blood–tissue exchange dynamics by probing the T2 relaxation of the labelled spins. In this study, we provide a recipe for robust assessment of exchange time (Texch) as a proxy measure of blood–brain barrier (BBB) integrity based on a test-retest analysis. This includes a novel scan protocol and an extension of the two-compartment model with an “intra-voxel transit time” (ITT) to address tissue transit effects. With the extended model, we intend to separate the underlying two distinct mechanisms of tissue transit and exchange. The performance of the extended model in comparison with the two-compartment model was evaluated in simulations. Multi-TE ASL sequence with two different bolus durations was used to acquire in vivo data (n = 10). Cerebral blood flow (CBF), arterial transit time (ATT) and Texch were fitted with the two models, and mean grey matter values were compared. Additionally, the extended model also extracted ITT parameter. The test-retest reliability of Texch was assessed for intra-session, inter-session and inter-visit pairs of measurements. Intra-class correlation coefficient (ICC) and within-subject coefficient of variance (CoV) for grey matter were computed to assess the precision of the method. Mean grey matter Texch and ITT values were found to be 227.9 ± 37.9 ms and 310.3 ± 52.9 ms, respectively. Texch estimated by the extended model was 32.6 ± 5.9% lower than the two-compartment model. A significant ICC was observed for all three measures of Texch reliability (P < 0.05). Texch intra-session CoV, inter-session CoV and inter-visit CoV were found to be 6.6%, 7.9%, and 8.4%, respectively. With the described improvements addressing intra-voxel transit effects, multi-TE ASL shows good reproducibility as a non-invasive measure of BBB permeability. These findings offer an encouraging step forward to apply this potential BBB permeability biomarker in clinical research.

Assessing Computational Methods to Quantify Mother-Child Brain Synchrony in Naturalistic Settings Based on fNIRS Signals

Mother-child brain-to-brain synchrony captures the temporal similarities in brain signals between dyadic partners, and has been shown to emerge during the display of joint behaviours. Despite the rise in the number of studies that investigate synchrony in naturalistic contexts, the use of varying methodological approaches to compute synchrony remains a central problem. When dyads engage in unstructured social interactions, the wide range of behavioural cues they display contribute to the use of varying lengths of signals to compute synchrony. The present functional Near-infrared Spectroscopy (fNIRS) study investigates how different methods to quantify brain signals during joint and non-joint portions of dyadic play affect the outcome of brain-to-brain synchrony. Three strategies to cope with unstructured data are tested and different signal lengths of 15, 20, 25, 30, 35, 40, 45s were used to determine the optimal method to sensitively capture synchrony. Results showed that using all available portions of the signals generated a greater number of less conservative results compared to the other two strategies, which were to compute the average synchrony for the joint and non-joint signals portions and to compute the difference between the average synchrony of joint and non-joint portions. From the different signal durations, only length portions of 25s to 35s generated significant results. These findings demonstrate that differences in computational approaches and signal lengths affect synchrony measurements and should be considered in naturalistic synchrony studies.

Dysfunction of the blood-brain barrier in Alzheimer's disease: evidence from human studies

The pathological processes leading to synapse loss, neuronal loss, brain atrophy and gliosis in Alzheimer´s disease (AD) and their relation to vascular disease and immunological changes are yet to be fully explored. Amyloid-β (Aβ) aggregation, vascular damage and altered immune response interact at the blood-brain-barrier (BBB), affecting the brain endothelium and fuelling neurodegeneration. The aim of the present systematic literature review was to critically appraise and to summarise the published evidence on the clinical correlations and pathophysiological concepts of BBB damage in AD, focusing on human data. The PubMed, Cochrane, Medline and Embase databases were searched for original research articles, systematic reviews and meta-analyses, published in English language from 01/2000 to 07/2021, using the keywords Alzheimer*, amyloid-β or β-amyloid or abeta and brain-blood barrier or BBB. This review shows that specific changes of intercellular structures, reduced expression of transendothelial carriers, induction of vasoactive mediators and activation of both astroglia and monocytes/macrophages characterise blood-brain barrier damage in human AD and AD models. BBB dysfunction on magnetic resonance imaging takes place early in the disease course in AD-specific brain regions. The toxic effects of Aβ and apolipoprotein E (ApoE) are likely to induce a non-cerebral-amyloid-angiopathy-related degeneration of endothelial cells, independently of cerebrovascular disease; however, some of the observed structural changes may just arise with age. Small vessel disease, ApoE, loss of pericytes, pro-inflammatory signalling and cerebral amyloid angiopathy enhance blood-brain-barrier damage. Novel therapeutic approaches for AD, including magnetic resonance-guided focused ultrasound, aim to open the BBB, potentially leading to an improved drainage of Aβ along perivascular channels and increased elimination from the brain. In vitro treatments with ApoE-modifying agents yielded promising effects on modulating BBB function. Reducing cardiovascular risk factors represents one of the most promising interventions for dementia prevention at present. However, further research is needed to elucidate the connection of BBB damage and tau pathology, the role of pro-inflammatory mediators in draining macromolecules and cells from the cerebral parenchyma, including their contribution to cerebral amyloid angiopathy. Improved insight into these pathomechanisms may allow to shed light on the role of Aβ deposition as a primary vs. a secondary event in the complex pathogenesis of AD.

Understanding the brain uptake and permeability of small molecules through the BBB: A technical overview

The brain is the most important organ in our body requiring its unique microenvironment. By the virtue of its function, the blood-brain barrier poses a significant hurdle in drug delivery for the treatment of neurological diseases. There are also different theories regarding how molecules are typically effluxed from the brain. In this review, we comprehensively discuss how the different pharmacokinetic techniques used for measuring brain uptake/permeability of small molecules have evolved with time. We also discuss the advantages and disadvantages associated with these different techniques as well as the importance to utilize the right method to properly assess CNS exposure to drug molecules. Even though very strong advances have been made we still have a long way to go to ensure a reduction in failures in central nervous system drug development programs.

Advanced Human BBB-on-a-Chip: A New Platform for Alzheimer's Disease Studies

The blood-brain barrier (BBB) is a unique vascular structure that serves as a molecular transport gateway for the maintenance of brain homeostasis. Chronic disruption or breakdown of the BBB reportedly leads to neurodegenerative diseases. Nonetheless, research on human BBB pathophysiology and drug development remains highly dependent on studies using inherently different animals. Moreover, more studies have shown that animal models are not appropriate in modeling Alzheimer's disease (AD), underlining the importance of in vitro models of the human BBB with physiological relevance. In this review, recent advances in human BBB-on-a-chip technologies are highlighted and their potential for pathogenesis studies and drug prescreening for AD treatment are discussed.

Omega-3 fatty acids are associated with blood-brain barrier integrity in a healthy aging population

In aging populations, omega-3 polyunsaturated fatty acids (PUFAs) have been associated with better cognitive function, slower rates of cognitive decline, and lower risk of developing dementia. Animal studies have shown that diets rich in omega-3 PUFAs reduce blood-brain barrier (BBB) disruption associated with aging, but this has yet to be observed in humans. Forty-five healthy subjects (mean age, 76 years) were recruited and underwent cognitive assessment (verbal learning and memory, language, processing speed, executive function, and motor control) and measurement of PUFAs. Forty of the same subjects also underwent magnetic resonance imaging (MRI) to measure BBB integrity (K_trans using dynamic contrast-enhanced MRI). The long chain omega-3 score (DHA+EPA) was negatively correlated with K_trans values in the internal capsule, indicating higher omega-3 levels were associated with greater BBB integrity in this region (r = -0.525, p = .004). Trends were observed for a positive correlation between the long chain omega-3 score and both memory and language scores, but not with executive function, speed, or motor control. The omega-6 score was not significantly correlated with any cognitive scores or K_trans values. The significant correlations between long chain omega-3 levels and BBB integrity provide a possible mechanism by which omega-3 PUFAs are associated with brain health.

Age-related changes in cerebrovascular health and their effects on neural function and cognition: A comprehensive review

The process of aging includes changes in cellular biology that affect local interactions between cells and their environments and eventually propagate to systemic levels. In the brain, where neurons critically depend on an efficient and dynamic supply of oxygen and glucose, age-related changes in the complex interaction between the brain parenchyma and the cerebrovasculature have effects on health and functioning that negatively impact cognition and play a role in pathology. Thus, cerebrovascular health is considered one of the main mechanisms by which a healthy lifestyle, such as habitual cardiorespiratory exercise and a healthful diet, could lead to improved cognitive outcomes with aging. This review aims at detailing how the physiology of the cerebral vascular system changes with age and how these changes lead to differential trajectories of cognitive maintenance or decline. This provides a framework for generating specific mechanistic hypotheses about the efficacy of proposed interventions and lifestyle covariates that contribute to enhanced cognitive well-being. Finally, we discuss the methodological implications of age-related changes in the cerebral vasculature for human cognitive neuroscience research and propose directions for future experiments aimed at investigating age-related changes in the relationship between physiology and cognitive mechanisms.

7T dynamic contrast-enhanced MRI for the detection of subtle blood-brain barrier leakage

Background and Purpose

Dynamic contrast‐enhanced MRI (DCE‐MRI) can be employed to assess the blood–brain barrier (BBB) integrity. Detection of BBB leakage at lower field strengths (≤3T) is cumbersome as the signal is noisy, while leakage can be subtle. Utilizing the increased signal‐to‐noise ratio at higher field strengths, we explored the application of 7T DCE‐MRI for assessing BBB leakage.

Methods

A dual‐time resolution DCE‐MRI method was implemented at 7T and a slow injection rate (0.3 ml/s) and low dose (3 mmol) served to obtain signal changes linearly related to the gadolinium concentration, that is, minimized for T₂^* degradation effects. With the Patlak graphical approach, the leakage rate (K_i) and blood plasma volume fraction (v_p) were calculated. The method was evaluated in 10 controls, an ischemic stroke patient, and a patient with a transient ischemic attack.

Results

K_i and v_p were significantly higher in gray matter compared to white matter of all participants. These K_i values were higher in both patients compared to the control subjects. Finally, for the lesion identified in the ischemic stroke patient, higher leakage values were observed compared to normal‐appearing tissue.

Conclusion

We demonstrate how a dual‐time resolution DCE‐MRI protocol at 7T, with administration of half the clinically used contrast agent dose, can be used for assessing subtle BBB leakage. Although the feasibility of DCE‐MRI for assessing the BBB integrity at 3T is well known, we showed that a continuous sampling DCE‐MRI method tailored for 7T is also capable of assessing leakage with a high sensitivity over a range of K_i values.

Assessment of Blood-Brain Barrier Integrity in Tuberculous Meningitis Using Dynamic Contrast-Enhanced MR Perfusion

Objective Tuberculous meningitis (TBM) is the most common form of central nervous system tuberculosis. The aim of the study was to quantitatively evaluate blood-brain barrier (BBB) perfusion changes in TBM patients using dynamic contrast-enhanced (DCE) MR perfusion. Methods and Material Thirty untreated patients of TBM and 10 healthy controls were prospectively evaluated by conventional imaging and DCE MR perfusion. Mean permeability indices- K trans and Ve-were calculated from multiple regions of interest (ROIs) placed in basal cisterns and comparison was done between the patients and controls. Results The permeability indices were significantly higher (where p < 0.001) in cisterns of TBM patients who showed basal meningeal enhancement when compared with healthy controls. Significant differences in permeability were observed between "enhancing" cases and controls as well as in "enhancing" cases when compared with the "non-enhancing" cases. However, no significant difference was observed in the mean cisternal value between "non-enhancing" cases and the controls. K trans with a cutoff value of > 0.0838 had 81.6% sensitivity and 78.6% specificity in differentiating cases and controls while V e mean value with a cutoff value of 0.0703 showed 86.8% sensitivity and 91.4% specificity in predicting the permeability difference between the cases and controls. Conclusion DCE MR perfusion is useful in the quantitative measurement of disruption of BBB and perfusion alterations in patients of TBM.

The Microbiota-Gut-Brain Axis and Alzheimer Disease. From Dysbiosis to Neurodegeneration: Focus on the Central Nervous System Glial Cells

The microbiota-gut system can be thought of as a single unit that interacts with the brain via the "two-way" microbiota-gut-brain axis. Through this axis, a constant interplay mediated by the several products originating from the microbiota guarantees the physiological development and shaping of the gut and the brain. In the present review will be described the modalities through which the microbiota and gut control each other, and the main microbiota products conditioning both local and brain homeostasis. Much evidence has accumulated over the past decade in favor of a significant association between dysbiosis, neuroinflammation and neurodegeneration. Presently, the pathogenetic mechanisms triggered by molecules produced by the altered microbiota, also responsible for the onset and evolution of Alzheimer disease, will be described. Our attention will be focused on the role of astrocytes and microglia. Numerous studies have progressively demonstrated how these glial cells are important to ensure an adequate environment for neuronal activity in healthy conditions. Furthermore, it is becoming evident how both cell types can mediate the onset of neuroinflammation and lead to neurodegeneration when subjected to pathological stimuli. Based on this information, the role of the major microbiota products in shifting the activation profiles of astrocytes and microglia from a healthy to a diseased state will be discussed, focusing on Alzheimer disease pathogenesis.

Modifications of physical and functional integrity of the blood-brain barrier in an inducible mouse model of neurodegeneration

The inducible p25 overexpression mouse model recapitulate many hallmark features of Alzheimer's disase including progressive neuronal loss, elevated Aβ, tau pathology, cognitive dysfunction, and impaired synaptic plasticity. We chose p25 mice to evaluate the physical and functional integrity of the blood-brain barrier (BBB) in a context of Tau pathology (pTau) and severe neurodegeneration, at an early (3 weeks ON) and a late (6 weeks ON) stage of the pathology. Using in situ brain perfusion and confocal imaging, we found that the brain vascular surface area and the physical integrity of the BBB were unaltered in p25 mice. However, there was a significant 14% decrease in cerebrovascular volume in 6 weeks ON mice, possibly explained by a significant 27% increase of collagen IV in the basement membrane of brain capillaries. The function of the BBB transporters GLUT1 and LAT1 was evaluated by measuring brain uptake of d-glucose and phenylalanine, respectively. In 6 weeks ON p25 mice, d-glucose brain uptake was significantly reduced by about 17% compared with WT, without any change in the levels of GLUT1 protein or mRNA in brain capillaries. The brain uptake of phenylalanine was not significantly reduced in p25 mice compared with WT. Lack of BBB integrity, impaired BBB d-glucose transport have been observed in several mouse models of AD. In contrast, reduced cerebrovascular volume and an increased basement membrane thickness may be more specifically associated with pTau in mouse models of neurodegeneration.

Application of contrast-enhanced magnetic resonance imaging in the assessment of blood-cerebrospinal fluid barrier integrity

VERHEGGEN, I.C.M., W. Freeze, J. de Jong, J. Jansen, A. Postma, M. van Boxtel, F. Verhey and W. Backes. The application of contrast-enhanced MRI in the assessment of blood-cerebrospinal fluid barrier integrity. Choroid plexus epithelial cells form a barrier that enables active, bidirectional exchange between the blood plasma and cerebrospinal fluid (CSF), known as the blood-CSF barrier (BCSFB). Through its involvement in CSF composition, the BCSFB maintains homeostasis in the central nervous system. While the relation between blood-brain barrier disruption, aging and neurodegeneration is extensively studied using contrast-enhanced MRI, applying this technique to investigate BCSFB disruption in age-related neurodegeneration has received little attention. This review provides an overview of the current status of contrast-enhanced MRI to assess BCSFB permeability. Post-contrast ventricular gadolinium enhancement has been used to indicate BCSFB permeability. Moreover, new techniques highly sensitive to low gadolinium concentrations in the CSF, for instance heavily T2-weighted imaging with cerebrospinal fluid suppression, seem promising. Also, attempts are made at using other contrast agents, such as manganese ions or very small superparamagnetic iron oxide particles, that seem to be cleared from the brain at the choroid plexus. Advancing and applying new developments such as these could progress the assessment of BCSFB integrity.

Sesame Oil-Based Nanostructured Lipid Carriers of Nicergoline, Intranasal Delivery System for Brain Targeting of Synergistic Cerebrovascular Protection

Nicergoline (NIC) is a semisynthetic ergot alkaloid derivative applied for treatment of dementia and other cerebrovascular disorders. The efficacy of sesame oil to slow and reverse the symptoms of neurodegenerative cognitive disorders has been proven. This work aimed to formulate and optimize sesame oil-based NIC-nanostructured lipid carriers (NIC–NLCs) for intranasal (IN) delivery with expected synergistic and augmented neuroprotective properties. The NIC–NLC were prepared using sesame oil as a liquid lipid. A three-level, three-factor Box–Behnken design was applied to statistically optimize the effect of sesame oil (%) of the total lipid, surfactant concentration, and sonication time on particle size, zeta potential, and entrapment efficacy as responses. Solid-state characterization, release profile, and ex vivo nasal permeation in comparison to NIC solution (NIC–SOL) was studied. In vivo bioavailability from optimized NIC–NLC and NIC–SOL following IN and IV administration was evaluated and compared. The optimized NIC–NLC formula showed an average particle size of 111.18 nm, zeta potential of −15.4 mV, 95.11% entrapment efficacy (%), and 4.6% loading capacity. The NIC–NLC formula showed a biphasic, extended-release profile (72% after 48 h). Permeation of the NIC–NLC formula showed a 2.3 enhancement ratio. Bioavailability studies showed a 1.67 and 4.57 fold increase in plasma and brain following IN administration. The results also indicated efficient direct nose-to-brain targeting properties with the brain-targeting efficiency (BTE%) and direct transport percentage (DTP%) of 187.3% and 56.6%, respectively, after IN administration. Thus, sesame oil-based NIC–NLC can be considered as a promising IN delivery system for direct and efficient brain targeting with improved bioavailability and expected augmented neuroprotective action for the treatment of dementia.

Noncontrast assessment of blood-brain barrier permeability to water: Shorter acquisition, test-retest reproducibility, and comparison with contrast-based method

PURPOSE

Assessment of the blood-brain barrier (BBB) permeability without the need for contrast agent is desirable, and the ability to measure the permeability to small molecules such as water may further increase the sensitivity in detecting diseases. This study proposed a time-efficient, noncontrast method to measure BBB permeability to water, evaluated its test-retest reproducibility, and compared it with a contrast agent-based method.

METHODS

A single-delay water extraction with phase-contrast arterial spin tagging (WEPCAST) method was devised in which spatial profile of the signal along the superior sagittal sinus was used to estimate bolus arrival time, and the WEPCAST signal at the corresponding location was used to compute water extraction fraction, which was combined with global cerebral blood flow to estimate BBB permeability surface area product to water. The reliability of WEPCAST sequence was examined in terms of intrasession, intersession, and inter-vendor (Philips [Ingenia, Best, the Netherlands] and Siemens [Prisma, Erlangen, Germany]) reproducibility. Finally, we compared this new technique to a contrast agent-based method.

RESULTS

Single-delay WEPCAST reduced the scan duration from approximately 20 min to 5 min. Extract fraction values estimated from single-delay WEPCAST showed good consistency with the multi-delay method (R = 0.82, P = .004). Group-averaged permeability surface area product values were found to be 137.5 ± 9.3 mL/100 g/min. Intrasession, intersession, and inter-vendor coefficient of variation of the permeability surface area product values were 6.6 ± 4.5%, 6.9 ± 3.7%, and 8.9 ± 3.0%, respectively. Finally, permeability surface area product obtained from WEPCAST MRI showed a significant correlation with that from the contrast-based method (R = .73, P = .02).

CONCLUSION

Single-delay WEPCAST MRI can measure BBB permeability to water within 5 min with an intrasession, intersession, and inter-vendor test-retest reproducibility of 6% to 9%. This method may provide a useful marker of BBB breakdown in clinical studies.

Vasculotide restores the blood-brain barrier after focused ultrasound-induced permeability in a mouse model of Alzheimer's disease

Focused ultrasound (FUS) is used to locally and transiently induce blood-brain barrier (BBB) permeability, allowing targeted drug delivery to the brain. The purpose of the current study is to evaluate the potential of Vasculotide to accelerate the recovery of the BBB following FUS disruption in the TgCRND8 mouse model of amyloidosis, characteristic of Alzheimer's disease (AD). Accelerating the restoration of the BBB post-FUS would represent an additional safety procedure, which could be beneficial for clinical applications. Methods: TgCRND8 mice and their non-transgenic littermates were treated with Vasculotide (250 ng, intraperitoneal) every 48 hours for 3 months. BBB permeability was induced using FUS, in presence of intravenously injected microbubbles, in TgCRND8 and non-transgenic mice, and confirmed at time 0 by MRI enhancement using the contrast agent gadolinium. BBB closure was assessed at 6, 12 and 20 hours by MRI. In a separate cohort of animals, BBB closure was assessed at 24-hours post-FUS using Evans blue injected intravenously and followed by histological evaluation. Results: Chronic Vasculotide administration significantly reduces the ultra-harmonic threshold required for FUS-induced BBB permeability in the TgCRND8 mice. In addition, Vasculotide treatment led to a faster restoration of the BBB following FUS in TgCRND8 mice. BBB closure after FUS is not significantly different between TgCRND8 and non-transgenic mice. BBB permeability was assessed by gadolinium up to 20-hours post-FUS, demonstrating 87% closure in Vasculotide treated TgCRND8 mice, as opposed to 52% in PBS treated TgCRND8 mice, 58% in PBS treated non-transgenic mice, and 74% in Vasculotide treated non-transgenic mice. In both TgCRND8 mice and non-transgenic littermates the BBB was impermeable to Evans blue dye at 24-hours post-FUS. Conclusion: Vasculotide reduces the pressure required for microbubble ultra-harmonic onset for FUS-induced BBB permeability and it accelerates BBB restoration in a mouse model of amyloidosis, suggesting its potential clinical utility to promote vascular health, plasticity and repair in AD.

Blood-Brain Barrier Leakage Is Increased in Parkinson's Disease

Background

Blood–brain barrier (BBB) disruption has been noted in animal models of Parkinson’s disease (PD) and forms the basis of the vascular hypothesis of neurodegeneration, yet clinical studies are lacking.

Objective

To determine alterations in BBB integrity in PD, with comparison to cerebrovascular disease.

Methods

Dynamic contrast enhanced magnetic resonance images were collected from 49 PD patients, 15 control subjects with cerebrovascular disease [control positive (CP)] and 31 healthy control subjects [control negative (CN)], with all groups matched for age. Quantitative maps of the contrast agent transfer coefficient across the BBB (K^trans) and plasma volume (v_p) were produced using Patlak analysis. Differences in K^trans and v_p were assessed with voxel-based analysis as well as in regions associated with PD pathophysiology. In addition, the volume of white matter lesions (WMLs) was obtained from T₂-weighted fluid attenuation inversion recovery (FLAIR) images.

Results

Higher K^trans, reflecting higher BBB leakage, was found in the PD group than in the CN group using voxel-based analysis; differences were most prominent in the posterior white matter regions. Region of interest analysis confirmed K^trans to be significantly higher in PD than in CN, predominantly driven by differences in the substantia nigra, normal-appearing white matter, WML and the posterior cortex. WML volume was significantly higher in PD compared to CN. K^trans values and WML volume were similar in PD and CP, suggesting a similar burden of cerebrovascular disease despite lower cardiovascular risk factors.

Conclusion

These results show BBB disruption in PD.

Combining T2 measurements and crusher gradients into a single ASL sequence for comparison of the measurement of water transport across the blood-brain barrier

Purpose

Arterial spin labeling can be used to assess the transition time of water molecules across the blood–brain barrier when combined with sequence modules, which allow a separation of intravascular from tissue signal. The bipolar gradient technique measures the intravascular fraction by removing flowing spins. The T₂‐relaxation‐under‐spin‐tagging (TRUST) technique modulates the TE to differentiate between intravascular and extravascular spins based on T₂. These modules were combined into a single time‐encoded pseudo‐continuous arterial spin labeling sequence to compare their mechanisms of action as well as their assessment of water transition across the blood–brain barrier.

Methods

This protocol was acquired on a scanner with 9 healthy volunteers who provided written, informed consent. The sequence consisted of a Hadamard‐encoded pseudo‐continuous arterial spin labeling module, followed by the TRUST module (effective TEs of 0, 40, and 80 ms) and bipolar flow‐crushing gradients (2, 4, and ∞ cm/s). An additional experiment was performed with TRUST and a 3D gradient and spin‐echo readout.

Results

Gradients imperfectly canceled the intravascular signal, as evidenced by the presence of residual signal in the arteries at early postlabeling delays as well as the underestimation of the intravascular fraction as compared with the TRUST method. The TRUST module allowed us to detect the transport of water deeper into the vascular tree through changes in T₂ than the used crusher gradients could, with their limited b‐value.

Conclusion

Of the implemented techniques, TRUST allowed us to follow intravascular signal deeper into the vascular tree than the approach with (relatively weak) crusher gradients when quantifying the transport time of water across the blood–brain barrier.

Treatment of Alzheimer's Disease and Blood-Brain Barrier Drug Delivery

Despite the enormity of the societal and health burdens caused by Alzheimer's disease (AD), there have been no FDA approvals for new therapeutics for AD since 2003. This profound lack of progress in treatment of AD is due to dual problems, both related to the blood-brain barrier (BBB). First, 98% of small molecule drugs do not cross the BBB, and ~100% of biologic drugs do not cross the BBB, so BBB drug delivery technology is needed in AD drug development. Second, the pharmaceutical industry has not developed BBB drug delivery technology, which would enable industry to invent new therapeutics for AD that actually penetrate into brain parenchyma from blood. In 2020, less than 1% of all AD drug development projects use a BBB drug delivery technology. The pathogenesis of AD involves chronic neuro-inflammation, the progressive deposition of insoluble amyloid-beta or tau aggregates, and neural degeneration. New drugs that both attack these multiple sites in AD, and that have been coupled with BBB drug delivery technology, can lead to new and effective treatments of this serious disorder.

An Overview on the Differential Interplay Among Neurons-Astrocytes-Microglia in CA1 and CA3 Hippocampus in Hypoxia/Ischemia

Neurons have been long regarded as the basic functional cells of the brain, whereas astrocytes and microglia have been regarded only as elements of support. However, proper intercommunication among neurons-astrocytes-microglia is of fundamental importance for the functional organization of the brain. Perturbation in the regulation of brain energy metabolism not only in neurons but also in astrocytes and microglia may be one of the pathophysiological mechanisms of neurodegeneration, especially in hypoxia/ischemia. Glial activation has long been considered detrimental for survival of neurons, but recently it appears that glial responses to an insult are not equal but vary in different brain areas. In this review, we first take into consideration the modifications of the vascular unit of the glymphatic system and glial metabolism in hypoxic conditions. Using the method of triple-labeling fluorescent immunohistochemistry coupled with confocal microscopy (TIC), we recently studied the interplay among neurons, astrocytes, and microglia in chronic brain hypoperfusion. We evaluated the quantitative and morpho-functional alterations of the neuron-astrocyte-microglia triads comparing the hippocampal CA1 area, more vulnerable to ischemia, to the CA3 area, less vulnerable. In these contiguous and interconnected areas, in the same experimental hypoxic conditions, astrocytes and microglia show differential, finely regulated, region-specific reactivities. In both areas, astrocytes and microglia form triad clusters with apoptotic, degenerating neurons. In the neuron-astrocyte-microglia triads, the cell body of a damaged neuron is infiltrated and bisected by branches of astrocyte that create a microscar around it while a microglial cell phagocytoses the damaged neuron. These coordinated actions are consistent with the scavenging and protective activities of microglia. In hypoxia, the neuron-astrocyte-microglia triads are more numerous in CA3 than in CA1, further indicating their protective effects. These data, taken from contiguous and interconnected hippocampal areas, demonstrate that glial response to the same hypoxic insult is not equal but varies significantly. Understanding the differences of glial reactivity is of great interest to explain the differential susceptibility of hippocampal areas to hypoxia/ischemia. Further studies may evidence the differential reactivity of glia in different brain areas, explaining the higher or lower sensitivity of these areas to different insults and whether glia may represent a target for future therapeutic interventions.