Blood-Brain Barrier Leakage in Patients with Early Alzheimer Disease

Evaluation of dietary and lifestyle changes as modifiers of S100β levels in Alzheimer's disease

There is a significant body of research undertaken in order to elucidate the mechanisms underlying the pathology of Alzheimer's disease (AD), as well as to discover early detection biomarkers and potential therapeutic strategies. One such proposed biomarker is the calcium binding protein S100β, which, depending on its local concentration, is known to exhibit both neurotrophic and neuroinflammatory properties in the central nervous system. At present, relatively little is known regarding the effect of chronic S100β disruption in AD. Dietary intake has been identified as a modifiable risk factor for AD. Preliminary in vitro and animal studies have demonstrated an association between S100β expression and dietary intake which links to AD pathophysiology. This review describes the association of S100β to fatty acids, ketone bodies, insulin, and botanicals as well as the potential impact of physical activity as a lifestyle factor. We also discuss the prospective implications of these findings, including support of the use of a Mediterranean dietary pattern and/or the ketogenic diet as an approach to modify AD risk.

CSF/serum albumin ratio in dementias: a cross-sectional study on 1861 patients

A connection between dementias and blood-brain barrier (BBB) dysfunction has been suggested, but previous studies have yielded conflicting results. We examined cerebrospinal fluid (CSF)/serum albumin ratio in a large cohort of patients diagnosed with Alzheimer's disease (AD, early onset [EAD, n = 130], late onset AD [LAD, n = 666]), vascular dementia (VaD, n = 255), mixed AD and VaD (MIX, n = 362), Lewy body dementia (DLB, n = 50), frontotemporal dementia (FTD, n = 56), Parkinson's disease dementia (PDD, n = 23), other dementias (other, n = 48), and dementia not otherwise specified (NOS, n = 271). We compared CSF/serum albumin ratio to 2 healthy control groups (n = 292, n = 20), between dementia diagnoses, and tested biomarker associations. Patients in DLB, LAD, VaD, MIX, other, and NOS groups had higher CSF/serum albumin ratio than controls. CSF/serum albumin ratio correlated with CSF neurofilament light in LAD, MIX, VaD, and other groups but not with AD biomarkers. Our data show that BBB leakage is common in dementias. The lack of association between CSF/serum albumin ratio and AD biomarkers suggests that BBB dysfunction is not inherent to AD but might represent concomitant cerebrovascular pathology.

Subtle blood-brain barrier leakage rate and spatial extent: Considerations for dynamic contrast-enhanced MRI

PURPOSE

Dynamic contrast-enhanced (DCE) MRI can be used to measure blood-brain barrier (BBB) leakage. In neurodegenerative disorders such as small vessel disease and dementia, the leakage can be very subtle and the corresponding signal can be rather noisy. For these reasons, an optimized DCE-MRI measurement and study design is required. To this end, a new measure indicative of the spatial extent of leakage is introduced and the effects of scan time and sample size are explored.

METHODS

Dual-time resolution DCE-MRI was performed in 16 patients with early Alzheimer's disease (AD) and 17 healthy controls. The leakage rate (K_i ) and volume fraction of detectable leaking tissue (v_L ) to quantify the spatial extent of BBB leakage were calculated in cortical gray matter and white matter using noise-corrected histogram analysis of leakage maps. Computer simulations utilizing realistic K_i histograms, mimicking the strong effect of noise and variation in K_i values, were performed to understand the influence of scan time on the estimated leakage.

RESULTS

The mean K_i was very low (order of 10^-4 min^-1 ) and highly influenced by noise, causing the K_i to be increasingly overestimated at shorter scan times. In the white matter, the K_i was not different between patients with early AD and controls, but was higher in the cortex for patients, reaching significance after 14.5 min of scan time. To detect group differences, v_L proved more suitable, showing significantly higher values for patients compared with controls in the cortex after 8 minutes of scan time, and in white matter after 15.5 min.

CONCLUSIONS

Several ways to improve the sensitivity of a DCE-MRI experiment to subtle BBB leakage were presented. We have provided v_L as an attractive and potentially more time-efficient alternative to detect group differences in subtle and widespread blood-brain barrier leakage compared with leakage rate K_i . Recommendations on group size and scan time are made based on statistical power calculations to aid future research.

Pericortical Enhancement on Delayed Postgadolinium Fluid-Attenuated Inversion Recovery Images in Normal Aging, Mild Cognitive Impairment, and Alzheimer Disease

BACKGROUND AND PURPOSE

Breakdown of BBB integrity occurs in dementia and may lead to neurodegeneration and cognitive decline. We assessed whether extravasation of gadolinium chelate could be visualized on delayed postcontrast FLAIR images in older individuals with and without cognitive impairment.

MATERIALS AND METHODS

Seventy-four individuals participated in this study (15 with Alzheimer disease, 33 with mild cognitive impairment, and 26 with normal cognition). We assessed the appearance of pericortical enhancement after contrast administration, MR imaging markers of cerebrovascular damage, and medial temporal lobe atrophy. Three participants who were positive for pericortical enhancement (1 with normal cognition and 2 with mild cognitive impairment) were followed up for approximately 2 years. In vitro experiments with a range of gadolinium concentrations served to elucidate the mechanisms underlying the postcontrast FLAIR signals.

RESULTS

Postcontrast pericortical enhancement was observed in 21 participants (28%), including 6 individuals with Alzheimer disease (40%), 10 with mild cognitive impairment (30%), and 5 with normal cognition (19%). Pericortical enhancement was positively associated with age (P < .02) and ischemic stroke (P < .05), but not with cognitive status (P = .3). Foci with enhanced signal remained stable across time in all follow-up cases. The in vitro measurements confirmed that FLAIR imaging is highly sensitive for the detection of low gadolinium concentrations in CSF, but not in cerebral tissue.

CONCLUSIONS

Postcontrast pericortical enhancement on FLAIR images occurs in older individuals with normal cognition, mild cognitive impairment, and dementia. It may represent chronic focal superficial BBB leakage. Future longitudinal studies are needed to determine its clinical significance.

Cerebral blood flow regulation and neurovascular dysfunction in Alzheimer disease

Cerebral blood flow (CBF) regulation is essential for normal brain function. The mammalian brain has evolved a unique mechanism for CBF control known as neurovascular coupling. This mechanism ensures a rapid increase in the rate of CBF and oxygen delivery to activated brain structures. The neurovascular unit is composed of astrocytes, mural vascular smooth muscle cells and pericytes, and endothelia, and regulates neurovascular coupling. This Review article examines the cellular and molecular mechanisms within the neurovascular unit that contribute to CBF control, and neurovascular dysfunction in neurodegenerative disorders such as Alzheimer disease.

Advanced Neuroimaging of Cerebral Small Vessel Disease

Cerebral small vessel disease (SVD) is characterised by damage to deep grey and white matter structures of the brain and is responsible for a diverse range of clinical problems that include stroke and dementia. In this review, we describe advances in neuroimaging published since January 2015, mainly with magnetic resonance imaging (MRI), that, in general, are improving quantification, observation and investigation of SVD focussing on three areas: quantifying the total SVD burden, imaging brain microstructural integrity and imaging vascular malfunction. Methods to capture ‘whole brain SVD burden’ across the spectrum of SVD imaging changes will be useful for patient stratification in clinical trials, an approach that we are already testing. More sophisticated imaging measures of SVD microstructural damage are allowing the disease to be studied at earlier stages, will help identify specific factors that are important in development of overt SVD imaging features and in understanding why specific clinical consequences may occur. Imaging vascular function will help establish the precise blood vessel and blood flow alterations at early disease stages and, together with microstructural integrity measures, may provide important surrogate endpoints in clinical trials testing new interventions. Better knowledge of SVD pathophysiology will help identify new treatment targets, improve patient stratification and may in future increase efficiency of clinical trials through smaller sample sizes or shorter follow-up periods. However, most of these methods are not yet sufficiently mature to use with confidence in clinical trials, although rapid advances in the field suggest that reliable quantification of SVD lesion burden, tissue microstructural integrity and vascular dysfunction are imminent.

Efflux proteins at the blood-brain barrier: review and bioinformatics analysis

1. Efflux proteins at the blood-brain barrier provide a mechanism for export of waste products of normal metabolism from the brain and help to maintain brain homeostasis. They also prevent entry into the brain of a wide range of potentially harmful compounds such as drugs and xenobiotics. 2. Conversely, efflux proteins also hinder delivery of therapeutic drugs to the brain and central nervous system used to treat brain tumours and neurological disorders. For bypassing efflux proteins, a comprehensive understanding of their structures, functions and molecular mechanisms is necessary, along with new strategies and technologies for delivery of drugs across the blood-brain barrier. 3. We review efflux proteins at the blood-brain barrier, classified as either ATP-binding cassette (ABC) transporters (P-gp, BCRP, MRPs) or solute carrier (SLC) transporters (OATP1A2, OATP1A4, OATP1C1, OATP2B1, OAT3, EAATs, PMAT/hENT4 and MATE1). 4. This includes information about substrate and inhibitor specificity, structural organisation and mechanism, membrane localisation, regulation of expression and activity, effects of diseases and conditions and the principal technique used for in vivo analysis of efflux protein activity: positron emission tomography (PET). 5. We also performed analyses of evolutionary relationships, membrane topologies and amino acid compositions of the proteins, and linked these to structure and function.

Neurovascular Alterations in Alzheimer's Disease: Transporter Expression Profiles and CNS Drug Access

Despite a century of steady and incremental progress toward understanding the underlying biochemical mechanisms, Alzheimer's disease (AD) remains a complicated and enigmatic disease, and greater insight will be necessary before substantive clinical success is realised. Over the last decade in particular, a large body of work has highlighted the cerebral microvasculature as an anatomical region with an increasingly apparent role in the pathogenesis of AD. The causative interplay and temporal cascade that manifest between the brain vasculature and the wider disease progression of AD are not yet fully understood, and further inquiry is required to properly characterise these relationships. The purpose of this review is to highlight the recent advancements in research implicating neurovascular factors in AD, at both the molecular and anatomical levels. We begin with a brief introduction of the biochemical and genetic aspects of AD, before reviewing the essential concepts of the blood-brain barrier (BBB) and the neurovascular unit (NVU). In detail, we then examine the evidence demonstrating involvement of BBB dysfunction in AD pathogenesis, highlighting the importance of neurovascular components in AD. Lastly, we include within this review research that focuses on how altered properties of the BBB in AD impact upon CNS exposure of therapeutic agents and the potential clinical impact that this may have on people with this disease.

Brain metabolism in health, aging, and neurodegeneration

Brain cells normally respond adaptively to bioenergetic challenges resulting from ongoing activity in neuronal circuits, and from environmental energetic stressors such as food deprivation and physical exertion. At the cellular level, such adaptive responses include the "strengthening" of existing synapses, the formation of new synapses, and the production of new neurons from stem cells. At the molecular level, bioenergetic challenges result in the activation of transcription factors that induce the expression of proteins that bolster the resistance of neurons to the kinds of metabolic, oxidative, excitotoxic, and proteotoxic stresses involved in the pathogenesis of brain disorders including stroke, and Alzheimer's and Parkinson's diseases. Emerging findings suggest that lifestyles that include intermittent bioenergetic challenges, most notably exercise and dietary energy restriction, can increase the likelihood that the brain will function optimally and in the absence of disease throughout life. Here, we provide an overview of cellular and molecular mechanisms that regulate brain energy metabolism, how such mechanisms are altered during aging and in neurodegenerative disorders, and the potential applications to brain health and disease of interventions that engage pathways involved in neuronal adaptations to metabolic stress.

Alzheimer's disease: Innate immunity gone awry?

Inflammation is an immune activity designed to protect the host from pathogens and noxious agents. In its low-intensity form, presence of an inflammatory process must be inferred from appropriate biomarkers. Occult neuroinflammation is not just secondary to Alzheimer's disease (AD) but may contribute to its pathogenesis and promote its progression. A leaky blood-brain barrier (BBB) has been observed in early AD and may play a role in its initiation and development. Studies of the temporal evolution of AD's biomarkers have shown that, in AD, the brain's amyloid burden correlates poorly with cognitive decline. In contrast, cognitive deficits in AD correlate well with synapse loss. Oligomeric forms of amyloid-beta (oAβs) can be synaptotoxic and evidence of their deposition inside synaptic terminals of cognition-associated neurons explains early memory loss in AD better than formation of extracellular Aβ plaques. Among innate immune cells that reside in the brain, microglia sense danger signals represented by proteins like oAβ and become activated by neuronal damage such as that caused by bacterial endotoxins. The resulting reactive microgliosis has been implicated in generating the chronic form of microglial activation believed to promote AD's development. Genome-wide association studies (GWASs) have yielded data from patients with sporadic AD indicating that its causes include genetic variation in the innate immune system. Recent preclinical studies have reported that β-hydroxybutyrate (βOHB) may protect the brain from the adverse effects of both the nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome and the deacetylation of histone. Consequently, there is an urgent need for clinical investigations designed to test whether an orally administered βOHB preparation, such as a ketone ester, can have a similar beneficial effect in human subjects.

Measuring subtle leakage of the blood-brain barrier in cerebrovascular disease with DCE-MRI: Test-retest reproducibility and its influencing factors

PURPOSE

Increased blood-brain barrier (BBB) permeability has been shown to play a significant role in the pathophysiology of cerebrovascular disease and it may provide an early functional marker of progression or treatment effects. The aim of the study was to investigate the test-retest reproducibility and influencing factors of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) in measuring subtle leakage in patients with cerebrovascular disease.

MATERIAL AND METHODS

DCE-MRI (3T) was performed on two separate days in 16 patients (age 66 ± 9 years) with cerebrovascular disease, prospectively. The leakage rate was quantified for white matter (WM) and gray matter (GM) using the Patlak graphical approach with individual vascular input functions (VIFs). Furthermore, the influence of session-averaged VIFs, the average of the VIFs obtained on two days, and shorter scan times (range 5-25 minutes) on the reproducibility were evaluated in WM and GM.

RESULTS

Coefficients of variation (CV) ≤14.4% (WM and GM), intraclass correlation coefficients (ICCs) of 0.77 (WM) and 0.49 (GM), were observed for the leakage rate. Session-averaged VIFs hardly affected these results (CV ≤13.4%). The repeatability coefficients (RCs) of the leakage rate decreased from 2.7·10^-3 to 0.4·10^-3 min^-1 in WM (P < 0.01) and 4.4·10^-3 to 0.9·10^-3 min^-1 in GM (P < 0.01) with increasing scan time (range 5-25 minutes).

CONCLUSION

Based on the moderate CVs and moderate-to-excellent ICCs, we demonstrate that measuring subtle BBB leakage using DCE-MRI is moderate-to-excellent reproducible. Longer scan times improve the reproducibility. The provided RCs at various scan times may assist future clinical studies investigating BBB leakage using DCE-MRI.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;46:159-166.

Reversal of high fat diet-induced obesity improves glucose tolerance, inflammatory response, β-amyloid accumulation and cognitive decline in the APP/PSEN1 mouse model of Alzheimer's disease

This study assessed the extent to which high fat diet (HFD)-induced β-amyloid accumulation and cognitive decline in APP/PSEN1 mice are reversible through control of fat intake. Ten months of HFD (60% calories from fat) led to significant deficits in a 2-trial Y maze task, and nest building assay, and decreased voluntary locomotor activity. The HFD induced an inflammatory response, indicated by increased expression of several inflammatory markers. Substituting a low fat diet led to pronounced weight loss and correction of glucose intolerance, decreases in the inflammatory response, and improved performance on behavioral tasks in both wild-type and APP/PSEN1 transgenic mice. Insoluble β-amyloid levels, and extent of tau phosphorylation were also lower following dietary reversal in APP/PSEN1 mice compared to high fat-fed animals, indicating that the inflammatory response may have contributed to key pathogenic pathways in the Alzheimer's disease model. The data suggest that weight loss can be a vital strategy for cognitive protection, but also highlight potential mechanisms for intervention when sustained weight loss is not possible.

Blood-brain barrier leakage is more widespread in patients with cerebral small vessel disease

OBJECTIVE

As blood-brain barrier (BBB) dysfunction may occur in normal aging but may also play a pivotal role in the pathophysiology of cerebral small vessel disease (cSVD), we used dynamic contrast-enhanced (DCE)-MRI to quantify the rate and the spatial extent of BBB leakage in patients with cSVD and age- and sex-matched controls to discern cSVD-related BBB leakage from aging-related leakage.

METHODS

We performed structural brain MRI and DCE-MRI in 80 patients with clinically overt cSVD and 40 age- and sex-matched controls. Using the Patlak pharmacokinetic model, we calculated the leakage rate. The mean leakage rate and relative leakage volume were calculated using noise-corrected histogram analysis. Leakage rate and leakage volume were compared between patients with cSVD and controls for the normal-appearing white matter (NAWM), white matter hyperintensities (WMH), cortical gray matter (CGM), and deep gray matter.

RESULTS

Multivariable linear regression analyses adjusting for age, sex, and cardiovascular risk factors showed that the leakage volume of the NAWM, WMH, and CGM was significantly larger in patients with cSVD compared with controls. No significant difference was found for leakage rate in any of the tissue regions.

CONCLUSION

We demonstrated a larger tissue volume with subtle BBB leakage in patients with cSVD than in controls. This was shown in the NAWM, WMH, and CGM, supporting the generalized nature of cSVD.

Antioxidants and Dementia Risk: Consideration through a Cerebrovascular Perspective

A number of natural and chemical compounds that exert anti-oxidative properties are demonstrated to be beneficial for brain and cognitive function, and some are reported to reduce the risk of dementia. However, the detailed mechanisms by which those anti-oxidative compounds show positive effects on cognition and dementia are still unclear. An emerging body of evidence suggests that the integrity of the cerebrovascular blood-brain barrier (BBB) is centrally involved in the onset and progression of cognitive impairment and dementia. While recent studies revealed that some anti-oxidative agents appear to be protective against the disruption of BBB integrity and structure, few studies considered the neuroprotective effects of antioxidants in the context of cerebrovascular integrity. Therefore, in this review, we examine the mechanistic insights of antioxidants as a pleiotropic agent for cognitive impairment and dementia through a cerebrovascular axis by primarily focusing on the current available data from physiological studies. Conclusively, there is a compelling body of evidence that suggest antioxidants may prevent cognitive decline and dementia by protecting the integrity and function of BBB and, indeed, further studies are needed to directly examine these effects in addition to underlying molecular mechanisms.

The Effects of Long-Term Saturated Fat Enriched Diets on the Brain Lipidome

The brain is highly enriched in lipids, where they influence neurotransmission, synaptic plasticity and inflammation. Non-pathological modulation of the brain lipidome has not been previously reported and few studies have investigated the interplay between plasma lipid homeostasis relative to cerebral lipids. This study explored whether changes in plasma lipids induced by chronic consumption of a well-tolerated diet enriched in saturated fatty acids (SFA) was associated with parallel changes in cerebral lipid homeostasis. Male C57Bl/6 mice were fed regular chow or the SFA diet for six months. Plasma, hippocampus (HPF) and cerebral cortex (CTX) lipids were analysed by LC-ESI-MS/MS. A total of 348 lipid species were determined, comprising 25 lipid classes. The general abundance of HPF and CTX lipids was comparable in SFA fed mice versus controls, despite substantial differences in plasma lipid-class abundance. However, significant differences in 50 specific lipid species were identified as a consequence of SFA treatment, restricted to phosphatidylcholine (PC), phosphatidylethanolamine (PE), alkyl-PC, alkenyl-PC, alkyl-PE, alkenyl-PE, cholesterol ester (CE), diacylglycerol (DG), phosphatidylinositol (PI) and phosphatidylserine (PS) classes. Partial least squares regression of the HPF/CTX lipidome versus plasma lipidome revealed the plasma lipidome could account for a substantial proportion of variation. The findings demonstrate that cerebral abundance of specific lipid species is strongly associated with plasma lipid homeostasis.

Bacteroides fragilis Lipopolysaccharide and Inflammatory Signaling in Alzheimer's Disease

The human microbiome consists of ~3.8 × 10¹³ symbiotic microorganisms that form a highly complex and dynamic ecosystem: the gastrointestinal (GI) tract constitutes the largest repository of the human microbiome by far, and its impact on human neurological health and disease is becoming increasingly appreciated. Bacteroidetes, the largest phylum of Gram-negative bacteria in the GI tract microbiome, while generally beneficial to the host when confined to the GI tract, have potential to secrete a remarkably complex array of pro-inflammatory neurotoxins that include surface lipopolysaccharides (LPSs) and toxic proteolytic peptides. The deleterious effects of these bacterial exudates appear to become more important as GI tract and blood-brain barriers alter or increase their permeability with aging and disease. For example, presence of the unique LPSs of the abundant Bacteroidetes species Bacteroides fragilis (BF-LPS) in the serum represents a major contributing factor to systemic inflammation. BF-LPS is further recognized by TLR2, TLR4, and/or CD14 microglial cell receptors as are the pro-inflammatory 42 amino acid amyloid-beta (Aβ42) peptides that characterize Alzheimer's disease (AD) brain. Here we provide the first evidence that BF-LPS exposure to human primary brain cells is an exceptionally potent inducer of the pro-inflammatory transcription factor NF-kB (p50/p65) complex, a known trigger in the expression of pathogenic pathways involved in inflammatory neurodegeneration. This 'Perspectives communication' will in addition highlight work from recent studies that advance novel and emerging concepts on the potential contribution of microbiome-generated factors, such as BF-LPS, in driving pro-inflammatory degenerative neuropathology in the AD brain.

Neurovascular unit impairment in early Alzheimer's disease measured with magnetic resonance imaging

The neurovascular unit, which protects neuronal cells and supplies them with essential molecules, plays an important role in the pathophysiology of Alzheimer's Disease (AD). The aim of this study was to noninvasively investigate 2 linked functional elements of the neurovascular unit, blood-brain barrier (BBB) permeability and cerebral blood flow (CBF), in patients with early AD and healthy controls. Therefore, both dynamic contrast-enhanced magnetic resonance imaging and arterial spin labeling magnetic resonance imaging were applied to measure BBB permeability and CBF, respectively. The patients with early AD showed significantly lower CBF and local blood volume in the gray matter, compared with controls. In the patients, we also found that a reduction in CBF is correlated with an increase in leakage rate. This finding supports the hypothesis that neurovascular damage, and in particular impairment of the neurovascular unit constitutes the pathophysiological link between CBF reduction and BBB impairment in AD.

The Ticking of the Epigenetic Clock: Antipsychotic Drugs in Old Age

BACKGROUND

Exposed to antipsychotic drugs (APDs), older individuals with dementing illness are at risk of cerebrovascular adverse effects (CVAE), including sudden death. Transient microvascular dysfunctions are known to occur in younger persons exposed to APDs; however, they seldom progress to CVAE, suggesting that APDs alone are insufficient for engendering this untoward effect. It is, therefore, believed that a preexistent microvascular damage is necessary for CVAE to take place, but the exact nature of this lesion remains unclear. CNS small vessel disease (SVD) is a well-known age-related risk factor for strokes, dementia, and sudden death, which may constitute the initial CVAE-predisposing pathology. Therefore, we propose the two strikes CVAE paradigm, in which SVD represents the first strike, while exposure to APDs, the second. In this model, both strikes must be present for CVAE to take place, and the neuroimaging load of white matter hyperintensities may be directly proportional with the CVAE risk. To investigate this hypothesis at the molecular level, we focused on a seemingly unrelated phenomenon: both APDs and SVD were found protective against a similar repertoire of cancers and their spread to the brain (1-4). Since microRNA-29 has shown efficacy against the same malignancies and has been associated with small vessels pathology, we narrowed our search down to this miR, hypothesizing that the APDs mechanism of action includes miR-29 upregulation, which in turn facilitates the development of SVD.

AIM

To assess whether miR-29 can be utilized as a peripheral blood biomarker for SVD and CVAE risk.

METHOD

We conducted a search of experimentally verified miR-29 target genes utilizing the public domain tools miRanda, RNA22 and Weizemann Institute of Science miRNA Analysis. We identified in total 67 experimentally verified target genes for miR-29 family, 18 of which correlate with microvascular integrity and may be relevant for CVAE.

CONCLUSION

Upregulated microRNA-29 silences the expression of 18 genes connected with capillary stability, engendering a major vulnerability for SVD (first strike) which in turn increases the risk for CVAE after exposure to APDs (second strike).