Arterial spin labeling imaging reveals widespread and Aβ-independent reductions in cerebral blood flow in elderly apolipoprotein epsilon-4 carriers

Towards a better diagnosis and treatment of dementia: Identifying common and distinct neuropathological mechanisms in Alzheimer's and vascular dementia

Alzheimer's disease (AD) and vascular dementia (VaD) together contribute to almost 90 % of all dementia cases leading to major health challenges of our time with a substantial global socioeconomic burden. While in AD, the improved understanding of Amyloid beta (Aß) mismetabolism and tau hyperphosphorylation as pathophysiological hallmarks has led to significant clinical breakthroughs, similar advances in VaD are lacking. After comparing the clinical presentation, including risk factors, disease patterns, course of diseases and further diagnostic parameters for both forms of dementia, we highlight the importance of shared pathomechanisms found in AD and VaD: Endothelial damage, blood brain barrier (BBB) breakdown and hypoperfusion inducing oxidative stress and inflammation and thus trophic uncoupling in the neurovascular unit. A dysfunctional endothelium and BBB lead to the accumulation of neurotoxic molecules and Aß through impaired clearance, which in turn leads to neurodegeneration. In this context we discuss possible neuropathological parameters, which might serve as biomarkers and thus improve diagnostic accuracy or reveal targets for novel therapeutic strategies for both forms of dementia.

Uncovering cerebral blood flow patterns corresponding to Amyloid-beta accumulations in patients across the Alzheimer's disease continuum using the arterial spin labeling

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder ranging from mild cognitive impairment (MCI) to AD dementia. Abnormal cerebral perfusion alterations, influenced by amyloid-beta (Aβ) accumulations, have been implicated in cognitive decline along this spectrum.

OBJECTIVE

This study investigates the relationship between cerebrospinal fluid (CSF) Aβ1-42 levels and regional cerebral blood flow (CBF) changes across the AD continuum using the Arterial Spin Labeling (ASL) technique.

METHODS

We analyzed data from 229 participants extracted from the ADNI cohort, comprising of 50 cognitively normal (CN), 13 subjective memory complaints (SMC), 83 early MCI (EMCI), 52 late MCI (LMCI), and 31 AD participants with complete ASL and CSF data. Correlations between Aβ1-42 levels and regional mean CBF values were assessed. Multiple linear regression models accounted for confounders, including age, gender, and education level.

RESULTS

Preliminary unadjusted analyses revealed strong positive correlations between Aβ1-42 levels and CBF in multiple regions, predominantly in the AD group. After adjusting for confounders, significant correlations in AD participants emerged in the left pars triangularis and left caudal middle frontal cortex. In the LMCI group, significant associations were identified in the right lateral occipital cortex, right inferior parietal cortex, and left amygdala.

CONCLUSION

These findings highlight the critical role of Aβ-driven CBF alterations in regions associated with higher cognitive functions and suggest that these patterns may serve as potential biomarkers for diagnosing and monitoring disease progression.

New evidence of vascular defects in neurodegenerative diseases revealed by single cell RNA sequencing

Neurodegenerative diseases (NDs) involve the progressive loss of neuronal structure or function in the brain and spinal cord. Despite their diverse etiologies, NDs manifest similar pathologies. Emerging research identifies vascular defects as a previously neglected hallmark of NDs. The development and popularization of single-cell RNA sequencing (scRNA-seq) technologies have significantly advanced our understanding of brain vascular cell types and their molecular characteristics, including gene expression changes at the single-cell level in NDs. These unprecedented insights deepen our understanding of the pathogenic mechanisms underlying NDs. However, the occurrence and role of vascular defects in disease progression remain largely unexplored. In this paper, we systematically summarize recent advances in the structure and organization of the central nervous system vasculature in mice, healthy individuals, and patients with NDs, focussing primarily on disease-specific alterations in vascular cell types or subtypes. Combining scRNA-seq with pathology evidence, we propose that vascular defects, characterized by disruptions in cell types and structural integrity, may serve as common early features of NDs. Finally, we discuss several pathways through which vascular defects in NDs lead to neuronal degeneration. A deeper understanding of the causes and contributions of vascular defects to NDs aids in elucidating the pathogenic mechanisms and developing meaningful therapeutic interventions.

Elevated cerebral blood flow proxy with increased beta-amyloid burden in Alzheimer's disease preclinical phase evaluated by dual-phase 18F-florbetaben positron emission tomography

This study investigated the earliest change of cerebral blood flow (CBF) and its relationship with β-amyloid (Aβ) burden in preclinical Alzheimer's disease (AD) employing dual-phase 18F-florbetaben (FBB) PET. Seventy-one cognitively normal (NC) individuals were classified as Aβ negative (Aβ-NC) or positive (Aβ+NC) based on two different cutoff values: an SUVR of > 1.08 and a Centiloid scale of > 20. The PET scans were acquired in two phases: an early phase (0-10 min, eFBB) and a delayed phase (90-110 min, dFBB), which were averaged to generate single-frame images for each phase. Furthermore, an R1 parametric map was generated from the early phase data using a simplified reference tissue model. We conducted regional and voxel-based analyses to compare the eFBB, dFBB, and R1 images between the Aβ positive and negative groups. In addition, the correlations between the CBF proxy R1 and the dFBB SUVR were analyzed. The Aβ+NC group showed significantly higher dFBB SUVR in both the global cerebral cortex and target regions compared to the Aβ-NC group, while no significant differences were observed in eFBB SUVR between the two groups. Furthermore, the Aβ+NC group exhibited significantly higher R1 values, a proxy for cerebral perfusion, in both the global cerebral cortex and target regions compared to the Aβ-NC group. Significant positive correlations were observed between R1 and dFBB SUVR in both the global cerebral cortex and target regions, which remained significant after controlling for demographics and cognitive profiles, except for the medial temporal and occipital cortices. The findings reveal increased CBF in preclinical AD and a positive correlation between CBF and amyloid pathology. The positive correlation between R1 and amyloid burden may indicate a compensatory mechanism in the preclinical stage of Alzheimer's disease, but to elucidate this hypothesis, further longitudinal observational studies are necessary.

Arteriolar degeneration and stiffness in cerebral amyloid angiopathy are linked to β-amyloid deposition and lysyl oxidase

Cerebral amyloid angiopathy (CAA) is a vasculopathy characterized by vascular β-amyloid (Aβ) deposition on cerebral blood vessels. CAA is closely linked to Alzheimer's disease (AD) and intracerebral hemorrhage. CAA is associated with the loss of autoregulation in the brain, vascular rupture, and cognitive decline. To assess morphological and molecular changes associated with the degeneration of penetrating arterioles in CAA, we analyzed post-mortem human brain tissue from 26 patients with mild, moderate, and severe CAA end neurological controls. The tissue was optically cleared for three-dimensional light sheet microscopy, and morphological features were quantified using surface volume rendering. We stained Aβ, vascular smooth muscle (VSM), lysyl oxidase (LOX), and vascular markers to visualize the relationship between degenerative morphological features, including vascular dilation, dolichoectasia (variability in lumenal diameter) and tortuosity, and the volumes of VSM, Aβ, and LOX in arterioles. Atomic force microscopy (AFM) was used to assess arteriolar wall stiffness, and we identified a pattern of morphological features associated with degenerating arterioles in the cortex. The volume of VSM associated with the arteriole was reduced by around 80% in arterioles with severe CAA and around 60% in cases with mild/moderate CAA. This loss of VSM correlated with increased arteriolar diameter and variability of diameter, suggesting VSM loss contributes to arteriolar laxity. These vascular morphological features correlated strongly with Aβ deposits. At sites of microhemorrhage, Aβ was consistently present, although the morphology of the deposits changed from the typical organized ring shape to sharply contoured shards with marked dilation of the vessel. AFM showed that arteriolar walls with CAA were more than 400% stiffer than those without CAA. Finally, we characterized the association of vascular degeneration with LOX, finding strong associations with VSM loss and vascular degeneration. These results show an association between vascular Aβ deposition, microvascular degeneration, and increased vascular stiffness, likely due to the combined effects of replacement of VSM by β-amyloid, cross-linking of extracellular matrices (ECM) by LOX, and possibly fibrosis. This advanced microscopic imaging study clarifies the association between Aβ deposition and vascular fragility. Restoration of physiologic ECM properties in penetrating arteries may yield a novel therapeutic strategy for CAA.

Associations of apolipoprotein E ε4 allele, regional cerebral blood flow, and serum liver function markers in patients with cognitive impairment

Introduction

The ε4 allele of the apolipoprotein E gene (APOE4) is expressed abundantly in both the brain and peripheral circulation as a genetic risk factor for Alzheimer's disease (AD). Cerebral blood flow (CBF) dysfunction is an essential feature of AD, and the liver plays an important role in the pathogenesis of dementia. However, the associations of APOE4 with CBF and liver function markers in patients with cognitive impairment remains unclear. We aimed to evaluate the associations of APOE4 with CBF measured by arterial spin labeling (ASL) magnetic resonance imaging (MRI) and serum liver function markers in participants who were diagnosed with cognitive impairment.

Methods

Fourteen participants with AD and sixteen with amnestic mild cognitive impairment (MCI) were recruited. In addition to providing comprehensive clinical information, all patients underwent laboratory tests and MRI. All participants were divided into carriers and noncarriers of the ε4 allele, and T-tests and Mann-Whitney U tests were used to observe the differences between APOE4 carriers and noncarriers in CBF and liver function markers.

Results

Regarding regional cerebral blood flow (rCBF), APOE4 carriers showed hyperperfusion in the bilateral occipital cortex, bilateral thalamus, and left precuneus and hypoperfusion in the right lateral temporal cortex when compared with noncarriers. Regarding serum liver function markers, bilirubin levels (including total, direct, and indirect) were lower in APOE4 carriers than in noncarriers.

Conclusion

APOE4 exerts a strong effect on CBF dysfunction by inheritance, representing a risk factor for AD. APOE4 may be related to bilirubin metabolism, potentially providing specific neural targets for the diagnosis and treatment of AD.

mTOR signaling and Alzheimer's disease: What we know and where we are?

Despite the great body of research done on Alzheimer's disease, the underlying mechanisms have not been vividly investigated. To date, the accumulation of amyloid-beta plaques and tau tangles constitutes the hallmark of the disease; however, dysregulation of the mammalian target of rapamycin (mTOR) seems to be significantly involved in the pathogenesis of the disease as well. mTOR, as a serine-threonine protein kinase, was previously known for controlling many cellular functions such as cell size, autophagy, and metabolism. In this regard, mammalian target of rapamycin complex 1 (mTORC1) may leave anti-aging impacts by robustly inhibiting autophagy, a mechanism that inhibits the accumulation of damaged protein aggregate and dysfunctional organelles. Formation and aggregation of neurofibrillary tangles and amyloid-beta plaques seem to be significantly regulated by mTOR signaling. Understanding the underlying mechanisms and connection between mTOR signaling and AD may suggest conducting clinical trials assessing the efficacy of rapamycin, as an mTOR inhibitor drug, in managing AD or may help develop other medications. In this literature review, we aim to elaborate mTOR signaling network mainly in the brain, point to gaps of knowledge, and define how and in which ways mTOR signaling can be connected with AD pathogenesis and symptoms.

Pulse pressure and APOE ε4 dose interact to affect cerebral blood flow in older adults without dementia

This study assessed whether the effect of vascular risk on cerebral blood flow (CBF) varies by gene dose of apolipoprotein (APOE) ε4 alleles. 144 older adults without dementia from the Alzheimer's Disease Neuroimaging Initiative underwent arterial spin labeling and T1-weighted MRI, APOE genotyping, fluorodeoxyglucose positron emission tomography (FDG-PET), lumbar puncture, and blood pressure (BP) assessment. Vascular risk was assessed using pulse pressure (systolic BP - diastolic BP). CBF was examined in six AD-vulnerable regions: entorhinal cortex, hippocampus, inferior temporal cortex, inferior parietal cortex, rostral middle frontal gyrus, and medial orbitofrontal cortex. Linear regressions tested the interaction between APOE ε4 dose and pulse pressure on CBF in each region, adjusting for age, sex, cognitive classification, antihypertensive medication use, FDG-PET, reference CBF region, and AD biomarker positivity. There was a significant interaction between pulse pressure and APOE ɛ4 dose on CBF in the entorhinal cortex, hippocampus, and inferior parietal cortex, such that higher pulse pressure was associated with lower CBF only among ε4 homozygous participants. These findings demonstrate that the association between pulse pressure and regional CBF differs by APOE ε4 dose, suggesting that targeting modifiable vascular risk factors may be particularly important for those genetically at risk for AD.

APOE4 is a Risk Factor and Potential Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease, the main pathological hallmark of which is the loss of neurons, resulting in cognitive and memory impairments. Sporadic late-onset AD is a prevalent form of the disease and the apolipoprotein E4 (APOE4) genotype is the strongest predictor of the disease development. The structural variations of APOE isoforms affect their roles in synaptic maintenance, lipid trafficking, energy metabolism, inflammatory response, and BBB integrity. In the context of AD, APOE isoforms variously control the key pathological elements of the disease, including Aβ plaque formation, tau aggregation, and neuroinflammation. Taking into consideration the limited number of therapy choices that can alleviate symptoms and have little impact on the AD etiology and progression to date, the precise research strategies guided by apolipoprotein E (APOE) polymorphisms are required to assess the potential risk of age-related cognitive decline in people carrying APOE4 genotype. In this review, we summarize the evidence implicating the significance of APOE isoforms on brain functions in health and pathology with the aim to identify the possible targets that should be addressed to prevent AD manifestation in individuals with the APOE4 genotype and to explore proper treatment strategies.

Emerging Links between Cerebral Blood Flow Regulation and Cognitive Decline: A Role for Brain Microvascular Pericytes

Cognitive impairment associated with vascular etiology has been of considerable interest in the development of dementia. Recent studies have started to uncover cerebral blood flow deficits in initiating cognitive deterioration. Brain microvascular pericytes, the only type of contractile cells in capillaries, are involved in the precise modulation of vascular hemodynamics due to their ability to regulate resistance in the capillaries. They exhibit potential in maintaining the capillary network geometry and basal vascular tone. In addition, pericytes can facilitate better blood flow supply in response to neurovascular coupling. Their dysfunction is thought to disturb cerebral blood flow causing metabolic imbalances or structural injuries, leading to consequent cognitive decline. In this review, we summarize the characteristics of microvascular pericytes in brain blood flow regulation and outline the framework of a two-hit hypothesis in cognitive decline, where we emphasize how pericytes serve as targets of cerebral blood flow dysregulation that occurs with neurological challenges, ranging from genetic factors, aging, and pathological proteins to ischemic stress.

Amyloid and tau pathology are associated with cerebral blood flow in a mixed sample of nondemented older adults with and without vascular risk factors for Alzheimer's disease

Identification of biomarkers for the early stages of Alzheimer's disease (AD) is an imperative step in developing effective treatments. Cerebral blood flow (CBF) is a potential early biomarker for AD; generally, older adults with AD have decreased CBF compared to normally aging peers. CBF deviates as the disease process and symptoms progress. However, further characterization of the relationships between CBF and AD risk factors and pathologies is still needed. We assessed the relationships between CBF quantified by arterial spin-labeled magnetic resonance imaging, hypertension, APOEε4, and tau and amyloid positron emission tomography in 77 older adults: cognitively normal, subjective cognitive decline, and mild cognitive impairment. Tau and amyloid aggregation were related to altered CBF, and some of these relationships were dependent on hypertension or APOEε4 status. Our findings suggest a complex relationship between risk factors, AD pathologies, and CBF that warrants future studies of CBF as a potential early biomarker for AD.

Regional cerebral blood flow and brain atrophy in mild cognitive impairment and Alzheimer's disease

Objectives

A decline in the regional cerebral blood flow (CBF) is proposed to be one of the initial changes in the Alzheimer's disease process. To date, there are limited data on the correlation between CBF decline and gray matter atrophy in mild cognitive impairment (MCI) and AD patients. to investigate the association between CBF with the gray matter structural parameters such as cortical volume, surface area, and thickness in AD, MCI, and healthy controls (HC).

Methods

Data from three groups of participants including 39 HC, 82 MCI, and 28 AD subjects were obtained from the Alzheimer's disease Neuroimaging Initiative (ADNI). One-way ANOVA and linear regression were used to compare data and find a correlation between structural parameters such as cortical volume, surface area, and thickness and CBF which measured by arterial spin labeling (ASL)-MRI.

Results

Our findings revealed a widespread significant correlation between the CBF and structural parameters in temporal, frontal, parietal, occipital, precentral gyrus, pericalcarine cortex, entorhinal cortex, supramarginal gyrus, fusiform, precuneus, and pallidum.

Conclusion

CBF decline may be a useful biomarker for MCI and AD and accurately reflect the structural changes related to AD. According to the present results, CBF decline, as measured by ASL-MRI, is correlated with lower measures of structural parameters in AD responsible regions. It means that CBF decline may reflect AD-associated atrophy across disease progression and is also used as an early biomarker for AD and MCI diagnosis.

Cerebral blood flow, amyloid burden, and cognition in cognitively normal individuals

PURPOSE

The role of cerebral blood flow (CBF) in the early stages of Alzheimer's disease is complex and largely unknown. We investigated cross-sectional and longitudinal associations between CBF, amyloid burden, and cognition, in cognitively normal individuals with subjective cognitive decline (SCD).

METHODS

We included 187 cognitively normal individuals with SCD from the SCIENCe project (65 ± 8 years, 39% F, MMSE 29 ± 1). Each underwent a dynamic (0-70 min) [18F]florbetapir PET and T1-weighted MRI scan, enabling calculation of mean binding potential (BPND; specific amyloid binding) and R1 (measure of relative (r)CBF). Eighty-three individuals underwent a second [18F]florbetapir PET (2.6 ± 0.7 years). Participants annually underwent neuropsychological assessment (follow-up time 3.8 ± 3.1 years; number of observations n = 774).

RESULTS

A low baseline R1 was associated with steeper decline on tests addressing memory, attention, and global cognition (range betas 0.01 to 0.27, p < 0.05). High BPND was associated with steeper decline on tests covering all domains (range betas - 0.004 to - 0.70, p < 0.05). When both predictors were simultaneously added to the model, associations remained essentially unchanged. Additionally, we found longitudinal associations between R1 and BPND. High baseline BPND predicted decline over time in R1 (all regions, range betasBP×time - 0.09 to - 0.14, p < 0.05). Vice versa, low baseline R1 predicted increase in BPND in frontal, temporal, and composite ROIs over time (range betasR1×time - 0.03 to - 0.08, p < 0.05).

CONCLUSION

Our results suggest that amyloid accumulation and decrease in rCBF are two parallel disease processes without a fixed order, both providing unique predictive information for cognitive decline and each process enhancing the other longitudinally.

Cerebral blood flow, tau imaging, and memory associations in cognitively unimpaired older adults

Objective

Cerebral blood flow (CBF) has been independently linked to cognitive impairment and traditional Alzheimer's disease (AD) pathology (e.g., amyloid-beta [Aβ], tau) in older adults. However, less is known about the possible interactive effects of CBF, Aβ, and tau on memory performance. The present study examined whether CBF moderates the effect of Aβ and tau on objective and subjective memory within cognitively unimpaired (CU) older adults.

Methods

Participants included 54 predominately white CU older adults from the Alzheimer's Disease Neuroimaging Initiative. Multiple linear regression models examined meta-temporal CBF associations with (1) meta-temporal tau PET adjusting for cortical Aβ PET and (2) and cortical Aβ PET adjusting for tau PET. The CBF and tau meta region was an average of 5 distinct temporal lobe regions. CBF interactions with Aβ or tau PET on memory performance were also examined. Covariates for all models included age, sex, education, pulse pressure, APOE-ε4 positivity, and imaging acquisition date differences.

Results

CBF was significantly negatively associated with tau PET (t = -2.16, p = .04) but not Aβ PET (t = 0.98, p = .33). Results revealed a CBF by tau PET interaction such that there was a stronger effect of tau PET on objective (t = 2.51, p = .02) and subjective (t = -2.67, p = .01) memory outcomes among individuals with lower levels of CBF.

Conclusions

Cerebrovascular and tau pathologies may interact to influence cognitive performance. This study highlights the need for future vascular risk interventions, which could offer a scalable and cost-effective method for AD prevention.

Aberrant pattern of regional cerebral blood flow in mild cognitive impairment: A meta-analysis of arterial spin labeling magnetic resonance imaging

In mild cognitive impairment (MCI), cognitive decline is associated with abnormal changes of cerebral blood flow (CBF). Arterial spin labeling magnetic resonance imaging (ASL-MRI) is an effective method for assessing regional cerebral blood flow (rCBF). However, the CBF estimated via ASL-MRI in MCI often differs between studies, and the consistency of CBF changes in MCI is unclear. In this study, 13 ASL-MRI studies with 495 MCI patients and 441 health controls were screened out from PubMed, Embase, Cochrane, Web of Science, Wanfang, and CNKI. An activation likelihood estimation (ALE) meta-analysis was performed to explore the brain regions with abnormal CBF in MCI. It showed that the decreased CBF in MCI was identified in the precuneus, inferior parietal lobule (IPL), superior occipital gyrus (SOG), middle temporal gyrus (MTG), and middle occipital gyrus (MOG), while the increased CBF in MCI was identified in the lentiform nucleus (LN) compared with healthy controls. The study characterized the abnormal pattern of regional CBF in MCI, which would promote our knowledge of MCI and might be used as a biomarker in clinic.

Exploring the mechanism of YangXue QingNao Wan based on network pharmacology in the treatment of Alzheimer’s disease

It is clinical reported that YangXue QingNao Wan (YXQNW) combined with donepezil can significantly improve the cognitive function of AD patients. However, the mechanism is not clear. A network pharmacology approach was employed to predict the protein targets and affected pathways of YXQNW in the treatment of AD. Based on random walk evaluation, the correlation between YXQNW and AD was calculated; while a variety of AD clinical approved Western drugs were compared. The targets of YXQNW were enriched and analyzed by using the TSEA platform and MetaCore. We proved that the overall correlation between YXQNW and AD is equivalent to clinical Western drugs, but the mechanism of action is very different. Firstly, YXQNW may promote cerebral blood flow velocity by regulating platelet aggregation and the vasoconstriction/relaxation signal pathway, which has been verified by clinical meta-analysis. Secondly, YXQNW may promote Aβ degradation in the liver by modulating the abnormal glucose and lipid metabolisms via the adiponectin-dependent pathway, RXR/PPAR-dependent lipid metabolism signal pathway, and fatty acid synthase activity signal pathway. We also verified whether YXQNW indeed promoted Aβ degradation in hepatic stellate cells. This work provides a novel scientific basis for the mechanism of YXQNW in the treatment of AD.

Mediation of the APOE Associations With Cognition Through Cerebral Blood Flow: The CIBL Study

Background

The ε4 allele of the apolipoprotein E (APOE) gene is a strong genetic risk factor for aging-related cognitive decline. However, the causal connection between ε4 alleles and cognition is not well understood. The objective of this study was to identify the roles of cerebral blood flow (CBF) in cognitive-related brain areas in mediating the associations of APOE with cognition.

Methods

The multiple linear regression analyses were conducted on 369 subjects (mean age of 68.8 years; 62.9% of women; 29.3% of APOE ε4 allele carriers). Causal mediation analyses with 5,000 bootstrapped iterations were conducted to explore the mediation effects.

Result

APOE ε4 allele was negatively associated with cognition (P < 0.05) and CBF in the amygdala, hippocampus, middle temporal gyrus, posterior cingulate, and precuneus (all P < 0.05). The effect of the APOE genotype on cognition was partly mediated by the above CBF (all P < 0.05).

Conclusion

CBF partially mediates the potential links between APOE genotype and cognition. Overall, the APOE ε4 allele may lead to a dysregulation of the vascular structure and function with reduced cerebral perfusion, which in turn leads to cognitive impairment.

Impact of sex and APOE ε4 on age-related cerebral perfusion trajectories in cognitively asymptomatic middle-aged and older adults: A longitudinal study

Cerebral hypoperfusion is thought to contribute to cognitive decline in Alzheimer's disease, but the natural trajectory of cerebral perfusion in cognitively healthy adults has not been well-studied. This longitudinal study is consisted of 950 participants (40-89 years), who were cognitively unimpaired at their first visit. We investigated the age-related changes in cerebral perfusion, and their associations with APOE-genotype, biological sex, and cardiometabolic measurements. During the follow-up period (range 0.13-8.24 years), increasing age was significantly associated with decreasing cerebral perfusion, in total gray-matter (β=-1.43), hippocampus (-1.25), superior frontal gyrus (-1.70), middle frontal gyrus (-1.99), posterior cingulate (-2.46), and precuneus (-2.14), with all P-values < 0.01. Compared with male-ɛ4 carriers, female-ɛ4 carriers showed a faster decline in global and regional cerebral perfusion with increasing age, whereas the age-related decline in cerebral perfusion was similar between male- and female-ɛ4 non-carriers. Worse cardiometabolic profile (i.e., increased blood pressure, body mass index, total cholesterol, and blood glucose) was associated with lower cerebral perfusion at all the visits. When time-varying cardiometabolic measurements were adjusted in the model, the synergistic effect of sex and APOE-ɛ4 on age-related cerebral perfusion-trajectories became largely attenuated. Our findings demonstrate that APOE-genotype and sex interactively impact cerebral perfusion-trajectories in mid- to late-life. This effect may be partially explained by cardiometabolic alterations.

[11C]PBR28 radiotracer kinetics are not driven by alterations in cerebral blood flow

The positron emission tomography (PET) radiotracer [¹¹C]PBR28 has been increasingly used to image the translocator protein (TSPO) as a marker of neuroinflammation in a variety of brain disorders. Interrelatedly, similar clinical populations can also exhibit altered brain perfusion, as has been shown using arterial spin labelling in magnetic resonance imaging (MRI) studies. Hence, an unsolved debate has revolved around whether changes in perfusion could alter delivery, uptake, or washout of the radiotracer [¹¹C]PBR28, and thereby influence outcome measures that affect interpretation of TSPO upregulation. In this simultaneous PET/MRI study, we demonstrate that [¹¹C]PBR28 signal elevations in chronic low back pain patients are not accompanied, in the same regions, by increases in cerebral blood flow (CBF) compared to healthy controls, and that areas of marginal hypoperfusion are not accompanied by decreases in [¹¹C]PBR28 signal. In non-human primates, we show that hypercapnia-induced increases in CBF during radiotracer delivery or washout do not alter [¹¹C]PBR28 outcome measures. The combined results from two methodologically distinct experiments provide support from human data and direct experimental evidence from non-human primates that changes in CBF do not influence outcome measures reported by [¹¹C]PBR28 PET imaging studies and corresponding interpretations of the biological meaning of TSPO upregulation.

From Neurodevelopmental to Neurodegenerative Disorders: The Vascular Continuum

Structural and functional integrity of the cerebral vasculature ensures proper brain development and function, as well as healthy aging. The inability of the brain to store energy makes it exceptionally dependent on an adequate supply of oxygen and nutrients from the blood stream for matching colossal demands of neural and glial cells. Key vascular features including a dense vasculature, a tightly controlled environment, and the regulation of cerebral blood flow (CBF) all take part in brain health throughout life. As such, healthy brain development and aging are both ensured by the anatomical and functional interaction between the vascular and nervous systems that are established during brain development and maintained throughout the lifespan. During critical periods of brain development, vascular networks remodel until they can actively respond to increases in neural activity through neurovascular coupling, which makes the brain particularly vulnerable to neurovascular alterations. The brain vasculature has been strongly associated with the onset and/or progression of conditions associated with aging, and more recently with neurodevelopmental disorders. Our understanding of cerebrovascular contributions to neurological disorders is rapidly evolving, and increasing evidence shows that deficits in angiogenesis, CBF and the blood-brain barrier (BBB) are causally linked to cognitive impairment. Moreover, it is of utmost curiosity that although neurodevelopmental and neurodegenerative disorders express different clinical features at different stages of life, they share similar vascular abnormalities. In this review, we present an overview of vascular dysfunctions associated with neurodevelopmental (autism spectrum disorders, schizophrenia, Down Syndrome) and neurodegenerative (multiple sclerosis, Huntington's, Parkinson's, and Alzheimer's diseases) disorders, with a focus on impairments in angiogenesis, CBF and the BBB. Finally, we discuss the impact of early vascular impairments on the expression of neurodegenerative diseases.

Convergent and Discriminant Validity of Default Mode Network and Limbic Network Perfusion in Amnestic Mild Cognitive Impairment Patients

BACKGROUND

Previous studies reported default mode network (DMN) and limbic network (LIN) brain perfusion deficits in patients with amnestic mild cognitive impairment (aMCI), frequently a prodromal stage of Alzheimer's disease (AD). However, the validity of these measures as AD markers has not yet been tested using MRI arterial spin labeling (ASL).

OBJECTIVE

To investigate the convergent and discriminant validity of DMN and LIN perfusion in aMCI.

METHODS

We collected core AD markers (amyloid-β 42 [Aβ42], phosphorylated tau 181 levels in cerebrospinal fluid [CSF]), neurodegenerative (hippocampal volumes and CSF total tau), vascular (white matter hyperintensities), genetic (apolipoprotein E [APOE] status), and cognitive features (memory functioning on Paired Associate Learning test [PAL]) in 14 aMCI patients. Cerebral blood flow (CBF) was extracted from DMN and LIN using ASL and correlated with AD features to assess convergent validity. Discriminant validity was assessed carrying out the same analysis with AD-unrelated features, i.e., somatomotor and visual networks' perfusion, cerebellar volume, and processing speed.

RESULTS

Perfusion was reduced in the DMN (F = 5.486, p = 0.039) and LIN (F = 12.678, p = 0.004) in APOE ɛ4 carriers compared to non-carriers. LIN perfusion correlated with CSF Aβ42 levels (r = 0.678, p = 0.022) and memory impairment (PAL, number of errors, r = -0.779, p = 0.002). No significant correlation was detected with tau, neurodegeneration, and vascular features, nor with AD-unrelated features.

CONCLUSION

Our results support the validity of DMN and LIN ASL perfusion as AD markers in aMCI, indicating a significant correlation between CBF and amyloidosis, APOE ɛ4, and memory impairment.

Aerobic exercise improves hippocampal blood flow for hypertensive Apolipoprotein E4 carriers

Cerebrovascular dysfunction likely contributes causally to Alzheimer's disease (AD). The strongest genetic risk factor for late-onset AD, Apolipoprotein E4 (APOE4), may act synergistically with vascular risk to cause dementia. Therefore, interventions that improve vascular health, such as exercise, may be particularly beneficial for APOE4 carriers. We assigned cognitively normal adults (65-87 years) to an aerobic exercise intervention or education only. Arterial spin labeling MRI measured hippocampal blood flow (HBF) before and after the 52-week intervention. We selected participants with hypertension at enrollment (n = 44). For APOE4 carriers, change in HBF (ΔHBF) was significantly (p = 0.006) higher for participants in the exercise intervention (4.09 mL/100g/min) than the control group (-2.08 mL/100g/min). There was no difference in ΔHBF between the control (-0.32 mL/100g/min) and exercise (-0.54 mL/100g/min) groups for non-carriers (p = 0.918). Additionally, a multiple regression showed an interaction between change in systolic blood pressure (ΔSBP) and APOE4 carrier status on ΔHBF (p = 0.035), with reductions in SBP increasing HBF for APOE4 carriers only. Aerobic exercise improved HBF for hypertensive APOE4 carriers only. Additionally, only APOE4 carriers exhibited an inverse relationship between ΔSBP and ΔHBF. This suggests exercise interventions, particularly those that lower SBP, may be beneficial for individuals at highest genetic risk of AD.ClinicalTrials.gov Identifier: NCT02000583.

Causes and consequences of baseline cerebral blood flow reductions in Alzheimer's disease

Reductions of baseline cerebral blood flow (CBF) of ∼10-20% are a common symptom of Alzheimer's disease (AD) that appear early in disease progression and correlate with the severity of cognitive impairment. These CBF deficits are replicated in mouse models of AD and recent work shows that increasing baseline CBF can rapidly improve the performance of AD mice on short term memory tasks. Despite the potential role these data suggest for CBF reductions in causing cognitive symptoms and contributing to brain pathology in AD, there remains a poor understanding of the molecular and cellular mechanisms causing them. This review compiles data on CBF reductions and on the correlation of AD-related CBF deficits with disease comorbidities (e.g. cardiovascular and genetic risk factors) and outcomes (e.g. cognitive performance and brain pathology) from studies in both patients and mouse models, and discusses several potential mechanisms proposed to contribute to CBF reductions, based primarily on work in AD mouse models. Future research aimed at improving our understanding of the importance of and interplay between different mechanisms for CBF reduction, as well as at determining the role these mechanisms play in AD patients could guide the development of future therapies that target CBF reductions in AD.

Apolipoprotein E4 Moderates the Association Between Vascular Risk Factors and Brain Pathology

BACKGROUND

The strongest genetic risk factor for late-onset Alzheimer disease (AD), Apolipoprotein E4 (APOE4), increases cardiovascular disease risk and may also act synergistically with vascular risk factors to contribute to AD pathogenesis. Here, we assess the interaction between APOE4 and vascular risk on cerebrovascular dysfunction and brain pathology.

METHODS

This is an observational study of cognitively normal older adults, which included positron emission tomography imaging and vascular risk factors. We measured beat-to-beat blood pressure and middle cerebral artery velocity at rest and during moderate-intensity exercise. Cerebrovascular measures included cerebrovascular conductance index and the cerebrovascular response to exercise.

RESULTS

There was a significant interaction between resting cerebrovascular conductance index and APOE4 carrier status on β-amyloid deposition (P=0.026), with poor conductance in the cerebrovasculature associated with elevated β-amyloid for the APOE4 carriers only. There was a significant interaction between non-high-density lipoprotein cholesterol and APOE4 carrier status (P=0.014), with elevated non-high-density lipoprotein cholesterol predicting a blunted cerebrovascular response to exercise in APOE4 carriers and the opposite relationship in noncarriers.

CONCLUSIONS

Both cerebral and peripheral vascular risk factors are preferentially associated with brain pathology in APOE4 carriers. These findings provide insight into pathogenic vascular risk mechanisms and target strategies to potentially delay AD onset.

APOE4 accelerates advanced-stage vascular and neurodegenerative disorder in old Alzheimer’s mice via cyclophilin A independently of amyloid-β

Apolipoprotein E4 (APOE4), the main susceptibility gene for Alzheimer's disease (AD), leads to vascular dysfunction, amyloid-β pathology, neurodegeneration and dementia. How these different pathologies contribute to advanced-stage AD remains unclear. Using aged APOE knock-in mice crossed with 5xFAD mice, we show that, compared to APOE3, APOE4 accelerates blood-brain barrier (BBB) breakdown, loss of cerebral blood flow, neuronal loss and behavioral deficits independently of amyloid-β. BBB breakdown was associated with activation of the cyclophilin A-matrix metalloproteinase-9 BBB-degrading pathway in pericytes. Suppression of this pathway improved BBB integrity and prevented further neuronal loss and behavioral deficits in APOE4;5FAD mice while having no effect on amyloid-β pathology. Thus, APOE4 accelerates advanced-stage BBB breakdown and neurodegeneration in Alzheimer's mice via the cyclophilin A pathway in pericytes independently of amyloid-β, which has implication for the pathogenesis and treatment of vascular and neurodegenerative disorder in AD.

Hippocampal blood-brain barrier permeability is related to the APOE4 mutation status of elderly individuals without dementia

Blood-brain barrier (BBB) disruption, modulated by APOE4 mutation, is implicated in the pathogenesis of cognitive decline. We determined whether BBB permeability differed according to cognitive functioning and APOE4 status in elderly subjects without dementia. In this prospective study, 33 subjects with mild cognitive impairment (MCI) and 33 age-matched controls (normal cognition [NC]) underwent 3 T brain magnetic resonance imaging. The Patlak model was used to calculate tissue permeability (K_trans). A region-of interest analysis of K_trans was performed to compare relevant brain regions. Effects of K_trans on cognitive functioning were evaluated with linear regression analysis adjusted for confounding factors. NC and MCI groups did not differ in terms of vascular risk factors or hippocampal K_trans, except for hippocampal volume. Hippocampal K_trans was significantly higher in APOE4 carriers than in non-carriers (p = 0.007). Factors which predicted cognitive functioning included hippocampal volume (beta=-0.445, standard error [SE]=0.137, p = 0.003) and hippocampal BBB permeability (beta = 0.142, SE = 0.050, p = 0.008) after correcting for age, education, and APOE4 status. This suggests that hippocampal BBB permeability is associated with APOE4 mutation, and may predict cognitive functioning. BBB permeability imaging represents a distinct imaging biomarker for APOE4 mutations in NC and MCI subjects and for determining the degree of APOE4-related pathology.

Astrocyte-Neuron Metabolic Crosstalk in Neurodegeneration: A Mitochondrial Perspective

Converging evidence made clear that declining brain energetics contribute to aging and are implicated in the initiation and progression of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Indeed, both pathologies involve instances of hypometabolism of glucose and oxygen in the brain causing mitochondrial dysfunction, energetic failure and oxidative stress. Importantly, recent evidence suggests that astrocytes, which play a key role in supporting neuronal function and metabolism, might contribute to the development of neurodegenerative diseases. Therefore, exploring how the neuro-supportive role of astrocytes may be impaired in the context of these disorders has great therapeutic potential. In the following, we will discuss some of the so far identified features underlining the astrocyte-neuron metabolic crosstalk. Thereby, special focus will be given to the role of mitochondria. Furthermore, we will report on recent advancements concerning iPSC-derived models used to unravel the metabolic contribution of astrocytes to neuronal demise. Finally, we discuss how mitochondrial dysfunction in astrocytes could contribute to inflammatory signaling in neurodegenerative diseases.

EEG-fMRI Signal Coupling Is Modulated in Subjects With Mild Cognitive Impairment and Amyloid Deposition

Cognitive impairment indicates disturbed brain physiology which can be due to various mechanisms including Alzheimer's pathology. Combined functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) recordings (EEG-fMRI) can assess the interplay between complementary measures of brain activity and EEG changes to be localized to specific brain regions. We used a two-step approach, where we first examined changes related to a syndrome of mild cognitive impairment irrespective of pathology and then studied the specific impact of amyloid pathology. After detailed clinical and neuropsychological characterization as well as a positron emission tomography (PET) scans with the tracer 11-[C]-Pittsburgh Compound B to estimate cerebral amyloid deposition, 14 subjects with mild cognitive impairment (MCI) (mean age 75.6 SD: 8.9) according to standard criteria and 21 cognitively healthy controls (HCS) (mean age 71.8 SD: 4.2) were assessed with EEG-fMRI. Thalamo-cortical alpha-fMRI signal coupling was only observed in HCS. Additional EEG-fMRI signal coupling differences between HCS and MCI were observed in parts of the default mode network, salience network, fronto-parietal network, and thalamus. Individuals with significant cerebral amyloid deposition (amyloid-positive MCI and HCS combined compared to amyloid-negative HCS) displayed abnormal EEG-fMRI signal coupling in visual, fronto-parietal regions but also in the parahippocampus, brain stem, and cerebellum. This finding was paralleled by stronger absolute fMRI signal in the parahippocampus and weaker absolute fMRI signal in the inferior frontal gyrus in amyloid-positive subjects. We conclude that the thalamocortical coupling in the alpha band in HCS more closely reflects previous findings observed in younger adults, while in MCI there is a clearly aberrant coupling in several networks dominated by an anticorrelation in the posterior cingulate cortex. While these findings may broadly indicate physiological changes in MCI, amyloid pathology was specifically associated with abnormal fMRI signal responses and disrupted coupling between brain oscillations and fMRI signal responses, which especially involve core regions of memory: the hippocampus, para-hippocampus, and lateral prefrontal cortex.

Lower cerebral perfusion is associated with tau-PET in the entorhinal cortex across the Alzheimer's continuum

Alzheimer's disease (AD) is associated with reduced temporo-parietal cerebral blood flow (CBF). However, a substantial variability in CBF across the clinical spectrum of AD has been reported, possibly due to differences in primary AD pathologies. Here, we assessed CBF (ASL-MRI), tau (AV1451-PET) and amyloid (AV45/FBB-PET) in 156 subjects across the AD continuum. Using mixed-effect regression analyses, we assessed the local associations between amyloid-PET, tau-PET and CBF in a hypothesis-driven way focusing on each pathology's predilection areas. The contribution of Apolipoprotein E (APOE) genotype, and MRI markers of small vessel disease (SVD) to alterations in CBF were assessed as well. Tau-PET was associated with lower CBF in the entorhinal cortex, independent of Aβ. Amyloid-PET was associated with lower CBF in temporo-parietal regions. No associations between MRI markers of SVD and CBF were observed. These results provide evidence that in addition to Aβ, pathologic tau is a major correlate of CBF in early Braak stages, independent of Aβ, APOE genotype and SVD markers.

Cerebral blood flow decrease as an early pathological mechanism in Alzheimer's disease

Therapies targeting late events in Alzheimer's disease (AD), including aggregation of amyloid beta (Aβ) and hyperphosphorylated tau, have largely failed, probably because they are given after significant neuronal damage has occurred. Biomarkers suggest that the earliest event in AD is a decrease of cerebral blood flow (CBF). This is caused by constriction of capillaries by contractile pericytes, probably evoked by oligomeric Aβ. CBF is also reduced by neutrophil trapping in capillaries and clot formation, perhaps secondary to the capillary constriction. The fall in CBF potentiates neurodegeneration by upregulating the BACE1 enzyme that makes Aβ and by promoting tau hyperphosphorylation. Surprisingly, therefore, CBF reduction may play a crucial role in driving cognitive decline by initiating the amyloid cascade itself, or being caused by and amplifying Aβ production. Here, we review developments in this area that are neglected in current approaches to AD, with the aim of promoting novel mechanism-based therapeutic approaches.

Targeted genetic analysis of cerebral blood flow imaging phenotypes implicates the INPP5D gene

The vascular hypothesis of Alzheimer's disease (AD) has proposed the involvement of brain hypoperfusion in AD pathogenesis, where cognitive decline and dysfunction result from dwindling cerebral blood flow (CBF). Based on the vascular hypothesis of Alzheimer's disease, we focused on exploring how genetic factors influence AD pathogenesis via the cerebrovascular system. To investigate the role of CBF endophenotypes in AD pathogenesis, we performed a targeted genetic analysis of 258 subjects from the Alzheimer's Disease Neuroimaging Initiative cohort to examine associations between 4033 single-nucleotide polymorphisms of 24 AD genes and CBF measures in 4 brain regions. A novel association with CBF measure in the left angular gyrus was identified in an INPP5D single-nucleotide polymorphism (i.e., rs61068452; p = 1.48E-7; corrected p = 2.39E-3). The gene-based analysis discovered both INPP5D and CD2AP associated with the left angular gyrus CBF. Further analyses on nonoverlapping samples revealed that rs61068452-G was associated with lower CSF t-tau/Aβ1-42 ratio. Our findings suggest a protective role of rs61068452-G in an AD-relevant cerebrovascular endophenotype, which has the potential to provide novel insights for better mechanistic understanding of AD.

Approaches to CNS Drug Delivery with a Focus on Transporter-Mediated Transcytosis

Drug delivery to the central nervous system (CNS) conferred by brain barriers is a major obstacle in the development of effective neurotherapeutics. In this review, a classification of current approaches of clinical or investigational importance for the delivery of therapeutics to the CNS is presented. This classification includes the use of formulations administered systemically that can elicit transcytosis-mediated transport by interacting with transporters expressed by transvascular endothelial cells. Neurotherapeutics can also be delivered to the CNS by means of surgical intervention using specialized catheters or implantable reservoirs. Strategies for delivering drugs to the CNS have evolved tremendously during the last two decades, yet, some factors can affect the quality of data generated in preclinical investigation, which can hamper the extension of the applications of these strategies into clinically useful tools. Here, we disclose some of these factors and propose some solutions that may prove valuable at bridging the gap between preclinical findings and clinical trials.

Increased cerebral blood flow with increased amyloid burden in the preclinical phase of alzheimer's disease

BACKGROUND

Arterial spin labeling (ASL) is an emerging MRI technique for noninvasive measurement of cerebral blood flow (CBF) that has been used to show hemodynamic changes in the brains of people with Alzheimer's disease (AD). CBF changes have been measured using positron emission tomography (PET) across the AD spectrum, but ASL showed limited success in measuring CBF variations in the preclinical phase of AD, where amyloid β (Aβ) plaques accumulate in the decades prior to symptom onset.

PURPOSE

To investigate the relationship between CBF measured by multiphase-pseudocontinuous-ASL (MP-PCASL) and Aβ burden as measured by ¹¹ C-PiB PET imaging in a study of cognitively normal (CN) subjects age over 65.

STUDY TYPE

Cross-sectional.

POPULATION

Forty-six CN subjects including 33 with low levels of Aβ burden and 13 with high levels of Aβ.

FIELD STRENGTH/SEQUENCE

3T/3D MP-PCASL.

ASSESSMENT

The MP-PCASL method was chosen because it has a high signal-to-noise ratio. Furthermore, the data were analyzed using an efficient processing pipeline consisting of motion correction, ASL motion correction imprecision removal, temporal and spatial filtering, and partial volume effect correction.

STATISTICAL TESTS

General Linear Model.

RESULTS

In CN subjects positive for Aβ burden (n = 13), we observed a positive correlation between CBF and Aβ burden in the hippocampus, amygdala, caudate (P < 0.01), frontal, temporal, and insula (P < 0.05).

DATA CONCLUSION

To the best of our knowledge, this is the first study using MP-PCASL in the study of AD, and the results suggest a potential compensatory hemodynamic mechanism that protects against pathology in the early stages of AD.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2020;51:505-513.

Age‑ and brain region‑associated alterations of cerebral blood flow in early Alzheimer's disease assessed in AβPPSWE/PS1ΔE9 transgenic mice using arterial spin labeling

It has been suggested that cerebral blood flow (CBF) alterations may be involved in the pathogenesis of Alzheimer's disease (AD). However, how CBF changes with age has not been detailed in AD, particularly in its early stages. The objective of the present study was to evaluate CBF in four brain regions (the hippocampus, entorhinal cortex, frontoparietal cortex and thalamus) of mice in four age groups, to mimic the respective stages of AD in humans [2 months (pre-clinical), 3.5 months (sub-clinical), 5 months (early-clinical) and 8 months (mid-clinical)], to understand the age-associated changes in selected brain regions and to elucidate the underlying vascular mechanisms. CBF was measured using magnetic resonance imaging-arterial spin labelling (ASL) under identical conditions across the age groups of AβPP^SWE/PS1^ΔE9 (APP/PS1) transgenic mice with AD. The results indicated age- and brain region-associated changes in CBF were associated with early AD. More precisely, an age-dependent increase in CBF (in the pre- and sub-clinical AD groups) was observed in the frontoparietal cortex and thalamus. Conversely, increased CBF demonstrated an age-dependent decline (in the early- and mid-clinical AD groups) in all examined brain regions. Among the regions, the thalamus had the greatest increase in CBF in the 2 and 3.5 months age groups, which was substantially different compared with the age-matched controls. An extension of vessel area was also noted to be age- and brain region-dependent. In particular, correlation analysis revealed significant associations of CBF with vessel area in the frontoparietal cortex and thalamus of APP/PS1 mice at ages 2 and 3.5 months, indicating that CBF increase may arise from vessel extension. The results of the present study suggested that ASL can detect age- and brain region-associated changes in CBF in mice with AD, and that ASL-measured CBF increase may be a potential diagnostic biomarker for early AD. The observation that CBF increase resulted from vessel extension may aid in the understanding of the vascular role in age-associated development of AD pathology, and provide preclinical evidence for AD patient management.

Increased cerebral blood volume in small arterial vessels is a correlate of amyloid-β-related cognitive decline

The protracted accumulation of amyloid-β (Aβ) is a major pathologic hallmark of Alzheimer's disease and may trigger secondary pathological processes that include neurovascular damage. This study was aimed at investigating long-term effects of Aβ burden on cerebral blood volume of arterioles and pial arteries (CBVa), possibly present before manifestation of dementia. Aβ burden was assessed by 11C Pittsburgh compound-B positron emission tomography in 22 controls and 18 persons with mild cognitive impairment (MCI), [ages: 75(±6) years]. After 2 years, inflow-based vascular space occupancy at ultra-high field strength of 7-Tesla was administered for measuring CBVa, and neuropsychological testing for cognitive decline. Crushing gradients were incorporated during MR-imaging to suppress signals from fast-flowing blood in large arteries, and thereby sensitize inflow-based vascular space occupancy to CBVa in pial arteries and arterioles. CBVa was significantly elevated in MCI compared to cognitively normal controls and regional CBVa related to local Aβ deposition. For both MCI and controls, Aβ burden and follow-up CBVa in several brain regions synergistically predicted cognitive decline over 2 years. Orbitofrontal CBVa was positively associated with apolipoprotein E e4 carrier status. Increased CBVa may reflect long-term effects of region-specific pathology associated with Aβ deposition. Additional studies are needed to clarify the role of the arteriolar system and the potential of CBVa as a biomarker for Aβ-related vascular downstream pathology.

Vascular dysfunction-The disregarded partner of Alzheimer's disease

Increasing evidence recognizes Alzheimer's disease (AD) as a multifactorial and heterogeneous disease with multiple contributors to its pathophysiology, including vascular dysfunction. The recently updated AD Research Framework put forth by the National Institute on Aging-Alzheimer's Association describes a biomarker-based pathologic definition of AD focused on amyloid, tau, and neuronal injury. In response to this article, here we first discussed evidence that vascular dysfunction is an important early event in AD pathophysiology. Next, we examined various imaging sequences that could be easily implemented to evaluate different types of vascular dysfunction associated with, and/or contributing to, AD pathophysiology, including changes in blood-brain barrier integrity and cerebral blood flow. Vascular imaging biomarkers of small vessel disease of the brain, which is responsible for >50% of dementia worldwide, including AD, are already established, well characterized, and easy to recognize. We suggest that these vascular biomarkers should be incorporated into the AD Research Framework to gain a better understanding of AD pathophysiology and aid in treatment efforts.

The role of brain vasculature in neurodegenerative disorders

Adequate supply of blood and structural and functional integrity of blood vessels are key to normal brain functioning. On the other hand, cerebral blood flow shortfalls and blood-brain barrier dysfunction are early findings in neurodegenerative disorders in humans and animal models. Here we first examine molecular definition of cerebral blood vessels, as well as pathways regulating cerebral blood flow and blood-brain barrier integrity. Then we examine the role of cerebral blood flow and blood-brain barrier in the pathogenesis of Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and multiple sclerosis. We focus on Alzheimer's disease as a platform of our analysis because more is known about neurovascular dysfunction in this disease than in other neurodegenerative disorders. Finally, we propose a hypothetical model of Alzheimer's disease biomarkers to include brain vasculature as a factor contributing to the disease onset and progression, and we suggest a common pathway linking brain vascular contributions to neurodegeneration in multiple neurodegenerative disorders.

The Age-Related Perfusion Pattern Measured With Arterial Spin Labeling MRI in Healthy Subjects

Aim: To analyze age-related cerebral blood flow (CBF) using arterial spin labeling (ASL) MRI in healthy subjects with multivariate principal component analysis (PCA). Methods: 50 healthy subjects (mean age 45.8 ± 18.5 years, range 21-85) had 3D structural MRI and pseudo-continuous ASL MRI at resting state. The relationship between CBF and age was examined with voxel-based univariate analysis using multiple regression and two-sample t-test (median age 41.8 years as a cut-off). An age-related CBF pattern was identified using multivariate PCA. Results: Age correlated negatively with CBF especially anteriorly and in the cerebellum. After adjusting by global value, CBF was relatively decreased with aging in certain regions and relatively increased in others. The age-related CBF pattern showed relative reductions in frontal and parietal areas and cerebellum, and covarying increases in temporal and occipital areas. Subject scores of this pattern correlated negatively with age (R2 = 0.588; P < 0.001) and discriminated between the older and younger subgroups (P < 0.001). Conclusion: A distinct age-related CBF pattern can be identified with multivariate PCA using ASL MRI.

Anserine/Carnosine Supplementation Preserves Blood Flow in the Prefrontal Brain of Elderly People Carrying APOE e4

In a previously reported double-blind, randomized controlled trial (RCT), we demonstrated that daily supplementation with anserine (750 mg) and carnosine (250 mg) improves brain blood flow and memory function in elderly people. Here, we conducted a sub-analysis of MRI data and test scores from the same RCT to determine whether anserine/carnosine supplementation specifically benefits elderly people carrying the APOE e4 allele, which is a risk gene for accelerated brain aging and for the onset of Alzheimer’s Disease. We collected data from 68 participants aged 65 years or older who received anserine/carnosine supplementation (ACS) or placebo for 12 months. Subjects were assessed at the start and end of the trial using several neuropsychological tests, including the Wechsler Memory Scale-Logical Memory (WMS-LM). We also collected two types of MRI data, arterial spin labeling (ASL) and diffusion tensor imaging (DTI) at the start and end of the trial. We found that ACS significantly preserved verbal memory (WMS-LM, F[1,65] = 4.2003, p = 0.0445) and blood flow at frontal areas of the brain (FWE_{cluster level}, p < 0.001). Sub-analysis based on the APOE4 genotype showed a significant preservation of blood flow (p = 0.002, by ASL analysis) and white-matter microstructure (p = 0.003, by DTI analysis) at prefrontal areas in APOE4⁺ subjects in the active group, while there was no significant difference between APOE4^- subjects in the active and placebo groups. The effect of ACS in preserving brain structure and function in elderly people carrying APOE4 should be verified by further studies.

Intracranial Arterial 4D Flow in Individuals with Mild Cognitive Impairment is Associated with Cognitive Performance and Amyloid Positivity

It is becoming increasingly recognized that cerebrovascular disease is a contributing factor in the pathogenesis of Alzheimer's disease (AD). A unique 4D-Flow magnetic resonance imaging (MRI) technique, phase contrast vastly undersampled isotropic projection imaging (PC VIPR), enables examination of angiographic and quantitative metrics of blood flow in the arteries of the Circle of Willis within a single MRI acquisition. Thirty-eight participants with mild cognitive impairment (MCI) underwent a comprehensive neuroimaging protocol (including 4D-Flow imaging) and a standard neuropsychological battery. A subset of participants (n = 22) also underwent lumbar puncture and had cerebrospinal fluid (CSF) assayed for AD biomarkers. Cut-offs for biomarker positivity in CSF resulting from a receiver operating characteristic curve analysis of AD cases and controls from the larger Wisconsin Alzheimer's Disease Research Center cohort were used to classify MCI participants as biomarker positive or negative on amyloid-β (Aβ42), total-tau and total-tau/Aβ42 ratio. Internal carotid artery (ICA) and middle cerebral artery (MCA) mean flow were associated with executive functioning performance, with lower mean flow corresponding to worse performance. MCI participants who were biomarker positive for Aβ42 had lower ICA mean flow than did those who were Aβ42 negative. In sum, mean ICA and MCA arterial flow was associated with cognitive performance in participants with MCI and lower flow in the ICA was associated with amyloid positivity. This provides further evidence for vascular health as a contributing factor in the etiopathogenesis of AD, and could represent a point to intervene in the disease process.

Cerebral Hemodynamics in Mild Cognitive Impairment: A Systematic Review

BACKGROUND

The incidence of dementia is projected to rise over the coming decades, but with no sensitive diagnostic tests available. Vascular pathology precedes the deposition of amyloid and is an attractive early target.

OBJECTIVE

The aim of this review was to investigate the use of cerebral hemodynamics and oxygenation as a novel biomarker for mild cognitive impairment (MCI), focusing on transcranial Doppler ultrasonography (TCD) and near-infrared spectroscopy (NIRS).

METHODS

2,698 articles were identified from Medline, Embase, PsychINFO, and Web of Science databases. 306 articles were screened and quality assessed independently by two reviewers; 26 met the inclusion criteria. Meta-analyses were performed for each marker with two or more studies and limited heterogeneity.

RESULTS

Eleven studies were TCD, 8 NIRS, 5 magnetic resonance imaging, and 2 positron/single photon emission tomography. Meta-analyses showed reduced tissue oxygenation index, cerebral blood flow and velocity, with higher pulsatility index, phase and cerebrovascular resistance in MCI compared to controls. The majority of studies found reduced CO2 reactivity in MCI, with mixed findings in neuroactivation studies.

CONCLUSION

Despite small sample sizes and heterogeneity, meta-analyses demonstrate clear abnormalities in cerebral hemodynamic and oxygenation parameters, even at an early stage of cognitive decline. Further work is required to investigate the use of cerebral hemodynamic and oxygenation parameters as a sensitive biomarker for dementia.

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.

Cerebral Blood Flow and Amyloid-β Interact to Affect Memory Performance in Cognitively Normal Older Adults

Cerebral blood flow (CBF) alterations and amyloid-β (Aβ) accumulation have been independently linked to cognitive deficits in older adults at risk for dementia. Less is known about how CBF and Aβ may interact to affect cognition in cognitively normal older adults. Therefore, we examined potential statistical interactions between CBF and Aβ status in regions typically affected in Alzheimer's disease (AD) within a sample of older adults from the Alzheimer's Disease Neuroimaging Initiative (ADNI) study. Sixty-two cognitively normal participants (mean age = 72 years) underwent neuroimaging and memory testing. Arterial spin labeling magnetic resonance imaging was used to quantify CBF and florbetapir PET amyloid imaging was used to measure Aβ deposition. Aβ status (i.e., positivity versus negativity) was determined based on established cutoffs (Landau et al., 2013). The Rey Auditory Verbal Learning Test was used to assess memory. Linear regression models adjusted for age, education, and sex, demonstrated significant interactions between CBF and Aβ status on memory performance. Among Aβ positive older adults, there were significant negative associations between higher CBF in hippocampus, posterior cingulate, and precuneus and poorer memory performance. In contrast, among Aβ negative older adults, there were no significant associations between CBF and cognition. Our findings extend previous CBF studies of dementia risk by reporting interactions between Aβ status and CBF on memory performance in a sample of well-characterized, cognitively normal older adults. Results suggest that differential CBF-cognition associations can be identified in healthy, asymptomatic Aβ positive older adults relative to Aβ negative individuals. Associations between higherCBF and poorer memory among Aβ positive older adults may reflect a cellular and/or vascular compensatory response to pathologic processes whereby higher CBF is needed to maintain normal memory abilities. Findings indicate that CBF and its associations with cognition may have utility as a reliable marker of brain function early in the AD process when interventions are likely to be beneficial.

Divergent regional patterns of cerebral hypoperfusion and gray matter atrophy in mild cognitive impairment patients

Reductions of cerebral blood flow and gray matter structure have been implicated in early pathogenesis of Alzheimer's disease, potentially providing complementary information. The present study evaluated regional patterns of cerebral hypoperfusion and atrophy in patients with mild cognitive impairment and healthy older adults. In each participant, cerebral perfusion and gray matter structure were extracted within selected brain regions vulnerable to Alzheimer's disease using magnetic resonance imaging. Measures were compared between diagnostic groups with/without adjustment for covariates. In mild cognitive impairment patients, cerebral blood flow was significantly reduced in comparison with healthy controls in temporo-parietal regions and the basal ganglia in the absence of local gray matter atrophy. By contrast, gray matter structure was significantly reduced in the hippocampus in the absence of local hypoperfusion. Both, cerebral perfusion and gray matter structure were significantly reduced in the entorhinal and isthmus cingulate cortex in mild cognitive impairment patients compared with healthy older adults. Our results demonstrated partly divergent patterns of temporo-parietal hypoperfusion and medial-temporal atrophy in mild cognitive impairment patients, potentially indicating biomarker sensitivity to dissociable pathological mechanisms. The findings support applicability of cerebral perfusion and gray matter structure as complementary magnetic resonance imaging-based biomarkers in early Alzheimer's disease detection, a hypothesis to be further evaluated in longitudinal studies.