The ART of loss: Abeta imaging in the evaluation of Alzheimer's disease and other dementias

Preclinical and clinical imaging techniques as pathophysiological biomarkers - A preface to the special issue "Brain Imaging"

Molecular imaging techniques have become important tools to characterize and measure biological processes at the cellular and molecular levels. Nowadays, molecular imaging techniques are widely used in preclinical and clinical studies to assess the molecular dynamics under physiological conditions and during pathological processes. This special issue on Brain Imaging (https://onlinelibrary.wiley.com/doi/toc/10.1111/[ISSN]1471-4159.brain-imaging) will highlight some of the recent advances in developing new tools and applying molecular imaging techniques to understand biomarker dynamics in health and diseases.

Somatic Mutations and Alzheimer’s Disease

Alzheimer’s disease (AD) represents a global health challenge, with an estimated 55 million people suffering from the non-curable disease across the world. While amyloid-β plaques and tau neurofibrillary tangles in the brain define AD proteinopathy, it has become evident that diverse coding and non-coding regions of the genome may significantly contribute to AD neurodegeneration. The diversity of factors associated with AD pathogenesis, coupled with age-associated damage, suggests that a series of triggering events may be required to initiate AD. Since somatic mutations accumulate with aging, and aging is a major risk factor for AD, there is a great potential for somatic mutational events to drive disease. Indeed, recent data from the Gozes team/laboratories as well as other leading laboratories correlated the accumulation of somatic brain mutations with the progression of tauopathy. In this review, we lay the current perspectives on the principal genetic factors associated with AD and the potential causes, highlighting the contribution of somatic mutations to the pathogenesis of late onset Alzheimer’s disease. The roles that artificial intelligence and big data can play in accelerating the progress of causal somatic mutation markers/biomarkers identification, and the associated drug discovery/repurposing, have been highlighted for future AD and other neurodegenerative studies, with the aim to bring hope for the vulnerable aging population.

Molecular Imaging Approaches in Dementia

The increasing prevalence of dementia worldwide places a high demand on healthcare providers to perform a diagnostic work-up in relatively early stages of the disease, given that the pathologic process usually begins decades before symptoms are evident. Structural imaging is recommended to rule out other disorders and can only provide diagnosis in a late stage with limited specificity. Where PET imaging previously focused on the spatial pattern of hypometabolism, the past decade has seen the development of novel tracers to demonstrate characteristic protein abnormalities. Molecular imaging using PET/SPECT is able to show amyloid and tau deposition in Alzheimer disease and dopamine depletion in parkinsonian disorders starting decades before symptom onset. Novel tracers for neuroinflammation and synaptic density are being developed to further unravel the molecular pathologic characteristics of dementia disorders. In this article, the authors review the current status of established and emerging PET tracers in a diagnostic setting and also their value as prognostic markers in research studies and outcome measures for clinical trials in Alzheimer disease.

Advances in Brain Amyloid Imaging

Amyloid-β (Aβ) PET imaging has now been available for over 15 years. The ability to detect Aβ in vivo has greatly improved the clinical and research landscape of Alzheimer's disease (AD) and other neurodegenerative conditions. Aβ imaging provides very reliable, accurate, and reproducible measurements of regional and global Aβ burden in the brain. It has proved invaluable in anti-Aβ therapy trials, and is now recognized as a powerful diagnostic tool. The appropriate use of Aβ PET, when combined with comprehensive clinical evaluation by a dementia-trained specialist, can improve the accuracy of a clinical diagnosis of AD and substantially alter management. It can assist in differentiating AD from other neurodegenerative conditions, often by its ability to rule out the presence of Aβ. When combined with tau imaging, further increase in specificity for the diagnosis of AD can be achieved. The integration of Aβ PET, in conjunction with biomarkers of tau, neurodegeneration and neuroinflammation, into large, longitudinal, observational cohort studies continues to increase our understanding of the development of AD. Its incorporation into clinical trials has been pivotal in defining the most effective anti-Aβ biological therapies and optimal dosing so that effective disease modifying therapy now appears imminent. Aβ deposition is a gradual and protracted process, permitting a wide treatment window for anti-Aβ therapies and Aβ PET has made trials in this preclinical AD period feasible. Continuing improvement in Aβ tracer target to background ratio is allowing trials in earlier AD that tailor drug dosage to Aβ level. The quest to standardize quantification and define universally applicable thresholds for all Aβ tracers has produced the Centiloid method. Centiloid values that correlate well with neuropathologic findings and prognosis have been identified. Rapid cloud-based automated individual scan analysis is now possible and does not require MRI. Challenges remain, particularly around cross camera standardized uptake value ratio variation that need to be addressed. This review will compare available Aβ radiotracers, discuss approaches to quantification, as well as the clinical and research applications of Aβ PET.

11C-PIB PET in subjective cognitive impairment

People with Subjective Cognitive Impairment (SCI) may be at increased risk of dementia. In this study we examined amyloid load in 5 SCI subjects and 14 controls using PIB PET scanning. One SCI subject had significantly increased PIB retention in the cortical areas of interest. Larger, longitudinal studies are indicated.

Temporal and anatomical distribution of 18F-flutemetamol uptake in canine brain using positron emission tomography

BACKGROUND

Positron emission tomography (PET) is increasingly being used as an imaging modality for clinical and research applications in veterinary medicine. Amyloid PET has become a useful tool for diagnosing Alzheimer's disease (AD) in humans, by accurately identifying amyloid-beta (Aβ) plaques. Cognitive dysfunction syndrome in dogs shows cognitive and pathophysiologic characteristics similar to AD. Therefore, we assessed the physiologic characteristics of uptake of 18F-flutemetamol, an Aβ protein-binding PET tracer in clinical development, in normal dog brains, for distinguishing an abnormal state. Static and dynamic PET images of six adult healthy dogs were acquired after 18F-flutemetamol was administered intravenously at approximately 3.083 MBq/kg. For static images, PET data were acquired at 30, 60, and 90 min after injection. One week later, dynamic images were acquired for 120 min, from the time of tracer injection. PET data were reconstructed using an iterative technique, and corrections for attenuation and scatter were applied. Regions of interest were manually drawn over the frontal, parietal, temporal, occipital, anterior cingulate, posterior cingulate, and cerebellar cortices, cerebral white matter, midbrain, pons, and medulla oblongata. After calculating standardized uptake values with an established formula, standardized uptake value ratios (SUVRs) were obtained, using the cerebellar cortex as a reference region.

RESULTS

Among the six cerebral cortical regions, the cingulate cortices and frontal lobe showed the highest SUVRs. The lowest SUVR was observed in the occipital lobe. The average values of the cortical SUVRs were 1.25, 1.26, and 1.27 at 30, 60, and 90 min post-injection, respectively. Tracer uptake on dynamic scans was rapid, peaking within 4 min post-injection. After reaching this early maximum, cerebral cortical regions showed a curve with a steep descent, whereas cerebral white matter demonstrated a curve with a slow decline, resulting in a large gap between cerebral cortical regions and white matter.

CONCLUSION

This study provides normal baseline data of 18F-flutemetamol PET that can facilitate an objective diagnosis of cognitive dysfunction syndrome in dogs in future.

Calcium Signaling in ß-cell Physiology and Pathology: A Revisit

Pancreatic beta (β) cell dysfunction results in compromised insulin release and, thus, failed regulation of blood glucose levels. This forms the backbone of the development of diabetes mellitus (DM), a disease that affects a significant portion of the global adult population. Physiological calcium (Ca²⁺) signaling has been found to be vital for the proper insulin-releasing function of β-cells. Calcium dysregulation events can have a dramatic effect on the proper functioning of the pancreatic β-cells. The current review discusses the role of calcium signaling in health and disease in pancreatic β-cells and provides an in-depth look into the potential role of alterations in β-cell Ca²⁺ homeostasis and signaling in the development of diabetes and highlights recent work that introduced the current theories on the connection between calcium and the onset of diabetes.

Bax inhibitor 1 is a γ-secretase-independent presenilin-binding protein

β-amyloid is regarded by some scientists to be the cause of Alzheimer’s disease (AD). One of the strongest arguments against this hypothesis is the presence of hundreds of AD-causing mutations in presenilin, but none in the other three components of γ-secretase. This observation implies a γ-secretase–independent function of presenilin. To understand such a putative function, discovery of presenilin-binding proteins represents an important first step. In this study, we report the identification of Bax-inhibitor 1 (BI1) as a stable interacting partner of presenilin 1 (PS1), but not the intact γ-secretase. Our results link PS1 to BI1, a protein thought to play a role in apoptosis and calcium channel regulation. This finding opens a range of possibilities for the investigation of PS1 function and AD genesis.

Presenilin is the catalytic subunit of γ-secretase, a four-component intramembrane protease responsible for the generation of β-amyloid (Aβ) peptides. Over 200 Alzheimer’s disease-related mutations have been identified in presenilin 1 (PS1) and PS2. Here, we report that Bax-inhibitor 1 (BI1), an evolutionarily conserved transmembrane protein, stably associates with PS1. BI1 specifically interacts with PS1 in isolation, but not with PS1 in the context of an assembled γ-secretase. The PS1–BI1 complex exhibits no apparent proteolytic activity, as judged by the inability to produce Aβ40 and Aβ42 from the substrate APP-C99. At an equimolar concentration, BI1 has no impact on the proteolytic activity of γ-secretase; at a 200-fold molar excess, BI1 reduces γ-secretase activity nearly by half. Our biochemical study identified BI1 as a PS1-interacting protein, suggesting additional functions of PS1 beyond its involvement in γ-secretase.

Presenilin mutations deregulate mitochondrial Ca2+ homeostasis and metabolic activity causing neurodegeneration in Caenorhabditis elegans

Mitochondrial dysfunction and subsequent metabolic deregulation is observed in neurodegenerative diseases and aging. Mutations in the presenilin (PSEN) encoding genes (PSEN1 and PSEN2) cause most cases of familial Alzheimer’s disease (AD); however, the underlying mechanism of pathogenesis remains unclear. Here, we show that mutations in the C. elegans gene encoding a PSEN homolog, sel-12 result in mitochondrial metabolic defects that promote neurodegeneration as a result of oxidative stress. In sel-12 mutants, elevated endoplasmic reticulum (ER)-mitochondrial Ca²⁺ signaling leads to an increase in mitochondrial Ca²⁺ content which stimulates mitochondrial respiration resulting in an increase in mitochondrial superoxide production. By reducing ER Ca²⁺ release, mitochondrial Ca²⁺ uptake or mitochondrial superoxides in sel-12 mutants, we demonstrate rescue of the mitochondrial metabolic defects and prevent neurodegeneration. These data suggest that mutations in PSEN alter mitochondrial metabolic function via ER to mitochondrial Ca²⁺ signaling and provide insight for alternative targets for treating neurodegenerative diseases.

Alzheimer's disease is the most common type of dementia. A hallmark of this condition is progressive loss of memory, accompanied by a buildup of hard clumps of protein between the brain cells. These protein clumps, known as amyloid plaques, are a key focus of research into Alzheimer's disease. They are likely to be toxic to brain cells, but their role in the development and progression of the disease is not yet known.

Though the cause of Alzheimer's disease remains unclear, an inherited form of the disease may hold some clues. Mutations in genes for proteins called presenilins cause an earlier onset form of Alzheimer's disease, in which symptoms can develop in people who are in their 40s or 50s. The presenilin proteins appear in a cell structure called the endoplasmic reticulum, which plays many roles in the normal activities of a cell. Among other things, this structure stores and releases calcium ions, and cells use these ions to send and process many signals.

The cell's energy-producing powerhouses, the mitochondria, use calcium to boost their metabolic activity. This allows them to make more energy for the cell, but in the process they also make damaging byproducts. These byproducts include oxygen-containing chemicals, known as reactive oxygen species (ROS), which react strongly with other molecules. While low levels of ROS are a normal part of cell activity, if the levels get too high, these chemicals can attack and damage structures within the cell.

Untangling the effects of amyloid plaques and presenilins on brain cells in humans is challenging. But, a nematode worm called Caenorhabditis elegans does not form plaques, making it possible to look at presenilins on their own. Previous work in these worms has shown that presenilin mutations affect the endoplasmic reticulum and change the appearance of mitochondria. Here, Sarasija et al. extend this work to find out more about the effects presenilin mutations have on living cells.

Presenilin mutations in young adult worms increased the amount of calcium released by the endoplasmic reticulum. This increased the activity of the mitochondria and caused ROS levels to rise to damaging levels. This caused stress inside the cells, and the worms started to show early signs damage to their nervous systems. Mutations that decreased the movement of calcium from the endoplasmic reticulum to the mitochondria helped to prevent the damage. Treating the mitochondria with antioxidants to mop up the extra ROS also protected the cells.

This kind of damage to brain cells did not depend on amyloid plaques. Whilst the plaques are likely to be toxic, these new findings highlights the role that other chemical and biological processes might play in Alzheimer's disease. Further work to reveal the underlying cause of Alzheimer's disease may lead to new therapies to treat this condition in the future.

Analysis of 138 pathogenic mutations in presenilin-1 on the in vitro production of Aβ42 and Aβ40 peptides by γ-secretase

A hallmark of Alzheimer's disease (AD) is the aggregation of β-amyloid peptides (Aβ) into amyloid plaques in patient brain. Cleavage of amyloid precursor protein (APP) by the intramembrane protease γ-secretase produces Aβ of varying lengths, of which longer peptides such as Aβ42 are thought to be more harmful. Increased ratios of longer Aβs over shorter ones, exemplified by the ratio of Aβ42 over Aβ40, may lead to formation of amyloid plaques and consequent development of AD. In this study, we analyzed 138 reported mutations in human presenilin-1 (PS1) by individually reconstituting the mutant PS1 proteins into anterior-pharynx-defective protein 1 (APH-1)aL-containing γ-secretases and examining their abilities to produce Aβ42 and Aβ40 in vitro. About 90% of these mutations lead to reduced production of Aβ42 and Aβ40. Notably, 10% of these mutations result in decreased Aβ42/Aβ40 ratios. There is no statistically significant correlation between the Aβ42/Aβ40 ratio produced by a γ-secretase variant containing a specific PS1 mutation and the mean age at onset of patients from whom the mutation was isolated.

Neuroimaging biomarkers in Alzheimer's disease and other dementias

In vivo imaging of β-amyloid (Aβ) has transformed the assessment of Aβ pathology and its changes over time, extending our insight into Aβ deposition in the brain by providing highly accurate, reliable, and reproducible quantitative statements of regional or global Aβ burden in the brain. This knowledge is essential for therapeutic trial recruitment and for the evaluation of anti-Aβ treatments. Although cross sectional evaluation of Aβ burden does not strongly correlate with cognitive impairment, it does correlate with cognitive (especially memory) decline and with a higher risk for conversion to AD in the aging population and MCI subjects. This suggests that Aβ deposition is a protracted pathological process starting well before the onset of symptoms. Longitudinal observations, coupled with different disease-specific biomarkers to assess potential downstream effects of Aβ are required to confirm this hypothesis and further elucidate the role of Aβ deposition in the course of Alzheimer's disease.

Untangling tau imaging

In vivo imaging of tau deposits is providing a better understanding of the temporal and spatial tau deposition in the brain, allowing a more comprehensive insight into the causes, diagnoses, and potentially treatment of tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. The assessment of tau deposition in the brain over time will allow a deeper understanding of the relationship between tau and other variables such as cognition, genotype, and neurodegeneration, as well as assessing the role tau plays in ageing. Preliminary human studies suggest that tau imaging could also be used as a diagnostic, prognostic, and theranostic biomarker, as well as a surrogate marker for target engagement, patient recruitment, and efficacy monitoring for disease-specific therapeutic trials.

Insight into the Molecular Imaging of Alzheimer's Disease

Alzheimer's disease is a complex neurodegenerative disease affecting millions of individuals worldwide. Earlier it was diagnosed only via clinical assessments and confirmed by postmortem brain histopathology. The development of validated biomarkers for Alzheimer's disease has given impetus to improve diagnostics and accelerate the development of new therapies. Functional imaging like positron emission tomography (PET), single photon emission computed tomography (SPECT), functional magnetic resonance imaging (fMRI), and proton magnetic resonance spectroscopy provides a means of detecting and characterising the regional changes in brain blood flow, metabolism, and receptor binding sites that are associated with Alzheimer's disease. Multimodal neuroimaging techniques have indicated changes in brain structure and metabolic activity, and an array of neurochemical variations that are associated with neurodegenerative diseases. Radiotracer-based PET and SPECT potentially provide sensitive, accurate methods for the early detection of disease. This paper presents a review of neuroimaging modalities like PET, SPECT, and selected imaging biomarkers/tracers used for the early diagnosis of AD. Neuroimaging with such biomarkers and tracers could achieve a much higher diagnostic accuracy for AD and related disorders in the future.

A γ-Secretase Independent Role for Presenilin in Calcium Homeostasis Impacts Mitochondrial Function and Morphology in Caenorhabditis elegans

Mutations in the presenilin (PSEN) encoding genes (PSEN1 and PSEN2) occur in most early onset familial Alzheimer's Disease. Despite the identification of the involvement of PSEN in Alzheimer's Disease (AD) ∼20 years ago, the underlying role of PSEN in AD is not fully understood. To gain insight into the biological function of PSEN, we investigated the role of the PSEN homolog SEL-12 in Caenorhabditis elegans. Using genetic, cell biological, and pharmacological approaches, we demonstrate that mutations in sel-12 result in defects in calcium homeostasis, leading to mitochondrial dysfunction. Moreover, consistent with mammalian PSEN, we provide evidence that SEL-12 has a critical role in mediating endoplasmic reticulum (ER) calcium release. Furthermore, we found that in SEL-12-deficient animals, calcium transfer from the ER to the mitochondria leads to fragmentation of the mitochondria and mitochondrial dysfunction. Additionally, we show that the impact that SEL-12 has on mitochondrial function is independent of its role in Notch signaling, γ-secretase proteolytic activity, and amyloid plaques. Our results reveal a critical role for PSEN in mediating mitochondrial function by regulating calcium transfer from the ER to the mitochondria.

Tau imaging in the study of ageing, Alzheimer's disease, and other neurodegenerative conditions

In vivo tau imaging allows a deeper understanding of tau deposition in the brain, providing insights into the causes, diagnosis and treatment of primary and secondary tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. The cross-sectional and longitudinal assessment of the temporal and spatial patterns of tau deposition in the brain will allow a better understanding of the role tau plays in ageing as well as its relationship with cognition, genotype, and neurodegeneration. It is likely that selective tau imaging could be used as a diagnostic and prognostic biomarker of disease progression, as well as a surrogate marker for monitoring of efficacy and patient recruitment for disease-specific therapeutic trials.

Recommendations for the use of PET imaging biomarkers in the diagnosis of neurodegenerative conditions associated with dementia: SEMNIM and SEN consensus

The new diagnostic criteria for Alzheimer's disease (AD) acknowledges the interest given to biomarkers to improve the specificity in subjects with dementia and to facilitate an early diagnosis of the pathophysiological process of AD in the prodromal or pre-dementia stage. The current availability of PET imaging biomarkers of synaptic dysfunction (PET-FDG) and beta amyloid deposition using amyloid-PET provides clinicians with the opportunity to apply the new criteria and improve diagnostic accuracy in their clinical practice. Therefore, it seems essential for the scientific societies involved to use the new clinical diagnostic support tools to establish clear, evidence-based and agreed set of recommendations for their appropriate use. The present work includes a systematic review of the literature on the utility of FDG-PET and amyloid-PET for the diagnosis of AD and related neurodegenerative diseases that occur with dementia. Thus, we propose a series of recommendations agreed on by the Spanish Society of Nuclear Medicine and Spanish Society of Neurology as a consensus statement on the appropriate use of PET imaging biomarkers.

In vivo tau imaging: obstacles and progress

The military conflicts of the last decade have highlighted the growing problem of traumatic brain injury in combatants returning from the battlefield. The considerable evidence pointing at the accumulation of tau aggregates and its recognition as a risk factor in neurodegenerative conditions such as Alzheimer's disease have led to a major effort to develop selective tau ligands that would allow research into the physiopathologic underpinnings of traumatic brain injury and chronic traumatic encephalopathy in military personnel and the civilian population. These tracers will allow new insights into tau pathology in the human brain, facilitating research into causes, diagnosis, and treatment of traumatic encephalopathy and major neurodegenerative dementias, such as Alzheimer's disease and some variants of frontotemporal lobar degeneration, in which tau plays a role. The field of selective tau imaging has to overcome several obstacles, some of them associated with the idiosyncrasies of tau aggregation and others related to radiotracer design. A worldwide effort has focused on the development of imaging agents that will allow selective tau imaging in vivo. Recent progress in the development of these tracers is enabling the noninvasive assessment of the extent of tau pathology in the brain, eventually allowing the quantification of changes in tau pathology over time and its relation to cognitive performance, brain volumetrics, and other biomarkers, as well as assessment of efficacy and patient recruitment for antitau therapeutic trials.

Tau imaging: early progress and future directions

Use of selective in-vivo tau imaging will enable improved understanding of tau aggregation in the brain, facilitating research into causes, diagnosis, and treatment of major tauopathies such as Alzheimer's disease, progressive supranuclear palsy, corticobasal syndrome, chronic traumatic encephalopathy, and some variants of frontotemporal lobar degeneration. Neuropathological studies of Alzheimer's disease show a strong association between tau deposits, decreased cognitive function, and neurodegenerative changes. Selective tau imaging will allow the in-vivo exploration of such associations and measure the global and regional changes in tau deposits over time. Such imaging studies will comprise non-invasive assessment of the spatial and temporal pattern of tau deposition over time, providing insight into the role tau plays in ageing and helping to establish the relation between cognition, genotype, neurodegeneration, and other biomarkers. Once validated, selective tau imaging might be useful as a diagnostic, prognostic, and progression biomarker, and a surrogate marker for the monitoring of efficacy and patient recruitment for anti-tau therapeutic trials.

Differences in functional brain connectivity alterations associated with cerebral amyloid deposition in amnestic mild cognitive impairment

Despite potential implications for the early detection of impending Alzheimer’s disease (AD), very little is known about the differences of large-scale brain networks between amnestic mild cognitive impairment (aMCI) with high cerebral amyloid-beta protein (Aβ) deposition (i.e., aMCI+) and aMCI with no or very little Aβ deposition (i.e., aMCI−). We first aimed to extend the current literature on altering intrinsic functional connectivity (FC) of the default mode network (DMN) and salience network (SN) from cognitively normal (CN) to AD dementia. Second, we further examined the differences of the DMN and the SN between aMCI−, aMCI+, and CN. Forty-three older adult (12 CN, 10 aMCI+, 10 aMCI−, and 11 AD dementia) subjects were included. All participants received comprehensive clinical and neuropsychological assessment, resting-state functional magnetic resonance imaging, structural MRI, and Pittsburgh compound-B-PET scans. FC data were preprocessed using multivariate exploratory linear optimized decomposition into independent components of FMRIB’s Software Library. Group comparisons were carried out using the “dual-regression” approach. In addition, to verify presence of gray matter volume changes with intrinsic functional network alterations, voxel-based morphometry was performed on the acquired T1-weighted data. As expected, AD dementia participants exhibited decreased FC in the DMN compared to CN (particularly in the precuneus and cingulate gyrus). The degree of alteration in the DMN in aMCI+ compared to CN was intermediate to that of AD. In contrast, aMCI− exhibited increased FC in the DMN compared to CN (primarily in the precuneus) as well as aMCI+. In terms of the SN, aMCI− exhibited decreased FC compared to both CN and aMCI+ particularly in the inferior frontal gyrus. FC within the SN in aMCI+ and AD did not differ from CN. Compared to CN, aMCI− showed atrophy in bilateral superior temporal gyri whereas aMCI+ showed atrophy in right precuneus. The results indicate that despite the similarity in cross-sectional cognitive features, aMCI− has quite different functional brain connectivity compared to aMCI+.

Molecular imaging of neuropsychiatric symptoms in Alzheimer's and Parkinson's disease

Neuropsychiatric symptoms (NPS) are very common in neurodegenerative diseases and are a major contributor to disability and caregiver burden. There is accumulating evidence that NPS may be a prodrome and/or a "risk factor" of neurodegenerative diseases. The medications used to treat these symptoms in younger patients are not very effective in patients with neurodegenerative disease and may have serious side effects. An understanding of the neurobiology of NPS is critical for the development of more effective intervention strategies. Targeting these symptoms may also have implications for prevention of cognitive or motor decline. Molecular brain imaging represents a bridge between basic and clinical observations and provides many opportunities for translation from animal models and human post-mortem studies to in vivo human studies. Molecular brain imaging studies in Alzheimer's disease (AD) and Parkinson's disease (PD) are reviewed with a primary focus on positron emission tomography studies of NPS. Future directions for the field of molecular imaging in AD and PD to understand the neurobiology of NPS will be discussed.

An overview of PET neuroimaging

Over the past 35 years or so, PET brain imaging has allowed powerful and unique insights into brain function under normal conditions and in disease states. Initially, as PET instrumentation continued to develop, studies were focused on brain perfusion and glucose metabolism. This permitted refinement of brain imaging for important, non-oncologic clinical indications. The ability of PET to not only provide spatial localization of metabolic changes but also to accurately and consistently quantify their distribution proved valuable for applications in the clinical setting. Specifically, glucose metabolism brain imaging using (F-18) fluorodeoxyglucose continues to be invaluable for evaluating patients with intractable seizures for identifying seizure foci and operative planning. Cerebral glucose metabolism also contributes to diagnosis of neurodegenerative diseases that cause dementia. Alzheimer disease, dementia with Lewy bodies, and the several variants of frontotemporal lobar degeneration have differing typical patterns of hypometabolism. In Alzheimer disease, hypometabolism has furthermore been associated with poorer cognitive performance and ensuing cognitive and functional decline. As the field of radiochemistry evolved, novel radioligands including radiolabeled flumazenil, dopamine transporter ligands, nicotine receptor ligands, and others have allowed for further understanding of molecular changes in the brain associated with various diseases. Recently, PET brain imaging reached another milestone with the approval of (F-18) florbetapir imaging by the United States Federal Drug Administration for detection of amyloid plaque accumulation in brain, the major histopathologic hallmark of Alzheimer disease, and efforts have been made to define the clinical role of this imaging agent in the setting of the currently limited treatment options. Hopefully, this represents the first of many new radiopharmaceuticals that would allow improved diagnostic and prognostic information in these and other clinical applications, including Parkinson disease and traumatic brain injury.

A survey of FDG- and amyloid-PET imaging in dementia and GRADE analysis

PET based tools can improve the early diagnosis of Alzheimer's disease (AD) and differential diagnosis of dementia. The importance of identifying individuals at risk of developing dementia among people with subjective cognitive complaints or mild cognitive impairment has clinical, social, and therapeutic implications. Within the two major classes of AD biomarkers currently identified, that is, markers of pathology and neurodegeneration, amyloid- and FDG-PET imaging represent decisive tools for their measurement. As a consequence, the PET tools have been recognized to be of crucial value in the recent guidelines for the early diagnosis of AD and other dementia conditions. The references based recommendations, however, include large PET imaging literature based on visual methods that greatly reduces sensitivity and specificity and lacks a clear cut-off between normal and pathological findings. PET imaging can be assessed using parametric or voxel-wise analyses by comparing the subject's scan with a normative data set, significantly increasing the diagnostic accuracy. This paper is a survey of the relevant literature on FDG and amyloid-PET imaging aimed at providing the value of quantification for the early and differential diagnosis of AD. This allowed a meta-analysis and GRADE analysis revealing high values for PET imaging that might be useful in considering recommendations.

Assessing THK523 selectivity for tau deposits in Alzheimer's disease and non-Alzheimer's disease tauopathies

Introduction

The introduction of tau imaging agents such as ¹⁸F-THK523 offers new hope for the in vivo assessment of tau deposition in tauopathies such as Alzheimer’s disease (AD), where preliminary ¹⁸F-THK523-PET studies have demonstrated significantly higher cortical retention of ¹⁸F-THK523 in AD compared to age-matched healthy individuals. In addition to AD, tau imaging with PET may also be of value in assessing non-AD tauopathies, such as corticobasal degeneration (CBD), progressive supranuclear palsy (PSP) and Pick’s disease (PiD).

Methods

To further investigate the ability of THK523 to recognize tau lesions, we undertook immunohistochemical and fluorescence studies in serial brain sections taken from individuals with AD (n = 3), CBD (n = 2), PSP (n = 1), PiD (n = 2) and Parkinson’s disease (PD; n = 2). In addition to the neuropathological analysis, one PSP patient had undergone a ¹⁸F-THK523 PET scan 5 months before death.

Results

Although THK523 labelled tau-containing lesions such as neurofibrillary tangles and neuropil threads in the hippocampus and frontal regions of AD brains, it failed to label tau-containing lesions in non-AD tauopathies. Furthermore, though THK523 faintly labelled dense-cored amyloid-β plaques in the AD frontal cortex, it failed to label α-synuclein-containing Lewy bodies in PD brain sections.

Conclusion

The results of this study suggest that ¹⁸F-THK523 selectively binds to paired helical filament tau in AD brains but does not bind to tau lesions in non-AD tauopathies, or to α-synuclein in PD brains.

Enabling a multidisciplinary approach to the study of ageing and Alzheimer's disease: an update from the Australian Imaging Biomarkers and Lifestyle (AIBL) study

The Australian Imaging Biomarkers and Lifestyle (AIBL) study is a longitudinal study of 1,112 volunteers from healthy, mild cognitive impairment (MCI) and Alzheimer's disease (AD) populations who are assessed at 18-month intervals in order to enable prospective research into ageing and AD. Using a multidisciplinary battery, AIBL assessments comprise the extensive study of clinical factors and cognitive function, collection of blood and cerebrospinal fluid (CSF) samples for biomarker discovery, structural and β-amyloid (Aβ) neuroimaging, and obtaining information on diet and physical activity patterns of the cohort. Now in its seventh year, AIBL is part of a substantial international effort to prospectively study the relationships between clinical characteristics and putative AD biomarkers in groups who carry different risk factors for AD. The identification of biomarkers would provide a window of opportunity to assess AD risk in individuals prior to the onset of advanced clinical symptoms, in addition to facilitating testing of therapeutic and lifestyle interventions likely to emerge within the next decade that prevent or delay symptom emergence in those at high risk for developing AD. In this paper, we present key findings from the AIBL study and discuss how they contribute to our understanding of AD pathogenesis and diagnosis.

Unexpected finding of cerebral meningioma on (11)C-PiB PET

A 69-year-old man with no history of neurologic or psychiatric disorders underwent C-PiB PET as a healthy volunteer for amyloid imaging study. C-PiB PET images demonstrated an unexpected increased PiB binding in the left frontal region. MR images demonstrated an extra-axial left frontal mass lesion with mild enhancement, highly suggestive of meningioma. There is a limited amount of published information on the detection of meningioma with C-PiB PET.

Resting-state functional connectivity in normal brain aging

The world is aging and, as the elderly population increases, age-related cognitive decline emerges as a major concern. Neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), allow the investigation of the neural bases of age-related cognitive changes in vivo. Typically, fMRI studies map brain activity while subjects perform cognitive tasks, but such paradigms are often difficult to implement on a wider basis. Resting-state fMRI (rs-fMRI) has emerged as an important alternative modality of fMRI data acquisition, during which no specific task is required. Due to such simplicity and the reliability of rs-fMRI data, this modality presents increased feasibility and potential for clinical application in the future. With rs-fMRI, fluctuations in regional brain activity can be detected across separate brain regions and the patterns of intercorrelation between the functioning of these regions are measured, affording quantitative indices of resting-state functional connectivity (RSFC). This review article summarizes the results of recent rs-fMRI studies that have documented a variety of aging-related RSFC changes in the human brain, discusses the neurophysiological hypotheses proposed to interpret such findings, and provides an overview of the future, highly promising perspectives in this field.

Is the amyloid hypothesis of Alzheimer's disease therapeutically relevant?

The conventional view of AD (Alzheimer's disease) is that much of the pathology is driven by an increased load of β-amyloid in the brain of AD patients (the 'Amyloid Hypothesis'). Yet, many therapeutic strategies based on lowering β-amyloid have so far failed in clinical trials. This failure of β-amyloid-lowering agents has caused many to question the Amyloid Hypothesis itself. However, AD is likely to be a complex disease driven by multiple factors. In addition, it is increasingly clear that β-amyloid processing involves many enzymes and signalling pathways that play a role in a diverse array of cellular processes. Thus the clinical failure of β-amyloid-lowering agents does not mean that the hypothesis itself is incorrect; it may simply mean that manipulating β-amyloid directly is an unrealistic strategy for therapeutic intervention, given the complex role of β-amyloid in neuronal physiology. Another possible problem may be that toxic β-amyloid levels have already caused irreversible damage to downstream cellular pathways by the time dementia sets in. We argue in the present review that a more direct (and possibly simpler) approach to AD therapeutics is to rescue synaptic dysfunction directly, by focusing on the mechanisms by which elevated levels of β-amyloid disrupt synaptic physiology.

The challenges of tau imaging

In vivo imaging of tau pathology will provide new insights into tau deposition in the human brain, thus facilitating research into causes, diagnosis and treatment of major dementias, such as Alzheimer’s disease, or some variants of frontotemporal lobar degeneration, in which tau plays a role. Tau imaging poses several challenges, some related to the singularities of tau aggregation, and others related to radiotracer design. Several groups around the world are working on the development of imaging agents that will allow the in vivo assessment of tau deposition in aging and in neurodegeneration. Development of a tau imaging tracer will enable researchers to noninvasively examine the degree and extent of tau pathology in the brain, quantify changes in tau deposition over time, evaluate its relation to cognition and assess the efficacy of anti-tau therapy.

Environmental enrichment prevents behavioral deficits and oxidative stress caused by chronic cerebral hypoperfusion in the rat

AIMS

The aim of the present study was to evaluate the neuroprotective effects of environmental enrichment (EE), assessed by cognitive activity in the Morris water maze, and on brain oxidative status, through measurement of macromolecules damage, lipid peroxidation levels, total cellular thiols and antioxidant enzymes in hippocampus, striatum and cerebral cortex.

MAIN METHODS

Adult male Wistar rats were submitted to the modified permanent bilateral occlusion of the common carotid arteries (2VO) method, with right common carotid artery being first occluded, and tested three months after the ischemic event. Cognitive and physical stimulation, named Environmental Enrichment, consisted of one-hour sessions run 3 times per week during 12weeks, following two different stimulation protocols: pre-ischemia and pre+post-ischemia. Rats were then tested for both reference and working spatial memory tasks in the water maze and later sacrificed for measurement of oxidative stress parameters.

KEY FINDINGS

A significant cognitive deficit was found in both spatial tasks after hypoperfusion; this effect was reversed in the 2VO enriched group. Moreover, hippocampal oxidative damage and antioxidant enzyme activity were decreased by environmental enrichment.

SIGNIFICANCE

These results suggest that both stimulation protocols exert a neuroprotective effect against the cognitive impairment and the reduction of biomarkers for oxidative damage caused by chronic cerebral hypoperfusion.

The profile of executive functioning in amnestic mild cognitive impairment: disproportionate deficits in inhibitory control

Amnestic mild cognitive impairment (aMCI) represents a group of individuals who are highly likely to develop Alzheimer's disease (AD). Although aMCI is typically conceptualized as involving predominantly deficits in episodic memory, recent studies have demonstrated that deficits in executive functioning may also be present, and thorough categorization of cognitive functioning in MCI may improve early diagnosis and treatment of AD. We first provide an extensive review of neuropsychology studies that examined executive functioning in MCI. We then present data on executive functioning across multiple sub-domains (divided attention, working memory, inhibitory control, verbal fluency, and planning) in 40 aMCI patients (single or multiple domain) and 32 normal elderly controls (NECs). MCI patients performed significantly worse than NECs in all 5 sub-domains, and there was impairment (>1.0 SD below the mean of NECs) in all sub-domains. Impairment on each test was frequent, with 100% of MCI patients exhibiting a deficit in at least one sub-domain of executive functioning. Inhibitory control was the most frequently and severely impaired. These results indicate that executive dysfunction in multiple sub-domains is common in aMCI and highlights the importance of a comprehensive neuropsychological evaluation for fully characterizing the nature and extent of cognitive deficits in MCI.

The prognostic value of amyloid imaging

Mild cognitive impairment is characterized by a decline in cognitive performance without interference with activities of daily living. The amnestic subtype of mild cognitive impairment progresses to Alzheimer's disease at a rate of 10-15% per year and in the majority the neuropathology is intermediate between the neuropathological changes of typical ageing and Alzheimer's disease. Amyloid deposition occurs over a decade before the development of noticeable cognitive symptoms in a continuous process that starts in healthy elderly individuals. Newly developed PET amyloid imaging agents provide noninvasive biomarkers for the early in vivo detection of Alzheimer's pathology in healthy elderly individuals and those with mild cognitive impairment. Exclusion of amyloid pathology should allow a more accurate prognosis to be given and ensure appropriate recruitment into clinical trials testing the efficacy of new putative antiamyloid agents at an earlier disease stage. The development of (18)F-labelled amyloid imaging agents has increased the availability of this new technology for clinical and research use since they can be used in PET centres where a cyclotron and radiochemistry are not available. This review discusses the role of PET imaging for assessing the amyloid load in cognitively normal elderly subjects and subjects with mild cognitive impairment at risk of conversion to Alzheimer's disease.

Comparison of 11C-PiB and 18F-florbetaben for Aβ imaging in ageing and Alzheimer's disease

PURPOSE

Amyloid imaging with (18)F-labelled radiotracers will allow widespread use of this technique, facilitating research, diagnosis and therapeutic development for Alzheimer's disease (AD). The purpose of this analysis was to compare data on cortical Aβ deposition in subjects who had undergone both (11)C-PiB (PiB) and (18)F-florbetaben (FBB) PET imaging.

METHODS

We identified ten healthy elderly controls (HC) and ten patients with AD who had undergone PET imaging after intravenous injection of 370 MBq of PiB and 300 MBq of FBB under separate research protocols. PiB and FBB images were coregistered so that placement of regions of interest was identical on both scans and standard uptake value ratios (SUVR) using the cerebellar cortex as reference region were calculated between 40 and 70 min and between 90 and 110 min after injection for PiB and FBB, respectively.

RESULTS

Significantly higher SUVR values (p < 0.0001) in most cortical areas were observed in AD patients when compared with HC with both radiotracers. Global SUVR values in AD patients were on average 75% higher than in HC with PiB and 56% higher with FBB. There was an excellent linear correlation between PiB and FBB global SUVR values (r = 0.97, p < 0.0001) with similar effect sizes for distinguishing AD from HC subjects for both radiotracers (Cohen's d 3.3 for PiB and 3.0 for FBB).

CONCLUSION

FBB, while having a narrower dynamic range than PiB, clearly distinguished HC from AD patients, with a comparable effect size. FBB seems a suitable (18)F radiotracer for imaging AD pathology in vivo.

Amyloid imaging for Alzheimer's disease

INTRODUCTION

Biomarkers for Alzheimer's disease (AD) may facilitate early and accurate diagnosis and offer a way to track the effectiveness of new therapies. Amyloid imaging offers an unrivaled ability to visualize the presence and anatomical distribution of fibrillar beta-amyloid (Aβ) pathology non-invasively in living humans. New diagnostic criteria for AD incorporate the use of such biomarkers as amyloid imaging, highlighting the importance of this technology going forward.

AREAS COVERED

Here, the author reviews individual amyloid tracers, including the (11)C-labeled tracer Pittsburgh compound B and the (18)F-labeled tracers nearing FDA approval. While these tracers are reliable at detecting fibrillar Aβ pathology, there is a weaker connection with true AD pathology, reflecting the complex relationship between Aβ and AD. The author discusses apolipoprotein E genotyping and spinal fluid Aβ levels in relation to amyloid imaging, and considers the possible uses of amyloid imaging as a diagnostic instrument in the clinic, or as a clinical trials tool for selecting patients or monitoring outcomes. Selected literature retrieved by Medline searches for 'amyloid imaging' and related terms from 2001 to present is discussed.

EXPERT OPINION

Amyloid imaging accurately portrays the presence of fibrillar Aβ pathology in the brain. However, fundamental questions about the role of Aβ in AD must be answered before the full potential of amyloid imaging can be realized.

Amyloid imaging

Molecular neuroimaging techniques such as PET are proving valuable in the early and differential diagnosis of Alzheimer's disease (AD).With the advent of new therapeutic strategies aimed at reducing β-amyloid (Aβ) burden in the brain to potentially prevent or delay functional and irreversible cognitive loss, there is increased interest in developing agents that allow assessment of Aβ burden in vivo. Amyloid imaging with PET has proven useful in the discrimination of dementias, showing significantly higher Aβ burden in the gray matter of AD patients when compared with healthy controls or patients with frontotemporal dementia. ApoE ε4 carriers, independent of diagnosis or disease severity, present with higher Aβ burden than non-ε4 carriers. Amyloid imaging matches histopathological reports in aging and dementia, reflecting the true regional density of Aβ plaques in cortical areas. It also appears to be more sensitive than FDG-PET for the diagnosis of AD. In healthy older people there is an increasing prevalence of amyloid positive scans with age, rising from 20% in the seventh decade to 60% in the ninth decade. Of people with mild cognitive impairment (MCI), 40-60% present with detectable cortical Aβ deposition. In both groups, Aβ deposition is associated with a higher risk for cognitive decline and dementia due to AD. These observations suggest that Aβ deposition is not part of normal aging, supporting the hypothesis that it occurs well before the onset of symptoms and is likely to represent preclinical AD in asymptomatic persons and prodromal AD in MCI. Further longitudinal observations, coupled with different disease-specific tracers and biomarkers, are required to confirm this hypothesis and further elucidate the precise role of Aβ deposition in the course of AD.

Design and evaluation of a 6-mer amyloid-beta protein derived phage display library for molecular targeting of amyloid plaques in Alzheimer's disease: Comparison with two cyclic heptapeptides derived from a randomized phage display library

Amyloid plaques are the main molecular hallmark of Alzheimer's disease. Specific carriers are needed for molecular imaging and for specific drug delivery. In order to identify new low molecular weight amyloid plaque-specific ligands, the phage display technology was used to design short peptides that bind specifically to amyloid-beta protein, which is the principal component of amyloid plaques. For this purpose, a phage display library was designed from the amino acid sequence of amyloid-beta 1-42. Then, the diversity was increased by soft oligonucleotide-directed mutagenesis. This library was screened against amyloid-beta 1-42 and several phage clones were isolated. Their genomes were sequenced to identify the displayed peptides and their dissociation constants for amyloid-beta 1-42 binding were evaluated by ELISA. The two best peptides, which are derived from the C-terminus hydrophobic domain of amyloid-beta 1-42 that forms a beta-strand in amyloid fibers, were synthesized and biotinylated. After confirming their binding affinity for amyloid-beta 1-42 by ELISA, the specific interaction with amyloid plaques was validated by immunohistochemistry on brain sections harvested from a mouse model of Alzheimer's disease. The thioflavin T aggregation assay has furthermore shown that our peptides are able to inhibit the amyloid fiber formation. They are not toxic for neurons, and some of them are able to cross the blood-brain barrier after grafting to a magnetic resonance imaging contrast agent. To conclude, these peptides have high potential for molecular targeting of amyloid plaques, either as carriers of molecular imaging and therapeutic compounds or as amyloid fiber disrupting agents.

Advances in structural and molecular neuroimaging in Alzheimer's disease

Longer life expectancies lead to increases in the prevalence of age-associated illnesses. The number of Australians with dementia is predicted to rise, from 234,000 in 2009 to over 1 million by 2050, as a result of the increased prevalence of Alzheimer's disease (AD), the leading cause of dementia in the elderly. Early diagnosis of AD will become more important as disease-modifying therapies emerge within the next decade. Advances in molecular neuroimaging with amyloid-β-specific radioligands for positron emission tomography, aided by magnetic resonance imaging techniques, allow detection of AD years before symptoms of dementia develop. Longitudinal prospective studies, such as the Australian Imaging Biomarkers and Lifestyle (AIBL) study of ageing, will determine the sensitivity and specificity of these analysis techniques for diagnosing AD and predicting cognitive decline.

Assessment of mild dementia with amyloid and dopamine terminal positron emission tomography

We assessed the relationship between consensus clinical diagnostic classification and neurochemical positron emission tomography imaging of striatal vesicular monoamine transporters and cerebrocortical deposition of aβ-amyloid in mild dementia. Seventy-five subjects with mild dementia (Mini-Mental State Examination score≥18) underwent a conventional clinical evaluation followed by 11C-dihydrotetrabenazine positron emission tomography imaging of striatal vesicular monoamine transporters and 11C-Pittsburgh compound-B positron emission tomography imaging of cerebrocortical aβ-amyloid deposition. Clinical classifications were assigned by consensus of an experienced clinician panel. Neuroimaging classifications were assigned as Alzheimer's disease, frontotemporal dementia or dementia with Lewy bodies on the basis of the combined 11C-dihydrotetrabenazine and 11C-Pittsburgh compound-B results. Thirty-six subjects were classified clinically as having Alzheimer's disease, 25 as having frontotemporal dementia and 14 as having dementia with Lewy bodies. Forty-seven subjects were classified by positron emission tomography neuroimaging as having Alzheimer's disease, 15 as having dementia with Lewy bodies and 13 as having frontotemporal dementia. There was only moderate agreement between clinical consensus and neuroimaging classifications across all dementia subtypes, with discordant classifications in ∼35% of subjects (Cohen's κ=0.39). Discordant classifications were least frequent in clinical consensus Alzheimer's disease (17%), followed by dementia with Lewy bodies (29%) and were most common in frontotemporal dementia (64%). Accurate clinical classification of mild neurodegenerative dementia is challenging. Though additional post-mortem correlations are required, positron emission tomography imaging likely distinguishes subgroups corresponding to neurochemically defined pathologies. Use of these positron emission tomography imaging methods may augment clinical classifications and allow selection of more uniform subject groups in disease-modifying therapeutic trials and other prospective research involving subjects in the early stages of dementia.

Biomarkers in Alzheimer's disease drug development

Developing new therapies for Alzheimer's disease (AD) is critically important to avoid the impending public health disaster imposed by this common disorder. Means must be found to prevent, delay the onset, or slow the progression of AD. These goals will be achieved by identifying disease-modifying therapies and testing them in clinical trials. Biomarkers play an increasingly important role in AD drug development. In preclinical testing, they assist in decisions to develop an agent. Biomarkers in phase I provide insights into toxic responses and drug metabolism and in Phase II proof-of-concept trials they facilitate go/no-go decisions and dose finding. Biomarkers can play a role in identifying presymptomatic patients or specific patient subgroups. They can provide evidence of target engagement before clinical changes can be expected. Brain imaging can serve as a primary outcome in Phase II trials and as a key secondary outcome in Phase III trials. Magnetic resonance imaging is currently best positioned for use in large multicenter clinical trials. Cerebrospinal fluid (CSF) measures of amyloid beta protein (Aβ), tau protein, and hyperphosphorylated tau (p-tau) protein are sensitive and specific to the diagnosis of AD and may serve as inclusion criteria and possibly as outcomes in clinical trials targeting relevant pathways. Plasma measures of Aβ are of limited diagnostic value but may provide important information as a measure of treatment response. A wide variety of measures of detectable products of cellular processes are being developed as possible biomarkers accessible in the cerebrospinal fluid and plasma or serum. Surrogate markers that can function as outcomes in pivotal trials and reliably predict clinical outcomes are needed to facilitate primary prevention trials of asymptomatic persons where clinical measures may be of limited value. Fit-for-purpose biomarkers are increasingly available to guide AD drug development decisions.

Cognition and beta-amyloid in preclinical Alzheimer's disease: data from the AIBL study

The 'preclinical' phase of Alzheimer's disease is a future target for treatment, but additional research is essential to understand the relationship between β-amyloid burden and cognition during this time. We investigated this relationship using a large sample of apparently healthy older adults (N=177), which also enabled examination of whether the relationship differed according to age, gender, years of education, apolipoprotein E status, and the presence of subjective memory complaints. In addition to episodic memory, a range of cognitive measures (global cognition, semantic memory, visuospatial performance, and executive function) were examined. Participants were aged over 60 years with no objective cognitive impairment and came from the imaging arm of the Australian Imaging, Biomarkers, and Lifestyle (AIBL) study of ageing. (11)C-PiB PET was used to measure β-amyloid burden and a PiB 'cut-off' level of 1.5 was used to separate participants with low PiB retention from those with high PiB retention. Thirty-three percent of participants had a PiB positive scan. PiB positive participants were 5 years older, twice as likely to carry an apolipoprotein E ɛ4 allele, and their composite episodic memory was 0.26 SD worse than PiB negative volunteers. Linear regressions with β-amyloid burden as a dichotomous predictor, revealed an interaction between β-amyloid burden and gender, as well as age and education effects, in predicting episodic memory and visuospatial performance. In females, but not in males, increased β-amyloid was related to worse episodic memory and visuospatial performance. Furthermore, an interaction between β-amyloid burden and APOE status was found in predicting visuospatial performance, whereby there was a trend for increased β-amyloid to relate to worse visuospatial performance for those without an APOE ɛ4 allele. There were no other main or interaction effects of β-amyloid on any of the other composite cognitive measures. These cross-sectional findings suggest that β-amyloid burden does not have a large effect on cognition in this subset of apparently healthy older people. The finding of gender differences deserves further research to answer definitively the important question of gender susceptibility to adverse cognitive effects from β-amyloid.