PET sheds light on Alzheimer's disease genetic risk

Broadening the definition of brain insulin resistance in aging and Alzheimer's disease

It has been >20 years since studies first revealed that the brain is insulin sensitive, highlighted by the expression of insulin receptors in neurons and glia, the presence of circulating brain insulin, and even localized insulin production. Following these discoveries, evidence of decreased brain insulin receptor number and function was reported in both clinical samples and animal models of aging and Alzheimer's disease, setting the stage for the hypothesis that neuronal insulin resistance may underlie memory loss in these conditions. The development of therapeutic insulin delivery to the brain using intranasal insulin administration has been shown to improve aspects of memory or learning in both humans and animal models. However, whether this approach functions by compensating for poorly signaling insulin receptors, for reduced insulin levels in the brain, or for reduced trafficking of insulin into the brain remains unclear. Direct measures of insulin's impact on cellular physiology and metabolism in the brain have been sparse in models of Alzheimer's disease, and even fewer studies have analyzed these processes in the aged brain. Nevertheless, recent evidence supports the role of brain insulin as a mediator of glucose metabolism through several means, including altering glucose transporters. Here, we provide a review of contemporary literature on brain insulin resistance, highlight the rationale for improving memory function using intranasal insulin, and describe initial results from experiments using a molecular approach to more directly measure the impact of insulin receptor activation and signaling on glucose uptake in neurons.

Hippocampal calcium dysregulation at the nexus of diabetes and brain aging

Recently it has become clear that conditions of insulin resistance/metabolic syndrome, obesity and diabetes, are linked with moderate cognitive impairment in normal aging and elevated risk of Alzheimer's disease. It appears that a common feature of these conditions is impaired insulin signaling, affecting the brain as well as peripheral target tissues. A number of studies have documented that insulin directly affects brain processes and that reduced insulin signaling results in impaired learning and memory. Several studies have also shown that diabetes induces Ca(2+) dysregulation in neurons. Because brain aging is associated with substantial Ca(2+) dyshomeostasis, it has been proposed that impaired insulin signaling exacerbates or accelerates aging-related Ca(2+) dyshomeostasis. However, there have been few studies examining insulin interactions with Ca(2+) regulation in aging animals. We have been testing predictions of the Ca(2+) dysregulation/diabetes/brain aging hypothesis and have found that insulin and insulin-sensitizers (thiazolidinediones) target several hippocampal Ca(2+)-related processes affected by aging. The drugs appear able to reduce the age-dependent increase in Ca(2+) transients and the Ca(2+) -sensitive afterhyperpolarization. Thus, while additional testing is needed, the results to date are consistent with the view that strategies that enhance insulin signaling can counteract the effect of aging on Ca(2+) dysregulation.

Evaluation of Early Dementia (Mild Cognitive Impairment)

Early diagnosis of Alzheimer disease (AD) is one of the major challenges for the prevention of this dementia. The pathologic lesions associated with AD develop many years before the clinical manifestations of the disease become evident, during a likely transitional period between normal aging and the appearance of first cognitive symptoms. AD biomarkers are needed not only to reveal these early pathologic changes but also to monitor progression in cognitive and behavioral decline and brain lesions. PET neuroimaging can reliably assess indirect and direct aspects of the molecular biology and neuropathology of AD. This article reviews the use of [18F] 2-fluoro-2-deoxy-D-glucose-PET and amyloid PET imaging in the early detection of AD.

Association of parental dementia with cognitive and brain MRI measures in middle-aged adults

OBJECTIVES

Studies of autosomal dominant Alzheimer disease (AD) have shown structural and cognitive changes in mutation carriers decades prior to clinical disease. Whether such changes are detectable in offspring of persons with sporadic dementia remains unknown. We related prospectively verified parental dementia to brain MRI and cognitive testing in the offspring, within a 2-generational community-based cohort.

METHODS

A total of 717 Framingham offspring (mean age: 59 +/- 8 years) were studied. In multivariate analyses, we compared offspring with and without verified parental dementia (and AD) for 1) performance on tests of memory, abstract reasoning, and cognitive flexibility, and 2) volumetric brain MRI measures of total cerebral brain volume (TCBV), hippocampal volume (HV), and white matter hyperintensity volume (WMHV), assessed cross-sectionally and longitudinally.

RESULTS

When testing the association of parental dementia and AD with baseline cognitive performance, we observed an interaction of parental dementia and AD with APOE epsilon4 status (p < 0.002). In APOE epsilon4 carriers only (n = 165), parental dementia was associated with poorer scores on tests of verbal memory (beta = -1.81 +/- 0.53, p < 0.001) and visuospatial memory (beta = -1.73 +/- 0.47, p < 0.001). These associations were stronger for parental AD (beta = -1.97 +/- 0.52, p < 0.001, beta = -1.95 +/- 0.48, p < 0.001), equivalent to 14-16 years of brain aging. Among APOE epsilon4 carriers, offspring of participants with dementia were also more likely to show an annual decline in TCBV in the top quartile (odds ratio = 4.67 [1.26-17.30], p = 0.02). Regardless of APOE epsilon4 status, participants with parental dementia were more likely to be in the highest quartile of decline in executive function test scores (odds ratio = 1.61 [1.02-2.53], p = 0.04).

CONCLUSIONS

Among middle-aged carriers of the APOE epsilon4 allele, parental dementia and Alzheimer disease were associated with poorer verbal and visuospatial memory and a higher rate of global brain atrophy.

Chapter 4 - Applications of nanotechnology in molecular imaging of the brain

Rapid advances in the field of nanotechnology promise revolutionary improvements in the diagnosis and therapy of neuroinflammatory disorders. An array of iron oxide nano- and microparticle agents have been developed for in vivo molecular magnetic resonance imaging (mMRI) of cerebrovascular endothelial targets, such as vascular cell adhesion molecule-1 (VCAM-1), E-selectin, and the glycoprotein receptor GP IIb/IIIa expressed on activated platelets. Molecular markers of glioma cells, such as matrix metalloproteinase-2 (MMP-2), and markers for brain tumor angiogenesis, such as alpha (v) beta (3) integrin (alpha(v)beta(3)), have also been successfully targeted using nanoparticle imaging probes. This chapter provides an overview of targeted, iron oxide nano- and microparticles that have been applied for in vivo mMRI of the brain in experimental models of multiple sclerosis (MS), brain ischemia, cerebral malaria (CM), brain cancer, and Alzheimer's disease. The potential of targeted nanoparticle agents for application in clinical imaging is also discussed, including multimodal and therapeutic approaches.

2-(2'-((Dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine for positron emission tomography imaging of serotonin transporters

INTRODUCTION

A new (18)F ligand, 2-(2'-((dimethylamino)methyl)-4'-(3-[(18)F]fluoropropoxy)-phenylthio)benzenamine ([(18)F]1), for positron emission tomography (PET) imaging of serotonin transporters (SERT) was evaluated.

METHODS

Binding affinity was determined through in vitro binding assays with LLC-PK1 cells overexpressing SERT, NET or DAT (LLC-SERT, LLC-NET and LLC-DAT) and with rat cortical homogenates. Localization and selectivity of [(18)F]1 binding in vivo were evaluated by biodistribution, autoradiography and A-PET imaging studies in rats.

RESULTS

This compound displayed excellent binding affinity for SERT in vitro with K(i)=0.33 and 0.24 nM in LLC-SERT and rat cortical homogenates, respectively. Biodistribution studies with [(18)F]1 showed good brain uptake (1.61% dose/g at 2 min postinjection), high uptake into the hypothalamus (1.22% dose/g at 30 min) and a high target-to-nontarget (hypothalamus to cerebellum) ratio of 9.66 at 180 min postinjection. Pretreatment with a SERT selective inhibitor considerably inhibited [(18)F]1 binding in biodistribution studies. Ex vivo autoradiography reveals [(18)F]1 localization to brain regions with high SERT density, and this binding was blocked by pretreatment with SERT selective inhibitors. Small animal PET (A-PET) imaging in rats provided clear images of tracer localization in the thalamus, midbrain and striatum. In A-PET chasing experiments, injecting a SERT selective inhibitor 75 min post-tracer injection causes a dramatic reduction in regional radioactivity and the target-to-nontarget ratio.

CONCLUSION

The results of the biological studies and the ease of radiosynthesis with moderately good radiochemical yield (RCY=10-35%) make [(18)F]1 an excellent candidate for SERT PET imaging.