P2-031 Amyloid deposits in transgenic PS1/APP mice do not bind the amyloid pet tracer, PIB, in the same manner as human brain amyloid

Physical and structural basis for polymorphism in amyloid fibrils

As our understanding of the molecular structures of amyloid fibrils has matured over the past 15 years, it has become clear that, while amyloid fibrils do have well-defined molecular structures, their molecular structures are not uniquely determined by the amino acid sequences of their constituent peptides and proteins. Self-propagating molecular-level polymorphism is a common phenomenon. This article reviews current information about amyloid fibril structures, variations in molecular structures that underlie amyloid polymorphism, and physical considerations that explain the development and persistence of amyloid polymorphism. Much of this information has been obtained through solid state nuclear magnetic resonance measurements. The biological significance of amyloid polymorphism is also discussed briefly. Although this article focuses primarily on studies of fibrils formed by amyloid-β peptides, the same principles apply to many amyloid-forming peptides and proteins.

Molecular polymorphism of Abeta in Alzheimer's disease

Alzheimer's disease is defined pathologically by the presence of senile plaques, which consist primarily of extracellular aggregates of fibrillar Abeta peptide, and neurofibrillary tangles, which are abnormal, intracellular bundles of fibrillar tau protein. The advent of amyloid binding agents as diagnostic imaging probes for Alzheimer's disease (AD) has made it imperative to understand at a molecular and disease level what these ligands are reporting. In addition to improving the accuracy of diagnosis, we argue that these selective ligands can serve as probes for molecular polymorphisms that may govern the pathogenicity of abnormal protein aggregates.

Imaging noradrenergic influence on amyloid pathology in mouse models of Alzheimer's disease

INTRODUCTION

Molecular imaging aims towards the non-invasive characterization of disease-specific molecular alterations in the living organism in vivo. In that, molecular imaging opens a new dimension in our understanding of disease pathogenesis, as it allows the non-invasive determination of the dynamics of changes on the molecular level. IMAGING OF AD CHARACTERISTIC CHANGES BY microPET: The imaging technology being employed includes magnetic resonance imaging (MRI) and nuclear imaging as well as optical-based imaging technologies. These imaging modalities are employed together or alone for disease phenotyping, development of imaging-guided therapeutic strategies and in basic and translational research. In this study, we review recent investigations employing positron emission tomography and MRI for phenotyping mouse models of Alzheimer's disease by imaging. We demonstrate that imaging has an important role in the characterization of mouse models of neurodegenerative diseases.

Mechanism of Aβ(1−40) Fibril-Induced Fluorescence of (trans,trans)-1-Bromo-2,5-bis(4-hydroxystyryl)benzene (K114)

K114, (trans,trans)-1-bromo-2,5-bis(4-hydroxystyryl)benzene, is a fluorescent Congo Red analogue that binds tightly to amyloid fibrils, but not the monomeric proteins, with a concomitant enhancement in fluorescence. The mechanism for the low aqueous fluorescence and the subsequent enhancement by A beta(1-40) fibrils was investigated by fluorescence spectroscopy and binding analysis. K114's unusually low buffer fluorescence is due to self-quenching in sedimentable aggregates or micelles which upon interacting with amyloid fibrils undergo an enhancement in fluorescence intensity and shifts in the excitation and emission spectra. These spectral changes are suggestive of a stabilization of the phenolate anion, perhaps by hydrogen bonding, rather than an increase in the microenvironment dielectric constant or dye immobilization. 1,4-Bis(4-aminophenylethenyl)-2-methoxybenzene, which lacks the phenol moiety, and X-34, which contains a stabilized phenol (pK approximately 13.4), do not display the phenolate anion fluorescence in the presence of fibrils. The apparent affinity of K114 for fibril binding is 20-30 nM with a stoichiometry of 2.2 mol of K114/mol of A beta(1-40) monomer. Competition studies indicate that K114 and Congo Red share a site, but K114 does not bind to sites on A beta(1-40) fibrils for neutral benzothiazole (BTA-1), cationic thioflavin T, or the hydrophobic (S)-naproxen and (R)-ibuprofen molecules. Comparison of benzothiazole binding stoichiometry which has been suggested to reflect disease-relevant amyloid structures to that of Congo Red analogues which reflect total fibril content may be useful in defining biologically pertinent conformational forms of amyloid.

Kinetic modeling of amyloid binding in humans using PET imaging and Pittsburgh Compound-B

A valid quantitative imaging method for the measurement of amyloid deposition in humans could improve Alzheimer's disease (AD) diagnosis and antiamyloid therapy assessment. Our group developed Pittsburgh Compound-B (PIB), an amyloid-binding radiotracer, for positron emission tomography (PET). The current study was aimed to further validate PIB PET through quantitative imaging (arterial input) and inclusion of subjects with mild cognitive impairment (MCI). Pittsburgh Compound-B studies were performed in five AD, five MCI, and five control subjects and five subjects were retested within 20 days. Magnetic resonance images were acquired for partial volume correction and region-of-interest definition (e.g., posterior cingulate: PCG; cerebellum: CER). Data were analyzed using compartmental and graphical approaches. Regional distribution volume (DV) values were normalized to the reference region (CER) to yield DV ratios (DVRs). Good agreement was observed between compartmental and Logan DVR values (e.g., PCG: r=0.89, slope=0.91); the Logan results were less variable. Nonspecific PIB retention was similar across subjects (n=15, Logan CER DV: 3.63+/-0.48). Greater retention was observed in AD cortical areas, relative to controls (P<0.05). The PIB retention in MCI subjects appeared either 'AD-like' or 'control-like'. The mean test/retest variation was approximately 6% in primary areas-of-interest. The Logan analysis was the method-of-choice for the PIB PET data as it proved stable, valid, and promising for future larger studies and voxel-based statistical analyses. This study also showed that it is feasible to perform quantitative PIB PET imaging studies that are needed to validate simpler methods for routine use across the AD disease spectrum.

PET imaging of brain with the β-amyloid probe, [11C]6-OH-BTA-1, in a transgenic mouse model of Alzheimer’s disease

PURPOSE

The purpose of this study was to evaluate the capacity of [11C]6-OH-BTA-1 and positron emission tomography (PET) to quantify beta-amyloid (Abeta) plaques in the Tg2576 mouse model of Alzheimer's disease (AD).

METHODS

PET imaging was performed with the NIH ATLAS small animal scanner in six elderly transgenic mice (Tg2576; age 22.0+/-1.8 months; 23.6+/-2.6 g) overexpressing a mutated form of human beta-amyloid precursor protein (APP) known to result in the production of Abeta plaques, and in six elderly wild-type litter mates (age 21.8+/-1.6 months; 29.5+/-4.7 g). Dynamic PET scans were performed for 30 min in each mouse under 1% isoflurane inhalation anesthesia after a bolus injection of 13-46 MBq of [11C]6-OH-BTA-1. PET data were reconstructed with 3D OSEM. On the coronal PET image, irregular regions of interest (ROIs) were placed on frontal cortex (FR), parietal cortex (PA), striatum (ST), thalamus (TH), pons (PO), and cerebellum (CE), guided by a mouse stereotaxic atlas. Time-activity curves (TACs) (expressed as percent injected dose per gram normalized to body weight: % ID-kg/g) were obtained for FR, PA, ST, TH, PO, and CE. ROI-to-CE radioactivity ratios were also calculated. Following PET scans, sections of mouse brain prepared from anesthetized and fixative-perfused mice were stained with thioflavin-S.

RESULTS

TACs for [11C]6-OH-BTA-1 in all ROIs peaked early (at 30-55 s), with radioactivity washing out quickly thereafter in both transgenic and wild-type mice. Peak uptake in all regions was significantly lower in transgenic mice than in wild-type mice. During the later part of the washout phase (12-30 min), the mean FR/CE and PA/CE ratios were higher in transgenic than in wild-type mice (1.06+/-0.04 vs 0.98+/-0.07, p=0.04; 1.06+/-0.09 vs 0.93+/-0.08 p=0.02) while ST/CE, TH/CE, and PO/CE ratios were not. Ex vivo staining revealed widespread Abeta plaques in cortex, but not in cerebellum of transgenic mice or in any brain regions of wild-type mice.

CONCLUSION

Marked reductions in brain uptake of this radioligand in transgenic mice may be due to reduced cerebral blood flow relative to that in wild-type mice. Specific [11C]6-OH-BTA-1 binding to Abeta plaques, if any, is probably very low, as reflected in the small FR/CE and PA/CE ratio differences. FR/CE and PA/CE ratios are considerably higher in AD patients while Abeta plaque densities in 22-month-old transgenic mice may be expected to show essentially the same density as is observed in the AD brain. This implies that the absence of tracer retention in 22-month-old transgenic mice may be due to the smaller number of Abeta plaque binding sites and/or to lower affinity of the binding sites for [11C]6-OH-BTA-1 as compared with AD patients. [11C]6-OH-BTA-1 shows excellent brain uptake in mice.

Biphenyls labeled with technetium 99m for imaging β-amyloid plaques in the brain

Formation and accumulation of excess aggregates of beta-amyloid (Abeta) plaques in the brain are critical factors contributing to the development and progression of Alzheimer's disease (AD). There is an urgent need for in vivo imaging agents that can specifically demonstrate the location and density of Abeta plaques in the brain. The aim of this study was to develop potential technetium 99m (99mTc)-labeled diagnostic imaging agents specific for the detection of Abeta plaques. Based on previously obtained Abeta plaque-specific biphenyls containing a p-N, N-dimethylaminophenyl group, a series of 99mTc and Re-N2S2-biphenyl derivatives was prepared. The stable neutral and lipophilic 99mTc complexes, [99mTc]19 and [99mTc]23, A+B, were successfully obtained. As surrogates for the 99mTc complexes, the corresponding surrogates, Re complexes of 23, were also prepared. Surprisingly, it was found that the Re complexes showed distinctively different retention profiles as compared with the corresponding 99mTc complexes. Biodistribution studies indicated that [99mTc]23A readily passed through the blood-brain barrier (1.18% dose/brain at 2 min) in contrast to the low brain penetration of [99mTc]19 (0.29% dose/brain at 2 min). Initial results suggested that [99mTc]23A showed selective binding to the Abeta plaque-like structures in the brain sections from transgenic mice but not in the postmortem human brain tissue of patients with confirmed AD. The results provide encouraging evidence that development of a 99mTc-labeled agent for imaging Abeta plaques in the brain may be feasible. Caution should be taken when comparing these 99mTc complexes with the corresponding surrogates--the Re complexes.