Quantitative high-resolution microradiographic imaging of amyloid deposits in a novel murine model of AA amyloidosis

A transgenic mouse model reproduces human hereditary systemic amyloidosis

Amyloidoses are rare life-threatening diseases caused by protein misfolding of normally soluble proteins. The fatal outcome is predominantly due to renal failure and/or cardiac dysfunction. Because amyloid fibrils formed by all amyloidogenic proteins share structural similarity, amyloidoses may be studied in transgenic models expressing any amyloidogenic protein. Here we generated transgenic mice expressing an amyloidogenic variant of human apolipoprotein AII, a major protein of high density lipoprotein. According to amyloid nomenclature this variant was termed STOP78SERApoAII. STOP78SER-APOA2 expression at the physiological level spontaneously induced systemic amyloidosis in all mice with full-length mature STOP78SER-ApoAII identified as the amyloidogenic protein. Amyloid deposits stained with Congo red were extracellular, and consisted of fibrils of approximately 10 nm diameter. Renal glomerular amyloidosis was a major feature with onset of renal insufficiency occurring in mice older than six months of age. The liver, heart and spleen were also greatly affected. Expression of STOP78SER-APOA2 in the liver and intestine in mice of the K line but not in other amyloid-laden organs showed they present systemic amyloidosis. The amyloid burden was a function of STOP78SER-APOA2 expression and age of the mice with amyloid deposition starting in two-month-old high-expressing mice that died from six months onwards. Because STOP78SER-ApoAII conserved adequate lipid binding capacity as shown by high STOP78SER-ApoAII amounts in high density lipoprotein of young mice, its decrease in circulation with age suggests preferential deposition into preformed fibrils. Thus, our mouse model faithfully reproduces early-onset hereditary systemic amyloidosis and is ideally suited to devise and test novel therapies.

Animal models for prion-like diseases

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species being Creutzfeldt-Jacob Disease (CJD) the most representative in human beings, scrapie in ovine, Bovine Spongiform Encephalopathy (BSE) in bovine and Chronic Wasting Disease (CWD) in cervids. As stated by the "protein-only hypothesis", the causal agent of TSEs is a self-propagating aberrant form of the prion protein (PrP) that through a misfolding event acquires a β-sheet rich conformation known as PrP(Sc) (from scrapie). This isoform is neurotoxic, aggregation prone and induces misfolding of native cellular PrP. Compelling evidence indicates that disease-specific protein misfolding in amyloid deposits could be shared by other disorders showing aberrant protein aggregates such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic lateral sclerosis (ALS) and systemic Amyloid A amyloidosis (AA amyloidosis). Evidences of shared mechanisms of the proteins related to each disease with prions will be reviewed through the available in vivo models. Taking prion research as reference, typical prion-like features such as seeding and propagation ability, neurotoxic species causing disease, infectivity, transmission barrier and strain evidences will be analyzed for other protein-related diseases. Thus, prion-like features of amyloid β peptide and tau present in AD, α-synuclein in PD, SOD-1, TDP-43 and others in ALS and serum α-amyloid (SAA) in systemic AA amyloidosis will be reviewed through models available for each disease.

GATE validation of standard dual energy corrections in small animal SPECT-CT

This paper addresses 123I and 125I dual isotope SPECT imaging, which can be challenging because of spectrum overlap in the low energy spectrums of these isotopes. We first quantify the contribution of low-energy photons from each isotope using GATE-based Monte Carlo simulations for the MOBY mouse phantom. We then describe and analyze a simple, but effective method that uses the ratio of detected low and high energy 123I activity to separate the mixed low energy 123I and 125I activities. Performance is compared with correction methods used in conventional tissue biodistribution techniques. The results indicate that the spectrum overlap effects can be significantly reduced, if not entirely eliminated, when attenuation and scatter is either absent or corrected for using standard methods. In particular, we show that relative activity levels of the two isotopes can be accurately estimated for a wide range of organs and provide quantitative validation that standard methods for spectrum overlap correction provide reasonable estimates for reasonable corrections in small-animal SPECT/CT imaging.

Dynamic PET and SPECT imaging with radioiodinated, amyloid-reactive peptide p5 in mice: a positive role for peptide dehalogenation

Dynamic molecular imaging provides bio-kinetic data that is used to characterize novel radiolabeled tracers for the detection of disease. Amyloidosis is a rare protein misfolding disease that can affect many organs. It is characterized by extracellular deposits composed principally of fibrillar proteins and hypersulfated proteoglycans. We have previously described a peptide, p5, which binds preferentially to amyloid deposits in a murine model of reactive (AA) amyloidosis. We have determined the whole body distribution of amyloid by molecular imaging techniques using radioiodinated p5. The loss of radioiodide from imaging probes due to enzymatic reaction has plagued the use of radioiodinated peptides and antibodies. Therefore, we studied iodine-124-labeled p5 by using dynamic PET imaging of both amyloid-laden and healthy mice to assess the rates of amyloid binding, the relevance of dehalogenation and the fate of the radiolabeled peptide. Rates of blood pool clearance, tissue accumulation and dehalogenation of the peptide were estimated from the images. Comparisons of these properties between the amyloid-laden and healthy mice provided kinetic profiles whose differences may prove to be indicative of the disease state. Additionally, we performed longitudinal SPECT/CT imaging with iodine-125-labeled p5 up to 72h post injection to determine the stability of the radioiodinated peptide when bound to the extracellular amyloid. Our data show that amyloid-associated peptide, in contrast to the unbound peptide, is resistant to dehalogenation resulting in enhanced amyloid-specific imaging. These data further support the utility of this peptide for detecting amyloidosis and monitoring potential therapeutic strategies in patients.

Phagocyte depletion inhibits AA amyloid accumulation in AEF-induced huIL-6 transgenic mice

OBJECTIVE

Determine the role of phagocytosis in the deposition of acute phase SAA protein in peripheral organs as AA amyloid.

METHODS

AA amyloidosis was induced by injection of amyloid enhancing factor (AEF) in huIL-6 transgenic mice. Clodronate liposomes were injected at different times, and the amyloid load evaluated by Congo red birefringence staining and monitoring with the amyloid specific probe (125)I-labeled peptide p5R.

RESULTS

Injection of clodronate containing liposomes depleted Iba-1 positive and F4/80 positive phagocytic cells in liver and spleen for up to 5 days. Treatment prior to administration of intravenous AEF did not alter the pattern of deposition of the AEF in spleen, but inhibited the catabolism of the (125)I-labeled AEF. Clodronate treatment 1 day before or 1 day after AEF administration had little effect on AA amyloid accumulation at 2 weeks; however, mice treated with clodronate liposomes 5 days after AEF induction and evaluated at 2 weeks post-AEF induction showed reduced amyloid load relative to controls. At 6 weeks post-AEF there was no significant effect on amyloid load following a single clodronate treatment.

CONCLUSION

Macrophages have been shown to be instrumental in both accumulation and clearance of AA amyloid after cessation of inflammation. Our data indicate that when SAA protein is continuously present, depletion of phagocytic cells during the early course of the disease progression temporarily reduces amyloid load.

Comparative analysis of peptide p5 and serum amyloid P component for imaging AA amyloid in mice using dual-isotope SPECT

PURPOSE

I-labeled human serum amyloid P component (SAP) is used clinically only in the UK for imaging visceral amyloidosis to assist with diagnosis, disease staging, and monitoring response to therapy. We compare a new amyloid-reactive probe, peptide p5, with SAP for imaging amyloidosis.

PROCEDURES

Dual-energy SPECT/CT images were acquired of (125)I-labeled SAP and (99m)Tc-labeled p5 in mice with systemic AA amyloidosis (n = 3). Twelve organs and tissues were harvested for radiotracer biodistribution assessment and for micro-autoradiographic analysis.

RESULTS

I-SAP and (99m)Tc-p5 localized equivalently in amyloid deposits in liver (∼10% injected dose (ID)/g) whereas (125)I-SAP was twofold higher in the spleen (∼20% ID/g; (99m)Tc-p5, ∼10% ID/g). In contrast, (99m)Tc-p5 was bound to pancreatic and intestinal amyloid approximately fivefold more efficiently as evidenced in biodistribution data.

CONCLUSIONS

Radiolabeled p5 is an effective amyloid-imaging radiotracer as compared to SAP in the murine model of amyloidosis and may be rapidly translated for imaging patients with visceral amyloidosis in the USA.

SPECT imaging of peripheral amyloid in mice by targeting hyper-sulfated heparan sulfate proteoglycans with specific scFv antibodies

INTRODUCTION

Amyloid deposits are associated with a broad spectrum of disorders including monoclonal gammopathies, chronic inflammation, and Alzheimer's disease. In all cases, the amyloid pathology contains, in addition to protein fibrils, a plethora of associated molecules, including high concentrations of heparan sulfate proteoglycans (HSPGs).

METHODS

We have evaluated radioiodinated scFvs that bind HS for their ability to image amyloid deposits in vivo. scFv's with different binding characteristics were isolated by phage display using HS extracted from bovine kidney or mouse and human skeletal muscle glycosaminoglycans (GAGs). Following purification and radioiodination, the biodistribution of (125)I-scFv's was assessed in mice with inflammation-associated AA amyloidosis or in amyloid-free mice by using SPECT imaging, biodistribution measurements and tissue autoradiography.

RESULTS

Four different scFv's all showed binding in vivo to amyloid in the spleen, liver and kidney of diseased mice; however, three of the scFv's also bound to sites within these organs in disease free mice. One scFv specific for hypersulfated HSPGs preferentially bound amyloid and did not accumulate in healthy tissues.

CONCLUSIONS

These data indicate that HS expressed in amyloid deposits has unique qualities that can be distinguished from HS in normal tissues. A scFv specific for rare hypersulfated HS was used to selectively image AA amyloid in mice with minimal retention in normal tissue.

Generation and characterization of anti-AA amyloid-specific monoclonal antibodies

AA amyloidosis results from the pathologic deposition in the kidneys and other organs of fibrils composed of N-terminal fragments of serum amyloid A protein (SAA). Given that there are only limited means to visualize these deposits, we have developed a series of mAbs, 2A4, 7D8, and 8G9, that bind specifically with nanomolar affinity to a carboxy-terminal epitope generated following proteolysis of SAA that yields the predominant component of AA amyloid deposits. Notably, these antibodies do not recognize native SAA, they retain their immunoreactivity when radiolabeled with I-125 and, after injection into AA amyloidotic mice, localize, as evidenced by autoradiography and micro-single photon emission computed tomography imaging, to histologically confirmed areas of amyloid deposition; namely, spleen, liver, and pancreas. The results of our in vitro and in vivo studies demonstrate the AA fibril-selectivity of mAbs 2A4, 7D8, and 8G9 and warrant further investigation into their role as novel diagnostic agents for patients with AA amyloidosis.

The heparan sulfate motif (GlcNS6S-IdoA2S)3, common in heparin, has a strict topography and is involved in cell behavior and disease

Heparan sulfate (HS) is a structurally complex polysaccharide that interacts with a broad spectrum of extracellular effector ligands and thereby is thought to regulate a diverse array of biologic processes. The specificity of HS-ligand interactions is determined by the arrangement of sulfate groups on HS, which creates distinct binding motifs. Biologically important HS motifs are expected to exhibit regulated expression, yet there is a profound lack of tools to identify such motifs; consequently, little is known of their structures and functions. We have identified a novel phage display-derived antibody (NS4F5) that recognizes a highly regulated HS motif (HS(NS4F5)), which we have rigorously identified as (GlcNS6S-IdoA2S)(3). HS(NS4F5) exhibits a restricted expression in healthy adult tissues. Blocking HS(NS4F5) on cells in culture resulted in reduced proliferation and enhanced sensitivity to apoptosis. HS(NS4F5) is up-regulated in tumor endothelial cells, consistent with a role in endothelial cell activation. Indeed, TNF-α stimulated endothelial expression of HS(NS4F5), which contributed to leukocyte adhesion. In a mouse model of severe systemic amyloid protein A amyloidosis, HS(NS4F5) was expressed within amyloid deposits, which were successfully detected by microSPECT imaging using NS4F5 as a molecularly targeted probe. Combined, our results demonstrate that NS4F5 is a powerful tool for elucidating the biological function of HS(NS4F5) and can be exploited as a probe to detect novel polysaccharide biomarkers of disease processes.

A 3D level sets method for segmenting the mouse spleen and follicles in volumetric microCT images

We present a semi-automatic, 3D approach for segmenting the mouse spleen, and its interior follicles, in volumetric microCT imagery. Based upon previous 2D level sets work, we develop a fully 3D implementation and provide the corresponding finite difference formulas. We incorporate statistical and proximity weighting schemes to improve segmentation performance. We also note an issue with the original algorithm and propose a solution that proves beneficial in our experiments. Experimental results are provided for artificial and real data.

Amyloidogenic potential of foie gras

The human cerebral and systemic amyloidoses and prion-associated spongiform encephalopathies are acquired or inherited protein folding disorders in which normally soluble proteins or peptides are converted into fibrillar aggregates. This is a nucleation-dependent process that can be initiated or accelerated by fibril seeds formed from homologous or heterologous amyloidogenic precursors that serve as an amyloid enhancing factor (AEF) and has pathogenic significance in that disease may be transmitted by oral ingestion or parenteral administration of these conformationally altered components. Except for infected brain tissue, specific dietary sources of AEF have not been identified. Here we report that commercially available duck- or goose-derived foie gras contains birefringent congophilic fibrillar material composed of serum amyloid A-related protein that acted as a potent AEF in a transgenic murine model of secondary (amyloid A protein) amyloidosis. When such mice were injected with or fed amyloid extracted from foie gras, the animals developed extensive systemic pathological deposits. These experimental data provide evidence that an amyloid-containing food product hastened the development of amyloid protein A amyloidosis in a susceptible population. On this basis, we posit that this and perhaps other forms of amyloidosis may be transmissible, akin to the infectious nature of prion-related illnesses.

Micro-imaging of amyloid in mice

Scintigraphic imaging of radioiodinated serum amyloid P-component is a proven method for the clinical detection of peripheral amyloid deposits (Hawkins et al., 1990). However, the inability to perform comparably high-resolution studies in experimental animal models of amyloid disease has impacted not only basic studies into the pathogenesis of amyloidosis but also in the preclinical in vivo evaluation of potential anti-amyloid therapeutic agents. We have developed microimaging technologies, implemented novel computational methods, and established protocols to generate high-resolution images of amyloid deposits in mice. (125)I-labeled serum amyloid P component (SAP) and an amyloid-fibril reactive murine monoclonal antibody (designated 11-1F4) have been used successfully to acquire high-resolution single photon emission computed tomographic (SPECT) images that, when fused with x-ray computed tomographic (CT) data, have provided precise anatomical localization of secondary (AA) and primary (AL) amyloid deposits in mouse models of these diseases. This chapter will provide detailed protocols for the radioiodination and purification of amyloidophilic proteins and the generation of mouse models of AA and AL amyloidosis. A brief description of the available hardware and the parameters used to acquire high-resolution microSPECT and CT images is presented, and the tools used to perform image reconstruction and visualization that permit the analysis and presentation of image data are discussed. Finally, we provide established methods for measuring organ- and tissue-specific activities with which to corroborate the microSPECT and CT images.