A review of amyloid staining: methods and artifacts

Ex situ and in situ demonstration of amyloid fibrils for confirmation of amyloidosis using transmission electron microscopy

Confirmation of extracellular amyloid deposition across various animal species and tissue types has been a long-standing challenge in veterinary diagnostic pathology. Transmission electron microscopy (TEM) has historically been used to advance the understanding of amyloid fibril morphology and confirm amyloid fibril deposition when histologic methods provide unclear results. We assessed the feasibility of utilizing TEM for routine confirmation of amyloidosis as an addition to histology. We analyzed ex situ amyloid fibrils with direct, negative-contrast TEM and in situ amyloid fibrils with aldehyde-fixed, plastic-embedding TEM to confirm amyloidosis in a variety of cases in which amorphous extracellular amyloid deposits had been identified by H&E and Congo red staining. We compared the 2 TEM methods and documented amyloid fibril morphology and morphometry in 7 species (goat, guinea pig, kudu, fox, sheep, flamingo, and duck). Ex situ fibrils had helical morphology and widths of 15-18 nm across all species. Fibril crossover distances had more interspecies variation of 60-130 nm, and species could be grouped based on pitch (twist size). Twisting patterns of in situ fibrils could not be visualized, but in situ widths of 10 nm were measured across all species. In 4 different chicken cases, fibrils differing morphologically from amyloid were consistently detected via both TEM methods, suggesting the possibility of a non-amyloid deposit that is commonly diagnosed as amyloidosis based on its histologic appearance. When available, we recommend routine confirmation of amyloid fibril deposition by TEM.

Leveraging Optical Anisotropy of the Morpho Butterfly Wing for Quantitative, Stain-Free, and Contact-Free Assessment of Biological Tissue Microstructures

Changes in the density and organization of fibrous biological tissues often accompany the progression of serious diseases ranging from fibrosis to neurodegenerative diseases, heart disease and cancer. However, challenges in cost, complexity, or precision faced by existing imaging methodologies and materials pose barriers to elucidating the role of tissue microstructure in disease. Here, we leverage the intrinsic optical anisotropy of the Morpho butterfly wing and introduce Morpho-Enhanced Polarized Light Microscopy (MorE-PoL), a stain- and contact-free imaging platform that enhances and quantifies the birefringent material properties of fibrous biological tissues. We develop a mathematical model, based on Jones calculus, which describes fibrous tissue density and organization. As a representative example, we analyzed collagen-dense and collagen-sparse human breast cancer tissue sections and leverage our technique to assess the microstructural properties of distinct regions of interest. We compare our results with conventional Hematoxylin and Eosin (H&E) staining procedures and second harmonic generation (SHG) microscopy for fibrillar collagen detection. Our findings demonstrate that our MorE-PoL technique provides a robust, quantitative, and accessible route toward analyzing biological tissue microstructures, with great potential for application to a broad range of biological materials.

Crystal Violet Selectively Detects Aβ Oligomers but Not Fibrils In Vitro and in Alzheimer's Disease Brain Tissue

Deposition of extracellular Amyloid Beta (Aβ) and intracellular tau fibrils in post-mortem brains remains the only way to conclusively confirm cases of Alzheimer's Disease (AD). Substantial evidence, though, implicates small globular oligomers instead of fibrils as relevant biomarkers of, and critical contributors to, the clinical symptoms of AD. Efforts to verify and utilize amyloid oligomers as AD biomarkers in vivo have been limited by the near-exclusive dependence on conformation-selective antibodies for oligomer detection. While antibodies have yielded critical evidence for the role of both Aβ and tau oligomers in AD, they are not suitable for imaging amyloid oligomers in vivo. Therefore, it would be desirable to identify a set of oligomer-selective small molecules for subsequent development into Positron Emission Tomography (PET) probes. Using a kinetics-based screening assay, we confirm that the triarylmethane dye Crystal Violet (CV) is oligomer-selective for Aβ42 oligomers (AβOs) grown under near-physiological solution conditions in vitro. In postmortem brains of an AD mouse model and human AD patients, we demonstrate that A11 antibody-positive oligomers but not Thioflavin S (ThioS)-positive fibrils colocalize with CV staining, confirming in vitro results. Therefore, our kinetic screen represents a robust approach for identifying new classes of small molecules as candidates for oligomer-selective dyes (OSDs). Such OSDs, in turn, provide promising starting points for the development of PET probes for pre-mortem imaging of oligomer deposits in humans.

Development of fluorophores for the detection of oligomeric aggregates of amyloidogenic proteins found in neurodegenerative diseases

Alzheimer's disease and Parkinson's disease are the two most common neurodegenerative diseases globally. These neurodegenerative diseases have characteristic late-stage symptoms allowing for differential diagnosis; however, they both share the presence of misfolded protein aggregates which appear years before clinical manifestation. Historically, research has focused on the detection of higher-ordered aggregates (or amyloids); however, recent evidence has shown that the oligomeric state of these protein aggregates plays a greater role in disease pathology, resulting in increased efforts to detect oligomers to aid in disease diagnosis. In this review, we summarize some of the exciting new developments towards the development of fluorescent probes that can detect oligomeric aggregates of amyloidogenic proteins present in Alzheimer's and Parkinson's disease patients.

Engineering of a Fluorescent Protein for a Sensing of an Intrinsically Disordered Protein through Transition in the Chromophore State

Intrinsically disordered proteins (IDPs) not only play important roles in biological processes but are also linked with the pathogenesis of various human diseases. Specific and reliable sensing of IDPs is crucial for exploring their roles but remains elusive due to structural plasticity. Here, we present the development of a new type of fluorescent protein for the ratiometric sensing and tracking of an IDP. A β-strand of green fluorescent protein (GFP) was truncated, and the resulting GFP was further engineered to undergo the transition in the absorption maximum upon binding of a target motif within amyloid-β (Aβ) as a model IDP through rational design and directed evolution. Spectroscopic and structural analyses of the engineered truncated GFP demonstrated that a shift in the absorption maximum is driven by the change in the chromophore state from an anionic (460 nm) state into a neutral (390 nm) state as the Aβ binds, allowing a ratiometric detection of Aβ. The utility of the developed GFP was shown by the efficient and specific detection of an Aβ and the tracking of its conformational change and localization in astrocytes.

The application of alcian blue to identify astrocyte-associated amyloid plaques by using fluorescence and confocal microscopy

BACKGROUND

Astrocytes play an essential role in the normal functioning of the nervous system and are active contributors to the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). Therefore, to comprehend the astrocytes and amyloid plaques relationship there is a need for imaging techniques providing simultaneous visualization of astrocytes using fluorescence and amyloid plaques revealed by transmitted light microscopy.

NEW METHOD

The possibility of simultaneous detection of astrocytes by immunocytochemistry (fluorescent) and amyloid plaques by cytochemical Alcian Blue (transparent) using confocal microscopy in 8-month-old 5хFAD mice samples shown.

RESULTS

The described method supposes performing astrocytes fluorescent labelling by GFAP or S100beta and amyloid plaques staining by Alcian Blue.

COMPARISON WITH EXISTING METHODS

Proposed approach circumvents some limitations of fluorescence microscopy, such as weak fluorescence, low contrast, fluorophore broad excitation/emission profile and chemical instability.

CONCLUSIONS

The proposed technique provides high-quality resulting images of GFAP/s100beta- labelled astrocytes and Alcian Blue-stained amyloid plaques. These images are appliable for prospective qualitative and quantitative three-dimensional analysis due to the z-axis scanning. Moreover, it demonstrated the formation of stable Alcian Blue staining.

A guide to studying protein aggregation

Disrupted protein folding or decreased protein stability can lead to the accumulation of (partially) un- or misfolded proteins, which ultimately cause the formation of protein aggregates. Much of the interest in protein aggregation is associated with its involvement in a wide range of human diseases and the challenges it poses for large-scale biopharmaceutical manufacturing and formulation of therapeutic proteins and peptides. On the other hand, protein aggregates can also be functional, as observed in nature, which triggered its use in the development of biomaterials or therapeutics as well as for the improvement of food characteristics. Thus, unmasking the various steps involved in protein aggregation is critical to obtain a better understanding of the underlying mechanism of amyloid formation. This knowledge will allow a more tailored development of diagnostic methods and treatments for amyloid-associated diseases, as well as applications in the fields of new (bio)materials, food technology and therapeutics. However, the complex and dynamic nature of the aggregation process makes the study of protein aggregation challenging. To provide guidance on how to analyse protein aggregation, in this review we summarize the most commonly investigated aspects of protein aggregation with some popular corresponding methods.

Protein misfolding and related human diseases: A comprehensive review of toxicity, proteins involved, and current therapeutic strategies

Most of the cell's chemical reactions and structural components are facilitated by proteins. But proteins are highly dynamic molecules, where numerous modifications or changes in the cellular environment can affect their native conformational fold leading to protein aggregation. Various stress conditions, such as oxidative stress, mutations and metal toxicity may cause protein misfolding and aggregation by shifting the conformational equilibrium towards more aggregation-prone states. Most of the protein misfolding diseases (PMDs) involve aggregation of protein. We have discussed such proteins like Aβ peptide, α-synuclein, amylin and lysozyme involved in Alzheimer's, Parkinson's, type II diabetes and non-neuropathic systemic amyloidosis respectively. Till date, all advances in PMDs therapeutics help symptomatically but do not prevent the root cause of the disease, i.e., the aggregation of protein involved in the diseases. Current efforts focused on developing therapies for PMDs have employed diverse strategies; repositioning pre-existing drugs as it saves time and money; natural compounds that are touted as potential drug candidates have an advantage of being taken in diet normally and will induce lesser side effects. This review also covers recently developed therapeutic strategies like antisense drugs and disaggregases which has yielded therapeutic agents that have transitioned from preclinical studies into human clinical trials.

Abdominal fat pad biopsies exhibit good diagnostic accuracy in patients with suspected transthyretin amyloidosis

Background

The diagnostic accuracy of histopathological detection of transthyretin amyloid (ATTR) by Congo red staining of abdominal fat samples has been questioned since low sensitivity has been reported, especially for patients with ATTR cardiomyopathy. However, the outcome of surgically obtained fat pad biopsies has not yet been evaluated. The aim was to evaluate the diagnostic accuracy of skin punch biopsies from abdominal fat in patients with suspected ATTR amyloidosis.

Material and methods

Data were evaluated from patients who had undergone abdominal fat pad biopsies using a skin punch due to suspected amyloidosis from 2006 to 2015. The biopsies had been analysed using Congo red staining to determine the presence of amyloid, and immunohistochemistry or Western blot to determine the type of amyloidosis. The final diagnosis was based on the clinical picture, biopsy results and DNA sequencing. Minimum follow-up after the initial biopsy was 3 years.

Results

Two hundred seventy-four patients (61% males) were identified, and in 132 (48%), a final diagnosis of amyloidosis had been settled. The majority (93%) had been diagnosed with hereditary transthyretin (ATTRv) amyloidosis, and therefore subsequent analyses were focused on these patients. Overall, our data showed a test specificity of 99% and a sensitivity of 91%. Ninety-eight (94%) of the patients had neuropathic symptoms at diagnosis, whereas 57 (55%) had signs of amyloid cardiomyopathy. Subgroup analyses showed that patients with merely neuropathic symptoms displayed the highest test sensitivity of 91%, whereas patients with pure cardiomyopathy displayed the lowest sensitivity of 83%. However, no significant differences in sensitivity were found between patients with or without cardiomyopathy or between the sexes.

Conclusions

Abdominal fat pad biopsies exhibit good diagnostic accuracy in patients with suspect ATTRv amyloidosis, including patients presenting with cardiomyopathy. In addition, the method enables typing not only of the precursor protein but also of the amyloid fibril type, which is related to the phenotype and to the outcome of the disease.

Systemic AL Amyloidosis: Current Approaches to Diagnosis and Management

AL amyloidosis is characterized by a low-level expansion of an indolent, small plasma cell clone that produces amyloidogenic light chains. Amyloid aggregates or preceding intermediaries cause direct cell damage through their proteotoxicity, and amyloid deposits distort tissue architecture, and, eventually, lead to organ impairment. It is a rare, underdiagnosed disease with a diverse clinical presentation depending on the organ tropism of the amyloid fibrils; cardiac and renal involvement is most common, but any organ can be affected, excluding the central nervous system. A high level of awareness and a systematic approach using newly emerging screening biomarkers is required to achieve early diagnosis. Management should be multidisciplinary as supportive management tailored to management of organ dysfunction is paramount to survival and minimization of treatment-associated toxicity. The initial therapeutic aim is to rapidly eliminate the clonal plasma cell that produces the circulating amyloid precursor and achieve a complete hematologic response, and if possible with undetectable minimal residual disease as assessed by next-generation methods (flow and sequencing), with minimal toxicity. Treatment is tailored to the initial risk assessment of the patients. Treatments are based on regimens adapted from the expanding options that are available for multiple myeloma patients and hematological response rates have improved. Organ response rates are strongly associated with deeper hematologic response but usually lag behind hematological response and are also dependent on the initial organ function reserve. Agents directed against the amyloid deposits have been explored to aid amyloid clearance and improve organ function, but data are still negative.

Congo Red and amyloids: history and relationship

Staining with Congo Red (CR) is a qualitative method used for the identification of amyloids in vitro and in tissue sections. However, the drawbacks and artefacts obtained when using this dye can be found both in vitro and in vivo. Analysis of scientific data from previous studies shows that CR staining alone is not sufficient for confirmation of the amyloid nature of protein aggregates in vitro or for diagnosis of amyloidosis in tissue sections. In the present paper, we describe the characteristics and limitations of other methods used for amyloid studies. Our historical review on the use of CR staining for amyloid studies may provide insight into the pitfalls and caveats related to this technique for researchers considering using this dye.

Analysing the microenvironment of 2-p-toluidinylnaphthalene-6-sulfonate (TNS) in solvents and in different conformational states of proteins in relation to its fluorescence properties: a computational study

Characterization of different conformational states of proteins is essential to understand their stability and activity. Biophysical techniques aid in analysing these conformational states and molecular fluorescence is one of the most reliable and quickly accessible methods. Apart from the intrinsic fluorescence of proteins, external fluorescence dyes such as TNS, ANS, nile red and thioflavin are also used to characterize partially unfolded, aggregated and fibrillar states of proteins, though their exact molecular-level interactions with proteins are yet to be completely unravelled. The present study attempts to investigate the binding of TNS molecules on different conformational states of proteins. Unconstrained molecular dynamics simulation of 50 molecules of TNS with the native state of BSA, native and two partially unfolded states of RNase A and α-lactalbumin in water was carried out. Dynamics simulation of TNS alone in different solvents such as water, ethanol, DMF and DMSO was also performed. Binding environments in all the proteins and the solvents were analysed in terms of H-bonding interactions, order of contacts, amino acid specificity and conformational changes of TNS, and correlated with experimentally observed fluorescence changes of the dye. The results suggest that TNS forms aggregates in water whereas in non-aqueous solvents the order of aggregates is lower which might result in an enhancement of its fluorescence intensity. Further, TNS preferably interacts with basic and aromatic amino acid residues of the proteins. In RNase A and α-lactalbumin, most of the TNS molecules tend to form aggregates even with the unfolded conformations of the proteins. However in BSA, the number of aggregated TNS molecules is less and TNS molecules in monomeric form are found in the hydrophobic crevices of the protein. This might result in an enhancement of the fluorescence in BSA compared to the other proteins. The distributions of angles and dihedrals of TNS in different environments suggest that the bending movement between the naphthyl and tolyl rings is constrained whereas significant planar rotations could be observed both in solvents and in protein-bound states.

Imaging mass spectrometry analysis of renal amyloidosis biopsies reveals protein co-localization with amyloid deposits

Amyloidosis is a heterogeneous group of protein misfolding diseases characterized by deposition of amyloid proteins. The kidney is frequently affected, especially by immunoglobulin light chain (AL) and serum amyloid A (SAA) amyloidosis as the most common subgroups. Current diagnosis relies on histopathological examination, Congo red staining, or electron microscopy. Subtyping is done by immunohistochemistry; however, commercially available antibodies lack specificity. The purpose of this study was to identify and map amyloid proteins in formalin-fixed paraffin-embedded tissue sections using matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis in an integrated workflow. Renal amyloidosis and non-amyloidosis biopsies were processed for histological and MS analysis. Mass spectra corresponding to the congophilic areas were directly linked to the histological and MS images for correlation studies. Peptides for SAA and AL were detected by MALDI IMS associated to Congo red-positive areas. Sequence determination of amyloid peptides by LC-MS/MS analysis provided protein distribution and identification. Serum amyloid P component, apolipoprotein E, and vitronectin proteins were identified in both AA and AL amyloidosis, showing a strong correlation with Congo red-positive regions. Our findings highlight the utility of MALDI IMS as a new method to type amyloidosis in histopathological routine material and characterize amyloid-associated proteins that may provide insights into the pathogenetic process of amyloid formation.

Homozygosity for the V122I mutation in transthyretin is associated with earlier onset of cardiac amyloidosis in the African American population in the seventh decade of life

Individuals heterozygous for the V122I mutation in transthyretin (TTR) tend to develop cardiac amyloidosis, often after the seventh decade of life. Although homozygotes have been reported, these have typically been single case reports. We report a cohort of 13 V122I homozygotes. TTR gene sequencing results from the Mayo Clinic Molecular Genetics Laboratory between September 2004 and January 2013 were reviewed; 177 heterozygotes and 13 homozygotes for the V122I alteration were identified. Detailed clinical history was available for the 24 heterozygotes seen at Mayo Clinic. We compared age at onset of disease for this group to homozygotes, both alone and pooled with the 11 homozygotes from the literature. Individuals with homozygous V122I manifested symptoms a mean of 10 years earlier than heterozygotes (63.8 ± 5.7 versus 72 ± 8.1 yrs, P = 0.0002). Further, males were significantly overrepresented in both heterozygous and homozygous individuals. There was a trend for an even higher male bias in the homozygous group. All 24 homozygotes were African American, whereas four of the heterozygotes were reported as white. Two novel V122I compound heterozygotes were also identified, with clinical presentation in the late fifth or early sixth decade of life. This study is the largest homozygous V122I cohort reported and demonstrates association with earlier age at onset. It also highlights the uncertain penetrance, particularly with respect to sex.