Raman Imaging Reveals Accumulation of Hemoproteins in Plaques from Alzheimer's Diseased Tissues

Oligomer sensitive in-situ detection and characterization of gold colloid aggregate formations observed within the hippocampus of the Alzheimer's disease rat

In order to better understand the dynamics governing the formation of pathological oligomers leading to Alzheimer's disease (AD) in a rat model the present study examined the protein aggregates accumulating on gold colloids in the hippocampus. Sections of the hippocampus of the Long Evans Cohen's AD(+) rat model were mixed with gold colloids and the resulting aggregates were examined by Surface Enhanced Raman Scattering (SERS) imaging. Compared to AD(-) rat tissues, the AD(+) rat hippocampal tissues produced a larger sized gold colloid aggregates. The SERS spectrum of each hippocampal section exhibited similar spectral patterns in the Amide I, II, and III band regions, but showed distinct spectral patterns in the region between 300 cm-1 - 1250 cm-1 in AD(+) rat tissues, respectively. Amyloid fibrils with a β-sheet conformation were previously reported to form gold colloid aggregates in mouse and human AD brain tissues. The gold colloid aggregates in the AD (+) rat hippocampal brain sections showed distinct morphological traits compared to those observed in AD(-) rats. This suggests that there is a spatial distribution of oligomer concentration in the hippocampus, which induces fibril formation to disrupt neuronal networks within the hippocampus and between other parts of the brain.

Targeting Reactive Oxygen Species for Diagnosis of Various Diseases

Reactive oxygen species (ROS) are generated predominantly during cellular respiration and play a significant role in signaling within the cell and between cells. However, excessive accumulation of ROS can lead to cellular dysfunction, disease progression, and apoptosis that can lead to organ dysfunction. To overcome the short half-life of ROS and the relatively small amount produced, various imaging methods have been developed, using both endogenous and exogenous means to monitor ROS in disease settings. In this review, we discuss the molecular mechanisms underlying ROS production and explore the methods and materials that could be used to detect ROS overproduction, including iron-based materials, ROS-responsive chemical bond containing polymers, and ROS-responsive molecule containing biomaterials. We also discuss various imaging and imaging techniques that could be used to target and detect ROS overproduction. We discuss the ROS imaging potentials of established clinical imaging methods, such as magnetic resonance imaging (MRI), sonographic imaging, and fluorescence imaging. ROS imaging potentials of other imaging methods, such as photoacoustic imaging (PAI) and Raman imaging (RI) that are currently in preclinical stage are also discussed. Finally, this paper focuses on various diseases that are associated with ROS overproduction, and the current and the future clinical applications of ROS-targeted imaging. While the most widely used clinical condition is cardiovascular diseases, its potential extends into non-cardiovascular clinical conditions, such as neurovascular, neurodegenerative, and other ROS-associated conditions, such as cancers, skin aging, acute kidney injury, and inflammatory arthritis.

Biological Oxidations and Nitrations Promoted by the Hemin-Aβ16 Complex

Both β-amyloid (Aβ) peptides and oxidative stress conditions play key roles in Alzheimer's disease. Hemin contributes to the development of the disease as it possesses redox properties and its level increases in pathological conditions or traumatic brain injuries. The aim of this work was to deepen the investigation of the reactivity of the hemin-Aβ₁₆ complex, considering its ability to catalyze oxidation and nitration reactions. We performed kinetic studies in the presence of hydrogen peroxide and nitrite with phenolic and catechol substrates, as well as mass spectrometry studies to investigate the modifications occurring on the peptide itself. The kinetic constants were similar for oxidation and nitration reactions, and their values suggest that the hemin-Aβ₁₆ complex binds negatively charged substrates with higher affinity. Mass spectrometry studies showed that tyrosine residue is the endogenous target of nitration. Hemin degradation analysis showed that hemin bleaching is only partly prevented by the coordinated peptide. In conclusion, hemin has rich reactivity, both in oxidation and nitration reactions on aromatic substrates, that could contribute to redox equilibrium in neurons. This reactivity is modulated by the coordination of the Aβ₁₆ peptide and is only partly quenched when oxidative and nitrative conditions lead to hemin degradation.

A review: Exploring the metabolic and structural characterisation of beta pleated amyloid fibril in human tissue using Raman spectrometry and SAXS

Amyloidosis is a deleterious condition caused by abnormal amyloid fibril build-up in living tissues. To date, 42 proteins that are linked to amyloid fibrils have been discovered. Amyloid fibril structure variation can affect the severity, progression rate, or clinical symptoms of amyloidosis. Since amyloid fibril build-up is the primary pathological basis for various neurodegenerative illnesses, characterization of these deadly proteins, particularly utilising optical techniques have been a focus. Spectroscopy techniques provide significant non-invasive platforms for the investigation of the structure and conformation of amyloid fibrils, offering a wide spectrum of analyses ranging from nanometric to micrometric size scales. Even though this area of study has been intensively explored, there still remain aspects of amyloid fibrillization that are not fully known, a matter hindering progress in treating and curing amyloidosis. This review aims to provide recent updates and comprehensive information on optical techniques for metabolic and proteomic characterization of β-pleated amyloid fibrils found in human tissue with thorough literature analysis of publications. Raman spectroscopy and SAXS are well established experimental methods for study of structural properties of biomaterials. With suitable models, they offer extended information for valid proteomic analysis under physiologically relevant conditions. This review points to evidence that despite limitations, these techniques are able to provide for the necessary output and proteomics indication in order to extrapolate the aetiology of amyloid fibrils for reliable diagnostic purposes. Our metabolic database may also contribute to elucidating the nature and function of the amyloid proteome in development and clearance of amyloid diseases.

An Approach for In-Situ Detection of Gold Colloid Aggregates Amyloid Formations Within The Hippocampus of The Cohen's Alzheimer's Disease Rat Model By Surface Enhanced Raman Scattering Methods

BACKGROUND

Amyloid beta (Aβ) peptides, such as Aβ_1-40 or Aβ_1-42 are regarded as hallmark neuropathological biomarkers associated with Alzheimer's disease (AD). The formation of an aggregates by Aβ_1-40 or Aβ_1-42-coated gold nano-particles are hypothesized to contain conformation of Aβ oligomers, which could exist only at an initial stage of fibrillogenesis.

NEW METHOD

The attempt of in-situ detection of externally initiated gold colloid (ca. 80nm diameter) aggregates in the middle section of the hippocampus of the Long Evans Cohen's Alzheimer's disease rat model was conducted through the Surface Enhanced Raman Scattering (SERS) method.

RESULTS

The SERS spectral features contained modes associated with β-sheet interactions and a significant number of modes that were previously reported in SERS shifts for Alzheimer diseased rodent and human brain tissues; thereby, strongly implying a containment of amyloid fibrils. The spectral patterns were further examined and compared with those collected from in-vitro gold colloid aggregates which were formed from Aβ_1-40 - or Aβ_1-42 -coated 80nm gold colloid under pH ~4, pH ~7, and pH ~10, and the best matched datasets were found with that of the aggregates of Aβ_1-42 -coated 80nm gold colloid at ~pH 4.0. The morphology and physical size of this specific gold colloid aggregate was clearly different from those found in-vitro.

COMPARISON WITH EXISTING METHOD(S)

The amyloid fibril with a β-sheet conformation identified in previously reported in AD mouse/human brain tissues was involved in a formation of the gold colloid aggregates. However, to our surprise, best explanation for the observed SERS spectral features was possible with those in vitro Aβ_1-42 -coated 80nm gold colloid under pH ~4.

CONCLUSIONS

A formation of gold colloid aggregates was confirmed in the AD rat hippocampal brain section with unique physical morphology compared to those observed in in-vitro Aβ_1-42 or Aβ_1-40 mediated gold colloid aggregates. It was concluded that a β-sheet conformation identified in previously reported in AD mouse/human brain tissues was in volved in a formation of the gold colloid aggregates.

The Role of Heme and Copper in Alzheimer's Disease and Type 2 Diabetes Mellitus

Beyond the well-explored proposition of protein aggregation or amyloidosis as the central event in amyloidogenic diseases like Alzheimer's Disease (AD), and Type 2 Diabetes Mellitus (T2Dm); there are alternative hypotheses, now becoming increasingly evident, which suggest that the small biomolecules like redox noninnocent metals (Fe, Cu, Zn, etc.) and cofactors (Heme) have a definite influence in the onset and extent of such degenerative maladies. Dyshomeostasis of these components remains as one of the common features in both AD and T2Dm etiology. Recent advances in this course reveal that the metal/cofactor-peptide interactions and covalent binding can alarmingly enhance and modify the toxic reactivities, oxidize vital biomolecules, significantly contribute to the oxidative stress leading to cell apoptosis, and may precede the amyloid fibrils formation by altering their native folds. This perspective highlights this aspect of amyloidogenic pathology which revolves around the impact of the metals and cofactors in the pathogenic courses of AD and T2Dm including the active site environments, altered reactivities, and the probable mechanisms involving some highly reactive intermediates as well. It also discusses some in vitro metal chelation or heme sequestration strategies which might serve as a possible remedy. These findings might open up a new paradigm in our conventional understanding of amyloidogenic diseases. Moreover, the interaction of the active sites with small molecules elucidates potential biochemical reactivities that can inspire designing of drug candidates for such pathologies.

Spin state dependent peroxidase activity of heme bound amyloid β peptides relevant to Alzheimer's disease

The colocalization of heme rich deposits in the senile plaque of Aβ in the cerebral cortex of the Alzheimer's disease (AD) brain along with altered heme homeostasis and heme deficiency symptoms in AD patients has invoked the association of heme in AD pathology. Heme bound Aβ complexes, depending on the concentration of the complex or peptide to heme ratio, exhibit an equilibrium between a high-spin mono-His bound peroxidase-type active site and a low-spin bis-His bound cytochrome b type active site. The high-spin heme-Aβ complex shows higher peroxidase activity than free heme, where compound I is the reactive oxidant. It is also capable of oxidizing neurotransmitters like serotonin in the presence of peroxide, owing to the formation of compound I. The low-spin bis-His heme-Aβ complex on the other hand shows enhanced peroxidase activity relative to high-spin heme-Aβ. It reacts with H2O2 to produce two stable intermediates, compound 0 and compound I, which are characterized by absorption, EPR and resonance Raman spectroscopy. The stability of compound I of low-spin heme-Aβ is accountable for its enhanced peroxidase activity and oxidation of the neurotransmitter serotonin. The effect of the second sphere Tyr10 residue of Aβ on the formation and stability of the intermediates of low-spin heme-Aβ has also been investigated. The higher stability of compound I for low-spin heme-Aβ is likely due to H-bonding interactions involving Tyr10 in the distal pocket.

In vitro coordination of Fe-protoheme with amyloid β is non-specific and exhibits multiple equilibria

In addition to copper and zinc, heme is thought to play a role in Alzheimer's disease and its metabolism is strongly affected during the course of this disease. Amyloid β, the peptide associated with Alzheimer's disease, was shown to bind heme in vitro with potential catalytic activity linked to oxidative stress. To date, there is no direct determination of the structure of this complex. In this work, we studied the binding mode of heme to amyloid β in different conditions of pH and redox state by using isotopically labelled peptide in combination with advanced magnetic and vibrational spectroscopic methods. Our results show that the interaction between heme and amyloid β leads to a variety of species in equilibrium. The formation of these species seems to depend on many factors suggesting that the binding site is neither very strong nor highly specific. In addition, our data do not support the currently accepted model where a water molecule is bound to the ferric heme as sixth ligand. They also exclude structural models mimicking a peroxidatic site in the amyloid β-Fe-protoheme complexes.