Highly Selective Tau-SPECT Imaging Probes for Detection of Neurofibrillary Tangles in Alzheimer's Disease

Micromotor-based electrochemical immunoassays for reliable determination of amyloid-β (1-42) in Alzheimer's diagnosed clinical samples

Alzheimer's disease (AD), in addition to being the most common cause of dementia, is very difficult to diagnose, with the 42-amino acid form of Aβ (Aβ-42) being one of the main biomarkers used for this purpose. Despite the enormous efforts made in recent years, the technologies available to determine Aβ-42 in human samples require sophisticated instrumentation, present high complexity, are sample and time-consuming, and are costly, highlighting the urgent need not only to develop new tools to overcome these limitations but to provide an early detection and treatment window for AD, which is a top-challenge. In recent years, micromotor (MM) technology has proven to add a new dimension to clinical biosensing, enabling ultrasensitive detections in short times and microscale environments. To this end, here an electrochemical immunoassay based on polypyrrole (PPy)/nickel (Ni)/platinum nanoparticles (PtNPs) MM is proposed in a pioneering manner for the determination of Aβ-42 in left prefrontal cortex brain tissue, cerebrospinal fluid, and plasma samples from patients with AD. MM combines the high binding capacity of their immunorecognition external layer with self-propulsion through the catalytic generation of oxygen bubbles in the internal layer due to decomposition of hydrogen peroxide as fuel, allowing rapid bio-detection (15 min) of Aβ-42 with excellent selectivity and sensitivity (LOD = 0.06 ng/mL). The application of this disruptive technology to the analysis of just 25 μL of the three types of clinical samples provides values concordant with the clinical values reported, thus confirming the potential of the MM approach to assist in the reliable, simple, fast, and affordable diagnosis of AD by determining Aβ-42.

Proteomics analysis of prefrontal cortex of Alzheimer's disease patients revealed dysregulated proteins in the disease and novel proteins associated with amyloid-β pathology

BACKGROUND

Alzheimer's disease (AD) is a progressive, chronic, and neurodegenerative disease, and the most common cause of dementia worldwide. Currently, the mechanisms underlying the disease are far from being elucidated. Thus, the study of proteins involved in its pathogenesis would allow getting further insights into the disease and identifying new markers for AD diagnosis.

METHODS

We aimed here to analyze protein dysregulation in AD brain by quantitative proteomics to identify novel proteins associated with the disease. 10-plex TMT (tandem mass tags)-based quantitative proteomics experiments were performed using frozen tissue samples from the left prefrontal cortex of AD patients and healthy individuals and vascular dementia (VD) and frontotemporal dementia (FTD) patients as controls (CT). LC-MS/MS analyses were performed using a Q Exactive mass spectrometer.

RESULTS

In total, 3281 proteins were identified and quantified using MaxQuant. Among them, after statistical analysis with Perseus (p value < 0.05), 16 and 155 proteins were defined as upregulated and downregulated, respectively, in AD compared to CT (Healthy, FTD and VD) with an expression ratio ≥ 1.5 (upregulated) or ≤ 0.67 (downregulated). After bioinformatics analysis, ten dysregulated proteins were selected as more prone to be associated with AD, and their dysregulation in the disease was verified by qPCR, WB, immunohistochemistry (IHC), immunofluorescence (IF), pull-down, and/or ELISA, using tissue and plasma samples of AD patients, patients with other dementias, and healthy individuals.

CONCLUSIONS

We identified and validated novel AD-associated proteins in brain tissue that should be of further interest for the study of the disease. Remarkably, PMP2 and SCRN3 were found to bind to amyloid-β (Aβ) fibers in vitro, and PMP2 to associate with Aβ plaques by IF, whereas HECTD1 and SLC12A5 were identified as new potential blood-based biomarkers of the disease.

The Sensitivity of Tau Tracers for the Discrimination of Alzheimer's Disease Patients and Healthy Controls by PET

Hyperphosphorylated tau aggregates, also known as neurofibrillary tangles, are a hallmark neuropathological feature of Alzheimer's disease (AD). Molecular imaging of tau by positron emission tomography (PET) began with the development of [18F]FDDNP, an amyloid β tracer with off-target binding to tau, which obtained regional specificity through the differing distributions of amyloid β and tau in AD brains. A concerted search for more selective and affine tau PET tracers yielded compounds belonging to at least eight structural categories; 18F-flortaucipir, known variously as [18F]-T807, AV-1451, and Tauvid®, emerged as the first tau tracer approved by the American Food and Drug Administration. The various tau tracers differ concerning their selectivity over amyloid β, off-target binding at sites such as monoamine oxidase and neuromelanin, and degree of uptake in white matter. While there have been many reviews of molecular imaging of tau in AD and other conditions, there has been no systematic comparison of the fitness of the various tracers for discriminating between AD patient and healthy control (HC) groups. In this narrative review, we endeavored to compare the binding properties of the various tau tracers in vitro and the effect size (Cohen's d) for the contrast by PET between AD patients and age-matched HC groups. The available tracers all gave good discrimination, with Cohen's d generally in the range of two-three in culprit brain regions. Overall, Cohen's d was higher for AD patient groups with more severe illness. Second-generation tracers, while superior concerning off-target binding, do not have conspicuously higher sensitivity for the discrimination of AD and HC groups. We suppose that available pharmacophores may have converged on a maximal affinity for tau fibrils, which may limit the specific signal imparted in PET studies.

"Turn-On" Quinoline-Based Fluorescent Probe for Selective Imaging of Tau Aggregates in Alzheimer's Disease: Rational Design, Synthesis, and Molecular Docking

Tau aggregation is believed to have a strong association with the level of cognitive deficits in Alzheimer's disease (AD). Thus, optical brain imaging of tau aggregates has recently gained substantial attention as a promising tool for the early diagnosis of AD. However, selective imaging of tau aggregates is a major challenge due to sharing similar β-sheet structures with homologous Aβ fibrils. Herein, four quinoline-based fluorescent probes (Q-tau) were judiciously designed using the donor-acceptor architecture for selective imaging of tau aggregates. In particular, probe Q-tau 4 exhibited a strong intramolecular charge transfer and favorable photophysical profile, such as a large Stokes' shift and fluorescence emission wavelength of 630 nm in the presence of tau aggregates. The probe also displayed a "turn-on" fluorescence behavior toward tau fibrils with a 3.5-fold selectivity versus Aβ fibrils. In addition, Q-tau 4 exhibited nanomolar binding affinity to tau aggregates (K_d = 16.6 nM), which was 1.4 times higher than that for Aβ fibrils. The mechanism of "turn-on" fluorescence was proposed to be an environment-sensitive molecular rotor-like response. Moreover, ex vivo labeling of human AD brain sections demonstrated favorable colocalization of Q-tau 4 and the phosphorylated tau antibody, while comparable limited staining was observed with Aβ fibrils. Molecular docking was conducted to obtain insights into the tau-binding mode of the probe. Collectively, Q-tau 4 has successfully been used as a tau-specific fluorescent imaging agent with lower background interference.

Characterization and Optimization of Benzimidazopyrimidine and Pyridoimidazopyridine Derivatives as Tau-SPECT Probes

The accumulation of hyperphosphorylated tau protein in the brain is regarded as one of the hallmarks of Alzheimer's disease (AD). In vivo imaging of tau aggregates is helpful for diagnosis and monitoring of the progression of AD. In this study, we designed and synthesized novel radioiodinated benzimidazopyrimidine (BIPM) and pyridoimidazopyridine (PIP) derivatives with a monomethylamino, monoethylamino, monopropylamino, or diethylamino group as tau imaging probes for single-photon-emission computed tomography (SPECT). On in vitro autoradiography with AD brain sections, [¹²⁵I]PIP-NHMe showed the highest selective binding affinity for tau aggregates among the radioiodinated BIPM and PIP derivatives. In a biodistribution study using normal mice, [¹²⁵I]PIP-NHMe and [¹²⁵I]PIP-NHEt displayed high initial uptake (6.62 and 6.86% ID/g, respectively, at 2 min postinjection) into and rapid clearance from the brain, with brain_2 min/brain_30 min ratios of 38.9 and 28.6, respectively. These results suggest that [¹²³I]PIP-NHMe may be a novel SPECT probe that is useful for detecting tau aggregates in the AD brain.

Structure-Activity and Brain Kinetics Relationships of 18F-Labeled Benzimidazopyridine Derivatives as Tau PET Tracers

Noninvasive imaging of tau aggregates with a positron emission tomography (PET) tracer is useful for the diagnosis and staging of Alzheimer's disease (AD). Recently, we found that benzimidazopyridine (BIP) is an attractive scaffold for developing PET and single photon computed emission tomography tracers targeting tau aggregates. In this study, we designed and synthesized five novel 18F-labeled compounds with various substituted groups or atoms at the 7-position of the BIP scaffold. In in vitro autoradiographic studies, all 18F-labeled BIP derivatives selectively bound to tau aggregates deposited in AD brain sections. On the other hand, the initial brain uptake of these compounds was affected by the type of substituted group or halogen atom introduced into the 7-position of the BIP scaffold. Among these compounds, [18F]Me-BIPF showed the highest brain uptake (6.79% ID/g at 2 min postinjection) and 2 min/60 min ratio (3.59). These results suggest that appropriate introduction of the substituted group or atom into the 7-position of the BIP scaffold may be effective for developing useful tau PET tracers.

Fluorescent Diagnostic Probes in Neurodegenerative Diseases

Neurodegenerative diseases are debilitating disorders that feature progressive and selective loss of function or structure of anatomically or physiologically associated neuronal systems. Both chronic and acute neurodegenerative diseases are associated with high morbidity and mortality along with the death of neurons in different areas of the brain; moreover, there are few or no effective curative therapy options for treating these disorders. There is an urgent need to diagnose neurodegenerative disease as early as possible, and to distinguish between different disorders with overlapping symptoms that will help to decide the best clinical treatment. Recently, in neurodegenerative disease research, fluorescent-probe-mediated biomarker visualization techniques have been gaining increasing attention for the early diagnosis of neurodegenerative diseases. A survey of fluorescent probes for sensing and imaging biomarkers of neurodegenerative diseases is provided. These imaging probes are categorized based on the different potential biomarkers of various neurodegenerative diseases, and their advantages and disadvantages are discussed. Guides to develop new sensing strategies, recognition mechanisms, as well as the ideal features to further improve neurodegenerative disease fluorescence imaging are also explored.

The clinical utilities of multi-pinhole single photon emission computed tomography

Single photon emission computed tomography (SPECT) is an important imaging modality for various applications in nuclear medicine. The use of multi-pinhole (MPH) collimators can provide superior resolution-sensitivity trade-off when imaging small field-of-view compared to conventional parallel-hole and fan-beam collimators. Besides the very successful application in small animal imaging, there has been a resurgence of the use of MPH collimators for clinical cardiac and brain studies, as well as other small field-of-view applications. This article reviews the basic principles of MPH collimators and introduces currently available and proposed clinical MPH SPECT systems.

Structure-Activity Relationships of Radioiodinated 6,5,6-Tricyclic Compounds for the Development of Tau Imaging Probes

Tau aggregate, which is the main component of the neurofibrillary tangle, is an attractive imaging target for diagnosing and monitoring the progression of Alzheimer's disease (AD). In this study, we designed and synthesized six radioiodinated 6,5,6-tricyclic compounds to explore novel scaffolds for tau imaging probes. On in vitro autoradiography of AD brain sections, pyridoimidazopyridine and benzimidazopyrimidine derivatives ([¹²⁵I]21 and [¹²⁵I]22, respectively) showed selective binding affinity for tau aggregates, whereas carbazole, pyrrolodipyridine, and pyridoimidazopyrimidine derivatives ([¹²⁵I]10, [¹²⁵I]12, and [¹²⁵I]23, respectively) bound β-amyloid aggregates. In a biodistribution study using normal mice, [¹²⁵I]21 and [¹²⁵I]22 showed high initial uptakes (5.73 and 5.66% ID/g, respectively, at 2 min postinjection) into and rapid washout (0.14 and 0.10% ID/g, respectively, at 60 min postinjection) from the brain. Taken together, two novel scaffolds including pyridoimidazopyridine and benzimidazopyrimidine may be applied to develop useful tau imaging probes.

18F-labeled benzimidazopyridine derivatives for PET imaging of tau pathology in Alzheimer's disease

Hyperphosphorylated tau proteins are one of the neuropathological hallmarks in the Alzheimer's disease (AD) brain. The in vivo imaging of tau aggregates with nuclear medical imaging probes is helpful for the further comprehension of and medical intervention in the AD pathology. For tau-selective PET imaging, we newly designed and synthesized ¹⁸F-labeled benzimidazopyridine (BIP) derivatives with fluoroalkylamino groups, [¹⁸F]IBIPF1 and [¹⁸F]IBIPF2, and evaluated their utilities as tau imaging probes. They both bound selectively to tau against amyloid β (Aβ) aggregates in AD brain sections in vitro, and showed good pharmacokinetics in mouse brains in vivo. Notably, [¹⁸F]IBIPF1 exhibited high tau-selectivity (Tau/Aβ ratio = 34.8), high brain uptake (6.22% ID/g at 2 min postinjection), and subsequent washout (2.77% ID/g at 30 min postinjection). In vivo analysis of radiometabolites indicated that [¹⁸F]IBIPF1 was stable against metabolism in the mouse brain. These encouraging preclinical results suggest that further structural optimization based on the BIP scaffold may lead to the development of more useful tau imaging probes.

Synthesis and bioevaluation of technetium-99 m / rhenium labeled phenylquinoxaline derivatives as Tau imaging probes

Based on our previous research on the fluorinated phenylquinoxaline scaffold, in this study, different positions of N,N-dimethyl amino group, and alkyl linkers with various lengths were introduced into this scaffold to regulate their lipophilicity and binding affinity to Tau. Four novel ^99mTc/Re complexes with diethyl iminodiacetate chelator were synthesized and evaluated as Tau imaging tracers in the brain of Alzheimer's disease. Their specific binding to neurofibrillary tangles was verified by in vitro fluorescence staining and further confirmed by the results of immunofluorescence staining on the same brain sections from AD patient and Tg-tau mice. From in vitro binding assay using recombinant Tau aggregates, complex 4.2 with 6-N(CH₃)₂ and longer carbon chain (n = 4) displayed the highest affinity (K_d = 59.95 nM). [^99mTc]4.2 was achieved by the ligand exchange reaction between dicarboxylic precursor and [^99mTc(CO)₃(H₂O)₃]⁺ intermediate with radiochemical yield over 45%. Ex vivo biodistribution studies on normal ICR mice revealed that [^99mTc]4.2 exhibited moderate initial brain uptake (0.61% ID/g) and more structure optimizations are still required to improve the blood-brain barrier permeability.

Characterization of Novel 18F-Labeled Phenoxymethylpyridine Derivatives as Amylin Imaging Probes

Deposition of islet amyloid consisting of amylin constitutes one of pathological hallmarks of type 2 diabetes mellitus (T2DM), and it may be involved in the development and progression of T2DM. However, the details about the relationship between the deposition of islet amyloid and the pathology of T2DM remain unclear, since no useful imaging tracer enabling the visualization of pancreatic amylin is available. In the present study, we synthesized and evaluated six novel ¹⁸F-labeled phenoxymethylpyridine (PMP) derivatives as amylin imaging probes. All ¹⁸F-labeled PMP derivatives showed not only affinity for islet amyloid in the post-mortem T2DM pancreatic sections but also excellent pharmacokinetics in normal mice. Furthermore, ex vivo autoradiographic studies demonstrated that [¹⁸F]FPMP-5 showed intense labeling of islet amyloids in the diabetes model mouse pancreas in vivo. The preclinical studies suggested that [¹⁸F]FPMP-5 may have potential as an imaging probe that targets amylin aggregates in the T2DM pancreas.

Structure-Activity Relationships of Radioiodinated Benzoimidazopyridine Derivatives for Detection of Tau Pathology

It is generally accepted that neurofibrillary tangles consisting of tau proteins are involved in the pathogenesis of Alzheimer's disease (AD). For selective detection of tau pathology, we synthesized and evaluated radioiodinated benzoimidazopyridine (BIP) derivatives with an alkylamino group as tau imaging probes. In vitro selectivity to tau aggregates and in vivo pharmacokinetics of BIP derivatives varied markedly, being strongly dependent on the alkylamino group. In in vitro autoradiography with AD brain sections, the BIP derivative with a dimethylamino group (BIP-NMe₂) showed the highest selectivity to tau aggregates. Regarding the biodistribution using normal mice, the BIP derivative with an ethylamino group (BIP-NHEt) showed the highest uptake (6.04% ID/g at 2 min postinjection) into and rapid washout (0.12% ID/g at 60 min postinjection) from the brain. These results suggest that the introduction of an optimal alkylamino group into the BIP scaffold may lead to the development of more potential tau imaging probes.

Identification of Circulating miR-125b as a Potential Biomarker of Alzheimer's Disease in APP/PS1 Transgenic Mouse

Alzheimer's disease (AD) is the most common form of dementia, characterized by progressive decline in cognitive abilities of the affected individuals. Biological markers are essential to identify individuals at early stages of the disease for timely therapeutic intervention. Currently, pathological biomarkers are detected either through cerebrospinal fluid analysis or brain imaging, or postmortem, all of which are expensive, invasive, or time consuming. Recently, some studies have shown that circulating miR-125b, miR-181c, miR-9, miR-191-5p, miR-26b-3p, and miR-28-3p may be biomarkers of AD. However, those potential biomarkers are not validated in an AD mouse model. In the current study, we found that circulating miR-125b, miR-9, and miR-191-5p are downregulated, and miR-28-3p is upregulated in an APP/PS1 transgenic mouse model of AD. Furthermore, the correlation analysis shows a positive correlation between the expression of miR-125b and cognitive function of the APP/PS1 transgenic mouse. Moreover, we also determined that the level of serum miR-125b, miR-9, and miR-191-5p were reversed in EGCG-treated APP/PS1 transgenic mouse models. Finally, the expression of miR-125b was significantly downregulated in EGCG-treated SH-SY5Y cells.

Raman Spectrometric Detection Methods for Early and Non-Invasive Diagnosis of Alzheimer's Disease

The continuous increasing rate of patients suffering of Alzheimer's disease (AD) worldwide requires the adoption of novel techniques for non-invasive early diagnosis and monitoring of the disease. Here we review the various Raman spectroscopic techniques, including Fourier Transform-Raman spectroscopy, surface-enhanced Raman scattering spectroscopy, coherent anti-Stokes Raman scattering spectroscopy, and confocal Raman microspectroscopy, that could be used for the diagnosis of AD. These techniques have shown the potential to detect AD biomarkers, such as the amyloid-β peptide and the tau protein, or the neurotransmitters involved in the disease (e.g., Glutamate and γ-Aminobutyric acid), or the typical structural alterations in specific brain areas. The possibility to detect the specific biomarkers in liquid biopsies and to obtain high resolution 3D microscope images of the affected area make the Raman spectroscopy a valuable ally in the early diagnosis and monitoring of AD.

A Difluoroboron β-Diketonate Probe Shows "Turn-on" Near-Infrared Fluorescence Specific for Tau Fibrils

Tau aggregation in neuronal cells has recently received significant attention as a robust predictor of the progression of Alzheimer's disease (AD) because of its proven correlation with the degree of cognitive impairment in AD patients. Accordingly, noninvasive imaging of tau aggregates has been highlighted as a promising diagnostic tool for AD. We have previously identified a tau-specific "turn-on" near-infrared fluorescent (NIRF) probe (1), and, in this study, structural modification was performed to optimize its physicochemical as well as fluorescence properties. Thus, a series of fluorescent dyes (2a-2j) composed of a variously substituted difluoroboron β-diketonate and an N,N-dimethylaniline moiety linked by a length-extendable π-bridge were prepared. Among those, isobutyl-substituted difluoroboron β-ketonate with a π-conjugated 1,4-butadienyl linker (2e) showed the most promising properties as a tau-specific NIRF probe. Compared with 1, the "turn-on" fluorescence of 2e was more specific to tau fibrils, and it showed 8.8- and 6.2-times higher tau-over-Aβ and tau-over-BSA specificity, respectively. Also, the fluorescence intensity of 2e upon binding to tau fibrils was substantially higher (∼2.9 times) than that observed from 1. The mechanism for tau-specificity of 2e was investigated, which suggested that the molecular rotor-like property of 2e enables specific recognition of the microenvironment of tau aggregates to emit strong fluorescence. In transgenic cell lines stably expressing GFP-tagged tau proteins, 2e showed good colocalization with tau-GFP. Moreover, the fluorescence from 2e exhibited almost complete overlap with p-Tau antibody staining in the human AD brain tissue section. Collectively, these observations demonstrate the potential of 2e as a tau-specific fluorescent dye in both in vitro and ex vivo settings.

Current Opinion on the Role of Neurogenesis in the Therapeutic Strategies for Alzheimer Disease, Parkinson Disease, and Ischemic Stroke; Considering Neuronal Voiding Function

Neurological diseases such as Alzheimer, Parkinson, and ischemic stroke have increased in occurrence and become important health issues throughout the world. There is currently no effective therapeutic strategy for addressing neurological deficits after the development of these major neurological disorders. In recent years, it has become accepted that adult neural stem cells located in the subventricular and subgranular zones have the ability to proliferate and differentiate in order to replace lost or damaged neural cells. There have been many limitations in the clinical application of both endogenous and exogenous neurogenesis for neurological disorders. However, many studies have investigated novel mechanisms in neurogenesis and have shown that these limitations can potentially be overcome with appropriate stimulation and various approaches. We will review concepts related to possible therapeutic strategies focused on the perspective of neurogenesis for the treatment of patients diagnosed with Alzheimer disease, Parkinson disease, and ischemic stroke based on current reports.