Multimodal Imaging in Rat Model Recapitulates Alzheimer's Disease Biomarkers Abnormalities

Cryo-EM structure of Alzheimer's disease tau filaments with PET ligand MK-6240

Positron Emission Tomography (PET) ligands have advanced Alzheimer's disease (AD) diagnosis and treatment. Using autoradiography and cryo-EM, we identify AD brain tissue with elevated tau burden, purify filaments, and determine the structure of second-generation high avidity PET ligand MK-6240 at 2.31 Å resolution, which bound at a 1:1 ratio within the cleft of tau paired-helical filament (PHF), engaging with glutamine 351, lysine 353, and isoleucine 360. This information elucidates the basis of MK-6240 PET in quantifying PHF deposits in AD and may facilitate the structure-based design of superior ligands against tau amyloids.

Differential effect of an evolving amyloid and tau pathology on brain phospholipids and bioactive lipid mediators in rat models of Alzheimer-like pathology

BACKGROUND

Brain inflammation contributes significantly to the pathophysiology of Alzheimer's disease, and it is manifested by glial cell activation, increased production of cytokines/chemokines, and a shift in lipid mediators from a pro-homeostatic to a pro-inflammatory profile. However, whether the production of bioactive lipid mediators is affected at earlier stages, prior to the deposition of Aβ plaques and tau hyperphosphorylation, is unknown. The differential contribution of an evolving amyloid and tau pathology on the composition and abundance of membrane phospholipids and bioactive lipid mediators also remains unresolved.

METHODS

In this study, we examined the cortical levels of DHA- and AA-derived bioactive lipid mediators and of membrane phospholipids by liquid chromatography with tandem mass spectrometry in transgenic rat models of the Alzheimer's-like amyloid and tau pathologies at early and advanced pathological stages.

RESULTS

Our findings revealed a complex balance between pro-inflammatory and pro-resolving processes in which tau pathology has a more pronounced effect compared to amyloid pathology. At stages preceding tau misfolding and aggregation, there was an increase in pro-resolving lipid mediators (RVD6 and NPD1), DHA-containing phospholipids and IFN-γ levels. However, in advanced tau pathology displaying NFT-like inclusions, neuronal death, glial activation and cognitive deficits, there was an increase in cytokine and PGD2, PGE2, and PGF2α generation accompanied by a drop in IFN-γ levels. This pathology also resulted in a marked increase in AA-containing phospholipids. In comparison, pre-plaque amyloid pathology already presented high levels of cytokines and AA-containing phospholipids together with elevated RVD6 and NPD1 levels. Finally, Aβ plaque deposition was accompanied by a modest increase in prostaglandins, increased AA-containing phospholipids and reduced DHA-containing phospholipids.

CONCLUSIONS

Our findings suggest a dynamic trajectory of inflammatory and lipid mediators in the evolving amyloid and tau pathologies and support their differing roles on membrane properties and, consequentially, on signal transduction.

Brain PET Imaging in Small Animals: Tracer Formulation, Data Acquisition, Image Reconstruction, and Data Analysis

Positron emission tomography (PET) is a noninvasive functional imaging modality that involves in vivo detection of spatiotemporal changes in the binding of radioactive pharmaceuticals (a.k.a. PET tracers) to their target sites in different organs. The development of new PET tracers commonly involves their preclinical evaluation in small rodents. Moreover, laboratory animal PET research is now being used with progressively greater frequency to complement human PET studies, to investigate in greater depth the underlying pathophysiology of human diseases, and to monitor the efficiency of novel therapeutic interventions. Here we describe the steps toward a successful small animal PET study, from tracer formulation and image acquisition to data reconstruction and analysis of the acquired scans, with a particular focus on its utility for the brain.

Cryo-EM structure of Alzheimer's disease tau filaments with PET ligand MK-6240

Positron Emission Tomography (PET) ligands have advanced Alzheimer's disease (AD) diagnosis and treatment. Using autoradiography and cryo-EM, we identified AD brain tissue with elevated tau burden, purified filaments, and determined the structure of second-generation high avidity PET ligand MK-6240 at 2.31 Å resolution, which bound at a 1:1 ratio within the cleft of tau paired-helical filament (PHF), engaging with glutamine 351, lysine K353, and isoleucine 360. This information elucidates the basis of MK-6240 PET in quantifying PHF deposits in AD and may facilitate the structure-based design of superior ligands against tau amyloids.

Emerging concepts towards a translational framework in Alzheimer's disease

Over the past decades, significant efforts have been made to understand the precise mechanisms underlying the pathogenesis of Alzheimer's disease (AD), the most common cause of dementia. However, clinical trials targeting AD pathological hallmarks have consistently failed. Refinement of AD conceptualization, modeling, and assessment is key to developing successful therapies. Here, we review critical findings and discuss emerging ideas to integrate molecular mechanisms and clinical approaches in AD. We further propose a refined workflow for animal studies incorporating multimodal biomarkers used in clinical studies - delineating critical paths for drug discovery and translation. Addressing unresolved questions with the proposed conceptual and experimental framework may accelerate the development of effective disease-modifying strategies for AD.

Progressive human-like tauopathy with downstream neurodegeneration and neurovascular compromise in a transgenic rat model

Tauopathies, including frontotemporal dementia (FTD) and Alzheimer's disease (AD), clinically present with progressive cognitive decline and the deposition of neurofibrillary tangles (NFTs) in the brain. Neurovascular compromise is also prevalent in AD and FTD however the relationship between tau and the neurovascular unit is less understood relative to other degenerative phenotypes. Current animal models confer the ability to recapitulate aspects of the CNS tauopathies, however, existing models either display overaggressive phenotypes, or do not develop neuronal loss or genuine neurofibrillary lesions. In this report, we communicate the longitudinal characterization of brain tauopathy in a novel transgenic rat model, coded McGill-R955-hTau. The model expresses the longest isoform of human P301S tau. Homozygous R955-hTau rats displayed a robust, progressive accumulation of mutated human tau leading to the detection of tau hyperphosphorylation and cognitive deficits accelerating from 14 months of age. This model features extensive tau hyperphosphorylation with endogenous tau recruitment, authentic neurofibrillary lesions, and tau-associated neuronal loss, ventricular dilation, decreased brain volume, and gliosis in aged rats. Further, we demonstrate how neurovascular integrity becomes compromised at aged life stages using a combination of electron microscopy, injection of the tracer horseradish peroxidase and immunohistochemical approaches.

Exploitation of Quercetin's Antioxidative Properties in Potential Alternative Therapeutic Options for Neurodegenerative Diseases

Oxidative stress (OS) is a condition in which there is an excess of reactive oxygen species (ROS) in the body, which can lead to cell and tissue damage. This occurs when there is an overproduction of ROS or when the body's antioxidant defense systems are overwhelmed. Quercetin (Que) is part of a group of compounds called flavonoids. It is found in high concentrations in vegetables, fruits, and other foods. Over the past decade, a growing number of studies have highlighted the therapeutic potential of flavonoids to modulate neuronal function and prevent age-related neurodegeneration. Therefore, Que has been shown to have antioxidant, anticancer, and anti-inflammatory properties, both in vitro and in vivo. Due to its antioxidant character, Que alleviates oxidative stress, thus improving cognitive function, reducing the risk of neurodegenerative diseases. On the other hand, Que can also help support the body's natural antioxidant defense systems, thus being a potentially practical supplement for managing OS. This review focuses on experimental studies supporting the neuroprotective effects of Que in Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and epilepsy.

High-dimensional investigation of the cerebrospinal fluid to explore and monitor CNS immune responses

The cerebrospinal fluid (CSF) features a unique immune cell composition and is in constant contact with the brain borders, thus permitting insights into the brain to diagnose and monitor diseases. Recently, the meninges, which are filled with CSF, were identified as a neuroimmunological interface, highlighting the potential of exploring central nervous system (CNS) immunity by studying CNS border compartments. Here, we summarize how single-cell transcriptomics of such border compartments advance our understanding of neurological diseases, the challenges that remain, and what opportunities novel multi-omic methods offer. Single-cell transcriptomics studies have detected cytotoxic CD4+ T cells and clonally expanded T and B cells in the CSF in the autoimmune disease multiple sclerosis; clonally expanded pathogenic CD8+ T cells were found in the CSF and in the brain adjacent to β-amyloid plaques of dementia patients; in patients with brain metastases, CD8+ T cell clonotypes were shared between the brain parenchyma and the CSF and persisted after therapy. We also outline how novel multi-omic approaches permit the simultaneous measurements of gene expression, chromatin accessibility, and protein in the same cells, which remain to be explored in the CSF. This calls for multicenter initiatives to create single-cell atlases, posing challenges in integrating patients and modalities across centers. While high-dimensional analyses of CSF cells are challenging, they hold potential for personalized medicine by better resolving heterogeneous diseases and stratifying patients.

PET Imaging in Preclinical Anti-Aβ Drug Development

Positron emission tomography (PET), a medical imaging technique allowing for studies of the living human brain, has gained an important role in clinical trials of novel drugs against Alzheimer’s disease (AD). For example, PET data contributed to the conditional approval in 2021 of aducanumab, an antibody directed towards amyloid-beta (Aβ) aggregates, by showing a dose-dependent reduction in brain amyloid after treatment. In parallel to clinical studies, preclinical studies in animal models of Aβ pathology may also benefit from PET as a tool to detect target engagement and treatment effects of anti-Aβ drug candidates. PET is associated with a high level of translatability between species as similar, non-invasive protocols allow for longitudinal rather than cross-sectional studies and can be used both in a preclinical and clinical setting. This review focuses on the use of preclinical PET imaging in genetically modified animals that express human Aβ, and its present and potential future role in the development of drugs aimed at reducing brain Aβ levels as a therapeutic strategy to halt disease progression in AD.

Preclinical in vivo longitudinal assessment of KG207-M as a disease-modifying Alzheimer's disease therapeutic

In vivo biomarker abnormalities provide measures to monitor therapeutic interventions targeting amyloid-β pathology as well as its effects on downstream processes associated with Alzheimer's disease pathophysiology. Here, we applied an in vivo longitudinal study design combined with imaging and cerebrospinal fluid biomarkers, mirroring those used in human clinical trials to assess the efficacy of a novel brain-penetrating anti-amyloid fusion protein treatment in the McGill-R-Thy1-APP transgenic rat model. The bi-functional fusion protein consisted of a blood-brain barrier crossing single domain antibody (FC5) fused to an amyloid-β oligomer-binding peptide (ABP) via Fc fragment of mouse IgG (FC5-mFc2a-ABP). A five-week treatment with FC5-mFc2a-ABP (loading dose of 30 mg/Kg/iv followed by 15 mg/Kg/week/iv for four weeks) substantially reduced brain amyloid-β levels as measured by positron emission tomography and increased the cerebrospinal fluid amyloid-β_42/40 ratio. In addition, the 5-week treatment rectified the cerebrospinal fluid neurofilament light chain concentrations, resting-state functional connectivity, and hippocampal atrophy measured using magnetic resonance imaging. Finally, FC5-mFc2a-ABP (referred to as KG207-M) treatment did not induce amyloid-related imaging abnormalities such as microhemorrhage. Together, this study demonstrates the translational values of the designed preclinical studies for the assessment of novel therapies based on the clinical biomarkers providing tangible metrics for designing early-stage clinical trials.

Recent Developments in Positron Emission Tomography Tracers for Proteinopathies Imaging in Dementia

An early detection and intervention for dementia represent tremendous unmet clinical needs and priorities in society. A shared feature of neurodegenerative diseases causing dementia is the abnormal accumulation and spreading of pathological protein aggregates, which affect the selective vulnerable circuit in a disease-specific pattern. The advancement in positron emission tomography (PET) biomarkers has accelerated the understanding of the disease mechanism and development of therapeutics for Alzheimer's disease and Parkinson's disease. The clinical utility of amyloid-β PET and the clinical validity of tau PET as diagnostic biomarker for Alzheimer's disease continuum have been demonstrated. The inclusion of biomarkers in the diagnostic criteria has introduced a paradigm shift that facilitated the early and differential disease diagnosis and impacted on the clinical management. Application of disease-modifying therapy likely requires screening of patients with molecular evidence of pathological accumulation and monitoring of treatment effect assisted with biomarkers. There is currently still a gap in specific 4-repeat tau imaging probes for 4-repeat tauopathies and α-synuclein imaging probes for Parkinson's disease and dementia with Lewy body. In this review, we focused on recent development in molecular imaging biomarkers for assisting the early diagnosis of proteinopathies (i.e., amyloid-β, tau, and α-synuclein) in dementia and discussed future perspectives.

Magnetic Resonance Imaging in Animal Models of Alzheimer's Disease Amyloidosis

Amyloid-beta (Aβ) plays an important role in the pathogenesis of Alzheimer's disease. Aberrant Aβ accumulation induces neuroinflammation, cerebrovascular alterations, and synaptic deficits, leading to cognitive impairment. Animal models recapitulating the Aβ pathology, such as transgenic, knock-in mouse and rat models, have facilitated the understanding of disease mechanisms and the development of therapeutics targeting Aβ. There is a rapid advance in high-field MRI in small animals. Versatile high-field magnetic resonance imaging (MRI) sequences, such as diffusion tensor imaging, arterial spin labeling, resting-state functional MRI, anatomical MRI, and MR spectroscopy, as well as contrast agents, have been developed for preclinical imaging in animal models. These tools have enabled high-resolution in vivo structural, functional, and molecular readouts with a whole-brain field of view. MRI has been used to visualize non-invasively the Aβ deposits, synaptic deficits, regional brain atrophy, impairment in white matter integrity, functional connectivity, and cerebrovascular and glymphatic system in animal models of Alzheimer's disease amyloidosis. Many of the readouts are translational toward clinical MRI applications in patients with Alzheimer's disease. In this review, we summarize the recent advances in MRI for visualizing the pathophysiology in amyloidosis animal models. We discuss the outstanding challenges in brain imaging using MRI in small animals and propose future outlook in visualizing Aβ-related alterations in the brains of animal models.

Positron Emission Tomography in Animal Models of Alzheimer's Disease Amyloidosis: Translational Implications

Animal models of Alzheimer's disease amyloidosis that recapitulate cerebral amyloid-beta pathology have been widely used in preclinical research and have greatly enabled the mechanistic understanding of Alzheimer's disease and the development of therapeutics. Comprehensive deep phenotyping of the pathophysiological and biochemical features in these animal models is essential. Recent advances in positron emission tomography have allowed the non-invasive visualization of the alterations in the brain of animal models and in patients with Alzheimer's disease. These tools have facilitated our understanding of disease mechanisms and provided longitudinal monitoring of treatment effects in animal models of Alzheimer's disease amyloidosis. In this review, we focus on recent positron emission tomography studies of cerebral amyloid-beta accumulation, hypoglucose metabolism, synaptic and neurotransmitter receptor deficits (cholinergic and glutamatergic system), blood-brain barrier impairment, and neuroinflammation (microgliosis and astrocytosis) in animal models of Alzheimer's disease amyloidosis. We further propose the emerging targets and tracers for reflecting the pathophysiological changes and discuss outstanding challenges in disease animal models and future outlook in the on-chip characterization of imaging biomarkers towards clinical translation.

Characterization of an APP/tau rat model of Alzheimer's disease by positron emission tomography and immunofluorescent labeling

BACKGROUND

To better understand the etiology and pathomechanisms of Alzheimer's disease, several transgenic animal models that overexpress human tau or human amyloid-beta (Aβ) have been developed. In the present study, we generated a novel transgenic rat model by cross-breeding amyloid precursor protein (APP) rats with tau rats. We characterized this model by performing positron emission tomography scans combined with immunofluorescent labeling and cerebrospinal fluid analyses.

METHODS

APP/Tau rats were generated by cross-breeding male McGill-R-Thy1-APP transgenic rats with female hTau-40/P301L transgenic rats. APP/Tau double transgenic rats and non-transgenic (ntg) littermates aged 7, 13, and 21 months were subjected to dynamic [11C] PiB scan and dynamic [18F]THK-5317 scans. For regional brain analysis, a template was generated from anatomical MR images of selected animals, which was co-registered with the PET images. Regional analysis was performed by application of the simplified reference tissue model ([11C]PiB data), whereas [18F]THK-5317 data were analyzed using a 2-tissue compartment model and Logan graphical analysis. In addition, immunofluorescent labeling (tau, amyloid) and cerebrospinal fluid analyses were performed.

RESULTS

[11C]PiB binding potential (BPND) and [18F]THK-5317 volume of distribution (VT) showed an increase with age in several brain regions in the APP/Tau group but not in the ntg control group. Immunohistochemical analysis of brain slices of PET-scanned animals revealed a positive correlation between Aβ labeling and [11C]PiB regional BPND. Tau staining yielded a trend towards higher levels in the cortex and hippocampus of APP/Tau rats compared with ntg littermates, but without reaching statistical significance. No correlation was found between tau immunofluorescence labeling results and the respective [18F]THK-5317 VT values.

CONCLUSIONS

We thoroughly characterized a novel APP/Tau rat model using combined PET imaging and immunofluorescence analysis. We observed an age-related increase in [11C]PiB and [18F]THK-5317 binding in several brain regions in the APP/Tau group but not in the ntg group. Although we were able to reveal a positive correlation between amyloid labeling and [11C]PiB regional brain uptake, we observed relatively low human tau and amyloid fibril expression levels and a somewhat unstable brain pathology which questions the utility of this animal model for further studies.

Exploring the multifunctional role of melatonin in regulating autophagy and sleep to mitigate Alzheimer's disease neuropathology

Melatonin (MLT) is a neurohormone that is regulated by the circadian clock and plays multifunctional roles in numerous neurodegenerative disorders, such as Alzheimer's disease (AD). AD is the most common form of dementia and is associated with the degradation of axons and synapses resulting in memory loss and cognitive impairment. Despite extensive research, there is still no effective cure or specific treatment to prevent the progression of AD. The pathogenesis of AD involves atrophic alterations in the brain that also result in circadian alterations, sleep disruption, and autophagic dysfunction. In this scenario, MLT and autophagy play a central role in removing the misfolded protein aggregations. MLT also promotes autophagy through inhibiting methamphetamine toxicity to protect against neuronal cell death in AD brain. Besides, MLT plays critical roles as either a pro-autophagic indicator or anti-autophagic regulator depending on the phase of autophagy. MLT also has antioxidant properties that can counteract mitochondrial damage, oxidative stress, and apoptosis. Aging, a major risk factor for AD, can change sleep patterns and sleep quality, and MLT can improve sleep quality through regulating sleep cycles. The primary purpose of this review is to explore the putative mechanisms of the beneficial effects of MLT in AD patients. Furthermore, we also summarize the findings from preclinical and clinical studies on the multifunctional roles of MLT on autophagic regulation, the control of the circadian clock-associated genes, and sleep regulation.

Anti-Neuroinflammatory Potential of Polyphenols by Inhibiting NF-κB to Halt Alzheimer's Disease

Alzheimer's disease (AD) is an irrevocable chronic brain disorder featured by neuronal loss, microglial accumulation, and progressive cognitive impairment. The proper pathophysiology of this life-threatening disorder is not completely understood and no exact remedies have been found yet. Over the last few decades, research on AD has mainly highlighted pathomechanisms linked to a couple of the major pathological hallmarks, including extracellular senile plaques made of amyloid-β (Aβ) peptides, and intracellular neurofibrillary tangles (NFTs) made of tau proteins. Aβ can induce apoptosis, trigger an inflammatory response, and inhibit the synaptic plasticity of the hippocampus, which ultimately contributes to reducing cognitive functions and memory impairment. Recently, a third disease hallmark, the neuroinflammatory reaction that is mediated by cerebral innate immune cells, has become a spotlight in the current research area, assured by pre-clinical, clinical, and genetic investigations. Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), a cytokine producer, is significantly associated with physiological inflammatory proceedings and thus shows a promising candidate for inflammation- based AD therapy. Recent data reveal that phytochemicals, mainly polyphenol compounds, exhibit potential neuroprotective functions and these may be considered as a vital resource for discovering several drug candidates against AD. Interestingly, phytochemicals can easily interfere with the signaling pathway of NF-κB. This review represents the anti-neuroinflammatory potential of polyphenols as inhibitors of NF-κB to combat AD pathogenesis.

Improvement of Oxidative Stress and Mitochondrial Dysfunction by β-Caryophyllene: A Focus on the Nervous System

Mitochondrial dysfunction results in a series of defective cellular events, including decreased adenosine triphosphate (ATP) production, enhanced reactive oxygen species (ROS) output, and altered proteastasis and cellular quality control. An enhanced output of ROS may damage mitochondrial components, such as mitochondrial DNA and elements of the electron transport chain, resulting in the loss of proper electrochemical gradient across the mitochondrial inner membrane and an ensuing shutdown of mitochondrial energy production. Neurons have an increased demand for ATP and oxygen, and thus are more prone to damage induced by mitochondrial dysfunction. Mitochondrial dysfunction, damaged electron transport chains, altered membrane permeability and Ca²⁺ homeostasis, and impaired mitochondrial defense systems induced by oxidative stress, are pathological changes involved in neurodegenerative disorders. A growing body of evidence suggests that the use of antioxidants could stabilize mitochondria and thus may be suitable for preventing neuronal loss. Numerous natural products exhibit the potential to counter oxidative stress and mitochondrial dysfunction; however, science is still looking for a breakthrough in the treatment of neurodegenerative disorders. β-caryophyllene is a bicyclic sesquiterpene, and an active principle of essential oils derived from a large number of spices and food plants. As a selective cannabinoid receptor 2 (CB2) agonist, several studies have reported it as possessing numerous pharmacological activities such as antibacterial (e.g., Helicobacter pylori), antioxidant, anti-inflammatory, analgesic (e.g., neuropathic pain), anti-neurodegenerative and anticancer properties. The present review mainly focuses on the potential of β-caryophyllene in reducing oxidative stress and mitochondrial dysfunction, and its possible links with neuroprotection.

Cognitive Reserve in Model Systems for Mechanistic Discovery: The Importance of Longitudinal Studies

The goal of this review article is to provide a resource for longitudinal studies, using animal models, directed at understanding and modifying the relationship between cognition and brain structure and function throughout life. We propose that forthcoming longitudinal studies will build upon a wealth of knowledge gleaned from prior cross-sectional designs to identify early predictors of variability in cognitive function during aging, and characterize fundamental neurobiological mechanisms that underlie the vulnerability to, and the trajectory of, cognitive decline. Finally, we present examples of biological measures that may differentiate mechanisms of the cognitive reserve at the molecular, cellular, and network level.

Amyloid-beta modulates the association between neurofilament light chain and brain atrophy in Alzheimer's disease

Neurofilament light chain (NFL) measurement has been gaining strong support as a clinically useful neuronal injury biomarker for various neurodegenerative conditions. However, in Alzheimer's disease (AD), its reflection on regional neuronal injury in the context of amyloid pathology remains unclear. This study included 83 cognitively normal (CN), 160 mild cognitive impairment (MCI), and 73 AD subjects who were further classified based on amyloid-beta (Aβ) status as positive or negative (Aβ+ vs Aβ-). In addition, 13 rats (5 wild type and 8 McGill-R-Thy1-APP transgenic (Tg)) were examined. In the clinical study, reduced precuneus/posterior cingulate cortex and hippocampal grey matter density were significantly associated with increased NFL concentrations in cerebrospinal fluid (CSF) or plasma in MCI Aβ+ and AD Aβ+. Moreover, AD Aβ+ showed a significant association between the reduced grey matter density in the AD-vulnerable regions and increased NFL concentrations in CSF or plasma. Congruently, Tg rats recapitulated and validated the association between CSF NFL and grey matter density in the parietotemporal cortex, entorhinal cortex, and hippocampus in the presence of amyloid pathology. In conclusion, reduced grey matter density and elevated NFL concentrations in CSF and plasma are associated in AD-vulnerable regions in the presence of amyloid positivity in the AD clinical spectrum and amyloid Tg rat model. These findings further support the NFL as a neuronal injury biomarker in the research framework of AD biomarker classification and for the evaluation of therapeutic efficacy in clinical trials.

Utility of Animal Models to Understand Human Alzheimer's Disease, Using the Mastermind Research Approach to Avoid Unnecessary Further Sacrifices of Animals

To diagnose and treat early-stage (preclinical) Alzheimer’s disease (AD) patients, we need body-fluid-based biomarkers that reflect the processes that occur in this stage, but current knowledge on associated processes is lacking. As human studies on (possible) onset and early-stage AD would be extremely expensive and time-consuming, we investigate the potential value of animal AD models to help to fill this knowledge gap. We provide a comprehensive overview of processes associated with AD pathogenesis and biomarkers, current knowledge on AD-related biomarkers derived from on human and animal brains and body fluids, comparisons of biomarkers obtained in human AD and frequently used animal AD models, and emerging body-fluid-based biomarkers. In human studies, amyloid beta (Aβ), hyperphosphorylated tau (P-tau), total tau (T-tau), neurogranin, SNAP-25, glial fibrillary acidic protein (GFAP), YKL-40, and especially neurofilament light (NfL) are frequently measured. In animal studies, the emphasis has been mostly on Aβ. Although a direct comparison between human (familial and sporadic) AD and (mostly genetic) animal AD models cannot be made, still, in brain, cerebrospinal fluid (CSF), and blood, a majority of similar trends are observed for human AD stage and animal AD model life stage. This indicates the potential value of animal AD models in understanding of the onset and early stage of AD. Moreover, animal studies can be smartly designed to provide mechanistic information on the interrelationships between the different AD processes in a longitudinal fashion and may also include the combinations of different conditions that may reflect comorbidities in human AD, according to the Mastermind Research approach.

Differential glucose and beta-hydroxybutyrate metabolism confers an intrinsic neuroprotection to the immature brain in a rat model of neonatal hypoxia ischemia

Neonatal hypoxia ischemia (HI) is the main cause of newborn mortality and morbidity. Preclinical studies have shown that the immature rat brain is more resilient to HI injury, suggesting innate mechanisms of neuroprotection. During neonatal period brain metabolism experience changes that might greatly affect the outcome of HI injury. Therefore, the aim of the present study was to investigate how changes in brain metabolism interfere with HI outcome in different stages of CNS development. For this purpose, animals were divided into 6 groups: HIP3, HIP7 and HIP11 (HI performed at postnatal days 3, 7 and 11, respectively), and their respective shams. In vivo [¹⁸F]FDG micro positron emission tomography (microPET) imaging was performed 24 and 72 h after HI, as well as ex-vivo assessments of glucose and beta-hydroxybutyrate (BHB) oxidation. At adulthood behavioral tests and histology were performed. Behavioral and histological analysis showed greater impairments in HIP11 animals, while HIP3 rats were not affected. Changes in [¹⁸F]FDG metabolism were found only in the lesion area of HIP11, where a substantial hypometabolism was detected. Furthermore, [¹⁸F]FDG hypometabolism predicted impaired cognition and worst histological outcomes at adulthood. Finally, substrate oxidation assessments showed that glucose oxidation remained unaltered and higher level of BHB oxidation found in P3 animals, suggesting a more resilient metabolism. Overall, present results show [¹⁸F]FDG microPET predicts long-term injury outcome and suggests that higher BHB utilization is one of the mechanisms that confer the intrinsic neuroprotection to the immature brain and should be explored as a therapeutic target for treatment of HI.

Multi-target PET evaluation in APP/PS1/tau mouse model of Alzheimer's disease

Positron emission tomography (PET) has great benefits for developing therapeutics and quantifying pathological markers in neuropsychiatric disorders. This study aimed to firstly demonstrate the feasibility of PET imaging for glucagon-like peptide-1 receptor (GLP-1R) in Alzheimer's disease (AD) and evaluate the GLP-1R expression. Besides, microglial activation, dopamine D2 receptor (D2R) expression, and glucose metabolism in the brain of APP/PS1/tau transgenic model of AD (3×Tg-AD) were also investigated by PET. [¹⁸F]FBEM-Cys³⁹-exendin-4, [¹⁸F]DPA-714, [¹⁸F]fallypride, and [¹⁸F]FDG were prepared and PET imaging acquisitions for 3×Tg-AD mice and wild-type (WT) mice were performed at 15, 30, and 60 min post-injection. Fifteen regions of interest (ROIs) were selected and %ID/g was calculated. The results showed that the uptake of [¹⁸F]FBEM-Cys³⁹-exendin-4 in 10 ROIs of 3×Tg-AD mice at 60 min post-injection was significantly lower than that of WT mice (p < 0.05). Besides, 3×Tg-AD mice showed significantly higher [¹⁸F]DPA-714 uptake in 7 ROIs and lower [¹⁸F]fallypride uptake in 4 ROIs compared to WT mice. [¹⁸F]FDG PET showed no significant differences in any ROIs between the two groups. A positive correlation between the uptake of [¹⁸F]fallypride and [¹⁸F]FBEM-Cys³⁹-exendin-4 could be found in the whole brain. In summary, these results validated the feasibility of GLP-1R PET in AD and demonstrated the reduced GLP-1R and D2R expression as well as increased microglial activation caused by AD.

Longitudinal Characterization and Biomarkers of Age and Sex Differences in the Decline of Spatial Memory

The current longitudinal study examined factors (sex, physical function, response to novelty, ability to adapt to a shift in light/dark cycle, brain connectivity), which might predict the emergence of impaired memory during aging. Male and female Fisher 344 rats were tested at 6, 12, and 18 months of age. Impaired spatial memory developed in middle-age (12 months), particularly in males, and the propensity for impairment increased with advanced age. A reduced response to novelty was observed over the course of aging, which is inconsistent with cross-sectional studies. This divergence likely resulted from differences in the history of environmental enrichment/impoverishment for cross-sectional and longitudinal studies. Animals that exhibited lower level exploration of the inner region on the open field test exhibited better memory at 12 months. Furthermore, males that exhibited a longer latency to enter a novel environment at 6 months, exhibited better memory at 12 months. For females, memory at 12 months was correlated with the ability to behaviorally adapt to a shift in light/dark cycle. Functional magnetic resonance imaging of the brain, conducted at 12 months, indicated that the decline in memory was associated with altered functional connectivity within different memory systems, most notably between the hippocampus and multiple regions such as the retrosplenial cortex, thalamus, striatum, and amygdala. Overall, some factors, specifically response to novelty at an early age and the capacity to adapt to shifts in light cycle, predicted spatial memory in middle-age, and spatial memory is associated with corresponding changes in brain connectivity. We discuss similarities and differences related to previous longitudinal and cross-sectional studies, as well as the role of sex differences in providing a theoretical framework to guide future longitudinal research on the trajectory of cognitive decline. In addition to demonstrating the power of longitudinal studies, these data highlight the importance of middle-age for identifying potential predictive indicators of sexual dimorphism in the trajectory in brain and cognitive aging.

Neuroprotective Effects of Quercetin in Alzheimer's Disease

Quercetin is a flavonoid with notable pharmacological effects and promising therapeutic potential. It is widely distributed among plants and found commonly in daily diets predominantly in fruits and vegetables. Neuroprotection by quercetin has been reported in several in vitro studies. It has been shown to protect neurons from oxidative damage while reducing lipid peroxidation. In addition to its antioxidant properties, it inhibits the fibril formation of amyloid-β proteins, counteracting cell lyses and inflammatory cascade pathways. In this review, we provide a synopsis of the recent literature exploring the relationship between quercetin and cognitive performance in Alzheimer's disease and its potential as a lead compound in clinical applications.

Resting State Networks in the TgF344-AD Rat Model of Alzheimer's Disease Are Altered From Early Stages

A better and non-invasive characterization of the preclinical phases of Alzheimer's disease (AD) is important to advance its diagnosis and obtain more effective benefits from potential treatments. The TgF344-AD rat model has been well characterized and shows molecular, behavioral and brain connectivity alterations that resemble the silent period of the pathology. Our aim was to longitudinally investigate functional brain connectivity in established resting-state networks (RSNs) obtained by independent component analysis (ICA) in a cohort of TgF344-AD and control rats every 3 months, from 5 to 18 months of age, to cover different stages of the disease. Before each acquisition, working memory performance was evaluated by the delayed non match-to-sample (DNMS) task. Differences in the temporal evolution were observed between groups in the amplitude and shape of the somatosensorial and sensorimotor networks but not in the whole default mode network (DMN). Subsequent high dimensional ICA analysis showed early alterations in the anterior DMN subnetwork activity of TgF344-AD rats compared to controls. Performance of DNMS task was positively correlated with somatosensorial network at 5 months of age in the wild-type (WT) animals but not in the Tg-F344 rats. At different time points, DMN showed negative correlation with cognitive performance in the control group while in the transgenic group the correlation was positive. In addition, behavioral differences observed at 5 months of age correlated with alterations in the posterior DMN subnetwork. We have demonstrated that functional connectivity using ICA represents a useful biomarker also in animal models of AD such as the TgF344AD rats, as it allows the identification of alterations associated with the progression of the disease, detecting differences in specific networks even at very early stages.

Consequences of Metabolic Disruption in Alzheimer's Disease Pathology

Alzheimer's disease (AD) is an irreversible, progressive disease that slowly destroys cognitive function, such as thinking, remembering, and reasoning, to a level that one cannot carry out a daily living. As people live longer, the risk of developing AD has increased to 1 in 10 among people who are older than 65 and to almost 1 in 2 among those who are older than 85 according to a 2019 Alzheimer's Association report. As a most common cause of dementia, AD accounts for 60-80% of all dementia cases. AD is characterized by amyloid plaques and neurofibrillary tangles, composed of extracellular aggregates of amyloid-β peptides and intracellular aggregates of hyperphosphorylated tau, respectively. Besides plaques and tangles, AD pathology includes synaptic dysfunction including loss of synapses, inflammation, brain atrophy, and brain hypometabolism, all of which contribute to progressive cognitive decline. Recent genetic studies of sporadic cases of AD have identified a score of risk factors, as reported by Hollingworth et al. (Nat Genet 43:429-435, 2001) and Lambert et al. (Nat Genet 45:1452-1458, 2013). Of all these genes, apolipoprotein E4 (APOE4) still presents the biggest risk factor for sporadic cases of AD, as stated in Saunders et al. (Neurology 43:1467-1472, 1993): depending on whether you have 1 or 2 copies of APOE4 allele, the risk increases from 3- to 12-fold, respectively, in line with Genin et al. (Mol Psychiatry 16:903-907, 2011). Besides these genetic risk factors, having type 2 diabetes (T2D), a chronic metabolic disease, is known to increase the AD risk by at least 2-fold when these individuals age, conforming to Sims-Robinson et al. (Nat Rev Neurol 6:551-559, 2010). Diabetes is reaching a pandemic scale with over 422 million people diagnosed worldwide in 2014 according to World Health Organization. Although what proportion of these diabetic patients develop AD is not known, even if 10% of diabetic patients develop AD later in their life, it would double the number of AD patients in the world. Better understanding between T2D and AD is of paramount of importance for the future. The goal of this review is to examine our current understanding on metabolic dysfunction in AD, so that a potential target can be identified in the near future.

Neuropathological changes and cognitive deficits in rats transgenic for human mutant tau recapitulate human tauopathy

The assembly of tau protein into abnormal filaments and brain cell degeneration are characteristic of a number of human neurodegenerative diseases, including Alzheimer's disease and frontotemporal dementia and parkinsonism linked to chromosome 17. Several murine models have been generated to better understand the mechanisms contributing to tau assembly and neurodegeneration. Taking advantage of the more elaborate central nervous system and higher cognitive abilities of the rat, we generated a model expressing the longest human tau isoform (2N4R) with the P301S mutation. This transgenic rat line, R962-hTau, exhibits the main features of human tauopathies, such as: age-dependent increase in inclusions comprised of aggregated-tau, neuronal loss, global neurodegeneration as reflected by brain atrophy and ventricular dilation, alterations in astrocytic and microglial morphology, and myelin loss. In addition, substantial deficits across multiple memory and learning paradigms, including novel object recognition, fear conditioning and Morris water maze tasks, were observed at the time of advanced tauopathy. These results support the concept that progressive tauopathy correlates with brain atrophy and cognitive impairment.

Targeting Alzheimer's Disease at the Right Time and the Right Place: Validation of a Personalized Approach to Diagnosis and Treatment

Cautious optimism is appropriate for a near future (five years) time frame for a number of drugs acting on the different pathophysiological components of Alzheimer’s disease (amyloid deposition, tau hyperphosphorylation, neuroinflammation, vascular changes, to name the most important known so far). Since the relative weight of these components will be different between individuals and will even change over time for each individual, a ‘one drug fit for all’ approach is no longer defensible. Precision medicine using biomarkers in the diagnosis and treatment of Alzheimer’s disease is the new strategy.

Experimental Pharmacology in Transgenic Rodent Models of Alzheimer's Disease

This Mini Review discusses the merits and shortfalls of transgenic (tg) rodents modeling aspects of the human Alzheimer’s disease (AD) pathology and their application to evaluate experimental therapeutics. It addresses some of the differences between mouse and rat tg models for these investigations. It relates, in a condensed fashion, the experience of our research laboratory with the application of anti-inflammatory compounds and S-adenosylmethionine (SAM) at the earliest stages of AD-like amyloid pathology in tg mice. The application of SAM was intended to revert the global brain DNA hypomethylation unleashed by the intraneuronal accumulation of amyloid-β-immunoreactive material, an intervention that restored levels of DNA methylation including of the bace1 gene. This review also summarizes experimental pharmacology observations made in the McGill tg rat model of AD-like pathology by applying “nano-lithium” or a drug with allosteric M1 muscarinic and sigma 1 receptor agonistic properties (AF710B). Extremely low doses of lithium (up to 400 times lower than used in the clinic) had remarkable beneficial effects on lowering pathology and improving cognitive functions in tg rats. Likewise, AF710B treatment, even at advanced stages of the pathology, displayed remarkable beneficial effects. This drug, in experimental conditions, demonstrated possible “disease-modifying” properties as pathology was frankly diminished and cognition improved after a month of “wash-out” period. The Mini-Review ends with a discussion on the predictive value of similar experimental pharmacological interventions in current rodent tg models. It comments on the validity of some of these approaches for early interventions at preclinical stages of AD, interventions which may be envisioned once definitive diagnosis of AD before clinical presentation is made possible.

Ablation of amyloid precursor protein increases insulin-degrading enzyme levels and activity in brain and peripheral tissues

The amyloid precursor protein (APP) is a type I transmembrane glycoprotein widely studied for its role as the source of β-amyloid peptide, accumulation of which is causal in at least some cases of Alzheimer's disease (AD). APP is expressed ubiquitously and is involved in diverse biological processes. Growing bodies of evidence indicate connections between AD and somatic metabolic disorders related to type 2 diabetes, and App^-/- mice show alterations in glycemic regulation. We find that App^-/- mice have higher levels of insulin-degrading enzyme (IDE) mRNA, protein, and activity compared with wild-type controls. This regulation of IDE by APP was widespread across numerous tissues, including liver, skeletal muscle, and brain as well as cell types within neural tissue, including neurons, astrocytes, and microglia. RNA interference-mediated knockdown of APP in the SIM-A9 microglia cell line elevated IDE levels. Fasting levels of blood insulin were lower in App^-/- than App^+/+ mice, but the former showed a larger increase in response to glucose. These low basal levels may enhance peripheral insulin sensitivity, as App^-/- mice failed to develop impairment of glucose tolerance on a high-fat, high-sucrose ("Western") diet. Insulin levels and insulin signaling were also lower in the App^-/- brain; synaptosomes prepared from App^-/- hippocampus showed diminished insulin receptor phosphorylation compared with App^+/+ mice when stimulated ex vivo. These findings represent a new molecular link connecting APP to metabolic homeostasis and demonstrate a novel role for APP as an upstream regulator of IDE in vivo.

Platelets Bioenergetics Screening Reflects the Impact of Brain Aβ Plaque Accumulation in a Rat Model of Alzheimer

Alzheimer's disease (AD) is associated to depressed brain energy supply and impaired cortical and hippocampal synaptic function. It was previously reported in McGill-R-Thy1-APP transgenic (Tg(+/+)) rats that Aβ deposition per se is sufficient to cause abnormalities in glucose metabolism and neuronal connectivity. These data support the utility of this animal model as a platform for the search of novel AD biomarkers based on bioenergetic status. Recently, it has been proposed that energy dysfunction can be dynamically tested in platelets (PLTs) of nonhuman primates. PLTs are good candidates to find peripheral biomarkers for AD because they may reflect in periphery the bioenergetics deficits and the inflammatory and oxidative stress processes taking place in AD brain. In the present study, we carried out a PLTs bioenergetics screening in advanced-age (12-14 months old) control (WT) and Tg(+/+) rats. Results indicated that thrombin-activated PLTs of Tg(+/+) rats showed a significantly lower respiratory rate, as compared to that measured in WT animals, when challenged with the same dose of FCCP (an uncoupler of oxidative phosphorylation). In summary, our results provide original evidence that PLTs bioenergetic profiling may reflect brain bioenergetics dysfunction mediated by Aβ plaque accumulation. Further studies on human PLTs from control and AD patients are required to validate the usefulness of PLTs bioenergetics as a novel blood-based biomarker for AD.

A guide to using functional magnetic resonance imaging to study Alzheimer's disease in animal models

Alzheimer's disease is a leading healthcare challenge facing our society today. Functional magnetic resonance imaging (fMRI) of the brain has played an important role in our efforts to understand how Alzheimer's disease alters brain function. Using fMRI in animal models of Alzheimer's disease has the potential to provide us with a more comprehensive understanding of the observations made in human clinical fMRI studies. However, using fMRI in animal models of Alzheimer's disease presents some unique challenges. Here, we highlight some of these challenges and discuss potential solutions for researchers interested in performing fMRI in animal models. First, we briefly summarize our current understanding of Alzheimer's disease from a mechanistic standpoint. We then overview the wide array of animal models available for studying this disease and how to choose the most appropriate model to study, depending on which aspects of the condition researchers seek to investigate. Finally, we discuss the contributions of fMRI to our understanding of Alzheimer's disease and the issues to consider when designing fMRI studies for animal models, such as differences in brain activity based on anesthetic choice and ways to interrogate more specific questions in rodents beyond those that can be addressed in humans. The goal of this article is to provide information on the utility of fMRI, and approaches to consider when using fMRI, for studies of Alzheimer's disease in animal models.