Plaque-Associated Oligomeric Amyloid-Beta Drives Early Synaptotoxicity in APP/PS1 Mice Hippocampus: Ultrastructural Pathology Analysis

Dietary salt promotes cognitive impairment through repression of SIRT3/PINK1-mediated mitophagy and fission

Dietary salt is increasingly recognized as an independent risk factor for cognitive impairment. However, the exact mechanisms are not yet fully understood. Mitochondria, which play a crucial role in energy metabolism, are implicated in cognitive function through processes such as mitochondrial dynamics and mitophagy. While mitochondrial dysfunction is acknowledged as a significant determinant of cognitive function, the specific relationship between salt-induced cognitive impairment and mitochondrial health has yet to be fully elucidated. Here, we explored the underlying mechanism of cognitive impairment of mice and N2a cells treated with high-salt focusing on the mitochondrial homeostasis with western blotting, immunofluorescence, electron microscopy, RNA sequencing, and more. We further explored the potential role of SIRT3 in salt-induced mitochondrial dysfunction and synaptic alteration through plasmid transfection and siRNA. High salt diet significantly inhibited mitochondrial fission and blocked mitophagy, leading to dysfunctional mitochondria and impaired synaptic plasticity. Our findings demonstrated that SIRT3 not only promote mitochondrial fission by modulating phosphorylated DRP1, but also rescue mitophagy through promoting PINK1/Parkin-dependent pathway. Overall, our data for the first time indicate that mitochondrial homeostasis imbalance is a driver of impaired synaptic plasticity in a cognitive impairment phenotype that is exacerbated by a long-term high-salt diet, and highlight the protective role of SIRT3 in this process.

Correlative light and electron microscopy imaging of proteinaceous deposits in cell cultures and brain tissues

Identifying protein deposits and associated components is crucial for understanding the pathogenesis of neurodegenerative disorders with intracellular or extracellular deposits. Correlative light and electron microscopy (CLEM) has emerged as a powerful tool to accurately study tissue and cellular pathology by examination of the same target at both microstructural and ultrastructural levels. However, the technical challenges with CLEM have limited its application to neuropathology. Here, we developed a simplified efficient CLEM method and applied it to a cell model that produces a high proportion of α-synuclein (αS) inclusions with immunopositivity to phosphorylated αS and the synaptic vesicle marker SV2A and synaptophysin. This approach incorporates modifications in sample processing and innovative fiducial marking techniques, which enhance antigen preservation and improve target registration, respectively. These advancements achieve an optimal balance in sensitivity, accuracy, efficiency, and cost-effectiveness compared to current CLEM methods employing different strategies. Using this method, we identified and analyzed αS inclusions in cell cultures, as well as various pathological protein deposits in postmortem brain tissues from individuals with a range of neurodegenerative disorders. Our findings replicate recently reported new features of αS pathology and also reveal unrecognized a variety forms of small αS inclusions in human brain, which provide valuable insights into mechanisms underlying Lewy-related pathology. Application of this enhanced CLEM method is a powerful tool in research on neurodegenerative disorders, including αS-opathies.

Investigating the Impact of NMDA Receptor Organization and Biological Sex in the APPswe/PS1dE9 Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by memory loss and cognitive decline, with women being disproportionately affected in both prevalence and severity. A key feature of AD is synaptic loss, particularly around amyloid-β (Aβ) aggregates, which correlates strongly with the severity of dementia. Oligomeric Aβ is believed to be the primary driver of synaptic dysfunction by impairing excitatory neurotransmission through interactions with synaptic receptors, including N-methyl-D-aspartate (NMDA) receptors. However, the influence of sex on these synaptic changes and NMDA receptor mislocalization in AD is not well understood. This study examined potential sex-specific differences in synaptotoxicity and the role of extrasynaptic GluN2B-containing NMDA receptors in AD pathogenesis using the APP/PS1 double transgenic mouse model. Although both male and female mice showed a similar amyloid burden and cognitive impairments, synaptic alterations were slightly less severe in females, suggesting subtle sex differences in synaptic pathology. Both sexes exhibited the mislocalization of GluN2B subunits to extrasynaptic areas, which was linked to reduced PSD-95 levels and the synaptic accumulation of Aβ1-42. Intrahippocampal injections of DL-TBOA confirmed the role of extrasynaptic GluN2B-containing NMDA receptors in memory dysfunction. These findings emphasize the importance of targeting synaptic receptor trafficking to address AD-related memory deficits, potentially offering a therapeutic approach for both sexes.

Mapping cellular stress and lipid dysregulation in Alzheimer-related progressive neurodegeneration using label-free Raman microscopy

Aβ plaques are a main feature of Alzheimer's disease, and pathological alterations especially in their microenvironment have recently come into focus. However, a holistic imaging approach unveiling these changes and their biochemical nature is still lacking. In this context, we leverage confocal Raman microscopy as unbiased tool for non-destructive, label-free differentiation of progressive biomolecular changes in the Aβ plaque microenvironment in brain tissue of a murine model of cerebral amyloidosis. By developing a detailed approach, overcoming many challenges of chemical imaging, we identify spatially-resolved molecular signatures of disease-associated structures. Specifically, our study reveals nuclear condensation, indicating cellular degeneration, and increased levels of cytochrome c, showing mitochondrial dysfunction, in the vicinity of Aβ plaques. Further, we observe severe accumulation of especially unsaturated lipids. Thus, our study contributes to a comprehensive understanding of disease progression in the Aβ plaque microenvironment, underscoring the prospective of Raman imaging in neurodegenerative disorder research.

Synaptic changes in psychiatric and neurological disorders: state-of-the art of in vivo imaging

Synapses are implicated in many neuropsychiatric illnesses. Here, we provide an overview of in vivo techniques to index synaptic markers in patients. Several positron emission tomography (PET) tracers for synaptic vesicle glycoprotein 2 A (SV2A) show good reliability and selectivity. We review over 50 clinical studies including over 1700 participants, and compare findings in healthy ageing and across disorders, including addiction, schizophrenia, depression, posttraumatic stress disorder, and neurodegenerative disorders, including tauopathies, Huntington's disease and α-synucleinopathies. These show lower SV2A measures in cortical brain regions across most of these disorders relative to healthy volunteers, with the most well-replicated findings in tauopathies, whilst changes in Huntington's chorea, Parkinson's disease, corticobasal degeneration and progressive supranuclear palsy are predominantly subcortical. SV2A PET measures are correlated with functional connectivity across brain networks, and a number of other measures of brain function, including glucose metabolism. However, the majority of studies found no relationship between grey matter volume measured with magnetic resonance imaging and SV2A PET measures. Cognitive dysfunction, in domains including working memory and executive function, show replicated inverse relationships with SV2A measures across diagnoses, and initial findings also suggest transdiagnostic relationships with mood and anxiety symptoms. This suggests that synaptic abnormalities could be a common pathophysiological substrate underlying cognitive and, potentially, affective symptoms. We consider limitations of evidence and future directions; highlighting the need to develop postsynaptic imaging markers and for longitudinal studies to test causal mechanisms.

Stress and the risk of Alzheimer dementia: Can deconstructed engrams be rebuilt?

The exact neuropathological mechanism by which the dementia process unfolds is under intense scrutiny. The disease affects about 38 million people worldwide, 70% of which are clinically diagnosed with Alzheimer's disease (AD). If the destruction of synapses essential for learning, planning and decision-making is part of the problem, must the restoration of previously lost synapses be part of the solution? It is plausible that neuronal capacity to restitute information corresponds with the adaptive capacity of its connectivity reserve. A challenge will be to promote the functional connectivity that can compensate for the lost one. This will require better clarification of the remodeling of functional connectivity during the progression of AD dementia and its reversal upon experimental treatment. A major difficulty is to promote the neural pathways that are atrophied in AD dementia while suppressing others that are bolstered. Therapeutic strategies should aim at scaling functional connectivity to a just balance between the atrophic and hypertrophic systems. However, the exact factors that can help reach this objective are still unclear. Similarities between the effects of chronic stress and some neuropathological mechanisms underlying AD dementia support the idea that common components deserve prime attention as therapeutic targets.

Damaged mitochondria coincide with presynaptic vesicle loss and abnormalities in alzheimer's disease brain

Loss of synapses is the most robust pathological correlate of Alzheimer's disease (AD)-associated cognitive deficits, although the underlying mechanism remains incompletely understood. Synaptic terminals have abundant mitochondria which play an indispensable role in synaptic function through ATP provision and calcium buffering. Mitochondrial dysfunction is an early and prominent feature in AD which could contribute to synaptic deficits. Here, using electron microscopy, we examined synapses with a focus on mitochondrial deficits in presynaptic axonal terminals and dendritic spines in cortical biopsy samples from clinically diagnosed AD and age-matched non-AD control patients. Synaptic vesicle density within the presynaptic axon terminals was significantly decreased in AD cases which appeared largely due to significantly decreased reserve pool, but there were significantly more presynaptic axons containing enlarged synaptic vesicles or dense core vesicles in AD. Importantly, there was reduced number of mitochondria along with significantly increased damaged mitochondria in the presynapse of AD which correlated with changes in SV density. Mitochondria in the post-synaptic dendritic spines were also enlarged and damaged in the AD biopsy samples. This study provided evidence of presynaptic vesicle loss as synaptic deficits in AD and suggested that mitochondrial dysfunction in both pre- and post-synaptic compartments contribute to synaptic deficits in AD.

An aptamer-based magnetic resonance imaging contrast agent for detecting oligomeric amyloid-β in the brain of an Alzheimer's disease mouse model

The oligomeric amyloid-β (oAβ) is a reliable feature for an early diagnosis of Alzheimer's disease (AD). Therefore, the objective of this study was to demonstrate imaging of oAβ deposits using our developed DNA aptamer called ob5 conjugated with gadolinium (Gd)-dodecane tetraacetic acid (DOTA) as a contrast agent for early diagnosis of AD using MRI. An oAβ-specific aptamer was developed by amide bond formation and conjugated to Gd-DOTA MRI contrast agent and/or cyanine5 (cy5). We verified the performance of our new contrast agent with an AD mouse model using in vivo and ex vivo fluorescent imaging and animal MRI experiments. The presence of soluble Aβ in 3xTg AD mice was detected using GdDOTA-ob5-cy5 probe ex vivo. Fluorescence intensities of the GdDOTA-ob5-cy5 contrast agent were high in the brains of 3xTg-AD mice, but relatively low in the brains of control mice. The GdDOTA-ob5 contrast agent had higher relaxivity than a clinically available contrast agent. T1-weighted MRI signals in 5-month-old 3xTg AD mice increased at 5 min, were prolonged until 10 min, then decreased 15 min after injecting the GdDOTA-ob5 contrast agent. Our targeted DNA aptamer GdDOTA-ob5 contrast agent could be potentially useful for validating the efficacy of a novel diagnostic contrast agent for selectively targeting neurotoxic oAβ. It could ultimately be used for early diagnosis of AD.

Linking Social Cognition, Parvalbumin Interneurons, and Oxytocin in Alzheimer's Disease: An Update

Finding a cure for Alzheimer's disease (AD) has been notoriously challenging for many decades. Therefore, the current focus is mainly on prevention, timely intervention, and slowing the progression in the earliest stages. A better understanding of underlying mechanisms at the beginning of the disease could aid in early diagnosis and intervention, including alleviating symptoms or slowing down the disease progression. Changes in social cognition and progressive parvalbumin (PV) interneuron dysfunction are among the earliest observable effects of AD. Various AD rodent models mimic these early alterations, but only a narrow field of study has considered their mutual relationship. In this review, we discuss current knowledge about PV interneuron dysfunction in AD and emphasize their importance in social cognition and memory. Next, we propose oxytocin (OT) as a potent modulator of PV interneurons and as a promising treatment for managing some of the early symptoms. We further discuss the supporting evidence on its beneficial effects on AD-related pathology. Clinical trials have employed the use of OT in various neuropsychiatric diseases with promising results, but little is known about its prospective impacts on AD. On the other hand, the modulatory effects of OT in specific structures and local circuits need to be clarified in future studies. This review highlights the connection between PV interneurons and social cognition impairment in the early stages of AD and considers OT as a promising therapeutic agent for addressing these early deficits.

Ultrastructural characterization of dark microglia during aging in a mouse model of Alzheimer's disease pathology and in human post-mortem brain samples

A diverse heterogeneity of microglial cells was previously described in Alzheimer's disease (AD) pathology, including dark microglia, a state characterized by ultrastructural markers of cellular stress. To provide novel insights into the roles of dark microglia during aging in the context of AD pathology, we performed a quantitative density and ultrastructural analysis of these cells using high-throughput scanning electron microscopy in the ventral hippocampus CA1 stratum lacunosum-moleculare of 20-month-old APP-PS1 vs C57BL/6J male mice. The density of dark microglia was significantly higher in APP-PS1 vs C57BL/6J mice, with these cells accounting for nearly half of all microglia observed near amyloid-beta (Aβ) plaques. This dark microglial state interacted more with dystrophic neurites compared to other APP-PS1 microglia and possessed glycogen granules, associated with a metabolic shift toward glycolysis, which provides the first ultrastructural evidence of their presence in microglia. Dark microglia were further observed in aging human post-mortem brain samples showing similar ultrastructural features as in mouse. Overall, our results provide a quantitative ultrastructural characterization of a microglial state associated with cellular stress (i.e., dark microglia) that is primarily restricted near Aβ plaques and dystrophic neurites. The presence of this microglial state in the aging human post-mortem brain is further revealed.

Distribution and inter-regional relationship of amyloid-beta plaque deposition in a 5xFAD mouse model of Alzheimer’s disease

Alzheimer’s disease (AD) is the most common form of dementia. Although previous studies have selectively investigated the localization of amyloid-beta (Aβ) deposition in certain brain regions, a comprehensive characterization of the rostro-caudal distribution of Aβ plaques in the brain and their inter-regional correlation remain unexplored. Our results demonstrated remarkable working and spatial memory deficits in 9-month-old 5xFAD mice compared to wildtype mice. High Aβ plaque load was detected in the somatosensory cortex, piriform cortex, thalamus, and dorsal/ventral hippocampus; moderate levels of Aβ plaques were observed in the motor cortex, orbital cortex, visual cortex, and retrosplenial dysgranular cortex; and low levels of Aβ plaques were located in the amygdala, and the cerebellum; but no Aβ plaques were found in the hypothalamus, raphe nuclei, vestibular nucleus, and cuneate nucleus. Interestingly, the deposition of Aβ plaques was positively associated with brain inter-regions including the prefrontal cortex, somatosensory cortex, medial amygdala, thalamus, and the hippocampus. In conclusion, this study provides a comprehensive morphological profile of Aβ deposition in the brain and its inter-regional correlation. This suggests an association between Aβ plaque deposition and specific brain regions in AD pathogenesis.

Terminal complement pathway activation drives synaptic loss in Alzheimer's disease models

Complement is involved in developmental synaptic pruning and pathological synapse loss in Alzheimer's disease. It is posited that C1 binding initiates complement activation on synapses; C3 fragments then tag them for microglial phagocytosis. However, the precise mechanisms of complement-mediated synaptic loss remain unclear, and the role of the lytic membrane attack complex (MAC) is unexplored. We here address several knowledge gaps: (i) is complement activated through to MAC at the synapse? (ii) does MAC contribute to synaptic loss? (iii) can MAC inhibition prevent synaptic loss? Novel methods were developed and optimised to quantify C1q, C3 fragments and MAC in total and regional brain homogenates and synaptoneurosomes from WT and AppNL-G-F Alzheimer's disease model mouse brains at 3, 6, 9 and 12 months of age. The impact on synapse loss of systemic treatment with a MAC blocking antibody and gene knockout of a MAC component was assessed in Alzheimer's disease model mice. A significant increase in C1q, C3 fragments and MAC was observed in AppNL-G-F mice compared to controls, increasing with age and severity. Administration of anti-C7 antibody to AppNL-G-F mice modulated synapse loss, reflected by the density of dendritic spines in the vicinity of plaques. Constitutive knockout of C6 significantly reduced synapse loss in 3xTg-AD mice. We demonstrate that complement dysregulation occurs in Alzheimer's disease mice involving the activation (C1q; C3b/iC3b) and terminal (MAC) pathways in brain areas associated with pathology. Inhibition or ablation of MAC formation reduced synapse loss in two Alzheimer's disease mouse models, demonstrating that MAC formation is a driver of synapse loss. We suggest that MAC directly damages synapses, analogous to neuromuscular junction destruction in myasthenia gravis.

Loss of glucocorticoid receptor phosphorylation contributes to cognitive and neurocentric damages of the amyloid-β pathway

Aberrant cortisol and activation of the glucocorticoid receptor (GR) play an essential role in age-related progression of Alzheimer's disease (AD). However, the GR pathways required for influencing the pathobiology of AD dementia remain unknown. To address this, we studied an early phase of AD-like progression in the well-established APP/PS1 mouse model combined with targeted mutations in the BDNF-dependent GR phosphorylation sites (serines 134/267) using molecular, behavioral and neuroimaging approaches. We found that disrupting GR phosphorylation (S134A/S267A) in mice exacerbated the deleterious effects of the APP/PS1 genotype on mortality, neuroplasticity and cognition, without affecting either amyloid-β deposition or vascular pathology. The dynamics, maturation and retention of task-induced new dendritic spines of cortical excitatory neurons required GR phosphorylation at the BDNF-dependent sites that amyloid-β compromised. Parallel studies in postmortem human prefrontal cortex revealed AD subjects had downregulated BDNF signaling and concomitant upregulated cortisol pathway activation, which correlated with cognitive decline. These results provide key evidence that the loss of neurotrophin-mediated GR phosphorylation pathway promotes the detrimental effects of the brain cortisol response that contributes to the onset and/or progression of AD dementia. These findings have important translational implications as they provide a novel approach to treating AD dementia by identifying drugs that increase GR phosphorylation selectively at the neurotrophic sites to improve memory and cognition.

Transgenic Mouse Models of Alzheimer's Disease: An Integrative Analysis

Alzheimer's disease (AD) constitutes the most prominent form of dementia among elderly individuals worldwide. Disease modeling using murine transgenic mice was first initiated thanks to the discovery of heritable mutations in amyloid precursor protein (APP) and presenilins (PS) genes. However, due to the repeated failure of translational applications from animal models to human patients, along with the recent advances in genetic susceptibility and our current understanding on disease biology, these models have evolved over time in an attempt to better reproduce the complexity of this devastating disease and improve their applicability. In this review, we provide a comprehensive overview about the major pathological elements of human AD (plaques, tauopathy, synaptic damage, neuronal death, neuroinflammation and glial dysfunction), discussing the knowledge that available mouse models have provided about the mechanisms underlying human disease. Moreover, we highlight the pros and cons of current models, and the revolution offered by the concomitant use of transgenic mice and omics technologies that may lead to a more rapid improvement of the present modeling battery.

Neuronal-Derived EV Biomarkers Track Cognitive Decline in Alzheimer’s Disease

The hallmarks of Alzheimer’s disease (AD) pathology are senile plaques containing amyloid-beta (Aβ) and neurofibrillary tangles containing hyperphosphorylated tau. Additional pathologies often co-exist, whereas multiple pathogenic mechanisms are involved in AD, especially synaptic degeneration, which necessitate the need for synaptic integrity-related biomarkers alongside Aβ- and tau-related biomarkers. Plasma neuron-derived Extracellular Vesicles EVs (NDEVs) provide biomarkers related to Aβ and tau and synaptic degeneration. Here, to further establish the latter as a “liquid biopsy” for AD, we examined their relationship with ante-mortem cognition in pathologically-confirmed AD cases. We immunoprecipitated NDEVs by targeting neuronal marker L1CAM from ante-mortem plasma samples from 61 autopsy-confirmed cases of pure AD or AD with additional pathologies and measured Aβ₄₂, p181-Tau, total Tau, synaptophysin, synaptopodin and three canonical EV markers, CD63, CD81 and CD9. Higher NDEV Aβ₄₂ levels were consistently associated with better cognitive status, memory, fluency, working memory and executive function. Higher levels of NDEV synaptic integrity-related biomarkers were associated with better performance on executive function tasks. Our findings motivate the hypothesis that releasing Aβ₄₂-laden NDEVs may be an adaptive mechanism in AD.

Muscarinic Acetylcholine Receptor Localization on Distinct Excitatory and Inhibitory Neurons Within the ACC and LPFC of the Rhesus Monkey

Acetylcholine (ACh) can act on pre- and post-synaptic muscarinic receptors (mAChR) in the cortex to influence a myriad of cognitive processes. Two functionally-distinct regions of the prefrontal cortex-the lateral prefrontal cortex (LPFC) and the anterior cingulate cortex (ACC)-are differentially innervated by ascending cholinergic pathways yet, the nature and organization of prefrontal-cholinergic circuitry in primates are not well understood. Using multi-channel immunohistochemical labeling and high-resolution microscopy, we found regional and laminar differences in the subcellular localization and the densities of excitatory and inhibitory subpopulations expressing m1 and m2 muscarinic receptors, the two predominant cortical mAChR subtypes, in the supragranular layers of LPFC and ACC in rhesus monkeys (Macaca mulatta). The subset of m1+/m2+ expressing SMI-32+ pyramidal neurons labeled in layer 3 (L3) was denser in LPFC than in ACC, while m1+/m2+ SMI-32+ neurons co-expressing the calcium-binding protein, calbindin (CB) was greater in ACC. Further, we found between-area differences in laminar m1+ dendritic expression, and m2+ presynaptic localization on cortico-cortical (VGLUT1+) and sub-cortical inputs (VGLUT2+), suggesting differential cholinergic modulation of top-down vs. bottom-up inputs in the two areas. While almost all inhibitory interneurons-identified by their expression of parvalbumin (PV+), CB+, and calretinin (CR+)-expressed m1+, the localization of m2+ differed by subtype and area. The ACC exhibited a greater proportion of m2+ inhibitory neurons compared to the LPFC and had a greater density of presynaptic m2+ localized on inhibitory (VGAT+) inputs targeting proximal somatodendritic compartments and axon initial segments of L3 pyramidal neurons. These data suggest a greater capacity for m2+-mediated cholinergic suppression of inhibition in the ACC compared to the LPFC. The anatomical localization of muscarinic receptors on ACC and LPFC micro-circuits shown here contributes to our understanding of diverse cholinergic neuromodulation of functionally-distinct prefrontal areas involved in goal-directed behavior, and how these interactions maybe disrupted in neuropsychiatric and neurological conditions.

Early Memory Impairment is Accompanied by Changes in GluA1/ p-GluA1 in APP/PS1 Mice

AIMS

Exploring the neurobiological mechanisms of early AD damage.

BACKGROUND

The early diagnosis of Alzheimer's disease (AD) has a very important impact on the prognosis of AD. However, the early symptoms of AD are not obvious and difficult to diagnose. Existing studies have rarely explored the mechanism of early AD. AMPARs are early important learning memory-related receptors. However, it is not clear how the expression levels of AMPARs change in early AD.

OBJECTIVE

We explored learning memory abilities and AMPAR expression changes in APP/PS1 mice at 4 months, 8 months, and 12 months.

METHODS

We used the classic Morris water maze to explore the learning and memory impairment of APP/PS1 mice and used western blotting to explore the changes in AMPARs in APP/PS1 mice.

RESULTS

We found that memory impairment occurred in APP/PS1 mice as early as 4 months of age, and the impairment of learning and memory gradually became serious with age. The changes in GluA1 and p-GluA1 were most pronounced in the early stages of AD in APP/PS1 mice.

CONCLUSION

Our study found that memory impairment in APP/PS1 mice could be detected as early as 4 months of age, and this early injury may be related to GluA1.