Synaptic density and cognitive performance in Alzheimer's disease: A PET imaging study with [11 C]UCB-J

An approach based on a combination of toxicological experiments and in silico predictions to investigate the adverse outcome pathway (AOP) of paraquat neuro-immunotoxicity

Paraquat (PQ) exposure is strongly associated with neurotoxicity. However, research on the neurotoxicity mechanisms of PQ varies in terms of endpoints of toxic assessment, resulting in a great challenge to understand the early neurotoxic effects of PQ. In this study, we developed an adverse outcome pathway (AOP) to investigate PQ-induced neuro-immunotoxicity from an immunological perspective, combining of traditional toxicology methods and computer simulations. In vivo, PQ can microstructurally lead to an early synaptic loss in the brain mice, which is a large degree regarded as a main reason for cognitive impairment to mice behavior. Both in vitro and in vivo demonstrated synapse loss is caused by excessive activation of the complement C1q/C3-CD11b pathway, which mediates microglial phagocytosis dysfunction. Additionally, the interaction between PQ and C1q was validated by molecular simulation docking. Our findings extend the AOP framework related to PQ neurotoxicity from a neuro-immunotoxic perspective, highlighting C1q activation as the initiating event for PQ-induced neuro-immunotoxicity. In addition, downstream complement cascades induce abnormal microglial phagocytosis, resulting in reduced synaptic density and subsequent non-motor dysfunction. These findings deepen our understanding of neurotoxicity and provide a theoretical basis for ecological risk assessment of PQ.

PET brain imaging in neurological disorders

Brain disorders are a series of conditions with damage or loss of neurons, such as Parkinson's disease (PD), Alzheimer's disease (AD), or drug dependence. These individuals have gradual deterioration of cognitive, motor, and other central nervous system functions affected. This degenerative trajectory is intricately associated with dysregulations in neurotransmitter systems. Positron Emission Tomography (PET) imaging, employing radiopharmaceuticals and molecular imaging techniques, emerges as a crucial tool for detecting brain biomarkers. It offers invaluable insights for early diagnosis and distinguishing brain disorders. This article comprehensively reviews the application and progress of conventional and novel PET imaging agents in diagnosing brain disorders. Furthermore, it conducts a thorough analysis on merits and limitations. The article also provides a forward-looking perspective in the future development directions of PET imaging agents for diagnosing brain disorders and proposes potential innovative strategies. It aims to furnish clinicians and researchers with an all-encompassing overview of the latest advancements and forthcoming trends in the utilization of PET imaging for diagnosing brain disorders.

Rational Design of Dual-Functionalized Gd@C82 Nanoparticles to Relieve Neuronal Cytotoxicity in Alzheimer's Disease via Inhibition of Aβ Aggregation

The accumulation of amyloid-β (Aβ) peptides is a major hallmark of Alzheimer's disease (AD) and plays a crucial role in its pathogenesis. Particularly, the structured oligomeric species rich in β-sheet formations were implicated in neuronal organelle damage. Addressing this formidable challenge requires identifying candidates capable of inhibiting peptide aggregation or disaggregating preformed oligomers for effective antiaggregation-based AD therapy. Here, we present a dual-functional nanoinhibitor meticulously designed to target the aggregation driving force and amyloid fibril spatial structure. Leveraging the exceptional structural stability and facile tailoring capability of endohedral metallofullerene Gd@C82, we introduce desired hydrogen-binding sites and charged groups, which are abundant on its surface for specific designs. Impressively, these designs endow the resultant functionalized-Gd@C82 nanoparticles (f-Gd@C82 NPs) with high capability of redirecting peptide self-assembly toward disordered, off-pathway species, obstructing the early growth of protofibrils, and disaggregating the preformed well-ordered protofibrils or even mature Aβ fibrils. This results in considerable alleviation of Aβ peptide-induced neuronal cytotoxicity, rescuing neuronal death and synaptic loss in primary neuron models. Notably, these modifications significantly improved the dispersibility of f-Gd@C82 NPs, thus substantially enhancing its bioavailability. Moreover, f-Gd@C82 NPs demonstrate excellent cytocompatibility with various cell lines and possess the ability to penetrate the blood-brain barrier in mice. Large-scale molecular dynamics simulations illuminate the inhibition and disaggregation mechanisms. Our design successfully overcomes the limitations of other nanocandidates, which often overly rely on hydrophobic interactions or photothermal conversion properties, and offers a viable direction for developing anti-AD agents through the inhibition and even reversal of Aβ aggregation.

Polygenic Score for Conscientiousness Is a Protective Factor for Reversion from Mild Cognitive Impairment to Normal Cognition

Spontaneous reversion from mild cognitive impairment (MCI) to normal cognition (NC) is little known. Based on the data of the Genetics of Personality Consortium and MCI participants from Alzheimer's Disease Neuroimaging Initiative, the authors investigate the effect of polygenic scores (PGS) for personality traits on the reversion of MCI to NC and its underlying neurobiology. PGS analysis reveals that PGS for conscientiousness (PGS-C) is a protective factor that supports the reversion from MCI to NC. Gene ontology enrichment analysis and tissue-specific enrichment analysis indicate that the protective effect of PGS-C may be attributed to affecting the glutamatergic synapses of subcortical structures, such as hippocampus, amygdala, nucleus accumbens, and caudate nucleus. The structural covariance network (SCN) analysis suggests that the left whole hippocampus and its subfields, and the left whole amygdala and its subnuclei show significantly stronger covariance with several high-cognition relevant brain regions in the MCI reverters compared to the stable MCI participants, which may help illustrate the underlying neural mechanism of the protective effect of PGS-C.

Cerebrospinal Fluid Proteomics Identifies Potential Biomarkers for Early-Onset Alzheimer's Disease

Background

Early-onset Alzheimer's disease (EOAD) exhibits a notable degree of heterogeneity as compared to late-onset Alzheimer's disease (LOAD). The proteins and pathways contributing to the pathophysiology of EOAD still need to be completed and elucidated.

Objective

Using correlation network analysis and machine learning to analyze cerebrospinal fluid (CSF) proteomics data to identify potential biomarkers and pathways associated with EOAD.

Methods

We employed mass spectrometry to conduct CSF proteomic analysis using the data-independent acquisition method in a Chinese cohort of 139 CSF samples, including 40 individuals with normal cognition (CN), 61 patients with EOAD, and 38 patients with LOAD. Correlation network analysis of differentially expressed proteins was performed to identify EOAD-associated pathways. Machine learning assisted in identifying crucial proteins differentiating EOAD. We validated the results in an Western cohort and examined the proteins expression by enzyme-linked immunosorbent assay (ELISA) in additional 9 EOAD, 9 LOAD, and 9 CN samples from our cohort.

Results

We quantified 2,168 CSF proteins. Following adjustment for age and sex, EOAD exhibited a significantly greater number of differentially expressed proteins than LOAD compared to CN. Additionally, our data indicates that EOAD may exhibit more pronounced synaptic dysfunction than LOAD. Three potential biomarkers for EOAD were identified: SH3BGRL3, LRP8, and LY6 H, of which SH3BGRL3 also accurately classified EOAD in the Western cohort. LY6 H reduction was confirmed via ELISA, which was consistent with our proteomic results.

Conclusions

This study provides a comprehensive profile of the CSF proteome in EOAD and identifies three potential EOAD biomarker proteins.

Neuropsychological Test Performance Differentiates Subgroups of Individuals With Adult Moyamoya Disease: A Cross-Sectional Clinical Study

BACKGROUND AND OBJECTIVES

Moyamoya disease (MMD) is a rare noninflammatory disorder involving progressive intracranial vasculopathy and impaired cerebral blood flow in the anterior circulation, resulting in stroke and cognitive impairment. We aimed to characterize cognitive impairment and the possible predictive value of sociodemographic and clinical characteristics of adults with MMD.

METHODS

This cross-sectional study examined neurocognitive performance in a group of 42 consecutive adult patients (mean age = 40.52 years; 69% female) referred for a presurgical neuropsychological evaluation. Neuropsychological functioning was assessed with a comprehensive battery, and cognitive dysfunction was defined as 1.5 SDs below the mean. Neurocognitive performance correlated with clinical/demographic characteristics and disease markers.

RESULTS

Most patients (91%) had a history of stroke, and 45% had cognitive deficits, most notably on measures of attention/speed (48%), executive functioning (47%), visuoconstruction (41%), and memory (31%-54%). Only higher educational attainment and poor collateral blood flow in the right hemisphere differentiated cognitively impaired (n = 19) and intact groups (n = 23), and MMD-related characteristics (eg, disease duration, stroke history) did not differentiate the 2 groups.

CONCLUSION

Consistent with previous work, frontal-subcortical cognitive deficits (eg, deficits in mental speed, attention, executive functioning) were found in nearly half of patients with MMD and better cognitive performance was associated with factors related to cognitive reserve. Angiographic metrics of disease burden (eg, Suzuki rating, collateral flow) and hemodynamic reserve were not consistently associated with poorer cognitive outcomes, suggesting that cognition is a crucial independent factor to assess in MMD and has relevance for treatment planning and functional status.

Differentiating Prodromal Dementia with Lewy Bodies from Prodromal Alzheimer's Disease: A Pragmatic Review for Clinicians

This pragmatic review synthesises the current understanding of prodromal dementia with Lewy bodies (pDLB) and prodromal Alzheimer's disease (pAD), including clinical presentations, neuropsychological profiles, neuropsychiatric symptoms, biomarkers, and indications for disease management. The core clinical features of dementia with Lewy bodies (DLB)-parkinsonism, complex visual hallucinations, cognitive fluctuations, and REM sleep behaviour disorder are common prodromal symptoms. Supportive clinical features of pDLB include severe neuroleptic sensitivity, as well as autonomic and neuropsychiatric symptoms. The neuropsychological profile in mild cognitive impairment attributable to Lewy body pathology (MCI-LB) tends to include impairment in visuospatial skills and executive functioning, distinguishing it from MCI due to AD, which typically presents with impairment in memory. pDLB may present with cognitive impairment, psychiatric symptoms, and/or recurrent episodes of delirium, indicating that it is not necessarily synonymous with MCI-LB. Imaging, fluid and other biomarkers may play a crucial role in differentiating pDLB from pAD. The current MCI-LB criteria recognise low dopamine transporter uptake using positron emission tomography or single photon emission computed tomography (SPECT), loss of REM atonia on polysomnography, and sympathetic cardiac denervation using meta-iodobenzylguanidine SPECT as indicative biomarkers with slowing of dominant frequency on EEG among others as supportive biomarkers. This review also highlights the emergence of fluid and skin-based biomarkers. There is little research evidence for the treatment of pDLB, but pharmacological and non-pharmacological treatments for DLB may be discussed with patients. Non-pharmacological interventions such as diet, exercise, and cognitive stimulation may provide benefit, while evaluation and management of contributing factors like medications and sleep disturbances are vital. There is a need to expand research across diverse patient populations to address existing disparities in clinical trial participation. In conclusion, an early and accurate diagnosis of pDLB or pAD presents an opportunity for tailored interventions, improved healthcare outcomes, and enhanced quality of life for patients and care partners.

Decreased Synaptic Vesicle Glycoprotein 2A Binding in the Human Postmortem Essential Tremor Cerebellum: Evidence of Reduction in Synaptic Density

OBJECTIVE

Despite being one of the most prevalent neurological diseases, the pathophysiology of essential tremor (ET) is not fully understood. Neuropathological studies have identified numerous degenerative changes in the cerebellum of ET patients, however. These data align with considerable clinical and neurophysiological data linking ET to the cerebellum. While neuroimaging studies have variably shown mild atrophy in the cerebellum, marked atrophy is not a clear feature of the cerebellum in ET and a search for a more suitable neuroimaging signature of neurodegeneration is in order. Postmortem studies in ET have examined different neuropathological alterations in the cerebellum, but as of yet have not focused on measures of generalized synaptic markers. This pilot study focuses on synaptic vesicle glycoprotein 2A (SV2A), a protein expressed in practically all synapses in the brain, as a measure of synaptic density in postmortem ET cases.

METHODS

The current study utilized autoradiography with the SV2A radioligand [18F]SDM-16 to assess synaptic density in the cerebellar cortex and dentate nucleus in three ET cases and three age-matched controls.

RESULTS

Using [18F]SDM-16, SV2A was 53% and 46% lower in the cerebellar cortex and dentate nucleus, respectively, in ET cases compared to age-matched controls.

CONCLUSION

In this pilot study, using in vitro SV2A autoradiography, we have observed significantly lower synaptic density in the cerebellar cortex and dentate nucleus of ET cases. Future research could expand on our sample size and focus on in vivo imaging in ET to explore whether SV2A imaging could serve as a much-needed disease biomarker.

Association of synaptic density and cognitive performance in temporal lobe epilepsy: Humans and animals PET imaging study with [18F]SynVesT-1

AIM

Cognitive impairment is a common comorbidity in individuals with temporal lobe epilepsy (TLE), yet the underlying mechanisms remain unknown. This study explored the putative association between in vivo synaptic loss and cognitive outcomes in TLE patients by PET imaging of synaptic vesicle glycoprotein 2A (SV2A).

METHODS

We enrolled 16 TLE patients and 10 cognitively normal controls. All participants underwent SV2A PET imaging using [18F]SynVesT-1 and cognitive assessment. Lithium chloride-pilocarpine-induced rats with status epilepticus (n = 20) and controls (n = 6) rats received levetiracetam (LEV, specifically binds to SV2A), valproic acid (VPA), or saline for 14 days. Then, synaptic density was quantified by [18F]SynVesT-1 micro-PET/CT. The novel object recognition and Morris water maze tests evaluated TLE-related cognitive function. SV2A expression was examined and confirmed by immunohistochemistry.

RESULTS

Temporal lobe epilepsy patients showed significantly reduced synaptic density in hippocampus, which was associated with cognitive performance. In the rat model of TLE, the expression of SV2A and synaptic density decreased consistently in a wider range of brain regions, including the entorhinal cortex, insula, hippocampus, amygdala, thalamus, and cortex. We treated TLE animal models with LEV or VPA to explore whether synaptic loss contributes to cognitive deficits. It was found that LEV significantly exerted protective effects against brain synaptic deficits and cognitive impairment.

CONCLUSION

This is the first study to link synaptic loss to cognitive deficits in TLE, suggesting [18F]SynVesT-1 PET could be a promising biomarker for monitoring synaptic loss and cognitive dysfunction. LEV might help reverse synaptic deficits and ameliorate learning and memory impairments in TLE patients.

Latest advances and clinical application prospects of resveratrol therapy for neurocognitive disorders

Neurocognitive disorders, such as Alzheimer's disease, vascular dementia, and postoperative cognitive dysfunction, are non-psychiatric brain syndromes in which a significant decline in cognitive function causes great trauma to the mental status of the patient. The lack of effective treatments for neurocognitive disorders imposes a considerable burden on society, including a substantial economic impact. Over the past few decades, the identification of resveratrol, a natural plant compound, has provided researchers with an opportunity to formulate novel strategies for the treatment of neurocognitive disorders. This is because resveratrol effectively protects the brain of those with neurocognitive disorders by targeting some mechanisms such as inflammation and oxidative stress. This article reviews the status of recent research investigating the use of resveratrol for the treatment of different neurocognitive disorders. By examining the possible mechanisms of action of resveratrol and the shared mechanisms of different neurocognitive disorders, treatments for neurocognitive disorders may be further clarified.

Perivascular space diffusivity and brain microstructural measures are associated with circadian time and sleep quality

The glymphatic system is centred around brain cerebrospinal fluid flow and is enhanced during sleep, and the synaptic homeostasis hypothesis proposes that sleep acts on brain microstructure by selective synaptic downscaling. While so far primarily studied in animals, we here examine in humans if brain diffusivity and microstructure is related to time of day, sleep quality and cognitive performance. We use diffusion weighted images from 916 young healthy individuals, aged between 22 and 37 years, collected as part of the Human Connectome Project to assess diffusion tensor image analysis along the perivascular space index, white matter fractional anisotropy, intra-neurite volume fraction and extra-neurite mean diffusivity. Next, we examine if these measures are associated with circadian time of acquisition, the Pittsburgh Sleep Quality Index (high scores correspond to low sleep quality) and age-adjusted cognitive function total composite score. Consistent with expectations, we find that diffusion tensor image analysis along the perivascular space index and orbitofrontal grey matter extra-neurite mean diffusivity are negatively and white matter fractional anisotropy positively correlated with circadian time. Further, we find that grey matter intra-neurite volume fraction correlates positively with Pittsburgh Sleep Quality Index, and that this correlation is driven by sleep duration. Finally, we find positive correlations between grey matter intra-neurite volume fraction and cognitive function total composite score, as well as negative interaction effects between cognitive function total composite score and Pittsburgh Sleep Quality Index on grey matter intra-neurite volume fraction. Our findings propose that perivascular flow is under circadian control and that sleep downregulates the intra-neurite volume in healthy adults with positive impact on cognitive function.

Linking white matter hyperintensities to regional cortical thinning, amyloid deposition, and synaptic density loss in Alzheimer's disease

INTRODUCTION

We investigated the association between white matter hyperintensities (WMH) and regional cortical thickness, amyloid and tau deposition, and synaptic density in the WMH-connected cortex using multimodal images.

METHODS

We included 107 participants (59 with Alzheimer's disease [AD]; 27 with mild cognitive impairment; 21 cognitively normal controls) with amyloid beta (Aβ) positivity on amyloid positron emission tomography (PET). The cortex connected to WMH was identified using probabilistic tractography.

RESULTS

We found that WMH connected to the cortex with more severe regional degeneration as measured by cortical thickness, Aβ and tau deposition, and synaptic vesicle glycoprotein 2 A (SV2A) density using 18F-SynVesT-1 PET. In addition, higher ratios of Aβ in the deep WMH-connected versus WMH-unconnected cortex were significantly related to lower cognitive scores. Last, the cortical thickness of WMH-connected cortex reduced more than WMH-unconnected cortex over 12 months.

DISCUSSION

Our results suggest that WMH may be associated with AD-intrinsic processes of degeneration, in addition to vascular mechanisms.

HIGHLIGHTS

We studied white matter hyperintensities (WMHs) and WMH-connected cortical changes. WMHs are associated with more severe regional cortical degeneration. Findings suggest WMHs may be associated with Alzheimer's disease-intrinsic processes of degeneration.

SV2A PET imaging in human neurodegenerative diseases

This manuscript presents a thorough review of synaptic vesicle glycoprotein 2A (SV2A) as a biomarker for synaptic integrity using Positron Emission Tomography (PET) in neurodegenerative diseases. Synaptic pathology, characterized by synaptic loss, has been linked to various brain diseases. Therefore, there is a need for a minimally invasive approach to measuring synaptic density in living human patients. Several radiotracers targeting synaptic vesicle protein 2A (SV2A) have been created and effectively adapted for use in human subjects through PET scans. SV2A is an integral glycoprotein found in the membranes of synaptic vesicles in all synaptic terminals and is widely distributed throughout the brain. The review delves into the development of SV2A-specific PET radiotracers, highlighting their advancements and limitations in neurodegenerative diseases. Among these tracers, 11C-UCB-J is the most used so far. We summarize and discuss an increasing body of research that compares measurements of synaptic density using SV2A PET with other established indicators of neurodegenerative diseases, including cognitive performance and radiological findings, thus providing a comprehensive analysis of SV2A's effectiveness and reliability as a diagnostic tool in contrast to traditional markers. Although the literature overall suggests the promise of SV2A as a diagnostic and therapeutic monitoring tool, uncertainties persist regarding the superiority of SV2A as a biomarker compared to other available markers. The review also underscores the paucity of studies characterizing SV2A distribution and loss in human brain tissue from patients with neurodegenerative diseases, emphasizing the need to generate quantitative neuropathological maps of SV2A density in cases with neurodegenerative diseases to fully harness the potential of SV2A PET imaging in clinical settings. We conclude by outlining future research directions, stressing the importance of integrating SV2A PET imaging with other biomarkers and clinical assessments and the need for longitudinal studies to track SV2A changes throughout neurodegenerative disease progression, which could lead to breakthroughs in early diagnosis and the evaluation of new treatments.

Synaptic vesicle glycoprotein 2 A in serum is an ideal biomarker for early diagnosis of Alzheimer's disease

BACKGROUND

Previous studies have demonstrated that early intervention was the best plan to inhibit the progression of Alzheimer's disease (AD), which relied on the discovery of early diagnostic biomarkers. In this study, synaptic vesicle glycoprotein 2 A (SV2A) was examined to improve the early diagnostic efficiency in AD.

METHODS

In this study, biomarker testing was performed through the single-molecule array (Simoa). A total of 121 subjects including cognitively unimpaired controls, amnestic mild cognitive impairment (aMCI), AD and other types of dementia underwent cerebrospinal fluid (CSF) SV2A testing; 430 subjects including health controls, aMCI, AD and other types of dementia underwent serum SV2A, glial fibrillary acidic protein (GFAP), neurofilament light chain (NfL) and p-tau217 testing; 92 subjects including aMCI and AD underwent both CSF SV2A and serum SV2A testing; 115 cognitively unimpaired subjects including APOE ε4 carriers and APOE ε4 non-carriers were tested for serum SV2A, GFAP, NfL and p-tau217. Then, the efficacy of SV2A for the early diagnosis of AD and its ability to identify those at high risk of AD from a cognitively unimpaired population were further analyzed.

RESULTS

Both CSF and serum SV2A significantly and positively correlated with cognitive performance in patients with AD, and their levels gradually decreased with the progression of AD. Serum SV2A demonstrated excellent diagnostic efficacy for aMCI, with a sensitivity of 97.8%, which was significantly higher than those of NfL, GFAP, and p-tau217. The SV2A-positive rates ranged from 92.86 to 100% in aMCI cases that were negative for the above three biomarkers. Importantly, of all the biomarkers tested, serum SV2A had the highest positivity rate (81.82%) in individuals at risk for AD.

CONCLUSIONS

Serum SV2A was demonstrated to be a novel and ideal biomarker for the early diagnosis of AD, which can effectively distinguish those at high risk of AD in cognitively unimpaired populations.

Another Use for a Proven Drug: Experimental Evidence for the Potential of Artemisinin and Its Derivatives to Treat Alzheimer's Disease

Plant-derived multitarget compounds may represent a promising therapeutic strategy for multifactorial diseases, such as Alzheimer's disease (AD). Artemisinin and its derivatives were indicated to beneficially modulate various aspects of AD pathology in different AD animal models through the regulation of a wide range of different cellular processes, such as energy homeostasis, apoptosis, proliferation and inflammatory pathways. In this review, we aimed to provide an up-to-date overview of the experimental evidence documenting the neuroprotective activities of artemi-sinins to underscore the potential of these already-approved drugs for treating AD also in humans and propose their consideration for carefully designed clinical trials. In particular, the benefits to the main pathological hallmarks and events in the pathological cascade throughout AD development in different animal models of AD are summarized. Moreover, dose- and context-dependent effects of artemisinins are noted.

Tau pathology is associated with synaptic density and longitudinal synaptic loss in Alzheimer's disease

The associations of synaptic loss with amyloid-β (Aβ) and tau pathology measured by positron emission tomography (PET) and plasma analysis in Alzheimer's disease (AD) patients are unknown. Seventy-five participants, including 26 AD patients, 19 mild cognitive impairment (MCI) patients, and 30 normal controls (NCs), underwent [18F]SynVesT-1 PET/MR scans to assess synaptic density and [18F]florbetapir and [18F]MK6240 PET/CT scans to evaluate Aβ plaques and tau tangles. Among them, 19 AD patients, 12 MCI patients, and 29 NCs had plasma Aβ42/40 and p-tau181 levels measured by the Simoa platform. Twenty-three individuals, 6 AD patients, 4 MCI patients, and 13 NCs, underwent [18F]SynVesT-1 PET/MRI and [18F]MK6240 PET/CT scans during a one-year follow-up assessment. The associations of Aβ and tau pathology with cross-sectional and longitudinal synaptic loss were investigated using Pearson correlation analyses, generalized linear models and mediation analyses. AD patients exhibited lower synaptic density than NCs and MCI patients. In the whole cohort, global Aβ deposition was associated with synaptic loss in the medial (r = -0.431, p < 0.001) and lateral (r = -0.406, p < 0.001) temporal lobes. Synaptic density in almost all regions was related to the corresponding regional tau tangles independent of global Aβ deposition in the whole cohort and stratified groups. Synaptic density in the medial and lateral temporal lobes was correlated with plasma Aβ42/40 (r = 0.300, p = 0.020/r = 0.289, p = 0.025) and plasma p-tau 181 (r = -0.412, p = 0.001/r = -0.529, p < 0.001) levels in the whole cohort. Mediation analyses revealed that tau tangles mediated the relationship between Aβ plaques and synaptic density in the whole cohort. Baseline tau pathology was positively associated with longitudinal synaptic loss. This study suggested that tau burden is strongly linked to synaptic density independent of Aβ plaques, and also can predict longitudinal synaptic loss.

Synaptic density patterns in early Alzheimer's disease assessed by independent component analysis

Synaptic loss is a primary pathology in Alzheimer's disease and correlates best with cognitive impairment as found in post-mortem studies. Previously, we observed in vivo reductions of synaptic density with [11C]UCB-J PET (radiotracer for synaptic vesicle protein 2A) throughout the neocortex and medial temporal brain regions in early Alzheimer's disease. In this study, we applied independent component analysis to synaptic vesicle protein 2A-PET data to identify brain networks associated with cognitive deficits in Alzheimer's disease in a blinded data-driven manner. [11C]UCB-J binding to synaptic vesicle protein 2A was measured in 38 Alzheimer's disease (24 mild Alzheimer's disease dementia and 14 mild cognitive impairment) and 19 cognitively normal participants. [11C]UCB-J distribution volume ratio values were calculated with a whole cerebellum reference region. Principal components analysis was first used to extract 18 independent components to which independent component analysis was then applied. Subject loading weights per pattern were compared between groups using Kruskal-Wallis tests. Spearman's rank correlations were used to assess relationships between loading weights and measures of cognitive and functional performance: Logical Memory II, Rey Auditory Verbal Learning Test-long delay, Clinical Dementia Rating sum of boxes and Mini-Mental State Examination. We observed significant differences in loading weights among cognitively normal, mild cognitive impairment and mild Alzheimer's disease dementia groups in 5 of the 18 independent components, as determined by Kruskal-Wallis tests. Only Patterns 1 and 2 demonstrated significant differences in group loading weights after correction for multiple comparisons. Excluding the cognitively normal group, we observed significant correlations between the loading weights for Pattern 1 (left temporal cortex and the cingulate gyrus) and Clinical Dementia Rating sum of boxes (r = -0.54, P = 0.0019), Mini-Mental State Examination (r = 0.48, P = 0.0055) and Logical Memory II score (r = 0.44, P = 0.013). For Pattern 2 (temporal cortices), significant associations were demonstrated between its loading weights and Logical Memory II score (r = 0.34, P = 0.0384). Following false discovery rate correction, only the relationship between the Pattern 1 loading weights with Clinical Dementia Rating sum of boxes (r = -0.54, P = 0.0019) and Mini-Mental State Examination (r = 0.48, P = 0.0055) remained statistically significant. We demonstrated that independent component analysis could define coherent spatial patterns of synaptic density. Furthermore, commonly used measures of cognitive performance correlated significantly with loading weights for two patterns within only the mild cognitive impairment/mild Alzheimer's disease dementia group. This study leverages data-centric approaches to augment the conventional region-of-interest-based methods, revealing distinct patterns that differentiate between mild cognitive impairment and mild Alzheimer's disease dementia, marking a significant advancement in the field.

Correlation of Presynaptic and Postsynaptic Proteins with Pathology in Alzheimer's Disease

Synaptic transmission is essential for nervous system function and the loss of synapses is a known major contributor to dementia. Alzheimer's disease dementia (ADD) is characterized by synaptic loss in the mesial temporal lobe and cerebral neocortex, both of which are brain areas associated with memory and cognition. The association of synaptic loss and ADD was established in the late 1980s, and it has been estimated that 30-50% of neocortical synaptic protein is lost in ADD, but there has not yet been a quantitative profiling of different synaptic proteins in different brain regions in ADD from the same individuals. Very recently, positron emission tomography (PET) imaging of synapses is being developed, accelerating the focus on the role of synaptic loss in ADD and other conditions. In this study, we quantified the densities of two synaptic proteins, the presynaptic protein Synaptosome Associated Protein 25 (SNAP25) and the postsynaptic protein postsynaptic density protein 95 (PSD95) in the human brain, using enzyme-linked immunosorbent assays (ELISA). Protein was extracted from the cingulate gyrus, hippocampus, frontal, primary visual, and entorhinal cortex from cognitively unimpaired controls, subjects with mild cognitive impairment (MCI), and subjects with dementia that have different levels of Alzheimer's pathology. SNAP25 is significantly reduced in ADD when compared to controls in the frontal cortex, visual cortex, and cingulate, while the hippocampus showed a smaller, non-significant reduction, and entorhinal cortex concentrations were not different. In contrast, all brain areas showed lower PSD95 concentrations in ADD when compared to controls without dementia, although in the hippocampus, this failed to reach significance. Interestingly, cognitively unimpaired cases with high levels of AD pathology had higher levels of both synaptic proteins in all brain regions. SNAP25 and PSD95 concentrations significantly correlated with densities of neurofibrillary tangles, amyloid plaques, and Mini Mental State Examination (MMSE) scores. Our results suggest that synaptic transmission is affected by ADD in multiple brain regions. The differences were less marked in the entorhinal cortex and the hippocampus, most likely due to a ceiling effect imposed by the very early development of neurofibrillary tangles in older people in these brain regions.

Neural stem cells promote neuroplasticity: a promising therapeutic strategy for the treatment of Alzheimer's disease

Recent studies have demonstrated that neuroplasticity, such as synaptic plasticity and neurogenesis, exists throughout the normal lifespan but declines with age and is significantly impaired in individuals with Alzheimer's disease. Hence, promoting neuroplasticity may represent an effective strategy with which Alzheimer's disease can be alleviated. Due to their significant ability to self-renew, differentiate, and migrate, neural stem cells play an essential role in reversing synaptic and neuronal damage, reducing the pathology of Alzheimer's disease, including amyloid-β, tau protein, and neuroinflammation, and secreting neurotrophic factors and growth factors that are related to plasticity. These events can promote synaptic plasticity and neurogenesis to repair the microenvironment of the mammalian brain. Consequently, neural stem cells are considered to represent a potential regenerative therapy with which to improve Alzheimer's disease and other neurodegenerative diseases. In this review, we discuss how neural stem cells regulate neuroplasticity and optimize their effects to enhance their potential for treating Alzheimer's disease in the clinic.

C5aR1 signaling promotes region- and age-dependent synaptic pruning in models of Alzheimer's disease

INTRODUCTION

Synaptic loss is a hallmark of Alzheimer's disease (AD) that correlates with cognitive decline in AD patients. Complement-mediated synaptic pruning has been associated with this excessive loss of synapses in AD. Here, we investigated the effect of C5aR1 inhibition on microglial and astroglial synaptic pruning in two mouse models of AD.

METHODS

A combination of super-resolution and confocal and tridimensional image reconstruction was used to assess the effect of genetic ablation or pharmacological inhibition of C5aR1 on the Arctic48 and Tg2576 models of AD.

RESULTS

Genetic ablation or pharmacological inhibition of C5aR1 partially rescues excessive pre-synaptic pruning and synaptic loss in an age and region-dependent fashion in two mouse models of AD, which correlates with improved long-term potentiation (LTP).

DISCUSSION

Reduction of excessive synaptic pruning is an additional beneficial outcome of the suppression of C5a-C5aR1 signaling, further supporting its potential as an effective targeted therapy to treat AD.

HIGHLIGHTS

C5aR1 ablation restores long-term potentiation in the Arctic model of AD. C5aR1 ablation rescues region specific excessive pre-synaptic loss. C5aR1 antagonist, PMX205, rescues VGlut1 loss in the Tg2576 model of AD. C1q tagging is not sufficient to induce VGlut1 microglial ingestion. Astrocytes contribute to excessive pre-synaptic loss at late stages of the disease.

Alzheimer's therapeutic development: shifting neurodegeneration to neuroregeneration

Alzheimer's disease (AD), similar to AD-related dementias, is characterized by impaired/lost neuronal structures and functions due to a long progression of neurodegeneration. Derailed endogenous signal pathways and disease processes have critical roles in neurodegeneration and are pharmacological targets in inducing neuroregeneration. Pharmacologically switching/shifting the brain status from neurodegeneration to neuroregeneration is emerging as a new therapeutic concept, one that is not only achievable, but also essential for effective therapy for AD. The results of the pharmacological-induced shift from neurodegeneration to neuroregeneration are twofold: arresting cognitive deterioration (and directing the brain toward cognitive recovery) in established AD, and preventing neurodegeneration through building up cognitive resilience in patients with preclinical or probable AD. In this review, we discuss these new developments in AD pharmacology and relevant clinical trials.

In vivo PET of synaptic density as potential diagnostic marker for cognitive disorders: prospective comparison with current imaging markers for neuronal dysfunction and relation to symptomatology - study protocol

BACKGROUND

18F-FDG brain PET is clinically used for differential diagnosis in cognitive dysfunction of unclear etiology and for exclusion of a neurodegenerative cause in patients with cognitive impairment in late-life psychiatric disorders. 18F-FDG PET measures regional glucose metabolism, which represents a combination of neuronal/synaptic activity but also astrocytic activity and neuroinflammation. Recently, imaging of synaptic vesicle protein 2 A (SV2A) has become available and was shown to be a proxy of synaptic density. This prospective study will investigate the use of 18F-SynVesT-1 for imaging SV2A and its discriminative power for differential diagnosis in cognitive disorders in a head-to-head comparison to 18F-FDG PET. In addition, simultaneous PET/MR allows an evaluation of contributing factors and the additional value of advanced MRI imaging to FDG/SV2A PET imaging will be investigated. In this work, the study design and protocol are depicted.

METHODS

In this prospective, multimodal imaging study, 110 patients with uncertain diagnosis of cognitive impairment who are referred for 18F-FDG PET brain imaging in their diagnostic work-up in a tertiary memory clinic will be recruited. In addition, 40 healthy volunteers (HV) between 18 and 85 years (M/F) will be included. All study participants will undergo simultaneous 18F-SynVesT-1 PET/MR and an extensive neuropsychological evaluation. Amyloid status will be measured by PET using 18FNAV4694, in HV above 50 years of age. Structural T1-weighted and T2-weighted fluid-attenuated inversion recovery MR images, triple-tagging arterial spin labeling (ASL) and resting-state functional MRI (rs-fMRI) will be obtained. The study has been registered on ClinicalTrials.gov (NCT05384353) and is approved by the local Research Ethics Committee.

DISCUSSION

The main endpoint of the study will be the comparison of the diagnostic accuracy between 18F-SynVesT-1 and 18F-FDG PET in cognitive disorders with uncertain etiology and in exclusion of a neurodegenerative cause in patients with cognitive impairment in late-life psychiatric disorders. The strength of the relationship between cognition and imaging data will be assessed, as well as the potential incremental diagnostic value of including MR volumetry, ASL perfusion and rs-fMRI.

Complement-mediated synapse loss in Alzheimer's disease: mechanisms and involvement of risk factors

The complement system is increasingly recognized as a key player in the synapse loss and cognitive impairments observed in Alzheimer's disease (AD). In particular, the process of complement-dependent synaptic pruning through phagocytosis is over-activated in AD brains, driving detrimental excessive synapse elimination and contributing to synapse loss, which is the strongest neurobiological correlate of cognitive impairments in AD. Herein we review recent advances in characterizing complement-mediated synapse loss in AD, summarize the underlying mechanisms, and discuss the possible involvement of AD risk factors such as aging and various risk genes. We conclude with an overview of key questions that remain to be addressed.

In vivo imaging of synaptic density in neurodegenerative disorders with positron emission tomography: A systematic review

Positron emission tomography (PET) with radiotracers that bind to synaptic vesicle glycoprotein 2 A (SV2A) enables quantification of synaptic density in the living human brain. Assessing the regional distribution and severity of synaptic density loss will contribute to our understanding of the pathological processes that precede atrophy in neurodegeneration. In this systematic review, we provide a discussion of in vivo SV2A PET imaging research for quantitative assessment of synaptic density in various dementia conditions: amnestic Mild Cognitive Impairment and Alzheimer's disease, Frontotemporal dementia, Progressive supranuclear palsy and Corticobasal degeneration, Parkinson's disease and Dementia with Lewy bodies, Huntington's disease, and Spinocerebellar Ataxia. We discuss the main findings concerning group differences and clinical-cognitive correlations, and explore relations between SV2A PET and other markers of pathology. Additionally, we touch upon synaptic density in healthy ageing and outcomes of radiotracer validation studies. Studies were identified on PubMed and Embase between 2018 and 2023; last searched on the 3rd of July 2023. A total of 36 studies were included, comprising 5 on normal ageing, 21 clinical studies, and 10 validation studies. Extracted study characteristics were participant details, methodological aspects, and critical findings. In summary, the small but growing literature on in vivo SV2A PET has revealed different spatial patterns of synaptic density loss among various neurodegenerative disorders that correlate with cognitive functioning, supporting the potential role of SV2A PET imaging for differential diagnosis. SV2A PET imaging shows tremendous capability to provide novel insights into the aetiology of neurodegenerative disorders and great promise as a biomarker for synaptic density reduction. Novel directions for future synaptic density research are proposed, including (a) longitudinal imaging in larger patient cohorts of preclinical dementias, (b) multi-modal mapping of synaptic density loss onto other pathological processes, and (c) monitoring therapeutic responses and assessing drug efficacy in clinical trials.

A pilot study to evaluate the effect of CT1812 treatment on synaptic density and other biomarkers in Alzheimer's disease

BACKGROUND

Effective, disease-modifying therapeutics for the treatment of Alzheimer's disease (AD) remain a large unmet need. Extensive evidence suggests that amyloid beta (Aβ) is central to AD pathophysiology, and Aβ oligomers are among the most toxic forms of Aβ. CT1812 is a novel brain penetrant sigma-2 receptor ligand that interferes with the binding of Aβ oligomers to neurons. Preclinical studies of CT1812 have demonstrated its ability to displace Aβ oligomers from neurons, restore synapses in cell cultures, and improve cognitive measures in mouse models of AD. CT1812 was found to be generally safe and well tolerated in a placebo-controlled phase 1 clinical trial in healthy volunteers and phase 1a/2 clinical trials in patients with mild to moderate dementia due to AD. The unique objective of this study was to incorporate synaptic positron emission tomography (PET) imaging as an outcome measure for CT1812 in AD patients.

METHODS

The present phase 1/2 study was a randomized, double-blind, placebo-controlled, parallel-group trial conducted in 23 participants with mild to moderate dementia due to AD to primarily evaluate the safety of CT1812 and secondarily its pharmacodynamic effects. Participants received either placebo or 100 mg or 300 mg per day of oral CT1812 for 24 weeks. Pharmacodynamic effects were assessed using the exploratory efficacy endpoints synaptic vesicle glycoprotein 2A (SV2A) PET, fluorodeoxyglucose (FDG) PET, volumetric MRI, cognitive clinical measures, as well as cerebrospinal fluid (CSF) biomarkers of AD pathology and synaptic degeneration.

RESULTS

No treatment differences relative to placebo were observed in the change from baseline at 24 weeks in either SV2A or FDG PET signal, the cognitive clinical rating scales, or in CSF biomarkers. Composite region volumetric MRI revealed a trend towards tissue preservation in participants treated with either dose of CT1812, and nominally significant differences with both doses of CT1812 compared to placebo were found in the pericentral, prefrontal, and hippocampal cortices. CT1812 was safe and well tolerated.

CONCLUSIONS

The safety findings of this 24-week study and the observed changes on volumetric MRI with CT1812 support its further clinical development.

TRIAL REGISTRATION

The clinical trial described in this manuscript is registered at clinicaltrials.gov (NCT03493282).

Assessment of Gray Matter Microstructure and Synaptic Density in Alzheimer's Disease: A Multimodal Imaging Study With DTI and SV2A PET

OBJECTIVE

Multimodal imaging techniques have furthered our understanding of how different aspects of Alzheimer's disease (AD) pathology relate to one another. Diffusion tensor imaging (DTI) measures such as mean diffusivity (MD) may be a surrogate measure of the changes in gray matter structure associated with AD. Positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A) has been used to quantify synaptic loss, which is the major pathological correlate of cognitive impairment in AD. In this study, we investigated the relationship between gray matter microstructure and synaptic density.

METHODS

DTI was used to measure MD and [11C]UCB-J PET to measure synaptic density in 33 amyloid-positive participants with AD and 17 amyloid-negative cognitively normal (CN) participants aged 50-83. Univariate regression analyses were used to assess the association between synaptic density and MD in both the AD and CN groups.

RESULTS

Hippocampal MD was inversely associated with hippocampal synaptic density in participants with AD (r = -0.55, p <0.001, df = 31) but not CN (r = 0.13, p = 0.62, df = 15). Exploratory analyses across other regions known to be affected in AD suggested widespread inverse associations between synaptic density and MD in the AD group.

CONCLUSION

In the setting of AD, an increase in gray matter MD is inversely associated with synaptic density. These co-occurring changes may suggest a link between synaptic loss and gray matter microstructural changes in AD. Imaging studies of gray matter microstructure and synaptic density may allow important insights into AD-related neuropathology.

Targeting synapse function and loss for treatment of neurodegenerative diseases

Synapse dysfunction and loss are hallmarks of neurodegenerative diseases that correlate with cognitive decline. However, the mechanisms and therapeutic strategies to prevent or reverse synaptic damage remain elusive. In this Review, we discuss recent advances in understanding the molecular and cellular pathways that impair synapses in neurodegenerative diseases, including the effects of protein aggregation and neuroinflammation. We also highlight emerging therapeutic approaches that aim to restore synaptic function and integrity, such as enhancing synaptic plasticity, preventing synaptotoxicity, modulating neuronal network activity and targeting immune signalling. We discuss the preclinical and clinical evidence for each strategy, as well as the challenges and opportunities for developing effective synapse-targeting therapeutics for neurodegenerative diseases.

Synaptic density affects clinical severity via network dysfunction in syndromes associated with frontotemporal lobar degeneration

There is extensive synaptic loss from frontotemporal lobar degeneration, in preclinical models and human in vivo and post mortem studies. Understanding the consequences of synaptic loss for network function is important to support translational models and guide future therapeutic strategies. To examine this relationship, we recruited 55 participants with syndromes associated with frontotemporal lobar degeneration and 24 healthy controls. We measured synaptic density with positron emission tomography using the radioligand [11C]UCB-J, which binds to the presynaptic vesicle glycoprotein SV2A, neurite dispersion with diffusion magnetic resonance imaging, and network function with task-free magnetic resonance imaging functional connectivity. Synaptic density and neurite dispersion in patients was associated with reduced connectivity beyond atrophy. Functional connectivity moderated the relationship between synaptic density and clinical severity. Our findings confirm the importance of synaptic loss in frontotemporal lobar degeneration syndromes, and the resulting effect on behaviour as a function of abnormal connectivity.

Recent progress of small-molecule-based theranostic agents in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia. As a multifactorial disease, AD involves several etiopathogenic mechanisms, in which multiple pathological factors are interconnected with each other. This complicated and unclear pathogenesis makes AD lack effective diagnosis and treatment. Theranostics, exerting the synergistic effect of diagnostic and therapeutic functions, would provide a promising strategy for exploring AD pathogenesis and developing drugs for combating AD. With the efforts in small drug-like molecules for both diagnosis and treatment of AD, small-molecule-based theranostic agents have attracted significant attention owing to their facile synthesis, high biocompatibility and reproducibility, and easy clearance from the body through the excretion systems. In this review, the small-molecule-based theranostic agents reported in the literature for anti-AD are classified into four groups according to their diagnostic modalities. Their design rationales, chemical structures, and working mechanisms for theranostics are summarized. Finally, the opportunities for small-molecule-based theranostic agents in AD are also proposed.

Evaluation of cerebrospinal fluid levels of synaptic vesicle protein, VAMP-2, across the sporadic Alzheimer's disease continuum

BACKGROUND

Synapse loss is an early event that precedes neuronal death and symptom onset and is considered the best neuropathological correlate of cognitive decline in Alzheimer's disease (AD). Vesicle-associated membrane protein 2 (VAMP-2) has emerged as a promising biomarker of AD-related synapse degeneration in cerebrospinal fluid (CSF). The aim of this study was to explore the CSF profile of VAMP-2 across the AD continuum in relation to core AD biomarkers, other synaptic proteins, neurogranin (Ng) and synaptosomal-associated Protein-25 kDa (SNAP-25) and cognitive performance.

METHODS

We developed a digital immunoassay on the Single Molecule Array platform to quantify VAMP-2 in CSF and used existing immunoassays to quantify Ng, SNAP-25 and core CSF AD biomarkers. The clinical study included 62 cognitively unimpaired AD biomarker-negative subjects and 152 participants across the AD continuum from the SPIN cohort (Sant Pau Initiative on Neurodegeneration). Cognitive measures of episodic, semantic, executive and visuospatial domains and global cognition were included. Statistical methods included χ2 tests, spearman correlation, and ANCOVA analyses.

RESULTS

The VAMP-2 assay had a good analytical performance (repeatability 8.9%, intermediate precision 10.3%). Assay antibodies detected native VAMP-2 protein in human brain homogenates. CSF concentrations of VAMP-2, neurogranin and SNAP-25 were lower in preclinical AD stage 1 compared to controls and higher at later AD stages compared to AD stage 1 and were associated with core AD biomarkers, particularly total tau (adj. r2 = 0.62 to 0.78, p < 0.001). All three synaptic proteins were associated with all cognitive domains in individuals on the AD continuum (adj. r2 = 0.04 to 0.19, p < 0.05).

CONCLUSIONS

Our novel digital immunoassay accurately measures VAMP-2 changes in CSF, which reflect AD biomarkers and cognitive performance across multiple domains.

Alzheimer's disease: an axonal injury disease?

Alzheimer's disease (AD) is the primary cause of dementia and is anticipated to impose a substantial economic burden in the future. Over a significant period, the widely accepted amyloid cascade hypothesis has guided research efforts, and the recent FDA approval of an anti- amyloid-beta (Aβ) protofibrils antibody, believed to decelerate AD progression, has further solidified its significance. However, the excessive emphasis placed on the amyloid cascade hypothesis has overshadowed the physiological nature of Aβ and tau proteins within axons. Axons, specialized neuronal structures, sustain damage during the early stages of AD, exerting a pivotal influence on disease progression. In this review, we present a comprehensive summary of the relationship between axonal damage and AD pathology, amalgamating the physiological roles of Aβ and tau proteins, along with the impact of AD risk genes such as APOE and TREM2. Furthermore, we underscore the exceptional significance of axonal damage in the context of AD.

C5aR1 signaling promotes region and age dependent synaptic pruning in models of Alzheimer's Disease

INTRODUCTION

Synaptic loss is a hallmark of Alzheimer's disease (AD) that correlates with cognitive decline in AD patients. Complement-mediated synaptic pruning has been associated with this excessive loss of synapses in AD. Here, we investigated the effect of C5aR1 inhibition on microglial and astroglial synaptic pruning in two mouse models of AD.

METHODS

A combination of super-resolution and confocal and tridimensional image reconstruction was used to assess the effect of genetic ablation or pharmacological inhibition of C5aR1 on the Arctic48 and Tg2576 models of AD.

RESULTS

Genetic ablation or pharmacological inhibition of C5aR1 rescues the excessive pre-synaptic pruning and synaptic loss in an age and region dependent fashion in two mouse models of AD, which correlates with improved long-term potentiation (LTP).

DISCUSSION

Reduction of excessive synaptic pruning is an additional beneficial outcome of the suppression of C5a-C5aR1 signaling, further supporting its potential as an effective targeted therapy to treat AD.

Advanced brain imaging for the diagnosis of Alzheimer disease

PURPOSE OF REVIEW

The purpose is to review the latest advances of brain imaging for the diagnosis of Alzheimer disease (AD).

RECENT FINDINGS

Brain imaging techniques provide valuable and complementary information to support the diagnosis of Alzheimer disease in clinical and research settings. The recent FDA accelerated approvals of aducanumab, lecanemab and donanemab made amyloid-PET critical in helping determine the optimal window for anti-amyloid therapeutic interventions. Tau-PET, on the other hand, is considered of key importance for the tracking of disease progression and for monitoring therapeutic interventions in clinical trials. PET imaging for microglial activation, astrocyte reactivity and synaptic degeneration are still new techniques only used in the research field, and more studies are needed to validate their use in the clinical diagnosis of AD. Finally, artificial intelligence has opened new prospective in the early detection of AD using MRI modalities.

SUMMARY

Brain imaging techniques using PET improve our understanding of the different AD-related pathologies and their relationship with each other along the course of disease. With more robust validation, machine learning and deep learning algorithms could be integrated with neuroimaging modalities to serve as valuable tools for clinicians to make early diagnosis and prognosis of AD.

Effects of escitalopram on synaptic density in the healthy human brain: a randomized controlled trial

Selective serotonin reuptake inhibitors (SSRIs) are widely used for treating neuropsychiatric disorders. However, the exact mechanism of action and why effects can take several weeks to manifest is not clear. The hypothesis of neuroplasticity is supported by preclinical studies, but the evidence in humans is limited. Here, we investigate the effects of the SSRI escitalopram on presynaptic density as a proxy for synaptic plasticity. In a double-blind placebo-controlled study (NCT04239339), 32 healthy participants with no history of psychiatric or cognitive disorders were randomized to receive daily oral dosing of either 20 mg escitalopram (n = 17) or a placebo (n = 15). After an intervention period of 3-5 weeks, participants underwent a [11C]UCB-J PET scan (29 with full arterial input function) to quantify synaptic vesicle glycoprotein 2A (SV2A) density in the hippocampus and the neocortex. Whereas we find no statistically significant group difference in SV2A binding after an average of 29 (range: 24-38) days of intervention, our secondary analyses show a time-dependent effect of escitalopram on cerebral SV2A binding with positive associations between [11C]UCB-J binding and duration of escitalopram intervention. Our findings suggest that brain synaptic plasticity evolves over 3-5 weeks in healthy humans following daily intake of escitalopram. This is the first in vivo evidence to support the hypothesis of neuroplasticity as a mechanism of action for SSRIs in humans and it offers a plausible biological explanation for the delayed treatment response commonly observed in patients treated with SSRIs. While replication is warranted, these results have important implications for the design of future clinical studies investigating the neurobiological effects of SSRIs.

The Q/R editing site of AMPA receptor GluA2 subunit acts as an epigenetic switch regulating dendritic spines, neurodegeneration and cognitive deficits in Alzheimer's disease

BACKGROUND

RNA editing at the Q/R site of GluA2 occurs with ~99% efficiency in the healthy brain, so that the majority of AMPARs contain GluA2(R) instead of the exonically encoded GluA2(Q). Reduced Q/R site editing infcreases AMPA receptor calcium permeability and leads to dendritic spine loss, neurodegeneration, seizures and learning impairments. Furthermore, GluA2 Q/R site editing is impaired in Alzheimer's disease (AD), raising the possibility that unedited GluA2(Q)-containing AMPARs contribute to synapse loss and neurodegeneration in AD. If true, then inhibiting expression of unedited GluA2(Q), while maintaining expression of GluA2(R), may be a novel strategy of preventing synapse loss and neurodegeneration in AD.

METHODS

We engineered mice with the 'edited' arginine codon (CGG) in place of the unedited glutamine codon (CAG) at position 607 of the Gria2 gene. We crossbred this line with the J20 mouse model of AD and conducted anatomical, electrophysiological and behavioural assays to determine the impact of eliminating unedited GluA2(Q) expression on AD-related phenotypes.

RESULTS

Eliminating unedited GluA2(Q) expression in AD mice prevented dendritic spine loss and hippocampal CA1 neurodegeneration as well as improved working and reference memory in the radial arm maze. These phenotypes were improved independently of Aβ pathology and ongoing seizure susceptibility. Surprisingly, our data also revealed increased spine density in non-AD mice with exonically encoded GluA2(R) as compared to their wild-type littermates, suggesting an unexpected and previously unknown role for unedited GluA2(Q) in regulating dendritic spines.

CONCLUSION

The Q/R editing site of the AMPA receptor subunit GluA2 may act as an epigenetic switch that regulates dendritic spines, neurodegeneration and memory deficits in AD.

Comprehensive analysis of N6-methyladenosine-related RNA methylation in the mouse hippocampus after acquired hearing loss

BACKGROUND

The mechanism underlying cognitive impairment after hearing loss (HL) remains unclear. N6-methyladenosine (m6A) is involved in many neurodegenerative diseases; however, its role in cognitive impairment after HL has not yet been investigated. Therefore, we aimed to analyze the m6A modification profile of the mouse hippocampus after HL exposure. A mouse model of neomycin-induced HL was established. An auditory brainstem-response test was utilized for detecting hearing threshold. The passive avoidance test was served as the mean for evaluating cognitive function. The m6A-regulated enzyme expression levels were analyzed by using reverse transcription quantitative real-time polymerase chain reaction and western blot analyses. RNA sequencing (RNA-Seq) and methylated RNA immunoprecipitation sequencing (MeRIP-Seq) were performed with the aim of investigating gene expression differences and m6A modification in the mouse hippocampus.

RESULTS

Neomycin administration induced severe HL in mice. At four months of age, the mice in the HL group showed poorer cognitive performance than the mice in the control group. METTL14, WTAP, and YTHDF2 mRNA levels were downregulated in the hippocampi of HL mice, whereas ALKBH5 and FTO mRNA levels were significantly upregulated. At the protein level, METTL3 and FTO were significantly upregulated. Methylated RNA immunoprecipitation sequencing analysis revealed 387 and 361 m6A hypermethylation and hypomethylation peaks, respectively. Moreover, combined analysis of mRNA expression levels and m6A peaks revealed eight mRNAs with significantly changed expression levels and methylation.

CONCLUSIONS

Our findings revealed the m6A transcriptome-wide profile in the hippocampus of HL mice, which may provide a basis for understanding the association between HL and cognitive impairment from the perspective of epigenetic modifications.

Looking Behind the Curtain: Patient Stratification According to Genetic or Demographic Factors May Yield Unexpected Results in Studies of Neurodegenerative Diseases

Amyloid-PET studies of neurodegenerative diseases may yield inconclusive findings due to lacking stratification according to genetic or demographic variants. APOEɛ4 alleles are the major variants to increase disease susceptibility and cause earlier onset and more behavioral features in patients with late-onset Alzheimer's disease, but have no linear effects on cognitive or functional decline; thus, sample stratification according to APOEɛ4 carrier status may be the best option. Interactions among APOEɛ4 alleles, sex, and age on amyloid-β deposition may reveal even more innovative findings with sufficiently large samples, suggesting variable genomic effects of cognitive reserve, sex differences, and cerebrovascular risk on neurodegeneration.

Principal component analysis of synaptic density measured with [11C]UCB-J PET in early Alzheimer's disease

BACKGROUND

Synaptic loss is considered an early pathological event and major structural correlate of cognitive impairment in Alzheimer's disease (AD). We used principal component analysis (PCA) to identify regional patterns of covariance in synaptic density using [11C]UCB-J PET and assessed the association between principal components (PC) subject scores with cognitive performance.

METHODS

[11C]UCB-J binding was measured in 45 amyloid + participants with AD and 19 amyloid- cognitively normal participants aged 55-85. A validated neuropsychological battery assessed performance across five cognitive domains. PCA was applied to the pooled sample using distribution volume ratios (DVR) standardized (z-scored) by region from 42 bilateral regions of interest (ROI).

RESULTS

Parallel analysis determined three significant PCs explaining 70.2% of the total variance. PC1 was characterized by positive loadings with similar contributions across the majority of ROIs. PC2 was characterized by positive and negative loadings with strongest contributions from subcortical and parietooccipital cortical regions, respectively, while PC3 was characterized by positive and negative loadings with strongest contributions from rostral and caudal cortical regions, respectively. Within the AD group, PC1 subject scores were positively correlated with performance across all cognitive domains (Pearson r = 0.24-0.40, P = 0.06-0.006), PC2 subject scores were inversely correlated with age (Pearson r = -0.45, P = 0.002) and PC3 subject scores were significantly correlated with CDR-sb (Pearson r = 0.46, P = 0.04). No significant correlations were observed between cognitive performance and PC subject scores in CN participants.

CONCLUSIONS

This data-driven approach defined specific spatial patterns of synaptic density correlated with unique participant characteristics within the AD group. Our findings reinforce synaptic density as a robust biomarker of disease presence and severity in the early stages of AD.

Synaptic density in aging mice measured by [18F]SynVesT-1 PET

Synaptic alterations in certain brain structures are related to cognitive decline in neurodegeneration and in aging. Synaptic loss in many neurodegenerative diseases can be visualized by positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A). However, the use of SV2A PET for studying synaptic changes during aging is not particularly explored. Thus, in the present study, PET ligand [¹⁸F]SynVesT-1, which binds to SV2A, was used to investigate synaptic density at different ages in healthy mice. Wild type C57BL/6 mice divided into three age groups (4-5 months (n = 7), 12-14 months (n = 11), 17-19 months (n = 7)) were PET scanned with [¹⁸F]SynVesT-1. Brain retention of [¹⁸F]SynVesT-1 expressed as the volume of distribution (V_IDIF) was calculated using an image-derived input function. Estimates of V_IDIF were derived using either a one-tissue compartment model (1TCM), a two-tissue compartment model (2TCM), or the Logan plot with blood input to find the best-fit model for [¹⁸F]SynVesT-1. After the PET scans, tissue sections were immunostained for the detection of SV2A and neuronal markers. We found that [¹⁸F]SynVesT-1 data acquired 60 min post intravenously injection and analyzed with 1TCM described the brain pharmacokinetics of the radioligand in mice well. [¹⁸F]SynVesT-1 brain retention was lower in the oldest group of mice, indicating a decrease in synaptic density in this age group. However, no gradual age-dependent decrease in synaptic density at a region-specific level was observed. Immunostaining indicated that SV2A expression and neuron numbers were similar across all three age groups. In general, these data obtained in healthy aging mice are consistent with previous findings in humans where synaptic density appeared stable during aging up to a certain age, after which a small decrease is observed.

Innate immune activation and aberrant function in the R6/2 mouse model and Huntington's disease iPSC-derived microglia

Huntington's disease (HD) is an inherited autosomal dominant neurodegenerative disease caused by CAG repeats in exon 1 of the HTT gene. A hallmark of HD along with other psychiatric and neurodegenerative diseases is alteration in the neuronal circuitry and synaptic loss. Microglia and peripheral innate immune activation have been reported in pre-symptomatic HD patients; however, what "activation" signifies for microglial and immune function in HD and how it impacts synaptic health remains unclear. In this study we sought to fill these gaps by capturing immune phenotypes and functional activation states of microglia and peripheral immunity in the R6/2 model of HD at pre-symptomatic, symptomatic and end stages of disease. These included characterizations of microglial phenotypes at single cell resolution, morphology, aberrant functions such as surveillance and phagocytosis and their impact on synaptic loss in vitro and ex vivo in R6/2 mouse brain tissue slices. To further understand how relevant the observed aberrant microglial behaviors are to human disease, transcriptomic analysis was performed using HD patient nuclear sequencing data and functional assessments were conducted using induced pluripotent stem cell (iPSC)-derived microglia. Our results show temporal changes in brain infiltration of peripheral lymphoid and myeloid cells, increases in microglial activation markers and phagocytic functions at the pre-symptomatic stages of disease. Increases in microglial surveillance and synaptic uptake parallel significant reduction of spine density in R6/2 mice. These findings were mirrored by an upregulation of gene signatures in the endocytic and migratory pathways in disease-associated microglial subsets in human HD brains, as well as increased phagocytic and migratory functions of iPSC-derived HD microglia. These results collectively suggest that targeting key and specific microglial functions related to synaptic surveillance and pruning may be therapeutically beneficial in attenuating cognitive decline and psychiatric aspects of HD.

Alzheimer's Disease: Significant Benefit from the Yeast-Based Models

Alzheimer's disease (AD) is an age-related, multifaceted neurological disorder associated with accumulation of aggregated proteins (amyloid Aβ and hyperphosphorylated tau), loss of synapses and neurons, and alterations in microglia. AD was recognized by the World Health Organization as a global public health priority. The pursuit of a better understanding of AD forced researchers to pay attention to well-defined single-celled yeasts. Yeasts, despite obvious limitations in application to neuroscience, show high preservation of basic biological processes with all eukaryotic organisms and offer great advantages over other disease models due to the simplicity, high growth rates on low-cost substrates, relatively simple genetic manipulations, the large knowledge base and data collections, and availability of an unprecedented amount of genomic and proteomic toolboxes and high-throughput screening techniques, inaccessible to higher organisms. Research reviewed above clearly indicates that yeast models, together with other, more simple eukaryotic models including animal models, C. elegans and Drosophila, significantly contributed to understanding Aβ and tau biology. These models allowed high throughput screening of factors and drugs that interfere with Aβ oligomerization, aggregation and toxicity, and tau hyperphosphorylation. In the future, yeast models will remain relevant, with a focus on creating novel high throughput systems to facilitate the identification of the earliest AD biomarkers among different cellular networks in order to achieve the main goal-to develop new promising therapeutic strategies to treat or prevent the disease.

In vivo synaptic density loss correlates with impaired functional and related structural connectivity in Alzheimer's disease

Synapse loss has been considered as a major pathological change in Alzheimer's disease (AD). It remains unclear about whether and how synapse loss relates to functional and structural connectivity dysfunction in AD. We measured synaptic vesicle glycoprotein 2 A (SV2A) binding using 18F-SynVesT-1 PET to evaluate synaptic alterations in 33 participants with AD, 31 with mild cognitive impairment (MCI), and 30 controls. We examined the correlation between synaptic density and cognitive function. Functional MRI was performed to analyze functional connectivity in lower synaptic density regions. We tracked the white matter tracts between impaired functional connectivity regions using Diffusion MRI. In AD group, lower synaptic density in bilateral cortex and hippocampus was found when compared with controls. The synaptic density changes in right insular cortex and bilateral caudal middle frontal gyrus (MFG) were correlated with cognitive decline. Among them, right MFG synaptic density was positively associated with right MFG - bilateral superior frontal gyrus (SFG) functional connectivity. AD had lower probability of tract (POT) between right MFG and SFG than controls, which was significantly associated with global cognition. These findings provide evidence supporting synapse loss contributes to functional and related structural connectivity alterations underlying cognitive impairment of AD.

Astrocyte Signature in Alzheimer's Disease Continuum through a Multi-PET Tracer Imaging Perspective

Reactive astrogliosis is an early event in the continuum of Alzheimer's disease (AD). Current advances in positron emission tomography (PET) imaging provide ways of assessing reactive astrogliosis in the living brain. In this review, we revisit clinical PET imaging and in vitro findings using the multi-tracer approach, and point out that reactive astrogliosis precedes the deposition of Aβ plaques, tau pathology, and neurodegeneration in AD. Furthermore, considering the current view of reactive astrogliosis heterogeneity-more than one subtype of astrocyte involved-in AD, we discuss how astrocytic body fluid biomarkers might fit into trajectories different from that of astrocytic PET imaging. Future research focusing on the development of innovative astrocytic PET radiotracers and fluid biomarkers may provide further insights into the heterogeneity of reactive astrogliosis and improve the detection of AD in its early stages.

Synaptic vesicle glycoprotein 2A (SV2A) levels in the cerebral cortex in patients with Alzheimer's disease: a radioligand binding study in postmortem brains

Histological and biochemical analyses in postmortem tissues have demonstrated neurodegenerative changes in the cerebral cortex in patients with Alzheimer's disease (AD), and it has been suggested that this represents a loss of synapses. PET imaging of the (pre)synaptic vesicular glycoprotein 2A (SV2A) has demonstrated a reduction in synapse density in AD in the hippocampus but not consistently in the neocortex. This investigation examines the level of [³H]UCB-J binding in postmortem cortical tissue from patients with AD and matched healthy controls using autoradiography. Among the neocortical areas examined, the binding was significantly lower only in the middle frontal gyrus in AD compared to matched controls. No differences were observed in the parietal, temporal, or occipital cortex. The binding levels in the frontal cortex in the AD cohort displayed large variability among subjects, and this revealed a highly significant negative association with the age of the patient. These results demonstrate low UCB-J binding in the frontal cortex of patients with AD, and this biomarker correlates negatively with age, which may further indicate that SV2A could be an important biomarker in AD patients.

Decreased Synaptic Vesicle Glycoprotein 2A Binding in the Human Postmortem Essential Tremor Cerebellum: Evidence of Reduction in Synaptic Density

Objective Despite being one of the most prevalent neurological diseases, the pathophysiology of essential tremor (ET) is not fully understood. Neuropathological studies have identified numerous degenerative changes in the cerebellum of ET patients, however. These data align with considerable clinical and neurophysiological data linking ET to the cerebellum. While neuroimaging studies have variably shown mild atrophy in the cerebellum, marked atrophy is not a clear feature of the cerebellum in ET and that a search for a more suitable neuroimaging signature of neurodegeneration is in order. Postmortem studies in ET have examined different neuropathological alterations in the cerebellum, but as of yet have not focused on measures of generalized synaptic markers. This pilot study focuses on synaptic vesicle glycoprotein 2A (SV2A), a protein expressed in practically all synapses in the brain, as a measure of synaptic density in postmortem ET cases. Methods The current study utilized autoradiography with the SV2A radioligand [ 18 F]SDM-16 to assess synaptic density in the cerebellar cortex and dentate nucleus in three ET cases and three age-matched controls. Results Using [ 18 F]SDM-16, SV2A was 53% and 46% lower in the cerebellar cortex and dentate nucleus, respectively, in ET cases compared to age-matched controls. Conclusion For the first time, using in vitro SV2A autoradiography, we have observed significantly lower synaptic density in the cerebellar cortex and dentate nucleus of ET cases. Future research could focus on in vivo imaging in ET to explore whether SV2A imaging could serve as a much-needed disease biomarker.

Recent Development in the Understanding of Molecular and Cellular Mechanisms Underlying the Etiopathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that leads to dementia and patient death. AD is characterized by intracellular neurofibrillary tangles, extracellular amyloid beta (Aβ) plaque deposition, and neurodegeneration. Diverse alterations have been associated with AD progression, including genetic mutations, neuroinflammation, blood-brain barrier (BBB) impairment, mitochondrial dysfunction, oxidative stress, and metal ion imbalance.Additionally, recent studies have shown an association between altered heme metabolism and AD. Unfortunately, decades of research and drug development have not produced any effective treatments for AD. Therefore, understanding the cellular and molecular mechanisms underlying AD pathology and identifying potential therapeutic targets are crucial for AD drug development. This review discusses the most common alterations associated with AD and promising therapeutic targets for AD drug discovery. Furthermore, it highlights the role of heme in AD development and summarizes mathematical models of AD, including a stochastic mathematical model of AD and mathematical models of the effect of Aβ on AD. We also summarize the potential treatment strategies that these models can offer in clinical trials.

3D models of neurodegeneration: implementation in drug discovery

A lack of in vitro models that robustly represent the complex cellular pathologies underlying neurodegeneration has resulted in a translational gap between in vitro and in vivo results, creating a bottleneck in the development of new therapeutics. In the past decade, new and complex 3D models of the brain have been published at an exponential rate. However, many novel 3D models of neurodegeneration overlook the validation and throughput requirements for implementation in drug discovery. This therefore represents a knowledge gap that could hinder the translation of these models to drug discovery efforts. We review the recent progress in the development of 3D models of neurodegeneration, examining model design benefits and validation techniques, and discuss opportunities and standards for 3D models of neurodegeneration to be implemented in drug discovery and development.

Loss of Extrasynaptic Inhibitory Glycine Receptors in the Hippocampus of an AD Mouse Model Is Restored by Treatment with Artesunate

Alzheimer's disease (AD) is characterized by synaptic failure and neuronal loss. Recently, we demonstrated that artemisinins restored the levels of key proteins of inhibitory GABAergic synapses in the hippocampus of APP/PS1 mice, a model of cerebral amyloidosis. In the present study, we analyzed the protein levels and subcellular localization of α2 and α3 subunits of GlyRs, indicated as the most abundant receptor subtypes in the mature hippocampus, in early and late stages of AD pathogenesis, and upon treatment with two different doses of artesunate (ARS). Immunofluorescence microscopy and Western blot analysis demonstrated that the protein levels of both α2 and α3 GlyRs are considerably reduced in the CA1 and the dentate gyrus of 12-month-old APP/PS1 mice when compared to WT mice. Notably, treatment with low-dose ARS affected GlyR expression in a subunit-specific way; the protein levels of α3 GlyR subunits were rescued to about WT levels, whereas that of α2 GlyRs were not affected significantly. Moreover, double labeling with a presynaptic marker indicated that the changes in GlyR α3 expression levels primarily involve extracellular GlyRs. Correspondingly, low concentrations of artesunate (≤1 µM) also increased the extrasynaptic GlyR cluster density in hAPPswe-transfected primary hippocampal neurons, whereas the number of GlyR clusters overlapping presynaptic VIAAT immunoreactivities remained unchanged. Thus, here we provide evidence that the protein levels and subcellular localization of α2 and α3 subunits of GlyRs show regional and temporal alterations in the hippocampus of APP/PS1 mice that can be modulated by the application of artesunate.

Anti-LINGO-1 antibody protects neurons and synapses in the medial prefrontal cortex of APP/PS1 transgenic mice

The medial prefrontal cortex (mPFC), one of the most vulnerable brain regions in Alzheimer's disease (AD), plays a critical role in cognition. Leucine-rich repeat and immunoglobulin-like domain-containing nogo receptor-interacting protein-1 (LINGO-1) negatively affects nerve growth in the central nervous system; however, its role in the pathological damage to the mPFC remains to be studied in AD. In this study, an anti-LINGO-1 antibody was administered to 10-month-old APP/PS1 mice, and behavioral tests, stereological methods, immunohistochemistry and immunofluorescence were used to answer this question. Our results revealed that LINGO-1 was highly expressed in the neurons of the mPFC of AD mice, and the anti-LINGO-1 antibody improved prefrontal cortex-related function and reduced the protein level of LINGO-1, atrophy of the volume, Aβ deposition and massive losses of synapses and neurons in the mPFC of AD mice. Antagonizing LINGO-1 could effectively alleviate the pathological damage in the mPFC of AD mice, which might be an important structural basis for improving prefrontal cortex-related function. Abnormal expression of LINGO-1 in the mPFC may be one of the key targets of AD, and the effect initiated by the anti-LINGO-1 antibody may provide an important basis in the search for drugs for the prevention and treatment of AD.

Decreased synaptic vesicle glycoprotein 2A binding in a rodent model of familial Alzheimer's disease detected by [18F]SDM-16

Introduction

Synapse loss is one of the hallmarks of Alzheimer's disease (AD) and is associated with cognitive decline. In this study, we tested [18F]SDM-16, a novel metabolically stable SV2A PET imaging probe, in the transgenic APPswe/PS1dE9 (APP/PS1) mouse model of AD and age-matched wild-type (WT) mice at 12 months of age.

Methods

Based on previous preclinical PET imaging studies using [11C]UCB-J and [18F]SynVesT-1 in the same strain animals, we used the simplified reference tissue model (SRTM), with brain stem as the pseudo reference region to calculate distribution volume ratios (DVRs).

Results

To simplify and streamline the quantitative analysis, we compared the standardized uptake value ratios (SUVRs) from different imaging windows to DVRs and found that the averaged SUVRs from 60-90 min post-injection (p.i.) are most consistent with the DVRs. Thus, we used averaged SUVRs from 60-90 min for group comparisons and found statistically significant differences in the tracer uptake in different brain regions, e.g., hippocampus (p = 0.001), striatum (p = 0.002), thalamus (p = 0.003), and cingulate cortex (p = 0.0003).

Conclusions

In conclusion, [18F]SDM-16 was used to detect decreased SV2A levels in the brain of APP/PS1 AD mouse model at one year old. Our data suggest that [18F]SDM-16 has similar statistical power in detecting the synapse loss in APP/PS1 mice as [11C]UCB-J and [18F]SynVesT-1, albeit later imaging window (60-90 min p.i.) is needed when SUVR is used as a surrogate for DVR for [18F]SDM-16 due to its slower brain kinetics.