Encephalopathy induced by Alzheimer brain inoculation in a non-human primate

Transmissible long-term neuroprotective and pro-cognitive effects of 1-42 beta-amyloid with A2T icelandic mutation in an Alzheimer's disease mouse model

The amyloid cascade hypothesis assumes that the development of Alzheimer's disease (AD) is driven by a self-perpetuating cycle, in which β-amyloid (Aβ) accumulation leads to Tau pathology and neuronal damages. A particular mutation (A673T) of the amyloid precursor protein (APP) was identified among Icelandic population. It provides a protective effect against Alzheimer- and age-related cognitive decline. This APP mutation leads to the reduced production of Aβ with A2T (position in peptide sequence) change (Aβice). In addition, Aβice has the capacity to form protective heterodimers in association with wild-type Aβ. Despite the emerging interest in Aβice during the last decade, the impact of Aβice on events associated with the amyloid cascade has never been reported. First, the effects of Aβice were evaluated in vitro by electrophysiology on hippocampal slices and by studying synapse morphology in cortical neurons. We showed that Aβice protects against endogenous Aβ-mediated synaptotoxicity. Second, as several studies have outlined that a single intracerebral administration of Aβ can worsen Aβ deposition and cognitive functions several months after the inoculation, we evaluated in vivo the long-term effects of a single inoculation of Aβice or Aβ-wild-type (Aβwt) in the hippocampus of transgenic mice (APPswe/PS1dE9) over-expressing Aβ1-42 peptide. Interestingly, we found that the single intra-hippocampal inoculation of Aβice to mice rescued synaptic density and spatial memory losses four months post-inoculation, compared with Aβwt inoculation. Although Aβ load was not modulated by Aβice infusion, the amount of Tau-positive neuritic plaques was significantly reduced. Finally, a lower phagocytosis by microglia of post-synaptic compounds was detected in Aβice-inoculated animals, which can partly explain the increased density of synapses in the Aβice animals. Thus, a single event as Aβice inoculation can improve the fate of AD-associated pathology and phenotype in mice several months after the event. These results open unexpected fields to develop innovative therapeutic strategies against AD.

Evaluating the inter-species transmission risk of amyloid beta peptide aggregates via ingestion

BACKGROUND

Recent reports suggest that amyloid beta (Aβ) peptides can exhibit prion-like pathogenic properties. Transmission of Aβ peptide and the development of associated pathologies after surgeries with contaminated instruments and intravenous or intracerebral inoculations have now been reported across fish, rodents, primates, and humans. This raises a worrying prospect of Aβ peptides also having other characteristics typical of prions, such as evasion of the digestive process. We asked if such transmission of Aβ aggregates via ingestion was possible.

METHODS

We made use of a transgenic Drosophila melanogaster line expressing human Aβ peptide prone to aggregation. Fly larvae were fed to adult zebrafish under two feeding schemes. The first was a short-term, high-intensity scheme over 48 h to determine transmission and retention in the gut. The second, long-term scheme specifically examined retention and accumulation in the brain. The gut and brain tissues were examined by histology, western blotting, and mass spectrometric analyses.

RESULTS

None of the analyses could detect Aβ aggregates in the guts of zebrafish following ingestion, despite being easily detectable in the feed. Additionally, there was no detectable accumulation of Aβ in the brain tissue or development of associated pathologies after prolonged feeding.

CONCLUSIONS

While human Aβ aggregates do not appear to be readily transmissible by ingestion across species, two prospects remain open. First, this mode of transmission, if occurring, may stay below a detectable threshold and may take much longer to manifest. A second possibility is that the human Aβ peptide is not able to trigger self-propagation or aggregation in other species. Either possibility requires further investigation, taking into account the possibility of such transmission from agricultural species used in the food industry.

Neurodegenerative Disease Tauopathies

Tauopathies are a diverse group of progressive and fatal neurodegenerative diseases characterized by aberrant tau inclusions in the central nervous system. Tau protein forms pathologic fibrillar aggregates that are typically closely associated with neuronal cell death, leading to varied clinical phenotypes including dementia, movement disorders, and motor neuron disease. In this review, we describe the clinicopathologic features of tauopathies and highlight recent advances in understanding the mechanisms that lead to spread of pathologic aggregates through interconnected neuronal pathways. The cell-to-cell propagation of tauopathy is then linked to posttranslational modifications, tau fibril structural variants, and the breakdown of cellular protein quality control.

Tau seeds from patients induce progressive supranuclear palsy pathology and symptoms in primates

Progressive supranuclear palsy is a primary tauopathy affecting both neurons and glia and is responsible for both motor and cognitive symptoms. Recently, it has been suggested that progressive supranuclear palsy tauopathy may spread in the brain from cell to cell in a 'prion-like' manner. However, direct experimental evidence of this phenomenon, and its consequences on brain functions, is still lacking in primates. In this study, we first derived sarkosyl-insoluble tau fractions from post-mortem brains of patients with progressive supranuclear palsy. We also isolated the same fraction from age-matched control brains. Compared to control extracts, the in vitro characterization of progressive supranuclear palsy-tau fractions demonstrated a high seeding activity in P301S-tau expressing cells, displaying after incubation abnormally phosphorylated (AT8- and AT100-positivity), misfolded, filamentous (pentameric formyl thiophene acetic acid positive) and sarkosyl-insoluble tau. We bilaterally injected two male rhesus macaques in the supranigral area with this fraction of progressive supranuclear palsy-tau proteopathic seeds, and two other macaques with the control fraction. The quantitative analysis of kinematic features revealed that progressive supranuclear palsy-tau injected macaques exhibited symptoms suggestive of parkinsonism as early as 6 months after injection, remaining present until euthanasia at 18 months. An object retrieval task showed the progressive appearance of a significant dysexecutive syndrome in progressive supranuclear palsy-tau injected macaques compared to controls. We found AT8-positive staining and 4R-tau inclusions only in progressive supranuclear palsy-tau injected macaques. Characteristic pathological hallmarks of progressive supranuclear palsy, including globose and neurofibrillary tangles, tufted astrocytes and coiled bodies, were found close to the injection sites but also in connected brain regions that are known to be affected in progressive supranuclear palsy (striatum, pallidum, thalamus). Interestingly, while glial AT8-positive lesions were the most frequent near the injection site, we found mainly neuronal inclusions in the remote brain area, consistent with a neuronal transsynaptic spreading of the disease. Our results demonstrate that progressive supranuclear palsy patient-derived tau aggregates can induce motor and behavioural impairments in non-human primates related to the prion-like seeding and spreading of typical pathological progressive supranuclear palsy lesions. This pilot study paves the way for supporting progressive supranuclear palsy-tau injected macaque as a relevant animal model to accelerate drug development targeting this rare and fatal neurodegenerative disease.

Long term worsening of amyloid pathology, cerebral function, and cognition after a single inoculation of beta-amyloid seeds with Osaka mutation

Alzheimer's disease (AD) is characterized by intracerebral deposition of abnormal proteinaceous assemblies made of amyloid-β (Aß) peptides or tau proteins. These peptides and proteins induce synaptic dysfunctions that are strongly correlated with cognitive decline. Intracerebral infusion of well-defined Aβ seeds from non-mutated Aβ_1-40 or Aβ_1-42 peptides can increase Aβ depositions several months after the infusion. Familial forms of AD are associated with mutations in the amyloid precursor protein (APP) that induce the production of Aβ peptides with different structures. The Aβ Osaka (Aβ_osa mutation (E693Δ)) is located within the Aβ sequence and thus the Aβ_osa peptides have different structures and properties as compared to non-mutated Aβ_1-42 peptides (Aβ_wt). Here, we wondered if a single exposure to this mutated Aβ can worsen AD pathology as well as downstream events including cognition, cerebral connectivity and synaptic health several months after the inoculation. To answer this question we inoculated Aβ_1-42-bearing Osaka mutation (Aβ_osa) in the dentate gyrus of APP_swe/PS1_dE9 mice at the age of two months. Their cognition and cerebral connectivity were analyzed at 4 months post-inoculation by behavioral evaluation and functional MRI. Aβ pathology as well as synaptic density were evaluated by histology. The impact of Aβ_osa peptides on synaptic health was also measured on primary cortical neurons. Remarkably, the intracerebral administration of Aβ_osa induced cognitive and synaptic impairments as well as a reduction of functional connectivity between different brain regions, 4 months post-inoculation. It increased Aβ plaque depositions and increased Aβ oligomers. This is the first study showing that a single, sporadic event as Aβ_osa inoculation can worsen the fate of the pathology and clinical outcome several months after the event. It suggests that a single inoculation of Aβ regulates a large cascade of events for a long time.

Cell numbers in the reflected blade of CA3 and their relation to other hippocampal principal cell populations across seven species

The hippocampus of many mammals contains a histoarchitectural region that is not present in laboratory mice and rats-the reflected blade of the CA3 pyramidal cell layer. Pyramidal cells of the reflected blade do not extend dendrites into the hippocampal molecular layer, and recent evidence indicates that they, like the proximal CA3 pyramids in laboratory rats and mice, partially integrate functionally with the dentate circuitry in pattern separation. Quantitative assessments of phylogenetic or disease-related changes in the hippocampal structure and function treat the reflected blade heterogeneously. Depending on the ease with which it can be differentiated, it is either assigned to the dentate hilus or to the remainder of CA3. Here, we investigate the impact that the differential assignment of reflected blade neurons may have on the outcomes of quantitative comparisons. We find it to be massive. If reflected blade neurons are treated as a separate entity or pooled with dentate hilar cells, the quantitative makeup of hippocampal cell populations can differentiate between species in a taxonomically sensible way. Assigning reflected blade neurons to CA3 greatly diminishes the differentiating power of all hippocampal principal cell populations, which may point towards a quantitative hippocampal archetype. A heterogeneous assignment results in a differentiation pattern with little taxonomic semblance. The outcomes point towards the reflected blade as either a major potential player in hippocampal functional and structural differentiation or a region that may have cloaked that hippocampi are more similarly organized across species than generally believed.

A general deep learning framework for neuron instance segmentation based on Efficient UNet and morphological post-processing

Recent studies have demonstrated the superiority of deep learning in medical image analysis, especially in cell instance segmentation, a fundamental step for many biological studies. However, the excellent performance of the neural networks requires training on large, unbiased dataset and annotations, which is labor-intensive and expertise-demanding. This paper presents an end-to-end framework to automatically detect and segment NeuN stained neuronal cells on histological images using only point annotations. Unlike traditional nuclei segmentation with point annotation, we propose using point annotation and binary segmentation to synthesize pixel-level annotations. The synthetic masks are used as the ground truth to train the neural network, a U-Net-like architecture with a state-of-the-art network, EfficientNet, as the encoder. Validation results show the superiority of our model compared to other recent methods. In addition, we investigated multiple post-processing schemes and proposed an original strategy to convert the probability map into segmented instances using ultimate erosion and dynamic reconstruction. This approach is easy to configure and outperforms other classical post-processing techniques. This work aims to develop a robust and efficient framework for analyzing neurons using optical microscopic data, which can be used in preclinical biological studies and, more specifically, in the context of neurodegenerative diseases. Code is available at: https://github.com/MIRCen/NeuronInstanceSeg.

Seeding, maturation and propagation of amyloid β-peptide aggregates in Alzheimer’s disease

Alzheimer’s disease is neuropathologically characterized by the deposition of the amyloid β-peptide (Aβ) as amyloid plaques. Aβ plaque pathology starts in the neocortex before it propagates into further brain regions. Moreover, Aβ aggregates undergo maturation indicated by the occurrence of post-translational modifications. Here, we show that propagation of Aβ plaques is led by presumably non-modified Aβ followed by Aβ aggregate maturation. This sequence was seen neuropathologically in human brains and in amyloid precursor protein transgenic mice receiving intracerebral injections of human brain homogenates from cases varying in Aβ phase, Aβ load and Aβ maturation stage. The speed of propagation after seeding in mice was best related to the Aβ phase of the donor, the progression speed of maturation to the stage of Aβ aggregate maturation. Thus, different forms of Aβ can trigger propagation/maturation of Aβ aggregates, which may explain the lack of success when therapeutically targeting only specific forms of Aβ.

Photocatalytic Inactivation of Viruses and Prions: Multilevel Approach with Other Disinfectants

Ag, Cu, Zn, Ti, and Au nanoparticles show enhanced photocatalytic properties. Efficient indoor disinfection strategies are imperative to manage the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. Virucidal agents, such as ethanol, sodium hypochlorite, 222-nm UV light, and electrolyzed water inactivate SARS-CoV-2 in indoor environments. Tungsten trioxide (WO3) photocatalyst and visible light disinfect abiotic surfaces against SARS-CoV-2. The titanium dioxide (TiO2)/UV system inactivates SARS-CoV-2 in aerosols and on deliberately contaminated TiO2-coated glass slide surfaces in photocatalytic chambers, wherein 405-nm UV light treatment for 20 min sterilizes the environment and generates reactive oxygen species (ROS) that inactivate the virus by targeting S and envelope proteins and viral RNA. Mesoscopic calcium bicarbonate solution (CAC-717) inactivates pathogens, such as prions, influenza virus, SARS-CoV-2, and noroviruses, in fluids; it presumably acts similarly on human and animal skin. The molecular complexity of cementitious materials promotes the photocatalysis of microorganisms. In combination, the two methods can reduce the pathogen load in the environment. As photocatalysts and CAC-717 are potent disinfectants for prions, disinfectants against prionoids could be developed by combining photocatalysis, gas plasma methodology, and CAC-717 treatment, especially for surgical devices and instruments.

BioSimia, France CNRS network for nonhuman primate biomedical research in infectiology, immunology, and neuroscience

Research and developments based on nonhuman primate models have a specific place in biomedical sciences, and nonhuman primate species also have a specific place in the public opinion on the use of animal in research. While nonhuman primates are used in very limited number compared to other animal models, they are rightly the focus of deep ethical concerns. The importance of nonhuman primates in neuroscientific fundamental and preclinical discoveries together with the targeting of such research by activist groups well illustrate this fact. Nonhuman primates also highly contribute to other biomedical fields including immunology, virology, or metabolic and respiratory physiology. In all these fields, researchers, engineers and technicians face similar matters and share the same needs for optimal animal welfare, handling, and veterinary care, the same quest for first-rate research infrastructure and funding, and the same yearning for more public understanding and support. In this article, we give an overview of the evolution of human-animal relationships and public attitudes to animal research in France, and we recount the creation of BioSimia, France network for nonhuman primate biomedical research which now links all academic laboratories nationwide in all the domains for which nonhuman primates remain essential. We explain the principles as well as the outcomes of networking across disciplines. As a perspective, we outline the potential benefits of extending such network to a European scale.

Pathological changes induced by Alzheimer's brain inoculation in amyloid-beta plaque-bearing mice

Alzheimer's disease (AD) is characterized by intracerebral accumulations of extracellular amyloid-β (Aβ) plaques and intracellular tau pathology that spread in the brain. Three types of tau lesions occur in the form of neuropil threads, neurofibrillary tangles, and neuritic plaques i.e. tau aggregates within neurites surrounding Aβ deposits. The cascade of events linking these lesions and synaptic or memory impairments are still debated. Intracerebral infusion of human AD brain extracts in Aβ plaque-bearing mice that do not overexpress pathological tau proteins induces tau pathologies following heterotopic seeding of mouse tau protein. There is however little information regarding the downstream events including synaptic or cognitive repercussions of tau pathology induction in these models. In the present study, human AD brain extracts (AD_be) and control-brain extracts (Ctrl_be) were infused into the hippocampus of Aβ plaque-bearing APP_swe/PS1_dE9 mice. Memory, synaptic density, as well as Aβ plaque and tau aggregate loads, microgliosis, astrogliosis at the inoculation site and in connected regions (perirhinal/entorhinal cortex) were evaluated 4 and 8 months post-inoculation. AD_be inoculation produced the following effects: (i) memory deficit; (ii) increased Aβ plaque deposition in proximity to the inoculation site; (iii) tau pathology induction; (iv) appearance of neuropil threads and neurofibrillary tangles next to the inoculation site with a spreading to connected regions. Neuritic plaque pathology was detected in both AD_be- and Ctrl_be-inoculated animals but AD_be inoculation increased the severity close to and at distance of the inoculation site. (v) Finally, AD_be inoculation reduced synaptic density in the vicinity to the inoculation site and in connected regions as the perirhinal/entorhinal cortex. Synaptic impairments were correlated with increased severity of neuritic plaques but not to other tau lesions or Aβ lesions, suggesting that neuritic plaques are a culprit for synaptic loss. Synaptic density was also associated with microglial load.

Non-human primates in prion diseases

The fascinating history of prion diseases is intimately linked to the use of nonhuman primates as experimental models, which brought so fundamental and founding information about transmissibility, pathogenesis, and resistance of prions. These models are still of crucial need for risk assessment of human health and may contribute to pave a new way towards the moving field of prion-like entities which now includes the main human neurodegenerative diseases (especially Alzheimer's and Parkinson's diseases).

Emerging Electroencephalographic Biomarkers to Improve Preclinical to Clinical Translation in Alzheimer’s Disease

Continually emerging data indicate that sub-clinical, non-convulsive epileptiform activity is not only prevalent in Alzheimer’s disease (AD) but is detectable early in the course of the disease and predicts cognitive decline in both humans and animal models. Epileptiform activity and other electroencephalographic (EEG) measures may hold powerful, untapped potential to improve the translational validity of AD-related biomarkers in model animals ranging from mice, to rats, and non-human primates. In this review, we will focus on studies of epileptiform activity, EEG slowing, and theta-gamma coupling in preclinical models, with particular focus on its role in cognitive decline and relevance to AD. Here, each biomarker is described in the context of the contemporary literature and recent findings in AD relevant animal models are discussed.

Relationships between endogenous circadian period, physiological and cognitive parameters and sex in aged gray mouse lemurs (Microcebus murinus)

The biological clock generates circadian rhythms, with an endogenous period tau close to 24 h. The circadian resonance theory proposes that lifespan is reduced when endogenous period goes far from 24 h. It has been suggested that daily resetting of the circadian clock to the 24 h external photoperiod might induce marginal costs that would accumulate over time and forward accelerate aging and affect fitness. In this study, we aimed to evaluate the link between the endogenous period and biomarkers of aging in order to investigate the mechanisms of the circadian resonance theory. We studied 39 middle-aged and aged Microcebus murinus, a nocturnal non-human primate whose endogenous period is about 23.1 h, measuring the endogenous period of locomotor activity, as well as several physiological and behavioral parameters (rhythm fragmentation and amplitude, energetic expenditure, oxidative stress, insulin-like growth factor-1 (IGF-1) concentrations and cognitive performances) in both males and females. We found that aged males with tau far from 24 h displayed increased oxidative stress. We also demonstrated a positive correlation between tau and IGF-1 concentrations, as well as learning performances, in males and females. Together these results suggest that a great deviation of tau from 24 h leads to increased biomarkers of age-related impairments.

Modelling behaviors relevant to brain disorders in the nonhuman primate: Are we there yet?

Recent years have seen a profound resurgence of activity with nonhuman primates (NHPs) to model human brain disorders. From marmosets to macaques, the study of NHP species offers a unique window into the function of primate-specific neural circuits that are impossible to examine in other models. Examining how these circuits manifest into the complex behaviors of primates, such as advanced cognitive and social functions, has provided enormous insights to date into the mechanisms underlying symptoms of numerous neurological and neuropsychiatric illnesses. With the recent optimization of modern techniques to manipulate and measure neural activity in vivo, such as optogenetics and calcium imaging, NHP research is more well-equipped than ever to probe the neural mechanisms underlying pathological behavior. However, methods for behavioral experimentation and analysis in NHPs have noticeably failed to keep pace with these advances. As behavior ultimately lies at the junction between preclinical findings and its translation to clinical outcomes for brain disorders, approaches to improve the integrity, reproducibility, and translatability of behavioral experiments in NHPs requires critical evaluation. In this review, we provide a unifying account of existing brain disorder models using NHPs, and provide insights into the present and emerging contributions of behavioral studies to the field.

Transmission of amyloid-beta and tau pathologies is associated with cognitive impairments in a primate

Amyloid-β (Aβ) pathology transmission has been described in patients following iatrogenic exposure to compounds contaminated with Aβ proteins. It can induce cerebral Aβ angiopathy resulting in brain hemorrhages and devastating clinical impacts. Iatrogenic transmission of tau pathology is also suspected but not experimentally proven. In both scenarios, lesions were detected several decades after the putatively triggering medico-surgical act. There is however little information regarding the cognitive repercussions in individuals who do not develop cerebral hemorrhages. In the current study, we inoculated the posterior cingulate cortex and underlying corpus callosum of young adult primates (Microcebus murinus) with either Alzheimer's disease or control brain extracts. This led to widespread Aβ and tau pathologies in all of the Alzheimer-inoculated animals following a 21-month-long incubation period (n = 12) whereas none of the control brain extract-inoculated animals developed such lesions (n = 6). Aβ deposition affected almost all cortical regions. Tau pathology was also detected in Aβ-deposit-free regions distant from the inoculation sites (e.g. in the entorhinal cortex), while some regions adjacent, but not connected, to the inoculation sites were spared (e.g. the occipital cortex). Alzheimer-inoculated animals developed cognitive deficits and cerebral atrophy compared to controls. These pathologies were induced using two different batches of Alzheimer brain extracts. This is the first experimental demonstration that tau can be transmitted by human brain extracts inoculations in a primate. We also showed for the first time that the transmission of widespread Aβ and tau pathologies can be associated with cognitive decline. Our results thus reinforce the need to organize a systematic monitoring of individuals who underwent procedures associated with a risk of Aβ and tau iatrogenic transmission. They also provide support for Alzheimer brain-inoculated primates as relevant models of Alzheimer pathology.

Transmission of cerebral amyloid pathology by peripheral administration of misfolded Aβ aggregates

Previous reports showed that brain Aβ amyloidosis can be induced in animal models by exogenous administration of pre-formed aggregates. To date, only intra-peritoneal and intra-venous administrations are described as effective means to peripherally accelerate brain Aβ amyloidosis by seeding. Here, we show that cerebral accumulation of Aβ can be accelerated after exposing mouse models of Alzheimer's disease (AD) to Aβ seeds by different peripheral routes of administration, including intra-peritoneal and intra-muscular. Interestingly, animals receiving drops of brain homogenate laden with Aβ seeds in the eyes were efficiently induced. On the contrary, oral administration of large quantities of brain extracts from aged transgenic mice and AD patients did not have any effect in brain pathology. Importantly, pathological induction by peripheral administration of Aβ seeds generated a large proportion of aggregates in blood vessels, suggesting vascular transport. This information highlights the role of peripheral tissues and body fluids in AD-related pathological changes.

Nonhuman primates at the intersection of aging biology, chronic disease, and health: An introduction to the American journal of primatology special issue on aging, cognitive decline, and neuropathology in nonhuman primates

Aging across the Primate Order is poorly understood because ages of individuals are often unknown, there is a dearth of aged animals available for study, and because aging is best characterized by longitudinal studies which are difficult to carry out in long-lived species. The human population is aging rapidly, and advanced age is a primary risk factor for several chronic diseases and conditions that impact healthspan. As lifespan has increased, diseases and disorders of the central nervous system (CNS) have become more prevalent, and Alzheimer's disease and related dementias have become epidemic. Nonhuman primate (NHP) models are key to understanding the aging primate CNS. This Special Issue presents a review of current knowledge about NHP CNS aging across the Primate Order. Similarities and differences to human aging, and their implications for the validity of NHP models of aging are considered. Topics include aging-related brain structure and function, neuropathologies, cognitive performance, social behavior and social network characteristics, and physical, sensory, and motor function. Challenges to primate CNS aging research are discussed. Together, this collection of articles demonstrates the value of studying aging in a breadth of NHP models to advance our understanding of human and nonhuman primate aging and healthspan.

Amyloid-type Protein Aggregation and Prion-like Properties of Amyloids

This review will focus on the process of amyloid-type protein aggregation. Amyloid fibrils are an important hallmark of protein misfolding diseases and therefore have been investigated for decades. Only recently, however, atomic or near-atomic resolution structures have been elucidated from various in vitro and ex vivo obtained fibrils. In parallel, the process of fibril formation has been studied in vitro under highly artificial but comparatively reproducible conditions. The review starts with a summary of what is known and speculated from artificial in vitro amyloid-type protein aggregation experiments. A partially hypothetic fibril selection model will be described that may be suitable to explain why amyloid fibrils look the way they do, in particular, why at least all so far reported high resolution cryo-electron microscopy obtained fibril structures are in register, parallel, cross-β-sheet fibrils that mostly consist of two protofilaments twisted around each other. An intrinsic feature of the model is the prion-like nature of all amyloid assemblies. Transferring the model from the in vitro point of view to the in vivo situation is not straightforward, highly hypothetic, and leaves many open questions that need to be addressed in the future.

Prion Diseases: A Unique Transmissible Agent or a Model for Neurodegenerative Diseases?

The accumulation and propagation in the brain of misfolded proteins is a pathological hallmark shared by many neurodegenerative diseases such as Alzheimer's disease (Aβ and tau), Parkinson's disease (α-synuclein), and prion disease (prion protein). Currently, there is no epidemiological evidence to suggest that neurodegenerative disorders are infectious, apart from prion diseases. However, there is an increasing body of evidence from experimental models to suggest that other pathogenic proteins such as Aβ and tau can propagate in vivo and in vitro in a prion-like mechanism, inducing the formation of misfolded protein aggregates such as amyloid plaques and neurofibrillary tangles. Such similarities have raised concerns that misfolded proteins, other than the prion protein, could potentially transmit from person-to-person as rare events after lengthy incubation periods. Such concerns have been heightened following a number of recent reports of the possible inadvertent transmission of Aβ pathology via medical and surgical procedures. This review will provide a historical perspective on the unique transmissible nature of prion diseases, examining their impact on public health and the ongoing concerns raised by this rare group of disorders. Additionally, this review will provide an insight into current evidence supporting the potential transmissibility of other pathogenic proteins associated with more common neurodegenerative disorders and the potential implications for public health.

Current and future applications of induced pluripotent stem cell-based models to study pathological proteins in neurodegenerative disorders

Neurodegenerative disorders emerge from the failure of intricate cellular mechanisms, which ultimately lead to the loss of vulnerable neuronal populations. Research conducted across several laboratories has now provided compelling evidence that pathogenic proteins can also contribute to non-cell autonomous toxicity in several neurodegenerative contexts, including Alzheimer's, Parkinson's, and Huntington's diseases as well as Amyotrophic Lateral Sclerosis. Given the nearly ubiquitous nature of abnormal protein accumulation in such disorders, elucidating the mechanisms and routes underlying these processes is essential to the development of effective treatments. To this end, physiologically relevant human in vitro models are critical to understand the processes surrounding uptake, release and nucleation under physiological or pathological conditions. This review explores the use of human-induced pluripotent stem cells (iPSCs) to study prion-like protein propagation in neurodegenerative diseases, discusses advantages and limitations of this model, and presents emerging technologies that, combined with the use of iPSC-based models, will provide powerful model systems to propel fundamental research forward.

Induction of amyloid-β deposits from serially transmitted, histologically silent, Aβ seeds issued from human brains

In humans, iatrogenic transmission of cerebral amyloid-β (Aβ)-amyloidosis is suspected following inoculation of pituitary-derived hormones or dural grafts presumably contaminated with Aβ proteins as well as after cerebral surgeries. Experimentally, intracerebral inoculation of brain homogenate extracts containing misfolded Aβ can seed Aβ deposition in transgenic mouse models of amyloidosis or in non-human primates. The transmission of cerebral Aβ is governed by the host and by the inoculated samples. It is critical to better characterize the propensities of different hosts to develop Aβ deposition after contamination by an Aβ-positive sample as well as to better assess which biological samples can transmit this lesion. Aβ precursor protein (huAPP_wt) mice express humanized non-mutated forms of Aβ precursor protein and do not spontaneously develop Aβ or amyloid deposits. We found that inoculation of Aβ-positive brain extracts from Alzheimer patients in these mice leads to a sparse Aβ deposition close to the alveus 18 months post-inoculation. However, it does not induce cortical or hippocampal Aβ deposition. Secondary inoculation of apparently amyloid deposit-free hippocampal extracts from these huAPP_wt mice to APP_swe/PS1_dE9 mouse models of amyloidosis enhanced Aβ deposition in the alveus 9 months post-inoculation. This suggests that Aβ seeds issued from human brain samples can persist in furtive forms in brain tissues while maintaining their ability to foster Aβ deposition in receptive hosts that overexpress endogenous Aβ. This work emphasizes the need for high-level preventive measures, especially in the context of neurosurgery, to prevent the risk of iatrogenic transmission of Aβ lesions from samples with sparse amyloid markers.

The art of growing old: environmental manipulation, physiological rhythms, and the advent of Microcebus murinus as a primate model of aging

In the early 1990s, Microcebus murinus, a small primate endemic to Madagascar, emerged as a potential animal model for the study of aging and Alzheimer's disease. This paper traces the use of the lesser mouse lemur in research on aging and associated neurodegenerative diseases, focusing on a basic material precondition that made this possible, namely, the conversion of a wild animal into an experimental organism that lives, breeds, and survives in the laboratory. It argues that the "old" mouse lemur model can be considered as an eco-zootechnical acquisition. This is shown by examining how, since the early 1970s, French mouse lemur researchers have articulated colony productivity and viability with the influence of environmental factors on the demographics and physiology of the species. The appearance and maintenance of a growing number of old mouse lemurs in French research facilities are related to three developments: the application of the ecological notion of "social stress" to the understanding and management of the behavior of the captive population; the experimental demonstration that a variety of seasonal physiological changes in the species were influenced by the photoperiod; and the related attempt to accelerate aging in mouse lemurs through the manipulation of annual light conditions.

Evidence for the spread of human-derived mutant huntingtin protein in mice and non-human primates

In recent years, substantial evidence has emerged to suggest that spreading of pathological proteins contributes to disease pathology in numerous neurodegenerative disorders. Work from our laboratory and others have shown that, despite its strictly genetic nature, Huntington's disease (HD) may be another condition in which this mechanism contributes to pathology. In this study, we set out to determine if the mutant huntingtin protein (mHTT) present in post-mortem brain tissue derived from HD patients can induce pathology in mice and/or non-human primates. For this, we performed three distinct sets of experiments where homogenates were injected into the brains of adult a) Wild-type (WT) and b) BACHD mice or c) non-human primates. Neuropathological assessments revealed that, while changes in the endogenous huntingtin were not apparent, mHTT could spread between cellular elements and brain structures. Furthermore, behavioural differences only occurred in the animal model of HD which already overexpressed mHTT. Taken together, our results indicate that mHTT derived from human brains has only a limited capacity to propagate between cells and does not depict prion-like characteristics. This contrasts with recent work demonstrating that other forms of mHTT - such as fibrils of a pathological polyQ length or fibroblasts and induced pluripotent stem cells derived from HD cases - can indeed disseminate disease throughout the brain in a prion-like fashion.

Intracerebral seeding of amyloid-β and tau pathology in mice: Factors underlying prion-like spreading and comparisons with α-synuclein

Alzheimer's disease (AD) is characterized neuropathologically by progressive neurodegeneration and by the presence of amyloid plaques and neurofibrillary tangles. These plaques and tangles are composed, respectively, of amyloid-beta (Aβ) and tau proteins. While long recognized as hallmarks of AD, it remains unclear what causes the formation of these insoluble deposits. One theory holds that prion-like templated misfolding of Aβ and tau induces these proteins to form pathological aggregates, and propagation of this misfolding causes the stereotyped progression of pathology commonly seen in AD. Supporting this theory, numerous studies have been conducted in which aggregated Aβ, tau, or α-synuclein is injected intracerebrally into pathology-free host animals, resulting in robust formation of pathology. Here, we review this literature, focusing on in vivo intracerebral seeding of Aβ and tau in mice. We compare the results of these experiments to what is known about the seeding and spread of α-synuclein pathology, and we discuss how this research informs our understanding of the factors underlying the onset, progression, and outcomes of proteinaceous pathologies.

Strengths and Weaknesses of the Gray Mouse Lemur (Microcebus murinus) as a Model for the Behavioral and Psychological Symptoms and Neuropsychiatric Symptoms of Dementia

To face the load of the prevalence of Alzheimer’s disease in the aging population, there is an urgent need to develop more translatable animal models with similarities to humans in both the symptomatology and physiopathology of dementia. Due to their close evolutionary similarity to humans, non-human primates (NHPs) are of primary interest. Of the NHPs, to date, the gray mouse lemur (Microcebus murinus) has shown promising evidence of its translatability to humans. The present review reports the known advantages and limitations of using this species at all levels of investigation in the context of neuropsychiatric conditions. In this easily bred Malagasy primate with a relatively short life span (approximately 12 years), age-related cognitive decline, amyloid angiopathy, and risk factors (i.e., glucoregulatory imbalance) are congruent with those observed in humans. More specifically, analogous behavioral and psychological symptoms and neuropsychiatric symptoms of dementia (BPSD/NPS) to those in humans can be found in the aging mouse lemur. Aged mouse lemurs show typical age-related alterations of locomotor activity daily rhythms such as decreased rhythm amplitude, increased fragmentation, and increased activity during the resting-sleeping phase of the day and desynchronization with the light-dark cycle. In addition, sleep deprivation successfully induces cognitive deficits in adult mouse lemurs, and the effectiveness of approved cognitive enhancers such as acetylcholinesterase inhibitors or N-methyl-D-aspartate antagonists is demonstrated in sleep–deprived animals. This result supports the translational potential of this animal model, especially for unraveling the mechanisms underlying dementia and for developing novel therapeutics to prevent age-associated cognitive decline. In conclusion, actual knowledge of BPSD/NPS-like symptoms of age-related cognitive deficits in the gray mouse lemur and the recent demonstration of the similarity of these symptoms with those seen in humans offer promising new ways of investigating both the prevention and treatment of pathological aging.