Tauopathy with paired helical filaments in an aged chimpanzee

Might Diet, APOE-APOA1 Axis, and Iron Metabolism Provide Clues About the Discrepancy in Alzheimer's Disease Occurrence Between Humans and Chimpanzees? A Bioinformatics-Based Re-Analysis of Gene Expression Data on Mice Fed with Human and Chimpanzee Diets

The emergence of conflicting reports on the natural occurrence of Alzheimer's disease (AD) in non-human primates has prompted research on the comparison of the role of diet-associated changes in gene expression between humans and non-human primates. This article analyzes the effects of different human and chimpanzee diets and their link with apolipoproteins, lipid, and iron (Fe) metabolism, starting from available data, to find out any gap in the existing knowledge. By using a system biology approach, we have re-analyzed the liver and brain RNA seq data of mice fed with either human or chimpanzee diet for 2 weeks to look for genetic differences that may explain the differences in AD occurrence between those two classes. In liver samples of mice fed with the chimpanzee diet in comparison to the human diet, apolipoprotein A-1, ceruloplasmin, and 10 other genes were upregulated while 21 genes were downregulated. However, brain apolipoprotein E4 gene expression was not changed upon diet. Genetic, structural, and functional differences in apolipoprotein E protein, along with differences in Fe metabolisms and a longer lifespan of humans during evolution may account for the observed disparity.

Cognitive Deficits and Alzheimer's Disease-Like Pathologies in the Aged Chinese Tree Shrew

Alzheimer's disease (AD) is the most common chronic progressive neurodegenerative disease in the elderly. It has an increasing prevalence and a growing health burden. One of the limitations in studying AD is the lack of animal models that show features of Alzheimer's pathogenesis. The tree shrew has a much closer genetic affinity to primates than to rodents and has great potential to be used for research into aging and AD. In this study, we aimed to investigate whether tree shrews naturally develop cognitive impairment and major AD-like pathologies with increasing age. Pole-board and novel object recognition tests were used to assess the cognitive performance of adult (about 1 year old) and aged (6 years old or older) tree shrews. The main AD-like pathologies were assessed by Western blotting, immunohistochemical staining, immunofluorescence staining, and Nissl staining. Our results showed that the aged tree shrews developed an impaired cognitive performance compared to the adult tree shrews. Moreover, the aged tree shrews exhibited several age-related phenotypes that are associated with AD, including increased levels of amyloid-β (Aβ) accumulation and phosphorylated tau protein, synaptic and neuronal loss, and reactive gliosis in the cortex and the hippocampal tissues. Our study provides further evidence that the tree shrew is a promising model for the study of aging and AD.

Bearded capuchin monkeys as a model for Alzheimer's disease

The absence of a natural animal model is one of the main challenges in Alzheimer's disease research. Despite the challenges of using nonhuman primates in studies, these animals can bridge mouse models and humans, as nonhuman primates are phylogenetically closer to humans and can spontaneously develop AD-type pathology. The capuchin monkey, a New World primate, has recently attracted attention due to its skill in creating and using instruments. We analyzed one capuchin brain using structural 7 T MRI and performed a neuropathological evaluation of three animals. Alzheimer-type pathology was found in the two of the capuchins. Widespread β-amyloid pathology was observed, mainly in focal deposits with variable morphology and a high density of mature plaques. Notably, plaque-associated dystrophic neurites associated with disruption of axonal transport and early cytoskeletal alteration were frequently found. Unlike in other species of New World monkeys, cerebral arterial angiopathy was not the predominant form of β-amyloid pathology. Additionally, abnormal aggregates of hyperphosphorylated tau, resembling neurofibrillary pathology, were observed in the temporal and frontal cortex. Astrocyte hypertrophy surrounding plaques was found, suggesting a neuroinflammatory response. These findings indicate that aged capuchin monkeys can spontaneously develop Alzheimer-type pathology, indicating that they may be an advantageous animal model for research in Alzheimer's disease.

Addressing the Discrepancies Between Animal Models and Human Alzheimer's Disease Pathology: Implications for Translational Research

Animal models, particularly transgenic mice, are extensively used in Alzheimer's disease (AD) research to emulate key disease hallmarks, such as amyloid plaques and neurofibrillary tangles formation. Although these models have contributed to our understanding of AD pathogenesis and can be helpful in testing potential therapeutic interventions, their reliability is dubious. While preclinical studies have shown promise, clinical trials often yield disappointing results, highlighting a notable gap and disparity between animal models and human AD pathology. Existing models frequently overlook early-stage human pathologies and other key AD characteristics, thereby limiting their application in identifying optimal therapeutic interventions. Enhancing model reliability necessitates rigorous study design, comprehensive behavioral evaluations, and biomarker utilization. Overall, a nuanced understanding of each model's neuropathology, its fidelity to human AD, and its limitations is essential for accurate interpretation and successful translation of findings. This article analyzes the discrepancies between animal models and human AD pathology that complicate the translation of findings from preclinical studies to clinical applications. We also delve into AD pathogenesis and attributes to propose a new perspective on this pathology and deliberate over the primary limitations of key experimental models. Additionally, we discuss several fundamental problems that may explain the translational failures and suggest some possible directions for more effective preclinical studies.

Senile plaques and phosphorylated tau deposition in a super-aged rhesus monkey (Macaca mulatta)

The brain of a rhesus monkey that died at 43 years of age with symptoms of suspected cognitive dysfunction was analyzed. pathological analyses revealed characteristic Alzheimer's disease-related lesions: the aggregation of amyloid β (Aβ) in the form of senile plaques and phosphorylated tau proteins. We also revealed that Aβ43, which is prone to aggregation and toxicity in humans, is involved in senile plaques in the brain of the rhesus monkey, as well as several other Aβ species. Comparative studies of neuropathology using aged nonhuman primates lack behavioral descriptions compared to human medicine. This case report showed behavioral abnormalities and the detailed pathological changes that may have caused it in a super-aged rhesus monkey.

Tauopathy promotes spinal cord-dependent production of toxic amyloid-beta in transgenic monkeys

Tauopathy, characterized by the hyperphosphorylation and accumulation of the microtubule-associated protein tau, and the accumulation of Aβ oligomers, constitute the major pathological hallmarks of Alzheimer's disease. However, the relationship and causal roles of these two pathological changes in neurodegeneration remain to be defined, even though they occur together or independently in several neurodegenerative diseases associated with cognitive and movement impairment. While it is widely accepted that Aβ accumulation leads to tauopathy in the late stages of the disease, it is still unknown whether tauopathy influences the formation of toxic Aβ oligomers. To address this, we generated transgenic cynomolgus monkey models expressing Tau (P301L) through lentiviral infection of monkey embryos. These monkeys developed age-dependent neurodegeneration and motor dysfunction. Additionally, we performed a stereotaxic injection of adult monkey and mouse brains to express Tau (P301L) via AAV9 infection. Importantly, we found that tauopathy resulting from embryonic transgenic Tau expression or stereotaxic brain injection of AAV-Tau selectively promoted the generation of Aβ oligomers in the monkey spinal cord. These Aβ oligomers were recognized by several antibodies to Aβ1-42 and contributed to neurodegeneration. However, the generation of Aβ oligomers was not observed in other brain regions of Tau transgenic monkeys or in the brains of mice injected with AAV9-Tau (P301L), suggesting that the generation of Aβ oligomers is species- and brain region-dependent. Our findings demonstrate for the first time that tauopathy can trigger Aβ pathology in the primate spinal cord and provide new insight into the pathogenesis and treatment of tauopathy.

Transcription, structure, and organoids translate time across the lifespan of humans and great apes

How the neural structures supporting human cognition developed and arose in evolution is an enduring question of interest. Yet, we still lack appropriate procedures to align ages across primates, and this lacuna has hindered progress in understanding the evolution of biological programs. We generated a dataset of unprecedented size consisting of 573 time points from abrupt and gradual changes in behavior, anatomy, and transcription across human and 8 nonhuman primate species. We included time points from diverse human populations to capture within-species variation in the generation of cross-species age alignments. We also extracted corresponding ages from organoids. The identification of corresponding ages across the lifespan of 8 primate species, including apes (e.g., orangutans, gorillas) and monkeys (i.e., marmosets, macaques), reveals that some biological pathways are extended in humans compared with some nonhuman primates. Notably, the human lifespan is unusually extended relative to studied nonhuman primates demonstrating that very old age is a phase of life in humans that does not map to other studied primate species. More generally, our work prompts a reevaluation in the choice of a model system to understand aging given very old age in humans is a period of life without a clear counterpart in great apes.

Comparative physiological anthropogeny: exploring molecular underpinnings of distinctly human phenotypes

Anthropogeny is a classic term encompassing transdisciplinary investigations of the origins of the human species. Comparative anthropogeny is a systematic comparison of humans and other living nonhuman hominids (so-called "great apes"), aiming to identify distinctly human features in health and disease, with the overall goal of explaining human origins. We begin with a historical perspective, briefly describing how the field progressed from the earliest evolutionary insights to the current emphasis on in-depth molecular and genomic investigations of "human-specific" biology and an increased appreciation for cultural impacts on human biology. While many such genetic differences between humans and other hominids have been revealed over the last two decades, this information remains insufficient to explain the most distinctive phenotypic traits distinguishing humans from other living hominids. Here we undertake a complementary approach of "comparative physiological anthropogeny," along the lines of the preclinical medical curriculum, i.e., beginning with anatomy and considering each physiological system and in each case considering genetic and molecular components that are relevant. What is ultimately needed is a systematic comparative approach at all levels from molecular to physiological to sociocultural, building networks of related information, drawing inferences, and generating testable hypotheses. The concluding section will touch on distinctive considerations in the study of human evolution, including the importance of gene-culture interactions.

Going beyond established model systems of Alzheimer's disease: companion animals provide novel insights into the neurobiology of aging

Alzheimer's disease (AD) is characterized by brain plaques, tangles, and cognitive impairment. AD is one of the most common age-related dementias in humans. Progress in characterizing AD and other age-related disorders is hindered by a perceived dearth of animal models that naturally reproduce diseases observed in humans. Mice and nonhuman primates are model systems used to understand human diseases. Still, these model systems lack many of the biological characteristics of Alzheimer-like diseases (e.g., plaques, tangles) as they grow older. In contrast, companion animal models (cats and dogs) age in ways that resemble humans. Both companion animal models and humans show evidence of brain atrophy, plaques, and tangles, as well as cognitive decline with age. We embrace a One Health perspective, which recognizes that the health of humans is connected to those of animals, and we illustrate how such a perspective can work synergistically to enhance human and animal health. A comparative biology perspective is ideally suited to integrate insights across veterinary and human medical disciplines and solve long-standing problems in aging.

Age differences in cortical thickness and their association with cognition in chimpanzee (Pan troglodytes)

Humans and chimpanzees are genetically similar and share a number of life history, behavioral, cognitive and neuroanatomical similarities. Notwithstanding, our understanding of age-related changes in cognitive and motor functions in chimpanzees remains largely unstudied despite recent evident demonstrating that chimpanzees exhibit many of the same neuropathological features of Alzheimer's disease observed in human postmortem brains. Here, we examined age-related differences in cognition and cortical thickness measured from magnetic resonance images in a sample of 215 chimpanzees ranging in age between 9 and 54 years. We found that chimpanzees showed global and region-specific thinning of cortex with increasing age. Further, within the elderly cohort, chimpanzees that performed better than average had thicker cortex in frontal, temporal and parietal regions compared to chimpanzees that performed worse than average. Independent of age, we also found sex differences in cortical thickness in 4 brain regions. Males had higher adjusted cortical thickness scores for the caudal anterior cingulate, rostral anterior cingulate, and medial orbital frontal while females had higher values for the inferior parietal cortex. We found no evidence that increasing age nor sex was associated with asymmetries in cortical thickness. Moreover, age-related differences in cognitive function were only weakly associated with asymmetries in cortical thickness. In summary, as has been reported in humans and other primates, elderly chimpanzees show thinner cortex and variation in cortical thickness is associated with general cognitive functions.

The association of astrogliosis and microglial activation with aging and Alzheimer's disease pathology in the chimpanzee brain

Aging and neurodegenerative disorders, such as Alzheimer's disease (AD), trigger an immune response known as glial activation in the brain. Recent evidence indicates species differences in inflammatory responses to AD pathology, highlighting the need for additional comparative studies to further understand human-specific neuropathologies. In the present study, we report on the occurrence of astrogliosis, microglial activation, and their relationship with age and AD-like pathology in a cohort of male and female chimpanzees (Pan troglodytes). Chimpanzees with severe astrogliosis exhibited widespread upregulation of hypertrophic astrocytes immunoreactive for glial fibrillary acidic protein (GFAP) throughout all layers of the dorsolateral prefrontal cortex and a loss of the interlaminar palisade. In addition, extreme astrogliosis was associated with increased astrocyte density in the absence of significant microglial activation and AD lesions. A shift from decreased resting to increased phagocytotic microglia occurred with aging, although proliferation was absent and no changes in astrogliosis was observed. Vascular amyloid correlated with decreased astrocyte and microglia densities, while tau lesions were associated with morphological changes in microglia and greater total glia density and glia: neuron ratio. These results further our understanding of inflammatory processes within the chimpanzee brain and provide comparative data to improve our understanding of human aging and neuropathological processes.

The common marmoset as a model of neurodegeneration

Human life expectancy has increased over the past few centuries, and the incidence of dementia in the older population is also projected to continue to rise. Neurodegenerative diseases are complex multifactorial conditions for which no effective treatments are currently available. Animal models are necessary to understand the causes and progression of neurodegeneration. Nonhuman primates (NHPs) offer significant advantages for the study of neurodegenerative disease. Among them, the common marmoset, Callithrix jacchus, stands out due to its easy handling, complex brain architecture, and occurrence of spontaneous beta-amyloid (Aβ) and phosphorylated tau aggregates with aging. Furthermore, marmosets present physiological adaptations and metabolic alterations associated with the increased risk of dementia in humans. In this review, we discuss the current literature on the use of marmosets as a model of aging and neurodegeneration. We highlight aspects of marmoset physiology associated with aging, such as metabolic alterations, which may help understand their vulnerability to developing a neurodegenerative phenotype that goes beyond normal aging.

Evolutionary and genomic perspectives of brain aging and neurodegenerative diseases

This chapter utilizes genomic concepts and evolutionary perspectives to further understand the possible links between typical brain aging and neurodegenerative diseases, focusing on the two most prevalent of these: Alzheimer's disease and Parkinson's disease. Aging is the major risk factor for these neurodegenerative diseases. Researching the evolutionary and molecular underpinnings of aging helps to reveal elements of the typical aging process that leave individuals more vulnerable to neurodegenerative pathologies. Very little is known about the prevalence and susceptibility of neurodegenerative diseases in nonhuman species, as only a few individuals have been observed with these neuropathologies. However, several studies have investigated the evolution of lifespan, which is closely connected with brain size in mammals, and insights can be drawn from these to enrich our understanding of neurodegeneration. This chapter explores the relationship between the typical aging process and the events in neurodegeneration. First, we examined how age-related processes can increase susceptibility to neurodegenerative diseases. Second, we assessed to what extent neurodegeneration is an accelerated form of aging. We found that while at the phenotypic level both neurodegenerative diseases and the typical aging process share some characteristics, at the molecular level they show some distinctions in their profiles, such as variation in genes and gene expression. Furthermore, neurodegeneration of the brain is associated with an earlier onset of cellular, molecular, and structural age-related changes. In conclusion, a more integrative view of the aging process, both from a molecular and an evolutionary perspective, may increase our understanding of neurodegenerative diseases.

Amyloidogenesis: What Do We Know So Far?

The study of protein aggregation, and amyloidosis in particular, has gained considerable interest in recent times. Several neurodegenerative diseases, such as Alzheimer's (AD) and Parkinson's (PD) show a characteristic buildup of proteinaceous aggregates in several organs, especially the brain. Despite the enormous upsurge in research articles in this arena, it would not be incorrect to say that we still lack a crystal-clear idea surrounding these notorious aggregates. In this review, we attempt to present a holistic picture on protein aggregation and amyloids in particular. Using a chronological order of discoveries, we present the case of amyloids right from the onset of their discovery, various biophysical techniques, including analysis of the structure, the mechanisms and kinetics of the formation of amyloids. We have discussed important questions on whether aggregation and amyloidosis are restricted to a subset of specific proteins or more broadly influenced by the biophysiochemical and cellular environment. The therapeutic strategies and the significant failure rate of drugs in clinical trials pertaining to these neurodegenerative diseases have been also discussed at length. At a time when the COVID-19 pandemic has hit the globe hard, the review also discusses the plausibility of the far-reaching consequences posed by the virus, such as triggering early onset of amyloidosis. Finally, the application(s) of amyloids as useful biomaterials has also been discussed briefly in this review.

Characterization of microtubule-associated protein tau isoforms and Alzheimer's disease-like pathology in normal sheep (Ovis aries): relevance to their potential as a model of Alzheimer's disease

Alzheimer's disease is a chronic neurodegenerative disease that accounts for up to 80% of all dementias. Characterised by deteriorations of memory and cognitive function, the key neuropathological features are accumulations of β-amyloid and hyperphosphorylated tau, as 'plaques' and 'tangles', respectively. Despite extensive study, however, the exact mechanism underlying aggregate formation in Alzheimer's disease remains elusive, as does the contribution of these aggregates to disease progression. Importantly, a recent evaluation of current Alzheimer's disease animal models suggested that rodent models are not able to fully recapitulate the pathological intricacies of the disease as it occurs in humans. Therefore, increasing attention is being paid to species that might make good alternatives to rodents for studying the molecular pathology of Alzheimer's disease. The sheep (Ovis aries) is one such species, although to date, there have been few molecular studies relating to Alzheimer's disease in sheep. Here, we investigated the Alzheimer's disease relevant histopathological characteristics of 22 sheep, using anti-β-amyloid (Abcam 12267 and mOC64) and phosphorylation specific anti-tau (AT8 and S396) antibodies. We identified numerous intraneuronal aggregates of both β-amyloid and tau that are consistent with early Alzheimer's disease-like pathology. We confirmed the expression of two 3-repeat (1N3R, 2N3R) and two 4-repeat (1N4R, 2N4R) tau isoforms in the ovine brain, which result from the alternative splicing of two tau exons. Finally, we investigated the phosphorylation status of the serine396 residue in 30 sheep, and report that the phosphorylation of this residue begins in sheep aged as young as 2 years. Together, these data show that sheep exhibit naturally occurring β-amyloid and tau pathologies, that reflect those that occur in the early stages of Alzheimer's disease. This is an important step towards the validation of the sheep as a feasible large animal species in which to model Alzheimer's disease.

Brain aging, Alzheimer's disease, and the role of stem cells in primate comparative studies

Alzheimer's disease (AD) is a progressive neurodegenerative disease that is ultimately fatal. Currently, millions of Americans are living with AD, and this number is predicted to grow with increases in the aging population. Interestingly, despite the prevalence of AD in human populations, the full AD phenotype has not been observed in any nonhuman primate (NHP) species, and it has been suggested that NHPs are immune to neurodegenerative diseases such as AD. Here, we review the typical age-related changes and pathologies in humans along with the neuropathologic changes associated with AD, and we place this information in the context of the comparative neuropathology of NHPs. We further propose the use of induced pluripotent stem cell technology as a way of addressing initial molecular processes and changes that occur in neurons and glia (in both humans and NHPs) when exposed to AD-inducing pathology prior to cell death.

The evolution of microtubule associated proteins - a reference proteomic perspective

Microtubule associated proteins (MAPs), defined as proteins that bind microtubules but are not molecular motors or severing enzymes, play a key role in regulating microtubule stability in neurons. Existing studies of the evolutionary relationships between these proteins are limited to genomic data from a small number of species. We therefore used a large collection of publicly available reference-quality eukaryotic proteomes to carry out a phylogenetic analysis of microtubule associated proteins in both vertebrates and invertebrates. Complete or near-complete reference quality proteomes were obtained from Uniprot. Microtubule associated proteins were identified using InterProtScan, aligned using MUSCLE and then phylogenetic trees constructed using the WAG algorithm. We identified 889 proteins with tubulin binding domains, of which 663 were in eukaryotes, including 168 vertebrates and 64 invertebrates. The vertebrate proteins separated into three families, resembling human MAP 2, MAP4 and MAPT, respectively, while invertebrate MAPs clustered separately. We found significant variation in number of microtubule associated proteins and number of microtubule binding domains between taxa, with fish and mollusks having an unexpectedly high number of MAPs and binding domains, respectively. Our findings represent a novel analysis of the evolution of microtubule associated proteins based on publicly available proteomics data sets. We were able to confirm the phylogeny of MAPs identified based on more limited genomic analyses, and in addition, derived several novel insights on the structure and function of MAPs.

A Review of the Current Mammalian Models of Alzheimer's Disease and Challenges That Need to Be Overcome

Alzheimer's disease (AD) is one of the looming health crises of the near future. Increasing lifespans and better medical treatment for other conditions mean that the prevalence of this disease is expected to triple by 2050. The impact of AD includes both the large toll on individuals and their families as well as a large financial cost to society. So far, we have no way to prevent, slow, or cure the disease. Current medications can only alleviate some of the symptoms temporarily. Many animal models of AD have been created, with the first transgenic mouse model in 1995. Mouse models have been beset by challenges, and no mouse model fully captures the symptomatology of AD without multiple genetic mutations and/or transgenes, some of which have never been implicated in human AD. Over 25 years later, many mouse models have been given an AD-like disease and then 'cured' in the lab, only for the treatments to fail in clinical trials. This review argues that small animal models are insufficient for modelling complex disorders such as AD. In order to find effective treatments for AD, we need to create large animal models with brains and lifespan that are closer to humans, and underlying genetics that already predispose them to AD-like phenotypes.

Predicting their past: Machine language learning can discriminate the brains of chimpanzees with different early-life social rearing experiences

Early life experiences, including separation from caregivers, can result in substantial, persistent effects on neural, behavioral, and physiological systems as is evidenced in a long-standing literature and consistent findings across species, populations, and experimental models. In humans and other animals, differential rearing conditions can affect brain structure and function. We tested for whole brain patterns of morphological difference between 108 chimpanzees reared typically with their mothers (MR; N = 54) and those reared decades ago in a nursery with peers, human caregivers, and environmental enrichment (NR; N = 54). We applied support vector machine (SVM) learning to archival MRI images of chimpanzee brains to test whether we could, with any degree of significant probability, retrospectively classify subjects as MR and NR based on variation in gray matter within the entire brain. We could accurately discriminate MR and NR chimpanzee brains with nearly 70% accuracy. The combined brain regions discriminating the two rearing groups were widespread throughout the cortex. We believe this is the first report using machine language learning as an analytic method for discriminating nonhuman primate brains based on early rearing experiences. In this sense, the approach and findings are novel, and we hope they stimulate application of the technique to studies on neural outcomes associated with early experiences. The findings underscore the potential for infant separation from caregivers to leave a long-term mark on the developing brain.

Comparative neuropathology in aging primates: A perspective

While humans exhibit a significant degree of neuropathological changes associated with deficits in cognitive and memory functions during aging, non-human primates (NHP) present with more variable expressions of pathological alterations among individuals and species. As such, NHP with long life expectancy in captivity offer an opportunity to study brain senescence in the absence of the typical cellular pathology caused by age-related neurodegenerative illnesses commonly seen in humans. Age-related changes at neuronal population, single cell, and synaptic levels have been well documented in macaques and marmosets, while age-related and Alzheimer's disease-like neuropathology has been characterized in additional species including lemurs as well as great apes. We present a comparative overview of existing neuropathologic observations across the primate order, including classic age-related changes such as cell loss, amyloid deposition, amyloid angiopathy, and tau accumulation. We also review existing cellular and ultrastructural data on neuronal changes, such as dendritic attrition and spine alterations, synaptic loss and pathology, and axonal and myelin pathology, and discuss their repercussions on cellular and systems function and cognition.

Synthetic amyloid-β oligomers drive early pathological progression of Alzheimer's disease in nonhuman primates

As an insidious and slowly progressive neurodegenerative disorder, Alzheimer’s disease (AD) uniquely develops in humans but fails in other species. Therefore, it has been challenged to rebuild human AD in animals, including in non-human primates. Here, we bilaterally delivered synthetic Aβ oligomers (AβOs) into the cerebral parenchyma of cynomolgus monkeys, which rapidly drove the formation of massive Aβ plaques and concomitant neurofibrillary tangles in the cynomolgus brain. The amyloid and tau pathology as well as their co-occurrence in AβO-monkeys were reminiscent of those in patients with AD. In addition, the activated astrocytes and microglia surrounding Aβ plaques indicated the triggered neuroinflammation. The degenerative neurons and synapses around Aβ plaques also emerged in cynomolgus brain. Together, soluble AβOs caused the cascade of pathologic events associated with AD in monkeys as occurred in patients at the early phase, which could facilitate the development of a promising animal model for human AD in non-human primates.

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The Aβ oligomers (AβOs) drive to develop massive Aβ plaque in the monkey brain

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Neurofibrillary tangles form in multiple brain regions of AβO-monkeys

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The co-occurrence of amyloid and tau pathology in AβO-monkeys as in patients with AD

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The neuroinflammation and neurodegeneration are triggered in AβO-monkeys

What is the gold standard model for Alzheimer's disease drug discovery and development?

Introduction: Alzheimer's disease models (ADMs) are currently used for drug development (DD). More than 20,000 molecules were screened for AD treatment over decades, with only one drug (Aducanumab)FDA-approved over the past 18 years. A revision of pathogenic concepts and ADMs are needed.Areas covered: The authors discuss herein preclinical models including: (i) in vitro models (cell lines, primary neuron cell cultures, iPSC-derived brain cells), (ii) ex vivo models, and (iii) in vivo models (artificial, transgenic, non-transgenic and induced).Expert opinion: The following types of ADMs have been reported: Mouse models (45.08%), Rat models (15.04%), Non-human Primate models (0.76%), Rabbit models (0.46%), Cat models (0.53%), Pig models (0.30%), Guinea pig models (0.15%), Octodon degu models (0.02%), Dog models (0.54%), Drosophila melanogaster models (1.79%), Zebrafish models (0.50%), Caenorhabditis elegans (1.21%), Cell culture models (3.31%), Cholinergic models (8.26%), Neurotoxic models (6.79%), Neuroinflammation models (6.92%), Neurovascular models (7.88%), and Microbiome models (0.45%).No single ADM faithfully reproduces all the pathogenic events in the human AD phenotype spectrum. ADMs should be different for (i) pathogenic studies vs basic DD, and (ii) preventive interventions vs symptomatic treatments. There cannot be an ideal ADM for DD, because AD is a spectrum of syndromes. DD can integrate pathogenic, mechanistic, metabolic, transporter and pleiotropic genes in a multisystem model.

Proteopathic lesions in the brain of a super-aged chimpanzee (Pan troglodytes)

The brain of a chimpanzee estimated to be 68 years old, the oldest reported so far, has been examined. Pathological analyses revealed the formation of mild tau-positive neuritic clusters and cytoplasmic α-synuclein aggregates, in addition to severe cerebral amyloid angiopathy and diffuse plaques, but no tangle lesions were observed.

Microglia in Aging and Alzheimer's Disease: A Comparative Species Review

Microglia are the primary immune cells of the central nervous system that help nourish and support neurons, clear debris, and respond to foreign stimuli. Greatly impacted by their environment, microglia go through rapid changes in cell shape, gene expression, and functional behavior during states of infection, trauma, and neurodegeneration. Aging also has a profound effect on microglia, leading to chronic inflammation and an increase in the brain's susceptibility to neurodegenerative processes that occur in Alzheimer's disease. Despite the scientific community's growing knowledge in the field of neuroinflammation, the overall success rate of drug treatment for age-related and neurodegenerative diseases remains incredibly low. Potential reasons for the lack of translation from animal models to the clinic include the use of a single species model, an assumption of similarity in humans, and ignoring contradictory data or information from other species. To aid in the selection of validated and predictive animal models and to bridge the translational gap, this review evaluates similarities and differences among species in microglial activation and density, morphology and phenotype, cytokine expression, phagocytosis, and production of oxidative species in aging and Alzheimer's disease.

Age- and cognition-related differences in the gray matter volume of the chimpanzee brain (Pan troglodytes): A voxel-based morphometry and conjunction analysis

Several primate species have been shown to exhibit age-related changes in cognition, brain, and behavior. However, severe neurodegenerative illnesses, such as Alzheimer's disease (AD), were once thought to be uniquely human. Recently, some chimpanzees naturally were documented to develop both neurofibrillary tangles and amyloid plaques, the main characteristics of AD pathology. In addition, like humans and other primates, chimpanzees show similar declines in cognition and motor function with age. Here, we used voxel-based morphometry to examine the relationships among gray matter volume, age, and cognition using magnetic resonance imaging scans previously acquired from chimpanzees (N = 216). We first determined the relationship between age and gray matter volume, identifying the regions that declined with age. With a subset of our sample (N = 103), we also determined differences in gray matter volume between older chimpanzees with higher cognition scores than expected for their age, and older chimpanzees with lower than expected scores. Finally, we ran a conjunction analysis to determine any overlap in brain regions between these two analyses. We found that as chimpanzees age, they lose gray matter in regions associated with cognition. In addition, cognitively healthy older chimpanzees (those performing better for their age) have greater gray matter volume in many brain regions compared with chimpanzees who underperform for their age. Finally, the conjunction analysis revealed that regions of age-related decline overlap with the regions that differ between cognitively healthy chimpanzees and those who underperform. This study provides further evidence that chimpanzees are an important model for research on the neurobiology of aging. Future studies should investigate the effects of cognitive stimulation on both cognitive performance and brain structure in aging nonhuman primates.

Integrated phylogeny of the human brain and pathobiology of Alzheimer's disease: A unifying hypothesis

The disproportionate evolutionary expansion of the human cerebral cortex with reinforcement of cholinergic innervations warranted a major rise in the functional and metabolic load of the conserved basal forebrain (BF) cholinergic system. Given that acetylcholine (ACh) regulates properties of the microtubule-associated protein (MAP) tau and promotes non-amyloidogenic processing of amyloid precursor protein (APP), growing neocortex predicts higher demands for ACh, while the emerging role of BF cholinergic projections in Aβ clearance infers greater exposure of source neurons and their innervation fields to amyloid pathology. The higher exposure of evolutionary most recent cortical areas to the amyloid pathology of Alzheimer's disease (AD) with synaptic impairments and atrophy, therefore, might involve attenuated homeostatic effects of BF cholinergic projections, in addition to fall-outs of inherent processes of expanding association areas. This unifying model, thus, views amyloid pathology and loss of cholinergic cells as a quid pro quo of the allometric evolution of the human brain, which in combination with increase in life expectancy overwhelm the fine homeostatic balance and trigger the disease process.

Racemization in Post-Translational Modifications Relevance to Protein Aging, Aggregation and Neurodegeneration: Tip of the Iceberg

Homochirality of DNA and prevalent chirality of free and protein-bound amino acids in a living organism represents the challenge for modern biochemistry and neuroscience. The idea of an association between age-related disease, neurodegeneration, and racemization originated from the studies of fossils and cataract disease. Under the pressure of new results, this concept has a broader significance linking protein folding, aggregation, and disfunction to an organism's cognitive and behavioral functions. The integrity of cognitive function is provided by a delicate balance between the evolutionarily imposed molecular homo-chirality and the epigenetic/developmental impact of spontaneous and enzymatic racemization. The chirality of amino acids is the crucial player in the modulation the structure and function of proteins, lipids, and DNA. The collapse of homochirality by racemization is the result of the conformational phase transition. The racemization of protein-bound amino acids (spontaneous and enzymatic) occurs through thermal activation over the energy barrier or by the tunnel transfer effect under the energy barrier. The phase transition is achieved through the intermediate state, where the chirality of alpha carbon vanished. From a thermodynamic consideration, the system in the homo-chiral (single enantiomeric) state is characterized by a decreased level of entropy. The oscillating protein chirality is suggesting its distinct significance in the neurotransmission and flow of perceptual information, adaptive associative learning, and cognitive laterality. The common pathological hallmarks of neurodegenerative disorders include protein misfolding, aging, and the deposition of protease-resistant protein aggregates. Each of the landmarks is influenced by racemization. The brain region, cell type, and age-dependent racemization critically influence the functions of many intracellular, membrane-bound, and extracellular proteins including amyloid precursor protein (APP), TAU, PrP, Huntingtin, α-synuclein, myelin basic protein (MBP), and collagen. The amyloid cascade hypothesis in Alzheimer's disease (AD) coexists with the failure of amyloid beta (Aβ) targeting drug therapy. According to our view, racemization should be considered as a critical factor of protein conformation with the potential for inducing order, disorder, misfolding, aggregation, toxicity, and malfunctions.

Age-related changes in chimpanzee (Pan troglodytes) cognition: Cross-sectional and longitudinal analyses

Chimpanzees are the species most closely related to humans, yet age-related changes in brain and cognition remain poorly understood. The lack of studies on age-related changes in cognition in chimpanzees is particularly unfortunate in light of the recent evidence demonstrating that this species naturally develops Alzheimer's disease (AD) neuropathology. Here, we tested 213 young, middle-aged, and elderly captive chimpanzees on the primate cognitive test battery (PCTB), a set of 13 tasks that assess physical and social cognition in nonhuman primates. A subset of these chimpanzees (n = 146) was tested a second time on a portion of the PCTB tasks as a means of evaluating longitudinal changes in cognition. Cross-sectional analyses revealed a significant quadratic association between age and cognition with younger and older chimpanzees performing more poorly than middle-aged individuals. Longitudinal analyses showed that the oldest chimpanzees at the time of the first test showed the greatest decline in cognition, although the effect was mild. The collective data show that chimpanzees, like other nonhuman primates, show age-related decline in cognition. Further investigations into whether the observed cognitive decline is associated with AD pathologies in chimpanzees would be invaluable in understanding the comparative biology of aging and neuropathology in primates.

Amyloid β and tau pathology in brains of aged pinniped species (sea lion, seal, and walrus)

Alzheimer’s disease (AD) is characterized by the accumulation of amyloid-β (Aβ) as senile plaques and cerebral amyloid angiopathy, and hyperphosphorylated tau (hp-tau) as neurofibrillary tangles in the brain. The AD-related pathology has been reported in several non-human animals, and most animals develop only the Aβ or tau pathology. We herein describe the Aβ and hp-tau pathology in the brains of aged pinniped species (seal, sea lion, and walrus). Molecular analyses revealed that the sequence of pinniped Aβ was identical to that of human Aβ. Histopathological examinations detected argyrophilic plaques composed of Aβ associated with dystrophic neurites in the cerebral cortex of aged pinnipeds. Astrogliosis and microglial infiltration were identified around Aβ plaques. Aβ deposits were observed in the blood vessel walls of the meninges and cerebrum. Pinniped tau protein was physiologically subjected to alternative splicing at exons 2, 3, and 10, and presented as five isoforms: two 3-repeat tau isoforms (1N3R, 2N3R) and three 4-repeat tau isoforms (0N4R, 1N4R, 2N4R); 0N3R tau isoform was absent. Histopathological examinations revealed argyrophilic fibrillar aggregates composed of hp-tau in the neuronal somata and neurites of aged pinniped brains. Few hp-tau aggregates were found in oligodendrocytes and microglia. Biochemically, hp-tau of the 3-repeat and 4-repeat isoforms was detected in brain sarkosyl-insoluble fractions. Aβ and hp-tau both predominantly accumulated in the neocortex, particularly the frontal cortex. Furthermore, the activation of GSK-3β was detected within cells containing hp-tau aggregates, and activated GSK-3β was strongly expressed in cases with severe hp-tau pathologies. The present results suggest that, in association with Aβ deposition, the activation of GSK-3β contributes to hp-tau accumulation in pinniped brains. Here, we report that pinniped species naturally accumulate Aβ and tau with aging, similar to the human AD pathology.

Microglial peri-somatic abutments classify two novel types of GABAergic neuron in the inferior colliculus

Emerging evidence suggests functional roles for microglia in the healthy, mature nervous system. However, we know little of the cellular density and ramified morphology of microglia in sensory systems, and even less of their inter-relationship with inhibitory neurons. We therefore conducted fluorescent multi-channel immunohistochemistry and confocal microscopy in guinea pigs of both sexes for Iba1, GAD67, GFAP, calbindin, and calretinin. We explored these markers in the inferior colliculi (IC), which contain sub-regions specialized for different aspects of auditory processing. First, we found that while the density of Iba1+ somata is similar throughout the IC parenchyma, Iba1+ microglia in dorsal cortex are significantly more ramified than those in the central nucleus or lateral cortex. Conversely, Iba1+ ramifications in ventral central nucleus, a region with the highest density of GAD67+ (putative GABAergic) neurons in IC, are longer with fewer ramifications. Second, we observed extensive abutments of ramified Iba1+ processes onto GAD67+ somata throughout the whole IC and developed novel measures to quantify these. Cluster analyses revealed two novel sub-types of GAD67+ neuron that differ in the quantity of Iba1+ somatic abutments they receive. Unlike previous classification schemes for GAD67+ neurons in IC, these clusters are not related to GAD67+ soma size. Taken together, these data demonstrate that microglial ramifications vary between IC sub-regions in the healthy, adult IC, possibly related to the ongoing demands of their niche. Furthermore, Iba1+ abutments onto neuronal somata are a novel means by which GAD67+ neurons can be classified.

Neuron loss associated with age but not Alzheimer's disease pathology in the chimpanzee brain

In the absence of disease, ageing in the human brain is accompanied by mild cognitive dysfunction, gradual volumetric atrophy, a lack of significant cell loss, moderate neuroinflammation, and an increase in the amyloid beta (Aβ) and tau proteins. Conversely, pathologic age-related conditions, particularly Alzheimer's disease (AD), result in extensive neocortical and hippocampal atrophy, neuron death, substantial Aβ plaque and tau-associated neurofibrillary tangle pathologies, glial activation and severe cognitive decline. Humans are considered uniquely susceptible to neurodegenerative disorders, although recent studies have revealed Aβ and tau pathology in non-human primate brains. Here, we investigate the effect of age and AD-like pathology on cell density in a large sample of postmortem chimpanzee brains (n = 28, ages 12-62 years). Using a stereologic, unbiased design, we quantified neuron density, glia density and glia:neuron ratio in the dorsolateral prefrontal cortex, middle temporal gyrus, and CA1 and CA3 hippocampal subfields. Ageing was associated with decreased CA1 and CA3 neuron densities, while AD pathologies were not correlated with changes in neuron or glia densities. Differing from cerebral ageing and AD in humans, these data indicate that chimpanzees exhibit regional neuron loss with ageing but appear protected from the severe cell death found in AD. This article is part of the theme issue 'Evolution of the primate ageing process'.

Amyloid-beta peptide and phosphorylated tau in the frontopolar cerebral cortex and in the cerebellum of toothed whales: aging versus hypoxia

Hypoxia could be a possible risk factor for neurodegenerative alterations in cetaceans' brain. Among toothed whales, the beaked whales are particularly cryptic and routinely dive deeper than 1000 m for about 1 h in order to hunt squids and fishes. Samples of frontal cerebral and cerebellar cortex were collected from nine animals, representing six different species of the suborder Odontoceti. Immunohistochemical analysis employed anti-β-amyloid (Aβ) and anti-neurofibrillary tangle (NFT) antibodies. Six of nine (67%) animals showed positive immunolabeling for Aβ and/or NFT. The most striking findings were intranuclear Aβ immunopositivity in cerebral cortical neurons and NFT immunopositivity in cerebellar Purkinje neurons with granulovacuolar degeneration. Aβ plaques were also observed in one elderly animal. Herein, we present immunohistopathological findings classic of Alzheimer's and other neurodegenerative diseases in humans. Our findings could be linked to hypoxic phenomena, as they were more extensive in beaked whales. Despite their adaptations, cetaceans could be vulnerable to sustained and repetitive brain hypoxia.

Heritability of Gray Matter Structural Covariation and Tool Use Skills in Chimpanzees (Pan troglodytes): A Source-Based Morphometry and Quantitative Genetic Analysis

Nonhuman primates, and great apes in particular, possess a variety of cognitive abilities thought to underlie human brain and cognitive evolution, most notably, the manufacture and use of tools. In a relatively large sample (N = 226) of captive chimpanzees (Pan troglodytes) for whom pedigrees are well known, the overarching aim of the current study was to investigate the source of heritable variation in brain structure underlying tool use skills. Specifically, using source-based morphometry (SBM), a multivariate analysis of naturally occurring patterns of covariation in gray matter across the brain, we investigated (1) the genetic contributions to variation in SBM components, (2) sex and age effects for each component, and (3) phenotypic and genetic associations between SBM components and tool use skill. Results revealed important sex- and age-related differences across largely heritable SBM components and associations between structural covariation and tool use skill. Further, shared genetic mechanisms appear to account for a heritable link between variation in both the capacity to use tools and variation in morphology of the superior limb of the superior temporal sulcus and adjacent parietal cortex. Findings represent the first evidence of heritability of structural covariation in gray matter among nonhuman primates.

Quantification of neurons in the hippocampal formation of chimpanzees: comparison to rhesus monkeys and humans

The hippocampal formation is important for higher brain functions such as spatial navigation and the consolidation of memory, and it contributes to abilities thought to be uniquely human, yet little is known about how the human hippocampal formation compares to that of our closest living relatives, the chimpanzees. To gain insight into the comparative organization of the hippocampal formation in catarrhine primates, we quantified neurons stereologically in its major subdivisions-the granular layer of the dentate gyrus, CA4, CA2-3, CA1, and the subiculum-in archival brain tissue from six chimpanzees ranging from 29 to 43 years of age. We also sought evidence of Aβ deposition and hyperphosphorylated tau in the hippocampus and adjacent neocortex. A 42-year-old animal had moderate cerebral Aβ-amyloid angiopathy and tauopathy, but Aβ was absent and tauopathy was minimal in the others. Quantitatively, granule cells of the dentate gyrus were most numerous, followed by CA1, subiculum, CA4, and CA2-3. In the context of prior investigations of rhesus monkeys and humans, our findings indicate that, in the hippocampal formation as a whole, the proportions of neurons in CA1 and the subiculum progressively increase, and the proportion of dentate granule cells decreases, from rhesus monkeys to chimpanzees to humans. Because CA1 and the subiculum engender key hippocampal projection pathways to the neocortex, and because the neocortex varies in volume and anatomical organization among these species, these findings suggest that differences in the proportions of neurons in hippocampal subregions of catarrhine primates may be linked to neocortical evolution.

Oligomeric Aβ in the monkey brain impacts synaptic integrity and induces accelerated cortical aging

As the average age of the population continues to rise, the number of individuals affected with age-related cognitive decline and Alzheimer's disease (AD) has increased and is projected to cost more than $290 billion in the United States in 2019. Despite significant investment in research over the last decades, there is no effective treatment to prevent or delay AD progression. There is a translational gap in AD research, with promising drugs based on work in rodent models failing in clinical trials. Aging is the leading risk factor for developing AD and understanding neurobiological changes that affect synaptic integrity with aging will help clarify why the aged brain is vulnerable to AD. We describe here the development of a rhesus monkey model of AD using soluble oligomers of the amyloid beta (Aβ) peptide (AβOs). AβOs infused into the monkey brain target a specific population of spines in the prefrontal cortex, induce neuroinflammation, and increase AD biomarkers in the cerebrospinal fluid to similar levels observed in patients with AD. Importantly, AβOs lead to similar dendritic spine loss to that observed in normal aging in monkeys, but so far without detection of amyloid plaques or tau pathology. Understanding the basis of synaptic impairment is the most effective route to early intervention and prevention or postponement of age-related cognitive decline and transition to AD. These initial findings support the use of monkeys as a platform to understand age-related vulnerabilities of the primate brain and may help develop effective disease-modifying therapies for treatment of AD and related dementias.

Evolvability and Neurodegenerative Disease: Antagonistic Pleiotropy Phenomena Derived from Amyloid Aggregates

At present, the precise physiological role of neurodegenerative disease-related amyloidogenic proteins (APs), including α-synuclein in Parkinson’s disease and β-amyloid in Alzheimer’s disease, remains unclear. Because of similar adaptability of both human brain neurons and yeast cells to diverse environmental stressors, we previously proposed that the concept of evolvability in yeast prion could also be applied to APs in human brain. However, the mechanistic relevance of evolvability to neurodegenerative disorders is elusive. Therefore, our objective is to discuss our hypothesis that evolvability and neurodegenerative disease may represent a form of antagonistic pleiotropy derived from the aggregates of APs. Importantly, such a perspective may provide an outlook of the entire course of sporadic neurodegenerative diseases.

A cross-species approach to disorders affecting brain and behaviour

Structural and functional elements of biological systems are highly conserved across vertebrates. Many neurological and psychiatric conditions affect both humans and animals. A cross-species approach to the study of brain and behaviour can advance our understanding of human disorders via the identification of unrecognized natural models of spontaneous disorders, thus revealing novel factors that increase vulnerability or resilience, and via the assessment of potential therapies. Moreover, diagnostic and therapeutic advances in human neurology and psychiatry can often be adapted for veterinary patients. However, clinical and research collaborations between physicians and veterinarians remain limited, leaving this wealth of comparative information largely untapped. Here, we review pain, cognitive decline syndromes, epilepsy, anxiety and compulsions, autoimmune and infectious encephalitides and mismatch disorders across a range of animal species, looking for novel insights with translational potential. This comparative perspective can help generate novel hypotheses, expand and improve clinical trials and identify natural animal models of disease resistance and vulnerability.

Canine Cognitive Dysfunction and Alzheimer's Disease - Two Facets of the Same Disease?

Neurodegenerative diseases present a major and increasing burden in the societies worldwide. With aging populations, the prevalence of neurodegenerative diseases is increasing, yet there are no effective cures and very few treatment options are available. Alzheimer’s disease is one of the most prevalent neurodegenerative conditions and although the pathology is well studied, the pathogenesis of this debilitating illness is still poorly understood. This is, among other reasons, also due to the lack of good animal models as laboratory rodents do not develop spontaneous neurodegenerative diseases and human Alzheimer’s disease is only partially mimicked by transgenic rodent models. On the other hand, older dogs commonly develop canine cognitive dysfunction, a disease that is similar to Alzheimer’s disease in many aspects. Dogs show cognitive deficits that could be paralleled to human symptoms such as disorientation, memory loss, changes in behavior, and in their brains, beta amyloid plaques are commonly detected both in extracellular space as senile plaques and around the blood vessels. Dogs could be therefore potentially a very good model for studying pathological process and novel treatment options for Alzheimer’s disease. In the present article, we will review the current knowledge about the pathogenesis of canine cognitive dysfunction, its similarities and dissimilarities with Alzheimer’s disease, and developments of novel treatments for these two diseases with a focus on canine cognitive dysfunction.

Astrocytic changes with aging and Alzheimer's disease-type pathology in chimpanzees

Astrocytes are the main homeostatic cell of the central nervous system. In addition, astrocytes mediate an inflammatory response when reactive to injury or disease known as astrogliosis. Astrogliosis is marked by an increased expression of glial fibrillary acidic protein (GFAP) and cellular hypertrophy. Some degree of astrogliosis is associated with normal aging and degenerative conditions such as Alzheimer's disease (AD) and other dementing illnesses in humans. The recent observation of pathological markers of AD (amyloid plaques and neurofibrillary tangles) in aged chimpanzee brains provided an opportunity to examine the relationships among aging, AD-type pathology, and astrocyte activation in our closest living relatives. Stereologic methods were used to quantify GFAP-immunoreactive astrocyte density and soma volume in layers I, III, and V of the prefrontal and middle temporal cortex, as well as in hippocampal fields CA1 and CA3. We found that the patterns of astrocyte activation in the aged chimpanzee brain are distinct from humans. GFAP expression does not increase with age in chimpanzees, possibly indicative of lower oxidative stress loads. Similar to humans, chimpanzee layer I astrocytes in the prefrontal cortex are susceptible to AD-like changes. Both prefrontal cortex layer I and hippocampal astrocytes exhibit a high degree of astrogliosis that is positively correlated with accumulation of amyloid beta and tau proteins. However, unlike humans, chimpanzees do not display astrogliosis in other cortical layers. These results demonstrate a unique pattern of cortical aging in chimpanzees and suggest that inflammatory processes may differ between humans and chimpanzees in response to pathology.

Synaptic distributions of pS214-tau in rhesus monkey prefrontal cortex are associated with spine density, but not with cognitive decline

Female rhesus monkeys and women are subject to age- and menopause-related deficits in working memory, an executive function mediated by the dorsolateral prefrontal cortex (dlPFC). Long-term cyclic administration of 17β-estradiol improves working memory, and restores highly plastic axospinous synapses within layer III dlPFC of aged ovariectomized monkeys. In this study, we tested the hypothesis that synaptic distributions of tau protein phosphorylated at serine 214 (pS214-tau) are altered with age or estradiol treatment, and couple to working memory performance. First, ovariectormized young and aged monkeys received vehicle or estradiol treatment, and were tested on the delayed response (DR) test of working memory. Serial section electron microscopic immunocytochemistry was then performed to quantitatively assess the subcellular synaptic distributions of pS214-tau. Overall, the majority of synapses contained pS214-tau immunogold particles, which were predominantly localized to the cytoplasm of axon terminals. pS214-tau was also abundant within synaptic and cytoplasmic domains of dendritic spines. The density of pS214-tau immunogold within the active zone, cytoplasmic, and plasmalemmal domains of axon terminals, and subjacent to the postsynaptic density within the subsynaptic domains of dendritic spines, were each reduced with age. None of the variables examined were directly linked to cognitive status, but a high density of pS214-tau immunogold particles within presynaptic cytoplasmic and plasmalemmal domains, and within postsynaptic subsynaptic and plasmalemmal domains, accompanied high synapse density. Together, these data support a possible physiological, rather than pathological, role for pS214-tau in the modulation of synaptic morphology in monkey dlPFC.

Microglia changes associated to Alzheimer's disease pathology in aged chimpanzees

In Alzheimer's disease (AD), the brain's primary immune cells, microglia, become activated and are found in close apposition to amyloid beta (Aβ) protein plaques and neurofibrillary tangles (NFT). The present study evaluated microglia density and morphology in a large group of aged chimpanzees (n = 20, ages 37-62 years) with varying degrees of AD-like pathology. Using immunohistochemical and stereological techniques, we quantified the density of activated microglia and morphological variants (ramified, intermediate, and amoeboid) in postmortem chimpanzee brain samples from prefrontal cortex, middle temporal gyrus, and hippocampus, areas that show a high degree of AD pathology in humans. Microglia measurements were compared to pathological markers of AD in these cases. Activated microglia were consistently present across brain areas. In the hippocampus, CA3 displayed a higher density than CA1. Aβ42 plaque volume was positively correlated with higher microglial activation and with an intermediate morphology in the hippocampus. Aβ42-positive vessel volume was associated with increased hippocampal microglial activation. Activated microglia density and morphology were not associated with age, sex, pretangle density, NFT density, or tau neuritic cluster density. Aged chimpanzees displayed comparable patterns of activated microglia phenotypes as well as an association of increased microglial activation and morphological changes with Aβ deposition similar to AD patients. In contrast to human AD brains, activated microglia density was not significantly correlated with tau lesions. This evidence suggests that the chimpanzee brain may be relatively preserved during normal aging processes but not entirely protected from neurodegeneration as previously assumed.

Astrogliopathy in Tauopathies

Astrocytes are involved in many diseases of the central nervous system, not only as reactive cells to neuronal damage but also as primary actors in the pathological process. Astrogliopathy is a term used to designate the involvement of astrocytes as key elements in the pathogenesis and pathology of diseases and injuries of the central nervous system. Astrocytopathy is utilized to name non-reactive astrogliosis covering hypertrophy, atrophy and astroglial degeneration with loss of function in astrocytes and pathological remodeling, as well as senescent changes. Astrogliopathy and astrocytopathy are hallmarks of tauopathies—neurodegenerative diseases with abnormal hyper-phosphorylated tau aggregates in neurons and glial cells. The involvement of astrocytes covers different disease-specific types such as tufted astrocytes, astrocytic plaques, thorn-shaped astrocytes, granular/fuzzy astrocytes, ramified astrocytes and astrocytes with globular inclusions, as well as others which are unnamed but not uncommon in familial frontotemporal degeneration linked to mutations in the tau gene. Knowledge of molecular differences among tau-containing astrocytes is only beginning, and their distinct functional implications remain rather poorly understood. However, tau-containing astrocytes in certain conditions have deleterious effects on neuronal function and nervous system integrity. Moreover, recent studies have shown that tau-containing astrocytes obtained from human brain tauopathies have a capacity for abnormal tau seeding and spreading in wild type mice. Inclusive conceptions include a complex scenario involving neurons, glial cells and local environmental factors that potentiate each other and promote disease progression in tauopathies.

The aged rhesus macaque manifests Braak stage III/IV Alzheimer's-like pathology

INTRODUCTION

An animal model of late-onset Alzheimer's disease is needed to research what causes degeneration in the absence of dominant genetic insults and why the association cortex is particularly vulnerable to degeneration.

METHODS

We studied the progression of tau and amyloid cortical pathology in the aging rhesus macaque using immunoelectron microscopy and biochemical assays.

RESULTS

Aging macaques exhibited the same qualitative pattern and sequence of tau and amyloid cortical pathology as humans, reaching Braak stage III/IV. Pathology began in the young-adult entorhinal cortex with protein kinase A-phosphorylation of tau, progressing to fibrillation with paired helical filaments and mature tangles in oldest animals. Tau pathology in the dorsolateral prefrontal cortex paralleled but lagged behind the entorhinal cortex, not afflicting the primary visual cortex.

DISCUSSION

The aging rhesus macaque provides the long-sought animal model for exploring the etiology of late-onset Alzheimer's disease and for testing preventive strategies.

Lack of human-like extracellular sortilin neuropathology in transgenic Alzheimer's disease model mice and macaques

BACKGROUND

Alzheimer's disease (AD) is a devastating neurodegenerative disorder bearing multiple pathological hallmarks suggestive of complex cellular/molecular interplay during pathogenesis. Transgenic mice and nonhuman primates are used as disease models for mechanistic and translational research into AD; the extent to which these animal models recapitulate AD-type neuropathology is an issue of importance. Putative C-terminal fragments from sortilin, a member of the vacuolar protein sorting 10 protein (Vps10p) family, have recently been shown to deposit in the neuritic β-amyloid (Aβ) plaques in the human brain.

METHODS

We set out to explore if extracellular sortilin neuropathology exists in AD-related transgenic mice and nonhuman primates. Brains from different transgenic strains and ages developed overt cerebral Aβ deposition, including the β-amyloid precursor protein and presenilin 1 double-transgenic (APP/PS1) mice at ~ 14 months of age, the five familial Alzheimer's disease mutations transgenic (5×FAD) mice at ~ 8 months, the triple-transgenic Alzheimer's disease (3×Tg-AD) mice at ~ 22 months, and aged monkeys (Macaca mulatta and Macaca fascicularis) were examined. Brain samples from young transgenic mice, middle-aged/aged monkeys, and AD humans were used as negative and positive pathological controls.

RESULTS

The C-terminal sortilin antibody, which labeled senile plaques in the AD human cerebral sections, did not display extracellular immunolabeling in the transgenic mouse or aged monkey brain sections with Aβ deposition. In Western blot analysis, sortilin fragments ~ 15 kDa were not detectable in transgenic mouse cortical lysates, but they occurred in control AD lysates.

CONCLUSIONS

In reference to their human brain counterparts, neuritic plaques seen in transgenic AD model mouse brains represent an incomplete form of this AD pathological hallmark. The species difference in neuritic plaque constituents also indicates more complex secondary proteopathies in the human brain relative to rodents and nonhuman primates during aging and in AD.

Neurodegenerative disease biomarkers Aβ1-40, Aβ1-42, tau, and p-tau181 in the vervet monkey cerebrospinal fluid: Relation to normal aging, genetic influences, and cerebral amyloid angiopathy

Background

The Caribbean vervet monkey (Chlorocebus aethiops sabaeus) is a potentially valuable animal model of neurodegenerative disease. However, the trajectory of aging in vervets and its relationship to human disease is incompletely understood.

Methods

To characterize biomarkers associated with neurodegeneration, we measured cerebrospinal fluid (CSF) concentrations of Aβ1-40, Aβ1-42, total tau, and p-tau181 in 329 members of a multigenerational pedigree. Linkage and genome-wide association were used to elucidate a genetic contribution to these traits.

Results

Aβ1-40 concentrations were significantly correlated with age, brain total surface area, and gray matter thickness. Levels of p-tau181 were associated with cerebral volume and brain total surface area. Among the measured analytes, only CSF Aβ1-40 was heritable. No significant linkage (LOD > 3.3) was found, though suggestive linkage was highlighted on chromosomes 4 and 12. Genome-wide association identified a suggestive locus near the chromosome 4 linkage peak.

Conclusions

Overall, these results support the vervet as a non-human primate model of amyloid-related neurodegeneration, such as Alzheimer's disease and cerebral amyloid angiopathy, and highlight Aβ1-40 and p-tau181 as potentially valuable biomarkers of these processes.

Aged chimpanzees exhibit pathologic hallmarks of Alzheimer's disease

Alzheimer's disease (AD) is a uniquely human brain disorder characterized by the accumulation of amyloid-beta protein (Aβ) into extracellular plaques, neurofibrillary tangles (NFT) made from intracellular, abnormally phosphorylated tau, and selective neuronal loss. We analyzed a large group of aged chimpanzees (n = 20, age 37-62 years) for evidence of Aβ and tau lesions in brain regions affected by AD in humans. Aβ was observed in plaques and blood vessels, and tau lesions were found in the form of pretangles, NFT, and tau-immunoreactive neuritic clusters. Aβ deposition was higher in vessels than in plaques and correlated with increases in tau lesions, suggesting that amyloid build-up in the brain's microvasculature precedes plaque formation in chimpanzees. Age was correlated to greater volumes of Aβ plaques and vessels. Tangle pathology was observed in individuals that exhibited plaques and moderate or severe cerebral amyloid angiopathy, a condition in which amyloid accumulates in the brain's vasculature. Amyloid and tau pathology in aged chimpanzees suggests these AD lesions are not specific to the human brain.

The Alzheimer's disease-protective CD33 splice variant mediates adaptive loss of function via diversion to an intracellular pool

The immunomodulatory receptor Siglec-3/CD33 influences risk for late-onset Alzheimer's disease (LOAD), an apparently human-specific post-reproductive disease. CD33 generates two splice variants: a full-length CD33M transcript produced primarily by the "LOAD-risk" allele and a shorter CD33m isoform lacking the sialic acid-binding domain produced primarily from the "LOAD-protective" allele. An SNP that modulates CD33 splicing to favor CD33m is associated with enhanced microglial activity. Individuals expressing more protective isoform accumulate less brain β-amyloid and have a lower LOAD risk. How the CD33m isoform increases β-amyloid clearance remains unknown. We report that the protection by the CD33m isoform may not be conferred by what it does but, rather, from what it cannot do. Analysis of blood neutrophils and monocytes and a microglial cell line revealed that unlike CD33M, the CD33m isoform does not localize to cell surfaces; instead, it accumulates in peroxisomes. Cell stimulation and activation did not mobilize CD33m to the surface. Thus, the CD33m isoform may neither interact directly with amyloid plaques nor engage in cell-surface signaling. Rather, production and localization of CD33m in peroxisomes is a way of diminishing the amount of CD33M and enhancing β-amyloid clearance. We confirmed intracellular localization by generating a CD33m-specific monoclonal antibody. Of note, CD33 is the only Siglec with a peroxisome-targeting sequence, and this motif emerged by convergent evolution in toothed whales, the only other mammals with a prolonged post-reproductive lifespan. The CD33 allele that protects post-reproductive individuals from LOAD may have evolved by adaptive loss-of-function, an example of the less-is-more hypothesis.

Is sporadic Alzheimer's disease a developmental disorder?

Alzheimer's disease (AD) is a neurodegenerative disorder of higher age that specifically occurs in human. Its clinical phase, characterized by a decline in physiological, psychological, and social functioning, is preceded by a long clinically silent phase of at least several decades that might perhaps even start very early in life. Overall, key functional abilities in AD patients decline in reverse order of the development of these abilities during normal childhood and adolescence. Early symptoms of AD, thus, typically affect mental functions that have been acquired only during very recent hominid evolution and as such are specific to human. Neurofibrillar degeneration, a typical neuropathological lesion of the disease and one of the most robust pathological correlates of cognitive impairment, is rarely seen in non-primate mammals and even non-human primates hardly develop a pathology comparable to those seen in AD patients. Neurofibrillar degeneration is not randomly distributed throughout the AD brain. It preferentially affects brain areas that become increasingly predominant during the evolutionary process of encephalization. During progression of the disease, it affects cortical areas in a stereotypic sequence that inversely recapitulates ontogenetic brain development. The specific distribution of cortical pathology in AD, moreover, appears to be determined by the modular organization of the cerebral cortex which basically is a structural reflection of its ontogeny. Here, we summarize recent evidence that phylogenetic and ontogenetic dimensions of brain structure and function provide the key to our understanding of AD. More recent molecular biological studies of the potential pathogenetic role of a genomic mosaic in the brains of patients with AD might even provide arguments for a developmental origin of AD. This article is part of a series "Beyond Amyloid".