Histopathological studies of senile plaques and cerebral amyloidosis in cynomolgus monkeys

Alzheimer disease-like neuropathologic changes in a geriatric baboon (Papio hamadryas)

IMPORTANCE

Alzheimer's disease (AD) is the most common cause of dementia in the elderly with the incidence rising exponentially after the age of 65 years. Unfortunately, effective treatments are extremely limited and definite diagnosis can only be made at autopsy. This is in part due to our limited understanding of the complex pathophysiology, including the various genetic, environmental, and metabolic contributing factors. In an effort to better understand this complex disease, researchers have employed nonhuman primates as translational models.

CASE PRESENTATION

This report aims to describe the AD-like neuropathology in the brain of a 37-year-old female baboon (Papio hamadryas), which at the time of her death made her the oldest hamadryas baboon at any member institution of the Association of Zoos and Aquariums. A diagnostic necropsy was performed, and the brain was evaluated for neurodegenerative disease. Frequent amyloid-β deposits were identified, consistent with what has been described in other geriatric nonhuman primates. Phospho-tau pathology, including neurofibrillary tangles, a feature not well-described in other primate models, was also abundant.

CONCLUSIONS AND RELEVANCE

Our results suggest that more detailed, prospective, longitudinal studies are warranted utilizing this particular species to see if they represent a viable model for human brain aging.

Amyloid-β Pathology Is the Common Nominator Proteinopathy of the Primate Brain Aging

Senile plaques, mainly diffuse, and cerebral amyloid-β (Aβ) angiopathy are prevalent in the aging brain of non-human primates, from lemurs to non-human Hominidae. Aβ but not hyper-phosphorylated tau (HPtau) pathology is the common nominator proteinopathy of non-human primate brain aging. The abundance of Aβ in the aging primate brain is well tolerated, and the impact on cognitive functions is usually limited to particular tasks. In contrast, human brain aging is characterized by the early appearance of HPtau pathology, mainly forming neurofibrillary tangles, dystrophic neurites of neuritic plaques, and neuropil threads, preceding Aβ deposits by several decades and by its severity progressing from selected nuclei of the brain stem, entorhinal cortex, and hippocampus to the limbic system, neocortex, and other brain regions. Neurofibrillary tangles correlate with cognitive impairment and dementia in advanced cases. Aβ pathology is linked in humans to altered membrane protein and lipid composition, particularly involving lipid rafts. Although similar membrane alterations are unknown in non-human primates, membrane senescence is postulated to cause the activated β-amyloidogenic pathway, and Aβ pathology is the prevailing signature of non-human and human primate brain aging.

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.

Biology and postnatal development of organ systems of cynomolgus monkeys (Macaca fascicularis)

BACKGROUND

The cynomolgus macaque has become the most used non-human primate species in nonclinical safety assessment during the past decades.

METHODS

This review summarizes the biological data and organ system development milestones of the cynomolgus macaque available in the literature.

RESULTS

The cynomolgus macaque is born precocious relative to humans in some organ systems (e.g., nervous, skeletal, respiratory, and gastrointestinal). Organ systems develop, refine, and expand at different rates after birth. In general, the respiratory, gastrointestinal, renal, and hematopoietic systems mature at approximately 3 years of age. The female reproductive, cardiovascular and hepatobiliary systems mature at approximately 4 years of age. The central nervous, skeletal, immune, male reproductive, and endocrine systems complete their development at approximately 5 to 9 years of age.

CONCLUSIONS

The cynomolgus macaque has no meaningful developmental differences in critical organ systems between 2 and 3 years of age for use in nonclinical safety assessment.

Blood-brain barrier leakage in Alzheimer's disease: From discovery to clinical relevance

Alzheimer's disease (AD) is the most common form of dementia. AD brain pathology starts decades before the onset of clinical symptoms. One early pathological hallmark is blood-brain barrier dysfunction characterized by barrier leakage and associated with cognitive decline. In this review, we summarize the existing literature on the extent and clinical relevance of barrier leakage in AD. First, we focus on AD animal models and their susceptibility to barrier leakage based on age and genetic background. Second, we re-examine barrier dysfunction in clinical and postmortem studies, summarize changes that lead to barrier leakage in patients and highlight the clinical relevance of barrier leakage in AD. Third, we summarize signaling mechanisms that link barrier leakage to neurodegeneration and cognitive decline in AD. Finally, we discuss clinical relevance and potential therapeutic strategies and provide future perspectives on investigating barrier leakage in AD. Identifying mechanistic steps underlying barrier leakage has the potential to unravel new targets that can be used to develop novel therapeutic strategies to repair barrier leakage and slow cognitive decline in AD and AD-related dementias.

Characterization of Early Alzheimer's Disease-Like Pathological Alterations in Non-Human Primates with Aging: A Pilot Study

BACKGROUND

Sporadic or late onset Alzheimer's disease (LOAD) is a multifactorial neurodegenerative disease with aging the most known risk factor. Non-human primates (NHPs) may serve as an excellent model to study LOAD because of their close similarity to humans in many aspects including neuroanatomy and neurodevelopment. Recent studies reveal AD-like pathology in old NHPs.

OBJECTIVE

In this pilot study, we took advantage of brain samples from 6 Cynomolgus macaques that were divided into two groups: middle aged (average age 14.81 years) and older (average age 19.33 years). We investigated whether AD-like brain pathologies are present in the NHPs.

METHODS

We used immunohistochemical method to examine brain Aβ pathology and neuron density. We applied biochemical assays to measure tau phosphorylation and multiple signaling pathways indicated in AD. We performed electron microscopy experiments to study alterations of postsynaptic density and mitochondrial morphology in the brain of NHPs.

RESULTS

We found multiple AD-like pathological alteration in the prefrontal cortex (but not in the hippocampus) of the older NHPs including tau hyperphosphorylation, increased activity of AMP-activated protein kinase (AMPK), decreased expression of protein phosphatase 2A (PP2A), impairments in mitochondrial morphology, and postsynaptic densities formation.

CONCLUSION

These findings may provide insights into the factors contributing to the development of LOAD, particularly during the early stage transitioning from middle to old age. Future endeavors are warranted to elucidate mechanisms underlying the regional (and perhaps cellular) vulnerability with aging and the functional correlation of such pathological changes in NHPs.

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.

International Harmonization of Nomenclature and Diagnostic Criteria (INHAND): Non-proliferative and Proliferative Lesions of the Non-human Primate (M. fascicularis)

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions Project (www.toxpath.org/inhand.asp) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in most tissues and organs from the nonhuman primate used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals will provide a common language among regulatory and scientific research organizations in different countries and increase and enrich international exchanges of information among toxicologists and pathologists.

Aging induces abnormal accumulation of Aβ in extracellular vesicle and/or intraluminal membrane vesicle-rich fractions in nonhuman primate brain

Aβ metabolism in the brain is mediated by endocytosis, one part of the intracellular membrane trafficking system. We previously showed that aging attenuates the interaction of dynein with dynactin, which disrupts the endosomal/lysosomal trafficking pathway involved in Aβ metabolism, resulting in intracellular accumulation of Aβ. Several studies have shown that in Alzheimer's disease (AD), intraneuronal accumulation of Aβ precedes extracellular Aβ depositions. However, it is unclear what accounts for this transition from intracellular to extracellular depositions. Accumulating evidence suggest that autophagy has an important role in AD pathology, and we observed that autophagy-related protein levels began to decrease before amyloid plaque formation in cynomolgus monkey brains. Surprisingly, experimental induction of autophagosome formation in Neuro2a cells significantly increased intracellular Aβ and decreased extracellular release of Aβ, accompanied by the prominent reduction of extracellular vesicle (EV) secretion. RNAi study confirmed that EV secretion affected intracellular and extracellular Aβ levels, and siRNA-induced downregulation of autophagosome formation enhanced EV secretion to ameliorate intracellular Aβ accumulation induced by dynein knockdown. In aged cynomolgus monkeys, Aβ levels in EV/intraluminal membrane vesicle (ILV)-rich fractions isolated from temporal lobe parenchyma were drastically increased. Moreover, EV/ILV marker proteins overlapped spatially with amyloid plaques. These findings suggest that EV would be an important carrier of Aβ in brain and abnormal accumulation of Aβ in EVs/ILVs may be involved in the transition of age-dependent Aβ pathology.

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.

Generation of Transgenic Cynomolgus Monkeys Overexpressing the Gene for Amyloid-β Precursor Protein

Alzheimer’s disease (AD) is the most common cause of dementia and understanding its pathogenesis should lead to improved therapeutic and diagnostic methods. Although several groups have developed transgenic mouse models overexpressing the human amyloid-β precursor protein (APP) gene with AD mutations, with and without presenilin mutations, as well as APP gene knock-in mouse models, these animals display amyloid pathology but do not show neurofibrillary tangles or neuronal loss. This presumably is due to differences between the etiology of the aged-related human disease and the mouse models. Here we report the generation of two transgenic cynomolgus monkeys overexpressing the human gene for APP with Swedish, Artic, and Iberian mutations, and demonstrated expression of gene tagged green fluorescent protein marker in the placenta, amnion, hair follicles, and peripheral blood. We believe that these nonhuman primate models will be very useful to study the pathogenesis of dementia and AD. However, generated Tg monkeys still have some limitations. We employed the CAG promoter, which will promote gene expression in a non-tissue specific manner. Moreover, we used transgenic models but not knock-in models. Thus, the inserted transgene destroys endogenous gene(s) and may affect the phenotype(s). Nevertheless, it will be of great interest to determine whether these Tg monkeys will develop tauopathy and neurodegeneration similar to human AD.

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.

Current state of research on non-human primate models of Alzheimer's disease

With the increasingly serious aging of the global population, dementia has already become a severe clinical challenge on a global scale. Dementia caused by Alzheimer's disease (AD) is the most common form of dementia observed in the elderly, but its pathogenetic mechanism has still not been fully elucidated. Furthermore, no effective treatment strategy has been developed to date, despite considerable efforts. This can be mainly attributed to the paucity of animal models of AD that are sufficiently similar to humans. Among the presently established animal models, non-human primates share the closest relationship with humans, and their neural anatomy and neurobiology share highly similar characteristics with those of humans. Thus, there is no doubt that these play an irreplaceable role in AD research. Considering this, the present literature on non-human primate models of AD was reviewed to provide a theoretical basis for future research.

Type II Diabetes Mellitus Accelerates Age-Dependent Aβ Pathology in Cynomolgus Monkey Brain

Accumulating evidence suggests that diabetes mellitus (DM) is one of the strongest risk factors for developing Alzheimer's disease (AD). However, it remains unclear how DM accelerates AD pathology in the brain. Cynomolgus monkey (Macaca fascicularis) is one of the nonhuman primates used for biomedical research, and we can observe spontaneous formation of AD pathology, such as senile plaques (SPs) and neurofibrillary tangles (NFTs), with the advance of aging. Furthermore, obesity is occasionally observed and frequently leads to development of type II DM (T2DM) in laboratory-housed cynomolgus monkeys. These findings suggest that cynomolgus monkey is a useful species to study the relationship between T2DM and AD pathology. In T2DM-affected monkey brains, SPs were observed in frontal and temporal lobe cortices almost 5 years earlier than healthy control monkeys. Moreover, age-related endocytic pathology, such as intraneuronal accumulation of enlarged endosomes, was exacerbated in T2DM-affected monkey brains. Since accumulating evidences suggest that endocytic dysfunction is involved in Aβ pathology, T2DM may aggravate age-related endocytic dysfunction, leading to the acceleration of Aβ pathology.

Granulovacuolar Degeneration in Brains of Senile Cynomolgus Monkeys

Neurons with histopathological changes consistent with granulovacuolar degeneration (GVD) were found in brain sections from aged cynomolgus monkeys (Macaca fascicularis) with clinical and pathological signs of cognitive aging. To our knowledge, this is the first reported description of GVD in non-human primates. GVD-like lesions were found also in age-matched cognitively healthy subjects, albeit in lower numbers, suggesting that they may relate to aging and the increase may have tendency to increase with the memory deficits. The increased incidence of GVD-like lesions in memory-impaired subjects with pahological backgrounds of senile plaques (SPs) and tauopathy is, however, an interesting observation of relevance to the characterization of pathologies in the spontaneous cynomolgus monkey model of human Alzheimer's type of brain pathology.

Ganglioside-Mediated Assembly of Amyloid β-Protein: Roles in Alzheimer's Disease

Assembly and deposition of amyloid β-protein (Aβ) is an early and invariable pathological event of Alzheimer's disease (AD), a chronic neurodegenerative disease affecting the neurons in the brain of aging population. Thus, clarification of the molecular mechanism underlying Aβ assembly is crucial not only for understanding the pathogenesis of AD, but also for developing disease-modifying remedies. In 1995, ganglioside-bound Aβ (GAβ), with unique molecular characteristics, including its altered immunoreactivity and its conspicuous ability to accelerate Aβ assembly, was discovered in an autopsied brain showing early pathological changes of AD. Based on these findings, it was hypothesized that GAβ is an endogenous seed for amyloid fibril formation in the AD brain. A body of evidence that supports the GAβ hypothesis has been growing for over 20years as follows. First, the conformational changes of Aβ from a random coil to an α-helix, and then to a β-sheet in the presence of ganglioside were validated by several techniques. Second, the seed activity of GAβ to accelerate the assembly of soluble Aβ into amyloid fibrils was confirmed by various in vitro and in vivo experiments. Third, it was found that the Aβ binding to ganglioside to form GAβ occurs under limited conditions, which were provided by the lipid environment surrounding ganglioside. Fourth, the region-specific Aβ deposition in the brain appeared to be dependent on the presence of the lipid environment that was in favor of GAβ generation. In this chapter, further progress of the study of ganglioside-mediated Aβ assembly, especially from the aspects of physicochemistry, structural biology, and neuropathology, is reviewed.

Animal models of cerebral amyloid angiopathy

Cerebral amyloid angiopathy (CAA), due to vascular amyloid β (Aβ) deposition, is a risk factor for intracerebral haemorrhage and dementia. CAA can occur in sporadic or rare hereditary forms, and is almost invariably associated with Alzheimer's disease (AD). Experimental (animal) models are of great interest in studying mechanisms and potential treatments for CAA. Naturally occurring animal models of CAA exist, including cats, dogs and non-human primates, which can be used for longitudinal studies. However, due to ethical considerations and low throughput of these models, other animal models are more favourable for research. In the past two decades, a variety of transgenic mouse models expressing the human Aβ precursor protein (APP) has been developed. Many of these mouse models develop CAA in addition to senile plaques, whereas some of these models were generated specifically to study CAA. In addition, other animal models make use of a second stimulus, such as hypoperfusion or hyperhomocysteinemia (HHcy), to accelerate CAA. In this manuscript, we provide a comprehensive review of existing animal models for CAA, which can aid in understanding the pathophysiology of CAA and explore the response to potential therapies.

Dynein Dysfunction Reproduces Age-Dependent Retromer Deficiency: Concomitant Disruption of Retrograde Trafficking Is Required for Alteration in β-Amyloid Precursor Protein Metabolism

It is widely accepted that β-amyloid (Aβ) protein plays a pivotal role in Alzheimer disease pathogenesis, and accumulating evidence suggests that endocytic dysfunction is involved in Aβ pathology. Retromer, a conserved multisubunit complex, mediates the retrograde transport of numerous kinds of cargo from endosomes to the trans-Golgi network. Several studies have found that retromer deficiency enhances Aβ pathology both in vitro and in vivo. Cytoplasmic dynein, a microtubule-based motor protein, mediates minus-end-directed vesicle transport via interactions with dynactin, another microtubule-associated protein that also interacts with retromer. Aging attenuates the dynein-dynactin interaction, and dynein dysfunction reproduces age-dependent endocytic disturbance, resulting in the intracellular accumulation of beta-amyloid precursor protein (APP) and its β-cleavage products, including Aβ. Here, we report that aging itself affects retromer trafficking in cynomolgus monkey brains. In addition, dynein dysfunction reproduces this type of age-dependent retromer deficiency (ie, the endosomal accumulation of retromer-related proteins and APP. Moreover, we found that knockdown of Rab7, Rab9, or Rab11 did not alter endogenous APP metabolism, such as that observed in aged monkey brains and in dynein-depleted cells. These findings suggest that dynein dysfunction can cause retromer deficiency and that concomitant disruption of retrograde trafficking may be the key factor underlying age-dependent Aβ pathology.

Lessons from the analysis of nonhuman primates for understanding human aging and neurodegenerative diseases

Animal models are necessary tools for solving the most serious challenges facing medical research. In aging and neurodegenerative disease studies, rodents occupy a place of choice. However, the most challenging questions about longevity, the complexity and functioning of brain networks or social intelligence can almost only be investigated in nonhuman primates. Beside the fact that their brain structure is much closer to that of humans, they develop highly complex cognitive strategies and they are visually-oriented like humans. For these reasons, they deserve consideration, although their management and care are more complicated and the related costs much higher. Despite these caveats, considerable scientific advances have been possible using nonhuman primates. This review concisely summarizes their role in the study of aging and of the mechanisms involved in neurodegenerative disorders associated mainly with cognitive dysfunctions (Alzheimer's and prion diseases) or motor deficits (Parkinson's and related diseases).

Diabetes mellitus accelerates Aβ pathology in brain accompanied by enhanced GAβ generation in nonhuman primates

Growing evidence suggests that diabetes mellitus (DM) is one of the strongest risk factors for developing Alzheimer’s disease (AD). However, it remains unclear why DM accelerates AD pathology. In cynomolgus monkeys older than 25 years, senile plaques (SPs) are spontaneously and consistently observed in their brains, and neurofibrillary tangles are present at 32 years of age and older. In laboratory-housed monkeys, obesity is occasionally observed and frequently leads to development of type 2 DM. In the present study, we performed histopathological and biochemical analyses of brain tissue in cynomolgus monkeys with type 2 DM to clarify the relationship between DM and AD pathology. Here, we provide the evidence that DM accelerates Aβ pathology in vivo in nonhuman primates who had not undergone any genetic manipulation. In DM-affected monkey brains, SPs were observed in frontal and temporal lobe cortices, even in monkeys younger than 20 years. Biochemical analyses of brain revealed that the amount of GM1-ganglioside-bound Aβ (GAβ)—the endogenous seed for Aβ fibril formation in the brain—was clearly elevated in DM-affected monkeys. Furthermore, the level of Rab GTPases was also significantly increased in the brains of adult monkeys with DM, almost to the same levels as in aged monkeys. Intraneuronal accumulation of enlarged endosomes was also observed in DM-affected monkeys, suggesting that exacerbated endocytic disturbance may underlie the acceleration of Aβ pathology due to DM.

Dynein dysfunction disrupts β-amyloid clearance in astrocytes through endocytic disturbances

We showed previously that aging attenuates the interaction between dynein-dynactin complexes in cynomolgus monkey brain and that dynein dysfunction reproduces age-dependent endocytic disturbances, resulting in intracellular β-amyloid (Aβ) accumulation, synaptic vesicle transport deficits, and neuritic swelling. It remains unclear whether such endocytic disturbances also occur in glial cells. Here, we show that endocytic pathology, such as intracellular accumulation of enlarged endosomes, occurs in astrocytes of aged monkey brains. Also, Aβ accumulates in these enlarged endosomes. RNA interference studies have shown that dynein dysfunction reproduces astroglial endocytic pathology and disrupts Aβ clearance in astrocytes through endocytic disturbances. These findings suggest that endocytic disturbances can alter astroglial functions and may also be involved in age-dependent Aβ pathology.

Amyloid Beta1-42 and the Phoshorylated Tau Threonine 231 in Brains of Aged Cynomolgus Monkeys (Macaca fascicularis)

Pathological hallmarks indicative of Alzheimer’s disease (AD), which are the plaques of amyloid beta_1–42 and neurofibrillary tangles, were found in brain of aged cynomolgus monkey. The aim of this study was to investigate if aged monkeys exhibiting spatial memory impairment and levels of biomarkers indicative of AD, had brain lesions similar to human patients suffering from senile dementia. Generating immunohistochemistry technique to biomarkers of amyloid beta_1–42 and the phosphorylated tau 231, our study assessed the amyloidopathy, such as indicative to the senile plaques and cerebral amyloid angiopathy, and the tauopathy, to possible neurofibrillary tangles. Six aged monkeys were selected based on their spatial memory performance and profile of biomarkers of AD, divided equally to affected aged subject – with Memory-affected and low amyloid level, and aged with higher performance in memory and amyloid, as the age-matched subjects. Using immunohistochemistry, plaques of amyloid beta_1–42 were observed in two out of three brains of aged subjects with memory impairment and biomarkers indicative of AD. The cerebral amyloid angiopathy was observed in both aged monkey groups, and unlike in the human, the amyloids were found to deposit in the small veins and capillaries. In one of the affected individuals, phosphorylated tau was positively stained intracellularly of the neurons, indicating a possibility of an early stage of the formation of tangles. These findings add to the body of evidence of the utility of the aged cynomolgus monkeys as a spontaneous model for Alzheimer-related disease.

Age-associated changes of cerebrospinal fluid amyloid-β and tau in cynomolgus monkeys

Nonhuman primates (NHPs) are useful for the study of age-associated changes in the brain as a model that is biologically closely related to humans. For example, with age, all NHPs analyzed to date, develop β-amyloid (Aβ) plaques as seen in humans. Nevertheless, it is still unclear if NHPs have human-like age-associated changes in Aβ and tau protein in cerebrospinal fluid. The present study was an attempt to specifically address these issues. Cerebrospinal fluid levels of Aβ and phosphorylated tau were measured in 37 and 22 cynomolgus monkeys, respectively, with ages ranging from 4 to 22-year-old. The result from the present study revealed significant age-associated declines in Aβ42 levels but not in Aβ40 and phosphorylated tau levels. This finding appears to parallel changes seen with human aging, in which decreased levels of Aβ42 can be seen in normal older adults, and supporting that cynomolgus monkeys would be a useful model for studying age-related neurologic disorders associated with Alzheimer-like cerebral proteopathy.

Delayed response task performance as a function of age in cynomolgus monkeys (Macaca fascicularis)

We compared delayed response task performance in young, middle-aged, and old cynomolgus monkeys using three memory tests that have been used with non-human primates. Eighteen cynomolgus monkeys—6 young (4–9 years), 6 middle-aged (10–19 years), and 6 old (above 20 years)—were tested. In general, the old monkeys scored significantly worse than did the animals in the two other age groups. Longer delays between stimulus presentation and response increased the performance differences between the old and younger monkeys. The old monkeys in particular showed signs of impaired visuo-spatial memory and deteriorated memory consolidation and executive functioning. These results add to the body of evidence supporting the utility of Macaca fascicularis in studies of cognition and as a potential translational model for age-associated memory impairment/dementia-related disorders.

Neurofibrillary tangles and the deposition of a beta amyloid peptide with a novel N-terminal epitope in the brains of wild Tsushima leopard cats

Beta amyloid (Aβ) deposits are seen in aged individuals in many of the mammalian species that possess the same Aβ amino acid sequence as humans. Conversely, neurofibrillary tangles (NFT), the other hallmark lesion of Alzheimer's disease (AD), are extremely rare in these animals. We detected Aβ deposits in the brains of Tsushima leopard cats (Prionailurus bengalensis euptilurus) that live exclusively on Tsushima Island, Japan. Aβ42 was deposited in a granular pattern in the neuropil of the pyramidal cell layer, but did not form argyrophilic senile plaques. These Aβ deposits were not immunolabeled with antibodies to the N-terminal of human Aβ. Sequence analysis of the amyloid precursor protein revealed an amino acid substitution at the 7th residue of the Aβ peptide. In a comparison with other mammalian animals that do develop argyrophilic senile plaques, we concluded that the alternative Aβ amino acid sequence displayed by leopard cats is likely to be related to its distinctive deposition pattern. Interestingly, most of the animals with these Aβ deposits also developed NFTs. The distributions of hyperphosphorylated tau-positive cells and the two major isoforms of aggregated tau proteins were quite similar to those seen in Alzheimer's disease. In addition, the unphosphorylated form of GSK-3β colocalized with hyperphosphorylated tau within the affected neurons. In conclusion, this animal species develops AD-type NFTs without argyrophilic senile plaques.

Hyperpolarization-activated cyclic nucleotide gated channels: a potential molecular link between epileptic seizures and Aβ generation in Alzheimer's disease

Background

One of the best-characterized causative factors of Alzheimer’s disease (AD) is the generation of amyloid-β peptide (Aβ). AD subjects are at high risk of epileptic seizures accompanied by aberrant neuronal excitability, which in itself enhances Aβ generation. However, the molecular linkage between epileptic seizures and Aβ generation in AD remains unclear.

Results

X11 and X11-like (X11L) gene knockout mice suffered from epileptic seizures, along with a malfunction of hyperpolarization-activated cyclic nucleotide gated (HCN) channels. Genetic ablation of HCN1 in mice and HCN1 channel blockage in cultured Neuro2a (N2a) cells enhanced Aβ generation. Interestingly, HCN1 levels dramatically decreased in the temporal lobe of cynomolgus monkeys (Macaca fascicularis) during aging and were significantly diminished in the temporal lobe of sporadic AD patients.

Conclusion

Because HCN1 associates with amyloid-β precursor protein (APP) and X11/X11L in the brain, genetic deficiency of X11/X11L may induce aberrant HCN1 distribution along with epilepsy. Moreover, the reduction in HCN1 levels in aged primates may contribute to augmented Aβ generation. Taken together, HCN1 is proposed to play an important role in the molecular linkage between epileptic seizures and Aβ generation, and in the aggravation of sporadic AD.

Dynein dysfunction disrupts intracellular vesicle trafficking bidirectionally and perturbs synaptic vesicle docking via endocytic disturbances a potential mechanism underlying age-dependent impairment of cognitive function

Although genetic studies have demonstrated that β-amyloid protein (Aβ) plays a pivotal role in Alzheimer's disease (AD) pathogenesis, how aging contributes to AD onset remains unclear. Moreover, growing evidence suggests that Aβ-independent mechanisms, such as altered intracellular signaling cascades and impaired neurotransmitter release, also are likely involved in this process. Cytoplasmic dynein, a microtubule-based motor protein, mediates minus end-directed vesicle transport via interactions with dynactin, another microtubule-associated protein. We previously showed that normal aging attenuates the interaction between dynein-dynactin complexes in monkey brain and that dynein dysfunction reproduces age-dependent endocytic disturbances, resulting in intracellular Aβ accumulation. In this study, we report that dynein dysfunction disrupts not only retrograde transport of neurotrophic receptors but also anterograde transport of synaptic vesicles, which occurs concomitantly with an increase in Rab3 GTPase levels. Additionally, synaptic vesicle docking was perturbed via enhanced endocytosis. Dynein dysfunction also induced neuritic swelling, which is accompanied by a significant accumulation of neurofilaments. Moreover, we also confirmed that the dynein dysfunction-related disturbances are associated with aging in monkey brains and that age-dependent endocytic disturbances precede Aβ abnormality. These findings suggest that dynein dysfunction can alter neuronal activity via endocytic disturbances and may underlie age-dependent impairment of cognitive function. Moreover, in the presence of other risk factors, such as intracellular Aβ accumulation, dynein dysfunction may contribute to the development of AD.

Binding of curcumin to senile plaques and cerebral amyloid angiopathy in the aged brain of various animals and to neurofibrillary tangles in Alzheimer's brain

The binding of curcumin to senile plaques (SPs) and cerebral amyloid angiopathy (CAA) was examined in the aged brain of various animal species and a human patient with Alzheimer's disease (AD), together with its binding to neurofibrillary tangles (NFTs). Brain sections were immunostained with anti-amyloid β protein 1-42 (Aβ42) and anti-amyloid β protein 1-40 (Aβ40) antibodies. These sections were also stained with alkaline Congo red, periodic acid-methenamine silver (PAM), and curcumin (0.009% curcumin solution) with or without formic acid pretreatment. The sections from the AD brain were also immunostained for anti-paired helical filament-tau (PHF-tau), and were stained with Gallyas silver for NFTs. Some SPs in the AD, monkey, dog, bear, and amyloid precursor protein transgenic mouse (APP Tg-mouse) brains contained congophilic materials, and were intensely positive for curcumin. In addition, curcumin labeled some diffuse SPs negative for Congo red in the AD, monkey, bear, and APP Tg-mouse brains. In all animals, CAA was intensely positive for both Congo red and curcumin. The specific curcumin staining activity was lost by formic acid pretreatment. In the AD brain, NFTs positive for PHF-tau and Gallyas silver were moderately stained with curcumin. These findings indicate that curcumin specifically binds to the aggregated Aβ molecules in various animals, and further to phosphorylated tau protein, probably according to its conformational nature.

[Does Alzheimer's disease exist in all primates? Alzheimer pathology in non-human primates and its pathophysiological implications (II)]

INTRODUCTION

In the ageing process there are some species of non-human primates which can show some of the defining characteristics of the Alzheimer's disease (AD) of man, both in neuropathological changes and cognitive-behavioural symptoms. The study of these species is of prime importance to understand AD and develop therapies to combat this neurodegenerative disease.

DEVELOPMENT

In this second part of the study, these AD features are discussed in the most important non-experimental AD models (Mouse Lemur -Microcebus murinus, Caribbean vervet -Chlorocebus aethiops, and the Rhesus and stump-tailed macaque -Macaca mulatta and M. arctoides) and experimental models (lesional, neurotoxic, pharmacological, immunological, etc.) non-human primates. In all these models cerebral amyloid neuropathology can occur in senility, although with different levels of incidence (100% in vervets;<30% in macaques). The differences between normal and pathological (Alzheimer's) senility in these species are difficult to establish due to the lack of cognitive-behavioural studies in the many groups analysed, as well as the controversy in the results of these studies when they were carried out. However, in some macaques, a correlation between a high degree of functional brain impairment and a large number of neuropathological changes ("possible AD") has been found.

CONCLUSIONS

In some non-human primates, such as the macaque, the existence of a possible continuum between "normal" ageing process, "normal" ageing with no deep neuropathological and cognitive-behavioural changes, and "pathological ageing" (or "Alzheimer type ageing"), may be considered. In other cases, such as the Caribbean vervet, neuropathological changes are constant and quite marked, but its impact on cognition and behaviour does not seem to be very important. This does assume the possible existence in the human senile physiological regression of a stable phase without dementia even if neuropathological changes appeared.

Concomitant detection of beta-amyloid peptides with N-terminal truncation and different C-terminal endings in cortical plaques from cases with Alzheimer's disease, senile monkeys and triple transgenic mice

The disturbed metabolism of beta-amyloid peptides generated from amyloid precursor protein is widely considered as a main factor during the pathogenesis of Alzheimer's disease. A neuropathological hallmark in the brains from cases with Alzheimer's disease are senile plaques mainly composed of hardly soluble beta-amyloid peptides comprising up to 43 amino acids. Age-dependent cortical beta-amyloidosis was also shown in several transgenic mice and old individuals from various mammalian species, e.g., non-human primates. Beta-amyloid(1-42) is believed to be the main component in the core of senile plaques, whereas less hydrophobic beta-amyloid(1-40) predominantly occurs in the outer rim of plaques. Amino-terminally truncated pyroglutamyl-beta-amyloid(pE3-x) was recently found to be a beta-amyloid species of high relevance to the progression of the disease. While a few biochemical studies provided data on the co-occurrence of several beta-amyloid forms, their concomitant histochemical detection is still lacking. Here, we present a novel triple immunofluorescence labelling of amino- and differently carboxy-terminally truncated beta-amyloid peptides in cortical plaques from a case with Alzheimer's disease, senile macaques and baboons, and triple transgenic mice with age-dependent beta-amyloidosis and tau hyperphosphorylation. Additionally, beta-amyloid(pE3-x) and total beta-amyloid were concomitantly detected with beta-amyloid peptides ending with amino acid 40 or 42, respectively. Simultaneous staining of several beta-amyloid species reveals for instance vascular amyloid containing beta-amyloid(pE3-x) in Alzheimer's disease and monkeys, and may contribute to the further elucidation of beta-amyloidosis in neurodegenerative disorders and animal models.

Age-Related Lesions in the Cerebrum in Middle-Aged Female Cynomolgus Monkeys

Alzheimer’s disease (AD) in humans is a progressive neurogenic disease that can be linked with such characteristic pathological findings in the cerebrum as senile plaques (SPs), neurofibrillary tangles (NFTs), cerebral amyloid angiopathy (CAA), and neuronal loss. In the present study, the authors investigated the age-related morphological changes in 12 middle-aged and 12 young cynomolgus monkeys. Low numbers of neurons and astrocytes in the hippocampal region in cynomolgus monkeys accompanied ageing, and there was a high number of microglial cells; however, no clearly neurotoxic abnormalities due to β-amyloid were noted before the age of 20 years. The onset of SPs and CAA in the cerebrum in cynomolgus monkeys can occur before the age of 20 years. SPs were almost all categorized as diffuse plaques (DPs); they did not have amyloid cores and were unaccompanied by neuritic degeneration. In cynomolgus monkeys, SPs (DPs) occur before the appearance of CAA. From the above, it was concluded that cynomolgus monkeys showed pathological changes due to ageing similar to those related to Alzheimer’s disease in humans, even before they were 20 years old.

Neuronal apoptosis in Alzheimer's disease: the role of age-related mitochondrial metabolic competence

To assess the role of the mitochondrial metabolic competence (MMC) in the development of age-related changes, we measured the levels of immunohistochemically stained (IH) mitochondrial- and nuclear-encoded subunits of cytochrome oxidase (COX II and COX IV, respectively) and compared these data with mRNA in situ hybridization (ISH) of the same subunits and with cytochemically evidenced COX activity in the temporal (TC) and frontal (FC) cortex of adult and late-adult monkeys. Quantitative cytochemistry of COX activity was performed by calculating the ratio (R) of the area of the cytochemical precipitate to the area of the respective organelle. Although ISH studies showed reduced gene expression of both subunits in FC of late-adult monkeys, no significant age-related difference was found either in TC or FC when considering the IH data. R was significantly increased in FC of late-adult animals, and a quartile distribution of the mitochondrial area showed that R is higher in the FC of older animals independent of the organelle size. The assessment of COX genetic and phenotypic parameters reliably reports on MMC because this enzyme is the terminal complex of the electron transport chain. Taken together, the present IH, ISH, and R findings suggest that, with advancing age, compensating mechanisms are activated to preserve the mitochondrial functional metabolic capacities. Although significant mitochondrial defects are currently reported to occur in Alzheimer's disease pathogenesis, our data document that MMC is actively involved in the physiological rearrangement of the age-related neuronal network and may provide substantial metabolic support for the energy demand of neuronal apoptosis.

Dynein dysfunction induces endocytic pathology accompanied by an increase in Rab GTPases: a potential mechanism underlying age-dependent endocytic dysfunction

Growing evidence suggests that endocytic dysfunction is intimately involved in early stage Alzheimer disease pathology, such as the accumulation of beta-amyloid precursor protein in enlarged early endosomes. However, it remains unclear how endocytic dysfunction is induced in an age-dependent manner. Cytoplasmic dynein, a microtubule-based motor protein, interacts with another microtubule-associated protein, dynactin. The resulting dynein-dynactin complex mediates minus end-directed vesicle transport, including endosome trafficking. We have previously shown that the interaction between dynein-dynactin complexes is clearly attenuated in aged monkey brains, suggesting that dynein-mediated transport dysfunction exists in aged brains. Our immunohistochemical analyses revealed that age-dependent endocytic pathology was accompanied by an increase in Rab GTPases in aged monkey brains. Here, we demonstrated that siRNA-induced dynein dysfunction reproduced the endocytic pathology accompanied by increased Rab GTPases seen in aged monkey brains and significantly disrupted exosome release. Moreover, it also resulted in endosomal beta-amyloid precursor protein accumulation characterized by increased beta-site cleavage. These findings suggest that dynein dysfunction may underlie age-dependent endocytic dysfunction via the up-regulation of Rab GTPases. In addition, this vicious circle may worsen endocytic dysfunction, ultimately leading to Alzheimer disease pathology.

Endosomal accumulation of GM1 ganglioside-bound amyloid beta-protein in neurons of aged monkey brains

We performed an immunohistochemical analysis of the GM1 ganglioside-bound amyloid beta-protein (GAbeta), an endogenous seed of Alzheimer amyloids, in sections of cerebral cortices of cynomolgus monkeys of different ages from 4 to 36 years old. Here, we show that neuronal GAbeta immunostaining significantly increases in the sections obtained from animals at ages below 19 years, even without senile plaque formation, and that GAbeta accumulation exclusively occurs in organelles involved in the endocytic pathway, including early, late, and recycling endosomes, not in those involved in the secretory pathway. Together with previous findings that Abeta generation likely occurs in early endosomes and that GM1 accumulation in early endosomes is induced by endocytic pathway abnormalities, our results provide further evidence that endosomes are intimately involved in the Abeta-associated pathology of Alzheimer's disease.

Immunohistochemical and biochemical analyses of LGI3 in monkey brain: LGI3 accumulates in aged monkey brains

Leucine-rich glioma inactivated (LGI) 3 encodes a leucine-rich repeat protein. The precise function of LGI3, however, remains unknown. We have previously shown that amyloid-beta peptide (Abeta) upregulates LGI3 and that Abeta and LGI3 colocalize on plasma membranes of cultured rat astrocytes. In the present study, we performed immunohistochemical and biochemical analyses of LGI3 using various aged monkey brains. Immunohistochemistry showed that LGI3 was present in almost all neural cells and mainly localized at plasma membranes and nuclei. In aged monkey brains, we found that LGI3 accumulated on or near the plasma membranes of neurons, and colocalized with endocytosis-associated proteins and lipid raft markers. Double immunohistochemistry also showed that LGI3 colocalized with Abeta in astrocytes of aged brains. Moreover, Western blot analyses revealed that LGI3 may be cleaved in brain. Additionally, in aged monkeys LGI3 accumulated in microsomal and nuclear brain fractions.

Preservation of mitochondrial volume homeostasis at the early stages of age-related synaptic deterioration

A morphometric study on synaptic mitochondria was performed in the frontal (FC) and temporal (TC) cortex of adult and aged monkeys to seek ultrastructural alterations due to age. The overall volume covered by mitochondria (volume density: Vv), the number of mitochondria/microm(3) of tissue (numeric density: Nv), the average mitochondrial size (average volume: V), and the average mitochondrial shape (average length: Fmax) were calculated. Either in FC and TC, no significant age-related differences were revealed for any of the above-mentioned morphometric parameters. Namely, in FC of aged monkeys, a decrease of Vv (2%) and Nv (6%) was observed, whereas V and Fmax were increased by 5% and 2%, respectively. In TC of aged animals, both Vv and Nv increased by 7%, V decreased by 2%, and Fmax increased by 1%. The above morphometric parameters account for changes in single aspects of mitochondrial ultrastructure; nonetheless, when considered together per experimental group, they provide information regarding the structural rearrangements occurring on discrete populations of organelles. Considering these assumptions, the present findings document a preservation of the mitochondrial volume homeostasis in the brain of aged monkeys. Because our data from a previous investigation on the same animals showed early signs of synaptic deterioration in FC and TC during aging, this seems to be in contrast with the results of the present study. However, the clear age-related preservation of the mitochondrial potential for structural dynamics may be interpreted as a reactive response to early signs of synaptic deterioration.

Quantitation of sleep and spinal curvature in an unusually longevous owl monkey (Aotus azarae)

BACKGROUND

A table summarizing the primary literature on 19 species of longevous non-human primates, other than owl monkey, is presented.

METHODS

We prospectively quantitated the sleep of a longevous female owl monkey (Aotus azarae), aged >30 years, longitudinally for 2 years and also evaluated the senility-induced change in spinal curvature.

RESULTS

The mean daily total sleep time (TST) of this monkey ranged between 790 and 1106 minutes, and was markedly higher in comparison with its female progeny (aged 16 years and used as a control) whose daily TST during the same experimental period ranged between 612 and 822 minutes.

CONCLUSIONS

The calculated kyphotic index (KI) of 2.27 for this monkey, compared with the KIs 4.83 and 5.42, for its progeny and female grandprogeny (aged 1 year) respectively, confirmed the prominent spinal curvature.

Aging attenuates dynactin–dynein interaction: Down-regulation of dynein causes accumulation of endogenous tau and amyloid precursor protein in human neuroblastoma cells

Impaired axonal transport may promote pathogenesis in neurodegenerative disorders, such as Alzheimer's disease (AD). We previously showed that tau, amyloid precursor protein (APP), and intracellular amyloid beta-protein (Abeta) accumulate in the nerve-ending fraction of aged monkey brains, perhaps because of impaired axonal transport. In the present study, we assessed age-related changes of axonal transport motor proteins in aged monkey brains. Western blotting showed that kinesin, dynein, and dynactin (DYN) localizations dramatically changed with aging, and dynein level in nerve-ending fractions increased significantly. Coimmunoprecipitation analyses showed that DYN-dynein intermediate chain (DIC) interactions decreased, suggesting that age-related attenuation of this interaction may cause the impairment of dynein function. Moreover, RNAi-induced down-regulation of DIC in human neuroblastoma cells caused endogenous tau and APP to accumulate, and their subcellular localizations were also affected. Our findings suggest that aging attenuates DYN-DIC interaction, representing one of the risk factors for age-related impaired dynein function and even for accumulation of disease proteins.

Alterations of Synaptic Turnover Rate in Aging May Trigger Senile Plaque Formation and Neurodegeneration

The changes of synaptic ultrastructure were investigated by morphometry in the frontal (FC) and temporal (TC) cortex of adult and aged monkeys, to assess the potential role of age-related synaptic deterioration in neurodegeneration. The average synaptic size (S), the synaptic numeric density (Nv: number of synapses/microm(3) of tissue), the synaptic surface density (Sv: overall area of synaptic junctional zones/microm(3) of tissue), and the number of synapses/neuron (Syn/Neur) were calculated. In FC, significant differences of Nv and Sv due to age were not revealed, while the S value was significantly increased in the aged animals. In TC, Sv did not change in relation to age, whereas Nv was significantly decreased and S significantly increased in aged monkeys. A percent distribution of S showed that the fraction of enlarged synapses (>0.20 microm(2)) was higher in TC than in FC, regardless of the age of the animals (21.3% versus 16.9% in adult and 33.9% versus 26.0% in aged monkeys, respectively). In aged animals, Syn/Neur was not significantly decreased in TC and not significantly increased in FC (4.4%). The above morphometric parameters account for the ongoing rearrangements of synaptic ultrastructure, reacting to the environmental stimuli. Our findings provide evidence of an age-related decline of synaptic plasticity in the brain of aged monkeys that is statistically significant in TC. According to current literature data on synaptic structural dynamics, this decay may represent an early and subtle alteration able to trigger the development of senile plaques and neurodegenerative events.

Synaptic Pathology in the Brain Cortex of Old Monkeys as an Early Alteration in Senile Plaque Formation

Synaptic numeric density (Nv), average size (area: S), surface density (Sv) and number of synapses/neurone (Syn/Neur) were morphometrically measured in frontal (FC) and temporal (TC) cortex of adult and old monkeys. Sv was constant, a clear age-related trend to decrease by Nv and increase by S were observed in both areas investigated. Syn/Neur significantly decreased in TC of aged animals (-21.1%), whereas FC showed a not significant reduction (-2.6%). The present data support the hypothesis of an increased sensitivity to deterioration of TC synapses in aged monkeys, which might constitute a predisposing condition to the development of senile plaques.

Age-related changes of intracellular Abeta in cynomolgus monkey brains

To confirm the intracellular accumulation of amyloid beta-protein (Abeta), we carefully performed immunohistochemistry using brains of cynomolgus monkeys of various ages. Cortical neurones and their large neurites were immunostained with antibodies against Abeta in young monkey brains. In aged monkey brains, intracellular Abeta localized within cortical neurones; no clear association was found between the presence of intracellular Abeta and senile plaques (SPs). Interestingly, we did not observe Abeta-immunoreactive cortical neurones in brains fixed with neutral buffered formalin. Western blot analyses of microsomal and nerve ending fractions derived from the brains of young to aged monkeys revealed that intracellular Abeta generation changed with age. In the microsomal fraction, the amount of Abeta42 significantly increased in brains from older monkeys (>30 years of age), and the amount of Abeta43 significantly decreased with age in the microsomal fraction. The amount of Abeta40 remained the same regardless of age. Biochemical analyses also showed that intracellular levels of each of these Abeta molecules significantly increased with age in nerve ending fractions. As we previously observed that a similar accumulation of presenilin1, beta-amyloid precursor protein (APP) and APP C-terminal fragment cleaved by beta-secretase in the nerve ending fractions obtained from brains with SPs, the accumulation of intracellular Abeta in this fraction may be closely related to formation of spontaneous SPs with age. Taken together, these results suggest that intensive investigation of age-related changes in the nerve ending will contribute to a better understanding of the pathogenesis of age-related neurodegenerative disorders such as sporadic Alzheimer's disease.

A comparative study of age-related brain pathology--are neurodegenerative diseases present in nonhuman animals ?

Although some aged dogs definitely have dementia-like conditions, they have rather different brain histopathology from that seen in Alzheimer's disease including the shape of senile plaques, severity of neuron loss and absence of neurofibrillary tangles. Aged wild-type mice never show such brain lesions at all. In addition, no cases of Parkinson's disease have been reported in nonhuman animals yet. The reason for this might be non-parallel aging of the whole body and brain. If such nonhuman animals had a longer life span, like humans, typical Alzheimer's and Parkinson's lesions would be formed in the brain. As the rate of deposition of the misfolded proteins causing the lesions might be slow, nonhuman animals normally die before the lesions appear.