Age-related accumulation of Marinesco bodies and lipofuscin in rhesus monkey midbrain dopamine neurons: relevance to selective neuronal vulnerability

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.

Natural Variation in Age-Related Dopamine Neuron Degeneration is Glutathione-Dependent and Linked to Life Span

Aging is the biggest risk factor for Parkinson's disease (PD), suggesting that age-related changes in the brain promote dopamine neuron vulnerability. It is unclear, however, whether aging alone is sufficient to cause significant dopamine neuron loss and if so, how this intersects with PD-related neurodegeneration. Here, through examining a large collection of naturally varying Drosophila strains, we find a strong relationship between life span and age-related dopamine neuron loss. Naturally short-lived strains exhibit a loss of dopamine neurons but not generalized neurodegeneration, while long-lived strains retain dopamine neurons across age. Metabolomic profiling reveals lower glutathione levels in short-lived strains which is associated with elevated levels of reactive oxygen species (ROS), sensitivity to oxidative stress and vulnerability to silencing the familial PD gene parkin . Strikingly, boosting neuronal glutathione levels via glutamate-cysteine ligase (GCL) overexpression is sufficient to normalize ROS levels, extend life span and block dopamine neurons loss in short-lived backgrounds, demonstrating that glutathione deficiencies are central to neurodegenerative phenotypes associated with short longevity. These findings may be relevant to human PD pathogenesis, where glutathione depletion is frequently reported in idiopathic PD patient brain. Building on this evidence, we detect reduced levels of GCL catalytic and modulatory subunits in brain from PD patients harboring the LRRK2 G2019S mutation, implicating possible glutathione deficits in familial LRRK2-linked PD. Our study across Drosophila and human PD systems suggests that glutathione plays an important role in the influence of aging on PD neurodegeneration.

Inosine monophosphate dehydrogenase intranuclear inclusions are markers of aging and neuronal stress in the human substantia nigra

We explored mechanisms involved in the age-dependent degeneration of human substantia nigra (SN) dopamine (DA) neurons. Owing to its important metabolic functions in post-mitotic neurons, we investigated the developmental and age-associated changes in the purine biosynthetic enzyme inosine monophosphate dehydrogenase (IMPDH). Tissue microarrays prepared from post-mortem samples of SN from 85 neurologically intact participants humans spanning the age spectrum were immunostained for IMPDH combined with other proteins. SN DA neurons contained two types of IMPDH structures: cytoplasmic IMPDH filaments and intranuclear IMPDH inclusions. The former were not age-restricted and may represent functional units involved in sustaining purine nucleotide supply in these highly metabolically active cells. The latter showed age-associated changes, including crystallization, features reminiscent of pathological inclusion bodies, and spatial associations with Marinesco bodies; structures previously associated with SN neuron dysfunction and death. We postulate dichotomous roles for these two subcellularly distinct IMPDH structures and propose a nucleus-based model for a novel mechanism of SN senescence that is independent of previously known neurodegeneration-associated proteins.

Neuronal Senescence in the Aged Brain

Cellular senescence is a highly complicated cellular state that occurs throughout the lifespan of an organism. It has been well-defined in mitotic cells by various senescent features. Neurons are long-lived post-mitotic cells with special structures and functions. With age, neurons display morphological and functional changes, accompanying alterations in proteostasis, redox balance, and Ca2+ dynamics; however, it is ambiguous whether these neuronal changes belong to the features of neuronal senescence. In this review, we strive to identify and classify changes that are relatively specific to neurons in the aging brain and define them as features of neuronal senescence through comparisons with common senescent features. We also associate them with the functional decline of multiple cellular homeostasis systems, proposing the possibility that these systems are the main drivers of neuronal senescence. We hope this summary will serve as a steppingstone for further inputs on a comprehensive but relatively specific list of phenotypes for neuronal senescence and in particular their underlying molecular events during aging. This will in turn shine light on the association between neuronal senescence and neurodegeneration and lead to the development of strategies to perturb the processes.

Neuronal Dot1l Activity Acts as a Mitochondrial Gene-Repressor Associated with Human Brain Aging via H3K79 Hypermethylation

Methylation of histone 3 at lysine 79 (H3K79) and its catalyst, a disrupter of telomeric silencing (DOT1l), have been coupled to multiple forms of stress, such as bioenergetic and ER challenges. However, studies on H3K79 methylation and Dot1l in the (aging) brain and neurons are limited. This, together with the increasing evidence of a dynamic neuroepigenome, made us wonder if H3K79 methylation and its activator Dot1l could play important roles in brain aging and associated disorders. In aged humans, we found strong and consistent global hypermethylation of H3K79 in neurons. Specific in dopaminergic neurons, we found a strong increase in H3K79 methylation in lipofucsin positive neurons, which are linked to pathology. In animals, where we conditionally removed Dot1l, we found a rapid loss of H3K79 methylation. As a consequence, we found some decrease in specific dopaminergic genes, and surprisingly, a clear up-regulation of almost all genes belonging to the family of the respiratory chain. These data, in relation to the observed increase in global H3K79 methylation, suggest that there is an inverse relationship between H3K79 methylation and the capacity of energy metabolism in neuronal systems.

DNA Methylation Signature of Aging: Potential Impact on the Pathogenesis of Parkinson's Disease

Regulation of gene expression by epigenetic modifications means lasting and heritable changes in the function of genes without alterations in the DNA sequence. Of all epigenetic mechanisms identified thus far, DNA methylation has been of particular interest in both aging and age-related disease research over the last decade given the consistency of site-specific DNA methylation changes during aging that can predict future health and lifespan. An increasing line of evidence has implied the dynamic nature of DNA (de)methylation events that occur throughout the lifespan has a role in the pathophysiology of aging and age-associated neurodegenerative conditions, including Parkinson's disease (PD). In this regard, PD methylome shows, to some extent, similar genome-wide changes observed in the methylome of healthy individuals of matching age. In this review, we start by providing a brief overview of studies outlining global patterns of DNA methylation, then its mechanisms and regulation, within the context of aging and PD. Considering diverging lines of evidence from different experimental and animal models of neurodegeneration and how they combine to shape our current understanding of tissue-specific changes in DNA methylome in health and disease, we report a high-level comparison of the genomic methylation landscapes of brain, with an emphasis on dopaminergic neurons in PD and in natural aging. We believe this will be particularly useful for systematically dissecting overlapping genome-wide alterations in DNA methylation during PD and healthy aging, and for improving our knowledge of PD-specific changes in methylation patterns independent of aging process.

Biology of aging: Oxidative stress and RNA oxidation

The prevalence of aged people has increased rapidly in recent years and brings profound demographic changes worldwide. The multi-level progression of aging occurs at diverse stages of complexity, from cell to organ systems and eventually to the human as a whole. The cellular and molecular damages are usually regulated by the cells; repair or degrade mechanisms. However, these mechanisms are not entirely functional; their effectiveness decreases with age due to influence from endogenous sources like oxidative stress, which all contribute to the aging process. The hunt for novel strategies to increase the man's longevity since ancient times needs better understandings of the biology of aging, oxidative stress, and their roles in RNA oxidation. The critical goal in developing new strategies to increase the man's longevity is to compile the novel developed knowledge on human aging into a single picture, preferably able to understand the biology of aging and the contributing factors. This review discusses the biology of aging, oxidative stress, and their roles in RNA oxidation, leading to aging in humans.

GSK-3 mediates nuclear translocation of p62/SQSTM1 in MPTP-induced mouse model of Parkinson's disease

p62/SQSTM1 is a multifunctional, cytoplasmic protein with fundamental roles in autophagy and antioxidant responses. Here we showed that p62 translocated from the cytoplasm to the nucleus in nigral dopaminergic neurons in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrid (MPTP)-induced mouse model of Parkinson's disease (PD). We found that p62 was physically associated with glycogen synthase kinase (GSK)-3β, a serine/threonine protein kinase implicated in dopaminergic neurodegeneration in PD, and that MPTP treatment promoted dissociation of the complex in mice. Conditional knockout of GSK-3 prevented nuclear translocation of p62, suggesting that this translocation was detrimental to dopaminergic neurons. p62 knockout mice were used to investigate the role of p62 in MPTP-induced neuronal death. Knockout of p62 aggravated neuronal injury induced by MPTP intoxication, suggesting that p62 plays an important role in dopaminergic cell survival in stress conditions. Together, our data demonstrate that GSK-3 mediates nuclear translocation of p62 during MPTP-induced parkinsonian neurodegeneration. These findings shed new light on the role of the cytoplasmic-nuclear shuttling of p62 and the mechanism underlying GSK-3-depedent neuronal death in PD pathogenesis.

Evidence of distinct α-synuclein strains underlying disease heterogeneity

Synucleinopathies are a group of neurodegenerative disorders caused by the misfolding and self-templating of the protein α-synuclein, or the formation of α-synuclein prions. Each disorder differs by age of onset, presenting clinical symptoms, α-synuclein inclusion morphology, and neuropathological distribution. Explaining this disease-specific variability, the strain hypothesis postulates that each prion disease is encoded by a distinct conformation of the misfolded protein, and therefore, each synucleinopathy is caused by a unique α-synuclein structure. This review discusses the current data supporting the role of α-synuclein strains in disease heterogeneity. Several in vitro and in vivo models exist for evaluating strain behavior, however, as the focus of this article is to compare strains across synucleinopathy patients, our discussion predominantly focuses on the two models most commonly used for this purpose: the α-syn140*A53T-YFP cell line and the TgM83^+/- mouse model. Here we define each strain based on biochemical stability, ability to propagate in α-syn140-YFP cell lines, and incubation period, inclusion morphology and distribution, and neurological signs in TgM83^+/- mice.

Region-specific vulnerability in neurodegeneration: lessons from normal ageing

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Why neurodegenerative disease pathology is regionally restricted remains elusive.

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Regions selectively prone to neurodegeneration are also vulnerable to normal ageing.

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Nervous system tissue, cellular and molecular ageing may determine regional vulnerability.

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Differential ageing can conceptually extend from an individual to subcellular scale.

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An understanding of region-specific vulnerability might guide therapeutic advances.

A number of age-associated neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD) and amyotrophic lateral sclerosis (ALS), possess a shared characteristic of region-specific neurodegeneration. However, the mechanisms which determine why particular regions within the nervous system are selectively vulnerable to neurodegeneration, whilst others remain relatively unaffected throughout disease progression, remain elusive. Here, we review how regional susceptibility to the ubiquitous physiological phenomenon of normal ageing might underlie the vulnerability of these same regions to neurodegeneration, highlighting three regions archetypally associated with AD, PD and ALS (the hippocampus, substantia nigra pars compacta and ventral spinal cord, respectively), as especially prone to age-related alterations. Placing particular emphasis on these three regions, we comprehensively explore differential regional susceptibility to nervous system tissue, cellular and molecular level ageing to provide an integrated perspective on why age-related neurodegenerative diseases exhibit region-selective vulnerability. Combining these principles with increasingly recognised differences between chronological and biological ageing (termed differential or ‘delta’ ageing) might ultimately guide therapeutic approaches for these devastating neurodegenerative diseases, for which a paucity of disease modifying and/or life promoting treatments currently exist.

Neuronal and Glial Distribution of Tau Protein in the Adult Rat and Monkey

Tau is a microtubule-associated protein for which the physiological functions remain a topic of vigorous investigation. Additionally, tau is a central player in the pathogenesis of several diseases such as Alzheimer's disease and several frontotemporal dementias. A critical variable to understanding tau in physiological and disease contexts is its normal localization within cells of the adult CNS. Tau is often described as an axon-specific (or enriched) and neuron-specific protein with little to no expression in glial cells, all of which are untrue. Understanding normal tau distribution also impacts interpretation of experimental results and hypotheses regarding its role in disease. Thus, we set out to help clarify the normal localization of tau in the adult CNS of middle-aged rats and rhesus macaque using the hippocampus as a representative brain structure. The physiological concentration of tau in the rat hippocampus was 6.6 μM and in white matter was 3.6 μM as determined by quantitative sandwich ELISAs. We evaluated the cellular localization of tau using multiple tau-specific antibodies with epitopes to different regions, including Tau1, Tau5, Tau7, R1, and two novel primate-specific antibodies NT9 and NT15. In the rat and monkey, tau was localized within the somatodendritic and axonal compartments, as well as a subset of neuronal nuclei. Semi-quantitative fluorescence intensity measurements revealed that depending on the specific reagent used the somatodendritic tau is relatively equal to, higher than, or lower than axonal tau, highlighting differential labeling of tau with various antibodies despite its distribution throughout the neuron. Tau was strongly expressed in mature oligodendrocytes and displayed little to no expression in oligodendrocyte precursor cells, astrocytes or microglia. Collectively, the data indicate tau is ∼3 - 7 μM under physiological conditions, is not specifically enriched in axons, and is normally found in both neurons and mature oligodendrocytes in the adult CNS. The full landscape of tau distribution is not revealed by all antibodies suggesting availability of the epitopes is different within specific neuronal compartments. These findings set the stage for better understanding normal tau distributions and interpreting data regarding the presence of tau in different compartments or cell types within disease conditions.

A geroscience approach for Parkinson's disease: Conceptual framework and design of PROPAG-AGEING project

Advanced age is the major risk factor for idiopathic Parkinson's disease (PD), but to date the biological relationship between PD and ageing remains elusive. Here we describe the rationale and the design of the H2020 funded project "PROPAG-AGEING", whose aim is to characterize the contribution of the ageing process to PD development. We summarize current evidences that support the existence of a continuum between ageing and PD and justify the use of a Geroscience approach to study PD. We focus in particular on the role of inflammaging, the chronic, low-grade inflammation characteristic of elderly physiology, which can propagate and transmit both locally and systemically. We then describe PROPAG-AGEING design, which is based on the multi-omic characterization of peripheral samples from clinically characterized drug-naïve and advanced PD, PD discordant twins, healthy controls and "super-controls", i.e. centenarians, who never showed clinical signs of motor disability, and their offspring. Omic results are then validated in a large number of samples, including in vitro models of dopaminergic neurons and healthy siblings of PD patients, who are at higher risk of developing PD, with the final aim of identifying the molecular perturbations that can deviate the trajectories of healthy ageing towards PD development.

Aging and age-related diseases: from mechanisms to therapeutic strategies

Aging is a physiological process mediated by numerous biological and genetic pathways, which are directly linked to lifespan and are a driving force for all age-related diseases. Human life expectancy has greatly increased in the past few decades, but this has not been accompanied by a similar increase in their healthspan. At present, research on aging biology has focused on elucidating the biochemical and genetic pathways that contribute to aging over time. Several aging mechanisms have been identified, primarily including genomic instability, telomere shortening, and cellular senescence. Aging is a driving factor of various age-related diseases, including neurodegenerative diseases, cardiovascular diseases, cancer, immune system disorders, and musculoskeletal disorders. Efforts to find drugs that improve the healthspan by targeting the pathogenesis of aging have now become a hot topic in this field. In the present review, the status of aging research and the development of potential drugs for aging-related diseases, such as metformin, rapamycin, resveratrol, senolytics, as well as caloric restriction, are summarized. The feasibility, side effects, and future potential of these treatments are also discussed, which will provide a basis to develop novel anti-aging therapeutics for improving the healthspan and preventing aging-related diseases.

A role for glia maturation factor dependent activation of mast cells and microglia in MPTP induced dopamine loss and behavioural deficits in mice

The molecular mechanism mediating degeneration of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) is not yet fully understood. Previously, we have shown the contribution of glia maturation factor (GMF), a proinflammatory protein in dopaminergic neurodegeneration mediated by activation of mast cells (MCs). In this study, methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced nigrostriatal neurodegeneration and astro-glial activations were determined by western blot and immunofluorescence techniques in wild type (WT) mice, MC-deficient (MC-KO) mice and GMF-deficient (GMF-KO) mice, with or without MC reconstitution before MPTP administration. We show that GMF-KO in the MCs reduces the synergistic effects of MC and Calpain1 (calcium-activated cysteine protease enzyme)-dependent dopaminergic neuronal loss that reduces motor behavioral impairments in MPTP-treated mouse. Administration of MPTP increase in calpain-mediated proteolysis in nigral dopaminergic neurons further resulting in motor decline in mice. We found that MPTP administered WT mice exhibits oxidative stress due to significant increases in the levels of malondialdehyde, superoxide dismutase and reduction in the levels of reduced glutathione and glutathione peroxidase activity as compared with both MC-KO and GMF-KO mice. The number of TH-positive neurons in the ventral tegmental area, substantia nigra and the fibers in the striatum were significantly reduced while granulocyte macrophage colony-stimulating factor (GM-CSF), MC-Tryptase, GFAP, IBA1, Calpain1 and intracellular adhesion molecule 1 expression were significantly increased in WT mice. Similarly, tyrosine hydroxylase, dopamine transporters and vesicular monoamine transporters 2 proteins expression were significantly reduced in the SN of MPTP treated WT mice. The motor behavior as analyzed by rotarod and hang test was significantly reduced in WT mice as compared with both the MC-KO and GMF-KO mice. We conclude that GMF-dependent MC activation enhances the detrimental effect of astro-glial activation-mediated oxidative stress and neuroinflammation in the midbrain, and its inhibition may slowdown the progression of PD.

The loss of dopaminergic neurons in DEC1 deficient mice potentially involves the decrease of PI3K/Akt/GSK3β signaling

Here we study the effects of differentiated embryonic chondrocyte gene 1(DEC1) deficiency on midbrain dopaminergic(DA) neurons in the substantia nigra pars compacta(SNpc) through behavioral, histological and molecular analysis. We have found that compared to the age-matched WT mice, DEC1 deficient mice show a decrease in locomotor activity and motor coordination, which shows the main features of Parkinson's disease(PD). But there is no significant difference in spatial learning and memory skills between WT and DEC1 KO mice. Compared to the age-matched WT mice, DEC1 deficient mice exhibit the loss of DA neurons in the SNpc and reduction of dopamine and its metabolites in the striatum. The activated caspase-3 and TH/TUNEL⁺ cells increase in the SNpc of 6- and 12-month-old DEC1 KO mice compared to those of the age-matched WT mice. But we haven't found any NeuN/TUNEL⁺ cell increase in the hippocampus of the above two types of mice at the age of 6 months. Furthermore, DEC1 deficiency leads to a significant inhibition of PI3K/Akt/GSK3β signaling pathway. Additionally, LiCl could rescue the DA neuron loss of midbrain in the 6-month-old DEC1 KO mice. Taken together, the loss of DA neurons in the DEC1 deficient mice potentially involves the downregulation of PI3K/Akt/GSK3β signaling.

Intrastriatal alpha-synuclein fibrils in monkeys: spreading, imaging and neuropathological changes

Several studies have demonstrated that intrastriatal injections of fibrillar α-synuclein in rodent brain induced a Parkinson's disease-like propagation of Lewy body pathology with significant nigrostriatal neurodegeneration. This study evaluated the pathological features when exogenous α-synuclein preformed fibrils were injected into the putamen of non-human primates. Eight cynomolgus monkeys received unilateral intraputamen injections of α-synuclein preformed fibrils and four monkeys received sham surgery. Monkeys were assessed with 123I-PE2I single-photon emission computerized tomography scans targeting the dopamine transprter at baseline, 3, 6, 9, 12, and 15 months. Imaging revealed a robust increase in dopamine transporter binding, an effect confirmed by port-mortem immunohistochemical analyses, suggesting that upregulation of dopamine transporter occurs as part of an early pathological process. Histochemistry and immunohistochemistry revealed that α-synuclein preformed fibrils injections into the putamen induced intraneuronal inclusions positive for phosphorylated α-synuclein in ipsilateral substantia nigra and adjacent to the injection site. α-Synuclein inclusions were thioflavin-S-positive suggesting that the inclusions induced by α-synuclein preformed fibrils exhibited pathological properties similar to amyloid-like Lewy body pathology in Parkinson's disease brains. The α-synuclein preformed fibrils resulted in Lewy pathology in the ipsilateral substantia nigra with significant reduction (-29.30%) of dopaminergic neurons as compared with controls. Nigral neurons with α-synuclein inclusions exhibited a phenotypic downregulation of the dopamine markers tyrosine hydroxylase and Nurr1. Taken together, our findings demonstrate that α-synuclein preformed fibrils induce a synucleinopathy in non-human primates with authentic Lewy pathology and nigrostriatal changes indicative of early Parkinson's disease.

The functional microscopic neuroanatomy of the human subthalamic nucleus

The subthalamic nucleus (STN) is successfully used as a surgical target for deep brain stimulation in the treatment of movement disorders. Interestingly, the internal structure of the STN is still incompletely understood. The objective of the present study was to investigate three-dimensional (3D) immunoreactivity patterns for 12 individual protein markers for GABA-ergic, serotonergic, dopaminergic as well as glutamatergic signaling. We analyzed the immunoreactivity using optical densities and created a 3D reconstruction of seven postmortem human STNs. Quantitative modeling of the reconstructed 3D immunoreactivity patterns revealed that the applied protein markers show a gradient distribution in the STN. These gradients were predominantly organized along the ventromedial to dorsolateral axis of the STN. The results are of particular interest in view of the theoretical underpinning for surgical targeting, which is based on a tripartite distribution of cognitive, limbic and motor function in the STN.

The interplay of aging, genetics and environmental factors in the pathogenesis of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is characterized by dopaminergic neuronal loss in the substantia nigra pars compacta and intracellular inclusions called Lewy bodies (LB). During the course of disease, misfolded α-synuclein, the major constituent of LB, spreads to different regions of the brain in a prion-like fashion, giving rise to successive non-motor and motor symptoms. Etiology is likely multifactorial, and involves interplay among aging, genetic susceptibility and environmental factors.

MAIN BODY

The prevalence of PD rises exponentially with age, and aging is associated with impairment of cellular pathways which increases susceptibility of dopaminergic neurons to cell death. However, the majority of those over the age of 80 do not have PD, thus other factors in addition to aging are needed to cause disease. Discovery of neurotoxins which can result in parkinsonism led to efforts in identifying environmental factors which may influence PD risk. Nevertheless, the causality of most environmental factors is not conclusively established, and alternative explanations such as reverse causality and recall bias cannot be excluded. The lack of geographic clusters and conjugal cases also go against environmental toxins as a major cause of PD. Rare mutations as well as common variants in genes such as SNCA, LRRK2 and GBA are associated with risk of PD, but Mendelian causes collectively only account for 5% of PD and common polymorphisms are associated with small increase in PD risk. Heritability of PD has been estimated to be around 30%. Thus, aging, genetics and environmental factors each alone is rarely sufficient to cause PD for most patients.

CONCLUSION

PD is a multifactorial disorder involving interplay of aging, genetics and environmental factors. This has implications on the development of appropriate animal models of PD which take all these factors into account. Common converging pathways likely include mitochondrial dysfunction, impaired autophagy, oxidative stress and neuroinflammation, which are associated with the accumulation and spread of misfolded α-synuclein and neurodegeneration. Understanding the mechanisms involved in the initiation and progression of PD may lead to potential therapeutic targets to prevent PD or modify its course.

An Alternative to Dye-Based Approaches to Remove Background Autofluorescence From Primate Brain Tissue

Brain tissue contains autofluorescing elements that potentially impede accurate identification of neurons when visualized with fluorescent microscopy. Age-related accumulation of molecules with autofluorescent properties, such as lipofuscin, can possess spectral profiles that invade the typical emission range of fluorophores commonly utilized in fluorescent microscopy. The traditional method for accounting for this native fluorescence is to apply lipophilic dyes that are able to sequester these unwanted signals. While effective, such dyes can present a range of problems including the obstruction of fluorescent probe emissions. The present study utilizes aged primate midbrain tissue stained for tyrosine hydroxylase and calbindin to investigate an image processing approach for removing autofluorescence utilizing spectral imaging and linear unmixing. This technique is then compared against the traditional, dye-based autofluorescence sequestration method using Sudan Black B (SBB). Spectral imaging and linear unmixing yielded significantly higher cell numbers than SBB treatment. This finding suggests that computational approaches for removing autofluorescence in neural tissue are both viable and preferential to dye-based approaches for estimation of cell body numbers.

Pretangle pathology within cholinergic nucleus basalis neurons coincides with neurotrophic and neurotransmitter receptor gene dysregulation during the progression of Alzheimer's disease

Cholinergic basal forebrain neurons of the nucleus basalis of Meynert (nbM) regulate attentional and memory function and are exquisitely prone to tau pathology and neurofibrillary tangle (NFT) formation during the progression of Alzheimer's disease (AD). nbM neurons require the neurotrophin nerve growth factor (NGF), its cognate receptor TrkA, and the pan-neurotrophin receptor p75NTR for their maintenance and survival. Additionally, nbM neuronal activity and cholinergic tone are regulated by the expression of nicotinic (nAChR) and muscarinic (mAChR) acetylcholine receptors as well as receptors modulating glutamatergic and catecholaminergic afferent signaling. To date, the molecular and cellular relationships between the evolution of tau pathology and nbM neuronal survival remain unknown. To address this knowledge gap, we profiled cholinotrophic pathway genes within nbM neurons immunostained for pS422, a pretangle phosphorylation event preceding tau C-terminal truncation at D421, or dual-labeled for pS422 and TauC3, a later stage tau neo-epitope revealed by this same C-terminal truncation event, via single-population custom microarray analysis. nbM neurons were obtained from postmortem tissues from subjects who died with an antemortem clinical diagnosis of no cognitive impairment (NCI), mild cognitive impairment (MCI), or mild/moderate AD. Quantitative analysis revealed significant downregulation of mRNAs encoding TrkA as well as TrkB, TrkC, and the Trk-mediated downstream pro-survival kinase Akt in pS422+ compared to unlabeled, pS422-negative nbM neurons. In addition, pS422+ neurons displayed a downregulation of transcripts encoding NMDA receptor subunit 2B, metabotropic glutamate receptor 2, D2 dopamine receptor, and β1 adrenoceptor. By contrast, transcripts encoding p75NTR were downregulated in dual-labeled pS422+/TauC3+ neurons. Appearance of the TauC3 epitope was also associated with an upregulation of the α7 nAChR subunit and differential downregulation of the β2 nAChR subunit. Notably, we found that gene expression patterns for each cell phenotype did not differ with clinical diagnosis. However, linear regression revealed that global cognition and Braak stage were predictors of select transcript changes within both unlabeled and pS422+/TauC3- neurons. Taken together, these cell phenotype-specific gene expression profiling data suggest that dysregulation of neurotrophic and neurotransmitter signaling is an early pathogenic mechanism associated with NFT formation in vulnerable nbM neurons and cognitive decline in AD, which may be amenable to therapeutic intervention early in the disease process.

Expression and localization of CB1R, NAPE-PLD, and FAAH in the vervet monkey nucleus accumbens

Extensive rodent literature suggests that the endocannabinoid (eCB) system present in the nucleus accumbens (NAc) modulates dopamine (DA) release in this area. However, expression patterns of the cannabinoid receptor type 1 (CB1R), the synthesizing enzyme N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD), and the degradation enzyme fatty acid amide hydrolase (FAAH) in the NAc have not yet been described in non-human primates. The goal of this study is therefore to characterize the expression and localization of the eCB system within the NAc of vervet monkeys (Chlorocebus sabaeus) using Western blots and immunohistochemistry. Results show that CB1R, NAPE-PLD, and FAAH are expressed across the NAc rostrocaudal axis, both in the core and shell. CB1R, NAPE-PLD, and FAAH are localized in medium spiny neurons (MSNs) and fast-spiking GABAergic interneurons (FSIs). Dopaminergic projections and astrocytes did not express CB1R, NAPE-PLD, or FAAH. These data show that the eCB system is present in the vervet monkey NAc and supports its role in the primate brain reward circuit.

Tau Oligomer Pathology in Nucleus Basalis Neurons During the Progression of Alzheimer Disease

Although tau is the primary constituent of neurofibrillary tangles (NFTs), evidence suggests that its toxic moiety is oligomeric in Alzheimer disease (AD). In this regard, tau oligomers correlate more strongly with neuronal loss than NFTs and exhibit neurotoxicity in preclinical AD models. Here, we investigated the spatiotemporal progression of oligomeric tau accumulation within the highly vulnerable cholinergic neurons of the nucleus basalis of Meynert (nbM) in AD. Tissue from subjects who died with a clinical diagnosis of no cognitive impairment, mild cognitive impairment, or AD was immunostained with the tau oligomeric complex 1 (TOC1) antibody, a marker of tau oligomers, and p75NTR, a cholinergic cell marker. Stereological estimates revealed a significant increase in the number of TOC1 nbM immunopositive (+) neurons with a concomitant decrease in p75NTR+ nbM neurons during the transition from mild cognitive impairment to AD. Immunofluorescence identified TOC1+ neurons that colocalized with the pretangle tau marker phospho-Ser422, which persisted into late stage NFTs immunoreactive for MN423. Analysis of the nbM subfields revealed a topographical caudal to rostral gradient of TOC1+ neurons during disease progression. Taken together, these data suggest that toxic tau oligomers accumulate caudorostrally in selectively vulnerable nbM neurons during the onset of AD.

George Marinesco in the Constellation of Modern Neuroscience

George Marinesco is the founder of Romanian School of Neurology and one of the most remarkable neuroscientists of the last century. He was the pupil of Jean-Martin Charcot in Salpêtrière Hospital in Paris, France, but visited many other neurological centers where he met the entire constellation of neurologists of his time, including Camillo Golgi and Santiago Ramón y Cajal. The last made the preface of Nervous Cell, written in French by Marinesco. The original title was “La Cellule Nerveuse” and is considered even now a basic reference book for specialists in the field. He was a refined clinical observer with an integrative approach, as could be seen from the multitude of his discoveries. The descriptions of the succulent hand in syringomyelia, senile plaque in old subjects, palmar jaw reflex known as Marinesco-Radovici sign, or the application of cinematography in medicine are some of his important contributions. He was the first who described changes of locus niger in a patient affected by tuberculosis, as a possible cause in Parkinson disease. Before modern genetics, Marinesco and Sjögren described a rare and complex syndrome bearing their names. He was a hardworking man, focused on his scientific research, did not accepted flattering of others and was a great fighter against the injustice of the time.

Marinesco bodies and substantia nigra neuron density in Parkinson's disease

AIM

Marinesco bodies (MB) are intranuclear inclusions in pigmented neurons of the substantia nigra (SN). While rare in children, frequency increases with normal ageing and is high in Alzheimer's disease, dementia with Lewy bodies and other neurodegenerative disorders. Coinciding with the age-related rise in MB frequency is initiation of cell death among SN neurons. Whether MB have a role in this process is unknown. Our aim is to examine the association of MB with SN neuron density in Parkinson's disease (PD) in the Honolulu-Asia Aging Study.

METHODS

Data on MB and neuron density were measured in SN transverse sections in 131 autopsied men aged 73-99 years at the time of death from 1992 to 2007.

RESULTS

Marinesco body frequency was low in the presence vs. absence of PD (2.3% vs. 6.6%, P < 0.001). After PD onset, MB frequency declined as duration of PD increased (P = 0.006). Similar patterns were observed for SN neuron density. When MB frequency was low, neuron density was noticeably reduced in the SN ventrolateral quadrant, the region most vulnerable to PD neurodegeneration. Low MB frequency was unique to PD as its high frequency in non-PD cases was unrelated to parkinsonian signs and incidental Lewy bodies. Frequency was high in the presence of Alzheimer's disease and apolipoprotein ε4 alleles.

CONCLUSIONS

While findings confirm that MB frequency is low in PD, declines in MB frequency continue with PD duration. The extent to which MB have a distinct relationship with PD warrants clarification. Further studies of MB could be important in understanding PD processes.

Aging and Parkinson's disease: Different sides of the same coin?

Despite abundant epidemiological evidence in support of aging as the primary risk factor for PD, biological correlates of a connection have been elusive. In this article, we address the following question: does aging represent biology accurately characterized as pre-PD? We present evidence from our work on midbrain dopamine neurons of aging nonhuman primates that demonstrates that markers of known correlates of dopamine neuron degeneration in PD, including impaired proteasome/lysosome function, oxidative/nitrative damage, and inflammation, all increase with advancing age and are exaggerated in the ventral tier substantia nigra dopamine neurons most vulnerable to degeneration in PD. Our findings support the view that aging-related changes in the dopamine system approach the biological threshold for parkinsonism, actively producing a vulnerable pre-parkinsonian state. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

The effect of manganese exposure in Atp13a2-deficient mice

Loss of function mutations in the P₅-ATPase ATP13A2 are associated with Kufor-Rakeb Syndrome and Neuronal Ceroid Lipofuscinosis. While the function of ATP13A2 is unclear, in vitro studies suggest it is a lysosomal protein that interacts with the metals manganese (Mn) and zinc and the presynaptic protein alpha-synuclein. Loss of ATP13A2 function in mice causes sensorimotor deficits, enhanced autofluorescent storage material, and accumulation of alpha-synuclein. The present study sought to determine the effect of Mn administration on these same outcomes in ATP13A2-deficient mice. Wildtype and ATP13A2-deficient mice received saline or Mn at 5-9 or 12-19 months for 45days. Sensorimotor function was assessed starting at day 30. Autofluorescence was quantified in multiple brain regions and alpha-synuclein protein levels were determined in the ventral midbrain. Brain Mn, iron, zinc, and copper concentrations were measured in 5-9 month old mice. The results show Mn enhanced sensorimotor function, increased autofluorescence in the substantia nigra, and increased insoluble alpha-synuclein in the ventral midbrain in older ATP13A2-deficient mice. In addition, the Mn regimen used increased Mn concentration in the brain and levels were higher in Mn-treated mutants than controls. These results indicate loss of ATP13A2 function leads to increased sensitivity to Mn in vivo.

α-Synuclein nonhuman primate models of Parkinson's disease

Proper understanding of the mechanism(s) by which α-synuclein misfolds and propagates may hold the key to unraveling the complex pathophysiology of Parkinson's disease. A more complete understanding of the disease itself, as well as establishing animal models that fully recapitulate pathological and functional disease progression, are needed to develop treatments that will delay, halt or reverse the disease course. Traditional neurotoxin-based animal models fail to mimic crucial aspects of Parkinson's and thus are not relevant for the study of neuroprotection and disease-modifying therapies. Therefore, a new era of animal models centered on α-synuclein has emerged with the utility of nonhuman primates in these studies beginning to become important. Indeed, disease modeling in nonhuman primates offers a more similar anatomical and genetic background to humans, and the ability to assess complex behavioral impairments that are difficult to test in rodents. Furthermore, results obtained from monkey studies translate better to applications in humans. In this review, we highlight the importance of α-synuclein in Parkinson's disease and discuss the development of α-synuclein based nonhuman primate models.

Pseudophosphorylation of tau at S422 enhances SDS-stable dimer formation and impairs both anterograde and retrograde fast axonal transport

In Alzheimer's disease (AD), tau undergoes numerous modifications, including increased phosphorylation at serine-422 (pS422). In the human brain, pS422 tau protein is found in prodromal AD, correlates well with cognitive decline and neuropil thread pathology, and appears associated with increased oligomer formation and exposure of the N-terminal phosphatase-activating domain (PAD). However, whether S422 phosphorylation contributes to toxic mechanisms associated with disease-related forms of tau remains unknown. Here, we report that S422-pseudophosphorylated tau (S422E) lengthens the nucleation phase of aggregation without altering the extent of aggregation or the types of aggregates formed. When compared to unmodified tau aggregates, the S422E modification significantly increased the amount of SDS-stable tau dimers, despite similar levels of immunoreactivity with an oligomer-selective antibody (TOC1) and another antibody that reports PAD exposure (TNT1). Vesicle motility assays in isolated squid axoplasm further revealed that S422E tau monomers inhibited anterograde, kinesin-1 dependent fast axonal transport (FAT). Unexpectedly, and unlike unmodified tau aggregates, which selectively inhibit anterograde FAT, aggregates composed of S422E tau were found to inhibit both anterograde and retrograde FAT. Highlighting the relevance of these findings to human disease, pS422 tau was found to colocalize with tau oligomers and with a fraction of tau showing increased PAD exposure in the human AD brain. This study identifies novel effects of pS422 on tau biochemical properties, including prolonged nucleation and enhanced dimer formation, which correlate with a distinct inhibitory effect on FAT. Taken together, these findings identify a novel mechanistic basis by which pS422 confers upon tau a toxic effect that may directly contribute to axonal dysfunction in AD and other tauopathies.

Characterization of Early Pathological Tau Conformations and Phosphorylation in Chronic Traumatic Encephalopathy

Chronic traumatic encephalopathy (CTE) is a neurodegenerative tauopathy that develops after repetitive head injury. Several lines of evidence in other tauopathies suggest that tau oligomer formation induces neurotoxicity and that tau oligomer-mediated neurotoxicity involves induction of axonal dysfunction through exposure of an N-terminal motif in tau, the phosphatase-activating domain (PAD). Additionally, phosphorylation at serine 422 in tau occurs early and correlates with cognitive decline in patients with Alzheimer disease (AD). We performed immunohistochemistry and immunofluorescence on fixed brain sections and biochemical analysis of fresh brain extracts to characterize the presence of PAD-exposed tau (TNT1 antibody), tau oligomers (TOC1 antibody), tau phosphorylated at S422 (pS422 antibody), and tau truncated at D421 (TauC3 antibody) in the brains of 9-11 cases with CTE and cases of nondemented aged controls and AD (Braak VI) (n = 6, each). All 3 early tau markers (ie, TNT1, TOC1, and pS422) were present in CTE and displayed extensive colocalization in perivascular tau lesions that are considered diagnostic for CTE. Notably, the TauC3 epitope, which is abundant in AD, was relatively sparse in CTE. Together, these results provide the first description of PAD exposure, TOC1 reactive oligomers, phosphorylation of S422, and TauC3 truncation in the tau pathology of CTE.

Quantitative and semi-quantitative measurements of axonal degeneration in tissue and primary neuron cultures

BACKGROUND

Axon viability is critical for maintaining neural connectivity, which is central to neural functionality. Many neurodegenerative diseases (e.g., Parkinson's disease (PD) and Alzheimer's disease) appear to involve extensive axonal degeneration that often precedes somatic loss in affected neural populations. Axonal degeneration involves a number of intracellular pathways and characteristic changes in axon morphology (i.e., swelling, fragmentation, and loss).

NEW METHOD

We describe a relatively simple set of methods to quantify the axonal degeneration using the 6-hydroxydopamine neurotoxin model of PD in rats and a colchicine-induced model in primary rat neurons. Specifically, approaches are described that use the spaceballs stereological probe for tissue sections and petrimetrics stereological probe for cultured neurons, and image analysis techniques in both tissue sections and cultured neurons.

RESULTS

These methods provide a mechanism for obtaining quantitative and semi-quantitative data to track the extent of axonal degeneration and may prove useful as outcome measures in studies aimed at preventing or slowing axonal degeneration in disease models.

COMPARISON WITH EXISTING METHODS

Existing methods of quantification of axonal degeneration use densitometry and manual counts of axonal projections, but they do not utilize the random, unbiased systematic sampling approaches that are characteristic of stereological methods. The ImageJ thresholding analyses described here provide a descriptive method for quantifying the state of axonal degeneration.

CONCLUSIONS

These methods provide an efficient and effective means to quantify the extent and state of axonal degeneration in animal tissue and cultured neurons and can be used in other models for the same purposes.

Heroin abuse exaggerates age-related deposition of hyperphosphorylated tau and p62-positive inclusions

The observation of increased hyperphosphorylated tau levels correlating with microglial activation in opiate abusers has been interpreted as predisposition to accelerated Alzheimer disease-related changes. The present study focused on evaluating additional neurodegeneration-related proteins, including α-synuclein and TDP-43, and p62-positive deposits. We performed a systematic mapping of protein deposits in the brains of 27 individuals with documented heroin addiction (age: 19-40 years) and compared with 11 controls (age: 15-40 years). We confirm previous findings that heroin addiction associates with tau hyperphosphorylation in predilection brain areas for aging and Alzheimer disease. Furthermore, we show that this occurs also in areas implicated in the molecular disturbances and in vivo neuronal networks related to heroin abuse. There was, however, no presence of amyloid-beta deposits. We extend previous findings by showing the lack of TDP-43 or α-synuclein pathology and emphasize the independent effect of the duration of drug use on the appearance of age-related p62-positive neuritic profiles. These observations provide unique insights about neuropathological alterations in the brains of young heroin addicts and have implications about brain aging and the influences of environmental and toxic factors.

Parkinson's disease as a result of aging

It is generally considered that Parkinson's disease is induced by specific agents that degenerate a clearly defined population of dopaminergic neurons. Data commented in this review suggest that this assumption is not as clear as is often thought and that aging may be critical for Parkinson's disease. Neurons degenerating in Parkinson's disease also degenerate in normal aging, and the different agents involved in the etiology of this illness are also involved in aging. Senescence is a wider phenomenon affecting cells all over the body, whereas Parkinson's disease seems to be restricted to certain brain centers and cell populations. However, reviewed data suggest that Parkinson's disease may be a local expression of aging on cell populations which, by their characteristics (high number of synaptic terminals and mitochondria, unmyelinated axons, etc.), are highly vulnerable to the agents promoting aging. The development of new knowledge about Parkinson's disease could be accelerated if the research on aging and Parkinson's disease were planned together, and the perspective provided by gerontology gains relevance in this field.

Interrogating the aged striatum: robust survival of grafted dopamine neurons in aging rats produces inferior behavioral recovery and evidence of impaired integration

Advanced age is the primary risk factor for Parkinson's disease (PD). In PD patients and rodent models of PD, advanced age is associated with inferior symptomatic benefit following intrastriatal grafting of embryonic dopamine (DA) neurons, a pattern believed to result from decreased survival and reinnervation provided by grafted neurons in the aged host. To help understand the capacity of the aged, parkinsonian striatum to be remodeled with new DA terminals, we used a grafting model and examined whether increasing the number of grafted DA neurons in aged rats would translate to enhanced behavioral recovery. Young (3months), middle-aged (15months), and aged (22months) parkinsonian rats were grafted with proportionately increasing numbers of embryonic ventral mesencephalic (VM) cells to evaluate whether the limitations of the graft environment in subjects of advancing age can be offset by increased numbers of transplanted neurons. Despite robust survival of grafted neurons in aged rats, reinnervation of striatal neurons remained inferior and amelioration of levodopa-induced dyskinesias (LID) was delayed or absent. This study demonstrates that: 1) counter to previous evidence, under certain conditions the aged striatum can support robust survival of grafted DA neurons; and 2) unknown factors associated with the aged striatum result in inferior integration of graft and host, and continue to present obstacles to full therapeutic efficacy of DA cell-based therapy in this model of aging.

Dopaminergic control of autophagic-lysosomal function implicates Lmx1b in Parkinson's disease

The role of developmental transcription factors in maintenance of neuronal properties and in disease remains poorly understood. Lmx1a and Lmx1b are key transcription factors required for the early specification of ventral midbrain dopamine (mDA) neurons. Here we show that conditional ablation of Lmx1a and Lmx1b after mDA neuron specification resulted in abnormalities that show striking resemblance to early cellular abnormalities seen in Parkinson's disease. We found that Lmx1b was required for the normal execution of the autophagic-lysosomal pathway and for the integrity of dopaminergic nerve terminals and long-term mDA neuronal survival. Notably, human LMX1B expression was decreased in mDA neurons in brain tissue affected by Parkinson's disease. Thus, these results reveal a sustained and essential requirement of Lmx1b for the function of midbrain mDA neurons and suggest that its dysfunction is associated with Parkinson's disease pathogenesis.

Molecular determinants of selective dopaminergic vulnerability in Parkinson's disease: an update

Numerous disorders of the central nervous system (CNS) are attributed to the selective death of distinct neuronal cell populations. Interestingly, in many of these conditions, a specific subset of neurons is extremely prone to degeneration while other, very similar neurons are less affected or even spared for many years. In Parkinson’s disease (PD), the motor manifestations are primarily linked to the selective, progressive loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc). In contrast, the very similar DA neurons in the ventral tegmental area (VTA) demonstrate a much lower degree of degeneration. Elucidating the molecular mechanisms underlying the phenomenon of differential DA vulnerability in PD has proven extremely challenging. Moreover, an increasing number of studies demonstrate that considerable molecular and electrophysiologic heterogeneity exists among the DA neurons within the SNpc as well as those within the VTA, adding yet another layer of complexity to the selective DA vulnerability observed in PD. The discovery of key pathways that regulate this differential susceptibility of DA neurons to degeneration holds great potential for the discovery of novel drug targets and the development of promising neuroprotective treatment strategies. This review provides an update on the molecular basis of the differential vulnerability of midbrain DA neurons in PD and highlights the most recent developments in this field.

Calcium signaling in Parkinson's disease

Calcium (Ca(2+)) is an almost universal second messenger that regulates important activities of all eukaryotic cells. It is of critical importance to neurons, which have developed extensive and intricate pathways to couple the Ca(2+) signal to their biochemical machinery. In particular, Ca(2+) participates in the transmission of the depolarizing signal and contributes to synaptic activity. During aging and in neurodegenerative disease processes, the ability of neurons to maintain an adequate energy level can be compromised, thus impacting on Ca(2+) homeostasis. In Parkinson's disease (PD), many signs of neurodegeneration result from compromised mitochondrial function attributable to specific effects of toxins on the mitochondrial respiratory chain and/or to genetic mutations. Despite these effects being present in almost all cell types, a distinguishing feature of PD is the extreme selectivity of cell loss, which is restricted to the dopaminergic neurons in the ventral portion of the substantia nigra pars compacta. Many hypotheses have been proposed to explain such selectivity, but only recently it has been convincingly shown that the innate autonomous activity of these neurons, which is sustained by their specific Cav1.3 L-type channel pore-forming subunit, is responsible for the generation of basal metabolic stress that, under physiological conditions, is compensated by mitochondrial buffering. However, when mitochondria function becomes even partially compromised (because of aging, exposure to environmental factors or genetic mutations), the metabolic stress overwhelms the protective mechanisms, and the process of neurodegeneration is engaged. The characteristics of Ca(2+) handling in neurons of the substantia nigra pars compacta and the possible involvement of PD-related proteins in the control of Ca(2+) homeostasis will be discussed in this review.

Alterations in axonal transport motor proteins in sporadic and experimental Parkinson's disease

The progressive loss of the nigrostriatal pathway is a distinguishing feature of Parkinson's disease. As terminal field loss seems to precede cell body loss, we tested whether alterations of axonal transport motor proteins would be early features in Parkinson's disease. There was a decline in axonal transport motor proteins in sporadic Parkinson's disease that preceded other well-known nigral cell-related pathology such as phenotypic downregulation of dopamine. Reductions in conventional kinesin levels precede the alterations in dopaminergic phenotypic markers (tyrosine hydroxylase) in the early stages of Parkinson's disease. This reduction was significantly greater in nigral neurons containing α-synuclein inclusions. Unlike conventional kinesin, reductions in the levels of the cytoplasmic dynein light chain Tctex type 3 subunit were only observed at late Parkinson's disease stages. Reductions in levels of conventional kinesin and cytoplasmic dynein subunits were recapitulated in a rat genetic Parkinson's disease model based on over-expression of human mutant α-synuclein (A30P). Together, our data suggest that α-synuclein aggregation is a key feature associated with reductions of axonal transport motor proteins in Parkinson's disease and support the hypothesis that dopaminergic neurodegeneration following a 'dying-back' pattern involving axonal transport disruption.

Increased α-synuclein phosphorylation and nitration in the aging primate substantia nigra

Post-translational modifications of α-synuclein occur in the brain of patients affected by Parkinson's disease and other α-synucleinopathies, as indicated by the accumulation of Lewy inclusions containing phosphorylated (at serine 129) and nitrated α-synuclein. Here we found that phospho-Ser 129 and nitrated α-synuclein are also formed within dopaminergic neurons of the monkey substantia nigra as a result of normal aging. Dopaminergic cell bodies immunoreactive for phospho-Ser 129 and nitrated α-synuclein were rarely seen in adult mature animals but became significantly more frequent in the substantia nigra of old primates. Dual labeling with antibodies against phospho-Ser 129 and nitrated α-synuclein revealed only limited colocalization and mostly stained distinct sub-populations of dopaminergic neurons. Age-related elevations of modified protein paralleled an increase in the number of neurons immunoreactive for unmodified α-synuclein, supporting a relationship between higher levels of normal protein and enhanced phosphorylation/nitration. Other mechanisms were also identified that likely contribute to α-synuclein modifications. In particular, increased expression of Polo-like kinase 2 within neurons of older animals could contribute to phospho-Ser 129 α-synuclein production. Data also indicate that a pro-oxidant environment characterizes older neurons and favors α-synuclein nitration. Aging is an unequivocal risk factor for human α-synucleinopathies. These findings are consistent with a mechanistic link between aging, α-synuclein abnormalities and enhanced vulnerability to neurodegenerative processes.

Age-related appearance of dendritic inclusions in catecholaminergic brainstem neurons

We identified p62-immunoreactive inclusions in dendrites of catecholaminergic brainstem projection neurons using antibodies against p62, ubiquitin, α-synuclein, hyperphosphorylated tau, and tyrosine hydroxylase in 100-μm sections through the brainstem dorsal vagal area, locus coeruleus, and substantia nigra of 149 autopsy cases staged for intraneuronal Alzheimer's and Parkinson's disease-associated lesions. The inclusions resembled Marinesco bodies within cell nuclei of catecholaminergic neurons as well as the dot-like structures previously described by Dickson in specific neuropil areas in humans. The p62-positive inclusions were confined to dendrites of catecholaminergic neurons, lacked neuromelanin granules, and were tau- and α-synuclein-negative. Their immunoreactivity for ubiquitin varied and their prevalence significantly increased with advancing age. The presence or absence of Alzheimer's and/or Parkinson's disease-associated pathology did not influence their existence. There was a strong association between the presence of p62-positive inclusions and Marinesco bodies (p < 0.0001). Our results reveal a hitherto unknown alteration within specific neuronal types of the human brainstem that may be independent of the sequestosome-ubiquitin-proteasomal pathway and unrelated to proteinaceous aggregate-formation of neurodegenerative diseases.

Progression of tau pathology in cholinergic Basal forebrain neurons in mild cognitive impairment and Alzheimer's disease

Tau is a microtubule-associated protein that forms neurofibrillary tangles (NFTs) in the selective vulnerable long projection neurons of the cholinergic basal forebrain (CBF) in Alzheimer's disease (AD). Although CBF neurodegeneration correlates with cognitive decline during AD progression, little is known about the temporal changes of tau accumulation in this region. We investigated tau posttranslational modifications during NFT evolution within the CBF neurons of the nucleus basalis (NB) using tissue from subjects with no cognitive impairment, mild cognitive impairment, and AD. The pS422 antibody was used as an early tau pathology marker that labels tau phosphorylated at Ser422; the TauC3 antibody was used to detect later stage tau pathology. Stereologic evaluation of NB tissue immunostained for pS422 and TauC3 revealed an increase in neurons expressing these tau epitopes during disease progression. We also investigated the occurrence of pretangle tau events within cholinergic NB neurons by dual staining for the cholinergic cell marker, p75(NTR), which displays a phenotypic down-regulation within CBF perikarya in AD. As pS422+ neurons increased in number, p75(NTR)+ neurons decreased, and these changes correlated with both AD neuropathology and cognitive decline. Also, NFTs developed slower in the CBF compared with previously examined cortical regions. Taken together, these results suggest that changes in cognition are associated with pretangle events within NB cholinergic neurons before frank NFT deposition.

Pathogenic forms of tau inhibit kinesin-dependent axonal transport through a mechanism involving activation of axonal phosphotransferases

Aggregated filamentous forms of hyperphosphorylated tau (a microtubule-associated protein) represent pathological hallmarks of Alzheimer's disease (AD) and other tauopathies. While axonal transport dysfunction is thought to represent a primary pathogenic factor in AD and other neurodegenerative diseases, the direct molecular link between pathogenic forms of tau and deficits in axonal transport remain unclear. Recently, we demonstrated that filamentous, but not soluble, forms of wild-type tau inhibit anterograde, kinesin-based fast axonal transport (FAT) by activating axonal protein phosphatase 1 (PP1) and glycogen synthase kinase 3 (GSK3), independent of microtubule binding. Here, we demonstrate that amino acids 2-18 of tau, comprising a phosphatase-activating domain (PAD), are necessary and sufficient for activation of this pathway in axoplasms isolated from squid giant axons. Various pathogenic forms of tau displaying increased exposure of PAD inhibited anterograde FAT in squid axoplasm. Importantly, immunohistochemical studies using a novel PAD-specific monoclonal antibody in human postmortem tissue indicated that increased PAD exposure represents an early pathogenic event in AD that closely associates in time with AT8 immunoreactivity, an early marker of pathological tau. We propose a model of pathogenesis in which disease-associated changes in tau conformation lead to increased exposure of PAD, activation of PP1-GSK3, and inhibition of FAT. Results from these studies reveal a novel role for tau in modulating axonal phosphotransferases and provide a molecular basis for a toxic gain-of-function associated with pathogenic forms of tau.

Chronic Akt activation accentuates aging-induced cardiac hypertrophy and myocardial contractile dysfunction: role of autophagy

Aging is often accompanied with geometric and functional changes in the heart, although the underlying mechanisms remain unclear. Recent evidence has described a potential role of Akt and autophagy in aging-associated organ deterioration. This study was to examine the impact of cardiac-specific Akt activation on aging-induced cardiac geometric and functional changes and underlying mechanisms involved. Cardiac geometry, contractile and intracellular Ca(2+) properties were evaluated using echocardiography, edge-detection and fura-2 techniques. Level of insulin signaling and autophagy was evaluated by western blot. Our results revealed cardiac hypertrophy (enlarged chamber size, wall thickness, myocyte cross-sectional area), fibrosis, decreased cardiac contractility, prolonged relengthening along with compromised intracellular Ca(2+) release and clearance in aged (24-26 month-old) mice compared with young (3-4 month-old) mice, the effects of which were accentuated by chronic Akt activation. Aging enhanced Akt and mTOR phosphorylation while reducing that of PTEN, AMPK and ACC with a more pronounced response in Akt transgenic mice. GSK3β phosphorylation and eNOS levels were unaffected by aging or Akt overexpression. Levels of beclin-1, Atg5 and LC3-II-to-LC3-I ratio were decreased in aged hearts, the effect of which with the exception of Atg 5 was exacerbated by Akt overactivation. Levels of p62 were significantly enhanced in aged mice with a more pronounced increase in Akt mice. Neither aging nor Akt altered β-glucuronidase activity and cathepsin B although aging reduced LAMP1 level. In addition, rapamycin reduced aging-induced cardiomyocyte contractile and intracellular Ca(2+) dysfunction while Akt activation suppressed autophagy in young but not aged cardiomyocytes. In conclusion, our data suggest that Akt may accentuate aging-induced cardiac geometric and contractile defects through a loss of autophagic regulation.

Phosphorylation in the amino terminus of tau prevents inhibition of anterograde axonal transport

Alzheimer's disease (AD) and other tauopathies are characterized by fibrillar inclusions composed of the microtubule-associated protein, tau. Recently, we demonstrated that the N-terminus of tau (amino acids [aa] 2-18) in filamentous aggregates or N-terminal tau isoforms activate a signaling cascade involving protein phosphatase 1 and glycogen synthase kinase 3 that results in inhibition of anterograde fast axonal transport (FAT). We have termed the functional motif comprised of aa 2-18 in tau the phosphatase-activating domain (PAD). Here, we show that phosphorylation of tau at tyrosine 18, which is a fyn phosphorylation site within PAD, prevents inhibition of anterograde FAT induced by both filamentous tau and 6D tau. Moreover, Fyn-mediated phosphorylation of tyrosine 18 is reduced in disease-associated forms of tau (e.g., tau filaments). A novel PAD-specific monoclonal antibody revealed that exposure of PAD in tau occurs before and more frequently than tyrosine 18 phosphorylation in the evolution of tangle formation in AD. These results indicate that N-terminal phosphorylation may constitute a regulatory mechanism that controls tau-mediated inhibition of anterograde FAT in AD.

Aged monkeys as a partial model for Parkinson's disease

Parkinson's Disease (PD) and the natural aging process share a number of biochemical mechanisms, including reduced function of dopaminergic systems. The present study aims to determine the extent that motor and behavioral changes in aged monkeys resemble parkinsonism induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The behavioral and physiological changes in PD are believed to result largely from selective depletion of dopamine in the nigrostriatal system. In the present study, ten aged female monkeys were compared with three groups: 9 untreated young adult female monkeys, 10 young adult male monkeys and 13 older male monkeys that had been exposed to MPTP. Trained observers, blind as to age and drug condition and without knowledge of the hypotheses, scored the monkeys using the Parkinson's factor score (Parkscore), which has been validated by a high correlation with post mortem striatal dopamine (DA) concentrations. The aged animals had higher scores on the Parkscore compared with the young adults, with most of its component behavioral items showing significance (tremor, Eating Problems, Delayed initiation of movement, and Poverty of Movement). L-Dopa and DA-agonists did not clearly reverse the principal measure of parkinsonism. DA concentrations post mortem were 63% lower in 3 aged monkeys in the ventral putamen compared with 4 young adults, with greater reductions in putamen than in caudate (45%). We conclude that aged monkeys, unexposed to MPTP, show a similar profile of parkinsonism to that seen after the neurotoxin exposure to MPTP in young adult monkeys. The pattern of greater DA depletion in putamen than in caudate in aged monkeys is the same as in human Parkinson's disease and contrasts with the greater depletion in caudate seen after MPTP. Aged monkeys of this species reflect many facets of Parkinson's disease, but like older humans do not improve with standard dopamine replacement pharmacotherapies.

Ageing as a primary risk factor for Parkinson's disease: evidence from studies of non-human primates

Ageing is the greatest risk factor for the development of Parkinson's disease. However, the current dogma holds that cellular mechanisms that are associated with ageing of midbrain dopamine neurons and those that are related to dopamine neuron degeneration in Parkinson's disease are unrelated. We propose, based on evidence from studies of non-human primates, that normal ageing and the degeneration of dopamine neurons in Parkinson's disease are linked by the same cellular mechanisms and, therefore, that markers of cellular risk factors accumulate with age in a pattern that mimics the pattern of degeneration observed in Parkinson's disease. We contend that ageing induces a pre-parkinsonian state, and that the cellular mechanisms of dopamine neuron demise during normal ageing are accelerated or exaggerated in Parkinson's disease through a combination of genetic and environmental factors.

Characterization of prefibrillar Tau oligomers in vitro and in Alzheimer disease

Neurofibrillary tangles, composed of insoluble aggregates of the microtubule-associated protein Tau, are a pathological hallmark of Alzheimer disease (AD) and other tauopathies. However, recent evidence indicates that neuronal dysfunction precedes the formation of these insoluble fibrillar deposits, suggesting that earlier prefibrillar Tau aggregates may be neurotoxic. To determine the composition of these aggregates, we have employed a photochemical cross-linking technique to examine intermolecular interactions of full-length Tau in vitro. Using this method, we demonstrate that dimerization is an early event in the Tau aggregation process and that these dimers self-associate to form larger oligomeric aggregates. Moreover, using these stabilized Tau aggregates as immunogens, we generated a monoclonal antibody that selectively recognizes Tau dimers and higher order oligomeric aggregates but shows little reactivity to Tau filaments in vitro. Immunostaining indicates that these dimers/oligomers are markedly elevated in AD, appearing in early pathological inclusions such as neuropil threads and pretangle neurons as well as colocalizing with other early markers of Tau pathogenesis. Taken as a whole, the work presented herein demonstrates the existence of alternative Tau aggregates that precede formation of fibrillar Tau pathologies and raises the possibility that these hierarchical oligomeric forms of Tau may contribute to neurodegeneration.

The origins of oxidant stress in Parkinson's disease and therapeutic strategies

Parkinson's disease (PD) is a major world-wide health problem afflicting millions of the aged population. Factors that act on most or all cell types (pan-cellular factors), particularly genetic mutations and environmental toxins, have dominated public discussions of disease etiology. Although there is compelling evidence supporting an association between disease risk and these factors, the pattern of neuronal pathology and cell loss is difficult to explain without cell-specific factors. This article focuses on recent studies showing that the neurons at greatest risk in PD-substantia nigra pars compacta dopamine neurons-have a distinctive physiological phenotype that could contribute to their vulnerability. The opening of L-type calcium channels during autonomous pacemaking results in sustained calcium entry into the cytoplasm of substantia nigra pars compacta dopamine neurons, resulting in elevated mitochondrial oxidant stress and susceptibility to toxins used to create animal models of PD. This cell-specific stress could increase the negative consequences of pan-cellular factors that broadly challenge either mitochondrial or proteostatic competence. The availability of well-tolerated, orally deliverable antagonists for L-type calcium channels points to a novel neuroprotective strategy that could complement current attempts to boost mitochondrial function in the early stages of the disease.