Mitochondrial degeneration in dystrophic neurites of senile plaques may lead to extracellular deposition of fine filaments

Neuritic Plaques - Gateways to Understanding Alzheimer's Disease

Extracellular deposits of amyloid-β (Aβ) in the form of plaques are one of the main pathological hallmarks of Alzheimer's disease (AD). Over the years, many different Aβ plaque morphologies such as neuritic plaques, dense cored plaques, cotton wool plaques, coarse-grain plaques, and diffuse plaques have been described in AD postmortem brain tissues, but correlation of a given plaque type with AD progression or AD symptoms is not clear. Furthermore, the exact trigger causing the development of one Aβ plaque morphological subtype over the other is still unknown. Here, we review the current knowledge about neuritic plaques, a subset of Aβ plaques surrounded by swollen or dystrophic neurites, which represent the most detrimental and consequential Aβ plaque morphology. Neuritic plaques have been associated with local immune activation, neuronal network dysfunction, and cognitive decline. Given that neuritic plaques are at the interface of Aβ deposition, tau aggregation, and local immune activation, we argue that understanding the exact mechanism of neuritic plaque formation is crucial to develop targeted therapies for AD.

Neuronal ageing is promoted by the decay of the microtubule cytoskeleton

Natural ageing is accompanied by a decline in motor, sensory, and cognitive functions, all impacting quality of life. Ageing is also the predominant risk factor for many neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. We need to therefore gain a better understanding of the cellular and physiological processes underlying age-related neuronal decay. However, gaining this understanding is a slow process due to the large amount of time required to age mammalian or vertebrate animal models. Here, we introduce a new cellular model within the Drosophila brain, in which we report classical ageing hallmarks previously observed in the primate brain. These hallmarks include axonal swellings, cytoskeletal decay, a reduction in axonal calibre, and morphological changes arising at synaptic terminals. In the fly brain, these changes begin to occur within a few weeks, ideal to study the underlying mechanisms of ageing. We discovered that the decay of the neuronal microtubule (MT) cytoskeleton precedes the onset of other ageing hallmarks. We showed that the MT-binding factors Tau, EB1, and Shot/MACF1, are necessary for MT maintenance in axons and synapses, and that their functional loss during ageing triggers MT bundle decay, followed by a decline in axons and synaptic terminals. Furthermore, genetic manipulations that improve MT networks slowed down the onset of neuronal ageing hallmarks and confer aged specimens the ability to outperform age-matched controls. Our work suggests that MT networks are a key lesion site in ageing neurons and therefore the MT cytoskeleton offers a promising target to improve neuronal decay in advanced age.

Real-time imaging of mitochondrial redox reveals increased mitochondrial oxidative stress associated with amyloid β aggregates in vivo in a mouse model of Alzheimer's disease

BACKGROUND

Reactive oxidative stress is a critical player in the amyloid beta (Aβ) toxicity that contributes to neurodegeneration in Alzheimer's disease (AD). Damaged mitochondria are one of the main sources of reactive oxygen species and accumulate in Aβ plaque-associated dystrophic neurites in the AD brain. Although Aβ causes neuronal mitochondria reactive oxidative stress in vitro, this has never been directly observed in vivo in the living mouse brain. Here, we tested for the first time whether Aβ plaques and soluble Aβ oligomers induce mitochondrial oxidative stress in surrounding neurons in vivo, and whether this neurotoxic effect can be abrogated using mitochondrial-targeted antioxidants.

METHODS

We expressed a genetically encoded fluorescent ratiometric mitochondria-targeted reporter of oxidative stress in mouse models of the disease and performed intravital multiphoton microscopy of neuronal mitochondria and Aβ plaques.

RESULTS

For the first time, we demonstrated by direct observation in the living mouse brain exacerbated mitochondrial oxidative stress in neurons after both Aβ plaque deposition and direct application of soluble oligomeric Aβ onto the brain, and determined the most likely pathological sequence of events leading to oxidative stress in vivo. Oxidative stress could be inhibited by both blocking calcium influx into mitochondria and treating with the mitochondria-targeted antioxidant SS31. Remarkably, the latter ameliorated plaque-associated dystrophic neurites without impacting Aβ plaque burden.

CONCLUSIONS

Considering these results, combination of mitochondria-targeted compounds with other anti-amyloid beta or anti-tau therapies hold promise as neuroprotective drugs for the prevention and/or treatment of AD.

Transformation of non-neuritic into neuritic plaques during AD progression drives cortical spread of tau pathology via regenerative failure

Extracellular amyloid-β (Aβ) plaques and intracellular aggregates of tau protein in form of neurofibrillary tangles (NFT) are pathological hallmarks of Alzheimer's disease (AD). The exact mechanism how these two protein aggregates interact in AD is still a matter of debate. Neuritic plaques (NP), a subset of Aβ plaques containing dystrophic neurites (DN), are suggested to be unique to AD and might play a role in the interaction of Aβ and tau. Quantifying NP and non-NP in postmortem brain specimens from patients with increasing severity of AD neuropathological changes (ADNC), we demonstrate that the total number of Aβ plaques and NP increase, while the number of non-NP stagnates. Furthermore, investigating the correlation between NP and NFT, we identified unexpected brain region-specific differences when comparing cases with increasingly more severe ADNC. In neocortical regions NFT counts increase in parallel with NP counts during the progression of ADNC, while this correlation is not observed in hippocampus. These data support the notion that non-NP are transformed into NP during the progression of ADNC and indicate that NP might drive cortical NFT formation. Next, using spatial transcriptomics, we analyzed the gene expression profile of the microenvironment around non-NP and NP. We identified an upregulation of neuronal systems and Ca-dependent event pathways around NP compared to non-NP. We speculate that the upregulation of these transcripts may hint at a compensatory mechanism underlying NP formation. Our studies suggest that the transformation of non-NP to NP is a key event in ADNC progression and points to regenerative failure as a potential driving force of this process.

How neurons maintain their axons long-term: an integrated view of axon biology and pathology

Axons are processes of neurons, up to a metre long, that form the essential biological cables wiring nervous systems. They must survive, often far away from their cell bodies and up to a century in humans. This requires self-sufficient cell biology including structural proteins, organelles, and membrane trafficking, metabolic, signalling, translational, chaperone, and degradation machinery-all maintaining the homeostasis of energy, lipids, proteins, and signalling networks including reactive oxygen species and calcium. Axon maintenance also involves specialised cytoskeleton including the cortical actin-spectrin corset, and bundles of microtubules that provide the highways for motor-driven transport of components and organelles for virtually all the above-mentioned processes. Here, we aim to provide a conceptual overview of key aspects of axon biology and physiology, and the homeostatic networks they form. This homeostasis can be derailed, causing axonopathies through processes of ageing, trauma, poisoning, inflammation or genetic mutations. To illustrate which malfunctions of organelles or cell biological processes can lead to axonopathies, we focus on axonopathy-linked subcellular defects caused by genetic mutations. Based on these descriptions and backed up by our comprehensive data mining of genes linked to neural disorders, we describe the 'dependency cycle of local axon homeostasis' as an integrative model to explain why very different causes can trigger very similar axonopathies, providing new ideas that can drive the quest for strategies able to battle these devastating diseases.

Neuropathology of the Common Forms of Dementia

Dementias encompass a range of debilitating neurologic conditions. Here, we summarize the neuropathology of common forms of dementia, focusing on Alzheimer disease (AD) and related dementias. AD is part of a spectrum of neurodegenerative diseases that consists of various protein inclusions (ie, proteinopathies) but other brain abnormalities are also related to dementia. Beta-amyloid and tau aggregates are hallmarks of AD. Other tissue substrates include Lewy bodies, TDP-43 inclusions, vascular brain lesions, and mixed pathologies. This review highlights the complexity of neurodegenerative and other disease substrates and summarizes topography of these lesions and concepts of mixed brain pathologies, resistance, and resilience.

Maintaining essential microtubule bundles in meter-long axons: a role for local tubulin biogenesis?

Axons are the narrow, up-to-meter long cellular processes of neurons that form the biological cables wiring our nervous system. Most axons must survive for an organism's lifetime, i.e. up to a century in humans. Axonal maintenance depends on loose bundles of microtubules that run without interruption all along axons. The continued turn-over and the extension of microtubule bundles during developmental, regenerative or plastic growth requires the availability of α/β-tubulin heterodimers up to a meter away from the cell body. The underlying regulation in axons is poorly understood and hardly features in past and contemporary research. Here we discuss potential mechanisms, particularly focussing on the possibility of local tubulin biogenesis in axons. Current knowledge might suggest that local translation of tubulin takes place in axons, but far less is known about the post-translational machinery of tubulin biogenesis involving three chaperone complexes: prefoldin, CCT and TBC. We discuss functional understanding of these chaperones from a range of model organisms including yeast, plants, flies and mice, and explain what is known from human diseases. Microtubules across species depend on these chaperones, and they are clearly required in the nervous system. However, most chaperones display a high degree of functional pleiotropy, partly through independent functions of individual subunits outside their complexes, thus posing a challenge to experimental studies. Notably, we found hardly any studies that investigate their presence and function particularly in axons, thus highlighting an important gap in our understanding of axon biology and pathology.

Advances in Alzheimer's disease's pharmacological treatment

Alzheimer's disease (AD) is the most common type of dementia in the elderly. Several hypotheses emerged from AD pathophysiological mechanisms. However, no neuronal protective or regenerative drug is available nowadays. Researchers still work in drug development and are finding new molecular targets to treat AD. Therefore, this study aimed to summarize main advances in AD pharmacological therapy. Clinical trials registered in the National Library of Medicine database were selected and analyzed accordingly to molecular targets, therapeutic effects, and safety profile. The most common outcome was the lack of efficacy. Only seven trials concluded that tested drugs were safe and induced any kind of therapeutic improvement. Three works showed therapeutic effects followed by toxicity. In addition to aducanumab recent FDA approval, antibodies against amyloid-β (Aβ) showed no noteworthy results. 5-HT6 antagonists, tau inhibitors and nicotinic agonists' data were discouraging. However, anti-Aβ vaccine, BACE inhibitor and anti-neuroinflammation drugs showed promising results.

Dendritic Spine Density Scales with Microtubule Number in Rat Hippocampal Dendrites

Microtubules deliver essential resources to and from synapses. Three-dimensional reconstructions in rat hippocampus reveal a sampling bias regarding spine density that needs to be controlled for dendrite caliber and resource delivery based on microtubule number. The strength of this relationship varies across dendritic arbors, as illustrated for area CA1 and dentate gyrus. In both regions, proximal dendrites had more microtubules than distal dendrites. For CA1 pyramidal cells, spine density was greater on thicker than thinner dendrites in stratum radiatum, or on the more uniformly thin terminal dendrites in stratum lacunosum moleculare. In contrast, spine density was constant across the cone shaped arbor of tapering dendrites from dentate granule cells. These differences suggest that thicker dendrites supply microtubules to subsequent dendritic branches and local dendritic spines, whereas microtubules in thinner dendrites need only provide resources to local spines. Most microtubules ran parallel to dendrite length and associated with long, presumably stable mitochondria, which occasionally branched into lateral dendritic branches. Short, presumably mobile, mitochondria were tethered to microtubules that bent and appeared to direct them into a thin lateral branch. Prior work showed that dendritic segments with the same number of microtubules had elevated resources in subregions of their dendritic shafts where spine synapses had enlarged, and spine clusters had formed. Thus, additional microtubules were not required for redistribution of resources locally to growing spines or synapses. These results provide new understanding about the potential for microtubules to regulate resource delivery to and from dendritic branches and locally among dendritic spines.

Ataxin-2 is essential for cytoskeletal dynamics and neurodevelopment in Drosophila

Ataxin-2 (Atx2) is a highly conserved RNA binding protein. Atx2 undergoes polyglutamine expansion leading to amyotrophic lateral sclerosis (ALS) or spinocerebellar ataxia type 2 (SCA2). However, the physiological functions of Atx2 in neurons remain unknown. Here, using the powerful genetics of Drosophila, we show that Atx2 is essential for normal neuronal cytoskeletal dynamics and organelle trafficking. Upon neuron-specific Atx2 loss, the microtubule and actin networks were abnormally stabilized and cargo transport was drastically inhibited. Depletion of Atx2 caused multiple morphological defects in the nervous system of third instar larvae. These include reduced brain size, impaired axon development, and decreased dendrite outgrowth. Defects in the nervous system caused loss of the ability to crawl and lethality at the pupal stage. Taken together, these data mark Atx2 as a major regulator of cytoskeletal dynamics and denote Atx2 as an essential gene in neurodevelopment, as well as a neurodegenerative factor.

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Atx2 is a major regulator of the cytoskeleton in neurons

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Atx2 is responsible for maintaining dynamic cytoskeletal networks

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Atx2 depletion in the Drosophila larval CNS severely impairs organelle transport

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Atx2 is necessary for correct neurite outgrowth and CNS development in Drosophila

Mitostasis, Calcium and Free Radicals in Health, Aging and Neurodegeneration

Mitochondria play key roles in ATP supply, calcium homeostasis, redox balance control and apoptosis, which in neurons are fundamental for neurotransmission and to allow synaptic plasticity. Their functional integrity is maintained by mitostasis, a process that involves mitochondrial transport, anchoring, fusion and fission processes regulated by different signaling pathways but mainly by the peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). PGC-1α also favors Ca²⁺ homeostasis, reduces oxidative stress, modulates inflammatory processes and mobilizes mitochondria to where they are needed. To achieve their functions, mitochondria are tightly connected to the endoplasmic reticulum (ER) through specialized structures of the ER termed mitochondria-associated membranes (MAMs), which facilitate the communication between these two organelles mainly to aim Ca²⁺ buffering. Alterations in mitochondrial activity enhance reactive oxygen species (ROS) production, disturbing the physiological metabolism and causing cell damage. Furthermore, cytosolic Ca²⁺ overload results in an increase in mitochondrial Ca²⁺, resulting in mitochondrial dysfunction and the induction of mitochondrial permeability transition pore (mPTP) opening, leading to mitochondrial swelling and cell death through apoptosis as demonstrated in several neuropathologies. In summary, mitochondrial homeostasis is critical to maintain neuronal function; in fact, their regulation aims to improve neuronal viability and to protect against aging and neurodegenerative diseases.

A common theme for axonopathies? The dependency cycle of local axon homeostasis

The number of acquired or inherited conditions leading to axon degeneration (from now on referred to as axonopathies) is vast. To diagnose patients, clinicians use a range of indicators including physiology, morphology, family and patient history, as well as genetics, with the specific location of the lesion within the nervous system being a prominent feature. For the neurobiologist, these criteria are often unsatisfactory, and key questions remain unanswered. For example, does it make sense that different axonopathies affect distinct neuron groups through distinct mechanisms? Would it not be more likely that there are common routes to axon degeneration? In this opinion piece, I shall pose this fundamental question and try to find answers that are hopefully thought-provoking and trigger some conceptual rethinking in the field. I will conclude by describing the 'dependency cycle of axon homeostasis' as a new approach to make sense of the intricate connections of axon biology and physiology, also suggesting that different axonopathies might share common paths to axon degeneration.

Three-dimensional analysis of synaptic organization in the hippocampal CA1 field in Alzheimer's disease

Alzheimer's disease is the most common form of dementia, characterized by a persistent and progressive impairment of cognitive functions. Alzheimer's disease is typically associated with extracellular deposits of amyloid-β peptide and accumulation of abnormally phosphorylated tau protein inside neurons (amyloid-β and neurofibrillary pathologies). It has been proposed that these pathologies cause neuronal degeneration and synaptic alterations, which are thought to constitute the major neurobiological basis of cognitive dysfunction in Alzheimer's disease. The hippocampal formation is especially vulnerable in the early stages of Alzheimer's disease. However, the vast majority of electron microscopy studies have been performed in animal models. In the present study, we performed an extensive 3D study of the neuropil to investigate the synaptic organization in the stratum pyramidale and radiatum in the CA1 field of Alzheimer's disease cases with different stages of the disease, using focused ion beam/scanning electron microscopy (FIB/SEM). In cases with early stages of Alzheimer's disease, the synapse morphology looks normal and we observed no significant differences between control and Alzheimer's disease cases regarding the synaptic density, the ratio of excitatory and inhibitory synapses, or the spatial distribution of synapses. However, differences in the distribution of postsynaptic targets and synaptic shapes were found. Furthermore, a lower proportion of larger excitatory synapses in both strata were found in Alzheimer's disease cases. Individuals in late stages of the disease suffered the most severe synaptic alterations, including a decrease in synaptic density and morphological alterations of the remaining synapses. Since Alzheimer's disease cases show cortical atrophy, our data indicate a reduction in the total number (but not the density) of synapses at early stages of the disease, with this reduction being much more accentuated in subjects with late stages of Alzheimer's disease. The observed synaptic alterations may represent a structural basis for the progressive learning and memory dysfunctions seen in Alzheimer's disease cases.

The Locus Coeruleus- Norepinephrine System in Stress and Arousal: Unraveling Historical, Current, and Future Perspectives

Arousal may be understood on a spectrum, with excessive sleepiness, cognitive dysfunction, and inattention on one side, a wakeful state in the middle, and hypervigilance, panic, and psychosis on the other side. However, historically, the concepts of arousal and stress have been challenging to define as measurable experimental variables. Divergent efforts to study these subjects have given rise to several disciplines, including neurobiology, neuroendocrinology, and cognitive neuroscience. We discuss technological advancements that chronologically led to our current understanding of the arousal system, focusing on the multifaceted nucleus locus coeruleus. We share our contemporary perspective and the hypotheses of others in the context of our current technological capabilities and future developments that will be required to move forward in this area of research.

An overview of glutaminyl cyclase inhibitors for Alzheimer’s disease

A diverse range of N-terminally truncated and modified forms of amyloid-β (Aβ) oligomers have been discovered in Alzheimer’s disease brains, including the pyroglutamate-Aβ (AβpE3). AβpE3 species are shown to be more neurotoxic when compared with the full-length Aβ peptide. Findings visibly suggest that glutaminyl cyclase (QC) catalyzed the generation of cerebral AβpE3, and therapeutic effects are achieved by reducing its activity. In recent years, efforts to effectively develop QC inhibitors have been pursued worldwide. The inhibitory activity of current QC inhibitors is mainly triggered by zinc-binding groups that coordinate Zn2+ ion in the active site and other common features. Herein, we summarized the current state of discovery and evolution of QC inhibitors as a potential Alzheimer’s disease-modifying strategy.

The model of local axon homeostasis - explaining the role and regulation of microtubule bundles in axon maintenance and pathology

Axons are the slender, cable-like, up to meter-long projections of neurons that electrically wire our brains and bodies. In spite of their challenging morphology, they usually need to be maintained for an organism's lifetime. This makes them key lesion sites in pathological processes of ageing, injury and neurodegeneration. The morphology and physiology of axons crucially depends on the parallel bundles of microtubules (MTs), running all along to serve as their structural backbones and highways for life-sustaining cargo transport and organelle dynamics. Understanding how these bundles are formed and then maintained will provide important explanations for axon biology and pathology. Currently, much is known about MTs and the proteins that bind and regulate them, but very little about how these factors functionally integrate to regulate axon biology. As an attempt to bridge between molecular mechanisms and their cellular relevance, we explain here the model of local axon homeostasis, based on our own experiments in Drosophila and published data primarily from vertebrates/mammals as well as C. elegans. The model proposes that (1) the physical forces imposed by motor protein-driven transport and dynamics in the confined axonal space, are a life-sustaining necessity, but pose a strong bias for MT bundles to become disorganised. (2) To counterbalance this risk, MT-binding and -regulating proteins of different classes work together to maintain and protect MT bundles as necessary transport highways. Loss of balance between these two fundamental processes can explain the development of axonopathies, in particular those linking to MT-regulating proteins, motors and transport defects. With this perspective in mind, we hope that more researchers incorporate MTs into their work, thus enhancing our chances of deciphering the complex regulatory networks that underpin axon biology and pathology.

Mathematical Models to Shed Light on Amyloid-Beta and Tau Protein Dependent Pathologies in Alzheimer's Disease

The number of patients suffering from dementia due to Alzheimer's disease (AD) is constantly rising worldwide. This has accordingly resulted in huge burdens on the health systems and involved families. Lack of profound understanding of neural networking in normal brain and their interruption in AD makes the treatment of this neurodegenerative multifaceted disease a challenging issue. In recent years, mathematical and computational methods have paved the way towards a better understanding of the brain functional connectivity. Thus, much attention has been paid to this matter from both basic science researchers and clinicians with an interdisciplinary approach to determine what is not functioning properly in AD patients and how this malfunctioning can be addressed. In this review, a number of AD-related articles and well-studied pathophysiologic topics (e.g., amyloid-beta, neurofibrillary tangles, Ca²⁺ dysregulation, and synaptic plasticity alterations) has been literally surveyed from a computational and systems biology point of view. The neural networks were discussed from biological and mathematical point of views and their alterations in recent findings were further highlighted. Application of the graph theoretical analysis in the brain imaging was reviewed, depicting the relations between brain structure and function, without diving into mathematical details. Moreover, differential rate equations were briefly articulated, emphasizing the potential use of these equations in simplifying complex processes in relevance to pathologies of AD. Comprehensive insights were given into the AD progression from neural networks perspective, which may lead us towards potential strategies for early diagnosis and effective treatment of AD.

Association Between Axonopathy and Amyloid Plaques in the Spinal Cord of the Transgenic Mice of Alzheimer's Disease

Axonopathy manifested by axon swellings might constitute one of the earliest pathological features of Alzheimer's disease. It has been proposed that axonopathy might be associated with the origin of Aβ plaques. However, how axonopathy leads to Aβ plaque pathogenesis remains elusive. Our previous studies have shown that Aβ neuropathology (mainly diffuse plaques) selectively occurred in the regions of corticospinal tract (CST) pathway and its innervated region in the spinal cord of TgCRND8 mice. In this study, we investigated the occurrence and progression of axonopathy and the possible implication in Aβ plaque pathogenesis in the spinal cord of TgCRND8 mice. By anterograde labeling of CST system with a neuroanatomical tracer, we found that dilated corticospinal axons started to appear at 7 months, then exhibited an age-dependent increase. These abnormal structures appear before any plaque deposits are visible in the spinal cord of the mice. Importantly, they colocalized with Aβ plaques in either the white matter or gray matter of the spinal cord at later stages, suggesting that these axonal swellings might represent the initial stages of Aβ plaque formation, and could play a role in Aβ plaque pathogenesis. Furthermore, using ultrastructural analysis we demonstrated that intracellular contents in the axonal dystrophies such as various dense vesicles leaked out into the extracellular matrix under a condition of axon swelling rupture in CST pathways of spinal cord. This provided precise structural evidence that how the Aβ leaks out from the axonal dystrophies into extracellular matrix and how an axonal swelling might serve as a nidus of amyloid plaque formation.

Mitochondrial therapeutic interventions in Alzheimer's disease

Alzheimer's Disease (AD) is one of the most common age-related neurodegenerative diseases in the developed world. Treatment of AD is particularly challenging as the drug must overcome the blood brain barrier (BBB) before it can reach its target. Mitochondria are recognized as one of the most important targets for neurological drugs as the organelle is known to play a critical role in diverse cellular processes such as energy production and apoptosis regulation. Mitochondrial targeting was originally developed to study mitochondrial dysfunction and the organelles interaction with other sub-cellular organelles. The purpose of this review is to provide an overview of mitochondrial dysfunction and its role in late onset AD pathology. We then highlight recent antioxidant and enzymatic treatments used to alleviate mitochondrial dysfunction. Finally, we describe current applications of targeted mitochondrial delivery in the treatment of AD.

Defining the earliest pathological changes of Alzheimer's disease

The prospects for effectively treating well-established dementia, such as Alzheimer's disease (AD), are slim, due to the destruction of key brain pathways that underlie higher cognitive function. There has been a substantial shift in the field towards detecting conditions such as AD in their earliest stages, which would allow preventative or therapeutic approaches to substantially reduce risk and/or slow the progression of disease. AD is characterized by hallmark pathological changes such as extracellular Aβ plaques and intracellular neurofibrillary pathology, which selectively affect specific subclasses of neurons and brain circuits. Current evidence indicates that Aβ plaques begin to form many years before overt dementia, a gradual and progressive pathology which offers a potential target for early intervention. Early Aβ changes in the brain result in localized damage to dendrites, axonal processes and synapses, to which excitatory synapses and the processes of projection neurons are highly vulnerable. Aβ pathology is replicated in a range of transgenic models overexpressing mutant human familial AD genes (e.g. APP and presenilin 1). Studying the development of aberrant regenerative and degenerative changes in neuritic processes associated with Aβ plaques may represent the best opportunity to understand the relationship between the pathological hallmarks of AD and neuronal damage, and to develop early interventions to prevent, slow down or mitigate against Aβ pathology and/or the neuronal alterations that leads to cognitive impairment.

Age and Long-Term Hormone Treatment Effects on the Ultrastructural Morphology of the Median Eminence of Female Rhesus Macaques

The median eminence (ME) of the hypothalamus comprises the hypothalamic nerve terminals, glia (especially tanycytes) and the portal capillary vasculature that transports hypothalamic neurohormones to the anterior pituitary gland. The ultrastructure of the ME is dynamically regulated by hormones and undergoes organizational changes during development and reproductive cycles in adult females, but relatively little is known about the ME during aging, especially in nonhuman primates. Therefore, we used a novel transmission scanning electron microscopy technique to examine the cytoarchitecture of the ME of young and aged female rhesus macaques in a preclinical monkey model of menopausal hormone treatments. Rhesus macaques were ovariectomized and treated for 2 years with vehicle, estradiol (E2), or estradiol + progesterone (E2 + P4). While the overall cytoarchitecture of the ME underwent relatively few changes with age and hormones, changes to some features of neural and glial components near the portal capillaries were observed. Specifically, large neuroterminal size was greater in aged compared to young adult animals, an effect that was mitigated or reversed by E2 alone but not by E2 + P4 treatment. Overall glial size and the density and tissue fraction of the largest subset of glia were greater in aged monkeys, and in some cases reversed by E2 treatment. Mitochondrial size was decreased by E2, but not E2 + P4, only in aged macaques. These results contrast substantially with work in rodents, suggesting that the ME of aging macaques is less vulnerable to age-related disorganization, and that the effects of E2 on monkeys' ME are age specific.

A resource from 3D electron microscopy of hippocampal neuropil for user training and tool development

Resurgent interest in synaptic circuitry and plasticity has emphasized the importance of 3D reconstruction from serial section electron microscopy (3DEM). Three volumes of hippocampal CA1 neuropil from adult rat were imaged at X-Y resolution of ~2 nm on serial sections of ~50-60 nm thickness. These are the first densely reconstructed hippocampal volumes. All axons, dendrites, glia, and synapses were reconstructed in a cube (~10 μm(3)) surrounding a large dendritic spine, a cylinder (~43 μm(3)) surrounding an oblique dendritic segment (3.4 μm long), and a parallelepiped (~178 μm(3)) surrounding an apical dendritic segment (4.9 μm long). The data provide standards for identifying ultrastructural objects in 3DEM, realistic reconstructions for modeling biophysical properties of synaptic transmission, and a test bed for enhancing reconstruction tools. Representative synapses are quantified from varying section planes, and microtubules, polyribosomes, smooth endoplasmic reticulum, and endosomes are identified and reconstructed in a subset of dendrites. The original images, traces, and Reconstruct software and files are freely available and visualized at the Open Connectome Project (Data Citation 1).

Impaired mitochondrial energy metabolism in Alzheimer's disease: Impact on pathogenesis via disturbed epigenetic regulation of chromatin landscape

The amyloid cascade hypothesis for the pathogenesis of Alzheimer's disease (AD) was proposed over twenty years ago. However, the mechanisms of neurodegeneration and synaptic loss have remained elusive delaying the effective drug discovery. Recent studies have revealed that amyloid-β peptides as well as phosphorylated and fragmented tau proteins accumulate within mitochondria. This process triggers mitochondrial fission (fragmentation) and disturbs Krebs cycle function e.g. by inhibiting the activity of 2-oxoglutarate dehydrogenase. Oxidative stress, hypoxia and calcium imbalance also disrupt the function of Krebs cycle in AD brains. Recent studies on epigenetic regulation have revealed that Krebs cycle intermediates control DNA and histone methylation as well as histone acetylation and thus they have fundamental roles in gene expression. DNA demethylases (TET1-3) and histone lysine demethylases (KDM2-7) are included in the family of 2-oxoglutarate-dependent oxygenases (2-OGDO). Interestingly, 2-oxoglutarate is the obligatory substrate of 2-OGDO enzymes, whereas succinate and fumarate are the inhibitors of these enzymes. Moreover, citrate can stimulate histone acetylation via acetyl-CoA production. Epigenetic studies have revealed that AD is associated with changes in DNA methylation and histone acetylation patterns. However, the epigenetic results of different studies are inconsistent but one possibility is that they represent both coordinated adaptive responses and uncontrolled stochastic changes, which provoke pathogenesis in affected neurons. Here, we will review the changes observed in mitochondrial dynamics and Krebs cycle function associated with AD, and then clarify the mechanisms through which mitochondrial metabolites can control the epigenetic landscape of chromatin and induce pathological changes in AD.

Highly energized inhibitory interneurons are a central element for information processing in cortical networks

Gamma oscillations (∼30 to 100 Hz) provide a fundamental mechanism of information processing during sensory perception, motor behavior, and memory formation by coordination of neuronal activity in networks of the hippocampus and neocortex. We review the cellular mechanisms of gamma oscillations about the underlying neuroenergetics, i.e., high oxygen consumption rate and exquisite sensitivity to metabolic stress during hypoxia or poisoning of mitochondrial oxidative phosphorylation. Gamma oscillations emerge from the precise synaptic interactions of excitatory pyramidal cells and inhibitory GABAergic interneurons. In particular, specialized interneurons such as parvalbumin-positive basket cells generate action potentials at high frequency ('fast-spiking') and synchronize the activity of numerous pyramidal cells by rhythmic inhibition ('clockwork'). As prerequisites, fast-spiking interneurons have unique electrophysiological properties and particularly high energy utilization, which is reflected in the ultrastructure by enrichment with mitochondria and cytochrome c oxidase, most likely needed for extensive membrane ion transport and γ-aminobutyric acid metabolism. This supports the hypothesis that highly energized fast-spiking interneurons are a central element for cortical information processing and may be critical for cognitive decline when energy supply becomes limited ('interneuron energy hypothesis'). As a clinical perspective, we discuss the functional consequences of metabolic and oxidative stress in fast-spiking interneurons in aging, ischemia, Alzheimer's disease, and schizophrenia.

Activation of Dopamine D1 Receptors Regulates Dendritic Morphogenesis Through Rac1 and RhoA in Prefrontal Cortex Neurons

Dopamine (DA) is an important regulator of neuronal plasticity in the prefrontal cortex (PFC) and plays a critical role in addiction-related neuroadaptation. The Rho GTPases, including Rac1, RhoA and Cdc42, are key regulators of actin cytoskeleton rearrangement that play important roles in dendritic morphogenesis. The goal of the current study was to use cultures of primary PFC neurons to gain a better understanding of the molecular mechanisms underlying DA-induced dendritic morphogenesis, a phenomenon that mimics the increase in DA synaptic transmission observed in the PFC of in vivo cocaine administration. We investigated the effects of repeated DA treatments on dendritic morphology changes in PFC neurons, and identified Rac1 and RhoA as downstream effectors of D1 receptors during the regulation of dendritic morphogenesis. Importantly, we found that D1 receptor-regulated Rac1 and RhoA have distinct roles in the regulation of dendritic morphogenesis after repeated DA treatments. Our data provide the first evidence that Rac1 and RhoA are effectors of D1 receptor signaling during dendritic morphogenesis and represent new signaling molecules involved in long-lasting neuroadaptation in the PFC.

Beyond counts and shapes: studying pathology of dendritic spines in the context of the surrounding neuropil through serial section electron microscopy

Because dendritic spines are the sites of excitatory synapses, pathological changes in spine morphology should be considered as part of pathological changes in neuronal circuitry in the forms of synaptic connections and connectivity strength. In the past, spine pathology has usually been measured by changes in their number or shape. A more complete understanding of spine pathology requires visualization at the nanometer level to analyze how the changes in number and size affect their presynaptic partners and associated astrocytic processes, as well as organelles and other intracellular structures. Currently, serial section electron microscopy (ssEM) offers the best approach to address this issue because of its ability to image the volume of brain tissue at the nanometer resolution. Renewed interest in ssEM has led to recent technological advances in imaging techniques and improvements in computational tools indispensable for three-dimensional analyses of brain tissue volumes. Here we consider the small but growing literature that has used ssEM analysis to unravel ultrastructural changes in neuropil including dendritic spines. These findings have implications in altered synaptic connectivity and cell biological processes involved in neuropathology, and serve as anatomical substrates for understanding changes in network activity that may underlie clinical symptoms.

The intricate relationship between microtubules and their associated motor proteins during axon growth and maintenance

The hallmarks of neurons are their slender axons which represent the longest cellular processes of animals and which act as the cables that electrically wire the brain, and the brain to the body. Axons extend along reproducible paths during development and regeneration, and they have to be maintained for the lifetime of an organism. Both axon extension and maintenance essentially depend on the microtubule (MT) cytoskeleton. For this, MTs organize into parallel bundles that are established through extension at the leading axon tips within growth cones, and these bundles then form the architectural backbones, as well as the highways for axonal transport essential for supply and intracellular communication. Axon transport over these enormous distances takes days or even weeks and is a substantial logistical challenge. It is performed by kinesins and dynein/dynactin, which are molecular motors that form close functional links to the MTs they walk along. The intricate machinery which regulates MT dynamics, axonal transport and the motors is essential for nervous system development and function, and its investigation has huge potential to bring urgently required progress in understanding the causes of many developmental and degenerative brain disorders. During the last years new explanations for the highly specific properties of axonal MTs and for their close functional links to motor proteins have emerged, and it has become increasingly clear that motors play active roles also in regulating axonal MT networks. Here, I will provide an overview of these new developments.

Reelin immunoreactivity in neuritic varicosities in the human hippocampal formation of non-demented subjects and Alzheimer's disease patients

BACKGROUND

Reelin and its downstream signaling members are important modulators of actin and microtubule cytoskeleton dynamics, a fundamental prerequisite for proper neurodevelopment and adult neuronal functions. Reductions in Reelin levels have been suggested to contribute to Alzheimer's disease (AD) pathophysiology. We have previously reported an age-related reduction in Reelin levels and its accumulation in neuritic varicosities along the olfactory-limbic tracts, which correlated with cognitive impairments in aged mice. Here, we aimed to investigate whether a similar Reelin-associated neuropathology is observed in the aged human hippocampus and whether it correlated with dementia status.

RESULTS

Our immunohistochemical stainings revealed the presence of N- and C-terminus-containing Reelin fragments in corpora amylacea (CAm), aging-associated spherical deposits. The density of these deposits was increased in the molecular layer of the subiculum of AD compared to non-demented individuals. Despite the limitation of a small sample size, our evaluation of several neuronal and glial markers indicates that the presence of Reelin in CAm might be related to aging-associated impairments in neuronal transport leading to accumulation of organelles and protein metabolites in neuritic varicosities, as previously suggested by the findings and discussions in rodents and primates.

CONCLUSIONS

Our results indicate that aging- and disease-associated changes in Reelin levels and proteolytic processing might play a role in the formation of CAm by altering cytoskeletal dynamics. However, its presence may also be an indicator of a degenerative state of neuritic compartments.

Using fly genetics to dissect the cytoskeletal machinery of neurons during axonal growth and maintenance

The extension of long slender axons is a key process of neuronal circuit formation, both during brain development and regeneration. For this, growth cones at the tips of axons are guided towards their correct target cells by signals. Growth cone behaviour downstream of these signals is implemented by their actin and microtubule cytoskeleton. In the first part of this Commentary, we discuss the fundamental roles of the cytoskeleton during axon growth. We present the various classes of actin- and microtubule-binding proteins that regulate the cytoskeleton, and highlight the important gaps in our understanding of how these proteins functionally integrate into the complex machinery that implements growth cone behaviour. Deciphering such machinery requires multidisciplinary approaches, including genetics and the use of simple model organisms. In the second part of this Commentary, we discuss how the application of combinatorial genetics in the versatile genetic model organism Drosophila melanogaster has started to contribute to the understanding of actin and microtubule regulation during axon growth. Using the example of dystonin-linked neuron degeneration, we explain how knowledge acquired by studying axonal growth in flies can also deliver new understanding in other aspects of neuron biology, such as axon maintenance in higher animals and humans.

Decisive role of Reelin signaling during early stages of Alzheimer's disease

Alzheimer's disease (AD) is one of the largest unmet medical concerns of our society. Around 25 million patients worldwide together with their families are still waiting for an effective treatment. We have recently initiated a re-evaluation of our knowledge of the molecular and cellular mechanisms underlying sporadic AD. Based on the existing literature, we have proposed a mechanistic explanation of how the late-onset form of the disease may evolve on the cellular level. Here, we expand this hypothesis by addressing the pathophysiological changes underlying the early and almost invariant appearance of the neurofibrillary tangles, the only reliable correlate of the cognitive status, in distinct brain areas and their consistent "spread" along interconnected neurons as the disease advances. In this review we present and discuss novel evidence that the extracellular signaling protein Reelin, expressed along the olfactory and limbic pathways in the adult brain, might hold a key to understand the earliest steps of the disease, highlighting the olfactory pathway as the brain's Achilles heel involved in the initiation of the pathophysiological characteristic of late-onset AD.

Automated transmission-mode scanning electron microscopy (tSEM) for large volume analysis at nanoscale resolution

Transmission-mode scanning electron microscopy (tSEM) on a field emission SEM platform was developed for efficient and cost-effective imaging of circuit-scale volumes from brain at nanoscale resolution. Image area was maximized while optimizing the resolution and dynamic range necessary for discriminating key subcellular structures, such as small axonal, dendritic and glial processes, synapses, smooth endoplasmic reticulum, vesicles, microtubules, polyribosomes, and endosomes which are critical for neuronal function. Individual image fields from the tSEM system were up to 4,295 µm² (65.54 µm per side) at 2 nm pixel size, contrasting with image fields from a modern transmission electron microscope (TEM) system, which were only 66.59 µm² (8.160 µm per side) at the same pixel size. The tSEM produced outstanding images and had reduced distortion and drift relative to TEM. Automated stage and scan control in tSEM easily provided unattended serial section imaging and montaging. Lens and scan properties on both TEM and SEM platforms revealed no significant nonlinear distortions within a central field of ∼100 µm² and produced near-perfect image registration across serial sections using the computational elastic alignment tool in Fiji/TrakEM2 software, and reliable geometric measurements from RECONSTRUCT™ or Fiji/TrakEM2 software. Axial resolution limits the analysis of small structures contained within a section (∼45 nm). Since this new tSEM is non-destructive, objects within a section can be explored at finer axial resolution in TEM tomography with current methods. Future development of tSEM tomography promises thinner axial resolution producing nearly isotropic voxels and should provide within-section analyses of structures without changing platforms. Brain was the test system given our interest in synaptic connectivity and plasticity; however, the new tSEM system is readily applicable to other biological systems.

Neuropathological alterations in Alzheimer disease

The neuropathological hallmarks of Alzheimer disease (AD) include "positive" lesions such as amyloid plaques and cerebral amyloid angiopathy, neurofibrillary tangles, and glial responses, and "negative" lesions such as neuronal and synaptic loss. Despite their inherently cross-sectional nature, postmortem studies have enabled the staging of the progression of both amyloid and tangle pathologies, and, consequently, the development of diagnostic criteria that are now used worldwide. In addition, clinicopathological correlation studies have been crucial to generate hypotheses about the pathophysiology of the disease, by establishing that there is a continuum between "normal" aging and AD dementia, and that the amyloid plaque build-up occurs primarily before the onset of cognitive deficits, while neurofibrillary tangles, neuron loss, and particularly synaptic loss, parallel the progression of cognitive decline. Importantly, these cross-sectional neuropathological data have been largely validated by longitudinal in vivo studies using modern imaging biomarkers such as amyloid PET and volumetric MRI.

Deciphering the mechanism underlying late-onset Alzheimer disease

Despite tremendous investments in understanding the complex molecular mechanisms underlying Alzheimer disease (AD), recent clinical trials have failed to show efficacy. A potential problem underlying these failures is the assumption that the molecular mechanism mediating the genetically determined form of the disease is identical to the one resulting in late-onset AD. Here, we integrate experimental evidence outside the 'spotlight' of the genetic drivers of amyloid-β (Aβ) generation published during the past two decades, and present a mechanistic explanation for the pathophysiological changes that characterize late-onset AD. We propose that chronic inflammatory conditions cause dysregulation of mechanisms to clear misfolded or damaged neuronal proteins that accumulate with age, and concomitantly lead to tau-associated impairments of axonal integrity and transport. Such changes have several neuropathological consequences: focal accumulation of mitochondria, resulting in metabolic impairments; induction of axonal swelling and leakage, followed by destabilization of synaptic contacts; deposition of amyloid precursor protein in swollen neurites, and generation of aggregation-prone peptides; further tau hyperphosphorylation, ultimately resulting in neurofibrillary tangle formation and neuronal death. The proposed sequence of events provides a link between Aβ and tau-related neuropathology, and underscores the concept that degenerating neurites represent a cause rather than a consequence of Aβ accumulation in late-onset AD.

Primate aging in the mammalian scheme: the puzzle of extreme variation in brain aging

At later ages, humans have high risk of developing Alzheimer disease (AD) which may afflict up to 50% by 90 years. While prosimians and monkeys show more substantial changes, the great apes brains examined show mild neurodegenerative changes. Compared with rodents, primates develop and reproduce slowly and are long lived. The New World primates contain some of the shortest as well as some of the longest-lived monkey species, while the prosimians develop the most rapidly and are the shortest lived. Great apes have the largest brains, slowest development, and longest lives among the primates. All primates share some level of slowly progressive, age-related neurodegenerative changes. However, no species besides humans has yet shown regular drastic neuron loss or cognitive decline approaching clinical grade AD. Several primates accumulate extensive deposits of diffuse amyloid-beta protein (Aβ) but only a prosimian-the gray mouse lemur-regularly develops a tauopathy approaching the neurofibrillary tangles of AD. Compared with monkeys, nonhuman great apes display even milder brain-aging changes, a deeply puzzling observation. The genetic basis for these major species differences in brain aging remains obscure but does not involve the Aβ coding sequence which is identical in nonhuman primates and humans. While chimpanzees merit more study, we note the value of smaller, shorter-lived species such as marmosets and small lemurs for aging studies. A continuing concern for all aging studies employing primates is that relative to laboratory rodents, primate husbandry is in a relatively primitive state, and better husbandry to control infections and obesity is needed for brain aging research.

Beneficial effect of human anti-amyloid-beta active immunization on neurite morphology and tau pathology

Anti-amyloid-beta immunization leads to amyloid clearance in patients with Alzheimer's disease, but the effect of vaccination on amyloid-beta-induced neuronal pathology has not been quantitatively examined. The objectives of this study were to address the effects of anti-amyloid-beta active immunization on neurite trajectories and the pathological hallmarks of Alzheimer's disease in the human hippocampus. Hippocampal sections from five patients with Alzheimer's disease enrolled in the AN1792 Phase 2a trial were compared with those from 13 non-immunized Braak-stage and age-matched patients with Alzheimer's disease, and eight age-matched non-demented controls. Analyses included neurite curvature ratio as a quantitative measure of neuritic abnormalities, amyloid and tau loads, and a quantitative characterization of plaque-associated neuritic dystrophy and astrocytosis. Amyloid load and density of dense-core plaques were decreased in the immunized group compared to non-immunized patients (P < 0.01 and P < 0.001, respectively). The curvature ratio in non-immunized patients with Alzheimer's disease was elevated compared to non-demented controls (P < 0.0001). In immunized patients, however, the curvature ratio was normalized when compared to non-immunized patients (P < 0.0001), and not different from non-demented controls. In the non-immunized patients, neurites close to dense-core plaques (within 50 microm) were more abnormal than those far from plaques (i.e. beyond 50 microm) (P < 0.0001). By contrast, in the immunized group neurites close to and far from the remaining dense-core plaques did not differ, and both were straighter compared to the non-immunized patients (P < 0.0001). Compared to non-immunized patients, dense-core plaques remaining after immunization had similar degree of astrocytosis (P = 0.6060), more embedded dystrophic neurites (P < 0.0001) and were more likely to have mitochondrial accumulation (P < 0.001). In addition, there was a significant decrease in the density of paired helical filament-1-positive neurons in the immunized group as compared to the non-immunized (P < 0.05), but not in the density of Alz50 or thioflavin-S positive tangles, suggesting a modest effect of anti-amyloid-beta immunization on tangle pathology. Clearance of amyloid plaques upon immunization with AN1792 effectively improves a morphological measure of neurite abnormality in the hippocampus. This improvement is not just attributable to the decrease in plaque load, but also occurs within the halo of the remaining dense-core plaques. However, these remaining plaques still retain some of their toxic potential. Anti-amyloid-beta immunization might also ameliorate the hippocampal tau pathology through a decrease in tau phosphorylation. These data agree with preclinical animal studies and further demonstrate that human anti-amyloid-beta immunization does not merely clear amyloid from the Alzheimer's disease brain, but reduces some of the neuronal alterations that characterize Alzheimer's disease.

Laminar and modular organization of prefrontal projections to multiple thalamic nuclei

The prefrontal cortex projects to many thalamic nuclei, in pathways associated with cognition, emotion, and action. We investigated how multiple projection systems to the thalamus are organized in prefrontal cortex after injection of distinct retrograde tracers in the principal mediodorsal (MD), the limbic anterior medial (AM), and the motor-related ventral anterior/ventral lateral (VA/VL) thalamic nuclei in rhesus monkeys. Neurons projecting to these nuclei were organized in interdigitated modules extending vertically within layers VI and V. Projection neurons were also organized in layers. The majority of projection neurons to MD or AM originated in layer VI ( approximately 80%), but a significant proportion ( approximately 20%) originated in layer V. In contrast, prefrontal neurons projecting to VA/VL were equally distributed in layers V and VI. Neurons directed to VA/VL occupied mostly the upper part of layer V, while neurons directed to MD or AM occupied mostly the deep part of layer V. The highest proportions of projection neurons in layer V to each nucleus were found in dorsal and medial prefrontal areas. The laminar organization of prefrontal cortico-thalamic projections differs from sensory systems, where projections originate predominantly or entirely from layer VI. Previous studies indicate that layer V cortico-thalamic neurons innervate through some large terminals thalamic neurons that project widely to superficial cortical layers. The large population of prefrontal projection neurons in layer V may drive thalamic neurons, triggering synchronization by recruiting several cortical areas through widespread thalamo-cortical projections to layer I. These pathways may underlie the synthesis of cognition, emotion and action.

Autophagy in neurite injury and neurodegeneration: in vitro and in vivo models

Recent advances indicate that maintaining a balanced level of autophagy is critically important for neuronal health and function. Pathologic dysregulation of macroautophagy has been implicated in synaptic dysfunction, cellular stress, and neuronal cell death. Autophagosomes and autolysosomes are induced in acute and chronic neurological disorders including stroke, brain trauma, neurotoxin injury, Parkinson's, Alzheimer's, Huntington's, motor neuron, prion, lysosomal storage, and other neurodegenerative diseases. Compared to other cell types, neuronal autophagy research presents particular challenges that may be addressed through still evolving techniques. Neuronal function depends upon maintenance of axons and dendrites (collectively known as neurites) that extend for great distances from the cell body. Both autophagy and mitochondrial content have been implicated in regulation of neurite length and function in physiological (plasticity) and pathological remodeling. Here, we highlight several molecular cell biological and imaging methods to study autophagy and mitophagy in neuritic and somatic compartments of differentiated neuronal cell lines and primary neuron cultures, using protocols developed in toxic and genetic models of parkinsonian neurodegeneration. In addition, mature neurons can be studied using in vivo protocols for modeling ischemic and traumatic injuries. Future challenges include application of automated computer-assisted image analysis to the axodendritic tree of individual neurons and improving methods for measuring neuronal autophagic flux.

Dystrophic neurites of senile plaques in Alzheimer's disease are deficient in cytochrome c oxidase

Double-labeling immunofluorescence and confocal microscopy have been used to learn about the local relationship between amyloid, mitochondria, and cytochrome c oxidase (COX) in dystrophic neurites of senile plaques in the frontal cortex in Alzheimer's disease (AD). Dystrophic neurites surrounding amyloid plaques are filled with mitochondrial porin-immunoreactive structures. In contrast with tangle-bearing and non-tangle-bearing neurons, which express mitochondrial porin and COX subunit 4, porin-immunoreactive neurites of senile plaques lack COX subunit 4. Parallel western blot studies in mitochondria-enriched fractions of the frontal cortex in the same cases disclosed reduced expression levels of COX, but not of prohibitin, in AD stages VB/C of Braak. Co-localization of porin and lysosomal associated protein 1, as revealed by double-labeling immunofluorescence and confocal microscopy, suggests that mitochondria may be engulfed by lysosomes in dystrophic neurites. These findings support a local link between amyloid deposition, abnormal mitochondria and impaired respiratory chain function (resulting from decrease of COX expression) in dystrophic neurites of senile plaques in AD.

Mechanisms of amyloid plaque pathogenesis

The first ultrastructural investigations of Alzheimer's disease noted the prominence of degenerating mitochondria in the dystrophic neurites of amyloid plaques, and speculated that this degeneration might be a major contributor to plaque pathogenesis. However, the fate of these organelles has received scant consideration in the intervening decades. A number of hypotheses for the formation and progression of amyloid plaques have since been suggested, including glial secretion of amyloid, somal and synaptic secretion of amyloid-beta protein from neurons, and endosomal-lysosomal aggregation of amyloid-beta protein in the cell bodies of neurons, but none of these hypotheses fully account for the focal accumulation of amyloid in plaques. In addition to Alzheimer's disease, amyloid plaques occur in a variety of conditions, and these conditions are all accompanied by dystrophic neurites characteristic of disrupted axonal transport. The disruption of axonal transport results in the autophagocytosis of mitochondria without normal lysosomal degradation, and recent evidence from aging, traumatic injury, Alzheimer's disease and transgenic mice models of Alzheimer's disease, suggests that the degeneration of these autophagosomes may lead to amyloid production within dystrophic neurites. The theory of amyloid plaque pathogenesis has thus come full circle, back to the intuitions of the very first researchers in the field.