Amyloid-β may be released from non-junctional varicosities of axons generated from abnormal tau-containing brainstem nuclei in sporadic Alzheimer's disease: a hypothesis

A nonhuman primate model with Alzheimer's disease-like pathology induced by hippocampal overexpression of human tau

BACKGROUND

Alzheimer's disease (AD) is one of the most burdening diseases of the century with no disease-modifying treatment at this time. Nonhuman primates (NHPs) share genetic, anatomical, and physiological similarities with humans, making them ideal model animals for investigating the pathogenesis of AD and potential therapies. However, the use of NHPs in AD research has been hindered by the paucity of AD monkey models due to their long generation time, ethical considerations, and technical challenges in genetically modifying monkeys.

METHODS

Here, we developed an AD-like NHP model by overexpressing human tau in the bilateral hippocampi of adult rhesus macaque monkeys. We evaluated the pathological features of these monkeys with immunostaining, Nissl staining, cerebrospinal fluid (CSF) analysis, magnetic resonance imaging (MRI), positron emission tomography (PET), and behavioural tests.

RESULTS

We demonstrated that after hippocampal overexpression of tau protein, these monkeys displayed multiple pathological features of AD, including 3-repeat (3R)/4-repeat (4R) tau accumulation, tau hyperphosphorylation, tau propagation, neuronal loss, hippocampal atrophy, neuroinflammation, Aβ clearance deficits, blood vessel damage, and cognitive decline. More interestingly, the accumulation of both 3R and 4R tau is specific to NHPs but not found in adult rodents.

CONCLUSIONS

This work establishes a tau-induced AD-like NHP model with many key pathological and behavioural features of AD. In addition, our model may potentially become one of the AD NHP models adopted by researchers worldwide since it can be generated within 2 ~ 3 months through a single injection of AAVs into the monkey brains. Hence, our model NHPs may facilitate mechanistic studies and therapeutic treatments for AD.

Emerging therapeutics agents and recent advances in drug repurposing for Alzheimer's disease

Alzheimer's disease (AD) is a multivariate and diversified disease and affects the most sensitive areas of the brain, the cerebral cortex, and the hippocampus. AD is a progressive age-related neurodegenerative disease most often associated with memory deficits and cognition that get more worsen over time. The central theory on the pathophysiological hallmark features of AD is characterized by the accumulation of amyloid β (Aβ) peptides, also associated with tau proteins (τ) dysfunctioning which leads to distorted microtubular structure, affects the cholinergic system, and mitochondrial biogenesis. This review emphasizes how simple it is to find novel treatments for AD and focuses on several recently developed medications through repurposing that can speed up traditional drug development.

Protective Effects of Polysaccharides in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are characterized by progressive degeneration and necrosis of neurons, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease and others. There are no existing therapies that correct the progression of these diseases, and current therapies provide merely symptomatic relief. The use of polysaccharides has received significant attention due to extensive biological activities and application prospects. Previous studies suggest that the polysaccharides as a candidate participate in neuronal protection and protect against NDs. In this review, we demonstrate that various polysaccharides mediate NDs, and share several common mechanisms characterized by autophagy, apoptosis, neuroinflammation, oxidative stress, mitochondrial dysfunction in PD and AD. Furthermore, this review reveals potential role of polysaccharides in vitro and in vivo models of NDs, and highlights the contributions of polysaccharides and prospects of their mechanism studies for the treatment of NDs. Finally, we suggest some remaining questions for the field and areas for new development.

Comparing the Clinical Utility and Diagnostic Performance of CSF P-Tau181, P-Tau217, and P-Tau231 Assays

Background and Objectives

Phosphorylated tau (p-tau) in CSF is considered an important biomarker in Alzheimer disease (AD) and has been incorporated in recent diagnostic criteria. Several variants exist, including p-tau at threonines 181 (p-tau181), 217 (p-tau217), and 231 (p-tau231). However, no studies have compared their diagnostic performance or association to β-amyloid (Aβ) and tau-PET. Understanding which p-tau variant to use remains an important yet answered question. We aimed to compare the diagnostic accuracy of p-tau181, p-tau217, and p-tau231 in CSF for AD and their association with Aβ and tau-PET.

Methods

A total of 629 participants in the Swedish BioFINDER-2 study were included (cognitively unimpaired, n = 334; Aβ-positive mild cognitive impairment, n = 84; AD dementia, n = 119; and non-AD disorders, n = 92). In addition to p-tau181 and p-tau217 measured using assays with the same detector antibodies from Eli Lilly (p-tau181_Lilly, p-tau217_Lilly) and p-tau231, we also included p-tau181 measurements from 2 commonly used assays (Innotest and Elecsys).

Results

Although all p-tau variants increased across the AD continuum, p-tau217_Lilly showed the greatest dynamic range (13-fold increase vs 1.9–5.4-fold increase for other p-tau variants for AD dementia vs non-AD). P-Tau217_Lilly showed stronger correlations with Aβ- and tau-PET (p < 0.0001). P-Tau217_Lilly exhibited higher accuracy than other p-tau variants for separating AD dementia from non-AD (area under the curve [AUC], 0.98 vs 0.88 [p < 0.0001] - 0.96 [p < 0.05]) and for identifying Aβ-PET (AUC, 0.86 vs 0.74 [p < 0.0001] and 0.83 [p < 0.001]) and tau-PET positivity (AUC, 0.94 vs 0.80—0.92, p < 0.0001). Finally, p-Tau181_Lilly generally performed better than the other p-tau181 assays (e.g., AD dementia vs non-AD, AUC, 0.96 vs 0.88 [p-tau181_Innotest] and 0.89 [p-tau181_Elecsys]; p < 0.0001).

Discussion

CSF p-tau217_Lilly seems to be more useful than other included p-tau assays in the workup of AD. Varied results across p-tau181 assays highlights the importance of anti-tau antibodies for biomarker performance.

Classification of Evidence

This study provides Class II evidence that p-tau217 provides higher diagnostic accuracy for diagnosis of AD dementia than p-tau181 or p-tau231.

Neuropathology of the Brainstem to Mechanistically Understand and to Treat Alzheimer's Disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder as yet without effective therapy. Symptoms of this disorder typically reflect cortical malfunction with local neurohistopathology, which biased investigators to search for focal triggers and molecular mechanisms. Cortex, however, receives massive afferents from caudal brain structures, which do not only convey specific information but powerfully tune ensemble activity. Moreover, there is evidence that the start of AD is subcortical. The brainstem harbors monoamine systems, which establish a dense innervation in both allo- and neocortex. Monoaminergic synapses can co-release neuropeptides either by precisely terminating on cortical neurons or, when being “en passant”, can instigate local volume transmission. Especially due to its early damage, malfunction of the ascending monoaminergic system emerges as an early sign and possible trigger of AD. This review summarizes the involvement and cascaded impairment of brainstem monoaminergic neurons in AD and discusses cellular mechanisms that lead to their dysfunction. We highlight the significance and therapeutic challenges of transmitter co-release in ascending activating system, describe the role and changes of local connections and distant afferents of brainstem nuclei in AD, and summon the rapidly increasing diagnostic window during the last few years.

Alterations and interactions of subcortical modulatory systems in Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is not fully understood. Here we summarize current knowledge on the involvement of the serotonergic, noradrenergic, dopaminergic, cholinergic, and opioid systems in AD, emphasizing the importance of interactions between the serotonergic and the other subcortical modulatory systems during the progression of AD. In physiological conditions, all neurotransmitter systems function in concert and are interdependent at both the neuroanatomical and molecular levels. Through their early involvement in AD, cognitive and behavioral abilities that rely on their interactions also become disrupted. Considering that serotonin (5HT) regulates the release of noradrenaline (NA), dopamine (DA) and acetylcholine (ACh), any alteration in 5HT levels leads to disturbance of NA, DA, and ACh homeostasis in the brain. One of the earliest pathological changes during the prodromal phase of AD is a decrease of serotonergic transmission throughout the brain, with serotonergic receptors being also affected. Additionally, serotonergic and noradrenergic as well as serotonergic and dopaminergic nuclei are reciprocally interconnected. As the serotonergic dorsal raphe nucleus (DRN) is affected by pathological changes early in AD, and the noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental area (VTA) exhibit AD-related pathological changes, their connectivity also becomes altered in AD. Such disrupted interactions among neurotransmitter systems in AD can be used in the development of multi-target drugs. Some of the potential AD therapeutics (such as ASS234, RS67333, tropisetron) target multiple neurotransmitter systems to achieve the best possible improvement of cognitive and behavioral deficits observed in AD. Here, we review how serotonergic system interacts with other subcortical modulatory systems (noradrenergic, dopaminergic, cholinergic, and opioid systems) during AD.

Hypothesis: Tau pathology is an initiating factor in sporadic Alzheimer's disease

The etiology of the common, sporadic form of Alzheimer's disease (sAD) is unknown. We hypothesize that tau pathology within select projection neurons with susceptible microenvironments can initiate sAD. This postulate rests on extensive data demonstrating that in human brains tau pathology appears about a decade before the formation of Aβ plaques (Aβps), especially targeting glutamate projection neurons in the association cortex. Data from aging rhesus monkeys show abnormal tau phosphorylation within vulnerable neurons, associated with calcium dysregulation. Abnormally phosphorylated tau (pTau) on microtubules traps APP‐containing endosomes, which can increase Aβ production. As Aβ oligomers increase abnormal phosphorylation of tau, this would drive vicious cycles leading to sAD pathology over a long lifespan, with genetic and environmental factors that may accelerate pathological events. This hypothesis could be testable in the aged monkey association cortex that naturally expresses characteristics capable of promoting and sustaining abnormal tau phosphorylation and Aβ production.

Human subiculo-fornico-mamillary system in Alzheimer's disease: Tau seeding by the pillar of the fornix

In Alzheimer's disease (AD), Tau and Aβ aggregates involve sequentially connected regions, sometimes distantly separated. These alterations were studied in the pillar of the fornix (PoF), an axonal tract, to analyse the role of axons in their propagation. The PoF axons mainly originate from the subicular neurons and project to the mamillary body. Forty-seven post-mortem cases at various Braak stages (Tau) and Thal phases (Aβ) were analysed by immunohistochemistry. The distribution of the lesions showed that the subiculum was affected before the mamillary body, but neither Tau aggregation nor Aβ deposition was consistently first. The subiculum and the mamillary body contained Gallyas positive neurofibrillary tangles, immunolabelled by AT8, TG3, PHF1, Alz50 and C3 Tau antibodies. In the PoF, only thin and fragmented threads were observed, exclusively in the cases with neurofibrillary tangles in the subiculum. The threads were made of Gallyas negative, AT8 and TG3 positive Tau. They were intra-axonal and devoid of paired helical filaments at electron microscopy. We tested PoF homogenates containing Tau AT8 positive axons in a Tau P301S biosensor HEK cell line and found a seeding activity. There was no Aβ immunoreactivity detected in the PoF. We could follow microcryodissected AT8 positive axons entering the mamillary body; contacts between Tau positive endings and Aβ positive diffuse or focal deposits were observed in CLARITY-cleared mamillary body. In conclusion, we show that non-fibrillary, hyperphosphorylated Tau is transported by the axons of the PoF from the subiculum to the mamillary body and has a seeding activity. Either Tau aggregation or Aβ accumulation may occur first in this system: this inconstant order is incompatible with a cause-and-effects relationship. However, both pathologies were correlated and intimately associated, indicating an interaction of the two processes, once initiated.

Blood-brain barrier and innate immunity in the pathogenesis of Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is only partly understood. This is the probable reason why significant efforts to treat or prevent AD have been unsuccessful. In fact, as of April 2019, there have been 2094 studies registered for AD on the clinicaltrials.gov U.S. National Library of Science web page, of which only a few are still ongoing. In AD, abnormal accumulation of amyloid and tau proteins in the brain are thought to begin 10-20 years before the onset of overt symptoms, suggesting that interventions designed to prevent pathological amyloid and tau accumulation may be more effective than attempting to reverse a pathology once it is established. However, to be successful, such early interventions need to be selectively administered to individuals who will likely develop the disease long before the symptoms occur. Therefore, it is critical to identify early biomarkers that are strongly predictive of AD. Currently, patients are diagnosed on the basis of a variety of clinical scales, neuropsychological tests, imaging and laboratory modalities, but definitive diagnosis can be made only by postmortem assessment of underlying neuropathology. People suffering from AD thus may be misdiagnosed clinically with other primary causes of dementia, and vice versa, thereby also reducing the power of clinical trials. The amyloid cascade hypothesis fits well for the familial cases of AD with known mutations, but is not sufficient to explain sporadic, late-onset AD (LOAD) that accounts for over 95% of all cases. Since the earliest descriptions of AD there have been neuropathological features described other than amyloid plaques (AP) and neurofibrillary tangles (NFT), most notably gliosis and neuroinflammation. However, it is only recently that genetic and experimental studies have implicated microglial dysfunction as a causal factor for AD, as opposed to a merely biological response of its accumulation around AP. Additionally, many studies have suggested the importance of changes in blood-brain barrier (BBB) permeability in the pathogenesis of AD. Here we suggest how these less investigated aspects of the disease that have gained increased attention in recent years may contribute mechanistically to the development of lesions and symptoms of AD.

Longitudinal structural cerebral changes related to core CSF biomarkers in preclinical Alzheimer's disease: A study of two independent datasets

Alzheimer's disease (AD) is characterized by an accumulation of β-amyloid (Aβ₄₂) accompanied by brain atrophy and cognitive decline. Several recent studies have shown that Aβ₄₂ accumulation is associated with gray matter (GM) changes prior to the development of cognitive impairment, in the so-called preclinical stage of the AD (pre-AD). It also has been proved that the GM atrophy profile is not linear, both in normal ageing but, especially, on AD. However, several other factors may influence this association and may have an impact on the generalization of results from different samples. In this work, we estimate differences in rates of GM volume change in cognitively healthy elders in association with baseline core cerebrospinal fluid (CSF) AD biomarkers, and assess to what these differences are sample dependent. We report the dependence of atrophy rates, measured in a two-year interval, on Aβ₄₂, computed both over continuous and categorical values of Aβ₄₂, at voxel-level (p < 0.001; k < 100) and corrected for sex, age and education. Analyses were performed jointly and separately, on two samples. The first sample was formed of 31 individuals (22 Ctrl and 9 pre-AD), aged 60–80 and recruited at the Hospital Clinic of Barcelona. The second sample was a replica of the first one with subjects selected from the ADNI dataset. We also investigated the dependence of the GM atrophy rate on the basal levels of continuous p-tau and on the p-tau/Aβ₄₂ ratio. Correlation analyses on the whole sample showed a dependence of GM atrophy rates on Aβ₄₂ in medial and orbital frontal, precuneus, cingulate, medial temporal regions and cerebellum. Correlations with p-tau were located in the left hippocampus, parahippocampus and striatal nuclei whereas correlation with p-tau/Aβ₄₂ was mainly found in ventral and medial temporal areas. Regarding analyses performed separately, we found a substantial discrepancy of results between samples, illustrating the complexities of comparing two independent datasets even when using the same inclusion criteria. Such discrepancies may lead to significant differences in the sample size needed to detect a particular reduction on cerebral atrophy rates in prevention trials. Higher cognitive reserve and more advanced pathological progression in the ADNI sample could partially account for the observed discrepancies. Taken together, our findings in these two samples highlight the importance of comparing and merging independent datasets to draw more robust and generalizable conclusions on the structural changes in the preclinical stages of AD.

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GM atrophy rates depends differently on values of CSF Aβ₄₂ than on CSF p-tau in the preclinical stage of AD.

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Discrepant results were obtained. Although nominally equivalent, samples might reflect different time-windows in the AD continuum.

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It is necessary a further effort to standardize CSF-biomarkers measures and thresholds to make different samples to be directly comparable.

How the formation of amyloid plaques and neurofibrillary tangles may be related: a mathematical modelling study

We develop a mathematical model that enables us to investigate possible mechanisms by which two primary markers of Alzheimer's disease (AD), extracellular amyloid plaques and intracellular tangles, may be related. Our model investigates the possibility that the decay of anterograde axonal transport of amyloid precursor protein (APP), caused by toxic tau aggregates, leads to decreased APP transport towards the synapse and APP accumulation in the soma. The developed model thus couples three processes: (i) slow axonal transport of tau, (ii) tau misfolding and agglomeration, which we simulated by using the Finke-Watzky model and (iii) fast axonal transport of APP. Because the timescale for tau agglomeration is much larger than that for tau transport, we suggest using the quasi-steady-state approximation for formulating and solving the governing equations for these three processes. Our results suggest that misfolded tau most likely accumulates in the beginning of the axon. The analysis of APP transport suggests that APP will also likely accumulate in the beginning of the axon, causing an increased APP concentration in this region, which could be interpreted as a 'traffic jam'. The APP flux towards the synapse is significantly reduced by tau misfolding, but not due to the APP traffic jam, which can be viewed as a symptom, but rather due to the reduced affinity of kinesin-1 motors to APP-transporting vesicles.

Monoaminergic neuropathology in Alzheimer's disease

None of the proposed mechanisms of Alzheimer's disease (AD) fully explains the distribution patterns of the neuropathological changes at the cellular and regional levels, and their clinical correlates. One aspect of this problem lies in the complex genetic, epigenetic, and environmental landscape of AD: early-onset AD is often familial with autosomal dominant inheritance, while the vast majority of AD cases are late-onset, with the ε4 variant of the gene encoding apolipoprotein E (APOE) known to confer a 5-20 fold increased risk with partial penetrance. Mechanisms by which genetic variants and environmental factors influence the development of AD pathological changes, especially neurofibrillary degeneration, are not yet known. Here we review current knowledge of the involvement of the monoaminergic systems in AD. The changes in the serotonergic, noradrenergic, dopaminergic, histaminergic, and melatonergic systems in AD are briefly described. We also summarize the possibilities for monoamine-based treatment in AD. Besides neuropathologic AD criteria that include the noradrenergic locus coeruleus (LC), special emphasis is given to the serotonergic dorsal raphe nucleus (DRN). Both of these brainstem nuclei are among the first to be affected by tau protein abnormalities in the course of sporadic AD, causing behavioral and cognitive symptoms of variable severity. The possibility that most of the tangle-bearing neurons of the LC and DRN may release amyloid β as well as soluble monomeric or oligomeric tau protein trans-synaptically by their diffuse projections to the cerebral cortex emphasizes their selective vulnerability and warrants further investigations of the monoaminergic systems in AD.

Epigenetic dysregulation of brainstem nuclei in the pathogenesis of Alzheimer's disease: looking in the correct place at the right time?

Even though the etiology of Alzheimer's disease (AD) remains unknown, it is suggested that an interplay among genetic, epigenetic and environmental factors is involved. An increasing body of evidence pinpoints that dysregulation in the epigenetic machinery plays a role in AD. Recent developments in genomic technologies have allowed for high throughput interrogation of the epigenome, and epigenome-wide association studies have already identified unique epigenetic signatures for AD in the cortex. Considerable evidence suggests that early dysregulation in the brainstem, more specifically in the raphe nuclei and the locus coeruleus, accounts for the most incipient, non-cognitive symptomatology, indicating a potential causal relationship with the pathogenesis of AD. Here we review the advancements in epigenomic technologies and their application to the AD research field, particularly with relevance to the brainstem. In this respect, we propose the assessment of epigenetic signatures in the brainstem as the cornerstone of interrogating causality in AD. Understanding how epigenetic dysregulation in the brainstem contributes to AD susceptibility could be of pivotal importance for understanding the etiology of the disease and for the development of novel diagnostic and therapeutic strategies.

Greater specificity for cerebrospinal fluid P-tau231 over P-tau181 in the differentiation of healthy controls from Alzheimer's disease

Cerebrospinal fluid (CSF) measures of phosphorylated-tau (P-tau) 231 and P-tau181 are two biomarkers for the identification of tau pathology as related to Alzheimer's disease (AD). While both are pathologically validated, their relative diagnostic performances are not well known. This cross-sectional diagnostic study of 87 normal (NL) subjects and 28 AD subjects compared CSF P-tau231 with CSF P-tau181. Logistic regression modeling demonstrated that the P-tau231 was superior to the P-tau181 in the diagnostic classifications. At a fixed 85% sensitivity cutoff, the ROC analysis shows that P-tau231 has greater overall specificity than P-tau181. While both P-tau analytes demonstrated equivalent negative predictive accuracies, P-tau231 yielded significantly fewer false positives. Moreover, P-tau231, but not P-tau181, demonstrated sensitivity to the E4 genotype. A postmortem validation with 9 AD subjects confirmed the superiority of the CSF P-tau231 specificity. This study suggests that P-tau231 has the potential to improve the CSF tau biomarker diagnosis of AD.

Pupillary responses and memory-guided visual search reveal age-related and Alzheimer's-related memory decline

Episodic memory - composed of memory for unique spatiotemporal experiences - is known to decline with aging, and even more severely in Alzheimer 's disease (AD). Memory for trial-unique objects in spatial scenes depends on the integrity of the hippocampus and interconnected structures that are among the first areas affected in AD. We reasoned that memory for objects-in-scenes would be impaired with aging, and that further impairments would be observed in AD. We asked younger adults, healthy older adults, older adults at-risk for developing cognitive impairments, and older adults with probable early AD to find changing items ('targets') within images of natural scenes, measuring repeated-trial changes in search efficiency and pupil diameter. Compared to younger adults, older adults took longer to detect target objects in repeated scenes, they required more fixations and those fixations were more dispersed. Whereas individuals with AD showed some benefit of memory in this task, they had substantially longer detection times, and more numerous, dispersed fixations on repeated scenes compared to age-matched older adults. Correspondingly, pupillary responses to novel and repeated scenes were diminished with aging and further in AD, and the memory-related changes were weaker with aging and absent in AD. Our results suggest that several nonverbal measures from memory-guided visual search tasks can index aging and Alzheimer's disease status, including pupillary dynamics. The task measurements are sensitive to the integrity of brain structures that are associated with Alzheimer's-related neurodegeneration, the task is well tolerated across a range of abilities, and thus, it may prove useful in early diagnostics and longitudinal tracking of memory decline.

Molecular Mechanisms in the Pathogenesis of Alzheimer's disease and Tauopathies-Prion-Like Seeded Aggregation and Phosphorylation

Neurofibrillary tau pathology (tangles and threads) and extracellular amyloid-β (Aβ) pathology are defining features of Alzheimer’s disease. For 25 years, most research has focused on the amyloid hypothesis of AD pathogenesis and progression. But, because of failures in clinical trials of Aβ-targeted therapies and the new concept of prion-like propagation of intracellular abnormal proteins, tau has come back into the spotlight as a candidate therapeutic target in AD. Tau pathologies are found in a range of neurodegenerative disorders, but extensive analyses of pathological tau in diseased brains has demonstrated that the abnormal tau protein in each disease is structurally distinct, supporting the idea that progression of the diverse but characteristic tau pathologies occurs through prion-like seed-dependent aggregation. Therefore, intervention in the conversion of normal tau to abnormal forms and in cell-to-cell transmission of tau may be the key to development of disease-modifying therapies for AD and other dementing disorders.

The Locus Coeruleus: Essential for Maintaining Cognitive Function and the Aging Brain

Research on cognitive aging has focused on how decline in various cortical and hippocampal regions influence cognition. However, brainstem regions play essential modulatory roles, and new evidence suggests that, among these, the integrity of the locus coeruleus (LC)-norepinephrine (NE) system plays a key role in determining late-life cognitive abilities. The LC is especially vulnerable to toxins and infection and is often the first place Alzheimer's-related pathology appears, with most people showing at least some tau pathology by their mid-20s. On the other hand, NE released from the LC during arousing, mentally challenging, or novel situations helps to protect neurons from damage, which may help to explain how education and engaging careers prevent cognitive decline in later years.

Propagation of Aß pathology: hypotheses, discoveries, and yet unresolved questions from experimental and human brain studies

In brains of patients with Alzheimer's disease (AD), Aβ peptides accumulate in parenchyma and, almost invariably, also in the vascular walls. Although Aβ aggregation is, by definition, present in AD, its impact is only incompletely understood. It occurs in a stereotypical spatiotemporal distribution within neuronal networks in the course of the disease. This suggests a role for synaptic connections in propagating Aβ pathology, and possibly of axonal transport in an antero- or retrograde way-although, there is also evidence for passive, extracellular diffusion. Striking, in AD, is the conjunction of tau and Aβ pathology. Tau pathology in the cell body of neurons precedes Aβ deposition in their synaptic endings in several circuits such as the entorhino-dentate, cortico-striatal or subiculo-mammillary connections. However, genetic evidence suggests that Aβ accumulation is the first step in AD pathogenesis. To model the complexity and consequences of Aβ aggregation in vivo, various transgenic (tg) rodents have been generated. In rodents tg for the human Aβ precursor protein, focal injections of preformed Aβ aggregates can induce Aβ deposits in the vicinity of the injection site, and over time in more distant regions of the brain. This suggests that Aβ shares with α-synuclein, tau and other proteins the property to misfold and aggregate homotypic molecules. We propose to group those proteins under the term "propagons". Propagons may lack the infectivity of prions. We review findings from neuropathological examinations of human brains in different stages of AD and from studies in rodent models of Aβ aggregation and discuss putative mechanisms underlying the initiation and spread of Aβ pathology.

Effects of ketone bodies in Alzheimer's disease in relation to neural hypometabolism, β-amyloid toxicity, and astrocyte function

Diet supplementation with ketone bodies (acetoacetate and β-hydroxybuturate) or medium-length fatty acids generating ketone bodies has consistently been found to cause modest improvement of mental function in Alzheimer's patients. It was suggested that the therapeutic effect might be more pronounced if treatment was begun at a pre-clinical stage of the disease instead of well after its manifestation. The pre-clinical stage is characterized by decade-long glucose hypometabolism in brain, but ketone body metabolism is intact even initially after disease manifestation. One reason for the impaired glucose metabolism may be early destruction of the noradrenergic brain stem nucleus, locus coeruleus, which stimulates glucose metabolism, at least in astrocytes. These glial cells are essential in Alzheimer pathogenesis. The β-amyloid peptide Aβ interferes with their cholinergic innervation, which impairs synaptic function because of diminished astrocytic glutamate release. Aβ also reduces glucose metabolism and causes hyperexcitability. Ketone bodies are similarly used against seizures, but the effectively used concentrations are so high that they must interfere with glucose metabolism and de novo synthesis of neurotransmitter glutamate, reducing neuronal glutamatergic signaling. The lower ketone body concentrations used in Alzheimer's disease may owe their effect to support of energy metabolism, but might also inhibit release of gliotransmitter glutamate. Alzheimer's disease is a panglial-neuronal disorder with long-standing brain hypometabolism, aberrations in both neuronal and astrocytic glucose metabolism, inflammation, hyperexcitability, and dementia. Relatively low doses of β-hydroxybutyrate can have an ameliorating effect on cognitive function. This could be because of metabolic supplementation or inhibition of Aβ-induced release of glutamate as gliotransmitter, which is likely to reduce hyperexcitability and inflammation. The therapeutic β-hydroxybutyrate doses are too low to reduce neuronally released glutamate.

Models of β-amyloid induced Tau-pathology: the long and "folded" road to understand the mechanism

The amyloid cascade hypothesis has been the prevailing hypothesis in Alzheimer’s Disease research, although the final and most wanted proof i.e. fully successful anti-amyloid clinical trials in patients, is still lacking. This may require a better in depth understanding of the cascade. Particularly, the exact toxic forms of Aβ and Tau, the molecular link between them and their respective contributions to the disease process need to be identified in detail. Although the lack of final proof has raised substantial criticism on the hypothesis per se, accumulating experimental evidence in in vitro models, in vivo models and from biomarkers analysis in patients supports the amyloid cascade and particularly Aβ-induced Tau-pathology, which is the focus of this review. We here discuss available models that recapitulate Aβ-induced Tau-pathology and review some potential underlying mechanisms. The availability and diversity of these models that mimic the amyloid cascade partially or more complete, provide tools to study remaining questions, which are crucial for development of therapeutic strategies for Alzheimer’s Disease.

Can BACE1 inhibition mitigate early axonal pathology in neurological diseases?

β-Secretase-1 (BACE1) is the rate-limiting enzyme for the genesis of amyloid-β (Aβ) peptides, the main constituents of the amyloid plaques in the brains of Alzheimer's disease (AD) patients. BACE1 is being evaluated as an anti-Aβ target for AD therapy. Recent studies indicate that BACE1 elevation is associated with axonal and presynaptic pathology during plaque development. Evidence also points to a biological role for BACE1 in axonal outgrowth and synapse formation during development. Axonal, including presynaptic, pathology exists in AD as well as many other neurological disorders such as Parkinson's disease, epilepsy, stroke, and trauma. In this review, we discuss pharmaceutical BACE1 inhibition as a therapeutic option for axonal pathogenesis, in addition to amyloid pathology. We first introduce the amyloidogenic processing of amyloid-β protein precursor and describe the normal expression pattern of the amyloidogenic proteins in the brain, with an emphasis on BACE1. We then address BACE1 elevation relative to amyloid plaque development, followed by updating recent understanding of a neurotrophic role of BACE1 in axon and synapse development. We further elaborate the occurrence of axonal pathology in some other neurological conditions. Finally, we propose pharmacological inhibition of excessive BACE1 activity as an option to mitigate early axonal pathology occurring in AD and other neurological disorders.