Amyloid precursor protein-induced axonopathies are independent of amyloid-beta peptides

Swedish Alzheimer's disease variant perturbs activity of retrograde molecular motors and causes widespread derangement of axonal transport pathways

Experimental studies in flies, mice, and humans suggest a significant role of impaired axonal transport in the pathogenesis of Alzheimer's disease (AD). The mechanisms underlying these impairments in axonal transport, however, remain poorly understood. Here we report that the Swedish familial AD mutation causes a standstill of the amyloid precursor protein (APP) in the axons at the expense of its reduced anterograde transport. The standstill reflects the perturbed directionality of the axonal transport of APP, which spends significantly more time traveling in the retrograde direction. This ineffective movement is accompanied by an enhanced association of dynactin-1 with APP, which suggests that reduced anterograde transport of APP is the result of enhanced activation of the retrograde molecular motor dynein by dynactin-1. The impact of the Swedish mutation on axonal transport is not limited to the APP vesicles since it also reverses the directionality of a subset of early endosomes, which become enlarged and aberrantly accumulate in distal locations. In addition, it also reduces the trafficking of lysosomes due to their less effective retrograde movement. Altogether, our experiments suggest a pivotal involvement of retrograde molecular motors and transport in the mechanisms underlying impaired axonal transport in AD and reveal significantly more widespread derangement of axonal transport pathways in the pathogenesis of AD.

Exploring the efficient natural products for Alzheimer's disease therapy via Drosophila melanogaster (fruit fly) models

Alzheimer's disease (AD) is a grievous neurodegenerative disorder and a major form of senile dementia, which is partially caused by abnormal amyloid-beta peptide deposition and Tau protein phosphorylation. But until now, the exact pathogenesis of AD and its treatment strategy still need to investigate. Fortunately, natural products have shown potential as therapeutic agents for treating symptoms of AD due to their neuroprotective activity. To identify the excellent lead compounds for AD control from natural products of herbal medicines, as well as, detect their modes of action, suitable animal models are required. Drosophila melanogaster (fruit fly) is an important model for studying genetic and cellular biological pathways in complex biological processes. Various Drosophila AD models were broadly used for AD research, especially for the discovery of neuroprotective natural products. This review focused on the research progress of natural products in AD disease based on the fruit fly AD model, which provides a reference for using the invertebrate model in developing novel anti-AD drugs.

Cuprizone drives divergent neuropathological changes in different mouse models of Alzheimer's disease

Myelin degradation is a normal feature of brain aging that accelerates in Alzheimer's disease (AD). To date, however, the underlying biological basis of this correlation remains elusive. The amyloid cascade hypothesis predicts that demyelination is caused by increased levels of the β-amyloid (Aβ) peptide. Here we report on work supporting the alternative hypothesis that early demyelination is upstream of amyloid. We challenged two different mouse models of AD (R1.40 and APP/PS1) using cuprizone-induced demyelination and tracked the responses with both neuroimaging and neuropathology. In oppose to amyloid cascade hypothesis, R1.40 mice, carrying only a single human mutant APP (Swedish; APP SWE ) transgene, showed a more abnormal changes of magnetization transfer ratio and diffusivity than in APP/PS1 mice, which carry both APP SWE and a second PSEN1 transgene (delta exon 9; PSEN1 dE9 ). Although cuprizone targets oligodendrocytes (OL), magnetic resonance spectroscopy and targeted RNA-seq data in R1.40 mice suggested a possible metabolic alternation in axons. In support of alternative hypotheses, cuprizone induced significant intraneuronal amyloid deposition in young APP/PS1, but not in R1.40 mice, and it suggested the presence of PSEN deficiencies, may accelerate Aβ deposition upon demyelination. In APP/PS1, mature OL is highly vulnerable to cuprizone with significant DNA double strand breaks (53BP1 + ) formation. Despite these major changes in myelin, OLs, and Aβ immunoreactivity, no cognitive impairment or hippocampal pathology was detected in APP/PS1 mice after cuprizone treatment. Together, our data supports the hypothesis that myelin loss can be the cause, but not the consequence, of AD pathology.

SIGNIFICANCE STATEMENT

The causal relationship between early myelin loss and the progression of Alzheimer's disease remains unclear. Using two different AD mouse models, R1.40 and APP/PS1, our study supports the hypothesis that myelin abnormalities are upstream of amyloid production and deposition. We find that acute demyelination initiates intraneuronal amyloid deposition in the frontal cortex. Further, the loss of oligodendrocytes, coupled with the accelerated intraneuronal amyloid deposition, interferes with myelin tract diffusivity at a stage before any hippocampus pathology or cognitive impairments occur. We propose that myelin loss could be the cause, not the consequence, of amyloid pathology during the early stages of Alzheimer's disease.

Glycogen synthase kinase 3β (GSK3β) and presenilin (PS) are key regulators of kinesin-1-mediated cargo motility within axons

It has been a quarter century since the discovery that molecular motors are phosphorylated, but fundamental questions still remain as to how specific kinases contribute to particular motor functions, particularly in vivo, and to what extent these processes have been evolutionarily conserved. Such questions remain largely unanswered because there is no cohesive strategy to unravel the likely complex spatial and temporal mechanisms that control motility in vivo. Since diverse cargoes are transported simultaneously within cells and along narrow long neurons to maintain intracellular processes and cell viability, and disruptions in these processes can lead to cancer and neurodegeneration, there is a critical need to better understand how kinases regulate molecular motors. Here, we review our current understanding of how phosphorylation can control kinesin-1 motility and provide evidence for a novel regulatory mechanism that is governed by a specific kinase, glycogen synthase kinase 3β (GSK3β), and a scaffolding protein presenilin (PS).

Overexpression of alpha synuclein disrupts APP and Endolysosomal axonal trafficking in a mouse model of synucleinopathy

Mutations or triplication of the alpha synuclein (ASYN) gene contribute to synucleinopathies including Parkinson's disease (PD), Dementia with Lewy bodies (DLB) and multiple system atrophy (MSA). Recent evidence suggests that ASYN also plays an important role in amyloid-induced neurotoxicity, although the mechanism(s) remains unknown. One hypothesis is that accumulation of ASYN alters endolysosomal pathways to impact axonal trafficking and processing of the amyloid precursor protein (APP). To define an axonal function for ASYN, we used a transgenic mouse model of synucleinopathy that expresses a GFP-human ASYN (GFP-hASYN) transgene and an ASYN knockout (ASYN-/-) mouse model. Our results demonstrate that expression of GFP-hASYN in primary neurons derived from a transgenic mouse impaired axonal trafficking and processing of APP. In addition, axonal transport of BACE1, Rab5, Rab7, lysosomes and mitochondria were also reduced in these neurons. Interestingly, axonal transport of these organelles was also affected in ASYN-/- neurons, suggesting that ASYN plays an important role in maintaining normal axonal transport function. Therefore, selective impairment of trafficking and processing of APP by ASYN may act as a potential mechanism to induce pathological features of Alzheimer's disease (AD) in PD patients.

Inefficient quality control of ribosome stalling during APP synthesis generates CAT-tailed species that precipitate hallmarks of Alzheimer's disease

Amyloid precursor protein (APP) metabolism is central to Alzheimer's disease (AD) pathogenesis, but the key etiological driver remains elusive. Recent failures of clinical trials targeting amyloid-β (Aβ) peptides, the proteolytic fragments of amyloid precursor protein (APP) that are the main component of amyloid plaques, suggest that the proteostasis-disrupting, key pathogenic species remain to be identified. Previous studies suggest that APP C-terminal fragment (APP.C99) can cause disease in an Aβ-independent manner. The mechanism of APP.C99 pathogenesis is incompletely understood. We used Drosophila models expressing APP.C99 with the native ER-targeting signal of human APP, expressing full-length human APP only, or co-expressing full-length human APP and β-secretase (BACE), to investigate mechanisms of APP.C99 pathogenesis. Key findings are validated in mammalian cell culture models, mouse 5xFAD model, and postmortem AD patient brain materials. We find that ribosomes stall at the ER membrane during co-translational translocation of APP.C99, activating ribosome-associated quality control (RQC) to resolve ribosome collision and stalled translation. Stalled APP.C99 species with C-terminal extensions (CAT-tails) resulting from inadequate RQC are prone to aggregation, causing endolysosomal and autophagy defects and seeding the aggregation of amyloid β peptides, the main component of amyloid plaques. Genetically removing stalled and CAT-tailed APP.C99 rescued proteostasis failure, endolysosomal/autophagy dysfunction, neuromuscular degeneration, and cognitive deficits in AD models. Our finding of RQC factor deposition at the core of amyloid plaques from AD brains further supports the central role of defective RQC of ribosome collision and stalled translation in AD pathogenesis. These findings demonstrate that amyloid plaque formation is the consequence and manifestation of a deeper level proteostasis failure caused by inadequate RQC of translational stalling and the resultant aberrantly modified APP.C99 species, previously unrecognized etiological drivers of AD and newly discovered therapeutic targets.

Widespread Reduced Density of Noradrenergic Locus Coeruleus Axons in the App Knock-In Mouse Model of Amyloid-β Amyloidosis

BACKGROUND

The locus coeruleus (LC), a brainstem nucleus comprising noradrenergic neurons, is one of the earliest regions affected by Alzheimer's disease (AD). Amyloid-β (Aβ) pathology in the cortex in AD is thought to exacerbate the age-related loss of LC neurons, which may lead to cortical tau pathology. However, mechanisms underlying LC neurodegeneration remain elusive.

OBJECTIVE

Here, we aimed to examine how noradrenergic neurons are affected by cortical Aβ pathology in AppNL-G-F/NL-G-F knock-in mice.

METHODS

The density of noradrenergic axons in LC-innervated regions and the LC neuron number were analyzed by an immunohistochemical method. To explore the potential mechanisms for LC degeneration, we also examined the occurrence of tau pathology in LC neurons, the association of reactive gliosis with LC neurons, and impaired trophic support in the brains of AppNL-G-F/NL-G-F mice.

RESULTS

We observed a significant reduction in the density of noradrenergic axons from the LC in aged AppNL-G-F/NL-G-F mice without neuron loss or tau pathology, which was not limited to areas near Aβ plaques. However, none of the factors known to be related to the maintenance of LC neurons (i.e., somatostatin/somatostatin receptor 2, brain-derived neurotrophic factor, nerve growth factor, and neurotrophin-3) were significantly reduced in AppNL-G-F/NL-G-F mice.

CONCLUSION

This study demonstrates that cortical Aβ pathology induces noradrenergic neurodegeneration, and further elucidation of the underlying mechanisms will reveal effective therapeutics to halt AD progression.

GSK3β Impairs KIF1A Transport in a Cellular Model of Alzheimer's Disease but Does Not Regulate Motor Motility at S402

Impairment of axonal transport is an early pathologic event that precedes neurotoxicity in Alzheimer’s disease (AD). Soluble amyloid-β oligomers (AβOs), a causative agent of AD, activate intracellular signaling cascades that trigger phosphorylation of many target proteins, including tau, resulting in microtubule destabilization and transport impairment. Here, we investigated how KIF1A, a kinesin-3 family motor protein required for the transport of neurotrophic factors, is impaired in mouse hippocampal neurons treated with AβOs. By live cell imaging, we observed that AβOs inhibit transport of KIF1A-GFP similarly in wild-type and tau knock-out neurons, indicating that tau is not required for this effect. Pharmacological inhibition of glycogen synthase kinase 3β (GSK3β), a kinase overactivated in AD, prevented the transport defects. By mass spectrometry on KIF1A immunoprecipitated from transgenic AD mouse brain, we detected phosphorylation at S402, which conforms to a highly conserved GSK3β consensus site. We confirmed that this site is phosphorylated by GSK3β in vitro. Finally, we tested whether a phosphomimic of S402 could modulate KIF1A motility in control and AβO-treated mouse neurons and in a Golgi dispersion assay devoid of endogenous KIF1A. In both systems, transport driven by mutant motors was similar to that of WT motors. In conclusion, GSK3β impairs KIF1A transport but does not regulate motor motility at S402. Further studies are required to determine the specific phosphorylation sites on KIF1A that regulate its cargo binding and/or motility in physiological and disease states.

The contribution of glial cells to Huntington's disease pathogenesis

Glial cells play critical roles in the normal development and function of neural circuits, but in many neurodegenerative diseases, they become dysregulated and may contribute to the development of brain pathology. In Huntington's disease (HD), glial cells both lose normal functions and gain neuropathic phenotypes. In addition, cell-autonomous dysfunction elicited by mutant huntingtin (mHTT) expression in specific glial cell types is sufficient to induce both pathology and Huntington's disease-related impairments in motor and cognitive performance, suggesting that these cells may drive the development of certain aspects of Huntington's disease pathogenesis. In support of this imaging studies in pre-symptomatic HD patients and work on mouse models have suggested that glial cell dysfunction occurs at a very early stage of the disease, prior to the onset of motor and cognitive deficits. Furthermore, selectively ablating mHTT from specific glial cells or correcting for HD-induced changes in their transcriptional profile rescues some HD-related phenotypes, demonstrating the potential of targeting these cells for therapeutic intervention. Here we review emerging research focused on understanding the involvement of different glial cell types in specific aspects of HD pathogenesis. This work is providing new insight into how HD impacts biological functions of glial cells in the healthy brain as well as how HD induced dysfunction in these cells might change the way they integrate into biological circuits.

Genetic Dissection of Alzheimer's Disease Using Drosophila Models

Alzheimer’s disease (AD), a main cause of dementia, is the most common neurodegenerative disease that is related to abnormal accumulation of the amyloid β (Aβ) protein. Despite decades of intensive research, the mechanisms underlying AD remain elusive, and the only available treatment remains symptomatic. Molecular understanding of the pathogenesis and progression of AD is necessary to develop disease-modifying treatment. Drosophila, as the most advanced genetic model, has been used to explore the molecular mechanisms of AD in the last few decades. Here, we introduce Drosophila AD models based on human Aβ and summarize the results of their genetic dissection. We also discuss the utility of functional genomics using the Drosophila system in the search for AD-associated molecular mechanisms in the post-genomic era.

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.

The presenilin loop region is essential for glycogen synthase kinase 3 β (GSK3β) mediated functions on motor proteins during axonal transport

Neurons require intracellular transport of essential components for function and viability and defects in transport has been implicated in many neurodegenerative diseases including Alzheimer's disease (AD). One possible mechanism by which transport defects could occur is by improper regulation of molecular motors. Previous work showed that reduction of presenilin (PS) or glycogen synthase kinase 3 beta (GSK3β) stimulated amyloid precursor protein vesicle motility. Excess GSK3β caused transport defects and increased motor binding to membranes, while reduction of PS decreased active GSK3β and motor binding to membranes. Here, we report that functional PS and the catalytic loop region of PS is essential for the rescue of GSK3β-mediated axonal transport defects. Disruption of PS loop (PSΔE9) or expression of the non-functional PS variant, PSD447A, failed to rescue axonal blockages in vivo. Further, active GSK3β associated with and phosphorylated kinesin-1 in vitro. Our observations together with previous work that showed that the loop region of PS interacts with GSK3β propose a scaffolding mechanism for PS in which the loop region sequesters GSK3β away from motors for the proper regulation of motor function. These findings are important to uncouple the complex regulatory mechanisms that likely exist for motor activity during axonal transport in vivo.

Prion protein stabilizes amyloid-β (Aβ) oligomers and enhances Aβ neurotoxicity in a Drosophila model of Alzheimer's disease

The cellular prion protein (PrP^C) can act as a cell-surface receptor for β-amyloid (Aβ) peptide; however, a role for PrP^C in the pathogenesis of Alzheimer's disease (AD) is contested. Here, we expressed a range of Aβ isoforms and PrP^C in the Drosophila brain. We found that co-expression of Aβ and PrP^C significantly reduces the lifespan, disrupts circadian rhythms, and increases Aβ deposition in the fly brain. In contrast, under the same conditions, expression of Aβ or PrP^C individually did not lead to these phenotypic changes. In vitro studies revealed that substoichiometric amounts of PrP^C trap Aβ as oligomeric assemblies and fragment-preformed Aβ fibers. The ability of membrane-anchored PrP^C to trap Aβ as cytotoxic oligomers at the membrane surface and fragment inert Aβ fibers suggests a mechanism by which PrP^C exacerbates Aβ deposition and pathogenic phenotypes in the fly, supporting a role for PrP^C in AD. This study provides a second animal model linking PrP^C expression with Aβ toxicity and supports a role for PrP^C in AD pathogenesis. Blocking the interaction of Aβ and PrP^C represents a potential therapeutic strategy.

A molecular dynamics study of the binary complexes of APP, JIP1, and the cargo binding domain of KLC

Mutations in the amyloid precursor protein (APP) are responsible for the formation of amyloid-β peptides. These peptides play a role in Alzheimer's and other dementia-related diseases. The cargo binding domain of the kinesin-1 light chain motor protein (KLC1) may be responsible for transporting APP either directly or via interaction with C-jun N-terminal kinase-interacting protein 1 (JIP1). However, to date there has been no direct experimental or computational assessment of such binding at the atomistic level. We used molecular dynamics and free energy estimations to gauge the affinity for the binary complexes of KLC1, APP, and JIP1. We find that all binary complexes (KLC1:APP, KLC1:JIP1, and APP:JIP1) contain conformations with favorable binding free energies. For KLC1:APP the inclusion of approximate entropies reduces the favorability. This is likely due to the flexibility of the 42-residue APP protein. In all cases we analyze atomistic/residue driving forces for favorable interactions. Proteins 2017; 85:221-234. © 2016 Wiley Periodicals, Inc.

Hippocampal to basal forebrain transport of Mn2+ is impaired by deletion of KLC1, a subunit of the conventional kinesin microtubule-based motor

Microtubule-based motors carry cargo back and forth between the synaptic region and the cell body. Defects in axonal transport result in peripheral neuropathies, some of which are caused by mutations in KIF5A, a gene encoding one of the heavy chain isoforms of conventional kinesin-1. Some mutations in KIF5A also cause severe central nervous system defects in humans. While transport dynamics in the peripheral nervous system have been well characterized experimentally, transport in the central nervous system is less experimentally accessible and until now not well described. Here we apply manganese-enhanced magnetic resonance (MEMRI) to study transport dynamics within the central nervous system, focusing on the hippocampal-forebrain circuit, and comparing kinesin-1 light chain 1 knock-out (KLC-KO) mice with age-matched wild-type littermates. We injected Mn²⁺ into CA3 of the posterior hippocampus and imaged axonal transport in vivo by capturing whole-brain 3D magnetic resonance images (MRI) in living mice at discrete time-points after injection. Precise placement of the injection site was monitored in both MR images and in histologic sections. Mn²⁺-induced intensity progressed along fiber tracts (fimbria and fornix) in both genotypes to the medial septal nuclei (MSN), correlating in location with the traditional histologic tract tracer, rhodamine dextran. Pairwise statistical parametric mapping (SPM) comparing intensities at successive time-points within genotype revealed Mn²⁺-enhanced MR signal as it proceeded from the injection site into the forebrain, the expected projection from CA3. By region of interest (ROI) analysis of the MSN, wide variation between individuals in each genotype was found. Despite this statistically significant intensity increases in the MSN at 6h post-injection was found in both genotypes, albeit less so in the KLC-KO. While the average accumulation at 6h was less in the KLC-KO, the difference between genotypes did not reach significance. Projections of SPM T-maps for each genotype onto the same grayscale image revealed differences in the anatomical location of significant voxels. Although KLC-KO mice had smaller brains than wild-type, the gross anatomy was normal with no apparent loss of septal cholinergic neurons. Hence anatomy alone does not explain the differences in SPM maps. We conclude that kinesin-1 defects may have only a minor effect on the rate and distribution of transported Mn²⁺ within the living brain. This impairment is less than expected for this abundant microtubule-based motor, yet such defects could still be functionally significant, resulting in cognitive/emotional dysfunction due to decreased replenishments of synaptic vesicles or mitochondria during synaptic activity. This study demonstrates the power of MEMRI to observe and measure vesicular transport dynamics in the central nervous system that may result from or lead to brain pathology.

Defective Transcytosis of APP and Lipoproteins in Human iPSC-Derived Neurons with Familial Alzheimer's Disease Mutations

We investigated early phenotypes caused by familial Alzheimer's disease (fAD) mutations in isogenic human iPSC-derived neurons. Analysis of neurons carrying fAD PS1 or APP mutations introduced using genome editing technology at the endogenous loci revealed that fAD mutant neurons had previously unreported defects in the recycling state of endocytosis and soma-to-axon transcytosis of APP and lipoproteins. The endocytosis reduction could be rescued through treatment with a β-secretase inhibitor. Our data suggest that accumulation of β-CTFs of APP, but not Aβ, slow vesicle formation from an endocytic recycling compartment marked by the transcytotic GTPase Rab11. We confirm previous results that endocytosis is affected in AD and extend these to uncover a neuron-specific defect. Decreased lipoprotein endocytosis and transcytosis to the axon suggest that a neuron-specific impairment in endocytic axonal delivery of lipoproteins and other key materials might compromise synaptic maintenance in fAD.

The Ubiquitin-Proteasome System: Potential Therapeutic Targets for Alzheimer's Disease and Spinal Cord Injury

The ubiquitin-proteasome system (UPS) is a crucial protein degradation system in eukaryotes. Herein, we will review advances in the understanding of the role of several proteins of the UPS in Alzheimer’s disease (AD) and functional recovery after spinal cord injury (SCI). The UPS consists of many factors that include E3 ubiquitin ligases, ubiquitin hydrolases, ubiquitin and ubiquitin-like molecules, and the proteasome itself. An extensive body of work links UPS dysfunction with AD pathogenesis and progression. More recently, the UPS has been shown to have vital roles in recovery of function after SCI. The ubiquitin hydrolase (Uch-L1) has been proposed to increase cellular levels of mono-ubiquitin and hence to increase rates of protein turnover by the UPS. A low Uch-L1 level has been linked with Aβ accumulation in AD and reduced neuroregeneration after SCI. One likely mechanism for these beneficial effects of Uch-L1 is reduced turnover of the PKA regulatory subunit and consequently, reduced signaling via CREB. The neuron-specific F-box protein Fbx2 ubiquitinates β-secretase thus targeting it for proteasomal degradation and reducing generation of Aβ. Both Uch-L1 and Fbx2 improve synaptic plasticity and cognitive function in mouse AD models. The role of Fbx2 after SCI has not been examined, but abolishing ß-secretase reduces neuronal recovery after SCI, associated with reduced myelination. UBB+1, which arises through a frame-shift mutation in the ubiquitin gene that adds 19 amino acids to the C-terminus of ubiquitin, inhibits proteasomal function and is associated with increased neurofibrillary tangles in patients with AD, Pick’s disease and Down’s syndrome. These advances in understanding of the roles of the UPS in AD and SCI raise new questions but, also, identify attractive and exciting targets for potential, future therapeutic interventions.

The calcineurin inhibitor Sarah (Nebula) exacerbates Aβ42 phenotypes in a Drosophila model of Alzheimer's disease

Expression of the Down syndrome critical region 1 (DSCR1) protein, an inhibitor of the Ca²⁺-dependent phosphatase calcineurin, is elevated in the brains of individuals with Down syndrome (DS) or Alzheimer's disease (AD). Although increased levels of DSCR1 were often observed to be deleterious to neuronal health, its beneficial effects against AD neuropathology have also been reported, and the roles of DSCR1 on the pathogenesis of AD remain controversial. Here, we investigated the role of sarah (sra; also known as nebula), a Drosophila DSCR1 ortholog, in amyloid-β42 (Aβ42)-induced neurological phenotypes in Drosophila. We detected sra expression in the mushroom bodies of the fly brain, which are a center for learning and memory in flies. Moreover, similar to humans with AD, Aβ42-expressing flies showed increased Sra levels in the brain, demonstrating that the expression pattern of DSCR1 with regard to AD pathogenesis is conserved in Drosophila. Interestingly, overexpression of sra using the UAS-GAL4 system exacerbated the rough-eye phenotype, decreased survival rates and increased neuronal cell death in Aβ42-expressing flies, without modulating Aβ42 expression. Moreover, neuronal overexpression of sra in combination with Aβ42 dramatically reduced both locomotor activity and the adult lifespan of flies, whereas flies with overexpression of sra alone showed normal climbing ability, albeit with a slightly reduced lifespan. Similarly, treatment with chemical inhibitors of calcineurin, such as FK506 and cyclosporin A, or knockdown of calcineurin expression by RNA interference (RNAi), exacerbated the Aβ42-induced rough-eye phenotype. Furthermore, sra-overexpressing flies displayed significantly decreased mitochondrial DNA and ATP levels, as well as increased susceptibility to oxidative stress compared to that of control flies. Taken together, our results demonstrating that sra overexpression augments Aβ42 cytotoxicity in Drosophila suggest that DSCR1 upregulation or calcineurin downregulation in the brain might exacerbate Aβ42-associated neuropathogenesis in AD or DS.

Drosophila Collection: Chronically increased levels of Sarah (Nebula), a calcineurin inhibitor, cause mitochondria dysfunction and subsequently increased Aβ42-induced cytotoxicity in Drosophila.

Drosophila as an In Vivo Model for Human Neurodegenerative Disease

With the increase in the ageing population, neurodegenerative disease is devastating to families and poses a huge burden on society. The brain and spinal cord are extraordinarily complex: they consist of a highly organized network of neuronal and support cells that communicate in a highly specialized manner. One approach to tackling problems of such complexity is to address the scientific questions in simpler, yet analogous, systems. The fruit fly, Drosophila melanogaster, has been proven tremendously valuable as a model organism, enabling many major discoveries in neuroscientific disease research. The plethora of genetic tools available in Drosophila allows for exquisite targeted manipulation of the genome. Due to its relatively short lifespan, complex questions of brain function can be addressed more rapidly than in other model organisms, such as the mouse. Here we discuss features of the fly as a model for human neurodegenerative disease. There are many distinct fly models for a range of neurodegenerative diseases; we focus on select studies from models of polyglutamine disease and amyotrophic lateral sclerosis that illustrate the type and range of insights that can be gleaned. In discussion of these models, we underscore strengths of the fly in providing understanding into mechanisms and pathways, as a foundation for translational and therapeutic research.

Modeling the complex pathology of Alzheimer's disease in Drosophila

Alzheimer's disease (AD) is the leading cause of dementia and the most common neurodegenerative disorder. AD is mostly a sporadic disorder and its main risk factor is age, but mutations in three genes that promote the accumulation of the amyloid-β (Aβ42) peptide revealed the critical role of amyloid precursor protein (APP) processing in AD. Neurofibrillary tangles enriched in tau are the other pathological hallmark of AD, but the lack of causative tau mutations still puzzles researchers. Here, we describe the contribution of a powerful invertebrate model, the fruit fly Drosophila melanogaster, to uncover the function and pathogenesis of human APP, Aβ42, and tau. APP and tau participate in many complex cellular processes, although their main function is microtubule stabilization and the to-and-fro transport of axonal vesicles. Additionally, expression of secreted Aβ42 induces prominent neuronal death in Drosophila, a critical feature of AD, making this model a popular choice for identifying intrinsic and extrinsic factors mediating Aβ42 neurotoxicity. Overall, Drosophila has made significant contributions to better understand the complex pathology of AD, although additional insight can be expected from combining multiple transgenes, performing genome-wide loss-of-function screens, and testing anti-tau therapies alone or in combination with Aβ42.

Differential roles of Aβ processing in hypoxia-induced axonal damage

Axonopathy is a common and early phase in neurodegenerative and traumatic CNS diseases. Recent work suggests that amyloid β (Aβ) produced from amyloid precursor protein (APP) may be a critical downstream mediator of CNS axonopathy in CNS diseases, particularly those associated with hypoxia. We critically tested this hypothesis in an adult retinal explant system that preserves the three-dimensional organization of the retina while permitting direct imaging of two cardinal features of early-stage axonopathy: axonal structural integrity and axonal transport capacity. Using this system, we found via pharmacological inhibition and genetic deletion of APP that production of Aβ is a necessary step in structural compromise of retinal ganglion cell (RGC) axons induced by the disease-relevant stressor hypoxia. However, identical blockade of Aβ production was not sufficient to protect axons from associated hypoxia-induced reduction in axonal transport. Thus, Aβ mediates distinct facets of hypoxia-induced axonopathy and may represent a functionally selective pharmacological target for therapies directed against early-stage axonopathy in CNS diseases.

Dendritic and axonal mechanisms of Ca2+ elevation impair BDNF transport in Aβ oligomer-treated hippocampal neurons

Intracellular Ca²⁺ dysregulation and transport disruption precede cell death in Alzheimer's disease. Mechanisms of AβO-induced Ca²⁺ elevation are identified that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects. The results challenge dogmatic views on mechanisms of AβO toxicity and subcellular sites of action.

Disruption of fast axonal transport (FAT) and intracellular Ca²⁺ dysregulation are early pathological events in Alzheimer's disease (AD). Amyloid-β oligomers (AβOs), a causative agent of AD, impair transport of BDNF independent of tau by nonexcitotoxic activation of calcineurin (CaN). Ca²⁺-dependent mechanisms that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects from dendritic and axonal AβO binding sites are unknown. Here we show that BDNF transport defects in dendrites and axons are induced simultaneously but exhibit different rates of decline. The spatiotemporal progression of FAT impairment correlates with Ca²⁺ elevation and CaN activation first in dendrites and subsequently in axons. Although many axonal pathologies have been described in AD, studies have primarily focused only on the dendritic effects of AβOs despite compelling reports of presynaptic AβOs in AD models and patients. Indeed, we observe that dendritic CaN activation converges on Ca²⁺ influx through axonal voltage-gated Ca²⁺ channels to impair FAT. Finally, FAT defects are prevented by dantrolene, a clinical compound that reduces Ca²⁺ release from the ER. This work establishes a novel role for Ca²⁺ dysregulation in BDNF transport disruption and tau-independent Aβ toxicity in early AD.

α-secretase cleaved amyloid precursor protein (APP) accumulates in cholinergic dystrophic neurites in normal, aged hippocampus

AIMS

Dystrophic neurites are associated with β-amyloid (Aβ) plaques in the brains of Alzheimer's disease (AD) patients and are also found in some specific areas of normal, aged brains. This study assessed the molecular characteristics of dystrophic neurites in normal ageing and its difference from AD.

METHODS

We compared the dystrophic neurites in normal aged human brains (age 20-70 years) and AD brains (Braak stage 4-6) by immunostaining against ChAT, synaptophysin, γ-tubulin, cathepsin-D, Aβ1-16, Aβ17-24, amyloid precursor protein (APP)-CT695 and APP-NT. We then tested the reproducibility in C57BL/6 mice neurone cultures.

RESULTS

In normal, aged mice and humans, we found an increase in clustered dystrophic neurites of cholinergic neurones in CA1 regions of the hippocampus and layer II and III regions of the entorhinal cortex, which are the major and earliest affected areas in AD. These dystrophic neurites showed accumulation of sAPPα peptides cleaved from the amyloid precursor protein by α-secretase rather than Aβ or C-terminal fragments. In contrast, Aβ and APP-CTFs accumulated in the dystrophic neurites in and around Aβ plaques of AD patients. Several experiments suggested that the accumulation of sAPPα resulted from ageing-related proteasomal dysfunction.

CONCLUSIONS

Ageing-associated impairment of the proteasomal system and accumulation of sAPPα at cholinergic neurites in specific areas of brain regions associated with memory could be associated with the normal decline of memory in aged individuals. In addition, these age-related changes might be the most vulnerable targets of pathological insults that result in pathological accumulation of Aβ and/or APP-CTFs and lead to neurodegenerative conditions such as AD.

Disruption of axonal transport perturbs bone morphogenetic protein (BMP)--signaling and contributes to synaptic abnormalities in two neurodegenerative diseases

Formation of new synapses or maintenance of existing synapses requires the delivery of synaptic components from the soma to the nerve termini via axonal transport. One pathway that is important in synapse formation, maintenance and function of the Drosophila neuromuscular junction (NMJ) is the bone morphogenetic protein (BMP)-signaling pathway. Here we show that perturbations in axonal transport directly disrupt BMP signaling, as measured by its downstream signal, phospho Mad (p-Mad). We found that components of the BMP pathway genetically interact with both kinesin-1 and dynein motor proteins. Thick vein (TKV) vesicle motility was also perturbed by reductions in kinesin-1 or dynein motors. Interestingly, dynein mutations severely disrupted p-Mad signaling while kinesin-1 mutants showed a mild reduction in p-Mad signal intensity. Similar to mutants in components of the BMP pathway, both kinesin-1 and dynein motor protein mutants also showed synaptic morphological defects. Strikingly TKV motility and p-Mad signaling were disrupted in larvae expressing two human disease proteins; expansions of glutamine repeats (polyQ77) and human amyloid precursor protein (APP) with a familial Alzheimer's disease (AD) mutation (APPswe). Consistent with axonal transport defects, larvae expressing these disease proteins showed accumulations of synaptic proteins along axons and synaptic abnormalities. Taken together our results suggest that similar to the NGF-TrkA signaling endosome, a BMP signaling endosome that directly interacts with molecular motors likely exist. Thus problems in axonal transport occurs early, perturbs BMP signaling, and likely contributes to the synaptic abnormalities observed in these two diseases.

Axonal degeneration in an Alzheimer mouse model is PS1 gene dose dependent and linked to intraneuronal Aβ accumulation

Abnormalities and impairments in axonal transport are suggested to strongly contribute to the pathological alterations underlying AD. The exact mechanisms leading to axonopathy are currently unclear, but it was recently suggested that APP expression itself triggers axonal degeneration. We used APP transgenic mice and crossed them on a hemi- or homozygous PS1 knock-in background (APP/PS1KI). Depending on the mutant PS1 dosage, we demonstrate a clear aggravation in both plaque-associated and plaque-distant axonal degeneration, despite of an unchanged APP expression level. Amyloid-β (Aβ) peptides were found to accumulate in axonal swellings as well as in axons and apical dendrites proximate to neurons accumulating intraneuronal Aβ in their cell bodies. This suggests that Aβ can be transported within neurites thereby contributing to axonal deficits. In addition, diffuse extracellular Aβ deposits were observed in the close vicinity of axonal spheroids accumulating intracellular Aβ, which might be indicative of a local Aβ release from sites of axonal damage.

Axonal transport and neurodegenerative disease: vesicle-motor complex formation and their regulation

The process of axonal transport serves to move components over very long distances on microtubule tracks in order to maintain neuronal viability. Molecular motors - kinesin and dynein - are essential for the movement of neuronal cargoes along these tracks; defects in this pathway have been implicated in the initiation or progression of some neurodegenerative diseases, suggesting that this process may be a key contributor in neuronal dysfunction. Recent work has led to the identification of some of the motor-cargo complexes, adaptor proteins, and their regulatory elements in the context of disease proteins. In this review, we focus on the assembly of the amyloid precursor protein, huntingtin, mitochondria, and the RNA-motor complexes and discuss how these may be regulated during long-distance transport in the context of neurodegenerative disease. As knowledge of these motor-cargo complexes and their involvement in axonal transport expands, insight into how defects in this pathway contribute to the development of neurodegenerative diseases becomes evident. Therefore, a better understanding of how this pathway normally functions has important implications for early diagnosis and treatment of diseases before the onset of disease pathology or behavior.

Presenilins regulate neurotrypsin gene expression and neurotrypsin-dependent agrin cleavage via cyclic AMP response element-binding protein (CREB) modulation

Presenilins, the catalytic components of the γ-secretase complex, are upstream regulators of multiple cellular pathways via regulation of gene transcription. However, the underlying mechanisms and the genes regulated by these pathways are poorly characterized. In this study, we identify Tequila and its mammalian ortholog Prss12 as genes negatively regulated by presenilins in Drosophila larval brains and mouse embryonic fibroblasts, respectively. Prss12 encodes the serine protease neurotrypsin, which cleaves the heparan sulfate proteoglycan agrin. Altered neurotrypsin activity causes serious synaptic and cognitive defects; despite this, the molecular processes regulating neurotrypsin expression and activity are poorly understood. Using γ-secretase drug inhibitors and presenilin mutants in mouse embryonic fibroblasts, we found that a mature γ-secretase complex was required to repress neurotrypsin expression and agrin cleavage. We also determined that PSEN1 endoproteolysis or processing of well known γ-secretase substrates was not essential for this process. At the transcriptional level, PSEN1/2 removal induced cyclic AMP response element-binding protein (CREB)/CREB-binding protein binding, accumulation of activating histone marks at the neurotrypsin promoter, and neurotrypsin transcriptional and functional up-regulation that was dependent on GSK3 activity. Upon PSEN1/2 reintroduction, this active epigenetic state was replaced by a methyl CpG-binding protein 2 (MeCP2)-containing repressive state and reduced neurotrypsin expression. Genome-wide analysis revealed hundreds of other mouse promoters in which CREB binding is similarly modulated by the presence/absence of presenilins. Our study thus identifies Tequila and neurotrypsin as new genes repressed by presenilins and reveals a novel mechanism used by presenilins to modulate CREB signaling based on controlling CREB recruitment.

The airbag problem-a potential culprit for bench-to-bedside translational efforts: relevance for Alzheimer's disease

For the last 20 years, the "amyloid cascade hypothesis" has dominated research aimed at understanding, preventing, and curing Alzheimer's disease (AD). During that time researchers have acquired an enormous amount of data and have been successful, more than 300 times, in curing the disease in animal model systems by treatments aimed at clearing amyloid deposits. However, to date similar strategies have not been successful in human AD patients. Hence, before rushing into further clinical trials with compounds that aim at lowering amyloid-beta (Aβ) levels in increasingly younger people, it would be of highest priority to re-assess the initial assumption that accumulation of Aβ in the brain is the primary pathological event driving AD. Here we question this assumption by highlighting experimental evidence in support of the alternative hypothesis suggesting that APP and Aβ are part of a neuronal stress/injury system, which is up-regulated to counteract inflammation/oxidative stress-associated neurodegeneration that could be triggered by a brain injury, chronic infections, or a systemic disease. In AD, this protective program may be overridden by genetic and other risk factors, or its maintenance may become dysregulated during aging. Here, we provide a hypothetical example of a hypothesis-driven correlation between car accidents and airbag release in analogy to the evolution of the amyloid focus and as a way to offer a potential explanation for the failure of the AD field to translate the success of amyloid-related therapeutic strategies in experimental models to the clinic.

Modeling Alzheimer's disease: from past to future

Alzheimer’s disease (AD) is emerging as the most prevalent and socially disruptive illness of aging populations, as more people live long enough to become affected. Although AD is placing a considerable and increasing burden on society, it represents the largest unmet medical need in neurology, because current drugs improve symptoms, but do not have profound disease-modifying effects. Although AD pathogenesis is multifaceted and difficult to pinpoint, genetic and cell biological studies led to the amyloid hypothesis, which posits that amyloid β (Aβ) plays a pivotal role in AD pathogenesis. Amyloid precursor protein (APP), as well as β- and γ-secretases are the principal players involved in Aβ production, while α-secretase cleavage on APP prevents Aβ deposition. The association of early onset familial AD with mutations in the APP and γ-secretase components provided a potential tool of generating animal models of the disease. However, a model that recapitulates all the aspects of AD has not yet been produced. Here, we face the problem of modeling AD pathology describing several models, which have played a major role in defining critical disease-related mechanisms and in exploring novel potential therapeutic approaches. In particular, we will provide an extensive overview on the distinct features and pros and contras of different AD models, ranging from invertebrate to rodent models and finally dealing with computational models and induced pluripotent stem cells.

Presenilin controls kinesin-1 and dynein function during APP-vesicle transport in vivo

Neurons and other cells require intracellular transport of essential components for viability and function. Previous work has shown that while net amyloid precursor protein (APP) transport is generally anterograde, individual vesicles containing APP move bi-directionally. This discrepancy highlights our poor understanding of the in vivo regulation of APP-vesicle transport. Here, we show that reduction of presenilin (PS) or suppression of gamma-secretase activity substantially increases anterograde and retrograde velocities for APP vesicles. Strikingly, PS deficiency has no effect on an unrelated cargo vesicle class containing synaptotagmin, which is powered by a different kinesin motor. Increased velocities caused by PS or gamma-secretase reduction require functional kinesin-1 and dynein motors. Together, our findings suggest that a normal function of PS is to repress kinesin-1 and dynein motor activity during axonal transport of APP vesicles. Furthermore, our data suggest that axonal transport defects induced by loss of PS-mediated regulatory effects on APP-vesicle motility could be a major cause of neuronal and synaptic defects observed in Alzheimer's Disease (AD) pathogenesis. Thus, perturbations of APP/PS transport could contribute to early neuropathology observed in AD, and highlight a potential novel therapeutic pathway for early intervention, prior to neuronal loss and clinical manifestation of disease.

Alzheimer's disease in a dish: promises and challenges of human stem cell models

Human pluripotent stem cells can differentiate into disease-relevant cell types, which capture the unique genome of an individual patient and provide insight into pathological mechanisms of human disease. Recently, human stem cell models for Alzheimer's disease (AD), the most common neurodegenerative dementia, have been described. Stem cell-derived neurons from patients with familial and sporadic AD and Down's syndrome recapitulate human disease phenotypes such as amyloid β peptide production, hyperphosphorylation of tau protein and endosomal abnormalities. Treatment of human neurons with small molecules can modulate these phenotypes, demonstrating the utility of this system for drug development and screening. This review will highlight the current AD stem cell models and discuss the remaining challenges and potential future directions of this field.

Enhanced β-secretase processing alters APP axonal transport and leads to axonal defects

Alzheimer's disease (AD) is a neurodegenerative disease pathologically characterized by amyloid plaques and neurofibrillary tangles in the brain. Before these hallmark features appear, signs of axonal transport defects develop, though the initiating events are not clear. Enhanced amyloidogenic processing of amyloid precursor protein (APP) plays an integral role in AD pathogenesis, and previous work suggests that both the Aβ region and the C-terminal fragments (CTFs) of APP can cause transport defects. However, it remains unknown if APP processing affects the axonal transport of APP itself, and whether increased APP processing is sufficient to promote axonal dystrophy. We tested the hypothesis that β-secretase cleavage site mutations of APP alter APP axonal transport directly. We found that the enhanced β-secretase cleavage reduces the anterograde axonal transport of APP, while inhibited β-cleavage stimulates APP anterograde axonal transport. Transport behavior of APP after treatment with β- or γ-secretase inhibitors suggests that the amount of β-secretase cleaved CTFs (βCTFs) of APP underlies these transport differences. Consistent with these findings, βCTFs have reduced anterograde axonal transport compared with full-length, wild-type APP. Finally, a gene-targeted mouse with familial AD (FAD) Swedish mutations to APP, which enhance the β-cleavage of APP, develops axonal dystrophy in the absence of mutant protein overexpression, amyloid plaque deposition and synaptic degradation. These results suggest that the enhanced β-secretase processing of APP can directly impair the anterograde axonal transport of APP and are sufficient to lead to axonal defects in vivo.

Simple model systems: a challenge for Alzheimer's disease

The success of biomedical researches has led to improvement in human health and increased life expectancy. An unexpected consequence has been an increase of age-related diseases and, in particular, neurodegenerative diseases. These disorders are generally late onset and exhibit complex pathologies including memory loss, cognitive defects, movement disorders and death. Here, it is described as the use of simple animal models such as worms, fishes, flies, Ascidians and sea urchins, have facilitated the understanding of several biochemical mechanisms underlying Alzheimer's disease (AD), one of the most diffuse neurodegenerative pathologies. The discovery of specific genes and proteins associated with AD, and the development of new technologies for the production of transgenic animals, has helped researchers to overcome the lack of natural models. Moreover, simple model systems of AD have been utilized to obtain key information for evaluating potential therapeutic interventions and for testing efficacy of putative neuroprotective compounds.

Axonal transport and neurodegenerative disease: can we see the elephant?

Although it is well established that axonal transport defects are part of the initiation or progression of some neurodegenerative diseases, the precise role of these defects in disease development is poorly understood. Thus, in this article, rather than enumerate the already well-reviewed evidence that there are transport deficits in disease, I will focus on a discussion of two crucial and unanswered questions about the possible role of axonal transport defects in HD and AD. (1) Are alterations in axonal transport caused by changes in the normal function of proteins mutated or altered in HD and AD and/or do such alterations in transport occur as a result of the formation of toxic aggregates of peptides or proteins? (2) Do alterations in axonal transport contribute to the causes of HD and AD or are they early, or late, secondary consequences of other cellular defects caused by disease-induction?

Imaging amyloid precursor protein in vivo: an axonal transport assay

Fusion of fluorescent probes to axonally transported proteins represents an established approach that enables live imaging of axonal transport. In this approach, in vivo examination of fluorescent particle dynamics provides information about the length, directionality, and the velocity by which axonally transported proteins travel along axons. Analysis of these parameters provides information about the distribution of axonal proteins and their dynamics in and between different subcellular compartments. Establishing the movement behavior of amyloid precursor protein within axons indicated that live imaging approaches offer the opportunity to significantly enhance our understanding of the biology as well as pathology of axonal transport. This chapter provides a fluorescence-based procedure for measuring axonal transport of APP in cultured newborn mouse hippocampal neurons.

Absence of the Cdk5 activator p35 causes adult-onset neurodegeneration in the central brain of Drosophila

Altered function of Cdk5 kinase is associated with many forms of neurodegenerative disease in humans. We show here that inactivating the Drosophila Cdk5 ortholog, by mutation of its activating subunit, p35, causes adult-onset neurodegeneration in the fly. In the mutants, a vacuolar neuropathology is observed in a specific structure of the central brain, the 'mushroom body', which is the seat of olfactory learning and memory. Analysis of cellular phenotypes in the mutant brains reveals some phenotypes that resemble natural aging in control flies, including an increase in apoptotic and necrotic cell death, axonal fragmentation, and accumulation of autophagosomes packed with crystalline-like depositions. Other phenotypes are unique to the mutants, notably age-dependent swellings of the proximal axon of mushroom body neurons. Many of these phenotypes are also characteristic of mammalian neurodegenerative disease, suggesting a close relationship between the mechanisms of Cdk5-associated neurodegeneration in fly and human. Together, these results identify the cellular processes that are unleashed in the absence of Cdk5 to initiate the neurodegenerative program, and they provide a model that can be used to determine what part each process plays in the progression to ultimate degeneration.

Fiber Tracts Anomalies in APPxPS1 Transgenic Mice Modeling Alzheimer's Disease

Amyloid beta (Aβ) peptides are known to accumulate in the brain of patients with Alzheimer's disease (AD). However, the link between brain amyloidosis and clinical symptoms has not been elucidated and could be mediated by secondary neuropathological alterations such as fiber tracts anomalies. In the present study, we have investigated the impact of Aβ overproduction in APPxPS1 transgenic mice on the integrity of forebrain axonal bundles (corpus callosum and anterior commissure). We found evidence of fiber tract volume reductions in APPxPS1 mice that were associated with an accelerated age-related loss of axonal neurofilaments and a myelin breakdown. The severity of these defects was neither correlated with the density of amyloid plaques nor associated with cell neurodegeneration. Our data suggest that commissural fiber tract alterations are present in Aβ-overproducing transgenic mice and that intracellular Aβ accumulation preceding extracellular deposits may act as a trigger of such morphological anomalies.

Lysosomal proteolysis inhibition selectively disrupts axonal transport of degradative organelles and causes an Alzheimer's-like axonal dystrophy

In the hallmark neuritic dystrophy of Alzheimer's disease (AD), autophagic vacuoles containing incompletely digested proteins selectively accumulate in focal axonal swellings, reflecting defects in both axonal transport and autophagy. Here, we investigated the possibility that impaired lysosomal proteolysis could be a basis for both of these defects leading to neuritic dystrophy. In living primary mouse cortical neurons expressing fluorescence-tagged markers, LC3-positive autophagosomes forming in axons rapidly acquired the endo-lysosomal markers Rab7 and LAMP1 and underwent exclusive retrograde movement. Proteolytic clearance of these transported autophagic vacuoles was initiated after fusion with bidirectionally moving lysosomes that increase in number at more proximal axon levels and in the perikaryon. Disrupting lysosomal proteolysis by either inhibiting cathepsins directly or by suppressing lysosomal acidification slowed the axonal transport of autolysosomes, late endosomes, and lysosomes and caused their selective accumulation within dystrophic axonal swellings. Mitochondria and other organelles lacking cathepsins moved normally under these conditions, indicating that the general functioning of the axonal transport system was preserved. Dystrophic swellings induced by lysosomal proteolysis inhibition resembled in composition those in several mouse models of AD and also acquired other AD-like features, including immunopositivity for ubiquitin, amyloid precursor protein, and hyperphosphorylated neurofilament proteins. Restoration of lysosomal proteolysis reversed the affected movements of proteolytic Rab7 vesicles, which in turn essentially cleared autophagic substrates and reversed the axonal dystrophy. These studies identify the AD-associated defects in neuronal lysosomal proteolysis as a possible basis for the selective transport abnormalities and highly characteristic pattern of neuritic dystrophy associated with AD.

AD synapses contain abundant Aβ monomer and multiple soluble oligomers, including a 56-kDa assembly

Much evidence indicates that soluble amyloid beta (Aβ) oligomers are key mediators of early cognitive loss, but the localization and key peptide species remain unclear. We have used flow cytometry analysis to demonstrate that surviving Alzheimer's disease (AD) synapses accumulate both Aβ and phosphorylated tau (p-tau). The present experiments use peptide-specific X-map assays and Western blot analyses to identify the Aβ peptide species in synaptosome-enriched samples from normal human subjects, neurologic controls, and AD cases. Aβ40 peptide levels did not vary, but both Aβ42 and Aβ oligomers were increased in soluble AD extracts, with oligomer levels 20-fold higher in aqueous compared with detergent extracts. In Western blot analysis, a ladder of sodium dodecyl sulfate (SDS)-stable oligomers was observed in AD cases, varying in size from monomer, the major peptide observed, to larger assemblies up to about 200 kDa and larger. Multiple oligomers, including monomer, small oligomers, a 56-kDa assembly, and amyloid precursor protein (APP) were correlated with the Aβ level measured in flow cytometry-purified synaptosomes. These results suggest that multiple amyloid precursor protein processing pathways are active in AD synapses and multiple soluble oligomeric assemblies may contribute to synaptic dysfunction.

Characterization of a Drosophila Alzheimer's disease model: pharmacological rescue of cognitive defects

Transgenic models of Alzheimer's disease (AD) have made significant contributions to our understanding of AD pathogenesis, and are useful tools in the development of potential therapeutics. The fruit fly, Drosophila melanogaster, provides a genetically tractable, powerful system to study the biochemical, genetic, environmental, and behavioral aspects of complex human diseases, including AD. In an effort to model AD, we over-expressed human APP and BACE genes in the Drosophila central nervous system. Biochemical, neuroanatomical, and behavioral analyses indicate that these flies exhibit aspects of clinical AD neuropathology and symptomology. These include the generation of Aβ₄₀ and Aβ₄₂, the presence of amyloid aggregates, dramatic neuroanatomical changes, defects in motor reflex behavior, and defects in memory. In addition, these flies exhibit external morphological abnormalities. Treatment with a γ-secretase inhibitor suppressed these phenotypes. Further, all of these phenotypes are present within the first few days of adult fly life. Taken together these data demonstrate that this transgenic AD model can serve as a powerful tool for the identification of AD therapeutic interventions.

Cytoplasmic dynein in neurodegeneration

Cytoplasmic dynein 1 (later referred to as dynein) is the major molecular motor moving cargoes such as mitochondria, organelles and proteins towards the minus end of microtubules. Dynein is involved in multiple basic cellular functions, such as mitosis, autophagy and structure of endoplasmic reticulum and Golgi, but also in neuron specific functions in particular retrograde axonal transport. Dynein is regulated by a number of protein complexes, notably by dynactin. Several studies have supported indirectly the involvement of dynein in neurodegeneration associated with Alzheimer's disease, Parkinson's disease, Huntington's disease and motor neuron diseases. First, axonal transport disruption represents a common feature occurring in neurodegenerative diseases. Second, a number of dynein-dependent processes, including autophagy or clearance of aggregation-prone proteins, are found defective in most of these diseases. Third, a number of mutant genes in various neurodegenerative diseases are involved in the regulation of dynein transport. This includes notably mutations in the P150Glued subunit of dynactin that are found in Perry syndrome and motor neuron diseases. Interestingly, gene products that are mutant in Huntington's disease, Parkinson's disease, motor neuron disease or spino-cerebellar ataxia are also involved in the regulation of dynein motor activity or of cargo binding. Despite a constellation of indirect evidence, direct links between the motor itself and neurodegeneration are few, and this might be due to the requirement of fully active dynein for development. Here, we critically review the evidence of dynein involvement in different neurodegenerative diseases and discuss potential underlying mechanisms.

Familial amyloid precursor protein mutants cause caspase-6-dependent but amyloid β-peptide-independent neuronal degeneration in primary human neuron cultures

Although familial Alzheimer disease (AD)-associated autosomal dominant mutants have been extensively studied, little is known about the underlying molecular mechanisms of neurodegeneration induced by these mutants in AD. Wild-type, Swedish or London amyloid precursor protein (APP) transfection in primary human neurons induced neuritic beading, in which several co-expressed proteins, such as enhanced green fluorescent protein, red fluorescent protein (RFP)-tau and RFP-ubiquitin, accumulated. APP-induced neuritic beading was dependent on caspase-6 (Casp6), because it was inhibited with 5 μM z-VEID-fmk or with dominant-negative Casp6. Neuritic beading was independent from APP-mediated amyloid β-peptide (Aβ) production, because the APPM596V (APP(MV)) mutant, which cannot generate Aβ, still induced Casp6-dependent neuritic beading. However, the beaded neurons underwent Casp6- and Aβ-dependent cell death. These results indicate that overexpression of wild-type or mutant APP causes Casp6-dependent but Aβ-independent neuritic degeneration in human neurons. Because Casp6 is activated early in AD and is involved in axonal degeneration, these results suggest that the inhibition of Casp6 may represent an efficient early intervention against familial forms of AD. Furthermore, these results indicate that removing Aβ without inhibiting Casp6 may have little effect in preventing the progressive dementia associated with sporadic or familial AD.

Drosophila melanogaster in the study of human neurodegeneration

Human neurodegenerative diseases are devastating illnesses that predominantly affect elderly people. The majority of the diseases are associated with pathogenic oligomers from misfolded proteins, eventually causing the formation of aggregates and the progressive loss of neurons in the brain and nervous system. Several of these proteinopathies are sporadic and the cause of pathogenesis remains elusive. Heritable forms are associated with genetic defects, suggesting that the affected protein is causally related to disease formation and/or progression. The limitations of human genetics, however, make it necessary to use model systems to analyse affected genes and pathways in more detail. During the last two decades, research using the genetically amenable fruitfly has established Drosophila melanogaster as a valuable model system in the study of human neurodegeneration. These studies offer reliable models for Alzheimer's, Parkinson's, and motor neuron diseases, as well as models for trinucleotide repeat expansion diseases, including ataxias and Huntington's disease. As a result of these studies, several signalling pathways including phosphatidylinositol 3-kinase (PI3K)/Akt and target of rapamycin (TOR), c-Jun N-terminal kinase (JNK) and bone morphogenetic protein (BMP) signalling, have been shown to be deregulated in models of proteinopathies, suggesting that two or more initiating events may trigger disease formation in an age-related manner. Moreover, these studies also demonstrate that the fruitfly can be used to screen chemical compounds for their potential to prevent or ameliorate the disease, which in turn can directly guide clinical research and the development of novel therapeutic strategies for the treatment of human neurodegenerative diseases.

Kinesin-1 transport reductions enhance human tau hyperphosphorylation, aggregation and neurodegeneration in animal models of tauopathies

Neurodegeneration induced by abnormal hyperphosphorylation and aggregation of the microtubule-associated protein tau defines neurodegenerative tauopathies. Destabilization of microtubules by loss of tau function and filament formation by toxic gain of function are two mechanisms suggested for how abnormal tau triggers neuronal loss. Recent experiments in kinesin-1 deficient mice suggested that axonal transport defects can initiate biochemical changes that induce activation of axonal stress kinase pathways leading to abnormal tau hyperphosphorylation. Here we show using Drosophila and mouse models of tauopathies that reductions in axonal transport can exacerbate human tau protein hyperphosphorylation, formation of insoluble aggregates and tau-dependent neurodegeneration. Together with previous work, our results suggest that non-lethal reductions in axonal transport, and perhaps other types of minor axonal stress, are sufficient to induce and/or accelerate abnormal tau behavior characteristic of Alzheimer's disease and other neurodegenerative tauopathies.

Drosophila as a model to study age-related neurodegenerative disorders: Alzheimer's disease

Alzheimer's Disease (AD) is the most common cause of dementia in the aging population. Although a variety of drug treatments can delay the onset of disease or temporarily reduce its severity, there is currently no cure or effective long-term treatment. This therapeutic void in part reflects an incomplete understanding of the biochemical pathogenesis of this disease. Model organisms, including invertebrates, have been extensively utilized to gain insight into the molecular and cellular mechanisms underlying disease. Here, we will describe how Drosophila has been used to study the function of genes associated with AD and to develop models of this devastating disease.

Amyloid precursor protein and tau transgenic models of Alzheimer's disease: insights from the past and directions for the future

During the last 20 years, our understanding of the mechanisms underlying Alzheimer's disease (AD) has considerably improved, in part owing to both in vitro and in vivo model systems. Studies in mice expressing both human amyloid precursor protein and human tau have provided clear evidence that amyloid-beta and tau interact in the pathogenesis of AD. Moreover, amyloid-beta toxicity has been shown to be tau-dependent since reducing tau levels prevents behavioral deficits and sudden death in amyloid precursor protein transgenic mice. As tau pathology preferentially develops in specific sites and spreads in a predictable manner across the brain, understanding the mechanism underlying tau dysfunction should be a focus in AD mouse modeling. A defined effort must be made to develop therapies that directly address the impact of tau dysfunction in the pathogenesis of AD. Finally, early diagnosis of AD is essential and this must be made possible by identification of early biomarkers, behavioral changes or use of novel imaging techniques.

Loss of tau elicits axonal degeneration in a mouse model of Alzheimer's disease

A central issue in the pathogenesis of tauopathy is the question of how tau protein dysfunction leads to neurodegeneration. We have previously demonstrated that the absence of tau protein is associated with destabilization of microtubules and impaired neurite outgrowth (Dawson et al., 2001; Rapoport et al., 2002). We now hypothesize that the absence of functional tau protein may render the central nervous system more vulnerable to secondary insults such as the overexpression of mutated beta amyloid precursor protein (APP) and traumatic brain injury. We therefore crossed tau knockout mice (Dawson et al., 2001) to mice overexpressing a mutated human APP (APP(670,671), A(sw)) (Hsiao et al., 1996) and created a mouse model (A(sw)/mTau(-/-)) that provides evidence that the loss of tau function causes degeneration of neuronal processes. The overexpression of APP(670,671) in tau knockout mice, elicits the extensive formation of axonal spheroids. While spheroids are only found associated with Abeta plaques in mice expressing APP(670,671) on an endogenous mouse tau background (Irizarry et al., 1997), A(sw)/mTau(-/-) mice have spheroids not only surrounding Abeta plaques but also in white matter tracks and in the neuropil. Plaque associated and neuropil dystrophic neurites and spheroids are prominent features of Alzheimer's disease (Masliah et al., 1993; Terry, 1996; Stokin et al., 2005), and our current data suggests that loss of tau function may lead to neurodegeneration.

White matter diffusivity predicts memory in patients with subjective and mild cognitive impairment and normal CSF total tau levels

Subjective and mild cognitive impairment (SCI and MCI) are etiologically heterogeneous conditions. This poses problems for assessment of pathophysiological mechanisms and risk of conversion to dementia. Neuropsychological, imaging, and cerebrospinal fluid (CSF) findings serve to distinguish Alzheimer's disease (AD) and other etiological subgroups. Tau-molecules stabilize axonal microtubuli; high CSF total tau (T-tau) reflects ongoing axonal damage consistent with AD. Here, we stratify patients by CSF T-tau pathology to determine if memory network diffusion tensor imaging (DTI) predicts memory performance in the absence of elevated T-tau. We analyzed neuropsychological test results, hippocampus volume (HcV) and white matter diffusivity in 45 patients (35 with normal T-tau). The T-tau pathology group showed more hippocampus atrophy and memory impairment than the normal T-tau group. In the T-tau normal group: (1) memory was related with white matter diffusivity [fractional anisotropy (FA) and radial diffusivity (DR)], and (2) FA of the genu corpus callosum was a unique predictor of variance for verbal learning, and HcV did not contribute to this prediction. The smaller sample size in the T-tau pathology group precludes firm conclusions. In the normal T-tau group, white matter tract and memory changes may be associated with normal aging, or with non-tau related pathological mechanisms.