Gene expression and cellular content of cathepsin D in Alzheimer's disease brain: evidence for early up-regulation of the endosomal-lysosomal system

A combinational treatment of carotenoids decreases Aβ secretion in human neurons via β-secretase inhibition

Alzheimer's disease (AD) is the most common cause of dementia and is characterized neuropathologically by the presence of amyloid plaques and neurofibrillary tangles. Amyloid-β (Aβ) peptides, major components of amyloid plaques and crucial pathogenic molecules in terms of the amyloid hypothesis, are derived from successive proteolytic processing of amyloid-β precursor protein (APP). In this study, we established a human neuronal culture system using induced pluripotent stem cells (iPSCs) to evaluate the possible effects of natural compounds on the amyloid phenotype. Unexpectedly, we found that combinational treatment of carotenoids, but not docosahexaenoic acid, significantly decreased Aβ secretion from iPSC-derived human cortical neurons. Importantly, the effects of the carotenoids resulted from specific inhibition of BACE1 activity and not from expression changes in APP or BACE1. Therefore, these results indicate a novel beneficial function of carotenoids in the anti-amyloidogenic processing of APP. Collectively, this study will shed light on neuronal protection by a novel mechanism during the pathogenesis of AD.

Endo-lysosomal proteins and ubiquitin CSF concentrations in Alzheimer's and Parkinson's disease

BACKGROUND

Increasing evidence implicates dysfunctional proteostasis and the involvement of the autophagic and endo-lysosomal system and the ubiquitin-proteasome system in neurodegenerative diseases. In Alzheimer's disease (AD), there is an accumulation of autophagic vacuoles within the neurons. In Parkinson's disease (PD), susceptibility has been linked to genes encoding proteins involved in autophagy and lysosomal function, as well as mutations causing lysosomal disorders. Furthermore, both diseases are characterized by the accumulation of protein aggregates.

METHODS

Proteins associated with endocytosis, lysosomal function, and the ubiquitin-proteasome system were identified in the cerebrospinal fluid (CSF) and targeted by combining solid-phase extraction and parallel reaction monitoring mass spectrometry. In total, 50 peptides from 18 proteins were quantified in three cross-sectional cohorts including AD (N = 61), PD (N = 21), prodromal AD (N = 10), stable mild cognitive impairment (N = 15), and controls (N = 68).

RESULTS

A pilot study, including subjects selected based on their AD CSF core biomarker concentrations, showed increased concentrations of several targeted proteins in subjects with core biomarker levels indicating AD pathology compared to controls. Next, in a clinically characterized cohort, lower concentrations in CSF of proteins in PD were found compared to subjects with prodromal AD. Further investigation in an additional clinical study again revealed lower concentrations in CSF of proteins in PD compared to controls and AD.

CONCLUSION

In summary, significantly different peptide CSF concentrations were identified from proteins AP2B1, C9, CTSB, CTSF, GM2A, LAMP1, LAMP2, TCN2, and ubiquitin. Proteins found to have altered concentrations in more than one study were AP2B1, CTSB, CTSF, GM2A, LAMP2, and ubiquitin. Interestingly, given the genetic implication of lysosomal function in PD, we did identify the CSF concentrations of CTSB, CTSF, GM2A, and LAMP2 to be altered. However, we also found differences in proteins associated with endocytosis (AP2B1) and the ubiquitin-proteasome system (ubiquitin). No difference in any peptide CSF concentration was found in clinically characterized subjects with AD compared to controls. In conclusion, CSF analyses of subjects with PD suggest a general lysosomal dysfunction, which resonates well with recent genetic findings, while such changes are minor or absent in AD.

Mitochondrial Dysfunction and Stress Responses in Alzheimer's Disease

Alzheimer's disease (AD) patients display widespread mitochondrial defects. Brain hypometabolism occurs alongside mitochondrial defects, and correlates well with cognitive decline. Numerous theories attempt to explain AD mitochondrial dysfunction. Groups propose AD mitochondrial defects stem from: (1) mitochondrial-nuclear DNA interactions/variations; (2) amyloid and neurofibrillary tangle interactions with mitochondria, and (3) mitochondrial quality control defects and oxidative damage. Cells respond to mitochondrial dysfunction through numerous retrograde responses including the Integrated Stress Response (ISR) involving eukaryotic initiation factor 2α (eIF2α), activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP). AD brains activate the ISR and we hypothesize mitochondrial defects may contribute to ISR activation. Here we review current recognized contributions of the mitochondria to AD, with an emphasis on their potential contribution to brain stress responses.

Lysosomal Dysfunction in Down Syndrome Is APP-Dependent and Mediated by APP-βCTF (C99)

Lysosomal failure underlies pathogenesis of numerous congenital neurodegenerative disorders and is an early and progressive feature of Alzheimer's disease (AD) pathogenesis. Here, we report that lysosomal dysfunction in Down ayndrome (trisomy 21), a neurodevelopmental disorder and form of early onset AD, requires the extra gene copy of amyloid precursor protein (APP) and is specifically mediated by the β cleaved carboxy terminal fragment of APP (APP-βCTF, C99). In primary fibroblasts from individuals with DS, lysosomal degradation of autophagic and endocytic substrates is selectively impaired, causing them to accumulate in enlarged autolysosomes/lysosomes. Direct measurements of lysosomal pH uncovered a significant elevation (0.6 units) as a basis for slowed LC3 turnover and the inactivation of cathepsin D and other lysosomal hydrolases known to be unstable or less active when lysosomal pH is persistently elevated. Normalizing lysosome pH by delivering acidic nanoparticles to lysosomes ameliorated lysosomal deficits, whereas RNA sequencing analysis excluded a transcriptional contribution to hydrolase declines. Cortical neurons cultured from the Ts2 mouse model of DS exhibited lysosomal deficits similar to those in DS cells. Lowering APP expression with siRNA or BACE1 inhibition reversed cathepsin deficits in both fibroblasts and neurons. Deleting one Bace1 allele from adult Ts2 mice had similar rescue effects in vivo The modest elevation of endogenous APP-βCTF needed to disrupt lysosomal function in DS is relevant to sporadic AD where APP-βCTF, but not APP, is also elevated. Our results extend evidence that impaired lysosomal acidification drives progressive lysosomal failure in multiple forms of AD.SIGNIFICANCE STATEMENT Down syndrome (trisomy 21) (DS) is a neurodevelopmental disorder invariably leading to early-onset Alzheimer's disease (AD). We showed in cells from DS individuals and neurons of DS models that one extra copy of a normal amyloid precursor protein (APP) gene impairs lysosomal acidification, thereby depressing lysosomal hydrolytic activities and turnover of autophagic and endocytic substrates, processes vital to neuronal survival. These deficits, which were reversible by correcting lysosomal pH, are mediated by elevated levels of endogenous β-cleaved carboxy-terminal fragment of APP (APP-βCTF). Notably, similar endosomal-lysosomal pathobiology emerges early in sporadic AD, where neuronal APP-βCTF is also elevated, underscoring its importance as a therapeutic target and underscoring the functional and pathogenic interrelationships between the endosomal-lysosomal pathway and genes causing AD.

Extracellular Vesicles: Intercellular Mediators in Alcohol-Induced Pathologies

Though alcoholic liver injury plays the primary role in direct alcohol-related morbidity, alcohol consumption is also interlinked with many other diseases in extra-hepatic tissues/organs. The mechanism of alcoholic tissue injury is well documented, however the mechanisms that affect extra-hepatic tissues have not yet been well defined. Extracellular vesicles (EVs) such as exosomes and microvesicles, have been identified as key components of alcohol-induced extra-hepatic effects. We have reviewed the recent findings on the potential impact of alcohol-modified EVs/exosomes production and their downstream effects on extra-hepatic tissues. In this review, we discuss the available information on the cross-talk between hepatocytes and immune cells via EV/exosomal cargos (miRNA, mRNA, protein, etc.) in alcoholic liver diseases. We also discuss the effects of alcohol exposure on the contents of EVs/exosomes derived from various extra-hepatic tissues and their associated pathological consequences on recipient cells. Finally, we speculate on other potential EV/exosomal agents that may mediate alcohol-induced tissue damage. Graphical Abstract Alcohol can alter contents of extracellular vesicles (EVs) (e.g. exosomes) such as miRNAs, protein, cytokines, etc. in hepatic and extra-hepatic cells. The transfer of these alcohol modified EVs to nearby or distant cells can play vital role in inflammatory pathways in alcohol induced pathogenesis/comorbidities.

Molecular Network Analysis of the Urinary Proteome of Alzheimer's Disease Patients

Background/Aims

The identification of predictive biomarkers for Alzheimer's disease (AD) from urine would aid in screening for the disease, but information about biological and pathophysiological changes in the urine of AD patients is limited. This study aimed to explore the comprehensive profile and molecular network relations of urinary proteins in AD patients.

Methods

Urine samples collected from 18 AD patients and 18 age- and sex-matched cognitively normal controls were analyzed by mass spectrometry and semiquantified with the normalized spectral index method. Bioinformatics analyses were performed on proteins which significantly increased by more than 2-fold or decreased by less than 0.5-fold compared to the control (p < 0.05) using DAVID bioinformatics resources and KeyMolnet software.

Results

The levels of 109 proteins significantly differed between AD patients and controls. Among these, annotation clusters related to lysosomes, complement activation, and gluconeogenesis were significantly enriched. The molecular relation networks derived from these proteins were mainly associated with pathways of lipoprotein metabolism, heat shock protein 90 signaling, matrix metalloproteinase signaling, and redox regulation by thioredoxin.

Conclusion

Our findings suggest that changes in the urinary proteome of AD patients reflect systemic changes related to AD pathophysiology.

TFEB-mediated Enhancement of the Autophagy-lysosomal Pathway Dually Modulates the Process of Amyloid β-Protein Generation in Neurons

Abnormalities of the autophagy-lysosomal pathway (ALP) have been implicated in the pathology of Alzheimer's disease (AD). Activation of TFEB (transcription factor EB), a master regulator of the ALP, leads to ALP facilitation. The present study sought to clarify whether TFEB-mediated ALP facilitation influences the process of amyloid β-protein (Aβ) generation in neurons. TFEB was overexpressed in mature rat primary cortical neurons via recombinant adenoviruses, without (basal conditions) or with co-overexpression of wild-type amyloid precursor protein (APP) or its β-C-terminal fragment (β-CTF). We confirmed that TFEB overexpression upregulated the lysosomal proteins, cathepsin D and LAMP-1. In TFEB-expressing neurons, protein levels of ADAM10 were profoundly increased, whereas those of APP, BACE1, or γ-secretase complex proteins were unaffected. However, TFEB did not affect ADAM10 mRNA levels. TFEB overexpression had different effects on Aβ production depending on the expression level of APP or β-CTF: TFEB slightly decreased Aβ secretion under basal conditions; clearly increased α-CTF levels and marginally increased β-CTF levels with modest increases in secreted Aβ in APP-expressing neurons; and caused a remarkable increase in β-CTF levels with a significant increase in secreted Aβ in β-CTF-expressing neurons. Inhibition of proteasomes, but not lysosomes, markedly increased β-CTF levels in β-CTF-expressing neurons. These results collectively indicate that TFEB modulates Aβ production not only by increasing α-secretase processing of APP through ADAM10 upregulation but also by augmenting β-CTF levels possibly via altered proteasome-mediated catabolism. Thus, TFEB-mediated ALP enhancement appears to have dual, but opposite, effects on Aβ production in neurons.

Autophagic dysfunction in Alzheimer's disease: Cellular and molecular mechanistic approaches to halt Alzheimer's pathogenesis

Autophagy is a preserved cytoplasmic self-degradation process and endorses recycling of intracellular constituents into bioenergetics for the controlling of cellular homeostasis. Functional autophagy process is essential in eliminating cytoplasmic waste components and helps in the recycling of some of its constituents. Studies have revealed that neurodegenerative disorders may be caused by mutations in autophagy-related genes and alterations of autophagic flux. Alzheimer's disease (AD) is an irrevocable deleterious neurodegenerative disorder characterized by the formation of senile plaques and neurofibrillary tangles (NFTs) in the hippocampus and cortex. In the central nervous system of healthy people, there is no accretion of amyloid β (Aβ) peptides due to the balance between generation and degradation of Aβ. However, for AD patients, the generation of Aβ peptides is higher than lysis that causes accretion of Aβ. Likewise, the maturation of autophagolysosomes and inhibition of their retrograde transport creates favorable conditions for Aβ accumulation. Furthermore, increasing mammalian target of rapamycin (mTOR) signaling raises tau levels as well as phosphorylation. Alteration of mTOR activity occurs in the early stage of AD. In addition, copious evidence links autophagic/lysosomal dysfunction in AD. Compromised mitophagy is also accountable for dysfunctional mitochondria that raises Alzheimer's pathology. Therefore, autophagic dysfunction might lead to the deposit of atypical proteins in the AD brain and manipulation of autophagy could be considered as an emerging therapeutic target. This review highlights the critical linkage of autophagy in the pathogenesis of AD, and avows a new insight to search for therapeutic target for blocking Alzheimer's pathogenesis.

Lysosomal cathepsin D is upregulated in Alzheimer's disease neocortex and may be a marker for neurofibrillary degeneration

Alzheimer's disease (AD) is characterized by accumulation of β-amyloid plaques (AP) and neurofibrillary tangles (NFT) in the cortex, together with synaptic loss and amyloid angiopathy. Perturbations in the brain lysosomal system, including the cathepsin family of proteases, have been implicated in AD where they may be involved in proteolytic clearance of misfolded and abnormally aggregated peptides. However, the status of cathepsin D (catD) is unclear in Lewy body dementia, the second most common form of neurodegenerative dementia after AD, and characterized by Lewy bodies (LB) containing aggregated α-synuclein. Furthermore, earlier reports of catD changes in AD have not been entirely consistent. We measured CatD immunoreactivities in the temporal (Brodmann area BA21) and parietal (BA40) cortices of well characterized AD brains as well as two clinical subtypes of Lewy body dementia, namely Parkinson disease dementia (PDD) and dementia with Lewy bodies (DLB), known to show varying degrees of concomitant AD pathology. Increased catD immunoreactivities in AD were found for both neocortical regions measured, where they also correlated with neuropathological NFT scores and phosphorylated pSer396 tau burden, and appeared to co-localize at least partly to NFT-containing neurons. In contrast, catD was increased only in BA40 in DLB and not at all in PDD, did not correlate with LB scores, and did not appreciably co-localize with α-synuclein inclusions. Our study suggests that catD upregulation may be an adaptive response to AD-related processes leading to neurofibrillary degeneration, but may not be directly associated with formation of α-synuclein inclusions in Lewy body dementia.

Impaired hepatic amyloid-beta degradation in Alzheimer's disease

Extensive research strongly suggests that amyloid beta (Aβ) aggregates in the brain have a central role in Alzheimer's disease (AD) pathogenesis. Pathological Aβ deposition is likely due to an altered balance between overproduction and elimination. Rodent studies have suggested that the liver has a major role in Aβ degradation. It is possible alterations of liver function could affect brain Aβ levels through changes in blood Aβ concentration. In this study, we hypothesized hepatic Aβ degradation to be impaired in AD subjects. To test our hypothesis, an Aβ degradation assay was developed using synthetic fluorescein-labeled Aβ40 and Aβ42 spiked into human liver homogenates. Aβ degradation rates were lower in AD-derived homogenates as compared with those from non-demented (ND) control subjects, even after accounting for such covariates as age, sex, and APOE genotype. The protein expression of potential Aβ-degrading enzymes were also examined. Neprilysin levels were not different in AD liver samples, while cathepsin D and insulin-degrading enzyme were significantly altered in AD subjects. The results support the possibility that impaired hepatic Aβ degradation could be a factor contributing to increased brain Aβ accumulation and AD.

Exploring the putative role of kallikrein-6, calpain-1 and cathepsin-D in the proteolytic degradation of α-synuclein in multiple system atrophy

AIMS

There is evidence that accumulation of α-synuclein (α-syn) in Parkinson's disease (PD) and dementia with Lewy bodies (DLB) results from impaired removal of α-syn rather than its overproduction. Kallikrein-6 (KLK6), calpain-1 (CAPN1) and cathepsin-D (CTSD) are among a small number of proteases that cleave α-syn and are dysregulated in PD and DLB. Our aim in this study was to determine whether protease activity is altered in another α-synucleinopathy, multiple system atrophy (MSA), and might thereby modulate the regional distribution of α-syn accumulation.

METHODS

mRNA and protein level and/or activity of KLK6, CAPN1 and CTSD were measured and assessed in relation to α-syn load in multiple brain regions (posterior frontal cortex, caudate nucleus, putamen, occipital cortex, pontine base and cerebellar white matter), in MSA (n = 20) and age-matched postmortem control tissue (n = 20).

RESULTS

CTSD activity was elevated in MSA in the pontine base and cerebellar white matter. KLK6 and CAPN1 levels were elevated in MSA in the putamen and cerebellar white matter. However, the activity or level of these proteolytic enzymes did not correlate with the regional distribution of α-syn.

CONCLUSIONS

Accumulation of α-syn in MSA is not due to reduced activity of the proteases we have studied. We suggest that their upregulation is likely to be a compensatory response to increased α-syn in MSA.

Using Drosophila Models of Amyloid Toxicity to Study Autophagy in the Pathogenesis of Alzheimer's Disease

Autophagy is a conserved catabolic pathway that involves the engulfment of cytoplasmic components such as large protein aggregates and organelles that are delivered to the lysosome for degradation. This process is important in maintaining neuronal function and raises the possibility of a role for autophagy in neurodegenerative diseases. Alzheimer's disease (AD) is the most prevalent form of these diseases and is characterized by the accumulation of amyloid plaques in the brain which arise due to the misfolding and aggregation of toxic peptides, including amyloid beta (Aβ). There is substantial evidence from both AD patients and animal models that autophagy is dysregulated in this disease. However, it remains to be determined whether this is protective or pathogenic as there is evidence that autophagy can act to promote the degradation as well as function in the generation of toxic Aβ peptides. Understanding the molecular details of the extensive crosstalk that occurs between the autophagic and endolysosomal cellular pathways is essential for identifying the molecular details of amyloid toxicity. Drosophila models that express the toxic proteins that aggregate in AD have been generated and have been shown to recapitulate hallmarks of the disease. Here we focus on what is known about the role of autophagy in amyloid toxicity in AD from mammalian models and how Drosophila models can be used to further investigate AD pathogenesis.

Lysosomal response in relation to α-synuclein pathology differs between Parkinson's disease and multiple system atrophy

Intracellular deposition of pathologically altered α-synuclein mostly in neurons characterises Parkinson's disease (PD), while its accumulation predominantly in oligodendrocytes is a feature of multiple system atrophy (MSA). Recently a prion-like spreading of pathologic α-synuclein has been suggested to play a role in the pathogenesis of PD and MSA. This implicates a role of protein processing systems, including lysosomes, supported also by genetic studies in PD. However, particularly for MSA, the mechanism of cell-to-cell propagation of α-synuclein is yet not fully understood. To evaluate the significance of lysosomal response, we systematically compared differently affected neuronal populations in PD, MSA, and non-diseased brains using morphometric immunohistochemistry (cathepsin D), double immunolabelling (cathepsin D/α-synuclein) laser confocal microscopy, and immunogold electron microscopy for the disease associated α-synuclein. We found that i) irrespective of the presence of neuronal inclusions, the volume density of cathepsin D immunoreactivity significantly increases in affected neurons of the pontine base in MSA brains; ii) volume density of cathepsin D immunoreactivity increases in nigral neurons in PD without inclusions and with non-ubiquitinated pre-aggregates of α-synuclein, but not in neurons with Lewy bodies; iii) cathepsin D immunoreactivity frequently colocalises with α-synuclein pre-aggregates in nigral neurons in PD; iv) ultrastructural observations confirm disease-associated α-synuclein in neuronal and astrocytic lysosomes in PD; v) lysosome-associated α-synuclein is observed in astroglia and rarely in oligodendroglia and in neurons in MSA. Our observations support a crucial role for the neuronal endosomal-lysosomal system in the processing of α-synuclein in PD. We suggest a distinct contribution of lysosomes to the pathogenesis of MSA, including the possibility of oligodendroglial and eventually neuronal uptake of exogenous α-synuclein in MSA.

The Role of Free Radicals in Autophagy Regulation: Implications for Ageing

Reactive oxygen and nitrogen species (ROS and RNS, resp.) have been traditionally perceived solely as detrimental, leading to oxidative damage of biological macromolecules and organelles, cellular demise, and ageing. However, recent data suggest that ROS/RNS also plays an integral role in intracellular signalling and redox homeostasis (redoxtasis), which are necessary for the maintenance of cellular functions. There is a complex relationship between cellular ROS/RNS content and autophagy, which represents one of the major quality control systems in the cell. In this review, we focus on redox signalling and autophagy regulation with a special interest on ageing-associated changes. In the last section, we describe the role of autophagy and redox signalling in the context of Alzheimer's disease as an example of a prevalent age-related disorder.

Apolipoprotein E4 Elicits Lysosomal Cathepsin D Release, Decreased Thioredoxin-1 Levels, and Apoptosis

The major genetic risk factor for Alzheimer’s disease (AD), apolipoprotein E4 (ApoE4), has been suggested to have detrimental effects on neurons, including direct toxicity via apoptosis. Thioredoxin-1 (Trx1) is an endogenous antioxidant protein important for redox regulation and participates in the regulation of apoptosis through the inhibition of apoptosis signal-regulating kinase-1 (Ask-1). In this study, we have investigated the effects of ApoE on Trx1 in the brain. Our results showed that the protein levels of Trx1 were reduced in the hippocampus of ApoE4 targeted replacement (TR) mice compared to ApoE3 TR mice. The reduction was also seen in vitro after treatment of both human primary cortical neurons and neuroblastoma cells with human recombinant ApoE4 (rApoE4). Furthermore, ApoE4 caused a disruption of lysosomal integrity and a shift in the localization of Cathepsin D, an enzyme known to degrade Trx1. ApoE4 treatment induced in addition apoptosis through translocation of Death-domain associated protein-6 (Daxx) from the nucleus to the cytosol, suggesting an activation of the Ask-1 pathway. This toxicity was prevented by overexpression of Trx1 and other endogenous Ask-1 inhibitors. Our data suggests that down-regulation of Trx1 is involved in the toxicity caused by ApoE4. An activated ASK-1 pathway might indeed make cells more vulnerable to other insults such as amyloid-β, which could partially explain the mechanism behind the strongest genetic risk factor for AD.

N-glycome of the Lysosomal Glycocalyx is Altered in Niemann-Pick Type C Disease (NPC) Model Cells

Increasing evidence implicates lysosomal dysfunction in the pathogenesis of neurodegenerative diseases, including the rare inherited lysosomal storage disorders (LSDs) and the most common neurodegenerative diseases, such as Alzheimer's and Parkinson's disease (AD and PD). Although the triggers of the lysosomal impairment may involve the accumulated macromolecules or dysfunction of the lysosomal enzymes, the role of the lysosomal glycocalyx in the lysosomal (dys)function has not been studied. The goal of this work was to analyze whether there are changes in the lysosomal glycocalyx in a cellular model of a LSD Niemann-Pick type C disease (NPC). Using the ferrofluid nanoparticles we isolated lysosomal organelles from NPC1-null and CHOwt cells. The magnetically isolated lysosomal fractions were enriched with the lysosomal marker protein LAMP1 and showed the key features of NPC disease: 3-fold higher cholesterol content and 4-5 fold enlarged size of the particles compared with the lysosomal fractions of wt cells. These lysosomal fractions were further processed to isolate lysosomal membrane proteins using Triton X-114 and their N-glycome was analyzed by HILIC-UPLC. N-glycans presented in each chromatographic peak were elucidated using MALDI-TOF/TOF-MS. We detected changes in the N-glycosylation pattern of the lysosomal glycocalyx of NPC1-null versus wt cells which involved high-mannose and sialylated N-glycans. To the best of our knowledge this study is the first to report N-glycome profiling of the lysosomal glycocalyx in NPC disease cellular model and the first to report the specific changes in the lysosomal glycocalyx in NPC1-null cells. We speculate that changes in the lysosomal glycocalyx may contribute to lysosomal (dys)function. Further glycome profiling of the lysosomal glycocalyx in other LSDs as well as the most common neurodegenerative diseases, such as AD and PD, is necessary to better understand the role of the lysosomal glycocalyx and to reveal its potential contribution in lysosomal dysfunction leading to neurodegeneration.

Tau Proteolysis in the Pathogenesis of Tauopathies: Neurotoxic Fragments and Novel Biomarkers

With predictions showing that 131.5 million people worldwide will be living with dementia by 2050, an understanding of the molecular mechanisms underpinning disease is crucial in the hunt for novel therapeutics and for biomarkers to detect disease early and/or monitor disease progression. The metabolism of the microtubule-associated protein tau is altered in different dementias, the so-called tauopathies. Tau detaches from microtubules, aggregates into oligomers and neurofibrillary tangles, which can be secreted from neurons, and spreads through the brain during disease progression. Post-translational modifications exacerbate the production of both oligomeric and soluble forms of tau, with proteolysis by a range of different proteases being a crucial driver. However, the impact of tau proteolysis on disease progression has been overlooked until recently. Studies have highlighted that proteolytic fragments of tau can drive neurodegeneration in a fragment-dependent manner as a result of aggregation and/or transcellular propagation. Proteolytic fragments of tau have been found in the cerebrospinal fluid and plasma of patients with different tauopathies, providing an opportunity to develop these fragments as novel disease progression biomarkers. A range of therapeutic strategies have been proposed to halt the toxicity associated with proteolysis, including reducing protease expression and/or activity, selectively inhibiting protease-substrate interactions, and blocking the action of the resulting fragments. This review highlights the importance of tau proteolysis in the pathogenesis of tauopathies, identifies putative sites during tau fragment-mediated neurodegeneration that could be targeted therapeutically, and discusses the potential use of proteolytic fragments of tau as biomarkers for different tauopathies.

Dysfunction of autophagy and endosomal-lysosomal pathways: Roles in pathogenesis of Down syndrome and Alzheimer's Disease

Individuals with Down syndrome (DS) have an increased risk of early-onset Alzheimer's Disease (AD), largely owing to a triplication of the APP gene, located on chromosome 21. In DS and AD, defects in endocytosis and lysosomal function appear at the earliest stages of disease development and progress to widespread failure of intraneuronal waste clearance, neuritic dystrophy and neuronal cell death. The same genetic factors that cause or increase AD risk are also direct causes of endosomal-lysosomal dysfunction, underscoring the essential partnership between this dysfunction and APP metabolites in AD pathogenesis. The appearance of APP-dependent endosome anomalies in DS beginning in infancy and evolving into the full range of AD-related endosomal-lysosomal deficits provides a unique opportunity to characterize the earliest pathobiology of AD preceding the classical neuropathological hallmarks. Facilitating this characterization is the authentic recapitulation of this endosomal pathobiology in peripheral cells from people with DS and in trisomy mouse models. Here, we review current research on endocytic-lysosomal dysfunction in DS and AD, the emerging importance of APP/βCTF in initiating this dysfunction, and the potential roles of additional trisomy 21 genes in accelerating endosomal-lysosomal impairment in DS. Collectively, these studies underscore the growing value of investigating DS to probe the biological origins of AD as well as to understand and ameliorate the developmental disability of DS.

The Endosomal-Lysosomal Pathway Is Dysregulated by APOE4 Expression in Vivo

Possession of the ε4 allele of apolipoprotein E (APOE) is the major genetic risk factor for late-onset Alzheimer's disease (AD). Although numerous hypotheses have been proposed, the precise cause of this increased AD risk is not yet known. In order to gain a more comprehensive understanding of APOE4's role in AD, we performed RNA-sequencing on an AD-vulnerable vs. an AD-resistant brain region from aged APOE targeted replacement mice. This transcriptomics analysis revealed a significant enrichment of genes involved in endosomal–lysosomal processing, suggesting an APOE4-specific endosomal–lysosomal pathway dysregulation in the brains of APOE4 mice. Further analysis revealed clear differences in the morphology of endosomal–lysosomal compartments, including an age-dependent increase in the number and size of early endosomes in APOE4 mice. These findings directly link the APOE4 genotype to endosomal–lysosomal dysregulation in an in vivo, AD pathology-free setting, which may play a causative role in the increased incidence of AD among APOE4 carriers.

Nutrient-driven O-GlcNAc in proteostasis and neurodegeneration

Proteostasis is essential in the mammalian brain where post-mitotic cells must function for decades to maintain synaptic contacts and memory. The brain is dependent on glucose and other metabolites for proper function and is spared from metabolic deficits even during starvation. In this review, we outline how the nutrient-sensitive nucleocytoplasmic post-translational modification O-linked N-acetylglucosamine (O-GlcNAc) regulates protein homeostasis. The O-GlcNAc modification is highly abundant in the mammalian brain and has been linked to proteopathies, including neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's. C. elegans, Drosophila, and mouse models harboring O-GlcNAc transferase- and O-GlcNAcase-knockout alleles have helped define the role O-GlcNAc plays in development as well as age-associated neurodegenerative disease. These enzymes add and remove the single monosaccharide from protein serine and threonine residues, respectively. Blocking O-GlcNAc cycling is detrimental to mammalian brain development and interferes with neurogenesis, neural migration, and proteostasis. Findings in C. elegans and Drosophila model systems indicate that the dynamic turnover of O-GlcNAc is critical for maintaining levels of key transcriptional regulators responsible for neurodevelopment cell fate decisions. In addition, pathways of autophagy and proteasomal degradation depend on a transcriptional network that is also reliant on O-GlcNAc cycling. Like the quality control system in the endoplasmic reticulum which uses a 'mannose timer' to monitor protein folding, we propose that cytoplasmic proteostasis relies on an 'O-GlcNAc timer' to help regulate the lifetime and fate of nuclear and cytoplasmic proteins. O-GlcNAc-dependent developmental alterations impact metabolism and growth of the developing mouse embryo and persist into adulthood. Brain-selective knockout mouse models will be an important tool for understanding the role of O-GlcNAc in the physiology of the brain and its susceptibility to neurodegenerative injury.

Cathepsin D Polymorphism C224T in Childhood-Onset Neurodegenerative Disorders: No Impact for Childhood Dementia

Compromised lysosomal functioning has been identified as a major risk factor for neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Furthermore, the association between a defined cathepsin D ( CTSD ) polymorphism and a higher risk of sporadic Alzheimer's disease has been established for particular populations. Here, we analyzed 189 children with rare neurodegenerative disease for carrying the T-allele by polymerase chain reaction-restriction fragment length polymorphism. We found no statistical differences in genotype and allele frequencies between the neurodegenerative group and European descent participants of genetic studies using the Cochran-Armitage's trend test. In contrast to adult-onset neurodegenerative diseases, analysis of clinical datasets of children carrying the T-allele did not demonstrate differences to the general disease group.

Amyloid precursor protein and endosomal-lysosomal dysfunction in Alzheimer's disease: inseparable partners in a multifactorial disease

Abnormalities of the endosomal-lysosomal network (ELN) are a signature feature of Alzheimer's disease (AD). These include the earliest known cytopathology that is specific to AD and that affects endosomes and induces the progressive failure of lysosomes, each of which are directly linked by distinct mechanisms to neurodegeneration. The origins of ELN dysfunction and β-amyloidogenesis closely overlap, which reflects their common genetic basis, the established early involvement of endosomes and lysosomes in amyloid precursor protein (APP) processing and clearance, and the pathologic effect of certain APP metabolites on ELN functions. Genes that promote β-amyloidogenesis in AD (APP, PSEN1/2, and APOE4) have primary effects on ELN function. The importance of primary ELN dysfunction to pathogenesis is underscored by the mutations in more than 35 ELN-related genes that, thus far, are known to cause familial neurodegenerative diseases even though different pathogenic proteins may be involved. In this article, I discuss growing evidence that implicates AD gene-driven ELN disruptions as not only the antecedent pathobiology that underlies β-amyloidogenesis but also as the essential partner with APP and its metabolites that drive the development of AD, including tauopathy, synaptic dysfunction, and neurodegeneration. The striking amelioration of diverse deficits in animal AD models by remediating ELN dysfunction further supports a need to integrate APP and ELN relationships, including the role of amyloid-β, into a broader conceptual framework of how AD arises, progresses, and may be effectively therapeutically targeted.-Nixon, R. A. Amyloid precursor protein and endosomal-lysosomal dysfunction in Alzheimer's disease: inseparable partners in a multifactorial disease.

Divergent Metabolic Regulation of Autophagy and mTORC1-Early Events in Alzheimer's Disease?

Alzheimer's disease (AD) is a progressive disease associated with the production and deposition of amyloid β-peptide (Aβ) aggregates and neurofibrillary tangles, which lead to synaptic and neuronal damage. Reduced autophagic flux has been widely associated with the accumulation of autophagic vacuoles (AV), which has been proposed to contribute to aggregate build-up observed in AD. As such, targeting autophagy regulation has received wide review, where an understanding as to how this mechanism can be controlled will be important to neuronal health. The mammalian target of rapamycin complex 1 (mTORC1), which was found to be hyperactive in AD brain, regulates autophagy and is considered to be mechanistically important to aberrant autophagy in AD. Hormones and nutrients such as insulin and leucine, respectively, positively regulate mTORC1 activation and are largely considered to inhibit autophagy. However, in AD brain there is a dysregulation of nutrient metabolism, linked to insulin resistance, where a role for insulin treatment to improve cognition has been proposed. Recent studies have highlighted that mitochondrial proteins such as glutamate dehydrogenase and the human branched chain aminotransferase protein, through metabolism of leucine and glutamate, differentially regulate mTORC1 and autophagy. As the levels of the hBCAT proteins are significantly increased in AD brain relative to aged-matched controls, we discuss how these metabolic pathways offer new potential therapeutic targets. In this review article, we highlight the core regulation of autophagy through mTORC1, focusing on how insulin and leucine will be important to consider in particular with respect to our understanding of nutrient load and AD pathogenesis.

Haplodeficiency of Cathepsin D does not affect cerebral amyloidosis and autophagy in APP/PS1 transgenic mice

Autophagy and lysosomal function are important for protein homeostasis and their dysfunction have been associated with Alzheimer's disease (AD). Increased immunoreactivities of an important lysosomal protease, cathepsin D (Cat D), are evident in amyloid plaques and neurons in patients with AD. This study tests the hypothesis that deleting one allele of the cathepsin D gene (Ctsd) impacts cerebral β-amyloidosis in amyloid-β precursor protein (APP)sw/PS1dE9 (APP/PS1) double transgenic mice. Despite a significant 38% decrease in Cat D level in APP/PS1/Ctsd+/- compared with APP/PS1/Ctsd+/+ mice, no changes in steady state levels and deposition of Aβ were found in the brain. There were also no differences in APP processing, the levels of two other Aβ-degrading proteases, the levels of autophagy related protein, such as LAMP2, P62, LC3-I, LC3-II, and Beclin-1, or the markers of neuroinflammation, observed between the APP/PS1/Ctsd+/+ and APP/PS1/Ctsd+/- mice. Our findings demonstrate that in wild-type mice, Cat D protein levels are either in excess or redundant with other factors in the brain, and at least one allele of Ctsd is dispensable for cerebral β-amyloidosis and autophagy in APP/PS1 transgenic mice.

Evidence for Mitochondrial UPR Gene Activation in Familial and Sporadic Alzheimer's Disease

Mitochondrial perturbations such as oxidative stress, increased fission/fusion dysfunction, and mitophagy are consistent features of Alzheimer's disease (AD), yet the mechanisms that initiate these perturbations are unclear. One potential source for mitochondrial defects could be an imbalance in mitochondrial proteostasis. In this regard, studies indicate that a specialized mitochondrial unfolded protein response (mtUPR) is activated upon the aberrant accumulation of damaged or unfolded proteins in the mitochondrial matrix, resulting in the up-regulation of key genes involved in mitochondrial stabilization. To test whether mtUPR activation occurs in AD, we performed real-time quantitative PCR on postmortem frontal cortex samples from subjects classified as sporadic AD, familial AD linked to presenilin-1 mutations, or cognitively intact controls. Compared to controls, sporadic AD subjects exhibited a significant ~40-60% increase in expression levels of select genes activated by the mtUPR, including mitochondrial chaperones dnaja3, hspd1, and hspe1, mitochondrial proteases clpp and yme1l1, and txn2, a mitochondrial-specific oxidoreductase. Furthermore, levels of all six mtUPR genes were significantly up-regulated by ~70-90% in familial AD compared to controls, and these expression levels were significantly higher compared to sporadic AD. The increase in hspd1 (Hsp60) was validated by western blotting. These data support the concept that both sporadic and familial AD are characterized by mtUPR gene activation. Understanding the physiological consequences of this response may provide subcellular mechanistic clues to selective neuronal vulnerability or endogenous compensatory mechanisms during the progression of AD.

Taking out the garbage: cathepsin D and calcineurin in neurodegeneration

Cellular homeostasis requires a tightly controlled balance between protein synthesis, folding and degradation. Especially long-lived, post-mitotic cells such as neurons depend on an efficient proteostasis system to maintain cellular health over decades. Thus, a functional decline of processes contributing to protein degradation such as autophagy and general lysosomal proteolytic capacity is connected to several age-associated neurodegenerative disorders, including Parkinson's, Alzheimer's and Huntington's diseases. These so called proteinopathies are characterized by the accumulation and misfolding of distinct proteins, subsequently driving cellular demise. We recently linked efficient lysosomal protein breakdown via the protease cathepsin D to the Ca²⁺/calmodulin-dependent phosphatase calcineurin. In a yeast model for Parkinson's disease, functional calcineurin was required for proper trafficking of cathepsin D to the lysosome and for recycling of its endosomal sorting receptor to allow further rounds of shuttling. Here, we discuss these findings in relation to present knowledge about the involvement of cathepsin D in proteinopathies in general and a possible connection between this protease, calcineurin signalling and endosomal sorting in particular. As dysregulation of Ca²⁺ homeostasis as well as lysosomal impairment is connected to a plethora of neurodegenerative disorders, this novel interplay might very well impact pathologies beyond Parkinson's disease.

Co-Localization of Glia Maturation Factor with NLRP3 Inflammasome and Autophagosome Markers in Human Alzheimer's Disease Brain

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by the presence of intracellular neurofibrillary tangles (NFTs) containing hyperphosphorylated tau, and the extracellular deposition of amyloid plaques (APs) with misfolded amyloid-β (Aβ) peptide. Glia maturation factor (GMF), a highly conserved pro-inflammatory protein, isolated and cloned in our laboratory, has been shown to activate glial cells leading to neuroinflammation and neurodegeneration in AD. We hypothesized that inflammatory reactions promoted by NLRP3-Caspase-1inflammasome pathway trigger dysfunction in autophagy and accumulation of Aβ which is amplified and regulated by GMF in AD. In this study, using immunohistochemical techniques we analyzed components of the NLRP3 inflammasome and autophagy- lysosomal markers in relation to Aβ, p-tau and GMF in human postmortem AD and age-matched non-AD brains. Tissue sections were prepared from the temporal cortex of human postmortem brains. Here, we demonstrate an increased expression of the inflammasome components NLRP3 and Caspase-1 and the products of inflammasome activation IL-1β and IL-18 along with GMF in the temporal cortex of AD brains. These inflammasome components and the pro-inflammatory cytokines co-localized with GMF in the vicinity and periphery of the APs and NFTs. Moreover, using double immunofluorescence staining, AD brain displayed an increase in the autophagy SQSTM1/p62 and LC3 positive vesicles and the lysosomal marker LAMP1 that also co-localized with GMF, Aβ and hyperphosphorylated p-tau. Our results indicate that in AD, the neuroinflammation promoted by the NLRP3 inflammasome may be amplified and regulated by GMF, which further impairs clearance of protein aggregates mediated by the auto-phagosomal pathway.

The endocytic pathway in microglia during health, aging and Alzheimer's disease

Microglia, the main phagocytes of the central nervous system (CNS), are involved in the surveillance and maintenance of nervous tissue. During normal tissue homeostasis, microglia migrates within the CNS, phagocytose dead cells and tissue debris, and modulate synapse pruning and spine formation via controlled phagocytosis. In the event of an invasion by a foreign body, microglia are able to phagocytose the invading pathogen and process it proteolytically for antigen presentation. Internalized substrates are incorporated and sorted within the endocytic pathway and thereafter transported via complex vesicular routes. When targeted for degradation, substrates are delivered to acidic late endosomes and lysosomes. In these, the enzymatic degradation relies on pH and enzyme content. Endocytosis, sorting, transport, compartment acidification and degradation are regulated by complex signaling mechanisms, and these may be altered during aging and pathology. In this review, we discuss the endocytic pathway in microglia, with insight into the mechanisms controlling lysosomal biogenesis and pH regulation. We also discuss microglial lysosome function associated with Alzheimer's disease (AD) and the mechanisms of amyloid-beta (Aβ) internalization and degradation. Finally, we explore some therapies currently being investigated to treat AD and their effects on microglial response to Aβ, with insight in those involving enhancement of lysosomal function.

Loss of Cathepsin B and L Leads to Lysosomal Dysfunction, NPC-Like Cholesterol Sequestration and Accumulation of the Key Alzheimer's Proteins

Proper function of lysosomes is particularly important in neurons, as they cannot dilute accumulated toxic molecules and aggregates by cell division. Thus, impairment of lysosomal function plays an important role in neuronal degeneration and in the pathogenesis of numerous neurodegenerative diseases. In this work we analyzed how inhibition and/or loss of the major lysosomal proteases, the cysteine cathepsins B and L (CtsB/L), affects lysosomal function, cholesterol metabolism and degradation of the key Alzheimer's disease (AD) proteins. Here, we show that cysteine CtsB/L, and not the aspartyl cathepsin D (CtsD), represent a major lysosomal protease(s) that control lysosomal function, intracellular cholesterol trafficking and AD-like amyloidogenic features. Intriguingly, accumulation of free cholesterol in late endosomes/lysosomes upon CtsB/L inhibition resembled a phenotype characteristic for the rare neurodegenerative disorder Niemann-Pick type C (NPC). CtsB/L inhibition and not the inhibition of CtsD led to lysosomal impairment assessed by decreased degradation of EGF receptor, enhanced LysoTracker staining and accumulation of several lysosomal proteins LC3II, NPC1 and NPC2. By measuring the levels of NPC1 and ABCA1, the two major cholesterol efflux proteins, we showed that CtsB/L inhibition or genetic depletion caused accumulation of the NPC1 in lysosomes and downregulation of ABCA1 protein levels and its expression. Furthermore, we revealed that CtsB/L are involved in degradation of the key Alzheimer's proteins: amyloid-β peptides (Aβ) and C-terminal fragments of the amyloid precursor protein (APP) and in degradation of β-secretase (BACE1). Our results imply CtsB/L as major regulators of lysosomal function and demonstrate that CtsB/L may play an important role in intracellular cholesterol trafficking and in degradation of the key AD proteins. Our findings implicate that enhancing the activity or levels of CtsB/L could provide a promising and a common strategy for maintaining lysosomal function and for preventing and/or treating neurodegenerative diseases.

Autophagy flux in CA1 neurons of Alzheimer hippocampus: Increased induction overburdens failing lysosomes to propel neuritic dystrophy

Defective autophagy contributes to Alzheimer disease (AD) pathogenesis although evidence is conflicting on whether multiple stages are impaired. Here, for the first time, we have comprehensively evaluated the entire autophagic process specifically in CA1 pyramidal neurons of hippocampus from early and late-stage AD subjects and nondemented controls. CA1 neurons aspirated by laser capture microdissection were analyzed using a custom-designed microarray comprising 578 neuropathology- and neuroscience-associated genes. Striking upregulation of autophagy-related genes, exceeding that of other gene ontology groups, reflected increases in autophagosome formation and lysosomal biogenesis beginning at early AD stages. Upregulated autophagosome formation was further indicated by elevated gene and protein expression levels for autophagosome components and increased LC3-positive puncta. Increased lysosomal biogenesis was evidenced by activation of MiTF/TFE family transcriptional regulators, particularly TFE3 (transcription factor binding to IGHM enhancer 3) and by elevated expression of their target genes and encoded proteins. Notably, TFEB (transcription factor EB) activation was associated more strongly with glia than neurons. These findings establish that autophagic sequestration is both competent and upregulated in AD. Autophagosome-lysosome fusion is not evidently altered. Despite this early disease response, however, autophagy flux is progressively impeded due to deficient substrate clearance, as reflected by autolysosomal accumulation of LC3-II and SQSTM1/p62 and expansion of autolysosomal size and total area. We propose that sustained induction of autophagy in the face of progressively declining lysosomal clearance of substrates explains the uncommonly robust autophagic pathology and neuritic dystrophy implicated in AD pathogenesis.

Alzheimer's Disease: From Mitochondrial Perturbations to Mitochondrial Medicine

Age-related neurodegenerative diseases such as Alzheimer's disease (AD) are distressing conditions causing countless levels of suffering for which treatment is often insufficient or inexistent. Considered to be the most common cause of dementia and an incurable, progressive neurodegenerative disorder, the intricate pathogenic mechanisms of AD continue to be revealed and, consequently, an effective treatment needs to be developed. Among the diverse hypothesis that have been proposed to explain AD pathogenesis, the one concerning mitochondrial dysfunction has raised as one of the most discussed with an actual acceptance in the field. It posits that manipulating mitochondrial function and understanding the deficits that result in mitochondrial injury may help to control and/or limit the development of AD. To achieve such goal, the concept of mitochondrial medicine places itself as a promising gathering of strategies to directly manage the major insidious disturbances of mitochondrial homeostasis as well as attempts to directly or indirectly manage its consequences in the context of AD. The aim of this review is to summarize the evolution that occurred from the establishment of mitochondrial homeostasis perturbation as masterpieces in AD pathogenesis up until the development of mitochondrial medicine. Following a brief glimpse in the past and current hypothesis regarding the triad of aging, mitochondria and AD, this manuscript will address the major mechanisms currently believed to participate in above mentioned events. Both pharmacological and lifestyle interventions will also be reviewed as AD-related mitochondrial therapeutics.

Mitochondria as a target for neuroprotection: implications for Alzheimer´s disease

INTRODUCTION

Alzheimer's disease (AD), the most common form of dementia, is marked by progressive loss of memory and impairment of cognitive ability. Despite decades of intensive research and scientific advances, the intricate pathogenic mechanisms of AD are still not fully understood and, consequently, an effective treatment is yet to be developed. As widely accepted, the alterations of mitochondrial function are actively engaged in a plethora of neurodegenerative diseases, including AD. With growing interest in the mitochondria as a potential target for understanding AD, it has even been hypothesized that deficits in these organelles may be at the heart of the progression of AD itself. Areas covered: The purpose of this review is to summarize relevant studies that suggest a role for mitochondrial (dys)function in AD and to provide a survey on latest developments regarding AD-related mitochondrial therapeutics. Expert commentary: As outlined in a plethora of studies, there is no doubt that mitochondria play a major role in several stages of AD progression. Even though more in-depth studies are needed before pharmaceutical industry can apply such knowledge to human medicine, the continuous advances in AD research field will certainly facilitate and accelerate the development of more effective preventive or therapeutic strategies to fight this devastating disease.

Kallikrein-8 inhibition attenuates Alzheimer's disease pathology in mice

INTRODUCTION

Memory loss and increased anxiety are clinical hallmarks of Alzheimer's disease (AD). Kallikrein-8 is a protease implicated in memory acquisition and anxiety, and its mRNA is known to be up-regulated in AD-affected human hippocampus. Therefore, an involvement of Kallikrein-8 in Alzheimer's pathogenesis is conceivable but remains to be proved.

METHODS

We determined the cerebral expression of Kallikrein-8 mRNA and protein during the course of AD in patients and in transgenic mice and tested the impact of Kallikrein-8 inhibition on AD-related pathology in mice and in primary glial cells.

RESULTS

Kallikrein-8 mRNA and protein were up-regulated in both species at incipient stages of AD. Kallikrein-8 inhibition impeded amyloidogenic amyloid-precursor-protein processing, facilitated amyloid β (Aβ) clearance across the blood-brain-barrier, boosted autophagy, reduced Aβ load and tau pathology, enhanced neuroplasticity, reversed molecular signatures of anxiety, and ultimately improved memory and reduced fear.

DISCUSSION

Kallikrein-8 is a promising new therapeutic target against AD.

Synaptic Amyloid-β Oligomers Precede p-Tau and Differentiate High Pathology Control Cases

Amyloid-β (Aβ) and hyperphosphorylated tau (p-tau) aggregates form the two discrete pathologies of Alzheimer disease (AD), and oligomeric assemblies of each protein are localized to synapses. To determine the sequence by which pathology appears in synapses, Aβ and p-tau were quantified across AD disease stages in parietal cortex. Nondemented cases with high levels of AD-related pathology were included to determine factors that confer protection from clinical symptoms. Flow cytometric analysis of synaptosome preparations was used to quantify Aβ and p-tau in large populations of individual synaptic terminals. Soluble Aβ oligomers were assayed by a single antibody sandwich enzyme-linked immunosorbent assay. Total in situ Aβ was elevated in patients with early- and late-stage AD dementia, but not in high pathology nondemented controls compared with age-matched normal controls. However, soluble Aβ oligomers were highest in early AD synapses, and this assay distinguished early AD cases from high pathology controls. Overall, synapse-associated p-tau did not increase until late-stage disease in human and transgenic rat cortex, and p-tau was elevated in individual Aβ-positive synaptosomes in early AD. These results suggest that soluble oligomers in surviving neocortical synaptic terminals are associated with dementia onset and suggest an amyloid cascade hypothesis in which oligomeric Aβ drives phosphorylated tau accumulation and synaptic spread. These results indicate that antiamyloid therapies will be less effective once p-tau pathology is developed.

Stabilization of Microtubule-Unbound Tau via Tau Phosphorylation at Ser262/356 by Par-1/MARK Contributes to Augmentation of AD-Related Phosphorylation and Aβ42-Induced Tau Toxicity

Abnormal accumulation of the microtubule-interacting protein tau is associated with neurodegenerative diseases including Alzheimer’s disease (AD). β-amyloid (Aβ) lies upstream of abnormal tau behavior, including detachment from microtubules, phosphorylation at several disease-specific sites, and self-aggregation into toxic tau species in AD brains. To prevent the cascade of events leading to neurodegeneration in AD, it is essential to elucidate the mechanisms underlying the initial events of tau mismetabolism. Currently, however, these mechanisms remain unclear. In this study, using transgenic Drosophila co-expressing human tau and Aβ, we found that tau phosphorylation at AD-related Ser262/356 stabilized microtubule-unbound tau in the early phase of tau mismetabolism, leading to neurodegeneration. Aβ increased the level of tau detached from microtubules, independent of the phosphorylation status at GSK3-targeted SP/TP sites. Such mislocalized tau proteins, especially the less phosphorylated species, were stabilized by phosphorylation at Ser262/356 via PAR-1/MARK. Levels of Ser262 phosphorylation were increased by Aβ42, and blocking this stabilization of tau suppressed Aβ42-mediated augmentation of tau toxicity and an increase in the levels of tau phosphorylation at the SP/TP site Thr231, suggesting that this process may be involved in AD pathogenesis. In contrast to PAR-1/MARK, blocking tau phosphorylation at SP/TP sites by knockdown of Sgg/GSK3 did not reduce tau levels, suppress tau mislocalization to the cytosol, or diminish Aβ-mediated augmentation of tau toxicity. These results suggest that stabilization of microtubule-unbound tau by phosphorylation at Ser262/356 via the PAR-1/MARK may act in the initial steps of tau mismetabolism in AD pathogenesis, and that such tau species may represent a potential therapeutic target for AD.

Alzheimer’s disease (AD) is the most common cause of dementia resulting from progressive neuron loss. Two proteins, β-amyloid (Aβ) and tau, accumulate in AD brains and are involved in AD pathogenesis. In healthy neurons, tau binds to microtubules to regulate its stability; in AD brains, however, tau is detached from microtubules and phosphorylated at multiple sites. Such abnormal tau behavior, which is likely to be triggered by Aβ, results in generation of pathological tau species that mediate neuron loss. However, the detailed mechanisms underlying this event remain incompletely understood. Using transgenic flies expressing human tau and Aβ as a model system, we found that tau phosphorylation at specific AD-related sites stabilized microtubule-unbound tau in the early phase of tau mismetabolism to generate toxic tau species. Moreover, this process is critical for Aβ to promote subsequent tau phosphorylation and neurodegeneration. Our results reveal a critical step in the initiation of tau mismetabolism, and this process may represent a potential therapeutic target for AD.

Autophagic and lysosomal defects in human tauopathies: analysis of post-mortem brain from patients with familial Alzheimer disease, corticobasal degeneration and progressive supranuclear palsy

Introduction

The accumulation of insoluble proteins within neurons and glia cells is a pathological hallmark of several neurodegenerative diseases. Abnormal aggregation of the microtubule-associated protein tau characterizes the neuropathology of tauopathies, such as Alzheimer disease (AD), corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP). An impairment of the lysosomal degradation pathway called macroautophagy, hereafter referred to as autophagy, could contribute to the accumulation of aggregated proteins. The role of autophagy in neurodegeneration has been intensively studied in the context of AD but there are few studies in other tauopathies and it is not known if defects in autophagy is a general feature of tauopathies. In the present study, we analysed autophagic and lysosomal markers in human post-mortem brain samples from patients with early-onset familial AD (FAD) with the APP Swedish mutation (APPswe), CBD and PSP and control individuals.

Results

FAD, CBD and PSP patients displayed an increase in LC3-positive vesicles in frontal cortex, indicating an accumulation of autophagic vesicles. Moreover, using double-immunohistochemistry and in situ proximity ligation assay, we observed colocalization of hyperphosphorylated tau with the autophagy marker LC3 in FAD, CBD and PSP patients but not in control individuals. Increased levels of the lysosomal marker LAMP1 was detected in FAD and CBD, and in addition Cathepsin D was diffusely spread in the cytoplasm in all tauopathies suggesting an impaired lysosomal integrity.

Conclusion

Taken together, our results indicate an accumulation of autophagic and lysosomal markers in human brain tissue from patients with primary tauopathies (CBD and PSP) as well as FAD, suggesting a defect of the autophagosome-lysosome pathway that may contribute to the development of tau pathology.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-016-0292-9) contains supplementary material, which is available to authorized users.

Targeting LAMP2 in human cerebrospinal fluid with a combination of immunopurification and high resolution parallel reaction monitoring mass spectrometry

Background

Alzheimer’s disease is the most common form of dementia. An increasing body of evidence suggests that endo-lysosomal dysfunction is a pathogenic mechanism of Alzheimer’s disease. Thus there is a potential for proteins involved in the normal function of endo-lysosomal vesicles to act as biomarkers of disease. Herein we focused on the lysosomal protein LAMP2 that is involved in chaperone mediated autophagy.

Results

Using a combination of immunoprecipitation, digestion and nano-liquid chromatography tandem mass spectrometry we targeted and identified six tryptic LAMP2 peptides in human cerebrospinal fluid. Employing the identified proteotypic tryptic peptides a hybrid immunoprecipitation high resolution parallel reaction monitoring mass spectrometric method was developed for the relative quantitation of LAMP2. The method was evaluated in a number of experiments which defined the overall methodological as well as the analytical micro-liquid chromatography mass spectrometric intra- and inter-day variability. We identified an overall methodological peptide dependent intra-day variability of 8–16 %. The inter-day experiments showed similar results. The analytical contribution to the variation was minor with a coefficient of variation of 0.5–2.1 %, depending on the peptide. Using the developed method, with defined and limited variability, we report increased cerebrospinal fluid levels of three LAMP2 peptides in Alzheimer’s disease subjects (n = 14), as compared to non-Alzheimer’s disease controls (n = 14).

Conclusion

Altered LAMP2 levels in cerebrospinal fluid may indicate endo-lysosomal dysfunction in Alzheimer’s disease. However, further studies in larger cohorts comprised of well-defined patient materials are required. We here present a tool which can be used for exploring the relevance of the level of LAMP2 as a potential measure of lysosomal dysfunction in Alzheimer’s disease or other neurodegenerative diseases.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-016-9104-2) contains supplementary material, which is available to authorized users.

Lysosomal dysfunction and impaired autophagy in a novel mouse model deficient for the lysosomal membrane protein Cln7

CLN7 disease is an autosomal recessive, childhood-onset neurodegenerative lysosomal storage disorder caused by the defective lysosomal membrane protein CLN7. We have disrupted the Cln7/Mfsd8 gene in mice by targeted deletion of exon 2 generating a novel knockout (KO) mouse model for CLN7 disease, which recapitulates key features of human CLN7 disease pathology. Cln7 KO mice showed increased mortality and a neurological phenotype including hind limb clasping and myoclonus. Lysosomal dysfunction in the brain of mutant mice was shown by the storage of autofluorescent lipofuscin-like lipopigments, subunit c of mitochondrial ATP synthase and saposin D and increased expression of lysosomal cathepsins B, D and Z. By immunohistochemical co-stainings, increased cathepsin Z expression restricted to Cln7-deficient microglia and neurons was found. Ultrastructural analyses revealed large storage bodies in Purkinje cells of Cln7 KO mice containing inclusions composed of irregular, curvilinear and rectilinear profiles as well as fingerprint profiles. Generalized astrogliosis and microgliosis in the brain preceded neurodegeneration in the olfactory bulb, cerebral cortex and cerebellum in Cln7 KO mice. Increased levels of LC3-II and the presence of neuronal p62- and ubiquitin-positive protein aggregates suggested that impaired autophagy represents a major pathomechanism in the brain of Cln7 KO mice. The data suggest that loss of the putative lysosomal transporter Cln7 in the brain leads to lysosomal dysfunction, impaired constitutive autophagy and neurodegeneration late in disease.

From rare to common and back again: 60years of lysosomal dysfunction

Sixty years after its discovery, the lysosome is no longer considered as cell's waste bin but as an organelle playing a central role in cell metabolism. Besides its well known association with lysosomal storage disorders (mostly rare and life-threatening diseases), recent data have shown that the lysosome is also a player in some of the most common conditions of our time; and, perhaps even most important, it is not only a target for orphan drugs (rare disease therapeutic approaches) but also a putative target to treat patients suffering from common complex diseases worldwide. Here we review the striking associations linking rare lysosomal storage disorders such as the well-known Gaucher disease, or even the recently discovered, extremely rare Neuronal Ceroid Lipofuscinosis-11 and some of the most frequent, multifaceted and complex disorders of modern society such as cancer, Parkinson's disease and frontotemporal lobar degeneration.

Hippocampal plasticity during the progression of Alzheimer's disease

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aβ) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

Cln1 gene disruption in mice reveals a common pathogenic link between two of the most lethal childhood neurodegenerative lysosomal storage disorders

Neurodegeneration is a devastating manifestation in the majority of >50 lysosomal storage disorders (LSDs). Neuronal ceroid lipofuscinoses (NCLs) are the most common childhood neurodegenerative LSDs. Mutations in 13 different genes (called CLNs) underlie various types of NCLs, of which the infantile NCL (INCL) and congenital NCL (CNCL) are the most lethal. Although inactivating mutations in the CLN1 gene encoding palmitoyl-protein thioesterase-1 (PPT1) cause INCL, those in the CLN10 gene encoding cathepsin D (CD) underlie CNCL. PPT1 is a lysosomal thioesterase that cleaves the thioester linkage in S-acylated proteins required for their degradation by lysosomal hydrolases like CD. Thus, PPT1 deficiency causes lysosomal accumulation of these lipidated proteins (major constituents of ceroid) leading to INCL. We sought to determine whether there is a common pathogenic link between INCL and CNCL. Using biochemical, histological and confocal microscopic analyses of brain tissues and cells from Cln1(-/-) mice that mimic INCL, we uncovered that Cln10/CD is overexpressed. Although synthesized in the endoplasmic reticulum, the CD-precursor protein (pro-CD) is transported through endosome to the lysosome where it is proteolytically processed to enzymatically active-CD. We found that despite Cln10 overexpression, the maturation of pro-CD to enzymatically active-CD in lysosome was disrupted. This defect impaired lysosomal degradative function causing accumulation of undegraded cargo in lysosome leading to INCL. Notably, treatment of intact Cln1(-/-) mice as well as cultured brain cells derived from these animals with a thioesterase-mimetic small molecule, N-tert-butyl-hydroxylamine, ameliorated the CD-processing defect. Our findings are significant in that they define a pathway in which Cln1 mutations disrupt the maturation of a major degradative enzyme in lysosome contributing to neuropathology in INCL and suggest that lysosomal CD deficiency is a common pathogenic link between INCL and CNCL.