Altered levels and distribution of amyloid precursor protein and its processing enzymes in Niemann-Pick type C1-deficient mouse brains

A Potential Role for the Amyloid Precursor Protein in the Regulation of Interferon Signaling, Cholesterol Homeostasis, and Tau Phosphorylation in Niemann-Pick Disease Type C

Niemann-Pick disease type C (NPC) is a rare and fatal neurological disorder caused by mutations in Npc1 or Npc2, with Npc1 accounting for 95% of cases. These mutations result in the functional loss of their respective proteins, causing cellular abnormalities characterized by disrupted lipid dysregulation, calcium dysfunction, elevated damage associated molecular patterns (DAMPs), and a pro-inflammatory environment. This cellular pathology ultimately triggers neurodegeneration, with the cerebellum being the earliest and most affected region. We have recently shown atypical activation of interferon signaling in the presymptomatic Npc1-/- mouse cerebellum and, to a lesser extent, in the cerebral cortex. In addition, we reported that the Amyloid Precursor Protein (APP) is an NPC disease modifier. Loss of APP function leads to widespread neurodegeneration in the NPC brain, including exacerbated interferon signaling in the cerebellum. To better understand the role of APP as a disease modifier throughout the NPC brain, here we carried out a transcriptomic analysis of the cerebral cortex and cerebellum from 3-week-old Npc1-/- mice as well as age-matched controls in the presence and absence of APP. We report differential effects of APP loss of function in the cerebral cortex and cerebellum, including cholesterol and tau dysregulation, in both brain regions. Our findings demonstrate a novel link between APP loss and early pathogenic mechanisms in NPC.

Comparative Hippocampal Proteome and Phosphoproteome in a Niemann-Pick, Type C1 Mouse Model Reveal Insights into Disease Mechanisms

Niemann-Pick disease, type C (NPC) is a neurodegenerative, lysosomal storage disorder in individuals carrying two mutated copies of either the NPC1 or NPC2 gene. Consequently, impaired cholesterol recycling and an array of downstream events occur. Interestingly, in NPC, the hippocampus displays lysosomal lipid storage but does not succumb to progressive neurodegeneration as significantly as other brain regions. Since defining the neurodegeneration mechanisms in this disease is still an active area of research, we use mass spectrometry to analyze the overall proteome and phosphorylation pattern changes in the hippocampal region of a murine model of NPC. Using 3 week old mice representing an early disease time point, we observed changes in the expression of 47 proteins, many of which are consistent with the previous literature. New to this study, changes in members of the SNARE complex, including STX7, VTI1B, and VAMP7, were identified. Furthermore, we identified that phosphorylation of T286 on CaMKIIα and S1303 on NR2B increased in mutant animals, even at the late stage of the disease. These phosphosites are crucial to learning and memory and can trigger neuronal death by altering protein-protein interactions.

Reexamining the Causes and Effects of Cholesterol Deposition in the Brains of Patients with Alzheimer's Disease

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. Numerous studies have shown that imbalances in cholesterol homeostasis in the brains of AD patients precede the onset of clinical symptoms. In addition, cholesterol deposition has been observed in the brains of AD patients even though peripheral cholesterol does not enter the brain through the blood‒brain barrier (BBB). Studies have demonstrated that cholesterol metabolism in the brain is associated with many pathological conditions, such as amyloid beta (Aβ) production, Tau protein phosphorylation, oxidative stress, and inflammation. In 2022, some scholars put forward a new hypothesis of AD: the disease involves lipid invasion and its exacerbation of the abnormal metabolism of cholesterol in the brain. In this review, by discussing the latest research progress, the causes and effects of cholesterol retention in the brains of AD patients are analyzed and discussed. Additionally, the possible mechanism through which AD may be improved by targeting cholesterol is described. Finally, we propose that improving the impairments in cholesterol removal observed in the brains of AD patients, instead of further reducing the already impaired cholesterol synthesis in the brain, may be the key to preventing cholesterol deposition and improving the corresponding pathological symptoms.

Bisphenol A has a sex-dependent disruptive effect on hepatic lipid metabolism in zebrafish

Bisphenol A (BPA) is an endocrine disruptor that has adverse effects on lipid metabolism. However, most of the current studies on the effects of BPA on lipid metabolism in fish have focused on middle- and short-term exposure tests. The aim of this study was to investigate the effects of long-term BPA exposure on liver lipid metabolism in zebrafish. Post-fertilization embryos were exposed to environmentally relevant concentrations of BPA for 120 days, and the changes in triglyceride (TG), total cholesterol (TC) levels, and gene expression related to liver lipid metabolism were investigated in both male and female fish. The results showed that long-term exposure to BPA led to lipid deposition in liver, and there was a sex difference. In the liver of female fish, there was higher lipid transport and synthesis at low concentration of BPA, while overall metabolic levels were increased at high concentration of BPA. In contrast, BPA showed a dose-dependent effect on the lipid deposition in male fish. The expression of mRNA of TG transport-related and lipid synthesis-related genes was significantly up-regulated and the expression of genes related to lipid catabolism, was significantly down-regulated with increasing BPA dose. Taken together, our results indicate that long-term exposure to BPA can increase lipid deposition in a gender-specific manner. This may be due to the different responses of lipid metabolism related genes to BPA in male and female zebrafish. These results will provide a new reference for a deeper understanding of the ecotoxicological effects of BPA on aquatic animals.

The Niemann-Pick type diseases – A synopsis of inborn errors in sphingolipid and cholesterol metabolism

Disturbances of lipid homeostasis in cells provoke human diseases. The elucidation of the underlying mechanisms and the development of efficient therapies represent formidable challenges for biomedical research. Exemplary cases are two rare, autosomal recessive, and ultimately fatal lysosomal diseases historically named "Niemann-Pick" honoring the physicians, whose pioneering observations led to their discovery. Acid sphingomyelinase deficiency (ASMD) and Niemann-Pick type C disease (NPCD) are caused by specific variants of the sphingomyelin phosphodiesterase 1 (SMPD1) and NPC intracellular cholesterol transporter 1 (NPC1) or NPC intracellular cholesterol transporter 2 (NPC2) genes that perturb homeostasis of two key membrane components, sphingomyelin and cholesterol, respectively. Patients with severe forms of these diseases present visceral and neurologic symptoms and succumb to premature death. This synopsis traces the tortuous discovery of the Niemann-Pick diseases, highlights important advances with respect to genetic culprits and cellular mechanisms, and exposes efforts to improve diagnosis and to explore new therapeutic approaches.

The role of DHCR24 in the pathogenesis of AD: re-cognition of the relationship between cholesterol and AD pathogenesis

Previous studies show that 3β-hydroxysterol-Δ24 reductase (DHCR24) has a remarked decline in the brain of AD patients. In brain cholesterol synthetic metabolism, DHCR24 is known as the heavily key synthetase in cholesterol synthesis. Moreover, mutations of DHCR24 gene result in inhibition of the enzymatic activity of DHCR24, causing brain cholesterol deficiency and desmosterol accumulation. Furthermore, in vitro studies also demonstrated that DHCR24 knockdown lead to the inhibition of cholesterol synthesis, and the decrease of plasma membrane cholesterol and intracellular cholesterol level. Obviously, DHCR24 could play a crucial role in maintaining cholesterol homeostasis via the control of cholesterol synthesis. Over the past two decades, accumulating data suggests that DHCR24 activity is downregulated by major risk factors for AD, suggesting a potential link between DHCR24 downregulation and AD pathogenesis. Thus, the brain cholesterol loss seems to be induced by the major risk factors for AD, suggesting a possible causative link between brain cholesterol loss and AD. According to previous data and our study, we further found that the reduced cholesterol level in plasma membrane and intracellular compartments by the deficiency of DHCR24 activity obviously was involved in β-amyloid generation, tau hyperphosphorylation, apoptosis. Importantly, increasing evidences reveal that the brain cholesterol loss and lipid raft disorganization are obviously linked to neuropathological impairments which are associated with AD pathogenesis. Therefore, based on previous data and research on DHCR24, we suppose that the brain cholesterol deficiency/loss might be involved in the pathogenesis of AD.

Implications of exosomes derived from cholesterol-accumulated astrocytes in Alzheimer's disease pathology

Amyloid β (Aβ) peptides generated from the amyloid precursor protein (APP) play a critical role in the development of Alzheimer's disease (AD) pathology. Aβ-containing neuronal exosomes, which represent a novel form of intercellular communication, have been shown to influence the function/vulnerability of neurons in AD. Unlike neurons, the significance of exosomes derived from astrocytes remains unclear. In this study, we evaluated the significance of exosomes derived from U18666A-induced cholesterol-accumulated astrocytes in the development of AD pathology. Our results show that cholesterol accumulation decreases exosome secretion, whereas lowering cholesterol increases exosome secretion, from cultured astrocytes. Interestingly, exosomes secreted from U18666A-treated astrocytes contain higher levels of APP, APP-C-terminal fragments, soluble APP, APP secretases and Aβ1-40 than exosomes secreted from control astrocytes. Furthermore, we show that exosomes derived from U18666A-treated astrocytes can lead to neurodegeneration, which is attenuated by decreasing Aβ production or by neutralizing exosomal Aβ peptide with an anti-Aβ antibody. These results, taken together, suggest that exosomes derived from cholesterol-accumulated astrocytes can play an important role in trafficking APP/Aβ peptides and influencing neuronal viability in the affected regions of the AD brain.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

Identification of Novel Pathways Associated with Patterned Cerebellar Purkinje Neuron Degeneration in Niemann-Pick Disease, Type C1

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by progressive cerebellar ataxia. In NPC1, a defect in cholesterol transport leads to endolysosomal storage of cholesterol and decreased cholesterol bioavailability. Purkinje neurons are sensitive to the loss of NPC1 function. However, degeneration of Purkinje neurons is not uniform. They are typically lost in an anterior-to-posterior gradient with neurons in lobule X being resistant to neurodegeneration. To gain mechanistic insight into factors that protect or potentiate Purkinje neuron loss, we compared RNA expression in cerebellar lobules III, VI, and X from control and mutant mice. An unexpected finding was that the gene expression differences between lobules III/VI and X were more pronounced than those observed between mutant and control mice. Functional analysis of genes with anterior to posterior gene expression differences revealed an enrichment of genes related to neuronal cell survival within the posterior cerebellum. This finding is consistent with the observation, in multiple diseases, that posterior Purkinje neurons are, in general, resistant to neurodegeneration. To our knowledge, this is the first study to evaluate anterior to posterior transcriptome-wide changes in gene expression in the cerebellum. Our data can be used to not only explore potential pathological mechanisms in NPC1, but also to further understand cerebellar biology.

U18666A inhibits classical swine fever virus replication through interference with intracellular cholesterol trafficking

The level of cholesterol in host cells has been demonstrated to affect viral infection. Our previous studies showed that cholesterol-rich membrane rafts mediated the entry of classical swine fever virus (CSFV) into PK-15 or 3D4/21 cells, but the role of cholesterol post entry was still not clear. In this study, we found that CSFV replication before fusion was affected when the cholesterol trafficking in infected cells was disrupted using a cholesterol transport inhibitor, U18666A. Our data showed that U18666A affected both the fusion and replication steps in the life cycle of the virus, but not its binding and entry steps. The subsequent experiments confirmed that niemann-pick C1 (NPC1), a lysosomal membrane protein that helps cholesterol to leave the lysosome, was affected by U18666A, which led to the accumulation of cholesterol in lysosomes and inhibition of CSFV replication. Imipramine, a cationic hydrophobic amine similar to U18666A, also inhibited CSFV replication via similar mechanism. Surprisingly, the antiviral effect of U18666A was restored by the histone deacetylase inhibitor (HDACi), Vorinostat, which suggested that HDACi reverted the dysfunction of NPC1, and intra-cellular cholesterol accumulation disappeared and CSFV replicability resumed. Together, these data indicated that CSFV transformed from early endosome and late endosome into lysosome after endocytosis for further replication and that U18666A was a potential drug candidate for anti-pestivirus treatment.

Hepatic and neuronal phenotype of NPC1-/- mice

Niemann-Pick type C disease (NPC) is a fatal autosomal recessive disorder characterized by a defect in the intracellular transport of lipoproteins leading to the accumulation of lipids in diverse tissues. A visceral and neuronal phenotype mimicking human NPC1 disease has been described in NPC1 mutant mice. These mice are by now the most widely used NPC1 rodent model to study NPC and developmental compounds against this devastating disease. Here we characterized NPC1-/- mice for their hepatic and neuronal phenotype to confirm the stability of the phenotype, provide a characterization of disease progression and pinpoint the age of robust phenotype onset. Animals of 4-10 weeks of age were analyzed for general health, motor deficits as well as hepatic and neuronal alterations with a special focus on cerebellar pathology. Our results show that NPC1-/- mice have a reduced general health at the age of 9-10 weeks. Robust motor deficits can be observed even earlier at 8 weeks of age. Hepatic changes included increased organ weight and cholesterol levels at 6 weeks of age accompanied by severely increased liver enzyme levels. Analysis of NPC1-/- brain pathology showed decreased cholesterol and increased Aβ levels in the hippocampus at the age of 6 weeks. Further analysis revealed a decrease of the cytokine IL-12p70 in the cerebellum along with a very early increase of astrocytosis. Hippocampal IL-12p70 levels were increased at the age of 6 weeks followed by increased activated microglia levels. By the age of 10 weeks, also cerebellar Aβ levels were increased along with strongly reduced Calbindin D-28k levels. Our results validate and summarize the progressive development of the hepatic and neuronal phenotype of NPC1-/- mice that starts with cerebellar astrocytosis, making this mouse model a valuable tool for the development of new compounds against NPC.

The sterol regulatory element-binding protein 2 is dysregulated by tau alterations in Alzheimer disease

Disturbed neuronal cholesterol homeostasis has been observed in Alzheimer disease (AD) and contributes to the pathogenesis of AD. As the master switch of cholesterol biosynthesis, the sterol regulatory element-binding protein 2 (SREBP-2) translocates to the nucleus after cleavage/activation, but its expression and activation have not been studied in AD which is the focus of the current study. We found both a significant decrease in the nuclear translocation of N-terminal SREBP-2 accompanied by a significant accumulation of C-terminal SREBP-2 in NFT-containing pyramidal neurons in AD. N-terminal- SREBP-2 is also found in dystrophic neurites around plaques in AD brain. Western blot confirmed a significantly reduced nuclear translocation of mature SREBP-2 (mSREBP-2) in AD brain. Interestingly, reduced nuclear mSREBP-2 was only found in animal models of tauopathies such as 3XTg AD mice and P301L Tau Tg mice but not in CRND8 APP transgenic mice, suggesting that tau alterations likely are involved in the changes of mSREBP-2 distribution and activation in AD. Altogether, our study demonstrated disturbed SREBP-2 signaling in AD and related models, and proved for the first time that tau alterations contribute to disturbed cholesterol homeostasis in AD likely through modulation of nuclear mSREBP-2 translocation.

Kainate Receptor Activation Enhances Amyloidogenic Processing of APP in Astrocytes

Kainic acid (KA) is an analogue of the excitatory neurotransmitter glutamate that, when injected systemically into adult rats, can trigger seizures and progressive neuronal loss in a manner that mirrors the neuropathology of human mesial temporal lobe epilepsy. However, biomolecular mechanisms responsible for the neuronal loss that occurs as a consequence of this treatment remains elusive. We have recently reported that toxicity induced by KA can partly be mediated by astrocyte-derived amyloid β (Aβ) peptides, which are critical in the development of Alzheimer's disease (AD). Nonetheless, little is known how KA can influence amyloid precursor protein (APP) levels and processing in astrocytes. Thus, in the present study using human U-373 astrocytoma and rat primary astrocytes, we evaluated the role of KA on APP metabolism. Our results revealed that KA treatment increased the levels of APP and its cleaved products (α-/β-CTFs) in cultured U-373 astrocytoma and primary astrocytes, without altering the cell viability. The cellular and secretory levels of Aβ_1-40/Aβ_1-42 were markedly increased in KA-treated astrocytes. We also demonstrated that the steady-state levels of APP-secretases were not altered but the activity of γ-secretase is enhanced in KA-treated U-373 astrocytoma. Furthermore, using selective receptor antagonists, we showed that the effects of KA is mediated by activation of kainate receptors and not NMDA or AMPA receptors. These results suggest that KA can enhance amyloidogenic processing of APP by activating its own receptor leading to increased production/secretion of Aβ-related peptides from activated astrocytes which may contribute to the pathogenesis of temporal lobe epilepsy.

BACE1-cleavage of Sez6 and Sez6L is elevated in Niemann-Pick type C disease mouse brains

It is intriguing that a rare, inherited lysosomal storage disorder Niemann-Pick type C (NPC) shares similarities with Alzheimer’s disease (AD). We have previously reported an enhanced processing of β-amyloid precursor protein (APP) by β-secretase (BACE1), a key enzyme in the pathogenesis of AD, in NPC1-null cells. In this work, we characterized regional and temporal expression and processing of the recently identified BACE1 substrates seizure protein 6 (Sez6) and seizure 6-like protein (Sez6L), and APP, in NPC1^-/- (NPC1) and NPC1^+/+ (wt) mouse brains. We analysed 4-weeks old brains to detect the earliest changes associated with NPC, and 10-weeks of age to identify changes at terminal disease stage. Sez6 and Sez6L were selected due to their predominant cleavage by BACE1, and their potential role in synaptic function that may contribute to presentation of seizures and/or motor impairments in NPC patients. While an enhanced BACE1-cleavage of all three substrates was detected in NPC1 vs. wt-mouse brains at 4-weeks of age, at 10-weeks increased proteolysis by BACE1 was observed for Sez6L in the cortex, hippocampus and cerebellum of NPC1-mice. Interestingly, both APP and Sez6L were found to be expressed in Purkinje neurons and their immunostaining was lost upon Purkinje cell neurodegeneration in 10-weeks old NPC1 mice. Furthermore, in NPC1- vs. wt-mouse primary cortical neurons, both Sez6 and Sez6L showed increased punctuate staining within the endolysosomal pathway as well as increased Sez6L and BACE1-positive puncta. This indicates that a trafficking defect within the endolysosomal pathway may play a key role in enhanced BACE1-proteolysis in NPC disease. Overall, our findings suggest that enhanced proteolysis by BACE1 could be a part of NPC disease pathogenesis. Understanding the basic biology of BACE1 and the functional impact of cleavage of its substrates is important to better evaluate the therapeutic potential of BACE1 against AD and, possibly, NPC disease.

A role for astrocyte-derived amyloid β peptides in the degeneration of neurons in an animal model of temporal lobe epilepsy

Kainic acid, an analogue of the excitatory neurotransmitter glutamate, can trigger seizures and neurotoxicity in the hippocampus and other limbic structures in a manner that mirrors the neuropathology of human temporal lobe epilepsy (TLE). However, the underlying mechanisms associated with the neurotoxicity remain unclear. Since amyloid-β (Aβ) peptides, which are critical in the development of Alzheimer's disease, can mediate toxicity by activating glutamatergic NMDA receptors, it is likely that the enhanced glutamatergic transmission that renders hippocampal neurons vulnerable to kainic acid treatment may involve Aβ peptides. Thus, we seek to establish what role Aβ plays in kainic acid-induced toxicity using in vivo and in vitro paradigms. Our results show that systemic injection of kainic acid to adult rats triggers seizures, gliosis and loss of hippocampal neurons, along with increased levels/processing of amyloid precursor protein (APP), resulting in the enhanced production of Aβ-related peptides. The changes in APP levels/processing were evident primarily in activated astrocytes, implying a role for astrocytic Aβ in kainic acid-induced toxicity. Accordingly, we showed that treating rat primary cultured astrocytes with kainic acid can lead to increased Aβ production/secretion without any compromise in cell viability. Additionally, we revealed that kainic acid reduces neuronal viability more in neuronal/astrocyte co-cultures than in pure neuronal culture, and this is attenuated by precluding Aβ production. Collectively, these results indicate that increased production/secretion of Aβ-related peptides from activated astrocytes can contribute to neurotoxicity in kainic acid-treated rats. Since kainic acid administration can lead to neuropathological changes resembling TLE, it is likely that APP/Aβ peptides derived from astrocytes may have a role in TLE pathogenesis.

Endosomal-Lysosomal Cholesterol Sequestration by U18666A Differentially Regulates Amyloid Precursor Protein (APP) Metabolism in Normal and APP-Overexpressing Cells

Amyloid β (Aβ) peptide, derived from amyloid precursor protein (APP), plays a critical role in the development of Alzheimer's disease. Current evidence indicates that altered levels or subcellular distribution of cholesterol can regulate Aβ production and clearance, but it remains unclear how cholesterol sequestration within the endosomal-lysosomal (EL) system can influence APP metabolism. Thus, we evaluated the effects of U18666A, which triggers cholesterol redistribution within the EL system, on mouse N2a cells expressing different levels of APP in the presence or absence of extracellular cholesterol and lipids provided by fetal bovine serum (FBS). Our results reveal that U18666A and FBS differentially increase the levels of APP and its cleaved products, the α-, β-, and η-C-terminal fragments, in N2a cells expressing normal levels of mouse APP (N2awt), higher levels of human wild-type APP (APPwt), or "Swedish" mutant APP (APPsw). The cellular levels of Aβ_1-40/Aβ_1-42 were markedly increased in U18666A-treated APPwt and APPsw cells. Our studies further demonstrate that APP and its cleaved products are partly accumulated in the lysosomes, possibly due to decreased clearance. Finally, we show that autophagy inhibition plays a role in mediating U18666A effects. Collectively, these results suggest that altered levels and distribution of cholesterol and lipids can differentially regulate APP metabolism depending on the nature of APP expression.

Bisphosphonates: Future perspective for neurological disorders

Neurodegenerative disorders and osteoporosis share some common underlying pathological features including calcium overload, accumulation of toxic chemicals, inflammation and impaired protein prenylation by isoprenoids (farnesyl pyrophosphate and geranylgeranyl pyrophosphate) appear later stage of life. Substantial number of pre-clinical and clinical reports as well as in vitro data univocally acknowledged the negative impact of altered post-translational modification (prenylation) of proteins like small GTPases (Rffhes, Rho, Rac etc.) and cholesterol levels in both serum and brain on CNS integrity. Bisphosphonates (BPs), referred to as gold standard for osteoporosis treatment, have well established role in attenuation of bone resorption and osteoclast apoptosis by inhibition of farnesyl pyrophosphate synthase enzyme (FPPS) in mevalonate pathway. BPs mainly nitrogen containing BPs (NBPs) have potential to offer new therapeutic targets for neurological disorders and received increasing attention in recent years. A year back clinical and pre-clinical studies revealed that NBPs have the potential to alleviate the symptoms of neurological disorders like brain calcification, Alzheimer's disease and Huntington's disease by targeting mevalonate pathway. Though these drugs have well developed role in inhibition of isoprenoids synthesis, these were demonstrated to inhibit acetyl cholinesterase enzyme and cholesterol synthesis in brain that are considered as the critical factors for impairment of cognitive functions which is the hallmark of several neurological disorders. Still the current understanding of BPs' effect in CNS is limited due to lack of studies focusing the molecular and cellular mechanism. The present review aims to reveal the updated discussion on the mechanism contributing BPs' effect in CNS disorders.

Effects of cholesterol transport inhibitor U18666A on APP metabolism in rat primary astrocytes

Amyloid β (Aβ) peptides generated from the amyloid precursor protein (APP) play an important role in the degeneration of neurons and development of Alzheimer's disease (AD). Current evidence indicates that high levels of cholesterol-which increase the risk of developing AD-can influence Aβ production in neurons. However, it remains unclear how altered level/subcellular distribution of cholesterol in astrocytes can influence APP metabolism. In this study, we evaluated the effects of cholesterol transport inhibitor U18666A-a class II amphiphile that triggers redistribution of cholesterol within the endosomal-lysosomal (EL) system-on APP levels and metabolism in rat primary cultured astrocytes. Our results revealed that U18666A increased the levels of the APP holoprotein and its cleaved products (α-/β-/η-CTFs) in cultured astrocytes, without altering the total levels of cholesterol or cell viability. The cellular levels of Aβ1-40 were also found to be markedly increased, while secretory levels of Aβ1-40 were decreased in U18666A-treated astrocytes. We further report a corresponding increase in the activity of the enzymes regulating APP processing, such as α-secretase, β-secretase, and γ-secretase as a consequence of U18666A treatment. Additionally, APP-cleaved products are partly accumulated in the lysosomes following cholesterol sequestration within EL system possibly due to decreased clearance. Interestingly, serum delipidation attenuated enhanced levels of APP and its cleaved products following U18666A treatment. Collectively, these results suggest that cholesterol sequestration within the EL system in astrocytes can influence APP metabolism and the accumulation of APP-cleaved products including Aβ peptides, which can contribute to the development of AD pathology.

The lysosomal storage disease continuum with ageing-related neurodegenerative disease

Lysosomal storage diseases and diseases of ageing share many features both at the physiological level and with respect to the mechanisms that underlie disease pathogenesis. Although the exact pathophysiology is not exactly the same, it is astounding how many similar pathways are altered in all of these diseases. The aim of this review is to provide a summary of the shared disease mechanisms, outlining the similarities and differences and how genetics, insight into rare diseases and functional research has changed our perspective on the causes underlying common diseases of ageing. The lysosome should no longer be considered as just the stomach of the cell or as a suicide bag, it has an emerging role in cellular signalling, nutrient sensing and recycling. The lysosome is of fundamental importance in the pathophysiology of diseases of ageing and by comparing against the LSDs we not only identify common pathways but also therapeutic targets so that ultimately more effective treatments can be developed for all neurodegenerative diseases.

Lysosomal Storage Diseases-Regulating Neurodegeneration

Autophagy is a complex pathway regulated by numerous signaling events that recycles macromolecules and can be perturbed in lysosomal storage diseases (LSDs). The concept of LSDs, which are characterized by aberrant, excessive storage of cellular material in lysosomes, developed following the discovery of an enzyme deficiency as the cause of Pompe disease in 1963. Great strides have since been made in better understanding the biology of LSDs. Defective lysosomal storage typically occurs in many cell types, but the nervous system, including the central nervous system and peripheral nervous system, is particularly vulnerable to LSDs, being affected in two-thirds of LSDs. This review provides a summary of some of the better characterized LSDs and the pathways affected in these disorders.

Neuronal-Targeted TFEB Accelerates Lysosomal Degradation of APP, Reducing Aβ Generation and Amyloid Plaque Pathogenesis

In AD, an imbalance between Aβ production and removal drives elevated brain Aβ levels and eventual amyloid plaque deposition. APP undergoes nonamyloidogenic processing via α-cleavage at the plasma membrane, amyloidogenic β- and γ-cleavage within endosomes to generate Aβ, or lysosomal degradation in neurons. Considering multiple reports implicating impaired lysosome function as a driver of increased amyloidogenic processing of APP, we explored the efficacy of targeting transcription factor EB (TFEB), a master regulator of lysosomal pathways, to reduce Aβ levels. CMV promoter-driven TFEB, transduced via stereotactic hippocampal injections of adeno-associated virus particles in APP/PS1 mice, localized primarily to neuronal nuclei and upregulated lysosome biogenesis. This resulted in reduction of APP protein, the α and β C-terminal APP fragments (CTFs), and in the steady-state Aβ levels in the brain interstitial fluid. In aged mice, total Aβ levels and amyloid plaque load were selectively reduced in the TFEB-transduced hippocampi. TFEB transfection in N2a cells stably expressing APP695, stimulated lysosome biogenesis, reduced steady-state levels of APP and α- and β-CTFs, and attenuated Aβ generation by accelerating flux through the endosome-lysosome pathway. Cycloheximide chase assays revealed a shortening of APP half-life with exogenous TFEB expression, which was prevented by concomitant inhibition of lysosomal acidification. These data indicate that TFEB enhances flux through lysosomal degradative pathways to induce APP degradation and reduce Aβ generation. Activation of TFEB in neurons is an effective strategy to attenuate Aβ generation and attenuate amyloid plaque deposition in AD.

SIGNIFICANCE STATEMENT

A key driver for AD pathogenesis is the net balance between production and clearance of Aβ, the major component of amyloid plaques. Here we demonstrate that lysosomal degradation of holo-APP influences Aβ production by limiting the availability of APP for amyloidogenic processing. Using viral gene transfer of transcription factor EB (TFEB), a master regulator of lysosome biogenesis in neurons of APP/PS1 mice, steady-state levels of APP were reduced, resulting in decreased interstitial fluid Aβ levels and attenuated amyloid deposits. These effects were caused by accelerated lysosomal degradation of endocytosed APP, reflected by reduced APP half-life and steady-state levels in TFEB-expressing cells, with resultant decrease in Aβ production and release. Additional studies are needed to explore the therapeutic potential of this approach.

The Effect of Aβ₁₋₄₂ Oligomers on APP Processing and Aβ₁₋₄₀ Generation in Cultured U-373 Astrocytes

BACKGROUND

Amyloid-β (Aβ) peptides are a family of proteins that are considered to be a principal aspect of Alzheimer's disease (AD), the most common cause of senile dementia affecting elderly individuals. These peptides result from the proteolytic processing of amyloid precursor protein (APP) by sequential cleavage mediated via β- and x03B3;-secretases. Evidence suggests that an overproduction and/or a lack of degradation may increase brain Aβ levels which, in turn, contribute to neuronal loss and development of AD.

OBJECTIVES

In this study, we seek to determine what effect Aβ has on APP processing in cultured astrocytes.

METHODS

Using the human astrocytoma cell line U-373, we investigated the effects induced by oligomeric Aβ1-42 treatment on the cellular levels/expression of APP and its products, C-terminal fragments αCTF and βCTF, and Aβ1-40. In conjunction with these experiments, we examined the relative levels and activity of β- and x03B3;-secretases in Aβ-treated astrocytes.

RESULTS

We report here that Aβ1-42 treatment of astrocytes increased the expression of APP and its cleaved products including Aβ1-40 in a time-dependent manner.

CONCLUSIONS

These results suggest that activated astrocytes can contribute to the development of AD by enhancing levels and processing of APP leading to an increased production/secretion of Aβ-related peptides.

APP overexpression in the absence of NPC1 exacerbates metabolism of amyloidogenic proteins of Alzheimer's disease

Amyloid-β (Aβ) peptides originating from β-amyloid precursor protein (APP) are critical in Alzheimer's disease (AD). Cellular cholesterol levels/distribution can regulate production and clearance of Aβ peptides, albeit with contradictory outcomes. To better understand the relationship between cholesterol homeostasis and APP/Aβ metabolism, we have recently generated a bigenic ANPC mouse line overexpressing mutant human APP in the absence of Niemann-Pick type C-1 protein required for intracellular cholesterol transport. Using this unique bigenic ANPC mice and complementary stable N2a cells, we have examined the functional consequences of cellular cholesterol sequestration in the endosomal-lysosomal system, a major site of Aβ production, on APP/Aβ metabolism and its relation to neuronal viability. Levels of APP C-terminal fragments (α-CTF/β-CTF) and Aβ peptides, but not APP mRNA/protein or soluble APPα/APPβ, were increased in ANPC mouse brains and N2a-ANPC cells. These changes were accompanied by reduced clearance of peptides and an increased level/activity of γ-secretase, suggesting that accumulation of APP-CTFs is due to decreased turnover, whereas increased Aβ levels may result from a combination of increased production and decreased turnover. APP-CTFs and Aβ peptides were localized primarily in early-/late-endosomes and to some extent in lysosomes/autophagosomes. Cholesterol sequestration impaired endocytic-autophagic-lysosomal, but not proteasomal, clearance of APP-CTFs/Aβ peptides. Moreover, markers of oxidative stress were increased in vulnerable brain regions of ANPC mice and enhanced β-CTF/Aβ levels increased susceptibility of N2a-ANPC cells to H2O2-induced toxicity. Collectively, our results show that cellular cholesterol sequestration plays a key role in APP/Aβ metabolism and increasing neuronal vulnerability to oxidative stress in AD-related pathology.

Cholesterol-induced astrocyte activation is associated with increased amyloid precursor protein expression and processing

Cholesterol is essential for maintaining lipid raft integrity and has been regarded as a crucial regulatory factor for amyloidogenesis in Alzheimer's disease (AD). The vast majority of studies on amyloid precursor protein (APP) metabolism and amyloid β-protein (Aβ) production have focused on neurons. The role of astrocytes remains largely unexplored, despite the presence of activated astrocytes in the brains of most patients with AD and in transgenic models of the disease. The role of cholesterol in Aβ production has been thoroughly studied in neurons and attributed to the participation of lipid rafts in APP metabolism. Thus, in this study, we analyzed the effect of cholesterol loading in astrocytes and analyzed the expression and processing of APP. We found that cholesterol exposure induced astrocyte activation, increased APP content, and enhanced the interaction of APP with BACE-1. These effects were associated with an enrichment of ganglioside GM1-cholesterol patches in the astrocyte membrane and with increased ROS production. GLIA 2015;63:2010-2022.

Astrocytes and lysosomal storage diseases

Lysosomal storage diseases (LSDs) encompass a wide range of disorders characterized by inborn errors of lysosomal function. The majority of LSDs result from genetic defects in lysosomal enzymes, although some arise from mutations in lysosomal proteins that lack known enzymatic activity. Neuropathological abnormalities are a feature of several LSDs and when severe, represent an important determinant in disease outcome. Glial dysfunction, particularly in astrocytes, is also observed in numerous LSDs and has been suggested to impact neurodegeneration. This review will discuss the potential role of astrocytes in LSDs and highlight the possibility of targeting glia as a beneficial strategy to counteract the neuropathology associated with LSDs.

Bidirectional links between Alzheimer's disease and Niemann-Pick type C disease

Alzheimer's disease (AD) and Niemann-Pick type C (NPC) disease are progressive neurodegenerative diseases with very different epidemiology and etiology. AD is a common cause of dementia with a complex polyfactorial etiology, including both genetic and environmental risk factors, while NPC is a very rare autosomal recessive disease. However, the diseases share some disease-related molecular pathways, including abnormal cholesterol metabolism, and involvement of amyloid-β (Aβ) and tau pathology. Here we review recent studies on these pathological traits, focusing on studies of Aβ and tau pathology in NPC, and the importance of the NPC1 gene in AD. Further studies of similarities and differences between AD and NPC may be useful to increase the understanding of both these devastating neurological diseases.

Metals and cholesterol: two sides of the same coin in Alzheimer's disease pathology

Alzheimer's disease (AD) is a multifactorial neurodegenerative disease. It begins years prior to the onset of clinical symptoms, such as memory loss and cognitive decline. Pathological hallmarks of AD include the accumulation of β-amyloid in plaques and hyperphosphorylated tau in neurofibrillary tangles. Copper, iron, and zinc are abnormally accumulated and distributed in the aging brain. These metal ions can adversely contribute to the progression of AD. Dysregulation of cholesterol metabolism has also been implicated in the development of AD pathology. To date, large bodies of research have been carried out independently to elucidate the role of metals or cholesterol on AD pathology. Interestingly, metals and cholesterol affect parallel molecular and biochemical pathways involved in AD pathology. The possible links between metal dyshomeostasis and altered brain cholesterol metabolism in AD are reviewed.

Hyperactive glial cells contribute to axonal pathologies in the spinal cord of Npc1 mutant mice

Niemann-Pick disease type C1 (NPC1) is a neurodegenerative disease with various progressive pathological features, for example, neuronal loss, dysmyelination, abnormal axon swelling, and gliosis, in the brain. Pathological activation of p38-mitogen-activated protein kinase (MAPK) results in hyperphosphorylation of tau protein, which contributes to the development of neurodegenerative diseases. In this study, axonal varicosities or spheroids and presynaptic aggregates in the spinal cord of the Npc1 mutant mice were found from postnatal day (P) 35 onwards, as indicated by the increased hyperphosphorylated neurofilament and synaptophysin immunoreactivity as well as the findings from electron microscopy. However, activities of astrocytes and microglia in the Npc1 mutant spinal cord were progressively increased earlier from P10 onwards, accompanied by increased expression of interleukin-1β and apolipoprotein E, as well as up-regulated p38-MAPK activity and enhanced phosphorylated tau protein, but not cyclin-dependent kinase 5/p35 complex and glycogen synthase kinase-3β. Taken together, our data suggest that the axonal pathologies in the Npc1 mutant spinal cord are strongly correlated with the increase of activated glial cells, which produce IL-1β and ApoE, resulting in the activation of p38-MAPK signaling pathway and enhanced phosphorylated tau protein.

Disruption in connexin-based communication is associated with intracellular Ca²⁺ signal alterations in astrocytes from Niemann-Pick type C mice

Reduced astrocytic gap junctional communication and enhanced hemichannel activity were recently shown to increase astroglial and neuronal vulnerability to neuroinflammation. Moreover, increasing evidence suggests that neuroinflammation plays a pivotal role in the development of Niemann-Pick type C (NPC) disease, an autosomal lethal neurodegenerative disorder that is mainly caused by mutations in the NPC1 gene. Therefore, we investigated whether the lack of NPC1 expression in murine astrocytes affects the functional state of gap junction channels and hemichannels. Cultured cortical astrocytes of NPC1 knock-out mice (Npc1^−/−) showed reduced intercellular communication via gap junctions and increased hemichannel activity. Similarly, astrocytes of newborn Npc1^−/− hippocampal slices presented high hemichannel activity, which was completely abrogated by connexin 43 hemichannel blockers and was resistant to inhibitors of pannexin 1 hemichannels. Npc1^−/− astrocytes also showed more intracellular Ca²⁺ signal oscillations mediated by functional connexin 43 hemichannels and P2Y₁ receptors. Therefore, Npc1^−/− astrocytes present features of connexin based channels compatible with those of reactive astrocytes and hemichannels might be a novel therapeutic target to reduce neuroinflammation in NPC disease.

APP/PS1 mice overexpressing SREBP-2 exhibit combined Aβ accumulation and tau pathology underlying Alzheimer's disease

Current evidence indicates that excess brain cholesterol regulates amyloid-β (Aβ) deposition, which in turn can regulate cholesterol homeostasis. Moreover, Aβ neurotoxicity is potentiated, in part, by mitochondrial glutathione (mGSH) depletion. To better understand the relationship between alterations in cholesterol homeostasis and Alzheimer's disease (AD), we generated a triple transgenic mice featuring sterol regulatory element-binding protein-2 (SREBP-2) overexpression in combination with APPswe/PS1ΔE9 mutations (APP/PS1) to examine key biochemical and functional characteristics of AD. Unlike APP/PS1 mice, APP/PS1/SREBP-2 mice exhibited early mitochondrial cholesterol loading and mGSH depletion. Moreover, β-secretase activation and Aβ accumulation, correlating with oxidative damage and neuroinflammation, were accelerated in APP/PS1/SREBP-2 mice compared with APP/PS1 mice. Triple transgenic mice displayed increased synaptotoxicity reflected by loss of synaptophysin and neuronal death, resulting in early object-recognition memory impairment associated with deficits in spatial memory. Interestingly, tau pathology was present in APP/PS1/SREBP-2 mice, manifested by increased tau hyperphosphorylation and cleavage, activation of tau kinases and neurofibrillary tangle (NFT) formation without expression of mutated tau. Importantly, in vivo treatment with the cell permeable GSH ethyl ester, which restored mGSH levels in APP/PS1/SREBP-2 mice, partially prevented the activation of tau kinases, reduced abnormal tau aggregation and Aβ deposition, resulting in attenuated synaptic degeneration. Taken together, these results show that cholesterol-mediated mGSH depletion is a key event in AD progression, accelerating the onset of key neuropathological hallmarks of the disease. Thus, therapeutic approaches to recover mGSH may represent a relevant strategy in the treatment of AD.

Alterations in gene expression in mutant amyloid precursor protein transgenic mice lacking Niemann-Pick type C1 protein

Niemann-Pick type C (NPC) disease, a rare autosomal recessive disorder caused mostly by mutation in NPC1 gene, is pathologically characterized by the accumulation of free cholesterol in brain and other tissues. This is accompanied by gliosis and loss of neurons in selected brain regions, including the cerebellum. Recent studies have shown that NPC disease exhibits intriguing parallels with Alzheimer’s disease, including the presence of neurofibrillary tangles and increased levels of amyloid precursor protein (APP)-derived β-amyloid (Aβ) peptides in vulnerable brain neurons. To evaluate the role of Aβ in NPC disease, we determined the gene expression profile in selected brain regions of our recently developed bigenic ANPC mice, generated by crossing APP transgenic (Tg) mice with heterozygous Npc1-deficient mice. The ANPC mice exhibited exacerbated neuronal and glial pathology compared to other genotypes [i.e., APP-Tg, double heterozygous (Dhet), Npc1-null and wild-type mice]. Analysis of expression profiles of 86 selected genes using real-time RT-PCR arrays showed a wide-spectrum of alterations in the four genotypes compared to wild-type controls. The changes observed in APP-Tg and Dhet mice are limited to only few genes involved mostly in the regulation of cholesterol metabolism, whereas Npc1-null and ANPC mice showed alterations in the expression profiles of a number of genes regulating cholesterol homeostasis, APP metabolism, vesicular trafficking and cell death mechanism in both hippocampus and cerebellum compared to wild-type mice. Intriguingly, ANPC and Npc1-null mice, with some exceptions, exhibited similar changes, although more genes were differentially expressed in the affected cerebellum than the relatively spared hippocampus. The altered gene profiles were found to match with the corresponding protein levels. These results suggest that lack of Npc1 protein can alter the expression profile of selected transcripts as well as proteins, and APP overexpression influences cerebral pathology by enhancing changes triggered by Npc1 deficiency in the bigenic line.

Neuroprotection by cyclodextrin in cell and mouse models of Alzheimer disease

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There is extensive evidence that cholesterol and membrane lipids play a key role in Alzheimer disease (AD) pathogenesis. Cyclodextrins (CD) are cyclic oligosaccharide compounds widely used to bind cholesterol. Because CD exerts significant beneficial effects in Niemann-Pick type C disease, which shares neuropathological features with AD, we examined the effects of hydroxypropyl-β-CD (HP-β-CD) in cell and mouse models of AD. Cell membrane cholesterol accumulation was detected in N2a cells overexpressing Swedish mutant APP (SwN2a), and the level of membrane cholesterol was reduced by HP-β-CD treatment. HP-β-CD dramatically lowered the levels of Aβ42 in SwN2a cells, and the effects were persistent for 24 h after withdrawal. 4 mo of subcutaneous HP-β-CD administration significantly improved spatial learning and memory deficits in Tg19959 mice, diminished Aβ plaque deposition, and reduced tau immunoreactive dystrophic neurites. HP-β-CD lowered levels of Aβ42 in part by reducing β cleavage of the APP protein, and it also up-regulated the expression of genes involved in cholesterol transport and Aβ clearance. This is the first study to show neuroprotective effects of HP-β-CD in a transgenic mouse model of AD, both by reducing Aβ production and enhancing clearance mechanisms, which suggests a novel therapeutic strategy for AD.

Amyloid-β metabolism in Niemann-Pick C disease models and patients

Niemann-Pick type C (NPC) is a progressive neurodegenerative lysosomal disease with altered cellular lipid trafficking. The metabolism of amyloid-β (Aβ) - previously mainly studied in Alzheimer's disease - has been suggested to be altered in NPC. Here we aimed to perform a detailed characterization of metabolic products from the amyloid precursor protein (APP) in NPC models and patients. We used multiple analytical technologies, including immunoassays and immunoprecipitation followed by mass spectrometry (IP-MS) to characterize Aβ peptides and soluble APP fragments (sAPP-α/β) in cell media from pharmacologically (U18666A) and genetically (NPC1 ( -/- ) ) induced NPC cell models, and cerebrospinal fluid (CSF) from NPC cats and human patients. The pattern of Aβ peptides and sAPP-α/β fragments in cell media was differently affected by NPC-phenotype induced by U18666A treatment and by NPC1 ( -/- ) genotype. U18666A treatment increased the secreted media levels of sAPP-α, AβX-40 and AβX-42 and reduced the levels of sAPP-β, Aβ1-40 and Aβ1-42, while IP-MS showed increased relative levels of Aβ5-38 and Aβ5-40 in response to treatment. NPC1 ( -/- ) cells had reduced media levels of sAPP-α and Aβ1-16, and increased levels of sAPP-β. NPC cats had altered CSF distribution of Aβ peptides compared with normal cats. Cats treated with the potential disease-modifying compound 2-hydroxypropyl-β-cyclodextrin had increased relative levels of short Aβ peptides including Aβ1-16 compared with untreated cats. NPC patients receiving β-cyclodextrin had reduced levels over time of CSF Aβ1-42, AβX-38, AβX-40, AβX-42 and sAPP-β, as well as reduced levels of the axonal damage markers tau and phosphorylated tau. We conclude that NPC models have altered Aβ metabolism, but with differences across experimental systems, suggesting that NPC1-loss of function, such as in NPC1 ( -/- ) cells, or NPC1-dysfunction, seen in NPC patients and cats as well as in U18666A-treated cells, may cause subtle but different effects on APP degradation pathways. The preliminary findings from NPC cats suggest that treatment with cyclodextrin may have an impact on APP processing pathways. CSF Aβ, sAPP and tau biomarkers were dynamically altered over time in human NPC patients.

Cholesterol-dependent energy transfer between fluorescent proteins-insights into protein proximity of APP and BACE1 in different membranes in Niemann-Pick type C disease cells

Förster resonance energy transfer (FRET) -based techniques have recently been applied to study the interactions between β-site APP-cleaving enzyme-GFP (BACE1-GFP) and amyloid precursor protein-mRFP (APP-mRFP) in U373 glioblastoma cells. In this context, the role of APP-BACE1 proximity in Alzheimer’s disease (AD) pathogenesis has been discussed. FRET was found to depend on intracellular cholesterol levels and associated alterations in membrane stiffness. Here, NPC1 null cells (CHO-NPC1^−/−), exhibiting increased cholesterol levels and disturbed cholesterol transport similar to that observed in Niemann-Pick type C disease (NPC), were used to analyze the influence of altered cholesterol levels on APP-BACE1 proximity. Fluorescence lifetime measurements of whole CHO-wild type (WT) and CHO-NPC1^−/− cells (EPI-illumination microscopy), as well as their plasma membranes (total internal reflection fluorescence microscopy, TIRFM), were performed. Additionally, generalized polarization (GP) measurements of CHO-WT and CHO-NPC1^−/− cells incubated with the fluorescence marker laurdan were performed to determine membrane stiffness of plasma- and intracellular-membranes. CHO-NPC1^−/− cells showed higher membrane stiffness at intracellular- but not plasma-membranes, equivalent to cholesterol accumulation in late endosomes/lysosomes. Along with higher membrane stiffness, the FRET efficiency between BACE1-GFP and APP-mRFP was reduced at intracellular membranes, but not within the plasma membrane of CHO-NPC1^−/−. Our data show that FRET combined with TIRF is a powerful technique to determine protein proximity and membrane fluidity in cellular models of neurodegenerative diseases.

Role of cholesterol in APP metabolism and its significance in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a complex multifactorial neurodegenerative disorder believed to be initiated by accumulation of amyloid β (Aβ)-related peptides derived from proteolytic processing of amyloid precursor protein (APP). Research over the past two decades provided a mechanistic link between cholesterol and AD pathogenesis. Genetic polymorphisms in genes regulating the pivotal points in cholesterol metabolism have been suggested to enhance the risk of developing AD. Altered neuronal membrane cholesterol level and/or subcellular distribution have been implicated in aberrant formation, aggregation, toxicity, and degradation of Aβ-related peptides. However, the results are somewhat contradictory and we still do not have a complete understanding on how cholesterol can influence AD pathogenesis. In this review, we summarize our current understanding on the role of cholesterol in regulating the production/function of Aβ-related peptides and also examine the therapeutic potential of regulating cholesterol homeostasis in the treatment of AD pathology.

Mutant human APP exacerbates pathology in a mouse model of NPC and its reversal by a β-cyclodextrin

Niemann-Pick type C (NPC) disease, an autosomal recessive disorder caused primarily by loss-of-function mutations in NPC1 gene, is characterized neuropathologically by intracellular cholesterol accumulation, gliosis and neuronal loss in selected brain regions. Recent studies have shown that NPC disease exhibits intriguing parallels with Alzheimer's disease (AD), including the presence of tau-positive neurofibrillary tangles (NFTs) and β-amyloid (Aβ)-related peptides in vulnerable brain regions. Since enhanced cholesterol level, which acts as a risk factor for AD, can increase Aβ production by regulating amyloid precursor protein (APP) metabolism, it is possible that APP overexpression can influence cholesterol-regulated NPC pathology. We have addressed this issue in a novel bigenic mice (ANPC) generated by crossing heterozygous Npc1-deficient mice with mutant human APP transgenic mice. These mice exhibited decreased lifespan, early object memory and motor impairments, and exacerbated glial pathology compared with other littermates. Neurodegeneration observed in the cerebellum of ANPC mice was found to be accelerated along with a selective increase in the phosphorylation/cleavage of tau protein. Additionally, enhanced levels/activity of cytosolic cathepsin D together with cytochrome c and Bcl-2-associated X protein suggest a role for the lysosomal enzyme in the caspase-induced degeneration of neurons in ANPC mice. The reversal of cholesterol accretion by 2-hydroxypropyl-β-cyclodextrin (2-HPC) treatment increased longevity and attenuated behavioral/pathological abnormalities in ANPC mice. Collectively, our results reveal that overexpression of APP in Npc1-deficient mice can negatively influence longevity and a wide spectrum of behavioral/neuropathological abnormalities, thus raising the possibility that APP and NPC1 may interact functionally to regulate the development of AD and NPC pathologies.

Cholesterol-depletion corrects APP and BACE1 misstrafficking in NPC1-deficient cells

Cholesterol accumulation in Niemann-Pick type C disease (NPC) causes increased levels of the amyloid-precursor-protein C-terminal fragments (APP-CTFs) and intracellular amyloid-β peptide (Aβ), the two central molecules in Alzheimer's disease (AD) pathogenesis. We previously reported that cholesterol accumulation in NPC-cells leads to cholesterol-dependent increased APP processing by β-secretase (BACE1) and decreased APP expression at the cell surface (Malnar et al. Biochim Biophys Acta. 1802 (2010) 682-691.). We hypothesized that increased formation of APP-CTFs and Aβ in NPC disease is due to cholesterol-mediated altered endocytic trafficking of APP and/or BACE1. Here, we show that APP endocytosis is prerequisite for enhanced Aβ levels in NPC-cells. Moreover, we observed that NPC cells show cholesterol dependent sequestration and colocalization of APP and BACE1 within enlarged early/recycling endosomes which can lead to increased β-secretase processing of APP. We demonstrated that increased endocytic localization of APP in NPC-cells is likely due to both its increased internalization and its decreased recycling to the cell surface. Our findings suggest that increased cholesterol levels, such as in NPC disease and sporadic AD, may be the upstream effector that drives amyloidogenic APP processing characteristic for Alzheimer's disease by altering endocytic trafficking of APP and BACE1.

Miglustat treatment may reduce cerebrospinal fluid levels of the axonal degeneration marker tau in niemann-pick type C

INTRODUCTION

Niemann-Pick disease type C (NPC) is a lysosomal storage disorder that leads to progressive neurodegeneration. The glucosylceramide synthase blocker miglustat is being used to treat NPC, but monitoring of disease progression and treatment response is difficult. NPC patients have elevated cerebrospinal fluid (CSF) levels of total-tau (T-tau) indicating axonal degeneration, and increased CSF amyloid β (Aβ) indicating abnormal brain amyloid metabolism, but it is unknown if start of miglustat treatment affects these biomarker levels.

METHODS

Biomarkers were measured in serial CSF samples from NPC patients who started miglustat between samplings (N=5), were untreated at both samplings (N=5) or received treatment during the whole study (N=6) (median time between samplings 309 days [range 175-644]). CSF was analyzed for Aβ(38), Aβ(40), Aβ(42), α-cleaved soluble APP, β-cleaved soluble APP, T-tau and phospho-tau.

RESULTS

T-tau levels decreased in patients who started miglustat treatment (median 955 [range 338-1,271]ng/L at baseline vs. 382 [187-736]ng/L at follow-up, p=0.043). Untreated patients and continuously treated patients had stable levels (p>0.05). No changes were seen in the other biomarkers.

CONCLUSION

Reduced CSF T-tau suggests that miglustat treatment might affect axonal degeneration in NPC. However, the results must be interpreted with caution and verified in future studies, since this pilot study was small, treatment was not randomized, and patients starting treatment had higher baseline CSF T-tau than untreated patients.

Rab GTPases implicated in inherited and acquired disorders

The endocytotic machinery imports, transports and exports receptors and associated molecules between the plasma membrane and various cytoplasmic chambers resulting in selective recycling, degradation, or secretion of molecules and signaling complexes. Trafficking of receptors, growth factors, nutrients, cytokines, integrins as well as pathogens dictates the kinetics and magnitude of signal transduction cascades. Understandably, alterations in the 'fate' of such cargo complexes have profound physiologic and pathophysiologic implications. Rab GTPases regulate endocytosis by decorating intracellular vesicles and targeting these vesicles along with their cargoes to appropriate subcellular compartments. In the last decade, the number of genetic diseases driven by germline mutations in Rab GTPases or their interacting proteins, has increased and there is growing evidence of aberrant Rab GTPase function in acquired pathophysiologies such as immune deficiency, infection, obesity, diabetes and cancer.