Neuroprotection by cyclodextrin in cell and mouse models of Alzheimer disease

Defective macroautophagic turnover of brain lipids in the TgCRND8 Alzheimer mouse model: prevention by correcting lysosomal proteolytic deficits

Autophagy, the major lysosomal pathway for the turnover of intracellular organelles is markedly impaired in neurons in Alzheimer's disease and Alzheimer mouse models. We have previously reported that severe lysosomal and amyloid neuropathology and associated cognitive deficits in the TgCRND8 Alzheimer mouse model can be ameliorated by restoring lysosomal proteolytic capacity and autophagy flux via genetic deletion of the lysosomal protease inhibitor, cystatin B. Here we present evidence that macroautophagy is a significant pathway for lipid turnover, which is defective in TgCRND8 brain where lipids accumulate as membranous structures and lipid droplets within giant neuronal autolysosomes. Levels of multiple lipid species including several sphingolipids (ceramide, ganglioside GM3, GM2, GM1, GD3 and GD1a), cardiolipin, cholesterol and cholesteryl esters are elevated in autophagic vacuole fractions and lysosomes isolated from TgCRND8 brain. Lipids are localized in autophagosomes and autolysosomes by double immunofluorescence analyses in wild-type mice and colocalization is increased in TgCRND8 mice where abnormally abundant GM2 ganglioside-positive granules are detected in neuronal lysosomes. Cystatin B deletion in TgCRND8 significantly reduces the number of GM2-positive granules and lowers the levels of GM2 and GM3 in lysosomes, decreases lipofuscin-related autofluorescence, and eliminates giant lipid-containing autolysosomes while increasing numbers of normal-sized autolysosomes/lysosomes with reduced content of undigested components. These findings have identified macroautophagy as a previously unappreciated route for delivering membrane lipids to lysosomes for turnover, a function that has so far been considered to be mediated exclusively through the endocytic pathway, and revealed that autophagic-lysosomal dysfunction in TgCRND8 brain impedes lysosomal turnover of lipids as well as proteins. The amelioration of lipid accumulation in TgCRND8 by removing cystatin B inhibition on lysosomal proteases suggests that enhancing lysosomal proteolysis improves the overall environment of the lysosome and its clearance functions, which may be possibly relevant to a broader range of lysosomal disorders beyond Alzheimer's disease.

Unraveling the mechanism of cell death induced by chemical fibrils

We previously discovered a small-molecule inducer of cell death, named 1541, that noncovalently self-assembles into chemical fibrils ('chemi-fibrils') and activates procaspase-3 in vitro. We report here that 1541-induced cell death is caused by the fibrillar rather than the soluble form of the drug. A short hairpin RNA screen reveals that knockdown of genes involved in endocytosis, vesicle trafficking and lysosomal acidification causes partial 1541 resistance. We confirm the role of these pathways using pharmacological inhibitors. Microscopy shows that the fluorescent chemi-fibrils accumulate in punctae inside cells that partially colocalize with lysosomes. Notably, the chemi-fibrils bind and induce liposome leakage in vitro, suggesting they may do the same in cells. The chemi-fibrils induce extensive proteolysis including caspase substrates, yet modulatory profiling reveals that chemi-fibrils form a distinct class from existing inducers of cell death. The chemi-fibrils share similarities with proteinaceous fibrils and may provide insight into their mechanism of cellular toxicity.

Cholesterol in brain disease: sometimes determinant and frequently implicated

Cholesterol is essential for neuronal physiology, both during development and in the adult life: as a major component of cell membranes and precursor of steroid hormones, it contributes to the regulation of ion permeability, cell shape, cell-cell interaction, and transmembrane signaling. Consistently, hereditary diseases with mutations in cholesterol-related genes result in impaired brain function during early life. In addition, defects in brain cholesterol metabolism may contribute to neurological syndromes, such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD), and even to the cognitive deficits typical of the old age. In these cases, brain cholesterol defects may be secondary to disease-causing elements and contribute to the functional deficits by altering synaptic functions. In the first part of this review, we will describe hereditary and non-hereditary causes of cholesterol dyshomeostasis and the relationship to brain diseases. In the second part, we will focus on the mechanisms by which perturbation of cholesterol metabolism can affect synaptic function.

Blockage of GSK3β-mediated Drp1 phosphorylation provides neuroprotection in neuronal and mouse models of Alzheimer's disease

It is well established that mitochondrial fragmentation plays a key role in the pathogenesis of Alzheimer's disease (AD). Mitochondrial fission is mediated by dynamin-related protein 1 (Drp1), which is highly expressed in nervous system and regulated by various posttranslational modifications including phosphorylation. We identified glycogen synthase kinase (GSK)3β-dependent Drp1 phosphorylation at Ser(40) and Ser(44), which increases Drp1 GTPase activity and its mitochondrial distribution and could induce mitochondrial fragmentation. Moreover, neurons transfected with Ser(40)Ser(44) phosphomimic Drp1 showed increased mitochondria fragmentation and were more vulnerable to amyloid-β (Aβ)-induced apoptosis. Therefore, blocking GSK3β-induced Drp1 phosphorylation may be an effective way to protect neurons from Aβ toxicity. To address this, we designed and synthesized an artificial polypeptide named TAT-Drp1-SpS, which could specifically block GSK3β-induced Drp1 phosphorylation. Our results demonstrated that TAT-Drp1-SpS treatment could significantly reduce Aβ-induced neuronal apoptosis in cultured neurons. Notably, TAT-Drp1-SpS administration in hippocampus Cornu Ammonis 1 (CA1) region significantly reduced Aβ burden and rescued the memory deficits in AD transgenic mice. Although Aβ has multiple targets to exert its neurotoxicity, our findings suggested that GSK3β-induced mitochondrial fragmentation was, at least partially, mediated by Aβ toxicity and contribute to the pathogenesis of AD. Taken together, GSK3β-induced Drp1 phosphorylation provides a novel mechanism for mitochondrial fragmentation in AD, and our findings suggested a novel therapeutic strategy for AD.

Role of cholesterol metabolism in the pathogenesis of Alzheimer's disease

PURPOSE OF REVIEW

Cholesterol has been shown to stimulate the cleavage of amyloid precursor protein (APP) into amyloid peptides involved in Alzheimer's disease. However, high level of peripheral cholesterol as a risk factor for Alzheimer's disease is still debated. This current review provides an update of the recent literature on cholesterol and APP metabolisms in the brain.

RECENT FINDINGS

First, a new relationship between neuronal APP and cholesterol has been shown in which this protein controls cholesterol turnover required for neuronal activity. Second, oxysterols are able to stimulate the synthesis of ATP-binding cassette transporters involved in the exchange of amyloid peptides between the blood and the brain. Third, changes in APP targeting to lipid rafts and/or their composition in cholesterol regulate amyloid peptide production.

SUMMARY

These recent findings open new areas of investigations to control the neuronal activity and to decrease the amyloid peptide levels in brain, opening on new preventive and therapeutic strategies for Alzheimer's disease.

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.

Development and validation of sensitive LC-MS/MS assays for quantification of HP-β-CD in human plasma and CSF

2-Hydroxypropyl-β-cyclodextrin (HP-β-CD), a widely used excipient for drug formulation, has emerged as an investigational new drug for the treatment of Niemann-Pick type C1 (NPC1) disease, a neurodegenerative cholesterol storage disorder. Development of a sensitive quantitative LC-MS/MS assay to monitor the pharmacokinetics (PKs) of HP-β-CD required for clinical trials has been challenging owing to the dispersity of the HP-β-CD. To support a phase 1 clinical trial for ICV delivery of HP-β-CD in NPC1 patients, novel methods for quantification of HP-β-CD in human plasma and cerebrospinal fluid (CSF) using LC-MS/MS were developed and validated: a 2D-LC-in-source fragmentation-MS/MS (2D-LC-IF-MS/MS) assay and a reversed phase ultra performance LC-MS/MS (RP-UPLC-MS/MS) assay. In both assays, protein precipitation and "dilute and shoot" procedures were used to process plasma and CSF, respectively. The assays were fully validated and in close agreement, and allowed determination of PK parameters for HP-β-CD. The LC-MS/MS methods are ∼100-fold more sensitive than the current HPLC assay, and were successfully employed to analyze HP-β-CD in human plasma and CSF samples to support the phase 1 clinical trial of HP-β-CD in NPC1 patients.

Multifunctional 8-hydroxyquinoline-appended cyclodextrins as new inhibitors of metal-induced protein aggregation

Mounting evidence suggests a pivotal role of metal imbalances in protein misfolding and amyloid diseases. As such, metal ions represent a promising therapeutic target. In this context, the synthesis of chelators that also contain complementary functionalities to combat the multifactorial nature of neurodegenerative diseases is a highly topical issue. We report two new 8-hydroxyquinoline-appended cyclodextrins and highlight their multifunctional properties, including their Cu(II) and Zn(II) binding abilities, and capacity to act as antioxidants and metal-induced antiaggregants. In particular, the latter property has been applied in the development of an effective assay that exploits the formation of amyloid fibrils when β-lactoglobulin A is heated in the presence of metal ions.

Cholesterol as a causative factor in Alzheimer's disease: a debatable hypothesis

High serum/plasma cholesterol levels have been suggested as a risk factor for Alzheimer's disease (AD). Some reports, mostly retrospective epidemiological studies, have observed a decreased prevalence of AD in patients taking the cholesterol lowering drugs, statins. The strongest evidence causally linking cholesterol to AD is provided by experimental studies showing that adding/reducing cholesterol alters amyloid precursor protein (APP) and amyloid beta-protein (Ab) levels. However, there are problems with the cholesterol-AD hypothesis. Cholesterol levels in serum/plasma and brain of AD patients do not support cholesterol as a causative factor in AD.Prospective studies on statins and AD have largely failed to show efficacy. Even the experimental data are open to interpretation given that it is well-established that modification of cholesterol levels has effects on multiple proteins, not only amyloid precursor protein and Ab. The purpose of this review, therefore, was to examine the above-mentioned issues, discuss the pros and cons of the cholesterol-AD hypothesis, involvement of other lipids in the mevalonate pathway, and consider that AD may impact cholesterol homeostasis.

Beta cyclodextrins bind, stabilize, and remove lipofuscin bisretinoids from retinal pigment epithelium

Accumulation of lipofuscin bisretinoids (LBs) in the retinal pigment epithelium (RPE) is the alleged cause of retinal degeneration in genetic blinding diseases (e.g., Stargardt) and a possible etiological agent for age-related macular degeneration. Currently, there are no approved treatments for these diseases; hence, agents that efficiently remove LBs from RPE would be valuable therapeutic candidates. Here, we show that beta cyclodextrins (β-CDs) bind LBs and protect them against oxidation. Computer modeling and biochemical data are consistent with the encapsulation of the retinoid arms of LBs within the hydrophobic cavity of β-CD. Importantly, β-CD treatment reduced by 73% and 48% the LB content of RPE cell cultures and of eyecups obtained from Abca4-Rdh8 double knock-out (DKO) mice, respectively. Furthermore, intravitreal administration of β-CDs reduced significantly the content of bisretinoids in the RPE of DKO animals. Thus, our results demonstrate the effectiveness of β-CDs to complex and remove LB deposits from RPE cells and provide crucial data to develop novel prophylactic approaches for retinal disorders elicited by LBs.

Niemann-Pick C disease and mobilization of lysosomal cholesterol by cyclodextrin

Niemann-Pick type C (NPC) disease is a lysosomal storage disease in which endocytosed cholesterol becomes sequestered in late endosomes/lysosomes (LEs/Ls) because of mutations in either the NPC1 or NPC2 gene. Mutations in either of these genes can lead to impaired functions of the NPC1 or NPC2 proteins and progressive neurodegeneration as well as liver and lung disease. NPC1 is a polytopic protein of the LE/L limiting membrane, whereas NPC2 is a soluble protein in the LE/L lumen. These two proteins act in tandem and promote the export of cholesterol from LEs/Ls. Consequently, a defect in either NPC1 or NPC2 causes cholesterol accumulation in LEs/Ls. In this review, we summarize the molecular mechanisms leading to NPC disease, particularly in the CNS. Recent exciting data on the mechanism by which the cholesterol-sequestering agent cyclodextrin can bypass the functions of NPC1 and NPC2 in the LEs/Ls, and mobilize cholesterol from LEs/Ls, will be highlighted. Moreover, the possible use of cyclodextrin as a valuable therapeutic agent for treatment of NPC patients will be considered.

Collaborative development of 2-hydroxypropyl-β-cyclodextrin for the treatment of Niemann-Pick type C1 disease

In 2010, the National Institutes of Health (NIH) established the Therapeutics for Rare and Neglected Diseases (TRND) program within the National Center for Advancing Translational Sciences (NCATS), which was created to stimulate drug discovery and development for rare and neglected tropical diseases through a collaborative model between the NIH, academic scientists, nonprofit organizations, and pharmaceutical and biotechnology companies. This paper describes one of the first TRND programs, the development of 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) for the treatment of Niemann-Pick disease type C1 (NPC1). NPC is a neurodegenerative, autosomal recessive rare disease caused by a mutation in either the NPC1 (about 95% of cases) or the NPC2 gene (about 5% of cases). These mutations affect the intracellular trafficking of cholesterol and other lipids, which leads to a progressive accumulation of unesterified cholesterol and glycosphingolipids in the CNS and visceral organs. Affected individuals typically exhibit ataxia, swallowing problems, seizures, and progressive impairment of motor and intellectual function in early childhood, and usually die in adolescence. There is no disease modifying therapy currently approved for NPC1 in the US. A collaborative drug development program has been established between TRND, public and private partners that has completed the pre-clinical development of HP-β-CD through IND filing for the current Phase I clinical trial that is underway. Here we discuss how this collaborative effort helped to overcome scientific, clinical and financial challenges facing the development of new drug treatments for rare and neglected diseases, and how it will incentivize the commercialization of HP-β-CD for the benefit of the NPC patient community.

Altered Cholesterol Intracellular Trafficking and the Development of Pathological Hallmarks of Sporadic AD

Compared to the rare familial early onset Alzheimer's disease (AD) that results from gene mutations in AbPP and presenilin-1, the pathogenesis of sporadic AD is much more complex and is believed to result from complex interactions between nutritional, environmental, epigenetic and genetic factors. Among those factors, the presence APOE4 is still the single strongest genetic risk factor for sporadic AD. However, the exact underlying mechanism whereby apoE4 contributes to the pathogenesis of sporadic AD remains unclear. Here, we discuss how altered cholesterol intracellular trafficking as a result of apoE4 might contribute to the development of pathological hallmarks of AD including brain deposition of amyloid beta (Ab), neurofibrillary tangles, and synaptic dysfunction.

The role of autophagy in neurodegenerative disease

Autophagy is a lysosomal degradative process used to recycle obsolete cellular constituents and eliminate damaged organelles and protein aggregates. These substrates reach lysosomes by several distinct mechanisms, including delivery within endosomes as well as autophagosomes. Completion of digestion involves dynamic interactions among compartments of the autophagic and endocytic pathways. Neurons are particularly vulnerable to disruptions of these interactions, especially as the brain ages. Not surprisingly, mutations of genes regulating autophagy cause neurodegenerative diseases across the age spectrum with exceptional frequency. In late-onset disorders such as Alzheimer's disease, amyotrophic lateral sclerosis and familial Parkinson's disease, defects arise at different stages of the autophagy pathway and have different implications for pathogenesis and therapy. This Review provides an overview of the role of autophagy in neurodegenerative disease, focusing particularly on less frequently considered lysosomal clearance mechanisms and their considerable impact on disease. Various therapeutic strategies for modulating specific stages of autophagy and the current state of drug development for this purpose are also evaluated.

Protein folding and aggregation into amyloid: the interference by natural phenolic compounds

Amyloid aggregation is a hallmark of several degenerative diseases affecting the brain or peripheral tissues, whose intermediates (oligomers, protofibrils) and final mature fibrils display different toxicity. Consequently, compounds counteracting amyloid aggregation have been investigated for their ability (i) to stabilize toxic amyloid precursors; (ii) to prevent the growth of toxic oligomers or speed that of fibrils; (iii) to inhibit fibril growth and deposition; (iv) to disassemble preformed fibrils; and (v) to favor amyloid clearance. Natural phenols, a wide panel of plant molecules, are one of the most actively investigated categories of potential amyloid inhibitors. They are considered responsible for the beneficial effects of several traditional diets being present in green tea, extra virgin olive oil, red wine, spices, berries and aromatic herbs. Accordingly, it has been proposed that some natural phenols could be exploited to prevent and to treat amyloid diseases, and recent studies have provided significant information on their ability to inhibit peptide/protein aggregation in various ways and to stimulate cell defenses, leading to identify shared or specific mechanisms. In the first part of this review, we will overview the significance and mechanisms of amyloid aggregation and aggregate toxicity; then, we will summarize the recent achievements on protection against amyloid diseases by many natural phenols.

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.