Irfan Y. Tamboli

Inhibition of Glycosphingolipid Biosynthesis Reduces Secretion of the β-Amyloid Precursor Protein and Amyloid β-Peptide

Alzheimer disease is associated with extracellular deposits of amyloid β-peptides in the brain. Amyloid β-peptides are generated by proteolytic processing of the β-amyloid precursor protein by β- and γ-secretases. The cleavage by secretases occurs predominantly in post-Golgi secretory and endocytic compartments and is influenced by cholesterol, indicating a role of the membrane lipid composition in proteolytic processing of the β-amyloid precursor protein. To analyze the role of glycosphingolipids in these processes we inhibited glycosyl ceramide synthase, which catalyzes the first step in glycosphingolipid biosynthesis. The depletion of glycosphingolipids markedly reduced the secretion of endogenous β-amyloid precursor protein in different cell types, including human neuroblastoma SH-SY5Y cells. Importantly, secretion of amyloid β-peptides was also strongly decreased by inhibition of glycosphingolipid biosynthesis. Conversely, the addition of exogenous brain gangliosides to cultured cells reversed these effects. Biochemical and cell biological experiments demonstrate that the pharmacological reduction of cellular glycosphingolipid levels inhibited maturation and cell surface transport of the β-amyloid precursor protein. In the glycosphingolipid-deficient cell line GM95, cellular levels and maturation of β-amyloid precursor protein were also significantly reduced as compared with normal B16 cells. Together, these data demonstrate that glycosphingolipids are implicated in the regulation of the subcellular transport of the β-amyloid precursor protein in the secretory pathway and its proteolytic processing. Thus, enzymes involved in glycosphingolipid metabolism might represent targets to inhibit the production of amyloid β-peptides.

Statins Promote the Degradation of Extracellular Amyloid β-Peptide by Microglia via Stimulation of Exosome-associated Insulin-degrading Enzyme (IDE) Secretion

Epidemiological studies indicate that intake of statins decrease the risk of developing Alzheimer disease. Cellular and in vivo studies suggested that statins might decrease the generation of the amyloid β-peptide (Aβ) from the β-amyloid precursor protein. Here, we show that statins potently stimulate the degradation of extracellular Aβ by microglia. The statin-dependent clearance of extracellular Aβ is mainly exerted by insulin-degrading enzyme (IDE) that is secreted in a nonconventional pathway in association with exosomes. Stimulated IDE secretion and Aβ degradation were also observed in blood of mice upon peripheral treatment with lovastatin. Importantly, increased IDE secretion upon lovastatin treatment was dependent on protein isoprenylation and up-regulation of exosome secretion by fusion of multivesicular bodies with the plasma membrane. These data demonstrate a novel pathway for the nonconventional secretion of IDE via exosomes. The modulation of this pathway could provide a new strategy to enhance the extracellular clearance of Aβ.

Histone deacetylase inhibitor valproic acid inhibits cancer cell proliferation via down-regulation of the alzheimer amyloid precursor protein.

The β-amyloid precursor protein (APP) represents a type I transmembrane glycoprotein that is ubiquitously expressed. In the brain, it is a key player in the molecular pathogenesis of Alzheimer disease. Its physiological function is however less well understood. Previous studies showed that APP is up-regulated in prostate, colon, pancreatic tumor, and oral squamous cell carcinoma. In this study, we show that APP has an essential role in growth control of pancreatic and colon cancer. Abundant APP staining was found in human pancreatic adenocarcinoma and colon cancer tissue. Interestingly, treating pancreatic and colon cancer cells with valproic acid (VPA, 2-propylpentanoic acid), a known histone deacetylase (HDAC) inhibitor, leads to up-regulation of GRP78, an endoplasmic reticulum chaperone immunoglobulin-binding protein. GRP78 is involved in APP maturation and inhibition of tumor cell growth by down-regulation of APP and secreted soluble APPα. Trichostatin A, a pan-HDAC inhibitor, also lowered APP and increased GRP78 levels. In contrast, treating cells with valpromide, a VPA derivative lacking HDAC inhibitory properties, had no effect on APP levels. VPA did not modify the level of epidermal growth factor receptor, another type I transmembrane protein, and APLP2, a member of the APP family, demonstrating the specificity of the VPA effect on APP. Small interfering RNA-mediated knockdown of APP also resulted in significantly decreased cell growth. Based on these observations, the data suggest that APP down-regulation via HDAC inhibition provides a novel mechanism for pancreatic and colon cancer therapy.

Sphingolipid Storage Affects Autophagic Metabolism of the Amyloid Precursor Protein and Promotes Aβ Generation

Deposition of amyloid β peptides (Aβs) in extracellular amyloid plaques within the human brain is a hallmark of Alzheimers disease (AD). Aβ derives from proteolytic processing of the amyloid precursor protein (APP) by β- and γ-secretases. The initial cleavage by β-secretase results in shedding of the APP ectodomain and generation of APP C-terminal fragments (APP-CTFs), which can then be further processed within the transmembrane domain by γ-secretase, resulting in release of Aβ. Here, we demonstrate that accumulation of sphingolipids (SLs), as occurs in lysosomal lipid storage disorders (LSDs), decreases the lysosome-dependent degradation of APP-CTFs and stimulates γ-secretase activity. Together, this results in increased generation of both intracellular and secreted Aβ. Notably, primary fibroblasts from patients with different SL storage diseases show strong accumulation of potentially amyloidogenic APP-CTFs. By using biochemical, cell biological, and genetic approaches, we demonstrate that SL accumulation affects autophagic flux and impairs the clearance of APP-CTFs. Thus, accumulation of SLs might not only underlie the pathogenesis of LSDs, but also trigger increased generation of Aβ and contribute to neurodegeneration in sporadic AD.

Presenilin-1 L166P Mutant Human Pluripotent Stem Cell–Derived Neurons Exhibit Partial Loss of γ-Secretase Activity in Endogenous Amyloid-β Generation

Alzheimers disease (AD) is the most frequent cause of dementia. There is compelling evidence that the proteolytic processing of the amyloid precursor protein (APP) and accumulation of amyloid-β (Aβ) peptides play critical roles in AD pathogenesis. Due to limited access to human neural tissue, pathogenetic studies have, so far, mostly focused on the heterologous overexpression of mutant human APP in non-human cells. In this study, we show that key steps in proteolytic APP processing are recapitulated in neurons generated from human embryonic and induced pluripotent stem cell-derived neural stem cells (NSC). These human NSC-derived neurons express the neuron-specific APP(695) splice variant, BACE1, and all members of the γ-secretase complex. The human NSC-derived neurons also exhibit a differentiation-dependent increase in Aβ secretion and respond to the pharmacotherapeutic modulation by anti-amyloidogenic compounds, such as γ-secretase inhibitors and nonsteroidal anti-inflammatory drugs. Being highly amenable to genetic modification, human NSCs enable the study of mechanisms caused by disease-associated mutations in human neurons. Interestingly, the AD-associated PS1(L166P) variant revealed a partial loss of γ-secretase function, resulting in the decreased production of endogenous Aβ40 and an increased Aβ42/40 ratio. The PS1(L166P) mutant is also resistant to γ-secretase modulation by nonsteroidal anti-inflammatory drugs. Pluripotent stem cell-derived neurons thus provide experimental access to key steps in AD pathogenesis and can be used to screen pharmaceutical compounds directly in a human neuronal system.

Loss of gamma-secretase function impairs endocytosis of lipoprotein particles and membrane cholesterol homeostasis.

Presenilins (PSs) are components of the γ-secretase complex that mediates intramembranous cleavage of type I membrane proteins. We show that γ-secretase is involved in the regulation of cellular lipoprotein uptake. Loss of γ-secretase function decreased endocytosis of low-density lipoprotein (LDL) receptor. The decreased uptake of lipoproteins led to upregulation of cellular cholesterol biosynthesis by increased expression of CYP51 and enhanced metabolism of lanosterol. Genetic deletion of PS1 or transgenic expression of PS1 mutants that cause early-onset Alzheimers disease led to accumulation of γ-secretase substrates and mistargeting of adaptor proteins that regulate endocytosis of the LDL receptor. Consistent with decreased endocytosis of these receptors, PS1 mutant mice have elevated levels of apolipoprotein E in the brain. Thus, these data demonstrate a functional link between two major genetic factors that cause early-onset and late-onset Alzheimers disease.

Cerebral small vessel disease-induced Apolipoprotein E leakage is associated with Alzheimer disease and the accumulation of amyloid [beta]-protein in perivascular astrocytes

Apolipoprotein E (apoE) plays a role in the pathogenesis of Alzheimer disease (AD). It is involved in the receptor-mediated cellular clearance of the amyloid &bgr;-protein (A&bgr;) and in the perivascular drainage of the extracellular fluid. Microvascular changes are also associated with AD and have been discussed as a possible reason for altered perivascular drainage. To further clarify the role of apoE in the perivascular and vascular pathology in AD patients, we studied its occurrence and distribution in the perivascular space, the perivascular neuropil, and in the vessel wall of AD and control cases with and without small vessel disease (SVD). Apolipoprotein E was found in the perivascular space and in the neuropil around arteries of the basal ganglia from control and AD cases disclosing no major differences. Western blot analysis of basal ganglia tissue also revealed no significant differences pertaining to the amount of full-length and C-terminal truncated apoE in AD cases compared with controls. In contrast, A&bgr; occurred in apoE-positive perivascular astrocytes in AD cases but not in controls. In blood vessels, apoE and immunoglobulin G were detected within the SVD-altered vessel wall. The severity of SVD was associated with the occurrence of apoE in the vessel wall and with that of A&bgr; in perivascular astrocytes. These results point to an important role of apoE in the perivascular clearance of A&bgr; in the human brain. The occurrence of apoE and immunoglobulin G in SVD lesions and in the perivascular space suggests that the presence of SVD results in plasma-protein leakage into the brain. It is therefore tempting to speculate that apoE represents a pathogenetic link between SVD and AD.

Adaptation of neuronal cells to chronic oxidative stress is associated with altered cholesterol and sphingolipid homeostasis and lysosomal function

Chronic oxidative stress has been causally linked to several neurodegenerative disorders. As sensitivity for oxidative stress greatly differs between brain regions and neuronal cell types, specific cellular mechanisms of adaptation to chronic oxidative stress should exist. Our objective was to identify molecular mechanisms of adaptation of neuronal cells after applying chronic sublethal oxidative stress. We demonstrate that cells resistant to oxidative stress exhibit altered cholesterol and sphingomyelin metabolisms. Stress‐resistant cells showed reduced levels of molecules involved in cholesterol trafficking and intracellular accumulation of cholesterol, cholesterol precursors, and metabolites. Moreover, stress‐resistant cells exhibited reduced SMase activity. The altered lipid metabolism was associated with enhanced autophagy. Treatment of stress‐resistant cells with neutral SMase reversed the stress‐resistant phenotype, whereas it could be mimicked by treatment of neuronal cells with a specific inhibitor of neutral SMase. Analysis of hippocampal and cerebellar tissue of mouse brains revealed that the obtained cell culture data reflect the in vivo situation. Stress‐resistant cells in vitro showed similar features as the less vulnerable cerebellum in mice, whereas stress‐sensitive cells resembled the highly sensitive hippocampal area. These findings suggest an important role of the cell type‐specific lipid profile for differential vulnerabilities of different brain areas toward chronic oxidative stress.

Deficiency of Sphingosine-1-phosphate Lyase Impairs Lysosomal Metabolism of the Amyloid Precursor Protein

Background: Sphingolipid metabolism is functionally linked to the proteolytic processing of APP. Results: Inhibition of S1P-lyase decreases APP degradation in lysosomes, and mobilization of Ca2+ can partially rescue the accumulation of APP. Conclusion: S1P-lyase is critically involved in the regulation of lysosomal activity and degradation of APP. Significance: Alterations in S1P metabolism could play important roles in the pathogenesis of Alzheimer disease. Progressive accumulation of the amyloid β protein in extracellular plaques is a neuropathological hallmark of Alzheimer disease. Amyloid β is generated during sequential cleavage of the amyloid precursor protein (APP) by β- and γ-secretases. In addition to the proteolytic processing by secretases, APP is also metabolized by lysosomal proteases. Here, we show that accumulation of intracellular sphingosine-1-phosphate (S1P) impairs the metabolism of APP. Cells lacking functional S1P-lyase, which degrades intracellular S1P, strongly accumulate full-length APP and its potentially amyloidogenic C-terminal fragments (CTFs) as compared with cells expressing the functional enzyme. By cell biological and biochemical methods, we demonstrate that intracellular inhibition of S1P-lyase impairs the degradation of APP and CTFs in lysosomal compartments and also decreases the activity of γ-secretase. Interestingly, the strong accumulation of APP and CTFs in S1P-lyase-deficient cells was reversed by selective mobilization of Ca2+ from the endoplasmic reticulum or lysosomes. Intracellular accumulation of S1P also impairs maturation of cathepsin D and degradation of Lamp-2, indicating a general impairment of lysosomal activity. Together, these data demonstrate that S1P-lyase plays a critical role in the regulation of lysosomal activity and the metabolism of APP.

Sphingolipid storage impairs autophagic clearance of Alzheimer-associated proteins

Recent work from our laboratory demonstrates that the accumulation of sphingolipids (SLs) decreases the capacity of cells to clear potentially amyloidogenic fragments of the amyloid precursor protein (APP) during autophagy. APP is a type I membrane protein and could undergo sequential proteolytic processing by β- and γ-secretase resulting in the generation of the amyloid β-peptide (Aβ). Genetic, molecular and biochemical evidence indicates that the accumulation of toxic Aβ aggregates plays a critical role in the degeneration of neurons during the pathogenesis of Alzheimer disease (AD). Thus, SL storage could promote the accumulation of Ab in endosomal and lysosomal compartments and thereby induce characteristic cytopathological changes of AD.