Daniel Sevlever

Cathepsin D is the main lysosomal enzyme involved in the degradation of α-synuclein and generation of its carboxy-terminally truncated species

Alpha-synuclein is likely to play a key role in the development of Parkinsons disease as well as other synucleinopathies. In animal models, overexpression of full-length or carboxy-terminally truncated alpha-synuclein has been shown to produce pathology. Although the proteosome and lysosome have been proposed to play a role in the degradation of alpha-synuclein, the enzyme(s) involved in alpha-synuclein clearance and generation of its carboxy-terminally truncated species have not been identified. In this study, the role of cathepsin D and calpain I in these processes was analyzed. In vitro experiments, using either recombinant or endogenous alpha-synuclein as substrates and purified cathepsin D or lysosomes, demonstrated that cathepsin D degraded alpha-synuclein very efficiently, and that limited proteolysis resulted in the generation of carboxy-terminally truncated species. Purified calpain I also cleaved alpha-synuclein, but carboxy-terminally truncated species were not the main cleavage products, and calpain I activity present in cellular lysates was not able to degrade the protein. Knockdown of cathepsin D in cells overexpressing wild-type alpha-synuclein increased total alpha-synuclein levels by 28% and lysosomal alpha-synuclein by 2-fold. In in vitro experiments, pepstatin A completely blocked the degradation of alpha-synuclein in purified lysosomes. Furthermore, lysosomes isolated from cathepsin D knockdown cells showed a marked reduction in alpha-synuclein degrading activity, indicating that cathepsin D is the main lysosomal enzyme involved in alpha-synuclein degradation. Our findings suggest that upregulation of cathepsin D could be an additional therapeutic strategy to lessen alpha-synuclein burden in synucleinopathies.

Oxidative stress-induced phosphorylation, degradation and aggregation of α-synuclein are linked to upregulated CK2 and cathepsin D

Intracellular accumulation of α‐synuclein (α‐Syn) as filamentous aggregates is a pathological feature shared by Parkinsons disease, dementia with Lewy bodies and multiple system atrophy, referred to as synucleinopathies. To understand the mechanisms underlying α‐Syn aggregation, we established a tetracycline‐off inducible transfectant (3D5) of neuronal lineage overexpressing human wild‐type α‐Syn. α‐Syn aggregation was initiated by exposure of 3D5 cells to FeCl2. The exposure led to formation of α‐Syn inclusions and oligomers of 34, 54, 68 kDa and higher molecular weights. The oligomers displayed immunoreactivity with antibodies to the amino‐, but not to the carboxyl(C)‐, terminus of α‐Syn, indicating that C‐terminally truncated α‐Syn is a major component of oligomers. FeCl2 exposure also promoted accumulation of S129 phosphorylated monomeric α‐Syn (Pα‐Syn) and casein kinase 2 (CK2); however, G‐protein‐coupled receptor kinase 2 was reduced. Treatment of FeCl2‐exposed cells with CK2 inhibitors (DRB or TBB) led to decreased formation of α‐Syn inclusions, oligomers and Pα‐Syn. FeCl2 exposure also enhanced the activity/level of cathepsin D. Treatment of the FeCl2‐exposed cells with pepstatin A or NH4Cl led to reduced formation of oligomers/inclusions as well as of ∼ 10 and 12 kDa truncated α‐Syn. Our results indicate that α‐Syn phosphorylation caused by FeCl2 is due to CK2 upregulation, and that lysosomal proteases may have a role in producing truncated α‐Syn for oligomer assembly.

Glycosylphosphatidylinositol-anchored Proteins Play an Important Role in the Biogenesis of the Alzheimer’s Amyloid β-Protein

The Alzheimer’s amyloid protein (Aβ) is released from the larger amyloid β-protein precursor (APP) by unidentified enzymes referred to as β- and γ-secretase. β-Secretase cleaves APP on the amino side of Aβ producing a large secreted derivative (sAPPβ) and an Aβ-bearing C-terminal derivative that is subsequently cleaved by γ-secretase to release Aβ. Alternative cleavage of the APP by α-secretase at Aβ16/17 releases the secreted derivative sAPPα. In yeast, α-secretase activity has been attributed to glycosylphosphatidylinositol (GPI)-anchored aspartyl proteases. To examine the role of GPI-anchored proteins, we specifically removed these proteins from the surface of mammalian cells using phosphatidylinositol-specific phospholipase C (PI-PLC). PI-PLC treatment of fetal guinea pig brain cultures substantially reduced the amount of Aβ40 and Aβ42 in the medium but had no effect on sAPPα. A mutant CHO cell line (gpi85), which lacks GPI-anchored proteins, secreted lower levels of Aβ40, Aβ42, and sAPPβ than its parental line (GPI+). When this parental line was treated with PI-PLC, Aβ40, Aβ42, and sAPPβ decreased to levels similar to those observed in the mutant line, and the mutant line was resistant to these effects of PI-PLC. These findings provide strong evidence that one or more GPI-anchored proteins play an important role in β-secretase activity and Aβ secretion in mammalian cells. The cell-surface GPI-anchored protein(s) involved in Aβ biogenesis may be excellent therapeutic target(s) in Alzheimer’s disease.

ABCA7 Deficiency Accelerates Amyloid-β Generation and Alzheimer's Neuronal Pathology

In Alzheimers disease (AD), the accumulation and deposition of amyloid-β (Aβ) peptides in the brain is a central event. Aβ is cleaved from amyloid precursor protein (APP) by β-secretase and γ-secretase mainly in neurons. Although mutations in APP, PS1, or PS2 cause early-onset familial AD, ABCA7 encoding ATP-binding cassette transporter A7 is one of the susceptibility genes for late-onset AD (LOAD), in which its loss-of-function variants increase the disease risk. ABCA7 is homologous to a major lipid transporter ABCA1 and is highly expressed in neurons and microglia in the brain. Here, we show that ABCA7 deficiency altered brain lipid profile and impaired memory in ABCA7 knock-out (Abca7−/−) mice. When bred to amyloid model APP/PS1 mice, plaque burden was exacerbated by ABCA7 deficit. In vivo microdialysis studies indicated that the clearance rate of Aβ was unaltered. Interestingly, ABCA7 deletion facilitated the processing of APP to Aβ by increasing the levels of β-site APP cleaving enzyme 1 (BACE1) and sterol regulatory element-binding protein 2 (SREBP2) in primary neurons and mouse brains. Knock-down of ABCA7 expression in neurons caused endoplasmic reticulum stress highlighted by increased level of protein kinase R-like endoplasmic reticulum kinase (PERK) and increased phosphorylation of eukaryotic initiation factor 2α (eIF2α). In the brains of APP/PS1;Abca7−/− mice, the level of phosphorylated extracellular regulated kinase (ERK) was also significantly elevated. Together, our results reveal novel pathways underlying the association of ABCA7 dysfunction and LOAD pathogenesis. SIGNIFICANCE STATEMENT Gene variants in ABCA7 encoding ATP-binding cassette transporter A7 are associated with the increased risk for late-onset Alzheimers disease (AD). Importantly, we found the altered brain lipid profile and impaired memory in ABCA7 knock-out mice. The accumulation of amyloid-β (Aβ) peptides cleaved from amyloid precursor protein (APP) in the brain is a key event in AD pathogenesis and we also found that ABCA7 deficit exacerbated brain Aβ deposition in amyloid AD model APP/PS1 mice. Mechanistically, we found that ABCA7 deletion facilitated the processing of APP and Aβ production by increasing the levels of β-secretase 1 (BACE1) in primary neurons and mouse brains without affecting the Aβ clearance rate in APP/PS1 mice. Our study demonstrates a novel mechanism underlying how dysfunctions of ABCA7 contribute to the risk for AD.

Glycosylphosphatidylinositol-anchor intermediates associate with triton-insoluble membranes in subcellular compartments that include the endoplasmic reticulum.

Glycosylphosphatidylinositol (GPI)-anchored proteins are resistant to solubilization with Triton X-100 at 4 degrees C, and they can be recovered in Triton-insoluble membranes (TIMs) that float to a characteristic buoyant density. Because the GPI structure itself has been shown to target GPI-anchored proteins to TIMs, we investigated the association of GPI-anchor intermediates with TIMs. GPI-anchor biosynthesis involves a pathway of some 10 steps that take place in the endoplasmic reticulum (ER). These intermediates include glucosaminyl-acylphosphatidylinositol [GlcN-(acyl)PI] and later mannosylated GPIs, denoted H6, H7 and H8, that are present not only in the ER but also in other cell compartments, including the plasma membrane. At least two-thirds of the GlcN-(acyl)PI in HeLa D cells and mannosylated GPIs in K562 cells were found in TIMs. Although previous reports have considered TIMs to be derived primarily from the plasma membrane, we recovered TIMs from subcellular fractions enriched in ER membranes. The ER marker calnexin and GPI-anchored proteins as well as N-acetylglucosaminyl-phosphatidylinositol and mannosylated GPIs were present in ER-TIMs. Interestingly, GlcN-PI and H7 were less enriched in ER-TIM than the other GPIs, suggesting that ER-TIMs might reflect a compartmentalization of the GPI-anchor biosynthetic pathway in the ER.

Altered expression of zonula occludens-2 precedes increased blood-brain barrier permeability in a murine model of fulminant hepatic failure.

Brain edema secondary to increased blood–brain barrier (BBB) permeability is a lethal complication in fulminant hepatic failure (FHF). Intact tight junctions (TJ) between brain capillary endothelial cells are critical for normal BBB function. However, the role of TJ in FHF has not been explored. We hypothesized that alterations in the composition of TJ proteins would result in increased BBB permeability in FHF. In this study, FHF was induced in C57BL/6J mice by using azoxymethane. BBB permeability was assessed with sodium fluorescein. Expression of TJ proteins was determined by Western blot, and their cellular distribution was examined using immunofluorescent microscopy. Comatose FHF mice had significant cerebral sodium fluorescein extravasation compared with control and precoma FHF mice, indicating increased BBB permeability. Western blot analysis showed a significant decrease in zonula occludens (ZO)-2 expression starting in the precoma stage. Immunofluorescent microscopy showed a significantly altered distribution pattern of ZO-2 in isolated microvessels from precoma FHF mice. These changes were more prominent in comatose FHF animals. Significant alterations in ZO-2 expression and distribution in the tight junctions preceded the increased BBB permeability in FHF mice. These results suggest that ZO-2 may play an important role in the pathogenesis of brain edema in FHF.

Effect of glycosylphosphatidylinositol (GPI)-phospholipase D overexpression on GPI metabolism

GPI-PLD [glycosylphosphatidylinositol (GPI)-specific phospholipase D (PLD)] is a secreted mammalian enzyme that specifically cleaves GPI-anchored proteins. In addition, the enzyme has been shown to cleave GPI anchor intermediates in cell lysates. The biosynthesis of the GPI anchor is well characterized; however, the mechanisms by which the levels of GPI anchor intermediates are regulated are still unknown. To investigate whether GPI-PLD plays a role in this regulation, we isolated stable HeLa cells overexpressing the enzyme. GPI-PLD-HeLa (GPI-PLD-transfected HeLa) cells showed a 3-fold increase in intracellular GPI-PLD activity and drastically decreased the levels of GPI-anchored proteins when compared with untransfected HeLa controls. Intracellular cleavage of GPI-anchored proteins has been suggested to occur early in the secretory pathway and, in agreement with this proposal, GPI-PLD activity in GPI-PLD-HeLa cells was detected not only in the endoplasmic reticulum and Golgi apparatus, but also in the plasma membrane. The enzyme was also active in lipid rafts, membrane microdomains in which GPI-anchored proteins and GPI anchor intermediates are concentrated, indicating that intracellular GPI-PLD cleavage may also occur in this compartment. Pulse-chase paradigms revealed the turnover rate of the last intermediate of the GPI anchor pathway in GPI-PLD-HeLa cells to be accelerated compared with the controls. Furthermore, 1,10-phenanthroline, a GPI-PLD inhibitor, reversed this effect. Our studies demonstrated that GPI-PLD can cleave not only GPI-anchored proteins, but also GPI anchor intermediates intracellularly. This observation opens the possibility that GPI-PLD can influence the steady-state levels of GPI-anchored proteins by hydrolysing the anchor before and after its attachment to proteins.

Expression and processing analyses of wild type and p.R47H TREM2 variant in Alzheimer's disease brains.

BackgroundGenetic analyses showed that the triggering receptor expressed in myeloid cells 2 (TREM2) p.R47H variant increases the risk for Alzheimer’s disease (AD). The question of whether the p.R47H mutation affects expression or function of the receptor remains unanswered. To address this question we quantified mRNA and analyzed protein profiles of WT and p.R47H TREM2 in human brains.MethodsQuantitative real-time PCR (qPCR) was performed using 2 sets of primers one that detects all TREM2 mRNA isoforms and one specific for the alternative spliced isoform (TREM2alt) that encodes for the extracellular domain (soluble TREM2). Because in the brain TREM2 is expressed primarily in microglial cells, we also assessed the levels of IBA1 to control for microglial variability across samples. For TREM2 protein quantitation and N-glycosylation processing, RIPA brain extracts were analyzed by Western blot before and after EndoH and PNGaseF treatments.ResultsWe identified statistically significant increased levels of TREM2 transcripts in the temporal cortex of AD subjects when compared with controls; TREM2alt was likewise higher in AD cases, but was not significant after adjustment for covariates. Quantitative analysis of TREM2 protein confirmed qPCR results that showed higher levels in AD than in control brains. Among AD subjects, we observed a trend towards higher mRNA and protein TREM2 levels in carriers of the p.R47H risk allele. Analysis of individual TREM2 species found no difference in the relative amounts of mature and immature species, and carboxyl terminal fragments between non carriers and p.R47H samples. Furthermore, TREM2 species from either non carriers or p.R47H brains were equally susceptible to EndoH and PNGaseF treatments.ConclusionsOur results suggest that TREM2 expression is increased in AD. Furthermore, we provide evidence indicating that p.R47H mutation does not affect the levels of TREM2 either directly by altering expression or indirectly by affecting processing of the protein. Our data support previous findings that suggest that p.R47H variant affects TREM2 function by altering binding properties of the receptor rather than expression.

Genetically-controlled Vesicle-Associated Membrane Protein 1 expression may contribute to Alzheimer’s pathophysiology and susceptibility

BackgroundAlzheimer’s disease is a neurodegenerative disorder in which extracellular deposition of β-amyloid (Aβ) oligomers causes synaptic injury resulting in early memory loss, altered homeostasis, accumulation of hyperphosphorylated tau and cell death. Since proteins in the SNAP (Soluble N-ethylmaleimide-sensitive factor Attachment Protein) REceptors (SNARE) complex are essential for neuronal Aβ release at pre-synaptic terminals, we hypothesized that genetically controlled SNARE expression could alter neuronal Aß release at the synapse and hence play an early role in Alzheimer’s pathophysiology.ResultsHere we report 5 polymorphisms in Vesicle-Associated Membrane Protein 1 (VAMP1), a gene encoding a member of the SNARE complex, associated with bidirectionally altered cerebellar VAMP1 transcript levels (all p < 0.05). At the functional level, we demonstrated that control of VAMP1 expression by heterogeneous knockdown in mice resulted in up to 74% reduction in neuronal Aβ exocytosis (p < 0.001). We performed a case-control association study of the 5 VAMP1 expression regulating polymorphisms in 4,667 Alzheimer’s disease patients and 6,175 controls to determine their contribution to Alzheimer’s disease risk. We found that polymorphisms associated with increased brain VAMP1 transcript levels conferred higher risk for Alzheimer’s disease than those associated with lower VAMP1 transcript levels (p = 0.03). Moreover, we also report a modest protective association for a common VAMP1 polymorphism with Alzheimer’s disease risk (OR = 0.88, p = 0.03). This polymorphism was associated with decreased VAMP1 transcript levels (p = 0.02) and was functionally active in a dual luciferase reporter gene assay (p < 0.01).ConclusionsGenetically regulated VAMP1 expression in the brain may modify both Alzheimer’s disease risk and may contribute to Alzheimer’s pathophysiology.

A proteomic study identifies different levels of light chain ferritin in corticobasal degeneration and progressive supranuclear palsy

Clinical and pathological evidence supports the notion that corticobasal degeneration (CBD) and progressive supranuclear palsy (PSP) are distinct, but overlapping neurodegenerative tauopathies. Although both disorders are characterized by abnormal accumulation of 4-repeat tau, they display distinct proteolytic profiles of tau species and they have distinct astrocytic lesions, astrocytic plaques in CBD and tufted astrocytes in PSP. To investigate other differences between these two disorders at the molecular level, we compared the profiles of proteins from caudate nucleus of CBD and PSP by quantitative two-dimensional difference gel electrophoresis. Twenty-one protein spots differentially expressed in CBD and PSP were dissected for mass spectrometry (MS). One of the spots was identified by MS to contain light chain (LC) ferritin. Western blot analysis verified the presence of LC ferritin in this spot and showed that this protein was two-fold higher in caudate of CBD than that of PSP samples. These results were confirmed by LC ferritin immunohistochemistry. Co-labeling of caudate nucleus with tau and LC ferritin antibodies showed the presence of LC ferritin immunoreactivity in astrocytic plaques of CBD, but minimal labeling of tufted astrocytes in PSP. This difference did not reflect the extent of gliosis. Analysis of other brain regions in CBD and PSP showed no difference in LC ferritin levels. Together the data suggest that LC ferritin is a unique marker of astrocytic lesions in CBD, adding further support to the notion that CBD and PSP are distinct clinicopathologic entities.