Martin Citron

Secreted amyloid β-protein similar to that in the senile plaques of Alzheimer's disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease

To determine whether the presenilin 1 (PS1), presenilin 2 (PS2) and amyloid β-protein precursor (APP) mutations linked to familial Alzheimers disease (FAD) increase the extracellular concentration of amyloid β–protein (Aβ) ending at Aβ42(43) in vivo, we performed a blinded comparison of plasma Aβ levels in carriers of these mutations and controls. Aβ1 –42(43) was elevated in plasma from subjects with FAD–linked PS1 (P < 0.0001), PS2N141I (P = 0.009), APPK670N,M671L (P < 0.0001), and APPV717I (one subject) mutations. Aβ ending at Aβ42(43) was also significantly elevated in fibroblast media from subjects with PS1 (P < 0.0001) or P52 (P = 0.03) mutations. These findings indicate that the FAD–linked mutations may all cause Alzheimers disease by increasing the extracellular concentration of Aβ42(43), thereby fostering cerebral deposition of this highly amyloidogenic peptide.

Mice deficient in BACE1, the Alzheimer's β-secretase, have normal phenotype and abolished β-amyloid generation

Mice deficient in BACE1 (beta-site APP cleaving enzyme 1) are healthy, fertile and appear normal in gross anatomy, tissue histology, hematology and clinical chemistry. BACE1−/− mice also hemizygous for an amyloid precursor protein (APP) transgene lack brain β-amyloid (Aβ) and β-secretase-cleaved APP C-terminal fragments (CTFs). These results provide validation of BACE1 as the major β-secretase in vivo and suggest that therapeutic inhibition of BACE1 for the treatment of Alzheimers disease may be free of mechanism-based toxicity.

BOTH FAMILIAL PARKINSON'S DISEASE MUTATIONS ACCELERATE ALPHA -SYNUCLEIN AGGREGATION

Parkinson’s disease (PD) is a neurodegenerative disorder that is pathologically characterized by the presence of intracytoplasmic Lewy bodies, the major component of which are filaments consisting of α-synuclein. Two recently identified point mutations in α-synuclein are the only known genetic causes of PD, but their pathogenic mechanism is not understood. Here we show that both wild type and mutant α-synuclein form insoluble fibrillar aggregates with antiparallel β-sheet structure upon incubation at physiological temperature in vitro. Importantly, aggregate formation is accelerated by both PD-linked mutations. Under the experimental conditions, the lag time for the formation of precipitable aggregates is about 280 h for the wild type protein, 180 h for the A30P mutant, and only 100 h for the A53T mutant protein. These data suggest that the formation of α-synuclein aggregates could be a critical step in PD pathogenesis, which is accelerated by the PD-linked mutations.

Elevated β-secretase expression and enzymatic activity detected in sporadic Alzheimer disease [1]

Elevated β-secretase expression and enzymatic activity detected in sporadic Alzheimer disease

alpha-synuclein fibrillogenesis is nucleation-dependent. Implications for the pathogenesis of Parkinson's disease.

Parkinson’s disease (PD) is a neurodegenerative disorder that is pathologically characterized by the presence of intracytoplasmic Lewy bodies, the major components of which are filaments consisting of α-synuclein. Two recently identified point mutations in α-synuclein are the only known genetic causes of PD. α-Synuclein fibrils similar to the Lewy body filaments can be formedin vitro, and we have shown recently that both PD-linked mutations accelerate their formation. This study addresses the mechanism of α-synuclein aggregation: we show that (i) it is a nucleation-dependent process that can be seeded by aggregated α-synuclein functioning as nuclei, (ii) this fibril growth follows first-order kinetics with respect to α-synuclein concentration, and (iii) mutant α-synuclein can seed the aggregation of wild type α-synuclein, which leads us to predict that the Lewy bodies of familial PD patients with α-synuclein mutations will contain both, the mutant and the wild type protein. Finally (iv), we show that wild type and mutant forms of α-synuclein do not differ in their critical concentrations. These results suggest that differences in aggregation kinetics of α-synucleins cannot be explained by differences in solubility but are due to different nucleation rates. Consequently, α-synuclein nucleation may be the rate-limiting step for the formation of Lewy body α-synuclein fibrils in Parkinson’s disease.

BACE1 Deficiency Rescues Memory Deficits and Cholinergic Dysfunction in a Mouse Model of Alzheimer's Disease

beta-site APP cleaving enzyme 1 (BACE1) is the beta-secretase enzyme required for generating pathogenic beta-amyloid (Abeta) peptides in Alzheimers disease (AD). BACE1 knockout mice lack Abeta and are phenotypically normal, suggesting that therapeutic inhibition of BACE1 may be free of mechanism-based side effects. However, direct evidence that BACE1 inhibition would improve cognition is lacking. Here we show that BACE1 null mice engineered to overexpress human APP (BACE1(-/-).Tg2576(+)) are rescued from Abeta-dependent hippocampal memory deficits. Moreover, impaired hippocampal cholinergic regulation of neuronal excitability found in the Tg2576 AD model is ameliorated in BACE1(-/-).Tg2576(+) bigenic mice. The behavioral and electrophysiological rescue of deficits in BACE1(-/-).Tg2576(+) mice is correlated with a dramatic reduction of cerebral Abeta40 and Abeta42 levels and occurs before amyloid deposition in Tg2576 mice. Our gene-based approach demonstrates that lower Abeta levels are beneficial for AD-associated memory impairments, validating BACE1 as a therapeutic target for AD.

Strategies for disease modification in Alzheimer's disease

Treating Alzheimers disease (AD) is the biggest unmet medical need in neurology. Current drugs improve symptoms, but do not have profound disease-modifying effects. Three main classes of disease-modification approaches can be defined: one that is broadly neurotrophic or neuroprotective, one that targets specific aspects of AD pathology, and one that is based on epidemiological observation. This review discusses all three approaches, with particular emphasis on anti-amyloid strategies — currently the most active area of investigation. The approaches that are reviewed include secretase inhibition, amyloid-β aggregation inhibition, immunotherapy and strategies that might indirectly affect the amyloid pathway.

A loss of function mutation of presenilin-2 interferes with amyloid beta-peptide production and notch signaling.

Presenilin-1 (PS1) facilitates γ-secretase cleavage of the β-amyloid precursor protein and the intramembraneous cleavage of Notch1. Although Alzheimer’s disease-associated mutations in the homologous presenilin (PS2) gene elevate amyloid β-peptide (Aβ42) production like PS1 mutations, here we demonstrate that a gene ablation of PS2 (unlike that of PS1) in mice does not result in a severe phenotype resembling that of Notch-ablated animals. To investigate the amyloidogenic function of PS2 more directly, we mutagenized a conserved aspartate at position 366 to alanine, because the corresponding residue of PS1 is known to be required for its amyloidogenic function. Cells expressing the PS2 D366A mutation exhibit significant deficits in proteolytic processing of β-amyloid precursor protein indicating a defect in γ-secretase activity. The reduced γ-secretase activity results in the almost complete inhibition of Aβ and p3 production in cells stably expressing PS2 D366A, whereas cells overexpressing the wild-type PS2 cDNA produce robust levels of Aβ and p3. Using highly sensitive in vivo assays, we demonstrate that the PS2 D366A mutation not only blocks γ-secretase activity but also inactivates PS2 activity in Notch signaling by inhibiting the proteolytic release of the cytoplasmic Notch1 domain. These data suggest that PS2 is functionally involved in Aβ production and Notch signaling by facilitating similar proteolytic cleavages.

Presenilin Proteins Undergo Heterogeneous Endoproteolysis between Thr291and Ala299and Occur as Stable N- and C-Terminal Fragments in Normal and Alzheimer Brain Tissue

Humans inheriting missense mutations in the presenilin (PS)1 and -2 genes undergo progressive cerebral deposition of the amyloid beta-protein at an early age and develop a clinically and pathologically severe form of familial Alzheimers disease (FAD). Because PS1 mutations cause the most aggressive known form of AD, it is important to elucidate the structure and function of this multitransmembrane protein in the brain. Using a panel of region-specific PS antibodies, we characterized the presenilin polypeptides in mammalian tissues, including brains of normal, AD, and PS1-linked FAD subjects, and in transfected and nontransfected cell lines. Very little full-length PS1 or -2 was detected in brain and untransfected cells; instead the protein occurred as a heterogeneous array of stable N- and C-terminal proteolytic fragments that differed subtly among cell types and mammalian tissues. Sequencing of the major C-terminal fragment from PS1-transfected human 293 cells showed that the principal endoproteolytic cleavage occurs at and near Met298 in the proximal portion of the large hydrophilic loop. Full-length PS1 in these cells is quickly turned over (T1/2 approximately 60 min), in part to the two major fragments. The sizes and amounts of the PS fragments were not significantly altered in four FAD brains with the Cys410Tyr PS1 missense mutation. Our results indicate that presenilins are rapidly processed to N- and C-terminal fragments in both neural and nonneural cells and that interference with this processing is not an obligatory feature of FAD-causing mutations.

Enhanced Production and Oligomerization of the 42-residue Amyloid β-Protein by Chinese Hamster Ovary Cells Stably Expressing Mutant Presenilins

Mutations in the presenilin 1 (PS1) and presenilin 2 (PS2) genes cause the most common and aggressive form of early onset familial Alzheimers disease. To elucidate their pathogenic mechanism, wild-type (wt) or mutant (M146L, C410Y) PS1 and wt or mutant (M239V) PS2 genes were stably transfected into Chinese hamster ovary cells that overexpress the β-amyloid precursor protein (APP). The identity of the 43-45-kDa PS1 holoproteins was confirmed by N-terminal radiosequencing. PS1 was rapidly processed (t1/2 = 40 min) in the endoplasmic reticulum into stable fragments. Wild-type and mutant PS2 holoproteins exhibited similar half lives (1.5 h); however, their endoproteolytic fragments showed both mutation-specific and cell type-specific differences. Mutant PS1 or PS2 consistently induced a 1.4-2.5-fold increase (p < 0.001) in the relative production of the highly amyloidogenic 42-residue form of amyloid β-protein (Aβ42) as determined by quantitative immunoprecipitation and by enzyme-linked immunosorbent assay. In mutant PS1 and PS2 cell lines with high increases in Aβ42/Aβtotal ratios, spontaneous formation of low molecular weight oligomers of Aβ42 was observed in media, suggesting enhanced Aβ aggregation from the elevation of Aβ42. We conclude that mutant PS1 and PS2 proteins enhance the proteolysis of β-amyloid precursor protein by the γ-secretase cleaving at Aβ residue 42, thereby promoting amyloidogenesis.