Cryo-EM structures of amyloid-β 42 filaments from human brains

Hi-res view of human Aβ42 filaments Alzheimer’s disease is characterized by a loss of memory and other cognitive functions and the filamentous assembly of Aβ and tau in the brain. The assembly of Aβ peptides into filaments that end at residue 42 is a central event. Yang et al . used electron cryo–electron microscopy to determine the structures of Aβ42 filaments from human brain (see the Perspective by Willem and Fändrich). They identified two types of related S-shaped filaments, each consisting of two identical protofilaments. These structures will inform the development of better in vitro and animal models, inhibitors of Aβ42 assembly, and imaging agents with increased specificity and sensitivity. —SMH

Turning the tide on Alzheimer’s disease: modulation of γ-secretase

Alzheimer’s disease (AD) is the most common type of neurodegenerative disorder. Amyloid-beta (Aβ) plaques are integral to the “amyloid hypothesis,” which states that the accumulation of Aβ peptides triggers a cascade of pathological events leading to neurodegeneration and ultimately AD. While the FDA approved aducanumab, the first Aβ-targeted therapy, multiple safe and effective treatments will be needed to target the complex pathologies of AD. γ-Secretase is an intramembrane aspartyl protease that is critical for the generation of Aβ peptides. Activity and specificity of γ-secretase are regulated by both obligatory subunits and modulatory proteins. Due to its complex structure and function and early clinical failures with pan inhibitors, γ-secretase has been a challenging drug target for AD. γ-secretase modulators, however, have dramatically shifted the approach to targeting γ-secretase. Here we review γ-secretase and small molecule modulators, from the initial characterization of a subset of NSAIDs to the most recent clinical candidates. We also discuss the chemical biology of γ-secretase, in which small molecule probes enabled structural and functional insights into γ-secretase before the emergence of high-resolution structural studies. Finally, we discuss the recent crystal structures of γ-secretase, which have provided valuable perspectives on substrate recognition and molecular mechanisms of small molecules. We conclude that modulation of γ-secretase will be part of a new wave of AD therapeutics.

Comprehensive review on Alzheimer's Disease: Pathophysiology and its Treatment

Neurodegenerative disorders are marked by the loss of brain neuron activity, resulting in gradual cognitive impairment. The effects of neurodegenerative diseases are severe in terms of pathology and the cost of patient care. The aged, in general, are the most vulnerable. Alzheimer's disease (AD) is a brain ailment that causes cell degradation and is the leading cause of dementia, identified by a loss of thinking ability and independence in daily tasks. The amyloid cascade hypothesis, which attributes clinical signs/symptoms to an abundance of amyloid-beta (Aβ) peptides, enhanced deposition into amyloid plaques, and eventually neuronal destruction, is one theory for pathogenesis AD. The use of acetylcholinesterase inhibitors in AD treatment is based on their favorable effects on the disease's functional, cognitive and behavioral symptoms. However, their involvement in AD pathogenesis is uncertain. This comprehensive review will provide an overview of AD, including the pathophysiology, causes, treatments, and future treatment.

Control of β-Site Amyloid Precursor Protein-Cleaving Enzyme-1 Expression by Protein Kinase C-λ/ι and Nuclear Factor κ-B

Βackground: β-Amyloid precursor protein-cleaving enzyme-1 (BACE1) initiates the production of Aβ-peptides that form Aβ-plaque in Alzheimer’s disease. Methods: Reportedly, acute insulin treatment in normal mice, and hyperinsulinemia in high-fat-fed (HFF) obese/diabetic mice, increase BACE1 activity and levels of Aβ-peptides and phospho- -thr-231-tau in the brain; moreover, these effects are blocked by PKC-λ/ι inhibitors. However, as chemical inhibitors may affect unsuspected targets, we presently used knockout methodology to further examine PKC-λ/ι requirements. We found that total-body heterozygous PKC-λ knockout reduced acute stimulatory effects of insulin and chronic effects of hyperinsulinemia in HFF/obese/diabetic mice, on brain PKC-λ activity and production of Aβ1-40/42 and phospho-thr-231-tau. This protection in HFF mice may reflect that hepatic PKC-λ haploinsufficiency prevents the development of glucose intolerance and hyperinsulinemia. Results: On the other hand, heterozygous knockout of PKC-λ markedly reduced brain levels of BACE1 protein and mRNA, and this may reflect diminished activation of nuclear factor kappa-B (NFκB), which is activated by PKC-λ and increases BACE1 and proinflammatory cytokine transcription. Accordingly, whereas intravenous administration of aPKC inhibitor diminished aPKC activity and BACE1 levels by 50% in the brain and 90% in the liver, nasally-administered inhibitor reduced aPKC activity and BACE1 mRNA and protein levels by 50-70% in the brain while sparing the liver. Additionally, 24-hour insulin treatment in cultured human-derived neurons increased NFκB activity and BACE1 levels, and these effects were blocked by various PKC-λ/ι inhibitors. Conclusion: PKC-λ/ι controls NFκB activity and BACE1 expression; PKC-λ/ι inhibitors may be used nasally to target brain PKC-λ/ι or systemically to block both liver and brain PKC-λ/ι, to regulate NFκB-dependent BACE1 and proinflammatory cytokine expression.

Probing the Influence of Single-Site Mutations in the Central Cross-β Region of Amyloid β (1–40) Peptides

Amyloid β (Aβ) is a peptide known to form amyloid fibrils in the brain of patients suffering from Alzheimer’s disease. A complete mechanistic understanding how Aβ peptides form neurotoxic assemblies and how they kill neurons has not yet been achieved. Previous analysis of various Aβ40 mutants could reveal the significant importance of the hydrophobic contact between the residues Phe19 and Leu34 for cell toxicity. For some mutations at Phe19, toxicity was completely abolished. In the current study, we assessed if perturbations introduced by mutations in the direct proximity of the Phe19/Leu34 contact would have similar relevance for the fibrillation kinetics, structure, dynamics and toxicity of the Aβ assemblies. To this end, we rationally modified positions Phe20 or Gly33. A small library of Aβ40 peptides with Phe20 mutated to Lys, Tyr or the non-proteinogenic cyclohexylalanine (Cha) or Gly33 mutated to Ala was synthesized. We used electron microscopy, circular dichroism, X-ray diffraction, solid-state NMR spectroscopy, ThT fluorescence and MTT cell toxicity assays to comprehensively investigate the physicochemical properties of the Aβ fibrils formed by the modified peptides as well as toxicity to a neuronal cell line. Single mutations of either Phe20 or Gly33 led to relatively drastic alterations in the Aβ fibrillation kinetics but left the global, as well as the local structure, of the fibrils largely unchanged. Furthermore, the introduced perturbations caused a severe decrease or loss of cell toxicity compared to wildtype Aβ40. We suggest that perturbations at position Phe20 and Gly33 affect the fibrillation pathway of Aβ40 and, thereby, influence the especially toxic oligomeric species manifesting so that the region around the Phe19/Leu34 hydrophobic contact provides a promising site for the design of small molecules interfering with the Aβ fibrillation pathway.

STAT3 inhibitor mitigates cerebral amyloid angiopathy and parenchymal amyloid plaques while improving cognitive functions and brain networks

Previous reports indicate a potential role for signal transducer and activator of transcription 3 (STAT3) in amyloid-β (Aβ) processing and neuritic plaque pathogenesis. In the present study, the impact of STAT3 inhibition on cognition, cerebrovascular function, amyloid pathology, oxidative stress, and neuroinflammation was studied using in vitro and in vivo models of Alzheimer’s disease (AD)-related pathology. For in vitro experiments, human brain vascular smooth muscle cells (HBVSMC) and human brain microvascular endothelial cells (HBMEC) were used, and these cultured cells were exposed to Aβ peptides followed by measurement of activated forms of STAT3 expression and reactive oxygen species (ROS) generation. Further, 6 months old 5XFAD/APOE4 (5XE4) mice and age-matched negative littermates were used for in vivo experiments. These mice were treated with STAT3 specific inhibitor, LLL-12 for 2 months followed by neurobehavioral and histopathological assessment. In vitro experiments showed exposure of cerebrovascular cells to Aβ peptides upregulated activated forms of STAT3 and produced STAT3-mediated vascular oxidative stress. 5XE4 mice treated with the STAT3-specific inhibitor (LLL-12) improved cognitive functions and functional connectivity and augmented cerebral blood flow. These functional improvements were associated with a reduction in neuritic plaques, cerebral amyloid angiopathy (CAA), oxidative stress, and neuroinflammation. Reduction in amyloid precursor protein (APP) processing and attenuation of oxidative modification of lipoprotein receptor related protein-1 (LRP-1) were identified as potential underlying mechanisms. These results demonstrate the broad impact of STAT3 on cognitive functions, parenchymal and vascular amyloid pathology and highlight the therapeutic potential of STAT3 specific inhibition for treatment of AD and CAA.

Rab35 and glucocorticoids regulate APP and BACE1 trafficking to modulate Aβ production

Chronic stress and elevated glucocorticoids (GCs), the major stress hormones, are risk factors for Alzheimer’s disease (AD) and promote AD pathomechanisms, including overproduction of toxic amyloid-β (Aβ) peptides and intraneuronal accumulation of hyperphosphorylated Tau protein. The latter is linked to downregulation of the small GTPase Rab35, which mediates Tau degradation via the endolysosomal pathway. Whether Rab35 is also involved in Aβ overproduction remains an open question. Here, we find that hippocampal Rab35 levels are decreased not only by stress/GC but also by aging, another AD risk factor. Moreover, we show that Rab35 negatively regulates Aβ production by sorting amyloid precursor protein (APP) and β-secretase (BACE1) out of the endosomal network, where they interact to produce Aβ. Interestingly, Rab35 coordinates distinct intracellular trafficking steps for BACE1 and APP, mediated by its effectors OCRL and ACAP2, respectively. Finally, we demonstrate that Rab35 overexpression prevents the amyloidogenic trafficking of APP and BACE1 induced by high GC levels. These studies identify Rab35 as a key regulator of APP processing and suggest that its downregulation may contribute to stress-related and AD-related amyloidogenesis.

Down-regulation of cyclin D2 in amyloid β toxicity, inflammation, and Alzheimer’s disease

In the current study, we analyzed the effects of the systemic inflammatory response (SIR) and amyloid β (Aβ) peptide on the expression of genes encoding cyclins and cyclin-dependent kinase (Cdk) in: (i) PC12 cells overexpressing human beta amyloid precursor protein (βAPP), wild-type (APPwt-PC12), or carrying the Swedish mutantion (APPsw-PC12); (ii) the murine hippocampus during SIR; and (iii) Alzheimer’s disease (AD) brain. In APPwt-PC12 expression of cyclin D2 (cD2) was exclusively reduced, and in APPsw-PC12 cyclins cD2 and also cA1 were down-regulated, but cA2, cB1, cB2, and cE1 were up-regulated. In the SIR cD2, cB2, cE1 were found to be significantly down-regulated and cD3, Cdk5, and Cdk7 were significantly up-regulated. Cyclin cD2 was also found to be down-regulated in AD neocortex and hippocampus. Our novel data indicate that Aβ peptide and inflammation both significantly decreased the expression of cD2, suggesting that Aβ peptides may also contribute to downregulation of cD2 in AD brain.