Ayano Kimura

Alternative Selection of β-Site APP-Cleaving Enzyme 1 (BACE1) Cleavage Sites in Amyloid β-Protein Precursor (APP) Harboring Protective and Pathogenic Mutations within the Aβ Sequence.

β-Site APP-cleaving enzyme 1 (BACE1) cleaves amyloid β-protein precursor (APP) at the bond between Met671 and Asp672 (β-site) to generate the carboxyl-terminal fragment (CTFβ/C99). BACE1 also cleaves APP at another bond between Thr681 and Gln682 (β′-site), yielding CTFβ′/C89. Cleavage of CTFβ/C99 by γ-secretase generates Aβ(1-XX), whereas cleavage of CTFβ′/C89 generates Aβ(11-XX). Thus, β′-site cleavage by BACE1 is amyloidolytic rather than amyloidogenic. β′ cleavage of mouse APP is more common than the corresponding cleavage of human APP. We found that the H684R substitution within human Aβ, which replaces the histidine in the human protein with the arginine found at the corresponding position in mouse, facilitated β′ cleavage irrespective of the species origin of BACE1, thereby significantly increasing the level of Aβ(11-XX) and decreasing the level of Aβ(1-XX). Thus, amino acid substitutions within the Aβ sequence influenced the selectivity of alternative β- or β′-site cleavage of APP by BACE1. In familial Alzheimers disease (FAD), the APP gene harbors pathogenic variations such as the Swedish (K670N/M671L), Leuven (E682K), and A673V mutations, all of which decrease Aβ(11–40) generation, whereas the protective Icelandic mutation (A673T) increases generation of Aβ(11–40). Thus, A673T promotes β′ cleavage of APP and protects subjects against AD. In addition, CTFβ/C99 was cleaved by excess BACE1 activity to generate CTFβ′/C89, followed by Aβ(11–40), even if APP harbored pathogenic mutations. The resultant Aβ(11–40) was more metabolically labile in vivo than Aβ(1–40). Our analysis suggests that some FAD mutations in APP are amyloidogenic and/or amyloidolytic via selection of alternative BACE1 cleavage sites.

Cytoplasmic Fragment of Alcadein α Generated by Regulated Intramembrane Proteolysis Enhances Amyloid β-Protein Precursor (APP) Transport into the Late Secretory Pathway and Facilitates APP Cleavage

Background: Alcadein α (Alcα) forms a ternary complex with APP and X11L. Results: Transport into the nerve terminus and metabolism of APP were facilitated in Alcα CTF transgenic mice, along with an increase in Aβ. Conclusion: Alcα ICD, a product of γ-secretase cleavage of Alcα CTF, enhanced APP trafficking from the ternary complex into a late secretory pathway. Significance: Novel function of Alcadein α results from regulated intramembrane proteolysis. The neural type I membrane protein Alcadein α (Alcα), is primarily cleaved by amyloid β-protein precursor (APP) α-secretase to generate a membrane-associated carboxyl-terminal fragment (Alcα CTF), which is further cleaved by γ-secretase to secrete p3-Alcα peptides and generate an intracellular cytoplasmic domain fragment (Alcα ICD) in the late secretory pathway. By association with the neural adaptor protein X11L (X11-like), Alcα and APP form a ternary complex that suppresses the cleavage of both Alcα and APP by regulating the transport of these membrane proteins into the late secretory pathway where secretases are active. However, it has not been revealed how Alcα and APP are directed from the ternary complex formed largely in the Golgi into the late secretory pathway to reach a nerve terminus. Using a novel transgenic mouse line expressing excess amounts of human Alcα CTF (hAlcα CTF) in neurons, we found that expression of hAlcα CTF induced excess production of hAlcα ICD, which facilitated APP transport into the nerve terminus and enhanced APP metabolism, including Aβ generation. In vitro cell studies also demonstrated that excess expression of Alcα ICD released both APP and Alcα from the ternary complex. These results indicate that regulated intramembrane proteolysis of Alcα by γ-secretase regulates APP trafficking and the production of Aβ in vivo.

Mechanism of Intramembrane Cleavage of Alcadeins by γ-Secretase

Background Alcadein proteins (Alcs; Alcα, Alcβand Alcγ) are predominantly expressed in neurons, as is Alzheimers β-amyloid (Aβ) precursor protein (APP). Both Alcs and APP are cleaved by primary α- or β-secretase to generate membrane-associated C-terminal fragments (CTFs). Alc CTFs are further cleaved by γ-secretase to secrete p3-Alc peptide along with the release of intracellular domain fragment (Alc ICD) from the membrane. In the case of APP, APP CTFβ is initially cleaved at the ε-site to release the intracellular domain fragment (AICD) and consequently the γ-site is determined, by which Aβ generates. The initial ε-site is thought to define the final γ-site position, which determines whether Aβ40/43 or Aβ42 is generated. However, initial intracellular ε-cleavage sites of Alc CTF to generate Alc ICD and the molecular mechanism that final γ-site position is determined remains unclear in Alcs. Methodology Using HEK293 cells expressing Alcs plus presenilin 1 (PS1, a catalytic unit of γ-secretase) and the membrane fractions of these cells, the generation of p3-Alc possessing C-terminal γ-cleavage site and Alc ICD possessing N-terminal ε-cleavage site were analysed with MALDI-TOF/MS. We determined the initial ε-site position of all Alcα, Alcβ and Alcγ, and analyzed the relationship between the initially determined ε-site position and the final γ-cleavage position. Conclusions The initial ε-site position does not always determine the final γ-cleavage position in Alcs, which differed from APP. No additional γ-cleavage sites are generated from artificial/non-physiological positions of ε-cleavage for Alcs, while the artificial ε-cleavage positions can influence in selection of physiological γ-site positions. Because alteration of γ-secretase activity is thought to be a pathogenesis of sporadic Alzheimers disease, Alcs are useful and sensitive substrate to detect the altered cleavage of substrates by γ-secretase, which may be induced by malfunction of γ-secretase itself or changes of membrane environment for enzymatic reaction.

Cytoplasmic Fragment of Alcadeinα Generated by Regulated Intramembrane Proteolysis Enhances APP Transport into the Late-Secretory Pathway and Facilitates APP Cleavage

Background: Alcadein α (Alcα) forms a ternary complex with APP and X11L. Results: Transport into the nerve terminus and metabolism of APP were facilitated in Alcα CTF transgenic mice, along with an increase in Aβ. Conclusion: Alcα ICD, a product of γ-secretase cleavage of Alcα CTF, enhanced APP trafficking from the ternary complex into a late secretory pathway. Significance: Novel function of Alcadein α results from regulated intramembrane proteolysis. The neural type I membrane protein Alcadein α (Alcα), is primarily cleaved by amyloid β-protein precursor (APP) α-secretase to generate a membrane-associated carboxyl-terminal fragment (Alcα CTF), which is further cleaved by γ-secretase to secrete p3-Alcα peptides and generate an intracellular cytoplasmic domain fragment (Alcα ICD) in the late secretory pathway. By association with the neural adaptor protein X11L (X11-like), Alcα and APP form a ternary complex that suppresses the cleavage of both Alcα and APP by regulating the transport of these membrane proteins into the late secretory pathway where secretases are active. However, it has not been revealed how Alcα and APP are directed from the ternary complex formed largely in the Golgi into the late secretory pathway to reach a nerve terminus. Using a novel transgenic mouse line expressing excess amounts of human Alcα CTF (hAlcα CTF) in neurons, we found that expression of hAlcα CTF induced excess production of hAlcα ICD, which facilitated APP transport into the nerve terminus and enhanced APP metabolism, including Aβ generation. In vitro cell studies also demonstrated that excess expression of Alcα ICD released both APP and Alcα from the ternary complex. These results indicate that regulated intramembrane proteolysis of Alcα by γ-secretase regulates APP trafficking and the production of Aβ in vivo.

Facilitation of brain mitochondrial activity by 5-aminolevulinic acid in a mouse model of Alzheimer's disease.

The activities of mitochondrial enzymes, which are essential for neural function, decline with age and in age-related disease. In particular, the activity of cytochrome c oxidase (COX/complex IV) decreases in patients with Alzheimers disease (AD). COX, a mitochondrial inner membrane protein complex that contains heme, plays an essential role in the electron transport chain that generates ATP. Heme synthesis begins with 5-aminolevulinic acid (5-ALA) in mitochondria. 5-ALA synthetase is the rate-limiting enzyme in heme synthesis, suggesting that supplementation with 5-ALA might help preserve mitochondrial activity in the aged brain. We administered a diet containing 5-ALA to triple-transgenic AD (3xTg-AD) model mice for 6 months, starting at 3 months of age. COX activity and protein expression, as well as mitochondrial membrane potential, were significantly higher in brains of 5-ALA-fed mice than in controls. Synaptotagmin protein levels were also significantly higher in 5-ALA-fed mice, suggesting improved preservation of synapses. Although brain Aβ levels tended to decrease in 5-ALA-fed mice, we observed no other significant changes in other biochemical and pathological hallmarks of AD. Nevertheless, our study suggests that daily oral administration of 5-ALA could preserve mitochondrial enzyme activities in the brains of aged individuals, thereby contributing to the preservation of neural activity.

Stabilization of intracellular trafficking and metabolism of amyloid β-protein precursor and Alcadein β by apolipoprotein E.

Intracellular metabolism of amyloid β‐protein precursor (APP) is important for the pathogenesis of Alzheimers disease (AD). Alcadeins (Alcα, Alcβ, and Alcγ) are neural membrane proteins similar to APP in their localization, metabolism, and cellular function. Isoform ε4 of apolipoprotein E (ApoE) is a major risk factor for AD. We found that ApoE expression attenuated intracellular trafficking of APP and Alcβ, resulting in metabolic stabilization of both proteins. By contrast, Alcα intracellular proteolysis was facilitated by ApoE expression, which was not due to an increase in the primary cleavage of Alcα. This difference may result from binding of ApoE to membrane proteins.

Brain Aging as a Cause of Alzheimer’s Disease

Alzheimer’s disease (AD) is the most common form of senile dementia. Identification of genes causally associated with familial Alzheimer’s disease (FAD) advanced our understanding of the molecular mechanisms of AD pathogenesis. However, FAD is much less common than sporadic Alzheimer’s disease (SAD), which constitutes the majority of cases. Despite its similar pathology (albeit at a later age of onset), SAD is not linked to mutations in FAD-associated genes. In both FAD and SAD, the generation and oligomerization of amyloid β (Aβ) peptide play central roles in neurotoxicity, but it remains unclear how qualitative and quantitative alterations in Aβ occur in SAD patients in the absence of causative mutations. The predominant risk factor for SAD is aging, suggesting that some as-yet-unknown alterations in the aged brain augment the amyloidogenic metabolism of APP and promote the neural toxicity of Aβ oligomers. In this chapter, we discuss potential biochemical changes in amyloid β precursor protein (APP) and proteins related to APP metabolism and function in the aged brain. APP axonal transport, membrane microlocalization and metabolism, including generation of Aβ in neurons, are regulated by interactions with several cytoplasmic proteins and phosphorylation of the APP cytoplasmic region. Age-related decline or aberration in the regulation of APP transport, localization and metabolism may induce generation of altered Aβ. Here, we focus on APP phosphorylation at threonine 668 in the cytoplasmic domain and the roles of APP regulatory proteins, including X11-like (X11L), JIP1, kinesin-1, and Alcadein, on the regulation of APP metabolism and intracellular trafficking.