Fe65 does not stabilize AICD during activation of transcription in a luciferase assay

Amyloid Precursor Protein (APP) Controls the Expression of the Transcriptional Activator Neuronal PAS Domain Protein 4 (NPAS4) and Synaptic GABA Release

The amyloid precursor protein (APP) has been extensively studied as the precursor of the β-amyloid (Aβ) peptide, the major component of the senile plaques found in the brain of Alzheimer’s disease (AD) patients. However, the function of APP per se in neuronal physiology remains to be fully elucidated. APP is expressed at high levels in the brain. It resembles a cell adhesion molecule or a membrane receptor, suggesting that its function relies on cell-cell interaction and/or activation of intracellular signaling pathways. In this respect, the APP intracellular domain (AICD) was reported to act as a transcriptional regulator. Here, we used a transcriptome-based approach to identify the genes transcriptionally regulated by APP in the rodent embryonic cortex and on maturation of primary cortical neurons. Surprisingly, the overall transcriptional changes were subtle, but a more detailed analysis pointed to genes clustered in neuronal-activity dependent pathways. In particular, we observed a decreased transcription of neuronal PAS domain protein 4 (NPAS4) in APP−/− neurons. NPAS4 is an inducible transcription factor (ITF) regulated by neuronal depolarization. The downregulation of NPAS4 co-occurs with an increased production of the inhibitory neurotransmitter GABA and a reduced expression of the GABA_A receptors α1. CRISPR-Cas-mediated silencing of NPAS4 in neurons led to similar observations. Patch-clamp investigation did not reveal any functional decrease of GABA_A receptors activity, but long-term potentiation (LTP) measurement supported an increased GABA component in synaptic transmission of APP−/− mice. Together, NPAS4 appears to be a downstream target involved in APP-dependent regulation of inhibitory synaptic transmission.

Presenilin transmembrane domain 8 conserved AXXXAXXXG motifs are required for the activity of the γ-secretase complex

Understanding the molecular mechanisms controlling the physiological and pathological activity of γ-secretase represents a challenging task in Alzheimer disease research. The assembly and proteolytic activity of this enzyme require the correct interaction of the 19 transmembrane domains (TMDs) present in its four subunits, including presenilin (PS1 or PS2), the γ-secretase catalytic core. GXXXG and GXXXG-like motifs are critical for TMDs interactions as well as for protein folding and assembly. The GXXXG motifs on γ-secretase subunits (e.g. APH-1) or on γ-secretase substrates (e.g. APP) are known to be involved in γ-secretase assembly and in Aβ peptide production, respectively. We identified on PS1 and PS2 TMD8 two highly conserved AXXXAXXXG motifs. The presence of a mutation causing an inherited form of Alzheimer disease (familial Alzheimer disease) in the PS1 motif suggested their involvement in the physiopathological configuration of the γ-secretase complex. In this study, we targeted the role of these motifs on TMD8 of PSs, focusing on their role in PS assembly and catalytic activity. Each motif was mutated, and the impact on complex assembly, activity, and substrate docking was monitored. Different amino acid substitutions on the same motif resulted in opposite effects on γ-secretase activity, without affecting the assembly or significantly impairing the maturation of the complex. Our data suggest that AXXXAXXXG motifs in PS TMD8 are key determinants for the conformation of the mature γ-secretase complex, participating in the switch between the physiological and pathological functional conformations of the γ-secretase.

The physiology of the β-amyloid precursor protein intracellular domain AICD

The amyloid-β precursor protein (βAPP) undergoes several cleavages by enzymatic activities called secretases. Numerous studies aimed at studying the biogenesis and catabolic fate of Aβ peptides, the proteinaceous component of the senile plaques that accumulate in Alzheimer's disease-affected brains. Relatively recently, another secretase-mediated β-APP-derived catabolite called APP IntraCellular Domain (AICD) entered the game. Whether AICD corresponded to a biologically inert by-pass product of βAPP processing or whether it could harbor its own function remained questionable. In this study, we review the mechanisms by which AICD is generated and how its production is regulated. Furthermore, we discuss the degradation mechanism underlying its rapid catabolic fate. Finally, we review putative AICD-related functions and more particularly, the numerous studies indicating that AICD could translocate to the nucleus and control at a transcriptional level, the expression of a series of proteins involved in various functions including the control of cell death and Aβ degradation.

In vitro screening on β-amyloid peptide production of plants used in traditional medicine for cognitive disorders

AIM OF THE STUDY

The aim of the study was to investigate the activity on β-amyloid peptide production of crude extracts of 9 plant species traditionally used in Benin or in Madagascar for the treatment of cognitive disorders, in order to select candidates for Alzheimer's disease treatment.

MATERIALS AND METHODS

For each species, hexane, dichloromethane, ethyl-acetate and water extracts were tested, at non-toxic concentrations, on CHO cells overexpressing the human neuronal β-amyloid peptide precursor (APP695) to measure variations of APP processing (by Western-blotting) and, for the most active, of Aβ-amyloid production (by ECLIA).

RESULTS

We observed, at non-toxic concentrations, a significant increase in CTF/APP ratio with Oldenlandia affinis cyclotide-enriched fraction, Prosopis africana EtOAc extract, Pterocarpus erinaceus aqueous extract and Trichilia emetica hexane extract. We also showed that the Pterocarpus erinaceus extract significantly decreased Aβ production, displaying effects similar to those of DAPT (γ-secretase inhibitor) on APP processing, but may act on another inhibition site.

CONCLUSION

These active extracts are worth further studies to isolate the compounds responsible for the observed activities, to analyze their mode of action and determine their clinical potentials.

Epigenetic control of aquaporin 1 expression by the amyloid precursor protein

Cellular processing of the amyloid precursor protein (APP) has been extensively studied, but its precise function remains elusive. The intracellular domain of APP has been proposed to regulate expression of several genes by mechanisms that are largely unknown. We report that APP regulates expression of the aquaporin 1 (AQP1) gene in mouse embryonic fibroblasts and in transgenic mice. AQP1 mRNA and protein were down-regulated in fibroblasts lacking APP or presenilin 2 in which AQP1 expression was restored by stable expression of full-length APP or presenilin 2 but not by APP deleted from its carboxy-terminal domain. The transcriptional activity of the AQP1 gene promoter and the stability of AQP1 mRNA were identical in fibroblasts expressing or not expressing APP. Control of AQP1 expression by APP was sensitive to trichostatin A, an histone deacetylase inhibitor, and histone deacetylase activity coimmunoprecipitated with APP. Altogether, these data show that a presenilin-2-dependent gamma-secretase activity releases the intracellular domain of APP involved in the epigenetic control of AQP1 expression. Since AQP1 is found in astrocytes surrounding senile plaques, this epigenetic control of AQP1 expression could have important implications in Alzheimer disease.

A helix-to-coil transition at the epsilon-cut site in the transmembrane dimer of the amyloid precursor protein is required for proteolysis

Processing of amyloid precursor protein (APP) by gamma-secretase is the last step in the formation of the Abeta peptides associated Alzheimer's disease. Solid-state NMR spectroscopy is used to establish the structural features of the transmembrane (TM) and juxtamembrane (JM) domains of APP that facilitate proteolysis. Using peptides corresponding to the APP TM and JM regions (residues 618-660), we show that the TM domain forms an alpha-helical homodimer mediated by consecutive GxxxG motifs. We find that the APP TM helix is disrupted at the intracellular membrane boundary near the epsilon-cleavage site. This helix-to-coil transition is required for gamma-secretase processing; mutations that extend the TM alpha-helix inhibit epsilon cleavage, leading to a low production of Abeta peptides and an accumulation of the alpha- and beta-C-terminal fragments. Our data support a progressive cleavage mechanism for APP proteolysis that depends on the helix-to-coil transition at the TM-JM boundary and unraveling of the TM alpha-helix.

The amyloid precursor protein intracellular domain (AICD) as modulator of gene expression, apoptosis, and cytoskeletal dynamics-relevance for Alzheimer's disease

Since the discovery of the amyloid precursor protein (APP) in 1987, extensive research has been conducted analyzing the APP-derived beta-amyloid (Abeta) which is found in massive quantities in senile plaques of Alzheimer disease (AD) patients. Numerous studies over the last two decades have demonstrated the neurotoxic properties of Abeta. However, it is still unclear whether Abeta neurotoxicity is an initial cause or rather a late event in the pathophysiology of AD. The understanding of preclinical AD-related pathophysiological mechanisms is of significant interest in the identification of potential pharmacological targets. In this context another APP-derived cleavage product, the amyloid precursor protein intracellular domain (AICD), has sparked considerable research interest over the last 7 years. Different AICD levels as a result of gamma-secretase activity may contribute to early pathophysiological mechanisms in AD. However, the relevance of AICD is being discussed highly controversially amongst AD researchers. This review summarizes recent findings in terms of the origin of AICD by regulated intramembrane proteolysis; its structure, binding factors, and post-translational modifications; and its putative role in gene transcription, apoptosis, and cytoskeletal dynamics.

Amyloidogenic processing but not amyloid precursor protein (APP) intracellular C-terminal domain production requires a precisely oriented APP dimer assembled by transmembrane GXXXG motifs

The beta-amyloid peptide (Abeta) is the major constituent of the amyloid core of senile plaques found in the brain of patients with Alzheimer disease. Abeta is produced by the sequential cleavage of the amyloid precursor protein (APP) by beta- and gamma-secretases. Cleavage of APP by gamma-secretase also generates the APP intracellular C-terminal domain (AICD) peptide, which might be involved in regulation of gene transcription. APP contains three Gly-XXX-Gly (GXXXG) motifs in its juxtamembrane and transmembrane (TM) regions. Such motifs are known to promote dimerization via close apposition of TM sequences. We demonstrate that pairwise replacement of glycines by leucines or isoleucines, but not alanines, in a GXXXG motif led to a drastic reduction of Abeta40 and Abeta42 secretion. beta-Cleavage of mutant APP was not inhibited, and reduction of Abeta secretion resulted from inhibition of gamma-cleavage. It was anticipated that decreased gamma-cleavage of mutant APP would result from inhibition of its dimerization. Surprisingly, mutations of the GXXXG motif actually enhanced dimerization of the APP C-terminal fragments, possibly via a different TM alpha-helical interface. Increased dimerization of the TM APP C-terminal domain did not affect AICD production.