Although the role of APP and PSEN genes in genetic Alzheimer's disease (AD) cases is well established, fairly little is known about the molecular mechanisms affecting A␤ generation in sporadic AD. Deficiency in A␤ clearance is certainly a possibility, but increased expression of proteins like APP or BACE1/␤-secretase may also be associated with the disease. We therefore investigated changes in microRNA (miRNA) expression profiles of sporadic AD patients and found that several miRNAs potentially involved in the regulation of APP and BACE1 expression appeared to be decreased in diseased brain. We show here that miR-29a, -29b-1, and -9 can regulate BACE1 expression in vitro. The miR-29a/b-1 cluster was significantly (and AD-dementia-specific) decreased in AD patients displaying abnormally high BACE1 protein. Similar correlations between expression of this cluster and BACE1 were found during brain development and in primary neuronal cultures. Finally, we provide evidence for a potential causal relationship between miR-29a/b-1 expression and A␤ generation in a cell culture model. We propose that loss of specific miRNAs can contribute to increased BACE1 and A␤ levels in sporadic AD.neurodegeneration ͉ amyloid ͉ noncoding RNA M utations in the APP and PSEN genes cause A␤ accumulation and familial Alzheimer's disease (AD) (1-4). However, little is known about the mechanisms that contribute to A␤ accumulation in the vast majority of sporadic AD cases. BACE1/␤-secretase cleavage of APP is the rate-limiting step for A␤ peptide production. Increased BACE1 expression is observed in patients with sporadic AD (5-8), and several mechanisms for this up-regulation have been proposed (9, 10). A link between BACE1 levels, A␤ load, and AD pathology has been reported (11), suggesting that increased BACE1 expression is indeed an important risk factor for sporadic AD.miRNAs are small noncoding RNAs that control gene expression at the posttranscriptional level by binding to the 3Ј untranslated region (3ЈUTR) of target mRNAs leading to their translational inhibition or sometimes degradation. Several miRNAs are specifically expressed or enriched in the brain (12-15), and some have been associated with neuronal differentiation, synaptic plasticity, and memory formation (16,17). The hypothesis that miRNA pathways could contribute to neurodegeneration is appealing (18) and has been tested to a certain degree in Drosophila (19) and mouse models (18,20,21) in which all miRNAs are lacking. Recently, Kim et al. (21) identified a subgroup of miRNAs, normally enriched in the midbrain, which expression is altered in sporadic Parkinson's disease (PD). One of the affected miRNAs, miR-133b, controls the differentiation and function of dopaminergic neurons (which are lost in PD). Here, we sought to investigate whether changes in miRNA expression exist in sporadic AD, and whether these changes could contribute to A␤ pathology. ResultsmiRNA Profile Analysis of Sporadic AD Brain. In a pilot study, we assessed the expression profiles of 328 human miRNAs f...

Mutations in human presenilin (PS) genes cause aggressive forms of familial Alzheimer's disease. Presenilins are polytopic proteins that harbour the catalytic site of the c-secretase complex and cleave many type I transmembrane proteins including b-amyloid precursor protein (APP), Notch and syndecan 3. Contradictory results have been published concerning whether PS mutations cause 'abnormal' gain or (partial) loss of function of c-secretase. To avoid the possibility that wild-type PS confounds the interpretation of the results, we used presenilin-deficient cells to analyse the effects of different clinical mutations on APP, Notch, syndecan 3 and N-cadherin substrate processing, and on c-secretase complex formation. A loss in APP and Notch substrate processing at e and S3 cleavage sites was observed with all presenilin mutants, whereas APP processing at the c site was affected in variable ways. PS1-D9 and PS1-L166P mutations caused a reduction in b-amyloid peptide (Ab) 40 production whereas PS1-G384A mutant significantly increased Ab 42 . Interestingly PS2, a close homologue of PS1, appeared to be a less efficient producer of Ab than PS1. Finally, subtle differences in c-secretase complex assembly were observed. Overall, our results indicate that the different mutations in PS affect c-secretase structure or function in multiple ways.

Alzheimer's disease (AD)-linked mutations in Presenilins (PSEN) and the amyloid precursor protein (APP) lead to production of longer amyloidogenic Aβ peptides. The shift in Aβ length is fundamental to the disease; however, the underlying mechanism remains elusive. Here, we show that substrate shortening progressively destabilizes the consecutive enzyme-substrate (E-S) complexes that characterize the sequential γ-secretase processing of APP. Remarkably, pathogenic PSEN or APP mutations further destabilize labile E-S complexes and thereby promote generation of longer Aβ peptides. Similarly, destabilization of wild-type E-S complexes by temperature, compounds, or detergent promotes release of amyloidogenic Aβ. In contrast, E-Aβ stabilizers increase γ-secretase processivity. Our work presents a unifying model for how PSEN or APP mutations enhance amyloidogenic Aβ production, suggests that environmental factors may increase AD risk, and provides the theoretical basis for the development of γ-secretase/substrate stabilizing compounds for the prevention of AD.

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The γ-secretase complex plays a role in Alzheimer’s disease (AD) and cancer progression. The development of clinical useful inhibitors, however, is complicated by the role of the γ-secretase complex in regulated intramembrane proteolysis of Notch and other essential proteins. Different γ-secretase complexes containing different Presenilin or Aph1 protein subunits are present in various tissues. Here we show that these complexes have heterogeneous biochemical and physiological properties. Specific inactivation of the Aph1B γ-secretase in a murine Alzheimer’s disease model led to improvements of Alzheimer’s disease-relevant phenotypic features without any Notch-related side effects. The Aph1B complex contributes to total γ-secretase activity in the human brain, thus specific targeting of Aph1B-containing γ-secretase complexes may be helpful in generating less toxic therapies for Alzheimer’s disease.

␥-Secretase is the protease responsible for amyloid ␤ peptide release and is needed for Notch, N-Cadherin, and possibly other signaling pathways. The protease complex consists of at least four subunits, i.e., Presenilin, Aph1, Pen2, and Nicastrin. Two different genes encode Aph1A and Aph1B in man. A duplication of Aph1B in rodents has given rise to a third gene, Aph1C. Different mixes of ␥-secretase subunits assemble in at least four human and six rodent complexes but it is not known whether they have different activities in vivo. We report here the inactivation of the three Aph1 genes in mice. Aph1A ؊/؊ embryos show a lethal phenotype characterized by angiogenesis defects in the yolk sac, neuronal tube malformations, and mild somitogenesis defects. Aph1B ؊/؊ or C ؊/؊ or the combined Aph1BC ؊/؊ mice (which can be considered as a model for total Aph1B loss in human) survive into adulthood. However, Aph1BC ؊/؊ deficiency causes a mild but significant reduction in amyloid ␤ percursor protein processing in selective regions of the adult brain. We conclude that the biochemical and physiological repercussions of genetically reducing ␥-secretase activity via the different Aph1 components are quite divergent and tissue specific. Our work provides in vivo evidence for the concept that different ␥-secretase complexes may exert different biological functions. In the context of Alzheimer's disease therapy, this implies the theoretical possibility that targeting specific ␥-secretase subunit combinations could yield less toxic drugs than the currently available general inhibitors of ␥-secretase activity.Alzheimer ͉ intramembrane cleavage ͉ Presenilin ͉ knockout T he multimolecular complex ␥-secretase cleaves proteins in their transmembrane domain. The complex consists of at least four subunits called Presenilin (Psen), Nicastrin (Nct), Pen2, and Aph1 (1-3). The Psens provide the catalytic subunits of the complex (4), although the precise functional contribution of the other subunits remains to be clarified. Mutations in the genes encoding presenilin 1 (PSEN1) or its homologue presenilin 2 (PSEN2) cause familial Alzheimer's disease (5, 6). Besides amyloid ␤ (A␤) precursor protein (APP), ␥-secretase cleaves an increasing list of type I transmembrane proteins including Notch (7) and N-Cadherin (8) (for a full review, see ref. 9).Until now, ␥-secretase has largely been considered as a homogenous activity, but especially in mammals the situation is probably more complicated (10). Two different Psen genes and two (human) or three (rodent) Aph1 genes that can be alternatively spliced have been identified. Aph1A or Aph1B and Psen1 or Psen2 are incorporated in a mutually exclusive way into different complexes as demonstrated recently, providing formal proof that at least four different complexes in man (and six in mouse) can be generated (11,12). The question remains however whether those different complexes have also different physiological functions. Because ␥-secretase is considered a potential drug target in Alzheimer's disease, a...

The metalloproteinase ADAM10 is of importance for Notch-dependent cortical brain development. The protease is tightly linked with ␣-secretase activity toward the amyloid precursor protein (APP) substrate. Increasing ADAM10 activity is suggested as a therapy to prevent the production of the neurotoxic amyloid ␤ (A␤) peptide in AlzheimerЈs disease. To investigate the function of ADAM10 in postnatal brain, we generated Adam10 conditional knock-out (A10cKO) mice using a CaMKII␣-Cre deleter strain. The lack of ADAM10 protein expression was evident in the brain cortex leading to a reduced generation of sAPP␣ and increased levels of sAPP␤ and endogenous A␤ peptides. The A10cKO mice are characterized by weight loss and increased mortality after weaning associated with seizures. Behavioral comparison of adult mice revealed that the loss of ADAM10 in the A10cKO mice resulted in decreased neuromotor abilities and reduced learning performance, which were associated with altered in vivo network activities in the hippocampal CA1 region and impaired synaptic function. Histological and ultrastructural analysis of ADAM10-depleted brain revealed astrogliosis, microglia activation, and impaired number and altered morphology of postsynaptic spine structures. A defect in spine morphology was further supported by a reduction of the expression of NMDA receptors subunit 2A and 2B. The reduced shedding of essential postsynaptic cell adhesion proteins such as N-Cadherin, Nectin-1, and APP may explain the postsynaptic defects and the impaired learning, altered network activity, and synaptic plasticity of the A10cKO mice. Our study reveals that ADAM10 is instrumental for synaptic and neuronal network function in the adult murine brain.

β-arrestins are associated with numerous aspects of G protein-coupled receptor (GPCR) signaling and regulation and accordingly influence diverse physiological and pathophysiological processes. Here we report that β-arrestin 2 expression is elevated in two independent cohorts of individuals with Alzheimer's disease. Overexpression of β-arrestin 2 leads to an increase in amyloid-β (Aβ) peptide generation, whereas genetic silencing of Arrb2 (encoding β-arrestin 2) reduces generation of Aβ in cell cultures and in Arrb2(-/-) mice. Moreover, in a transgenic mouse model of Alzheimer's disease, genetic deletion of Arrb2 leads to a reduction in the production of Aβ(40) and Aβ(42). Two GPCRs implicated previously in Alzheimer's disease (GPR3 and the β(2)-adrenergic receptor) mediate their effects on Aβ generation through interaction with β-arrestin 2. β-arrestin 2 physically associates with the Aph-1a subunit of the γ-secretase complex and redistributes the complex toward detergent-resistant membranes, increasing the catalytic activity of the complex. Collectively, these studies identify β-arrestin 2 as a new therapeutic target for reducing amyloid pathology and GPCR dysfunction in Alzheimer's disease.