Roberta Epis

Synapse-Associated Protein-97 Mediates α-Secretase ADAM10 Trafficking and Promotes Its Activity

Alzheimers disease (AD) is a chronic neurodegenerative disorder caused by a combination of events impairing normal neuronal function. Here we found a molecular bridge between key elements of primary and secondary pathogenic events in AD, namely the elements of the amyloid cascade and synaptic dysfunction associated with the glutamatergic system. In fact, we report that synapse-associated protein-97 (SAP97), a protein involved in dynamic trafficking of proteins to the excitatory synapse, is responsible for driving ADAM10 (a disintegrin and metalloproteinase 10, the most accredited candidate for α-secretase) to the postsynaptic membrane, by a direct interaction through its Src homology 3 domain. NMDA receptor activation mediates this event and positively modulates α-secretase activity. Furthermore, perturbing ADAM10/SAP97 association in vivo by cell-permeable peptides impairs ADAM10 localization in postsynaptic membranes and consequently decreases the physiological amyloid precursor protein (APP) metabolism. Our findings indicate that glutamatergic synapse activation through NMDA receptor promotes the non-amyloidogenic APP cleavage, strengthening the correlation between APP metabolism and synaptic plasticity.

Synaptic Localization and Activity of ADAM10 Regulate Excitatory Synapses through N-Cadherin Cleavage

N-Cadherin has an important role during dendrite arborization, axon guidance, and synaptogenesis. In particular, at synaptic sites, N-cadherin is involved in the regulation of cell–cell adhesion and in morphology and plasticity control. Recent studies have shown that N-cadherin can be cleaved by the metalloproteinase ADAM10. Here we demonstrate that impairing ADAM10 localization and activity at synaptic sites decreases its processing of N-cadherin. This leads to an accumulation of the full-length form of N-cadherin, to an increase in spine head width, and to modifications of the number and function of glutamate receptors of AMPA type, both in vitro and in vivo. Our results indicate a key role for ADAM10 in the complex sequence of events through which N-cadherin affects spine maturation and controls structure and function of glutamatergic synapses.

Synaptic Dysfunction in Alzheimer’s Disease

Generation of amyloid peptide (Aβ) is at the beginning of a cascade that leads to Alzheimers disease (AD). Amyloid precursor protein (APP), as well as β- and γ-secretases, is the principal player involved in Aβ production, while α-secretase cleavage on APP prevents Aβ deposition. Recent studies suggested that soluble assembly states of Aβ peptides can cause cognitive problems by disrupting synaptic function in the absence of significant neurodegeneration. Therefore, current research investigates the relative importance of these various soluble Aβ assemblies in causing synaptic dysfunction and cognitive deficits. Several Aβ oligomers targets and cellular mechanisms responsible of Aβ-induced synaptic failure have been identified. The first and most important mechanism impugns a toxic gain of function for Aβ which results due to self-association and attainment of new structures capable of novel interactions that lead to impaired plasticity. Other scenarios predicate that Aβ has a normal physiological role. On the one hand, insufficient Aβ could lead to a loss of normal function, whereas excess Aβ may precipitate dysfunction. How this occurs and which the main target/s is/are for the synaptic action of Aβ remains to be fully understood and would certainly represent one of the main challenges to future AD research.

Amyloid flirting with synaptic failure : Towards a comprehensive view of Alzheimer's disease pathogenesis

Many neurological disorders accompanied by cognitive deficits exhibit abnormal synaptic function. This emerging concept is exemplified by Alzheimers disease. According to the amyloid hypothesis, Alzheimers disease is thought to be caused by the progressive accumulation and deposition of neurotoxic Amyloid beta-peptide in amyloid plaques and aggregates in brain. Now new theories are emerging associating synaptic and neuronal loss to Amyloid beta monomers and Amyloid beta oligomers. In particular, Amyloid beta oligomers have been described as the earliest effectors to adversely affect synaptic structure and plasticity. In this way, they compromise aspects of learning and memory, including long-term potentiation. Local inflammatory changes, neurofibrillary degeneration, and neurotransmitter deficits all contribute to the memory impairment, but available evidence suggests that these alterations develop as a consequence of early Amyloid beta accumulation. Even more recently, different studies have focused on the capability of neuronal activity itself to influence Amyloid Precursor Protein (APP) metabolism. Neuronal activity modulates, in fact, the formation and secretion of Amyloid beta peptides. The identification of both the mechanism through which Amyloid beta can modify neuronal activity and the way by which neuronal activity can alter APP metabolism is becoming more and more important. And the challenge for the future is, therefore, to find the linkage between these two processes.

Blocking ADAM10 synaptic trafficking generates a model of sporadic Alzheimer’s disease

We describe here an innovative, non-transgenic animal model of Alzheimers disease. This model mimics early stages of sporadic disease, which represents the vast majority of cases. The model was obtained by interfering with the complex between a disintegrin and metalloproteinase domain containing protein 10 (ADAM10), the main α-secretase candidate, and its partner, synapse-associated protein 97, a protein of the postsynaptic density-membrane associated guanylate kinase family. Association of ADAM10 with synapse-associated protein 97 governs enzyme trafficking and activity at synapses. Interfering with the ADAM10/synapse-associated protein 97 complex for 2 weeks by means of a cell-permeable peptide strategy is sufficient to shift the metabolism of the amyloid precursor protein towards amyloidogenesis and allows the reproduction of initial phases of sporadic Alzheimers disease. After 2 weeks of treatment, we detected progressive Alzheimers disease-like neuropathology, with an increase of β-amyloid aggregate production and of tau hyperphosphorylation, and a selective alteration of N-methyl-d-aspartic acid receptor subunit composition in the postsynaptic compartment of mouse brain. Behavioural and electrophysiological deficits were also induced by peptide treatment.

Searching for new animal models of Alzheimer′s disease

The pathophysiology of chronic neurodegenerative diseases, as Alzheimers diseases, has remained inaccessible till recently. But this situation is changing quickly. In the past decades, genes causing familiar forms of the disease have been identified and provided the genetic framework for the emerging amyloid hypothesis. On the basis of these findings, engineered mouse models have been developed and have allowed the understanding of crucial information about the pathogenic process. Certain observations obtained by transgenic mice, however, do not easily fit with the simplest version of the amyloid hypothesis. Even if there are transgenic lines that offer robust and relatively faithful reproductions of a subset of Alzheimers diseases features, a mouse model that recapitulates all aspects of the disease has not yet been produced. Several still not completely known factors combine to produce highly variability across transgenic mouse models. Discrepancies in neuropathology and behaviour between transgenic mouse models and human Alzheimers disease, and among different transgenic-lines, suggest caution in the interpretation of the results. Here we try to analyze critically some of the information provided by transgenic mice but ascertaining which elements of the neuropathological and behavioural phenotype of these various strains of transgenic mice are relevant to that observed in Alzheimers disease continues to be a challenge.

Modulatory effect of acetyl-L-carnitine on amyloid precursor protein metabolism in hippocampal neurons.

Alzheimer Disease is the most common chronic neurodegenerative disorder associated with aging. Nevertheless, its pharmacological therapy is still an unresolved issue. In double-blind controlled studies, acetyl-L-carnitine (ALC) demonstrated beneficial effects on Alzheimers disease. However, the mechanisms behind its neuroprotective ability remain to be fully established. In this study, the effect of acetyl-L-carnitine on amyloid precursor protein (APP) metabolism was investigated by in vitro models, both in a neuroblastoma cell line and in primary hippocampal cultures. We found that ALC treatment stimulates alpha-secretase activity and physiological APP metabolism. In particular, ALC favors the delivery of ADAM10 (a disintegrin and metalloproteinase 10, the most accredited candidate for alpha-secretase) to the post-synaptic compartment, and consequently positively modulates its enzymatic activity towards APP. Our findings suggest that the benefits of ALC reported in previous clinical studies are underscored by the specific biological mechanism of this compound on APP metabolism. In fact, ALC can directly influence the primary event in Alzheimers disease pathogenesis, i.e. the Amyloid beta cascade, promoting alpha-secretase activity and directly affecting the release of the non amyloidogenic metabolite.

Alpha, beta-and gamma-secretases in Alzheimer's disease.

Generation of Amyloid peptide (Abeta) is at the beginning of a cascade that leads to Alzheimers disease. Currenty, the mechanisms of Abeta generation and Abeta prevention are subject of intensive research. Amyloid precursor protein (APP), as well as beta- and gamma-secretases are the principal players involved in Abeta production, while alpha-secretase cleavage on APP prevents Abeta deposition. Inhibitors or modulators that target beta- and gamma-secretases as well as alpha-secretase activators are promising candidates for treatment of Alzheimers disease. A deep knowledge of the secretases is required to develop disease modifying drugs that target them. The most challenging quest is to translate the growing knowledge about the cell biology of secretases and their mechanisms of action into effective therapeutics. Here, we review the main features of the secretases.

Merging amyloid-beta cascade and activity-dependent plasticity in hippocampus: A possible role for ADAM10

Background: The alpha-secretase is a proteolytic activity, which cleaves within the amyloid ß (Aß) sequence and, thus, prevents Aß generation. The identity of alpha-secretase is unknown, but several proteases, including ‘‘a disintegrin and metalloproteases’’, have been proposed to be candidate alpha-secretases. Methods: To systematically characterize proteases with alpha-secretase activity, we first mapped the alpha-secretase cleavage sites within the Aß domain of APP and then screened for proteases cleaving at these sites. Results: We found that the main cleavage site is between amino acids 16 and 17 of the Aß sequence, which we refer to as major alpha-secretase cleavage site. Additional cleavage sites were identified at nearby peptide bonds, but only represent a smaller amount of total APP alphasecretase shedding. In order to selectively detect the secreted APP ending at the major alpha-secretase cleavage site, we generated a monoclonal antibody detecting the neoepitope arising through this cleavage. Next, we individually knocked-down different proteases and evaluated their contribution to the major alpha-secretase cleavage site of APP in neuronal cells. Surprisingly, we found that the knock-down of a single metalloprotease was sufficient to completely suppress APP shedding at the major cleavage site. This reveals that other proteases were not able to compensate for this loss of cleavage. Conclusions: In summary, our data show that a) distinct proteases contribute to total alpha-secretase cleavage of APP, but that b) only a single metalloprotease seems to mediate APP shedding at the major cleavage site.