Effects of neuron-specific ADAM10 modulation in an in vivo model of acute excitotoxic stress

Molecular Genetics of Acquired Temporal Lobe Epilepsy

An epilepsy diagnosis reduces a patient's quality of life tremendously, and it is a fate shared by over 50 million people worldwide. Temporal lobe epilepsy (TLE) is largely considered a nongenetic or acquired form of epilepsy that develops in consequence of neuronal trauma by injury, malformations, inflammation, or a prolonged (febrile) seizure. Although extensive research has been conducted to understand the process of epileptogenesis, a therapeutic approach to stop its manifestation or to reliably cure the disease has yet to be developed. In this review, we briefly summarize the current literature predominately based on data from excitotoxic rodent models on the cellular events proposed to drive epileptogenesis and thoroughly discuss the major molecular pathways involved, with a focus on neurogenesis-related processes and transcription factors. Furthermore, recent investigations emphasized the role of the genetic background for the acquisition of epilepsy, including variants of neurodevelopmental genes. Mutations in associated transcription factors may have the potential to innately increase the vulnerability of the hippocampus to develop epilepsy following an injury-an emerging perspective on the epileptogenic process in acquired forms of epilepsy.

Epilepsy and demyelination: Towards a bidirectional relationship

Demyelination stands out as a prominent feature in individuals with specific types of epilepsy. Concurrently, individuals with demyelinating diseases, such as multiple sclerosis (MS) are at a greater risk of developing epilepsy compared to non-MS individuals. These bidirectional connections raise the question of whether both pathological conditions share common pathogenic mechanisms. This review focuses on the reciprocal relationship between epilepsy and demyelination diseases. We commence with an overview of the neurological basis of epilepsy and demyelination diseases, followed by an exploration of how our comprehension of these two disorders has evolved in tandem. Additionally, we discuss the potential pathogenic mechanisms contributing to the interactive relationship between these two diseases. A more nuanced understanding of the interplay between epilepsy and demyelination diseases has the potential to unveiling the molecular intricacies of their pathological relationships, paving the way for innovative directions in future clinical management and treatment strategies for these diseases.

Neuroprotective effect and potential of cellular prion protein and its cleavage products for treatment of neurodegenerative disorders part II: strategies for therapeutics development

Introduction: The cellular prion protein (PrP^C), some of its derivatives (especially PrP N-terminal N1 peptide and shed PrP), and PrP^C-containing exosomes have strong neuroprotective activities, which have been reviewed in the companion article (Part I) and are briefly summarized here.Areas covered: We propose that elevating the extracellular levels of a protective PrP form using gene therapy and other approaches is a very promising novel avenue for prophylactic and therapeutic treatments against prion disease, Alzheimer's disease, and several other neurodegenerative diseases. We will dissect the pros and cons of various potential PrP-based treatment options and propose a few strategies that are more likely to succeed. The cited references were obtained from extensive PubMed searches of recent literature, including peer-reviewed original articles and review articles.Expert opinion: Concurrent knockdown of celllular PrP expression and elevation of the extracellular levels of a neuroprotective PrP N-terminal peptide via optimized gene therapy vectors is a highly promising broad-spectrum prophylactic and therapeutic strategy against several neurodegenerative diseases, including prion diseases, Alzheimer's disease and Parkinson's disease.

ADAM10 suppresses demyelination and reduces seizure susceptibility in cuprizone-induced demyelination model

The metalloproteinase ADAM10 is the most important amyloid precursor protein (APP) α-secretase, preventing the deposit of neurotoxic amyloid β (Aβ) peptide and generating a soluble APP fragment (sAPP_α) with neurotrophic functions. Recent studies have suggested that ADAM10 also play a role in the pathogenesis of inflammatory CNS diseases, such as multiple sclerosis (MS). Demyelination is the hallmarks of MS but the mechanisms involved remain unclear. Here in this study, we examined the role that ADAM10 might play in the cuprizone-induced demyelination model. Our results demonstrated that ADAM10 expression and sAPP_α production were significantly reduced in the corpus callosum in response to cuprizone treatment. Overexpression of ADAM10 increased sAPP_α production and suppressed demyelination as well as neuroinflammation and oxidative stress in cuprizone-induced demyelination model. Pharmacological inhibition of ADAM10 activity, however, abrogates the protective effect of ADAM10 against demyelination, neuroinflammation and oxidative stress. It has been reported that CNS demyelination may induce seizure activity. Here, we found that overexpression of ADAM10 reduced seizure susceptibility in cuprizone-induced demyelination model, suggesting that ADAM10-derived sAPP_α suppresses demyelination and reduces seizure susceptibility via ameliorating neuroinflammation and oxidative stress in cuprizone-induced demyelination model.

Pharmacologic Inhibition of ADAM10 Attenuates Brain Tissue Loss, Axonal Injury and Pro-inflammatory Gene Expression Following Traumatic Brain Injury in Mice

The α-secretase A disintegrin and metalloprotease 10 (ADAM10) regulates various physiological and pathophysiological processes. Despite its broad functional implications during development, plasticity, and disease, no pharmacological approaches to inhibit ADAM10 in acute brain injury have been reported. Here, we examined the effects of the ADAM10 inhibitor GI254023X on the neurological and histopathological outcome after experimental traumatic brain injury (TBI). C57BL/6N mice were subjected to the controlled cortical impact (CCI) model of TBI or sham procedure and received GI254023X or vehicle during the acute phase of injury (n = 40, 100 mg/kg, 25% DMSO, 0.1 M Na2CO3, intraperitoneal, 30 min and 24 h after TBI). GI254023X treatment did not improve neurological deficits from 1 to 7 days post-injury (dpi) but animals treated with GI254023X exhibited smaller brain lesions compared to vehicle treatment. Determination of brain mRNA expression by quantitative PCR showed that TBI-induced up-regulation of Adam10 and Adam17 was not influenced by GI254023X but the up-regulation of the matrix metalloproteinase genes Mmp2 and Mmp9 was attenuated. GI254023X treatment further increased the T cell marker Cd247 but did not affect blood brain barrier integrity, as assessed by Occludin mRNA expression and IgG brain extravasation. However, in agreement with neuroprotective effects of ADAM10 inhibition, GI254023X treatment attenuated axonal injury, as indicated by decreased generation of spectrin breakdown products (SBDPs) and decreased immunostaining using anti-non-phosphorylated neurofilament (SMI-32). Interestingly, reduced axonal injury in GI254023X-treated animals coincided with subtle mRNA dysregulation in the glutamate receptor subunit genes Gria1 and Grin2b. Quantitative PCR also revealed that GI254023X mitigated up-regulation of the pro-inflammatory markers Il6, Tnfa, and Lcn2 but not the up-regulation of the pan-microglia marker Aif1, the M2 microglia marker Arg1 and the reactive astrocyte marker Gfap. Taken together, the ADAM10 inhibitor GI254023X attenuates brain tissue loss, axonal injury and pro-inflammatory gene expression in the CCI model of TBI. These results suggest that ADAM10 may represent a therapeutic target in the acute phase of TBI.

Genetic Effects of the Schizophrenia-Related Gene DTNBP1 in Temporal Lobe Epilepsy

Recent studies have reported patients who concurrently exhibit conditions of epilepsy and schizophrenia, indicating certain shared pathologies between them. This study aimed to investigate the genetic effects of the schizophrenia-related gene DTNBP1 in temporal lobe epilepsy (TLE). A total of 496 TLE patients and 528 healthy individuals were successfully genotyped for six DTNBP1 polymorphisms (rs760665, rs1011313, rs2619528, rs2619522, rs909706, and rs2619538), including 335 TLE patients and 325 healthy controls in cohort 1, and 161 TLE patients and 203 healthy controls in cohort 2. The frequency of the TT genotype at rs909706 T > C was lower in TLE patients than in normal controls in the initial cohort (cohort 1), which was confirmed in an independent cohort (cohort 2). However, the intronic T allele failed to be in linkage disequilibrium (LD) with any functional variations nearby; thus, together with the CCAC and TCAT haplotypes (rs1011313-rs2619528-rs2619522-rs909706) observed in the study, this allele acts only as a protective factor against susceptibility to TLE. Meanwhile, a novo mutant allele rs2619538 T > A was exclusively observed in TLE patients, and a dual-luciferase assay revealed that the mutant allele was increased by approximately 22% in the DTNBP2 promoter compared with the wild-type allele. Together with the trend of increasing DTNBP1 expression in epilepsy patients and animal models in this study, these are the first findings to demonstrate the genetic association of DTNBP1 with TLE. Homozygous mutation of rs2619538 T > A likely promotes DTNBP1 expression and facilitates subsequent processes in epilepsy pathologies. Thus, the role of DTNBP1 in TLE deserves further exploration in the future.

Autophagy-Dependent Increased ADAM10 Mature Protein Induced by TFEB Overexpression Is Mediated Through PPARα

Nonamyloidogenic processing of amyloid precursor protein (APP) by augmenting ADAM10 is a promising therapeutic strategy for Alzheimer's disease (AD). Therefore identification of molecular pathways that regulate ADAM10 expression is crucial. Autophagy is strongly dysregulated in AD, and TFEB was recently shown to be a master regulator of autophagy-lysosome pathway (ALP). Here, we report that TFEB expression in HeLa cells increased ADAM10 mature form by 72% (p < 0.01, n = 4), while TFEB knockdown by CRISPR strategy reduced ADAM10 mature form by 36% (p < 0.05, n = 4). Autophagy inhibition by 3-methyladenine (3-MA), but not bafilomycin A1 (BAF1), reduced ADAM10 mature form by 49% (p < 0.05, n = 4) in the TFEB expressing HeLa cells. Autophagy activation by 3 h of starvation increased ADAM10 to 91% (p < 0.001, n = 6) relative to 51% (p < 0.01, n = 6) in the nutrient-fed cells. Further, siRNAs targeted against PPARα in HeLa cells decreased ADAM10 levels by 28% (p < 0.05, n = 6) relative to the cells treated with scrambled siRNAs. Further, incubation of EGFP-TFEB expressing HeLa cells with PPARα antagonist, but not PPARβ or PPARγ antagonists, prevented TFEB-induced increase in ADAM10 levels. Importantly, flag-TFEB expression in the brain also increased ADAM10 by 60% (p < 0.05, n = 3) in the cortical and 34% (p < 0.001, n = 3) in the hippocampal homogenates. ADAM10 activity also increased by 57% (p < 0.01, n = 3) in the HeLa cells. Finally, TFEB-induced ADAM10 potentiation led to increased secretion of sAPPα by 154% (p < 0.001, n = 3) in the cortex and 62% (p < 0.001, n = 3) in the hippocampus. Thus, TFEB expression enhances nonamyloidogenic processing of APP. In conclusion, TFEB expression induces ADAM10 in an autophagy-dependent manner through PPARα.

Stage-dependent involvement of ADAM10 and its significance in epileptic seizures

The prevalence of epileptic seizures in Alzheimer's disease (AD) has attracted an increasing amount of attention in recent years, and many cohort studies have found several risk factors associated with the genesis of seizures in AD. Among these factors, young age and severe dementia are seemingly contradictory and independent risk factors, indicating that the pathogenesis of epileptic seizures is, to a certain extent, stage‐dependent. A disintegrin and metalloproteinase domain‐containing protein 10 (ADAM10) is a crucial α‐secretase responsible for ectodomain shedding of its substrates; thus, the function of this protein depends on the biological effects of its substrates. Intriguingly, transgenic models have demonstrated ADAM10 to be associated with epilepsy. Based on the biological effects of its substrates, the potential pathogenic roles of ADAM10 in epileptic seizures can be classified into amyloidogenic processes in the ageing stage and cortical dysplasia in the developmental stage. Therefore, ADAM10 is reviewed here as a stage‐dependent modulator in the pathogenesis of epilepsy. Current data regarding ADAM10 in epileptic seizures were collected and reviewed for potential pathogenic roles (ie amyloidogenic processes and cortical dysplasia) and regulatory mechanisms (ie transcriptional and posttranscriptional regulation). These findings are then discussed in terms of the significance of the stage‐dependent functions of ADAM10 in epilepsy. Several potential targets for seizure control, such as candidate transcription factors and microRNAs that regulate ADAM10, as well as potential genetic screening tools for the early recognition of cortical dysplasia, have been suggested but must be studied in more detail.

Inhibiting pathologically active ADAM10 rescues synaptic and cognitive decline in Huntington's disease

A disintegrine and metalloproteinase 10 (ADAM10) is implicated in synaptic function through its interaction with postsynaptic receptors and adhesion molecules. Here, we report that levels of active ADAM10 are increased in Huntington's disease (HD) mouse cortices and striata and in human postmortem caudate. We show that, in the presence of polyglutamine-expanded (polyQ-expanded) huntingtin (HTT), ADAM10 accumulates at the postsynaptic densities (PSDs) and causes excessive cleavage of the synaptic protein N-cadherin (N-CAD). This aberrant phenotype is also detected in neurons from HD patients where it can be reverted by selective silencing of mutant HTT. Consistently, ex vivo delivery of an ADAM10 synthetic inhibitor reduces N-CAD proteolysis and corrects electrophysiological alterations in striatal medium-sized spiny neurons (MSNs) of 2 HD mouse models. Moreover, we show that heterozygous conditional deletion of ADAM10 or delivery of a competitive TAT-Pro-ADAM10709-729 peptide in R6/2 mice prevents N-CAD proteolysis and ameliorates cognitive deficits in the mice. Reduction in synapse loss was also found in R6/2 mice conditionally deleted for ADAM10. Taken together, these results point to a detrimental role of hyperactive ADAM10 at the HD synapse and provide preclinical evidence of the therapeutic potential of ADAM10 inhibition in HD.

Pathophysiology of Trans-Synaptic Adhesion Molecules: Implications for Epilepsy

Chemical synapses are specialized interfaces between neurons in the brain that transmit and modulate information, thereby integrating cells into multiplicity of interacting neural circuits. Cell adhesion molecules (CAMs) might form trans-synaptic complexes that are crucial for the appropriate identification of synaptic partners and further for the establishment, properties, and dynamics of synapses. When affected, trans-synaptic adhesion mechanisms play a role in synaptopathies in a variety of neuropsychiatric disorders including epilepsy. This review recapitulates current understanding of trans-synaptic interactions in pathophysiology of interneuronal connections. In particular, we discuss here the possible implications of trans-synaptic adhesion dysfunction for epilepsy.

Metalloprotease Adam10 suppresses epilepsy through repression of hippocampal neuroinflammation

Background

Mice with pilocarpine-induced temporal lobe epilepsy (TLE) are characterized by intense hippocampal neuroinflammation, a prominent pathological hallmark of TLE that is known to contribute to neuronal hyperexcitability. Recent studies indicate that Adam10, a member of a disintegrin and metalloproteinase domain-containing protein (Adam) family, has been involved in the neuroinflammation response. However, it remains unclear whether and how Adam10 modulates neuroinflammation responses in the context of an epileptic brain or whether Adam10 affects epileptogenesis via the neuroinflammation pathway.

Methods

Adult male C57BL/6J mice were subjected to intraperitoneal injection of pilocarpine to induce TLE. Adeno-associated viral (AAV) vectors carrying Adam10 (AAV-Adam10) or lentiviral vectors carrying short hairpin RNA, which is specific to the mouse Adam10 mRNA (shRNA-Adam10), were bilaterally injected into the hippocampus to induce overexpression or knockdown of Adam10, respectively. The specific anti-inflammatory agent minocycline was administered following status epilepticus (SE) to block hippocampal neuroinflammation. Continuous video EEG recording was performed to analyze epileptic behavior. Western blot, immunofluorescence staining, and ELISA were performed to determine Adam10 expression as well as hippocampal neuroinflammation.

Results

In this study, we demonstrate that overexpression of Adam10 in the hippocampus suppresses neuroinflammation and reduces seizure activity in TLE mice, whereas knockdown of Adam10 exacerbates hippocampal neuroinflammation and increases seizure activity. Furthermore, increased seizure activity in Adam10 knockdown TLE mice is dependent on hippocampal neuroinflammation.

Conclusion

These results suggest that Adam10 suppresses epilepsy through repression of hippocampal neuroinflammation. Our findings provide new insights into the Adam10 regulation of development of epilepsy via the neuroinflammation pathway and identify a potential therapeutic target for epilepsy.

Structural and mechanistic aspects influencing the ADAM10-mediated shedding of the prion protein

Background

Proteolytic processing of the prion protein (PrPC) by endogenous proteases generates bioactive membrane-bound and soluble fragments which may help to explain the pleiotropic roles of this protein in the nervous system and in brain diseases. Shedding of almost full-length PrPC into the extracellular space by the metalloprotease ADAM10 is of peculiar relevance since soluble PrP stimulates axonal outgrowth and is protective in neurodegenerative conditions such as Alzheimer’s and prion disease. However, molecular determinates and mechanisms regulating the shedding of PrP are entirely unknown.

Methods

We produced an antibody recognizing the neo-epitope of shed PrP generated by ADAM10 in biological samples and used it to study structural and mechanistic aspects affecting the shedding. For this, we investigated genetically modified cellular and murine models by biochemical and morphological approaches.

Results

We show that the novel antibody specifically detects shed PrP in cell culture supernatants and murine brain. We demonstrate that ADAM10 is the exclusive sheddase of PrPC in the nervous system and reveal that the glycosylation state and type of membrane-anchorage of PrPC severely affect its shedding. Furthermore, we provide evidence that PrP shedding can be modulated by pharmacological inhibition and stimulation and present data suggesting that shedding is a relevant part of a compensatory network ensuring PrPC homeostasis of the cell.

Conclusions

With the new antibody, our study introduces a new tool to reliably investigate PrP-shedding. In addition, this study provides novel and important insight into the regulation of this cleavage event, which is likely to be relevant for diagnostic and therapeutic approaches even beyond neurodegeneration.

Regulation of Alpha-Secretase ADAM10 In vitro and In vivo: Genetic, Epigenetic, and Protein-Based Mechanisms

ADAM10 (A Disintegrin and Metalloproteinase 10) has been identified as the major physiological alpha-secretase in neurons, responsible for cleaving APP in a non-amyloidogenic manner. This cleavage results in the production of a neuroprotective APP-derived fragment, APPs-alpha, and an attenuated production of neurotoxic A-beta peptides. An increase in ADAM10 activity shifts the balance of APP processing toward APPs-alpha and protects the brain from amyloid deposition and disease. Thus, increasing ADAM10 activity has been proposed an attractive target for the treatment of neurodegenerative diseases and it appears to be timely to investigate the physiological mechanisms regulating ADAM10 expression. Therefore, in this article, we will (1) review reports on the physiological regulation of ADAM10 at the transcriptional level, by epigenetic factors, miRNAs and/or protein interactions, (2) describe conditions, which change ADAM10 expression in vitro and in vivo, (3) report how neuronal ADAM10 expression may be regulated in humans, and (4) discuss how this knowledge on the physiological and pathophysiological regulation of ADAM10 may help to preserve or restore brain function.

APP as a Protective Factor in Acute Neuronal Insults

Despite its key role in the molecular pathology of Alzheimer’s disease (AD), the physiological function of amyloid precursor protein (APP) is unknown. Increasing evidence, however, points towards a neuroprotective role of this membrane protein in situations of metabolic stress. A key observation is the up-regulation of APP following acute (stroke, cardiac arrest) or chronic (cerebrovascular disease) hypoxic-ischemic conditions. While this mechanism may increase the risk or severity of AD, APP by itself or its soluble extracellular fragment APPsα can promote neuronal survival. Indeed, different animal models of acute hypoxia-ischemia, traumatic brain injury (TBI) and excitotoxicity have revealed protective effects of APP or APPsα. The underlying mechanisms involve APP-mediated regulation of calcium homeostasis via NMDA receptors (NMDAR), voltage-gated calcium channels (VGCC) or internal calcium stores. In addition, APP affects the expression of survival- or apoptosis-related genes as well as neurotrophic factors. In this review, we summarize the current understanding of the neuroprotective role of APP and APPsα and possible implications for future research and new therapeutic strategies.

Promoter Variants of the ADAM10 Gene and Their Roles in Temporal Lobe Epilepsy

Previous evidence has indicated that downregulated ADAM10 gives rise to epileptic seizures in Alzheimer's disease, and this study investigated the association of ADAM10 with temporal lobe epilepsy (TLE) from a genetic perspective. A total of 496 TLE patients and 528 healthy individuals were enrolled and genotyped for ADAM10 promoter variants (rs653765 G > A and rs514049 A > C). The alleles, genotypes, and haplotypes were then compared with clarify the association of these variants with TLE and their impacts upon age at onset, initial seizure types before treatments, and responses to drug treatments. In cohorts I, II, and I + II, the frequencies of the A allele and AA genotype at rs514049 were consistently increased in the cases compared with the controls (p = 0.020 and p = 0.009; p = 0.008 and p = 0.009; p = 0.000 and p = 0.000; q = 0.003 and q = 0.002, respectively). In contrast, the frequency of the AC haplotype (rs653765-rs514049) decreased in cohorts I + II (p = 0.013). Further analyses of the TLE patients indicated that the AA genotype functioned as a predisposing factor to drug-resistant TLE and the AC haplotype as a protective factor against generalized tonic-clonic seizures (GTCS) and drug-resistant TLE. This study is the first to demonstrate an association of the ADAM10 promoter variants with TLE. In particular, the AA genotype and AC haplotype showed their effects upon GTCS and drug-resistant TLE.

The alpha secretase ADAM10: A metalloprotease with multiple functions in the brain

Proteins belonging to the 'A Disintegrin And Metalloproteinase' (ADAM) family are membrane-anchored proteases that are able to cleave the extracellular domains of several membrane-bound proteins in a process known as 'ectodomain shedding'. In the central nervous system, ADAM10 has attracted the most attention, since it was described as the amyloid precursor protein α-secretase over ten years ago. Despite the excitement over the potential of ADAM10 as a novel drug target in Alzheimer disease, the physiological functions of ADAM10 in the brain are not yet well understood. This is largely because of the embryonic lethality of ADAM10-deficient mice, which results from the loss of cleavage and signaling of the Notch receptor, another ADAM10 substrate. However, the recent generation of conditional ADAM10-deficient mice and the identification of further ADAM10 substrates in the brain has revealed surprisingly numerous and fundamental functions of ADAM10 in the development of the embryonic brain and also in the homeostasis of adult neuronal networks. Mechanistically, ADAM10 controls these functions by utilizing unique postsynaptic substrates in the central nervous system, in particular synaptic cell adhesion molecules, such as neuroligin-1, N-cadherin, NCAM, Ephrin A2 and A5. Consequently, a dysregulation of ADAM10 activity is linked to psychiatric and neurological diseases, such as epilepsy, fragile X syndrome and Huntington disease. This review highlights the recent progress in understanding the substrates and function as well as the regulation and cell biology of ADAM10 in the central nervous system and discusses the value of ADAM10 as a drug target in brain diseases.

Time dependent integration of matrix metalloproteinases and their targeted substrates directs axonal sprouting and synaptogenesis following central nervous system injury

Over the past two decades, many investigators have reported how extracellular matrix molecules act to regulate neuroplasticity. The majority of these studies involve proteins which are targets of matrix metalloproteinases. Importantly, these enzyme/substrate interactions can regulate degenerative and regenerative phases of synaptic plasticity, directing axonal and dendritic reorganization after brain insult. The present review first summarizes literature support for the prominent role of matrix metalloproteinases during neuroregeneration, followed by a discussion of data contrasting adaptive and maladaptive neuroplasticity that reveals time-dependent metalloproteinase/substrate regulation of postinjury synaptic recovery. The potential for these enzymes to serve as therapeutic targets for enhanced neuroplasticity after brain injury is illustrated with experiments demonstrating that metalloproteinase inhibitors can alter adaptive and maladaptive outcome. Finally, the complexity of metalloproteinase role in reactive synaptogenesis is revealed in new studies showing how these enzymes interact with immune molecules to mediate cellular response in the local regenerative environment, and are regulated by novel binding partners in the brain extracellular matrix. Together, these different examples show the complexity with which metalloproteinases are integrated into the process of neuroregeneration, and point to a promising new angle for future studies exploring how to facilitate brain plasticity.

"Untangling" Alzheimer's disease and epilepsy

There is a substantial body of evidence that spontaneous recurrent seizures occur in a subset of patients with Alzheimer disease (AD), especially the familial forms that have an early onset. In transgenic mice that simulate these genetic forms of AD, seizures or reduced seizure threshold have also been reported. Mechanisms underlying the seizures or reduced seizure threshold in these mice are not yet clear and are likely to be complex, because the synthesis of amyloid β (Aβ) involves many peptides and proteases that influence excitability. Based on transgenic mouse models of AD where Aβ and its precursor are elevated, it has been suggested that seizures are caused by the downregulation of the Nav1.1 sodium channel in a subset of GABAergic interneurons, leading to a reduction in GABAergic inhibition. Another mechanism of hyperexcitability appears to involve tau, because deletion of tau reduces seizures in some of the same transgenic mouse models of AD. Therefore, altered excitability may be as much a characteristic of AD as plaques and tangles-especially for the familial forms of AD.

Amyloid precursor proteins are protective in Drosophila models of progressive neurodegeneration

The processing of Amyloid Precursor Proteins (APPs) results in several fragments, including soluble N-terminal ectodomains (sAPPs) and C-terminal intracellular domains (AICD). sAPPs have been ascribed neurotrophic or neuroprotective functions in cell culture, although β-cleaved sAPPs can have deleterious effects and trigger neuronal cell death. Here we describe a neuroproprotective function of APP and fly APPL (Amyloid Precursor Protein-like) in vivo in several Drosophila mutants with progressive neurodegeneration. We show that expression of the N-terminal ectodomain is sufficient to suppress the progressive degeneration in these mutants and that the secretion of the ectodomain is required for this function. In addition, a protective effect is achieved by expressing kuzbanian (which has α-secretase activity) whereas expression of fly and human BACE aggravates the phenotypes, suggesting that the protective function is specifically mediated by the α-cleaved ectodomain. Furthermore, genetic and molecular studies suggest that the N-terminal fragments interact with full-length APPL activating a downstream signaling pathway via the AICD. Because we show protective effects in mutants that affect different genes (AMP-activated protein kinase, MAP1b, rasGAP), we propose that the protective effect is not due to a genetic interaction between APPL and these genes but a more general aspect of APP proteins. The result that APP proteins and specifically their soluble α-cleaved ectodomains can protect against progressive neurodegeneration in vivo provides support for the hypothesis that a disruption of the physiological function of APP could play a role in the pathogenesis of Alzheimer's Disease.

Identification and biology of α-secretase

'Secretase' is a generic term coined more than 20 years ago to refer to a group of proteases responsible for the cleavage of a vast number of membrane proteins. These endoproteolytic events result in the extracellular or intracellular release of soluble metabolites associated with a broad range of intrinsic physiological functions. α-Secretase refers to the activity targeting the amyloid precursor protein (APP) and generating sAPPα, a soluble extracellular fragment potentially associated with neurotrophic and neuroprotective functions. Several proteases from the a disintegrin and metalloproteinase (ADAM) family, including ADAM10 and ADAM17, have been directly or indirectly associated with the constitutive and regulated α-secretase activities. Recent evidence in primary neuronal cultures indicates that ADAM10 may represent the genuine constitutive α-secretase. Mainly because α-secretase cleaves APP within the sequence of Aβ, the core component of the cerebral amyloid plaques in Alzheimer's disease, α-secretase activation is considered to be of therapeutic value. In this article, we will provide a historical perspective on the characterization of α-secretase and review the recent literature on the identification and biology of the current α-secretase candidates.

α-secretase in Alzheimer's disease: molecular identity, regulation and therapeutic potential

Ectodomain shedding of the amyloid precursor protein (APP) by the metalloprotease activity α-secretase is a key regulatory event preventing the generation of the Alzheimer's disease (AD) amyloid β peptide. Proteases similar to α-secretase are essential for diverse physiological processes, such as embryonic development, cell adhesion and neuronal guidance. Previously, several proteases were suggested as candidate α-secretases for APP, in particular members of the ADAM family (a disintegrin and metalloprotease). Two recent studies analyzed primary neurons, which are the cell type affected in AD, and finally demonstrated that the constitutively cleaving α-secretase activity is selectively mediated by ADAM10. An increase in α-secretase cleavage is considered a therapeutic approach for AD. However, the molecular mechanisms regulating α-secretase cleavage remain only partly understood. Signaling pathways activating protein kinase C and MAP kinase play a central role in stimulating α-secretase cleavage of APP. Additionally, several recent publications demonstrate that ADAM10 expression and α-secretase cleavage of APP are tightly controlled at the level of transcription, e.g. by retinoic acid receptors and sirtuins, and at the level of translation and protein trafficking. This review focuses on the recent progress made in unraveling the molecular identity, regulation and therapeutic potential of α-secretase in Alzheimer's disease.

An overview of APP processing enzymes and products

The generation of amyloid beta-peptide (A beta) by enzymatic cleavages of the beta-amyloid precursor protein (APP) has been at the center of Alzheimer's disease (AD) research. While the basic process of beta- and gamma-secretase-mediated generation of A beta is text book knowledge, new aspects of A beta and other cleavage products have emerged in recent years. Also our understanding of the enzymes involved in APP proteolysis has increased dramatically. All of these discoveries contribute to a more complete understanding of APP processing and the physiologic and pathologic roles of its secreted and intracellular protein products. Understanding APP processing is important for any therapeutic strategy aimed at reducing A beta levels in AD. In this review, we provide a concise description of the current state of understanding the enzymes involved in APP processing, the cleavage products generated by different processing patterns, and the potential functions of those cleavage products.

Upregulation of the alpha-secretase ADAM10--risk or reason for hope?

A decade ago, a disintegrin and metalloproteinase 10 (ADAM10) was identified as an alpha-secretase and as a key proteinase in the processing of the amyloid precursor protein. Accordingly, the important role that it plays in Alzheimer's disease was manifested. Animal models with an overexpression of ADAM10 revealed a beneficial profile of the metalloproteinase with respect to learning and memory, plaque load and synaptogenesis. Therefore, ADAM10 presents a worthwhile target with respect to the treatment of a neurodegenerative disease such as Morbus Alzheimer. Initially, ADAM10 was suggested to be an enzyme, shaping the extracellular matrix by cleavage of collagen type IV, or to be a tumour necrosis factor alpha convertase. In a relatively short time, a wide variety of additional substrates (with amyloid precursor protein probably being the most prominent) has been identified and the search is still ongoing. Hence, any side effects concerning the therapeutic enhancement of ADAM10 alpha-secretase activity have to be considered. The present review summarizes our knowledge about the structure and function of ADAM10 and highlights the opportunities for enhancing the expression and/or activity of the alpha-secretase as a therapeutic target.

Deprivation-induced dendritic shrinkage might be oppositely affected by the expression of wild-type and mutated human amyloid precursor protein

The physiological role of the amyloid precursor protein (APP) and its proteolytic fragments in the brain is associated with neuronal survival, neurite outgrowth, synaptic formation, and neuronal plasticity. However, malregulation of APP processing leads to disordered balance of fragments, which may results in opposite, degenerative neuronal effects. In the present study, we analyzed in vivo effects of the expression of wild-type or mutated human APP on afferent deprivation-induced changes of dendritic morphology. After vibrissectomy, expression of wild-type human APP prevented diameter shrinkage of dendritic segments as well as dendritic rarefaction of apical arbors. In contrast, mutant human APP expression exacerbated degenerative changes of deprived barrel neurons. Degradation of apical arbors was especially pronounced. Results demonstrate for the first time opposite effects of the expression of wild-type and mutated human APP on deprivation-induced dendritic restructuring in vivo.

Influence of ADAM10 on prion protein processing and scrapie infectiosity in vivo

Both the cellular prion protein (PrP(c)) and the amyloid precursor protein (APP) are physiologically subjected to complex proteolytic processing events. While for APP the proteinases involved--alpha-, beta- and gamma-secretase--have been identified in vitro and in vivo, the cleavage of PrP(c) by now has been linked only to the shedding activity of the metalloproteinase ADAM10 and/or ADAM17 in cell culture. Here we show that neuronal overexpression of the alpha-secretase ADAM10 in mice reduces all PrP(c) species detected in the brain instead of leading to enhanced amounts of specific cleavage products of PrP(c). Additionally, the incubation time of mice after scrapie infection is significantly increased in mice moderately overexpressing ADAM10. This indicates that overexpression of ADAM10 rather influences the amount of the cellular prion protein than its processing in vivo.

Potential late-onset Alzheimer's disease-associated mutations in the ADAM10 gene attenuate {alpha}-secretase activity

ADAM10, a member of a disintegrin and metalloprotease family, is an alpha-secretase capable of anti-amyloidogenic proteolysis of the amyloid precursor protein. Here, we present evidence for genetic association of ADAM10 with Alzheimer's disease (AD) as well as two rare potentially disease-associated non-synonymous mutations, Q170H and R181G, in the ADAM10 prodomain. These mutations were found in 11 of 16 affected individuals (average onset age 69.5 years) from seven late-onset AD families. Each mutation was also found in one unaffected subject implying incomplete penetrance. Functionally, both mutations significantly attenuated alpha-secretase activity of ADAM10 (>70% decrease), and elevated Abeta levels (1.5-3.5-fold) in cell-based studies. In summary, we provide the first evidence of ADAM10 as a candidate AD susceptibility gene, and report two potentially pathogenic mutations with incomplete penetrance for late-onset familial AD.

The effects of alpha-secretase ADAM10 on the proteolysis of neuregulin-1

Although ADAM10 is a major alpha-secretase involved in non-amyloidogenic processing of the amyloid precursor protein, several additional substrates have been identified, most of them in vitro. Thus, therapeutical approaches for the prevention of Alzheimer's disease by upregulation of this metalloproteinase may have severe side effects. In the present study, we examined whether the ErbB receptor ligand neuregulin-1, which is essential for myelination and other important neuronal functions, is cleaved by ADAM10. Studies with beta- and gamma-secretase inhibitors, as well as with the metalloproteinase inhibitor GM6001, revealed an inhibition of neuregulin-1 processing in human astroglioma cell line U373; however, specific RNA interference-induced knockdown of ADAM10 remained without effect. In vivo investigations of mice overexpressing either ADAM10 or dominant negative ADAM10 showed unaltered cleavage of neuregulin-1 compared to wild-type animals. As a consequence, the myelin sheath thickness of peripheral nerves was unaffected in mice with altered ADAM10 activity. Thus, although the beta-secretase BACE-1 acts as a neuregulin-1 sheddase, ADAM10 does not lead to altered neuregulin-1 processing either in cell culture or in vivo. Adverse reactions of an ADAM10-based therapy of Alzheimer's disease due to neuregulin-1 cleavage are therefore unlikely.

Differential gene expression in ADAM10 and mutant ADAM10 transgenic mice

Background

In a transgenic mouse model of Alzheimer disease (AD), cleavage of the amyloid precursor protein (APP) by the α-secretase ADAM10 prevented amyloid plaque formation, and alleviated cognitive deficits. Furthermore, ADAM10 overexpression increased the cortical synaptogenesis. These results suggest that upregulation of ADAM10 in the brain has beneficial effects on AD pathology.

Results

To assess the influence of ADAM10 on the gene expression profile in the brain, we performed a microarray analysis using RNA isolated from brains of five months old mice overexpressing either the α-secretase ADAM10, or a dominant-negative mutant (dn) of this enzyme. As compared to non-transgenic wild-type mice, in ADAM10 transgenic mice 355 genes, and in dnADAM10 mice 143 genes were found to be differentially expressed. A higher number of genes was differentially regulated in double-transgenic mouse strains additionally expressing the human APP_[V717I] mutant.

Overexpression of proteolytically active ADAM10 affected several physiological pathways, such as cell communication, nervous system development, neuron projection as well as synaptic transmission. Although ADAM10 has been implicated in Notch and β-catenin signaling, no significant changes in the respective target genes were observed in adult ADAM10 transgenic mice.

Real-time RT-PCR confirmed a downregulation of genes coding for the inflammation-associated proteins S100a8 and S100a9 induced by moderate ADAM10 overexpression. Overexpression of the dominant-negative form dnADAM10 led to a significant increase in the expression of the fatty acid-binding protein Fabp7, which also has been found in higher amounts in brains of Down syndrome patients.

Conclusion

In general, there was only a moderate alteration of gene expression in ADAM10 overexpressing mice. Genes coding for pro-inflammatory or pro-apoptotic proteins were not over-represented among differentially regulated genes. Even a decrease of inflammation markers was observed. These results are further supportive for the strategy to treat AD by increasing the α-secretase activity.

Evaluation of potential pro-survival pathways regulated by melatonin in a murine senescence model

We examined the effect of melatonin on pro-survival processes in three groups of mice. Untreated senescence-accelerated mice (SAMP8), melatonin-treated SAMP8 and untreated senescence-accelerated resistant mice (SAMR1) of 10 months old were studied. Melatonin (10 mg/kg) or vehicle (ethanol at 0.066%) was supplied in the drinking water from the end of the first month until the end of the ninth month of life. Differences in the Akt/Erk1-2 pathway and downstream targets were examined and no significant changes were observed, except for beta-catenin. However, sirtuin 1 expression was significantly lower in SAMP8 than in SAMR1. In addition, acetylated p53 and NFkappaB expression were lower in SAMP8 than in SAMR1. These changes were prevented by melatonin. Moreover, the concentration/expression of alpha-secretase was lower and that of amyloid beta aggregates (Abeta) was higher in untreated SAMP8 than in SAMR1. Likewise, the levels of Bid were higher, whereas Bcl-2(XL) levels were lower in SAMP8 than in SAMR1. Melatonin reduced all these changes. We conclude that melatonin improves pro-survival signals and reduces pro-death signals in age-related impairments of neural processes.