Hermann Altmeppen

Lack of a-disintegrin-and-metalloproteinase ADAM10 leads to intracellular accumulation and loss of shedding of the cellular prion protein in vivo

BackgroundThe cellular prion protein (PrPC) fulfils several yet not completely understood physiological functions. Apart from these functions, it has the ability to misfold into a pathogenic scrapie form (PrPSc) leading to fatal transmissible spongiform encephalopathies. Proteolytic processing of PrPC generates N- and C-terminal fragments which play crucial roles both in the pathophysiology of prion diseases and in transducing physiological functions of PrPC. A-disintegrin-and-metalloproteinase 10 (ADAM10) has been proposed by cell culture experiments to be responsible for both shedding of PrPC and its α-cleavage. Here, we analyzed the role of ADAM10 in the proteolytic processing of PrPCin vivo.ResultsUsing neuron-specific Adam10 knockout mice, we show that ADAM10 is the sheddase of PrPC and that its absence in vivo leads to increased amounts and accumulation of PrPC in the early secretory pathway by affecting its posttranslational processing. Elevated PrPC levels do not induce apoptotic signalling via p53. Furthermore, we show that ADAM10 is not responsible for the α-cleavage of PrPC.ConclusionOur study elucidates the proteolytic processing of PrPC and proves a role of ADAM10 in shedding of PrPCin vivo. We suggest that ADAM10 is a mediator of PrPC homeostasis at the plasma membrane and, thus, might be a regulator of the multiple functions discussed for PrPC. Furthermore, identification of ADAM10 as the sheddase of PrPC opens the avenue to devising novel approaches for therapeutic interventions against prion diseases.

Postnatal Disruption of the Disintegrin/Metalloproteinase ADAM10 in Brain Causes Epileptic Seizures, Learning Deficits, Altered Spine Morphology, and Defective Synaptic Functions

The metalloproteinase ADAM10 is of importance for Notch-dependent cortical brain development. The protease is tightly linked with α-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 β (Aβ) peptide in Alzheimer′s disease. To investigate the function of ADAM10 in postnatal brain, we generated Adam10 conditional knock-out (A10cKO) mice using a CaMKIIα-Cre deleter strain. The lack of ADAM10 protein expression was evident in the brain cortex leading to a reduced generation of sAPPα and increased levels of sAPPβ and endogenous Aβ 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.

High molecular mass assemblies of amyloid-β oligomers bind prion protein in patients with Alzheimer’s disease

Alzheimers disease is the most common form of dementia and the generation of oligomeric species of amyloid-β is causal to the initiation and progression of it. Amyloid-β oligomers bind to the N-terminus of plasma membrane-bound cellular prion protein (PrP(C)) initiating a series of events leading to synaptic degeneration. Composition of bound amyloid-β oligomers, binding regions within PrP(C), binding affinities and modifiers of this interaction have been almost exclusively studied in cell culture or murine models of Alzheimers disease and our knowledge on PrP(C)-amyloid-β interaction in patients with Alzheimers disease is limited regarding occurrence, binding regions in PrP(C), and size of bound amyloid-β oligomers. Here we employed a PrP(C)-amyloid-β binding assay and size exclusion chromatography on neuropathologically characterized Alzheimers disease and non-demented control brains (n = 15, seven female, eight male, average age: 79.2 years for Alzheimers disease and n = 10, three female, seven male, average age: 66.4 years for controls) to investigate amyloid-β-PrP(C) interaction. PrP(C)-amyloid-β binding always occurred in Alzheimers disease brains and was never detected in non-demented controls. Neither expression level of PrP(C) nor known genetic modifiers of Alzheimers disease, such as the PrP(C) codon 129 polymorphism, influenced this interaction. In Alzheimers disease brains, binding of amyloid-β to PrP(C) occurred via the PrP(C) N-terminus. For synthetic amyloid-β42, small oligomeric species showed prominent binding to PrP(C), whereas in Alzheimers disease brains larger protein assemblies containing amyloid-β42 bound efficiently to PrP(C). These data confirm Alzheimers disease specificity of binding of amyloid-β to PrP(C) via its N-terminus in a large cohort of Alzheimers disease/control brains. Differences in sizes of separated protein fractions between synthetic and brain-derived amyloid-β binding to PrP(C) suggest that larger assemblies of amyloid-β or additional non-amyloid-β components may play a role in binding of amyloid-β42 to PrP(C) in Alzheimers disease.

LIMP-2 expression is critical for β-glucocerebrosidase activity and α-synuclein clearance

Significance Our report highlights, for the first time to our knowledge, a distinct relationship between lysosomal integral membrane protein type-2 (LIMP-2) expression, β-glucocerebrosidase (GC) activity, and clearance of α-synuclein. In LIMP-2–deficient mice, increased levels of endogenous α-synuclein led to severe neurological deficits and premature death. We found that loss of LIMP-2 reduced lysosomal GC activity, resulting in lipid storage, disturbed autophagic/lysosomal function, and α-synuclein accumulation leading to neurotoxicity of dopaminergic neurons as well as apoptotic cell death and inflammation. Furthermore, heterologous overexpression of functional LIMP-2 enhanced α-synuclein clearance and improved lysosomal activity of GC. Our results suggest that lysosomal GC activity can be influenced via its interaction with LIMP-2, which could be a promising strategy for the treatment of synucleinopathies. Mutations within the lysosomal enzyme β-glucocerebrosidase (GC) result in Gaucher disease and represent a major risk factor for developing Parkinson disease (PD). Loss of GC activity leads to accumulation of its substrate glucosylceramide and α-synuclein. Since lysosomal activity of GC is tightly linked to expression of its trafficking receptor, the lysosomal integral membrane protein type-2 (LIMP-2), we studied α-synuclein metabolism in LIMP-2–deficient mice. These mice showed an α-synuclein dosage-dependent phenotype, including severe neurological impairments and premature death. In LIMP-2–deficient brains a significant reduction in GC activity led to lipid storage, disturbed autophagic/lysosomal function, and α-synuclein accumulation mediating neurotoxicity of dopaminergic (DA) neurons, apoptotic cell death, and inflammation. Heterologous expression of LIMP-2 accelerated clearance of overexpressed α-synuclein, possibly through increasing lysosomal GC activity. In surviving DA neurons of human PD midbrain, LIMP-2 levels were increased, probably to compensate for lysosomal GC deficiency. Therefore, we suggest that manipulating LIMP-2 expression to increase lysosomal GC activity is a promising strategy for the treatment of synucleinopathies.

Exosomal cellular prion protein drives fibrillization of amyloid beta and counteracts amyloid beta-mediated neurotoxicity.

Alzheimers disease is a common neurodegenerative, progressive, and fatal disorder. Generation and deposition of amyloid beta (Aβ) peptides associate with its pathogenesis and small soluble Aβ oligomers show the most pronounced neurotoxic effects and correlate with disease initiation and progression. Recent findings showed that Aβ oligomers bind to the cellular prion protein (PrPC) eliciting neurotoxic effects. The role of exosomes, small extracellular vesicles of endosomal origin, in Alzheimers disease is only poorly understood. Besides serving as disease biomarkers they may promote Aβ plaque formation, decrease Aβ‐mediated synaptotoxicity, and enhance Aβ clearance. Here, we explore how exosomal PrPC connects to protective functions attributed to exosomes in Alzheimers disease. To achieve this, we generated a mouse neuroblastoma PrPC knockout cell line using transcription activator‐like effector nucleases. Using these, as well as SH‐SY5Y human neuroblastoma cells, we show that PrPC is highly enriched on exosomes and that exosomes bind amyloid beta via PrPC. Exosomes showed highest binding affinity for dimeric, pentameric, and oligomeric Aβ species. Thioflavin T assays revealed that exosomal PrPC accelerates fibrillization of amyloid beta, thereby reducing neurotoxic effects imparted by oligomeric Aβ. Our study provides further evidence for a protective role of exosomes in Aβ‐mediated neurodegeneration and highlights the importance of exosomal PrPC in molecular mechanisms of Alzheimers disease.

Roles of endoproteolytic α‐cleavage and shedding of the prion protein in neurodegeneration

The cellular prion protein (PrPC) plays important roles in neurodegenerative diseases. First, it is the well‐established substrate for the conformational conversion into its pathogenic isoform (PrPSc) giving rise to progressive and fatal prion diseases. Moreover, several recent reports highlight important roles of PrPC in other neurodegenerative conditions such as Alzheimers disease. Since PrPC is subject to proteolytic processing, here we discuss the two main cleavage events under physiological conditions, α‐cleavage and shedding. We focus on how these cleavages and the resulting fragments may impact prion diseases as well as other neurodegenerative proteinopathies. Finally, we discuss the recently identified sheddase of PrPC, namely the metalloprotease ADAM10, with regard to therapeutic potential against neurodegenerative diseases.

The sheddase ADAM10 is a potent modulator of prion disease

The prion protein (PrPC) is highly expressed in the nervous system and critically involved in prion diseases where it misfolds into pathogenic PrPSc. Moreover, it has been suggested as a receptor mediating neurotoxicity in common neurodegenerative proteinopathies such as Alzheimers disease. PrPC is shed at the plasma membrane by the metalloprotease ADAM10, yet the impact of this on prion disease remains enigmatic. Employing conditional knockout mice, we show that depletion of ADAM10 in forebrain neurons leads to posttranslational increase of PrPC levels. Upon prion infection of these mice, clinical, biochemical, and morphological data reveal that lack of ADAM10 significantly reduces incubation times and increases PrPSc formation. In contrast, spatiotemporal analysis indicates that absence of shedding impairs spread of prion pathology. Our data support a dual role for ADAM10-mediated shedding and highlight the role of proteolytic processing in prion disease. DOI: http://dx.doi.org/10.7554/eLife.04260.001

The GPI-anchoring of PrP: Implications in sorting and pathogenesis

The cellular prion protein (PrPC) is an N-glycosylated GPI-anchored protein usually present in lipid rafts with numerous putative functions. When it changes its conformation to a pathological isoform (then referred to as PrPSc), it is an essential part of the prion, the agent causing fatal and transmissible neurodegenerative prion diseases. There is growing evidence that toxicity and neuronal damage on the one hand and propagation/infectivity on the other hand are two distinct processes of the disease and that the GPI-anchor attachment of PrPC and PrPSc plays an important role in protein localization and in neurotoxicity. Here we review how the signal sequence of the GPI-anchor matters in PrPC localization, how an altered cellular localization of PrPC or differences in GPI-anchor composition can affect prion infection, and we discuss through which mechanisms changes on the anchorage of PrPC can modify the disease process.

Cellular Composition of Cerebrospinal Fluid in HIV-1 Infected and Uninfected Subjects

In order to characterize the cellular composition of cerebrospinal fluid (CSF) in a healthy state and in the setting of chronic pleocytosis associated with HIV-1 (HIV) infection, multi-parameter flow cytometry was used to identify and quantitate cellular phenotypes in CSF derived from HIV-uninfected healthy controls and HIV-infected subjects across a spectrum of disease and treatment. CD4+ T cells were the most frequent CSF population and the CD4:CD8 ratio was significantly increased in the CSF compared to blood (p = 0.0232), suggesting preferential trafficking of CD4+ over CD8+ T cells to this compartment. In contrast, in HIV-infection, CD8+ T cells were the major cellular component of the CSF and were markedly increased compared to HIV-uninfected subjects (p<0.001). As with peripheral blood, the CSF CD4:CD8 ratio was reversed in HIV-infected subjects compared to HIV-uninfected subjects. Monocytes, B cells and NK cells were rare in the CSF in both groups, although absolute counts of CSF NK cells and B cells were significantly increased in HIV-infected subjects (p<0.05). Our studies show that T cells are the major cellular component of the CSF in HIV-infected and uninfected subjects. The CSF pleocytosis characteristic of HIV infection involves all lymphocyte subsets we measured, except for CD4+ T cells, but is comprised primarily of CD8+ T cells. The reduced proportion of CD4+ T cells in the CSF may reflect both HIV-related peripheral loss and changes in trafficking patterns in response to HIV infection in the central nervous system.

Generation of aggregation prone N-terminally truncated amyloid β peptides by meprin β depends on the sequence specificity at the cleavage site

BackgroundThe metalloprotease meprin β cleaves the Alzheimer’s Disease (AD) relevant amyloid precursor protein (APP) as a β-secretase reminiscent of BACE-1, however, predominantly generating N-terminally truncated Aβ2-x variants.ResultsHerein, we observed increased endogenous sAPPα levels in the brains of meprin β knock-out (ko) mice compared to wild-type controls. We further analyzed the cellular interaction of APP and meprin β and found that cleavage of APP by meprin β occurs prior to endocytosis. The N-terminally truncated Aβ2-40 variant shows increased aggregation propensity compared to Aβ1-40 and acts even as a seed for Aβ1-40 aggregation. Additionally, we observed that different APP mutants affect the catalytic properties of meprin β and that, interestingly, meprin β is unable to generate N-terminally truncated Aβ peptides from Swedish mutant APP (APPswe).ConclusionConcluding, we propose that meprin β may be involved in the generation of N-terminally truncated Aβ2-x peptides of APP, but acts independently from BACE-1.