• Imbalance of synaptic actin dynamics as a key to fragile X syndrome?

      Michaelsen-Preusse, Kristin; Feuge, Jonas; Korte, Martin; Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2018-01-30)
      Our experiences and memories define who we are, and evidence has accumulated that memory formation is dependent on functional and structural adaptations of synaptic structures in our brain. Especially dendritic spines, the postsynaptic compartments of synapses show a strong structure-to-function relationship and a high degree of structural plasticity. Although the molecular mechanisms are not completely understood, it is known that these modifications are highly dependent on the actin cytoskeleton, the major cytoskeletal component of the spine. Given the crucial involvement of actin in these mechanisms, dysregulations of spine actin dynamics (reflected by alterations in dendritic spine morphology) can be found in a variety of neurological disorders ranging from schizophrenia to several forms of autism spectrum disorders such as fragile X syndrome (FXS). FXS is caused by a single mutation leading to an inactivation of the X-linked fragile X mental retardation 1 gene and loss of its gene product, the RNA-binding protein fragile X mental retardation protein 1 (FMRP), which normally can be found both pre- and postsynaptically. FMRP is involved in mRNA transport as well as regulation of local translation at the synapse, and although hundreds of FMRP-target mRNAs could be identified only a very few interactions between FMRP and actin-regulating proteins have been reported and validated. In this review we give an overview of recent work by our lab and others providing evidence that dysregulated actin dynamics might indeed be at the very base of a deeper understanding of neurological disorders ranging from cognitive impairment to the autism spectrum.
    • A guiding map for inflammation.

      Netea, Mihai G; Balkwill, Frances; Chonchol, Michel; Cominelli, Fabio; Donath, Marc Y; Giamarellos-Bourboulis, Evangelos J; Golenbock, Douglas; Gresnigt, Mark S; Heneka, Michael T; Hoffman, Hal M; Hotchkiss, Richard; Joosten, Leo A B; Kastner, Daniel L; Korte, Martin; Latz, Eicke; Libby, Peter; Mandrup-Poulsen, Thomas; Mantovani, Alberto; Mills, Kingston H G; Nowak, Kristen L; O'Neill, Luke A; Pickkers, Peter; van der Poll, Tom; Ridker, Paul M; Schalkwijk, Joost; Schwartz, David A; Siegmund, Britta; Steer, Clifford J; Tilg, Herbert; van der Meer, Jos W M; van de Veerdonk, Frank L; Dinarello, Charles A; Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2017-07-19)
      Biologists, physicians and immunologists have contributed to the understanding of the cellular participants and biological pathways involved in inflammation. Here, we provide a general guide to the cellular and humoral contributors to inflammation as well as to the pathways that characterize inflammation in specific organs and tissues.
    • APLP1 Is a Synaptic Cell Adhesion Molecule, Supporting Maintenance of Dendritic Spines and Basal Synaptic Transmission.

      Schilling, Sandra; Mehr, Annika; Ludewig, Susann; Stephan, Jonathan; Zimmermann, Marius; August, Alexander; Strecker, Paul; Korte, Martin; Koo, Edward H; Müller, Ulrike C; Kins, Stefan; Eggert, Simone; Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2017-05-24)
      The amyloid precursor protein (APP), a key player in Alzheimer's disease, belongs to the family of synaptic adhesion molecules (SAMs) due to its impact on synapse formation and synaptic plasticity. These functions are mediated by both the secreted APP ectodomain that acts as a neurotrophic factor and full-length APP forming trans-cellular dimers. Two homologs of APP exist in mammals: the APP like proteins APLP1 and APLP2, exhibiting functions that partly overlap with those of APP. Here we tested whether APLP1 and APLP2 also show features of SAMs. We found that all three family members were upregulated during postnatal development coinciding with synaptogenesis. We observed presynaptic and postsynaptic localization of all APP family members and could show that heterologous expression of APLP1 or APLP2 in non-neuronal cells induces presynaptic differentiation in contacting axons of cocultured neurons, similar to APP and other SAMs. Moreover, APP/APLPs all bind to synaptic-signaling molecules, such as MINT/X11. Furthermore, we report that aged APLP1 knock-out mice show impaired basal transmission and a reduced mEPSC frequency, likely resulting from reduced spine density. This demonstrates an essential nonredundant function of APLP1 at the synapse. Compared to APP, APLP1 exhibits increased trans-cellular binding and elevated cell-surface levels due to reduced endocytosis. In conclusion, our results establish that APLPs show typical features of SAMs and indicate that increased surface expression, as observed for APLP1, is essential for proper synapse formation in vitro and synapse maintenance in vivoSIGNIFICANCE STATEMENT According to the amyloid-cascade hypothesis, Alzheimer's disease is caused by the accumulation of Aβ peptides derived from sequential cleavage of the amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) and γ-secretase. Here we show that all mammalian APP family members (APP, APLP1, and APLP2) exhibit synaptogenic activity, involving trans-synaptic dimerization, similar to other synaptic cell adhesion molecules, such as Neuroligin/Neurexin. Importantly, our study revealed that the loss of APLP1, which is one of the major substrates of BACE1, causes reduced spine density in aged mice. Because some therapeutic interventions target APP processing (e.g., BACE inhibitors), those strategies may alter APP/APLP physiological function. This should be taken into account for the development of pharmaceutical treatments of Alzheimer's disease.
    • Metaplasticity mechanisms restore plasticity and associativity in an animal model of Alzheimer's disease.

      Li, Qin; Navakkode, Sheeja; Rothkegel, Martin; Soong, Tuck Wah; Sajikumar, Sreedharan; Korte, Martin; Helmholtz-Zentrum für Infektionsforschung GmbH, Inhoffenstr.7, 38124 Braunschweig, Germany. (2017-05-23)
      Dynamic regulation of plasticity thresholds in a neuronal population is critical for the formation of long-term plasticity and memory and is achieved by mechanisms such as metaplasticity. Metaplasticity tunes the synapses to undergo changes that are necessary prerequisites for memory storage under physiological and pathological conditions. Here we discovered that, in amyloid precursor protein (APP)/presenilin-1 (PS1) mice (age 3-4 mo), a prominent mouse model of Alzheimer's disease (AD), late long-term potentiation (LTP; L-LTP) and its associative plasticity mechanisms such as synaptic tagging and capture (STC) were impaired already in presymptomatic mice. Interestingly, late long-term depression (LTD; L-LTD) was not compromised, but the positive associative interaction of LTP and LTD, cross-capture, was altered in these mice. Metaplastic activation of ryanodine receptors (RyRs) in these neurons reestablished L-LTP and STC. We propose that RyR-mediated metaplastic mechanisms can be considered as a possible therapeutic target for counteracting synaptic impairments in the neuronal networks during the early progression of AD.
    • Novel Insights into the Physiological Function of the APP (Gene) Family and Its Proteolytic Fragments in Synaptic Plasticity.

      Ludewig, Susann; Korte, Martin; Hemholtz Centre for infection research, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2016)
      The amyloid precursor protein (APP) is well known to be involved in the pathophysiology of Alzheimer's disease (AD) via its cleavage product amyloid ß (Aß). However, the physiological role of APP, its various proteolytic products and the amyloid precursor-like proteins 1 and 2 (APLP1/2) are still not fully clarified. Interestingly, it has been shown that learning and memory processes represented by functional and structural changes at synapses are altered in different APP and APLP1/2 mouse mutants. In addition, APP and its fragments are implicated in regulating synaptic strength further reinforcing their modulatory role at the synapse. While APLP2 and APP are functionally redundant, the exclusively CNS expressed APLP1, might have individual roles within the synaptic network. The proteolytic product of non-amyloidogenic APP processing, APPsα, emerged as a neurotrophic peptide that facilitates long-term potentiation (LTP) and restores impairments occurring with age. Interestingly, the newly discovered η-secretase cleavage product, An-α acts in the opposite direction, namely decreasing LTP. In this review we summarize recent findings with emphasis on the physiological role of the APP gene family and its proteolytic products on synaptic function and plasticity, especially during processes of hippocampal LTP. Therefore, we focus on literature that provide electrophysiological data by using different mutant mouse strains either lacking full-length or parts of the APP proteins or that utilized secretase inhibitors as well as secreted APP fragments.
    • The APP Intracellular Domain Is Required for Normal Synaptic Morphology, Synaptic Plasticity, and Hippocampus-Dependent Behavior.

      Klevanski, Maja; Herrmann, Ulrike; Weyer, Sascha W; Fol, Romain; Cartier, Nathalie; Wolfer, David P; Caldwell, John H; Korte, Martin; Müller, Ulrike C; Helmholtz Centre for infection research, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2015-12-09)
      The amyloid precursor protein family (APP/APLPs) has essential roles for neuromuscular synapse development and for the formation and plasticity of synapses within the CNS. Despite this, it has remained unclear whether APP mediates its functions primarily as a cell surface adhesion and signaling molecule or via its numerous proteolytic cleavage products. To address these questions, we followed a genetic approach and used APPΔCT15 knockin mice lacking the last 15 amino acids of APP, including the highly conserved YENPTY protein interaction motif. To circumvent functional compensation by the closely related APLP2, these mice were bred to an APLP2-KO background to generate APPΔCT15-DM double mutants. These APPΔCT15-DM mice were partially viable and displayed defects in neuromuscular synapse morphology and function with impairments in the ability to sustain transmitter release that resulted in muscular weakness. In the CNS, we demonstrate pronounced synaptic deficits including impairments in LTP that were associated with deficits in spatial learning and memory. Thus, the APP-CT15 domain provides essential physiological functions, likely via recruitment of specific interactors. Together with the well-established role of APPsα for synaptic plasticity, this shows that multiple domains of APP, including the conserved C-terminus, mediate signals required for normal PNS and CNS physiology. In addition, we demonstrate that lack of the APP-CT15 domain strongly impairs Aβ generation in vivo, establishing the APP C-terminus as a target for Aβ-lowering strategies.
    • Cannabinoid CB1 Receptor Calibrates Excitatory Synaptic Balance in the Mouse Hippocampus

      Monory, K.; Polack, M.; Remus, A.; Lutz, B.; Korte, M.; Helmholtz Centre for infection research, Inhoffenstr. 7, D-38124 Braunschweig, Germany. (2015-03-04)
    • Two-Photon Correlation Spectroscopy in Single Dendritic Spines Reveals Fast Actin Filament Reorganization during Activity-Dependent Growth.

      Chen, Jian-Hua; Kellner, Yves; Zagrebelsky, Marta; Grunwald, Matthias; Korte, Martin; Walla, Peter Jomo; Helmholtz Centre for infection research, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2015)
      Two-photon fluorescence correlation spectroscopy (2P-FCS) within single dendritic spines of living hippocampal pyramidal neurons was used to resolve various subpopulations of mobile F-actin during activity-dependent structural changes such as potentiation induced spine head growth. Two major classes of mobile F-actin were discovered: very dynamic and about a hundred times less dynamic F-actin. Spine head enlargement upon application of Tetraethylammonium (TEA), a protocol previously used for the chemical induction of long-term potentiation (cLTP) strictly correlated to changes in the dynamics and filament numbers in the different actin filament fractions. Our observations suggest that spine enlargement is governed by a mechanism in which longer filaments are first cut into smaller filaments that cooperate with the second, increasingly dynamic shorter actin filament population to quickly reorganize and expand the actin cytoskeleton within the spine head. This process would allow a fast and efficient spine head enlargement using a major fraction of the actin filament population that was already present before spine head growth.
    • NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice.

      Heneka, Michael T; Kummer, Markus P; Stutz, Andrea; Delekate, Andrea; Schwartz, Stephanie; Vieira-Saecker, Ana; Griep, Angelika; Axt, Daisy; Remus, Anita; Tzeng, Te-Chen; Gelpi, Ellen; Halle, Annett; Korte, Martin; Latz, Eicke; Golenbock, Douglas T (2013-01-31)
      Alzheimer's disease is the world's most common dementing illness. Deposition of amyloid-β peptide drives cerebral neuroinflammation by activating microglia. Indeed, amyloid-β activation of the NLRP3 inflammasome in microglia is fundamental for interleukin-1β maturation and subsequent inflammatory events. However, it remains unknown whether NLRP3 activation contributes to Alzheimer's disease in vivo. Here we demonstrate strongly enhanced active caspase-1 expression in human mild cognitive impairment and brains with Alzheimer's disease, suggesting a role for the inflammasome in this neurodegenerative disease. Nlrp3(-/-) or Casp1(-/-) mice carrying mutations associated with familial Alzheimer's disease were largely protected from loss of spatial memory and other sequelae associated with Alzheimer's disease, and demonstrated reduced brain caspase-1 and interleukin-1β activation as well as enhanced amyloid-β clearance. Furthermore, NLRP3 inflammasome deficiency skewed microglial cells to an M2 phenotype and resulted in the decreased deposition of amyloid-β in the APP/PS1 model of Alzheimer's disease. These results show an important role for the NLRP3/caspase-1 axis in the pathogenesis of Alzheimer's disease, and suggest that NLRP3 inflammasome inhibition represents a new therapeutic intervention for the disease.