Entwicklung von NMR-Methoden zur Untersuchung der Proteinfaltung

The focus of this thesis has been to further advance and develop existing NMR techniques for the study of protein folding. In order to do so, experimental as well as theoretical approaches have been pursued. From the theoretical side, a successful attempt to the development of a general theory for the treatment of residual dipolar couplings in the case of unfolded proteins has been undertaken. Information contained in residual dipolar couplings is especially valuable due to its long-range nature. The dynamic character of unfolded states of proteins, which may be composed of distinct subsets of conformations, renders reliable interpretation of data a non-trivial task. Statistical-coil-based approaches have been shown to be powerful in data interpretation. A consistent theory based on fundamental polymer physics, however, had not been presented so far. The herein presented model addresses this problem building on the original work by Annila and co-workers. In this work, several shortcomings have been identified. These shortcomings have been corrected here leading to a general approach for the treatment of residual dipolar couplings of unfolded proteins. More specifically, it is shown that, in the case of fully unfolded proteins aligned by a steric mechanism, basic dependencies of dipolar couplings such as on chain length and location with in the chain can be analysed in simple analytical terms. The main predictions of the model are compared to experimental data showing reasonable agreement. The presented mathematical framework is principally suited for various improvements which could include the treatment of long-range interactions and of the actual geometry of the given aligment medium. From the experimental side, bovine alpha-lactalbumin has been chosen as a model system for the development of improved time-resolved 1D NMR methods aiming at the observation of conformational transitions by kinetic means. The presented results show that high-quality data can now be obtained at protein concentrations as low as 100uM. Rate constants characterising distinct conformational transitions of up to 8/s have been measured. These are the fastest rate constants which have been reported so far for protein folding events. The NMR data supplemented by complementary biophysical data furthermore demonstrate that the folding of bovine alpha-lactalbumin is more complex than has been anticipated. All data are consistent with a triangular folding mechanism involving parallel pathways of folding for formation of the native state of the protein. Interestingly, such a folding mechanism has also been found for the highly structurally homologous protein lysoyzme from hen egg white. Evidence is presented that the guiding role of long-range interactions in the unfolded state of lysoyzme for mediating intersubdomain interactions during folding is replaced in the case of bovine alpha-lactalbumin by the Ca2+ binding site.Der Schwerpunkt der vorliegenden Arbeit besteht darin, dass bestehende NMR-Techniken zur Charakterisierung der Proteinfaltung weiter entwickelt wurden. Das erste Teil ist rein theoretischer Natur und entwickelt ein mathematisches Gerüst, das die Vorhersage dipolarer Restkopplungen für entfaltete Proteine auf der Grundlage polymerphysikalischer Prinzipien ermöglicht. Auf Grund der Tatsache, dass die im Experiment gemessenen Kopplungen das Ergebnis einer populationsgewichteten Mittelung darstellen, ist ihre Interpretation im strukturellen und dynamischen Sinne schwieriger im Vergleich zu gefalteten Proteinen. Für eine verlässliche Interpretation der Daten ist die Ausarbeitung einer Theorie für die partielle Ausrichtung entfalteter Proteine demzufolge unerlässlich. Als Startpunkt wurde hier der von Annila und Mitarbeitern vorgeschlagene Ansatz gewählt. Eine ausführliche Auseinandersetzung mit der Originalpublikation ergab, dass sie Defizite aufweist. Diese Defizite wurden hier aufgegriffen und korrigiert. Konnte in der Originalpublikation das zweite Legendre-Polynom für ein Kettensegment, das die Ausrichtung relativ zum Magnetfeld wiedergibt, nur numerisch ermittelt werden, so war es hier möglich, eine analytische Lösung zu erarbeiten. Die Vorhersagen auf der Basis des ausgearbeiteten Modells sind, dass (i) dipolare Restkopplungen in der Mitte der Kette größer sind als an den Enden der Kette, (ii) bei einer definierten Konzentration des Ausrichtungsmediums (Barrierendistanz) für kürzere Ketten größer ausfallen als für längere Ketten und (iii) für eine abnehmende Konzentration des Ausrichtungsmediums kleiner werden. Ein ausführlicher Vergleich mit Literaturdaten zeigt gute Übereinstimmung mit den Vorhersagen des Modells. Im zweiten Teil der Arbeit wurden NMR-Experimente zur Untersuchung der Kinetik vorgenommen, mit der sich bovines alpha-Lactalbumin (BLA) vom entfalteten Zustand (U) in den gefalteten Zustand (N) umwandelt. Die Faltung von BLA ist mit verschiedenen biophysikalischen Methoden untersucht worden und das gängige Modell für die Faltung des Proteins, das auf der Basis der Daten erstellt wurde, beschreibt die Bildung eines molten-globule-Intermediates (MG) aus dem entfalteten Zustand und die Umwandlung des Intermediates in den nativen Zustand als einen schrittweisen Prozess: U->MG->N Im Rahmen der vorgenommenen kinetischen Messungen konnten zeitaufgelöste NMR-Methoden derart weiter entwickelt werden, dass (i) die Proteinkonzentration auf 100uM abgesenkt werden konnte, (ii) Faltungsereignisse auf der Millisekundenzeitskala mit Ratenkonstanten von bis zu 8/s detektiert werden konnten und (iii) die kinetischen Daten eindeutig zeigten, dass die Faltung von BLA komplexer abläuft als bisher angenommen wurde. Gestützt durch stopped-flow-Doppelsprung-Fluoreszenzmessungen konnte der Triangular-Faltungsmechanismus für BLA abgeleitet werden. Demnach werden bei der Bildung des gefalteten Zustands von BLA parallele Faltungswege beschritten. I bezeichnet ein Faltungsintermediat, das auf einem der beiden Faltungswege populiert wird und von dem MG-Faltungsintermediat verschieden ist. Besonders interessant hierbei ist, dass für das Hühnereiweißlysozym, das eine hohe Strukturhomologie zu BLA zeigt, ebenfalls dieser sogenannte Triangular-Faltungsmechanismus gefunden wurde. Ein Vergleich der Strukturen des Proteins in Abwesenheit bzw. Anwesenheit des Cofaktors Calcium unter Berücksichtigung der Faltungshomologie legt den Schluss nahe, dass der Calciumbindestelle bei der Faltung von BLA eine tragende Rolle bei der Ausbildung von Kontakten zwischen den Subdomänen zukommt, eine Funktion, die im Falle des Lysozyms durch langreichweitige Wechselwirkungen zwischen den Subdomänen im entfalteten Zustand erfüllt wird

An Alzheimer-associated TREM2 variant occurs at the ADAM cleavage site and affects shedding and phagocytic function

Sequence variations occurring in the gene encoding the triggering receptor expressed on myeloid cells 2 (TREM2) support an essential function of microglia and innate immunity in the pathogenesis of Alzheimer's disease (AD) and other neurodegenerative disorders. TREM2 matures within the secretory pathway, and its ectodomain is shed on the plasma membrane. Missense mutations in the immunoglobulin (Ig)-like domain such as p.T66M and p.Y38C retain TREM2 within the endoplasmic reticulum and reduce shedding as well as TREM2-dependent phagocytosis. Using mass spectrometry, we have now determined the cleavage site of TREM2. TREM2 is shed by proteases of the ADAM (a disintegrin and metalloproteinase domain containing protein) family C-terminal to histidine 157, a position where an AD-associated coding variant has been discovered (p.H157Y) in the Han Chinese population. Opposite to the characterized mutations within the Ig-like domain, such as p.T66M and p.Y38C, the p.H157Y variant within the stalk region leads to enhanced shedding of TREM2. Elevated ectodomain shedding reduces cell surface full-length TREM2 and lowers TREM2-dependent phagocytosis. Therefore, two seemingly opposite cellular effects of TREM2 variants, namely reduced versus enhanced shedding, result in similar phenotypic outcomes by reducing cell surface TREM2

Pathways linking aging and atheroprotection in Mif‐deficient atherosclerotic mice

Atherosclerosis is a chronic inflammatory condition of our arteries and the main underlying pathology of myocardial infarction and stroke. The pathogenesis is age-dependent, but the links between disease progression, age, and atherogenic cytokines and chemokines are incompletely understood. Here, we studied the chemokine-like inflammatory cytokine macrophage migration inhibitory factor (MIF) in atherogenic Apoe−/− mice across different stages of aging and cholesterol-rich high-fat diet (HFD). MIF promotes atherosclerosis by mediating leukocyte recruitment, lesional inflammation, and suppressing atheroprotective B cells. However, links between MIF and advanced atherosclerosis across aging have not been systematically explored. We compared effects of global Mif-gene deficiency in 30-, 42-, and 48-week-old Apoe−/− mice on HFD for 24, 36, or 42 weeks, respectively, and in 52-week-old mice on a 6-week HFD. Mif-deficient mice exhibited reduced atherosclerotic lesions in the 30/24- and 42/36-week-old groups, but atheroprotection, which in the applied Apoe−/− model was limited to lesions in the brachiocephalic artery and abdominal aorta, was not detected in the 48/42- and 52/6-week-old groups. This suggested that atheroprotection afforded by global Mif-gene deletion differs across aging stages and atherogenic diet duration. To characterize this phenotype and study the underlying mechanisms, we determined immune cells in the periphery and vascular lesions, obtained a multiplex cytokine/chemokine profile, and compared the transcriptome between the age-related phenotypes. We found that Mif deficiency promotes lesional macrophage and T-cell counts in younger but not aged mice, with subgroup analysis pointing toward a role for Trem2+ macrophages. The transcriptomic analysis identified pronounced MIF- and aging-dependent changes in pathways predominantly related to lipid synthesis and metabolism, lipid storage, and brown fat cell differentiation, as well as immunity, and atherosclerosis-relevant enriched genes such as Plin1, Ldlr, Cpne7, or Il34, hinting toward effects on lesional lipids, foamy macrophages, and immune cells. Moreover, Mif-deficient aged mice exhibited a distinct plasma cytokine/chemokine signature consistent with the notion that mediators known to drive inflamm'aging are either not downregulated or even upregulated in Mif-deficient aged mice compared with the corresponding younger ones. Lastly, Mif deficiency favored formation of lymphocyte-rich peri-adventitial leukocyte clusters. While the causative contributions of these mechanistic pillars and their interplay will be subject to future scrutiny, our study suggests that atheroprotection due to global Mif-gene deficiency in atherogenic Apoe−/− mice is reduced upon advanced aging and identifies previously unrecognized cellular and molecular targets that could explain this phenotype shift. These observations enhance our understanding of inflamm'aging and MIF pathways in atherosclerosis and may have implications for translational MIF-directed strategies

Early increase of CSF sTREM2 in Alzheimer's disease is associated with tau related-neurodegeneration but not with amyloid- pathology

BackgroundTREM2 is a transmembrane receptor that is predominantly expressed by microglia in the central nervous system. Rare variants in the TREM2 gene increase the risk for late-onset Alzheimer's disease (AD). Soluble TREM2 (sTREM2) resulting from shedding of the TREM2 ectodomain can be detected in the cerebrospinal fluid (CSF) and is a surrogate measure of TREM2-mediated microglia function. CSF sTREM2 has been previously reported to increase at different clinical stages of AD, however, alterations in relation to Amyloid -peptide (A) deposition or additional pathological processes in the amyloid cascade (such as tau pathology or neurodegeneration) remain unclear. In the current cross-sectional study, we employed the biomarker-based classification framework recently proposed by the NIA-AA consensus guidelines, in combination with clinical staging, in order to examine the CSF sTREM2 alterations at early asymptomatic and symptomatic stages of AD.MethodsA cross-sectional study of 1027 participants of the Alzheimer's Disease Imaging Initiative (ADNI) cohort, including 43 subjects carrying TREM2 rare genetic variants, was conducted to measure CSF sTREM2 using a previously validated enzyme-linked immunosorbent assay (ELISA). ADNI participants were classified following the A/T/N framework, which we implemented based on the CSF levels of A(1-42) (A), phosphorylated tau (T) and total tau as a marker of neurodegeneration (N), at different clinical stages defined by the clinical dementia rating (CDR) score.ResultsCSF sTREM2 differed between TREM2 variants, whereas the p.R47H variant had higher CSF sTREM2, p.L211P had lower CSF sTREM2 than non-carriers. We found that CSF sTREM2 increased in early symptomatic stages of late-onset AD but, unexpectedly, we observed decreased CSF sTREM2 levels at the earliest asymptomatic phase when only abnormal A pathology (A+) but no tau pathology or neurodegeneration (TN-), is present.ConclusionsA pathology (A) and tau pathology/neurodegeneration (TN) have differing associations with CSF sTREM2. While tau-related neurodegeneration is associated with an increase in CSF sTREM2, A pathology in the absence of downstream tau-related neurodegeneration is associated with a decrease in CSF sTREM2

Enhancing protective microglial activities with a dual function TREM2 antibody to the stalk region

Triggering receptor expressed on myeloid cells 2 (TREM2) is essential for the transition of homeostatic microglia to a disease‐associated microglial state. To enhance TREM2 activity, we sought to selectively increase the full‐length protein on the cell surface via reducing its proteolytic shedding by A Disintegrin And Metalloproteinase (i.e., α‐secretase) 10/17. We screened a panel of monoclonal antibodies against TREM2, with the aim to selectively compete for α‐secretase‐mediated shedding. Monoclonal antibody 4D9, which has a stalk region epitope close to the cleavage site, demonstrated dual mechanisms of action by stabilizing TREM2 on the cell surface and reducing its shedding, and concomitantly activating phospho‐SYK signaling. 4D9 stimulated survival of macrophages and increased microglial uptake of myelin debris and amyloid β‐peptide in vitro. In vivo target engagement was demonstrated in cerebrospinal fluid, where nearly all oluble TREM2 was 4D9‐bound. Moreover, in a mouse model for Alzheimer's disease‐related pathology, 4D9 reduced amyloidogenesis, enhanced microglial TREM2 expression, and reduced a homeostatic marker, suggesting a protective function by driving microglia toward a disease‐associated state

A TREM2-activating antibody with a blood-brain barrier transport vehicle enhances microglial metabolism in Alzheimer's disease models

van Lengerich et al. developed a human TREM2 antibody with a transport vehicle (ATV) that improves brain exposure and biodistribution in mouse models. ATV:TREM2 promotes microglial energetic capacity and metabolism via mitochondrial pathways. Loss-of-function variants of TREM2 are associated with increased risk of Alzheimer's disease (AD), suggesting that activation of this innate immune receptor may be a useful therapeutic strategy. Here we describe a high-affinity human TREM2-activating antibody engineered with a monovalent transferrin receptor (TfR) binding site, termed antibody transport vehicle (ATV), to facilitate blood-brain barrier transcytosis. Upon peripheral delivery in mice, ATV:TREM2 showed improved brain biodistribution and enhanced signaling compared to a standard anti-TREM2 antibody. In human induced pluripotent stem cell (iPSC)-derived microglia, ATV:TREM2 induced proliferation and improved mitochondrial metabolism. Single-cell RNA sequencing and morphometry revealed that ATV:TREM2 shifted microglia to metabolically responsive states, which were distinct from those induced by amyloid pathology. In an AD mouse model, ATV:TREM2 boosted brain microglial activity and glucose metabolism. Thus, ATV:TREM2 represents a promising approach to improve microglial function and treat brain hypometabolism found in patients with AD

Tandem Phosphorylation of Serines 221 and 318 by Protein Kinase Cδ Coordinates mRNA Binding and Nucleocytoplasmic Shuttling of HuR▿

Stabilization of mRNA by the ubiquitous RNA binding protein human antigen R (HuR), a member of the embryonic lethal abnormal vision (ELAV) protein family, requires canonical binding to AU-rich element (ARE)-bearing target mRNA and export of nuclear HuR-mRNA complexes to the cytoplasm. In human mesangial cells (HMC) both processes are induced by angiotensin II (AngII) via protein kinase Cδ (PKCδ)-triggered serine phosphorylation of HuR. By testing different point-mutated Flag-tagged HuR proteins, we found that Ser 318 within RNA recognition motif 3 (RRM3) is essential for AngII-induced binding to ARE-bearing mRNA but irrelevant for nucleocytoplasmic HuR shuttling. Conversely, mutation at Ser 221 within the HuR hinge region prevents AngII-triggered HuR export without affecting mRNA binding of HuR. Using phosphorylation state-specific antibodies, we found a transient increase in HuR phosphorylation at both serines by AngII. Functionally, PKCδ mediates the AngII-induced stabilization of prominent HuR target mRNAs, including those of cyclin A, cyclin D1, and cyclooxygenase-2 (COX-2), and is indispensable for AngII-triggered migration and wound healing of HMC. Our data suggest a regulatory paradigm wherein a simultaneous phosphorylation at different domains by PKCδ coordinates mRNA binding and nucleocytoplasmic shuttling of HuR, both of which events are essentially involved in the stabilization of HuR target mRNAs and relevant cell functions

γ‐Secretase cleavage of the Alzheimer risk factor TREM

Sequence variants of the microglial expressedTREM2 (triggering receptor expressed on myeloid cells 2) are a major risk factor for late onset Alzheimer's disease.TREM2 requires a stable interaction withDAP12 in the membrane to initiate signaling, which is terminated byTREM2 ectodomain shedding and subsequent intramembrane cleavage by gamma-secretase. To understand the structural basis for the specificity of the intramembrane cleavage event, we determined the solution structure of theTREM2 transmembrane helix (TMH). Caused by the presence of a charged amino acid in the membrane region, theTREM2-TMHadopts a kinked structure with increased flexibility. Charge removal leads toTMHstabilization and reduced dynamics, similar to its structure in complex withDAP12. Strikingly, these dynamical features match with the site of the initial gamma-secretase cleavage event. These data suggest an unprecedented cleavage mechanism by gamma-secretase where flexibleTMHregions act as key determinants of substrate cleavage specificity