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Das Wachstum von M. acetivorans mit Kohlenmonoxid (CO) als Energiesubstrat unterscheidet sich deutlich von dem anderer methanogener Archaeen. M. acetivorans kann während des Wachstums auf CO zwischen Methanogenese und Azetogenese wechseln. Ein wichtiges Enzym, welches den Kohlenstoff-Fluss im Energiestoffwechsel von M. acetivorans reguliert, ist die N5-Methyl-H4SPT:HS-CoM-Methyltransferase Mtr. Durch die Deletion dieses Enzyms konnte ein Stamm, MKOmtr3, erhalten werden, welcher prinzipiell azetogen auf CO wächst, aber geringe Mengen an zusätzlichen Methylgruppen benötigt. Es zeigte sich, dass das bei der Methylgruppen-Reduktion gebildete Heterodisulfid aus HS-CoB und HS-CoM höchstwahrscheinlich für (eine) essentielle anabole Reaktion(en) benötigt wird. In Zellsuspensionen mit MKOmtr3 wurde festgestellt, dass M. acetivorans aus CO kein Methan bildet, d.h. die Mtr-Reaktion nicht umgangen werden kann. Demgegenüber wurde bei Supplementation mit einem Methylgruppen-Donor CO2 gebildet. Mit Hilfe von [13C]-Markierung konnte gezeigt werden, dass das CO2 nicht aus dem Methylgruppen-Donor, sondern aus intrazellulären Speicherstoffen gebildet wird und die Methylgruppen als Elektronenakzeptor fungieren. Somit konnte in dieser Arbeit die von anderer Seite aufgestellte Hypothese, dass es einen cytoplasmatischen Mtr-Bypass gibt, widerlegt werden. Durch Selektion konnten mehrere Suppressoren von MKOmtr3 erhalten werden, welche in der Lage waren mit CO ohne zusätzliche Methylgruppen zu wachsen. Dabei wurde festgestellt, dass diese Stämme nur noch Spuren von Methan bildeten und somit ausschließlich azetogen wuchsen. Dem Suppressor MKOmtrSF fehlte HdrD, das aktive Zentrum der membranständigen Heterodisulfid-Reduktase (HdrED). Das Fehlen der bisher als essentiell bezeichneten HdrED konnte durch Quantifizierung der Enzymaktivität und gezielter Deletion der codierenden Gene bestätigt werden. Trotz der azetogenen Lebensweise von MKOmtrSF und MKOmtrhdr blieb Mcr essentiell, da es weder gelang das Enzym zu inhibieren, ohne die Lebensfähigkeit des Stammes zu verlieren, noch die codierenden Gene zu deletieren. Um einen maximalen Fluss der geringen Mengen an Heterodisulfid, welche von Mcr noch produziert wird, zum Anabolismus zu gewährleisten, wird anscheinend der Haupt-“Verbraucher“, HdrED, ausgeschaltet. In dieser Arbeit wurde somit erstmals gezeigt, dass die Methanogenese nicht zwangsläufig essentiell in methanogenen Archaeen ist, sondern durch eine Variante des reduktiven Azetyl-CoA-Weges ersetzt werden kann.:Abkürzungsverzeichnis 4 1. Einleitung 7 1.1 Methan 7 1.2 Methanogene und Methanogenese 7 1.2.1 Hydrogenotrophe Methanogenese 9 1.2.2 Methylotrophe Methanogenese 12 1.2.3 Azetiklastische Methanogenese 14 1.2.4 Carboxidotrophe Methanogenese 16 1.3 M. acetivorans als Modellorganismus 20 1.4 Ziele dieser Arbeit 23 2. Material und Methoden 25 2.1 Chemikalien 25 2.2 Anaerobes Arbeiten 25 2.3 Verwendete Mikroorganismen 26 2.4 Mikrobiologische Methoden 27 2.4.1 Kultivierung von E. coli 27 2.4.2 Kultivierung von M. acetivorans 28 2.4.3 Bestimmung der Zellausbeute 31 2.4.4 Herstellung von Zellsuspensionen von M. acetivorans 32 2.4.5 Herstellung elektroporationskompetenter E. coli-Zellen 33 2.4.6 Transformation von E. coli 33 2.4.7 Polyethylenglykol-vermittelte Transformation von M. acetivorans 34 2.4.8 Liposomen-vermittelte Transformation von M. acetivorans 35 2.5 Molekularbiologische Methoden 36 2.5.1 Plasmide 36 2.5.2 Oligonukleotide 38 2.5.3 Isolierung von Plasmiden 43 2.5.4 Isolierung von genomischer DNA 43 2.5.5 Restriktionsverdau 44 2.5.6 Ligation mehrerer DNA-Fragmente 44 2.5.7 Cointegration von pAMG40 in pDN201-Derivate 45 2.5.8 Polymerase-Ketten-Reaktion (PCR) 45 2.5.9 Agarose-Gelelektrophorese 46 2.5.10 Sequenzierung von DNA 47 2.6 Analyse von Proteinen 47 2.6.1 Auftrennung von Proteinen mittels SDS-Polyacrylamid-Gelelektrophorese 47 2.6.2 Bestimmung der Proteinkonzentration 48 2.6.3 Proteomanalyse 49 2.6.4 Bestimmung der Fumarat-Reduktase-Aktivität 51 2.6.5 Bestimmung der Heterodisulfid-Reduktase-Aktivität 52 2.7 Quantifizierung von Metaboliten 53 2.7.1 Quantifizierung gasförmiger Metabolite 53 2.7.2 Quantifizierung gelöster Metabolite 55 3. Ergebnisse 57 3.1 Wachstum von M. acetivorans M42 57 3.2 Deletion der Molybdän-abhängigen Formylmethanofuran-Dehydrogenasen 61 3.2.1 Deletion von Fmd1 61 3.2.2 Phänotypische Analyse von MKOfmd1 62 3.2.3 Deletion von Fmd2 64 3.3 Deletion der Formyltransferase Ftr 64 3.4 Deletion der Methyltransferase Mtr 67 3.4.1 Wachstum von MKOmtr3 mit Azetat + MeOH 67 3.4.2 Wachstum von MKOmtr3 mit CO + MeOH 70 3.5 Analyse von möglichen Mtr-Bypässen 74 3.5.1 Metabolitenbildung in Zellsuspensionen aus CO, MeOH und Trimethylamin 74 3.5.2 Deletion von Mtr in DmtsDFH 77 3.5.3 Markierung mit [13C]-MeOH 79 3.6 Überwindung der Methylgruppen-Abhängigkeit von MKOmtr3 82 3.6.1 Ersatz von MeOH bei Wachstum auf CO 82 3.6.2 Genom-Analyse von ausgewählten Suppressoren 85 3.6.3 Proteom-Analyse von ausgewählten Suppressoren 91 3.6.4 Physiologie von MKOmtrSF 96 3.6.5 Komplementation von MKOmtrSF mit MA2238 100 3.6.6 Fumarat-Reduktase-Aktivität in M. acetivorans? 101 3.6.7 Abhängigkeit der Azetogenese vom Elektronentransport über die Zellmembran 101 3.7 Deletion der Heterodisulfid-Reduktase HdrED1 in MKOmtrSF 103 3.7.1 Anpassung der PEG-Transformationsmethode für MKOmtrSF 103 3.7.2 Konstruktion des Stammes MKOmtrhdr und dessen Wachstum 104 3.7.3 Heterodisulfid-Reduktase-Aktivität in M. acetivorans 105 3.8 Untersuchung der Essentialität von Mcr in MKOmtrSF 107 3.8.1 Inhibierung mit Bromethansulfonat und 3-Nitrooxy-Propanol 107 3.8.2 Deletion der Methyl-S-CoM Reduktase Mcr 109 4. Diskussion 113 4.1 Die Formiatbildung von M. acetivorans beim Wachstum auf CO 113 4.2 Methanogenese oder Azetogenese – die Rolle von Mtr als „Schalter“ 116 4.3 In M. acetivorans gibt es keinen cytoplasmatischen Mtr-Bypass 117 4.4 Die Abhängigkeit von MKOmtr3 von Methylgruppen und wie diese umgangen werden konnte 118 4.4.1 Der Verlust von MA2238 118 4.4.2 Der Verlust von MA2285 in MKOmtrSF 119 4.5 Katabole Methyl-Reduktion von MKOmtr3 120 4.6 Eine anabole Rolle von CoB-S-S-CoM in M. acetivorans? 120 4.7 Woher kommt CH3-S-CoM in MKOmtrSF? 122 4.8 Benötigt M. acetivorans für die Azetogenese Elektronentransport über die Membran? 123 4.9 Nichtmethanogenes Wachstum von M. acetivorans 127 4.10 Warum findet sich bisher kein ausschließlich mit Azetogenese wachsendes Archaeon? 128 4.11 Anwendungsmöglichkeiten für ein azetogenes Archaeon 132 Kurzfassung 137 Literaturverzeichnis 138 Anhang 154 Lebenslauf & Publikationen 163 Danksagung 164 Erklärung 165

In dieser Broschüre sind umfassende Kenntnisse über unsere heimischen Schwalbenarten, ihre Lebensweise, ihre Verbreitung in Sachsen und Möglichkeiten zu ihrem Schutz zusammengetragen

Die Versorgung mit Nährstoffen und Sauerstoff in artifiziellen, 3D Scaffolds für das Tissue Engineering ist Grundvoraussetzung für die Viabilität darin kultivierter Zellen. Die Kombination aus definierter Architektur und biomimetischer Zusammensetzung ist dabei entscheidend, sodass neben dem Stofftransport auch die Adhäsion, Proliferation, Migration und Morphologie von Zellen gesteuert werden können. Die Verwendung von Komponenten der nativen Extrazellulären Matrix, wie Collagen Typ I (COL I), als Scaffold-Material wird dafür als optimal angesehen. Jedoch stellt die Gestaltung der Architektur und Bereitstellung instruktiver Elemente zur Steuerung der Zellverhaltens bei COL I-basierten Scaffolds, aufgrund der mechanischen und biochemischen Eigenschaften des COL I, besondere Ansprüche an die Bearbeitungsverfahren. Ziel dieser Arbeit war es daher, Verfahren zur Ausstattung von modularisierbaren, COL I-basierten Membranen mit einer instruktiven biochemischen Zusammensetzung, Mikroarchitekturen als Basis für ein initiales vaskuläres System und Gradienten des Vascular Endothelial Growth Factors (VEGF) zu entwickeln. Diese Scaffolds sollten anschließend hinsichtlich der Beeinflussung des Wachstums, der Orientierung und der gerichteten Migration von humanen Endothelzellen aus der Nabelschnurvene (HUVEC) charakterisiert werden. COL I-Scaffolds konnten durch Plastic Compression in Membranform hergestellt und mit Fibronectin (FN) und Hyaluronsäure (HA, mit hohem Molekulargewicht) ausgestattet werden. Die Inkorporation von FN erhöhte die HUVEC-Proliferation, während die Proliferationsrate bei der verwendeten HA konstant blieb. Anisotrope Mikrostrukturen als Basis für ein vaskuläres System und zur HUVECOrientierung wurden durch einen 3-stufigen Prozess auf den COL I-Membranen erzeugt. Dabei wurde die Mikrostrukturübertragung durch mikrostrukturierte Polystyren-Stempel in einem Druckumformprozess realisiert. Die erhaltenen Mikrogräben mit Breiten von 10-40 µm beeinflussten die Orientierung der HUVEC deutlich. Für die Erzeugung von VEGF-Gradienten zur Steuerung der gerichteten Migration von HUVEC wurde ein Immersionsverfahren mit Hilfe der Modellproteine Bovines Serumalbumin und Hyaluronidase entwickelt. Dieses ermöglichte die Erzeugung eines linearen, graduellen Konzentrationsverlaufs der Modellproteine auf der Membranoberfläche. Der entwickelte Prozess konnte auf VEGF mit einem minimalen Gradientenprofil von 3,8-11,8 pg/mm² nach Immersion in 100 ng/mL VEGF-Lösung übertragen werden. Erste Versuche zur Induktion der HUVEC-Migration zeigten, dass ein erfolgreiches Gradientenprofil jedoch noch gefunden werden muss. Die erzeugten COL I-Membranen bilden eine gute Ausgangsposition für die Bereitstellung modularer, biomimetischer Tissue-Engineering-Scaffolds, mit initialer Vaskularisierung und zellinstruktiven Elementen. Außerdem besitzen sie das Potential, durch eine Vielzahl von Bio-Engineering-Methoden modifiziert und biochemisch den Anforderungen an das zelluläre Mikromilieu ausgestattet zu werden. / The supply of nutrients and oxygen in artificial, three-dimensional scaffolds for tissue engineering is a basic prerequisite for the viability of cells cultivated therein. The combination of defined architecture and biomimetic composition are crucial in this context, so that besides the metabolic transport, the adhesion, proliferation, migration and morphology of cells can also be controlled. The use of native extracellular matrix components, such as collagen type I (COL I), as scaffold material is considered optimal for this purpose. However, designing the architecture and providing instructive elements to control cell behavior in COL I-based scaffolds poses special challenges to processing methods due to the mechanical and biochemical properties of COL I. Therefore, the aim of this work was to develop basic procedures to equip modularizable COL I-based membranes with instructive biochemical composition, microarchitectures as initial vascular system, and vascular endothelial growth factor (VEGF) gradients and to characterize the obtained scaffolds with respect to influencing human umbilical vein endothelial cell (HUVEC) growth, orientation, and migration. It was shown that COL I scaffolds can be prepared by Plastic Compression in membrane form and equipped with fibronectin (FN) and hyaluronic acid (HA, with high molecular weight). Incorporation of FN increased HUVEC proliferation, whereas the proliferation rate remained constant with the HA used. Anisotropic microstructures as a basis for a vascular system and for HUVEC orientation were generated by a 3-step process on the COL I membranes. In this process, the microstructure transfer was realized by microstructured polystyrene stamps in a pressure forming process. The obtained microtrenches with widths of 10-40 µm significantly affected the orientation of HUVEC. For the generation of VEGF gradients to control the directional migration of HUVEC, an immersion technique was developed using the model proteins bovine serum albumin and hyaluronidase. This allowed the generation of a linear, gradual concentration gradient of the model proteins on the membrane surface. The developed process could be applied to VEGF with a minimum gradient profile of 3.8-11.8 pg/mm² after immersion in 100 ng/mL VEGF solution. Initial experiments to induce HUVEC migration showed that a successful gradient profile still needs to be found, however. The generated COL I membranes provide a good starting point for the provision of modular, biomimetic tissue engineering scaffolds, with initial vascularization and cell-instructive elements. They also possess the potential to be modified by a variety of bioengineering methods and biochemically equipped to meet the requirements of the cellular microenvironment.

This thesis investigates the variability of ontogenetic maxillary bone modeling patterns in humans (Homo sapiens) and chimpanzees (Pan troglodytes). Along with sutural growth, bone modeling is the microscopic process by which bones grow in size and model their shape. It results from the simultaneous cellular activities of bone formation (produced by the osteoblasts) and bone resorption (produced by the osteoclasts) on bone surfaces. The study of these activities can bring new insights into our understanding of maxillary, and, more generally, facial ontogeny. However, bone modeling variability remains poorly understood. Using surface histology, we developed quantitative methods to objectively compare and visualize bone modeling patterns. In parallel, geometric morphometric methods were used to capture and quantify maxillary shape changes. Both methods were used for the first time together in an integrative approach. A large sample of H. sapiens individuals ranging from birth to adulthood, and originating from three geographically distinct areas (Greenland, Western Europe and South Africa), was used to infer the variation in maxillary bone modeling at the intraspecific level. We found that human populations express similar bone modeling patterns, with only subtle differences in the location of bone resorption. Moreover, differences in developmental trajectories were identified. This suggests that population differences in maxillary morphology stem from changes in timing and/or rates of the osteoblastic and osteoclastic activities. Adult individuals show similar maxillary bone modeling patterns to subadults, with both cellular activities expressed at reduced intensities. All human populations express high amounts of bone resorption throughout ontogeny, and high inter-individual variation. In contrast, we find low amounts of bone resorption and a low inter-individual variation in chimpanzees, which results in the anterior projection of their maxilla. In chimpanzees, resorption is predominant in the premaxilla, which has been found in some species of Australopithecus and Paranthropus. Other similarities in the location of bone resorption, mostly close to the sutures, suggest the preservation of shared ontogenetic patterns between the humans and chimpanzees. The low intraspecific variation in the location of bone resorption found in both species suggests that species-specific bone modeling patterns can be inferred from a limited number of individuals. This will allow future studies to discuss the bone modeling patterns in fossils for which subadult individuals are scarce.

TRPM3 (melastatin-related transient receptor potential 3) is a calcium-permeable nonselective cation channel that is expressed in various tissues, including insulin-secreting β-cells and a subset of sensory neurons from trigeminal and dorsal root ganglia (DRG). TRPM3 can be activated by the neurosteroid pregnenolone sulphate (PregS) or heat. TRPM3α2 mice display an impaired sensation of noxious heat and inflammatory thermal hyperalgesia. A calcium-based screening of a compound library identified four natural compounds as TRPM3 blockers. Three of the natural compounds belong to the citrus fruit flavanones (hesperetin, eriodictyol and naringenin), the forth compound is a deoxybenzoin that can be synthesized from an isoflavone of the root of Ononis spinosa (ononetin). The IC50 for the substances ranged from upper nanomolar to lower micromolar concentrations. Electrophysiological whole-cell measurements as well as calcium measurements confirmed the potency of the compounds to block TRPM3 in DRG neurones. To further improve the potency and the selectivity of TRPM3 block and to identify the pharmacophore within the flavanone structure, we conducted a hit optimisation procedure by re-screening a focussed library. The library composed of several flavanones with different substitutions on relevant chemical positions and of representatives from different flavonoid subgroups. Within this secondary screen, we identified isosakuranetin and liquiritigenin as active blockers of PregS-induced Ca2+ entry through TRPM3. Isosakuranetin, a flavanone that can be found in blood oranges and grapefruits, displayed an IC50 of 50 nM, and is the most potent inhibitor of TRPM3 identified so far. The novel compounds exhibited a marked specificity for TRPM3 compared with other thermosensitive TRP channels, and blocked PregS-induced [Ca2+]i signals and ionic currents in freshly isolated DRG neurones. Furthermore, isosakuranetin and hesperetin reduced the sensitivity of mice to noxious heat and PregS-induced chemical pain. Since the physiological functions of TRPM3 channels are still poorly defined, the development and validation of potent and selective blockers is expected to contribute to clarifying the role of TRPM3 in vivo. Considering the involvement of TRPM3 in nociception, TRPM3 blockers may represent a novel concept for analgesic treatment.

During fetal development, the lung is filled with fluid that is secreted by an active Cltransport promoting lung growth. The basolateral Na+,K+,2Cl- cotransporter (NKCC1) participates in Cl- secretion. The apical Cl- channels responsible for secretion are unknown but studies suggest an involvement of the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is developmentally regulated with a high expression in early fetal development and a decline in late gestation. Perinatal lung transition is triggered by hormones that stimulate alveolar Na+ channels resulting in fluid absorption. Little is known on how hormones affect pulmonary Cl- channels. Since the rise of fetal cortisol levels correlates with the decrease in fetal CFTR expression, a causal relation may be assumed. The aim of this study was to analyze the influence of glucocorticoids on pulmonary Cl- channels. Alveolar cells from fetal and adult rats, A549 cells, bronchial Calu-3 and 16HBE14o- cells, and primary rat airway cells were studied with real-time quantitative PCR and Ussing chambers. In fetal and adult alveolar cells, glucocorticoids strongly reduced Cftr expression and channel activity, which was prevented by mifepristone. In bronchial and primary airway cells CFTR mRNA expression was also reduced, whereas channel activity was increased which was prevented by LY-294002 in Calu-3 cells. Therefore, glucocorticoids strongly reduce CFTR expression while their effect on CFTR activity depends on the physiological function of the cells. Another apical Cl- channel, anoctamin 1 showed a glucocorticoid-induced reduction of mRNA expression in alveolar cells and an increase in bronchial cells. Furthermore, voltage-gated chloride channel 5 and anoctamine 6 mRNA expression were increased in alveolar cells. NKCC1 expression was reduced by glucocorticoids in alveolar and bronchial cells alike. The results demonstrate that glucocorticoids differentially modulate pulmonary Clchannels and are likely causing the decline of CFTR during late gestation in preparation for perinatal lung transition.

Ramisyllis multicaudata is a member of Syllidae (Annelida, Errantia, Phyllodocida) with a remarkable branching body plan. Using a next-generation sequencing approach, the complete mitochondrial genomes of R. multicaudata and Trypanobia sp. are sequenced and analysed, representing the first ones from Syllidae. The gene order in these two syllids does not follow the order proposed as the putative ground pattern in Errantia. The phylogenetic relationships of R. multicaudata are discerned using a phylogenetic approach with the nuclear 18S and the mitochondrial 16S and cox1 genes. Ramisyllis multicaudata is the sister group of a clade containing Trypanobia species. Both genera, Ramisyllis and Trypanobia, together with Parahaplosyllis, Trypanosyllis, Eurysyllis, and Xenosyllis are located in a long branched clade. The long branches are explained by an accelerated mutational rate in the 18S rRNA gene. Using a phylogenetic backbone, we propose a scenario in which the postembryonic addition of segments that occurs in most syllids, their huge diversity of reproductive modes, and their ability to regenerate lost parts, in combination, have provided an evolutionary basis to develop a new branching body pattern as realised in Ramisyllis.

The present thesis investigated memory load and memory decay in auditory working memory. Alpha power as a marker for memory load served as the primary indicator for load and decay fluctuations hypothetically reflecting functional inhibition of irrelevant information. Memory load was induced by presenting auditory signals (syllables and pure-tone sequences) in noise because speech-in-noise has been shown before to increase memory load. The aim of the thesis was to assess with magnetoencephalography whether a-priori temporal expectations for the onset-time of a to-be-remembered stimulus reduces memory load. It was reported previously that top-down modulations such as spatial expectations reduce memory load and improve memory performance. However, this effect has neither been investigated with temporal expectations nor in the auditory domain. The present thesis showed that temporal expectations for a syllable in noise reduced memory load. Reduced alpha power during stimulus maintenance as well as improved performance indicated the decrease in memory load. Alpha power effects emerged from the right cingulo-opercular network, presumably reflecting a reduced need for functional inhibition. Critically, symbolic cues induced temporal expectations. This effect could not be replicated for clear speech. However, more implicit temporal expectations based on the passage of time elicited a similar decrease in alpha power for clear speech reflecting reduced memory load. Memory decay was assessed with variable delay phases in an auditory sensory memory task with pure-tone sequences. Similarly to memory performance, alpha power decreased with longer delay phases. Critically, temporal expectations counteracted memory decay and led to more sustained performance as well as alpha power across different delay phases. These alpha-power effects were localized to frontal and parietal attention networks as well as primary auditory and visual sensory areas. This implies the involvement of different brain regions relevant for encoding and maintenance in auditory memory and questions a parsimonious functional inhibition explanation. A correlation of alpha power and behavioral performance underpinned the importance of alpha power for auditory working memory. Altogether, the results of the present thesis provide evidence for a beneficial effect of a-priori temporal expectations for an auditory signal on working memory. Moreover, alpha dynamics were shown to be a distinct marker for the neural efficiency of managing working memory limitations.

This thesis investigates how the neural system instantiates selective attention to speech in challenging acoustic conditions, such as spectral degradation and the presence of background noise. Four studies using behavioural measures, magneto- and electroencephalography (M/EEG) recordings were conducted in younger (20–30 years) and older participants (60–80 years). The overall results can be summarized as follows. An EEG experiment demonstrated that slow negative potentials reflect participants’ enhanced allocation of attention when they are faced with more degraded acoustics. This basic mechanism of attention allocation was preserved at an older age. A follow-up experiment in younger listeners indicated that attention allocation can be further enhanced in a context of increased task-relevance through monetary incentives. A subsequent study focused on brain oscillatory dynamics in a demanding speech comprehension task. The power of neural alpha oscillations (~10 Hz) reflected a decrease in demands on attention with increasing acoustic detail and critically also with increasing predictiveness of the upcoming speech content. Older listeners’ behavioural responses and alpha power dynamics were stronger affected by acoustic detail compared with younger listeners, indicating that selective attention at an older age is particularly dependent on the sensory input signal. An additional analysis of listeners’ neural phase-locking to the temporal envelopes of attended speech and unattended background speech revealed that younger and older listeners show a similar segregation of attended and unattended speech on a neural level. A dichotic listening experiment in the MEG aimed at investigating how neural alpha oscillations support selective attention to speech. Lateralized alpha power modulations in parietal and auditory cortex regions predicted listeners’ focus of attention (i.e., left vs right). This suggests that alpha oscillations implement an attentional filter mechanism to enhance the signal and to suppress noise. A final behavioural study asked whether acoustic and semantic aspects of task-irrelevant speech determine how much it interferes with attention to task-relevant speech. Results demonstrated that younger and older adults were more distracted when acoustic detail of irrelevant speech was enhanced, whereas predictiveness of irrelevant speech had no effect. All findings of this thesis are integrated in an initial framework for the role of attention for speech comprehension under demanding acoustic conditions.