Biochemistry and Molecular Biology - Theses
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In vitro investigation of dipeptide repeat proteins in C9ORF72-associated motor neuron disease and frontotemporal dementia
Mutations that expand a hexanucleotide (GGGGCC) tandem repeat sequence (HTRS) in intron-1 of the C9ORF72 gene is the major cause of familial motor neuron disease MND and frontotemporal dementia (FTD). The expanded HTRSs trigger abnormal bi-directional transcription, which generates both sense and antisense RNA transcripts that form foci. The subsequent translation of all reading frames of HTRSs generates five types of dipeptide repeat proteins (DRPs) via Repeat-Associated Non-ATG (RAN) translation. The sense (G4C2) RNA encodes poly-glycine-alanine (poly-GA), poly-glycine-proline (poly-GP), and poly-glycine-arginine (poly-GR), while the antisense (C4G2) RNA generates poly-GP, poly-proline-arginine (poly-PR), and poly-alanine-proline (poly-AP). The presumably toxic RAN proteins aggregate in neurons suggesting that they play a crucial role(s) in NMD and FTD pathogenesis. The mechanism underlying the DPR protein-induced neuronal toxicity have not been fully elucidated. The current thesis has employed a quantitative proteomics approach aimed at understanding how these DRPs, as putative “junk” proteins, confer damage to the cellular biochemistry via first identifying the protein interactome of each DRP and secondly by determining the impact of these DRP sequences on the global proteome. To this end, we have developed artificial cDNA constructs engineered by a randomized codon strategy to express short (10x) and long (101x) DPR proteins avoiding GC repeat sequences. When transiently expressed in mouse neuroblastoma cells, the soluble poly-PA and poly-GP were well tolerated at the lengths tested. Clear length-dependent neurotoxicity was seen for poly-GA that formed cytoplasmic aggregates only at the long length. When compared to 97Q-expanded huntingtin exon 1 aggregates, GA101 exhibited faster aggregation kinetics and stronger proteasomal recruitment as revealed in the aggregate proteome. Conversely, the arginine-rich DPRs (Poly-PR and Poly-GR) formed discrete nuclear aggregates and were profoundly toxic at both short and long repeat lengths. Data presented within this thesis has also provided evidence that the arginine valency in poly-PR and poly-GR triggers their pernicious binding to the proteome, compared to the sparse binding of the more inert poly-AP and poly-GA. The poly-PR and poly-GR interactome included proteins involved in ribosome biogenesis, translation and RNA splicing machinery. Using stalling reporters, long R-rich DPRs stalled translation, through a putative electrostatic jamming mechanism. These findings also indicate that Arg-rich dipeptide polymers are mimetics of endogenous arginine methylase substrates and recruit these methylase proteins. A consequence is hypomethylation of the proteome, including key proteins known to be involved in ALS mechanisms. The mechanism explains the previously reported abnormalities in arginine methylation patterns in the brains of ALS patients. Finally, this work has unravelled how Arg-rich dipeptide polymers impair the assembly of the actin cytoskeleton. This finding provides important mechanistic insight into the previously reported actin polymerization defects in multiple forms of amyotrophic lateral sclerosis.
Investigations of the mechanisms of action of human small heat-shock proteins against amyloid fibril formation and pre-formed amyloid fibrils
Small heat-shock proteins (sHSPs) are ubiquitously expressed molecular chaperones that protect the proteome against proteostasis failure. sHSPs comprise a conserved alpha-crystallin domain (ACD) flanked by variable N- and C-terminal regions (NTR and CTR, respectively). Heterogenous intermolecular interactions involving the NTR and CTR of sHSPs give rise to polydisperse ensembles of oligomers that readily exchange subunits. This polydispersity renders sHSPs refractory to biophysical characterization by X-ray diffraction and cryogenic electron microscopy, however the ACD can be crystallized. As such, much of the structural information regarding sHSPs has been derived from biophysical studies of this domain. The ACD adopts a beta-sandwich fold and forms antiparallel dimers via an extended beta-strand, creating a shared beta-sheet between the two subunits. sHSP oligomers are widely regarded to act as storage forms of these chaperone-active, exchangeable dimers. Since ACD-only constructs form dimers, they have become widely used tools for investigating the chaperone properties of sHSP dimers. However, it is becoming evident that the NTR and CTR are also required for chaperone activity, and the exchange of sHSP monomers from oligomers has been observed. The chaperone function(s) of the monomers are largely unknown. We and others have begun to characterize the ACD monomer via NMR and sedimentation velocity analytical ultracentrifugation. The first part of this thesis investigates the functional contributions of the three regions of sHSPs, the NTR, ACD and CTR. This was achieved by comparing the chaperone activities of various constructs of human alphaB-crystallin (alphaB-C, HSPB5) and heat-shock 27 kDa protein (Hsp27, HSPB1) during amyloid formation by alpha-synuclein and apolipoprotein C-II. Inhibition of the nucleation phase of amyloid fibril formation and disaggregation of mature fibrils was found to be dependent on the NTR and/or CTR of alphaB-C and the NTR of Hsp27. Both the NTR and CTR are required for forming strong interactions with fibrils and the CTR appears to play an important role in mediating lateral fibril-fibril association, which sequesters pre-formed fibrils into larger aggregates, a function that is believed to be cytoprotective. We conclude that important chaperone activities in inhibiting amyloid formation and in mediating interactions of sHSPs with amyloid fibrils are conferred by the NTR and CTR of these two sHSPs and that subunit exchange from sHSP oligomers likely regulates some of these chaperone functions. Despite this, the ACD also possesses some chaperone activity against amyloid formation and this was investigated in the second part of this thesis. In the second part of this thesis, we investigated the interaction of alphaB-C ACD with apolipoprotein C-II and alpha-synuclein amyloid fibrils using NMR. Significant resonance broadening was observed in the presence of amyloid fibrils, indicative of an ACD-fibril interaction. We subsequently demonstrated, by varying the concentration of fibril ends, that the ACD exhibits a preference for binding to fibril ends. We also demonstrate that both the ACD dimer and the ACD monomer can be observed via solution NMR under conditions that are close to physiological. NMR revealed that that for apolipoprotein C-II, the primary fibril-interacting species is monomeric and implicated the intra-dimer interface as an interactive site. Interestingly, alphaB-C ACD exhibited distinct modes of binding to monomeric and fibrillar alpha-synuclein. The alphaB-C ACD dimer was implicated in binding monomeric alpha-synuclein via a substrate interactive site distal to the dimer interface, while the alphaB-C ACD monomer appeared to interact with fibrillar alpha-synuclein. Fibril end binding by the ACD monomer was supported by our mutagenesis studies in which disulfide cross-linked ACD dimers were less effective at inhibiting amyloid fibril elongation and naturally occurring fibril end-to-end joining than non-covalent dimers. Overall, the results presented here demonstrate the multiple mechanisms sHSPs use to protect cells from the process of amyloid fibril formation and the associated toxicity. The multiple chaperone activities are dependent on all three regions of the protein and the accessibility of these substrate binding sites were shown to depend on the ability of these proteins to dissociate from oligomers into various sub-oligomeric forms including monomers and dimers. Our results therefore provide further evidence for the dynamic nature of sHSP assemblies and the importance of this dynamism for substrate binding and chaperone function.
The role of Complement and MHC in innate and adaptive immunity cooperation
Major histocompatibility complex class II (MHC II) molecules are expressed by antigen presenting cells and essential for the development and activation of CD4+ T cells. Complement component 3 (C3) is the central component of the complement system and its activation is known to induce protective functions as part of the innate immune system. In this study, we have identified the unknown cooperation of these two central molecules of the adaptive and innate immune system, by describing their covalent association on the surface of conventional dendritic cells (cDCs). This in turn led to their ablation and enriched trogocytosis by marginal zone (MZ) B cells. We therefore describe two novel instances of intersection between the innate and adaptive immune systems, one at the molecular and the other at the cellular level, namely the covalent binding of activated C3 to MHC II and the cellular interaction between cDCs and MZ B cells based on a novel molecular communication axis. Chapter 3 investigates the biochemical mechanisms of MARCHF1-mediated C3 deposition in cDCs and describes the binding of activated C3 to surface MHC II molecules in scenarios where its surface expression is enriched due to impaired ubiquitination. Biochemical analyses revealed the covalent association of C3dg and C3d, the two smallest C3 fragments, specifically to the N-linked carbohydrate of MHC IIalpha. This novel function of surface MHC II molecules as an acceptor for the covalent association of complement C3 was cDC-specific and also observed in human cDCs from blood samples. In Chapter 4 we explore the physiological role/consequence of enriched MHC II-dependent C3 fixation on cDCs and identified the specific and significant loss of splenic cDCs, associated with enriched C3 deposition. Furthermore, we identified the appearance of cDC-specific markers at the surface of MZ B cells that lacked the transcriptional expression for these proteins, also associated with enriched C3 deposition. Chapter 5 demonstrates C3-mediated trogocytosis of cDCs by MZ B cells, in which the latter acquire membrane fractions, as well as surface proteins and peptide-loaded MHC II molecules from cDCs. Importantly this intercellular exchange of membrane proteins was unidirectional and dependent on complement receptor 2 (CR2) expression in B cells, which indicates that C3-mediated trogocytosis is a coordinated cellular process. In summary, this thesis provides comprehensive analyses on the interplay of innate and adaptive immunity, both on a molecular and cellular level. The data presented in this thesis indicate fundamental new roles for both MHC II and C3, in which surface MHC II serves as an acceptor for covalent C3 binding, which in turn is a driving mediator for close cell-cell interactions between cDCs and MZ B cells and unidirectional trogocytosis.
Targeting the mitochondrion in Coxiella burnetii infection
Mitochondria are essential organelles, fundamental to eukaryotic cell function and survival. Perhaps best known for their role in energy production, mitochondria are also central to many cellular processes, including calcium homeostasis, immune and cell death signalling. With such diverse cellular roles, it is no surprise that microbial virulence factors target the mitochondrion during infection. Coxiella burnetii is a unique intracellular bacterial pathogen and the causative agent of Q fever. The bacterium infects alveolar macrophages and replicates within a highly acidic, lysosome-like vacuole, termed the Coxiella-containing vacuole (CCV). C. burnetii translocates over 150 bacterial effector proteins into the host cytosol via a Type 4 Secretion System (T4SS). Effector proteins translocated into the cell modulate cellular functions to facilitate CCV development and bacterial replication. This study has aimed to deepen our understanding of the host-pathogen interactions occurring between C. burnetii and the host cell mitochondrion during infection. In doing so, we have developed new methods and adapted current technologies to the study of this fascinating bacterial pathogen. We uncovered the effector protein CBU0077 (later renamed Mitochondrial Coxiella effector A (MceA)) that localised to the mitochondrial network during infection. This established the mitochondrion is a bona fide target during C. burnetii infection. To investigate whether additional C. burnetii proteins associate with the organelle during infection, we analysed mitochondria purified from infected cells by high-sensitivity mass spectrometry. This unbiased, proteomic approach identified an additional 7 effector proteins associated with mitochondria in the context of infection and we further went on to biochemically characterise CBU1425. We were able to demonstrate that CBU1425 is imported into the mitochondria and interacts with key components of the organelle quality control pathway, revealing a new aspect of C. burnetii biology. A complex interplay exists between the host cell and the pathogenic agent during infection. To disentangle the impact of C. burnetii infection on the host cell and the mitochondrion, we utilised SILAC quantitative proteomics to investigate changes to the cellular and organellar proteome. This revealed a global effect on cellular and mitochondrial pathways. Our findings compare and contrast these organelle-wide changes between cell types and another distinct pathogen, providing a deeper understanding of the role of the mitochondrion during C. burnetii infection and, more broadly in the host cell response to this assault. This research has contributed comprehensive new insights into our understanding of the interaction between the host cell mitochondria and C. burnetii during infection. With a foundational knowledge of the host-pathogen interactions occurring during infection, we can begin to further probe the molecular requirements and outcomes of our own cellular pathways in both health and disease.
A structural understanding of interleukin- 11 signalling
Interleukin (IL)-11 is an IL-6 family cytokine with several described roles in normal human physiology, haematopoiesis and disease. IL-11 signals by forming a hexameric signalling complex with its two receptors, the IL-11 specific receptor IL- 11R alpha, and the receptor glycoprotein (gp) 130, which is also utilised by approximately 15 other cytokines. IL-11 signalling has been shown to have roles in several diseases, including gastrointestinal cancer and cardiovascular fibrosis. Despite the growing importance that has been assigned to IL-11 signalling in disease, little is known about the structure of IL-11, the structural mechanisms of IL-11 signalling, or the structural basis of existing IL-11 signalling inhibitors. Overall, this thesis aims to develop a structural and mechanistic understanding of the formation of the IL-11 signalling complex, and the mechanism of action of an existing IL-11 signalling inhibitor. Here, we present the structures of IL-11 and IL-11R alpha. The new high-resolution structure of IL-11 uncovers details not resolved in previous structures, including in a loop which has previously been directly implicated in binding to IL-11R alpha. The structure of IL-11R alpha shows structural differences to related cytokine receptors. Combined with biophysical analysis of the interaction between IL-11 and IL-11R alpha, our structures allow us to propose a model for the recruitment of IL-11 by IL-11R alpha. We also used our structure to investigate the mechanism of action of known disease mutations in IL-11R alpha. These mutations were distal to cytokine-binding regions in IL-11R alpha and appear to act through destabilisation of IL-11R alpha. We also solved the structure of the IL-11 signalling complex using cryo-electron microscopy and X-ray crystallography. The complex is a hexameric complex consisting of two copies each of IL-11, IL-11R alpha and gp130. The resolution of the map is sufficiently high to define in detail of the binding sites on IL-11 and the receptors and uncovers mechanistic differences in complex formation by IL-11 and the related cytokines IL-6 and viral IL-6. Using solution methods, we also showed that the IL-11 signalling complex is hexameric in solution, and forms in three high- affinity steps, combined with our knowledge of the IL-11/IL-11R alpha interaction, this allows us a detailed, mechanistic understanding of the formation of the IL-11 signalling complex. We also studied the lead IL-11 signalling inhibitor, IL-11 Mutein. IL-11 Mutein was developed from an existing antagonistic mutant of IL-11, IL-11 W147A, which removed a key residue required for hexameric complex formation. IL-11 Mutein was developed from IL-11 W147A using phage display, to identify novel mutations which increased the potency of IL-11 W147A. We solved structures of IL-11 Mutein and the IL-11 W147A mutant, showing that the mutations in IL-11 Mutein disrupt a loop in proximity to the IL-11R alpha binding site. We showed that IL-11 Mutein is an effective IL-11 antagonist and prevents the assembly of the IL-11 signalling complex. Supported with extensive biochemical and biophysical analysis, we propose a model for the antagonistic action of IL-11 Mutein, which will guide the development of novel cytokine-like inhibitors. Overall, the work presented here allows a detailed understanding of the structural mechanisms underpinning IL-11 signalling. The results will allow the development of novel antagonists targeting the IL-11 signalling complex, which we hope will be of therapeutic benefit in cancers and other diseases.
Investigating the Epitranscriptome of Plasmodium falciparum
Recent years have seen an increasing identification and awareness of post-transcriptional modifications of RNA. Nucleotide modifications have long been known to be important in diverse non-coding RNAs, but post-transcriptional modifications of mRNA are now also recognised as playing an important role in regulation of gene expression. One of the most abundant mRNA modifications is N6-methyladenosine (m6 A), the presence of which guides mRNA metabolism including maturation, nuclear export, translation and decay. In model eukaryotes, m6A is produced by the action of METTL-family methyltransferases and recognised by a family of YTH proteins. These enzymes play a key role in cellular differentiation and development. I have identified several putative METTL methyltransferases and several YTH reader proteins in the Plasmodium falciparum genome, and have investigated the m6A modification in the transcriptome of P. falciparum. I have characterised two methyltransferase proteins, Mettl3 and Mettl14-like proteins and two reader YTHDC1-like proteins (PfYTHDC1-a and PfYTHDC1-b)in P. falciparum through localisation, overexpression experiments and knock-sideways studies. Initial studies using epitope tagging and GFP fusion proteins revealed a predominantly nuclear localisation for both the methyltransferase and reader proteins, with some signal in the cytoplasm. Knock-sideways studies revealed complete inducible mis-localisation by 24 hours for PfMETTL3 and within 7 hours for PfYTHDC1-a. Additionally, under induced mis-localisation, both transfectant lines underwent morphological changes as evident by microscopy, with clear spots within the digestive vacuole, changes not seen in the 3D7 parental line. A pilot RNA-seq study was conducted, wherein differential transcript expression was analysed as a means to characterise the impact of perturbing m6A modifications by overexpressing and disrupting METTL methyltransferases. Future experiments should analyse the distribution of m6A in mRNA by nanopore direct-RNA sequencing. Additional investigations are also warranted on the influence of the METTL and YTHDC1-like proteins on splicing patterns in the parasite as a means to test the role of these modifications on proliferation and differentiation.
Neuroinflammation, microglia and the cell biology of Alzheimer's Disease
The pathology of Alzheimer’s disease (AD) is characterised by progressive accumulation of misfolded proteins, which form senile plaques and neurofibrillary tangles, and chronic inflammation in the brain associated with inflammatory mediators by the activation of innate immune responses. There has been considerable interest in the role of neuroinflammation in directly contributing to the progression of AD. Studies in mice and humans have identified a role for microglial cells, the resident innate immune cells of the CNS, in AD. Activated microglia are a key hallmark of the disease and the secretion of pro-inflammatory cytokines by microglia may result in a positive feedback loop between neurons and microglia, resulting in ongoing low-grade inflammation and associated neurotoxicity. The underlying mechanisms however are poorly understood. Here the role of microglia was investigated, especially their link to ApoE – the strongest risk factor for late onset Alzheimer’s disease – and the relationship between microglia, neurons and neuroinflammation. Target replacement mice were used, where the human ApoE2, ApoE3 or ApoE4 allele replaces the mouse ApoE allele. Microglia were activated in a two- step setup. Initially cells were primed with LPS, followed by a secondary stimulus, such as ATP or Ab. The system was used to characterise the cytokines secreted by activated microglia and to assess the impact of conditioned medium from stimulated and Ab treated microglia on neuronal morphology. The first results Chapter (Chapter 3) established the system – mouse microglia were isolated from brains of neonatal mice and characterised by CD11b staining. Microglia from all three ApoE genotypes were directly compared and the data from ELISA and mass spectrometry revealed an enhanced pro- inflammatory response by ApoE4 microglia and the least efficient at internalizing amyloid b. Chapter 4 analysed the impact of conditional medium from the microglia ApoE variants on neurons and the results showed an ApoE-dependent effect on dendrite morphology. Conditioned media from immunostimulated and Ab microglia were incubated with cortical neurons from wt animals. Both the dendrite length and number of dendrites were significantly reduced in neurons treated with conditioned medium from ApoE4 microglia. TNFa was identified as a major cytokine and was responsible for modifying neuron morphology in cell assays. Neutralising the cytokine, with an anti-TNFa antibody abrogated the majority of morphological changes induced by the conditioned media from activated microglia. Hence the data suggests that TNFa may have a major role in mediating neuroinflammation. The third results Chapter (Chapter 5) compared aspects of macrophage function with microglia. Here it was shown, that microglia do not require SNX5 for macropinocytosis, and most likely utilise peripheral mediated macropinocytosis as the main form of macropinocytic internalisation. A major finding was the ability of sodium chloride to augment a pro-inflammatory response not only by immunostimulated macrophages by also microglia. Inhibition of the p38 MAPK signalling pathway partially ameliorated the NaCl- induced inflammatory responses in both macrophages and microglia, together with high levels of secreted IL-1b, indicating activation of the NLRP3 inflammasome. Overall the studies highlight a role for APoE4 allele to promote an enhanced inflammatory response by microglia cells.
Delayed death by plastid inhibition in Plasmodium falciparum
Apicomplexan parasites possess a plastid organelle called the apicoplast. Inhibitors that selectively target apicoplast housekeeping functions, including DNA replication and protein translation, are lethal for the parasite, and several (doxycycline, clindamycin, and azithromycin) are in clinical use as antimalarials. A major limitation of such drugs is that treated parasites only arrest one intraerythrocytic development cycle (approximately 48 hours) after treatment commences, a phenotype known as the ‘delayed death’ effect. The molecular basis of delayed death is a long-standing mystery in parasitology and establishing the mechanism would aid rational clinical implementation of apicoplast-targeted drugs. Parasites undergoing delayed death transmit defective apicoplasts to their daughter cells and cannot produce the sole, blood-stage essential metabolic product of the apicoplast: the isoprenoid precursor isopentenyl pyrophosphate. How the isoprenoid precursor depletion kills the parasite remains unknown. We investigated the requirements for the range of isoprenoids in the human malaria parasite Plasmodium falciparum and characterised the molecular and morphological phenotype of parasites experiencing delayed death. Metabolomic profiling reveals disruption of digestive vacuole function in the absence of apicoplast derived isoprenoids. Three-dimensional electron microscopy reveals digestive vacuole fragmentation and the accumulation of cytostomal invaginations, characteristics common in digestive vacuole disruption. We show that DV disruption results from a defect in the trafficking of vesicles to the digestive vacuole. The loss of prenylation of vesicular trafficking proteins abrogates their membrane attachment and function and prevents the parasite from feeding. Our data show that the proximate cause of delayed death is an interruption of protein prenylation and consequent cellular trafficking defects.
Teaching STEM for conceptual change: Lessons from large classes in biomedical science
Despite the best efforts of academics, some students leave the undergraduate science courses with uncorrected misconceptions. There are several possible explanations for the persistence of misconceptions through and beyond undergraduate education, one of which is due to large class sizes. It seems, however, that large class teaching will be a continued response to the expansion of higher education. Therefore, it is imperative for academics to introduce teaching methods that are suited to large class settings and that will arrest problems that lead to uncorrected misconceptions. Conceptual change theory is one of the prevailing theoretical approaches to transforming misconceptions into conventional scientific understanding by reorganising or modifying the ideas underlying the misconception. One of the best approaches academics can take to fostering conceptual change is to infuse active learning into their teaching practice. In this thesis, I propose an active learning intervention that potentially fosters conceptual change in a large class setting, as well as presenting the theoretical underpinnings of the development of this intervention. I started by enquiring into the current pedagogical practices of Australian biomedical science academics in order to understand how they cope with the mass education context, and whether there are specific obstacles to the introduction of active learning into their classrooms. Findings indicate that, despite identifying themselves as traditional teachers, the current pedagogical practices of Australian biomedical science academics exhibit potential in ‘teaching for conceptual change’ by being reflective teachers, by shifting to non-traditional teaching, and by collaborating with their colleagues to design the curriculum. Next, I probed the precursors for conceptual change learning experienced by interviewing students. By capitalising on the cognitive disequilibrium generated by misconceptions, I was able to further inform the development of the intervention. A contradictory understanding of current knowledge triggers cognitive disequilibrium because students are unable to bridge the gap between their prior knowledge and an incoming information. Findings show that the amount of prior knowledge, regardless of the quality, appeared to predict students’ degree of confidence in it. In turn, the degree of confidence in prior knowledge appeared to predict a student’s decision to change their mind in a collaborative setting; and the degree of confidence in prior knowledge also appeared to predict learning and retention of concepts. Finally, drawing from the notion that cognitive disequilibrium, if properly induced, can lead to conceptual change, I propose a misconception-driven intervention that potentially induces cognitive disequilibrium by using intentionally flawed scientific manuscripts. This active learning intervention showed comparable improvements with a didactic approach in terms of students’ conceptual understanding and student self-reported engagement, however, independent classroom observers observed that this intervention improved student engagement. A review of the extant literature on student engagement suggests that engagement is predominantly linked to improved learning outcomes. I would argue further that for the learning process to ultimately lead to conceptual change, the responsibility does not only depend on the improved pedagogical practices of academics, but also more importantly, on the students themselves. Our role as academics is to integrate active learning methods that promote self-regulation, which will enable students to reflect on their own understanding and become more metaconceptually aware for them to recognise a potential state of cognitive disequilibrium. To be able to do this, academics should attend education conferences or fora and professional development opportunities to help them improve their pedagogical practice.
Immune defense mechanisms against Legionella longbeachae
The pulmonary epithelial barrier is the first line of defense against pathogens invading the lungs. If those are able to overcome this first barrier, myeloid cells of the innate immune system are instrumental for the antimicrobial defense and can directly eliminate invading microorganisms. This work aimed to identify novel mechanisms by which pulmonary epithelial cells and myeloid cells eliminate invading bacteria from the lungs. For this, infections with Legionella longbeachae were used to investigate a severe and often fatal form of pneumonia in humans known as Legionnaires’ disease in a mouse model. Following infection, infiltration of immune cells was dominated by neutrophils and, to a lesser extent, by monocytes. In addition to this, a significantly higher fraction of neutrophils contained L. longbeachae bacteria compared with other myeloid immune cells. Within host cells, bacteria translocated effector proteins mostly into neutrophils, and were residing in a vacuole resembling the Legionella-containing vacuole, as known from infections with L. pneumophila. However, neutrophils played an important role in the in vivo clearance of L. longbeachae, as mice depleted of this cell type exhibited significantly higher bacterial burden in the lungs. Besides neutrophils, monocytes also contributed to the control of pulmonary L. longbeachae infections, while lymphoid immune cells had no effect on the clearance of the bacteria. Molecularly, it is well known that IL18 is important in anti-bacterial defense by inducing lymphocytes to release IFN gamma. However, IL18 receptor (IL18R) expression on lymphoid cells did apparently not promote L. longbeachae clearance. Instead, expression by pulmonary stromal cells was required and sufficient for elimination of the bacteria. Stromal expression of the IL18 receptor was almost confined to the ciliated epithelial cell compartment in the bronchioles. IL18R signaling in those cells did not promote mucus production but it rather enhanced the anti-bactericidal activity of neutrophils. Therefore, these results indicate a non-canonical role of IL18 in the defense against pulmonary L. longbeachae infection, linking non-immune pulmonary epithelial cells with inflammatory neutrophils.
Structure and conformational dynamics studies of α1A-adrenoceptor
G-Protein Coupled Receptors (GPCRs) are integral membrane proteins representing one of the most important class of drug targets. alpha1-adrenoceptors (alpha1-ARs) comprise three GPCR subtypes that stimulate smooth muscle contraction in response to binding adrenaline and noradrenaline. alpha1A-AR and alpha1B-AR are clinically targeted for treating hypertension and benign prostatic hyperplasia but are putative drug targets for neurodegenerative diseases. The subtype-selective antibodies and tool compounds are required to probe the role of these receptors in the brain and to validate them as drug targets for neurodegenerative diseases, where the structure studies of alpha1-ARs would assist. So far, no structures of alpha1-ARs have been published which may due to their low stabilities when purified. Thus, in this study, we selected an ultra-stable alpha1A-AR through directed evolution to pave way for the structure determination of alpha1A-AR. GPCRs typically bind to an extracellular ligand and transmit signals across the cell membrane via an allosteric network from the ligand-binding site to the G-protein binding site via a series of conserved microswitches. Crystal structures of GPCRs provide snapshots of inactive and active states, but poorly describe the conformational dynamics of the allosteric network that underlies GPCR activation. We analyse the structural correlation between ligand binding and the allosteric network of the alpha1A-AR. NMR of 13CH3-methionine labelled alpha1A-AR mutants, each exhibiting differing signalling capacities, revealed how different ligands modulate receptor conformational equilibria. 13CH3-methionine residues near the microswitches revealed distinct states that correlated with ligand efficacies, supporting a conformational selection mechanism. Restoration of functional microswitches revealed that while the conformational states of individual microswitches are loosely independent, for alpha1A-AR activation, complete flexibility of all microswitches is necessary for full receptor function. Our work deepens the understanding of the activation mechanism of alpha1A-AR and how it works in human body. This is important for the development of novel drugs to target alpha1A-AR and to understand its physiological functions.
Mechanism of activation of the relaxin family peptide receptors RXFP1 and RXFP2
The relaxin family peptide receptors RXFP1 and RXFP2 are the cognate receptors for the peptide hormones relaxin and INSL3 respectively, best known for their roles in reproductive biology. Being GPCRs, these receptors are members of the largest family of membrane-bound receptors known in the human genome, but they are unique within this family due to the existence of a single low-density lipoprotein type A (LDLa) module on the N-terminus of their large ectodomain. The LDLa module is imperative to normal receptor signalling and is hypothesised to be a tethered ligand, interacting with the receptor transmembrane domain (TMD) to bring about an active conformation. This module is connected to the leucine-rich repeats that make up the remainder of their extracellular domain by a stretch of amino acids 32 long in RXFP1 and 25 in RXFP2. These linking residues have been termed the linker, and a series of accidental and intentional discoveries led to the notion that the linker plays an important role within the activation mechanism of both RXFP1 and RXFP2 in response to their peptide ligands. The work presented within this thesis delves into the details of this activity, exploring the various regions of the linker, as well as the LDLa modules, with the use of mutagenesis and the construction of chimeras of the two receptors. The mutants and chimeras were systematically tested in stably or transiently transfected HEK293T cells in a series of ligand binding, activity (focussing on their ability to prompt an accumulation of intracellular cAMP) and cell surface/total expression assays. By comparing the behaviour of the mutant and chimeric receptors to that of their wild-type counterparts we have been able to paint a detailed picture of the binding and activation mechanisms of RXFP1 and RXFP2 in response to either active peptide ligand, adding our findings to a growing understanding of their activity quickly emerging from the lab and the field at large. Complementing the data generated from cell-based assays are nuclear magnetic resonance (NMR) studies performed concurrently using recombinantly expressed and purified RXFP1 and RXFP2 LDLa-linker proteins in titrations with relaxin as well as the receptor extracellular loops, as expressed on a soluble scaffold based on thermostabilized protein GB1. While the NMR experiments were carried out by a collaborating student, the RXFP2 LDLa-linker and soluble scaffold proteins were designed and characterized for use in NMR during this PhD project, and the details are outlined herein. In Chapter 2 the LDLa modules of the two receptors were swapped, such that RXFP1 contained the LDLa module from RXFP2 and vice versa. We found that while ligand-induced activity was weakened in the chimeric receptors, they were able to produce a robust signal, and for RXFP2 (but not RXFP1) the signal was slightly closer to wild-type levels upon subsequent swapping of the TMDs, such that they would match the non-native LDLa module . The result contradicted previous findings in which RXFP1 with the LDLa module from RXFP2 was shown to be incapable of signalling upon relaxin stimulation. We rationalized this discrepancy as being due to the differing design in the cloning of the chimeric constructs. While our versions swapped only the LDLa modules themselves, the previous versions had also swapped a large portion of the neighbouring linker residues. This alerted us to a possible function being carried out by the linker and guided our future work. We proceeded to mutate residues of the linker to alanine, carrying out an extensive scan of the RXFP1 linker that is presented in Chapter 3. We discovered that the initial residues formed something of a motif with the sequence GDNNGW, and when mutated – the residues Asp2, Gly5 and Trp6 in particular – the receptors lost their ability to stimulate cAMP production and bind relaxin effectively. This information along with NMR data led to the conclusion that this motif made up an activation region that was involved in the tethered ligand activation mechanism of RXFP1. Of note, we observed that the fold-difference in affinity for relaxin exhibited by the mutants of the activation domain was not commensurate with the enormous weakening in potency in relaxin signalling assays shown by the same mutants. This contrasted with mutants of the central part of the linker, in particular Phe54 and Tyr58, which when mutated individually or together displayed similar fold-differences from wild-type in relaxin potency and affinity. Coupled with compelling NMR data we concluded from this evidence that the central portion of the linker constituted a relaxin binding site that had hitherto never been described . We pursued a similar investigation focussing on the linker of RXFP2 in Chapter 4. The supposed activation region is largely conserved in the similar receptor, with the initial sequence of the linker GDTSGW. Indeed, mutation to alanine of these residues and the Phe found three residues along showed that both relaxin and INSL3 activation was dependent on equivalent residues to those identified in the RXFP1 mutagenesis campaign. Similarly, relaxin binding was severely compromised in the mutant receptors, while INSL3 binding was not. This result mirrored prior knowledge coming from mutagenesis and A-chain truncations of the relaxin and INSL3 peptides and highlighted a differing mode of action on RXFP2 in response to the two similar agonists. We postulated that the activation region of RXFP2 consisted of the linker residues from the GDxxGW motif found in both receptors, but they were assisted or accompanied by the actions of the N-terminal residues of the INSL3 A-chain . We further investigated the relaxin binding site from the RXFP1 linker by creating another two chimeras, in which the implicated residues were inserted into an equivalent position in the RXFP2 linker. The linker of RXFP2 is shorter than that of RXFP1 and hence a relaxin binding site had not been identified at a similar position, but by including the residues we supposed to be contributing to this binding site we were able to increase the potency and affinity for relaxin of the chimera to more closely resemble the behaviour of RXFP1 with its native ligand. In addition, the dissociation kinetics of the interaction, measured using a NanoBRET assay, resembled more closely the case of RXFP1 than RXFP2. This information helped us to confirm the existence of the relaxin binding site and highlight major differences between the two receptors with a focus on the linker . The work presented in this thesis gives a deep and detailed look at an integral part of the binding and activation mechanisms of RXFP1 and RXFP2 in response to the peptide hormones relaxin and INSL3, paving the way for their use in a therapeutic setting. The resolution of the role that the LDLa module plays has altered the prevailing view about receptor activation in a number of aspects. Firstly, the complex binding mode of both relaxin and INSL3 has been defined more thoroughly, and secondly, we now know to focus on the linker when defining TM interactions. It therefore highlights the utility of using a combination of approaches – cell-based assays partnered with mutagenesis and chimeras alongside protein expression and NMR – to reach valid conclusions about molecular systems.