Oxytocin receptor heteromerization-a novel convergence of central molecular signalling and G-protein coupled receptor crosstalk

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Wallace Fitzsimons, Shauna E.
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University College Cork
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G-protein coupled receptors (GPCR) are the largest family of cell membrane receptors, eliciting a multitude of effects. Due to their wide array of function, GPCRs have become widely studied, specifically within the pharmaceutical market; where over 30% of currently approved drugs target such GPCRs. It is now over three decades since the concept of a GPCR-GPCR interaction emerged, leading to the discovery of GPCR heterodimers which is the interaction of two different GPCRS, homodimers the interaction of two of the same GPCRS and high-order oligomers, the interaction of 3 or more GPCRs. Intriguingly these heterocomplex formations led to differential signalling of the GPCRs within these complexes compared to their monomer state. Such studies ushered in an exciting new field in GPCR research. Yet despite the large therapeutic effect produced from targeting GPCRs, the full ability of GPCR heterocomplex signalling remains largely unexplored. The oxytocin receptor (OTR), is one of the best studied central GPCRs located in multiple regions throughout the central nervous system and periphery. In addition to its ability to facilitate social behaviours it also known to impact multiple other interrelated behaviours such as food intake and mood. So, the question can be asked how such a well-studied receptor in its monomer state, with only one known endogenous ligand can elicit such a multitude of responses? Is it doing so in its monomer state or does it have the ability to form GPCR heterocomplexes subsequently altering its signalling ability? Therefore, this thesis focuses on the ability of the OTR to form a multitude of heterocomplexes which may impact its multifaceted signalling abilities. With a focus on the interplay between social behaviour, appetite and mood multiple GPCRs associated with such functions were screened for co-localisation with the OTR in HEK293A cells. Due to previous studies linking their signalling pathways and known co-expression in different brain regions associated with social behaviour, appetite and mood, a cellular screening platform was established to aid in the identification of an OTR heterocomplex with receptors such as the ghrelin 1a receptor (GHSR) (Chapter 2), glucagon like peptide 1 receptor (GLP-1r) (Chapter 3), serotonin 2a receptor (5-HT2A) (Chapter 4) and the serotonin 2c receptor (5-HT2C) (Chapter 5), receptors known too to signal in either social behaviour, food intake and mood. Chapter 1 will further expand on the current knowledge and impact of GPCR heterodimerization on GPCR signalling and where it stands as pharmacotherapeutic target. Moreover, introducing these receptors, there signalling abilities and central expression patterns, finally, finishing on the link between these receptors and how they may play a role in the intricate interplay between social behaviour, food intake and mood. An interplay possibly driven by such heterocomplex formations? Chapters 2 through 5 focuses on these heterocomplex formations, using a multitude of assays, including heterocomplex identification (co-localization and FRET, PLA) and functional assays (calcium mobilization, inositol monophosphate and ligand mediated trafficking, cyclic adenosine monophosphate) the ability of the OTR to form functional heterocomplexes was studied. Excitingly, data revealed the OTR to be a hub for GPCR heterocomplex formations, each leading to unique signalling pathways associated with these heterocomplex formation. Lastly, chapter 6 focuses on the current scientific knowledge of OTR heterodimerization and the impact of these dimer formations on such knowledge, moreover the possible impact these heterocomplexes may have in future therapeutics. In conclusion, this thesis highlights the ability of the OTR to heterodimerize with a number of other GPCRs, possibly implicating these heterodimer pairs in the ability of the OTR to regulate multiple behaviours. Thus, fully understanding the full extent of OTR heterocomplex signalling will aid in a better understanding of current therapies and may lead to the development of much needed novel, more potent and selective pharmacotherapies. An area which is already proving promising with the use of bivalent ligands.
Oxytocin receptor , Heteromer , GPCR , Crosstalk
Wallace Fitzsimons, S. 2019. Oxytocin receptor heteromerization-a novel convergence of central molecular signalling and G-protein coupled receptor crosstalk. PhD Thesis, University College Cork.