Host defense against pathogenic organisms represents one of the most important and highly-conserved biological processes across the evolutionary timescale. The ability to detect, engulf and destroy particulates for either nutrition or host defense is conserved from single-celled protists to complex multicellular organisms. A central component of host defense is the recognition of invariant, conserved patterns on pathogenic organisms through the use of pattern recognition receptors (PRRs), such as macrophage receptor with collagenous structure (MARCO). MARCO modulates binding and phagocytosis of unopsonized microorganisms and particulates, tethers ligands to signalling complexes and enhances cellular adhesion. Current literature suggests these functions are mediated via the C-terminal scavenger receptor cysteine rich (SRCR) domain. The relative importance of this domain remains unclear, as other, closely-related scavenger receptors function independently of the SRCR domain via a shared lysine-rich motif.
In chapter 3.1, we discovered and cloned a naturally-occurring transcript variant of MARCO which lacks the SRCR domain, termed MARCOII. We demonstrated that the SRCR domain is required for ligand binding and internalization and that MARCOII can form heteromeric complexes with MARCO and reduce receptor function. Furthermore, the SRCR domain enhanced TLR2/CD14-mediated pro-inflammatory responses to Streptococcus pneumoniae. Finally, it was demonstrated that the SRCR domain modulates MARCO-mediated cellular adhesion.
In chapter 3.2, we used comparative phylogenetics to identify human specific mutations and residues undergoing positive selection in human MARCO. We demonstrated that humans possess a unique phenylalanine residue at position 282 that is polymorphic, with some humans encoding an ancestral serine residue. We also demonstrated that glutamine at position 452 is found in Denisovans, Neanderthals, and extant humans, but all other non-primate, terrestrial, and aquatic mammals possess an aspartic acid residue. We cloned the ancestral residues and loss-of-function mutations and demonstrated that these residues enhance ligand association and phagocytosis of Escherichia coli. / Thesis / Doctor of Philosophy (PhD) / Some of the most ancient mechanisms of host defense rely on invariant recognition of pathogens through the use of pattern recognition receptors, such as the macrophage receptor with collagenous structure (MARCO). MARCO plays an integral role to allow for specialized subsets of white blood cells to bind pathogens, activate signalling complexes and to bring pathogens inside the white blood cell for destruction. Current literature suggests the C-terminal Scavenger Receptor Cysteine Rich (SRCR) domain of MARCO is required for these processes. This remains under scrutiny, as other closely-related receptors have been shown to operate independently of the SRCR domain. Herein, we utilized a variant of MARCO which lacks the SRCR domain and patterns of evolution to confirm both that the SRCR domain is critical for receptor function and to discover novel sites within the human MARCO protein that play indirect, but important roles in receptor function.
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/22802 |
| Date | January 2018 |
| Creators | Novakowski, Kyle E |
| Contributors | Bowdish, Dawn, Medical Sciences |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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