Understanding Polymer-Lipid Bilayer Interactions

Abstract

Cell membrane instability is a common feature to Duchenne Muscular Dystrophy, heart attacks, strokes, and traumatic brain injury, which together affect over a million people in the United States every year and currently have no clinical treatment. In 1992, it was discovered that poloxamers, a class of biocompatible block polymer amphiphile stabilized cell membranes under stress, thereby having therapeutic potential. Unfortunately, the stabilization mechanism is not fully understood, hindering the engineering of more effective treatments.Bottlebrush polymers have a wide parameter space and known relationships between architectural parameters and polymer properties, enabling their use as a tool for mechanistic investigations of polymer−lipid bilayer interactions. In this thesis, I report a synthetic strategy for making grafted block polymers with poly(propylene oxide) and poly(ethylene oxide) side chains, “bottlebrush poloxamers (BBPs).” Combined anionic and sequential ring-opening metathesis polymerization yielded low dispersity polymers, at full conversion of the macromonomers, with control over graft length, graft end-groups, and overall molecular weight. Dynamic light scattering and transmission electron microscopy were used to characterize micelle formation in aqueous buffer. The critical micelle concentration scales exponentially with overall molecular weight for both linear and bottlebrush poloxamers; however, the scaling coefficient is two orders of magnitude smaller in the bottlebrush architecture compared to the linear architecture, suggesting that micellization of BBPs is less sensitive to molecular weight. I then employed this synthetic platform to create a set of BBPs over a range of molecular weight, with two PEO block side chain lengths, and with block and statistical architectures. Then, this set of molecules was used to interrogate the effects of bottlebrush architectural parameters on binding to, and protection of, phospholipid bilayers using pulsed-field-gradient NMR and an in vitro osmotic stress assay, respectively. I found that the binding affinity of a bottlebrush poloxamer (BBP) ("B-" "E" _"10" ^"43" "P" _"5" ^"15" , Mn = 26 kDa) is about 3 times higher than a linear poloxamer with a similar composition and number of PPO units (L-E93P54E93, Mn = 11 kDa). Furthermore, BBP binding is sensitive to overall molecular weight, side-chain length, and architecture (statistical versus block). Finally, all tested BBPs exhibit a protective effect on cell membranes under stress at sub-μM concentrations. As the factors controlling membrane affinity and protection efficacy of bottlebrush poloxamers are not understood, these results provide important insight into how they adhere to and stabilize a lipid bilayer surface. The final two chapters of this thesis return to commercially available, linear poloxamers and seek to understand the effect of temperature and the role of lipid phase coexistence on poloxamer-liposome interactions. Hydrogen bonding between water and oxygen atoms in PEO and PPO units results in thermoresponsive behavior because the bound water shell around both blocks dehydrates as temperature increases. This motivates an investigation of poloxamer-lipid bilayer interactions as a function of temperature and thermal history. Pulsed-field gradient NMR spectroscopy measurements revealed that the fraction of chains bound to 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) liposomes increased by 11 (± 3)× at 37 °C relative to 27 °C. Moreover, following incubation at 37 °C, it takes weeks for the system to re-equilibrate at 25 °C. Such slow desorption kinetics suggests that at elevated temperatures polymer chains can pass through the bilayer and access the interior of the liposomes, a mechanism that is inaccessible at lower temperatures. We propose a molecular mechanism to explain this effect, which could have important ramifications on the cellular distribution of ABPs and could be exploited to modulate mechanical and surface properties of liposomes and cell membranes. The lipid raft and picket fence models assert that the cell membrane contains liquid ordered domains (Lo) among a matrix of liquid disordered domains (Ld). These domains have different structural and physical properties, affecting protein conformation, cell signaling, and cellular processes. Therefore, I employed a liposome model consisting of a saturated lipid, an unsaturated lipid, and cholesterol that has a well-documented phase space to explore how lipid phase behavior affects polymer binding. I found that polymer binding is maximized in a window of the phase space coinciding with coexistence of the two liquid domains. This is likely because the borders between the Lo and Ld domains are attractive binding sites. The proximity between bound polymer and lipid rafts could provide a non-specific mechanism by which flexible, non-polar amphiphilic block polymers affect cell signaling.