Liposomes for Controlled Release and Artificial Membrane Signal Transduction

Liposomes are artificially engineered bilayer vesicles with remarkable versatility in their composition and functionality. Ongoing advancements in liposome-based technologies continue to enhance our understanding of fundamental biological processes while also improving clinical applications, particularly as model biological membranes and drug delivery vehicles. As drug carriers, liposomes improve the pharmacokinetics of encapsulated therapeutics by providing a protective barrier between the payload and the surrounding biological environment. Furthermore, they exploit the enhanced permeation and retention (EPR) effect to facilitate passive targeting of tumors and other diseased tissues characterized by poor vasculature. However, the ability to achieve precise and spatiotemporally controlled release of the encapsulated drug molecules remains a technical challenge, as does the reliable real-time monitoring of release dynamics. In response to these limitations, emerging research has focused on engineering liposome compositions that undergo controlled payload release upon bioorthogonal chemistry. Additionally, fluorescence-based assays have gained traction as a preferred method for tracking liposomal content release due to their sensitivity, biocompatibility, cost-effectiveness, and user-friendly nature compared to imaging techniques such as MRI or CT. All known cellular life—including bacteria, archaea, and eukaryotes—relies on at least one lipid bilayer membrane (except for certain viruses), which serves as the structural foundation for cellular integrity, molecular transport regulation, and intracellular communication. Although extremely useful as model membranes, liposomes lack the full complexity of natural biological membranes, which limits their applicability for studies of intricate transmembrane signal transduction mechanisms. Consequently, there is a need to incorporate artificial signal transduction processes within liposomal membranes and use these systems as advanced models to better elucidate the underlying principles governing cellular communication across membranes. This dissertation presents research aimed at addressing some of these challenges. Chapter 1 provides an overview of liposomal applications in both fundamental membrane studies and drug delivery strategies, particularly for localized therapeutic interventions such as cancer treatment. Chapter 2 describes the design, characterization, and solution-state analysis of a novel liposome formulation containing Alloc-PE, a synthetic lipid cleaved by palladium-catalyzed bioorthogonal chemistry to generate DOPE, thereby enabling controlled aqueous payload release. Chapter 3 introduces a three-component ratiometric fluorescence assay capable of real-time monitoring of aqueous content release in both solution-state and live-cell microscopy experiments. Additionally, this system functions independently of pH or concentration fluctuations. Finally, Chapter 4 details the systematic development of enzyme-triggered artificial signal transduction systems within liposome bilayers. One such system, T5-Glu, employs the MDAP-CB[8] supramolecular pair to detect enzymatic activity via a fluorescence “lights-off” response. Another system, T-Me-SA-Glu, utilizes terbium luminescence for a “lights-on” mechanism to report enzymatic hydrolysis, which in both systems triggers molecular translocation across the liposomal bilayer. Collectively, these studies expand the functional utility of liposomes in controlled drug delivery, fluorescence-based monitoring, and artificial membrane signal transduction.