Colloidal Probe Force Spectroscopy

Abstract

This thesis demonstrates how colloidal probe force spectroscopy, in connection with reconstituted biological model systems and theoretical models, allows us to investigate and understand the behavior of a wide variety of physical-biological systems like membrane fusion of vesicles or cells throughout nuclear transport events. We explore the use of colloidal particles of various sizes, which, coupled to different springs, serve to measure forces and break down physical quantities into distributions. We show how these colloidal probes (CP) can examine distinct biology areas and address different time and size scales. This extends from a global cellular level over vesicular orders of magnitude to individual molecular transport sizes. We applied force spectroscopy techniques based on colloidal particles to study mechanical behavior in three different model systems: transport through nuclear pore complexes, myogenesis of myoblast cells, and SNARE-mediated membrane fusion. In chapter 4, the nuclear pore complex-inspired system shows how one can visuali- ze mechanically small and hypothetical porous pathways for biomolecules hidden from optical observation. This approach implies being appropriate for nuclear pore complexes and hydrogels in general, membrane-less organelles, or liquid-liquid-phase- separated (LLPS) systems overall. In chapter 5, it shows myoblast cellular fusion and the cellular membrane’s mechanical properties in different states of myogenesis. To study it as a small functional system with the influence of the myomerger-ectodomain in detail, we isolated it from the biochemical signals sent by parts of the cell by employing model membrane system components on colloidal particles. It allowed us to distinguish the intrinsic physics of membrane fusion from this biological system apart from the cell’s regulatory processes. Lastly, in chapter 6, we employ colloidal probes attached to distinct spring-potentials to measure nanometer-scale interactions of micrometer-sized colloids (1-15 um) to facilitate areas of weak, specific protein-bilayer interactions in the context of SNARE- mediated membrane fusion. Performed in the presence and absence of bilayer-anchored f-Synaptotagmin-1 as Calcium ions were present in solution. We perform these ex- periments based on Atomic Force Microscopy (AFM), Optical Tweezers (OT), and Holographic Video Particle Tracking (HVPT) as they enable us to first discriminate intuitively intermediate states from step heights and corresponding lifetimes of the fusion pathway. Based on this, we resolve them by colloidal particle motion fluctuation with even greater detail. AFM and OT have been used to measure weak, kT scale inter-actions between membrane coated colloids and target bilayer surface as we measure separation-dependent interactions between protein-bilayer binding partners. Our ulti- mate results demonstrate the use of optical microscopy methods and particle tracking to quantify the connections between potentials of mean force and the crucial behavior of specific protein-bilayer interactions. Our knowledge from AFM and OT allowed us to separate these from fusion events. Throughout these different systems, colloidal probes connected with optical microscopy, optical tweezers, or atomic force microscopy provide a resourceful means to study biological systems on various length scales and visualize dynamic biological processes with high throughput and minimal reliance on fluorescent labels. Colloidal probes have many potentials to provide further insights into the details of Membrane Fusion of vesicles or cells and Nuclear Transport.