Investigating spatial distribution and dynamics of membrane proteins in polymer-tethered lipid bilayer systems using single molecule-sensitive imaging techniques

Yifan Ge, Purdue University


Plasma membranes are complex supramolecular assemblies comprised of lipids and membrane proteins. Both types of membrane constituents are organized in highly dynamic patches with profound impact on membrane functionality, illustrating the functional importance of plasma membrane fluidity. Exemplary, dynamic processes of membrane protein oligomerization and distribution are of physiological and pathological importance. However, due to the complexity of the plasma membrane, the underlying regulatory mechanisms of membrane protein organization and distribution remain elusive. To address this shortcoming, in this thesis work, different mechanisms of dynamic membrane protein assembly and distribution are examined in a polymer-tethered lipid bilayer system using complementary confocal optical detection techniques, including 2D confocal imaging and single molecule-sensitive confocal fluorescence intensity analysis methods [fluorescence correlation spectroscopy (FCS) autocorrelation analysis and photon counting histogram (PCH) method]. Specifically, this complementary methodology was applied to investigate mechanisms of membrane protein assembly and distribution, which are of significance in the areas of membrane biophysics and cellular mechanics. From the membrane biophysics perspective, the role of lipid heterogeneities in the distribution and function of membrane proteins in the plasma membrane has been a long-standing problem. One of the most well-known membrane heterogeneities are known as lipid rafts, which are domains enriched in sphingolipids and cholesterol (CHOL). A hallmark of lipid rafts is that they are important regulators of membrane protein distribution and function in the plasma membrane. Unfortunately, progress in deciphering the mechanisms of raft-mediated regulation of membrane protein distribution has been sluggish, largely due to the small size and transient nature of raft domains in cellular membranes. To overcome this challenge, the current thesis explored the distribution and oligomerization of membrane proteins in raft-mimicking lipid mixtures, which form stable coexisting CHOL-enriched and CHOL-deficient lipid domains of micron-size, which can easily be visualized using optical microscopy techniques. In particular, model membrane experiments were designed, which provided insight into the role of membrane CHOL level versus binding of native ligands on the oligomerization state and distribution of GPI-anchored urokinase plasminogen activator receptor (uPAR) and the transmembrane protein αvβ3 integrin. Experiments on uPAR showed that receptor oligomerization and raft sequestration are predominantly influenced by the binding of natural ligands, but are largely independent of CHOL level changes. In contrast, through a presumably different mechanism, the sequestration of αvβ3 integrin in raft-mimicking lipid mixtures is dependent on both ligand binding and CHOL content changes without altering protein oligomerization state. In addition, the significance of membrane-embedded ligands as regulators of integrin sequestration in raft-mimicking lipid mixtures was explored. One set of experiments showed that ganglioside GM3 induces dimerization of α5β 1 integrins in a CHOL-free lipid bilayer, while addition of CHOL suppresses such a dimerization process. Furthermore, GM3 was found to recruit α 5β1 integrin into CHOL-enriched domains, illustrating the potential significance of GM3 as a membrane-associated ligand of α 5β1 integrin. Similarly, uPAR was observed to form complexes with αvβ3 integrin in a CHOL dependent manner, thereby causing the translocation of the complex into CHOL-enriched domains. Moreover, using a newly developed dual color FCS and PCH assay, the composition of uPAR and integrin within complexes was determined for the first time. From the perspective of cell mechanics, the characterization of the dynamic assembly of membrane proteins during formation of cell adhesions represents an important scientific problem. Cell adhesions play an important role as force transducers of cellular contractile forces. They may be formed between cell and extracellular matrix, through integrin-based focal adhesions, as well as between different cells, through cadherin-based adherens junctions (AJs). Importantly, both types of cell adhesions act as sensitive force sensors, which change their size and shape in response to external mechanical signals. Traditionally, the correlation between adhesion linker assembly and external mechanical cues was investigated by employing polymeric substrates of adjustable substrate stiffness containing covalently attached linkers. Such systems are well suited to mimic the mechanosensitive assembly of focal adhesions (FAs), but fail to replicate the rich dynamics of cell-cell linkages, such as treadmilling of adherens junctions, during cellular force sensing. To overcome this limitation, the 2D confocal imaging methodology was applied to investigate the dynamic assembly of N-cadherin-chimera on the surface of a polymer-tethered lipid multi-bilayer in the presence of plated cells. Here, the N-cadherin chimera-functionalized polymer-tethered lipid bilayer acts as a cell surface-mimicking cell substrate, which: (i) allows the adjustment of substrate stiffness by changing the degree of bilayer stacking and (ii) enables the free assembly of N-cadherin chimera linkers into clusters underneath migrating cells, thereby forming highly dynamic cell-substrate linkages with remarkable parallels to adherens junctions. By applying the confocal methodology, the dynamic assembly of dye-labeled N-cadherin chimera into clusters was monitored underneath adhered cells. Moreover, the long-range mobility of N-cadherin chimera clusters was analyzed by tracking the cluster positions over time using a MATLAB-based multiple-particle tracking method. Disruption of the cytoskeleton organization of plated cells confirmed the disassembly of N-cadherin chimera clusters, emphasizing the important role of the cytoskeleton of migrating cells during formation of cadherin-based cell-substrate linkages. Size and dynamics of N-cadherin chimera clusters were also analyzed as a function of substrate stiffness.




Naumann, Purdue University.

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