Biology subjects

Wisser, K.

Publications and source records attributed to Wisser, K..

3 recordsLinked to original sources

bioRxiv2022

Chemical potential measurements constrain models of cholesterol-phosphatidylcholine interactions

Bilayer membranes composed of cholesterol and phospholipids exhibit diverse forms of non-ideal mixing. In particular, many previous studies document macroscopic liquid-liquid phase separation as well as nanometer-scale heterogeneity in membranes of phosphatidylcholine (PC) lipids and cholesterol. Here, we present experimental measurements of cholesterol chemical potential (c) in binary membranes containing dioleoyl PC (DOPC), 1-palmitoyl-2-oleoyl PC (POPC), or dipalmitoyl PC (DPPC), and in ternary membranes of DOPC and DPPC, adapting a calibrated experimental protocol developed to measure c in cells (Ayuyan and Cohen, Biophys. J. 114:904-918). c is the thermodynamic quantity that dictates the availability of cholesterol to bind other factors, and notably must be equal between coexisting phases of a phase-separated mixture. It is simply related to concentration under conditions of ideal mixing but is found to be far from ideal for the majority of lipid mixtures investigated. Here we perform experimental measurements of c, constraining thermodynamic models of membrane interactions. Our measurements are consistent with models involving cholesterol-phospholipid complexes, but only if complexes are more weakly bound than has been assumed in previous reports. Experimental measurements are also well described by regular solution theory and lattice models with pairwise interactions between components. We find that c can vary by ~1.5 kBT at constant cholesterol mole-fraction implying a more than five-fold change in its availability for binding receptors and other reactions. These findings reinforce that c depends on membrane composition overall, suggesting avenues for cells to alter the availability of cholesterol without varying cholesterol concentration. SIGNIFICANCEThe chemical potential of cholesterol (c) reflects its availability to interact with other molecules. In a complex mixtures this chemical potential can vary dramatically even at fixed cholesterol concentration. In this report, we present measurements of c in bilayer membranes composed of mixtures of cholesterol with one or two phospholipids. We find that c in these mixtures depends strongly on the phospholipids that are present, with activity varying by a factor of more than five at fixed cholesterol concentration. This suggests that the availability of cholesterol in biological membranes could be tuned without altering cholesterol concentration directly, by adjusting the concentration of other lipid or protein components.

biophysics↗

bioRxiv2021

Membrane phase separation drives organization at B cell receptor clusters

Heterogeneity in intact cell plasma membranes has been explained by analogy to coexisting liquid-ordered and liquid-disordered phases, although models based on this idea fall short of describing the rich structure within cell membranes. Here, a new framework of lipid-driven plasma membrane heterogeneity is presented, drawing on quantitative measurements of protein partitioning and dynamics within B cell receptor clusters in live B lymphocyte plasma membranes, compared to coexisting phases in isolated plasma membranes. We propose that membrane domains in cells integrate the thermodynamic state of the membrane and the magnitude of the applied stimulus to give rise to a tunable response. This framework is supported through functional observations of B cell receptor phosphorylation in perturbed systems.

biophysics↗

bioRxiv2019

Membrane domains and phosphatase exclusion produce robust signaling responses in B cells engaged with natural ligands and artificial cross-linkers presented on lipid bilayers.

B cells respond to a wide variety of antigens with varying valiancy and mode of presentation to the highly expressed B cell receptor (BCR). We previously demonstrated that clustering the IgM isotype of BCR with an artificial soluble cross-linker led to the stabilization of an ordered phase-like domain. This domain sorted minimal peptides and full-length proteins to generate local hot-spots for tyrosine phosphorylation at receptor clusters, facilitating receptor activation. BCR can also be activated through interactions with natural ligands or artificial cross-linkers presented at bilayer surfaces, where it is proposed that alternate mechanisms play important roles in receptor clustering and activation, including one mechanism involving the exclusion phosphatases due to their bulky extracellular domains. The goals of the current study are to determine if markers of membrane phases are sorted by BCR clusters formed through engagement with bilayer-presented natural ligands or cross-linkers, and to estimate the contribution that membrane phase partitioning plays in organizing regulatory proteins with respect to BCR clusters under these stimulation conditions. We use super-resolution fluorescence localization microscopy to find that BCR engagement with either a bilayer-presented natural ligand or artificial cross-linker generates ordered phase-like domains that are more robust than those observed with soluble cross-linkers. In addition, we provide evidence that interactions between regulatory proteins and BCR are partially determined through their preference for ordered membrane domains and present a minimal model of receptor activation that incorporates both ordered domains and steric exclusion mechanisms to produce a more sensitive response. Overall, this work highlights that cells are capable of integrating multiple interaction modalities to give rise to cellular functions, likely conferring flexibility and robustness to cellular responses.

biophysics↗