Research
Elastic Biomembrane Responses to Compositional Adaptations
Biomembranes, or cell membranes, are a remarkable class of molecular self-assemblies that regulate the functions of life with unparalleled precision and efficiency – features that scientists and engineers strive to emulate. They are primarily formed of a 2-molecule thick assembly of lipids and sterols which provided the structural scaffolds of protocells and gradually evolved to support all present life forms. Most intriguingly, biomembranes exhibit material properties that seem rather contradictory: they must be sturdy enough to ensure cell stability yet soft enough to allow for fluctuations, molecular diffusion, and signaling events. What is more, biomembranes constantly adapt to cell growth conditions and external cues by modulating their lipid and sterol compositions to maintain an optimal biophysical environment for protein activity. But what triggers these compositional adaptations and how they modulate membrane dynamics and function remains an open question.
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We address this fundamental question by focusing on membrane elasticity as an allosteric function regulator. This is important on a fundamental biophysical level and for practical applications of membranes as configurable materials in biosensing platforms and artificial cell technologies. More importantly, our approach aims to uncover the design rules evolved by nature to regulate membrane elasticity and dynamics. Major questions include: How do lipid and sterol architectures dictate elasticity? What biophysical rules underlie compositional adaptations? Can these rules be captured by unified physical laws?
To answer these questions, we use a suite of experimental methods including neutron and X-ray scattering methods, fluorescence and optical microscopy, as well as Langmuir isotherms and other characterization tools. We often combine our findings with synergistic results from deuterium NMR relaxometry (in collaboration with the Brown Lab at U. Arizona) and molecular dynamics simulations (through computing time proposals or through many wonderful collaborations with expert simulators like A. Sodt and M. Doktorova).
Related publications:
“How cholesterol stiffens unsaturated lipid membranes”, PNAS (2020)
“Cholesterol stiffening of lipid membranes”, J. Membr. Biol. (2022)
Stay tuned for more!
Dynamic Signatures of Membrane-Protein Interactions
Cell membranes are responsible for a range of functional processes that require coordinated interactions between membranes and proteins. While the effects of membrane structure (thickness, curvature) are becoming better understood, our knowledge of the dynamic synergy between the two remains rather rudimentary. This is especially exacerbated on mesoscopic timescales, which are still underexplored despite their functional significance. It is on these scales that membrane fluctuations and protein conformational changes take place. Therefore, understanding the dynamic cooperativity occuring in lipid-protein complexes requires access to membrane dynamics on relevant scales.
Our lab addresses these key questions by exploring how membrane dynamics change with protein conformations on unprecedented length and time scales. To do this, we use tunable model proteins such as the pH Low Insertion Peptide (pHLIP) which changes conformations from a surface associated state at neutral pH to a transmembrane state at acidic conditions (in collaboration with the Barrera group at UTK). Utilizing neutron spectroscopy methods, we can now probe how membranes and proteins dynamically interact and how these interactions can be tuned in technological applications including membrane-based biosensors and synthetic cells.
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Related publications:
“Neutron spin echo shows pHLIP is capable of retarding membrane thickness fluctuations”, Biochim. Biophys. Acta (BBA) – Biomembranes (2024)
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Predictive Design Rules for Stable Liposomal Nanocarriers
Designing stable, biocompatible liposomal nanoparticles for drug and vaccine delivery is more pressing than ever. With the increasing demand for mRNA vaccines, like the ones recently developed for coronavirus, it has become clear that shortcomings in current formulations of liposomal nanocarriers need to be addressed for more effective administration of mRNA vaccines and other target-specific drugs. Current challenges include degradation or early uptake of liposomal particles before they reach their target, and toxic buildup from polymeric materials traditionally used to stabilize liposomes. We address these challenges by answering fundamental questions like: What structure-property relations can we use to guide the molecular designs of liposomes for various applications? Can liposomal properties be enhanced using engineered molecules? How does the functionalization of liposomes with biocompatible polysaccharides affect their stability and targeting properties?
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Our lab addresses these questions using novel engineered molecules synthesized through the chemical conjugation of lipids and sterols, referred to as sterol-modified lipids (SMLs). We also investigate the effects of functionalizing lipids and cholesterol with dextran biopolymer for advanced liposomal designs (in collaboration with the Edgar group at Virginia Tech). Our guiding principle is establishing physical laws that bridge the gap between molecular structures and liposomal properties using compatible experimental tools (e.g. small-angle X-ray scattering, neutron spin echo spectroscopy, leakage assays, etc.) combined with new synthetic approaches and molecular dynamics simulations (in collaboration with the Deshmukh group at Virginia Tech).
Related publications:
coming soon!
Tuning Membrane Phase Separation by Engineered Molecules
Lateral phase separation is a hallmark of cellular membranes and a desirable feature in synthetic membranes with site-specific functionality. While the repertoire of natural lipids enables lateral membrane organization into distinct phases and domains, the similarity of lipid structures does not afford a large parameter space for a wide range of organizational tunability. To address this challenge, our lab exploits engineered molecules with modified chain structures or headgroups for controlled membrane patterning. This includes the use of sterol modified lipids (SMLs) to regulate the formation and growth of raft-like lipid domains, as well as nicotinic acid Gemini surfactants (NAGS) with biocompatible linkers to modulate the shape and stability of the formed domains (in collaboration with the Eftaiha group at the Hashemite University).
To do this, we employ a range of experimental tools including X-ray and neutron scattering, calorimetry, atomic force microscopy, and fluorescence imaging. These methods allow us to examine the phase separation phenomena on nanoscopic to macroscopic length scales, shedding light on the underlying molecular mechanisms and their utilization in the designs of controllably patterned membranes.
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Related publications:
coming soon!
Curvature-Induced Membrane Remodeling and Reorganization
Cell membranes adopt various shapes and curvatures, much of which are driven by dynamic cytoskeletal deformations and are critical to the cell function. While very little is known about membrane restructuring during such events, numerous observations allude to the possibility that cells use membrane reshaping as a machinery to translate mechanical signals into compositional rearrangements and subsequent biochemical processes. To address this pressing question, we utilize topographically tunable nanopatterned hydrogel scaffolds that emulate the cell cytoskeleton. This system allows real-time observations of lateral membrane organization and protein-membrane interactions in response to membrane topography. These studies provide insights into critical membrane functions, namely curvature-driven domain stabilization and protein recruitment. Our system also offers a promising route to thermally switchable membrane-based biosensors.
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To address these questions, we utilize experimental characterization methods including AFM, fluorescence microscopy, and neutron reflectometry as well as molecular dynamics simulations. These methods are ideally suited to answering these critical questions: How does imposed curvature dictate lateral domain formation, stabilization, and localization? Would dynamic changes in scaffold topography influence membrane remodeling? Does peripheral protein binding respond to local changes in the membrane curvature?
To do this, we employ a range of experimental tools including X-ray and neutron scattering, calorimetry, atomic force microscopy, and fluorescence imaging. These methods allow us to examine the phase separation phenomena on nanoscopic to macroscopic length scales, shedding light on the underlying molecular mechanisms and their utilization in the designs of controllably patterned membranes.
Related publications:
“The Influence of Curvature on Domain Distribution in Binary Mixture Membranes”, Soft Matter (2019)
Stay tuned for more!
Smart Nanostructured Polymeric Interfaces
Structured nanomaterials with functional interfaces are attractive candidates for a wide range of applications, including nanofluidics, molecular sorting, biological sensors, and lab-on-chip devices. These materials often involve nanoscale features that are critical for envisioned functionality but are typically challenging for traditional characterization methods. In order to incorporate functionality, responsive and tunable features are often desired. In our lab, we are particularly interested in the use of stimuli-responsive polymer coatings for functional nanostructured interfaces.
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Our focus is on periodic interfaces, commonly found in a wide range of applications. These studies leverage an amazing network of collaborators who are experts in polymer synthesis and surface functionalization (e.g. Ober group at Cornell, Ryan group at the University of Sheffield, Toomey group at the University of South Florida).
To understand the responsive nature of these materials in-situ, we adopt a rather unique approach founded on a synergistic combination of neutron or X-ray scattering measurements and theoretical data modeling. The scattering signals from these materials are rich in 3D structural information that can be elegantly extracted using a Dynamical Theory developed and refined by our group.
Related publications:
“Synthesis and Characterization of Stimuli-Responsive Polymer Brushes in Nanofluidic Channels”, ACS Appl. Mater. Interfaces (2023)
“Profile retrieval of a buried periodic structure using spin echo grazing incidence neutron scattering”, Appl. Phys. Lett. (2020)
Stay tuned for more!
