The project will develop full size, integrated, dynamical models of lipid membranes. Lipid bilayers are found in the cytosolic membrane of cells and in the barriers that isolate their internal organelles. They are highly dynamic systems able to rearrange their topology in response to mechanical and chemical stimuli. In addition to the biological interest, the dynamic compartmentalization they feature can be exploited for biomimetic systems for, e.g., enzyme synthesis, membrane based sensors, desalination and water purification and personalized medicine. Based on their multiscale nature - nanometer thickness and micrometer lateral extension - lipid bilayers are considered continuous surfaces endowed with curvature elasticity. This approach cannot handle topological modifications like vesicles fusion or detachment with/from the bilayer, see, e.g., endo- and exocytosis. On the other hand, molecular modeling is limited to portions of the membrane of tens of nanometers which cannot capture the global dynamics. The present project overcomes these limitations exploiting phase-field methods and leveraging the interdisciplinary PI's expertise in developing innovative mesoscale models able to capture topological transitions. The proposal is structured into complementary parts: 1) mesoscale lipid bilayer dynamics, implemented in the context of phase field models with an original approach that includes thermal fluctuations in the spirit of fluctuating hydrodynamics and accounts for actin and membrane proteins; 2) molecular dynamics simulations aimed at extracting information from the microscopic dynamics; 3) experiments on artificial bilayers for direct measure of the membrane parameters and validation. An innovative proposal to quantify the elusive Gaussian contribution to the bending rigidity, crucial in establishing the free energy barrier for topology rearrangement, will require an additional personnel resource to be hired on the project.
As anticipated in the previous sections, the models currently available for lipid bilayers focus on two widely distinct ranges of scales: on one hand atomistic models deal with microscopic dynamics. They are mostly used to address the interaction of membrane and single membrane-embedded proteins like, e.g., ionic channels and aquaporins. On the opposite side of the scale range, continuum models describe the membrane as a surface immersed in three-dimensional space endowed with bending rigidity (Canham-Helfrich model and recent variants). These models accounts for the large scale dynamics of the bilayer and describe its overall equilibrium shape and associated energies.
In fact, a lipid bilayer is an intrinsically multiscale system. The bilayer is formed by two leaflets of amphiphilic molecules, with hydrophilic heads exposed to the surrounding aqueous environment and inner-facing hydrophobic tails. When dispersed in aqueous environment, the lipids aggregate to form different structures, bilayers and vesicles being those of interest here. These soft-matter structures are highly dynamic, substantially fluid with respect to deformations in the bilayer tangent plane and elastic with respect to bending deformations. The possibility of rearranging lipid positions and orientations allow for topological changes like vesicle detachment from mother membranes, fusion of membranes and poration. All these processes hinge on the molecular nature of the bilayer but are strongly conditioned/controlled by the large scale dynamics of the membrane. Typically, substantial free energy barriers, presumably induced by global topological constraints, separate states of different topology implying that topological changes need to be assisted by dedicated proteins diffusing (diffusion is driven by large scale gradients) on the membrane and from/to the surrounding environment. It is clear that the overall dynamics depends on a subtle interplay of small and large scale features that are completely out of reach of current models. At variance with currently available descriptions, the present proposal, leveraging the strongly interdisciplinary expertise of PI and coworkers, will build mesoscale models designed to capture this multiscale phenomena including a thermodynamically correct description of thermal fluctuations which are crucial for many membrane related processes.
Beyond state of the art will be the conception and realization of modular elements to be integrated together to account for the interaction of membrane embedded objects among themselves, with the membrane and with the surrounding fluid, which is most often non-Newtonian. An example among others are caveolins and clathrins which mediate the invaginations that initiate endocytic processes, or the polymerization of actin filaments at their so-called barbed ends on the membrane.
Being able to account for these mesoscale features will allow to describe important and incompletely understood processes like, for example, ultrasound-membrane interactions, important for establishing protocols for sonoporesis of outer and inner cell membranes or for augmented permeabilization of biological barriers (e.g. brain-blood barrier opening) of relevance for drag and gene delivery. Advancement to our current understanding of bilayer dynamics will be contributed in view of controlling dynamic compartmentalization processes. Integrated membrane, cytosol and actin network modeling will allow for controlling diffusion process occurring in living cells that can be potentially exploited in artificial vesicles.
The integrated and modular models for natural and bio-inspired bilayers will be implemented on highly parallel computational architectures. They will constitute unprecedented tools for simulating and predicting the behavior of full scale lipid bilayers subject to external and internal stimuli. The project will provide basic knowledge and in silico test benches for controlled vesicle compartmentalization. The new paradigm for mechanical/chemical coupling able to dynamically induce structural modifications will be of significant importance for realizing artificial lipid systems, contributing to pave the way to a number of innovative high-tech applications.
On the experimental side novel techniques for lipid membrane assembly will be made available ready for use to biology and medicine researchers, leveraging on the microfabrication and microfluidics expertise available in the PI's research group. The interplay between engineering, physics and biology fostered by the strict interaction with the Center for Life Nano Science is expected to contribute innovative experimental set-ups for quantifying elusive mechanical properties of lipid systems.
Many papers on prestigious international journals are expected to be published and, last but not least, among the major outcomes one should mention fostering bright young scientists fully formed for high-tech, interdisciplinary achievements.