DMS/NIGMS 1: Viscoelasticity and Flow of Biological Condensates via Continuum Descriptions - How Droplets Coalesce and Wet Cellular Surfaces

Princeton University, Princeton NJ

Investigators

Abstract

Standard textbooks in biology, at all levels, illustrate that membranes cover the distinct compartments inside cells, where all the amazing chemical processing occurs that makes life possible. However, it is now apparent that the cell interior has a richer structure that provides for more and membrane-independent ways to reorganize cellular components and thus enable cellular functions. In particular, in recent years it has been recognized that a single aqueous phase of cellular proteins can transition into two distinct phases, typically a phase of liquid droplets rich in protein suspended in a dilute solution of proteins, i.e., liquid-liquid phase separation occurs, comparable to oil droplets in water. Observations of this behavior have been made for animal, bacterial, and plant cells. Consequently, these so-called membrane-less compartments, or biomolecular condensates, are important to characterize since they help explain fundamental cell biology and are starting to be linked to possible disease states. The physical chemistry of the solution, including the electrolyte concentration and ion type, pH, and osmotic strength, can dictate the nature and scale of these changes in solution properties and so influence cell function, or dysfunction. The research in this project includes both experiments and theory, used together, to better characterize and understand the physicochemical features of these biomolecular condensates including how they interact with nearby surfaces, such as membranes. In addition, this project will provide support and research opportunities for undergraduate and graduate students. Liquid-liquid phase separation (LLPS) and related phase transitions of proteins in the cellular milieu were recognized recently as a generic mechanism in living cells for the formation of membrane-less compartments, or biomolecular condensates. Biological condensates flow and age, which has been suggested to interrelate the chemical and mechanical responses of the cell. Consequently, recent studies have provided measurements of the rheology of the protein solutions, including approximate viscosities, surface tension, because they are immiscible with the cytoplasm, and relaxation times for the viscoelastic characterization. Salt concentration, because it influences the polymer conformation, affects the rheological response and surface tension of the condensates and may also influence how the condensates wet a substrate. The research in this project will develop continuum viscoelastic models, familiar from the polymer physics literature, to address questions associated with flow and wetting of biological condensates, including their behavior on surfaces (membranes, microtubules, etc.); the approach will recognize the microstructural variables specific to condensates and address important mathematical questions of rearrangements of cytoplasmic components. The work will include electrostatic and electrokinetic effects within the framework of physicochemical hydrodynamics to account for unique features of condensates that impact cellular flows. Thus, via three interconnected research themes, we will provide a mathematical framework for dynamics of biological condensates, from (1) constitutive modeling of the stress versus strain and strain rate behavior, to (2) simulations of model flows, and (3) experiments testing these descriptions. The results will be given both at a level useful to an experimentalist and biologist and at a mathematical level that consistently integrates the thermodynamics, mechanics, and physical chemistry of soft material responses. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.