Bulletin of the American Physical Society
Mid-Atlantic Section 2022 Meeting
Volume 67, Number 20
Friday–Sunday, December 2–4, 2022; University Park, PA, Pennsylvania State University
Session C01: Bio I |
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Chair: Alexandre Morozov, Rutgers University Room: Pennsylvania State University Osmond 103 |
Saturday, December 3, 2022 9:00AM - 9:35AM |
C01.00001: Differential stress in asymmetric membranes Invited Speaker: Markus Deserno Lipid membranes are called "asymmetric" if their two leaflets differ in at least one physical property, the most widely considered one being lipid composition. Many biological membranes are asymmetric, as has been known since the 1970s, but our ability to replicate asymmetry in artificial model membranes is a much more recent achievement, and it has led to surprising discoveries regarding their thermodynamic and mechanical properties. In this presentation I investigate the implications of a particular type of asymmetry, namely, a difference in lateral tension between the two leaflets, a situation I refer to as differential stress. Being a source of torque, it obviously couples to membrane shape and other drivers of bending, such as spontaneous curvature. But differential stress also affects lipid phase behavior, for instance by modifying the fluid-gel transition, and it is one of several factors that determine the equilibrium distribution of cholesterol between the leaflets. I will present a simple model for this cholesterol balance and, as an example, apply it to the human red blood cell. |
Saturday, December 3, 2022 9:35AM - 10:10AM |
C01.00002: Active-microrheology of actin–intermediate filaments composite networks Invited Speaker: Bekele J Gurmessa The cytoskeleton--a highly dynamic and complex network of biopolymers comprising actin, microtubules, and intermediate filaments--plays a vital role in several cellular processes ranging from the stability and rigidity of biological cells to cell motility and shape change. Central to this multifunctionality is the inherent stiffness of each filament and the myriad binding proteins that serve to crosslink, bundle, assemble, and disassemble these filaments. Exploring the mechanics and dynamics of in vitro reconstituted networks consisting of only one of these cytoskeletal components has been the subject of extensive experimental and theoretical work. However, the role each cytoskeletal component plays in their composite networks' mechanical properties and structural dynamics is yet to be well understood. In this talk, I will present our recent work on the microscale mechanics of in vitro reconstituted filamentous actin (F-actin)--vimentin intermediate filaments (VIFs) composite networks using active microrheology coupled with microfluidics techniques. Our measurements shed new light on how VIFs and F-actin work synergistically during the various cellular processes. |
Saturday, December 3, 2022 10:10AM - 10:22AM |
C01.00003: Nano-Scale "Sticky Tape" Stabilizes Open-Edge Boundary Conditions in MD Simulations of Asymmetric Membranes Samuel L Foley, Markus Deserno Recent advances in membrane biophysics have driven deeper investigations into lipid bilayer asymmetry and its biological implications. Molecular dynamics simulations of asymmetric membranes have recently highlighted the potential role of differential stress in regulating membrane elastic moduli and phase behavior, in addition to the already known composition asymmetries. The vast majority of MD simulations of membranes are flat, tensionless, fully periodic geometries, which artificially suppress membrane bending even in the presence of a non-vanishing net torque. In order to allow a membrane to relax to its true equilibrium curvature state determined by a sensitive balance of monolayer area strains and spontaneous curvatures, it is desirable to simulate with periodicity in only one direction with open edges along the other. However, lipid flip-flop over the open edges rapidly destroys any membrane asymmetry. We present a technique for maintaining membrane asymmetry with open edges by applying "sticky tape" to the membrane edges parallel to the direction of periodicity to prevent flip-flop. By varying lipid number asymmetry at fixed monolayer compositions, this free-curvature technique can be used to determine the voluntarily flat state of an asymmetric membrane, which can be transferred to fully periodic conditions for production. Additionally, it can be exploited to determine appropriate lipid numbers to place in the inner and outer leaflets of tether simulations. Importantly, this method does not significantly impact the structure of the fluid lipid phase in the bulk. We present this technique using the ultra-coarse-grained Cooke lipid model, but the concept is easily transferable to more finely-resolved models, allowing for simultaneously area- and curvature-relaxed membrane simulations. |
Saturday, December 3, 2022 10:22AM - 10:57AM |
C01.00004: Theory of cell size homeostasis Invited Speaker: Andrew Mugler Cells maintain a stable size as they grow and divide. But how they do so in the face of internal and external noise sources has proven to be a surprisingly difficult question to answer. In this talk, I will review the standard models of bacterial cell size control and highlight experimental observations that cannot be explained by these models. In particular, recent evidence from our collaborating group suggests that cell size correlations can last for many generations, rather than one or two as commonly assumed. I will introduce a theory that explains these long-lived correlations and why they may not have been observed before. Furthermore, I will demonstrate that the properties of so-called timer, adder, and sizer models change when intracellular molecule number noise is taken into account. Surprisingly, for example, the sizer model no longer produces the tightest cell size distribution. This work generalizes and advances our understanding of the minimal ingredients needed for stable cell growth. |
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