[Paper Review] Physics of Solutions and Networks of Semiflexible Macromolecules and the Control of Cell Function

Erwin Frey, Klaus Kroy+1 more|arXiv (Cornell University)|Aug 3, 1998

Cellular Mechanics and Interactions6 references7 citations

TL;DR

This paper investigates the physics of semiflexible biopolymers—such as actin filaments and microtubules—in cellular contexts, emphasizing how their mechanical and dynamic properties arise from a balance of elasticity and thermal fluctuations. Using statistical mechanics and simulations, the authors show that cytoskeletal networks exhibit unique mechanical responses critical for cell shape maintenance and motility, revealing a new class of soft matter systems with emergent biological functions.

ABSTRACT

Living cells are soft bodies of a characteristic form, but endowed with a capacity for a steady turnover of their structures. Both of these material properties, i.e. recovery of the shape after an external stress has been imposed and dynamic structural reorganization, are essential for many cellular phenomena. Examples are mechanical properties of tissue, cell motility, cell growth and division, and active intracellular transport. Numerous experiments in vivo and in vitro have shown that the structural element responsible for the extraordinary mechanical and dynamical properties of eukaryotic cells is the cytoskeleton, a three-dimensional assembly of protein fibers such as actin filaments and microtubules. In addition to those biopolymers various proteins with structural and regulatory functions have a major influence on the mechanical properties. At the relevant length-scales (a few microns at most) the building blocks of these biomaterials are very different from conventional polymeric material. In contrast to flexible polymers the persistence length is of the same order of magnitude as their total contour length or even larger. This implies that the physics of such a system is determined by a subtle interplay between energetic and entropic contributions. We review our present understanding of the physics of biopolymers using concepts from macromolecular and statistical physics complemented by computer simulation. These systems open up a new field of soft condensed matter research, which to date is only poorly understood but has a great potential for interesting new physical phenomena.

Motivation & Objective

• To understand the mechanical and dynamic behavior of semiflexible biopolymers in eukaryotic cells, particularly in relation to cell shape and motility.
• To identify how the interplay between elasticity and thermal fluctuations governs the physical properties of cytoskeletal networks.
• To bridge concepts from soft condensed matter physics with experimental observations in living cells.
• To explore how structural proteins and regulatory factors modulate the mechanical response of cytoskeletal networks.
• To establish a theoretical and computational framework for studying active, dynamic biomaterials in biological systems.

Proposed method

• Application of statistical mechanics to model semiflexible polymers with persistence lengths comparable to their contour length.
• Use of the worm-like chain (WLC) model to describe the elastic and entropic contributions to polymer conformation.
• Incorporation of thermal fluctuations and bending rigidity in the analysis of single-filament mechanics.
• Development of network models to simulate the collective mechanical response of cross-linked semiflexible filaments.
• Integration of computer simulations to explore the emergent mechanical properties of dense cytoskeletal networks.
• Use of continuum elasticity theory and scaling arguments to describe network-level behavior under external stress.

Experimental results

Research questions

• RQ1 How do semiflexible biopolymers like actin filaments contribute to the mechanical stability and dynamic remodeling of the cytoskeleton?
• RQ2 What is the role of thermal fluctuations and bending rigidity in determining the mechanical response of individual filaments?
• RQ3 How do cross-linking proteins and network architecture influence the viscoelastic properties of cytoskeletal networks?
• RQ4 In what way do the physical properties of semiflexible polymers enable cellular functions such as motility and division?
• RQ5 What emergent physical phenomena arise in dense, active networks of semiflexible filaments that are not present in flexible polymers?

Key findings

• Semiflexible biopolymers exhibit a unique mechanical response due to the competition between bending elasticity and thermal fluctuations, leading to non-Gaussian fluctuations and long-range correlations.
• The persistence length being comparable to or larger than the contour length results in a stiff, yet flexible, mechanical behavior distinct from flexible polymers.
• Cytoskeletal networks display a nonlinear elastic response and enhanced resistance to deformation due to the geometric constraints and cross-linking of semiflexible filaments.
• Thermal fluctuations significantly influence the mechanical stability of individual filaments, especially at micron-scale lengths relevant to cellular structures.
• Theoretical modeling and simulations reveal that network mechanics are governed by a delicate balance of filament stiffness, cross-link density, and thermal noise.
• The study identifies a new class of soft matter systems—active, semiflexible biopolymer networks—where emergent physical phenomena underlie essential cellular functions.