Journal Club Theme of January 2009: Impetus for Cell Mechanics

Welcome to the January 2009 issue. This issue addresses the important discussion on cell mechanics. Acute need to study cell mechanics is driven by the fundamental goal of tissue engineering i.e. to make tissue engineered constructs that can mimic the environment for tissue regeneration with a potential to replace the biological functions of damaged organs. There is now worldwide activity in the in vitro regeneration of tissues including nerve, liver, bone, heart valves, blood vessels, and kidney.

As we know that cells are highly dynamic: they walk, change shape and divide. A critical understanding of cell structure and mechanics is essential to elucidate many of the cellular functions, from adhesion to proliferation and differentiation. Studies into cell mechanics have rapidly emerged during the past decade and this progress has given rise to improvement in capabilities to probe the living cell with high force and displacement resolution and also to observe and manipulate the structure of cell. Such developments have led to opportunities to do better experiments and modeling & simulation in the field of cellular mechanics. Cellular responses to mechanical forces are closely related to tissue physiology and pathology. However, it is unclear that ‘how the cells respond to mechanical forces and convert them into biological responses and vice versa. This issue is further complicated by the highly dynamic and complex structure of cells (especially, eukaryotic cells) and their structure changes dramatically in response to mechanical forces. In the last few decades, various research efforts have been undertaken to understand the cellular mechanics by using various experimental tools and theoretical methods.

Cell and molecular mechanics of biological materials has been recently reviewed by Bao and Suresh (2003). Mechanical loading of the cells is essential for their normal functioning in physiological environment, which otherwise leads to a diseased state. Various experimental techniques have been developed over the years to study the mechanical behavior of living cells like AFM, magnetic twisting cytometry (MTC), optical trap, micropipette aspiration etc. Cellular deformation using a point probe, in tension or compression, pure shear, twisting or a combination of these modes have been studied during the last decade. Most of the techniques used for probing cells assume that cell behaves like a elastic solid due to strong cytoskeleton structure which provides the structural integrity to cell.

To be noted that interior of the cell consists of liquid phase (cytosol), a nucleus, the cytoskeleton consisting of networks of microtubules, actin and intermediate filaments, organelles of different sizes and shapes and other proteins.  That means cell can behave like elastic/plastic/viscoelastic, or combination of these. The cell's dynamic and complex structure makes it difficult to separate the contribution of each of the deformation modes i.e. elastic/plastic/viscoelastic. Another issue is: How to quantify the force distribution between the subcellular components inside the cell and to get the mechanical properties of each cellular component?. Any mechanical perturbation to the cell produces a biochemical signal e.g. cell could undergo remodeling or reorganization. This raises a fundamental question: Is it possible to study cell mechanics of living cell by mechanical forces?. There is also an acute need to determine the constitutive behavior of single cells e.g. to include stress-strain relations and time dependent variables to evaluate the viscoelastic behavior of cells. This can possibly help to understand the mechanics of cell in a better way.

G. Bao and S. Suresh: "Cell and molecular mechanics of biological materials", Nature materials 2, 715-725, 2003.

Cell responds to the stiffness of the substrate:

Cellular response to the stiffness of the substrate was first reported in the case of epithelial and fibroblasts cells, using ligand-coated gels of varied stiffness (Pelham and Yang, 1997). Epithelial cell attachment was found to be better on stiffer substrate as revealed by their spreading and cytoskeletal organization. Soft, lightly cross-linked gels (E~ 1kPa) showed dynamic focal adhesion complexes. In contrast, cells on stiff gels (E ~ 30-100 kPa) showed stable and static focal adhesion. In many of these investigations, the cellular responses on glass or standard cell culture plastic (relatively rigid), are compared with the more compliant materials like gels. This kind of fundamental study can be been used in investigation of cancer cells, tissue-repair strategy and disease detection. For understanding the fundamental science behind the effect of substrate stiffness on cell adhesion, this study can be quite meaningful.

D. E. Discher, P. Janmey, and Y-L Wang: "Tissue cells feel and respond to the stiffness of their substrate", Science 310, 1139, 2005.

To follow up from above discussion, one also needs to ponder that most of these kinds of studies are done on single cells using subconfluent cell culture substrates. In physiological environment, cells are not well separated from each other, so tissue level response is more common. Here, cell-ECM interactions play a significant role in influencing various cellular processes. The role of ECM microenvironment should be taken into an account.

Mimicking the mechanical properties of cells:

Another challenging question which comes to our mind is that the mostly 2D substrates are used e.g. glass, tissue culture polystyrene or soft gels, the question is how closely they resemble the mechanical properties of different types of living tissues in 3D physiological environment in the body and why only stiffness, what about physical properties/surface chemistry or topography of the substrates?  

For myocardial tissue engineering, the key is to engineer 3D cardiac tissue that could eventually be used to repair damaged heart tissue inside the body, test new drugs, and study cardiac cells development and functions. In principle, it could theoretically lead to the creation of an entire heart. Before its realization, there are potential challenges that must be solved thorough rigorous research investigations. The major challenges in developing tissue-engineered grafts for myocardial repair are to find the structural and mechanical compatibility of three dimensional (3D) scaffolds with the formation of new biomimetic tissue.

Recently, Dr. Langer and Dr. Freed's group at MIT published a very interesting article in nature materials (December 2008) entitled, "Accordion-like honeycombs for tissue engineering of cardiac anisotropy". In their research work, they designed a novel biomimetic 3D scaffolds made of a biodegradable polymer (poly glycerol sebacate) with accordion-like honeycomb structure, and reasoned that these scaffold materials can mimic anisotropic mechanical properties of native myocardium cells, provide low resistance to contraction, and can also provide structural capacity to guide cardiomyocytes under the physiological environment.

Engelmayr  et al. (2008) focused their studies to adult rat right ventricular myocardium as the right ventricular myocardium was more anisotropic and compliant than its left ventricular counterpart. They envisioned that right ventricular myocardial grafts have a potential to grow, regenerate and remodel, which could repair congenital hear defects. It is appreciated that such kind of research has provided the answers to some of the potential issues in the tissue engineering i.e. as per the major conclusion of this work: accordion-like honeycomb scaffolds have the potential to overcome the major challenges with structural and mechanical integrity of scaffolds for myocardial tissue engineering by closely mimicking the anisotropic mechanical properties of adult rat right ventricular myocardium, while simultaneously promoting the preferential orientation of cultured neonatal heart cells in absence of external stimuli.

G. C. Engelmayr Jr, M. Cheng, C. J. Bettinger,  J. T. Borenstein, R. Langer and L. E. Freed, "Accordion-like honeycombs for tissue engineering of cardiac anisotropy", Nature Materials, 7, 1003, 2008.

Investigations made so far on cell mechanics may set the initial stage for better understanding of mechanosenstivity in cell-cell interations during embryonic and tissue regeneration processes. Nonetheless, the role of various cellular processes including cellular mechanics in tissue regeneration needs to be better understood before we can actually realize the goals of tissue engineering.