The cytoskeleton is the name of a filamentous network spanning the cytosol. The initial understanding of the cytoskeleton was o f a scaffolding structure that helps the cell resist external pressures and maintain its shape. This idea was broadened as more of the cytoskeleton’s function came into light.
By regulating polymerization and depolymerization of its components the cytoskeleton plays a major role in cell locomotion. The cytoskeleton also takes part in one of the most intriguing parts of the cell life cycle, which is cell division. Apparently microtubules (one of the kinds of filaments that compose the cytoskeleton) are anchored to the chromosomes during the mitotic phase and are responsible to pull them apart. Without this, cell division will not take place. The macroscopic muscle contraction is a direct result of thousands of myosin molecules pulling simultaneously on two actin filaments in muscle cells. This leads to the realization that there is more than meets the eye when it comes to cytoskeleton function. With new experimental techniques, this field is enjoying increased interest in the scientific community.
The cytoskeleton is composed of three groups of chain-like proteins and we describe their main characteristics below:
Actin. Actin is one of the polymers that constitute the cytoskeleton. The basic monomer of actin is a chain of 375 amino acids, known as G-actin. X-ray diffraction methods have shown that the G-actin is packed into a helix shaped polymer known as F-actin with a persistence length of 15 µm. The exact concentration of actin in the cell is between 2 mg/ml to 10 mg/ml. These figures for G-actin and F-actin combined, make actin the most abundant protein in the cell.
Polymerization of actin occurs at both ends of the filament but with different rates. At one end the rate of polymerization is faster than in the other end. The fast growing end is called the “barbed” end, while the other is called the “pointed” end. Though actin is abundant throughout the cell, it is found mostly in the cell periphery, called the “cell cortex”.
Microtubule. Microtubule is a much more complex molecule. The building block of this protein is a dimer called tubulin, which is composed of two sub units: α-tubulin and β-tubulin. α-tubulin and β- tubulin form a filamentous chain called “protofilament”. Microtubules are built by arranging 13 such protofilaments around an empty core. This gives rise to a tube-like construction (hence the name microtubule), which is stiffer, longer and wider than actin. Microtubules have a distinct organizing site called the “centrosome”. Microtubule polymerization begins at this organelle. The end where faster polymerization occurs is called the plus terminus. The end where slower polymerization takes place is called the minus end. Microtubules grow from the centrosome towards the membrane, by anchoring their minus end to the organelle. Once microtubules reach the membrane they detach from the centrosome and create a highly dynamic network. The formation of this network is assisted by a group of proteins with microtubule binding domains called Microtubule Associated Proteins (MAP).
Fiber-like proteins. In contrast to the other two cytoskeletal components, intermediate filaments (IF) refer to a group (and not to single filaments) of fiber-like proteins. These are more flexible than actin or microtubules, and are abundant in all regions of the cell. Their diameter is about 10 nm, which is between actin and microtubules (therefore: “intermediate” filaments). The four subgroups of proteins that comprise the set of IF are Type I (acidic, neutral and basic keratins), Type II (vimentin, desmin and glial fibrillary acidic protein), Type III (neurofilaments) and Type IV (nuclear lamins). All groups have the same basic structure of a rod-like molecule with carboxyl and amino terminals.
Extracellular Cell Matrix (ECM)
Interactions between cells and the surrounding extracellular matrix (ECM) are critical for cell migration, differentiation, and tissue morphogenesis. Cells assemble ECM proteins into insoluble, rope-like fibrils to form a complex three-dimensional matrix. This requires coordinated contractions of the cytoskeleton and transmission of these contractile forces to the ECM. Cells also adhere to existing matrix and build plaques of proteins at sites of attachment. These protein plaques, known as focal adhesions, serve as a physical link between the cytoskeleton and surrounding ECM, but also act as a hub of biochemical signaling cascades that respond to mechanical signals. Thus, a reciprocal relationship exists between cells and ECM in which cells use contractile forces to mediate matrix assembly, and then respond to mechanical signals through transmembrane and intracellular signaling events.
Fibrillogenesis of fibronectin is recognized as a first step in the assembly of ECM. This area of investigation is highly applicable to the advancement of tissue engineering and the development of de novo tissues and organs. By studying the mechanisms that link cytoskeletal contractility to the assembly of ECM, a significant contribution can be made to the understanding of how cells construct ECM and drive tissue morphogenesis.
Fibronectin. Fibronectin is a well-characterized, multifunetional ECM and plasma protein. The soluble protomeric molecule, synthesized by liver hepatocytes, circulates in the blood (300 µg/ml in plasma) and other body fluids. Fibronectin can also be locally synthesized by cells in tissues during development, inflammation or in response to tissue injury. Fibronectin found within the extracellular spaces of conneetive tissue, basement membranes and cultured cells is eomprised mainly of insoluble multimeric fibrils.
Fibronectin is a 450-500 kDa glycoprotein consisting of two similar 220-250 kDa subunits that are disulfide-bonded at their carboxyl terminus. The molecule, based upon the amino acid sequence, is organized into three types of homology units of type I, II, III. There are 12 type I modules each containing about 45 amino acids, that are located at the amino and carboxyl terminus of each subunit. Two 60 amino acid type II repeats disrupt a row of nine type I repeats at the amino terminus. The remaining central region of the molecule is comprised of 15-17 type III repeats, each containing about 90 amino acids. In total, approximately 2500 amino acids make up the fibronectin subunit.