Microfilaments, microtubules

Nonmuscle cells perform mechanical work, including self-propulsion, morphogenesis, cleavage, endocytosis, exocytosis, intracellular transport, and changing cell shape. These cellular functions are carried out by an extensive intracellular network of filamentous structures constimting the cytoskeleton. The cell cytoplasm is not a sac of fluid, as once thought. Essentially all eukaryotic cells contain three types of filamentous struc-mres actin filaments (7-9.5 nm in diameter also known as microfilaments), microtubules (25 nm), and intermediate filaments (10-12 nm). Each type of filament can be distinguished biochemically and by the electron microscope. 

Intermediate Filaments Differ From Microfilaments Microtubules... 

The cellular cytoskeleton, primarily composed of microfilaments, microtubules, and intermediate filaments, provides structural support and enables cell motility. The cytoskeleton is composed of biological polymers and is not static. Rather, it is capable of dynamic reassembly in less than a minute [136], The cytoskeleton is built from three key components, the actin filaments, the intermediate filaments, and the microtubules. The filaments are primarily responsible for maintaining cell shape, whereas the microtubules can be seen as the load-bearing elements that prevent a cell from collapsing [136], The cytoskeleton protects cellular structures and connects mechanotransductive pathways. Along with mechanical support, the cytoskeleton plays a critical role in many biological processes. 

Like microfilaments, microtubules are dynamic structures with (+) and (-) ends. The (-) end is usually stabilized by bonding to the centrosome. The (+) end shows dynamic instability, it can either grow slowly or shorten rapidly. GTR which is bound by the microtubules and gradually hydrolyzed into GDR plays a role in this. Various proteins can also be associated with microtubules. 

Exposure of lymphocytes to authentic N=0 did not induce a global inhibition of lymphocyte function. The cytolytic activity of alloactivated mouse splenocytes as well as a murine allospecific cytolytic T lymphocyte clone were unaffected by exposure to N=0 immediately prior to the cytotoxicity assay. Additionally, the motility of various lymphocyte populations, including Con A stimulated splenocytes, alloactivated mouse splenocytes, and a T lymphocyte clone were also unaffected by previous N=0 exposure (R. A. Hoffman, J. M. Langrehr, and R. L. Simmons, unpublished observations, 1994). Thus, brief exposure to -N=0 does not affect T lymphocyte microfilament-microtubule function nor the enzymes required to induce target cell destruction. 

The liver cell is a polar unit. The resorptive processes take place at the sinusoidal and lateral membrane, the secretory processes on the surface of the canaliculi. The cytoskeleton (microfilaments, microtubules, intracellular membranes) maintains the polar orientation of the hepatocyte. (24, 33, 42, 44, 71,80) (s. fig. 13.1)... 

The ability of a cell to hold or to change shape and to move organelles within it depends on the existence of a cytoskeleton, comprising actin filaments (microfilaments), microtubules, and intermediate filaments (IPs) capable of transmitting force. Actin filaments and microtubules contain predominantly actin and tubulin, respectively. (Table 21-1). The diameter of intermediate filaments (10 nm) is between that of actin filaments (6-7 nm) and microtubules (25 nm). They are structural proteins not directly involved in motion. 

The cytoskeleton is the remaining fibrous framework following the treatment of eukaryotic cells with non-ionic detergents under conditions in which most of the cellular proteins are extracted. The cytoskeleton consists of at least three distinct cytoplasmic systems of aggregated protein microfilaments, microtubules and intermediate filaments (Figure 9.2). These structures may be identified by the diameter of their fibres microfilaments 5-7 nm, microtubules about 25 nm and intermediate filaments 8-10 nm. 

Microfilaments and Microtubules. There are two important classes of fibers found in the cytoplasm of many plant and animal ceUs that are characterized by nematic-like organization. These are the microfilaments and microtubules which play a central role in the determination of ceU shape, either as the dynamic element in the contractile mechanism or as the basic cytoskeleton. Microfilaments are proteinaceous bundles having diameters of 6—10 nm that are chemically similar to actin and myosin muscle ceUs. Microtubules also are formed from globular elements, but consist of hoUow tubes that are about 30 nm in diameter, uniform, and highly rigid. Both of these assemblages are found beneath the ceU membrane in a linear organization that is similar to the nematic Hquid crystal stmcture. 

The major types of cytoskeletal filaments are 7-nm-thick microfilaments. 25-nm-thick microtubules, and 10-nm-thick intermediate filaments (IPs). These are respectively composed of actin, tubulin, and a variety of interrelated sparsely soluble fibrous proteins termed intermediate filament proteins. In addition, thick myosin filaments are present in large numbers in skeletal and heart muscle cells and in small numbers in many other types of eukaryotic cells. 

The cytoskeleton also contains different accessory proteins, which, in accordance with their affinities and functions, are designated as microtubule-associated proteins (MAPs), actin-binding proteins (ABPs), intermediate-filament-associated proteins (IFAPs), and myosin-binding proteins. This chapter is focused on those parts of the cytoskeleton that are composed of microfilaments and microtubules and their associated proteins. The subject of intermediate filaments is dealt with in detail in Volume 2. 

In this chapter we describe the distribution, assembly, and interaction of microfilaments and microtubules and their functional roles in cell movement and in the maintenance of the spatial organization of the cytoplasm. Also, the relative roles... 

The Cytoskeleton—Microtubules and Microfilaments Drug Effects on Microtubules... 

The Cytoskeleton—Microtubules and Microfilaments Patterns of Arrangement of Actin Filaments in Animal Cells... 

The Locomotion of Amoeba The Locomotion of Fibroblastic Cell Types The Locomotion of Leukocytes The Behavior of Locomoting Cells The Role of the Cytoskeleton in Cell Locomotion The Microtubule-Based Cytoskeleton The Intermediate Filament-Based Cytoskeleton The Microfilament-Based Cytoskeleton The Organization of Microfilaments in Cells Microfilament Dynamics and Cell Locomotion Sites of Lamellar Protrusion May Be Determined by the Nucleation of Actin Polymerization... 

Microtubule-Based Motor Proteins The Meiotic and Mitotic Spindles Microfilament-Based Intracellular Motility Cytokinesis... 

Intracellular motility is also of vital importance in the lives of cells and the organisms they form. Material and organelles are transported within cells along microtubules and microfilaments an extreme example of this are the axons of nerve cells which transport materials to the synapses where they are secreted—another motile event. Other examples of intracellular motility include phagocytosis, pino-cytosis, the separating of chromosomes and cells in cell division, and maintenance of cell polarity. 

Just as myosins are able to move along microfilaments, there are motor proteins that move along microtubules. Microtubules, like microfilaments, are polar polymeric assemblies, but unlike actin-myosin interactions, microtubule-based motors exist that move along microtubules in either direction. A constant traffic of vesicles and organelles is visible in cultured cells especially using time-lapse photography. The larger part of this movement takes place on micrombules and is stimulated by phorbol ester (an activator of protein kinase C), and over-expression of N-J aj oncoprotein (Alexandrova et al., 1993). 

Cytokinesis is the separation of daughter cells at the completion of the microtubule directed separation of duplicate chromosomes at mitosis or meiosis. This is usually accomplished by a purse string mechanism, whereby daughter cells become separated by the gradual constriction of a structure composed of actin microfilaments and myosins-II (Fujiwara and Pollard, 1976 Satterwhite and Pollard, 1992). 

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