|
ScienceWeek
DEVELOPMENTAL BIOLOGY: ON SLOW VS. FAST MUSCLE DIFFERENTIATION
Vertebrate striated muscle is of two types: a) Slow-contracting "tonic" fibers that are often involved in postural control. Tonic fibers do not propagate action potentials and require multiple stimulation to contract, their contraction graded rather than all-or-nothing. b) "Phasic" fibers contract more quickly than tonic fibers, with phasic fibers in turn divided into two groups: slow-phasic (e.g., "red muscle") and fast-phasic (e.g., "white muscle"). Slow-phasic fibers are slow to fatigue.
The following points are made by Simon M. Hughes (Current Biology 2004 14:R156):
1) Successful marathon runners have higher numbers of slow muscle fibers than members of the general population, who in turn have more slow fibers than Olympic sprinters [1]. The extent to which genetics or training determines this character is unclear, but two arguments suggest genotype will turn out to play a significant role. First, during embryonic development, the broad pattern of slow and fast fibre types arises prior to birth or hatching. Indeed, the slow/fast muscle pattern can develop in the complete absence of innervation [2]. So muscle activity has little role in establishing pattern. Second, genetic selection should work particularly well on muscle fibre type, because catching prey or escaping predators are good determinants of survival to breeding age in most species. Until recently, however, the molecular mechanisms controlling fibre type pattern were obscure. Baxendale et al.[3] have recently reported a candidate regulator of slow myogenesis.
2) The first hint of where to look for genetic control of muscle pattern came from studies in chickens, which showed that distinct clones of myoblasts differ in their propensity to express slow myosin [4]. Myoblasts are the proliferative precursors of muscle fibers that do not yet express genes encoding the apparatus required for muscle contraction. The so-called "slow" myoblast clone only begins to express slow myosin after cells exit the cell cycle and undergo terminal differentiation. An intracellular memory mechanism must therefore commit some myoblasts to slow fibre formation in the future when they terminally differentiate. To date, searches for molecules that distinguish slow and fast myoblasts prior to their differentiation and might constitute such a memory mechanism have been unsuccessful.
3) The embryological origin of slow and fast myoblasts is equally obscure. Myoblasts arise at several distinct sites in the somites of mouse and chicken embryos, as determined by studies of the expression of myogenic regulatory transcription factors (MRFs), such as myf5 and myoD, which commit cells to myogenesis [5]. Each site seems to depend on specific extracellular signalling molecules from neighboring embryonic tissues. It is unclear how, or if, these early myoblast populations relate to the slow and fast myoblasts observed in older amniote embryos. These considerations led to a search for simpler systems in which to study muscle patterning.
4) Fish have a relatively simple musculature which is adapted to the aquatic environment. In zebrafish, the first muscle fibers are slow and form adjacent to the notochord, but subsequently migrate laterally. These early slow fibers form in the medial somite because midline-derived Hedgehog (Hh) proteins drive MRF expression, and hence myogenesis, in slow muscle precursors. Hh also drives the earliest myogenesis in mouse and chicken embryos. But MRF expression is not restricted to slow muscle -- it occurs in all muscle, including zebrafish fast muscle. So MRF expression is unlikely to account for the differences between slow and fast fate.
References (abridged):
1 Dirix, A., Knuttgen, H.G., and Tittel, K. (1988). The Olympic Book of Sports Medicine. (Oxford: Blackwell Scientific Publications)
2 Butler, J., Cosmos, E., and Brierley, J. (1982). Differentiation of muscle fiber types in aneurogenic brachial muscles of the chick embryo. J. Exp. Zool. 224, 65-80
3 Baxendale, S., Davison, C., Muxworthy, C., Wolff, C., Ingham, P.W., and Roy, S. (2004). The SET domain transcription factor Blimp1 is a Hedgehog activated switch that drives slow-twitch fibre-type muscle differentiation in the zebrafish embryo. Nat. Genet. 36, 88-93
4 Miller, J.B. and Stockdale, F.E. (1986). Developmental origins of skeletal muscle fibers: clonal analysis of myogenic cell lineages based on expression of fast and slow myosin heavy chains. Proc. Natl. Acad. Sci. USA 83, 3860-3864
5 Hadchouel, J., Carvajal, J.J., Daubas, P., Bajard, L., Chang, T., Rocancourt, D., Cox, D., Summerbell, D., Tajbakhsh, S., Rigby, P.W., and Buckingham, M. (2003). Analysis of a key regulatory region upstream of the Myf5 gene reveals multiple phases of myogenesis, orchestrated at each site by a combination of elements dispersed throughout the locus. Development 130, 3415-3426
Current Biology http://www.current-biology.com
--------------------------------
REVERSE ENGINEERING OF THE GIANT MUSCLE PROTEIN TITIN
The term "myofibril" refers to one of the long cylindrical contractile elements that constitute the major component of the muscle fiber, extending for the entire length of the muscle fiber, with each myofibril composed in turn of numerous myofilaments.
The term "sarcomere" refers to the functional unit of a myofibril of vertebrate muscle, each sarcomere approximately 2 to 3 microns long. In striated muscle, the sarcomeres of many parallel myofibrils are positioned such that myosin thick filaments are aligned in register across the myofibril and have a dense appearance in microscope preparations. The actin thin filaments have a lighter appearance, with a resulting alteration of light isotropic bands called "I bands" and dark anisotropic bands called "A bands". The I band is bisected by a dense narrow "Z line", while the central less dense region of the A band is known as the "H zone", which in turn is bisected by the dark "M line" (or midline), the locus of specific proteins that link adjacent thick filaments to each other. The Z lines are due to attachment sites for thin filaments. The various sarcomere bands are thus classical light-microscope features which have only in the past few decades been analyzed in terms of various macromolecular structures.
The term "titin" (connectin) refers to a giant protein that forms a single-molecule elastic filament extending from the M line to the Z line in the striated muscle sarcomere. Titin is one of the largest polypeptides yet described. Its amino acid sequence consists mainly of repeats of two types of approximately 100-amino-acid motifs, known as class I and class II. There is also a domain characteristic of protein kinases near the C terminus. Titin is believed to play an important role in sarcomere alignment during muscle contraction. A single molecule of titin measures approximately 1 micron in length and has a molecular weight of approximately 3 million daltons.
The following points are made by H. Li et al (Nature 2002 418:998):
1) Individual titin molecules span both the A-band and I-band regions of muscle sarcomeres. The I-band part of titin has been identified as the region that is functionally elastic. The authors report a study of the shortest titin isoform, the N2B isoform found in cardiac-muscle sarcomeres. The elastic I-band region of N2B-titin can be subdivided into four structurally distinct regions: a proximal immunoglobulin region containing 15 tandem immunoglobulin-like (Ig) domains; a middle N2B segment that contains a 572-residue amino-acid sequence of unknown structure; a 186-amino-acid-long segment rich in proline (P), glutamate (E), valine (V) and lysine (K) residues, named the PEVK region; and a distal Ig region that contains 22 tandem Ig modules. The authors use polyprotein engineering and single-molecule force spectroscopy to dissect the individual mechanical elements of the I-band of cardiac titin and reconstruct the elasticity of cardiac muscle. Polyproteins, when mechanically stretched by single-molecule atomic force microscopy (AFM) give distinctive mechanical fingerprints as their modules unfold sequentially (sawtooth patterns in the force extension curve), and can be used to positively identify the mechanical features of a single molecule.
2) In summary: Through the study of single molecules it has become possible to explain the function of many of the complex molecular assemblies found in cells(1-5). The protein titin provides muscle with its passive elasticity. Each titin molecule extends over half a sarcomere, and its extensibility has been studied both in situ and at the level of single molecules. These studies suggested that titin is not a simple entropic spring but has a complex structure-dependent elasticity. The authors use protein engineering and single-molecule atomic force microscopy to examine the mechanical components that form the elastic region of human cardiac titin. The authors demonstrate that when these mechanical elements are combined, they explain the macroscopic behavior of titin in intact muscle. The authors suggest their studies demonstrate the functional reconstitution of a protein from the sum of its parts.
References (abridged):
1. Sigworth, F. J. & Neher, E. Single Na+ channel currents observed in cultured rat muscle cells. Nature 287, 447-449 (1980)
2. Bustamante, C., Smith, S. B., Liphardt, J. & Smith, D. Single-molecule studies of DNA mechanics. Curr. Opin. Struct. Biol. 10, 279-285 (2000)
3. Smith, D. E. et al. The bacteriophage 29 portal motor can package DNA against a large internal force. Nature 413, 748-752 (2001)
4. Finer, J. T., Simmons, R. M. & Spudich, J. A. Single myosin molecule mechanics: piconewton forces and nanometer steps. Nature 368, 113-119 (1994)
5. Lu, H. & Schulten, K. Steered molecular dynamics simulations of force-induced protein domain unfolding. Proteins Struct. Funct. Genet. 35, 453-463 (1999)
Nature http://www.nature.com/nature
--------------------------------
GENE THERAPY FOR AGING-RELATED LOSS OF MUSCLE FUNCTION
One of the primary consequences of aging, a consequence which leads to significantly impaired function in the elderly population, is the loss of *skeletal muscle strength and mass. Both of these decrease up to one-third in humans between the ages of 30 and 80 years. In addition, loss of the fastest and most powerful muscle fiber types has been documented. Similar aging-related muscle alterations have been observed in rats and mice, indicating that the trend is maintained in other mammalian species. The mechanisms underlying this aging-related muscle loss have remained unclear, but there is some evidence that so-called "insulin-like growth factor-I" may be involved. The term "insulin-like growth factor" refers to a group of polypeptides structurally homologous to *insulin, and which share many of the biological activities of insulin, but which are apparently biochemically distinct from it. These substances are "mitogens", i.e., they enhance or induce cell division (mitosis). Insulin-like growth factor-I (insulin-like growth factor type I) is a monomer of 70 amino acids.
The following points are made by E.R. Barton-Davis et al (Proc. Nat. Acad. Sci. 1998 95:15603):
1) The authors report an attempt to moderate the aging-related loss of muscle in mice by increasing the regenerative capacity of muscle. The study involved the injection of a genetically engineered virus to direct overexpression (i.e., genome-based protein overproduction) of insulin-like growth factor-I in adult muscle.
2) The authors report that insulin-like growth factor-I expression promotes an average increase of 15 percent in muscle mass and a 14 percent increase in strength in young adult mice, and prevents aging-related muscle changes in old adult mice. In old adult mice, muscle mass and fiber type distributions were maintained at levels similar to those in young adults.
3) The authors propose that these effects are primarily due to stimulation of muscle regeneration via the activation of *satellite cells by insulin-like growth factor-I. The authors suggest this supports the hypothesis that the primary cause of aging-related impairment of muscle function is a cumulative failure to repair damage sustained during muscle utilization.
4) The authors further suggest that gene transfer of insulin-like growth factor-I into muscle could form the basis of a human gene therapy for preventing the loss of muscle function associated with aging, and may be of benefit in diseases where the rate of damage to skeletal muscle is pathologically accelerated.
Proc. Nat. Acad. Sci. http://www.pnas.org
--------------------------------
Notes by ScienceWeek:
skeletal muscle: (striated muscle, voluntary muscle) Muscle in which cross striations occur in the fibers as a result of regular overlapping of thick and thin filament structures. Although cardiac muscle is not "voluntary" muscle, it is also striated in appearance.
insulin: A protein hormone that promotes uptake by body cells of free glucose and/or amino acids, depending on target cell type.
satellite cells: The satellite cells of skeletal muscle are cells associated with muscle fibers that are believed to play a role in muscle repair and regeneration.
ScienceWeek http://scienceweek.com
|