Sci. Aging Knowl. Environ., 18 January 2006
Vol. 2006, Issue 3, p. pe3
[DOI: 10.1126/sageke.2006.3.pe3]

PERSPECTIVES

Dividing to Keep Muscle Together: The Role of Satellite Cells in Aging Skeletal Muscle

Russell T. Hepple

The author is a member of the Faculty of Kinesiology and Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada. E-mail: hepple{at}ucalgary.ca

http://sageke.sciencemag.org/cgi/content/full/2006/3/pe3

Key Words: sarcopenia • muscle fiber • myocyte • satellite cells • nuclear domain • MyoD

Introduction

Skeletal muscle mass declines progressively with increasing age, beginning in approximately the fourth decade of life (1), a process also known as sarcopenia (2) (see Hepple Perspective). Skeletal muscle cells, also called myocytes or muscle fibers, decline in number (1) and, to a more variable degree, size (1, 3) with aging. Although debate remains whether cells that are lost entirely can be replaced through de novo fiber synthesis, it is clear that mechanisms must and do exist to facilitate repair of damaged myocytes throughout the life span of an organism. Because failure to repair a damaged region of muscle fiber could result in severe cell atrophy and even cell death, an understanding of how skeletal muscle is repaired is central to understanding the basis of sarcopenia. In this respect, there is a growing appreciation for the role of a population of cells in skeletal muscle known as satellite cells.

Satellite Cells in Muscle Repair and Regeneration

Satellite cells are relatively undifferentiated cells that reside in indentations of myocytes between the basal lamina and plasma (also known as the sarcolemma) membranes (4). Whether these cells are pluripotent stem cells, or whether only a subpopulation of these cells are pluripotent stem cells, continues to be explored (5). However, the important point is that the satellite cells are vital to repair damage to the myocytes they surround. In response to muscle injury, the satellite cells proliferate and migrate to the point of injury, fusing with the damaged region and each other to form myotubes and eventually mature myocytes. A picture is emerging suggesting that sarcopenia is associated with an impaired regenerative response (6-8), which implies that changes in satellite cell proliferative responses likely play a key role in sarcopenia. In support of this idea, there appear to be fewer satellite cells in aged skeletal muscles (9, 10), and satellite cells isolated from the skeletal muscles of older rats show a slower rate of population doubling when cultured in vitro (8). Synergistic ablation, an example of which involves surgical removal of the gastrocnemius muscle (an ankle extensor muscle of the distal hind leg) to dramatically overload the remaining muscles, results in marked hypertrophy of plantaris muscle (a second ankle extensor muscle of the distal hind leg and a synergist to the gastrocnemius muscle) in young adult animals. On the other hand, synergistic ablation in senescent rats resulted in no gain in plantaris muscle mass and an actual decrease in myocyte cross-sectional area (implying muscle fiber splitting) (11), responses that implicate an impaired satellite cell regenerative response. Despite these changes, it is also being recognized that the impairment of satellite cell proliferative capacity can be reversed and, thus, it is unlikely to be the result of cell senescence (12).

Barton-Davis et al. previously showed that overexpression of the mitogen, insulin-like growth factor-1 (IGF-1), in skeletal muscle using viral transfection of a construct containing an IGF-1 transgene results in marked attenuation of sarcopenia in extensor digitorum longus muscle (a toe extensor muscle of the distal hind leg) of the rat (13), although it should be pointed out that this muscle normally undergoes relatively minor atrophy with aging as compared with other limb skeletal muscles (14). Notwithstanding this point, the viral-mediated IGF-1 overexpression was associated with larger myocytes containing centralized nuclei (the nuclei of myocytes are located normally at the periphery of the myofiber), indicative of fusion of satellite cells with pre-existing fibers (13). In addition, Chakravarthy et al. (8) showed that the recovery of muscle mass following hindlimb casting in aged rats was dramatically improved when IGF-1 was continuously infused into the hindlimb vasculature by implantation of a mini-osmotic pump. Furthermore, the proliferative capacity of the satellite cells in culture, although dramatically reduced with aging, was restored following IGF-1 infusion in these older animals (8). More recently, Conboy and colleagues (15) have implicated impaired Notch signaling in the impaired proliferative responses of aged satellite cells (see Miller Perspective and "Many Roads to Ruin"). A follow-up study showed that some element in the blood is limiting to the satellite cell regenerative response (16) (see "Buddy System"), and a likely candidate for this is myostatin (17, 18), a member of the transforming growth factor beta superfamily that inhibits satellite cell replicative capacity (4).

Satellite Cells and Myonuclear Domain Size: Implications for Sarcopenia

Adult skeletal myocytes are multinucleated. This situation arises because, during skeletal muscle development in utero, mesenchymal cells in the somite differentiate into myoblasts, and it is the proliferation and fusion of multiple myoblasts that form the myotubes that eventually differentiate into mature myocytes (19). Interestingly, it appears that the ratio of nuclear number to myocyte volume and/or surface area (i.e., nuclear domain size) is maintained within fairly narrow limits (20). Thus, in response to myocyte hypertrophy, nuclei must be added to the fiber (by fusion of satellite cells to the myocyte), and during myocyte atrophy, nuclei must be removed (likely by apoptosis) (20). The process(es) by which nuclear domain size is regulated is not clear. However, it is well established that satellite cells are essential to this process (20).

Recently, Brack and colleagues (21) endeavored to examine the role of satellite cells in the decline of nuclear number evident in larger myocytes of aged mouse skeletal muscles as a means of explaining how such changes could lead to age-related atrophy of individual myocytes seen with sarcopenia. They isolated single myocytes from hindlimb skeletal muscles of normal CBAj mice (a common laboratory strain) aged 2 months, 12 months, 22 to 24 months, and 26 months, or from homozygous MyoD deficient (MyoD-/-) mice (22) aged 2 months and 24 months. MyoD is a myogenic growth factor required for normal satellite cell differentiation and, thus, MyoD-/- mice exhibit abnormal satellite cell function (23). Whole tibialis anterior muscles (an ankle flexor muscle of the distal hind leg) were first placed in 4% paraformaldehyde at a fixed muscle length for 2 days, and then bundles of almost pure Type IIB muscle fibers (these are the fastest contracting and least fatigue-resistant of skeletal muscle fibers; see Hepple Perspective) were dissected from the anterior superficial surface. After single fibers were isolated, they were stained to identify nuclei [using 4',6-diamidino-2-phenylindole-2HCl (DAPI), a fluorescent probe for DNA], satellite cells (using antibodies that recognize the proteins Pax7 and Syn4, which are markers for these cells) and MyoD expression. Fiber diameter was determined at four locations over ≥500 µm fiber length, and all nuclei were counted. From these measures, the fiber surface area per nucleus and volume per nucleus were calculated as estimates of nuclear domain size. These researchers observed that mean fiber diameter decreased with aging in normal mice and that this atrophy was associated with a reduction in the number of nuclei per fiber length. The result of the maintained nuclear number was a relative maintenance of the surface area and volume per nuclei with aging (i.e., maintained nuclear domain size). One issue that bears particular attention here is that Brack and colleagues noted that nuclei on aged myocytes became elongated, which they reasoned could account for some studies showing an increase in nuclear domain size with aging when using nuclear counts made in cross sections of muscle [e.g., (24, 25)], that is, nuclear number may have been overestimated in previous studies.

Although the importance of nuclear domain size is not entirely clear, some have suggested that an increased nuclear domain size would put additional stress on the signaling and protein synthetic roles of the nucleus, which could impair a cell's response to damage (20, 21). Thus, it was interesting that, when fibers were classified on the basis of their size, it was revealed that larger fibers exhibited an increased nuclear domain size (i.e., a reduction in the surface and volume per nucleus) with aging, due to a reduction in the number of nuclei per unit fiber length in these larger fibers with age. These authors hypothesized that this result was caused by inadequate replacement of myocyte nuclei by satellite cells with aging. To test this idea, they examined changes in the number of satellite cells per myocyte with aging (based on Pax7 and Syn4 immunopositive labeling) and compared the degree of nuclear loss with aging in MyoD-/- mice to that seen in normal mice. Consistent with their hypothesis, there was a significant reduction in the number of Pax7- and Syn4-positive satellite cells on the surface of the myocytes with aging, and this appeared to be more severe in larger fibers.

Furthermore, in MyoD-/- mice the loss of nuclei per unit fiber length with aging was even more severe than in wild-type mice. Because MyoD is essential for normal satellite cell differentiation (23), these results support the idea that impaired satellite cell differentiation can lead to an increase in nuclear domain size consequent to inadequate replacement of myocyte nuclei with aging. On the other hand, it is important to point out that the effect of aging on MyoD and other myogenic growth factors in skeletal muscles remains unclear. Although the current study (21) found that MyoD protein concentrations within single myocytes tended to increase with development and remain relatively constant with aging, Alway and colleagues have previously reported dramatically reduced MyoD and myogenin protein concentrations in plantaris and gastrocnemius muscles from senescent rats (26), although this latter study did not differentiate MyoD levels in myocytes versus satellite cells. Notwithstanding this issue, the current study (21) shows that impaired satellite cell replicative responses can exacerbate the increase in nuclear domain size seen with aging. In this scenario, the authors propose a model wherein fiber atrophy with aging could be an attempt to restore nuclear domain size and/or occur as a consequence of failing intracellular homeostasis resulting from overtaxed nuclei (21). This, of course, raises the question of how some myocytes come to have inadequate nuclear number relative to their size. Although this issue remains to be clarified, it is possible that apoptotic loss of nuclei along the length of myocytes with aging (27) is not accompanied by sufficient replacement of lost nuclei secondary to inadequate satellite cell responses in aged muscles. This, and other issues, will need to be explored in future investigations.


January 18, 2006
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Citation: R. T. Hepple, Dividing to Keep Muscle Together: The Role of Satellite Cells in Aging Skeletal Muscle. Sci. Aging Knowl. Environ. 2006 (3), pe3 (2006).








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