Sarcopenia Age-Related Muscle Wasting and Weakness: Mechanisms and Treatments P19 pot

166 L.E. Gosselin
same recruitment history due to its innate membrane fragility (Menke and Jockusch
1991; Petrof et al. 1993). Therefore, the same factors released fleetingly by normal
muscle to promote wound healing are present chronically in dystrophic muscle and
may have pathologic consequences.
The presence of inflammatory cells is increased in skeletal muscle from patients
with DMD and in mdx mice. The major infiltrating cell types in dystrophin-
deficient muscle are macrophages (Engel and Arahata 1986; Spencer et al. 1997),
T cells (Engel and Arahata 1986; Spencer et al. 1997), and eosinophils (Cai et al.
2000). Nguyen and Tidball (Nguyen and Tidball 2003) demonstrated that
macrophages caused significant myotube lysis when co-cultured together.
Furthermore, Wehling et al. (2001) reported that macrophage depletion from mdx
muscles significantly reduced the concentration of regenerative muscle fibers.
These findings support the hypothesis that macrophage accumulation secondary to
inflammation can promote muscle injury. Given the persistent inflammatory
response in dystrophic muscle, it is possible that an altered extracellular environment
exists that promotes muscle fibrosis. Both TNF and TGF-b1 are produced by
macrophages and are known to stimulate collagen metabolism. Moreover, their
levels have been reported to be increased in muscular dystrophy (Bernasconi et al.
1995; Iannaccone et al. 1995; Lundberg et al. 1995; Tews and Goebel 1996;
Murakami et al. 1999; Porreca et al. 1999; Hartel et al. 2001; Andreetta et al. 2006;
Zhou et al. 2006). Given that the extracellular environment contains increased
levels of and these cytokines, and because of their biologic actions observed
in vitro, these cytokines may have prominent yet unknown in vivo roles in the
pathogenesis of fibrosis in DMD.
6 Summary
Regulation of collagen metabolism in normal and damaged skeletal muscle is com-
plex and likely involves the interaction of several cell types and growth factors.
Moreover, within a given organism, muscles with different activation patterns
exhibit marked differences in collagen mRNA levels as well as collagen character-
istics – indicative that mechanical load mediates collagen biosynthesis. Injured

skeletal muscle contains elevated levels of inflammatory cells, which are known to
secrete pro- and anti-inflammatory cytokines such as TNF-a and TGF-b1.
Moreover, the expression of bFGF is also up-regulated in damaged and/or dystro-
phic skeletal muscle. Significant evidence exists to suggest chronic inflammation
plays a key role in the development of fibrosis in dystrophic muscle, though the
mechanisms that regulate this process are not well understood. Both neutrophils
and macrophages play important roles in the regulation of collagen remodeling
post-injury by releasing various cytokines that mediate the behavior of inflamma-
tory cells, fibroblasts and satellite cells. Moreover, the behavior of these cells can
be affected by extrinsic factors such as basal levels of growth hormone, which
changes with age.
167Skeletal Muscle Collagen: Age, Injury and Disease
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G.S. Lynch (ed.), Sarcopenia – Age-Related Muscle Wasting and Weakness,
DOI 10.1007/978-90-481-9713-2_9, © Springer Science+Business Media B.V. 2011
Abstract Apoptosis is a well-conserve cellular disassembly process, which has
been implicated in a variety of diseases. Unlike cells with a single nucleus, apop-
totic signaling can target individual nuclei in multi-nucleated skeletal muscle cells
without necessarily eliminating the entire cell (muscle fiber). This targeted apop-
tosis or “nuclear apoptosis” appears to have a role in regulating aging-induced
muscle loss (sarcopenia) by reducing the myofiber volume (i.e. cytoplasm) that
can be supported in a single muscle fibre. Recent investigations indicate that apop-
totic signaling in aged skeletal muscles occurs through three apoptotic pathways.
The intrinsic or mitochondria apoptotic pathway has been most widely studied in
muscle. Mitochondria dysfunction and increased mitochondria permeability lead
to activation of cysteine-aspartic acid proteases (caspases) and eventually DNA
fragmentation in sarcopenia. The death receptor (extrinsic) apoptotic pathway has
been strongly implicated in sarcopenia and other conditions of muscle loss with
aging or disuse. TNF-a is thought to initiate apoptotic signaling via the death
receptor, and this can proceed to activate the effort proteases (e.g., caspase 3)
independent from mitochondria signaling. Nevertheless, there is some cross-talk
between the intrinsic and the extrinsic apoptotic pathways. Finally, a few studies
have shown data to suggest that the endoplasmic reticulum-stress apoptotic path-
way may also have a role in sarcopenia, although the importance of this pathway
relative to the other two pathways is less clear. Both myonuclei and satellite cells
appear to be susceptible to nuclear apoptosis in sarcopenia.
S.E. Alway (*)
Department of Exercise Physiology, and Center for Cardiovascular and Respiratory Sciences,
West Virginia University School of Medicine, Robert C Byrd Health Sciences Center,
1 Medical Center Drive, Morgantown, WV 26506, USA

e-mail:
P.M. Siu
Department of Health Technology and Informatics, The Hong Kong Polytechnic University,
Hung Hom, Kowloon, Hong Kong, China
e-mail:
Nuclear Apoptosis and Sarcopenia
Stephen E. Alway and Parco M. Siu
174 S.E. Alway and P.M. Siu
Keywords Nuclear cell death • Apoptosis • Skeletal myofiber • Satellite cell
• Mitochondria • Muscle atrophy
1 Apoptosis
Apoptosis is a fundamental biological process that is highly conserved among
species ranging from worm to human (Ellis et al. 1991; Yuan 1996) for elimination
of cells from tissues in an energy dependent manner. The term “apoptosis” origi-
nates from Greek (apo – from; ptosis – falling) which means “falling off”. The
phenomenon of apoptosis was first systematically described in nematode
Caenorhabditis elegans by Kerr and colleagues (Kerr et al. 1972). The distinctive
morphological characteristics of apoptosis include cell shrinkage, cell membrane
blebbing, chromatin condensation, internucleosomal degradation of chromosomal
DNA, and formation of membrane-bound fragments called apoptotic bodies (Kerr
et al. 1972). The morphological and biochemical characteristics of apoptosis are
unique and clearly distinguish it from necrotic cell death. Homologous apoptotic
regulatory death genes have been identified in a variety of organisms including
mammals and humans (Sulston and Horvitz 1977).
In the past several decades, there has been a better understanding of the biologi-
cal role and the regulatory mechanisms of apoptosis in life science and disease and
aging. Apoptosis is necessary for the elimination of damaged, aberrant, or harmful
cells. Apoptosis also participates in normal embryonic development, tissue
turnover, and immunological function (Thompson 1995). Apoptosis coordinates
the balance among cell proliferation, differentiation, and cell death in multicellular

organisms. Therefore, it is reasonable to conclude that health would be threatened
if apoptosis is not adequately maintained or if it is disrupted. In fact, aberrant regu-
lation of apoptosis has been demonstrated to contribute to the pathogenesis of
severe diseases including viral infections, cancers, autoimmune diseases (e.g.,
systemic lupus erythematosus and rheumatoid arthritis), loss of pancreatic beta-cell
in diabetes mellitus, toxin-induced liver disease, and acquired immune deficiency
syndrome (AIDS), myocardial and cerebral ischemic injuries and neurodegenera-
tive diseases and muscle loss associated with aging such as Alzheimer’s and
Parkinson’s diseases (Williams 1991; Thompson 1995; Duke et al. 1996; Yuan and
Yankner 2000; Lee and Pervaiz 2007; McMullen et al. 2009; Cacciapaglia et al.
2009; Campisi and Sedivy 2009).
2 Muscle Specific Apoptotic Signalling – Nuclear Apoptosis
Apoptosis was initially described as a process that was responsible for elimination of
entire cells, and this was essential for maintaining the homeostasis of cell growth and
death especially in cells with a high proliferative rate. In the context of single cells,
the term apoptosis has a clearly defined process leading to elimination of the nucleus
and therefore the cell. However, the better term to describe this same process in
175Nuclear Apoptosis and Sarcopenia
multinucleated post mitotic cell populations including cardiomyocytes and skeletal
myofibres is “nuclear apoptosis”. This is because elimination of a single nucleus can
occur without the death of the entire (multinucleated) muscle cell although this may
result in smaller cells. We propose that the process of apoptotic loss of myonuclei in
skeletal muscle should be best described as “nuclear apoptosis”. Nuclear apoptosis
can occur without inflammation or disturbing adjacent proteins or organelles.
The concept of “nuclear apoptosis” (i.e., death of a nucleus without death of the
entire cell) is intriguing and exciting. By definition, nuclear apoptosis involves cell
signalling that is so precise that specific individual nuclei can be targeted for elimi-
nation in a multinucleated skeletal myofiber without targeting other nuclei. Thus,
nuclear apoptosis requires amazingly precise targeting of some nuclei but not others
within a single muscle fibre.

Evidence accumulated over the last several years has shown that apoptosis is a
significant contributor to muscle degeneration (Primeau et al. 2002; Adhihetty and
Hood 2003; Dirks and Leeuwenburgh 2005; Tews 2005; Siu and Alway 2005a,
2006b; Siu et al. 2006; Pistilli et al. 2006b; Adhihetty et al. 2008, 2009; Marzetti
et al. 2008c, 2009b; Lees et al. 2009; Smith et al. 2009). However, apoptosis in
skeletal muscle is unique for several reasons. First, skeletal muscle is multi-nucle-
ated. Thus, the removal of one myonucleus by apoptosis will not produce “whole-
sale” muscle cell death, but it does result in a loss of gene expression within the
local myonuclear domain, potentially leading to cellular atrophy. Second, muscle
contains two morphologically and biochemically distinct subfractions of mitochon-
dria (subsarcolemmal, SS and intermyofibrillar, IMF) that exist in different regions
of the fibre could produce regional differences in the sensitivity to apoptotic stimuli
within the cell (Adhihetty et al. 2007a, 2008, 2009). Third, skeletal muscle is a
malleable tissue capable of changing its mitochondrial content and/or composition
in response to chronic alterations in muscle use or disuse. Such variations in mito-
chondrial content and/or composition can undoubtedly influence the degree of
organelle-directed apoptotic signalling in skeletal muscle.
Evidence that not all myonuclei in a single myofiber become apoptotic during
muscle loss has been observed in experimental denervation and denervation-asso-
ciated disease (e.g., infantile spinal muscular atrophy). This further supports the
hypothesis of “nuclear apoptosis” in modulating the myofiber volume by control-
ling the successive myofiber segments. The hypothesis of nuclear apoptosis is
consistent with the proposed “nuclear domain hypothesis” which explains the phe-
nomenon of cell size remodelling of myofiber by adding or subtracting nuclei
because each nucleus controls a specifically defined cytoplasmic area (Fig. 1). The
skeletal myofiber is a differentiated but highly plastic cell type which adapts to
loading and unloading. The nuclear domain hypothesis predicts that a nucleus con-
trols a defined volume of cellular territory in each myofiber. Therefore, addition of
extra nuclei (from satellite cells) into the myofiber is required to support the incre-
ment of cell size in order to achieve muscle hypertrophy and removal of the myo-

nuclei is needed to allow the muscle to atrophy. If fewer nuclei are available, less
cytoplasmic area could be supported. Generally, there is a tight relationship
between nuclear number and muscle fibre cross-sectional area and volume.
Nevertheless, this relationship is not perfect, because the nuclear domain increases