J Appl Physiol 93: 407–417, 2002;
10.1152/japplphysiol.01242.2001.
invited review
Functional characteristics of dystrophic skeletal
muscle: insights from animal models
JON F. WATCHKO,1 TERRENCE L. O’DAY,1 AND ERIC P. HOFFMAN2
Department of Pediatrics, Magee-Women’s Research Institute, Duchenne Muscular Dystrophy
Research Center, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15213;
and 2Research Center for Genetic Medicine, Children’s National Medical Center, Duchenne
Muscular Dystrophy Research Center, Washington, DC 20010
1
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Watchko, Jon F., Terrence L. O’Day, and Eric P. Hoffman.
Functional characteristics of dystrophic skeletal muscle: insights from animal models. J Appl Physiol 93: 407–417, 2002; 10.1152/japplphysiol.01242.
2001.—Muscular dystrophies are a clinically and genetically heterogeneous group of disorders that show myofiber degeneration and regeneration. Identification of animal models of muscular dystrophy has been
instrumental in research on the pathogenesis, pathophysiology, and
treatment of these disorders. We review our understanding of the functional status of dystrophic skeletal muscle from selected animal models
with a focus on 1) the mdx mouse model of Duchenne muscular dystrophy, 2) the Bio 14.6 ␦-sarcoglycan-deficient hamster model of limb-girdle
muscular dystrophy, and 3) transgenic null mutant murine lines of
sarcoglycan (␣, ␤, ␦, and ␥) deficiencies. Although biochemical data from
these models suggest that the dystrophin-sarcoglycan-dystroglycanlaminin network is critical for structural integrity of the myofiber plasma
membrane, emerging studies of muscle physiology suggest a more complex picture, with specific functional deficits varying considerably from
muscle to muscle and model to model. It is likely that changes in muscle
structure and function, downstream of the specific, primary biochemical
deficiency, may alter muscle contractile properties.
dystrophin; mdx mouse; sarcoglycanopathies
MUSCULAR DYSTROPHIES ARE A GROUP of human and animal
disorders that show myofiber degeneration and regeneration, typically associated with progressive muscle
weakness. Clinically and genetically, they are a heterogeneous group of inherited diseases. Mutations in
several individual genes are now known to underlie the
pathogenesis of different types of muscular dystrophy
(Table 1) (37, 45). Some of these gene mutations involve proteins expressed throughout the body, yet appear to preferentially affect skeletal muscle. These
disorders include defects in nuclear envelope and
mRNA metabolism. Specifically, defects in two membrane components of the nuclear envelope cause different forms of Emery-Dreifus muscular dystrophy: 1)
Address for reprint requests and other correspondence: J. F.
Watchko, Division of Neonatology and Developmental Biology, Dept.
of Pediatrics, Magee-Women’s Hospital, 300 Halket St., Pittsburgh,
PA 15213 (E-mail: [email protected]).
http://www.jap.org
emerin (X-linked recessive Emery-Dreifuss muscular
dystrophy) and 2) lamin A/C (autosomal dominant Emery-Dreifuss muscular dystrophy) (8). Three genes involved in generalized mRNA metabolism cause specific
dystrophies, again despite the fact that the biochemical
deficiency should be expressed throughout the body.
Mutations in poly(A) binding protein 2 cause oculopharyngeal muscular dystrophy (87), whereas expression
of long trinucleotide repeats in specific mRNAs disrupts mRNA splicing and regulation and leads to two
different forms of myotonic dystrophy (55). The mechanisms underlying the muscle-specific phenotype of
these generically expressed proteins are not understood, and this question remains a challenge for researchers.
Other types of muscular dystrophies are caused by
genes expressed specifically in muscle and involve
myofiber plasma membrane function (dysferlin, calpain III, calveolin) or structural support (see below).
8750-7587/02 $5.00 Copyright © 2002 the American Physiological Society
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Table 1. Genes associated with selected muscular dystrophies
Gene/Protein Product
Cellular/Localization Function
Ememin
Nuclear envelope
Lamin A/C
Nuclear envelope
Poly(A) binding protein 2
Expanded trinucleotide repeats
in 3⬘ UTR of cAMPdependent protein kinase
Expanded trinucleotide repeats
in 3⬘ UTR of cAMPdependent protein kinase
Dysferlin
Calpain III
Caveolin
␣-2 Laminin
Sarcoglycan complex
Ref.
8
mRNA metabolism
mRNA metabolism
X-linked recessive Emery-Dreifus muscular
dystrophy
Autosomal dominant Emery-Dreifus muscular
dystrophy
Oculopharyngeal muscular dystrophy
Congential myotonic dystrophy
mRNA metabolism
Myotonic dystrophy
55
Myofibril plasma membrane
maintenance
Myofibril plasma membrane
maintenance
Myofibril plasma membrane
maintenance
Membrane cytoskeleton
structural support
Membrane cytoskeleton
structural support
Membrane cytoskeleton
structural support
Miyoshi/LGMD-2B
59
LGMD-2A
81
LGMD-1C
65
Congenital muscular dystrophy
31, 74
Duchenne muscular dystrophy, Becker
muscular dystrophy
LGMD-2C, -2D, -2E, -2F
46, 48,
54
4
8
87
55
LGMD, limb-girdle muscular dystrophy.
Three proteins, dysferlin, calpain III, and calveolin,
appear to be involved in plasma membrane maintenance and/or traffic and cause Miyoshi limb-girdle
muscular dystrophy (LGMD) 2B, LGMD-2A, and
LGMD-1C, respectively (59, 65, 81). A mouse model for
dysferlin deficiency (SJL mice) has been used as a
model for both heart failure and autoimmune disease,
depending on the background strain harboring the
mutation (3). The limited knowledge of the role of these
proteins in muscle cell biology has hindered the understanding of the pathophysiology of these types of muscular dystrophies, and no physiological studies of muscle function have been reported.
The most common and devastating types of muscular
dystrophies are also those that are most fully characterized; each involves a defect of the muscle membrane
cytoskeleton. There are thought to be at least two
membrane cytoskeleton systems: 1) the vinculin-integrin-laminin network and 2) the dystrophin-dystroglycan-laminin network (45). These two networks appear
to be at least in part functionally redundant, and this
may be why patients with mutations in any one of
these components survive to birth, yet evidence muscle
abnormalities during postnatal development (11, 43).
The one component shared by both networks is laminin. Laminins are a major component of the basal
lamina, and the functional protein is a heterotrimer
containing alpha, beta, and gamma subunits. There
are different genes for each of the subunit types, and
these show tissue- and cell-type-specific expression.
Muscle laminin is composed of alpha-2, beta-1, and
gamma-1 proteins, with this trimer often called “merosin.” Mutations in the alpha-2 gene cause a very
severe form of muscular dystrophy that shows a neonatal or even prenatal onset (congenital muscular dystrophy) (31, 74). The loss of this muscle protein in
J Appl Physiol • VOL
patients is associated with large-scale degeneration of
myofibers at or around the time of birth, with subsequent failure of muscle regeneration and profound
weakness. A mouse model for this disorder has been
known for many years [dy/dy mice (96)], as well as
recently characterized murine laminin alpha-2 chain
null mutants, all of which similarly show early lethality (66). One would expect that both the dystrophindystroglycan-laminin and vinculin-integrin-laminin
networks would be defective in these patients and
mice, and this may explain the severe nature of this
disorder.
The most common and extensively studied muscular
dystrophies are those that involve mutations of the
dystrophin-dystroglycan-laminin network. Considerable effort has been expended on understanding the
physiology and cellular biology of this network, and the
remainder of this review discusses the proteins specifically affecting the dystrophin-based membrane cytoskeleton. The organization of the dystrophin-based
membrane cytoskeleton of muscle fibers is shown in
Fig. 1 and demonstrates the relationship between dystrophin and the other dystrophin-associated proteins
within the cytoskeletal oligomeric protein complex.
The dystrophin-associated proteins include the dystroglycan (␣-dystroglycan and ␤-dystroglycan), sarcoglycan (SG; ␣-, ␤-, ␥-, ␦-SG), and cytoplasmic (syntrophin)
subcomplexes. The dystroglycan complex consists of
␣-dystroglycan, which associates with the basal-lamina protein merosin in the extracellular matrix, and
␤-dystroglycan, a transsarcolemmal protein that binds
␣-dystroglycan and dystrophin. The SG complex consists of four transsarcolemmal proteins, ␣-, ␤-, ␥-, and
␦-SG, that associate with each other and likely function
as a single unit, with ␦-SG serving as the link to
␤-dystroglycan of the dystroglycan complex. Within the
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Dystrophin
Disease
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INVITED REVIEW
cytoplasmic subcomplex, the COOH-terminal region of
dystrophin is bound to syntrophin, whereas the aminoterminus of dystrophin binds F-actin.
Abnormalities of dystrophin are the most common
cause of muscular dystrophy (27, 45), accounting for
both the Duchenne and Becker phenotypes. Duchenne
muscular dystrophy (DMD) is a devastating muscular
dystrophy and the most common inherited childhoodlethal disorder of humans worldwide (45). DMD is
caused by mutations in the dystrophin gene that precludes the production of stable dystrophin molecules
and results in sarcolemmal instability and contractioninduced myofiber necrosis. Dystrophin is highly conserved through vertebrate evolution, the dystrophin
gene is X-linked in placental mammals, and dystrophin
deficiency appears to be completely specific for DMD
(46, 48). Thus any animal that manifests dystrophin
deficiency as an X-linked inherited trait is a candidate
model of DMD. Among other causes of muscular dystrophy are 1) Becker muscular dystrophy, a disorder
known to be allelic (due to a different mutation in the
same gene) to DMD (54), and 2) genetic defects of the
SG subcomplex (␣, ␤, ␥, ␦) of the myofiber cytoskeleton
(sarcoglycanopathies), a common cause of LGMD-2C,
-2D, -2E, and -2F (1, 27, 37, 40, 86).
Identification of animal models for DMD and the
sarcoglycanopathies has been instrumental in research
on the pathogenesis, pathophysiology, and treatment
of these disorders. Indeed, the past 15 yr have been
witness to a virtual explosion of knowledge regarding
the molecular pathology of DMD, Becker muscular
dystrophy, and LGMD in humans and their mammaJ Appl Physiol • VOL
lian counterparts (45). This knowledge, in turn, is
being used to develop novel approaches to the treatment of these disorders, including gene therapy (47, 93,
95), myoblast transfer (71), and new pharmacological
interventions (38). There remains, however, a gap between our knowledge of the molecular pathology of
muscular dystrophy and downstream biomechanical
events. Given that most muscular dystrophies are associated with primary defects of the muscle membrane
cytoskeleton and characterized phenotypically by muscle weakness, a host of potential functional targets
exist beyond those involved in the cascade leading to
muscle cell death (45, 78). Targets include the metabolic pathways involved in intracellular calcium regulation (t tubule system, sarcoplasmic reticulum, and
excitation-contraction coupling), myosin molecular
motor function, and bioenergetics (mitochondria and
phosphocreatine-creatine kinase energy shuttle), among
others. An expanded knowledge of these secondary
consequences of muscular dystrophy will enhance our
understanding of these disorders and dystrophic skeletal muscle dysfunction. Moreover, the aforementioned
novel therapeutic approaches will ultimately be judged
by their ability to enhance dystrophic skeletal muscle
function. To date, such therapies have been almost
exclusively applied and tested in animal models. Below, we discuss our current understanding of the functional status of dystrophic skeletal muscle from selected animal models used in the study of muscular
dystrophy with a specific focus on 1) the mdx mouse
model of DMD, 2) the Bio 14.6 ␦-SG-deficient hamster
model of LGMD, and 3) transgenic null mutant murine
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Fig. 1. Schematic organization of the
dystrophin membrane cytoskeleton of
muscle fibers. Dystrophin-associated
proteins include the dystroglycan (␣dystroglycan and ␤-dystroglycan), sarcoglycan (␣-, ␤-, ␥-, and ␦-sarcoglycan),
and cytoplasmic (syntrophin) subcomplexes. The dystroglycan complex consists of ␣-dystroglycan, which associates with the basal-lamina protein
laminin-2 (merosin) in the extracellular matrix, and ␤-dystroglycan, a
transsarcolemmal protein that binds
␣-dystroglycan and dystrophin. The
sarcoglycan complex consists of four
transsarcolemmal proteins (␣-, ␤-, ␥-,
and ␦-sarcoglycan) that associate with
each other and likely function as a single unit with ␦-sarcoglycan serving as
the link to ␤-dystroglycan of the dystroglycan complex. Within the cytoplasmic subcomplex, the COOH-terminal region of dystrophin is bound to
syntrophin, whereas the NH2 terminus
of dystrophin binds F-actin.
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lines of SG (␣, ␤, ␦, and ␥) deficiencies. In addition to
characterizing the force-generating capacity of dystrophic muscle in the isometric and miometric (shortening) modes, dystrophic muscle function assessed during repetitive lengthening activations, a paradigm of
mechanical stress, is detailed (23, 36, 75). The latter
measurement is thought to be particularly important
because evidence suggests that a function of the dystrophin-based membrane skeleton is to protect against
stress-induced muscle damage (75).
ANIMAL MODELS OF DMD: MDX MOUSE
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The most widely used animal model of muscular
dystrophy is the mdx mouse, the first animal model of
DMD identified (9, 45, 48). This mouse line occurred
spontaneously on a C57BL/10 background and was
fortuitously identified during a screen for red blood cell
defects in a C57BL/10 colony (9). Serum creatine kinase levels were strikingly elevated in this line, suggesting a dystrophic process. Dystrophin deficiency in
the mdx muscle was subsequently identified (46) and
shown to result from a stop codon mutation in exon 23
(89) of the dystrophin gene. More recently, a series of
new alleles of the mdx strain were induced by mutagenesis; all demonstrate a phenotype consistent with
the original mdx allele (52).
Isometric contractile characteristics. Most studies of
mdx mouse muscle have been performed on limb and
diaphragm muscle in the isometric mode. These studies generally evidence twitch contraction kinetics that
approximate those of age-matched C57BL/10 control
muscle (29, 72, 76, 80). Some studies have reported a
slowing in twitch contraction and half-relaxation times
associated with an increase in the proportion of type I
fibers (72, 76), whereas others have shown the opposite
(83). Absolute peak twitch force (Pt) approximates that
observed in C57BL/10 control muscle (77) whereas
specific twitch force (Pt/CSA), i.e., Pt normalized for an
estimate of muscle cross-sectional area (CSA) is consistently decreased in mdx muscle relative to control
(76, 77). Similarly, absolute peak tetanic force (Po) is
comparable to or exceeds that observed in C57BL/10
muscle (32, 60), but specific peak tetanic force (Po/
CSA), i.e., Po normalized for CSA, is consistently lower
in mdx mouse vs. C57BL/10 muscle (7, 20, 21, 29, 60,
61, 63, 77, 83). Fatigue resistance during repetitive
isometric contractions is either at control levels or
greater (76, 83), the latter response presumably related to a shift in myosin heavy chain phenotype to
slower isoforms with greater oxidative capacity and
lower myofibrillar ATPase activity (76).
The lower Pt/CSA and Po/CSA in mdx muscle likely
reflects an increasing contribution of degenerating
and/or necrotic fibers to mdx muscle CSA (32) compared with control C57BL/10 muscle. Indeed, DMD in
humans is characterized by increased bulk in selected
muscles that, early in the disease process, results from
true hypertrophy of muscle fibers (45). This hypertrophy in DMD patients later evolves into pseudohypertrophy as the muscle is gradually replaced by first
fibrotic tissue and then fat (45). Skeletal muscle from
mdx mice generally shows increased mass (77, 83),
although this can be later in onset and variable across
specific muscle and age groups (72, 73). Thus cycles of
muscle fiber degeneration typically seen as early as
3–5 wk of life in mdx mice are apparently balanced by
fiber regeneration, leading to hypertrophic changes
followed later in life in selected muscles by atrophy
(73). Muscle hypertrophy in mdx limb muscle is effective in maintaining control values of absolute force
(60). In contrast to mdx limb muscle, the mdx diaphragm demonstrates more progressive cycles of fiber
degneneration and fiber loss throughout the lifespan of
the mdx mouse. The proportion of viable fibers in
diaphragm decreases from ⬃70% in 4- to 6-mo-old
animals to 40% in 24-mo-old animals (63, 76). Thus one
difference between mdx limb muscle and mdx diaphragm is that mdx diaphagm evinces greater and
more progressive decrements in Po/CSA with age (29).
In addition to injured or necrotic fibers, other potential
mechanism(s) that could contribute to a reduced Po/
CSA in mdx muscle includes adverse changes in excitation-contraction coupling, reduced calcium sensitivity (altered thin-filament regulation), lower contractile
protein concentration, altered cross-bridge cycling kinetics, and/or lower average force per cross bridge.
These possible mechanisms merit further investigation
because there are limited, and at times conflicting,
data on such phenomenon in mdx muscle (15, 35, 36).
Dynamic shortening activations. A more limited
number of studies have characterized the dynamic
shortening properties, i.e., force velocity, power, and
work functions of mdx muscle. These studies consistently report a downward shifted force-velocity relationship and diminished maximum power and work
output in mdx muscle (15, 20, 21, 32, 60, 61, 63, 91).
Unfortunately, there are limited data with respect to
the mechanism(s) that may underlie these functional
deficits. Investigators have reported a shift in myosin
heavy chain expression to slower isoforms (76) and
abnormalities in cross-bridge properties characterized
by both a reduction in total number of cross bridges
and the force produced per cross bridge in mdx muscle
to account for these deficits (15). The latter, in turn,
may be related to defective energy transport in mdx
muscle (6) and/or altered calcium handling due to
chronic calcium overload (35, 47, 79). The observed
changes in myosin heavy chain isoform composition
(and presumably actomyosin ATPase activity), however, do not appear to be of sufficient degree to fully
account for the observed reduced force per cross bridge
in mdx muscle (15). Changes in cross-bridge cycling
kinetics could contribute to the impaired dynamic
shortening properties of mdx muscle (15). In this regard, Coirault et al. (15) report that both the duration
of the cross-bridge cycle and the rate constant for
cross-bridge detachment are significantly lower in mdx
mice compared with C57BL/10 controls. These authors
conclude that impaired dynamic shortening in mdx
muscle is a result of myosin molecular motor dysfunction (15). Additional study is necessary to confirm and
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J Appl Physiol • VOL
occurring anywhere from the neonatal period to adulthood (17, 56). Independent alleles have been identified
in a series of breeds, including rottweilers, wire-hair
fox terriers, shelties, and German short-haired pointers (45). Varying degrees of muscle involvement and
marked temporal differences in the dystrophic phenotype across individual dogs have limited the utility of
this model for functional studies. In addition, at least
two independent isolates of dystrophinopathy have
also been identified in domestic cats (12, 34, 94). Dystrophin-deficient cats show marked muscular hypertrophy that becomes so severe that they succumb to
dehydration because of lingual hypertrophy or to starvation because of crural diaphragm hypertrophy (33,
34). Cats also demonstrate considerable skeletal muscle stiffness and cardiac involvement but never lose
mass and thus are better able to conserve muscle
force-generating capacity.
The “appropriateness” of these animal dystrophinopathies for providing insights regarding human
DMD has generated considerable debate within the
scientific community. Clearly, the clinical phenotype of
each animal model differs from the human disease and
from each other. Nevertheless, all of the aforementioned models have loss-of-function mutations in the
same highly conserved dystrophin gene, so they are
certainly genetically homologous. They all lack the
highly conserved dystrophin protein in muscle tissue,
so they are biochemically homologous. Muscle tissue
itself is exceedingly highly conserved through evolution, and, in fact, all animal models share many of the
histological features of human DMD, including fiber
size variation and the degeneration/regeneration of
myofibers. All have striking elevations in serum creatine kinase. The phenotypic differerences reside in the
progressive aspects of the disease, particularly with
respect to connective tissue proliferation in muscle,
and failure of regeneration over time. All dystrophindeficient species show marked hypertrophy at a young
age, but, in dogs and humans, the hypertrophy changes
to loss of muscle (wasting) and loss of strength. There
is general consensus that the dystrophin-deficient
mouse, dog, and cat are outstanding genetic and biochemical models for DMD despite phenotypic differences. The fact that they show differences in phenotype
is valuable in that they provide insight into the secondary consequences of primary dystrophinopathy.
However, it can be more difficult to use these animals
as therapeutic models since the downstream consequences of dystrophin deficiency may vary considerably from species to species.
SARCOGLYCANOPATHIES
The autosomal-recessive LGMDs are a heterogeneous group of muscular diseases characterized by
progressive muscle weakness and, in severe cases,
death in the second or third decade of life (1). The more
severe phenotypes are often caused by mutations in the
SG genes ␣ (LGMD-2D), ␤ (LGMD-2E), ␦ (LGMD-2F),
and ␥ (LGMD-2C) (4, 5, 27, 28, 53, 58, 69, 86). A
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further define the mechanism(s) of impaired dynamic
shortening activation in dystrophic muscle.
Response to repetitive lengthening activations. Several studies have assessed the contractile response of
mdx muscle to repetitive dynamic lengthening activations (i.e., “eccentric contractions”), a paradigm of mechanical stress (23, 44, 75). The rationale for such
study resides in 1) reports demonstrating that skeletal
muscle injury is greatest after exercise that involves
lengthening of activated muscle (greater than that
which follows intense isometric or shortening contractions) and 2) evidence suggesting that a function of the
dystrophin-based membrane cytoskeleton is to protect
against stressed-induced muscle damage (75). As predicted, several reports demonstrate that mdx muscle
does demonstrate an enhanced susceptibility to injury
during repetitive lengthening activation, as assessed
by both contraction-induced sarcolemmal rupture and
force deficits (7, 22, 23, 36, 44, 67, 75). Indeed, the
literature would suggest that enhanced susceptibility
to lengthening-activation injury, as assessed by force
deficit, is a hallmark of dystrophin-deficient muscle
function (36, 75).
Until recently, it has been unclear why mdx muscle
is so susceptible to lengthening-activation force deficits. Previous study has shown that mdx diaphragm
evidences enhanced passive stiffness relative to controls (90, 91). Whether this characteristic predisposes
or protects against lengthening-activation injury remains undocumented. Recent reports taken together,
however, confirm that the locus of contraction-induced
injury in mdx muscle resides in the sarcolemma (62)
and suggest that reduced elasticity of mdx muscle itself
may predispose muscle to an enhanced susceptibility of
lengthening-activation force decrements. Specifically,
the amount of negative work done during the lengthening phase of activation has been identified as an
important factor in modulating the extent of lengthening-activation force deficits (51). Moreover, investigators have reported that the amount of negative work
performed by mdx diaphragm increases dramatically
with increases in muscle length in association with its
enhanced passive stiffness (91), significantly more so
than control muscle. These findings suggest that reduced muscle elasticity may predispose mdx muscle to
injury during lengthening activations. This area of
study merits further investigation, particularly in light
of data (see below) suggesting that dystrophic muscle
from the closely biochemically related sarcoglycanopathies do not evidence similar heightened susceptibility
to lengthening-activation force deficits (39).
Other animal models of DMD. Other animal models
of DMD have been identified, although a relative paucity of functional studies on their respective limb or
respiratory musculature exist. These include a canine
X-linked or golden retriever muscular dystrophy (17,
18, 24, 56) that show dystrophin deficiency as an Xlinked trait (17, 18) associated with a splicing mutation
in exon 8 (88). Dystrophin-deficient dogs show a rapidly
progressive course of muscle weakness, wasting, and
contracture, with onset at ⬃3 mo of age and death
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primary deficiency of any single SG leads to partial or
complete absence of all other sarcoglycans on the sarcolemma, suggesting that sarcoglycans are mutually
dependent on each other for assembly and stability in
the plasma membrane (28, 70, 84, 92). Phenotypically,
the SG deficiencies are indistinguishable from the dystrophin deficiencies (27). However, dystrophinopathies
show secondary deficiencies of all SG proteins, whereas
sarcoglycanopathies usually show normal levels of dystrophin (42, 57). LGMDs, although less common than
DMD, represent an important group in which to study
the dystrophic process. Until the recent discovery of a
␦-SG mutation in the Bio 14.6 inbred dystrophic hamster (70, 84) and the generation of ␣- (26), ␤- (2, 30), ␦(41), and ␥-SG (42, 85) null mutant transgenic mouse
lines, study of LGMDs has been hindered by the lack of
an animal model.
BIO 14.6 HAMSTER MODEL OF ␦-SG DEFICIENCY
Fig. 2. Relative changes (% baseline) in isometric
force generation of control (F1B) and ␦-sarcoglycandeficient (Bio 14.6) hamster tibialis anterior muscle at
three ages (9 wk, 17 wk, and 22 wk of life) during
repetitive isovelocity lengthening activations in vitro.
Muscle activation (75 Hz) was initiated at 90% of
optimal length (Lo), and for the first 300 ms of each
500-ms train, length was not changed. During the
remaining 200 ms of each stimulus train, the muscle
was lengthened at a constant velocity (1.0 Lo/s) from
90 to 110% of Lo. Repeated-measures analysis of variance did not reveal any significant differences across
groups in the decline in force observed during lengthening activations.
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The inbred Bio 14.6 hamster was identified in 1962
as a recessively inherited dystrophic myopathy model
that affected both cardiac and skeletal muscle (50). The
skeletal muscle of Bio 14.6 hamster displays classical
signs of muscular dystrophy, including necrosis, central nucleation, and variable size of muscle fibers (10,
49, 50, 57, 92). The genetic cause for the dystrophic
disease in the Bio 14.6 hamster was recently identified
as a loss-of-function mutation of the ␦-SG gene (70, 84).
␦-SG is a 35-kDa transmembrane glycoprotein component of the dystrophin-based membrane cytoskeleton
of muscle fibers. It is part of a functional tetrameric
complex that includes ␣-, ␤-, ␥-, and ␦-SG proteins in
equal stoichiometry. As with their human counterpart,
primary loss of ␦-SG in Bio 14.6 hamster also results in
the secondary deficiency of the other skeletal muscle
SGs (␣, ␤, and ␥) (49, 57, 92). The shared muscle
histopathology and protein biochemistry between Bio
14.6 hamster and human patients with LGMD-2F renders this animal as an excellent model to study the
functional consequences of ␦-SG deficiency. The clinical
picture, however, deviates considerably between af-
fected humans and hamsters, with the ␦-SG deficient
hamster showing muscle hypertrophy as a notable
finding and early death from cardiac failure. Humans
with the same defect generally show more limited
cardiac involvement (64). Although the cardiac features of the Bio 14.6 hamster have been extensively
studied, there is relatively limited data available on
skeletal muscle function (10, 68).
Isometric contractile characteristics. Bio 14.6 hamster dystrophic muscle evidence twitch-contraction kinetics that approximate those of age-matched F1B
control muscle (10, 68). Absolute peak Pt is generally
comparable to that observed in F1B hindlimb muscle
(10, 68, 95), whereas Pt/CSA is decreased in Bio 14.6
dystrophic hamster muscle relative to control (10, 95).
Similarly, absolute Po is comparable to that of F1B
hamster, whereas Po/CSA is consistently lower in Bio
14.6 dystrophic hamster vs. F1B muscle (10, 95). The
lower Pt/CSA and Po/CSA in Bio 14.6 hamster muscle
may reflect a combination of a greater muscle mass and
an increasing contribution of degenerating and/or necrotic fibers to dystrophic muscle CSA compared with
control F1B muscle.
There are no reported data on Bio 14.6 hamster
dystrophic-shortening activations and only unpublished observations (T. L. O’Day, personal communication) regarding the response of ␦-SG-deficient Bio 14.6
muscle to repetitive lengthening activations. The latter
is of interest, however, in that the Bio 14.6 muscle does
not demonstrate increased susceptibility to force deficit
when compared with F1B control muscle (Fig. 2). This
finding runs counter to the notion that dystrophic muscle always shows enhanced susceptibility to lengthening-activation injury and suggests that nonmechanical
events may be operative in the pathogenesis of the
dystrophic process in SG-deficient states (see below)
(39). It is of interest, in this regard, that Bio 14.6
hamster does not show the decreased passive stiffness
observed in the mdx mouse but in fact the opposite as
recently reported by Coirault et al. (16). The later
study demonstrates increased muscle compliance in
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the ␦-SG-deficient hamster, results that contrast
sharply with those found in mdx muscle, suggesting
that muscle viscoelastic properties are critically dependent on the presence or absence of specific cytoskeletal
proteins and the myopathic animal model considered
(see below) (16). Relevant to these observations is the
fact that dystrophin strongly binds intracellular actin
filaments at multiple sites along the protein (82). The
connection between the intracellular actin cytoskeleton and the dystrophin-based membrane cytoskeleton
is still intact in SG deficiency but is lost in dystrophin
deficiency.
␥-SG TRANSGENIC MURINE NULL MUTANT
␦-SG MURINE NULL MUTANT LINE
␦-SG null mutant transgenic mice (dsg ⫺/⫺) have
also been generated (41). This model is analogous to
␣-SG NULL MUTANT MICE
␣-SG-deficient null mutant transgenic mice have
also been generated by targeted disruption of the ␣-SG
gene (26). These mice show the complete absence of
␣-SG transcript and protein, a complete loss of the SG
complex and sarcospan, and demonstrate a progressive
muscular dystrophy (26) similar to its human homologue, LGMD-2D (27). ␣-SG ⫺/⫺ mice demonstrate
significant muscle hypertrophy and muscle-specific
changes in contractile properties. Hypertrophic slowtwitch weight-bearing soleus muscle demonstrates Po
values that are greater than control with a resultant
specific force that is not different from control. In
contrast, the hypertrophic fast-twitch EDL (26) and
Table 2. Mechanical characteristics of dystrophic skeletal muscle in vitro vs. control
Muscle mass
Lo
Pt
Po
Po/CSA
Passive stiffness
Lengthening activations
Bio 14.6
␣-SG [⫺/⫺]
␤-SG [⫺/⫺]
␦-SG [⫺/⫺]
␥-SG [⫺/⫺]
mdx
_
7
7
7
+
+
7
_
7
7
7
+
_
7
_
7
7
7
+
N/A
N/A
7
7
7
7
7
N/A
+
7
7
7
7
7
7
7
_
7
7
7
+
_
+
Lo, optimal muscle length; Pt, peak twitch tension; Po, peak tetanic tension; Po/CSA, specific force generation or peak tetanic tension
normalized for an estimate of muscle cross-sectional area; Bio 14.6, ␦-sarcoglycan-deficient hamster; ␣-SG [⫺/⫺], ␣-sarcoglycan-deficient
transgenic null mutant mouse; ␤-SG [⫺/⫺], ␤-sarcoglycan-deficient transgenic null mutant mouse; ␦-SG [⫺/⫺], ␦-sarcoglycan-deficient
transgenic null mutant mouse; ␥-SG [⫺/⫺], ␥-sarcoglycan-deficient transgenic null mutant mouse; mdx, dystrophin-deficient mouse; 7, not
different from control; _, increased relative to control; +, decreased relative to control; N/A, not available.
J Appl Physiol • VOL
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Recent years have seen the genesis of SG-deficient
transgenic null mutant lines. Table 2 shows the mechanical characteristics of these null mutant lines and
how they compare with those observed in the Bio 14.6
␦-SG-deficient hamster and dystrophin-deficient mdx
mouse. ␥-SG null mutant transgenic mice (gsg ⫺/⫺)
(39, 42, 85) show contractile properties in vitro that do
not differ from their wild-type counterparts (39). Specifically, muscle [extensor digitorum longus (EDL)]
from gsg ⫺/⫺ demonstrates comparable muscle mass,
CSA, Pt, Po, and Po/CSA as muscles from wild-type
counterparts. Importantly, gsg ⫺/⫺ EDL also showed
force output decline and Procion orange uptake (a
marker of sarcolemmal integrity) at levels comparable
to control muscle during repetitive lengthening activations. These findings suggest that ␥-SG deficiency does
not enhance the susceptibility of gsg ⫺/⫺ muscle to
lengthening-activation injury. The aforementioned
functional response to repetitive lengthening activations is comparable to that seen in the Bio 14.6 ␦-SGdeficient hamster and suggests that nonmechanical
events may be important in the pathophysiology of
muscular dystrophy within the context of SG deficiency
(39). As pointed out by Hack and colleagues (39, 40),
these findings further suggest that the loss of SGs may
contribute to muscle degeneration in both the dystrophin- and SG-associated muscular dystrophies.
the Bio 14.6 ␦-SG-deficient hamster, but dsg ⫺/⫺ skeletal muscle shows some differences in isolated muscle
mechanics when compared with the Bio 14.6 hamster
and other transgenic SG-deficient null mutant lines.
As with both the Bio 14.6 hamster and gsg ⫺/⫺ null
mutant, dsg ⫺/⫺ mice demonstrate classic histological
findings of dystrophy, including cell death, muscle regeneration, inflammation, and fibrosis as well as reduced survival (41). Dsg ⫺/⫺ muscle mass, Pt, Po, and
Po/CSA, however, do not differ from wild-type muscle
(41). Interestingly, dsg ⫺/⫺ muscle does show an enhanced susceptibility to lengthening-activation force
deficits (41). These findings suggest ␦- and ␥-SG deficiency have different molecular consequences and resultant functional impacts that may also be species
specific. Hack et al. (39) have proposed that the absence of lengthening-activation deficits in gsg ⫺/⫺
muscle suggests a nonmechanical role for ␥-SG in myocyte survival. The role of ␦-SG remains less clear given
the species-specific response of ␦-SG deficiency on muscle mechanics, both in terms of force generation and in
susceptibility to lengthening-activation force deficits.
As stated by Hack and colleagues (41), however, these
functional differences underscore the important nature
of the SG complex and the need to understand the role
of each component in the pathogenesis of muscular
dystrophy. In this regard, recent work suggests that
␦-SG deficiency can adversely impact vascular smooth
muscle function, induce ischemic injury in skeletal
muscle, and exacerbate muscular dystrophy (19).
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INVITED REVIEW
␤-SG-DEFICIENT NULL MUTANT MICE
␤-SG-deficient mice have also been generated by
targeted mutagenesis (2, 30). Skeletal muscle from
␤-SG-deficient mice, like their ␣- and ␦-SG-deficient
transgenic null mutant counterparts, demonstrates increased muscle mass (2) and reduced Po/CSA (30).
There are no data on the passive stiffness of ␤-SGdeficient muscle nor on the response of such muscles to
repetitive lengthening activations.
SUMMARY
Mutations in components of the dystrophin-SG-dystroglycan-laminin network lead to devastating and
common muscular dystrophies in humans. The current
models suggest that this network is critical for structural integrity of the myofiber plasma membrane. Although considerable data, such as dye-exclusion studies, support this model, emerging studies of muscle
physiology in the animal models suggest a more complex picture, with specific functional deficits varying
considerably from muscle to muscle and model to
model. It is likely that changes of muscle structure and
function, downstream of the specific primary biochemical deficiency, may alter muscle contractile properties
(45, 47, 78). A full understanding of the apparently
variable consequences of single biochemical defects on
muscle contractile function may require genome-wide
approaches to understanding the response of muscle to
injury and compensatory molecular mechanisms invoked by the muscle. Expression profiling and proteomic approaches will likely soon provide us with a
more complete picture (14).
J Appl Physiol • VOL
This work was supported by National Institute of Arthritis and
Musculoskeletal and Skin Diseases Grant AR-45925, the Parent
Project USA, the Mario Lemieux Centers for Patient Care and
Research, and the 25 Club of Magee-Women’s Hospital.
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