* Department of Molecular Genetics and Cell Biology, Department of Medicine, Section of Cardiology, and § Department of
Pathology, University of Chicago, Chicago, Illinois 60637
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Abstract |
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-Sarcoglycan is a transmembrane, dystrophin-associated protein expressed in skeletal and cardiac muscle. The murine
-sarcoglycan gene was disrupted using homologous recombination. Mice lacking
-sarcoglycan showed pronounced dystrophic muscle
changes in early life. By 20 wk of age, these mice developed cardiomyopathy and died prematurely. The loss
of
-sarcoglycan produced secondary reduction of
-
and
-sarcoglycan with partial retention of
- and
-sarcoglycan, suggesting that
-,
-, and
-sarcoglycan
function as a unit. Importantly, mice lacking
-sarco-
glycan showed normal dystrophin content and local-
ization, demonstrating that myofiber degeneration
occurred independently of dystrophin alteration. Furthermore,
-dystroglycan and laminin were left intact,
implying that the dystrophin-dystroglycan-laminin
mechanical link was unaffected by sarcoglycan deficiency. Apoptotic myonuclei were abundant in skeletal
muscle lacking
-sarcoglycan, suggesting that programmed cell death contributes to myofiber degeneration. Vital staining with Evans blue dye revealed that
muscle lacking
-sarcoglycan developed membrane disruptions like those seen in dystrophin-deficient muscle.
Our data demonstrate that sarcoglycan loss was sufficient, and that dystrophin loss was not necessary to
cause membrane defects and apoptosis. As a common
molecular feature in a variety of muscular dystrophies,
sarcoglycan loss is a likely mediator of pathology.
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Introduction |
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PROGRESS in human genetics has identified a number
of proteins at the muscle membrane that are associated with muscular dystrophy. Dystrophin, the
protein product of the Duchenne muscular dystrophy (DMD)1 gene, is an elongated cytoskeletal protein that
binds actin and a complex of dystrophin-associated proteins, forming the dystrophin-glycoprotein complex (DGC;
Burghes et al., 1987; Campbell and Kahl, 1989
; Ervasti and Campbell, 1991
; Koenig et al., 1987
; Monaco et al.,
1986
; Rybakova et al., 1996
; Yoshida and Ozawa, 1990
).
More recently, a subcomplex of the DGC, sarcoglycan,
was associated with mutations that result in human autosomal recessive muscular dystrophy (Bönnemann et al.,
1996
a; Campbell, 1995
; Duggan et al., 1997
; Straub and Campbell, 1997
). There are at least five subunits of
sarcoglycan:
,
,
,
, and
(Ettinger et al., 1997
; Lim et al.,
1995
; McNally et al., 1998
; Nigro et al., 1996
; Noguchi et al.,
1995
; Roberds et al., 1993
). Three of these sarcoglycans
,
, and
share homology as type II transmembrane
proteins with cysteine-rich EGF-like repeats in their extracellular domains. Mutations in any single sarcoglycan gene
result in varying degrees of secondary reduction of the
other sarcoglycan units, suggesting interaction between
the sarcoglycan subunits.
-sarcoglycan is a type I transmembrane protein expressed exclusively in skeletal and
cardiac muscle. A widely expressed fifth sarcoglycan,
-sarcoglycan, was recently identified with significant homology to
-sarcoglycan (Ettinger et al., 1997
; McNally et
al., 1998
).
-sarcoglycan is present in skeletal and cardiac
muscle, but is also expressed in most tissues studied, demonstrating that sarcoglycan-like proteins are present in
nonmuscle tissues.
In the presence of -D-octyl-glycoside, the sarcoglycan
subunits can be separated from the remainder of the
DGC, but the exact interaction between sarcoglycan and
the remainder of the DGC, particularly dystrophin, is ill-defined (Ozawa et al., 1995
; Yoshida et al., 1994
). Studies
of human muscle biopsies have provided limited information regarding the nature of the sarcoglycan-dystrophin interaction. Quantitative assessment of dystrophin content
showed a secondary decrease of dystrophin content in patients with mutations in
-,
-, and
-sarcoglycan; the reduction in dystrophin content ranged from 40 to 90% of
normal (Vainzof et al., 1996
). This finding suggests that a
secondary decrease in dystrophin may be responsible for
the pathology of muscular dystrophy in patients with primary sarcoglycan mutations.
In addition to the sarcoglycans, the DGC includes dystroglycan, a highly glycosylated protein whose subunit
directly binds the extracellular matrix (ECM) protein
laminin (Ibraghimov-Beskrovnaya et al., 1992
). Dystroglycan is widely expressed, and mice homozygously lacking
dystroglycan have an early embryonic lethality (day 6.5 after conception) arising from the role of dystroglycan in
nonmuscle tissue, particularly the extraembryonic structure Reichert's membrane (Williamson et al., 1997
). Dystroglycan binds laminin-
2, an extracellular matrix protein
expressed in peripheral nerve, skeletal, and cardiac muscle
(Ervasti and Campbell, 1993
; Yamada et al., 1994
). In humans, laminin-
2 (merosin) mutations have been associated with a severe phenotype, congenital muscular dystrophy (Helbling-Leclerc et al., 1995
; Nissinen et al., 1996
). A
murine model for laminin-
2 deficiency, the dy mouse,
demonstrates a severe phenotype resulting in marked skeletal muscle degeneration and early death (Sunada et al.,
1994
; Xu et al., 1994
). In contrast, the murine model for
dystrophin deficiency, the mdx mouse (Sicinski et al.,
1989
), has a milder phenotype than its human counterpart.
mdx mice have a normal lifespan, but demonstrate many
of the histologic features seen in human muscular dystrophy, including fiber size variation and centrally placed nuclei. Human DMD patients exhibit replacement of myofibers with fatty and fibrous infiltration within the first decade, yet mdx mice show progressive fibrosis relatively
later in life. Although the mdx mouse has a milder phenotype than human DMD patients, it has proved useful for
studies identifying the nature of the membrane defect in
muscular dystrophy. Vital staining studies of mdx and dy
mice suggested different mechanisms of membrane degeneration since dystrophin deficiency produces membrane
defects, while laminin deficiency did not (Matsuda et al.,
1995
; Straub et al., 1997
). This observation and its cytoskeletal localization has lead to the hypothesis that dystrophin participates in a mechanical link that stabilizes muscle membrane. Contraction-induced force exerted on a
membrane defective in dystrophin is thought to lead directly to membrane disruptions and myofiber degeneration.
To investigate the role of sarcoglycan in muscular dystrophy, we targeted the murine -sarcoglycan gene. In
early life, mice lacking
-sarcoglycan developed muscular
dystrophy that preferentially affected the proximal musculature. Mice lacking
-sarcoglycan developed cardiomyopathy that affected both the right and left ventricle. Vital
staining of mice lacking
-sarcoglycan revealed membrane
disruptions demonstrating that sarcoglycan deficiency has
a similar mechanism to dystrophin-deficiency and is unlike laminin deficiency. Membrane disruptions in
-sarcoglycan-deficient muscle were primarily observed in proximal
skeletal muscles similar to humans with limb-girdle muscular dystrophy (LGMD). Moreover, apoptotic myonuclei
were abundant in
-sarcoglycan-deficient muscle. Importantly, dystrophin, dystroglycan, and laminin were intact and normally present at the sarcolemma in
-sarcoglycan-
deficient muscle. Therefore,
-sarcoglycan deficiency was
sufficient to cause muscle membrane instability and the
dystrophic process independently of dystrophin, placing
sarcoglycan genetically downstream of dystrophin.
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Materials and Methods |
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Gene Targeting
Genomic phage encoding exon 2 of -sarcoglycan was isolated from a murine 129SVJ library (Stratagene, La Jolla, CA) and characterized by restriction mapping, nucleotide sequencing, long-range PCR, and Southern
hybridization. 5 kb of the first intron and 1.7 kb of the second intron were
cloned into pPNT (Tybulewicz et al., 1991
). Targeted RW4 ES cells
were selected as described previously (Scott et al., 1994
). Recombinants
were screened by PCR and confirmed by Southern blotting of EcoRI-digested genomic DNA using a probe shown in Fig. 1 a. Recombinant ES
cell clones were injected into C57BL/6 blastocysts and implanted into
pseudopregnant foster mothers. Chimeric males were identified by coat
color, and were mated to C57BL/6 females. Germline transmission was
identified by coat color, and was confirmed by PCR and Southern blotting. PCR genotyping of progeny was performed on genomic DNA isolated from tail clippings using the primers shown in Fig. 1 a. Animals were
housed and treated according to standards set by the University of Chicago Institutional Animal Care and Use Committee.
|
Immunoblotting
Immunoblots to detect -sarcoglycan protein were performed as described previously (Nicholson et al., 1989
), using either affinity-purified
polyclonal (McNally et al., 1996
) or monoclonal (NCL-35DAG; NovoCastra Laboratories, Newcastle upon Tyne, UK) anti-
-sarcoglycan.
General Characterization, Vital Staining, and Terminal Deoxynucleotidyl Transferase-mediated dUTP Nick End-labeling (TUNEL) Assay
Animals were observed for gait abnormalities weekly through 24 wk of
age. Serum creatine kinase (CK) levels were measured on blood drawn
from tail veins at 4 (n = 24) and 8 (n = 24) wk of age. Blood was collected
in a Microstainer Serum Separator tube (Becton Dickinson, Franklin
Lakes, NJ) and analyzed using a Vitros DT60II discrete chemistry analyzer (Eastman Kodak, Rochester, NY). Vital staining was performed as
described previously using filter-sterilized Evans blue dye (Sigma-Aldrich,
St. Louis, MO) resuspended in PBS at 1 mg/0.1 ml/10 g body weight (Matsuda et al., 1995; Straub et al., 1997
). Intraperitoneal injections were performed on mice at 2 (n = 5), 4 (n = 4) and 8 (n = 4) wk of age; mice were
killed 12 h after injection. TUNEL was performed on fixed, paraffin-embedded muscle using the ApopTagTM-fluorescein kit (Oncor Inc.,
Gaithersburg, MD). Double-labeling with polyclonal anti-dystrophin antibodies was performed after TUNEL-labeling, as described below.
Histology and Immunocytochemistry
Muscle from 4-, 8-, and 20-wk-old animals was fixed for 24-48 h in 10%
neutral buffered formalin and embedded in paraffin. 10-µM sections were
cut and stained with Masson trichrome. Immunostaining was performed
as described (McNally et al., 1996). The following primary antibodies
were used: polyclonal anti-dystrophin AB6-10 (Lidov et al., 1990
), polyclonal anti-
-sarcoglycan (McNally et al., 1996
), polyclonal anti-
-sarcoglycan (Bönnemann et al., 1996
b), monoclonal anti-
and
-sarcoglycans
(NCL-50DAG and NCL-35DAG, respectively), monoclonal anti-
-dystroglycan (NCL-
-DG), monoconal anti-utrophin (NCL-DRP2), and monoclonal anti-merosin (NCL-merosin; NovoCastra Laboratories). Polyclonal
anti-
-sarcoglycan was raised to a glutathione-S-transferase fusion protein
containing amino acids 85-291 of the human protein (Genbank number
X95191), as previously described (McNally et al., 1996
). Polyclonal anti-
-sarcoglycan was raised similarly using amino acid residues 31-413 of the
human
-sarcoglycan sequence (Genbank no. AF036364). Cy3-conjugated secondary antibodies (Jackson ImmunoResearch Laboratories, West Grove, PA) were used. Integrin-
1 antibody (MAB1997) was from Chemicon International, Inc. (Temecula, CA). Counterstaining with 4',6-diamidino-2-phenylindole (DAPI) was performed at 20 µg/ml for 1 min.
Sections were photographed on a Zeiss Axiophot epifluorescence microscope.
Electron Microscopy
Mice were perfusion-fixed with 2% paraformaldehyde and 2% gluteraldehyde. Skeletal muscle was processed for EM by postfixation with 1% osmium tetrozide in water for 1 h, washed in water, and dehydrated with ethanol and propylene oxide. Dehydrated tissues were embedded in plastic, sectioned at 60 nm, and stained with 5% uranyl acetate and 1% lead. Sections were viewed and photographed on a Hitachi 600 EM.
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Results |
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Targeted Disruption of -Sarcoglycan and Generation
of Null Mice
Given the large size of the -sarcoglycan gene, exon 2 was
chosen for targeting. Exon 2 specifies the initiator methionine, the cytoplasmic tail, and the transmembrane domain. Homologous recombination replaces exon 2 with
the phosphoglycerate kinase promoter/neomycin phosphotransferase cDNA (Fig. 1 a). A representative Southern blot of a targeted ES cell clone is shown in Fig. 1 b. Heterozygous gsg+/
mice were viable, fertile, and appeared healthy. gsg+/
mice were bred to obtain gsg
/
mice (Fig. 1 c). Immunoblot analysis showed no detectable
-sarcoglycan protein in homozygous mutant gsg
/
muscle (Fig. 1 d), indicating that this mutation created a null allele. Notably, immunoblotting for dystrophin revealed
that normal amounts of dystrophin remain in
-sarcoglycan-deficient muscle (Fig. 1 d).
-Sarcoglycan-deficient Mice Develop
Muscular Dystrophy
Most mutant gsg/
mice were of normal size and appearance, but displayed a variably abnormal gait that was characterized by a stiff walking posture and a widened hind-leg
spacing (Fig. 2 a). Less than 10% (4 of 50) of mutant gsg
/
mice had stunted growth or died as early as 3 wk of age.
By 5 mo of age, 50% of gsg
/
animals died (Fig. 2 b). The
average weight of mutant, heterozygous, and wild-type
mice did not differ significantly at 8 wk of age. However,
by 20 wk of age mutant mice weighed less than wild-type (28.3 ± 1.9 vs. 34.1 ± 0.3 g). Mutant mice were less active
than wild-type littermates, exhibited less climbing and burrowing activity, and were slow to initiate walking or running from a sitting position. Elevation of serum CK, indicating disruption of muscle membrane integrity, was
detected at 2 wk of age (1.7× wild-type controls). By 4 wk
of age, serum CK levels were elevated in gsg
/
mice to an
average of ten times that seen in normal mice (Fig. 2 c).
This elevation continued in 8-wk-old gsg
/
mice and
ranged from 3-240× the serum CK levels of normal controls (Fig. 2 c).
|
Diaphragm muscle from gsg/
mice appeared hypertrophied and opaque when compared with gsg+/+ and gsg+/
littermates. The thickness of gsg
/
diaphragm muscle was
greater than that of wild-type controls without an increase
in myofiber size or content (Fig. 3 a). Such pseudohypertrophy is a hallmark of progressive proximal muscular dystrophy, and commonly affects the gastrocnemius muscles
of patients with LGMD and DMD (Dubowitz, 1995
). Histologic analyses of muscle from gsg
/
mice revealed severe dystrophic changes, including wide variation in fiber
size, inflammatory infiltrate, and abnormal calcification with fatty and fibrous replacement. Dystrophic changes
were evident throughout the entire diaphragm muscle of
gsg
/
mice (Fig. 3 a). Quadriceps and gastrocnemius muscle also showed pronounced dystrophic changes (Fig. 3, b
and c). In these muscles, unlike the diaphragm, regions of
marked degeneration were juxtaposed with segments of
intact myofibers. Most of these intact myofibers, however,
had centrally placed nuclei consistent with regeneration having occurred. The number of centrally placed nuclei in
8-wk-old gsg
/
quadriceps muscle was greatly increased
(45×) above wild-type littermate controls (Figs. 2 d and 3 d).
|
Cardiomyopathy in gsg/
Mice
The hearts of 20-wk-old gsg/
mice were examined. Both
the right and left ventricular walls appeared thickened, resulting in near obliteration of the ventricular cavities. gsg
/
hearts weighed less than wild-type hearts (167 ± 17 vs. 190 ± 0 mg, but total body mass was decreased in gsg
/
mice).
The heart-to-body mass index was increased (5.91 ± 0.30 mg/g for gsg
/
vs. 5.57 ± 0.05 mg/g for wild-type animals).
Mice examined at earlier time points, including 4 and 8 wk
of age, did not show the same degree of cardiomyopathy
(data not shown). Fig. 4 a shows representative hearts
from 20-wk-old gsg
/
animals that were killed for the
study. Mutant hearts showed marked thickening of the
ventricular walls; the heart in Fig. 4 a also showed significant fibrosis. Fibrosis was evident throughout the right and
left ventricle, but a prominent fibrotic area was present in
the posterior segment of the left ventricle (Fig. 4 c).
Perivascular regions of fibrosis were also seen (Fig. 4 d). Fibrosis was not seen in gsg
/
hearts at 4 and 8 wk of age
(data not shown). Of a cohort of ten gsg
/
mice, five died
before 5 mo of age. Postmortem analysis of these gsg
/
mice revealed areas of ventricular fibrosis that were visible on gross examination (data not shown).
|
Sarcoglycan Loss Causes Muscular Dystrophy Without Alteration in Dystrophin
Immunostaining of -sarcoglycan-deficient and dystrophin-deficient skeletal muscle is shown in Fig. 5 a. mdx
muscle showed a marked secondary deficiency of
-,
-,
and
-sarcoglycan in addition to the primary defect of dystrophin loss. In gsg
/
muscle, dystrophin staining is intact,
but
- and
-sarcoglycan show severe secondary reduction
at the sarcolemma. In mdx muscle, revertant fibers are
commonly seen that reexpress dystrophin. Dystrophin-positive revertant fibers in mdx also reexpress sarcoglycan as shown for
-sarcoglycan (Fig. 5 a, bottom middle). No
such revertant phenomenon is seen in gsg
/
muscle. Partial retention of
-sarcoglycan at the sarcolemma was seen
(Fig. 5 b, top). Immunoblotting for
-sarcoglycan shows that 10-30% of
-sarcoglycan is expressed in gsg
/
muscle (Fig. 5 c). Normal levels of
-sarcoglycan are present in
gsg
/
muscle, yet
-sarcoglycan is completely absent in mdx
muscle.
-dystroglycan and laminin-
2 (merosin) staining
was intact at the sarcolemma of gsg
/
muscle. In muscle
lacking dystrophin, the dystrophin-related protein utrophin is upregulated to compensate for the lack of dystrophin protein (Helliwell et al., 1992
; Karpati et al., 1993
; Love et al., 1989
). Utrophin staining in gsg
/
muscle is
unchanged from wild-type controls, consistent with a normal dystrophin content in
-sarcoglycan-deficient animals. Integrin-
1 was also unchanged in gsg
/
muscle.
|
Membrane Defects in Muscular Dystrophy
Evans blue dye (EBD) is a small molecular mass tracer
that tightly complexes with serum albumin. Normal muscle fibers are impermeable to the EBD-albumin complex,
while mdx muscle fibers are positively stained, reflecting a
loss of membrane integrity (Matsuda et al., 1995; Straub
et al., 1997
). EBD was injected into mutant and normal
mice to determine the extent sarcolemmal disruption as a
result of sarcoglycan deficiency. No EBD staining was
seen in gsg+/+ and gsg+/
animals at any age (data not
shown). In contrast, 2-wk-old gsg
/
mice showed EBD
staining of the diaphragm, shoulder girdle, and pectoralis
major muscles on gross examination (data not shown). By
4 wk of age, gsg
/
animals displayed significant staining in
nearly all muscle groups examined. Bands of EBD-positive tissue gave the gsg
/
diaphragm a striking, radially
streaked appearance not seen in wild-type littermate controls (Fig. 6 a). The adductor muscles of the leg were commonly EBD-positive, as were the pectoralis major, latissimus dorsi, rectus abdominus, external oblique, and triceps
brachii muscles. Mutant muscles lacking macroscopic evidence of EBD uptake showed microscopic evidence of
EBD-positive fiber clusters demonstrating the extensive
nature of the dystrophic process in gsg
/
mice (Fig. 6 b).
|
Myofiber Loss is Mediated by Apoptosis
To investigate the mechanism of myofiber degeneration in
-sarcoglycan deficiency, TUNEL was performed on
quadriceps muscle from 8-wk-old gsg+/+, gsg+/
, and gsg
/
mice. No TUNEL-positive myonuclei were detected in
gsg+/+ and gsg+/
skeletal muscle (Fig. 6 c, left). In contrast, TUNEL-positive nuclei were commonly found in
gsg
/
muscle (Fig. 6 c, right), indicating that apoptosis
contributed to muscle degeneration in
-sarcoglycan-deficient muscular dystrophy. TUNEL-positive nuclei were
most commonly seen in degenerating areas of gsg
/
muscle, but were seen also in intact or partially degenerated muscle fibers (Fig. 6 d). The presence of TUNEL-positive
myonuclei in intact, dystrophin-positive muscle fibers suggests that sarcoglycan deficiency leads to apoptosis as an
early event in the dystrophic process.
Normal Sarcomeres, Neuromuscular Junctions, and
Basement Membrane in -Sarcoglycan Deficiency
The sarcoglycans are found throughout the sarcolemma,
and are not excluded at the neuromuscular junctions. The
sarcomeres, membrane, and neuromuscular junctions of
gsg/
mice were examined by EM and compared with
wild-type littermates. No ultrastructural differences were
seen in the sarcomeres, membrane, or neuromuscular
junctions of gsg
/
mice (Fig. 7), suggesting that membrane disruptions visualized by EBD staining are below
the limit of resolution of EM. Gross ultrastructural abnormalities are not a feature of sarcoglycan deficiency.
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Discussion |
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-Sarcoglycan is Critical for Cardiac and
Skeletal Muscle
We have developed mice lacking -sarcoglycan through
homologous recombination in embryonic stem cells. Unlike mdx mice that display a more mild phenotype than
DMD patients, mice lacking
-sarcoglycan develop a phenotype that closely parallels the human disease exhibiting
the clinical and histopathologic features of LGMD, including pseudohypertrophy, elevated serum CK, degeneration, and regeneration of skeletal muscle. Furthermore, gsg
/
mice develop marked cardiomyopathy that is likely to play
a role in their early death. The degree of left ventricular
wall thickening is consistent with an intrinsic cardiomyopathy that is likely hypertrophic in nature. The presence of
marked fibrosis in the posterior segment of the left ventricle likely arises from similar pathogenetic mechanisms
that affect skeletal muscle in sarcoglycan deficiency. It is
possible that wall thickening seen in the right ventricle of
gsg
/
hearts arises in part as a secondary consequence of
restrictive lung disease and elevated pulmonary vascular
resistance. The marked dystrophic process that affects the
respiratory muscles, particularly the diaphragm, may lead
to a restrictive lung disease and hence, a secondary cardiomyopathy.
The phenotype in mice lacking -sarcoglycan is similar
to that seen in the Syrian hamster, BIO 14.6, that displays
both cardiomyopathy and muscular dystrophy (Homburger et al., 1962
). The BIO 14.6 hamster is a spontaneously
arising mutant that has a large, not fully defined deletion
that encompasses part of an alternative first exon of the
hamster
-sarcoglycan gene (Nigro et al., 1997
; Sakamoto
et al., 1997
). Recently, membrane defects as observed by
EBD staining were described in the BIO 14.6 hamster
(Holt et al., 1998
). The similar phenotype in gsg
/
mice and
the BIO 14.6 hamster argues that both
- and
-sarcoglycan have critical and nonredundant roles for cardiac and
skeletal muscle. The cardiomyopathic phenotype of gsg
/
mice is strikingly different from that seen in mdx mice
since mdx mice display little to no cardiomyopathy. This
result further suggests that the sarcoglycans, particularly
- and
-sarcoglycan, are critical for cardiac muscle function.
gsg/
mice display a more severe skeletal muscle phenotype than mdx mice since gsg
/
mice show a greater serum CK elevation, early and more extensive fibrosis, and a
higher percentage of centrally placed nuclei (McArdle et al.,
1994
; Deconinck et al., 1997
). mdx mice show a period of
intense muscle necrosis at 3-8 wk of age followed by a relative plateau period. gsg
/
mice show marked necrosis at
3-8 wk of age. Areas of necrosis were commonly seen in
gsg
/
muscle from older animals, but regions of necrosis
appeared less often than in 8-wk animals. Serum CK in
gsg
/
animals peaks at 8 wk of age, but remain grossly elevated at 16 wk of age at levels twice that seen in mdx
(data not shown).
gsg/
mice die at a similar age as the recently described
double knockout mice that lack both dystrophin and utrophin (Deconinck et al., 1997
; Grady et al., 1997
). However,
gsg
/
mice do not show gross skeletal abnormalities such
as kyphoscoliosis described in the double knockout mice.
Interactions Within the DGC
The absence of -sarcoglycan produces a nearly complete
secondary reduction of both
- and
-sarcoglycan.
- and
-sarcoglycan mRNA was normally produced in gsg
/
myofibers (data not shown). Thus,
-sarcoglycan is necessary for the proper assembly and membrane localization
of
- and
-sarcoglycan proteins. Curiously,
-sarcoglycan
appears to be partially retained in gsg
/
muscle, suggesting that other components can stabilize
-sarcoglycan at
the membrane.
-sarcoglycan is completely absent in mdx
muscle, while it is produced at normal levels in
-sarcoglycan deficiency. Since
-sarcoglycan expression mirrors dystrophin expression in mdx and gsg
/
muscle,
-sarcoglycan may be more closely associated with dystrophin.
Most importantly, the normal placement of dystrophin,
-dystroglycan, and laminin-
2 (merosin) in
-sarcoglycan-deficient muscle indicates that this cytoskeleton-
membrane-ECM interaction is intact and unaffected by
-sarcoglycan deficiency. The normal ultrastructural appearance of gsg
/
sarcomeres, basement membrane, and
neuromuscular junctions may favor alternative cellular defects such as aberrant cell signaling as a mediator of muscular dystrophy. Sarcoglycan has the primary structure
suggestive of a cell surface receptor. The cytoplasmic domain of
-sarcoglycan has five tyrosine residues, and recent studies imply bidirectional signaling with integrins
may involve the sarcoglycan subunits (Yoshida et al., 1998
).
Identifying the extracellular and intracellular ligands that
normally bind the subunits of sarcoglycan will be an important step in defining any potential cytoskeleton-
sarcoglycan-ECM complex or signaling cascade.
Sarcoglycan Deficiency Mediates Pathology in DMD
In DMD and mdx, primary dystrophin mutations lead to a
secondary reduction or mislocalization of a number of dystrophin-associated proteins, including the sarcoglycan subunits (Ohlendieck and Campbell, 1992; Ohlendieck et al.,
1993
). In mice lacking
-sarcoglycan, the normal content
and location of dystrophin suggests that sarcoglycan deficiency is sufficient to cause muscular dystrophy. It is clear that in gsg
/
muscle, the presence of dystrophin is insufficient to stabilize sarcoglycan, and cannot prevent the dystrophic process in both cardiac and skeletal muscle. Given
the macromolecular nature of the DGC, the absence of
sarcoglycan may lead to abnormal interactions within the
DGC and the presence of muscular dystrophy. Whether restoration of the sarcoglycan complex, specifically
-,
-,
and
-sarcoglycan will be sufficient to prevent the dystrophic phenotype is unknown. Previous data suggested that
restoration of the DGC is not sufficient to prevent the dystrophic phenotype (Cox et al., 1994
; Greenberg et al.,
1994
). However, DGC analysis in those studies was before
the discovery of
-,
-, and
-sarcoglycan. Since sarcoglycan deficiency is the common feature in both DMD and
LGMD, it is a likely mediator of the dystrophic process in both these disorders. Moreover, inhibiting apoptosis or
developing new strategies to stabilize the sarcoglycan subunits, particularly
-,
-, and
-sarcoglycan, are reasonable
therapeutic approaches to the treatment of Duchenne and
the Duchenne-like muscular dystrophies.
![]() |
Footnotes |
---|
Received for publication 20 May 1998 and in revised form 31 July 1998.
A.A. Hack is supported by the Medical Scientist Training Program and the Training Program in Development Biology. E.M. McNally is supported by the National Institues of Health, Muscular Dystrophy Association, Heart Research Foundation, and a Research Resources award from the Howard Hughes Medical Institute to the University of Chicago Biological Sciences Division.We thank Celeste Simon, Renée Hackenmiller, and Jeff Leiden for advice and comments, and Lisa Gottschalk for her help preparing figures.
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Abbreviations used in this paper |
---|
DMD, Duchenne muscular dystrophy; LGMD, limb girdle muscular dystrophy; CK, creatine kinase; EBD, Evans blue dye; DGC, dystrophin-glycoprotein complex; ECM, extracellular matrix.
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References |
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