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From the Servizio Malattie Neuro-Muscolari,*
Dipartimento di Pediatria, Università di Genova, Istituto G.
Gaslini, Genova, Italy; Department of Molecular
Pharmacology,
Albert Einstein College of
Medicine, Bronx, New York; Dipartimento di Medicina Sperimentale e
Patologia,
Università di Roma "La
Sapienza," Roma, Italy; Dipartimento di Medicina
Sperimentale,
Università dell Aquila,
LAquila, Italy; Departments of Neurology and
Pathology,¶
Columbia University, New York,
New York
| Abstract |
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Caveolins are members of a gene family of 21- to 25-kd integral membrane proteins, that are the principal protein components of caveolar membranes. Caveolins play an important structural role in the formation of caveolar membranes, by acting as scaffolding proteins to organize and concentrate specific caveolin-interacting lipids and proteins within caveolae microdomains.5-9 So far, three different caveolins have been recognized, termed caveolin-1, -2 and -3, which are the products of three different genes.10-14
Caveolin-3, the most recently recognized member of caveolin gene family, shows a muscle-specific tissue distribution and is the major caveolar protein of differentiated skeletal muscle cells. The expression of caveolin-3 is developmentally regulated, and the protein localizes to the sarcolemma in fully differentiated muscle fibers, where it interacts with dystrophin and with the dystrophin-associated glycoproteins.12-14
We have recently identified an autosomal dominant form of limb girdle muscular dystrophy (LGMD-1C) that is due to caveolin-3 deficiency and CAV3 mutations. Analysis of genomic DNA revealed two distinct mutations in CAV3: a 9-base pair microdeletion that removes the sequence 63TFT65 from the caveolin scaffolding domain and a missense mutation that changes a proline to a leucine (P105L) in the transmembrane domain. Both mutations leaded to a severe deficiency (95%) of the caveolin-3 protein.15
It has long been hypothesized that muscle cell caveolae may play a role in the formation or organization of the T-tubule system. In agreement with this concept, recent studies on developing muscle have demonstrated that caveolin-3 associates with vescicles which eventually fuse to form the T-tubule openings at the surface of developing myotubes.16
To gain further insight into the distribution of caveolae and to evaluate the organization of the T-tubule system in caveolin-3 deficient muscle, we applied electron microscopic techniques, including freeze-fracture and lanthanum staining, to muscle biopsies from four patients with LGMD-1C and caveolin-3 deficiency.
| Materials and Methods |
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We studied muscle biopsies (quadriceps) from four patients (case
1, age 45; case 2, age 15; case 3, age 13; case 4, age 10) from two
different families with different LGMD-1C mutations. The clinical and
molecular genetic aspects of these patients, together with other
members of their families, were previously reported.15
Two
different mutations in CAV3 gene were found, a microdeletion in the
scaffolding domain (patients 1 and 2) and a missense mutation in the
membrane-spanning region (patients 3 and 4), leading to a severe
caveolin-3 deficiency (
95%) in muscle fibers. As controls, we used
samples of quadriceps muscle from four subjects of comparable age, with
normal muscle histology. All samples were frozen in liquid
nitrogen-cooled isopentane, sectioned for diagnostic purposes,
and stored in liquid nitrogen until this study. A portion of each
sample was fixed in glutaraldehyde and processed for ultrastructural
examination as described below.
Immunohistochemistry
We used a monoclonal antibody directed against caveolin-3 (cl26) that was generated with a synthetic peptide corresponding to amino acids 3 to 24 of the rat caveolin-3 protein sequence. The immunological and immunohistochemical characteristics of this antibody have been previously described in detail.14 For immunohistochemistry, 4 nm-thick unfixed serial frozen muscle sections were incubated with anti-caveolin-3 mAb diluted 1:1000 in phosphate-buffered saline, and processed for immunofluorescence microscopy as previously described.15
Ultrastructural Study
For routine electron microscopy, samples were fixed in 2.5% glutaraldheyde, processed and embedded in epon-araldite. Ultrathin sections were cut using a Leica Ultracut UCT ultramicrotome, stained with uranyl acetate and lead citrate, and observed by a Zeiss 110 electron microscope. For lanthanum staining, small biopsy samples were fixed in 2.5% glutaraldheyde-2% paraformaldheyde in 0.1 mol/L cacodylate buffer, pH 7.2 for 30 minutes. Specimens were then reduced to 1 mm x 2 mm pieces, washed three times in buffer, and rinsed overnight at 4°C in 0.5 mol/L cacodylate buffer. The specimens were postfixed at room temperature by vibratory agitation for 2 hours in a medium containing 1.3% osmiun tetroxide in 0.2 s-collidine buffer at pH 7.2 and 2% lanthanum nitrate. They were rapidly dehydrated, washed in propylene oxide 100%, and finally embedded in epon-araldite.
Freeze-Fracture Analysis
The specimens were removed at rest length in a U-shaped muscle clamp or attached to a tick and fixed immediately in 3% glutaraldehyde in 0.1 mol/L phosphate buffer. The samples were cut into small blocks and gradually infiltrated with glycerol up to a concentration of 30% and then frozen in freon-liquid nitrogen. The specimens were fractured at -110°C in a freeze-fracture apparatus at a vacuum of 6 x 10-7 mmHg and immediately replicated with platinum and carbon using electron beam guns. The tissue was digested in sodium hypoclorite. The detached replicas were washed three times in distilled water and finally picked up on Formvar coated grids and examined/photographed using a Zeiss EM-10 transmission electron microscope.
Image Analysis and Quantitation
Tissue sections were examined using a Leitz Diaplan optical microscope equipped with epi-illumination. Images were obtained with an Optronics DEI-750 digital camera connected to a Power Macintosh. Image analysis was performed using Image Pro-Plus software. For electron microscopy and freeze fractures studies, pictures were scanned and analyzed using an Image Pro-Plus program. The density of caveolae was determined by counting the number of caveolae on the P face in at least 30 different fields from different fibers for each sample; 10 different samples were freeze-fractured from each muscle biopsy. The number of caveolae was expressed per square micrometer. Statistical analysis was performed using the two-tailed paired t-test.
| Results |
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In cross sections, immunofluorescent staining of normal control
muscle showed that caveolin-3 is localized at the sarcolemma of human
muscle fibers (Figure 1B)
. In
longitudinal sections and using high-resolution immunofluorescence,
caveolin-3 showed an organized network at the cell surface. This
network consisted of a set of major bands running along the
longitudinal axis of the fibers and a set of minor interconnecting
transverse bands (Figure 1A)
. In contrast, muscle sections prepared
from LGMD-1C patients showed a severe reduction in caveolin-3 staining
at the sarcolemma (Figure 1C)
. This caveolin-3 deficiency was
previously confirmed by immunoblot analysis (data not shown, see
Reference 15).
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In normal control muscle fibers, caveolae appeared as
flask-shaped profiles located beneath the plasma membrane (Figure 2A)
. Muscle samples from LGMD-1C patients
showed that caveolae were virtually absent from most muscle fibers.
Only in a few areas we could detect isolated caveolae-like structures
adjacent to the muscle plasma membrane. Quantitative analysis of more
than 30 different fields revealed that the number of caveolae was less
than 5% in muscle fibers from LGMD-1C patients. Furthermore, LGMD-1C
fibers contained in several areas large vacuoles close to the cell
surface. These vacuolated structures revealed the presence of a
membrane at the outskirt and were generally empty, or partially
containing amorphous material (Figure 2, B and C)
.
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To evaluate the number and distribution of caveolae in the plasma
membrane of the muscle fibers, we examined the two leaflets of the
surface membrane, the protoplasmic leaflet (P face) and the external
leaflet (E face). In normal muscle plasma membrane, both the P and E
faces contained caveolae which appeared relatively uniform in size and
shape, and they were characteristically distributed in rows of
longitudinal and transverse bands (Figure 3A)
. This highly organized array of
caveolae seen in freeze-fracture preparations appeared to correspond to
the pattern of longitudinal and transverse bands that we observed by
high-resolution immunofluorescence (Figure 1B)
. The mean density of
caveolae in normal muscle plasma membrane was 9.68 (SD ± 2.05)
per square micron. In sharp contrast, in LGMD-1C muscle plasma membrane
we observed very few caveolae in both P and E faces of the membrane.
(Figure 3, B and C)
. Quantitative analysis showed that the number of
caveolae in LGMD-1C muscle fibers was less than 5%, as compared to
normal control muscle.
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To study the organization of the T-tubule system in muscle fibers, we used lanthanum nitrate staining combined with transmission electron microscopy. Lanthanum is an electron-dense tracer that, in muscle, stains the extracellular space, and also outlines the caveolae and the T-system for ultrastructural studies.17,18 Previous reports have shown that lanthanum does not penetrate the sarcoplasmic reticulum, except in specific pathological conditions such as thermal injury.19
In our experiments, muscle samples from normal controls and from
LGMD-1C patients were pre-treated with lanthanum nitrate, and then
processed for electron microscopy. In normal muscle, the extracellular
space as well as the caveolae were labeled with electron-dense
material. The T-tubules were also stained by lanthanum and were
observed at the I and A band junction (Figure 4A)
.
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For comparative purposes and to exclude the possibility that our observations may be due to technical artifacts, we also examined by lanthanum staining muscle biopsies from two patients affected with dermatomyositis. It has been reported that in this disorder, the T-system is generally abnormal and that it proliferates in affected muscle fibers.20 In these biopsies, we found disorganization and proliferation of the T-tubules, but we did not detect lanthanum-positive large vacuoles beneath the plasma membrane, similar to the structures we noted in LGMD-1C muscle fibers (data not shown).
| Discussion |
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During the process of caveolae formation, caveolin undergoes two stages of self-association or oligomerization. First, shortly after caveolin synthesis, caveolin oligomerizes in the endoplasmic reticulum to form homo-oligomers of 300 to 350 kd, each containing approximately 14 to 16 individual caveolin monomers. Second, at a later stage, these caveolin homo-oligomers can interact with each other to form clusters of caveolin oligomers that are approximately 25 to 50 nm in diameter. Thus, through the interaction of caveolin with itself and the caveolin-mediated selection of endogenous lipid components, a caveolae-sized vesicle is generated.6-8
We demonstrated that heterozygous mutations in CAV3 gene are associated with a severe caveolin-3 deficiency at the cell surface.15 The molecular mechanisms underlying this deficiency have not yet been elucidated. A possible mechanism is that the heterozygous mutations exert a dominant-negative effect that induces rapid degradation of both the wild-type and mutant caveolin-3 proteins. Consistent with this hypothesis, we investigated the phenotypic behavior of these caveolin-3 mutations using heterologous expression in NIH 3T3 cells. The results show that LGMD-1C mutants of caveolin-3 form unstable high molecular mass aggregates of caveolin-3, which are retained within the Golgi complex and are not targeted to the plasma membrane.21 These data provide a molecular explanation for why caveolin-3 levels are down-regulated in patients with LGMD-1C.
Caveolin-3 deficient mutant mice, designed to disrupt exon 2 of the caveolin-3 gene, show a reduction of caveolae at the plasma membrane.22 Interestingly, the pathogenetic mechanisms leading to the impairment of caveolae formation at the cell surface seem to be different in the mutant mice as compared with humans. In fact, heterozygous mutant mice do not behave in a dominant negative fashion and only homozygous mutants reveal a severe impairment of caveolae formation. Furthermore, no apparent muscle degeneration was observed in these mutants.22 In contrast, here we demonstrate that heterozygous mutations of the CAV-3 gene, involving the caveolin scaffolding and membrane spanning domains, exert in a significant down-regulation of caveolin-3 protein, leading to a severe impairment of the formation of caveolae at the cell membrane of muscle fibers from LGMD-1C patients.
Electron microscopic studies have shown that caveolae-like structures are associated with forming T-tubules at an early developmental stage.23 Several morphological studies have demonstrated that caveolae initially form tubular structures that later associate to develop into a larger three-dimensional tubular network.23,24 Furthermore, it has been recently reported that caveolin-3 transiently associates with T-tubules during development and may be involved in the early development of the T-tubule system in muscle.16,25,26 In mature muscle fibers, however, caveolin-3 is no longer detectable within the T-tubule system, but is highly concentrated in sarcolemmal caveolae.25 These results suggest that a functional relationship may exist between caveolin-3 expression, caveolae formation, and T-tubule biogenesis.
We have recently reported that caveolin-3 null mice, lacking caveolin-3 protein expression and sarcolemmal caveolae membranes, show T-tubule abnormalities in muscle fibers.27 Thus, we examined here the possible consequences of caveolin-3 deficiency on the formation of T-tubule domains in caveolin-3 deficient muscle fibers of LGMD-1C patients. We found the presence of abnormal large vacuolated and membranous structures in the subsarcolemmal area, associated with an abnormal proliferation of T-tubule like structures. These alterations were independent from the age of the patients, varying from 10 to 45 years, and were more severe than those we observed in muscle from caveolin-3 null mice.
In conclusion, our data suggest that LGMD-1C muscle fibers with severe caveolin-3 deficiency show an impairment of caveolae formation at the cell surface and a disorganization of the T-system openings at the subsarcolemmal level. Because the T-system plays an important role in muscle contraction and relaxation by regulating the distribution of the Ca2+ ions, this alteration may explain, at least in part, the pathogenesis of muscle weakness in LGMD-1C patients.
It has been recently reported that mutations in the CAV3 gene may cause the mechanical hyperirritability of skeletal muscle seen in rippling muscle disease.28 However, the pathogenetic mechanism(s) underlying this disorder still remain to be elucidated. Further studies will be necessary to evaluate if the alterations we find in LGMD1C muscle fibers are also present in other clinical phenotypes due to CAV3 mutations, such as isolated hyperCKemia and rippling muscle disease.
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| Footnotes |
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Supported by grants from Telethon-Italy (GP0271/01), and the Italian Ministry of Health (G. Gaslini Institute, Ricerca Finalizzata). F.S. was supported by a fellowship from Telethon-Italy. M.P.L. was supported by grants from the National Institutes of Health, the Komen Breast Cancer Foundation, the American Heart Association, and The Muscular Dystrophy Association. M.P.L. is a the recipient of a Scholar award from the Irma T. Hirschl/Monique Weil-Caulier Trust.
Accepted for publication October 9, 2001.
| References |
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