INTRODUCTION

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

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) 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). 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 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 dystrophin-dystroglycan-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 cytoplasmic subcomplex, the COOH-terminal region of dystrophin is bound to syntrophin, whereas the amino-terminus of dystrophin binds F-actin.

View larger version (42K): [in this window] [in a new window]   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.

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 childhood-lethal 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 contraction-induced 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 mammalian 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 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

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

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 (P_t) approximates that observed in C57BL/10 control muscle (77) whereas specific twitch force (P_t/CSA), i.e., P_t 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 (P_o) is comparable to or exceeds that observed in C57BL/10 muscle (32, 60), but specific peak tetanic force (P_o/CSA), i.e., P_o 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).

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 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).

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 X-linked 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 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.

	SARCOGLYCANOPATHIES

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

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 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

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

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 affected 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 P_t is generally comparable to that observed in F1B hindlimb muscle (10, 68, 95), whereas P_t/CSA is decreased in Bio 14.6 dystrophic hamster muscle relative to control (10, 95). Similarly, absolute P_o is comparable to that of F1B hamster, whereas P_o/CSA is consistently lower in Bio 14.6 dystrophic hamster vs. F1B muscle (10, 95). The lower P_t/CSA and P_o/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.

View larger version (57K): [in this window] [in a new window]   Fig. 2.   Relative changes (% baseline) in isometric force generation of control (F1B) and -sarcoglycan-deficient (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.

	-SG TRANSGENIC MURINE NULL MUTANT

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

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, P_t, P_o, and P_o/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 -SG-deficient 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.

	-SG MURINE NULL MUTANT LINE

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

-SG null mutant transgenic mice (dsg /) have also been generated (41). This model is analogous to 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, P_t, P_o, and P_o/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).

	-SG NULL MUTANT MICE

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

-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 slow-twitch weight-bearing soleus muscle demonstrates P_o 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 tibialis anterior (25) muscles demonstrate P_o values that are comparable to control levels and a resultant P_o/CSA that is significantly lower than control. Fast- and slow-twitch -SG-deficient skeletal muscle also evidences disparate passive stiffness; soleus muscle shows normal resistance to passive stretch, whereas the EDL shows reduced elasticity when compared with control EDL (26). Moreover, -SG-deficient tibialis anterior muscle does not demonstrate enhanced susceptibility to lengthening-activation force deficits (T. L. O'Day, personal observation). A recent preliminary report on -SG-deficient diaphragm demonstrates decreased P_o/CSA under both uniaxial and biaxial loading compared with control muscle (13). Moreover, in contrast to control muscle, biaxial loading was not associated with an increase in maximal diaphragm force, indicating an inability of -SG / diaphragm sarcolemma to transmit forces in the transverse fiber direction (13). This finding suggests that the SG complex is important in transmitting force in the transverse direction to the long axis of muscle. Taken together, these data suggest that -SG is an important component of the SG-sarcospan complex and that the functional consequences of the hypertrophic response in this model are muscle specific. The mechanisms responsible for the hypertrophic response and its muscle-specific nature merit further study.

	-SG-DEFICIENT NULL MUTANT MICE

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

-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 P_o/CSA (30). There are no data on the passive stiffness of -SG-deficient muscle nor on the response of such muscles to repetitive lengthening activations.

	SUMMARY

TOP ABSTRACT INTRODUCTION ANIMAL MODELS OF DMD:... SARCOGLYCANOPATHIES BIO 14.6 HAMSTER MODEL... -SG TRANSGENIC MURINE NULL... -SG MURINE NULL MUTANT... -SG NULL MUTANT MICE -SG-DEFICIENT NULL MUTANT MICE SUMMARY REFERENCES

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).