Aquaporin-4 expression is severely reduced in human sarcoglycanopathies and dysferlinopathies

Muscular and Neurodegenerative Disease Unit, Department of Pediatrics of University of Genova, Department of Neuroscience and Rehabilitation, G. Gaslini Institute, Genoa, Italy.
Cell cycle (Georgetown, Tex.) (Impact Factor: 4.57). 07/2008; 7(14):2199-207. DOI: 10.4161/cc.7.14.6272
Source: PubMed


Aquaporin-4 (AQP4) is the major water channel expressed in fast-twitch skeletal muscle fibers. AQP4 is reduced in Duchenne and Becker Muscular Dystrophies, but not in caveolinopathies, thus suggesting an interaction with dystrophin or with members of the dystrophin-glycoprotein complex (DGC) rather than a nonspecific effect due to muscle membrane damage. To establish the role of sarcoglycans in AQP4 decrease occurring in muscular dystrophy, AQP4 expression was analyzed in muscle biopsies from patients affected by Limb Girdle Muscular Dystrophies (LGMDs) 2C-F genetically confirmed. In all the LGMD 2C-F (2alpha-, 1beta-, 2gamma-, 1delta-deficiency), AQP4 was severely decreased. This effect was associated to a marked reduction in alpha1-syntrophin levels. In control muscle AQP4 did not show a direct interaction with any of the four sarcoglycans but, it co-immunoprecipitated with alpha1-syntrophin, indicating that this modular protein may link AQP4 levels with the DGC complex. To determine whether AQP4 expression could be affected in other LGMDs due to the defect of a membrane protein not associated to the dystrophin complex, we examined AQP4 expression in 6 patients affected by dysferlin deficiency genetically confirmed. All the patients displayed a reduction of the water channel, and AQP4 expression appeared to correlate with the severity of the muscle histopathological lesions. However, differently from what observed in the sarcoglycans, alpha1-syntrophin expression was normal or just slightly reduced. These results seem to indicate an additional mechanism of regulation of AQP4 levels in muscle cells. In accordance with a specific effect of membrane muscle disorders, AQP4 protein levels were not changed in 3 mitochondrial and 3 metabolic myopathies. In conclusion, AQP4 expression and membrane localization are markedly reduced in LGMD 2B-2F. The role of AQP4 in the degenerative mechanism occurring in these diseases will be the object of our future research.

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Available from: Claudio Bruno
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    • "AQP4 is expressed as two major isoforms of 32 kDa (AQP4-M1) and 30 kDa (AQP4-M23), which differ by 22 amino acids in the N-terminus [3]. These two major AQP4 isoforms are organized in the plasma membrane in higher order structures called Orthogonal Array of Particles (OAPs) [4], [5], [6] whose expression is affected in several muscular dystrophies [7], [8], [9], [10], [11]. The dimension of an OAP is tightly associated to the M1/M23 AQP4 isoform ratio, given that M23 is the OAPs-forming isoform, and M1 alone is unable to form OAPs [5]. "
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    ABSTRACT: In this study we assess the functional role of Aquaporin-4 (AQP4) in the skeletal muscle by analyzing whether physical activity modulates AQP4 expression and whether the absence of AQP4 has an effect on osmotic behavior, muscle contractile properties, and physical activity. To this purpose, rats and mice were trained on the treadmill for 10 (D10) and 30 (D30) days and tested with exercise to exhaustion, and muscles were used for immunoblotting, RT-PCR, and fiber-type distribution analysis. Taking advantage of the AQP4 KO murine model, functional analysis of AQP4 was performed on dissected muscle fibers and sarcolemma vesicles. Moreover, WT and AQP4 KO mice were subjected to both voluntary and forced activity. Rat fast-twitch muscles showed a twofold increase in AQP4 protein in D10 and D30 rats compared to sedentary rats. Such increase positively correlated with the animal performance, since highest level of AQP4 protein was found in high runner rats. Interestingly, no shift in muscle fiber composition nor an increase in AQP4-positive fibers was found. Furthermore, no changes in AQP4 mRNA after exercise were detected, suggesting that post-translational events are likely to be responsible for AQP4 modulation. Experiments performed on AQP4 KO mice revealed a strong impairment in osmotic responses as well as in forced and voluntary activities compared to WT mice, even though force development amplitude and contractile properties were unvaried. Our findings definitively demonstrate the physiological role of AQP4 in supporting muscle contractile activity and metabolic changes that occur in fast-twitch skeletal muscle during prolonged exercise.
    Full-text · Article · Jun 2013 · PLoS ONE
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    • "For example, AQP4 is strongly reduced in skeletal muscle of Duchenne muscular dystrophy (DMD) patients [8]–[11] as well as in the sarcolemma of the mdx mouse, an animal model of the disease [2], [8], [9], [12]. Moreover, AQP4 down-regulation was observed in human patients affected by Limb Girdle Muscular Dystrophies (LGMDs) [13], in which defects in several isoforms of sarcoglycan (SG) occur. Reduction of AQP4 expression has often been associated to a marked reduction in α1-syntrophin (α1-syn) level, because of the close association between them [14], [15]. "
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    ABSTRACT: Aquaporin-4 (AQP4) is a water channel expressed at the sarcolemma of fast-twitch skeletal muscle fibers, whose expression is altered in several forms of muscular dystrophies. However, little is known concerning the physiological role of AQP4 in skeletal muscle and its functional and structural interaction with skeletal muscle proteome. Using AQP4-null mice, we analyzed the effect of the absence of AQP4 on the morphology and protein composition of sarcolemma as well as on the whole skeletal muscle proteome. Immunofluorescence analysis showed that the absence of AQP4 did not perturb the expression and cellular localization of the dystrophin-glycoprotein complex proteins, aside from those belonging to the extracellular matrix, and no alteration was found in sarcolemma integrity by dye extravasation assay. With the use of a 2DE-approach (BN/SDS-PAGE), protein maps revealed that in quadriceps, out of 300 Coomassie-blue detected and matched spots, 19 proteins exhibited changed expression in AQP4(-/-) compared to WT mice. In particular, comparison of the protein profiles revealed 12 up- and 7 down-regulated protein spots in AQP4-/- muscle. Protein identification by MS revealed that the perturbed expression pattern belongs to proteins involved in energy metabolism (i.e. GAPDH, creatine kinase), as well as in Ca(2+) handling (i.e. parvalbumin, SERCA1). Western blot analysis, performed on some significantly changed proteins, validated the 2D results. Together these findings suggest AQP4 as a novel determinant in the regulation of skeletal muscle metabolism and better define the role of this water channel in skeletal muscle physiology.
    Full-text · Article · Apr 2011 · PLoS ONE
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    • "So they suggested that the reduced expression of AQP4 in DMD muscles was secondary to dystrophic process. Reduced AQP4 expression was also reported in severe muscular dystrophies such as FCMD [84], limb girdle muscular dystrophy (LGMD) type 2B [68, 85], sarcoglycanopathies [85], and animal models of sarcoglycanopathies [86]. However, in immunofluorescence analysis, AQP4 was reported to be expressed normally at the sarcolemma of biopsy samples from LGMD type 1C and facioscapulohumeral muscular dystrophy [80]. "
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    ABSTRACT: Freeze-fracture electron microscopy enabled us to observe the molecular architecture of the biological membranes. We were studying the myofiber plasma membranes of health and disease by using this technique and were interested in the special assembly called orthogonal arrays (OAs). OAs were present in normal myofiber plasma membranes and were especially numerous in fast twitch type 2 myofibers; while OAs were lost from sarcolemmal plasma membranes of severely affected muscles with dystrophinopathy and dysferlinopathy but not with caveolinopathy. In the mid nineties of the last century, the OAs turned out to be a water channel named aquaporin 4 (AQP4). Since this discovery, several groups of investigators have been studying AQP4 expression in diseased muscles. This review summarizes the papers which describe the expression of OAs, AQP4, and other AQPs at the sarcolemma of healthy and diseased muscle and discusses the possible role of AQPs, especially that of AQP4, in normal and pathological skeletal muscles.
    Full-text · Article · Mar 2010 · BioMed Research International
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