Aquaporin 4 Expression in the mdx Mouse Diaphragm

Non-coding RNAs and Aquaporin 4: Their Role in the Pathogenesis of Neurological Disorders

Neurological disorders are a major group of non-communicable diseases affecting quality of life. Non-Coding RNAs (ncRNAs) have an important role in the etiology of neurological disorders. In studies on the genesis of neurological diseases, aquaporin 4 (AQP4) expression and activity have both been linked to ncRNAs. The upregulation or downregulation of several ncRNAs leads to neurological disorder progression by targeting AQP4. The role of ncRNAs and AQP4 in neurological disorders is discussed in this review.

Proteome Profiling of the Dystrophic mdx Mice Diaphragm

Mdx mice with a spontaneous mutation in exon 23 of the Dmd gene represent the most common model to investigate the pathophysiology of Duchenne muscular dystrophy (DMD). The disease, caused by the lack of functional dystrophin, is characterized by irreversible impairment of muscle functions, with the diaphragm affected earlier and more severely than other skeletal muscles. We applied a label-free (LF) method and the more thorough tandem mass tag (TMT)-based method to analyze differentially expressed proteins in the diaphragm of 6-week-old mdx mice. The comparison of both methods revealed 88 commonly changed proteins. A more in-depth analysis of the TMT-based method showed 953 significantly changed proteins, with 867 increased and 86 decreased in dystrophic animals (q-value < 0.05, fold-change threshold: 1.5). Consequently, several dysregulated processes were demonstrated, including the immune response, fibrosis, translation, and programmed cell death. Interestingly, in the dystrophic diaphragm, we found a significant decrease in the expression of enzymes generating hydrogen sulfide (H2S), suggesting that alterations in the metabolism of this gaseous mediator could modulate DMD progression, which could be a potential target for pharmacological intervention.

The expression of aquaporin-4 is regulated based on innervation in skeletal muscles

Aquaporin-4 (AQP4) is a selective water channel, which expresses on the plasma membrane of myofibers and regulates the osmotic pressure, energy metabolism and morphological changes in myofibers by modulating water transport across sarcolemma in skeletal muscles. Although the physiological roles of AQP4 have been gradually clarified in skeletal muscles, the regulatory mechanisms of AQP4 expression have been poorly understood in skeletal muscles. Recently, it was reported that the expression of AQP4 decreased in atrophied skeletal muscles following sciatic nerve transection, but not tail-suspension. Therefore, expecting that the nerve supply to myofibers would be one of the major regulatory factors regulating AQP4 expression in skeletal muscles, we investigated whether the expression patterns of AQP4 were changed in skeletal muscles by denervation and subsequent reinnervation. As a result, while the APQ4 expression levels were significantly decreased by sciatic nerve freezing-induced denervation, subsequently the expression levels of AQP4 were fully restored during reinnervation in skeletal muscles (p < 0.05, respectively). On the other hand, the expression levels of α1-syntrophin and AQP1, which are respectively structural and functional related AQP4 factors, were stably maintained during the denervation and subsequent reinnervation. Therefore, the present study demonstrated that the expression of AQP4 may be regulated depending on the innervation to skeletal muscles. Moreover, AQP4 regulatory mechanisms may be fundamentally different to those of AQP1 in skeletal muscles.

Marked decrease of aquaporin-4 protein is independent of the changes in α1-syntrophin and TRPV4 levels in response to denervation-induced muscle atrophy in vivo

Aquaporin-4 (AQP4) is a selective water channel mediating water transport across cell membranes in skeletal muscles. Recently, it was noted that AQP4 is one of the key molecules regulating muscle morphology. Indeed, the AQP4 accumulation level was stably maintained in hypertrophied skeletal muscles. On the other hand, whether the AQP4 accumulation level is stably maintained in atrophied muscles remains poorly understood. The present study investigated the changes in the AQP4 accumulation level in the atrophied muscles at 2 weeks after denervation. As a result, the accumulation level of AQP4 in the atrophied muscle was significantly decreased compared with that in the control muscle (p < 0.05). Interestingly, the accumulation level of α1-syntrophin, which is an essential factor in regulating the stable accumulation level of AQP4, was stably maintained in the atrophied muscles. On the other hand, the accumulation level of the transient receptor potential vanilloid 4 (TRPV4), which contributes to cell volume control via interaction with AQP4, was significantly increased in the atrophied muscles compared with that in the control muscle (p < 0.05). Therefore, the present study suggested that the imbalance between the AQP4 accumulation level and skeletal muscle volume may be induced in the atrophied muscles by denervation, and the decrease in the accumulation level of AQP4 may be accompanied by defects in the functional and structural relationships with α1-syntrophin and TRPV4.

Aquaporin-4 Protein Is Stably Maintained in the Hypertrophied Muscles by Functional Overload

Aquaporin-4 (AQP4) is a selective water channel that is located on the plasma membrane of myofibers in skeletal muscle and is bound to α1-syntrophin. It is considered that AQP4 is involved in the modulation of homeostasis in myofibers through the regulation of water transport and osmotic pressure. However, it remains unclear whether AQP4 expression is altered by skeletal muscle hypertrophy to modulate water homeostasis in myofibers. The present study investigated the effect of muscle hypertrophy on the changes in AQP4 and α1-syntrophin expression patterns in myofibers. Novel findings indicated in the present study were as follows: 1) Expression levels of AQP4 and α1-syntrophin were stably maintained in hypertrophied muscles, and 2) AQP4 was not expressed in the myofibers containing the slow-type myosin heavy chain isoform (MHC) with or without the presence of fast-type MHC. The present study suggests that AQP4 may regulate the efficiency of water transport in hypertrophied myofibers through its interaction with α1-syntrophin. In addition, this study suggests that AQP4 expression may be inhibited by a regulatory mechanism activated under physiological conditions that induces the expression of slow-type MHC in skeletal muscles.

Tranilast administration reduces fibrosis and improves fatigue resistance in muscles of mdx dystrophic mice

BACKGROUND

Duchenne muscular dystrophy (DMD) is a severe and progressive muscle-wasting disorder caused by mutations in the dystrophin gene that result in the absence of the membrane-stabilising protein dystrophin. Dystrophic muscle fibres are susceptible to injury and degeneration, and impaired muscle regeneration is associated with fibrotic deposition that limits the efficacy of potential pharmacological, cell- and gene-based therapies. Novel treatments that can prevent or attenuate fibrosis have important clinical merit for DMD and related neuromuscular diseases. We investigated the therapeutic potential for tranilast, an orally bioavailable anti-allergic agent, to prevent fibrosis in skeletal muscles of mdx dystrophic mice.

RESULTS

Three-week-old C57Bl/10 and mdx mice received tranilast (~300 mg/kg) in their food for 9 weeks, after which fibrosis was assessed through histological analyses, and functional properties of tibialis anterior muscles were assessed in situ and diaphragm muscle strips in vitro. Tranilast administration did not significantly alter the mass of any muscles in control or mdx mice, but it decreased fibrosis in the severely affected diaphragm muscle by 31% compared with untreated mdx mice (P < 0.05). A similar trend of decreased fibrosis was observed in the tibialis anterior muscles of mdx mice (P = 0.10). These reductions in fibrotic deposition were not associated with improvements in maximum force-producing capacity, but we did observe small but significant improvements in the resistance to fatigue in both the diaphragm and TA muscles of mdx mice treated with tranilast.

CONCLUSION

Together these findings demonstrate that administration of potent antifibrotic compounds such as tranilast could help preserve skeletal muscle structure, which could ultimately increase the efficacy of pharmacological, cell and gene replacement/correction therapies for muscular dystrophy and related disorders.