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Aquaporin-4 expression in distal myopathy with rimmed vacuoles
© Hoshi et al.; licensee BioMed Central Ltd. 2012
Received: 29 December 2011
Accepted: 27 April 2012
Published: 27 April 2012
Distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy is clinically characterized by the early involvement of distal leg muscles. The striking pathological features of the myopathy are muscle fibers with rimmed vacuoles. To date, the role of aquaporin-4 water channel in distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy has not been studied.
Here, we studied the expression of aquaporin-4 in muscle fibers of a patient with distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy. Immunohistochemical and immunofluorescence analyses showed that sarcolemmal aquaporin-4 immunoreactivity was reduced in many muscle fibers of the patient. However, the intensity of aquaporin-4 staining was markedly increased at rimmed vacuoles or its surrounding areas and in some muscle fibers. The fast-twitch type 2 fibers were predominantly involved with the strong aquaporin-4-positive rimmed vacuoles and TAR-DNA-binding protein-43 aggregations. Rimmed vacuoles with strong aquaporin-4 expression seen in the distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy patient were not found in control muscles without evidence of neuromuscular disorders and the other disease-controls.
Aquaporin-4 might be crucial in determining the survival or degeneration of fast-twitch type 2 fibers in distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy.
Autosomal recessive distal myopathy with rimmed vacuoles (DMRV)/hereditary inclusion body myopathy (hIBM) is clinically characterized by preferential involvement of the distal leg muscles in the very early stage, and later of most proximal muscles. One of the striking pathological features of this myopathy is muscle fibers with rimmed vacuoles (RVs) [1, 2]. DMRV/hIBM is caused by a mutation in the uridine diphosphate-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene, which encodes a bifunctional enzyme catalyzing the 2 exclusive rate-limiting reactions of sialic acid synthesis in the cytosol [3, 4]. However, why these mutations produce a myopathy with RVs remains to be determined.
Aquaporin-4 (AQP4) is the main water channel of the neuromuscular system. In the skeletal muscle, AQP4 is predominantly localized to the sarcolemma of fast-twitch type 2 fibers [5–7]. AQP4 expression in the muscles is markedly reduced in patients with dystrophinopathy, dysferlinopathy, and amyotrophic lateral sclerosis (ALS) [8–10], but the pathophysiology underlying the reduction in expression is unclear. Changes in AQP4 expression, however, have not been studied in DMRV/hIBM thus far. In this communication, we aimed to characterize AQP4 expressions in the muscle fibers of a patient with DMRV/hIBM associated with GNE mutation. Furthermore, we investigated accumulation of TAR-DNA-binding protein-43 (TDP-43), a pathological hallmark of vacuolar myopathies [11, 12], in the muscle fibers of DMRV/hIBM patients.
Muscle biopsy samples were taken from the left quadriceps femoris, and the tissue specimens were immediately frozen in isopentane chilled with liquid nitrogen. Serial 10-μm-thick transverse sections cut using a cryostat were stained by routine muscle histochemical methods. In addition, we performed immunohistochemical and immunofluorescence studies (Additional file 1 shows these methods in more detail).
To the best of our knowledge, this is the first report on AQP4 expression in the skeletal muscle of a DMRV/hIBM patient. We showed that sarcolemmal AQP4 immunoreactivity was reduced in many muscle fibers of the DMRV/hIBM patient, a female carrier of DMD, and an ALS patient. It is interesting to note that AQP4 staining was markedly increased in the areas of the RVs and in some muscle fibers in DMRV/hIBM. The most striking findings were that the fast-twitch type 2 fibers were predominantly involved with the strong AQP4-positive RVs and TDP-43 aggregations in DMRV/hIBM.
Our DMRV/hIBM case showed unusual involvement of the quadriceps muscles. In general, even at the advanced stage of DMRV/hIBM, quadriceps muscles are relatively spared. However, an earlier study has described a DMRV/hIBM patient with weakness of quadriceps . Intriguingly, the study also reported that a non-DMRV/hIBM patient with predominant proximal muscle weakness of the lower extremities had GNE mutations . On the other hand, quadriceps muscles of GNE knockout mouse are preferentially involved . It remains unknown why clinical variations are observed in GNE-mutated DMRV/hIBM. Further studies are needed to clarify the genotype/phenotype correlations in DMRV/hIBM patients.
Our results about downregulation of AQP4 are consistent with the results reported in other studies [8–10]. We suppose that AQP4 downregulation occurs as a common pathway in muscle degeneration at a late stage. An important finding of our study is the result of AQP4-positive staining. We show that AQP4 staining was markedly increased at the RVs or the areas surrounding the RVs and in some muscle fibers in our DMRV/hIBM patient. Furthermore, fast-twitch type 2 fibers were predominantly involved with the strong AQP4-positive RVs, while these findings were not observed in a control muscle samples without evidence of neuromuscular disorders and in disease-control muscles. The lysosomal system is thought to be activated in DMRV/hIBM muscle because of accumulation of various lysosome-related proteins in the RVs [14, 15]. Moreover, several sarcolemmal proteins, α-dystroglycan, β-dystroglycan, and α-sarcoglycan are also accumulated in the myofibers of the DMRV/hIBM mouse model, presumably because of abnormal protein misfolding/aggregation [14, 15]. We surmise that the AQP4-positive aggregates in the sarcoplasm and intense AQP4 expression with RVs are associated with the lysosomal autophagic process. On the other hand, the slow- to fast-twitch conversion of soleus fibers under muscle unloading is associated with AQP4 expression in rats . The fact that the modulation of AQP4 expression is associated with the transition of muscle fiber type indicates that AQP4 is an important muscle protein involved in muscle plasticity. In addition, AQP4 may protect against muscle damage, by maintaining muscle volume regulation and muscle osmolarity . Thus, we consider that muscle adaptation from type 1 to type 2 fibers is associated with the change in AQP4 expression against muscle degeneration in DMRV/hIBM. A critical feature in terms of involvement of a specific type of muscle fiber in DMRV/hIBM is that the fast-twitch fibers are predominantly affected . As shown in our DMRV/hIBM patient, the fast-twitch type 2 fibers would be more involved with TDP-43 accumulation in the late stage. TDP-43 positive aggregates have been observed in various vacuolar myopathies, which suggests that TDP-43 accumulation is more likely to be a common endpoint of vacuolar muscle degeneration .
In conclusion, we found that many muscle fibers of a patient with DMRV/hIBM showed a reduction of sarcolemmal AQP4 immunoreactivity, while fast-twitch type 2 fibers were predominantly involved with strong AQP4-positive RVs and TDP-43 aggregation. Although the functional role of AQP4 in skeletal muscle is not well understood, AQP4 might be crucial in determining the survival or degeneration of fast-twitch type 2 fibers in DMRV/hIBM.
Written informed consent was obtained from the patient for publication of this Case report and any accompanying images.
We would like to thank Ichizo Nishino (Department of Neuromuscular Research, National Institute of Neuroscience, National Center of Neurology and Psychiatry, Japan) for GNE mutation analysis and Hisae Kayama for her technical assistance. Part of this work was supported by the followings: Research Project Grants-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No. 22390181); grants from the Uehara Memorial Foundation; NOVARTIS Foundation (Japan) for the promotion of Science.
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