The decrease of the cytoskeleton tubulin follows the decrease of the associating molecular chaperone αB‐crystallin in unloaded soleus muscle atrophy without stretch

• Published in: The FASEB journal 2005-07, Vol.19 (9), p.1199-1201
• Main Authors: Sakurai, Takashi, Fujita, Yoshinobu, Ohto, Eri, Oguro, Asami, Atomi, Yoriko
• Format: Article
• Language: English
• Subjects: hindlimb suspension; mechanical stress; microtubule; small heat shock protein
• ISSN: 0892-6638; 1530-6860
• DOI: 10.1096/fj.04-3060fje

ABSTRACTThe cytoskeletal component tubulin/microtubule commonly allows the cell to respond mechanically to the environment. The concentration of free tubulin dimer is autoregulated in the balance of free dimer and polymeric forms of microtubule (MT) protein, having an intrinsic property of “dynamic instability”, and through cotranslational β‐tubulin mRNA degradation. Recently, we have demonstrated that αB‐crystallin is a key molecule of muscle atrophy, since αB‐crystallin has a chaperone‐like‐activity that suppresses tubulin aggregation and protects the MT disassembly against both Ca2+ and depolymelizing alkaloid in vitro. Most of the small heat‐shock proteins (sHsps), including αB‐crystallin, are expressed in skeletal muscle. However, no report to date has studied the changes of tubulin/MT during muscle adaptation. Here, we examined changes in tubulin content in rat soleus muscles after hindlimb suspension (HS) with/without passive stretch and the recovery. HS induced rapid decreases of soleus muscle mass, most Hsps (αB‐crystallin, Hsp90, Hsp70, Hsp27, and p20) and tubulin contents in soleus muscle, while heat‐shock cognate 70‐kDa protein (Hsc70) did not decrease. Soleus muscle mass, most Hsps, and tubulin were maintained with passive stretch. After 5 days' recovery, the levels of tubulin and Hsps, but not Hsc70, were restored to control levels. The interactions of αB‐crystallin and tubulin/MT were observed with immunoprecipitation with an anti‐α‐tubulin antibody and taxol‐dependent MT assembly. Other sHsps were also associated with αB‐crystallin and MT, whereas Hsp90 and Hsp70 did not co‐precipitate with them. These data imply an interaction and close relationship between αB‐crystallin and tubulin/MTs in muscle tissues. The amount of mRNA of αB‐crystallin decreased with the muscle atrophy level, whereas the gene expression level of βI‐tubulin was maintained during HS. This means a significant role of post‐transcriptional regulation in tubulin/MT system in muscle adaptation, whereas αB‐crystallin and most sHsps are regulated at the transcriptional level. Additional functional contribution of αB‐crystallin to tubulin/MTs during myotube formation was examined using C2C12 myoblast cultured cells, the αB‐crystallin expression of which was decreased or increased. It indicated the necessity of αB‐crystallin during microtubule reorganization. In conclusion, tubulin/MTs were revealed to be one of the substrates of αB‐crystallin, and also serial decreases of αB‐cr