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Timing appearance and integration of actin-organizing palladin protein in dynamic myofibril assembly

The involvement of actin-associated protein palladin in myogenesis has been elucidated, however, palladin distribution in a functional myotube remains to be identified. Since actin is required for myofibrillogenesis, it is of great interest to enhance our understanding of the spatial arrangements of palladin during sarcomeric assembly. Surprisingly, palladin was found to be discretely organized in different stages of myofibrillogenesis. Palladin revealed stress-fiber-like structures at undifferentiated stages, subsequently displayed chaotic expression and strongly co-distributed with actin, α-actinin, and myosin heavy chain of premyofibrils. At late stages, aggregates of palladin were spaced in a regular dot-like structure. On the other hand, palladin presents at I-Z-I bands of adult muscle. These observations suggest that palladin engages with sarcomeric proteins during the process of myoblast differentiation and that these interactions might occur in a temporally regulated fashion. In addition, transient overexpression of 140-kDa-palladin resulted in nonfilamentous actin arresting mature myotube formation. 200-kDa-palladin overexpression led to the early formation of Z-lines. Collectively, these findings suggest that palladin might serve a role in myofibrillogenesis by guiding and positioning sarcomeric proteins at the appropriate time and place. Our results highlight the involvement of palladin protein and the discrete functions of palladin isoforms in sarcomeric development in vitro.