Animals may have different types of muscles but they all have one thing in common: molecular machines called sarcomeres that produce a pulling force. Conserved from fruit flies to humans, these structures line up end-to-end inside muscle cells, forming long cables called myofibrils. Some of the myofibrils in a human can reach several centimetres in length, which is longer than those in a fruit fly. However, individual sarcomeres are the same length in both humans and flies.
To assemble the parts of the sarcomere, an animal cell first copies the relevant genes into intermediate molecules known as mRNAs, which are then translated to build new sarcomere proteins. Developing muscle cells later tune their sarcomeres to make them sensitive to stretching. This tweaks the power and force of the mature muscle, but the details of this developmental process are not fully understood.
In a new publication by the Schnorrer lab in eLife, Spletter et al. have now counted all the mRNAs in the developing flight muscles of fruit flies, with the aim of generating a resource that catalogues the changes in gene activity, or expression, that occur as muscles develop. This revealed that sarcomeres form in three phases. First, muscles assemble all their myofibrils containing short immature sarcomeres. Then, they add additional sarcomeres to these myofibrils and thus the muscles grow in length. Finally, all sarcomeres mature to their full length and diameter, and become sensitive to stretching.
This work identified that fruit fly muscles display 40 different temporal patterns of gene expression during development, with most of the sarcomere components having only one of two specific patterns. The expression of these genes dramatically rises after the young muscle cells have assembled all their immature myofibrils, suggesting muscles express different genes when their sarcomeres mature. A protein called spalt-major helps the muscles to know when to make the transition, allowing the sarcomeres to grow in length and width. Losing spalt-major late in development reduces sarcomere growth and prevents the tuning process.
The similarities between fruit fly and human sarcomeres suggest this developmental sequence may also occur in human muscles. Understanding these steps may help to improve repair after injury or muscle growth during exercise.
To know more :
Elife. 2018 May 30;7. pii: e34058. doi: 10.7554/eLife.34058. Spletter ML, Barz C, Yeroslaviz A, Zhang X, Lemke SB, Bonnard A, Brunner E, Cardone G, Basler K, Habermann BH, Schnorrer F.