Composite biopolymer scaffolds shape muscle nucleus: Insights and perspectives from Drosophila.

Contractile muscle fibers produce enormous intrinsic forces during contraction/relaxation waves. These forces are directly applied to their cytoplasmic organelles including mitochondria, sarcoplasmic reticulum, and multiple nuclei. Data from our analysis of Drosophila larval somatic muscle fibers suggest that an intricate network of organized microtubules (MT) intermingled with Spectrin-Repeat-Containing Proteins (SRCPs) are major structural elements that protect muscle organelles and maintain their structure and position during muscle contraction. Whereas the perinuclear MT network provides structural rigidity to the myonucleus, the SRCPs Nesprin and Spectraplakin form semiflexible filamentous biopolymer networks, providing nuclei with the elasticity required to resist the contractile cytoplasmic forces produced by the muscle. Spectrin repeats are domains found in numerous structural proteins, which are able to unfold under tension and are subject to mechanical stresses in the cell. This unique composite scaffold combines rigidity and resilience in order to neutralize the oscillating cellular forces occurring during muscle contraction/relaxation waves and thereby protect myonuclei. We suggest that the elastic properties of SRCPs are critical for nuclear protection and proper function in muscle fibers.