In vivo imaging of myonuclei during spontaneous muscle contraction reveals non-uniform nuclear mechanical dynamics in Nesprin/klar mutants

Abstract

Muscle contractions produce reiterated cytoplasmic mechanical variations, which impact the nuclear membrane and potentially influence nuclear mechanotransduction. It is unclear, however, whether the mechanical dynamics of individual myonuclei changes during each contractile wave, and whether mutants whose connection to the cytoskeleton is impaired are subjected to different mechanical input.\n\nTo monitor nuclear mechanical dynamics in vivo, we imaged myonuclei along muscles during multiple spontaneous muscle contractile events, within live and intact Drosophila wild-type or Nesprin/klar mutant larvae. The data were subsequently analyzed and quantified aiming to reveal potential changes in the mechanical parameters of nuclear dynamics during muscle contraction in the Nesprin/klar mutant. Our results show that all myonuclei in control larvae exhibited comparable dynamics in the course of multiple contractile events. In contrast, myonuclei of homozygous mutant larvae lacking the Nesprin-like gene klar displayed differential dynamics relative to wild type, and higher variability between myonuclei at distinct positions along individual myofibers. Estimation of the drag force applied on individual myonuclei revealed that force fluctuations in time were considerably higher in the Nesprin/klar mutant myonuclei relative to control, reflecting a significant variability in the mechanical dynamics of individual myonuclei during contractile waves. The variable mechanical dynamics along the muscle fiber and the higher variance of Nesprin/klar mutant myonuclei may lead to altered nuclear mechanotransduction. Since mutations in Nesprin genes lead to devastating muscle and cardiac human diseases, our findings may provide new insight into the mechanism underlying these pathologies.