Calcium transients regulate titin organization during myofibrillogenesis

Titin has a Ca2+-dependent kinase domain and may act as a molecular template for myofibrillogenesis. Therefore, we examined the relationship between endogenous Ca2+ transients and titin organization in embryonic myocytes. When transients were blocked during sarcomere assembly, titin organization was disrupted. Titin was distributed in punctate aggregates on an otherwise diffuse background, resulting in a 66% decrease in organization. Myosin, as reported previously, was also disrupted in a similar manner (75% decrease). In titin-actin-myosin triple-labeling experiments, myosin and titin were highly colocalized, although titin aggregates without significant myosin accumulation were also observed. This suggests that myosin-titin association is not dependent on Ca2+ transients, although terminal aspects of titin-myosin organization require transients. We also examined whether titin organization is dependent on actin filament dynamics. The data indicate that (1) the normal sarcomeric arrangement of titin depends on Ca2+ transients, (2) titin-myosin association does not require Ca2+ transients, and (3) titin filament organization does not depend on barbed-end actin dynamics.