Antisense oligonucleotide experiments elucidate the essential role of titin in sarcomerogenesis in adult rat cardiomyocytes in long-term culture

An essential role of titin as a molecular ruler in sarcomerogenesis has been frequently discussed. In this study, we tested the hypothesis that the expression of titin is a prerequisite for thick filament incorporation into sarcomeres by using an antisense oligonucleotide approach to interfere with titin translation in the de-/redifferentiation model of adult rat cardiomyocytes (ARC) in long-term culture. As a first step, the growth pattern ranging from rod shape to round and later to spreading cells and the cell surface area of ARC were quantitatively evaluated and standardized. This represents the basis for experiments interfering with sarcomere formation using three different antisense phosphorothioate oligonucleotides (S-ODN) at a dosage of 10 microM specific for titin mRNA. Presence of fluorescein labeled S-ODN in ARC indicated cellular uptake and both, antisense and random S-ODN, induced a significant increase in cell size as compared with control untreated ARC. At days 12 and 16 in culture, antisense S-ODN treatment resulted in reduced expression of titin and disturbance of myosin incorporation into sarcomeres, evident by diffuse myosin labeling and a significantly decreased area of regular myosin cross-striation (control 75%, day 12 S-ODN 20%, day 16 14%) shown by laser scanning confocal microscopy. Cellular integrity indicated by presence of alpha-actinin was not disturbed. These findings provide evidence for the role of titin as a template for myosin incorporation and therefore as a prerequisite for sarcomerogenesis.