Genes expressed by high frequency pacing and the
influence of an impaired Ca⁺² signaling on the function
of cardiomyocytes

M. Cohen-Armon and M. Eldar,
The Neufeld Cardiac Research Institute, Sackler School of Medicine, Tel-Aviv university and Sheba Medical Center, Tel-Hashomer, Israel.

Cardiomyocytes functioning has been shown to be regulated by elevated concentration of cytoplasmic free calcium, [Ca⁺²+]i. In hypertrophy, cardiomyocyte cell size is increased. This involves an enhanced protein synthesis, up-regulation of fetal cardiac genes and induction of immediate early genes (1). Signaling pathways leading to cardiac hypertrophy are all associated with an increase in intracellular Ca⁺²concentration. (1). Also, changes in cytosolic Ca⁺² play a central role in the regulation of key nuclear events, including regulation of gene expression (2). In addition, numerous data highlight the role of Ca⁺² release in inflammatory heart disease, heart failure myocardial ischemia, reperfusion and apoptosis (2).

Cohen–Armon and collaborators found recently that the highly conserved and abundant chromatin-bound protein PARP-1 (polyADP-ribose polymerase-1) is rapidly activated in depolarized brain cortical neurons in response to Ca⁺² release into the nucleoplasm (3). PARP-1 is known to be activated by nicked DNA promoting cell death under stress conditions, thereby catalyzing a transient (t_1/2=1 min) post-translational modification of nuclear proteins by polyADP-ribosylation. This modification is initiated by ADP-ribose binding to glutamic and aspartic moieties of nuclear proteins and proceeds by polymerization of ADP-riboses via glycoside bonds into high polymers. NAD-derived ADP-ribose polymers interfere with the binding of proteins to DNA (4). Therefore, PARP-1 activation and polyADP-ribosylation of histones, transcription factors, RNA-polymerase II, DNA-polymerases, topoisomerases and ligases affects chromatin structure, DNA transcription and repair (4).

In this work we show that in cardiomyocytes, beating is accompanied by a rhythmic release of Ca⁺² into the nucleoplasm. PARP-1 is rapidly and dose-dependenly activated by applying Ca⁺² to isolated nuclei of cardiomyocytes, or by agents promoting Ca⁺²-release from intracellular Ca⁺² stores. In addition, prolonged inhibition of PARP-1 activity reduces most significantly their beating rate. These results suggest a possible physiological role of polyADP-ribosylation in normal functioning of cardiomyocytes. The rhythmic release of Ca⁺² into the nucleus, and dose dependent activation of PARP-1 apparently take part in a mechanism controlling the activity of transcription factors in the nucleus of cardiomyocytes. The results of this research are prepared for submission to J Biol Chem.

References:

1. Bers D.M., Calcium and cardiac rhythm: Physiological and pathophysiological. Circ Res. 90:14 (2002).
2. Frey N, et.al. Decoding calcium signals involved in cardiac growth and function. Nature Medicine 6:1221 (2000)
3. Homburg S, et al. A fast signal- induced activation of poly(ADP-ribose) polymerase: A novel downstream target of phospholipase C. J. Cell Biol. 150:293(2000).
4. D’Amours D. et al. Poly(ADP-ribosyl) ation reactions in the regulation of nuclear functions. Biochem. J. 342:249 (1999).