Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) gene products are regulated post-transcriptionally during rat cardiac development

Abstract

Objective: The Sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) plays a major role in the contraction-relaxation cycle and is responsible for transporting calcium into the lumen of the sarcoplasmic reticulum. This study was performed to determine if the increase in SERCA2 messenger RNA (mRNA) abundance during the perinatal period is regulated transcriptionally. Methods: Transcriptional activity was determined by nuclear run-on assays and mRNA and protein abundances were determined during late fetal and early neonatal cardiac development in rat. Results: From nuclear run-on assays, SERCA2 gene transcription at 17/18 embryonic days (139±41 parts per million (ppm), n=7) did not differ from that at 20 neonatal days (139±37 ppm, n=6) after birth. No increase in transcriptional activity could be demonstrated during the time frame examined. In contrast, both alpha and beta myosin heavy chains showed significant changes in measured transcriptional activity. SERCA2 mRNA normalized to 18S RNA levels are very low in the fetus (9.8±1.9 to 13.4±4.9 arbitrary units (A.U.) from 17/18 to 19/20 embryonic days) and significantly increase from birth (15±3.8 A.U.) to reach a maximum at 20 days of age (29.1±9.5 to 48.3±7.0 in 15 to 20 neonatal days rats respectively). Similarly, SR Ca2+-ATPase protein levels are less abundant in the fetus (0.82±0.08 to 1.13±0.13 A.U./μg total protein) and reach a maximum at 15-20 neonatal days (3.08±0.58 to 2.98±0.17). Ca2+ uptake in the fetal heart is about one sixth the level seen in the adult, reaches the highest observed value at 5 days after birth (6.05±0.77 pmole Ca2+ per μg/min) and remains relatively constant over the next 15 days. The activity increases even though phospholamban protein increases in abundance. Conclusions: Since the transcriptional activity of this gene is unchanged whereas the mRNA, protein abundance and activity increase, we conclude that the abundance of SERCA2 gene products is regulated primarily through post- transcriptional mechanisms during the perinatal period.