Regulation of mitochondrial Ca2+ uniporter expression and calcium signalling by lncRNA Tug1 in cardiomyocytes
Cardiomyocyte Ca2+ is buffered by mitochondria via the mitochondrial calcium uniporter (MCU) complex. The MCU complex consists of pore-forming proteins including the mitochondrial calcium uniporter (MCU), and regulatory proteins such as mitochondrial calcium uptake proteins 1 and 2 (MICU1/2). The stoichiometry of these proteins influences the sensitivity to Ca2+ and activity of the complex. However, the factors that regulate their gene expression remain incompletely understood. Long non-coding RNAs (lncRNAs) regulate gene expression through various mechanisms, and we recently found that the lncRNA Tug1 affected the expression of MCU-associated genes. To further explore this, we knocked down Tug1 (Tug1 KD) in H9c2 rat cardiomyocytes using antisense LNA oligo. This led to increased MCU protein expression yet this did not enhance a marker of mitochondrial Ca2+ uptake. RNA-seq revealed that Tug1 KD increased Mcu and led to differential expression of genes and pathways related to Ca2+ regulation in the heart. To understand the effect of this on Ca2+ signalling, we measured phosphorylation of Ca2+/calmodulin-dependent protein kinase II (CaMKII) and its downstream target cAMP Response Element-Binding protein (CREB), a transcription factor known to promote Mcu gene expression. Tug1 KD attenuated the increase in CAMKII and CREB phosphorylation in response to ionomycin, a Ca2+ ionophore. Inhibition of CaMKII, but not CREB, partially prevented the Tug1 KD mediated increase in Mcu. Together, these data suggest that Tug1 modulates MCU expression via a mechanism that may involve CAMKII and CREB. The Tug1 mediated regulation of MCU on mitochondrial Ca2+ uptake, may have functional consequences for cellular Ca2+ handling which could have implications for cardiac disease. Overall design: Comparative gene expression profiling analysis of RNA-seq data for H9c2 cells with or without antisense LNA-mediated knockdown of Tug1