Threonine-5 at the N-terminus can modulate sarcolipin function in cardiac myocytes

Abstract

Sarcolipin (SLN) has emerged as an important regulator of the atrial sarcoplasmic reticulum (SR) Ca 2+ transport. The inhibitory effect of SLN on cardiac SR Ca 2+ ATPase (SERCA) pump can be relieved by β-adrenergic stimulation, which indicates that SLN is a reversible inhibitor. However, the mechanism of this reversible regulation of SERCA pump by SLN is yet to be determined. In the current study using adult rat ventricular myocytes we provide evidence that the threonine 5 (T5) residue at the N-terminus of SLN which is conserved among various species, critically regulates the SLN function. Point mutation of T5→alanine exerts an inhibitory effect on myocyte contractility and calcium transients similar to that of wild-type SLN, whereas mutation of T5→glutamic acid which mimics the phosphorylation abolished the inhibitory function of SLN. Our results showed that T5 can be phosphorylated in vitro by calcium-calmodulin dependent protein kinase II (CaMKII). Blocking the CaMKII activity in WT-SLN overexpressing myocytes using autocamtide inhibitory peptide completely abolished the β-adrenergic response. Taken together, our data suggest that T5 is the key amino acid which modulates SLN function via phosphorylation/dephosphorylation mechanisms through CaMKII pathway.

Enhanced Ca2+ transport and muscle relaxation in skeletal muscle from sarcolipin-null mice

“…Recently, phosphorylation of Thr 5 by Ca 2ϩ /calmodulin-dependent protein kinase II (CaMKII) was shown to be a key mechanism involved in the regulation of SLN function in cardiac myocytes (8). Although the precise signaling pathway(s) involved in the lusitropic response to repeated contractions in soleus are unknown, given that CaMKII is enriched in the SR membrane in skeletal muscle (11), that it becomes activated during repeated muscle contractions (28), and that Thr 5 is a target of CaMKII signaling, we propose that CaMKII is of primary importance (9).…”

Oligomeric Interactions of Sarcolipin and the Ca-ATPase

“…SERCA is a 110-kDa membrane protein that relaxes muscle by pumping calcium out of the cytoplasm using energy derived from ATP hydrolysis (3). Transgenic mouse studies have demonstrated that knock-out of SLN enhances atrial contractility (4), cross-expression of SLN inhibits ventricular contractility (2,5,6), and ␤-adrenergic stimulation relieves SLN inhibition of contractility (2,6,7). Phosphorylation/dephosphorylation of SLN is responsible for SERCA regulation, controlling the rate and amount of calcium loading in SR, which in turn determines the rate of muscle relaxation and the force of subsequent contraction (2, 4 -7).…”

Role of abnormal sarcoplasmic reticulum function in atrial fibrillation

“…SLN inhibits SERCA2a activity by decreasing the apparent Ca 2+ affinity of the pump [36], a process that is dynamically regulated by phosphorylation. Similar to PLN, stimulation of β-adrenoceptors phosphorylates SLN at T5 and enhances contractility by decreasing SERCA2a inhibition [37]. Thus, SLN plays an important regulatory role in the atria.…”