Nothing Regular about the Regulins: Distinct Functional Properties of SERCA Transmembrane Peptide Regulatory Subunits

Abstract

The sarco-endoplasmic reticulum calcium ATPase (SERCA) is responsible for maintaining calcium homeostasis in all eukaryotic cells by actively transporting calcium from the cytosol into the sarco-endoplasmic reticulum (SR/ER) lumen. Calcium is an important signaling ion, and the activity of SERCA is critical for a variety of cellular processes such as muscle contraction, neuronal activity, and energy metabolism. SERCA is regulated by several small transmembrane peptide subunits that are collectively known as the “regulins”. Phospholamban (PLN) and sarcolipin (SLN) are the original and most extensively studied members of the regulin family. PLN and SLN inhibit the calcium transport properties of SERCA and they are required for the proper functioning of cardiac and skeletal muscles, respectively. Myoregulin (MLN), dwarf open reading frame (DWORF), endoregulin (ELN), and another-regulin (ALN) are newly discovered tissue-specific regulators of SERCA. Herein, we compare the functional properties of the regulin family of SERCA transmembrane peptide subunits and consider their regulatory mechanisms in the context of the physiological and pathophysiological roles of these peptides. We present new functional data for human MLN, ELN, and ALN, demonstrating that they are inhibitors of SERCA with distinct functional consequences. Molecular modeling and molecular dynamics simulations of SERCA in complex with the transmembrane domains of MLN and ALN provide insights into how differential binding to the so-called inhibitory groove of SERCA—formed by transmembrane helices M2, M6, and M9—can result in distinct functional outcomes.

Calcium Cycling as a Mediator of Thermogenic Metabolism in Adipose Tissue

“…Phospholamban (PLN) was the first regulin to be identified. It's expressed primarily in slow-twitch and cardiac muscle and, along with sarcolipin (SLN; expressed primarily in fasttwitch skeletal muscle), is the most well-studied of the regulins (Davis et al, 1983;Rathod et al, 2021). Both SLN and PLN have been shown to reduce the calcium affinity of SERCA through the distortion of the two calcium transport sites, but SLN has been shown to uncouple calcium transport from ATP hydrolysis through a unique structural mechanism in which the calcium bound in the transport site is not transferred to the SR in every catalytic cycle.…”

“…Therefore, while SLN binding has no effect on the ATP hydrolysis activity of SERCA, it reduces the efficiency of SERCA calcium transport activity and has unsurprisingly been shown to be necessary for NST in skeletal muscle (Bal et al, 2012;Rowland et al, 2015). While SLN and PLN expression are best characterized in skeletal and cardiac muscle, respectively, additional SERCA-mediating regulins have been identified, including myoregulin (MLN), endoregulin (ELN), and another-regulin (ALN) in nonmuscle tissues (Anderson et al, 2016;Bal and Periasamy, 2020;Rathod et al, 2021). Of the regulins, ALN and PLN are the most prominently expressed in WAT (Fig.…”

“…This regulin-mediated inhibition of SERCA calcium transport is reversable through their phosphorylation by PKA or CaMKII, both of which are activated by stimulation of b-adrenergic receptors on the cell membrane. However, exploration into the structural and functional properties of newly identified regulins raises the possibility of distinct mechanisms of SERCA modulation by isoform, thus suggesting that each regulin may operate differently to modulate calcium affinity and transport activity of SERCA (Rathod et al, 2021) (Table 1). Although regulin-mediated ATPase activity has been extensively studied in myocytes, to date there has been no investigation into the expression or activity of SERCA regulins in BAT.…”

“…Like SERCA, numerous pharmacological agents exist that are known to modulate RyR activity, though RyR targeting appears to be less pursued therapeutically compared with its SERCA pump counterparts, most likely due to its Odermatt et al, 1997;Xie et al, 2012;Bal et al, 2012;Bhupathy et al, 2009;Rathod et al, Tada et al,1975;Davis et al, 1983;Gorski et al, 2015;Verboomen et al, 1992;Rathod et al, 2021;Schmitt et al, 2003;Simmerman et al, involvement in several diseases. In addition to its role in SR calcium leak in heart failure mentioned above, dysfunctional RyRs have been linked to neurodegenerative disorders (Sun and Wei, 2021) and RyR mutations have been linked to multiple myopathies, including cardiac ventricular tachycardia and central core disease (Betzenhauser and Marks, 2010).…”

Excitation-contraction coupling in mammalian skeletal muscle: Blending old and last-decade research

“…The final mechanism responsible for restoring the sarcoplasmic resting [Ca 2+ ] and keeping it low is the SERCA. SERCA is a high molecular weight, highly regulated pump, enriched in the longitudinal region of the SR and the nonjunctional membrane of the terminal cisternae, which transports Ca 2+ into the SR against its concentration gradient (Hasselbach, 1964;Jorgensen and Jones, 1986;Hasselbach, 1998;Odermatt et al, 1998;Periasamy and Kalyanasundaram, 2007;Rathod et al, 2021). This protein has three large cytoplasmic domains, N, P and A, attached to a domain consisting of 10 hydrophobic trans-SR-membrane helices (M1 to M10) Constructed at scale based on microscopy and protein measurements, the cartoon considers many current observations and addressed the question if there is space for all ECC proteins in the triadic space.…”