Regulation of the cardiac sodium pump

Abstract

In cardiac muscle, the sarcolemmal sodium/potassium ATPase is the principal quantitative means of active transport at the myocyte cell surface, and its activity is essential for maintaining the trans-sarcolemmal sodium gradient that drives ion exchange and transport processes that are critical for cardiac function. The 72-residue phosphoprotein phospholemman regulates the sodium pump in the heart: unphosphorylated phospholemman inhibits the pump, and phospholemman phosphorylation increases pump activity. Phospholemman is subject to a remarkable plethora of post-translational modifications for such a small protein: the combination of three phosphorylation sites, two palmitoylation sites, and one glutathionylation site means that phospholemman integrates multiple signaling events to control the cardiac sodium pump. Since misregulation of cytosolic sodium contributes to contractile and metabolic dysfunction during cardiac failure, a complete understanding of the mechanisms that control the cardiac sodium pump is vital. This review explores our current understanding of these mechanisms.

Fig. 1

Sequence alignment of phospholemman from different species. The phosphorylation sites and palmitoylation/glutathionylation sites plus surrounding residues are conserved across vertebrate classes

Fig. 2

Sequence divergence between α1 and α2 subunits as a basis of differential regulation by PLM? Na pump α1 and α2 sequences from the species indicated were aligned with Clustal (for full alignment, see Supplement 1). A heat map was generated using the porcine crystal structure (3B8E.pdb [3]) to indicate positions of surface conservation and divergence between α subunits using a 1–5 scale (annotated on the Clustal alignment). The β1 subunit is shown in magenta, and PLM (phosphorylated at S63, S68, and T69) is shown in green, positioned according to [15]. Color coding of the α subunit is as follows: (1) Dark Blue 85+ % conserved (where both α1 and α2 are the same, allowing only one outlier in both groups). (2) Light blue either no consensus for α1 and α2, or used if α1 and α2 cannot be discriminated significantly. (3) Yellow conservative change, represented by ‘;’ in the Clustal alignment. Also used even if there is a single outlier in one group. (4) Orange moderate change, represented by ‘.’ in the Clustal alignment. (5) Red major change. Also used even if a single outlier is present. If multiple outliers are present, this is downgraded to yellow. a Surface divergence between α1 and α2 is particularly notable in the N domain. b Major changes between α1 and α2 that may be influenced by phosphorylation of PLM are highlighted

Fig. 3

The many faces of phospholemman. The NMR structure of PLM (2JO1.pdb [137, 138]) is in the center, and the post-translational modifications of PLM discussed in the text are shown. The functional effect of each modification on pump activity (compared to unmodified PLM) is indicated by green (for activation), red (for inhibition), or both red and green where this remains to be determined

Fig. 4

Position of the palmitoylation sites of phospholemman relative to the α subunit. PLM is shown in green, α subunit transmembrane region in blue, N domain in cyan, P domain in yellow, and A domain in purple. The β subunit is red. Sodium (purple spheres) is shown in its proposed binding sites (reviewed in [6]). Helices H2, H3, and H4 of PLM are labeled. C42 of PLM is orientated towards the α subunit, and C40 away, meaning palmitoylation of C42 may pull PLM H3 across the intracellular mouth of a sodium-binding site in the α subunit. Conversely, palmitoylation of C40 would oppose such a movement by pulling H3 in the opposite direction

Fig. 5

Summary of signaling pathways leading to sodium pump regulation in the heart. Green arrows indicate activation of the next step in the pathway, red arrows indicate inhibition. Full details are discussed in “Phospholemman the kinase target” to “Phospholemman: integrating multiple post-translational modifications?”