The importance of the Thr17 residue of phospholamban as a phosphorylation site under physiological and pathological conditions

Abstract

The sarcoplasmic reticulum (SR) Ca 2+ -ATPase (SERCA2a) is under the control of an SR protein named phospholamban (PLN). Dephosphorylated PLN inhibits SERCA2a, whereas phosphorylation of PLN at either the Ser 16 site by PKA or the Thr 17 site by CaMKII reverses this inhibition, thus increasing SERCA2a activity and the rate of Ca 2+ uptake by the SR. This leads to an increase in the velocity of relaxation, SR Ca 2+ load and myocardial contractility. In the intact heart, ß-adrenoceptor stimulation results in phosphorylation of PLN at both Ser 16 and Thr 17 residues. Phosphorylation of the Thr 17 residue requires both stimulation of the CaMKII signaling pathways and inhibition of PP1, the major phosphatase that dephosphorylates PLN. These two prerequisites appear to be fulfilled by ß-adrenoceptor stimulation, which as a result of PKA activation, triggers the activation of CaMKII by increasing intracellular Ca 2+ , and inhibits PP1. Several pathological situations such as ischemia-reperfusion injury or hypercapnic acidosis provide the required conditions for the phosphorylation of the Thr 17 residue of PLN, independently of the increase in PKA activity, i.e., increased intracellular Ca 2+ and acidosis-induced phosphatase inhibition. Our results indicated that PLN was phosphorylated at Thr 17 at the onset of reflow and immediately after hypercapnia was established, and that this phosphorylation contributes to the mechanical recovery after both the ischemic and acidic insults. Studies on transgenic mice with Thr 17 mutated to Ala (PLN-T17A) are consistent with these results. Thus, phosphorylation of the Thr 17 residue of PLN probably participates in a protective mechanism that favors Ca 2+ handling and limits intracellular Ca 2+ overload in pathological situations.

Designing Heart Performance by Gene Transfer

“…PLN, a small pentameric protein complex comprised of 6-kDa monomers (55), dynamically regulates SERCA2a function on a beat-to-beat basis by its phosphorylation state. It is the interplay between kinases and phosphatases that determines PLN's phosphorylation status (539). In the unphosphorylated state, PLN lowers the affinity of SERCA2a for Ca 2ϩ , thereby inhibiting Ca 2ϩ transport (25, 451).…”

“…Phosphorylation of PLN by PKA at Ser-16 or by CAMKII at Thr-17 reverses the PLN-mediated inhibition of SERCA2a (see also sect. VI) (539). PLN phosphorylation is an important contributor to the hastening of myocardial relaxation during ␤-adrenergic stimulation as this PLN modification increases Ca 2ϩ reuptake into the SR (950).…”

“…PLN phosphorylation status is contingent on the activity of PP1 (106,539), the phosphatase involved in counteracting PKA-mediated phosphorylation of PLN. PP1 dephosphorylates PLN which in turn inhibits SERCA2a activity.…”

“…PP1 dephosphorylates PLN which in turn inhibits SERCA2a activity. An additional level of regulation is due to the actions of inhibitor 1 (I-1) protein (539). When activated by PKA, I-1 negatively controls the phosphatase activity of PP1 (539).…”

“…An additional level of regulation is due to the actions of inhibitor 1 (I-1) protein (539). When activated by PKA, I-1 negatively controls the phosphatase activity of PP1 (539). The resulting I-1-dependent inhibition of PP1 maximizes the phosphorylation state of PLN, thereby increasing SERCA2a function.…”