Evidence that the H1‐H2 domain of α1 subunit of (Na++K+)‐ATPase participates in the regulation of cardiac contraction

Sensational site: the sodium pump ouabain-binding site and its ligands

Cardiotonic steroids (CTS), used by certain insects, toads, and rats for protection from predators, became, thanks to Withering's trailblazing 1785 monograph, the mainstay of heart failure (HF) therapy. In the 1950s and 1960s, we learned that the CTS receptor was part of the sodium pump (NKA) and that the Na+/Ca2+ exchanger was critical for the acute cardiotonic effect of digoxin- and ouabain-related CTS. This "settled" view was upended by seven revolutionary observations. First, subnanomolar ouabain sometimes stimulates NKA while higher concentrations are invariably inhibitory. Second, endogenous ouabain (EO) was discovered in the human circulation. Third, in the DIG clinical trial, digoxin only marginally improved outcomes in patients with HF. Fourth, cloning of NKA in 1985 revealed multiple NKA α and β subunit isoforms that, in the rodent, differ in their sensitivities to CTS. Fifth, the NKA is a cation pump and a hormone receptor/signal transducer. EO binding to NKA activates, in a ligand- and cell-specific manner, several protein kinase and Ca2+-dependent signaling cascades that have widespread physiological effects and can contribute to hypertension and HF pathogenesis. Sixth, all CTS are not equivalent, e.g., ouabain induces hypertension in rodents while digoxin is antihypertensinogenic ("biased signaling"). Seventh, most common rodent hypertension models require a highly ouabain-sensitive α2 NKA and the elevated blood pressure is alleviated by EO immunoneutralization. These numerous phenomena are enabled by NKA's intricate structure. We have just begun to understand the endocrine role of the endogenous ligands and the broad impact of the ouabain-binding site on physiology and pathophysiology.

Antibody engineering and therapeutics, The Annual Meeting of the Antibody Society: December 8-12, 2013, Huntington Beach, CA

The 24^th Antibody Engineering & Therapeutics meeting brought together a broad range of participants who were updated on the latest advances in antibody research and development. Organized by IBC Life Sciences, the gathering is the annual meeting of The Antibody Society, which serves as the scientific sponsor. Preconference workshops on 3D modeling and delineation of clonal lineages were featured, and the conference included sessions on a wide variety of topics relevant to researchers, including systems biology; antibody deep sequencing and repertoires; the effects of antibody gene variation and usage on antibody response; directed evolution; knowledge-based design; antibodies in a complex environment; polyreactive antibodies and polyspecificity; the interface between antibody therapy and cellular immunity in cancer; antibodies in cardiometabolic medicine; antibody pharmacokinetics, distribution and off-target toxicity; optimizing antibody formats for immunotherapy; polyclonals, oligoclonals and bispecifics; antibody discovery platforms; and antibody-drug conjugates.

Dual activity of the H1-H2 domain of the (Na(+)+K+)-ATPase

(Na(+)+K(+))-ATPase is a target receptor of digitalis (cardiac glycoside) drugs. It has been demonstrated that the H1-H2 domain of the alpha-subunit of the (Na(+)+K(+))-ATPase is one of the digitalis drug interaction sites of the enzyme. Despite the extensive studies of the inhibitory effect of digitalis on the (Na(+)+K(+))-ATPase, the functional property of the H1-H2 domain of the enzyme and its role in regulating enzyme activity is not completely understood. Here we report a surprise finding: instead of inhibiting the enzyme, binding of a specific monoclonal antibody SSA78 to the H1-H2 domain of the (Na(+)+K(+))-ATPase elevates the catalytic activity of the enzyme. In the presence of low concentration of ouabain, monoclonal antibody SSA78 significantly protects enzyme function against ouabain-induced inhibition. However, higher concentration of ouabain completely inactivates the (Na(+)+K(+))-ATPase even in the presence of SSA78. These results suggest that the H1-H2 domain of the (Na(+)+K(+))-ATPase is capable of regulating enzyme function in two distinct ways for both ouabain-sensitive and -resistant forms of the enzyme: it increases the activity of the (Na(+)+K(+))-ATPase during its interaction with an activator; it also participates in the mechanism of digitalis or ouabain-induced inhibition of the enzyme. Understanding the dual activity of the H1-H2 domain will help better understand the structure-function relationships of the (Na(+)+K(+))-ATPase and the biological processes mediated by the enzyme.

Activation of (Na++K+)-ATPase induces positive inotropy in intact mouse heart in vivo

OBJECTIVE

We have recently identified an activation site on (Na+ + K+)-ATPase and found that binding of antibody SSA412 to this specific site of the enzyme markedly augments (Na+ + K+)-ATPase catalytic activity. Demonstration of whether activation of (Na+ + K+)-ATPase affects heart function in animal in vivo was the object of this investigation.

METHODS

Male wild-type CD-1 mouse and specific antibody SSA412 were used for the study. A pressure-volume micromanometer-conductance catheter in anesthetized mouse assessed in vivo cardiac functions.

RESULTS

Specific antibody SSA412 infusion in mouse shifted pressure-volume loop leftward with increased stroke volume and enhanced end-systolic elastance. Global systolic parameters such as ejection fraction and cardiac output, and load independent contractile parameters including dP/dtmax/IP, PMX/EDV, Ees, and PRSW, were all increased without any effect on relaxation following administration of SSA412. Cardiac preload indexed by EDV and afterload by ESP did not alter, suggesting that SSA412-enhanced myocardial performance is a direct cardiac effect caused by the activation of (Na+ + K+)-ATPase.

CONCLUSION

Our study provides the first in vivo physiological evidence to demonstrate that activation of (Na+ + K+)-ATPase induces significant positive inotropic effect in intact animal heart. The finding may lead to new therapeutic strategies for the treatment of heart failure.

Activation of (Na+ + K+)-ATPase

Enzymes catalyze essential chemical reactions needed for living processes. (Na+ +K+)-ATPase (NKA) is one of the key enzymes that control intracellular ion homeostasis and regulate cardiac function. Little is known about activation of NKA and its biological impact. Here we show that native activity of NKA is markedly elevated when protein-protein interaction occurs at the extracellular DVEDSYGQQWTYEQR (D-R) region in the alpha-subunit of the enzyme. The apparent catalytic turnover of NKA is approximately twice as fast as the controls for both ouabain-resistant and ouabain-sensitive enzymes. Activation of NKA not only markedly protects enzyme function against denaturing, but also directly affects cellular activities by regulating intracellular Ca2+ transients and inducing a positive inotropic effect in isolated rat cardiac myocytes. Immunofluorescent labeling indicates that the D-R region of NKA is not a conventional digitalis-binding site. Our findings uncover a novel activation site of NKA that is capable of promoting the catalytic function of the enzyme and establish a new concept that activating of NKA mediates cardiac contraction.