Na,K-ATPase as signaling transducer

Abstract

It is now generally agreed that Na,K-ATPase (NKA), in addition to its role in the maintenance of Na+ and K+ gradients across the cell membrane, is a signal transducer. Our group has identified a novel signaling pathway where NKA interact with IP3R to form a signaling microdomain. Ouabain, a specific ligand of NKA, activates this pathway, triggers slow Ca2+ oscillations and activates NF-κB. In current study, the molecular mechanisms and some important downstream effects of NKA signaling are demonstrated.

The critical binding motifs in NKA/IP3R signaling microdomain and the role of ankyrin B (Ank- B) were demonstrated. N-terminal tail of the NKA α-subunit (αNT-t) binds directly to N terminus of IP3R. Three amino acid residues, LKK, conserved in most species and α-isoforms of NKA, are essential for binding. Ank-B expressed in most mammalian cells, plays a pivotal role for NKA/IP3R signalosome. The N-terminal tails of NKA a subunit and IP3R are novel binding sites for Ank-B. In Ank-B silenced cells, the interaction between NKA and IP3R is decreased.

The role of NKA/IP3R signaling for activation of NF-κB was elucidated. Cells overexpressing a peptide corresponding to αNT-t, which binds to IP3R and blocks NKA/IP3R binding, suppresses ouabain’s effect. Knockdown of Ank-B abolishes the ouabain effect on NF-κB.

The downstream effects of this signaling pathway include protection from apoptosis and stimulation of cell proliferation. Ouabain (nM) completely abolishes serum deprivation induced apoptosis. Ouabain protection from apoptosis is not observed in cells overexpressing a mutant NKA α subunit with deletion of the N-terminal tail or a peptide corresponding to αNT-t. Both of them block the interaction between NKA and IP3R. Inhibition of Ca2+release from intracellular stores via IP3R or inhibition of NF-κB activity abolishes the anti-apoptotic effect of ouabain. Ouabain stimulates cell proliferation which depends on Ca2+release via IP3R.

Activation of this signaling pathway rescues nephrogenesis in growth factor deprived embryonic rat kidney. Exposure to ouabain triggers Ca2+oscillations and activates NF-κB in embryonic kidney cells. Growth factor deprivation retards formation of new glomeruli and increases apoptotic index. Ouabain (nM) completely prevents these effects. The protective effects of ouabain are abolished by depletion of intracellular Ca2+ stores and by inhibition of NF-κB. The expression of the inductive factors Wt1 and Pax2 activated by NF-κB, are increased in ouabain exposed growth factor deprived kidneys. Thus we have identified a novel mechanism by which kidney development can be protected under adverse intrauterine circumstances.

In conclusion, this thesis demonstrates that NKA directly binds to IP3R to form a signaling microdomain and Ank-B tethers this binding. Ouabain activates this signaling pathway that results in NF-κB activation, the downstream effects of which are stimulation of cell proliferation, protection from apoptosis and rescue of growth factor deprivation-induced inhibition of embryonic kidney nephrogenesis.