Na,K-beta – a master chaperone for ion transporters and adhesion molecules

Selva, Erica M.Barwe, Sonali P.Cell membranes are enriched with a variety of proteins that play various roles in maintaining cell architecture and function. Structural and functional integrity of epithelial cells are maintained by synergistic interaction between adherent and tight junctions. Extensive studies have been carried out to understand the architecture of these complexes and the signal transducers that aid in maintaining the morphology of epithelia. Na,K-ATPase is a very well established membrane protein involved in maintaining ion homeostasis by mediating [Na+] efflux and formation of tight junctions. It is a hetero-oligomer with a catalytic α-subunit (Na,K-α) and a regulatory β-subunit (Na,K-β). Historically, Na,K-β was identified as a chaperone for the transport and stabilization of its cognate partner Na,K-α (Geering, 1997). Na,KATPase expression governs intracellular Na+ and Ca2+ concentration thus assisting the formation and maintenance of tight junctions, governing epithelial polarity (Rajasekaran et al., 2001b). Further studies have reported that Na,K-β functions synergistically with well-known cell adhesion molecule, E-cadherin regulating epithelial architecture. Moreover Na,K-β plays an important role in cell-cell adhesion and suppression of cell motility (Barwe et al., 2005; Rajasekaran et al., 2001b; Shoshani et al., 2005; Tokhtaeva et al., 2011; Vagin et al., 2006). In my study, I explore the molecular basis of the functional synergism between E-cadherin and Na,K-β, and provide evidence that Na,K-β associates with E-cadherin and regulates its membrane localization, acting as a chaperone (Chapter 2). Recent studies have demonstrated that Na,K-β is a binding partner essential for membrane localization of other ion transporters apart from Na,K-α. 1) Na,K-β interacts and targets the transport of large conductance Ca2+ activated K+ channels (BKCa) to specific regions of plasma membrane (Jha and Dryer, 2009) and also 2) Na,K-β regulates Na,K,2Cl co-transporter (NKCC2) channel exocytosis and membrane expression (Carmosino et al., 2014). Though Na,K-α is known to functionally couple with Na+, Ca2+ exchanger 1 (NCX1), the role of Na,K-β in the regulation of membrane expression of NCX1 remained unexplored. My study focused on elucidating the role of Na,K-β in governing NCX1 expression. The results indicate that Na,K-β mediates membrane trafficking of NCX1 that is required for suppression of Ca2+-dependent epithelial cell migration (Chapter 3) (Balasubramaniam et al., 2015b). Taken together these studies suggest that in addition to its auxiliary role in aiding the trafficking of these proteins, Na,K-β also stabilizes their membrane expression. The stabilization of these proteins at the membrane is essential for their steady-state expression and function. These interesting observations suggest the collaboration of ion transporters and adhesion proteins regulate the epithelial phenotype. Loss of Na,K-β expression has been associated with EMT that leads to cancer progression and renal fibrosis (Rajasekaran et al., 1999; Rajasekaran et al., 2010). My study indicates that the loss of Na,K-β involves the changes in the expression and function of other membrane proteins, in addition to having a direct correlation between expression and cancer prognosis. Thus it can be inferred that during the loss of Na,K-β in pre-neoplastic lesions, a plethora of events are unleashed by downregulation of adhesion molecules and ion transporters, culminating in cancer.University of Delaware, Department of Biological SciencesPh.D