The β2-Subunit (AMOG) of Human Na+, K+-ATPase Is a Homophilic Adhesion Molecule

Abstract

The β2 subunit of Na+, K+-ATPase was originally identified as the adhesion molecule on glia (AMOG) that mediates the adhesion of astrocytes to neurons in the central nervous system and that is implicated in the regulation of neurite outgrowth and neuronal migration. While β1 isoform have been shown to trans-interact in a species-specific mode with the β1 subunit on the epithelial neighboring cell, the β2 subunit has been shown to act as a recognition molecule on the glia. Nevertheless, none of the works have identified the binding partner of β2 or described its adhesion mechanism. Until now, the interactions pronounced for β2/AMOG are heterophilic cis-interactions. In the present report we designed experiments that would clarify whether β2 is a cell–cell homophilic adhesion molecule. For this purpose, we performed protein docking analysis, cell–cell aggregation, and protein–protein interaction assays. We observed that the glycosylated extracellular domain of β2/AMOG can make an energetically stable trans-interacting dimer. We show that CHO (Chinese Hamster Ovary) fibroblasts transfected with the human β2 subunit become more adhesive and make large aggregates. The treatment with Tunicamycin in vivo reduced cell aggregation, suggesting the participation of N-glycans in that process. Protein–protein interaction assay in vivo with MDCK (Madin-Darby canine kidney) or CHO cells expressing a recombinant β2 subunit show that the β2 subunits on the cell surface of the transfected cell lines interact with each other. Overall, our results suggest that the human β2 subunit can form trans-dimers between neighboring cells when expressed in non-astrocytic cells, such as fibroblasts (CHO) and epithelial cells (MDCK).

Mesenchymal Stem/Stromal Cells in Three-Dimensional Cell Culture: Ion Homeostasis and Ouabain-Induced Apoptosis

“…It is also established that the Na + ,K + -ATPase promotes adhesion and the formation of tight junctions due to the self-adhesive properties of the β-subunit, and E-cadherin binds to the β-subunit, forming a complex for adhesion [49][50][51]. Recent studies also suggest that the different isoforms of the β-subunit of Na,K-ATPase are implicated in regulating cell motility during cancer progression [52,53]. We propose that cell-cell interactions during spheroid formation can contribute to the optimization of ion pump activity and the maintenance of transmembrane gradients of Na + and K + , which are necessary for the normal functioning of eMSCs-in particular, to enhance the paracrine activity of cells.…”

Na+/K+-ATPase: More than an Electrogenic Pump

“…Early research indicated that neuro–glial interactions are mediated by the adhesion between the AMOG/β2-subunit and an unidentified partner on the neuron membrane [ 104 , 105 , 106 ]. More recently, studies on heterologous expression systems have suggested the possibility of a homophilic β2–β2 interaction, although evidence for such interactions within the nervous system is lacking [ 107 ]. Based on early findings, Schachner’s group has proposed that astrocytes do not adhere to each other through AMOG/β2 [ 108 ], suggesting a potential β2–β2 interaction between astrocytes and cerebellar granular cells.…”

Recent Advances in Protein–Protein Interactions

“…Shoshani et al [ 1 ] consider the challenging task to identify the binding partner of β 2 subunit of human Na + , K + -ATPase mediating the adhesion of astrocytes to neurons. Surprisingly, their results allow to suggest that this protein is a homophilic cell-adhesion molecule when expressed on the plasma membrane of non-astrocytic cells such as fibroblasts or epithelial cells.…”