NaBC1 Acts as a Mechanosensitive Co-regulator of Fibronectin-binding Integrins Adhesion and Myoblast Polarization

Cell–matrix interactions are central to the regulation of cell mechanics, polarity, and signal transduction. In this study, we investigate the contribution of the borate transporter NaBC1 to myoblast adhesion dynamics and mechanotransductive responses. Using C2C12 myoblasts cultured on fibronectin-coated substrates, we show that NaBC1 activation rapidly reinforces cell–substrate attachment, leading to accelerated spreading and the establishment of polarized cell morphologies. These changes are associated with the assembly of enlarged focal adhesions and a marked slowdown of actin retrograde flow, consistent with increased force transmission across adhesion sites. NaBC1 stimulation also induces a coordinated increase in the expression of fibronectin-binding integrins, including α 5 β 1 and α v β 3 , at both transcriptional and protein levels. Proximity ligation assays reveal an enhanced spatial association between NaBC1 and these integrins at the cell membrane, supporting the formation of cooperative adhesion complexes. In parallel, fluorescently labelled boron accumulates at focal adhesions and within intracellular compartments such as mitochondria, lysosomes, and the endoplasmic reticulum, suggesting a link between NaBC1-dependent adhesion signaling and subcellular organization. Importantly, the adhesive and mechanotransductive effects driven by NaBC1 are strictly dependent on fibronectin integrity and are lost on mutant fibronectins lacking RGD or synergy motifs, as well as on laminin-111 substrates that do not support molecular clutch engagement. Together, these findings identify NaBC1 as an integral component of fibronectin–integrin adhesion systems, contributing to the regulation of myoblast mechanics and polarity through cooperative interactions with fibronectin-binding integrins.