Na /H exchanger NHE1 is active at cell-cell contacts and facilitates cell dissemination during collective migration of melanoma cells.

Tumour cell detachment from the primary tumour is an early and crucial step of the metastatic cascade. At the single cell level, it was already shown that migrating melanoma cells establish both intra- and extracellular pH gradients and that the Na /H exchanger NHE1 accumulates at the leading edges to strengthen cell-matrix interactions. However, less is known about the role of NHE1 in collective cell migration and the specific pH microenvironment at tumour cell-cell contacts. We used MV3 melanoma cells transfected with a NHE1-expressing vector or a control vector. NHE1 localization at cell-cell contacts was assessed via immunofluorescence imaging. Collective migration was analysed by live-cell imaging. The NHE1 activity and the perimembranous pH were measured both intra- and extracellularly by ratiometric fluorescence microscopy. NHE1 clearly localizes at cell-cell contacts. Its overexpression further increases migratory speed and translocation in multidirectional pathway analyses. NHE1 overexpressing MV3 cells also move further away from their neighbouring cells during wound closure assays. pH measurements revealed that the NHE1 is highly active at cell-cell contacts of melanoma cells. NHE1-mediated pH dynamics at such contact sites are more prominent in NHE1-overexpressing melanoma cells. Our findings highlight the contribution of the NHE1 towards modulation and plasticity of melanoma cell-cell contacts. We propose that its localization and functional activity at cell-cell contacts promotes evasion of single melanoma cells from the primary tumour.