Their glycolytic metabolism imposes an increased acid load upon tumour cells. The surplus protons are extruded by the Na + /H + exchanger (NHE) which causes an extracellular acidification. It is not yet known by what mechanism extracellular pH (pH e ) and NHE activity affect tumour cell migration and thus metastasis. We studied the impact of pH e and NHE activity on the motility of human melanoma (MV3) cells. Cells were seeded on/in collagen I matrices. Migration was monitored employing time lapse video microscopy and then quantified as the movement of the cell centre. Intracellular pH (pH i ) was measured fluorometrically. Cell-matrix interactions were tested in cell adhesion assays and by the displacement of microbeads inside a collagen matrix. Migration depended on the integrin α2β1. Cells reached their maximum motility at pH e ∼7.0. They hardly migrated at pH e 6.6 or 7.5, when NHE was inhibited, or when NHE activity was stimulated by loading cells with propionic acid. These procedures also caused characteristic changes in cell morphology and pH i . The changes in pH i , however, did not account for the changes in morphology and migratory behaviour. Migration and morphology more likely correlate with the strength of cell-matrix interactions. Adhesion was the strongest at pH e 6.6. It weakened at basic pH e , upon NHE inhibition, or upon blockage of the integrin α2β1. We propose that pH e and NHE activity affect migration of human melanoma cells by modulating cell-matrix interactions. Migration is hindered when the interaction is too strong (acidic pH e ) or too weak (alkaline pH e or NHE inhibition).

Migration and morphology of human melanoma cells (MV3) depend on extracellular pH (pHe) and the activity of the Na+/H+ exchanger NHE1. To distinguish effects of NHE1 activity per se from effects of pHe we compared an NHE1-deficient mutant with rescued and wild-type cells. Time lapse video microscopy was used to investigate migratory and morphological effects caused by pHe and NHE1 activity, and a membrane-bound fluorescein conjugate was employed for ratiometric pH measurements at the outer leaflet of the cell membrane. As long as NHE1 remained inactive due to deficiency or inhibition by cariporide (HOE642) neither migration nor morphology was affected by changes in pHe. Under these conditions pH at the outer leaflet of the plasma membrane was uniform all over the cell surface. The typical pH dependence of MV3 cell migration and morphology could be reconstituted by restoring NHE1 activity. At the same time the proton gradient at the outer leaflet of the plasma membrane with the higher proton concentration at the leading edge and the lower one at the cell rear was re-established as well. Hence, NHE1 activity generates a proton gradient at the cell surface accompanied by the cells' ability to respond to changes in pHe (bulk pH). We conclude that NHE1 activity contributes to the generation of a well-defined cell surface pH by creating a proton gradient at the outer leaflet of the plasma membrane that is needed for (i) the development of a variety of morphologies including a distinct polarity and (ii) migration. A missing proton gradient at the cell surface cannot be compensated for by varying pHe.

Extracellular pH and the Na⁺/H⁺ exchanger (NHE1) modulate tumor cell migration. Yet, the pH nanoenvironment at the outer surface of the cell membrane (pH_em) where cell/matrix interaction occurs and matrix metalloproteinases work was never measured. We present a method to measure this pH nanoenvironment using proton-sensitive dyes to label the outer leaflet of the plasma membrane or the glycocalyx of human melanoma cells. Polarized cells generate an extracellular proton gradient at their surface that increases from the rear end to the leading edge of the lamellipodium along the direction of movement. This gradient collapses upon NHE1 inhibition by HOE642. NHE1 stimulation by intracellular acidification increases the difference in pH_em between the tips of lamellipodia and the cell body in a Na⁺ dependent way. Thus, cells create a pH nanoenvironment that promotes cell migration by facilitating cell adhesion at their front and the release of cell/matrix contacts at their rear part.

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SummaryIn fibroblasts, platelet-derived growth factor receptor alpha (PDGFRa) is upregulated during growth arrest and compartmentalized to the primary cilium. PDGF-AA mediated activation of the dimerized ciliary receptor produces a phosphorylation cascade through the PI3K-AKT and MEK1/2-ERK1/2 pathways leading to the activation of the Na + /H + exchanger, NHE1, cytoplasmic alkalinization and actin nucleation at the lamellipodium that supports directional cell migration. We here show that AKT and MEK1/2-ERK1/2-p90 RSK inhibition reduced PDGF-AA-induced cell migration by distinct mechanisms: AKT inhibition reduced NHE1 activity by blocking the translocation of NHE1 to the cell membrane. MEK1/2 inhibition did not affect NHE1 activity but influenced NHE1 localization, causing NHE1 to localize discontinuously in patches along the plasma membrane, rather than preferentially at the lamellipodium. We also provide direct evidence of NHE1 translocation through the cytoplasm to the leading edge. In conclusion, signals initiated at the primary cilium through the PDGFRaa cascade reorganize the cytoskeleton to regulate cell migration differentially through the AKT and the MEK1/2-ERK1/2-p90 RSK pathways. The AKT pathway is necessary for initiation of NHE1 translocation, presumably in vesicles, to the leading edge and for its activation. In contrast, the MEK1/2-ERK1/2-p90 RSK pathway controls the spatial organization of NHE1 translocation and incorporation, and therefore specifies the direction of the leading edge formation.