Tightly regulated cell migration is pivotal in development, immune responses, wound healing, and tissue homeostasis, and dysregulation of migration is associated with severe pathophysiological conditions such as cancer and developmental defects. Activation of Platelet-Derived Growth Factor receptors (PDGFR) -a and -ß by their ligands plays important roles in control of directional migration. PDGFRa is targeted to the primary cilium, an essential sensory organelle found on most growth-arrested mammalian cells. We recently showed that the primary cilium coordinates PDGFRa-mediated directional migration in growth-arrested fibroblasts (1). PDGFR stimulation also activates the plasma membrane Na⁺/H⁺ exchanger, NHE1, which we and others have shown to play central roles in cell migration (2). This raises the question of whether there is a functional relationship between ciliary PDGFRa signaling and NHE1 in directional migration, and the aim of this study was to explore this possibility. NHE1 mRNA and protein levels were doubled and tripled, respectively, in NIH3T3 cells after 48 h serum starvation (growth arrest), and a similar pattern was seen in mouse embryonic fibroblasts (MEFs). NHE1 upregulation was concomitant with the formation of the primary cilium, yet was not dependent on its presence, as upregulation was similar in Tg737^orpk MEFs, which exhibit no or only very short primary cilia. In contrast, NHE1-dependent recovery after an acute acid load was strongly attenuated in growth-arrested compared to interphase NIH3T3 cells, presumably reflecting stringent posttranslational control of NHE1 activity. NHE1-dependent pH_i recovery was rescued by the specific PDGFRa ligand, PDGF-AA (50 ng/ml), thus, NHE1 is activated through PDGFRa signaling. Cell migration in 5 h wound healing assays was quantified as (i) translocation, i.e. distance covered by the cell, and (ii) directional migration, the velocity of migration in direction of the wound. PDGF-AA stimulated translocation and directional migration of growth-arrested NIH3T3 cells and wt MEFs, but not of Tg737^orpk MEFs. NHE1 inhibition by EIPA potently reduced PDGF-AA-mediated translocation, and even more dramatically directional migration, in growth-arrested NIH3T3 cells and wt MEFs. PDGF-AA failed to stimulate migration in NHE1-null fibroblasts. EIPA affected neither cilia formation nor PDGFRa signalling, and a similar pattern was seen after NHE1 knockdown. The inhibitory effect of EIPA was strongly attenuated in interphase NIH3T3 cells, which are devoid of primary cilia, and in Tg737^orpk MEFs, suggesting that the role of NHE1 in cell migration is dependent on signalling via the primary cilium. In conclusion, we show here that directional migration stimulated by ciliary PDGFRa signaling is specifically dependent on NHE1 activity.

1. Schneider,L.et al. J Cell Biol. submitted, 2008

2. Stock, C. and A.Schwab. 2006. Acta Physiol 187:149–157