Sciatic nerve conditioned medium depleted of pro-NGF modulates sodium currents and neurite outgrowth in PC12 cells.

Excitability and axon/dendrite specification are the most distinctive features in the establishment of neuronal polarization. Conditioned medium from rat sciatic nerve (CM) induced a neuronal-like morphology in PC12 cells. Here we show that CM neuritogenic activity is limited to the induction of dendrites in PC12 cells. However, treatment of these cells with CM in combination with a generic inhibitor for tyrosine kinase receptors (k252a) promoted the enhancement of neurite length, development of axons and induction of sodium currents. On the other hand, specific inhibition of TrkA and p75(NTR) receptors in CM-treated cells reduced the neurite length in comparison with cells treated only with CM, although the effect over the induction of sodium currents was continuously observed. These results suggested that CM had some components that, even though are able to start the morphological cell differentiation and produce short neurites (likely acting through TrkA and p75(NTR)), can restrain further neurite extension. Depletion of pro-NGF isoforms from CM produced a similar effect as the exerted by k252a, TrkA and p75(NTR) receptor inhibitors in CM-treated cells, inducing the elicitation of sodium currents. These results suggested that the effect of CM might be mediated through pro-NGF. The difference between the results obtained with the generic inhibitor for Trk receptors and the specific inhibitors for TrkA and p75(NTR) receptors in CM-treated cells, suggested that alternative pathways could be used to regulate neurite elongation, axon specification and sodium currents in PC12 cells. These findings represent important clues to improve the understanding of the initiation of neuronal polarity.