Mitogenic factors regulate ion channels in Schwann cells cultured from newborn rat sciatic nerve

Abstract

1. Patch clamp studies were carried out in Schwann cells cultured from newborn rat sciatic nerve to determine the effects of mitogens on voltage-gated currents without the confounding influences of axonal contact and myelin present in vivo. The relevance of the various Schwann cell currents to proliferation was assessed using assays of [3H]thymidine incorporation. 2. Treatment of cultured Schwann cells with known mitogens, namely axon fragments (AF), myelin fragments (MF), or glial growth factor in combination with forskolin (GGF+F), increased the magnitudes of delayed rectifying potassium (K+) and sodium (Na+) currents. 3. In both control and mitogen-treated cells, the magnitude of net outward current paralleled clearly the magnitude of the cells' proliferative response. 4. The K+ channel-blocking quaternary ammonium ions, tetrabutylammonium (TBuA), tetrapentylammonium (TPeA) and tetrahexylammonium (THeA), but not the Na+ channel blocker tetrodotoxin (TTX), reduced proliferation in a dose-dependent fashion offering further evidence for a role for K+ channels in Schwann cell proliferation. 5. Voltage-gated chloride (Cl-) currents were observed in both control and mitogen-treated cells. Addition of the Cl- channel blockers, 4-acetamido-4'-isocyanatostilbene-2,2'-disulphonate (SITS) or 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS), to the culture media enhanced proliferation. 6. The possible intermediary role of the Schwann cell resting potential was explored in ion substitution experiments by increasing the K+ concentration of the media and by adding ouabain. Both manipulations inhibited Schwann cell mitosis. 7. Comparison of the expression of functional ion channels in vitro with that previously described for Schwann cells in vivo suggests a difference in the Schwann cell response to the membrane fragment mitogens and their intact counterparts in regard to the regulation of ion channels. MF up-regulates the number of functional channels, whereas the elaboration of myelin (or a factor related to its presence) in vivo appears to down-regulate channel expression, at the cell soma of myelinating Schwann cells. In addition, axonal contact may be required for normal expression of functional inwardly rectifying K+ channels.