Acidification of the cytosol inhibits the uptake of tetanus toxin in NG108-15 and NBr-10A neurohybridoma cells

The influence of cytosol acidification on the uptake of two-chain tetanus toxin (TeTX)1 by neurohybridoma cells NG 108-15 and NBr-l0A was investigated with two established techniques, the NH4C1 pulse method and the pH-clamp method. With the former, the extracellular pH is maintained at its physiological value, but is set to different values with the latter. - Acidification of the cytoplasm with an NH4Cl pulse retarded the uptake of TeTX by both NG 108-15 and NBr-l0A cells. This result provides further evidence for a vesicular endocytotic uptake of TeTX. In contrast, acidification of both the external medium and the cytoplasm (pH-clamp method) resulted in a net increase of toxin uptake. This result is explained as follows: Acidification of the extracellular environment has been shown to facilitate the uptake of tetanus toxin, and under pH clamp conditions, this effect is stronger than the simultaneous retardation of the toxin uptake by acidification also of the cytosol.