Isolated plasma membranes regulate neurotransmitter expression and facilitate effects of a soluble brain cholinergic factor

The choice of which transmitter will be expressed by a neuron is influenced by diffusible differentiating factors produced by a variety of nonneuronal cells. In this study we show that human, bovine, and rat brains contain a soluble heparin-binding factor that stimulates cholinergic and peptidergic expression. We also show that neuronal contact with other cell membranes influences neurotransmitter development and acts synergistically with the effects of the soluble brain factor. Exposure of cultured sympathetic neurons to purified plasma membranes derived either from cultured Schwann cells or from cultured sympathetic neurons promoted expression of choline acetyltransferase (CAT), a cholinergic trait, and of substance P (SP). CAT activity and SP were also stimulated by a 50-kDa soluble protein that was purified 14,000-fold from human, calf, and rat brain by heparin-Sepharose affinity chromatography. CAT activity after concurrent treatment with plasma membranes and the soluble factor far exceeded the sum of the enzyme activities after the individual treatments, suggesting that membrane molecules and the factor facilitated each other. Thus, cell-surface molecules, which have been shown previously to influence neuronal morphogenesis and neurite elongation, may also help determine the transmitter phenotype of the neuron. Moreover, cell-surface molecules may modulate the effects of diffusible differentiating factors.