Neuronal aggregation and neurotransmitter regulation: Partial purification and characterization of a membrane‐derived factor

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Abstract
Cell membrane contact induces marked differential changes in neurotransmitter expression. In cultures of virtually pure dissociated sympathetic neurons, when such contact is provided by either high cell densities or addition of membranes derived from specific tissues, there is a marked increase in cell‐specific content of substance P and de novo induction of choline acetyltransferase. To identify molecular mechanisms underlying regulation of transmitter expression by neuronal aggregation and membrane contact, we have begun to isolate and characterize a membrane‐associated factor responsible for stimulation of choline acetyltransferase activity. The factor was found in substantial quantities in membranes from adult rat spinal cord as well as from sympathetic and sensory ganglia. Ionic mechanisms were employed to extract transmitter‐inducing activity from spinal cord membranes in soluble form. The solubilized factor was then partially purified by ion exchange and gel filtration chromatography. It appears to be an extrinsic (non‐integral) protein with an apparent molecular weight of 27. It is inactivated by trypsin and chymotrypsin, but is only moderately sensitive to heat inactivation, retaining activity at 60°C but not at 90°C. Neuronal perikaryal contact via aggregation represents a critical mechanism by which neurons themselves may influence phenotypic expression. Membrane localization of the factor provides a means by which cell contact may regulate transmitter expression.