SEROTONIN-ACTIVATED ALPHA-2-MACROGLOBULIN INHIBITS NEURITE OUTGROWTH AND SURVIVAL OF EMBRYONIC SENSORY AND CEREBRAL CORTICAL-NEURONS

Methylamine-modified alpha-2-macroglobulin (MA-alpha2M) has been recen tly shown to inhibit the biological activity of beta-nerve growth fact or (NGF) in promoting neurite outgrowth by embryonic dorsal root gangl ia in culture (Koo PH, Liebl DJ, J Neurosci Res 31:678-692, 1992). The objectives of this study are to determine whether alpha2M can also be modified by larger aromatic biogenic amines such as 5-hydroxytryptami ne (5HT; serotonin), the nature of interaction between NGF and 5HT-mod ified alpha-2-M (5HT-alpha2M), and the effect of 5HT-alpha2M on the ne urite extension and the growth of embryonic sensory and cholinergic ne urons in 2 disparate animal species (chicken and rats). This study dem onstrates that each mole of alpha2M can combine with 15.2 +/- 1.8 mole s of 5HT, in which up to 4.5 +/- 0.4 moles may be covalently bonded. A s determined by gel filtration and polyacrylamide gel electrophoresis studies, both 5HT-alpha2M and normal alpha2M combine noncovalently wit h NGF, but 5HT-alpha2M by comparison can combine with NGF somewhat mor e effectively. In contrast to normal alpha2M, 5HT-alpha2M at concentra tions greater than about 0.17 muM exerts a dose-dependent inhibition o n the NGF-stimulated neurite outgrowth by embryonic dorsal root gangli a and dissociated cells in culture, and the inhibitory effect can be o vercome by higher NGF concentrations. Both 5HT-alpha2M and MA-alpha2M at 1.0 muM inhibit neurite extension by embryonic rat cerebral cortica l cells and seriously damage these cells in culture. Such neurite-inhi bitory activity, however, can only be partially blocked by extraneousl y added NGF alone. Normal alpha2M (at 1.0 muM) and 5HT (at 188 muM), o n the other hand, under the identical conditions produce very little o r no effect on the normal cellular and axonal growth of these cells. W e conclude that alpha2M can potentially interact with nucleophilic mon oamines, including neurotransmitters, to form inhibitory complexes whi ch may inhibit/regulate NGF-promoted neurite outgrowth and neuronal su rvival. In addition, higher concentrations of such complexes can serio usly damage certain CNS neurons which do not depend solely on NGF for survival.