Retinoic acid responsive gene product, midkine, has neurotrophic functions for mouse spinal cord and dorsal root ganglion neurons in culture

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Abstract
Midkine (MK) is the product of a retinoic acid responsive gene and is a member of a new family of heparin‐binding growth factors. Neurotrophic effects of MK were examined using cultured spinal cord and dorsal root ganglion (DRG) neurons derived from fetal mouse. MK, which was added to the culture medium at concentrations of 1–100 ng/ml, promoted survival of both types of neurons approximately 5‐fold after 7 days in culture. For spinal cord neurons, the increased survival was reflected in an increase of choline acetyltransferase activity. MK also promoted neurite extension in spinal cord (2‐fold) and DRG (1.7‐fold) neurons. The survival‐promoting activity of MK to these neurons was comparable to that of basic fibroblast growth factor (bFGF) and leukemia inhibitory factor (LIF). In spite of its significant effects on fetal neurons, MK was ineffective in sustaining survival of DRG neurons derived from postnatal mice. From these results, we conclude that MK is a neurotrophic factor to embryonic spinal cord and DRG neurons, and we propose that MK plays a significant role in embryogenesis of the nervous system.