Structure and expression of the nerve growth factor gene inXenopus oocytes and embryos

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Abstract
A large part of the coding portion of the Xenopus nerve growth factor (NGF) gene has been identified and cloned by the use of a chicken cDNA probe and its sequence has been determined. Comparison of the derived amino acid sequence of mature Xenopus NGF with that of other species showed a high conservation, whereas comparison of the prepropeptide showed large divergent regions alternated with short conserved regions. Expression of the NGF gene was examined during development of oocytes and embryos. Surprisingly, NGF mRNA was found in the oocyte; it is present in small previtellogenic as well as in fully grown oocytes. NGF mRNA, passed to the embryo at fertilization, is degraded before the gastrula stage and starts accumulating again around the stage of the neurula. The association of NGF mRNA with polysomes is indicative of NGF synthesis during oogenesis. In fact, by using antibodies against mouse NGF it was possible to reveal NGF molecules present as precursors. These molecules accumulate during oogenesis and are maintained in the embryos up to the blastula stage; a very faint band corresponding to a smaller size peptide is sometimes detected. A maternal role for the NGF can be proposed, although a possible activity of NGF in the oocyte cannot be ruled out.