Page 9 - 71_04

VOL. 71 (4), 765-782, 2005 OLD HORMONES OF THE INSULIN...

digestive endoderm (Fig. 2F). IGF-I protein was detected in a few
ectoderm cells in the dorsal part of the neural tube and a few cells
in the digestive endoderm (Fig. 2G). IGF-II was more broadly
present, decorating multiple basal membranes (Fig. 2H).

Overall, members of the insulin family are therefore present at
very early stages of vertebrate development, and the prohormone
proinsulin, previously believed to be nearly inactive, was expressed
in a tissue- and time-specific manner in the chick embryo. We had
proposed that there was an endocrinization of the early embryo (14),
but these hormone-like factors were there before circulation was
established, thus acting in more of a short-range, paracrine or
autocrine manner, rather than in the canonical endocrine, longe-
range form.

Embryonic proinsulin transcripts: more surprises

The multiple studies performed on pancreatic insulin regulation
aimed at understanding the physiopathology of diabetes and
approaches for therapy had demonstrated that glucose was the major
stimulus for proinsulin gene expression and insulin secretion (41).
Our studies with cultured prepancreatic chick embryos showed that
proinsulin mRNA at that stage was not regulated by glucose (31).
The analysis of the embryonic transcript provided further surprises,
since at least two different embryonic mRNAs were identified,
distinct from pancreatic proinsulin mRNA.

The chick embryo contains a single proinsulin gene, as in the
human, and in contrast to the mouse, in which the proinsulin gene
has been duplicated (see later). The protein coding region is
contained in exon 2 and exon 3, whereas exon 1 is untranslated
(5’UTR). The product obtained by rapid amplification of the cDNA
end (5’RACE) was different in the pancreas than in the E1.5 chick
embryo (Fig. 3).

771