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

dependent manner. Interestingly, insulin or proinsulin had no effect
on proliferation, and only a small cell survival effect if added alone,
but they were synergistic with EGF and FGF2 when added together
with these two mitogens. When high insulin or low IGF-I was present
and the mitogens were withdrawn, the stem cells stopped
proliferating and underwent differentiation into the three types of
cells characteristic of the central nervous system: neurons, astrocytes
and oligodendrocytes (Fig. 5). Compared to other types of embryo-
derived neural stem cells reported in the literature, the isolated
olfactory bulb stem/precursor cells showed a remarkable capacity to
generate neurons (52% of the differentiated cells after 3-4 days)
(Fig. 5).

If instead of preparing neurospheres from olfactory bulbs of wild
type mice, we used IGF-I knockout mice, the cells were able to
proliferate in culture normally, whereas their ability to differentiate
into neurons and glia were impaired (40). This finding agreed with
in vivo data, since olfactory bulb mitral neurons were decreased in
the IGF-I knockout embryos at E18.5 (32).

FIGURE 6. Expression of proinsulin II mRNA in the O.B. stem cells. A) RT-PCR
of a common region of proinsulin mRNA I and II followed by Southern blot with
a nested oligonucleotide in pancreas and proliferative neural stem cells (NSC).
B) Restriction enzyme digestion with MspI of the PCR products generated in A,

followed by Southern blot shows the presence of proinsulin II in NSC.

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