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the ARC region [
42
] including the cortex (Robert Steiner, personal communication).
Thus, it seems likely that some neurons in the cortex express
Kiss1
during develop-
ment and this expression is switched off in the adult.
Conditional Gene Ablation
One of the limitations of complete disruption of a gene is that the corresponding
protein is absent throughout development, which may produce pleiotropic changes
in gene expression and result in compensatory developmental changes. To overcome
this problem, conditional mutant alleles are usually generated where expression of
the gene can be ablated in a tissue-specifi c or temporal way. This is normally
achieved by creating a targeted allele where a coding exon is fl anked by sequences
that can be induced to undergo recombination to delete the intervening exon. This is
usually achieved by CRE-mediated recombination between LoxP sites or FLP-
mediated recombination between FRT sites.
Conditional gene ablation has been used to study the role of
Kiss1
neurons in
mediating the effects of leptin on puberty in mice. Leptin is produced by white adi-
pose tissue and provides a signal to the hypothalamus about peripheral energy
reserves [
45
]. Mice lacking leptin (
ob
/
ob
) or the leptin receptor (
db
/
db
) develop
obesity and are sterile. Leptin administration advances the onset of puberty in
rodents [
46
-
48
] and restores fertility to
ob/ob
mice [
49
]. To examine how leptin
mediates its action on the reproductive axis, mice lacking the leptin receptor (
LepR
)
in
Kiss1
neurons were generated by breeding
Tg
(
Kiss1
-
Cre
)
J2
-
4Cfe
mice with mice
carrying a LoxP-modifi ed
LepR
allele [
50
]. Loss of functional leptin receptors in
Kiss1
neurons was confi rmed by treating fasted mice with leptin and examining
brain sections for leptin-induced phosphorylation of the STAT3 protein. Quantitation
of pSTAT3 immunoreactivity showed a reduction of 90-95% in the number of
leptin-responsive
Kiss1
neurons [
50
]. Mice of either sex showed no problems with
the timing of puberty or fertility suggesting that leptin signalling in
Kiss1
neurons
is not required for these processes. Moreover, the authors also showed that LepR
neurons in the ventral premammillary nucleus seem to be more important in mediat-
ing the action of leptin on the reproductive axis. One caveat to these data, however,
is that there are still a small number of
Kiss1
neurons (5-10%) that can still respond
to leptin signalling in the LepR deleted mice, and it is not known if this would be
suffi cient to maintain functional inputs into GnRH neurons.
Genetic Ablation of GnRH Neurons
The
Kiss1r
tm1.1
(
cre
)
Uboe
mice have been used to genetically ablate GnRH neurons by
crossing with a CRE-inducible Diphtheria toxin A transgene [
39
]. The Diphtheria
toxin A inhibits protein synthesis by reducing the activity of elongation factor-2
(EF-2) causing cell death. The number of GnRH neurons in these transgenic mice
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