Download Kisspeptin Expert review (Bloom) FINAL - Spiral

Title: Does Kisspeptin signalling offer a new way to treat infertility?
Channa N Jayasena, Waljit S Dhillo, Stephen R Bloom
Department of Investigative Medicine, Faculty of Medicine, Imperial College London,
Hammersmith Hospital, London, UK
Corresponding author for proof and reprints:
Prof. S.R. Bloom
Department of Investigative Medicine,
Faculty of Medicine,
6th Floor, Commonwealth Building,
Imperial College London,
Hammersmith Hospital,
Du Cane Road,
London W12 ONN.
Tel: +44 208 383 3242
Fax: +44 208 383 3142
[email protected]
Word count: 1758
Conflict of interests: none declared
Financial disclosure
C.N.J. is supported by a Wellcome Trust Research Training Fellowship. W.S.D. is
supported by a Department of Health Clinician Scientist Award. This work is funded by
an MRC Experimental Medicine project grant. The department is funded by an
Integrative Mammalian Biology (IMB) Capacity Building Award and the NIHR
Biomedical Research Centre Funding Scheme.
Over the last 5 years, the field of reproductive biology has seen a surge of
interest in the group of peptides termed Kisspeptin. These peptides are notably
distributed in the placenta, pituitary gland, as well as specialised hypothalamic
neurons [1;2], and have recently emerged as important regulatory factors for
control of the mammalian hypothalamo-pituitary-gonadal (HPG) axis [3;4]. There
has been much speculation over the therapeutic potential of Kisspeptin and its
receptor, G-protein coupled receptor 54 (GPR54). But could future infertility
therapies which target GPR54, ever become reality?
Kisspeptin: an emerging role in reproduction
In 2003, researchers made the ground-breaking observation that inactivating
mutations of GPR54 in humans and mice lead to pubertal failure together with
hypogonadotrophic hypogonadism (low circulating gonadotrophins and sex
steroids) [3;4]. Mutant mice lacking GPR54 had normal gonadotrophin releasing
hormone (GnRH) neuronal migration, and responded to exogenous GnRH
injection [4]. Kiss1 mutant mice display a similar albeit less severe phenotype of
hypogonadotrophic hypogonadism [5;6]. Furthermore central administration of a
GPR54 antagonist suppresses GnRH release in female pubertal monkeys,
reduces both mean LH (luteinising hormone) levels and LH pulse amplitude in
adult ovariectomised sheep, and abolishes the post-castration rise in LH seen in
adult mice, rats and sheep [7]. Therefore GPR54 signalling plays an important
role in pubertal development and adult reproductive physiology.
Numerous investigators have demonstrated that central or peripheral injection of
Kisspeptin to all mammalian species studied to date (including rats [8-11], mice
[12;13], sheep [13-15], hamsters [16], monkeys [17], pigs [18], cows [19] and
humans [20;21]), potently stimulates gonadotrophin release. This phenomenon is
abolished by co-administration of the GnRH antagonist acyline [9;17]. Most
rodent hypothalamic GnRH neurons express GPR54 [8;22]. Furthermore
Kisspeptin induces GnRH release from hypothalamic rat explants [11], and
increases electrophysiological activation of green fluorescent protein (GFP)tagged GnRH neurons within mouse brain slices [22]. Kisspeptin administration
to rodents also induces c-fos (a marker of neuronal activation) immunoreactivity
in GnRH cell bodies [8]. Taken together, this evidence suggests that Kisspeptin
administration stimulates endogenous hypothalamic GnRH release. Kisspeptin
may also act within the pituitary gland. GPR54 is expressed in the pituitary gland
[1;23], and some researchers have found that Kisspeptin administration can
directly stimulate gonadotrophin release in vitro [24;25]. Hence the primary site of
Kisspeptin’s action appears to be the hypothalamic GnRH neuron, although it
remains a possibility that an additional (perhaps modulatory) action exists in the
pituitary gland.
How does peripherally administered Kisspeptin gain access to the brain in order
to stimulate GnRH release? An action at the median eminence of the
hypothalamus seems likely; as one of the circumventricular organs, the median
eminence has an incomplete blood-brain barrier and is the location where GnRH
nerve terminals release GnRH into the hypophyseal portal circulation [26].
Kisspeptin stimulates GnRH from isolated mediobasal hypothalamus explants
from wild-type but not GPR54 knockout mice [27]. Since this effect is not
abolished by tetrodotoxin (a potent inhibitor action of neuronal potential
formation), this suggests a direct action on GnRH nerve terminals within the
median eminence [27]. Fos expression in the median eminence also increases
following peripheral Kisspeptin administration. Hence a GPR54-mediated action
on GnRH nerve terminals within the median eminence, may explain why
peripherally
administered
Kisspeptin
effectively
stimulates
gonadotrophin
release.
The neuroanatomy of Kisspeptin neurons suggests a dual function for Kisspeptin
signalling within the central nervous system. In rodents, primates and sheep a
population of specialised neurons expressing Kisspeptin is located in the
hypothalamic arcuate nucleus (Arc) (equivalent to the infundibular nucleus in
primates) [12;28;29]. These Arc Kisspeptin neurons are negatively regulated by
circulating sex steroids. In rodents, levels of Arc Kiss1 expression fall during the
evening of proestrous (coincident with the LH surge) or during an oestrogen and
progesterone-induced LH surge [30]; however levels of Arc Kiss1 expression rise
after gonadectomy, and subsequently fall if sex steroid replacement is given to
these animals [31]. A further population of Kisspeptin neurons is located in the
anteroventral periventricular (AVPV) nucleus of rodents and sheep [12;29], which
is implicated in generation of the LH surge [32]. AVPV neurons are positively
regulated by circulating sex steroids, in contrast to neurons of the Arc. In rodents,
AVPV Kiss1 expression falls after ovariectomy [31], and rises during the evening
of proestrous or during an oestrogen and progesterone-induced LH surge [30].
Thus AVPV Kisspeptin neurons may play a role in generation of the LH surge in
female rodents. Although no functional equivalent of the rodent AVPV is found in
primates [33], Kisspeptin may also play a role in the human pre-ovulatory surge
[21]. Subcutaneous Kisspeptin administration to healthy women stimulates
gonadotrophin release 20-fold more potently in the pre-ovulatory phase of the
menstrual cycle than in the follicular phase [21]. The basis for this increased
sensitivity to Kisspeptin is currently not known.
Clinical studies: where are we now?
Four years ago it was first demonstrated that exogenous Kisspeptin could
stimulate gonadotrophin release in humans [20]. Healthy human males were
intravenously infused with Kisspeptin-54 in a double-blinded, placebo controlled
crossover study [20]. Dose-dependent increases in plasma gonadotrophins were
observed. For example, a Kisspeptin-54 infusion rate of 4 pmol/kg/min more than
doubled 90 minute mean plasma LH, and increased mean plasma FSH by 18%
when compared with saline. Subsequently, the first studies of Kisspeptin-54
administration to women were performed [21]. Subcutaneous bolus injections of
Kisspeptin-54 at doses between 0.2 and 6.4nmol/kg dose-dependently increased
plasma LH and FSH secretion. LH responses to a given dose of kisspeptin were
about seven times higher than corresponding FSH responses, which is
consistent with rodent data from showing the greater effect on LH than FSH [21].
Preliminary results of a study of Kisspeptin administration within a model of
human infertility were presented earlier this year [34]. Patients with hypothalamic
amenorrhoea (HA) received subcutaneous bolus injections of either Kisspeptin54 (6.4nmol/kg) or saline. Kisspeptin potently increased mean luteinising
hormone (LH) 48-fold and follicle stimulating hormone (FSH) 16-fold when
compared with saline injection. Fascinatingly, subjects with HA were over 4-fold
more responsive to Kisspeptin injection than previously studied healthy females
in the follicular phase of the menstrual cycle [34].
These initial clinical studies demonstrate the efficacy of Kisspeptin in stimulating
gonadotrophin release in healthy and infertile human subjects. Hence Kisspeptin
can be regarded as a potential novel therapy for human infertility.
How safe is exogenous Kisspeptin likely to be?
Levels of circulating endogenous Kisspeptin rise to over 9000pmol/l during the
third trimester of normal human pregnancy due to placental production, which is
in sharp contrast to the barely detectable plasma levels of Kisspeptin (1.3pmol/l)
observed in men and non-pregnant women [35]. It has been suggested that
placental Kisspeptin negatively regulates trophoblast invasion of uterine tissue;
indeed levels of KISS1 expression are significantly lower in invasive forms of
gestational trophoblastic disease such as choriocarcinoma, when compared with
hydatiform mole or normal placental tissue [36]. Regardless of the function of
placental kisspeptin, the fact that women are exposed to markedly raised
endogenous levels throughout pregnancy provides some reassurance about its
safety. Consistent with this observation, administration of single injections of
Kisspeptin to healthy male and female subjects temporarily raised plasma
Kisspeptin to levels comparable to those seen in pregnancy, and caused no
adverse or behavioural effects such as changes in libido (assessed by
questionnaire) [20;21]. No significant changes in heart rate or blood pressure
were observed during these studies [20;21], which is noteworthy given that
vasoconstrictor properties of Kisspeptin have been demonstrated in vitro [37].
It is important to consider the specificity of Kisspeptin’s actions. Until recently the
only known humoral action of Kisspeptin was on the HPG axis. However
evidence from cows [19;38;39] and rats [24] is emerging of a stimulatory action
on pituitary growth hormone release. In addition to the pituitary, hypothalamus
and placenta, GPR54 is expressed in other regions of the brain as well as the
spleen, liver, pancreas and gonads [1;2]. We must therefore remain open to the
possibility that additional biological actions of Kisspeptin exist; indeed, effects on
insulin secretion [40] and hippocampal synaptic function [41] have been
suggested.
What advantages might Kisspeptin have over existing therapies?
Since clinical studies are at their early stage, we are limited in our ability to judge
whether Kisspeptin is likely to provide a novel means of treating human infertility.
However the available evidence does at least point to several potential
advantages of a therapeutic which targeted GPR54.
As previously discussed, Kisspeptin appears to primarily act by through
stimulation of endogenous GnRH release, which in turn triggers endogenous
gonadotrophin release [8;9;11;17;22]. GPR54 agonists are therefore predicted to
confer a lower risk of excessive ovarian stimulation than gonadotrophin injections
[42;43].
No orally bioavailable GPR54 agonist currently exists, but its future discovery
might offer a clear practical advantage over GnRH and gonadotrophin-based
therapies. A series of pentapeptide-based GPR54 agonists have been described
[44;45]. Furthermore small molecule GPR54 agonists have been identified,
although detailed results of these have not been published [46].
GPR54 super-agonists may offer a novel means of inducing medical castration.
Continuous exposure of monkeys [47] and rodents [48] to exogenous Kisspeptin
leads to desensitisation of gonadotrophin responses after 3 hours and 1 day of
commencement of administration, respectively. Such desensitisation is probably
occurring at the GPR54 receptor since monkeys remained responsive to
exogenous GnRH at the end of continuous Kisspeptin infusion [47]. Reduced
Kiss1 expression is associated with metastasis in a number of solid tumours [49].
Furthermore KISS1 expression in purified human trophoblasts is significantly
lower in choriocarcinoma when compared with hydatidifom mole or normal
placental tissue [36]. It would be fascinating to see if Kisspeptin functions as an
anti-metastatic factor in hormone-sensitive tumours; an agent which inhibited
tumour metastasis as well as sex-steroid secretion would clearly have
therapeutic potential.
It is important to recognise that Kisspeptin would offer no benefit to certain
individuals, including those with incompetent GnRH function (such as the
defective GnRH neuronal migration observed in Kallman’s syndrome [50]) or a
structural pituitary lesion. Furthermore Kisspeptin would be predicted to offer no
benefit in post-menopausal women, who already have raised circulating levels of
gonadotrophins.
Concluding remarks
We are now beginning to understand the importance of GPR54 signalling. It is
obligatory for pubertal development [3;4] and it plays an important role in adult
reproductive physiology [7;9;12;13;17]. Initial clinical studies suggest that
Kisspeptin
potently and
safely stimulates
gonadotrophin
release
when
administered to healthy volunteers [20;21]. Furthermore a preliminary report has
emerged of its efficacy in a human model of infertility [34]. The development and
future availability of GPR54-based fertility and oncology therapeutics has
become very real possibility. Further studies are needed to determine if such
agents will offer a distinct advantage over existing therapies.
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