The Emerging Therapeutic Potential of Kisspeptin and Neurokinin B

Abstract

Kisspeptin (KP) and neurokinin B (NKB) are neuropeptides that govern the reproductive endocrine axis through regulating hypothalamic gonadotropin-releasing hormone (GnRH) neuronal activity and pulsatile GnRH secretion. Their critical role in reproductive health was first identified after inactivating variants in genes encoding for KP or NKB signaling were shown to result in congenital hypogonadotropic hypogonadism and a failure of pubertal development. Over the past 2 decades since their discovery, a wealth of evidence from both basic and translational research has laid the foundation for potential therapeutic applications. Beyond KP's function in the hypothalamus, it is also expressed in the placenta, liver, pancreas, adipose tissue, bone, and limbic regions, giving rise to several avenues of research for use in the diagnosis and treatment of pregnancy, metabolic, liver, bone, and behavioral disorders. The role played by NKB in stimulating the hypothalamic thermoregulatory center to mediate menopausal hot flashes has led to the development of medications that antagonize its action as a novel nonsteroidal therapeutic agent for this indication. Furthermore, the ability of NKB antagonism to partially suppress (but not abolish) the reproductive endocrine axis has supported its potential use for the treatment of various reproductive disorders including polycystic ovary syndrome, uterine fibroids, and endometriosis. This review will provide a comprehensive up-to-date overview of the preclinical and clinical data that have paved the way for the development of diagnostic and therapeutic applications of KP and NKB.

Keywords: behavior; bone; kisspeptin; metabolism; neurokinin B; reproduction.

Graphical abstract

Figure 1.

KP and NKB in the regulation of the HPG axis. KP is released from the POA (equivalent to rostral periventricular area of the third ventricle, RP3V, in nonhumans) and infundibular nucleus (arcuate, ARC, nucleus in nonhumans) of the hypothalamus. The KP neurons in the infundibular nucleus coexpress NKB and dynorphin (known as KNDy neurons) and are involved in the autosynaptic regulation of pulsatile KP secretion via the NKB receptor (NK3R) and kappa opioid peptide receptor (KOR), respectively. Dynorphin inhibits, whereas NKB stimulates, KP release. Following KP's release from the hypothalamus, KP stimulates the hypothalamic GnRH neurons to release GnRH in a pulsatile manner, which stimulates anterior pituitary production of gonadotropins (LH, FSH) and subsequent production of gonadal (testicular/ovarian) sex-steroids (E2, T). The gonadotropins’ effect on the ovary stimulates follicular development, oocyte maturation, and ovulation. The KNDy neurons in the infundibular nucleus mainly receive negative feedback (E2, T) from sex-steroids, whereas KP neurons in the POA receive positive feedback from estrogen in females (high E2), which is involved in the preovulatory LH surge. Sex-steroid communication with the POA has not yet been fully established in males. E2, estrogen; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; HPG, hypothalamic-pituitary-gonadal; KISS1R, kisspeptin receptor; KOR, kappa opioid peptide receptor; KP, kisspeptin; LH, luteinizing hormone; NK3R, neurokinin 3 receptor; NKB, neurokinin B; P, progesterone; POA, preoptic area; RP3V, rostral periventricular area of the third ventricle; T, testosterone. Figure created with BioRender.com.

Figure 2.

KISS1 and KISS1R human gene expression in areas where kisspeptin signaling has well-identified roles. Expression is abundant in other areas of the human body, not illustrated in Fig. 2, in which the full role of KP signaling has yet to be elucidated. This widespread distribution of KISS1 and KISS1R reflects the pleiotropic action of KP, beyond reproduction. In humans, the tissue distribution of KISS1 and KISS1R has been identified using RT-PCR methods. KISS1 mRNA is predominantly expressed in the placenta, with the next highest level in the testis, and moderate levels in the pancreas, liver, uterus, gonads, and small intestine. KISS1 mRNA is also strongly expressed in bone, in particular the osteoblasts. KISS1R expression is particularly abundant in the placenta, pituitary, spinal cord, liver, pancreas, and bone (osteoblasts and osteoclasts), but expressed at lower levels in other tissues, such as the stomach, uterus, small intestine, thymus, spleen, lung, gonads, heart, kidney, adrenal gland, bone, and fetal liver. Both KISS1 and KISS1R are also expressed in the brain, and in particular the human hypothalamus, as well as extrahypothalamic regions, such as the amygdala, caudate nucleus, cerebellum, cingulate gyrus, globus pallidus, hippocampus, medial frontal gyrus, nucleus accumbens, parahippocampal gyrus, putamen, spinal cord, striatum, substantia nigra, superior frontal gyrus and thalamus, as localized by RT-PCR. KISS1, kisspeptin gene; KISS1R, kisspeptin receptor gene; KP, kisspeptin; mRNA, messenger RNA; RT-PCR, reverse transcription polymerase chain reaction. Figure created with BioRender.com.

Figure 3.

KP receptor induces differential responses in downstream signaling. KP has a high-affinity binding site for the human KP receptor and induces a biphasic response in downstream signaling, with an acute (lasting ∼5 minutes) and prolonged response (lasting >30 minutes). KISS1R (coupled to Gαq/11) triggers the activation of PLC and subsequent recruitment of secondary intracellular messengers, IP3 and DAG, which in turn mediate intracellular calcium release. DAG additionally activates PKC and induces downstream phosphorylation of ERK 1 and 2. Kisspeptin binding results in the recruitment of β-arrestin and GPCR serine/threonine kinases (GRK2), which leads to desensitization and internalization of the kisspeptin receptor (through uncoupling of Gαq/11). β-Arrestin traffics the desensitized KISS1R to the clathrin-coated pit resulting in sequestration, which results in β-arrestin–dependent signaling. Internalized KISS1R eventually dissociates from β-arrestin and the majority of kisspeptin receptors become resensitized and traffic back to the cell surface, thus maintaining a continuous pool of receptors at the cell surface which are ready to signal while a lesser population of KISS1R are targeted for degradation. DAG; diacylglycerol; ERK, extracellular signal-related kinase; GRK2, GPCR serine/threonine kinases; IP3, inositol triphosphate; KISS1R, kisspeptin receptor gene; KP, kisspeptin; PKC; protein kinase C; PLC, phospholipase C. Figure created with BioRender.com.

Figure 4.

Role of KP in disorders of puberty. Puberty is triggered by the pulsatile secretion of GnRH and subsequent downstream activation of the HPG reproductive axis. The pulsatile secretion of GnRH requires adequate development and migration of GnRH neurons from the olfactory bulb to the hypothalamus. The HPG axis is transiently activated at 2 distinct phases: during early developmental life, termed “mini puberty,” and at the onset of puberty. KP stimulates an LH response during the later stages of puberty (Tanner stage 5) thus suggesting KISS1R sensitivity on GnRH neurons develops during the later part of puberty. KISS1 gain-in-function variants can lead to premature activation of the HPG axis resulting in early central precocious puberty (CPP). KP levels are increased in CPP vs age-matched healthy controls and thus KP has potential in aiding in the diagnosis of early puberty. KISS1 loss-of-function variants cause aberrations in GnRH neuronal development or migration and impair GnRH secretion resulting in congenital hypogonadotropic hypogonadism (CHH) and delayed puberty. Constitutional delay of growth and puberty (CDGP) is another common cause of delayed puberty and can be challenging to accurately differentiate from CHH. KP, a potent stimulator of GnRH and LH release, induces differential responses in CDGP (increased LH) and CHH (absent/reduced LH) and thus can aid in the diagnosis of delayed puberty. CDGP, constitutional delay of growth and puberty; CHH, congenital hypogonadotropic hypogonadism; CPP, central precocious puberty; GnRH, gonadotropin-releasing hormone; HPG, hypothalamic-pituitary-gonadal; KP, kisspeptin; LH, luteinizing hormone. Figure created with BioRender.com.

Figure 5.

Therapeutic potential of KP and NKB in female reproductive disorders. Activation of hypothalamic KP neurons directly stimulates GnRH release and regulates reproductive hormone secretion. Absent or reduced GnRH and LH pulses observed in HA and hyperprolactinemia can be restored using exogenous KP. While GnRH/ LH pulsatility is retained in patients with endometriosis/uterine fibroids, patients with PCOS have high pulsatility. During the menopause, increased KNDy neuronal activity results in very high GnRH/LH pulses and induction of vasomotor symptoms through dysregulation of the thermoregulatory center. Considering NKB antagonism partially suppresses (but does not abolish) the reproductive endocrine axis, NK3R antagonists have been developed for the therapeutic potential of these disorders. NK3R antagonism can be used to treat endometriosis/ uterine fibroids (by reducing E2), PCOS (by reducing androgens), and menopausal hot flashes (by reducing vasomotor symptoms). E2, estradiol; GnRH, gonadotropin-releasing hormone; HA, hypothalamic amenorrhea; KP, kisspeptin; LH, luteinizing hormone; NK3R, neurokinin 3 receptor; NKB, neurokinin B; PCOS, polycystic ovary syndrome; KNDy, kisspeptin-neurokinin B-dynorphin. Figure created with BioRender.com.

Figure 6.

The utility of KP in the prediction of pregnancy complications. KP regulates trophoblast invasion and placentation during pregnancy and has emerged as a promising biomarker to predict several adverse pregnancy complications. The KISS1 gene is abundantly expressed in syncytiotrophoblasts, whereas its receptor (KISS1R) is expressed in both cytotrophoblasts and syncytiotrophoblasts. Circulating KP levels increase linearly in healthy pregnancy but are reduced in miscarriage during early pregnancy. KP can accurately predict the risk of miscarriage with average/above average levels of KP being associated with a less than 1% risk of miscarriage. KP levels are reduced in FGR and GDM and raised in PET during the later stages of pregnancy. FGR, fetal growth restriction; GDM, gestational diabetes mellitus; KISS1, kisspeptin gene; KISS1R, kisspeptin receptor; KP, kisspeptin; PET, preeclampsia. Figure created with BioRender.com.

Figure 7.

Effects of liver-specific Kiss1r KO and enhanced KP signaling. Liver-specific Kiss1r KO mice model placed on a high-fat diet exhibited increased lipogenesis, TG synthesis, and reduced mitochondrial β oxidation compared to controls. This resulted in increased TG levels, serum ALT levels (indicating hepatocellular injury), and hepatic steatosis. Increased body weight and reduced energy expenditure were observed. Higher fasting glucose and basal insulin levels, indicating glucose intolerance and insulin resistance, were also observed. Moreover, markers of inflammation and early stages of fibrosis were upregulated. Effects of enhanced KP signaling: wild-type mice were placed on a high-fat diet for 6 weeks prior to administration of MVT-602, a KP receptor agonist, for 5 weeks on a high-fat diet. MVT-602 alleviated hepatic steatosis and metabolic deterioration through improvements in insulin sensitivity, lower basal insulin levels, reduced TGs, and ALT levels. MVT-602–treated mice had slightly lower body weight compared to controls with increased energy expenditure in the light phase. Mechanistically MVT-602 treatment under high-fat diet conditions significantly reduced TG synthesis, increased lipolysis, and mitochondrial β oxidation compared to controls. Markers of inflammation and early stages of fibrosis were downregulated. ALT, alanine transaminase; Kiss1r, kisspeptin receptor; KO, knockout; KP, kisspeptin; TGs, triglycerides. Figure created with BioRender.com.

Figure 8.

The effects of KP signaling on key reproductive behaviors in rodents, sheep, and humans, including olfactory processing, sexual partner preference, copulatory behavior, and arousal and bonding. KP is widely expressed in limbic and paralimbic regions of the brain, which are involved in reproductive behaviors. Olfaction: KP expression and activity increased in several brain regions in response to opposite sex olfactory cues in male mice and female mice and ewes. In humans, KP administration enhanced limbic brain activity when men were exposed to a pleasant feminine scent. Sexual partner preference and bonding: Studies showed that the KP MePD neurons regulate partner preference in male mice, whereas intraperitoneal and intranasal administration of KP to male rats increases sexual motivation. When peripheral KP was administered to Kiss KO female mice, normal male-directed sexual preference was restored. In healthy heterosexual men, peripheral KP administration increased brain activity in aesthetic brain regions in response to viewing female faces. Copulatory behavior and arousal: KP stimulation in the MePD resulted in erections in male rats, whereas both peripheral and central KP administration to female mice robustly stimulated lordosis. In healthy heterosexual men, peripheral KP administration enhanced limbic brain activity when exposed to visual sexual stimuli. KP administration to males with HSDD deactivated brain regions involved in self- monitoring and introspection, and increased brain activity in sexual arousal centers, in response to watching erotic videos in the fMRI scanner. KP administration also led to increases in penile tumescence. KP administration to pre-menopausal women with HSDD, deactivated brain regions involved in inhibitory control and activated areas known to be activated in the context of sexual arousal in response to erotic visual cues. AVPV; anteroventral periventricular nucleus; fMRI, functional magnetic resonance imaging; GnRH, gonadotropin-releasing hormone; HSDD, hypoactive sexual desire disorder; KO, knockout; KP, kisspeptin; MePD, posterodorsal subnucleus of medial amygdala. Figure created with BioRender.com.