**4. Kisspeptin in the luteal phase and implantation**

Kisspeptin is not only expressed in the central nervous system – it also performs peripheral functions. Expression of kisspeptin and its receptor KISS-1 has been demonstrated in the human ovary, fallopian tube, uterus, and placenta [22]. It is thought that kisspeptin primarily functions in the hypothalamus, but also interacts between the signaling pathways of the central and peripheral reproductive systems [23]. In fact, several studies have supported the idea that kisspeptin exerts direct effects on ovarian tissue via ovarian kisspeptin receptors [24–26].

A number of studies have demonstrated that kisspeptin is expressed at the maternal-fetal interface of many species, including humans [27]. In the human uterus, kisspeptin is expressed in the endometrial epithelial and stromal cells, but not in the myometrium [28]. In the early placenta, kisspeptin is initially produced by villous cytotrophoblast cells, then villous syncytiotrophoblast cells and the placental bed [29, 30]. As pregnancy progresses, placental production of kisspeptin declines [31, 32].

Kisspeptin expression in the endometrium is absent during the proliferative and early secretory phases but becomes abundant during the late secretory phase [27, 33]. This indicates a potential role of kisspeptin in the preparation of endometrial tissue for implantation. Kisspeptin knockout mice exhibit thin, weak uteri with almost no endometrial glands, suggesting kisspeptin is an important regulator of normal endometrial development [34]. Kisspeptin may also act as a mediator that facilitates implantation of the growing embryo to the uterine wall. It has been shown that exogenous kisspeptin administration strengthens adhesion of kisspeptin-expressing trophoblast cells to collagen present in uterine tissue [34]. Immediately after implantation, kisspeptin levels are known to rise; this suggests involvement of kisspeptin during the decidualization process [35]. A study by Wu et al. demonstrated a dose-dependent relationship between kisspeptin expression and inhibition of cell invasion/migration in human decidualized endometrial cells [29]. In contrast, a kisspeptin antagonist called kisspeptin 234 stimulates the process of decidual invasion and migration [29]. Similarly, when small interfering RNAs that antagonize kisspeptin are introduced, stromal decidualization is impaired [35]. In a study by Calder et al., ablation of the KISS-1 gene and subsequent absence of kisspeptin expression resulted in infertile mice [36]. Even in mice that received rescue gonadotropins and estradiol, which restored ovulation, the mice embryos could not implant in the mice that lacked KISS-1. These embryos were, however, able to implant in wildtype mice [36].

Kisspeptin was originally identified as a suppressor of cancer metastasis; its function in the regulation of cellular proliferation and growth is also integral to the process of placentation. The early placenta expresses high levels of kisspeptin, perhaps to tame the invasive and migratory capability of trophoblasts [32]. Kisspeptin decreases the activity of collagenases, matrix metalloproteinases, and vascular endothelial growth factor, which are all signaling proteins involved in trophoblast proliferation [31, 37]. Kisspeptin also supports the adhesion of extravillous trophoblasts to the endometrium, which inhibits migration [38]. This careful balance between invasion and the prevention of invasion is essential to the placentation process as well as the appropriate remodeling of the maternal uterine wall [34]. As the placenta develops throughout pregnancy, it exhibits a pattern of kisspeptin expression that follows a circadian rhythm [39]. The term placenta demonstrates kisspeptin surges at 0400 and 1200 daily. This rhythm correlates with circadian expression of other proteins involved in placental physiology, including TNFα, melatonin, and oxytocin [39].

#### **5. Kisspeptin in pregnancy**

Maternal kisspeptin levels rise dramatically during pregnancy, then return to normal within 15 days of delivery [28]. Unlike β-hCG, kisspeptin levels rise steadily and do not plateau [40]. It is thought that the primary source of maternal kisspeptin is placental tissue [27], and that maternal kisspeptin levels reflect the volume of viable placental tissue [41]. Kisspeptin may be useful as a biomarker of pregnancy due to its association with placental invasion and apoptosis [42]. It also has potential utility as a biomarker of miscarriage.

Spontaneous abortion (SAB) is a common experience, seen in 10–20% of clinically recognized pregnancies [43]. A study by Jayasena et al. showed that maternal plasma kisspeptin is significantly lower in women with early pregnancies who later develop SAB compared to women who have a viable intrauterine pregnancy (IUP) [44]. Maternal kisspeptin levels also had higher diagnostic performance than β-hCG in detecting SAB [44]. Wu et al. demonstrated that women with recurrent SAB have decreased decidual kisspeptin expression compared to women with IUP [45]. Kavvasoglu also showed decreased maternal kisspeptin levels in women with SAB compared to healthy IUPs [46]. Sullivan et al. validated a serum kisspeptin-54 assay as well as confirmed that maternal kisspeptin levels are positively correlated with fetal gestational age and pregnancy viability [40].

There is currently no established clinical test for early miscarriage; diagnosis relies on serial β-hCG measurements and correlation with ultrasound. This requires multiple maternal encounters with the healthcare system, a prolonged timeframe, and can involve considerable distress of the patient and partner. Jayasena et al. describes the current diagnostic pathway for establishing fetal viability as having limited clinical utility due to delay and a high degree of uncertainty [44]. Thus, there is interest in establishing a more accurate and streamlined diagnostic marker of viable IUP vs. SAB.

Kisspeptin has been shown to be massively downregulated in the case of ectopic pregnancy [47]. Ectopic pregnancy occurs when a fertilized ovum implants and develops outside the uterine cavity. Similarly to SAB, ectopic pregnancy is currently diagnosed by serial β-hCG measurements in correlation with ultrasound. Definitive diagnosis may require direct visualization via laparoscopy [48]. A study by Romero-Ruiz et al. explored the roll of kisspeptin in individuals with ectopic pregnancy. They found that maternal circulating kisspeptins gradually increased during the first trimester of pregnancy in healthy controls. However, in those with ectopic

*The Role of Kisspeptin in the Ovarian Cycle, Pregnancy, and Fertility DOI: http://dx.doi.org/10.5772/intechopen.98446*

pregnancy, kisspeptin levels were suppressed. The study correlated these results to levels of microRNAs (miRNA) (small noncoding RNAs that can modulate gene and protein expression). In particular, miR-324-3p is known to inhibit kisspeptin function. Romero-Ruiz et al. found that in ectopic pregnancies, miR-324-3p was significantly increased in placental tissue (though maternal circulating levels were low). This finding suggests defective export of the miRNA from its embryonic/placental source in ectopic pregnancy, which may further contribute to the local suppression of kisspeptin. The authors suggested that correlation of maternal miR-324-3p with kisspeptin and β-hCG levels could provide a better modality for timely diagnosis of ectopic pregnancy, especially considering the stability of miRNA in maternal serum [46].

Kisspeptin could also have diagnostic utility in identifying women at risk of preeclampsia. A study by Qaio showed that the placentas of term preeclamptic pregnancies express significantly lower kisspeptin levels compared to healthy pregnancies [49]. These findings were reproduced by Farina et al., which demonstrated lower KISS-1 expression in preeclamptic patients compared to healthy pregnant patients [50]. The study also suggested KISS-1 cell-free mRNA has potential to serve as a predictive biomarker of preeclampsia [50]. Matjila et al. investigated the relationship between maternal kisspeptin levels and placental kisspeptin expression in preeclamptic pregnancies – they found that preeclamptic placentas demonstrated high kisspeptin expression but low maternal kisspeptin levels [30]. It is speculated that elevated kisspeptin expression in diseased placentas may inhibit trophoblast invasion and contribute to the development of preeclampsia [30, 34]. Kisspeptin therefore has potential to offer predictive information about the risk of preeclampsia.
