Puberty is a milestone in a child’s development on a mental and physical level. The onset of puberty can be influenced by genetic, nutritional, environmental, and socioeconomic factors (Teles et al., 2008). Due to the many factors that play a role in the initiation of puberty, the study of the mechanisms involved in puberty onset has been proven to be more complicated than initially anticipated (Windsor-Engnell, Kasuya, Mizuno, Keen & Terasawa, 2007). In females, puberty starts at the age around 10 or 11. In males, puberty starts slightly later, at the age of 12, on average (American Medical Association, 2001). In girls, the initiation of puberty is featured by breast development and in boys, it is marked by the testicular enlargement. Like any process in the body, initiation of puberty might go wrong, for example early puberty (precocious puberty), late puberty or the complete absence of puberty. Therefore, it is important to study and to understand the mechanisms related to the initiation of puberty, on both a physiological level and endocrinological level. In the 1950s, the first evidence of an hormone was found to play a role in the onset of puberty. This hormone is the gonadotropin-releasing hormone (GnRH). This finding was the start of many studies related to puberty onset. In 2003, kisspeptin was found to play a major role in the initiation of puberty. Kisspeptin is the peptide product of the KiSS1 gene and its receptor is the KiSS1R, also called GPR54 receptor (Franssen & Tena-Sempere, 2018). The KiSS1/GPR54 system has been found to be one of the major pathways in puberty onset. Therefore, in this paper, kisspeptin and its role in the KiSS1/GPR54 system will be discussed.
Gonadotropin-releasing Hormone
In the late 1948, the presence of hypothalamic releasing hormones to control the synthesis and secretion of pituitary hormones such as gonadotropins was predicted in a study by Harris (1948). This led to numerous studies to hypothalamic releasing hormones. Two independent groups were able to isolate luteinizing hormone-releasing hormone (LHRH), a neuropeptide consisting of ten amino acids, Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 (Yen, 1975), isolated from the hypothalamus of sheep and pigs. LHRH was found to stimulate the secretion of both luteinizing hormone (LH) and follicle-stimulating hormone (FSH), and was therefore named gonadotropin-releasing hormone (GnRH) (Uenoyama, Pheng, Tsukamura & Maeda, 2016), because LH and FSH are hormones called gonadotrophins.
GnRH is one of the most important hormones of the reproductive axis. It is secreted by several neurons in the hypothalamus, GnRH in turn stimulates the pituitary gland, which stimulates the synthesis of the gonadotrophins in the testes and ovaries (gonads). When the gonads are stimulated by gonadotrophins, they mature and produce sperm or egg cells. During the first months after birth, the GnRH system is slightly activated, but remains quiet until reactivation signals the initiation of puberty (Messager, 2005).
Belchetz, Plant, Nakai, Keogh and Knobil (1978) studied the effect of GnRH in ovariectomized rhesus monkeys. The monkeys bore hypothalamic lesions, which caused the abolishment of gonadotropin secretion. However, by administering GnRH in a pulsatile fashion, FSH and LH levels were recovered, showing the importance of GnRH. Additionally, the administered GnRH pulses caused regular menstrual cycles, but stopping the administration of GnRH caused reversal in prepubertal female monkeys to the immature state. Another significant finding of Belchetz et al. (1978) was the importance of the pulsatile administration of GnRH. If GnRH levels were kept at a constant high or low level, gonadotropin secretion was abolished.
Discovery of Kisspeptin
The role of kisspeptin in relation to sexual reproduction, or puberty, was discovered in 2003 by two independent research teams. In these studies was observed that inactivation due to mutations on the gene for the GPR54 receptor led to idiopathic hypogonadotropic hypogonadism in mice and humans (Smith & Clarke, 2007). Hypogonadotropic hypogonadism is characterized by delayed or absent puberty (Seminara & Crowley, 2008). It is the clinical syndrome that is caused by gonadal failure, caused by abnormal pituitary gonadotropin levels. Hypogonadotropic hypogonadism is a result of reduced hypothalamic GnRH secretion or the absence of GnRH secretion (Silveira & Latronico, 2013). Since this discovery, kisspeptins have been thoroughly studied as regulators of GnRH function and its role in the onset of puberty (Smith & Clarke, 2007).
Kisspeptin was initially found to play a major role in suppressing metastasis in humans. It is the 54 amino-acid peptide product of the KiSS-1 gene, and it’s the natural ligand of the G-protein-coupled receptor GPR54. Kisspeptin has the ability to suppress tumor metastasis and was therefore named metastasin (Ohtaki et al. 2001). The amino acid sequence of Kisspeptin consists of 52-54 amino acids and is very well conserved in most mammals and non-mammalian vertebrates (Biran, Ben-Dor & Levavi-Sivan, 2008), especially the 10 amino acids at the C terminal which bind to the GPR54 receptor (Terao, Kumano, Takatsu, Hattori, Nishimura, Ohtaki & Shintani, 2004). This suggest that kisspeptin plays a significant role in a variety of vertebrates (Roseweir & Millar, 2009). The initial product of the KiSS1 gene is a peptide of 145 amino acids long. This kisspeptin precursor peptide is cut in kisspeptin-54, the above described 54-amino-acid peptide. Kisspeptin’s name is derived from where the KiSS1 gene was discovered. KiSS1 was cloned in Hershey Pennsylvania, which is a city that is known for its chocolate kisses (Popa, Clifton & Steiner, 2008).
The GPR54 receptor
In 2001, kisspeptin was found to be the natural ligand for the orphan G-protein coupled receptor 54 (GPR54). It was first found in the rat brain and later in humans as well (Ohtaki et al., 2001). In humans it was named AXOR12 and hOT7T175, later it was named KiSS1R. The KiSS1R gene can be found on chromosome 19p13.3. It has five exons, which encode a 398 amino acid long protein. Even though GPR54 does not bind galanin or galanin-like peptides, it’s amino acid sequence is related to the sequence of the galanin receptor family. When kisspeptin binds to the GPR54 receptor, an intracellular second messenger system is activated. Upon binding, phospholipase C is activated, which recruits the second messengers inositol triphosphate and diacylglycerol. These second messengers mediate calcium release and protein kinase C activation to mediate the functioning of kisspeptin (Constantin, Caligioni, Stojilkovic & Wray, 2009), (Skorupskaite, George & Anderson, 2014).
Kisspeptin as an Initiation Factor for Puberty
The discovery of kisspeptin and its role in the onset of puberty, has been an exciting finding. After the role of GnRH was discovered, multiple hormones were found to play a role in sexual maturation. However, none of these hormones proved to have such an significant effect as kisspeptin. This effect is demonstrated in the studies discussed above. In addition, another research showed these effect by genetically modifying a mouse that lacks the KiSS1 gene, this a technique called ‘knockout’. This mouse, with a mutated GPR54, did not undergo puberty. The gonads were observed to be small and sex hormone, estrogen and testosterone, levels were low, as well as the gonadotrophin levels. In this study was also shown that by hormonal treatment, the mouse could still go through puberty and develop in a healthy rodent. This is the same observation as shown by Belchetz et al. (1978). Humans with the same mutation, could also be treated with hormones. These observations show that kisspeptin is important for puberty and to reach a sexual mature state, but the absence of kisspeptin does not harm the reproductive organs neither causes it a developmental defect. Eventually, the pituitary gland and the gonads are still able to respond to hormonal stimulation and develop into mature reproductive organs even though the normal age of puberty has passed (Messager, 2005).
Kisspeptin and GnRH Secretion
The above mentioned observation that inactivation of the GPR54 receptor led to idiopathic hypogonadotropic hypogonadism (iHH) raised the question whether kisspeptin might stimulate GnRH secretion by acting through GPR54 (kisspeptin and GPR54 were already linked in metastasis suppression). Since then, studies have shown “that central administration or peripheral administration of kisspeptin stimulates gonadotropin secretion through a GnRH dependent mechanism” (Popa et al., 2008, p. 215). However, GnRH cannot be measured in the peripheral circulation. Therefore, LH pulse frequencies are measured, because it has been found that each GnRH pulse is associated with an LH pulse, which makes LH pulse frequency a good surrogate for hypophyseal GnRH pulse frequency (Clarke and Cummins, 1985).
Han et al. (2005) found that the hypothalamic GnRH system is directly stimulated by kisspeptin and secretes gonadotrophin. In vitro, it was found that kisspeptin depolarize GnRH neurones and increase the frequency of depolarization. This was also found by Zhang, Roepke, Kelly and Ronnekleiv (2008).