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| The KiSS1 gene, encodes a 145 amino acid (aa) long propeptide, from which a 54aa sequence, known as Kp-54, is cleaved (Kauffman, et al., 2007). In the precursor, Kp-145, the 54aa sequence is surrounded pairs of basic residue, which is the site of cleavage. It is thought that furin, a prohormone convertase enzyme, which transforms inactive precursors into active molecules, is responsible for this split. Shorter sequences has also been discovered, kisspeptin-10, -13, -14, respectively, which all have the same Agr-Phe-NH2 pattern at the C-terminus. Kp is the natural ligand for the KISS1-derived peptide receptor (KiSS1r), formerly known as GPR54 (G-protein receptor54) (Pineda, et al., 2010). | The KiSS1 gene, encodes a 145 amino acid (aa) long propeptide, from which a 54aa sequence, known as Kp-54, is cleaved (Kauffman, ''et al''., 2007). In the precursor, Kp-145, the 54aa sequence is surrounded pairs of basic residue, which is the site of cleavage. It is thought that furin, a prohormone convertase enzyme, which transforms inactive precursors into active molecules, is responsible for this split. Shorter sequences has also been discovered, kisspeptin-10, -13, -14, respectively, which all have the same Agr-Phe-NH2 pattern at the C-terminus. Kp is the natural ligand for the KISS1-derived peptide receptor (KiSS1r), formerly known as GPR54 (G-protein receptor54) (Pineda, ''et al''., 2010). |
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| The relevance of these short peptides is not known, but as all kisspeptins show equal affinity for; and efficiency at their corresponding receptor. It has thus been concluded that the C-terminal of the peptide is responsible for binding and activation at the receptor (Oakley, et al., 2010). | The relevance of these short peptides is not known, but as all kisspeptins show equal affinity for; and efficiency at their corresponding receptor. It has thus been concluded that the C-terminal of the peptide is responsible for binding and activation at the receptor (Oakley, ''et al''., 2010). |
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| GnRH neurons are located mainly in the preoptic area of the hypothalamus (Oakley, et al., 2010), which, when activated, releases GnRH that travels via the hypophyseal portal system to the pituitary gland, where LH and FSH are secreted by the gonadotropes. These two hormones are regulated by a negative feedback loop of sex steroids, oestrogen and progesterone, produced by the gonads (Roa, et al., 2011; Oakley, et al., 2010). | GnRH neurons are located mainly in the preoptic area of the hypothalamus (Oakley, ''et al''., 2010), which, when activated, releases GnRH that travels via the hypophyseal portal system to the pituitary gland, where LH and FSH are secreted by the gonadotropes. These two hormones are regulated by a negative feedback loop of sex steroids, oestrogen and progesterone, produced by the gonads (Roa, ''et al''., 2011; Oakley, ''et al''., 2010). |
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| The KiSS1 gene is expressed in many areas of the body, but is predominantly found in the hypothalamus, more specifically in two main populations; one in the arcuate nucleus (ARC) and one in the anteroventral periventricular nucleus (AVPV), (D’Anglemont de Tassigny & Colledge, 2010). The distribution also shows interspecies differences and, furthermore, disparity between sexes. A larger number of cells expressing the gene are found in the AVPV of females (Oakley, et al., 2010). This is most likely linked to the organizing effect of sex steroids during neonatal sexual maturation of the brain (Pineda, et al., 2010). | The KiSS1 gene is expressed in many areas of the body, but is predominantly found in the hypothalamus, more specifically in two main populations; one in the arcuate nucleus (ARC) and one in the anteroventral periventricular nucleus (AVPV), (D’Anglemont de Tassigny & Colledge, 2010). The distribution also shows interspecies differences and, furthermore, disparity between sexes. A larger number of cells expressing the gene are found in the AVPV of females (Oakley, ''et al''., 2010). This is most likely linked to the organizing effect of sex steroids during neonatal sexual maturation of the brain (Pineda, ''et al''., 2010). |
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| Kp can act directly to depolarize and increase GnRH neuron’s firing rate, which increases the secretion of GnRH. The ligand is also thought to be able to act on intermediary neurons, like GABAergic neurons, to regulate the GnRH release (Oakley, et al., 2010). It is commonly concluded that kisspeptins effect the LH and FSH secretion indirectly via the GnRH pathway, but some studies have given us reason to believe that in addition to working on a hypothalamic level, Kp can act directly on the gonadotropes in the pituitary gland; stimulating LH and FSH secretion directly. In 2007, Kauffman et al, mentioned the expression of KiSS1r in the pituitary gland, but discarded the idea of Kp having a direct effect on a pituitary level. They concluded that GnRH secretion caused by Kp was a caused by direct activation of GnRH neurons. Three years later, Oakley, et al, (2010) reopened the possibility for Kp activation directly through the gonadotropes. They based this on the presence of functional KiSS1r in the pituitary, in addition to detecting Kp in ovine hypophyseal portal blood. Nevertheless, there is still ongoing controversy about this dual activation, as GnRH secretion via direct Kp stimuli is proven to be relatively low (Oakley, et al., 2010). Administration of exogenous Kp has shown a significant increase in plasma LH and FSH levels shortly after infusion. This surge has proven to be sufficient to induce estrous cycles in prepubescent animals and cause ovulation. In 2009, Sébert, et al, conducted a study on Kisspeptins effect on LH secretion and ovulation in anoestrous ewes. 75% of the treated animals ovulated after a 24hour infusion of Kp. Similar increases in LH/FSH secretion was proved by Pineda, et al. (2010) when they administered Kp intra-cerebrally to rats. |
Kp can act directly to depolarize and increase GnRH neuron’s firing rate, which increases the secretion of GnRH. The ligand is also thought to be able to act on intermediary neurons, like GABAergic neurons, to regulate the GnRH release (Oakley, ''et al''., 2010). It is commonly concluded that kisspeptins effect the LH and FSH secretion indirectly via the GnRH pathway, but some studies have given us reason to believe that in addition to working on a hypothalamic level, Kp can act directly on the gonadotropes in the pituitary gland; stimulating LH and FSH secretion directly. In 2007, Kauffman ''et al''., mentioned the expression of KiSS1r in the pituitary gland, but discarded the idea of Kp having a direct effect on a pituitary level. They concluded that GnRH secretion caused by Kp was a caused by direct activation of GnRH neurons. Three years later, Oakley, ''et al''., (2010) reopened the possibility for Kp activation directly through the gonadotropes. They based this on the presence of functional KiSS1r in the pituitary, in addition to detecting Kp in ovine hypophyseal portal blood. Nevertheless, there is still ongoing controversy about this dual activation, as GnRH secretion via direct Kp stimuli is proven to be relatively low (Oakley, ''et al''., 2010). Administration of exogenous Kp has shown a significant increase in plasma LH and FSH levels shortly after infusion. This surge has proven to be sufficient to induce estrous cycles in prepubescent animals and cause ovulation. In 2009, Sébert, et al, conducted a study on Kisspeptins effect on LH secretion and ovulation in anoestrous ewes. 75% of the treated animals ovulated after a 24hour infusion of Kp. Similar increases in LH/FSH secretion was proved by Pineda, ''et al''. (2010) when they administered Kp intra-cerebrally to rats. |
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| It was found in an experiment on rats, that it takes a 100 times higher dose of Kp to secrete FSH than LH (Roseweir & Millar, 2008). After the gonadotrophin surge, levels of LH and FSH decrease due to the negative feedback loop caused by sex steroids. However, LH plasma levels are still higher than before infusion, whilst FSH levels drop back to basal levels (Sébert, et al., 2009; Navarro, et al., 2005). Studies also show that prolonged exogenous Kp administration can lead to a down-regulation of the GnRH receptors and in turn LH and FSH secretion, due to desensitization of the KiSS1r (Oakley, et al., 2010; Mead, et al., 2007). | It was found in an experiment on rats, that it takes a 100 times higher dose of Kp to secrete FSH than LH (Roseweir & Millar, 2008). After the gonadotrophin surge, levels of LH and FSH decrease due to the negative feedback loop caused by sex steroids. However, LH plasma levels are still higher than before infusion, whilst FSH levels drop back to basal levels (Sébert, ''et al''., 2009; Navarro, et al., 2005). Studies also show that prolonged exogenous Kp administration can lead to a down-regulation of the GnRH receptors and in turn LH and FSH secretion, due to desensitization of the KiSS1r (Oakley, ''et al''., 2010; Mead, ''et al''., 2007). |
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| The pulse frequency of GnRH secretion has an impact on which gonadotropin hormone will be released. Low GnRH pulse frequency is shown to favor FSH secretion, whilst high GnRH pulse frequency favors LH secretion (Millar, et al., 2010). | The pulse frequency of GnRH secretion has an impact on which gonadotropin hormone will be released. Low GnRH pulse frequency is shown to favor FSH secretion, whilst high GnRH pulse frequency favors LH secretion (Millar, ''et al''., 2010). |
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| GnRH neurons do not exhibit any androgen receptors (AR) or oestrogen α receptors (ERα), thus for a long time it was suspected that another component was involved in regulating GnRH secretion via the negative/positive feedback system of sex steroids. Kp neurons express both AR and ER(α), and via their intimate contact with GnRH neurons, have a crucial role in relaying feedback information (D’Anglemont de Tassigny & Colledge, 2010; Oakley, et al., 2010). | GnRH neurons do not exhibit any androgen receptors (AR) or oestrogen α receptors (ERα), thus for a long time it was suspected that another component was involved in regulating GnRH secretion via the negative/positive feedback system of sex steroids. Kp neurons express both AR and ER(α), and via their intimate contact with GnRH neurons, have a crucial role in relaying feedback information (D’Anglemont de Tassigny & Colledge, 2010; Oakley, ''et al''., 2010). |
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| Kp is also believed to be linked to the suppression of GnRH/LH pulse during lactation, due to lower KiSS1 gene expression in the ARC region and lower KiSS1 receptor expression in the AVPV region of the hypothalamus (Mead et al., 2007). | Kp is also believed to be linked to the suppression of GnRH/LH pulse during lactation, due to lower KiSS1 gene expression in the ARC region and lower KiSS1 receptor expression in the AVPV region of the hypothalamus (Mead, ''et al''., 2007). |
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| It has also been discovered that Kp directly communicates with neurons containing nNOS (neuronal Nitric Oxide synthase), and that NO release is essential for the preovulatory GnRH surge, which is mediated by Kp (Hanchate, et al., 2012). | It has also been discovered that Kp directly communicates with neurons containing nNOS (neuronal Nitric Oxide synthase), and that NO release is essential for the preovulatory GnRH surge, which is mediated by Kp (Hanchate, ''et al''., 2012). |
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| As previously mentioned, Kp has a pivotal role in many of the bodies functions and of particular interest to recent research, the reproductive (HPG) axis. Both, Mead, et al. (2007) and Navarro, et al. (2005) refer to Kp as the ‘molecular switch for puberty’. Thus, if Kiss1 or Kiss1r has mutated it will have serious repercussions for the individual. These mutations may be spontaneous or genetically targeted: missense (point mutations) deletions or insertions, (Roseweir & Millar, 2009), (Kauffman, et al., 2007), (Tena-Sempere, 2006). Often such mutations reveal themselves as disorders such as Idiopathic Hypogonadotrophic Hypogonadism (IHH) (decreased activity of the gonads), delayed puberty, hypothalamic secondary amenorrhea (cessation of menstruation) and even precocious puberty (early onset puberty), (Sonigo, & Binart, 2012), (D’Anglemont de Tassigny, & Colledge, 2010), (Oakley, et al., 2009). Furthermore, mutations may cause infertility, poor gonadal growth and impaired gametogenesis; which, in females, results in the disruption of the normal estrus cycle leading to ovulation issues and lack of Corpus Lutea development (D’Anglemont de Tassigny, & Colledge, 2010). Roseweir and Millar (2009) explain that the affected individuals have ‘small ovaries and uteri, delayed vaginal opening, no maturation of follicles, decreased sexual behaviour... and low gonadotrophin levels’. Accordingly, these mutations can have varying severity from loss of function to gain of function, whereas some are so mild that they decrease GnRH levels but allow normal bodily functioning. (Roa, et al., 2011), (D’Anglemont de Tassigny & Colledge, 2010), (Millar, et al., 2010), (Oakley, et al., 2009), (Tena-Sempere, 2006) |
As previously mentioned, Kp has a pivotal role in many of the bodies functions and of particular interest to recent research, the reproductive (HPG) axis. Both, Mead, ''et al''. (2007) and Navarro, ''et al''. (2005) refer to Kp as the ‘molecular switch for puberty’. Thus, if Kiss1 or Kiss1r has mutated it will have serious repercussions for the individual. These mutations may be spontaneous or genetically targeted: missense (point mutations) deletions or insertions, (Roseweir & Millar, 2009; Kauffman, ''et al''., 2007; Tena-Sempere, 2006). Often such mutations reveal themselves as disorders such as Idiopathic Hypogonadotrophic Hypogonadism (IHH) (decreased activity of the gonads), delayed puberty, hypothalamic secondary amenorrhea (cessation of menstruation) and even precocious puberty (early onset puberty), (Sonigo, & Binart, 2012; D’Anglemont de Tassigny, & Colledge, 2010; Oakley, ''et al''., 2009). Furthermore, mutations may cause infertility, poor gonadal growth and impaired gametogenesis; which, in females, results in the disruption of the normal estrus cycle leading to ovulation issues and lack of Corpus Lutea development (D’Anglemont de Tassigny, & Colledge, 2010). Roseweir and Millar (2009) explain that the affected individuals have ‘small ovaries and uteri, delayed vaginal opening, no maturation of follicles, decreased sexual behaviour... and low gonadotrophin levels’. Accordingly, these mutations can have varying severity from loss of function to gain of function, whereas some are so mild that they decrease GnRH levels but allow normal bodily functioning (Roa, ''et al''., 2011; D’Anglemont de Tassigny & Colledge, 2010; Millar, ''et al''., 2010; Oakley, ''et al''., 2009; Tena-Sempere, 2006). |
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| There are many polymorphisms that exist for Kp in the female; resulting in the phenotypes described previously. Mutations of Kiss1 receptor gene, such as R331X and X399R, cause IHH that results in decreased GnRH production (Mead, et al., 2007). These mutations are the result of the stop codon having been ‘moved’, thus elongating the sequence. Further Kiss1r gene mutations resulting in IHH are missense mutations C223R, R297L and L102P. These mutations, however, affect the receptor in a different way, resulting in impaired GnRH signaling. Of the three, C223R has the most severe effects). There are two other mutations of the Kiss1r known to cause IHH: a 155 nucleotide deletion and L1485 which is a single nucleotide deletion. Less is known of the 155 nucleotide deletion but the deletions do have different effects. L1475 has a similar effect to C223R, R297L and L102P (Roseweir & Millar, 2009), (Mead, et al., 2007). | There are many polymorphisms that exist for Kp in the female; resulting in the phenotypes described previously. Mutations of Kiss1 receptor gene, such as R331X and X399R, cause IHH that results in decreased GnRH production (Mead, ''et al''., 2007). These mutations are the result of the stop codon having been ‘moved’, thus elongating the sequence. Further Kiss1r gene mutations resulting in IHH are missense mutations C223R, R297L and L102P. These mutations, however, affect the receptor in a different way, resulting in impaired GnRH signaling. Of the three, C223R has the most severe effects. There are two other mutations of the Kiss1r known to cause IHH: a 155 nucleotide deletion and L1485 which is a single nucleotide deletion. Less is known of the 155 nucleotide deletion but the deletions do have different effects. L1475 has a similar effect to C223R, R297L and L102P (Roseweir & Millar, 2009; Mead, ''et al''., 2007). |
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| Additionally there are two other gene mutations which cause IHH: TAC3 and TAC3R. Although these are not strictly Kp mutations they are thought to have a direct effect on Kp and thus GnRH secretion. These mutations are of the gene which encodes a protein called NKB and its receptors. NKB is believed to regulate the secretion of Kp along with another protein called Dyn (Pineda, et al., 2010). | Additionally there are two other gene mutations which cause IHH: TAC3 and TAC3R. Although these are not strictly Kp mutations they are thought to have a direct effect on Kp and thus GnRH secretion. These mutations are of the gene which encodes a protein called NKB and its receptors. NKB is believed to regulate the secretion of Kp along with another protein called Dyn (Pineda, ''et al''., 2010). |
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| There are two known mutations of the Kiss1r gene that cause precocious puberty: R386P and P110T (Roseweir & Millar, 2009). Oakley, et al. (2009) explains that these mutations cause ‘prolonged intracellular Kiss1r signalling in response to Kp’. Both IHH and precocious puberty caused by such mutations present other symptoms also; such as hypothalamic secondary amenorrhea, (Roseweir & Millar, 2009). | There are two known mutations of the Kiss1r gene that cause precocious puberty: R386P and P110T (Roseweir & Millar, 2009). Oakley, ''et al''. (2009) explains that these mutations cause ‘prolonged intracellular Kiss1r signalling in response to Kp’. Both IHH and precocious puberty caused by such mutations present other symptoms also; such as hypothalamic secondary amenorrhea, (Roseweir & Millar, 2009). |
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| The maintenance of hormonal equilibrium within the HPG-axis, is under both direct and indirect influence of Kisspeptins. Gondatropin releasing hormone of the hypothalamus is under direct duress from Kp, which, acting indirectly through GnRH, induces the release of both LH and FSH gonadotropins (Roa, et al., 2011; Oakley, et al., 2010). It is evident, from conclusive research, that interference in this mechanism, via insertion, deletion or point mutations, can dramatically alter the physiological reproductive development of an individual. Additional research, linked to the successful functioning of Kp, is predicted on the proteins NKB and Dyn. These proteins are thought to work as “auto-regulators of Kp output onto GnRH neurons” (Pineda, et al., 2010). With a more thorough understanding not just of the effects of Kp on the HPG-axis, but also the factors that control and regulate Kp expression in an individual, possible therapeutic applications of this gene may be comprehended. |
The maintenance of hormonal equilibrium within the HPG-axis, is under both direct and indirect influence of Kisspeptins. Gondatropin releasing hormone of the hypothalamus is under direct duress from Kp, which, acting indirectly through GnRH, induces the release of both LH and FSH gonadotropins (Roa, ''et al''., 2011; Oakley, ''et al''., 2010). It is evident, from conclusive research, that interference in this mechanism, via insertion, deletion or point mutations, can dramatically alter the physiological reproductive development of an individual. Additional research, linked to the successful functioning of Kp, is predicted on the proteins NKB and Dyn. These proteins are thought to work as “auto-regulators of Kp output onto GnRH neurons” (Pineda, ''et al''., 2010). With a more thorough understanding not just of the effects of Kp on the HPG-axis, but also the factors that control and regulate Kp expression in an individual, possible therapeutic applications of this gene may be comprehended. |
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| Surmounting the basic understanding, mapping and function of Kp, researchers believe that the regulation of reproductive cycles can thus be manipulated. For example, earlier estrus cycles in domestic mammals which could have a potential constructive impact on agricultural economy (Caraty, et al., 2012). Studies were conducted with the aim of inducing well-timed and coordinated ovulations in exogenously Kp-treated animals. An LH surge was noted in all treated animals, with an understanding that Kp had the capability to be used to “activate reproductive function in acyclic animals” (Caraty, et al., 2012). | Surmounting the basic understanding, mapping and function of Kp, researchers believe that the regulation of reproductive cycles can thus be manipulated. For example, earlier estrus cycles in domestic mammals which could have a potential constructive impact on agricultural economy (Caraty, ''et al''., 2012). Studies were conducted with the aim of inducing well-timed and coordinated ovulations in exogenously Kp-treated animals. An LH surge was noted in all treated animals, with an understanding that Kp had the capability to be used to “activate reproductive function in acyclic animals” (Caraty, ''et al''., 2012). |
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| Ansel, L., et al., 2011. Peripheral Kisspeptin Reverses Short Photoperiod-induced Gonadal Regression in Syrian Hamsters by Promoting GnRH Release, The Journal of the Society for Reproduction and fertility, [e-journal], 144 (5), [Online] Available at: <http://www.reproduction-online.org/content/142/3/417.short> [Accessed: 07 November 2012]. | Ansel, L., ''et al''., 2011. Peripheral Kisspeptin Reverses Short Photoperiod-induced Gonadal Regression in Syrian Hamsters by Promoting GnRH Release, The Journal of the Society for Reproduction and fertility, [e-journal], 144 (5), [Online] Available at: <http://www.reproduction-online.org/content/142/3/417.short> [Accessed: 07 November 2012]. |
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| Caraty, A, et al., 2012. Domestic Animal Endocrinology - Kisspeptins and the Reproductive Axis: Potential Applications to Manage Reproduction in Farm Animals, Elsevier, [e-journal] 43 (2), Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0739724012000446> [Accessed: 07 November 2012]. | Caraty, A, ''et al''., 2012. Domestic Animal Endocrinology - Kisspeptins and the Reproductive Axis: Potential Applications to Manage Reproduction in Farm Animals, Elsevier, [e-journal] 43 (2), Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0739724012000446> [Accessed: 07 November 2012]. |
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| Hanchate, N.K., et al., 2012. Kisspeptin- GPR54 Signaling in Mouse NO-Synthesizing Neurons Participates in the Hypothalamic Control of Ovulation, The Journal of Neuroscience, [e-journal], 32 (3), [Online] Available at: <http://www.jneurosci.org/content/32/3/932.long> [Accessed: 06 November 2012]. | Hanchate, N.K., ''et al''., 2012. Kisspeptin- GPR54 Signaling in Mouse NO-Synthesizing Neurons Participates in the Hypothalamic Control of Ovulation, The Journal of Neuroscience, [e-journal], 32 (3), [Online] Available at: <http://www.jneurosci.org/content/32/3/932.long> [Accessed: 06 November 2012]. |
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| Kauffman, A.S., et al., 2007. Emerging Ideas about Kisspeptin-GPR54 Signaling in the Neuroendocrine Regulation of Reproduction, Cell Press, [e-journals], 30 (10), Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0166223607002123> [Accessed: 07 November 2012]. | Kauffman, A.S., ''et al''., 2007. Emerging Ideas about Kisspeptin-GPR54 Signaling in the Neuroendocrine Regulation of Reproduction, Cell Press, [e-journals], 30 (10), Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0166223607002123> [Accessed: 07 November 2012]. |
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| Mead, E.J., et al., 2007. Kisspeptins: a multifunctional peptide system with a role in reproduction, cancer and the cardiovascular system, British Journal of Pharmacology, [e-journal] 151 (8), Available through Wiley Online Library at: <http://onlinelibrary.wiley.com/doi/10.1038/sj.bjp.0707295/full> [Accessed: 07 November 2012]. | Mead, E.J., ''et al''., 2007. Kisspeptins: a multifunctional peptide system with a role in reproduction, cancer and the cardiovascular system, British Journal of Pharmacology, [e-journal] 151 (8), Available through Wiley Online Library at: <http://onlinelibrary.wiley.com/doi/10.1038/sj.bjp.0707295/full> [Accessed: 07 November 2012]. |
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| Millar, R.P., et al., 2010. Kisspeptin Antagonists: Unraveling the Role of Kisspeptin in Reproductive Physiology, Elsevier, [e-journal], 1364, Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0006899310020494> [Accessed: 07 November 2012]. | Millar, R.P., ''et al''., 2010. Kisspeptin Antagonists: Unraveling the Role of Kisspeptin in Reproductive Physiology, Elsevier, [e-journal], 1364, Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0006899310020494> [Accessed: 07 November 2012]. |
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| Navarro, V.M., et al., 2005. Effects of KISS-1 Peptide, the Natural Ligand of GPR54, on Follicle-Stimulating Hormone Secretion in the Rat, The Endocrine Society, [e-journal], 146 (4), [Online] Available at: <http://endo.endojournals.org/content/146/4/1689.long> [Accessed: 07 November 2012]. | Navarro, V.M., ''et al''., 2005. Effects of KISS-1 Peptide, the Natural Ligand of GPR54, on Follicle-Stimulating Hormone Secretion in the Rat, The Endocrine Society, [e-journal], 146 (4), [Online] Available at: <http://endo.endojournals.org/content/146/4/1689.long> [Accessed: 07 November 2012]. |
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| Oakley, A.E., et al., 2009. Kisspeptin Signaling in the Brain, The Endocrine Society, [e-journal], 30 (6), [Online] Available at: <http://edrv.endojournals.org/content/30/6/713.long> Accessed: 07 November 2012]. | Oakley, A.E., ''et al''., 2009. Kisspeptin Signaling in the Brain, The Endocrine Society, [e-journal], 30 (6), [Online] Available at: <http://edrv.endojournals.org/content/30/6/713.long> Accessed: 07 November 2012]. |
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| Pineda, R, et al., 2010. Progress in Brain Research: Chapter 5, Physiological Roles of Kisspeptin/GPR54 System in the Neuroendocrine Control of Reproduction, Elsevier, [e-journals] 181, Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0079612308810059> [Accessed: 07 November 2012]. | Pineda, R, ''et al''., 2010. Progress in Brain Research: Chapter 5, Physiological Roles of Kisspeptin/GPR54 System in the Neuroendocrine Control of Reproduction, Elsevier, [e-journals] 181, Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0079612308810059> [Accessed: 07 November 2012]. |
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| Pratheesh, M.D., et al., 2011. Role of Kisspeptin in Puberty and Reproduction, Online Veterinary Journal, [e-journal], 6 (2), [Online], Available at: <http://vetscan.co.in/v6n2/87-Role-of-Kisspeptin-in-Puberty-Reproduction.htm> [Accessed: 06 November 2012]. | Pratheesh, M.D., ''et al''., 2011. Role of Kisspeptin in Puberty and Reproduction, Online Veterinary Journal, [e-journal], 6 (2), [Online], Available at: <http://vetscan.co.in/v6n2/87-Role-of-Kisspeptin-in-Puberty-Reproduction.htm> [Accessed: 06 November 2012]. |
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| Roa, J, et al., 2011. Kisspeptins Reproductive Biology: Consensus Knowledge and recent Developments, Biology of Reproduction, [ e-journal] 85 (4), [Online] Available at: <http://www.biolreprod.org/content/85/4/650.full> [Accessed: 07 November 2012]. | Roa, J, ''et al''., 2011. Kisspeptins Reproductive Biology: Consensus Knowledge and recent Developments, Biology of Reproduction, [ e-journal] 85 (4), [Online] Available at: <http://www.biolreprod.org/content/85/4/650.full> [Accessed: 07 November 2012]. |
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| Sébert, M.E., et al., 2010. Domestic Animal Endocrinology - Insight into the Mechanism by which Kisspeptin Stimulates a Preovulatory LH Surge and Ovulation in Seasonally Acyclic Ewes: Potential role of Estradiol, Elsevier, [e-journals] 38 (4), Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0739724010000044> [Accessed: 07 November 2012]. | Sébert, M.E., ''et al''., 2010. Domestic Animal Endocrinology - Insight into the Mechanism by which Kisspeptin Stimulates a Preovulatory LH Surge and Ovulation in Seasonally Acyclic Ewes: Potential role of Estradiol, Elsevier, [e-journals] 38 (4), Available through SciVerse Science Direct at: <http://www.sciencedirect.com/science/article/pii/S0739724010000044> [Accessed: 07 November 2012]. |
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| Tsutsui, K, et al., 2010. Discovery and Evolutionary History of Gonadotrophin-Inhibitory Hormone and Kisspeptin: New Key Neuropeptides Controlling Reproduction, Journal of Neuroendocrinology, [e-journals] 22 (7), Available through Wiley Online Library at:< http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2826.2010.02018.x/full> [Accessed: 07 November 2012]. | Tsutsui, K, ''et al''., 2010. Discovery and Evolutionary History of Gonadotrophin-Inhibitory Hormone and Kisspeptin: New Key Neuropeptides Controlling Reproduction, Journal of Neuroendocrinology, [e-journals] 22 (7), Available through Wiley Online Library at:< http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2826.2010.02018.x/full> [Accessed: 07 November 2012]. |
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| Wu, M, et al., 2009. Melanin – Concentrating Hormone Directly Inhibits GnRH Neurons and Blocks Kisspeptin Activation, Linking Energy Balance To Reproduction, Proceedings of the National Academy of Sciences of the United States of America, [e-journals] 106 (40), [Online] Available at: <http://www.pnas.org/content/106/40/17217.short> [Accessed: 07 November 2012]. | Wu, M, ''et al''., 2009. Melanin – Concentrating Hormone Directly Inhibits GnRH Neurons and Blocks Kisspeptin Activation, Linking Energy Balance To Reproduction, Proceedings of the National Academy of Sciences of the United States of America, [e-journals] 106 (40), [Online] Available at: <http://www.pnas.org/content/106/40/17217.short> [Accessed: 07 November 2012]. |
Introduction
In 1996, Lee et al. identified kisspeptins as a product the KiSS1 gene, a metastasis suppressor gene, in malignant melanomas (Pratheesh, et al., 2011).The term kisspeptins (Kp), is comprehensively used to communally describe a family of neuropeptides that have an Arg-Phe-NH2 pattern at the C-terminus, which are primarily expressed in distinct neuronal populations of the hypothalamus (Pratheesh, et al., 2011; Roa, et al., 2011).
Kisspeptins act as mediators of various physiological and hormonal signals in both male and female mammalian and non-mammalian vertebrates (De Tassigny & Colledge, 2010). They play an essential role as regulators of crucial aspects of development and function of the reproductive axis, also termed the hypothalamic-pituitary-gonadal axis (HPG-axis) (Roa, et al., 2011). The importance of kisspeptin and its associated receptor, G-protein receptor – 54 (GPR54),was demonstrated in 2003 following mutations of the peptide or receptor causing precocious puberty (Sonigo & Binart, 2012) or delayed perhaps absent puberty; hypogonadotropichypogonadism (Roseweir & Miller, 2009).
Mechanisms of kisspeptin action as a molecular switch for puberty and reproductive development are a current source of research. Kisspeptins stimulate the secretion of gonadotropins from the pituitary through exciting the release of GnRH from the forebrain after the activation of Gpr-54 by GnRH neurons (Pratheesh, et al., 2011).
== Kisspeptins ==
The hypothalamic-pituitary-gonad axis is a complex system governing a number of bodily functions, but is mainly responsible for development, reproduction and ageing in animals and humans (Klein, 2012).This system regulates reproduction in the body by pulsatile gonadotropin-releasing hormone (GnRH) secretion. This main neurotransmitter of the HPG-axis induces a corresponding pulsatile release of gonadotropins, namely luteinizing hormone (LH) and follicle stimulating hormone (FSH), from the anterior pituitary gland (Roa, et al., 2011; Oakley, et al., 2010). Both ofthese gonadotropins are of crucial importance in female reproductive physiology, initiating ovarian follicle development, and ovulation (Hanache, et al., 2012; Klein, et al., 2012; Oakley, et al., 2010).
The KiSS1 gene, encodes a 145 amino acid (aa) long propeptide, from which a 54aa sequence, known as Kp-54, is cleaved (Kauffman, et al., 2007). In the precursor, Kp-145, the 54aa sequence is surrounded pairs of basic residue, which is the site of cleavage. It is thought that furin, a prohormone convertase enzyme, which transforms inactive precursors into active molecules, is responsible for this split. Shorter sequences has also been discovered, kisspeptin-10, -13, -14, respectively, which all have the same Agr-Phe-NH2 pattern at the C-terminus. Kp is the natural ligand for the KISS1-derived peptide receptor (KiSS1r), formerly known as GPR54 (G-protein receptor54) (Pineda, et al., 2010).
The relevance of these short peptides is not known, but as all kisspeptins show equal affinity for; and efficiency at their corresponding receptor. It has thus been concluded that the C-terminal of the peptide is responsible for binding and activation at the receptor (Oakley, et al., 2010).
GnRH neurons are located mainly in the preoptic area of the hypothalamus (Oakley, et al., 2010), which, when activated, releases GnRH that travels via the hypophyseal portal system to the pituitary gland, where LH and FSH are secreted by the gonadotropes. These two hormones are regulated by a negative feedback loop of sex steroids, oestrogen and progesterone, produced by the gonads (Roa, et al., 2011; Oakley, et al., 2010).
The KiSS1 gene is expressed in many areas of the body, but is predominantly found in the hypothalamus, more specifically in two main populations; one in the arcuate nucleus (ARC) and one in the anteroventral periventricular nucleus (AVPV), (D’Anglemont de Tassigny & Colledge, 2010). The distribution also shows interspecies differences and, furthermore, disparity between sexes. A larger number of cells expressing the gene are found in the AVPV of females (Oakley, et al., 2010). This is most likely linked to the organizing effect of sex steroids during neonatal sexual maturation of the brain (Pineda, et al., 2010). Most GnRH neurons in the hypothalamus express KiSS1-receptors, again with different data across the species. Using double immunofluorescence, a staining method to observe several antigens in one sample, it was discovered that 85% of GnRH neurons in the medial eminence of rat’s brains express KiSS1r (Roseweir & Millar, 2009). Other studies show that between 70-90% of GnRH neurons in sheep, mice, and rhesus monkeys express KiSS1r.
Kp can act directly to depolarize and increase GnRH neuron’s firing rate, which increases the secretion of GnRH. The ligand is also thought to be able to act on intermediary neurons, like GABAergic neurons, to regulate the GnRH release (Oakley, et al., 2010). It is commonly concluded that kisspeptins effect the LH and FSH secretion indirectly via the GnRH pathway, but some studies have given us reason to believe that in addition to working on a hypothalamic level, Kp can act directly on the gonadotropes in the pituitary gland; stimulating LH and FSH secretion directly. In 2007, Kauffman et al., mentioned the expression of KiSS1r in the pituitary gland, but discarded the idea of Kp having a direct effect on a pituitary level. They concluded that GnRH secretion caused by Kp was a caused by direct activation of GnRH neurons. Three years later, Oakley, et al., (2010) reopened the possibility for Kp activation directly through the gonadotropes. They based this on the presence of functional KiSS1r in the pituitary, in addition to detecting Kp in ovine hypophyseal portal blood. Nevertheless, there is still ongoing controversy about this dual activation, as GnRH secretion via direct Kp stimuli is proven to be relatively low (Oakley, et al., 2010). Administration of exogenous Kp has shown a significant increase in plasma LH and FSH levels shortly after infusion. This surge has proven to be sufficient to induce estrous cycles in prepubescent animals and cause ovulation. In 2009, Sébert, et al, conducted a study on Kisspeptins effect on LH secretion and ovulation in anoestrous ewes. 75% of the treated animals ovulated after a 24hour infusion of Kp. Similar increases in LH/FSH secretion was proved by Pineda, et al. (2010) when they administered Kp intra-cerebrally to rats.
The majority of research on Kp effect on the HPG-axis has focused on the LH secretion, but recently efforts to chart the role of Kp on FSH secretion have greatly increased.
It was found in an experiment on rats, that it takes a 100 times higher dose of Kp to secrete FSH than LH (Roseweir & Millar, 2008). After the gonadotrophin surge, levels of LH and FSH decrease due to the negative feedback loop caused by sex steroids. However, LH plasma levels are still higher than before infusion, whilst FSH levels drop back to basal levels (Sébert, et al., 2009; Navarro, et al., 2005). Studies also show that prolonged exogenous Kp administration can lead to a down-regulation of the GnRH receptors and in turn LH and FSH secretion, due to desensitization of the KiSS1r (Oakley, et al., 2010; Mead, et al., 2007).
The pulse frequency of GnRH secretion has an impact on which gonadotropin hormone will be released. Low GnRH pulse frequency is shown to favor FSH secretion, whilst high GnRH pulse frequency favors LH secretion (Millar, et al., 2010).
GnRH neurons do not exhibit any androgen receptors (AR) or oestrogen α receptors (ERα), thus for a long time it was suspected that another component was involved in regulating GnRH secretion via the negative/positive feedback system of sex steroids. Kp neurons express both AR and ER(α), and via their intimate contact with GnRH neurons, have a crucial role in relaying feedback information (D’Anglemont de Tassigny & Colledge, 2010; Oakley, et al., 2010).
Kp is also believed to be linked to the suppression of GnRH/LH pulse during lactation, due to lower KiSS1 gene expression in the ARC region and lower KiSS1 receptor expression in the AVPV region of the hypothalamus (Mead, et al., 2007).
It has also been discovered that Kp directly communicates with neurons containing nNOS (neuronal Nitric Oxide synthase), and that NO release is essential for the preovulatory GnRH surge, which is mediated by Kp (Hanchate, et al., 2012).
As previously mentioned, Kp has a pivotal role in many of the bodies functions and of particular interest to recent research, the reproductive (HPG) axis. Both, Mead, et al. (2007) and Navarro, et al. (2005) refer to Kp as the ‘molecular switch for puberty’. Thus, if Kiss1 or Kiss1r has mutated it will have serious repercussions for the individual. These mutations may be spontaneous or genetically targeted: missense (point mutations) deletions or insertions, (Roseweir & Millar, 2009; Kauffman, et al., 2007; Tena-Sempere, 2006). Often such mutations reveal themselves as disorders such as Idiopathic Hypogonadotrophic Hypogonadism (IHH) (decreased activity of the gonads), delayed puberty, hypothalamic secondary amenorrhea (cessation of menstruation) and even precocious puberty (early onset puberty), (Sonigo, & Binart, 2012; D’Anglemont de Tassigny, & Colledge, 2010; Oakley, et al., 2009). Furthermore, mutations may cause infertility, poor gonadal growth and impaired gametogenesis; which, in females, results in the disruption of the normal estrus cycle leading to ovulation issues and lack of Corpus Lutea development (D’Anglemont de Tassigny, & Colledge, 2010). Roseweir and Millar (2009) explain that the affected individuals have ‘small ovaries and uteri, delayed vaginal opening, no maturation of follicles, decreased sexual behaviour... and low gonadotrophin levels’. Accordingly, these mutations can have varying severity from loss of function to gain of function, whereas some are so mild that they decrease GnRH levels but allow normal bodily functioning (Roa, et al., 2011; D’Anglemont de Tassigny & Colledge, 2010; Millar, et al., 2010; Oakley, et al., 2009; Tena-Sempere, 2006).
There are many polymorphisms that exist for Kp in the female; resulting in the phenotypes described previously. Mutations of Kiss1 receptor gene, such as R331X and X399R, cause IHH that results in decreased GnRH production (Mead, et al., 2007). These mutations are the result of the stop codon having been ‘moved’, thus elongating the sequence. Further Kiss1r gene mutations resulting in IHH are missense mutations C223R, R297L and L102P. These mutations, however, affect the receptor in a different way, resulting in impaired GnRH signaling. Of the three, C223R has the most severe effects. There are two other mutations of the Kiss1r known to cause IHH: a 155 nucleotide deletion and L1485 which is a single nucleotide deletion. Less is known of the 155 nucleotide deletion but the deletions do have different effects. L1475 has a similar effect to C223R, R297L and L102P (Roseweir & Millar, 2009; Mead, et al., 2007).
Additionally there are two other gene mutations which cause IHH: TAC3 and TAC3R. Although these are not strictly Kp mutations they are thought to have a direct effect on Kp and thus GnRH secretion. These mutations are of the gene which encodes a protein called NKB and its receptors. NKB is believed to regulate the secretion of Kp along with another protein called Dyn (Pineda, et al., 2010).
There are two known mutations of the Kiss1r gene that cause precocious puberty: R386P and P110T (Roseweir & Millar, 2009). Oakley, et al. (2009) explains that these mutations cause ‘prolonged intracellular Kiss1r signalling in response to Kp’. Both IHH and precocious puberty caused by such mutations present other symptoms also; such as hypothalamic secondary amenorrhea, (Roseweir & Millar, 2009).
Kisspeptins regulate various aspects of the female reproductive pathways, many of which are not yet fully understood. It is a vigorously researched topic exploring the mechanisms and mutations of the Kp gene and its associated receptors.
The maintenance of hormonal equilibrium within the HPG-axis, is under both direct and indirect influence of Kisspeptins. Gondatropin releasing hormone of the hypothalamus is under direct duress from Kp, which, acting indirectly through GnRH, induces the release of both LH and FSH gonadotropins (Roa, et al., 2011; Oakley, et al., 2010). It is evident, from conclusive research, that interference in this mechanism, via insertion, deletion or point mutations, can dramatically alter the physiological reproductive development of an individual. Additional research, linked to the successful functioning of Kp, is predicted on the proteins NKB and Dyn. These proteins are thought to work as “auto-regulators of Kp output onto GnRH neurons” (Pineda, et al., 2010). With a more thorough understanding not just of the effects of Kp on the HPG-axis, but also the factors that control and regulate Kp expression in an individual, possible therapeutic applications of this gene may be comprehended.
Surmounting the basic understanding, mapping and function of Kp, researchers believe that the regulation of reproductive cycles can thus be manipulated. For example, earlier estrus cycles in domestic mammals which could have a potential constructive impact on agricultural economy (Caraty, et al., 2012). Studies were conducted with the aim of inducing well-timed and coordinated ovulations in exogenously Kp-treated animals. An LH surge was noted in all treated animals, with an understanding that Kp had the capability to be used to “activate reproductive function in acyclic animals” (Caraty, et al., 2012).
Furthermore, it is thought that this exogenous administration of Kp could potentially be a therapeutic aid to treat “infertility, premature and delayed puberty, and prostatic or metastatic cancers” (D’Anglemont de Tassigny & Colledge, 2010). The prospective therapeutic biological applications of the study of Kisspeptins’ regulatory pathway to both veterinary and human medicine is undeniable.
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Wu, M, et al., 2009. Melanin – Concentrating Hormone Directly Inhibits GnRH Neurons and Blocks Kisspeptin Activation, Linking Energy Balance To Reproduction, Proceedings of the National Academy of Sciences of the United States of America, [e-journals] 106 (40), [Online] Available at: <http://www.pnas.org/content/106/40/17217.short> [Accessed: 07 November 2012].