Kisspeptin

Kisspeptin (formerly known as metastin) is a is_associated_with::protein that is encoded by the KISS1 is_associated_with::gene in humans. Kisspeptin is a is_associated_with::G-protein coupled receptor ligand for GPR54. Kiss1 was originally identified as a human is_associated_with::metastasis suppressor gene that has the ability to suppress is_associated_with::melanoma and is_associated_with::breast cancer is_associated_with::metastasis. Kisspeptin-GPR54 signaling has an important role in initiating secretion of is_associated_with::gonadotropin-releasing hormone (GnRH) at puberty, the extent of which is an area of ongoing research. The release of gonadotropin-releasing hormone is due to an action on the is_associated_with::anterior pituitary and also involves the release of is_associated_with::luteinizing hormone (LH), and is_associated_with::follicle stimulating hormone (FSH). These gonadotropic hormones lead to sexual maturation and is_associated_with::gametogenesis. Disrupting GPR54 signaling can cause is_associated_with::hypogonadotrophic hypogonadism in rodents and man. The Kiss1 gene is located on chromosome 1. It is transcribed in the brain, adrenal gland, and pancreas.

History
In 1996, Dr. Danny Welch’s lab in Hershey, Pennsylvania isolated a is_associated_with::cDNA from a cancer cell that was not able to undergo is_associated_with::metastasis after the human is_associated_with::chromosome 6 was added to the cell. This is_associated_with::gene was named KISS1 because of the location of where it was discovered (Hershey, Pennsylvania, home of is_associated_with::Hershey's Kiss). Introduction of this chromosome into the once active cancer cell inhibited it from spreading and the cDNA responsible was taken from that cell. The fact that KISS1 was responsible for this was proved when it was transfected into is_associated_with::melanoma cells and yet again, metastasis was suppressed. Later, a breakthrough would occur not involving Kisspeptin, but with its receptor.

Three years later in is_associated_with::1999, a is_associated_with::G protein coupled receptor was identified in rat, cloned, and termed GPR54. Additionally, two years later, this receptor’s is_associated_with::ortholog in humans would be isolated. Using the identified receptors, is_associated_with::endogenous is_associated_with::ligands were isolated from cells (HEK293, B16-BL6, and CHO-K1 cells) that had these receptors inserted into them. The next step in the history of Kisspeptin involved revealing more of its pathways and the mechanism involved. Kisspeptin was found to play a role in is_associated_with::hypogonadotropic hypogonadism in 2003, which was supported by several independent lab groups. A is_associated_with::mutation in GPR54 was considered responsible for this abnormality because those who held this mutation, or were missing GPR54 altogether, had problems in gonadal development during is_associated_with::puberty. Several other is_associated_with::phenotypes related to this mutation included a smaller is_associated_with::sex steroid and is_associated_with::gonadotropin is_associated_with::concentration in the circulating blood and even sterility. These observations prompted the research on how kisspeptin is involved during the beginning of puberty. This research led to the discovery that kisspeptin stimulates the neurons that were involved in the release of is_associated_with::gonadotropin-releasing hormone (GnRH) and possibly may have some impact on the release of is_associated_with::luteinizing hormone (LH) and is_associated_with::follicle-stimulating hormone (FSH).

Today, much effort is being made to characterize the regulation of kisspeptin and its is_associated_with::gene expression, as well as to more specifically determine the mechanism behind kisspeptin’s action on GnRH and LH release.

Hippocampal dentate gyrus
Kisspeptin is most notably expressed in the is_associated_with::hypothalamus, but is also found in other areas of the is_associated_with::brain including the hippocampal is_associated_with::dentate gyrus. The hippocampus is known to integrate information on a person’s spatial environment and is_associated_with::memory. KISS1 is known to be expressed in the is_associated_with::hippocampus. However, the levels of KISS1 is_associated_with::mRNA expressed are decidedly lower than in the is_associated_with::hypothalamus and is_associated_with::amygdala. Studies have shown that the levels of KISS1 mRNA expressed in the hippocampus are proportional to less than half of the levels found in the hypothalamus. Despite this, it is suggested that expression of KISS1 is influenced by the is_associated_with::gonad hormones similar to the hypothalamus. There is a high degree of expression of GPR54 in the hippocampus. The density of GPR54 is not discernable in is_associated_with::pyramidal cells, but has high levels of expression in the is_associated_with::granule cell layer. It is known to be found in specific nuclei and neurons.

Adrenal gland
The neuropeptide kisspeptin plays an important role in is_associated_with::reproduction, but also stimulates is_associated_with::aldosterone secretion from the is_associated_with::adrenal cortex. Kisspeptin is distributed from the adrenal cortex and it is transcribed in the is_associated_with::neocortex. The exact nature of the expression of kisspeptins in human adrenal glands unfortunately has not been fully clarified yet and remains a large topic of research among many scientists.

Genomics
Kisspeptin is a product of the KISS1 gene, which is cleaved from an initial 145 is_associated_with::amino acid peptide to a 54 amino acid long protein. This gene is located on the long arm of is_associated_with::chromosome 1 (1q32) and has four exons of which the 5' and 3' exons only partly undergo is_associated_with::translation. The KISS1 gene was first isolated as a tumor spreading gene by investigators and named metastin. is_associated_with::Metastin is derived from the protein kisspeptin and is a natural is_associated_with::ligand of the receptor known as GPR54 Kisspeptin expression in the brain: Catalyst for the initiation of puberty. Different types made up of 14 and 13 amino acids have been isolated and they each share a common is_associated_with::C-terminal sequence. These N-terminally truncated peptides are known as the kisspeptins and belong to a larger family of peptides known as RFamides which all share a common is_associated_with::arginine-is_associated_with::phenylalanine-NH2 motif at their C-terminus. Among these conserved amino acids are is_associated_with::arginine and is_associated_with::phenylalanine residues, which are paired in this family of peptides. Also within this conserved family is a C-terminus that has an is_associated_with::amide added to it. This family which kisspeptin includes is_associated_with::prolactin releasing peptide and gonadotropin releasing inhibiting hormone.

A polymorphism in the terminal is_associated_with::exon of this mRNA results in two is_associated_with::protein isoforms. An adenosine present at the polymorphic site represents the third position in a stop codon. When the is_associated_with::adenosine is absent, a downstream is_associated_with::stop codon is utilized and the encoded protein extends for an additional seven amino acid residues.

Kisspeptin
The is_associated_with::gene for kisspeptin codes for a is_associated_with::peptide that can be cleaved into several pieces. In humans, one of these pieces is made up of 54 amino acids, while in is_associated_with::mice it is made up of 52 is_associated_with::amino acids. This fragment is then proteolytically processed into several smaller fragments that have been isolated in humans composed of 13 and 14 amino acids (kisspeptin-13 and kisspeptin-14 respectively). Each of these fragments has a similar conserved region at the is_associated_with::C-terminal sequence consisting of ten is_associated_with::aromatic amino acids. Specifically, positions 2, 4, 6, 7, 8, and 9 in this region are completely conserved where any variation seen is due to random mutations. The sequence on the carboxy terminal side of the conserved region is a well-known site for cleavage in is_associated_with::neuropeptides.

GPR54
The structure for GPR54 is very similar throughout many different is_associated_with::vertebrates. It is composed of 398 amino acids that form seven is_associated_with::transmembrane domains, like most is_associated_with::G-protein coupled receptors. Sequences found in transmembrane spanning regions one, four, and seven are all very highly conserved throughout species. Variation appears in the around the amino and C-terminal domains, which accounts for the different types of Kisspeptin receptors seen in various is_associated_with::species.

GnRH release
Kisspeptin-54 interacts with is_associated_with::G protein-coupled receptors, specifically GPR54 (Kiss1R). Other versions of Kisspeptin are also able to interact with Kiss1R. Research in both rats and humans has provided evidence that the binding of Kisspeptin stimulates PIP2 is_associated_with::hydrolysis, Ca 2+ mobilization, is_associated_with::arachidonic acid release, extracellular signal-regulated is_associated_with::protein kinase 1 (ERK1), ERK2, and p38 MAP kinase phosphorylation. Although GnRH is located in many areas such as the is_associated_with::pituitary gland and the GnRH neurons, research proves that is_associated_with::GnRH is highly dependent upon GnRH neuron activation and less dependent on the is_associated_with::pituitary gonadotropes. Many studies show that kisspeptin has the ability to not only cause is_associated_with::depolarization, but also excite many GnRH neurons, leading to high expression of kisspeptin in these is_associated_with::genes. But, it is hypothesized that there are two different types of GFP-GnRH neurons due to expression in some is_associated_with::neurons but not others, only one of which responds to kisspeptin. The neurons response to kisspeptin is also hypothesized to be related to age and is_associated_with::puberty. The binding of Kisspeptin to the GnRH receptor can have effects on puberty, is_associated_with::tumor suppression and is_associated_with::reproduction.

Biological Function
Kisspeptin can stimulate secretion of is_associated_with::aldosterone and the release of is_associated_with::insulin.

Kisspeptin appears to directly activate GnRH neurons. Evidence for this involves the persistence of a neural response to kisspeptin levels even in the presence of TTX, a neurotoxin that blocks nerve signals.
 * is_associated_with::Gramicidin-perforated patch recordings: approx. 30% of GnRH neurons respond to kisspeptin administration in prepubertal males, whereas 60% of GnRH neurons in adult mice responded.
 * Because only adult mice respond to low doses of kisspeptin, it appears that GnRH neurons become developmentally activated by kisspeptin over the course of puberty.
 * Kisspeptin induces production of LH and FSH, which are required for female's menstruation. Athletes may not undergo menstruation due to low fat levels; fat produces the hormone "Leptin", which induces production of "Kisspeptin".

Role in puberty
The onset of is_associated_with::puberty is marked by an increase in is_associated_with::gonadotropin secretion, which leads to sexual maturity and the ability to reproduce. Puberty can also be affected by a range of environmental factors, and is known to be affected by a person’s metabolic capacity. Gonadotropin secretion is brought about and regulated by is_associated_with::gonadotropin releasing hormone (GnRH). GnRH leads to the release of is_associated_with::luteinizing hormone (LH) and is_associated_with::follicle stimulating hormone (FSH), which primarily target the is_associated_with::gonads to trigger puberty and is_associated_with::reproduction. The primary event that leads to the beginning of puberty is the activation of GnRH neurons. This event is thought to involve kisspeptin/GPR54 signaling, which leads to the eventual activation of GnRH is_associated_with::neurons. Several studies have confirmed that addition of kisspeptin to biological systems including rat, mouse, and sheep are able to bring about the release of LH and FSH. In addition to this, the release of these gonadotropins has proven to be dose dependent. A greater addition of kisspeptin peptide resulted in greater release of LH and FSH. Kisspeptin was found to evoke one of the strongest effects on the gonadotropin system.



Kisspeptin’s ability to stimulate the release of GnRH and is_associated_with::gonadotropins is the result of its effect GnRH release at the is_associated_with::hypothalamus. In rat hypothalamus, it was found that over three-fourths of GnRH neurons coexpress the receptor for kisspeptin, GPR54, in their RNA. Kisspeptin was also able to bring about the release of GnRH both ex vivo and in vivo in rat and sheep. It can be concluded that by activating GnRH neurons in the hypothalamus, kisspeptin causes GnRH release which leads to the release of FSH and LH. The major role kisspeptin/GPR54 plays in is_associated_with::sexual development was initially found in sexually immature humans and mice who had is_associated_with::mutations that blocked the expression of the GPR54 gene. In rats, the initiation of puberty accompanied a greater presence of KISS1 and GPR54 in mRNA. The same events were later observed in is_associated_with::mammals, where KISS1 and GPR54 mRNA increased more than twofold in the is_associated_with::hypothalamus. This suggests that there is greater expression of KISS1 and potentially even GPR54 at the onset of puberty leading to an increase in kisspeptin/GPR54 signaling that results in the activation of the is_associated_with::gonadotropin pathway. The addition of kisspeptin to female rats who had yet to mature led to the initiation of gonadotropin pathway. In humans, it was shown that females at the beginning stages of puberty had much higher kisspeptin levels than those females of the same age who had yet to begin puberty. It has been concluded that the activation of the GPR54/kisspeptin pathway is a catalyst that leads to puberty onset.

Role in tumor suppression
Kisspeptin plays a role in is_associated_with::tumor suppression. In a study where is_associated_with::malignant tumor cells were injected into a model system, the system was then tested for genes involved in the injected chromosome 6. KISS1 was discovered to be the only gene expressed in non-metastatic cells and absent in metastatic, metastatic meaning the ability for cancer to spread to unconnected areas. This suggested that there Kisspeptin is an essential is_associated_with::regulation factor in whether or not a cell will be is_associated_with::metastatic or not. Additional experimentation identified CRSP3 as the exact gene responsible for KISS1 regulation within chromosome 6. In clinical evidence studies, KISS1 and Kisspeptin were found in primary, metastatic tumors, and growing tumors showing decreased levels of KISS1 and Kisspeptin. In conclusion, kisspeptin plays a large role in is_associated_with::tumor suppression. When it is active in cells the tumor stays consolidated and does not spread or grow.

Role in reproduction
Kisspeptin is highly expressed during is_associated_with::pregnancy. In early-term placentas, GPR54 was expressed at a higher rate than placentas at-term. The expression of kisspeptin, however, remains unchanged in the placenta throughout pregnancy. The increased expression of GPR54 in early-term placentas is due to the increased presence of intrusive is_associated_with::trophoblasts during the beginning of pregnancy. Term cells, by comparison are less invasive. When measuring kisspeptin-54 during pregnancy, a 1000x increase was observed in early pregnancy with a 10 000x increase seen by the third trimester. Following birth, kisspeptin-54 levels returned to normal, showing the is_associated_with::placenta as the source of these increased kisspeptin levels.

Role in kidney function
Kisspeptin and its receptor was found in various sites in the kidney, including in the is_associated_with::collecting duct, is_associated_with::vascular smooth muscle, and in the is_associated_with::renal tubule cells. Much of the impact on the kidney deals with the increased production of is_associated_with::aldosterone in the adrenals glands stimulated by kisspeptin. Kisspeptin directly increases release of aldosterone by several means, the first being through these receptors leading to a direct route to is_associated_with::aldosterone release. Secondly, the is_associated_with::H295R adrenal cells stimulated by kisspeptin can synthesize aldosterone by breaking down is_associated_with::pregnenolone more efficiently. Lastly, the kisspeptin-angiotensin II pathway of producing aldosterone is increased. Aldosterone that comes from the neighboring is_associated_with::adrenal glands causes reabsorption of filtrate in order to retain water, leading to an increased is_associated_with::blood pressure.

Kisspeptin neurons
Kisspeptin expressing neurons are located in:
 * Anteroventral is_associated_with::periventricular nucleus (AVPV)
 * is_associated_with::Periventricular nucleus (PeN)
 * Anterodorsal preoptic nucleus (ADP)
 * The is_associated_with::arcuate nucleus (Arc)

Kisspeptin-expressing neurons reside in the anteroventral periventricular nucleus and the arcuate nucleus, among others, and send projections into the is_associated_with::MPOA, where there is an abundance of GnRH cell bodies. This anatomical evidence suggests that Kisspeptin fibers appear in close anatomical relationship to GnRH (parvicellular) neurons. In fact, Kisspeptin appears to act directly on GnRH neurons (via GPR54) to stimulate the secretion of GnRH.

However, for kisspeptin to be involved in the regulation of GnRH release, it must also be sensitive to circulating sex steroid levels, as it is established that steroids produced by the gonads exert regulatory effects on FSH and LH levels through GnRH mediation. Thus, there are at least two possible scenarios: that either kisspeptin neurons express sex steroid receptors themselves, or they receive input about circulating sex steroid levels from a different mechanism.

Coexpression imaging of KISS1 mRNA (using vector red) and steroid receptors determined that neurons that express KISS1 mRNA are targets for the action of sex steroids in both male and female mice.