Nomen est omen - let's get to know kisspeptin!

A good story is always good. Here, for example, is the case of kisspeptin, which belongs to the group of peptides, the smaller building blocks of proteins. It was discovered less than thirty years ago, but it quickly gained fame, and even those who had only heard of it remembered its name. True, a chocolate bar contributed to this. The story begins with an American chocolate factory founded in 1894 by Milton S. Hershey, originally of Derry Church, Pennsylvania, for whom the town was later - deservedly - renamed. But let Balázs Göcz, one of the first authors of this article, continue the story:

- Hershey, the town of the popular Hershey's Kisses, was the home of Danny Welch's research team's laboratory. In 1996, a cDNA was isolated from a cancer cell line that, after the addition of human chromosome 6, was unable to metastasize. Further studies also identified the gene responsible for the tumour suppressor effect, which they named KISS1 after their city's famous product. The products of this gene are peptides called kisspeptin. 

- So the discovery of kisspeptin was not linked to reproductive research! 

- That was seven years in the making. It has been published in two thousand three that kisspeptin plays a key role in the development of hypogonadotropic hypogonadism - a condition in which levels of gonadotropins (LH, FSH) and sex hormones are low. This finding was made possible by the study of the gene encoding the kisspeptin receptor, GPR54/KISS1R. Mutations in this gene have been shown to cause puberty and the development of reproductive failure and infertility. 

- According to sources on the internet, there are several areas of the brain where neurons that produce kisspeptin are located. What do these nerve cells do? 

 - Nerve cells that produce kisspeptin play a key role in the brain's regulation of reproduction. To understand this, we first need to understand the broad outline of the hypothalamic-pituitary-gonadal (testes, ovaries) axis that controls reproduction. 

GnRH neurons in the hypothalamus produce gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to secrete luteinising hormone (LH) and follicle-stimulating hormone (FSH). The latter also regulate the production of sex steroids (oestrogen, progesterone, testosterone) in the reproductive organs. These sex steroid hormones, in addition to regulating the target cells, can also have a feedback effect on the higher levels of the axis.

- Why is kisspeptin needed? 

- Because this neuropeptide plays a prominent role in stimulating GnRH neurons. Meanwhile, it can also sense the levels of sex hormones circulating in the blood through its steroid receptor content and transmit this information to GnRH neurons. Similar hormonal feedback is also present in other endocrine organs. Remaining with reproduction, kisspeptin neurons play a key role in the initiation of puberty and in regulating the relationships between nutrition, stress and reproduction. There are two main populations of them in the hypothalamus: one responsible for negative and one for positive estrogen feedback. 

- When is the earliest time to notice that something is wrong with the kisspeptin neurons? 

- If puberty starts too early (before the age of 8 in girls and 9 in boys) or too late (after the age of 13 in girls and 14 in boys), there may be a disorder of the kisspeptin system, although other factors and disorders of several other genes may also play a role in pubertal disorders. The question highlights one of the most difficult differential diagnostic problems: whether the cases are late puberty or a gene defect that is not expected to resolve over time. In other words, how long can the paediatric endocrinologist wait, how long should he or she wait, before triggering puberty with hormone replacement, before inducing the development of secondary sexuality. Screening tests for known genes that cause pubertal disorders do exist, but they are only justified if there is a history of a similar gene defect on the side of the family. Detection of newly emerging mutations is much more difficult and, in some cases of pubertal defects, it is not even known which mutation in which gene is responsible for the clinical picture.   

- Besides the hipotalamus, where else are many kisspeptin-producing neurons found? Do they have any particular distribution? 

- There are also kisspeptin-producing neurons in several brain regions outside the hypothalamus. They occur in the amygdala, hippocampus, periaqueductal grey matter and lateral septum. Although the exact role of these neurons is not yet fully understood, they are considered an important area of research. 

- How does the number-activity of the kisspeptin neurons you are now studying vary?

- The focus of our research was on kisspeptin neurons in the lateral septum, which are extremely sensitive to the hormone oestrogen. Remarkably, these neurons only start to produce kisspeptin in the presence of oestrogen, so their hormonal regulation is of paramount importance. However, the study of these neurons is so difficult that we finally used a genetically modified mouse model in which the cells producing kisspeptin show green fluorescence, thus facilitating the study of the system using anatomical, electrophysiological and molecular biological approaches. 

The number and activity of the lateral septal kisspeptin neurons undergoes significant changes during life. Before puberty, which in mice starts around day 28-30, these neurons do not yet produce kisspeptin. However, as estrogen levels begin to rise, peptide production begins. The number of cells continues to increase with age, suggesting that changes in estrogen levels play a key role in the development and function of this neuron population.

- Were there any observable differences in this respect in the brains of male and female mice?

- Yes. Interestingly, these neurons appear earlier in females, as early as 33-36 days after birth, whereas in males they appear later, on days 40-45. As I mentioned earlier, the number of fluorescent cells increases steadily until adulthood, but they are significantly more abundant in females: two to three times more of these neurons are found in females than in males.

- Didn't making the KP neurons luminous change anything significant about them? How can you check something like that? (Did the mouse behave the same way in everything etc.?)

- This is a valid question, as it is important to check that the genetic modification itself does not significantly affect the physiology and behaviour of the animals.

The mouse model was not created by us, but was obtained from a Cambridge laboratory in a collaborative effort and is being bred in the KOKI animal house. These experimental animals carry the genetic modification in heterozygous form and are still able to produce cat peptin. They are reproductively capable, go through normal puberty and females show normal cycles. This suggests that the genetic modification does not cause a significant difference from wild-type mice, so it is likely that our studies in this model reflect the physiological state.

- You have shown that lateral septal kisspeptin neurons are directly connected to GnRH neurons. 

- The direct connection to GnRH-producing neurons suggests that kisspeptin cells may directly regulate central processes of reproduction, rather than only through mediating systems. This sheds new light on the role of the lateral septum in hormonal regulation and also raises the possibility that kisspeptin may contribute significantly to neuroendocrine regulation of reproduction through hitherto unknown pathways outside the hypothalamus.

- Why is it a significant discovery that the human lateral septum also contains kisspeptin neurons? 

- Firstly, because it suggests that the role of the estrogen-dependent system is evolutionarily conserved, and that kisspeptin neurons are not only important in the reproductive mechanism of rodents, but may also play a role in the central regulation of human reproduction. On the other hand, the study of rodent models may also provide relevant results for understanding the brain regulation of human reproduction. 

- You mentioned that kisspeptin neurons may also play a role in other physiological and behavioural processes besides reproduction. What do you mean?

- Mood, stress response, and social behavior are just a few of the functions in which the lateral septum is involved in regulating. Exploring the role of the kisspeptin neurons in all these functions will be a task for further research.