Male infertility is a clinical condition described as the inability of a man to impregnate a fertile woman despite regular and unprotected sexual intercourse for at least one year. There are several reasons behind male infertility, which accounts for almost half of all infertility cases [14]. These factors include endocrinological defects (GnRH deficiency, HPG axis disorders, etc.), genetic factors (GnRH1/GNRHR, PROK2/PROK2R gene deficiencies, chromosomal anomalies, Kallman syndrome, etc.), sexual dysfunction (premature ejaculation, erectile dysfunction), certain drugs, and idiopathic factors [15].
The hypothalamic-pituitary-gonadal axis regulates the reproductive functions in mammals. The axis entails the coordinated secretion of peripheral gonadal hormones, GnRH, and the secretion of hypophyseal gonadotropins (FSH and LH) in the hypothalamus [16]. GnRH neurons, found in the hypothalamus, play a role in the control of the HPG axis. PROK2 plays a role in the migration and survival of GnRH neurons. Various abnormalities were observed in reproductive system organs such as testes, ovaries, vagina, and mammary glands in PROKR2 knockout mice. It was also determined that the mice did not have GnRH neurons in the hypothalamus (preoptic and medians). Thus, low serum FSH, testosterone, and mRNA expression levels of hypothalamic GnRH and pituitary gonadotropins were reported [17]. Pittelau et al., [6] reported that the decrease in hypothalamic GnRH neurons in PROK2 and PROKR2 knockout mice also led to problems in hypothalamic GnRH secretion. They reported that problems in GnRH secretion led to low gonadotropin levels, and impaired sexual development and fertility in male and female rats. Particularly in male rats, small seminiferous tubules without haploid spermatocytes and spermatids were noted. In another study, it was determined that the neuronal projection of GnRH was impaired in PROK2 and PROKR2 knockout mice, and there was a significant decrease in the GnRH-expressing cell count, especially in the median preoptic region. It was observed that these mice exhibited problems in olfactory bulb neurogenesis [18].
The reports on mice required in-depth analysis of the role of PROK2 and PROKR2 in GnRH deficiency in humans. Initially, a homozygous deletion that led to function losses in the PROK2 ligand was identified in three siblings with GnRH deficiency., which in turn led to the development of a truncated protein with 27 amino acids, which was biologically inactive [6]. Then, several researchers reported heterozygous loss-of-function mutations in PROK2 and PROKR2 in Kallman syndrome and normosmic idiopathic hypogonatropic hypogonadism patients [19–22].
The known angiogenic effect of PROK2 on the testicles and its presence in primary spermatocytes suggested that this protein could play a role in spermatogenesis. It was reported that testicular PROK2 mRNA expression increased in varicocele, and considering the role of prokineticin in endothelial cell renewal, it was reported that the increase was important for infertility risk [23]. Li et al., [24] emphasized that high varicocele PROK2 expression induced spermatocyte apoptosis and PROK2 could be a varicocele biomarker.
The findings reported by previous animal and human studies suggested that PROK2 could play a key role in the reproductive system. However, further studies are required to elucidate these reports. In our study, since icv PROK2 infusion increased GnRH mRNA expression in hypothalamus tissues, there could be a correlation between PROK2 and GnRH secretion. It could be suggested that direct and central PROK2 infusion every hour would allow the analysis of GnRH levels without the impact of the fluctuations induced by the pulsatile secretion of GnRH. Furthermore, it was determined that icv PROK2 infusion increased FSH and LH secretion in the pituitary gland, which are HPG axis components, and increased testicular testosterone production. High GnRH expressions in the hypothalamus due to PROK2 infusion also led to an increase in these hormones in the HPG axis. There was a significant increase in seminiferous tubule thickness in the PROK2 groups and the increase in sperm concentration was not different between the groups.