Intracerebroventricular asprosin administration strongly stimulates hypothalamic-pituitary-testicular axis in rats
Graphical abstract
Introduction
Infertility is an important condition that affects an estimated 70 million people worldwide. The World Health Organization estimates that 9% of couples worldwide struggle with fertility problems, and the male factor contributes to 50% of problems (Fainberg and Kashanian, 2019). Approximately a quarter of infertility cases in men are characterized by a decrease in sperm quality (impaired spermatogenesis, low sperm count, abnormal sperm morphology, etc.) (Cooper et al., 2010; Templeton, 1995). The main mission of the hypothalamic-pituitary-testicular (HPT) axis is to control the secretion of hormones involved in the male reproductive system, to ensure the formation and maturation of healthy sperm cells, and to perform the reproductive function (Kuiri-Hänninen et al., 2014). The hypothalamus, localized in the HPT axis, secretes gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland (Tobari et al., 2014). It performs the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are gonadotropic hormones from the anterior pituitary gland that stimulate through the portal system (Kaprara and Huhtaniemi, 2018). The FSH and LH secreted from the anterior pituitary gland activate the second messenger system of cyclic adenosine monophosphate in target cells in the testes and shows its effect at the cellular level. The hormone FSH stimulates Sertoli cells in the testicles, performing sex hormone binding globulin secretion and spermatogenesis. The LH secretion stimulates testosterone release by stimulating Leydig cells (Kaprara and Huhtaniemi, 2018). Disruption of the HPT axis causes gonads to be unable to produce testosterone and sperm at a physiological level (Tobari et al., 2014). Although adipose tissue was once considered only an energy storage area, today's data has revealed that fat tissue acts as an active endocrine organ and is involved in the secretion of adipokine based proteins (Carobbio et al., 2017; Stephens, 2012). Some protein-based hormones that effect the physiological structure of the HPT axis are produced by the adipose tissue (Reverchon et al., 2013). For many adipokines of white adipose tissue (WAT) origin, a new axis is now defined as the “Adipose-Brain Axis” in the literature (Zheng et al., 2014).
Asprosin, a new member of the adipokine family, is encoded by two exons (exon 65 and exon 66) of the fibrillin 1 gene (FBN1) and is mainly synthesized and released by WAT during hunger (Romere et al., 2016; Yuan et al., 2020). It is known that the FBN1 gene in which the asprosin hormone is produced is mostly localized in the WAT, lungs, liver, kidneys, heart, stomach, testicles, and brains in rodents (Kocaman and Kuloglu, 2020). This hormone is expressed in adipose, brain, liver and testicular tissues in humans. Asprosin increases in mouse, rat, and human serum samples due to hunger (Romere et al., 2016). Moreover, asprosin administered subcutaneously to mice reaches a peak level of 40 nM 30 min after administration (Duerrschmid et al., 2017). Asprosin, which has the half life of approximately 145 min, is also present in the cerebrospinal fluid (CSF) (Duerrschmid et al., 2017). Asprosin present in the arcuate nucleus of the hypothalamus contributes to the regulation of appetite through central receptors. 34 nM asprosin, which is administered intracerebroventricularly to mice, increases the activity of AgRP neurons, which leads to increased appetite in the hypothalamus (Duerrschmid et al., 2017). Asprosin binds to olfactory receptor 734 (OLFR734) (Li. Erwei et al., 2019; Liu et al., 2020). Based on the localization of asprosin and its receptor OLFR734 in the testicles, it was reported that asprosin increased sperm progressive movement in a study that investigated its effect on male fertility (Wei et al., 2019). However, the available information is insufficient to explain the effects of asprosin on the reproductive axis in male. That both asprosin and its receptor are localized in the testis and hypothalamus tissue (Duerrschmid et al., 2017; Kocaman and Kuloglu, 2020; Romere et al., 2016), which are important for the HPT axis, give the first signals that asprosin can stimulate the HPT axis. This study was conducted to investigate the effects of intracerebroventricular (ICV) asprosin infusion on hormones and sperm cells involved in the HPT axis in male rats.
Section snippets
Animal ethics and the research center
The study was carried out with the approval of the Local Ethics Committee of Animal Experiments at Inonu University Faculty of Medicine (Date 10.03.2020, Protocol No. 2020/4-3), in Inonu University Faculty of Medicine Experimental Animals Production and Research Center, Faculty of Medicine, Department of Physiology laboratories and Bartin University Faculty of Science Molecular Biology and Genetics Laboratory.
Assignment of experimental animals to groups
The number of rats used in the experiment was determined by power analysis. With the
Asprosin infusion increased GnRH mRNA and protein expression in the hypothalamus
The effect of asprosin application on GnRH mRNA level in hypothalamus tissue is shown in Fig. 2A. It was determined that there was no statistically significant difference between the control and sham groups when the groups were compared in terms of GnRH mRNA level (p > 0.05). The GnRH mRNA levels in asprosin administered groups were found to be high compared to the control and sham groups (p < 0.05). This increase was approximately 3 fold in the high dose group compared to the control group.
Discussion
Asprosin, which recently joined this adipokine family, reveals the first signals that it may have effects on the HPT axis with its presence in testicular and brain tissues (Duerrschmid et al., 2017; Kocaman and Kuloglu, 2020; Romere et al., 2016). Studies determined that there was a positive correlation between serum asprosin level and serum testosterone and FSH levels (Alan et al., 2019; Deniz et al., 2020; Liao et al., 2018). In another study, OLFR734 (asprosin receptor) was detected in the
Funding sources
This study was supported by the Scientific Research Projects unit of Inonu University (Project number: TYL-2020-2185).
Credit author statement
TK and ST designed the study and conducted animal experiments. YE analyzed the tissues. TK, YE, and ST prepared the manuscript together.
Declaration of competing interest
The authors declare that there are no conflicts of interest.
Acknowledgement
The authors would like to thank Prof. Suleyman Sandal (Department of Physiology, Faculty of Medicine, Inonu University, Malatya, Turkey) for technical assistance.
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