Kisspeptin Receptor, GPR54, as a Candidate for the Regulation of Testicular Activity in the Frog Rana esculenta1

Abstract

Kisspeptins, acting via GPR54, are new players in the control of reproductive axis. They have the ability to communicate with GnRH neurons sending environmental, metabolic, and gonadal signals, with the induction of GnRH and LH secretion as final effect. At present, the physiological significance of kisspeptin signaling in the gonad is poorly investigated. We cloned GPR54 receptor from the anuran amphibian Rana esculenta testis and investigated its expression in several tissues (brain, spinal cord, ovary, muscle, and kidney). In particular, the expression analysis was carried out in pituitary and testis during the annual sexual cycle. Pituitary and testicular GPR54 mRNA increased at the end of the winter stasis (February) and reached high levels during the breeding season (April). The analysis of GPR54 expression in testis was reinforced by in situ hybridization that revealed GPR54 presence in the interstitial compartment and in proliferating germ cells. Testicular GPR54 expression in February and in June was indicated to be estradiol dependent. Furthermore, in February, kisspeptin-10 (Kp-10) induced the testicular expression of both GPR54 and estrogen receptor alpha (ERalpha) in a dose-dependent manner. Conversely, in March, Kp-10 had a biphasic effect on the expression of ERalpha, being inhibitory at short (1 h) and stimulatory at longer (4 h) incubation time. In conclusion, our results demonstrate that frog testis expresses GPR54 in an estradiol-dependent manner and that Kp-10 modulates the testicular expression of ERalpha; thus, the kisspeptin/GPR54 system might be locally involved in the regulation of estrogen-dependent testicular functions such as germ cell proliferation and steroidogenesis.

MOLECULAR EVOLUTION OF GPCRS: Kisspeptin/kisspeptin receptors

“…Phylogenetic and syntenic analyses support the absence of KissR1, KissR2, and KissR3 genes in birds; however, the possible existence of Kissr4 in birds cannot be ruled out due to the incomplete sequencing of bird genomes in the putative region of KissR4 (Pasquier et al 2014). Among tetrapods, amphibians present the largest number of KissR, with three KissR in X. tropicalis (KissR1, KissR2, and KissR3) (Lee et al 2009b), while only the KissR2 transcript has been identified in the bullfrog (Moon et al 2009) and the edible frog (Chianese et al 2013).…”

Differential activation of kiss receptors by Kiss1 and Kiss2 peptides in the sea bass

“…Expression levels of kiss and kissr in the brain and, to a lesser extent, in the gonad have been analyzed in several non-mammalian species, including amphibians (Chianese et al 2013) and fish (Mohamed et al 2007, Nocillado et al 2007, Biran et al 2008, Filby et al 2008, van Aerle et al 2008, Kitahashi et al 2009, Mechaly et al 2009, Migaud et al 2012, Alvarado et al 2013, Ohga et al 2013, suggesting a putative role for kisspeptins in controlling reproduction. Moreover, in vivo studies in some teleosts have found that the systemic administration of kisspeptin forms induces detectable biological responses at brain and pituitary levels, with potencies depending on the species (Filby et al 2008, Felip et al 2009, Kitahashi et al 2009, Li et al 2009, Shi et al 2010, Espigares et al 2015.…”

Kisspeptins: New Local Modulators of Testis Activity

“…Lastly, the expression of kisspeptin system has also been reported in post meiotic germ cells [Meccariello 2013]. In particular, in human kisspeptin signalling affects sperm motility [23] and lower plasma levels of kisspeptin have been observed in infertile males as compared to fertile males [24][25].…”

“…In this seasonal breeder spermatogenesis starts during late winter-spring with mitotic division of spermatogonia and proceeds during summer with meiotic events; in autumn massive spermiogenesis occurs followed by the winter stasis in which the testis simply contains quiescent primary spermatogonia and newly formed spermatozoa attached to Sertoli cells. During the annual cycle of P. esculentus, Gpr54 fluctuates and is expressed in both Leydig cells and mitotic spermatogonia [19]. Furthermore, in spite no kiss gene has been cloned or characterized in this experimental model, kisspeptin signalling locally induces a wide set of events critical for the upsurge of spermatogenesis onset and the progression of spermatogenesis [20] (Figure 1).…”

“…Furthermore, in spite no kiss gene has been cloned or characterized in this experimental model, kisspeptin signalling locally induces a wide set of events critical for the upsurge of spermatogenesis onset and the progression of spermatogenesis [20] (Figure 1). In fact, at the onset of a new reproductive cycle, Kp-10 administration in ex vivo testis is accomplished by i) the increase of the number of tubules containing cysts of mitotic stages in parallel to the decrease of empty tubules containing quiescent spermatogonia and spermatozoa only [20]; ii) the increased expression of PCNA (Proliferating Cell Nuclear Antigen), a well known marker of DNA biosynthesis during cell cycle [20], iii) the increased expression of local GnRH/GnRH receptors and estrogen receptors (ER) α and ß [19][20], two master signalling systems strongly involved in the recruitment of quiescent spermatogonia for proliferation [21]. In addition, the existence of mutual enhancement between kisspeptin and estradiol may be suggested since Kp-10 increases the expression of ERα/ß and 17β-estradiol increases the expression rate of Gpr54 [19][20] (Figure 2).…”