Relationships of gonadotropins, testosterone, and cortisol in response to GnRH and GnRH antagonist in boars selected for high and low follicle-stimulating hormone levels

Gene expression profiles underlying aggressive behavior in the prefrontal cortex of cattle

BACKGROUND

Aggressive behavior is an ancient and conserved trait, habitual for most animals in order to eat, protect themselves, compete for mating and defend their territories. Genetic factors have been shown to play an important role in the development of aggression both in animals and humans, displaying moderate to high heritability estimates. Although such types of behaviors have been studied in different animal models, the molecular architecture of aggressiveness remains poorly understood. This study compared gene expression profiles of 16 prefrontal cortex (PFC) samples from aggressive and non-aggressive cattle breeds: Lidia, selected for agonistic responses, and Wagyu, selected for tameness.

RESULTS

A total of 918 up-regulated and 278 down-regulated differentially expressed genes (DEG) were identified, representing above-chance overlap with genes previously identified in studies of aggression across species, as well as those implicated in recent human evolution. The functional interpretation of the up-regulated genes in the aggressive cohort revealed enrichment of pathways such as Alzheimer disease-presenilin, integrins and the ERK/MAPK signaling cascade, all implicated in the development of abnormal aggressive behaviors and neurophysiological disorders. Moreover, gonadotropins, are up-regulated as natural mechanisms enhancing aggression. Concomitantly, heterotrimeric G-protein pathways, associated with low reactivity mental states, and the GAD2 gene, a repressor of agonistic reactions associated with PFC activity, are down-regulated, promoting the development of the aggressive responses selected for in Lidia cattle. We also identified six upstream regulators, whose functional activity fits with the etiology of abnormal behavioral responses associated with aggression.

CONCLUSIONS

These transcriptional correlates of aggression, resulting, at least in part, from controlled artificial selection, can provide valuable insights into the complex architecture that underlies naturally developed agonistic behaviors. This analysis constitutes a first important step towards the identification of the genes and metabolic pathways that promote aggression in cattle and, providing a novel model species to disentangle the mechanisms underlying variability in aggressive behavior.

Production of a gonadotropin-releasing hormone 2 receptor knockdown (GNRHR2 KD) swine line

Swine are the only livestock species that produce both the second mammalian isoform of gonadotropin-releasing hormone (GNRH2) and its receptor (GNRHR2). Previously, we reported that GNRH2 and GNRHR2 mediate LH-independent testosterone secretion from porcine testes. To further explore this ligand-receptor complex, a pig model with reduced GNRHR2 expression was developed. Small hairpin RNA sequences targeting porcine GNRHR2 were subcloned into a lentiviral-based vector, lentiviral particles were generated and microinjected into the perivitelline space of zygotes, and embryos were transferred into a recipient. One GNRHR2 knockdown (KD) female was born that subsequently produced 80 piglets from 6 litters with 46 hemizygous progeny (57% transgenic). Hemizygous GNRHR2 KD (n = 10) and littermate control (n = 7) males were monitored at 40, 100, 150, 190, 225 and 300 days of age; body weight and testis size were measured and serum was isolated and assayed for testosterone and luteinizing hormone (LH) concentrations. Body weight of GNRHR2 KD boars was not different from littermate controls (P = 0.14), but testes were smaller (P < 0.05; 331.8 vs. 374.8 cm³, respectively). Testosterone concentrations tended (P = 0.06) to be reduced in GNRHR2 KD (1.6 ng/ml) compared to littermate control (4.2 ng/ml) males, but LH levels were similar (P = 0.47). The abundance of GNRHR2 mRNA was reduced (P < 0.001) by 69% in testicular tissue from mature GNRHR2 KD (n = 5) versus littermate control (n = 4) animals. These swine represent the first genetically-engineered model to elucidate the function of GNRH2 and its receptor in mammals.

Expression and Role of Gonadotropin-Releasing Hormone 2 and Its Receptor in Mammals

Gonadotropin-releasing hormone 1 (GnRH1) and its receptor (GnRHR1) drive mammalian reproduction via regulation of the gonadotropins. Yet, a second form of GnRH (GnRH2) and its receptor (GnRHR2) also exist in mammals. GnRH2 has been completely conserved throughout 500 million years of evolution, signifying high selection pressure and a critical biological role. However, the GnRH2 gene is absent (e.g., rat) or inactivated (e.g., cow and sheep) in some species but retained in others (e.g., human, horse, and pig). Likewise, many species (e.g., human, chimpanzee, cow, and sheep) retain the GnRHR2 gene but lack the appropriate coding sequence to produce a full-length protein due to gene coding errors; although production of GnRHR2 in humans remains controversial. Certain mammals lack the GnRHR2 gene (e.g., mouse) or most exons entirely (e.g., rat). In contrast, old world monkeys, musk shrews, and pigs maintain the coding sequence required to produce a functional GnRHR2. Like GnRHR1, GnRHR2 is a 7-transmembrane, G protein-coupled receptor that interacts with G_αq/11 to mediate cell signaling. However, GnRHR2 retains a cytoplasmic tail and is only 40% homologous to GnRHR1. A role for GnRH2 and its receptor in mammals has been elusive, likely because common laboratory models lack both the ligand and receptor. Uniquely, both GnRH2 and GnRHR2 are ubiquitously expressed; transcript levels are abundant in peripheral tissues and scarcely found in regions of the brain associated with gonadotropin secretion, suggesting a divergent role from GnRH1/GnRHR1. Indeed, GnRH2 and its receptor are not physiological modulators of gonadotropin secretion in mammals. Instead, GnRH2 and GnRHR2 coordinate the interaction between nutritional status and sexual behavior in the female brain. Within peripheral tissues, GnRH2 and its receptor are novel regulators of reproductive organs. GnRH2 and GnRHR2 directly stimulate steroidogenesis within the porcine testis. In the female, GnRH2 and its receptor may help mediate placental function, implantation, and ovarian steroidogenesis. Furthermore, both the GnRH2 and GnRHR2 genes are expressed in human reproductive tumors and represent emerging targets for cancer treatment. Thus, GnRH2 and GnRHR2 have diverse functions in mammals which remain largely unexplored.

LH-Independent Testosterone Secretion Is Mediated by the Interaction Between GNRH2 and Its Receptor Within Porcine Testes

Unlike classic gonadotropin-releasing hormone 1 (GNRH1), the second mammalian isoform (GNRH2) is an ineffective stimulant of gonadotropin release. Species that produce GNRH2 may not maintain a functional GNRH2 receptor (GNRHR2) due to coding errors. A full-length GNRHR2 gene has been identified in swine, but its role in reproduction requires further elucidation. Our objective was to examine the role of GNRH2 and GNRHR2 in testicular function of boars. We discovered that GNRH2 levels were higher in the testis than in the anterior pituitary gland or hypothalamus, corresponding to greater GNRHR2 abundance in the testis versus the anterior pituitary gland. Moreover, GNRH2 immunostaining was most prevalent within seminiferous tubules, whereas GNRHR2 was detected in high abundance on Leydig cells. GNRH2 pretreatment of testis explant cultures elicited testosterone secretion similar to that of human chorionic gonadotropin stimulation. Treatment of mature boars with GNRH2 elevated testosterone levels similar to those of GNRH1-treated males, despite minimal GNRH2-induced release of luteinizing hormone (LH). When pretreated with a GNRHR1 antagonist (SB-75), subsequent GNRH2 treatment stimulated low levels of testosterone secretion despite a pattern of LH release similar to that in the previous trial, suggesting that SB-75 inhibited testicular GNRHR2s. Given that pigs lack testicular GNRHR1, these data may indicate that GNRH2 and its receptor are involved in autocrine or paracrine regulation of testosterone secretion. Notably, our data are the first to suggest a biological function of a novel GNRH2-GNRHR2 system in the testes of swine.

Free and sulfated steroids secretion in postpubertal boars (Sus scrofa domestica)

Sulfated steroids have been traditionally regarded as inactive metabolites. However, they may also serve as precursors for the production of active free steroids in target cells. In this study, we used the boar as a model to study the metabolism, transport, and function of steroid sulfates due to their high production in the porcine testicular–epididymal compartment, of which the role is unknown. To characterize the secretion of free and sulfated steroids, plasma samples were collected from six postpubertal boars over 6 h every 20 min from the jugular vein. Long-term secretion profiles were also established in seven boars stimulated with human chorionic gonadotropin. To directly characterize the testicular output, samples were collected from superficial testicular arterial and venous blood vessels. Testosterone, androstenedione and sulfated pregnenolone, DHEA, estrone (E1), and estradiol-17β (E2) were determined by liquid chromatography–tandem mass spectrometry. Free E1 and E2 were measured by RIA. Irrespective of a high variability between individuals, the results suggest that i) all steroids assessed are primarily produced in the testis, ii) they exhibit similar profiles pointing to a pulsatile secretion with low frequency (three to five pulses per day), and iii) after synthesis at least a major proportion is immediately released into peripheral circulation. The fact that all steroid sulfates assessed are original testicular products and their high correlations with one another suggest their role as being intermediates of testicular steroidogenesis rather than as being inactivated end products. Moreover, a substantial use of sulfated steroids in porcine testicular steroidogenesis would assign a crucial regulatory role to steroid sulfatase, which is highly expressed in Leydig cells.

Contributions of the maternal uterine environment and piglet genotype on weaning survivability potential: I. Development of neonatal piglets after reciprocal embryo transfers between Meishan and White crossbred gilts

In commercial pigs, the greatest susceptibility for pre-weaning mortality occurs in low birth-weight piglets. Despite their overall decreased birth weight, Meishan (MS) piglets have decreased pre-weaning mortality rates compared with contemporary Western breeds. The objective of the current study was to determine the contributions of the maternal uterine environment, piglet genotype, and their interaction on the development of neonatal piglets pertaining to pre-weaning survivability using reciprocal embryo transfer between MS and White crossbred (WC) pigs. Twenty-five successful pregnancies were produced from 2 farrowing seasons, generating litters of maternal uterine environment (MUE) by piglet genotype (PigG) combinations; MS × MS (n = 4 litters), MS × WC (n = 7 litters), WC × MS (n = 7 litters), and WC × WC (n = 7 litters). At approximately 24 h of age (Day 1), piglets (n = 173) were weighed and a blood sample was taken. Hematocrit, hemoglobin, glucose, plasma urea nitrogen, albumin, NEFA, lactate, and cortisol were measured in all blood samples. Representative piglets (n = 46) from each litter were harvested and body measurements (i.e., organ weights, tissue glycogen content, and body composition) were determined. Piglet data were analyzed by ANOVA using MIXED model procedures. Both MUE (P < 0.001) and PigG (P < 0.01) affected piglet BW, illustrating that piglets gestated in WC gilts were heavier than piglets gestated in MS gilts, and WC piglets were heavier than MS piglets. Serum albumin concentrations were increased (P < 0.05) in MS piglets compared with WC piglets, indicating greater liver maturity. Significant MUE × PigG interactions were observed for hematocrit and hemoglobin, in which the greatest concentrations were observed in MS piglets gestated in MS and WC gilts, and the lowest concentrations were observed in WC piglets gestated in WC gilts, demonstrating increased oxygen-carrying capability. The percentage of fat and nitrogen, as well as the GE of the body, were greater (P < 0.05) in MS piglets, indicating greater energy stores. Liver, bicep femoris, and LM glycogen concentrations were greater (P < 0.01) in WC piglets compared with MS piglets, demonstrating increased glycogen catabolism in MS piglets. This study demonstrated limited interactions between the maternal uterine environment and piglet genotype on weaning survivability potential, suggesting that the MS piglet is a viable model for pre-weaning survivability.

Lamprey GnRH-III Stimulates FSH Secretion in Barrows

Although studies have indicated that follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release can be dissociated in the pig, the underlying mechanisms are still to be answered. Since it was demonstrated that lamprey gonadotropin-releasing hormone (l-GnRH-III) has preferential FSH-releasing potency in several mammalian species, we have investigated the gonadotropin-releasing activity of l-GnRH-III in barrows. Each of nine barrows (body weight: 85-90 kg; age: 207 days) received 2 ml saline (S-barrow), followed by 150 microg l-GnRH-III (1.6-1.7 microg/kg body weight) dissolved in 2 ml saline intramuscularly 7 days later. Three pre-treatment and 13 post-treatment blood samples were taken at intervals of 30 min to 8 h to assess basal and treatment-associated concentrations of FSH and LH, respectively, by radioimmunoassay. Animals were defined as having responded to treatment if, 2 h post-treatment, plasma FSH and/or LH levels were >3 SD of the respective basal concentrations. There was no treatment-associated FSH response after saline treatment, but a clear FSH response in all l-GnRH-III-injected barrows. On average, the maximum FSH level (205% of the basal concentration) was observed at 1 h post-treatment. Mean FSH values were elevated until 10 h post-treatment. There was no LH response either to saline or to l-GnRH-III. In conclusion, this study demonstrates a selective FSH-releasing activity of 150 microg l-GnRH-III in barrows. Further studies are needed to investigate whether this effect is ubiquitous in the pig and what the physiological relevance is.

Innate immune traits differ between Meishan and Large White pigs

A panel of innate immune traits were compared between Meishan and Large White pigs. These pigs were of similar age and kept under the same environmental conditions to reduce non-genetically derived variation in immune traits. The animals were all apparently healthy and were not experimentally challenged with any pathogen during the study. The measures only required a small blood sample. Total white cell counts were similar between the pig breeds. However, the numbers of lymphocytes, neutrophils and monocytes differed significantly, with Meishans having higher neutrophil and monocyte counts and lower lymphocyte counts. Flow cytometric methods were used to determine quantitatively the characteristics and function of neutrophils and monocytes. Meishan neutrophils were smaller and less complex than Large White neutrophils, and phagocytosis of Escherichia coli and the ensuing oxidative burst was lower in Meishan neutrophils compared to Large White neutrophils. Monocyte phagocytosis of E. coli was significantly less than that of neutrophils in both breeds but the function of Meishan monocytes as measured by phagocytosis and tumour necrosis factor alpha (TNFalpha) release did not differ from that of Large White monocytes. Levels of acute phase proteins also differed between the breeds with a significantly higher proportion of Meishans having elevated serum amyloid A levels. However, Meishans had lower alpha(1)-acid glycoprotein levels than Large Whites and haptoglobin levels were similar. Such differences in innate immune traits may have implications in the resistance to infection by a broad range of pathogens and subsequent disease effects in these breeds. Further studies are warranted to investigate the genes underlying these traits.

Relationships of testicular iron and ferritin concentrations with testicular weight and sperm production in boars

The inverse relationship of testicular size and circulating follicle-stimulating hormone (FSH) concentrations has been documented, and accompanying this relationship is the change in color of the parenchymal tissue of the testes. Large testes (300 to 400 g) are pink to light red and small testes (100 g) are dark maroon with color gradations for weights in between. It was hypothesized that this color most likely represented an iron protein. Chromatographic analysis of testicular tissue indicated that the Fe was associated primarily with ferritin, and immunohistochemistry showed that Leydig cells were the primary location of ferritin storage within the testes. Concentrations of Fe and ferritin were higher in small testes and decreased as testes weight increased (P < 0.05). As testicular Fe concentrations increased, daily sperm production (DSP) and total DSP declined (P < 0.05). Genotyping six generations of Meishan x White composite boars (n = 288) for a quantitative trait locus that is indicative of elevated FSH and small testes in boars indicated that the Meishan genotype had elevated testicular iron concentrations and darker color in conjunction with reduced total DSP (P < 0.01). It is not thought the elevated iron concentrations affect testicular weights but are probably a result of elevated FSH and FSH inducement of Fe transport. The storage of Fe in Leydig cells may provide a reservoir of Fe for easy access by Sertoli and germ cells, but still provide a degree of protection to germ cells from ionic iron.

Regulation of microsomal P450, redox partner proteins, and steroidogenesis in the developing testes of the neonatal pig

Testicular growth and plasma androgen concentrations increase markedly in the first weeks of neonatal life of pigs. The regulation of steroidogenesis through this period was examined by measuring total microsomal cytochromes P450 (P450), 17alpha-hydroxylase/17,20-lyase P450 (P450c17) and aromatase P450 (P450arom) enzyme activities, and the redox partner proteins nicotinamide adenine dinucleotide phosphate, reduced form (NADPH)-cytochrome P450 reductase (reductase) and cytochrome b(5) in testicular microsomes. Testes were collected from 1-24 d of age, and testicular development was suppressed by a GnRH antagonist in some animals from d 1-14. Both 17/20-lyase and aromatase activities increased from d 1-7 but not thereafter, and 17-20-lyase activity was always at least 200-fold higher than aromatase activity. Reductase decreased in wk 1, then increased to d 24. No changes were seen in cytochrome b(5) expression. GnRH antagonist treatment suppressed plasma LH, testosterone and testes growth to d 14. 17,20-Lyase and aromatase activities in testicular microsomes were reduced by 20% and 50%, respectively. Total microsomal P450 concentration was reduced by 50% on d 7, but there was no effect of treatment on reductase or cytochrome b(5) expression. These data support the hypothesis that the rise in neonatal testicular androgen secretion is more likely due to gonadotropin-stimulated gonadal growth, rather than specific P450c17 expression. Neither P450c17 nor P450arom can account for the decline in total microsomal P450. Reductase and cytochrome b(5) expression appears to be constitutive, but reductase levels saturate both P450c17 and P450arom.