Puberty in beef heifers: effects of prenatal and postnatal nutrition on the development of the neuroendocrine axis

Reproductive maturation is a complex physiological process controlled by the neuroendocrine system and is characterized by an increase in gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) pulsatile secretion. Nutrition during early development is a key factor regulating puberty onset, which is defined as first ovulation in females. In heifers, nutrient restriction after weaning delays puberty, whereas elevated levels of nutrition and energy reserves advance reproductive maturation. Recent studies in cattle and other animal models have shown that the dam's nutrition during gestation can also program the neuroendocrine system in the developing fetus and has the potential to alter timing of puberty in the offspring. Among the metabolic signals that modulate brain development and control timing of puberty is leptin, a hormone produced primarily by adipocytes that communicates energy status to the brain. Leptin acts within the arcuate nucleus of the hypothalamus to regulate GnRH secretion via an upstream network of neurons that includes neurons that express neuropeptide Y (NPY), an orexigenic peptide with inhibitory effects on GnRH secretion, and alpha melanocyte-stimulating hormone (αMSH), an anorexigenic peptide with excitatory effects on GnRH neurons. Another important population of neurons are KNDy neurons, neurons in the arcuate nucleus that co-express the neuropeptides kisspeptin, neurokinin B, and dynorphin and have strong stimulatory effects on GnRH secretion. Our studies in beef heifers indicate that increased nutrition between 4 to 8 months of age advances puberty by diminishing NPY inhibitory tone and by increasing excitatory inputs of αMSH and kisspeptin, which collectively lead to increased GnRH/LH pulsatility. Our ongoing studies indicate that different planes of nutrition during gestation can alter maternal leptin concentrations and promote changes in the fetal brain. Nonetheless, at least in Bos indicus-influenced heifers, deficits programmed prenatally can be overcome by adequate postnatal nutrition without negatively impacting age at puberty or subsequent fertility.

Interaction of pre- and postnatal nutrition on expression of leptin receptor variants and transporter molecules, leptin transport, and functional response to leptin in heifers†

Perinatal nutrition modulates the hypothalamic neurocircuitries controlling GnRH release, thus programming pubertal maturation in female mammals. Objectives of experiments reported here were to test the hypotheses that prenatal nutrition during mid- to late gestation interacts with postnatal nutrition during the juvenile period in heifer offspring to alter expression of leptin receptor (LepR) variants (ObRa, ObRb, ObRc, ObRt), and lipoprotein transporter molecules (LRP1 and 2) in the choroid plexus, leptin transport across the blood-brain barrier, and hypothalamic-hypophyseal responsiveness to exogenous ovine leptin (oleptin) during fasting. Nutritional programming of heifers employed a 3 × 2 factorial design of maternal (high, H; low, L; and moderate, M) × postnatal (H and L) dietary treatments. Results (Expt. 1) demonstrated that prepubertal heifers born to L dams, regardless of postnatal diet, had reduced expression of the short isoform of ObRc compared to H and M dams, with sporadic effects of undernutrition (L or LL) on ObRb, ObRt, and LRP1. Intravenous administration of oleptin to a selected postpubertal group (HH, MH, LL) of ovariectomized, estradiol-implanted heifers fasted for 56 h (Expt. 2) did not create detectable increases in third ventricle cerebrospinal fluid but increased gonadotropin secretion in all nutritional groups tested. Previous work has shown that leptin enhances gonadotropin secretion during fasting via effects at both hypothalamic and anterior pituitary levels in cattle. Given the apparent lack of robust transfer of leptin across the blood-brain barrier in the current study, effects of leptin at the adenohypophyseal level may predominate in this experimental model.

Early milk-feeding regimes in calves exert long-term effects on the development of ovarian granulosa cells

BACKGROUND

Nutrition has not only an impact on the general wellbeing of an animal but can also affect reproductive processes. In cattle, feeding regimes can influence the age of puberty onset and alter gonadal development. We analyzed effects of different milk replacer (MR) feeding regimes during rearing on ovarian physiology with specific emphasis on the numbers as well as gene expression characteristics of granulosa cells (GCs) at the age of puberty onset. Two groups of calves received either 10% or 20% of bodyweight MR per day during their first 8 weeks. After weaning, both groups were fed the same mixed ration ad libitum until slaughter at 8 months.

RESULTS

Animals of the 20% feeding group had a significantly higher body weight, but the proportion of animals having a corpus luteum at the time of slaughter was not different between groups, suggesting a similar onset of puberty. Calves of the 10% group showed a constant GC count regardless of the number of follicles (r = 0.23) whereas in the 20% group increasing numbers of GCs were detected with a higher follicle count (r = 0.71). As a first effort to find a possible molecular explanation for this unexpected limitation of GC numbers in the 10% group, we comparatively analyzed GC transcriptomes in both diet groups. The mRNA microarray analysis revealed a total of 557 differentially expressed genes comparing both groups (fold change > |1.5| and p < 0.05). OAS1X, MX2 and OAS1Z were among the top downregulated genes in the 20% vs. the 10% group, whereas top upregulated genes comprised BOLA and XCL1. All of these genes are known to be regulated by interferon. Subsequent signaling pathway analysis revealed the involvement of several immune response mechanisms in accordance with a number of interferons as upstream regulators.

CONCLUSIONS

The results indicate that the plane of MR feeding in early life has an impact on the number and physiology of GCs later in life. This might influence the overall reproductive life initiated by the onset of puberty in cattle. In addition, the observed alterations in GCs of calves fed less MR might be a consequence of interferon regulated immunological pathways.

Review: Gestational and postnatal nutritional effects on the neuroendocrine control of puberty and subsequent reproductive performance in heifers

Pubertal attainment is an intricate biological process that involves maturation of the reproductive neuroendocrine axis and increased pulsatile release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone. Nutrition is a critical environmental factor controlling the timing of puberty attainment. Nutrient restriction during early postnatal development delays puberty, whereas increased feed intake and adiposity during this period hasten pubertal maturation by imprinting the hypothalamus. Moreover, the dam's nutrition during gestation can program the neuroendocrine system in the developing fetus and has the potential to advance or delay puberty in the offspring. Leptin, a hormone produced primarily by adipose cells, plays an important role in communicating energy status to the brain and regulating sexual maturation. Leptin's regulation of GnRH release is mediated by an upstream neuronal network since GnRH neurons do not contain the leptin receptor. Two groups of neurons located in the arcuate nucleus of the hypothalamus that express neuropeptide Y (NPY), an orexigenic peptide, and alpha melanocyte-stimulating hormone (αMSH), an anorexigenic peptide, are central elements of the neural circuitry that relay inhibitory (NPY) and excitatory (αMSH) inputs to GnRH neurons. Moreover, KNDy neurons, neurons in the arcuate nucleus that co-express kisspeptin, neurokinin B (NKB), and dynorphin, also play a role in the metabolic regulation of puberty. Our studies in beef heifers demonstrate that increased rates of BW gain during early postweaning (4-9 mo of age) result in reduced expression of NPY mRNA, increased expression of proopiomelanocortin and kisspeptin receptor mRNA, reduced NPY inhibitory inputs to GnRH neurons, and increased excitatory αMSH inputs to KNDy neurons. Finally, our most recent data demonstrate that nutrition of the cow during the last two trimesters of gestation can also induce transcriptional and structural changes in hypothalamic neurocircuitries in the heifer progeny that likely persist long-term after birth. Managerial approaches, such as supplementation of the dam during gestation (fetal programming), creep feeding, early weaning, and stair-step nutritional regimens have been developed to exploit brain plasticity and advance pubertal maturation in heifers.

Review: Role of early life nutrition in regulating sexual development in bulls

The availability of high-quality semen from genetically elite bulls is essential to support continued genetic gain and the sustainability of cattle production worldwide. While reducing the age at which usable semen is available also reduces the generation interval, it is dependent on timely onset of puberty in young bulls. There is now good evidence that hastened sexual development in bulls is achieved through enhancing nutrition in early life. This review will cover the physiological and molecular-based response to prevailing diet in key organs that orchestrate the ontogeny of sexual development in the bull calf. Given the central importance of the interaction between metabolic status and neuronal function to the progression of sexual development, we will discuss how communication between metabolic organs, reproductive organs and the brain are mediated via molecular and physiological processes. The availability of high-throughput nucleic acid and protein sequencing technologies and innovative data analytics have allowed us to improve our understanding of molecular regulation of puberty and sexual development. Analysing data from a number of organs, simultaneously, allows for a better understanding of the underlying biology and biochemical interactions that are influencing sexual development. Specifically, we can determine how early life nutritional interventions augment changes in potential key molecules regulating sexual development. Ultimately, a greater understanding of the inherent regulation of postnatal sexual development in the bull calf and how strategically targeted nutritional management can advance the ontogeny of this process, will facilitate the timely availability of high-quality semen from genetically elite animals, thus supporting more economically and environmentally sustainable beef and dairy production systems.

Investigation of Sperm and Seminal Plasma Candidate MicroRNAs of Bulls with Differing Fertility and In Silico Prediction of miRNA-mRNA Interaction Network of Reproductive Function

Simple Summary

The objective of this study was to identify differentially expressed (DE) sperm and seminal plasma microRNAs (miRNAs) in high- and low-fertile Holstein bulls (four bulls per group), integrate miRNAs to their target genes, and categorize target genes based on predicted biological processes. Out of 84 bovine-specific, prioritized miRNAs analyzed by RT-PCR, 30 were differentially expressed in high-fertile sperm and seminal plasma compared to low-fertile sperm and seminal plasma, respectively (p ≤ 0.05, fold regulation ≥5 magnitudes). Interestingly, expression levels of DE-miRNAs in sperm and seminal plasma followed a similar pattern. Highly scored integrated genes of DE-miRNAs predicted various biological and molecular functions, cellular process, and pathways. Further in silico analysis revealed categorized genes may have a plausible association with pathways regulating sperm structure and function, fertilization, and embryo and placental development. In conclusion, highly DE-miRNAs in bovine sperm and seminal plasma could be used as a tool for predicting reproductive functions. Since the identified miRNA-mRNA interactions were mostly based on predictions from public databases, the causal regulations of miRNA-mRNA and the underlying mechanisms require further functional characterization in future studies.

Abstract

Recent advances in high-throughput in silico techniques portray experimental data as exemplified biological networks and help us understand the role of individual proteins, interactions, and their biological functions. The objective of this study was to identify differentially expressed (DE) sperm and seminal plasma microRNAs (miRNAs) in high- and low-fertile Holstein bulls (four bulls per group), integrate miRNAs to their target genes, and categorize the target genes based on biological process predictions. Out of 84 bovine-specific, prioritized miRNAs analyzed by RT-PCR, 30 were differentially expressed in high-fertile sperm and seminal plasma compared to low-fertile sperm and seminal plasma, respectively (p ≤ 0.05, fold regulation ≥ 5 magnitudes). The expression levels of DE-miRNAs in sperm and seminal plasma followed a similar pattern. Highly scored integrated genes of DE-miRNAs predicted various biological and molecular functions, cellular process, and pathways. Further, analysis of the categorized genes showed association with pathways regulating sperm structure and function, fertilization, and embryo and placental development. In conclusion, highly DE-miRNAs in bovine sperm and seminal plasma could be used as a tool for predicting reproductive functions. Since the identified miRNA-mRNA interactions were mostly based on predictions from public databases, the causal regulations of miRNA-mRNA and the underlying mechanisms require further functional characterization in future studies.

Early juvenile but not mid to late prenatal nutrition controls puberty in heifers but neither impact adult reproductive function

Objectives were to test the hypothesis that pre and postnatal nutrition in the bovine female, independently or interactively, affect age at puberty and functional characteristics of the estrous cycle of sexually mature offspring. Brangus and Braford (n = 97) beef cows bearing a female fetus were fed to achieve body condition scores of 7.5-8 (H, obese), 5.5-6 (M, moderate) or 3-3.5 (L, thin) by the start of the third trimester and maintained until parturition. Heifer offspring were weaned and fed to gain weight at either a high (H; 1 kg/d) or low (L; 0.5 kg/d) rate between 4 and 8 months of age, then fed the same diet during a common feeding period until puberty which resulted in compensatory growth of heifers in the L group. Heifers (n = 95) from the H postnatal diet reached puberty two months earlier (12 ± 0.4 months; P = 0.0002) than those from the L postnatal diet (14 ± 0.4 months). Estrous cycles of a subgroup of postpubertal heifers (n = 53) were synchronized to evaluate antral follicle count (AFC), rate of growth and size of the pre-ovulatory follicle, size of corpus luteum and ovary, endometrial thickness, and plasma concentrations of progesterone and estradiol-17β (E2). Although there was a trend for postnatal H heifers to have greater AFC and plasma concentrations of E2 compared to L heifers, neither pre nor postnatal nutrition affected any other physiological or hormonal variables, including short-term fertility. Postnatal nutritional effects on pubertal age remained the dominant observed feature.

Effect of temporary weaning and creep feeding on calf growth and the reproductive efficiency of their Hereford dams

Objective

The objective was to test if creep feeding (CF) improves the average daily gain (ADG) and weaning weight of calves submitted to temporary weaning (TW) and if the combination of CF and TW improves conception and pregnancy rates of cows.

Methods

Primiparous (n = 74) and primiparous and multiparous (n = 104) cows grazing native grasslands were used in experiment 1 and 2; respectively. The experimental design was in plots divided into complete random blocks with two replications. The CF was the big plot and TW the small plot, thus four experimental groups were formed: i) −CF−TW (n = 21 and 27); ii) −CF+TW (n = 16 and 24); iii) +CF−TW (n = 20 and 26); iv) +CF+TW (n = 17 and 27) with cow-calf pairs for experiments 1 and 2; respectively. Nose plate application for TW had a duration of 14 and 15 days for experiment 1 and 2: respectively. In experiment 1, calves were fed at 1% of live weight for 112 days using a commercial supplement with 18.4% crude protein. In experiment 2, the supplementation lasted 98 days, and was carried out with corn dried distillers grains with soluble (DDGS) at 40% of the potential intake on a daily basis.

Results

The TW reduced ADG during the TW period and the following 14 days, but the negative effect of TW was maintained until the final weaning only in experiment 2. The CF increased ADG during TW period in both experiments. The TW promoted an earlier conception of the dams (12 days in −CF treatment and 19 days in +CF treatment, p<0.01) and CF increased pregnancy rate in experiment 1, being the effects not consistent between experiments.

Conclusion

The CF consistently promoted an increase in ADG during the period of TW and increased final weaning weight of calves, therefore it is economically viable.

Impacts of Nutritional Management During Early Postnatal Life on Long-Term Physiological and Productive Responses of Beef Cattle

Effective early postnatal nutritional management is a crucial component of livestock production systems, and nutrient manipulation during this period has been shown to exert long-term consequences on beef cattle growth and physiology. Metabolic imprinting defines these biological responses to a nutritional intervention early in life that permanently alter physiological outcomes later in life. Early weaning has been used to study metabolic imprinting effects, given that it allows for nutritional manipulation of animals at a young age. This practice has been shown to enhance carcass characteristics in feedlot cattle and accelerate reproductive development of females. Another strategy to study the effects of metabolic imprinting without the need for early weaning is to provide supplements via creep feeding. Providing creep feed to nursing cattle has resulted in transient and long-term alterations in cattle metabolism, contributing to increased reproductive performance of developing heifers and enhanced carcass quality of feeder cattle. Collectively, results described herein demonstrate nutrient manipulation during early postnatal life exerts long-term consequences on beef cattle productivity and may be a strategy to optimize production efficiency in beef cattle systems.

A high plane of nutrition during early life alters the hypothalamic transcriptome of heifer calves

The aim was to examine the effect of rapid body weight gain during early calfhood consistent with earlier sexual development on the transcriptional profile of the hypothalamus. Angus X Holstein-Friesian heifer calves (19 ± 5 days of age) were offered a high (HI, n = 14) or moderate (MOD, n = 15) plane of nutrition from 3 to 21 weeks of age to achieve a growth rate of 1.2 kg/d and 0.5 kg/d, respectively. Following euthanasia at 21 weeks, the arcuate nucleus (ARC) region was separated from the remainder of the hypothalamus and both were subjected to RNA-Seq. HI calves exhibited altered expression of 80 and 39 transcripts in the ARC and the remaining hypothalamus, respectively (P < 0.05) including downregulation of AGRP and NPY and upregulation of POMC, previously implicated in precocious sexual development. Stress-signaling pathways were amongst the most highly dysregulated. Organ morphology, reproductive system development and function, and developmental disorder were amongst the networks derived from differentially expressed genes (DEGs) in the ARC. Gene co-expression analysis revealed DEGs within the ARC (POMC, CBLN2, CHGA) and hypothalamus (PENK) as hub genes. In conclusion, enhanced nutrition during early calfhood alters the biochemical regulation of the hypothalamus consistent with advanced sexual development in the prepubertal heifer.

Nutritional control of puberty in the bovine female: prenatal and early postnatal regulation of the neuroendocrine system

Puberty is a complex biological event that requires maturation of the reproductive neuroendocrine axis and subsequent initiation of high-frequency, episodic release of GnRH and LH. Nutrition is a critical factor affecting the neuroendocrine control of puberty. Although nutrient restriction during juvenile development delays puberty, elevated rates of body weight gain during this period facilitate pubertal maturation by programming hypothalamic centers that underlie the pubertal process. Recent findings suggest that maternal nutrition during gestation can also modulate the development of the fetal neuroendocrine axis, thus influencing puberty and subsequent reproductive function. Among the several metabolic signals, leptin plays a critical role in conveying metabolic information to the brain and, consequently, controlling puberty. The effects of leptin on GnRH secretion are mediated via an upstream neuronal network because GnRH neurons do not express the leptin receptor. Two neuronal populations located in the arcuate nucleus that express the orexigenic peptide neuropeptide Y (NPY), and the anorexigenic peptide alpha melanocyte-stimulating hormone (αMSH), are key components of the neurocircuitry that conveys inhibitory (NPY) and excitatory (αMSH) inputs to GnRH neurons. In addition, neurons in the arcuate nucleus that coexpress kisspeptin, neurokinin B, and dynorphin (termed KNDy neurons) are also involved in the metabolic control of puberty. Our studies in the bovine female demonstrate that increased planes of nutrition during juvenile development lead to organizational and functional changes in hypothalamic pathways comprising NPY, proopiomelanocortin (POMC, the precursor of αMSH), and kisspeptin neurons. Changes include alterations in the abundance of NPY, POMC, and Kiss1 mRNA and in plasticity of the neuronal projections to GnRH neurons. Our studies also indicate that epigenetic mechanisms, such as modifications in the DNA methylation pattern, are involved in this process. Finally, our most recent data demonstrate that maternal nutrition during gestation can also induce morphological and functional changes in the hypothalamic NPY system in the heifer offspring that are likely to persist long after birth. These organizational changes occurring during fetal development have the potential to not only impact puberty but also influence reproductive performance throughout adulthood in the bovine female.

Review: Pro-inflammatory cytokines and hypothalamic inflammation: implications for insufficient feed intake of transition dairy cows

Improvements in feed intake of dairy cows entering the early lactation period potentially decrease the risk of metabolic disorders, but before developing approaches targeting the intake level, mechanisms controlling and dysregulating energy balance and feed intake need to be understood. This review focuses on different inflammatory pathways interfering with the neuroendocrine system regulating feed intake of periparturient dairy cows. Subacute inflammation in various peripheral organs often occurs shortly before or after calving and is associated with increased pro-inflammatory cytokine levels. These cytokines are released into the circulation and sensed by neurons located in the hypothalamus, the key brain region regulating energy balance, to signal reduction in feed intake. Besides these peripheral humoral signals, glia cells in the brain may produce pro-inflammatory cytokines independent of peripheral inflammation. Preliminary results show intensive microglia activation in early lactation, suggesting their involvement in hypothalamic inflammation and the control of feed intake of dairy cows. On the other hand, pro-inflammatory cytokine-induced activation of the vagus nerve transmits signalling to the brain, but this pathway seems not exclusively necessary to signal feed intake reduction. Yet, less studied in dairy cows so far, the endocannabinoid system links inflammation and the hypothalamic control of feed intake. Distinct endocannabinoids exert anti-inflammatory action but also stimulate the posttranslational cleavage of neuronal proopiomelanocortin towards β-endorphin, an orexigen promoting feed intake. Plasma endocannabinoid concentrations and hypothalamic β-endorphin levels increase from late pregnancy to early lactation, but less is known about the regulation of the hypothalamic endocannabinoid system during the periparturient period of dairy cows. Dietary fatty acids may modulate the formation of endocannabinoids, which opens new avenues to improve metabolic health and immune status of dairy cows.

Effect of Early Calf-Hood Nutrition on the Transcriptional Regulation of the Hypothalamic-Pituitary-Testicular axis in Holstein-Friesian Bull Calves

The aim of this study was to investigate the effect of early calf-hood nutrition on the transcriptomic profile of the arcuate nucleus of the hypothalamus, anterior pituitary and testes in Holstein-Friesian bulls. Holstein-Friesian bull calves with a mean (±S.D.) age and bodyweight of 19 (±8.2) days and 47.5 (±5.3) kg, respectively, were offered a high (n = 10) or low (n = 10) plane of nutrition in order to achieve an overall growth rate of 1.2 and 0.5 kg/day. At 126 (±3) days of age, calves were euthanized, hypothalamus (arcuate region), anterior pituitary and testicular parenchyma samples were harvested and RNAseq analysis was performed. There were 0, 49 and 1,346 genes differentially expressed in the arcuate nucleus, anterior pituitary and testicular tissue of bull calves on the low relative to the high plane of nutrition, respectively (P < 0.05; False Discovery Rate <0.05). Cell cycle processes in the anterior pituitary were down regulated in the low relative to the high plane of nutrition; there was no differential expression of genes related to reproductive processes. Gene expression involved in cholesterol and androgen biosynthesis in the testes were down regulated in animals on the low plane of nutrition. This study provides insight into the effect of early life plane of nutrition on the regulation of the HPT axis.

Interaction of dietary energy source and body weight gain during the juvenile period on metabolic endocrine status and age at puberty in beef heifers

Using a previously established model for nutritional acceleration of puberty, beef heifers ( = 48; 1/2 Angus × 1/4 Hereford × 1/4 Brahman) were used in a replicated 2 × 2 factorial design to examine the effects of diet type (high forage [HF] vs. high concentrate [HC]) and rate of BW gain (low gain [LG], 0.45 kg/d, vs. high gain [HG], 0.91 kg/d) on key metabolic hormones and age at puberty. After weaning at 14 ± 1 wk of age, heifers were assigned randomly to be fed HC-HG, HC-LG, HF-HG, or HF-LG ( = 12/group) beginning at 4 mo of age for 14 wk. Heifers were then switched to a common growth diet until puberty. Average daily gain was greater ( < 0.04) during the dietary treatment phase in HG heifers (0.81 ± 0.06 kg/d) than in LG heifers (0.43 ± 0.06 kg/d), and there was no diet type × rate of gain interaction. Puberty was achieved at a younger age (54.5 ± 1.8 wk) in both HG groups than in LG groups (60.2 ± 1.9 wk; < 0.04), but dietary energy source (HC vs. HF) did not influence this variable. Moreover, mean BW at puberty did not differ by diet type or rate of gain during the dietary treatment phase. Nonetheless, heifers fed HC-HG exhibited a striking increase ( < 0.0001) in serum leptin beginning at 26 ± 1 wk of age and remained elevated ( < 0.01) throughout the remainder of the experimental feeding phase compared to all other treatments. However, serum leptin in HC-HG dropped precipitously when heifers were switched to the common growth diet and did not differ from that of other groups thereafter. Overall mean concentrations of serum glucose were greater ( < 0.006) in HG heifers than in LG during the dietary treatment phase, with serum insulin also greater ( < 0.04) in HG than in LG only during weeks 20, 22, and 30. Mean serum IGF-1 was not affected by dietary type or rate of BW gain. We speculate that failure of the marked increase in serum leptin observed in HC-HG heifers during the dietary treatment phase to further accelerate puberty compared to HF-HG occurred because of its abrupt decline at the onset of the common growth phase, thus attenuating the temporal cue for activation of the reproductive neuroendocrine system.

Effect of short term diet restriction on gene expression in the bovine hypothalamus using next generation RNA sequencing technology

BACKGROUND

Negative energy balance (NEB) is an imbalance between energy intake and energy requirements for lactation and body maintenance affecting high-yielding dairy cows and is of considerable economic importance due to its negative impact on fertility and health in dairy herds. It is anticipated that the cow hypothalamus experiences extensive biochemical changes during the early post partum period in an effort to re-establish metabolic homeostasis. However, there is variation in the tolerance to NEB between individual cows. In order to understand the genomic regulation of ovulation in hypothalamic tissue during NEB, mRNA transcriptional patterns between tolerant and sensitive animals were examined. A short term dietary restriction heifer model was developed which induced abrupt onset of anoestrus in some animals (Restricted Anovulatory; RA) while others maintained oestrous cyclicity (Restricted Ovulatory; RO). A third control group (C) received a higher level of normal feeding.

RESULTS

A total of 15,295 genes were expressed in hypothalamic tissue. Between RA and C groups 137 genes were differentially expressed, whereas between RO and C, 32 genes were differentially expressed. Differentially expressed genes were involved in the immune response and cellular motility in RA and RO groups, respectively, compared to C group. The largest difference between groups was observed in the comparison between RA and RO heifers, with 1094 genes shown to be significantly differentially expressed (SDE). Pathway analysis showed that these SDE genes were associated with 6 canonical pathways (P < 0.01), of which neuroactive ligand-receptor interaction was the most significant. Within the comparisons the main over-represented pathway functions were immune response including neuroprotection (CXCL10, Q1KLR3, IFIH1, IL1 and IL8; RA v C and RA v RO); energy homeostasis (AgRP and NPY; RA v RO); cell motility (CADH1, DSP and TSP4; RO v C) and prevention of GnRH release (NTSR1 IL1α, IL1β, NPY and PACA; RA v RO).

CONCLUSIONS

This information will assist in understanding the genomic factors regulating the influence of diet restriction on fertility and may assist in optimising nutritional and management systems for the improvement in reproductive performance.

Endogenous and dietary lipids influencing feed intake and energy metabolism of periparturient dairy cows

The high metabolic priority of the mammary gland for milk production, accompanied by limited feed intake around parturition results in a high propensity to mobilize body fat reserves. Under these conditions, fuel selection of many peripheral organs is switched, for example, from carbohydrate to fat utilization to spare glucose for milk production and to ensure partitioning of tissue- and dietary-derived nutrients toward the mammary gland. For example, muscle tissue uses nonesterified fatty acids (NEFA) but releases lactate and amino acids in a coordinated order, thereby providing precursors for milk synthesis or hepatic gluconeogenesis. Tissue metabolism and in concert, nutrient partitioning are controlled by the endocrine system involving a reduction in insulin secretion and systemic insulin sensitivity and orchestrated changes in plasma hormones such as insulin, adiponectin, insulin growth factor-I, growth hormone, glucagon, leptin, glucocorticoids, and catecholamines. However, the endocrine system is highly sensitive and responsive to an overload of fatty acids no matter if excessive NEFA supply originates from exogenous or endogenous sources. Feeding a diet containing rumen-protected fat from late lactation to calving and beyond exerts similar negative effects on energy intake, glucose and insulin concentrations as does a high extent of body fat mobilization around parturition in regard to the risk for ketosis and fatty liver development. High plasma NEFA concentrations are thought not to act directly at the brain level, but they increase the energy charge of the liver which is, signaled to the brain to diminish feed intake. Cows differing in fat mobilization during the transition phase differ in their hepatic energy charge, whole body fat oxidation, glucose metabolism, plasma ghrelin, and leptin concentrations and in feed intake several week before parturition. Hence, a high lipid load, no matter if stored, mobilized or fed, affects the endocrine system, metabolism, and feed intake, and increases the risk for metabolic disorders. Future research should focus on a timely parallel increase in feed intake and milk yield during early lactation to reduce the impact of body fat on feed intake, metabolic health, and negative energy balance.

Factors affecting puberty in replacement beef heifers

Puberty is defined as when ovulation is accompanied by visual signs of estrus and subsequent normal luteal function. Age at puberty is an important trait in relation to reproductive success, productive life span, and profitability in beef operations. Although puberty and initiation of normal estrous cycles are complex events that require maturation of the hypothalamic-pituitary-ovarian axis, it has been well documented that nutrition, age, and genetics are regulators of age at puberty. However, their role is mainly as regulators of the endocrine maturation that must occur for sustained ovarian cyclicity to be initiated. Increased growth rate between 4 and 7 months of age is apparently sufficient to induce early puberty, and this increased growth rate decreased the negative feedback of estradiol on LH secretion during the prepubertal period. As puberty approaches, a progressive decrease in the negative feedback of estradiol on GnRH secretion allows increased pulse frequency of LH, thus stimulating follicular growth and increased estradiol production. In addition, expression of estrogen receptors in the anterior hypothalamus and ventromedial nucleus is negatively correlated with LH pulse frequency. Although a significant number of genes and pathways are involved in neuromaturation for the initiation of normal estrous cycles, the inhibitory effects of neuropeptide Y on GnRH/LH release appear to decrease, and the stimulatory effect of melanocyte-stimulating hormone alpha on GnRH appears to increase as puberty approaches. Thus, a thorough understanding of the metabolic and neuroendocrine changes that occur to initiate normal estrous cycles is needed to facilitate management of the important reproductive event.

Nutritional Programming of Accelerated Puberty in Heifers: Involvement of Pro-Opiomelanocortin Neurones in the Arcuate Nucleus

The timing of puberty and subsequent fertility in female mammals are dependent on the integration of metabolic signals by the hypothalamus. Pro-opiomelanocortin (POMC) neurones in the arcuate nucleus (ARC) comprise a critical metabolic-sensing pathway controlling the reproductive neuroendocrine axis. α-Melanocyte-stimulating hormone (αMSH), a product of the POMC gene, has excitatory effects on gonadotrophin-releasing hormone (GnRH) neurones and fibres containing αMSH project to GnRH and kisspeptin neurones. Because kisspeptin is a potent stimulator of GnRH release, αMSH may also stimulate GnRH secretion indirectly via kisspeptin neurones. In the present work, we report studies conducted in young female cattle (heifers) aiming to determine whether increased nutrient intake during the juvenile period (4-8 months of age), a strategy previously shown to advance puberty, alters POMC and KISS1 mRNA expression, as well as αMSH close contacts on GnRH and kisspeptin neurones. In Experiment 1, POMC mRNA expression, detected by in situ hybridisation, was greater (P < 0.05) in the ARC in heifers that gained 1 kg/day of body weight (high-gain, HG; n = 6) compared to heifers that gained 0.5 kg/day (low-gain, LG; n = 5). The number of KISS1-expressing cells in the middle ARC was reduced (P < 0.05) in HG compared to LG heifers. In Experiment 2, double-immunofluorescence showed limited αMSH-positive close contacts on GnRH neurones, and the magnitude of these inputs was not influenced by nutritional status. Conversely, a large number of kisspeptin-immunoreactive cells in the ARC were observed in close proximity to αMSH-containing varicosities. Furthermore, HG heifers (n = 5) exhibited a greater (P < 0.05) percentage of kisspeptin neurones in direct apposition to αMSH fibres and an increased (P < 0.05) number of αMSH close contacts per kisspeptin cell compared to LG heifers (n = 6). These results indicate that the POMC-kisspeptin pathway may be important in mediating the nutritional acceleration of puberty in heifers.

Reciprocal changes in leptin and NPY during nutritional acceleration of puberty in heifers

Feeding a high-concentrate diet to heifers during the juvenile period, resulting in increased body weight (BW) gain and adiposity, leads to early-onset puberty. In this study, we tested the hypothesis that the increase in GnRH/LH release during nutritional acceleration of puberty is accompanied by reciprocal changes in circulating leptin and central release of neuropeptide Y (NPY). The heifers were weaned at 3.5 months of age and fed to gain either 0.5 (Low-gain; LG) or 1.0 kg/day (High-gain; HG) for 30 weeks. A subgroup of heifers was fitted surgically with third ventricle guide cannulas and was subjected to intensive cerebrospinal fluid (CSF) and blood sampling at 8 and 9 months of age. Mean BW was greater in HG than in LG heifers at week 6 of the experiment and remained greater thereafter. Starting at 9 months of age, the percentage of pubertal HG heifers was greater than that of LG heifers, although a replicate effect was observed. During the 6-h period in which CSF and blood were collected simultaneously, all LH pulses coincided with or shortly followed a GnRH pulse. At 8 months of age, the frequency of LH pulses was greater in the HG than in the LG group. Beginning at 6 months of age, concentrations of leptin were greater in HG than in LG heifers. At 9 months of age, concentrations of NPY in the CSF were lesser in HG heifers. These observations indicate that increased BW gain during juvenile development accelerates puberty in heifers, coincident with reciprocal changes in circulating concentrations of leptin and hypothalamic NPY release.

Environmental stressors influencing hormones and systems physiology in cattle

Environmental stressors undoubtedly influence organismal biology, specifically the endocrine system that, in turn, impact cattle at the systems physiology level. Despite the significant advances in understanding the genetic determinants of the ideal dairy or beef cow, there is a grave lack of understanding of the systems physiology and effects of the environmental stressors that interfere with the endocrine system. This is a major problem because the lack of such knowledge is preventing advances in understanding gene-environment interactions and developing science-based solutions to these challenges. In this review, we synthesize the current knowledge on the nature of the major environmental stressors, such as climate (heat, cold, wind, and humidity), nutrition (feeds, feeding systems, and endocrine disruptors) and management (housing density and conditions, transportation, weaning practices). We summarize the impact of each one of these factors on cattle at the systems level, and provide solutions for the challenges.

Reproduction Symposium: hypothalamic neuropeptides and the nutritional programming of puberty in heifers

Nutrition during the juvenile period has a major impact on timing reproductive maturity in heifers. Restricted growth delays puberty, whereas elevated BW gain advances the onset of puberty. The initiation of high-frequency episodic release of GnRH and, consequently, LH during the peripubertal period is crucial for maturation of the reproductive axis and establishment of normal estrous cycles. Nutritional signals are perceived by metabolic-sensing cells in the hypothalamus, which interact with estradiol-receptive neurons to regulate the secretory activity of GnRH neurons. The orexigenic peptide, neuropeptide Y (NPY), and the anorexigenic peptide derived from the proopiomelanocortin (POMC) gene, melanocyte-stimulating hormone α (αMSH), are believed to be major afferent pathways that transmit inhibitory (NPY) and excitatory (αMSH) inputs to GnRH neurons. The neuropeptide kisspeptin is considered a major stimulator of GnRH secretion and has been shown to mediate estradiol's effect on GnRH neuronal activity. Kisspeptin may also integrate the neuronal pathways mediating the metabolic and gonadal steroid hormone control of gonadotropin secretion. Recent studies in our laboratories indicate that functional and structural changes in the pathways involving NPY, POMC, and kisspeptin neurons occur in response to high rates of BW gain during the juvenile period in heifers. Changes include regulation of expression in NPY, POMC, and KISS1 and plasticity in the neuronal projections to GnRH neurons and within the neuronal network comprising these cells. Moreover, an intricate pattern of differential gene expression in the arcuate nucleus of the hypothalamus occurs in response to feeding high concentrate diets that promote elevated BW gain. Genes involved include those controlling feeding intake and cell metabolism, neuronal growth and remodeling, and synaptic transmission. Characterizing the cellular pathways and molecular networks involved in the mechanisms that control the timing of pubertal onset will assist in improving existing strategies and facilitate the development of novel approaches to program puberty in heifers. These include the use of diets that elevate BW gain during strategic periods of prepubertal development.

Use of a stair-step compensatory gain nutritional regimen to program the onset of puberty in beef heifers

It was hypothesized that metabolic programming of processes underlying puberty can be shifted temporally through the use of a stair-step compensatory growth model such that puberty is optimally timed to occur at 11 to 12 mo of age. Forty crossbred beef heifers were weaned at approximately 3.5 mo of age and, after a 2-wk acclimation period, were assigned randomly to 1 of 4 nutritional groups: 1) low control (LC), restricted feed intake of a forage-based diet to promote BW gain of 0.5 kg/d until 14 mo of age, 2) high control (HC), controlled feed intake of a high-concentrate diet to promote BW gain of 1 kg/d until 14 mo of age, 3) stair-step 1 (SS-1), ad libitum feed intake of a high-concentrate diet until 6.5 mo of age followed by restricted access to a high-forage diet to promote BW gain of 0.35 kg/d until 9 mo of age, ad libitum feed intake of a high-concentrate diet until 11.5 mo of age, and restricted intake of a high-forage diet to promote BW gain of 0.35 kg/d until 14 mo of age, and 4) stair-step 2 (SS-2), reverse sequence of SS-1, beginning with restricted access to a high-forage diet. Body weight (every 2 wk) and circulating concentrations of leptin (monthly) were determined throughout the experiment. Concentrations of progesterone in blood samples collected twice weekly beginning at 8 mo of age were used to determine pubertal status. Body weight gain followed a pattern similar to that proposed in our experimental design. Circulating concentrations of leptin increased following distinct elevations in BW but decreased abruptly after feed intake restriction. Survival analysis indicated that the percentage of pubertal heifers in the LC group was lower (P < 0.05) than all other groups throughout the experiment. Although heifers in SS-1 were nutritionally restricted between 6.5 and 9 mo of age, the proportion pubertal by 12 mo of age did not differ (P = 0.36) from that of the HC group, with 80% and 70% pubertal in SS-1 and HC, respectively. In contrast, the proportion of heifers pubertal by 12 mo of age in the SS-2 group (40%) was lower (P < 0.05) than both HC and SS-1. However, by 14 mo of age, 90% of heifers in the SS-2 group had also attained puberty compared to only 40% of the LC group. In summary, these data provide evidence that changes in the nutritional and metabolic status during the early juvenile period can program the onset of puberty that occurs months later, allowing optimal timing of sexual maturation in replacement beef heifers.

Application of Top-Down and Bottom-up Systems Approaches in Ruminant Physiology and Metabolism

Systems biology is a computational field that has been used for several years across different scientific areas of biological research to uncover the complex interactions occurring in living organisms. Applications of systems concepts at the mammalian genome level are quite challenging, and new complimentary computational/experimental techniques are being introduced. Most recent work applying modern systems biology techniques has been conducted on bacteria, yeast, mouse, and human genomes. However, these concepts and tools are equally applicable to other species including ruminants (e.g., livestock). In systems biology, both bottom-up and top-down approaches are central to assemble information from all levels of biological pathways that must coordinate physiological processes. A bottom-up approach encompasses draft reconstruction, manual curation, network reconstruction through mathematical methods, and validation of these models through literature analysis (i.e., bibliomics). Whereas top-down approach encompasses metabolic network reconstructions using ‘omics’ data (e.g., transcriptomics, proteomics) generated through DNA microarrays, RNA-Seq or other modern high-throughput genomic techniques using appropriate statistical and bioinformatics methodologies. In this review we focus on top-down approach as a means to improve our knowledge of underlying metabolic processes in ruminants in the context of nutrition. We also explore the usefulness of tissue specific reconstructions (e.g., liver and adipose tissue) in cattle as a means to enhance productive efficiency.