1. Introduction
In cattle, early weaning consists of separating the calves from cows at less than 150 days of age [
1] and is aimed at reducing the stress of suckling and nutritional requirements of the cow, thus enabling early body condition recovery and return to estrus [
2]. However, attention must be paid to early weaned calves in order to avoid malnutrition [
3,
4]. Furthermore, energy supplementation of early weaned calves can be used to enhance characteristics of interest in animal production [
5,
6]. In breeds of European origin, early weaning has been shown to exert a major effect on the physiology of calves destined for slaughter when compared to those conventionally weaned at 210 days of age [
6,
7].
Studies have shown that early weaned calves supplemented with energy sources can achieve superior performance in systems that employ a slaughter age between 18 and 24 months, such as higher body weight, greater subcutaneous fat thickness, greater
Longissimus thoracis muscle area, and higher intramuscular fat levels [
5,
6,
8]. Among the key mechanisms underlying these metabolic changes are epigenetic modifications that can affect gene modulation and expression in response to environmental and nutritional stimuli [
9] and, consequently, phenotypes.
DNA methylation, which is an important epigenetic modification, can alter gene expression through the formation of a physical barrier that inhibits the binding of general transcription factors or affects the state of chromatin condensation; in addition, DNA methylation is involved in the recruitment of methyl-binding proteins that form repressive complexes [
10,
11]. The diet is known to play a fundamental role in the determination of the DNA methylation pattern by providing substrates necessary for methylation and cofactors that modulate the enzymatic activity of DNA methyltransferase, as well as in the activity of enzymes that regulate one-carbon metabolism [
12]. S-adenosylmethionine (SAM), which is synthesized in the methionine cycle from various dietary precursors, is a potent aminopropyl and methyl donor within the one-carbon cycle and serves as the main substrate for the methylation of DNA, associated proteins, and RNA [
13,
14].
Epigenetic programming is sensitive to nutritional factors, especially during the prenatal period and early phases of life of animals [
12,
15]. Thus, nutritional alterations can induce “metabolic imprinting”, a mechanism whereby adequate nutrition in the first months after birth alters glucose metabolism and causes long-term beneficial effects on the health and performance of animals [
16]. This mechanism proposes critical periods of development, known as “nutritional windows”, in which changes in the DNA methylation pattern and gene expression can permanently modify the animal’s metabolism [
17].
At present, there are no omics studies that have associated epigenetics, particularly DNA methylation, with gene expression in Nellore cattle submitted to early weaning and energy supplementation. In this context, we examined the effects of distinct nutritional management protocols on DNA methylation patterns in the Longissimus thoracis muscle of Nelore calves during the perinatal phase, and how these epigenetic modifications may have influenced the gene expression profile.
3. Discussion
In the present study, the comparison of groups of Nellore calves submitted to EW and energy supplementation and calves submitted to CW revealed an important correlation between methylated DNA regions and gene expression, with many of the genes being associated with fat metabolism. It is known that the epigenome is sensitive to nutritional factors [
19]. Comparison of the EW vs CW groups showed a low percent variation in methylation. This finding can be explained by the conserved methylation profile of cattle [
19], as well as by the fact that the animals were exposed to the same management conditions from birth to 120 days of age. However, the exposure of EW animals to a period of “nutritional window” (121 to 205 days) under energy supplementation exerted an epigenetic effect, with the identification of 485 DMRs. This finding might be related to the presence of so-called epigenetic modifiers – molecules that are important methyl donors and/or cofactors in DNA methylation reactions such as methionine, choline, folate, and vitamin B12 [
20]. Thus, the hypermethylation of genes during the postnatal metabolic programming that occurred in the EW group may be the result of nutritional factors. Since these animals received a high-protein diet (20%), the availability of the compounds mentioned above, which are recognized as methyl donors for DNA methylation, was greater. These compounds enter one-carbon metabolism at different sites but are ultimately converted to the methyl group donor SAM, which provides one methyl group for DNA methylation [
21]. In one-carbon metabolism, most methyl groups are derived from choline (60%), followed by methionine (20%) and folate (10-20%) [
22]. Methionine is an essential amino acid that is regenerated from homocysteine. In addition to its importance for protein synthesis, when converted to SAM, methionine participates in more than 100 methylation reactions in the body [
23]. Thus, variations in dietary methionine may affect DNA methylation [
24]. However, some studies suggest that there is no simple correlation between the concentration of methyl donors and DNA methylation and that complex mechanisms of competitive inhibition of DNA methyltransferases may contribute to DNA imprinting [
12,
25].
Bos indicus breeds are known for their lower meat tenderness and intramuscular fat deposition when compared to
Bos taurus [
26]. Thus, the identification of gene pathways that could improve fat deposition and meat tenderness in
Bos indicus is an important strategy for this animal production sector [
26]. Within this context, studies of European cattle breeds suggested that early weaning management leads to changes in DNA methylation patterns and in the expression of genes involved in metabolism [
6,
8]. In the study by [
6], metabolic programming due to early weaning resulted in significant changes in carcass fat deposition, suggesting that early postnatal metabolic imprinting events can be explored as a management tool to improve the value and quality of meat based on increased marbling. In our study, EW animals exhibited 53 DMGs/DEGs. These results are consistent with the effects of early postnatal metabolic programming on the expression of genes levels involved in numerous biological processes, including those related to the positive regulation of lipid metabolic processes.
Among the main genes exhibiting significant epigenetic changes and alteration in expression due to early weaning, we highlight the
ADCY3 and
AKT1 genes. The
ADCY3 gene is involved in the regulation of different processes, including embryogenesis, hormone secretion, glycogen degradation, smooth muscle relaxation, cardiac muscle contraction, and olfaction [
27,
28,
29]. In general, adenylyl cyclases (ADCYs) are enzymes that catalyze the synthesis of cAMP from adenosine triphosphate (ATP) [
30]. The present results showed that the cAMP signaling pathway was enriched in the EW group. Cyclic AMP is a second messenger involved in intracellular signal transduction that is associated with the function of kinases in different biochemical processes, including the regulation of carbohydrate and lipid metabolism [
30,
31]. Increases in cAMP ultimately activate a number of enzymes and transcription factors that regulate satiety and metabolism [
32].
In this scenario, we evaluated some pathways linked to energy metabolism and regulated by overexpression of
ADCY3 regarding genes differentially expressed in the EW group. The altered expression of
ADCY3 is related to an increase in protein kinase cAMP-activated catalytic subunit alpha (PRKACA). This kinase signals changes in adipogenesis, thermogenesis, fatty acid oxidation, and insulin resistance pathways [
28]. However, the evaluation of DEGs in this study did not provide solid data on changes in adipogenesis, thermogenesis, or fatty acid oxidation; however, genes involved in de novo lipogenesis were upregulated.
Adipogenesis consists of the differentiation of mesenchymal stem cells into preadipocytes and the proliferation of these cells, the differentiation of preadipocytes into adipocytes, and the conversion to lipid-assimilating cells [
33,
34,
35]. The mechanisms that control adipogenesis in skeletal muscle are largely regulated by transcription factors of the enhancer binding protein (CEBP) and peroxisome proliferator-activated receptor γ (
PPARg) family [
35,
36,
37]. The CEBP and
PPARg genes were found to be overexpressed but not differentially methylated in the EW group. However, various other genes that are important for adipogenesis did not show consistent changes in expression, including bone morphogenetic protein 4 (
BMP4), PR domain containing protein 16 (
PRDM16), fibroblast growth factor 10 (
FGF10), and fatty acid binding protein 4 adipocyte (
FABP4). For example,
PRDM16 acts as a switch between the myogenic lineage and brown adipocytes [
38,
39]. Cells expressing
PRDM16 do not undergo differentiation to the myogenic lineage. Thus, although the EW group exhibited a significant change in
ADCY3, the alteration in the expression of genes involved in adipogenesis was less consistent. In fact, it has been proposed that adipogenesis starts in the second half of gestation in ruminants and reaches is highest activity during the perinatal period [
35]. In the present study, the postnatal period was the nutritional window, i.e., after the main phase of adipocyte differentiation.
Beta-oxidation and thermogenesis are other processes that can be influenced by excess ADCY3 and PRKAC
A [
28]. Although the expression of these regulators was altered in the EW group, no significant changes were observed in the expression of the hormone-sensitive lipase (HSL or LIPE) or carnitine palmitoyltransferase (
CPT1 and
CPT2) gene, indicating the lack of a difference in beta-oxidation between groups. Cyclic AMP-responsive element-binding protein 1 (
CREB1) is regulated by PRKACA during thermogenesis. Likewise, neither the expression of
CREB1 nor of other genes involved in thermogenesis was significantly altered in the EW group.
The insulin resistance pathway has also been suggested to respond to cAMP signals [
28]. According to [
6], an increase in gluconeogenic substrates for ruminal fermentation and in the concentration of insulin in response to circulating glucose leads to greater cellular uptake and utilization of glucose for fat deposition in intramuscular adipocytes proliferated by imprinting, increasing marbling during the growth phase. The gene expression data obtained for the EW group showed increased expression of protein kinase AMP-activated catalytic subunit alpha 1 (PRKAA1 or AMPK), an important mediator of the insulin resistance pathway, along with phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit beta (PI3KCB) and AKT serine/threonine kinase 1 (AKT1) [
28]. AKT1 is responsible for the regulation of glucose uptake by mediating the insulin-induced translocation of solute carrier family 2 member 4 or glucose transporter type 4 (SLC2A4 or GLUT4) to the cell surface [
40].
AKT1 was hypermethylated in the promoter and intron regions in the EW group; however, no significant alteration in gene expression was observed. Furthermore, the analysis of DEGs in this study revealed no alteration.
AKT signaling has been reported to prevent the degradation of sterol regulatory element binding transcription factor 1 (
SREBP1) and thus to promote de novo lipid synthesis [
41]. This gene belongs to a family of transcription factors [
41] and encodes an important transcriptional regulator of genes involved in lipid synthesis and metabolism, including fatty acid synthase (
FASN) and ATP-citrate lyase (
ACLY) [
42,
43].
SREBP1 was overexpressed, demonstrating that
AKT1 hypermethylation did not compromise gene expression. Other factors may be involved in the positive regulation of de novo lipid synthesis in the EW group.
Indeed, the DEG data demonstrated overexpression of genes involved in the de novo lipid synthesis pathway, including acetyl-CoA synthetase (
ACSS2),
ACLY, acetyl-CoA carboxylase (
ACACA), stearoyl-CoA desaturase (
SCD), and
FASN, as well as the transcription factor SREBF-1, as also reported by [
44]. An intriguing question raised here is how the postnatal metabolic programming performed in this study has stimulated lipogenesis. According to the review by [
35], fat synthesis requires the incorporation of triglycerides into the adipose tissue of the animal after absorption of dietary fatty acids or de novo synthesis of other fatty acids. Acetate – a volatile fatty acid produced during ruminal fermentation – is the main precursor molecule for the synthesis of fatty acids in ruminants [
45]. After ruminal absorption and already in the adipocyte, acetyl-CoA synthetase (ACSS2) converts acetate into acetyl-CoA [
46] which, in the presence of reduced nicotinamide adenine dinucleotide phosphate (NADPH), is crucial for de novo fatty acid synthesis [
35].
Interestingly, the cAMP signaling pathway enriched in this study regulated various other processes through ADCYs. One such process involves the sodium/potassium-transporting ATPase subunits gamma, alpha-1 and beta-2 (KEGG
Bos taurus: 281773). These proteins, in turn, participate in digestion and absorption pathways (bta04973 and bta04974) in the intestinal epithelium, including acetate absorption in colonic epithelial cells. Although the expression of ATPases was not obtained for intestinal cells in this study, indirect evidence of increased acetate absorption and de novo lipid synthesis stems from the overexpression of
FABP4. Expressed mainly in adipocytes, the product of the
FABP4 gene is a cytoplasmic protein that binds to long-chain fatty acids and other hydrophobic ligands and plays a role in uptake, transport, and hydrolysis [
47,
48]. Increased expression of this gene has been associated with greater intramuscular fat deposition, carcass weight, and fatty acid composition in beef cattle [
44,
49,
50].
Taken together, the data reinforce the proposals of greater fat deposition in cattle submitted to early weaning [
6,
8,
44]. Furthermore, emerging evidence indicates that postnatal metabolic programming through epigenetic modification mainly exerts an effect on the ADCY
3 receptor which, in turn, leads to numerous changes in pathways linked to cAMP signaling.
Authors should discuss the results and how they can be interpreted from the perspective of previous studies and of the working hypotheses. The findings and their implications should be discussed in the broadest context possible. Future research directions may also be highlighted.