METTL3 Regulated the Meat Quality of Rex Rabbits by Controlling PCK2 Expression via a YTHDF2–N6-Methyladenosine Axis

N6-methyladenosine (m6A) is the most prevalent internal mRNA modification in eukaryotes. The M6A modification plays an important role in transcription and cell function. The mechanism by which m6A modification regulates meat quality remains elusive. In this study, gene knockout and overexpression were used to explore m6A-modified regulation of meat quality. The content of PCK2 in blood increased significantly with the increase of Rex rabbits’ age. PCK2 expression levels in the longissimus lumborum and liver also increased significantly with the increase of Rex rabbits’ age. However, the expression level of PCK2 showed no significant difference in adipose tissue. In cell experiments, we found that METTL3 inhibited adipocyte differentiation by targeting the PCK2 gene via the recognition function of YTHDF2. Finally, the results of correlation analysis showed that PCK2 expression was positively correlated with intramuscular fat, whereas PCK2 expression was negatively correlated with total water loss rate at three different stages. In addition, PCK2 expression was also negatively correlated with reduced pH value at 75 and 165 days. Intramuscular fat content, pH and muscle water holding capacity are the main factors affecting the taste and flavor of muscle. Therefore, N6-methyladenosine regulated muscle quality by targeting the PCK2 gene.


Introduction
Rabbit meat has been widely accepted as a healthy food worldwide due to its high nutritional value and easy digestibility, which make it especially suitable for children and elderly people. It contains not only a high content of protein, essential amino acids, mineral and trace element components such as selenium and cobalt but also a low fat and cholesterol level [1]. However, the low intramuscular fat content reduced rabbit meat consumption and hindered the development of the rabbit industry. Appropriately increased IMF (intramuscular fat) content can improve the meat quality, including color, tenderness, flavor and juiciness [2][3][4][5]. In addition, greater intramuscular fat can lead to an increased postmortem pH [6]. Besides, studies on pork showed that the higher the pH value, the smaller the cooking loss [7]; the pH value has a significant effect on the tenderness, color, water retention and moisture distribution of muscle fibers [8]. Thus, the formation of fat has a great impact on the taste, flavor and juiciness of the muscle. Obesity or adipogenesis is characterized by the process of cell proliferation and differentiation at the cellular level. Therefore, since the manipulation of adipocyte differentiation could be a promising strategy for the flavor improvement of rabbit meat, improving our understanding of the molecular mechanism of adipogenesis is of major significance.
N6-methyladenosine (m 6 A) occurs in the sixth nitrogen atom of the RNA adenylate and is the most abundant internal modification in eukaryotic mRNAs [9]. M 6 A is installed by a multicomponent methyltransferase complex consisting of methyltransferaselike 3 (METTL3), METTL14 and Wilms tumor 1-associated protein (WTAP) and erased by

Cell Culture and Transfection
The perirenal fat of 0-day-old rabbits was isolated immediately after slaughter and placed in PBS containing 4% double antibody. Subsequently, the fat was digested with 0.1% collagenase type I (GIBCO, Carlsbad, CA, USA) in a water bath at 37 • C, and the growth medium (DM/F12, 10% fetal bovine serum, 2% penicillin-streptomycin) was used to culture preadipocytes. The cells were cultured in the incubator (5% CO 2 , 37 • C) using culture bottles after filtration with 70 and 40 nm cell sieves. Cell transfection was achieved by using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA) for siRNA following the manufacturer's protocols when cells grow to 80-90% in the culture bottle or cell dish. Then, an adipogenic cocktail (0.5 mM 3-isobutyl-1-methylxanthine, 10% fetal bovine serum, 1 µM dexamethasone and 1.7 µM insulin) was added into the growth medium to induce differentiation. After 3 days, the culture medium was changed to maintain differentiation. After another 3 days, the growth medium (DM/F12, 10% fetal bovine serum, 2% penicillinstreptomycin and 1.7 µM insulin) was changed to maintain differentiation.

RT-qPCR
Total RNA was extracted using TRIzol reagent (Invitrogen, CA, USA) and reverse transcribed into cDNA by the Prime Script RT Reagent Kit with genomic DNA Eraser (Takara Bio, Saint-Germain-en Laye, France). The QRT-PCR experiments were carried out with 2.5 ng/µL cDNA template, SYBR Premix Ex Taq II and Rox Plus and 10 pmol/µL forward/reverse primer on a Quant Studio TM (Applied Biosystems by Thermo Fisher Scientific, Waltham, MA, USA). β-Actin was used as an internal control. All the primers used for qPCR are listed in Table 1.

Measurement of PCK2 in Blood
Concentrations of PCK2 were determined according to the manufacturer's instructions using PCK2 ELISA kits (Ruixinbio, Quanzhou, China). The actual sensitivities of PCK2 were typically <0.1 ng/mL. This PCK2 ELISA kits only recognizes PCK2 and is not disturbed by other similar substances.

Gene-Specific m 6 A qPCR
Total RNA was extracted using TRIzol regent (Invitrogen, CA, USA). Then, the Magna MeRIP m 6 A Kit (Millipore) was used to examine m 6 A modifications on individual genes according to the manufacturer's instructions. mRNA was purified by using mRNA miniprep kit (SIGMA, Cat. No. MRN10). Briefly, 0.5 µg of mRNA was removed and placed into a new microcentrifuge tube labeled "RNA input". The input sample was then kept at −80 • C. The sample was later used for comparison in RT-PCR methods. Subsequently, RNase inhibitor and 5 × IP buffer were used to enrich the m 6 A modification in mRNA. Finally, PCR analysis was performed after elution with eluent.

Western Blotting
Cells were harvested using RIPA lysis buffer (CWBIO, Jiangsu, China) containing protease inhibitor (CWBIO, Jiangsu, China) and incubated on ice for 30 min. Subsequently, WB was performed using the method used in the previous study [24]. The membranes were incubated with the following primary antibodies: PCK2 (Shenggong, Shanghai, China), METTL3 (Shenggong, Shanghai, China), FTO (Fitzgerald, Acton, USA) and β-actin (Absin, Shanghai, China). Finally, the images were obtained with a Bio-Rad GelDoc system equipped with a Universal Hood III (Bio-Rad, California, USA), and the integrated optical density (IOD) was calculated using Gel-Pro Analyzer 4.0.0.4. Actin was used as an internal control.

Measurement of Triglyceride Content and Oil Red-O Staining
The contents of triglyceride (TG) in adipocytes were measured using the TG Assay Kit (Applygen, Beijing, China) according to the manufacturer's protocol. The method of oil red-O staining was the same as that used in previous articles [25]. Finally, oil red-O was eluted from the stained cells with isopropanol and quantified by measuring the optical density values at 510 nm wavelength.

Determination of Meat Quality Traits
Intramuscular fat was measured by Soxhlet extraction method [26]. Briefly, the meat sample was dehydrated to a constant amount in the oven and then crushed. Then, the sample weight, sample and paper package weight and the total weight after extraction were recorded as M1, M2 and M3, respectively. The formula for intramuscular fat content (w) is as follows: w = (M2 − M3)/M1 × 100%. The longissimus lumborum was used to measure the pH values at 45 min and 24 h after rabbits were sacrificed by an insert electrode pH-star according to Blasco's method [23]. A meat moisture meter was used to determine the total water content according to the instructions. For the determination methods of the cooked meat rate and the drip loss, refer to the previous study [27].

Statistical Analysis
All data are presented as the mean ± standard deviation (SD). Differences in the mean values between 2 groups were tested for significance with a Student's t-test, and between 3 groups, the significance was determined with one-way ANOVA using GraphPad Prism7 (GraphPad Software, La Jolla, CA, USA). p < 0.05 and p < 0.01 were deemed to be significant and highly significant, respectively. Correlation analyses were performed using SAS9.2 (Statistical Analysis System; SAS Institute Inc., Raleigh, NC, USA), the method used was Pearson's correlation coefficient. Two-tail Student's t-test was used to analyze the significance of the different levels. p-values of less than 0.05 and 0.01 were considered to be significantly and extremely significantly correlated.

Analysis of the Content of PCK2 in Blood, the Expression of the PCK2 Gene and m 6 A Modification of the PCK2 Gene in Different Tissues of Rex Rabbits
First, by measuring the content of PCK2 in the blood of Rex rabbits at different stages, it was found that the PCK2 content increased with age ( Figure 1A). To investigate whether the expression level of the PCK2 gene is related to fat deposition, the expression levels of PCK2 were detected in the perirenal fat, the longissimus lumborum and the liver of 35-day-old, 75-day-old and 165-day-old Rex rabbits. The results showed that the expression level of PCK2 was not significantly different in the perirenal fat of different stages ( Figure 1B), while the expression levels of PCK2 in the longissimus lumborum and liver of old Rex rabbits were significantly higher than that of young Rex rabbits (p < 0.01) ( Figure 1C,D). Finally, we detected the methylation of the PCK2 gene in the longissimus lumborum and perirenal fat of newborn rabbits and found that the modification of the PCK2 gene was significantly different between the longissimus lumborum and the perirenal fat ( Figure 1E). Together, these results showed that the expression of the PCK2 gene may be modified by m 6 A and related to fat deposition.
Prism7 (GraphPad Software, La Jolla, CA, USA). p < 0.05 and p < 0.01 were deemed to be significant and highly significant, respectively. Correlation analyses were performed using SAS9.2 (Statistical Analysis System; SAS Institute Inc., NC, Raleigh, USA), the method used was Pearson's correlation coefficient. Two-tail Student's t-test was used to analyze the significance of the different levels. p-values of less than 0.05 and 0.01 were considered to be significantly and extremely significantly correlated.

Analysis of the Content of PCK2 in Blood, the Expression of the PCK2 Gene and m 6 A Modification of the PCK2 Gene in Different Tissues of Rex Rabbits
First, by measuring the content of PCK2 in the blood of Rex rabbits at different stages, it was found that the PCK2 content increased with age ( Figure 1A). To investigate whether the expression level of the PCK2 gene is related to fat deposition, the expression levels of PCK2 were detected in the perirenal fat, the longissimus lumborum and the liver of 35day-old, 75-day-old and 165-day-old Rex rabbits. The results showed that the expression level of PCK2 was not significantly different in the perirenal fat of different stages ( Figure  1B), while the expression levels of PCK2 in the longissimus lumborum and liver of old Rex rabbits were significantly higher than that of young Rex rabbits (p < 0.01) ( Figure 1C,D). Finally, we detected the methylation of the PCK2 gene in the longissimus lumborum and perirenal fat of newborn rabbits and found that the modification of the PCK2 gene was significantly different between the longissimus lumborum and the perirenal fat ( Figure  1E). Together, these results showed that the expression of the PCK2 gene may be modified by m 6 A and related to fat deposition.

Knockout of YTHDF2 Gene Partially Restored the Effect of METTL3 Knockout on Adipocyte Differentiation
To investigate the regulation of YTHDF2 and METTL3 on adipocyte differentiation, we simultaneously interfered with YTHDF2 and METTL3 in preadipocytes. The results indicated that METTL3 and YTHDF2 were successfully knocked out in the experimental group (Figure 2A-D). Knockout of the METTL3 gene promoted the differentiation of preadipocytes. However, YTHDF2 knockout inhibited the promoting effect of METTL3 knockout on differentiation of preadipocytes. Downexpression of YTHDF2 in METTL3 knockdown preadipocytes significantly reduced the lipid droplets (p < 0.01) ( Figure 2E,F), accompanied by significantly decreased TG content (p < 0.01) ( Figure 2G). Similarly, knockout of YTHDF2 partly decreased the expression of adipogenic marker PPARγ, C/EBPa and FABP4 genes in METTL3 knockdown preadipocytes (p < 0.01) ( Figure 2H-J). Consistently, YTHDF2 partly influenced the regulation of METTL3 on the differentiation of preadipocytes.

Effect of METTL3 and YTHDF2 on the Expression Level of the PCK2 Gene
To determine whether METTL3 regulates the PCK2 gene m 6 A-YTHDF2 dependently, we first detected the expression of PCK2 in METTL3 knockdown adipocytes and found that the expression levels of PCK2 were significantly higher than those of the NC group (p < 0.01) ( Figure 3A-C). However, the fold enrichment of PCK2 methylation modification decreased significantly in the METTL3-depleted adipocytes (p < 0.01) ( Figure 3D). Next, we assessed the role of YTHDF2 by knocking down METTL3 and YTHDF2. Knockdown of YTHDF2 significantly decreased the mRNA and protein levels of the PCK2 gene in METTL3 knockdown cells (p < 0.01) ( Figure 3E-G). In addition, knockdown of YTHDF2 partly recovered the fold enrichment of PCK2 methylation modification ( Figure 3H). These results demonstrated that METTL3 m 6 A-YTHDF2 dependently regulates the PCK2 gene.

Effect of PCK2 on the Differentiation of Preadipocytes
To understand the mechanism by which PCK2 regulates the differentiation of preadipocytes, we first interfered with the PCK2 gene in preadipocytes ( Figure 4A-C). In PCK2knockdown adipocytes, we found that the number and volume of lipid droplets was significantly reduced using oil red-O staining and determination of TG content ( Figure 4D-

Effect of PCK2 on the Differentiation of Preadipocytes
To understand the mechanism by which PCK2 regulates the differentiation of preadipocytes, we first interfered with the PCK2 gene in preadipocytes ( Figure 4A-C). In PCK2knockdown adipocytes, we found that the number and volume of lipid droplets was significantly reduced using oil red-O staining and determination of TG content ( Figure 4D-F). At the same time, the expression of adipogenic marker PPARγ, C/EBPa and FABP4 genes was lower in PCK2 knockdown preadipocytes than those in the NC group (p < 0.01) ( Figure 4G-I). Overall, PCK2 played an important role in regulating the differentiation of preadipocytes.

Correlation Analysis between Expression Level of the PCK2 Gene in the Longissimus Lumborum and Meat Quality Traits
The correlation analysis is plotted in Table 2. The correlation coefficients between intramuscular fat content and PCK2 expression in 35-day-old, 75-day-old and 165-day-old

Correlation Analysis between Expression Level of the PCK2 Gene in the Longissimus Lumborum and Meat Quality Traits
The correlation analysis is plotted in Table 2. The correlation coefficients between intramuscular fat content and PCK2 expression in 35-day-old, 75-day-old and 165-day-old rabbits were 0.94 (p < 0.01), 0.85 (p < 0.05) and 0.82 (p < 0.05), respectively. Reduced pH value was negatively correlated with PCK2 expression only in 75-day-old and 165-day-old rabbits, and the correlation coefficients were 0.85 (p < 0.05) and 0.88 (p < 0.05), respectively. The total water loss rate was negatively correlated with the expression level of the PCK2 gene, and the correlation coefficients were 0.87 (p < 0.05), 0.79 (p < 0.05) and 0.74 (p < 0.05) in 35-day-old, 75-day-old and 165-day-old rabbits, respectively.

Discussion
In this study, we found that the level of PCK2 in blood also increased with age ( Figure 1A). Previous studies showed that there was a positive correlation between fat deposition and age in growing rabbits [27]. It indicated that PCK2 may regulate fat deposition. To investigate whether PCK2 is associated with fat deposition, we detected the expression levels of PCK2 in the perirenal fat, longissimus lumborum and liver of Rex rabbits and found that the expression level of PCK2 increased with age in the longissimus lumborum and liver tissues ( Figure 1C,D). However, there was no significant difference in the expression level of PCK2 in the perirenal fat at different stages ( Figure 1B). The liver is the main site of fat synthesis [28]. Due to the need of adaptability, many animals deposit a large amount of fat in the liver; the liver returns to its original state after the stored fat is consumed by energy supply, and the whole change process is reversible [29,30]. In addition, the study showed that the volume of adipocytes gradually increased and the rate of fat deposition increased rapidly in lambs entering the fattening stage [30]. These results further indicated that PCK2 played an important role in fat deposition. At the same time, we found that the PCK2 gene was modified by m 6 A in the longissimus lumborum and perirenal adipose tissues of Rex rabbits.
The profiles and function of METTL3-mediated mRNA m 6 A are different depending on the tissue and the developmental stage [31][32][33]. Indeed, it has also been reported that the expression of many genes is regulated by either METTL3 or FTO alone [34]. A previous study also suggested the functional importance of the heterodimer formation of METTL3 and METTL14 in adipocyte differentiation [35].
In this study, we found that interference with METTL3 can promote the differentiation of adipocytes (Figure 2) and the expression level of the PCK2 gene ( Figure 3A-C). METTL3 inhibits hepatic insulin sensitivity and promotes fatty acid metabolism [36]. A loss of the initial insulin suppression of glucose output in vitro and an associated increased activity of hepatic PCK2 has been reported [37]. It was recently demonstrated that hepatic PCK2 mRNA expression increased following the induction of chronic hypoglycemia in fetal sheep and that this was associated with a decrease in circulating insulin and an increase in plasma cortisol [38]. PPARγ agonists may sensitize the response of peripheral tissues to insulin by inhibiting the expression of 11β-HSD1 in adipocytes [39]. 11β-HSD1-knockout mice lost the ability to convert 11-dehydrocortisone into cortisol and inhibited the activity of PEPCK [40]. Cortisol has been shown to play a key role in the upregulation of hepatic PCK2 activity and expression in the late-gestation fetus [41,42], so the increase in hepatic HSDL1 mRNA expression and intrahepatic cortisol production may stimulate increased PCK2 mRNA expression. RNA N6-methyladenosine methyltransferase METTL3 facilitates colorectal cancer by activating the m 6 A-GLUT1-mTORC1 axis [43]. There is a positive correlation between the expression of GLUT-1 and PCK2 in laryngeal squamous cell carcinoma [44]. These results indicated METTL3 can regulate the expression of the PCK2 gene. However, the specific regulatory mechanism is still unclear. In this study, we found that mRNA and protein levels of the PCK2 gene increased significantly, whereas m 6 A methylation of the PCK2 gene decreased significantly after transfection with si-METTL3. Based on the above results, we inferred that METTL3 regulated the expression level of PCK2 by m 6 A methylation.
METTL3-mediated mRNA m 6 A modification can be recognized by the individual m 6 A reader proteins, which play key roles in controlling gene expression. Early studies showed that YTHDF2-binding increases mRNA decay [45]. In addition, other roles for these reader proteins are rapidly emerging in the regulation of specific mRNAs in different types of cells during different stimuli [46][47][48]. Moreover, the study revealed a novel mechanism wherein downregulated spinal cord METTL3 in coordination with YTHDF2 contributes to the modulation of inflammatory pain by stabilizing the upregulation of TET1 in spinal neurons [49]. METTL3 epigenetically repressed YPEL5 in an m 6 A-YTHDF2-dependent manner by targeting the m 6 A site in the coding sequence region of the YPEL5 transcript [50]. In this study, we found that YTHDF2 silencing could partially reverse the phenotype induced by METTL3 knockdown, including differentiation of adipocytes and methylation level of the PCK2 gene ( Figure 2, Figure 3). These results suggested METTL3 regulates adipocyte differentiation by targeting the PCK2 gene in an m 6 A-YTHDF2-dependent manner.
In order to explore the mechanism by which PCK2 regulates adipogenesis, we knocked out the PCK2 gene in adipocytes and found that the differentiation of adipocytes was significantly inhibited (Figure 4). It has been found that PCK1 and PCK2 can be used as candidate genes of obesity, which can participate in the synthesis of glycerophosphate and promote the accumulation of fat in human body [51]. In addition, the loss of PCK2 in islets can damage insulin secretion, which suggested that the activity of PCK2 is related to insulin secretion [52]. In the majority of these studies [53][54][55], the degree of obesity and not the direct measurement of insulin resistance was used to identify individuals as insulin resistant vs. insulin sensitive. PCK2 was upregulated by effectors of this pathway by recruiting ATF4 to a consensus AARE site located at the PCK2 proximal promoter [56]. A clue to the mechanism for the integrative role of PCK2 in cancer metabolism arises from its transcription regulation by ATF4 under nutrient stress [57,58]. ATF4 regulates age-related and diet-induced obesity as well as glucose homeostasis in mammals and has conserved metabolic functions in flies [59]. In addition, the expression level of PCK2 was higher in Rex rabbits with morefat deposition. These results indicated that PCK2 can promoted adipocyte differentiation.
Adipocyte differentiation is a necessary process of fat deposition. In order to explore the relationship between the expression level of PCK2 and intramuscular fat, we analyzed the correlation between the expression level of PCK2 and intramuscular fat and found that they are positively correlated (Table 2). Previous study has shown that PCK2 can promote adipogenesis. In summary, PCK2 can regulate the production of intramuscular fat. Studies have shown that intramuscular fat is positively correlated with the pH of muscle [60]. The faster the pH value drops, the faster the meat spoils. Therefore, a reduced pH value is an important indicator of good meat quality. In this study, we found that the expression level of PCK2 was negatively correlated with reduced pH value in rabbits at 75 and 165 days of age. At the same time, there was a strong correlation between intramuscular fat and muscle system hydraulics [61]. Water in muscle also affects meat production. In this study, we found the expression level of PCK2 was negatively correlated with the total water loss rate. Meat with low drip loss is more easily accepted by consumers [62]. A study reported that consumers are attracted to rabbit meat according to its healthiness, sensory properties and hedonistic quality (variability on visual appeal) [63]. Intramuscular fat, pH and muscle water holding capacity are important indicators affecting muscle flavor. At the same time, intramuscular fat is positively correlated with sensory tenderness, juiciness and flavor [64]. Therefore, it can be concluded that PCK2 plays an important role in muscle flavor.

Conclusions
METTL3 decreased the expression level of the PCK2 gene in an m 6 A-YTHDF2dependent manner. The expression level of the PCK2 gene was negatively correlated with reduced pH value and total water loss rate. However, intramuscular fat was positively correlated with the expression of the PCK2 gene. As shown in Figure 5, METTL3 affects meat quality by regulating the expression of PCK2. The study provides a scientific, molecular theoretical basis for cultivating rabbits with good meat quality. Intramuscular fat, pH and muscle water holding capacity are important indicators affecting muscle flavor. At the same time, intramuscular fat is positively correlated with sensory tenderness, juiciness and flavor [64]. Therefore, it can be concluded that PCK2 plays an important role in muscle flavor.

Conclusion
METTL3 decreased the expression level of the PCK2 gene in an m 6 A-YTHDF2-dependent manner. The expression level of the PCK2 gene was negatively correlated with reduced pH value and total water loss rate. However, intramuscular fat was positively correlated with the expression of the PCK2 gene. As shown in Figure 5, METTL3 affects meat quality by regulating the expression of PCK2. The study provides a scientific, molecular theoretical basis for cultivating rabbits with good meat quality.