1 Introduction

It has been well documented that low crude protein (CP), amino acid (AA)-supplemented diets may reduce feed costs and nitrogen excretion (Kerr et al., 2003; Zarate et al., 2003; Fairbrother et al., 2005; Deng et al., 2007a; 2007b; Lallès et al., 2007; Hou et al., 2008; Kang et al., 2008; Yue and Qiao, 2008; Deng et al., 2009; Opapeju et al., 2009; Gallo et al., 2014; Gloaguen et al., 2014; Liu et al., 2014; Recktenwald et al., 2014). Therefore, the CP level in the diet of weaned pigs has been suggested to be one of the main factors that affect their growth performance (Gallo et al., 2014), feed efficiency, and gastrointestinal health (Yin and Tan, 2010; Acciaioli et al., 2011). An increase in the entry of nutrients (e.g. dipeptides, tripeptides, arginine, lysine, and histidine) from the lumen of the small intestine into the enterocyte can enhance tissue protein synthesis and improve the efficiency of utilization of dietary nutrients, and it can selectively modulate the gene expression of AA transporters in intestinal cells (Wang et al., 2009). Some studies have reported that improving the efficiency of protein use and maintaining performance can be achieved by reducing the CP content of the diet while ensuring the AA balance (Kerr and Easter, 1995; Le Bellego and Noblet, 2002; Kerr et al., 2003; Opapeju et al., 2008; Gloaguen et al., 2014). However, Nyachoti et al. (2006) demonstrated that piglet performance may be impaired when dietary CP levels are reduced from 230 to 190 or 170 g/kg, as the AA deficiency may change the efficiency of AA absorption and transportation (Tuitoek et al., 1997; Tan et al., 2009; 2011; Gallo et al., 2014). The present study investigated the effects of reducing dietary CP while supplementing with essential AA on the gene expression of AA transporters in weaned piglets.

2 Materials and methods

2.1 Animals and experimental treatment

The experimental protocol was approved by the Protocol Management and Review Committee of the Institute of Subtropical Agriculture, Chinese Academy of Sciences (CAS). Pigs were cared for according to the guidelines of the Institute of Subtropical Agriculture on Animal Care, CAS.

Eighteen piglets (Duroc×Landrace×Yorkshire, female), from the experimental field of the animal observation station in southern center of China, were weaned at 28 d of age, but the experiment started after a 7-d adaptation period when the piglets were weighed to determine an initial mean body weight (BW; mean±standard deviation (SD), (9.57±0.64) kg), and randomly assigned to three different groups (six pigs per group). Each of the groups was fed diets with different levels of CP (14%, 17%, and 20%), respectively. Their ingredients are shown in Tables 1 and 2. The dietary treatment met the National Research Council (NRC; 2012) nutrient specifications for 11 to 20 kg BW pigs. All of the animals were housed individually in cages, and had free access to feed and drinking water at all times throughout the experimental period.

Table 1
figure Tab1

Composition and nutrient levels of diets a

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Table 2
figure Tab2

CP content and AA composition of feed ingredients used in formulating the experiment diets a

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2.2 Relative quantification of mRNA expression of AA transporters

Primers for the selected genes (Table 3) were designed using Oligo 6.0 software (Molecular Biology Insights, CO, USA). After the quantity of complementary DNA (cDNA) was determined by a NanoDrop® ND-1000 (NanoDrop Technologies, Rockland, DE, USA), real-time quantitative polymerase chain reaction (PCR) analyses were performed on an ABI 7900HT Fast Real-Time PCR System (Applied Biosystems, CA, USA) with a total volume of 10 μl containing 5 ng of cDNA, 5 μl SYBR Green mix, 0.2 μl ROX Reference Dye (50×), and 0.2 μl each of the forward and reverse primers (Yao et al., 2012). The following protocol was used: (1) pre-denaturation (30 s at 95 °C); (2) amplification and quantification, repeated 40 cycles (5 s at 95 °C, 30 s at 60 °C); and (3) a melting curve program (extension at 72 °C). β-Actin was used as an internal reference gene to normalize target gene transcript levels. The efficiency of the real-time reverse-transcription PCR was determined by the amplification of a dilution series of cDNA according to the equation 10(-1/slope). Target mRNA and β-actin mRNA were amplified with comparable efficiencies (Wang et al., 2009; He et al., 2013). Negative controls, in which cDNA was replaced by water, were also tested (Liu et al., 2012).

Table 3
figure Tab3

Primers used for real-time PCR analysis

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2.3 Statistical analysis

All data were subjected to a one-way analysis of variance (ANOVA; SAS Version 9.2) and data were presented as mean±standard error of the mean (SEM). Differences with P values of <0.05 were considered to be statistically significant.

3 Results

3.1 Growth performance of piglets fed diets with different levels of CP (internal data)

The growth performance of weaned piglets fed diets with different CP levels is unpublished data. After the animals were fed different diets for 45 d, significant differences were noted in the average daily gain (ADG) among the three groups (P<0.01), and the 20% CP group had a significantly higher ADG than the other groups (P<0.01). Average daily feed intake (ADFI) showed the same trend as ADG: the 20% CP group had the highest ADFI and ADG, and 14% CP group had the lowest ADFI and ADG. However, there was no significant difference in ADFI between the 17% and 20% CP groups. While the 20% CP group had the lowest feed to gain (F/G) ratio, there were no significant differences between the 14% and 17% CP groups.

3.2 mRNA expression of AA transporters

Changes in the abundance of ASCT2, EAAC1, B0+AT, y+LAT1, 4F2hc, and ATB0 mRNA in the jejunum of the piglets are shown in Table 4. While the abundance of ASCT2, EAAC1, B0+AT, and ATB0 mRNA significantly differed among the three groups (P<0.05), there were no differences in the expression of y+LAT1 and 4F2hc mRNA (P>0.05). Compared with the values in the 14% CP group, the expression of ASCT2, 4F2hc, and ATB0 mRNA in the jejunum was increased by 23.00%, 12.00%, 6.00% and 48.00%, 47.00%, 56.00% in the 17% and 20% CP groups, respectively. The 17% CP group showed the lowest expression of EAAC1 mRNA, and the highest expression of B0+AT mRNA. Furthermore, while the expression of y+LAT1 in the 17% and 20% CP groups was decreased by 13.00% and 8.00%, respectively, compared with that in the 14% CP group, there were no significant differences among the three groups (P>0.05).

Table 4
figure Tab4

Effects of diets with different levels of CP on jejunal intestinal ASCT2, EAAC1, B 0+ AT, y + LAT1, 4F2hc , and ATB 0 mRNA relative abundance in weaned piglets ( n =6)

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4 Discussion

AAs regulate key metabolic pathways that are crucial for the maintenance, health, and growth of animals (Wu, 1998; Hu et al., 2008). The absorption of AA requires many transporter systems that differ with respect to their substrate specificity and driving force (Wang et al., 2009). It has been reported that some genes (such as ASCT2, EAAC1, B0+AT, y+LAT1, 4F2hc, and ATB0) that are involved in the control of growth or AA metabolism are regulated by AA availability (Wu, 1998; Lallès et al., 2007; Zhang et al., 2012; Abdulhussein and Wallace, 2014). In our study, we found that, with an increase in the dietary CP level, the expression of AA transporter genes (ASCT2, EAAC1, B0+AT, 4F2hc, and ATB0) tended to increase, while an opposite trend was seen related to the abundance of y+LAT1 mRNA. A possible reason was that y+LAT1 and EAAC1 have differential expression in our study because the diets with different CP levels related with absorption in piglets. The transport of AA in the small intestine is critical for the supply of AA to all tissues and the homeostasis of serum AA levels (Zhang et al., 2012). Since dietary CP can provide an ideal AA level, the genes of AA transporters that regulate protein metabolism in piglets should be more activated. The present results indicate that the expression of ASCT2, EAAC1, B0+AT, and ATB0 mRNA in the jejunum was influenced by dietary CP levels. Our results regarding the expression of y+LAT1 mRNA were similar to those of He et al. (2013). Under conditions of nutritional stress, the expression of some AA transporters (y+LAT1) may be increased to maintain normal growth of animals under adverse conditions (He et al., 2013). Skalli et al. (2014) suggest that dietary protein levels regulate the expression of AA transporter genes by a complex regulatory system, which may also affect the energy balance and endocrine system. AAs and hormones can play an important role in the regulation of gene expression. Overall, the use of low-CP (14%) AA-supplemented diets in piglets may decrease the transport capacity of AA in the small intestine. However, the absorption and transport of AA is a complex physiological process that is influenced by many factors (Wang et al., 2009; He et al., 2013; Zhang et al., 2013; Chen et al., 2014), including the dietary CP level and the AA balance. Thus, the transport of these nutrients is a key regulatory step in the use of dietary protein by weaned piglets. An excess decrease in dietary CP with AA supplementation resulted in significant reductions in the expression of AA transporter genes in the jejunum. This result suggests that there is an appropriate range within which CP in the diet may be reduced, and any further decrease may dramatically reduce the efficiency of energy and AA utilization in the small intestine (Jensen, 1998). This understanding may also make it possible to increase productivity and reduce the impact of livestock on the environment (Franklin et al., 2002). While this finding has important practical implications, further research will be required to elucidate the underlying mechanisms.

Compliance with ethics guidelines

Li WU, Liu-qin HE, Zhi-jie CUI, Gang LIU, Kang YAO, Fei WU, Jun LI, and Tie-jun LI declare that they have no conflict of interest.

All institutional and national guidelines for the care and use of laboratory animals were followed.