Author(s): Ting Gu 1 , Xi Su 1 , Quanyong Zhou 2 , Xinyun Li 1 , Mei Yu 1 , Yi Ding 1 , Shuhong Zhao 1 , Changchun Li 1 , *
Imprinted genes are a special category of genes that imprinted one allele in the early embryo development decided by the parental origin. The theory raised by Morre and Haig  was widely demonstrated that imprinting evolved in mammals because of the conflicting interests of maternal and paternal genes in transferring of nutrients from the mother to her offspring. For example, maternally imprinted genes Mest and Grb10 play important roles in the placental and fetal development of mammals. Mest knockout mice were viable and characterized by growth retardation . Mice with a disrupted maternal copy of Grb10 produced larger embryos and placentas, while mutant mice were 30% larger than normal mice .
Chinese Meishan pigs produce 3 to 4 more piglets than Yorkshire pigs in each litter. Numerous investigations have focused on the mechanisms behind this difference. Early investigators believed that factors regulating developmental rate and uniformity of the conceptus were the primary determinants of prolificacy . Further study showed that the weight of Yorkshire placentas dramatically increased from day90 of gestation to term, while in Meishan pigs, the weight of the fetus, not the placenta, increased during this period . These studies indicated that Meishan and Yorkshire pig placentas have different nutrient transport capacities.
We detected differentially expressed genes in the placentas of Meishan and Yorkshire pigs on day75 and day90 of gestation by Affymetrix Porcine Expression Microarray. A total of 226 transcripts on day75 and 577 transcripts on day90 were differentially expressed between placentas from the two divergent breeds. The differentially expressed transcripts included genes involved in angiogenesis, placental development, nutrient transportation and imprinted genes, such as PEG1 , PEG3 , PEG10 , PLAGL1 , SLC38A4 , and DIRAS3 . Neuronatin (NNAT) was found to be one of the imprinted genes differentially expressed in Meishan and Yorkshire pig placentas. NNAT , also known as paternally expressed gene 5 (PEG5 ), was first discovered in the rat neonatal brain and has significant roles in the differentiation of neurons . Other studies in adipose tissue and pancreatic [beta] cells showed that NNAT is involved in adipocyte differentiation and in regulating glucose-mediated insulin secretion , .
NNAT was paternally expressed in human and mouse brains , . In the pig, a study showed that NNAT was imprinted in 11 tissues, including heart, liver, spleen, lung, kidney, stomach, small intestine, skeletal muscle, fat, uterus, ovary, and pituitary gland , but in pig placenta, the expression status inherited from parents was still uncovered to our knowledge.
In this study, we determined the NNAT allelic expression status and confirmed the differential expression of NNAT in placentas of Meishan and Yorkshire pigs on day75 and day90 of gestation. The published literature was used to create a network model showing the possible relationships between NNAT and glucose transporter genes. The crucial promoter region of NNAT in JEG-3 and PK-15...