Identification and characterization of a novel bovine oocyte-specific secreted protein gene.

The oocyte transcriptome and proteome largely remain a mystery and the important roles of these genes, especially in the bovine, are poorly understood. To better understand specific developmental roles of the bovine oocyte, we sought to characterize a novel oocyte gene. We have sequenced the full-length mRNAs of bovine oocyte-secreted protein 1 (OOSP1) and discovered two splice variants, namely OOSP1_v1 (variant 1) and OOSP1_v2 (variant 2). The OOSP1 gene is located on chromosome 15 and is composed of 5 exons, which span over nearly 7.9 kb. Bovine OOSP1_v1 encodes a predicted protein of 163 aa in length that shares amino acid identity with mouse OOSP1, it also bears a putative signal peptide and is probably glycosylated. Bovine OOSP1_v2 protein bears the same signal peptide as OOSP1_v1 but due to the skipping of exon 2 that introduces a frameshift, it is only 35 aa in length. Expression analysis of both OOSP1 transcripts showed that this gene is exclusively expressed in oocytes, contrasting with the mouse where Oosp1 is also expressed in the spleen and liver. OOSP1_v1 and OOSP1_v2 mRNA expression profiling using real-time PCR revealed that their mRNA levels were high in germinal vesicle (GV) stage oocytes and gradually decreased until the blastocyst stage, at which time they were undetectable. For the first time, we have identified the bovine OOSP1 gene as being one of the few known oocyte-specific markers.