Region-specific gene expression in the epididymis of Yak

Immature spermatozoa undergo series of events in the epididymis to acquire motility and fertilizing ability. These events are a direct result of exposure to, and interaction with, the luminal environment created by the epididymal epithelium. The three conventional regions of the epididymis namely; caput, corpus and cauda have been identified to play specific roles in the epididymal maturation process of the spermatozoa; their respective roles have been associated with specific gene expression patterns that account for the composition of the luminal fluid that bathe the spermatozoa as they transit through the epididymal lumen and ensure their maturation. The identification of genes expressed in a region-specific manner provides valuable insight into the functional differences among the regions. Microarray technology has previously been employed in region-specific gene expression studies using the epididymis as a model in different species such as mouse, rat, boar and human. However, to characterize gene expression in the different regions of the epididymis, RNA-seq analysis was used in our study to examine gene expressions in the caput, corpus, and cauda of yak epididymis. Comparative transcriptomic analysis was performed between region pairs in the order; caput vs corpus, caput vs cauda and corpus vs cauda. DEGs among the various region pairs were detected and functional analysis were performed for the detected DEGs. Overall, the caput vs cauda epididymidis pair produced the highest number of DEGs (49.4%) while the corpus vs cauda pair produced the least number of DEGs (19.3%). The caput segment demonstrated relatively high expression of Sal1, LCN6, PTDS, DEFB109, DEFB 119, DEFB 123, SPAG11, PROC, CST3, ADAM28, KCNJ12 and SLC13A2; corpus epididymis demonstrated relatively high expression of MAN2B2, ELP, ZFYVE21, GLB1L, BMP4, DEFB125, PPP1R10, RIOX2, TKDP1, DEFB106A, NPBWR1 and SLC28A1; and the cauda epididymis, demonstrated relatively high expressions of MCT7, PAG4, OAS1, TGM3 and PRSS45. Gene Ontology results showed that DEGs in the caput vs corpus and corpus vs cauda pairs were mostly enriched in the cell/cell part GO term. On the other hand, DEGs in the caput vs cauda pair was were mostly enriched in the cellular process term. KEGG pathway annotation was also performed for DEGs among the various groups. AMPK signaling pathway, which is characterized by the ratio between cellular AMP and ATP and also determines cellular energy state, was selected from among the top five KEGG pathways for DEGs in the caput vs corpus pair. Our results showed that some down-regulated DEGs in the caput and corpus pair such as HN4a, eEF2K and CFTR were present and played significant roles in the AMPK signaling pathway. In the corpus vs cauda pair, our results showed that up-regulated DEGs such as XDH, TRMP2 and ENTPD were involved in the purine metabolism KEGG pathway, which was among top five KEGG pathways for DEGs in this pair. Pentose phosphate pathway functions in antioxidation to protect both the spermatozoa and epididymis from oxidative damage; it was among top five KEGG pathways for DEGs in the caput vs cauda pair. Our results also showed that down-regulated genes in the caput vs cauda pair such as TALDO1 was found to be involved in the Pentose phosphate pathway. The significance of the upregulated and downregulated genes on the pathways were elucidated. SAL1, which showed high expression in the caput, had previously not been demonstrated in the epididymis, needs further investigation to establish its unique role in the yak epididymis.