Alternative titles; symbols
HGNC Approved Gene Symbol: SPAG11B
Cytogenetic location: 8p23.1 Genomic coordinates (GRCh38): 8:7,447,754-7,463,542 (from NCBI)
Sperm undergo maturational changes in the epididymis that are necessary for motility, capacitation, and sperm-egg interaction. Proteins that affect sperm maturation are secreted, often in a testosterone-dependent manner, and include epididymis-specific secretory proteins encoded by the SPAG11 gene.
By differential screening of an epididymis cDNA library against cDNAs from brain, liver, and testis, followed by rescreening with an epididymis-specific fragment, Osterhoff et al. (1994) isolated a cDNA encoding SPAG11, which they called HE2. Sequence analysis predicted that the 103-amino acid SPAG11 protein contains a signal peptide and an N-glycosylation site, as well as numerous positively charged residues. Northern blot analysis revealed expression of an approximately 0.7-kb transcript. Immunohistochemical analysis demonstrated expression in the apical region of cells lining the epididymal epithelium. In situ hybridization analysis showed expression in the epididymal duct but not in the duct lumen. Immunofluorescent microscopy demonstrated expression on the acrosomal domain of motile sperm.
Hamil et al. (2000) identified several HE2 splice variants, which they termed HE2-alpha (the sequence identified by Osterhoff et al. (1994)), HE2-beta (a deleted form), and HE2-gamma (a form with a larger deletion). All the variants have a common 71-residue N terminus but various C termini. The 2 beta isoforms encode proteins of 133 (HE2-beta-1) and 108 (HE2-beta-2) amino acids, while the 2 gamma isoforms have 82 (HE2-gamma-1) and 74 (HE2-gamma-2) amino acids. Northern blot analysis revealed expression of a predominant 0.9-kb transcript, as well as less abundant 1.4-, 2.5-, and 4.4-kb transcripts, in caput; expression was much lower in corpus and nearly undetectable in cauda. Immunohistochemical analysis indicated that the HE2-beta-1 form is highly expressed in principal cells of the initial segment and caput, from which it is secreted into the lumen and binds to the sperm surface in the postacrosomal and neck regions.
HE2B1
In the course of a database search for beta-defensin homologs in the defensin gene cluster on chromosome 8p23-p22, Jia et al. (2001) determined that one of the 6 SPAG11 isoforms, termed HE2B1, contains the 6-cysteine beta-defensin motif. The motif is located in exon 4 of SPAG11, or exon 3 of the 3-exon HE2B1 isoform. Jia et al. (2001) noted that other beta-defensin genes have only 2 exons. RT-PCR analysis detected expression in both gingival keratinocytes and pulmonary epithelia, the only tissues examined. Jia et al. (2001) proposed that if HE2B1 indeed has antimicrobial activity, it may play a role in host defense in the genital tract. Li et al. (2001) cloned a rat epididymal beta-defensin, Bin1b, with such activity and determined that it is 29% identical to human HE2B1. They proposed that the peptide may have a role in both contraception and treatment of sexually transmitted diseases.
Zhou et al. (2004) showed that rat epididymal cells or human colonic epithelial cells transfected with rat Bin1b could induce sperm motility in immotile immature sperm. The induction of motility was mediated by Bin1b-induced Ca(2+) uptake, a mechanism that was less prominent in maintaining motility in mature sperm. Injection of antisense Bin1b into the caput epididymis of 3-month-old rats reduced the binding of Bin1b to caput sperm and attenuated sperm motility. Zhou et al. (2004) concluded that the beta-defensin Bin1b is important for the acquisition of sperm motility and the initiation of sperm maturation.
By Southern blot and genomic sequence analyses, Frohlich et al. (2001) determined that the SPAG11 gene, which they termed EP2, is 19 kb long, including 3 kb of proximal promoter region. The gene contains 8 exons and 2 promoters. Promoter A, located at the beginning of the gene, has no TATA box but has a GC-rich region. Promoter B, located in intron 3, lacks sequence similarity with promoter A and has a TATA box. Both promoters drive androgen-dependent, epididymis-specific gene expression. Frohlich et al. (2001) proposed that the EP2 gene was derived from ancestral beta-defensin genes (e.g., DEFB2; 602215).
By YAC analysis, Frohlich et al. (2001) mapped the SPAG11 gene to 8p23-p22, in close proximity to the DEFB2 gene.
Frohlich, O., Po, C., Young, L. G. Organization of the human gene encoding the epididymis-specific EP2 protein variants and its relationship to defensin genes. Biol. Reprod. 64: 1072-1079, 2001. [PubMed: 11259252] [Full Text: https://doi.org/10.1095/biolreprod64.4.1072]
Hamil, K. G., Sivashanmugam, P., Richardson, R. T., Grossman, G., Ruben, S. M., Mohler, J. L., Petrusz, P., O'rand, M. G., French, F. S., Hall, S. H. HE2-beta and HE2-gamma, new members of an epididymis-specific family of androgen-regulated proteins in the human. Endocrinology 141: 1245-1253, 2000. [PubMed: 10698202] [Full Text: https://doi.org/10.1210/endo.141.3.7389]
Jia, H. P., Schutte, B. C., Schudy, A., Linzmeier, R., Guthmiller, J. M., Johnson, G. K., Tack, B. F., Mitros, J. P., Rosenthal, A., Ganz, T., McCray, P. B., Jr. Discovery of new human beta-defensins using a genomics-based approach. Gene 263: 211-218, 2001. [PubMed: 11223260] [Full Text: https://doi.org/10.1016/s0378-1119(00)00569-2]
Li, P., Chan, H. C., He, B., So, S. C., Chung, Y. W., Shang, Q., Zhang, Y.-D., Zhang, Y.-L. An antimicrobial peptide gene found in the male reproductive system of rats. Science 291: 1783-1785, 2001. [PubMed: 11230693] [Full Text: https://doi.org/10.1126/science.1056545]
Osterhoff, C., Kirchhoff, C., Krull, N., Ivell, R. Molecular cloning and characterization of a novel human sperm antigen (HE2) specifically expressed in the proximal epididymis. Biol. Reprod. 50: 516-525, 1994. [PubMed: 8167223] [Full Text: https://doi.org/10.1095/biolreprod50.3.516]
Zhou, C. X., Zhang, Y.-L., Xiao, L., Zheng, M., Leung, K. M., Chan, M. Y., Lo, P. S., Tsang, L. L., Wong, H. Y., Ho, L. S., Chung, Y. W., Chan, H. C. An epididymis-specific beta-defensin is important for the initiation of sperm maturation. Nature Cell Biol. 6: 458-464, 2004. [PubMed: 15122269] [Full Text: https://doi.org/10.1038/ncb1127]