Studies fromDr. Long Miao’s laboratory at the Institute of Biophysics (IBP), CAS, and Dr. Mengqiu Dong’s laboratory at the National Institute of Biological Sciences (NIBS) have revealed that the activation of nematode sperm is regulated by a serine protease secreted by the vas deferens and a serine protease inhibitor released from the sperm. This work, entitled “Nematode sperm maturation triggered by protease involves sperm-secreted serine protease inhibitor (Serpin),” was published in the Proceedings of the National Academy of Sciences in January 2012.

Nematode sperm undergo distinct morphological changes during activation, changing from spherical, immotile spermatids into crawling spermatozoa with pseudopods. Inactive nematode sperm such as those of Ascaris suum or C. elegans are packed with membranous organelles (MOs) whose exocytosis is coupled with and required by sperm activation. However, it was not previously known which molecules are released during sperm activation or the purpose of their release. Combining cell biology, biochemical purification, and a novel de novo peptide sequencing method, Dr. Miao and colleagues identified a serine protease inhibitor (As_SRP-1, a member of the Serpin superfamily) that is secreted by spermatids during sperm activation. They showed that As_SRP-1 has two well-coordinated functions: (1) As_SRP-1 functions in cis to support Major Sperm Protein (MSP)-based cytoskeletal assembly in the spermatid that releases it, thereby facilitating sperm motility and enhancing the competitiveness of the resulting spermatozoon; (2) As_SRP-1 released from an activated sperm inhibits, in trans, the activation of surrounding spermatids by blocking vas-deferens-secreted As_TRY-5, a trypsin-like serine protease necessary for spermiogenesis. These dual functions of As_SRP-1 facilitate the success of the activated sperm in sperm cooperation (in the male body to prevent premature activation), competition (in the female reproductive tract) and subsequent fertilization.

Sperm competition is a topic of broad interest and intense research activity. It has been widely recognized as one of most potent driving forces in the evolution of physiological, morphological, and behavioral adaptations. However, research on the mechanism of sperm competition has primarily focused on some of the physical traits of sperm, such as the size, quality, morphology, speed, and number of sperm inseminated, and the seminal fluid produced by several accessory glands in the male body. This study is the first to show that, besides components secreted from accessory glands in the male body, sperm also contribute a protein (As_SRP-1) to the seminal fluid which alters the immediate environment around the sperm to enhance its competitiveness and outcompete its rivals. Such a regulatory mechanism is most likely shared by C. elegans, and possibly other species both within and outside the nematode phylum, as similar proteins have been found in the seminal fluids of other animals including mice.

Among the authors are Dr. Yanmei Zhao (the first author), Wei Sun, Dr. Xuan Ma, Yunlong Shang, Dr. Bin Wang, Youqiao Hu, and Dr. Fanxia Meng from IBP; Pan Zhang, Mei-Jun Zhang, and Chun-Qing Song from NIBS; Hao Chi and Dr. Si-Min He from the Institute of Computing Technology, CAS; Drs. Zhiqi Hao and Andreas F. Huhmer from ThermoFisher Scientific; and Dr. Steven W. L’Hernault from Emory University. Drs. Long Miao and Meng-Qiu Dong are corresponding authors. This study was supported by grants from the Ministry of Science and Technology of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Municipal Government of Beijing, and the National Institutes of Health of the United States.

The link to this article: http://www.pnas.org/content/109/5/1542

Figure 1: Cartoon depiction showing that sperm competition is regulated by the vas deferens-secreted protease (As_TRY-5) and sperm-released protease inhibitor (As_SRP-1) during sperm maturation.