The combined human sperm proteome: cellular pathways and implications for basic and clinical science

BACKGROUND: The human sperm cell is very well suited for proteomic studies, as it is accessible, can be easily purified and is believed to be transcriptionally and translationally silent. The recent use of advanced proteomic approaches is clearly challenging the understanding of sperm biology. The aims of this review are to discuss the various human sperm proteomic studies, to create a compiled list of all the sperm proteins described to date and to re-assess the potential functional implications. METHODS: A search of the scientific literature available in the PubMed/Medline database at 31 December 2012 was conducted for studies on human sperm proteomics. The complete list of proteins obtained was carefully analysed using different bioinformatics tools, including Reactome, a knowledgebase of biological pathways. RESULTS: A total of 30 studies were identified. The proteomics studies have resulted in the identification of 6198 different proteins, an important proportion of which (around 30%) are known to be expressed in the testis. The proteins were assigned to various functional pathways, including metabolism, apoptosis, cell cycle, meiosis and membrane trafficking, among others. Unexpectedly, the sperm cell also contains a range of proteins involved in RNA metabolism and translational regulation, as well as proteins usually located in organelles believed to be absent in sperm, such as cytoplasmatic ribosomes and peroxisomes. Additionally, some proteins whose levels seem to be altered in low quality sperm might have clinical relevance. CONCLUSIONS: The analysis of the most complete sperm proteome available to date indicates the presence of several cellular protein pathways previously ignored in the male gamete. Confirming the activity of each of these pathways and understanding their biological significance will certainly boost the knowledge of human sperm and male fertility and infertility in the next years.