Molecular basis of sperm quality

Abstract

Molecular basis of sperm quality

Vanesa Robles

Spanish Institute of Oceanography (IEO) Promontorio de San Martín s/n 39004, Santander, Spain and INDEGSAL, University of León, 24071, León, Spain. Tel.+34 942348397, vanesa.robles@st.ieo.es

INTRODUCTION

It is generally accepted that the best indicator for sperm quality is fertilization ability. Other parameters have been correlated with fertilization such as motility and sperm viability. Plasma membrane composition, mitochondrial functionality, spermatozoa metabolic activity, spermatozoa morphology and analysis of chromatin integrity have been also used in order to provide a general picture of sperm quality. But, taking into account the inherent ability of germ cells to transmit their genetic and epigenetic information to the progeny, it is necessary to introduce a molecular component in the equation of sperm quality assessment.

RNAs IN THE CONTROL OF SPERM PRODUCTION AND SPERM QUALITY

The specialized spermatozoa are the result of a complex differentiation process tightly controlled at transcriptional and epigenetic levels but also controlled at post-transcriptional level. Matured spermatozoa are transcriptionally inactive cells, but their nucleus have a pool of miRNAs and mRNAs remnant from spermatogenesis that have been pointed as crucial factors not only for fertility but also for a proper embryo development.

Low sperm quality could be reflecting a problem in spermatogenesis and differences in remnant molecules from such process could be good indicators of failures at crucial differentiation steps. In this sense, the study of RNAs in the control of sperm production and sperm quality takes special interest. It is known that the Chromatoid Body (CB) that it is found in the cytoplasm of hapoid male germ cells is crucial in the control of postmeiotic differentiation of these cells, having an important function in mRNA storage, metabolism and even in the mRNA transport between nucleus and cytoplasm. CB moves during spermatogenesis stages. Some times it is associated with the nuclear pores, but it also moves perpendicularly to nuclear envelope and it also appears around the Golgi complex. Some reports even suggested the possibility of CB transfer through cytoplasmic bridges to the neighbouring spermatids. It has been found that microtubule depolymerization disturbs its movement. Therefore microtubular network via kinesin interaction has been proposed as a possible via for its mobility. The composition of CB includes RNA-binding proteins such as VASA (ATP dependent DEAD-box RNA helicase) (Yuan et al. 2014) PIWI proteins (which bind to piRNAs) and RNAs (small RNAs and mRNAs) (Wang et al. 2009). Other proteins such as the kinesin K1F17b also accumulates in CBs (Nagamori and Sassone-Corsi 2008). The alteration of these CB components could result in the arrest of spermatogenesis.

Interestingly, the non-coding RNAs could also play an important role in epigenetic inheritance. It is well known that epigenetic information can be transgenerational transmited via germ cells, and piRNAs have been proposed as potential mediators of epigenetic transgenerational inheritance (Yadav and Kotaja 2014).

NEW TOOLS AND TECHNIQUES AT THE SERVICE OF SPERM QUALITY EVALUATION AND SPERM SUBPOPULATIONS SELECTION

The importance of the RNAs stored in the spermatozoa has been highlighted by recent studies, which confirmed (using microarrays and/or quantitative PCR) that the presence of certain RNAs is crucial for fertilization and for successful early embryo development (Guerra et al 2013). Quantitative PCR has been also used for lesion quantification in specific genes or genome regions (Cartón-García et al 2012). This methodology provides the perfect complementary tool for global DNA integrity assays. All these techniques provide novel and rigorous methods to define molecular aspects leading to high sperm quality. But in addition to this, we need methods that provide, from a practical point of view, an efficient tool for selecting high quality spermatozoa. The presence of different spermatozoa subpopulations within the same seminal sample is an important point to take into account, particularly in low quality samples. Magnetic Activated Cell sorting has been recently used for selecting non-apoptotic spermatozoa from Solea senegalensis F1 low quality seminal samples. This method is based in the fact that Annexin V binds to phosphatidylserine, which is externalized to the outer surface of the sperm membrane in early apoptosis. Those spermatozoa linked to Annexin V-MACS beads are retained in the column, and non-apoptotic spermatozoa are recovered.

Finally, it is important to mention that new approaches, such as subrogate production, could be also understood as potential techniques to discover new key players in spermatogenesis. The possibility of xenotransplanting germinal cells, could allow the study within the same gonad of the interaction of germ cells and somatic cells from different species. To know these molecular interactions will undoubtedly help in the understanding of the molecular basis of sperm quality.

ACKNOWLEDGEMENTS AQUAGAMETE FA 1205 COST

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