Presence of RNA in the sperm nucleus

A novel acrosomal protein, IQCF1, involved in sperm capacitation and the acrosome reaction

On the basis of the unknown tags in the mature human sperm serial analysis of gene expression library constructed by our laboratory, some transcripts were cloned, including Iqcf1 (IQ motif containing F1). To investigate the function of sperm-retained Iqcf1 in spermatogenesis and fertilization of mice, we investigated the spatial and temporal expression of IQCF1. By using the (transcription activator-like effector nuclease) strategy, Iqcf1-knockout mice were produced, and the phenotypes of the Iqcf1(-/-) mice were analyzed. The results showed that IQCF1 was localized in the acrosome of spermatozoa and spermatids; the expression of IQCF1 in testes was associated with spermatogenic capacity. The Iqcf1(-/-) mice were significantly less fertile than the wild-type mice (p = 0.0057) because of reduced sperm motility (p = 0.0094) and the acrosome reaction (AR) (p = 0.0093). In spermatozoa, IQCF1 interacted with calmodulin (CaM) and possibly participated in the tyrosine phosphorylation of sperm proteins during capacitation. In conclusion, a newly identified acrosomal protein, IQCF1, is closely related to sperm capacitation and AR; in particular, it is involved in tyrosine phosphorylation of sperm proteins through interaction with CaM. Research into the function of IQCF1 during fertilization could facilitate the investigation of the molecular mechanism of capacitation, which is unclear.

Sperm RNA as a Mediator of Genomic Plasticity

Sperm RNA has been linked recently to trans-generational, non-Mendelian patterns of inheritance. Originally dismissed as “residual” to spermatogenesis, some sperm RNA may have postfertilization functions including the transmission of acquired characteristics. Sperm RNA may help explain how trans-generational effects are transmitted and it may also have implications for assisted reproductive technologies (ART) where sperm are subjected to considerable, ex vivo manual handling. The presence of sperm RNA was originally a controversial topic because nuclear gene expression is switched off in the mature mammalian spermatozoon. With the recent application of next generation sequencing (NGS), an unexpectedly rich and complex repertoire of RNAs has been revealed in the sperm of several species that makes its residual presence counterintuitive. What follows is a personal survey of the science behind our understanding of sperm RNA and its functional significance based on experimental observations from my laboratory as well as many others who have contributed to the field over the years and are continuing to contribute today. The narrative begins with a historical perspective and ends with some educated speculation on where research into sperm RNA is likely to lead us in the next 10 years or so.

Sperm epigenomics: challenges and opportunities

Sperm is a highly differentiated cell type whose function is to deliver a haploid genome to the oocyte. The sperm “epigenomes” were traditionally considered to be insignificant – the sperm is transcriptionally inactive, its genome is packaged in sperm-specific protamine toroids instead of nucleosomes, and its DNA methylation profile is erased immediately post-fertilization. Yet, in recent years there has been an increase in the number of reported cases of apparent epigenetic inheritance through the male germline, suggesting that the sperm epigenome may transmit information between generations. At the same time, technical advances have made the genome-wide profiling of different layers of the sperm epigenome feasible. As a result, a large number of datasets have been recently generated and analyzed with the aim to better understand what non-genetic material is contained within the sperm and whether it has any function post-fertilization. Here, we provide an overview of the current knowledge of the sperm epigenomes as well as the challenges in analysing them and the opportunities in understanding the potential non-genetic carriers of information in sperm.

Global characterization and target identification of piRNAs and endo-siRNAs in mouse gametes and zygotes

A set of small RNAs known as rasRNAs (repeat-associated small RNAs) have been related to the down-regulation of Transposable Elements (TEs) to safeguard genome integrity. Two key members of the rasRNAs group are piRNAs and endo-siRNAs. We have performed a comparative analysis of piRNAs and endo-siRNAs present in mouse oocytes, spermatozoa and zygotes, identified by deep sequencing and bioinformatic analysis. The detection of piRNAs and endo-siRNAs in the spermatozoa and revealed also in zygotes, hints to their potential delivery to oocytes during fertilization. However, a comparative assessment of the three cell types indicates that both piRNAs and endo-siRNAs are mainly maternally inherited. Finally, we have assessed the role of the different rasRNA molecules in connection with amplification processes by way of the "ping-pong cycle". Our results suggest that the ping-pong cycle can act on other rasRNAs, such as tRNA- and rRNA-derived fragments, thus not only being restricted to TEs during gametogenesis.

A promoter fragment of the sycp1 gene is sufficient to drive transgene expression in male and female meiotic germ cells in zebrafish

The synaptonemal complex protein 1 (Sycp1) is required for the formation of crossovers that occurs during the meiotic prophase. The tissue and cell-specific expression pattern of the Sycp1 protein have been studied in mammals and fish, but data on the corresponding transcript remain scarce. In this report, we described for the first time in zebrafish the tissue- and cell-specific expression pattern of the sycp1 gene. In ovary, the expression of the sycp1 transcript was restricted to the early primary oocytes. In testis, the sycp1 transcript was observed in primary spermatocytes in agreement with a previous report describing the localization of the Sycp1 protein in those cells. Unexpectedly, sycp1 transcript expression remained high in spermatids. To gain insight on the genomic region responsible for the sycp1 gene expression pattern, we generated four independent Dr_sycp1:eGFP transgenic zebrafish lines carrying the -1482/+338 gene fragment fused to the enhanced green fluorescent protein reporter gene. We demonstrate that this promoter fragment contains the information required for the cell-specific expression of the endogenous sycp1 gene in males and in females. However, the GFP protein and its associated fluorescence persist in spermatozoa and maturing oocytes. The Dr_sycp1:eGFP zebrafish lines have the potential to be valuable models to trace meiosis onset in zebrafish and constitute the first transgenic lines expressing the GFP reporter protein only in the male meiotic and postmeiotic cells in fish.

Evaluation of the effectiveness of semen storage and sperm purification methods for spermatozoa transcript profiling

Different semen storage and sperm purification methods may affect the integrity of isolated spermatozoal RNA. RNA-Seq was applied to determine whether semen storage methods (pelleted vs. liquefied) and somatic cell lysis buffer (SCLB) vs. PureSperm (PS) purification methods affect the quantity and quality of sperm RNA. The results indicate that the method of semen storage does not markedly impact RNA profiling whereas the choice of purification can yield significant differences. RNA-Seq showed that the majority of mitochondrial and mid-piece associated transcripts were lost after SCLB purification, which indicated that the mid-piece of spermatozoa may have been compromised. In addition, the number of stable transcript pairs from SCLB-samples was less than that from the PS samples. This study supports the view that PS purification better maintains the integrity of spermatozoal RNAs.

The presence, role and clinical use of spermatozoal RNAs

BACKGROUND Spermatozoa are highly differentiated, transcriptionally inert cells characterized by a compact nucleus with minimal cytoplasm. Nevertheless they contain a suite of unique RNAs that are delivered to oocyte upon fertilization. They are likely integrated as part of many different processes including genome recognition, consolidation-confrontation, early embryonic development and epigenetic transgenerational inherence. Spermatozoal RNAs also provide a window into the developmental history of each sperm thereby providing biomarkers of fertility and pregnancy outcome which are being intensely studied. METHODS Literature searches were performed to review the majority of spermatozoal RNA studies that described potential functions and clinical applications with emphasis on Next-Generation Sequencing. Human, mouse, bovine and stallion were compared as their distribution and composition of spermatozoal RNAs, using these techniques, have been described. RESULTS Comparisons highlighted the complexity of the population of spermatozoal RNAs that comprises rRNA, mRNA and both large and small non-coding RNAs. RNA-seq analysis has revealed that only a fraction of the larger RNAs retain their structure. While rRNAs are the most abundant and are highly fragmented, ensuring a translationally quiescent state, other RNAs including some mRNAs retain their functional potential, thereby increasing the opportunity for regulatory interactions. Abundant small non-coding RNAs retained in spermatozoa include miRNAs and piRNAs. Some, like miR-34c are essential to the early embryo development required for the first cellular division. Others like the piRNAs are likely part of the genomic dance of confrontation and consolidation. Other non-coding spermatozoal RNAs include transposable elements, annotated lnc-RNAs, intronic retained elements, exonic elements, chromatin-associated RNAs, small-nuclear ILF3/NF30 associated RNAs, quiescent RNAs, mse-tRNAs and YRNAs. Some non-coding RNAs are known to act as epigenetic modifiers, inducing histone modifications and DNA methylation, perhaps playing a role in transgenerational epigenetic inherence. Transcript profiling holds considerable potential for the discovery of fertility biomarkers for both agriculture and human medicine. Comparing the differential RNA profiles of infertile and fertile individuals as well as assessing species similarities, should resolve the regulatory pathways contributing to male factor infertility. CONCLUSIONS Dad delivers a complex population of RNAs to the oocyte at fertilization that likely influences fertilization, embryo development, the phenotype of the offspring and possibly future generations. Development is continuing on the use of spermatozoal RNA profiles as phenotypic markers of male factor status for use as clinical diagnostics of the father's contribution to the birth of a healthy child.

Stability, delivery and functions of human sperm RNAs at fertilization

Increasing attention has focused on the significance of RNA in sperm, in light of its contribution to the birth and long-term health of a child, role in sperm function and diagnostic potential. As the composition of sperm RNA is in flux, assigning specific roles to individual RNAs presents a significant challenge. For the first time RNA-seq was used to characterize the population of coding and non-coding transcripts in human sperm. Examining RNA representation as a function of multiple methods of library preparation revealed unique features indicative of very specific and stage-dependent maturation and regulation of sperm RNA, illuminating their various transitional roles. Correlation of sperm transcript abundance with epigenetic marks suggested roles for these elements in the pre- and post-fertilization genome. Several classes of non-coding RNAs including lncRNAs, CARs, pri-miRNAs, novel elements and mRNAs have been identified which, based on factors including relative abundance, integrity in sperm, available knockout data of embryonic effect and presence or absence in the unfertilized human oocyte, are likely to be essential male factors critical to early post-fertilization development. The diverse and unique attributes of sperm transcripts that were revealed provides the first detailed analysis of the biology and anticipated clinical significance of spermatozoal RNAs.

Stallion sperm transcriptome comprises functionally coherent coding and regulatory RNAs as revealed by microarray analysis and RNA-seq

Mature mammalian sperm contain a complex population of RNAs some of which might regulate spermatogenesis while others probably play a role in fertilization and early development. Due to this limited knowledge, the biological functions of sperm RNAs remain enigmatic. Here we report the first characterization of the global transcriptome of the sperm of fertile stallions. The findings improved understanding of the biological significance of sperm RNAs which in turn will allow the discovery of sperm-based biomarkers for stallion fertility. The stallion sperm transcriptome was interrogated by analyzing sperm and testes RNA on a 21,000-element equine whole-genome oligoarray and by RNA-seq. Microarray analysis revealed 6,761 transcripts in the sperm, of which 165 were sperm-enriched, and 155 were differentially expressed between the sperm and testes. Next, 70 million raw reads were generated by RNA-seq of which 50% could be aligned with the horse reference genome. A total of 19,257 sequence tags were mapped to all horse chromosomes and the mitochondrial genome. The highest density of mapped transcripts was in gene-rich ECA11, 12 and 13, and the lowest in gene-poor ECA9 and X; 7 gene transcripts originated from ECAY. Structural annotation aligned sperm transcripts with 4,504 known horse and/or human genes, rRNAs and 82 miRNAs, whereas 13,354 sequence tags remained anonymous. The data were aligned with selected equine gene models to identify additional exons and splice variants. Gene Ontology annotations showed that sperm transcripts were associated with molecular processes (chemoattractant-activated signal transduction, ion transport) and cellular components (membranes and vesicles) related to known sperm functions at fertilization, while some messenger and micro RNAs might be critical for early development. The findings suggest that the rich repertoire of coding and non-coding RNAs in stallion sperm is not a random remnant from spermatogenesis in testes but a selectively retained and functionally coherent collection of RNAs.

Novel insights into the genetic and epigenetic paternal contribution to the human embryo

The integrity of the sperm genome and epigenome are critical for normal embryonic development. The advent of assisted reproductive technology has led to an increased understanding of the role of sperm in fertilization and embryogenesis. During fertilization, the sperm transmits not only nuclear DNA to the oocyte but also activation factor, centrosomes, and a host of messenger RNA and microRNAs. This complex complement of microRNAs and other non-coding RNAs is believed to modify important post-fertilization events. Thus, the health of the sperm genome and epigenome is critical for improving assisted conception rates and the birth of healthy offspring.

RETRACTED ARTICLE: Transcriptome profile alterations in asthenozoospermic and idiopathic infertile spermatozoa uncovered by microarray

This article has been retracted at the request of the Editor-in- Chief. The original submission was made without the approval of the previously listed co-author Dr. Rajender Singh. In addition, the article is being retracted because the corresponding author is not the owner of the data and has no right to publication.

Progesterone through progesterone receptors affects survival and metabolism of pig sperm

Progesterone receptors (PR) through interaction with the specific ligand progesterone (PRG), play a central coordinate role in different reproductive events. In this study conventional PR were identified in boar spermatozoa by Western blotting. Immunofluorescence techniques indicate that PR are located at sperm acrosomal region, suggesting a possible role in the oocyte fertilization events. Treatment with 17-hydroxyprogesterone (17-OHP) enhanced viability and induced cholesterol efflux, serine and tyrosine phosphorylation, p-Bcl2, p-Akt that are known hallmarks of capacitation in sperm. The analysis of the triglyceride contents, lipase and acyl-CoA dehydrogenase activities, as well as the G6PDH activity, was conducted so as to address whether there was an increase in energy expenditure via 17-OHP through the PR. Taken together these results, particularly the 17-OHP action on boar sperm lipid and glucose metabolism, give emphasis to the role of PR in sperm physiology within the oviductal environment. Moreover the present study highlights, the effect of PRG via PR on boar sperm survival, renewing the role of this hormone in male reproduction as candidate for boar fertility. Thus the present research contributes to further elucidating the role of progesterone on the physiological regulation of the most relevant spermatozoa functions for a successful fertilization.

Human spermatozoal RNAs

OBJECTIVE

To provide a focused review of the scientific literature pertaining to spermatozoal RNA.

DESIGN

Review of the literature and appraisal of relevant articles.

SETTING

Not applicable.

PATIENT(S)

Infertile male.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Spermatozoal RNAs as potential epigenetic modifiers in early embryo development and as clinical markers of male infertility.

RESULT(S)

The nucleus of mature spermatozoa contains a complex population of mRNAs and miRNAs despite its transcriptionally inert state.

CONCLUSION(S)

A specific set of functional RNAs are delivered into oocytes during fertilization and are thought to contribute extragenomically to early embryonic development. Even if spermatozoal RNAs is merely residual, it still has the potential to greatly improve the investigative and diagnostic potential of male infertility.

Spermatozoal RNAs: what about their functions?

The profound architectural changes that transform spermatids into spermatozoa result in a high degree of DNA packaging within the sperm head. However, the mature sperm chromatin that harbors imprinted genes exhibits a dual nucleoprotamine/nucleohistone structure with DNase-sensitive regions, which could be implicated in the establishment of efficient epigenetic information in the developing embryo. Despite its apparent transcriptionally inert state, the sperm nucleus contains diverse RNA populations, mRNAs, antisense and miRNAs, that have been transcribed throughout spermatogenesis. There is also an endogenous reverse transcriptase that may be activated under certain circumstances. It is now commonly accepted that sperm can deliver some RNAs to the ovocyte at fertilization. This review presents potential links between male-specific genomic imprinting, chromatin organization, and the presence of diverse RNA populations within the sperm nucleus and discusses the functional significance of these RNAs in the spermatozoon itself and in the early embryo following fertilization. Some recent data are provided, supporting the view that analyzing the profile of spermatozoal RNAs could be useful for assessment of male fertility.

Transcript expression profiles of Takifugu rubripes spermatozoa and eggs by expressed sequence tag analysis

Two cDNA libraries from Takifugu rubripes spermatozoa and eggs were constructed and a total of 620 expressed sequence tag (EST) clones were generated from the two libraries: 300 clones are from the spermatozoa library and 320 clones are from the eggs library. The most abundant cDNA clones in the two libraries were identified. A total of 207 'contigs' (or single) EST clones were found to share significant sequence identity with known sequences in the GenBank database, representing at least 51 different genes. In order to understand the two types of germ cells further, the expression profiles of the identified clones in these cDNA libraries were analyzed. Furthermore, the presence of specific messenger RNAs in the spermatozoa and eggs has been demonstrated with BLAST analysis; the spermatozoa and egg library can supply unique and novel cDNA sequences in the Takifugu rubripes EST project. Another aim of this study is to identify cDNA clones that can be used as molecular markers for the analysis of the spermatogenesis and oogenesis in Takifugu rubripes. Six potential clones (S1-3 from spermatozoa and E1-3 from eggs) were selected to analyze their expression patterns by reverse transcription (RT)-PCR analyses. Half of these showed a specific expression in the expected tissue. Two of the clones were found by RT-PCR and in situ hybridization to be expressed specifically in the testis or ovary, and they maybe suitable molecular markers for the analysis of spermatogenesis and oogenesis.

Paternal contributions: new functional insights for spermatozoal RNA

Whereas the presence of RNA in mature ejaculate spermatozoa is now established, its functional significance, if any, is still a matter of debate. This reflects the accepted description that spermatozoa are highly differentiated, specialized cells of minimal cytoplasm and compacted nucleus that are transcriptionally inactive. A significant proportion of the RNA required for the later, haploid stages of terminal spermatogenic differentiation (spermiogenesis) is synthesized prior to transcriptional arrest then stably stored until its translation during spermiogenesis. Spermatozoal RNAs, including messenger RNAs (mRNAs) are therefore considered to be stored remnants. Any role in fertilization and early development has, until recently, seemed unlikely, since the oocyte contains large stores of maternal mRNAs known to be required for early embryonic development prior to zygotic genome activation. Although the spermatozoon can deliver its RNA to the oocyte at fertilization, it has been generally assumed that compared to the oocyte RNA reserve, the spermatozoan payload is too small to be functional in embryo development. However, the debate continues as recent studies suggest that in specific instances sperm RNA is functional. This review presents and discusses the functional significance of spermatozoal RNA in relation to some recent advances in the field.

Birth of mice after intracytoplasmic injection of single purified sperm nuclei and detection of messenger RNAs and MicroRNAs in the sperm nuclei

We have developed a method that effectively removes all of the perinuclear materials of a mouse sperm head, including the acrosome, plasma membrane, perinuclear theca, and nuclear envelope. By injection of a single purified sperm head into a metaphase II mouse oocyte followed by activation with strontium chloride, 93% of the zygotes developed into two-cell embryos. Although only approximately 17% of the transferred two-cell embryos were born alive, all live pups developed into adults, and they appeared to be normal in reproduction and behavior. We detected RNA species, including mRNAs and miRNAs from the purified sperm heads. Our data demonstrate that pure membrane-free sperm heads are sufficient to produce normal offspring through intracytoplasmic sperm injection and that at least part of the RNA molecules are deeply embedded in the sperm nucleus.

Protein synthesis in sperm: dialog between mitochondria and cytoplasm

Ejaculated sperm are capable of using mRNAs transcripts for protein translation during the final maturation steps before fertilization. In a capacitation-dependent process, nuclear-encoded mRNAs are translated by mitochondrial-type ribosomes while the cytoplasmic translation machinery is not involved. Our findings suggest that new proteins are synthesized to replace degraded proteins while swimming and waiting in the female reproductive tract before fertilization, or produced due to the specific needs of the capacitating spermatozoa. In addition, a growing number of articles have reported evidence for the correlation of nuclear-encoded mRNA and protein synthesis in somatic mitochondria. It is known that all of the proteins necessary for the replication, transcription and translation of the genes encoded in mtDNA are now encoded in the nuclear genome. This genetic investment is far out of proportion to the number of proteins involved, as there have been multiple movements and duplications of genes. However, the evolutionary retention (or secondary uptake) of the mitochondrial machinery for translation of nuclear-encoded mRNAs may shed light on this paradox.

Relationship between chromatin organization, mRNAs profile and human male gamete quality

Spermiogenesis is a complex process leading to the formation of motile spermatozoa characterized by a highly stable chromatin compaction that transfers the paternal genome into the oocyte. It is commonly held that these haploid cells are devoid of transcriptional and translational activities and that the transcripts represent remnants of stored mRNAs. Recently, the chromatin organization of mature spermatozoa has been revisited as a double nucleoprotamine-nucleohistone structure possessing less-condensed regions sensitive to nuclease activity, which could be implicated in the expression of genes involved in the early embryo development. The existence of a complex population of mRNAs in human sperm is well-documented, but their role is not yet elucidated. Evidence for a latent transcriptional capacity and/or a potential de novo translation in mature spermatozoa from fertile men are essential for understanding the last steps of sperm maturation, such as capacitation and acrosome reaction. As such, we have documented the relationship between sperm quality and the distribution of sperm RNAs by showing divergent levels of transcripts encoding for proteins involved in either nuclear condensation (protamines 1 and 2) or in capacitation (eNOS and nNOS, c-myc) or in motility and sperm survival (aromatase) between low and high motile sperm issued from the same sample. Therefore, analyzing the profile of mRNAs could be helpful either as a diagnostic tool for evaluating male fertility after spermatogenesis or for prognosis use for fertilization.

A molecular analysis of the population of mRNA in bovine spermatozoa

Spermiogenesis represents the transition from haploid spermatids to spermatozoa. This process entails an extreme condensation of the nucleus and a loss of nearly all cytoplasmic content. The presence of messenger RNAs in the spermatozoa has previously been shown. Generally, these transcripts are considered to be remnants of spermiogenesis. However, it has recently been proposed that there may exist a function for these sperm-associated RNAs. To address the possibility of a functional role for these transcripts, we sought to investigate and characterize the RNA pool found in bovine spermatozoa. The main goals of this study were to examine RNA integrity and survey the mRNA found in spermatids and spermatozoa. Assessment of mRNAs integrity was performed by three approaches: microelectrophoresis, comparative smearing after global amplification, and PCR amplification of target sequences located either in the 5' or the 3' ends, while mRNAs survey was performed by microarray hybridizations. RNA integrity studies in the spermatozoa showed a majority of low molecular size fragments indicating a natural segmentation of the mRNA population. The mRNA survey indicated that the sperm transcriptome harbors a complex mixture of messengers implicated in a wide array of cell functions and representing a large subset of transcripts found in spermatids. Subsequently, such sperm RNA profiling could allow the molecular diagnosis of male gamete quality.

Isolation of intact RNA from cytometrically sorted Saccharomyces cerevisiae for the analysis of intrapopulation diversity of gene expression

Characterizing and understanding the functional heterogeneity in a given population on the cellular and molecular level is a great challenge in microbiology. Each microorganism contributes differently to the overall performance of the community and responds differently to changing microenvironmental conditions. Here, we present a method for isolation of intact RNA out of small subpopulations of live Saccharomyces cerevisiae cells for differential gene expression analysis. The protocol includes fluorescence staining, flow cytometric analysis and sorting of live yeast cells, subsequent isolation of RNA from the resulting subpopulations and finally RNA quantification and integrity check. The isolated RNA can be transcribed into cDNA and successfully used for microarray analysis. This aids in relating molecular regulation processes within subpopulations with the dynamics and functioning of the entire population. The procedure can be accomplished in 2 d.

Spermatozoal RNA as reservoir, marker and carrier of epigenetic information: implications for cloning

The mammalian male gamete is transcriptionally silent as a consequence of the highly condensed architecture of its chromatin and there is also little or no cytoplasm capable of supporting translation; however, we now understand that under certain conditions, spermatozoa can translate their mRNAs de novo and that spermatozoal RNA can potentially affect phenotypic traits in offspring. This epigenetic phenomenon may involve the transmission of extra-chromosomal episomal elements. Recent evidence indicates that spermatozoal RNA may play a role in the progressive shutdown of transcription during spermiogenesis. The presence of RNA in the sperm nucleus and its potential as a carrier of epigenetic information to the egg may prove insightful with regard to the abysmal success rates for cloning of domestic species by somatic nuclear transfer procedures.

A possible role for sperm RNA in early embryo development

Recently it has been demonstrated that, along with sperm, some of its RNA can be introduced into the oocyte during fertilization, which stays stable until the activation of the embryonic genome. Originally it was thought that RNA present in semen relates to contamination from somatic cells and/or immature sperm both containing substantially higher amounts of RNA than the fertilizing sperm. However, RNA is still found after stringent washing through density gradients resulting in a sperm fraction that is translational silenced and devoid of cytosolic rRNA and thus of potential RNA contamination-which is not transferable to the oocyte. Sperm only delivers a relatively small amount of paternal RNA (5-10 fg) into the fertilized oocyte when compared to the amount of maternal RNA (approximately 1 ng). Pooled human sperm contains about 5000 different mRNA sequences of which half are common between ejaculates. Besides mRNA sperm also contains small sperm RNA molecules that might interfere in gene expression (iRNA). In human sperm already more than 68 putative iRNAs have been identified and 15 of them may specifically inhibit genes that are only active during early embryonic development. The composition and quantity of sperm RNA is considered to be a valuable diagnostic tool for male fertility. However, only a subpopulation of the purified mature sperm fraction (with a yet unknown composition and quantity of RNA) will appropriately respond to capacitation media to become competent to fertilize the oocyte. In this review the origin and function of sperm borne RNA transferred into the oocyte is discussed along with their putative role in early embryogenesis, which still needs to be experimentally proven.

Expression and localization of delta-, kappa-, and mu-opioid receptors in human spermatozoa and implications for sperm motility

CONTEXT

Endogenous opioid peptides signal through delta-, kappa-, and mu-opioid receptors. Some of these peptides such as endorphins and enkephalins are present in the male reproductive tract, but the presence of the corresponding receptors in human sperm cells has not yet been reported.

OBJECTIVE

Our objective was to study the expression and localization of delta-, kappa-, and mu-opioid receptors on human spermatozoa and the implication in sperm motility.

METHODS

The expression of receptors was studied by RT-PCR, Western blot, and immunofluorescence techniques. We evaluated the effects of activation of each opioid receptor by specific agonist and antagonist.

RESULTS

Human spermatozoa express delta-, kappa-, and mu-opioid receptors. These receptors were located in different parts of the head, in the middle region, and in the tail of the sperm. Progressive motility of spermatozoa, an important parameter to evaluate male fertility, was found to be significantly reduced after incubation with the mu-receptor agonist morphine, whereas this effect was antagonized in the presence of the corresponding antagonist naloxone. The delta-receptor antagonist naltrindole significantly reduced progressive motility immediately after its addition. However, the delta-receptor agonist DPDPE had no significant effect. Finally, neither the kappa-receptor agonist U50488 nor its antagonist nor-binaltorphimine significantly affected the progressive motility of human spermatozoa.

CONCLUSION

We report for first time the presence of functional delta-, kappa-, and mu-opioid receptors in human sperm membranes. These findings are indicative of a role for the opioid system in the regulation of sperm physiology.

Spermatozoal RNA: why is it there and what does it do?

The mammalian male gamete is transcriptionally silent as a consequence of the highly condensed architecture of its chromatin and there is also little or no cytoplasm capable of supporting translation; however, we now understand that these cells carry a full complement of mRNAs and that under certain conditions, spermatozoa can translate their mRNAs de novo. Moreover, there is now good evidence that spermatozoal RNA can potentially affect phenotypic traits in offspring. This epigenetic phenomenon may involve the transmission of extra-chromosomal episomal elements. These data indicate that the spermatozoon is well matched to its role of delivering the paternal genome, but that additional male-benefits may also be accrued by the delivery of spermatozoal RNA.

Towards a better understanding of RNA carriage by ejaculate spermatozoa

Research on spermatozoal RNA has made considerable progress since the original reports on its presence appeared in the late 1950s and early 1960s. Through the use of stringent procedures aimed at eliminating contamination artefacts, we now appreciate that a complex cohort of mRNAs persists in the ejaculate cell but that 80S (cytoplasmic) ribosomal complexes are not present in sufficient quantities to support cytoplasmic mRNA translation. Despite this, under certain conditions, at least some cytoplasmic mRNAs can apparently be translated de novo, possibly on mitochondrial polysomes. The detection of mRNA translation by mature spermatozoa essentially supports the earliest research reports on spermatozoal gene expression although the suggested relationship with protein turnover and capacitation is wholly unexpected. We also examine some alternative explanations and roles for RNA carriage, including the RNAs passive retention as a consequence of nuclear shutdown and a more active role in chromatin repackaging, genomic imprinting, gene silencing and post-fertilization requirements of essential paternal RNAs. The recent report of an RNA-mediated epigenetic alteration to phenotype that is likely to be sperm derived is of particular interest in this regard. We finally show that regardless of the biological role(s) of spermatozoal RNA, its utility in infertility studies, particularly when coupled with modern techniques in gene-expression analysis (e.g. microarrays), is obvious. As a wholly non-invasive proxy for the testis, this RNA offers considerable potential as a marker for fertility status and the genetic and environmental influences that could make all the difference between a fertile and an infertile phenotype.

Characterization and quantification of mRNA transcripts in ejaculated spermatozoa of fertile men by serial analysis of gene expression

BACKGROUND

Accumulated evidence proves that mature spermatozoa contain a complex yet specific array of mRNA, which could provide information on the past events of spermatogenesis.

OBJECTIVE

To quantitatively microdissect these mRNA transcripts by a digital approach.

METHODS

Serial analysis of gene expression (SAGE) was used to study the mRNA transcripts from ejaculate of a fertile individual and of a pool of 10 fertile men. Online DAVID software suite was also utilized to cluster the UniGene data.

RESULTS

A SAGE library from the individual produced 20,237 raw tags representing 2459 unique tags and that from pooled 10 men generated 21,052 raw tags representing 2712 unique tags. When the unique tags occurring > or = 4 times were analysed, 564 overlapping tags were produced by 638 unique tags from the individual and 682 from the pooled library. Fifty-four of these overlapped tags were considered to be novel genes. Online analysis of the overlapping tags revealed 25 functional gene groups, with the dominant one comprising 96 nuclear protein genes involving transcription and transcription regulation and also a group with 84 ribosomal subunit genes involving protein synthesis.

CONCLUSION

A SAGE analysis of ejaculate from fertile men has revealed a large number of transcripts, which occur in steady frequencies and probably have important roles in spermatogenesis and fertilization.

Arguments raised by the recent discovery that insulin and leptin are expressed in and secreted by human ejaculated spermatozoa

The recent findings demonstrating that insulin and leptin are expressed in and secreted by human ejaculated spermatozoa raise the controversial issue related to mRNA function in male gamete. Capacitated sperm display an increased metabolism and overall energy expenditure presumably to affect the changes in sperm signaling and function during capacitation. However the relationship between the signaling events associated with capacitation and the change in sperm metabolism energy is poorly understood. It emerges from the findings here reported that both leptin and insulin may be crucial in ejaculated spermatozoa to manage their energy status. Immunoistochemical analysis revealed that in uncapacitated sperm insulin was located at the subacrosomial level, in the midpiece and through the tail while leptin was immunodetected at the equatorial segment and at the midpiece. Capacitated sperm display an overall decrease and a more uniform distribution in the signal for both hormones and this is in agreement with their enhanced release in the medium. Both hormones in ejaculated sperm somehow recapitulate the cross-talk between their signalling transductional pathways in somatic cells, resulting in the increase of phosphoinositide 3-kinase (PI3K) activity, AKT S473 and Glycogen synthase kinase 3 (GSK-3)-S9 phosphorylations. During capacitation GSK-3 phosphorylation was abolished suggesting how in capacitating sperm there is a block in glycogen synthesis. This reasonably indicates how during capacitation glycogen reserve is mobilized and this makes the glucose as energy substrate available. For instance insulin dismissed by ejaculated spermatozoa up-regulates Glucose 6-Phosphate Dehydrogenase (G6PDH), the rate-limiting enzyme in the pentose phosphate pathway (PPP), which has be shown to be crucial in the acquisition of fertilizing capability as well as to mediate gamete fusion. Insulin immunoneutralization or blockage of its release, dramatically down regulated G6PDH. Interestingly, in the presence of a disruptor of insulin signaling wortmannin, an inhibitor of PI3K, the intrinsic activity of G6PDH drops. Leptin appears to play similar action to that of insulin on G6PDH in sperm (data in progress). The enhanced activity of this enzyme induced by both hormones produces an increase of NADPH that is essential for fatty acid synthesis from acetyl CoA. These fatty acids have two possible fates: beta-oxidation to produce ATP or reesterification back into triacylglycerol. Inter-relationships of the classes of substrates of free fatty acids (FFA) and glucose utilized for energy, has been long established [Randle, P.J., 1964. The interrelationships of hormones, fatty acid and glucose in the provision of energy. Postgrad. Med. J. 40, 457-463]. The authors observed in ejaculated spermatozoa what it occurs in somatic cells: FFA beta-oxidation tested utilizing the octanoil-CoA as substrate, appears to be stimulated by leptin and down-regulated by the contemporaneous presence of insulin in uncapacitated sperms. FFA beta-oxidation activity dramatically increases when capacitation starts, so it may be assumed the possibility that leptin may work to stimulate such enzymatic activity providing additional metabolic fuel to triggering capacitation process. The autonomous capability of sperm to release insulin and leptin suggests that they through an autocrine short loop may provide the recruitment of energy substrate according to sperm metabolic needs. This occurs independently by the systemic regulation and may represent a protective mechanism which preserves sperm fertilizing capability by any detrimental effects produced by long calorie restriction or by alterations occurring in the energy homeostasis at systemic level.

Paternal contribution: new insights and future challenges

It has been widely held that all that fathers essentially contribute to the next generation is half their genome. However, recent progress towards understanding biological processes such as sperm maturation and fertilization now indicates that the paternal contribution has been underestimated. To tackle some of the misconceptions surrounding the paternal contribution, the factors that are actually delivered by the sperm at fertilization and their potential developmental functions will be discussed using data from humans and animal models. Although still in their infancy, the practical applications of using sperm RNAs have already emerged in reproductive medicine as markers that are indicative of successful vasectomy. They are also beginning to appear in the forensic sciences and, within the next decade, might appear in the environmental sciences.

The controversy, potential and roles of spermatozoal RNA

The majority of cellular and molecular andrologists endorse the view that the sperm is a vessel for transporting the paternal genome to the waiting egg and nothing more. Any requirement for additional spermatozoal components that enter the ooplasm apart from the paternal centriole and the soluble egg-activating factor is generally dismissed. Many studies, however, have reported RNAs in ejaculate spermatozoa and we now know that mRNAs are delivered to the egg on fertilisation. The function and utility of sperm mRNA remains essentially unexplored. Here, we examine the controversy surrounding spermatozoal mRNA carriage, the evidence refuting its presence as an artefact and how spermatozoal mRNA is leading us to suspect that, quite apart from its undoubted diagnostic potential, it might have an important role in the establishment and maintenance of a viable paternal genome.

RNA in human sperm

We have yet to develop a fundamental understanding of the molecular complexities of human spermatozoa. This encompasses the unique packaging and structure of the sperm genome along with their paternally derived RNAs in preparation for their delivery to the egg. The diversity of these transcripts is vast, including several anti-sense molecules resembling known regulatory micro-RNAs. The field is still grasping with its delivery to the oocyte at fertilization and possible significance. It remains tempting to analogize them to maternally-derived transcripts active in early embryo patterning. Irrespective of their role in the embryo, their use as a means to assess male factor infertility is promising.

Identification of transcripts by macroarrays, RT–PCR and in situ hybridization in human ejaculate spermatozoa

Round spermatids contain high levels of extremely varied mRNAs that are synthesized either throughout early spermatogenesis or during spermiogenesis from the haploid genome. Concomitantly, with major changes in the chromatin organization, arrest of transcription occurs at midspermiogenesis. However, previous investigations using RT-PCR have revealed the persistence of numerous and different transcripts in ejaculated spermatozoa. In the present study, a step-by-step analysis by means of macroarray hybridization, RT-PCR and in situ hybridization was performed to identify more accurately the different mRNA species found in the human ejaculated spermatozoa. The data showed an extended pattern of various transcripts encoding a diverse range of proteins involved in signal transduction and cell proliferation. For the first time, they demonstrated that mRNAs coding for the transcription factors NFkappaB, HOX2A, ICSBP, protein kinase JNK2, growth factor HBEGF and receptors RXRbeta and ErbB3 accumulate within the sperm nucleus. The origin and fate of the sperm transcripts remain subject to discussion.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.

An alternative internal splicing site defines new Ikaros isoforms in Pleurodeles waltl

The Ikaros gene encodes a family of transcription factors which plays a crucial role in hematopoiesis. To improve our knowledge about the immune system of Pleurodeles waltl, we sequenced the cDNA coding for the Ik-1 isoform of that salamander and analyzed its tissue expression by semi-quantitative RT-PCR. Ikaros transcripts are abundant in the thymus and the spleen, thereby confirming that these organs are, respectively, the primary and secondary lymphoid tissues of Pleurodeles. Analysis of the isoforms produced by this animal revealed two isoforms characteristic of amphibians in which an alternative internal splicing site deletes the 3' half of exon 3 which interestingly comprises the first Zn finger of Ikaros. Ikaros transcripts were found at the earliest stages of development of Pleurodeles indicating that Ikaros has a function at the very early lymphopoietic stages. Moreover, the presence of Ikaros transcripts in spermatozoa suggests that this protein could have another and yet unknown function.

A novel chimeric mitochondrial RNA localized in the nucleus of mouse sperm

Six identical cDNA clones corresponding to an RNA of 1685 nucleotides that is enriched in mouse sperm compared with testis were isolated from a mouse testis cDNA library. The sequence of these clones corresponds to the 16S mitochondrial RNA plus an inverted repeat of 120 bp covalently joined to the 5' end of the RNA. By RT-PCR, it was demonstrated that this transcript, referred to as chimeric RNA, was present in mouse sperm, testis, liver, kidney, brain, and spleen. The absence of an equivalent sequence in mitochondrial DNA or as a mitochondrial pseudogene in total DNA extracted from sperm, testis, and somatic tissues suggests that the chimeric RNA is a post-transcriptional product, maybe resulting from a trans splicing reaction. The chimeric RNA was found by RT-PCR in total RNA extracted from purified sperm heads. This result was confirmed by in situ hybridization, which showed clear staining of the sperm nucleus with probes corresponding to sequences of the mitochondrial 16S RNA and the inverted repeat.