Human sperm tail proteome suggests new endogenous metabolic pathways

Proteomic studies are contributing greatly to our understanding of the sperm cell, and more detailed descriptions are expected to clarify additional cellular and molecular sperm attributes. The aim of this study was to characterize the subcellular proteome of the human sperm tail and, hopefully, identify less concentrated proteins (not found in whole cell proteome studies). Specifically, we were interested in characterizing the sperm metabolic proteome and gaining new insights into the sperm metabolism issue. Sperm were isolated from normozoospermic semen samples and depleted of any contaminating leukocytes. Tail fractions were obtained by means of sonication followed by sucrose-gradient ultracentrifugation, and their purity was confirmed via various techniques. Liquid chromatography and tandem mass spectrometry of isolated sperm tail peptides resulted in the identification of 1049 proteins, more than half of which had not been previously described in human sperm. The categorization of proteins according to their function revealed two main groups: proteins related to metabolism and energy production (26%), and proteins related to sperm tail structure and motility (11%). Interestingly, a great proportion of the metabolic proteome (24%) comprised enzymes involved in lipid metabolism, including enzymes for mitochondrial beta-oxidation. Unexpectedly, we also identified various peroxisomal proteins, some of which are known to be involved in the oxidation of very long chain fatty acids. Analysis of our data using Reactome suggests that both mitochondrial and peroxisomal pathways might indeed be active in sperm, and that the use of fatty acids as fuel might be more preponderant than previously thought. In addition, incubation of sperm with the fatty acid oxidation inhibitor etomoxir resulted in a significant decrease in sperm motility. Contradicting a common concept in the literature, we suggest that the male gamete might have the capacity to obtain energy from endogenous pools, and thus to adapt to putative exogenous fluctuations.

Effects of different storage protocols on cat testis tissue potential for xenografting and recovery of spermatogenesis

The loss of genetic diversity due to premature death of valuable individuals is a significant problem in animal conservation programs, including endangered felids. Testis tissue xenografting has emerged as a system to obtain spermatozoa from dead immature animals, however protocols to store this tissue before xenografting are still lacking. This study focused on testis tissue cryopreservation and storage from the domestic cat (Felis catus) classified as "pre-pubertal" and "pubertal" according to spermatogenesis development. Grafts from testis tissue cryopreserved with DMSO 1.4M, recovered after 10 weeks xenografting, presented seminiferous tubules with no germ cells. On the contrary, testis tissue from pre-pubertal animals preserved in ice-cold medium for 2 to 5 days presented no loss of viability or spermatogenic potential, while the number of grafts of pubertal cat testis tissue with germ cells after 10 weeks of xenografting decreased with increasing storage time. Nevertheless, even grafts from pre-pubertal cat testis tissue presented lower anti-DDX4 and anti-BOULE staining (proteins necessary for the meiosis completion), when compared with adult cat testis. Finally, a strong correlation found between testis weight and xenograft outcome may help choose good candidates for xenografting.

Energy metabolism in human pluripotent stem cells and their differentiated counterparts

Background

Human pluripotent stem cells have the ability to generate all cell types present in the adult organism, therefore harboring great potential for the in vitro study of differentiation and for the development of cell-based therapies. Nonetheless their use may prove challenging as incomplete differentiation of these cells might lead to tumoregenicity. Interestingly, many cancer types have been reported to display metabolic modifications with features that might be similar to stem cells. Understanding the metabolic properties of human pluripotent stem cells when compared to their differentiated counterparts can thus be of crucial importance. Furthermore recent data has stressed distinct features of different human pluripotent cells lines, namely when comparing embryo-derived human embryonic stem cells (hESCs) and induced pluripotent stem cells (IPSCs) reprogrammed from somatic cells.

Methodology/Principal Findings

We compared the energy metabolism of hESCs, IPSCs, and their somatic counterparts. Focusing on mitochondria, we tracked organelle localization and morphology. Furthermore we performed gene expression analysis of several pathways related to the glucose metabolism, including glycolysis, the pentose phosphate pathway and the tricarboxylic acid (TCA) cycle. In addition we determined oxygen consumption rates (OCR) using a metabolic extracellular flux analyzer, as well as total intracellular ATP levels by high performance liquid chromatography (HPLC). Finally we explored the expression of key proteins involved in the regulation of glucose metabolism.

Conclusions/Findings

Our results demonstrate that, although the metabolic signature of IPSCs is not identical to that of hESCs, nonetheless they cluster with hESCs rather than with their somatic counterparts. ATP levels, lactate production and OCR revealed that human pluripotent cells rely mostly on glycolysis to meet their energy demands. Furthermore, our work points to some of the strategies which human pluripotent stem cells may use to maintain high glycolytic rates, such as high levels of hexokinase II and inactive pyruvate dehydrogenase (PDH).

Human procreation in unchartered territory: new twists in ethical discussions

Since their validation in mammals, there have been profound ethical discussions on the possible applications of somatic cell nuclear transfer, human embryonic stem cells and induced pluripotent stem cells to reproductive medicine. This has been the case whether these technologies were considered as direct (i.e. when procreation is the ultimate goal) or indirect applications. In most countries, the majority of these approaches have been either stringently regulated, or regulation has been strongly and consensually suggested. However, this is not necessarily the case for possibilities such as same-sex chimaeras or the direct differentiation of gametes from somatic cells, skipping a pluripotent cell intermediate. The author suggests that the field of reproductive medicine should be more proactive in discussing both current and emerging developments with possible implications for human reproduction, even those reaching beyond current paradigms.

Not all sperm are equal: functional mitochondria characterize a subpopulation of human sperm with better fertilization potential

Human sperm samples are very heterogeneous and include a low amount of truly functional gametes. Distinct strategies have been developed to characterize and isolate this specific subpopulation. In this study we have used fluorescence microscopy and fluorescence-activated cell sorting to determine if mitochondrial function, as assessed using mitochondrial-sensitive probes, could be employed as a criterion to obtain more functional sperm from a given ejaculate. We first determined that mitochondrial activity correlated with the quality of distinct human samples, from healthy donors to patients with decreased semen quality. Furthermore, using fluorescence-activated cell sorting to separate sperm with active and inactive mitochondria we found that this was also true within samples. Indeed, sperm with active mitochondria defined a more functional subpopulation, which contained more capacitated and acrosome intact cells, sperm with lower chromatin damage, and, crucially, sperm more able to decondense and participate in early development using both chemical induction and injection into mature bovine oocytes. Furthermore, cell sorting using mitochondrial activity produced a more functional sperm subpopulation than classic swim-up, both in terms of improvement in a variety of functional sperm parameters and in statistical significance. In conclusion, whatever the true biological role of sperm mitochondria in fertilization, mitochondrial activity is a clear hallmark of human sperm functionality.

Xenografting as a tool to preserve endangered species: outcomes and challenges in model systems

The use of testis tissue xenografting as a valuable tool to rescue endangered and genetically valuable individuals that die young or otherwise fail to produce sperm has been the subject of much interest. Although the technique has been successfully applied to a wide variety of species, little is known about what determines the outcome. Furthermore, to improve the applicability of xenografting, new methods to preserve and transport testis tissue from valuable animals are emerging. However, one major issue remains: the application of xenografting implies the development of subsequent ART techniques to produce offspring from the recovered material. This paper focuses on these three aspects of testis tissue xenografting as a tool for rescuing endangered and valuable genetic pools.

Proton leak modulation in testicular mitochondria affects reactive oxygen species production and lipid peroxidation

Mitochondrial proton leak can account for almost 20% of oxygen consumption and it is generally accepted that this process contributes to basal metabolism. In order to clarify the role of basal proton leak in testicular mitochondria, we performed a comparative study with kidney and liver mitochondrial fractions. Proton leak stimulated by linoleic acid and inhibited by guanosine diphosphate (GDP) was detected, in a manner that was correlated with protein levels for uncoupling protein 2 (UCP2) in the three fractions. Modulation of proton leak had an effect on reactive oxygen species production as well as on lipid peroxidation, and this effect was also tissue-dependent. However, a possible role for the adenine nucleotide transporter (ANT) in testicular mitochondria proton leak could not be excluded. The modulation of proton leak appears as a possible and attractive target to control oxidative stress with implications for male gametogenesis.

Aging, mitochondria and male reproductive function

The rise in life expectancy over the last century, together with higher maternal and paternal ages and have highlighted the issue of reduced fertility with advancing age. Aging of the male reproductive system is incited by multi-factorial changes at molecular, cellular and regulatory levels, and individual characteristics are highly variable, although strongly influenced by lifestyle and environmental factors. Damage accumulated with age leads to progressive deregulation of the hypothalamic-pituitary-gonadal axis and of local auto/paracrine interactions, thereby inducing changes in target organs such as the testis, penis and prostate. Elderly human males produce less testosterone, have fewer motile sperm and a higher incidence of erectile dysfunction and prostate disorders, all of which contribute to lower fertility. Cellular aging can manifest itself at several levels. Aging cells progressively accumulate "waste" products, resulting in a decreased functionally. Changes to mitochondria are among the most remarkable features observed in aging cells and several theories place mitochondria at the hub of cellular events related to aging, namely in terms of the accumulation of oxidative damage to cells and tissues, a process in which these organelles may play a prominent role, although alternative theories have also emerged. Furthermore, mitochondrial energy metabolism is also crucial for male reproductive function and mitochondria may therefore constitute a common link between aging and fertility loss.

LOCALIZATION OF SNARES, NSF AND CAVEOLIN 1 IN HUMAN SPERMATOZOA: RELATIONSHIP WITH SEMINAL PARAMETERS

Membrane fusion is a very important process in gametes. The mechanism of membrane fusion during the AR has been proposed to involve SNAREs. Our aim is to quantify patterns of localization of Caveolin 1, SNAREs (Syntaxin 1A, Syntaxin 2 and VAMP 1) and NSF on human sperm, to determine how the differential distribution of these proteins might be interdependent and to evaluate if this distribution is related with seminal parameters. These proteins are present in different regions of the head of human sperm: anterior, equatorial and posterior regions and that Syntaxin 2 and Syntaxin 1A had a slightly different pattern of labelling. The presence and localization of SNAREs, NSF and Caveolin 1 do not correlate with seminal parameters. There is significant correlation between NSF and SNAREs, which may indicate a cooperation of these proteins in membrane fusion mechanisms of human sperm.

PRESENCE OF N-ETHYL MALEIMIDE SENSITIVE FACTOR (NSF) ON THE ACROSOME OF MAMMALIAN SPERM

We have extended previous observations to show that the ATPase N-ethyl maleimide sensitive factor (NSF) an important regulator of membrane trafficking and fusion in somatic cells, is present on bovine, murine and rhesus macaque sperm. However, NSFs main effectors, alfa- and beta-SNAP, although present in the developing acrosome, could not be detected in the mature organelle. The fact that NSF localizes mainly to the acrosome suggests that this protein, together with other factors such as rabs and SNAREs, may be a common feature in the triggering/regulation of membrane merging during the mammalian acrosome reaction.