Differential expression of succinyl CoA transferase (SCOT) genes in somatic and germline cells of the mouse testis

Ketone Body β-Hydroxy-Butyrate Sustains Progressive Motility in Capacitated Human Spermatozoa: A Possible Role in Natural Fertility

Background Calorie restriction is recognized as a useful nutritional approach to improve the endocrine derangements and low fertility profile associated with increased body weight. This is particularly the case for dietary regimens involving ketosis, resulting in increased serum levels of ketone bodies such as β-hydroxy-butyrate (β-HB). In addition to serum, β-HB is detected in several biofluids and β-HB levels in the follicular fluid are strictly correlated with the reproductive outcome in infertile females. However, a possible direct role of ketone bodies on sperm function has not been addressed so far. Methods Semen samples were obtained from 10 normozoospermic healthy donors attending the University Andrology Unit as participants in an infertility survey programme. The effect of β-HB on cell motility in vitro was evaluated on isolated spermatozoa according to their migratory activity in a swim-up selection procedure. The effect of β-HB on spermatozoa undergone to capacitation was also assessed. Results Two hours of exposure to β-HB, 1 mM or 4 mM, proved to be ineffective in modifying the motility of freshly ejaculated spermatozoa isolated according to the migratory activity in a swim-up procedure (all p values > 0.05). Differently, sperm maintenance in 4 mM β-HB after capacitation was associated with a significantly higher percentage of sperm cells with progressive motility compared to β-HB-lacking control (respectively, 67.6 ± 3.5% vs. 55.3 ± 6.5%, p = 0.0158). Succinyl-CoA transferase inhibitor abolished the effect on motility exerted by β-HB, underpinning a major role for this enzyme. Conclusion Our results suggest a possible physiological role for β-HB that could represent an energy metabolite in support of cell motility on capacitated spermatozoa right before encountering the oocyte.

Reproductive Suppression Caused by Spermatogenic Arrest: Transcriptomic Evidence from a Non-Social Animal

Reproductive suppression is an adaptive strategy in animal reproduction. The mechanism of reproductive suppression has been studied in social animals, providing an essential basis for understanding the maintenance and development of population stability. However, little is known about it in solitary animals. The plateau zokor is a dominant, subterranean, solitary rodent in the Qinghai-Tibet Plateau. However, the mechanism of reproductive suppression in this animal is unknown. We perform morphological, hormonal, and transcriptomic assays on the testes of male plateau zokors in breeders, in non-breeders, and in the non-breeding season. We found that the testes of non-breeders are smaller in weight and have lower serum testosterone levels than those of breeders, and the mRNA expression levels of the anti-Müllerian hormone (AMH) and its transcription factors are significantly higher in non-breeder testes. Genes related to spermatogenesis are significantly downregulated in both meiotic and post-meiotic stages in non-breeders. Genes related to the meiotic cell cycle, spermatogenesis, flagellated sperm motility, fertilization, and sperm capacitation are significantly downregulated in non-breeders. Our data suggest that high levels of AMH may lead to low levels of testosterone, resulting in delayed testicular development, and physiological reproductive suppression in plateau zokor. This study enriches our understanding of reproductive suppression in solitary mammals and provides a basis for the optimization of managing this species.

Motility Assessment of Ram Spermatozoa

For successful fertilisation to occur, spermatozoa need to successfully migrate through the female reproductive tract and penetrate the oocyte. Predictably, poor sperm motility has been associated with low rates of fertilisation in many mammalian species, including the ram. As such, motility is one of the most important parameters used for in vitro evaluation of ram sperm quality and function. This review aims to outline the mechanical and energetic processes which underpin sperm motility, describe changes in motility which occur as a result of differences in sperm structure and the surrounding microenvironment, and assess the effectiveness of the various methods used to assess sperm motility in rams. Methods of subjective motility estimation are convenient, inexpensive methods widely used in the livestock industries, however, the subjective nature of these methods can make them unreliable. Computer-assisted sperm analysis (CASA) technology accurately and objectively measures sperm motility via two-dimensional tracing of sperm head motion, making it a popular method for sperm quality assurance in domesticated animal production laboratories. Newly developed methods of motility assessment including flagellar tracing, three-dimensional sperm tracing, in vivo motility assessment, and molecular assays which quantify motility-associated biomarkers, enable analysis of a new range of sperm motion parameters with the potential to reveal new mechanistic insights and improve ram semen assessment. Experimental application of these technologies is required to fully understand their potential to improve semen quality assessment and prediction of reproductive success in ovine artificial breeding programs.

Frataxin controls ketone body metabolism through regulation of OXCT1

Friedreich's ataxia (FRDA) is an autosomal recessive neurodegenerative disease caused by the deficiency of mitochondrial protein frataxin, which plays a crucial role in iron-sulphur cluster formation and ATP production. The cellular function of frataxin is not entirely known. Here, we demonstrate that frataxin controls ketone body metabolism through regulation of 3-Oxoacid CoA-Transferase 1 (OXCT1), a rate limiting enzyme catalyzing the conversion of ketone bodies to acetoacetyl-CoA that is then fed into the Krebs cycle. Biochemical studies show a physical interaction between frataxin and OXCT1 both in vivo and in vitro. Frataxin overexpression also increases OXCT1 protein levels in human skin fibroblasts while frataxin deficiency decreases OXCT1 in multiple cell types including cerebellum and skeletal muscle both acutely and chronically, suggesting that frataxin directly regulates OXCT1. This regulation is mediated by frataxin-dependent suppression of ubiquitin-proteasome system (UPS)-dependent OXCT1 degradation. Concomitantly, plasma ketone bodies are significantly elevated in frataxin deficient knock-in/knockout (KIKO) mice with no change in the levels of other enzymes involved in ketone body production. In addition, ketone bodies fail to be metabolized to acetyl-CoA accompanied by increased succinyl-CoA in vitro in frataxin deficient cells, suggesting that ketone body elevation is caused by frataxin-dependent reduction of OXCT1 leading to deficits in tissue utilization of ketone bodies. Considering the potential role of metabolic abnormalities and deficiency of ATP production in FRDA, our results suggest a new role for frataxin in ketone body metabolism and also suggest modulation of OXCT1 may be a potential therapeutic approach for FRDA.

Sex hormones regulate lipid metabolism in adult Sertoli cells: A genome-wide study of estrogen and androgen receptor binding sites

Optimal functioning of Sertoli cells is crucial for spermatogenesis which is under tight regulation of sex hormones, estrogen and androgen. Adult rat Sertoli cells expresses estrogen receptor beta (ERβ) and androgen receptor (AR), both of which regulate gene transcription by binding to the DNA. The present study is aimed to acquire a genome-wide map of estrogen- and androgen-regulated genes in adult Sertoli cells. ChIP-Seq was performed for ERβ and AR in Sertoli cells under physiological conditions. 30,859 peaks in ERβ and 9,594 peaks in AR were identified with a fold enrichment >2 fold. Pathway analysis for the genes revealed metabolic pathways to be significantly enriched. Since Sertoli cells have supportive functions and provide energy substrates to germ cells during spermatogenesis, significantly enriched metabolic pathways were explored further. Peaks of the genes involved in lipid metabolism, like fatty acid, glyceride, leucine, and sphingosine metabolism were validated. Motif analysis confirmed the presence of estrogen- and androgen-response elements (EREs and AREs). Moreover, transcript levels of enzymes involved in the lipid metabolic pathways were significantly altered in cultured Sertoli cells treated with estrogen and androgen receptor agonists, demonstrating functional significance of these binding sites. This study elucidates a mechanism by which sex hormones regulate lipid metabolism in Sertoli cells by transcriptionally controlling the expression of these genes, thereby shedding light on the roles of these hormones in male fertility.

A new hyperpolarized 13C ketone body probe reveals an increase in acetoacetate utilization in the diabetic rat heart

Emerging studies have recently shown the potential importance of ketone bodies in cardio-metabolic health. However, techniques to determine myocardial ketone body utilization in vivo are lacking. In this work, we developed a novel method to assess myocardial ketone body utilization in vivo using hyperpolarized [3-¹³C]acetoacetate and investigated the alterations in myocardial ketone body metabolism in diabetic rats. Within a minute upon injection of [3-¹³C]acetoacetate, the production of [5-¹³C]glutamate and [1-¹³C] acetylcarnitine can be observed real time in vivo. In diabetic rats, the production of [5-¹³C]glutamate was elevated compared to controls, while [1-¹³C]acetylcarnitine was not different. This suggests an increase in ketone body utilization in the diabetic heart, with the produced acetyl-CoA channelled into the tricarboxylic acid cycle. This observation was corroborated by an increase activity of succinyl-CoA:3-ketoacid-CoA transferase (SCOT) activity, the rate-limiting enzyme of ketone body utilization, in the diabetic heart. The increased ketone body oxidation in the diabetic hearts correlated with cardiac hypertrophy and dysfunction, suggesting a potential coupling between ketone body metabolism and cardiac function. Hyperpolarized [3-¹³C]acetoacetate is a new probe with potential for non-invasive and real time monitoring of myocardial ketone body oxidation in vivo, which offers a powerful tool to follow disease progression or therapeutic interventions.

Cardiac metabolic modulation upon low-carbohydrate low-protein ketogenic diet in diabetic rats studied in vivo using hyperpolarized 13 C pyruvate, butyrate and acetoacetate probes

AIM

To investigate the effects of long-term low-carbohydrate low-protein ketogenic diet (KD) on cardiac metabolism and diabetic cardiomyopathy status in lean diabetic Goto-Kakizaki (GK) rats.

MATERIALS AND METHODS

Diabetic GK rats were fed with KD for 62 weeks. Cardiac function and metabolism were assessed using magnetic resonance imaging and ¹³ C magnetic resonance spectroscopy (¹³ C-MRS), at rest and under dobutamine stress. ¹³ C-MRS was performed following injection of hyperpolarized [3-¹³ C]acetoacetate, [1-¹³ C]butyrate or [1-¹³ C]pyruvate to assess ketone body, short-chain fatty acid or glucose utilization, respectively. Protein expression and cardiomyocyte structure were determined via Western blotting and histology, respectively.

RESULTS

KD lowered blood glucose, triglyceride and insulin levels while increasing blood ketone body levels. In KD-fed diabetic rats, myocardial ketone body and glucose oxidation were lower than in chow-fed diabetic rats, while myocardial glycolysis and short-chain fatty acid oxidation were unaltered. Dobutamine stress revealed an increased cardiac preload and reduced cardiac compliance in KD-fed diabetic rats. Dobutamine-induced stimulation of myocardial glycolysis was more enhanced in KD-fed diabetic rats than in chow-fed diabetic rats, which was potentially facilitated via an upregulation in basal expression of proteins involved in glucose transport and glycolysis in the hearts of KD-fed rats. The metabolic profile induced by KD was accompanied by cardiac hypertrophy, a trend for increased myocardial lipid and collagen content, and an increased marker of oxidative stress.

CONCLUSION

KD seems to exacerbate diabetic cardiomyopathy in GK rats, which may be associated with maladaptive cardiac metabolic modulation and lipotoxicity.

The role of OXCT1 in the pathogenesis of cancer as a rate-limiting enzyme of ketone body metabolism

Cancer cells are well documented to reprogram their metabolism in order to support the maintenance and reproduction. 3-oxoacid CoA-transferase 1 (OXCT1) is a key enzyme in ketone body metabolism that catalyzes the first and rate-determining step of ketolysis. The product of OXCT1 converts to acetyl-CoA and finally fed into the tricarboxylic acid cycle for oxidation and ATP production. However, little is known of its regulation right now. Recently, some studies suggested that OXCT1 participates in tumorigenesis and signaling in cancer cells. Furthermore, our recent work showed that a marked elevation of OXCT1 expression in different categories of cancer cells. Here we review the metabolic functions of OXCT1 and its surprising roles in supporting the biological hallmarks of malignancy. We also review recent efforts in exploring the mechanism responsible for the tumor promoting effect of OXCT1 and suggest a novel therapeutic target for cancer therapy.

Differences in ATP Generation Via Glycolysis and Oxidative Phosphorylation and Relationships with Sperm Motility in Mouse Species

Mouse sperm produce enough ATP to sustain motility by anaerobic glycolysis and respiration. However, previous studies indicated that an active glycolytic pathway is required to achieve normal sperm function and identified glycolysis as the main source of ATP to fuel the motility of mouse sperm. All the available evidence has been gathered from the studies performed using the laboratory mouse. However, comparative studies of closely related mouse species have revealed a wide range of variation in sperm motility and ATP production and that the laboratory mouse has comparatively low values in these traits. In this study, we compared the relative reliance on the usage of glycolysis or oxidative phosphorylation as ATP sources for sperm motility between mouse species that exhibit significantly different sperm performance parameters. We found that the sperm of species with higher oxygen consumption/lactate excretion rate ratios were able to produce higher amounts of ATP, achieving higher swimming velocities. Additionally, we show that the species with higher respiration/glycolysis ratios have a higher degree of dependence upon active oxidative phosphorylation. Moreover, we characterize for the first time two mouse species in which sperm depend on functional oxidative phosphorylation to achieve normal performance. Finally, we discuss that sexual selection could promote adaptations in sperm energetic metabolism tending to increase the usage of a more efficient pathway for the generation of ATP (and faster sperm).

A novel transcriptional factor Nkapl is a germ cell-specific suppressor of Notch signaling and is indispensable for spermatogenesis

Spermatogenesis is an elaborately regulated system dedicated to the continuous production of spermatozoa via the genesis of spermatogonia. In this process, a variety of genes are expressed that are relevant to the differentiation of germ cells at each stage. Although Notch signaling plays a critical role in germ cell development in Drosophila and Caenorhabditis elegans, its function and importance for spermatogenesis in mammals is controversial. We report that Nkapl is a novel germ cell-specific transcriptional suppressor in Notch signaling. It is also associated with several molecules of the Notch corepressor complex such as CIR, HDAC3, and CSL. It was expressed robustly in spermatogonia and early spermatocytes after the age of 3 weeks. Nkapl-deleted mice showed complete arrest at the level of pachytene spermatocytes. In addition, apoptosis was observed in this cell type. Overexpression of NKAPL in germline stem cells demonstrated that Nkapl induced changes in spermatogonial stem cell (SSC) markers and the reduction of differentiation factors through the Notch signaling pathway, whereas testes with Nkapl deleted showed inverse changes in those markers and factors. Therefore, Nkapl is indispensable because aberrantly elevated Notch signaling has negative effects on spermatogenesis, affecting SSC maintenance and differentiation factors. Notch signaling should be properly regulated through the transcriptional factor Nkapl.

Metabolic substrates exhibit differential effects on functional parameters of mouse sperm capacitation

Although substantial evidence exists that sperm ATP production via glycolysis is required for mammalian sperm function and male fertility, conflicting reports involving multiple species have appeared regarding the ability of individual glycolytic or mitochondrial substrates to support the physiological changes that occur during capacitation. Several mouse models with defects in the signaling pathways required for capacitation exhibit reductions in sperm ATP levels, suggesting regulatory interactions between sperm metabolism and signal transduction cascades. To better understand these interactions, we conducted quantitative studies of mouse sperm throughout a 2-h in vitro capacitation period and compared the effects of single substrates assayed under identical conditions. Multiple glycolytic and nonglycolytic substrates maintained sperm ATP levels and comparable percentages of motility, but only glucose and mannose supported hyperactivation. These monosaccharides and fructose supported the full pattern of tyrosine phosphorylation, whereas nonglycolytic substrates supported at least partial tyrosine phosphorylation. Inhibition of glycolysis impaired motility in the presence of glucose, fructose, or pyruvate but not in the presence of hydroxybutyrate. Addition of an uncoupler of oxidative phosphorylation reduced motility with pyruvate or hydroxybutyrate as substrates but unexpectedly stimulated hyperactivation with fructose. Investigating differences between glucose and fructose in more detail, we demonstrated that hyperactivation results from the active metabolism of glucose. Differences between glucose and fructose appeared to be downstream of changes in intracellular pH, which rose to comparable levels during incubation with either substrate. Sperm redox pathways were differentially affected, with higher levels of associated metabolites and reactive oxygen species generated during incubations with fructose than during incubations with glucose.

Identification of ORF sequences and exercise-induced expression change in thoroughbred horse OXCT1 gene

In the mitochondrial matrix, the OXCT1 gene catalyzes the reversible transfer of coenzyme A from succinyl-CoA to acetoacetate in a reaction related to energy production from ketone bodies. Here, horse OXCT1 gene containing coenzyme A transferase domain was identified in the transcriptome analysis of cDNAs derived from skeletal muscles. Horse OXCT1 gene consisted of 1761 [corrected] nucleotide sequences with an open reading frame of 1560 nucleotides encoding a protein of 520 putative amino acid residues.The number of non-synonymous substitutions was lower than the number of synonymous substitutions in the OXCT1 genes of other species, indicating that purifying selection occurred in the OXCT1 genes during evolutionary radiation. Quantitative real-time RT-RCR analysis showed a dominant expression pattern of horse OXCT1 gene in the cerebrum, heart, and skeletal muscle. Different expression levels of horse OXCT1 transcripts between before- and after-exercise samples were also measured in the skeletal muscles of six horses. These data could be of great use for further investigation of the relationship between energy products and horse OXCT1 gene.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Cloning and characterization of a mouse spergen-1 localized in sperm mitochondria

Development of spermatozoa is a complex process involving specific morphological formation of flagella, nucleus and mitochondria. Although detailed morphological observations of these events are available, the molecular mechanisms remain to be fully elucidated. We report here the molecular cloning and characterization of mouse spergen-1 encoding a sperm specific mitochondrial protein, from a haploid germ cell-specific subtracted cDNA library of mouse testis. Isolated cDNA is c. 0.7 kb and contains a 465 bp ORF that encodes mouse spergen-1, a sperm mitochondrial protein consisting of 154 predicted amino acids. Antibodies raised against mouse Spergen-1 identified a testis-specific c. 18 Mr x 10(3) band in Western blot analysis. The protein was localized to the mitochondria of mouse sperm. Comparison of the mouse and human genomic sequences showed that 55 bps of the 5'-upstream region containing a CAAT box and binding sequence for NF-kappa B is conserved and could be important for specific expression of mouse spergen-1 in haploid germ cells.

Ketone bodies could support the motility but not the acrosome reaction of mouse sperm

Ketone bodies, D-beta-hydroxybutyrate and acetoacetate, produced by the metabolism of fatty acids, are an important energy source for many organs, especially the heart, kidney and brain. They are utilized by the body with the help of succinyl CoA transferase (SCOT), which is ubiquitously expressed in various organs. Previously, we identified a novel SCOT-t specifically expressed in testicular germ cells and sperm, substituting somatic cell-type SCOT, however the physiological role of SCOT-t had not then been clarified. In the present study, we investigated the effects of ketone bodies, the substrate of SCOT-t, on the motility and acrosome reaction of mouse sperm. D-beta-hydroxybutyrate and acetoacetate both stimulated the motility of sperm as glucose or pyruvate. The glycolysis inhibitor stopped the motility of sperm mediated by glucose but not by D-beta-hydroxybutyrate. In contrast, ketone bodies did not stimulate the activation of the acrosome reaction of sperm, different from the effect of glucose. These results indicate that ketone bodies could be involved in sperm movement but not the acrosome reaction and the SCOT-t enzyme we have identified in sperm mitochondria may have important roles in the activity of sperm, resulting in male infertility when its function is disabled.