Insulin-sperm interaction: effects on plasma membrane and binding to acrosome

Triglycerides/Glucose Index is Associated with Sperm Parameters and Sperm DNA Fragmentation in Primary Infertile Men: A Cross-Sectional Study

Study question: we aimed to investigate the relationship between the tyg index and both semen and hormonal characteristics in a cohort of primary infertile men. Summary answer: almost one in two primary infertile men presented with a triglycerides/glucose index (tyg) suggestive of insulin resistance (ir). overall, patients with tyg suggestive of ir showed worse clinical, hormonal, and semen parameters. What is already known: male factor infertility (MFI) is often associated with metabolic disorders such as diabetes mellitus and metabolic syndrome, where insulin resistance (IR) plays a relevant pathological role. Recently, TyG has been suggested as a user-friendly IR marker. Study Design: serum hormones and the sperm DNA fragmentation index (SDF) were measured in every patient. The semen analysis was based on 2010 WHO reference criteria. Glucose and insulin levels were measured for every man after a 12-h overnight fast, and the homeostatic model assessment index (HOMA-IR) was then calculated and categorized using a 2.6 threshold. Similarly, fasting glucose and triglycerides levels were measured and the TyG index was calculated and categorized using an 8.1 threshold. Descriptive statistics and logistic regression models tested the association between the TyG and semen and hormonal characteristics. Participants: complete demographic, clinical, and laboratory data from 726 consecutive white European primary infertile men were considered for this analysis. Main results and the role of chance: the median (IQR) age was 39 (35–43) years. A TyG and HOMA suggestive for IR was found in 339 (46.6%) and 154 (21.2%) men, respectively. During the Spearman’s test, the TyG index was highly correlated with HOMA-IR (rho = 0.46, p < 0.001). Compared to men with a normal TyG, men with TyG > 8.1 were older, had greater BMI and CCI scores, and lower total testosterone and sperm concentration, but higher DFI, and presented a greater proportion of NOA (all p < 0.01). The multivariable logistic regression analysis showed that men with TyG > 8.1 were at higher risk of SDF > 30 (OR 1.92 (CI: 1.2–2.9)) and NOA (OR 1.78 (CI: 1.1–2.8)). Wider implications of the findings: the Tyng index may act as a reliable marker of IR in the clinical work-up of primary infertile men in real-life settings.

Evaluation of testicular glycogen storage, FGF21 and LDH expression and physiological parameters of sperm in hyperglycemic rats treated with hydroalcoholic extract of Securigera Securidaca seeds, and Glibenclamide

Structural and physiological changes in sperm and semen parameters reduce fertility in diabetic patients. Securigera Securidaca (S. Securidaca) seed is a herbal medicine with hypoglycemic, antioxidant, and anti-hypertensive effects. The question now is whether this herbal medicine improves fertility in diabetic males. The study aimed to evaluate the effects of hydroalcoholic extract of S. Securidaca seeds (HESS), glibenclamide and a combination of both on fertility in hyperglycemic rats by comparing histological and some biochemical changes in testicular tissue and sperm parameters. The treatment protocol included administration of three doses of HESS and one dose of glibenclamide, as well as treatment with both in diabetic Wistar diabetic rats and comparison of the results with untrated groups. The quality of the testicular tissue as well as histometric parameters and spermatogenesis indices were evaluated during histopathological examination. Epididymal sperm analysis including sperm motility, viability, abnormalities, maturity, and chromatin structure were studied. The effect of HESS on the expression of LDH and FGF21 genes and tissue levels of glycogen, lactate, and total antioxidant capacity in testicular tissue was investigated and compared with glibenclamide. HESS improved sperm parameters in diabetic rats but showed little restorative effect on damaged testicular tissue. In this regard, glibenclamide was more effective than the highest dose of HESS and its combination with HESS enhanced its effectiveness so that histological tissue characteristics and sperm parameters were were comparable to those of healthy rats. The expression level of testicular FGF21 gene increased in diabetic rats, which intensified after treatment with HESS as well as glibenclamide. The combination of HESS and glibenclamide restored the expression level of testicular LDH gene, as well as tissue storage of glycogen, lactate and LDH activity, and serum testosterone to the levels near healthy control. S. Securidaca seeds can be considered as an effective supplement in combination with hypoglycemic drugs to prevent infertility complications in diabetes.

A closer look at the role of insulin for the regulation of male reproductive function

While insulin demonstrates to have a considerable influence on the reproductive system, there are various unanswered questions regarding its precise sites, mechanisms of action, and roles for the developing and functioning of the adult male reproductive system. Apart from its effects on glucose level, insulin has an important role in the reproductive system directly by binding on insulin and IGF receptors in the brain and testis. To date, however, the effect of insulin or its alterations on blood-testis-barrier, as an important regulator of normal spermatogenesis and fertility, has not yet been studied. This review aimed to focus on the experimental and clinical studies to describe mechanisms by which insulin affects the hypothalamic-pituitary-gonadal (HPG) axis, testicular cells, spermatozoa, and sexual behavior. Moreover, we discussed the mechanism and impact of insulin changes in type 1 (insulin deficiency along with persisted or even increased sensitivity) and 2 (insulin resistance along with increased insulin level at the early stages of disease) diabetes and obesity on the male reproductive tract.

Diabetes mellitus and the impairment of male reproductive function: Possible signaling pathways

BACKGROUND AND AIMS

Today, it has been shown that diabetes mellitus (DM) can affect male fertility. Glucose metabolism is a vital process in spermatogenesis that is impacted by diabetes condition. But the mechanisms by which DM causes male infertility are not wholly clarified. The aim of this review is to provide brief information about the influence of hyperglycemia on male fertility and specific emphasis on the molecular signaling pathway that is involved.

METHODS

Broad literature search in the electronic database "Pubmed", "Google Scholar", the website of "World Health Organization" (WHO) and Control Disease and Prevention (CDC) took place. There was no time restriction. A key criterion for the selection of articles was English and language. Finally, one hundred thirty seven articles were included in the review.

RESULTS

Diabetes mellitus affects many signaling pathways that involved in the spermatogenesis. It seems that increased ROS and oxidative stress in the diabetes is the beginning of all fertility problems and affects all of involved signaling pathways in the spermatogenesis.

CONCLUSIONS

It seems that there was strong interconnected between oxidative stress and all of involved signaling pathways in the reproductive problems in diabetes. So, approaches that diminish oxidative stress in the testis can be effective in improving diabetes related infertility complications.

Male reproductive health and intergenerational metabolic responses from a small RNA perspective

The world has recently experienced a decline in male reproductive (e.g. sperm counts and motility) and metabolic (e.g. obesity and diabetes) health. Accumulated evidence from animal models also shows that the metabolic health of the father may influence the metabolic health in his offspring. Vectors for such paternal intergenerational metabolic responses (IGMRs) involve small noncoding RNAs (sncRNAs) that often increase in spermatozoa during the last days of maturation in the epididymis. We and others have shown that the metabolic state - depending on factors such as diet, obesity and physical exercise - may affect sperm quality and sperm sncRNA. Together, this suggests that there are overlapping aetiologies between the male metabolic syndrome, male factor infertility and intergenerational responses. In this review, we present a theoretical framework for an overlap of these aetiologies by exploring the advances in our understanding of the roles of sncRNA in spermatogenesis and offspring development. A special focus will lie on novel findings about tRNA-derived small RNA (tsRNA), rRNA-derived small RNA (rsRNA) and small mitochondrial RNA (mitoRNA), and their emerging roles in intergenerational metabolic and reproductive health.

Glucose, insulin, insulin receptor subunits α and β in normal and spontaneously diabetic and obese ob/ob and db/db infertile mouse testis and hypophysis

BACKGROUND

Type 2 diabetes touches young subjects of reproductive age in epidemic proportion. This study assesses glucose, total InsulinT, Insulin2 and insulin receptor subunits α and β in testis during mouse development then, in the spontaneously type 2 diabetes models associated with infertility db/db and ob/ob mice. IR-β and α were also assessed in spermatozoa (SPZ), anterior pituitary (AP) and serum.

METHODS

Serum and tissue glucose were measured with enzymatic colorimetric assays and InsulinT and Insulin2 by ELISAs in serum, interstitial tissue- (ITf) and seminiferous tubule (STf) fractions in14- > 60-day-old normal and db/db, ob/ob and wild type (WT) mice. IR subunits were assessed by immunoblotting in tissues and by immunoprecipitation followed by immunoblotting in serum.

RESULTS

Development: Glucose increased in serum, ITf and STf. InsulinT and Insulin2 dropped in serum; both were higher in STf than in ITf. In > 60-day-old mouse ITf, insulinT rose whereas Insulin2 decreased; InsulinT and Insulin2 rose concurrently in STf. Glucose and insulin were high in > 60-day-old ITf; in STf high insulin2 accompanied low glucose. One hundred ten kDa IR-β peaked in 28-day-old ITf and 14-day-old STf. One hundred thirty five kDa IR-α was high in ITf but decreased in STf. Glucose escalated in db/db and ob/ob sera. Glucose doubled in ITf while being halved in STf in db/db mice. Glucose significantly dropped in db/db and ob/ob mice spermatozoa. InsulinT and Insulin2 rose significantly in the serum, ITf and STf in db/db and ob/ob mice. One hundred ten kDa IR-β and 135 kDa IR-α decreased in db/db and ob/ob ITf. Only 110 kDa IR-β dropped in db/db and ob/ob STf and AP. One hundred ten kDa IR-β fell in db/db and ob/ob SPZ. One hundred ten kDa sIR-α rose in the db/db and ob/ob mouse sera.

CONCLUSION

Insulin regulates glucose in tubules not in the interstitium. The mouse interstitium contains InsulinT and Insulin2 whereas tubules contain Insulin2. Decreased 110 kDa IR-β and 135 kDa IR-α in the db/db and ob/ob interstitial tissue suggest a loss of active receptor sites that could alter the testicular cell insulin binding and response to the hormone. Decreased IR-β levels were insufficient to stimulate downstream effectors in AP and tubules. IR-α shedding increased in db/db and ob/ob mice.

Human sperm displays rapid responses to diet

The global rise in obesity and steady decline in sperm quality are two alarming trends that have emerged during recent decades. In parallel, evidence from model organisms shows that paternal diet can affect offspring metabolic health in a process involving sperm tRNA-derived small RNA (tsRNA). Here, we report that human sperm are acutely sensitive to nutrient flux, both in terms of sperm motility and changes in sperm tsRNA. Over the course of a 2-week diet intervention, in which we first introduced a healthy diet followed by a diet rich in sugar, sperm motility increased and stabilized at high levels. Small RNA-seq on repeatedly sampled sperm from the same individuals revealed that tsRNAs were up-regulated by eating a high-sugar diet for just 1 week. Unsupervised clustering identified two independent pathways for the biogenesis of these tsRNAs: one involving a novel class of fragments with specific cleavage in the T-loop of mature nuclear tRNAs and the other exclusively involving mitochondrial tsRNAs. Mitochondrial involvement was further supported by a similar up-regulation of mitochondrial rRNA-derived small RNA (rsRNA). Notably, the changes in sugar-sensitive tsRNA were positively associated with simultaneous changes in sperm motility and negatively associated with obesity in an independent clinical cohort. This rapid response to a dietary intervention on tsRNA in human sperm is attuned with the paternal intergenerational metabolic responses found in model organisms. More importantly, our findings suggest shared diet-sensitive mechanisms between sperm motility and the biogenesis of tsRNA, which provide novel insights about the interplay between nutrition and male reproductive health.

Can Antidiabetic Drugs Improve Male Reproductive (Dys)Function Associated with Diabetes?

The alarming increase in the number of diabetic patients worldwide raises concerns regarding the impact of the disease on global health, not to mention on social and economic aspects. Furthermore, the association of this complex metabolic disorder with male reproductive impairment is worrying, mainly due to the increasing chances that young individuals, at the apex of their reproductive window, could be affected by the disease, further contributing to the disturbing decline in male fertility worldwide. The cornerstone of diabetes management is glycemic control, proven to be effective in avoiding, minimizing or preventing the appearance or development of disease-related complications. Nonetheless, the possible impact of these therapeutic interventions on male reproductive function is essentially unexplored. To address this issue, we have made a critical assessment of the literature on the effects of several antidiabetic drugs on male reproductive function. While the crucial role of insulin is clear, as shown by the recovery of reproductive impairments in insulin-deficient individuals after treatment, the same clearly does not apply to other antidiabetic strategies. In fact, there is an abundance of controversial reports, possibly related to the various study designs, experimental models and compounds used, which include biguanides, sulfonylureas, meglitinides, thiazolidinediones/glitazones, bile acid sequestrants, amylin mimetics, as well as sodiumglucose co-transporter 2 (SGLT2) inhibitors, glucagon-like peptide 1 (GLP1), α-glucosidase inhibitors and dipeptidyl peptidase 4 (DPP4) inhibitors. These aspects constitute the focus of the current review.

Antidiabetic therapies and male reproductive function: where do we stand?

Diabetes mellitus has been increasing at alarming rates in recent years, thus jeopardizing human health worldwide. Several antidiabetic drugs have been introduced in the market to manage glycemic levels, and proven effective in avoiding, minimizing or preventing the appearance or development of diabetes mellitus-related complications. However, and despite the established association between such pathology and male reproductive dysfunction, the influence of these therapeutic interventions on such topics have been scarcely explored. Importantly, this pathology may contribute toward the global decline in male fertility, giving the increasing preponderance of diabetes mellitus in young men at their reproductive age. Therefore, it is mandatory that the reproductive health of diabetic individuals is maintained during the antidiabetic treatment. With this in mind, we have gathered the available information and made a critical analysis regarding the effects of several antidiabetic drugs on male reproductive function. Unlike insulin, which has a clear and fundamental role on male reproductive function, the other antidiabetic therapies' effects at this level seem incoherent. In fact, studies are highly controversial possibly due to the different experimental study approaches, which, in our opinion, suggests caution when it comes to prescribing such drugs to young diabetic patients. Overall, much is still to be determined and further studies are needed to clarify the safety of these antidiabetic strategies on male reproductive system. Aspects such as the effects of insulin levels variations, consequent of insulin therapy, as well as what will be the impact of the side effect hypoglycemia, common to several therapeutic strategies discussed, on the male reproductive system are still to be addressed.

Diabetes Mellitus and Infertility: Different Pathophysiological Effects in Type 1 and Type 2 on Sperm Function

Although the prevalence of sub-infertility in diabetic patients in childbearing age is known, the mechanisms by which diabetes mellitus (DM) causes male infertility are not completely explained. This detrimental effect is achieved with a variety of mechanisms that include pre-testicular, testicular, and post-testicular pathogenetic moments and can be different in type 1 diabetes mellitus (DM1) and type 2 diabetes mellitus (DM2) patients because of type of diabetes, duration of disease, and glycemic metabolic compensation. Aim of this study was to evaluate whether diabetic disease can be considered a risk factor for infertility considering the etiopathogenetic differences between DM1 and DM2 on sperm function. We enrolled 38 DM1 patients and 55 DM2 patients with idiopathic infertility history >12 months, and 100 healthy fertile subjects. The following outcomes were evaluated in optical microscopy and flow cytometry: sperm function (by conventional and biofunctional sperm parameters) and signs of urogenital infection/inflammation (by sperm leukocyte concentrations and indices of oxidative stress). Moreover, an andrological evaluation (by didymo-epididymal ultrasound evaluation, serum total testosterone, LH, and FSH measurements) was performed in DM1 and DM2 patients compared to controls. Diabetic patients showed a higher risk of becoming infertile and the pathophysiological mechanisms of damage were different in DM1 and DM2. Conventional sperm parameters of diabetic patients are worse than controls (p < 0.05). The DM2 caused an inflammatory condition with increased oxidative stress resulting in decreased sperm vitality and increased sperm DNA fragmentation. DM1 altered epididymal voiding causing low ejaculate volume and mitochondrial damage resulting in decreased sperm motility. These findings and evidences support the contention that DM could be regarded as cause of male infertility suggesting that the prevention of diabetic disease in DM2 and the follow-up of seminal parameters in DM1 could prevent fertility decline in these categories of patients.

The Expression of GLUT8, GLUT9a, and GLUT9b in the Mouse Testis and Sperm

The objective of this study is to confirm the precise glucose transporter (GLUT) 8 localization and determine the expression of GLUT9a and GLUT9b by Western blot and confocal and immunoelectron microscopy in the mouse testis and sperm. GLUT8, GLUT9a, and GLUT9b proteins are expressed in the most intraseminiferous tubula cells and Leydig cells. GLUT8 localizes in the midpiece and principal piece as well as in the acrosomal region of the sperm. Immunoelectron microscopic analysis shows that GLUT8 is strongly detectable at the acrosome and neck region of the sperm. In the midpiece, GLUT8 localizes at the outer dense fibers (odf) as well as at the circumference of the spiral mitochondria. In the principal piece, GLUT8 localizes at the odf. GLUT9a strictly localizes in the midpiece, but GLUT9b localizes in the acrosome, midpiece, and principal piece of the sperm. These results suggest that glucose uptake via GLUT8, GLUT9a, and GLUT9b likely affects normal spermatogenesis, steroidogenesis, and sperm function in the mouse.

Sperm glucose transport and metabolism in diabetic individuals

Individuals with diabetes mellitus (DM) present marked reduction in sperm quality and higher DNA damage in spermatozoa, evidencing that this metabolic disorder impairs male fertility. These effects are related to defective testicular metabolic pathways and signaling, resulting in altered sperm metabolism. Spermatozoa metabolize several substrates to ensure energy supplies and any alteration in this feature compromise sperm quality. For ATP production, spermatozoa require substrate availability and the involvement of specific hexose membrane carriers. DM is known to modulate the spermatozoa substrate consumption and/or production due to altered glycolytic behavior. In fact, glucose uptake and metabolism is highly deregulated in diabetic individuals. Herein, we present an overview of the implications of DM in sperm glucose uptake and metabolism. The understanding of these processes is essential to identify key mechanisms associated with DM-related male (in)fertility. Moreover, it may contribute to the development of therapeutics to counteract the dysfunction induced by DM in sperm metabolism.

Influence of male hyperinsulinaemia on IVF outcome

The IVF outcome of a group of hyperinsulinaemic men (group B) was compared with a group of IVF males with normal insulin levels (group A). The participating females in the study groups were younger than 38 years old, had blocked Fallopian tubes and/or endometriosis, had normal insulin levels and produced five or more ova on stimulation. The male participants in both groups were normozoospermic with motility above 50% and sperm morphology between 5 and 13% normal forms (G-pattern according to Tygerberg strict criteria). The two groups did not differ statistically in terms of age or semen parameters (P = 0.39; P < 0.05).The group of men that presented with normal insulin levels had a higher fertilisation rate (79.15% versus 74.57%) and superior embryo quality on day 3 (55.77% versus 50.39%) than the group that presented with hyperinsulinaemia, but these differences were not statistically significant (P = 0.28, P = 0.40; P < 0.05). The clinical pregnancy rate of the group with normal insulin was significantly higher than that of the hyperinsulinaemic group, 57.9% versus 31.8% respectively (P = 0.03). The results suggest that hyperinsulinaemia had a negative impact on IVF outcome and patients should be advised accordingly.

Molecular mechanisms beyond glucose transport in diabetes-related male infertility

Diabetes mellitus (DM) is one of the greatest public health threats in modern societies. Although during a few years it was suggested that DM had no significant effect in male reproductive function, this view has been challenged in recent years. The increasing incidence of DM worldwide will inevitably result in a higher prevalence of this pathology in men of reproductive age and subfertility or infertility associated with DM is expected to dramatically rise in upcoming years. From a clinical perspective, the evaluation of semen parameters, as well as spermatozoa deoxyribonucleic acid (DNA) integrity, are often studied due to their direct implications in natural and assisted conception. Nevertheless, recent studies based on the molecular mechanisms beyond glucose transport in testicular cells provide new insights in DM-induced alterations in male reproductive health. Testicular cells have their own glucose sensing machinery that react to hormonal fluctuations and have several mechanisms to counteract hyper- and hypoglycemic events. Moreover, the metabolic cooperation between testicular cells is crucial for normal spermatogenesis. Sertoli cells (SCs), which are the main components of blood-testis barrier, are not only responsible for the physical support of germ cells but also for lactate production that is then metabolized by the developing germ cells. Any alteration in this tied metabolic cooperation may have a dramatic consequence in male fertility potential. Therefore, we present an overview of the clinical significance of DM in the male reproductive health with emphasis on the molecular mechanisms beyond glucose fluctuation and transport in testicular cells.

Glucose and fructose as functional modulators of overall dog, but not boar sperm function

The main aim of the present work was to test the effects of glucose and fructose on the phosphorylation levels of proteins linked to the control of overall sperm function in two species with very different metabolic characteristics, dog and boar. Incubation of dog spermatozoa with 10mM glucose increased serine phosphorylation of proteins related to cell cycle and signal transduction including cyclins B and E, Cdk2, Cdk6, Cdc6, PYK2, c-kit, Raf-1, TRK and several protein phosphatases. Incubation of dog spermatozoa with 10mM fructose decreased serine phosphorylation levels of cyclins B and D3, Cdk1/Cdc2, Cdk2, Cdk6, Akt, PI3 kinase, ERK-1 and protein kinase C. Incubation of boar spermatozoa with glucose or fructose did not modify any of the phosphorylation patterns studied. Given that one important difference between dog and boar spermatozoa is the presence of glucokinase (GK) in dog but not in boar, GK-transfected COS7 cells were incubated with either 10mM glucose or 10mM fructose. Incubation of GK-transfected cells with fructose decreased serine phosphorylation of cyclin A, ERK-2 and Hsp-70. In contrast, incubation of control COS7 cells with fructose increased serine phosphorylation of Cdk6, Cdk1/Cdc2, protein kinase C and Hsp-70. Incubation with glucose did not induce any significant effect. Our results indicate that monosaccharides act as signalling compounds in dog spermatozoa after ejaculation through changes in the phosphorylation levels of specific proteins. One of the factors that may be related to the action of sugars is the equilibrium of the total sperm hexokinase activity, in which the presence or absence of GK appears to be relevant.

Proteomics: a subcellular look at spermatozoa

BACKGROUND

Male-factor infertility presents a vexing problem for many reproductively active couples. Many studies have focused on abnormal sperm parameters. Recent advances in proteomic techniques, especially in mass spectrometry, have aided in the study of sperm and more specifically, sperm proteins. The aim of this study was to review the current literature on the various proteomic techniques, and their usefulness in diagnosing sperm dysfunction and potential applications in the clinical setting.

METHODS

Review of PubMed database. Key words: spermatozoa, proteomics, protein, proteome, 2D-PAGE, mass spectrometry.

RESULTS

Recently employed proteomic methods, such as two-dimensional polyacrylamide gel electrophoresis, mass spectrometry, and differential in gel electrophoresis, have identified numerous sperm-specific proteins. They also have provided a further understanding of protein function involved in sperm processes and for the differentiation between normal and abnormal states. In addition, studies on the sperm proteome have demonstrated the importance of post-translational modifications, and their ability to bring about physiological changes in sperm function. No longer do researchers believe that in order for them to elucidate the biochemical functions of genes, mere knowledge of the human genome sequence is sufficient. Moreover, a greater understanding of the physiological function of every protein in the tissue-specific proteome is essential in order to unravel the biological display of the human genome.

CONCLUSION

Recent advances in proteomic techniques have provided insight into sperm function and dysfunction. Several multidimensional separation techniques can be utilized to identify and characterize spermatozoa. Future developments in bioinformatics can further assist researchers in understanding the vast amount of data collected in proteomic studies. Moreover, such advances in proteomics may help to decipher metabolites which can act as biomarkers in the detection of sperm impairments and to potentially develop treatment for infertile couples.Further comprehensive studies on sperm-specific proteome, mechanisms of protein function and its proteolytic regulation, biomarkers and functional pathways, such as oxidative-stress induced mechanisms, will provide better insight into physiological functions of the spermatozoa. Large-scale proteomic studies using purified protein assays will eventually lead to the development of novel biomarkers that may allow for detection of disease states, genetic abnormalities, and risk factors for male infertility. Ultimately, these biomarkers will allow for a better diagnosis of sperm dysfunction and aid in drug development.

Paternal effect on embryo quality in diabetic mice is related to poor sperm quality and associated with decreased glucose transporter expression

The objective of this study was to determine whether sperm quality, fertilization capacity, and subsequent embryo development are altered in diabetic male mice and whether differences in facilitative glucose transporter (GLUT; now known as solute carrier family 2, SLC2A) expression in the testis and sperm exist. Using two type 1 diabetic mouse models, SLC2A expression in the testis and sperm was determined by western immunoblotting and immunofluorescence staining. To address sperm quality and fertilization capacity, computer-assisted sperm analysis andin vitrofertilization were performed. SLC2A1, SLC2A3, and SLC2A5 did not change in expression in the testes or sperm between diabetic and non-diabetic mice. SLC2A8 and SLC2A9b were less expressed in the testes of both diabetic models versus controls. SLC2A9a was not expressed in the Akita testis or sperm when compared with strain-matched controls. 3β-hydroxysteroid dehydrogenase (HSD3B) expression was significantly decreased in the Leydig cells from the diabetic mice. Sperm concentration and motility were significantly lower in both the diabetics when compared with the control. These parameters normalized in Akita diabetic males treated with insulin. In addition, fertilization rates were significantly lower in the Akita group (17.9%) and the streptozotocin (STZ)-injected male group (43.6%) when compared with the normal group (88.8%). Interestingly, of the fertilized zygotes, embryo developmental rates to the blastocyst stage were lower in both diabetic models (7.1% Akita and 50.0% STZ) when compared with controls (71.7%). Male diabetes may cause male subfertility by altering steroidogenesis, sperm motility, and SLC2A expression. This is the first study to link a paternal metabolic abnormality to a sperm effect on cell division and subsequent embryonic development.

Differential expression of glucose transporter GLUT8 during mouse spermatogenesis

GLUT8 is a facilitative glucose transporter expressed at high levels in the testis. In this study, we analyzed the GLUT8 expression in mouse testis during spermatogenesis by RT–PCR, Western blot and immunohistochemistry methods. Our results show that GLUT8 expression is limited to spermatids and spermatozoa in the testis. Expression begins when round spermatids are formed at postnatal day 24. The expression persists throughout spermiogenesis, and it is also detected in spermatozoa, but it is absent in more immature germ cells, Sertoli cells and interstitial tissue. GLUT8 immunoreactivity is always restricted to the acrosomic system in a manner that matches the acrosome system formation. The GLUT8 expression is mainly associated with the acrosomic membrane in the acrosome, although significant immunoreactivity is also found inside the acrosomic lumen. The specific GLUT8 location suggests that this transporter plays a pivotal role in the fuel supply of spermatozoa, and in the traffic of sugars during the capacitation and fertilization processes.

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.

Autocrine regulation of insulin secretion in human ejaculated spermatozoa

A striking feature of insulin expression is its almost complete restriction to beta-cells of the pancreatic islet in normal mammals. Here we show that insulin is expressed in and secreted from human ejaculated spermatozoa. Both insulin transcript and protein were detected. In addition, the large differences in insulin secretion, assessed by RIA, between noncapacitated and capacitated sperm suggest a role for insulin in capacitation. Insulin had an oscillatory secretory pattern involving glucose dose-dependent increases and significant decreases during the blockage of an insulin autocrine effect. It appears that the effect of glucose on the fertilizing ability of sperm is mediated by glucose metabolism through the pentose phosphate pathway. Then we evaluated the autocrine effect of sperm insulin on glucose metabolism by studying the activity of glucose-6-phosphate dehydrogenase, the key rate-limiting enzyme in the pentose phosphate pathway. The simultaneous decrease in both insulin release and glucose-6-phosphate dehydrogenase activity induced by blocking the autocrine insulin effect with three different procedures (blockage of insulin release by nifedipine, immune neutralization of the released insulin by antiinsulin serum, and blockage of an insulin intracellular effector such as phosphotidylinositol 3-kinase by wortmannin) strongly suggests a physiological role of sperm insulin on these two events. Insulin secretion by spermatozoa may provide an autocrine regulation of glucose metabolism based on their energetic needs independent of systemic insulin. In conclusion, these data open a new area of study in male reproduction.

The cell biology of fertilization

Research into the cell biology of mammalian fertilization has been stimulated by the desire to provide a theoretical framework for the development of novel approaches to contraception and the need to understand the cellular basis of human infertility. The results of such studies have revealed a complex cascade of interactions initiated by the contact between capacitated spermatozoa on the oocyte-cumulus complex and culminating in sperm-oocyte fusion. In this review we shall examine our current understanding of the fertilization process, highlighting the strategic importance of recent findings and key areas where information is lacking.

Location of insulin receptors in the placenta and its progenitor tissues

The insulin receptor gene is constitutively expressed, so the presence of insulin receptor proteins might be expected on all mammalian tissues, with the plasma membrane as the predominant site of receptor location. Results reviewed here indicate that insulin receptors are also present in all placental tissues and the placenta's progenitor tissues and cells, i.e., oocytes, spermatozoa, and preimplantation embryos, in most of the species studied. Receptor densities, however, vary among individual cells and cell types and at various developmental stages. Three aspects deserve emphasis. 1) In human placenta, the insulin receptor distribution pattern is characterized by a spatiotemporal change between first trimester and term. At the beginning of pregnancy, insulin receptors are found predominantly on the maternal side (apical membrane of syncytiotrophoblast, low density on cytotrophoblast); at term, however, they are on the fetal side (lining the fetal vessels). This suggests that, in the first trimester, maternal insulin regulates insulin-dependent processes, whereas, at term, it must be fetal insulin mainly controlling these processes. 2) The majority of insulin receptors is expressed on structures that are currently assumed to drive placental growth, i.e., syncytial sprouts and mesenchymal villi in first-trimester placentas and fetal endothelium at term. Therefore, we hypothesize a growth-promoting function, among others, of insulin on the placenta. 3) At present, no histologic evidence is available to demonstrate insulin receptors in structures commonly associated with receptor-mediated endocytosis. Whether placental insulin receptors are internalized, therefore, awaits clarification.