AMP-activated kinase in human spermatozoa: identification, intracellular localization, and key function in the regulation of sperm motility

Mitophagy and spermatogenesis: Role and mechanisms

The mitophagy process, a type of macroautophagy, is the targeted removal of mitochondria. It is a type of autophagy exclusive to mitochondria, as the process removes defective mitochondria one by one. Mitophagy serves as an additional level of quality control by using autophagy to remove superfluous mitochondria or mitochondria that are irreparably damaged. During spermatogenesis, mitophagy can influence cell homeostasis and participates in a variety of membrane trafficking activities. Crucially, it has been demonstrated that defective mitophagy can impede spermatogenesis. Despite an increasing amount of evidence suggesting that mitophagy and mitochondrial dynamics preserve the fundamental level of cellular homeostasis, little is known about their role in developmentally controlled metabolic transitions and differentiation. It has been observed that male infertility is a result of mitophagy's impact on sperm motility. Furthermore, certain proteins related to autophagy have been shown to be present in mammalian spermatozoa. The mitochondria are the only organelle in sperm that can produce reactive oxygen species and finally provide energy for sperm movement. Furthermore, studies have shown that inhibited autophagy-infected spermatozoa had reduced motility and increased amounts of phosphorylated PINK1, TOM20, caspase 3/7, and AMPK. Therefore, in terms of reproductive physiology, mitophagy is the removal of mitochondria derived from sperm and the following preservation of mitochondria that are exclusively maternal.

Exploring correlations: Human seminal plasma and blood serum biochemistry in relation to semen quality

Male infertility is a pressing global issue, prompting the need for biomarkers correlating with seminal parameters for diagnosis. Our study investigated 10 biochemical and energetic parameters in the seminal plasma and blood sera of fertile (25 subjects) and infertile (88 subjects) Polish men, correlations between their levels in seminal plasma and semen quality, and correlations between blood sera and seminal plasma levels of examined parameters. Infertile men displayed elevated seminal plasma glucose and fructose but reduced HDL levels compared to fertile men. We observed also weak negative correlations between seminal plasma triglycerides and sperm concentration in both groups. Moreover, infertile men exhibited positive correlations between seminal plasma HDL/LDL concentrations and sperm concentration. Fertile men showed moderate negative correlations between glucose/triglycerides concentrations and sperm count and between seminal plasma triglycerides levels and sperm vitality. Semen volume correlated with triglycerides (negative) and fructose (positive) concentrations in infertile men. Sperm motility correlated negatively with total cholesterol, LDL, and triglycerides concentrations in fertile men, and weakly with AMP-activated protein kinase in infertile men. Weak negative correlations between seminal plasma fructose/AMP-activated protein kinase concentrations and sperm progressive motility were observed in infertile men, whereas in fertile men seminal plasma AMP-activated protein kinase levels were positively correlated with progressive motility. Correlation analysis between blood serum and seminal plasma parameters revealed intriguing connections, notably regarding LDL, AMP-activated protein kinase, and carnitine, suggesting systemic influences on seminal plasma composition. These findings emphasize the complex interplay between metabolic factors and sperm parameters, offering promising directions for future research in male infertility diagnostics and therapeutics.

Metformin improves sheep sperm cryopreservation via vitalizing the AMPK pathway

Adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) is a key regulator of sperm function and physiological metabolism. Metformin, an inexpensive and effective antioxidant, is known to play an important role in the activation of AMPK. Therefore metformin has potential to improve sperm cryopreservation. The aim of this study was to investigate the effect of metformin during semen cryopreservation of sheep and to find the most effective concentration in freezing extender. Semen were cryopreserved with extender containing different concentrations of metformin (0, 0.25, 0.5, 1.0, 2.0 and 4.0 mmol/L). Sperm motility, acrosome integrity and plasma membrane integrity were measured after semen freezing and thawing. All results showed that sperm quality was significantly increased in the 1.0 mmol/L metformin-treated group compared with the control group (P < 0.05). In addition, the study showed that metformin effectively reduced the content of malondialdehyde (MDA) and reactive oxygen species (ROS), and increased the activity of glutathione peroxidase (GSH-Px), superoxide dismutase (SOD), catalase (CAT) and total antioxidant capacity (T-AOC) of freeze-thawed sperm (P < 0.05). The optimal concentration of metformin was 1.0 mmol/L. Moreover, the results showed that AMPK was localized in the acrosome region, junction and midsection of sperm, and p-AMPK was distributed in the post-acrosomal region, junction and midsection. Western blot analysis indicated that 1.0 mmol/L metformin stimulated the phosphorylation of AMPK in sperm. Further results showed that 1.0 mmol/L metformin significantly increased the mitochondrial membrane potential (ΔΨm), ATP content, glucose uptake and lactate efflux of post-thawed sperm through the AMPK pathway, improved sperm quality, and increased the cleavage rate of in vitro fertilization (P < 0.05).

Role of Macroautophagy in Mammalian Male Reproductive Physiology

Physiologically, autophagy is an evolutionarily conserved and self-degradative process in cells. Autophagy carries out normal physiological roles throughout mammalian life. Accumulating evidence shows autophagy as a mechanism for cellular growth, development, differentiation, survival, and homeostasis. In male reproductive systems, normal spermatogenesis and steroidogenesis need a balance between degradation and energy supply to preserve cellular metabolic homeostasis. The main process of autophagy includes the formation and maturation of the phagophore, autophagosome, and autolysosome. Autophagy is controlled by a group of autophagy-related genes that form the core machinery of autophagy. Three types of autophagy mechanisms have been discovered in mammalian cells: macroautophagy, microautophagy, and chaperone-mediated autophagy. Autophagy is classified as non-selective or selective. Non-selective macroautophagy randomly engulfs the cytoplasmic components in autophagosomes that are degraded by lysosomal enzymes. While selective macroautophagy precisely identifies and degrades a specific element, current findings have shown the novel functional roles of autophagy in male reproduction. It has been recognized that dysfunction in the autophagy process can be associated with male infertility. Overall, this review provides an overview of the cellular and molecular basics of autophagy and summarizes the latest findings on the key role of autophagy in mammalian male reproductive physiology.

Antioxidant additive melatonin in tris-based egg yolk extender improves post-thaw sperm attributes in Hariana bull

In the study, melatonin, a known antioxidant pineal peptide was used as an additive in the tris-egg yolk glycerol-based semen extender in Hariana bull semen and post-thaw sperm characters were evaluated. In the study, Group I was a control without melatonin; and Group II, III, and IV were having 0.5 mM, 1 mM, and 2 mM melatonin/80 × 10⁶ spermatozoa, respectively were treatment groups. Thirty-two semen ejaculates from 4 Hariana bulls were processed for freezing and post-thaw sperm characteristics were evaluated. Sperm motility, velocity, the viability with intact membrane, and total antioxidant capacity were markedly (P < 0.05) improved in Group IV compared to all other groups. The lipid peroxidation and total protein carbonylation were substantially (P < 0.05) decreased in Group IV compared to all other groups. The population of cryocapacitated, acrosome-reacted, and apoptotic-like spermatozoa were significantly (P < 0.05) decreased in Group IV. Further, the relative band intensity of 74 kDa protein and percent of spermatozoa showing positive immune reactivity to tyrosine-phosphorylated proteins was decreased in Group IV. The progesterone-receptor ligand binding, in vitro capacitation response, and Vanguard distance were markedly (P < 0.05) improved in Group IV. In summary- Group IV having 2 mM melatonin was found to be optimal in providing cryoprotective effects to Hariana bull spermatozoa after freezing-thawing and can be suitably used as a semen additive during semen cryopreservation.

F4-Neuroprostane Effects on Human Sperm

Swim-up selected human sperm were incubated with 7 ng F₄-neuroprostanes (F₄-NeuroPs) for 2 and 4 h. Sperm motility and membrane mitochondrial potential (MMP) were evaluated. The percentage of reacted acrosome was assessed by pisum sativum agglutinin (PSA). Chromatin integrity was detected using the acridine orange (AO) assay and localization of the ryanodine receptor was performed by immunofluorescence analysis. Sperm progressive motility (p = 0.02) and the percentage of sperm showing a strong MMP signal (p = 0.012) significantly increased after 2 h F₄-NeuroP incubation compared to control samples. The AO assay did not show differences in the percentage of sperm with dsDNA between treated or control samples. Meanwhile, a significantly higher number of sperm with reacted acrosomes was highlighted by PSA localization after 4 h F₄-NeuroP incubation. Finally, using an anti-ryanodine antibody, the immunofluorescence signal was differentially distributed at 2 and 4 h: a strong signal was evident in the midpiece and postacrosomal sheath (70% of sperm) at 2 h, whereas a dotted one appeared at 4 h (53% of sperm). A defined concentration of F₄-NeuroPs in seminal fluid may induce sperm capacitation via channel ions present in sperm cells, representing an aid during in vitro sperm preparation that may increase the positive outcome of assisted fertilization.

Metformin in therapeutic applications in human diseases: its mechanism of action and clinical study

Metformin, a biguanide drug, is the most commonly used first-line medication for type 2 diabetes mellites due to its outstanding glucose-lowering ability. After oral administration of 1 g, metformin peaked plasma concentration of approximately 20-30 μM in 3 h, and then it mainly accumulated in the gastrointestinal tract, liver and kidney. Substantial studies have indicated that metformin exerts its beneficial or deleterious effect by multiple mechanisms, apart from AMPK-dependent mechanism, also including several AMPK-independent mechanisms, such as restoring of redox balance, affecting mitochondrial function, modulating gut microbiome and regulating several other signals, such as FBP1, PP2A, FGF21, SIRT1 and mTOR. On the basis of these multiple mechanisms, researchers tried to repurpose this old drug and further explored the possible indications and adverse effects of metformin. Through investigating with clinical studies, researchers concluded that in addition to decreasing cardiovascular events and anti-obesity, metformin is also beneficial for neurodegenerative disease, polycystic ovary syndrome, aging, cancer and COVID-19, however, it also induces some adverse effects, such as gastrointestinal complaints, lactic acidosis, vitamin B12 deficiency, neurodegenerative disease and offspring impairment. Of note, the dose of metformin used in most studies is much higher than its clinically relevant dose, which may cast doubt on the actual effects of metformin on these disease in the clinic. This review summarizes these research developments on the mechanism of action and clinical evidence of metformin and discusses its therapeutic potential and clinical safety.

A comparative study between curcumin and curcumin nanoemulsion on high-fat, high-fructose diet-induced impaired spermatogenesis in rats

Objectives

Curcumin is a promising nutraceutical with reported diverse therapeutic properties, but of limited oral bioavailability. The current manuscript investigates the role of encapsulation of curcumin in nanoemulsion form in counteracting the adverse effect of chronic ingestion of a high-fat high-fructose diet (HFHF) by juvenile male rats regarding testicular abnormalities and declined spermatogenesis.

Methods

Curcumin nanoemulsion was administered orally to Wistar rats at a dose of 5 or 10 mg/kg and compared with curcumin powder, followed by a pharmacological and histological assessment.

Key findings

Results demonstrated that curcumin nanoemulsion was superior to curcumin powder, particularly in enhancing the percentage progressive motility of spermatozoa, normalization of essential and non-essential amino acids in semen, normalization of serum leptin and testosterone levels, as well as normalization of oxidative and nitrosative parameters. It was also proven to reduce testicular DNA fragmentation, while elevating testicular cellular energy. In addition, curcumin nanoemulsion administered at a dose of 10 mg/kg induced the highest level of spermatogenesis, delineated by histological examination of the seminiferous tubules.

Conclusions

It can be concluded that curcumin nanoemulsion administered at a dose of 10 mg/kg successfully ameliorates the adverse effects of a HFHF on spermatogenesis.

Bacterial Infections Affect Male Fertility: A Focus on the Oxidative Stress-Autophagy Axis

Numerous factors trigger male infertility, including lifestyle, the environment, health, medical resources and pathogenic microorganism infections. Bacterial infections of the male reproductive system can cause various reproductive diseases. Several male reproductive organs, such as the testicles, have unique immune functions that protect the germ cells from damage. In the reproductive system, immune cells can recognize the pathogen-associated molecular patterns carried by pathogenic microorganisms and activate the host's innate immune response. Furthermore, bacterial infections can lead to oxidative stress through multiple signaling pathways. Many studies have revealed that oxidative stress serves dual functions: moderate oxidative stress can help clear the invaders and maintain sperm motility, but excessive oxidative stress will induce host damage. Additionally, oxidative stress is always accompanied by autophagy which can also help maintain host homeostasis. Male reproductive system homeostasis disequilibrium can cause inflammation of the genitourinary system, influence spermatogenesis, and even lead to infertility. Here, we focus on the effect of oxidative stress and autophagy on bacterial infection in the male reproductive system, and we also explore the crosslink between oxidative stress and autophagy during this process.

Mitochondria: their role in spermatozoa and in male infertility

BACKGROUND

The best-known role of spermatozoa is to fertilize the oocyte and to transmit the paternal genome to offspring. These highly specialized cells have a unique structure consisting of all the elements absolutely necessary to each stage of fertilization and to embryonic development. Mature spermatozoa are made up of a head with the nucleus, a neck, and a flagellum that allows motility and that contains a midpiece with a mitochondrial helix. Mitochondria are central to cellular energy production but they also have various other functions. Although mitochondria are recognized as essential to spermatozoa, their exact pathophysiological role and their functioning are complex. Available literature relative to mitochondria in spermatozoa is dense and contradictory in some cases. Furthermore, mitochondria are only indirectly involved in cytoplasmic heredity as their DNA, the paternal mitochondrial DNA, is not transmitted to descendants.

OBJECTIVE AND RATIONAL

This review aims to summarize available literature on mitochondria in spermatozoa, and, in particular, that with respect to humans, with the perspective of better understanding the anomalies that could be implicated in male infertility.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed original articles and reviews pertaining to human spermatozoa and mitochondria. Searches were performed using keywords belonging to three groups: 'mitochondria' or 'mitochondrial DNA', 'spermatozoa' or 'sperm' and 'reactive oxygen species' or 'calcium' or 'apoptosis' or signaling pathways'. These keywords were combined with other relevant search phrases. References from these articles were used to obtain additional articles.

OUTCOMES

Mitochondria are central to the metabolism of spermatozoa and they are implicated in energy production, redox equilibrium and calcium regulation, as well as apoptotic pathways, all of which are necessary for flagellar motility, capacitation, acrosome reaction and gametic fusion. In numerous cases, alterations in one of the aforementioned functions could be linked to a decline in sperm quality and/or infertility. The link between the mitochondrial genome and the quality of spermatozoa appears to be more complex. Although the quantity of mtDNA, and the existence of large-scale deletions therein, are inversely correlated to sperm quality, the effects of mutations seem to be heterogeneous and particularly related to their pathogenicity.

WIDER IMPLICATIONS

The importance of the role of mitochondria in reproduction, and particularly in gamete quality, has recently emerged following numerous publications. Better understanding of male infertility is of great interest in the current context where a significant decline in sperm quality has been observed.

Metformin improves boar sperm quality via 5'-AMP-activated protein kinase-mediated energy metabolism in vitro

Sperm are specialized cells that require adenosine triphosphate (ATP) to support their function. Maintaining sperm energy homeostasis in vitro is vitally important to improve the efficacy of boar sperm preservation. Metformin can activate 5′-AMP-activated protein kinase (AMPK) to improve metabolic flexibility and maintain energy homeostasis. Thus, the aim of the present study was to investigate whether metformin can improve boar sperm quality through AMPK mediation of energy metabolism. Sperm motility parameters, membrane integrity, acrosome integrity, mitochondrial membrane potential (ΔΨ_m), ATP content, glucose uptake, and lactate efflux were analyzed. Localization and expression levels of AMPK and phospho-Thr¹⁷²-AMPK (p-AMPK) were also detected by immunofluorescence and western blotting. We found that metformin treatment significantly increased sperm motility parameters, ΔΨ_m, and ATP content during storage at 17 °C. Moreover, results showed that AMPK was localized at the acrosomal region, connecting piece, and midpiece of sperm and p-AMPK was distributed at the post-acrosomal region, connecting piece, and midpiece. When sperm were incubated with metformin for 4 h at 37 °C, sperm motility parameters, ΔΨ_m, ATP content, p-AMPK, glucose uptake, and lactate efflux all significantly increased, whereas the addition of Compound C treatment, an inhibitor of AMPK, counteracted these positive effects. Together, our results suggest that metformin promotes AMPK activation, which contributes to the maintenance of energy hemostasis and mitochondrial activity, thereby maintaining boar sperm functionality and improving the efficacy of semen preservation.

Improvement Effect of Metformin on Female and Male Reproduction in Endocrine Pathologies and Its Mechanisms

Metformin (MF), a first-line drug to treat type 2 diabetes mellitus (T2DM), alone and in combination with other drugs, restores the ovarian function in women with polycystic ovary syndrome (PCOS) and improves fetal development, pregnancy outcomes and offspring health in gestational diabetes mellitus (GDM) and T2DM. MF treatment is demonstrated to improve the efficiency of in vitro fertilization and is considered a supplementary drug in assisted reproductive technologies. MF administration shows positive effect on steroidogenesis and spermatogenesis in men with metabolic disorders, thus MF treatment indicates prospective use for improvement of male reproductive functions and fertility. MF lacks teratogenic effects and has positive health effect in newborns. The review is focused on use of MF therapy for restoration of female and male reproductive functions and improvement of pregnancy outcomes in metabolic and endocrine disorders. The mechanisms of MF action are discussed, including normalization of metabolic and hormonal status in PCOS, GDM, T2DM and metabolic syndrome and restoration of functional activity and hormonal regulation of the gonadal axis.

Rosmarinic acid improves boar sperm quality, antioxidant capacity and energy metabolism at 17°C via AMPK activation

Boar sperm are susceptible to oxidative damage caused by reactive oxygen species (ROS) during storage. Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is an important therapeutic target, because it is a cellular metabolism energy sensor and key signalling kinase in spermatozoa. We evaluated the effects of rosmarinic acid (RA), an antioxidant, on boar sperm during liquid storage to determine whether it protects boar sperm via AMPK activation. Boar ejaculates were diluted with Modena extender with different concentrations of RA and stored at 17°C for 9 days. Sperm quality parameters, antioxidant capacity, energy metabolism, AMPK phosphorylation and fertility were analysed. Compared with the control, 40 μmol/L significantly improved sperm motility, plasma membrane integrity and acrosome integrity (p < .05). The effective storage time of boar sperm was up to 9 days. On the third and seventh days, the sperm with RA exhibited increased total antioxidant capacity (T-AOC), superoxide dismutase (SOD) activity, adenosine triphosphate (ATP) content, mitochondrial membrane potential (ΔΨm) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activity, whereas malondialdehyde (MDA) content was significantly decreased (p < .05). Western blot showed that RA, as well as AICAR (AMPK activator), promoted AMPK phosphorylation, whereas Compound C (AMPK inhibitor) inhibited this effect. The sperm-zona pellucida binding experiment showed that 40 μmol/L RA increased the number of sperm attached to the zona pellucida (p < .05). These findings suggest meaningful methods for improved preservation of boar sperm in vitro and provide new insights into the mechanism by which RA protects sperm cells from oxidative damage via AMPK activation.

Role of Selective Autophagy in Spermatogenesis and Male Fertility

Autophagy is a "self-eating" process that engulfs cellular contents for their subsequent digestion in lysosomes to engage the metabolic need in response to starvation or environmental insults. According to the contents of degradation, autophagy can be divided into bulk autophagy (non-selective autophagy) and selective autophagy. Bulk autophagy degrades non-specific cytoplasmic materials in response to nutrient starvation while selective autophagy targets specific cargoes, such as damaged organelles, protein aggregates, and intracellular pathogens. Selective autophagy has been documented to relate to the reproductive processes, especially for the spermatogenesis, fertilization, and biosynthesis of testosterone. Although selective autophagy is vital in the field of reproduction, its role and the underlying mechanism have remained unclear. In this review, we focus on selective autophagy to discuss the recent advances in our understanding of the mechanism and role of selective autophagy on spermatogenesis and male fertility in mammals. Understanding the role of selective autophagy during spermatogenesis will promote the recognition of genetic regulation in male infertility, and shed light on therapies of infertile patients.

In vitro exposure to metformin activates human spermatozoa at therapeutically relevant concentrations

BACKGROUND

Metformin, a drug used to treat type 2 diabetes, has gained attention for its multiple therapeutic applications. However, little is known about its effects on human sperm function at therapeutically relevant concentration.

OBJECTIVES

The aim of this study was to elucidate the in vitro actions of metformin on human sperm function and explore the underlying mechanism of any effects.

MATERIALS AND METHODS

Human ejaculated spermatozoa were treated with therapeutically relevant concentrations (0.25, 5, 10, 20, 40, and 80 µM) of metformin in vitro. Fertilization-essential functions of spermatozoa were examined, including viability, motility, capacitation, acrosome reaction, hyperactivation, and penetration ability. The signaling pathways mediated by 5'-AMP-activated protein kinase (AMPK), intracellular calcium concentration ([Ca²⁺ ]_i ), and tyrosine phosphorylation of spermatozoa were also measured.

RESULTS

Although metformin did not affect sperm viability, motility, and [Ca²⁺ ]_i , it significantly increased the percentages of capacitated spermatozoa, acrosomal-reacted spermatozoa, and hyperactivated spermatozoa as well as penetration ability of human spermatozoa at the concentrations of 40 and 80 µM (P < .05). These concentrations of metformin also elevated the levels of phosphorylated AMPK and tyrosine phosphorylation in human spermatozoa. In addition, activation of AMPK by A769662 (an AMPK activator) had similar effects to metformin on human spermatozoa, while inhibition of AMPK by Compound C (an AMPK inhibitor) suppressed the enhancement of metformin on human spermatozoa.

CONCLUSION

Our findings indicate that metformin activates human sperm function through an AMPK-related mechanism which increases tyrosine phosphorylation at therapeutically relevant concentrations, thereby suggesting its improvement on human sperm function when treating subfertile males of type 2 diabetes.

Metformin inhibits human spermatozoa motility and signalling pathways mediated by protein kinase A and tyrosine phosphorylation without affecting mitochondrial function

Metformin is a leading antidiabetic drug that is used worldwide in the treatment of diabetes mellitus. This biguanide exerts metabolic and pleiotropic effects in somatic cells, although its invitro actions on human spermatozoa remain unknown. The present study investigated the effects of metformin on human sperm function. Human spermatozoa were incubated in the presence or absence of 10mM metformin for 8 or 20h, and motility was measured by computer-aided sperm analysis (CASA); other parameters were evaluated by flow cytometry. Metformin significantly reduced the percentage of motile, progressive and rapid spermatozoa and significantly decreased sperm velocity. Metformin did not affect viability, mitochondrial membrane potential (MMP) or mitochondrial superoxide anion generation of human spermatozoa at any time studied. However, metformin clearly inhibited the protein kinase (PK) A pathway and protein tyrosine phosphorylation at 8 and 20h, key regulatory pathways for correct sperm function. In summary, metformin treatment of human spermatozoa had a detrimental effect on motility and inhibited essential sperm signalling pathways, namely PKA and protein tyrosine phosphorylation, without affecting physiological parameters (viability, MMP, mitochondrial superoxide anion generation). Given the growing clinical use of metformin in different pathologies in addition to diabetes, this study highlights an adverse effect of metformin on spermatozoa and its relevance in terms of human fertility in patients who potentially could be treated with metformin in the future.

Activation of adenosine monophosphate-activated protein kinase (AMPK) enhances energy metabolism, motility, and fertilizing ability of cryopreserved spermatozoa in domestic cat model

PURPOSE

Increasing intracellular energy storage by chemically activating adenosine monophosphate-activated protein kinase (AMPKα) prior to sperm cryopreservation may improve post-thawed sperm function. Using the domestic cat as a biomedical model, the objectives were to (1) confirm the expression of AMPKα and its regulatory kinases in epididymal spermatozoa and (2) assess the influence of AMPK activator, 5'-aminoimidasole-4-carboxamide-1-β-d-ribofuranoside (AICAR) on epididymal sperm function before and after cryopreservation.

METHODS

In study I, sperm samples of different qualities were obtained from cauda epididymides of domestic cats and evaluated for AMPKα expression. In study II, epididymal spermatozoa were equilibrated for either 30 or 60 min in the presence of 0 (control), 0.5, 2.0, and 5.0 mM AICAR and sperm functions were assessed before and after cryopreservation. In study III, epididymal spermatozoa were treated as in study II and evaluated for AMPKα signaling protein expressions (phospho-AMPKα Thr172 and GLUT1) as well as ATP levels.

RESULTS

AMPKα protein expression was higher in high-motility vs poor-motility samples. Thirty-minute equilibration with 0.5 mM AICAR improved motion characteristics and fertilizing ability of cryopreserved sperm to the control. Increased expressions of phospho-AMPKα Thr172 and GLUT1 as well as intracellular ATP level were confirmed in sperm samples equilibrated with 0.5 or 2.0 mM AICAR for 30 min.

CONCLUSIONS

Presence and role of AMPKα protein in cat regulating sperm function were demonstrated before and after cryopreservation. Findings could be used to potentially enhance cryopreserved sperm function in sub-fertile men.

Role of AMPK in mammals reproduction: Specific controls and whole-body energy sensing

AMP-activated protein kinase (AMPK) is a key enzyme involved in linking the energy sensing to metabolic pathways. As such, it plays a central role at the whole-body level to translate endocrine communications into adapted responses aimed either at saving energy when food is scarce or at allocating it to various functions, particularly reproduction, when food is available. AMPK also plays major roles in the energy individual cells use in order to realize their specific functions. This is of course especially true for all cells involved in the reproductive function (gonads, gametes) or in its control (hypothalamus, pituitary). In the present review, I report a survey of the various roles of AMPK functions in reproduction, either directly in reproductive organs, or indirectly in organs controlling reproduction, particularly at hypothalamus level.

Insights into leptin signaling and male reproductive health: the missing link between overweight and subfertility?

Obesity stands as one of the greatest healthcare challenges of the 21st century. Obesity in reproductive-age men is ever more frequent and is reaching upsetting levels. At the same time, fertility has taken an inverse direction and is decreasing, leading to an increased demand for fertility treatments. In half of infertile couples, there is a male factor alone or combined with a female factor. Furthermore, male fertility parameters such as sperm count and concentration went on a downward spiral during the last few decades and are now approaching the minimum levels established to achieve successful fertilization. Hence, the hypothesis that obesity and deleterious effects in male reproductive health, as reflected in deterioration of sperm parameters, are somehow related is tempting. Most often, overweight and obese individuals present leptin levels directly proportional to the increased fat mass. Leptin, besides the well-described central hypothalamic effects, also acts in several peripheral organs, including the testes, thus highlighting a possible regulatory role in male reproductive function. In the last years, research focusing on leptin effects in male reproductive function has unveiled additional roles and molecular mechanisms of action for this hormone at the testicular level. Herein, we summarize the novel molecular signals linking metabolism and male reproductive function with a focus on leptin signaling, mitochondria and relevant pathways for the nutritional support of spermatogenesis.

AMPK Function in Mammalian Spermatozoa

AMP-activated protein kinase AMPK regulates cellular energy by controlling metabolism through the inhibition of anabolic pathways and the simultaneous stimulation of catabolic pathways. Given its central regulator role in cell metabolism, AMPK activity and its regulation have been the focus of relevant investigations, although only a few studies have focused on the AMPK function in the control of spermatozoa's ability to fertilize. This review summarizes the known cellular roles of AMPK that have been identified in mammalian spermatozoa. The involvement of AMPK activity is described in terms of the main physiological functions of mature spermatozoa, particularly in the regulation of suitable sperm motility adapted to the fluctuating extracellular medium, maintenance of the integrity of sperm membranes, and the mitochondrial membrane potential. In addition, the intracellular signaling pathways leading to AMPK activation in mammalian spermatozoa are reviewed. We also discuss the role of AMPK in assisted reproduction techniques, particularly during semen cryopreservation and preservation (at 17 °C). Finally, we reinforce the idea of AMPK as a key signaling kinase in spermatozoa that acts as an essential linker/bridge between metabolism energy and sperm's ability to fertilize.

Cellular stress and AMPK activation as a common mechanism of action linking the effects of metformin and diverse compounds that alleviate accelerated aging defects in Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is a rare genetic disorder characterized by an accelerated aging phenotype that typically leads to death via stroke or myocardial infarction at approximately 14.6 years of age. Most cases of HGPS have been linked to the extensive use of a cryptic splice donor site located in the LMNA gene due to a de novo mutation, generating a truncated and toxic protein known as progerin. Progerin accumulation in the nuclear membrane and within the nucleus distorts the nuclear architecture and negatively effects nuclear processes including DNA replication and repair, leading to accelerated cellular aging and premature senescence. The serine-arginine rich splicing factor SRSF1 (also known as ASF/SF2) has recently been shown to modulate alternative splicing of the LMNA gene, with SRSF1 inhibition significantly reducing progerin at both the mRNA and protein levels. In 2014, we hypothesized for the first time that compounds including metformin that induce activation of AMP-activated protein kinase (AMPK), a master metabolic regulator activated by cellular stress (e.g. increases in intracellular calcium, reactive oxygen species, and/or an AMP(ADP)/ATP ratio increase, etc.), will beneficially alter gene splicing in progeria cells by inhibiting SRSF1, thus lowering progerin levels and altering the LMNA pre-mRNA splicing ratio. Recent evidence has substantiated this hypothesis, with metformin significantly reducing the mRNA and protein levels of both SRSF1 and progerin, activating AMPK, and alleviating pathological defects in HGPS cells. Metformin has also recently been shown to beneficially alter gene splicing in normal humans. Interestingly, several chemically distinct compounds, including rapamycin, methylene blue, all-trans retinoic acid, MG132, 1α,25-dihydroxyvitamin D3, sulforaphane, and oltipraz have each been shown to alleviate accelerated aging defects in patient-derived HGPS cells. Each of these compounds has also been independently shown to induce AMPK activation. Because these compounds improve accelerated aging defects in HGPS cells either by enhancing mitochondrial functionality, increasing Nrf2 activity, inducing autophagy, or by altering gene splicing and because AMPK activation beneficially modulates each of the aforementioned processes, it is our hypothesis that cellular stress-induced AMPK activation represents an indirect yet common mechanism of action linking such chemically diverse compounds with the beneficial effects of those compounds observed in HGPS cells. As normal humans also produce progerin at much lower levels through a similar mechanism, compounds that safely induce AMPK activation may have wide-ranging implications for both normal and pathological aging.

Transposable elements, placental development, and oocyte activation: Cellular stress and AMPK links jumping genes with the creation of human life

Transposable elements (TEs), also known as "jumping genes", are DNA sequences first described by Nobel laureate Barbara McClintock that comprise nearly half of the human genome and are able to transpose or move from one genomic location to another. As McClintock also noted that a genome "shock" or stress may induce TE activation and transposition, accumulating evidence suggests that cellular stress (e.g. mediated by increases in intracellular reactive oxygen species [ROS] and calcium [Ca2+], etc.) induces TE mobilization in several model organisms and L1s (a member of the retrotransposon class of TEs) are active and capable of retrotransposition in human oocytes, human sperm, and in human neural progenitor cells. Cellular stress also plays a critical role in human placental development, with cytotrophoblast (CTB) differentiation leading to the formation of the syncytiotrophoblast (STB), a cellular layer that facilitates nutrient and gas exchange between the mother and the fetus. Syncytin-1, a protein that promotes fusion of CTB cells and is necessary for STB formation, and its receptor is found in human sperm and human oocytes, respectively, and increases in ROS and Ca2+ promote trophoblast differentiation and syncytin-1 expression. Cellular stress is also essential in promoting human oocyte maturation and activation which, similar to TE mobilization, can be induced by compounds that increase intracellular Ca2+ and ROS levels. AMPK is a master metabolic regulator activated by increases in ROS, Ca2+, and/or an AMP(ADP)/ATP ratio increase, etc. as well as compounds that induce L1 mobilization in human cells. AMPK knockdown inhibits trophoblast differentiation and AMPK-activating compounds that promote L1 mobility also enhance trophoblast differentiation. Cellular stressors that induce TE mobilization (e.g. heat shock) also promote oocyte maturation in an AMPK-dependent manner and the antibiotic ionomycin activates AMPK, promotes TE activation, and induces human oocyte activation, producing normal, healthy children. Metformin promotes AMPK-dependent telomerase activation (critical for telomere maintenance) and induces activation of the endonuclease RAG1 (promotes DNA cleavage and transposition) via AMPK. Both RAG1 and telomerase are derived from TEs. It is our hypothesis that cellular stress and AMPK links TE activation and transposition with placental development and oocyte activation, facilitating both human genome evolution and the creation of all human life. We also propose the novel observation that various cellular stress-inducing compounds (e.g. metformin, resveratrol, etc.) may facilitate beneficial TE activation and transposition and enhance fertilization and embryological development through a common mechanism of AMPK activation.

Impact of 5'-amp-activated Protein Kinase on Male Gonad and Spermatozoa Functions

As we already know, the male reproductive system requires less energetic investment than the female one. Nevertheless, energy balance is an important feature for spermatozoa production in the testis and for spermatozoa properties after ejaculation. The 5'-AMP-activated protein kinase, AMPK, is a sensor of cell energy, that regulates many metabolic pathways and that has been recently shown to control spermatozoa quality and functions. It is indeed involved in the regulation of spermatozoa quality through its action on the proliferation of testicular somatic cells (Sertoli and Leydig), on spermatozoa motility and acrosome reaction. It also favors spermatozoa quality through the management of lipid peroxidation and antioxidant enzymes. I review here the most recent data available on the roles of AMPK in vertebrate spermatozoa functions.