Alessandra Ferramosca

The role of mitochondria in energy production for human sperm motility.

Mitochondria of spermatozoa are different from the corresponding organelles of somatic cells, in both their morphology and biochemistry. The biochemical differences are essentially related to the existence of specific enzyme isoforms, which are characterized by peculiar kinetic and regulatory properties. As mitochondrial energy metabolism is a key factor supporting several sperm functions, these organelles host critical metabolic pathways during germ cell development and fertilization. Furthermore, spermatozoa can use different substrates, and therefore activate different metabolic pathways, depending on the available substrates and the physico-chemical conditions in which they operate. This versatility is critical to ensure fertilization success. However, the most valuable aspect of mitochondria function in all types of cells is the production of chemical energy in the form of ATP which can be used, in the case of spermatozoa, for sustaining sperm motility. The latter, on the other hand, represents one of the major determinants of male fertility. Accordingly, the presence of structural and functional alterations in mitochondria from asthenozoospermic subjects confirms the important role played by these organelles in energy maintenance of sperm motility. The present study gives an overview of the current knowledge on the energy-producing metabolic pathways operating inside human sperm mitochondria and critically analyse the differences with respect to somatic mitochondria. Such a comparison has also been carried out between the functional characteristics of human sperm mitochondria and those of other mammalian species. A deeper understanding of mitochondrial energy metabolism could open up new avenues of investigation in bioenergetics of human sperm mitochondria, both in physiological and pathological conditions.

A Krill Oil Supplemented Diet Suppresses Hepatic Steatosis in High-Fat Fed Rats

Krill oil (KO) is a dietary source of n-3 polyunsaturated fatty acids, mainly represented by eicosapentaenoic acid and docosahexaenoic acid bound to phospholipids. The supplementation of a high-fat diet with 2.5% KO efficiently prevented triglyceride and cholesterol accumulation in liver of treated rats. This effect was accompanied by a parallel reduction of the plasma levels of triglycerides and glucose and by the prevention of a plasma insulin increase. The investigation of the molecular mechanisms of KO action in high-fat fed animals revealed a strong decrease in the activities of the mitochondrial citrate carrier and of the cytosolic acetyl-CoA carboxylase and fatty acid synthetase, which are both involved in hepatic de novo lipogenesis. In these animals a significant increase in the activity of carnitine palmitoyl-transferase I and in the levels of carnitine was also observed, suggesting a concomitant stimulation of hepatic fatty acid oxidation. The KO supplemented animals also retained an efficient mitochondrial oxidative phosphorylation, most probably as a consequence of a KO-induced arrest of the uncoupling effects of a high-fat diet. Lastly, the KO supplementation prevented an increase in body weight, as well as oxidative damage of lipids and proteins, which is often found in high-fat fed animals.

Mitochondrial carrier protein biogenesis: role of the chaperones Hsc70 and Hsp90

Metabolite carrier proteins of the mitochondrial inner membrane share homology in their transmembrane domains, which also carries their targeting information. In addition, some carriers have cleavable presequences which are not essential for targeting, but have some other function before import. The cytosolic chaperones Hsc70 (heat-shock cognate 70) and Hsp90 (heat-shock protein 90) complex with carrier precursors and interact specifically with the Tom (translocase of the mitochondrial outer membrane) 70 import receptor to promote import. We analysed how the presequences of the PiC (phosphate carrier) and CIC (citrate carrier) relate to the mechanisms of chaperone-mediated import. Deletion of the PiC presequence reduced the efficiency of import but, notably, not by causing aggregation. Instead, binding of the protein to Hsc70 was reduced, as well as the dependence on Hsc70 for import. Hsp90 binding and function in import was not greatly affected, but it could not entirely compensate for the lack of Hsc70 interaction. Deletion of the presequence from CIC was shown to cause its aggregation, but had little effect on the contribution to import of either Hsc70 or Hsp90. The presequence of PiC, but not that of CIC, conferred Hsc70 binding to dihydrofolate reductase fusion proteins. In comparison, OGC (oxoglutarate carrier) lacks a presequence and was more soluble, though it is still dependent on both Hsc70 and Hsp90. We propose that carrier presequences evolved to improve targeting competence by different mechanisms, depending on physical properties of the precursors in the cytosolic targeting environment.

Bioenergetics of Mammalian Sperm Capacitation

After ejaculation, the mammalian male gamete must undergo the capacitation process, which is a prerequisite for egg fertilization. The bioenergetics of sperm capacitation is poorly understood despite its fundamental role in sustaining the biochemical and molecular events occurring during gamete activation. Glycolysis and mitochondrial oxidative phosphorylation (OXPHOS) are the two major metabolic pathways producing ATP which is the primary source of energy for spermatozoa. Since recent data suggest that spermatozoa have the ability to use different metabolic substrates, the main aim of this work is to present a broad overview of the current knowledge on the energy-producing metabolic pathways operating inside sperm mitochondria during capacitation in different mammalian species. Metabolism of glucose and of other energetic substrates, such as pyruvate, lactate, and citrate, is critically analyzed. Such knowledge, besides its obvious importance for basic science, could eventually translate into the development of novel strategies for treatment of male infertility, artificial reproduction, and sperm selection methods.

A krill oil supplemented diet reduces the activities of the mitochondrial tricarboxylate carrier and of the cytosolic lipogenic enzymes in rats

The mitochondrial tricarboxylate carrier supplies cytosol with the carbon units necessary for hepatic lipogenesis. The activities of cytosolic acetyl-CoA carboxylase and fatty acid synthetase are therefore strictly connected to the function of mitochondrial tricarboxylate carrier. Dietary polyunsaturated fatty acids (PUFA) are potent modulators of hepatic lipogenesis. In rats fed with a diet enriched with 2.5% krill oil (KO), a novel source of dietary n-3 PUFA, a time-dependent decrease in the activities of the mitochondrial tricarboxylate carrier and of the lipogenic enzymes was found. The KO induced inhibition of hepatic lipogenesis was more pronounced than that found in fish oil (FO)-fed rats, at least at short feeding times. The decrease in the activity of the mitochondrial tricarboxylate carrier caused by KO was due to a reduced expression of the protein. Furthermore, in the KO-fed animals a greater reduction in the levels of hepatic triglycerides and cholesterol was found in comparison to FO-fed rats.

Oxidative Stress Negatively Affects Human Sperm Mitochondrial Respiration

OBJECTIVE To correlate the level of oxidative stress in serum and seminal fluid and the level of sperm deoxyribonucleic acid (DNA) fragmentation with sperm mitochondrial respiratory efficiency. METHODS Sperm mitochondrial respiratory activity was evaluated with a polarographic assay of oxygen consumption carried out in hypotonically treated sperm cells. A possible relationship between sperm mitochondrial respiratory efficiency, the level of oxidative stress, and the level of sperm DNA fragmentation was investigated. RESULTS Sperm motility was positively correlated with mitochondrial respiration but negatively correlated with oxidative stress and DNA fragmentation. Interestingly, sperm mitochondrial respiratory activity was negatively affected by oxidative stress and DNA fragmentation. CONCLUSION Our data indicate that sperm mitochondrial respiration is decreased in patients with high levels of reactive oxygen species by an uncoupling between electron transport and adenosine triphosphate synthesis. This reduction in mitochondrial functionality might be 1 of the reasons responsible for the decrease in spermatozoa motility.

Olive Oil Increases the Hepatic Triacylglycerol Content in Mice by a Distinct Influence on the Synthesis and Oxidation of Fatty Acids

Diet supplementation with olive oil exerts beneficial effects on an organism, even if an increase in the level of hepatic lipids has been concomitantly observed. This study was therefore designed to investigate whether the stimulation of lipogenesis was responsible for the olive oil-induced hepatic fat accumulation. In mice fed for 8 weeks with an olive oil-enriched diet, an increase of about 2.6 fold in the level of liver triglycerides was found in comparison to animals fed with a corn oil-containing diet. Despite that, no increase in the activities of cytosolic lipogenic enzymes or of the mitochondrial tricarboxylate carrier was found; on the contrary, a decrease in the activity of carnitine palmitoyltransferase I was observed. This impairment of fatty acid oxidation, which was not apparent in corn oil-fed animals, may have had a role in the increase of hepatic lipid content found in the olive oil-fed mice.

Biogenesis of mitochondrial carrier proteins: Molecular mechanisms of import into mitochondria

Mitochondrial metabolite carriers are hydrophobic proteins which catalyze the flux of several charged or hydrophilic substrates across the inner membrane of mitochondria. These proteins, like most mitochondrial proteins, are nuclear encoded and after their synthesis in the cytosol are transported into the inner mitochondrial membrane. Most metabolite carriers, differently from other nuclear encoded mitochondrial proteins, are synthesized without a cleavable presequence and contain several, poorly characterized, internal targeting signals. However, an interesting aspect is the presence of a positively charged N-terminal presequence in a limited number of mitochondrial metabolite carriers. Over the last few years the molecular mechanisms of import of metabolite carrier proteins into mitochondria have been thoroughly investigated. This review summarizes the present knowledge and discusses recent advances on the import and sorting of mitochondrial metabolite carriers.

Mitochondrial Respiratory Efficiency is Positively Correlated With Human Sperm Motility

OBJECTIVE To correlate sperm mitochondrial respiratory efficiency with variations in sperm motility and with sperm morphologic anomalies. METHODS Sperm mitochondrial respiratory activity was evaluated with a polarographic assay of oxygen consumption carried out in hypotonically-treated sperm cells. A possible relationship among sperm mitochondrial respiratory efficiency, sperm motility, and morphologic anomalies was investigated. RESULTS Mitochondrial respiratory efficiency was positively correlated with sperm motility and negatively correlated with the percentage of immotile spermatozoa. Moreover, midpiece defects impaired mitochondrial functionality. CONCLUSION Our data indicate that an increase in sperm motility requires a parallel increase in mitochondrial respiratory capacity, thereby supporting the fundamental role played by mitochondrial oxidative phosphorylation in sperm motility of normozoospermic subjects. These results are of physiopathological relevance because they suggest that disturbances of sperm mitochondrial function and of energy production could be responsible for asthenozoospermia.

Evaluation of mitochondrial respiratory efficiency during in vitro capacitation of human spermatozoa

The role of mitochondria in sperm motility was the subject of several investigations. However, different views on this topic emerged among scientists. In particular, very little is known on the mechanisms of energy production occurring during human sperm capacitation and related processes. In this study, we have investigated the mitochondrial respiratory efficiency in human sperm samples from normozoospermic subjects before and after swim-up selection and incubation under capacitating condition. Sperm cells, selected by swim-up treatment, were incubated up to 24 h and then demembranated by hypotonic swelling at selected times. The oxygen uptake rate was measured in both basal and swim-up selected samples by a polarographic assay. Mitochondria of swim-up selected cells showed an impressive oxygen consumption rate, which was about 20 times higher than that measured in basal samples. The high mitochondrial respiratory efficiency remained stable up to 24 h after the swim-up treatment. The respiration control ratio, the substrate specificity and the inhibitor sensitivity in the swim-up selected samples were similar to those of basal samples thereby suggesting that the physiology of mitochondria was preserved after the swim-up treatment. Furthermore, the remarkably high mitochondrial respiration in swim-up selected samples allowed the oxygraphic analysis of just 200,000 sperm cells. Sperm selection and incubation under capacitating condition are therefore associated with a high activity of the mitochondrial respiratory chain. The sperm oxygen consumption rate could be useful to exclude mitochondria malfunctioning in male infertility.