Testis-Specific Lactate Dehydrogenase (LDH-C4) in Skeletal Muscle Enhances a Pika's Sprint-Running Capacity in Hypoxic Environment

Lactate as the sole energy substrate induces spontaneous acrosome reaction in viable stallion spermatozoa

BACKGROUND

Equine spermatozoa appear to differ from spermatozoa of other species in using oxidative phosphorylation preferentially over glycolysis. However, there is little information regarding effects of different energy sources on measured parameters in equine spermatozoa.

OBJECTIVE

To determine the effect of three individual energy substrates, glucose, pyruvate, and lactate, on motion characteristics, membrane integrity, and acrosomal status of stallion spermatozoa.

MATERIALS AND METHODS

Freshly ejaculated stallion spermatozoa were incubated with combinations of glucose (5 mm), pyruvate (10 mm), and lactate (10 mm) for 0.5 to 4 h. Response to calcium ionophore A23187 (5 μm) was used to evaluate capacitation status. Motility was evaluated using computer-assisted sperm analysis, and plasma membrane and acrosomal integrity were evaluated by flow cytometry.

RESULTS

Incubation with lactate alone for 2 h increased acrosomal sensitivity to A23187. Notably, incubation with lactate alone for 4 h induced a significant spontaneous increase in acrosome-reacted, membrane-intact (viable) spermatozoa, to approximately 50% of the live population, whereas no increase was seen with incubation in glucose or pyruvate alone. This acrosomal effect was observed in spermatozoa incubated at physiological pH as well as under alkaline conditions (medium pH approximately 8.5). Sperm motility declined concomitantly with the increase in acrosome-reacted spermatozoa. Sperm motility was significantly higher in pyruvate-only medium than in glucose or lactate. The addition of pyruvate to lactate-containing medium increased sperm motility but reduced the proportion of live acrosome-reacted spermatozoa in a dose-dependent fashion.

DISCUSSION

This is the first study to demonstrate that incubation with a specific energy substrate, lactate, is associated with spontaneous acrosome reaction in spermatozoa. The proportion of live, acrosome-reacted spermatozoa obtained is among the highest reported for equine spermatozoa.

CONCLUSION

These findings highlight the delicate control of key sperm functions, and may serve as a basis to increase our understanding of stallion sperm physiology.

The hypoxia adaptation of small mammals to plateau and underground burrow conditions

Oxygen is one of the important substances for the survival of most life systems on the earth, and plateau and underground burrow systems are two typical hypoxic environments. Small mammals living in hypoxic environments have evolved different adaptation strategies, which include increased oxygen delivery, metabolic regulation of physiological responses and other physiological responses that change tissue oxygen utilization. Multi-omics predictions have also shown that these animals have evolved different adaptations to extreme environments. In particular, vascular endothelial growth factor (VEGF) and erythropoietin (EPO), which have specific functions in the control of O2 delivery, have evolved adaptively in small mammals in hypoxic environments. Naked mole-rats and blind mole-rats are typical hypoxic model animals as they have some resistance to cancer. This review primarily summarizes the main living environment of hypoxia tolerant small mammals, as well as the changes of phenotype, physiochemical characteristics and gene expression mode of their long-term living in hypoxia environment.

Lactate dehydrogenase C4 (LDH-C4) is essential for the sperm count and motility: A case-control study

Background: The lactate dehydrogenase C4 (LDH-C4) isoenzyme is an important enzyme involved in metabolic processes that are needed for spermatogenesis and sperm motility. Objectives: This study aims to assess the activity and kinetic parameters (maximum velocity, Vmax and Michaelis constant, Km) of LDH-C4 in fertile and infertile (azoospermia and oligospermia) men in Baghdad City, Iraq. Methods: A total of 120 participants (80 infertile and 40 healthy fertile men) were included in the current study. The patients were sub-grouped into: 40 infertile men with oligospermia, and 40 infertile men with azoospermia. The oligospermia patients were further subdivided into subgroups based on sperm count and motility. Semen samples were obtained by masturbation after 3-5 days abstain for seminal fluid analysis. The microscopic test included the assessment of the count, motility and morphology of the sperms. In addition, the coulometric assay was used for measuring the activity and kinetic parameters of LDH-C4 enzyme. Results: The activity of LDH-C4 is significantly higher in fertile men when compared with infertile subjects (fertile: 403.13±189.90, oligospermia: 110.01±58.13, azoospermia: 39.06±28.15; p≤0.01). Statistically significant differences in LDH-C4 activity were also noted among patients with oligospermia based on sperms count and motility. Based on sperms’ count in patients with oligospermia, a significantly higher LDH-C4 activity (p≤0.01) was noted in those with higher sperm count (10-15 million/ml) when compared to others who have lesser count. Significant elevation in enzyme activity (p≤0.01) was also observed in oligospermia patients with higher percentages of motile sperms when compared with others who have fewer percentages of motile sperms. Moreover, the highest Vmax value (0.483 mmol/L.min) and the lowest Km value (0.39 mmol/L) were recorded in fertile men. While, the lowest Vmax value (0.174 mmol/L.min) and the lowest Km value (0.75 mmol/L) were detected in azoospermia patients. Conclusions: Our results suggest that LDH-C4 is essential for the count and motility of sperm and may be considered as a therapeutic approach for infertility.

Diverse energy metabolism patterns in females in Neodon fuscus, Lasiopodomys brandtii, and Mus musculus revealed by comparative transcriptomics under hypoxic conditions

The effects of global warming and anthropogenic disturbance force animals to migrate from lower to higher elevations to find suitable new habitats. As such migrations increase hypoxic stress on the animals, it is important to understand how plateau- and plain-dwelling animals respond to low-oxygen environments. We used comparative transcriptomics to explore the response of Neodon fuscus, Lasiopodomys brandtii, and Mus musculus skeletal muscle tissues to hypoxic conditions. Results indicate that these species have adopted different oxygen transport and energy metabolism strategies for dealing with a hypoxic environment. N. fuscus promotes oxygen transport by increasing hemoglobin synthesis and reduces the risk of thrombosis through cooperative regulation of genes, including Fga, Fgb, Alb, and Ttr; genes such as Acs16, Gpat4, and Ndufb7 are involved in regulating lipid synthesis, fatty acid β-oxidation, hemoglobin synthesis, and electron-linked transmission, thereby maintaining a normal energy supply in hypoxic conditions. In contrast, the oxygen-carrying capacity and angiogenesis of red blood cells in L. brandtii are promoted by genes in the CYP and COL families; this species maintains its bodily energy supply by enhancing the pentose phosphate pathway and mitochondrial fatty acid synthesis pathway. However, under hypoxia, M. musculus cannot effectively transport additional oxygen; thus, its cell cycle, proliferation, and migration are somewhat affected. Given its lack of hypoxic tolerance experience, M. musculus also shows significantly reduced oxidative phosphorylation levels under hypoxic conditions. Our results suggest that the glucose capacity of M. musculus skeletal muscle does not provide sufficient energy during hypoxia; thus, we hypothesize that it supplements its bodily energy by synthesizing ketone bodies. For the first time, we describe the energy metabolism pathways of N. fuscus and L. brandtii skeletal muscle tissues under hypoxic conditions. Our findings, therefore, improve our understanding of how vertebrates thrive in high altitude and plain habitats when faced with hypoxic conditions.

MitoKATP channels promote the proliferation of hypoxic human pulmonary artery smooth muscle cells via the ROS/HIF/miR-210/ISCU signaling pathway

Previous results have indicated that mitochondrial ATP-sensitive potassium (mitoK_ATP) channels are associated with the hypoxic proliferation of pulmonary artery smooth muscle cells (PASMCs). However, the mechanism underlying the promotive effects of mitoK_ATP channels on cell proliferation in response to hypoxia remains unknown. mitoK_ATP channel opening results in a collapse of mitochondrial membrane potential and generation of mitochondrial reactive oxygen species (ROS). As hypoxia-inducible factor-1α (HIF-1α) is a critical oxygen sensor and major transcriptional regulator of the hypoxic adaptive response, the current study assessed whether mitoK_ATP opening contributes to the chronic proliferation of human PASMCs (hPASMCs) in collaboration with HIF-1α and its downstream targets under hypoxic conditions. The present study demonstrated that there was crosstalk between mitoK_ATP channels and HIF-1α signaling in PASMCs under hypoxic conditions. The results suggest that mitoK_ATP channels are involved in the proliferation of PASMCs during hypoxia through upregulation of the ROS/HIF/microRNA-210/iron-sulfur cluster protein signaling pathway.

The expression of Ldh-c in the skeletal muscle of plateau pika (Ochotona curzoniae) enhances adaptation to a hypoxic environment

The plateau pika (Ochotona curzoniae) is a species of sprint-running alpine animals in the Qinghai-Tibet Plateau, which is a harsh highland hypoxic environment. Ldh-c is expressed in the testis, sperm and somatic tissues of plateau pika. To reveal the role and physiological mechanisms of sperm-specific lactate dehydrogenase (LDH-C₄), in plateau pika to adapt to hypoxic environment, an adenoviral line of pMultiRNAi-Ldhc was constructed and injected into the bilateral biceps femoris of the hind legs. The swimming times of the pikas, and the Ldh-c expression levels, total LDH activities and ATP levels in skeletal muscle, were measured after the pikas were raised in the trapped site for 5 days. Our results showed that after Ldh-c was silenced, the sprint-running ability (swimming time) of the plateau pikas was significant decreased, and the total LDH activities and ATP levels were reduced by 28.21% and 27.88%, respectively. Our results indicated that expression of Ldh-c in the skeletal muscle of plateau pika increased anaerobic glycolysis and enhanced adaptation to highland hypoxic environments.

Summary: The expression of Ldh-c in the skeletal muscle of plateau pika (Ochotona curzoniae) increases anaerobic glycolysis, resulting in enhanced sprint-running capacity and adaptation to a hypoxic environment.

Effect of Hypoxia on Ldh-c Expression in Somatic Cells of Plateau Pika

Sperm specific lactate dehydrogenases (LDH-C₄) is a lactate dehydrogenase that catalyzes the conversion of pyruvate to lactate. In mammals, Ldh-c was originally thought to be expressed only in testes and spermatozoa. Plateau pika (Ochotona curzoniae), which belongs to the genus Ochotona of the Ochotonidea family, is a hypoxia-tolerant mammal living 3000–5000 m above sea level on the Qinghai-Tibet Plateau, an environment which is strongly hypoxic. Ldh-c is expressed not only in testes and sperm, but also in the somatic tissues of plateau pika. To reveal the effect of hypoxia on pika Ldh-c expression, we investigated the mRNA and protein level of Ldh-c as well as the biochemical index of anaerobic glycolysis in pika somatic tissues at the altitudes of 2200 m, 3200 m and 3900 m. Our results showed that mRNA and protein expression levels of Ldh-c in the tissues of pika’s heart, liver, brain and skeletal muscle were increased significantly from 2200 m to 3200 m, but had no difference from 3200 m to 3900 m; the activities of LDH and the contents of lactate showed no difference from 2200 m to 3200 m, but were increased significantly from 3200 m to 3900 m. Hypoxia up-regulated and maintained the expression levels of Ldh-c in the pika somatic cells. Under the hypoxia condition, plateau pikas increased anaerobic glycolysis in somatic cells by LDH-C₄, and that may have reduced their dependence on oxygen and enhanced their adaptation to the hypoxic environment.

Inhibitory effects of ionic liquids on the lactic dehydrogenase activity

Ionic liquids (ILs) were widely used in scientific and industrial application and have been reported to possess potential toxicity to the environment and human health. The effects of six typical N-methylimidazolium-based ILs ([Cnmim]X, n=4, 6, 8; X=Br(-), Cl(-), BF4(-), CF3SO3(-)) on the lactic dehydrogenase (LDH) activity and the molecular interaction mechanism of ILs and the LDH were investigated with the aid of spectroscopic techniques. Experimental results showed that the LDH activity was inhibited in the presence of ILs. For the ILs with the same anion but different cations, their inhibitory ability on the LDH activity increased with increasing the alkyl chain length on the IL cation. Thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) were obtained by analyzing the fluorescence behavior of LDH with the addition of ILs. Both positive ΔH and ΔS suggested that hydrophobicity was the major driven force in the interaction process as expected.