Interaction between basigin and monocarboxylate transporter 2 in the mouse testes and spermatozoa

Cytosolic and Acrosomal pH Regulation in Mammalian Sperm

As in most cells, intracellular pH regulation is fundamental for sperm physiology. Key sperm functions like swimming, maturation, and a unique exocytotic process, the acrosome reaction, necessary for gamete fusion, are deeply influenced by pH. Sperm pH regulation, both intracellularly and within organelles such as the acrosome, requires a coordinated interplay of various transporters and channels, ensuring that this cell is primed for fertilization. Consistent with the pivotal importance of pH regulation in mammalian sperm physiology, several of its unique transporters are dependent on cytosolic pH. Examples include the Ca2+ channel CatSper and the K+ channel Slo3. The absence of these channels leads to male infertility. This review outlines the main transport elements involved in pH regulation, including cytosolic and acrosomal pH, that participate in these complex functions. We present a glimpse of how these transporters are regulated and how distinct sets of them are orchestrated to allow sperm to fertilize the egg. Much research is needed to begin to envision the complete set of players and the choreography of how cytosolic and organellar pH are regulated in each sperm function.

In vitro storage of boar spermatozoa increases the demand of ATP for reactivation of motility

BACKGROUND

Prolonging the shelf-life of liquid-preserved semen without compromising its fertilizing capacity may increase the efficiency of artificial insemination in pigs. Many fertilization-relevant processes are ATP dependent. The impact of semen storage and rewarming to body temperature on the energy status of sperm are as yet unknown.

OBJECTIVES

To investigate the energy status of boar spermatozoa during storage and subsequent rewarming, and to reveal the potential role of mitochondrial function for reactivation and maintenance of sperm motility.

MATERIALS AND METHODS

Extended semen samples (n = 7 boars) were used. Spermatozoa were challenged by storage at 17°C for seven days and incubation at 38°C for 180 minutes. The ATP concentration and energy charge (EC) in semen samples and lactate concentration in the extracellular medium were assessed. Viability and mitochondrial activity were determined by flow cytometry, and clustered single cell analysis of motility parameters were performed.

RESULTS

The energy status was not affected by semen storage (p>0.05). Rewarming resulted in a net reduction in ATP concentration which increased with storage time (maximum Day 5: -24.2±10.3 %), but was not accompanied by a loss in viability, motility or mitochondrial activity. Blocking glycolysis with 2-Deoxy-D-glucose prevented re-establishing of motility and mitochondrial activity after rewarming. Mitochondrial activity gradually subsided in virtually all spermatozoa during incubation at 38°C, while ATP and EC remained high. Concomitantly, extracellular lactate levels rose and sperm populations with lower velocity, increased linearity, and low lateral head-displacement grew larger. Size changes for major sperm subpopulations correlated with the percentage of viable sperm with high mitochondrial activity (r = 0.44 to 0.70 for individual subpopulations, p<0.01).

CONCLUSION

Storage of boar spermatozoa increases the demand of ATP for reactivation of sperm towards fast, non-linear and hyperactivation-like motility patterns upon rewarming. Maintenance of glycolysis seems to be decisive for sperm function after long-term storage in vitro. This article is protected by copyright. All rights reserved.

The current perspective on genetic and epigenetic factors in sperm maturation in the epididymis

Male infertility affects approximately 30% of infertile couples. As spermatozoa mature in the epididymal lumen, their potential for mobility increases, and their protein, lipid and small RNA (sRNA) content changes, whereas capacitation and fertilisation take place in the female reproductive tract. Both of the latter processes are affected by maturation, because impaired maturation causes premature capacitation and fertilization. The epididymis produces a suitable environment for sperm maturation via ion transport, vesicle secretion and protein matrix formation. The microenvironment for sperm maturation varies in three broad segments: the caput, the corpus and the cauda epididymis. Epididymosomes transfer proteins, lipids and sRNAs from the epididymal epithelium to spermatozoa and genetic alterations of epididymal genes can lead to decreased sperm motility, morphological abnormalities of spermatozoa and subfertility. Genetic factors are involved in all aetiological categories in male infertility. However, studies conducted on the genes involved in epididymal functions are limited. The sRNA content of spermatozoa changes during epididymal migration, and these sRNAs play a role in embryo development and epigenetic inheritance. This review aims to clarify the role of the epididymal epithelium in the maturation of spermatozoa in light of the current molecular genomic knowledge.

Roles of glycogen synthase kinase 3 alpha and calcineurin in regulating the ability of sperm to fertilize eggs

Glycogen synthase kinase 3 (GSK3) was identified as an enzyme regulating sperm protein phosphatase. The GSK3α paralog, but not GSK3β, is essential for sperm function. Sperm lacking GSK3α display altered motility and are unable to undergo hyperactivation, which is essential for fertilization. Male mice lacking sperm-specific calcineurin (PP2B), a calcium regulated phosphatase, in testis and sperm, are also infertile. Loss of PP2B results in impaired epididymal sperm maturation and motility. The phenotypes of GSK3α and PP2B knockout mice are similar, prompting us to examine the interrelationship between these two enzymes in sperm. High calcium levels must exist to permit catalytically active calcineurin to function during epididymal sperm maturation. Total and free calcium levels are high in immotile compared to motile epididymal sperm. Inhibition of calcineurin by FK506 results in an increase in the net phosphorylation and a consequent decrease in catalytic activity of sperm GSK3. The inhibitor FK506 and an isoform-selective inhibitor of GSK3α, BRD0705, also inhibited fertilization of eggs in vitro. Interrelated functions of GSK3α and sperm PP2B are essential during epididymal sperm maturation and during fertilization. Our results should enable the development of male contraceptives targeting one or both enzymes.

pH and male fertility: making sense on pH homeodynamics throughout the male reproductive tract

In the male reproductive tract, ionic equilibrium is essential to maintain normal spermatozoa production and, hence, the reproductive potential. Among the several ions, HCO3- and H+ have a central role, mainly due to their role on pH homeostasis. In the male reproductive tract, the major players in pH regulation and homeodynamics are carbonic anhydrases (CAs), HCO3- membrane transporters (solute carrier 4-SLC4 and solute carrier 26-SLC26 family transporters), Na+-H+ exchangers (NHEs), monocarboxylate transporters (MCTs) and voltage-gated proton channels (Hv1). CAs and these membrane transporters are widely distributed throughout the male reproductive tract, where they play essential roles in the ionic balance of tubular fluids. CAs are the enzymes responsible for the production of HCO3- which is then transported by membrane transporters to ensure the maturation, storage, and capacitation of the spermatozoa. The transport of H+ is carried out by NHEs, Hv1, and MCTs and is essential for the electrochemical balance and for the maintenance of the pH within the physiological limits along the male reproductive tract. Alterations in HCO3- production and transport of ions have been associated with some male reproductive dysfunctions. Herein, we present an up-to-date review on the distribution and role of the main intervenient on pH homeodynamics in the fluids throughout the male reproductive tract. In addition, we discuss their relevance for the establishment of the male reproductive potential.

Immunohistochemical localization of GLUT3, MCT1, and MCT2 in the testes of mice and rats: the use of different energy sources in spermatogenesis

Lactate represents a preferential energy substrate of germ cells rather than glucose. Testicular Sertoli cells are believed to produce lactate and pyruvate and to supply these to germ cells, particularly spermatocytes and spermatids. Monocarboxylate transporter (MCT), responsible for the transport of lactate and other monocarboxylates via the cell membrane, is abundant in the testes and sperm (MCT1, MCT2, and MCT4). For the uptake of glucose, germ cells within the seminiferous tubules and sperm have been known to intensely express GLUT3. The present study investigated expression profiles of MCTs and GLUTs and revealed their cellular and subcellular localization in the mouse and rat testis. An in situ hybridization analysis showed significant expressions of MCT1, MCT2, and GLUT3 mRNA in the testis. Immunohistochemically, spermatogonia, spermatocytes, and spermatids expressed MCT1 on their cell surfaces in a stage-dependent manner: in some seminiferous tubules, an intense expression of MCT1 was unique to the spermatogonia. MCT2 was restricted to the tails of elongated spermatids and sperm. An intense immunoreactivity for GLUT3 was shared by spermatocytes, spermatids, and sperm. Sertoli cells were devoid of any immunoreactivities for MCT1, MCT2, and GLUT3. The predominant energy source of germ cells may be lactate and other monocarboxylates--especially for spermatogonia, but glucose and other hexoses may be responsible for an energy supply to spermatocytes and spermatids.