A Sea Urchin Sperm Flagellar Adenylate Kinase with Triplicated Catalytic Domains

Deficiency in AK9 causes asthenozoospermia and male infertility by destabilising sperm nucleotide homeostasis

BACKGROUND

Asthenozoospermia is the primary cause of male infertility; however, its genetic aetiology remains poorly understood. Adenylate kinase 9 (AK9) is highly expressed in the testes of humans and mice and encodes a type of adenosine kinase that is functionally involved in cellular nucleotide homeostasis and energy metabolism. We aimed to assess whether AK9 is involved in asthenozoospermia.

METHODS

One-hundred-and-sixty-five Chinese men with idiopathic asthenozoospermia were recruited. Whole-exome sequencing (WES) and Sanger sequencing were performed for genetic analyses. Papanicolaou staining, Haematoxylin and eosin staining, scanning electron microscopy, and transmission electron microscopy were used to observe the sperm morphology and structure. Ak9-knockout mice were generated using CRISPR-Cas9. Sperm adenosine was detected by liquid chromatography-mass spectrometry. Targeted sperm metabolomics was performed. Intracytoplasmic sperm injection (ICSI) was used to treat patients.

FINDINGS

We identified five patients harbouring bi-allelic AK9 mutations. Spermatozoa from men harbouring bi-allelic AK9 mutations have a decreased ability to sustain nucleotide homeostasis. Moreover, bi-allelic AK9 mutations inhibit glycolysis in sperm. Ak9-knockout male mice also presented similar phenotypes of asthenozoospermia. Interestingly, ICSI was effective in bi-allelic AK9 mutant patients in achieving good pregnancy outcomes.

INTERPRETATION

Defects in AK9 induce asthenozoospermia with defects in nucleotide homeostasis and energy metabolism. This sterile phenotype could be rescued by ICSI.

FUNDING

The National Natural Science Foundation of China (82071697), Medical Innovation Project of Fujian Province (2020-CXB-051), open project of the NHC Key Laboratory of Male Reproduction and Genetics in Guangzhou (KF202004), Medical Research Foundation of Guangdong Province (A2021269), Guangdong Provincial Reproductive Science Institute Innovation Team grants (C-03), and Outstanding Young Talents Program of Capital Medical University (B2205).

Energy sources that fuel metabolic processes in protruding finger-like organelles

Cells possess a variety of organelles with characteristic structure and subcellular localization intimately linked to their specific function. While most are intracellular and found in virtually all eukaryotic cells, there is a small group of organelles of elongated cylindrical shapes in highly specialized cells that protrude into the extracellular space, such as cilia, flagella, and microvilli. The ATP required by intracellular organelles is amply available in the cytosol, largely generated by mitochondria. However, such is not the case for cilia and flagella, whose slender structures cannot accommodate mitochondria. These organelles consume massive amounts of ATP to carry out high energy-demanding functions, such as sensory transduction or motility. ATP from the nearest mitochondria or other reactions within the cell body is severely limited by diffusion and generally insufficient to fuel the entire length of cilia and flagella. These organelles overcome this fuel restriction by local generation of ATP, using mechanisms that vary depending on the nutrients that are available in their particular external environment. Here, we review, with emphasis in mammals, the remarkable adaptations that cilia and flagella use to fuel their metabolic needs. Additionally, we discuss how a decrease in nutrients surrounding olfactory cilia might impair olfaction in COVID-19 patients.

Asthenozoospermia and membrane remodeling enzymes: a new role for phospholipase A2

Phosholipase A2 (PLA2 ) activity in the seminal plasma and in sperm heads is closely related to sperm motility and male fertility. Therefore, the purpose of this study was to investigate the possible involvement of different isoforms of phospholipase in asthenozoospermia. To accomplish this, cPLA2 , phospho-cPLA2 , iPLA2 , and sPLA2 were evaluated by immunofluorescence and immunoblot analyses in spermatozoa obtained from 22 normozoospermic men and 28 asthenozoospermic patients. We found significant differences in cPLA2 and its phosphorylated/activated form, iPLA2 , and sPLA2 content and distribution in normal and asthenozoospermic patients. cPLA2 was localized in heads, midpieces, and tails of all spermatozoa as constitutive enzyme, less expressed in the tail of spermatozoa with low progressive motility. While active phospho-cPLA2 distribution was homogeneous throughout the cell body of control-donor spermatozoa, lower levels were detected in the tails of asthenozoospermic patients, as opposed to its strong presence in heads. Low immunofluorescence signal for iPLA2 was found in astenozoospermic patients, whereas sPLA2 was significantly lower in the heads of asthenozoospermic patients. Spermatozoa with low progressive motility showed differences both in terms of total specific activity and of intracellular distribution. cPLA2 , iPLA2 , and sPLA2 specific activities correlated positively and in a significantly manner with sperm progressive motility both in normozoospermic men and asthenozoospermic patients. In conclusion, PLA2 s are expressed in different areas of human spermatozoa. Spermatozoa with low motility showed differences in total specific activity and enzyme distributions. We speculated that PLA2 expression and/or different distribution could be potential biomarkers of asthenozoospermia, one of the major causes of male factor infertility.

Soluble adenylyl cyclase of sea urchin spermatozoa

Fertilization, a key step in sexual reproduction, requires orchestrated changes in cAMP concentrations. It is notable that spermatozoa (sperm) are among the cell types with extremely high adenylyl cyclase (AC) activity. As production and consumption of this second messenger need to be locally regulated, the discovery of soluble AC (sAC) has broadened our understanding of how such cells deal with these requirements. In addition, because sAC is directly regulated by HCO(3)(-) it is able to translate CO₂/HCO(3)(-)/pH changes into cAMP levels. Fundamental sperm functions such as maturation, motility regulation and the acrosome reaction are influenced by cAMP; this is especially true for sperm of the sea urchin (SU), an organism that has been a model in the study of fertilization for more than 130 years. Here we summarize the discovery and properties of SU sperm sAC, and discuss its involvement in sperm physiology. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Adenine nucleotide metabolism and a role for AMP in modulating flagellar waveforms in mouse sperm

While most ATP, the main energy source driving sperm motility, is derived from glycolysis and oxidative phosphorylation, the metabolic demands of the cell require the efficient use of power stored in high-energy phosphate bonds. In times of high energy consumption, adenylate kinase (AK) scavenges one ATP molecule by transphosphorylation of two molecules of ADP, simultaneously yielding one molecule of AMP as a by-product. Either ATP or ADP supported motility of detergent-modeled cauda epididymal mouse sperm, indicating that flagellar AKs are functional. However, the ensuing flagellar waveforms fueled by ATP or ADP were qualitatively different. Motility driven by ATP was rapid but restricted to the distal region of the sperm tail, whereas ADP produced slower and more fluid waves that propagated down the full flagellum. Characterization of wave patterns by tracing and superimposing the images of the flagella, quantifying the differences using digital image analysis, and computer-assisted sperm analysis revealed differences in the amplitude, periodicity, and propagation of the waves between detergent-modeled sperm treated with either ATP or ADP. Surprisingly, addition of AMP to the incubation medium containing ATP recapitulated the pattern of sperm motility seen with ADP alone. In addition to AK1 and AK2, which we previously demonstrated are present in outer dense fibers and mitochondrial sheath of the mouse sperm tail, we show that another AK, AK8, is present in a third flagellar compartment, the axoneme. These results extend the known regulators of sperm motility to include AMP, which may be operating through an AMP-activated protein kinase.

Spontaneous generation of reactive oxygen species and effect on motility and fertilizability of sea urchin spermatozoa

We investigated the generation of reactive oxygen species (ROS) by spermatozoa in two species of sea urchin. ROS generation was accompanied by the initiation of motility and respiration and influenced the motility and fertilizability of spermatozoa. The sea urchin performs external fertilization in aerobic seawater. Sperm motility was initiated after spawning through Na+/H+ exchange. ROS generation was dependent on the respiration and sperm concentration and its generation was first observed at initiation of motility, via activation of respiration through ATP/ADP transport. The ROS generation rate increased at higher dilution ratios of spermatozoa, in a manner that was synchronous with the respiratory rate. This phenomenon resembled the previously defined ‘sperm dilution effect’ on respiration. The loss of motility and fertilizability was induced not only by treatment with hydrogen peroxide but also by sperm dilution. Storage of spermatozoa with a higher dilution ratio also accelerated the decrease in fertilizability. Thus, optimum sea urchin fertilizability is maintained by storage of undiluted spermatozoa on ice, in order to minimize oxidative stress and to maximize longevity.

Rho-kinase in sea urchin eggs and embryos

The activation of sea urchin eggs at fertilization provides an ideal system for studying the molecular events involved in cellular activation. Rho GTPases, which are key signaling enzymes in eukaryotes, are involved in sustaining the activation of sea urchin eggs; however, their downstream effectors have not yet been characterized. In somatic cells, RhoA regulates a serine/threonine kinase known as Rho-kinase (ROCK). The activity of ROCK in early sea urchin development has been inferred, but not tested directly. A ROCK gene was identified in the sea urchin (Strongylocentrotus purpuratus) genome and the sequence of its cDNA determined. The sea urchin ROCK (SpROCK) sequence predicts a protein of 158 kDa with >72% and 45% identities with different protein orthologues of the kinase catalytic domain and the complete protein sequence, respectively. SpROCK mRNA levels are high in unfertilized eggs and decrease to 35% after 15 min postfertilization and remain low up to the 4 cell stage. Antibodies to the human ROCK-I kinase domain revealed SpROCK to be concentrated in the cortex of eggs and early embryos. Co-immunoprecipitation assays indicate that RhoA and SpROCK are physically associated. This association is destroyed by treatment with the C3 exoenzyme and with the ROCK antagonist H-1152. H-1152 also inhibited DNA synthesis in embryos. We conclude that the Rho-dependent signaling pathway, via SpROCK, is essential for early embryonic development.

The characterization of human adenylate kinases 7 and 8 demonstrates differences in kinetic parameters and structural organization among the family of adenylate kinase isoenzymes

Differences in expression profiles, substrate specificities, kinetic properties and subcellular localization among the AK (adenylate kinase) isoenzymes have been shown to be important for maintaining a proper adenine nucleotide composition for many different cell functions. In the present study, human AK7 was characterized and its substrate specificity, kinetic properties and subcellular localization determined. In addition, a novel member of the human AK family, with two functional domains, was identified and characterized and assigned the name AK8. AK8 is the second known human AK with two complete and active AK domains within its polypeptide chain, a feature that has previously been shown for AK5. The full-length AK8, as well as its two domains AK8p1 and AK8p2, all showed similar AK enzyme activity. AK7, full-length AK8, AK8p1 and AK8p2 phosphorylated AMP, CMP, dAMP and dCMP with ATP as the phosphate donor, and also AMP, CMP and dCMP with GTP as the phosphate donor. Both AK7 and full-length AK8 showed highest affinity for AMP with ATP as the phosphate donor, and proved to be more efficient in AMP phosphorylation as compared with the major cytosolic isoform AK1. Expression of the proteins fused with green fluorescent protein demonstrated a cytosolic localization for both AK7 and AK8.

Sex-specific biochemical and histological differences in gonads of sea urchins (Psammechinus miliaris) and their response to phenanthrene exposure

Female and male individuals of the same species often differ with respect to their susceptibility to toxicant stress. In the present study, sea urchins (Psammechinus miliaris) of both sexes were exposed to high (150 μg L⁻¹) and environmentally relevant (5 μg L⁻¹) concentrations of phenanthrene over 10 days. While food intake was significantly decreased following exposure to 150 μg L⁻¹ phenanthrene, histological indices (lipofuscin accumulation, fibrosis, oocyte atresia), energetic status (energy charge, sum adenylates, AMP/ATP ratio) as well as ascorbate levels in the gonads showed either little or no effect upon phenanthrene exposure. However, most parameters (vitamin C, energy charge, sum adenylates, AMP/ATP ratio, ATP and ADP concentrations, lipofuscin content, fibrosis) significantly differed between male and female animals. This study illustrates the difficulties to identify toxic injury in reproductive tissue as it may be superimposed by gametogenesis and spawning of gametes.

Role of adenylate kinase type 7 expression on cilia motility: possible link in primary ciliary dyskinesia

BACKGROUND

Adenylate kinase 7 (AK7) mediates the reaction 2ADP <--> ATP + AMP, providing energy for the beating of cilia. A study recently showed that AK7 expression may be correlated with the primary ciliary dyskinesia (PCD) phenotype in mice. In this study, we characterized AK7 expression in vitro in an air-liquid interface (ALI) model and in middle nasal turbinate biopsy specimens from a cohort of patients with PCD to elucidate whether AK7 expression is correlated with ciliary malfunction.

METHODS

AK7 expression was measured by real-time reverse-transcription polymerase chain reaction and Western blotting. In vitro differentiated nasal human epithelial cell siRNA experiments were performed to investigate the effect of AK7 expression on ciliary beat frequency (CBF). Ciliary motility and ultrastructure were evaluated in a cohort of 29 patients with PCD (PCD, n = 17; Kartagener's syndrome, n = 12) and 26 healthy control donors.

RESULTS

AK7 expression was mainly located on the apical surface of differentiated nasal ALI cells, and targeted suppression of the AK7 gene decreased CBF by 41%. AK7 expression was diminished significantly in patients with PCD (0.54 +/- 0.1-fold; p < 0.05) compared with healthy controls (1.1 +/- 0.08-fold). Furthermore, AK7 expression was correlated with CBF in patients with PCD (r = 0.5; p = 0.009).

CONCLUSION

AK7 expression was correlated with CBF in vitro and in nasal biopsy specimens from patients with PCD, which may have contributed to the ciliary malfunction observed in our patients with PCD.

Analysis of the role of nucleotides in axonemal dynein function

Axonemal dynein in flagella and cilia is a motor molecule that produces microtubule sliding, powered by the energy of ATP hydrolysis. Our goal is to understand how dynein motile activity is controlled to produce the characteristic oscillatory movement of flagella. ATP, the energy source for dynein, is also important as a regulator of dynein activity. Among the four nucleotide-binding sites of a dynein heavy chain, one is the primary ATP hydrolyzing site while the others are noncatalytic sites and thought to perform regulatory functions. Stable binding of both ATP and ADP to these regulatory sites is probably essential for the chemomechanical energy transduction in dynein. Although the ATP concentration in beating flagella is physiologically high and constant, at any moment in the oscillatory cycle some dynein molecules are active while others are not, and the motile activity of dynein oscillates temporally and spatially in the axoneme. It is likely that the basic mechanism underlying the highly dynamic control of dynein activity involves the ATP-dependent inhibition and ADP-dependent activation (or release of inhibition) of dynein. How the inhibition and activation can be induced in beating flagella is still unknown. It seems, however, that the mechanical force of bending is involved in the activation of dynein, probably through the control of noncatalytic nucleotide binding to dynein. This chapter provides an overview of several approaches, using sea urchin sperm flagella, to studying the roles of ATP and ADP in the regulation of dynein activity with or without the mechanical signal of bending.

Adenylate kinase and AMP signaling networks: metabolic monitoring, signal communication and body energy sensing

Adenylate kinase and downstream AMP signaling is an integrated metabolic monitoring system which reads the cellular energy state in order to tune and report signals to metabolic sensors. A network of adenylate kinase isoforms (AK1-AK7) are distributed throughout intracellular compartments, interstitial space and body fluids to regulate energetic and metabolic signaling circuits, securing efficient cell energy economy, signal communication and stress response. The dynamics of adenylate kinase-catalyzed phosphotransfer regulates multiple intracellular and extracellular energy-dependent and nucleotide signaling processes, including excitation-contraction coupling, hormone secretion, cell and ciliary motility, nuclear transport, energetics of cell cycle, DNA synthesis and repair, and developmental programming. Metabolomic analyses indicate that cellular, interstitial and blood AMP levels are potential metabolic signals associated with vital functions including body energy sensing, sleep, hibernation and food intake. Either low or excess AMP signaling has been linked to human disease such as diabetes, obesity and hypertrophic cardiomyopathy. Recent studies indicate that derangements in adenylate kinase-mediated energetic signaling due to mutations in AK1, AK2 or AK7 isoforms are associated with hemolytic anemia, reticular dysgenesis and ciliary dyskinesia. Moreover, hormonal, food and antidiabetic drug actions are frequently coupled to alterations of cellular AMP levels and associated signaling. Thus, by monitoring energy state and generating and distributing AMP metabolic signals adenylate kinase represents a unique hub within the cellular homeostatic network.

Contributions to catalysis and potential interactions of the three catalytic domains in a contiguous trimeric creatine kinase

Three separate creatine kinase (CK) isoform families exist in animals. Two of these (cytoplasmic and mitochondrial) are obligate oligomers. A third, flagellar, is monomeric but contains the residues for three complete CK domains. It is not known whether the active sites in each of the contiguous flagellar domains are catalytically competent, and, if so, whether they are capable of acting independently. Here we have utilized site-directed mutagenesis to selectively disable individual active sites and all possible combinations thereof. Kinetic studies showed that these mutations had minimal impact on substrate binding and synergism. Interestingly, the active sites were not catalytically equivalent, and were in fact interdependent, a phenomenon that has previously been reported only in the oligomeric CK isoforms.

Inhibition by ATP and activation by ADP in the regulation of flagellar movement in sea urchin sperm

ATP and ADP are known to play inhibitory and activating roles, respectively, in the regulation of dynein motile activity of flagella. To elucidate how these nucleotide functions are related to the regulation of normal flagellar beating, we examined their effects on the motility of reactivated sea urchin sperm flagella at low pH. At pH 7.0-7.2 which is lower than the physiological pH of 8, about 90% of reactivated flagella were motionless at 1 mM ATP, while about 60% were motile at 0.02 mM ATP. The motionless flagella at 1 mM ATP maintained a single large bend or an S-shaped bend, indicating formation of dynein crossbridges in the axoneme. The ATP-dependent inhibition of flagellar movement was released by ADP, and was absent in outer arm-depleted flagella. Similar inhibition was also observed at 0.02 mM ATP when demembranated flagella were reactivated in the presence of Li+ or pretreated with protein phosphatase 1 (PP1). ADP also released this type of ATP-inhibition. In PP1-pretreated axonemes the binding of a fluorescent analogue of ADP to dynein decreased. Under elastase-treatment at pH 8.0, the beating of demembranated flagella at 1 mM ATP and 0.02 mM ATP lasted for approximately 100 and 45 s, respectively. The duration of beating at 0.02 mM ATP was prolonged by Li+, and that at 1 mM ATP was shortened by removal of outer arms. These results indicate that the regulation of on/off switching of dynein motile activity of flagella involves ATP-induced inhibition and ADP-induced activation, probably through phosphorylation/dephosphorylation of outer arm-linked protein(s).