Enzymic characteristics of an ecto-cyclic AMP-dependent protein kinase in rat epididymal spermatozoa

Understanding sperm motility mechanisms and the implication of sperm surface molecules in promoting motility

Background

It is estimated that approximately 8–12% of couples globally face problems associated with infertility. A large number of men exhibit suboptimal sperm parameters. Sperm motility is one of the factors that is measured when analysing sperm parameters. The indication of several crucial sperm surface molecules, having the ability to modulate motility, has opened new avenues in understanding the complex processes involved in motility.

Main body of the abstract

There are various mechanisms that regulate and enhance sperm motility. Several surface molecules on sperm cells can also regulate motility, thus showing their possible application as a treatment for infertility caused by impaired motility. Sperm motility is regulated by intracellular and extracellular pH, along with calcium ions (Ca2+) and carbonate ion (HCO3−) concentrations. Moreover, sperm cells have an array of surface proteins which play a critical role in their function and motility. The indication of surface molecules presented new opportunities for understanding sperm motility and the possibility of treating infertility caused by impaired sperm function. Infertility and problems associated with conception can cause underlying stress and mental trauma. Although there are several methods for treating infertility, most are complex, invasive, and expensive.

Conclusion

It is important to understand how surface molecules and proteins on the sperm cell regulate motility. This will enable us to treat anomalies associated with proper sperm function. This review highlights the general mechanisms that regulate sperm motility, and it stresses the importance and relevance of sperm surface molecules in regulating sperm motility.

Extracellular phosphorylation and phosphorylated proteins: not just curiosities but physiologically important

Mining of the literature and high-throughput mass spectrometry data from both healthy and diseased tissues and from body fluids reveals evidence that various extracellular proteins can exist in phosphorylated states. Extracellular kinases and phosphatases (ectokinases and ectophosphatases) are active in extracellular spaces during times of sufficiently high concentrations of adenosine triphosphate. There is evidence for a role of extracellular phosphorylation in various physiological functions, including blood coagulation, immune cell activation, and the formation of neuronal networks. Ectokinase activity is increased in some diseases, including cancer, Alzheimer's disease, and some microbial infections. We summarize the literature supporting the physiological and pathological roles of extracellularly localized protein kinases, protein phosphatases, and phosphorylated proteins and provide an analysis of the available mass spectrometry data to annotate potential extracellular phosphorylated proteins.

Cell surface phosphorylation by a novel ecto-protein kinase: a key regulator of cellular functions in spermatozoa

Since 1976 many studies have been reported on the occurrence and functional significance of ecto-protein kinases in a variety of cell types although their precise biochemical identity is largely unknown. This study reports for the first time purification to apparent homogeneity of an ecto-protein kinase (ecto-CIK) and some of its characteristics using caprine sperm as the cell model. The ecto-CIK is a unique membrane-specific serine/threonine protein kinase. It is a strongly basic 115 kDa protein made up of two subunits: 63 and 55 kDa. The ecto-kinase undergoes a remarkable lateral movement on the outer cell surface culminating in capping on the sperm acrosomal tip. MPS, its major protein substrate is also located on the acrosomal tip. Both ecto CIK and MPS serve as potential regulators of flagellar motility. This novel enzyme appears to be major kinase responsible for the reported regulation of mammalian cellular functions by modulating phosphorylation of the membrane-bound proteins.

Cyclic AMP phosphodiesterase: a regulator of forward motility initiation during epididymal sperm maturation

The concentrations of cAMP, cAMP phosphodiesterase (PDE) activity, and the effect of theophylline in vitro on the forward motility (FM) of maturing goat epididymal sperm have been analyzed. cAMP levels increase slowly during transit of the cells from the caput to the proximal cauda, although they acquired a minimal degree of forward progression. The last phase of sperm transit (proximal to distal cauda) was associated with a concomitant sharp rise in the level of both cAMP as well as flagellar motility. PDE activity progressively decreased (approximately threefold) during epididymal maturation, being minimal in mature cauda sperm. Theophylline (30 mM), a specific inhibitor of PDE, markedly activated (10-fold or greater) motility of the sperm derived from proximal-corpus, mid-corpus, distal-corpus, and proximal-cauda epididymides. FM of the native mature caudal sperm was similar to that of the theophylline-treated proximal-cauda sperm. The terminal stage of sperm maturity (proximal to distal cauda) was associated with a markedly reduced level of theophylline-dependent motility activation (approximately 50%). The data are consistent with the view that PDE plays an important role in the initiation of motility during epididymal sperm maturation.

Cauda epididymal sperm interactions with seminal vesicle fluid

The interaction of rat cauda epididymal sperm cAMP-dependent protein kinase (PKA) with seminal vesicle fluid (SVF) proteins was examined. Specific proteins in SVF act as substrates for the sperm cell PKA. The apparent molecular weights of these proteins are 45.0, 31.5, 17.2, 14.7, and 13.3 kDa. The phosphorylation of one low-molecular-weight cauda sperm protein is blocked in the presence of SVF. There is no PKA enzyme activity in SVF. The presence of phosphate transfer activity between the sperm cell enzyme and the SVF proteins is species dependent. For example, mouse and rat SVF proteins are efficient phosphate acceptors, but there is no phosphorylation activity when hamster SVF is used as the enzyme substrate. The sperm cell samples were also assessed for membrane integrity. Specifically, cauda sperm cells used in these assays were judged to be intact when examined microscopically using the fluorescent vital dye carboxyfluorodiacetate. Although there was enzyme activity in the supernatants of the rat sperm cell samples, in the protein kinase assay it required three times as much supernatant volume (compared with intact cell sample volume) to measure the activity. Supernatant enzyme activity did not increase with washing, indicating that the cells were not damaged by this procedure. The enzyme itself does not adhere to the sperm cells, so the PKA enzyme activity is most likely oriented on the external surface of the sperm cell.

Seminal vesicles: development, secretory products, and fertility

The development of the seminal vesicle from the mesonephric duct is described. Particular attention is given to the recent biochemistry of seminal vesicle proteins. Proteins in the seminal vesicle fluid are few in number, may be insoluble at certain pH, and frequently form large macromolecular aggregates. Although not an absolute requirement for fertility, seminal vesicle fluid assists in a number of ways to insure fertility. A biochemical model is presented that demonstrates that cAMP dependent phosphorylation may be an important interaction between sperm and certain seminal vesicle proteins.

Biochemical parameters of initiation and regulation of sperm motility

Studies of in vitro models demonstrate that a forward motility protein (FMP) is required for the initiation of forward motility in the immature epididymal spermatozoa. FMP is a heat-stable glycoprotein derived from epididymal plasma. During the epididymal maturation of spermatozoa in vivo, there is a marked increase of intrasperm pH and level of cyclic adenosine monophosphate (cAMP). Several studies suggest that exogenous FMP in concert with elevated intrasperm pH and level of cAMP initiates flagellar motility during the epididymal transit of sperm. cAMP activates sperm cytosolic cAMP-dependent protein kinases, which in turn phosphorylate multiple intrasperm phosphoproteins that may regulate flagellar motility. Exogenous calcium ion activates intact sperm motility, although it inhibits motility of demembranated cells on reactivation. Occurrence of cAMP-dependent type I and II protein kinases, a novel cAMP-independent protein kinase, and a phosphoprotein phosphatase has been demonstrated on the external surface of spermatozoa. The sperm surface has a coupled-enzyme system: ecto-cAMP-independent protein kinase and phosphoprotein phosphatase that regulate the phosphorylation and dephosphorylation of endogenous sperm ectophosphoproteins. The specific activities of these ecto-enzymes increase markedly during forward progression, suggesting that they may have a role in regulating flagellar motility.

Phosphocreatine, an intracellular high-energy compound, is found in the extracellular fluid of the seminal vesicles in mice and rats

High levels of phosphocreatine, a compound known to serve as an intracellular energy reserve, were found in the fluid contained in seminal vesicle glands. The concentrations of phosphocreatine in the extracellular fluid in the mouse and rat were found to be 5.6 +/- 1.6 and 2.2 +/- 0.8 mumol/g, respectively, which are higher than the intracellular levels reported for smooth muscles. The creatine concentrations in the seminal vesicular fluid from these two species were 22.8 +/- 3.1 and 13.0 +/- 5.3 mumol/g, respectively. These creatine levels are approximately 100 and 65 times higher than the creatine levels in mammalian blood. Smaller amounts of ATP (phosphocreatine/ATP ratio of 20-40) and traces of ADP were also found. Comparison of the pattern of distribution of macromolecules (proteins and DNA) with the distribution of phosphocreatine between the cells and the fluid of the seminal vesicle indicates that cell lysis did not account for the phosphocreatine in the seminal vesicle fluid. Rather, the available evidence strongly suggests that this high-energy compound is actively secreted. We found that in the testes, the sperm are exposed to the highest known creatine concentration in any mammalian tissue studied. Based on these results and other recent reports, we propose that the extracellular phosphocreatine, ATP, and creatine are involved in sperm metabolism.

Ecto-cyclic AMP-receptor in goat epididymal intact spermatozoa and its change in activity during forward motility

Goat epididymal intact spermatozoa have been shown to possess on the external surface specific receptors that bind with high affinity to exogenous [8-3H]cyclic AMP. The ecto-cyclic AMP-receptor activity was not due to contamination of broken or "leaky" cells, if any. The binding reaction of [3H]cyclic AMP with the receptors was extremely rapid. Uptake of the labeled cyclic AMP to the sperm cytosolic fraction was undetectable. There was little leakage of cyclic AMP-receptors from intact spermatozoa during the binding assays. The binding reaction was proportional to cell concentration, specific and saturable at 250 nM cyclic AMP. The binding of the labelled cyclic nucleotide was nearly completely displaced at saturating concentrations (2.5 microM) of the unlabelled nucleotide. The ecto-receptors showed high specificity for binding to cyclic AMP. The Kd of the binding sites was approximately 1.7 X 10(-8) M. The binding interaction was highly sensitive to treatment with proteolytic enzymes: trypsin, chymotrypsin, or pronase (125 micrograms/ml). Sonication caused a nearly 450% increase of the ecto-receptor activity. The specific activity of the ecto-cyclic AMP-receptor was approximately twofold higher in the vigorously forwardly motile spermatozoa than in the "composite" cells, suggesting that the ecto-receptors may have a role in modulating flagellar motility.

Phosphorylation of external cell-surface proteins by an endogenous ecto-protein kinase of goat epididymal intact spermatozoa

Intact spermatozoa from goat cauda epididymides possess an ecto-(cyclic AMP-independent protein kinase) activity that causes transfer of the terminal phosphate of exogenously added [gamma-32P]ATP to the serine and threonine residues of several endogenous plasma-membrane phosphoproteins located on the external cell surface. Cyclic AMP, cyclic GMP, calmodulin and muscle cyclic AMP-dependent protein kinases I and II had no appreciable effect on the rate of phosphorylation of ecto-proteins by the intact cells. The ecto-enzyme is not derived from the catalytic subunit of a cyclic AMP-dependent kinase. Sperm ecto-kinase activity is not due to contamination of broken cells or any possible cell damage during incubation and isolation of spermatozoa. The phosphorylation reaction was linear for approx. 1 min and there was no detectable uptake of ATP by these cells. The activity of the ecto-kinase was strongly inhibited by proteinases and by the membrane-nonpenetrating surface probes. The products of the reaction were associated with the intact cells and the 32P of the labelled cells was largely lost when treated with Triton X-100 or proteinases: trypsin and pronase. These data are consistent with the view that the observed protein kinase and the phosphoproteins are located on the external surface of spermatozoa. Vigorously forward-motile whole spermatozoa showed a relatively high capacity to phosphorylate ecto-proteins that undergo rapid turnover. The results suggest the occurrence of a novel coupled-enzyme system (ecto-protein kinase and phosphoprotein phosphatase) on the sperm external surface that may modulate sperm physiology by determining the phosphorylated states of the ecto-proteins.

Defining erythrocyte internal labeling by phosphorylation

When erythrocytes are incubated with 32Pi, incorporation of label into phosphoproteins is a gradual process, increasing for at least 2 hours. Membrane phospholipids also are labeled. Exogenous protein kinase substrates are unlabeled in these incubations. This suggests that labeling by 32Pi occurs into polypeptides inside the erythrocytes. When erythrocytes are incubated with [gamma-32P]ATP and active protein kinase, membrane polypeptides are not labeled. Only exogenously added protein kinase substrates and the regulatory subunit of protein kinase (and its contaminants) are labeled. This suggests that labeling from [gamma-32P]ATP and active protein kinase occurs in the compartment outside the erythrocytes. Apyrase (EC 3.6.1.5) eliminates such labeling, demonstrating that it was occurring in the compartment external to the erythrocytes. However, in incubations of cells with 32Pi, apyrase has no effect on the incorporation into membrane polypeptides and phospholipids, demonstrating that this labeling occurs on the inside of the membrane. Thus, additions of apyrase to intact particles incubated with protein kinase substrates and 32Pi provides a method for identifying internally exposed polypeptides in the plasma membranes of a variety of systems.