Primate sperm contain protein phosphatase 1, a biochemical mediator of motility

Inhibition of motility of hamster spermatozoa by protein tyrosine kinase inhibitors

Genistein, tyrphostin and piceatannol, which are specific inhibitors of protein tyrosine kinase, were screened for their effects on the motility of intact and demembranated hamster spermatozoa. Of the three inhibitors only piceatannol inhibited the motility of intact spermatozoa. None of the inhibitors had any inhibitory effect on the reactivation of motility of demembranated hamster spermatozoa. Taken together these results indicated that a protein tyrosine kinase associated with the membrane of hamster spermatozoa was probably involved in sustenance of hamster sperm motility. Therefore in the present study a membrane-associated protein tyrosine kinase was purified from a detergent-soluble extract of plasma membranes of mature hamster spermatozoa. The purification involved cation exchange chromatography on fast protein liquid chromatography (FPLC) followed by affinity chromatography either on an antiphosphotyrosine antibody agarose or poly glu-tyr agarose column. The pure protein tyrosine kinase had an apparent molecular mass of 45 kDa. The enzyme was not inhibited by genistein or herbimycin but was inhibited by piceatannol. This is the first report on the purification of a sperm plasma membrane-associated protein tyrosine kinase, an enzyme which has also been implicated in hamster sperm motility.

cAMP-dependent protein kinase control of plasma membrane lipid architecture in boar sperm

Bicarbonate/CO(2), a physiological effector of sperm capacitation, has been shown to induce a rapid and reversible change in the lipid architecture of the plasma membrane of live boar sperm: the change is detectable as an increase in the cells' ability to bind the fluorescent dye merocyanine, a characteristic which implied an increase in lipid packing disorder (Harrison et al. 1996. Mol Reprod Dev 45:378-391). Evidence suggested that cAMP may act as a second messenger in the system, and we have therefore investigated this cAMP-dependency in more detail. Bicarbonate stimulates cAMP levels within 1 min in a dose-dependent fashion, prior to parallel increases in merocyanine binding. Although the potent somatic cell adenylyl cyclase activator forskolin is unable to induce significant increases in cAMP or merocyanine binding, increases in merocyanine binding are inducible in a dose-dependent fashion by 5, 6-dichloro-1-beta-D-ribofuranosylbenzimidazole 3',5'-cyclic monophosphothioate, a cAMP analogue highly specific in its ability to stimulate protein kinase A; moreover, the bicarbonate-induced membrane change is inhibited by H89, a specific protein kinase A inhibitor. Neither bisindolylmaleimide I (protein kinase C inhibitor) nor lavendustin A (protein tyrosine kinase inhibitor) are inhibitory. In the presence of low levels of the potent phosphodiesterase inhibitor papaverine, increases in merocyanine binding are enhanced by okadaic acid and (more effectively) by calyculin (both protein phosphatase inhibitors). We conclude that boar sperm plasma membrane lipid architecture is controlled via a target protein that is dynamically phosphorylated by cAMP-dependent protein kinase and dephosphorylated by protein phosphatase type 1. Mol. Reprod. Dev. 55:220-228, 2000.

Role of tyrosine phosphorylation of flagellar proteins in hamster sperm hyperactivation

Despite extensive study of sperm motility, little is known of the mechanism of mammalian sperm hyperactivation. Here we describe a novel method for preparation of rodent sperm flagella and use it to show a correlation between tyrosine phosphorylation of flagellar proteins and hyperactivation of hamster sperm. When hyperactivation was produced by a 3.5-h incubation in a medium supporting capacitation, four major tyrosine-phosphorylated peptides of 90-, 80-, 62-, and 48-kDa mass were detected in flagellar extracts. Incubation with calyculin A, an inhibitor of protein phosphatases 1 and 2A, produced hyperactivation within 40 min but only a single 80-kDa phosphotyrosine-containing flagellar component. Conversely, incubation with inhibitors of either protein kinase A (H8) or protein tyrosine kinase (tyrphostin 47) prevented both hyperactivation and the production of tyrosine-phosphorylated flagellar peptides. These results indicate a strong correlation of hyperactivation with the tyrosine phosphorylation of sperm flagellar peptides, and they strongly implicate an 80-kDa component as a major mediator of the mechanism that produces hyperactivated motility of hamster sperm.

Hyperactivation of hamster sperm motility by temperature-dependent tyrosine phosphorylation of an 80-kDa protein

Protein phosphorylation and dephosphorylation are believed to play key roles in regulation of sperm motility. Here we examine the effect of temperature on hamster sperm motility and protein tyrosine phosphorylation status. As in previous work, a decrease from 37 degrees C to 22 degrees C caused loss of hyperactivated motility. We now find that cooling also produces a dephosphorylation of several 48-80-kDa flagellar peptides. A return to 37 degrees C restored hyperactivation but resulted in rephosphorylation of only an 80-kDa protein. Conversely, hyperactivation and phosphorylation of the 80-kDa component were insensitive to incubation temperature for sperm incubated with the protein phosphatase inhibitor, calyculin A, or for sperm demembranated by detergent extraction. These results strongly indicate that the temperature-sensitive tyrosine phosphorylation status of an 80-kDa sperm flagellar peptide explains the sensitivity of hyperactivation to temperature.

Plasma membrane-associated protein tyrosine phosphatase activity in hamster spermatozoa

Plasma membranes of caput and cauda epididymal spermatozoa of hamster exhibited protein phosphatase activity. This membrane-associated protein phosphatase was identified as a protein tyrosine phosphatase based on its ability to hydrolyse a substrate specific for PTPase, by inhibition of its activity with a specific inhibitor of PTPase (sodium orthovanadate) and by the inability to inhibit its activity with calyculin, okadaic acid, trifluoperazine, calcium, EGTA, and EDTA, which are specific inhibitors of other protein phosphatases, namely PP-1, PP-2A, PP-2B, and PP-2C respectively. The specific activity of the protein tyrosine phosphatase both in the caput and cauda epididymal sperm plasma membranes was similar, implying that the enzyme may not be solely responsible for the differential phosphorylation of membrane proteins observed during maturation (Uma Devi et al. 1997. Mol Reprod Dev 47:341-350). Thus the significance of the PTPase activity in epididymal maturation still remains to be determined. The membrane-associated PTPase may not be essential for acquisition of motility. However, it appears that the activity is essential for the sustenance of motility since sodium orthovanadate, which specifically inhibits PTPase activity, also inhibits motility of spermatozoa and decreases the overall velocity of the spermatozoa by decreasing the average path velocity, straight line velocity, curvilinear velocity, and amplitude of lateral head displacement of the treated spermatozoa.

Characterization of protein phosphatases in mouse oocytes

Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indicating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is involved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesicle-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1alpha and PP1gamma. In addition, the transcript for PP2A was also present. At the protein level only PP1alpha and PP2A were recognized in fully grown GVI oocytes by Western blot analysis. Neither of the PP1gamma spliced variant proteins, PP1gamma1 and PP1gamma2, was detectable. Immunohistochemical analysis of ovarian tissue from gonadotropin-stimulated adult mice resulted in subcellular localization of both PP1alpha and PP2A, but not PP1gamma, in oocytes from all stages of folliculogenesis. In primordial oocytes, PP1alpha and PP2A were present in the cytoplasm. In more advanced stages of oogenesis, PP1alpha, although still present in the cytoplasm, was highly concentrated in the nucleus, whereas PP2A was predominantly cytoplasmic with a distinct reduction in the nuclear area. Both PP1alpha and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesicle breakdown (GVB)-incompetent, displayed both PP1alpha and PP2A predominantly in the cytoplasm. By 15 days of age mouse oocytes, which are beginning to acquire OA sensitivity and GVB competence, showed a relocation of PP1alpha into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1alpha and PP2A in mouse oocytes. The differential localization of PP1alpha and PP2A, in addition to the relocation of PP1alpha during the acquisition of meiotic competence, suggests that these PPs have distinct regulatory roles during the resumption of meiosis.

A maturation-related differential phosphorylation of the plasma membrane proteins of the epididymal spermatozoa of the hamster by endogenous protein kinases

When the plasma membranes of caput and cauda epididymal spermatozoa of hamster were evaluated for their ability to undergo phosphorylation, a differential phosphorylation of the membrane proteins was observed. In the plasma membranes of the caput epididymal spermatozoa (immature), twelve proteins were phosphorylated (100, 76, 67, 65, 55, 52, 47, 42, 38, 32, 30, and 20 kD), whereas in the plasma membranes of cauda epididymal spermatozoa (mature), a differential phosphorylation pattern was observed with respect to the 94, 67, 52, and 47 kD proteins. The 94 kD protein was found to be phosphorylated and the 67 kD protein was found to be not phosphorylated in cauda spermatozoal plasma membrane (Cd SPM) in contrast to this protein in caput spermatozoal plasma membrane (Cpt SPM). The 52 and 47 kD proteins were also more intensely phosphorylated in Cd SPM than Cpt SPM. The 100 kilodalton protein, although present in both Cpt and Cd sperm plasma membranes, was found to be phosphorylated at the tyrosine residues only in the Cd SPM, as indicated by the Western blot using antiphosphotyrosine antibody. Further, a differential phosphorylation of the substrate proteins present in the Cpt and Cd SPM was seen when Mg2+ in the assay buffer was replaced by other divalent cations. For instance, Zn2+ stimulated the phosphorylation of an 85 kD protein in cauda SPM and not in the caput SPM and Ca2+ stimulated the phosphorylation of a 76 kD protein only in the cauda SPM. The phosphoproteins in both the plasma membranes were found to be phosphorylated predominantly at the tyrosine residue. The differential phosphorylation at a 100 kD protein at tyrosine in the Cd SPM (Western blot), which is absent in the immature Cpt SPM, also indicated that certain proteins in the hamster spermatozoa are phosphorylated in a maturation-specific manner.

Regulation of fowl sperm flagellar motility by protein phosphatase type 1 and its relationship with dephosphorylation of axonemal and/or accessory cytoskeletal proteins

The motility of demembranated fowl spermatozoa was vigorous at 30 degrees C in the presence of ATP, but decreased markedly following the addition of recombinant protein phosphatase type 1 (PP-1) supplemented with Mn2+. This inhibition was not restored by the addition of cAMP, within the range 1-1000 microM, but instantly restored by the addition of 50 ng/ml trypsin. Phosphorylation of demembranated fowl sperm proteins during incubation with [gamma-32P]ATP at 30 degrees C was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). A marked difference in phosphorylation status was observed in approximately 116, 86, 79, 50 and 29-kDa proteins. These proteins were dephosphorylated in the presence of PP-1 and Mn2+ compared with those in control samples. These results suggest that PP-1-mediated dephosphorylation of some of these proteins of the axoneme and/or accessory cytoskeletal components of fowl spermatozoa may be involved in the inhibition of motility.

Protein kinase A-anchoring inhibitor peptides arrest mammalian sperm motility

Cyclic AMP-dependent protein kinase (PKA) is anchored at specific subcellular sites through the interaction of the regulatory subunit (R) with protein kinase A-anchoring proteins (AKAPs) via an amphipathic helix binding motif. Synthetic peptides containing this amphipathic helix domain competitively disrupt PKA binding to AKAPs and cause a loss of PKA modulation of cellular responses. In this report we use S-Ht31, a cell-permeant anchoring inhibitor peptide, to study the role of PKA anchoring in sperm. Our analysis of three species of mammalian sperm detected three isoforms of PKA (RIIalpha, RIIbeta, and RIbeta) and one 110-kDa AKAP. The addition of S-Ht31 to bovine caudal epididymal sperm inhibits motility in a time- and concentration-dependent manner. A control peptide, S-Ht31-P, identical to S-Ht31 except for a proline for isoleucine substitution to prevent amphipathic helix formation, had no effect on motility. The inhibition of motility by S-Ht31 is reversible but only if calcium is present in the suspension buffer, suggesting a role for PKA anchoring in regulating cellular calcium homeostasis. Surprisingly, inhibition of PKA catalytic activity had little effect on basal motility or motility stimulated by agents previously thought to work via PKA activation. These data suggest that the interaction of the regulatory subunit of PKA with sperm AKAPs, independent of PKA catalytic activity, is a key regulator of sperm motility and that disruption of this interaction using cell-permeable anchoring inhibitor peptides may form the basis of a sperm-targeted contraceptive.