122 OVARIAN FOLLICULAR DYNAMICS IN CROSS-BRED EWES DURING THE RAINY SEASON UNDER TROPICAL CONDITIONS

Small ruminants are an important livestock species on many island nations of the Eastern Caribbean region and serve as a vital source of food and income. This study was designed to characterise follicular development during the inter-ovulatory interval (interval between 2 consecutive ovulations) under tropical conditions. Non-lactating, 2- to 4-year-old crossbred Barbados-Black Belly × White Virgin Island ewes weighing a mean 30.4 kg with mean body condition score 2.9 (score 1–5) were used during September–November for the study. Ewes were maintained in a paddock of 100 m2 at the research farm in St. Kitts and Nevis (17.3°N, 62.7°W). Animals were fed daily with 100 g of concentrate (17% crude protein) and had free access to elephant grass (Pennisetum purpureum), mineral salt, water, and shade. Ovaries of 19 females were examined daily for 46 days by transrectal B-mode ultrasonography using a 7.5-MHz lineal array transducer (Honda HS-2200UV, MA, USA) to detect ovulations and growth and regression of antral follicles. Data were analysed using chi-square, Student t-test, or one-way ANOVA. Proportion of females with 2, 3, and 4 follicular waves was 2/19 (10.5%), 10/19 (52.6%), and 7/19 (36.8%), respectively. Mean (± SEM) length of the inter-ovulatory intervals did not differ (P = 0.9) among females with 2 (16.5 ± 0.5 days), 3 (16.3 ± 0.4 days), and 4 (16.6 ± 0.2 days) follicular waves, respectively. Mean diameter of ovulatory follicles was 4.4 ± 0.1 mm, proportion of multiple ovulations was 11/19 (57.9%), and mean number of follicular waves was 3.3 ± 0.1. Wave emergence was normalized to the first ovulation of the inter-ovulatory interval (ovulation = Day 0). Mean days of wave emergence for 2 follicular waves were Days 1.0 ± 0 and 10.0 ± 0.0; for 3 follicular waves were Days 0.7 ± 0.1, 5.6 ± 0.5, and 10.4 ± 0.3; and 4 follicular waves were Days 0.8 ± 0.3, 4.7 ± 0.6, 8.4 ± 0.5, and 11.7 ± 0.3, respectively. The inter-wave intervals were 9.0 ± 0.0 days for 2 waves; 5.1 ± 0.5 and 4.8 ± 0.3 days for 3 waves; and 3.8 ± 0.5, 3.7 ± 0.3, and 3.3 ± 0.4 days for 4 waves. Growth rate (mm/day) of the largest follicle/wave did not differ (P = 07) among animals with 2 follicular waves (1.2 ± 0.7 v. 1.1 ± 0.0) or 3 follicular waves (0.9 ± 0.1 v. 0.9 ± 0.1 v. 1.2 ± 0.2). However, in animals with 4 follicular waves, the largest follicle grew faster (P < 0.05) for wave 1 (1.3 ± 0.2) than that of waves 2 (0.5 ± 0.2), 3 (0.6 ± 0.1), and 4 (0.6 ± 0.1). In conclusion, length of inter-ovulatory intervals, emergence of antral follicles, growth, and regression follicles in a wave-like pattern, inter-wave intervals, and diameter of ovulatory follicles in cross-bred ewes under tropical conditions are comparable to that described in sheep from temperate climates.

Src Kinase Is the Connecting Player between Protein Kinase A (PKA) Activation and Hyperpolarization through SLO3 Potassium Channel Regulation in Mouse Sperm

Plasma membrane hyperpolarization is crucial for mammalian sperm to acquire acrosomal responsiveness during capacitation. Among the signaling events leading to mammalian sperm capacitation, the immediate activation of protein kinase A plays a pivotal role, promoting the subsequent stimulation of protein tyrosine phosphorylation that associates with fertilizing capacity. We have shown previously that mice deficient in the tyrosine kinase cSrc are infertile and exhibit improper cauda epididymis development. It is therefore not clear whether lack of sperm functionality is due to problems in epididymal maturation or to the absence of cSrc in sperm. To further address this problem, we investigated the kinetics of cSrc activation using anti-Tyr(P)-416-cSrc antibodies that only recognize active cSrc. Our results provide evidence that cSrc is activated downstream of PKA and that inhibition of its activity blocks the capacitation-induced hyperpolarization of the sperm plasma membrane without blocking the increase in tyrosine phosphorylation that accompanies capacitation. In addition, we show that cSrc inhibition also blocks the agonist-induced acrosome reaction and that this inhibition is overcome by pharmacological hyperpolarization. Considering that capacitation-induced hyperpolarization is mediated by SLO3, we evaluated the action of cSrc inhibitors on the heterologously expressed SLO3 channel. Our results indicate that, similar to SLO1 K(+) channels, cSrc blockers significantly decreased SLO3-mediated currents. Together, these results are consistent with findings showing that hyperpolarization of the sperm plasma membrane is necessary and sufficient to prepare the sperm for the acrosome reaction and suggest that changes in sperm membrane potential are mediated by cSrc activation.

Inhibition of Ser/Thr phosphatases induces capacitation-associated signaling in the presence of Src kinase inhibitors

Signaling events leading to mammalian sperm capacitation rely on activation/deactivation of proteins by phosphorylation. This cascade includes soluble adenylyl cyclase, an atypical bicarbonate-stimulated adenylyl cyclase, and is mediated by protein kinase A and the subsequent stimulation of protein tyrosine phosphorylation. Recently, it has been proposed that the capacitation-associated increase in tyrosine phosphorylation is governed by Src tyrosine kinase activity. This conclusion was based mostly on the observation that Src is present in sperm and that the Src kinase family inhibitor SU6656 blocked the capacitation-associated increase in tyrosine phosphorylation. Results in the present manuscript confirmed these observations and provided evidence that these inhibitors were also able to inhibit protein kinase A phosphorylation, sperm motility, and in vitro fertilization. However, the block of capacitation-associated parameters was overcome when sperm were incubated in the presence of Ser/Thr phosphatase inhibitors such as okadaic acid and calyculin-A at concentrations reported to affect only PP2A. Altogether, these data indicate that Src is not directly involved in the observed increase in tyrosine phosphorylation. More importantly, this work presents strong evidence that capacitation is regulated by two parallel pathways. One of them requiring activation of protein kinase A and the second one involving inactivation of Ser/Thr phosphatases.

Identification of proteins undergoing tyrosine phosphorylation during mouse sperm capacitation

Mammalian sperm are not able to fertilize immediately upon ejaculation; they become fertilization-competent after undergoing changes in the female reproductive tract collectively termed capacitation. Although it has been established that capacitation is associated with an increase in tyrosine phosphorylation, little is known about the role of this event in sperm function. In this work we used a combination of two dimensional gel electrophoresis and mass spectrometry to identify proteins that undergo tyrosine phosphorylation during capacitation. Some of the identified proteins are the mouse orthologues of human sperm proteins known to undergo tyrosine phosphorylation. Among them we identified VDAC, tubulin, PDH E1 beta chain, glutathione S-transferase, NADH dehydrogenase (ubiquinone) Fe-S protein 6, acrosin binding protein precursor (sp32), proteasome subunit alpha type 6b and cytochrome b-c1 complex. In addition to previously described proteins, we identified two testis-specific aldolases as substrates for tyrosine phosphorylation. Genomic and EST analyses suggest that these aldolases are retroposons expressed exclusively in the testis, as has been reported elsewhere. Because of the importance of glycolysis for sperm function, we hypothesize that tyrosine phosphorylation of these proteins can play a role in the regulation of glycolysis during capacitation. However, neither the Km nor the Vmax of aldolase changed as a function of capacitation when its enzymatic activity was assayed in vitro, suggesting other levels of regulation for aldolase function.

Signalling pathways involved in sperm capacitation

After ejaculation, mammalian sperm have not yet acquired full fertilising capacity. They will require a finite period of residence in the female reproductive tract before they become fertilisation competent. The molecular, biochemical, and physiological changes that occur to sperm while in the female tract are collectively referred to as capacitation. During capacitation, changes in membrane properties, enzyme activities, and motility render spermatozoa responsive to stimuli that induce the acrosome reaction and prepare spermatozoa for penetration of the egg investments prior to fertilisation. These changes are facilitated by the activation of cell signalling cascades in the female reproductive tract in vivo or in defined media in vitro. The purposes of this review are to consider some recent contributions towards our understanding of capacitation, to summarise open questions in this field, and to discuss future avenues of research.

Evidence for the involvement of proline-directed serine/threonine phosphorylation in sperm capacitation

To become fertilization competent, mammalian sperm undergo changes in the female reproductive tract termed capacitation. Capacitation correlates with an increase in tyrosine phosphorylation; however, less is known about the role of serine/threonine phosphorylation in this process. Proline-directed phosphorylation is one of the major regulatory phosphorylation events in many cellular processes such as cell proliferation and differentiation. Using mitotic phosphoprotein monoclonal-2 (MPM-2) antibody in this study, we observed that several mouse sperm proteins in the range of 70-250 kDa underwent increased serine/threonine-proline phosphorylation during capacitation. In contrast to the time course of tyrosine phosphorylation, proline-directed phosphorylation could be observed at shorter time points of sperm incubation, and it was found to be independent of NaHCO(3) and adenosine 3'5'-cyclic monophosphate (cAMP). Similar to the regulation of the increase in tyrosine phosphorylation, cholesterol acceptors such as bovine serum albumin (BSA) or 2-hydroxypropyl-beta-cyclodextrin (2-OH-propyl-beta-CD) were essential for the regulation of proline-directed phosphorylation in mouse sperm. Furthermore, it was also found to be BSA dependent in human sperm. Among proline-directed kinases, extracellular signal-regulated kinase 1/2 (ERK1/2) is present in mammalian sperm; nevertheless, U0126 and PD098059, two inhibitors of the ERK pathway, did not block this phosphorylation in mouse sperm. In conclusion, capacitation is associated with an increase in proline-directed phosphorylation linked to cholesterol efflux in the sperm.