Chapter17 140331233521-phpapp01

Editor's Notes

• #3: Reassessing the role of progesterone in fertilization—compartmentalized calcium signalling in human spermatozoa? Claire V. Harper1,2 and Stephen J. Publicover1,3 1 School of Biosciences, University of Birmingham, Birmingham B15 2TT, UK 2 Present address: School of Biological Sciences, Biosciences Building, Crown Street, University of Liverpool, Liverpool L69 7ZB, UK 3 Corresponding author. E-mail: [email_address]    Abstract Top Abstract Sperm responses to oocyte... Effect of progesterone on... Response of human spermatozoa... Generation of [Ca2+]i... Role of progesterone-induced... Compartmentalized [Ca2+]I... References Progesterone is present at micromolar concentrations in the vicinity of the oocyte. Human spermatozoa generate a biphasic rise in intracellular calcium concentration ([Ca2+]i) and undergo the acrosome reaction upon progesterone stimulation, suggesting that the hormone acts as a secondary inducer or ‘primer’ of the acrosome reaction in association with the zona pellucida. However, the sensitivity of human spermatozoa to progesterone is such that many cells may undergo the acrosome reaction prematurely, compromising their ability to fertilize. We have shown that exposing human spermatozoa to a progesterone gradient, simulating the stimulus encountered as sperm approach the oocyte, results in a novel response. A slow rise in [Ca2+]i occurs, upon which, in many cells, [Ca2+]i oscillations are superimposed. Cells showing this pattern of response do not undergo the acrosome reaction, but instead show an alternating pattern of flagellar activity associated with peaks and troughs of [Ca2+]i. A Ca2+ store in the rear of the sperm head apparently generates this complex signal, functioning as an ‘[Ca2+]i oscillator’. We propose that: (i) the acrosome reaction and flagellar beat are regulated by separate Ca2+ stores; (ii) these stores are mobilized through different mechanisms by different agonists; and (iii) progesterone in vivo acts as a switch for the oscillator which regulates the flagellar beat mode. Key words: calcium/motility/oscillation/progesterone/sperm    Sperm responses to oocyte-derived factors Top Abstract Sperm responses to oocyte... Effect of progesterone on... Response of human spermatozoa... Generation of [Ca2+]i... Role of progesterone-induced... Compartmentalized [Ca2+]I... References After deposition in the female tract, a mammalian sperm must swim to the oocyte, requiring both motility through a viscous environment and (probably) chemotactic control of that motility. On reaching the oocyte–cumulus complex, the cell has to penetrate the layers of cumulus and zona pellucida, a process dependent upon both the acrosome reaction (AR) and regulation of motility. The sperm must, therefore, detect and respond appropriately both ‘remotely’ (to factors derived from the oocyte) and also to stimuli presented upon direct contact with the egg. These stimuli must elicit intracellular signals that achieve complex regulation of the flagellum (both beat mode and directional) and appropriately timed activation of the AR. Sperm responses to factors derived from the egg and its vestments are primarily mediated through [Ca2+]i, which is known to control flagellar activity both during regulation of flagellar beat mode (Ho et al., 2002; Harper et al., 2004 ) and during chemotactic responses (Spehr et al., 2003 , 2004 ). Ca2+ is also pivotal to induction of the AR (Publicover and Barratt, 1999). At least two components of the egg vestments, zona pellucida and progesterone (secreted at high concentrations by the cumulus cells), activate Ca2+ signalling in mammalian spermatozoa (Florman et al., 1989 ; Blackmore et al., 1990) . The induction of the AR by solubilized zona pellucida has been studied in detail in the mouse model and involves activation of a T-type voltage-operated calcium chanel (VOCC) followed by store mobilization and prolonged (probably) capacitative Ca2+ influx (Florman et al., 1992; Evans and Florman, 2002 ). In contrast, the significance of progesterone is much less clear. The responsiveness of human spermatozoa to progesterone correlates with the fertilization rate at IVF (Krausz et al., 1996 ; Forti et al., 1999 ; Giojalas et al., 2004 ), and removal of cumulus cells from oocytes significantly reduces the success rate of IVF in most mammals (Tanghe et al., 2002 ; van Soom et al., 2002). However, the most commonly observed effect of progesterone-induced Ca2+ influx, stimulation of the AR, is arguably not an adaptive response. Human spermatozoa respond with elevation of [Ca2+]i and the AR to doses well below the 1–10 µmol/l that is believed to occur adjacent to the oocyte (Osman et al., 1989 ; Baldi et al., 1991 ; Harper et al., 2003) . The AR should therefore occur in many cells before they enter the cumulus, potentially compromising their ability subsequently to penetrate the zona. Identification of a Testicular Odorant Receptor Mediating Human Sperm Chemotaxis Marc Spehr,1 Günter Gisselmann,1 Alexandra Poplawski,1 Jeffrey A. Riffell,2 Christian H. Wetzel,1 Richard K. Zimmer,23 Hanns Hatt1* Although it has been known for some time that olfactory receptors (ORs) reside in spermatozoa, the function of these ORs is unknown. Here, we identified, cloned, and functionally expressed a previously undescribed human testicular OR, hOR17-4. With the use of ratiofluorometric imaging, Ca2+ signals were induced by a small subset of applied chemical stimuli, establishing the molecular receptive fields for the recombinantly expressed receptor in human embryonic kidney (HEK) 293 cells and the native receptor in human spermatozoa. Bourgeonal was a powerful agonist for both recombinant and native receptor types, as well as a strong chemoattractant in subsequent behavioral bioassays. In contrast, undecanal was a potent OR antagonist to bourgeonal and related compounds. Taken together, these results indicate that hOR17-4 functions in human sperm chemotaxis and may be a critical component of the fertilization process.