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Involvement of Bicarbonate-Induced Radical Signaling in Oxysterol Formation and Sterol Depletion of Capacitating Mammalian Sperm During In Vitro Fertilization

Biology of Reproduction (Impact Factor: 3.32). 10/2012; 88(1). DOI: 10.1095/biolreprod.112.101253
Source: PubMed

ABSTRACT

This study demonstrates for the first time that porcine and mouse sperm incubated in capacitation media supplemented with bicarbonate produce oxysterols. The production is dependent on a reactive oxygen species (ROS) signaling pathway that is activated by bicarbonate and can be inhibited or blocked by addition of vitamin E or vitamin A or induced in absence of bicarbonate with pro-oxidants. The oxysterol formation was required to initiate albumin dependent depletion of 30% of the total free sterol and >50% of the formed oxysterols. Incubation of bicarbonate treated sperm with oxysterol binding proteins (ORP-1 or -2) caused a reduction of >70% of the formed oxysterols in the sperm pellet but no free sterol depletion. Interestingly, both ORP and albumin treatments led to similar signs of sperm capacitation: hyper-activated motility, tyrosin phosphorylation, aggregation of flotillin in the apical ridge area of the sperm head. However, only albumin incubations led to high in vitro fertilization rates of the oocytes whereas the ORP-1 and -2 incubations did not. A pretreatment of sperm with vitamin E or A caused reduced in vitro fertilization rates with 47% and 100%, respectively. Artificial depletion of sterols mediated by methyl-beta cyclodextrin bypasses the bicarbonate ROS oxysterol signaling pathway but resulted only in low in vitro fertilization rates and oocyte degeneration. Thus bicarbonate induced ROS formation causes at the sperm surface oxysterol formation and a simultaneous activation of reverse sterol transport from the sperm surface which appears to be required for efficient oocyte fertilization.

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Available from: Chris Van de Lest, Jun 06, 2014
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    • "Importantly, in vitro capacitation of sperm in the presence of albumin did not result in cholesterol depletion in the DRM fraction (in contrast to MBCD treatment; van Gestel et al. 2005b). Nevertheless, MBCD treatment allows cholesterol depletion in sperm and this does result in increased zona binding of stallion sperm (Bromfield and Nixon 2013a, b) and to some extent in vitro fertilization of porcine oocytes (Boerke et al. 2013). Note that sperm capacitation is a process essential for sperm to become competent to fertilize the oocyte (Gadella et al. 2008; Aitken and Nixon 2013). "
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    • "While bicarbonate and calcium fluxes are required for rapid cAMP production and increases in membrane fluidity (Harrison et al. 1993), albumin-mediated sterol depletion is critical to achieve increases in protein tyrosine phosphorylation, often considered a hallmark of sperm functional competence in model species such as the mouse (Visconti et al. 1995b, Galantino-Homer et al. 1997). Sterol loss occurs either through the contribution of an active cholesterol transporter that provides free cholesterol to the hydrophobic pocket of albumin (Flesch et al. 2001) or possibly through the oxidation of membrane sterols and the subsequent scavenging of the hydrophilic oxidation products by albumin (Boerke et al. 2013). "
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