Human sperm chemotaxis is a critical component of the fertilization process, but the molecular basis for this behavior remains unclear. Recent evidence shows that chemotactic responses depend on activation of the sperm olfactory receptor, hOR17-4. Certain floral scents, including bourgeonal, activate hOR17-4, trigger pronounced Ca 2؉ fluxes, and evoke chemotaxis. Here, we provide evidence that hOR17-4 activation is coupled to a cAMP-mediated signaling cascade. Multidimensional protein identification technology was used to identify potential components of a G-protein-coupled cAMP transduction pathway in human sperm. These products included various membrane-associated adenylate cyclase (mAC) isoforms and the G olf -subunit. Using immunocytochemistry, specific mAC isoforms were localized to particular cell regions. Whereas mAC III occurred in the sperm head and midpiece, mAC VIII was distributed predominantly in the flagellum. In contrast, G olf was found mostly in the flagellum and midpiece. The observed spatial distribution patterns largely correspond to the spatiotemporal character of hOR17-4-induced Ca 2؉ changes. Behavioral and Ca 2؉ signaling responses of human sperm to bourgeonal were bioassayed in the presence, or absence, of the adenylate cyclase antagonist SQ22536. This specific agent inhibits particulate AC, but not soluble AC, activation. Upon incubation with SQ22536, cells ceased to exhibit Ca 2؉ signaling, chemotaxis, and hyperactivation (faster swim speed and flagellar beat rate) in response to bourgeonal. Particulate AC is therefore required for induction of hOR17-4-mediated human sperm behavior and represents a promising target for future design of contraceptive drugs.

In the female reproductive tract, mammalian sperm undergo a regulated sequence of prefusion changes that "prime" sperm for fertilization. Among the least understood of these complex processes are the molecular mechanisms that underlie sperm guidance by environmental chemical cues. A "hard-wired" Ca 2؉ signaling strategy that orchestrates specific motility patterns according to given functional requirements is an emerging concept for regulation of sperm swimming behavior. The molecular players involved, the spatiotemporal characteristics of such motility-associated Ca 2؉ dynamics, and the relation between a distinct Ca 2؉ signaling pattern and a behavioral sperm phenotype, however, remain largely unclear. Here, we report the functional characterization of two human sperm chemoreceptors. Using complementary molecular, physiological, and behavioral approaches, we comparatively describe sperm Ca 2؉ responses to specific agonists of these novel receptors and bourgeonal, a known sperm chemoattractant. We further show that individual receptor activation induces specific Ca 2؉ signaling patterns with unique spatiotemporal dynamics. These distinct Ca 2؉ dynamics are correlated to a set of stimulus-specific stereotyped behavioral responses that could play vital roles during various stages of prefusion sperm-egg chemical communication.On their journey to locate the oocyte, navigating mammalian sperm depend on both chemical (1) and physical (2, 3) cues to regulate flagellar motion, switch flagellar beat modes, and, thus, direct their movement. How such environmental signals are detected and translated into changes in motility, however, remains largely unknown.Sperm acquire motility upon vaginal deposition (4). This initial swimming behavior is referred to as activated motility and characterized by a relatively low amplitude/high frequency sinusoidal flagellar motion (5). Motile sperm that pass the cervical mucus barrier must locate and enter the oviduct. Only a fraction of sperm present in the uterus (ϳ10% in humans (6)) accomplishes this task. Within the oviductal isthmus, sperm frequently attach to the highly convoluted epithelial surface (3,7,8), forming a sperm reservoir in close proximity to the fertilization site. Upon ovulation, capacitated sperm that adopt a hyperactivated state (i.e. displaying asymmetrical and relatively high amplitude/low frequency flagellar beating) (4), generate sufficient propulsion force to detach from the epithelium, enter the ampulla, and eventually penetrate the cumulus and zona pellucida surrounding the egg (9).Given the small fraction of sperm that reaches the oviduct, a "competitive race" scenario has fallen out of favor in recent years. Instead, effective sperm guidance mechanisms are required to assure a synchronized arrival and encounter of both gametes at the fertilization site (1). Current models propose a complex multistep process of sperm navigation along thermal (2) and chemical (10) gradients. Accordingly, chemical guidance cues are secreted by both the oocyte and the surroundi...

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