Chemical communication is not only critical in mediating macroorganismal interactions, but plays a profound importance in controlling processes that occur at the level of the single cell. In contrast to the neural approach used for moth olfaction, the relatively simple system of the sperm cell permits a molecular approach towards understanding the mechanisms controlling chemosensory behavior. Despite a century of research, fertilization remains one of the least understood fundamental biological processes. Chemical communication between sperm and egg is critical in sexual reproduction, yet the contribution of soluble egg factors is still unclear. Sperm activation and chemotaxis have been demonstrated in marine broadcast spawners, as well as in terrestrial animals (including humans) with internal fertilization. Chemically mediated behavior thus is a key component of sperm-egg dynamics, in environments ranging from the turbulent ocean to the relatively benign mammalian reproductive tract. To determine the influence sperm chemoattractantion has on reproductive success, my studies have examined 2 different aspects of this process:
Sperm chemoattractants are found in a diverse array of taxa, including broadcast spawning marine invertebrates and internal fertilizers, like humans. Despite the potential importance in reproduction, the ecological and evolutionary consequences of these signal molecules remain a mystery, and the molecular mechanisms controlling sperm chemotaxis are unknown. In collaboration with Dr. Marc Spehr, we helped establish the physiological mechanisms controlling sperm chemotaxis. Results from the human genome project were used to establish that human sperm chemotaxis is mediated by an olfactory receptor (OR) protein, hOR-17-4 (Spehr et al., 2003, Science 299:2054-2058). This OR is found in live human sperm cells, and activated by the compound bourgeonal. Moreover, hOR17-4 activation is coupled to a cAMP transduction pathway through particulate adenylate cyclase, which controls flagellar beating and sperm turning behavior (Spehr et al., 2004, J. Biol. Chem. 279:40194-40203). Thus, sperm cells respond to olfactory stimuli in an equivalent manner to that of olfactory neurons (Spehr et al., 2006, Mol. Cell. Endocrinol. 242). Moreover, the ORs are located on the located on the flagellar midpiece, and when activated, induce a rapid (<1 s) signal transduction cascade mediating rapid flagellar beating and cell orientation to the chemical gradient.
Fig. 1. Proposed model of a human odorant receptor. hOR17-4 (OR1D2) is activated by the synthetic floral odorant bourgeonal and competitively inhibited by undecanal. Receptor activation triggers a cAMP-dependent signaling cascade. In sperm, however, the identity of the G protein and mAC isoform involved as well as the nature of downstream signaling components remains elusive. Image courtesy of M. Spehr.
Although work with human sperm allowed us to determine the genetic and physiological
basis for sperm chemoattractants, the chemoattractant effects and evolutionary consequences
on fertilization were untenable due to ethical considerations. Therefore, in the
laboratory of Dr. Richard K Zimmer, we targeted a broadcast spawning marine invertebrate,
the red abalone, Haliotis rufescens, to determine the role of sperm chemotaxis on
fertilization and how these chemical signals operate under natural (flowing seawater)
conditions. Live red abalone eggs produce a sperm chemoattractant, L-tryptophan,
at sufficient enough quantities to attractant sperm from a distance (Riffell et al.,
2002, J. Exp. Biol. 205:1459-1470). The presence of tryptophan is critical in mediating
gamete encounters and increasing fertilization rates (Riffell et al., 2004, P. Natl.
Acad. Sci., USA 101:4501-4506). Moreover, tryptophan is species-specific in its effect,
causing only homospecific red abalone sperm to navigate to eggs. We next examined
the effects of water motion (fluid shear) on fertilization success and gamete interactions.
Through a combination of field measurements of water motion in kelp-forest micro-habitats
and simulating important aspects of water flow in the laboratory, we found that the
greatest fertilization success occurred in conditions closest to the natural environment
(cracks and crevices) where the abalone are found (Riffell and Zimmer, 2007, J. Exp.
Biol. 210: 3644-3660). These results suggest that for the endangered abalone, the
best location to transplant them will be based on an environment's water flow, which
would maximize fertilization success and therefore survival. Together, these studies
were the first to determine the evolutionary consequences and reproductive benefits
of sperm chemotactic behavior and how fluid motion influences gamete interactions.
Ecological and environmental consequences
of sperm chemoattractants
Theoretical plume in steady shear and sperm behavior around an abalone egg at a shear flow of 1.0/s. Open circles correspond to video images captured at intervals of 0.016 s, and arrowheads indicate directions of travel of individual cells. The attractant plume was modeled numerically, integrating with respect to a time period of 240s. The colorbar represents the tryptophan concentration.
Physiological basis of sperm chemotaxis
Physiological basis of
sperm chemotaxis
