For some animals living in the sea, fertilization does not require that males and females meet. Instead, both genders release their gametes into the seawater and leave them on their own. The ocean is vast. To increase the chances that gametes encounter each other, eggs of many species release molecules (“chemoattractants”); these molecules serve as signposts for sperm to locate the egg. Sperm can “read” or measure the spatial distribution of chemoattractant molecules and adjust their swimming path accordingly. How do sperm accomplish this task?
Sperm of the sea urchin Arbacia punctulata swim on a circular path. In a chemoattractant gradient, owing to the circular movement, sperm are exposed to a periodic stream of chemoattractant molecules that impinge on their flagellum and bind to their cognate receptors. The periodic stimulation is translated by a cellular signalling pathway into Ca2+ oscillations that control the steering response and guide sperm to the egg. Although the basics underlying chemotaxis have been unfolded, fundamental questions still await a definitive answer: For how long do sperm sample a chemical gradient before they respond? What happens when additional molecules are delivered during an ongoing Ca2+ signal or steering response? How do sperm cope with the large range of concentrations that they encounter during their sojourn? Do sperm adjust their sensitivity? From how far away can eggs attract sperm?
Cells or whole organisms employ two different strategies to find out where to go. They measure the chemoattractant concentration along their body (spatial sampling) or they measure the concentration at two different time points while moving in the gradient (temporal sampling). Spatial sampling is favorable for large cells moving slowly, whereas temporal sampling is favorable for small and fast moving cells. Sperm are large (roughly 50 µm) and move fast (100-200 µm/s), thus it is not obvious, which sampling strategy they follow. Our work shows that sperm, in fact, use a temporal mechanism; they sample the chemoattractant for a period of 0.2-0.6 seconds, before a Ca2+ signal is produced. During this sampling period, sperm swim about one body length; this is an optimal compromise between spatial precision and sensitivity.
A second fundamental question that we addressed is: What happens when a sperm cell made a decision, initiated a cellular response, and then receives a new chemotactic stimulus? Intuitively, one would predict that the second stimulus will reinforce the ongoing cellular response. However, we observed a different, quite unexpected result: sperm terminate the ongoing Ca2+ response and then initiate a new one (Figure 1). In other words, the cell resets its cellular program, like a computer. During resetting of the Ca2+ signal, sperm straighten their swimming path and thereby prolong their swimming up the gradient.
Figure 1: A second stimulus produces a Ca2+ drop followed by a new Ca2+rise. A. Ca2+ signals evoked by one stimulus (blue) or two different stimuli one second apart (red). Inset: The second Ca2+ signal was shifted by one second to the left and superposed on the first Ca2+ signal. B. and C. Representative swimming paths from individual cells after receiving a single stimulus (B, yellow box and black arrow) or two stimuli 800 ms apart (C, yellow boxes and black arrows). The sperm head (blue trace) wiggles around the average path (black trace); red arrow indicates the swimming direction. After the second flash (C), the swimming path transiently bent in a clockwise direction, whereas the control cell swam undeviated (B, grey arrow).
A third question that we addressed is: How can sperm reconcile a wide dynamical range with sensitivity at the physical limit of quantal detection? Sperm are exquisitely sensitive and can respond to a single chemoattractant molecule. Sperm can even count single molecules delivered in succession to produce a sum response. While sperm approach the egg, they are exposed to an ever increasing concentration of chemoattractant. By exposing sperm to different background levels of the attractant and testing their sensitivity, we show that sperm escape saturation by adjusting their sensitivity; they shift the sensitivity range to higher chemoattractant concentrations and compress the Ca2+ signal.
Figure 2:Adaptation of Ca2+ signals. A. Shift in the dynamic range of Ca2+signals. Test stimuli were delivered either in the absence (black) or presence of adapting chemoattractant (in nM): 0.25 (red), 2.5 (blue), and 25 (green). Ca2+signal amplitudes produced by a test stimulus were plotted; B. Apparent K1/2 andC. response compression at different background concentrations.
Although sperm chemotaxis has been studied for a century, it is unknown how far the gradient reaches out to attract sperm. We calculated the spatial distribution of the chemoattractant concentration in time and estimated a distance of 4.7 mm from which eggs attract sperm. While this distance does not seem to be impressive, it increases the "effective" volume of the egg to capture sperm by many orders of magnitude.
Figure 3: Range of sperm attraction. A. Chemoattractant gradient (gchem; black) and the estimated minimal gradient to attract sperm (gmin; red) for 1 or 10 minutes after egg release. For the effective range of sperm attraction, the chemoattractant gradient must be larger than gmin. B. Effective range of sperm attraction vs. time after egg release. At t > 30 min, gchem < gmin in the very vicinity of the egg. Hence, we observe two boundaries, a short one and a long one for the effective gradient.
Our work provides fundamental insights about gradient sensing. Many microorganisms explore gradients along periodic paths and translate the spatialdistribution of the stimulus into a temporal pattern of cell responses. The basic principles outlined here - temporal sampling, resetting, and adaptation - might control gradient sensing in other organisms and cells as well.
Kashikar, N., Alvarez, L., Seifert, R., Gregor, I., Jäckle, O., Beyermann, M., Krause, E. & Kaupp, U. B. (2012) "Temporal sampling, resetting, and adaptation orchestrate gradient sensing in sperm" J. Cell. Biol. 198, 1075-1091
This article in focus
Mitch, L. (2012) “Unlocking the sperm’s internal compass” J. Cell Biol. 198, 955