Published September 17, 2012
Text by Mitch Leslie
[email protected]
In Focus
Unlocking the sperm’s internal compass
E
In Focus • THE JOURNAL OF CELL BIOLOGY
Downloaded from on June 16, 2017
ven without GPS units to guide
FOCAL POINT
them to the egg, sperm perform
some impressive navigational
feats. Kashikar et al. reveal several responses that enable sperm to get their
bearings and steer to the egg (1).
A sperm tracks down the egg by following a gradient of chemical attractants. In
the sea urchin species Arbacia punctulata,
the attractant is the peptide resact, which
sticks to receptors on the sperm’s flagellum
and triggers an influx of calcium (2). Sperm
set their course based on these calcium
surges (3). For example, a recent study by Nachiket Kashikar (left), U. Benjamin Kaupp (center), and colleagues (not pictured) put swimming
the team found that the rate of change in sperm through their paces to determine how they set their course. They found that sperm test
their environment for a limited time but remain attentive to stimuli that arrive after that period.
calcium levels dictates whether sea urchin
They also showed that sperm respond to individual molecules of attractant but reduce their
sperm make sharp or gentle turns (4). By sensitivity as they approach their target. The diagram (right) of the swimming path of sperm
continually “sniffing” its surroundings and in a gradient of attractant (indicated by shades of gray) reveals that sperm tend to swim in
recalibrating its course, a sperm follows a loops (green) where the gradient is steepest and in tighter circles (orange) where concentration
circuitous path to the egg. But many of the changes are more gradual.
details of how sperm navigate using resact
levels remain mysterious. Researchers the sperm would ignore late-arriving stimuli Results of these experiments suggest that
don’t know, for example, how long a sperm or add them to previous ones. Instead, the sperm reduce their sensitivity as the backsamples its surroundings before heading in team showed that a pulse of resact or cyclic ground concentration increases, probably by
a particular direction or how it adjusts its GMP one second after the original stim- tweaking the resact receptor. The scientists
sensitivity to concentrations of attractant ulus—when the calcium influx was at its tested that possibility by prodding sperm
that can vary by orders of magnitude.
peak—caused the sperm to reset. The cal- with cyclic GMP, which stimulates the cells
Kashikar et al. set up experiments to cium surge fizzled, and a new one began. without activating resact receptors. That
answer questions like these. They exposed When the researchers tracked the swimming procedure spurred less desensitization, supsea urchin sperm to caged versions of resact paths of the sperm, they discovered that porting the notion that sperm somehow dull
and cyclic GMP—which
plunging calcium levels the responsiveness of their resact receptors
spurs calcium surges—that
caused the cells to stay on as the concentration of the attractant rises.
“We identified
they could free with flashes
a straight path for a longer
“We identified the different computaof light. In this way they
period
of
time.
The
sperm
tional
steps that sperm perform in order to
the different
could stimulate the sperm at
then turned as calcium navigate successfully,” says senior author
computational
specific time intervals. The
levels began to rise after U. Benjamin Kaupp. By sampling their sursteps that sperm the second stimulus.
first question the researchers
roundings, remaining responsive to latetried to answer was how long perform in order
By altering the con- arriving stimuli, and adjusting their sensithe sperm samples its surcentration of caged resact tivity as they near their target, sperm can
to navigate
roundings. The team comand the strength of the home in on an egg that is 4.7 millimeters
successfully.”
pared responses to a single
light pulse, Kashikar et away, or about 100 times their length.
full-strength flash and two
al. determined that a Among the questions researchers still need
half-strength flashes delivered varying times sperm can detect and count single mole- to answer are how sperm adjust the sensitivapart. The results suggest that a sperm tests cules of the attractant. But that finding ity of their resact receptors and how the cells
its environment for between 0.2 and 0.6 raises a question: as the sperm approaches deal with noise that could send them astray.
seconds, integrating the stimuli it receives in the egg, resact’s concentration soars, so
1. Kashikar, N.D., et al. 2012. J. Cell Biol. http://
that time and then computing its direction.
why aren’t the resact receptors swamped
dx.doi.org/10.1083/jcb.201204024.
But what if another stimulus arrives after at close range? To find out, the research- 2. Strünker, T., et al. 2006. Nat. Cell Biol. 8:1149–1154.
a sperm has already begun to respond? The ers added resact to solutions with differ- 3. Böhmer, M., et al. 2005. EMBO J. 24:2741–2752.
researchers thought they knew the answer: ent background levels of the attractant. 4. Alvarez, L., et al. 2012. J. Cell Biol. 196:653–663.
PHOTOS COURTESY OF PETER GOLDMANN (KASHIKAR)
AND U. BENJAMIN KAUPP
Study shows how a sperm’s behavior shapes its swimming path.
955