Ionic Channels in Sea Urchin Sperm Physiology 
Alberto Darszon, Ph.D. — International Research Scholar 
Dr. Darszon is Professor of Biochemistry at the Biotechnology Institute, National Autonomous University 
of Mexico, Cuernavaca, and Adjunct Professor of Biochemistry at the Center for Research and Advanced 
Studies, National Polytechnic Institute, Mexico City. He received his undergraduate degree in chemistry 
from the Universidad Iberoamericana in Mexico City, and his Ph.D. degree in biochemistry from the 
Center for Research and Advanced Studies. His postdoctoral research was done with Mauricio Montal at 
the University of California, San Diego. His honors include the National Science Award of the Mexican 
Academy of Scientific Research and both Guggenheim and World Health Organization fellowships. 
THE molecular mechanisms involved in cell 
communication are at the forefront of re- 
search in biology today, since they play a key role 
in determining the behavior of organisms. For the 
past decade the main goal in our laboratory has 
been to understand the vital egg signals that allow 
sperm to fuse with the egg and fertilize it. 
The sea urchin has proved to be an excellent 
model in which to study reproduction. From this 
animal enormous quantities of sperm (10^°/ 
male) can be collected that respond to environ- 
mental stimuli rapidly, synchronously, and in a 
compulsory order. It has been shown that the 
flow of ions through the plasma membrane of sea 
urchin sperm participates crucially in the events 
leading to fertilization. Indeed, these are excit- 
able cells that quickly respond to components 
from the outer layer of the egg, the jelly, with 
changes in their plasma membrane permeability. 
Ionic fluxes play a fundamental role in the acti- 
vation of respiration and motility, in chemotaxis, 
and in triggering the sperm acrosome reaction 
(AR) . This latter reaction occurs within seconds 
after sea urchin sperm reach the egg and interact 
with its jelly coat. The AR involves important mor- 
phological changes that allow sperm to fuse with 
the egg. Participating in AR induction is an in- 
crease in the uptake of Ca^"^ and Na^ and efflux of 
K"^ and H^. We would like to learn how these 
fluxes are related and elucidate the molecular 
mechanisms that orchestrate them to trigger 
the AR. 
Previous results indicated indirectly that some 
ion fluxes occur through ionic channels. These 
are integral membrane proteins capable of form- 
ing hydrophilic pores through the membrane that 
allow the passive diffusion of ions at high rates 
(10^"^ ions/s). We have used model membranes 
(planar bilayers) formed from sperm compo- 
nents, and patch-clamp techniques in whole 
cells, to detect for the first time the activity of 
single channels in the plasma membrane of sea 
urchin sperm. These techniques, together with 
studies of membrane potential, intracellular Ca^^ 
([Ca^+]j), and intracellular pH (pHj) in whole 
sperm, have established the presence of K^, Ca^^, 
and Cl~ channels in this cell and are allowing us 
to explore their participation in the AR. 
These findings have led us to propose a work- 
ing hypothesis of how egg jelly-induced changes 
in ionic permeability might trigger the AR in sea 
urchin sperm. A receptor (to the inducing egg 
factor) in the plasma membrane of the sperm cell 
opens a Ca^^ channel, which deactivates in a few 
seconds. The activation of this channel modu- 
lates the opening of a second Ca^^ channel by 
poorly understood mechanisms that include a 
Ca^^-dependent change in pHj. At the same time 
or immediately after the first Ca^"^ channel opens, 
a channel is activated, hyperpolarizing the 
cell and activating a voltage-dependent Na^/H^ 
exchange that increases pHj. This latter change in 
pHj is linked to the opening of the second Ca^"^ 
channel and to a large depolarization. 
In light of this working hypothesis, we thought 
it should be possible to hyperpolarize sperm arti- 
ficially and induce an increase in [Ca^"^]! after a 
depolarization and AR. Valinomycin-induced hy- 
perpolarization of sperm from the sea urchin Ly- 
techinus pictus in K^-free sea water raised pHj, 
caused a small increase in ^'Ca^"^ uptake, and trig- 
gered some AR. When the cells were depolarized 
with 30 mM KCl 40-60 seconds after the hyper- 
polarization, the pHj decreased and there was a 
significant increase in ''^Ca^^ uptake, [Ca^"^];, and 
AR. Therefore the jelly-induced hyperpolariza- 
tion may lead to the intracellular alkalinization 
required to trigger the AR and may modulate, on 
its own or via pHj, Ca^^ channels involved in this 
process. 
The sea urchin sperm offers distinct advantages 
over more complex cell types as a basic model for 
chemotaxis. As the spermatid matures, many in- 
tracellular organelles and macromolecules not 
involved in fertilization are eliminated. Sperm 
are incapable of division and devoid of the ma- 
chinery for genome expression. However, they 
retain systems for sensing, swimming toward, and 
fusing with the egg, as described above. 
The sperm of the sea urchin Arbacia punctu- 
lata is attracted at nanomolar concentrations to a 
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