Mechanically Activated Ion Channels 
suggested that adaptation is not a mechanical ad- 
justment, but comes about through calcium entry 
through the transduction channels. Calcium in- 
side the cell would close channels, to create a 
similar kind of negative feedback. It was clearly 
important to distinguish these models. 
In the past two years, we have obtained good 
evidence for the mechanical adjustment hypoth- 
esis. A key feature of this hypothesis is that the 
putative adjustment of tension in the tip links 
would also be felt by the cilia themselves and 
would act to move the bundle. We found, first, 
that we could change the rate of adaptation by 
changing the voltage across the cell membrane. 
Because voltage affected the relaxing more than 
the tightening, we could then predict that posi- 
tive voltage would increase the tension, and how 
much and how quickly. Judging from the activa- 
tion of transduction channels, these predictions 
were borne out. Third, we could then predict 
how much the tension change should move the 
bundles. The predicted change was only about 
100 nanometers — a quarter the wavelength of 
light — so we worked out a video microscopy sys- 
tem to measure the motion with a resolution of 5 
nanometers. When the voltage was changed, the 
bundles did move, with the predicted amplitude 
and time course. Thus a quantitative theory that 
describes the adaptation also describes the move- 
ment, strongly suggesting that adaptation is 
mechanical. 
How could tension on the tip links be adjusted? 
Others have speculated that the adaptation comes 
about by a movement of the points where the tip 
links are attached to the structure of the cilia. 
When the tip links are stretched to open chan- 
nels, the attachment points might slip to allow 
the links to shorten. Conversely, if the tip links 
are relaxed, the attachment points move up to 
stretch the links to restore the resting tension. 
This past year, we began to test this structural 
theory, by measuring the position of the attach- 
ment points before and after deflections that 
cause adaptation. Bundles were given calibrated 
deflections and then fixed with glutaraldehyde in 
the deflected position. Measurements from trans- 
mission electron micrographs suggest that the at- 
tachment points do move in response to deflec- 
tion, although we still need to test whether the 
movement is quantitatively consistent with the 
adaptation. 
What could move the attachment point? Some 
circumstantial evidence suggests that the "mo- 
tor" molecule is like the myosin protein that 
causes the contraction of muscle: The stiff cores 
of the cilia are composed largely of actin, on 
which myosin moves in muscle; the motor mole- 
cule in these cilia moves in the same direction 
and at the same rate as myosin; and glass beads 
coated with muscle myosin do move on the actin 
cores of these cilia. We are now seeking further 
evidence as to whether the motor is a form of 
myosin. 
Our ultimate aim is to describe each link in the 
mechanical chain from cilia to channels, in terms 
of the protein identity of the links, their biophysi- 
cal properties, and their relationship to each 
other. Answers to these specific questions will 
contribute to the long-range goal of a compre- 
hensive theory of mechanically activated chan- 
nels, not only in the ear but in the many other cell 
types that display a mechanical sensitivity. 
In the past two years, we have started to use 
some of what has been learned about ion chan- 
nels to understand human disease. Hyperkalemic 
periodic paralysis is a dominantly inherited mus- 
cle disease that causes sporadic weakness or paral- 
ysis. Exercise or certain foods that raise the level 
of potassium in the blood can bring on a paralytic 
attack. Colleagues at Massachusetts General Hos- 
pital had found that the voltage-sensitive sodium 
channel in muscle was genetically linked to this 
disease, suggesting that a defect in the channel 
might be the cause. Last year we were able to 
show that the channels are in fact defective. In 
the presence of excess potassium, they fail to in- 
activate normally, which allows sodium to enter 
the muscle and (indirectly) causes the paralysis. 
A puzzle, however, is that paralytic episodes 
are often preceded in patients by myotonia — an 
excessive contraction of the muscle. Could the 
same defect cause both myotonia and paralysis? 
This year we have used a toxin derived from sea 
anenomes, which mimics the genetic defect in 
sodium channels when applied to normal mus- 
cle. Toxin-treated muscle did, in fact, show clas- 
sic myotonia, with both increased electrical activ- 
ity and increased tension. Thus a single genetic 
defect can have a graded effect. Induced only 
slightly, it causes myotonia; induced to a greater 
extent by potassium, it causes paralysis. A com- 
puter model of electrical activity in muscle 
shows the same result with a single defect. Ex- 
plaining the pathology at the molecular level 
gives hints for effective drug treatment of the 
disorder. 
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