Molecular Studies of Ca Channels and the 
Dystrophin-Glycoprotein Complex 
Kevin p. Campbell, Ph.D. — Investigator 
Dr. Campbell is also Professor of Physiology and Biophysics at the University of Iowa. He received his B.S. 
degree in physics from Manhattan College, his master's degree from the University of Rochester School of 
Medicine and Dentistry, and his Ph.D. degree from the Department of Radiation Biology and Biophysics 
at the University of Rochester. He did postdoctoral studies in the laboratory of David MacLennan at the 
Banting and Best Department of Medical Research, University of Toronto, before coming to Iowa. 
IN a wide variety of cellular responses — in- 
cluding contraction, secretion, and prolifera- 
tion — Ca^^ functions as a ubiquitous intracellu- 
lar messenger. Two common pathways by which 
intracellular Ca^^ transients can be triggered 
have been identified in cells. The first involves 
the influx of Ca^"^ into the cytoplasm from the 
extracellular medium through plasma membrane 
Ca^^ channels, and the second involves the re- 
lease of Ca^^ into the cytoplasm from intracellu- 
lar stores through Ca^^ release channels. 
Over the last nine years, we have investigated 
the structure and function of the membrane 
components involved in Ca^^ fluxes across mem- 
branes. In particular, we have focused on identi- 
fying, purifying, and characterizing the mem- 
brane proteins that function as surface Ca^^ 
channels and intracellular Ca^"*^ release channels 
in excitable cells. 
Excitation-Contraction Coupling 
in Skeletal Muscle 
Muscle contraction is initiated by a depolariza- 
tion of the transverse tubular membrane, which 
in turn signals the release of Ca^^ from the junc- 
tional sarcoplasmic reticulum. One specific aim 
of our research is to understand the structure and 
function of protein components of the sarcoplas- 
mic reticulum membrane. We have purified the 
ryanodine receptor of rabbit muscle sarcoplas- 
mic reticulum and have shown that it can mediate 
single-channel activity identical to that of the sar- 
coplasmic reticulum's Ca^^ release channels. The 
morphology of the purified receptor has revealed 
that it is identical to the "SR feet" and thus would 
seem to play a dual role in excitation-contraction 
coupling as the Ca^^ release channel and the 
bridging structure in the junctional gap. 
A second specific aim of this research concerns 
the dihydropyridine-sensitive Ca^^ channel of 
skeletal muscle and its dual role as a voltage sen- 
sor for excitation-contraction coupling and a 
Ca^"^ channel. The dihydropyridine receptor has 
been purified from rabbit skeletal muscle and 
shown to consist of four subunits — a,, 012, and 
7. In the last year we determined the structure of 
the 7-subunit by isolating cDNAs from an expres- 
sion library. We deduced that the primary struc- 
ture is a 25,058-dalton protein containing four 
transmembrane domains and two N-linked glyco- 
sylation sites. This description is consistent with 
biochemical analyses showing that the 7-subunit 
is a glycosylated hydrophobic protein. 
We have also analyzed the dihydropyridine re- 
ceptor's a2-subunit and associated b peptides. 
The results have shown that the b peptides are not 
true subunits, but instead represent the carboxyl- 
terminal peptide of a proteolytically processed 
a2-subunit. The goal of this project now is to de- 
termine how the receptors for dihydropyridine 
and ryanodine are coupled in the excitation- 
contraction process. 
Neuronal Ca^"^ Release Channels 
A second area of research in our laboratory con- 
cerns Ca^^ release channels in neuronal cells. Ino- 
sitol 1,4,5-trisphosphate (IP3), produced after 
receptor activation, is an important second mes- 
senger in the release of stored Ca^^ from intracel- 
lular compartments. Although IP3 appears to have 
a major role in Ca^"*^ regulation within neurons, 
physiological and pharmacological evidence has 
indicated the presence of non-IPj-gated Ca^^ 
pools. We have identified a high-afifinity ryano- 
dine receptor in rabbit brain membranes as a 
400-kDa protein that cross-reacts with antibodies 
against the sarcoplasmic reticulum ryanodine re- 
ceptor and migrates as a large oligomeric com- 
plex of about 30s on sucrose density gradients. 
Although we have not identified the exact physio- 
logical role of the brain ryanodine receptor, we 
are now attempting to purify it so that it may be 
shown responsible for the non-IPj-gated Ca^^ re- 
lease in neurons. 
Molecular Pathogenesis 
of Muscular Dystrophy 
A third area of research concerns the structure 
and function of dystrophin, the high-molecular- 
weight protein product of the human Duchenne 
muscular dystrophy (DMD) gene. Dystrophin oc- 
curs in the sarcolemma (fiber membrane) of nor- 
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