Regulation of Gene Expression During Cell Differentiation and Activation 
designed to determine the role of each of these 
transcription factors in normal T cell develop- 
ment and activation, as well as in T cell tumors. 
Molecular Characterization of the Cardiac 
and Skeletal Muscle Troponin C Genes 
^ Normal heart muscle contraction depends on 
the phenomenon of excitation-contraction cou- 
pling — i.e., the transmission of the electrical de- 
polarization signal into the biochemical events 
involved in muscle cell contraction. Cardiac tro- 
ponin C (cTnC) is the calcium-binding subunit 
of the muscle fiber that regulates excitation-con- 
traction coupling. Cardiac or skeletal muscle con- 
traction is initiated by electrical depolarization 
of the muscle cell membrane, resulting in a dra- 
matic rise in intracellular calcium, which in turn 
binds to the troponin C molecule, causing a con- 
formational change in the structure of the poly- 
peptide. This signal is transduced across the 
muscle fiber, facilitating the formation of actin- 
myosin cross bridges, and ultimately resulting in 
the generation of tension in the muscle fiber. 
Cardiac and skeletal muscle each express a dif- 
ferent form of troponin C. These two forms dis- 
play markedly different biophysical properties 
that, in part, account for the different properties 
of cardiac and skeletal muscle contraction. To 
clarify the function and regulation of the cTnC 
and skeletal muscle troponin C (sTnC) mole- 
cules, we have recently cloned cDNAs and genes 
that encode both of these proteins. Structural 
studies have revealed that these genes belong to a 
common multigene family that contains at least 
8-10 closely related members. Although the two 
proteins are highly related, we have identified 
three areas that display significant structural di- 
versity and that may account for the functional 
differences. Studies of the expression of the two 
genes have shown that the cTnC gene is ex- 
pressed only in embryonic (not adult) fast skele- 
tal muscle and in adult cardiac and slow skeletal 
muscle, while the fast sTnC gene is expressed 
only in embryonic and adult fast skeletal muscle. 
Although neither gene is expressed in cultured 
myoblasts (muscle cell progenitors), the expres- 
sion of both is markedly induced following chem- 
ical differentiation of these myoblasts into embry- 
onic myotubes. 
Recently we have focused on identifying the 
genetic elements that regulate the tissue-specific 
expression of the two TnC genes during muscle 
cell development. These studies have dem- 
onstrated that the cTnC gene contains two dis- 
tinct tissue-specific transcriptional enhancer ele- 
ments: 1) a first-intron skeletal muscle-specific 
enhancer that is required for expression of the 
cTnC gene in embryonic skeletal myotubes and 
2) an upstream cardiac-specific enhancer that is 
required for high-level cTnC gene expression in 
cardiac myocytes. Interestingly, the sTnC gene 
also contains a first-intron enhancer that is active 
in skeletal but not cardiac muscle cells. Ongoing 
studies are designed to dissect the molecular re- 
quirements for cTnC and sTnC enhancer function 
and to clone and characterize the enhancer- 
binding proteins that control the expression of 
these three enhancers. 
More recently we have developed a method for 
introducing foreign genes directly into heart 
muscle cells in living animals. These inserted 
genes are expressed in the heart muscle for as 
long as six months. This simple technique, which 
involves injection of recombinant genes directly 
into the beating hearts of rats and mice, holds 
promise for the treatment of a number of human 
acquired and inherited cardiovascular diseases. 
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