Molecular Analysis of Muscle Contraction 
Bernardo Nadal-Ginard, M.D., Ph.D. — Investigator 
Dr. Nadal-Ginard is also the Alexander S. Nadas Professor of Pediatrics and Professor of Cellular and Mo- 
lecular Physiology at Harvard Medical School and Cardiologist-in- Chief at the Children 's Hospital, Boston. 
He received his M.D. degree from the University of Barcelona, Spain, and his Ph.D. degree in biology from 
Yale University. After training in internal medicine and cardiology, he was a student and postdoctoral 
fellow with Clement Markert at Yale. He was a professor of cell biology at Albert Einstein College of Medi- 
cine before assuming his present position. 
OUR laboratory is interested in the molecular 
mechanisms that regulate the production 
and function of the contractile system in muscle 
cells. This apparatus is the molecular motor for 
locomotion and for the heartbeat. Moreover, vari- 
ations of this contractile system are involved in 
maintaining the shape and function of nonmuscle 
cells. 
The functional unit of the contractile system is 
the sarcomere, with its precisely arranged compo- 
nents. Each of these sarcomeric proteins is the 
product of a small multigene family. Combina- 
tions of these protein isoforms can give rise to 
many different sarcomeres, and this capacity for 
generating diversity is further expanded by a pro- 
cess of alternative splicing. 
Among sarcomeres, significant functional dif- 
ferences are produced through two main mecha- 
nisms. One changes the components either at the 
transcriptional level or by alternative splicing; 
the other, by regulating the availability of ions in 
the muscle cells. These two aspects of contractil- 
ity continue to be the main focus of our research. 
Transcriptional Regulation of Contractile 
Protein Genes 
Which sarcomeres are assembled in a given cell 
at a particular time depends on which contractile 
protein genes are expressed in the cell. To ana- 
lyze the mechanisms involved in switching from 
one gene to another in the same gene family, we 
have focused on the genes coding for the myosin 
heavy chain (MHC) . Functionally this is the most 
important component of the sarcomere because 
it contains the ATPase activity that converts the 
chemical energy of ATP into mechanical force. 
We are currently exploring two main questions in 
the regulation of the MHC genes: What deter- 
mines that a given MHC gene is expressed in a 
given cell type at a particular developmental or 
physiological stage? What determines the level of 
expression? For these analyses we have concen- 
trated on the two MHC genes expressed in the 
myocardium, a and |8 MHC. These genes are par- 
ticularly advantageous because their expression 
during development is modulated in response to 
different hormones and physiological conditions. 
We have identified many of the regulatory DNA 
sequences involved in the tissue-specific expres- 
sion of these two genes. We have shown that their 
expression is determined in large part by negative 
regulatory elements. These elements interact 
with nuclear proteins that are present in many 
different cell types but sparse in muscle cells. 
The transcriptional factors of the MyoD gene fam- 
ily do not seem to play a role in the regulation of 
these two MHC genes. 
In their expression, positive as well as negative 
factors are essential. The most important positive 
factor for a MHC is a thyroid-responsive element 
(TRE) . The interaction of this cis-acting element 
with different forms of receptor for thyroid hor- 
mone and retinoic acid has been analyzed in de- 
tail. These interactions are required and suffi- 
cient to produce most of the gene's known 
phenotypes. Interestingly, thyroid hormone re- 
presses the jS MHC gene. 
The main positive regulator of 0 MHC is a mus- 
cle-specific enhancer of complex structure, com- 
posed of binding sites for muscle-specific and 
general transcriptional factors. Expression stud- 
ies of this binding site, together with mobility 
shift assays, demonstrate that the protein binding 
to the site is muscle-specific. Using this sequence 
as a probe to screen a cardiac expression cDNA 
library, we have cloned a novel transcription fac- 
tor that belongs to the family of the homeobox 
genes. Expression of this factor significantly en- 
hances the expression of constructs containing 
the |S MHC enhancer sequence. 
The finding of a muscle-specific transcription 
factor that does not belong to the MyoD family 
has long been a goal of workers in muscle dy- 
namics. This is so because the MyoD gene family 
is not expressed in the heart. However, this mus- 
cle expresses many of the skeletal muscle genes 
whose induction is dependent on MyoD. The 
search for the cardiac MyoD equivalent in many 
laboratories, including ours, has so far been un- 
successful. The identification and cloning of a 
cardiac transcription factor of the homeobox fara- 
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