Expression of alpha and beta integrins during terminal differentiation of cardiomyocytes.
BACKGROUND
In the myocardium, myocyte cell division is irreversibly blocked shortly after birth. The signal that initiates cell cycle withdrawal is unknown. The purpose of this study was to relate changes in expression of beta1 integrin and its associated alpha subunits to cardiomyocyte cell cycle progression during the fetal-to-neonatal developmental transition in rat.


RESULTS
The developmental expression pattern and function of beta 1 integrin and several of its associated alpha subunits were examined using reverse transcription (RT) polymerase chain reaction (PCR) and beta 1 blocking antibodies. During the fetal to neonatal transition, a dramatic shift occurred in the levels of beta1 and alpha isoforms. At the 17-day fetal stage only beta 1A was present, which remained relatively constant until immediately after birth then decreased by 30% at the adult stage. By contrast, beta 1D appeared at fetal day 18, increased at neonatal day 2, and afterwards remained constant. This resulted in a ratio of beta 1A to beta 1D of about 1:1 in the adult heart. The integrin beta 1-associated subunits, alpha 3, alpha 6, and alpha 7, were expressed at extremely low levels in 17-day fetal cardiomyocytes. After birth alpha 3 and alpha 6 transiently increased at the 2-day neonatal stage, while alpha 7 isoforms B, C, and X2 progressively increased to the adult stage. Unlike skeletal muscle cells, fluorescence-activated cell sorting analysis (FACS) showed no down regulation of the alpha 5 beta 1 fibronectin receptor during cell cycle withdrawal. Treatment of cultured cardiomyocytes with beta1 blocking antibody inhibited the cell cycle in fetal but not in neonatal cells.


CONCLUSIONS
These results suggest that progression through the cardiomyocyte cell cycle may be dependent upon cell attachment via integrin beta1 and correlate with changes that occur in beta1 spliced variants and their respective alpha isoforms.
Maitra
N
N
University of Arizona Sarver Heart Center and Department of Medicine, 1501 N. Campbell Avenue, University of Arizona, Tucson, AZ 85724, USA.
Flink
I L
IL
Bahl
J J
JJ
Morkin
E
E
D000818:Animals; D000831:Animals, Newborn; D000906:Antibodies / Q000494:pharmacology; D015703:Antigens, CD / Q000378:metabolism; D002453:Cell Cycle; D002454:Cell Differentiation; D002478:Cells, Cultured; D005314:Embryonic and Fetal Development; D005434:Flow Cytometry; D005865:Gestational Age; D015151:Immunoblotting; D039001:Integrin alpha Chains; D039201:Integrin alpha3beta1; D039503:Integrin alpha6; D019012:Integrin beta1 / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D016023:Integrins / Q000378:metabolism; D009024:Morphogenesis; D009206:Myocardium / Q000276:immunology*; D020033:Protein Isoforms / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017469:Receptors, Fibronectin / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction
D000906:Antibodies; D015703:Antigens, CD; D039001:Integrin alpha Chains; D039201:Integrin alpha3beta1; D039503:Integrin alpha6; D019012:Integrin beta1; D016023:Integrins; D020033:Protein Isoforms; D017469:Receptors, Fibronectin; C085725:integrin alpha7