Developmental stage-specific regulation of atrial natriuretic factor gene transcription in cardiac cells.

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Abstract
Cardiac myocytes undergo a major genetic switch within the first week of postnatal development, when cell division ceases terminally and many cardiac genes are either activated or silenced. We have developed stage-specific cardiocyte cultures to analyze transcriptional control of the rat atrial natriuretic factor (ANF) gene to identify the mechanisms underlying tissue-specific and developmental regulation of this gene in the heart. The first 700 bp of ANF flanking sequences was sufficient for cardiac muscle- and stage-specific expression in both atrial and ventricular myocytes, and a cardiac muscle-specific enhancer was localized between -136 and -700 bp. Deletion of this enhancer markedly reduced promoter activity in cardiac myocytes and derepressed ANF promoter activity in nonexpressing cells. Two distinct domains of the enhancer appeared to contribute differentially to cardiac specificity depending on the differentiation stage of the myocytes. DNase I footprinting of the enhancer domain active in differentiated cells revealed four putative regulatory elements including an A+T-rich region and a CArG element. Deletion mutagenesis and promoter reconstitution assays revealed an important role for the CArG-containing element exclusively in cardiac cells, where its activity was switched on in differentiated myocytes. Transcriptional activity of the ANF-CArG box correlated with the presence of a cardiac- and stage-specific DNA-binding complex which was not recognized by the c-fos serum response element. Thus, the use of this in vitro model system representing stage-specific cardiac development unraveled the presence of different regulatory mechanisms for transcription of the ANF gene during cardiac differentiation and may be useful for studying the regulatory pathways of other genes that undergo switching during cardiac myogenesis.