Details

Autor(en) / Beteiligte

• Lai, Donna Yang Li

Titel

The essential roles of neuregulin 1 in cardiac chamber formation

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2004

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• Vertebrate heart development is controlled by signals from the surrounding tissues and from within. Increasing evidence demonstrates that signals from cardiac endocardium, such as the neuregulin 1 (NRG1) family of ligands for ErbB receptor tyrosine kinases, are essential for normal cardiac development and growth. The NRG1-ErbB pathway is required for normal ventricular trabeculation, the first overt morphological sign of ventricular differentiation. Mutant mouse embryos lacking either NRG1, ErbB2 or ErbB4 genes exhibit no trabeculation and die in utero at 10.5dpc. However, the mechanism underlying this process remains unknown. The single NRG1 gene produces many NRG1 isoforms by alternative splicing and multiple promoter usage. Both secreted and membrane-bound NRG1s are present in the developing heart. To determine if different isoforms contribute to different functions, we generated a domain-specific knockout (NRG1TM) by gene targeting, which selectively disrupted transmembrane domain-containing NRG1 isoforms (TM-NRG1s). This thesis describes the creation and characterisation of the NRG1TM strain. Homozygous NRG1TM/TM embryos died at 10.5dpc and displayed defective ventricular trabeculation similar to those carrying the NRG1 null mutation. TUNEL assay and BrdU incorporation experiments showed that deletion of TM-NRG1s failed to cause an increase in cardiomyocyte death and did not dramatically alter proliferation of cardiomyocytes in mutant hearts. However, differentiation of cardiomyocytes and development of the cardiac conduction system (CCS) were severely interrupted in NRG1TM/TM hearts. Expression of a number of cardiac genes was down-regulated in mutant hearts in a region-specific and dose-dependent manner. Several transcription factors (e.g. Nkx2.5, Hand1, Cited1, Tbx5 and Tbx20) are likely to act downstream of NRG1 in the ventricles. In addition, experiments in primary cultured cardiomyocytes demonstrated that NRG1 regulated cardiomyocyte proliferation and differentiation in a dose-dependent manner. In this system, different concentrations of NRGI resulted in transient versus sustained activation of p42/44MAPKs, which in turn regulated distinct responses in cell cycle proteins. Immunohistochemistrical data demonstrated that the p42/44MAPK pathway is indeed active in cardiac chamber primordia and is regulated, at least in part, by NRG1. These findings suggest that NRG1 plays essential roles in cardiac chamber formation by regulating the expression of cardiac transcription factor genes, potentially via activation of a p42/44MAPK pathway.