Zebrafish early cardiac connexin, Cx36.7/Ecx, regulates myofibril orientation and heart morphogenesis by establishing Nkx2.5 expression

Abstract

Heart development is a precisely coordinated process of cellular proliferation, migration, differentiation, and integrated morphogenetic interactions, and therefore it is highly susceptible to developmental anomalies such as the congenital heart disease (CHD). One of the major causes of CHD has been shown to be the mutations in key cardiac transcription factors, including nkx2.5. Here, we report the analysis of zebrafish mutant ftk that showed a progressive heart malformation in the later stages of heart morphogenesis. Our analyses revealed that the cardiac muscle maturation and heart morphogenesis in ftk mutants were impaired because of the disorganization of myofibrils. Notably, we found that the expression of nkx2.5 was down-regulated in the ftk heart despite the normal expression of gata4 and tbx5, suggesting a common mechanism for the occurrence of ftk phenotype and CHD. We identified ftk to be a loss-of-function mutation in a connexin gene, cx36.7/early cardiac connexin (ecx), expressed during early heart development. We further showed by a rescue experiment that Nkx2.5 is the downstream mediator of Ecx-mediated signaling. From these results, we propose that the cardiac connexin Ecx and its downstream signaling are crucial for establishing nkx2.5 expression, which in turn promotes unidirectional, parallel alignment of myofibrils and the subsequent proper heart morphogenesis.cardiomyocyte ͉ congenital heart disease ͉ trabeculation ͉ futka ͉ positional cloning N ormal development and function of the heart are indispensable for the survival of vertebrates. Cardiogenesis involves a precisely coordinated process of cellular proliferation, migration, and differentiation, and integrated morphogenetic interactions. Because of the complexity of these embryonic processes, heart development is highly susceptible to developmental anomalies collectively known as congenital heart diseases (CHD). The syndrome threatens the lives of as many as 1% of newborns (1, 2). Studies have uncovered that several transcription factors, including TBX5, NKX2.5, GATA4, and SALL4, cooperate to direct the cardiac cell lineage commitment and/or morphogenesis through the regulation of proteins characteristic of cardiomyocytes and that mutations in these transcription factors are responsible for many cases of CHD (3-7). Recent data have also suggested the significant roles of these transcription factors for maintenance of the functional heart in postnatal life as well (8-12). In the past decade, zebrafish (Danio rerio) has emerged as a valuable model system to study many developmental processes, including heart development, and a number of important insights have been gained from analyses of zebrafish mutants (13)(14)(15)(16)(17). In addition to the early determination and differentiation of heart primordium, zebrafish mutations are also expected to contribute to the clarification of a molecular mechanism that governs myofibrillogenesis and morphogenesis of the heart. The unidirectional alignment of myofibrils is essential for proper functio...

JunB-CBFβ signaling is essential to maintain sarcomeric Z-disc structure and when defective leads to heart failure

“…Besides its structural role at the Z-disc, CBF also regulates cardiac and skeletal sarcomerogenesis, because sarcomeres are significantly thinner in CBF morphants. Similar changes in myofilament structure can be observed in other models of heart failure and range from almost completely disrupted sarcomeric units, for example in the Troponin-T-deficient zebrafish mutant silent heart (tnnt2a), the Titin-deficient mutant pickwick and the cMLC2-deficient zebrafish mutant tell tale heart, to minor changes in the content, size or organization of sarcomeres in Tropomyosin-4-or connexin Cx36.7-deficient zebrafish Sehnert et al, 2002;Sultana et al, 2008;Xu et al, 2002;Zhao et al, 2008).…”

Embryo exposure to elevated cortisol level leads to cardiac performance dysfunction in zebrafish

“…The mechanism behind these observed deformations requires further study; however, we have identified some key genes involved in cardiovascular development that are downregulated in cortisol-injected embryos. The cardiac transcription factor nkx2.5, which is involved in heart field definition and myofibril development (Chen and Fishman, 1996;Sultana et al, 2008), is suppressed at both 36 and 48 hpf. Knockdown of nkx2.5 has been shown to cause a variety of cardiac defects in heart chamber formation as well as pericardial edema (Targoff et al, 2008), similar to the phenotype observed in our cortisol-injected embryos.…”

The N Terminus of Connexin37 Contains an α-Helix That Is Required for Channel Function

“…A D12V mutation was identified in zebrafish CX36.7 (55); fish carrying this mutation show disorganization of myofibrils and cardiac malformations. This residue (which aligns with Asp-12 in human CX37) may be important for gap junction plaque formation and function, because the CX36.7D12V mutant shows impaired trafficking to the plasma membrane in HeLa cells (55) and a different substitution in mouse CX43 (D12S) causes loss of Lucifer yellow transfer, although CX43D12SGFP does form gap junction plaques in baby hamster kidney cells (23). Many of the mutants within the NT associated with oculodentodigital dysplasia (CX43) or with cataracts (CX50) occur in amino acid residues beyond the predominant helical structure (after amino acid 16).…”