Myocardial Mycn is essential for mouse ventricular wall morphogenesis

Abstract

MYCN is a highly conserved transcription factor with multifaceted roles in development and disease. Mutations in MYCN are associated with Feingold syndrome, a developmental disorder characterized in part by congenital heart defects. Mouse models have helped elucidate MYCN functions; however its cardiac-specific roles during development remain unclear. We employed a Cre/loxp strategy to uncover the specific activities of MYCN in the developing mouse myocardium. Myocardial deletion of Mycn resulted in a thin-myocardial wall defect with dramatically reduced trabeculation. The mutant heart defects strongly resemble the phenotype caused by disruption of BMP10 and Neuregulin-1 (NRG1) signaling pathways, two central mediators of myocardial wall development. Our further examination showed that expression of MYCN is regulated by both BMP and NRG1 signaling. The thin-wall defect in mutant hearts is caused by a reduction in both cell proliferation and cell size. MYCN promotes cardiomyocyte proliferation through regulating expression of cell cycle regulators (including CCND1, CCND2, and ID2) and promotes cardiomyocyte growth through regulating expression of p70S6K. In addition, expression of multiple sarcomere proteins is altered in Mycn myocardial-inactivation embryos, indicating its essential role for proper cardiomyocyte differentiation. In summary, Mycn acts downstream of BMP and NRG1 cardiogenic signaling pathways to promote normal myocardial wall morphogenesis.

Mycis dispensable for cardiac development in the mouse but rescuesMycn-deficient hearts through functional replacement and cell competition

“…We established crosses to obtain embryos simultaneously deleted for a conditional Mycn allele (cKO-Mycn) and activated for iMOS T1Myc with the Nkx2.5-Cre driver ( Fig 3B). Embryos in which Mycn has been deleted but the iMOS mosaic is not activated are not viable past E10.5-E11.5 (Fig 3B), in accordance with the phenotype previously reported for Mycn deletion in cardiomyocytes 15 . Similarly, iMOS T1Myc activation on a wild type or Mycn-heterozygous background (iMOS-Myc) does not produce any phenotypic alteration 12 .…”

“…An alternative view would be that Mycn-deficient cells are not actively eliminated but just diluted out due to their limited ability to proliferate. To discriminate between these possibilities, given that Mycn-deficient cardiomyocytes do not autonomously display an increased apoptotic rate 15 and that apoptosis is a hallmark of Myc-induced cell competition in the developing mouse heart, we performed TUNEL analysis.…”

“…Furthermore, the conditional deletion of Myc in the blood/endothelial lineage produces heart defects similar to those observed in the complete Myc elimination 14 raising the possibility that the cardiac defects observed in the global mutant do not result from a primary function in cardiomyocytes. In contrast, Mycn is essential for cardiomyocyte development in conditional deletion models 15 . Myc and Mycn show high sequence and structure homology and this translates into a highly conserved function, as exemplified by Mycn full rescue of the Myc global knockout in a knock-in replacement mouse model 16 .…”

N-myc regulates growth and fiber cell differentiation in lens development

“…Tissue-specific inactivation studies revealed organ-specific functions of both c-myc and N-myc . Roles of N-myc were demonstrated in brain (Knoepfler et al, 2002), lung (Okubo et al, 2005), retina (Martins et al, 2008), hematopoietic stem cells (Laurenti et al, 2008), ear (Dominguez-Frutos et al, 2011), heart (Harmelink et al, 2013), and olfactory development (Wittmann et al, 2014). Even though, these studies did not analyze how N-myc regulates global gene expression during morphogenesis, few of these demonstrated functional compensation or redundancy between Myc family members.…”

Control of Vertebrate Development by MYC

“…And mice engineered to express 15% MYCN showed severe cardiac muscle hypoplasia and lethality between E12.5 and E14.5 (Moens et al 1993). The essential roles of MYCN in lung and heart development (Okubo et al 2005;Harmelink and Jiao 2010;Harmelink et al 2013), as well as in the development of the central nervous system (CNS) (Knoepfler et al 2002), limbs (Ota et al 2007), kidneys (Nakhai et al 2008), and inner ear (Domínguez-Frutos et al 2011) were confirmed and/or further defined by conditional MYCN deletion at these sites (see below for further discussion).…”