Control of cardiac jelly dynamics by NOTCH1 and NRG1 defines the building plan for trabeculation

Abstract

In vertebrate hearts, the ventricular trabecular myocardium develops as a sponge-like network of cardiomyocytes that is critical for contraction and conduction, ventricular septation, papillary muscle formation and wall thickening through the process of compaction . Defective trabeculation leads to embryonic lethality or non-compaction cardiomyopathy (NCC) . There are divergent views on when and how trabeculation is initiated in different species. In zebrafish, trabecular cardiomyocytes extrude from compact myocardium , whereas in chicks, chamber wall thickening occurs before overt trabeculation . In mice, the onset of trabeculation has not been described, but is proposed to begin at embryonic day 9.0, when cardiomyocytes form radially oriented ribs . Endocardium-myocardium communication is essential for trabeculation, and numerous signalling pathways have been identified, including Notch and Neuregulin (NRG) . Late disruption of the Notch pathway causes NCC . Whereas it has been shown that mutations in the extracellular matrix (ECM) genes Has2 and Vcan prevent the formation of trabeculae in mice and the matrix metalloprotease ADAMTS1 promotes trabecular termination , the pathways involved in ECM dynamics and the molecular regulation of trabeculation during its early phases remain unexplored. Here we present a model of trabeculation in mice that integrates dynamic endocardial and myocardial cell behaviours and ECM remodelling, and reveal new epistatic relationships between the involved signalling pathways. NOTCH1 signalling promotes ECM degradation during the formation of endocardial projections that are critical for individualization of trabecular units, whereas NRG1 promotes myocardial ECM synthesis, which is necessary for trabecular rearrangement and growth. These systems interconnect through NRG1 control of Vegfa, but act antagonistically to establish trabecular architecture. These insights enabled the prediction of persistent ECM and cardiomyocyte growth in a mouse NCC model, providing new insights into the pathophysiology of congenital heart disease.

Development of the human heart

“…The endocardium becomes sculptured and forms domes filled with cardiac jelly. At the side where the endocardium contacts the myocardium, Nrg1 expressed by endocardial cells can interact with ErbB2/ErbB4 expressed by the cardiomyocytes (Del Monte‐Nieto et al, ). Notch1 regulates the proliferation of the cardiomyocytes that are added to the base of the forming trabeculations in the domes (Del Monte‐Nieto et al, ; Grego‐Bessa and others, ).…”

“…At the side where the endocardium contacts the myocardium, Nrg1 expressed by endocardial cells can interact with ErbB2/ErbB4 expressed by the cardiomyocytes (Del Monte‐Nieto et al, ). Notch1 regulates the proliferation of the cardiomyocytes that are added to the base of the forming trabeculations in the domes (Del Monte‐Nieto et al, ; Grego‐Bessa and others, ). Bmp10 expression in the trabecular cardiomyocytes is an important regulator of trabecular growth, shown by the facts that (a) deletion of Bmp10 results in hypotrabeculation (Chen et al, ) and (b) overexpression of Bmp10 in hypertrabeculation (Pashmforoush et al, ).…”

MyocardialNotch-Rbpjdeletion does not affect heart development or function

“…These results demonstrate that myocardial inactivation of Rbpj in cTnT-Cre;Rbpj flox mice does not affect heart development and structure, nor does impair adult heart function, as it occurs with NOTCH signaling inactivation in the endocardium (4, 17, 18, 26, 30, 57). In contrast, Rbpj deletion driven by the Nkx2.5-Cre driver leads to VSD, DORV and BAV, phenotypes due to Nkx2.5 -mediated CRE activity in valve endocardial cells in which RBPJ mediates NOTCH signaling, with VSD being the likely cause of perinatal lethality of these mutants.…”

“…DLL4-NOTCH1 signaling is reflected by the endocardial expression of the CBF:H2B-Venus transgenic NOTCH reporter in mice (17). Conditional inactivation of Dll4, Notch1 or Rbpj in the endocardium, results in very similar phenotypes (more severe in Rbpj mutants) consisting of ventricular hypoplasia and impaired trabeculation (4, 17, 26), while myocardial deletion of Jag1 does not affect trabeculation (17). Later, NOTCH1 signaling in the endocardium is activated in a temporal sequence from the myocardium by the JAGGED1/2 ligands in a MIB1-dependent manner, to sustain ventricular compaction and maturation (17, 27).…”

The transmembrane protein Crb2a regulates cardiomyocyte apicobasal polarity and adhesion in zebrafish

“…Previously, we have reported that CMs in zebrafish and mouse are polarized in the apicobasal axis and that at least in zebrafish CMs undergo apical constriction and depolarization as they delaminate and seed the trabecular layer (Jiménez-Amilburu et al, 2016;Li et al, 2016;del Monte-Nieto et al, 2018). However, how this polarity is regulated in CMs remains unknown.…”