CCBE1 in Cardiac Development and Disease

Abstract

The collagen- and calcium-binding EGF-like domains 1 (CCBE1) is a secreted protein extensively described as indispensable for lymphangiogenesis during development enhancing VEGF-C signaling. In human patients, mutations in CCBE1 have been found to cause Hennekam syndrome, an inherited disease characterized by malformation of the lymphatic system that presents a wide variety of symptoms such as primary lymphedema, lymphangiectasia, and heart defects. Importantly, over the last decade, an essential role for CCBE1 during heart development is being uncovered. In mice, Ccbe1 expression was initially detected in distinct cardiac progenitors such as first and second heart field, and the proepicardium. More recently, Ccbe1 expression was identified in the epicardium and sinus venosus (SV) myocardium at E11.5-E13.5, the stage when SV endocardium-derived (VEGF-C dependent) coronary vessels start to form. Concordantly, CCBE1 is required for the correct formation of the coronary vessels and the coronary artery stem in the mouse. Additionally, Ccbe1 was found to be enriched in mouse embryonic stem cells (ESC) and revealed as a new essential gene for the differentiation of ESC-derived early cardiac precursor cell lineages. Here, we bring an up-to-date review on the role of CCBE1 in cardiac development, function, and human disease implications. Finally, we envisage the potential of this molecule's functions from a regenerative medicine perspective, particularly novel therapeutic strategies for heart disease.

Keywords: CCBE1; Hennekam syndrome; cardiogenesis; coronary vessels; cvd; lymphangiogenesis; proliferation.

FIGURE 1

Schematic representation of CCBE1 domains: signal peptide (SP) in red, EGF and calcium-binding EGF (Ca-EGF) domains in green, and collagen repeats (Collagen 1, Collagen 2) in cyan. The triangles indicate the CCBE1 mutations screened in Hennekam syndrome patients. The N-terminal region is described to bind to the ECM and important for the mobilization of pro-VEGF-C. The C-terminal region of CCBE1 interacts with ADAMTS3 promoting the proteolytic cleavage of pro-VEGF-C.

FIGURE 2

Schematic of Ccbe1 and Vegfc expression and heart phenotypes. (A): Ccbe1 and Vegfc expressions are colocalized in the epicardium from E10.5 onward. Ccbe1 expression is also present in the SV. Vegfc is expressed in the vessel wall of the aorta and pulmonary artery, whereas Ccbe1 is restricted to the aortic epicardium. (B): Schematic representation of heart phenotype in Ccbe1 KO vs Vegfc KO mice shows that both mutant lines display underdeveloped dorsal subepicardial coronary vessels, however, Ccbe1 phenotype extends to defective dorsal and ventral intramyocardial vessels. Green draws represent subepicardial vessels. Gray dots represent intramyocardial vessels.

FIGURE 3

The VEGF family and receptor activation in the presence and absence of CCBE1. Left panel: CCBE1-ADAMTS3 complex promotes the processing of the mature form of VEGF-C, which positively regulates vasculogenesis via VEGFR-2 and -3 activation. Right panel: absence of CCBE1 leads to an accumulation of the unprocessed form of VEGF-C. Unprocessed VEGF-C binding to VEGFR-3 does not increase VEGFR-3 signaling but displaces mature VEGF-C from VEGFR-3. Displaced mature form of VEGF-C competes with VEGF-A for binding to VEGFR-2 and enhancing the binding-affinity of VEGFR-1 for VEGF-A, which translates in negatively regulation of vasculogenesis.