Maine Medical Center, Medical Education, Maine Medical Center Research Institute
Cilia, Genes, erbB, Signal Transduction, Heart Ventricles, Heart Defects, Congenital
The embryonic heart requires a unique, and highly-specialized group of progenitor cells called the cardiac neural crest (CNCC), which are known to contribute specifically to the developing valves, the interventricular septum, and the great vessels of the outflow tract. CNCC display primary cilia, which are tiny, plasma-membrane bound organelles that function to modulate cell signaling mechanisms involved in normal heart development as well as congenital heart disease (CHD). Our laboratory has modeled CNCC-specific ciliary loss, using a Wnt1:Cre-2, Ift88-targeted conditional elimination of primary cilia. Loss of cilia in CNCC of Ift88-homozygous mutants (MUT) was characterized by a variety of predicted CHDs in addition to a surprising disorganization of the ventricular endocardium, and pronounced noncompaction of the ventricular myocardium, collectively resulting in perinatal lethality. Extensive immunofluorescence analyses were combined with the use of a Td-Tomato (Tdt) reporter to track CNCC migration in the developing mouse heart. Loss of cilia in CNCC was observed beginning at embryonic day E9.5 in the MUT, with notable CNCC contribution to the ventricular myocardium, and pronounced disorganization of the endocardium at E10.5. To further explore the mechanistic aspects of ventricular noncompaction resulting from loss of cilia in CNCC, we focused on Neuregulin-1 (NRG-1) and its associated ErbB receptor family (specifically ErbB2, ErbB3, ErbB4). Dynamics of endocardial NRG-1 and ErbB signaling in cardiomyocytes are known modulators of myocardial development as well as the compaction process within maturing ventricular myocardium. Flow cytometry was performed on E12.5 embryonic whole hearts, where expression of cell surface proteins PECAM-1 marked endocardial cells (EC) and VCAM-1 marked cardiomyocytes (CM). Notably, loss of cilia in CNCC of MUT hearts resulted in a 43% reduction in VCAM-1-positive cells (p < 0.01) and a trend towards an increased PECAM-1-positive cell population (34%; p = 0.09) when compared to control hearts. While experiments to look at regional proliferation and rates of apoptosis are currently underway, quantification of cell density at E11.5 and E12.5 revealed no difference in the total number of cells between the MUT and controls. Further analysis of ErbB receptors Erb2-4 expression, revealed a slight decrease in ErbB2 and ErbB4 expression in VCAM-1-positive cardiomyocytes of MUT when compared to controls, with no change in ErbB3 expression. PECAM-1-positive endocardial cells (EC) showed a decrease in ErbB4 expression in the MUT when compared to controls. Although decreases in ErbB2/ErbB4 in MUT CM may be attributable to the relative overall decline in CM number, decreased ErbB4 expression in the expanding MUT EC population could serve as a mechanistic explanation for the pronounced hypertrabeculation and noncompaction of the ventricles seen in the MUT phenotype by E14.5. Collectively, our results indicate that loss of primary cilia from CNCC leads to noncompaction of the ventricular myocardium, with a shift in the relative proportion of the CM and EC subpopulations, which is potentially attributable to impaired ErbB signaling of the developing endocardium.
Fitzsimmons, Lindsey A.; Ryzhov, Sergey; and Tucker, Kerry L., "Primary cilia-mediated ventricular maturation in congenital heart disease: A role for ErbB signaling?" (2020). Costas T. Lambrew Research Retreat 2020. 28.