Fms-like tyrosine kinase 3 (Flt3) contributes to a receptor tyrosine kinase signature regulating the cardiac side population

Fms-like tyrosine kinase 3 (Flt3) is a regulator of hematopoietic progenitor cells. It is a target of tyrosine kinase inhibitors (TKIs) used for acute myeloid leukemia treatment. Flt3 and its ligand (Flt3L) are expressed in the heart and cardiac side effects occur under Flt3-targeting TKIs. Whether Flt3/Flt3L also regulate cardiac progenitor cells (CPCs), however, is not known. The cardiac side population (SP) is a pool of heterogenous CPCs that can give rise to all cardiac lineages, hence contributing to cardiovascular homeostasis. Here we show that SP-CPCs produce and are responsive to Flt3L. Compared to wild-type, SP-CPCs from flt3L-/- mice are less abundant, with less contribution of CD45-CD34+ cells, and lower expression of gene sets related to epithelial-to-mesenchymal transition, cardiovascular development and stem cell differentiation. Upon culturing and compared to wild-type, flt3L-/- Sca1+CD31- SP-CPCs show increased proliferation and less vasculogenic commitment, but differentiation can be induced in the presence of receptor tyrosine kinase-activating growth factors. The observed differences are associated with decreased microvascularisation and global systolic function of flt3L-/- hearts. Thus, Flt3 contributes to a receptor tyrosine kinase signature necessary for the maintenance and functionality of SP-CPCs. These findings have potential implications regarding cardiovascular side effects observed under TKI therapy. Comparison of gene expression in freshly isolated wt and flt3L-/- SP-CPCs.