Fibroblast growth factor (FGF)/FGF receptor (FGFR) signaling plays a crucial role in mesoderm formation and patterning.
Heartless
mutant studies in
Drosophila
suggest that FGFR1, among the different FGFRs, may play a role in cardiogenesis. However,
fgfr1−/−
mice die during gastrulation before heart formation. To establish the contribution of FGFR1 in cardiac development, we investigated the capacity of murine
fgfr1+/−
and
fgfr1−/−
embryonic stem (ES) cells to differentiate to cardiomyocytes in vitro. Clusters of pulsating cardiomyocytes were observed in >90% of 3-dimensional embryoid bodies (EBs) originated from
fgfr1+/−
ES cells at day 9 to 10 of differentiation. In contrast, 10% or less of
fgfr1−/−
EBs showed beating foci at day 16. Accordingly,
fgfr1−/−
EBs were characterized by impaired expression of early cardiac transcription factors
Nkx2.5
and
d-Hand
and of late structural cardiac genes
myosin heavy chain
(
MHC
)-α,
MHC
-β, and
ventricular myosin light chain
. Homozygous
fgfr1
mutation resulted also in alterations of the expression of mesoderm-related early genes, including
nodal
,
BMP2, BMP4
,
T
(
bra
), and
sonic hedgehog
. Nevertheless,
fgfr1+/−
and
fgfr1−/−
EBs similarly express cardiogenic precursor, endothelial, hematopoietic, and skeletal muscle markers, indicating that
fgfr1
-null mutation exerts a selective effect on cardiomyocyte development in differentiating ES cells. Accordingly, inhibitors of FGFR signaling, including the FGFR1 tyrosine kinase inhibitor SU 5402, the MEK1/2 inhibitor U0126, and the protein kinase C inhibitor GF109 all prevented cardiomyocyte differentiation in
fgfr1+/−
EBs without affecting the expression of the hematopoietic/endothelial marker
flk-1
. In conclusion, the data point to a nonredundant role for FGFR1-mediated signaling in cardiomyocyte development.