Bone morphogenetic proteins (Bmps) are members of the transforming growth factor beta (TGFbeta) superfamily that play critical roles during mouse embryogenesis. Signaling by Bmp receptors is mediated mainly by Smad proteins. In this study, we show that a targeted null mutation of Ecsit, encoding a signaling intermediate of the Toll pathway, leads to reduced cell proliferation, altered epiblast patterning, impairment of mesoderm formation, and embryonic lethality at embryonic day 7.5 (E7.5), phenotypes that mimic the Bmp receptor type1a (Bmpr1a) null mutant. In addition, specific Bmp target gene expression is abolished in the absence of Ecsit. Biochemical analysis demonstrates that Ecsit associates constitutively with Smad4 and associates with Smad1 in a Bmp-inducible manner. Together with Smad1 and Smad4, Ecsit binds to the promoter of specific Bmp target genes. Finally, knock-down of Ecsit with Ecsit-specific short hairpin RNA inhibits both Bmp and Toll signaling. Therefore, these results show that Ecsit functions as an essential component in two important signal transduction pathways and establishes a novel role for Ecsit as a cofactor for Smad proteins in the Bmp signaling pathway
Bone
morphogenetic protein activities are enhanced by 3',5'-cyclic
adenosine monophosphate through suppression of Smad6
expression in osteoprogenitor cells.
Sugama
R, Koike T, Imai Y, Nomura-Furuwatari C, Takaoka K.
Department of Orthopaedic Surgery,
Bone morphogenetic proteins (BMPs) belong to the
transforming growth factor (TGF)-beta superfamily,
and some display potent osteogenic activity both in
vivo and in vitro. The BMP signaling cascade involving BMP receptors at the
cell membrane and intracellular messengers (Smads)
has been elucidated, but the regulatory mechanisms of BMP signaling have not
been clarified. We previously found that pentoxifyline
(PeTx), a nonspecific inhibitor of phosphodiesterase (PDE), and rolipram,
a PDE-4-specific inhibitor, enhance BMP-4-induced osteogenic
differentiation of mesenchymal cells, probably
through the elevation of intracellular cyclic adenosine monophosphate
(cAMP) accumulation and modulation of BMP signaling
pathways as enhanced BMP-4 action was reproduced by addition of dibutylyl-cAMP (dbcAMP). However,
the precise mechanisms underlying the enhancing effects of those agents on BMP signaling
were not completely revealed. As already reported, BMPs
utilize a specific intracellular signaling cascade to target genes via R-Smads (Smad1,5,8), Co-Smad (Smad4) and I-Smads
(Smad6,7). One possibility for cAMP-mediated effects
on BMP signaling might be suppression of I-Smads
expression since these proteins form a negative feedback loop in BMP signaling.
To examine this possibility, changes in I-Smad
(Smad6) expression on addition of dbcAMP or PeTx were examined in a bone-marrow-derived osteogenic cell line (ST2). Alkaline phosphatase
activity in ST2 cells was consistently induced by BMP-4 treatment (300 ng/ml), and Smad6 mRNA expression was also induced by BMP-4
treatment. Although concurrent treatment of ST2 cells with BMP-4 and dbcAMP elicited further activation of alkaline phosphatase, addition of dbcAMP
reduced BMP-4-induced Smad6 expression in a dose-dependent manner. Furthermore,
detection of phosphorylated Smad1/5/8 on Western
blotting analysis was prolonged, suggesting prolonged kinase
activity of BMP receptors through suppressed expression of Smad6. Elevated
intracellular cAMP might thus enhance BMP signaling
by suppressing Smad6 induction and prolonging intracellular BMP signaling.
PMID: 16203197 [PubMed - as supplied by publisher]
BMP Enhances Transcriptional Responses to NGF During
PC12 Cell Differentiation.
Lonn P, Zaia K, Israelsson C, Althini S, Usoskin D, Kylberg A, Ebendal T.
Department of Neuroscience,
Bone morphogenetic proteins (BMPs) enhance neurite outgrowth in nerve growth factor (NGF)-stimulated
PC12 cells. To investigate the mechanism of this potentiating
effect, real-time PCR was used to analyze the expression of 45 selected genes.
A robust increase in expression of 10 immediate early genes including Egr1-4,
Hes1, Junb, Jun and Fos was observed already after 1 h treatment with NGF
alone. NGF plus BMP4 further increased these transcripts at 1 h and activated
18 additional genes. BMP4 alone induced Smad6, Mtap1b and Hes1. Egr3 was the
gene most strongly upregulated by NGF and BMP4.
However, luciferase assays showed that the cloned
Egr3 proximal promoter was not involved in the BMP4 potentiation.
Blocking Egr3 and Junb function by dominant-negative
constructs reduced neurite outgrowth under
stimulating conditions, proving that activation of members of both the Egr and Jun families is necessary for maximal PC12 cell
response to NGF and BMP4.
PMID: 16187211 [PubMed - in process]