Integrating Patterning Signals: Wnt/GSK3 Regulates the Duration of the BMP/Smad1 Signal

Abstract

BMP receptors determine the intensity of BMP signals via Smad1 C-terminal phosphorylations. Here we show that a finely controlled cell biological pathway terminates this activity. The duration of the activated pSmad1(Cter) signal was regulated by sequential Smad1 linker region phosphorylations at conserved MAPK and GSK3 sites required for its polyubiquitinylation and transport to the centrosome. Proteasomal degradation of activated Smad1 and total polyubiquitinated proteins took place in the centrosome. Inhibitors of the Erk, p38, and JNK MAPKs, as well as GSK3 inhibitors, prolonged the duration of a pulse of BMP7. Wnt signaling decreased pSmad1(GSK3) antigen levels and redistributed it from the centrosome to cytoplasmic LRP6 signalosomes. In Xenopus embryos, it was found that Wnts induce epidermis and that this required an active BMP-Smad pathway. Epistatic experiments suggested that the dorsoventral (BMP) and anteroposterior (Wnt/GSK3) patterning gradients are integrated at the level of Smad1 phosphorylations during embryonic pattern formation.

A direct role for Wnt8 in ventrolateral mesoderm patterning

“…While the BMP signaling domain is dorsally expanded in wnt8;chordin double morphants, lower levels of p-Smad1/5 staining at 80% epiboly stand in contrast to consistently elevated p-Smad1/5 staining in chordin morphants. Decreased pSmad1/5 staining in the double morphant is consistent with a recently proposed role for Wnt signaling in stabilization of activated Smads (Fuentealba et al, 2007), but it also correlates with the absence of bmp2b expression in the embryonic margin. However, bmp2b absence from the margin may not be relevant as bmp4 and bmp7 expression recovers in the double morphant.…”

“…However, one difference observed between chordin and wnt8;chordin morphants is that the staining intensity is slightly lower in the double knockdown (compare Fig. 3 C and D, K and L), perhaps reflecting a role for Wnt signaling in stabilizing phosphorylated Smad (Fuentealba et al, 2007). From these results, we deduce that reducing Chordin expression results in elevated BMP signaling in a wnt8 loss-of-function background.…”

“…Wnt and BMP activities can activate ventral genes independently of each other, but they also share some common targets (Hoppler and Moon, 1998;Marom et al, 1999;Ramel and Lekven, 2004;Szeto and Kimelman, 2004). While studies in zebrafish have highlighted a dynamic regulation of target genes by both pathways (Ramel and Lekven, 2004), it is still unclear what their relationship is: work in several model systems has shown that their relationship is complex and may involve cross-regulatory interactions at multiple levels (Hoppler and Moon, 1998;Marom et al, 1999;Fuentealba et al, 2007). Understanding the individual roles of Wnt and BMP signaling is critical for deciphering the regulation of vertebrate D/V patterning.…”

The benefits of local depletion: The centrosome as a scaffold for ubiquitin-proteasome-mediated degradation

“…The latter notion is further confirmed by localization of the proteasome subunit S20a5 and polyubiquitin to the centrosomes in this cell line. 72 Subsequent experiments revealed that phospho-Smad1 localized asymmetrically to mitotic centrosomes in human embryonic stem cells (hESCs). Phospho-Smad1 also colocalized with centrosomal microtubules.…”

Regulating the stability of TGFβ receptors and Smads

“…BMP-induced Smurf1 can, as in the steady-state condition, ubiquitinate and degrade Smad1 and Smad5 [40]. In the case of Smad1, it is known that after phosphorylation at the C-terminal motif by the BMP type I receptors, sequential phosphorylations follow in the linker region of Smad1 by MAPK and GSK3b [47,48]. These linker phosphorylations lead to poly-ubiquitination and degradation of activated Smad1 via Smurf1 (Figure 2).…”