Nuclear CDKs drive Smad transcriptional activation and turnover in BMP and TGF-beta pathways

Abstract

TGF-beta and BMP receptor kinases activate Smad transcription factors by C-terminal phosphorylation. We have identified a subsequent agonist-induced phosphorylation that plays a central dual role in Smad transcriptional activation and turnover. As receptor-activated Smads form transcriptional complexes, they are phosphorylated at an interdomain linker region by CDK8 and CDK9, which are components of transcriptional mediator and elongation complexes. These phosphorylations promote Smad transcriptional action, which in the case of Smad1 is mediated by the recruitment of YAP to the phosphorylated linker sites. An effector of the highly conserved Hippo organ size control pathway, YAP supports Smad1-dependent transcription and is required for BMP suppression of neural differentiation of mouse embryonic stem cells. The phosphorylated linker is ultimately recognized by specific ubiquitin ligases, leading to proteasome-mediated turnover of activated Smad proteins. Thus, nuclear CDK8/9 drive a cycle of Smad utilization and disposal that is an integral part of canonical BMP and TGF-beta pathways.

Figure 1. TGFβ and BMP induce Smad C-tail and linker phosphorylation

(A) Nuclear and cytoplasmic fractions of BMP or TGFβ treated (1h) HaCaT cells were analyzed by western blot with the indicated antibodies. (B) and (C) HaCaT cells treated with BMP, TGFβ or no addition (control) for 1 h were fixed and analyzed by immunofluorescence (IF) with antibodies against Smad1 (pLinker, Smad1 pS206; pTail, Smad1/5 pTail) and Smad3 (pLinker, Smad3 pS213; pTail, Smad3 pTail) including DAPI-staining of nuclei. (D) As in (B), with BMP treatment for the indicated times. (E–I) Images of E13.5 mouse embryo sections. (E) Adjacent sections through the telencephalic ventricular zone immunohistochemically stained with anti-Smad1 antibodies described in (B and C), with Hematoxylin counterstaining. Images at 40x magnification. Ventricle and ventricular zone (VZ) are indicated. (F) Confocal images of a section similar to (A) with double-immunofluorescence staining using the indicated antibodies and DAPI counterstaining. Images are at 20x magnification, with the far right panel at 200x. The ventricular zone is indicated by white dashed lines. (G) Immunohistochemistry on adjacent sections through dorsal root ganglia stained as in (E), with antibodies against Smad2 (pLinker Smad2, Smad2 pS245/250/245; pTail Smad2). Images at 80x magnification. (H) Confocal images of dorsal root ganglia with double-immunofluorescence staining with the antibodies used in (G). Magnification was 80x. (I) Merged confocal image of E13.5 testis from the same section as in (H).

Figure 2. Requirements for Smad ALP

(A) Immunoblot analysis of wild-type, Smad1C and Smad1L mutant knock-in MEFs treated with BMP or UV, with antibodies against the indicated proteins. (B, C) SW480 Smad4-null cells, or cells stably expressing Smad4, were treated with BMP (B) or TGFβ (C). Nuclear and cytoplasmic fractions were analyzed using the indicated antibodies. (D) Chromatin immunoprecipitation of BMP and TGFβ stimulated C2C12 cells with the indicated antibodies. Precipitates were subjected to qRT-PCR of the BMP and TGFβ responsive regions of the indicated genes with unresponsive regions of the same genes serving as negative controls. Data show the mean ± S.D. of triplicates and are representaive of at least two independent expreriments. (E) Immunoblot analysis of BMP treated HaCaT cells in the absence or after addition of α-amanitin for the indicated times. Total cell extracts were analyzed using the indicated antibodies. (F) HaCaT cells were stimulated with BMP for 1 h in the absence or presence of MG132 or of a siRNA against Smurf1. Cells were harvested at the indicated times after BMP removal and total cell extracts were analyzed by immunoblot. (G) As in (F) but cells were stimulated with TGFβ and siRNA against Nedd4L was used. (H) Schematic model of the sequential steps leading to Smad-ALP and binding of ubiquitin ligases.

Figure 3. Flavopiridol inhibits Smad ALP

(A) Cell extracts of BMP treated HaCaT cells with or without kinase inhibitors were analyzed with antibodies against the indicated proteins. SB, SB203580; SP, SP60125; combo, combination of the three MAPK inhibitors; Flavopir., Flavopiridol. (B, C) Nuclear and cytoplasmic fractions of HaCaT cells stimulated with BMP or TGFβ in the absence or presence of Flavopiridol were analyzed by immunoblotting. (D) HaCaT cells were stimulated with BMP for 1 h in the absence or presence of Flavopiridol (FP), or of Flavopiridol plus MG132. Incubations were continued for the indicated periods with media without BMP.

Figure 4. CDK8 and CDK9 as mediators of Smad ALP

(A, B) HaCaT cells were transfected with siRNAs against CDK7, CDK8, CDK9 or against FoxO4 as a negative control. Total extracts from cells stimulated with BMP or TGFβ for 1 h were analyzed by immunoblot. (C) Autoradiograph of bacterially expressed Smad1 wild-type or linker mutant, phosphorylated in vitro by the indicated CDK/Cyclin complexes with γ³²P-ATP as substrate. Purified CTD domain of RNA Pol II was used as a positive control for CDK activity. (D) In vitro kinase assay of the indicated CDK9/CyclinT, CDK8/CyclinC complexes or ERK, on bacterially expressed Smad1 and Smad3 wild-type or linker mutant. The numbers indicate the residue that was left as Ser/Thr in the linker region. (E) As in (C) but with Smad2 and Smad3 wild-type or linker mutants. (F) Co-immunoprecipitation of endogenous Smad1/5 and CDK8. Cell extracts from untreated or BMP-treated HaCaT cells were immunoprecipitated using an anti-CDK8 antibody and analyzed by western immunoblotting with antibodies against Smad1/5 pTail, or CDK8 as a loading control. (G) Flag immunoprecipitates of BMP-treated HaCaT cells stably expressing Flag-Smad1 were analyzed using antibodies against CDK8, or Flag as a loading control. (H) Schematic summary of CDK8/9 phosphorylation sites in Smad linker regions and their relationship to ubiquitin ligase recognition sites (Gao et al., 2009; Sapkota et al., 2007). Red dots, principal phosphorylation sites; green boxes, PY motifs.

Figure 5. Role of CDK8/9 and linker phosphorylation in Smad turnover

(A) Time course of Smad1/5 C-tail phosphorylation after 1 h of BMP stimulation as in Figure 2F. HaCaT cells transfected with siRNA against CDK8, CDK9 or the control FoxO4 were stimulated 48 h later cells and total lysates were analyzed by immunoblotting. The fourth panel shows a quantitation of the Smad1/5 pTail bands. (B) HaCaT cells with stable shRNA Smad1 knockdown and stably transduced with vectors encoding Smad1 wild-type or linker mutant were treated with BMP in the presence of Smurf1 or control siRNA and analyzed by immunoblotting. (C) The cell lines described in (B) were treated with BMP for 1 h and cells were harvested 2 h after BMP removal to test the expression of ID1 by qRT-PCR. Data show the mean ± S.D of quadruplicates and are representative of two independent experiments. (D) HeLa-S3 cells stably expressing Flag-tagged Smad3 (wild-type or linker mutant) were retrovirally infected with vectors encoding control shRNA or shRNA against Nedd4L. After treating cells with or without TGFβ for 3 h whole cell lysates were subjected to anti-Flag immunoprecipitation and analyzed by immunoblotting. (E) The cell lines used in (D) were treated with TGFβ for 3 h, and total RNA was isolated for qRT-PCR analysis of CTGF or SKIL levels. Values are normalized to the untreated levels, and shown as the fold induction by TGFβ in each cell line. Data are the mean ± S.D of quadruplicates and are representative of two independent experiments. (F) Schematic representation of the dual role of linker phopshorylation.

Figure 6. Smad1 linker phosphorylation mediates binding of YAP

(A) Schematic representation of Smurf1 and YAP. Orange boxes, WW domains; C2, calcium and lipid binding domain; HECT, ubiquitin ligase domain; TB, TEAD bindng domain, TX’N, transcription activation domain. (B) Co-immunoprecipitation of endogenous YAP and Smad1 in BMP-treated HaCaT cells. Complexes were analyzed by immunoblotting. (C) HEK293T cells transfected with vectors encoding Flag-tagged Smad1 or Smad3 (wild-type or linker mutant) were subjected to Flag immunoprecipitation and analyzed by immunoblotting. (D) HEK293T cells were transfected with vectors encoding HA-tagged YAP or Smurf1 and after BMP stimulation in the presence or absence of flavopiridol, HA immunoprecipitates were analyzed by immunoblotting using the indicated anti-Smad1 antibodies. (E) Dissociation constants of the interaction of a YAP (WW1+WW2) protein fragment with the indicated Smad1 linker peptides, as measured by isothermal titration calorimetry. (F) Drosophila S2 cells were transfected with Flag-Mad wild-type or linker mutant and Flag-immunoprecipitates were tested for the presence of endogenous Yorkie by immunoblot. (G) As in (E), with cotransfection of HA-Yorkie and immunoprecipitation of the HA-species. (H) As in (F), with transfection and immunoprecipitation of wild-type or WW domain mutant Yorkie. (I) Schematic representation of the recruitment of YAP upon CDK8/9-mediated linker phosphorylation of Smad1.

Figure 7. YAP enhances BMP-Smad responses in different biological contexts

(A) Chromatin immunoprecipitation of BMP-stimulated mESCs using antibodies against YAP and Smad1/5. BMP-responsive regions of Id1 and Id2 and an unresponsive control region of Id1 (control) were analyzed by qRT-PCR as in Figure 2D. (B) Wild-type YAP-knockdown mESCs were stimulated with BMP4 for 1 h. Total RNA was analyzed by qRT-PCR. Data show the mean ± S.D of quadruplicates and are representative of two independent experiments. (C) Gene expression analysis of the neural differentiation marker β-III tubulin (Tubb3), in differentiating wild-type and YAP-knockdown mESCs. Cells were cultured in N2B27 supplemented media in the presence or absence of BMP, harvested five days later and total RNA was subjected to qRT-PCR analysis for Tubb3 expression as in (B). (D) Confocal images of mESCs treated as in (C) and subjected to immunofluorescence staining with an anti-Tubb3 antibody (green) and DAPI counterstaining (blue). Tubb3 staining detects neuronal differentiation. (E) vgQE-lacZ expression (red) in Drosophila wing imaginal disc containing Yorkie-overexpressing clones (marked by GFP⁺). Confocal z-stack through the wing imaginal disc was projected into a single plane. Note that the ectopic vgQE-lacZ expression (asterisk) is discontinuous with the endogenous vgQE-lacZ expression domain (arrowhead) and only observed close to the anterior-posterior compartment boundary (arrow). (F) The Smad signaling cycle. Receptor-bound BMP and TGFβ ligands induce C-tail phosphorylation of R-Smads, which then accumulate in the nucleus. Nuclear R-Smads complexed with Smad4, bind to the regulatory elements of target genes and interact with other DNA binding cofactors (shown as a grey box), becoming linker-phosphorylated by CDK8/9 at some point during this process. This facilitates Smad1 binding to YAP, which is required for efficient transcription of BMP target genes. Linker phosphorylated Smad2/3 is thought to activate transcription of TGFβ target genes in a similar manner, but through as yet unknown cofactors. After fulfilling their transcriptional role, Smads may be targeted by linker and tail phosphatases that reset them to their ground state or linker phosphatases only that may allow further Smad activity. Alternatively, Smads can be recognized in a phospho-linker dependent manner, by ubiquitin ligases, such as Smurf1 in the case of Smad1, and Nedd4L in the case of Smad2/3, resulting in their eventual degradation. The R-Smad linker regions can also be phosphorylated by MAPK pathway kinases in response to antagonists, such as FGF, EGF, or stress signals, which ultimately leads to Smad degradation. By providing a platform for the regulation of the different and often opposing fates of the Smads, linker phosphorylation represents a focal point in the Smad signaling cycle.