Pin1 Plays a Critical Role as a Molecular Switch in Canonical BMP Signaling

Abstract

Pin1 is a peptidyl prolyl cis-trans isomerase that specifically binds to the phosphoserine-proline or phosphothreonine-proline motifs of numerous proteins. Previously, we reported that Pin1 deficiency resulted in defects in osteoblast differentiation during early bone development. In this study, we found that adult Pin1-deficient mice developed osteoporotic phenotypes compared to age-matched controls. Since BMP2 stored in the bone matrix plays a critical role in adult bone maintenance, we suspected that BMP R-Smads (Smad1 and Smad5) could be critical targets for Pin1 action. Pin1 specifically binds to the phosphorylated linker region of Smad1, which leads to structural modification and stabilization of the Smad1 protein. In this process, Pin1-mediated conformational modification of Smad1 directly suppresses the Smurf1 interaction with Smad1, thereby promoting sustained activation of the Smad1 molecule. Our data demonstrate that post-phosphorylational prolyl isomerization of Smad1 is a converging signal to stabilize the Smad1 molecule against the ubiquitination process mediated by Smurf1. Therefore, Pin1 is a critical molecular switch in the determination of Smad1 fate, opposing the death signal transmitted to the Smad1 linker region by phosphorylation cascades after its nuclear localization and transcriptional activation. Thus, Pin1 could be developed as a major therapeutic target in many skeletal diseases.

Pin1-mediated Modification Prolongs the Nuclear Retention of β-Catenin in Wnt3a-induced Osteoblast Differentiation

“…Genetic studies have shown that LRP5 can control bone mass (8,9), and conditional deletion of ␤-catenin in the embryonic limb and head mesenchyme results in the absence of mature osteoblasts in membranous bones (10,11). Our previous studies have shown the importance of Pin1 in bone development (25,26,30). Considering this, Wnt signaling, including all components of the canonical pathway, and Pin1 are deemed essential for bone formation.…”

“…Immunofluorescence and Immunohistochemistry-The detection of Pin1 and ␤-catenin by immunofluorescence and immunohistochemistry was performed as described previously (28,30).…”

Pin1, the Master Orchestrator of Bone Cell Differentiation

“…This balance between the cis and trans forms of the substrate has been proposed to be critical for Pin1‐mediated regulation of biological processes (Yaffe et al, ) (Fig. ), including bone cell differentiation (Yoon et al, ; Islam et al, ; Yoon et al, ). Pin1 also exerts an additional level of control after phosphorylation in the cascade of post‐translational events.…”

“…Smad2/3 was the first target of Pin1 identified to have a role in TGF‐β signaling (Nakano et al, ; Matsuura et al, ; Aragon et al, ; Shen et al, ). More recently, the TGF‐β subfamily ligand BMP (bone morphogenetic protein), which transmits signals in the canonical pathway through Smad1/5, was shown to be controlled by Pin1 (Shen et al, ; Yoon et al, ). Furthermore, Pin1‐mediated stabilization of Smad1/5 resulted in accelerated osteoblast differentiation and bone mineralization (Fig.…”

“…PIN1 has also been shown to enhance AP‐1 (Activator Protein‐1) activity (Monje et al, ). Moreover, PIN1 isomerase activity increased Smad1/5 activity upon BMP2 stimulation (Shen et al, ; Yoon et al, ). Our observations of Pin1 function in bone cells support these controversial aspects of crosstalk between FGF2, BMP2, and Wnt signaling, which all converge on Runx2 either directly or indirectly.…”