Schnurri-3 regulates ERK downstream of WNT signaling in osteoblasts

Abstract

Mice deficient in Schnurri-3 (SHN3; also known as HIVEP3) display increased bone formation, but harnessing this observation for therapeutic benefit requires an improved understanding of how SHN3 functions in osteoblasts. Here we identified SHN3 as a dampener of ERK activity that functions in part downstream of WNT signaling in osteoblasts. A D-domain motif within SHN3 mediated the interaction with and inhibition of ERK activity and osteoblast differentiation, and knockin of a mutation in Shn3 that abolishes this interaction resulted in aberrant ERK activation and consequent osteoblast hyperactivity in vivo. Additionally, in vivo genetic interaction studies demonstrated that crossing to Lrp5 -/-mice partially rescued the osteosclerotic phenotype of Shn3 -/-mice; mechanistically, this corresponded to the ability of SHN3 to inhibit ERK-mediated suppression of GSK3β. Inducible knockdown of Shn3 in adult mice resulted in a high-bone mass phenotype, providing evidence that transient blockade of these pathways in adults holds promise as a therapy for osteoporosis. IntroductionAdult bone mass reflects the balance between production of bone by osteoblasts and resorption of bone by osteoclasts, and disturbance of this balance results in bone pathology such as osteoporosis. As osteoblasts have a limited ability to directly perceive the thickness of the bone they overlay, they rely on the ability to integrate extracellular cues to appropriately adjust the rate of bone formation. One such extracellular cue, the WNT/ β-catenin pathway, is well established as a positive regulator of osteoblast differentiation, appearing to divert early progenitors away from a chondrocyte fate into becoming osteoblasts (1, 2). Deletion or inhibition of individual WNT signaling components results in dysregulation of osteoblast activity and differentiation and, by extension, altered anabolic bone formation in mice (3). Under basal conditions, cytosolic β-catenin undergoes constitutive ubiquitination and proteasome-dependent degradation via the destruction complex that consists of adenomatous polyposis coli (APS), Axin, CK1α, and glycogen synthase kinase 3-β (GSK3β) (4). A subset of ligands of the WNT family binds to the 7-pass transmembrane receptor Frizzled and the single-pass low-density lipoprotein receptor-related protein 5 (LRP5) or LRP6. This inhibits the constitutive phosphorylation of β-catenin by GSK3β, which in turn prevents β-catenin ubiquitination and proteasome-dependent degradation. Subsequently, β-catenin translocates into the nucleus to activate its transcrip-

MYC-dependent oxidative metabolism regulates osteoclastogenesis via nuclear receptor ERRα

“…Analysis of bone phenotype. μCT analysis (44) was performed as described previously (45), and all samples were included in the analysis, which was conducted in a blinded manner. For μCT analysis, a Scanco Medical μCT 35 System with an isotropic voxel size of 7 μm was used to image the distal femur.…”

Control of the Osteoblast Lineage by Mitogen-Activated Protein Kinase Signaling

“…The ERK/MAPK pathway indirectly activates the WNT pathway by phosphorylating and inactivating GSK-3β (Serine 9 phosphorylation) thereby allowing nuclear translocation of β-catenin and activation of osteoblast gene expression(64). This ERK-dependent activation of the WNT pathway is negatively regulated by the scaffolding protein, Schnurri-3 (SHN3), which selectively inhibits the phosphorylation and inactivation of GSK-3β by ERK, to inhibit WNT signaling(65). Consistent with this model, Shn3 -deficient mice have a high bone mass sclerotic phenotype that is partially rescued by crossing with Lrp5−/− mice.…”

The D Domain of LRRC4 anchors ERK1/2 in the cytoplasm and competitively inhibits MEK/ERK activation in glioma cells

“…ERK1/2 and other MAPKs contain the CD domain, which includes aspartate residues 316 and 319 (labeled for ERK2) that are located on the opposite side of the TXY activation loop [24, 39] and mediate interactions with the D domains [39–41]. The D domain within Schnurri-3 mediated the interaction with ERK and inhibition of ERK activity and osteoblast differentiation [42]. Human scribble (hScrib) interacts with ERK through two D domain docking sites and decreases activation of ERK [43].…”