Canonical and noncanonical TGF-β signaling regulate fibrous tissue differentiation in the axial skeleton

Abstract

Previously, we showed that embryonic deletion of TGF-β type 2 receptor in mouse sclerotome resulted in defects in fibrous connective tissues in the spine. Here we investigated how TGF-β regulates expression of fibrous markers: Scleraxis, Fibromodulin and Adamtsl2. We showed that TGF-β stimulated expression of Scleraxis mRNA by two hours and Fibromodulin and Adamtsl2 mRNAs by eight hours of treatment. Regulation of Scleraxis by TGF-β did not require new protein synthesis; however, protein synthesis was required for expression of Fibromodulin and Adamtsl2 indicating the necessity of an intermediate. We subsequently showed Scleraxis was a potential intermediate for TGF-β-regulated expression of Fibromodulin and Adamtsl2. The canonical effector Smad3 was not necessary for TGF-β-mediated regulation of Scleraxis. Smad3 was necessary for regulation of Fibromodulin and Adamtsl2, but not sufficient to super-induce expression with TGF-β treatment. Next, the role of several noncanonical TGF-β pathways were tested. We found that ERK1/2 was activated by TGF-β and required to regulate expression of Scleraxis, Fibromodulin, and Adamtsl2. Based on these results, we propose a model in which TGF-β regulates Scleraxis via ERK1/2 and then Scleraxis and Smad3 cooperate to regulate Fibromodulin and Adamtsl2. These results define a novel signaling mechanism for TGF-β-mediated fibrous differentiation in sclerotome.

Perspective Chapter: Valvular Interstitial Cells – Physiology, Isolation, and Culture

“…TGF-β forms a homodimer in the extracellular space and binds two type 2 TGF-β receptors (Tgfbr2), which then recruit two type 1 TGF-β receptors (Tgfbr1) forming an activated receptor-ligand complex [39]. The activated Tgfbr1 is then able to phosphorylate intracellular messenger proteins Smad2 and Smad3, which form a complex with Smad4 and move to the nucleus as transcription factors to activate the associated genes [39,41]. It is this process that is responsible for the activation of qVICs to myofibroblasts and for the development of dystrophic nodules [3].…”

“…Non-Smad pathways initiated by TGF-β are termed 'non canonical' . These include the ERK/MAPK pathway [41], which has been shown to regulate key aspects of aVIC expression in a manner independent of the canonical TGF-β pathway [42].…”