# Molecular Mechanisms of Topography Sensing by Osteoblasts: An Update
## 3 Molecular Mechanisms of Transduction of the Topographical Information
The presence of osteogenic topographies increases the expression of BMP2, BMP6 [86; 87] as well as BMP receptors such as BMP2R, suggesting that the BMP pathway may be triggered by the topographical cue. Consistently, BMP expression is associated with the increase in osteogenic markers such as Runx2, OSX, OPN, OCN, BSP and ALP activity and calcium deposition [87]. The importance of BMP in topography transduction is also illustrated by the reverse situation: topographies inhibiting osteogenic differentiation of MSCs lead to a downregulation of BMP receptors and a consistent decrease in Runx2 expression [75]. Importantly, the knock-down of the BMP receptor BMPR1A diminishes the osteogenic effect of nanorough surface [86]. Altogether, this demonstrates that the osteogenic effect of topography is mediated in part by the activation of the BMP pathway and the subsequent upregulation of osteogenesis-related genes such as Runx2. In addition of triggering the BMP pathway via the upregulation of BMP and BMP receptor expression, topographies can also enhance cell sensitivity to already present BMPs. For instance, the osteogenic effect of BMP9 gets stronger when MSC are cultured on rough substrates [88]. Although strong lines of evidence show that BMP are important in the osteogenic reaction to topography, the initial step causing the topography-induced increase in BMP expression remains unclear. To that respect, it is interesting to note that the BMP receptor BMPR1 and the integrin \(\alpha\)V \(\beta\)5 tend to cluster over osteogenic nanopit topography [89]. The inhibition of BMPR1 decreases the integrin chains' expression, suggesting a crosstalk between adhesion and BMP signaling. It is thus likely that the modification of integrin-based adhesions by the topography is also the starting point of the BMP-mediated response to topography. Figure 1: Schematic figure of the topography-induced osteogenesis. Some pathways and proteins are shown to be related to this activation, including MAPK pathway, Focal Adhesion Kinase (FAK) activation, BMPs and the Wnt/b-catenin pathway. An osteogenic topography (OT) can activate these pathways in several ways, such as: (1) in the MAPK pathway, activation occurs from the connection to integrins, leading to activation and translocation to the nucleus of p38 and ERK \(\gamma_{2}\); (2) BMPs, when connecting to a BMP receptor, trigger the activation and translocation of SMADs; (3) and the Wnt protein complex coupled to its specific receptor trigger the accumulation of beta-catenin and its translocation to the nucleus. Other forms of topography-induced osteogenic phenotype are not yet fully elucidated (dotted arrow), but it is known that the activation of the FAK and beta-catenin pathways may be involved and be activated through other pathways not yet known. The Wnt pathway is another pathway heavily involved in osteoblasts' maturation [90] (Figure 1). Nanoroughness induces the expression of several Wnt ligands including Wnt3a, Wnt5a and Wnt11, as well as Wnt-related transcription factors such as NFATs and the Wnt receptors of the Frizzled family [87; 91; 92]. As a consequence, the downstream Wnt effector beta-catenin is increasingly translocated to the nucleus in MSCs cultivated over rough surfaces [91; 93]. In all those conditions, the increased signaling through Wnt and beta-catenin correlates with expression of osteogenic markers and calcium deposition. Consistently, some studies using the MG63 osteoblast cell line cultivated on nanotextured titanium reported the diminution of the Wnt inhibitor DKK2, whereas exogenous addition of the inhibiting factor DKK1 downregulates the downstream expression of the osteogenic markers ALP, BMP and Collagen I [92]. As well as the BMP signaling, it is likely that integrin-mediated adhesion is involved at the starting point. Indeed, the silencing of the integrin \(\beta\)3 not only blocks the osteogenic behavior of MSCs cultivated on nanoroughness but also decreases the expression of elements from the Wnt pathway such as beta-catenin or Frizzled 5 [77].