Mechanobiology of TGFβ signaling in the skeleton

Abstract

Physical and biochemical cues play fundamental roles in the skeleton at both the tissue and cellular levels. The precise coordination of these cues is essential for skeletal development and homeostasis, and disruption of this coordination can drive disease progression. The growth factor TGFβ is involved in both the regulation of and cellular response to the physical microenvironment. It is essential to summarize the current findings regarding the mechanisms by which skeletal cells integrate physical and biochemical cues so that we can identify and address remaining gaps that could ultimately improve skeletal health. In this review, we describe the role of TGFβ in mechanobiological signaling in bone and cartilage at the tissue and cellular levels. We provide detail on how static and dynamic physical cues at the macro-level are transmitted to the micro-level, ultimately leading to regulation at each level of the TGFβ pathway and to cell differentiation. The continued integration of engineering and biological approaches is needed to answer many remaining questions, such as the mechanisms by which cells generate a coordinated response to physical and biochemical cues. We propose one such mechanism, through which the combination of TGFβ and an optimal physical microenvironment leads to synergistic induction of downstream TGFβ signaling.

FIGURE 1. Feedback loop integrating cytoskeletal tension and the TGFβ pathway

Cytoskeletal tension is dependent on many factors, including the material properties of the ECM (e.g. elastic modulus). In turn, cytoskeletal tension regulates the TGFβ pathway at several hierarchical levels, playing a role in TGFβ mRNA and protein expression and ligand activation; in receptor spatial organization and multimerization; in the choice among canonical Smad2/3 and non-canonical effectors; and in expression and function of lineage-specific transcription factors. These transcription factors bind to promoters of TGFβ-regulated lineage-specific ECM proteins which, through mechanisms that remain unclear, define the material properties of the ECM.

FIGURE 2. Proposed mechanism of interaction between physical cues and TGFβ in inducing skeletal cell differentiation

In a sub-optimal physical microenvironment (A, B), TGFβ receptors are segregated from each other at sites of adhesion. Due to lack of ideal cytoskeletal tension, integrins are unable to release activated TGFβ ligand from the ECM. This combination of cues leads to basal levels of downstream TGFβ signaling (A), unless exogenous TGFβ is added (B). Upon addition of TGFβ, TGFβ receptors away from sites of adhesions are able to bind the ligand and initiate downstream signaling (B). In an optimal physical microenvironment (C, D), the physical separation of TGFβ receptors at sites of adhesion is released. The receptors are able to bind active ligand that was released by integrin interactions with LAP (C). Addition of TGFβ to the optimal physical microenvironment leads to a synergistic induction of downstream TGFβ signaling (D).