Furthermore, other signaling cascades can modulate osteoblast activity. One of these is induced by PTH and its related peptide PTHrP. PTH(rP) signals via the
7‐transmembrane G protein coupled receptor PTHR1 and upon ligand binding, several intracellular signal- ing pathways can be activated, including the cAMP/
protein kinase A (PKA) and PKC pathways. Interestingly, whereas intermittent PTH administration induces bone formation, continuous treatment of PTH leads to bone loss. In humans, loss of function in PTH1R has been linked to Blomstrand lethal osteochondrodysplasia [35], characterized by advanced maturation and premature ossification of the skeleton. Different mechanisms have been suggested to explain the anabolic and catabolic effects of PTH; PTH may have diverse effects on the proliferation, commitment, differentiation, or apopto- sis of the osteoblasts.
Finally, also various growth factors, including IGF‐1 and FGFs, can affect osteoblast function by activating their specific receptor tyrosine kinases (RTKs). Activation of most RTKs results in activation of the phosphatidylin- ositol 3‐kinase (PI3K)‐Akt and Ras‐ERK MAP kinase pathways (Fig. 7.2). Interestingly, Akt1/Akt2 double‐
knockout mice show a phenotype resembling that of IGF‐1 receptor‐deficient mice, which includes impaired bone development [36]. Many human craniosynostosis disorders have been linked to activating mutations in FGF receptors [37]. FGFs affect both chondrogenesis and osteogenesis, and induce proliferation of immature oste- oblasts via the Ras/ERK MAPK pathway and via PKC stimulate Runx2 activity [37]. Disruption of FGFR2 sign- aling in skeletal tissues results in skeletal dwarfism and decreased bone density [38].
CONCLUSION
Because many of the signaling pathways mentioned here are activated subsequently or simultaneously, the ulti- mate effects they have on the osteoblast differentiation process is highly dependent on which signaling mole- cules are activated or inhibited, the magnitude and the duration of the responses, and the differentiation stage of the responding cells. Besides that regulation of Runx2 and Osterix activity are points of convergence of many signal transduction cascades, there is also a high degree of cross‐talk between various pathways, adding addi- tional degrees of complexity and providing further fine‐
tuning of the differentiation process. For example, TGF‐β can inhibit BMP‐induced osteoblast differentiation; how- ever, under specific conditions TGF‐β can also promote BMP‐induced osteoblast differentiation [39]. Apart from its C‐terminal phosphorylation by BMP type‐I receptors, Smad proteins can be phosphorylated by MAP kinases and GSK‐3 activated by RTKs and WNTs, resulting in cytoplasmic retention and proteosomal degradation and inhibition of signaling [5]. β‐catenin‐TCF/Lef1 can inter- act with Smad1 and ‐3 proteins to cooperate in inducing gene transcription [40], and Hh signaling is required for accurate β‐catenin‐mediated Wnt signaling in osteoblasts [18]. Thus, the combined action of the signal transduction
pathways induced by bone promoting cytokines deter- mines commitment of MSCs towards the osteoblast lineage and the efficiency of bone formation.
ACKNOWLEDGMENTS
We apologize to all authors whose primary work could not be cited owing to space constraints. DJJdG is sup- ported by the “Innovative Medizinische Forschung” of the Medical Faculty of Münster University. Bone research at the lab of PTD is supported by the LeDucq Foundation and Cancer Genomics Centre Netherlands. GSD is supported by Netherlands CardioVascular Research Initiative: the Dutch Heart Foundation, Dutch Federation of University Medical Centers, the Netherlands Organization for Health Research and Development, and the Royal Netherlands Academy of Sciences (CVON-RECONNECT).
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Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, Ninth Edition. Edited by John P. Bilezikian.
© 2019 American Society for Bone and Mineral Research. Published 2019 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/asbmrprimer
8
INTRODUCTION
BMPs were originally identified for their ability to induce ectopic bone formation [1]. Subsequent studies have demonstrated that the TGF‐β superfamily, which includes BMPs, possess pleiotropic functions. Phenotypic analy- ses of transgenic and gene knockout models reveal diverse functions of TGF‐β superfamily members and their downstream signaling effectors. These studies also emphasize the highly context‐dependent contribution of TGF‐β superfamily members in the skeletal system (Fig. 8.1). Detailed reviews of the mechanisms of signal- ing and the roles of TGF‐β and BMP pathways in skeletal tissues are available in recent comprehensive reviews [2].
This chapter summarizes some of the skeletal pheno- types found in genetically modified animals and dis- cusses how they relate to human conditions.
BASICS OF TGF‐ β AND BMP SIGNALING
The TGF‐β superfamily is the largest class of cytokines in vertebrates. A detailed review of the mechanics of signal transduction is available [2]. In brief, TGF‐β superfamily ligands signal as dimers by binding to specific serine/
threonine kinase receptors. Ligand binding triggers for- mation of heterotetrameric complexes composed of type I and type II receptors. There are two major arms: a TGF‐β/activin branch and a BMP branch. The majority of ligands in the superfamily fall into one of these branches.
TGF‐β/activin ligands bind to specific sets of type I/type
II receptors. Ligand binding triggers phosphorylation of the receptors, leading to creation of a docking site that enables phosphorylation of the intracellular proteins Smad2 and Smad3. Phosphorylated Smad2 and Smad3 then enter the nucleus, where they act as transcriptional regulators in conjunction with Smad4. Ligands in the BMP branch act through a distinct but structurally related set of receptors to activate the intracellular medi- ators Smad1, ‐5, and ‐9, which act in conjunction with Smad4. These proteins regulate expression of a distinct set of genes. In addition to these canonical pathways, both TGF‐β/activin and BMP ligands trigger a variety of noncanonical pathways. The relative contributions of Smad‐mediated and noncanonical pathways to the effects of TGF‐β/activin and BMP signaling in skeletal tissues is poorly understood.
TGF‐ β AND BONE DEVELOPMENT TG/KO phenotypes of ligands
There are three subtypes of TGF‐β in mammals (TGF‐β1,
‐2, and ‐3). These ligands bind to receptor complexes (dis- cussed later in this chapter) that activate the intracellular transducers and transcriptional regulators Smad2 and ‐3.
A second group of ligands, the activins, which bind to receptor complexes distinct from those that bind TGF‐β ligands, also activate Smad2 and ‐3. Very little is known about the role of activins in skeletal development [2].
However, previous studies have reported that TGF‐β1, ‐2, and ‐3 contribute to skeletogenesis.