Local versus systemic control of bone and skeletal muscle mass by components of the transforming growth factor-β signaling pathway

Abstract

Skeletal muscle and bone homeostasis are regulated by members of the myostatin/GDF-11/activin branch of the transforming growth factor-β superfamily, which share many regulatory components, including inhibitory extracellular binding proteins and receptors that mediate signaling. Here, we present the results of genetic studies demonstrating a critical role for the binding protein follistatin (FST) in regulating both skeletal muscle and bone. Using an allelic series corresponding to varying expression levels of endogenous Fst, we show that FST acts in an exquisitely dose-dependent manner to regulate both muscle mass and bone density. Moreover, by employing a genetic strategy to target Fst expression only in the posterior (caudal) region of the animal, we show that the effects of Fst loss are mostly restricted to the posterior region, implying that locally produced FST plays a much more important role than circulating FST with respect to regulation of muscle and bone. Finally, we show that targeting receptors for these ligands specifically in osteoblasts leads to dramatic increases in bone mass, with trabecular bone volume fraction being increased by 12- to 13-fold and bone mineral density being increased by 8- to 9-fold in humeri, femurs, and lumbar vertebrae. These findings demonstrate that bone, like muscle, has an enormous inherent capacity for growth that is normally kept in check by this signaling system and suggest that the extent to which this regulatory mechanism may be used throughout the body to regulate tissue mass may be more significant than previously appreciated.

Functional replacement of myostatin with GDF-11 in the germline of mice

“…One tissue that was substantially affected by replacing MSTN with GDF-11 was bone, with Mstn Gdf11/Gdf11 mice exhibiting significantly decreased BV/TV, trabecular number, trabecular thickness, and bone mineral density, at least in males. Although there are conflicting reports as to the role of GDF-11 in regulating bone, the phenotype of Mstn Gdf11/Gdf11 mice is consistent with the effects that have been observed upon blocking this pathway pharmacologically using decoy forms of activin type 2 receptors [68][69][70][71][72][73] or genetically targeting Acvr2/Acvr2b [74] or Alk4/Alk5 [45] in osteoblasts. The decreased bone mass seen in Mstn Gdf11/Gdf11 mice would be consistent with the possibility that GDF-11 made from the hybrid precursor protein is slightly more active than endogenous MSTN, as suggested by the slight decreases in muscle mass seen in the knock-in mice.…”

“…The role of GDF-11 in regulating bone homeostasis has been controversial, with one study reporting a decrease in bone density upon exogenous administration of GDF-11 to adult mice [66] and another study reporting decreased bone mass not only in newborn Gdf11 null mice but also in young adult mice in which tamoxifeninducible cre-mediated recombination was used to target a Gdf11 flox allele [67]. Whatever specific role GDF-11 may play in bone, targeting the MSTN/GDF-11/activin A signaling pathway, either pharmacologically using decoy forms of ACVR2 [68] or ACVR2B [69][70][71][72][73] or genetically by targeting Acvr2/Acvr2b [74] or Alk4/Alk5 [45] in osteoblasts, has been shown to cause significant increases in bone mass and density. To assess possible effects of the knock-in allele on bone, we first assessed bone density by DXA.…”

“…MSTN is regulated extracellularly by other binding proteins as well, including follistatin (FST) [36], FSTL-3 [41], GASP-1, and GASP-2 [42]. Genetic studies have shown that loss of FST [43][44][45] and/ or GASP-1/GASP-2 [46] results in decreases in muscle mass and fiber type shifts consistent with their roles in inhibiting MSTN in vivo. These binding proteins are also capable of inhibiting GDF-11, and mice lacking FST [43] or GASP-1/GASP-2 [46] also exhibit axial patterning defects consistent with increased activity of GDF-11.…”

Myostatin and its Regulation: A Comprehensive Review of Myostatin Inhibiting Strategies

“…Follistatin Follistatin (FST) is an extracellular cysteine-rich glycoprotein, which is structurally dissimilar to TGF-β family members, and it has been established that FST interrupts the activity of MSTN by binding to it and preventing MSTN binding to its receptor (Lee and McPherron, 2001;Liu et al, 2021). In vivo studies have reported that the overexpression of this glycoprotein has hypertrophic effects on mouse muscles similar to those observed in MSTN null mice (Winbanks et al, 2012).…”

“…In vivo studies have reported that the overexpression of this glycoprotein has hypertrophic effects on mouse muscles similar to those observed in MSTN null mice ( Winbanks et al, 2012 ). Moreover, a homozygous mutation in the FST gene reduces muscle mass, which suggests it plays an important role in the regulation of myogenesis ( Lee et al, 2010 ; Liu et al, 2021 ).…”