Divergent activities of osteogenic BMP2, and tenogenic BMP12 and BMP13 independent of receptor binding affinities

Abstract

Ectopic expression of recombinant human bone morphogenetic protein 2 (rhBMP2) induces osteogenesis, while ectopic expression of rhBMP12 and rhBMP13 induces the formation of tendon-like tissue. Despite their different in vivo activities, all three ligands bound to the type I bone morphogenic protein receptors (BMPRs), activin receptor-like kinase (ALK)-3 and ALK6, and to the type II BMPRs, activin receptor type-2A, activin receptor type-2B, and BMPR2, with similar affinities. Treatment of C3H10T1/2 cells with rhBMP2 activated SMAD signaling and induced expression of osteoblast markers including osteocalcin mRNA (Ocn). In contrast, treatment with rhBMP12 or rhBMP13 resulted in a dose-dependent induction of a tendon-specific gene (Thbs4) expression with no detectable activation of SMAD 1, 5, and 8. Differential regulation of Thbs4 and Ocn has potential utility as an in vitro biomarker for induction of tenogenic signaling. Such an assay also permits the ability to distinguish between the activities of different BMPs and may prove useful in studies on the molecular mechanisms of BMP tenogenic activity.

Loss of BMPR2 leads to high bone mass due to increased osteoblast activity

“…When BMPR2 is reduced or absent, increased BMP utilization of ACVR2A/B occurs at the expense of activin signaling. Although unexpected, this finding is supported by the fact that, in general, the affinity of BMPs for ACVR2A/B is in the same range as the affinity for BMPR2 (Allendorph et al, 2006;Berasi et al, 2011;Daly and Hearn, 2006;Greenwald et al, 2003;Greenwald et al, 2004;Heinecke et al, 2009;Hu et al, 2010;Isaacs et al, 2010;Kirsch et al, 2000;Knaus and Sebald, 2001;Koncarevic et al, 2010;Rosenzweig et al, 1995;Sako et al, 2010;Sengle et al, 2008), and BMPs also possess a flexible mode of receptor complex assembly, which might enhance their ability to compete with activins that have only a single mode of complex assembly (Hinck, 2012). Because bone cells have abundant type 1 BMP receptors (ALK2, ALK3 and ALK6, BioGPS Database), preformed receptor complexes might also be biased toward BMP binding.…”

BMPs are mediators in tissue crosstalk of the regenerating musculoskeletal system

“…All three ligands show similar affinity to the BMP type I receptors BMPRIa and BMPRIb and to the type II receptors ACTRIIa, ACTRIIb and BMPRII. Treatment of C3H10T1/2 with recombinant BMP2 induces Smad phosphorylation and the expression of osteogenic markers such as osteocalcin, whereas recombinant human (rh) BMP12 or BMP13 administration results in the expression of specific tenogenic genes such as thrombospondin 4 without the activation of Smads (Berasi et al 2011); this clearly demonstrates that the activation of the same receptors by different BMPs activates both Smad and non-Smad signaling to enable manifold differentiation processes. The current literature emphasizes Smad8 as a tendon-specific Smad; however, the way that the mechanism of action is regulated remains unclear.…”

The past, present and future in scaffold-based tendon treatments

“…Given there is currently no sufficiently effective pharmaceutical-based therapy, the use of more potent biological molecules was proposed as an alternative strategy. increases revascularisation of repairing tendon tissue, improving overall healing [169]; plateletderived growth factor (PDGF) has beneficial effects on the functional repair of tendon tissue in the canine model, increasing tendon glide, but not mechanical properties, over a 42 day period [170]; basic fibroblast growth factor (bFGF) stimulates both MSC proliferation and differentiation towards tenogenic lineage, leading to increased expression of tendon specific ECM proteins and increased collagen production from cells [171]; bone morphogenic protein 12 (BMP-12), also referred to as growth differentiation factor 7 (GDF-7) induces both in vitro and in vivo tenogenesis of MSCs in both human and equine cells [172][173][174]; BMP-13 (GDF-6) induces an increase in the expression of tendon specific proteins in rat MSCs along with increasing the characteristic wave like pattern found in tendon histological samples after 14 days implantation in a rat Achilles defect model [175]; BMP-14 (GDF-5) reduces adhesion formation between tendons and surrounding tissues, improving overall function and recovery [176]; early growth response protein 1 (EGR1) directs tendon differentiation in rat MSCs and improve tendon healing in a rat Achilles tendon injury model [177]; and transforming growth factor-β (TGF-β) is highly influential in the recruitment and maintenance of TC progenitor cells during injury [178]. While these growth factors have demonstrated efficacy, as assessed by increased cellular migration, matrix production and matrix mechanical properties over a short period of time (up to around 8 weeks), little difference has been documented in long term tissue integration, matrix composition and overall tissue strength over control groups [177,178].…”