Fibrillin-1 and -2 differentially modulate endogenous TGF-β and BMP bioavailability during bone formation

Area of Science:

• Biochemistry and Molecular Biology
• Skeletal Biology and Bone Remodeling
• Extracellular Matrix Biology

Background:

• Extracellular regulation of transforming growth factor-beta (TGF-β) family signaling is crucial for organogenesis and tissue repair.
• Fibrillins, structural components of extracellular microfibrils, are implicated in TGF-β bioavailability.
• The specific roles of fibrillin-1 and fibrillin-2 in bone formation and remodeling remain incompletely understood.

Purpose of the Study:

• To investigate the differential roles of fibrillin-1 and fibrillin-2 in regulating TGF-β and bone morphogenetic protein (BMP) bioavailability in bone.
• To elucidate the impact of fibrillin deficiency on osteoblast maturation and bone formation.
• To understand the molecular mechanisms by which microfibrils influence skeletal development.

Main Methods:

• Analysis of fibrillin-2-null (Fbn2(-/-)) and fibrillin-1-null (Fbn1(-/-)) mouse models.
• In vivo assessment of bone mass and bone formation rates.
• In vitro studies using cultured osteoblasts to evaluate cell maturation and signaling pathway activation (TGF-β, BMP).

Main Results:

• Fbn2(-/-) mice exhibit low bone mass, reduced bone formation, and impaired osteoblast maturation.
• The Fbn2(-/-) phenotype is linked to aberrant activation of latent TGF-β, suppressing osterix and collagen I expression.
• Fbn1(-/-) osteoblasts show altered TGF-β activation but faster maturation due to increased BMP availability; microfibrils do not directly support mineralization.

Conclusions:

• Extracellular microfibrils, specifically fibrillin-1 and -2, are critical regulators of bone formation.
• Differential regulation of TGF-β and BMP signaling by fibrillins controls osteoblast differentiation and bone accrual.
• These findings highlight microfibrils as key modulators of skeletal homeostasis through growth factor bioavailability.