The role of muscle loading on bone (Re)modeling at the developing enthesis

Area of Science:

• Musculoskeletal biology
• Bone and cartilage research
• Developmental biology

Background:

• Muscle forces are crucial for skeletal development and maintenance.
• Impaired musculoskeletal function and bone malformation result from unloading due to altered bone remodeling.
• The muscle-bone interface, or enthesis, is critical for load transmission and skeletal integrity.

Purpose of the Study:

• To investigate the impact of muscle unloading on enthesis development and bone formation.
• To determine if suppressing osteoclast activity can rescue unloading-induced bone defects at the enthesis.
• To explore the dose-dependent effects of alendronate on bone remodeling in unloaded conditions.

Main Methods:

• Botulinum toxin A was used to induce unilateral rotator cuff muscle paralysis in neonatal mice.
• Control groups included contralateral saline-injected shoulders and normal mice.
• Bone formation was assessed using fluorescent bone labeling and histomorphometry.
• Micro computed tomography evaluated bone volume and architecture.
• Osteoclast surface was quantified using tartrate-resistant acid phosphatase staining.

Main Results:

• Muscle unloading delayed endochondral ossification initiation but did not reduce bone formation rate.
• Unloaded entheses exhibited significant defects in bone volume and trabecular architecture.
• Alendronate treatment partially rescued these defects, with a dose-dependent effect.
• Bone formation rate increased with alendronate dosage across all loading conditions.

Conclusions:

• Unloading-induced bony defects at the enthesis are primarily driven by sustained high osteoclast activity.
• Alendronate effectively mitigates these defects, suggesting its therapeutic potential.
• These findings have implications for treating conditions like neonatal brachial plexus palsy, which cause muscle unloading and enthesis abnormalities.