OBJECTIVES: Molecular events occurring in the bone marrow microenvironment of an immobilized mouse limb after Botulinum toxin (BTX) injection haven't been characterized. BTX injection induces a localized disuse in which the tissue events have well been characterized.

METHODS: BTX injection was performed in the right quadriceps; saline injection in the left side was used as control. Mice were sacrificed at 0, 7, 14, 21 and 28 days; tibias were used for microCT analysis; bone marrow from femurs for RT-PCR analysis.

RESULTS: MicroCT revealed bone loss and microarchitectural damages on the immobilized side as from 7d; cortical area tended to be lower on the immobilized limb at 28d. Gene expression of formation factors was altered as from 7 days post-BTX: alkaline phosphatase, Tgfβ1, Lrp5, Sfrp2. Only Sfrp2 and Lrp5 were maintained altered until 28d. Expression of Dkk1 increased from 21d and represented a late inhibitor of formation. Gene expression of resorption markers increased as from 7d (Rankl, Tracp, Il1α, Il1β and Il6) and was maintained until 28d for Tracp and Il6.

CONCLUSION: A localized disuse induces rapid modifications in the bone marrow gene expression leading to bone loss due to an early decrease of formation associated with an increase in resorption.