Modest Effects of Osteoclast-Specific ERα Deletion after Skeletal Maturity

Area of Science:

• Endocrinology
• Bone Biology
• Skeletal Physiology

Background:

• Estrogen is vital for bone mass regulation, but its cellular mechanisms in bone are not fully understood.
• Estrogen receptor alpha (ERα) is the primary mediator of estrogen's effects on the skeleton.
• Previous studies on ERα deletion in bone cells had limitations, hindering clear understanding of ERα's role in adult bone.

Purpose of the Study:

• To investigate the role of ERα in osteoclasts using a novel inducible knockout mouse model.
• To compare the skeletal effects of ERα deletion in osteocytes versus osteoclasts in adult mice.
• To elucidate the specific cellular targets of estrogen action in maintaining adult bone homeostasis.

Main Methods:

• Generated and characterized mice with tamoxifen-inducible ERα deletion in osteoclasts (ERαΔOcl mice) using the Cathepsin K promoter.
• Administered tamoxifen to adult mice to induce ERα deletion specifically in osteoclasts.
• Assessed trabecular bone volume, osteoclast numbers, and serum C-terminal telopeptide (CTX) levels.

Main Results:

• Adult female ERαΔOcl mice exhibited a borderline reduction in trabecular bone volume and increased serum CTx levels, indicating enhanced osteoclast activity.
• No significant change in osteoclast numbers was observed in ERαΔOcl mice.
• These findings contrast with constitutive osteoclast-specific ERα deletion, suggesting timing and cell-type specificity are critical.

Conclusions:

• Estrogen action via ERα in osteocytes is more critical for maintaining adult bone mass than action via osteoclasts.
• Inducible deletion of ERα in adult osteoclasts has less dramatic skeletal effects compared to constitutive deletion from conception.
• These results highlight the differential importance of ERα in specific bone cell types for skeletal health in adulthood.