Unique Distal Enhancers Linked to the Mouse Tnfsf11 Gene Direct Tissue-Specific and Inflammation-Induced Expression...

Area of Science:

• Molecular Biology
• Immunology
• Skeletal Biology

Background:

• Receptor activator of nuclear factor κB ligand (RANKL) is vital for diverse physiological processes.
• Previous work identified 10 distal RANKL enhancers, including the multifunctional RL-D5.
• RL-D5 deletion impacts skeletal and lymphoid RANKL, leading to high bone mass.

Purpose of the Study:

• To investigate the physiological roles and lineage specificity of two additional RANKL enhancers: RL-D6 and RL-T1.
• To understand the regulatory mechanisms of RANKL expression in different cell types and under various conditions.

Main Methods:

• Generation of knockout mouse models lacking specific RANKL enhancers (RL-D6, RL-T1).
• Analysis of RANKL expression in skeletal and lymphoid tissues.
• Assessment of bone mineral density and lymphocyte counts.
• Evaluation of RANKL induction by inflammatory stimuli (oncostatin M, lipopolysaccharide) ex vivo and in vivo.

Main Results:

• Lack of RL-D6 or RL-T1 did not affect skeletal RANKL or bone density in adult mice.
• RL-T1 is the primary regulator of lymphocyte RANKL, with its absence causing drastically low levels and reduced circulating soluble RANKL.
• RL-D5 and RL-D6, but not RL-T1, are essential for inflammation-induced RANKL expression in osteocytes and osteoblasts.
• RL-D6 deletion did not affect RANKL induction during secondary hyperparathyroidism or lactation.

Conclusions:

• RL-T1 is the major regulator of lymphocyte RANKL, highlighting the role of lymphocytes as a source of circulating soluble RANKL.
• RL-D5 and RL-D6 cooperatively control inflammation-mediated RANKL induction in bone cells.
• RANKL expression is complex and cell type-specific, requiring distinct enhancers for regulation in different lineages and physiological contexts.