Disease-associated TNFRSF11B variants differentially disrupt osteoprotegerin secretion and function

Area of Science:

• Biochemistry
• Genetics
• Bone Biology

Background:

• Osteoprotegerin (OPG), encoded by TNFRSF11B, is crucial for inhibiting osteoclastogenesis by acting as a decoy receptor for RANKL.
• Loss-of-function mutations in TNFRSF11B lead to bone metabolic disorders like juvenile Paget's disease, but the precise mechanisms of OPG dysfunction are not fully understood.
• Understanding how specific TNFRSF11B variants affect OPG secretion and activity is essential for elucidating disease pathogenesis.

Purpose of the Study:

• To quantitatively assess the impact of disease-associated TNFRSF11B variants on OPG secretion and its osteoclastogenesis-inhibitory activity.
• To elucidate the molecular mechanisms underlying OPG dysfunction caused by specific mutations.
• To investigate the role of endoplasmic reticulum-associated degradation (ERAD) in regulating OPG secretion.

Main Methods:

• Expressed wild-type and variant OPG proteins as Gaussia luciferase fusion constructs to measure extracellular secretion.
• Evaluated functional activity using a mouse osteoclastogenesis assay to assess OPG's inhibitory capacity.
• Investigated the role of ERAD by employing proteasome inhibition and knockdown of ERAD components (HRD1, SEL1L).

Main Results:

• Cysteine substitution variants (C65F, C87Y) showed severe secretion defects.
• Variants T76P and D182del exhibited reduced secretion and inhibitory activity, while F117L had normal secretion but impaired function.
• Truncation variants (D323fs, R333Ter) had reduced secretion but retained significant anti-RANKL activity; ERAD inhibition increased OPG secretion for wild-type and these variants.

Conclusions:

• Mutation-specific mechanisms contribute to OPG dysfunction in bone metabolic disorders.
• Endoplasmic reticulum-associated degradation (ERAD) plays a significant role in regulating OPG secretion.
• These findings provide insights into the pathogenesis of TNFRSF11B-related bone diseases and potential therapeutic targets.