Different Lrp5-HBM mutations induce magnitude- and surface-selective effects that are not explained by receptor...

The Wnt co-receptor LRP5 plays a crucial role in skeletal physiology. Homozygous loss-of-function mutations in the Lrp5 gene produce severe osteopenia, whereas certain heterozygous point mutations produce extreme high bone mass (HBM). Different HBM-causing missense mutations are associated with variable effect magnitudes and sites of action in the skeleton. It is currently unknown whether the inclusion of 2 different HBM-causing mutations (eg, p.G171V and p.A214V), moved into the same genome, would potentiate bone mass beyond either mutation alone by manifesting their individual and divergent osteoanabolic effects. Further, a proposed unique HBM-causing mechanism of action for the G171V receptor was tested, which has poor plasma membrane localization and enriched intracellular presence. Conditional Lrp5-G171V and Lrp5-A214V were crossed to conditional mice deficient in the Wntless (Wls) gene; these mice are severely osteopenic and have impaired Wnt secretion but normal levels of intracellular (pre-released) Wnt. We tested whether the G171V mutation (which promotes intracellular localization of the receptor) would improve the skeletal phenotype induced by Wls deficiency, whereas the A214V mutation (which exhibits WT levels of plasma membrane localization) would not. The results suggest that the presence of both HBM mutations (Lrp5G171V/A214V) does not confer additional benefits to skeletal properties beyond the homozygous presence of individual HBM-causing receptors (Lrp5G171V/G171V or Lrp5A214V/A214V). Further, the severe osteopenia induced by Wls loss in bone was not affected by co-expression of the G171V mutant receptor, suggesting that intracellular retention of the G171V mutant receptor is unlikely to be the mechanism of enhanced skeletal properties in G171V mice and patients. The global knock-in studies suggest that therapeutic strategies aimed at mimicking the conformational changes in the LRP5 receptor should model the A214V (alone) rather than G171V mutations for maximizing fracture resistance. Further, the conditional studies indicate that the conundrum of G171V's ability to produce an HBM phenotype, despite very low plasma membrane expression/abundance, remains elusive.