Akhter et al. [17] report that in cortical bone female mice with the Lrp5HBM+ genotype showed greater increases in periosteal bone formation rates than WT controls in response to
5 days of tibial four-point bending. The preliminary data from Hackfort et al., who axially loaded the tibia of female Lrp5−/− mice [29], suggest that the absence of Lrp5 has no effect on the responsiveness of cortical bone to mechanical loading. These latter results are inconsistent with the data we generated for male mice, though the comparison to our female data is inconclusive. Our findings on male Lrp5−/− mice are consistent with the findings of Sawakami et al. who report that after 3 days MEK pathway of sequential loading of the ulna, male and female Lrp5−/− mice show an 88 to 99% lower response to loading in the cortical bone than WT controls [16]. Sawakami et al. also reported that male and
female Lrp5−/− mice are equally capable as WT+/+ mice at recruiting osteoblasts in response to a single period of mechanical loading and that absence of functional Lrp5 had little effect on early mediators of mechanical signalling, such as ATP Selleckchem RG-7204 and PGE2 release or ERK1/2 activation, that are detectable within seconds or minutes of mechanical stimulation. They attributed the deficiency of the fully osteogenic adaptive responses in their study to the inability of Lrp5−/− osteoblasts to synthesise the bone matrix protein osteopontin. This would explain the significantly reduced osteogenic response in male Lrp5−/− mice and supports the notion that the canonical Wnt signalling Acyl CoA dehydrogenase has a role in bone cells’ response to mechanical loading. However, other data suggest that the mechanism might not be so clear cut as indicated by Kato’s finding
that Wnt-signalling still partially occurs in osteoblasts from Lrp5−/− mice [15], by Robling’s finding that the sclerostin antibody can improve bone mass whether Lrp5 is present or not [30], or by the in vitro findings by Sunters et al. [31] and Case et al. [1] showing that during the early phase of the strain response, activation of the chief effector of the canonical Wnt pathway (β-catenin) is not contingent on Wnts interacting with the Lrp5 receptor. Thus, the required post-loading pathways in bone cells may also depend on other receptors, possibly Lrp4 [32] or Lrp6 [2]. The data we present here, at least in male mice, are consistent with the differences in bone mass between normal WT mice and those that lack Lrp5 function, being due to an altered responsiveness to bone loading. Karsenty and colleagues attribute the low bone mass of the Lrp5−/− related phenotype to the effect of Lrp5 on serotonin secretion in the duodenum [33]. However, this finding has not been replicated [34]. The Lrp5−/− mice in our study, as in that of Karsenty and colleagues, may have had high serotonin levels. However, Warden et al.