Activation of Wnt Signaling by Mechanical Loading Is Impaired in the Bone of Old Mice

Abstract

Aging diminishes bone formation engendered by mechanical loads, but the mechanism for this impairment remains unclear. Because Wnt signaling is required for optimal loading-induced bone formation, we hypothesized that aging impairs the load-induced activation of Wnt signaling. We analyzed dynamic histomorphometry of 5-month-old, 12-month-old, and 22-month-old C57Bl/6JN mice subjected to multiple days of tibial compression and corroborated an age-related decline in the periosteal loading response on day 5. Similarly, 1 day of loading increased periosteal and endocortical bone formation in young-adult (5-month-old) mice, but old (22-month-old) mice were unresponsive. These findings corroborated mRNA expression of genes related to bone formation and the Wnt pathway in tibias after loading. Multiple bouts (3 to 5 days) of loading upregulated bone formation–related genes, e.g., Osx and Col1a1, but older mice were significantly less responsive. Expression of Wnt negative regulators, Sost and Dkk1, was suppressed with a single day of loading in all mice, but suppression was sustained only in young-adult mice. Moreover, multiple days of loading repeatedly suppressed Sost and Dkk1 in young-adult, but not in old tibias. The age-dependent response to loading was further assessed by osteocyte staining for Sclerostin and LacZ in tibia of TOPGAL mice. After 1 day of loading, fewer osteocytes were Sclerostin-positive and, corroboratively, more osteocytes were LacZ-positive (Wnt active) in both 5-month-old and 12-month-old mice. However, although these changes were sustained after multiple days of loading in 5-month-old mice, they were not sustained in 12-month-old mice. Last, Wnt1 and Wnt7b were the most load-responsive of the 19 Wnt ligands. However, 4 hours after a single bout of loading, although their expression was upregulated threefold to 10-fold in young-adult mice, it was not altered in old mice. In conclusion, the reduced bone formation response of aged mice to loading may be due to failure to sustain Wnt activity with repeated loading.

Old age and the associated impairment of bones' adaptation to loading are associated with transcriptomic changes in cellular metabolism, cell-matrix interactions and the cell cycle

“…Detailed temporal analysis in the present study reinforces the conclusion that loading responses in the old are more transient than in the young, but this effect is unlikely to be limited to Wnt signalling, as indicated by more persistent activation of several of the loading responsive pathways in the SPEED database analysed in the young. The findings in the present study that a smaller overall number of genes are regulated by loading in the old than in the young and that bone-related genes such as Bglap1 and Col1a1 are suppressed at early time points selectively in the young, are also consistent with the findings of the Holguin study (Holguin et al, 2016). Differences between the studies, potentially due to differences in the loading protocols used and time points assayed, include differences in the Wnt ligands found to be regulated by loading.…”

“…Similar acute activation of Wnt signalling following a single episode of tibial loading has recently also been observed in a study by Holguin et al (2016) who concluded that “old mice have a normal Wnt response after a single loading bout” based on analysis of sclerostin down-regulation and TOPGAL reporter activation. In the Holguin study (Holguin et al, 2016), transcriptional analysis of tibiae from young and aged mice following repeated bouts of loading showed that activation of the Wnt pathway is more transient in the old and whereas young mice repeatedly (or persistently) activated Wnt signalling following repeated bouts of loading, old mice did not.…”

“…In the Holguin study (Holguin et al, 2016), transcriptional analysis of tibiae from young and aged mice following repeated bouts of loading showed that activation of the Wnt pathway is more transient in the old and whereas young mice repeatedly (or persistently) activated Wnt signalling following repeated bouts of loading, old mice did not. Detailed temporal analysis in the present study reinforces the conclusion that loading responses in the old are more transient than in the young, but this effect is unlikely to be limited to Wnt signalling, as indicated by more persistent activation of several of the loading responsive pathways in the SPEED database analysed in the young.…”

“…A further microarray study compared gene expression patterns in transiliac bone biopsies from control and osteoporotic patients (Jemtland et al, 2011), reporting differences in the expression of the Wnt pathways antagonists (Sost, Dkk1) and bone matrix proteins (MEPE, MMP13). More recently, transcriptomic analysis of the response to loading in young versus old mice revealed blunted activation of canonical Wnt signalling, a potently osteogenic pathway, in the old following repeated bouts of loading (Holguin et al, 2016). …”

Exploiting the WNT Signaling Pathway for Clinical Purposes

“…Recently there has been a focus on the role of the pathway with respect to aging associated changes in bone. Holguin et al [45] have shown impaired activation of β-catenin signaling in osteocytes in aged mice and Farr et al [46] have suggested that aging in human females is associated with changes in Wnt signaling. Thus, the role of β-catenin in skeletal homeostasis may change with age, and differences between sexes clearly exist.…”