The response of the skeleton to loading appears to be mediated through the activation of the Wnt/β-catenin signaling pathway and osteocytes have long been postulated to be the primary mechanosensory cells in bone. To examine the kinetics of the mechanoresponse of bone and cell types involved in the in vivo, we performed forearm loading of 17-week-old female TOPGAL mice. β-catenin signaling was observed only in embedded osteocytes, not osteoblasts, at 1 hour post loading, spreading to additional osteocytes and finally to cells on the bone surface by 24 hrs. This early activation at 1 hour appeared to be independent of receptor (Lrp5/6) mediated activation as it occurred in the presence of the inhibitors sclerostin and/or Dkk1. The COX-2 inhibitor, Carprofen, blocked the activation of β-catenin signaling and decline in sclerostin positive osteocytes post-loading implying an important role for prostaglandin. In vitro, PI3K/Akt activation was shown to be required for β-catenin nuclear translocation downstream from prostaglandin in MLO-Y4 osteocyte-like cells supporting this mechanism. Downstream targets of β-catenin signaling, sclerostin and Dkk1, were also examined and found to be significantly down regulated in osteocytes in vivo at 24 hours post-loading. The pattern of initially activated osteocytes appeared random and in order to understand this heterogeneous expression, a novel finite element model of the strain field in the ulna was developed, which predicts highly variable local magnitudes of strain experienced by osteocytes. In summary, both in vivo and in vitro models show the rapid activation of β-catenin in response to load through the early release of prostaglandin and that strain fields in the bone are extremely heterogeneous resulting in heterogeneous activation of the β-catenin pathway in osteocytes in vivo.

The osteocyte is hypothesized to be the mechanosensory cell in bone. However, osteoblastic cell models have been most commonly used to investigate mechanisms of mechanosensation in bone. Therefore, we sought to determine if differences might exist between osteocytic and osteoblastic cell models relative to the activation of β-catenin signaling in MLO-Y4 osteocytic, 2T3 osteoblastic and primary neonatal calvarial cells (NCCs) in response to pulsatile fluid flow shear stress (PFFSS). β-catenin nuclear translocation was observed in MLO-Y4 cells at 2 and 16 dynes/cm 2 PFFSS, but only at 16 dynes/cm 2 in the 2T3 or NCC cultures. MLO-Y4 cells released high amounts of PGE 2 into the media at all levels of PFFSS (2-24 dynes/cm 2 ) and we observed a biphasic pattern of relative to the level of PFFSS. In contrast PGE 2 release by 2T3 cells was only detected during 16 and 24 dynes/ cm 2 PFFSS starting at >1 hour and never reached the levels produced by MLO-Y4 cells. Exogenously added PGE 2 was able to induce β-catenin nuclear translocation in all cells suggesting that the differences between the cell lines observed for β-catenin nuclear translocation was associated with the differences in PGE 2 production. To investigate a possible mechanism for the differences in PGE 2 release by MLO-Y4 and 2T3 cells we examined the regulation of Ptgs2 (Cox-2) gene expression by PFFSS. 2T3 cell Ptgs2 mRNA levels at both 0 and 24 hours after 2 hours of PFFSS showed biphasic increases with peaks at 4 and 24 dynes/cm 2 and 24 hour levels were higher than 0 hour levels. MLO-Y4 cell Ptgs2 expression was similarly biphasic; however at 24 hours post flow Ptgs2 mRNA levels were lower. Our data suggest significant differences in the sensitivity and kinetics of the response mechanisms of 2T3 and neonatal calvarial osteoblastic versus MLO-Y4 osteocytic cells to PFFSS. Furthermore our data support a role for PGE 2 in mediating the activation of β-catenin signaling in response to fluid flow shear stress.

Understanding the role that the Wnt signaling pathways play in the regulation of bone metabolism offers great promise for the developmental of new paradigms and pharmaceutical strategies for the treatment of various diseases such as osteoporosis, rheumatoid arthritis and osteoarthritis. Progress in this regard has already been made.

The Nubian dragon tree Dracaena ombet, which is categorized as Endangered on the IUCN Red List, is found on the highest slopes of Gebel Elba National Park in Egypt, with scattered populations in Sudan, Djibouti, Ethiopia and Somalia. The Gebel Elba population is threatened by drought. Surveys were conducted in the Park to assess the condition and document the distribution of the species to prepare a baseline for conservation efforts. Eight sites were surveyed during 2007-2009: trees were tagged and their locations were recorded using a global positioning system, and tree density, diameter at breast height and population status were estimated. Of 353 trees recorded only 46% (161 individuals) were alive and only 27% (96 individuals) were in a healthy condition. Only 1% (2 individuals) were young trees, indicating a low regeneration level. Field-based observations suggest that 80% of the D. ombet population in Gebel Elba may soon be extinct. A conservation action plan is needed for this flagship species in Egypt and throughout its range.