REGULATION OF OSTEOBLAST MATURATION AND PROLIFERATION The conversion of mesenchymal stem cells into osteoblasts and the later maturation and proliferation are regulated by the hedgehog and Wnt signaling pathways.21,22 The Wnt family of proteins interacts with cellular surface receptors, frizzled and Lrp 5/6, inducing the canonic cytoplasmatic signaling pathway. Alternatively, the Wnt pathway can be activated by mechanical deformation of the osteoblast by external forces, via activation of cytoskeletal components,

i.e. non-canonical pathways Inhibitors,research,lifescience,medical that involve calcium flux into cells.23 Therefore, the dual ability of osteoblasts to activate the Wnt signaling, either humoral or mechanical, explains the sensitivity of these cells to mechanical stimuli and to biochemical agents (growth factors and cytokines). The extent of this dual regulation effect

is unique to osteoblasts. The hedgehog (Hh) signaling pathway functions upstream to the Wnt cellular effects, and its main role is in induction of initial Inhibitors,research,lifescience,medical maturation of MSCs toward an osteoblastic lineage.22 There are several Hh ligands that are involved in osteoblast maturation; the most investigated are Sonic hedgehog (Sh) and Indian hedgehog (Ih). These ligands release the inhibitory effect of Inhibitors,research,lifescience,medical the cell membrane protein Ptch1 on the Smo membrane protein. When uninhibited the DAPT latter induces the activation intracellular Inhibitors,research,lifescience,medical signaling pathway for activation of several genes (transcription factors) that are involved in cellular

maturation, e.g. gli1, hip1, and others.22 Therefore the Hh and Wnt ligands cause synergistic effect on MSCs’ maturation into osteoblasts. CONCLUSION Osteoblasts regulate directly the bone matrix synthesis and mineralization by their synthetic activities, and indirectly they regulate the bone resorption by paracrinic effects on osteoclasts. The overall synthetic and regulatory activities of osteoblasts govern bone tissue integrity and shape. Thus, in the development of treatment modalities for several serious pathologic conditions, e.g. osteoporosis, osteosarcoma, etc., the ability Inhibitors,research,lifescience,medical to intervene in the osteoblast metabolism, maturation, and proliferation is crucial. The above-presented humoral, mechanical, and cellular signaling pathways that regulate these activities in osteoblasts are the natural targets for the treatment intervention in pathological conditions. Abbreviations: ANT adenine nucleotide translocator; BMP2 bone morphogenic protein Histamine H2 receptor 2; BMU basic multicellular unit; FGF fibroblast growth factor; Hh hedgehog; Ih Indian hedgehog; M-CSF macrophage colony-stimulating factor; MPTP mitochondrial permeability transition pore; MSC mesenchymal stem cell; OPG osteoprotegerin; PTH parathyroid hormone; RANK receptor activator; RANKL receptor activator of nuclear factor (NF)-kappaB ligand; Sh Sonic hedgehog; TNF tumor necrosis factor; VDAC voltage-dependent anion channel.