Purinergic Receptor Involvement in Mesenchymal Stem Cell Differentiation Towards the Osteogenic Lineage
Bone defects after accidents and tumor surgery, or age-related bone degeneration such as osteoporosis, based on increasing life expectancy, are a main focus of current research.
Possible treatment for large and small bone defects is achieved by the use of bone grafts; surgical procedures where missing bone is replaced with autologous material or artificial, synthetic, or natural substitutes. These materials possess certain disadvantages: artificial and synthetic graft substitutes lack osteoinductive or osteogenic properties; natural osteoconductive substitutes such as bone extracellular matrix compounds purified from bovine material exhibit a risk of virus or prion transmission. The application of osteogenic materials including autologous bone marrow or bone shows limitations including site morbidity or graft availability if intended to apply for large bone defects.
In order to counteract these problems and to overcome the shortage of material, stem cells isolated from adipose tissue, a waste product of plastic surgery, provide a novel strategy. These cells have the potential to be differentiated into various lineages including osteoblasts.
Currently, processes involved during the differentiation of adipose-derived stem cells towards the osteogenic lineage are only partly understood. Therefore, the aim of this study is to examine the pathways of osteogenesis.
Since P2 purinergic receptors are known to be involved in cell proliferation, differentiation and to some extent in osteoregulation, they are investigated to explore their potential conducting osteogenesis.
P2 receptors are plasma membrane bound molecules, divided into the ionotrophic P2X and the metabotrophic P2Y subtype. The former are ligand-gated ion channels mediating influx and efflux of cations into and out of cells upon binding of extracellular ATP. The latter are G protein-coupled receptors that trigger numerous intracellular signalling cascades after activation by purine and/or pyrimidine nucleotides.
By the analysis of purinergic receptor presence possible signalling pathways that could be of major importance for the future application of autologous mesenchymal stem cells as a system for bone healing might be uncovered.
Activation of P2X receptors by extracellular ATP results in the influx of calcium (Ca2+) and sodium (Na+) ions and in the efflux of potassium (K+) ions, membrane depolarisation and the formation of a non-selective pore. Cytoplasmic calcium triggers intracellular events including the activation of ERK1/2 which results in the expression of genes involved in proliferation and differentiation among others.
P2Y receptors are activated upon ligand binding by ATP or UTP which induces possible MAPK-dependent or -independent pathways. Stimulation of PLC results in activation of tyrosine kinases, thus, the expression of c-jun or c-myc via the Ras-MAPK cascade or in an IP3-mediated release of calcium from intracellular stores coherent with initiation of calcium-dependent PKCs. Additionally, PLC-stimulation triggers the migration of CREB into the nucleus in a calcium-dependent manner or the activation of calcium-independent PKCs through DAG.
The transcription of c-fos is regulated by either MAPK-dependent mechanisms or by other events including PI3K action, both combined with relocation of the transcription factor NFkB into the nucleus. Arrows with solid lines indicate established signalling cascades. Dashed lines indicate migration into the nucleus.