Topographical Cues Of Direct Metal Laser Sintering Titanium Surfaces Facilitate Osteogenic Differentiation Of Bone Marrow Mesenchymal Stem Cells Through Epigenetic Regulation

ObjectivesHierarchical micro/nanoscale topography of intraosseous implants is similar to that of natural bone tissue and the extracellular microenvironment and thus has pronounced potential to modulate cell-surface interactions.However,an examination of the cell response to topographical cues and an analysis of the underlying mechanisms are still lacking.Here,we fabricated titanium surfaces with a hierarchical micro/nanoscale topography using direct metal laser sintering(DMLS)technology.And we investigated the role of topographical cues of DMLS titanium surfaces in osteogenic differentiation of bone marrow mesenchymal stem cells(BMSCs),as well as the possible underlying epigenetic mechanism.Methods1.Three groups of titanium specimens were prepared,including DMLS group,sandblasted,large-grit,acid-etched(SLA)group and smooth titanium(Ti)group.Then the surface characterization of surface morphology,hydrophilicity and protein adsorption on three groups of titanium surfaces were analysed.Field emission scanning electron microscope(FE-SEM)was used to observe surface morphology of titanium surfaces under different magnification;Drop contact angle measuring instrument was used to detect the difference of the contact angle of the titanium surfaces;Protein adsorption test was used to analyze the protein adsorption capacity of each group on the titanium surface.2.BMSCs were cultured on discs followed by surface characterization,and cell adhesion,activity and proliferation were detected at certain time points.The morphology and number of cells adhered to surface were observed by FE-SEM and laser confocal microscope(CLSM);the proliferation of the cells on the surface of each group was analyzed by CCK-8;After AO/EB staining,the activity and number of the proliferating cells on the titanium surface were observed by laser confocal microscope.3.In vivo and in vitro studies were performed to evaluate the effect of titanium surface morphology on osteogenic differentiation of BMSCs.Alkaline phosphatase activity of cells cultured on titanium surface in vitro was detected by alkaline phosphatase Kit(ALP-Kit);Real-time quantitative PCR was used to detect the mRNA level of osteogenic-related gene Runx2 and Osteocalcin(OC);The protein expression level of Runx2 was detected by immunofluorescence and western blot;An ectopic osteogenesis animal model was used to evaluate the osteogenic differentiation of BMSCs on the surface of each group.4.To investigate the underlying epigenetic mechanisms of surface morphology affecting osteogenic differentiation of BMSCs cultured on the SLA and DMLS titanium surface,immunofluorescence and western blot were performed to evaluate global level of H3K4me3 during osteogenesis.And the H3K4me3 and H3K27me3 level at the promoter area of the osteogenic gene Runx2 were detected by ChIP assay.Results1.The titanium surfaces with different surface morphology were successfully prepared by mechanical polishing,SLA and DMLS technology.DMLS surfaces with the hierarchical micro/nanoscale topography were observed by FE-SEM;Drop contact angle measurements showed that DMLS titanium surface is more hydrophilic than SLA and Ti surfaces;Protein adsorption assays revealed that DMLS surface has greater protein adsorption ability than the SLA and Ti surfaces.2.The results of FE-SEM and CLSM of BMSCs cultured on the titanium surface revealed that DMLS surface with hierarchical micro/nanoscale topography shows better pseudopodium extension and cell adhesion performance than SLA and smooth Ti surfaces;The CCK-8 growth curves and AO/EB staining results showed BMSCs have great activity on the titanium surface of each group,and DMLS group has a relatively large number of cells,which is more conducive to cell proliferation.3.Both in vitro and in vivo studies demonstrated that the DMLS titanium surface with hierarchical micro/nanoscale topography is more favorable for the osteogenic differentiation of BMSCs than SLA and smooth Ti surfaces.4.Immunofluorescence and CHIP results showed the osteogenesis-associated gene expression in BMSCs is efficiently induced by a rapid H3K27 demethylation and increase in H3K4me3 levels at gene promoters upon osteogenic differentiation on DMLS titanium surface.ConclusionsTopographical cues of DMLS titanium surfaces have greater potential for the induction of osteogenic differentiation of BMSCs than SLA and smooth titanium surfaces both in vitro and in vivo.A potential epigenetic mechanism is that the appropriate topography allows rapid H3K27 demethylation and an increased H3K4me3 level at the promoter region of osteogenesis-associated genes during the osteogenic differentiation of BMSCs.