Osteomacs Are Involved In Endplate Osteosclerosis Of Modic Changes Through OSM-STAT3/YAP1 Signaling Axis

Background:Low back pain(LBP)is one of the main causes of motor loss and chronic disability due to the high incidence of lumbar disc degenerative diseases.Modic changes(MCs)are strongly associated with clinical LBP,which is a kind of signal changes of the vertebral endplates and the adjacent subchondral vertebral regions seen on magnetic resonance imaging(MRI).Recent studies have shown that MCs are often associated with osteosclerosis in endplate regions,but the etiology is remaining unknown.Osteomacs,a kind of tissue macrophages,reside in osseous tissue and play an important role in bone formation and bone repair,but whether osteomacs participate in the endplate osteosclerosis in MCs remained unclear.Objective:To assess the changes of endplate subchondral bone mass and the number of osteomacs in different types of MCs.To explore the critical role of osteomacs in regulating osteogenesis in MCs and elucidate the underlying molecular mechanisms.Materials and methods:First,we collected lumbar endplate subchondral bone samples and MRI data from 7 patients with lumbar disc herniation and 23 patients with MCs.In addition,we also established a rat tail model of MCs induced by propionibacterium acnes(P.acnes).MCs within the endplate subchondral bone were confirmed 8 weeks postoperatively by MRI scan and then animals were euthanized.Next,micro-CT and immunohistochemistry were used to explore the endplate bone mass and detect osteomacs in both patient and rat samples.To further explore the functions of osteomacs in vitro,we isolated osteomacs using MACS technology and identified the isolated osteomacs using immunofluorescence staining and flow cytometry.Osteomacs-conditioned-medium was collected to detect the effects of osteomacs on osteoblasts and osteoclasts differentiation and we performed Western blot experiments to confirm osteomacs highly expressed oncostatin M(OSM).We also knocked down the expression of OSM in osteomacs by transfection with si RNA.In addition,the nuclear translocation and interaction of STAT3 and YAP1 were detected by nucleo-cytoplasmic protein separation technology and coimmunoprecipitation assay.Using Stattic and YAP1 si RNA to investigate the effects of STAT3 and YAP1 on OSM-mediated osteoblast differentiation.Finally,we verified the effect of OSM in endplate osteosclerosis in rat MCs model by intradiscal injection of antiOSM neutralizing antibody.Results:In patients' samples,all three types of MCs endplate subchondral bone presented different degrees of osteosclerosis,among which Modic type III endplate osteosclerotic phenotype was the most significant.The number of osteomacs in endplate subchondral bone increased in all types of MCs,among which the number of osteomacs in Modic type II was the largest.In rat MCs model,endplate subchondral bone mass in the bone hyperplasia area in the P.acnes group was significantly increased,where the number of osteomacs were also increased.Osteomacs-conditioned-media strongly promoted osteoblast differentiation rather than osteoclast.Osteomacs could highly express OSM,which promoted osteoblast differentiation.OSM knockdown osteomacs lost the ability of promoting osteoblast differentiation.OSM promoted tyrosine phosphorylation and nuclear translocation of STAT3 and YAP1.STAT3 phosphorylation inhibition or YAP1 knockdown attenuated OSM-mediated osteoblast differentiation.The bone mass of endplate was decreased in OSM blockade rat MCs model.Conclusion:Different degrees of endplate osteosclerosis accompanied with increased number of osteomacs in MCs.Osteomacs promoted osteoblast differentiation but had no significant effect on osteoclast differentiation through OSM-STAT3/YAP1 signaling axis.OSM blockade could attenuate endplate osteosclerosis in rat MCs model.