Senescence of human bone marrow mesenchymal stem cells impairs the osteoinductive properties of their derived apoptotic vesicles

To investigate the effect of the senescence state of parental human bone marrow mesenchymal stem cells (BMSCs) on the osteoinductive properties of their derived apoptotic vesicles (apoVs), and to provide an experimental basis and a quality control reference for the treatment of alveolar bone defects and osteoporosis based on apoVs.

A replicative senescence model of human aging BMSCs (A-BMSCs) was established via serial passaging, with young BMSCs (Y-BMSCs) serving as controls. Following the induction of apoptosis with staurosporine, apoVs were isolated from Y-BMSCs and A-BMSCs (termed Y-apoVs and A-apoVs, respectively) via differential centrifugation. The physicochemical properties (morphology, size, zeta potential, and yield) and protein markers of both apoV populations were systematically characterized. Subsequently, Y-BMSCs were divided into a proliferation medium (PM) group, osteogenic induction medium (OM) group, OM + Y-apoVs group, and OM + A-apoVs group. The osteoinductive efficacy in vitro was evaluated by alkaline phosphatase (ALP) staining, alizarin red S staining, and quantitative real-time PCR detection of key osteogenic genes, including runt-related transcription factor 2 (RUNX2), ALP, osteopontin (OPN), and osterix (OSX). Approved by the Institutional Animal Care and Use Committee, in vivo biodistribution (labelling apoVs with the red fluorescent dye PKH26) and bone regeneration efficacy were assessed in 18-month-old osteoporotic C57BL/6 mice. Mice were randomly divided into a control group (injected with phosphate-buffered saline), Y-apoVs group (injected with Y-apoVs), and A-apoVs group (injected with A-apoVs). Following tail-vein injection administration of apoVs for 8 weeks, bone regeneration was evaluated via micro-computed tomography and histological analysis.

Electron microscopy and particle size analysis revealed that both Y-apoVs and A-apoVs displayed typical biconcave discoid structures, with diameters mainly ranging from 100 to 500 nm. Western blot assays confirmed high expression of universal vesicle markers (CD9, CD63, CD81) and the apoptotic marker Fas in both groups. Y-apoVs and A-apoVs exhibited indistinguishable morphologies, size distributions, zeta potentials, and yields. In vitro experiments showed that, compared with the PM group, the OM, OM + A-apoVs, and OM + Y-apoVs groups significantly enhanced ALP activity, calcium nodule formation, and the expression of osteogenic genes (RUNX2, ALP, OPN, OSX) in recipient Y-BMSCs (P<0.05); moreover, compared with the OM and OM + A-apoVs groups, the OM + Y-apoVs group exhibited a more significant promoting effect (P<0.05). An in vivo analysis demonstrated that tail-vein-injected apoVs effectively homed to bone tissue. Moreover, the Y-apoVs group significantly improved trabecular microarchitecture, bone mineral density, and bone volume fraction in aged mice, exhibiting superior therapeutic efficacy over the control and A-apoVs groups (P<0.05).

The senescence state of parental cells significantly impairs the osteoinductive ability of BMSC-derived apoVs. Y-apoVs are effective biological agents for the treatment of age-related bone loss.