High Concentrations of Polyelectrolyte Complex Nanoparticles Decrease Activity of Osteoclasts

Abstract

Fracture treatment in osteoporotic patients is still challenging. Osteoporosis emerges when there is an imbalance between bone formation and resorption in favor of resorption by osteoclasts. Thus, new implant materials for osteoporotic fracture treatment should promote bone formation and reduce bone resorption. Nanoparticles can serve as drug delivery systems for growth factors like Brain-Derived Neurotrophic Factor (BDNF), which stimulated osteoblast differentiation. Therefore, polyelectrolyte complex nanoparticles (PEC-NPs) consisting of poly(l-lysine) (PLL) and cellulose sulfate (CS), with or without addition of BDNF, were used to analyze their effect on osteoclasts in vitro. Live cell images showed that osteoclast numbers decreased after application of high PLL/CS PEC-NPs concentrations independent of whether BDNF was added or not. Real-time RT-PCR revealed that relative mRNA expression of cathepsin K and calcitonin receptor significantly declined after incubation of osteoclasts with high concentrations of PLL/CS PEC-NPs. Furthermore, Enzyme-Linked Immunosorbent Assay indicated that tartrate-resistant acidic phosphatase 5b activity was significantly reduced in the presence of high PLL/CS PEC-NPs concentrations. Consistent with these results, the pit formation analysis showed that less hydroxyapatite was resorbed by osteoclasts after incubation with high concentrations of PLL/CS PEC-NPs. BDNF had no influence on osteoclasts. We conclude that highly concentrated PLL/CS PEC-NPs dosages decreased osteoclastogenesis and osteoclasts activity. Moreover, BDNF might be a promising growth factor for osteoporotic fracture treatment since it did not increase osteoclast activity.

Effects of BDNF and PEC Nanoparticles on Osteocytes

“…On the contrary, BDNF also doesn’t increase the expression of OPG to inhibit osteoclastogenesis and activity of osteoclasts. We suppose that BDNF does not affect osteoclasts in cell cultures of cells from healthy individuals as recently shown [ 16 , 17 ]. The situation in samples and cells of patients with multiple myeloma and myeloma animal models is different [ 13 , 15 ].…”

“…MLO-Y4 is able to proliferate as reported previously [ 23 ], but still holds the main characteristics of osteocytes [ 21 ]. Recent studies of our group have shown that the concentration of 40 ng/mL BDNF in the cell culture medium stimulated alkaline phosphatase (ALP) activity of osteoblasts and enhanced the cell number of MSC as progenitors of osteoblasts [ 9 ], whereas no alteration in the activity of osteoclasts and their differentiation was found [ 16 , 17 ]. Studies of other groups have reported positive effects with other concentrations ranging from 5 ng/mL to 100 ng/mL BDNF, and several cell types of different species to be affected when analyzed by a panel of functional assays [ 24 ].…”

“…The cytocompatibility of PECNP in human MSC has been proven in previous studies [ 20 , 32 ]. Only osteoclasts showed a reduced activity and delayed differentiation after addition of high concentrations of PECNP [ 17 ]. Here, we measured no significant changes in proliferation of MLO-Y4 after addition of PECNP compared to the controls without any treatment.…”

“…In patients with multiple myeloma BDNF stimulated bone destruction by promoting osteoclastogenesis, while the inhibition of the BDNF/TrkB axis blocked osteoclastogenesis and bone destruction [ 13 , 14 , 15 ]. The addition of BDNF to monocyte cultures during differentiation into osteoclasts did neither impair the osteoclastogenesis nor the activity of osteoclasts if they were harvested from healthy donors [ 16 , 17 ]. In non-tumorigenic animal models, e.g., in murine and rat fracture models, the application of BDNF revealed enhanced bone formation and fracture healing [ 18 , 19 ].…”

“…PECNP show good cytocompatibility with human mesenchymal stem cells (MSC) in vitro [ 20 ]. Their application to osteoclasts also resulted in good cytocompatibility at low concentrations, whereas higher concentrations reduced the formation and activity of osteoclasts [ 17 ]. To our knowledge, the cytocompatibility of PECNP with osteocytes has not been investigated so far.…”