Hui Zhang

Long cytoplasmic processes of osteocytes orchestrate bone activity by integration of biochemical and mechanical signals and regulate load-induced bone adaptation. Low-Intensity Pulsed Ultrasound (LIPUS) is a clinically used technique for fracture healing that delivers mechanical impulses to the damaged bone tissue in a non-invasive and non-ionizing manner. The mechanism of action of LIPUS is still controversially discussed in the scientific community. In this study, the effect of focused LIPUS (FLIPUS) on the survival of starved MLO-Y4 osteocytes was investigated in vitro. Osteocytes stimulated for 10 min with FLIPUS exhibited extended dendrites, which formed frequent connections to neighboring cells and spanned longer distances. The sonicated cells displayed thick actin bundles and experienced increase in expression of connexin 43 (Cx43) proteins, especially on their dendrites, and E11 glycoprotein, which is responsible for the elongation of cellular cytoplasmic processes. After stimulation, expression of cell growth and survival genes as well as genes related to cell–cell communication was augmented. In addition, cell viability was improved after the sonication, and a decrease in ATP release in the medium was observed. In summary, FLIPUS mitigated apoptosis of starved osteocytes, which is likely related to the formation of the extensive dendritic network that ensured cell survival.

Quickly developing precision medicine and patient-oriented treatment strategies urgently require novel technological solutions. The randomly cell-populated scaffolds usually used for tissue engineering often fail to mimic the highly anisotropic characteristics of native tissue. In this work, an ultrasound standing-wave-based tissue engineering acoustophoretic (TEA) set-up was developed to organize murine mesenchymal stromal cells (mMSCs) in an in situ polymerizing 3-D fibrin hydrogel. The resultant constructs, consisting of 17 cell layers spaced at 300 µm, were obtained by continuous wave ultrasound applied at a 2.5 MHz frequency. The patterned mMSCs preserved the structured behavior within 10 days of culturing in osteogenic conditions. Cell viability was moderately increased 1 day after the patterning; it subdued and evened out, with the cells randomly encapsulated in hydrogels, within 21 days of culturing. Cells in the structured hydrogels exhibited enhanced expression of certain osteogenic markers, i.e., Runt-related transcription factor 2 (RUNX2), osterix (Osx) transcription factor, collagen-1 alpha1 (COL1A1), osteopontin (OPN), osteocalcin (OCN), and osteonectin (ON), as well as of certain cell-cycle-progression-associated genes, i.e., Cyclin D1, cysteine-rich angiogenic inducer 61 (CYR61), and anillin (ANLN), when cultured with osteogenic supplements and, for ANLN, also in the expansion media. Additionally, OPN expression was also augmented on day 5 in the patterned gels cultured without the osteoinductive media, suggesting the pro-osteogenic influence of the patterned cell organization. The TEA set-up proposes a novel method for non-invasively organizing cells in a 3-D environment, potentially enhancing the regenerative properties of the designed anisotropic constructs for bone healing.