Extracellular vesicles meet mitochondria: Potential roles in regenerative medicine.

Extracellular vesicles (EVs), secreted by most cells, act as natural cell-derived carriers for delivering proteins, nucleic acids, and organelles between cells. Mitochondria are highly dynamic organelles responsible for energy production and cellular physiological processes. Recent evidence has highlighted the pivotal role of EVs in intercellular mitochondrial content transfer, including mitochondrial DNA (mtDNA), proteins, and intact mitochondria. Intriguingly, mitochondria are crucial mediators of EVs release, suggesting an interplay between EVs and mitochondria and their potential implications in physiology and pathology. However, in this expanding field, much remains unknown regarding the function and mechanism of crosstalk between EVs and mitochondria and the transport of mitochondrial EVs. Herein, we shed light on the physiological and pathological functions of EVs and mitochondria, potential mechanisms underlying their interactions, delivery of mitochondria-rich EVs, and their clinical applications in regenerative medicine.

Osteocytes autophagy mediated by mTORC2 activation controls osteoblasts differentiation and osteoclasts activities under mechanical loading.

Autophagy is an important mechanosensitive response for cellular homeostasis and survival in osteocytes. However, the mechanism and its effect on bone metabolism have not yet clarified. The objective of this study was to evaluate how compressive cyclic force (CCF) induced autophagic response in osteocytes and to determine the effect of mechanically induced-autophagy on bone cells including osteocytes, osteoblasts, and osteoclasts. Autophagic puncta observed in MLO-Y4 cells increased after exposure to CCF. The upregulated levels of the LC3-II isoform and the degradation of p62 further confirmed the increased autophagic flux. Additionally, ATP synthesis and release, osteocalcin (OCN) expression, and cell survival increased in osteocytes as well. The Murine osteoblasts MC3T3-E1 cells and RAW 264.7 macrophage cells were cultured in conditioned medium collected from MLO-Y4 cells subjected to CCF. The concentration of FGF23 increased and the concentrations of SOST and M-CSF and RANKL/OPG ratio decreased significantly in the conditioned medium. Moreover, the promotion of osteogenic differentiation in MC3T3-E1 cells and inhibition of osteoclastogenesis and function in RAW 264.7 cells were significantly attenuated when osteocytes autophagy was inhibited by siAtg7. Our findings suggested that CCF induced protective autophagy in osteocytes and subsequently enhanced osteocytes survival and osteoblasts differentiation and downregulated osteoclasts activities. Further study revealed that CCF induced autophagic response in osteocytes through mechanistic target of rapamycin complex 2 (mTORC2) activation. In conclusion, CCF-induced osteocytes autophagy upon mTORC2 activation promoted osteocytes survival and osteogenic response and decreased osteoclastic function. Thus, osteocytes autophagy will provide a promising target for better understanding of bone physiology and treatment of bone diseases.

Neuropeptide Y regulates osteocyte phenotype and function through AHNAK-Smad signalling.

Neuropeptide Y (NPY) is a widespread hormone in the central and peripheral nervous systems that maintains body homeostasis. Central actions of hypothalamic NPY have been identified in bone metabolism. Osteocytes are the main source of NPY in bone tissue, indicating that osteocytic NPY could be a local alternative pathway for hypothalamic mediated regulation of bone and bone cells. Here, we show that osteocytic NPY induces cell viability and proliferation. Osteocyte-derived factors are also closely associated with changes in cellular NPY mRNA levels. Furthermore, osteoblast mineralization was significantly induced by conditioned medium collected from NPY-overexpressing osteocytes (P < 0.05). Importantly, the NPY-AHNAK interaction was identified for the first time by co-immunoprecipitation, and significant inactivation of p-Smad1/5/9 was found in osteocytes with NPY or AHNAK insufficiency (P < 0.05). The activation of p-Smad1/5/9 reversed NPY insufficiency-caused decreases in the expression of osteocytic proliferating cell nuclear antigen and osteoblast markers including osteocalcin and Runx2 (P < 0.05); these findings showed an additional molecular mechanism by which NPY acts on cells through AHNAK-mediated Smad1/5/9 signalling. Collectively, our findings provide novel insights into the function of NPY in regulating osteocyte phenotype and function and provide new insights for further investigation into osteocytic NPY-mediated therapy.