H-type blood vessels participate in alveolar bone remodeling during murine tooth extraction healing.

OBJECTIVES: We aimed to investigate whether skeletal-specific H-type blood vessels exist in alveolar bone and how they function in alveolar bone remodeling. MATERIALS AND METHODS: H-type vessels with high expression of CD31 and Endomucin (CD31hi Emcnhi ) were immunostained in alveolar bone. Abundance and age-related changes in CD31hi Emcnhi endothelial cells (H-ECs) were detected by flow cytometry. Osteoprogenitors association with H-type vessels and bone mass were detected in tooth extraction model of alveolar bone remodeling by immunohistofluorescence and micro-CT, respectively. Transcription and expression of H-EC feature genes during in vitro Notch inhibition were measured by RT-qPCR and immunocytofluorescence. RESULTS: We verified that H-type vessels existed in alveolar bone, the abundance of which was highest at infancy age, then decreased but maintained a constant level during aging. In tooth extraction model, H-ECs significantly increased with concomitant perivascular accumulation of Runx2+ osteoprogenitors and gradually augmentation of bone mass. Notch inhibition of in vitro cultured H-ECs resulted in decreased expression levels of Emcn and hes1, but not Pecam1 or Kdr genes, with decreased expression levels of H-EC numbers, accordingly. CONCLUSIONS: The present study suggests that H-type vessels promote osteogenesis during alveolar bone remodeling. Notch signaling pathway regulates expression of Emcn and possibly determines fate and functions of alveolar H-ECs.

Reassessing endothelial-to-mesenchymal transition in mouse bone marrow: insights from lineage tracing models.

Endothelial cells (ECs) and bone marrow stromal cells (BMSCs) play crucial roles in supporting hematopoiesis and hematopoietic regeneration. However, whether ECs are a source of BMSCs remains unclear. Here, we evaluate the contribution of endothelial-to-mesenchymal transition to BMSC generation in postnatal mice. Single-cell RNA sequencing identifies ECs expressing BMSC markers Prrx1 and Lepr; however, this could not be validated using Prrx1-Cre and Lepr-Cre transgenic mice. Additionally, only a minority of BMSCs are marked by EC lineage tracing models using Cdh5-rtTA-tetO-Cre or Tek-CreERT2. Moreover, Cdh5+ BMSCs and Tek+ BMSCs show distinct spatial distributions and characteristic mesenchymal markers, suggestive of their origination from different progenitors rather than CDH5+ TEK+ ECs. Furthermore, myeloablation induced by 5-fluorouracil treatment does not increase Cdh5+ BMSCs. Our findings indicate that ECs hardly convert to BMSCs during homeostasis and myeloablation-induced hematopoietic regeneration, highlighting the importance of using appropriate genetic models and conducting careful data interpretation in studies concerning endothelial-to-mesenchymal transition.

Deficiency of Omentin-1 leads to delayed fracture healing through excessive inflammation and reduced CD31hiEmcnhi vessels.

Fracture healing is a complicated process affected by many factors, such as inflammatory responses and angiogenesis. Omentin-1 is an adipokine with anti-inflammatory properties, but whether omentin-1 affects the fracture healing process is still unknown. Here, by using global omentin-1 knockout (omentin-1-/-) mice, we demonstrated that omentin-1 deficiency resulted in delayed fracture healing in mice, accompanied by increased inflammation and osteoclast formation, and decreased production of platelet-derived growth factor-BB (PDGF-BB) and osteogenesis-promoting vessels that are strongly positive for CD31 and Endomucin (CD31hiEmcnhi) in the fracture area. In vitro, omentin-1 treatment suppressed the ability of the tumor necrosis factor-α (TNF-α)-activated macrophages to stimulate multi-nuclear osteoclast formation, resulting in a significant increase in the generation of mono-nuclear preosteoclasts and PDGF-BB, a pro-angiogenic protein that is abundantly secreted by preosteoclasts. PDGF-BB significantly augmented endothelial cell proliferation, tube formation and migration, whereas direct treatment with omentin-1 did not induce obvious effects on angiogenesis activities of endothelial cells. Our study suggests a positive role of omentin-1 in fracture healing, which may be associated with the inhibition of inflammation and stimulation of preosteoclast PDGF-BB-mediated promotion of CD31hiEmcnhi vessel formation.

Neuronal Induction of Bone-Fat Imbalance through Osteocyte Neuropeptide Y.

A differentiation switch of bone marrow mesenchymal stem/stromal cells (BMSCs) from osteoblasts to adipocytes contributes to age- and menopause-associated bone loss and marrow adiposity. Here it is found that osteocytes, the most abundant bone cells, promote adipogenesis and inhibit osteogenesis of BMSCs by secreting neuropeptide Y (NPY), whose expression increases with aging and osteoporosis. Deletion of NPY in osteocytes generates a high bone mass phenotype, and attenuates aging- and ovariectomy (OVX)-induced bone-fat imbalance in mice. Osteocyte NPY production is under the control of autonomic nervous system (ANS) and osteocyte NPY deletion blocks the ANS-induced regulation of BMSC fate and bone-fat balance. γ-Oryzanol, a clinically used ANS regulator, significantly increases bone formation and reverses aging- and OVX-induced osteocyte NPY overproduction and marrow adiposity in control mice, but not in mice lacking osteocyte NPY. The study suggests a new mode of neuronal control of bone metabolism through the ANS-induced regulation of osteocyte NPY.

Extracellular Vesicles from Child Gut Microbiota Enter into Bone to Preserve Bone Mass and Strength.

Recently, the gut microbiota (GM) has been shown to be a regulator of bone homeostasis and the mechanisms by which GM modulates bone mass are still being investigated. Here, it is found that colonization with GM from children (CGM) but not from the elderly (EGM) prevents decreases in bone mass and bone strength in conventionally raised, ovariectomy (OVX)-induced osteoporotic mice. 16S rRNA gene sequencing reveals that CGM reverses the OVX-induced reduction of Akkermansia muciniphila (Akk). Direct replenishment of Akk is sufficient to correct the OVX-induced imbalanced bone metabolism and protect against osteoporosis. Mechanistic studies show that the secretion of extracellular vesicles (EVs) is required for the CGM- and Akk-induced bone protective effects and these nanovesicles can enter and accumulate into bone tissues to attenuate the OVX-induced osteoporotic phenotypes by augmenting osteogenic activity and inhibiting osteoclast formation. The study identifies that gut bacterium Akk mediates the CGM-induced anti-osteoporotic effects and presents a novel mechanism underlying the exchange of signals between GM and host bone.