Neonatal Mouse Calvarial Cells Research Articles

Single-Cell RNA Sequencing of Calvarial and Long-Bone Endocortical Cells.

Single-cell RNA sequencing (scRNA-Seq) is emerging as a powerful technology to examine transcriptomes of individual cells. We determined whether scRNA-Seq could be used to detect the effect of environmental and pharmacologic perturbations on osteoblasts. We began with a commonly used in vitro system in which freshly isolated neonatal mouse calvarial cells are expanded and induced to produce a mineralized matrix. We used scRNA-Seq to compare the relative cell type abundances and the transcriptomes of freshly isolated cells to those that had been cultured for 12 days in vitro. We observed that the percentage of macrophage-like cells increased from 6% in freshly isolated calvarial cells to 34% in cultured cells. We also found that Bglap transcripts were abundant in freshly isolated osteoblasts but nearly undetectable in the cultured calvarial cells. Thus, scRNA-Seq revealed significant differences between heterogeneity of cells in vivo and in vitro. We next performed scRNA-Seq on freshly recovered long bone endocortical cells from mice that received either vehicle or sclerostin-neutralizing antibody for 1 week. We were unable to detect significant changes in bone anabolism-associated transcripts in immature and mature osteoblasts recovered from mice treated with sclerostin-neutralizing antibody; this might be a consequence of being underpowered to detect modest changes in gene expression, because only 7% of the sequenced endocortical cells were osteoblasts and a limited portion of their transcriptomes were sampled. We conclude that scRNA-Seq can detect changes in cell abundance, identity, and gene expression in skeletally derived cells. In order to detect modest changes in osteoblast gene expression at the single-cell level in the appendicular skeleton, larger numbers of osteoblasts from endocortical bone are required. © 2020 American Society for Bone and Mineral Research.

Calvarial Defect Healing Induced by Small Molecule Smoothened Agonist.

Hedgehog (Hh) signaling positively regulates both endochondral and intramembranous ossification. Use of small molecules for tissue engineering applications poses several advantages. In this study, we examined whether use of an acellular scaffold treated with the small molecule Smoothened agonist (SAG) could aid in critical-size mouse calvarial defect repair. First, we verified the pro-osteogenic effect of SAG in vitro, using primary neonatal mouse calvarial cells (NMCCs). Next, a 4 mm nonhealing defect was created in the mid-parietal bone of 10-week-old CD-1 mice. The scaffold consisted of a custom-fabricated poly(lactic-co-glycolic acid) disc with hydroxyapatite coating (measuring 4 mm diameter × 0.5 mm thickness). Treatment groups included dimethylsulfoxide control (n = 6), 0.5 mM SAG (n = 7) or 1.0 mM SAG (n = 7). Evaluation was performed at 4 and 8 weeks postoperative, by a combination of high-resolution microcomputed tomography, histology (H & E, Masson's Trichrome), histomorphometry, and immunohistochemistry (BSP, OCN, VEGF). In vivo results showed that SAG treatment induced a significant and dose-dependent increase in calvarial bone healing by all radiographic parameters. Histomorphometric analysis showed an increase in all parameters of bone formation with SAG treatment, but also an increase in blood vessel number and density. In summary, SAG is a pro-osteogenic, provasculogenic stimulus when applied locally in a bone defect environment.

Fibroblast Growth Factor-2 and Bone Morphogenetic Protein-2 Have a Synergistic Stimulatory Effect on Bone Formation in Cell Cultures From Elderly Mouse and Human Bone

Combined regimens of fibroblast growth factor-2 (FGF-2) and bone morphogenetic protein-2 (BMP-2) were investigated to stimulate osteogenic differentiation. In young mouse calvaria-derived cells, FGF-2 (0.16ng/mL) in combination with BMP-2 (50ng/mL) did not enhance mineralization, but in old mouse cells it resulted in more mineralization than BMP-2 alone. In young long bone mouse cultures, FGF-2 enhanced mineralization relative to BMP-2 alone, but in old cultures, lower dose of FGF-2 (0.016ng/mL) was necessary. In neonatal mouse calvarial cells, sequential delivery of low-dose FGF-2 and low-dose BMP-2 (5ng/mL) was more stimulatory than co-delivery. In young human cultures, 0.016ng/mL of FGF-2 did not enhance mineralization, in combination with 5ng/mL of BMP-2, but in older cultures, codelivery of FGF-2 and BMP-2 was superior to BMP-2 alone. In conclusion, BMP-2 treatment alone was sufficient for maximal mineralization in young osteoblast cultures. However, coadministration of FGF-2 and BMP-2 increases mineralization more than BMP-2 alone in cultures from old and young mouse long bones and old humans but not in young mouse calvarial cultures.

Mechanical stress induces COX-2 mRNA expression in bone cells from elderly women

Mechanical loading-induced fluid flow in the lacuno-canalicular network is a possible signal for bone cell adaptive responses. In an earlier study we found that pulsating fluid flow (PFF, 0.7±0.02 Pa, 5 Hz, 0.4 Pa/s) stimulates the production of prostaglandins by neonatal mouse calvarial cells. In addition, mRNA expression of the inducible form of cyclooxygenase (COX-2), but not the constitutive form (COX-1), the major enzymes in prostaglandin production, was increased by PFF. The present study was performed to determine whether human primary bone cells from the iliac crest, respond to mechanical stress in a similar way as neonatal mouse calvarial cells. We subjected bone cells originating from the iliac crest of nine elderly women, between 56 and 80 yr of age, for 1 h to PFF and measured prostaglandin production and COX-1 and COX-2 mRNA expression. One hour PFF treatment stimulated the release of PGE 2 by 3.5 fold and PGI 2 by 2.2 fold. PFF also increased the expression of COX-2 mRNA by 2.9 fold, but did not change COX-1 mRNA. No correlation was found between donor age and PFF effect, neither on prostaglandin production nor on COX-2 mRNA expression. This study shows that bone cells from the iliac crest of elderly women react to PFF treatment in a similar way as neonatal mouse calvarial cells, namely with increased production of prostaglandins and upregulation of COX-2 mRNA expression. These results suggest that human bone cells from the iliac crest and neonatal mouse calvarial cells share a similar mechanotransduction pathway.

Greater inhibition of in vitro bone mineralization with metabolic than respiratory acidosis

At a similar decrement in pH, acidosis produced by lowering the concentration of medium bicarbonate (metabolic acidosis) induces greater net calcium efflux from cultured neonatal mouse calvariae than acidosis produced by increasing the partial pressure of carbon dioxide (respiratory acidosis). This differential effect is due, at least in part, to enhanced cell-mediated bone mineral resorption during metabolic acidosis. To determine the effect of acidosis on osteoblastic bone formation we utilized primary cultures of neonatal mouse calvarial cells which produce calcified nodules in culture. Cells were plated at 4.5 x 10(4) cells/35 mm dish and incubated until confluent (day 9). Nodule formation was then induced by addition of beta-glycerophosphate and ascorbic acid and the cultures were randomly divided and then cultured in control (Ctl, N = 18) medium or in medium simulating metabolic (Met, N = 17) or respiratory (Resp, N = 19) acidosis. Medium was changed and calcium (Ca) measured every 48 hours until day 23. The mean initial medium pH of all Resp cultures (7.186 +/- 0.002) was lower than Met (7.243 +/- 0.006, P < 0.01), which was lower than Ctl (7.502 +/- 0.002, p < 0.01), yet the number of discrete nodules formed in Met (22 +/- 4 nodules/cm2) was lower than Resp (43 +2- 7, P < 0.01), and both were lower than Ctl (88 +/- 6, P < 0.01 vs. both Met and Resp).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of parathyroid hormone on protein phosphorylation in two osteoblastlike cell populations isolated from neonatal mouse calvaria

To further understand the mechanism of PTH effects on bone and bone cells, we have analyzed the effect of PTH on specific protein phosphorylation in cells isolated from neonatal mouse calvaria. Four populations of cells (I-IV), isolated by sequential digestion with chromatographically purified bacterial collagenase isozymes and neutral proteinase, were cultured overnight. Alkaline phosphatase activity was greater than acid phosphatase activity in all four populations. PTH stimulated cyclic AMP production in all four populations, although the effect was greatest in populations II and III. Cultured cells were treated with PTH for up to 15 minutes. Cytosolic and membrane fractions were obtained and assayed for in vitro protein phosphorylation. No hormonal effects were found in membrane fractions. In cytosol fractions, treatment of the population II cells for 10-15 minutes with 0.1 microM PTH decreased the subsequent protein phosphorylation of an 85,000 Mr protein. In contrast, PTH treatment increased in vitro phosphorylation of both the 85,000 and 35,000 Mr proteins in population III cells. Phosphorylation of the 35,000 Mr protein was cyclic AMP-dependent. All of the phosphoproteins appeared to be phosphorylated solely on serine or threonine residues except the 85,000 Mr protein which may also contain significant amounts of phosphotyrosine. Therefore, some of the effects of PTH are cyclic AMP-mediated and other effects may be mediated through tyrosine phosphorylation. These data indicate that PTH has differential effects on in vitro protein phosphorylation in two separable populations of isolated neonatal mouse calvarial cells and support a hypothesis that multiple osteoblastlike cells exist in vivo.

Parathyroid hormone stimulation of alkaline phosphatase activity in cultured neonatal mouse calvarial bone cells: Involvement of cyclic AMP and calcium

The involvement of cAMP and calcium in the rise in alkaline phosphatase (AP) activity observed when confluent, serum-free primary cultures of neonatal mouse calvarial cells are treated with parathyroid hormone (PTH) has been studied. Synthetic bovine PTH [bPTH-(1-34)] increased cellular cAMP at concentrations (10(-9) to 10(-7) M) previously found to elevate AP activity. Other substances that increase cAMP in these cells (forskolin, prostaglandin E2, 8-bromoadenosine cAMP and 3-isobutyl-1-methylxanthine) also increased enzyme activity. By comparison, increasing the concentration of calcium in the culture medium from 1.8 to 3.8 or 5.8 mM lowered the magnitude of the maximal AP response. In addition, treatment of cultures with the divalent cation ionophore A23187 caused a significant decrease in AP activity. These results suggest that: 1) cAMP mediates the rise in the specific activity of AP in cultured neonatal mouse calvarial cells treated with bPTH-(1-34) and 2) the concentration of calcium in the environment significantly influences the responsivity of bone cells to the hormone.

Stimulation of alkaline phosphatase activity in cultured neonatal mouse calvarial bone cells by parathyroid hormone

The effect of parathyroid hormone (PTH) on alkaline phosphatase activity was examined in confluent, serum-free primary cultures of neonatal mouse calvarial cells. It was found that synthetic bPTH-(1-34) caused an increase in the specific activity of skeletal alkaline phosphatase isoenzyme by 18 hours. Between 10 and 500 ng/ml, the magnitude of the change was directly related to peptide concentration. The change occurred in the absence of any effect on cell number, total cell protein, or DNA and was not the result of an effect on either proliferation or survival of a specific cell population. Results of histochemical studies indicate that bPTH-(1-34) caused an increase in the proportion of cells containing enzyme activity. The response was duplicated by intact bPTH-(1-84) and DBcAMP, but not by oxidized bPTH-(1-34) or insulin and did not require prostaglandin synthesis or hydroxylation of 25-hydroxyvitamin D3. These results demonstrate that bPTH has a direct effect on osteoblast maturation in vitro, that the effect is specific for PTH, and suggest that it is mediated by cAMP.