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Neighbor of Brca1 gene (Nbr1) functions as a negative regulator of postnatal osteoblastic bone formation and p38 MAPK activity.

ABSTRACT: The neighbor of Brca1 gene (Nbr1) functions as an autophagy receptor involved in targeting ubiquitinated proteins for degradation. It also has a dual role as a scaffold protein to regulate growth-factor receptor and downstream signaling pathways. We show that genetic truncation of murine Nbr1 leads to an age-dependent increase in bone mass and bone mineral density through increased osteoblast differentiation and activity. At 6 mo of age, despite normal body size, homozygous mutant animals (Nbr1(tr/tr)) have approximately 50% more bone than littermate controls. Truncated Nbr1 (trNbr1) co-localizes with p62, a structurally similar interacting scaffold protein, and the autophagosome marker LC3 in osteoblasts, but unlike the full-length protein, trNbr1 fails to complex with activated p38 MAPK. Nbr1(tr/tr) osteoblasts and osteoclasts show increased activation of p38 MAPK, and significantly, pharmacological inhibition of the p38 MAPK pathway in vitro abrogates the increased osteoblast differentiation of Nbr1(tr/tr) cells. Nbr1 truncation also leads to increased p62 protein expression. We show a role for Nbr1 in bone remodeling, where loss of function leads to perturbation of p62 levels and hyperactivation of p38 MAPK that favors osteoblastogenesis.

SUBMITTER: Whitehouse CA

PROVIDER: S-EPMC2919976 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundPeriodontal ligament stem cells (PDLSCs) are considered as candidate cells for the regeneration of periodontal and alveolar bone tissues. Antisense to the cerebellar degeneration-related protein 1 transcript (CDR1as), which is a newly discovered circular RNA (circRNA), has been reported to act as an miR-7 sponge and to be involved in many biological processes. Here, we investigated the potential roles of CDR1as and miR-7 in the osteogenic differentiation of PDLSCs.MethodsThe expression pattern of CDR1as and miR-7 in PDLSCs during osteogenesis was detected by quantitative reverse-transcription polymerase chain reaction (qRT-PCR). Then we overexpressed or knocked down CDR1as or miR-7 to confirm whether they were involved in the regulation of osteoblast differentiation in PDLSCs. Alkaline phosphatase (ALP) and alizarin red S (ARS) staining were used to detect the activity of osteoblasts and mineral deposition. Furthermore, a dual luciferase reporter assay was conducted to analyze the binding of miR-7 to growth differentiation factor (GDF)5. To further verify the role of CDR1as in osteoblast differentiation, we conducted animal experiments in vivo. New bone formation in specimens was analyzed by microcomputed tomography (micro-CT), hematoxylin and eosin staining, and immunofluorescence staining.ResultsWe observed that CDR1as was significantly upregulated during the osteogenic differentiation, whereas miR-7 was significantly downregulated. Moreover, knockdown of CDR1as and overexpression of miR-7 inhibited the ALP activity, ARS staining, and expression of osteogenic genes. Overexpression of miR-7 significantly reduced the activity of luciferase reporter vectors containing the wild-type, but not the mutant, 3' untranslated region (UTR) sequence of GDF5. Furthermore, knockdown of GDF5 partially reversed the effects of miR-7 inhibitor on osteoblast differentiation. Downregulation of CDR1as or GDF5 subsequently inhibited phosphorylation of Smad1/5/8 and p38 mitogen-activated protein kinases (MAPK), while upregulation of miR-7 decreased the level of phosphorylated Smad1/5/8 and p38 MAPK. In vivo, CDR1as knockdown lead to less bone formation compared with the control group as revealed by micro-CT and the histological analysis.ConclusionsOur results demonstrated that CDR1as acts as a miR-7 inhibitor, triggering the upregulation of GDF5 and subsequent Smad1/5/8 and p38 MAPK phosphorylation to promote osteogenic differentiation of PDLSCs. This study provides a novel understanding of the mechanisms of osteogenic differentiation, and suggests a potential method for promoting bone formation.

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Neighbor of Brca1 gene (Nbr1) functions as a negative regulator of postnatal osteoblastic bone formation and p38 MAPK activity.

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