Cbfb2 Isoform Dominates More Potent Cbfb1 and Is Required for Skeletal Development

Abstract

Cbfb is a cotranscription factor that forms a heterodimer with Runx proteins Runx1, Runx2, and Runx3. It is required for fetal liver hematopoiesis and skeletal development. Cbfb has two functional isoforms, Cbfb1 and Cbfb2, which are formed by alternative splicing. To address the biological functions of these isoforms in skeletal development, we examined Cbfb1 -/-and Cbfb2 -/-mouse embryos. Intramembranous and endochondral ossification was retarded and chondrocyte and osteoblast differentiation was inhibited in Cbfb2 -/-embryos but not in Cbfb1 -/-embryos. Cbfb2 mRNA was upregulated in calvariae, limbs, livers, thymuses, and hearts of Cbfb1 -/-embryos but Cbfb1 mRNA was not in those of Cbfb2 -/-embryos, and the total amount of Cbfb1 and Cbfb2 mRNA in Cbfb1 -/-embryos was similar to that in wild-type embryos but was severely reduced in Cbfb2 -/-embryos. The absolute numbers of Cbfb2 mRNA in calvariae, limbs, livers, thymuses, and brains in wild-type embryos were about three times higher than those of Cbfb1 in the respective tissue. The levels of Runx proteins were reduced in calvariae, limbs, and primary osteoblasts from Cbfb2 -/-embryos, but the reduction in Runx2 protein was very mild. Furthermore, the amounts of Runx proteins and Cbfb in Cbfb2 -/-embryos differed similarly among skeletal tissues, livers, and thymuses, suggesting that Runx proteins and Cbfb are mutually required for their stability. Although Cbfb1 -/-embryos developed normally, Cbfb1 induced chondrocyte and osteoblast differentiation and enhanced DNA binding of Runx2 more efficiently than Cbfb2. Our results indicate that modulations in the relative levels of the isoforms may adjust transcriptional activation by Runx2 to appropriate physiological levels. Cbfb2 was more abundant, but Cbfb1 was more potent for enhancing Runx2 activity. Although only Cbfb2 loss generated overt skeletal phenotypes, both may play major roles in skeletal development with functional redundancy.

Runx2 is required for the proliferation of osteoblast progenitors and induces proliferation by regulating Fgfr2 and Fgfr3

“…It may partly explain why open fontanelles and sutures are prominent phenotypes in cleidocranial dysplasia, which is caused by heterozygous mutation of RUNX2 48 . High dependency on the amount of Runx2 protein among Runx family transcription factors in calvarial bone development is also shown in the comparison of Runx2 +/– mice with conditional Cbfb knockout mice or Cbfb isoform knockout mice 49 , 50 . As osteoblast progenitors were scarce and the BrdU-positive cells were few in both regions of calvaria and mandible in Runx2 −/− mice (Figs 5 and 6 ), however, our findings also indicate that Runx2 is required for the proliferation of osteoblast progenitors in both calvaria and mandible.…”

YBX1 Promotes the Inclusion of RUNX2 Alternative Exon 5 in Dental Pulp Stem Cells

“…There are few studies about the role of pre-mRNA alternative splicing in the differentiation of DPSCs into odontoblasts, but there are some reports on the role of pre-mRNA alternative splicing in the differentiation of osteoblasts ( 32 , 33 ). Exon 5 of CBFB, the co-transcription factor of RUNX2, is alternatively spliced to produce two subtypes, CBFB1 and CBFB2.…”

“…Exon 5 of CBFB, the co-transcription factor of RUNX2, is alternatively spliced to produce two subtypes, CBFB1 and CBFB2. Jiang et al ( 33 ) found that CBFB1 and CBFB2 played different roles in bone development, and only the deletion of CBFB2 could lead to the inhibition of osteogenic differentiation. The pre-mRNA of the transcription factor TAF4 has complex alternative splicing and can produce at least 10 isoforms.…”

Cbfβ2 controls differentiation of and confers homing capacity to prethymic progenitors

“…However, the analysis of these two mouse strains also revealed tissue-specific nonredundant functions. In addition to impaired osteoblast differentiation caused by the lack of Cbfβ2 ( Jiang et al, 2016 ), we noticed the presence of small thymi in Cbfb 2m/2m but not Cbfb 1m/1m mice ( Fig. 1 C ).…”