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Translational regulation is a control point in RUNX2/Cbfa1 gene expression
1Geriatric Research, Education and Clinical Center, South Texas Veterans Health Care System, Audie L. Murphy Division, San Antonio, Texas 78229, USA.
Abstract:
Runt-related transcription factor-2 (RUNX2)/core binding factor a1 (Cbfa1) is implicated in the regulation of osteoblast differentiation and osteoblast-specific gene expression. Mutations in RUNX2 cause the bone disease cleidocranial dysplasia, which is characterized by multiple skeletal defects. RUNX2 is expressed as two isoforms (type-I and type-II) encoded by two different mRNAs. We report here the detection of both mRNAs in osteoblastic cells and osteoblast precursors as well as nonosteoblastic cells. Surprisingly, however, osteoblast precursors and nonosteoblastic cells express no RUNX2 protein; mature osteoblasts express both isoforms, while less mature osteoblastic cells express only type-I protein. Northern blot analysis of RNA isolated from polysomes and ribonucleoprotein particles demonstrated that RUNX2 mRNA is polysome-associated in osteoblastic cells but polysome-free in osteoblast precursors. These results suggest that (a) RUNX2 mRNAs are expressed but dormant in osteoblast precursors and nonosteoblastic cells, (b) RUNX2 gene expression is controlled at the translational level, and (c) the expression of individual protein isoforms of RUNX2 is differentiation stage specific. Thus, differentiation of cells along the osteoblast lineage appears to be regulated at the level of RUNX2 mRNA translation.
Insights
Runt-related transcription factor-2 (RUNX2) mRNA is present but inactive in early bone cells. RUNX2 protein expression is controlled by translation, regulating osteoblast differentiation and isoform-specific protein production.
Area of Science:
- Molecular Biology
- Developmental Biology
- Cell Biology
Background:
- Runt-related transcription factor-2 (RUNX2), also known as core binding factor a1 (Cbfa1), is crucial for osteoblast differentiation and gene expression.
- RUNX2 mutations lead to cleidocranial dysplasia, a skeletal disorder.
- Two RUNX2 mRNA isoforms (type-I and type-II) exist.
Purpose of the Study:
- To investigate the regulation of RUNX2 expression during osteoblast differentiation.
- To determine the translational control of RUNX2 gene expression.
- To examine the stage-specific expression of RUNX2 protein isoforms.
Main Methods:
- Detection of RUNX2 mRNAs in various cell types (osteoblastic cells, osteoblast precursors, nonosteoblastic cells).
- Analysis of RUNX2 protein expression in relation to cell differentiation stage.
- Northern blot analysis of polysome-associated and polysome-free RUNX2 mRNA.
Main Results:
- Both RUNX2 mRNA isoforms are detected in osteoblast precursors and nonosteoblastic cells, but no RUNX2 protein is expressed.
- Mature osteoblasts express both RUNX2 isoforms, while less mature osteoblasts express only type-I.
- RUNX2 mRNA is polysome-associated in osteoblastic cells but polysome-free in osteoblast precursors.
Conclusions:
- RUNX2 mRNAs are expressed but translationally repressed in osteoblast precursors and nonosteoblastic cells.
- RUNX2 gene expression is primarily regulated at the translational level.
- The expression of RUNX2 protein isoforms is specific to the differentiation stage, indicating translational control of osteoblast lineage differentiation.