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Microphthalmia-associated transcription factor interacts with PU.1 and c-Fos: determination of their subcellular
M Sato1, E Morii, K Takebayashi-Suzuki
1Department of Pathology, Department of Orthopaedic Surgery, Osaka University Medical School, Yamadaoka 2-2, Suita, 565-0871, Japan.
Abstract:
Marked osteopetrosis is observed only in mi/mi mutant mice although normal osteoclastgenesis is observed in other mutant mice including null mutants at the mi locus. Mutant microphthalmia-associated transcription factor (mi-MITF) has defective nuclear localization potential. We found normal MITF (+-MITF)-c-Fos and mi-MITF-c-Fos complexes in the cytoplasm by immunoblotting, and showed that PU.1 bound with both +-MITF and mi-MITF using an electrophoretic mobility shift assay. Furthermore, the nuclear localization of PU.1 and c-Fos was inhibited by over-expressed mi-MITF in WEHI-3 cells. These results indicate that mi-MITF expressing in osteoclasts specifically binds to c-Fos and PU.1 which are essential transcription factors of osteoclastgenesis and that mi-MITF blocks the nuclear localization of these other transcription factors, which may result in osteopetrosis in mi/mi mutant mice.
Insights
Mutant microphthalmia-associated transcription factor (mi-MITF) in mice prevents osteopetrosis by blocking essential transcription factors, PU.1 and c-Fos, from entering the nucleus, thus inhibiting osteoclastogenesis.
Area of Science:
- Genetics and Molecular Biology
- Developmental Biology
- Skeletal Biology
Background:
- Osteopetrosis is a rare genetic disorder characterized by increased bone density due to impaired osteoclast function.
- The microphthalmia-associated transcription factor (MITF) is crucial for osteoclast development and function.
- Mutations in the mi locus, specifically the mi-MITF variant, lead to severe osteopetrosis in mice, despite normal osteoclastogenesis in other related mutants.
Purpose of the Study:
- To investigate the molecular mechanism by which mutant microphthalmia-associated transcription factor (mi-MITF) causes osteopetrosis.
- To determine the interaction of mi-MITF with other transcription factors involved in osteoclastogenesis, such as PU.1 and c-Fos.
Main Methods:
- Immunoblotting was used to analyze protein complex formation between MITF variants and c-Fos in the cytoplasm.
- Electrophoretic mobility shift assay (EMSA) was employed to assess the binding of PU.1 to both wild-type (+-MITF) and mutant (mi-MITF) forms of MITF.
- Overexpression studies in WEHI-3 cells were conducted to evaluate the effect of mi-MITF on the nuclear localization of PU.1 and c-Fos.
Main Results:
- Normal MITF-c-Fos and mi-MITF-c-Fos complexes were detected in the cytoplasm, indicating cytoplasmic retention of mi-MITF.
- PU.1 was shown to bind to both +-MITF and mi-MITF, suggesting a direct interaction.
- Overexpression of mi-MITF significantly inhibited the nuclear localization of both PU.1 and c-Fos in WEHI-3 cells.
Conclusions:
- The mi-MITF protein specifically binds to osteoclastogenesis transcription factors c-Fos and PU.1.
- mi-MITF interferes with the nuclear translocation of c-Fos and PU.1, which are essential for osteoclast formation.
- This disruption of nuclear localization by mi-MITF is proposed as the underlying cause of osteopetrosis observed in mi/mi mutant mice.