beta 1D is a recently identified isoform of the beta 1 integrin subunit selectively expressed in skeletal and cardiac muscles. In the present study we determined the temporal expression of beta 1D and its association with alpha subunits during mouse development. By immunohistochemistry and western blot analysis we demonstrated that beta 1D begins to be expressed in skeletal muscles of 17 days embryo (stage E17). Its level progressively increases reaching maximal values few days after birth and remaining high in adult mice. At earlier stages of development (E11-E17) the beta 1A isoform is expressed in skeletal muscle cells. After E17 beta 1A is downregulated and disappears from muscle fibers few days after birth. In cardiac muscle the regulation of the beta 1D expression is different: beta 1D and beta 1A are coexpressed in the heart of E11 embryo. Subsequently expression of beta 1A declines, while beta 1D increases until it becomes the unique beta 1 isoform in cardiomyocytes few days after birth. Previous studies (Belkin et al J. Cell Biol. 132: 211-226, 1996) demonstrated that beta 1D in adult mouse cardiomyocytes is exclusively associated with alpha 7B. Western blot analysis shows that alpha 7B starts to be expressed in the heart only at stage E17, while beta 1D is expressed already at E11 embryo, indicating that alpha subunits other than alpha 7 should associate with beta 1D in early developmental stages. To investigate this aspect, beta 1 associated alpha subunits were identified by western blotting from cardiomyocytes integrin complexes immunoprecipitated with alpha subunit specific antibodies. We found that, during cardiomyocyte development, beta 1D associates with several alpha subunits namely with alpha 5, alpha 6A and alpha 7B. In conclusion these data show that the expression of the beta 1D muscle specific integrin during development occurs much earlier in heart than in skeletal muscle and it can dimerize with different alpha subunits.
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