β1D is a recently identified isoform of the β1 integrin subunit selectively expressed in skeletal and cardiac muscles. In the present study we determined the temporal expression of β1D and its association with α subunits during mouse development. By immunohistochemistry and western blot analysis we demonstrated that β1D 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 β1A isoform is expressed in skeletal muscle cells. After E17 β1A is downregulated and disappears from muscle fibers few days after birth.

In cardiac muscle the regulation of the β1D expression is different: β1D and β1A are coexpressed in the heart of E11 embryo. Subsequently expression of β1A declines, while β1D increases until it becomes the unique β1 isoform in cardiomyocytes few days after birth. Previous studies (Belkin et al J. Cell Biol. 132: 211–226, 1996) demonstrated that β1D in adult mouse cardiomyocytes is exclusively associated with α7B. Western blot analysis shows that α7B starts to be expressed in the heart only at stage E17, while β1D is expressed already at E11 embryo, indicating that α subunits other than α7 should associate with β1D in early developmental stages. To investigate this aspect, β1 associated α subunits were identified by western blotting from cardiomyocytes integrin complexes immunoprecipitated with α subunit specific antibodies. We found that, during cardiomyocyte development, β1D associates with several α subunits namely with α5, α6A and α7B.

In conclusion these data show that the expression of the β1D muscle specific integrin during development occurs much earlier in heart than in skeletal muscle and it can dimerize with different α subunits.