Up‐regulation of a novel integrin α‐chain (αmt) on human fetal myotubes

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Abstract
Integrin expression and distribution was studied in cloned human fetal G6 myoblasts and myotubes. Immunoprecipitation of β1 integrins from surface iodinated and metabolically labeled G6 cells typically showed a five‐fold induction of a β1 integrin associated protein upon differentiation. Under non‐reducing conditions this β1 associated protein migrated as 145 kD. No such β1 associated protein was observed in the myogenic L8 rat cell line, before or after differentiation. The β1 integrin associated cell surface protein present in G6 myotubes remained associated with the β1 subunit in the presence of 1% Triton X‐100 and 0.5 M NaCl. Like integrin α‐chains, the protein dissociated from the β1 integrin subunit at low pH. Immunoprecipitation of G6 myotubes further indicated the presence of α, α, α5, and αa integrins, and small amounts of α4 and α6 integrins. Immunodepletion with integrin α‐chain antibodies to α1, α3, α4, α5, α6, and αv integrin chains could not deplete the β1 integrin associated protein, indicating that it did not interact with any of these known integrin heterodimers. Upon treatment with reducing agents, the β1 integrin associated protein migrated in SDS‐PAGE as a 155 kD protein. The decreased mobility in SDS‐PAGE upon reduction is a feature shared with α1, α2, and α9 integrin α‐chains. Antibodies to α1 immunoprecipitated an integrin heterodimer distinct from the 155 kD protein. Antibodies to α2 and α9 failed to immunoprecipitate proteins from G6 myotubes and Northern blot analysis likewise failed to detect messages for these two integrin α‐chains. Immunohistochemistry with antibodies to different integrins of fetal thigh muscles showed staining of α6 integrin at the sarcolemma. Antibodies to α2 and α9 stained keratinocytes and other non‐muscle cells in the fetal leg, but no signal was detected for α2 and α9 integrin on fetal myotubes. Based on these characteristics, we propose that the induced protein is a hitherto unidentified integrin α‐chain on myotubes that we name αmt. Our finding of a novel integrin α‐chain that is induced upon myogenic differentiation supports the idea that different integrins are important for myogenesis during different developmental stages, and we suggest that αmt takes part in early human myotube formation.