Immunoglobulin M, a novel molecule of myocardial cells of mice

BackgroundImmunoglobulins(Igs)play an important role in host defence and were initially thought to be expressed solely by B cells. However, recent data suggest that Igs are also expressed in other lineages. Recently, Ig transcripts were detected in cardiomyocytes, but whether the functional Ig protein is expressed by cardiomyocytes has not been thoroughly elucidated.MethodsGene Expression Omnibus (GEO) microarray database analysis was used to analyse IgM heavy chain expression in the myocardium of mice. Immunohistochemistry (IHC), ELISA and Western blot were used to identify IgM in cardiomyocytes of both Balb/c mice and μMT mice (B cell-deficient mice), as well as in cultured cardiomyocytes of neonatal mice and in the myocardial cell line HL-1.Moreover, RT-PCR and cDNA sequencing were used to determine the VDJ rearrangement of the IgM heavy chain.ResultsIn this study, we first analysed transcription of the IgM heavy chain in heart tissue in mice by mining the GEO database, and we observed that IgM heavy chain transcripts were expressed in heart tissues. Subsequently, IgM was found to be expressed in cardiomyocytes in mice; the IgM was primarily localized on the cell membranes and intercalated discs of murine heart cells and in the cytoplasm and cell membranes of isolated cardiomyocytes and HL-1. Importantly, the functional IgM heavy chain transcripts exhibit a unique VDJ rearrangement pattern. Furthermore, IgM can be secreted and deposited in the extracellular space of the myocardium under ischaemic/hypoxic conditions.ConclusionsOur data indicate for the first time that IgM is expressed by cardiomyocytes in mice and suggest that its physiological function warrants further investigation.