Synchronous expression of LINE-1 RNA and protein in mouse embryonal carcinoma cells

L1, or LINE-1, is a repetitive DNA family found in all mammalian genomes that have been examined. At least a few individual members of the L1 family are functional transposable elements. Expression of these active elements leads to new insertions of L1 into the genomic DNA by the process of retrotransposition. We have detected coexpression of full-length, sense-strand L1 RNA transcripts and L1-encoded protein in mouse embryonal carcinoma cell lines. Both of these L1 expression products are candidates for intermediates in the retrotransposition process. L1 protein is found in what appear to be cytoplasmic aggregates and is not localized to any known cytoplasmic organelles. The six embryonal carcinoma cell lines tested were chosen to represent commitment to different developmental pathways in early mouse embryogenesis. The only two cell lines that express L1 are unique among the six in that they have a strong predilection to differentiate into extraembryonic endoderm. This observation is consistent with L1 expression and transposition in primordial germ cells of the mouse. An important implication of these studies is that L1 expression may provide a new marker for use in determining the origin of primordial germ cells during mouse embryogenesis.