UBIQUITOUS EXPRESSION OF A CLONED MURINE THYMOPOIETIN CDNA

Thymopoietin (TP) was originally isolated as a 5-kD 49-aa protein from bovine thymus and was subsequently observed to affect T-cell differen tiation and function. We report here the molecular cloning of a murine TP cDNA. The 2,514 bp fragment contains a 630 bp open reading frame t hat encodes for 210 aa, highly homologous to the first 220 aa of the h uman TP beta and TP gamma isoforms and to bovine TP. Southern blot ana lysis of genomic DNA revealed that in mouse, calf and human TP is enco ded by a single genomic locus. Recently, it was found that one of the TP isoforms designated TP beta is a homologous protein to the lamina-a ssociated polypeptide 2 (LAP2), which is thought to play an important role in the regulation of nuclear architecture by binding lamin B1 and chromosomes in a manner regulated by phosphorylation during mitosis. In this study we report the TP expression at the transcription lever i n 17 murine and rat tissues of different origins and 18 lymphoid and n onlymphoid cell lines. The assessment of TP mRNA expression by S1-nucl ease protection assays and in situ hybridizations revealed that its ex pression is not exclusive to thymus, but rather ubiquitous, higher in lymphatic tissues, but also in other cells characterized with a high r ate of proliferation. TP was also shown to be expressed in athymic and old animals, lacking a functional thymus gland. Further in situ hybri dization studies revealed that within the thymus, the highest levels o f TP mRNA are noted at the cortex. These results suggest a possible ro le for TP in proliferation and cell cycle control.