CELL-TYPE-SPECIFIC EXPRESSION OF HNRNP PROTEINS

HnRNP proteins are abundant nucleoplasmic pre-mRNA-binding proteins wh ich have important roles in the biogenesis of mRNA. Although hnRNP pro teins have been extensively characterized in cultured cell fines, litt le is known about their expression in animal tissues. Here, we have un dertaken a systematic survey of the expression of major hnRNP proteins in mouse tissues using specific monoclonal antibodies. Immunohistoche mical staining demonstrated that hnRNP proteins C, L, and U were local ized to nuclei in all tissues examined. However, cytoplasmic expressio n of hnRNP A1, D, F/H, and K was also detected in several tissues, sug gesting that these proteins have roles in the cytoplasm as well as the nucleus. Importantly, the relative amounts of different hnRNP protein s varied among cell types. This was especially striking in neuronal an d reproductive cells. In the brain, certain neuronal cell types contai ned more hnRNP proteins than glial cells, perhaps reflecting increased levels of neuronal transcription and RNA processing. In the ovary, oo cytes contained exceptionally high concentrations of hnRNP proteins as compared to follicular and stromal cells. In the testis, the expressi on of hnRNP proteins was generally high and was found to be tightly re gulated during spermatogenesis. Specifically, hnRNP A1 was highly expr essed only in early spermatogonia and absent in later stages. These fi ndings demonstrate that hnRNP proteins do not exist in a fixed stoichi ometry across different cell types. Furthermore, as the relative amoun ts of pre-mRNA-binding proteins (e.g., A1 and ASF/SF2) can affect alte rnative splicing patterns, the variations that we have observed could profoundly affect cell-specific gene expression. (C) 1995 Academic Pre ss, Inc.