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.