c-mos proto-oncogene RNA transcripts in mouse tissues: structural features, developmental regulation, and localization in specific cell types.

Abstract

c-mos RNA transcripts have been previously detected in mouse gonadal tissue and in late-term embryos. Here, we show that they are also present at low levels in placenta and in adult mouse brain, kidney, mammary gland, and epididymis. Marked differences are observed in the size of the mos RNA transcripts detected in different tissues. All transcripts appear to end at the same 3' position, and the tissue-specific size variations appear to be due to the use of different promoters. For example, the testicular and ovarian RNA transcripts initiate -280 and -70 base pairs, respectively, upstream from the first initiation codon, but both end at a common site downstream from the mos open reading frame. The expression of mos is developmentally regulated in gonadal tissue. Thus, the level of mos transcripts in testes is low for the first 3 weeks after birth, increases at least 10-fold around day 25, and reaches adult levels by day 30. In contrast, ovaries from preweaning mice contain a higher level of mos mRNA compared to ovaries from adult mice. In cell fractionation experiments we show that mos transcripts are present in haploid germ cells. We find that these transcripts are associated with monosomes and polysomes. The peculiar pattern of mos expression in mouse gonadal tissue suggests a role for the c-mos proto-oncogene in germ cell differentiation.The c-mos proto-oncogene was one of the first cellular homologs of a retroviral oncogene to be molecularly cloned (18,28). However, the analysis of c-mos RNA expression and the detection of protein products in normal tissues have been difficult. Early information about putative transcription regulation sequences at the c-mos locus was obtained from DNA transfer experiments. It was found that a cellular DNA sequence -1 kilobase (kb) upstream from the mos open reading frame has transcription termination functions and can inhibit expression of the c-mos gene transforming activity in NIH/3T3 cells (4,24,44). The presence of this cis-acting inhibitory sequence suggested that transcription of the c-mos gene may be tightly regulated and restricted to a few specific cell types (24, 32). Likewise, it emerged from similar transfection experiments that a low level (1 to 10 copies per cell) of mos transcripts (43, 45) and a very low level of mos protein (29) are sufficient to transform NIH/3T3 fibroblasts. These results prompted us to reexamine the question of c-mos expression in normal mouse tissues. Using a sensitive Si nuclease assay, we have been able to detect mos transcripts in 19-day embryos and gonads of adult mice (32). Subsequent Northern analysis revealed a 1.7-kb transcript in testes, a 1.4-kb transcript in ovaries, and -1.3-and 2.3-kb transcripts in male and female embryos (32).Here we report the detection of mos transcripts in additional tissues. We have mapped the mos RNA initiation sites in both testes and ovaries and examined the pattern of expression during postnatal development of the gonads. We also show that mos RNA transcripts in testes are expressed in haploid germ...

c-mos expression in mouse oocytes is controlled by initiator-related sequences immediately downstream of the transcription initiation site.

“…The size difference is apparently due to tissue-specific utilization of different promoters. In the testis, the transcription initiation sites are clustered around -280 bp, while in the ovary they have been mapped to around -70 bp with respect to the first ATG of the c-mos open reading frame (30). Neither of the two promoter regions contain TATA boxes, GC-rich tracts, or other identified transcriptional regulatory sequences.…”

“…Si nuclease analysis has indicated that c-mos produces 1.7-and 1.4-kb transcripts in mouse testis and ovary, respectively (30,31). The size difference is apparently due to tissue-specific utilization of different promoters.…”

“…c-mos, in particular, is of interest in that its major sites of expression appear to be limited to male and female germ cells of the mouse and several other species (11,20,26,30,33,35), suggesting a specific role for this proto-oncogene in meiosis. Indeed, microinjection of Xenopus and mouse oocytes with antisense oligonucleotides has demonstrated that c-mos is required for meiosis in both species (23,25,33), apparently acting as a cytostatic factor in Xenopus eggs (34) and functioning to maintain activity of maturation-promoting factor by stabilizing cyclin B in mouse eggs (22).…”

Microinjection of antisense c-mos oligonucleotides prevents meiosis II in the maturing mouse egg.

“…While some protooncogenes are expressed in many cell types, others display more restricted patterns of expression, suggesting that they play a role in particular developmental schemes. c-mos is unique among the protooncogenes in that its major sites of expression appear limited to male and female germ cells of the mouse and of several other species (1)(2)(3)(4)(5)(6)(7)(8), suggesting a specific function for c-mos in meiotic cell types.…”

Serum starved v-mos-transformed cells are unable to appropriately downregulate cyclins and CDKs

“…Its cellular homolog, c-mos, is also capable of transforming ®broblasts when placed under the control of long terminal repeat (LTR) sequences of Mo-MSV (Blair et al, 1981;Oskarsson et al, 1980). Relatively high levels of c-mos mRNA expression have been found in germ cells from a variety of vertebrates while much lower levels of expression have been detected in some somatic tissues (Goldman et al, 1987;Herzog et al, 1988Herzog et al, , 1989Keshet et al, 1988;Li et al, 1993;Mutter and Wolgemuth, 1987;Propst et al, 1987;Propst and Vande Woude, 1985;Sagata et al, 1988;Tsui et al, 1993). Mos has been shown to have a critical role in oocyte maturation as demonstrated by the phenotype of mos de®cient (knockout) mice (Colledge et al, 1994;Hashimoto et al, 1994).…”