Control of mouse U1 snRNA gene expression during in vitro differentiation of mouse embryonic stem cells.

Early in mouse development, two classes of U1 RNAs, mU1a and mU1b, are synthesized, but as development proceeds, transcription of the embryo-specific mU1b genes is selectively down-regulated to a barely detectable level. We show here that during in vitro differentiation of mouse embryonic stem (ES) cells, both exogenously introduced and endogenous U1b genes are subject to normal developmental regulation. Thus, ES cells represent a convenient isogenic system for studying the control of expression of developmentally regulated snRNA genes. Using this system, we have identified a region in the proximal 5'flanking region, located outside the PSE element, that is responsible for differential transcription of the mU1a and mU1b genes in both developing cells and transiently transfected NIH 3T3 cells.

Experimental metastasis of mouse embryonal carcinoma cell lines to specific locations.

Embryonal carcinoma cell lines produced tumors in highly specific and unusual sites when injected into mice. The pattern that developed when cells were injected into the left ventricle of the heart involved target organs related either to specific nerve patterns or neuronal outgrowth factors, or to pathways of primordial germ cell migration. Major sites included the ovary, testis, adrenal, iris, whiskers, and male submaxillary gland. Neither local growth responses, determined by direct injection of tumor cells into different organ parenchyma, nor initial attachments, observed upon injection of radiolabeled cells, appeared to sufficiently account for the specificity of tumor metastases occurring after arterial injection. However, tissue from uninjected target sites, but not other tissues, stimulated the in vitro migration of embryonal carcinoma cells. Conditioned medium from only target tissues had a similar effect. These results suggest that the specificity of this tumor pattern may depend on migration responses that are significant in the localization of embryonic germ and neural cells. The specific metastatic pattern observed following i.p. injection of embryonal carcinoma cells, involving only the ovary, appeared to require an additional component of high adhesivity to the target organ.

Nerve growth factor stimulation of mouse embryonal carcinoma cell migration.

Embryonal carcinoma cells localize to a specific array of target tissues including the male submaxillary gland following intracardiac injection. These target tissues or conditioned medium derived from them have been shown to stimulate the in vitro migration of embryonal carcinoma cells. Here we show that in vitro migration of mouse embryonal carcinoma cells is induced by the simultaneous presence of two different components. One of these is a specific requirement for a fibronectin substrate. The other component is present in male submaxillary conditioned medium and may be nerve growth factor (NGF). Migration of embryonal carcinoma cells on a fibronectin substrate could be induced by purified NGF, but not epidermal growth factor, and submaxillary conditioned medium contained very high levels of nerve outgrowth activity which could be blocked by anti-NGF antibody. Only the high molecular weight 7S NGF complex was active in inducing migration, while the low molecular weight 2.5S NGF inhibited migration. Neither type of NGF or submaxillary conditioned medium stimulated the in vitro growth of embryonal carcinoma cells. 7S NGF thus differentially affects murine embryonal carcinoma cells by inducing cell motility but not cell division. That embryonal carcinoma cells require 7S NGF for their migration in vitro raises the interesting possibility that these cells may respond similarly in vivo, and be stimulated by tissue-specific high molecular weight NGF molecules to migrate or extravasate into the parenchyma of target organs.