Down's syndrome: abnormal neuromuscular junction in tongue of transgenic mice with elevated levels of human Cu/Zn-superoxide dismutase

To investigate the possible involvement of Cu/Zn-superoxide dismutase (CuZnSOD) gene dosage in the neuropathological symptoms of Down's syndrome, we analyzed the tongue muscle of transgenic mice that express elevated levels of human CuZnSOD. The tongue neuromuscular junctions (NMJ) in the transgenic animals exhibited significant pathological changes, namely, withdrawal and destruction of some terminal axons and the development of multiple small terminals. The ratio of terminal axon area to postsynaptic membrane decreased, and secondary folds were often complex and hyperplastic. The morphological changes in the transgenic NMJ were similar to those previously seen in muscles of aging mice and rats as well as in tongue muscle of patients with Down's syndrome. The findings suggest that CuZnSOD gene dosage is involved in the pathological abnormalities of tongue NMJ observed in Down's syndrome patients.     

v-myc alters the response of a cloned mouse mammary epithelial cell line to lactogenic hormones

Several oncogenes have now been implicated in mammary carcinogenesis. We investigated the phenotypic effects of expressing three representative oncogenes in mammary epithelial cells. v-myc (coding for a nuclear protein), v-Ha-ras (a G-protein homologue) and v-fgr (a tyrosine kinase) genes were introduced into the nontumorigenic clone 14 of the mouse mammary epithelial cell line COMMA-1D. Their effects upon growth and differentiation were determined. Anchorage-independent growth was induced by all three oncogenes with low efficiency. v-Ha-ras and v-fgr induced tumorigenicity in nude mice. The effect of oncogenes upon parameters unique to mammary epithelial cells in vitro was assayed. Both v-myc and v-fgr abolished the ability of clone 14 to grow as three-dimensional branching structures in hydrated collagen gel. v-fgr completely and v-myc partially inhibited the expression of the epithelium specific cytokeratins. Clone 14 can be induced to produce the beta-casein milk protein by the combination of the lactogenic hormones, dexamethasone, insulin, and PRL. Introduction of v-myc into clone 14 cells resulted in an estimated 50-fold increased induction of beta-casein protein and at least a 60-fold increase in beta-casein mRNA. The number of cells stained with anti-beta casein antibodies also showed a 10-fold increase after v-myc introduction. This still required the synergistic action of all three lactogenic hormones. Thus v-myc can alter the normal response of mammary epithelial cells to lactogenic hormones.     

Oncogene expression in mammary epithelial cells.

Mouse strains which develop tumors at a high incidence with characteristics very similar to human cancers have been derived over the last 8 years. The tumors are caused by defined genetic alterations in the mouse genome. Three areas of research have contributed to the derivation of these mouse strains: (1) Molecular analysis of human tumors has shown that distinct oncogenes and tumor suppressor genes are consistently involved in a high percentage of primary tumors. (2) Regulatory enhancer-promoter sequences have been identified which direct gene expression to specific target cells, preferentially mammary epithelial cells. (3) The introduction of recombinant DNA molecules into fertilized mouse eggs by microinjection and integration of the injected DNA into the genome of injected cells has given rise to mutant mouse strains with unique and defined genetic alterations. Studies with different promoter-oncogene combinations introduced into transgenic mouse strains have led to the following general conclusions: (1) Oncogenes expressed in mammary gland cells predispose transgenic mice to mammary tumors. (2) The oncogenic potential of individual oncogenes in mammary epithelial cells differs. (3) Oncogene expression initially often causes a preneoplastic state affecting growth and differentiation parameters of cells. (4) The expression of different oncogenes synergizes to reduce tumor latency. Synergism can also be observed with physiological growth signals like estrogen or growth hormone. The oncogenes with a role in mammary carcinomas which have been investigated in transgenic mice will be described here. The phenotypic consequences of oncogene expression and the implications for the multistep carcinogenesis model will be discussed.     

Glucocorticoid hormone interactions with cloned proviral DNA of mouse mammary tumor virus

The molecular details of glucocorticoid hormone regulation of expression of the mouse mammary tumor virus (MMTV) proviral gene have been investigated. Cloned proviral DNA was introduced into cultured cells by a gene transfer procedure. DNA acquired by transfection was shown to be expressed in a hormone regulated fashion. The proviral DNA was fragmented and recombined in vitro with an indicator gene to delimit the hormone response sequence. Inducibility of the indicator gene (thymidine kinase gene from Herpes Simplex Virus, tk) was observed upon recombination with the long terminal repeat (LTR) sequence of MMTV. Further delimitation of the LTR DNA demonstrated that 202 nucleotides located 5' of the RNA initiation site are sufficient to confer glucocorticoid regulation. In vitro interaction of LTR DNA with glucocorticoid hormone receptor complex, showed a preferential affinity to the same sequence which mediated hormonal regulation in transfected cells. Evidence for a direct receptor gene interaction in the process of gene induction was gained by the measurement of the kinetics of induction and the use of a glucocorticoid antagonist (RU 486). The induction of the transfected gene is very rapid, independent of simultaneous protein synthesis and requires a functional glucocorticoid receptor hormone complex.     

Overproduction of human Cu/Zn-superoxide dismutase in transfected cells: extenuation of paraquat-mediated cytotoxicity and enhancement of lipid peroxidation.

The 'housekeeping' enzyme Cu/Zn-superoxide dismutase (SOD-1) is encoded by a gene residing on human chromosome 21, at the region 21q22 known to be involved in Down's syndrome. The SOD-1 gene and the SOD-1 cDNA were introduced into mouse L-cells and human HeLa cells, respectively as part of recombinant plasmids containing the neoR selectable marker. Human and mouse transformants were obtained that expressed elevated levels (up to 6-fold) of authentic, enzymatically active human SOD-1. This enabled us to examine the consequences of hSOD-1 gene dosage, apart from gene dosage effects contributed by other genes residing on chromosome 21. Human and mouse cell clones that overproduce the hSOD-1 had altered properties; they were more resistant to paraquat than the parental cells and showed an increase in lipid peroxidation. The data are consistent with the possibility that gene dosage of hSOD-1 contributes to some of the clinical symptoms associated with Down's syndrome.     

New mammary epithelial and fibroblastic cell clones in coculture form structures competent to differentiate functionally

We have established and characterized a spontaneously immortalized, nontumorigenic mouse mammary cell line, designated IM-2. IM-2 cells synthesize large amounts of the milk protein beta-casein upon addition of lactogenic hormones. The induction of beta-casein occurs rapidly and does not require any exogenous extracellular matrix components. The IM-2 cell line is morphologically heterogeneous and could be separated into cell clones with epithelial and fibroblastic characteristics. In monoculture, none of the epithelial clones could be induced to synthesize caseins. Coculture of epithelial and fibroblastic clones, however, rendered the epithelial cells competent to differentiate functionally; the addition of lactogenic hormones to these cocultures resulted in the synthesis of beta-casein in amounts comparable to that seen with the original IM-2 line. Using this unique cell system, we have investigated the interrelationships between different steps in differentiation leading to hormone-induced casein production. Independent of hormones, epithelial-fibroblastic cell contacts led to the formation of characteristic structures showing the deposition of laminin. We found that the epithelial cells located in these structures also exhibited significantly increased levels of cytokeratin intermediate filament polypeptides. Double immunofluorescence revealed that the cells inducible by hormones to synthesize casein, colocalized exactly with the areas of laminin deposition and with the cells showing greatly intensified cytokeratin expression. These results suggest that hormone-independent differentiation events take place in response to intercellular epithelial-mesenchymal contacts. These events in turn bring about a state of competence for functional differentiation after lactogenic hormonal stimulation.