Molecular basis of mouse microphthalmia (mi) mutations helps explain their developmental and phenotypic consequences.

Mutations in the mouse microphthalmia (mi) gene affect the development of a number of cell types including melanocytes, osteoclasts and mast cells. Recently, mutations in the human mi gene (MITF) were found in patients with Waardenburg Syndrome type 2 (WS2), a dominantly inherited syndrome associated with hearing loss and pigmentary disturbances. We have characterized the molecular defects associated with eight murine mi mutations, which vary in both their mode of inheritance and in the cell types they affect. These molecular data, combined with the extensive body of genetic data accumulated for murine mi, shed light on the phenotypic and developmental consequences of mi mutations and offer a mouse model for WS2.

Natural, genetically determined resistance toward influenza virus in hemopoietic mouse chimeras. Role of mononuclear phagocytes.

Radiation chimeras produced by crosswise transfers of bone-marrow cell among histocompatible mice susceptible, or genetically resistant, to lethal challenge by a number of myxoviruses were used to test whether macrophage resistance (as assessed in vitro) and resistance of the animal (as measured in vivo), both previously shown to be brought about by the gene Mx, were causally related. 49 chimeras were tested individually, both of resistance of their macrophages to in vitro challenge with M-TUR (a strain of avian influenza virus A/Turkey/England/63 adapted to grow in cultured mouse peritoneal macrophages), and for resistance of the animal in vivo upon challenge with pneumotropic, neurotropic, or hepatotropic influenza viruses. Cultivated Kupffer cells and peritoneal macrophages harvested from chimeric mice expressed the resistance phenotype of the bone-marrow donor irrespective of the host environment in which they had differentiated. However, susceptibility or resistance in vivo was according to the genotype of the host. Thus, inborn resistance of radiation chimeras was found to be independent of Mx-gene expression in cells of the hemopoietic system.

Hepatocyte-tumor cell interaction in vitro. I. Conditions for rosette formation and inhibition by anti-H-2 antibody.

Murine hepatocytes, isolated by an in situ collagenase-perfusion technique and cultured in Petri dishes, were shown to form rosettes with liver-metastasizing syngeneic tumor cells. Pretreatment of the tumor cells with neuraminidase generally increased the binding, whereas pretreatment of the liver cells with neuraminidase abolished the binding completely. The tumor-cell binding may be mediated by the previously described lectin-like receptor of hepatocytes that also was sensitive to neuraminidase treatment and that bound desialylated cells better than normal cells. Anti-H-2 sera could efficiently inhibit the rosette formation of metastatic tumor cells with the hepatocytes, which points to a possible role of H-2 molecules in this interaction of neoplastic and normal cells.

Genetically determined, interferon-dependent resistance to influenza virus in mice.

The genetically determined resistance towards orthomyxoviruses exhibited by mice homozygous (A2G) or heterozygous (A2G X A/J) for the gene Mx was abolished or greatly diminished by treatment with anti-interferon globulin (AIF). AIF induced increased susceptibility to challenge with hepatotropic, neurotropic, and pneumotropic strains of influenza A virus. Hepatotropic virus titers in blood and livers of AIF-treated, Mx-bearing mice were higher by a factor of 10(3)--10(6) than those in untreated mice of the same genotype, and were comparable to those in genetically susceptible (untreated or AIF-treated) mice. Peritoneal macrophages from Mx-bearing untreated mice were resistant to challenge with a macrophage-adapted strain of influenza A virus even in the presence of AIF. However, when macrophages were taken from resistant mice injected with AIF and also cultivated in the presence of AIF, they were as susceptible to the virus as macrophages taken from susceptible mice. We conclude that interferons is an important factor in resistance to orthomyxoviruses governed by the gene Mx.

Virus-specific interferon action. Protection of newborn Mx carriers against lethal infection with influenza virus.

The efficacy of interferon in antiviral protection of newborn mice differing at the Mx locus was investigated. Adult mice bearing the allele Mx exhibit a high degree of specific resistance toward lethal challenge with influenza viruses. In contrast, newborn Mx carriers are virtually as susceptible to influenza viruses as newborn mice devoid of Mx. Resistance can be abrogated by treating adult animals with anti-interferon serum. Here, we provide direct evidence of a virus-specific effect of interferon in vivo: newborn mice carrying the resistance gene Mx could be protected against lethal influenza virus infection with doses of interferon that were not protective in the absence of Mx. The efficacy of interferon towards a picornavirus (encephalomyocarditis virus) and a rhabdovirus (vesicular stomatitis virus) was independent of Mx.

Oct-3 and mammalian development: correction of discussion.

The title of the 5 June report on page 1445 by R. C. deL. Milton et al. should have been "Total chemical synthesis of a D-enzyme: The enantiomers of HIV-1 protease show reciprocal chiral substrate specificity." Figure 3 in the same report (p. 1447) was inadvertently printed upside down. The labels "L-HIV protease" and "D-HIV protease" were therefore under the wrong illustrations. The correct figure is printed below. [See figure in the PDF file]

The recessive phenotype displayed by a dominant negative microphthalmia-associated transcription factor mutant is a result of impaired nucleation potential.

In the DNA binding domain of microphthalmia-associated transcription factor (MITF), four mutations are reported: mi, Mi wh, mi ew, and mi or. MITFs encoded by the mi, Mi wh, mi ew, and Mi or mutant alleles (mi-MITF, Mi wh-MITF, Mi ew-MITF, and Mi or-MITF, respectively) interfered with the DNA binding of wild-type MITF, TFE3, and another basic helix-loop-helix leucine zipper protein in vitro. Polyclonal antibody against MITF was produced and used for investigating the subcellular localization of mutant MITFs. Immunocytochemistry and immunoblotting revealed that more than 99% of wild-type MITF and Mi wh-MITF located in nuclei of transfected NIH 3T3 and 293T cells. In contrast, mi-MITF predominantly located in the cytoplasm of cells transfected with the corresponding plasmid. When the immunoglobulin G (IgG)-conjugated peptides representing a part of the DNA binding domain containing mi and Mi wh mutations were microinjected into the cytoplasm of NRK49F cells, wild-type peptide and Mi wh-type peptide-IgG conjugate localized in nuclei but mi-type peptide-IgG conjugate was detectable only in the cytoplasm. It was also demonstrated that the nuclear translocation potential of Mi or-MITF was normal but that Mi ew-MITF was impaired as well as mi-MITF. In cotransfection assay, a strong dominant negative effect of Mi wh-MITF against wild-type MITF-dependent transactivation system on tyrosinase promoter was observed, but mi-MITF had a small effect. However, by the conjugation of simian virus 40 large-T-antigen-derived nuclear localization signal to mi-MITF, the dominant negative effect was enhanced. Furthermore, we demonstrated that the interaction between wild-type MITF and mi-MITF occurred in the cytoplasm and that mi-MITF had an inhibitory effect on nuclear localization potential of wild-type MITF.

Mutations in microphthalmia, the mouse homolog of the human deafness gene MITF, affect neuroepithelial and neural crest-derived melanocytes differently.

The mouse microphthalmia (Mitf) gene encodes a basic-helix-loop-helix-zipper transcription factor whose mutations are associated with abnormalities in neuroepithelial and neural crest-derived melanocytes. In wild type embryos, Mitf expression in neuropithelium and neural crest precedes that of the melanoblast marker Dct, is then co-expressed with Dct, and gradually fades away except in cells in hair follicles. In embryos with severe Mitf mutations, neural crest-derived Mitf-expressing cells are rare, lack Dct expression, and soon become undetectable. In contrast, the neuroepithelial-derived Mitf-expressing cells of the retinal pigment layer are retained, express Dct, but not the melanogenic enzyme genes tyrosinase and Tyrp1, and remain unpigmented. The results show that melanocyte development critically depends on functional Mitf and that Mitf mutations affect the neural crest and the neuroepithelium in different ways.

Inborn resistance of mice to mouse hepatitis virus type 3 (MHV3): liver parenchymal cells express phenotype in culture.

Primary monolayer cultures of hepatocytes isolated from adult resistant A/J or partially resistant A/Sn or C3H/HeJ mice exhibited resistance to MHV3 as the respective macrophages do: Compared to susceptible C57BL/6 hepatocyte cultures, cytopathic effect occurred later and was restricted to small foci, coinciding with areas specifically labelled by immunofluorescence. Production of infectious particles was delayed, titers being 100 to 1000 fold lower at the moment of maximal yields in susceptible cultures. Pretreatment with interferon could reduce the titers in susceptible cultures to a level as seen in resistant cultures not treated with interferon. Nevertheless, interferon was not responsible for the genetic resistance of hepatocytes: it reduced virus titers in susceptible and resistant cultures to the same extent and the addition of specific antibodies to interferon after infection did not augment susceptibility of resistant cultures. We assume that intrinsic resistance of liver parenchymal cells is an important facet of inborn resistance of mice in vivo.

Primary monolayer culture of adult mouse hepatocytes -- a model for the study of hepatotropic viruses.

Primary monolayer cultures of adult mouse hepatocytes isolated by collagenase perfusion of the liver in situ were exposed to 2 hepatotropic viruses, an avian influenza A virus adapted to grow in mouse liver in vivo and a herpes simplex type I virus. Influenza virus infection led to lysis ofindividual hepatocytes and total monolayer destruction within 18 to 120 hours after infection according to the virus dose used. Virus replication was evidenced by assaying hepatocyte supernates for hemagglutinin and infectivity, by immunofluorescent staining and by electron microscopy. Herpes virus infection resulted in polykaryocyte formation followed by nuclear pycnosis and cell lysis. Virus replication was assayed by titration of supernate infectivity.

Signaling and transcriptional regulation in early mammalian eye development: a link between FGF and MITF.

During vertebrate eye development, the optic vesicle is partitioned into a domain at its distal tip that will give rise to the neuroretina, and another at its proximal base that will give rise to the pigmented epithelium. Both domains are initially bipotential, each capable of giving rise to either neuroretina or pigmented epithelium. The partitioning depends on extrinsic signals, notably fibroblast growth factors, which emanate from the overlying surface ectoderm and induce the adjacent neuroepithelium to assume the neuroretinal fate. Using explant cultures of mouse optic vesicles, we demonstrate that bipotentiality of the optic neuroepithelium is associated with the initial coexpression of the basic-helix-loop-helix-zipper transcription factor MITF, which is later needed solely in the pigmented epithelium, and a set of distinct transcription factors that become restricted to the neuroretina. Implantation of fibroblast growth factor-coated beads close to the base of the optic vesicle leads to a rapid downregulation of MITF and the development of an epithelium that, by morphology, gene expression, and lack of pigmentation, resembles the future neuroretina. Conversely, the removal of the surface ectoderm results in the maintenance of MITF in the distal optic epithelium, lack of expression of the neuroretinal-specific CHX10 transcription factor, and conversion of this epithelium into a pigmented monolayer. This phenomenon can be prevented by the application of fibroblast growth factor alone. In Mitf mutant embryos, parts of the future pigment epithelium become thickened, lose expression of a number of pigment epithelium transcription factors, gain expression of neuroretinal transcription factors, and eventually transdifferentiate into a laminated second retina. The results support the view that the bipotential optic neuroepithelium is characterized by overlapping gene expression patterns and that selective gene repression, brought about by local extrinsic signals, leads to the separation into discrete expression domains and, hence, to domain specification.

Antiviral state against influenza virus neutralized by microinjection of antibodies to interferon-induced Mx proteins.

In mouse Mx+ cells, interferon alpha/beta induces the synthesis of the nuclear Mx protein, whose accumulation is correlated with specific inhibition of influenza viral protein synthesis. When Mx+ mouse cells are microinjected with the monoclonal anti-Mx antibody 2C12, interferon alpha/beta still induces Mx protein, but no longer inhibits efficiently the expression of influenza viral proteins as visualized by immunofluorescent labeling. However, interferon inhibition of an unrelated control virus, vesicular stomatitis virus, remains unchanged. Proteins with homology to mouse Mx protein are found in interferon-treated cells of a variety of mammalian species. In rat cells, for instance, rat interferon alpha/beta induces three Mx proteins which all cross-react with antibody 2C12 but differ in mol. wt and intracellular location, and it protects these cells well against influenza viruses. However, when rat cells are microinjected with antibody 2C12, interferon alpha/beta cannot induce an efficient antiviral state against influenza virus infection, whereas protection against vesicular stomatitis virus is not altered. These results show that both mouse and rat cells require functional Mx proteins for efficient protection against influenza virus. They further demonstrate that microinjection of antibodies is a promising way of elucidating the role of particular interferon-induced proteins in the intact cell.