Three-dimensional imaging of embryonic mouse kidney by two-photon microscopy.

Developing mammalian embryonic kidney becomes progressively more elaborate as the ureteric bud branches into undifferentiated mesenchyme. Morphological perturbations of nephrogenesis, such as those seen in inherited renal diseases or induced in transgenic animals, require careful and often tedious documentation by multiple methodologies. We have applied a relatively quick and simple approach combining two-photon microscopy and advanced three-dimensional (3-D) imaging techniques to visualize and evaluate these complex events. As compared with laser confocal microscopy, two-photon microscopy offers superior optical sectioning deep into biological tissues, permitting analysis of large, heterogeneous, 3-D structures such as developing mouse kidney. Embryonic and newborn mouse kidneys were fluorescently labeled with lectins, phalloidin, or antibody. Three-dimensional image volumes were then collected. The resulting volume data sets were processed using a novel 3-D visualization technique. Reconstructed image volumes demonstrate the dichotomous branching of ureteric bud as it progresses from a simple, symmetrical structure into an elaborate, asymmetrical collecting system of multiple branches. Detailed morphology of in situ cysts was elucidated in a transgene-induced mouse model of polycystic kidney disease. We expect this integration of two-photon microscopy with advanced 3-D image analysis will provide a powerful tool for illuminating a variety of complex developmental processes in multiple dimensions.

Rescue of the T-associated maternal effect in mice carrying null mutations in Igf-2 and Igf2r, two reciprocally imprinted genes.

In mice, only the paternal allele of the Igf2 gene, encoding insulin-like growth factor II (IGF-II) is expressed due to parental imprinting. Interestingly, the Igf2r gene, which encodes one of the two known receptors (IGF2R) to which IGF-II binds with high affinity is also subject to imprinting, but in a reciprocal fashion. This observation raises the possibility that imprinting of these loci serves to regulate the ratios of the gene products, since IGF2R provides a mechanism for IGF-II turnover. To test this hypothesis, we crossed mice mutant for Igf-2 with animals carrying the Thp chromosomal deletion, which encompasses the Igf2r locus. Inheritance of the Thp chromosome through the maternal germline results in a dominant lethal maternal effect (Tme). However, as we show here, Thp/+ embryos that inherit the Thp maternally are variably rescued to birth if they also lack IGF-II. Based on these data, the Tme phenotype can be viewed as a dominant effect resulting from an overabundance of IGF-II.

Tyrosine phosphorylation of a gap junction protein correlates with inhibition of cell-to-cell communication.

Cell-to-cell communication is achieved by passage of small molecules through gap junction membrane channels. The expression of the transforming gene from Rous sarcoma virus, v-src, induces a rapid and dramatic reduction in cell-to-cell communication in cultured cells. To determine whether connexin43, a major gap junction protein expressed in fibroblasts, is a target for the v-src protein tyrosine kinase activity, we examined the phosphorylation state of connexin43 in cells expressing variants of src. Using an antipeptide serum that recognizes connexin43, we demonstrate that this protein is phosphorylated on serine and tyrosine residues in avian and mammalian cells expressing activated src proteins. Connexin43 from control cells and cells expressing nonactivated variants of the src protein was phosphorylated solely on serine residues. In lysates from v-src-transformed cells, all phosphorylated connexin43 molecules were cleared from the lysate by sequential immunoprecipitations using the phosphotyrosine antibodies, suggesting that each molecule of phosphorylated connexin43 contains both phosphoserine and phosphotyrosine. We have also examined junctional permeability in cells expressing src variants and find that loss of cell-to-cell communication correlates with tyrosine phosphorylation of connexin43.

Detection of phosphotyrosine-containing proteins in polyomavirus middle tumor antigen-transformed cells after treatment with a phosphotyrosine phosphatase inhibitor.

Cells transformed with the middle tumor antigen (mT) of polyomavirus were treated with sodium orthovanadate (Na3VO4), an inhibitor of phosphotyrosine phosphatases, to enhance for the detection of cellular proteins which are phosphorylated on tyrosine. Na3VO4 treatment of mT-transformed rat F1-11 cells resulted in a 16-fold elevation in the level of phosphotyrosine associated with total cellular proteins. Parental F1-11 cells displayed only a twofold increase in phosphotyrosine following Na3VO4 treatment. The abundance of phosphotyrosine in Na3VO4-treated mT-transformed F1-11 cells was twofold higher than in untreated Rous sarcoma virus (RSV)-transformed F1-11 cells and 3.5-fold lower than in Na3VO4-treated RSV-transformed F1-11 cells. Tyrosine phosphorylation of many cellular proteins, including p36, the major substrate of the RSV pp60v-src protein, was detected in Na3VO4-treated mT-transformed F1-11 cells at levels comparable to those observed in RSV-transformed cells. Some of the major protein species recognized by antiphosphotyrosine antibodies in Na3VO4-treated mT-transformed cells displayed electrophoretic mobilities similar to those detected in RSV-transformed F1-11 cells. Tyrosine phosphorylation of p36 was also detected in fibroblasts infected with polyomavirus. There was no detectable difference in the kinase activity of pp60c-src:mT extracted from untreated and Na3VO4-treated mT-transformed cells; however, Na3VO4 treatment of F1-11 and mT-transformed F1-11 cells was shown to inhibit the activity of phosphotyrosine phosphatases in a crude assay of total cellular activity with pp60v-src as the substrate. Thus, Na3VO4 treatment may allow the detection of phosphotyrosine-containing proteins in mT-transformed cells by preventing the turnover of phosphate on substrates phosphorylated by activated cellular protein-tyrosine kinases associated with mT. These results suggest that tyrosine phosphorylation of cellular proteins may be involved in the events that are responsible for mT-induced cellular transformation.

Monoclonal antibodies prepared against the major Drosophila nuclear Matrix-pore complex-lamina glycoprotein bind specifically to the nuclear envelope in situ.

A high molecular weight glycoprotein found associated with a nuclear matrix-pore complex-lamina (NMPCL) preparation obtained from Drosophila melanogaster embryos has been shown by in vitro analyses to be largely confined to this subcellular fraction. In contrast with several of the NMPCL proteins, this glycoprotein remains completely insoluble after treatment with 5 M urea. It has, therefore, been possible to separate the glycoprotein from other NMPCL components by differential urea extraction. The glycoprotein in the 5 M urea-extracted pellet has been solubilized by boiling in sodium dodecyl sulfate and purified to near-homogeneity by sequential steps of chromatography on hydroxylapatite and Sephacryl S-300 (both run in the presence of 0.1% sodium dodecyl sulfate), followed by affinity chromatography on lentil lectin-Sepharose. Over 30 hybridoma cell lines producing antibodies against this glycoprotein have been obtained. Monoclonality has been established for two of these lines (designated AGP-26 and AGP-78), and the antibodies they secrete have been further characterized. Western blot analysis has shown both antibodies to be monospecific (with respect to other Drosophila embryo polypeptides) for the major NMPCL glycoprotein; in addition, antibody AGP-78 has been shown to be weakly cross-reactive with glycoproteins of similar or identical molecular weight found associated with isolated nuclear fractions obtained from Xenopus oocytes, as well as chicken, opossum, and rat livers. Finally, both antibodies AGP-26 and AGP-78 react exclusively with the Drosophila nuclear periphery (nuclear envelope) in situ as demonstrated by indirect immunofluorescence analysis of larval cryosections. Based on these results as well as upon those of biochemical studies reported previously (Berrios, M., Filson, A. J., Blobel, G, and Fisher, P. A. (1983) J. Biol. Chem. 258, 13384-13390), we conclude that the major Drosophila NMPCL glycoprotein is the specific homolog of the high molecular weight glycoprotein recently shown using immunoelectron microscopy to be a distinct component of the rat liver nuclear pore complex (Gerace, L., Ottaviano, Y., and Kondor-Koch, C. (1982) J. Cell Biol. 95, 826-837).

A 174-kilodalton ATPase/dATPase polypeptide and a glycoprotein of apparently identical molecular weight are common but distinct components of higher eukaryotic nuclear structural protein subfractions.

A 174-kilodalton polypeptide of the Drosophila nuclear matrix-pore complex-lamina fraction has been identified as an ATPase/dATPase on the basis of direct UV photoaffinity labeling studies; a polypeptide with similar properties has been found in nuclear envelope fractions prepared from several vertebrate sources (Berrios, M., Blobel, G., and Fisher, P. A. (1983) J. Biol. Chem. 258, 4548-4555). This ATPase/dATPase polypeptide co-migrates on sodium dodecyl sulfate (SDS)-polyacrylamide gels with a glycoprotein also found in all of these fractions. In rat liver, this glycoprotein has been localized to the nuclear pore complex by means of immunoelectron microscopy (Gerace, L., Ottaviano, Y., and Kondor-Koch, C. (1982) J. Cell Biol. 95, 826-837). Following SDS denaturation and reduction/alkylation, chromatography of the Drosophila nuclear matrix-pore complex-lamina fraction on hydroxylapatite columns run in the presence of SDS results in the separation of two quantitatively major 174-kilodalton polypeptides. The peak of glycoprotein elution from the SDS-hydroxylapatite column correlates exactly with that of the early eluting 174-kilodalton species while the photolabeled ATPase/dATPase polypeptide co-chromatographs with the late eluting one. Identical results have been obtained with the rat liver nuclear envelope fraction. The chromatographically separated 174-kilodalton species from both organisms have been further distinguished through the use of polypeptide-specific antisera; finally, the glycoprotein purified from Drosophila embryos is fully sensitive to limited degradation by endoglycosidase H whereas the ATPase/dATPase polypeptide is completely resistant. We have thus established, using material obtained from two widely divergent higher eukaryotes, that the 174-kilodalton ATPase/dATPase is a quantitatively major nuclear matrix-pore complex-lamina component distinct from the nuclear pore complex glycoprotein of apparently identical molecular weight.