Macrophages are required for cell death and tissue remodeling in the developing mouse eye

To identify and characterize tissue remodeling processes mediated by macrophages, we have generated transgenic mice in which diphtheria toxin is expressed from a macrophage-specific transgene. Expression of the transgene disrupts subsets of mature macrophages in both the eye and the peritoneal cavity and results in the persistence of two normally transient ocular tissues, the hyaloid vasculature and the pupillary membrane. Furthermore, the cells comprising the pupillary membrane appear alive up to 14 days after the structure is normally remodeled, suggesting that the macrophage actively elicits target cell death. Thus, these transgenic mice provide direct evidence for the active involvement of macrophages in developmentally programmed tissue remodeling and identify the hyaloid vessels and the pupillary membrane in the eye as targets of macrophage-mediated remodeling.

Predisposition to neoplastic transformation caused by gene replacement of H-ras1

Homologous recombination was used to introduce a nominally transforming mutation into an endogenous H-ras1 gene in Rat1 fibroblasts. Although both the mutant and the remaining normal allele were expressed equally, the heterozygous cells were not neoplastically transformed. Instead, spontaneously transformed cells arose from the heterozygotes at a low frequency, and the majority of these cells had amplified the mutant allele. Thus, the activated H-ras1 allele was not by itself dominant over the normal allele but predisposed cells to transformation by independent events, such as amplification of the mutant allele.

Torso, a receptor tyrosine kinase required for embryonic pattern formation, shares substrates with the sevenless and EGF-R pathways in Drosophila

The maternally expressed Drosophila gene torso (tor) is a receptor tyrosine kinase that, when activated, initiates a signal transduction cascade that is responsible for the proper differentiation of the terminal, nonsegmented regions of the embryo. l(1)pole hole, the Drosophila raf-1 serine-threonine kinase homolog, and corkscrew, a tyrosine phosphatase, have been shown previously to function in this signal transduction pathway. We have identified other products in this pathway by carrying out a mutagenesis screen for dominant suppressors of a tor gain-of-function allele. More than 40 mutations, some of which fall into seven complementation groups, have been characterized genetically. Two of these correspond to mutations in ras-1 and Son of sevenless (Sos), which also function in the sevenless and EGF receptor (Der) tyrosine kinase pathways. The phenotypes of several other Su(tor) mutations suggest that they also function in other receptor tyrosine kinase-activated pathways at different times during Drosophila development.

Domains of E1A that bind p105Rb, p130, and p300 are required to block nerve growth factor-induced neurite growth in PC12 cells

Nerve growth factor (NGF) causes PC12 cells to cease division and undergo sympathetic neuron-like differentiation, including neurite outgrowth. We have tested whether differentiation and division share overlapping control mechanisms in these cells. To do this, we have perturbed the activity of proteins known to participate in cell-cycle regulation by introducing the E1A oncogene or its mutant forms via microinjection into PC12 cells. The E1A protein binds to several putative cell cycle control proteins, including p105Rb (the product of the retinoblastoma susceptibility gene), as well as others of unknown function such as p130, p107, and p300. Similar to previous results, we find that wild-type E1A abrogates NGF-induced neurite extension. However, NGF does cause neurite outgrowth in the presence of E1A mutants known to have greatly reduced binding to either p105Rb and p130 or p300. Our experiments suggest that p105Rb, p130, and p300 may participate either in E1A-mediated inhibition of differentiation or in the NGF signal transduction pathway. We also report here that NGF affects phosphorylation of p105Rb, suggesting that Rb mediates at least some of NGF's effects. Our results raise the possibility that putative cell-cycle control proteins may participate not only in NGF-induced cessation of division but also in differentiation.

The human gene AHNAK encodes a large phosphoprotein located primarily in the nucleus

AHNAK is a newly identified human gene notable for the exceptional size (c.a. 700 kD) and structure of its product, and for the repression of its expression in human neuroblastoma cells. Here we report the identification and partial characterization of the protein encoded by AHNAK. The protein is located principally (but not exclusively) in the nucleus and is phosphorylated on both serine and threonine. The abundance of the protein increases appreciably when cells withdraw from the division cycle, in response to either withdrawal of serum (fibroblasts) or differentiation (neuroblastoma cells). By contrast, the amount of phosphorylation appears to diminish in those settings. The considerable abundance and conjectured fibrous structure of AHNAK protein suggest a role in cytoarchitecture, but no function can yet be discerned.

Directed establishment of rat brain cell lines with the phenotypic characteristics of type 1 astrocytes

Interest in obtaining cell lines for use in studies on the development and biochemistry of the central nervous system has motivated efforts to establish cells from primary brain cultures by the use of oncogene-transfer techniques. In previous reports, cell lines derived from astrocytes in this way have had immature or abnormal phenotypes. We have explored the possibility of specifically "targeting" expression of exogenous oncogenes to differentiated astrocytes by using the promoter of the gene encoding glial fibrillary acidic protein, which is expressed almost exclusively in such cells. We report here that cell lines displaying the phenotypic characteristics of type 1 astrocytes can be established reproducibly in this manner. Given the heterogeneity of primary cultures, the availability of clonal cell lines displaying characteristics of type 1 astrocytes should greatly facilitate our understanding of the biology of these cells.

A human gene (AHNAK) encoding an unusually large protein with a 1.2-microns polyionic rod structure

We report here the identification and partial characterization of a human gene (designated AHNAK) that encodes an unusually large protein (approximately 700 kDa). AHNAK is expressed by means of a 17.5-kilobase mRNA in diverse cellular lineages but is typically repressed in cell lines derived from human neuroblastomas and in several other types of tumors. Unique-sequence domains at the two ends of the protein flank a large internal domain (approximately 4300 amino acids) composed of highly conserved repeated elements, most of which are 128 amino acids in length. The repeated elements in turn display a redundant motif, marked by the recurrence of proline at every seventh residue. Within these sequences, hydrophobic and hydrophilic residues alternate in a manner that is incompatible with a helical coiled-coil structure. Instead, we propose a structure resembling a beta-strand but with a periodicity of 2.33. The structure would engender a polyionic rod approximately 1.2 microns long. Preliminary evidence indicates that the protein resides predominantly within the nucleus, but no function has yet been discerned.

Specific association of the proto-oncogene product pp60c-src with an intracellular organelle, the PC12 synaptic vesicle

The protein product of the proto-oncogene c-src is a membrane-associated tyrosine kinase of unknown function. Identification of pp60c-src target membranes may elucidate the function of the c-src protein. The available evidence indicates that pp60c-src associates with distinct membranes within single cell types and has different distributions in different cell types. Our experiments demonstrate targeting of pp60c-src to an isolatable and biochemically identified membrane fraction in the neuroendocrine cell line PC12. The c-src protein was found to be specifically associated with synaptic vesicles since: (a) the pp60c-src immunofluorescent pattern overlapped with a synaptic vesicle marker, synaptophysin; (b) a significant proportion (44%) of the pp60c-src from PC12 but not fibroblast postnuclear supernatants was recovered in a small vesicle fraction; (c) an anti-synaptophysin cytoplasmic domain antibody immunodepleted all of the pp60c-src vesicles in this fraction, and (d) pp60c-src copurified during a 100-fold purification of PC12 synaptic vesicles. These results suggest a role for the c-src protein in the regulation of synaptic vesicle function.

Human protein-tyrosine kinase gene HCK: expression and structural analysis of the promoter region

The vertebrate gene HCK encodes a protein-tyrosine kinase that is closely related to the product of the proto-oncogene SRC. HCK is expressed principally in monocytic and granulocytic hematopoietic cells, in coordination with differentiation of these cells. Here we report an initial description of the mechanisms by which expression of human HCK is controlled. Induction of the gene during differentiation was manifested by an increase in the steady-state levels of HCK RNA and protein product. The accumulation of RNA apparently resulted from modulation of transcription itself, since no change occurred in the stability of the transcripts. Transcription initiated at multiple sites, clustered c. 145 nucleotides upstream of the first intron of HCK. The sequence of 660 bp upstream of the major initiation site was determined, revealing candidate binding sites for Sp1 and AP-2 transcription factors, but neither TATA nor CAAT elements. Comparison to the same region of the mouse hck locus showed five small regions of similarity, only two of which were topographically analogous between the two sequences. It appears that expression of HCK is regulated primarily through control of transcription, but the mechanisms by which tissue-specific expression and increase of transcription during differentiation are achieved remain to be explored.

Premature termination of transcription from the P1 promoter of the mouse c-myc gene

Modulation of transcriptional elongation within the c-myc gene is thought to play a major role in determining levels of c-myc mRNA in both normal and tumor cells. A discrete site of blockage to transcriptional elongation has previously been localized at the 3' end of exon 1 of the mouse and human c-myc genes. We here identify an additional site of transcriptional attenuation that is located between the P1 and P2 promoters of the c-myc gene and that mediates premature termination of transcripts initiating from the P1 promoter. A 95-nucleotide DNA fragment derived from this region prematurely terminated transcription when placed downstream from the promoter of the H-2Kbm1 gene and assayed by expression in Xenopus oocytes. We also show that the previously identified attenuation signal in exon 1 of the mouse c-myc gene can mediate premature termination of P1-initiated transcripts. Premature termination of P1-initiated transcripts presumably increases transcription from the downstream P2 promoter; aberrant regulation of this termination may explain the increased use of the P1 promoter that is characteristic of certain tumors in which myc is overexpressed.

A 3' truncation of myc caused by chromosomal translocation in a human T-cell leukemia is tumorigenic when tested in established rat fibroblasts

We have previously identified in human T-leukemia cells a myc gene with an unusual 3' rearrangement, and we have shown that expression of the gene is deregulated by stabilization of mRNA. Here we report that the rearranged gene transforms established rat fibroblasts to a tumorigenic phenotype. In hybrid genes, the transforming capability segregates with the 3' rearrangement. Transformation is apparently due to a more than fivefold enhancement in myc expression, attributable to stabilization of mRNA. The rearranged allele of myc also contains a point mutation in a region upstream of the gene, identified previously as a potential negative regulator of myc expression. The mutation may increase expression of myc, but not sufficiently to cause cellular transformation. Our findings enlarge the variety of genetic lesions that may activate myc to an oncogene and sustain the view that augmented expression of an otherwise normal allele of myc can be pathogenic.

Dominant male sterility in mice caused by insertion of a transgene

While examining a series of transgenic mouse lines carrying the HCK protooncogene, we encountered one line in which males hemizygous for the transgene were sterile. The sterile males mated normally but failed to impregnate females. Light and electron microscopy revealed that spermatogenesis proceeds normally until nuclear condensation, which occurs but gives rise to a variety of abnormally shaped nuclei. Expression of the transgene was not detectable. Thus, the insertion itself probably caused the abnormal phenotype by disrupting a gene (or genes) important in spermatogenesis. The mutation is genetically dominant, causing an abnormal phenotype even though the sterile mice carry an ostensibly normal counterpart of the disrupted locus. The mutant phenotype is completely penetrant only in some genetic backgrounds, suggesting a modifying influence from a second locus. Junctions between the inserted transgene and adjoining cellular DNA were cloned, allowing us to confirm the heterozygous nature of the genetic disruption and to detect and associated deletion. We have designated the mutation Lvs (lacking vigorous sperm) and presume that it may define a previously undescribed locus important in spermatogenesis.

Expression of CD44 is repressed in neuroblastoma cells

We have used cDNA subtractive cloning to identify a group of human genes that are expressed in diverse differentiated derivatives of neural crest origin but not in neuroblastoma cell lines. One of these genes was identified as CD44, which encodes an integral membrane glycoprotein that serves as the principal receptor for hyaluronate and participates in specific cell-cell and cell-extracellular matrix interactions. The repression of CD44 expression in neuroblastoma cell lines might be relevant to their high metastatic potential. We have cloned full-length cDNAs corresponding to CD44 trancscripts and identified a novel splice variant of CD44 lacking 31 amino acids of the extracellular domain. As a first step toward analysis of CD44 downregulation in neuroblastoma cells, we have mapped the CD44 RNA initiation site and analyzed the structure of the upstream regulatory region. We constructed a series of plasmids containing different amounts of CD44 upstream regulatory region linked to the bacterial chloramphenicol acetyltransferase gene and then analyzed their ability to promote transcription in neuroblastoma and melanoma cells. We found that a DNA segment including about 150 bp of the CD44 upstream region and the 5' end of the gene itself was sufficient to induce substantial transcription of the chloramphenicol acetyltransferase gene in both neuroblastoma and melanoma cells. Several upstream cis-acting elements contribute to the downregulation of CD44 in neuroblastoma cells, the most prominent being a 120-bp DNA fragment located 450 bp upstream to the RNA initiation site. Our data suggest that multiple factors might be involved in downregulation of CD44 in neuroblastoma cells.

Identification of platelet membrane proteins that interact with amino-terminal peptides of pp60c-src

Platelets contain exceptionally high levels of pp60c-src and, thus, provide a convenient system for investigating the physiological function of this protein-tyrosine kinase. We have employed chemical cross-linking of myristylated amino-terminal peptides of pp60c-src to platelet membranes in order to identify platelet membrane components that interact with pp60c-src to regulate or mediate its activity. We detected specific binding of radioiodinated peptides to platelet membrane proteins of 32, 50, 92, and 105 kDa. The 32-kDa protein may be related to the putative src receptor component recently identified in fibroblast membranes. The most reactive platelet protein, however, is the 50-kDa protein, which is either absent or nonreactive in fibroblast membranes. Binding of src peptides to this protein was saturable, and we estimate the presence of approximately 1 x 10(6) of the 50-kDa binding sites per platelet. The specificity of the peptide binding to the 50- and 32-kDa platelet proteins was analyzed by competition with different peptides. The binding sites displayed an absolute requirement for an N-myristoyl moiety and a strong preference for pp60c-src amino-terminal sequences. The identification of these src-interacting proteins may help to decipher the biochemical pathways in which platelet pp60c-src is involved.

Isolation of Legionella maceachernii from an immunocompromised patient with severe underlying lung disease

OBJECTIVE

To present the first case in Australia in which Legionella maceachernii has been documented as a cause of infection.

CLINICAL FEATURES

A 71-year-old retired carpenter with underlying autoimmune disease and pulmonary fibrosis was admitted to hospital with increasing shortness of breath and fever, and pneumonia was diagnosed.

INTERVENTION AND OUTCOME

He was treated with ampicillin then gentamicin and flucloxacillin and later imipenem but his condition deteriorated with progression of the pneumonia and abscess formation and he died 25 days after admission. Subsequently, L. maceachernii was isolated from three sputum samples collected before the patient died.

CONCLUSION

There is a need for clinical awareness of the value of sputum culture in the diagnosis of infection due to Legionella spp. other than L. pneumophila.

The amino-terminal 14 amino acids of v-src can functionally replace the extracellular and transmembrane domains of v-erbB

The retroviral oncogene v-erbB encodes a truncated form of the receptor for epidermal growth factor, an integral membrane protein-tyrosine kinase. By contrast, the oncogene v-src encodes a protein-tyrosine kinase that is a peripheral membrane protein. The morphologies and spectra of cells transformed by these two oncogenes differ. In an effort to identify the functional determinant(s) of these differences, we constructed and tested first deletion mutants of v-erbB and then chimeras between v-src and v-erbB. As reported previously, the absence of any membrane anchorage eliminated transformation by v-erbB. Anchorage of the cytoplasmic kinase domain of v-erbB to membranes with amino-terminal portions of the v-src protein permitted transformation. The phenotype and spectrum of transformation were those expected for v-erbB rather than for v-src. The transforming chimeras lost their biological activity if the signal for myristylation at the amino terminus of v-src was compromised by mutation. Biochemical fractionations revealed a correlation between transforming activity and the association of chimeric gene products with the membrane fraction of the cell. For reasons not yet apparent, the combined presence of membrane anchorage domains of v-src, and the transmembrane domain of v-erbB in the same chimera typically (but not inevitably) impeded transformation. Our results suggest that the specificity of transformation by v-erbB resides in the selection of substrates by the cytoplasmic domain of the gene product. The protein retains access to those substrates even when anchored to the membrane in the manner of a peripheral rather than a transmembrane protein.