Gene transfer in fungi

Horizontal Transfer of Malignant Traits and the Involvement of Extracellular Vesicles in Metastasis

Metastases are responsible for the vast majority of cancer deaths, yet most therapeutic efforts have focused on targeting and interrupting tumor growth rather than impairing the metastatic process. Traditionally, cancer metastasis is attributed to the dissemination of neoplastic cells from the primary tumor to distant organs through blood and lymphatic circulation. A thorough understanding of the metastatic process is essential to develop new therapeutic strategies that improve cancer survival. Since Paget's original description of the "Seed and Soil" hypothesis over a hundred years ago, alternative theories and new players have been proposed. In particular, the role of extracellular vesicles (EVs) released by cancer cells and their uptake by neighboring cells or at distinct anatomical sites has been explored. Here, we will outline and discuss these alternative theories and emphasize the horizontal transfer of EV-associated biomolecules as a possibly major event leading to cell transformation and the induction of metastases. We will also highlight the recently discovered intracellular pathway used by EVs to deliver their cargoes into the nucleus of recipient cells, which is a potential target for novel anti-metastatic strategies.

In vivo rat model to study horizontal tumor progression

Most cancer deaths are due to metastases. Metastasis is an extraordinarily complex process by which cancer cells complete a sequential series of steps before they transform into a clinically detectable lesion. These steps typically include separation from the primary tumor, invasion through surrounding tissues and basement membranes, entry and survival in the circulation, lymphatic or peritoneal space, and arrest in a distant target organ and the formation of secondary tumors in distant organs.While proposed or accepted models and mechanisms of metastatic progression, have been demonstrated in experimental systems, none of them sufficiently explain all of the complexities associated with this process. These models can broadly be classified into two types, those occurring by vertical gene transfer (Darwinian) and those involving horizontal or lateral DNA transfer. Here, we describe an experimental system to study the metastatic process involving the horizontal transfer of circulating DNA.

Structural properties of double-stranded RNAs associated with biological control of chestnut blight fungus

Double-stranded RNAs (ds RNAs) are thought to be the cytoplasmic determinants responsible for the phenomenon of transmissible hypovirulence in the chestnut blight fungus Endothia parasitica [Murr.] Anderson. The three major ds RNA components associated with the North American hypovirulent strain, Grand Haven 2, were characterized with respect to molecular-hybridization specificity and RNase T1-digestion patterns. The large (L-RNA; approximately 9 kilobase pairs) and middle-sized (M-RNA; approximately 3.5 kilobase pairs) ds RNA components cross-hybridized under stringent conditions and exhibited indistinguishable partial and complete RNase T1 digestion patterns relative to their 5' and 3' termini. These results suggest that M-RNA was derived from L-RNA by an internal deletion event. The small (S-RNA; approximately 1 kilobase pair) RNA was unrelated to L- and M-RNA by these criteria. However, all three ds RNA components contained RNase T1-resistant oligonucleotides at one 5' terminus and at the corresponding 3' terminus of the complementary strand. These RNase T1-resistant species exhibited properties consistent with stretches of poly(uridylic acid) and poly(adenylic acid), respectively. The combined results are discussed in terms of the structural organization of hypovirulence-associated ds RNA molecules and their similarities to "double-stranded" RNA molecules observed in plant and animal cells infected with single-stranded RNA viruses.

Horizontal Transfer of DNA by the Uptake of Apoptotic Bodies

In this study we have raised the question of whether DNA can be transferred from one cell to another by phagocytosis of apoptotic bodies. We have used integrated copies of the Epstein-Barr virus (EBV) as a marker to follow the fate and expression pattern of apoptotic DNA in the phagocytotic host. Apoptosis was induced in EBV-carrying cell lines by irradiation before cultivation with either human fibroblasts, macrophages, or bovine aortic endothelial cells. Analysis of the expression pattern of EBV-encoded genes was performed by immunofluorescent staining as well as in situ hybridization. Cocultivation of apoptotic bodies from lymphoid cell lines containing integrated but not episomal copies of EBV resulted in expression of the EBV-encoded genes EBER and EBNA1 in the recipient cells at a high frequency. Fluorescence in situ hybridization analysis showed uptake of human chromatin as well as integrated EBV-DNA into the nuclei of bovine aortic endothelial cells. These data show that DNA may be rescued and reused from apoptotic bodies by somatic cells. In addition, our findings suggest that apoptotic bodies derived from EBV-carrying B lymphocytes may serve as the source of viral transfer to cells that lack receptors for the EBV virus in vivo.

Horizontal Transfer of DNA by the Uptake of Apoptotic Bodies

In this study we have raised the question of whether DNA can be transferred from one cell to another by phagocytosis of apoptotic bodies. We have used integrated copies of the Epstein-Barr virus (EBV) as a marker to follow the fate and expression pattern of apoptotic DNA in the phagocytotic host. Apoptosis was induced in EBV-carrying cell lines by irradiation before cultivation with either human fibroblasts, macrophages, or bovine aortic endothelial cells. Analysis of the expression pattern of EBV-encoded genes was performed by immunofluorescent staining as well as in situ hybridization. Cocultivation of apoptotic bodies from lymphoid cell lines containing integrated but not episomal copies of EBV resulted in expression of the EBV-encoded genes EBER and EBNA1 in the recipient cells at a high frequency. Fluorescence in situ hybridization analysis showed uptake of human chromatin as well as integrated EBV-DNA into the nuclei of bovine aortic endothelial cells. These data show that DNA may be rescued and reused from apoptotic bodies by somatic cells. In addition, our findings suggest that apoptotic bodies derived from EBV-carrying B lymphocytes may serve as the source of viral transfer to cells that lack receptors for the EBV virus in vivo.

Nucleotide and DNA uptake by Neurospora crassa: involvement of an uptake stimulating protein

The basal and DUSF (DNA-uptake-stimulating factor, described previously by Schablik and Szabó (1981) FEMS Microbiol. Lett. 10, 395-397) stimulated uptake of [3H]DNA and radioactive nucleotides by Neurospora crassa (FGSC 1118, slime) cell-wall-less strain was studied. The uptake of [3H]DNA by the cells is a saturable and time-dependent process. The pH and temperature optimum for [3H]DNA uptake are pH 7 and 27 degrees C, respectively. Both basal and DUSF-stimulated uptake of [3H]DNA are inhibited by 2,4-dinitrophenol and iodoacetic acid. The inhibition of the basal uptake of [3H]DNA by cycloheximide is greater than the DUSF-stimulated uptake. The DUSF enhanced not only DNA-uptake but also that of the oligonucleotides and mononucleotides. DUSF binds both macromolecular [3H]DNA and [14C]AMP, and there might be competition between nucleotides and DNA for the binding to DUSF. Polyclonal antibodies prepared against DUSF inhibited both basal and DUSF-enhanced [3H]DNA uptake. DUSF was detected by immunoblotting among the proteins isolated from purified N. crassa cell-membranes. DUSF might be a receptor protein for DNA and nucleotides in the cell-membrane of N. crassa and play a role in DNA and nucleotide uptake.

An ethidium bromide induced mutant of Neurospora crassa defective in mitochondrial DNA

Slow growing mutants of Neurospora crassa were obtained by ethidium bromide treatment of the wild type strain. A particular mutant ER-3 showed stopper phenotype accompanied by deficient cytochrome spectra. The mutant showed an altered restriction pattern of the mtDNA which indicated a deletion of 25,000 bp. The phenotype of the ethidium bromide induced mutant ER-3 seem to be related to the loss of several essential genes due to a deletion in its mtDNA.

Cloning of the PYR3 gene of Ustilago maydis and its use in DNA transformation

The Ustilago maydis PYR3 gene encoding dihydroorotase activity was cloned by direct complementation of Escherichia coli pyrC mutations. PYR3 transformants of E. coli pyrC mutants expressed homologous transcripts of a variety of sizes and regained dihydroorotase activity. PYR3 also complemented Saccharomyces cerevisiae ura4 mutations, and again multiple transcripts were expressed in transformants, and enzyme activity was regained. A 1.25-kilobase poly(rA)+ PYR3 transcript was detected in U. maydis itself. Linear DNA carrying the PYR3 gene transformed a U. maydis pyr3-1 pyrimidine auxotroph to prototrophy. Hybridization analysis revealed that three different types of transformants could be generated, depending on the structure of the transforming DNA used. The first type involved exchange of chromosomal mutant gene sequences with the cloned wild-type plasmid sequences. A second type had integrated linear transforming DNA at the chromosomal PYR3 locus, probably via a single crossover event. The third type had integrated transforming DNA sequences at multiple sites in the U. maydis genome. In the last two types, tandemly reiterated copies of the transforming DNA were found to have been integrated. All three types had lost the sensitivity of the parental pyr3-1 mutant to UV irradiation. They had also regained dihydroorotase activity, although its level did not correlate with the PYR3 gene copy number.

Cloning of the PYR3 gene of Ustilago maydis and its use in DNA transformation

The Ustilago maydis PYR3 gene encoding dihydroorotase activity was cloned by direct complementation of Escherichia coli pyrC mutations. PYR3 transformants of E. coli pyrC mutants expressed homologous transcripts of a variety of sizes and regained dihydroorotase activity. PYR3 also complemented Saccharomyces cerevisiae ura4 mutations, and again multiple transcripts were expressed in transformants, and enzyme activity was regained. A 1.25-kilobase poly(rA)+ PYR3 transcript was detected in U. maydis itself. Linear DNA carrying the PYR3 gene transformed a U. maydis pyr3-1 pyrimidine auxotroph to prototrophy. Hybridization analysis revealed that three different types of transformants could be generated, depending on the structure of the transforming DNA used. The first type involved exchange of chromosomal mutant gene sequences with the cloned wild-type plasmid sequences. A second type had integrated linear transforming DNA at the chromosomal PYR3 locus, probably via a single crossover event. The third type had integrated transforming DNA sequences at multiple sites in the U. maydis genome. In the last two types, tandemly reiterated copies of the transforming DNA were found to have been integrated. All three types had lost the sensitivity of the parental pyr3-1 mutant to UV irradiation. They had also regained dihydroorotase activity, although its level did not correlate with the PYR3 gene copy number.

Cotransformation of Aspergillus nidulans: a tool for replacing fungal genes

When a non-selected DNA sequence was added during the transformation of amdS320 deletion strains of Aspergillus nidulans with a vector containing the wild-type amdS gene the AmdS+ transformants were cotransformed at a high frequency. Cotransformation of an amdS320, trpC801 double mutant strain showed that both the molar ratio of the two vectors and the concentration of the cotransforming vector affected the cotransformation frequency. The maximum frequency obtained was defined by the gene chosen as selection marker for transformation. Cotransformation was used to induce a gene replacement in A. nidulans. An amdS320 strain was transformed to AmdS+ and cotransformed with a DNA fragment containing a fusion between a non-functional A. nidulans trpC gene and the Escherichia coli lacZ gene. Ten AmdS+, LacZ+ transformants with a Trp- mutant phenotype were selected. All of these strains could be transformed with a functional copy of the A. nidulans trpC gene, but only two strains yielded TrpC+ transformants which, with a low frequency, had a LacZ- phenotype. These latter transformants had also lost the AmdS+ phenotype. Southern blotting analysis of DNA from these transformants confirmed the inactivation of the wild-type trpC gene, but revealed that amdS vector sequences were also involved in the gene replacement events.

The ura5 gene of the filamentous fungus Podospora anserina: nucleotide sequence and expression in transformed strains

We have sequenced the ura5 gene of the filamentous fungus Podospora anserina. The deduced sequence for the orotidylic acid pyrophosphorylase (OMPppase) has been compared with the Escherichia coli enzyme which is the only known sequence for this enzyme. This comparison shows extensive blocks of homology. The expression of the ura5 gene has been studied in a ura5 mutant which has been transformed by a recombinant plasmid carrying the ura5 gene. We observed that strains carrying integrated multicopies of the transforming vector exhibit higher specific activity for OMPppase than wild type (wt). By recombination we have constructed a strain in which the level of this enzyme is 32 times higher than in the wt strain.

Recombinant DNA in filamentous fungi: progress and prospects

Recombinant DNA technology enables the creation of well-defined alterations in the genetic material of an organism. Methods to manipulate recombinant DNA in the filamentous fungi (a group of microorganisms that includes species of academic as well as commercial interest) have recently been developed. This has been the result of adaptation of procedures successfully employed in the manipulation of other microorganisms. There are a number of similarities in the behavior of recombinant DNA in different fungi, but a number of differences have also been observed between the filamentous and the nonfilamentous fungi. Such differences include the ability to identify DNA replication origins and the host range of expression of fungal genes.