Distinct hypomyelinated phenotypes in MBP-SV40 large T transgenic mice

SV40 TAg mouse models of cancer

The discovery of a number of viruses with the ability to induce tumours in animals and transform human cells has vastly impacted cancer research. Much of what is known about tumorigenesis today regarding tumour drivers and tumour suppressors has been discovered through experiments using viruses. The SV40 virus has proven extremely successful in generating transgenic models of many human cancer types and this review provides an overview of these models and seeks to give evidence as to their relevance in this modern era of personalised medicine and technological advancements.

Expression of SV40 T antigen gene in the oligodendroglia induced primitive neuroectodermal tumor-like tumors in the mouse brain

Primitive neuroectodermal tumors (PNET) are classified as the embryonal tumors developed in the brain, except for the cerebellum. Although many studies have been reported, the origin and pathogenesis of PNET are still unclear. In this study, we observed the development of undifferentiated tumors indistinguishable from PNET in the transgenic mice which expressed simian virus 40 T antigen (SV40-Tag) selectively in the oligodendroglia under the control of mouse myelin basic protein gene promoter. These PNET-like tumors reproducibly developed in the brain stem of the founder mice and the transgenic progeny derived from one founder mouse. Oligodendroglia-specific expression of SV40-Tag in these transgenic mice was observed by immunohistochemical analysis. Furthermore, expression of the oligodendroglia-specific marker genes was decreased in the tumors as well as in the transgenic brains. These findings suggested that tumors developed in transgenic mice were indistinguishable from PNET, and one of them showed oligodendroglia-like characteristics. Consequently, this transgenic line is a useful animal model to study the pathogenesis of undifferentiated tumor.

Fkbp8: novel isoforms, genomic organization, and characterization of a forebrain promoter in transgenic mice

The immunophilin homolog FKBP8 has been implicated in the regulation of apoptosis. Here we show that the 38-kDa form of FKBP8 (FKBP38) derives from a truncated ORF. The extended FKBP8 ORFs are 46 and 44 kDa in mouse and 45 kDa in human. Although the genomic organization of mouse and human FKBP8 is evolutionarily conserved, additional first exons are encoded by the murine locus. A 4.4-kb murine Fkbp8 gene fragment, containing a GC-rich potential promoter, directed expression of a LacZ reporter gene to forebrain neurons in transgenic mice. Expression of the transgene was observed in CA1 pyramidal neurons of the hippocampus in transgenic mice from three lines. One transgenic founder mouse exhibited widespread forebrain expression of the LacZ transgene that resembles the pattern for the endogenous Fkbp8 gene. Thus promoter/enhancer elements for forebrain expression are located around the first exons of the mouse Fkbp8 gene.

Characterization of two novel nuclear BTB/POZ domain zinc finger isoforms. Association with differentiation of hippocampal neurons, cerebellar granule cells, and macroglia

BTB/POZ (broad complex tramtrack bric-a-brac/poxvirus and zinc finger) zinc finger factors are a class of nuclear DNA-binding proteins involved in development, chromatin remodeling, and cancer. However, BTB/POZ domain zinc finger factors linked to development of the mammalian cerebral cortex, cerebellum, and macroglia have not been described previously. We report here the isolation and characterization of two novel nuclear BTB/POZ domain zinc finger isoforms, designated HOF(L) and HOF(S), that are specifically expressed in early hippocampal neurons, cerebellar granule cells, and gliogenic progenitors as well as in differentiated glia. During embryonic development of the murine cerebral cortex, HOF expression is restricted to the hippocampal subdivision. Expression coincides with early differentiation of presumptive CA1 and CA3 pyramidal neurons and dentate gyrus granule cells, with a sharp decline in expression at the CA1/subicular border. By using bromodeoxyuridine labeling and immunohistochemistry, we show that HOF expression coincides with immature non-dividing cells and is down-regulated in differentiated cells, suggesting a role for HOF in hippocampal neurogenesis. Consistent with the postulated role of the POZ domain as a site for protein-protein interactions, both HOF isoforms are able to dimerize. The HOF zinc fingers bind specifically to the binding site for the related promyelocytic leukemia zinc finger protein as well as to a newly identified DNA sequence.

Neurological disturbances, premature lethality, and central myelination deficiency in transgenic mice overexpressing the homeo domain transcription factor Oct-6

Pit, Oct, Unc (POU) homeo domain transcription factors have been implicated in various developmental processes, including cell division, differentiation, specification, and survival of specific cell types. Although expression of the transcription factor Oct-6 in oligodendroglia is confined to the promyelin stage and is downregulated at the myelin stage of development, the effect of Oct-6 overexpression on oligodendrocyte development has not been established. Here we show that transgenic animals overexpressing Oct-6 at late oligodendrocyte development develop a severe neurologic syndrome characterized by action tremors, recurrent seizures, and premature death. Axons in the central nervous system of Oct-6 transgenics were hypomyelinated, hypermyelinated, or dysmyelinated, and ultrastructural analyses suggested that myelin formation was premature. The vulnerability of developing oligodendroglia to Oct-6 deregulation provides evidence that the POU factor may play a direct role in myelin disease pathogenesis in the mammalian CNS.

A minimal human MBP promoter-lacZ transgene is appropriately regulated in developing brain and after optic enucleation, but not in shiverer mutant mice

Previous studies, both in vitro and in vivo, suggest that small portions of the mouse myelin basic protein (MBP) promoter are sufficient to activate regulated expression of MBP. To confirm our previous in vitro studies, we prepared transgenic mice with short regions of the human MBP promoter fused to the lacZ reporter gene. We found that 750 nucleotides of the proximal human MBP promoter is sufficient to activate oligodendrocyte-specific, developmentally regulated expression of lacZ in three independent lines. This promoter, however, does not activate expression of lacZ in Schwann cells in peripheral nerve or in adult mouse brain. The relative levels of beta-galactosidase specific activity, mRNA, and transcription parallel those of MBP mRNA during myelinogenesis. Thus, we exploited this transgene as a quantitative tool to evaluate the response to stimuli known to affect myelination. Transgene expression is reduced 75 % after optic enucleation, as previously reported for levels of MBP mRNA, indicating that axons signal to this portion of the proximal MBP promoter to fully activate MBP expression during myelinogenesis. Instead, in adult shiverer mice, another setting in which MBP transcription is modulated, transgene expression is not increased, in contrast to the increased transcriptional activation of MBP previously reported in these mice. These data suggest that the regulatory region that mediates transcriptional activation of the MBP gene is modular, since discrete subregions are required for activation in Schwann cells, during myelinogenesis in oligodendrocytes, during maintenance myelination in adult brain, and in the dysmyelinating mutant shiverer mouse.

Evidence for a mechanism of demyelination by human JC virus: negative transcriptional regulation of RNA and protein levels from myelin basic protein gene by large tumor antigen in human glioblastoma cells

Human JC virus (JCV) is a neurotropic human polyomavirus that was found in the plaques and oligodendroglial cells of the brains of patients with the fatal demyelinating disease, progressive multifocal leukoencephalopathy (PML). Transgenic mice expressing JCV large tumor (T)-antigen from integrated DNA showed dysmyelination in the central nervous system. However, the role of T-antigen from episomal DNA in the demyelination in PML remains unclear. In this report, we examined the effect of episomally expressed JCV T-antigen on the expression of myelin basic protein (MBP) in U-87 MG human glioblastoma cells to study the mechanism of demyelination. Expression assays of the MBP promoter in U-87 MG detected a 2.5-fold reduction in cells expressing intact T-antigen. Next, U-87 MG expressing T-antigen were examined by RNase protection assays for mRNA accumulation from the endogenous MBP promoter. Also, the expression of the MBP promoter plasmid was determined using in vitro transcription assays with extracts from T-antigen expressing cells. Both assays found a similar down-regulation of the MBP promoter by T-antigen, confirming that negative regulation occurred at the transcriptional level for the endogenous and exogenous MBP promoters. Furthermore, in situ immunofluorescence assays and quantitative Western blot analysis provided convincing evidence of a similar reduction in the level of MBP produced from the functional endogenous gene in U-87 MG glioblastoma cells expressing T-antigen. Thus, we provide evidence for the role of T-antigen in a transcriptional control mechanism for the demyelination that is caused by JCV in PML patients.

Myelin gene expression in glia treated with oligodendroglial trophic factor

Oligodendroglia synthesize myelin in the CNS. In vitro, oligodendroglia may be identified by the binding of monoclonal antibodies against galactocerebroside, a myelin-specific galactolipid. Oligodendroglial trophic factor is a protein mitogen for cells of the oligodendroglial lineage. When oligodendroglia in cerebral white matter cultures are treated with oligodendroglial trophic factor, galactocerebroside-positive cells undergo mitosis but fail to express the myelin structural proteins, myelin basic protein and proteolipid protein. Oligodendroglia treated with oligodendroglial trophic factor, however, do express 2',3'-cyclic nucleotide 3'-phosphodiesterase and myelin-associated glycoprotein in a manner similar to oligodendroglia treated with platelet-derived growth factor. Oligodendroglial trophic factor, therefore, generates a population of somewhat 'immature' oligodendroglia, which are galactocerebroside, myelin-associated glycoprotein and 2', 3'-cyclic nucleotide 3' phosphodiesterase positive but myelin basic protein and proteolipid protein negative.

Human COL2A1-directed SV40 T antigen expression in transgenic and chimeric mice results in abnormal skeletal development

The ability of SV40 T antigen to cause abnormalities in cartilage development in transgenic mice and chimeras has been tested. The cis-regulatory elements of the COL2A1 gene were used to target expression of SV40 T antigen to differentiating chondrocytes in transgenic mice and chimeras derived from embryonal stem (ES) cells bearing the same transgene. The major phenotypic consequences of transgenic (pAL21) expression are malformed skeleton, disproportionate dwarfism, and perinatal/neonatal death. Expression of T antigen was tissue specific and in the main characteristic of the mouse alpha 1(II) collagen gene. Chondrocyte densities and levels of alpha 1(II) collagen mRNAs were reduced in the transgenic mice. Islands of cells which express cartilage characteristic genes such as type IIB procollagen, long form alpha 1(IX) collagen, alpha 2(XI) collagen, and aggrecan were found in the articular and growth cartilages of pAL21 chimeric fetuses and neonates. But these cells, which were expressing T antigen, were not properly organized into columns of proliferating chondrocytes. Levels of alpha 1(II) collagen mRNA were reduced in these chondrocytes. In addition, these cells did not express type X collagen, a marker for hypertrophic chondrocytes. The skeletal abnormality in pAL21 mice may therefore be due to a retardation of chondrocyte maturation or an impaired ability of chondrocytes to complete terminal differentiation and an associated paucity of some cartilage matrix components.

Transgenic and knock-out mice: models of neurological disease

Besides providing useful model systems for basic science, studies based on modification of the mammalian germ line are changing our understanding of pathogenetic principles. In this article, we review the most popular techniques for generating specific germ line mutations in vivo and discuss the impact of various transgenic models on the study of neurodegenerative diseases. The "gain of function" approach, i.e., ectopic expression of exogenous genes in neural structures, has deepened our understanding of neurodegeneration resulting from infection with papova viruses, picorna viruses, and human retroviruses. Further, inappropriate expression of mutated cellular molecules in the nervous system of transgenic mice is proving very useful for studying conditions whose pathogenesis is controversial, such as Alzheimer's disease and motor neuron diseases. As a complementary approach, ablation of entire cell lineages by tissue-specific expression of toxins has been useful in defining the role of specific cellular compartments. Modeling of recessive genetic diseases, such as Lesch-Nyhan syndrome, was helped by the development of techniques for targeted gene deletion (colloquially termed "gene knock-out"). Introduction of subtle homozygous mutations in the mouse genome was made possible by the latter approach. Such "loss of function" mutants have been used for clarifying the role of molecules thought to be involved in development and structural maintenance of the nervous system, such as the receptors for nerve growth factor and the P0 protein of peripheral myelin. In addition, these models are showing their assets also in the study of enigmatic diseases such as spongiform encephalopathies.

Cyclic AMP has a differentiative effect on an immortalized oligodendrocyte cell line

We investigated the effects of increasing the concentration of intracellular cyclic adenosine monophosphate (cAMP) on genes associated with oligodendrocyte differentiation in an immortalized glial cell line, 6E12, derived from the spinal cord of an MBP-SV40 large T-antigen transgenic mouse. Raising intracellular levels of cAMP induced expression of oligodendrocyte differentiation antigens recognized by O4 and anti-galactocerebroside antibodies, up-regulated expression of the proteolipid protein (PLP) gene, and down-regulated glial fibrillary acidic protein (GFAP) expression. There was no treatment effect on myelin-associated glycoprotein (MAG) expression. These phenotypic changes are consistent with oligodendrocyte differentiation. Treatment of 6E12 cells with dibutyryl cyclic AMP (DBC) down-regulated myelin basic protein (MBP) gene expression, perhaps, because it also up-regulated expression of a putative MBP repressor SCIP/Tst-1. Moreover, the 6E12 cells expressed high levels of MBP mRNA but no MBP translation products were detected in the presence or absence of DBC. This immortalized glial cell line is proposed as a CNS model for cAMP-modulated myelin gene expression and for post-transcriptional regulation of MBP.

Transgenic mouse model for neurocristopathy: Schwannomas and facial bone tumors

We have characterized a strain of double transgenic mice with simian virus 40 large tumor antigen and prokaryotic lacZ under the control of the myelin basic protein promoter that develops spindle-cell sarcomas and osteogenic sarcomas at 5-7 months of age. Although poorly differentiated, the spindle-cell sarcomas were characterized as malignant Schwannomas based on their neural association, the presence of basal lamina, and expression of Schwann cell-specific genes. The osteogenic sarcomas were often multiple and appeared predominantly in the facial bones, less frequently in the ribs and vertebral column, and only rarely in the appendicular skeleton. Benign osteoblastic lesions were often observed adjacent to these sarcomas. Both the osteoblastic cells in the facial skeleton and Schwann cells are regarded as neural crest derivatives. The biological properties and anatomical location of these tumors suggest that they may share a common origin from the neural crest or its derivatives. R.P. Bolande [Hum. Pathol. (1974) 5, 409-429] introduced the term neurocristopathy as a unifying concept to describe such lesions arising from the neural crest or its derivatives. Cell lines established from both bone and Schwann cell tumors arising in these transgenic mice express simian virus 40 large tumor antigen mRNA as well as functional large tumor antigen. Such cell lines are potentially valuable in the search for markers that identify mammalian neural crest derivatives.