Dopaminergic neuronal loss and motor deficits in Caenorhabditis elegans overexpressing human alpha-synuclein

Overexpression of human alpha-synuclein in model systems, including cultured neurons, drosophila and mice, leads to biochemical and pathological changes that mimic synucleopathies including Parkinson's disease. We have overexpressed both wild-type (WT) and mutant alanine53-->threonine (A53T) human alpha-synuclein by transgenic injection into Caenorhabditis elegans. Motor deficits were observed when either WT or A53T alpha-synuclein was overexpressed with a pan-neuronal or motor neuron promoter. Neuronal and dendritic loss were accelerated in all three sets of C. elegans dopaminergic neurons when human alpha-synuclein was overexpressed under the control of a dopaminergic neuron or pan-neuronal promoter, but not with a motor neuron promoter. There were no significant differences in neuronal loss between overexpressed WT and A53T forms or between worms of different ages (4 days, 10 days or 2 weeks). These results demonstrate neuronal and behavioral perturbations elicited by human alpha-synuclein in C. elegans that are dependent upon expression in specific neuron subtypes. This transgenic model in C. elegans, an invertebrate organism with excellent experimental resources for further genetic manipulation, may help facilitate dissection of pathophysiologic mechanisms of various synucleopathies.

Subunit Selective Decrease of AMPA and Metabotropic Glutamate Receptor mRNA Expression in Rat Brain by Systemic Administration of the NMDA Receptor Blocker MK-801

Glutamate mediates its effects in mammals through both ionotropic and metabotropic receptors. Antagonists of ionotropic N-methyl-D-aspartate (NMDA) glutamate receptors elicit neuroprotective and neurotropic effects that have been attributed to Ca2+ block through the membrane ion channel. Nonetheless, molecular and biochemical effects of NMDA receptor antagonism on other glutamate receptor subunits remain poorly understood. We investigated the effects of acute administration of the noncompetitive NMDA receptor antagonist MK-801 on the mRNA expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and metabotropic glutamate receptor (mGluR) subunits to determine the contribution of different glutamate receptors in response to blockade of NMDA receptor channels. In situ hybridization to rat brain sections revealed that AMPAreceptor subunits GluR3 and GluR4, and mGluR3 were modestly but significantly decreased approximately 10-20%, 8 h following 5 mg/kg MK-801 administration. Atime course and dose response study revealed that the effect on mGluR3 was reversed by 24 h and occurred significantly at a dose range from 1 to 5 mg/kg. These results indicate that selected AMPA and mGluR subunit mRNAs respond at the RNA level to the blockade of NMDA receptors.

Antipsychotic drug treatment induces differential gene expression in the rat cortex

Antipsychotic drug treatment is known to modulate gene expression in experimental animals. In this study, candidate target genes for antipsychotic drug action were searched using microarrays after acute clozapine treatment (1, 6 and 24 h) in the rat prefrontal cortex. Microarray data clustering with a self-organizing map algorithm revealed differential expression of genes involved in presynaptic function following acute clozapine treatment. The differential expression of 35 genes most profoundly regulated in expression arrays was further examined using in situ hybridization following acute clozapine, and chronic clozapine and haloperidol treatments. Acute administration of clozapine regulated the expression of chromogranin A, synaptotagmin V and calcineurin A mRNAs in the cortex. Chronic clozapine treatment induced differential cortical expression of chromogranin A, son of sevenless (SoS) and Sec-1. Chronic treatment with haloperidol regulated the mRNA expression of inhibitor of DNA-binding 2 (ID-2) and Rab-12. Furthermore, the expression of visinin-like proteins-1, -2 and -3 was regulated by chronic drug treatments in various brain regions. Our data suggest that acute and chronic treatments with haloperidol and clozapine modulate the expression of genes involved in synaptic function and in regulation of intracellular Ca2+ in cortex.

Expression profiling to understand actions of NMDA/glutamate receptor antagonists in rat brain

Agents acting as noncompetitive N-methyl-D-aspartate (NMDA)/glutamate receptor antagonists induce the expression of several genes in limbic cortical regions, such as the cingulate, retrosplenial, and entorhinal cortices. These include important regulatory genes such as the neurotrophin brain-derived neurotrophic factor (BDNF), its receptor trkB, and c-fos. We applied expression profiling methods to find genes coregulated with BDNF following treatment with the prototypical NMDA/glutamate receptor antagonist MK-801. Expression profiling provides a useful technique for describing the molecular and transcriptional level events that follow various processes. We illustrate the utility of microarrays to find novel ESTs regulated by MK-801. We also used expression profiling with microarrays to characterize the levels of transcription factor cAMP response element modulator (CREM) and inducible cAMP early repressor (ICER) isoforms that are induced by MK-801. These factors may act as the eventual repressors for BDNF expression via competition and heterodimerization with phosphorylated CREB, a transcription factor important for BDNF expression. Finally, we find and confirm the regulation of Erp29, RTNI, and an ABC transporter by antagonism of NMDA/glutamate receptors as potential stress related molecules in brain. The emerging picture generated by using these expression profiling approaches, identifies several of what likely will be many molecules that take part in the complex events that occur during BDNF signaling mediated by blockade of NMDA/ glutamate receptors.

Syndecan-3-deficient mice exhibit enhanced LTP and impaired hippocampus-dependent memory

Syndecan-3 (N-syndecan) is a transmembrane heparan sulfate proteoglycan expressed predominantly in the nervous system in a developmentally regulated manner. Syndecan-3 has been suggested to play a role in the development and plasticity of neuronal connections by linking extracellular signals to the regulation of the cytoskeleton. To study its physiological functions, we produced mice deficient in syndecan-3 by gene targeting. The mutant animals are healthy, are fertile, and have no apparent defects in the structure of the brain. We focused on characterizing the functions of the hippocampus, a brain area where expression of syndecan-3 is prominent in adults. Mice lacking syndecan-3 exhibited an enhanced level of long-term potentiation (LTP) in area CA1, while basal synaptic transmission and short-term plasticity were similar to those in wild-type animals. Further, the mutant mice were not responsive to the syndecan-3 ligand heparin-binding growth-associated molecule, which inhibits LTP in area CA1 in wild-type animals. Behavioral testing of the syndecan-3-deficient mice revealed impaired performance in tasks assessing hippocampal functioning. We suggest that syndecan-3 acts as an important modulator of synaptic plasticity that influences hippocampus-dependent memory.

Synexpression analysis of ESTs in the rat brain reveals distinct patterns and potential drug targets

The gene expression profiles of 146 novel ESTs were characterized in newborn and adult rat brains via radioactive in situ hybridization. Using Euclidean metrics and hierarchical clustering tools the brain expression profiles obtained clustered into seven synexpression groups. The groups were: I, non-detectable expression (68 ESTs); II, low expression in hippocampus (40 ESTs); III, low expression in adult, high expression in newborn (two ESTs); IV, medium expression throughout brain (31 ESTs); V, high expression throughout brain (three ESTs); VI, selective high expression in hippocampus, caudate and putamen (one EST); VII, selective high expression in hippocampus (one EST). Five ESTs were expressed in the striatum and three responded transcriptionally to neuroleptic and neuroprotective drug treatments, suggesting that this approach could be used to detect novel drug targets. These results provide a useful starting point to explore the functional genomics of genes without known functions forthcoming from various genome projects.

Chronic antipsychotic drug treatment induces long-lasting expression of fos and jun family genes and activator protein 1 complex in the rat prefrontal cortex

We have characterized the effects of chronic clozapine and haloperidol treatments on the expression of fos (c-fos, fosB, fra-2) and jun (c-jun, junB, junD) family genes in the rat forebrain. The effects of chronic (17d) clozapine and haloperidol on mRNA expression were determined two hours, 24 hours, and six days after the last drug injection, and the DNA-binding activity of the activator protein-1 (AP-1) complex was studied after washout periods of 24 hours and six days. Chronic clozapine treatment with a 6 d washout period induced the expression of several fos and jun family genes in cortical regions, including the prefrontal cortex (PFC), and in the caudate putamen and nucleus accumbens. Moreover, the DNA-binding activity of the AP-1 complex was greatly increased in the anterior cingulate cortex-PFC in mobility shift assays already after 24 h, and remained increased after a 6d washout period. Chronic administration of haloperidol upregulated fos and jun family mRNA expression that was detectable 24 h and 6 d after cessation of the treatment mainly in the cortex. However, the DNA-binding activity of the AP-1 complex was not altered in the anterior cingulate cortex-PFC by chronic haloperidol administration at any of the time points studied. Thus, chronic treatments with clozapine and haloperidol induce a long-lasting enhancement of fos and jun family transcription factors that continues for several days after the cessation of the treatments in the cortex. These lasting effects might represent events that are potentially involved in the mechanisms of antipsychotic drug action.

The neuroprotective agent memantine induces brain-derived neurotrophic factor and trkB receptor expression in rat brain

Memantine is a medium-affinity uncompetitive N-methyl-d-aspartate receptor antagonist and has been clinically used as a neuroprotective agent to treat Alzheimer's and Parkinson's diseases. We have examined the effect of memantine (ip 5-50 mg/kg; 4 h) on the expression of brain-derived neurotrophic factor (BDNF) and trkB receptor mRNAs in rat brain by in situ hybridization. Memantine at a clinically relevant dose markedly increased BDNF mRNA levels in the limbic cortex, and this effect was more widespread and pronounced at higher doses. Effects of memantine on BDNF mRNA were also reflected in changes in BDNF protein levels. Moreover, memantine induced isoforms of the BDNF receptor trkB. Taken together, these data suggest that the neuroprotective properties of memantine could be mediated by the increased endogenous production of BDNF in the brain. These findings may open up new possibilities of pharmacologically regulating the expression of neurotrophic factors in the brain.

Increased expression of neuronal Src and tyrosine phosphorylation of NMDA receptors in rat brain after systemic treatment with MK-801

We have observed that systemic treatment with the uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 increases Src expression and NMDA receptor phosphorylation in rat brain. A partial cDNA encoding rat neuronal Src was isolated and its sequence was used to design specific oligonucleotide probes. Systemically administered MK-801 (5 mg/kg for 4 h) increased by 28+/-4% mRNA expression of neuronal Src in the superficial layers of the parietal cortex. This effect was observed at doses as low as 0.2 mg/kg. A similar, although more modest, induction was observed 6 h after phencyclidine (15 mg/kg) administration, but not after high doses of memantine and ketamine. The MK-801-induced effect was not blocked by pretreatment with clozapine. Consistent with the increase in mRNA levels, cortical Src protein was increased to 186 +/- 24% of control 24 h after MK-801 treatment. Total cellular Src activity was also increased in parietal cortex homogenates 4 h after MK-801 (5 mg/kg). Moreover, MK-801 treatment (0.5 mg/kg and 5 mg/kg for 4 h) increased tyrosine phosphorylation, but not protein levels, of the NMDA receptor subunit NR2A. These results provide evidence for a contribution of Src and tyrosine phosphorylation of NMDA receptors in the pharmacological actions of uncompetitive NMDA receptor antagonists.

Uncompetitive antagonists of the N-methyl-D-aspartate (NMDA) receptors alter the mRNA expression of proteins associated with the NMDA receptor complex

N-methyl-D-aspartate (NMDA) receptor function appears to be under complex control during physiological and pharmacological states. We have investigated the effects of acute administration of uncompetitive NMDA receptor antagonists on mRNA levels of NMDA receptor subunits and on molecules known to cluster or phosphorylate the receptor utilizing in situ hybridization on rat brain sections. A high dose (5 mg/kg; 4 hr) of dizocilpine (MK-801) decreased mRNA levels of NMDA receptor subunits NR2C and NR2B in the entorhinal and parietal cortices, respectively. MK-801 increased mRNA levels of synapse-associated protein-90/postsynaptic density-95 (SAP90/PSD-95) and a gamma-isoform of protein kinase C (PKCgamma) in cortical regions. Synapse-associated protein-97 (SAP97) mRNA levels were increased in the entorhinal cortex layer III after MK-801 or after relatively high doses of other uncompetitive NMDA receptor antagonists: phencyclidine (15 mg/kg; 6 hr) and memantine (50 mg/kg; 6 hr). Memantine also increased SAP97 mRNA expression in other cortical regions, but this effect was not observed with MK-801 or phencyclidine. NMDA receptor uncompetitive antagonists alter the expression of multiple receptor components and such events may ultimately play a role in adaptation or toxic responses.

A functional genomic study of the effects of antipsychotic agent chlorpromazine in PC12 cells

Expression profiling using methods of functional genomics can be used to investigate changes in gene transcription induced by drug treatment, which may lead to discovery of new potential drug targets. Antipsychotic agents alleviate symptoms of schizophrenia but the mechanism behind their clinical efficacy is unclear. We have used the PC12 cell line as a model to characterize effects of the antipsychotic drug chlorpromazine on gene expression using high-density complementary DNA array filters prepared from a rat brain entorhinal cortex complementary DNA library. Chlorpromazine treatment positively regulated the expression of several clones, five of which were selected for further characterization. Northern blotting experiments confirmed the increased expression of these genes after chlorpromazine treatment. Sequencing revealed that two clones were cytochrome c oxidase and three were novel genes. Characterization of the function of these genes could increase our understanding of the mechanisms of action of antipsychotic drugs, and might be beneficial for the development of more effective agents.

Induction of cAMP response element modulator (CREM) and inducible cAMP early repressor (ICER) expression in rat brain by uncompetitive N-methyl-D-aspartate receptor antagonists

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptor mediates fast excitatory neurotransmission, and agents that attenuate this function are neuroprotective, anesthetic, and psychotropic. To determine whether cAMP regulatable transcription factors play a role in the neurochemical actions of agents acting through NMDA receptors, the effects of the acute administration of uncompetitive and competitive antagonists on the expression of cAMP response element modulator (CREM) and inducible cAMP early repressor (ICER) transcription factors were examined. In situ hybridization to rat brain sections revealed that ICER mRNA expression was significantly increased by uncompetitive NMDA receptor antagonists (MK-801, phencyclidine, ketamine, memantine) but not by the competitive antagonist CPP [(+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid] or other psychotropic agents (clozapine, haloperidol, desipramine). Major brain regions where ICER transcripts were increased were the hippocampus, parietal cortex layers IV and VI, temporal cortex, cingulate cortex, thalamus, and granule cell layer of the olfactory bulb. Northern and Western blot analyses indicated that CREM mRNA and protein, respectively, were also increased after MK-801 treatment, but ICER isoforms predominate during both basal and induced conditions. MK-801 also transiently increased the binding of proteins to cAMP response element, which was supershifted by anti-CREM antibody, indicating that increased mRNA and protein levels have functional consequences on gene transcription. These results provide evidence for the involvement of CREM and ICER family transcription factors in the pharmacologic effects of uncompetitive NMDA receptor antagonists.

Molecular effects of the psychotropic NMDA receptor antagonist MK-801 in the rat entorhinal cortex: increases in AP-1 DNA binding activity and expression of Fos and Jun family members

Noncompetitive NMDA receptor antagonists such as phencyclidine and MK-801 produce psychotropic symptoms that closely resemble schizophrenic psychosis and induce the expression of immediate early genes in limbic cortical areas. We are concentrating on analyzing molecular and physiological effects that these drugs produce in the entorhinal cortex and on the potential connection between these effects and the psychotic symptoms. We show here that MK-801 increases the DNA binding activity of the activator protein-1 (AP-1) complex in the entorhinal cortex. We also observed increased expression of mRNAs for Fos and Jun transcription factor family members c-Fos, FosB, Fra-2, and JunB, as well as Fos family proteins in the entorhinal cortex after MK-801 administration. This suggests that the activated AP-1 complex consists of these transcription factors. Genes regulated by the AP-1 complex in the entorhinal cortex might be involved in the pathophysiology of psychotic behavior and are potential targets for new antipsychotic drugs.

Regulation of cell fate decision of undifferentiated spermatogonia by GDNF

The molecular control of self-renewal and differentiation of stem cells has remained enigmatic. Transgenic loss-of-function and overexpression models now show that the dosage of glial cell line-derived neurotrophic factor (GDNF), produced by Sertoli cells, regulates cell fate decisions of undifferentiated spermatogonial cells that include the stem cells for spermatogenesis. Gene-targeted mice with one GDNF-null allele show depletion of stem cell reserves, whereas mice overexpressing GDNF show accumulation of undifferentiated spermatogonia. They are unable to respond properly to differentiation signals and undergo apoptosis upon retinoic acid treatment. Nonmetastatic testicular tumors are regularly formed in older GDNF-overexpressing mice. Thus, GDNF contributes to paracrine regulation of spermatogonial self-renewal and differentiation.

Expression of neurotrophins BDNF and NT-3, and their receptors in rat brain after administration of antipsychotic and psychotrophic agents

We have investigated the potential role of neurotrophic factors in antipsychotic drug action by examining the effects of antipsychotic and psychotropic treatments on the mRNA expression of brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and their receptors, trkB and trkC, respectively, in rat brain. Neither acute nor chronic clozapine treatment significantly affected the expression of these mRNAs in any brain area investigated, except for a decrease in trkB expression in the granule cells of the olfactory bulb. We then examined the effects of the psychotropic agent MK-801. MK-801 (5 mg/kg; 4 h) significantly increased BDNF mRNA in the entorhinal cortex, but did not influence NT-3, trkB, or trkC expression in any brain area except for the olfactory bulb. The induction of BDNF mRNA by MK-801 was attenuated by pre-treatment (1 h prior to MK-801 administration) with the antipsychotics, clozapine (25 mg/kg) and haloperidol (2 mg/kg), but not with the antidepressant desipramine (15 mg/kg). Finally, we confirmed that the effects of MK-801 on BDNF mRNA were reflected in the respective changes in BDNF protein levels: MK-801 significantly increased anti-BDNF reactivity in the entorhinal cortex (126 +/- 7% of control) while concomitantly decreasing in the hippocampus (71 +/- 2% of control). These data do not support the hypothesis that neurotrophins play an important role in antipsychotic drug action, but rather suggest that induction of BDNF in the entorhinal cortex may play a significant role in the psychotropic action of MK-801.

Excitatory actions of NMDA receptor antagonists in rat entorhinal cortex and cultured entorhinal cortical neurons

We have characterized excitatory effects of non-competitive NMDA receptor antagonists MK-801, PCP, and ketamine in the rat entorhinal cortex and in cultured primary entorhinal cortical neurons using expression of immediate early gene c-fos as an indicator. NMDA receptor antagonists produced a strong and dose-dependent increase in c-fos mRNA and protein expression confined to neurons in the layer III of the caudal entorhinal cortex. Induction of c-fos mRNA is delayed and it is inhibited by antipsychotic drugs. Cultured entorhinal neurons are killed by high doses of MK-801 and PCP but c-fos expression is not induced in these neurons indicating that this in vitro model does not fully replicate the in vivo effects of PCP-like drugs in the entorhinal cortex. Excitatory effects of the NMDA receptor antagonists may be connected with the psychotropic side effects of these drugs and might become a useful model system to investigate neurobiology of psychosis.

Retarded growth and deficits in the enteric and parasympathetic nervous system in mice lacking GFR alpha2, a functional neurturin receptor

Glial cell line-derived neurotrophic factor (GDNF) and a related protein, neurturin (NTN), require a GPI-linked coreceptor, either GFR alpha1 or GFR alpha2, for signaling via the transmembrane Ret tyrosine kinase. We show that mice lacking functional GFR alpha2 coreceptor (Gfra2-/-) are viable and fertile but have dry eyes and grow poorly after weaning, presumably due to malnutrition. While the sympathetic innervation appeared normal, the parasympathetic cholinergic innervation was almost absent in the lacrimal and salivary glands and severely reduced in the small bowel. Neurite outgrowth and trophic effects of NTN at low concentrations were lacking in Gfra2-/- trigeminal neurons in vitro, whereas responses to GDNF were similar between the genotypes. Thus, GFR alpha2 is a physiological NTN receptor, essential for the development of specific postganglionic parasympathetic neurons.

Overexpression of VEGF in testis and epididymis causes infertility in transgenic mice: evidence for nonendothelial targets for VEGF

Vascular endothelial growth factor (VEGF) is a key regulator of endothelial growth and permeability. However, VEGF may also target nonendothelial cells, as VEGF receptors and responsiveness have been detected for example in monocytes, and high concentrations of VEGF have been reported in human semen. In this work we present evidence that overexpression of VEGF in the testis and epididymis of transgenic mice under the mouse mammary tumor virus (MMTV) LTR promoter causes infertility. The testes of the transgenic mice exhibited spermatogenic arrest and increased capillary density. The ductus epididymidis was dilated, containing areas of epithelial hyperplasia. The number of subepithelial capillaries in the epididymis was also increased and these vessels were highly permeable as judged by the detection of extravasated fibrinogen products. Intriguingly, the expression of VEGF receptor-1 (VEGFR-1) was detected in certain spermatogenic cells in addition to vascular endothelium, and both VEGFR-1 and VEGFR-2 were also found in the Leydig cells of the testis. The infertility of the MMTV-VEGF male mice could thus result from VEGF acting on both endothelial and nonendothelial cells of the male genital tract. Taken together, these findings suggest that the VEGF transgene has nonendothelial target cells in the testis and that VEGF may regulate male fertility.

Targeted expression of a toxin gene to D1 dopamine receptor neurons by cre-mediated site-specific recombination

Idiopathic Parkinson's disease involves the loss of midbrain dopaminergic neurons, resulting in the presynaptic breakdown of dopaminergic transmission in the striatum. Huntington's disease and some neurodegenerative diseases with Parkinsonian features have postsynaptic defects caused by striatal cell death. Mice were generated in which an attenuated form of the diphtheria toxin gene (tox-176) was expressed exclusively in D1 dopamine receptor (D1R)-positive cells with the aim of determining the effect of this mutation on development of the basal ganglia and on the locomotor phenotype. Transgenic mice expressing Cre, a site-specific DNA recombinase, were crossed with a second line in which a transcriptionally silenced tox-176 gene was inserted into the D1R gene locus by homologous recombination. Young doubly transgenic mutant mice expressing the tox-176 gene displayed bradykinesia, dystonia, and had falls caused by myoclonic jerks. The mutant brain had evidence of apoptosis and reactive gliosis and, consistent with the D1R expression pattern, the striatum was reduced in volume, and the Islands of Calleja were absent. In contrast, the cortex was of normal thickness. D1Rs were not detectable in mutants by in situ hybridization or ligand autoradiography, whereas D2 dopamine receptor (D2R) mRNA and protein was present in the striatum. In addition, substance P and dynorphin, neuropeptides known to be expressed in D1R-positive striatonigral projection neurons were not detectable. Enkephalin, a marker found in D2-positive striatopallidal projection neurons was expressed in the mutant brain. The mutant represents a novel neurodegenerative disease model with a dramatic extrapyramidal phenotype.

Osteoblast recruitment and bone formation enhanced by cell matrix-associated heparin-binding growth-associated molecule (HB-GAM)

Bone has an enormous capacity for growth, regeneration, and remodeling. This capacity is largely due to induction of osteoblasts that are recruited to the site of bone formation. The recruitment of osteoblasts has not been fully elucidated, though the immediate environment of the cells is likely to play a role via cell- matrix interactions. We show here that heparin-binding growth-associated molecule (HB-GAM), an extracellular matrix-associated protein that enhances migratory responses in neurons, is prominently expressed in the cell matrices that act as target substrates for bone formation. Intriguingly, N-syndecan, which acts as a receptor for HB-GAM, is expressed by osteoblasts/osteoblast precursors, whose ultrastructural phenotypes suggest active cell motility. The hypothesis that HB-GAM/N-syndecan interaction mediates osteoblast recruitment, as inferred from developmental studies, was tested using osteoblast-type cells that express N-syndecan abundantly. These cells migrate rapidly to HB-GAM in a haptotactic transfilter assay and in a migration assay where HB-GAM patterns were created on culture wells. The mechanism of migration is similar to that previously described for the HB-GAM-induced migratory response of neurons. Our hypothesis that HB-GAM/N-syndecan interaction participates in regulation of osteoblast recruitment was tested using two different in vivo models: an adjuvant-induced arthritic model and a transgenic model. In the adjuvant-induced injury model, the expression of HB-GAM and of N-syndecan is strongly upregulated in the periosteum accompanying the regenerative response of bone. In the transgenic model, the HB-GAM expression is maintained in mesenchymal tissues with the highest expression in the periosteum. The HB-GAM transgenic mice develop a phenotype characterized by an increased bone thickness. HB-GAM may thus play an important role in bone formation, probably by mediating recruitment and attachment of osteoblasts/osteoblast precursors to the appropriate substrates for deposition of new bone.

Hyperplasia of lymphatic vessels in VEGF-C transgenic mice

No growth factors specific for the lymphatic vascular system have yet been described. Vascular endothelial growth factor (VEGF) regulates vascular permeability and angiogenesis, but does not promote lymphangiogenesis. Overexpression of VEGF-C, a ligand of the VEGF receptors VEGFR-3 and VEGFR-2, in the skin of transgenic mice resulted in lymphatic, but not vascular, endothelial proliferation and vessel enlargement. Thus, VEGF-C induces selective hyperplasia of the lymphatic vasculature, which is involved in the draining of interstitial fluid and in immune function, inflammation, and tumor metastasis. VEGF-C may play a role in disorders involving the lymphatic system and may be of potential use in therapeutic lymphangiogenesis.

Efficient in vivo manipulation of mouse genomic sequences at the zygote stage

We describe a transgenic mouse line carrying the cre transgene under the control of the adenovirus EIIa promoter that targets expression of the Cre recombinase to the early mouse embryo. To assess the ability of this recombinase to excise loxP-flanked DNA sequences at early stages of development, we bred EIIa-cre transgenic mice to two different mouse lines carrying loxP-flanked target sequences: (i) a strain with a single gene-targeted neomycin resistance gene flanked by 1oxP sites and (ii) a transgenic line carrying multiple transgene copies with internal loxP sites. Mating either of these loxP-carrying mouse lines to EIIa-cre mice resulted in first generation progeny in which the loxP-flanked sequences had been efficiently deleted from all tissues tested, including the germ cells. Interbreeding of these first generation progeny resulted in efficient germ-line transmission of the deletion to subsequent generations. These results demonstrate a method by which loxP-flanked DNA sequences can be efficiently deleted in the early mouse embryo. Potential applications of this approach are discussed, including reduction of multicopy transgene loci to produce single-copy transgenic lines and introduction of a variety of subtle mutations into the line.

Timing of SV40 oncogene activation by site-specific recombination determines subsequent tumor progression during murine lens development

We generated mice that carry copies of a dormant transgene encoding the SV40 tumor antigens. The transgenes are specifically targeted to the lens and contain features that render their expression dependent on the action of Cre, a site-specific bacteriophage DNA recombinase. Timing of oncogene activation was controlled by making Cre available either prior to, or coincident with, the onset of primary fiber differentiation in the embryonic lens vesicle. Early expression of Cre resulted in oncogene activation in undifferentiated lens epithelial cells that rapidly proliferated inside the lens capsule. By contrast, when Cre accumulation was delayed to coincide with the onset of primary lens fiber differentiation, SV40 oncogenes were activated in cells that had begun to elongate and to accumulate lens-specific crystallins. During subsequent proliferation inside the lens capsule, transformed progeny cells maintained the profile of fiber differentiation that their parent cells had acquired at the time of oncogenic conversion. Developing lens tumors were confined within the capsule of the embryonic lens. However, if the capsule was perforated in an embryonic eye in organ culture, cells rapidly grew out while still maintaining features of differentiation. Our findings show that the differentiated state of the primary target cells is an important parameter of subsequent lens oncogenesis, and that an intact lens capsule can restrict invasive neoplastic growth.

Targeted oncogene activation by site-specific recombination in transgenic mice

An efficient and accurate method for controlled in vivo transgene modulation by site-directed recombination is described. Seven transgenic mouse founder lines were produced carrying the murine lens-specific alpha A-crystallin promoter and the simian virus 40 large tumor-antigen gene sequence, separated by a 1.3-kilobase-pair Stop sequence that contains elements preventing expression of the large tumor-antigen gene and Cre recombinase recognition sites. Progeny from two of these lines were mated with transgenic mice expressing the Cre recombinase under control of either the murine alpha A-crystallin promoter or the human cytomegalovirus promoter. All double-transgenic offspring developed lens tumors. Subsequent analysis confirmed that tumor formation resulted from large tumor-antigen activation via site-specific, Cre-mediated deletion of Stop sequences.