Genetic dissection of IGF1-dependent and -independent effects of permanent GH excess on postnatal growth and organ pathology of mice

To study insulin-like growth factor 1 (IGF1)-independent effects of permanent growth hormone (GH) excess on body and organ growth and pathology in vivo, hemizygous bovine GH transgenic mice with homozygous disruption of the Igf1 gene (Igf1(-/-)/GH) were generated, and examined in comparison to Igf1(-/-), Igf1(+/-), wild-type (WT), Igf1(+/-)/GH, and GH mice. GH mice and Igf1(+/-)/GH mice showed increased serum IGF1 levels and the well-known giant-phenotype of GH transgenic mice. In contrast, the typical dwarf-phenotype of Igf1(-/-) mice was only slightly ameliorated in Igf1(-/-)/GH mice. Similar to GH mice, Igf1(-/-)/GH mice displayed hepatocellular hypertrophy, glomerulosclerosis, and reduced volumes of acidophilic cells in the pituitary gland. However, GH excess associated skin lesions of male GH mice were not observed in Igf1(-/-)/GH mice. Therefore, development of GH excess induced liver-, kidney-, and pituitary gland-alterations in GH transgenic mice is independent of IGF1 whereas GH stimulated body growth depends on IGF1.

Lack of Pur-alpha alters postnatal brain development and causes megalencephaly

Pur-alpha (Purα) plays an important role in a variety of cellular processes including transcriptional regulation, cell proliferation and oncogenic transformation. To better understand the role of Purα in the developing and mature brain, we generated Purα-deficient mice, which we were able to raise to the age of six months. Purα(-/-) mice were born with no obvious pathological condition. We obtained convincing evidence that lack of Purα prolongs the postnatal proliferation of neuronal precursor cells both in the hippocampus and in the cerebellum, however, without affecting the overall number of postmitotic neurons. Independent of these findings, we observed alterations in the expression and distribution of the dendritic protein MAP2, the translation of which has been proposed previously to be Purα-dependent. At the age of 2 weeks, Purα(-/-) mice generated a continuous tremor which persisted throughout lifetime. Finally, adult Purα(-/-) mice displayed a megalencephaly and histopathological findings including axonal swellings and hyperphosphorylation of neurofilaments. Our studies underline the importance of Purα in the proliferation of neuronal precursor cells during postnatal brain development and suggest a role for Purα in the regulation of the expression and cellular distribution of dendritic and axonal proteins. Since recent studies implicate a link between Purα and the fragile X tremor/ataxia syndrome, our Purα(-/-) mouse model will provide new opportunities for understanding the mechanisms of neurodegeneration.

Specific transgene expression in mouse pancreatic beta-cells under the control of the porcine insulin promoter

The availability of regulatory sequences directing tissue-specific expression of transgenes in genetically modified mice and large animals is a prerequisite for the development of adequate models for human diseases. The rat insulin 2 gene (Ins2) promoter, widely used to achieve transgene expression in pancreatic beta-cells of mice, also directs expression to extrapancreatic tissues and performs poorly in isolated pancreatic islets of human, mouse, and pig. To evaluate whether the full 5' untranslated region (UTR) of the porcine insulin gene (INS) confers robust and specific expression in beta-cells we generated an expression cassette containing 1500bp of the porcine INS 5' UTR and the 3' UTR of the bovine growth hormone gene (GH). The cassette was designed to allow easy exchange of the sequences to be expressed and easy removal of the vector backbone from the expression cassette. To evaluate the properties of the cassette, we initially inserted a cDNA encoding human betacellulin, a growth factor known to affect structural and functional parameters of beta-cells. After confirming the functionality and specificity of the construct in vitro, transgenic mouse lines were generated by pronuclear DNA microinjection. Using RT-PCR, immunohistochemistry and immunofluorescence, we show that transgenic mice expressed human betacellulin exclusively in beta-cells. Confirming the proposed insulinotropic effect of betacellulin, transgenic mice showed improved glucose tolerance. We conclude that the newly designed expression cassette containing 1500bp of the porcine insulin promoter 5' UTR confers robust and specific transgene expression to beta-cells in vitro and in transgenic mice.

IGFBP-2 overexpression reduces the appearance of dysplastic aberrant crypt foci and inhibits growth of adenomas in chemically induced colorectal carcinogenesis

Colon cancer patients frequently show increased levels of serum insulin-like growth factor-binding protein-2 (IGFBP-2), however, the pathogenetic relevance of this phenomenon for colorectal cancer is unclear. Therefore, we have used IGFBP-2 transgenic animals which overexpress IGFBP-2 systemically and locally in the intestine to study its role in chemically induced colorectal carcinogenesis. Mice received intraperitoneal injections of 1,2-dimethylhydrazine (DMH) (40 mg/kg body weight) once a week for 6 weeks to selectively induce aberrant crypt foci (ACF) and tumors in the colon. While tumor incidence was comparable in transgenic and control mice, the volume of adenomas in IGFBP-2 transgenic mice was reduced more than 2-fold. Furthermore, serum IGFBP-2 levels negatively correlated with tumor volume in the IGFBP-2 transgenic group. Histological examination showed that IGFBP-2 transgenic mice developed significantly less dysplastic ACF with a high potential to progress to advanced stages. The reduced tumor volume in IGFBP-2 transgenic animals was due to significantly reduced proliferative capacity, evidenced by a lower proportion of cells positive for Ki67. Our results demonstrate for the first time in an experimental model that IGFBP-2 overabundance prior to the onset and during colorectal carcinogenesis reduces tumor growth by inhibition of cell proliferation.

Scrapie-infected transgenic mice expressing a laminin receptor decoy mutant reveal a prolonged incubation time associated with low levels of PrPres

The 37-kDa/67-kDa laminin receptor (LRP/LR) was identified as a cell surface receptor for prion proteins. The laminin receptor mutant LRP102-295::FLAG interfered with PrP(Sc) propagation in murine neuronal cells presumably acting as a decoy in a transdominant negative fashion by trapping PrP molecules in the extracellular matrix. Here, we generated hemizygous transgenic mice expressing LRP102-295::FLAG in the brain. Scrapie-infected transgenic mice exhibit a significantly prolonged incubation time in comparison to scrapie-infected wild-type (FVB) mice. At the terminal stage, transgenic mice revealed significantly reduced proteinase-K-resistant PrP levels by 71% compared to wild-type mice. Our results recommend the laminin receptor decoy mutant as an alternative therapeutic tool for treatment of transmissible spongiform encephalopathies.

Systemic overexpression of growth hormone (GH) in transgenic FVB/N inbred mice: an optimized model for holistic studies of molecular mechanisms underlying GH-induced kidney pathology

Transgenic mice overexpressing growth hormone (GH) display a plethora of phenotypic alterations and provide unique models for studying and influencing consequences of chronic GH excess. Since the first report on GH transgenic mice was published in 1982, many different mouse models overexpressing GH from various species at different levels and with different tissue specificities were established, most of them on random-bred or hybrid genetic background. We have generated a new transgenic mouse model on FVB/N inbred background, expressing bovine (b) GH under the control of the chicken beta-actin promoter (cbetaa). cbetaa-bGH transgenic mice exhibit ubiquitous expression of bGH mRNA and protein and circulating bGH levels in the range of several microg/ml, resulting in markedly stimulated growth and the characteristic spectrum of pathological lesions which were described in previous GH overexpressing mouse models. Importantly, a consistent sequence of renal alterations is observed, mimicking progressive kidney disease in human patients. The novel, genetically standardized GH transgenic mouse model is ideal for holistic transcriptome and proteome studies aiming at the identification of the molecular mechanisms underlying GH-induced pathological alterations especially in the kidney. Moreover, genetically defined cbetaa-bGH mice facilitate random mutagenesis screens for modifier genes which influence the effects of chronic GH excess and associated pathological lesions.

Effect of macrophage ApoE on atherosclerosis in LDL-receptor deficient mice

Apolipoprotein E (ApoE) plays an important role in the development of atherosclerosis. Previous studies provide evidence for an atheroprotective role of ApoE in mouse models on the ApoE deficient (ApoE-/-) background. However, it is not clear whether this is also true on the LDL-receptor deficient (LDLR-/-) background. Transgenic mice carrying hApoE coding sequences in a chicken lysozyme expression cassette were generated. Transgene expression was directed into macrophages, expressing low levels of hApoE. Expression of the hApoE transgene was not sufficient to correct hypercholesterolemia. However, lesion area at the brachiocephalic artery (BCA) was significantly reduced (-72%) in female hApoE transgenic mice on the LDLR-/- background. This was associated with increased cholesterol efflux in macrophages of transgenic animals on the ApoE-/- background. We conclude that over-expression of ApoE in macrophages might be useful as a therapeutic principle for the prevention of atherosclerosis.

A 470 bp WAP-promoter fragment confers lactation independent, progesterone regulated mammary-specific gene expression in transgenic mice

The ability of a 470 bp sub-fragment of the murine whey acidic protein (WAP) promoter in the context of a retroviral expression plasmid to direct gene expression to mammary epithelial cells was analysed in a number of independent transgenic mouse lines. In contrast to previous findings with the genuine 2.5 kb promoter fragment, our studies revealed a highly mammary gland-specific expression detectable only in non-lactating animals. This suggested a mainly progesterone-regulated activity of the short fragment. Therefore, transgene expression was examined in the progesterone-determined estrous cycle and during pregnancy. In accordance with in vitro data from stably transfected cell lines, in both situations expression was upregulated at stages associated with high progesterone levels. Taken together these data provide deeper insight into WAP-promoter regulation and stress the usefulness of the shortened fragment for a lactation independent mammary-targeted expression.

Adult Scl+/+Murine Hemangioblasts Persist in Allogeneic Mutant Blastocysts but Fail to Rescue the Scl−/−Phenotype

Isolated and expanded scl (+) adult murine progenitors show a strong endothelial and hematopoietic differentiation potential and have been considered to be the adult equivalent of the hemangioblast. These unique cells may provide effective therapeutic approaches to tissue damage resulting from hypoxemia or chronic ischemia. Here, we study the fate of adult scl (+/+) during development and their ability to reverse genetic defects in scl expression. scl (+/+) adult stem cells (clone RM26) did not persist during embryonic development after injection into blastocysts of allogeneic wild-type mice on day E 3.5. However, GFP(+)-marked scl (+/+) cells were detected in all possible genotypes from allogeneic scl (+/+) intercrosses (scl (+/+), scl (+/-), scl (-/-) on day E 9.5 after the cloned cells were injected into scl-mutant blastocysts on day E 3.5. Nevertheless, there was no indication of phenotypic rescue of the mutant blastocysts despite the continued presence of scl (+/+) RM26 cells in the allogeneic embryonic environment. The results show that differentiated stem cells providing a defective gene may exert effects during development when there is a reparative demand, but they are not capable of reversing the effects of a mutant phenotype during embryonic development. These effects should be considered when evaluating the efficacy of stem cells for therapeutic reversal of inborn errors of development.

Peri/nuclear localization of intact insulin-like growth factor binding protein-2 and a distinct carboxyl-terminal IGFBP-2 fragment in vivo

Insulin-like growth factor binding protein-2 (IGFBP-2) as one of the most important IGFBPs has never been assessed in the intracellular compartment in vivo. Since there is evidence for novel intracellular functions of distinct IGFBPs, we investigated the presence of IGFBP-2 inside the cell. In peri/nuclear fractions of various tissues isolated from IGFBP-2 transgenic and non-transgenic mice we were able to show the presence of intact IGFBP-2. In addition, we demonstrate the presence of a highly conserved carboxyl-terminal IGFBP-2 fragment in the peri/nuclear fraction by using different peptide-induced antibodies. In pancreatic sections, confocal microscopy revealed the presence of IGFBP-2 on the nuclear surface but not within the nucleus. Our findings suggest novel functions of intact IGFBP-2 and IGFBP-2 fragments within the cell.

Targeted disruption of the Walker-Warburg syndrome gene Pomt1 in mouse results in embryonic lethality

O-mannosylation is an important protein modification in eukaryotes that is initiated by an evolutionarily conserved family of protein O-mannosyltransferases. The first mammalian protein O-mannosyltransferase gene described was the human POMT1. Mutations in the hPOMT1 gene are responsible for Walker-Warburg syndrome (WWS), a severe recessive congenital muscular dystrophy associated with defects in neuronal migration that produce complex brain and eye abnormalities. During embryogenesis, the murine Pomt1 gene is prominently expressed in the neural tube, the developing eye, and the mesenchyme. These sites of expression correlate with those in which the main tissue alterations are observed in WWS patients. We have inactivated a Pomt1 allele by gene targeting in embryonic stem cells and produced chimeras transmitting the defect allele to offspring. Although heterozygous mice were viable and fertile, the total absence of Pomt1(-/-) pups in the progeny of heterozygous intercrosses indicated that this genotype is embryonic lethal. An analysis of the mutant phenotype revealed that homozygous Pomt1(-/-) mice suffer developmental arrest around embryonic day (E) 7.5 and die between E7.5 and E9.5. The Pomt1(-/-) embryos present defects in the formation of Reichert's membrane, the first basement membrane to form in the embryo. The failure of this membrane to form appears to be the result of abnormal glycosylation and maturation of dystroglycan that may impair recruitment of laminin, a structural component required for the formation of Reichert's membrane in rodents. The targeted disruption of mPomt1 represents an example of an engineered deletion of a known glycosyltransferase involved in O-mannosyl glycan synthesis.

Knock-Down of the 37-kDa/67-kDa Laminin Receptor in Mouse Brain by Transgenic Expression of Specific Antisense LRP RNA

The 37-kDa/67-kDa laminin receptor (LRP/LR) plays a major role in the propagation of PrPSc, the abnormal form of the prion protein. In order to ablate the expression of LRP/LR in mouse brain we generated transgenic mice ectopically expressing antisense LRP RNA in the brain under control of the neuron-specific enolase (NSE) promoter. Hemizygous transgenic mice TgN(NSEasLRP)2 showed a significant reduction of LRP/LR protein levels in hippocampal and cerebellar brain regions. These mice might act as powerful tools to investigate the role of the laminin receptor in scrapie pathogenesis.

Polyclonal anti-PrP auto-antibodies induced with dimeric PrP interfere efficiently with PrPSc propagation in prion-infected cells

Prion diseases are neurodegenerative infectious disorders for which no prophylactic regimens are known. In order to induce antibodies/auto-antibodies directed against surface-located PrP(c), we used a covalently linked dimer of mouse prion protein expressed recombinantly in Escherichia coli. Employing dimeric PrP as an immunogen we were able to effectively overcome autotolerance against murine PrP in PrP wild-type mice without inducing obvious side effects. Treatment of prion-infected mouse cells with polyclonal anti-PrP antibodies generated in rabbit or auto-antibodies produced in mice significantly inhibited endogenous PrP(Sc) synthesis. We show that polyclonal antibodies are binding to surface-located PrP(c), thereby interfering with prion biogenesis. This effect is much more pronounced in the presence of full IgG molecules, which, unlike Fab fragments, seem to induce a significant cross-linking of surface PrP. In addition, we found immune responses against different epitopes when comparing antibodies induced in rabbits and PrP wild-type mice. Only in the auto-antibody situation in mice an immune reaction against a region of PrP is found that was reported to be involved in the PrP(Sc) conversion process. Our data point to the possibility of developing means for an active immunoprophylaxis against prion diseases.

Hypocalcemia and osteopathy in mice with kidney-specific megalin gene defect

Megalin is an endocytic receptor highly expressed in the proximal tubules of the kidney. Recently, we demonstrated that this receptor is essential for the renal uptake and conversion of 25-OH vitamin D3 to 1,25-(OH)2 vitamin D3, a central step in vitamin D and bone metabolism. Unfortunately, the perinatal lethality of the conventional megalin knockout mouse model precluded the detailed analysis of the significance of megalin for calcium homeostasis and bone turnover in vivo. Here, we have generated a new mouse model with conditional inactivation of the megalin gene in the kidney by using Cre recombinase. Animals with a renal-specific receptor gene defect were viable and fertile. However, lack of receptor expression in the kidney results in plasma vitamin D deficiency, in hypocalcemia and in severe bone disease, characterized by a decrease in bone mineral content, an increase in osteoid surfaces, and a lack of mineralizing activity. These features are consistent with osteomalacia (softening of the bones) as a consequence of hypovitaminosis D and demonstrate the crucial importance of the megalin pathway for systemic calcium homeostasis and bone metabolism.

Mutation of the RGD sequence does not affect plasma membrane association and growth inhibitory effects of elevated IGFBP-2 in vivo

Using insulin-like growth factor-binding protein-2 (IGFBP-2) transgenic mice (D mice) as a model of elevated IGFBP-2 expression, which is often found in unphysiological conditions, we found association of IGFBP-2 to purified plasma membranes of many organs. To determine whether the RGD (Arg-Gly-Asp) motif of IGFBP-2 mediates cell surface binding in vivo, we mutated the RGD motif of IGFBP-2 into an RGE (Arg-Gly-Glu) sequence and produced transgenic mice (E mice) which express elevated amounts of mutated IGFBP-2. Our data demonstrate that in vivo IGFBP-2 cell surface association is not dependent on the RGD motif and that mutation of this sequence does not alter growth inhibitory effects of IGFBP-2.

Large-scale N-ethyl-N-nitrosourea mutagenesis of mice--from phenotypes to genes

The most important tool for obtaining insight into the function of genes is the use of mutant model organisms. Homologous recombination in embryonic stem cells allows the systematic production of mouse mutants for any gene that has been cloned. Gene trap strategies have been designed to interrupt even unknown genes which are tagged by the inserted vector and can be characterised structurally and functionally. Complementary to such 'gene-driven' approaches, 'phenotype-driven' approaches are necessary to identify new genes or gene products through a search for mutants with specific defects, uncovering the function of genetic pathways in physiological and pathological processes. Mutagenesis using the alkylating agent N-ethyl-N-nitrosourea (ENU) is a powerful approach for the production of such mouse mutants. Since ENU induces mainly point mutations in premeiotic spermatogonia, this strategy allows the production of multiple alleles of a particular gene, which is pivotal for a fine tuned analysis of its function.

Contrasting obesity phenotypes uncovered by partial leptin receptor gene deletion in transgenic mice

Non-insulin-dependent diabetes mellitus (type 2 diabetes) is known to be a polygenic and polyfactorial disorder. Here we describe the long-term examination of a transgenic mouse line showing the disruption of the leptin receptor (Lepr, Ob-R) gene caused by transgene insertion. The absence of the expression of the long isoform Ob-Rb uncovered a strong variation of the obesity and diabetes phenotype in the homozygous mutant mice of the outbred strain used. One part of the homozygous mice developed severe persistent early-onset obesity, whereas the other part developed cachexia after having shown initial obesity in the examination period up to 26 weeks p.p. The leptin-receptor-defective mice of this line might serve as a model for the investigation of genes modulating the development and mode of expression of diabetes.

Overgrowth of skin in growth hormone transgenic mice depends on the presence of male gonads

Growth hormone has been shown to possess stimulatory effects on various connective tissues. We observed that skin growth in male rat phosphoenolpyruvate carboxykinase-bovine growth hormone transgenic mice (serum growth hormone levels: 740-1940 ng per ml) is progressive with age, resulting in an "oversized coat" phenotype with a marked increase in absolute and relative skin weight and surface area, and in thickness of the dermis. Histologic changes include severe dermal fibrosis and replacement of subdermal adipose tissue by fibrous tissue. Apart from an increase in skin surface area, these changes were not noted in female transgenic mice, arguing for a specific interaction of growth hormone with male sex hormones. To clarify this point, 6 wk old male transgenic mice and control mice were castrated and compared with their noncastrated counterparts in parameters of skin growth at an age of 8 mo. The skin weight of castrated transgenic mice was smaller (p < 0.01) than that of intact transgenic mice both absolutely and relative to body weight. The relative skin weight of castrated transgenic mice was in the same range as in intact and castrated control mice. Absolute and relative skin area of castrated transgenic mice was greater (p < 0. 001 and p < 0.05) than in controls but lower than in intact transgenic mice (p < 0.001 and p < 0.05). When compared with control mice, intact transgenic mice displayed an increase (p < 0.01) in the thickness of dermis. In castrated transgenic mice the thickness of the dermis was in the same range as in control mice. Our findings demonstrate a specific interaction of growth hormone with male sex hormones resulting in a marked stimulation of skin growth.

CCR7 coordinates the primary immune response by establishing functional microenvironments in secondary lymphoid organs

The proper function of immune surveillance requires well-coordinated mechanisms in order to guide the patrolling immune cells through peripheral tissues and into secondary lymphoid organs. Analyzing gene-targeted mice, we identified the chemokine receptor CCR7 as an important organizer of the primary immune response. CCR7-deficient mice show severely delayed kinetics regarding the antibody response and lack contact sensitivity and delayed type hypersensitivity reactions. Due to the impaired migration of lymphocytes, these animals reveal profound morphological alterations in all secondary lymphoid organs. Upon activation, mature skin dendritic cells fail to migrate into the draining lymph nodes. Thus, in order to bring together lymphocytes and dendritic cells to form the characteristic microarchitecture of secondary lymphoid organs, CCR7 is required to rapidly initiate an adoptive immune response.

[Sex-specific analysis of bone mass in normal and growth hormone transgenic mice using dual energy x-ray absorptiometry (DXA)]

The aim of the present study was the non-invasive, sex-specific measurement of bone mass in bovine growth hormone (bGH) transgenic mice and normal controls with dual energy X-ray absorptiometry (DXA). The transgenic mouse constitutes a suitable animal model to study the influence of growth hormone on the skeletal system. We analysed 28 animals, aged 12 weeks (14 transgenic, 14 controls, 7 male and 7 female, respectively), using a peripheral DXA scanner that had been adapted to the measurement of small animals. At a measurement time of 20 min, the precision (RMS average CV%) was 4.4% for bone mass (BMC), 2.5% for areal bone density (BMD), 0.86% for total body weight and 4.5% for the percentage BMC (relative to body weight). While the absolute bone mass was not significantly different between male and female animals, we found a higher percentage of the BMC relative to the total bone mass in females (+21% in controls, +31% in transgenics; p < 0.01). The absolute bone mass was higher in the transgenic animals (+71% in females and +62% in males; p < 0.01), but relative to the body weight the transgenic females yielded similar and the transgenic males lower values (-7.2%; p < 0.05). Using DXA it is possible to non-invasively determine the mass of mineralised tissue in the mouse with relatively high precision and to effectively discriminate between different groups. Although a strong influence of growth hormone on the absolute bone mass is observed, the results show that this increase is not higher than that of the total body mass.

A non-destructive technique for 3-D microstructural phenotypic characterisation of bones in genetically altered mice: preliminary data in growth hormone transgenic animals and normal controls

A non-destructive, three-dimensional technique for microstructural phenotypic characterisation of skeletal elements in genetically altered mice is presented. Preliminary data in bovine growth-hormone transgenic animals and control littermates are shown. The technique is based on microcomputed tomography (microCT) and digital postprocessing and allows for a differential quantitative analysis of the cortical and trabecular bone compartments in the axial and peripheral skeleton. The distal femora and the first lumbar vertebral bodies of six animals were CT scanned in the axial plane with an isotropic resolution of 20 microm. The periostal surface and the marrow spaces were segmented fully automatically, and the trabecular and cortical compartments were separated interactively. After 3-D reconstruction, various regions of interest (diaphyseal, metaphyseal and epiphyseal) were selected for the analysis. The femora and vertebrae of the transgenic animals showed obvious differences in size, shape, and trabecular arrangement compared with the control animals. The total bone mass was increased by a factor of two to three, but the trabecular bone was increased much more (up to 12 times) than the cortical bone. The transgenic animals showed an increased ratio of trabecular vs cortical bone (0.90 to 1.27 vs 0.14 to 0.36 in the femoral diaphysis) and an elevated trabecular bone volume fraction (49% to 73% vs 18% to 43% in the femoral metaphysis). The mean 3-D cortical thickness was similar in the normal and transgenic animals (values between 93 microm and 232 microm in the dia- and metaphyses), but the minimal cortical thickness was lower in the transgenic animals (22 to 31 microm vs 54 microm to 110 microm in the diaphysis). The technique presented is suitable for phenotypic characterisation of bone structure in genetically altered mice.