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Advances in Brief |
1 Department of Biochemistry, Faculty of Medicine, University of Hong Kong, Hong Kong; 2 Division of Matrix Biology, Department of Medical Biochemistry and Biophysics, and 3 Microbiology and Tumorbiology Center, Karolinska Institute, Stockholm, Sweden; 4 Institute of Molecular Medicine, Department of Medicine, University of California at San Diego, La Jolla, California; and 5 Department of Medical Biochemistry and Molecular Biology, University of Oulu, Oulu, Finland
| ABSTRACT |
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3/
3) mice survived embryonic development and were apparently healthy as adults. However, mutant mice exhibited significantly delayed wound healing, retarded FGF-2-induced tumor growth, and defective angiogenesis. In the mouse corneal angiogenesis model, FGF-2-induced neovascularization was significantly impaired in Hspg2
3/
3 mutant mice. Our results suggest that HS in perlecan positively regulates the angiogenesis in vivo. | Introduction |
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The mouse Hspg2 gene encodes the large perlecan core protein. Hspg2 null mice were embryonic lethal, due to the disruption of basement membrane structure (5)
. To study the potential in vivo role of perlecan HS side chains, a 45-bp in-frame deletion was made to the Hspg2 gene to remove all three HS chain attachment sites from the domain I, resulting in the HS chains deficiency in a specific HSPG-perlecan HS deficiency. Hspg2
3/
3 mice survived embryonic development. Adult mice appear healthy and fertile and are grossly indistinguishable from their littermate controls. However, a careful examination showed that Hspg2
3/
3 mice had defects in lens development (6)
. The Hspg2
3/
3 mice provided the unique model to study the in vivo roles for specific HSPG HS chains in FGF-2 signaling and angiogenesis. The purpose of this study was to address the following questions: (a) Is angiogenesis in physiological and pathological conditions affected in Hspg2
3/
3 mice? (b) What is the role for perlecan HS in FGF-2-induced angiogenesis? Here, we show that deficiency in perlecan HS does not affect the structure or composition of the blood vessel basement membranes. We found that perlecan HS promoted angiogenesis in vivo, because the removal of perlecan HS side chains led to impaired FGF-2-mediated angiogenesis.
| Materials and Methods |
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3/
3 mice has been described elsewhere (6)
. In brief, exon 3 of mouse perlecan gene, Hspg2, which is 45 bp in size, was removed in-frame. After homologous recombination in embryonic stem cells, the mutated allele was transmitted through the germ line. Heterozygous mutant mice were back-crossed to C57BL/6 mice for seven generations. Homozygous mutants (Hspg2
3/
3) were obtained through mating between heterozygous animals. All animal experiments have been approved by the Stockholm Animal Ethics Committee.
Skin Wound-Healing Assay.
The skin wound-healing assay was performed essentially as described previously (7)
. Two 8-mm full-thickness punch wounds were created on the dorsal surface of each of the 6-week-old mice with a dermal biopsy punch (Miltex GmbH, Ludwigshafen, Germany). Twenty-four h after the surgery, the wounds were recorded as initial condition. The wounds were then recorded daily on transparent parafilm for 9 days. All recordings were scanned, and the wound areas were calculated with Scion Image software.
Tumor Experiments.
K1000, the NIH3T3 cells stably transfected with FGF-2 cDNA with a signal peptide (8)
, were maintained in DMEM supplied with 10% fetal bovine serum and 100 IU/ml penicillin-streptomycin (Life Technologies, Inc.). Before inoculation, cells were trypsinized, washed, and suspended in PBS. Cells of 5 x 106 were inoculated s.c. on the backs of newborn mice. Tumors were dissected and weighed at designated days after the inoculation.
Cornea Micropocket Assay.
The corneal micropocket assay was performed as described previously (9)
. In brief, a corneal micropocket was created on one eye of each of the 6-week-old Hspg2
3/
3 mice and controls. A micropellet containing 80 ng of human recombinant FGF-2 (Invitrogen) was implanted into each pocket. Corneal neovascularization was examined 5 days after the operation.
Immunohistochemistry.
Immunohistochemical staining was performed on cryostat sections. Fresh tumor samples and skin wound samples were collected and snap-frozen in OCT compound (Sakura, Finetek, the Netherlands). Cryostat sections (10 µm thick) were fixed in 1% paraformaldehyde and stained with biotin antimouse CD31 (PharMingen) or rat monoclonal antibodies against perlecan (NeoMarkers), nidogen (NeoMarkers), laminin
4 [a kind gift from Rupert Timpl (Max-Planck Institute for Biochemistry, Martinsried, Germany)], and collagen IV
1 chain.
Measurement of Blood Vessel Density.
To measure the blood vessel density, tumors of comparable size from control and Hspg2
3/
3 mice were used. Tumors from control mice were dissected 7 days after the inoculation, and tumors from Hspg2
3/
3 mice were dissected 10 days after the inoculation. Cryostat sections (10 µm thick) were immunostained with CD31 antibody. Five microscopic fields were randomly selected from each tumor section and recorded with a CCD camera. In wound-healing experiments, skin wound samples were harvested at day 5, and the sections were counterstained with hematoxyline after CD31 immunostaining. Two or three microscopic fields of granulation tissue were chosen from each section. The number of blood vessels was counted in sections of three animals from each group, and the density was calculated using Scion Image software.
Cell Surface Binding of 125I-FGF-2.
Binding of 125I-FGF-2 to murine embryonic fibroblasts (MEF) cells was determined as described previously (10)
, with the following modifications. MEF cells were seeded at a density of 2 x 105/well in 48-well plates and cultured until confluence in full medium. After washing with 1x PBS, cells were then incubated for an additional 24 h in serum-free DMEM for conditioned medium collection. A binding solution was prepared by mixing DMEM medium with various concentrations of 125I-FGF-2 and cooling to 4°C for 10 min. Confluent MEF cells were washed three times with ice-cold DMEM. The binding solution was added to the cells and incubated at 4°C for 2.5 h. The binding medium was then removed, and the cells were gently washed three times with ice-cold DMEM. The FGF-2 bound to cell surface was extracted with ice-cold 2 M NaCl/20 mM sodium acetate (pH 4.0) for 15 min, and the salt extracts were counted in a
counter. Nonspecific binding was considered as the value obtained in the presence of a 100-fold excess of nonradioactive FGF-2. The value was then normalized by the cell number.
Statistics.
Data were analyzed with the two-tailed Students t test.
| Results |
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3/
3 Mice.
3/
3 mice and their sex- and age-matched wild-type control mice from the same background were used in this study. Two full skin thickness punch wounds were created on the backs of these mice (Fig. 1A)
3/
3 mice. However, the extent of development of granulation tissue was less in Hspg2
3/
3 mice and appeared to be poorly vascularized (Fig. 1C)
3/
3 mice was significantly lower (182 ± 21/mm2, mean ± SE) in comparison with that in wild-type controls (287 ± 25/mm2, mean ± SE, P < 0.001; Fig. 1D
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3/
3 Mice.
3/
3 mice to further address the role of perlecan HS in angiogenesis. Basic FGF-transformed cells K1000 were used in this experiment. K1000 cells of 5 x 106 were inoculated s.c. on the backs of newborn Hspg2
3/
3 and control mice. Ten days after inoculation, tumors were dissected and weighed. Eight litters of newborn mice, amounting to 21 mutant and 37 control individuals, were used in this study. The tumor mass from Hspg2
3/
3 mice weighed 0.47 ± 0.10 g (mean ± SE) and was significantly less than that of their littermate controls (0.79 ± 0.17 g, mean ± SE, P < 0.01; Fig. 2A
3/
3 mice, we evaluated blood vessel formation in tumors. Tumors from control mice were sampled at day 7, and tumors from mutant mice were sampled at day 10, so the tumors from controls and mutants were comparable in size. Tumor sections were stained with CD31 (Fig. 2, C and D)
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3/
3 mice. To examine whether the loss of perlecan HS altered the composition of major basement membrane components in blood vessels, we immunostained tumor sections with antibodies against perlecan (Fig. 2, E and F)
4 (Fig. 2, G and H)
3/
3 and control mice.
Impaired FGF-2-Induced Corneal Angiogenesis in Hspg2
3/
3 Mice.
To further investigate the impaired angiogenesis in Hspg2
3/
3 mice, we used the corneal micropocket assay to study FGF-2-induced angiogenesis in vivo. An FGF-2-containing pellet was implanted into the cornea of each 6-week-old mouse. Three animals of each genotype group (Hspg2
3/
3, heterozygous, and control) were used in these experiments. Five days after pellet implantation, blood vessel formation was evident in all mice. No differences were found between wild-type and heterozygous mice. However, all Hspg2
3/
3 mice showed a severely impaired angiogenesis (Fig. 3)
.
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3/
3 mice, we isolated MEFs from Hspg2
3/
3 mice and wild-type mice. The role of perlecan HS in the ligand-receptor interaction was investigated in vitro. 125I-FGF-2 of different concentrations was incubated for 2.5 h at 4°C with wild-type MEFs and Hspg2
3/
3 MEFs. After washing, the cell surface-bound 125I-FGF-2 was extracted and measured by
counter. At the range of 520 ng/ml, wild-type MEFs exhibited a significant higher binding efficiency than Hspg2
3/
3 cells (P < 0.05), suggesting that in wild-type cells, the remaining wild-type perlecan HS on cell surface facilitates the binding of growth factor to cell surface (Fig. 4)
3/
3 cells.
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| Discussion |
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A number of studies have been carried out to understand the biochemical and biological roles of perlecan in growth factor signaling (14, 15, 16, 17) . Several functions of perlecan HS have been proposed based on in vitro studies: (a) Perlecan HS sequesters and traps growth factors, protecting them from proteolytic degradation; and (b) perlecan HS binds growth factors by acting as a low-affinity receptor and mediates high-affinity growth factor-receptor binding. Data from our in vitro binding support the notion that although a proportion of perlecan is secreted into the culture medium (17) , a significant amount of perlecan attached to cell surfaces (18 , 19) . Cell surface perlecan participated directly or indirectly in the presentation of growth factors to its high-affinity receptor. Both the ligand trapping and presenting functions of perlecan seemed to be carried out through its HS side chains.
Based on our and others results, it is likely that the major function of perlecan HS in the extracellular matrix is to bind FGF-2 and other growth factors, protect them from proteolytic degradation, and store the ligands in a dormant state. Perlecan on the cell surface may directly or indirectly present the growth factors to their receptor and facilitate the growth factor signaling. Perlecan without its HS chains failed to trap growth factors at the extracellular matrix of endothelial basement membranes and was unable to protect or present growth factors efficiently to the cells surface or higher affinity receptors. Therefore, impaired angiogenesis in the mutant mice was expected. Our results also indicate that the other membrane-associated HSPGs may also be important in the presentation of growth factors to their receptors. Lack of perlecan HS side chains did not result in severe developmental defects in neuronal and vascular tissues or total loss of mitotic response to FGF-2, indicating the functional redundancy in the regulation of growth factors receptors binding by HSPGs.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Zhongjun Zhou, Department of Biochemistry, Faculty of Medicine, University of Hong Kong, 21 Sassoon Road, Hong Kong. Phone: 852-28199542; Fax: 852-28551254; E-mail: zhongjun{at}hkucc.hku.hk
Received 3/ 5/04. Revised 5/ 4/04. Accepted 5/28/04.
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