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Previous Article | Next Article 
Molecular and Cellular Biology, March 2005, p. 2441-2449, Vol. 25, No. 6
0270-7306/05/$08.00+0 doi:10.1128/MCB.25.6.2441-2449.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Michael M. Halford,1,
Sally Roufail,1
Richard A. Williams,2
Margaret L. Hibbs,1
Dianne Grail,1
Hajime Kubo,3
Steven A. Stacker,1 and
Marc G. Achen1*
Ludwig Institute for Cancer Research, Royal Melbourne Hospital, Parkville, Victoria 3050,1 Department of Pathology, Faculty of Medicine, Dentistry, and Health Sciences, University of Melbourne, Parkville, Victoria 3052, Australia,2 Molecular and Cancer Research Unit, HMRO, Graduate School of Medicine, Kyoto University, Yoshida, Sakyo-ku, Kyoto, Japan3
Received 24 August 2004/ Returned for modification 10 October 2004/ Accepted 7 December 2004
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ABSTRACT |
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INTRODUCTION |
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Vascular endothelial growth factor receptor 3 (Vegfr-3), a cell surface receptor tyrosine kinase (also known as Flt4), is expressed on lymphatic endothelial cells in adult tissues (18, 27), and its activation is sufficient to stimulate lymphangiogenesis (45). The two known activating ligands for Vegfr-3 are Vegf-c (17) and Vegf-d (also known as c-fos-induced growth factor [Figf]) (1, 35), secreted homodimeric glycoproteins that are closely related in structure. In humans, both VEGF-C and VEGF-D activate VEGFR-3 and another receptor, VEGFR-2 (1, 17), which is implicated in signaling for angiogenesis (32); in mice, Vegf-c activates both receptors (23), but Vegf-d activates only Vegfr-3 (6). Thus, the receptor binding specificity of Vegf-d suggests that it may be lymphangiogenic but not angiogenic in the mouse. Indeed, the ectopic expression of VEGF-D in the skin of transgenic mice under the control of the keratin 14 gene promoter induced the growth of lymphatic vessels in the dermis, whereas blood vessels were unaffected (45). In addition, the expression of VEGF-D in a mouse tumor model stimulated lymphangiogenesis in solid tumors; this process promoted the metastatic spread of tumor cells via the lymphatics but could be blocked by a neutralizing VEGF-D antibody (2, 3, 42). The importance of Vegf-c and Vegf-d was further illustrated by the finding that the expression of a soluble form of VEGFR-3 that acts to sequester both growth factors in the skin of transgenic mice caused impaired fetal lymphatic development and a lymphedema-like phenotype (28).
During early embryonic development, Vegfr-3 is expressed widely on endothelial cells of blood vessels but subsequently becomes restricted to venous endothelium, at a stage prior to the emergence of lymphatic vessels, and then becomes restricted to the endothelium of lymphatic vessels (18, 27). A key role for VEGFR-3 in lymphatic development was demonstrated by the identification of mutations in VEGFR-3 in several cases of human congenital lymphedema (19). However, mutant mice deficient for Vegfr-3, generated by homologous recombination in embryonic stem (ES) cells, were not informative with respect to lymphatic vessel development or function, as these mice died at embryonic day 9.5 (E9.5) due to defective remodeling and maturation of blood vessels into larger vessels, prior to the emergence of lymphatic vessels from the veins (11). Further studies showed that the Vegfr-3 ligand Vegf-c is required for the initial steps of lymphatic development (20). In Vegf-c–/– mice, endothelial cells commit to the lymphatic lineage but do not sprout to form lymphatic vessels. The lack of lymphatic vessels results in prenatal death due to fluid accumulation in tissues. Vegf-c+/– mice develop cutaneous lymphatic hypoplasia and lymphedema, demonstrating that both Vegf-c alleles are required for normal lymphatic development (20).
To assess the role of Vegf-d in the development of the lymphatic system, we have generated mutant mice that lack Vegf-d by homologous recombination in ES cells. Vegf-d-deficient mice are viable and have a functional lymphatic system. The lymphatic vessels in these mice develop normally and are able to transport fluid to draining lymph nodes. Furthermore, Vegf-d-deficient mice do not display evidence of perturbed lymphatic vessel function. The lack of a profound phenotype associated with Vegf-d deficiency suggests that Vegf-d does not play an important role in lymphatic development or that there is compensation for Vegf-d function during lymphatic development by Vegf-c or another, as-yet-unknown ligand for Vegfr-3.
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MATERIALS AND METHODS |
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Generation of Vegf-d-deficient mice. Electroporation of W9.5 wild-type ES cells (2.5 x 107 cells) with 30 µg of pVDKO-construct that had been linearized with XhoI was performed essentially as described previously (9). ES cells were grown on a primary embryonic fibroblast (PEF) feeder layer in the presence of 1,000 U of leukemia inhibitory factor (Chemicon)/ml. Selection with G418 (Invitrogen) at a concentration of 175 µg/ml began 24 h following electroporation and was continued for 9 days, at which time individual colonies were picked and screened for targeting events by Southern hybridization. Genomic DNA from resistant colonies was digested with each of the enzymes HindIII, PvuII, and BglII individually and evaluated by Southern blotting. Blots were hybridized with a 681-nucleotide (nt) fragment of genomic DNA, denoted E1F2R1, that is homologous to a region of the mouse Vegf-d gene 5' to ShA (Fig. 1). E1F2R1 was amplified by PCR with the M18 genomic DNA clone as a template, a sense primer with the sequence 5'-TTTAGTTAGCCCTTTCCTCCA, and an antisense primer with the sequence 5'-TTCTGGGTTGTAGCATAGTCA. Targeted clones were identified at a frequency of 1 of 21, and 2 of them (clones 2.2.36 and 2.88) were injected into C57BL/6J blastocysts and transferred into pseudopregnant mice. Chimeric mice were mated, and the offspring were genotyped by Southern hybridization to identify females carrying a single targeted allele (denoted X–X+ because the Vegf-d gene is on the X chromosome) and Vegf-d-deficient males (X–Y). Subsequent mating predominantly involved the breeding of X+X+ and X+Y mice to generate litters consisting entirely of wild-type mice and the breeding of X–X– and X–Y mice to generate litters consisting entirely of Vegf-d-deficient mice. Such offspring were used for experimental phenotypic analyses. Mice designated breeders were the offspring of crosses of wild-type and Vegf-d-deficient mice or were the offspring of crosses of X+X– and either X+Y or X–Y mice. Breeders were generated in this way to prevent the genetic divergence of wild-type and Vegf-d-deficient mice.
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All experiments performed with mice were carried out in accordance with the guidelines set by the National Health and Medical Research Council of Australia.
Derivation of PEF cell lines. PEF cell lines for Northern and Western blot analyses were generated from litters of wild-type and Vegf-d-deficient mice. Embryos were collected at E13.5; the heads and livers were removed, digested with 0.1% trypsin-0.02% EDTA, and sheared by passage through needles of increasing gauge (18, 19, 21, 23, and 25 gauge). Cells were pelleted, resuspended in Dulbecco modified Eagle medium containing 10% fetal calf serum, 2 mM glutamine, and 10–6 M ß-mercaptoethanol, and transferred to gelatin-coated flasks. Cells were incubated at 37°C in 10% CO2, and PEFs were passaged 1:2 when a confluent, adherent monolayer had grown. For Northern and Western blot analyses, PEF supernatants and PEFs were collected when the cells were confluent at passage 5.
Northern blot analysis. RNA was extracted from mouse organs and PEF cell lines by the guanidine thiocyanate method essentially as described previously (38). Northern blots were hybridized with a 209-nt PCR product (VDEx7) that had been amplified from Vegf-d cDNA by using a sense primer specific to exon 7 (VEGFCM-87: 5'-CTGTGAGGACAGATGTCCTTTTC) and an antisense primer specific to the 3' untranslated region located in exon 7 (VEGFCM-8: 5'-GGTAGTGGGCAACAGTGACAG). In addition, Northern blots were hybridized with a 425-nt PCR product (VD-VHD) amplified from Vegf-d cDNA by using a sense primer specific to exon 3 (VEGFCM-22: 5'-GAAGAATGGCAGAGGACCCA) and an antisense primer specific to exon 5 (VEGFCM-17: 5'-CCAGGCAGTGGAGTCTC). VDEx7 and VD-VHD hybridized to transcripts of the same sizes. Northern blotting for Vegf-c mRNA was carried out essentially as described previously (23).
Western blot analysis.
Antiserum A2 (raised against a peptide in the VEGF homology domain [VHD] of human VEGF-D) (43), which binds to the VHD of mouse Vegf-d, or antiserum raised against the DokR protein (26) was incubated with protein A-Sepharose (Amersham Pharmacia Biotech) with rotation for 1 h at 4°C. PEF conditioned medium was added and incubated with rotation for 
1.5 h at 4°C. Protein A-Sepharose then was pelleted by centrifugation and washed three times with cold radioimmunoprecipitation buffer (20 mM Tris-HCl [pH 7.5], 150 mM NaCl, 1% Triton X-100, 1% sodium deoxycholate, 0.1% sodium dodecyl sulfate [SDS]). Proteins were eluted by the addition of reducing SDS sample buffer (2% SDS, 10% glycerol, 60 mM Tris-HCl [pH 6.8], 0.02% bromophenol blue, 2% ß-mercaptoethanol) and heating to 95°C for 5 min. Proteins were subjected to SDS-polyacrylamide gel electrophoresis (PAGE), transferred to Immobilon P membranes (Millipore), and analyzed by using biotinylated, affinity-purified antiserum (BAF469; 0.2 µg/ml; R&D Systems) raised against amino acids 98 to 206 of mouse Vegf-d (which corresponds to the VHD) and streptavidin-horseradish peroxidase conjugate (Roche Molecular Biochemicals). Development was carried out by chemiluminescence (SuperSignal System; Pierce).
Histological analysis. Excised embryos or mouse tissues were fixed for 3 to 24 h in 4% paraformaldehyde-phosphate-buffered saline, dehydrated, and embedded in paraffin wax. Paraffin sections (5 µm) were stained with hematoxylin and eosin or subjected to immunohistochemical staining with a rat anti-mouse monoclonal antibody to the extracellular domain of mouse Vegfr-3 (22), biotin-conjugated anti-rat immunoglobulin (Dako Corporation), and a tyramide signal amplification (TSA) system according to the manufacturer's instructions (NEN Life Science Products). As a control, the Vegfr-3 primary antibody was omitted from the procedure. The number of Vegfr-3-positive lymphatic vessels immediately adjacent to the external surface of the muscular layer surrounding bronchioles in age-matched adult lungs was counted by an investigator who was unaware of which sections were derived from wild-type or Vegf-d-deficient mice. The tissue sections used for this analysis were matched for position within the lungs. Three tissue sections were counted per mouse and encompassed at least 130 bronchiole sections per animal.
Measurement of lung fluid volume. Mouse lungs were excised and lightly blotted onto gauze to remove blood acquired during separation from the heart, and the mass was measured (lung wet mass). The lungs then were dried in an oven at 60°C for 3 days, and the mass was measured daily thereafter. The lung dry mass was recorded when the lung mass remained constant over two consecutive measurements. Lung fluid compositions were calculated by subtracting the lung dry mass from the lung wet mass.
Microlymphangiography.
Patent Blue 5 dye (Rhone Poulenc Rorer) was injected intradermally into mice anesthetized with xylazine (8 mg/kg)-ketamine (44 mg/kg). The injection site was lightly massaged for 15 min, and the mice were sacrificed by cervical dislocation and dissected to reveal the underlying surface of the ventral skin and the axillary, superficial inguinal, and paraaortic lymph nodes. Uptake of Patent Blue 5 dye into lymphatic vessels was monitored by using a Leica dissecting microscope (model MZ6) and a digital camera. Fluorescence microlymphangiography was performed on mouse tails essentially as described previously (15, 25). Hair was removed from tails 24 h prior to analysis by using depilatory cream. On the day of the experiment, mice were anesthetized with xylazine (8 mg/kg)-ketamine (44 mg/kg). Tetramethylrhodamine-dextran (rhodamine-dextran; molecular weight, 2,000,000; 8 mg/ml; Molecular Probes) was injected subdermally (50 µl) into the mouse tail tip by using a 30-gauge needle. Mice then were placed on the heated stage (32°C) of a Nikon confocal microscope (model Eclipse TE 300). Uptake of rhodamine-dextran into the superficial lymphatic networks of mouse tails was monitored by confocal microscopy for a period of 40 min per mouse. The width of the lymphatic vessels and the widest distance between the vessels in each component of the honeycomb lymphatic pattern were measured from the confocal images.
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RESULTS |
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To generate Vegf-d-deficient mice, a gene-targeting vector was used to replace the signal sequence for protein secretion, immediately downstream from the translation initiation site, and the remainder of the first coding exon (Fig. 1A). Correctly targeted ES cells were identified by Southern blot analysis with a DNA probe, E1F2R1, that hybridizes to HindIII, BglII, and PvuII fragments of diagnostic sizes in wild-type and Vegf-d-targeted ES cells (Fig. 1B). Eighteen correctly targeted ES cell colonies, out of a total of 384 screened, were identified (targeting frequency, 1 of 21); 2 of these were used to generate independent lines (2.2.36 and 2.88) (Fig. 1B). The 2.2.36 and 2.88 ES cell clones had incorporated a single copy of the neor gene in their genomes, as indicated by Southern blot analysis with a DNA probe homologous to a region of the neor gene (data not shown).
Molecular analysis of Vegf-d-deficient mice. To investigate whether replacement of the first coding exon with a PGK-neor cassette affected the generation of transcripts containing the remaining exons of the Vegf-d gene, Northern blot analysis was performed (Fig. 2A). The expression of Vegf-d in mice was previously shown to be high in E15.5 embryonic lungs (43). Total RNA was purified from the lungs of wild-type and Vegf-d-deficient E15.5 embryos and subjected to Northern blotting with a probe directed to the 3' end of the Vegf-d gene (VDEx7). The major Vegf-d transcripts, of 3.7 and 2.4 kb, in addition to the lower-abundance transcripts, of 3.3 and 2.0 kb, were detected in wild-type E15.5 mouse lungs but not in Vegf-d-deficient E15.5 lungs (Fig. 2A). Because the Vegf-d gene is also strongly expressed in PEFs, we confirmed this lack of expression in Vegf-d-deficient mice by generating multiple PEF lines from matings of X+X+ and X+Y mice (wild-type PEFs; this designation is used because the Vegf-d gene is on the X chromosome) and matings of X–X– and X–Y mice (Vegf-d-deficient PEFs). In agreement with the Northern analysis of embryonic lungs, a high level of Vegf-d expression was detected in all of the wild-type PEF lines, whereas no transcripts of any size could be detected in RNA from Vegf-d-deficient PEFs with this probe (Fig. 2A). Northern blot analysis with a probe homologous to a region of the VHD of Vegf-d (VD-VHD) also detected high levels of Vegf-d expression in wild-type E15.5 lungs and PEF lines but did not detect any transcripts in the RNA derived from Vegf-d-deficient E15.5 lungs or PEF lines (data not shown). These data were also consistent with the results of Northern blot analysis of RNA from mouse line 2.88 and PEFs derived from this line (data not shown).
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21-kDa fully processed subunit of Vegf-d, consisting of the VHD (31, 43), was present in the conditioned medium of wild-type PEFs but not in the conditioned medium of Vegf-d-deficient PEFs (Fig. 2B). In reducing SDS-PAGE, this fully processed derivative consisting of the VHD migrates as two species at 21 kDa, due to variations in glycosylation. Previous studies indicated that cleavage at the C terminus of the VHD is more rapid than that at the N terminus (43). Consequently, the partially processed derivative consisting of the VHD and the N-terminal propeptide should be relatively abundant compared to the fully processed VHD derivative and other partially processed forms. For human VEGF-D, the size of the polypeptide consisting of the N-terminal propeptide and the VHD is 31 kDa, consistent with the migration of the N-terminal propeptide at 10 kDa and of the VHD at 21 kDa under reducing conditions. This 31-kDa partially processed form of mouse Vegf-d was abundant in wild-type PEF conditioned medium but could not be detected in that of Vegf-d-deficient PEFs (Fig. 2B). The 21-kDa mature form and the 31-kDa partially processed form of Vegf-d were abundant in conditioned media of multiple wild-type PEF lines, whereas Vegf-d could not be detected in conditioned media of any of the Vegf-d-deficient PEFs derived from either of the Vegf-d-deficient mouse lines. The lack of expression of Vegf-d protein in Vegf-d-deficient mutant mice indicates that the gene-targeting strategy created a null Vegf-d allele. Vegf-d is closely related in primary structure to Vegf-c, and these proteins are both able to induce lymphangiogenesis via the activation of Vegfr-3 (45). It is therefore feasible that the up-regulation of Vegf-c could compensate for Vegf-d function. However, Vegf-d deficiency did not significantly alter the expression of the Vegf-c gene, as assessed by Nothern blot comparison of Vegf-c mRNA levels in livers, hearts, and lungs from wild-type and Vegf-d-deficient mice (Fig. 2C).
General observations of Vegf-d-deficient mice. Neither female nor male Vegf-d-deficient mice (denoted X–X– and X–Y, respectively) could be distinguished from wild-type mice by morphological appearance. Newborn Vegf-d-deficient mice were observed to suckle and, postweaning, to feed in a manner similar to that of control mice. This finding is reflected in the body masses, which were not different in wild-type and Vegf-d-deficient mice measured at up to 1 year of age (Table 1 shows measurements at 16 weeks). The similar body masses of Vegf-d-deficient and wild-type mice suggest that Vegf-d-deficient mice are able to effectively access, digest, and metabolize food to the same extent as wild-type mice. This suggestion is consistent with observations that Vegf-d-deficient mice displayed levels of activity similar to those of wild-type mice. Vegf-d-deficient mice were fertile, as visibly healthy mice were born from crosses of X–X– and X–Y mice in litters with a mean ± standard deviation number of offspring that was not significantly different (as assessed by the Student t test) from that in litters derived from wild-type matings (for wild-type mice, 8.0 ± 2.5 [n = 26]; for Vegf-d-deficient mice, 9.3 ± 3.0 [n = 34]) (P = 0.078). Perturbations in the development and function of the lymphatic system may be reflected in an alteration in interstitial fluid volume leading to swelling of tissue or accumulation of fluid in body cavities. However, there was no visual evidence of altered interstitial fluid volume in the subcutaneous, intraperitoneal, or thoracic regions of Vegf-d-deficient mice, nor was there any accumulation of chyle in the intestines or peritoneal cavity. None of the tissues or organs of Vegf-d-deficient mice appeared swollen or distended.
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Immunohistochemical staining with an antibody that was raised against the extracellular domain of mouse Vegfr-3 and that detects lymphatic endothelium in normal adult tissues indicated that the lymphatic vessels of Vegf-d-deficient mouse lungs were similar in structure, appearance, and location to those of wild-type mouse lungs (Fig. 3). Typically, Vegfr-3-positive lymphatic vessels are clustered adjacent to bronchioles and the blood vessels associated with the bronchioles in the lungs (Fig. 3E and F). Counting of Vegfr-3-positive vessels immediately adjacent to the external surface of the muscular layer of bronchioles revealed a small but statistically significant reduction (as assessed by the Student t test) in the number of Vegfr-3-positive vessels per bronchiole in Vegf-d-deficient mice in comparison to wild-type control mice (mean ± standard deviation for Vegf-d-deficient mice, 0.80 ± 0.16 [n = 10]; for wild-type mice, 1.15 ± 0.14 [n = 10]) (P = 0.00005).
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Structure and function of large lymphatic vessels in Vegf-d-deficient mice. The mouse Vegf-d gene is strongly expressed immediately under the skin during embryonic development (4, 5). We therefore analyzed the function of large lymphatic vessels associated with the skin in adult Vegf-d-deficient mice. Patent Blue 5 dye, which enters lymphatic vessels but not blood vessels (14), was intradermally injected into the ventral skin of wild-type mice. The dye was specifically taken up from the interstitium into a very large lymphatic vessel that is located on the underlying surface of the ventral skin (Fig. 4A). As expected, the dye did not enter the blood vascular system during the course of this experiment, as shown by the lack of dye in the accompanying blood vessel (Fig. 4A). Furthermore, this large lymphatic vessel drained to the superficial inguinal (Fig. 4C) and axillary (Fig. 4E) lymph nodes. The locations of this lymphatic vessel, the accompanying blood vessel, and the draining lymph node were similar in all of the wild-type mice studied (n = 12). The uptake of Patent Blue 5 dye into this large lymphatic vessel was also observed in all of the Vegf-d-deficient mice analyzed (n = 10) (Fig. 4B). Drainage of the dye to the superficial inguinal (Fig. 4D) and axillary (Fig. 4F) lymph nodes also occurred in the Vegf-d-deficient mice. Drainage to the paraaortic lymph nodes was also observed in wild-type and Vegf-d-deficient mice (Fig. 4G and H), suggesting that the drainage of lymph from large lymphatic vessels to lymph nodes is not significantly affected by Vegf-d deficiency.
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Structure and function of dermal lymphatic vessels. The dermal lymphatic plexus of the mouse tail is arranged in a regular honeycomb-like pattern that allows abnormalities of lymphatic vessel density, location, and function to be readily assessed by fluorescence microlymphangiography (15, 25, 44). Rhodamine-dextran, which is specifically taken up by lymphatic vessels, was injected into the tips of mouse tails, and its distribution was monitored by fluorescence microscopy. As expected, a regular superficial lymphatic network was observed in all wild-type mice examined (n = 20) (Fig. 5A), and rhodamine-dextran was transported at least 3 cm along the tail from the site of injection within 15 min. No difference was observed in the pattern or rate of dye uptake into the superficial lymphatic network in Vegf-d-deficient mice (n = 17) (Fig. 5D). In addition, lymphatic vessel width and the maximum distance between vessels in each component of the honeycomb lymphatic pattern were not significantly different in Vegf-d-deficient mice and control mice (the P value for both parameters, as determined by the Student t test, was >0.05). At higher magnification, comparably abundant precollectors or ducts were observed in wild-type and Vegf-d-deficient mice (Fig. 5C and F). These ducts drain to deeper precollectors that, in turn, drain to deep lymphatic vessels beneath the skin (25). These findings indicate that the dermal lymphatic vasculature of Vegf-d-deficient mice is arranged in an orderly, functional configuration that is comparable to that of wild-type mice.
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DISCUSSION |
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The lack of a profound lymphatic vessel defect in Vegf-d-deficient mice may reflect a subtle, redundant, or nonexistent role for this growth factor during embryonic development. Nonetheless, Vegf-d may induce lymphatic vessel growth in adult life in response to pathological conditions. Vegf-d-deficient mice represent an opportunity to study such putative roles by analyzing the effects of physiological or pathological challenges. This study also suggests that there may be functional redundancy within the VEGF family. The most likely candidate for compensating for Vegf-d deficiency is Vegf-c. Vegf-c is highly homologous to Vegf-d (1, 17), is the only other known activator of Vegfr-3 (17), has an expression pattern partly overlapping that of Vegf-d (1, 4, 5, 43, 23, 24), and is critical for the development of the lymphatic vasculature (20). We did not, however, find any evidence of Vegf-c up-regulation in the tissues of Vegf-d-deficient mice by Northern blotting, suggesting that if such compensation occurs, then it does so without a requirement for significantly enhanced levels of Vegf-c mRNA.
Inactivation of the Vegfr-3 gene in mice causes death at E9.5 in association with defective remodeling of blood vessels (11), whereas inactivation of the mouse Vegf-c gene leads to death of embryos between E15.5 and birth due to fluid accumulation in tissues as a consequence of a lack of lymphatic vessels (20). As Vegf-c and Vegf-d are the only known ligands for Vegfr-3, we anticipated that Vegf-d function might account for the difference between the phenotypes of Vegf-c–/– and Vegfr-3–/– mice. The lack of a profound phenotype in Vegf-d-deficient mice raises the possibility that Vegf-d may not function in Vegf-c-deficient mice to allow them to survive longer than Vegfr-3-deficient mice but rather that other, as-yet-unknown signaling mechanisms via Vegfr-3 could be involved during the early development of blood vessels. Such mechanisms could involve other soluble Vegfr-3 ligands or cell surface-ssociated molecules. The generation of mice doubly deficient for Vegf-c and Vegf-d will be required to explore this issue more conclusively.
The findings presented here indicate that Vegf-c-mediated signaling is more important for the development of the lymphatic vasculature during embryogenesis than is signaling induced by Vegf-d. It is noteworthy that a range of in vivo delivery approaches have demonstrated that Vegf-d can induce lymphangiogenesis and angiogenesis in normal tissues and pathological conditions, such as cancer (10, 36, 37, 42, 45). Therefore, the biological functions of Vegf-d may be revealed in the response to disease or tissue damage rather than in the context of embryonic development.
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ACKNOWLEDGMENTS |
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We thank Shin-Ichi Nishikawa for antibody against Vegfr-3, Patricia Lai, Dimitria Vranes, and Gillian Thornton for technical assistance, Peter Lock for DokR antiserum, and Tony Burgess for constructive comments.
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FOOTNOTES |
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Present address: Genentech Inc., South San Francisco, CA 94080-4990.
Present address: Center for Developmental Biology and Kent Waldrep Foundation Center for Basic Research on Nerve Growth and Regeneration, University of Texas Southwestern Medical Center, Dallas, TX 75390-9133.
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REFERENCES |
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