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*
Laboratory of Molecular Biology, Department of Biology, University of Dayton, Dayton, OH 45469; and
Protein Chemistry Laboratory, Department of Pathology and Laboratory Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA 19104
| Abstract |
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| Introduction |
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It is our belief that components of the immune system may also be important in the regeneration process. Macrophages, for example, might be able to assist in the dedifferentiation processes by helping defend the organism against injury-related exposure to pathogens; such a role has been described for muscle macrophages in injury (5). If macrophages are indeed involved in the regeneration process, there must be signals to attract them to the site of injury. In investigating the possible factors that might be playing roles in such phenomena, we concentrated on the complement component C3, a multifaceted molecule (6, 7) that is expressed in all vertebrate (8) as well in invertebrate species (9). In addition to its interactions with several complement proteins, C3 is known to interact with several proteins that are involved in dedifferentiation, such as fibronectin and integrins (10, 11, 12). The intersection of the immune system with the developmental program needed for limb regeneration has been the subject of speculation in the past (1, 2), but no concrete cellular or molecular data are available to elucidate this possible interrelationship. Cells that are not part of the immune system, such as myoblasts, have been found to express complement proteins in vitro (13, 14), which suggests that molecules of the immune system might also be involved in nonimmunologic functions. Most likely, the presence of C3 in muscle provides protection in response to injury; likewise, it is conceivable that C3 should function in urodeles to ensure normal muscle dedifferentiation and regeneration rather than necrosis in response to amputation. Given the expression of C3 in muscle we thought it useful to determine whether C3 is expressed in urodele limb muscle, especially in response to injury that is associated with limb regeneration.
In the present study we used a specific Ab recognizing axolotl C3 as well as a partial axolotl C3 cDNA clone to establish that C3 is expressed in the blastema cells of the amputated limb as well as in those that differentiate to reconstitute the limb. The presence of the mRNA was also observed in cultured blastema cells of the myogenic lineage. These findings support the involvement of complement proteins in regenerative processes and also provide evidence suggesting a role for C3 in muscle differentiation.
| Materials and Methods |
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Two urodeles, the axolotl Amblystoma mexicanum and the newt Notophthalmus viridescens, were used in this study. Axolotls were provided by the Indiana University Axolotl Colony (Bloomington, IN), and newts were purchased from Amphibia of North America (Nashville, TN). The limbs were amputated at the mid-ulna-radius level, and the regenerated portions were collected at various stages: early blastema (1 wk postamputation), mid-blastema (2 wk), or palette stage (3 wk). We also collected axolotl embryonic limb at the late bud stage. The collected limbs were either embedded in OCT (Miles, Elkhart, IN) and frozen at -70°C or embedded in paraffin.
Culture of limb cells
Blastema cells, especially those of myogenic lineage, can be cultured efficiently from muscle explants collected from an area immediately proximal to the amputation site. Dedifferentiated mononuclear cells grow out of these explants (15, 16). Such lines have previously been generated in our laboratory from the newt and were used in the present study. The cultures were kept in L-15 medium supplemented with 10% FCS and 2% CO2.
Preparation of anti-C3 Ab, Western blotting, and immunofluorescence
A polyclonal Ab recognizing axolotl C3 was raised in rabbits
using C3, which has been purified as previously described (17). The Ab
was purified using either protein A or Sepharose-C3 affinity
chromatography. This Ab was judged to be monospecific, since it reacted
only with C3 in Western blot analysis of axolotl serum, blastema,
liver, and kidney (Fig. 1
). For Western
blot analysis, tissues extracts were prepared as follows. First,
tissues were dissected and frozen in liquid nitrogen immediately. They
were then powdered while in N2 and homogenized in lysis
buffer (250 mM NaCl, 0.1% Nonidet P-40, 50 mM HEPES (pH 7.0), 5 mM
EDTA, 50 mM NaF, 0.1 mM sodium orthovanadate, 50 µg of PMSF/ml, 1
µg of leupeptin/ml, 1 µg of aprotinin/ml, and 1 mM DTT), passed
through a 21-gauge needle 10 times, and incubated on ice for 30 min.
The homogenates were spun at 10K for 3 min, and the supernatant was
subjected to SDS-PAGE on a 7.5% SDS-polyacrylamide gel under reducing
(2-ME) conditions followed by blotting onto polyvinylidene difluoride
membranes. The membranes were blocked in 1% milk and probed with the
affinity-purified anti-C3 Ab (2 µg/ml) followed by horseradish
peroxidase-congugated anti-rabbit Ig and enhanced chemiluminescence
(ECL, Amersham, Arlington Heights, IL).
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Cloning of axolotl C3
To isolate axolotl C3 cDNA, RT-PCR using total RNA isolated in this laboratory from axolotl liver was conducted essentially as described by Mavroidis et al. (18). Double-stranded cDNA was synthesized from 2 µg of total RNA and random hexanucleotide primers using the Super Script system cDNA synthesis kit (Life Technologies, Gaithersburg, MD) according to the manufacturers instructions. On the basis of Xenopus C3 amino acid sequences that are conserved in other species, we designed two degenerate oligonucleotides and used these as PCR primers: primer 1, 5'-(ctg)(cta)ctcca(tc)cca(ct)gtgtcctt-3' (extends downstream); and primer 2, 5'-gtgga(tc)t(at)(tc)gt(gt)tacaa(ga)(ga)c-3' (extends upstream).
The oligonucleotides were synthesized using an automated DNA synthesizer (Cyclone Plus, Millipore, Burlington, MA). The conditions for the PCR were denaturation at 95°C for 5 min, annealing at 45°C for 1 min, and polymerization at 72°C for 1 min. The reaction was initiated by adding 5 U of Taq DNA polymerase (Cetus, Northfolk, CT), after which 28 reaction cycles were conducted. The reaction products were separated by agarose gel electrophoresis, and the 266-bp PCR product was extracted from low melting point agarose and subcloned into a pCRII plasmid vector using the TA cloning kit (Invitrogen, San Diego, CA). Recombinant plasmid DNA was purified using the Qiagen kit (Qiagen, Stutio, CA) according to the procedure recommended by the supplier. DNA sequencing of both strands was performed according to Sanger et al. (19) using the Sequenase kit (U.S. Biochemical Corp., Cleveland, OH); each strand was sequenced twice. The PCR product was used in Northern blot analysis of samples from axolotl liver and blood RNA; we identified a 5.5-kb mRNA fragment that is similar in size those encoding C3 in other species.
In situ hybridization
This was conducted essentially as previously described for newt tissues (20, 21). Slides containing paraffin sections were deparaffinized in xylene and subsequently hydrated through an ethanol series. The slides were rinsed in 1x PBS and then fixed in 4% paraformaldehyde for 15 min. After a rinse with PBS, the slides were incubated with 250 µg/ml of pepsin at 37°C for 15 min, then rinsed again with PBS and treated with 0.1 M triethanolamine/0.25% acetic anhydride for 12 min. After a final wash with PBS, the slides were dehydrated through ethanol series, air dried for 1 h, and hybridized at 50°C for 16 h with hybridization solution (50% formamide, 1 mM EDTA, 10 mM Tris-HCl (pH 7.5), 600 mM NaCl, 0.25% SDS, 10% polyethylene glycol 6000, 1x Denharts, 200 µg/ml transfer RNA, and 250 ng/ml of digoxigenin-labeled probe); antisense and sense probes were labeled using the digoxigenin RNA labeling kit from Boehringer Mannheim (Indianapolis, IN). The next day the slides were washed with 4x SSC, then treated with 50 µg/ml RNase at 37°C for 1 h. Subsequently, the slides were incubated twice in 2x SSC at 50°C for 30 min each time, then twice in 0.1x SSC at 50°C for 30 min each time. For immunological detection, the slides were rinsed in buffer 1 (0.1 M Tris-HCl (pH 7.5), and 0.15 M NaCl) and then incubated in buffer 2 (buffer 1 with 1% blocking reagent (Boehringer Mannheim)) for 1 h at room temperature. The sections were then incubated with alkaline phosphatase conjugate-anti-digoxigenin Ab in buffer 2 at a 1/2500 dilution for 1 h at room temperature. After three washes with buffer 1 for 30 min each time, the slides were incubated in buffer 3 (0.1 M Tris-HCl (pH 9.5) with 0.1 M NaCl and 50 mM MgCl2) for 10 min and later incubated in the same solution with nitro blue tetrazolium/5'-bromo-4-chloro-3-indolylphosphite for 1624 h. The reaction was stopped with Tris-EDTA, pH 7.5, and the sections were mounted with Crystal Mount (Biomeda, Foster City, CA). Pictures were produced with a Sony video printer (Sony, Tokyo, Japan).
In situ hybridization was also used to examine C3 expression in cell
cultures. For this purpose we used the radioactive method, which
produces (in our experience) clearer results in cultured cells. The
cells were also hybridized with a muscle lineage-specific probe, myf-5,
that has been isolated from the newt (22). Cells attached to glass
coverslips were fixed in 4% paraformaldehyde for 10 min, then
incubated in 70% ethanol followed by 50% ethanol. The cells were
incubated in 1x PBS with 0.5% Triton X-100 and 5 mM MgCl2
for 10 min. The cells were then immersed in triethanolamine buffer for
2.5 min and in triethanolamine buffer plus 0.25% acetic anhydride for
another 10 min, after which they were rinsed in 2x SSC and gradually
dehydrated with ethanol. [35S]UTP-labeled probes at
1 x 107 cpm/ml were used to hybridize the cells
overnight at 5055°C; labeling of the probes (antisense and sense)
was performed using either T7 or SP6RNA polymerase and
[
-35S]thiol-UTP. The next day the cells were rinsed
once in 2x SSC, then twice in solution 1 (50% formamide with 1x SSC,
and 0.1% 2-ME) for 12 min each time at 5055°C. They were then
treated with 20 µg/ml of RNase A for 30 min at 37°C and washed
twice in solution 1 at 5055°C. Two more final washes were performed
in solution 2 (1x SSC with 0.1% 2-ME) for 12 min each. The cells were
dehydrated through a graded series of ethanol and allowed to air-dry
before being exposed to NBT-2 emulsion (Kodak, Rochester, NY).
Ten days later the samples were developed and observed under darkfield
microscopy.
| Results |
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As discussed in Materials and Methods we obtained via
PCR a fragment of 266 nucleotides (Fig. 2
). This fragment shows extensive
sequence similarity (94.36%) to its Xenopus counterpart
(23).
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Since we possessed both an Ab and a probe specific for axolotl (A. mexicanum) C3, our initial expression studies were performed using this urodele; protein A- and Sepharose-C3-purified Ab gave the same staining patterns. Once the pattern of expression of this C3 was established we also examined the expression of this molecule in the newt (N. viridescens). Both the Ab and the probe cross-reacted with newt C3, and the patterns of expression in the newt were identical with those in the axolotl. Therefore, we report here the expression data we obtained from the axolotl.
When an axolotl limb is amputated the wound epithelium covers the amputation site within a few days; this epithelium provides the underlying mesenchyme with signals to dedifferentiate. As the tissues of the mesenchyme (such as muscle and bone) dedifferentiate, they proliferate and form a blastema, from which the regenerated portion of the limb is derived. Muscle, for example, undergoes membrane lysis to give rise to embryonic-like mononucleated cells, which then begin active cell division. After blastema formation, these cells begin to redifferentiate to form the tissues that comprise the lost part, and an exact replica of the amputated portion of the limb is regenerated.
In our initial expression studies using immunofluorescent staining, C3
was detected in abundance during blastema formation. Positive cells
were observed in the wound epithelium and in the undifferentiated
blastema (Fig. 3
a). In the
later stages of blastema formation (such as the palette stage in which
redifferentiation has begun) we observed a strong reaction in the
presumptive future muscle, in areas where muscle fibers are
differentiating, as well as in the growing cartilage (Fig. 3
b). When we compared these staining patterns with those in
the normal intact (unamputated) limb, the epidermal and muscle tissue
showed weak staining; only blood cells were strongly positive (Fig. 3
c). Similarly, C3 was not detected in developing axolotl
limbs, in the ectoderm, or in the mesenchymal cells (Fig. 3
f). Thus, the expression patterns we observed were strongly
correlated with the process of limb regeneration.
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| Discussion |
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Another interesting potential role for C3 is as a participant in cellular events such as tissue remodeling and cell adhesion. C3 is perhaps the most multifaceted molecule in the complement system because it interacts with many proteins, including some that participate in or control cell adhesion and cell-to-cell communication. C3 has been found to interact with laminin and fibronectin and to bind to basement membranes in glomerulus and trophoblasts (12, 28). In addition, C3 binds to receptors that belong to the integrin family of proteins, which are involved in cell adhesion (29). It is possible that the interaction of C3 with proteins involved in cell adhesion is a part of the necessary remodeling that occurs during dedifferentiation. Several of these extracellular matrix proteins are regulated during dedifferentiation, including integrins and laminin (3, 4). Expression of laminin seems to be regulated during the synthesis of muscle membranes that occurs as part of the redifferentiation process (1), where C3 was particularly highly expressed as well.
Other complement factors show interesting sequence homologies to extracellular matrix proteins involved in cell adhesion. Complement factors B and C2 show homologies to cartilage matrix protein, von Willebrand factor, and the collagen binding domain of alkaline phosphatase (30, 31). The existence of these homologous domains strongly suggests that complement factors can, in fact, play a role in events that require cell adhesion and communication adjustments, such as differentiation and, in the present case, dedifferentiation. Such activities might also allow complement factors to act as growth factors. Indeed, this has been shown to be the case for C3 and C3 synthetic peptides, since they are known to support the growth of human CR-2-positive (EBV/C3d receptor-positive) lymphoblastoid B cells (32). Taken together, our results provide strong evidence for the involvement of C3 in the dedifferentiation and differentiation events that occur during limb regeneration.
Shedding light on the mechanisms of dedifferentiation would greatly enhance our understanding of the unique regenerative phenomena observed in some amphibia. In this sense the intersection of the immune system with regeneration could be particularly informative. At present, the role of the immune system in limb regeneration is largely obscure due to the lack of systematic studies and appropriate reagents. Our data provide the first concrete evidence for the specific expression of complement component C3 during the process of dedifferentiation and also suggest the involvement of this protein in muscle differentiation during limb regeneration.
| Acknowledgments |
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| Footnotes |
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2 The sequence described in this paper has been deposited in the GenBank database under accession number 3641563. ![]()
3 Current address: Department of Biology, University of Patras, Patra, Greece. ![]()
4 Address correspondence and reprint requests from both laboratories. ![]()
Received for publication April 30, 1998. Accepted for publication August 24, 1998.
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