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© The Rockefeller University Press,
0021-9525/2000//1071 $5.00
The Journal of Cell Biology, Volume 150, Number 5,
, 2000 1071-1084
Original Article |
Cd44 Enhances Neuregulin Signaling by Schwann Cells
lawrence.sherman{at}uc.edu
We describe a key role for the CD44 transmembrane glycoprotein in Schwann cell–neuron interactions. CD44 proteins have been implicated in cell adhesion and in the presentation of growth factors to high affinity receptors. We observed high CD44 expression in early rat neonatal nerves at times when Schwann cells proliferate but low expression in adult nerves, where CD44 was found in some nonmyelinating Schwann cells and to varying extents in some myelinating fibers. CD44 constitutively associated with erbB2 and erbB3, receptor tyrosine kinases that heterodimerize and signal in Schwann cells in response to neuregulins. Moreover, CD44 significantly enhanced neuregulin-induced erbB2 phosphorylation and erbB2–erbB3 heterodimerization. Reduction of CD44 expression in vitro resulted in loss of Schwann cell–neurite adhesion and Schwann cell apoptosis. CD44 is therefore crucial for maintaining neuron–Schwann cell interactions at least partly by facilitating neuregulin-induced erbB2–erbB3 activation.
Key Words: CD44 • erbB2 • erbB3 • Schwann cell • neuregulin
© 2000 The Rockefeller University Press
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Introduction |
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Axon-derived signals that influence Schwann cell proliferation and survival include members of the neuregulin protein family (for review see Mirsky and Jessen 1999; Topilko et al. 1996). Neuregulins are encoded by alternatively spliced transcripts of the neuregulin-1 (NRG-1) gene (Burden and Yarden 1997; for review see Gassmann and Lemke 1997). Neuregulins, including glial growth factors (GGFs), are either membrane bound or soluble, each with domains homologous to epidermal growth factor (EGF). Mice with targeted NRG-1 deletions have dramatically reduced numbers of Schwann cell precursors (Meyer and Birchmeier 1995). In vitro, neuregulin blocking antibodies inhibit the mitogenic effects of dorsal root ganglion (DRG) neurons on Schwann cells (Levi et al. 1995; Morrissey et al. 1995; Rosenbaum et al. 1997), while GGF and other neuregulins promote mitogenesis of mature Schwann cells and Schwann cell precursors (Baek and Kim 1998; Raff et al. 1978; Marchionni et al. 1993; Dong et al. 1995). In addition, neuregulins can rescue Schwann cell precursors (Dong et al. 1995; Syroid et al. 1996) and Schwann cells in damaged neonatal nerves (Trachtenberg and Thompson 1996; Grinspan et al. 1996; Kopp et al. 1997) from apoptosis. Collectively, these data indicate that neuregulins are critical for Schwann cell differentiation, survival, and proliferation at different stages of peripheral nerve development.
In Schwann cells, neuregulins function through the transmembrane receptor tyrosine kinases erbB2 and erbB3 (Morrissey et al. 1995; Vartanian et al. 1997; Rahmatullah et al. 1998). Mice with targeted mutations at erbB2 or erbB3 lack Schwann cells, underscoring the importance of these receptors in peripheral nerve development (Riethmacher et al. 1997; Britsch et al. 1998; Morris et al. 1999). Although erbB2 has no known ligands, neuregulins bind erbB3 with varying affinities (Peles et al. 1993; Carraway and Cantley 1994; Kita et al. 1994). However, erbB3 lacks intrinsic kinase activity (Guy et al. 1994). After ligand binding, erbB3 and erbB2 must heterodimerize in order to signal (Sliwkowski et al. 1994). It is unclear from these studies how erbB2–erbB3 heterodimerization is achieved. Furthermore, it is not known whether neuregulins reach erbB2 and erbB3 by simple diffusion, or if accessory proteins are required to sequester neuregulins to the Schwann cell membrane.
We have investigated the possibility that CD44 plays a role in mediating neuregulin signaling in the peripheral nervous system. The CD44 family of transmembrane glycoproteins has been implicated in cell–cell and cell–matrix adhesion, cell migration, and growth factor signaling (for review see Sherman et al. 1996; Naor et al. 1997). Different CD44 proteins are generated from a single gene by alternative RNA splicing of up to 10 variant ("v") exons and by extensive posttranslational modifications. These variant exons encode amino acid sequences in the extracellular portion of CD44, near the transmembrane domain (Screaton et al. 1992). Standard CD44 is an 85–90-kD protein that lacks variant sequences and is expressed in many cell types, whereas higher molecular weight variants are expressed in a limited number of tissues and in certain tumors. CD44 is expressed by subpopulations of rat neural crest cells (Ikeda et al. 1996) and by embryonic and neonatal rat (embryonic day [E]18 to postnatal day [P]2; Sherman et al. 1995) and adult human Schwann cells (Vogel et al. 1992; Sherman et al. 1997). However, the function of CD44 in the peripheral nervous system has not been studied.
Bourguignon et al. 1997 found that CD44 coimmunoprecipitated with erbB2 in an ovarian carcinoma cell line, suggesting that CD44 may be linked to erbB2 signaling. We investigated whether CD44 contributes to erbB2 and erbB3 function in Schwann cells. We found that CD44 associates with erbB2 and erbB3 in rat Schwann cells and that reducing CD44 expression prevents GGF-induced erbB2–erbB3 heterodimerization and signaling. Blocking CD44 expression also results in the release of Schwann cells from neurites in cocultures of Schwann cells and sensory neurons, and in Schwann cell apoptosis. These data indicate that CD44 facilitates neuregulin signaling in Schwann cells, and demonstrate a novel role for CD44 in mediating growth factor receptor function.
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Materials and Methods |
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Schwann Cell Culture
Primary Schwann cell cultures were established from neonatal Sprague-Dawley rat (Harlan) sciatic nerves as previously described (Kim et al. 1997). Cells were initially grown on poly-L-lysine–coated (Sigma-Aldrich) tissue culture plastic in DMEM supplemented with 10% FBS, 5 ng/ml recombinant human (rh)-GGF2, and 2 µM forskolin (Calbiochem-Novabiochem), and then either seeded onto neurons (see below) or switched for 24 h to a serum-free defined medium (N2; see Kleitman et al. 1991) either with or without rh-GGF2 and oligonucleotides as described. All assays were performed on cultures at passage 2 or 3. Apoptosis assays were performed using a Tdt-FragEL DNA fragmentation detection kit (Oncogene Research Products) according to the manufacturer's instructions.
Schwann Cell–Neuron Cocultures
Dissociated rat E15 DRG were cultured on collagen-coated 8-well chamber slides (Fisher Scientific) in the presence of antimitotic drugs to kill dividing cells (Kleitman et al. 1991). Neurons were maintained in DMEM plus 10% human placental serum and 50 ng/ml 2.5 S NGF (Harlan) for 14 d. Approximately 105 primary rat Schwann cells were then seeded onto the established neurons. 2 d later, cultures were analyzed by microscopy to confirm that the seeded cells preferentially bound to neurites. Cultures were then treated with AS or SAS oligonucleotides, then examined every 24 h by phase–contrast microscopy. In a separate set of experiments, Schwann cells were pretreated with AS or SAS CD44 oligonucleotides and then plated onto neurons, as described. In each culture, >50 microscopic fields of neurites were examined at each time point.
Immunocytochemistry and Laser Confocal Microscopy
For studies of nerve sections, sciatic nerves were dissected from Sprague-Dawley rat pups (P1, P3, P5, and P7; Harlan) and adults that had been perfused with 4% paraformaldehyde (in 0.1 M phosphate buffer). Nerves were post-fixed overnight and then incubated in 20% sucrose for 24 h. Frozen 5-µm sections were cut on a cryostat (Carl Zeiss, Inc.), fixed again in paraformaldehyde for 10 min, rinsed in phosphate buffer, and then incubated with 0.5% hydrogen peroxide to block endogenous peroxidases. Sections were blocked in 10% normal goat serum in phosphate buffer for 1 h, then incubated with the mouse anti–rat CD44 monoclonal antibody 5G8 (total hybridoma supernatant) overnight at room temperature (Sleeman et al. 1996). Slides were then developed using either the Vectastain ABC immunocytochemistry kit according to the manufacturer's instructions (Vector Laboratories) or by incubation with FITC-conjugated goat anti–mouse IgG (1:100; Jackson Immunoresearch Laboratories). Sections then either were rinsed three times with buffer and mounted in Fluoromount G (EM Sciences) or were processed further. For double labeling, CD44 antibody-labeled sections were fixed again with paraformaldehyde, rinsed, permeabilized in 0.1% Triton X-100 for 15 min, blocked in 10% goat serum for 1 h, and then incubated with rabbit antineurofilament (1:50; Parysek and Goldman 1987; provided by Linda Parysek, University of Cincinnati), rabbit anti-S100 (1:200; Dako), rabbit anti-erbB2 (1:10; Upstate Biotechnology), or rabbit anti-erbB3 (1:10; C-17; Santa Cruz Biotechnology) antibodies overnight. Next, sections were rinsed, incubated with goat anti–rabbit TRITC (1:100; Jackson Immunoresearch Laboratories) for 1 h, and mounted as above. Sections were analyzed either with a Zeiss Axiophot microscope (Carl Zeiss, Inc.) with epifluorescence or by confocal microscopy using a Zeiss LM-10 (Carl Zeiss, Inc.) or a Bio-Rad MRC-600 laser confocal microscope (Bio-Rad Laboratories).
For analysis of teased nerves, adult rats were killed and sciatic nerves were removed and placed into L15 medium. Nerves were cut to 0.55-mm lengths and teased to single fibers using 20-gauge needles in PBS, then dried onto gelatin-coated glass slides. Slides were stored at –80°C until used. For immunostaining, slides were warmed to room temperature, fixed in methanol at –20°C for 10 min, rinsed in PBS, and blocked in 10% normal goat serum for 1 h. Sections were incubated with mouse monoclonal anti-CD44 (5G8) at a 1:1 dilution overnight, rinsed, and incubated with goat anti–mouse FITC (1:100) for 1 h, then rinsed again. For double labeling with S-100 protein, nerves immunostained with the CD44 antibody were fixed in 4% paraformaldehyde, rinsed, permeabilized in Triton X-100, and blocked in normal goat serum. Nerves were then incubated in S-100 antibody (Dako) overnight, rinsed, incubated with goat anti–rabbit TRITC (1:100) for 1 h, rinsed again, and mounted in fluoromount G and analyzed as described above.
For immunocytochemical localization of CD44, erbB2, and erbB3 in cultured Schwann cells, primary rat Schwann cells were plated onto poly-L-lysine–coated 8-well chamber slides (Fisher Scientific) and fixed with 4% paraformaldehyde in PBS for 20 min at room temperature. Cells were permeabilized and blocked by incubation in PBS with 10% normal goat serum and 0.2% Triton X-100 for 30 min. Cells were then incubated with a mixture of the anti-CD44 mouse monoclonal antibody 5G8 (1:50) and either anti-erbB2 rabbit polyclonal antibody (1:100) or anti-erbB3 rabbit polyclonal antibody (1:100) as above for 1 h at room temperature. Next, cells were washed three times with blocking buffer and incubated for 30 min with a 1:50 dilution of FITC-conjugated goat anti–mouse IgG and a 1:50 dilution of rhodamine-conjugated goat anti–rabbit IgG (Jackson Immunoresearch Laboratories). Labeled cells were washed three times with blocking buffer and then mounted in Fluoromount G (EM Sciences). Cells were viewed and photographed as above.
Coimmunoprecipitation and Western Blotting
Cells were washed twice with ice-cold PBS and lysed in 50 mM Tris-HCl (pH 7.4), 150 mM NaCl, and 1% Triton X-100 buffer with 0.5 mM phenylmethylsulfonyl fluoride, 1 µg/ml each of aprotinin, leupeptin, and pepstatin, 50 mM NaF, and 1 mM sodium orthovanadate (all obtained from Sigma-Aldrich). Cellular debris was pelleted by centrifugation at 10,000 rpm for 10 min, then lysates were incubated overnight at 4°C with 5 µg/ml of anti-erbB2 (Upstate Biotechnology or Oncogene Research Products, Ab-4), anti-erbB3 (Santa Cruz Biotechnology, Inc.), or anti-CD44 antibodies (as described above), or isotype-matched control Igs (Jackson Immunoresearch Laboratories). By Western blotting, we observed only single bands at 185 kD with these erbB antibodies. Protein complexes were immunoprecipitated by adding a 50% slurry of protein A–Sepharose (Amersham Pharmacia Biotech), equilibrated in lysis buffer, for 1 h. Beads were washed four times in lysis buffer, then incubated with Laemmli buffer at 95°C for 10 min.
For Western blotting, proteins were separated on 7% SDS-PAGE gels, blotted onto nitrocellulose, then blocked in PBS with 0.1% Tween 20 and 3% nonfat dry milk for 1 h. Blots were incubated with 5G8 (1:250), anti-erbB2 (1:1,000), or anti-erbB3 (1:1,000) antibodies in blocking buffer, washed three times, then incubated with HRP-conjugated goat anti–mouse or anti–rabbit IgG antibodies (1:2,500; Bio-Rad Laboratories). Blots were washed an additional three times in blocking buffer, then developed using enhanced chemiluminescence (ECL)+ (Amersham Pharmacia Biotech). For assays of phosphorylated erbB2, the antiphospho erbB2 antibody (Upstate Biotechnology) was used at 1:1,000.
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Double labeling with erbB2 and CD44 antibodies suggested that much of the CD44 expression in adult peripheral nerves could be ascribed to Schwann cells. Indeed, in adult nerve cross-sections, CD44 did not colocalize with neurofilament (Fig. 2, middle). However, expression of the Schwann cell marker S100 protein only partially overlapped with CD44 expression (Fig. 2, middle right). To clarify the distribution of CD44 and S100 protein in adult nerves, individual teased adult sciatic nerve fibers were double labeled with antibodies against S100 protein and CD44. CD44 was easily detected in the outer, abaxonal membrane of a few brightly stained myelinated fibers (2 out of 40 counted; Fig. 2, bottom right). A larger percentage of myelinated fibers (38 out of 40) only weakly expressed CD44 (Fig. 2, bottom middle). As anticipated by the staining in cross-sections, S100 protein was predominantly detected in the adaxonal, inner cytoplasm and much less in the abaxonal Schwann cell surface where CD44 was found.
Although S100 protein is barely detectable in unmyelinating Schwann cells in cross-sections (Mata et al. 1990; see Fig. 2, middle right), anti-S100 antibodies did stain unmyelinated fibers in teased nerve preparations (Fig. 2, bottom left). Only some of these unmyelinated fibers expressed CD44 (11 out of 45; Fig. 2, bottom middle). These data demonstrate that subsets of both myelinating and unmyelinating Schwann cells express detectable levels of CD44 in adult nerve.
These studies confirm that CD44 is present in peripheral nerves and demonstrate that it is enriched in developing nerves. The data are consistent with colocalization of CD44 with erbB2 and/or erbB3 in specific membrane domains of Schwann cells. To confirm this, cultures containing >99% S100+ neonatal rat Schwann cells were prepared and stained with anti-CD44 and anti-erbB2 (Fig. 3) or erbB3 (data not shown) antibodies. Indeed, patches of membrane showed colocalization of the receptors with CD44, while adjacent membrane domains appeared to be enriched for either CD44 or erbB receptors (Fig. 3 C).
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Reducing CD44 Expression in Schwann Cell Cultures Results in Apoptosis
We next tested the effects of antisense CD44 oligonucleotides on purified cultures of primary rat Schwann cells grown in the presence of 5 ng/ml rh-GGF2. After 
48 h, 94 ± 5% (as determined by cell counts) of Schwann cells treated with the AS1 CD44 oligonucleotide became rounded, lifted off the culture dish, and died (Fig. 6 C). SAS1 oligonucleotide–treated and untreated control cultures grew to confluence during the same time period (Fig. 6A and Fig. B). Similar results were obtained using the AS2 and SAS2 oligonucleotides (Fig. 6, A–C, insets). After 36 h, many cells grown in the presence of the AS1 CD44 oligonucleotide were still attached to the culture substrate, but 20–30% (range of three separate experiments) of the cells were undergoing apoptosis as determined by a nuclear fragmentation assay (Fig. 7). These findings are consistent with the notion that CD44 plays a crucial role in promoting Schwann cell survival.
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Reducing CD44 Expression Inhibits erbB2 Phosphorylation in Schwann Cells
To test directly whether reducing CD44 expression influences erbB2–erbB3 signaling, we cultured Schwann cells in the presence of 5 ng/ml rh-GGF2 and either AS CD44 or SAS oligonucleotides for 24 h. 20 µg of protein from each condition were examined for levels of erbB2 phosphorylation by Western blotting with a phospho-specific erbB2 antibody. As expected, we observed a high level of phosphorylated erbB2 in cultures treated with 5 µM SAS oligonucleotides after GGF addition (Fig. 8 A). However, there was a dose-dependent decrease in erbB2 phosphorylation in cells treated with increasing concentrations (from 1 to 5µM) of AS CD44 oligonucleotides, such that phosphorylation was barely detectable in cells treated with 5 µM AS CD44 (the same concentration used in the experiments described above). The levels of total erbB2 were either unchanged in these experiments (using AS2) or reduced by 20–30% (using AS1; range in three separate experiments), as shown above for erbB3 (Fig. 5). As above, we believe that this reduction probably is due to the fact that a significant proportion of the Schwann cells are already undergoing apoptosis at this time point (Fig. 7). Nonetheless, the degree of reduction in phosphorylation is far greater (55–78%, range in three separate experiments) than the reduction in total erbB2 protein. These data suggest that CD44 is involved in signaling by erbB2–erbB3 receptor complexes in Schwann cells.
Reducing CD44 Expression Blocks erbB2–erbB3 Heterodimerization in Schwann Cells
In light of our finding that CD44 forms complexes with erbB2 and with erbB3, the observation that lowering CD44 expression blocks erbB2 phosphorylation suggested that CD44 might be required for efficient erbB2–erbB3 heterodimerization. To test this idea, we grew Schwann cells in defined medium (N2) alone with either SAS1 or AS1 CD44 oligonucleotides for 24 h, as above. Cells were then treated with 5 ng/ml rh-GGF2 for 30 min and assayed for erbB2–erbB3 heterodimerization by immunoprecipitation with an erbB2 antibody, followed by Western blotting with an erbB3 antibody. erbB3 coimmunoprecipitated with erbB2 in SAS1 oligonucleotide–treated cultures, but either barely or not at all in AS1 CD44–treated cultures (Fig. 8 B). These experiments were performed three times with identical results, and indicate that lowering CD44 expression in Schwann cells significantly interferes with erbB2–erbB3 heterodimerization and signaling in response to neuregulins.
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CD44 Facilitates Neuregulin Signaling through erbB Receptors
erbB2 is the preferred heterodimerization partner of the other erbB receptors (Graus-Porta et al. 1997). However, it is unclear how erbB2 specifically associates with erbB1, erbB3, or erbB4 in the presence of particular ligands. Our data are consistent with the notion that CD44 plays a critical accessory role in bringing erbB2 and erbB3 together in Schwann cells to form heterodimers in the presence of GGF. It is possible that CD44 achieves this function by binding GGF or other neuregulins, forming a ligand bridge between erbB2 and erbB3 that facilitates receptor interactions. CD44 splice variants containing sequences encoded by exon v3 can bind and sequester heparin binding growth factors and present these growth factors to their high affinity receptors (Brown et al. 1991; Faassen et al. 1992; Tanaka et al. 1993; Bennett et al. 1995; Jackson et al. 1995; Sherman et al. 1998; van der Voort et al. 1999). This function of CD44 depends on heparin sulfate modifications to amino acids within the v3 variant sequence. Certain neuregulins, including GGF2, are heparin binding growth factors (Ratner et al. 1988; Peles et al. 1993; Sudhalter et al. 1996). Furthermore, heparin sulfate proteoglycans on the surface of Schwann cells are required for neuregulin signaling (Sudhalter et al. 1996; Loeb et al. 1999). Heparin-binding neuregulins therefore could act to bridge CD44–erbB2 and CD44–erbB3 complexes, resulting in a functional signaling receptor heterodimer (Fig. 9).
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Our data indicate that CD44 facilitates Schwann cell erbB2–erbB3 heterodimerization and signaling in response to a neuregulin. However, we cannot rule out the possibility that CD44 has additional functions in Schwann cells, including mediating signaling by other growth factors, cell-–cell adhesion, or cell–matrix interactions. For example, Bourguignon et al. 1997 found that hyaluronate binding to CD44 could stimulate erbB2 phosphorylation in an ovarian carcinoma cell line. Hyaluronate could cross-link CD44–erbB2 complexes with other erbB family members also bound to CD44, resulting in erbB2 phosphorylation, or hyaluronate could stimulate cell signaling through CD44, directly influencing erbB2 activity. The relevance of these findings to Schwann cell biology remains to be determined.
CD44 binds a number of extracellular matrix components in addition to hyaluronate and may cooperate with integrins to mediate cell adhesion (Fujisaki et al. 1999; Katagiri et al. 1999). Numerous studies have implicated integrins and components of extracellular matrix in Schwann cell survival, differentiation and growth (for review see Mirsky and Jessen 1999). Our own preliminary studies indicate that Schwann cell apoptosis due to reduced CD44 expression is diminished when the cells are cultured on laminin instead of poly-L-lysine (our unpublished observations). The finding that CD44 is expressed at the abaxonal Schwann cell surface in adult myelinated nerves also suggests a role for CD44 in mediating Schwann cell interactions with components of the basal lamina that they themselves synthesize (for review see Bunge 1993). Furthermore, studies of a transient population of CD44-positive cells in the developing mouse optic chiasm suggested that CD44 may influence the function of the L1 cell adhesion molecule (Sretavan et al. 1994) that has been implicated in Schwann cell–axon adhesion in the peripheral nervous system (Seilheimer and Schachner 1988; Bixby et al. 1988; Haney et al. 1999). CD44 proteins therefore may function both in the mediation of neuregulin signaling and in Schwann cell–axon adhesion, accounting for the dramatic effects of CD44 antisense oligonucleotides in Schwann cell–neuron cocultures.
If CD44 is critical for erbB2–erbB3 heterodimerization and signaling, then one might predict that mice with targeted mutations in the CD44 gene would have at least some common phenotypes with mice lacking neuregulins, erbB2, or erbB3. However, mice with targeted mutations of the CD44 gene demonstrate only minor hematological abnormalities that include aberrant lymphocyte recirculation (Schmits et al. 1997; Protin et al. 1999). However, mice with such targeted mutations can compensate for the lack of one gene by upregulating the expression of related genes. For example, mice with targeted mutations in the cardiac alpha actin gene dramatically upregulate expression of vascular smooth muscle actin and skeletal alpha-actins (Kumar et al. 1997). Therefore, it is possible that embryos lacking CD44 from very early stages compensate for the lack of CD44. In agreement with this notion, transgenic mice expressing antisense CD44 under the control of the keratin-5-sulfate promoter have a dramatic skin phenotype, and keratinocytes from these animals fail to respond properly to particular growth factors (Kaya et al. 1997).
An alternative explanation for the phenotypic discrepancy between mice with targeted CD44 mutations and findings from studies, including this one, where CD44 was targeted with antisense strategies, is that additional gene transcripts are affected by antisense CD44. In the case of our study, we cannot exclude this explanation. However, several lines of evidence are consistent with CD44 being linked to the observed effects of the oligonucleotides: (i) the oligonucleotide sequences we used previously have been shown to specifically reduce CD44 expression in rat cells and to cause phenotypes that were predicted from independent, biochemical data (Lamb et al. 1997); (ii) the oligonucleotide sequences we used do not share homology with other known genes, including the recently cloned CD44 homologue LYVE-1 (Banerji et al. 1999); (iii) the effects of antisense CD44 on Schwann cells are consistent with our findings that CD44 colocalizes and interacts with erbB2 and erbB3; and (iv) antisense strategies have been used successfully to reduce CD44 in a number of in vitro and in vivo systems, often resulting in phenotypes that were predicted by independent means (Merzak et al. 1994; Lamb et al. 1997; Kaya et al. 1997, Kaya et al. 1999; Chow et al. 1998; Reeder et al. 1998).
CD44 May Be Required for Other Processes Linked to erbB Receptor Signaling
The observation that CD44 expression is highest in early postnatal peripheral nerves at times when Schwann cells are proliferating and then declines as Schwann cells become quiescent is consistent with the idea that CD44–erbB2/erbB3 interactions mediate Schwann cell proliferation during peripheral nerve development. CD44 may also play a role in conditions characterized by abnormal Schwann cell proliferation, such as Wallerian degeneration and Schwann cell tumorigenesis. Interestingly, NRG-1 transcripts, including GGF mRNAs, are induced in adult nerves during Wallerian degeneration (Carroll et al. 1997), and Schwann cells themselves produce neuregulins (Raabe et al. 1996; Rosenbaum et al. 1997; Cheng et al. 1998), suggesting that CD44 could be involved in a neuregulin autocrine signaling loop under certain circumstances. Furthermore, we found elevated CD44 expression in schwannomas with mutations in the NF2 gene (Sherman et al. 1997), whose protein product, merlin, associates with the cytoplasmic tail of CD44 (Sainio et al. 1997). Some of the abnormal growth and survival properties of schwannoma cells are consistent with aberrant cell adhesion and growth and survival signaling (Pelton et al. 1998; Rosenbaum et al. 1998). Therefore, it is intriguing to speculate that CD44–erbB receptor interactions contribute to Schwann cell tumorigenesis and other peripheral nerve pathologies.
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Acknowledgments |
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This work was supported by National Institutes of Health grants NS-28840 to N. Ratner and NS-10297 and NS-39550 to L.S. Sherman.
Submitted: 9 December 1999
Revised: 6 June 2000
Accepted: 11 July 2000
Abbreviations used in this paper: AS, antisense; DRG, dorsal root ganglion; E, embryonic day; GGF, glial growth factor; P, postnatal day; rh, recombinant human; SAS, scrambled antisense.
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