Preparation of a monoclonal antibody to a glycidic epitope of the epidermal growth factor receptor that recognizes inhibitors of astrocyte proliferation and reactive microglia

Secreted phospholipase A2-IIA-induced a phenotype of activated microglia in BV-2 cells requires epidermal growth factor receptor transactivation and proHB-EGF shedding

BACKGROUND

Activation of microglia, the primary component of the innate immune response in the brain, is a hallmark of neuroinflammation in neurodegenerative disorders, including Alzheimer's disease (AD) and other pathological conditions such as stroke or CNS infection. In response to a variety of insults, microglial cells produce high levels of inflammatory cytokines that are often involved in neuronal injury, and play an important role in the recognition, engulfment, and clearance of apoptotic cells and/or invading microbes. Secreted phospholipase A2-IIA (sPLA2-IIA), an enzyme that interacts with cells involved in the systemic immune/inflammatory response, has been found up-regulated in the cerebrospinal fluid and brain of AD patients. However, despite several approaches, its functions in mediating CNS inflammation remain unknown. In the present study, the role of sPLA2-IIA was examined by investigating its direct effects on microglial cells.

METHODS

Primary and immortalized microglial cells were stimulated by sPLA2-IIA in order to characterize the cytokine-like actions of the phospholipase. The hallmarks of activated microglia analyzed include: mitogenic response, phagocytic capabilities and induction of inflammatory mediators. In addition, we studied several of the potential molecular mechanisms involved in those events.

RESULTS

The direct exposure of microglial cells to sPLA2-IIA stimulated, in a time- and dose-dependent manner, their phagocytic and proliferative capabilities. sPLA2-IIA also triggered the synthesis of the inflammatory proteins COX-2 and TNFα. In addition, EGFR phosphorylation and shedding of the membrane-anchored heparin-binding EGF-like growth factor (pro-HB-EGF) ectodomain, as well as a rapid activation/phosphorylation of the classical survival proteins ERK, P70S6K and rS6 were induced upon sPLA2-IIA treatment. We further demonstrated that the presence of an EGFR inhibitor (AG1478), a matrix metalloproteinase inhibitor (GM6001), an ADAM inhibitor (TAPI-1), and a HB-EGF neutralizing antibody abrogated the phenotype of activated microglia induced by the sPLA2-IIA.

CONCLUSION

These results support the hypothesis that sPLA2-IIA may act as a potent modulator of microglial functions through its ability to induce EGFR transactivation and HB-EGF release. Accordingly, pharmacological modulation of EGFR might be a useful tool for treating neuroinflammatory diseases characterized by sPLA2-IIA accumulation.

Growth factor and cytokine regulation of chondroitin sulfate proteoglycans by astrocytes

After injury to the adult central nervous system (CNS), numerous cytokines and growth factors are released that contribute to reactive gliosis and extracellular matrix production. In vitro examination of these cytokines revealed that the presence of transforming growth factor-beta1 (TGF-beta1) and epidermal growth factor (EGF) greatly increased the production of several chondroitin sulfate proteoglycans (CSPG) by astrocytes. Treatment of astrocytes with other EGF-receptor (ErbB1) ligands, such as TGF-alpha and HB-EGF, produced increases in CSPG production similar to those observed with EGF. Treatment of astrocytes, however, with heregulin, which signals through other members of the EGF-receptor family (ErbB2, ErbB3, ErbB4), did not induce CSPG upregulation. The specificity of activation through the ErbB1 receptor was further verified by using a selective antagonist (AG1478) to this tyrosine kinase receptor. Western blot analysis of astrocyte supernatant pre-digested with chondroitinase ABC indicated the presence of multiple core proteins containing 4-sulfated or 6-sulfated chondroitin. To identify some of these CSPGs, Western blots were screened using antibodies to several known CSPG core proteins. These analyses showed that treatment of astrocytes with EGF increased phosphacan expression, whereas treatment with TGF-beta1 increased neurocan expression. Reverse transcription-polymerase chain reaction (RT-PCR) was used to examine the expression of these molecules in vivo, which result in increased expression of TGF-beta1, EGF-receptor, neurocan, and phosphacan after injury to the brain. These data begin to elucidate some of the injury-induced growth factors that regulate the expression of CSPGs which could be targeted in the future to modulate CSPG production after injury to the central nervous system.

MPP(+) injection into rat substantia nigra causes secondary glial activation but not cell death in the ipsilateral striatum

Injection of MPP(+) into the substantia nigra causes extensive necrosis and anterograde degeneration of pars compacta dopaminergic neurons. We studied secondary effects in the ipsilateral striatum by examining dopaminergic terminals, signs of neuronal damage, and glial reactivity at 1, 2, 3, and 7 days after injection of MPP(+) into the substantia nigra. Dopaminergic terminals and uptake sites were evaluated with [(3)H]GBR-12935 binding and tyrosine hydroxylase immunoreactivity. Glial reaction was examined with markers of astrocytes and microglia. Stereology was used to evaluate any changes in neuronal density. Tyrosine hydroxylase immunoreactivity and [(3)H]GBR-12935 binding markedly decreased (74%) from days 2 to 7. Loss of dopaminergic terminals in the ipsilateral striatum was accompanied by an intense astroglial and, to a lesser extent, microglial reaction. However, no signs of cell damage, neuronal loss, or disruption of the blood-brain barrier were found in the striatum. Resident astroglial and microglial cells showed a morphological shift and notable changes in protein expression typical of glial reactivity, yet the presence of macrophage-like cells was not detected. This study shows that injection of MPP(+) in the substantia nigra causes a secondary reaction within the ipsilateral striatum involving the transformation of quiescent glia to reactive glia. It is suggested that stimuli derived from damaged dopaminergic terminals within the striatum are able to activate resident glia and that this glial transformation may promote repair and regeneration.

A sandwich type acridinium-linked immunosorbent assay (ALISA) detects soluble ErbB1 (sErbB1) in normal human sera

The epidermal growth factor receptor (ErbB1) is overexpressed in various human tumor-derived cell lines and neoplasms, where it is believed that receptor dysregulation plays a role in oncogenic transformation and tumor progression. In addition to the ErbB1 holoreceptor, numerous studies demonstrate that cells synthesize soluble or secreted forms of ErbB1, i.e., sErbB1. Overexpression of ErbB1 in a variety of tumors has led us to hypothesize that sErbB levels also may be altered during oncogenesis, tumor progression, and/or metastasis; and that these molecules may be useful tumor biomarkers. To address this hypothesis we have developed an acridinium-linked immunosorbent assay (ALISA) specific for the extracellular domain of ErbB1 that can be used to quantify the levels of sErbB1 molecules in body fluids and conditioned culture media. This assay can also detect full-length ErbB1 in cell and tissue extracts. Our ALISA is characterized by high sensitivity (intra-assay LLD < 1 fmol/ml), a broad linear range (approximately 1 to 4000 fmol/ml), and good reproducibility (CVs < 10%). Specificity experiments show that this ALISA detects p170 ErbB1 and soluble forms of ErbB1 that embody extracellular subdomains I through IV, but not forms of sErbB1 lacking subdomain IV. Our ALISA does not detect full-length ErbB2, ErbB3, or ErbB4; or p105 soluble ErbB2. We report that serum sErbB1 levels of healthy women (median = 3716 fmol/ml), ranging in age from 43 to 76 years, differ significantly from those of healthy men (median = 24,512 fmol/ml), ranging in age from 25 to 79 years. Additional analyses do not indicate that serum sErbB1 levels change with age in either healthy men or women. Immunoprecipitation experiments show that monoclonal antibodies specific for extracellular epitopes of ErbB1 completely neutralize the detection of sErbB1 in normal human sera by ALISA. Finally, we show by immunoprecipitation and Western immunoblot analyses with monoclonal antibodies specific for the extracellular domain of ErbB1 that normal human female and male sera contain a approximately 110-kDa protein. We conclude that our ALISA is measuring the relative levels of this p110 sErbB1 analog in normal human sera. Our ALISA, therefore, should be useful for measuring the levels of ErbB1 and sErbB1 molecules in tumor biopsy specimens and body fluids, respectively, and for determining whether sErbB1, like ErbB1, is a useful tumor biomarker.

Control of glial number: purification from mammalian brain extracts of an inhibitor of astrocyte division

Inhibitors of astrocyte cell division, immunologically related to the sugar moiety of epidermal growth factor receptor and to blood group antigens, have been purified from mammalian brain extracts. Mass spectra, high resolution proton magnetic resonance spectra, and chemical and enzymic analysis of the purified fraction indicated that the compounds isolated were glycosphingolipids, although signals compatible with the presence of aminoacid residues were also observed. Lectin binding indicated the presence of NAc-Neuraminic acid, NAc-glucosamine, fucose, galactose, and glucose. The inhibitor was cytostatic for astrocytes, C6 glioma cells, and endothelial cells, with approximate ID50 of 250 nM. Primary cultures of fibroblasts or 3T3 cells were not affected up to concentrations of 800 nM. Concentrations of the inhibitor of 800 nM or higher, caused non-specific cytotoxicity. The biological and immunological properties of this brain inhibitor were identical to those of the EGF receptor-related inhibitor previously described with the acronym ERI. Because of its source and cytostatic action, the glial inhibitor has been renamed neurostatin. Rabbit antibodies to neurostatin immunostained astrocytes and neurons, both in culture and in tissue sections.

Epidermal growth factor receptor in proliferating reactive glia following transient focal ischemia in the rat brain

Severe transient focal cerebral ischemia causes brain infarction with a strong glial reaction. We have studied whether postischemic reactive glial cells express epidermal growth factor receptor (EGFR) following middle cerebral artery occlusion in the rat. We have also looked for signs of proliferating activity, as EGFR is known to be involved in cell growth and proliferation in certain non-neural cells. EGFR was studied using three different antibodies which were found to stain for a tyrosine-phosphorylated protein (p170) corresponding to the membrane-anchored EGFR. Neurons of the control brain were strongly immunoreactive to EGFR, but a decrease of EGFR-immunoreactivity was seen in the ipsilateral brain side from 24 h postischemia due to neuronal loss. However, the presence of abundant glial cells strongly immunoreactive to EGFR became apparent in this area from 4 days postischemia onward. The use of microglial (lectin or OX-42) and astroglial (GFAP) markers showed that these postischemic EGFR-stained cells were reactive microglia/macrophages and astroglia. The subcellular localization of EGFR in reactive microglia/macrophages was compatible with the network of the Golgi apparatus, as revealed with an antibody against a peripheral membrane-bound protein of the Golgi. The presence of abundant proliferating cells in the ischemic brain was detected from 4 days postischemia with an antibody against proliferating cell nuclear antigen. Proliferating reactive microglia/macrophages were abundant within the infarcted brain side, whereas proliferating astrocytes were found mainly in the immediate periphery of the infarct limiting the necrotic area from the undamaged tissue. These proliferating cells were immunoreactive to EGFR. The results show the presence of EGFR in postischemic reactive glial cells and suggest that EGFR-dependent pathways mediate signal transduction in reactive glia following transient focal cerebral ischemia.

Immunological aspects of kaolin-induced hydrocephalus

Adult female Sprague Dawley rats were administrated 0.1 ml Kaolin (250 mg/ml) into cisterna magna. One, 4 and 8 weeks later, brains were analyzed using antibodies against MHC class I (OX18), MHC class II (OX6), CD4 (OX38), CD8 (OX8), OX42, ED1, NF, GFAP, AChE and TH. Remarkably high numbers of T lymphocytes, and OX42- and ED1-positive macrophages were found aggregated in subarachnoid spaces, and in the third and fourth ventricles. Marked aggregations of ED1-positive reactive microglial cells were also found in paraventricular structures, medial septum, retrosplenic cortex and commissural structures. However, no such cells were found in hippocampus. ED1-positive areas were also positive for round cells with a rim of MHC I fluorescent cytoplasm as well as for OX42-positive cells and MHC II positive microglial cells. At week 1, in ventro-frontal areas of cortex, CD8-positive cells and MHC I positive astroglial fibers were detected. At week 1, MHC I positive ramified microglial cells were also recognized in almost the entire brain. These positive cells gradually decreased with time and finally remained rounded with a rim of fluorescent cytoplasm. In addition, ED1 positive partly ramified microglial cells could be recognized in corpus callosum, probably representing cells in transition between ramified and reactive microglia. CD8+ cells entered ventral brain structures, and were found in the horizontal diagonal band at week 4, and had disappeared at week 8. Finally in cortex, ED1 positive microglial cells could be identified only in the retrosplenic cortex, and there were also "dark shrunken neurons" in light microscopic stainings. However, there was only a moderate GFAP positive gliosis. In conclusion, kaolin-induced hydrocephalus leads to immune reactions in several defined areas such as cholinergic systems, corpus callosum, circumventricular organs, pontine cerebellar peduncles and the vestibular nucleus.

Monoclonal antibodies specific for peptide epitopes of the epidermal growth factor receptor's extracellular domain

The ErbB tyrosine kinase receptor family plays an important role in normal cellular growth and differentiation. In addition, ErbB receptor family members are commonly amplified and overexpressed in various human neoplasms and tumor-derived cell lines, where it is believed that increased signalling as a result of receptor overexpression may play an important role in oncogenesis. Consequently, ErbB receptor family members are being investigated rigorously as potential biomarkers of cancer and as therapeutic targets in malignant tissues. Numerous studies now demonstrate the existence of "soluble" ErbB (sErbB) analogs in normal and cancerous tissues. These sErbB proteins embody the extracellular domain (ECD) of the receptor only; they are generated by either proteolytic cleavage or from truncated, alternatively spliced mRNA transcripts. Recently, we have identified an alternate transcript of the human c-erbB1 (Epidermal Growth Factor Receptor) proto-oncogene from placenta that encodes a sErbB1 protein of 60-kDa. This protein, p60 sErbB1, is glycosylated and secreted when expressed in transfected tissue culture cells in vitro. Although "soluble" receptor analogs may play important physiological roles in intercellular communication, tissue morphogenesis, tissue regeneration and repair, and embryogenesis by inhibiting or stimulating specific mitogenic and pattern forming signals, their mechanism of action has not been thoroughly elucidated. To further characterize sErbB1 expression in human tissues and cell lines and to better understand their role in carcinogenesis and normal development, we have generated monoclonal antibodies (MAbs) toward specific peptide epitopes of ErbB1 extracellular subdomains III and IV. These antibody reagents are described here and should be useful experimental, preparative, analytical, diagnostic, and therapeutic reagents for the study of sErbB1 molecules in normal development and cancer.