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From the Department of Pathology,*
Haartman Institute,
University of Helsinki, Helsinki, Finland; Department of
Neurology,
Helsinki University Hospital,
Helsinki, Finland; R. W. Johnson Pharmaceutical Research
Institute,
San Diego, California; Human
Genome Science, Inc.,§
Rockville, Maryland; and
Department of Pathology,¶
Karolinska Institute,
Stockholm, Sweden
| Abstract |
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| Introduction |
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The cDNAs for human8 and mouse9 stc were recently cloned. Human STC was found to contain 273 amino acids in a 73% sequence similarity with fish STC. Northern blot analysis revealed the presence of stc mRNA in different human tissues, with the strongest signals in ovary, prostate, and thyroid. Mouse stc mRNA has in addition been found in spleen and in 13.5-day embryonic tissue.8,9 Immunostaining with antiserum raised against recombinant human STC demonstrated the presence of STC in distinct cells of the nephron tubule.10 The kidney may also be a physiological target of mammalian STC activity, given that administration of human recombinant STC was found to inhibit renal phosphate excretion in rats.11 Moreover, addition of STC to the serosal surface of rat and pig duodenal epithelium increased calcium fluxes and resulted in a net reduction in calcium absorption and an increased phosphate uptake.12
We have been investigating gene expression during terminal differentiation of neurons in a model consisting of a human cell line called Paju. This cell line was established from a malignant, neural crest-derived tumor. Paju cells respond to different stimuli, including phorbol 12-myristate 13-acetate (PMA), by vigorous neural sprouting, cessation of proliferation, and de novo expression of different markers of terminally differentiated neurons.13
To screen for gene expression in relation to neural differentiation, we performed a differential display (DD) reverse transcription-polymerase chain reaction (RT-PCR) on mRNA extracted from Paju cells before and after induced differentiation. Among genes with strongly upregulated expression during induced neural differentiation, we identified stc. Based on this observation, sections from human and mouse brain were stained by immunhistochemistry with antiserum to human STC. Here we report that the terminally differentiated neurons of the central nervous system express high levels of STC. STC was not found in the glial cells or in embryonic neurons. Given that influx of Ca2+ is considered a major pathogenic mechanism leading to neuron damage after hypoxia14,15 together with the documented antihypercalcemic effects of STC, we suggest that terminally differentiated neurons upregulate STC expression to increase their cellular integrity.
| Materials and Methods |
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DD16 was carried out as previously described in a report from this laboratory.17 For amplifying in the DD-RT-PCR, the 5' and 3' primers were 5'-CATTCAGCAC-3' and 5'-GVTTTTTTTTTTTT-3', respectively. The PCR products were separated on sequencing gels, and bands appearing differentially displayed were excised out and cloned into pCRII via TA cloning (Invitrogen; San Diego, CA), and sequencing of all cDNA clones was done on Applied Biosystems (Foster City, CA) 373 or 377 DNA sequencers according to the manufacturer's protocol.
Cell Culture and Reagents
The Paju tumor cell line was established by one of us (LCA) from the pleural metastases of a neural-crest-derived tumor in a young patient. Uninduced Paju cells grow surface adherent when cultivated in RPMI-1640 supplemented with 10% fetal calf serum, penicillin G (10 U/ml), streptomycin sulfate (50 mg/ml), and 1 mmol/L glutamine. For subculturing, the cells were detached by treatment with Versene/ethylenediaminetetra-acetic acid (Life Technologies, Inc., Grand Island, NY). PMA was obtained from Sigma Chemical Co. (St. Louis, MO), dissolved in ethanol, and used at an optimal concentration of 10 nmol/L. Human recombinant STC and rabbit antiserum against human STC were prepared as described.18
Western Blotting
Cells were collected after PMA treatment at the indicated time points and lysed in an ice-cold lysis buffer containing 20 mmol/L Tris/HCl pH 8.0, 0.2 mmol/L ethylenediaminetetra-acetic acid, 3% Nonidet P-40, 2 mmol/L orthovanadate, 50 mmol/L NaF, 10 mmol/L sodium pyrophosphate, 100 mmol/L NaCl and 10 mg/ml each of aprotinin and leupeptin. After incubation on ice for 10 minutes, the samples were centrifuged at 14,000 x g for 15 minutes, and the supernatants were collected. An aliquot was removed for total protein estimation using the Bio-Rad Protein Assay (Bio-Rad Laboratories, Hercules, CA). An aliquot corresponding to 30 µg of total protein of each sample was separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions and transferred electrophoretically to nitrocellulose filters. Nonspecific binding of antibody was blocked with 3% bovine serum albumin in 20 mmol/L Tris/HCl pH 7.5, 150 mmol/L NaCl, and Triton X-100 for 2 hours. Immunoblotting was carried out with the 1:2000 diluted rabbit STC antibody followed by peroxidase-conjugated secondary anti-immunoglobulin antibodies, and the blots were developed with the enhanced chemiluminescence method (ECL, Amersham, Little Chalfont, UK).
Immunohistochemistry and Immunofluorescence Staining
Brain tissue was fixed in 4% buffered formaldehyde for 12 to 48 hours, routinely processed, and embedded in paraffin. Sections 4 µm thick were mounted on slides coated with 3-aminopropyl-triethoxy-silane (Sigma Chemical Co.) and dried for 12 hours at 37°C. The sections were deparaffinized in xylene and rehydrated through graded concentrations of ethanol to distilled water, processed in a microwave oven,19 and treated with a methanol-perhydrol solution (0.5% hydrogen peroxide in absolute methanol) for 30 minutes at room temperature to block endogenous peroxidase activity. Immunohistochemical stainings were performed by using a commercial Elite ABC Kit (Vectastain, Vector Laboratories, Burlingame, CA). Blocking serum was applied for 15 minutes followed by a 60-minute incubation with the diluted primary antibody. The dilutions were made in phosphate-buffered saline (PBS; pH 7.2), and all incubations were done in a moist chamber at room temperature. Between the different staining steps, the slides were rinsed in three changes of PBS. The peroxidase staining was visualized with a 3-amino-9-ethylcarbazole (Sigma Chemical Co.) solution (0.2 mg/ml in 0.05 mol/L acetate buffer containing 0.03% perhydrol, pH 5.0) at room temperature for 15 minutes. Finally, the sections were lightly counterstained in Mayer's hematoxylin and mounted in an aqueous mounting medium (Aquamount, BDH, UK). Slides stained with STC antibodies preabsorbed with recombinant STC protein and slides stained with normal rabbit serum served as negative controls.
For immunofluorescence staining, Paju cells were grown on glass coverslips with or without addition of PMA. The coverslips were gently washed with PBS, fixed for 10 minutes in 3.5% freshly made paraformaldehyde, and permeabilized by treatment for 10 minutes with PBS containing 0.05% Nonidet P-40. Optimally diluted antibodies were added. Two types of control stainings were performed: coverslips were treated 1) with normal rabbit serum at the same dilution and 2) with the antiserum to STC preabsorbed with the recombinant protein. After incubation for 40 minutes, the coverslips were washed twice with PBS containing 5% fetal calf serum and 1:50 diluted fluorescein isothiocyanate-conjugated goat anti-rabbit immunoglobulin was added for 30 to 40 minutes. After two washes with PBS-5% fetal calf serum, the coverslips were mounted on object slides with 50% glycerol in PBS, pH 8.
Northern Blotting
mRNA was extracted by oligodeoxythymidylic acid chromatography. Eight µg of mRNA per sample was separated on 0.8% agarose-formaldehyde gels and transferred to Hybond-N filters (Amersham). Equal loading was confirmed by hybridization with a glyceraldehyde-3-phosphate dehydrogenase probe. cDNA probes were 32P-labeled by random priming. The blots were hybridized overnight at 42°C, washed in 2x saline-sodium citrate and 0.5% sodium dodecyl sulfate at 60°C for 30 minutes, and subjected to autoradiography.
| Results |
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Cultivation of Paju cells in the presence of 10 nmol/L PMA induced
neural differentiation, visualized as neural sprouting that was evident
after only 6 hours (data not shown). We have previously reported that
induced neural differentiation is accompanied by increased expression
of neuron-specific enolase and Bcl-2.13
mRNA was isolated
from Paju cells at different times of PMA treatment and from cells kept
in untreated parallel cultures and analyzed by DD-RT-PCR. Several
products were seen differentially expressed in PMA-treated cells.
Sequencing one of these differentially appearing bands revealed an
identity to stc (Figure 1A)
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Western blot analysis with antibodies to STC of lysates from
PMA-treated Paju cells revealed several strongly reactive bands of 20
to 30 kd after 6 hrs of PMA induction. The intensity of the bands of
lower molecular weight gradually declined, and after 72 hours of PMA
treatment, a major band of apparent molecular weight of 29 kd remained
(Figure 2)
. The initial variation in
apparent molecular weight of the bands seen by Western blotting during
early PMA treatment may be attributed to an initially immature
glycosylation. Treatment of the cell lysates with neuraminidase before
sodium dodecyl sulfate-polyacrylamide gel electrophoresis partially
removed the lower molecular weight bands in the Western blot, also
indicating the glycosylation of STC (data not shown).
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Based on the strong expression of STC in differentiated
Paju cells, we performed immunohistochemical staining with anti-STC
antibodies on sections from different parts of normal human brain
(Figure 4)
. A strong reactivity was
apparent in cortical neurons (Figure 4A)
, in Purkinje cells of the
cerebellum (Figure 4C)
, and in large neurons of the dentate nucleus
(Figure 4D)
. The strongest reactivity was seen in the perinuclear
cytoplasm. Also the nuclei, but not in the nucleoli, and the neuronal
processes frequently displayed positive staining. The STC reactivity
appeared frequently in a granular distribution like that seen in
differentiated Paju cells. In addition to the large neurons of the
dentate nucleus, weak nuclear staining was also seen in the small
neurons of the granular layer of the cerebellar cortex. No staining was
observed in the glial cells, and only a weak reactivity was seen in the
vascular endothelium. Preabsorption of the rabbit antibodies to STC
with recombinant protein completely removed the reactivity against
neurons (Figure 4B)
but not to the endothelium, suggesting a
nonspecific nature in the endothelial staining of this antibody.
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| Discussion |
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We used DD-RT-PCR to screen for gene expression in relation to neuronal differentiation in the human neural crest-derived cell line Paju and found a dramatically upregulated expression of stc mRNA coupled to neural sprouting by treatment with PMA. Immunostaining by indirect immunofluorescence with antibodies to STC did not show appreciable amounts of STC in untreated cultures of Paju cells, whereas, after 6 hours in the presence of PMA, a paranuclear accumulation of STC reactivity corresponding to the Golgi area was seen. A granular distribution of STC throughout the cytoplasm and also in the dendritic processes including the growth cones was seen in terminally differentiated cells. STC also gradually accumulated in the culture medium of PMA-treated Paju cells, but whether this results from active secretion or passive release is under investigation (Zhang et al, manuscript in preparation). Hypercalcemia triggers production of STC in fish.2,3 Cultivation of Paju cells in the presence of high Ca2+ concentrations or calcium ionophore A23187 did not, however, induce synthesis of detectable amounts of STC (data not shown), indicating that the expression of the stc gene in the Paju cell line is coupled to the genetic program of neural differentiation.
Immunostaining of sections from different parts of normal human brain revealed a strong expression of STC only in the neurons, whereas the glial cells did not contain detectable STC. The large neurons, including the Purkinje cells of the cerebellum, displayed a cytoplasmic granular reactivity extending to the axons with a similar pattern seen in differentiated Paju cells. Nuclear staining was particularly evident in the small neurons in the granular layer of the cerebellar cortex, but also some large neurons in the basal ganglia showed both cytoplasmic and nuclear staining.
The immunostaining of adult mouse brain gave a pattern similar to that seen in humans, with strong reactivity in the fully differentiated large neurons. Staining brain sections from fetal and newborn mice in which the neurons are still proliferating gave a very weak or absent reactivity. As in the Paju model, the expression of STC in brain neurons appears to be linked to their terminal maturation.
The only known function of STC is its regulatory influence on Ca2+ homeostasis. In fish, STC protects from dangerous hypercalcemia.20 Studies on STC in mammals so far have shown that addition of STC reduces the Ca2+ uptake and increases phosphate absorption in pig duodenal and in rat intestinal epithelium.12 Infusion of recombinant human STC in rats inhibits the renal phosphate excretion and reduces calcium reabsorption.11 Given this, it appears plausible that the high level of STC found in terminally differentiated brain neurons is associated with neuronal calcium homeostasis. It should, however, be emphasized that STC mRNA has also been found in ovary, prostate, and thyroid,8 which indicates that STC may have other endocrine functions unrelated to calcium homeostasis.
Calcium has a well established pleiotropic effect on the nervous system. In addition to the regulatory influence on ion fluxes in neural signal transmission,21-23 changes in intracellular calcium can modify events such as release of neurotransmitters,24 axon sprouting,25 and neuronal survival.26 Increased intracellular level of free Ca2+ is considered a major pathogenic mechanism in ischemic brain damage,15,27 and excessive Ca2+ influx is suggested to be important in excitotoxic neuronal cell death.28
It is conceivable that, as in intestinal and kidney tubular epithelia, STC may also regulate transmembrane Ca2+ fluxes in neurons and may contribute to the protection against hypercalcemia in terminally differentiated neurons. Our very recent observations lend some support to this notion. Immunohistochemical staining of human brain sections from areas of early (4-hour) ischemia revealed redistribution of STC in cortical neurones from a mainly perinuclear reactivity to membranes and axons (Lindsberg et al., manuscript in preparation). Moreover, Fierro and Llano29 recently reported a steep increase in the ability to buffer depolarization-evoked intracellular calcium changes in Purkinje cells from 15-day-old rats as compared with cells from 6-day-old animals. Given the increased expression of STC in relation to Purkinje cell maturation reported here, it is tempting to speculate that STC may contribute to the rising calcium buffering capacity observed in rat Purkinje cells during the neonatal period.
Together with our previous findings,13,30 these observations suggest that the generation of terminally maturated neuronal cells with limited proliferative potential may involve expression of genes conferring increased resistance to internal and external cell damage to ensure extended cell survival.
| Acknowledgements |
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| Footnotes |
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Supported by The Sigrid Jusélius Foundation, The Finnish Academy of Science, University of Helsinki, The Center for International Mobility Organisation, The Finnish Cancer Society, and The Cancer Society in Stockholm.
Accepted for publication May 2, 1998.
| References |
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