Cell Stress and Chaperones (2008) 13:73–84
DOI 10.1007/s12192-008-0013-9
ORIGINAL PAPER
Changes in the regulation of heat shock gene expression
in neuronal cell differentiation
Jay Oza & Jingxian Yang &
Kuang Yu Chen & Alice Y.-C. Liu
Received: 17 May 2007 / Revised: 1 August 2007 / Accepted: 9 August 2007 / Published online: 7 February 2008
# Cell Stress Society International 2008
Abstract Neuronal differentiation of the NG108-15 neuroblastoma–glioma hybrid cells is accompanied by a marked
attenuation in the heat shock induction of the Hsp70-firefly
luciferase reporter gene activity. Analysis of the amount and
activation of heat shock factor 1, induction of mRNAhsp, and
the synthesis and accumulation of heat shock proteins
(HSPs) in the undifferentiated and differentiated cells suggest a transcriptional mechanism for this attenuation. Concomitant with a decreased induction of the 72-kDa Hsp70
protein in the differentiated cells, there is an increased abundance of the constitutive 73-kDa Hsc70, a protein known to
function in vesicle trafficking. Assessment of sensitivity of
the undifferentiated and differentiated cells against stressinduced cell death reveals a significantly greater vulnerability of the differentiated cells toward the cytotoxic effects of
arsenite and glutamate/glycine. This study shows that
changes in regulation of the HSP and HSC proteins are
components of the neuronal cell differentiation program and
that the attenuated induction of HSPs likely contributes to
neuronal vulnerability whereas the increased expression of
Hsc70 likely has a role in neural-specific functions.
Keywords Heat shock gene expression .
Neuronal cell differentiation . Heat shock protein
J. Oza : J. Yang : A. Y.-C. Liu (*)
Department of Cell Biology and Neuroscience,
Division of Life Sciences, Rutgers State University of New Jersey,
604 Allison Road,
Piscataway, NJ 08854-8082, USA
e-mail: liu@biology.rutgers.edu
K. Y. Chen
Department of Chemistry and Chemical Biology,
Rutgers State University of New Jersey,
Piscataway, NJ, USA
Introduction
Induction of the heat shock response (HSR; a.k.a. stress
response) is a primary and evolutionarily conserved genetic
response to diverse stressors, mediated by activation of the
heat shock transcription factor HSF1, culminating in the
induction of a family of heat shock proteins (HSPs) that
function as chaperones to help in the folding/refolding of
nonnative protein, proteases to help in the degradation of
irreversibly damaged proteins, and other proteins essential
for the protection and recovery from cell damages associated with perturbation of protein homeostasis (Lis and Wu
1993; Morimoto 1993, 1998; Morimoto et al. 1994;
Voellmy 1994; Hendrick and Hartl 1995; Feige et al. 1996).
Evidence in the literature suggests that induction of the
HSR and ability to upregulate expression of the HSP
chaperones—mechanisms that provide important defense
against the dire consequences of protein mis-folding and
aberrant protein interactions—are decreased in various
brain and spinal cord neurons in vivo and in vitro
(Manzerra and Brown 1996; Marcuccilli et al. 1996;
Nishimura and Dwyer 1996; Guzhova et al. 2001; Batulan
et al. 2003; Chen and Brown 2007); in general, neurons, in
comparison with glial and ependymal cells, have a higher
threshold for induction of the HSR, requiring a greater
intensity or duration of stress for a diminished response.
Given the importance of protein mis-folding and aggregation in the pathogenesis of various neurodegenerative
diseases—including Alzheimer’s, Huntington’s, Parkinson’s,
Lou Gehrig’s, and prion diseases—it is clear that changes in
expression of the HSP chaperones in neurons would have
significant implications (Welch and Gambetti 1998; Sharp et
al. 1999; Sherman and Goldberg 2001; Bonini 2002;
Muchowski 2002; Benn and Brown 2004; Landsbury
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J. Oza et al.
2004; Westerheide and Morimoto 2005; Morimoto 2006;
Muchowski and Wacker 2005).
We commenced this study to determine if neural differentiation may be accompanied by changes in regulation of
heat shock gene expression. Using the NG108-15 tumor
neural progenitor cells as our model, we show in this study
that their differentiation into neuron-like cells is accompanied by a decreased induction of the heat-inducible HSPs and
an increased expression of the constitutive Hsc70 protein.
6 mg/ml glucose. Cells were plated at a density of 4×105
cells/35 mm plate. Experiments were done on cells after
12–15 days in culture, a time when the cells formed an
extensive and elaborate neuritic network.
Unless indicated otherwise, the condition for heat shock
was at 42°C for a specified time period. Cells were either
harvested immediately for analysis of HSF1 or mRNAhsp or
allowed to recover at 37°C for a specified time period for
analysis of Hsp70-firefly luciferase reporter gene expression and induction of HSP synthesis and accumulation.
Materials and methods
Assay of Hsp70 promoter-driven firefly luciferase reporter The
Hsp70 promoter-driven firefly luciferase reporter was
constructed by ligating a 1,036 bp KpnI and NcoI restriction enzyme fragment of the mouse Hsp70 promoterluciferase reporter, pLHSEU4 (Yanagida et al. 2000), to the
KpnI/NcoI digested pGL3E (5,006 bp; Promega Inc.). For
screening of the effects of heat shock on the Hsp70luciferase reporter gene activity, undifferentiated and
differentiated cells in either 35- or 60-mm plates were
transfected with the Hsp70-firefly luciferase reporter along
with the internal control of phRLSV40 (synthetic humanized Renilla luciferase DNA; Promega Inc. E6261). Unless
indicated otherwise, the amount of each DNA used was
0.5 μg/35-mm plate or 1.5 μg/60-mm plate, and the amount
of Lipofectamine 2000 used (in microliters) was three times
that of the total amount of DNA (in micrograms). Six hours
after DNA transfection, cells were plated into individual
wells of a 96 Stripwell™ plate (Corning/Costar 9102);
these identically transfected cells allowed for testing of the
effects of different times and temperature of heat shock on
reporter gene expression.
To evaluate heat shock induction of the Hsp70-luciferase
reporter gene, strips of eight wells or designated wells of
cells were placed in a 42°C incubator for 2 h followed by
recovery at 37°C for 4 h before harvesting. Undifferentiated
and differentiated cells were processed in parallel to
minimize experimental noise due to variation in incubator
temperature, quality/amount of the luciferase assay reagent,
and decay of the luciferase luminescence signal. The DualGlo luciferase assay reagent system from Promega Inc.
(E2920) was used to assay for first the firefly then the
Renilla luciferase activity according to manufacturer’s
instructions. We have also used the Bright-Glo luciferase
assay reagent (E2610) from Promega Inc.; qualitatively
similar results were obtained, although the Bright-Glo
reagent gave a stronger signal with a shorter half-life.
Luciferase activity was measured using the Perkin Elmer
Victor 2 multiplate reader equipped with dual injectors.
Result of the Hsp70-firefly luciferase activity was normalized against that of the Renilla luciferase, and, to facilitate
comparison across experiments for statistical analysis, this
ratio was set at 1 for the undifferentiated control. By
Cell culture and induction of neural differentiation Cells of
the NG108-15 mouse neuroblastoma–glioma hybrid lineage
(Nelson et al. 1976; Nirenberg et al. 1983, 1984) were
grown in Dulbecco’s modified Eagle’s medium (Mediatech
Inc.) supplemented with 10% fetal bovine serum (Atlanta
Biologicals, Inc.), 50 μg/ml streptomycin, and 50 U/ml of
penicillin. Cells were subcultured at or near confluency by
minimal trypsinization (0.25% trypsin; Mediatech Inc.) and
dispersion into single cell suspension in new growth
medium and plating onto new growing surfaces.
Differentiation of the NG108-15 cells was induced by
the subculturing of cells (1:4 split ratio) into a low serumcontaining medium (2%, as opposed to the normal 10%,
fetal bovine serum) supplemented with 1-mM dibutyryl
cAMP (Meyer et al. 1988). Differentiation, scored by % of
neurite-positive cells (neurite defined as processes>2×
soma diameter), was visible within hours, and >80% of
the cells was neurite-positive 2 days after induction with
dibutyryl cAMP, as compared to <10% of neurite-positive
cells in the undifferentiated culture. Two other parameters
used to confirm the neural differentiation phenotype were
(1) immunocytochemical staining for neural specific tubulin
βIII and neurofilament and (2) voltage clamp recording to
validate the presence of voltage-gated sodium channels in
the differentiated cells but not the undifferentiated cells
(data not shown). In previous studies, it was shown that the
differentiated NG108-15 cells form functional synapse with
muscle cells at relatively high frequency (Nelson et al.
1976; Nirenberg et al. 1983, 1984).
Primary hippocampal neuron culture was obtained from
embryonic day 16 rat embryos according to methods
described (Magby et al. 2006). Briefly, hippocampi were
dissected from surrounding brain tissue, and meninges were
removed. Hippocampi were dissociated by trypsinization,
followed by trituration through fire-polished Pasteur pipettes. Neurons were plated in poly-D-lysine-coated plates
and maintained in serum-free medium composed of a 1:1
mixture of Ham’s F12 and Eagle’s MEM supplemented
with 25 mg/ml insulin, 100 mg/ml transferrin, 60 mM
putrescine, 20 nM progesterone, 30 nM selenium, and
HSP chaperones and neural differentiation
normalizing the Hsp70-firefly luciferase activity against
that of the Renilla luciferase internal control, we effectively
minimized variations in experimental result due to possible
differences in transfection efficiency and cell viability as
well as nonselective and toxic effects of the treatment
conditions/reagents on gene expression.
Analysis of HSF1 by Western blotting and electrophoretic
mobility shift assay Whole cell extract was prepared as
previously described (Huang et al. 1994). Immuno-Western
blot probing for HSF1 was done using a 1:5,000–1:10,000
dilution of a rabbit polyclonal antibody, RTG88, we
generated against a recombinant histidine-tagged human
HSF1 protein. For assessment of the activation of HSF1
DNA-binding activity, electrophoretic mobility shift assay
was done according to methods described using 20 μg of
whole cell extract protein, 0.5 μg of poly(dI–dC).poly(dI–
dC), and [32P]labeled HSE in a total reaction volume of
10 μl (Huang et al. 1994). After 20 min of incubation at
room temperature, 2-μl aliquot of a five times loading
buffer was added and samples analyzed by electrophoresis
in 4% acrylamide gel.
Northern blot quantitation of HSP mRNAs RNA was
isolated from undifferentiated and differentiated cells
incubated under control (37°C) and heat shocked (42°C,
2 h) conditions after the Trizol reagent protocol for RNA
isolation from Invitrogen Inc. Concentration of the RNA
was determined spectrophotometrically. For Northern blotting, 20 μg of the RNA sample was used. The RNA
membrane was pre-hybridized at 60°C for 1 h in a prehybridization solution of 1% sodium dodecyl sulfate (SDS),
10% dextran sulfate, 1 M NaCl, and 100 μg/ml of sheared
salmon sperm DNA. Probing of the mRNAhsp89a ,
mRNAHsp70, and RNAhsp25 were done, respectively, by
hybridization with [32P]-labeled pHS801 (for Hsp89α),
pH2.3 (Hsp 70), and pHS208 (Hsp25) DNA at 60C
overnight in a hybridization oven (Hickey et al. 1986).
After extensive washing, the membrane was exposed to
X-ray film for signal detection.
Assessment of the synthesis of HSPs by [35S]methionine
incorporation Confluent cultures in 35-mm plates were
refurbished with serum-free medium. The condition for heat
shock was 42°C. To assess the induction of HSP synthesis
at various times of heat shock, cells were pulse labeled with
50–100 μCi/ml of [35S]methionine/cysteine (Amersham
Pro-Mix, a 70:30% mixture of [35S]methionine and [35S]
cysteine) for the last hour immediately before harvesting.
For example, for cells that were heat shocked for 6 h, [35S]
methionine was added at t=5 h, and cells were harvested at
t=6 h. Cells were harvested by first removing the [35S]containing medium, rinsed twice with ice cold phosphate-
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buffered saline (PBS), and scraped into 0.2 ml of a buffer of
10 mM Tris, pH 7.4 containing 1 mM ethylenediaminetetraacetic acid and 50 μg/ml of phenylmethylsulfonyl
fluoride. Cell homogenate was prepared by freezing and
thawing the cell suspension once and passing it through a 25G
needle. A 5-μl aliquot of the cell homogenate was used to
determine the amount of radioactivity incorporated into total
cellular protein (trichloroacetic acid-insoluble). Aliquots of
the cell extracts containing an equal amount of radioactivity
(50–100 K cpm) were subjected to analysis by sodium
dodecyl sulfate-polyacrylamide gel electrophoresis (SDSPAGE) and autoradiography.
Immuno-Western blot detection of the heat inducible Hsp70
and constitutive Hsc70 Immuno-Western blot detection and
quantitation of the heat-inducible Hsp70 and the constitutive Hsc70 were done using (1) the RTG76 rabbit
polyclonal antibody (1:5,000–1:10,000 dilution) that we
generated against a histidine-tagged human Hsp70-recombinant protein and that recognizes both the HSP and Hsc70
proteins and (2) a rabbit polyclonal antibody from
Stressgen (SPA816) that specifically recognizes the 73kDa Hsc70 protein. Membrane was incubated with the
primary antibody at 4°C overnight followed by horseradish
peroxidase-conjugated secondary antibody for 2 h at room
temperature. The antibodies were diluted in Tris-buffered
saline with 0.1% Tween 20 and 3% nonfat dry milk, and the
immunoblot was probed using Amersham ECL-plus or
Millipore Immobilon Western blot reagent.
Immunochemical staining for Hsc70 Undifferentiated and
differentiated cells in 60-mm plates were fixed with 4%
paraformaldehyde for 30 min at 4°C, permeabilized with
0.1% TritonX100 in PBS for 30 min at 4°C, and washed
three times with cold PBS. Wax pen circled areas (∼1 cm in
diameter) of the fixed and permeabilized cells were overlaid
with the Hsc70-specific antibody (Stressgen SPA816 at
1:50 dilution) and incubated at 4°C for 1 h. After washing
off the primary antibody, cells were overlaid with fluorescein
isothiocyanate (FITC)-conjugated goat anti-rabbit immunoglobulin G and incubated at 4°C for 1 h. Cells were viewed
using a Nikon Diaphot 300 microscope and phase and
fluorescent images captured with a SPOT camera system
(Diagnostic Instruments, Inc., Sterling Heights, MI, USA).
Assay for cell viability and activation of caspase 3/7 Cells
in 96-well plates were used. To test for vulnerability of
oxidative stress-induced cell death, sodium arsenite was
added to individual wells to final concentrations as
indicated and incubated for time periods specified (12–
24 h). The ability of glutamate to elicit excitotoxic cell
death was evaluated in the presence of 0-, 10-, and 50-μM
glycine and incubation at 37°C for time periods indicated
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J. Oza et al.
(12–24 h). Cell viability was determined using the
CellTiter-Glo luminescent cell viability assay reagent from
Promega Inc., and results were normalized against that of
the untreated control (100%). Caspase 3 and 7 activity was
determined using the Caspase-Glo™ 3/7 assay reagent from
Promega Inc., and the readouts were normalized against
signal from cell viability assay.
Results
Neural differentiation is associated with an attenuated
induction of the Hsp70-luciferase reporter gene
We used the Hsp70 promoter-firefly luciferase reporter
gene to assess induction of the HSR in the undifferentiated
versus the differentiated NG108-15 cells. Figure 1 presents
the average ± standard deviation of Hsp70-luciferase
reporter gene activity of the control- and heat shocked(42°C for 0.5, 1, 2 h) undifferentiated and differentiated
NG108-15 cells. Our results showed that heat shock elicited
a time-dependent increase in reporter gene activity. Furthermore, induction of the Hsp70-luciferase reporter gene
activity was significantly lower in the differentiated cells
when compared to that of the undifferentiated cells. The
fold of induction of the Hsp70-luciferase reporter by a
2-h heat shock at 42°C of the undifferentiated cells
ranged from 16–41 times over that of the control, and,
for the differentiated cells, the induction ranged from 4–10
times over that of the differentiated control. Such quantitative difference in induction of the Hsp70-luciferase
reporter gene activity of the undifferentiated versus the
differentiated cells was observed regardless of the time
and temperature of heat shock; the result was very reproducible over the course of a 2-year study. An alternative
approach we took to affirm this observation was to
transfect undifferentiated NG108-15 cells and divided the
transfected cells into two halves: induce half of the cells
to differentiate with dibutyryl cAMP (48 h) with the other
half serving as the undifferentiated control. Result similar
to that presented in Fig. 1 was obtained.
To validate that the attenuated induction of the Hsp70luciferase reporter gene is indeed a feature associated with
neural differentiation, we carried out two studies: (1) a
comparison of the control and heat-induced reporter gene
activity of the undifferentiated and differentiated NG108-15
cells with that of E16 (embryonic day 16) rat hippocampal
neurons. As shown in Fig. 2a, the control and heat-induced
Hsp70-luciferase reporter for the undifferentiated, differentiated NG108-15 cells, and the E16 hippocampal
neurons were 1 and 37, 0.9 and 7, and 0.2 and 1.5, respectively. (2) The attenuated induction of the Hsp70-luciferase
Fig. 1 Neural differentiation of the NG108-15 cells is associated with
an attenuated heat shock induction of the Hsp70-firefly luciferase
reporter gene. NG108-15 neuroblastoma–glioma hybrid cells were
induced to differentiate by subculturing of the cells into a Dulbecco’s
modified Eagle’s medium supplemented with 2% fetal bovine serum
and 1-mM dibutyryl cAMP for 2 days. Undifferentiated and
differentiated cells in 35-mm plates were transfected with the
Hsp70-firefly luciferase reporter DNA together with the Renilla
luciferase DNA as an internal control, and the transfected cells were
plated into wells of a 96 Stripwell plate. Cells were heat shocked at
42°C for time periods as indicated (0.5, 1, and 2 h) followed by
recovery at 37°C; all cells were harvested at 6 h. The relative
luminescence unit of the firefly luciferase readout was normalized
against that of the Renilla luciferase. To facilitate comparison across
experiments, this ratio was set at 1 for the undifferentiated control.
The result presented represents the average ± standard deviation, N=
8 (four separate experiments, each with two independent determinations). Result on Student’s t-tests of probability of difference
(probability <0.01, **highly significant; probability between 0.01
and 0.05, *significant) in the Hsp70-luciferase reporter gene activity
between paired samples of the undifferentiated and differentiated cells
is as illustrated
reporter is not a direct effect of dibutyryl cAMP. In Fig. 2b,
we show that the treatment of a near confluent culture of
the undifferentiated NG108-15 cells with 1-mM dibutyryl
cAMP for 2 days—when cells were mostly recalcitrant to
the neural inductive effect of dibutyryl cAMP (induced
undifferentiated)—failed to elicit a comparable decrease in
the heat-induced Hsp70-luciferase reporter. (Note: This
“recalcitrance” may be due to the need of cells to undergo
a round of quantal mitosis to commit to the differentiation
process [Macieira-Coelho 1995] and/or cell crowding that
block neurite extension. Our effort to determine the % of
neurite positive cells in the induced-undifferentiated culture
gave estimates between 25–35%.)
A transcriptional mechanism for the attenuated HSR
in neural differentiation
Induction of the HSR is initiated by the activation of HSF1—
a process that converts HSF1 from a cytosolic, latent
monomer to a nuclear localized, hyperphosphorylated,
HSP chaperones and neural differentiation
77
Fig. 2 a Comparison of the control and heat shock-induced Hsp70luciferase reporter activity in the undifferentiated and differentiated
NG108-15 cells and of E16 hippocampal neurons. The culturing and
differentiation condition of the NG108-15 cells were as described in
the text. Sprague–Dawley rat hippocampal neuron from E16 fetus at
14 days of culture was obtained as previously described (Magby et al.
2006). Cells were transfected with the Hsp70-firefly luciferase DNA
together with the Renilla luciferase internal control. Results of the
Hsp70-firefly luciferase activity (relative luminescence unit) were
normalized against that of the Renilla luciferase (relative luminescence
unit), and the ratio for the undifferentiated control was set at 1. The
results for the control and heat shocked cells were—undifferentiated: 1
and 37; differentiated: 0.9 and 7; hippocampal neuron (Hippo N): 0.2
and 1.5. Result represents the average ± standard deviation, N=4. b Hsp70-
luciferase reporter activity in the undifferentiated, differentiated, and
induced-undifferentiated NG108-15 cells. The culturing and differentiation condition of the NG108-15 cells were as described in the text. To test
if the attenuated induction of the Hsp70-reporter is a direct effect of
dibutyryl cAMP independent of neural differentiation, we treated a plate
of near confluent undifferentiated NG108-15 cells with 1-mM dibutyryl
cAMP for 48 h before DNA transfection, and this was designated as
“induced-undifferentiated.” The % of neurite-positive cells in the
undifferentiated, differentiated, and induced-undifferentiated cultures were
<10, >80, and ∼30%, respectively. Result of the Hsp70-firefly luciferase
activity was normalized against that of the Renilla luciferase, and this
ratio was set as 1 for the undifferentiated control. Result represents the
average ± standard deviation, N=4
DNA-binding trimer—and culminates in increased steadystate level of the HSP proteins. In experiments presented in
Fig. 3, we determined the amount and activation of HSF1
and the mRNA level of Hsp89α, Hsp70, and Hsp25 in the
undifferentiated versus the differentiated NG108-15 cells.
We show that, while there was little/no difference in the
abundance of HSF1 protein in extracts of the undifferentiated and differentiated NG108-15 cells (Fig. 3a), the DNAbinding activity of HSF1 in the differentiated cells was
resistant to stress-induced activation. Electrophoretic mobility shift assay of the DNA-binding activity of HSF1 in
Fig. 3b showed a much more robust activation in the
undifferentiated than the differentiated cells. Analysis by
Northern blot of the steady-state level of mRNA of HSPs in
Fig. 3c showed that heat shock induction of the mRNA of
Hsp89α, Hsp70, and Hsp25 was greater in the undifferentiated than the differentiated cells.
We also determined the induction of HSP synthesis in
the undifferentiated and differentiated NG108-15 cells by
the incorporation of [35S]methionine into newly synthesized proteins. The result in Fig. 4 on the profile of new
protein synthesis showed a heat shock time-dependent
increase in the synthesis of a number of proteins, marked
as Hsp98, Hsp89, Hsp72, Hsp50, and Hsp25. In particular,
we note that induction of the three major HSPs, Hsp98, 89,
and 72, starts at 2 h of heat shock, peaks at 6 h, and
decreases at 8 in the undifferentiated cells. The magnitude
of induction of the HSPs—as indicated by intensity of the
bands—was greater in the undifferentiated than in the
differentiated cells. Together, these results support a
transcriptional mechanism of the attenuated induction of
HSPs in the differentiated NG108-15 cells.
Increased expression of Hsc70 in neural differentiation
In Fig. 5, we used immuno-Western blot technique to affirm
the specificity and to evaluate changes of the Hsp70 versus
Hsc70 protein in neural differentiation. The experiment
shown in Fig. 5a was probed using the RTG76 antibody
that recognizes the inducible and constitutive Hsp70
proteins. We show that, while the heat shock induction of
the 72-kDa Hsp70 protein is markedly attenuated in the
differentiated NG108-15 cells, expression of the 73-kDa
Hsc70 protein was clearly upregulated in the differentiated
neural cells. Neuronal specificity of these changes in
expression of the Hsp70 versus Hsc70 protein in the differentiated NG108-15 cells was further evaluated using
extracts from normal and Hsp70 knockout murine embryo
fibroblasts (Hsp70−/− MEF). The identity of the 72-kDa
protein as the heat-inducible Hsp70 was validated by (1) its
induction by heat shock in NG108-15 (compare lanes 1 and 2)
and wild-type MEF (lane 5 and 6) and (2) its absence in extracts
of the Hsp70−/− MEF (lanes 9–12). That the attenuated
induction of the 72-kDa Hsp70 protein was specific to the
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J. Oza et al.
Fig. 3 Determination of the amount and activation of HSF1, and
induction of mRNA of HSPs in the undifferentiated and differentiated
NG108-15 cells. Cells in 100-mm plates were used. Condition for heat
shock was 2 h at 42°C. a Immuno-Western blot probing for HSF1 of
undifferentiated and differentiated NG108-15 cells. Ten-microgram
aliquots of whole cell extracts were loaded onto an 8% SDSacrylamide gel for analysis. b DNA-binding activity of HSF1 in
extracts from control and heat shocked (42°C, 1 h) cells was
determined by electrophoretic mobility shift assay. The relative
DNA-binding activity in the four samples (left to right) were 1, 40,
0.5, and 12, respectively. c Northern blot quantitation of mRNA of
Hsp89α, Hsp70, and Hsp25 in the undifferentiated and differentiated
NG108-15 cells. Cells were heat shocked at 42°C for 2 h, and RNA
was isolated according to methods described in the text. Probing of the
mRNA of Hsp89α, Hsp70, and Hsp25 were done by hybridization
with [32P]labeled Hsp89α cDNA (pHS801), Hsp70 DNA (pH2.3),
and Hsp25 cDNA (pHS208). The size of the transcripts are as indicated
(in kb). The relative abundance of the mRNA, quantitated by
densitometric scanning were Hsp89α (left to right): 6, 21, 4.3, 9;
Hsp70: not determined, 9.6, not determined, 2.8; Hsp25: 1.3, 6.8, 0.4, 1
differentiated neural cells (compare lanes 2 and 4 of Fig. 5a), as
opposed to effects of dibutyryl cAMP independent of neural
differentiation, was supported by the observation that treatment of MEF with 1-mM dibutyryl cAMP for 2 days failed to
produce the same effect; rather, dibutyryl cAMP boosted the
heat shock induction of the 72-kDa Hsp70 protein in MEF
(compare lanes 6 and 8, Fig. 5a). In a previous study, we
reported on effects of cAMP and cAMP-dependent protein
kinase in promoting Hsp70 gene expression (Choi et al.
1991). Neural specificity of the upregulation of Hsc70
expression was supported by the increase in 73-kDa Hsc70
protein in the differentiated NG108-15 cells (lanes 3 and 4
Fig. 4 Synthesis of heat shock proteins in the undifferentiated and
differentiated NG108-15 cells. Undifferentiated and differentiated
NG108-15 cells in 35-mm plates were used. Cells were heat shocked
at 42°C for time periods of 2, 4, 6, and 8 h. To monitor the induction
of HSP synthesis, [35S]methionine/cysteine (50 μCi/ml) was added to
the medium for the last hour before harvesting of the cells. The
amount of radioactivity incorporated into newly synthesized proteins
was determined by precipitation of proteins with trichloroacetic acid
followed by liquid scintillation counting. Aliquots of the cell
homogenate containing an identical amount radioactively labeled
protein (60,000 cpm of trichloroacetic acid-insoluble material) were
analyzed by SDS-PAGE and autoradiography. The positions of the
major HSPs, Hsp98, Hsp89, Hsp72, Hsp50, and Hsp25, are indicated
by arrowheads
HSP chaperones and neural differentiation
79
Fig. 5 Attenuated induction of the Hsp70 protein and increased
expression of the constitutive Hsc70 protein in the differentiated
NG108-15 cells. a Immuno-Western blot probing for Hsp70 and
Hsc70. Extracts from control- and heat shocked- (42°C, 2 h, followed
by recovery at 37°C for 6 h) undifferentiated and differentiated
NG108 cells were probed using the RTG76 antibody that detects the
72-kDa Hsp70 and the 74- and 73-kDa Hsc70 proteins. To validate the
identity of the protein bands and to assess the specificity of effects of
dibutyryl cAMP, we included in this experiment extracts from the wild
type and the Hsp70−/− MEF. Where indicated, MEF were treated with
1-mM dibutyryl cAMP for 48 h. The condition of the heat shock was
2 h at 42°C followed by recovery incubation at 37°C for 6 h. Aliquots
of whole cell lysate containing 10-μg protein were subjected to SDSPAGE (8%) after the transfer of proteins onto polyvinylidene fluoride
membrane and antibody probing. The positions of the 74- and 73-kDa
Hsc70 and the 72-kDa Hsp70 are as indicated. b Immuno-Western
blot probing for Hsc70. To unequivocally determine the increase in
Hsc70 expression in neural differentiation, extracts of the control- and
heat shocked-undifferentiated and differentiated NG108-15 cells, as
shown in lanes 1 through 4 of (a), were probed using an antibody
specific for the constitutive Hsc70 protein (Stressgen, SPA-816). The
relative abundance in the different samples determined by densitometry is shown at the bottom of the figure
versus 1 and 2) but not in the dibutyryl cAMP-treated MEF
(wild-type lanes 5–8; Hsp70−/−, lanes 9–12).
To validate the increased expression of Hsc70 in the
differentiated NG108-15 cells, we used a commercially
available Hsc70-specific antibody (Stressgen, SPA816) to
probe for Hsc70 by both immuno-Western blot and
immunocytochemistry. Result in Fig. 5b shows that this
antibody specifically recognized the 73-kDa Hsc70 protein.
Heat shock (42°C, 2 h followed by recovery at 37°C for
6 h) had a variable but insignificant effect on the expression
of Hsc70 (the relative abundance of the Hsc70 protein of
Fig. 5b as determined by densitometry is indicated at the
bottom of the figure). The average ± standard deviation of
Hsc70 from five determinations of two separate experiments for undifferentiated-control and undifferentiated-heat
shocked cells and differentiated-control and differentiatedheat shocked cells were 1, 1.1±0.3, 3.52±0.42, and 3.48±
0.5, respectively.
In Fig. 6, we used immunocytochemical techniques to
probe for the abundance and localization of the Hsc70
protein using the Stressgen Hsc70-specific antibody. In
general, the differentiated cells showed significantly stronger staining for Hsc70 than the undifferentiated cells, and
heat shock at 42°C for 2 h followed by recovery at 37°C for
6 h had no obvious effect either on the staining intensity or
the localization of Hsc70. The staining pattern revealed that
Hsc70 is located in the cytoplasm and the neuritic
processes. In the differentiated cells, we noticed structures
resembling neuronal varicosities (indicated by arrow heads
in the figure) at the terminus of or along the neuritic shafts
staining strongly for Hsc70. Furthermore, there appears to
be a correlation between morphological differentiation
(number and length of neurite) and the Hsc70 staining
intensity at the individual cell level. As shown in panels f
and h, the highly differentiated cells stained brightly for
Hsc70, whereas the less differentiated cells—less so (e.g.,
the three cells in the upper left hand corner of panel h and
cells in the lower left hand corner in panel f). Together, the
results in Figs. 5b and 6 demonstrate unequivocally an
increase expression of the constitutive Hsc70 protein in
neuronal cell differentiation.
Vulnerability of the differentiated NG108-15 cells
to stress-induced cell death
Induction of the HSPs provides a buffering capacity against
the toxic effects of mis-folded proteins; their activation
under conditions of stress is a powerful cyto-protective
mechanism for survival (Amin et al. 1996; Yenari et al.
1998, 1999; Akbar et al. 2003). These considerations suggest that the attenuated induction of HSPs in the differentiated may be associated with vulnerability to stress-induced
cell death.
To evaluate this possibility, we determined the effects of
increasing concentrations of arsenite (Fig. 7) and glutamate/
glucine (Fig. 8) on cell viability and activation of caspase
3/7. Arsenite was chosen for its ability to elicit oxidative
stress, and, indeed, the cytotoxic effects of arsenite were
80
J. Oza et al.
Fig. 6 Phase contrast and Hsc70 immuno-fluorescence photomicrographs of the control- and heat shocked-undifferentiated and differentiated NG108-15 cells. Undifferentiated and differentiated (1-mM
dibutyryl cAMP in a 2% fetal bovine serum supplemented medium for
3 days at 37°C) NG108-15 cells were incubated under control and
heat shocked conditions (42°C for 2 h followed by recovery at 37°C
for 6 h) and processed for immunocytochemical staining for Hsc70
according to methods described in the text. The phase contrast (a, c, e,
and g) and FITC fluorescence (b, d, f, and h) views of these cells are
illustrated. The arrowheads in e, f, g, and h point to examples of
varicosity-like structures at the terminus (h) of or along (f and h) the
neuritic shaft of the differentiated NG108-15 cells
negated by the transfection and expression of superoxide
dismutase 1 (data not shown). Glutamate/glycine was
chosen for its ability to bind to and activate the N-methylD-aspartate receptor (NMDAR) protein and, at appropriate
concentrations and time of incubation, elicit excitotoxic cell
death in NMDAR-positive neurons (Michaelis 1998;
Schubert and Piasecki 2001). We show in Fig. 7 that the
differentiated NG108-15 cells exhibited exquisite sensitivity toward the cytotoxic effects of arsenite. In the
differentiated cells, arsenite caused a significant and dosedependent loss of cell viability beginning at 10 μM and, at
50 μM, <15% of cells were viable (Fig. 7a). Under the
same condition, the undifferentiated NG108-15 cells were
more resistant against the cytotoxic effects of arsenite with
>90% of cells viable up to 50-μM arsenite, followed by a
steep decline in cell viability in the presence of 70- and
100-μM arsenite. The cause of cell death is likely due to
apoptosis, as there was a significant and arsenite dose-
Fig. 7 Differentiated NG108-15 cells exhibited greater sensitivity
toward oxidative stress-induced cell death and activation of caspase 3/
7 activity. Undifferentiated and differentiated NG108-15 cells in 96
Stripwell™ plate were used. To induce oxidative stress, sodium
arsenite was added to designated wells to final concentrations of 1, 5,
10, 20, 35, 50, 75, and 100 μM and incubated at 37°C for 16 h. a Cell
viability, relative to that of the untreated (i.e., without arsenite) control
of 100, is presented. Results represent average ± standard deviation,
N=4. b Caspase 3/7 activity (relative luminescence unit, normalized
against cell viability signal) was assayed using the Caspase3/7 Glo
reagent from Promega Inc. Results represent average ± standard
deviation, N=4
HSP chaperones and neural differentiation
Fig. 8 Susceptibility of the differentiated but not the undifferentiated
NG108-15 cells to the excitotoxic effects of glutamate/glycine.
Undifferentiated and differentiated NG108-15 cells in 96 Stripwell™
plate were used. To test for the effects of glutamate and glycine, cells
were refurbished with Dulbecco’s phosphate-buffered saline without
added amino acids. Glutamate was added to individual wells to final
concentrations of 0, 10, 20, 50, 100, 200, and 500 μM and 1 mM
either without (circle symbol) or with 10 (triangle symbol) and 50 μM
(square symbol) glycine (gly). Cells were incubated at 37°C overnight
(16 h). Cell viability was assayed using the CellTiter Glo luminescence reagent from Promega Inc. Results presented are relative to that
of the untreated (i.e., without glutamate or glycine) control of 100.
Results represent average ± standard deviation, N=4
dependent activation of caspase 3/7 activity particularly in
the differentiated cells (Fig. 7b). A maximal activation of
caspase 3/7 was observed after 16-h incubation at 37°C
with 50-μM sodium arsenite, and this activation was
approximately five times greater in the differentiated cells
than in the undifferentiated cells.
The excitotoxic effects of increasing concentrations of
glutamate and glycine (Fig. 8) appeared also to be selective
for the differentiated cells. Glutamate, without glycine, had
little or no effect on viability of the differentiated NG10815 cells; the addition of 10- and 50-μM glycine, however,
gave a glutamate dose-dependent decrease in viability of
the differentiated cells. Viability of the undifferentiated
cells was not statistically affected by the concentration and
combination of glutamate and glycine used.
Discussion
In our present study of the regulation of heat shock gene
expression in neural differentiation, we observed that
differentiation of the NG108-15 tumor neural progenitor
cells into neuron-like cells is associated with an attenuated
HSR. Our result is consistent with previous observations of
a reduced induction of Hsp70 during neuronal differentiation of the PC12 cells (Dwyer et al. 1996; Hatayama et al.
81
1997) and of differences in induction of the heat shock
genes in regions of the mammalian brain—a robust
response in glial and ependymal cells as compared to a
null, delayed, or diminished response in neurons (Manzerra
and Brown 1996; Marcuccilli et al. 1996; Nishimura and
Dwyer 1996; Tytell et al. 1996). Studies in various neuronal
systems noted a high threshold for induction for the stress
response, a defect attributed to the lack of activation of the
heat shock transcription factor, HSF1 (Marcuccilli et al.
1996; Nishimura and Dwyer 1996; Batulan et al. 2003).
Together, these observations strongly suggest that an attenuated HSR may be a common feature of the differentiated neuronal cell. This limited ability of neurons to
mount the protective HSR is likely to have dire consequences, as protein mis-folding and aberrant protein interactions are known to have fundamentally important roles in
the pathogenesis of various neurodegenerative conditions
(Welch and Gambetti 1998; Sharp et al. 1999; Sherman and
Goldberg 2001; Bonini 2002; Muchowski 2002; Benn and
Brown 2004; Landsbury 2004; Westerheide and Morimoto
2005; Morimoto 2006; Muchowski and Wacker 2005).
The molecular mechanism of this attenuated HSR in
differentiated neurons is not entirely clear. We showed that,
while the amount of HSF1 in the differentiated cells is not
significantly different from that of the undifferentiated cells,
the HSF1 of the differentiated cells was nonetheless
recalcitrant to heat-induced activation. In a previous study
on PC12 cells, neural differentiation was associated with a
marked increase in the HSF1 DNA-binding activity,
although induction of the HSP mRNA and protein was
markedly reduced (Hatayama et al. 1997). The cause of this
difference in regulation of HSF1 DNA-binding activity in
the PC12 versus NG108-15 cells is not entirely clear.
Studies on embryonic motor neurons showed that, while
the attenuated HSR in neurons cannot be rectified by the
transfection and expression of a wild-type HSF1, the
transfection and expression of a constitutively active form
of HSF1 were effective in reinstating the HSR (Batulan et
al. 2003). Together, these results suggest changes in the
sensing and/or signaling mechanism leading to the activation of HSF1 in the differentiated neuron.
The increased expression of Hsc70 protein in the
differentiated NG108-15 cells is of interest and, perhaps,
of significance. Hsc70 can, by interacting with various cochaperone proteins, guide the sequential restructuring of
stable or transient protein complexes to promote a temporal
and spatial regulation of the endo-and exocytotic machinery
and to ensure a vectorial passage through the vesicle cycle
(Zinsmaier and Bronk 2001; Young et al. 2003). In other
words, localized co-chaperones can harness the adenosine
triphosphate-dependent mechanisms of Hsc70 for conformational work in vesicle secretion and recycling, protein
transport, and the regulated assembly and/or disassembly of
82
protein complexes. Our observation that the differentiated
NG108-15 cells—notably, varicosity-like structures on
neuritic shafts—staining strongly for Hsc70, is consistent
with this suggested function of Hsc70. In neurons,
varicosities are known structures filled with synaptic
vesicles and release neurotransmitter by synaptic vesicle
exocytosis (Mandell et al. 1993; Cooper et al. 1995; Chiti
and Teschemacher 2007). In previous studies on PC12
cells, differentiation of these cells was not associated with
observable changes in expression of the constitutive Hsc70
protein, although there was a significant decrease in
induction of Hsp70 (Dwyer et al. 1996; Hatayama et al.
1997). The reason(s) for such difference in regulation of
HSC 70 expression upon differentiation of the PC12 versus
NG108-15 cells is not clear. Possibilities may include
differences in the cell model used or stages of differentiation attained in the different studies. To better understand
the mechanism and the functional significance of the
changes in heat shock gene expression in neural differentiation, we plan to evaluate if changes in expression of
Hsc70, by using sense and anti-sense vectors of Hsc70 DNA,
may modulate induction of the HSPs and/or differentiation
of the NG108-15 cells.
Unlike the stress-induced Hsp70, however, Hsc70 may
not afford significant protection against stress-induced pathologies. We show in Fig. 7 that the differentiated NG10815 cells are exquisitely sensitive to the cytotoxic effect of
arsenite. Given that arsenite is both an inducer of the HSR
and an elicitor of oxidative stress (Khalil et al. 2006), we
inferred that the limited induction of HSPs in the differentiated cells coupled with their increased sensitivity to oxidative stress-induced pathologies likely contributed to the
demise of the differentiated cells in the presence of arsenite.
The selective sensitivity of the differentiated NG108-15
cells to glutamate and glycine is of interest. The possibility
that this selective cytotoxic effect of glutamate and glycine
in the differentiated NG108-15 cells is due to activation of
the NMDAR protein is supported by our observation that,
whereas glutamate plus glycine gave dose-dependent
cytotoxic effects, glutamate alone was without effect.
Previous studies showed that NMDARs are heteromeric
composed of NR1 subunits, which binds glycine, and NR2
subunit, which binds glutamate; both NR1 and NR2
subunits are required to create a functional receptor
(Waxman and Lynch 2005). Importantly, expression and
function of the NMDAR protein appeared to be modulated
in neural differentiation: (1) Neurogenesis is correlated with
the expression of various NMDAR subunits (Varju et al.
2001; Pizzi et al. 2002), and (2) differentiation of the
NG108-15 cells is associated with an increase in the
NMDAR mRNA level (Beczkowska et al. 1996, 1997).
Therefore, it is most likely that the selective vulnerability of
the differentiated NG108-15 cells toward glutamate plus
J. Oza et al.
glycine, shown in Fig. 8, is due, at least in part, to the
increased expression and function of NMDAR as part of
the neural differentiation program. The possibility that expression of the HSP chaperones may afford protection
against the cytotoxic effects of glutamate and glycine is
supported by a previous observation that conditioning heat
shock and increased synthesis of HSPs protect cortical
neurons from glutamate toxicity (Rordorf et al. 1991). HSPs
can suppress stress-induced apoptosis by many and varied
mechanisms including blocking cytochrome c release from
mitochondria, preventing apoptosome formation, and inhibiting the activation of caspase 3 and downstream events
(Mosser et al. 2000; Gabai and Sherman 2002).
In summary, our study provides evidence that changes in
expression of the HSP and HSC proteins are components of
the neural differentiation program. It seems likely that the
attenuated induction of HSPs contributes to neuronal
vulnerability to stress-induced pathologies and death,
whereas the increased expression of Hsc70 may support
various neural-specific functions such as vesicle trafficking
in the differentiated cells.
Dibutyryl
cAMP
HSF1
HSR
HSP
Hsp70
Hsc70
Hsp70−/−
MEF
NMDA
NMDAR
PBS
N6,2′-O-dibutyryl adenosine 3′:5′-cyclic
mono-phosphate
heat shock factor 1
heat shock response
heat shock protein
the 72-kDa heat shock protein
the 74- and 73-kDa constitutively expressed
heat shock cognates
Hsp70 knockout
murine embryo fibroblasts
N-methyl-D-aspartate
NMDA receptor
phosphate-buffered saline (150 mM NaCl,
10 mM sodium phosphate, pH 7.4)
Acknowledgement We are grateful to Dr. Mark Plummer of the
Department of Cell Biology and Neuroscience for providing us with
the rat embryonic hippocampal neuron culture (Magby et al. 2006).
We thank Dr. Gutian Xiao for the Hsp70 knockout MEF. This work
was supported in part by grants from the NSF (MCB0240009) and NJ
Commission on Spinal Cord Research (05-3037-SCR-E-0).
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