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Constitutive activation of MAPK cascade in acute quadriplegic myopathy.

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Constitutive Activation of MAPK Cascade in
Acute Quadriplegic Myopathy
Simone Di Giovanni, MD,1 Annamaria Molon, PhD,1 Aldobrando Broccolini, MD,2 Gisela Melcon, MD,1
Massimiliano Mirabella, MD,2 Eric P. Hoffman, PhD,1 and Serenella Servidei, MD2
Acute quadriplegic myopathy (AQM; also called “critical illness myopathy”) shows acute muscle wasting and weakness
and is experienced by some patients with severe systemic illness, often associated with administration of corticosteroids
and/or neuroblocking agents. Key aspects of AQM include muscle atrophy and myofilament loss. Although these features
are shared with neurogenic atrophy, myogenic atrophy in AQM appears mechanistically distinct from neurogenic atrophy. Using muscle biopsies from AQM, neurogenic atrophy, and normal controls, we show that both myogenic and
neurogenic atrophy share induction of myofiber-specific ubiquitin/proteosome pathways (eg, atrogin-1). However, AQM
patient muscle showed a specific strong induction of transforming growth factor (TGF)–␤/MAPK pathways. Atrophic
AQM myofibers showed coexpression of TGF-␤ receptors, p38 MAPK, c-jun, and c-myc, including phosphorylated
active forms, and these same fibers showed apoptotic features. Our data suggest a model of AQM pathogenesis in which
stress stimuli (sepsis, corticosteroids, pH imbalance, osmotic imbalance) converge on the TGF-␤ pathway in myofibers.
The acute stimulation of the TGF-␤/MAPK pathway, coupled with the inactivity-induced atrogin-1/proteosome pathway,
leads to the acute muscle loss seen in AQM patients.
Ann Neurol 2004;55:195–206
Acute quadriplegic myopathy (AQM) or critical illness
myopathy (CIM) is a subacute muscle disorder characterized by generalized progressive muscle weakness and
atrophy that occurs in critically ill patients. Its pathogenesis is multifactorial and is associated with a patient
history of sepsis, severe pulmonary disorders, major
surgery, multiorgan failure, intensive care, and exposure to corticosteroids and neuroblocking agents.1–3
Electromyography has excluded a primary neurogenic
process and instead points to a primary myogenic disease.4 Diagnosis is possible with muscle biopsy, in
which histopathology shows a marked myogenic atrophy5 with or without selective loss of myosin filaments.
Classic necrosis is a rare event and limited to a minority of fibers. Thus, the presence of severe subacute atrophy in the absence of evidence of denervation or
nerve disease is strongly suggestive of a diagnosis of
AQM.
The molecular chain of events leading to myofiber
atrophy has been poorly understood. Ubiquitinmediated proteolysis and both cytoplasmic (calpains)
and lysosomal-dependent (cathepsins) proteolytic pathways have been associated with muscle wasting in
AQM patients and with cell atrophy in both in vivo
and in vitro models.6 –12 Lack of muscle excitability
due to sodium channel inactivation also has been described in a subset of patients and in an animal model
of AQM.13,14 Gene expression changes in animal models of AQM and CIM also have been reported and
showed repression of specific Na channels15 and induction of proteolytic machinery at the RNA transcription
level.16,17
We have described previously the occurrence of apoptotic features in atrophic fibers in human muscle biopsies from AQM patients, as shown by DNA fragmentation in situ by terminal deoxynucleotidyltransferase–mediated dUTP nick end labeling (TUNEL),
and nuclear margination and condensation by electron
microscopy.18 Many of the fibers were found to show
strong expression of calpain, cathepsin B, and caspase
3.18 We also have shown that these coordinated
changes occurred specifically in AQM muscles as opposed to biopsies from neurogenic myofiber atrophy.
These data suggested that stimulation of proapoptotic
pathways may be part of the underlying pathogenesis
of AQM.
From the 1Center for Genetic Medicine, Children’s National Medical Center and Genetics Program, George Washington University,
Washington, DC; and 2Institute of Neurology, Catholic University,
Rome, Italy.
Address correspondence to Dr Hoffman, Center for Genetic Medicine, Children’s National Medical Center, Washington, DC 20010.
E-mail: ehoffman@cnmcresearch.org
Received Aug 5, 2003, and in revised form Sep 15. Accepted for
publication Sep 15, 2003.
This article is a US Government work and, as such, is in the public domain in the United States of America.
Published 2003 by Wiley-Liss, Inc., through Wiley Subscription Services
195
To provide a more complete picture of the biochemical pathways activated in AQM, we defined a molecular fingerprint unique to myogenic atrophy (AQM) as
opposed to neurogenic atrophy by genomewide microarray analysis of patient muscle biopsies. We show
that unsupervised data analysis is easily able to differentiate myogenic atrophy from neurogenic atrophy via
expression profiles, and that constitutive stimulation of
the proapoptotic transforming growth factor (TGF)–␤/
MAPK pathway underlies myogenic atrophy. We also
show that the muscle-specific S␬p1-Cullin-F-box protein (SCF) ubiquitin ligase component, atrogin-1
(FBx32), is strongly induced in AQM, consistent with
the significant loss of myofilaments in this disorder.
We propose a model for AQM in which a combination
of cellular stress, inactivity of muscle, and drug stimulation (corticosteroids) all converge on the TGF-␤/
MAPK and atrogin-1 pathways, leading to acute stimulation of apoptosis and myofilament loss.
Patients and Methods
Patients and Biochemical and Morphological Analysis
of Muscle Biopsies
Diagnostic muscle biopsies were obtained, with informed
consent, from five patients, affected by acute quadriplegic
myopathy (Table 1). Muscle biopsies from all patients were
studied by standard histochemistry for morphological assessment, and for enzyme biochemistry for acid and neutral
maltase. Disease control muscle biopsies were five patients
with neurogenic disorders, including motor neuron disease,
lower motor neuron disease, spinal muscle atrophy type III,
and chronic axonal neuropathy. Seven normal control muscles were from patients with asymptomatic elevations of creatine kinase (hyperCKemia), with no evidence of histological
changes on muscle biopsy.
TUNEL, electron microscopy, and immunocytochemistry
for apoptosis-related proteins caspase 1, caspase 3, caplain,
and cathepsin B were done as previously described18 (see Table 1).
Expression Profiling
Expression profiling was done as we have described previously.19 In brief, RNA was extracted from each frozen mus-
cle biopsy using TRIzol reagent (GIBCO BRL, Gaithersburg, MD). Seven milligrams of total RNA from each tissue
sample (17 samples total) was processed accordingly to protocol (Affymetrix) to obtain fragmented cRNA that was hybridized on the chip.
Microarray (genechip) Quality Control, Data
Scrubbing, and Statistical Analysis
We used stringent quality control methods as previously
published19 and detailed on our Web site: (http://
microarray.cnmcresearch.org/pgaoutline-qcofsamples.asp).
Three of the neurogenic atrophy subject biopsies did not
meet our usual threshold for “present” calls. We attributed
this to the severe atrophy and fibrosis shown by muscle of
these patients. The scaling factors determinations were done
using default Affymetrix algorithms (MAS 5) with a target
intensity of chip sector fluorescence to 800. Both preamplification (s1) and postamplification with streptavidin/phycoerythrin (s2) scans were done, and the scans were compared
by scatterplots and correlation coefficients.19 Saturated probe
sets showing evidence of saturation of the PMT in s2 were
eliminated with our custom Array Data Manipulation software. We have shown recently that use of Affymetrix MAS
5.0 signal intensity values, together with a “present call”
noise filter achieves an excellent signal to noise balance for
human muscle relative to other probe set analysis methods
(RMA).20 Data analyses were limited to probe sets that
showed one or more “present” (P “calls”) in the 17 genechip
profiles in our complete data set. Experiment normalization
was performed by normalizing gene chips as described.19
Normalized data then were compared for differential gene
expression analysis between AQM patients and the two control groups. Genes that showed a Welch analysis of variance
(ANOVA) t test with p value less than 0.05 between groups
were retained for further analysis. Initial data analysis also
included a fold change filter of greater than 1.5 (50% difference) increase or decrease relative to control groups (Affymetrix MAS 5.0). Although a p value of less than 0.05
alone would give many false-positives, the combination of
present call filters, fold change thresholds, and p value
thresholds, eliminates most false-positives that are obtained
with only p values less than 0.05 (all confirmed by other
methods). We also confirmed nearly all expression changes at
the mRNA and/or protein level using independent techniques.
Table 1. Summary of Clinical, Electrophysiological, and Muscle Pathological Features of AQM patients
Pt. Age
No. (yr)
1
2
3
66
66
77
4
5
64
76
ICU
Systemic Illness
Surgery/
Atrophy/
Anesthesia Corticosteroids Weakness
No COPD-diabetes
Yes Sepsis-MD
No Lymphoma-hyper
cortisol.
Yes Sepsis-ARDS-MODS
Yes Sepsis-MODS
EMG
CK
Apoptotic
Signs
Myofiber
Atrophy (% fibers)
No
Yes
No
Yes
Yes
No
⫹⫹⫹
⫹⫹⫹
⫹⫹⫹
Myopathy 1/10 n.v.
Myopathy 1/2 n.v.
Myopathy 1/5 n.v.
⫹⫹⫹
⫹⫹⫹
⫹⫹
70
80
50
No
Yes
Yes
Yes
⫹⫹⫹
⫹⫹
Myopathy n.v.
Myopathy 1/20 n.v.
⫹⫹
⫹⫹
50
40
ICU ⫽ intensive care unit; EMG ⫽ electromyogram; CK ⫽ creatine kinase; COPD ⫽ chronic obstructive pulmonary disease; n.v. ⫽ normal
values; MD ⫽ myelodisplasia. ARDS ⫽ acute respiratory distress syndrome; MODS ⫽ multiorgan distress syndrome; AQM ⫽ acute quantitative myopathy.
196
Annals of Neurology
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Real-time Reverse Transcription Polymerase
Chain Reaction
We studied gene expression in muscle from AQM patients
and normal controls, by real-time polymerase chain reaction
(PCR) for both validation purposes (gadd45 ␤, p21, c-myc,
and c-jun) and to measure expression of genes not represented on the genechip (atrogin-1). Fluorophore-labeled
LUX primers (forward) and their unlabeled counterparts (reverse) were provided by Invitrogen (La Jolla, CA). LUX
primers were designed within Affymetrix probe sets sequences for each gene, and all primers were designed using
the software called LUX Designer (Invitrogen, www.invitrogen.com/lux). We performed multiplex PCR combining in
the same PCR mix each experimental gene with the housekeeping gene GAPDH.
For each sample, 20␮l PCR contained 2␮l cDNA (first
diluted 1:10 after reverse transcription [RT]), 200nM of
each gene-specific primer (two pairs for multiplex PCR) and
1 ⫻ Platinum Quantitative PCR SuperMix-UDG (Invitrogen), and 1 ⫻ ROX reference dye (500nM in SuperMix).
PCR conditions were standard (Invitrogen, www.invitrogen.com/lux), and reactions were conducted in a 96-well spectrofluorometric thermal cycler (ABI PRISM 7700 Sequence
detector system; Applied Biosystems, Foster City, CA). Fluorescence was monitored during every PCR cycle at the annealing or extension step and during the post-PCR temperature ramp. Fold changes were measured according to
manufacturer instructions (Invitrogen), and ANOVA t test
was used for statistical analysis.
Immunoblotting
Protein extracts were recovered from the lower organic phase
of TRIZOL after supernatant isolation for RNA extraction
(GIBCO). Proteins then were quantified by the Bradford
method, and 20␮g of protein solubilized in Laemmli sample
buffer supplemented with protease inhibitors (10mg of aprotinin per ml, 1mg of leupeptin per ml, 1mM phenylmethl
sulfonyl fluoride). Immunoblotting was done using standard
methods with 4 to 12% and 10% SDS-PAGE gels. For immunoblotting, we used the following antibodies: goat polyclonal anti–TGF-␤ receptor I and II, goat polyclonal anti–
TAK-1 (diluted 1:200; Santa Cruz Biotechnology, La Jolla,
CA); rabbit polyclonal anti-junB (diluted: 1:1,000; Oncogene, Cambridge, MA); and rabbit polyclonal anti–R-Ras,
anti–p38 MAPK, anti–phospho-p38 MAPK, anti–c-jun,
anti–phospho-c-jun, anti–c-myc, anti–phospho-c-myc, anti–
phosho-SMAD 1, 5, 8,anti-MKK3, anti–phspho-MKK3,
anti-MKK4, ant-Ask1, anti–phospho-Ask1, mouse monoclonal anti-JNK, and anti–phospho-JNK (Cell Signaling, Beverly, MA). Immunocomplexes were visualized with ECL
chemiluminescence (Amersham, Arlington Heights, IL).
TUNEL and Immunocytochemistry
The TUNEL technique was used for detection of nuclear
DNA fragmentation in situ. Frozen muscle sections (10␮m)
from patients and control subjects were incubated under the
same coverslip with TUNEL reaction mixture, and incorporated fluorescein-dUTP was detected by using alkaline
phosphatase-conjugated antifluorescein antibodies according
to the manufacturer’s instruction (In Situ Cell Death Detec-
tion Kit; Boehringer, Mannheim, Germany). Unfixed muscle
sections adjacent to those analyzed by means of TUNEL
were processed for immunocytochemistry accordingly to
standard procedures. The same primary antibodies used for
immunoblotting were used also for immunocytochemistry,
and incubated overnight at 4°C. Sections were developed
with peroxidase staining. The count of fibers positive by
TUNEL and immunocytochemistry was performed in at
least 100 muscle fibers per section.
Results
Clinical and Pathological Features of Acute
Quadriplegic Myopathy and Controls
Muscle biopsies from five patients, each showing a
clinical and histological pattern consistent with the diagnosis of AQM, were studied (see Table 1). Severe
systemic illness was present in all patients, variably including sepsis, intensive care unit treatment, surgery,
and corticosteroids administration. All patients had a
moderate/severe muscle weakness and atrophy. Electromyography showed myopathic changes compatible
with AQM. No clinical or electromyography evidence
of nerve involvement was seen, including lack of spontaneous activity (see Table 1). Apoptotic signs, as
shown by TUNEL and activated caspase 3 immunoreactivity were present in atrophic fibers in all patients
(see Table 1), as we have previously described.18
Both neurogenic and normal control muscles were
used to address the specificity of the changes in AQM
muscles. These age-matched controls included seven
biopsies from subjects with asymptomatic hyperCKemia (normal histopathology) and five biopsies from
patients from neurogenic disorders (motor neuron disease, lower motor neuron disease, spinal muscle atrophy type III, and chronic axonal neuropathy; all
showed neuropathic histopathology). A subset of neurogenic atrophic fibers showed also apoptotic features
as previously reported.18
Unsupervised Hierarchical Clustering Accurately
Diagnoses Acute Quadriplegic Myopathy from
Neurogenic Atrophy
Each skeletal muscle biopsy was processed individually
for expression profiling (five AQM patients, five neurogenic disorders, and seven normal muscles). We expression profiled approximately 12,000 transcripts using Affymetrix high-density oligonucleotide arrays Hu95 v2.
Data were processed according to bioinformatic methods
that we have previously shown provide good signal/noise
ratios for human muscle biopsies.20 Normalizations included per chip (50th percentile) and per sample normalization (to the median of each gene). We used several types of data analysis: unsupervised and supervised
hierarchical clustering; statistical analysis using analysis
of variance (Welch ANOVA t test); nucleation of dysregulated transcripts using a 1.5-fold change cutoff be-
Di Giovanni et al: MAPK Cascade in AQM In Vivo
197
tween the different groups of diseases (probe sets differing in 50% expression level); functional clustering with
implementation of the GeneSpring gene query tool; and
functional ontology visualizations using both GeneMapp,21 and David (http://apps1.niaid.nih.gov/david/
upload.jsp) software and databases.
Unsupervised hierarchical clustering based on standard distance metrics (GeneSpring) showed correct diagnosis of the three diagnostic groups (myogenic atrophy [AQM], neurogenic atrophy, normal controls) into
three specific branches on the dendrogram (Fig 1).
This analysis provides strong support for distinct molecular pathophysiological mechanisms for the two
types of atrophy and also indicates that the biological
variables were dominant over the sum of technical variables and interindividual noise.20
We then filtered data for dysregulated transcripts
based on a significant Welch ANOVA t test ( p ⬍
0.05) between AQM and neurogenic and AQM and
normal controls, combined with a 1.5-fold change
threshold. This resulted in 1,670 upregulated, and 709
downregulated transcripts (see http://microarray.
cnmcresearch.org/pgadatatable.asp for the complete
gene list). The AQM-specific transcripts then were filtered for functional ontologies and further analyzed
and verified, as described below.
Strong Induction of Oxidative Stress Response
and Protease/Proteasome Clusters in Acute
Quadriplegic Myopathy
Visualization of known functional clusters showed consistent activation of oxidative stress, protease (including
caspase 4 and 6), and ubiquitin pathways using “heat
map” methods (see Fig 1). Some of these pathway
members showed downregulation in neurogenic controls, consistent with distinct underlying molecular
pathophysiologies in the two types of atrophy. The
ubiquitin-dependent proteolytic pathway was particularly strongly induced in myogenic atrophy (see Fig 1),
including upregulation of several ubiquitin proteases
and ubiquitin activating enzymes. There was no evident upregulation of ubiquitin ligases; however, it is
known that many ubiquitin ligases are induced via activation of tissue specific F-box proteins of the SCF
ligase complex.11 The muscle-specific F-box, atrogin-1
(FBx32),11 was not on the U95v2A microarrays used
for our expression profiling studies. Therefore, we
studied expression of the ubiquitin ligase atrogin-1, by
quantitative real-time RT-PCR. We found atrogin-1
mRNA 10-fold increased in both AQM and neurogenic muscle relative to controls (Table 2). To our
knowledge, this is the first data on atrogin-1 in humans in vivo, although our data are consistent with in
vitro and animal models experiments showing
atrogin-1 induced in muscle atrophy under diverse
conditions and stimuli.
198
Annals of Neurology
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February 2004
The Transforming Growth Factor–␤/MAPK
Signaling Cascade Is Specifically Activated in Acute
Quadriplegic Myopathy Myogenic Atrophy
The GeneMapp and David databases were used for
functional classification of dysregulated transcripts to
identify potential signaling pathways specifically associated with myogenic atrophy. Both databases showed the
induction of several members of the MAPK signaling
cascade in atrophic AQM muscles. MAPK induction
plays an important role in the regulation of cell growth,
atrophy, mitosis, and apoptosis.22 This prompted us to
further evaluate with both the expression profiling data
set and RT-PCR the expression levels of all known
members of this pathway and of possible related pathways, including TGF-␤ and RAS signaling cascades. We
found the specific upregulation of TGF-␤ receptor II,
TAK-1, ASK-1, RAS family members, SMADs (Smad
1, 3, and 4), JNKK2, JNK, MKK3/6, p38 MAPK,
c-jun, junB, c-fos, c-myc, gadd-45␤, and p21 cell cycle
inhibitor. Fold changes and p values for each transcript
are reported (Table 2). Transcription of the extracellular
signal-repulsated kinase (ERK) branch of the large
MAPK pathway was inhibited or not differentially regulated in AQM muscles (data not shown).
Real time RT-PCR confirmation was performed for
a subset of the TGF-␤/MAPK pathway members (p21,
GADD45 ␤, c-myc, c-jun; see Table 2). In all transcripts tested, the RT-PCR data were consistent with
the microarray data, and all genes showed significant p
values by both independent assay methods.
Activation of the Transforming Growth Factor–␤/
MAPK Pathway Is Seen at the Protein Level
The majority of activation of the TGF-␤/MAPK pathway occurs at the protein level (phosphorylation, localization, stability), and mRNA studies are a relatively insensitive means of assessing the status of the entire
pathway. For this reason, we performed immunoblotting
for many members of the TGF-␤/MAPK cascade in
protein extracts from all AQM muscle biopsies and
compared them with the neurogenic and normal controls. We measured both total and phosphorylated (p)
forms for the following proteins: ASK-1, SMADs,
JNKKs, JNKs, MKK3/6, P38-MAPK, c-jun, c-fos,
c-myc. Protein levels also were assessed for r-RAS,
TGF-␤ receptors I and II, TAK-1, junB (Fig 2A). Our
immunoblot data were consistent with a wide activation
of the TGF-␤/MAPK pathway, with increases of both
total and phosphorylated protein forms in AQM atrophic muscles. These data were also in agreement with
the transcriptional upregulation seen by both microarray
and RT-PCR. Fold changes and p values from quantitated immunoblots were calculated for TGF-␤ receptor
II, ASK-1, p38-MAPK, phosphorylated p38-MAPK,
c-myc, phosphorylated c-myc, SMADs, and junB (see
Fig 2B); all demonstrated highly significant activation
Fig 1. Diagnostic and functional ontology analysis of
microarray data. (A) Unsupervised hierarchical clustering shows correct diagnosis of
acute quadriplegic myopathy
(AQM) subjects. Shown is
unsupervised hierarchical
clustering based on average
distance metrics (GeneSpring), with correct diagnosis of the three groups studied
(myogenic atrophy [AQM],
neurogenic atrophy, normal
controls) into three specific
branches on the dendrogram.
Specifically, each group of
patients is clustered into a
specific branch of the gene
tree, corresponding to genes
with similar expression levels.
Bar graph on the right shows
the color code for gene expression level (red: high; blue:
low). AQM patient numbers
refer to Table 1. Neurogenic
atrophy and controls are
numbered based on order in
the figure. (B) AQM shows
specific induction of stress
response and proteolysis pathways. Shown are supervised
hierarchical clusters obtained
after filtering for more than
one “present” call across the
17 profiles, p value less than
0.05 (Welch ANOVA t test),
and 1.5-fold changes in expression between AQM and
controls; these filters then
were combined with functional clustering of oxidative
stress and proteases and
ubiquitin-dependent pathway.
Bar graph on the right shows
the color code for gene expression level (red: high; blue:
low).
specific for AQM myogenic atrophy, including the ratio
of phosphorylated versus total p38-MAPK. We conclude
that the TGF-␤/MAPK cascade is coordinately activated
at both at the transcriptional and protein level, including
activated phosphorylated states of specific signaling proteins, in myogenic atrophy in AQM.
Di Giovanni et al: MAPK Cascade in AQM In Vivo
199
Table 2. TGF-␤ MAPK Signaling Cascade mRNA Expression Data Show Strong Upregulation Specific to Myogenic Atrophy
AQM vs Control
Genbank No.
D50683
V00568
AF078077
U59423
U59912
X89750
U59913
L35263
M14949
AF043938
J04111
U03106
X14885
L07594
L41690
AB009356
U71087
AF002715
U67156
U09759
M12174
L24564
L25080
U02570
M23379
U52522
Gene Name
TGFbeta type II receptor
mRNA Affymetrix
Protein in atrophic fibers
MYC
mRNA Affymetrix
mRNA real-time PCR
Protein in atrophic fibers
(p-c-MYC)
GADD45beta
mRNA Affymetrix
mRNA real-time PCR
Protein in atrophic fibers
SMAD1
mRNA Affymetrix
SMAD1
mRNA Affymetrix
SMAD2
mRNA Affymetrix
SMAD5
mRNA Affymetrix
SMAD 1, 5, 8
Protein in atrophic fibers
P38 MAPK
mRNA Affymetrix
Protein in atrophic fibers
R-RAS
mRNA Affymetrix
M-RAS R-RAS3
mRNA Affymetrix
RAS
Protein in atrophic fibers
JUN
mRNA Affymetrix
mRNA real-time PCR
CDKN1 P21
mRNA Affymetrix
mRNA real-time PCR
p-ASK
Protein in atrophic fibers
TGFbeta 3
mRNA Affymetrix
TGFbeta type III receptor
mRNA Affymetrix
TRADD
mRNA Affymetrix
TAK1
mRNA Affymetrix
MEK5 PRKMK5
mRNA Affymetrix
MAPKKK4 MEKK4
mRNA Affymetrix
MAPKKK5
mRNA Affymetrix
JNK2
mRNA Affymetrix
RHOH6
mRNA Affymetrix
RAD1
mRNA Affymetrix
RhoA
mRNA Affymetrix
CDC42GAP
mRNA Affymetrix
GAP RASA
mRNA Affymetrix
POR1
mRNA Affymetrix
Atrogin-1
mRNA real time PCR
Fold
Change
p
3
⬍0.01
9
8.8
⬍0.01
⬍0.01
4.3
7.1
⬍0.05
⬍0.01
2.1
AQM vs Neurogenic Atrophy
Percentage of Positive Fibers*
(⫾ SD)
90 ⴞ 5(AQM) 20 ⴞ 5(Neur)
Fold
Change
2.4
p
<0.01
12.6
5.1
⬍0.01
⬍0.05
<0.01
5.1
4.6
⬍0.05
⬍0.05
<0.01
⬍0.01
1.8
⬍0.01
1.9
⬍0.01
2.2
⬍0.01
4.1
⬍0.01
2.1
⬍0.05
2.8
⬍0.01
7.4
⬍0.01
82 ⴞ 3(AQM) 12 ⴞ 4(Neur)
86 ⴞ 5(AQM) 8 ⴞ 2(Neur)
88 ⴞ 4(AQM) 3 ⴞ 1(Neur)
1.9
⬍0.01
3
1.6
<0.01
1.8
⬍0.05
<0.01
⬍0.05
5.5
⬍0.01
⬍0.01
1.4
⬍0.05
65 ⴞ 10(AQM) 3 ⴞ 1(Neur)
95 ⴞ 5(AQM) 4 ⴞ 1(Neur)
<0.01
3.3
6
⬍0.01
⬍0.01
2.3
4.5
⬍0.05
⬍0.05
14.5
8
⬍0.01
⬍0.01
7.2
5
⬍0.01
⬍0.05
72 ⴞ 4(AQM) 3 ⴞ 1(Neur)
<0.01
3.1
⬍0.05
2.6
⬍0.05
2.6
⬍0.01
3.4
⬍0.01
2.1
⬍0.05
2.8
4.4
⬍0.01
1.8
⬍0.05
1.8
⬍0.01
3
⬍0.01
1.7
⬍0.01
1.4
ns
1.6
⬍0.05
1.3
ns
2.3
⬍0.01
2.3
⬍0.05
10.8
⬍0.01
3.1
⬍0.01
5.4
⬍0.05
11.5
⬍0.01
2.9
⬍0.01
4.1
⬍0.01
2.7
⬍0.01
4.3
⬍0.01
2.5
⬍0.01
2.1
⬍0.05
5.4
⬍0.01
6.1
⬍0.01
10.8
⬍0.01
No
change
ns
NS
Refers to immunohistochemistry experiments.
TGF ⫽ transforming growth factor; AQM ⫽ acute quadriplegic myopathy; PCR ⫽ polymerase chain reaction; NS ⫽ not significant.
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Annals of Neurology
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February 2004
Fig 2. Immunoblotting and quantitation of members of transforming growth factor (TGF)–␤/MAPK cascade. (A) Shown is protein
quantitation for several members of TGF-␤ MAPK cascade in acute quadriplegic myopathy (AQM), neurogenic atrophy, and normal control muscle. Both total and phosphorylated proteins were measured. Total proteins were measured using Bradford method,
and an equal amount of proteins (20␮g) was loaded for each sample. Both total and phosphorylated forms of TGF-␤/MAPK pathway members were markedly elevated in AQM. (B) Shown is quantitation of immunoblots for some representative members of
TGF-␤/MAPK cascade. Fold changes and p values from quantitated immunoblots are provided for TGF-␤ receptor II, Ask-1,
SMADs, p38-MAPK, phosphorylated p38-MAPK, c-myc, phosphorylated c-myc, and junB. All show highly significant activation in
AQM myogenic atrophy, including the ratio of phosphorylated versus total p38-MAPK.
Specific Key Members of Transforming Growth
Factor–␤/MAPK Cascade Are Expressed in Atrophic
Myofibers with Apoptotic Features in Acute
Quadriplegic Myopathy
We have shown previously the presence of apoptotic features (activated caspases 3 and TUNEL-positive fibers) in
atrophic myofibers in a subset of patients in AQM.18 To
extend these previous findings, we localized several members of the TGF-␤/MAPK cascade in cryosections of patient muscle biopsies by using immunohistochemistry.
We found TGF-␤, TGF-␤ receptor, p-ASK1, p38MAPK, GADD45 ␤, c-jun, and c-mycall strongly expressed specifically in atrophic myofibers with apoptotic
features (TUNEL-positive) and rare normotrophic fibers
in AQM muscle (Fig 3, 4). TGF-␤ receptor showed both
membrane and cytoplasmic staining, and p38-MAPK
showed diffuse cytoplasm localization, whereas c-jun
showed characteristic nuclear localization (see Fig 3). On
serial sections, TUNEL-positive picnotic nuclei in atrophic fibers strongly immunoreacted for this transcription
factor (see Fig 4). No or very faint staining was observed
in control muscles and in neurogenic muscle fibers.
Quantitation of the percentage of atrophic fibers (myogenic and neurogenic biopsies), immunoreactive for
TGF-␤ II receptor, phosphorylated c-myc, GADD45 ␤,
SMAD (1, 5, 8), P38, RAS, and phosphorylated ASK-1
Di Giovanni et al: MAPK Cascade in AQM In Vivo
201
Fig 3. Immunohistochemistry shows localization of transforming growth factor (TGF)–␤ MAPK cascade in atrophic fibers in acute
quadriplegic myopathy (AQM). Shown is specific localization in atrophic fibers in AQM (A, C, E, G, I, M) and neurogenic muscles (B, D, F, H, L, N) of TGF-␤ receptor (A, B), SMADs (C, D), p38 (E, F), Ras (G, H), p-c-Myc (I, L), and gadd45 beta
(M, N). Control muscles immunostained with TGF-␤ receptor (P), SMADs (Q), Ras (R), and p-c-Myc (S), show no signal. Intense immunostaining is present only in AQM atrophic muscles for all proteins. Several myofibers are positive for TGF-␤ receptor
also in neurogenic atrophy (B). Original magnification, ⫻250
was also performed. In all cases, the percentage of atrophic
fibers immunopositive for TGF-␤/MAPK members was
significantly higher in myogenic (AQM) compared with
neurogenic atrophy (see Table 2, boldface).
Discussion
Comparative Genomics of Myogenic and Neurogenic
Atrophy
We present protein and mRNA data in AQM myogenic
atrophy, neurogenic atrophy, and normal controls that
shows activation of TGF-␤/MAPK signaling cascade specifically in muscle in myogenic atrophy in AQM patients.
Neurogenic atrophy and myogenic atrophy shared similar
strong upregulation of the muscle-specific ubiquitin ligase
atrogin-1 (FBx32), consistent with the myofiber atrophy
seen in both conditions. The AQM-specific induction of
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the TGF-␤/MAPK pathway was demonstrated at both
the transcriptional and protein level, including protein
phosphorylation consistent with constitutive activation of
this signaling cascade. We also showed colocalization of
TGF-␤/MAPK pathway members with apoptotic atrophic myofibers in patient muscle, including TGF-␤ receptor II, ASK1, p38 MAPK, c-myc, and c-jun.
Transforming Growth Factor–␤/MAPK Signaling
Cascade: Mechanisms of Activation and Effectors
Atrophic myofibers in AQM patients showed strong
coordinated upregulation of TGF-␤ MAPK signaling
cascade, Ras family members, and cell cycle inhibitors
(Table 2, Fig 5). Oxidative and osmolar stress, pH imbalance, and cytokine release all are documented triggers for the TGF-␤/MAPK signaling cascade in diverse
Fig 4. TUNEL shows colocalization with members of MAPK cascade in atrophic fibers in acute quadriplegic myopathy (AQM).
Shown are TUNEL-positive fibers (B, F) that, respectively, co-localize with p38 (A, arrows) and c-jun (F, arrows show some representative fibers) in atrophic fibers in AQM. Shown are also control sections processed for p38 (C), c-jun (G), and TUNEL (D,
H) that do not show TUNEL or p38 positivity and express faint nuclear staining for c-jun. Original magnification, ⫻250.
Di Giovanni et al: MAPK Cascade in AQM In Vivo
203
Fig 5. Induction of transforming growth factor (TGF)–␤/Mapk signaling cascade in myogenic atrophy. Observed modulation of the
TGF-␤ MAPK cascade in acute quadriplegic myopathy (AQM).
cell types.22–28 Both the TGF-␤ and Ras pathways
converge on the MAPK pathway, likely leading to constitutive activation, and the resulting proteolysis and
apoptosis that are histological features of AQM. The
association of AQM with oxidative and osmolar stress
has been seen clinically; however, our data provide molecular confirmation of this via our observed upregulation of anti–oxidative stress enzymes HO-1, MnSOD,
and GPX (see Fig 2), and these changes were not
shared with neurogenic atrophy.
The upstream triggers, downstream regulation, and
cell-specific regulation of the TGF-␤/MAPK pathway
is complex; yet, it is recognized that this regulation is
critical in determining the biological response of many
cell types, toward either proliferation or atrophy/apoptosis.21–23,25,26 A few members of this cascade appear
to be key in influencing this biological outcome,
namely, p38 MAPK, JNKs, c-myc, and c-jun. These
specific pathway members have been shown to promote cell atrophy and death rather than mitosis, particularly in postmitotic tissues, including skeletal mus-
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cle.29 –38 Importantly, the MAPK and Ras pathways
have been shown to be necessary for muscle proteolysis
in Caenorhabditis elegans,39 and TGF-␤–mediated
MAPK activation was shown to antagonize muscle hypertrophy and the insulin-like growth factor–1 pathway, through activation of proapoptotic insulin-like
growth factor receptor binding proteins.40
Taken together, our data and data from the literature allow us to propose a model for muscle cell atrophy in AQM in which activation of TGF-␤ receptors,
exacerbated by stress response and corticosteroid/Ras
pathways,28,41 begins constitutive intracellular signaling
of the MAPK cascade in specific myofibers leading to
muscle atrophy with apoptotic features (see Fig 3, 4).
Consistent with this model, we show that many of the
pathway members are in the hyperphosphorylated, activated state, and that these pathway components colocalize with apoptotic cells in patient muscle biopsies.
To briefly describe the roles of some of the proteins
we found specifically activated in AQM, the most important TGF-␤ signal transducers are SMADs and
TAK-1 that after phosphorylation translocate to the
nucleus and promote transcription of downstream targets (c-jun, junD, and c-myc), and also phosphorylate
downstream MAPK members (see Fig 5). Transcription factors c-jun, junD, and c-myc can both promote
apoptosis and induce TGF-␤ transcription providing a
positive feedback to the entire pathway. They can be
activated at the transcriptional (activated by SMADs)
and protein level (phosphorylated by p38MAPK,
JNKs). Ras and Rho family members are typically triggered by TGF-␤ signaling, whereupon they mediate
actin reorganization, possibly participating in cytoskeleton reshaping. SMADs activation can also promote
transcription of p21 and gadd45 ␤, both of which induce cell growth arrest and interact with ASK-1.
ASK-1 and other MAPK pathway members promote
apoptosis and proteolysis.42– 45
In summary, this signaling cascade likely leads to
muscle actin-cytoskeleton reorganization, cell atrophy,
apoptosis, and proteolysis, all distinguishing features of
AQM muscle. Importantly, our data prove that neurogenic atrophy and myogenic atrophy (AQM) share the
ubiqutin ligase pathway, but only AQM activates the
TGF-␤/MAPK pathway. Note also that the TGF-␤/
MAPK cascade is not dysregulated in forms of inflammatory myopathies associated with necrotic features,
including dermatomyositis, polymyositis, and inclusion
body myopathy.46,47
Transforming Growth Factor–␤/MAPK Cascade:
Possible Therapeutic Targets
The novel pathophysiological cascades defined here for
AQM suggests new targets for potential therapies inhibiting muscle atrophy. As shown in Figure 5, expression of the ERK branch of MAPK cascade is either
inhibited or unchanged in AQM. ERKs family members are known for their progrowth and proregeneration potential in diverse conditions and cell types.
Therefore, upregulation of these members might shift
the cascade toward growth as opposed to atrophy. Another therapeutic target resides in the inhibition of key
players of the proatrophic pathway, including SMADs,
p38 MAPK, JNKs, ASK-1, and cell cycle inhibitor
p21. JNK inhibitors will soon be in clinical trials in
cancer and rheumatoid arthritis.22 Importantly, we
would anticipate that modulation of this signaling cascade might also show efficacy in diverse conditions
showing muscle atrophy, including cancer cachexia and
acquired immune deficiency syndrome.
This work was supported by grants from the National Institutes of
Health (National Heart Lung and Blood Institute U01
HL66614-01 “Programs in Genomic Applications” HOPGENE;
NINDS N01-NS-1-2339, NINDS 3R01 NS29525-09, E.P.H.) and
Telethon Italy (GGP02253, S.S.).
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