In comparison to wild-type muscle, the proportion of wild-type 1 slow fibers is significantly increased 6%, whereas the proportion of fast fibers is reduced in soleus, type 2A by 12%, and i
Trang 1Changes in acetylcholine receptor function induce shifts
in muscle fiber type composition
Tae-Eun Jin1,*, Anton Wernig2and Veit Witzemann1
1 Abt Zellphysiologie, Max-Planck-Institut fu¨r Medizinische Forschung, Heidelberg, Germany
2 Institut fu¨r Physiologie, Universita¨t Bonn, Germany
The impact of innervation on the establishment of
spe-cific muscle fiber types during embryonic and postnatal
development has been demonstrated in numerous
stud-ies [1], and has been attributed to the specific neural
impulse pattern [2] that can be mimicked partially by
electrical stimulation [3,4] Skeletal muscles adapt to
specific functions and have, throughout development,
the capacity to change their phenotype in response to
altered functional demands Their phenotypic profiles
are affected not only by innervation⁄ neuromuscular
activity, but also by exercise training, mechanical
load-ing⁄ unloading, hormones, and aging, causing transi-tions from fast-to-slow or slow-to-fast fiber types Muscle activity has also been shown to induce struc-tural and functional adaptations of the neuromuscular junction (NMJ), suggesting that muscle function, fiber type composition and plasticity of the NMJ may be linked [5] In order to identify the contributions of postsynaptic signaling to adaptation of muscle func-tion, it is necessary to modulate activity specifically
at endplate acetylcholine receptors (AChRs), leaving neuronal inputs unchanged and avoiding complex
Keywords
acetylcholine receptor; acetylcholine
receptor e-subunit knockout mice; fast and
slow muscle; fiber type; real-time PCR
Correspondence
V Witzemann, Abt Zellphysiologie,
Max-Planck-Institut fu¨r Medizinische
Forschung, Jahnstr 29, D-69120
Heidelberg, Germany
Fax: +49 6221 486459
Tel: +49 6221 486475
E-mail: witzeman@mpimf-heidelberg.mpg.de
*Present address
Center for Cell Signaling Research, Ewha
Woman’s University, Seoul, South Korea
(Received 9 January 2008, revised 12
February 2008, accepted 25 February 2008)
doi:10.1111/j.1742-4658.2008.06359.x
AChRe) ⁄ )mice lack e-subunits of the acetylcholine receptor and thus fail
to express adult-type receptors The expression of fetal-type receptors throughout postnatal life alters postsynaptic signal transduction and causes
a fast-to-slow fiber type transition, both in slow-twitch soleus muscle and
in fast-twitch extensor digitorum longus muscle In comparison to wild-type muscle, the proportion of wild-type 1 slow fibers is significantly increased (6%), whereas the proportion of fast fibers is reduced (in soleus, type 2A
by 12%, and in extensor digitorum longus, type 2B⁄ 2D by 10%) The increased levels of troponin Islow transcripts clearly support a fast-to-slow fiber type transition Shifts of protein and transcript levels are not restricted to ‘myogenic’ genes but also affect ‘synaptogenic’ genes Clear increases are observed for acetylcholine receptor a-subunits and the post-synaptically located utrophin Although the fast-to-slow fiber type transi-tion appears to occur in a coordinated manner in both muscle types, muscle-specific differences are retained Most prominently, the differential expression level of the synaptic regulator MuSK is significantly lower in extensor digitorum muscle than in soleus muscle The results show a new quality in muscle plasticity, in that changes in the functional properties of endplate receptors modulate the contractile properties of skeletal muscles Muscle thus represents a self-matching system that adjusts contractile prop-erties and synaptic function to variable functional demands
Abbreviations
AChR, acetylcholine receptor; BS, blocking solution; CSA, cross-sectional area; EDL, extensor digitorum longus muscle; GABP, growth-associated binding protein; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; MuSK, muscle, skeletal, receptor tyrosine kinase;
MyHC, myosin heavy chain; NFAT, nuclear factor of activated T cells; NMJ, neuromuscular junction; P, postnatal day; SOL, soleus muscle; Utrn, utrophin.
Trang 2treatments that affect both presynaptic and
postsynap-tic signaling, such as denervation, pharmacological
blockade, and exercise training
Mammalian AChRs are expressed in two forms:
Embryonic-type AChRc, composed of a2bcd subunits,
is replaced during postnatal development by adult-type
AChRe, composed of a2bed subunits [6,7] As a result,
endplate AChRs have reduced channel open times,
increased ion conductance, and higher Ca2+
perme-ability [6,8,9] Muscles of AChRe) ⁄ ) mice lack
adult-type AChRe and express instead embryonic-adult-type
AChRc throughout postnatal life Nevertheless,
mole-cular maturation of the postsynaptic apparatus
pro-ceeds in the absence of the AChRe, and all endplates
are apposed by nerve endings that appear to be normal
in structure and function despite progressive AChR
deficiency with increasing age [10,11] Thus, the
AChRe) ⁄ ) mice provide a model system for altered
postsynaptic signaling
We analyzed the muscle fiber type composition in
skeletal muscle of AChRe) ⁄ ) and wild-type mice, with
the aim of answering the following questions: (a) what
is the composition of muscle fiber type of the
slow-twitch soleus (SOL) muscle and the fast-slow-twitch
exten-sor digitorum longus (EDL) muscle; (b) is the fiber
type composition changed in AChRe) ⁄ ) mice; (c) are
changes in fiber type correlated with the mRNA
expression pattern of muscle-specific and
synapse-spe-cific genes; and (d) are changes in the contractile
machinery linked to changes in transcript levels of
myogenic genes and synaptogenic genes that regulate,
directly or indirectly, synaptic structure⁄ function? Our
results show that changes in the functional properties
of endplate AChRs modulate the contractile properties
of skeletal muscles and change the expression profile
of myogenic genes in a coordinated fashion
Results
Developmental changes of muscle fiber types in
muscle of wild-type and AChRe) ⁄ )mice
The heavy chain portion of the myosin molecule
(MyHC) determines the major functional characteristic
of distinct myosin isoforms and thus provides a
partic-ularly useful molecular marker for muscle fiber types
[12,13] The different MyHC isoforms correlate with
the functional characteristics of the respective fiber
type in the adult muscle [1], and fiber types are
classi-fied as: type 1 with MyHC1, type 2A with MyHC2A,
type 2D with MyHC2D, and type 2B with MyHC2B
As described in Experimental procedures, serial
cross-sections from SOL muscle of wild-type mice were
stained with hematoxylin⁄ eosin to visualize the individ-ual muscle fibers In addition, type 1, 2A and 2B⁄ 2D fibers were clearly identified by ATPase staining at
pH 4.6 (Fig 1A) and at pH 9.4 (Fig 1B) Type 1 and
2 fibers were also visualized by immunochemical stain-ing (Fig 1C,D) These staining procedures were employed to compare the fiber type composition of SOL and EDL muscles in wild-type and AChRe) ⁄ ) mice
Because the fiber type composition of muscle changes during postnatal development [14,15], we first determined the time when adult MyHC isoforms were expressed at constant levels in the SOL muscle of wild-type mice (Fig 2A–D) ATPase staining at pH 4.6 identified type 1(dark stain), 2A (light stain), and 2B⁄ 2D (intermediate stain) fibers, and showed that between postnatal day (P)15 and P20, the proportion
of slow type 1 and fast type 2A fibers was still vari-able After P20, from P60 up to P85, the fiber types remained at constant levels (Fig 2E,F) At all stages, a few fast fibers, type 2B⁄ 2D (0 £ 1% of the total fibers), were detectable Throughout the postnatal period analyzed here, the cross-sectional areas (CSAs) of single muscle fibers increased (Fig 2F,G)
Fig 1 Fiber type composition in SOL muscle of wild-type mice at P85 Serial cross-sections (10 lm) were analyzed by ATPase staining and immunochemically by using antibodies to MyHC The asterisk marks the position of identical muscle fibers in serial cross-sections (i) Type 1 muscle fiber (a) Type 2A muscle fiber (b) Type 2B ⁄ 2D muscle fiber Scale bar in (D) is 100 lm (A) ATPase staining at pH 4.6 identifies type 1 fibers (dark stain), type 2A fibers (light stain), and type 2B ⁄ 2D fibers (intermediate stain) (B) ATPase staining at pH 9.4 identifies type 1 fibers (light stain), and type 2 fibers (dark stain) (C) Immunochemical staining using antibody to MyHC1 (MY-32 at a 1 : 1000 dilution) identifies type 1 fibers (D) Immunochemical staining using antibody to MyHC2 (NOQ7.5.4.D at a 1 : 200 dilution) identifies type 2 fibers.
Trang 3Next, we analyzed SOL muscle from AChRe) ⁄ )
mice, and representative cross-sections are shown in
Fig 3A (P20) and Fig 3B (P58) At P20, muscle
type 1, 2A and 2B⁄ 2D fibers displayed a similar
com-position as in wild-type muscle at P15, suggesting that
postnatal differentiation in AChRe) ⁄ ) mice may be
delayed in comparison to wild-type mice Furthermore,
fiber types had not reached constant levels at P20 and
the profile displayed a moderate but steady
‘fast-to-slow’ transition throughout postnatal development
Until P60, the type 1 fiber level had increased by 10%,
whereas type 2A fibers decreased by 17% In addition,
there was a 7% increase in type 2B⁄ 2D fibers
(Fig 3C) The values shown in Fig 3D demonstrate
the significance of the observed changes
AChRe) ⁄ )mice develop severe muscle weakness and
muscle atrophy during postnatal age, which might
affect fiber number and⁄ or reduce muscle mass
There-fore, we not only followed myofiber type transitions,
but also counted the total number of muscle fibers and determined the CSAs in SOL mucle of AChRe) ⁄ )mice (Fig 4A–C) In spite of progressive muscle weakness and the observed fast-to-slow fiber type transition, the total number of fibers (Fig 4A,B) was comparable to that in wild-type SOL muscle (Fig 2F), and the CSAs increased until P60 (Fig 4A,C), as observed in wild-type mice (Fig 2G)
Muscle fiber types in SOL and EDL muscle from wild-type and AChRe) ⁄ )mice
To confirm the observed fast-to-slow shift in fiber type composition in SOL muscle from AChRe) ⁄ )and wild-type mice, we compared both muscles directly under identical experimental conditions In SOL muscle from AChRe) ⁄ ) mice, numbers of type 1 and 2B⁄ 2D fibers increased by 6%, whereas those of type 2A fibers decreased by about 12% (Fig 5A,C,E) The total fiber
A
D
E
G F
Postnatal days (P)
//
Type 1
Total fibers
Type 2a
C
B
Postnatal days (P)
Type 2b/2d
Postnatal days (P)
Type 2a Type 2b2d
80 60 40 20 0
Fig 2 Fiber type composition in SOL muscle of wild-type mice at increasing postnatal age Cross-sections of SOL at (A) P15, (B) P20, (C) P60, and (D) P85 (E) Developmentally regulated changes in fiber type composition in percent of total fibers (type 1 fibers, white bars; type 2A fibers, gray bars; type 2B ⁄ 2D fibers, black bars) Original values are given in the table below the diagram (F) Number of fiber types increases during postnatal development Data were collected from three different animals using cross-sections as indicated in the table (n) (G) CSAs of fiber types increase throughout postnatal development In each case, seven separate cross-sections were used to determine the CSA of 50–70 fibers ATPase staining, pH 4.6 Scale bar in (D), 100 lm.
Trang 4number and the CSAs were, as noted before,
compara-ble to those in wild-type SOL muscle (Tacompara-ble 1)
This raised the question of whether similar changes
were also induced in fast-twitch muscles, which differ
in their contractile properties and in their MyHC
expression profile from slow-twitch muscle Wild-type
EDL muscles have predominantly type 2A fibers mixed
with type 2D⁄ 2B fibers and a few type 1 fibers The
direct comparison with EDL muscles from AChRe) ⁄ )
mice showed a 6% increase in the proportion of type 1
fibers, which was similar to the increase observed in
slow-twitch muscle In contrast to SOL muscle, there
was a small increase of about 3% in type 2A fast
fibers, possibly at the expense of type 2B⁄ 2D fibers,
which decreased by 10% (Fig 5B,D,F) The CSAs of
type 2A and 2B⁄ 2D fibers in EDL muscle from
AChRe) ⁄ )mice were smaller than in wild-type muscle,
whereas type 1 fibers showed no significant difference
(Table 1) The total number of fibers was reduced in
EDL muscle of AChRe) ⁄ )mice in comparison to that
of wild-type mice (Table 1)
Direct comparison of the fiber type composition in EDL and SOL muscles from wild-type mice distin-guishes EDL muscle clearly as fast muscle, in that type 1 fibers are expressed in much lower numbers than
in SOL muscle, whereas type 2B⁄ 2D fibers are expressed much more abundantly (Fig 5G) The profile for EDL muscle versus SOL muscle in AChRe) ⁄ )mice still identifies EDL muscle as fast muscle in comparison
to SOL muscle However, the increased number of type 1 fibers and the reduced number of type 2B⁄ 2D fibers clearly reflects the fast-to-slow shift in fiber com-position in EDL muscle of AChRe) ⁄ )mice
Transcript levels in muscles from AChRe) ⁄ )and wild-type mice
The changing fiber type compositions led to the ques-tion of whether differences in MyHC protein profiles
in SOL and EDL muscles were reflected by changes in the transcript levels of the corresponding MyHC genes
In addition, we wanted to investigate whether these
‘AChR-mediated’ signals that change muscle fiber types cause changes in the expression of synaptically expressed genes We therefore selected, besides the
‘myogenic’ genes, several ‘synaptogenic’ genes that contribute directly or indirectly to synapse formation and⁄ or function and determined their respective mRNA expression levels
Comparing myogenic transcripts in SOL muscle of AChRe) ⁄ ) and wild-type mice (Fig 6A), we observed increased levels of MyHC1 and MyHC2A, whereas levels of MyHC2B and MyHC2D were decreased We also measured troponin I transcripts, as their fiber type-specific expression depends on ‘slow’ and ‘fast’ innervation [16] In accordance with a fast-to-slow transition, an increase was observed for troponin Islow, whereas troponin Ifast appeared to be unaffected
Ca2+-dependent calcineurin⁄ nuclear factor of activated
T cells (NFAT) signaling is also thought to contribute
to muscle activity-regulated fiber transformations [17] Therefore, we determined the transcript levels of the transcription factors NFATc1 and NFATc4, but observed no significant changes Synaptogenic tran-script levels (Fig 6B) were elevated for AChR a-sub-units, muscle, skeletal, receptor tyrosine kinase (MuSK) and utrophin (Utrn) transcripts, and were not significantly different (changes‡ 2-fold or £ 2-fold) for AChR c-subunit, dystrophin, rapsyn, growth-associ-ated binding protein (GABP)a, GABPb, dishevelled (Dvl1), and sodium channel (Scn4a) In AChRe) ⁄ ) mice, AChR e-subunit transcripts were not detected using primers recognizing sequences of exon 8 that had been deleted in AChRe) ⁄ ) mice AChRe-subunit
A
C
D
B
20 60 0
20
40
60
20 60 20 60 Postnatal days (P)
Fig 3 Fiber type composition in SOL muscle of AChRe) ⁄ )mice at
increasing postnatal age Cross-sections of SOL muscle from
AChRe) ⁄ )mice are shown, (A) at P20 and (B) at P58 ATPase
stain-ing, pH 4.6 Scale bar below (B), 100 lm (C) Columns represent
percentage of muscle fibers in SOL muscle (% of total) at P20 and
P60, respectively, for type 1, 2A and 2B ⁄ 2D fibers, as indicated.
(D) Original values were collected from 18–72 cross-sections (n).
For mice AChRe) ⁄ ) at P20, three different animals were used to
generate the cross-sections of SOL muscle The P60 values of
AChRe) ⁄ )mice were from four different animals, ranging between
50 and 60 days in age.
Trang 5transcripts, however, were identified using primers that
recognize 5¢-upstream sequences of exon 2 With these
primers, we observed that the transcriptional activities
of the e-subunit genes were similar in AChRe) ⁄ ) and
in wild-type muscle
Comparing EDL muscle of AChRe) ⁄ ) mice and
wild-type mice (Fig 6C), we found that expression of
myogenic gene transcripts was strongly increased for
MyHC1 and moderately increased for MyHC2A,
whereas no significant changes were observed for
MyHC2B and MyHC2D, reflecting the fast-to-slow
fiber shift Troponin Islow was clearly increased and
troponin Ifast was also elevated in this muscle Again,
no significant changes were seen for NFATc1 and
NFATc4 transcripts Synaptogenic transcripts
(Fig 6D) of AChR a-subunits were increased, whereas
AChR e-subunit transcripts were reduced and AChR
c-subunits were not significantly changed Rapsyn and
Utrn also appeared to be increased No significant
changes were observed for dystrophin, MuSK,
GABPa, GABPb, Dvl1, and Scn4a In Fig 6A,C,
arrows indicate increased or reduced expression of MyHC type 1, 2A and 2B⁄ 2D fibers A correlation with changes in the corresponding transcripts was seen only for MyHC1 in SOL and EDL muscle and for MyHC2A in EDL muscle The other transcript levels did not match fiber type expression
Differential expression of selected ‘myogenic’ and ‘synaptogenic’ transcripts in SOL and EDL muscle
Comparison of transcript levels in SOL and EDL muscle of wild-type mice and of AChRe) ⁄ )mice could reveal differences between slow and fast muscles and thus indicate whether altered AChR function would change the expression of myogenic and⁄ or synaptogenic transcripts In EDL muscle of wild-type mice, MyHC1 transcripts were strongly reduced and MyHC2B tran-scripts were strongly increased as compared to SOL muscle MyHC2A and MyHC2D transcripts showed
no significant difference Troponin Islow clearly stood
Fiber type 1
A
number
n
*(P < 0.05), **(P < 0.001)
20Postnatal days (P) 60 20 Postnatal days (P) 60
2)
Total fibers
Type 2A
Type 2A
Type 1
Type 1
Type 2B/2D
Type 2B/2D
Fig 4 Number and CSAs of muscle fibers in SOL muscle from AChRe) ⁄ )mice during postnatal development (A) Numbers of total fibers, type 1, 2A and 2B ⁄ 2D fibers, and CSAs, are shown for SOL muscle from AChRe) ⁄ )mice at P20 (18 cross-sections from three different ani-mals) and at P60 (72 cross-sections from four different animals between 50 and 60 days old) Fiber types were determined by ATPase stain-ing, pH 4.6 (B) Number of total fibers, type 1, 2A and 2B ⁄ 2D fibers, in SOL muscle at P20 and P60 from AChRe) ⁄ )mice are plotted as mean values ± SEM Arrows illustrate increase ⁄ decrease of fiber type numbers as indicated (C) CSAs (lm 2
± SEM) of type 1, 2A and 2B ⁄ 2D fibers in SOL muscle of AChRe) ⁄ )mice at P20 and P60 are plotted In each case, seven separate cross-sections were used to deter-mine the CSA of 50–70 fibers Arrows show that the CSA increases between P20 and P60.
Trang 6out as a marker for fast-to-slow transition, and was
accordingly reduced in EDL muscle, whereas
tropo-nin Ifast was expressed at similar levels in SOL and
EDL muscle The NFATc1 and NFATc4 transcripts
showed no significant difference (Fig 7A) Comparing
transcript levels of synaptogenic genes in SOL and
EDL muscles of wild-type mice, we observed no
changes ‡ 2-fold or £ 2-fold for the AChR e-subunit,
Dvl1, Utrn, and Scn4a transcripts Slightly reduced transcript levels were observed for the AChR a-subunit, rapsyn, dystrophin, GABPa and GABPb transcripts (Fig 7B) AChR c-subunit and MuSK transcripts were significantly reduced in EDL muscle The myogenic and synaptogenic transcript profiles
of SOL and EDL muscle in AChRe) ⁄ ) mice still reflected muscle-specific differences between SOL and EDL muscles A closer look at individual transcript levels, however, showed that MyHC1 transcripts in EDL muscle of AChRe) ⁄ )mice were elevated in com-parison to wild-type EDL muscle (Fig 7B), in accor-dance with the fast-to-slow fiber type transition in AChRe) ⁄ ) mice (Fig 5G) An increase was also seen for MyHC2B transcript levels, which is explained by the fact that MyHC2B transcripts were downregulated
in SOL muscle but upregulated in EDL muscle of AChRe) ⁄ ) mice Further support for a fast-to-slow transition was the shift of troponin Islowto higher levels
in EDL muscle in AChRe) ⁄ ) mice The synaptogenic transcript levels displayed no significant shifts when SOL and EDL muscles of AChRe) ⁄ ) mice and SOL and EDL muscles of wild-type mice were compared
As in EDL muscle of wild-type mice, the transcripts of the AChR c-subunit as well as the MuSK gene were reduced to similarly low levels (Fig 7D)
Discussion
AChRe) ⁄ ) mice were employed to investigate whether functional properties of endplate AChRs affect the fiber type composition in muscle In AChRe) ⁄ ) mice, embryonic-type AChRc is not replaced by adult-type AChRe and is expressed throughout postnatal life [10,11] The results show a new quality in muscle plasticity: postnatal expression of AChR with pro-longed channel open time but reduced Ca2+ perme-ability and ion conductance stimulates transitions from fast to slow fiber types, both in SOL muscle and in EDL muscle The AChR-induced changes in
‘myogenic’ and ‘synaptogenic’ gene expression indi-cate that AChR-mediated postsynaptic signaling is linked to signal pathways that regulate fiber type composition
MyHC isoforms in SOL muscle of wild-type and AChRe) ⁄ )mice during postnatal development Adult patterns of MyHC isoforms are expressed in
a species-specific and muscle-specific manner within 3–4 weeks after birth, and fiber type transitions depend
on neuronal, mechanical and ⁄ or hormonal signals [14,18] In agreement with a previous report [15], we
C
D
Fig 5 Comparison of fiber type composition in muscle sections
from SOL and EDL muscle of wild-type and AChRe) ⁄ )mice
Cross-sections of (A) SOL muscle and (B) EDL muscle of wild-type mice
at P75, and (C) SOL muscle and (D) EDL muscle of AChRe) ⁄ )mice
at P60 Cross-sections (10 lm) of three or four different animals
were subjected to ATPase staining, pH 4.6 (i) Type 1 fiber.
(a) Type 2A fiber (b) Type 2B ⁄ 2D fiber Scale bar in (D) is 100 lm.
(E) Fiber type composition in SOL muscle of wild-type mice (white
columns; 100 cross-sections) compared with fiber type composition
in AChRe) ⁄ ) mice (gray columns; 72 cross-sections) Columns
represent type 1, 2A and 2B ⁄ 2D fibers (% of total) (F) Fiber type
composition in EDL muscle of wild-type mice (white columns;
12 cross-sections) compared with fiber type composition in
AChRe) ⁄ )mice (gray columns; 12 cross-sections) Columns
repre-sent type 1, 2A and 2B⁄ 2D fibers (% of total) (G) Comparison of
fiber type composition of EDL muscle versus SOL muscle in
wild-type mice (white columns) and AChRe) ⁄ ) mice (gray columns).
EDL values were normalized to SOL values (fiber type EDL ⁄ fiber
type SOL) and plotted on a logarithmic scale The EDL⁄ SOL profile
of AChRe) ⁄ ) mice is similar to the wild-type profile, but type 1
fibers are increased, whereas type 2B ⁄ 2D fibers are reduced.
Trang 7observed constant expression levels of slow and fast
fibers in SOL muscle of wild-type mice (C57Bl⁄ 6) well
after P20 The early fiber type transitions occur during
a time when AChRc channels are replaced by AChRe
channels [6], suggesting that there is a link between
AChR conversion and fiber type transition In fact, it
has been reported that the c-to-e subunit transition is
delayed in slow-twitch muscle as compared to
fast-twitch muscles [19] The continuous fast-to-slow
speci-fication up to P60 in SOL muscle of AChRe) ⁄ ) mice
may thus be due to the lack of AChRe channels and
the persistence of AChRc channels More experiments
are now required to clarify whether the AChRc to
AChRe channel conversion affects early postnatal
MyHC isoform transitions
Fast-to-slow transition – correlation of muscle
fiber type and gene transcript levels in SOL and
EDL muscles from wild-type and AChRe) ⁄ )mice
The altered functional property⁄ density of the endplate
AChR stimulates, in SOL and EDL muscles of
AChRe) ⁄ ) mice, transitions from fast to slow fiber
types, as demonstrated by increased numbers of type 1
fibers The increase in type 1 fibers correlates with an
increase in MyHC1 transcript level Changes in
MyHC2A and MyHC2B⁄ 2D transcript levels,
how-ever, and changes in protein levels do not match
Dif-ferences in transcript and protein levels have been
attributed to translational or post-translational
pro-cessing events or expression of hybrid fibers in single
muscle fibers [20,21]
A reliable marker for AChR-mediated fast-to-slow
transition is troponin Islow Troponin I is the
regula-tory component of the troponin complex and probably
influences the rate of force generation and relaxation
during twitch [22] Troponin Islow and troponin Ifast
levels are regulated by electrical activity in a fiber-type-specific manner [16,23,24] Increased troponin Islow transcripts in muscle of AChRe) ⁄ ) mice suggest that the AChR-mediated signals that cause a fast-to-slow MyHC transition lead to an adaption of tropo-nin Islow Troponin Ifast, however, is not changed in a reciprocal manner, indicating that troponin Islow and troponin Ifast respond independently to distinct fast and slow signaling pathways [24]
The analyzed synaptogenic transcripts in SOL and EDL muscles of AChRe) ⁄ )mice are affected in a co-ordinated fashion, in that transcripts are moderately elevated or not significantly altered (changes were con-sidered significant only for values‡ 2-fold or £ 2-fold)
A clear increase is observed for AChR a-subunit and for MuSK (in SOL muscle) and rapsyn (in EDL mus-cle) transcripts As AChR a-subunit as well as MuSK transcripts respond to changes in muscle activity, the increase could be a compensatory reaction to progress-ing AChR deficiency On the other hand, fast-to-slow transitions induced by electrical stimulation have led
to an increase of postsynaptic AChR [25], suggesting that the myogenic and synaptogenic signaling path-ways are linked An exception to coordinated regula-tion is that the e-subunit transcripts appear to be significantly reduced in EDL muscle
The increased Utrn transcript levels provide further support for a fast-to-slow transition in SOL and EDL muscles of AChRe) ⁄ ) mice Slow fiber type specifica-tion is sensitive to nerve activity-induced intracellular
Ca2+[26], which regulates calcineurin⁄ NFAT signaling [17,27], and calcineurin⁄ NFAT signaling regulates the transcript levels of Utrn [28,29] The mRNAs of the transcription factors NFATc1 and NFATc4 are not altered dramatically Similarly, the transcription fac-tors GABPa and GABPb, which have been suggested
to contribute to synapse-specific gene expression
Table 1 The fiber number and CSAs of SOL and EDL muscles from wild-type and AChRe) ⁄ )mice Fiber type numbers and CSAs of muscle fibers were determined using cross-sections (Fig 5) of SOL and EDL muscles of wild-type (P68–P80) and AChRe) ⁄ )(P49–P57) mice CSAs (mean ± SEM) of fibers were measured at middle regions of each muscle In each case, seven separate cross-sections were used to deter-mine the CSA of 50–70 fibers Values with P < 0.05 were considered to be statistically significant ATPase staining, pH 4.6 WT, wild-type.
n
Total number
SOL
EDL
*P < 0.05; **P < 0.001.
Trang 8[30,31], are not significantly different between muscle
of wild-type and AChRe) ⁄ ) mice These results,
how-ever, do not exclude functional roles of these factors in
muscle fiber development and fiber type transitions as
observed here
Differential analysis of SOL and EDL muscles
The differential fiber type profile of SOL and EDL
muscles highlights similar muscle-specific differences
both in wild-type and in AChRe) ⁄ ) mice The
differ-ence between EDL and SOL muscles is less pronounced
in AChRe) ⁄ ) mice, because of the fast-to-slow shift, which causes a relative increase of type 1 fibers and a decrease of type 2B⁄ 2D fibers in EDL muscle Corre-sponding shifts of MyHC1 transcripts in differential SOL⁄ EDL transcript profiles as well as the increased troponin Islow transcript levels indicate that SOL and EDL muscles adjust to the altered AChR-mediated sig-naling in a muscle-specific manner The synaptogenic transcripts, on the other hand, display no significant differences between wild-type and AChRe) ⁄ ) SOL and
Fig 6 Gene expression of myogenic and synaptogenic genes in SOL and EDL muscles from AChRe) ⁄ )mice in relation to wild-type mice Transcript expression profiles in SOL and EDL muscles were quantified by real-time PCR The mean values (mean ± SEM) of SOL and EDL muscles from AChRe) ⁄ )mice were normalized to the values from wild-type SOL and EDL muscles, respectively, and are represented by gray bars Wild-type SOL and EDL muscle transcript levels are 1.0 (± SEM) Values were obtained by analyzing muscle from six different ani-mals Values with P < 0.05 were considered to be statistically significant The specific primers for the selected genes are listed in Table 2 (A, B) Relative expression levels of (A) myogenic and (B) synaptogenic transcripts in SOL muscle from AChRe) ⁄ )mice are compared to tran-script levels in SOL muscle from wild-type mice (C, D) Relative expression levels of (C) myogenic and (D) synaptogenic trantran-scripts in EDL muscle from AChRe) ⁄ )mice are compared to transcript levels in EDL muscle from wild-type mice Inserts in (A) and (C) indicate changes at the protein level for type 1, type 2A and type 2B⁄ 2D fibers Increase ⁄ decrease of fiber types in muscle from AChRe) ⁄ )mice as compared
to muscle from wild-type mice is schematically indicated by arrows.
Trang 9EDL muscles Generally, it appears that transcript
lev-els are reduced in EDL muscle, most prominently for
AChR c-subunit and MuSK transcripts In a previous
study of rat muscle, the majority of cytoskeletal
pro-teins were also found to be reduced in EDL muscle as
compared to SOL muscle [32] Comparison of fast and
slow muscle shows that MuSK, as a major synaptic
regulator [33–35], is expressed at significantly lower
lev-els in EDL muscle than in SOL muscle, and may
coor-dinate differentiation of the postsynaptic apparatus
differently in fast and slow muscle Compensatory
changes in MuSK expression levels as a consequence of
altered AChR function⁄ density are apparently similar
in SOL and EDL muscles The overall conservation of
muscle-specific expression patterns of myogenic and
synaptogenic transcripts reveals that AChR-induced changes affect contractile profiles of muscles in a coor-dinated fashion
Muscle fiber type and AChR function Fast-to-slow transitions are induced by enhanced neu-romuscular activity, e.g by chronic low-frequency stimulation [1], as well as by prolonged exercise [36], whole body exercise training [37,38] and hyperactivity
of rats [39] The mechanisms that cause these adaptive changes in wild-type muscle to increased muscle activ-ity seem unlikely to compensate for progressive muscle weakness in AChRe) ⁄ ) mice On the other hand, denervation, limb immobilization and unloading or
Table 2 List of TaqMan assay-on-demand products and our designed primer and probe set for quantitative real-time PCR.
dehydrogenase
Mm00431627_m1 FAM TTCTCTATAACAACGCAGACGGCGA AChRa Cholinergic receptor, nicotinic,
alpha polypeptide 1 (muscle)
Mm00437417_m1 FAM TGGAGAACAATGTGGACGGTGTCTT AChRc Cholinergic receptor, nicotinic,
gamma polypeptide
Mm00437411_m1 FAM ACTGCTGGGCAGGTATCTTATATTC AChRe Cholinergic receptor, nicotinic,
epsilon polypeptide
Custom designed FAM ATCTCACTGAACGAGAAAGAAGAAA AChRe2 Cholinergic receptor, nicotinic,
epsilon polypeptide
(Drosophila)
tyrosine kinase
Mm00485539_m1 FAM CTACGCCCAGGTCAAGGACTATGAG Rapsyn Receptor-associated protein of
the synapse, 43 kDa
cardiac muscle, beta
Mm00454991_m1 FAM CTGAGATCGACAGGAAGCCCGCAAT MyHC2A Myosin, heavy polypeptide 2,
skeletal muscle, adult
Mm01332531_g1 FAM GAATGCTGAAGGACACACAGCTGCA MyHC2B Myosin, heavy polypeptide 4,
skeletal muscle, adult
Mm01332500_gH FAM ACTTATCAAACTGAGGAAGACCGCA MyHC2D Myosin, heavy polypeptide 1,
skeletal muscle, adult
Mm00479445_m1 FAM GCAAGCCAAATTCCCTGGTGGTTGA Nfatc1 Nuclear factor of activated T cells,
cytoplasmic, calcineurin-dependent 1
Mm00452375_m1 FAM GGGTCCTGATGGAAAACTGCAGTGG Nfatc4 Nuclear factor of activated T cells,
cytoplasmic, calcineurin-dependent 4
Mm00500103_m1 FAM TATGGAGGAGCTGGAAGAGGCCCAT Scn4a Sodium channel, voltage-gated,
type IV, alpha polypeptide
Trang 10spinal cord transection all cause atrophy of slow and
fast extensor muscles The characteristic feature is the
transformation of muscle fibers from slow to fast [37]
Progressive muscle weakness and atrophy are
charac-teristic symptoms of the AChRe) ⁄ ) mice, and thus
would be expected to reduce the proportion of slow
fibers The overall fast-to-slow transition therefore
indicates that altered functional properties of the
end-plate AChRs mediate signals that dominate and
over-rule possible denervation⁄ atrophy-induced changes
It is not clear how changes in neuromuscular activity
or motor activity regulate transcriptional control
mechanisms of MyHC expression Major regulatory
signals are attributed to action potentials that are
transmitted to the fibers and change intracellular Ca2+
concentrations [40] Embryonic and adult-type AChRs
evoke different membrane potentials, which could affect the signal cascades regulating myofiber transfor-mation differently The AChR subtypes have, in addi-tion, different Ca2+ permeabilities, and thus could modulate the subsynaptic Ca2+ levels The challenge now is to determine whether and how the spatially restricted synaptic Ca2+ signals could be transmitted
to act more globally to alter myofiber expression
In wild-type mice, the functional change of AChRc to AChRe may fine-tune the postnatal expression of fiber type composition, e.g by increasing the inherent motor activity mediated by the higher ion conductance of AChRe In AChRe) ⁄ )mice, lack of AChRe may delay early postnatal fiber type transitions, and the persis-tence of AChRc reduces, rather than increases, motor activity At the same time, synaptic Ca2+ levels may
Fig 7 Gene expression of myogenic and synaptogenic genes in EDL muscle in relation to SOL muscle from wild-type and AChRe) ⁄ ) mice The transcript expression profile was quantified by real-time PCR The mean values (mean ± SEM) of EDL muscle from wild-type and AChRe) ⁄ ) mice were normalized to corresponding values from SOL muscle and are represented by gray bars SOL transcript levels are 1.0 (± SEM) The values were obtained by analyzing muscle from six different animals; values with P < 0.05 were considered to be statistically significant The specific primers for the selected genes are listed in Table 2 (A, B) Relative expression levels of (A) myogenic and (B) synaptogenic transcripts in EDL muscle from wild-type mice (C, D) Relative expression levels of (C) myogenic and (D) synapto-genic transcripts in EDL muscle from AChRe) ⁄ )mice The values are presented on a logarithmic scale and show the relative upregulation
or downregulation.