Cardiovascular Research (2013) 98, 269–276
doi:10.1093/cvr/cvt030
Ultrastructural uncoupling between T-tubules and
sarcoplasmic reticulum in human heart failure
Hai-Bo Zhang1†, Rong-Chang Li2†, Ming Xu3†, Shi-Ming Xu2, Ying-Si Lai2, Hao-Di Wu2,
Xian-Jin Xie4, Wei Gao3, Haihong Ye5, You-Yi Zhang3, Xu Meng1*, and Shi-Qiang Wang2*
1
Beijing Anzhen Hospital, Capital Medical University, Beijing 100092, China; 2State Key Laboratory of Biomembrane and Membrane Biotechnology, College of Life Sciences, Peking
University, Beijing 100871, China; 3Key Laboratory of Cardiovascular Molecular Biology and Regulatory Peptides, Key Laboratory of Molecular Cardiovascular Sciences, Third Hospital
of Peking University, Beijing 1000191, China; 4Department of Clinical Sciences and Simmons Cancer Center, University of Texas Southwestern Medical Center, Dallas TX 75390, USA;
and 5Department of Medical Genetics, Capital Medical University, Beijing 100069, China
Received 28 November 2012; revised 5 February 2013; accepted 6 February 2013; online publish-ahead-of-print 11 February 2013
Time for primary review: 65 days
Aims
Chronic heart failure is a complex clinical syndrome with impaired myocardial contractility. In failing cardiomyocytes,
decreased signalling efficiency between the L-type Ca2+ channels (LCCs) in the plasma membrane (including transverse tubules, TTs) and the ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) underlies the defective
excitation –contraction (E– C) coupling. It is therefore intriguing to know how the LCC –RyR signalling apparatus
is remodelled in human heart failure.
.....................................................................................................................................................................................
Methods
Stereological analysis of transmission electron microscopic images showed that the volume densities and the surface
and results
areas of TTs and junctional SRs were both decreased in heart failure specimens of dilated cardiomyopathy (DCM)
and ischaemic cardiomyopathy (ICM). The TT– SR junctions were reduced by 60%, with the remaining displaced
from the Z-line areas. Moreover, the spatial span of individual TT –SR junctions was reduced by 17% in both DCM
and ICM tissues. In accordance with these remodelling, junctophilin-2 (JP2), a structural protein anchoring SRs to TTs,
was down-regulated, and miR-24, a microRNA that suppresses JP2 expression, was up-regulated in both heart failure
tissues.
.....................................................................................................................................................................................
Conclusion
Human heart failure of distinct causes shared similar physical uncoupling between TTs and SRs, which appeared
attributable to the reduced expression of JP2 and increased expression of miR-24. Therapeutic strategy against
JP2 down-regulation would be expected to protect patients from cardiac E–C uncoupling.
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----------------------------------------------------------------------------Keywords
Heart failure † Myocardial remodelling † Ca
1. Introduction
Heart failure is the final stage of cardiac remodelling caused by hypertension, ischaemia, cardiomyopathy, and other chronic factors.1 Although numerous cellular and molecular changes have been
identified in failing hearts,1 – 6 a key symptom that threatens patients’
lives is the decreased myocardial contractility.7 The contraction of
the heart is initiated via the excitation –contraction (E–C) coupling
process, in which voltage-gated L-type Ca2+ channels (LCCs) in the
plasma membrane and transverse tubules (TTs) activate Ca2+
release from ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR) and initiate cell contraction.8 – 11 Ultrastructural studies
showed that the junctional SR meets the TT across a 15 nm cleft,
†
signalling † Excitation-contraction coupling
forming a Ca2+ signalling structure known as a dyad.12 Studies in
rodent models have shown that the volume densities of junctional
SRs and SR-coupled TTs were both decreased in failing heart
cells.13 The TT –SR junctions were displaced or missing from the
Z-line areas.13 Moreover, the spatial span of individual TT–SR junctions was markedly reduced in failing heart cells.13 The shrinkage
and eventual absence of TT –SR junctions have been proved to be important mechanisms underlying the desynchronized and inhomogeneous Ca2+ release and the decreased contractile strength in rodent
models.13
The maintenance of TT –SR dyadic junction is found to rely on a
family of proteins known as junctophilin.14 Junctophilin has multiple
membrane recognition nexus motifs and an SR transmembrane
These authors contributed equally.
* Corresponding author. Tel: +86 10 62755002; fax: +1 501 634 3556, Email: [email protected] (S.-Q.W.); [email protected] (X.M.).
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2013. For permissions please email: [email protected].
270
region that anchors SR membranes to TTs across a 15 nm junctional cleft.12 Junctophilin-2 (JP2) is the cardiac-specific isoform of
junctophilin,15 and its expression is found decreased in rodent
models of failing heart cells.16 – 18 Knockdown of JP2 indeed reproduced the structural and functional remodelling of E–C coupling.13,18 – 20 Therefore, the defective signalling between LCCs and
RyRs is attributed to the down-regulation of JP2.
Most of the above studies have been based on rodent models of
hypertrophy and heart failure. In human heart failure, many studies
have shown profound ultrastructural remodelling in TTs.21 – 25
However, how the structure of LCC –RyR signalling microdomain is
changed in different types of human heart failure and whether the remodelling comes with altered JP2 expression still call for in-depth investigation. In the present study, we utilized the transmission electron
microscopy (TEM) technology to characterize the ultrastructural
modification of TT– SR junctions in heart failure specimens of
dilated cardiomyopathy (DCM) and ischaemic cardiomyopathy
(ICM). We found that human heart failure of distinct causes shared
quite similar remodelling in TT –SR junctional structure and downregulation of JP2 expression.
2. Methods
2.1 Human specimens
Studies involving human tissues complied with the Helsinki Declaration of
the World Medical Association and were approved by Peking University
Ethical Review Board (IRB00001052-08034). For ultrastructural and biochemical analysis, left-ventricular wall samples were obtained from
explanted end-stage failing hearts from transplant recipients (eight DCM
patients of 41.1 + 5.3 years old and five ICM patients of 56.4 + 4.9
years old). Small pieces of left ventricle tissues from four adults of accidental traumatic brain-death (35.0 + 4.6 years old), who had no medical evidence of cardiac disease, were used as reference.
H.-B. Zhang et al.
miR-24 quantification, the first strand cDNA was first synthesized by
microRNA-specific reverse-transcription primers (RiboBio Co., Ltd). To
assess the expression of JP2 mRNA or miR-24, 10 ng cDNA product
was used for real-time PCR amplification using Brilliant II SYBR Green
QPCR master mix (Stratagene), and the fluorescent signals were monitored by an Mx3000p real-time PCR system (Stratagene). The thermocycling program was set as: 958C for 10 min, followed by 40 cycles at
958C for 15 s, 608C for 30 s, and 728C for 30 s, and finally an additional
dissociation step to ensure the specificity of amplification. The primers for
microRNA sample amplification were provided by RiboBio Co., Ltd, and
the primers for JP2 and GAPDH were: Human JP2 (forward: 5′ -ATG
GGC TGG GCA TAG AGA C-3′ ; reverse: 5′ -TTG AAG CCA TGT
GTC CAC TC-3′ ); and Human GAPDH (forward: 5′ -AGC TGA ACG
GGA AGC TCA CT-3′ ; reverse: 5′ -TAG GTC CAC CAC TGA CAC
GTT G-3′ ). GAPDH was used as control for JP2 mRNA quantification,
and the small nuclear RNA U6 was used as control for microRNA
quantification.
2.4 Western blot
Heart tissues from human left ventricles were homogenized in lysis buffer
containing 1% sodium deoxycholate, 10 mM Na4P2O7, 1% Triton-100,
10% glycerol, 150 mM NaCl, 5 mM EDTA.Na2, 20 mM Tris (pH 7.4),
0.1% SDS, 50 mM NaF, 1 mM Na3VO4, 1 mM PMSF, and protease
inhibitor cocktail (Roche). The sample lysate was separated on 10%
SDS – PAGE and then transferred to PVDF membrane. Membranes
were incubated with self-made rabbit polyclonal antibody against JP2
(1 mg/ml), which specifically recognized the JP2 p434-p447 peptide (QEILENSESLLEPR). A horseradish peroxidase-conjugated GAPDH antibody
(KangChen Bio-tech Inc., China) was used to measure the GAPDH
content as loading control.
2.5 Statistical analysis
Results are expressed as mean + SE. Statistical analysis was performed,
where appropriate, using Student’s t-test and the Mann– Whitney rank
sum test. Multiple comparisons were adjusted using Bonferroni correction. A value of P , 0.05 was considered significant.
2.2 TEM and stereological measurement
Heart tissues were first fixed in 2.5% glutaraldehyde and 1% paraformaldehyde in 0.1 M PBS buffer (pH 7.4). After rinsing three times with 0.1 M
PBS buffer, the samples were post-fixed in a mixture of 0.8% potassium
ferrocyanide and 2% osmium tetroxide in 0.1 M sodium cacodylate
buffer for 2 h. After dehydration in a graded series of acetone, the
samples were embedded in Spurr resin and sectioned with a glass knife
on a Leica Ultracut R cutter. Thin sections were stained with uranyl
acetate and lead citrate, then observed and randomly imaged under an
FEI Tecnai G2 20 Twin system. For stereological measurement of the
volume densities and surface areas of TTs and JSRs, we followed
Mobley’s stereological method.26 In brief, the volume density was calculated as the percentage of interested volume component (Vi) in all the
tested volume (Vtest) by Vi /Vtest = Pi /Ptest , where Pi is the number of
grid points that fell within the component of interest, and Ptest is the
total number of grid points in tested areas. The surface area of interest
(Si) in a tested volume (Vtest) was calculated as Si /Vtest = Ci /(d · Ptest ),
where Ci is the number of intersections between the grid and interested
membrane structures and d is the distance between parallel lines in the
grid.
2.3 JP2 mRNA and miR-24 expression assays
Total RNA was extracted from cardiac tissue using Trizol reagent (Invitrogen) according to the manufacturer’s instructions. For reverse transcription of total RNA, 2 mg total RNA was added to a 50 ml reaction
system containing oligo-dT(15), dNTP, RNAse OUT, and Superscript III
reverse transcriptase (Invitrogen), and incubated for 1 h at 508C. For
3. Results
Transmission electron microscopic (TEM) images were taken randomly to examine the TT–SR dyadic junctions (white squares and
inserts in Figure 1A) in left-ventricle specimens from DCM and ICM
heart failure patients during transplantation surgery and from agematched ostensibly healthy adults after traumatic brain-death
(Control). We found that the number of TTs per unit area
(Figure 1B) and the percentage of TTs coupled with SRs (Figure 1C)
were significantly lower in both DCM and ICM groups compared
with the Control group. These two factors led to a .60% decrease
in the number of TT– SR junctions per unit area in both heart failure
groups (Figure 1D).
To quantify the morphological changes objectively and accurately,
we used classic stereological analysis26 to measure volume densities
and surface areas of coupling structure between TTs and SRs
(Figure 2A). Compared with that in the Control group, the volume
density of TTs was reduced by 40% in DCM and ICM heart
tissues (Figure 2B). This reduction reflected the detachment
between TTs and SRs, because the volume density of SR-coupled
TTs was similarly reduced in both DCM and ICM groups while that
of TTs apparently not coupled to SRs was unchanged or even
increased. As a result, the volume density of junctional SRs was
reduced by 45.9 and 50.6%, respectively, in DCM and ICM groups.
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Ultrastructural remodelling in human heart failure
Figure 1 Ultrastructural remodelling of TT – SR junctions in human heart failure. (A) Typical TEM images showing TTs (white arrows) in myocardium from healthy Controls (left) and heart failure patients (DCM, right). The TT – SR dyadic junctions marked by the white squares are shown in the
inserts, which illustrate that the junctional SR was tightly attached to the TT in the Control group (left insert) but not in the DCM group (right insert).
(B – D) The density of TTs (B), the percentage of TTs coupled with SR (C) and the density of TT-SR junctions (D) were compared among myocardium
from four Controls of 35.0 + 4.6 years old (white), eight DCM patients of 41.1 + 5.3 years old (grey), and five ICM patients of 56.4 + 4.9 years old
(black). Data from 135, 386, and 192 TEM images in Control, DCM, and ICM, respectively. More than 25 images were randomly captured for each
specimen, and more than 4000 mm2 of image area were analysed in each group. **P , 0.01 vs. Control group after Bonferroni correction.
Statistics of the surface area of TTs, SR-coupled TTs, and junctional
SRs showed similar pattern of changes (Figure 2C).
We noted that the TT–SR junctions, which usually appear regularly at
Z-lines in the Control group, were missing from many of the ‘due’ areas
in heart failure tissues (Figure 1A). To quantify this change, we measured
the presence rate of TTs and TT–SR junctions in the non-myofilament
areas between two adjacent M-lines (inter-M areas, Figure 3A). We
found that the percentage of inter-M areas that displayed TTs was dramatically decreased in both DCM and ICM groups (Figure 3B). As the
percentage of TTs that coupled with SRs was also decreased
(Figure 1C), the percentage of inter-M areas that displayed TT–SR junctions was reduced even more dramatically (Figure 3C).
For those inter-M areas that displayed TT– SR junctions, the positioning of junctions was also modified. The perpendicular distance
from the centre of a TT–SR junction to its adjacent Z-line (Figure 4A)
was significantly increased in both DCM and ICM groups (Figure 4B),
with their distributions shifted to longer distances (Figure 4C).
Our recent work showed that the spatial span of individual TT –SR
junctions is an important factor in determining the efficiency of E– C
coupling.13 Measurements of the apparent curvilinear length of parallel TT and SR membranes (yellow line in Figure 5A) showed that the
junction size was 17% smaller in DCM and ICM groups than in the
Control group (Figure 5B), with the peak of its distribution shifted to
smaller size (Figure 5C). The 17% decrease in junction length would
predict a 30% decrease in junction size or RyR number per
junction.13
Maintenance of the TT–SR dyadic junction relies on JP2.13,14,20
To determine whether JP2 down-regulation characterizes different
types of human heart failure, we assessed JP2 expression in
human left-ventricle specimens. Real-time PCR assays showed that
JP2 was indeed significantly down-regulated in both DCM and ICM
groups (Figure 6A). Western blot assays confirmed that JP2 was
down-regulated at the protein levels in both heart failure groups
(Figure 6B).
Recently, we found that JP2 is regulated by miR-24, a microRNA
up-regulated in rodent models of heart failure.27 In order to know
whether miR-24 is modulated in human heart failure, we analysed
the expression of miR-24. We found that microRNA-24 expression
was increased by approximately two-folds in DCM and ICM groups
(Figure 6C). Because in vivo suppression of miR-24 is able to prevent
the TT–SR remodelling and protect the E– C coupling performance,28 we believe that the up-regulation of miR-24 and downregulation of JP2 have mechanistic implications in the ultrastructural
and functional remodelling in human heart failure.
4. Discussion
TT–SR junction is the structural basis for Ca2+-induced Ca2+ release
and E –C coupling in cardiomyocytes.12 The LCC –RyR signalling
across the TT–SR junctional cleft determines the contractile strength
of the heart.17 Therefore, examining the detailed structural remodelling of the Ca2+ signalling apparatus is essential for elucidating the
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H.-B. Zhang et al.
Figure 2 Stereological analysis of TT – SR junctions in human heart failure. (A) Representative TEM image illustrating the stereological analysis of
myocardium. The grid lines were spaced 0.167 mm apart as suggested by Mobley and Eisenberg.26 The closed and open circles denote examples
of point counts for volume density and intersection counts for surface area per unit volume, respectively. (B and C) The volume density (B) and
the surface area per unit volume (C) of total TTs, TTs coupled with SRs, TTs apparently not coupled with SRs, and junctional SRs (JSRs) were compared among Control (white), DCM (grey), and ICM (black) groups. Data from 162, 221, and 134 TEM images in four Controls, eight DCM, and five
ICM patients, respectively. More than 2500 mm2 of image area were analysed in each group. *P , 0.05 and **P , 0.01 vs. Control group after
Bonferroni correction.
mechanisms underlying the defective E–C coupling in heart failure. In
the present study, using TEM morphometric analysis, we systematically quantified the ultrastructural remodelling of TT–SR junctions that
governs the LCC –RyR Ca2+ signalling in human failing myocardium
from DCM and ICM patients. We found that, in both DCM and
ICM failing myocardium: (i) the volume densities and surface areas
per unit volume of junctional SRs and SR-coupled TTs were dramatically decreased, agreeing well with the decreased densities of TTs and
TT–SR junctions per unit area; (ii) the TT–SR junctions were displaced or missing from many of the Z-line areas; (iii) the spatial
span of individual TT–SR junctions was markedly reduced; (iv) JP2 expression was down-regulated; and (v) miR-24, which suppresses JP2
expression,27 was up-regulated. Compared with similar studies in
the aorta-constrained rat heart failure model,13 human heart failure
comes with relatively moderate shrinkage in individual TT–SR junction size but much more reduction in total TT –SR junctions. The
identification of miR-24 up-regulation and JP2 down-regulation in different types of human heart failure provides mechanistic interpretation for the ultrastructural remodelling of the TT –SR junctions.
4.1 Morphological changes of TTs
in hypertrophy and heart failure
In 1970s, the application of stereological quantification of intracellular
membranous structures29,30 promoted the ultrastructural study of
cardiac myocytes during hypertrophy and heart failure. Anversa
et al.31 demonstrated in rabbit myocardium with experimental
advanced hypertrophy that the TT system was dilated. This phenomenon was then observed in other animal models of hypertrophy32,33
and heart failure.34 Later on, dilated TTs were also found in human
heart tissues of hypertrophic cardiomyopathy, congestive cardiomyopathy, dilated cardiomyopathy, ischaemic cardiomyopathy, aortic
valvular disease, and congenital heart diseases associated with obstruction to right-ventricular outflow.24 Therefore, dilation of TTs is
a common feature of cardiac ultrastructural remodelling during hypertrophy and heart failure.
However, despite the dilation of TTs, the number of TTs per
unit area is markedly decreased in many animal models and
human specimens of hypertrophy and heart failure.22,35 Therefore,
the change of total TT surface would depend on the dominance
Ultrastructural remodelling in human heart failure
273
Figure 3 Rates of presence of TTs and TT – SR junctions in human heart failure. (A) An enlargement of the TEM example image in Figure 1A from the
DCM group showing the M-lines (white dashed lines) and non-myofilament areas between two adjacent M-lines (inter-M areas). (B and C ) The rates
were measured as the percentage of non-myofilament areas between adjacent M-lines that displayed TTs (B) or TT-SR junctions (C), respectively,
among Control (white), DCM (grey) ,and ICM (black) groups. Data from 135, 386, and 192 TEM images in four Controls, eight DCM, and five
ICM patients, respectively. More than 25 images were randomly captured for each specimen, and more than 4000 mm2 of image area were analysed
in each group. **P , 0.01 vs. Control group after Bonferroni correction.
Figure 4 Positioning of TT – SR junctions in human heart failure. (A) Typical TEM image showing the measurement of junction-Z distance (black
double arrow) between the centre of a junction cleft and its adjacent Z-line (white dashed lines). (B and C ) Comparison of junction-Z distance
(B) and its distribution (C) among Control (white), DCM (grey), and ICM (black) groups. Data from 135, 386, and 192 TEM images in four Controls,
eight DCM, and five ICM patients, respectively. More than 25 images were randomly captured for each specimen, and more than 4000 mm2 of image
area were analysed in each group. **P , 0.01 vs. Control group after Bonferroni correction.
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H.-B. Zhang et al.
Figure 5 Measurement of individual TT – SR junction size in human heart failure. (A) Representative TEM image illustrating the measurement of
junction length. TT, SR, and junctional cleft were marked red, green, and yellow, respectively. The junction length was measured as the curvilinear
length of the yellow line. (B and C ) Comparison of junction length (B) and its distribution (C ) among Control (white), DCM (grey), and ICM
(black) groups. Data from 228, 174, and 116 junctions in four Controls, eight DCM, and five ICM patients, respectively. **P , 0.01 vs. Control
group after Bonferroni correction.
Figure 6 JP2 and miR-24 expression in human heart failure. (A) Real-time PCR assessment of JP2 mRNA expression levels among Control (white,
n ¼ 4), DCM (grey, n ¼ 8), and ICM (black, n ¼ 5) groups. (B) Representative image of western blotting of JP2 (upper) and measurement of JP2
protein expression (lower) among Control, DCM, and ICM groups. (C ) Real-time PCR measurement of miR-24 expression among Control, DCM,
and ICM groups. *P , 0.05 and **P , 0.01 vs. Control group after Bonferroni correction.
275
Ultrastructural remodelling in human heart failure
of these two counteracting factors. Indeed, it has been shown that
the volume and surface area of TTs are increased in animal models
of cardiomyopathy,36 spontaneous hypertension,37 thyroxinestimulated hypertrophy,38 and hypertension/hypertrophy induced
by constriction of aorta or arteries.39,40 Most of these models are
in the compensated stage. In late stage of pressure-overload hypertrophy or heart failure models, in contrast, the volume and surface
area of TTs are reduced.13 These ultrastructural results agree well
with recent optical imaging studies, in which significant loss of TT
density and disorganization of TT network were detected in a
variety of animal models and human specimens of heart
failure.6,18,21,23,35,41 – 46
In human heart failure, our present study showed that the volume
density and surface area of TTs were reduced by 40% in both DCM
and ICM specimens. This percentage of reduction was higher compared with that in failing heart cells of rat models.13 Given that TTs
are invaginations from cell membranes, the reduction of TTs may
reflect either a shortening or a ‘pluck-off’ of the invaginations.
A recent study using scanning ion conductance microscopy has
shown that the TT holes on the cell surface are significantly
reduced in human specimens of heart failure,45 indicating that many
of the TTs are plucked-off from the cell membrane, although shortening of TTs is still reasonably expected.
4.2 Remodelling of TT – SR junctions
In cardiac cells, SR meets TT with a 15 nm junctional cleft,
forming a dyadic structure enabling the local control of Ca2+induced Ca2+ release.12 Similar to the remodelling patterns of
TTs, the volume and surface area of SRs (or smooth endoplasmic
reticulum) in heart cells are increased in hypertrophy.22,36,40,47 It
seems that TTs and SRs adapt themselves to enlarged cell size
mainly by swelling during the compensated hypertrophy stage.
However, in the decompensated/failing stage, the TT surface area
is reduced, and the junctional SR is also reduced.13 Our present
study confirmed that the volume density and surface area are
reduced in human heart failure of different causes. Taken together,
the remodelling of TT and SR structure occurs as early as compensated hypertrophy. While the TT and SR surfaces are increased in
the compensated stages, they are decreased in the decompensated/
failing stage.
Although many studies have characterized TT and SR structure in
a variety of animal models and human specimens of heart failure,
few studies quantified the remodelling of individual TT–SR couplons.
Our recent work in rat heart failure models shows that the TT –SR
junctions are displaced or missing from the Z-line areas. Moreover,
the spatial span of individual TT–SR junctions is markedly reduced
in failing heart cells.13 In the present study, we extended the quantification of TT–SR junction to human specimens. Our results
showed that similar ultrastructural remodelling of TT–SR junctions
occurred in both DCM and ICM failing hearts. Compared with the
changes in rat failing cells, although the percentage of junction size
reduction is relatively moderate in human heart failure, the loss of
TTs at their ‘due’ positions was much more pronounced. The
shrinkage and eventual loss of TT– SR junctions are among important mechanisms underlying the desynchronized/inhomogeneous
Ca2+ release and the decreased contractile strength in failing heart
cells.
4.3 Role of JP2 and miR-24 in E –C
coupling remodelling
JP2 is a structural protein linking SR to cell membrane/TTs,14 and plays
a key role in the nanoscopic signalling between LCCs and RyRs during
E–C coupling.13,14,19,20 In all heart failure models characterized so far,
including in DCM and ICM specimens measured in the present study,
JP2 expression is down-regulated without exception.13,16 – 19,27,46
Recent studies also show that JP2 mutations are associated with
human hypertrophic cardiomyopathy.48,49 Since JP2 knockdown
turns to disrupt the TT system, reduce the volume density, and
spatial span of TT-SR junctions, and decrease the efficiency of E–C
coupling,13,18 – 20 the down-regulation of JP2 is potentially a common
mechanism underlying the defective E– C coupling in heart failure of
different causes.
Given the determinant role of JP2 in forming the LCC –RyR signalling structure, to keep the E–C coupling in a healthy status, the JP2
level must be finely tuned within a certain dynamic range. Recent
study shows that transgenic over-expression of JP2 rescues the
effects of JP2 knockdown on TTs, junctional SRs, and E–C coupling
performances.20 Now, the only endogenous mechanism known to
regulate JP2 expression is miR-24, which suppresses JP2 expression
by binding to the 3′ -untranslated region of JP2 mRNA.27 In vivo suppression of miR-24 stabilizes JP2 expression and prevents the reductions in TTs, junctional SRs, and E–C coupling gain in cardiomyocytes
from aortic-constricted mice.28 Future exploration of the transcription control system of JP2, which counteracts the post-transcriptional
regulation by the microRNA system, is important in elucidating
the molecular mechanisms underlying the structural remodelling of
TT–SR junctions. Given that miR-24 is up-regulated in failing heart
cells27 and that in vivo suppression of miR-24 prevents the transition
from hypertrophy to heart failure,28 developing therapeutic treatments that target miR-24 and JP2 expression systems will be a new
strategy against heart failure.
Acknowledgements
We thank Drs Ying-Chun Hu and Xue-Mei Hao for technical
supports.
Conflict of interest: none declared.
Funding
This study was supported by the 973 Major State Basic Research Development Program of China (2011CB809101), the National Natural Science
Foundation of China (30800475, 81070196, 30730013, and 81030001),
the Program for New Century Excellent Talents in University, the
Beijing Talents Foundation, the Importation and Development of
High-Caliber Talents Project of Beijing Municipal Institutions
(CIT&TCD20130339), and Beijing Natural Science Foundation (7132074).
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