MOLECULAR AND CELLULAR BIOLOGY, Aug. 2011, p. 3424–3435
0270-7306/11/$12.00 doi:10.1128/MCB.05269-11
Copyright © 2011, American Society for Microbiology. All Rights Reserved.
Vol. 31, No. 16
PINCH Proteins Regulate Cardiac Contractility by Modulating
Integrin-Linked Kinase–Protein Kinase B Signaling䌤†
Benjamin Meder,1‡ Inken G. Huttner,2‡ Farbod Sedaghat-Hamedani,1‡ Steffen Just,3 Tillman Dahme,3
Karen S. Frese,1 Britta Vogel,1 Doreen Köhler,1 Wanda Kloos,1 Jessica Rudloff,3 Sabine Marquart,1
Hugo A. Katus,1 and Wolfgang Rottbauer3*
Department of Medicine III, University of Heidelberg, Heidelberg, Germany1; Victor Chang Cardiac Research Institute, Sydney,
Australia2; and Department of Medicine II, University of Ulm, Ulm, Germany3
Received 27 February 2011/Returned for modification 21 March 2011/Accepted 3 June 2011
Integrin-linked kinase (ILK) is an essential component of the cardiac mechanical stretch sensor and is
bound in a protein complex with parvin and PINCH proteins, the so-called ILK-PINCH-parvin (IPP) complex.
We have recently shown that inactivation of ILK or -parvin activity leads to heart failure in zebrafish via
reduced protein kinase B (PKB/Akt) activation. Here, we show that PINCH proteins localize at sarcomeric Z
disks and costameres in the zebrafish heart and skeletal muscle. To investigate the in vivo role of PINCH
proteins for IPP complex stability and PKB signaling within the vertebrate heart, we inactivated PINCH1 and
PINCH2 in zebrafish. Inactivation of either PINCH isoform independently leads to instability of ILK, loss of
stretch-responsive anf and vegf expression, and progressive heart failure. The predominant cause of heart
failure in PINCH morphants seems to be loss of PKB activity, since PKB phosphorylation at serine 473 is
significantly reduced in PINCH-deficient hearts and overexpression of constitutively active PKB reconstitutes
cardiac function in PINCH morphants. These findings highlight the essential function of PINCH proteins in
controlling cardiac contractility by granting IPP/PKB-mediated signaling.
Integrin-linked kinase (ILK) is an essential component of
the cardiac mechanical stretch sensor, modulating expression
of stretch-responsive genes such as anf and vegf and thereby
enabling the heart to adapt its force of contraction to changing
hemodynamic needs. Many of the currently known ILK-mediated functions require additional cofactors and interacting proteins. Increasing focus has therefore been put on the roles of
the ternary complex comprising the proteins ILK, ␣-, -, or
␥-parvin, and PINCH, PINCH1 or PINCH2 (5, 26, 30, 34).
The ILK-parvin-PINCH (IPP) complex provides physical
linkage between integrin transmembrane receptors and the
actin cytoskeleton and serves as a signaling mediator that
transduces mechanical signals to downstream effectors (32).
PINCH and parvins do not have catalytic activity but rather
function as adapters for other signaling molecules such as
Nck-2 and Ras suppressor 1 (RSU-1), through which PINCH
can regulate c-Jun N-terminal kinase (JNK) (7, 25).
In cardiomyocyte cell cultures, the IPP complex is known to
regulate integrin-mediated signaling pathways involved in hypertrophic and apoptotic responses (5). In vivo, a role of the
IPP complex and specifically of PINCH proteins in the control
of cardiac contractility was demonstrated in a recent study
using PINCH-deficient mice. Cardiac muscle-specific double
knockout of pinch1 and pinch2, but not single deletion of either
gene, resulted in heart failure and early postnatal lethality;
however, the underlying molecular pathway was ultimately not
elucidated (16).
We recently found that zebrafish ILK regulates the cardiac
stretch response and contractility via protein kinase B (PKB)VEGF signaling (2). To investigate if PINCH affects IPP-PKB
signaling, we analyzed PINCH1 and PINCH2 function in the
zebrafish heart. We show that PINCH localizes at sarcomeric
Z disks/costameres in heart and skeletal muscle. In contrast to
mice, knockdown of either PINCH1 or PINCH2 in zebrafish
independently leads to severe heart failure, similar to the phenotype observed in the ILK main squeeze (msq) mutant (2).
Loss of PINCH leads to ILK protein instability and consequently affects ILK-mediated downstream signaling, resulting
in reduced PKB phosphorylation and expression of stretchresponsive genes. Finally, we show that restoration of defective
PKB signaling is sufficient to reconstitute cardiac contractility
and the stretch-responsive gene program in PINCH morphants, demonstrating an important role of PINCH proteins in
IPP-PKB-dependent cardiac mechanosensing and transduction.
MATERIALS AND METHODS
Zebrafish strains. Care and breeding of zebrafish, Danio rerio, were as described previously (27). The present study was performed after appropriate
institutional approvals were secured. It conforms to the Guide for the Care and
Use of Laboratory Animals of the National Institutes of Health (24a). For all
mRNA and morpholino injection procedures, the TüAB wild-type strain was
used.
Histology, transmission electron microscopy (TEM), immunostaining, immunoblotting, real-time PCR, and RNA in situ hybridization. Histological analysis
and electron micrographs of fish embryos were performed as described previously (22). For whole-mount, sectioned hematoxylin-eosin (HE) staining and
immunostaining, embryos were fixed in 4% paraformaldehyde. Immunostaining
was performed on 8-m cryosections, using a polyclonal anti-PINCH1 IgG
antibody (15), a monoclonal anti-PINCH1 antibody raised against amino acids
* Corresponding author. Mailing address: Department of Medicine II,
University of Ulm, Albert-Einstein-Allee 23, D-89081 Ulm, Germany.
Phone: 0049 73150045001. Fax: 0049 73150045005. E-mail: wolfgang
.rottbauer@uniklinik-ulm.de.
† Supplemental material for this article may be found at http://mcb
.asm.org/.
‡ These authors contributed equally to this work.
䌤
Published ahead of print on 13 June 2011.
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PINCH PROTEINS REGULATE CARDIAC CONTRACTILITY
(aa) 1 to 108 of recombinant human PINCH1 (R&D Systems), an anti-ILK IgG
antibody (2), a polyclonal anti-vinculin antibody (Sigma), and a monoclonal
anti-␣-actinin IgG1 antibody (Sigma, Germany). Secondary antibodies included
goat anti-mouse IgG2b-tetramethyl rhodamine isocyanate (TRITC) and goat
anti-mouse IgG1-fluorescein isothiocyanate (FITC) antibodies (Southern Biotech), anti-rabbit IgG-TRITC antibody (Sigma, Germany), and anti-rabbit IgGFITC antibody (Molecular Probes). Western blots were probed with both the
monoclonal and polyclonal anti-PINCH1 antibodies, a polyclonal anti-PINCH2
antibody (28), or anti-ILK antibody as primary antibodies, and signals were
detected by chemiluminescence. Loading controls included mouse anti-rabbit
-actin (Sigma-Aldrich, Germany) and rabbit anti-human pan-cadherin (Abcam). anf mRNA whole-mount in situ hybridization and anf and vegf real-time
PCR were carried out as described previously (2). For real-time PCR, we used
the SYBR green method with the following primers: for anf, forward primer
5⬘-ACAGAGACCGAGAGGAAGCA-3⬘ and reverse primer 5⬘-AGGGTGCTG
GAAGACCCTAT-3⬘), and for vegf, forward primer 5⬘-ATGAACTTGGTTGT
TTATTTGA-3⬘ and reverse primer 5⬘-TTCTTTGCTTTGACTTCTGC-3⬘). ef1a
was used as the control gene.
Morpholino injection procedures and transient overexpression of protein
kinase B. Morpholino-modified antisense oligonucleotides (Gene Tools) were
directed against the splice-donor site of exon 5 (MO1-pinch1 [5⬘-CGCTGCTC
TACTGACCCGCAGTTGC-3⬘]) or the translational start site (MO2-pinch1 [5⬘GCCATGTTACTGCCCGTCATCTCTG-3⬘]) of zebrafish pinch1 (zpinch1).
zpinch2 was targeted using morpholinos directed against its translational start
site (MO2-pinch2 [5⬘-GAGCTTCTGCTTCTGCTTCCATCAT-3⬘]). zpinch1
and zpinch2 antisense oligonucleotides or a standard control oligonucleotide
(MO-control), diluted in 0.2 mol/liter KCl, were microinjected into one-cell-stage
zebrafish embryos. Transient overexpression of constitutive-active or kinasedeficient PKB was performed essentially as described previously (2).
Functional assessment and statistical analysis. Still images and video films
were recorded and digitized with a Zeiss MCU II microscope. The functional
assessment of cardiac contractility was carried out as described before (23).
Fractional shortening (FS) and atrial and ventricular diameters were assayed
with the help of the zebraFS software (http://www.benegfx.de). If not further
specified, results are expressed as means ⫾ standard deviations. Analyses were
performed using unpaired Student’s t test, and a P value of ⬍0.05 was accepted
as statistically significant.
RESULTS
PINCH1 localizes to the cardiac Z disk of the zebrafish heart.
We identified the zebrafish orthologs of PINCH1 (expressed sequence tag [EST]; GenBank accession number BQ130993) and
PINCH2 (GenBank accession number BC065908.1) by BLAST
analysis of human PINCH protein sequences against the NCBI
zebrafish EST database. Both zebrafish PINCH proteins are
highly conserved, showing 96% and 78% sequence identities with
human PINCH1 and PINCH2, respectively (Fig. 1A and B). Like
their mammalian counterparts, zebrafish PINCH1 and PINCH2
contain 5 LIM domains, followed by a short C-terminal tail.
In mice, PINCH1 is expressed in various tissues, including
the heart and skeletal muscle (4). To evaluate in detail the
subcellular expression of PINCH1 in heart and skeletal muscle
cells of zebrafish, we performed immunostainings with polyclonal (Fig. 2) and monoclonal (data not shown) antibodies
directed against PINCH1. As revealed by coimmunofluorescence imaging with ␣-actinin, an established marker of sarcomeric Z disks, zPINCH1 shows the specific stripy pattern of
sarcomeric Z-disk localization in both cardiomyocytes and
skeletal muscle cells of embryonic and adult zebrafish (Fig. 2A
to D). We furthermore find localization of PINCH1 at intercalated disks of zebrafish cardiomyocytes (Fig. 2B, arrowheads), whereas other sarcomeric structures are devoid of
PINCH1 expression (2, 22). Colocalization with vinculin (Fig.
2D), an established marker of Z disks and costameres (29),
which connect peripheral Z disks with adjoining sarcolemma,
3425
further supports our conclusion that PINCH1 localizes to Z
disks.
PINCH deficiency leads to heart failure in zebrafish. To
investigate the in vivo function of zebrafish PINCH proteins,
we inactivated PINCH1 or PINCH2 by blocking either the
translational start site of pinch1 (MO2-pinch1) or pinch2
(MO2-pinch2) or the splice donor site of exon 5 of pinch1
(MO1-pinch1) with morpholino-modified antisense oligonucleotides.
When either 4 ng of MO1-pinch1 or 4.5 ng of MO2-pinch1
was injected, up to 88% of injected embryos develop progressive heart failure (Fig. 3A to D). Starting at 48 hours postfertilization (hpf), ventricular contractility of PINCH1 morphants
decreases progressively and FS of the ventricular chamber
declines from 20% ⫾ 10% at 48 hpf to 5% ⫾ 4% at 72 hpf. By
96 hpf, the ventricular chamber becomes almost silent (Fig. 3E;
see Movie S1 in the supplemental material). Similarly, when
PINCH2 was targeted by injecting 1.6 ng of MO1-pinch2, 75%
of injected embryos develop the same heart failure phenotype
(Fig. 3A to E), with the ventricular FS declining from 30.5% ⫾
9% at 48 hpf to 9.8% ⫾ 5% at 72 hpf and 7.6% ⫾ 6.3% at 96
hpf (Fig. 3E; see Movie S2 in the supplemental material).
These findings demonstrate that in contrast to mice, loss of
either PINCH1 or PINCH2 independently leads to heart failure in zebrafish. To ensure that the PINCH morpholinos used
are indeed (i) functional in blocking PINCH expression and
(ii) specific to the respective PINCH protein targeted, we have
performed a number of control experiments (Fig. 4A to E and
5A and B). Isolation of mRNA from MO1-pinch1-injected
embryos confirmed the predicted effect on mRNA splicing,
namely, either skipping of exon 5 (192-bp product) or integration of intron 5 (418-bp product), both leading to premature
termination of protein translation. Wild-type zpinch1 cDNA
(342 bp product) was not detectable in the injected embryos
(Fig. 4A). This is confirmed by Western blot analysis of MO1pinch1-injected embryos, as neither wild-type PINCH1 protein (molecular mass, approximately 37 kDa) nor aberrant
PINCH1 protein (predicted molecular mass, 20.8 kDa) is detected when an antibody raised against the untargeted amino
acids 1 to 108 of PINCH1 is used (Fig. 4B). The start site
morpholinos MO2-pinch1 and MO2-pinch2 were chosen carefully to target cDNA sequences near the translational start
sites of both proteins, which are poorly conserved, to avoid
cross-reactivity (Fig. 4C). As shown in Fig. 5A and B, MO2pinch1 specifically blocks protein translation of PINCH1 but
not PINCH2, and vice versa, MO2-pinch2 specifically knocks
down PINCH2 but not PINCH1. In fact, our results show that
PINCH1 and PINCH2 are, rather, upregulated in zebrafish in
which the respective other isoform has been knocked down,
indicating an attempt to compensate for the loss of one isoform by upregulation of the other, which, however, fails to
prevent the heart failure phenotype observed in morphant
embryos. Lastly, the efficacy and specificity of PINCH1 knockdown via MO1-pinch1 injection were again confirmed by immunostaining of PINCH1 morphant tissue, showing significantly reduced fluorescence intensity in morphant versus
control muscle tissues (Fig. 4E).
To further investigate if PINCH proteins may also have
synergistic functions in zebrafish, we performed titrated knockdown experiments for dose-dependent inactivation of both
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FIG. 1. Identification of zebrafish PINCH1 and PINCH2. (A and B) Amino acid sequence alignments of zebrafish (z), human (h), and mouse
(m) PINCH1 (A) and PINCH2 (B), demonstrating the high cross-species homology of each PINCH ortholog. Black background, amino acid
identity; gray background, amino acids with similar chemical properties.
PINCH forms. As shown in Fig. 5C, injection of either 1.6 ng
MO1-pinch1 or 1.2 ng MO2-pinch2 does not induce the cardiomyopathy phenotype. However, after coinjection of 1.6 ng
MO1-pinch1 and 1.2 ng MO2-pinch2, a significant number
(52.8%) of zebrafish embryos display the PINCH-morphant
cardiomyopathic phenotype, indicating a synergistic function
of PINCH1 and -2.
In addition to the loss of cardiac contractility, skeletal muscle function is also impaired in PINCH-knockdown embryos.
On tactile stimulation, MO-pinch1-injected as well as MOpinch2-injected embryos show only an insufficient flight response and trembling skeletal muscle movements or even complete paralysis (see Movie S3 in the supplemental material).
In order to evaluate how PINCH1 or PINCH2 deficiency
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FIG. 2. PINCH1 is expressed at Z disks of the embryonic and adult zebrafish heart. In the embryonic (A and B) and the adult (C and D)
zebrafish heart, PINCH1 protein localizes specifically to the cardiac Z disks and intercalated disks (B, arrowheads). Localization of PINCH1 at
the sarcomeric Z disk of cardiomyocytes is demonstrated by coimmunostaining with ␣-actinin (A to C) and vinculin (D). ILK is also highly
expressed at the sarcomeric Z disk, as shown by coimmunostaining with ␣-actinin in zebrafish (E) and rat (F) hearts.
induces impairment of cardiac contractility, we next analyzed cardiac morphology and ultrastructure in PINCH1 and
PINCH2 morphant zebrafish embryos. Histological analysis of
PINCH1-deficient hearts shows that cardiac chambers are well
defined, with atrium and ventricle separated by the atrioven-
tricular ring (Fig. 6A and B), and that endocardial and myocardial layers of both chambers have developed properly with
a multilayered ventricular myocardium. In Drosophila melanogaster, loss of ILK function results in detachment of actin
filaments from muscle ends (33). Similarly, in PINCH1 and -2
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FIG. 3. Knockdown of PINCH1 or PINCH2 leads to cardiomyopathy and heart failure. (A to C) MO1-pinch1- and MO2-pinch2-injected embryos develop pericardial edema (*) and precardial blood
congestion due to disturbed cardiac contractility. Lateral views of MOcontrol-injected (A), MO1-pinch1-injected (B), and MO2-pinch2-injected (C) embryos at 72 hpf (v, ventricle; a, atrium). (D) After injection of MO1-pinch1 or MO2-pinch2, 88% and 75% of morphant
embryos, respectively, develop heart failure. (E) FS of the ventricular
chambers of MO-control-, MO1-pinch1-, and MO2-pinch2-injected
embryos measured at the indicated developmental stages. FS is significantly reduced in PINCH morphants after 48 hpf and further declines
by 96 hpf.
double-knockout mice, hearts exhibit disruptions of intercalated disks accompanied by interstitial fibrosis. Hence, to determine whether the contractility defects in PINCH1- or
PINCH2-depleted zebrafish hearts may be caused by disturbed
myofibrillogenesis or intercalated disk abnormalities and
consequent myocyte detachment, we analyzed PINCH1 and
PINCH2 morphant hearts and skeletal muscle by TEM. However, heart and skeletal muscle cells of PINCH1 (Fig. 6C to F)
or PINCH2 (data not shown) morphant embryos show normal,
highly organized arrays of maturated thick and thin myofilaments interconnected by Z disks and aligned in discernible A/I
and M bands. Furthermore, there are no apparent signs of
MOL. CELL. BIOL.
cardiac or skeletal muscle detachment. Similarly, in PINCH1
and -2 double-knockdown embryos, cardiac (Fig. 6G) and skeletal (Fig. 6H) muscle ultrastructure appears normal. Hence,
although PINCH morphants suffer from severe cardiac and
skeletal muscle dysfunction, there are no ultrastructural abnormalities, indicating that heart failure in zebrafish deficient in
either or both PINCH isoforms is not due to structural alterations but can, rather, be attributed to functional defects.
PINCH deficiency induces ILK protein instability and vice
versa. It has been previously reported that protein stability of
components of the IPP complex depends on IPP complex integrity. Accordingly, PINCH binding-deficient ILK variants
fail to localize to focal adhesions sites (35). Similarly, small
interfering RNA-mediated depletion of PINCH1 in HeLa cells
leads to downregulation of ILK protein levels in vitro (9).
Hence, to test whether zebrafish PINCH1 or PINCH2 is essential for ILK protein stability in vivo, we assayed ILK protein
levels in PINCH1- or PINCH2-deficient zebrafish embryos.
After knockdown of PINCH1 or PINCH2, protein levels of
PINCH1 or PINCH2 are significantly reduced in the respective
knockdown embryos (Fig. 5A and B). As shown in Fig. 7B and
C, PINCH deficiency leads to a significant reduction of ILK
protein levels; however, ILK is not completely abolished as in
embryos in which ILK has been knocked down using morpholino (Fig. 7A). To test whether ILK is also necessary for stability of PINCH, we next analyzed PINCH1 protein expression
in both ILK-depleted zebrafish embryos (Fig. 7D) and ILK
mutant msq embryos (Fig. 7E), which harbor a -parvin binding-deficient form of ILK. Interestingly, expression of PINCH1
is reduced in both, showing that the stability of PINCH1 is
strongly dependent on the presence and functionality of ILK.
These results demonstrate that zebrafish PINCH proteins are
necessary for ILK protein stability in vivo and vice versa. Furthermore, loss of PINCH cannot be compensated for by overexpression of ILK (Fig. 7F), indicating that a functional and
self-stabilizing complex consisting of ILK and PINCH is required for unconstrained cardiac function.
PINCH proteins control cardiac contractility via PKB-mediated signaling. We have previously shown that ILK regulates
cardiac contractility via PKB, modulating the expression of
stretch-responsive genes such as anf and vegf (2). To investigate whether disturbed ILK-PKB-mediated signaling accounts
for heart failure in PINCH morphants, we investigated PKB
phosphorylation at serine 473. As shown in Fig. 8A and C, the
amount of phosphorylated PKB in zebrafish embryos is significantly reduced after PINCH1 or PINCH2 knockdown in comparison to control-injected embryos, while overall PKB levels
remain unchanged. To test if reduction in PKB phosphorylation is per se causative for heart failure in PINCH morphants,
we transiently overexpressed a constitutively active PKB variant in PINCH1- or PINCH2-deficient embryos to bypass IPPPKB signaling. Indeed, injection of 380 pg mRNA encoding
constitutively active, myristoylated PKB (PKBmyr) restores cardiac contractility in PINCH1 and -2 morphants, while injection
of the same amount of kinase-dead PKB mRNA (PKBinact)
does not reconstitute heart function in PINCH1- and -2 morphants (Fig. 8B and D).
Next, we investigated the expression of stretch-responsive
genes anf and vegf by whole-mount mRNA antisense in situ
hybridization and quantitative real-time PCR in PINCH mor-
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FIG. 4. Morpholino-mediated knockdown of PINCH1 and PINCH2 is efficient and specific. (A) cDNA analysis of PINCH1 morphants after
injection of MO1-pinch1 shows the expected effect of the morpholino on splicing, namely, the integration of intron 5 (418-bp product) or skipping
of exon 5 (192-bp product). The wild-type form of pinch1 is undetectable in PINCH1-knockdown zebrafish. (B) Western blot analysis of
MO-control- and MO1-pinch1-injected embryos. While PINCH1 levels are normal in MO-control-injected embryos, they are severely reduced after
knockdown in the respective PINCH-morphant embryos. An aberrantly spliced PINCH1 fragment cannot be detected, indicating protein instability
of such a truncated form. n.s., not specific. (C) Although PINCH1 and -2 are highly conserved on the protein level, the start-site morpholinos
against PINCH1 (MO2-pinch1) and PINCH2 (MO2-pinch2) share only 8 overlapping homologous DNA bases, resulting in a high specificity of the
antisense probes. (D) As demonstrated by Western blot analysis, MO2-pinch1 targets Myc-tagged PINCH1 mRNA and MO2-pinch2 targets
Myc-tagged PINCH2 mRNA, as predicted. (E) Coimmunostaining of PINCH1 with ␣-actinin in MO-control- and MO1-pinch1-injected zebrafish
embryos. As shown, knockdown of PINCH1 (lower panels) leads to reduced immunofluorescence at the Z disk, whereas PINCH1 is normally
expressed and localized in control-injected zebrafish (upper panels).
phants (Fig. 9). While anf is highly expressed in atria and
ventricles of control embryos, abrogation of either ILK in msq
mutants (Fig. 9F) or PINCH1 and -2 via morpholino (Fig. 9D
and E) leads to a loss of anf and vegf expression in affected
zebrafish hearts. Importantly, we furthermore show that anf
expression in PINCH1 morphants is reconstituted to almost
normal levels in those zebrafish coinjected with constitutively
active PKBmyr but not kinase-dead PKBinact (Fig. 9G to I),
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FIG. 5. PINCH1 and PINCH2 have synergistic functions. (A and B) Western blot analysis of MO-control-, MO2-pinch1-, and MO2-pinch2injected embryos. While PINCH levels are normal in MO-control-injected embryos, they are severely reduced after knockdown in the respective
PINCH-morphant embryos. Interestingly, in each case the untargeted PINCH form is slightly increased, indicating a partially compensatory
mechanism. (C) Titrated double knockdown of PINCH1 and PINCH2. As shown, knockdown with 4.5 ng MO1-pinch1 or 1.6 ng MO2-pinch2 does
not lead to a cardiomyopathic phenotype. However, in double knockdowns of PINCH1 and PINCH2 using 1.6 ng MO1-pinch1 and 1.2 ng
MO2-pinch2, a high number of zebrafish embryos show the PINCH cardiomyopathic phenotype, indicating a synergistic effect of the two PINCH
forms.
suggesting that ILK and PINCH act in a common molecular
pathway.
DISCUSSION
The sensing of cellular mechanical stress and its transduction into molecular signals are vital for the vertebrate heart in
order to adapt its contractile force and energy metabolism to
changing hemodynamic needs. Several members of the cardiac
stretch-sensing apparatus, such as -integrin, melusin, and
muscle LIM protein (MLP), have been identified as regulators
of cardiac contractility and, when defective, lead to heart failure (3, 11). We have recently identified ILK, a 1-integrinbinding protein, to be a novel component of the cardiac stretch
sensor by studying the zebrafish heart failure main squeeze
(msq) mutant. ILK forms a link between integrin transmembrane receptors and sarcomeric structures at the costameres,
the mechanical integration sites of cardiomyocytes, and regulates cardiomyocyte mechanotransduction and contractile
force via PKB signaling. ILK contains four N-terminal ankyrin
domains, which are proposed to underlie its mechanosensing
ability by exhibiting tertiary structure-based elasticity and behaving as a nanospring (14).
We have investigated here the in vivo functions of PINCH1
and PINCH2 in zebrafish, which specifically bind to ILK via its
ankyrin repeat domain and colocalize with ILK at the Z disk
and costamere. We show that antisense oligonucleotide-mediated knockdown of PINCH1 or PINCH2 independently leads
to severe heart failure, resembling the phenotype of msq mutants. The effect of PINCH knockdown also mimics that of the
msq mutation on the molecular level by inducing ILK protein
instability, showing decreased PKB activation and severely reduced expression of stretch-responsive genes such as anf and
vegf.
PINCH and ILK not only interact with each other but also
form a ternary complex with ␣, , or ␥-parvin, the so-called IPP
complex. In mammals, this complex assembles in the cytoplasm
before it is recruited to integrin adhesion sites, such as the
intercalated disks of cardiomyocytes. The stability of the individual IPP components is ultimately dependent on the successful formation of the complex (9). We show here that knockdown of either PINCH1 or PINCH2 by itself leads to the
destabilization of ILK and consequent heart failure in zebrafish. This stands in contrast to the situation in mice, where
the single loss of either PINCH1 or PINCH2 does not immediately cause destabilization of the IPP complex and a cardiac
failure, because loss of one PINCH isoform can be compensated for by upregulation of the other, which is sufficient to
maintain IPP stability (16, 17, 28). While we observe a similar
upregulation of one PINCH isoform in the absence of the
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FIG. 6. PINCH is dispensable for sarcomere formation in heart and skeletal muscle cells. (A and B) Hematoxylin-eosin-stained sagittal
histological sections of MO-control and MO1-pinch1 morphant hearts at 72 hpf. Besides collapse of the ventricular chamber, morphants display
normal heart morphology with distinct endocardial and myocardial cell layers in atrium (a) and ventricle (v). (C through F) Ultrastructural analysis
of ventricular cardiomyocytes (C and D) and skeletal muscle cells (E and F) of MO-control- and MO1-pinch1-injected embryos at 72 hpf shows
highly organized sarcomeres with thin and thick myofilaments in well-aligned bundles and discernible A/I, M, and Z bands. (G and H) Even
PINCH1 and -2 double knockdowns show aligned heart (G) and skeletal muscle (H) sarcomeres, demonstrating that structural abnormalities are
not responsible for heart failure in PINCH-deficient embryos.
other, this does not prevent the development of cardiomyopathy in PINCH1 or PINCH2 morphant embryos.
Interestingly, we did not observe underlying ultrastructural
abnormalities in PINCH single- or double-morphant zebrafish
hearts and skeletal muscle cells. This is similar to observations
with the genetic ILK msq mutant, where the cardiac and skeletal muscle ultrastructures in msq mutants are also unaffected
(2), but stands in contrast to recent findings from cardiac
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FIG. 7. Knockdown of PINCH induces ILK protein instability and vice versa. (A to C) To determine the effect of PINCH deficiency on the IPP
complex and in particular on ILK protein stability, Western blot analysis of MO-control-, MO-ilk-, MO1-pinch1-, and MO1-pinch2-injected
embryos probed against an ILK antibody was carried out. ILK protein levels are normal in MO-control-treated embryos, while injection of MO-ilk
completely abolishes ILK protein expression (A). Interestingly, injection of either MO1-pinch1 (B) or MO1-pinch2 (C) also results in a significant
reduction of ILK levels, indicating that PINCH is needed to maintain ILK protein stability. (D and E) In ILK morphants as well as in ILK msq
mutant zebrafish, PINCH1 expression is also severely reduced, demonstrating that, vice versa, ILK is also mandatory for PINCH stability. (F) Bar
graphs representing ventricular fractional shortening of MO-ilk-, MO1-pinch1-, and MO2-pinch2-injected embryos. While coinjection of ILK
mRNA can compensate for knockdown of endogenous ILK, it cannot rescue the cardiomyopathic phenotype in PINCH morphants, underlining
the fact that only a functional IPP complex is able to control stretch-responsive signaling.
muscle-specific PINCH1 and -2 double-knockout mice, where
abnormal intercalated disk structure and sarcomeric disarray
were observed (16). This difference may be attributed to
PINCH proteins fulfilling primarily functional rather than
structural roles in the control of zebrafish heart and skeletal
muscle contraction. Hence, disruption of cell-cell adhesion and
sarcomere structure in PINCH-deficient mice may, at least in
part, be a secondary consequence of long-standing contractile
dysfunction (10), which we do not observe in PINCH-morphant
zebrafish embryos, which die within 5 days after fertilization. The
screening and generation of PINCH mutant zebrafish by targeting induced local lesions in genomes (TILLING) (24) or zinc-
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FIG. 8. PINCH proteins regulate PKB activation in the zebrafish heart. (A and C) Western blot analysis of serine 473-phosphorylated PKB
(pPKB S473) in comparison to whole PKB. In both PINCH1- and PINCH2-morphant zebrafish embryos, phosphorylation of PKB is strongly
reduced. (B and D) Ventricular fractional shortening measurements of wild-type zebrafish embryos injected with either MO-pinch1 or MO-pinch2
alone or MO-pinch1 and MO-pinch2 in combination with constitutive active PKB (PKBmyr) or kinase dead PKB (PKBinact) mRNA. As shown, only
constitutive active PKB restores cardiac contractility in PINCH morphants.
finger mediated gene silencing (1) will help to further unravel
the distinct structural and functional roles of the IPP complex
and its interaction partners in the beating heart.
The kinase domain of ILK lacks the highly conserved AspPhe-Gly (DFG) and His-Arg-Asp (HRD) motifs. Several studies, however, have shown that downstream factors such as
cardiac myosin light chain 2 (CMLC-2), glycogen synthase
kinase 3 beta (GSK-3), and PKB are substrates of ILK activity, at least in vitro (6). A study by Maydan and colleagues, for
example, provides the most recent evidence that ILK is a
functional Mn2⫹-dependent protein kinase in vitro (20). However, in vivo studies in D. melanogaster, Caenorhabditis elegans,
and mice showed that ILK-knockout models can be rescued
using kinase-dead versions of ILK, suggesting that ILK activity
may not be required in vivo and that ILK may, rather, act on its
targets indirectly via associated proteins (13, 18, 19, 31, 33).
Our own previous studies in zebrafish pointed toward zebrafish
ILK possessing kinase activity in vitro. Furthermore, in ILK
msq mutants, wild-type and constitutive kinase-active ILK variants could rescue the mutant phenotype, but ILK variants
containing mutations within the kinase domain could not, indicating possible kinase activity in vivo. However, an alternative explanation could be the defective interaction of ILK with
its binding partners, such as -parvin and PINCH, which potentially mediate the downstream effects. Results in this study
show the importance of the ILK-interacting PINCH proteins
on PKB phosphorylation at serine 473, which is strongly reduced in PINCH-deficient cardiomyopathic zebrafish. Accordingly, ectopic expression of constitutively active PKB rescues
the cardiomyopathy phenotype in PINCH morphants, showing
that PKB is the main downstream effector of PINCH function
in the control of cardiac contractility. Reduced phosphorylation of PKB in PINCH morphants could be due to destabilization of the IPP complex and, consequently, reduced ILK
levels and ILK activity. Alternatively, PINCH proteins closely
interact via their LIM5 domain (conserved KEVEF motif) with
protein phosphatase 1 alpha (PP1␣), a negative regulator of
PKB activation (8). PINCH is responsible for keeping PP1␣ in
an inactive state at focal adhesion sites, as shown in cell culture
experiments (8). However, after loss of PINCH, PP1␣ switches
to an activated state and dephosphorylates PKB at serine 473,
possibly explaining the decreased phosphorylation of PKB in
PINCH-morphant zebrafish.
Taken together, our results demonstrate that PINCH1 and
PINCH2 are essential for unconstrained myocardial function
by stabilizing the IPP complex and thereby regulating PKB
activity. Since PINCH-morphant zebrafish exhibit a cardiomyopathy phenotype that is functionally, morphologically, and
molecularly nearly identical to that of mutants with the ILK
msq mutation and ILK mutations were recently found to cause
3434
MEDER ET AL.
MOL. CELL. BIOL.
FIG. 9. Loss of PINCH leads to reduced expression of stretch-responsive anf and vegf. (A to C) RNA antisense in situ hybridization against
atrial natriuretic factor (anf) demonstrates normal expression of the stretch-response marker anf in control-injected zebrafish in comparison to
PINCH1 morphants or homozygous msq⫺/⫺ embryos, where loss of anf expression is observed. (D to F) Quantitative real-time PCR of anf or vegf
transcripts after injection of MO-control, MO-pinch1, or MO2-pinch2. In PINCH-morphant embryos, anf (D) as well as vegf (E) mRNA levels are
significantly reduced, as observed in msq⫺/⫺ mutant zebrafish (F and data not shown). (G to I) RNA antisense in situ hybridization demonstrates
restored expression of anf mRNA in PINCH1 morphants injected with constitutive active PKB but not kinase-dead PKB.
human dilated cardiomyopathy (DCM) (12, 21), it will be interesting to analyze whether PINCH mutations also contribute
to this heart muscle disease.
gramme INHERITANCE; and the Postdoc Fellowship of the Medical
Faculty of the University of Heidelberg.
We have no conflicts of interest to disclose.
ACKNOWLEDGMENTS
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Krijnse-Locker (Electron Scan Core Facility of the University Heidelberg) for their helpful support.
This work was supported by grants from the Bundesministerium für
Bildung und Forschung, NGFN-plus and NGFN-transfer (01GS0108,
01GS0420, 01GR0823, and 01GS0836); the European Union FP7 Pro-
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