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Giant Molecule Titin and Myocardial Stiffness
Stefan Hein, William H. Gaasch and Jutta Schaper
Circulation. 2002;106:1302-1304
doi: 10.1161/01.CIR.0000031760.65615.3B
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Editorial
Giant Molecule Titin and Myocardial Stiffness
Stefan Hein, MD; William H. Gaasch, MD; Jutta Schaper, MD
T
he beauty of the new titin hypothesis is that the heart
is able to change its resting length-tension relationship
under chronic stress conditions by isoform switching,
thereby rehabilitating the old clinical paradigm of “diastolic
tone.” The path from hypothesis to reality is rocky, however,
as the articles by Wu and Neagoe in this issue of Circulation
show.1,2
See pp 1333 and 1384
Wu et al found that the titin isoforms N2BA/N2B ratio
was decreased from a normal ratio of 1.0 to 0.8 in the paced
dog heart. Because N2B represents the isoform with stiffer
elastic properties, this resulted in an increased passive stiffness in isolated muscle strips. The authors suggest that this
“titin-based” stiffness is acting in concert with “collagenbased” stiffness and that it may counteract ventricular dilatation in canine hearts failing because of chronic pacing.
The results are somewhat in contrast with an earlier study
by the same group in which a switch to the more extensible
isoform N2BA was shown to occur in the subendocardium of
canine hearts after 2 weeks of pacing. This seemingly
contradictory result might reflect a different time course of
titin isoform expression.3
Neagoe et al2 report a titin isoform shift from 30:70 (ratio
0.42) of N2BA/N2B in normal human myocardium to 47:53
(ratio 0.89) in nonischemic regions from human hearts
transplanted because of coronary artery disease (CAD). Total
passive tension was reduced in CAD compared with control,
indicating a more compliant behavior of the myocardium.
The authors, referring to the well-known increase in cardiac
stiffness due to collagen in chronic heart failure, hypothesize
that this isoform shift occurs to counteract the global stiffening caused by chronic preload elevation in CAD patients.
1
Titin and Collagen
The giant molecule titin, the “third sarcomeric filament
system,” is the third most abundant (10%) of the cardiac
proteins, after myosin (40%) and actin (22%). Spanning as
one molecule from the Z-line to the M-band, it is indispensable in the structural integrity of developing and mature
The opinions expressed in this editorial are not necessarily those of the
editors or of the American Heart Association.
From the Department of Thoracic and Cardiaovascular Surgery,
Kerckhoff-Clinic, Bad Nauheim, Germany (S.H.); the Department of
Cardiovascular Medicine, Lahey Clinic, Burlington, Mass (W.H.G.); and
the Department of Experimental Cardiology, Max-Planck-Institute, Bad
Nauheim, Germany (J.S.).
Correspondence to Jutta Schaper, MD, Max-Planck-Institute, Department of Experimental Cardiology, Benekestr 2, D-61231 Bad Nauheim,
Germany. E-mail [email protected]
(Circulation. 2002;106:1302-1304.)
© 2002 American Heart Association, Inc.
Circulation is available at http://www.circulationaha.org
DOI: 10.1161/01.CIR.0000031760.65615.3B
sarcomeres.4 Titin provides elasticity to the sarcomere and
permits elastic recoil. Titin’s function is adapted by different
isoforms (alternative splicing) to the specific needs of striated
muscle. The most variable regions of the whole molecule
represent the molecular spring of the sarcomere localized in
the I-band. In cardiomyocytes, 2 titin isoforms, N2B and
N2BA, are coexpressed in each sarcomere in a speciesspecific ratio. For example, N2B, the smaller and stiffer
isoform, has been shown to be exclusively expressed in rat
and mouse ventricles, whereas N2BA is the dominating titin
found in bovine atria. Most species, however, possess a
mixture of both isoforms.5 In addition to the titin-based
regulation of myocardial stiffness that is important at low to
intermediate sarcomere lengths, collagen-based stiffness
plays a major role at sarcomere elongation.
The fibrillar collagen network consists mainly of collagens
I and III, which are coexpressed in a constant ratio. It is
important to the mechanical properties of tissue because it
builds a scaffold to keep the different cellular and noncellular
compartments in order. Under certain physiological conditions, the collagens contribute to a small extent to the
regulation of diastolic stiffness and sarcomere length (running as wavy chords, they prevent sarcomere lengths over 2.3
␮m), but they provide myocyte lateral alignment.6
Cardiac pathology is usually concerned with an increased
content, ie, fibrosis, or an inappropriate degradation of
collagen. This will influence left ventricular (LV) function in
a variety of fields, including diastolic stiffness, nutrition,
impulse propagation, arrhythmia, and dilatation resulting
finally in depressed systolic function. It is important to note
that quantitative changes in collagen expression are usually
accompanied by a switch in collagen type and arrangement;
this should be taken into consideration when describing
function-structure relationships. Increased collagen will lead
to an increased passive tension and myocardial stiffness.7
LV Chamber Stiffness
The pacing-induced tachycardia model of heart failure produces a dilated cardiomyopathy with little or no change in LV
mass. Over a period of several weeks, the end-diastolic
volume/mass (or radius/thickness) ratio increases, and despite
substantial remodeling and enlargement of the ventricle,
which might suggest a decrease in chamber stiffness, most
studies indicate that the slope of the end-diastolic pressurevolume curve remains normal or near normal. The stiffness of
a unit of the LV wall (defined by myocardial stress-strain
relations) has been found to be normal by some investigators
and increased by others. It is difficult to reconcile these
disparate findings except to say that different experimental
models and different periods of pacing might be responsible.8 –10 Unfortunately, Wu et al1 did not correlate their
findings with measurements of ventricular volume, mass,
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Hein et al
function, or structure in their pacing model. Changes in the
stiffness of the ventricle and the myocardium are thought to
be mediated largely by alterations in the collagenous matrix
that surrounds the cardiomyocyte. Wu et al1 suggest that the
large intrasarcomeric protein titin also plays a role in determining ventricular stiffness.
It should be emphasized, however, that Wu et al1 stated
that increased passive tension of the myocardium from failing
hearts was the result of a “coordinated change in both
collagen and titin, with titin dominating at short to intermediate sarcomere lengths and collagen at long sarcomere
lengths.” Because end-diastolic sarcomere lengths in failing
hearts are likely to be intermediate to long, LV size and
geometry must be affected primarily by the collagenous
matrix, with titin playing a lesser role.
The results of any study of the physical properties of the
ventricle are likely to be influenced by the nature of the
experimental model of heart failure as well as the duration of
the pathophysiological process. It is possible that the overall
effect of many interdependent and interacting factors is to
facilitate a dissolution of the extracellular matrix and allow an
early chamber dilatation, which is similar to the result seen in
mitral regurgitation.11 During this change in ventricular
geometry, titin limits sarcomere overstretch. Later, a “coordinated change in both collagen and titin” might act to limit
further ventricular dilatation.
The study by Neagoe et al,2 though carried out in human
hearts, is hampered by the fact that patients with CAD
represent an extremely variable cohort. The fact that hemodynamic data were available from only 2 CAD patients
makes it difficult to draw definitive conclusions as to changes
of chamber stiffness. The authors tried to compensate for the
lack of clinical information by providing data on cardiac
troponin J modifications as an indicator of preload. This,
however, is extremely artificial because this information was
obtained from short-time ischemia experiments in Langendorff perfused isolated rat hearts, a situation far removed
from that in ischemic human hearts. Nonischemic regions
from CAD hearts were found to show decreased total muscle
strip stiffness, which is proposed to counteract the global
stiffening caused by chronic preload elevation.
The functional consequences of an increased ventricular
stiffness include a limitation in the degree of enlargement of
the failing heart. This limitation protects the ventricle against
serious rises in systolic wall stresses and thereby attenuates
the downward spiral of progressive afterload excess and
systolic dysfunction. On the other hand, it should be recognized that chamber dilatation provides a mechanism for
maintaining an adequate stroke volume in the presence of a
low ejection fraction. This is in contrast to the “nonstructural
dilatation,” which was described as the consequence of
volume overload without renin-angiotensin-aldosterone system activation.8 Excessive stiffness may actually prevent
dilatation and result in a decline in stroke volume. Thus, an
early chamber enlargement may be a beneficial adaptation to
depressed myocyte and ventricular contractile function.
Later, a stiffening effect might prevent progressive ventricular dilatation, but excessive stiffening is maladaptive.
Giant Molecule Titin and Myocardial Stiffness
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Pacing-Induced Failure
The model of chronic high-rate pacing in dog or pig hearts
seems to resemble the situation in failing human hearts, but
its experimental period is short and the structural phenotype
is most probably different.
In failing human hearts with dilated cardiomyopathy, a
considerable amount of fibrosis is present (up to 40% versus
10% in normal human hearts). Furthermore, myocyte degeneration with reduction of contractile material and titin occurs,
and cell death and loss of myocytes are common.12 This may
leave the titin/myosin heavy chain quotient seemingly unaltered, when in fact absolute values may be significantly
reduced.
In pacing-induced dilated cardiomyopathy with heart failure, the structural situation is not clear. One group reported
massive loss of myocytes and significant replacement fibrosis,13 whereas others observed either a reduction of the
collagen fraction14 or collagen nonuniformity.9 Weber et al8
concluded that ventricular dilatation and failure after pacing
are on the basis of an architectural remodeling of the
myocardium, termed “structural dilatation.” Wu et al1,15 did
not report any changes in LV collagen content, which makes
interpretation of their data difficult.15 They assume that only
a slight increase in collagen occurs and that a change in the
collagen type I/III ratio may explain increased collagen-based
stiffness. Measurements of the absolute content of both
collagen and titin will provide data useful for the estimation
of LV total stiffness. This needs to be established in future
studies.
Studies in the Human Heart
Neagoe et al2 present measurements in patients with normal
hearts and in those with CAD. The tissue samples were from
the midmyocardium, as in the study by Wu et al.1 The
decrease of total muscle strip stiffness correlated with a shift
toward more N2BA, a result opposite from that obtained by
Wu et al.1 This result was confirmed in rat right ventricles
with LV infarction.
Collagen-based stiffness was not determined. On the other
hand, they claimed that collagen was increased on the basis of
immunofluorescent histological pictures that showed a very
small area of myocardium rather than on quantitative evaluation. Because studies of the myocyte structure are lacking as
well, it is difficult to put this work into perspective.
Discrepancy of Results
It is interesting to compare the conclusions of these 2 studies.
Wu et al1 show a contribution to myocardial stiffness by an
isoform shift toward N2B, and Neagoe et al2 propose the
concept that the shift toward N2BA is an attempt to counteract the global stiffening induced by collagen/desmin accumulation. Why these 2 studies, carried out by researchers who
used the same methods and who published together in
previous years,16 present divergent results is difficult to
explain. Discrepancies may have been caused by multiple
factors, including appropriateness of the pacing model and of
rat myocardial infarction, species differences, and heterogeneity of human pathology.
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Circulation
September 10, 2002
The shift to the stiffer titin isoform N2B described by Wu
et al1 may possibly be explained as a response to the sudden
and significant up-regulation of heart rate by pacing. This
may be similar to the situation in small rodents with a high
beating frequency that show an almost exclusive N2B
expression.5
Importance of the Present Studies and
Perspectives
The studies in the present issue by Wu et al1 and Neagoe et
al2 are important because they show that an isoform shift of
titin occurs under pathophysiological situations of cardiac
overload. The authors of both studies are well-known pioneers and experts in the field of titin, having clarified most of
the major issues about this interesting molecule, including
complete sequencing and multiple functional properties.17,18
We hope that the discrepancies of results will soon be
reconciled, and the relationship between disturbed (Wu et al1)
or tentatively preserved (Neagoe et al2) diastolic function and
isoform shifts of titin will be clarified. The results could bring
an improved understanding of the mechanisms underlying
systolic and diastolic heart failure in human beings.
References
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pacing-induced cardiac failure give rise to increased passive muscle
stiffness. Circulation. 2002;106:1384 –1389.
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human heart disease. Circulation. 2002;106:1333–1341.
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diastolic suction during pacing tachycardia. Circ Res. 2000;87:235–240.
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