In cardiac cells, Ca2+ signals appear as brief transients responsible for controlling both contraction and transcription. Information may be encoded in these digital signals through changes in both frequency and shape. An increase in Ca2+ signalling contributes to a process of phenotypic remodelling during hypertrophy. The increase in Ca2+ that drives the larger contractions may be responsible for switching on a second process of signalosome remodelling to down-regulate the Ca2+ signalling pathway. It is a change in the properties of the Ca2+ transient that seems to carry the information responsible for the remodelling of the cardiac gene transcription programme that leads first to hypertrophy and then to congestive heart failure.
Cells express a large number of Ca2+ signalling systems that are put in place when cells differentiate into specific cell types. There is an extensive toolkit of Ca2+ signalling components from which cell-specific Ca2+ signalosomes are selected . These different Ca2+ signalosomes deliver Ca2+ signals with the spatial and temporal characteristics necessary to control the function of specific cell types. Through a process of signalosome stability, the signalling system of each cell is maintained by ongoing transcriptional processes. However, signalosomes are highly plastic and can adapt to changing demands through a process of phenotypic remodelling to generate either reduced or elevated output signals.
Many conditions may occur through alterations that result in a modified signalosome that generates inappropriate output signals leading to disease states such as Alzheimer's disease, asthma, diabetes, hypertension, manic-depressive illness and heart disease. This review explores the mechanisms that are responsible for signalosome stability and how an alteration in these mechanisms can lead to the onset of cardiac disease.
During the final phase of development, differential gene transcription results in the expression of a cell-type-specific signalosome that defines the signalling phenotype of each specialized cell (Figure 1). One of the features of the differentiated state is the stability of its signalling systems. However, such signalosomes are not fixed in stone, but can be remodelled during both normal and pathological conditions. Indeed, abnormal remodelling of the signalosome is a major cause of disease. Therefore it is important to understand how cells regulate the transcription of their signalosome components in order to maintain the phenotypic stability of their signalling systems. Cells appear to operate a quality-assessment system whereby the properties of the output signals are constantly monitored and any deviations are fed back to the transcriptional system to make the necessary adjustments. Such autoregulatory mechanisms are particularly evident for the Ca2+ signalling system and form the basis of the many examples of compensation that have been observed when specific signalling components are either deleted or overexpressed. Overexpressing calsequestrin in mouse cardiac myocytes, which strongly reduces the amplitude of the Ca2+ spikes that activate contraction, results in a marked down-regulation in the proteins of the release mechanism [e.g. RYR2 (ryanodine receptor 2), triadin and junctin]. Likewise, when triadin 1 is overexpressed in mice, there is a compensatory decline in the expression of both RYR2 and junctin. Overexpression of the L-type channel in cardiac cells, which increases the amount of Ca2+ entering the cell, is counteracted by an up-regulation of the Na+/Ca2+ exchanger to increase Ca2+ extrusion . It can be argued that a process of Ca2+-induced transcription of Ca2+ signalling components is responsible for maintaining the stability of Ca2+ signalosomes.
Ca2+ regulates the phenotypic expression of the Ca2+ signalosome
The fact that the expression of Ca2+ signalling components can be altered to compensate for alterations in the signalling pathway implies that these Ca2+ signalling components must have promoter regions that respond to Ca2+-senstive transcription factors. There is already evidence that the Ca2+-dependent transcription factor NFAT (nuclear factor of activated T-cells) controls the transcription of components of the Ca2+ signalling toolkit such as endothelin, InsP3R1 (inositol 1,4,5-trisphosphate receptor type 1), NFAT2 and the DSCR1 (Down's syndrome critical region 1) gene, which encodes a protein that inhibits the activity of calcineurin. It is also apparent that these Ca2+-sensitive transcriptional systems are sensitive to subtle variations in the Ca2+ signalling system. Since most Ca2+ signals in cells are delivered as brief transients, it is necessary to propose that the transcriptional system is sensitive to either the frequency or the shape of such transients (see the next section).
Encoding and decoding information in Ca2+ transients
A characteristic feature of most Ca2+ signals is that they are presented as a brief Ca2+ transient. These transients can either be produced on demand, by periodic stimulation as occurs in muscle or neurons, or appear as part of an oscillation. Information may be encoded in such repetitive transients through changes in frequency, amplitude or shape. There are two main mechanisms for decoding these different bits of information. One is through digital tracking whereby downstream responses closely track each Ca2+ transient. Such digital tracking occurs in contractile cells and nerve terminals, where a Ca2+ transient triggers an all-or-none response. Another example is found in airway epithelial cells, where ciliary beat frequency closely tracks individual Ca2+ transients .
The other major decoding mechanism is integrative tracking whereby each transient has a small effect on some dynamic process that can adopt different equilibrium positions. Small changes in individual transients are then integrated over time to provide a significant change in some cellular process. An example of such integrative tracking is the NFAT shuttle. The basis of this shuttle is that NFAT is imported into the nucleus in response to an increase in Ca2+ and is exported back into the cytoplasm when Ca2+ returns to its resting level. The distribution of NFAT between the cytoplasm and the nucleus was found to vary depending on the frequency of Ca2+ transients .
Signalosome remodelling in cardiac hypertrophy and CHF (congestive heart failure)
There is increasing evidence that cardiac hypertrophy and CHF may arise from an inappropriate phenotypic remodelling of the signalosome. Heart disease is characterized by a decrease in the ability of the heart to perform its role of pumping blood around the body. This dysfunction is a major cause of human morbidity and mortality. CHF can be induced by multiple factors, many of which appear to act by increasing the workload of the heart. For example, the increase in blood pressure that occurs during hypertension causes an increased mechanical load on the heart. CHF can develop after ischaemic injury where a myocardial infarction has removed many of the functioning myocytes, so that the remaining cells have to work harder to maintain the pump. The onset of CHF can also be induced by endocrine disorders that increase the circulation of hormones such as catecholamines, endothelin-1 and angiotensin II. The last two hormones operate through the phosphoinositide signalling pathway (Figure 2), which plays an important role in the induction of hypertrophy. For example, the cardio-specific overexpression of Gαq can lead to hypertrophy . Conversely, the cardio-specific deletion of Gq prevents hypertrophy in response to a pressure overload .
Remodelling the cardiac signalosome during cardiac hypertrophy
The heart adapts to these various hypertrophic stimuli by increasing its size [7,8]. The reversible phenotypic remodelling responsible for cardiac hypertrophy is a compensatory mechanism, in that the heart will return to its original size if the abnormal inputs are reduced. If the stresses persist, this compensated hypertrophy shifts to the more irreversible state of CHF. Both cardiac hypertrophy and CHF are examples of phenotypic remodelling of the signalosome resulting in cardiac cells with altered physiological properties. It is therefore important to make the distinction between these two phases, i.e. the early stage of compensatory hypertrophy and the more terminal phase of CHF. The challenge is to find out how cardiac hypertrophy develops, in particular to understand the nature of the transition from compensated to decompensated hypertrophy.
Heart disease working hypothesis
The following working hypothesis, which has two components, provides a conceptual framework to understand how cardiac hypertrophy might develop and how it switches into CHF. The first concerns the reversible phenotypic remodelling of cardiac hypertrophy. This initial adaptive response is driven mainly by extrinsic factors that act through a variety of signalling mechanisms to induce the transcription events that are responsible for cardiac hypertrophy. The central tenet is that modification of the Ca2+ transient, by an increase in its frequency, amplitude or width, is the primary signal for hypertrophy. It is this up-regulation of the Ca2+ signalling pathway that may provide the connection to the second component, which is the irreversible phenotypic remodelling that results in CHF.
While the first component is driven primarily by extrinsic factors, the transition to CHF may depend upon intrinsic control mechanisms that maintain phenotypic stability as described above. This more speculative aspect of the working hypothesis proposes that the increase in Ca2+ signalling that occurs during the initial hypertrophy phase triggers a progressive down-regulation of the cardiac signalosome such that it fails to deliver the strong Ca2+ pulses that are necessary to maintain the cardiac pump cycle.
Remodelling gene transcription in cardiac hypertrophy
One of the characteristics of cardiac hypertrophy is that there appears to be a process of de-differentiation, in that the hypertrophic stimuli activate a programme of foetal cardiac gene transcription. It is proposed that phenotypic stability is maintained by the normal cardiac cell Ca2+ transients (Figure 3). However, a change in the properties of these transients may alter transcription to bring about the phenotypic remodelling that occurs during hypertrophy. A major problem with trying to understand cardiac hypertrophy is the fact that the heart is not quiescent but continues to contract regularly, driven by periodic Ca2+ signals that flood through the cytoplasm and nucleus every few seconds. The extrinsic factors that drive this early hypertrophic response (e.g. mechanical load, loss of myocytes and endocrine factors) act against this background of repetitive Ca2+ pulses. How do normal cardiac cells avoid triggering a hypertrophic response? It seems that subtle changes in the characteristics of the individual Ca2+ transients (e.g. increases in amplitude or width) induced by hypertrophic stimuli may be sufficient to activate the novel transcriptional events that are responsible for the phenotypic remodelling that leads to hypertrophy (Figure 3).
A hypothesis concerning the role of Ca2+ transients in cardiac hypertrophy
There is some evidence to show that there are alterations in the properties of Ca2+ transients in cardiac cells undergoing cardiac hypertrophy. In transgenic mice where FKBP12.6 (FK506-binding protein 12.6) has been deleted, the onset of hypertrophy is associated with a marked increase in the amplitude of the Ca2+ transient . Another example is the considerable prolongation of the Ca2+ transient that is observed in transgenic mice that develop hypertrophy following overexpressing of triadin 1 . Again there appears to be a correlation between a change in the shape of the Ca2+ transient and the onset of hypertrophy. Such observations indicate that Ca2+ transients may carry information to regulate both contraction and gene transcription. As described above, there are various ways in which information is encoded in Ca2+ transients. In the case of the cardiac cell, digital tracking may regulate contraction, whereas integrative tracking may enable the same transient to bring about a change in gene transcription (Figure 3). The integrative process may operate through altering dynamic processes such as the NFAT and HDAC (histone deacetylase) shuttles (Figure 2). One of the targets of Ca2+ signalling in cardiac cells is calcineurin that dephosphorylates NFAT3-P to NFAT3, which enters the nucleus. There are various NFAT kinases within the nucleus that phosphorylate NFAT, thus inactivating it by driving it back into the cytosol and hence setting up the dynamic process that constitutes the NFAT shuttle.
The phenotypic remodelling events that occur during compensatory hypertrophy described above are replaced by irreversible remodelling processes that result in a severe downregulation of the cardiac signalosome characteristic of CHF. The most noticeable change that occurs in the signalosome during CHF is a dramatic decline in the activity of the SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase) pump. At present, very little is known about this irreversible remodelling process. On the basis of the heart disease working hypothesis outlined above, it is proposed that the increase in Ca2+ signalling that induces the transcription events responsible for compensatory hypertrophy may in time sow the seeds for the irreversible remodelling. This down-regulation may be a direct consequence of the homoeostatic mechanisms whereby Ca2+ regulates the expression of its signalling components to maintain phenotypic stability. The excessive Ca2+ signalling that occurs during the onset of compensatory hypertrophy may trigger an adaptive response whereby the activities of various components of the signalosome are severely reduced.
Insulin, Calcium and the Control of Mammalian Metabolism: Focused Meeting to honour the retirement of Professor Dick Denton FRS, held at Willis Hall, University of Bristol, U.K., 22–23 September 2005. Organized and edited by A. Halestrap, G. Rutter and J.M. Tavaré (Bristol, U.K.).