MINI REVIEW article

Front. Physiol., 07 August 2018

Sec. Cardiac Electrophysiology

Volume 9 - 2018 | https://doi.org/10.3389/fphys.2018.01081

Caveolae-Specific CaMKII Signaling in the Regulation of Voltage-Dependent Calcium Channel and Cardiac Hypertrophy

  • Laboratory of Clinical Science and Biomedicine, Graduate School of Pharmaceutical Sciences, Osaka University, Osaka, Japan

Abstract

Cardiac hypertrophy is a major risk for the progression of heart failure; however, the underlying molecular mechanisms contributing to this process remain elusive. The caveolae microdomain plays pivotal roles in various cellular processes such as lipid homeostasis, signal transduction, and endocytosis, and also serves as a signaling platform. Although the caveolae microdomain has been postulated to have a major contribution to the development of cardiac pathologies, including cardiac hypertrophy, recent evidence has placed this role into question. Lack of direct evidence and appropriate methods for determining activation of caveolae-specific signaling has thus far limited the ability to obtain a definite answer to the question. In this review, we focus on the potential physiological and pathological roles of the multifunctional kinase Ca2+/calmodulin-dependent kinase II and voltage-dependent L-type calcium channel in the caveolae, toward gaining a better understanding of the contribution of caveolae-based signaling in cardiac hypertrophy.

Introduction

Caveolae are unique flask-like membrane invaginations of 50–80 nm in diameter, which are enriched in cholesterol and sphingolipids (, ; ). Currently, caveolae are considered to be involved in various cellular functions such as lipid homeostasis, signal transduction, endocytosis, and transcytosis (). The structure of caveolae is supported by two major component proteins: caveolins and cavins (; ). Owing to their specific lipid composition, caveolae are highly concentrated in multiple signaling molecules, including receptors, kinases, and ion channels. Those include endothelial nitric oxide synthase (), insulin (), epidermal growth factor (), transforming growth factor-beta (), P2X7 receptor (), and G-protein coupling signaling molecules (). Thus, the caveolae are considered to function as a signaling platform to facilitate efficient and specific cellular responses against stress (; ; ). However, determination of the specific physiological properties of caveolae signaling has been challenging due to the lack of efficient tools for direct visualization of kinase activation inside the caveolae. Understanding these signaling mechanisms of the caveolae can help to gain insight into their role in pathological mechanisms, particularly with regard to the contribution of cardiac hypertrophy, which remains controversial. Here, we review recent evidence on the signaling pathways and related molecules in the caveolae microdomain and their relation to cardiac pathogenesis, with a particular focus on Ca2+/calmodulin-dependent kinase II (CaMKII) and voltage-dependent L-type calcium channel (LTCC). This review can help to highlight targets of research focus and specific questions to tackle toward gaining a better understanding of the molecular mechanisms linking caveolae signaling and heart health, toward establishing new therapeutic strategies.

Caveolae Microdomain and Signal Transduction

There are two types of structural proteins in caveolae: caveolins and cavins. Caveolin is comprised of three isoforms, caveolin-1, caveolin-2, and caveolin-3 (; ; ), with specific cellular distributions. For instance, caveolin-1 is dominantly expressed in endothelial cells, whereas caveolin-3 shows abundant expression in skeletal muscle cells and cardiomyocytes (). These isoforms contain a common peptide sequence constituted by eight amino acids localized in the N-terminal cytosolic oligomerization domain (). As a monomer, caveolin is comprised of three domains, oligomerization domain localized in N-terminus, caveolin scaffolding domain (CSD), and intramembrane domain in C-terminal part of the protein. Caveolin is inserted into the plasma membrane through intramembrane domain and CSD. Caveolin monomers assemble and form a oligomer, and contribute to caveolae formation (). The CSD directly binds to a putative corresponding caveolin binding domain (CBD) identified in a number of signaling effectors localized in caveolae (; ; ; ). identified a peptide sequence “RNVPPIFNDVYWIAF” as a CBD, which strongly binds to the CSD of caveolin 1 or caveolin 3. Currently, the physiological implication of the binding between CSD and CBD remains controversial (). However, it is considered that CBD-fused protein binds to caveolin and demonstrates a specific localization in caveolae (). Caveolin deficiency in a genetically engineered mouse model results in loss of the caveolae structure, indicating that caveolin is indispensable for the formation of caveolae (). Cavin contains four isoforms comprised of cavin-1 or polymerase I transcript factor (PTRF) (), cavin-2 or serum deprivation protein response (SDPR) (), cavin-3 or SDR-related gene product that binds to C kinase (SRBC) (), and cavin-4 or muscle-related coiled-coiled protein (MURC) (). Similar to caveolins, the cavin protein family shows a specific cellular distribution, and cavin-4 is thought to be a muscle-specific isoform. Cavin-1 is required for caveolae assembly and regulates the functions of caveolae by determining the localization of activated receptors (; ). In contrast, cavin-4 is dispensable for caveolae formation in cardiomyocytes, whereas it facilitates ERK1/2 recruitment to the caveolae and supports effective α1-andrenic receptor (AR) signaling activation in the development of cardiomyocyte hypertrophy ().

Microdomain Signaling and Cardiac Hypertrophy

Cardiac hypertrophy is one of the predominant risks of heart failure (). The development of cardiac hypertrophy is governed by multiple intracellular protein signaling cascades from the plasma membrane to nuclei (). Subcellular compartmentalization is considered to allow signaling-related proteins to carry out multiple biological functions using a relatively small number of membrane receptors. However, the precise contribution of microdomain signaling in cardiomyocyte hypertrophy remains elusive. reported that caveolin-3 overexpression in the mouse heart attenuates cardiac hypertrophy via upregulation of natriuretic peptide, suggesting the involvement of caveolin-dependent signaling in the development of myocyte hypertrophy. reported that ARs and a component of the LTCC exist in caveolae microdomains. Ca2+-dependent signaling molecules such as calcineurin and CaMKII play vital roles in the development of cardiac hypertrophy, and activation of these molecules is associated with LTCC activity (; ). further demonstrated that caveolae-targeted inhibition of the LTCC mediates the attenuation of calcineurin activation induced by pacing stimulation without affecting Ca2+ influxes and transient in whole cells (). To inhibit Ca2+ influxes in caveolae, they generated a fusion protein comprised of the caveolin-binding domain and Rem protein, which specifically inhibits LTCC activity. They found that caveolae-localized LTCCs are not involved in excitation-contraction coupling or the regulation of Ca2+, which governs contractility in isolated cardiomyocytes. However, the same group failed to demonstrate similar effects in the mouse heart in which pressure-overload was applied to a genetically engineered model expressing the fused Rem protein with the caveolin-binding domain (). These results indicated that caveolae-related calcineurin/NFAT signaling alone is not sufficient for the development of cardiac hypertrophy.

CaMKII in the Heart

CaMKII is a serine-threonine (Ser/Thr) kinase that is activated in a Ca2+/calmodulin-dependent manner. The activation of CaMKII is also regulated by autophosphorylation (), oxidation (), and glycosylation (). CaMKII phosphorylates a vast number of substrates such as ion channels, calcium handling proteins, and transcription factors (). Activation of CaMKII in the heart has been observed in both experimental models of cardiac hypertrophy and dysfunction as well as in patients suffering from heart failure (; ). Genetic ablation of dominant CaMKII isoforms in the heart attenuates cardiac hypertrophy or the transition to cardiac dysfunction after pressure overload (; ; ). Thus, CaMKII is considered to play a pivotal role in the development of cardiac hypertrophy and in the transition from the adaptive responses to heart failure (). In addition, the location of CaMKII activation is critical for its biological effects (). Two isoforms of CaMKII, CaMKIIδ, and CaMKIIγ, are mainly expressed in the heart, and the splicing isoform CaMKIIδ shows a unique subcellular localization. Such differential localization of CaMKII activation has been demonstrated to lead to a distinct intracellular function and cardiac phenotype (, ). Moreover, the cardiac overexpression of the cytosolic CaMKIIδC isoform in mice impairs excitation-contraction coupling (), whereas activation of the nuclear isoform CaMKIIδB mediates hypertrophic gene induction (). Further, the mitochondrial inhibition of CaMKII was shown to attenuate necrotic cell death (). Collectively, these findings indicate that the subcellular localization of CaMKII determines its biological effect based on the availability of substrate molecules, and is closely related to cardiac pathogenesis (). However, the specific biological role of CaMKII in the caveolae microdomain remains to be elucidated. In particular, deciphering its role in the membrane, as central platform for signal transduction, is required to gain a better understanding of its contribution to cardiac hypertrophy.

Assessment of CaMKII Activation in the Caveolae Microdomain Using a Phosphor-Peptide Tag

One of the major obstacles in determining the precise pathophysiological role of CaMKII in the caveolae microdomain is the lack of efficient and simple methodology to assess microdomain-specific activation of the kinase. Conventionally, the activation of a kinase is determined biochemically by detecting the phosphorylation of its specific substrate using a radioisotope or fluorescence from a whole cell lysate. However, these methods are not suitable for the assessment of microdomain-specific signaling, since fraction preparation is complicated and time-consuming. Alternatively, the detection of phosphorylation using a phosphor-specific antibody is a simple, useful, and reliable method. The detection of mitogen-activated protein kinases (MAPKs) such as ERK and p38 MAPK is a representative example of the application of phosphor-specific antibodies for assessing signaling pathway activation.

We recently developed a novel tool to examine the caveolae-specific activation of CaMKII using a fusion protein comprising 22 amino acids of the cytosolic domain of phospholamban (PLN) fused to caveolin-3 (Figure 1) (). PLN is a 52-amino acid phospho-protein anchoring the membrane of the sarcoplasmic reticulum, which is comprised of a flexible cytosolic domain and an intramembrane domain (). The cytosolic domain of PLN contains two distinct phosphorylation sites: Ser16, which is mainly phosphorylated by cAMP-dependent kinase (PKA), and Thr17, which is specifically phosphorylated by CaMKII (; ; ). Notably, each phosphorylation is detectable using the corresponding phospho-specific antibody, and the phosphorylation state is considered to represent activation of the corresponding kinases in the cytosol (). Therefore, we took advantage of these properties of PLN to develop a novel tool for determining the caveolae-specific activation of CaMKII. Indeed, phosphorylation of Thr17 in tagged cPLN localized in the caveolae was successfully detected using the phospho-specific antibody. Moreover, the phosphorylation level was enhanced by caveolae-specific activation or was suppressed by the caveolae-specific inhibition of CaMKII, indicating the reliability of this method ().

FIGURE 1

LTCC and Cardiac Hypertrophy

The LTCC is a multi-protein complex composed of a pore-forming α-subunit and accessory subunits, including β-subunit proteins (). The β-subunits play important roles in regulation of channel activity as well as in channel membrane trafficking via interaction with the I-II intracellular loop of α-subunits (). Among the multiple splice variants of β-subunits, β2a is the dominant isoform in the heart. The LTCC serves as the primary source of Ca2+ influx for inducing contractions by triggering Ca2+-induced Ca2+ release (). However, enhanced Ca2+ influxes caused by the targeted expression of the cardiac α1 or β2a subunit mediates or enhances cardiac hypertrophy (; ). In addition, increased Ca2+ influxes caused by overexpression of the β2a subunit in feline cardiomyocytes or in the mouse heart driven by adenoviral expression or transgenesis induced pronounced myocardial Ca2+ overload that resulted in myocyte death (; ). Therefore, functional sequestration of the LTCC subpopulation could be an important strategy to regulate cardiac pathogenesis. The LTCC has been shown to localize not only in the T-tubules but also in the plasma membrane microdomains such as the caveolae, and its localization is assumed to contribute to the distinct biological roles of the channel (; ; ).

CaMKII and LTCC in the Caveolae Microdomain Mediate Cardiac Hypertrophy

Besides cardiomyocytes, several reports suggested the involvement of CaMK in caveolae-related biological effects, such as 1α,25(OH)2D3-dependent signaling or P2X3 receptor-mediated Ca2+ influx (; ). In cardiomyocytes, voltage-gated LTCC complex is a well-known substrate of CaMKII (). Phosphorylation of the β2 subunit by PKA or CaMKII has been proposed as an activation mechanism of LTCC mediated by extracellular stimuli (; ). Several studies have also shown that the CaMKII- and PKA-mediated phosphorylation of the α1C subunit of LTCC facilitates its activity (; ). However, the physiological importance of phosphorylation of the β2 subunit remains controversial. Mutant mice with a truncated β2 subunit lacking the phosphorylatable domain failed to show alteration of LTCC activity under physiological conditions (). By contrast, overexpression of a mutated β2 subunit resistant to CaMKII binding (L493A) and phosphorylation (T498A) resulted in attenuation of the cell death induced by delayed rapid-pacing (). Therefore, upregulation of β2 subunit phosphorylation is thought to play a role in cardiac pathogenesis. In support of this hypothesis, increased expression of the LTCC β2a subunit and enhanced CaMKII activation are frequently observed in cases of human heart failure (; ). Thus, sustained, excessive CaMKII activation is considered to be an upstream signaling event for increased LTCC opening probability, which is involved in excitation-contraction coupling dysfunction, myocardial hypertrophy, heart failure, and lethal arrhythmia (; ; ). However, whether the increased β2 subunit is phosphorylated by CaMKII and the subcellular location in which this critical event occurs remained unclear. Using our phospho-specific antibody, we demonstrated that the LTCC β2 subunit is phosphorylated by CaMKII in the caveolae to further induce CaMKII activation, possibly by increased Ca2+ influxes through the channel (Figure 2, ). This suggested the possibility of a positive feedback loop between the β2 subunit and CaMKII that specifically occurs in the caveolae microdomain. This activation mechanism would contribute to the promotion of cardiac hypertrophy caused by chronic α1 adrenergic stimulation in vivo, since overexpression of the non-phosphorylated mutant of the β2a subunit failed to display enhancement of cardiac hypertrophy (). However, further investigation is required to clarify the direct link between caveolae-specific CaMKII signaling and cardiac hypertrophy in vivo, using genetically engineered mouse models such as caveolae-specific expression of constitutive active CaMKII (activation) or a CaMKII-specific inhibitory peptide (inhibition).

FIGURE 2

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Conclusion and Prospects

We have here summarized the current knowledge on the effects of caveolae-specific signal activation in relation to the pathogenesis of cardiac hypertrophy. The related molecules such as CaMKII and LTCC have multiple cellular functions that seem to depend on the corresponding subcellular localization of the molecules, including intracellular organelles and microdomains. Thus, regulation of location-dependent signal activation is a potential therapeutic target for heart failure. For instance, inhibition of the specific population of CaMKII or LTCC, those which are involved in the development of cardiac hypertrophy or induction of cell death, could potentially improve the prognosis of patients with heart failure without disturbance of excitation-contraction coupling. However, several questions remain to be answered regarding the role of microdomain-specific signaling in the development of cardiac hypertrophy and heart failure. First, the contribution of other microdomains such as lipid rafts () or couplons () needs to be determined, which requires the development of novel and simple tools to assess these microdomain signals. Second, the physiological relevance of the regulation of these microdomain-specific signaling pathways should be determined in vivo. Third, the methodology for assessing signals other than CaMKII needs to be developed. Finally, the role of the specific activation of these signals in subcellular organelles such as the mitochondrion should be determined. Methods based on a phosphorylatable peptide-tag have great potential to help tackle these questions. Taken together, the evidence accumulated to date indicates that selective inhibition of target molecules involved in caveolae-specific signaling based on their subcellular location could be a promising therapeutic tool to treat cardiac hypertrophy and heart failure in the future.

Statements

Author contributions

ST and HN wrote the manuscript. YF checked and approved the manuscript.

Funding

This work was partially supported by MEXT/JSPS KAKENHI (Grant No. 17K09576) and Smoking Research Foundation to HN. This research was also supported by AMED under Grant No. 17am0101084j0001.

Acknowledgments

We thank Chiharu Tottori for the excellent secretarial work.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

Summary

Keywords

caveolae, caveolin, CaMKII, L-type calcium channel, cardiac hypertrophy

Citation

Tanaka S, Fujio Y and Nakayama H (2018) Caveolae-Specific CaMKII Signaling in the Regulation of Voltage-Dependent Calcium Channel and Cardiac Hypertrophy. Front. Physiol. 9:1081. doi: 10.3389/fphys.2018.01081

Received

30 April 2018

Accepted

19 July 2018

Published

07 August 2018

Volume

9 - 2018

Edited by

Alexey V. Glukhov, University of Wisconsin System, United States

Reviewed by

Di Lang, University of Wisconsin-Madison, United States; Jonathan Jaseth Hernandez, University of Michigan, United States

Updates

Copyright

*Correspondence: Hiroyuki Nakayama,

This article was submitted to Cardiac Electrophysiology, a section of the journal Frontiers in Physiology

Disclaimer

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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