PDB entry 3cln , shown here, has all four sites filled with calcium ions and the linker has formed a long alpha helix separating the two calcium-binding domains. Many different proteins are sensitive to calcium levels inside and outside cells. In the late 's, before the discovery of calmodulin, troponin C see, for instance, PDB entry 1tcf was the first protein shown to be sensitive to calcium. Troponin C senses rising calcium levels and triggers muscle contraction.
The structures of troponin C and calmodulin are remarkably similar, the major difference being the length of the linker connecting the two calcium-binding globular domains. The calcium-binding region of the protein, shown in detail in a later section, is almost identical. This motif has since been found in dozens of other calcium-sensitive proteins. Protein motion in calmodulin: without calcium left and with calcium right.
Sites that bind to target proteins are shown with stars. Calmodulin's target proteins come in various shapes, sizes and sequences and are involved in a wide array of functions. For example, calcium-bound calmodulin forms a critical subunit for the regulatory enzyme phosphorylase kinase, which in turn is a regulator for glycogen breakdown.
Calmodulin also binds and activates other kinases and phosphatases that play significant roles in cell signaling, ion transport and cell death. One common theme in the contact between calmodulin and its different target proteins is the use of non-polar interactions, in particular, through the interactions with the unusually abundant methionines of calmodulin. Calcium binding exposes these non-polar surfaces of calmodulin, which then bind to non-polar regions on the target proteins.
The structure shown on the left, from PDB entry 1cfd , shows calmodulin without calcium, and the structure on the right, from PDB entry 1cll , shows calmodulin after calcium binds.
The key nonpolar areas are colored with carbon atoms in green and the many methionine sulfur atoms in yellow. Notice how these non-polar amino acids form two neat grooves shown with red stars when calcium binds, waiting to grip the target protein. Because these non-polar grooves are generic in shape, calmodulin acts as a versatile regulatory protein and its targets are not required to possess any specific amino acid sequence or structural binding motifs.
Complexes of calmodulin with target proteins: with peptides from calmodulin-dependent protein kinase II-alpha top left and myosin light chain kinase top right , and with anthrax edema factor right. Examples of such abnormalities are conditions including atrial fibrillation, congenital heart disease, pericarditis constrictions, and chronic cardiac ischemia. Determining whether the CRS is a type 1 or 2 represents a challenge for clinicians since the majority of diagnostics are made when both organs are already injured.
The other two types of CRS 3 and 4 are described as nephron-cardiac syndromes, where the renal injury leads to cardiac dysfunction Damman et al. Type 3 CRS is defined as acute nephron-cardiac syndrome, occurring when acute renal failure leads to the development of acute cardiac injury. It is intimately related to events triggering increased inflammatory processes, such as oxidative stress and secretion of neurohormones Di Lullo et al.
Type 4 CRS is studied as chronic nephron-cardiac disease, initiated by chronic kidney disease CKD , leading to cardiovascular disease. Finally, CRS type 5 is a systemic disorder that reaches both organs simultaneously. Many factors have been suggested to contribute to these conditions, for instance, sepsis, infections, drugs, toxins, and diabetes. CRS type 5 results in cardiac and renal dysfunction coming from a larger and systematic situation.
The study of CRS is of great relevance for clinical treatment, considering that cardiovascular diseases represent the main cause of death in the United States for at least the last 15 years, according to the Centers for Disease Control and Prevention CDC 1. CaMKII has already been described as a cause of many heart dysfunctions, such as arrhythmia, hypertrophy, and infarction Yoo et al.
However, Alfazema and collaborators showed, recently, in a translational study, that deletion of CaMK2n1, diminishes CaMKII activity in the kidney and heart without affecting adipose tissue Alfazema et al.
Yet there are few studies involving both organs in a systemic profile. It is known that elements, such as the immune system, can mediate the communication between them. In an inflammatory process as observed during CKD and chronic heart failure CHF , cytokines are released by circulating and tissue-resident inflammatory cells monocytes mainly and play an important role in the progression of these diseases Yogasundaram et al.
In CRS, the tissue injury is strongly followed by inflammation. Many inflammatory cytokines are enhanced in experimental models of renal ischemia TNF-a, IL-1, and IL-6 and also markers, including the factor nuclear kappa B NF-kB , which is very important for cell signaling during inflammatory processes Trentin-Sonoda et al. During cardiac ischemia, myocytes release inflammatory cytokines Colombo et al. Colombo et al. Inflammation also causes progressive renal dysfunction and fibrosis, which continues to injure the organ, maintaining the cycle.
Additionally, inflammation leads to the release of renin, activating the renin-angiotensin-aldosterone system RAAS , which activates the sympathetic nervous system SNS by increasing serum norepinephrine concentrations and is the cause of ROS release from the inflammatory cells Bongartz et al. As mentioned above and summarized by Rusciano et al. There are many blockers and inhibitors used nowadays in research.
The first inhibitor described was KN in Tokumitsu et al. One year later, in , Sumi M and collaborators described a new and more selective inhibitor called KN Sumi et al. It was already discovered that KN can directly block the potassium current I Kr and potassium voltaged channels, preventing arrhythmic properties of CamKII Mustroph et al. Some studies have proven the efficiency of KN in heart pathologies in several animal models.
Under in vitro and in vivo stimulations with isoproterenol, arrhythmias have been abolished after using NK Sag et al. On some models, the KN does not seem to prevent but to slow the arrhythmia as longer cycle length without marked alterations in baseline ECG characteristics Hoeker et al. Another study shows the high capacity to inhibit the binding of CaM with Na V 1.
Experiments using TG RYR-mutant mice SD mutant are naturally more susceptible to atrial fibrillation, and because of this, it is used as a well-established model. It is the most used one in studies that require the blocking of CaMKII, and it is resistant even to proteolysis. As mentioned, AC3-I is derived from autocamtide As the research regarding CaMKII increases, its therapeutic use implicated in pharmaceuticals has been studied more.
Several studies, using these inhibitors mentioned above, impart a cardio protection. This is the main reason it is used more in the studies concerning the participation of CaMKII in many cell functions. Studies using this blocker also imply cardio protection: preventing hypertrophy, reducing ventricular arrhythmias, improving mechanical function, reducing RyR2 lacking, and decreasing mortality of diabetic mice Sag et al. Some studies have proven the efficiency of AC3-I in pathologies in some animal models.
CRS, already cited, seems to be one of them. Both kidneys and heart share many mechanisms of homeostasis, and any injury to one can lead to one in the other. The close relation between inflammation and oxidative stress in pathophysiological processes also makes the balance between oxidant and antioxidant forces and, therefore, oxidative stress, one of the most important mechanisms as has been demonstrated in heart and kidney injury studies Li et al. Some of these pathologies include left ventricle hypertrophy, atherosclerosis, endothelial dysfunction, and fibrosis in the heart while in the kidney ROS promotes interstitial fibrosis and increased inflammation Kumar et al.
Oxidative stress triggers an inflammatory response, and this response induces more oxidative stress. This stress may be maintaining the previously mentioned cycle of damage. Erickson et al. Some proteins maintain a redox sensor that regulates the cell response to oxidative stress Kim et al.
CaM is one of these proteins, and this oxidation leads to a regulatory cascade response with specific targets, including CaMKII Snijder et al. This isoform of NOS produces an excessive amount of NO that mediates impaired vasoconstriction, which may be further worsened by the decreased of eNOS activity Jian et al. In addition, studies have shown the role of NOS in the kidney, demonstrating that, when NOS activity is compromised, there are a series of renal dysfunctions that reduce glomerular perfusion and filtration, which may lead to a progressive scenario of hypertension and kidney injuries Carlstrom and Montenegro, On the other hand, Kong et al.
In addition to the redox balance, other factors indirectly contribute to cardiac and renal alterations. Many approaches have been studied in order to set a start point for the CRS. One of them is epigenetics factors. Epigenetics is the area of biology that studies changes in the functioning of a gene that is not caused by alterations in the DNA sequence and that perpetuate in the meiotic or mitotic cell divisions Wu and Morris, Epigenetic modifications are highly coordinated processes of change that are not restricted to a specific phase of life.
These characteristics are fundamental to diseases acquired throughout life. Studies have been developed to innovate the way to prevent CRS. Slowly, epigenetics is gaining space, and traditional mechanisms such as RAAS and inflammation are being replaced by other patterns of findings and prevention. Imaging the scale of modifications and mutations in a syndrome such as CRS, numerous cell lines may be altered and reprogrammed, in both heart and kidney.
Studies have pointed out the role of epigenetics in the development of CRS Gaikwad et al. In types 3 and 4, for example, renal failure increases cardiac histone H3 epigenetics, evidencing the crosstalk between renal failure and the transcription of cardiomyopathy-related genes Gaikwad et al. It is important to mention that epigenetics in CRS itself are little studied when compared to the traditional mechanisms even though it is very promising.
The focus of studies is linked to inflammation and oxidative stress, which we know to be the consequences of CRS. During HF independent of renal injury, we can note the expression of transcription factors, angiogenic factors, and natriuretic factors, often used as biomarkers of this condition. Epigenetic modifications regulate them.
Pathological hypertrophy and compromised contractility are described to increase DNA methylation levels. This connection can lead to CRS types 1 and 2 and can be strongly linked to the diagnosis of heart dysfunction. In addition, it is extremely important to highlight the role of micro RNA miR. To identify signaling pathways, there is a serviceable tool called gene set analysis GSA. It uses statistical analysis to predefine gene sets involved in a specific cellular process.
For example, miR has a key role during cardiac hypertrophy. Given that, miR is also a potent therapeutic target for cardiac diseases Brown et al. In addition, Kim et al. It is worth mentioning the role of miR-1, once alterations in its expression or inhibition have been discovered in many cardiac pathologies Yang et al. Recently, Zhang et al. Regarding inflammatory processes, miRp has been illustrated to inhibit inflammatory response in human bronchial epithelial cells and is downregulated in heart diseases once miRp is able to inhibit STAT3 and reduce the expression of CaMKII.
In relation to kidney disease, Park et al. In addition, recent studies have focused on MiR regulation and exosomes, specialized nanosized membranous vesicles, in different experimental models.
These membrane-bound vesicles 30— nm are released from different cell types and deliver bioactive molecules, including microRNAs miRs. Previous studies report that lncRNAs play critical roles in the modulation of heart development and cardiovascular diseases Wang et al. For example, Shao et al. The evidence in the literature suggests that CaMKII is a key molecule for understanding the physiology and physiopathology of cardiovascular diseases as well as a prominent target for new strategies of treatment.
Figure 2. CJ and MC-R proposed the idea and writing. All authors contributed to the article and approved the submitted version. 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. Alarcon, M. Alfazema, N. Camk2n1 is a negative regulator of blood pressure, left ventricular mass, insulin sensitivity, and promotes adiposity.
Hypertension 74, — Anavekar, N. Relation between renal dysfunction and cardiovascular outcomes after myocardial infarction. Anbanandam, A. Mediating molecular recognition by methionine oxidation: conformational switching by oxidation of methionine in the carboxyl-terminal domain of calmodulin. Biochemistry 44, — Awad, S. Backs, J. Beckendorf, J. Basic Res. Beckerman, P. Epigenetics: a new way to look at kidney diseases.
Bers, D. Bongartz, L. Heart J. Boubali, S. Brown, J. The Rac and Rho Hall of Fame. Bui, J. Cell , — Bussey, C. Calmodulin, or CaM , is a polypeptide that is ubiquitous in all eukaryotic cells. This protein is known as calmodulin because it is a cal cium- mod ulated prote in that plays a vital role in the process of calcium signal transduction. Calcium signal transduction is the process through which the interactions between calcium ions and numerous proteins mediate communication between cells.
The protein itself is amino acids in length with two globular regions containing 2 EF-hand motifs each, which are characteristic sites of calcium-mediated polypeptides. When calmodulin binds with the calcium ions, the protein opens from its apo form to its halo form, exposing an alpha helix that is known as the linker or central tether region.
Coined for its flexibility, the central tether region is the location of the protein on which partner proteins bind and contribute to the cascade that is the secondary messaging of calcium. Based on its structure and its need for calcium ions to function, calmodulin must be able to select for calcium ions in the cytoplasm, and the interactions between the ion and the ligands in the EF hand domains support this idea of selectivity Bertini et.
This image is a depiction of how an EF-hand motif resembles a hand. The ribbon diagram on the left shows one of the four characteristic helix-turn-helix EF hand motifs of calmodulin. EF-hand motifs are highly conserved structural regions of proteins involved in the binding of calcium.
This is demonstrated in the figure above. In a characteristic EF-hand motif, amino acids including glutamates, asparagines, aspartic acids, and glutamic acids bind to Ca, as well as water Bertini et. The typical EF-hand domain bonding sequence is shown in the figure below. This cartoon illustrates the coordinated amino acids in a typical EF hand domain The exact amino acid composition varies! The dashed line represents coordination of Ca to the oxygen of a backbone carbonyl, while solid lines indicate coordination to side chains or water.
The calmodulin binding site is somewhat different than the most typical EF hand domain shown above. The 6 ligands of calmodulin include the side chains of three asparatic acids D , 1 glutamic acid E that forms two coordinate covalent bonds with the ion, 1 water molecule from solution, and 1 carbonyl molecule from the backbone.
One can notice that the EF motif within calmodulin includes many of the same ligands, just in different places. This conformation may lend an idea of why calmodulin is selective for calcium and how only the presence of a calcium ion can satisfy the binding site that causes the protein to activate. This is related to the thermodynamics of calmodulin within the cell and how it responds to the presence of calcium ions. To understand the thermodynamics that couples the activation of calmodulin, one must first understand the concentration of ions such as calcium and magnesium within a eukaryotic cell, including that of a human.
The concentration of these ions must be closely regulated. Within in the cell, calcium signaling is accompanied by a temporary increase in the concentration of calcium ions, which is sensed by proteins such as calmodulin Bertini et. This calcium surge could be due to intracellular G-proteins that induce the rough and smooth reticulum to release calcium, or the calcium ions could be brought in from the extracellular space. In the case of calmodulin, it is usually responding to calcium being brought into the cell from the outside, which occurs during processes such as nerve signaling Bertini et.
Before the concentration of calcium is momentarily raised, the concentration within the cell is usually between nm, whereas during the brief influx of ions, the concentration increases to 1,, nm. The change in concentration causes the calmodulin to sense the calcium ions, bind them, and initiate further signal transduction Bertini et. How, then, does calmodulin definitively bind calcium ions and not, for example, magnesium ions?
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