Structural analysis of calmodulin binding to ion channels demonstrates the role of its plasticity in regulation

Advances in TRPV6 inhibitors for tumors by targeted therapies: Macromolecular proteins, synthetic small molecule compounds, and natural compounds

TRPV6, a Ca2+-selective member of the transient receptor potential vanilloid (TRPV) family, plays a key role in extracellular calcium transport, calcium ion reuptake, and maintenance of a local low calcium environment. An increasing number of studies have shown that TRPV6 is involved in the regulation of various diseases. Notably, overexpression of TRPV6 is closely related to the occurrence of various cancers. Research confirmed that knocking down TRPV6 could effectively reduce the proliferation and invasiveness of tumors by mainly mediating the calcium signaling pathway. Hence, TRPV6 has become a promising new drug target for numerous tumor treatments. However, the development of TRPV6 inhibitors is still in the early stage, and the existing TRPV6 inhibitors have poor selectivity and off-target effects. In this review, we focus on summarizing and describing the structure characters, and mechanisms of existing TRPV6 inhibitors to provide new ideas and directions for the development of novel TRPV6 inhibitors.

2'-deoxy-ADPR activates human TRPM2 faster than ADPR and thereby induces higher currents at physiological Ca2+ concentrations

TRPM2 is a Ca2+ permeable, non-selective cation channel in the plasma membrane that is involved in the innate immune response regulating, for example, chemotaxis in neutrophils and cytokine secretion in monocytes and macrophages. The intracellular adenine nucleotides ADP-ribose (ADPR) and 2'-deoxy-ADPR (2dADPR) activate the channel, in combination with their co-agonist Ca2+. Interestingly, activation of human TRPM2 (hsTRPM2) by 2dADPR is much more effective than activation by ADPR. However, the underlying mechanism of the nucleotides' differential effect on the channel is not yet fully understood. In this study, we performed whole-cell patch clamp experiments with HEK293 cells heterologously expressing hsTRPM2. We show that 2dADPR has an approx. 4-fold higher Ca2+ sensitivity than ADPR (EC50 = 190 and 690 nM). This allows 2dADPR to activate the channel at lower and thus physiological intracellular Ca2+ concentrations. Kinetic analysis of our data reveals that activation by 2dADPR is faster than activation by ADPR. Mutation in a calmodulin binding N-terminal IQ-like motif in hsTRPM2 completely abrogated channel activation by both agonists. However, mutation of a single amino acid residue (W1355A) in the C-terminus of hsTRPM2, at a site of extensive inter-domain interaction, resulted in slower activation by 2dADPR and neutralized the difference in rate of activation between the two agonists. Taken together, we propose a mechanism by which 2dADPR induces higher hsTRPM2 currents than ADPR by means of faster channel activation. The finding that 2dADPR has a higher Ca2+ sensitivity than ADPR may indicate that 2dADPR rather than ADPR activates hsTRPM2 in physiological contexts such as the innate immune response.

Megalencephalic leukoencephalopathy with subcortical cysts protein-1: A new calcium-sensitive protein functionally activated by endoplasmic reticulum calcium release and calmodulin binding in astrocytes

BACKGROUND

MLC1 is a membrane protein highly expressed in brain perivascular astrocytes and whose mutations account for the rare leukodystrophy (LD) megalencephalic leukoencephalopathy with subcortical cysts disease (MLC). MLC is characterized by macrocephaly, brain edema and cysts, myelin vacuolation and astrocyte swelling which cause cognitive and motor dysfunctions and epilepsy. In cultured astrocytes, lack of functional MLC1 disturbs cell volume regulation by affecting anion channel (VRAC) currents and the consequent regulatory volume decrease (RVD) occurring in response to osmotic changes. Moreover, MLC1 represses intracellular signaling molecules (EGFR, ERK1/2, NF-kB) inducing astrocyte activation and swelling following brain insults. Nevertheless, to date, MLC1 proper function and MLC molecular pathogenesis are still elusive. We recently reported that in astrocytes MLC1 phosphorylation by the Ca2+/Calmodulin-dependent kinase II (CaMKII) in response to intracellular Ca2+ release potentiates MLC1 activation of VRAC. These results highlighted the importance of Ca2+ signaling in the regulation of MLC1 functions, prompting us to further investigate the relationships between intracellular Ca2+ and MLC1 properties.

METHODS

We used U251 astrocytoma cells stably expressing wild-type (WT) or mutated MLC1, primary mouse astrocytes and mouse brain tissue, and applied biochemistry, molecular biology, video imaging and electrophysiology techniques.

RESULTS

We revealed that WT but not mutant MLC1 oligomerization and trafficking to the astrocyte plasma membrane is favored by Ca2+ release from endoplasmic reticulum (ER) but not by capacitive Ca2+ entry in response to ER depletion. We also clarified the molecular events underlining MLC1 response to cytoplasmic Ca2+ increase, demonstrating that, following Ca2+ release, MLC1 binds the Ca2+ effector protein calmodulin (CaM) at the carboxyl terminal where a CaM binding sequence was identified. Using a CaM inhibitor and generating U251 cells expressing MLC1 with CaM binding site mutations, we found that CaM regulates MLC1 assembly, trafficking and function, being RVD and MLC-linked signaling molecules abnormally regulated in these latter cells.

CONCLUSION

Overall, we qualified MLC1 as a Ca2+ sensitive protein involved in the control of volume changes in response to ER Ca2+ release and astrocyte activation. These findings provide new insights for the comprehension of the molecular mechanisms responsible for the myelin degeneration occurring in MLC and other LD where astrocytes have a primary role in the pathological process.

Structural basis of calmodulin modulation of the rod cyclic nucleotide-gated channel

Calmodulin (CaM) regulates many ion channels to control calcium entry into cells, and mutations that alter this interaction are linked to fatal diseases. The structural basis of CaM regulation remains largely unexplored. In retinal photoreceptors, CaM binds to the CNGB subunit of cyclic nucleotide-gated (CNG) channels and, thereby, adjusts the channel's Cyclic guanosine monophosphate (cGMP) sensitivity in response to changes in ambient light conditions. Here, we provide the structural characterization for CaM regulation of a CNG channel by using a combination of single-particle cryo-electron microscopy and structural proteomics. CaM connects the CNGA and CNGB subunits, resulting in structural changes both in the cytosolic and transmembrane regions of the channel. Cross-linking and limited proteolysis-coupled mass spectrometry mapped the conformational changes induced by CaM in vitro and in the native membrane. We propose that CaM is a constitutive subunit of the rod channel to ensure high sensitivity in dim light. Our mass spectrometry-based approach is generally relevant for studying the effect of CaM on ion channels in tissues of medical interest, where only minute quantities are available.

Regulation of Cardiac Cav1.2 Channels by Calmodulin

Cav1.2 Ca2+ channels, a type of voltage-gated L-type Ca2+ channel, are ubiquitously expressed, and the predominant Ca2+ channel type, in working cardiac myocytes. Cav1.2 channels are regulated by the direct interactions with calmodulin (CaM), a Ca2+-binding protein that causes Ca2+-dependent facilitation (CDF) and inactivation (CDI). Ca2+-free CaM (apoCaM) also contributes to the regulation of Cav1.2 channels. Furthermore, CaM indirectly affects channel activity by activating CaM-dependent enzymes, such as CaM-dependent protein kinase II and calcineurin (a CaM-dependent protein phosphatase). In this article, we review the recent progress in identifying the role of apoCaM in the channel ‘rundown’ phenomena and related repriming of channels, and CDF, as well as the role of Ca2+/CaM in CDI. In addition, the role of CaM in channel clustering is reviewed.

Biochemistry and pathophysiology of the Transient Potential Receptor Vanilloid 6 (TRPV6) calcium channel

TRPV6 is a Transient Receptor Potential Vanilloid (TRPV) cation channel with high selectivity for Ca²⁺ ions. First identified in 1999 in a search for the gene which mediates intestinal Ca²⁺ absorption, its far more extensive repertoire as a guardian of intracellular Ca²⁺ has since become apparent. Studies on TRPV6-deficient mice demonstrated additional important roles in placental Ca²⁺ transport, fetal bone development and male fertility. The first reports of inherited deficiency in newborn babies appeared in 2018, revealing its physiological importance in humans. There is currently strong evidence that TRPV6 also contributes to the pathogenesis of some common cancers. The recently reported association of TRPV6 deficiency with non-alcoholic chronic pancreatitis suggests a role in normal pancreatic function. Over time and with greater awareness of TRPV6, other disease-associations are likely to emerge. Powerful analytical tools have provided invaluable insights into the structure and operation of TRPV6. Its roles in Ca²⁺ signaling and carcinogenesis, and the use of channel inhibitors in cancer treatment are being intensively investigated. This review first briefly describes the biochemistry and physiology of the channel, and analytical methods used to investigate these. The focus subsequently shifts to the clinical disorders associated with abnormal expression and the underlying pathophysiology. The aims of this review are to increase awareness of this channel, and to draw together findings from a wide range of sources which may help to formulate new ideas for further studies.

Simultaneous binding of the N- and C-terminal cytoplasmic domains of aquaporin 4 to calmodulin

Aquaporin 4 (AQP4) is a water transporting, transmembrane channel protein that has important regulatory roles in maintaining cellular water homeostasis. Several other AQP proteins exhibit calmodulin (CaM)-binding properties, and CaM has recently been implicated in the cell surface localization of AQP4. The objective of the present study was to assess the CaM-binding properties of AQP4 in detail. Inspection of AQP4 revealed two putative CaM-binding domains (CBDs) in the cytoplasmic N- and C-terminal regions, respectively. The Ca²⁺-dependent CaM-binding properties of AQP4 CBD peptides were assessed using fluorescence spectroscopy, isothermal titration calorimetry, and two-dimensional ¹H, ¹⁵N-HSQC NMR with ¹⁵N-labeled CaM. The N-terminal CBD of AQP4 predominantly interacted with the N-lobe of CaM with a 1:1 binding ratio and a K_d of 3.4 μM. The C-terminal AQP4 peptide interacted with both the C- and N-lobes of CaM (2:1 binding ratio; K_d1: 3.6 μM, K_d2: 113.6 μM, respectively). A recombinant AQP4 protein domain (recAQP4^CT, containing the entire cytosolic C-terminal sequence) bound CaM in a 1:1 binding mode with a K_d of 6.1 μM. A ternary bridging complex could be generated with the N- and C-lobes of CaM interacting simultaneously with the N- and C-terminal CBD peptides. These data support a unique adapter protein binding mode for CaM with AQP4.

Calmodulin-Connexin Partnership in Gap Junction Channel Regulation-Calmodulin-Cork Gating Model

In the past four decades numerous findings have indicated that gap junction channel gating is mediated by intracellular calcium concentrations ([Ca²⁺_i]) in the high nanomolar range via calmodulin (CaM). We have proposed a CaM-based gating model based on evidence for a direct CaM role in gating. This model is based on the following: CaM inhibitors and the inhibition of CaM expression to prevent chemical gating. A CaM mutant with higher Ca²⁺ sensitivity greatly increases gating sensitivity. CaM co-localizes with connexins. Connexins have high-affinity CaM-binding sites. Connexin mutants paired to wild type connexins have a higher gating sensitivity, which is eliminated by the inhibition of CaM expression. Repeated trans-junctional voltage (Vj) pulses progressively close channels by the chemical/slow gate (CaM’s N-lobe). At the single channel level, the gate closes and opens slowly with on-off fluctuations. Internally perfused crayfish axons lose gating competency but recover it by the addition of Ca-CaM to the internal perfusion solution. X-ray diffraction data demonstrate that isolated gap junctions are gated at the cytoplasmic end by a particle of the size of a CaM lobe. We have proposed two types of CaM-driven gating: “Ca-CaM-Cork” and “CaM-Cork”. In the first, the gating involves Ca²⁺-induced CaM activation. In the second, the gating occurs without a [Ca²⁺]_i rise.

The X-ray structure of juvenile hormone diol kinase from the silkworm Bombyx mori

Insect juvenile hormones (JHs) are a family of sesquiterpenoid molecules that are secreted into the haemolymph. JHs have multiple roles in insect development, metamorphosis and sexual maturation. A number of pesticides work by chemically mimicking JHs, thus preventing insects from developing and reproducing normally. The haemolymph levels of JH are governed by the rates of its biosynthesis and degradation. One enzyme involved in JH catabolism is JH diol kinase (JHDK), which uses ATP (or GTP) to phosphorylate JH diol to JH diol phosphate, which can be excreted. The X-ray structure of JHDK from the silkworm Bombyx mori has been determined at a resolution of 2.0 Å with an R factor of 19.0% and an Rfree of 24.8%. The structure possesses three EF-hand motifs which are occupied by calcium ions. This is in contrast to the recently reported structure of the JHDK-like-2 protein from B. mori (PDB entry 6kth), which possessed only one calcium ion. Since JHDK is known to be inhibited by calcium ions, it is likely that our structure represents the calcium-inhibited form of the enzyme. The electrostatic surface of the protein suggests a binding site for the triphosphate of ATP close to the N-terminal end of the molecule in a cavity between the N- and C-terminal domains. Superposition with a number of calcium-activated photoproteins suggests that there may be parallels between the binding of JH diol to JHDK and the binding of luciferin to aequorin.

Gap Junction Channelopathies and Calmodulinopathies. Do Disease-Causing Calmodulin Mutants Affect Direct Cell-Cell Communication?

The cloning of connexins cDNA opened the way to the field of gap junction channelopathies. Thus far, at least 35 genetic diseases, resulting from mutations of 11 different connexin genes, are known to cause numerous structural and functional defects in the central and peripheral nervous system as well as in the heart, skin, eyes, teeth, ears, bone, hair, nails and lymphatic system. While all of these diseases are due to connexin mutations, minimal attention has been paid to the potential diseases of cell–cell communication caused by mutations of Cx-associated molecules. An important Cx accessory protein is calmodulin (CaM), which is the major regulator of gap junction channel gating and a molecule relevant to gap junction formation. Recently, diseases caused by CaM mutations (calmodulinopathies) have been identified, but thus far calmodulinopathy studies have not considered the potential effect of CaM mutations on gap junction function. The major goal of this review is to raise awareness on the likely role of CaM mutations in defects of gap junction mediated cell communication. Our studies have demonstrated that certain CaM mutants affect gap junction channel gating or expression, so it would not be surprising to learn that CaM mutations known to cause diseases also affect cell communication mediated by gap junction channels.

Differential modulation of SK channel subtypes by phosphorylation

Small-conductance Ca2+-activated K+ (SK) channels are voltage-independent and are activated by Ca2+ binding to the calmodulin constitutively associated with the channels. Both the pore-forming subunits and the associated calmodulin are subject to phosphorylation. Here, we investigated the modulation of different SK channel subtypes by phosphorylation, using the cultured endothelial cells as a tool. We report that casein kinase 2 (CK2) negatively modulates the apparent Ca2+ sensitivity of SK1 and IK channel subtypes by more than 5-fold, whereas the apparent Ca2+ sensitivity of the SK3 and SK2 subtypes is only reduced by ∼2-fold, when heterologously expressed on the plasma membrane of cultured endothelial cells. The SK2 channel subtype exhibits limited cell surface expression in these cells, partly as a result of the phosphorylation of its C-terminus by cyclic AMP-dependent protein kinase (PKA). SK2 channels expressed on the ER and mitochondria membranes may protect against cell death. This work reveals the subtype-specific modulation of the apparent Ca2+ sensitivity and subcellular localization of SK channels by phosphorylation in cultured endothelial cells.

Structural basis of cytoplasmic NaV1.5 and NaV1.4 regulation

Voltage-gated sodium channels (NaVs) are membrane proteins responsible for the rapid upstroke of the action potential in excitable cells. There are nine human voltage-sensitive NaV1 isoforms that, in addition to their sequence differences, differ in tissue distribution and specific function. This review focuses on isoforms NaV1.4 and NaV1.5, which are primarily expressed in skeletal and cardiac muscle cells, respectively. The determination of the structures of several eukaryotic NaVs by single-particle cryo-electron microscopy (cryo-EM) has brought new perspective to the study of the channels. Alignment of the cryo-EM structure of the transmembrane channel pore with x-ray crystallographic structures of the cytoplasmic domains illustrates the complementary nature of the techniques and highlights the intricate cellular mechanisms that modulate these channels. Here, we review structural insights into the cytoplasmic C-terminal regulation of NaV1.4 and NaV1.5 with special attention to Ca2+ sensing by calmodulin, implications for disease, and putative channel dimerization.

The LQT-associated calmodulin mutant E141G induces disturbed Ca2+-dependent binding and a flickering gating mode of the CaV1.2 channel

Calmodulin (CaM) mutations are associated with congenital long QT (LQT) syndrome (LQTS), which may be related to the dysregulation of the cardiac-predominant Ca²⁺ channel isoform Ca_V1.2. Among various mutants, CaM-E141G was identified as a critical missense variant. However, the interaction of this CaM mutant with the Ca_V1.2 channel has not been determined. In this study, by utilizing a semiquantitative pull-down assay, we explored the interaction of CaM-E141G with CaM-binding peptide fragments of the Ca_V1.2 channel. Using the patch-clamp technique, we also investigated the electrophysiological effects of the mutant on Ca_V1.2 channel activity. We found that the maximum binding (B_max) of CaM-E141G to the proximal COOH-terminal region, PreIQ-IQ, PreIQ, IQ, and NT (an NH₂-terminal peptide) was decreased (by 17.71-59.26%) compared with that of wild-type CaM (CaM-WT). In particular, the Ca²⁺-dependent increase in B_max became slower with the combination of CaM-E141G + PreIQ and IQ but faster in the case of NT. Functionally, CaM-WT and CaM-E141G at 500 nM Ca²⁺ decreased Ca_V1.2 channel activity to 24.88% and 55.99%, respectively, compared with 100 nM Ca²⁺, showing that the inhibitory effect was attenuated in CaM-E141G. The mean open time of the Ca_V1.2 channel was increased, and the number of blank traces with no channel opening was significantly decreased. Overall, CaM-E141G exhibits disrupted binding with the Ca_V1.2 channel and induces a flickering gating mode, which may result in the dysfunction of the Ca_V1.2 channel and, thus, the development of LQTS. The present study is the first to investigate the detailed binding properties and single-channel gating mode induced by the interaction of CaM-E141G with the Ca_V1.2 channel.

Ca2+/calmodulin-dependent regulation of polycystic kidney disease 2-like-1 by binding at C-terminal domain

Polycystic kidney disease 2-like-1 (PKD2L1), also known as polycystin-L or TRPP3, is a non-selective cation channel that regulates intracellular calcium concentration. Calmodulin (CaM) is a calcium binding protein, consisting of N-lobe and C-lobe with two calcium binding EF-hands in each lobe. In previous study, we confirmed that CaM is associated with desensitization of PKD2L1 and that CaM N-lobe and PKD2L1 EF-hand specifically are involved. However, the CaM-binding domain (CaMBD) and its inhibitory mechanism of PKD2L1 have not been identified. In order to identify CaM-binding anchor residue of PKD2L1, single mutants of putative CaMBD and EF-hand deletion mutants were generated. The current changes of the mutants were recorded with whole-cell patch clamp. The calmidazolium (CMZ), a calmodulin inhibitor, was used under different concentrations of intracellular. Among the mutants that showed similar or higher basal currents with that of the PKD2L1 wild type, L593A showed little change in current induced by CMZ. Co-expression of L593A with CaM attenuated the inhibitory effect of PKD2L1 by CaM. In the previous study it was inferred that CaM C-lobe inhibits channels by binding to PKD2L1 at 16 nM calcium concentration and CaM N-lobe at 100 nM. Based on the results at 16 nM calcium concentration condition, this study suggests that CaM C-lobe binds to Leu-593, which can be a CaM C-lobe anchor residue, to regulate channel activity. Taken together, our results provide a model for the regulation of PKD2L1 channel activity by CaM.

PIRT the TRP Channel Regulating Protein Binds Calmodulin and Cholesterol-Like Ligands

Transient receptor potential (TRP) ion channels are polymodal receptors that have been implicated in a variety of pathophysiologies, including pain, obesity, and cancer. The capsaicin and heat sensor TRPV1, and the menthol and cold sensor TRPM8, have been shown to be modulated by the membrane protein PIRT (Phosphoinositide-interacting regulator of TRP). The emerging mechanism of PIRT-dependent TRPM8 regulation involves a competitive interaction between PIRT and TRPM8 for the activating phosphatidylinositol 4,5-bisphosphate (PIP2) lipid. As many PIP2 modulated ion channels also interact with calmodulin, we investigated the possible interaction between PIRT and calmodulin. Using microscale thermophoresis (MST), we show that calmodulin binds to the PIRT C-terminal α-helix, which we corroborate with a pull-down experiment, nuclear magnetic resonance-detected binding study, and Rosetta-based computational studies. Furthermore, we identify a cholesterol-recognition amino acid consensus (CRAC) domain in the outer leaflet of the first transmembrane helix of PIRT, and with MST, show that PIRT specifically binds to a number of cholesterol-derivatives. Additional studies identified that PIRT binds to cholecalciferol and oxytocin, which has mechanistic implications for the role of PIRT regulation of additional ion channels. This is the first study to show that PIRT specifically binds to a variety of ligands beyond TRP channels and PIP2.

Structure and function of the calcium-selective TRP channel TRPV6

Epithelial calcium channel TRPV6 is a member of the vanilloid subfamily of TRP channels that is permeable to cations and highly selective to Ca²⁺ ; it shows constitutive activity regulated negatively by Ca²⁺ and positively by phosphoinositol and cholesterol lipids. In this review, we describe the molecular structure of TRPV6 and discuss how its structural elements define its unique functional properties. High Ca²⁺ selectivity of TRPV6 originates from the narrow selectivity filter, where Ca²⁺ ions are directly coordinated by a ring of anionic aspartate side chains. Divalent cations Ca²⁺ and Ba²⁺ permeate TRPV6 pore according to the knock-off mechanism, while tight binding of Gd³⁺ to the aspartate ring blocks the channel and prevents Na⁺ from permeating the pore. The iris-like channel opening is accompanied by an α-to-π helical transition in the pore-lining transmembrane helix S6. As a result of this transition, the intracellular halves of the S6 helices bend and rotate by about 100 deg, exposing different residues to the channel pore in the open and closed states. Channel opening is also associated with changes in occupancy of the transmembrane domain lipid binding sites. The inhibitor 2-aminoethoxydiphenyl borate (2-APB) binds to TRPV6 in a pocket formed by the cytoplasmic half of the S1-S4 transmembrane helical bundle and shifts open-closed channel equilibrium towards the closed state by outcompeting lipids critical for activation. Ca²⁺ inhibits TRPV6 via binding to calmodulin (CaM), which mediates Ca²⁺ -dependent inactivation. The TRPV6-CaM complex exhibits 1:1 stoichiometry; one TRPV6 tetramer binds both CaM lobes, which adopt a distinct head-to-tail arrangement. The CaM C-terminal lobe plugs the channel through a unique cation-π interaction by inserting the side chain of lysine K115 into a tetra-tryptophan cage at the ion channel pore intracellular entrance. Recent studies of TRPV6 structure and function described in this review advance our understanding of the role of this channel in physiology and pathophysiology and inform new therapeutic design.

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

The CaMKII inhibitor KN93-calmodulin interaction and implications for calmodulin tuning of NaV1.5 and RyR2 function

Here we report the structure of the widely utilized calmodulin (CaM)-dependent protein kinase II (CaMKII) inhibitor KN93 bound to the Ca²⁺-sensing protein CaM. KN93 is widely believed to inhibit CaMKII by binding to the kinase. The CaM-KN93 interaction is significant as it can interfere with the interaction between CaM and it's physiological targets, thereby raising the possibility of ascribing modified protein function to CaMKII phosphorylation while concealing a CaM-protein interaction. NMR spectroscopy, stopped-flow kinetic measurements, and x-ray crystallography were used to characterize the structure and biophysical properties of the CaM-KN93 interaction. We then investigated the functional properties of the cardiac Na⁺ channel (Na_V1.5) and ryanodine receptor (RyR2). We find that KN93 disrupts a high affinity CaM-Na_V1.5 interaction and alters channel function independent of CaMKII. Moreover, KN93 increases RyR2 Ca²⁺ release in cardiomyocytes independent of CaMKII. Therefore, when interpreting KN93 data, targets other than CaMKII need to be considered.

Mechanism of calmodulin inactivation of the calcium-selective TRP channel TRPV6

Calcium (Ca²⁺) plays a major role in numerous physiological processes. Ca²⁺ homeostasis is tightly controlled by ion channels, the aberrant regulation of which results in various diseases including cancers. Calmodulin (CaM)-mediated Ca²⁺-induced inactivation is an ion channel regulatory mechanism that protects cells against the toxic effects of Ca²⁺ overload. We used cryo-electron microscopy to capture the epithelial calcium channel TRPV6 (transient receptor potential vanilloid subfamily member 6) inactivated by CaM. The TRPV6-CaM complex exhibits 1:1 stoichiometry; one TRPV6 tetramer binds both CaM lobes, which adopt a distinct head-to-tail arrangement. The CaM carboxyl-terminal lobe plugs the channel through a unique cation-π interaction by inserting the side chain of lysine K115 into a tetra-tryptophan cage at the pore's intracellular entrance. We propose a mechanism of CaM-mediated Ca²⁺-induced inactivation that can be explored for therapeutic design.

Calmodulin: A Multitasking Protein in Kv7.2 Potassium Channel Functions

The ubiquitous calcium transducer calmodulin (CaM) plays a pivotal role in many cellular processes, regulating a myriad of structurally different target proteins. Indeed, it is unquestionable that CaM is the most relevant transductor of calcium signals in eukaryotic cells. During the last two decades, different studies have demonstrated that CaM mediates the modulation of several ion channels. Among others, it has been indicated that Kv7.2 channels, one of the members of the voltage gated potassium channel family that plays a critical role in brain excitability, requires CaM binding to regulate the different mechanisms that govern its functions. The purpose of this review is to provide an overview of the most recent advances in structure–function studies on the role of CaM regulation of Kv7.2 and the other members of the Kv7 family.

A two-step purification strategy using calmodulin as an affinity tag

Calmodulin (CaM) is a Ca²⁺-binding protein that plays an important role in cellular Ca²⁺-signaling. CaM interacts with diverse downstream target proteins and regulates their functions in a Ca²⁺-dependent manner. CaM changes its conformation and hydrophobicity upon [Ca²⁺] change and consequently changes its interaction with CaM-binding domains from the targets. Based on these special properties of CaM, it was used as an affinity tag to develop a novel purification strategy by using it for two sequential orthogonal purification steps: 1) an affinity purification step, in which CaM-tag interacts with an immobilized CaM-binding domain; and 2) a hydrophobic interaction chromatography step, during which CaM binds to a phenyl sepharose column. In both steps, the CaM-tagged protein binds in the presence of Ca²⁺ and unbinds in the presence of ethylenediaminetetraacetic acid (EDTA). An optional third step can be added to remove the CaM-tag if necessary. We used green fluorescent protein (GFP) as a test protein to demonstrate the effectiveness of the method. High yield and high purity of GFP with proper function was obtained using this novel strategy. We believe that this method can be applied to a wide range of protein targets for structural and functional studies.

Interaction of Calmodulin with the cSH2 Domain of the p85 Regulatory Subunit

Calmodulin (CaM) is a calcium sensor protein that directly interacts with the dual-specificity (lipid and protein) kinase PI3Kα through the SH2 domains of the p85 regulatory subunit. In adenocarcinomas, the CaM interaction removes the autoinhibition of the p110 catalytic subunit of PI3Kα, leading to activation of PI3Kα and promoting cell proliferation, survival, and migration. Here we demonstrate that the cSH2 domain of p85α engages its two CaM-binding motifs in the interaction with the N- and C-lobes of CaM as well as the flexible central linker, and our nuclear magnetic resonance experiments provide structural details. We show that in response to binding CaM, cSH2 exposes its tryptophan residue at the N-terminal region to the solvent. Because of the flexible nature of both CaM and cSH2, multiple binding modes of the interactions are possible. Binding of CaM to the cSH2 domain can help release the inhibition imposed on the p110 subunit, similar to the binding of the phosphorylated motif of RTK, or phosphorylated CaM (pCaM), to the SH2 domains. Amino acid sequence analysis shows that CaM-binding motifs are common in SH2 domains of non-RTKs. We speculate that CaM can also activate these kinases through similar mechanisms.

Calmodulin regulates Cav3 T-type channels at their gating brake

Calcium (Cav1 and Cav2) and sodium channels possess homologous CaM-binding motifs, known as IQ motifs in their C termini, which associate with calmodulin (CaM), a universal calcium sensor. Cav3 T-type channels, which serve as pacemakers of the mammalian brain and heart, lack a C-terminal IQ motif. We illustrate that T-type channels associate with CaM using co-immunoprecipitation experiments and single particle cryo-electron microscopy. We demonstrate that protostome invertebrate (LCav3) and human Cav3.1, Cav3.2, and Cav3.3 T-type channels specifically associate with CaM at helix 2 of the gating brake in the I-II linker of the channels. Isothermal titration calorimetry results revealed that the gating brake and CaM bind each other with high-nanomolar affinity. We show that the gating brake assumes a helical conformation upon binding CaM, with associated conformational changes to both CaM lobes as indicated by amide chemical shifts of the amino acids of CaM in 1H-15N HSQC NMR spectra. Intact Ca2+-binding sites on CaM and an intact gating brake sequence (first 39 amino acids of the I-II linker) were required in Cav3.2 channels to prevent the runaway gating phenotype, a hyperpolarizing shift in voltage sensitivities and faster gating kinetics. We conclude that the presence of high-nanomolar affinity binding sites for CaM at its universal gating brake and its unique form of regulation via the tuning of the voltage range of activity could influence the participation of Cav3 T-type channels in heart and brain rhythms. Our findings may have implications for arrhythmia disorders arising from mutations in the gating brake or CaM.

Synergy of cAMP and calcium signaling pathways in CFTR regulation

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene that encodes a chloride channel located in the apical membrane of epithelia cells. The cAMP signaling pathway and protein phosphorylation are known to be primary controlling mechanisms for channel function. In this study, we present an alternative activation pathway that involves calcium-activated calmodulin binding of the intrinsically disordered regulatory (R) region of CFTR. Beyond their potential therapeutic value, these data provide insights into the intersection of calcium signaling with control of ion homeostasis and the ways in which the local CFTR microdomain organizes itself.

Cystic fibrosis results from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, leading to defective apical chloride transport. Patients also experience overactivation of inflammatory processes, including increased calcium signaling. Many investigations have described indirect effects of calcium signaling on CFTR or other calcium-activated chloride channels; here, we investigate the direct response of CFTR to calmodulin-mediated calcium signaling. We characterize an interaction between the regulatory region of CFTR and calmodulin, the major calcium signaling molecule, and report protein kinase A (PKA)-independent CFTR activation by calmodulin. We describe the competition between calmodulin binding and PKA phosphorylation and the differential effects of this competition for wild-type CFTR and the major F508del mutant, hinting at potential therapeutic strategies. Evidence of CFTR binding to isolated calmodulin domains/lobes suggests a mechanism for the role of CFTR as a molecular hub. Together, these data provide insights into how loss of active CFTR at the membrane can have additional consequences besides impaired chloride transport.

Calmodulin Regulates Human Ether à Go-Go 1 (hEAG1) Potassium Channels through Interactions of the Eag Domain with the Cyclic Nucleotide Binding Homology Domain

The ether à go-go family of voltage-gated potassium channels is structurally distinct. The N terminus contains an eag domain (eagD) that contains a Per-Arnt-Sim (PAS) domain that is preceded by a conserved sequence of 25–27 amino acids known as the PAS-cap. The C terminus contains a region with homology to cyclic nucleotide binding domains (cNBHD), which is directly linked to the channel pore. The human EAG1 (hEAG1) channel is remarkably sensitive to inhibition by intracellular calcium (Ca²⁺_i) through binding of Ca²⁺-calmodulin to three sites adjacent to the eagD and cNBHD. Here, we show that the eagD and cNBHD interact to modulate Ca²⁺-calmodulin as well as voltage-dependent gating. Sustained elevation of Ca²⁺_i resulted in an initial profound inhibition of hEAG1 currents, which was followed by a phase when current amplitudes partially recovered, but activation gating was slowed and shifted to depolarized potentials. Deletion of either the eagD or cNBHD abolished the inhibition by Ca²⁺_i. However, deletion of just the PAS-cap resulted in a >15-fold potentiation in response to elevated Ca²⁺_i. Mutations of residues at the interface between the eagD and cNBHD have been linked to human cancer. Glu-600 on the cNBHD, when substituted with residues with a larger volume, resulted in hEAG1 currents that were profoundly potentiated by Ca²⁺_i in a manner similar to the ΔPAS-cap mutant. These findings provide the first evidence that eagD and cNBHD interactions are regulating Ca²⁺-dependent gating and indicate that the binding of the PAS-cap with the cNBHD is required for the closure of the channels upon CaM binding.

L-type calcium channels contribute to 5-HT3-receptor-evoked CaMKIIα and ERK activation and induction of emesis in the least shrew (Cryptotis parva)

Activation of serotonergic 5-HT3 receptors by its selective agonist 2-methyl serotonin (2-Me-5-HT) induces vomiting, which is sensitive to selective antagonists of both 5-HT3 receptors (palonosetron) and L-type calcium channels (LTCC) (amlodipine or nifedipine). Previously we demonstrated that 5-HT3 receptor activation also causes increases in a palonosetron-sensitive manner in: i) intracellular Ca(2+) concentration, ii) attachment of calmodulin (CaM) to 5-HT3 receptor, and iii) phosphorylation of Ca(2+)/calmodulin-dependent protein kinase IIα (CaMKIIα) and extracellular-signal-regulated kinase 1/2 (ERK1/2). Here, we investigate the role of the short-acting LTCC blocker nifedipine on 2-Me-5-HT-evoked intracellular Ca(2+) increase and on downstream intracellular emetic signaling, which have been shown to be coupled with 2-Me-5-HT׳s emetic effects in the least shrew. Using the cell-permeant Ca(2+) indicator fluo-4 AM, here we present evidence for the contribution of Ca(2+) influx through LTCCs (sensitive to nifedipine) in 2-Me-5-HT (1µM) -evoked rise in cytosolic Ca(2+) levels in least shrew brainstem slices. Nifedipine pretreatment (10mg/kg, s.c.) also suppressed 2-Me-5-HT-evoked interaction of 5-HT3 receptors with CaM as well as phosphorylation of CaMKIIα and ERK1/2 in the least shrew brainstem, and 5-HT3 receptors -CaM colocalization in jejunum of the small intestine. In vitro exposure of isolated enterochromaffin cells of the small intestine to 2-Me-5-HT (1µM) caused CaMKIIα phosphorylation, which was also abrogated by nifedipine pretreatment (0.1µM). In addition, pretreatment with the CaMKII inhibitor KN62 (10mg/kg, i.p.) suppressed emesis and also the activation of CaMKIIα, and ERK in brainstem caused by 2-Me-5-HT (5mg/kg, i.p.). This study provides further mechanistic explanation for our published findings that nifedipine can dose-dependently protect shrews from 2-Me-5-HT-induced vomiting.

Targeting the Small- and Intermediate-Conductance Ca-Activated Potassium Channels: The Drug-Binding Pocket at the Channel/Calmodulin Interface

The small- and intermediate-conductance Ca(2+)-activated potassium (SK/IK) channels play important roles in the regulation of excitable cells in both the central nervous and cardiovascular systems. Evidence from animal models has implicated SK/IK channels in neurological conditions such as ataxia and alcohol use disorders. Further, genome-wide association studies have suggested that cardiovascular abnormalities such as arrhythmias and hypertension are associated with single nucleotide polymorphisms that occur within the genes encoding the SK/IK channels. The Ca(2+) sensitivity of the SK/IK channels stems from a constitutively bound Ca(2+)-binding protein: calmodulin. Small-molecule positive modulators of SK/IK channels have been developed over the past decade, and recent structural studies have revealed that the binding pocket of these positive modulators is located at the interface between the channel and calmodulin. SK/IK channel positive modulators can potentiate channel activity by enhancing the coupling between Ca(2+) sensing via calmodulin and mechanical opening of the channel. Here, we review binding pocket studies that have provided structural insight into the mechanism of action for SK/IK channel positive modulators. These studies lay the foundation for structure-based drug discovery efforts that can identify novel SK/IK channel positive modulators.

Serotonin 5-HT3 receptor-mediated vomiting occurs via the activation of Ca2+/CaMKII-dependent ERK1/2 signaling in the least shrew (Cryptotis parva)

Stimulation of 5-HT₃ receptors (5-HT₃Rs) by 2-methylserotonin (2-Me-5-HT), a selective 5-HT₃ receptor agonist, can induce vomiting. However, downstream signaling pathways for the induced emesis remain unknown. The 5-HT₃R channel has high permeability to extracellular calcium (Ca²⁺) and upon stimulation allows increased Ca²⁺ influx. We examined the contribution of Ca²⁺/calmodulin-dependent protein kinase IIα (Ca²⁺/CaMKIIα), interaction of 5-HT₃R with calmodulin, and extracellular signal-regulated kinase 1/2 (ERK1/2) signaling to 2-Me-5-HT-induced emesis in the least shrew. Using fluo-4 AM dye, we found that 2-Me-5-HT augments intracellular Ca²⁺ levels in brainstem slices and that the selective 5-HT₃R antagonist palonosetron, can abolish the induced Ca²⁺ signaling. Pre-treatment of shrews with either: i) amlodipine, an antagonist of L-type Ca²⁺ channels present on the cell membrane; ii) dantrolene, an inhibitor of ryanodine receptors (RyRs) Ca²⁺-release channels located on the endoplasmic reticulum (ER); iii) a combination of their less-effective doses; or iv) inhibitors of CaMKII (KN93) and ERK1/2 (PD98059); dose-dependently suppressed emesis caused by 2-Me-5-HT. Administration of 2-Me-5-HT also significantly: i) enhanced the interaction of 5-HT₃R with calmodulin in the brainstem as revealed by immunoprecipitation, as well as their colocalization in the area postrema (brainstem) and small intestine by immunohistochemistry; and ii) activated CaMKIIα in brainstem and in isolated enterochromaffin cells of the small intestine as shown by Western blot and immunocytochemistry. These effects were suppressed by palonosetron. 2-Me-5-HT also activated ERK1/2 in brainstem, which was abrogated by palonosetron, KN93, PD98059, amlodipine, dantrolene, or a combination of amlodipine plus dantrolene. However, blockade of ER inositol-1, 4, 5-triphosphate receptors by 2-APB, had no significant effect on the discussed behavioral and biochemical parameters. This study demonstrates that Ca²⁺ mobilization via extracellular Ca²⁺ influx through 5-HT₃Rs/L-type Ca²⁺ channels, and intracellular Ca²⁺ release via RyRs on ER, initiate Ca²⁺-dependent sequential activation of CaMKIIα and ERK1/2, which contribute to the 5-HT₃R-mediated, 2-Me-5-HT-evoked emesis.

Lobe-specific calmodulin binding to different ryanodine receptor isoforms

Ryanodine receptors (RyRs) are large ion channels that are responsible for the release of Ca(2+) from the sarcoplasmic/endoplasmic reticulum. Calmodulin (CaM) is a Ca(2+) binding protein that can affect the channel open probability at both high and low Ca(2+) concentrations, shifting the Ca(2+) dependencies of channel opening in an isoform-specific manner. Here we analyze the binding of CaM and its individual domains to three different RyR regions using isothermal titration calorimetry. We compared binding to skeletal muscle (RyR1) and cardiac (RyR2) isoforms, under both Ca(2+)-loaded and Ca(2+)-free conditions. CaM can bind all three regions in both isoforms, but the binding modes differ appreciably in two segments. The results highlight a Ca(2+)/CaM and apoCaM binding site in the C-terminal fifth of the channel. This binding site is the target for malignant hyperthermia and central core disease mutations in RyR1, which affect the energetics and mode of CaM binding.

Conduits of life's spark: a perspective on ion channel research since the birth of neuron

Heartbeats, muscle twitches, and lightning-fast thoughts are all manifestations of bioelectricity and rely on the activity of a class of membrane proteins known as ion channels. The basic function of an ion channel can be distilled into, "The hole opens. Ions go through. The hole closes." Studies of the fundamental mechanisms by which this process happens and the consequences of such activity in the setting of excitable cells remains the central focus of much of the field. One might wonder after so many years of detailed poking at such a seemingly simple process, is there anything left to learn?