Activating and deactivating roles of lipid bilayers on the Ca(2+)-ATPase/phospholamban complex

Calcium Handling in Inherited Cardiac Diseases: A Focus on Catecholaminergic Polymorphic Ventricular Tachycardia and Hypertrophic Cardiomyopathy

Calcium (Ca²⁺) is the major mediator of cardiac contractile function. It plays a key role in regulating excitation-contraction coupling and modulating the systolic and diastolic phases. Defective handling of intracellular Ca²⁺ can cause different types of cardiac dysfunction. Thus, the remodeling of Ca²⁺ handling has been proposed to be a part of the pathological mechanism leading to electrical and structural heart diseases. Indeed, to ensure appropriate electrical cardiac conduction and contraction, Ca²⁺ levels are regulated by several Ca²⁺-related proteins. This review focuses on the genetic etiology of cardiac diseases related to calcium mishandling. We will approach the subject by focalizing on two clinical entities: catecholaminergic polymorphic ventricular tachycardia (CPVT) as a cardiac channelopathy and hypertrophic cardiomyopathy (HCM) as a primary cardiomyopathy. Further, this review will illustrate the fact that despite the genetic and allelic heterogeneity of cardiac defects, calcium-handling perturbations are the common pathophysiological mechanism. The newly identified calcium-related genes and the genetic overlap between the associated heart diseases are also discussed in this review.

Electrostatic interactions between single arginine and phospholipids modulate physiological properties of sarcoplasmic reticulum Ca2+-ATPase

Arg324 of sarcoplasmic reticulum Ca²⁺-ATPase forms electrostatic interactions with the phosphate moiety of phospholipids in most reaction states, and a hydrogen bond with Tyr122 in other states. Using site-directed mutagenesis, we explored the functional roles of Arg324 interactions, especially those with lipids, which at first glance might seem too weak to modulate the function of such a large membrane protein. The hydrogen bond forms transiently and facilitates Ca²⁺ binding from the cytoplasmic side. The contributions of the electrostatic interactions to the reaction steps were quantified using a rate vs activity coefficient plot. We found that the interaction between Arg324 and lipids decreases the affinity for luminal Ca²⁺. The transformation rate of the phosphoenzyme intermediate is facilitated by the electrostatic interactions, and the function of these interactions depends not only on the type but also on the composition of the phospholipids. The properties observed in microsomes could not be reproduced with any single phospholipid, but with a mixture of phospholipids that mimics the native membrane. These results suggest the importance of swapping of the lipid partners of different headgroups in the reaction step. This study shows that Arg324 plays a role in the reaction cycle via complex intra-protein and protein-lipid interactions.

Structural basis for sarcolipin's regulation of muscle thermogenesis by the sarcoplasmic reticulum Ca2+-ATPase

The sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) plays a central role in muscle contractility and nonshivering thermogenesis. SERCA is regulated by sarcolipin (SLN), a single-pass membrane protein that uncouples Ca2+ transport from ATP hydrolysis, promoting futile enzymatic cycles and heat generation. The molecular determinants for regulating heat release by the SERCA/SLN complex are unclear. Using thermocalorimetry, chemical cross-linking, and solid-state NMR spectroscopy in oriented phospholipid bicelles, we show that SERCA’s functional uncoupling and heat release rate are dictated by specific SERCA/SLN intramembrane interactions, with the carboxyl-terminal residues anchoring SLN to the SR membrane in an inhibitory topology. Systematic deletion of the carboxyl terminus does not prevent the SERCA/SLN complex formation but reduces uncoupling in a graded manner. These studies emphasize the critical role of lipids in defining the active topology of SLN and modulating the heat release rate by the SERCA/SLN complex, with implications in fat metabolism and basal metabolic rate.

The role of sigma 1 receptor in organization of endoplasmic reticulum signaling microdomains

Sigma 1 receptor (S1R) is a 223-amino-acid-long transmembrane endoplasmic reticulum (ER) protein. S1R modulates activity of multiple effector proteins and is a well-established drug target. However, signaling functions of S1R in cells are poorly understood. Here, we test the hypothesis that biological activity of S1R in cells can be explained by its ability to interact with cholesterol and to form cholesterol-enriched microdomains in the ER membrane. By performing experiments in reduced reconstitution systems, we demonstrate direct effects of cholesterol on S1R clustering. We identify a novel cholesterol-binding motif in the transmembrane region of human S1R. Mutations of this motif impair association of recombinant S1R with cholesterol beads, affect S1R clustering in vitro and disrupt S1R subcellular localization. We demonstrate that S1R-induced membrane microdomains have increased local membrane thickness and that increased local cholesterol concentration and/or membrane thickness in these microdomains can modulate signaling of inositol-requiring enzyme 1α in the ER. Further, S1R agonists cause disruption of S1R clusters, suggesting that biological activity of S1R agonists is linked to remodeling of ER membrane microdomains. Our results provide novel insights into S1R-mediated signaling mechanisms in cells.

Sigma-1 Receptor (S1R) Interaction with Cholesterol: Mechanisms of S1R Activation and Its Role in Neurodegenerative Diseases

The sigma-1 receptor (S1R) is a 223 amino acid-long transmembrane endoplasmic reticulum (ER) protein. The S1R modulates the activity of multiple effector proteins, but its signaling functions are poorly understood. S1R is associated with cholesterol, and in our recent studies we demonstrated that S1R association with cholesterol induces the formation of S1R clusters. We propose that these S1R-cholesterol interactions enable the formation of cholesterol-enriched microdomains in the ER membrane. We hypothesize that a number of secreted and signaling proteins are recruited and retained in these microdomains. This hypothesis is consistent with the results of an unbiased screen for S1R-interacting partners, which we performed using the engineered ascorbate peroxidase 2 (APEX2) technology. We further propose that S1R agonists enable the disassembly of these cholesterol-enriched microdomains and the release of accumulated proteins such as ion channels, signaling receptors, and trophic factors from the ER. This hypothesis may explain the pleotropic signaling functions of the S1R, consistent with previously observed effects of S1R agonists in various experimental systems.

Post-Translational Modification of Cysteines: A Key Determinant of Endoplasmic Reticulum-Mitochondria Contacts (MERCs)

Cells must adjust their redox state to an ever-changing environment that could otherwise result in compromised homeostasis. An obvious way to adapt to changing redox conditions depends on cysteine post-translational modifications (PTMs) to adapt conformation, localization, interactions and catalytic activation of proteins. Such PTMs should occur preferentially in the proximity of oxidative stress sources. A particular concentration of these sources is found near membranes where the endoplasmic reticulum (ER) and the mitochondria interact on domains called MERCs (Mitochondria-Endoplasmic Reticulum Contacts). Here, fine inter-organelle communication controls metabolic homeostasis. MERCs achieve this goal through fluxes of Ca²⁺ ions and inter-organellar lipid exchange. Reactive oxygen species (ROS) that cause PTMs of mitochondria-associated membrane (MAM) proteins determine these intertwined MERC functions. Chronic changes of the pattern of these PTMs not only control physiological processes such as the circadian clock but could also lead to or worsen many human disorders such as cancer and neurodegenerative diseases.

Linking Biochemical and Structural States of SERCA: Achievements, Challenges, and New Opportunities

Sarcoendoplasmic reticulum calcium ATPase (SERCA), a member of the P-type ATPase family of ion and lipid pumps, is responsible for the active transport of Ca²⁺ from the cytoplasm into the sarcoplasmic reticulum lumen of muscle cells, into the endoplasmic reticulum (ER) of non-muscle cells. X-ray crystallography has proven to be an invaluable tool in understanding the structural changes of SERCA, and more than 70 SERCA crystal structures representing major biochemical states (defined by bound ligand) have been deposited in the Protein Data Bank. Consequently, SERCA is one of the best characterized components of the calcium transport machinery in the cell. Emerging approaches in the field, including spectroscopy and molecular simulation, now help integrate and interpret this rich structural information to understand the conformational transitions of SERCA that occur during activation, inhibition, and regulation. In this review, we provide an overview of the crystal structures of SERCA, focusing on identifying metrics that facilitate structure-based categorization of major steps along the catalytic cycle. We examine the integration of crystallographic data with different biophysical approaches and computational methods to link biochemical and structural states of SERCA that are populated in the cell. Finally, we discuss the challenges and new opportunities in the field, including structural elucidation of functionally important and novel regulatory complexes of SERCA, understanding the structural basis of functional divergence among homologous SERCA regulators, and bridging the gap between basic and translational research directed toward therapeutic modulation of SERCA.

Intrinsically disordered HAX-1 regulates Ca2+ cycling by interacting with lipid membranes and the phospholamban cytoplasmic region

Hematopoietic-substrate-1 associated protein X-1 (HAX-1) is a 279 amino acid protein expressed ubiquitously. In cardiac muscle, HAX-1 was found to modulate the sarcoendoplasmic reticulum calcium ATPase (SERCA) by shifting its apparent Ca2+ affinity (pCa). It has been hypothesized that HAX-1 binds phospholamban (PLN), enhancing its inhibitory function on SERCA. HAX-1 effects are reversed by cAMP-dependent protein kinase A that phosphorylates PLN at Ser16. To date, the molecular mechanisms for HAX-1 regulation of the SERCA/PLN complex are still unknown. Using enzymatic, in cell assays, circular dichroism, and NMR spectroscopy, we found that in the absence of a binding partner HAX-1 is essentially disordered and adopts a partial secondary structure upon interaction with lipid membranes. Also, HAX-1 interacts with the cytoplasmic region of monomeric and pentameric PLN as detected by NMR and in cell FRET assays, respectively. We propose that the regulation of the SERCA/PLN complex by HAX-1 is mediated by its interactions with lipid membranes, adding another layer of control in Ca2+ homeostatic balance in the heart muscle.

Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein-lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

Hypoxia-induced remodelling of goldfish membranes

Hypoxia-tolerant animals use metabolic suppression as an essential strategy to survive low oxygen. Ectotherms can alter membrane lipid composition in response to changes in environmental temperature, but it is currently unknown whether chronic hypoxia can also elicit membrane restructuring. The goal of this study was to investigate a possible physiological link between membrane remodelling and metabolic suppression in goldfish exposed to prolonged hypoxia (4 weeks at 10% air saturation). We have tested the hypothesis that chronic hypoxia would modulate membrane lipid composition in ways that are consistent with known mechanisms of ion pump inhibition. Because homeoviscous membrane restructuring could interfere with the response to hypoxia, measurements were made at 2 temperatures. Results show that hypoxic goldfish suppress metabolic rate by 74% (at 13 °C) and 63% (at 20 °C). This study is the first to reveal that cold-acclimated animals undergo extensive, tissue-specific restructuring of membrane lipids as they reach minimal metabolic rates. However, hypoxia does not affect membrane composition in fish acclimated to 20 °C. The strong membrane response of cold-acclimated fish involves increases in cholesterol abundance (in white muscle and gills) and in fatty acid saturation, mainly caused by a reduction in %22:6 (docosahexaenoic acid in gills and liver). Major ion pumps like Na⁺/K⁺-ATPase are known to be inhibited by cholesterol and activated by 22:6. Because ion pumping by membrane-bound ATPases accounts for a large fraction of basal cellular energy use, we propose that the membrane responses reported here could be a novel mechanism to promote metabolic suppression in cold-acclimated animals.

Inactivation in the potassium channel KcsA

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C-type inactivation in potassium channels is a nearly universal regulatory mechanism.

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A major hypothesis states that C-type inactivation involves ion loss at the selectivity filter as an allosteric response to activation.

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NMR is used to probe protein conformational changes in response to pH and [K⁺], demonstrating that H⁺ and K⁺ binding are allosterically coupled in KcsA.

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The lipids are integrated parts of potassium channels in terms of structure, energetics and function.

Inactivation, the slow cessation of transmission after activation, is a general feature of potassium channels. It is essential for their function, and malfunctions in inactivation leads to numerous pathologies. The detailed mechanism for the C-type inactivation, distinct from the N-type inactivation, remains an active area of investigation. Crystallography, computational simulations, and NMR have greatly enriched our understanding of the process. Here we review the major hypotheses regarding C-type inactivation, particularly focusing on the key role played by NMR studies of the prokaryotic potassium channel KcsA, which serves as a good model for voltage gated mammalian channels.

General and specific interactions of the phospholipid bilayer with P-type ATPases

Protein structure and function are modulated via interactions with their environment, representing both the surrounding aqueous media and lipid membranes that have an active role in shaping the structural topology of membrane proteins. Compared to a decade ago, there is now an abundance of crystal structural data on membrane proteins, which together with their functional studies have enhanced our understanding of the salient features of lipid-protein interactions. It is now important to recognize that membrane proteins are regulated by both (1) general lipid-protein interactions, where the general physicochemical properties of the lipid environment affect the conformational flexibility of a membrane protein, and (2) by specific lipid-protein interactions, where lipid molecules directly interact via chemical interactions with specific lipid-binding sites located on the protein. However, due to local differences in membrane composition, thickness, and lipid packing, local membrane physical properties and hence the associated lipid-protein interactions also differ due to membrane location, even for the same protein. Such a phenomenon has been shown to be true for one family of integral membrane ion pumps, the P2-type adenosine triphosphatases (ATPases). Despite being highly homologous, individual members of this family have distinct structural and functional activity and are an excellent candidate to highlight how the local membrane physical properties and specific lipid-protein interactions play a vital role in facilitating the structural rearrangements of these proteins necessary for their activity. Hence in this review, we focus on both the general and specific lipid-protein interactions and will mostly discuss the structure-function relationships of the following P2-type ATPases, Na⁺,K⁺-ATPase (NKA), gastric H⁺,K⁺-ATPase (HKA), and sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), in concurrence with their lipid environment.

The Fluidity of Phosphocholine and Maltoside Micelles and the Effect of CHAPS

The dynamics of phosphocholine and maltoside micelles, detergents frequently used for membrane protein structure determination, were investigated using electron paramagnetic resonance of spin probes doped into the micelles. Specifically, phosphocholines are frequently used detergents in NMR studies, and maltosides are frequently used in x-ray crystallography structure determination. Beyond the structural and electrostatic differences, this study aimed to determine whether there are differences in the local chain dynamics (i.e., fluidity). The nitroxide probe rotational dynamics in longer chain detergents is more restricted than in shorter chain detergents, and maltoside micelles are more restricted than phosphocholine micelles. Furthermore, the micelle microviscosity can be modulated with mixtures, as demonstrated with mixtures of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate with n-dodecylphosphocholine, n-tetradecylphosphocholine, n-decyl-β-D-maltoside, or n-dodecyl-β-D-maltoside. These results indicate that observed differences in membrane protein stability in these detergents could be due to fluidity in addition to the already determined structural differences.

Reactions at Biomembrane Interfaces

Membranes surrounding the biological cell and its internal compartments host proteins that catalyze chemical reactions essential for the functioning of the cell. Rather than being a passive structural matrix that holds membrane-embedded proteins in place, the membrane can largely shape the conformational energy landscape of membrane proteins and impact the energetics of their chemical reaction. Here, we highlight the challenges in understanding how lipids impact the conformational energy landscape of macromolecular membrane complexes whose functioning involves chemical reactions including proton transfer. We review here advances in our understanding of how chemical reactions occur at membrane interfaces gleaned with both theoretical and experimental advances using simple protein systems as guides. Our perspective is that of bridging experiments with theory to understand general physicochemical principles of membrane reactions, with a long term goal of furthering our understanding of the role of the lipids on the functioning of complex macromolecular assemblies at the membrane interface.

Probing the effects of nonannular lipid binding on the stability of the calcium pump SERCA

The calcium pump SERCA is a transmembrane protein that is critical for calcium transport in cells. SERCA resides in an environment made up largely by the lipid bilayer, so lipids play a central role on its stability and function. Studies have provided insights into the effects of annular and bulk lipids on SERCA activation, but the role of a nonannular lipid site in the E2 intermediate state remains elusive. Here, we have performed microsecond molecular dynamics simulations to probe the effects of nonannular lipid binding on the stability and structural dynamics of the E2 state of SERCA. We found that the structural integrity and stability of the E2 state is independent of nonannular lipid binding, and that occupancy of a lipid molecule at this site does not modulate destabilization of the E2 state, a step required to initiate the transition toward the competent E1 state. We also found that binding of the nonannular lipid does not induce direct allosteric control of the intrinsic functional dynamics the E2 state. We conclude that nonannular lipid binding is not necessary for the stability of the E2 state, but we speculate that it becomes functionally significant during the E2-to-E1 transition of the pump.

Emerging Diversity in Lipid-Protein Interactions

Membrane lipids interact with proteins in a variety of ways, ranging from providing a stable membrane environment for proteins to being embedded in to detailed roles in complicated and well-regulated protein functions. Experimental and computational advances are converging in a rapidly expanding research area of lipid–protein interactions. Experimentally, the database of high-resolution membrane protein structures is growing, as are capabilities to identify the complex lipid composition of different membranes, to probe the challenging time and length scales of lipid–protein interactions, and to link lipid–protein interactions to protein function in a variety of proteins. Computationally, more accurate membrane models and more powerful computers now enable a detailed look at lipid–protein interactions and increasing overlap with experimental observations for validation and joint interpretation of simulation and experiment. Here we review papers that use computational approaches to study detailed lipid–protein interactions, together with brief experimental and physiological contexts, aiming at comprehensive coverage of simulation papers in the last five years. Overall, a complex picture of lipid–protein interactions emerges, through a range of mechanisms including modulation of the physical properties of the lipid environment, detailed chemical interactions between lipids and proteins, and key functional roles of very specific lipids binding to well-defined binding sites on proteins. Computationally, despite important limitations, molecular dynamics simulations with current computer power and theoretical models are now in an excellent position to answer detailed questions about lipid–protein interactions.

Perturbations of Native Membrane Protein Structure in Alkyl Phosphocholine Detergents: A Critical Assessment of NMR and Biophysical Studies

Membrane proteins perform a host of vital cellular functions. Deciphering the molecular mechanisms whereby they fulfill these functions requires detailed biophysical and structural investigations. Detergents have proven pivotal to extract the protein from its native surroundings. Yet, they provide a milieu that departs significantly from that of the biological membrane, to the extent that the structure, the dynamics, and the interactions of membrane proteins in detergents may considerably vary, as compared to the native environment. Understanding the impact of detergents on membrane proteins is, therefore, crucial to assess the biological relevance of results obtained in detergents. Here, we review the strengths and weaknesses of alkyl phosphocholines (or foscholines), the most widely used detergent in solution-NMR studies of membrane proteins. While this class of detergents is often successful for membrane protein solubilization, a growing list of examples points to destabilizing and denaturing properties, in particular for α-helical membrane proteins. Our comprehensive analysis stresses the importance of stringent controls when working with this class of detergents and when analyzing the structure and dynamics of membrane proteins in alkyl phosphocholine detergents.

Elevated whole muscle phosphatidylcholine: phosphatidylethanolamine ratio coincides with reduced SERCA activity in murine overloaded plantaris muscles

BACKGROUND

An increase in phosphatidylcholine:phosphatidylethanolamine (PC:PE) and a decrease in fatty acyl chain length, monounsaturated:polyunsaturated (MUFA:PUFA) fatty acyl ratio reduces SERCA activity in liposomes and in mouse models of obesity and muscular dystrophy. We have previously shown that maximal SERCA activity is significantly reduced in mechanically overloaded (OVL) plantaris, however, whether changes in PC:PE ratio or fatty acyl composition may contribute to the alterations in maximal SERCA activity remain unknown. Here, we tested the hypotheses that in OVL plantaris 1) PC:PE ratio would negatively correlate with maximal SERCA activity and 2) PC fatty acyl chain length (ACL) and/or MUFA:PUFA ratio would positively correlate with maximal SERCA activity.

METHODS

To overload plantaris in mice, we transected the soleus and gastrocnemius tendons from one leg, while the contralateral leg underwent a sham surgery. After two weeks, plantaris muscles were extracted, homogenized and processed for SERCA activity and lipid analyses. Specifically, we performed HPTLC densitometry to examine changes in PC, PE, and the ratio of PC:PE. We also performed gas chromatography to assess any potential changes to fatty acyl composition.

RESULTS

SERCA activity was significantly reduced in OVL plantaris compared with sham. Coinciding with this, we found a significant increase in PC but not PE in OVL plantaris. In turn, there was an increase in PC:PE but did not reach significance (p = 0.09). However, we found a significant negative correlation between PC:PE and maximal SERCA activity. Fatty acyl composition of PE remained similar between OLV and sham and PC demonstrated higher percent mole fraction of 17:1, 18:1, and ACL compared to sham. In addition, PC ACL, % MUFA, % PUFA, or MUFA:PUFA did not significantly correlate with maximal SERCA activity.

CONCLUSIONS

Our results indicate that the phospholipid headgroup PC:PE negatively correlated and could potentially contribute to reductions in SERCA activity seen in functionally overloaded plantaris. In contrast, fatty acyl chain (ACL, % MUFA, % PUFA, MUFA:PUFA) did not correlate with maximal SERCA activity. Future studies will determine whether altering PC:PE with genetic and dietary interventions can influence SERCA activity and ultimately change the physiological outcome in response to muscle overloading.

Effects of the Arg9Cys and Arg25Cys mutations on phospholamban's conformational equilibrium in membrane bilayers

Approximately, 70% of the Ca²⁺ ion transport into the sarcoplasmic reticulum is catalyzed by the sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), whose activity is endogenously regulated by phospholamban (PLN). PLN comprises a TM inhibitory region and a cytoplasmic regulatory region that harbors a consensus sequence for cAMP-dependent protein kinase (PKA). The inhibitory region binds the ATPase, reducing its apparent Ca²⁺ binding affinity. β-adrenergic stimulation activates PKA, which phosphorylates PLN at Ser 16, reversing its inhibitory function. Mutations and post-translational modifications of PLN may lead to dilated cardiomyopathy (DCM) and heart failure. PLN's cytoplasmic region interconverts between a membrane-associated T state and a membrane-detached R state. The importance of these structural transitions on SERCA regulation is emerging, but the effects of natural occurring mutations and their relevance to the progression of heart disease are unclear. Here we use solid-state NMR spectroscopy to investigate the structural dynamics of two lethal PLN mutations, R9C and R25C, which lead to DCM. We found that the R25C mutant enhances the dynamics of PLN and shifts the conformational equilibrium toward the R state confirmation, whereas the R9C mutant drives the amphipathic cytoplasmic domain toward the membrane-associate state, enriching the T state population. The changes in membrane interactions caused by these mutations may explain the aberrant regulation of SERCA.

Conformational memory in the association of the transmembrane protein phospholamban with the sarcoplasmic reticulum calcium pump SERCA

The sarcoplasmic reticulum Ca²⁺-ATPase SERCA promotes muscle relaxation by pumping calcium ions from the cytoplasm into the sarcoplasmic reticulum. SERCA activity is regulated by a variety of small transmembrane peptides, most notably by phospholamban in cardiac muscle and sarcolipin in skeletal muscle. However, how phospholamban and sarcolipin regulate SERCA is not fully understood. In the present study, we evaluated the effects of phospholamban and sarcolipin on calcium translocation and ATP hydrolysis by SERCA under conditions that mimic environments in sarcoplasmic reticulum membranes. For pre-steady-state current measurements, proteoliposomes containing SERCA and phospholamban or sarcolipin were adsorbed to a solid-supported membrane and activated by substrate concentration jumps. We observed that phospholamban altered ATP-dependent calcium translocation by SERCA within the first transport cycle, whereas sarcolipin did not. Using pre-steady-state charge (calcium) translocation and steady-state ATPase activity under substrate conditions (various calcium and/or ATP concentrations) promoting particular conformational states of SERCA, we found that the effect of phospholamban on SERCA depends on substrate preincubation conditions. Our results also indicated that phospholamban can establish an inhibitory interaction with multiple SERCA conformational states with distinct effects on SERCA's kinetic properties. Moreover, we noted multiple modes of interaction between SERCA and phospholamban and observed that once a particular mode of association is engaged it persists throughout the SERCA transport cycle and multiple turnover events. These observations are consistent with conformational memory in the interaction between SERCA and phospholamban, thus providing insights into the physiological role of phospholamban and its regulatory effect on SERCA transport activity.

Nanodisc-based kinetic assays reveal distinct effects of phospholipid headgroups on the phosphoenzyme transition of sarcoplasmic reticulum Ca2+-ATPase

Sarco(endo)plasmic reticulum Ca²⁺-ATPase catalyzes ATP-driven Ca²⁺ transport from the cytoplasm to the lumen and is critical for a range of cell functions, including muscle relaxation. Here, we investigated the effects of the headgroups of the 1-palmitoyl-2-oleoyl glycerophospholipids phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), and phosphatidylglycerol (PG) on sarcoplasmic reticulum (SR) Ca²⁺-ATPase embedded into a nanodisc, a lipid-bilayer construct harboring the specific lipid. We found that Ca²⁺-ATPase activity in a PC bilayer is comparable with that of SR vesicles and is suppressed in the other phospholipids, especially in PS. Ca²⁺ affinity at the high-affinity transport sites in PC was similar to that of SR vesicles, but 2-3-fold reduced in PE and PS. Ca²⁺ on- and off-rates in the non-phosphorylated ATPase were markedly reduced in PS. Rate-limiting phosphoenzyme (EP) conformational transition in 0.1 m KCl was as rapid in PC as in SR vesicles, but slowed in other phospholipids, especially in PS. Using kinetic plots of the logarithm of rate versus the square of mean activity coefficient of solutes in 0.1-1 m KCl, we noted that PC is optimal for the EP transition, but PG and especially PS had markedly unfavorable electrostatic effects, and PE exhibited a strong non-electrostatic restriction. Thus, the major SR membrane lipid PC is optimal for all steps and, unlike the other headgroups, contributes favorable electrostatics and non-electrostatic elements during the EP transition. Our analyses further revealed that the surface charge of the lipid bilayer directly modulates the transition rate.

We Know More Than We Can Tell About Diabetes and Vascular Disease: The 2016 Edwin Bierman Award Lecture

The Edwin Bierman Award Lecture is presented in honor of the memory of Edwin L. Bierman, MD, an exemplary scientist, mentor, and leader in the field of diabetes, obesity, hyperlipidemia, and atherosclerosis. The award and lecture recognizes a leading scientist in the field of macrovascular complications and contributing risk factors in diabetes. Clay F. Semenkovich, MD, the Irene E. and Michael M. Karl Professor and Chief of the Division of Endocrinology, Metabolism and Lipid Research at Washington University School of Medicine in St. Louis, St. Louis, MO, received the prestigious award at the American Diabetes Association's 76th Scientific Sessions, 10-14 June 2016, in New Orleans, LA. He presented the Edwin Bierman Award Lecture, "We Know More Than We Can Tell About Diabetes and Vascular Disease," on Sunday, 12 June 2016.Diabetes is a disorder of abnormal lipid metabolism, a notion strongly supported by the work of Edwin Bierman, for whom this eponymous lecture is named. This abnormal lipid environment continues to be associated with devastating vascular complications in diabetes despite current therapies, suggesting that our understanding of the pathophysiology of blood vessel disease in diabetes is limited. In this review, potential new insights into the nature of diabetic vasculopathy will be discussed. Recent observations suggest that while the concept of distinct macrovascular and microvascular complications of diabetes has been useful, vascular diseases in diabetes may be more interrelated than previously appreciated. Moreover, the intermediary metabolic pathway of de novo lipogenesis, which synthesizes lipids from simple precursors, is robustly sensitive to insulin and may contribute to these complications. De novo lipogenesis requires fatty acid synthase, and recent studies of this enzyme suggest that endogenously produced lipids are channeled to specific intracellular sites to affect physiology. These findings raise the possibility that novel approaches to treating diabetes and its complications could be based on altering the intracellular lipid milieu.

The critical role of phosphatidylcholine and phosphatidylethanolamine metabolism in health and disease

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in all mammalian cell membranes. In the 1950s, Eugene Kennedy and co-workers performed groundbreaking research that established the general outline of many of the pathways of phospholipid biosynthesis. In recent years, the importance of phospholipid metabolism in regulating lipid, lipoprotein and whole-body energy metabolism has been demonstrated in numerous dietary studies and knockout animal models. The purpose of this review is to highlight the unappreciated impact of phospholipid metabolism on health and disease. Abnormally high, and abnormally low, cellular PC/PE molar ratios in various tissues can influence energy metabolism and have been linked to disease progression. For example, inhibition of hepatic PC synthesis impairs very low density lipoprotein secretion and changes in hepatic phospholipid composition have been linked to fatty liver disease and impaired liver regeneration after surgery. The relative abundance of PC and PE regulates the size and dynamics of lipid droplets. In mitochondria, changes in the PC/PE molar ratio affect energy production. We highlight data showing that changes in the PC and/or PE content of various tissues are implicated in metabolic disorders such as atherosclerosis, insulin resistance and obesity. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Inhibitory action of linoleamide and oleamide toward sarco/endoplasmic reticulum Ca2+-ATPase

BACKGROUND

SERCA maintains intracellular Ca²⁺ homeostasis by sequestering cytosolic Ca²⁺ into SR/ER stores. Two primary fatty acid amides (PFAAs), oleamide (18:1^9-cis) and linoleamide (18:2^9,12-cis), induce an increase in intracellular Ca²⁺ levels, which might be caused by their inhibition of SERCA.

METHODS

Three major SERCA isoforms, rSERCA1a, hSERCA2b, and hSERCA3a, were individually overexpressed in COS-1 cells, and the inhibitory action of PFAAs on Ca²⁺-ATPase activity of SERCA was examined.

RESULTS

The Ca²⁺-ATPase activity of each SERCA was inhibited in a concentration-dependent manner strongly by linoleamide (IC₅₀ 15-53μM) and partially by oleamide (IC₅₀ 8.3-34μM). Inhibition by other PFAAs, such as stearamide (18:0) and elaidamide (18:1^9-trans), was hardly or slightly observed. With increasing dose, linoleamide decreased the apparent affinity for Ca²⁺ and the apparent maximum velocity of Ca²⁺-ATPase activity of all SERCAs tested. Oleamide also lowered these values for hSERCA3a. Meanwhile, oleamide uniquely reduced the apparent Ca²⁺ affinity of rSERCA1a and hSERCA2b: the reduction was considerably attenuated above certain concentrations of oleamide. The dissociation constants for SERCA interaction varied from 6 to 45μM in linoleamide and from 1.6 to 55μM in oleamide depending on the isoform.

CONCLUSIONS

Linoleamide and oleamide inhibit SERCA activity in the micromolar concentration range, and in a different manner. Both amides mainly suppress SERCA activity by lowering the Ca²⁺ affinity of the enzyme.

GENERAL SIGNIFICANCE

Our findings imply a novel role of these PFAAs as modulators of intracellular Ca²⁺ homeostasis via regulation of SERCA activity.

Reversal of Phospholamban Inhibition of the Sarco(endo)plasmic Reticulum Ca2+-ATPase (SERCA) Using Short, Protein-interacting RNAs and Oligonucleotide Analogs

The sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and phospholamban (PLN) complex regulates heart relaxation through its removal of cytosolic Ca²⁺ during diastole. Dysfunction of this complex has been related to many heart disorders and is therefore a key pharmacological target. There are currently no therapeutics that directly target either SERCA or PLN. It has been previously reported that single-stranded DNA binds PLN with strong affinity and relieves inhibition of SERCA in a length-dependent manner. In the current article, we demonstrate that RNAs and single-stranded oligonucleotide analogs, or xeno nucleic acids (XNAs), also bind PLN strongly (K_d <10 nm) and relieve inhibition of SERCA. Affinity for PLN is sequence-independent. Relief of PLN inhibition is length-dependent, allowing SERCA activity to be restored incrementally. The improved in vivo stability of XNAs offers more realistic pharmacological potential than DNA or RNA. We also found that microRNAs (miRNAs) 1 and 21 bind PLN strongly and relieve PLN inhibition of SERCA to a greater extent than a similar length random sequence RNA mixture. This may suggest that miR-1 and miR-21 have evolved to contain distinct sequence elements that are more effective at relieving PLN inhibition than random sequences.

Skeletal muscle phosphatidylcholine and phosphatidylethanolamine are related to insulin sensitivity and respond to acute exercise in humans

Several recent reports indicate that the balance of skeletal muscle phosphatidylcholine (PC) and phosphatidylethanolamine (PE) is a key determinant of muscle contractile function and metabolism. The purpose of this study was to determine relationships between skeletal muscle PC, PE and insulin sensitivity, and whether PC and PE are dynamically regulated in response to acute exercise in humans. Insulin sensitivity was measured via intravenous glucose tolerance in sedentary obese adults (OB; n = 14), individuals with type 2 diabetes (T2D; n = 15), and endurance-trained athletes (ATH; n = 15). Vastus lateralis muscle biopsies were obtained at rest, immediately after 90 min of cycle ergometry at 50% maximal oxygen consumption (V̇o2 max), and 2-h postexercise (recovery). Skeletal muscle PC and PE were measured via infusion-based mass spectrometry/mass spectrometry analysis. ATH had greater levels of muscle PC and PE compared with OB and T2D (P < 0.05), with total PC and PE positively relating to insulin sensitivity (both P < 0.05). Skeletal muscle PC:PE ratio was elevated in T2D compared with OB and ATH (P < 0.05), tended to be elevated in OB vs. ATH (P = 0.07), and was inversely related to insulin sensitivity among the entire cohort (r = -0.43, P = 0.01). Muscle PC and PE were altered by exercise, particularly after 2 h of recovery, in a highly group-specific manner. However, muscle PC:PE ratio remained unchanged in all groups. In summary, total muscle PC and PE are positively related to insulin sensitivity while PC:PE ratio is inversely related to insulin sensitivity in humans. A single session of exercise significantly alters skeletal muscle PC and PE levels, but not PC:PE ratio.

Ca(2+) ATPase Conformational Transitions in Lipid Bilayers Mapped by Site-directed Ethylation and Solid-State NMR

To transmit signals across cellular compartments, many membrane-embedded enzymes undergo extensive conformational rearrangements. Monitoring these events in lipid bilayers by NMR at atomic resolution has been challenging due to the large size of these systems. It is further exacerbated for large mammalian proteins that are difficult to express and label with NMR-active isotopes. Here, we synthesized and engineered (13)C ethyl groups on native cysteines to map the structural transitions of the sarcoplasmic reticulum Ca(2+)-ATPase, a 110 kDa transmembrane enzyme that transports Ca(2+) into the sarcoplasmic reticulum. Using magic angle spinning NMR, we monitored the chemical shifts of the methylene and methyl groups of the derivatized cysteine residues along the major steps of the enzymatic cycle. The methylene chemical shifts are sensitive to the ATPase conformational changes induced upon nucleotide and Ca(2+) ion binding and are ideal probes for active and inactive states of the enzyme. This new approach is extendable to large mammalian enzymes and signaling proteins with native or engineered cysteine residues in their amino acid sequence.

Skeletal Muscle Phospholipid Metabolism Regulates Insulin Sensitivity and Contractile Function

Skeletal muscle insulin resistance is an early defect in the development of type 2 diabetes. Lipid overload induces insulin resistance in muscle and alters the composition of the sarcoplasmic reticulum (SR). To test the hypothesis that skeletal muscle phospholipid metabolism regulates systemic glucose metabolism, we perturbed choline/ethanolamine phosphotransferase 1 (CEPT1), the terminal enzyme in the Kennedy pathway of phospholipid synthesis. In C2C12 cells, CEPT1 knockdown altered SR phospholipid composition and calcium flux. In mice, diet-induced obesity, which decreases insulin sensitivity, increased muscle CEPT1 expression. In high-fat diet-fed mice with skeletal muscle-specific knockout of CEPT1, systemic and muscle-based approaches demonstrated increased muscle insulin sensitivity. In CEPT1-deficient muscles, an altered SR phospholipid milieu decreased sarco/endoplasmic reticulum Ca(2+) ATPase-dependent calcium uptake, activating calcium-signaling pathways known to improve insulin sensitivity. Altered muscle SR calcium handling also rendered these mice exercise intolerant. In obese humans, surgery-induced weight loss increased insulin sensitivity and decreased skeletal muscle CEPT1 protein. In obese humans spanning a spectrum of metabolic health, muscle CEPT1 mRNA was inversely correlated with insulin sensitivity. These results suggest that high-fat feeding and obesity induce CEPT1, which remodels the SR to preserve contractile function at the expense of insulin sensitivity.

Detection of oxidative stress biomarkers in myricetin treated red blood cells

Effect of myricetin on RBC membrane enzymes (Na⁺, K⁺-ATPase and Ca²⁺-ATPase) and Na⁺, H⁺exchanger.

Diacylglycerol kinase-δ regulates AMPK signaling, lipid metabolism, and skeletal muscle energetics

Decrease of AMPK-related signal transduction and insufficient lipid oxidation contributes to the pathogenesis of obesity and type 2 diabetes. Previously, we identified that diacylglycerol kinase-δ (DGKδ), an enzyme involved in triglyceride biosynthesis, is reduced in skeletal muscle from type 2 diabetic patients. Here, we tested the hypothesis that DGKδ plays a role in maintaining appropriate AMPK action in skeletal muscle and energetic aspects of contraction. Voluntary running activity was reduced in DGKδ(+/-) mice, but glycogen content and mitochondrial markers were unaltered, suggesting that DGKδ deficiency affects skeletal muscle energetics but not mitochondrial protein abundance. We next determined the role of DGKδ in AMPK-related signal transduction and lipid metabolism in isolated skeletal muscle. AMPK activation and signaling were reduced in DGKδ(+/-) mice, concomitant with impaired lipid oxidation and elevated incorporation of free fatty acids into triglycerides. Strikingly, DGKδ deficiency impaired work performance, as evident by altered force production and relaxation dynamics in response to repeated contractions. In conclusion, DGKδ deficiency impairs AMPK signaling and lipid metabolism, thereby highlighting the deleterious role of excessive lipid metabolites in the development of peripheral insulin resistance and type 2 diabetes pathogenesis. DGKδ deficiency also influences skeletal muscle energetics, which may lead to low physical activity levels in type 2 diabetes.

Effect of Selection for High Activity-Related Metabolism on Membrane Phospholipid Fatty Acid Composition in Bank Voles

Endothermy, high basal metabolic rates (BMRs), and high locomotor-related metabolism were important steps in the evolution of mammals. It has been proposed that the composition of membrane phospholipid fatty acids plays an important role in energy metabolism and exercise muscle physiology. In particular, the membrane pacemaker theory of metabolism suggests that an increase in cell membrane fatty acid unsaturation would result in an increase in BMR. We aimed to determine whether membrane phospholipid fatty acid composition of heart, liver, and gastrocnemius muscles differed between lines of bank voles selected for high swim-induced aerobic metabolism-which also evolved an increased BMR-and unselected control lines. Proportions of fatty acids significantly differed among the organs: liver was the least unsaturated, whereas the gastrocnemius muscles were most unsaturated. However, fatty acid proportions of the heart and liver did not differ significantly between selected and control lines. In gastrocnemius muscles, significant differences between selection directions were found: compared to control lines, membranes of selected voles were richer in saturated C18:0 and unsaturated C18:2n-6 and C18:3n-3, whereas the pattern was reversed for saturated C16:0 and unsaturated C20:4n-6. Neither unsaturation index nor other combined indexes of fatty acid proportions differed between lines. Thus, our results do not support the membrane pacemaker hypothesis. However, the differences between selected and control lines in gastrocnemius muscles reflect chain lengths rather than number of double bonds and are probably related to differences in locomotor activity per se rather than to differences in the basal or routine metabolic rate.

Lipogenesis mitigates dysregulated sarcoplasmic reticulum calcium uptake in muscular dystrophy

Muscular dystrophy is accompanied by a reduction in activity of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) that contributes to abnormal Ca(2+) homeostasis in sarco/endoplasmic reticulum (SR/ER). Recent findings suggest that skeletal muscle fatty acid synthase (FAS) modulates SERCA activity and muscle function via its effects on SR membrane phospholipids. In this study, we examined muscle's lipid metabolism in mdx mice, a mouse model for Duchenne muscular dystrophy (DMD). De novo lipogenesis was ~50% reduced in mdx muscles compared to wildtype (WT) muscles. Gene expressions of lipogenic and other ER lipid-modifying enzymes were found to be differentially expressed between wildtype (WT) and mdx muscles. A comprehensive examination of muscles' SR phospholipidome revealed elevated phosphatidylcholine (PC) and PC/phosphatidylethanolamine (PE) ratio in mdx compared to WT mice. Studies in primary myocytes suggested that defects in key lipogenic enzymes including FAS, stearoyl-CoA desaturase-1 (SCD1), and Lipin1 are likely contributing to reduced SERCA activity in mdx mice. Triple transgenic expression of FAS, SCD1, and Lipin1 (3TG) in mdx myocytes partly rescued SERCA activity, which coincided with an increase in SR PE that normalized PC/PE ratio. These findings implicate a defect in lipogenesis to be a contributing factor for SERCA dysfunction in muscular dystrophy. Restoration of muscle's lipogenic pathway appears to mitigate SERCA function through its effects on SR membrane composition.

Regulation of the Ca(2+)-ATPase by cholesterol: a specific or non-specific effect?

Like other integral membrane proteins, the activity of the Sarco/Endoplasmic Reticulum Ca(2+)-ATPase (SERCA) is regulated by the membrane environment. Cholesterol is present in the endoplasmic reticulum membrane at low levels, and it has the potential to affect SERCA activity both through direct, specific interaction with the protein or through indirect interaction through changes of the overall membrane properties. There are experimental data arguing for both modes of action for a cholesterol-mediated regulation of SERCA. In the current study, coarse-grained molecular dynamics simulations are used to address how a mixed lipid-cholesterol membrane interacts with SERCA. Candidates for direct regulatory sites with specific cholesterol binding modes are extracted from the simulations. The binding pocket for thapsigargin, a nanomolar inhibitor of SERCA, has been suggested as a cholesterol binding site. However, the thapsigargin binding pocket displayed very little cholesterol occupation in the simulations. Neither did atomistic simulations of cholesterol in the thapsigargin binding pocket support any specific interaction. The current study points to a non-specific effect of cholesterol on SERCA activity, and offers an alternative interpretation of the experimental results used to argue for a specific effect.

Reduced efficiency of sarcolipin-dependent respiration in myocytes from humans with severe obesity

OBJECTIVE

Sarcolipin (SLN) regulates muscle energy expenditure through its action on sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump. It is unknown whether SLN-dependent respiration has relevance to human obesity, but whole-transcriptome gene expression profiling revealed that SLN was more highly expressed in myocytes from individuals with severe obesity (OB) than in lean controls (LN). The purpose of this study was to examine SLN-dependent cellular respiratory rates in LN and OB human muscles.

METHODS

Primary myocytes were isolated from muscle biopsy from seven LN and OB Caucasian females. Cellular respiration was assessed with and without lentivirus-mediated SLN knockdown in LN and OB myocytes.

RESULTS

SLN mRNA and protein abundance was greater in OB compared to LN cells. Despite elevated SLN levels in wild-type OB cells, respiratory rates among SLN-deficient cells were higher in OB compared to LN. Obesity-induced reduction in efficiency of SLN-dependent respiration was associated with altered sarcoplasmic reticulum phospholipidome.

CONCLUSIONS

SLN-dependent respiration is reduced in muscles from humans with severe obesity compared to lean controls. Identification of the molecular mechanism that affects SLN efficiency might lead to interventions that promote an increase in skeletal muscle energy expenditure.

Ecophysiology of Omega Fatty Acids: A Lid for Every Jar

Omega fatty acids affect various physiological functions, such as locomotion, cardiac function, and thermogenesis. We highlight evidence from animal models that points to pathways by which specific omega fatty acids exert differential effects. We suggest that optimizing the omega fatty acid composition of tissues involves trade-offs between costs and benefits of specific fatty acids.

Formalin evokes calcium transients from the endoplasmatic reticulum

The formalin test is the most widely used behavioral screening test for analgesic compounds. The cellular mechanism of action of formaldehyde, inducing a typically biphasic pain-related behavior in rodents is addressed in this study. The chemoreceptor channel TRPA1 was suggested as primary transducer, but the high concentrations used in the formalin test elicit a similar response in TRPA1 wildtype and knockout animals. Here we show that formaldehyde evokes a dose-dependent calcium release from intracellular stores in mouse sensory neurons and primary keratinocytes as well as in non-neuronal cell lines, and independent of TRPA1. The source of calcium is the endoplasmatic reticulum and inhibition of the sarco/endoplasmic reticulum calcium-ATPase has a major contribution. This TRPA1-independent mechanism may underlie formaldehyde-induced pan-neuronal excitation and subsequent inflammation.

Effects of naturally occurring arginine 14 deletion on phospholamban conformational dynamics and membrane interactions

Phospholamban (PLN) is a single-pass membrane protein that regulates the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA). Phosphorylation of PLN at Ser16 reverses its inhibitory function under β-adrenergic stimulation, augmenting Ca²⁺ uptake in the sarcoplasmic reticulum and muscle contractility. PLN exists in two conformations; a T state, where the cytoplasmic domain is helical and adsorbed on the membrane surface, and an R state, where the cytoplasmic domain is unfolded and membrane detached. Previous studies have shown that the PLN conformational equilibrium is crucial to SERCA regulation. Here, we used a combination of solution and solid-state NMR to compare the structural topology and conformational dynamics of monomeric PLN (PLN(AFA)) with that of the PLN(R14del), a naturally occurring deletion mutant that is linked to the progression of dilated cardiomyopathy. We found that the behavior of the inhibitory transmembrane domain of PLN(R14del) is similar to that of the native sequence. Conversely, the conformational dynamics of R14del both in micelles and lipid membranes are enhanced. We conclude that the deletion of Arg14 in the cytoplasmic region weakens the interactions with the membrane and shifts the conformational equilibrium of PLN toward the disordered R state. This conformational transition is correlated with the loss-of-function character of this mutant and is corroborated by SERCA's activity assays. These findings support our hypothesis that SERCA function is fine-tuned by PLN conformational dynamics and begin to explain the aberrant regulation of SERCA by the R14del mutant.

Comparing crystal structures of Ca(2+) -ATPase in the presence of different lipids

The activity of the sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) depends strongly on the lipid composition of the surrounding membrane. Yet, structural information on SERCA-lipid interaction is still relatively scarce, and the influence of different lipids on the enzyme is not well understood. We have analyzed SERCA crystal structures in the presence of four different phosphatidylcholine lipids of different lengths and double-bond compositions, and we find three different binding sites for lipid head groups, which are apparently independent of the acyl moiety of the lipids used. By comparison with other available SERCA structures with bound lipids, we find a total of five recurring sites, two of which are specific to certain conformational states of the enzyme, two others are state-independent, and one is a crucial site for crystal formation. Three of the binding sites overlap with or are in close vicinity to known binding sites for various SERCA-specific inhibitors and regulators, e.g. thapsigargin, sarcolipin/phospholamban and cyclopiazonic acid. Whereas the transient sites are amenable to a transient, regulatory influence of lipid molecules, the state-independent sites probably provide a flexible anchoring of the protein in the fluid bilayer.

Calcium signaling in extraembryonic domains during early teleost development

It is becoming recognized that the extraembryonic domains of developing vertebrates, that is, those that make no cellular contribution to the embryo proper, act as important signaling centers that induce and pattern the germ layers and help establish the key embryonic axes. In the embryos of teleost fish, in particular, significant progress has been made in understanding how signaling activity in extraembryonic domains, such as the enveloping layer, the yolk syncytial layer, and the yolk cell, might help regulate development via a combination of inductive interactions, cellular dynamics, and localized gene expression. Ca(2+) signaling in a variety of forms that include propagating waves and standing gradients is a feature found in all three teleostean extraembryonic domains. This leads us to propose that in addition to their other well-characterized signaling activities, extraembryonic domains are well suited (due to their relative stability and continuity) to act as Ca(2+) signaling centers and conduits.

Development of a Sensitive Assay for SERCA Activity Using FRET Detection of ADP

Various isoforms of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) regulate Ca2+ homeostatic balance in both the heart (SERCA2a) and skeletal muscle (SERCA1a). Ca2+ plays a key role in these tissues as an intracellular signal that controls contractility. Due to its key role in the contractility cycle, SERCA is emerging as a promising pharmacological target to modulate heart muscle function. SERCA function is regulated by its endogenous inhibitor phospholamban (PLN). Upon binding, PLN decreases SERCA's apparent affinity for Ca2+. Therefore the interaction between PLN and SERCA has an important role in determining both physiological and pathological conditions. Quantifying the inhibitory potency of PLN is of great importance in understanding the pathophysiology of heart muscle. Traditionally, SERCA activity assays have been performed using a PK/LDH-coupled enzyme reaction, which suffers from limited sensitivity. We have developed a new SERCA activity assay based on the direct detection of the product ADP via time resolved FRET (TR-FRET). Under optimized conditions, our assay reduced the amount of SERCA required to perform the assay 1,000-fold. Inter-day reproducibility was shown to be excellent for SERCA preparations in either detergent (C12E8) or reconstituted lipids. The inhibitory effect of PLN on SERCA measured under the low-concentration conditions of our assay allowed us to more accurately investigate the binding between PLN and SERCA. Significant inhibitory effects of PLN were observed even at mid-nanomolar concentrations significantly lower than previous Kd estimates for the SERCA-PLN complex.

Secondary structure, backbone dynamics, and structural topology of phospholamban and its phosphorylated and Arg9Cys-mutated forms in phospholipid bilayers utilizing 13C and 15N solid-state NMR spectroscopy

Phospholamban (PLB) is a membrane protein that regulates heart muscle relaxation rates via interactions with the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). When PLB is phosphorylated or Arg9Cys (R9C) is mutated, inhibition of SERCA is relieved. (13)C and (15)N solid-state NMR spectroscopy is utilized to investigate conformational changes of PLB upon phosphorylation and R9C mutation. (13)C═O NMR spectra of the cytoplasmic domain reveal two α-helical structural components with population changes upon phosphorylation and R9C mutation. The appearance of an unstructured component is observed on domain Ib. (15)N NMR spectra indicate an increase in backbone dynamics of the cytoplasmic domain. Wild-type PLB (WT-PLB), Ser16-phosphorylated PLB (P-PLB), and R9C-mutated PLB (R9C-PLB) all have a very dynamic domain Ib, and the transmembrane domain has an immobile component. (15)N NMR spectra indicate that the cytoplasmic domain of R9C-PLB adopts an orientation similar to P-PLB and shifts away from the membrane surface. Domain Ib (Leu28) of P-PLB and R9C-PLB loses the alignment. The R9C-PLB adopts a conformation similar to P-PLB with a population shift to a more extended and disordered state. The NMR data suggest the more extended and disordered forms of PLB may relate to inhibition relief.

Modulation of rabbit muscle sarcoplasmic reticulum Ca(2+)-ATPase activity by novel quercetin derivatives

Sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) is the pump crucial for calcium homeostasis and its impairment results in pathologies such as myopathy, heart failure or diabetes. Modulation of SERCA activity may represent an approach to the therapy of diseases with SERCA impairment involvment. Quercetin is flavonoid known to modulate SERCA activity. We examined the effect of nine novel quercetin derivatives on the activity of the pump. We found that 5-morpholinohydroxypoxyquercetin, di(prenylferuoyl)quercetin, di(diacetylcaffeoyl)-mono-(monoacetylcaffeoyl)quercetin and monoacetylferuloylquercetin stimulated the activity of SERCA. On the contrary, monochloropivaloylquercetin, tri(chloropivaloyl)quercetin, pentaacetylquercetin, tri(trimethylgalloyl)quercetin and diquercetin inhibited the activity of the pump. To identify compounds with a potential to protect SERCA against free radicals, we assessed the free radical scavenging activity of quercetin derivatives. We also related lipophilicity, an index of the ability to incorporate into the membrane of sarcoplasmic reticulum, to the modulatury effect of quercetin derivatives on SERCA activity. In addition to its ability to stimulate SERCA, di(prenylferuloyl)quercetin showed excellent radical scavenging activity.

Structural dynamics and topology of phosphorylated phospholamban homopentamer reveal its role in the regulation of calcium transport

Phospholamban (PLN) inhibits the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA), thereby regulating cardiac diastole. In membranes, PLN assembles into homopentamers that in both the phosphorylated and nonphosphorylated states have been proposed to form ion-selective channels. Here, we determined the structure of the phosphorylated pentamer using a combination of solution and solid-state nuclear magnetic resonance methods. We found that the pinwheel architecture of the homopentamer is preserved upon phosphorylation, with each monomer having an L-shaped conformation. The TM domains form a hydrophobic pore approximately 24 Å long and 2 Å in diameter, which is inconsistent with canonical Ca²⁺-selective channels. Phosphorylation, however, enhances the conformational dynamics of the cytoplasmic region of PLN, causing partial unwinding of the amphipathic helix. We propose that PLN oligomers act as storage for active monomers, keeping SERCA function within a physiological window.

Probing the interaction of Arg9Cys mutated phospholamban with phospholipid bilayers by solid-state NMR spectroscopy

Phospholamban (PLB) is a 52 amino acid integral membrane protein that interacts with the sarcoplasmic reticulum Ca(2+) ATPase (SERCA) and helps to regulate Ca(2+) flow. PLB inhibits SERCA impairing Ca(2+) translocation. The inhibition can be relieved upon phosphorylation of PLB. The Arg9 to Cys (R9C) mutation is a loss of function mutation with reduced inhibitory potency. The effect R9C PLB has on the membrane surface and the hydrophobic region dynamics was investigated by (31)P and (2)H solid-state NMR spectroscopy in multilamellar vesicles (MLVs). The (31)P NMR spectra indicate that, like the phosphorylated PLB (P-PLB), the mutated R9C-PLB protein has significantly less interaction with the lipid bilayer headgroup when compared to wild-type PLB (WT-PLB). Similar to P-PLB, R9C-PLB slightly decreases (31)P T1 values in the lipid headgroup region. (2)H SCD order parameters of (2)H nuclei along the lipid acyl chain decrease less dramatically for R9C-PLB and P-PLB when compared to WT-PLB. The results suggest that R9C-PLB interacts less with the membrane surface and hydrophobic region than WT-PLB. Detachment of the cytoplasmic domain of R9C-PLB from the membrane surface could be related to its loss of function.

Muscle lipogenesis balances insulin sensitivity and strength through calcium signaling

Exogenous dietary fat can induce obesity and promote diabetes, but endogenous fat production is not thought to affect skeletal muscle insulin resistance, an antecedent of metabolic disease. Unexpectedly, the lipogenic enzyme fatty acid synthase (FAS) was increased in the skeletal muscle of mice with diet-induced obesity and insulin resistance. Skeletal muscle-specific inactivation of FAS protected mice from insulin resistance without altering adiposity, specific inflammatory mediators of insulin signaling, or skeletal muscle levels of diacylglycerol or ceramide. Increased insulin sensitivity despite high-fat feeding was driven by activation of AMPK without affecting AMP content or the AMP/ATP ratio in resting skeletal muscle. AMPK was induced by elevated cytosolic calcium caused by impaired sarco/endoplasmic reticulum calcium ATPase (SERCA) activity due to altered phospholipid composition of the sarcoplasmic reticulum (SR), but came at the expense of decreased muscle strength. Thus, inhibition of skeletal muscle FAS prevents obesity-associated diabetes in mice, but also causes muscle weakness, which suggests that mammals have retained the capacity for lipogenesis in muscle to preserve physical performance in the setting of disrupted metabolic homeostasis.

Characterization of Ca(2+) signaling in the external yolk syncytial layer during the late blastula and early gastrula periods of zebrafish development

Preferential loading of the complementary bioluminescent (f-aequorin) and fluorescent (Calcium Green-1 dextran) Ca(2+) reporters into the yolk syncytial layer (YSL) of zebrafish embryos, revealed the generation of stochastic patterns of fast, short-range, and slow, long-range Ca(2+) waves that propagate exclusively through the external YSL (E-YSL). Starting abruptly just after doming (~4.5h post-fertilization: hpf), and ending at the shield stage (~6.0hpf) these distinct classes of waves propagated at mean velocities of ~50 and ~4μm/s, respectively. Although the number and pattern of these waves varied between embryos, their initiation site and arcs of propagation displayed a distinct dorsal bias, suggesting an association with the formation and maintenance of the nascent dorsal-ventral axis. Wave initiation coincided with a characteristic clustering of YSL nuclei (YSN), and their associated perinuclear ER, in the E-YSL. Furthermore, the inter-YSN distance (IND) appeared to be critical such that Ca(2+) wave propagation occurred only when this was <~8μm; an IND >~8μm was coincidental with wave termination at shield stage. Treatment with the IP3R antagonist, 2-APB, the Ca(2+) buffer, 5,5'-dibromo BAPTA, and the SERCA-pump inhibitor, thapsigargin, resulted in a significant disruption of the E-YSL Ca(2+) waves, whereas exposure to the RyR antagonists, ryanodine and dantrolene, had no significant effect. These findings led us to propose that the E-YSL Ca(2+) waves are generated mainly via Ca(2+) release from IP3Rs located in the perinuclear ER, and that the clustering of the YSN is an essential step in providing a CICR pathway required for wave propagation. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.