Biology subjects

Robia, S. L.

Publications and source records attributed to Robia, S. L..

13 recordsLinked to original sources

bioRxiv2026

Neprilysin mediated cleavage of phospholamban dysregulates SERCA in heart failure

BackgroundNeprilysin (NEP) is a zinc-dependent metalloprotease targeted in heart failure therapy to prevent it degrading circulating cardioprotective vasoactive peptides. NEP can also cleave sarcolipin (SLN), the skeletal- and atrial muscle-specific micropeptide regulator of the sarcoplasmic reticulum Ca2+-ATPase (SERCA). A direct pathophysiological role of NEP in ventricular muscle has not been established. MethodsProteomics and immunoblot analysis of human myocardial specimens were used to quantify NEP abundance in failing and non-failing hearts. Heterologous protein expression and biochemical binding assays assessed NEP-mediated cleavage of phospholamban (PLB) and its impact on PLB-SERCA interactions. Functional consequences of NEP expression or inhibition were evaluated in neonatal rat ventricular myocytes and in a human induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) model of heart failure. ResultsWe observed increased NEP abundance in failing human myocardium relative to non-failing controls. We demonstrated that NEP cleaves phospholamban (PLB), disrupting PLB-SERCA interactions. Mutation of PLB (V49A), prevented NEP cleavage and preserved PLB-SERCA binding, indicating V49 is critical for NEP substrate recognition. In neonatal rat ventricular myocytes, NEP expression was associated with faster Ca2+ transient decay kinetics and increased SR Ca2+ load, consistent with reduced SERCA inhibition. Inhibition of NEP in a hiPSC-CM heart failure model attenuated the hypertrophic transcriptional responses and reversed Ca2+-transport dysregulation. ConclusionsThese findings implicate increased NEP expression in the sarcoplasmic reticulum of cardiomyocytes as previously unrecognized maladaptive consequence of heart failure contributing to cardiac dysfunction. In this novel pathophysiological mechanism, increased NEP results in PLB cleavage and loss of regulation of SERCA. While this may relieve SERCA inhibition and augment cellular Ca2+ handling, loss of PLB chronically disrupts hearts dynamic response to adrenergic stress, changing heart rate, or other physiological challenges. The data provide new insight into the cardioprotective effects of pharmacological NEP inhibition in clinical practice, reveal a novel mechanism of action of neprilysin inhibition in cardiomyocytes and may help inform future therapeutic strategies for patients with heart failure. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=118 SRC="FIGDIR/small/732949v1_ufig1.gif" ALT="Figure 1"> View larger version (49K): org.highwire.dtl.DTLVardef@5ddc5dorg.highwire.dtl.DTLVardef@2e746corg.highwire.dtl.DTLVardef@1dd8273org.highwire.dtl.DTLVardef@19f5635_HPS_FORMAT_FIGEXP M_FIG C_FIG

physiology↗

bioRxiv2026

A Novel Mechanism of Cardiomyopathy: Toxic Peptides Dysregulate Calcium Transport

A hallmark of dilated cardiomyopathy (DCM) is calcium mishandling, including reduced transport activity of the SERCA calcium pump in cardiac muscle cells. This has focused attention on SERCA as mechanism of disease and potential therapeutic target. Previously, diminished SERCA activity has been attributed to decreased protein expression, but recent studies suggest SERCA levels are unchanged in DCM. Thus, another mechanism must be responsible for the deficit. Since proteolysis is increased and proteosome function is impaired in DCM, we reasoned that accumulation of toxic protein fragments may contribute to SERCA dysfunction. In particular, previous studies showed diverse species of hydrophobic -helices can inhibit SERCA, so we hypothesized that SERCA may become congested with transmembrane peptides that mimic endogenous regulatory partners. We purified cell membranes from non-failing and DCM human ventricles and subjected them to mass spectrometry to identify protein species upregulated in DCM. Select candidates were screened for binding and inhibition of SERCA. Several small membrane proteins and membrane protein fragments bound avidly to SERCA and significantly reduced cellular calcium stores. The data suggest a novel pathophysiological mechanism in which transmembrane protein debris obstructs SERCA function and regulation, contributing to cardiac muscle dysfunction in heart failure.

physiology↗

bioRxiv2023

Activation mechanism of the cardiac calcium pump by a small-molecule allosteric modulator

The discovery of small-molecule allosteric modulators is an emerging paradigm in drug discovery, and signal transduction is a subtle and dynamic process that is challenging to characterize. We developed a time-correlated single photon counting (TCSPC) imaging approach to investigate the activation mechanism of a druggable protein by a small-molecule allosteric modulator. We tested this approach using the cardiac sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), an important pharmacological target that transports Ca2+ at the expense of ATP hydrolysis in the heart. We found that CDN1163, a validated SERCA2a activator, does not dissociate the endogenous complex between SERCA2a and its regulator phospholamban (PLN) in the presence of either Ca2+ or AMP-PCP, a non-hydrolyzable ATP analog. CDN1163 does not influence SERCA2as affinity for Ca2+ ions at functionally relevant conditions. Global analysis of the fluorescence lifetimes showed that ATP is both a substrate and a modulator that populates competent SERCA2a conformations. Interestingly, CDN1163 alone does not significantly induce changes in the structural populations of SERCA2a. Instead, CDN1163 potentiates the effects of ATP to further shift the equilibrium toward a competent SERCA2a conformation. Importantly, this population shift occurs at sub-physiological conditions, and within physiological Ca2+ concentrations at which SERCA2a operates. We propose an activation mechanism whereby a small-molecule modulator synergizes with ATP to stabilize a conformation of SERCA2a primed for activation. This study demonstrates the power of TCSPC to reveal novel insights into how structural and biochemical states are coupled to allosterically activate a pharmacological target in the heart.

biophysics↗

bioRxiv2023

Phospholamban Inhibits the Cardiac Calcium Pump Through Reversing the Allosteric Enhancement of Calcium Affinity by ATP

Phospholamban (PLB) is a transmembrane micropeptide that regulates the Ca2+ pump SERCA in cardiac muscle, but the physical mechanism of this regulation remains poorly understood. PLB reduces the Ca2+ sensitivity of active SERCA, increasing the Ca2+ concentration required for pump cycling. However, PLB does not decrease Ca2+ binding to SERCA when ATP is absent, suggesting PLB does not inhibit SERCA Ca2+ affinity. The prevailing explanation for these seemingly conflicting results is that PLB slows the Ca2+-binding step in the SERCA enzymatic cycle, altering the Ca2+ dependence of cycling without affecting the true affinity of the Ca2+ binding sites. Here, we consider another hypothesis, that measurements of Ca2+ binding in the absence of ATP overlook important allosteric effects of nucleotide binding that increase SERCA Ca2+ binding affinity. We speculated that PLB inhibits SERCA by reversing this allostery. To test this, we used a fluorescent SERCA biosensor to quantify the Ca2+ affinity of non-cycling SERCA in the presence and absence of a non-hydrolyzable ATP-analog, AMPPCP. Nucleotide activation increased SERCA Ca2+ affinity, and this effect was reversed by co-expression of PLB. Interestingly, PLB had no effect on Ca2+ affinity in the absence of nucleotide. These results reconcile the previous conflicting observations from ATPase assays versus Ca2+ binding assays. Moreover, structural analysis of SERCA revealed a novel allosteric pathway connecting the ATP- and Ca2+-binding sites. We propose this pathway is interrupted by PLB binding. Thus, PLB reduces the true Ca2+ affinity of SERCA by reversing allosteric activation of the pump by ATP. O_TEXTBOXSignificance StatementThe micropeptide phospholamban (PLB) serves a vital role as the "adrenaline trigger" for the heart. PLB inhibits the calcium pump SERCA at rest, and relief of this inhibition increases cardiac performance during exercise. Still, the physical mechanism for PLB inhibition of SERCA remains poorly understood. Here, our results reveal that PLB regulates SERCA by reversing an allosteric effect of ATP that enhances SERCAs calcium affinity. We mapped a novel allosteric pathway that structurally couples SERCAs ATP- and calcium-binding sites. PLB binds residues along this pathway to disrupt communication between the ligand-binding sites. The data reveal an allosteric mechanism that may be conserved in other ATP-dependent ion pumps. This allosteric pathway may provide new opportunities for pharmacological targeting of SERCA. C_TEXTBOX

biochemistry↗

bioRxiv2023

RyR2 Binding of an Antiarrhythmic Cyclic Depsipeptide Mapped Using Confocal Fluorescence Lifetime Detection of FRET

Hyperactivity of cardiac sarcoplasmic reticulum (SR) ryanodine receptor (RyR2) Ca2+-release channels contributes to heart failure and arrhythmias. Reducing RyR2 activity, particularly during cardiac relaxation (diastole), is a desirable therapeutic goal. We previously reported that the unnatural enantiomer (ent) of an insect-RyR activator, verticilide, inhibits porcine and mouse RyR2 at diastolic (nanomolar) Ca2+ and has in vivo efficacy against atrial and ventricular arrhythmia. To determine the ent-verticilide structural mode of action on RyR2 and guide its further development via medicinal chemistry structure-activity relationship studies, here we used fluorescence lifetime (FLT)-measurements of Forster resonance energy transfer (FRET) in HEK293 cells expressing human RyR2. For these studies, we used an RyR-specific FRET molecular-toolkit and computational methods for trilateration (i.e., using distances to locate a point of interest). Multi-exponential analysis of FLT-FRET measurements between four donor-labeled FKBP12.6 variants and acceptor-labeled ent-verticilide, yielded distance relationships placing the acceptor probe at two candidate loci within the RyR2 cryo-EM map. One locus is within the Ry12 domain (at the corner periphery of the RyR2 tetrameric complex). The other locus is sandwiched at the interface between helical domain 1 and the SPRY3 domain. These findings document RyR2-target engagement by ent-verticilide, reveal new insight into the mechanism of action of this new class of RyR2-targeting drug candidate, and can serve as input in future computational determinations of the ent-verticilide binding site on RyR2 that will inform structure-activity studies for lead optimization.

biophysics↗

bioRxiv2023

Dilated cardiomyopathy variant R14del increases phospholamban pentamer stability, blunting dynamic regulation of cardiac calcium handling

The sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) is a membrane transporter that creates and maintains intracellular Ca2+ stores. In the heart, SERCA is regulated by an inhibitory interaction with the monomeric form of the transmembrane micropeptide phospholamban (PLB). PLB also forms avid homo-pentamers, and dynamic exchange of PLB between pentamers and the regulatory complex with SERCA is an important determinant of cardiac responsiveness to exercise. Here, we investigated two naturally occurring pathogenic mutations of PLB, a cysteine substitution of arginine 9 (R9C) and an in-frame deletion of arginine 14 (R14del). Both mutations are associated with dilated cardiomyopathy. We previously showed that the R9C mutation causes disulfide crosslinking and hyperstabilization of pentamers. While the pathogenic mechanism of R14del is unclear, we hypothesized that this mutation may also alter PLB homo-oligomerization and disrupt the PLB-SERCA regulatory interaction. SDS-PAGE revealed a significantly increased pentamer:monomer ratio for R14del-PLB when compared to WT-PLB. In addition, we quantified homo-oligomerization and SERCA-binding in live cells using fluorescence resonance energy transfer (FRET) microscopy. R14del-PLB showed an increased affinity for homo-oligomerization and decreased binding affinity for SERCA compared to WT, suggesting that, like R9C, the R14del mutation stabilizes PLB in its pentameric form, decreasing its ability to regulate SERCA. Moreover, the R14del mutation reduces the rate of PLB unbinding from the pentamer after a transient Ca2+ elevation, limiting the rate of re-binding to SERCA. A computational model predicted that hyperstabilization of PLB pentamers by R14del impairs the ability of cardiac Ca2+ handling to respond to changing heart rates between rest and exercise. We postulate that impaired responsiveness to physiological stress contributes to arrhythmogenesis in human carriers of the R14del mutation.

physiology↗

bioRxiv2021

STOICHIOMETRY OF THE SODIUM PUMP-PHOSPHOLEMMAN REGULATORY COMPLEX

The sodium-potassium ATPase (NKA) establishes ion gradients that facilitate many physiological processes. In the heart, NKA activity is regulated by its interaction with phospholemman (PLM, FXYD1). Here we used a novel fluorescence lifetime-based assay to investigate the structure, stoichiometry, and affinity of the NKA-PLM regulatory complex. We observed concentration dependent association of the subunits of NKA-PLM regulatory complex, with avid association of the alpha subunit with the essential beta subunit followed by lower affinity alpha-alpha and alpha-PLM interactions. The data provide the first evidence that the regulatory complex is composed of two alpha subunits associated with two beta subunits, decorated with two PLM regulatory subunits in intact cells. Docking and molecular dynamics simulations generated a structural model of the complex that is consistent with our experimental observations. We propose that alpha-alpha subunit interactions support conformational coupling of the catalytic subunits, which may enhance NKA turnover rate. These observations provide insight into the pathophysiology of heart failure, wherein low NKA expression may be insufficient to support formation of the complete regulatory complex with stoichiometry (alpha-beta-PLM)2.

biophysics↗

bioRxiv2021

Inhibitory and Stimulatory Micropeptides Preferentially Bind to Different Conformations of the Cardiac Calcium Pump

The ATP-dependent ion pump SERCA sequesters Ca2+ in the endoplasmic reticulum to establish a reservoir for cell signaling. Because of its central importance in physiology, this transporter is tightly controlled by physical interactions with tissue-specific regulatory micropeptides that tune SERCA function to match changing physiological conditions. In the heart, phospholamban (PLB) inhibits SERCA, while dwarf open reading frame (DWORF) stimulates SERCA. These competing interactions determine cardiac performance by modulating the amplitude of Ca2+ signals that drive the contraction/relaxation cycle. The distinct functions of these peptides may relate to their reciprocal preferences for SERCA binding. While SERCA binds PLB more avidly at low cytoplasmic Ca2+, it binds DWORF better at high Ca2+. In the present study, we determined that this opposing Ca2+ sensitivity is due to preferential binding of DWORF and PLB to different intermediate conformations that the pump samples during the Ca2+ transport cycle. The results suggest a mechanistic basis for inhibitory and stimulatory micropeptide function. In addition, fluorescence resonance energy transfer (FRET) measurements revealed dynamic shifts in SERCA-micropeptide binding equilibria during cellular Ca2+ elevations. The data suggest Ca2+-dependent dynamic exchange of inhibitory and stimulatory micropeptides from SERCA during the cardiac cycle. Together, these mechanisms provide beat-to-beat modulation of cardiac Ca2+ handling and contribute to the hearts adaptation to the increased physiological demands of exercise. O_TEXTBOXSignificance Interactions between SERCA and its regulatory micropeptides modulate cardiac performance. A previous study demonstrated that while the SERCA inhibitor, phospholamban, loses affinity during intracellular Ca2+ elevations, its competitor DWORF stimulates SERCA and binds better at elevated Ca2+. Here, we found this Ca2+-dependent difference in affinity is driven by preferential micropeptide binding to different SERCA conformations sampled during its enzymatic cycle. Tight binding to different conformations of the pump may underly the distinct mechanistic functions of these regulators. Lower affinity during alternating phases of the cardiac cycle may drive dynamic exchange of inhibitory and stimulatory micropeptides during contraction and relaxation. Our results reveal mechanisms that modulate cardiac Ca2+ handling on a beat-to-beat basis and help the heart adapt to exercise. C_TEXTBOX

biophysics↗

bioRxiv2021

A Kink in DWORF Helical Structure Controls the Activation of the Sarco-plasmic Reticulum Ca2+-ATPase

The sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) is a P-type ATPase embedded in the sarcoplasmic reticulum. For each enzymatic cycle, SERCA transports 2 Ca2+ ions per ATP hydrolyzed in exchange for 2 to 3 H+ ions. SERCA is responsible for approximately 70% of Ca2+ transport and plays a central role in muscle relaxation. SERCAs function is regulated by endogenous regulins, single-pass membrane proteins that bind the ATPase within the membrane. While most of the regulins, such as phospholamban and sarcolipin, inhibit SERCAs activity, a newly discovered protein DWarf Open Reading Frame (DWORF) has a unique activating effect. DWORF is a 3.8 kDa bitopic membrane protein expressed in cardiac muscle. In this work, we determine the structure, topology, and per-residue lipid interactions of DWORF in lipid bilayers using a combination of high-resolution oriented sample solid-state NMR (OS-ssNMR) spectroscopy and refinement by replica-averaged orientationally-restrained molecular dynamics (RAOR-MD). We found that DWORFs structural topology consists of a dynamic N-terminal domain, an amphipathic juxtamembrane helix that crosses the lipid groups at an angle of 64{degrees} and a transmembrane (TM) C-terminal helix with an angle of 32{degrees}. A kink induced by Pro15, unique to DWORF, separated the two helical domains. A single Pro15Ala mutant significantly decreases the kink and eliminates DWORFs activating effect on SERCA. Overall, our findings directly link DWORFs structural topology to its unique activating effect on SERCA.

biophysics↗

bioRxiv2020

Dwarf open reading frame (DWORF) peptide is a direct activator of the sarcoplasmic reticulum calcium pump SERCA.

The cardiac sarcoplasmic reticulum calcium pump, SERCA, sequesters calcium in the sarco-endoplasmic reticulum (SR/ER) and plays a critical role in the contraction-relaxation cycle of the heart. A well-known regulator of SERCA in cardiac muscle is phospholamban (PLN), which interacts with the pump and reduces its apparent calcium affinity. A newly discovered SERCA regulatory subunit in cardiac muscle, dwarf open reading frame (DWORF), has added a new level of SERCA regulation. In this report, we modeled the structure of DWORF and evaluated it using molecular dynamics simulations. DWORF structure was modeled as a discontinuous helix with an unwound region at Pro15. This model orients an N-terminal amphipathic helix along the membrane surface and leaves a relatively short C-terminal transmembrane helix. We determined the functional regulation of SERCA by DWORF using a membrane reconstitution system. Surprisingly, we observed that DWORF directly activated SERCA by increasing its turnover rate. Furthermore, in-cell imaging of calcium dynamics demonstrated that DWORF increased SERCA-dependent ER calcium load, calcium reuptake rate, and spontaneous calcium release. Together, these functional assays suggest opposing effects of DWORF and PLN on SERCA function. The results agree with fluorescence resonance energy transfer experiments, which revealed changes in the affinity of DWORF for SERCA at low versus high cytosolic calcium concentrations. We found that DWORF has a higher affinity for SERCA in the presence of calcium, while PLN had the opposite behavior, a higher affinity for SERCA in low calcium. We propose a new mechanism for DWORF regulation of cardiac calcium handling in which DWORF directly enhances SERCA turnover by stabilizing the conformations of SERCA that predominate during elevated cytosolic calcium.

biophysics↗

bioRxiv2020

Sarcolambans are phospholamban- and sarcolipin-like regulators of the sarcoplasmic reticulum calcium pump SERCA

From insects to humans, calcium signaling is essential for life. An important part of this process is the sarco-endoplasmic reticulum calcium pump SERCA, which maintains low cytosolic calcium levels required for intracellular calcium homeostasis. In higher organisms, this is a tightly controlled system where SERCA interacts with tissuespecific regulatory subunits such as phospholamban in cardiac muscle and sarcolipin in skeletal muscle. With the recent discovery of the sarcolambans, the family of calcium pump regulatory subunits also appears to be ancient, spanning more than 550 million years of evolutionary divergence from insects to humans. This evolutionary divergence is reflected in the peptide sequences, which vary enormously from one another and range from vaguely phospholamban-like to vaguely sarcolipin-like. Here, our goal was to investigate select sarcolamban peptides for their ability to regulate calcium pump activity. For a side-by-side comparison of diverse sarcolamban peptides, we tested them against mammalian skeletal muscle SERCA1a. This allowed us to determine if the sarcolamban peptides mimic phospholamban and sarcolipin in their regulatory activities. Four sarcolamban peptides were chosen from different invertebrate species. Of these, we were able to express and purify sarcolamban peptides from bumble bee, water flea, and tadpole shrimp. Sarcolamban peptides were co-reconstituted into proteoliposomes with mammalian SERCA1a and the effect of each peptide on the apparent calcium affinity and maximal activity of SERCA was measured. While all peptides were super-inhibitors of SERCA, they exhibited either phospholamban-like or sarcolipin-like characteristics. Molecular modeling, protein-protein docking, and molecular dynamics simulations were used to reveal novel features of insect versus mammalian calcium pumps and the sarcolamban regulatory subunits.

biochemistry↗

bioRxiv2020

Dynamics-driven allostery underlies pre-activation of the regulatory Ca2+-ATPase/phospholamban complex

Sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and phospholamban (PLB) are essential for intracellular Ca2+ transport in myocytes. Ca2+-dependent activation of SERCA-PLB provides a rheostat function that regulates cytosolic and SR Ca2+ levels. While experimental and computational studies alone have led to a greater insight into the mechanisms for SERCA-PLB regulation, the structural changes induced by Ca2+ binding and how those are communicated to couple enzymatic activity with active transport remain poorly understood. Therefore, we have performed atomistic simulations totaling 32.7 s and cell-based intramolecular fluorescence resonance energy transfer (FRET) experiments to determine structural changes of PLB-bound SERCA in response to Ca2+ binding. Complementary simulations and experiments showed structural disorder underlies PLB inhibition of SERCA, and Ca2+ binding is sufficient to shift the protein population toward a structurally ordered state of the complex. This structural transition results in a redistribution of structural states toward a partially closed conformation of SERCAs cytosolic headpiece. Closure is accompanied by functional interactions between the N-domain {beta}5-{beta}6 loop and the A-domain. Regulation of these key structural elements indicate that Ca2+ is a critical mediator of allosteric signaling that dictates structural changes and motions that pre-activate SERCA-PLB. These findings provide direct support that dynamically driven protein allostery underlies PLB regulation of SERCA. These functional insights at unprecedented spatiotemporal resolution suggest a general modular architecture mechanism for dynamic regulation of the SERCA-PLB complex. Understanding these mechanisms is of paramount importance to guide therapeutic modulation of SERCA and other evolutionarily related ion-motive ATPases.

biophysics↗

bioRxiv2019

Protein Docking and Steered Molecular Dynamics Reveal Alternative Regulatory Sites on the SERCA Calcium Transporter

The transport activity of the calcium ATPase SERCA is modulated by an inhibitory interaction with a 52-residue transmembrane peptide, phospholamban (PLB). Biochemical and structural studies have revealed the primary inhibitory site on SERCA, but PLB has been hypothesized to interact with alternative sites on SERCA that are distinct from the inhibitory site. The present study was undertaken to test these hypotheses and explore structural determinants of SERCA regulation by PLB. Steered molecular dynamics (SMD) and membrane protein-protein docking experiments were performed to investigate the apparent affinity of PLB interactions with candidate sites on SERCA. We modeled the relative binding of PLB to several different conformations of SERCA, representing different enzymatic states sampled during the calcium transport catalytic cycle. Overall, the SMD and docking experiments suggest that the canonical binding site is preferred, but also provide evidence for alternative sites that are favorable for certain conformational states of SERCA.

biophysics↗