Protein S-palmitoylation is a reversible lipid modification which regulates the localisation, stability, and activity of a substantial fraction of the proteome. In the heart S-palmitoylation controls the function of membrane proteins which regulate cardiac excitability and contractility. Recent work has moved the S-palmitoylation field beyond descriptive cataloguing towards mechanistic understanding, revealing how site-specific S-palmitoylation can modulate protein structure and function in a precise and quantifiable manner. These advances position S-palmitoylation as a tuneable regulator of cardiac proteins and consequently cardiac function, and a potential target for therapeutic modulation. In this review, we focus on the role of S-palmitoylation in cardiac function, highlighting its effects on key ion channels and transporters, including CaV1.2, NaV1.5 and NCX1, and the resulting consequences for cardiac electrophysiology and calcium handling. We also discuss emerging experimental tools and strategies to selectively manipulate S-palmitoylation and their limitations. Finally, we consider the translational potential of targeting S-palmitoylation, emphasising the need for selective intervention strategies informed by detailed molecular and structural insight.

Cellular Ca2+ homeostasis is critical for normal cell physiology and is regulated by several mechanisms. Two major players in intracellular Ca2+ homeostasis in multiple tissues belong to the SLC8 (Na+/Ca2+ exchangers (NCXs); NCX1-3) and SLC24 (K+ dependent Na+/Ca2+ exchangers (NCKXs); NCKX1-5) families. It has been established that NCXs and NCKX4 are palmitoylated, and that palmitoylation promotes NCX1 inactivation. However, there is still little known about NCKXs’ palmitoylation. We found that (1) NCKX3 and NCKX5, but not NCKX1, are palmitoylated, (2) Cys to Ala mutation at position 467 for NCXK3 and 325 for NCKX5 notably diminished palmitoylation and (3) reduced palmitoylation enhanced NCKX3 activity. Our findings bring novel insights into NCKX1, NCKX3 and NCKX5 palmitoylation and establish palmitoylation as an endogenous regulator of NCKX3 activity, paving the way for investigations evaluating the role of palmitoylation in NCKX3 function in health and disease.

The complexity of the cellular proteome is massively expanded by a repertoire of chemically distinct reversible post-translational modifications (PTMs) that control protein localisation, interactions, and function. The temporal and spatial control of these PTMs is central to organism physiology, and mis-regulation of PTMs is a hallmark of many diseases. Here we present an approach to manipulate PTMs on target proteins using nanobodies fused to enzymes that control these PTMs. Anti-GFP nanobodies fused to thioesterases (which depalmitoylate protein cysteines) depalmitoylate GFP tagged substrates. A chemogenetic approach to enhance nanobody affinity for its target enables temporal control of target depalmitoylation. Using a thioesterase fused to a nanobody directed against the Ca(v)1.2 beta subunit we reduce palmitoylation of the Ca(v)1.2 alpha subunit, modifying the channel's voltage dependence and arrhythmia susceptibility in stem cell derived cardiac myocytes. We conclude that nanobody enzyme chimeras represent an approach to specifically manipulate PTMs, with applications in both the laboratory and the clinic.

BACKGROUND:The fatty acid (FA) transporter CD36 (FA translocase/cluster of differentiation 36) is the gatekeeper of cardiac FA metabolism. Preferential localisation of CD36 to the sarcolemma is one of the initiating cellular responses in the development of muscle insulin resistance and in the type 2 diabetic heart. Post-translational S-acylation controls protein trafficking, and in this study we hypothesised that increased CD36 S-acylation may underpin the preferential sarcolemmal localisation of CD36, driving metabolic and contractile dysfunction in diabetes. METHODS:Type 2 diabetes was induced in the rat using high fat diet and a low dose of streptozotocin. Forkhead box O1 (FoxO1) transcriptional regulation of zDHHC4 (zinc finger DHHC-type palmitoyltransferase 4) and subsequent S-acylation of CD36 was assessed using chromatin immunoprecipitation (ChIP) sequencing, ChIP-quantitative polymerase chain reaction, luciferase assays, siRNA (small interfering RNA) and shRNA silencing. RESULTS:Type 2 diabetes increased cardiac CD36 S-acylation, CD36 sarcolemmal localisation, FA oxidation rates and triglyceride storage in the diabetic heart. CD36 S-acylation was increased in diabetic rats, db/db mice, diabetic pigs and insulin-resistant human iPSC-derived cardiomyocytes, demonstrating conservation between species. The enzyme responsible for S-acylating CD36, zDHHC4, was transcriptionally upregulated in the diabetic heart, and genetic silencing of zDHHC4 decreased CD36 S-acylation. We identified that zDHHC4 expression is under the regulation of the transcription factor FoxO1. Diabetic mice with cardiomyocyte-specific FoxO1 deletion had decreased cardiac zDHHC4 expression and decreased CD36 S-acylation, which was further confirmed using diabetic mice treated with the FoxO1 inhibitor AS1842856. Pharmacological inhibition of zDHHC enzymes in diabetic hearts decreased CD36 S-acylation, sarcolemmal CD36 content, FA oxidation rates and triglyceride storage, culminating in improved cardiac function in diabetes. Conversely, inhibiting the de-acylating enzymes in control hearts increased CD36 S-acylation, sarcolemmal CD36 content and FA metabolic rates in control hearts, recapitulating the metabolic phenotype seen in diabetic hearts. CONCLUSIONS:Activation of the FoxO1-zDHHC4-CD36 S-acylation axis drives metabolic and contractile dysfunction in the type 2 diabetic heart.

Collagen IV is a major constituent of basement membranes and mutations in the genes COL4A1 and COL4A2 present clinically as a variable, multi-system disorder called COL4A1 (Gould) syndrome. Evidence from case reports supports a cardiac component to this disease, but the phenotypic and functional implications affecting the heart, their progression and underlying mechanisms all remain poorly characterised. Indeed, the role of the basement membrane (BM) in adult cardiac disease remains underexplored. We set out to address these knowledge gaps by combining in-depth phenotypic and molecular analyses of a Col4a1 mutation on cardiac biology in a murine model (Col4a1+/svc) of Gould Syndrome. This revealed morphological cardiac defects including cardiomyocyte hypertrophy with myocardial and vascular fibrotic remodelling that impaired cardiac function. The Col4a1 mutation causes systolic and diastolic dysfunction with reduced left ventricular developed pressure. Mechanistically, we show these defects are due to secretion of mutant protein and BM defects rather than collagen misfolding and proteotoxic stress. The BM defects lead to a pro-fibrotic state with increased fibrillar collagen deposition, cardiac stiffness, and ECM compositional defects. These are accompanied by altered regulation of pathways involved in sarcomere formation, sarcolemma stability and cardiomyocyte metabolism, establishing a molecular signature of COL4A1-related cardiac disease. Intriguingly, aspects of this molecular signature including cardiac metabolic pathways, regulation of cardiac muscle contraction and BM component expression, are shared with common cardiomyopathies such as coronary micro-embolism, and dilated, ischemic and hypertrophic obstructive cardiomyopathies. By defining the molecular and phenotypic cardiac components of Gould syndrome these data show that the BM is essential for maintaining systolic and diastolic function and that alterations to the BM leads to a fibrotic response. These data increase insight into the role of the basement membrane and collagen IV in cardiac biology, and highlights mechanisms shared between Gould syndrome and common adult cardiac disease.

AbstractElevated intracellular sodium Nai adversely affects mitochondrial metabolism and is a common feature of heart failure. The reversibility of acute Na induced metabolic changes is evaluated in Langendorff perfused rat hearts using the Na/K ATPase inhibitor ouabain and the myosin-uncoupler para-aminoblebbistatin to maintain constant energetic demand. Elevated Nai decreases Gibb’s free energy of ATP hydrolysis, increases the TCA cycle intermediates succinate and fumarate, decreases ETC activity at Complexes I, II and III, and causes a redox shift of CoQ to CoQH2, which are all reversed on lowering Nai to baseline levels. Pseudo hypoxia and stabilization of HIF-1α is observed despite normal tissue oxygenation. Inhibition of mitochondrial Na/Ca-exchange with CGP-37517 or treatment with the mitochondrial ROS scavenger MitoQ prevents the metabolic alterations during Nai elevation. Elevated Nai plays a reversible role in the metabolic and functional changes and is a novel therapeutic target to correct metabolic dysfunction in heart failure.

Background Genetic variants in COL4A1 and COL4A2 (encoding collagen IV alpha chain 1/2) occur in genetic and sporadic forms of cerebral small vessel disease (CSVD), a leading cause of stroke, dementia and intracerebral haemorrhage (ICH). However, the molecular mechanisms of CSVD with ICH and COL4A1/COL4A2 variants remain obscure. Methods Vascular function and molecular investigations in mice with a Col4a1 missense mutation and heterozygous Col4a2 knock-out mice were combined with analysis of human brain endothelial cells harboring COL4A1/COL4A2 mutations, and brain tissue of patients with sporadic CSVD with ICH. Findings Col4a1 missense mutations cause early-onset CSVD independent of hypertension, with enhanced vasodilation of small arteries due to endothelial dysfunction, vascular wall thickening and reduced stiffness. Mechanistically, the early-onset dysregulated endothelium-dependent hyperpolarization (EDH) is due to reduced collagen IV levels with elevated activity and levels of endothelial Ca2+-sensitive K+ channels. This results in vasodilation via the Na/K pump in vascular smooth muscle cells. Our data support this endothelial dysfunction preceding development of CSVD-associated ICH is due to increased cytoplasmic Ca2+ levels in endothelial cells. Moreover, cerebral blood vessels of patients with sporadic CSVD show genotype-dependent mechanisms with wall thickening and lower collagen IV levels in those harboring common non-coding COL4A1/COL4A2 risk alleles. Interpretation COL4A1/COL4A2 variants act in genetic and sporadic CSVD with ICH via dysregulated EDH, and altered vascular wall thickness and biomechanics due to lower collagen IV levels and/or mutant collagen IV secretion. These data highlight EDH and collagen IV levels as potential treatment targets. Funding MRC, Wellcome Trust, BHF.

Caveolae are small flask-shaped invaginations of the surface membrane which are proposed to recruit and co-localise signalling molecules. The distinctive caveolar shape is achieved by the oligomeric structural protein caveolin, of which three isoforms exist. Aside from the finding that caveolin-3 is specifically expressed in muscle, functional differences between the caveolin isoforms have not been rigorously investigated. Caveolin-3 is relatively cysteine-rich compared to caveolins 1 and 2, so we investigated its cysteine post-translational modifications. We find that caveolin-3 is palmitoylated at 6 cysteines and becomes glutathiolated following redox stress. We map the caveolin-3 palmitoylation sites to a cluster of cysteines in its C terminal membrane domain, and the glutathiolation site to an N terminal cysteine close to the region of caveolin-3 proposed to engage in protein interactions. Glutathiolation abolishes caveolin-3 interaction with heterotrimeric G protein alpha subunits. Our results indicate that a caveolin-3 oligomer contains up to 66 palmitates, compared to up to 33 for caveolin-1. The additional palmitoylation sites in caveolin-3 therefore provide a mechanistic basis by which caveolae in smooth and striated muscle can possess unique phospholipid and protein cargoes. These unique adaptations of the muscle-specific caveolin isoform have important implications for caveolar assembly and signalling.

Summary: Phospholemman (PLM) regulates the cardiac sodium pump: PLM phosphorylation activates the pump whereas PLM palmitoylation inhibits its activity. Here, we show that the anti-oxidant protein peroxiredoxin 6 (Prdx6) interacts with and depalmitoylates PLM in a glutathione-dependent manner. Glutathione loading cells acutely reduce PLM palmitoylation; glutathione depletion significantly increases PLM palmitoylation. Prdx6 silencing abolishes these effects, suggesting that PLM can be depalmitoylated by reduced Prdx6. In vitro, only recombinant Prdx6, among several peroxiredoxin isoforms tested, removes palmitic acid from recombinant palmitoylated PLM. The broad-spectrum depalmitoylase inhibitor palmostatin B prevents Prdx6-dependent PLM depalmitoylation in cells and in vitro. Our data suggest that Prdx6 is a thioesterase that can depalmitoylate proteins by nucleophilic attack via its reactive thiol, linking PLM palmitoylation and hence sodium pump activity to cellular glutathione status. We show that protein depalmitoylation can occur via a catalytic cysteine in which substrate specificity is determined by a protein-protein interaction.

Reversible S-palmitoylation of protein cysteines, catalysed by a family of integral membrane zDHHC-motif containing palmitoyl acyl transferases (zDHHC-PATs), controls the localisation, activity, and interactions of numerous integral and peripheral membrane proteins. There are compelling reasons to want to inhibit the activity of individual zDHHC-PATs in both the laboratory and the clinic, but the specificity of existing tools is poor. Given the extensive conservation of the zDHHC-PAT active site, development of isoform-specific competitive inhibitors is highly challenging. We therefore hypothesised that proteolysis-targeting chimaeras (PROTACs) may offer greater specificity to target this class of enzymes. In proof-of-principle experiments we engineered cell lines expressing tetracycline-inducible Halo-tagged zDHHC5 or zDHHC20, and evaluated the impact of Halo-PROTACs on zDHHC-PAT expression and substrate palmitoylation. In HEK-derived FT-293 cells, Halo-zDHHC5 degradation significantly decreased palmitoylation of its substrate phospholemman, and Halo-zDHHC20 degradation significantly diminished palmitoylation of its substrate IFITM3, but not of the SARS-CoV-2 spike protein. In contrast, in a second kidney derived cell line, Vero E6, Halo-zDHHC20 degradation did not alter palmitoylation of either IFITM3 or SARS-CoV-2 spike. We conclude from these experiments that PROTAC-mediated targeting of zDHHC-PATs to decrease substrate palmitoylation is feasible. However, given the well-established degeneracy in the zDHHC-PAT family, in some settings the activity of non-targeted zDHHC-PATs may substitute and preserve substrate palmitoylation.

The cardiac Na+/Ca2+ Exchanger (NCX1) controls transmembrane calcium flux in numerous tissues. The only reversible post-translational modification established to regulate NCX1 is palmitoylation, which alters the ability of the exchanger to inactivate. Palmitoylation creates a binding site for the endogenous XIP domain, a region of the NCX1 intracellular loop established to inactivate NCX1. The binding site created by NCX1 palmitoylation sensitizes the transporter to XIP. Herein we summarize our recent knowledge on NCX1 palmitoylation and its association with cardiac pathologies, and discuss these findings in the light of the recent cryo-EM structures of human NCX1.

Mammalian voltage-activated L-type Ca2+ channels, such as Ca(v)1.2, control transmembrane Ca2+ fluxes in numerous excitable tissues. Here, we report that the pore-forming α1C subunit of Ca(v)1.2 is reversibly palmitoylated in rat, rabbit, and human ventricular myocytes. We map the palmitoylation sites to two regions of the channel: The N terminus and the linker between domains I and II. Whole-cell voltage clamping revealed a rightward shift of the Ca(v)1.2 current-voltage relationship when α1C was not palmitoylated. To examine function, we expressed dihydropyridine-resistant α1C in human induced pluripotent stem cell-derived cardiomyocytes and measured Ca2+ transients in the presence of nifedipine to block the endogenous channels. The transients generated by unpalmitoylatable channels displayed a similar activation time course but significantly reduced amplitude compared to those generated by wild-type channels. We thus conclude that palmitoylation controls the voltage sensitivity of Ca(v)1.2. Given that the identified Ca(v)1.2 palmitoylation sites are also conserved in most Ca(v)1 isoforms, we propose that palmitoylation of the pore-forming α1C subunit provides a means to regulate the voltage sensitivity of voltage-activated Ca2+ channels in excitable cells.

EDITORIAL article Front. Physiol., 30 October 2023Sec. Lipid and Fatty Acid Research Volume 14 - 2023 | https://doi.org/10.3389/fphys.2023.1317031

Objective: Reactive oxygen species (ROS) play a key role in the regulation of vascular smooth muscle cell (VSMC) phenotypic switching in hypertension. Nox5 is a major ROS-generating enzyme in vascular cells and is upregulated in hypertension. However, the effects of Nox5-derived ROS on VSMC proteome and phenotype in human hypertension are unknown. We aimed to characterize the global and oxidative proteomic profile and VSMC phenotype in human hypertension and the role of Nox5. Design and method: VSMC from resistance arteries from normotensive (NT) and hypertensive (HT) subjects were studied (n = 5). Proteins were labelled with isobaric tandem mass tags and identified by liquid chromatography tandem mass spectrometry. The oxidative proteome was assessed using stable isotope-labelled iodoacetamide to target cysteine thiols. Nox5 silencing was performed by siRNA. Protein expression was detected by western blotting. The inflammatory, pro-fibrotic and mitogenic phenotype of VSMCs was assessed by measuring pro-inflammatory cytokines (IL-6, IL-8), pro-collagen I in the culture media. Results: Proteomic analysis identified 207 proteins upregulated in HT subjects (fold change > 1.5, p < 0.05). Gene ontology enrichment analysis of upregulated proteins in HT showed most proteins belong to extracellular space and plasma membrane compartments and were involved in extracellular matrix organization (ECM), immune response and cell proliferation. ECM proteins COL1A1, COL9A1, COL10A1, FBN1, FBLN1 were increased in cells from HT, suggesting a switch to a fibroblast-like phenotype in hypertension. Expression of proteins related to interferon and IL-1β pathways (IFIT1, IFIT2, IFIT3, MX1, MX2, ABCA1, ABCA2, IL1RAP, CD36, ICAM1) were also increased in cells from HT subjects. The VSMC oxidative proteome analysis identified 130 significantly regulated cysteine-containing peptides, 88 showed increased oxidation in HT (fold change> 1.5, p < 0.05). Among the highly oxidized proteins in HT were ECM proteins, COL11A1, COL16A1, FBLN1 and FBLN2. VSMCs from HT subjects exhibit increased expression of the proliferation marker, PCNA (0.162 ± 0.3 vs NT:0.051 ± 0.04 relative fluorescence units, p < 0.05) and pro-collagen I (23.6 ± 2 vs NT:13.2 ± 0.3 ng/ml, p < 0.05). Production of pro-inflammatory cytokines IL-6 (501.8 ± 23.6 vs NT:121.7 ± 6.4 pg/mL) and IL-8 (373.6 ± 34.1 vs NT:262.5 ± 24.6 pg/mL, p < 0.05) were increased in HT. Nox5 silencing in VSMC from HT subjects reduced PCNA expression(43%), pro-collagen I release (8%), baseline and LPS-induced IL-6 (30% baseline, 43% LPS-induced) and IL-8 (21% baseline, 23% LPS-induced) release (p < 0.05). Conclusions: Our study provides new insights into the proteomic changes related to vascular phenotype in hypertension and demonstrated that Nox5 plays an important role in VSMC phenotypic switching associated with vascular injury and remodelling in hypertension.

The sinoatrial node (SAN) and subsidiary pacemakers in the cardiac conduction system generate spontaneous electrical activity which is indispensable for electrical and therefore contractile function of the heart. The hyperpolarisation-activated cyclic nucleotide-gated channel HCN4 is responsible for genesis of the pacemaker “funny” current during diastolic depolarisation. S-palmitoylation, the reversible conjugation of the fatty acid palmitate to protein cysteine sulfhydryls, regulates the activity of key cardiac Na+ and Ca2+ handling proteins, influencing their membrane microdomain localisation and function. We investigated HCN4 palmitoylation and its functional consequences in engineered human embryonic kidney 293T cells as well as endogenous HCN4 in neonatal rat ventricular myocytes. HCN4 was palmitoylated in all experimental systems investigated. We mapped the HCN4 palmitoylation sites to a pair of cysteines in the HCN4 intracellular amino terminus. A double cysteine-to-alanine mutation CC93A/179AA of full length HCN4 caused a ∼67% reduction in palmitoylation in comparison to wild type HCN4. We used whole-cell patch clamp to evaluate HCN4 current (IHCN4) in stably transfected 293T cells. Removal of the two N-terminal palmitoylation sites did not significantly alter half maximal activation voltage of IHCN4 or the activation slope factor. IHCN4 was significantly larger in cells expressing wild type compared to non-palmitoylated HCN4 across a range of voltages. Phylogenetic analysis revealed that although cysteine 93 is widely conserved across all classes of HCN4 vertebrate orthologs, conservation of cysteine 179 is restricted to placental mammals. Collectively, we provide evidence for functional regulation of HCN4 via palmitoylation of its amino terminus in vertebrates. We suggest that by recruiting the amino terminus to the bilayer, palmitoylation enhances the magnitude of HCN4-mediated currents, but does not significantly affect the kinetics.