Regulatory mechanisms controlling store-operated calcium entry

Calcium influx through plasma membrane ion channels is crucial for many events in cellular physiology. Cell surface stimuli lead to the production of inositol 1,4,5-trisphosphate (IP3), which binds to IP3 receptors (IP3R) in the endoplasmic reticulum (ER) to release calcium pools from the ER lumen. This leads to the depletion of ER calcium pools, which has been termed store depletion. Store depletion leads to the dissociation of calcium ions from the EF-hand motif of the ER calcium sensor Stromal Interaction Molecule 1 (STIM1). This leads to a conformational change in STIM1, which helps it to interact with the plasma membrane (PM) at ER:PM junctions. At these ER:PM junctions, STIM1 binds to and activates a calcium channel known as Orai1 to form calcium release-activated calcium (CRAC) channels. Activation of Orai1 leads to calcium influx, known as store-operated calcium entry (SOCE). In addition to Orai1 and STIM1, the homologs of Orai1 and STIM1, such as Orai2/3 and STIM2, also play a crucial role in calcium homeostasis. The influx of calcium through the Orai channel activates a calcium current that has been termed the CRAC current. CRAC channels form multimers and cluster together in large macromolecular assemblies termed "puncta". How CRAC channels form puncta has been contentious since their discovery. In this review, we will outline the history of SOCE, the molecular players involved in this process, as well as the models that have been proposed to explain this critical mechanism in cellular physiology.

Regulatory mechanisms controlling store-operated calcium entry

Calcium influx through plasma membrane ion channels is crucial for many events in cellular physiology. Cell surface stimuli lead to the production of inositol 1,4,5-trisphosphate (IP3), which binds to IP3 receptors (IP3R) in the endoplasmic reticulum (ER) to release calcium pools from the ER lumen. This leads to the depletion of ER calcium pools, which has been termed store depletion. Store depletion leads to the dissociation of calcium ions from the EF-hand motif of the ER calcium sensor Stromal Interaction Molecule 1 (STIM1). This leads to a conformational change in STIM1, which helps it to interact with the plasma membrane (PM) at ER:PM junctions. At these ER:PM junctions, STIM1 binds to and activates a calcium channel known as Orai1 to form calcium-release activated calcium (CRAC) channels. Activation of Orai1 leads to calcium influx, known as store-operated calcium entry (SOCE). In addition to Orai1 and STIM1, the homologs of Orai1 and STIM1, such as Orai2/3 and STIM2, also play a crucial role in calcium homeostasis. The influx of calcium through the Orai channel activates a calcium current that has been termed the CRAC current. CRAC channels form multimers and cluster together in large macromolecular assemblies termed "puncta". How CRAC channels form puncta has been contentious since their discovery. In this review, we will outline the history of SOCE, the molecular players involved in this process, as well as the models that have been proposed to explain this critical mechanism in cellular physiology.

Regulation of T cell function by protein S-acylation

S-acylation, the reversible lipidation of free cysteine residues with long-chain fatty acids, is a highly dynamic post-translational protein modification that has recently emerged as an important regulator of the T cell function. The reversible nature of S-acylation sets this modification apart from other forms of protein lipidation and allows it to play a unique role in intracellular signal transduction. In recent years, a significant number of T cell proteins, including receptors, enzymes, ion channels, and adaptor proteins, were identified as S-acylated. It has been shown that S-acylation critically contributes to their function by regulating protein localization, stability and protein-protein interactions. Furthermore, it has been demonstrated that zDHHC protein acyltransferases, the family of enzymes mediating this modification, also play a prominent role in T cell activation and differentiation. In this review, we aim to highlight the diversity of proteins undergoing S-acylation in T cells, elucidate the mechanisms by which reversible lipidation can impact protein function, and introduce protein acyltransferases as a novel class of regulatory T cell proteins.

Dynamic S-acylation of the ER-resident protein stromal interaction molecule 1 (STIM1) is required for store-operated Ca2+ entry

Many cell surface stimuli cause calcium release from endoplasmic reticulum (ER) stores to regulate cellular physiology. Upon ER calcium store depletion, the ER-resident protein stromal interaction molecule 1 (STIM1) physically interacts with plasma membrane protein Orai1 to induce calcium release–activated calcium (CRAC) currents that conduct calcium influx from the extracellular milieu. Although the physiological relevance of this process is well established, the mechanism supporting the assembly of these proteins is incompletely understood. Earlier we demonstrated a previously unknown post-translational modification of Orai1 with long-chain fatty acids, known as S-acylation. We found that S-acylation of Orai1 is dynamically regulated in a stimulus-dependent manner and essential for its function as a calcium channel. Here using the acyl resin–assisted capture assay, we show that STIM1 is also rapidly S-acylated at cysteine 437 upon ER calcium store depletion. Using a combination of live cell imaging and electrophysiology approaches with a mutant STIM1 protein, which could not be S-acylated, we determined that the S-acylation of STIM1 is required for the assembly of STIM1 into puncta with Orai1 and full CRAC channel function. Together with the S-acylation of Orai1, our data suggest that stimulus-dependent S-acylation of CRAC channel components Orai1 and STIM1 is a critical mechanism facilitating the CRAC channel assembly and function.

Simvastatin modulates estrogen signaling in uterine leiomyoma via regulating receptor palmitoylation, trafficking and degradation

Uterine leiomyomas or fibroids are the most common tumors of the female reproductive tract. Estrogen (E2), a steroid-derived hormone, and its receptors (ERs), particularly ER-α, are important drivers for the development and growth of leiomyomas. We previously demonstrated that simvastatin, a drug used for hyperlipidemia, also possesses anti-leiomyoma properties. The aim of this work is to investigate the impact of simvastatin on ER-α signaling in leiomyoma cells, including its expression, downstream signaling, transcriptional activity, post-translational modification, trafficking and degradation. Primary and immortalized human uterine leiomyoma (HuLM) cells were used for in vitro experiments. Immunodeficient mice xenografted with human leiomyoma tissue explants were used for in vivo studies. Leiomyoma samples were obtained from patients enrolled in an ongoing double-blinded, phase II, randomized controlled trial. Here, we found that simvastatin significantly reduced E2-induced proliferation and PCNA expression. In addition, simvastatin reduced total ER-α expression in leiomyoma cells and altered its subcellular localization by inhibiting its trafficking to the plasma membrane and nucleus. Simvastatin also inhibited E2 downstream signaling, including ERK and AKT pathways, E2/ER transcriptional activity and E2-responsive genes. To explain simvastatin effects on ER-α level and trafficking, we examined its effects on ER-α post-translational processing. We noticed that simvastatin reduced ER-α palmitoylation; a required modification for its stability, trafficking to plasma membrane, and signaling. We also observed an increase in ubiquitin-mediated ER-α degradation. Importantly, we found that the effects of simvastatin on ER-α expression were recapitulated in the xenograft leiomyoma mouse model and human tissues. Thus, our data suggest that simvastatin modulates several E2/ER signaling targets with potential implications in leiomyoma therapy and beyond.

Fatty acid mobilization from adipose tissue is mediated by CD36 post-translational modifications and intracellular trafficking

The mechanism controlling long-chain fatty acid (LCFA) mobilization from adipose tissue is not well understood. Here, we investigated how the LCFA transporter CD36 regulates this process. By using tissue-specific KO mouse models, we showed that CD36 in adipocytes and endothelial cells mediated both LCFA deposition into and release from adipose tissue. We demonstrated the role of adipocytic and endothelial CD36 in promoting tumor growth and chemoresistance conferred by adipose tissue–derived LCFAs. We showed that dynamic cysteine S-acylation of CD36 in adipocytes, endothelial cells, and cancer cells mediated intercellular LCFA transport. We demonstrated that lipolysis induction in adipocytes triggered CD36 deacylation and deglycosylation, as well as its dissociation from interacting proteins, prohibitin-1 (PHB) and annexin 2 (ANX2). Our data indicate that lipolysis triggers caveolar endocytosis and translocation of CD36 from the cell membrane to lipid droplets. This study suggests a mechanism for both outside-in and inside-out cellular LCFA transport regulated by CD36 S-acylation and its interactions with PHB and ANX2.

S-acylation of Orai1 regulates store-operated Ca2+ entry

Store-operated Ca2+ entry is a central component of intracellular Ca2+ signaling pathways. The Ca2+ release-activated channel (CRAC) mediates store-operated Ca2+ entry in many different cell types. The CRAC channel is composed of the plasma membrane (PM)-localized Orai1 channel and endoplasmic reticulum (ER)-localized STIM1 Ca2+ sensor. Upon ER Ca2+ store depletion, Orai1 and STIM1 form complexes at ER-PM junctions, leading to the formation of activated CRAC channels. Although the importance of CRAC channels is well described, the underlying mechanisms that regulate the recruitment of Orai1 to ER-PM junctions are not fully understood. Here, we describe the rapid and transient S-acylation of Orai1. Using biochemical approaches, we show that Orai1 is rapidly S-acylated at cysteine 143 upon ER Ca2+ store depletion. Importantly, S-acylation of cysteine 143 is required for Orai1-mediated Ca2+ entry and recruitment to STIM1 puncta. We conclude that store depletion-induced S-acylation of Orai1 is necessary for recruitment to ER-PM junctions, subsequent binding to STIM1 and channel activation.

Ca2+-dependent protein acyltransferase DHHC21 controls activation of CD4+ T cells

Despite the recognized significance of reversible protein lipidation (S-acylation) for T cell receptor signal transduction, the enzymatic control of this post-translational modification in T cells remains poorly understood. Here, we demonstrate that DHHC21 (also known as ZDHHC21), a member of the DHHC family of mammalian protein acyltransferases, mediates T cell receptor-induced S-acylation of proximal T cell signaling proteins. Using Zdhhc21dep mice, which express a functionally deficient version of DHHC21, we show that DHHC21 is a Ca2+/calmodulin-dependent enzyme critical for activation of naïve CD4+ T cells in response to T cell receptor stimulation. We find that disruption of the Ca2+/calmodulin-binding domain of DHHC21 does not affect thymic T cell development but prevents differentiation of peripheral CD4+ T cells into Th1, Th2 and Th17 effector T helper lineages. Our findings identify DHHC21 as an essential component of the T cell receptor signaling machinery and define a new role for protein acyltransferases in regulation of T cell-mediated immunity.

T cell receptor-dependent S-acylation of ZAP-70 controls activation of T cells

ZAP-70 is a tyrosine kinase essential for T cell immune responses. Upon engagement of the T cell receptor (TCR), ZAP-70 is recruited to the specialized plasma membrane domains, becomes activated, and is released to phosphorylate its laterally segregated targets. A shift in ZAP-70 distribution at the plasma membrane is recognized as a critical step in TCR signal transduction and amplification. However, the molecular mechanism supporting stimulation-dependent plasma membrane compartmentalization of ZAP-70 remains poorly understood. In this study, we identified previously uncharacterized lipidation (S-acylation) of ZAP-70 using Acyl-Biotin Exchange assay, a technique that selectively captures S-acylated proteins. We found that this posttranslational modification of ZAP-70 is dispensable for its enzymatic activity. However, the lipidation-deficient mutant of ZAP-70 failed to propagate the TCR pathway suggesting that S-acylation is essential for ZAP-70 interaction with its protein substrates. The kinetics of ZAP-70 S-acylation were consistent with TCR signaling events indicating that agonist-induced S-acylation is a part of the signaling mechanism controlling T cell activation and function. Taken together, our results suggest that TCR-induced S-acylation of ZAP-70 can serve as a critical regulator of T cell-mediated immunity.

Regulation of T cell receptor signaling by protein acyltransferase DHHC21

S-acylation reversible-post-translational lipidation of cysteine residues-is emerging as an important regulatory mechanism in T cell signaling. Dynamic S-acylation is critical for protein recruitment into the T cell receptor complex and initiation of the subsequent signaling cascade. However, the enzymatic control of protein S-acylation in T cells remains poorly understood. Here, we report a previously uncharacterized role of DHHC21, a member of the mammalian family of DHHC protein acyltransferases, in regulation of the T cell receptor pathway. We found that loss of DHHC21 prevented S-acylation of key T cell signaling proteins, resulting in disruption of the early signaling events and suppressed expression of T cell activation markers. Furthermore, downregulation of DHHC21 prevented activation and differentiation of naïve T cells into effector subtypes. Together, our study provides the first direct evidence that DHHC protein acyltransferases can play an essential role in regulation of T cell-mediated immunity.

Detection of Protein S-Acylation using Acyl-Resin Assisted Capture

Protein S-acylation, also referred to as S-palmitoylation, is a reversible post-translational modification of cysteine residues with long-chain fatty acids via a labile thioester bond. S-acylation, which is emerging as a widespread regulatory mechanism, can modulate almost all aspects of the biological activity of proteins, from complex formation to protein trafficking and protein stability. The recent progress in understanding of the biological function of protein S-acylation was achieved largely due to the development of novel biochemical tools allowing robust and sensitive detection of protein S-acylation in a variety of biological samples. Here, we describe acyl resin-assisted capture (Acyl-RAC), a recently developed method based on selective capture of endogenously S-acylated proteins by thiol-reactive Sepharose beads. Compared to existing approaches, Acyl-RAC requires fewer steps and can yield more reliable results when coupled with mass spectrometry for identification of novel S-acylation targets. A major limitation in this technique is the lack of ability to discriminate between fatty acid species attached to cysteines via the same thioester bond.

Aggregated and Hyperstable Damage-Associated Molecular Patterns Are Released During ER Stress to Modulate Immune Function

Chronic ER stress occurs when protein misfolding in the Endoplasmic reticulum (ER) lumen remains unresolved despite activation of the unfolded protein response. We have shown that traumatic injury such as a severe burn leads to chronic ER stress in vivo leading to systemic inflammation which can last for more than a year. The mechanisms linking chronic ER stress to systemic inflammatory responses are not clear. Here we show that induction of chronic ER stress leads to the release of known and novel damage-associated molecular patterns (DAMPs). The secreted DAMPs are aggregated and markedly protease resistant. ER stress-derived DAMPs activate dendritic cells (DCs) which are then capable of polarizing naïve T cells. Our findings indicate that induction of chronic ER stress may lead to the release of hyperstable DAMPs into the circulation resulting in persistent systemic inflammation and adverse outcomes.

DHHC5 Mediates β-Adrenergic Signaling in Cardiomyocytes by Targeting Gα Proteins

S-palmitoylation is a reversible posttranslational modification that plays an important role in regulating protein localization, trafficking, and stability. Recent studies have shown that some proteins undergo extremely rapid palmitoylation/depalmitoylation cycles after cellular stimulation supporting a direct signaling role for this posttranslational modification. Here, we investigated whether β-adrenergic stimulation of cardiomyocytes led to stimulus-dependent palmitoylation of downstream signaling proteins. We found that β-adrenergic stimulation led to rapidly increased Gαs and Gαi palmitoylation. The kinetics of palmitoylation was temporally consistent with the downstream production of cAMP and contractile responses. We identified the plasma membrane-localized palmitoyl acyltransferase DHHC5 as an important mediator of the stimulus-dependent palmitoylation in cardiomyocytes. Knockdown of DHHC5 showed that this enzyme is necessary for palmitoylation of Gαs, Gαi, and functional responses downstream of β-adrenergic stimulation. A palmitoylation assay with purified components revealed that Gαs and Gαi are direct substrates of DHHC5. Finally, we provided evidence that the C-terminal tail of DHHC5 can be palmitoylated in response to stimulation and such modification is important for its dynamic localization and function in the plasma membrane. Our results reveal that DHHC5 is a central regulator of signaling downstream of β-adrenergic receptors in cardiomyocytes.

Monitoring dynamic changes in mitochondrial calcium levels during apoptosis using a genetically encoded calcium sensor

Dynamic changes in intracellular calcium concentration in response to various stimuli regulates many cellular processes such as proliferation, differentiation, and apoptosis¹. During apoptosis, calcium accumulation in mitochondria promotes the release of pro-apoptotic factors from the mitochondria into the cytosol². It is therefore of interest to directly measure mitochondrial calcium in living cells in situ during apoptosis. High-resolution fluorescent imaging of cells loaded with dual-excitation ratiometric and non-ratiometric synthetic calcium indicator dyes has been proven to be a reliable and versatile tool to study various aspects of intracellular calcium signaling. Measuring cytosolic calcium fluxes using these techniques is relatively straightforward. However, measuring intramitochondrial calcium levels in intact cells using synthetic calcium indicators such as rhod-2 and rhod-FF is more challenging. Synthetic indicators targeted to mitochondria have blunted responses to repetitive increases in mitochondrial calcium, and disrupt mitochondrial morphology³. Additionally, synthetic indicators tend to leak out of mitochondria over several hours which makes them unsuitable for long-term experiments. Thus, genetically encoded calcium indicators based upon green fluorescent protein (GFP)⁴ or aequorin⁵ targeted to mitochondria have greatly facilitated measurement of mitochondrial calcium dynamics. Here, we describe a simple method for real-time measurement of mitochondrial calcium fluxes in response to different stimuli. The method is based on fluorescence microscopy of 'ratiometric-pericam' which is selectively targeted to mitochondria. Ratiometric pericam is a calcium indicator based on a fusion of circularly permuted yellow fluorescent protein and calmodulin⁴. Binding of calcium to ratiometric pericam causes a shift of its excitation peak from 415 nm to 494 nm, while the emission spectrum, which peaks around 515 nm, remains unchanged. Ratiometric pericam binds a single calcium ion with a dissociation constant in vitro of ~1.7 μM⁴. These properties of ratiometric pericam allow the quantification of rapid and long-term changes in mitochondrial calcium concentration. Furthermore, we describe adaptation of this methodology to a standard wide-field calcium imaging microscope with commonly available filter sets. Using two distinct agonists, the purinergic agonist ATP and apoptosis-inducing drug staurosporine, we demonstrate that this method is appropriate for monitoring changes in mitochondrial calcium concentration with a temporal resolution of seconds to hours. Furthermore, we also demonstrate that ratiometric pericam is also useful for measuring mitochondrial fission/fragmentation during apoptosis. Thus, ratiometric pericam is particularly well suited for continuous long-term measurement of mitochondrial calcium dynamics during apoptosis.

Chromatin remodeling of interleukin-17 (IL-17)-IL-17F cytokine gene locus during inflammatory helper T cell differentiation

During differentiation of naive CD4+ helper T (TH) cells into effector cells, specific cytokine gene loci undergo extensive changes in chromatin modification. A novel lineage of TH cells that is regulated by transforming growth factor-beta (TGFbeta) and interleukin-6 (IL-6) has been identified recently as promoting tissue inflammation. These inflammatory TH (THi) cells, also called TH17 or TH(IL-17), produce IL-17 and IL-17F, two highly homologous cytokines that have genes located in the same chromosomal region. Here, using chromatin immunoprecipitation techniques, we have demonstrated that similar to the regulation in TH1 and TH2 cell lineages, polarization of THi cells was accompanied by selective chromatin remodeling events. Histone H3 acetylation and Lys-4 tri-methylation were specifically associated with IL-17 and IL-17F gene promoters in THi lineage. At an early stage of T cell activation, histone acetylation on these promoters was greatly promoted by a combination of TGFbeta and IL-6, suggesting their synergistic role in initiating chromatin accessibility for transcription factors. Furthermore, we identified multiple noncoding sequences within the IL-17-IL-17F locus conserved across species. These elements were also associated with hyperacetylated histone 3 in a lineage-specific manner and may thus serve as potential regulatory regions. In summary, our results demonstrate for the first time that THi cell differentiation is associated with epigenetic changes in the IL-17-IL-17F locus, which suggests novel mechanisms in T cell functional regulation.

Human breast milk immunoglobulins G hydrolyze nucleotides

Catalytically active antibodies, abzymes, appear in the blood of mammals immunized with the analogs of transition state or in the case of autoimmune diseases. Until recently, it was not shown whether abzymes exist in the blood of apparently healthy subjects. We have discovered that secretory IgA (sIgA) from the milk of healthy mothers catalyze phosphorylation of a variety of proteins and that IgG can hydrolyze DNA and RNA. In this study for the first time it is shown that IgG from human milk (and their Fab-fragments) also catalyze hydrolysis of nucleoside mono-, di-, and triphosphates. The data meet certain stringent criteria, unambiguously indicating that the observed catalytic activity is associated with IgG rather than contaminating enzymes. Although the nucleotide-binding site of IgG is located in the light antibody chain, L-chains per se cannot hydrolyze NTP unlike the DNA-hydrolyzing abzymes. Kinetic and thermodynamic parameters that characterize the interaction of NTP and dNTP with IgG-abzymes were analyzed. Possible reasons for appearance of polyclonal abzymes with different catalytic activities in the milk of healthy mothers are considered.

Interaction of human milk lactoferrin with ATP

Human lactoferrin exhibits many unique properties. It is known as one of the most important factors that provide nonspecific defense of cells against bacteria, viruses, and carcinogenesis, as well as an important component of a specific system responsible for the passive immunity of newborns. As a compound with extremely broad spectrum of functions many of which were not elucidated so far, lactoferrin is intensely studied. In this study we obtained electrophoretically and immunologically homogenous preparations of lactoferrin from human milk. Using various methods, we were the first to show that the fraction of lactoferrin, which displays an increased affinity for Sepharose Blue, forms complexes with ATP with a stoichiometry of 1 mole ATP per mole protein. It is shown that the ATP-binding site is located in the C-terminal domain of the lactoferrin molecule. The binding of ATP results in the dissociation of tetrameric forms of the protein and a change in the mode of interaction of lactoferrin with polysaccharides and other proteins. The data may be used in analysis of the possible reasons for multifunctional properties of lactoferrin and possible ways of regulation of its functions.